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In Situ Scanning Transmission Electron Microscopy Study of MoS2 Formation on Graphene with a Deep-Learning Framework

  • Yeongdong Lee
    Yeongdong Lee
    Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
    Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
    More by Yeongdong Lee
    Orcidhttps://orcid.org/0000-0001-9749-5270
  • , 
  • Jongyeong Lee
    Jongyeong Lee
    Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
    Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
    More by Jongyeong Lee
    Orcidhttps://orcid.org/0000-0002-3456-0300
  • , 
  • Handolsam Chung
    Handolsam Chung
    Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
    Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
    More by Handolsam Chung
  • , 
  • Jaemin Kim
    Jaemin Kim
    Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
    Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
    More by Jaemin Kim
    Orcidhttps://orcid.org/0000-0001-8675-7271
  • , and 
  • Zonghoon Lee*
    Zonghoon Lee
    Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
    Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
    *Email: [email protected]
    More by Zonghoon Lee
    Orcidhttps://orcid.org/0000-0003-3246-4072
Cite this: ACS Omega 2021, 6, 33, 21623–21630
Publication Date (Web):August 10, 2021
https://doi.org/10.1021/acsomega.1c03002

Copyright © 2021 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Atomic-scale information is essential for understanding and designing unique structures and properties of two-dimensional (2D) materials. Recent developments in in situ transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) enable research to provide abundant insights into the growth of nanomaterials. In this study, 2D MoS2 is synthesized on a suspended graphene substrate inside a TEM column through thermolysis of the ammonium tetrathiomolybdate (NH4)2MoS4 precursor at 500 °C. To avoid misinterpretation of the in situ STEM images, a deep-learning framework, DeepSTEM, is developed. The DeepSTEM framework successfully reconstructs an object function in atomic-resolution STEM imaging for accurate determination of the atomic structure and dynamic analysis. In situ STEM imaging with DeepSTEM enables observation of the edge configuration, formation, and reknitting progress of MoS2 clusters with the formation of a mirror twin boundary. The synthesized MoS2/graphene heterostructure shows various twist angles, as revealed by atomic-resolution TEM. This deep-learning framework-assisted in situ STEM imaging provides atomic information for in-depth studies on the growth and structure of 2D materials and shows the potential use of deep-learning techniques in 2D material research.

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1. Introduction

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In recent years, researchers have explored two-dimensional (2D) materials and their heterostructures, focusing on their layered structure and unique properties that seem promising for future applications, including field-effect transistors, diffusion barriers, liners, sensors, batteries, and optoelectronic devices. (1−5) The nanostructures of 2D materials, such as atomic defects, edge configuration, and domain size, impact their physical and chemical properties and applications. Therefore, information about the nanostructure of 2D materials is essential for understanding and designing the required properties for a dedicated application.
Transition metal dichalcogenides (TMDs) exhibit attractive properties, and molybdenum disulfide (MoS2) is the most researched material among TMDs anticipated to use for electronics and photonics. (6) However, the low charge mobility and contact resistance with metal electrodes have limited their use. (7,8) One of the solutions is the MoS2/graphene heterostructure. The MoS2/graphene 2D heterostructure shows remarkable performance in electronic applications with low contact resistance with graphene as an electrode. (1,9) However, the fabrication and thermal budget of the MoS2/graphene heterostructure also matter for their use. The transferring processes for stacking MoS2 on graphene can adopt contamination introducing degradation of properties. The direct synthesis of MoS2 on graphene or vice versa is required. Also, the growth temperature is important to integrate the structure in the complementary metal-oxide-semiconductor (CMOS) system with extraordinary properties.
The thermolysis-driven MoS2 synthesis method is a promising pathway for the MoS2/graphene heterostructure. (10) The (NH4)2MoS4 thermally decomposes into MoS3 at 160–270 °C, with releases of NH3 and H2S. Then, the MoS2 forms at over 270 °C from MoS3 with the release of sulfur. (11,12) Its growth temperature is compatible with CMOS technology. Recent studies have achieved layer-number control and wafer-scale growth of MoS2 with spin coating of the (NH4)2MoS4 precursor and heating. (13,14)
Recent progress in in situ transmission electron microscopy (TEM) and in situ scanning transmission electron microscopy(STEM) have provided insights into the growth of 2D materials, with atomic resolution and the implementation of growth conditions of the existing methods. (15) In situ TEM/STEM is an effective tool for the manipulation of structures with an electron beam, simultaneously analyzing whether desired or novel nanostructures have been achieved. (16,17) The insights into the structure and formation are necessary for dedicated configuration synthesis of MoS2 by thermolysis. Several studies have reported efforts devoted to understanding thermolysis-driven growth using in situ TEM/STEM. (18−20) The thermolysis of the (NH4)2MoS4 precursor on silicon nitride leads to a vertical MoS2 structure at 400 °C. (18) On the other hand, a horizontal MoS2 structure was attained on a carbon film as a substrate. (19) However, an atomic-scale understanding of MoS2 formation on graphene by thermolysis has not yet been revealed.
In situ TEM/STEM images can often be misinterpreted owing to aberrations in the optic system of TEM. Hence, in the case of in situ STEM, light elements, specimen drift, and low-dose imaging for time resolution, as well as electron beam-sensitive materials, make it difficult to understand the exact atomic structure and evolution of 2D materials. Exit-wave reconstruction and object function reconstruction have been developed to avoid this misinterpretation. (21−24) It facilitates studies of rapid movement of atoms in atomic-resolution TEM. However, the high-angle annular dark-field STEM (HAADF-STEM) analysis relies on HAADF-STEM image simulation. The result of the HAADF-STEM image simulation is compared with the experimental raw data to match the expected and detected structures, relying on human know-how. Moreover, in the case of atomic-scale dynamics study, each frame needs to be examined using simulation. The object function reconstruction using a single image for STEM imaging is necessary for atomic-scale dynamics study. The most prominent method is Wiener filtering, which is a linear denoising solution. (25) However, it divides out the contrast transfer function in the reciprocal lattice, resulting in a constant transfer function without decay. (24) The constant transfer function from the Wiener filter can introduce artifacts in the STEM image. (24) Our solution is deep-learning-based object function reconstruction from the considered factors by the probe function. Deep learning has been solving challenges in electron microscopy. (26) Recently, deep learning of the simulation data set recognized the local atomic structure in high-resolution TEM (HRTEM) images. (27) Another deep learning using experimental HRTEM data set has been developed to reconstruct the exit wave from a single image with real microscope’s aberration parameters from the so-called Zemlin tableau method. (28) In this regard, a deep-learning framework in this study also shows the potential of deep learning in electron microscopy.
In this study, we investigated the formation of MoS2 on a suspended graphene substrate inside a TEM column through thermolysis of the (NH4)2MoS4 precursor at 500 °C with a heating holder. Atomic-resolution TEM/STEM shows the structure of the grown MoS2 and the configuration of the MoS2/graphene heterostructure. The dynamic structural evolution with thermal energy and electron-beam irradiation was investigated by in situ HAADF-STEM imaging. HAADF-STEM imaging can exclude a signal from the graphene substrate under incoherent conditions. In dynamics analysis, it can prevent the misinterpretation of object materials’ nonperiodic step structures. However, the experimental HAADF-STEM images were deteriorated by elevated temperature, specimen drift, and the low electron-dose rate for minimizing the effect of electron-beam irradiation on the structural evolution of MoS2 on graphene. In addition, the recognition of the sulfur atoms of MoS2, which are relatively light, was difficult with raw experimental data. We present the deep-learning framework for object function reconstruction for HAADF-STEM studies of the structure and dynamics of 2D materials. We conducted a fast Fourier transform (FFT)-based deconvolution with the deep-learning framework by including an HAADF-STEM image simulation equation with a precalculated probe function for the actual optical system conditions, which were measured as aberrations. With the assistance of the deep-learning framework, we elucidate the precise structure of MoS2 and investigate the structural evolution of MoS2 on graphene including edge configuration, formation, and reknitting progress. Atomic-resolution TEM shows a MoS2/graphene 2D heterostructure through thermolysis, exhibiting various twist angles without a specific orientation.

2. Results and Discussion

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We propose a deep-learning framework called the DeepSTEM framework. HAADF-STEM imaging has been used to accurately determine the structure and observe the dynamics of the nanostructure of MoS2 because HAADF-STEM imaging can directly identify the molybdenum and sulfur atoms with atomic number (Z)-dependence contrast, called Z contrast imaging, and an incoherent imaging feature. (29) However, the identification of the exact structure with dynamics is difficult because of the elevated temperature and specimen drift. To use in situ HAADF-STEM imaging for dynamical analysis, it is also necessary to decrease the electron-dose rate for better time resolution and minimizing the effects of electron-beam irradiation on the specimen. Hence, the relatively light sulfur atoms in MoS2 are not discernible owing to the low intensity. To observe the sulfur atoms using in situ STEM dynamics, object function reconstruction is essential.
The DeepSTEM framework consists of two main parts: an object function reconstruction part and a STEM image simulation (Figure 1). The object function reconstruction part, denoted by the red dashed line in Figure 1, has an encoder, decoder, and input image (X), which is an HAADF-STEM image obtained from the HAADF detector, and an object function (ψ). The object function reconstruction part is a variant of the convolutional autoencoder. (30) Each encoder and decoder consists of four sequential layers, and an activation function, rectified linear unit (ReLU), was used in each layer. The output of the decoder is the object function ψ. The object function ψ is the projected structure of the specimen. (23) An activation function of the decoder is the zero-centered hyperbolic tangent. At the end of the decoder, there is a convolutional layer of 1 × 1 size 1 kernel without the activation function. The details of the DeepSTEM are in Supporting Information.

Figure 1

Figure 1. Structure of the DeepSTEM framework. Input images are decoded to ψ, which corresponds to the object function of the STEM image. Ir is the result of the STEM image simulation when applying the PSF to the Fourier transform of the reconstructed object function.

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Then, this object function reconstruction subsystem finds the relationship between X and ψ. Ψ is defined as the FFT of ψ as follows
(1)
where u is a 2D position vector in the object plane and k is a 2D position vector in a reciprocal plane conjugate to u.
In the STEM image simulation part denoted by the blue dashed line, Ir represents the STEM image simulation results acquired from applying the precalculated probe function with the object function as follows
(2)
where |h(u)|2 is the point spread function (PSF). The PSF |h(u)|2 is precalculated with the experimental optic system conditions, aberrations being measured by the Zemlin tableau method. (31)
The DeepSTEM is a neural network that reconstructs the object function ψ as follows
(3)
We used the ADAM optimization algorithm, an extension to stochastic gradient descent, to update parameters that minimize the loss function in the DeepSTEM framework with an initial learning rate of 2 × 10–5 and default parameters. (32)
Figure 2a shows the HAADF-STEM image of a MoS2 monolayer grown on suspended graphene in the TEM column using an in situ heating holder. The monolayer graphene sheet was synthesized by chemical vapor deposition (CVD). (33,34) A heating chip mounted on the heating holder was perforated by a focused ion beam. Then, the monolayer graphene was transferred onto the perforated heating chip for suspended graphene (Figure S1). The (NH4)2MoS4 precursor was dispersed in N,N-dimethylformamide (DMF) with 0.1 wt % solution and dropped onto suspended graphene on the heating chip with a micropipette. The thermolysis of the precursor for the in situ STEM growth of MoS2 proceeded at a temperature of 500 °C. The temperature of the heating chip was elevated to 500 °C immediately (∼1 s). The synthesized MoS2 was recognized as a monolayer by Z-contrast imaging. Figure 2b shows an image of the reconstructed object function output by the DeepSTEM framework. Because MoS2 is a monolayer, the object function corresponds to the atomic structure of the specimen with Z-contrast. The image of the reconstructed object function made it possible to distinguish between Mo and S with high contrast, as shown in Figure 2c. Comparing the results obtained with the DeepSTEM framework with the experimental HAADF-STEM image, we concluded that the DeepSTEM framework reconstructed an object function that represents the projected atomic structure of the specimen (Figure 2c).

