Pair your accounts.

Export articles to Mendeley

Get article recommendations from ACS based on references in your Mendeley library.

Pair your accounts.

Export articles to Mendeley

Get article recommendations from ACS based on references in your Mendeley library.

You’ve supercharged your research process with ACS and Mendeley!

STEP 1:
Click to create an ACS ID

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

MENDELEY PAIRING EXPIRED
Your Mendeley pairing has expired. Please reconnect
ACS Publications. Most Trusted. Most Cited. Most Read
Fast Twist Angle Mapping of Bilayer Graphene Using Spectroscopic Ellipsometric Contrast Microscopy
More

OR SEARCH CITATIONS

My Activity
Publications

Figure 1Loading Img
Download Hi-Res ImageDownload to MS-PowerPointCite This:Nano Lett. 2023, 23, 12, 5506-5513
  • Open Access
Letter

Fast Twist Angle Mapping of Bilayer Graphene Using Spectroscopic Ellipsometric Contrast Microscopy
Click to copy article linkArticle link copied!

Open PDFSupporting Information (1)

Nano Letters

Cite this: Nano Lett. 2023, 23, 12, 5506–5513
Click to copy citationCitation copied!
https://doi.org/10.1021/acs.nanolett.3c00619
Published June 8, 2023

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY 4.0 .

Abstract

Click to copy section linkSection link copied!
High Resolution Image
Download MS PowerPoint Slide

Twisted bilayer graphene provides an ideal solid-state model to explore correlated material properties and opportunities for a variety of optoelectronic applications, but reliable, fast characterization of the twist angle remains a challenge. Here we introduce spectroscopic ellipsometric contrast microscopy (SECM) as a tool for mapping twist angle disorder in optically resonant twisted bilayer graphene. We optimize the ellipsometric angles to enhance the image contrast based on measured and calculated reflection coefficients of incident light. The optical resonances associated with van Hove singularities correlate well to Raman and angle-resolved photoelectron emission spectroscopy, confirming the accuracy of SECM. The results highlight the advantages of SECM, which proves to be a fast, nondestructive method for characterization of twisted bilayer graphene over large areas, unlocking process, material, and device screening and cross-correlative measurement potential for bilayer and multilayer materials.

This publication is licensed under

CC-BY 4.0 .
  • cc licence
  • by licence
Copyright © 2023 The Authors. Published by American Chemical Society

Subjects

what are subjects

Keywords

what are keywords

The band structure and thus electronic properties of twisted bilayer graphene can be tuned by the relative orientation or twist angle θt between the two layers. This results in novel properties such as topological transport, (1) enhanced photocurrent, (2) and correlated insulating phases (3,4) at particular twist angles. Higher energy interlayer interactions can be observed such as the formation of van Hove singularities (vHs) (2,5−9) in the electronic density of states (DOS) due to the superposition of bands from each of the two graphene layers. Optical resonances associated with transitions resonant with vHs occur with a twist angle-dependent energy separation continuously tunable from infrared (IR) to ultraviolet. (9) This enhanced absorption has been demonstrated to be beneficial to the performance of wavelength-selective photodetectors, (2,10,11) motivating the development of optically resonant twisted bilayer graphene for optoelectronic devices.

Twisted bilayer graphene can be created directly during growth, (12,13) by stacking graphene monolayers (14−16) or by folding them using atomic force microscope (AFM) tips. (17) Any contamination between layers during processing reduces the interlayer coupling which may destroy angle-dependent phenomena. (5) Atomically clean interlayer interfaces can be found in as-grown chemical vapor deposition (CVD) (18,19) bilayer graphene. (20) Significant effort has been made to tailor CVD growth parameters to selectively obtain bilayer graphene, (21) which is typically hindered by screening of carbon precursors from the growth catalyst by the primary graphene monolayer. A further challenge, specific to direct growth of twisted bilayer graphene, (12) is avoiding formation of the energetically favorable AB stacked and 30° rotated configurations which often make up the majority of CVD grown bilayer and multilayer graphene. (20,22)

Identifying bilayer graphene with a particular twist angle is challenging, particularly substrate-agnostic, fast, large-area mapping. Angle-dependent characterization of twisted bilayer graphene has been demonstrated through optical absorption (6,23) and reflection, (24−26) photoemission, (2,12,27−29) photoluminescence, (7,30) and Raman (5,13,31,32) spectroscopies, which are often correlated with higher resolution electron microscopy (20,25) or scanning probe microscopy. (33−36) Many of these techniques rely on specific substrate properties, such as transparent (6) or contrast-enhancing, (24,37) which may be incompatible with characterization during particular stages of manufacturing. (38) Spectroscopic imaging ellipsometry has emerged as a tool to determine the optical constants of graphene (39) and other two-dimensional materials (40) and provide thickness information with single-atomic layer precision (41,42) with a lateral resolution down to ∼1 μm on a wide range of substrates, including as-grown directly on metal catalyst foils. (43) Spectroscopic ellipsometry measures the change in polarized light upon reflection at a sample to determine the wavelength-, λ, dependent complex dielectric function (or refractive index n(λ) and extinction parameter k(λ) values) which provides insight into fundamental light–matter interactions crucial for understanding a range of optical phenomena. (44) To extract the full complex dielectric function or layer thicknesses of a given sample the ratio of perpendicular p and orthogonal s components of the reflected light represented by ρ = Rp/Rs = tan(ψ)eiΔ, where ψ is the amplitude ratio and Δ is the phase difference, is measured as a function of λ, angle of incidence (AOI), and polarizer (P), analyzer (A), and compensator (C) angles, and fitted to an optical model. (44)

Alternatively, ellipsometer settings (AOI, P, A, C) can be optimized for material and thickness contrast and fixed during imaging. This technique, often termed ellipsometric contrast microscopy (ECM), has been used across a variety of fields (43,45,46) and only images the intensity of reflected light, significantly improving characterization throughput. (43) Building on this technique, here we perform spectral ECM (SECM) of the chemical vapor deposition (CVD) grown monolayer, bilayer, and multilayer graphene transferred onto Si/SiO2. We demonstrate that, in addition to layer-number sensitivity and high material contrast, SECM provides wavelength-dependent contrast of optically resonant bilayer and multilayer regions allowing us to extract a map of the twist angle variation. The range of twist angles detectable is set by the spectral range of the image sensor. We validate the technique by correlating SECM data with Raman mapping and angle-resolved photoelectron emission spectroscopy (ARPES).

Figure 1a shows an optical micrograph of two merged bilayer and multilayer islands. Several bilayer grains within a 1 cm2 transferred graphene film showed colored regions in optical microscopy which typically appear as approximately radial sections in one or more “petals” within the bilayer graphene “flowers”. The region shown in Figure 1a was selected for further study as it showed blue, green, yellow, and red hues in close proximity to one another. Such colored regions are associated with enhanced absorption consistent with vHs-resonant transitions, (9) as schematically shown in Figure 1b. SECM reveals that a rich variety of twist angles can be observed in this region as described below.

Figure 1

Figure 1. (a) Optical microscopy image of a graphene flake on the Si/SiO2 substrate. Dashed square represents the region studied in SECM and Raman spectroscopy. (b) Schematic showing the Brillouin zone and twist angle θt, band structure for twisted bilayer graphene, and density of states of twisted bilayer graphene at different twist angles with the horizontal lines corresponding to the highest and lowest energy transitions for the wavelength range studied. Fitting of Δ (c) and ψ (d) of the monolayer (ML), bilayer (BL) and resonant bilayer (BLVHS) to the Si/SiO2/graphene (+ Gaussian) layer stack model. Imaging RCE mapping of Δ (e) and ψ (f) at λ = 440 nm. (g) Graphene thickness map of the flake at λ = 440 nm as fitted using a Si/SiO2/graphene model, without accounting for resonances.

High Resolution Image
Download MS PowerPoint Slide

To extract layer thickness for the graphene flake we perform imaging ellipsometry using a rotating compensator ellipsometry (RCE) method to measure ψ and Δ as a function of λ. We used the imaging RCE mode from 400 to 650 nm over 10 nm intervals, where a spatial map of Δ and ψ is measured for each wavelength interval, yielding 25 maps for both Δ and ψ. P and A were both fixed at 45°, with AOI at 50°. Examples of Δ and ψ maps at λ = 440 nm are shown in Figure 1e,f, respectively. We then performed map analysis to extract data averaged over the predefined region of interest for maps at all wavelengths within the range. We chose three different regions based on the optical microscope image in Figure 1a: monolayer (ML), bilayer (BL), and resonant bilayer (BLVHS), as labeled in Figure 1g. Figure 1c,d show the measured Δ and ψ, respectively, as a function of wavelength for the three regions of interest. We fitted the wavelength dependence of Δ and ψ for ML and BL with the Si/SiO2/graphene model described in the Methods section (see Supporting Information). To account for the presence of vHs, a Gaussian resonance term was added to the graphene model when fitting to the data extracted from BLVHS. The thickness of graphene tGr was found to be 0.57 ± 0.1 nm for monolayer and 0.75 ± 0.1 nm for bilayer, as detailed in Table S1 (see Supporting Information). Despite these values deviating from the expected monolayer thickness of 0.335 nm, (47,48) these values fall within the reported thickness deviations for copper foil-transferred graphene as measured with AFM (49−51) and ellipsometry, (41,52) likely due to polymer contamination or chemical interactions between graphene and the substrate. (49,53) It can be seen in both Figure 1c,d that the resonant bilayer region curve deviates from the monolayer and bilayer regions. For the resonant bilayer we observe an additional peak in both Δ and ψ curves centered at approximately 440 nm. Spatial maps of Δ and ψ (Figure 1e,f) show that the increased values at λ = 440 nm are consistent across a resonant bilayer region. Figure 1g shows a spatial map of the fitted graphene thickness determined from Δ and ψ values for a single wavelength (λ = 440 nm) where each pixel of the map was fitted with a Si/SiO2/graphene layer stack, i.e., without an additional Gaussian term, where the only free parameter was tGr. We see that under resonant illumination the extracted values of tGr for the region BLVHS, fitted using the simple graphene model, deviates from the adjacent bilayer regions, showing a higher apparent thickness.

To find the maximum contrast between monolayer graphene and resonant bilayer graphene on Si/SiO2 for ECM, we evaluate the choice of ellipsometer parameters, as shown in Figure 2. It has been shown that ellipsometric angles for optimized contrast rely heavily on the number of graphene layers, as well as the choice of substrate and range of wavelengths. (43) Figure 2a shows examples of ellipsometric contrast images as a function of AOI centered on another optically resonant bilayer region under λ = 480 nm, for P = A = C = 0°. We observe the strongest contrast for the resonant bilayer at AOI = 40°. We calculate the measured image contrast between the monolayer and bilayer regions using the Weber relation where contrast = I(BL)−I(ML)I(ML), and I(ML) and I(BL) are the average pixel intensity values of the monolayer background and the BL or BLVHS, respectively. We theoretically model this by calculating the reflection coefficient Rp (Figure 2b) as a function of AOI using the model parameters determined from the fitting results described above (Table S1), where Rp = 1 corresponds to 100% reflected intensity. For the resonant bilayer region, the measured image contrast follows the theoretically expected contrast (Rp(BL)−Rp(ML)Rp(ML)). For the nonresonant bilayer graphene region, the contrast is underestimated by the model, as the model does not account for nonidealities such as contamination (which increases the thickness of the layers). To further investigate the choice of optimum contrast for observing resonant bilayer graphene, we measure contrast at AOI = 40° and as a function of P and A for the resonant bilayer (λ = 480 nm) at two different C angles, C = 0° and C = 45°. The measured contrast for the resonant bilayer is shown in Figure 2c. It can be seen that the maximum contrast can be achieved when the polarization of light is parallel to the surface plane, with P = 90°, A = 90°, and C = 0°. We use these optimized parameters for the wavelength-dependent mapping described below.

Figure 2

Figure 2. Optimising ellipsometer parameters for bilayer graphene on Si/SiO2. (a) ECM images of graphene flake at different angles of incidence (AOI). (b) Rp coefficient as a function of AOI with calculated Weber contrast. (c) Normalized intensity as measured for P and A rotation on Si/SiO2/graphene at C = 0° (left) and C = 45° (right).

High Resolution Image
Download MS PowerPoint Slide

Figure 3a shows the Rp and Rs reflection coefficients as a function of wavelength for monolayer (ML), bilayer (BL), and resonant bilayer (BLVHS) regions as labeled in Figure 1g, with the corresponding Weber contrast calculated for bilayer and resonant bilayer regions with a background of monolayer graphene. The Rp and Rs coefficients were calculated using the layer-stack model parameters described above with AOI = 40°, with an additional Gaussian resonance centered at the corresponding wavelength of 440 nm applied to resonant bilayer region data. There is deviation noticeable in the reflection coefficient for the resonant bilayer region as compared to monolayer and bilayer regions. This results in a predicted enhancement of the contrast on resonance for BLVHS. We perform SECM on the bilayer graphene regions shown in Figure 1a. Under fixed ellipsometer angles we image the reflected intensity as a function of wavelength varied from 350 to 750 nm. To focus specifically on bilayer graphene, we implement a mask based on average pixel intensity to remove data points corresponding to areas of monolayer, or multilayer (≥3 layers), graphene, as indicated in Figure S1 (see Supporting Information). The spatial distribution of the resonant wavelength within the bilayer regions is shown in Figure 3b, with annotations for corresponding spectra at these regions shown in Figure 3c. We use EVHS=8πℏvF3asin⁡θt2 (54) to determine the twist angle from the resonant wavelength (Figure 4a, inset), with Fermi velocity vF = 1 × 106 m s–1, ℏ the reduced Planck’s constant, and a = 2.46 Å the graphene lattice constant.

Figure 3

Figure 3. (a) Rp and Rs reflection coefficients as a function of wavelength for monolayer (ML), bilayer (BL), and resonant bilayer (BLVHS) regions as labeled in Figure 1g with calculated Weber contrast above each plot. (b) Resonant wavelength map of graphene extracted by fitting a Gaussian peak to the intensity as a function of wavelength at each pixel measured in SECM. (c) Reflected intensity from SECM as a function of wavelength showing absorption resonances for the regions marked in (b) (i–iii). (d) Absorption peak as a function of twist angle.

High Resolution Image
Download MS PowerPoint Slide

Figure 4

Figure 4. (a) Raman spectra of graphene at different twist angles, normalized with respect to 2D peak intensity. (b) Raman 2D-peak width map of graphene. (c) Raman 2D-peak width of graphene as a function of bilayer twist angle. (d) Raman G peak amplitude map of graphene. (e) Raman G-peak amplitude as a function of bilayer twist angle. The error bars on the plots represent the standard deviation.

High Resolution Image
Download MS PowerPoint Slide

To validate our methodology, we correlate the twist angles determined from SECM with Raman spectra. The individual Raman spectra for different twist angle bilayer regions, compared to a reference monolayer region, are shown in Figure 4a. The Raman spectra show G and 2D peaks at ∼1580 cm–1 and ∼2700 cm–1, respectively, and their shapes and positions, as well as intensity ratios that vary with the number of graphene layers and twist angle. (55) The spectra correlate well with spectra from Figure 3c, which indicates that SECM data can aid the interpretation of Raman spectra. We also show a map of the graphene flake showing 2D width and G amplitude intensity (Figure 4b,d, respectively). Using image registration with the SECM data we apply the same mask to the Raman data to plot the resonant bilayer regions identified above. The widths of the 2D peaks decrease with larger bilayer twist angles, from 64 cm–1 for small twist angles to 30 cm–1 for larger twist angles, as also shown in Figure 4c. Similarly, we observe a variation in G peak intensity as a function of graphene bilayer twist angle, with a significant enhancement in G peak intensity (5) for bilayer graphene with twist angle close to 12° (Figure 4e). This corresponds to resonant absorption at 540 nm ±20 nm, which correlates well with resonant absorption from the Raman excitation wavelength (λ = 532 nm). The regions showing G peak resonance, shown in Figure 4d, were found to spatially correlate with the regions that show resonant contrast enhancement in ECM under similar wavelengths. The x (y) error bars in Figure 4c,e correspond to the standard deviation of the twist angle (Raman feature) measured within a particular spatially localized region.

The twist angle variation in bilayer graphene is often attributed to formation during graphene growth (12) or at wrinkles or folds within bilayer graphene films, which are one-dimensional defects that form as a result of mismatch in thermal expansion coefficients of graphene and the substrate. (56−58) To investigate the origins of twist angle domains, we perform AFM and IR scanning near-field microscopy (SNOM) on a region showing changes in stacking between resonant bilayer regions, and which showed lines of dark contrast in the optical microscopy indicating the presence of folded graphene (Figure 1a).

Figure 5a shows the AFM map of the studied graphene flake region, with the bright lines indicating the folded bilayer graphene. We examine the AFM height profile across one of the folds as indicated in Figure 5a and find the thickness at the center of the wrinkle of 1.2 nm, indicating 4 additional layers of graphene at that region (Figure 5b). The folds in the graphene flake can also be seen in the IR SNOM (59) as shown in the map of the second harmonic amplitude (s2) (Figure 5c), indicating enhanced scattering at these folds. The features in the SNOM amplitude at these twist angles (∼10°–12°) are colocated with topographic features observed in the AFM, such as folds and wrinkles.

Figure 5

Figure 5. (a) AFM map of the graphene flake on Si/SiO2 substrate indicating the region marked in (c). (b) AFM profile of graphene across the wrinkle in the AFM map in (a), indicating four layers of graphene. (c) Map of the second harmonic (s2) IR SNOM signal and the corresponding resonant energy map of the graphene flake showing different twist angle domains of graphene with annotations of the studied areas. (d, f) ARPES data obtained in the regions-of-interest indicated by circles in (c). In the inset, momentum–momentum cuts are shown for energies close to the Fermi level. The energy-momentum cut is taken along the black line indicated in the insets. (e, g) The vHs in the valence band can be identified when the second derivative along the energy axis is applied to the data (energy position highlighted by red arrows). Spectrum along the ky-direction, as shown in the inset momentum map, showing the band structure of twisted bilayer graphene regions with Ec = 1.77 eV (d) and Ec = 2.70 eV (g) as determined by SECM. The corresponding integrated intensity spectra ((e) and (g), respectively) each show a resonant feature at ∼Ec/2 (arrow).

High Resolution Image
Download MS PowerPoint Slide

We performed ARPES experiments to quantify the electronic band structure of twisted bilayer graphene. This experiment is especially helpful to identify the energy position of the vHs in the valence band and thus to corroborate the resonant energies obtained from ellipsometry for various twist angles. To determine the energy–momentum dispersive band structure around the vHs of different bilayer graphene regions, we use time-of-flight momentum microscopy. (60–62) This specialized ARPES setup is capable of probing the band structure with a spatial resolution of down to 10 μm, which allows spatial mapping of the twist angle. (63) Figure 5d,f shows energy–momentum and momentum–momentum (inset) cuts through the ARPES data taken at the two regions of interest indicated by circles in Figure 5c. In both areas, we find that the bilayer graphene flake is minimally doped, i.e., the Fermi-level is positioned within 100 meV of the Dirac point. Moreover, we only detect photoelectrons originating from the top layer (i.e., six Dirac points in the momentum–momentum cuts).

To evaluate the energy–momentum dispersive band structure and, in particular, the energy position of the vHs of the valence band in more detail, we apply the second derivative along the energy axis to the energy-momentum maps (Figure 5e,g). This data handling directly identifies abrupt changes in the photoemission intensity that we attribute to the energetic position of the vHs (highlighted by arrows). For the two regions in Figure 5e,g, we find that the vHs are located at E – EF = −0.9 ± 0.1 and −1.4 ± 0.1 eV, respectively. Having identified the energetic position of the vHs and measured that the bilayer graphene samples are minimally doped, we can determine the energy difference between the vHs of the valence and the conduction band to 1.8 ± 0.2 eV and 2.8 ± 0.2 eV, respectively. These energies match the optical experiments shown in Figure 3b and thus fully support the application of SECM as a technique for the fast identification of the twist angle in bilayer graphene.

In this work we perform SECM and correlated characterization to study the angle dependence of optically resonant twisted bilayer graphene. We compare the SECM results to full wavelength-dependent Δ and ψ maps obtained through imaging RCE to confirm the thickness of the bilayer regions. The ellipsometer wavelength range allows us to find twist angles between 9° and 17° and provides a typical twist angle accuracy of <1°. The range of twist angles could be expanded by using a spectroscopic imaging ellipsometer with an extended wavelength range such as commercial systems capable of measuring wavelengths of 190 to 2700 nm which would correspond to θt ∼ 30° to ∼2°, respectively. The SECM findings agree well with ARPES and Raman characterization. While Raman measurements typically require a contrast-enhancing substrate, like the 90 nm SiO2-on-Si substrates as used in this work, ellipsometry is substrate agnostic, which means it is applicable to a wider range of materials, and could also be used to screen different stages of the manufacturing process. For example, high contrast is observed between monolayer and bilayer graphene─imaged as-grown on Cu or after transfer onto Si─regardless of substrate (Figure S3). This capability enables wafer-scale mapping and fast identification of bilayers even on substrates without contrast enhancement. The origin of some regions of twist-angle disorder is attributed to graphene folds as confirmed by AFM and SNOM, whereas other regions show twist angle variations away from any obvious folds. This indicates that a combination of processes during growth and postgrowth (e.g., during cooling or transfer) are responsible for the twist angle disorder. The presence of optical resonances is confirmed with ARPES by analyzing the band structure at regions with different resonant wavelengths. This technique could be applicable to other material systems, (42) including transition metal dichalcogneides (64) and twisted heterostuctures. (65) With the advent of spectroscopic microellipsometry extensions for conventional optical microscopes, SECM has become even more affordable and accessible. (66) This work confirms that SECM is a powerful, fast, and nondestructive tool for material characterization, which unlocks the potential of material properties for a variety of research applications.

Data Availability

Click to copy section linkSection link copied!

Datasets related to this publication are available from the Cambridge University data repository at https://doi.org/10.17863/CAM.97020.

Supporting Information

Click to copy section linkSection link copied!

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.3c00619.

  • Methods, fitted parameters, intensity thresholding, comparison of techniques, and bilayer graphene on Cu and Si (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

Click to copy section linkSection link copied!
  • Corresponding Authors
  • Authors
    • Teja Potočnik - Department of Engineering, University of Cambridge, 9 JJ Thompson Avenue, Cambridge CB3 0FA, United Kingdom
    • Oliver Burton - Department of Engineering, University of Cambridge, 9 JJ Thompson Avenue, Cambridge CB3 0FA, United Kingdom;  Orcidhttps://orcid.org/0000-0002-2060-1714
    • Marcel Reutzel - I. Physikalisches Institut, Georg-August-Universität Göttingen, 37077 Göttingen, Germany;  Orcidhttps://orcid.org/0000-0002-1085-2931
    • David Schmitt - I. Physikalisches Institut, Georg-August-Universität Göttingen, 37077 Göttingen, Germany
    • Jan Philipp Bange - I. Physikalisches Institut, Georg-August-Universität Göttingen, 37077 Göttingen, Germany
    • Stefan Mathias - I. Physikalisches Institut, Georg-August-Universität Göttingen, 37077 Göttingen, Germany;  Orcidhttps://orcid.org/0000-0002-1255-521X
    • Fabian R. Geisenhof - Physics of Nanosystems, Department of Physics, Ludwig-Maximilians-Universität München, Geschwister-Scholl-Platz 1, Munich 80539, Germany;  Orcidhttps://orcid.org/0000-0002-3623-1906
    • R. Thomas Weitz - I. Physikalisches Institut, Georg-August-Universität Göttingen, 37077 Göttingen, Germany;  Physics of Nanosystems, Department of Physics, Ludwig-Maximilians-Universität München, Geschwister-Scholl-Platz 1, Munich 80539, Germany;  Orcidhttps://orcid.org/0000-0001-5404-7355
    • Linyuan Xin - Department of Engineering, University of Cambridge, 9 JJ Thompson Avenue, Cambridge CB3 0FA, United Kingdom
    • Hannah J. Joyce - Department of Engineering, University of Cambridge, 9 JJ Thompson Avenue, Cambridge CB3 0FA, United Kingdom;  Orcidhttps://orcid.org/0000-0002-9737-680X
  • Author Contributions

    (T.P. and O.B.) These authors contributed equally.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

Click to copy section linkSection link copied!

J.A.A.-W. acknowledges the support of his Royal Society Dorothy Hodgkin Research Fellowship and the EPSRC (EP/V055003/1). M.R., D.S., J.P.B, R.T.W., and S.M. acknowledge funding through the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - 217133147/SFB 1073, Projects B07 and B10. S.H. acknowledges funding from EPSRC (EP/P005152/1, EP/T001038/1) and Cambridge-LMU Strategic Partnership. H.J.J. acknowledges the support of the ERC (Grant No. 716471, ACrossWire).

References

Click to copy section linkSection link copied!

This article references 66 other publications.