Figure 2

Figure 2. (a) Atomic-resolution HAADF-STEM image of MoS2 on graphene. (b) Image of the reconstructed object function. (c) Intensity profiles of the experimental HAADF-STEM image and image of reconstructed object function are plotted as solid red lines from lower left to upper right in each image. The scale bar is 0.5 nm. Mo and S atoms are represented by cyan and orange spheres.

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Using the DeepSTEM framework, we identified the edge configuration of in situ STEM-grown MoS2 on suspended graphene. Hexagonal 2D materials primarily exhibit two types of edges: zigzag and armchair. Among them, in the case of MoS2, zigzag edges are commonly predicted and observed, with the direction of (101̅0) Mo-terminated zigzag (Mo-ZZ) and (1̅010) sulfur-terminated zigzag (S-ZZ) depending on growth conditions and chemical potential. (35−38) Because these edge structures can alter the electronic and magnetic properties of MoS2 clusters, (39) their precise identification is necessary. The corresponding object function reconstruction of HAADF-STEM images (Figure 3a–c) enables accurate identification of sulfur atoms, as shown in Figure 3d–f. The MoS2 clusters predominantly have Mo-ZZ edges (Figure 3d) and Mo-Klein edges (Mo atoms bound with the S-ZZ chain, Figure 3e,f) and rarely have S-ZZ (Figure 3f). Under Mo-rich conditions, the formation energies of Mo-ZZ and Mo-Klein edges are comparable, and these edges have lower formation energies than S-ZZ edges and sulfur-attached Mo-ZZ edges. (37) Therefore, our experimental growth conditions for in situ STEM MoS2 formation on graphene by thermolysis can be concluded to be Mo-rich.

Figure 3

Figure 3. (a–c) Atomic-resolution HAADF-STEM images showing the edge configurations. The image was taken at a temperature of 500 °C. (d–f) Images of the reconstructed object function of (a–c) showing (d) Mo-ZZ, (e) Mo-Klein, and (f) S-ZZ. The scale bar is 0.5 nm. (g–i) Schematic atomic models based on the interpretation of (d–f). Mo and S atoms are represented by cyan and yellow spheres, respectively.

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The structural evolution of MoS2 on graphene during formation was analyzed using in situ STEM imaging with the DeepSTEM framework. Figure 4 shows time-elapsed images of the reconstructed object function during the formation of MoS2 on graphene while holding temperature at 500 °C. First, a MoSx cluster is on the graphene substrate, as shown in Figure 4a. The Mo atoms do not show a periodic and ordered nanostructure. Then, the MoSx cluster reconstructs the hexagonal lattice structure of MoS2 with thermal energy under electron-beam irradiation, as shown in Figure 4b,c. Here, there was no merging of other MoSx clusters. The MoS2 cluster shows a truncated triangular shape with Mo-ZZ, Mo-Klein, and S-ZZ edges, as shown in Figure 4c. The remaining MoSx clusters, indicated by white arrows, continue to form a MoS2 cluster. After subsequent scans, the upside MoSx cluster continuously transformed into a MoS2 cluster, as shown in Figure 4d. The downside MoSx cluster finally transformed into a MoS2 cluster, as shown in Figure 4e. The MoS2 cluster in Figure 4e shows a triangular shape with Mo-ZZ edges. This is another indication of the Mo-rich growth condition discussed above. Our proposed method could reveal that the thermolysis at elevated temperature shows the detailed formation progress of MoS2 on graphene inside the TEM column.

Figure 4

Figure 4. Time-elapsed images of the reconstructed object function showing MoS2 formation progress on graphene with temperature holding at 500 °C. The elapsed time is shown in the lower left of each image. (a) MoSx cluster is adsorbed on the graphene substrates. (b) MoSx reconstructs the MoS2 cluster. (c) MoS2 cluster shows a truncated triangular shape with Mo-ZZ, Mo-Klein, and S-ZZ edges. The remaining MoSx clusters are denoted by white solid arrows. (d) Upside MoSx cluster transforms into MoS2. (e) Downside MoSx clusters also transformed into MoS2. The MoS2 cluster shows a triangular shape with Mo-ZZ edges. The scale bar is 0.5 nm.

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In addition, Figure 5 shows time-elapsed images of a reconstructed object function during the reknitting progress of a MoS2 cluster on graphene while holding temperature at 500 °C. In Figure 5a, a sub-nanometer scale hole defect exists between two MoS2 clusters. The hole defect is observed upon scan, confirming that it is not created by electron-beam irradiation. The hole defect shows Mo-ZZ and S-ZZ edges. Mo adatoms with bright intensity are noted by cyan dashed circles. Then, the hole defects start to reknit with diffusion of atoms and atomic reconstruction (Figure 5b). The images obtained show the MoS2 cluster, noted by an orange solid line, to be rearranged with roto-translational motion; however, a hole defect exists (Figure 5b). This shows that the roto-translational motion of MoS2 is possible on graphene as theoretically expected. (40) Also, the atoms and clusters on graphene are highly mobile (Figures S2 and S3). Thermal energy and the electron beam may mobilize the atoms and MoSx/MoS2 cluster to overcome the diffusion barrier. As shown in Figure 5c, the MoS2 cluster finally reknits the hole defect. The MoS2 cluster has been reknitted and merged with the mirror twin boundary, as denoted by a red dashed box. The mirror twin boundary is along the S-ZZ edge direction and is composed of four-membered rings. Note that the mirror twin boundary connecting sulfur edges is under a locally Mo-rich condition. Previous studies reported that chalcogen deficiency can affect the formation of mirror twin boundaries. It should be related to our growth condition. Also, among sulfur-deficient twin boundaries, the formation energy of the four-membered-ring twin boundary is the lowest. (41) We demonstrate that the revealed mirror twin boundary formation here originated because of Mo-rich growth conditions and minimization of the total energy of the hole and grain boundary. Recent studies have observed and predicted that the mirror twin boundary is formed by the merging of two grains. (41−43) To the best of our knowledge, this is the first in situ observation of mirror twin boundary formation by coalescence of MoS2 clusters and their atomic-scale progress. Our method revealed that the reknitting progress during in situ synthesis of MoS2 involves the removal of atomic defects and roto-translational displacement of clusters with in situ heating STEM experiments.

Figure 5

Figure 5. (a–c) Time-elapsed images of the reconstructed object function of MoS2 reknitting progress on graphene with temperature holding at 500 °C. The elapsed time is shown in the upper left of each image. (a) MoS2 clusters have a hole with Mo-ZZ and S-ZZ edges. Mo adatoms are noted by cyan dashed circles. (b) Hole starts to reknit with diffusion and atomic reconstruction. The orange solid line in (a,b) shows the roto-translational motion of MoS2 clusters. (c) Hole has reknitted and shows a mirror twin boundary, as denoted by a red dashed box. Mo and S atoms are represented by cyan and orange spheres, respectively. The scale bar is 0.5 nm..

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Because the electronic and optoelectronic properties of MoS2/graphene stack vary with the twist angle of the layers, (44,45) we analyzed the twist angles using TEM (Figure S4) TEM images of MoS2 monolayers grown on graphene using the in situ heating holder were taken at room temperature (RT) with an acceleration voltage of 80 kV. FFTs of the TEM images show mixed spot patterns of crystalline MoS2 and graphene (Figure S4a–e). As shown in the histogram of the observed misorientation angles between MoS2 and graphene (Figure S4f), MoS2 clusters are oriented randomly without preferential direction, indicating nonepitaxial growth of MoS2 on graphene. As observed in Figure 5, the roto-translational displacement of the MoS2/graphene heterostructure with thermal energy at elevated temperatures occurs and plays a critical role in the reorientation of MoS2 clusters to be relaxed to a thermodynamically stable orientation. (40,46) Nevertheless, growth conditions such as relatively low temperature and ultrahigh vacuum could result in nonepitaxial growth. (47,48) Also, since we rapidly cooled down the specimen to RT, it may be stuck as the thermal energy decreases before the MoS2 clusters relax to the corresponding graphene lattice. The effect of trapping of MoS2 clusters by contaminants or the precursor can also affect the nonepitaxial characteristic of MoS2 on graphene. (19) Considering that the MoS2 growth on graphene commonly exhibits van der Waals epitaxial growth, (10,49−51) the nonepitaxial growth of MoS2 on graphene here would be also useful for twistronics and their applications.

3. Conclusions

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In summary, we performed an in situ STEM study on the formation of MoS2 on graphene at 500 °C using the DeepSTEM framework. The MoS2 was synthesized directly on suspended graphene by thermolysis. The DeepSTEM framework successfully reconstructed the object function from HAADF-STEM imaging, which enabled the accurate determination of MoS2 structures and observation of the dynamics of in situ STEM. In situ STEM studies have been using conventional filtering interpretation methods to interpret atomic-scale behavior. However, these methods have limitations in terms of interpreting electron micrographs deteriorated by an imperfect optic system. Because the object function is the absolute value of the exit wave, the DeepSTEM framework to reconstruct the object function enables in situ STEM observation of atomic-scale behaviors of MoS2 clusters with correcting the imperfect optic system. The real-time structural formation and evolution from a MoSx cluster to the triangular MoS2 cluster were observed. In addition, the reknitting progress of MoS2 clusters with hole defects was observed. Furthermore, the formation of the mirror twin boundary was revealed during the merging of MoS2 clusters. Interestingly, note that the MoS2 clusters show nonepitaxial growth on graphene, as revealed by atomic-resolution TEM. This in situ study provides atomic-scale insights into the formation of MoS2/graphene heterostructures and the possible use of deep-learning techniques in 2D material research. The combination of in situ STEM experimental images and the deep-learning framework is promising for further exploring novel structures and properties.

4. Experimental Section

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4.1. Fabrication of Suspended Graphene and Drop Casting of the MoS2 Precursor

Graphene was synthesized by CVD on 25 μm-thick Cu foil (99.8%, Alfa-Aesar, US). The temperature was increased to 1000 °C under a hydrogen atmosphere (30 sccm) for 40 min, and then, the specimen was maintained for 20 min under a hydrogen atmosphere (30 sccm). After that, methane gas (5 sccm) was added into the chamber and heated for 25 min at 1000 °C. After the growth, the hot zone was opened to cool the specimen to RT. The transfer of the graphene films onto an MEMS-based heating chip was performed by wet etching of the copper substrate after a spin coating of poly(methyl methacrylate). A PMMA solution was spin-coated onto the surface of the as-grown graphene films on Cu foil at a speed of 3000 rpm for 100 s. To ensure the binding of PMMA onto graphene, it was baked at 130 °C for 10 min. Then, the specimen was placed into a solution of sodium persulfate (Na2S2O8 with a concentration of 0.2 g in 1 mL of distilled water). After etching the copper substrate and rinsing the PMMA/graphene stack, it was transferred onto a heating chip. To enhance the attachment, this heating chip was heated at 120 °C for 30 min under ambient conditions. It was then soaked in acetone for a day to remove the PMMA. Holes were etched by FIB for the suspended graphene support before the transfer progress. The diameter of the circular holes was 1.2 μm. (NH4)2MoS4 was dissolved in DMF with 0.1 wt % solution. The solution was sonicated for 1 h. Then, the solution of 0.5 μL was dropped onto the suspended graphene/heating chip and dried at 120 °C for 30 min under the ambient condition to vaporize the solvent.

4.2. In Situ STEM and MoS2 Orientation Analysis

In situ STEM was conducted using an aberration-corrected FEI Titan G2 cube 60–300 transmission electron microscope (Thermo Fisher Scientific, US) equipped with probe and image correctors and a monochromator. It was operated at 200 kV. The convergence angle was 26.6 mrad, and a beam current of 85 pA was used to obtain HAADF-STEM images. HAADF detector angles were 50.5–200 mrad, and dwell times of 4–16 μs were used. In situ formation of MoS2 by thermolysis was conducted using a Wildfire in situ heating holder (DENSsolutions, Netherlands). The heating chip with the MoS2 precursor on suspended graphene was loaded onto the in situ heating holder. The specimen was heated from RT to 500 °C in ∼1 s in the TEM column, and the elevated temperature was maintained during observation. The orientation between MoS2 and graphene was analyzed using the microscope with atomic-resolution TEM images.

4.3. Training Data

Atomic-resolution images of MoS2/Gr prepared by the abovementioned method were obtained from aberration-corrected STEM with an acceleration voltage of 200 kV. The original 700 atomic-resolution STEM images have 1024 × 1024 pixels. Images were cropped into 128 × 128 pixels images, yielding a data set of 44,800 images. Each image has several hundred atoms. Finally, over 4,480,000 samples were used to train the DeepSTEM framework because the same atoms cannot be identified. The architecture of the DeepSTEM framework is shown in Table S1.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03002.

  • Fabrication of suspended graphene and drop casting of the MoS2 precursor, in situ STEM and MoS2 orientation analysis, training data, images of the in situ heating holder and chip with suspended graphene, time-elapsed HAADF-STEM images of mobile Mo atoms on graphene at 500 °C, time-elapsed HAADF-STEM images of the roto-translational motion of MoS2 on graphene at 500 °C, atomic-resolution TEM image of MoS2 clusters on graphene and histogram of misorientation angles, HAADF-STEM image of MoS2 clusters, architecture of the DeepSTEM framework and Appendix A: autoencoder (PDF)

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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

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  • Corresponding Author
    • Zonghoon Lee - Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea;  Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea;  Orcidhttps://orcid.org/0000-0003-3246-4072;  Email: [email protected]
  • Authors
    • Yeongdong Lee - Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea;  Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea;  Orcidhttps://orcid.org/0000-0001-9749-5270
    • Jongyeong Lee - Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea;  Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea;  Orcidhttps://orcid.org/0000-0002-3456-0300
    • Handolsam Chung - Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea;  Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
    • Jaemin Kim - Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea;  Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea;  Orcidhttps://orcid.org/0000-0001-8675-7271
  • Author Contributions

    Y.L. and J.L. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the Institute for Basic Science (IBS-R019-D1).

References

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This article references 51 other publications.