  1. 1
    Rickhaus, P.; Wallbank, J.; Slizovskiy, S.; Pisoni, R.; Overweg, H.; Lee, Y.; Eich, M.; Liu, M. H.; Watanabe, K.; Taniguchi, T. Transport Through a Network of Topological Channels in Twisted Bilayer Graphene. Nano Lett. 2018, 18 (11), 6725– 6730,  DOI: 10.1021/acs.nanolett.8b02387
    Google Scholar
    1
    Transport Through a Network of Topological Channels in Twisted Bilayer Graphene
    Rickhaus, Peter; Wallbank, John; Slizovskiy, Sergey; Pisoni, Riccardo; Overweg, Hiske; Lee, Yongjin; Eich, Marius; Liu, Ming-Hao; Watanabe, Kenji; Taniguchi, Takashi; Ihn, Thomas; Ensslin, Klaus
    Nano Letters (2018), 18 (11), 6725-6730CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors explore a network of electronic quantum valley Hall states in the moir´e crystal of minimally twisted bilayer graphene. In the authors' transport measurements, the authors observe Fabry-P´erot and Aharanov-Bohm oscillations that are robust in magnetic fields ranging from 0 to 8 T, which is in strong contrast to more conventional two-dimensional systems where trajectories in the bulk are bent by the Lorentz force. This persistence in magnetic field and the linear spacing in d. indicate that charge carriers in the bulk flow in topol. protected, 1-dimensional channels. With this work, the authors demonstrate coherent electronic transport in a lattice of topol. protected states.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvFWkt7vI&md5=4b046eb3006aa02958bd1c52c0703243
  2. 2
    Yin, J.; Wang, H.; Peng, H.; Tan, Z.; Liao, L.; Lin, L.; Sun, X.; Koh, A. L.; Chen, Y.; Peng, H. Selectively Enhanced Photocurrent Generation in Twisted Bilayer Graphene with van Hove Singularity. Nat. Commun. 2016, 7, 10699,  DOI: 10.1038/ncomms10699
    Google Scholar
    2
    Selectively enhanced photocurrent generation in twisted bilayer graphene with van Hove singularity
    Yin, Jianbo; Wang, Huan; Peng, Han; Tan, Zhenjun; Liao, Lei; Lin, Li; Sun, Xiao; Koh, Ai Leen; Chen, Yulin; Peng, Hailin; Liu, Zhongfan
    Nature Communications (2016), 7 (), 10699CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Graphene with ultra-high carrier mobility and ultra-short photoresponse time has shown remarkable potential in ultrafast photodetection. However, the broad and weak optical absorption (∼2.3%) of monolayer graphene hinders its practical application in photodetectors with high responsivity and selectivity. Here we demonstrate that twisted bilayer graphene, a stack of two graphene monolayers with an interlayer twist angle, exhibits a strong light-matter interaction and selectively enhanced photocurrent generation. Such enhancement is attributed to the emergence of unique twist-angle-dependent van Hove singularities, which are directly revealed by spatially resolved angle-resolved photoemission spectroscopy. When the energy interval between the van Hove singularities of the conduction and valance bands matches the energy of incident photons, the photocurrent generated can be significantly enhanced (up to ∼80 times with the integration of plasmonic structures in our devices). These results provide valuable insight for designing graphene photodetectors with enhanced sensitivity for variable wavelength.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XjslGntr4%253D&md5=e04571669b7fcd0ca9aa62ae2c739ed7
  3. 3
    Padhi, B.; Setty, C.; Phillips, P. W. Doped Twisted Bilayer Graphene near Magic Angles: Proximity to Wigner Crystallization, Not Mott Insulation. Nano Lett. 2018, 18 (10), 6175– 6180,  DOI: 10.1021/acs.nanolett.8b02033
    Google Scholar
    3
    Doped Twisted Bilayer Graphene near Magic Angles: Proximity to Wigner Crystallization, Not Mott Insulation
    Padhi, Bikash; Setty, Chandan; Phillips, Philip W.
    Nano Letters (2018), 18 (10), 6175-6180CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors devise a model to explain why twisted bilayer graphene exhibits insulating behavior when ν = 2 or 3 charges occupy a unit moir´e cell, a feature attributed to Mottness per previous work but not for ν = 1, clearly inconsistent with Mott insulation. The authors compute rs = EU/EK, where EU and EK are the potential and kinetic energies, resp., and show that (i) the Mott criterion lies at a d. larger than exptl. values by a factor of 104 and (ii) a transition to Wigner cryst. states exists as a function of ν. For ν = 1, rs fails to cross the threshold (rs = 37) for the triangular lattice, and metallic transport ensues. However, for ν = 2 and ν = 3, the thresholds rs = 22 and rs = 17, resp., are satisfied for a transition to Wigner crystals (WCs) with a honeycomb (ν = 2) and a kagome (ν = 3) structure. The authors posit that such cryst. states form the correct starting point for analyzing supercond.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhs1Khur3L&md5=6575b6c6cd34e2d94575362414e362cf
  4. 4
    Cao, Y.; Fatemi, V.; Demir, A.; Fang, S.; Tomarken, S. L.; Luo, J. Y.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Kaxiras, E. Correlated Insulator Behaviour at Half-Filling in Magic-Angle Graphene Superlattices. Nature 2018, 556 (7699), 80– 84,  DOI: 10.1038/nature26154
    Google Scholar
    4
    Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
    Cao, Yuan; Fatemi, Valla; Demir, Ahmet; Fang, Shiang; Tomarken, Spencer L.; Luo, Jason Y.; Sanchez-Yamagishi, Javier D.; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Ashoori, Ray C.; Jarillo-Herrero, Pablo
    Nature (London, United Kingdom) (2018), 556 (7699), 80-84CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials. One such property is the 'twist' angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analog in other types of heterostructure, such as semiconductors grown using mol. beam epitaxy. For small twist angles, the moire pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concd. mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride. Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show exptl. that, as predicted theor., when this angle is close to the 'magic' angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moire´ pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through elec. tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liqs.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXntVGjtr8%253D&md5=1697ebd09a673171514a527c17acf5b4
  5. 5
    Havener, R. W.; Zhuang, H.; Brown, L.; Hennig, R. G.; Park, J. Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene. Nano Lett. 2012, 12 (6), 3162– 3167,  DOI: 10.1021/nl301137k
    Google Scholar
    5
    Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene
    Havener, Robin W.; Zhuang, Houlong; Brown, Lola; Hennig, Richard G.; Park, Jiwoong
    Nano Letters (2012), 12 (6), 3162-3167CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Few-layer graphene is a prototypical layered material, whose properties are detd. by the relative orientations and interactions between layers. Exciting elec. and optical phenomena were obsd. for the special case of Bernal-stacked few-layer graphene, but structure-property correlations in graphene which deviates from this structure are not well understood. Here, the authors combine 2 direct imaging techniques, dark-field TEM (DF-TEM) and widefield Raman imaging, to establish a robust, 1-to-one correlation between twist angle and Raman intensity in twisted bilayer graphene (tBLG). The Raman G band intensity is strongly enhanced due to a previously unreported singularity in the joint d. of states of tBLG, whose energy is exclusively a function of twist angle and whose optical transition strength is governed by interlayer interactions, enabling direct optical imaging of these parameters. Also, findings suggest future potential for novel optical and optoelectronic tBLG devices with angle-dependent, tunable characteristics.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XntlOit7Y%253D&md5=6fc537d164d3650a4dae99d646550c36
  6. 6
    Patel, H.; Havener, R. W.; Brown, L.; Liang, Y.; Yang, L.; Park, J.; Graham, M. W. Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons. Nano Lett. 2015, 15 (9), 5932– 5937,  DOI: 10.1021/acs.nanolett.5b02035
    Google Scholar
    6
    Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons
    Patel, Hiral; Havener, Robin W.; Brown, Lola; Liang, Yufeng; Yang, Li; Park, Jiwoong; Graham, Matt W.
    Nano Letters (2015), 15 (9), 5932-5937CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    When two sheets of graphene stack in a twisted bilayer graphene (tBLG) configuration, the resulting constrained overlap between interplanar 2p orbitals produce angle-tunable electronic absorption resonances. By applying a novel combination of multiphoton transient absorption (TA) microscopy and TEM, we resolve the electronic structure and ensuing relaxation by probing resonant excitations of single tBLG domains. Strikingly, we find that the transient electronic population in resonantly excited tBLG domains is enhanced many fold, forming a major electronic relaxation bottleneck. Two-photon TA microscopy shows this bottleneck effect originates from a strongly bound, dark exciton state lying ∼0.37 eV below the 1-photon absorption resonance. This stable coexistence of strongly bound excitons alongside free-electron continuum states has not been previously obsd. in a metallic, 2D material.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXht1Gnu7bI&md5=9069b3288557a5ee1b1bde593b0029cb
  7. 7
    Alencar, T. V.; von Dreifus, D.; Gabriela Cota Moreira, M.; Eliel, G. S. N.; Yeh, C.-H.; Chiu, P.-W.; Pimenta, M. A.; Malard, L. M.; Maria de Paula, A. Twisted Bilayer Graphene Photoluminescence Emission Peaks at van Hove Singularities. J. Phys.: Condens. Matter 2018, 30 (17), 175302,  DOI: 10.1088/1361-648X/aab64b
    Google Scholar
    7
    Twisted bilayer graphene photoluminescence emission peaks at van Hove singularities
    Alencar, Thonimar V.; von Dreifus, Driele; Moreira, Maria Gabriela Cota; Eliel, Gomes S. N.; Yeh, Chao-Hui; Chiu, Po-Wen; Pimenta, Marcos A.; Malard, Leandro M.; Maria de Paula, Ana
    Journal of Physics: Condensed Matter (2018), 30 (17), 175302/1-175302/5CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
    Luminescence emission imaging by fs laser excitation on twisted bilayer graphene samples is reported. The emission images are obtained by tuning the excitation laser energies in the near IR region. An increase of the luminescence emission at excitation energies that depends on the bilayer twist angle is demonstrated. The results show a peak for the light emission when the excitation is in resonance with transitions at the van Hove singularities in the electronic d. of states. The luminescence excitation peak position and width were measured for samples with various twist angles showing resonances in the energy range of 1.2 to 1.7 eV.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhslWlsrfL&md5=09a7d6476a5b545d6cc3019df8bba298
  8. 8
    Yao, W.; Wang, E.; Bao, C.; Zhang, Y.; Zhang, K.; Bao, K.; Chan, C. K.; Chen, C.; Avila, J.; Asensio, M. C. Quasicrystalline 30° Twisted Bilayer Graphene as an Incommensurate Superlattice with Strong Interlayer Coupling. Proc. Natl. Acad. Sci. U. S. A. 2018, 115 (27), 6928– 6933,  DOI: 10.1073/pnas.1720865115
    Google Scholar
    8
    Quasicrystalline 30° twisted bilayer graphene as an incommensurate superlattice with strong interlayer coupling
    Yao, Wei; Wang, Eryin; Bao, Changhua; Zhang, Yiou; Zhang, Kenan; Bao, Kejie; Chan, Chun Kai; Chen, Chaoyu; Avila, Jose; Asensio, Maria C.; Zhu, Junyi; Zhou, Shuyun
    Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (27), 6928-6933CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The interlayer coupling can be used to engineer the electronic structure of van der Waals heterostructures (superlattices) to obtain properties that are not possible in a single material. So far research in heterostructures has been focused on commensurate superlattices with a long-ranged Moire period. Incommensurate heterostructures with rotational symmetry but not translational symmetry (in analogy to quasicrystals) are not only rare in nature, but also the interlayer interaction has often been assumed to be negligible due to the lack of phase coherence. Here we report the successful growth of quasicryst. 30° twisted bilayer graphene (30°-tBLG), which is stabilized by the Pt(111) substrate, and reveal its electronic structure. The 30°-tBLG is confirmed by LEED and the intervalley double-resonance Raman mode at 1383 cm-1. Moreover, the emergence of mirrored Dirac cones inside the Brillouin zone of each graphene layer and a gap opening at the zone boundary suggest that these two graphene layers are coupled via a generalized Umklapp scattering mechanism - i.e., scattering of a Dirac cone in one graphene layer by the reciprocal lattice vector of the other graphene layer. Our work highlights the important role of interlayer coupling in incommensurate quasicryst. superlattices, thereby extending band structure engineering to incommensurate superstructures.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvFKhtrzN&md5=99792ffb225cc8a35deef0ebd592b7d3
  9. 9
    Moon, P.; Koshino, M. Optical Absorption in Twisted Bilayer Graphene. Phys. Rev. B 2013, 87 (20), 205404,  DOI: 10.1103/PhysRevB.87.205404
    Google Scholar
    9
    Optical absorption in twisted bilayer graphene
    Moon, Pilkyung; Koshino, Mikito
    Physical Review B: Condensed Matter and Materials Physics (2013), 87 (20), 205404/1-205404/11CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We theor. study the optical absorption property of twisted bilayer graphenes with various stacking geometries and demonstrate that the spectroscopic characteristics serve as a fingerprint to identify the rotation angle between two layers. We find that the absorption spectrum almost continuously evolves in changing the rotation angle, regardless of the lattice commensurability. The spectrum is characterized by series of peaks assocd. with the van Hove singularity, and the peak energies systematically shift with the rotation angle. We calc. the optical absorption in two frameworks: the tight-binding model and the effective continuum model based on the Dirac equation. For small rotation angles, less than 10°, the effective model well reproduces the low-energy band structure and the optical cond. of the tight-binding model and, also, explains the optical selection rule anal. in terms of the symmetry of the effective Hamiltonian.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtVGiur%252FM&md5=0fa20b9c5829e49ba6ab5426c40e8919
  10. 10
    Xin, W.; Chen, X.-D.; Liu, Z.-B.; Jiang, W.-S.; Gao, X.-G.; Jiang, X.-Q.; Chen, Y.; Tian, J.-G. Photovoltage Enhancement in Twisted-Bilayer Graphene Using Surface Plasmon Resonance. Adv. Opt. Mater. 2016, 4 (11), 1703– 1710,  DOI: 10.1002/adom.201600278
    Google Scholar
    10
    Photovoltage Enhancement in Twisted-Bilayer Graphene Using Surface Plasmon Resonance
    Xin, Wei; Chen, Xu-Dong; Liu, Zhi-Bo; Jiang, Wen-Shuai; Gao, Xiao-Guang; Jiang, Xiao-Qiang; Chen, Yongsheng; Tian, Jian-Guo
    Advanced Optical Materials (2016), 4 (11), 1703-1710CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The present article demonstrated the metal-graphene-metal (MGM) architectures based on a traditional Kretschmann configuration combining gold films and different graphene samples. It was found not only that the photoresponse approx. linearly increases with increasing graphene thickness but also that it exhibits a significant dependence on the graphene twist angle.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtF2htLvE&md5=bed22fbda2dec60d5d72fbb67c6432d4
  11. 11
    Xia, F.; Mueller, T.; Lin, Y.; Valdes-Garcia, A.; Avouris, P. Ultrafast Graphene Photodetector. Nat. Nanotechnol. 2009, 4 (12), 839– 843,  DOI: 10.1038/nnano.2009.292
    Google Scholar
    11
    Ultrafast graphene photodetector
    Xia, Fengnian; Mueller, Thomas; Lin, Yu-ming; Valdes-Garcia, Alberto; Avouris, Phaedon
    Nature Nanotechnology (2009), 4 (12), 839-843CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Graphene research so far has focused on electronic rather than photonic applications, in spite of its impressive optical properties. These include its ability to absorb ∼2% of incident light over a broad wavelength range despite being just one atom thick. Here, we demonstrate ultrafast transistor-based photodetectors made from single- and few-layer graphene. The photoresponse does not degrade for optical intensity modulations up to 40 GHz, and further anal. suggests that the intrinsic bandwidth may exceed 500 GHz. The generation and transport of photocarriers in graphene differ fundamentally from those in photodetectors made from conventional semiconductors as a result of the unique photonic and electronic properties of the graphene. This leads to a remarkably high bandwidth, zero source-drain bias and dark current operation, and good internal quantum efficiency.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsFagsrbN&md5=d3d0754001f70350c835921350521a92
  12. 12
    Sun, L.; Wang, Z.; Wang, Y.; Zhao, L.; Li, Y.; Chen, B.; Huang, S.; Zhang, S.; Wang, W.; Pei, D. Hetero-Site Nucleation for Growing Twisted Bilayer Graphene with a Wide Range of Twist Angles. Nat. Commun. 2021, 12 (1), 2391,  DOI: 10.1038/s41467-021-22533-1
    Google Scholar
    12
    Hetero-site nucleation for growing twisted bilayer graphene with a wide range of twist angles
    Sun, Luzhao; Wang, Zihao; Wang, Yuechen; Zhao, Liang; Li, Yanglizhi; Chen, Buhang; Huang, Shenghong; Zhang, Shishu; Wang, Wendong; Pei, Ding; Fang, Hongwei; Zhong, Shan; Liu, Haiyang; Zhang, Jincan; Tong, Lianming; Chen, Yulin; Li, Zhenyu; Rummeli, Mark H.; Novoselov, Kostya S.; Peng, Hailin; Lin, Li; Liu, Zhongfan
    Nature Communications (2021), 12 (1), 2391CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Twisted bilayer graphene (tBLG) has recently attracted growing interest due to its unique twist-angle-dependent electronic properties. The prepn. of high-quality large-area bilayer graphene with rich rotation angles would be important for the investigation of angle-dependent physics and applications, which, however, is still challenging. Here, we demonstrate a chem. vapor deposition (CVD) approach for growing high-quality tBLG using a hetero-site nucleation strategy, which enables the nucleation of the second layer at a different site from that of the first layer. The fraction of tBLGs in bilayer graphene domains with twist angles ranging from 0° to 30° was found to be improved to 88%, which is significantly higher than those reported previously. The hetero-site nucleation behavior was carefully investigated using an isotope-labeling technique. Furthermore, the clear Moire patterns and ultrahigh room-temp. carrier mobility of 68,000 cm2 V-1 s-1 confirmed the high cryst. quality of our tBLG. Our study opens an avenue for the controllable growth of tBLGs for both fundamental research and practical applications.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXpsVOgur0%253D&md5=a79ccfb5ea76a338671b92dceb0c3da5
  13. 13
    Kim, K.; Coh, S.; Tan, L. Z.; Regan, W.; Yuk, J. M.; Chatterjee, E.; Crommie, M. F.; Cohen, M. L.; Louie, S. G.; Zettl, A. Raman Spectroscopy Study of Rotated Double-Layer Graphene: Misorientation-Angle Dependence of Electronic Structure. Phys. Rev. Lett. 2012, 108 (24), 246103,  DOI: 10.1103/PhysRevLett.108.246103
    Google Scholar
    13
    Raman spectroscopy study of rotated double-layer graphene: misorientation-angle dependence of electronic structure
    Kim, Kwanpyo; Coh, Sinisa; Tan, Liang Z.; Regan, William; Yuk, Jong Min; Chatterjee, Eric; Crommie, M. F.; Cohen, Marvin L.; Louie, Steven G.; Zettl, A.
    Physical Review Letters (2012), 108 (24), 246103/1-246103/6CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We present a systematic Raman study of unconventionally stacked double-layer graphene, and find that the spectrum strongly depends on the relative rotation angle between layers. Rotation-dependent trends in the position, width and intensity of graphene 2D and G peaks are exptl. established and accounted for theor. Our theor. anal. reveals that changes in electronic band structure due to the interlayer interaction, such as rotational-angle dependent Van Hove singularities, are responsible for the obsd. spectral features. Our combined exptl. and theor. study provides a deeper understanding of the electronic band structure of rotated double-layer graphene, and leads to a practical way to identify and analyze rotation angles of misoriented double-layer graphene.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVamtrfM&md5=f00f0b706e3ab0b5be0045cf18647a13
  14. 14
    Jorio, A.; Kasperczyk, M.; Clark, N.; Neu, E.; Maletinsky, P.; Vijayaraghavan, A.; Novotny, L. Optical-Phonon Resonances with Saddle-Point Excitons in Twisted-Bilayer Graphene. Nano Lett. 2014, 14 (10), 5687– 5692,  DOI: 10.1021/nl502412g
    Google Scholar
    14
    Optical-Phonon Resonances with Saddle-Point Excitons in Twisted-Bilayer Graphene
    Jorio, Ado; Kasperczyk, Mark; Clark, Nick; Neu, Elke; Maletinsky, Patrick; Vijayaraghavan, Aravind; Novotny, Lukas
    Nano Letters (2014), 14 (10), 5687-5692CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Twisted-bilayer graphene (tBLG) exhibits van Hove singularities in the d. of states that can be tuned by changing the twisting angle θ. A θ-defined tBLG was produced and characterized with optical reflectivity and resonance Raman scattering. The θ-engineered optical response is consistent with persistent saddle-point excitons. Sep. resonances with Stokes and anti-Stokes Raman scattering components can be achieved due to the sharpness of the 2-dimensional saddle-point excitons, similar to what was previously obsd. for 1-dimensional C nanotubes. The excitation power dependence for the Stokes and anti-Stokes emissions indicate that the 2 processes are correlated and that they share the same phonon.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsFWjs7vI&md5=a69edfb8529494036ace9c29e1b8da6d
  15. 15
    Chen, X.-D.; Xin, W.; Jiang, W.-S.; Liu, Z.-B.; Chen, Y.; Tian, J.-G. High-Precision Twist-Controlled Bilayer and Trilayer Graphene. Adv. Mater. 2016, 28 (13), 2563– 2570,  DOI: 10.1002/adma.201505129
    Google Scholar
    15
    High-Precision Twist-Controlled Bilayer and Trilayer Graphene
    Chen, Xu-Dong; Xin, Wei; Jiang, Wen-Shuai; Liu, Zhi-Bo; Chen, Yongsheng; Tian, Jian-Guo
    Advanced Materials (Weinheim, Germany) (2016), 28 (13), 2563-2570CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The cutting-rotation-stacking (CRS) technique provides a method to investigate the elec. and optical properties of twisted multilayer graphene. The twist-control capability of this technique is important for the heterostructures because their phys. properties strongly depend on the relative orientation of the component layers. A more important contribution of the CRS technique is that double twisted trilayer graphene (DTTG) with arbitrary twist angles can be successfully fabricated by this method, which is an important breakthrough in the exptl. research of twisted TLG. Compared with a bilayer, an unprecedented degree of control of the electronic and optical properties is available for this DTTG.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhslSqsLc%253D&md5=13e4b740a36748f57f472db4782593ef
  16. 16
    Cao, Y.; Fatemi, V.; Fang, S.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P. Unconventional Superconductivity in Magic-Angle Graphene Superlattices. Nature 2018, 556 (7699), 43– 50,  DOI: 10.1038/nature26160
    Google Scholar
    16
    Unconventional superconductivity in magic-angle graphene superlattices
    Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo
    Nature (London, United Kingdom) (2018), 556 (7699), 43-50CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    The behavior of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theor. understanding has motivated the development of exptl. techniques for studying such behavior, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional supercond.-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a crit. temp. of up to 1.7 K. The temp.-carrier-d. phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to supercond. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting crit. temp. of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier d. of about 1011 per square centimeter), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-crit.-temp. superconductors and quantum spin liqs.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXntVGjsbc%253D&md5=686b029f989784d0fa11ede1bfcecdbe
  17. 17
    Carozo, V.; Almeida, C. M.; Ferreira, E. H. M.; Cançado, L. G.; Achete, C. A.; Jorio, A. Raman Signature of Graphene Superlattices. Nano Lett. 2011, 11 (11), 4527– 4534,  DOI: 10.1021/nl201370m
    Google Scholar
    17
    Raman Signature of Graphene Superlattices
    Carozo, Victor; Almeida, Clara M.; Ferreira, Erlon H. M.; Cancado, Luiz Gustavo; Achete, Carlos Alberto; Jorio, Ado
    Nano Letters (2011), 11 (11), 4527-4534CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    When 2 identical 2-dimensional periodic structures are superposed, a mismatch rotation angle between the structures generates a superlattice. This effect is commonly obsd. in graphite, where the rotation between graphene layers generates Moire patterns in scanning tunneling microscopy images. Here, a study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented. The peak properties depend on the mismatch rotation angle and can be used as an optical signature for superlattices in bilayer graphene. An at. force microscopy system is used to produce and identify sp. rotationally stacked bilayer graphenes that demonstrate the validity of model.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtlWjtL3N&md5=ced21b2e47402d42aaf3d06df9c20010
  18. 18
    Hofmann, S.; Braeuninger-Weimer, P.; Weatherup, R. S. CVD-Enabled Graphene Manufacture and Technology. J. Phys. Chem. Lett. 2015, 6 (14), 2714– 2721,  DOI: 10.1021/acs.jpclett.5b01052
    Google Scholar
    18
    CVD-Enabled Graphene Manufacture and Technology
    Hofmann, Stephan; Braeuninger-Weimer, Philipp; Weatherup, Robert S.
    Journal of Physical Chemistry Letters (2015), 6 (14), 2714-2721CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Integrated manufg. is arguably the most challenging task in the development of technol. based on graphene and other 2D materials, particularly with regard to the industrial demand for "electronic-grade" large-area films. To control the structure and properties of these materials at the monolayer level, their nucleation, growth and interfacing needs to be understood to a level of unprecedented detail compared to existing thin film or bulk materials. Chem. vapor deposition (CVD) has emerged as the most versatile and promising technique to develop graphene and 2D material films into industrial device materials and this Perspective outlines recent progress, trends, and emerging CVD processing pathways. A key focus is the emerging understanding of the underlying growth mechanisms, in particular on the role of the required catalytic growth substrate, which brings together the latest progress in the fields of heterogeneous catalysis and classic crystal/thin-film growth.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVKit7bP&md5=d91fed663bf815d23aaee5deaad27864
  19. 19
    De Fazio, D.; Purdie, D. G.; Ott, A. K.; Braeuninger-Weimer, P.; Khodkov, T.; Goossens, S.; Taniguchi, T.; Watanabe, K.; Livreri, P.; Koppens, F. H. L. High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition. ACS Nano 2019, 13 (8), 8926– 8935,  DOI: 10.1021/acsnano.9b02621
    Google Scholar
    19
    High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition
    De Fazio, Domenico; Purdie, David G.; Ott, Anna K.; Braeuninger-Weimer, Philipp; Khodkov, Timofiy; Goossens, Stijn; Taniguchi, Takashi; Watanabe, Kenji; Livreri, Patrizia; Koppens, Frank H. L.; Hofmann, Stephan; Goykhman, Ilya; Ferrari, Andrea C.; Lombardo, Antonio
    ACS Nano (2019), 13 (8), 8926-8935CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    We report high room-temp. mobility in single-layer graphene grown by chem. vapor deposition (CVD) after wet transfer on SiO2 and hexagonal boron nitride (hBN) encapsulation. By removing contaminations, trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to ∼70000 cm2 V-1 s-1 at room temp. and ∼120 000 cm2 V-1 s-1 at 9K. These are more than twice those of previous wet-transferred graphene and comparable to samples prepd. by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycryst. graphene, achieving room-temp. mobilities of ∼30 000 cm2 V-1 s-1. These results show that, with appropriate encapsulation and cleaning, room-temp. mobilities well above 10 000 cm2 V-1 s-1 can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsVSqsbnM&md5=9c06d8950fc9402af9d63dfc1a520686
  20. 20
    Lu, C.; Lin, Y.; Liu, Z.; Yeh, C.; Suenaga, K.; Chiu, P. Twisting Bilayer Graphene Superlattices. ACS Nano 2013, 7 (3), 2587– 2594,  DOI: 10.1021/nn3059828
    Google Scholar
    20
    Twisting Bilayer Graphene Superlattices
    Lu, Chun-Chieh; Lin, Yung-Chang; Liu, Zheng; Yeh, Chao-Hui; Suenaga, Kazu; Chiu, Po-Wen
    ACS Nano (2013), 7 (3), 2587-2594CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Bilayer graphene is an intriguing material in that its electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of low-dimensional carbon system. Twisted bilayer graphene can be obtained by (i) thermal decompn. of SiC; (ii) CVD on metal catalysts; (iii) folding graphene; or (iv) stacking graphene layers one atop the other, the latter of which suffers from interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycryst. structures. The present study studies bilayer graphene grown by ambient pressure CVD on polycryst. Cu. Controlling the nucleation in early stage growth allows the constituent layers to form single hexagonal crystals. New Raman active modes result from the twist, with the angle detd. by TEM. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXjt1Wlsb8%253D&md5=fc7e5afd6cb530217d653070c5325582
  21. 21
    Fang, W.; Hsu, A. L.; Song, Y.; Kong, J. A Review of Large-Area Bilayer Graphene Synthesis by Chemical Vapor Deposition. Nanoscale 2015, 7 (48), 20335– 20351,  DOI: 10.1039/C5NR04756K
    Google Scholar
    21
    A review of large-area bilayer graphene synthesis by chemical vapor deposition
    Fang, Wenjing; Hsu, Allen L.; Song, Yi; Kong, Jing
    Nanoscale (2015), 7 (48), 20335-20351CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Bilayer graphene has attracted considerable attention due to its potential as a tunable band gap in AB-stacked bilayers. Recently, great advancements have been made in the synthesis of chem.-vapor-deposited bilayer graphene. This featured article provides a detailed and up-to-date review of the synthesis of bilayer graphene by chem. vapor deposition (CVD). We will discuss various approaches to synthesize bilayer graphene and the corresponding growth dynamics. Methods for identifying the growth mechanism of bilayer graphene on Cu enclosures are highlighted for a deeper understanding of better control over uniformity and thickness.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhvVWmtLvK&md5=b3599f35bd6dac06122c298f5a43e5d6
  22. 22
    Zhao, H.; Lin, Y. C.; Yeh, C. H.; Tian, H.; Chen, Y. C.; Xie, D.; Yang, Y.; Suenaga, K.; Ren, T. L.; Chiu, P. W. Growth and Raman Spectra of Single-Crystal Trilayer Graphene with Different Stacking Orientations. ACS Nano 2014, 8 (10), 10766– 10773,  DOI: 10.1021/nn5044959
    Google Scholar
    22
    Growth and Raman Spectra of Single-Crystal Trilayer Graphene with Different Stacking Orientations
    Zhao, Haiming; Lin, Yung-Chang; Yeh, Chao-Hui; Tian, He; Chen, Yu-Chen; Xie, Dan; Yang, Yi; Suenaga, Kazu; Ren, Tian-Ling; Chiu, Po-Wen
    ACS Nano (2014), 8 (10), 10766-10773CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Understanding the growth mechanism of graphene layers in chem. vapor deposition (CVD) and their corresponding Raman properties is technol. relevant and of importance for the application of graphene in electronic and optoelectronic devices. Here, we report CVD growth of single-crystal trilayer graphene (TLG) grains on Cu and show that lattice defects at the center of each grain persist throughout the growth, indicating that the adlayers share the same nucleation site with the upper layers and these central defects could also act as a carbon pathway for the growth of a new layer. Statistics shows that ABA, 30-30, 30-AB, and AB-30 make up the major stacking orientations in the CVD-grown TLG, with distinctive Raman 2D characteristics. Surprisingly, a high level of lattice defects results whenever a layer with a twist angle of θ = 30° is found in the multiple stacks of graphene layers.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhs1OitLvM&md5=cf550a17ae7d981d116d20f952aa5814
  23. 23
    Yu, K.; Van Luan, N.; Kim, T.; Jeon, J.; Kim, J.; Moon, P.; Lee, Y. H.; Choi, E. J. Gate Tunable Optical Absorption and Band Structure of Twisted Bilayer Graphene. Phys. Rev. B 2019, 99 (24), 241405,  DOI: 10.1103/PhysRevB.99.241405
    Google Scholar
    23
    Gate tunable optical absorption and band structure of twisted bilayer graphene
    Yu, Kwangnam; Nguyen, Van Luan; Kim, Taesoo; Jeon, Jiwon; Kim, Jiho; Moon, Pilkyung; Lee, Young Hee; Choi, E. J.