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    The technol. evolution has been progressing for centuries and will possibly increase at a higher rate in the 21st century. Currently, in this age of nanotechnol., the discovery of more economical and sustainable novel materials has considerably increased. The abundance of two-dimensional (2D) materials has endowed them with a broad material platform in tech. studies and in the expansion of nano- and at.-level applications. The innovation of graphene has motivated considerable attention to the study of other novel 2D materials, known as modern day "alchemy", by which scientists are trying to convert most possible periodic table elements into 2D material structures and forms. 2D material devices with high quality and good optical encoder performance have a multitude of industrial applications. However, their stability and large size restrict their applications, but these problems can be overcome by functionalization and substrate-based formation of 2D materials. Therefore, via this review, first, basic attributes of 2D materials are described, and the mechanisms to further enhance their properties are also summarized. Second, the applications of 2D materials are discussed, along with their advantages and disadvantages. Finally, some effective device-fabrication approaches, such as heterostructure approaches, are applied to further enhance the properties of 2D materials; their novel device applications and opportunities are also presented. This updated review may provide new avenues for 2D material synthesis and development of more efficient devices compared to conventional devices in different fields.
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  2. 2
    Hong, S.; Lee, C.-S.; Lee, M.-H.; Lee, Y.; Ma, K. Y.; Kim, G.; Yoon, S. I.; Ihm, K.; Kim, K.-J.; Shin, T. J.; Kim, S. W.; Jeon, E.-c.; Jeon, H.; Kim, J.-Y.; Lee, H.-I.; Lee, Z.; Antidormi, A.; Roche, S.; Chhowalla, M.; Shin, H.-J.; Shin, H. S. Ultralow-dielectric-constant amorphous boron nitride. Nature 2020, 582, 511– 514,  DOI: 10.1038/s41586-020-2375-9
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    Ultralow-dielectric-constant amorphous boron nitride
    Hong, Seokmo; Lee, Chang-Seok; Lee, Min-Hyun; Lee, Yeongdong; Ma, Kyung Yeol; Kim, Gwangwoo; Yoon, Seong In; Ihm, Kyuwook; Kim, Ki-Jeong; Shin, Tae Joo; Kim, Sang Won; Jeon, Eun-chae; Jeon, Hansol; Kim, Ju-Young; Lee, Hyung-Ik; Lee, Zonghoon; Antidormi, Aleandro; Roche, Stephan; Chhowalla, Manish; Shin, Hyeon-Jin; Shin, Hyeon Suk
    Nature (London, United Kingdom) (2020), 582 (7813), 511-514CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Decrease in processing speed due to increased resistance and capacitance delay is a major obstacle for the down-scaling of electronics. Minimizing the dimensions of interconnects (metal wires that connect different electronic components on a chip) is crucial for the miniaturization of devices. Interconnects are isolated from each other by non-conducting (dielec.) layers. So far, research has mostly focused on decreasing the resistance of scaled interconnects because integration of dielecs. using low-temp. deposition processes compatible with complementary metal-oxide-semiconductors is tech. challenging. Interconnect isolation materials must have low relative dielec. consts. (κ values), serve as diffusion barriers against the migration of metal into semiconductors, and be thermally, chem. and mech. stable. Specifically, the International Road map for Devices and Systems recommends the development of dielecs. with κ values of less than 2 by 2028. Existing low-κ materials (such as silicon oxide derivs., org. compds. and aerogels) have κ values greater than 2 and poor thermo-mech. properties. Here, the authors report three-nanometer-thick amorphous boron nitride films with ultralow κ values of 1.78 and 1.16 (close to that of air, κ = 1) at operation frequencies of 100 kHz and 1 MHz, resp. The films are mech. and elec. robust, with a breakdown strength of 7.3 MV per cm, which exceeds requirements. Cross-sectional imaging reveals that amorphous boron nitride prevents the diffusion of cobalt atoms into silicon under very harsh conditions, in contrast to ref. barriers. The results demonstrate that amorphous boron nitride has excellent low-κ dielec. characteristics for high-performance electronics.
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  3. 3
    Lee, C.-S.; Shin, K. W.; Song, H.-J.; Park, H.; Cho, Y.; Im, D.-H.; Lee, H.; Lee, J.-H.; Won, J. Y.; Chung, J. G.; Kim, C.; Byun, K.-E.; Lee, E.-K.; Kim, Y.; Ko, W.; Lim, H. J.; Park, S.; Shin, H.-J. Fabrication of Metal/Graphene Hybrid Interconnects by Direct Graphene Growth and Their Integration Properties. Adv. Electron. Mater. 2018, 4, 1700624,  DOI: 10.1002/aelm.201700624
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    There is no corresponding record for this reference.
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    Lo, C.-L.; Helfrecht, B. A.; He, Y.; Guzman, D. M.; Onofrio, N.; Zhang, S.; Weinstein, D.; Strachan, A.; Chen, Z. Opportunities and challenges of 2D materials in back-end-of-line interconnect scaling. J. Appl. Phys. 2020, 128, 080903,  DOI: 10.1063/5.0013737
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    Opportunities and challenges of 2D materials in back-end-of-line interconnect scaling
    Lo, Chun-Li; Helfrecht, Benjamin A.; He, Yanbo; Guzman, David M.; Onofrio, Nicolas; Zhang, Shengjiao; Weinstein, Dana; Strachan, Alejandro; Chen, Zhihong
    Journal of Applied Physics (Melville, NY, United States) (2020), 128 (8), 080903CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    As the challenges in continued scaling of the integrated circuit technol. escalate every generation, there is an urgent need to find viable solns. for both the front-end-of-line (transistors) and the back-end-of-line (interconnects). For the interconnect technol., it is crucial to replace the conventional barrier and liner with much thinner alternatives so that the current driving capability of the interconnects can be maintained or even improved. Due to the inherent atomically thin body thicknesses, 2D materials have recently been proposed and explored as Cu diffusion barrier alternatives. In this Perspective article, a variety of 2D materials that have been studied, ranging from graphene, h-BN, MoS2, WSe2 to TaS2, will be reviewed. Their potentials will be evaluated based on several criteria, including fundamental material properties as well as the feasibility for technol. integration. Using TaS2 as an example, we demonstrate a large set of promising properties and point out that there remain challenges in the integration aspects with a few possible solns. waiting for validation. Applications of 2D materials for other functions in Cu interconnects and for different metal types will also be introduced, including electromigration, cobalt interconnects, and radio-frequency transmission lines. (c) 2020 American Institute of Physics.
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  5. 5
    Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. 2D materials and van der Waals heterostructures. Science 2016, 353, aac9439,  DOI: 10.1126/science.aac9439
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    Manzeli, S.; Ovchinnikov, D.; Pasquier, D.; Yazyev, O. V.; Kis, A. 2D transition metal dichalcogenides. Nat. Rev. Mater. 2017, 2, 17033,  DOI: 10.1038/natrevmats.2017.33
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    2D transition metal dichalcogenides
    Manzeli, Sajedeh; Ovchinnikov, Dmitry; Pasquier, Diego; Yazyev, Oleg V.; Kis, Andras
    Nature Reviews Materials (2017), 2 (2), 17033CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
    A review. Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of at.-scale thickness, direct bandgap, strong spin-orbit coupling and favorable electronic and mech. properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examd. and their properties are discussed, with particular attention to their charge d. wave, superconductive and topol. phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties.
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  7. 7
    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, 10451– 10453,  DOI: 10.1073/pnas.0502848102
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    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.
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  8. 8
    Kaushik, N.; Nipane, A.; Basheer, F.; Dubey, S.; Grover, S.; Deshmukh, M. M.; Lodha, S. Schottky barrier heights for Au and Pd contacts to MoS2. Appl. Phys. Lett. 2014, 105, 113505,  DOI: 10.1063/1.4895767
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    Schottky barrier heights for Au and Pd contacts to MoS2
    Kaushik, Naveen; Nipane, Ankur; Basheer, Firdous; Dubey, Sudipta; Grover, Sameer; Deshmukh, Mandar M.; Lodha, Saurabh
    Applied Physics Letters (2014), 105 (11), 113505/1-113505/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The search of a p-type metal contact on MoS2 has remained inconclusive, with high work function metals such as Au, Ni, and Pt showing n-type behavior and mixed reports of n as well as p-type behavior for Pd. In this work, we report quant. Schottky barrier heights for Au and Pd contacts to MoS2 obtained by analyzing low temp. transistor characteristics and contact resistance data obtained using the transfer length method. Both Au and Pd exhibit n-type behavior on multilayer as well as monolayer MoS2 transistors with Schottky barrier heights of 0.126 eV and 0.4 eV, and contact resistances of 42 Ω.mm and 18 × 104 Ω.mm, resp. Scanning photocurrent spectroscopy data is in agreement with the resulting energy band alignment in Au-MoS2-Pd devices further reinforcing the observation that the Fermi-level is pinned in the upper half of MoS2 bandgap. (c) 2014 American Institute of Physics.
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  9. 9
    Liu, Y.; Wu, H.; Cheng, H.-C.; Yang, S.; Zhu, E.; He, Q.; Ding, M.; Li, D.; Guo, J.; Weiss, N. O.; Huang, Y.; Duan, X. Toward barrier free contact to molybdenum disulfide using graphene electrodes. Nano Lett. 2015, 15, 3030– 3034,  DOI: 10.1021/nl504957p
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    Toward Barrier Free Contact to Molybdenum Disulfide Using Graphene Electrodes
    Liu, Yuan; Wu, Hao; Cheng, Hung-Chieh; Yang, Sen; Zhu, Enbo; He, Qiyuan; Ding, Mengning; Li, Dehui; Guo, Jian; Weiss, Nathan O.; Huang, Yu; Duan, Xiangfeng
    Nano Letters (2015), 15 (5), 3030-3034CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) have attracted tremendous interest as a new class of electronic materials. However, there are considerable challenges in making reliable contacts to these atomically thin materials. Here the authors present a new strategy by using graphene as the back electrodes to achieve ohmic contact to MoS2. With a finite d. of states, the Fermi level of graphene can be readily tuned by a gate potential to enable a nearly perfect band alignment with MoS2. The authors demonstrate for the 1st time a transparent contact to MoS2 with zero contact barrier and linear output behavior at cryogenic temps. (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, a metal-insulator transition can be obsd. in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barriers found in conventional metal-contacted MoS2 devices. With further passivation by boron nitride (BN) encapsulation, the authors demonstrate a record-high extrinsic (two-terminal) field effect mobility up to 1300 cm2/(V s) in MoS2 at low temp.
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  10. 10
    Shi, Y.; Zhou, W.; Lu, A.-Y.; Fang, W.; Lee, Y.-H.; Hsu, A. L.; Kim, S. M.; Kim, K. K.; Yang, H. Y.; Li, L.-J.; Idrobo, J.-C.; Kong, J. van der Waals Epitaxy of MoS2 Layers Using Graphene As Growth Templates. Nano Lett. 2012, 12, 2784– 2791,  DOI: 10.1021/nl204562j
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    van der Waals Epitaxy of MoS2 Layers Using Graphene As Growth Templates
    Shi, Yumeng; Zhou, Wu; Lu, Ang-Yu; Fang, Wenjing; Lee, Yi-Hsien; Hsu, Allen Long; Kim, Soo Min; Kim, Ki Kang; Yang, Hui Ying; Li, Lain-Jong; Idrobo, Juan-Carlos; Kong, Jing
    Nano Letters (2012), 12 (6), 2784-2791CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors present a method for synthesizing MoS2/Graphene hybrid heterostructures with a growth template of graphene-covered Cu foil. Compared to other recent reports, a much lower growth temp. of 400 °C is required for this procedure. The chem. vapor deposition of MoS2 on the graphene surface gives rise to single cryst. hexagonal flakes with a typical lateral size ranging from several hundred nanometers to several micrometers. The precursor (ammonium thiomolybdate) together with solvent was transported to graphene surface by a carrier gas at room temp., which was then followed by post annealing. At an elevated temp., the precursor self-assembles to form MoS2 flakes epitaxially on the graphene surface via thermal decompn. With higher amt. of precursor delivered onto the graphene surface, a continuous MoS2 film on graphene can be obtained. This simple chem. vapor deposition method provides a unique approach for the synthesis of graphene heterostructures and surface functionalization of graphene. The synthesized two-dimensional MoS2/Graphene hybrids possess great potential toward the development of new optical and electronic devices as well as a wide variety of newly synthesizable compds. for catalysts.
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    Wang, H. W.; Skeldon, P.; Thompson, G. E.; Wood, G. C. Synthesis and characterization of molybdenum disulphide formed from ammonium tetrathiomolybdate. J. Mater. Sci. 1997, 32, 497– 502,  DOI: 10.1023/a:1018538424373
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    Synthesis and characterization of molybdenum disulfide formed from ammonium tetrathiomolybdate
    Wang, H. W.; Skeldon, P.; Thompson, G. E.; Wood, G. C.
    Journal of Materials Science (1997), 32 (2), 497-502CODEN: JMTSAS; ISSN:0022-2461. (Chapman & Hall)
    An investigation was carried out on the possibility of in-situ formation of MoS2 within porous anodic films on aluminum, to improve subsequent tribol. behavior, by re-anodization in thiomolybdate electrolyte. Acidification of thiomolybdate was used to simulate the conditions for formation of the sulfide at the anodic film-electrolyte interface, followed by appropriate vacuum heat treatments to study possible temp. effects on the sulfide due to either friction or Joule heating during anodizing. The products of both acidification and heat treatment, characterized by x-ray powder diffraction and SEM, were compared with those formed by direct thermal decompn. of ammonium tetrathiomolybdate crystals. The ppt. formed by acidification was mainly amorphous molybdenum trisulfide (MoS3), which on heat treatment at 450° and 850° yielded 3R-MoS2 (identified by x-ray diffraction). 3R-MoS2 also formed by the thermal decompn. of thiomolybdate crystals. Thermogravimetric and differential thermal analyses showed that the decompn. of MoS3 to MoS2 occurred at 220-370° and revealed the sequence of reaction steps. Mainly amorphous MoS3 is formed as a consequence of pH changes in the film-electrolyte interface during re-anodizing but the product is relatively easily transformed to cryst. MoS2 upon moderate heating which may occur during wear processes.
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  12. 12
    Sygellou, L. An in-situ photoelectron spectroscopy study of the thermal processing of ammonium tetrathiomolybdate, (NH4)2MoS4, precursor. Appl. Surf. Sci. 2019, 476, 1079– 1085,  DOI: 10.1016/j.apsusc.2019.01.193
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    12
    An in-situ photoelectron spectroscopy study of the thermal processing of ammonium tetrathiomolybdate, (NH4)2MoS4, precursor
    Sygellou, Labrini
    Applied Surface Science (2019), 476 (), 1079-1085CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Nano scale molybdenum disulfide (MoS2) is a very promising material for the next generation electronics and energy storage devices. The active defect sites located at the periphery of the lattice may influence certain redox reactions, thus classifying it as an excellent non-noble metal catalyst. One effective approach to synthesize large scale MoS2 films is through simple thermolysis of (NH4)2MoS4 precursor. Herein, a combined in situ X-ray and Ultra-Violet photoelectron spectroscopies (XPS/UPS) study has been carried out to follow the evolution of at. compn., the surface chem. species and the electronic properties (Work Function and Ionization Potential) during the thermal decompn. of (NH4)2MoS4 salt in Ultra High Vacuum conditions. It was found that, up to 400C, thermal conversion of (NH4)2MoS4 salt to transient intermediates, across amorphous MoSx phase have been performed towards to MoS2 out of significant percent of sulfur active sites. The active sites have been decreased upon heating at higher temps. Assignment of the thermally evolved active species at defect sites and the correlation of surface at. concn. combined with either the Ionization Potential or the Work Function of annealed MoSx nanoflakes makes this study a knowledgeable factor of awareness through electrocatalytic properties of MoS2-based nanostructures. Acknowledgement: We acknowledge support of this work by the project "National Infrastructure in Nanotechnol., Advanced Materials and Micro-/ Nanoelectronics" (MIS 5002772) which is implemented under the Action "Reinforcement of the Research and Innovation Infrastructure", funded by the Operational Program "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).
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  13. 13
    Yang, J.; Gu, Y.; Lee, E.; Lee, H.; Park, S. H.; Cho, M.-H.; Kim, Y. H.; Kim, Y.-H.; Kim, H. Wafer-scale synthesis of thickness-controllable MoS2 films via solution-processing using a dimethylformamide/n-butylamine/2-aminoethanol solvent system. Nanoscale 2015, 7, 9311– 9319,  DOI: 10.1039/c5nr01486g
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    13
    Wafer-scale synthesis of thickness-controllable MoS2 films via solution-processing using a dimethylformamide/n-butylamine/2-aminoethanol solvent system
    Yang, Jaehyun; Gu, Yeahyun; Lee, Eunha; Lee, Hyangsook; Park, Sang Han; Cho, Mann-Ho; Kim, Yong Ho; Kim, Yong-Hoon; Kim, Hyoungsub
    Nanoscale (2015), 7 (20), 9311-9319CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    The wafer-scale synthesis of two-dimensional molybdenum disulfide (MoS2) films, with high layer-controllability and uniformity, remains a significant challenge in the fields of nano and optoelectronics. Here, we report the highly thickness controllable growth of uniform MoS2 thin films on the wafer-scale via a spin-coating route. Formulation of a dimethylformamide-based MoS2 precursor soln. mixed with addnl. amine- and amino alc.-based solvents (n-butylamine and 2-aminoethanol) allowed for the formation of a uniform coating of MoS2 thin films over a 2 in. wafer-scale SiO2/Si substrate. In addn., facile control of the av. no. of stacking layers is demonstrated by simply manipulating the concn. of the precursor soln. Various characterization results reveal that the synthesized MoS2 film has wafer-scale homogeneity with excellent cryst. quality and a stoichiometric chem. compn. To further demonstrate possible device applications, a mostly penta-layered MoS2 thin film was integrated into a top-gated field-effect transistor as the channel layer and we also successfully transferred our films onto transparent/flexible substrates.
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    Yang, H.; Giri, A.; Moon, S.; Shin, S.; Myoung, J.-M.; Jeong, U. Highly scalable synthesis of MoS2 thin films with precise thickness control via polymer-assisted deposition. Chem. Mater. 2017, 29, 5772– 5776,  DOI: 10.1021/acs.chemmater.7b01605
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    14
    Highly Scalable Synthesis of MoS2 Thin Films with Precise Thickness Control via Polymer-Assisted Deposition
    Yang, Heeseung; Giri, Anupam; Moon, Sungmin; Shin, Sangbae; Myoung, Jae-Min; Jeong, Unyong
    Chemistry of Materials (2017), 29 (14), 5772-5776CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The versatile applications of MoS2 thin films demand large-scale synthesis with high uniformity in thickness and compn. Among several synthetic approaches, the two-step process including soln.-based precursor coating followed by thermolysis is advantageous in terms of cost and scalability. In this study, we developed a polymer-assisted deposition process to prep. a highly scalable polymer-precursor complex thin film and apply thermolysis at a relatively low temp. without sulfurization. This process enables fine control of the precursor film, which leads to precise thickness control of the resulting MoS2 film (≥2 nm). In addn., we demonstrate the fabrication of a MoS2-based photodetector with a broad spectral response and excellent performance, as indicated by its fast response time (≤1.0 ms) and high on-off ratio (104).
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    Zhu, Y.; Yuan, D.; Zhang, H.; Xu, T.; Sun, L. Atomic-scale insights into the formation of 2D crystals from in situ transmission electron microscopy. Nano Res. 2021, 14, 1650– 1658,  DOI: 10.1007/s12274-020-3034-z
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    Mishra, R.; Ishikawa, R.; Lupini, A. R.; Pennycook, S. J. Single-atom dynamics in scanning transmission electron microscopy. MRS Bull. 2017, 42, 644– 652,  DOI: 10.1557/mrs.2017.187
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    Single-atom dynamics in scanning transmission electron microscopy
    Mishra, Rohan; Ishikawa, Ryo; Lupini, Andrew R.; Pennycook, Stephen J.
    MRS Bulletin (2017), 42 (9), 644-652CODEN: MRSBEA; ISSN:0883-7694. (Cambridge University Press)
    The correction of aberrations in the scanning transmission electron microscope (STEM) has simultaneously improved both spatial and temporal resoln., making it possible to capture the dynamics of single atoms inside materials, and resulting in new insights into the dynamic behavior of materials. In this article, we describe the different beam-matter interactions that lead to at. excitations by transferring energy and momentum. We review recent examples of sequential STEM imaging to demonstrate the dynamic behavior of single atoms both within materials, at dislocations, at grain and interface boundaries, and on surfaces. We also discuss the effects of such dynamic behavior on material properties. We end with a summary of ongoing instrumental and algorithm developments that we anticipate will improve the temporal resoln. significantly, allowing unprecedented insights into the dynamic behavior of materials at the at. scale.
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  17. 17
    Su, C.; Tripathi, M.; Yan, Q.-B.; Wang, Z.; Zhang, Z.; Hofer, C.; Wang, H.; Basile, L.; Su, G.; Dong, M.; Meyer, J. C.; Kotakoski, J.; Kong, J.; Idrobo, J.-C.; Susi, T.; Li, J. Engineering single-atom dynamics with electron irradiation. Sci. Adv. 2019, 5, eaav2252  DOI: 10.1126/sciadv.aav2252
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    There is no corresponding record for this reference.
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    Fei, L.; Lei, S.; Zhang, W.-B.; Lu, W.; Lin, Z.; Lam, C. H.; Chai, Y.; Wang, Y. Direct TEM observations of growth mechanisms of two-dimensional MoS2 flakes. Nat. Commun. 2016, 7, 12206,  DOI: 10.1038/ncomms12206
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    Direct TEM observations of growth mechanisms of two-dimensional MoS2 flakes
    Fei, Linfeng; Lei, Shuijin; Zhang, Wei-Bing; Lu, Wei; Lin, Ziyuan; Lam, Chi Hang; Chai, Yang; Wang, Yu
    Nature Communications (2016), 7 (), 12206CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    A microscopic understanding of the growth mechanism of two-dimensional materials is of particular importance for controllable synthesis of functional nanostructures. Because of the lack of direct and insightful observations, how to control the orientation and the size of two-dimensional material grains is still under debate. Here we discern distinct formation stages for MoS2 flakes from the thermolysis of ammonium thiomolybdates using in situ transmission electron microscopy. In the initial stage (400 °C), vertically aligned MoS2 structures grow in a layer-by-layer mode. With the increasing temp. of up to 780 °C, the orientation of MoS2 structures becomes horizontal. When the growth temp. reaches 850 °C, the cryst. size of MoS2 increases by merging adjacent flakes. Our study shows direct observations of MoS2 growth as the temp. evolves, and sheds light on the controllable orientation and grain size of two-dimensional materials.
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  19. 19
    Sang, X.; Li, X.; Puretzky, A. A.; Geohegan, D. B.; Xiao, K.; Unocic, R. R. Atomic Insight into Thermolysis-Driven Growth of 2D MoS2. Adv. Funct. Mater. 2019, 29, 1902149,  DOI: 10.1002/adfm.201902149
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    Atomic Insight into Thermolysis-Driven Growth of 2D MoS2
    Sang, Xiahan; Li, Xufan; Puretzky, Alexander A.; Geohegan, David B.; Xiao, Kai; Unocic, Raymond R.
    Advanced Functional Materials (2019), 29 (52), 1902149CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Understanding and controlling the transformations of transition metal dichalcogenides (TMDs) from amorphous precursors into 2-dimensional (2D) materials is important for guiding synthesis, directing fabrication, and tailoring functional properties. Here, the combined effects of thermal energy and electron beam irradn. are explored on the structural evolution of 2-dimensional MoS2 flakes through the thermal decompn. of a (NH4)2MoS4 precursor inside an ultrahigh vacuum (10-9 Torr) scanning transmission electron microscope (STEM). The influence of reaction temp., growth substrate, and the initial precursor morphol. on the resulting 2-dimensional MoS2 flake morphol., edge structures, and point defects are explored. Although thermal decompn. occurs extremely fast at elevated temps. and is difficult to capture using current STEM techniques, electron beam irradn. can induce local transformations at lower temps., enabling direct observation and interpretation of crit. growth steps including oriented attachment and transition from single- to multilayer structures at at. resoln. An increase in the no. of layers of the MoS2 flakes from island growth is studied using electron beam irradn. These findings provide insight into the growth mechanisms and factors that control the synthesis of few-layer MoS2 flakes through thermolysis and toward the prospect of atomically precise control and growth of 2-dimensional TMDs.
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  20. 20
    Kondekar, N.; Boebinger, M. G.; Tian, M.; Kirmani, M. H.; McDowell, M. T. The Effect of Nickel on MoS2 Growth Revealed with in Situ Transmission Electron Microscopy. ACS Nano 2019, 13, 7117– 7126,  DOI: 10.1021/acsnano.9b02528
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    The Effect of Nickel on MoS2 Growth Revealed with in Situ Transmission Electron Microscopy
    Kondekar, Neha; Boebinger, Matthew G.; Tian, Mengkun; Kirmani, Mohammad Hamza; McDowell, Matthew T.
    ACS Nano (2019), 13 (6), 7117-7126CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    MoS2 has important applications in (electro)catalysis and as a semiconductor for electronic devices. Other chem. species are commonly added to MoS2 to increase catalytic activity or to alter electronic properties through substitutional or adsorption-based doping. While ground breaking work has been devoted to detg. the at.-scale structure of MoS2 and other layered transition-metal dichalcogenides (TMDCs), there is a lack of understanding of the dynamic processes that govern the evolution of these materials during synthesis. Here, in situ transmission electron microscopy (TEM) heating, in combination with larger length scale ex situ expts., is used to investigate the effects of added Ni on the growth of MoS2 during the thermolysis of the solid-state (NH4)2MoS4 precursor. Low concns. of Ni are obsd. to cause significant differences in the MoS2 crystn. and growth process, leading to an increase in MoS2 crystal size. This is likely a result of the altered mobility of interfaces between crystals during growth. These findings demonstrate the important role of addnl. elements in controlling the evolution of TMDCs during synthesis, which should be considered when designing these materials for a variety of applications.
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    Morgan, A. J.; Martin, A. V.; D’Alfonso, A. J.; Putkunz, C. T.; Allen, L. J. Direct exit-wave reconstruction from a single defocused image. Ultramicroscopy 2011, 111, 1455– 1460,  DOI: 10.1016/j.ultramic.2011.07.005
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    Direct exit-wave reconstruction from a single defocused image
    Morgan, A. J.; Martin, A. V.; D'Alfonso, A. J.; Putkunz, C. T.; Allen, L. J.
    Ultramicroscopy (2011), 111 (9-10), 1455-1460CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)
    We propose a direct, non-iterative method for the exact recovery of the complex wave in the exit-surface plane of a coherently illuminated object from a single defocused image. The method is applicable for a wide range of illumination conditions. The defocus range is subject to certain conditions, which if satisfied allow the complex exit-surface wave to be directly recovered by solving a set of linear equations. These linear equations, whose coeffs. depend on the incident illumination, are obtained by analyzing the autocorrelation function of an auxiliary wave which is related to the exit-surface wave in a simple way. This autocorrelation is constructed by taking the inverse Fourier transform of the defocused image. We present an exptl. proof of concept by recovering the exit-surface wave of a microfiber illuminated by a plane wave formed using a HeNe laser.
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    Kirkland, E. J. Improved high resolution image processing of bright field electron micrographs: I. Theory. Ultramicroscopy 1984, 15, 151– 172,  DOI: 10.1016/0304-3991(84)90037-8
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    Accurate structure determination from image reconstruction in ADF STEM
    Nellist, P. D.; Pennycook, S. J.
    Journal of Microscopy (Oxford) (1998), 190 (1/2), 159-170CODEN: JMICAR; ISSN:0022-2720. (Blackwell Science Ltd.)
    Annular dark-field (ADF) imaging in a scanning transmission electron microscope results in direct structure images of the at. configuration of the specimen. Since such images are almost perfectly incoherent they can be treated as a convolution between a point-spread function, which is simply the intensity of the illuminating electron probe, and a sharply peaked object function that represents the projected structure of the specimen. Knowledge of the object function for an image region of perfect crystal allows the point-spread function to be directly detd. for that image. We examine how the object function for an image can then be reconstructed using a Wiener filter, the CLEAN algorithm and a max. entropy reconstruction. Prior information is required to perform a reconstruction, and we discuss what nature of prior information is suitable for ADF imaging.