    Physical Review B (2019), 99 (24), 241405CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    We report the IR transmission measurement on elec. gated twisted bilayer graphene. The optical absorption spectrum clearly manifests dramatic changes such as the splitting of the interlinear-band absorption step, the shift of the inter-van Hove singularity transition peak, and the emergence of a very strong intravalence (intraconduction) band transition. These anomalous optical behaviors demonstrate consistently a nonrigid band structure modification created by ion-gel gating through layer-dependent Coulomb screening. We propose that this screening-driven band modification is a universal phenomenon that persists to other bilayer crystals in general, establishing elec. gating as a versatile technique to engineer band structures and to create different types of optical absorptions that can be exploited in electro-optical device applications.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhs12ksr3N&md5=d0c00b3e2b9938c63ef216bcab807486
  24. 24
    Wang, Y.; Ni, Z.; Liu, L.; Liu, Y.; Cong, C.; Yu, T.; Wang, X.; Shen, D.; Shen, Z. Stacking-Dependent Optical Conductivity of Bilayer Graphene. ACS Nano 2010, 4 (7), 4074– 4080,  DOI: 10.1021/nn1004974
    Google Scholar
    24
    Stacking-Dependent Optical Conductivity of Bilayer Graphene
    Wang, Yingying; Ni, Zhenhua; Liu, Lei; Liu, Yanhong; Cong, Chunxiao; Yu, Ting; Wang, Xiaojun; Shen, Dezhen; Shen, Zexiang
    ACS Nano (2010), 4 (7), 4074-4080CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The optical conductivities of graphene layers are strongly dependent on their stacking orders. Our first-principle calcns. show that, while the optical conductivities of single-layer graphene (SLG) and bilayer graphene (BLG) with Bernal stacking are almost frequency-independent in the visible region, the optical cond. of twisted bilayer graphene (TBG) is frequency-dependent, giving rise to addnl. absorption features due to the band folding effect. Exptl., we obtain from contrast spectra the optical cond. profiles of BLG with different stacking geometries. Some TBG samples show addnl. features in their cond. spectra, in full agreement with our calcn. results, while a few samples give universal cond. values similar to that of SLG. We propose that those variations of optical cond. spectra of TBG samples originate from the difference between the commensurate and incommensurate stackings. Our results reveal that the optical cond. measurements of graphene layers indeed provide an efficient way to select graphene films with desirable electronic and optical properties, which would greatly help the future application of those large-scale misoriented graphene films in photonic devices.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXmvVSlur4%253D&md5=1aa96414ffecfcd1f5a31a04ee7c18ea
  25. 25
    Robinson, J. T.; Schmucker, S. W.; Diaconescu, C. B.; Long, J. P.; Culbertson, J. C.; Ohta, T.; Friedman, A. L.; Beechem, T. E. Electronic Hybridization of Large-Area Stacked Graphene Films. ACS Nano 2013, 7 (1), 637– 644,  DOI: 10.1021/nn304834p
    Google Scholar
    25
    Electronic Hybridization of Large-Area Stacked Graphene Films
    Robinson, Jeremy T.; Schmucker, Scott W.; Diaconescu, C. Bogdan; Long, James P.; Culbertson, James C.; Ohta, Taisuke; Friedman, Adam L.; Beechem, Thomas E.
    ACS Nano (2013), 7 (1), 637-644CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Direct, tunable coupling between individually assembled graphene layers is a next step toward designer two-dimensional (2D) crystal systems, with relevance for fundamental studies and technol. applications. Here the authors describe the fabrication and characterization of large-area (>cm2), coupled bilayer graphene on SiO2/Si substrates. Stacking two graphene films leads to direct electronic interactions between layers, where the resulting film properties are detd. by the local twist angle. Polycryst. bilayer films have a stained-glass window appearance explained by the emergence of a narrow absorption band in the visible spectrum that depends on twist angle. Direct measurement of layer orientation via electron diffraction, together with Raman and optical spectroscopy, confirms the persistence of clean interfaces over large areas. Finally, interlayer coupling can be reversibly turned off through chem. modification, enabling optical-based chem. detection schemes. Together, these results suggest that 2-dimensional crystals can be individually assembled to form electronically coupled systems suitable for large-scale applications.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVersLbI&md5=58494e296d1fa9645cfb094d2e6f436e
  26. 26
    Campos-Delgado, J.; Algara-Siller, G.; Santos, C. N.; Kaiser, U.; Raskin, J. P. Twisted Bi-Layer Graphene: Microscopic Rainbows. Small 2013, 9 (19), 3247– 3251,  DOI: 10.1002/smll.201300050
    Google Scholar
    26
    Twisted Bi-Layer Graphene: Microscopic Rainbows
    Campos-Delgado, J.; Algara-Siller, G.; Santos, C. N.; Kaiser, U.; Raskin, J.-P.
    Small (2013), 9 (19), 3247-3251CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Blue, pink, and yellow colorations appear from twisted bi-layer graphene (tBLG) when transferred to a SiO2/Si substrate (SiO2 = 100 nm-thick). Raman and electron microscope studies reveal that these colorations appear for twist angles in the 9-15° range. Optical contrast simulations confirm that the obsd. colorations are related to the angle-dependent electronic properties of tBLG combined with the reflection that results from the layered structure tBLG/100 nm-thick SiO2/Si.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXmtlartLs%253D&md5=4be3a86e255e927e7ab669d0d9aed594
  27. 27
    Ohta, T.; Beechem, T. E.; Robinson, J. T.; Kellogg, G. L. Long-Range Atomic Ordering and Variable Interlayer Interactions in Two Overlapping Graphene Lattices with Stacking Misorientations. Phys. Rev. B 2012, 85 (7), 75415,  DOI: 10.1103/PhysRevB.85.075415
    Google Scholar
    27
    Long-range atomic ordering and variable interlayer interactions in two overlapping graphene lattices with stacking misorientations
    Ohta, Taisuke; Beechem, Thomas E.; Robinson, Jeremy T.; Kellogg, G. L.
    Physical Review B: Condensed Matter and Materials Physics (2012), 85 (7), 075415/1-075415/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The low-energy electronic dispersion of graphene is extremely sensitive to the nearest layer interaction and thus the stacking sequence. Here, we report a method to examine the effect of stacking misorientation in bilayer graphene by transferring chem. vapor deposited (CVD) graphene onto monolithic graphene epitaxially grown on silicon carbide (SiC) (0001). The resulting hybrid bilayer graphene displays long-range Moire diffraction patterns having various misorientations even as it exhibits electron reflectivity spectra nearly identical to epitaxial bilayer graphene grown directly on SiC. These varying twist angles affect the 2D (G')-band shape of the Raman spectrum, indicating regions of both a monolayer-like single π state and Bernal-like split π states brought about by the differing interlayer interactions. This hybrid bilayer graphene fabricated via a transfer process therefore offers a way to systematically study the electronic properties of bilayer graphene films as a function of stacking misorientation angle.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XkslCms78%253D&md5=a63fe7515800d98c6a7a3a6d51188646
  28. 28
    Ahn, S. J.; Moon, P.; Kim, T. H.; Kim, H. W.; Shin, H. C.; Kim, E. H.; Cha, H. W.; Kahng, S. J.; Kim, P.; Koshino, M. Dirac Electrons in a Dodecagonal Graphene Quasicrystal. Science (80-.). 2018, 361 (6404), 782– 786,  DOI: 10.1126/science.aar8412
    Google Scholar
    There is no corresponding record for this reference.
  29. 29
    Düvel, M.; Merboldt, M.; Bange, J. P.; Strauch, H.; Stellbrink, M.; Pierz, K.; Schumacher, H. W.; Momeni, D.; Steil, D.; Jansen, G. S. M. Far-from-Equilibrium Electron–Phonon Interactions in Optically Excited Graphene. Nano Lett. 2022, 22 (12), 4897– 4904,  DOI: 10.1021/acs.nanolett.2c01325
    Google Scholar
    29
    Far-from-Equilibrium Electron-Phonon Interactions in Optically Excited Graphene
    Duevel, Marten; Merboldt, Marco; Bange, Jan Philipp; Strauch, Hannah; Stellbrink, Michael; Pierz, Klaus; Schumacher, Hans Werner; Momeni, Davood; Steil, Daniel; Jansen, G. S. Matthijs; Steil, Sabine; Novko, Dino; Mathias, Stefan; Reutzel, Marcel
    Nano Letters (2022), 22 (12), 4897-4904CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Comprehending far-from-equil. many-body interactions is one of the major goals of current ultrafast condensed matter physics research. Here, a particularly interesting but barely understood situation occurs during a strong optical excitation, where the electron and phonon systems can be significantly perturbed and the quasiparticle distributions cannot be described with equil. functions. In this work, we use time- and angle-resolved photoelectron spectroscopy to study such far-from-equil. many-body interactions for the prototypical material graphene. In accordance with theor. simulations, we find remarkable transient renormalizations of the quasiparticle self-energy caused by the photoinduced nonequil. conditions. These observations can be understood by ultrafast scatterings between nonequil. electrons and strongly coupled optical phonons, which signify the crucial role of ultrafast nonequil. dynamics on many-body interactions. Our results advance the understanding of many-body physics in extreme conditions, which is important for any endeavor to optically manipulate or create non-equil. states of matter.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhsVSktrvP&md5=bf18cc58fc82a686a3eda718fe36a18b
  30. 30
    Patel, H.; Huang, L.; Kim, C.-J.; Park, J.; Graham, M. W. Stacking Angle-Tunable Photoluminescence from Interlayer Exciton States in Twisted Bilayer Graphene. Nat. Commun. 2019, 10 (1), 1445,  DOI: 10.1038/s41467-019-09097-x
    Google Scholar
    30
    Stacking angle-tunable photoluminescence from interlayer exciton states in twisted bilayer graphene
    Patel Hiral; Graham Matt W; Huang Lujie; Park Jiwoong; Kim Cheol-Joo
    Nature communications (2019), 10 (1), 1445 ISSN:.
    Twisted bilayer graphene (tBLG) is a metallic material with two degenerate van Hove singularity transitions that can rehybridize to form interlayer exciton states. Here we report photoluminescence (PL) emission from tBLG after resonant 2-photon excitation, which tunes with the interlayer stacking angle, θ. We spatially image individual tBLG domains at room-temperature and show a five-fold resonant PL-enhancement over the background hot-electron emission. Prior theory predicts that interlayer orbitals mix to create 2-photon-accessible strongly-bound (~0.7 eV) exciton and continuum-edge states, which we observe as two spectral peaks in both PL excitation and excited-state absorption spectra. This peak splitting provides independent estimates of the exciton binding energy which scales from 0.5-0.7 eV with θ = 7.5° to 16.5°. A predicted vanishing exciton-continuum coupling strength helps explain both the weak resonant PL and the slower 1 ps(-1) exciton relaxation rate observed. This hybrid metal-exciton behavior electron thermalization and PL emission are tunable with stacking angle for potential enhancements in optoelectronic and fast-photosensing graphene-based applications.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BB3cbps1GisQ%253D%253D&md5=acb092f08335edc72820eb43a0f94928
  31. 31
    Ribeiro, H. B.; Sato, K.; Eliel, G. S. N.; De Souza, E. A. T.; Lu, C. C.; Chiu, P. W.; Saito, R.; Pimenta, M. A. Origin of van Hove Singularities in Twisted Bilayer Graphene. Carbon N. Y. 2015, 90, 138– 145,  DOI: 10.1016/j.carbon.2015.04.005
    Google Scholar
    There is no corresponding record for this reference.
  32. 32
    Schäpers, A.; Sonntag, J.; Valerius, L.; Pestka, B.; Strasdas, J.; Watanabe, K.; Taniguchi, T.; Wirtz, L.; Morgenstern, M.; Beschoten, B. Raman Imaging of Twist Angle Variations in Twisted Bilayer Graphene at Intermediate Angles. 2D Mater. 2022, 9 (4), 045009,  DOI: 10.1088/2053-1583/ac7e59
    Google Scholar
    There is no corresponding record for this reference.
  33. 33
    Li, G.; Luican, a.; Lopes dos Santos, J. M. B.; Castro Neto, a. H.; Reina, a.; Kong, J.; Andrei, E. Y. Observation of Van Hove Singularities in Twisted Graphene Layers. Nat. Phys. 2010, 6 (2), 109– 113,  DOI: 10.1038/nphys1463
    Google Scholar
    33
    Observation of Van Hove singularities in twisted graphene layers
    Li, Guohong; Luican, A.; Lopes dos Santos, J. M. B.; Castro Neto, A. H.; Reina, A.; Kong, J.; Andrei, E. Y.
    Nature Physics (2010), 6 (2), 109-113CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)
    Electronic instabilities at the crossing of the Fermi energy with a Van Hove singularity in the d. of states often lead to new phases of matter such as supercond., magnetism or d. waves. However, in most materials this condition is difficult to control. In the case of single-layer graphene, the singularity is too far from the Fermi energy and hence difficult to reach with std. doping and gating techniques. Here we report the observation of low-energy Van Hove singularities in twisted graphene layers seen as two pronounced peaks in the d. of states measured by scanning tunneling spectroscopy. We demonstrate that a rotation between stacked graphene layers can generate Van Hove singularities, which can be brought arbitrarily close to the Fermi energy by varying the angle of rotation. This opens intriguing prospects for Van Hove singularity engineering of electronic phases.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtlCrtbs%253D&md5=ccd3756af24ea751c3ff22a7ee229574
  34. 34
    Brihuega, I.; Mallet, P.; González-Herrero, H.; Trambly de Laissardière, G.; Ugeda, M. M.; Magaud, L.; Gómez-Rodríguez, J. M.; Ynduráin, F.; Veuillen, J.-Y. Unraveling the Intrinsic and Robust Nature of van Hove Singularities in Twisted Bilayer Graphene by Scanning Tunneling Microscopy and Theoretical Analysis. Phys. Rev. Lett. 2012, 109 (19), 196802,  DOI: 10.1103/PhysRevLett.109.196802
    Google Scholar
    34
    Unraveling the intrinsic and robust nature of van Hove singularities in twisted bilayer graphene by scanning tunneling microscopy and theoretical analysis
    Brihuega, I.; Mallet, P.; Gonzalez-Herrero, H.; Trambly de Laissardiere, G.; Ugeda, M. M.; Magaud, L.; Gomez-Rodriguez, J. M.; Yndurain, F.; Veuillen, J.-Y.
    Physical Review Letters (2012), 109 (19), 196802/1-196802/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Extensive scanning tunneling microscopy and spectroscopy expts. complemented by first-principles and parametrized tight binding calcns. provide a clear answer to the existence, origin, and robustness of van Hove singularities (vHs) in twisted graphene layers. Our results are conclusive: vHs due to interlayer coupling are ubiquitously present in a broad range (from 1° to 10°) of rotation angles in our graphene on 6H-SiC(000-1) samples. From the variation of the energy sepn. of the vHs with the rotation angle we are able to recover the Fermi velocity of a graphene monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by expts., which show that they survive in the presence of a third graphene layer, and by calcns., which test the role of the periodic modulation and abs. value of the interlayer distance. Finally, we clarify the role of the layer topog. corrugation and of electronic effects in the apparent moire contrast measured on the STM images.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xhslylsr7I&md5=29a40447f8181ab1f69fb09544ffc90b
  35. 35
    Jeong, G.; Choi, B.; Kim, D. S.; Ahn, S.; Park, B.; Kang, J. H.; Min, H.; Hong, B. H.; Kim, Z. H. Mapping of Bernal and Non-Bernal Stacking Domains in Bilayer Graphene Using Infrared Nanoscopy. Nanoscale 2017, 9 (12), 4191– 4195,  DOI: 10.1039/C7NR00713B
    Google Scholar
    35
    Mapping of Bernal and non-Bernal stacking domains in bilayer graphene using infrared nanoscopy
    Jeong, Gyouil; Choi, Boogeon; Kim, Deok-Soo; Ahn, Seongjin; Park, Baekwon; Kang, Jin Hyoun; Min, Hongki; Hong, Byung Hee; Kim, Zee Hwan
    Nanoscale (2017), 9 (12), 4191-4195CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Bilayer graphene (BLG) shows great potential as a new material for opto-electronic devices because its bandgap can be controlled by varying the stacking orders, as well as by applying an external elec. field. An imaging technique that can visualize and characterize various stacking domains in BLG may greatly help in fully utilizing such properties of BLG. Here we demonstrate that IR (IR) scattering-type scanning near-field optical microscopy (sSNOM) can visualize Bernal and non-Bernal stacking domains of BLG, based on the stacking-specific inter- and intra-band optical conductivities. The method enables nanometric mapping of stacking domains in BLG on dielec. substrates, augmenting current limitations of Raman spectroscopy and electron microscopy techniques for the structural characterization of BLG.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXjtl2gtLo%253D&md5=74dfacc58126afc8f02cabe5d7023745
  36. 36
    Uri, A.; Grover, S.; Cao, Y.; Crosse, J. A.; Bagani, K.; Rodan-Legrain, D.; Myasoedov, Y.; Watanabe, K.; Taniguchi, T.; Moon, P. Mapping the Twist-Angle Disorder and Landau Levels in Magic-Angle Graphene. Nature 2020, 581 (7806), 47– 52,  DOI: 10.1038/s41586-020-2255-3
    Google Scholar
    36
    Mapping the twist-angle disorder and Landau levels in magic-angle graphene
    Uri, A.; Grover, S.; Cao, Y.; Crosse, J. A.; Bagani, K.; Rodan-Legrain, D.; Myasoedov, Y.; Watanabe, K.; Taniguchi, T.; Moon, P.; Koshino, M.; Jarillo-Herrero, P.; Zeldov, E.
    Nature (London, United Kingdom) (2020), 581 (7806), 47-52CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Abstr.: The recently discovered flat electronic bands and strongly correlated and superconducting phases in magic-angle twisted bilayer graphene (MATBG)1,2 crucially depend on the interlayer twist angle, θ. Although control of the global θ with a precision of about 0.1 degrees has been demonstrated1-7, little information is available on the distribution of the local twist angles. Here we use a nanoscale on-tip scanning superconducting quantum interference device (SQUID-on-tip)8 to obtain tomog. images of the Landau levels in the quantum Hall state9 and to map the local θ variations in hexagonal boron nitride (hBN)-encapsulated MATBG devices with relative precision better than 0.002 degrees and a spatial resoln. of a few moire´ periods. We find a correlation between the degree of θ disorder and the quality of the MATBG transport characteristics and show that even state-of-the-art devices-which exhibit correlated states, Landau fans and supercond.-display considerable local variation in θ of up to 0.1 degrees, exhibiting substantial gradients and networks of jumps, and may contain areas with no local MATBG behavior. We observe that the correlated states in MATBG are particularly fragile with respect to the twist-angle disorder. We also show that the gradients of θ generate large gate-tunable in-plane elec. fields, unscreened even in the metallic regions, which profoundly alter the quantum Hall state by forming edge channels in the bulk of the sample and may affect the phase diagram of the correlated and superconducting states. We thus establish the importance of θ disorder as an unconventional type of disorder enabling the use of twist-angle gradients for band structure engineering, for realization of correlated phenomena and for gate-tunable built-in planar elec. fields for device applications.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXoslygtLY%253D&md5=c84f30a17dab4c276634346d7920ea61
  37. 37
    Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Making Graphene Visible. Appl. Phys. Lett. 2007, 91 (6), 063124,  DOI: 10.1063/1.2768624
    Google Scholar
    37
    Making graphene visible
    Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K.
    Applied Physics Letters (2007), 91 (6), 063124/1-063124/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Microfabrication of graphene devices used in many exptl. studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepd. on top of Si wafers with a certain thickness of SiO2. The authors study graphene's visibility and show that it depends strongly on both thickness of SiO2 and light wavelength. By using monochromatic illumination, graphene can be isolated for any SiO2 thickness, albeit 300 nm (the current std.) and, esp., ≈100 nm are most suitable for its visual detection. By using a Fresnel-law-based model, they quant. describe the exptl. data.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXpsFGntrg%253D&md5=a8c99c400588bc70b7a1741de9646d09
  38. 38
    Hofmann, S.; Braeuninger-Weimer, P.; Weatherup, R. S. CVD-Enabled Graphene Manufacture and Technology. J. Phys. Chem. Lett. 2015, 6 (14), 2714– 2721,  DOI: 10.1021/acs.jpclett.5b01052
    Google Scholar
    38
    CVD-Enabled Graphene Manufacture and Technology
    Hofmann, Stephan; Braeuninger-Weimer, Philipp; Weatherup, Robert S.
    Journal of Physical Chemistry Letters (2015), 6 (14), 2714-2721CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Integrated manufg. is arguably the most challenging task in the development of technol. based on graphene and other 2D materials, particularly with regard to the industrial demand for "electronic-grade" large-area films. To control the structure and properties of these materials at the monolayer level, their nucleation, growth and interfacing needs to be understood to a level of unprecedented detail compared to existing thin film or bulk materials. Chem. vapor deposition (CVD) has emerged as the most versatile and promising technique to develop graphene and 2D material films into industrial device materials and this Perspective outlines recent progress, trends, and emerging CVD processing pathways. A key focus is the emerging understanding of the underlying growth mechanisms, in particular on the role of the required catalytic growth substrate, which brings together the latest progress in the fields of heterogeneous catalysis and classic crystal/thin-film growth.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVKit7bP&md5=d91fed663bf815d23aaee5deaad27864
  39. 39
    Wurstbauer, U.; Röling, C.; Wurstbauer, U.; Wegscheider, W.; Vaupel, M.; Thiesen, P. H.; Weiss, D. Imaging Ellipsometry of Graphene. Appl. Phys. Lett. 2010, 97 (23), 231901,  DOI: 10.1063/1.3524226
    Google Scholar
    39
    Imaging ellipsometry of graphene
    Wurstbauer, Ulrich; Roeling, Christian; Wurstbauer, Ursula; Wegscheider, Werner; Vaupel, Matthias; Thiesen, Peter H.; Weiss, Dieter
    Applied Physics Letters (2010), 97 (23), 231901/1-231901/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Imaging ellipsometry studies of graphene on SiO2/Si and cryst. GaAs are presented. Imaging ellipsometry is a powerful tool to detect and characterize graphene on any flat substrate. Variable angle spectroscopic ellipsometry is used to explore the dispersion of the optical consts. of graphene in the visible range with high lateral resoln. In this way, the influence of the substrate on graphene's optical properties can be studied. (c) 2010 American Institute of Physics.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsFagsLzO&md5=3c054e08652ee727519885f4dcd61ed2
  40. 40
    Funke, S.; Miller, B.; Parzinger, E.; Thiesen, P.; Holleitner, A. W.; Wurstbauer, U. Imaging Spectroscopic Ellipsometry of MoS 2. J. Phys.: Condens. Matter 2016, 28 (38), 385301,  DOI: 10.1088/0953-8984/28/38/385301
    Google Scholar
    40
    Imaging spectroscopic ellipsometry of MoS2
    Funke, S.; Miller, B.; Parzinger, E.; Thiesen, P.; Holleitner, A. W.; Wurstbauer, U.
    Journal of Physics: Condensed Matter (2016), 28 (38), 385301/1-385301/12CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
    Micromechanically exfoliated mono- and multilayers of molybdenum disulfide (MoS2) are investigated by spectroscopic imaging ellipsometry. In combination with knife edge illumination, MoS2 flakes can be detected and classified on arbitrary flat and also transparent substrates with a lateral resoln. down to 1-2 μm. The complex dielec. functions from mono- and trilayer MoS2 are presented. They are extd. from a multilayer model to fit the measured ellipsometric angles employing an anisotropic and an isotropic fit approach. We find that the energies of the crit. points of the optical consts. can be treated to be independent of the utilized model, whereas the magnitude of the optical consts. varies with the used model. The anisotropic model suggests a max. absorbance for a MoS2 sheet supported by sapphire of about 14% for monolayer and of 10% for trilayer MoS2. Furthermore, the lateral homogeneity of the complex dielec. function for monolayer MoS2 is investigated with a spatial resoln. of 2 μm. Only minor fluctuations are obsd. No evidence for strain, for a significant amt. of disorder or lattice defects can be found in the wrinkle-free regions of the MoS2 monolayer from complementary μ-Raman spectroscopy measurements. We assume that the minor lateral variation in the optical consts. are caused by lateral modification in the van der Waals interaction presumably caused by the prepn. using micromech. exfoliation and viscoelastic stamping.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVyqsrfK&md5=9095d41ecd14f8fc6e1ebd08045169fa
  41. 41
    Matković, A.; Beltaos, A.; Milićević, M.; Ralević, U.; Vasić, B.; Jovanović, D.; Gajić, R. Spectroscopic Imaging Ellipsometry and Fano Resonance Modeling of Graphene. J. Appl. Phys. 2012, 112 (12), 123523,  DOI: 10.1063/1.4771875
    Google Scholar
    41
    Spectroscopic imaging ellipsometry and Fano resonance modeling of graphene
    Matkovic, Aleksandar; Beltaos, Angela; Milicevic, Marijana; Ralevic, Uros; Vasic, Borislav; Jovanovic, Djordje; Gajic, Rados
    Journal of Applied Physics (Melville, NY, United States) (2012), 112 (12), 123523/1-123523/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    The optical properties were examd. of exfoliated graphene on an Si/SiO2 substrate using imaging ellipsometry in the visible range (360-800 nm). Measured spectra were analyzed by an optical model based on the Fresnel coeff. equations. The optical model was supported by correlated Raman and at. force microscopy measurements. The complex refractive index of graphene was obtained by inversion of the measured ellipsometry data. The Fano line-shape was used to parameterize the optical properties. Measurements were highly reliable due to the numerous advantages of the spectroscopic imaging ellipsometric technique combined with the proper choice of substrate and exptl. set-up. Thickness maps of the graphene sample were obtained from spatially resolved imaging ellipsometry spectra with a spot size of 1 μm. The data showed a H2O layer on the surface of the sample, and the thickness was mapped showing the distribution of H2O over graphene in ambient conditions. (c) 2012 American Institute of Physics.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvFSnur7P&md5=0bbe8d67044cf7f4d43c5657d845d24c
  42. 42
    Funke, S.; Wurstbauer, U.; Miller, B.; Matković, A.; Green, A.; Diebold, A.; Röling, C.; Thiesen, P. H. Spectroscopic Imaging Ellipsometry for Automated Search of Flakes of Mono- and n-Layers of 2D-Materials. Appl. Surf. Sci. 2017, 421, 435– 439,  DOI: 10.1016/j.apsusc.2016.10.158
    Google Scholar
    42
    Spectroscopic imaging ellipsometry for automated search of flakes of mono- and n-layers of 2D-materials
    Funke, S.; Wurstbauer, U.; Miller, B.; Matkovic, A.; Green, A.; Diebold, A.; Roeling, C.; Thiesen, P. H.
    Applied Surface Science (2017), 421 (Part_B), 435-439CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Spectroscopic imaging ellipsometry (SIE) is used to localize and characterize flakes of conducting, semi-conducting and insulating 2D-materials. Although the research in the field of monolayers of 2D-materials increased the last years, it is still challenging to look for small flakes and distinguish between different layer nos. Special substrates are used to enhance optical contrast for the conventional light microscopy (LM). In case when other functional support from the substrate is essential, an addnl. transfer step needs to be employed, bringing the drawbacks as contamination, cracking and wrinkling of the 2D materials. Furthermore it is time-consuming and not yet fully automatically to search for monolayers by contrast with the LM. Here we present a method, that is able to automatically localize regions with desired thicknesses, e.g. monolayers, of the different materials on arbitrary substrates.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVOgsrvF&md5=017f82239009e181ce68559b2eb3bb49
  43. 43
    Braeuninger-Weimer, P.; Funke, S.; Wang, R.; Thiesen, P.; Tasche, D.; Viöl, W.; Hofmann, S. Fast, Noncontact, Wafer-Scale, Atomic Layer Resolved Imaging of Two-Dimensional Materials by Ellipsometric Contrast Micrography. ACS Nano 2018, 12 (8), 8555– 8563,  DOI: 10.1021/acsnano.8b04167
    Google Scholar
    43
    Fast, Noncontact, Wafer-Scale, Atomic Layer Resolved Imaging of Two-Dimensional Materials by Ellipsometric Contrast Micrography
    Braeuninger-Weimer, Philipp; Funke, Sebastian; Wang, Ruizhi; Thiesen, Peter; Tasche, Daniel; Viol, Wolfgang; Hofmann, Stephan
    ACS Nano (2018), 12 (8), 8555-8563CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Adequate characterization and quality control of atomically thin layered materials (2DM) has become a serious challenge particularly given the rapid advancements in their large area manufg. and numerous emerging industrial applications with different substrate requirements. Here, the authors focus on ellipsometric contrast microg. (ECM), a fast intensity mode within spectroscopic imaging ellipsometry, and show that it can be effectively used for noncontact, large area characterization of 2DM to map coverage, layer no., defects and contamination. The authors demonstrate at. layer resolved, quant. mapping of chem. vapor deposited graphene layers on Si/SiO2-wafers, but also on rough Cu catalyst foils, highlighting that ECM is applicable to all application relevant substrates. The optimization of ECM parameters for high throughput characterization are discussed. While the lateral resoln. can be <1 μm, the authors particularly explore fast scanning and demonstrate imaging of a 4'' graphene wafer in 47 min at 10 μm lateral resoln., i.e., an imaging speed of 1.7 cm2/min. Also, the authors show ECM of monolayer hexagonal BN (h-BN) and of h-BN/graphene bilayers, highlighting that ECM is applicable to a wide range of 2-dimensional layered structures that were previously very challenging to characterize and thereby fills an important gap in 2DM metrol.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsVCqtrzK&md5=9b7aab8f43a3fdd82d0615a31cb5aae6
  44. 44
    Fujiwara, H. Spectroscopic Ellipsometry: Principles and Applications; Maruzen Co. Ltd.: Tokyo, Japan, 2003.
    Google Scholar
    There is no corresponding record for this reference.
  45. 45
    Dicke, J.; Rotermund, H.-H.; Lauterbach, J. Ellipsomicroscopy for Surface Imaging: Contrast Mechanism, Enhancement, and Application to CO Oxidation on Pt(110). J. Opt. Soc. Am. A 2000, 17 (1), 135,  DOI: 10.1364/JOSAA.17.000135
    Google Scholar
    45
    Ellipsomicroscopy for surface imaging: contrast mechanism, enhancement, and application to CO oxidation on Pt(110)
    Dicke, Jan; Rotermund, Harm-Hinrich; Lauterbach, Jochen
    Journal of the Optical Society of America A: Optics, Image Science, and Vision (2000), 17 (1), 135-141CODEN: JOAOD6; ISSN:0740-3232. (Optical Society of America)
    Ellipsomicroscopy for surface imaging (EMSI) is a powerful new tool for studying spatiotemporal adsorbate pattern formation on catalyst surfaces. It is a surface-sensitive technique that is able to measure submonolayer coverage of adsorbates. The imaging of the sample's surface achieves a spatial sensitivity, making it possible to measure nonuniformity of adsorbate coverage. The image contrast, however, depends strongly on the setup of the instrument. The optimum setup can be calcd. from the ellipsometric properties of the catalyst/adsorbate system and the intrinsic parameters of the EMSI instrument. Optimizing the setup of the EMSI instrument permitted enhancement of the image contrast over the previous setup. As a result, new features in CO oxidn. on Pt(110) were discovered.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3cXks1yksw%253D%253D&md5=6de5c0f3292ecae1af1d127c51a50f64
  46. 46
    Ducret, A.; Valignat, M.-P.; Mouhamar, F.; Mignot, T.; Theodoly, O. Wet-Surface–Enhanced Ellipsometric Contrast Microscopy Identifies Slime as a Major Adhesion Factor during Bacterial Surface Motility. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (25), 10036– 10041,  DOI: 10.1073/pnas.1120979109
    Google Scholar
    46
    Wet-surface-enhanced ellipsometric contrast microscopy identifies slime as a major adhesion factor during bacterial surface motility
    Ducret, Adrien; Valignat, Marie-Pierre; Mouhamar, Fabrice; Mignot, Tam; Theodoly, Olivier