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    Nellist, P. D.; Pennycook, S. J. Probe and object function reconstruction in incoherent scanning transmission electron microscope imaging. Scanning Microsc. 1997, 11, 81– 90
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    Madsen, J.; Liu, P.; Kling, J.; Wagner, J. B.; Hansen, T. W.; Winther, O.; Schiøtz, J. A Deep Learning Approach to Identify Local Structures in Atomic-Resolution Transmission Electron Microscopy Images. Adv. Theory Simul. 2018, 1, 1800037,  DOI: 10.1002/adts.201800037
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    Lee, J.; Lee, Y.; Kim, J.; Lee, Z. Contrast Transfer Function-Based Exit-Wave Reconstruction and Denoising of Atomic-Resolution Transmission Electron Microscopy Images of Graphene and Cu Single Atom Substitutions by Deep Learning Framework. Nanomaterials 2020, 10, 1977,  DOI: 10.3390/nano10101977
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    Contrast transfer function-based exit-wave reconstruction and denoising of atomic-resolution transmission electron microscopy images of graphene and Cu single atom substitutions by deep learning framework
    Lee, Jongyeong; Lee, Yeongdong; Kim, Jaemin; Lee, Zonghoon
    Nanomaterials (2020), 10 (10), 1977CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
    The exit wave is the state of a uniform plane incident electron wave exiting immediately after passing through a specimen and before the at.-resoln. transmission electron microscopy (ARTEM) image is modified by the aberration of the optical system and the incoherence effect of the electron. Although exit-wave reconstruction has been developed to prevent the misinterpretation of ARTEM images, there have been limitations in the use of conventional exit-wave reconstruction in ARTEM studies of the structure and dynamics of two-dimensional materials. In this study, we propose a framework that consists of the convolutional dual-decoder autoencoder to reconstruct the exit wave and denoise ARTEM images. We calcd. the contrast transfer function (CTF) for real ARTEM and assigned the output of each decoder to the CTF as the amplitude and phase of the exit wave. We present exit-wave reconstruction expts. with ARTEM images of monolayer graphene and compare the findings with those of a simulated exit wave. Cu single atom substitution in monolayer graphene was, for the first time, directly identified through exit-wave reconstruction expts. Our exit-wave reconstruction expts. show that the performance of the denoising task is improved when compared to the Wiener filter in terms of the signal-to-noise ratio, peak signal-to-noise ratio, and structural similarity index map metrics.
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    Nellist, P. D.; Pennycook, S. J. The principles and interpretation of annular dark-field Z-contrast imaging. Adv. Imaging Electron Phys. 2000, 113, 147– 203,  DOI: 10.1016/s1076-5670(00)80013-0
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    Nonlinear principal component analysis using autoassociative neural networks
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    AIChE Journal (1991), 37 (2), 233-43CODEN: AICEAC; ISSN:0001-1541.
    Nonlinear-principal-component anal. (NLPCA), is a novel technique for multivariate data anal., similar to the method of principal-component anal. (PCA). NLPCA like PCA, is used to identify and remove correlations among problem variables as an aid to dimensionality redn., visualization, and exploratory data anal. While PCA identifies only linear correlations between variables, NLPCA uncovers both linear and nonlinear correlations, without restriction on the character of the nonlinearities present in the data. NLPCA operates by training a feedforward neural network to perform the identity mapping, where the network inputs are reproduced at the output layer. The network contains an internal bottleneck layer (contg. fewer nodes than input or output layers), which forces the network to develop a compact representation of the input data and 2 addnl. hidden layers. The NLPCA method is demonstrated by using time-dependent, simulated batch-reaction data. NLPCA can reduce dimensionality and produce a feature space map resembling the actual distribution of the underlying system parameters.
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    Zemlin, F.; Weiss, K.; Schiske, P.; Kunath, W.; Herrmann, K.-H. Coma-free alignment of high resolution electron microscopes with the aid of optical diffractograms. Ultramicroscopy 1978, 3, 49– 60,  DOI: 10.1016/s0304-3991(78)80006-0
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    The rise of graphene
    Geim, A. K.; Novoselov, K. S.
    Nature Materials (2007), 6 (3), 183-191CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    A review. Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when com. products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top expts. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.
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    Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312– 1314,  DOI: 10.1126/science.1171245
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    Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils
    Li, Xuesong; Cai, Weiwei; An, Jinho; Kim, Seyoung; Nah, Junghyo; Yang, Dongxing; Piner, Richard; Velamakanni, Aruna; Jung, Inhwa; Tutuc, Emanuel; Banerjee, Sanjay K.; Colombo, Luigi; Ruoff, Rodney S.
    Science (Washington, DC, United States) (2009), 324 (5932), 1312-1314CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Graphene was attracting great interest because of its distinctive band structure and phys. properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. The authors grew large-area graphene films of the order of centimeters on Cu substrates by CVD using methane. The films are predominantly single-layer graphene, with a small percentage (<5%) of the area having few layers, and are continuous across Cu surface steps and grain boundaries. The low soly. of C in Cu appears to help make this growth process self-limiting. The authors also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities ≤4050 cm2/V-s at room temp.
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    van der Zande, A. M.; Huang, P. Y.; Chenet, D. A.; Berkelbach, T. C.; You, Y.; Lee, G.-H.; Heinz, T. F.; Reichman, D. R.; Muller, D. A.; Hone, J. C. Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide. Nat. Mater. 2013, 12, 554– 561,  DOI: 10.1038/nmat3633
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    Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide
    van der Zande, Arend M.; Huang, Pinshane Y.; Chenet, Daniel A.; Berkelbach, Timothy C.; You, YuMeng; Lee, Gwan-Hyoung; Heinz, Tony F.; Reichman, David R.; Muller, David A.; Hone, James C.
    Nature Materials (2013), 12 (6), 554-561CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Recent progress in large-area synthesis of monolayer MoS2, a new 2D direct-band gap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chem. vapor deposition synthesis to grow highly cryst. islands of monolayer MoS2 up to 120 μm in size with optical and elec. properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphol. and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror twin boundaries that are stitched together by lines of 8- and 4-membered rings. D. functional theory reveals localized mid-gap states arising from these 8-4 defects. Mirror twin boundaries cause strong photoluminescence quenching whereas tilt boundaries cause strong enhancement. Meanwhile, mirror twin boundaries slightly increase the measured in-plane elec. cond., whereas tilt boundaries slightly decrease the cond.
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    Xu, W.; Li, S.; Zhou, S.; Lee, J. K.; Wang, S.; Sarwat, S. G.; Wang, X.; Bhaskaran, H.; Pasta, M.; Warner, J. H. Large dendritic monolayer MoS2 grown by atmospheric pressure chemical vapor deposition for electrocatalysis. ACS Appl. Mater. Interfaces 2018, 10, 4630– 4639,  DOI: 10.1021/acsami.7b14861
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    Large Dendritic Monolayer MoS2 Grown by Atmospheric Pressure Chemical Vapor Deposition for Electrocatalysis
    Xu, Wenshuo; Li, Sha; Zhou, Si; Lee, Ja Kyung; Wang, Shanshan; Sarwat, Syed Ghazi; Wang, Xiaochen; Bhaskaran, Harish; Pasta, Mauro; Warner, Jamie H.
    ACS Applied Materials & Interfaces (2018), 10 (5), 4630-4639CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The edge sites of MoS2 are catalytically active for the H evolution reaction (HER), and growing monolayer structures that are edge-rich is desirable. Here, the authors show the prodn. of large-area highly branched MoS2 dendrites on amorphous SiO2/Si substrates using an atm. pressure CVD and explore their use in electrocatalysis. By tailoring the substrate construction, the monolayer MoS2 evolves from triangular to dendritic morphol. because of the change of growth conditions. The rough edges endow dendritic MoS2 with a fractal dimension down to 1.54. The highly cryst. basal plane and the edge of the dendrites are visualized at at. resoln. using an annular dark field scanning transmission electron microscope. The monolayer dendrites exhibit strong photoluminescence, which is indicative of the direct band gap emission, which is preserved after being transferred. Post-transfer S annealing restores the structural defects and decreases the n-type doping in MoS2 monolayers. The annealed MoS2 dendrites show good and highly durable HER performance on the glassy C with a large exchange c.d. of 32 μA cmgeo-2, demonstrating its viability as an efficient HER catalyst.
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    Schweiger, H.; Raybaud, P.; Kresse, G.; Toulhoat, H. Shape and edge sites modifications of MoS2 catalytic nanoparticles induced by working conditions: A theoretical study. J. Catal. 2002, 207, 76– 87,  DOI: 10.1006/jcat.2002.3508
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    Shape and Edge Sites Modifications of MoS2 Catalytic Nanoparticles Induced by Working Conditions: A Theoretical Study
    Schweiger, Hannes; Raybaud, Pascal; Kresse, Georg; Toulhoat, Herve
    Journal of Catalysis (2002), 207 (1), 76-87CODEN: JCTLA5; ISSN:0021-9517. (Academic Press)
    Detn. of the morphol. and electronic properties of single-layer nanosize MoS2 particles is of considerable interest for a better understanding of the active phase of hydrotreating catalysts. We propose an original approach, based on d. functional calcns. applied to various types of MoS2 clusters contg. up to 200 atoms, to evaluate accurately the surface energies of the Mo-edge and S-edge terminated surfaces. The results are expressed as a function of the chem. potential of sulfur, which is in turn controlled by the temp. and the ratio of partial pressures of H2S and H2. Gibbs-Curie-Wulff equil. morphologies reveal that a high chem. potential of sulfur leads to triangular-shaped particles terminated by the Mo-edge surface, giving an interpretation of the observations made recently by scanning tunneling microscopy (STM). Simulations of the STM images for the most stable clusters reveal a qual. good agreement with expts. Moreover, our approach predicts that varying the potential of sulfur modifies the local Mo-edge structure and the shape of the nanoparticles. Finally, the thermodn. diagram for a MoS2 particle with a realistic size shows that the creation of one sulfur vacancy at the corner and on the edge is possible under HDS working conditions. (c) 2002 Academic Press.
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    Zhu, D.; Shu, H.; Jiang, F.; Lv, D.; Asokan, V.; Omar, O.; Yuan, J.; Zhang, Z.; Jin, C. Capture the growth kinetics of CVD growth of two-dimensional MoS2. npj 2D Mater. Appl. 2017, 1, 8,  DOI: 10.1038/s41699-017-0010-x
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    Pan, H.; Zhang, Y.-W. Edge-dependent structural, electronic and magnetic properties of MoS2 nanoribbons. J. Mater. Chem. 2012, 22, 7280– 7290,  DOI: 10.1039/c2jm15906f
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    Edge-dependent structural, electronic and magnetic properties of MoS2 nanoribbons
    Pan, Hui; Zhang, Yong-Wei
    Journal of Materials Chemistry (2012), 22 (15), 7280-7290CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    Two-dimensional materials have various applications in next-generation nanodevices because of their easy fabrication and particular properties. In this work, we studied the effects of edge structures on the edge stability, and electronic and magnetic properties of MoS2 nanoribbons by first-principles calcns. We predicted that S-terminated zigzag nanoribbons are the most stable even without hydrogen satn. because of their low and neg. edge energies, although hydrogen satn. of the edge states can stabilize other nanoribbons with different edge structures. MoS2 zigzag nanoribbons are metallic and ferromagnetic. Importantly, their cond. may be semiconducting (n- or p-type) or half metallic by controlling the edge structures satd. with H atoms. The magnetic states of the MoS2 zigzag nanoribbons are enhanced by H-satn. and are much stronger than those of graphene zigzag nanoribbons. The armchair nanoribbons are semiconducting, with bandgaps increased by the hydrogen satn. of their edge states, and are nonmagnetic. These MoS2 nanoribbons with versatile functions may have applications in spintronics, nanodevices, and energy harvesting.
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    Wang, L.; Zhou, X.; Ma, T.; Liu, D.; Gao, L.; Li, X.; Zhang, J.; Hu, Y.; Wang, H.; Dai, Y.; Luo, J. Superlubricity of a graphene/MoS2 heterostructure: a combined experimental and DFT study. Nanoscale 2017, 9, 10846– 10853,  DOI: 10.1039/c7nr01451a
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    Superlubricity of a graphene/MoS2 heterostructure: a combined experimental and DFT study
    Wang, Linfeng; Zhou, Xiang; Ma, Tianbao; Liu, Dameng; Gao, Lei; Li, Xin; Zhang, Jun; Hu, Yuanzhong; Wang, Hui; Dai, Yadong; Luo, Jianbin
    Nanoscale (2017), 9 (30), 10846-10853CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Graphene and other two-dimensional materials have been proved to be able to offer low friction. Here we assembled van der Waals heterostructures with graphene and molybdenum disulfide monolayers. The Raman spectrum together with a modified linear chain model indicate a two-orders-of-magnitude decrease in the interlayer lateral force const., as compared with their homogeneous bilayers, indicating a possible routine to achieve superlubricity. The decrease in the interlayer lateral force const. is consistent with the ultrasmall potential energy corrugation during sliding, which is derived from d. functional theory calcns. The potential energy corrugation is found to be detd. by the sliding-induced interfacial charge d. fluctuation, suggesting a new perspective to understand the phys. origin of the at. scale friction of two-dimensional materials.
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    Batzill, M. Mirror twin grain boundaries in molybdenum dichalcogenides. J. Phys.: Condens. Matter 2018, 30, 493001,  DOI: 10.1088/1361-648x/aae9cf
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    Mirror twin grain boundaries in molybdenum dichalcogenides
    Batzill, Matthias
    Journal of Physics: Condensed Matter (2018), 30 (49), 493001/1-493001/15CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