    Proceedings of the National Academy of Sciences of the United States of America (2012), 109 (25), 10036-10041, S10036/1-S10036/8CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    In biol., the extracellular matrix (ECM) promotes both cell adhesion and specific recognition, which is essential for central developmental processes in both eukaryotes and prokaryotes. However, live studies of the dynamic interactions between cells and the ECM, for example during motility, have been greatly impaired by imaging limitations: mostly the ability to observe the ECM at high resoln. in absence of specific staining by live microscopy. To solve this problem, we developed a unique technique, wet-surface enhanced ellipsometry contrast (Wet-SEEC), which magnifies the contrast of transparent org. materials deposited on a substrate (called Wet-surf) with exquisite sensitivity. We show that Wet-SEEC allows both the observation of unprocessed nanofilms as low as 0.2 nm thick and their accurate 3D topog. reconstructions, directly by std. light microscopy. We next used Wet-SEEC to image slime secretion, a poorly defined property of many prokaryotic and eukaryotic organisms that move across solid surfaces in absence of obvious extracellular appendages (gliding). Using combined Wet-SEEC and fluorescent-staining expts., we obsd. slime deposition by gliding Myxococcus xanthus cells at unprecedented resoln. Altogether, the results revealed that in this bacterium, slime assocs. preferentially with the outermost components of the motility machinery and promotes its adhesion to the substrate on the ventral side of the cell. Strikingly, analogous roles have been proposed for the extracellular proteoglycans of gliding diatoms and apicomplexa, suggesting that slime deposition is a general means for gliding organisms to adhere and move over surfaces.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVKqtbjK&md5=b94477ca134f9cbc9033ead9efd546b6
  47. 47
    Affoune, A.; Prasad, B. L.; Sato, H.; Enoki, T.; Kaburagi, Y.; Hishiyama, Y. Experimental Evidence of a Single Nano-Graphene. Chem. Phys. Lett. 2001, 348 (1–2), 17– 20,  DOI: 10.1016/S0009-2614(01)01066-1
    Google Scholar
    47
    Experimental evidence of a single nano-graphene
    Affoune, A. M.; Prasad, B. L. V.; Sato, Hirohiko; Enoki, Toshiaki; Kaburagi, Yutaka; Hishiyama, Yoshihiro
    Chemical Physics Letters (2001), 348 (1,2), 17-20CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
    A single nano-sized graphene sheet is prepd. by a combination of electrophoretic deposition (EPD) and heat-treatment of diamond nano-particles on a highly oriented pyrolytic graphite (HOPG) substrate. Heat-treatment at 1600 °C converts diamond nano-particles to single nano-graphenes, where the mean in-plane size and the inter-layer distance from the substrate are estd. at 10-15 and 0.35-0.37 nm, resp. The considerably large inter-layer distance compared to bulk graphite suggests a large redn. of inter-layer interaction, although a nano-graphene is placed epitaxially on the substrate. The isolated single nano-graphene provides an important model of nano-sized π-electron system, for which recent theory predicts unconventional electronic structure of edge-inherited non-bonding state.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXovFejsro%253D&md5=c4fffe79604fa3263391caf0c8e9030e
  48. 48
    Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science (80-.). 2004, 306 (5696), 666– 669,  DOI: 10.1126/science.1102896
    Google Scholar
    48
    Electric Field Effect in Atomically Thin Carbon Films
    Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A.
    Science (Washington, DC, United States) (2004), 306 (5696), 666-669CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The authors describe monocryst. graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar elec. field effect such that electrons and holes in concns. up to 1013 per square centimeter and with room-temp. mobilities of ∼10,000 square centimeters per V-second can be induced by applying gate voltage.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXos1Kqt70%253D&md5=488da13500bf24e8fc419052dc1a9e84
  49. 49
    Nemes-Incze, P.; Osváth, Z.; Kamarás, K.; Biró, L. P. Anomalies in Thickness Measurements of Graphene and Few Layer Graphite Crystals by Tapping Mode Atomic Force Microscopy. Carbon N. Y. 2008, 46 (11), 1435– 1442,  DOI: 10.1016/j.carbon.2008.06.022
    Google Scholar
    49
    Anomalies in thickness measurements of graphene and few layer graphite crystals by tapping mode atomic force microscopy
    Nemes-Incze, P.; Osvath, Z.; Kamaras, K.; Biro, L. P.
    Carbon (2008), 46 (11), 1435-1442CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
    AFM in the tapping (intermittent contact) mode is a commonly used tool to measure the thickness of graphene and few layer graphene (FLG) flakes on silicon oxide surfaces. It is a convenient tool to quickly det. the thickness of individual FLG films. However, reports from literature show a large variation of the measured thickness of graphene layers. This paper is focused on the imaging mechanism of tapping mode AFM (TAFM) when measuring graphene and FLG thickness, and it is shown that at certain measurement parameters significant deviations can be introduced in the measured thickness of FLG flakes. An increase of as much as 1 nm can be obsd. in the measured height of FLG crystallites, when using an improperly chosen range of free amplitude values of the tapping cantilever. Comparative Raman spectroscopy and TAFM measurements on selected single and multilayer graphene films are presented, based on which ways are suggested to correctly measure graphene and FLG thickness using TAFM.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXpvFyhtLs%253D&md5=f1dbceb0716fd161e532d7ce544ddb30
  50. 50
    Eigler, S.; Hof, F.; Enzelberger-Heim, M.; Grimm, S.; Müller, P.; Hirsch, A. Statistical Raman Microscopy and Atomic Force Microscopy on Heterogeneous Graphene Obtained after Reduction of Graphene Oxide. J. Phys. Chem. C 2014, 118 (14), 7698– 7704,  DOI: 10.1021/jp500580g
    Google Scholar
    50
    Statistical Raman Microscopy and Atomic Force Microscopy on Heterogeneous Graphene Obtained after Reduction of Graphene Oxide
    Eigler, Siegfried; Hof, Ferdinand; Enzelberger-Heim, Michael; Grimm, Stefan; Mueller, Paul; Hirsch, Andreas
    Journal of Physical Chemistry C (2014), 118 (14), 7698-7704CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Graphene oxide can be used as a precursor to graphene, but the quality of graphene flakes is highly heterogeneous. Scanning Raman microscopy (SRM) is used to characterize films of graphene derived from flakes of graphene oxide with an almost intact carbon framework (ai-GO). The defect d. of these flakes is visualized in detail by analyzing the intensity and full width at half-max. of the most pronounced Raman peaks. In addn., the authors superimpose the SRM results with AFM images and correlate the spectroscopic results with the morphol. Furthermore, the authors use the SRM technique to display the amt. of defects in a film of graphene. An area of 250 × 250 μm2 of graphene was probed with a step-size increment of 1 μm. The position of graphene flakes, edges, and the substrate were visualized. Finally, the authors alter parameters of measurement to analyze the quality of graphene in a fast and reliable way. The described method can be used to probe and visualize the quality of graphene films.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXksVWisr4%253D&md5=10eb129030930a8b56ff82179676f9bb
  51. 51
    Obraztsova, E. A.; Osadchy, A. V.; Obraztsova, E. D.; Lefrant, S.; Yaminsky, I. V. Statistical Analysis of Atomic Force Microscopy and Raman Spectroscopy Data for Estimation of Graphene Layer Numbers. Phys. status solidi 2008, 245 (10), 2055– 2059,  DOI: 10.1002/pssb.200879657
    Google Scholar
    51
    Statistical analysis of atomic force microscopy and Raman spectroscopy data for estimation of graphene layer numbers
    Obraztsova, Ekaterina A.; Osadchy, Alexander V.; Obraztsova, Elena D.; Lefrant, Serge; Yaminsky, Igor V.
    Physica Status Solidi B: Basic Solid State Physics (2008), 245 (10), 2055-2059CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We present the results on prepn. and anal. of structural and electronic properties of graphene flakes with different no. of layers. Since the material electronic properties depend strongly on the layer no., a simple and reliable way to est. this parameter should be found. We propose to perform a quant. anal. based on the at. force microscopy (AFM) which was exploited by another authors rather as an illustrative technique. The graphene flakes were prepd. by a micromech. cleavage of highly oriented pyrolytic graphite (HOPG). Numerous flakes were studied with AFM to est. the no. of graphene layers in each of them. The thickness distribution over the individual graphene flake, averaged over the all flakes investigated, has exhibited a row of maxima: 0.7 nm, 1.1 nm, 1.4 nm, 1.7 nm, 2.1 nm, etc. These values correspond perfectly to the one-, 2-, 3-, and etc layers of graphene. The flakes characterized by AFM were studied afterwards with a 2-phonon Raman spectroscopy. This method is known to be sensitive to the no. of layers due to a "double resonance" mechanism. To interpret the Raman spectra the electronic structure of the graphene flakes of different thickness was calcd. using the d. functional method. A perfect correspondence between the no. of graphene layers in different flakes measured by AFM and by Raman techniques was obtained. This demonstrates a possibility to est. correctly the graphene layer no. using the at. force microscopy supplied with the statistical anal.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXht12itr3N&md5=59362456f5f3efb311700f170a2def27
  52. 52
    Darakchieva, V.; Boosalis, A.; Zakharov, A. A.; Hofmann, T.; Schubert, M.; Tiwald, T. E.; Iakimov, T.; Vasiliauskas, R.; Yakimova, R. Large-Area Microfocal Spectroscopic Ellipsometry Mapping of Thickness and Electronic Properties of Epitaxial Graphene on Si- and C-Face of 3C-SiC(111). Appl. Phys. Lett. 2013, 102 (21), 213116,  DOI: 10.1063/1.4808379
    Google Scholar
    52
    Large-area microfocal spectroscopic ellipsometry mapping of thickness and electronic properties of epitaxial graphene on Si- and C-face of 3C-SiC(111)
    Darakchieva, V.; Boosalis, A.; Zakharov, A. A.; Hofmann, T.; Schubert, M.; Tiwald, T. E.; Iakimov, T.; Vasiliauskas, R.; Yakimova, R.
    Applied Physics Letters (2013), 102 (21), 213116/1-213116/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Microfocal spectroscopic ellipsometry mapping of the electronic properties and thickness of epitaxial graphene grown by high-temp. sublimation on 3C-SiC (111) substrates is reported. Growth of one monolayer graphene is demonstrated on both Si- and C-polarity of the 3C-SiC substrates and it is shown that large area homogeneous single monolayer graphene can be achieved on the Si-face substrates. Correlations between the no. of graphene monolayers on one hand and the main transition assocd. with an exciton enhanced van Hove singularity at ∼4.5 eV and the free-charge carrier scattering time, on the other are established. It is shown that the interface structure on the Si- and C-polarity of the 3C-SiC(111) differs and has a detg. role for the thickness and electronic properties homogeneity of the epitaxial graphene. (c) 2013 American Institute of Physics.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXosFeksb4%253D&md5=5f04a1565c0024b1d4993e080fd92e25
  53. 53
    Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102 (30), 10451– 10453,  DOI: 10.1073/pnas.0502848102
    Google Scholar
    53
    Two-dimensional atomic crystals
    Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K.
    Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (30), 10451-10453CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The authors report free-standing at. crystals that are strictly 2-dimensional and can be viewed as individual at. planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromech. cleavage, the authors prepd. and studied a variety of 2-dimensional crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2-dimensional mols. unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXntVSit7g%253D&md5=1ce9e5f5eb0f7b9abb033d4a690d49c3
  54. 54
    Lopes dos Santos, J. M. B.; Peres, N. M. R.; Castro Neto, A. H. Graphene Bilayer with a Twist: Electronic Structure. Phys. Rev. Lett. 2007, 99 (25), 256802,  DOI: 10.1103/PhysRevLett.99.256802
    Google Scholar
    54
    Graphene Bilayer with a Twist: Electronic Structure
    Lopes dos Santos, J. M. B.; Peres, N. M. R.; Castro Neto, A. H.
    Physical Review Letters (2007), 99 (25), 256802/1-256802/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We consider a graphene bilayer with a relative small angle rotation between the layers-a stacking defect often seen in the surface of graphite-and calc. the electronic structure near zero energy in a continuum approxn. Contrary to what happens in an AB stacked bilayer and in accord with observations in epitaxial graphene, we find: (a) the low energy dispersion is linear, as in a single layer, but the Fermi velocity can be significantly smaller than the single-layer value; (b) an external elec. field, perpendicular to the layers, does not open an electronic gap.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXhsVOlt7bL&md5=8d685de8e04798b9a6cac63aca485df7
  55. 55
    Ferrari, A. C. Raman Spectroscopy of Graphene and Graphite: Disorder, Electron–Phonon Coupling, Doping and Nonadiabatic Effects. Solid State Commun. 2007, 143 (1–2), 47– 57,  DOI: 10.1016/j.ssc.2007.03.052
    Google Scholar
    55
    Raman spectroscopy of graphene and graphite: Disorder, electron-phonon coupling, doping and nonadiabatic effects
    Ferrari, Andrea C.
    Solid State Communications (2007), 143 (1-2), 47-57CODEN: SSCOA4; ISSN:0038-1098. (Elsevier Ltd.)
    The authors review recent work on Raman spectroscopy of graphite and graphene. The authors focus on the origin of the D and G peaks and the 2nd order of the D peak. The G and 2 D Raman peaks change in shape, position and relative intensity with no. of graphene layers. This reflects the evolution of the electronic structure and electron-phonon interactions. The authors then consider the effects of doping on the Raman spectra of graphene. The Fermi energy is tuned by applying a gate-voltage. This induces a stiffening of the Raman G peak for both holes and electrons doping. Thus Raman spectroscopy can be efficiently used to monitor no. of layers, quality of layers, doping level and confinement.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXmt1yhtr0%253D&md5=b67986a7f5f92c4a5ab64950f3330d7a
  56. 56
    Deng, S.; Berry, V. Wrinkled, Rippled and Crumpled Graphene: An Overview of Formation Mechanism, Electronic Properties, and Applications. Mater. Today 2016, 19 (4), 197– 212,  DOI: 10.1016/j.mattod.2015.10.002
    Google Scholar
    56
    Wrinkled, rippled and crumpled graphene: an overview of formation mechanism, electronic properties, and applications
    Deng, Shikai; Berry, Vikas
    Materials Today (Oxford, United Kingdom) (2016), 19 (4), 197-212CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
    Distinctive from their 1D and 0D counterparts, 2D nanomaterials (2DNs) exhibit surface corrugations (wrinkles and ripples) and crumples. Thermal vibrations, edge instabilities, thermodynamically unstable (interat.) interactions, strain in 2D crystals, thermal contraction, dislocations, solvent trapping, pre-strained substrate-relaxation, surface anchorage and high solvent surface tension during transfer cause wrinkles or ripples to form on graphene. These corrugations on graphene can modify its electronic structure, create polarized carrier puddles, induce pseudomagnetic field in bilayers and alter surface properties. This review outlines the different mechanisms of wrinkle, ripple and crumple formation, and the interplay between wrinkles' and ripples' attributes (wavelength/width, amplitude/height, length/size, and bending radius) and graphene's electronic properties and other mech., optical, surface, and chem. properties. Also included are brief discussions on corrugation-induced reversible wettability and transmittance in graphene, modulation of its chem. potential, enhanced energy storage and strain sensing via relaxation of corrugations. Finally, the review summarizes the future areas of research for 2D corrugations and crumples.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhs1OntrnF&md5=562c71f24c7b2b4c7b2bac0aab2b4755
  57. 57
    Hattab, H.; N’Diaye, A. T.; Wall, D.; Klein, C.; Jnawali, G.; Coraux, J.; Busse, C.; van Gastel, R.; Poelsema, B.; Michely, T. Interplay of Wrinkles, Strain, and Lattice Parameter in Graphene on Iridium. Nano Lett. 2012, 12 (2), 678– 682,  DOI: 10.1021/nl203530t
    Google Scholar
    57
    Interplay of Wrinkles, Strain, and Lattice Parameter in Graphene on Iridium
    Hattab, Hichem; N'Diaye, Alpha T.; Wall, Dirk; Klein, Claudius; Jnawali, Giriraj; Coraux, Johann; Busse, Carsten; van Gastel, Raoul; Poelsema, Bene; Michely, Thomas; Meyer-zu Heringdorf, Frank-J.; Horn-von Hoegen, Michael
    Nano Letters (2012), 12 (2), 678-682CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Following graphene growth by thermal decompn. of ethylene on Ir(111) at high temps. we analyzed the strain state and the wrinkle formation kinetics as function of temp. Using the moire spot sepn. in a LEED pattern as a magnifying mechanism for the difference in the lattice parameters between Ir and graphene, we achieved an unrivaled relative precision of ±0.1 pm for the graphene lattice parameter. Our data reveals a characteristic hysteresis of the graphene lattice parameter that is explained by the interplay of reversible wrinkle formation and film strain. We show that graphene on Ir(111) always exhibits residual compressive strain at room temp. Our results provide important guidelines for strategies to avoid wrinkling.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhs1aqu7rK&md5=dc01208579b550f5dece4384a508ca80
  58. 58
    Yan, W.; He, W.-Y.; Chu, Z.-D.; Liu, M.; Meng, L.; Dou, R.-F.; Zhang, Y.; Liu, Z.; Nie, J.-C.; He, L. Strain and Curvature Induced Evolution of Electronic Band Structures in Twisted Graphene Bilayer. Nat. Commun. 2013, 4 (1), 2159,  DOI: 10.1038/ncomms3159
    Google Scholar
    58
    Strain and curvature induced evolution of electronic band structures in twisted graphene bilayer
    Yan Wei; He Wen-Yu; Chu Zhao-Dong; Liu Mengxi; Meng Lan; Dou Rui-Fen; Zhang Yanfeng; Liu Zhongfan; Nie Jia-Cai; He Lin
    Nature communications (2013), 4 (), 2159 ISSN:.
    It is well established that strain and geometry could affect the band structure of graphene monolayer dramatically. Here we study the evolution of local electronic properties of a twisted graphene bilayer induced by a strain and a high curvature, which are found to strongly affect the local band structures of the twisted graphene bilayer. The energy difference of the two low-energy van Hove singularities decreases with increasing lattice deformation and the states condensed into well-defined pseudo-Landau levels, which mimic the quantization of massive chiral fermions in a magnetic field of about 100 T, along a graphene wrinkle. The joint effect of strain and out-of-plane distortion in the graphene wrinkle also results in a valley polarization with a significant gap. These results suggest that strained graphene bilayer could be an ideal platform to realize the high-temperature zero-field quantum valley Hall effect.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC3sfgslWktQ%253D%253D&md5=dcfe69882c5f7f0082881e0258abeca8
  59. 59
    Geisenhof, F. R.; Winterer, F.; Seiler, A. M.; Lenz, J.; Martin, I.; Weitz, R. T. Interplay between Topological Valley and Quantum Hall Edge Transport. Nat. Commun. 2022, 13 (1), 4187,  DOI: 10.1038/s41467-022-31680-y
    Google Scholar
    59
    Interplay between topological valley and quantum Hall edge transport
    Geisenhof, Fabian R.; Winterer, Felix; Seiler, Anna M.; Lenz, Jakob; Martin, Ivar; Weitz, R. Thomas
    Nature Communications (2022), 13 (1), 4187CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
    An established way of realizing topol. protected states in a two-dimensional electron gas is by applying a perpendicular magnetic field thus creating quantum Hall edge channels. In electrostatically gapped bilayer graphene intriguingly, even in the absence of a magnetic field, topol. protected electronic states can emerge at naturally occurring stacking domain walls. While individually both types of topol. protected states have been investigated, their intriguing interplay remains poorly understood. Here, we focus on the interplay between topol. domain wall states and quantum Hall edge transport within the eight-fold degenerate zeroth Landau level of high-quality suspended bilayer graphene. We find that the two-terminal conductance remains approx. const. for low magnetic fields throughout the distinct quantum Hall states since the conduction channels are traded between domain wall and device edges. For high magnetic fields, however, we observe evidence of transport suppression at the domain wall, which can be attributed to the emergence of spectral minigaps. This indicates that stacking domain walls potentially do not correspond to a topol. domain wall in the order parameter.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhvFWnsL%252FL&md5=de57f46ac5662edc0eff9c88ca51dae9
  60. 60
    Keunecke, M.; Reutzel, M.; Schmitt, D.; Osterkorn, A.; Mishra, T. A.; Möller, C.; Bennecke, W.; Jansen, G. S. M.; Steil, D.; Manmana, S. R. Electromagnetic Dressing of the Electron Energy Spectrum of Au(111) at High Momenta. Phys. Rev. B 2020, 102 (16), 161403,  DOI: 10.1103/PhysRevB.102.161403
    Google Scholar
    60
    Electromagnetic dressing of the electron energy spectrum of Au(111) at high momenta
    Keunecke, Marius; Reutzel, Marcel; Schmitt, David; Osterkorn, Alexander; Mishra, Tridev A.; Moeller, Christina; Bennecke, Wiebke; Jansen, G. S. Matthijs; Steil, Daniel; Manmana, Salvatore R.; Steil, Sabine; Kehrein, Stefan; Mathias, Stefan
    Physical Review B (2020), 102 (16), 161403CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Light-engineering of quantum materials via electromagnetic dressing is considered an on-demand approach for tailoring electronic band dispersions and even inducing topol. phase transitions. For probing such dressed bands, photoemission spectroscopy is an ideal tool, and we employ here a novel expt. based on ultrafast photoemission momentum microscopy. Using this setup, we measure the in-plane momentum-dependent intensity fingerprints of the electromagnetically-dressed sidebands from a Au(111) surface for s- and p-polarized IR driving. We find that at metal surfaces, due to screening of the driving laser, the contribution from Floquet-Bloch bands is negligible, and the dressed bands are dominated by the laser-assisted photoelec. effect. Also, we find, from calcns., that in contrast to general expectations, s-polarized light can dress free-electron states at large photoelectron momenta. Our results show that the dielec. response of the material must carefully be taken into account when using photoemission for the identification of light-engineered electronic band structures.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitlChtrbP&md5=403293d2a71cd6ad51acaf06a1fb7377
  61. 61
    Keunecke, M.; Möller, C.; Schmitt, D.; Nolte, H.; Jansen, G. S. M.; Reutzel, M.; Gutberlet, M.; Halasi, G.; Steil, D.; Steil, S. Time-Resolved Momentum Microscopy with a 1 MHz High-Harmonic Extreme Ultraviolet Beamline. Rev. Sci. Instrum. 2020, 91 (6), 063905,  DOI: 10.1063/5.0006531
    Google Scholar
    61
    Time-resolved momentum microscopy with a 1 MHz high-harmonic extreme ultraviolet beamline
    Keunecke, Marius; Moeller, Christina; Schmitt, David; Nolte, Hendrik; Jansen, G. S. Matthijs; Reutzel, Marcel; Gutberlet, Marie; Halasi, Gyula; Steil, Daniel; Steil, Sabine; Mathias, Stefan
    Review of Scientific Instruments (2020), 91 (6), 063905CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
    A setup for time-resolved momentum microscopy driven by a 1 MHz fs extreme UV table-top light source optimized for the generation of 26.5 eV photons is presented. The setup provides simultaneous access to the temporal evolution of the photoelectron's kinetic energy and in-plane momentum. Opportunities and limitations of the new expt. based on static and time-resolved measurements on graphene are discussed. (c) 2020 American Institute of Physics.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhtlShsr7N&md5=6d822e6192cc76b35fdd9c00f7632fed
  62. 62
    Medjanik, K.; Fedchenko, O.; Chernov, S.; Kutnyakhov, D.; Ellguth, M.; Oelsner, A.; Schonhense, B.; Peixoto, T. R. F.; Lutz, P.; Min, C.-H.; Reinert, F.; Daster, S.; Acremann, Y.; Viefhaus, J.; Wurth, W.; Elmers, H. J.; Schonhense, G. Direct 3D Mapping of the Fermi Surface and Fermi Velocity. Nat. Mater. 2017, 16 (6), 615– 621,  DOI: 10.1038/nmat4875
    Google Scholar
    6150
    Direct 3D mapping of the Fermi surface and Fermi velocity
    Medjanik, K.; Fedchenko, O.; Chernov, S.; Kutnyakhov, D.; Ellguth, M.; Oelsner, A.; Schoenhense, B.; Peixoto, T. R. F.; Lutz, P.; Min, C.-H.; Reinert, F.; Daester, S.; Acremann, Y.; Viefhaus, J.; Wurth, W.; Elmers, H. J.; Schoenhense, G.
    Nature Materials (2017), 16 (6), 615-621CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    The authors performed a full mapping of the bulk electronic structure including the Fermi surface and Fermi-velocity distribution vF(kF) of tungsten. The 4D spectral function ρ(EB; k) in the entire bulk Brillouin zone and 6 eV binding-energy (EB) interval was acquired in ∼3 h thanks to a new multidimensional photoemission data-recording technique (combining full-field k-microscopy with time-of-flight parallel energy recording) and the high brilliance of the soft x-rays used. A direct comparison of bulk and surface spectral functions (taken at low photon energies) reveals a time-reversal-invariant surface state in a local band gap in the (110)-projected bulk band structure. The surface state connects hole and electron pockets that would otherwise be sepd. by an indirect local band gap. The authors confirmed its Dirac-like spin texture by spin-filtered momentum imaging. The measured 4D data array enables extn. of the 3D dispersion of all bands, all energy isosurfaces, electron velocities, hole or electron cond., effective mass and inner potential by simple algorithms without approxns. The high-Z bcc metals with large spin-orbit-induced band gaps are discussed as candidates for topol. non-trivial surface states.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXlt12jtbc%253D&md5=cb6897e75ca0a56a1de5d17f69c38f77
  63. 63
    Schmitt, D.; Bange, J. P.; Bennecke, W.; AlMutairi, A.; Meneghini, G.; Watanabe, K.; Taniguchi, T.; Steil, D.; Luke, D. R.; Weitz, R. T. Formation of Moiré Interlayer Excitons in Space and Time. Nature 2022, 608 (7923), 499– 503,  DOI: 10.1038/s41586-022-04977-7
    Google Scholar
    62
    Formation of moir´e interlayer excitons in space and time
    Schmitt, David; Bange, Jan Philipp; Bennecke, Wiebke; AlMutairi, AbdulAziz; Meneghini, Giuseppe; Watanabe, Kenji; Taniguchi, Takashi; Steil, Daniel; Luke, D. Russell; Weitz, R. Thomas; Steil, Sabine; Jansen, G. S. Matthijs; Brem, Samuel; Malic, Ermin; Hofmann, Stephan; Reutzel, Marcel; Mathias, Stefan
    Nature (London, United Kingdom) (2022), 608 (7923), 499-503CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
    To exploit the full potential of correlated moir´e and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement is indispensable. Femtosecond photoemission momentum microscopy provides quant. access to these key properties of the moir´e interlayer excitons. Interlayer excitons are dominantly formed through femtosecond exciton-phonon scattering and subsequent charge transfer at the interlayer-hybridized Σ valleys. Interlayer excitons exhibit a momentum fingerprint that is a direct hallmark of the superlattice moir´e modification. The wavefunction distribution of the electronic part of the exciton are reconstructed, and the size with the real-space moir´e superlattice are compared. The work provides direct access to interlayer exciton formation dynamics in space and time and reveals opportunities to study correlated moir´e and exciton physics for the future realization of exotic quantum phases of matter.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XitFGlsrfE&md5=7e9fade5c584dfb037f493eaeac5bb69
  64. 64
    Magnozzi, M.; Pflug, T.; Ferrera, M.; Pace, S.; Ramó, L.; Olbrich, M.; Canepa, P.; Ağircan, H.; Horn, A.; Forti, S. Local Optical Properties in CVD-Grown Monolayer WS 2 Flakes. J. Phys. Chem. C 2021, 125 (29), 16059– 16065,  DOI: 10.1021/acs.jpcc.1c04287
    Google Scholar
    63
    Local Optical Properties in CVD-Grown Monolayer WS2 Flakes
    Magnozzi, Michele; Pflug, Theo; Ferrera, Marzia; Pace, Simona; Ramo, Lorenzo; Olbrich, Markus; Canepa, Paolo; Agircan, Hasret; Horn, Alexander; Forti, Stiven; Cavalleri, Ornella; Coletti, Camilla; Bisio, Francesco; Canepa, Maurizio
    Journal of Physical Chemistry C (2021), 125 (29), 16059-16065CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably ext. information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielec. function of chem. vapor deposition (CVD)-grown WS2 flakes with a microscopic lateral resoln., thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS2. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was obsd., along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modulations across the flakes, having a different influence on the absorption and re-emission of light, are deemed responsible for the effect.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhsFelu7bF&md5=a1177ae4d8e39978b77e2e85a96d98c6
  65. 65
    Peci, E.; Magnozzi, M.; Ramó, L.; Ferrera, M.; Convertino, D.; Pace, S.; Orlandini, G.; Sharma, A.; Milekhin, I.; Salvan, G. Dielectric Function of 2D Tungsten Disulfide in Homo- and Heterobilayer Stacking. Adv. Mater. Interfaces 2023, 10 (3), 2201586,  DOI: 10.1002/admi.202201586
    Google Scholar
    64
    Dielectric Function of 2D Tungsten Disulfide in Homo- and Heterobilayer Stacking
    Peci, Ermes; Magnozzi, Michele; Ramo, Lorenzo; Ferrera, Marzia; Convertino, Domenica; Pace, Simona; Orlandini, Giorgio; Sharma, Apoorva; Milekhin, Ilya; Salvan, Georgeta; Coletti, Camilla; Zahn, Dietrich R. T.; Bisio, Francesco; Canepa, Maurizio
    Advanced Materials Interfaces (2023), 10 (3), 2201586CODEN: AMIDD2; ISSN:2196-7350. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The opto-electronic properties of semiconducting 2D materials can be flexibly manipulated by engineering the at.-scale environment. This can be done by including 2D materials in tailored van der Waals (vdW) stacks, whose optical response is a function of the no. and the type of adjacent 2D layers. This work reports a systematic investigation of the dielec. function of 2D semiconducting WS2 in various stacking configurations: monolayer, 3R/2H homobilayer, and WS2/MoS2 heterobilayer. Reliable, Kramers-Kronig-consistent dielec. functions are obtained for WS2 in each configuration by means of spectroscopic ellipsometry (SE) and related parametric optical modeling in a wide spectral range (1.55-3.10 eV). The results of SE are combined with photoluminescence and absorbance spectra to identify the spectral position of the main excitonic features in WS2, which manifest sizable red shifts depending on the stacking configuration. These results represent a consistent ref. set for the dielec. function of WS2 in vdW stacking configurations of particular interest for the scientific and technol. field, and can be fruitfully exploited for reliable predictions of the optical response of WS2-contg. systems.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XjtVehsbjL&md5=1749143037903b79d2283a0a8b1b74bd
  66. 66
    Kenaz, R.; Rapaport, R. Mapping Spectroscopic Micro-Ellipsometry with Sub-5 Microns Lateral Resolution and Simultaneous Broadband Acquisition at Multiple Angles. Rev. Sci. Instrum. 2023, 94 (2), 023908,  DOI: 10.1063/5.0123249
    Google Scholar
    65
    Mapping spectroscopic micro-ellipsometry with sub-5 microns lateral resolution and simultaneous broadband acquisition at multiple angles
    Kenaz, Ralfy; Rapaport, Ronen
    Review of Scientific Instruments (2023), 94 (2), 023908CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
    Spectroscopic ellipsometry is a widely used optical technique in both industry and research for detg. the optical properties and thickness of thin films. The effective use of spectroscopic ellipsometry on micro-structures is inhibited by tech. limitations on the lateral resoln. and data acquisition rate. Here, we introduce a spectroscopic micro-ellipsometer (SME), capable of recording spectrally resolved ellipsometric data simultaneously at multiple angles of incidence in a single measurement of a few seconds, with a lateral resoln. down to 2μm in the visible spectral range. The SME can be easily integrated into generic optical microscopes by the addn. of a few std. optical components. We demonstrate complex refractive index and thickness measurements by using the SME, which are in excellent agreement with a com. spectroscopic ellipsometer. The high lateral resoln. is displayed by complex refractive index and thickness maps over micron-scale areas. As an application for its accuracy and high lateral resoln., the SME can characterize the optical properties and no. of layers of exfoliated transition-metal dichalcogenides and graphene, for structures that are a few microns in size. (c) 2023 American Institute of Physics.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3sXjvFCntLs%253D&md5=779991e82edd6434df18e752ceeb037f