    A review. Mirror twin grain boundaries (MTBs) exist at the interface between 2 grains of 60° rotated hexagonal transition metal dichalcogenides (TMDC). These grain boundaries form a regular at. structure that extends in one dimension and thus may be described as a one-dimensional (1D) lattice embedded in the 2-dimensional TMDC. In this review, the different at. structures and compns. of these MTBs are discussed. The obvious formation of MTBs is by coalescence of 2 twinned grains. However, in MoSe2 and MoTe2 a different formation mechanism was revealed for the formation of Mo-rich MTBs. Excess Mo can be incorporated into the TMDC lattices. These excess Mo atoms can then reorganize into closed, triangular MTB-loops that can grow in size by adding more Mo atoms to them. This mechanism gave dense MTB networks in MoSe2 and MoTe2. Such MTB networks were obsd. in samples grown by MBE and consequently their presence needs to be considered in understanding the properties of MBE grown MoSe2 and MoTe2. D. functional theory as well as photoemission spectroscopy of MTB networks showed that MTBs exhibit dispersing 1-dimensional-bands that intersect the Fermi-level, thus suggesting that these are 1-dimensional electron systems. Consequently, exptl. data were interpreted to reveal a charge d. wave (or Peierls) instability, as well as a Tomonaga-Luttinger liq. behavior for electrons confined in 1D. These observations and the controversies that remain in the interpretation of some data are discussed. The metallic properties of the MTBs and their formation in dense networks also sparked the potential use of such crystal modifications for making metallic contacts to MoTe2 or MoSe2. Also, these crystal modifications may also boost the catalytic properties of these materials.
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    Kim, J. H.; Kim, S. Y.; Park, S. O.; Jung, G. Y.; Song, S.; Sohn, A.; Kim, S. W.; Kwak, S. K.; Kwon, S. Y.; Lee, Z. Antiphase Boundaries as Faceted Metallic Wires in 2D Transition Metal Dichalcogenides. Adv. Sci. 2020, 7, 2000788,  DOI: 10.1002/advs.202000788
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    Antiphase Boundaries as Faceted Metallic Wires in 2D Transition Metal Dichalcogenides
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    Advanced Science (Weinheim, Germany) (2020), 7 (15), 2000788CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Antiphase boundaries (APBs) in 2D transition metal dichalcogenides have attracted wide interest as 1D metallic wires embedded in a semiconducting matrix, which could be exploited in fully 2D-integrated circuits. Here, the anisotropic morphologies of APBs (i.e., linear and saw-toothed APBs) in the nanoscale are investigated. The exptl. and computational results show that despite their anisotropic nanoscale morphologies, all APBs adopt a predominantly chalcogen-oriented dense structure to maintain the energetically most stable at. configuration. Moreover, the effect of the nanoscale morphol. of an APB on electron transport from two-probe field effect transistor measurements is investigated. A saw-toothed APB has a considerably lower electron mobility than a linear APB, indicating that kinks between facets are the main factors of scattering. The observations contribute to the systematical understanding of the faceted APBs and its impact on elec. transport behavior and it could potentially extend the applications of 2D materials through defect engineering to achieve the desired properties.
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    Zhou, S.; Wang, S.; Shi, Z.; Sawada, H.; Kirkland, A. I.; Li, J.; Warner, J. H. Atomically sharp interlayer stacking shifts at anti-phase grain boundaries in overlapping MoS2 secondary layers. Nanoscale 2018, 10, 16692– 16702,  DOI: 10.1039/c8nr04486d
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    Atomically sharp interlayer stacking shifts at anti-phase grain boundaries in overlapping MoS2 secondary layers
    Zhou, Si; Wang, Shanshan; Shi, Zhe; Sawada, Hidetaka; Kirkland, Angus I.; Li, Ju; Warner, Jamie H.
    Nanoscale (2018), 10 (35), 16692-16702CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    When secondary domains nucleate and grow on the surface of monolayer MoS2, they can extend across grain boundaries in the underlying monolayer MoS2 and form overlapping sections. We present an at. level study of overlapping antiphase grain boundaries (GBs) in MoS2 monolayer-bilayers using aberration-cor. annular dark field scanning transmission electron microscopy. In particular we focus on the antiphase GB within a monolayer and track its propagation through an overlapping bilayer domain. We show that this leads to an atomically sharp interface between 2H and 3R interlayer stacking in the bilayer region. We have studied the micro-nanoscale "meandering" of the antiphase GB in MoS2, which shows a directional dependence on the d. of 4 and 8 member ring defects, as well as sharp turning angles 90°-100° that are mediated by a special 8-member ring defect. D. functional theory has been used to explore the overlapping interlayer stacking around the antiphase GBs, confirming our exptl. findings. These results show that overlapping secondary bilayer MoS2 domains cause at. structure modification to underlying anti-phase GB sites to accommodate the van der Waals interactions.
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    Du, L.; Yu, H.; Liao, M.; Wang, S.; Xie, L.; Lu, X.; Zhu, J.; Li, N.; Shen, C.; Chen, P.; Yang, R.; Shi, D.; Zhang, G. Modulating PL and electronic structures of MoS2/graphene heterostructures via interlayer twisting angle. Appl. Phys. Lett. 2017, 111, 263106,  DOI: 10.1063/1.5011120
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    Applied Physics Letters (2017), 111 (26), 263106/1-263106/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Stacking two-dimensional materials into van der Waals heterostructures with distinct interlayer twisting angles opens up new strategies for electronic structure and phys. property engineering. Here, we investigate how the interlayer twisting angles affect the photoluminescence (PL) and Raman spectra of the MoS2/graphene heterostructures. Based on a series of heterostructure samples with different interlayer twisting angles, we found that the PL and Raman spectra of the monolayer MoS2 in these heterostructures are strongly twisting angle dependent. When the interlayer twisting angle evolves from 0° to 30°, both the PL intensity and emission energy increase, while the splitting of the E2g Raman mode decreases gradually. The obsd. phenomena are attributed to the twisting angle dependent interlayer interaction and misorientation-induced lattice strain between MoS2 and graphene. (c) 2017 American Institute of Physics.
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    Van der Waals heterostructures stacked from different two-dimensional materials offer a unique platform for addressing many fundamental physics and construction of advanced devices. Twist angle between the two individual layers plays a crucial role in tuning the heterostructure properties. Here we report the experimental investigation of the twist angle-dependent conductivities in MoS2/graphene van der Waals heterojunctions. We found that the vertical conductivity of the heterojunction can be tuned by ∼5 times under different twist configurations, and the highest/lowest conductivity occurs at a twist angle of 0°/30°. Density functional theory simulations suggest that this conductivity change originates from the transmission coefficient difference in the heterojunctions with different twist angles. Our work provides a guidance in using the MoS2/graphene heterojunction for electronics, especially on reducing the contact resistance in MoS2 devices as well as other TMDCs devices contacted by graphene.
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    Azizi, Amin; Eichfeld, Sarah; Geschwind, Gayle; Zhang, Kehao; Jiang, Bin; Mukherjee, Debangshu; Hossain, Lorraine; Piasecki, Aleksander F.; Kabius, Bernd; Robinson, Joshua A.; Alem, Nasim
    ACS Nano (2015), 9 (5), 4882-4890CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Vertical stacking of two-dimensional (2D) crystals has recently attracted substantial interest due to unique properties and potential applications they can introduce. However, little is known about their microstructure because fabrication of the 2-dimensional heterostructures on a rigid substrate limits one's ability to directly study their at. and chem. structures using electron microscopy. This study demonstrates a unique approach to create atomically thin freestanding van der Waals heterostructures-WSe2/graphene and MoS2/graphene-as ideal model systems to study the nucleation and growth mechanisms in heterostructures. The authors use TEM imaging and diffraction to show epitaxial growth of the freestanding WSe2/graphene heterostructure, while no epitaxy is maintained in the MoS2/graphene heterostructure. Ultra-high-resoln. aberration-cor. scanning TEM (STEM) shows growth of monolayer WSe2 and MoS2 triangles on graphene membranes and reveals their edge morphol. and crystallinity. Photoluminescence measurements indicate a significant quenching of the photoluminescence response for the transition metal dichalcogenides on freestanding graphene, compared to those on a rigid substrate, such as sapphire and epitaxial graphene. Using a combination of (S)TEM imaging and electron diffraction anal., this study also reveals the significant role of defects on the heterostructure growth. The direct growth technique applied here enables the authors to study the heterostructure nucleation and growth mechanisms at the at. level without sample handling and transfer. Importantly, this approach can be used to study a wide spectrum of van der Waals heterostructures.
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    All Chemical Vapor Deposition Synthesis and Intrinsic Bandgap Observation of MoS2/Graphene Heterostructures
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    By exploiting an all-CVD approach, the authors have successfully fabricated large area, thickness uniform, and high quality MoS2 /graphene vertical heterostructures with clean interfaces on conventional Au foil substrates. Spectroscopic characterizations reveal that the in-between graphene weakens the interface interaction between MoS2 and Au. This to some extend provide us a unique pathway for constructing a nearly freestanding MoS2 monolayer, which is evidenced by its very weak n-doping level and its intrinsic band feature according to STM/STS characterizations. Moreover, the MoS2/Graphene heterostructure is transferable onto arbitrary substrates, which will stimulate further in-depth investigations on the intrinsic electronic properties of monolayer MoS2 , and facilitate the development of ground breaking optoelectronic devices.
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    Miwa, J. A.; Dendzik, M.; Grønborg, S. S.; Bianchi, M.; Lauritsen, J. V.; Hofmann, P.; Ulstrup, S. Van der Waals Epitaxy of Two-Dimensional MoS2–Graphene Heterostructures in Ultrahigh Vacuum. ACS Nano 2015, 9, 6502– 6510,  DOI: 10.1021/acsnano.5b02345
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    Van der Waals Epitaxy of Two-Dimensional MoS2-Graphene Heterostructures in Ultrahigh Vacuum
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    ACS Nano (2015), 9 (6), 6502-6510CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    In this work, we demonstrate direct van der Waals epitaxy of MoS2-graphene heterostructures on a semiconducting silicon carbide (SiC) substrate under ultrahigh vacuum conditions. Angle-resolved photoemission spectroscopy (ARPES) measurements show that the electronic structure of free-standing single-layer (SL) MoS2 is retained in these heterostructures due to the weak van der Waals interaction between adjacent materials. The MoS2 synthesis is based on a reactive phys. vapor deposition technique involving Mo evapn. and sulfurization in a H2S atm. on a template consisting of epitaxially grown graphene on SiC. Using scanning tunneling microscopy, we study the seeding of Mo on this substrate and the evolution from nanoscale MoS2 islands to SL and bilayer (BL) MoS2 sheets during H2S exposure. Our ARPES measurements of SL and BL MoS2 on graphene reveal the coexistence of the Dirac states of graphene and the expected valence band of MoS2 with the band max. shifted to the corner of the Brillouin zone at ‾K in the SL limit. We confirm the 2D character of these electronic states via a lack of dispersion with photon energy. The growth of epitaxial MoS2-graphene heterostructures on SiC opens new opportunities for further in situ studies of the fundamental properties of these complex materials, as well as perspectives for implementing them in various device schemes to exploit their many promising electronic and optical properties.
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    Controlled van der Waals Epitaxy of Monolayer MoS2 Triangular Domains on Graphene
    Ago, Hiroki; Endo, Hiroko; Solis-Fernandez, Pablo; Takizawa, Rina; Ohta, Yujiro; Fujita, Yusuke; Yamamoto, Kazuhiro; Tsuji, Masaharu
    ACS Applied Materials & Interfaces (2015), 7 (9), 5265-5273CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Multilayered heterostructures of two-dimensional materials have recently attracted increased interest because of their unique electronic and optical properties. Here, the authors present CVD growth of triangular crystals of monolayer MoS2 on single-cryst. hexagonal graphene domains which are also grown by CVD. MoS2 grows selectively on the graphene domains rather than on the bare supporting SiO2 surface. Reflecting the heteroepitaxy of the growth process, the MoS2 domains grown on graphene present two preferred equiv. orientations. The interaction between the MoS2 and the graphene induced an upshift of the Raman G and 2-dimensional bands of the graphene, while significant photoluminescence quenching was obsd. for the monolayer MoS2. Also, photoinduced current modulation along with an optical memory effect was demonstrated for the MoS2-graphene heterostructure. The authors' work highlights that heterostructures synthesized by CVD offer an effective interlayer van der Waals interaction which can be developed for large-area multilayer electronic and photonic devices.
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    Liu, X.; Balla, I.; Bergeron, H.; Campbell, G. P.; Bedzyk, M. J.; Hersam, M. C. Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene. ACS Nano 2016, 10, 1067– 1075,  DOI: 10.1021/acsnano.5b06398
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    Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene
    Liu, Xiaolong; Balla, Itamar; Bergeron, Hadallia; Campbell, Gavin P.; Bedzyk, Michael J.; Hersam, Mark C.
    ACS Nano (2016), 10 (1), 1067-1075CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Atomically thin MoS2/graphene heterostructures are promising candidates for nanoelectronic and optoelectronic technologies. Among different graphene substrates, epitaxial graphene (EG) on SiC provides several potential advantages for such heterostructures, including high electronic quality, tunable substrate coupling, wafer-scale processability, and cryst. ordering that can template commensurate growth. Exploiting these attributes, the authors demonstrate here the thickness-controlled van der Waals epitaxial growth of MoS2 on EG via CVD, giving rise to transfer-free synthesis of a two-dimensional heterostructure with registry between its constituent materials. The rotational commensurability obsd. between the MoS2 and EG is driven by the energetically favorable alignment of their resp. lattices and results in nearly strain-free MoS2, as evidenced by synchrotron x-ray scattering and at.-resoln. scanning tunneling microscopy (STM). The electronic nature of the MoS2/EG heterostructure is elucidated with STM and scanning tunneling spectroscopy, which reveals bias-dependent apparent thickness, band bending, and a reduced band gap of ∼0.4 eV at the monolayer MoS2 edges.
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  • Abstract