Cited By

Click to copy section linkSection link copied!

This article has not yet been cited by other publications.

Download PDF
Open PDF
Go to Nano Letters
Get e-Alerts
Get e-Alerts

Nano Letters

Cite this: Nano Lett. 2023, 23, 12, 5506–5513
Click to copy citationCitation copied!
https://doi.org/10.1021/acs.nanolett.3c00619
Published June 8, 2023

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY 4.0 .

Article Views

2742

Altmetric

-

Citations

-
Learn about these metrics

Article Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.

Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.

The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated.

  • Abstract

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 1

    Figure 1. (a) Optical microscopy image of a graphene flake on the Si/SiO2 substrate. Dashed square represents the region studied in SECM and Raman spectroscopy. (b) Schematic showing the Brillouin zone and twist angle θt, band structure for twisted bilayer graphene, and density of states of twisted bilayer graphene at different twist angles with the horizontal lines corresponding to the highest and lowest energy transitions for the wavelength range studied. Fitting of Δ (c) and ψ (d) of the monolayer (ML), bilayer (BL) and resonant bilayer (BLVHS) to the Si/SiO2/graphene (+ Gaussian) layer stack model. Imaging RCE mapping of Δ (e) and ψ (f) at λ = 440 nm. (g) Graphene thickness map of the flake at λ = 440 nm as fitted using a Si/SiO2/graphene model, without accounting for resonances.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 2

    Figure 2. Optimising ellipsometer parameters for bilayer graphene on Si/SiO2. (a) ECM images of graphene flake at different angles of incidence (AOI). (b) Rp coefficient as a function of AOI with calculated Weber contrast. (c) Normalized intensity as measured for P and A rotation on Si/SiO2/graphene at C = 0° (left) and C = 45° (right).

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 3

    Figure 3. (a) Rp and Rs reflection coefficients as a function of wavelength for monolayer (ML), bilayer (BL), and resonant bilayer (BLVHS) regions as labeled in Figure 1g with calculated Weber contrast above each plot. (b) Resonant wavelength map of graphene extracted by fitting a Gaussian peak to the intensity as a function of wavelength at each pixel measured in SECM. (c) Reflected intensity from SECM as a function of wavelength showing absorption resonances for the regions marked in (b) (i–iii). (d) Absorption peak as a function of twist angle.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 4

    Figure 4. (a) Raman spectra of graphene at different twist angles, normalized with respect to 2D peak intensity. (b) Raman 2D-peak width map of graphene. (c) Raman 2D-peak width of graphene as a function of bilayer twist angle. (d) Raman G peak amplitude map of graphene. (e) Raman G-peak amplitude as a function of bilayer twist angle. The error bars on the plots represent the standard deviation.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 5

    Figure 5. (a) AFM map of the graphene flake on Si/SiO2 substrate indicating the region marked in (c). (b) AFM profile of graphene across the wrinkle in the AFM map in (a), indicating four layers of graphene. (c) Map of the second harmonic (s2) IR SNOM signal and the corresponding resonant energy map of the graphene flake showing different twist angle domains of graphene with annotations of the studied areas. (d, f) ARPES data obtained in the regions-of-interest indicated by circles in (c). In the inset, momentum–momentum cuts are shown for energies close to the Fermi level. The energy-momentum cut is taken along the black line indicated in the insets. (e, g) The vHs in the valence band can be identified when the second derivative along the energy axis is applied to the data (energy position highlighted by red arrows). Spectrum along the ky-direction, as shown in the inset momentum map, showing the band structure of twisted bilayer graphene regions with Ec = 1.77 eV (d) and Ec = 2.70 eV (g) as determined by SECM. The corresponding integrated intensity spectra ((e) and (g), respectively) each show a resonant feature at ∼Ec/2 (arrow).

    High Resolution Image
    Download MS PowerPoint Slide

  • References

    This article references 66 other publications.

    1. 1
      Rickhaus, P.; Wallbank, J.; Slizovskiy, S.; Pisoni, R.; Overweg, H.; Lee, Y.; Eich, M.; Liu, M. H.; Watanabe, K.; Taniguchi, T. Transport Through a Network of Topological Channels in Twisted Bilayer Graphene. Nano Lett. 2018, 18 (11), 6725– 6730,  DOI: 10.1021/acs.nanolett.8b02387
      1
      Transport Through a Network of Topological Channels in Twisted Bilayer Graphene
      Rickhaus, Peter; Wallbank, John; Slizovskiy, Sergey; Pisoni, Riccardo; Overweg, Hiske; Lee, Yongjin; Eich, Marius; Liu, Ming-Hao; Watanabe, Kenji; Taniguchi, Takashi; Ihn, Thomas; Ensslin, Klaus
      Nano Letters (2018), 18 (11), 6725-6730CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The authors explore a network of electronic quantum valley Hall states in the moir´e crystal of minimally twisted bilayer graphene. In the authors' transport measurements, the authors observe Fabry-P´erot and Aharanov-Bohm oscillations that are robust in magnetic fields ranging from 0 to 8 T, which is in strong contrast to more conventional two-dimensional systems where trajectories in the bulk are bent by the Lorentz force. This persistence in magnetic field and the linear spacing in d. indicate that charge carriers in the bulk flow in topol. protected, 1-dimensional channels. With this work, the authors demonstrate coherent electronic transport in a lattice of topol. protected states.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvFWkt7vI&md5=4b046eb3006aa02958bd1c52c0703243
    2. 2
      Yin, J.; Wang, H.; Peng, H.; Tan, Z.; Liao, L.; Lin, L.; Sun, X.; Koh, A. L.; Chen, Y.; Peng, H. Selectively Enhanced Photocurrent Generation in Twisted Bilayer Graphene with van Hove Singularity. Nat. Commun. 2016, 7, 10699,  DOI: 10.1038/ncomms10699
      2
      Selectively enhanced photocurrent generation in twisted bilayer graphene with van Hove singularity
      Yin, Jianbo; Wang, Huan; Peng, Han; Tan, Zhenjun; Liao, Lei; Lin, Li; Sun, Xiao; Koh, Ai Leen; Chen, Yulin; Peng, Hailin; Liu, Zhongfan
      Nature Communications (2016), 7 (), 10699CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Graphene with ultra-high carrier mobility and ultra-short photoresponse time has shown remarkable potential in ultrafast photodetection. However, the broad and weak optical absorption (∼2.3%) of monolayer graphene hinders its practical application in photodetectors with high responsivity and selectivity. Here we demonstrate that twisted bilayer graphene, a stack of two graphene monolayers with an interlayer twist angle, exhibits a strong light-matter interaction and selectively enhanced photocurrent generation. Such enhancement is attributed to the emergence of unique twist-angle-dependent van Hove singularities, which are directly revealed by spatially resolved angle-resolved photoemission spectroscopy. When the energy interval between the van Hove singularities of the conduction and valance bands matches the energy of incident photons, the photocurrent generated can be significantly enhanced (up to ∼80 times with the integration of plasmonic structures in our devices). These results provide valuable insight for designing graphene photodetectors with enhanced sensitivity for variable wavelength.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XjslGntr4%253D&md5=e04571669b7fcd0ca9aa62ae2c739ed7
    3. 3
      Padhi, B.; Setty, C.; Phillips, P. W. Doped Twisted Bilayer Graphene near Magic Angles: Proximity to Wigner Crystallization, Not Mott Insulation. Nano Lett. 2018, 18 (10), 6175– 6180,  DOI: 10.1021/acs.nanolett.8b02033
      3
      Doped Twisted Bilayer Graphene near Magic Angles: Proximity to Wigner Crystallization, Not Mott Insulation
      Padhi, Bikash; Setty, Chandan; Phillips, Philip W.
      Nano Letters (2018), 18 (10), 6175-6180CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The authors devise a model to explain why twisted bilayer graphene exhibits insulating behavior when ν = 2 or 3 charges occupy a unit moir´e cell, a feature attributed to Mottness per previous work but not for ν = 1, clearly inconsistent with Mott insulation. The authors compute rs = EU/EK, where EU and EK are the potential and kinetic energies, resp., and show that (i) the Mott criterion lies at a d. larger than exptl. values by a factor of 104 and (ii) a transition to Wigner cryst. states exists as a function of ν. For ν = 1, rs fails to cross the threshold (rs = 37) for the triangular lattice, and metallic transport ensues. However, for ν = 2 and ν = 3, the thresholds rs = 22 and rs = 17, resp., are satisfied for a transition to Wigner crystals (WCs) with a honeycomb (ν = 2) and a kagome (ν = 3) structure. The authors posit that such cryst. states form the correct starting point for analyzing supercond.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhs1Khur3L&md5=6575b6c6cd34e2d94575362414e362cf
    4. 4
      Cao, Y.; Fatemi, V.; Demir, A.; Fang, S.; Tomarken, S. L.; Luo, J. Y.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Kaxiras, E. Correlated Insulator Behaviour at Half-Filling in Magic-Angle Graphene Superlattices. Nature 2018, 556 (7699), 80– 84,  DOI: 10.1038/nature26154
      4
      Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
      Cao, Yuan; Fatemi, Valla; Demir, Ahmet; Fang, Shiang; Tomarken, Spencer L.; Luo, Jason Y.; Sanchez-Yamagishi, Javier D.; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Ashoori, Ray C.; Jarillo-Herrero, Pablo
      Nature (London, United Kingdom) (2018), 556 (7699), 80-84CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials. One such property is the 'twist' angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analog in other types of heterostructure, such as semiconductors grown using mol. beam epitaxy. For small twist angles, the moire pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concd. mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride. Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show exptl. that, as predicted theor., when this angle is close to the 'magic' angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moire´ pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through elec. tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liqs.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXntVGjtr8%253D&md5=1697ebd09a673171514a527c17acf5b4
    5. 5
      Havener, R. W.; Zhuang, H.; Brown, L.; Hennig, R. G.; Park, J. Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene. Nano Lett. 2012, 12 (6), 3162– 3167,  DOI: 10.1021/nl301137k
      5
      Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene
      Havener, Robin W.; Zhuang, Houlong; Brown, Lola; Hennig, Richard G.; Park, Jiwoong
      Nano Letters (2012), 12 (6), 3162-3167CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Few-layer graphene is a prototypical layered material, whose properties are detd. by the relative orientations and interactions between layers. Exciting elec. and optical phenomena were obsd. for the special case of Bernal-stacked few-layer graphene, but structure-property correlations in graphene which deviates from this structure are not well understood. Here, the authors combine 2 direct imaging techniques, dark-field TEM (DF-TEM) and widefield Raman imaging, to establish a robust, 1-to-one correlation between twist angle and Raman intensity in twisted bilayer graphene (tBLG). The Raman G band intensity is strongly enhanced due to a previously unreported singularity in the joint d. of states of tBLG, whose energy is exclusively a function of twist angle and whose optical transition strength is governed by interlayer interactions, enabling direct optical imaging of these parameters. Also, findings suggest future potential for novel optical and optoelectronic tBLG devices with angle-dependent, tunable characteristics.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XntlOit7Y%253D&md5=6fc537d164d3650a4dae99d646550c36
    6. 6
      Patel, H.; Havener, R. W.; Brown, L.; Liang, Y.; Yang, L.; Park, J.; Graham, M. W. Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons. Nano Lett. 2015, 15 (9), 5932– 5937,  DOI: 10.1021/acs.nanolett.5b02035
      6
      Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons
      Patel, Hiral; Havener, Robin W.; Brown, Lola; Liang, Yufeng; Yang, Li; Park, Jiwoong; Graham, Matt W.
      Nano Letters (2015), 15 (9), 5932-5937CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      When two sheets of graphene stack in a twisted bilayer graphene (tBLG) configuration, the resulting constrained overlap between interplanar 2p orbitals produce angle-tunable electronic absorption resonances. By applying a novel combination of multiphoton transient absorption (TA) microscopy and TEM, we resolve the electronic structure and ensuing relaxation by probing resonant excitations of single tBLG domains. Strikingly, we find that the transient electronic population in resonantly excited tBLG domains is enhanced many fold, forming a major electronic relaxation bottleneck. Two-photon TA microscopy shows this bottleneck effect originates from a strongly bound, dark exciton state lying ∼0.37 eV below the 1-photon absorption resonance. This stable coexistence of strongly bound excitons alongside free-electron continuum states has not been previously obsd. in a metallic, 2D material.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXht1Gnu7bI&md5=9069b3288557a5ee1b1bde593b0029cb
    7. 7
      Alencar, T. V.; von Dreifus, D.; Gabriela Cota Moreira, M.; Eliel, G. S. N.; Yeh, C.-H.; Chiu, P.-W.; Pimenta, M. A.; Malard, L. M.; Maria de Paula, A. Twisted Bilayer Graphene Photoluminescence Emission Peaks at van Hove Singularities. J. Phys.: Condens. Matter 2018, 30 (17), 175302,  DOI: 10.1088/1361-648X/aab64b
      7
      Twisted bilayer graphene photoluminescence emission peaks at van Hove singularities
      Alencar, Thonimar V.; von Dreifus, Driele; Moreira, Maria Gabriela Cota; Eliel, Gomes S. N.; Yeh, Chao-Hui; Chiu, Po-Wen; Pimenta, Marcos A.; Malard, Leandro M.; Maria de Paula, Ana
      Journal of Physics: Condensed Matter (2018), 30 (17), 175302/1-175302/5CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
      Luminescence emission imaging by fs laser excitation on twisted bilayer graphene samples is reported. The emission images are obtained by tuning the excitation laser energies in the near IR region. An increase of the luminescence emission at excitation energies that depends on the bilayer twist angle is demonstrated. The results show a peak for the light emission when the excitation is in resonance with transitions at the van Hove singularities in the electronic d. of states. The luminescence excitation peak position and width were measured for samples with various twist angles showing resonances in the energy range of 1.2 to 1.7 eV.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhslWlsrfL&md5=09a7d6476a5b545d6cc3019df8bba298
    8. 8
      Yao, W.; Wang, E.; Bao, C.; Zhang, Y.; Zhang, K.; Bao, K.; Chan, C. K.; Chen, C.; Avila, J.; Asensio, M. C. Quasicrystalline 30° Twisted Bilayer Graphene as an Incommensurate Superlattice with Strong Interlayer Coupling. Proc. Natl. Acad. Sci. U. S. A. 2018, 115 (27), 6928– 6933,  DOI: 10.1073/pnas.1720865115
      8
      Quasicrystalline 30° twisted bilayer graphene as an incommensurate superlattice with strong interlayer coupling
      Yao, Wei; Wang, Eryin; Bao, Changhua; Zhang, Yiou; Zhang, Kenan; Bao, Kejie; Chan, Chun Kai; Chen, Chaoyu; Avila, Jose; Asensio, Maria C.; Zhu, Junyi; Zhou, Shuyun
      Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (27), 6928-6933CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The interlayer coupling can be used to engineer the electronic structure of van der Waals heterostructures (superlattices) to obtain properties that are not possible in a single material. So far research in heterostructures has been focused on commensurate superlattices with a long-ranged Moire period. Incommensurate heterostructures with rotational symmetry but not translational symmetry (in analogy to quasicrystals) are not only rare in nature, but also the interlayer interaction has often been assumed to be negligible due to the lack of phase coherence. Here we report the successful growth of quasicryst. 30° twisted bilayer graphene (30°-tBLG), which is stabilized by the Pt(111) substrate, and reveal its electronic structure. The 30°-tBLG is confirmed by LEED and the intervalley double-resonance Raman mode at 1383 cm-1. Moreover, the emergence of mirrored Dirac cones inside the Brillouin zone of each graphene layer and a gap opening at the zone boundary suggest that these two graphene layers are coupled via a generalized Umklapp scattering mechanism - i.e., scattering of a Dirac cone in one graphene layer by the reciprocal lattice vector of the other graphene layer. Our work highlights the important role of interlayer coupling in incommensurate quasicryst. superlattices, thereby extending band structure engineering to incommensurate superstructures.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvFKhtrzN&md5=99792ffb225cc8a35deef0ebd592b7d3
    9. 9
      Moon, P.; Koshino, M. Optical Absorption in Twisted Bilayer Graphene. Phys. Rev. B 2013, 87 (20), 205404,  DOI: 10.1103/PhysRevB.87.205404
      9
      Optical absorption in twisted bilayer graphene
      Moon, Pilkyung; Koshino, Mikito
      Physical Review B: Condensed Matter and Materials Physics (2013), 87 (20), 205404/1-205404/11CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We theor. study the optical absorption property of twisted bilayer graphenes with various stacking geometries and demonstrate that the spectroscopic characteristics serve as a fingerprint to identify the rotation angle between two layers. We find that the absorption spectrum almost continuously evolves in changing the rotation angle, regardless of the lattice commensurability. The spectrum is characterized by series of peaks assocd. with the van Hove singularity, and the peak energies systematically shift with the rotation angle. We calc. the optical absorption in two frameworks: the tight-binding model and the effective continuum model based on the Dirac equation. For small rotation angles, less than 10°, the effective model well reproduces the low-energy band structure and the optical cond. of the tight-binding model and, also, explains the optical selection rule anal. in terms of the symmetry of the effective Hamiltonian.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtVGiur%252FM&md5=0fa20b9c5829e49ba6ab5426c40e8919
    10. 10
      Xin, W.; Chen, X.-D.; Liu, Z.-B.; Jiang, W.-S.; Gao, X.-G.; Jiang, X.-Q.; Chen, Y.; Tian, J.-G. Photovoltage Enhancement in Twisted-Bilayer Graphene Using Surface Plasmon Resonance. Adv. Opt. Mater. 2016, 4 (11), 1703– 1710,  DOI: 10.1002/adom.201600278
      10
      Photovoltage Enhancement in Twisted-Bilayer Graphene Using Surface Plasmon Resonance
      Xin, Wei; Chen, Xu-Dong; Liu, Zhi-Bo; Jiang, Wen-Shuai; Gao, Xiao-Guang; Jiang, Xiao-Qiang; Chen, Yongsheng; Tian, Jian-Guo
      Advanced Optical Materials (2016), 4 (11), 1703-1710CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The present article demonstrated the metal-graphene-metal (MGM) architectures based on a traditional Kretschmann configuration combining gold films and different graphene samples. It was found not only that the photoresponse approx. linearly increases with increasing graphene thickness but also that it exhibits a significant dependence on the graphene twist angle.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtF2htLvE&md5=bed22fbda2dec60d5d72fbb67c6432d4
    11. 11
      Xia, F.; Mueller, T.; Lin, Y.; Valdes-Garcia, A.; Avouris, P. Ultrafast Graphene Photodetector. Nat. Nanotechnol. 2009, 4 (12), 839– 843,  DOI: 10.1038/nnano.2009.292
      11
      Ultrafast graphene photodetector
      Xia, Fengnian; Mueller, Thomas; Lin, Yu-ming; Valdes-Garcia, Alberto; Avouris, Phaedon
      Nature Nanotechnology (2009), 4 (12), 839-843CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Graphene research so far has focused on electronic rather than photonic applications, in spite of its impressive optical properties. These include its ability to absorb ∼2% of incident light over a broad wavelength range despite being just one atom thick. Here, we demonstrate ultrafast transistor-based photodetectors made from single- and few-layer graphene. The photoresponse does not degrade for optical intensity modulations up to 40 GHz, and further anal. suggests that the intrinsic bandwidth may exceed 500 GHz. The generation and transport of photocarriers in graphene differ fundamentally from those in photodetectors made from conventional semiconductors as a result of the unique photonic and electronic properties of the graphene. This leads to a remarkably high bandwidth, zero source-drain bias and dark current operation, and good internal quantum efficiency.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsFagsrbN&md5=d3d0754001f70350c835921350521a92
    12. 12
      Sun, L.; Wang, Z.; Wang, Y.; Zhao, L.; Li, Y.; Chen, B.; Huang, S.; Zhang, S.; Wang, W.; Pei, D. Hetero-Site Nucleation for Growing Twisted Bilayer Graphene with a Wide Range of Twist Angles. Nat. Commun. 2021, 12 (1), 2391,  DOI: 10.1038/s41467-021-22533-1
      12
      Hetero-site nucleation for growing twisted bilayer graphene with a wide range of twist angles
      Sun, Luzhao; Wang, Zihao; Wang, Yuechen; Zhao, Liang; Li, Yanglizhi; Chen, Buhang; Huang, Shenghong; Zhang, Shishu; Wang, Wendong; Pei, Ding; Fang, Hongwei; Zhong, Shan; Liu, Haiyang; Zhang, Jincan; Tong, Lianming; Chen, Yulin; Li, Zhenyu; Rummeli, Mark H.; Novoselov, Kostya S.; Peng, Hailin; Lin, Li; Liu, Zhongfan
      Nature Communications (2021), 12 (1), 2391CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Twisted bilayer graphene (tBLG) has recently attracted growing interest due to its unique twist-angle-dependent electronic properties. The prepn. of high-quality large-area bilayer graphene with rich rotation angles would be important for the investigation of angle-dependent physics and applications, which, however, is still challenging. Here, we demonstrate a chem. vapor deposition (CVD) approach for growing high-quality tBLG using a hetero-site nucleation strategy, which enables the nucleation of the second layer at a different site from that of the first layer. The fraction of tBLGs in bilayer graphene domains with twist angles ranging from 0° to 30° was found to be improved to 88%, which is significantly higher than those reported previously. The hetero-site nucleation behavior was carefully investigated using an isotope-labeling technique. Furthermore, the clear Moire patterns and ultrahigh room-temp. carrier mobility of 68,000 cm2 V-1 s-1 confirmed the high cryst. quality of our tBLG. Our study opens an avenue for the controllable growth of tBLGs for both fundamental research and practical applications.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXpsVOgur0%253D&md5=a79ccfb5ea76a338671b92dceb0c3da5
    13. 13
      Kim, K.; Coh, S.; Tan, L. Z.; Regan, W.; Yuk, J. M.; Chatterjee, E.; Crommie, M. F.; Cohen, M. L.; Louie, S. G.; Zettl, A. Raman Spectroscopy Study of Rotated Double-Layer Graphene: Misorientation-Angle Dependence of Electronic Structure. Phys. Rev. Lett. 2012, 108 (24), 246103,  DOI: 10.1103/PhysRevLett.108.246103
      13
      Raman spectroscopy study of rotated double-layer graphene: misorientation-angle dependence of electronic structure
      Kim, Kwanpyo; Coh, Sinisa; Tan, Liang Z.; Regan, William; Yuk, Jong Min; Chatterjee, Eric; Crommie, M. F.; Cohen, Marvin L.; Louie, Steven G.; Zettl, A.
      Physical Review Letters (2012), 108 (24), 246103/1-246103/6CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      We present a systematic Raman study of unconventionally stacked double-layer graphene, and find that the spectrum strongly depends on the relative rotation angle between layers. Rotation-dependent trends in the position, width and intensity of graphene 2D and G peaks are exptl. established and accounted for theor. Our theor. anal. reveals that changes in electronic band structure due to the interlayer interaction, such as rotational-angle dependent Van Hove singularities, are responsible for the obsd. spectral features. Our combined exptl. and theor. study provides a deeper understanding of the electronic band structure of rotated double-layer graphene, and leads to a practical way to identify and analyze rotation angles of misoriented double-layer graphene.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVamtrfM&md5=f00f0b706e3ab0b5be0045cf18647a13
    14. 14
      Jorio, A.; Kasperczyk, M.; Clark, N.; Neu, E.; Maletinsky, P.; Vijayaraghavan, A.; Novotny, L. Optical-Phonon Resonances with Saddle-Point Excitons in Twisted-Bilayer Graphene. Nano Lett. 2014, 14 (10), 5687– 5692,  DOI: 10.1021/nl502412g
      14
      Optical-Phonon Resonances with Saddle-Point Excitons in Twisted-Bilayer Graphene
      Jorio, Ado; Kasperczyk, Mark; Clark, Nick; Neu, Elke; Maletinsky, Patrick; Vijayaraghavan, Aravind; Novotny, Lukas
      Nano Letters (2014), 14 (10), 5687-5692CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Twisted-bilayer graphene (tBLG) exhibits van Hove singularities in the d. of states that can be tuned by changing the twisting angle θ. A θ-defined tBLG was produced and characterized with optical reflectivity and resonance Raman scattering. The θ-engineered optical response is consistent with persistent saddle-point excitons. Sep. resonances with Stokes and anti-Stokes Raman scattering components can be achieved due to the sharpness of the 2-dimensional saddle-point excitons, similar to what was previously obsd. for 1-dimensional C nanotubes. The excitation power dependence for the Stokes and anti-Stokes emissions indicate that the 2 processes are correlated and that they share the same phonon.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsFWjs7vI&md5=a69edfb8529494036ace9c29e1b8da6d
    15. 15
      Chen, X.-D.; Xin, W.; Jiang, W.-S.; Liu, Z.-B.; Chen, Y.; Tian, J.-G. High-Precision Twist-Controlled Bilayer and Trilayer Graphene. Adv. Mater. 2016, 28 (13), 2563– 2570,  DOI: 10.1002/adma.201505129
      15
      High-Precision Twist-Controlled Bilayer and Trilayer Graphene
      Chen, Xu-Dong; Xin, Wei; Jiang, Wen-Shuai; Liu, Zhi-Bo; Chen, Yongsheng; Tian, Jian-Guo
      Advanced Materials (Weinheim, Germany) (2016), 28 (13), 2563-2570CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The cutting-rotation-stacking (CRS) technique provides a method to investigate the elec. and optical properties of twisted multilayer graphene. The twist-control capability of this technique is important for the heterostructures because their phys. properties strongly depend on the relative orientation of the component layers. A more important contribution of the CRS technique is that double twisted trilayer graphene (DTTG) with arbitrary twist angles can be successfully fabricated by this method, which is an important breakthrough in the exptl. research of twisted TLG. Compared with a bilayer, an unprecedented degree of control of the electronic and optical properties is available for this DTTG.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhslSqsLc%253D&md5=13e4b740a36748f57f472db4782593ef
    16. 16
      Cao, Y.; Fatemi, V.; Fang, S.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P. Unconventional Superconductivity in Magic-Angle Graphene Superlattices. Nature 2018, 556 (7699), 43– 50,  DOI: 10.1038/nature26160
      16
      Unconventional superconductivity in magic-angle graphene superlattices
      Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo
      Nature (London, United Kingdom) (2018), 556 (7699), 43-50CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      The behavior of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theor. understanding has motivated the development of exptl. techniques for studying such behavior, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional supercond.-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a crit. temp. of up to 1.7 K. The temp.-carrier-d. phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to supercond. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting crit. temp. of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier d. of about 1011 per square centimeter), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-crit.-temp. superconductors and quantum spin liqs.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXntVGjsbc%253D&md5=686b029f989784d0fa11ede1bfcecdbe
    17. 17
      Carozo, V.; Almeida, C. M.; Ferreira, E. H. M.; Cançado, L. G.; Achete, C. A.; Jorio, A. Raman Signature of Graphene Superlattices. Nano Lett. 2011, 11 (11), 4527– 4534,  DOI: 10.1021/nl201370m
      17
      Raman Signature of Graphene Superlattices
      Carozo, Victor; Almeida, Clara M.; Ferreira, Erlon H. M.; Cancado, Luiz Gustavo; Achete, Carlos Alberto; Jorio, Ado
      Nano Letters (2011), 11 (11), 4527-4534CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      When 2 identical 2-dimensional periodic structures are superposed, a mismatch rotation angle between the structures generates a superlattice. This effect is commonly obsd. in graphite, where the rotation between graphene layers generates Moire patterns in scanning tunneling microscopy images. Here, a study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented. The peak properties depend on the mismatch rotation angle and can be used as an optical signature for superlattices in bilayer graphene. An at. force microscopy system is used to produce and identify sp. rotationally stacked bilayer graphenes that demonstrate the validity of model.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtlWjtL3N&md5=ced21b2e47402d42aaf3d06df9c20010
    18. 18
      Hofmann, S.; Braeuninger-Weimer, P.; Weatherup, R. S. CVD-Enabled Graphene Manufacture and Technology. J. Phys. Chem. Lett. 2015, 6 (14), 2714– 2721,  DOI: 10.1021/acs.jpclett.5b01052
      18
      CVD-Enabled Graphene Manufacture and Technology
      Hofmann, Stephan; Braeuninger-Weimer, Philipp; Weatherup, Robert S.
      Journal of Physical Chemistry Letters (2015), 6 (14), 2714-2721CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Integrated manufg. is arguably the most challenging task in the development of technol. based on graphene and other 2D materials, particularly with regard to the industrial demand for "electronic-grade" large-area films. To control the structure and properties of these materials at the monolayer level, their nucleation, growth and interfacing needs to be understood to a level of unprecedented detail compared to existing thin film or bulk materials. Chem. vapor deposition (CVD) has emerged as the most versatile and promising technique to develop graphene and 2D material films into industrial device materials and this Perspective outlines recent progress, trends, and emerging CVD processing pathways. A key focus is the emerging understanding of the underlying growth mechanisms, in particular on the role of the required catalytic growth substrate, which brings together the latest progress in the fields of heterogeneous catalysis and classic crystal/thin-film growth.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVKit7bP&md5=d91fed663bf815d23aaee5deaad27864
    19. 19
      De Fazio, D.; Purdie, D. G.; Ott, A. K.; Braeuninger-Weimer, P.; Khodkov, T.; Goossens, S.; Taniguchi, T.; Watanabe, K.; Livreri, P.; Koppens, F. H. L. High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition. ACS Nano 2019, 13 (8), 8926– 8935,  DOI: 10.1021/acsnano.9b02621
      19
      High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition
      De Fazio, Domenico; Purdie, David G.; Ott, Anna K.; Braeuninger-Weimer, Philipp; Khodkov, Timofiy; Goossens, Stijn; Taniguchi, Takashi; Watanabe, Kenji; Livreri, Patrizia; Koppens, Frank H. L.; Hofmann, Stephan; Goykhman, Ilya; Ferrari, Andrea C.; Lombardo, Antonio
      ACS Nano (2019), 13 (8), 8926-8935CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      We report high room-temp. mobility in single-layer graphene grown by chem. vapor deposition (CVD) after wet transfer on SiO2 and hexagonal boron nitride (hBN) encapsulation. By removing contaminations, trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to ∼70000 cm2 V-1 s-1 at room temp. and ∼120 000 cm2 V-1 s-1 at 9K. These are more than twice those of previous wet-transferred graphene and comparable to samples prepd. by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycryst. graphene, achieving room-temp. mobilities of ∼30 000 cm2 V-1 s-1. These results show that, with appropriate encapsulation and cleaning, room-temp. mobilities well above 10 000 cm2 V-1 s-1 can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsVSqsbnM&md5=9c06d8950fc9402af9d63dfc1a520686
    20. 20
      Lu, C.; Lin, Y.; Liu, Z.; Yeh, C.; Suenaga, K.; Chiu, P. Twisting Bilayer Graphene Superlattices. ACS Nano 2013, 7 (3), 2587– 2594,  DOI: 10.1021/nn3059828
      20
      Twisting Bilayer Graphene Superlattices
      Lu, Chun-Chieh; Lin, Yung-Chang; Liu, Zheng; Yeh, Chao-Hui; Suenaga, Kazu; Chiu, Po-Wen
      ACS Nano (2013), 7 (3), 2587-2594CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Bilayer graphene is an intriguing material in that its electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of low-dimensional carbon system. Twisted bilayer graphene can be obtained by (i) thermal decompn. of SiC; (ii) CVD on metal catalysts; (iii) folding graphene; or (iv) stacking graphene layers one atop the other, the latter of which suffers from interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycryst. structures. The present study studies bilayer graphene grown by ambient pressure CVD on polycryst. Cu. Controlling the nucleation in early stage growth allows the constituent layers to form single hexagonal crystals. New Raman active modes result from the twist, with the angle detd. by TEM. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXjt1Wlsb8%253D&md5=fc7e5afd6cb530217d653070c5325582
    21. 21
      Fang, W.; Hsu, A. L.; Song, Y.; Kong, J. A Review of Large-Area Bilayer Graphene Synthesis by Chemical Vapor Deposition. Nanoscale 2015, 7 (48), 20335– 20351,  DOI: 10.1039/C5NR04756K
      21
      A review of large-area bilayer graphene synthesis by chemical vapor deposition
      Fang, Wenjing; Hsu, Allen L.; Song, Yi; Kong, Jing
      Nanoscale (2015), 7 (48), 20335-20351CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Bilayer graphene has attracted considerable attention due to its potential as a tunable band gap in AB-stacked bilayers. Recently, great advancements have been made in the synthesis of chem.-vapor-deposited bilayer graphene. This featured article provides a detailed and up-to-date review of the synthesis of bilayer graphene by chem. vapor deposition (CVD). We will discuss various approaches to synthesize bilayer graphene and the corresponding growth dynamics. Methods for identifying the growth mechanism of bilayer graphene on Cu enclosures are highlighted for a deeper understanding of better control over uniformity and thickness.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhvVWmtLvK&md5=b3599f35bd6dac06122c298f5a43e5d6
    22. 22
      Zhao, H.; Lin, Y. C.; Yeh, C. H.; Tian, H.; Chen, Y. C.; Xie, D.; Yang, Y.; Suenaga, K.; Ren, T. L.; Chiu, P. W. Growth and Raman Spectra of Single-Crystal Trilayer Graphene with Different Stacking Orientations. ACS Nano 2014, 8 (10), 10766– 10773,  DOI: 10.1021/nn5044959
      22
      Growth and Raman Spectra of Single-Crystal Trilayer Graphene with Different Stacking Orientations
      Zhao, Haiming; Lin, Yung-Chang; Yeh, Chao-Hui; Tian, He; Chen, Yu-Chen; Xie, Dan; Yang, Yi; Suenaga, Kazu; Ren, Tian-Ling; Chiu, Po-Wen
      ACS Nano (2014), 8 (10), 10766-10773CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Understanding the growth mechanism of graphene layers in chem. vapor deposition (CVD) and their corresponding Raman properties is technol. relevant and of importance for the application of graphene in electronic and optoelectronic devices. Here, we report CVD growth of single-crystal trilayer graphene (TLG) grains on Cu and show that lattice defects at the center of each grain persist throughout the growth, indicating that the adlayers share the same nucleation site with the upper layers and these central defects could also act as a carbon pathway for the growth of a new layer. Statistics shows that ABA, 30-30, 30-AB, and AB-30 make up the major stacking orientations in the CVD-grown TLG, with distinctive Raman 2D characteristics. Surprisingly, a high level of lattice defects results whenever a layer with a twist angle of θ = 30° is found in the multiple stacks of graphene layers.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhs1OitLvM&md5=cf550a17ae7d981d116d20f952aa5814
    23. 23
      Yu, K.; Van Luan, N.; Kim, T.; Jeon, J.; Kim, J.; Moon, P.; Lee, Y. H.; Choi, E. J. Gate Tunable Optical Absorption and Band Structure of Twisted Bilayer Graphene. Phys. Rev. B 2019, 99 (24), 241405,  DOI: 10.1103/PhysRevB.99.241405
      23
      Gate tunable optical absorption and band structure of twisted bilayer graphene
      Yu, Kwangnam; Nguyen, Van Luan; Kim, Taesoo; Jeon, Jiwon; Kim, Jiho; Moon, Pilkyung; Lee, Young Hee; Choi, E. J.
      Physical Review B (2019), 99 (24), 241405CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      We report the IR transmission measurement on elec. gated twisted bilayer graphene. The optical absorption spectrum clearly manifests dramatic changes such as the splitting of the interlinear-band absorption step, the shift of the inter-van Hove singularity transition peak, and the emergence of a very strong intravalence (intraconduction) band transition. These anomalous optical behaviors demonstrate consistently a nonrigid band structure modification created by ion-gel gating through layer-dependent Coulomb screening. We propose that this screening-driven band modification is a universal phenomenon that persists to other bilayer crystals in general, establishing elec. gating as a versatile technique to engineer band structures and to create different types of optical absorptions that can be exploited in electro-optical device applications.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhs12ksr3N&md5=d0c00b3e2b9938c63ef216bcab807486
    24. 24
      Wang, Y.; Ni, Z.; Liu, L.; Liu, Y.; Cong, C.; Yu, T.; Wang, X.; Shen, D.; Shen, Z. Stacking-Dependent Optical Conductivity of Bilayer Graphene. ACS Nano 2010, 4 (7), 4074– 4080,  DOI: 10.1021/nn1004974
      24
      Stacking-Dependent Optical Conductivity of Bilayer Graphene
      Wang, Yingying; Ni, Zhenhua; Liu, Lei; Liu, Yanhong; Cong, Chunxiao; Yu, Ting; Wang, Xiaojun; Shen, Dezhen; Shen, Zexiang
      ACS Nano (2010), 4 (7), 4074-4080CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The optical conductivities of graphene layers are strongly dependent on their stacking orders. Our first-principle calcns. show that, while the optical conductivities of single-layer graphene (SLG) and bilayer graphene (BLG) with Bernal stacking are almost frequency-independent in the visible region, the optical cond. of twisted bilayer graphene (TBG) is frequency-dependent, giving rise to addnl. absorption features due to the band folding effect. Exptl., we obtain from contrast spectra the optical cond. profiles of BLG with different stacking geometries. Some TBG samples show addnl. features in their cond. spectra, in full agreement with our calcn. results, while a few samples give universal cond. values similar to that of SLG. We propose that those variations of optical cond. spectra of TBG samples originate from the difference between the commensurate and incommensurate stackings. Our results reveal that the optical cond. measurements of graphene layers indeed provide an efficient way to select graphene films with desirable electronic and optical properties, which would greatly help the future application of those large-scale misoriented graphene films in photonic devices.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXmvVSlur4%253D&md5=1aa96414ffecfcd1f5a31a04ee7c18ea
    25. 25
      Robinson, J. T.; Schmucker, S. W.; Diaconescu, C. B.; Long, J. P.; Culbertson, J. C.; Ohta, T.; Friedman, A. L.; Beechem, T. E. Electronic Hybridization of Large-Area Stacked Graphene Films. ACS Nano 2013, 7 (1), 637– 644,  DOI: 10.1021/nn304834p
      25
      Electronic Hybridization of Large-Area Stacked Graphene Films
      Robinson, Jeremy T.; Schmucker, Scott W.; Diaconescu, C. Bogdan; Long, James P.; Culbertson, James C.; Ohta, Taisuke; Friedman, Adam L.; Beechem, Thomas E.
      ACS Nano (2013), 7 (1), 637-644CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Direct, tunable coupling between individually assembled graphene layers is a next step toward designer two-dimensional (2D) crystal systems, with relevance for fundamental studies and technol. applications. Here the authors describe the fabrication and characterization of large-area (>cm2), coupled bilayer graphene on SiO2/Si substrates. Stacking two graphene films leads to direct electronic interactions between layers, where the resulting film properties are detd. by the local twist angle. Polycryst. bilayer films have a stained-glass window appearance explained by the emergence of a narrow absorption band in the visible spectrum that depends on twist angle. Direct measurement of layer orientation via electron diffraction, together with Raman and optical spectroscopy, confirms the persistence of clean interfaces over large areas. Finally, interlayer coupling can be reversibly turned off through chem. modification, enabling optical-based chem. detection schemes. Together, these results suggest that 2-dimensional crystals can be individually assembled to form electronically coupled systems suitable for large-scale applications.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVersLbI&md5=58494e296d1fa9645cfb094d2e6f436e
    26. 26
      Campos-Delgado, J.; Algara-Siller, G.; Santos, C. N.; Kaiser, U.; Raskin, J. P. Twisted Bi-Layer Graphene: Microscopic Rainbows. Small 2013, 9 (19), 3247– 3251,  DOI: 10.1002/smll.201300050
      26
      Twisted Bi-Layer Graphene: Microscopic Rainbows
      Campos-Delgado, J.; Algara-Siller, G.; Santos, C. N.; Kaiser, U.; Raskin, J.-P.
      Small (2013), 9 (19), 3247-3251CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Blue, pink, and yellow colorations appear from twisted bi-layer graphene (tBLG) when transferred to a SiO2/Si substrate (SiO2 = 100 nm-thick). Raman and electron microscope studies reveal that these colorations appear for twist angles in the 9-15° range. Optical contrast simulations confirm that the obsd. colorations are related to the angle-dependent electronic properties of tBLG combined with the reflection that results from the layered structure tBLG/100 nm-thick SiO2/Si.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXmtlartLs%253D&md5=4be3a86e255e927e7ab669d0d9aed594
    27. 27
      Ohta, T.; Beechem, T. E.; Robinson, J. T.; Kellogg, G. L. Long-Range Atomic Ordering and Variable Interlayer Interactions in Two Overlapping Graphene Lattices with Stacking Misorientations. Phys. Rev. B 2012, 85 (7), 75415,  DOI: 10.1103/PhysRevB.85.075415
      27
      Long-range atomic ordering and variable interlayer interactions in two overlapping graphene lattices with stacking misorientations
      Ohta, Taisuke; Beechem, Thomas E.; Robinson, Jeremy T.; Kellogg, G. L.
      Physical Review B: Condensed Matter and Materials Physics (2012), 85 (7), 075415/1-075415/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The low-energy electronic dispersion of graphene is extremely sensitive to the nearest layer interaction and thus the stacking sequence. Here, we report a method to examine the effect of stacking misorientation in bilayer graphene by transferring chem. vapor deposited (CVD) graphene onto monolithic graphene epitaxially grown on silicon carbide (SiC) (0001). The resulting hybrid bilayer graphene displays long-range Moire diffraction patterns having various misorientations even as it exhibits electron reflectivity spectra nearly identical to epitaxial bilayer graphene grown directly on SiC. These varying twist angles affect the 2D (G')-band shape of the Raman spectrum, indicating regions of both a monolayer-like single π state and Bernal-like split π states brought about by the differing interlayer interactions. This hybrid bilayer graphene fabricated via a transfer process therefore offers a way to systematically study the electronic properties of bilayer graphene films as a function of stacking misorientation angle.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XkslCms78%253D&md5=a63fe7515800d98c6a7a3a6d51188646
    28. 28
      Ahn, S. J.; Moon, P.; Kim, T. H.; Kim, H. W.; Shin, H. C.; Kim, E. H.; Cha, H. W.; Kahng, S. J.; Kim, P.; Koshino, M. Dirac Electrons in a Dodecagonal Graphene Quasicrystal. Science (80-.). 2018, 361 (6404), 782– 786,  DOI: 10.1126/science.aar8412
      There is no corresponding record for this reference.
    29. 29
      Düvel, M.; Merboldt, M.; Bange, J. P.; Strauch, H.; Stellbrink, M.; Pierz, K.; Schumacher, H. W.; Momeni, D.; Steil, D.; Jansen, G. S. M. Far-from-Equilibrium Electron–Phonon Interactions in Optically Excited Graphene. Nano Lett. 2022, 22 (12), 4897– 4904,  DOI: 10.1021/acs.nanolett.2c01325
      29
      Far-from-Equilibrium Electron-Phonon Interactions in Optically Excited Graphene
      Duevel, Marten; Merboldt, Marco; Bange, Jan Philipp; Strauch, Hannah; Stellbrink, Michael; Pierz, Klaus; Schumacher, Hans Werner; Momeni, Davood; Steil, Daniel; Jansen, G. S. Matthijs; Steil, Sabine; Novko, Dino; Mathias, Stefan; Reutzel, Marcel
      Nano Letters (2022), 22 (12), 4897-4904CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Comprehending far-from-equil. many-body interactions is one of the major goals of current ultrafast condensed matter physics research. Here, a particularly interesting but barely understood situation occurs during a strong optical excitation, where the electron and phonon systems can be significantly perturbed and the quasiparticle distributions cannot be described with equil. functions. In this work, we use time- and angle-resolved photoelectron spectroscopy to study such far-from-equil. many-body interactions for the prototypical material graphene. In accordance with theor. simulations, we find remarkable transient renormalizations of the quasiparticle self-energy caused by the photoinduced nonequil. conditions. These observations can be understood by ultrafast scatterings between nonequil. electrons and strongly coupled optical phonons, which signify the crucial role of ultrafast nonequil. dynamics on many-body interactions. Our results advance the understanding of many-body physics in extreme conditions, which is important for any endeavor to optically manipulate or create non-equil. states of matter.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhsVSktrvP&md5=bf18cc58fc82a686a3eda718fe36a18b
    30. 30
      Patel, H.; Huang, L.; Kim, C.-J.; Park, J.; Graham, M. W. Stacking Angle-Tunable Photoluminescence from Interlayer Exciton States in Twisted Bilayer Graphene. Nat. Commun. 2019, 10 (1), 1445,  DOI: 10.1038/s41467-019-09097-x
      30
      Stacking angle-tunable photoluminescence from interlayer exciton states in twisted bilayer graphene
      Patel Hiral; Graham Matt W; Huang Lujie; Park Jiwoong; Kim Cheol-Joo
      Nature communications (2019), 10 (1), 1445 ISSN:.
      Twisted bilayer graphene (tBLG) is a metallic material with two degenerate van Hove singularity transitions that can rehybridize to form interlayer exciton states. Here we report photoluminescence (PL) emission from tBLG after resonant 2-photon excitation, which tunes with the interlayer stacking angle, θ. We spatially image individual tBLG domains at room-temperature and show a five-fold resonant PL-enhancement over the background hot-electron emission. Prior theory predicts that interlayer orbitals mix to create 2-photon-accessible strongly-bound (~0.7 eV) exciton and continuum-edge states, which we observe as two spectral peaks in both PL excitation and excited-state absorption spectra. This peak splitting provides independent estimates of the exciton binding energy which scales from 0.5-0.7 eV with θ = 7.5° to 16.5°. A predicted vanishing exciton-continuum coupling strength helps explain both the weak resonant PL and the slower 1 ps(-1) exciton relaxation rate observed. This hybrid metal-exciton behavior electron thermalization and PL emission are tunable with stacking angle for potential enhancements in optoelectronic and fast-photosensing graphene-based applications.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BB3cbps1GisQ%253D%253D&md5=acb092f08335edc72820eb43a0f94928
    31. 31
      Ribeiro, H. B.; Sato, K.; Eliel, G. S. N.; De Souza, E. A. T.; Lu, C. C.; Chiu, P. W.; Saito, R.; Pimenta, M. A. Origin of van Hove Singularities in Twisted Bilayer Graphene. Carbon N. Y. 2015, 90, 138– 145,  DOI: 10.1016/j.carbon.2015.04.005
      There is no corresponding record for this reference.
    32. 32
      Schäpers, A.; Sonntag, J.; Valerius, L.; Pestka, B.; Strasdas, J.; Watanabe, K.; Taniguchi, T.; Wirtz, L.; Morgenstern, M.; Beschoten, B. Raman Imaging of Twist Angle Variations in Twisted Bilayer Graphene at Intermediate Angles. 2D Mater. 2022, 9 (4), 045009,  DOI: 10.1088/2053-1583/ac7e59
      There is no corresponding record for this reference.
    33. 33
      Li, G.; Luican, a.; Lopes dos Santos, J. M. B.; Castro Neto, a. H.; Reina, a.; Kong, J.; Andrei, E. Y. Observation of Van Hove Singularities in Twisted Graphene Layers. Nat. Phys. 2010, 6 (2), 109– 113,  DOI: 10.1038/nphys1463
      33
      Observation of Van Hove singularities in twisted graphene layers
      Li, Guohong; Luican, A.; Lopes dos Santos, J. M. B.; Castro Neto, A. H.; Reina, A.; Kong, J.; Andrei, E. Y.
      Nature Physics (2010), 6 (2), 109-113CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)
      Electronic instabilities at the crossing of the Fermi energy with a Van Hove singularity in the d. of states often lead to new phases of matter such as supercond., magnetism or d. waves. However, in most materials this condition is difficult to control. In the case of single-layer graphene, the singularity is too far from the Fermi energy and hence difficult to reach with std. doping and gating techniques. Here we report the observation of low-energy Van Hove singularities in twisted graphene layers seen as two pronounced peaks in the d. of states measured by scanning tunneling spectroscopy. We demonstrate that a rotation between stacked graphene layers can generate Van Hove singularities, which can be brought arbitrarily close to the Fermi energy by varying the angle of rotation. This opens intriguing prospects for Van Hove singularity engineering of electronic phases.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtlCrtbs%253D&md5=ccd3756af24ea751c3ff22a7ee229574
    34. 34
      Brihuega, I.; Mallet, P.; González-Herrero, H.; Trambly de Laissardière, G.; Ugeda, M. M.; Magaud, L.; Gómez-Rodríguez, J. M.; Ynduráin, F.; Veuillen, J.-Y. Unraveling the Intrinsic and Robust Nature of van Hove Singularities in Twisted Bilayer Graphene by Scanning Tunneling Microscopy and Theoretical Analysis. Phys. Rev. Lett. 2012, 109 (19), 196802,  DOI: 10.1103/PhysRevLett.109.196802
      34
      Unraveling the intrinsic and robust nature of van Hove singularities in twisted bilayer graphene by scanning tunneling microscopy and theoretical analysis
      Brihuega, I.; Mallet, P.; Gonzalez-Herrero, H.; Trambly de Laissardiere, G.; Ugeda, M. M.; Magaud, L.; Gomez-Rodriguez, J. M.; Yndurain, F.; Veuillen, J.-Y.
      Physical Review Letters (2012), 109 (19), 196802/1-196802/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Extensive scanning tunneling microscopy and spectroscopy expts. complemented by first-principles and parametrized tight binding calcns. provide a clear answer to the existence, origin, and robustness of van Hove singularities (vHs) in twisted graphene layers. Our results are conclusive: vHs due to interlayer coupling are ubiquitously present in a broad range (from 1° to 10°) of rotation angles in our graphene on 6H-SiC(000-1) samples. From the variation of the energy sepn. of the vHs with the rotation angle we are able to recover the Fermi velocity of a graphene monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by expts., which show that they survive in the presence of a third graphene layer, and by calcns., which test the role of the periodic modulation and abs. value of the interlayer distance. Finally, we clarify the role of the layer topog. corrugation and of electronic effects in the apparent moire contrast measured on the STM images.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xhslylsr7I&md5=29a40447f8181ab1f69fb09544ffc90b
    35. 35
      Jeong, G.; Choi, B.; Kim, D. S.; Ahn, S.; Park, B.; Kang, J. H.; Min, H.; Hong, B. H.; Kim, Z. H. Mapping of Bernal and Non-Bernal Stacking Domains in Bilayer Graphene Using Infrared Nanoscopy. Nanoscale 2017, 9 (12), 4191– 4195,  DOI: 10.1039/C7NR00713B
      35
      Mapping of Bernal and non-Bernal stacking domains in bilayer graphene using infrared nanoscopy
      Jeong, Gyouil; Choi, Boogeon; Kim, Deok-Soo; Ahn, Seongjin; Park, Baekwon; Kang, Jin Hyoun; Min, Hongki; Hong, Byung Hee; Kim, Zee Hwan
      Nanoscale (2017), 9 (12), 4191-4195CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Bilayer graphene (BLG) shows great potential as a new material for opto-electronic devices because its bandgap can be controlled by varying the stacking orders, as well as by applying an external elec. field. An imaging technique that can visualize and characterize various stacking domains in BLG may greatly help in fully utilizing such properties of BLG. Here we demonstrate that IR (IR) scattering-type scanning near-field optical microscopy (sSNOM) can visualize Bernal and non-Bernal stacking domains of BLG, based on the stacking-specific inter- and intra-band optical conductivities. The method enables nanometric mapping of stacking domains in BLG on dielec. substrates, augmenting current limitations of Raman spectroscopy and electron microscopy techniques for the structural characterization of BLG.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXjtl2gtLo%253D&md5=74dfacc58126afc8f02cabe5d7023745
    36. 36
      Uri, A.; Grover, S.; Cao, Y.; Crosse, J. A.; Bagani, K.; Rodan-Legrain, D.; Myasoedov, Y.; Watanabe, K.; Taniguchi, T.; Moon, P. Mapping the Twist-Angle Disorder and Landau Levels in Magic-Angle Graphene. Nature 2020, 581 (7806), 47– 52,  DOI: 10.1038/s41586-020-2255-3
      36
      Mapping the twist-angle disorder and Landau levels in magic-angle graphene
      Uri, A.; Grover, S.; Cao, Y.; Crosse, J. A.; Bagani, K.; Rodan-Legrain, D.; Myasoedov, Y.; Watanabe, K.; Taniguchi, T.; Moon, P.; Koshino, M.; Jarillo-Herrero, P.; Zeldov, E.
      Nature (London, United Kingdom) (2020), 581 (7806), 47-52CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Abstr.: The recently discovered flat electronic bands and strongly correlated and superconducting phases in magic-angle twisted bilayer graphene (MATBG)1,2 crucially depend on the interlayer twist angle, θ. Although control of the global θ with a precision of about 0.1 degrees has been demonstrated1-7, little information is available on the distribution of the local twist angles. Here we use a nanoscale on-tip scanning superconducting quantum interference device (SQUID-on-tip)8 to obtain tomog. images of the Landau levels in the quantum Hall state9 and to map the local θ variations in hexagonal boron nitride (hBN)-encapsulated MATBG devices with relative precision better than 0.002 degrees and a spatial resoln. of a few moire´ periods. We find a correlation between the degree of θ disorder and the quality of the MATBG transport characteristics and show that even state-of-the-art devices-which exhibit correlated states, Landau fans and supercond.-display considerable local variation in θ of up to 0.1 degrees, exhibiting substantial gradients and networks of jumps, and may contain areas with no local MATBG behavior. We observe that the correlated states in MATBG are particularly fragile with respect to the twist-angle disorder. We also show that the gradients of θ generate large gate-tunable in-plane elec. fields, unscreened even in the metallic regions, which profoundly alter the quantum Hall state by forming edge channels in the bulk of the sample and may affect the phase diagram of the correlated and superconducting states. We thus establish the importance of θ disorder as an unconventional type of disorder enabling the use of twist-angle gradients for band structure engineering, for realization of correlated phenomena and for gate-tunable built-in planar elec. fields for device applications.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXoslygtLY%253D&md5=c84f30a17dab4c276634346d7920ea61
    37. 37
      Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Making Graphene Visible. Appl. Phys. Lett. 2007, 91 (6), 063124,  DOI: 10.1063/1.2768624
      37
      Making graphene visible
      Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K.
      Applied Physics Letters (2007), 91 (6), 063124/1-063124/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Microfabrication of graphene devices used in many exptl. studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepd. on top of Si wafers with a certain thickness of SiO2. The authors study graphene's visibility and show that it depends strongly on both thickness of SiO2 and light wavelength. By using monochromatic illumination, graphene can be isolated for any SiO2 thickness, albeit 300 nm (the current std.) and, esp., ≈100 nm are most suitable for its visual detection. By using a Fresnel-law-based model, they quant. describe the exptl. data.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXpsFGntrg%253D&md5=a8c99c400588bc70b7a1741de9646d09
    38. 38
      Hofmann, S.; Braeuninger-Weimer, P.; Weatherup, R. S. CVD-Enabled Graphene Manufacture and Technology. J. Phys. Chem. Lett. 2015, 6 (14), 2714– 2721,  DOI: 10.1021/acs.jpclett.5b01052
      38
      CVD-Enabled Graphene Manufacture and Technology
      Hofmann, Stephan; Braeuninger-Weimer, Philipp; Weatherup, Robert S.
      Journal of Physical Chemistry Letters (2015), 6 (14), 2714-2721CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Integrated manufg. is arguably the most challenging task in the development of technol. based on graphene and other 2D materials, particularly with regard to the industrial demand for "electronic-grade" large-area films. To control the structure and properties of these materials at the monolayer level, their nucleation, growth and interfacing needs to be understood to a level of unprecedented detail compared to existing thin film or bulk materials. Chem. vapor deposition (CVD) has emerged as the most versatile and promising technique to develop graphene and 2D material films into industrial device materials and this Perspective outlines recent progress, trends, and emerging CVD processing pathways. A key focus is the emerging understanding of the underlying growth mechanisms, in particular on the role of the required catalytic growth substrate, which brings together the latest progress in the fields of heterogeneous catalysis and classic crystal/thin-film growth.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVKit7bP&md5=d91fed663bf815d23aaee5deaad27864
    39. 39
      Wurstbauer, U.; Röling, C.; Wurstbauer, U.; Wegscheider, W.; Vaupel, M.; Thiesen, P. H.; Weiss, D. Imaging Ellipsometry of Graphene. Appl. Phys. Lett. 2010, 97 (23), 231901,  DOI: 10.1063/1.3524226
      39
      Imaging ellipsometry of graphene
      Wurstbauer, Ulrich; Roeling, Christian; Wurstbauer, Ursula; Wegscheider, Werner; Vaupel, Matthias; Thiesen, Peter H.; Weiss, Dieter
      Applied Physics Letters (2010), 97 (23), 231901/1-231901/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Imaging ellipsometry studies of graphene on SiO2/Si and cryst. GaAs are presented. Imaging ellipsometry is a powerful tool to detect and characterize graphene on any flat substrate. Variable angle spectroscopic ellipsometry is used to explore the dispersion of the optical consts. of graphene in the visible range with high lateral resoln. In this way, the influence of the substrate on graphene's optical properties can be studied. (c) 2010 American Institute of Physics.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsFagsLzO&md5=3c054e08652ee727519885f4dcd61ed2
    40. 40
      Funke, S.; Miller, B.; Parzinger, E.; Thiesen, P.; Holleitner, A. W.; Wurstbauer, U. Imaging Spectroscopic Ellipsometry of MoS 2. J. Phys.: Condens. Matter 2016, 28 (38), 385301,  DOI: 10.1088/0953-8984/28/38/385301
      40
      Imaging spectroscopic ellipsometry of MoS2
      Funke, S.; Miller, B.; Parzinger, E.; Thiesen, P.; Holleitner, A. W.; Wurstbauer, U.
      Journal of Physics: Condensed Matter (2016), 28 (38), 385301/1-385301/12CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
      Micromechanically exfoliated mono- and multilayers of molybdenum disulfide (MoS2) are investigated by spectroscopic imaging ellipsometry. In combination with knife edge illumination, MoS2 flakes can be detected and classified on arbitrary flat and also transparent substrates with a lateral resoln. down to 1-2 μm. The complex dielec. functions from mono- and trilayer MoS2 are presented. They are extd. from a multilayer model to fit the measured ellipsometric angles employing an anisotropic and an isotropic fit approach. We find that the energies of the crit. points of the optical consts. can be treated to be independent of the utilized model, whereas the magnitude of the optical consts. varies with the used model. The anisotropic model suggests a max. absorbance for a MoS2 sheet supported by sapphire of about 14% for monolayer and of 10% for trilayer MoS2. Furthermore, the lateral homogeneity of the complex dielec. function for monolayer MoS2 is investigated with a spatial resoln. of 2 μm. Only minor fluctuations are obsd. No evidence for strain, for a significant amt. of disorder or lattice defects can be found in the wrinkle-free regions of the MoS2 monolayer from complementary μ-Raman spectroscopy measurements. We assume that the minor lateral variation in the optical consts. are caused by lateral modification in the van der Waals interaction presumably caused by the prepn. using micromech. exfoliation and viscoelastic stamping.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVyqsrfK&md5=9095d41ecd14f8fc6e1ebd08045169fa
    41. 41
      Matković, A.; Beltaos, A.; Milićević, M.; Ralević, U.; Vasić, B.; Jovanović, D.; Gajić, R. Spectroscopic Imaging Ellipsometry and Fano Resonance Modeling of Graphene. J. Appl. Phys. 2012, 112 (12), 123523,  DOI: 10.1063/1.4771875
      41
      Spectroscopic imaging ellipsometry and Fano resonance modeling of graphene
      Matkovic, Aleksandar; Beltaos, Angela; Milicevic, Marijana; Ralevic, Uros; Vasic, Borislav; Jovanovic, Djordje; Gajic, Rados
      Journal of Applied Physics (Melville, NY, United States) (2012), 112 (12), 123523/1-123523/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The optical properties were examd. of exfoliated graphene on an Si/SiO2 substrate using imaging ellipsometry in the visible range (360-800 nm). Measured spectra were analyzed by an optical model based on the Fresnel coeff. equations. The optical model was supported by correlated Raman and at. force microscopy measurements. The complex refractive index of graphene was obtained by inversion of the measured ellipsometry data. The Fano line-shape was used to parameterize the optical properties. Measurements were highly reliable due to the numerous advantages of the spectroscopic imaging ellipsometric technique combined with the proper choice of substrate and exptl. set-up. Thickness maps of the graphene sample were obtained from spatially resolved imaging ellipsometry spectra with a spot size of 1 μm. The data showed a H2O layer on the surface of the sample, and the thickness was mapped showing the distribution of H2O over graphene in ambient conditions. (c) 2012 American Institute of Physics.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvFSnur7P&md5=0bbe8d67044cf7f4d43c5657d845d24c
    42. 42
      Funke, S.; Wurstbauer, U.; Miller, B.; Matković, A.; Green, A.; Diebold, A.; Röling, C.; Thiesen, P. H. Spectroscopic Imaging Ellipsometry for Automated Search of Flakes of Mono- and n-Layers of 2D-Materials. Appl. Surf. Sci. 2017, 421, 435– 439,  DOI: 10.1016/j.apsusc.2016.10.158
      42
      Spectroscopic imaging ellipsometry for automated search of flakes of mono- and n-layers of 2D-materials
      Funke, S.; Wurstbauer, U.; Miller, B.; Matkovic, A.; Green, A.; Diebold, A.; Roeling, C.; Thiesen, P. H.
      Applied Surface Science (2017), 421 (Part_B), 435-439CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Spectroscopic imaging ellipsometry (SIE) is used to localize and characterize flakes of conducting, semi-conducting and insulating 2D-materials. Although the research in the field of monolayers of 2D-materials increased the last years, it is still challenging to look for small flakes and distinguish between different layer nos. Special substrates are used to enhance optical contrast for the conventional light microscopy (LM). In case when other functional support from the substrate is essential, an addnl. transfer step needs to be employed, bringing the drawbacks as contamination, cracking and wrinkling of the 2D materials. Furthermore it is time-consuming and not yet fully automatically to search for monolayers by contrast with the LM. Here we present a method, that is able to automatically localize regions with desired thicknesses, e.g. monolayers, of the different materials on arbitrary substrates.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVOgsrvF&md5=017f82239009e181ce68559b2eb3bb49
    43. 43
      Braeuninger-Weimer, P.; Funke, S.; Wang, R.; Thiesen, P.; Tasche, D.; Viöl, W.; Hofmann, S. Fast, Noncontact, Wafer-Scale, Atomic Layer Resolved Imaging of Two-Dimensional Materials by Ellipsometric Contrast Micrography. ACS Nano 2018, 12 (8), 8555– 8563,  DOI: 10.1021/acsnano.8b04167
      43
      Fast, Noncontact, Wafer-Scale, Atomic Layer Resolved Imaging of Two-Dimensional Materials by Ellipsometric Contrast Micrography
      Braeuninger-Weimer, Philipp; Funke, Sebastian; Wang, Ruizhi; Thiesen, Peter; Tasche, Daniel; Viol, Wolfgang; Hofmann, Stephan
      ACS Nano (2018), 12 (8), 8555-8563CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Adequate characterization and quality control of atomically thin layered materials (2DM) has become a serious challenge particularly given the rapid advancements in their large area manufg. and numerous emerging industrial applications with different substrate requirements. Here, the authors focus on ellipsometric contrast microg. (ECM), a fast intensity mode within spectroscopic imaging ellipsometry, and show that it can be effectively used for noncontact, large area characterization of 2DM to map coverage, layer no., defects and contamination. The authors demonstrate at. layer resolved, quant. mapping of chem. vapor deposited graphene layers on Si/SiO2-wafers, but also on rough Cu catalyst foils, highlighting that ECM is applicable to all application relevant substrates. The optimization of ECM parameters for high throughput characterization are discussed. While the lateral resoln. can be <1 μm, the authors particularly explore fast scanning and demonstrate imaging of a 4'' graphene wafer in 47 min at 10 μm lateral resoln., i.e., an imaging speed of 1.7 cm2/min. Also, the authors show ECM of monolayer hexagonal BN (h-BN) and of h-BN/graphene bilayers, highlighting that ECM is applicable to a wide range of 2-dimensional layered structures that were previously very challenging to characterize and thereby fills an important gap in 2DM metrol.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsVCqtrzK&md5=9b7aab8f43a3fdd82d0615a31cb5aae6
    44. 44
      Fujiwara, H. Spectroscopic Ellipsometry: Principles and Applications; Maruzen Co. Ltd.: Tokyo, Japan, 2003.
      There is no corresponding record for this reference.
    45. 45
      Dicke, J.; Rotermund, H.-H.; Lauterbach, J. Ellipsomicroscopy for Surface Imaging: Contrast Mechanism, Enhancement, and Application to CO Oxidation on Pt(110). J. Opt. Soc. Am. A 2000, 17 (1), 135,  DOI: 10.1364/JOSAA.17.000135
      45
      Ellipsomicroscopy for surface imaging: contrast mechanism, enhancement, and application to CO oxidation on Pt(110)
      Dicke, Jan; Rotermund, Harm-Hinrich; Lauterbach, Jochen
      Journal of the Optical Society of America A: Optics, Image Science, and Vision (2000), 17 (1), 135-141CODEN: JOAOD6; ISSN:0740-3232. (Optical Society of America)
      Ellipsomicroscopy for surface imaging (EMSI) is a powerful new tool for studying spatiotemporal adsorbate pattern formation on catalyst surfaces. It is a surface-sensitive technique that is able to measure submonolayer coverage of adsorbates. The imaging of the sample's surface achieves a spatial sensitivity, making it possible to measure nonuniformity of adsorbate coverage. The image contrast, however, depends strongly on the setup of the instrument. The optimum setup can be calcd. from the ellipsometric properties of the catalyst/adsorbate system and the intrinsic parameters of the EMSI instrument. Optimizing the setup of the EMSI instrument permitted enhancement of the image contrast over the previous setup. As a result, new features in CO oxidn. on Pt(110) were discovered.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3cXks1yksw%253D%253D&md5=6de5c0f3292ecae1af1d127c51a50f64
    46. 46
      Ducret, A.; Valignat, M.-P.; Mouhamar, F.; Mignot, T.; Theodoly, O. Wet-Surface–Enhanced Ellipsometric Contrast Microscopy Identifies Slime as a Major Adhesion Factor during Bacterial Surface Motility. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (25), 10036– 10041,  DOI: 10.1073/pnas.1120979109
      46
      Wet-surface-enhanced ellipsometric contrast microscopy identifies slime as a major adhesion factor during bacterial surface motility
      Ducret, Adrien; Valignat, Marie-Pierre; Mouhamar, Fabrice; Mignot, Tam; Theodoly, Olivier
      Proceedings of the National Academy of Sciences of the United States of America (2012), 109 (25), 10036-10041, S10036/1-S10036/8CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      In biol., the extracellular matrix (ECM) promotes both cell adhesion and specific recognition, which is essential for central developmental processes in both eukaryotes and prokaryotes. However, live studies of the dynamic interactions between cells and the ECM, for example during motility, have been greatly impaired by imaging limitations: mostly the ability to observe the ECM at high resoln. in absence of specific staining by live microscopy. To solve this problem, we developed a unique technique, wet-surface enhanced ellipsometry contrast (Wet-SEEC), which magnifies the contrast of transparent org. materials deposited on a substrate (called Wet-surf) with exquisite sensitivity. We show that Wet-SEEC allows both the observation of unprocessed nanofilms as low as 0.2 nm thick and their accurate 3D topog. reconstructions, directly by std. light microscopy. We next used Wet-SEEC to image slime secretion, a poorly defined property of many prokaryotic and eukaryotic organisms that move across solid surfaces in absence of obvious extracellular appendages (gliding). Using combined Wet-SEEC and fluorescent-staining expts., we obsd. slime deposition by gliding Myxococcus xanthus cells at unprecedented resoln. Altogether, the results revealed that in this bacterium, slime assocs. preferentially with the outermost components of the motility machinery and promotes its adhesion to the substrate on the ventral side of the cell. Strikingly, analogous roles have been proposed for the extracellular proteoglycans of gliding diatoms and apicomplexa, suggesting that slime deposition is a general means for gliding organisms to adhere and move over surfaces.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVKqtbjK&md5=b94477ca134f9cbc9033ead9efd546b6
    47. 47
      Affoune, A.; Prasad, B. L.; Sato, H.; Enoki, T.; Kaburagi, Y.; Hishiyama, Y. Experimental Evidence of a Single Nano-Graphene. Chem. Phys. Lett. 2001, 348 (1–2), 17– 20,  DOI: 10.1016/S0009-2614(01)01066-1
      47
      Experimental evidence of a single nano-graphene
      Affoune, A. M.; Prasad, B. L. V.; Sato, Hirohiko; Enoki, Toshiaki; Kaburagi, Yutaka; Hishiyama, Yoshihiro
      Chemical Physics Letters (2001), 348 (1,2), 17-20CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
      A single nano-sized graphene sheet is prepd. by a combination of electrophoretic deposition (EPD) and heat-treatment of diamond nano-particles on a highly oriented pyrolytic graphite (HOPG) substrate. Heat-treatment at 1600 °C converts diamond nano-particles to single nano-graphenes, where the mean in-plane size and the inter-layer distance from the substrate are estd. at 10-15 and 0.35-0.37 nm, resp. The considerably large inter-layer distance compared to bulk graphite suggests a large redn. of inter-layer interaction, although a nano-graphene is placed epitaxially on the substrate. The isolated single nano-graphene provides an important model of nano-sized π-electron system, for which recent theory predicts unconventional electronic structure of edge-inherited non-bonding state.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXovFejsro%253D&md5=c4fffe79604fa3263391caf0c8e9030e
    48. 48
      Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science (80-.). 2004, 306 (5696), 666– 669,  DOI: 10.1126/science.1102896
      48
      Electric Field Effect in Atomically Thin Carbon Films
      Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A.
      Science (Washington, DC, United States) (2004), 306 (5696), 666-669CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The authors describe monocryst. graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar elec. field effect such that electrons and holes in concns. up to 1013 per square centimeter and with room-temp. mobilities of ∼10,000 square centimeters per V-second can be induced by applying gate voltage.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXos1Kqt70%253D&md5=488da13500bf24e8fc419052dc1a9e84
    49. 49
      Nemes-Incze, P.; Osváth, Z.; Kamarás, K.; Biró, L. P. Anomalies in Thickness Measurements of Graphene and Few Layer Graphite Crystals by Tapping Mode Atomic Force Microscopy. Carbon N. Y. 2008, 46 (11), 1435– 1442,  DOI: 10.1016/j.carbon.2008.06.022
      49
      Anomalies in thickness measurements of graphene and few layer graphite crystals by tapping mode atomic force microscopy
      Nemes-Incze, P.; Osvath, Z.; Kamaras, K.; Biro, L. P.
      Carbon (2008), 46 (11), 1435-1442CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
      AFM in the tapping (intermittent contact) mode is a commonly used tool to measure the thickness of graphene and few layer graphene (FLG) flakes on silicon oxide surfaces. It is a convenient tool to quickly det. the thickness of individual FLG films. However, reports from literature show a large variation of the measured thickness of graphene layers. This paper is focused on the imaging mechanism of tapping mode AFM (TAFM) when measuring graphene and FLG thickness, and it is shown that at certain measurement parameters significant deviations can be introduced in the measured thickness of FLG flakes. An increase of as much as 1 nm can be obsd. in the measured height of FLG crystallites, when using an improperly chosen range of free amplitude values of the tapping cantilever. Comparative Raman spectroscopy and TAFM measurements on selected single and multilayer graphene films are presented, based on which ways are suggested to correctly measure graphene and FLG thickness using TAFM.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXpvFyhtLs%253D&md5=f1dbceb0716fd161e532d7ce544ddb30
    50. 50
      Eigler, S.; Hof, F.; Enzelberger-Heim, M.; Grimm, S.; Müller, P.; Hirsch, A. Statistical Raman Microscopy and Atomic Force Microscopy on Heterogeneous Graphene Obtained after Reduction of Graphene Oxide. J. Phys. Chem. C 2014, 118 (14), 7698– 7704,  DOI: 10.1021/jp500580g
      50
      Statistical Raman Microscopy and Atomic Force Microscopy on Heterogeneous Graphene Obtained after Reduction of Graphene Oxide
      Eigler, Siegfried; Hof, Ferdinand; Enzelberger-Heim, Michael; Grimm, Stefan; Mueller, Paul; Hirsch, Andreas
      Journal of Physical Chemistry C (2014), 118 (14), 7698-7704CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Graphene oxide can be used as a precursor to graphene, but the quality of graphene flakes is highly heterogeneous. Scanning Raman microscopy (SRM) is used to characterize films of graphene derived from flakes of graphene oxide with an almost intact carbon framework (ai-GO). The defect d. of these flakes is visualized in detail by analyzing the intensity and full width at half-max. of the most pronounced Raman peaks. In addn., the authors superimpose the SRM results with AFM images and correlate the spectroscopic results with the morphol. Furthermore, the authors use the SRM technique to display the amt. of defects in a film of graphene. An area of 250 × 250 μm2 of graphene was probed with a step-size increment of 1 μm. The position of graphene flakes, edges, and the substrate were visualized. Finally, the authors alter parameters of measurement to analyze the quality of graphene in a fast and reliable way. The described method can be used to probe and visualize the quality of graphene films.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXksVWisr4%253D&md5=10eb129030930a8b56ff82179676f9bb
    51. 51
      Obraztsova, E. A.; Osadchy, A. V.; Obraztsova, E. D.; Lefrant, S.; Yaminsky, I. V. Statistical Analysis of Atomic Force Microscopy and Raman Spectroscopy Data for Estimation of Graphene Layer Numbers. Phys. status solidi 2008, 245 (10), 2055– 2059,  DOI: 10.1002/pssb.200879657
      51
      Statistical analysis of atomic force microscopy and Raman spectroscopy data for estimation of graphene layer numbers
      Obraztsova, Ekaterina A.; Osadchy, Alexander V.; Obraztsova, Elena D.; Lefrant, Serge; Yaminsky, Igor V.
      Physica Status Solidi B: Basic Solid State Physics (2008), 245 (10), 2055-2059CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We present the results on prepn. and anal. of structural and electronic properties of graphene flakes with different no. of layers. Since the material electronic properties depend strongly on the layer no., a simple and reliable way to est. this parameter should be found. We propose to perform a quant. anal. based on the at. force microscopy (AFM) which was exploited by another authors rather as an illustrative technique. The graphene flakes were prepd. by a micromech. cleavage of highly oriented pyrolytic graphite (HOPG). Numerous flakes were studied with AFM to est. the no. of graphene layers in each of them. The thickness distribution over the individual graphene flake, averaged over the all flakes investigated, has exhibited a row of maxima: 0.7 nm, 1.1 nm, 1.4 nm, 1.7 nm, 2.1 nm, etc. These values correspond perfectly to the one-, 2-, 3-, and etc layers of graphene. The flakes characterized by AFM were studied afterwards with a 2-phonon Raman spectroscopy. This method is known to be sensitive to the no. of layers due to a "double resonance" mechanism. To interpret the Raman spectra the electronic structure of the graphene flakes of different thickness was calcd. using the d. functional method. A perfect correspondence between the no. of graphene layers in different flakes measured by AFM and by Raman techniques was obtained. This demonstrates a possibility to est. correctly the graphene layer no. using the at. force microscopy supplied with the statistical anal.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXht12itr3N&md5=59362456f5f3efb311700f170a2def27
    52. 52
      Darakchieva, V.; Boosalis, A.; Zakharov, A. A.; Hofmann, T.; Schubert, M.; Tiwald, T. E.; Iakimov, T.; Vasiliauskas, R.; Yakimova, R. Large-Area Microfocal Spectroscopic Ellipsometry Mapping of Thickness and Electronic Properties of Epitaxial Graphene on Si- and C-Face of 3C-SiC(111). Appl. Phys. Lett. 2013, 102 (21), 213116,  DOI: 10.1063/1.4808379
      52
      Large-area microfocal spectroscopic ellipsometry mapping of thickness and electronic properties of epitaxial graphene on Si- and C-face of 3C-SiC(111)
      Darakchieva, V.; Boosalis, A.; Zakharov, A. A.; Hofmann, T.; Schubert, M.; Tiwald, T. E.; Iakimov, T.; Vasiliauskas, R.; Yakimova, R.
      Applied Physics Letters (2013), 102 (21), 213116/1-213116/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Microfocal spectroscopic ellipsometry mapping of the electronic properties and thickness of epitaxial graphene grown by high-temp. sublimation on 3C-SiC (111) substrates is reported. Growth of one monolayer graphene is demonstrated on both Si- and C-polarity of the 3C-SiC substrates and it is shown that large area homogeneous single monolayer graphene can be achieved on the Si-face substrates. Correlations between the no. of graphene monolayers on one hand and the main transition assocd. with an exciton enhanced van Hove singularity at ∼4.5 eV and the free-charge carrier scattering time, on the other are established. It is shown that the interface structure on the Si- and C-polarity of the 3C-SiC(111) differs and has a detg. role for the thickness and electronic properties homogeneity of the epitaxial graphene. (c) 2013 American Institute of Physics.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXosFeksb4%253D&md5=5f04a1565c0024b1d4993e080fd92e25
    53. 53
      Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102 (30), 10451– 10453,  DOI: 10.1073/pnas.0502848102
      53
      Two-dimensional atomic crystals
      Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K.
      Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (30), 10451-10453CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The authors report free-standing at. crystals that are strictly 2-dimensional and can be viewed as individual at. planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromech. cleavage, the authors prepd. and studied a variety of 2-dimensional crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2-dimensional mols. unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXntVSit7g%253D&md5=1ce9e5f5eb0f7b9abb033d4a690d49c3
    54. 54
      Lopes dos Santos, J. M. B.; Peres, N. M. R.; Castro Neto, A. H. Graphene Bilayer with a Twist: Electronic Structure. Phys. Rev. Lett. 2007, 99 (25), 256802,  DOI: 10.1103/PhysRevLett.99.256802
      54
      Graphene Bilayer with a Twist: Electronic Structure
      Lopes dos Santos, J. M. B.; Peres, N. M. R.; Castro Neto, A. H.
      Physical Review Letters (2007), 99 (25), 256802/1-256802/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      We consider a graphene bilayer with a relative small angle rotation between the layers-a stacking defect often seen in the surface of graphite-and calc. the electronic structure near zero energy in a continuum approxn. Contrary to what happens in an AB stacked bilayer and in accord with observations in epitaxial graphene, we find: (a) the low energy dispersion is linear, as in a single layer, but the Fermi velocity can be significantly smaller than the single-layer value; (b) an external elec. field, perpendicular to the layers, does not open an electronic gap.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXhsVOlt7bL&md5=8d685de8e04798b9a6cac63aca485df7
    55. 55
      Ferrari, A. C. Raman Spectroscopy of Graphene and Graphite: Disorder, Electron–Phonon Coupling, Doping and Nonadiabatic Effects. Solid State Commun. 2007, 143 (1–2), 47– 57,  DOI: 10.1016/j.ssc.2007.03.052
      55
      Raman spectroscopy of graphene and graphite: Disorder, electron-phonon coupling, doping and nonadiabatic effects
      Ferrari, Andrea C.
      Solid State Communications (2007), 143 (1-2), 47-57CODEN: SSCOA4; ISSN:0038-1098. (Elsevier Ltd.)
      The authors review recent work on Raman spectroscopy of graphite and graphene. The authors focus on the origin of the D and G peaks and the 2nd order of the D peak. The G and 2 D Raman peaks change in shape, position and relative intensity with no. of graphene layers. This reflects the evolution of the electronic structure and electron-phonon interactions. The authors then consider the effects of doping on the Raman spectra of graphene. The Fermi energy is tuned by applying a gate-voltage. This induces a stiffening of the Raman G peak for both holes and electrons doping. Thus Raman spectroscopy can be efficiently used to monitor no. of layers, quality of layers, doping level and confinement.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXmt1yhtr0%253D&md5=b67986a7f5f92c4a5ab64950f3330d7a
    56. 56
      Deng, S.; Berry, V. Wrinkled, Rippled and Crumpled Graphene: An Overview of Formation Mechanism, Electronic Properties, and Applications. Mater. Today 2016, 19 (4), 197– 212,  DOI: 10.1016/j.mattod.2015.10.002
      56
      Wrinkled, rippled and crumpled graphene: an overview of formation mechanism, electronic properties, and applications
      Deng, Shikai; Berry, Vikas
      Materials Today (Oxford, United Kingdom) (2016), 19 (4), 197-212CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
      Distinctive from their 1D and 0D counterparts, 2D nanomaterials (2DNs) exhibit surface corrugations (wrinkles and ripples) and crumples. Thermal vibrations, edge instabilities, thermodynamically unstable (interat.) interactions, strain in 2D crystals, thermal contraction, dislocations, solvent trapping, pre-strained substrate-relaxation, surface anchorage and high solvent surface tension during transfer cause wrinkles or ripples to form on graphene. These corrugations on graphene can modify its electronic structure, create polarized carrier puddles, induce pseudomagnetic field in bilayers and alter surface properties. This review outlines the different mechanisms of wrinkle, ripple and crumple formation, and the interplay between wrinkles' and ripples' attributes (wavelength/width, amplitude/height, length/size, and bending radius) and graphene's electronic properties and other mech., optical, surface, and chem. properties. Also included are brief discussions on corrugation-induced reversible wettability and transmittance in graphene, modulation of its chem. potential, enhanced energy storage and strain sensing via relaxation of corrugations. Finally, the review summarizes the future areas of research for 2D corrugations and crumples.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhs1OntrnF&md5=562c71f24c7b2b4c7b2bac0aab2b4755
    57. 57
      Hattab, H.; N’Diaye, A. T.; Wall, D.; Klein, C.; Jnawali, G.; Coraux, J.; Busse, C.; van Gastel, R.; Poelsema, B.; Michely, T. Interplay of Wrinkles, Strain, and Lattice Parameter in Graphene on Iridium. Nano Lett. 2012, 12 (2), 678– 682,  DOI: 10.1021/nl203530t
      57
      Interplay of Wrinkles, Strain, and Lattice Parameter in Graphene on Iridium
      Hattab, Hichem; N'Diaye, Alpha T.; Wall, Dirk; Klein, Claudius; Jnawali, Giriraj; Coraux, Johann; Busse, Carsten; van Gastel, Raoul; Poelsema, Bene; Michely, Thomas; Meyer-zu Heringdorf, Frank-J.; Horn-von Hoegen, Michael
      Nano Letters (2012), 12 (2), 678-682CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Following graphene growth by thermal decompn. of ethylene on Ir(111) at high temps. we analyzed the strain state and the wrinkle formation kinetics as function of temp. Using the moire spot sepn. in a LEED pattern as a magnifying mechanism for the difference in the lattice parameters between Ir and graphene, we achieved an unrivaled relative precision of ±0.1 pm for the graphene lattice parameter. Our data reveals a characteristic hysteresis of the graphene lattice parameter that is explained by the interplay of reversible wrinkle formation and film strain. We show that graphene on Ir(111) always exhibits residual compressive strain at room temp. Our results provide important guidelines for strategies to avoid wrinkling.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhs1aqu7rK&md5=dc01208579b550f5dece4384a508ca80
    58. 58
      Yan, W.; He, W.-Y.; Chu, Z.-D.; Liu, M.; Meng, L.; Dou, R.-F.; Zhang, Y.; Liu, Z.; Nie, J.-C.; He, L. Strain and Curvature Induced Evolution of Electronic Band Structures in Twisted Graphene Bilayer. Nat. Commun. 2013, 4 (1), 2159,  DOI: 10.1038/ncomms3159
      58
      Strain and curvature induced evolution of electronic band structures in twisted graphene bilayer
      Yan Wei; He Wen-Yu; Chu Zhao-Dong; Liu Mengxi; Meng Lan; Dou Rui-Fen; Zhang Yanfeng; Liu Zhongfan; Nie Jia-Cai; He Lin
      Nature communications (2013), 4 (), 2159 ISSN:.
      It is well established that strain and geometry could affect the band structure of graphene monolayer dramatically. Here we study the evolution of local electronic properties of a twisted graphene bilayer induced by a strain and a high curvature, which are found to strongly affect the local band structures of the twisted graphene bilayer. The energy difference of the two low-energy van Hove singularities decreases with increasing lattice deformation and the states condensed into well-defined pseudo-Landau levels, which mimic the quantization of massive chiral fermions in a magnetic field of about 100 T, along a graphene wrinkle. The joint effect of strain and out-of-plane distortion in the graphene wrinkle also results in a valley polarization with a significant gap. These results suggest that strained graphene bilayer could be an ideal platform to realize the high-temperature zero-field quantum valley Hall effect.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC3sfgslWktQ%253D%253D&md5=dcfe69882c5f7f0082881e0258abeca8
    59. 59
      Geisenhof, F. R.; Winterer, F.; Seiler, A. M.; Lenz, J.; Martin, I.; Weitz, R. T. Interplay between Topological Valley and Quantum Hall Edge Transport. Nat. Commun. 2022, 13 (1), 4187,  DOI: 10.1038/s41467-022-31680-y
      59
      Interplay between topological valley and quantum Hall edge transport
      Geisenhof, Fabian R.; Winterer, Felix; Seiler, Anna M.; Lenz, Jakob; Martin, Ivar; Weitz, R. Thomas
      Nature Communications (2022), 13 (1), 4187CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
      An established way of realizing topol. protected states in a two-dimensional electron gas is by applying a perpendicular magnetic field thus creating quantum Hall edge channels. In electrostatically gapped bilayer graphene intriguingly, even in the absence of a magnetic field, topol. protected electronic states can emerge at naturally occurring stacking domain walls. While individually both types of topol. protected states have been investigated, their intriguing interplay remains poorly understood. Here, we focus on the interplay between topol. domain wall states and quantum Hall edge transport within the eight-fold degenerate zeroth Landau level of high-quality suspended bilayer graphene. We find that the two-terminal conductance remains approx. const. for low magnetic fields throughout the distinct quantum Hall states since the conduction channels are traded between domain wall and device edges. For high magnetic fields, however, we observe evidence of transport suppression at the domain wall, which can be attributed to the emergence of spectral minigaps. This indicates that stacking domain walls potentially do not correspond to a topol. domain wall in the order parameter.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhvFWnsL%252FL&md5=de57f46ac5662edc0eff9c88ca51dae9
    60. 60
      Keunecke, M.; Reutzel, M.; Schmitt, D.; Osterkorn, A.; Mishra, T. A.; Möller, C.; Bennecke, W.; Jansen, G. S. M.; Steil, D.; Manmana, S. R. Electromagnetic Dressing of the Electron Energy Spectrum of Au(111) at High Momenta. Phys. Rev. B 2020, 102 (16), 161403,  DOI: 10.1103/PhysRevB.102.161403
      60
      Electromagnetic dressing of the electron energy spectrum of Au(111) at high momenta
      Keunecke, Marius; Reutzel, Marcel; Schmitt, David; Osterkorn, Alexander; Mishra, Tridev A.; Moeller, Christina; Bennecke, Wiebke; Jansen, G. S. Matthijs; Steil, Daniel; Manmana, Salvatore R.; Steil, Sabine; Kehrein, Stefan; Mathias, Stefan
      Physical Review B (2020), 102 (16), 161403CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Light-engineering of quantum materials via electromagnetic dressing is considered an on-demand approach for tailoring electronic band dispersions and even inducing topol. phase transitions. For probing such dressed bands, photoemission spectroscopy is an ideal tool, and we employ here a novel expt. based on ultrafast photoemission momentum microscopy. Using this setup, we measure the in-plane momentum-dependent intensity fingerprints of the electromagnetically-dressed sidebands from a Au(111) surface for s- and p-polarized IR driving. We find that at metal surfaces, due to screening of the driving laser, the contribution from Floquet-Bloch bands is negligible, and the dressed bands are dominated by the laser-assisted photoelec. effect. Also, we find, from calcns., that in contrast to general expectations, s-polarized light can dress free-electron states at large photoelectron momenta. Our results show that the dielec. response of the material must carefully be taken into account when using photoemission for the identification of light-engineered electronic band structures.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitlChtrbP&md5=403293d2a71cd6ad51acaf06a1fb7377
    61. 61
      Keunecke, M.; Möller, C.; Schmitt, D.; Nolte, H.; Jansen, G. S. M.; Reutzel, M.; Gutberlet, M.; Halasi, G.; Steil, D.; Steil, S. Time-Resolved Momentum Microscopy with a 1 MHz High-Harmonic Extreme Ultraviolet Beamline. Rev. Sci. Instrum. 2020, 91 (6), 063905,  DOI: 10.1063/5.0006531
      61
      Time-resolved momentum microscopy with a 1 MHz high-harmonic extreme ultraviolet beamline
      Keunecke, Marius; Moeller, Christina; Schmitt, David; Nolte, Hendrik; Jansen, G. S. Matthijs; Reutzel, Marcel; Gutberlet, Marie; Halasi, Gyula; Steil, Daniel; Steil, Sabine; Mathias, Stefan
      Review of Scientific Instruments (2020), 91 (6), 063905CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
      A setup for time-resolved momentum microscopy driven by a 1 MHz fs extreme UV table-top light source optimized for the generation of 26.5 eV photons is presented. The setup provides simultaneous access to the temporal evolution of the photoelectron's kinetic energy and in-plane momentum. Opportunities and limitations of the new expt. based on static and time-resolved measurements on graphene are discussed. (c) 2020 American Institute of Physics.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhtlShsr7N&md5=6d822e6192cc76b35fdd9c00f7632fed
    62. 62
      Medjanik, K.; Fedchenko, O.; Chernov, S.; Kutnyakhov, D.; Ellguth, M.; Oelsner, A.; Schonhense, B.; Peixoto, T. R. F.; Lutz, P.; Min, C.-H.; Reinert, F.; Daster, S.; Acremann, Y.; Viefhaus, J.; Wurth, W.; Elmers, H. J.; Schonhense, G. Direct 3D Mapping of the Fermi Surface and Fermi Velocity. Nat. Mater. 2017, 16 (6), 615– 621,  DOI: 10.1038/nmat4875
      6150
      Direct 3D mapping of the Fermi surface and Fermi velocity
      Medjanik, K.; Fedchenko, O.; Chernov, S.; Kutnyakhov, D.; Ellguth, M.; Oelsner, A.; Schoenhense, B.; Peixoto, T. R. F.; Lutz, P.; Min, C.-H.; Reinert, F.; Daester, S.; Acremann, Y.; Viefhaus, J.; Wurth, W.; Elmers, H. J.; Schoenhense, G.
      Nature Materials (2017), 16 (6), 615-621CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      The authors performed a full mapping of the bulk electronic structure including the Fermi surface and Fermi-velocity distribution vF(kF) of tungsten. The 4D spectral function ρ(EB; k) in the entire bulk Brillouin zone and 6 eV binding-energy (EB) interval was acquired in ∼3 h thanks to a new multidimensional photoemission data-recording technique (combining full-field k-microscopy with time-of-flight parallel energy recording) and the high brilliance of the soft x-rays used. A direct comparison of bulk and surface spectral functions (taken at low photon energies) reveals a time-reversal-invariant surface state in a local band gap in the (110)-projected bulk band structure. The surface state connects hole and electron pockets that would otherwise be sepd. by an indirect local band gap. The authors confirmed its Dirac-like spin texture by spin-filtered momentum imaging. The measured 4D data array enables extn. of the 3D dispersion of all bands, all energy isosurfaces, electron velocities, hole or electron cond., effective mass and inner potential by simple algorithms without approxns. The high-Z bcc metals with large spin-orbit-induced band gaps are discussed as candidates for topol. non-trivial surface states.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXlt12jtbc%253D&md5=cb6897e75ca0a56a1de5d17f69c38f77
    63. 63
      Schmitt, D.; Bange, J. P.; Bennecke, W.; AlMutairi, A.; Meneghini, G.; Watanabe, K.; Taniguchi, T.; Steil, D.; Luke, D. R.; Weitz, R. T. Formation of Moiré Interlayer Excitons in Space and Time. Nature 2022, 608 (7923), 499– 503,  DOI: 10.1038/s41586-022-04977-7
      62
      Formation of moir´e interlayer excitons in space and time
      Schmitt, David; Bange, Jan Philipp; Bennecke, Wiebke; AlMutairi, AbdulAziz; Meneghini, Giuseppe; Watanabe, Kenji; Taniguchi, Takashi; Steil, Daniel; Luke, D. Russell; Weitz, R. Thomas; Steil, Sabine; Jansen, G. S. Matthijs; Brem, Samuel; Malic, Ermin; Hofmann, Stephan; Reutzel, Marcel; Mathias, Stefan
      Nature (London, United Kingdom) (2022), 608 (7923), 499-503CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
      To exploit the full potential of correlated moir´e and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement is indispensable. Femtosecond photoemission momentum microscopy provides quant. access to these key properties of the moir´e interlayer excitons. Interlayer excitons are dominantly formed through femtosecond exciton-phonon scattering and subsequent charge transfer at the interlayer-hybridized Σ valleys. Interlayer excitons exhibit a momentum fingerprint that is a direct hallmark of the superlattice moir´e modification. The wavefunction distribution of the electronic part of the exciton are reconstructed, and the size with the real-space moir´e superlattice are compared. The work provides direct access to interlayer exciton formation dynamics in space and time and reveals opportunities to study correlated moir´e and exciton physics for the future realization of exotic quantum phases of matter.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XitFGlsrfE&md5=7e9fade5c584dfb037f493eaeac5bb69
    64. 64
      Magnozzi, M.; Pflug, T.; Ferrera, M.; Pace, S.; Ramó, L.; Olbrich, M.; Canepa, P.; Ağircan, H.; Horn, A.; Forti, S. Local Optical Properties in CVD-Grown Monolayer WS 2 Flakes. J. Phys. Chem. C 2021, 125 (29), 16059– 16065,  DOI: 10.1021/acs.jpcc.1c04287
      63
      Local Optical Properties in CVD-Grown Monolayer WS2 Flakes
      Magnozzi, Michele; Pflug, Theo; Ferrera, Marzia; Pace, Simona; Ramo, Lorenzo; Olbrich, Markus; Canepa, Paolo; Agircan, Hasret; Horn, Alexander; Forti, Stiven; Cavalleri, Ornella; Coletti, Camilla; Bisio, Francesco; Canepa, Maurizio
      Journal of Physical Chemistry C (2021), 125 (29), 16059-16065CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably ext. information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielec. function of chem. vapor deposition (CVD)-grown WS2 flakes with a microscopic lateral resoln., thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS2. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was obsd., along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modulations across the flakes, having a different influence on the absorption and re-emission of light, are deemed responsible for the effect.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhsFelu7bF&md5=a1177ae4d8e39978b77e2e85a96d98c6
    65. 65
      Peci, E.; Magnozzi, M.; Ramó, L.; Ferrera, M.; Convertino, D.; Pace, S.; Orlandini, G.; Sharma, A.; Milekhin, I.; Salvan, G. Dielectric Function of 2D Tungsten Disulfide in Homo- and Heterobilayer Stacking. Adv. Mater. Interfaces 2023, 10 (3), 2201586,  DOI: 10.1002/admi.202201586
      64
      Dielectric Function of 2D Tungsten Disulfide in Homo- and Heterobilayer Stacking
      Peci, Ermes; Magnozzi, Michele; Ramo, Lorenzo; Ferrera, Marzia; Convertino, Domenica; Pace, Simona; Orlandini, Giorgio; Sharma, Apoorva; Milekhin, Ilya; Salvan, Georgeta; Coletti, Camilla; Zahn, Dietrich R. T.; Bisio, Francesco; Canepa, Maurizio
      Advanced Materials Interfaces (2023), 10 (3), 2201586CODEN: AMIDD2; ISSN:2196-7350. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The opto-electronic properties of semiconducting 2D materials can be flexibly manipulated by engineering the at.-scale environment. This can be done by including 2D materials in tailored van der Waals (vdW) stacks, whose optical response is a function of the no. and the type of adjacent 2D layers. This work reports a systematic investigation of the dielec. function of 2D semiconducting WS2 in various stacking configurations: monolayer, 3R/2H homobilayer, and WS2/MoS2 heterobilayer. Reliable, Kramers-Kronig-consistent dielec. functions are obtained for WS2 in each configuration by means of spectroscopic ellipsometry (SE) and related parametric optical modeling in a wide spectral range (1.55-3.10 eV). The results of SE are combined with photoluminescence and absorbance spectra to identify the spectral position of the main excitonic features in WS2, which manifest sizable red shifts depending on the stacking configuration. These results represent a consistent ref. set for the dielec. function of WS2 in vdW stacking configurations of particular interest for the scientific and technol. field, and can be fruitfully exploited for reliable predictions of the optical response of WS2-contg. systems.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XjtVehsbjL&md5=1749143037903b79d2283a0a8b1b74bd
    66. 66
      Kenaz, R.; Rapaport, R. Mapping Spectroscopic Micro-Ellipsometry with Sub-5 Microns Lateral Resolution and Simultaneous Broadband Acquisition at Multiple Angles. Rev. Sci. Instrum. 2023, 94 (2), 023908,  DOI: 10.1063/5.0123249
      65
      Mapping spectroscopic micro-ellipsometry with sub-5 microns lateral resolution and simultaneous broadband acquisition at multiple angles
      Kenaz, Ralfy; Rapaport, Ronen
      Review of Scientific Instruments (2023), 94 (2), 023908CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
      Spectroscopic ellipsometry is a widely used optical technique in both industry and research for detg. the optical properties and thickness of thin films. The effective use of spectroscopic ellipsometry on micro-structures is inhibited by tech. limitations on the lateral resoln. and data acquisition rate. Here, we introduce a spectroscopic micro-ellipsometer (SME), capable of recording spectrally resolved ellipsometric data simultaneously at multiple angles of incidence in a single measurement of a few seconds, with a lateral resoln. down to 2μm in the visible spectral range. The SME can be easily integrated into generic optical microscopes by the addn. of a few std. optical components. We demonstrate complex refractive index and thickness measurements by using the SME, which are in excellent agreement with a com. spectroscopic ellipsometer. The high lateral resoln. is displayed by complex refractive index and thickness maps over micron-scale areas. As an application for its accuracy and high lateral resoln., the SME can characterize the optical properties and no. of layers of exfoliated transition-metal dichalcogenides and graphene, for structures that are a few microns in size. (c) 2023 American Institute of Physics.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3sXjvFCntLs%253D&md5=779991e82edd6434df18e752ceeb037f

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.3c00619.

    • Methods, fitted parameters, intensity thresholding, comparison of techniques, and bilayer graphene on Cu and Si (PDF)


    Terms & Conditions

    Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.