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    Figure 1

    Figure 1. Structure of the DeepSTEM framework. Input images are decoded to ψ, which corresponds to the object function of the STEM image. Ir is the result of the STEM image simulation when applying the PSF to the Fourier transform of the reconstructed object function.

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    Figure 2

    Figure 2. (a) Atomic-resolution HAADF-STEM image of MoS2 on graphene. (b) Image of the reconstructed object function. (c) Intensity profiles of the experimental HAADF-STEM image and image of reconstructed object function are plotted as solid red lines from lower left to upper right in each image. The scale bar is 0.5 nm. Mo and S atoms are represented by cyan and orange spheres.

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    Figure 3

    Figure 3. (a–c) Atomic-resolution HAADF-STEM images showing the edge configurations. The image was taken at a temperature of 500 °C. (d–f) Images of the reconstructed object function of (a–c) showing (d) Mo-ZZ, (e) Mo-Klein, and (f) S-ZZ. The scale bar is 0.5 nm. (g–i) Schematic atomic models based on the interpretation of (d–f). Mo and S atoms are represented by cyan and yellow spheres, respectively.

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    Figure 4

    Figure 4. Time-elapsed images of the reconstructed object function showing MoS2 formation progress on graphene with temperature holding at 500 °C. The elapsed time is shown in the lower left of each image. (a) MoSx cluster is adsorbed on the graphene substrates. (b) MoSx reconstructs the MoS2 cluster. (c) MoS2 cluster shows a truncated triangular shape with Mo-ZZ, Mo-Klein, and S-ZZ edges. The remaining MoSx clusters are denoted by white solid arrows. (d) Upside MoSx cluster transforms into MoS2. (e) Downside MoSx clusters also transformed into MoS2. The MoS2 cluster shows a triangular shape with Mo-ZZ edges. The scale bar is 0.5 nm.

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    Figure 5

    Figure 5. (a–c) Time-elapsed images of the reconstructed object function of MoS2 reknitting progress on graphene with temperature holding at 500 °C. The elapsed time is shown in the upper left of each image. (a) MoS2 clusters have a hole with Mo-ZZ and S-ZZ edges. Mo adatoms are noted by cyan dashed circles. (b) Hole starts to reknit with diffusion and atomic reconstruction. The orange solid line in (a,b) shows the roto-translational motion of MoS2 clusters. (c) Hole has reknitted and shows a mirror twin boundary, as denoted by a red dashed box. Mo and S atoms are represented by cyan and orange spheres, respectively. The scale bar is 0.5 nm..

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    This article references 51 other publications.

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      Decrease in processing speed due to increased resistance and capacitance delay is a major obstacle for the down-scaling of electronics. Minimizing the dimensions of interconnects (metal wires that connect different electronic components on a chip) is crucial for the miniaturization of devices. Interconnects are isolated from each other by non-conducting (dielec.) layers. So far, research has mostly focused on decreasing the resistance of scaled interconnects because integration of dielecs. using low-temp. deposition processes compatible with complementary metal-oxide-semiconductors is tech. challenging. Interconnect isolation materials must have low relative dielec. consts. (κ values), serve as diffusion barriers against the migration of metal into semiconductors, and be thermally, chem. and mech. stable. Specifically, the International Road map for Devices and Systems recommends the development of dielecs. with κ values of less than 2 by 2028. Existing low-κ materials (such as silicon oxide derivs., org. compds. and aerogels) have κ values greater than 2 and poor thermo-mech. properties. Here, the authors report three-nanometer-thick amorphous boron nitride films with ultralow κ values of 1.78 and 1.16 (close to that of air, κ = 1) at operation frequencies of 100 kHz and 1 MHz, resp. The films are mech. and elec. robust, with a breakdown strength of 7.3 MV per cm, which exceeds requirements. Cross-sectional imaging reveals that amorphous boron nitride prevents the diffusion of cobalt atoms into silicon under very harsh conditions, in contrast to ref. barriers. The results demonstrate that amorphous boron nitride has excellent low-κ dielec. characteristics for high-performance electronics.
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    3. 3
      Lee, C.-S.; Shin, K. W.; Song, H.-J.; Park, H.; Cho, Y.; Im, D.-H.; Lee, H.; Lee, J.-H.; Won, J. Y.; Chung, J. G.; Kim, C.; Byun, K.-E.; Lee, E.-K.; Kim, Y.; Ko, W.; Lim, H. J.; Park, S.; Shin, H.-J. Fabrication of Metal/Graphene Hybrid Interconnects by Direct Graphene Growth and Their Integration Properties. Adv. Electron. Mater. 2018, 4, 1700624,  DOI: 10.1002/aelm.201700624
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      There is no corresponding record for this reference.
    4. 4
      Lo, C.-L.; Helfrecht, B. A.; He, Y.; Guzman, D. M.; Onofrio, N.; Zhang, S.; Weinstein, D.; Strachan, A.; Chen, Z. Opportunities and challenges of 2D materials in back-end-of-line interconnect scaling. J. Appl. Phys. 2020, 128, 080903,  DOI: 10.1063/5.0013737
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      Opportunities and challenges of 2D materials in back-end-of-line interconnect scaling
      Lo, Chun-Li; Helfrecht, Benjamin A.; He, Yanbo; Guzman, David M.; Onofrio, Nicolas; Zhang, Shengjiao; Weinstein, Dana; Strachan, Alejandro; Chen, Zhihong
      Journal of Applied Physics (Melville, NY, United States) (2020), 128 (8), 080903CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      As the challenges in continued scaling of the integrated circuit technol. escalate every generation, there is an urgent need to find viable solns. for both the front-end-of-line (transistors) and the back-end-of-line (interconnects). For the interconnect technol., it is crucial to replace the conventional barrier and liner with much thinner alternatives so that the current driving capability of the interconnects can be maintained or even improved. Due to the inherent atomically thin body thicknesses, 2D materials have recently been proposed and explored as Cu diffusion barrier alternatives. In this Perspective article, a variety of 2D materials that have been studied, ranging from graphene, h-BN, MoS2, WSe2 to TaS2, will be reviewed. Their potentials will be evaluated based on several criteria, including fundamental material properties as well as the feasibility for technol. integration. Using TaS2 as an example, we demonstrate a large set of promising properties and point out that there remain challenges in the integration aspects with a few possible solns. waiting for validation. Applications of 2D materials for other functions in Cu interconnects and for different metal types will also be introduced, including electromigration, cobalt interconnects, and radio-frequency transmission lines. (c) 2020 American Institute of Physics.
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    5. 5
      Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. 2D materials and van der Waals heterostructures. Science 2016, 353, aac9439,  DOI: 10.1126/science.aac9439
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      Manzeli, S.; Ovchinnikov, D.; Pasquier, D.; Yazyev, O. V.; Kis, A. 2D transition metal dichalcogenides. Nat. Rev. Mater. 2017, 2, 17033,  DOI: 10.1038/natrevmats.2017.33
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      2D transition metal dichalcogenides
      Manzeli, Sajedeh; Ovchinnikov, Dmitry; Pasquier, Diego; Yazyev, Oleg V.; Kis, Andras
      Nature Reviews Materials (2017), 2 (2), 17033CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
      A review. Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of at.-scale thickness, direct bandgap, strong spin-orbit coupling and favorable electronic and mech. properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examd. and their properties are discussed, with particular attention to their charge d. wave, superconductive and topol. phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties.
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    7. 7
      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, 10451– 10453,  DOI: 10.1073/pnas.0502848102
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      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.
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    8. 8
      Kaushik, N.; Nipane, A.; Basheer, F.; Dubey, S.; Grover, S.; Deshmukh, M. M.; Lodha, S. Schottky barrier heights for Au and Pd contacts to MoS2. Appl. Phys. Lett. 2014, 105, 113505,  DOI: 10.1063/1.4895767
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      Schottky barrier heights for Au and Pd contacts to MoS2
      Kaushik, Naveen; Nipane, Ankur; Basheer, Firdous; Dubey, Sudipta; Grover, Sameer; Deshmukh, Mandar M.; Lodha, Saurabh
      Applied Physics Letters (2014), 105 (11), 113505/1-113505/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The search of a p-type metal contact on MoS2 has remained inconclusive, with high work function metals such as Au, Ni, and Pt showing n-type behavior and mixed reports of n as well as p-type behavior for Pd. In this work, we report quant. Schottky barrier heights for Au and Pd contacts to MoS2 obtained by analyzing low temp. transistor characteristics and contact resistance data obtained using the transfer length method. Both Au and Pd exhibit n-type behavior on multilayer as well as monolayer MoS2 transistors with Schottky barrier heights of 0.126 eV and 0.4 eV, and contact resistances of 42 Ω.mm and 18 × 104 Ω.mm, resp. Scanning photocurrent spectroscopy data is in agreement with the resulting energy band alignment in Au-MoS2-Pd devices further reinforcing the observation that the Fermi-level is pinned in the upper half of MoS2 bandgap. (c) 2014 American Institute of Physics.
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    9. 9
      Liu, Y.; Wu, H.; Cheng, H.-C.; Yang, S.; Zhu, E.; He, Q.; Ding, M.; Li, D.; Guo, J.; Weiss, N. O.; Huang, Y.; Duan, X. Toward barrier free contact to molybdenum disulfide using graphene electrodes. Nano Lett. 2015, 15, 3030– 3034,  DOI: 10.1021/nl504957p
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      Toward Barrier Free Contact to Molybdenum Disulfide Using Graphene Electrodes
      Liu, Yuan; Wu, Hao; Cheng, Hung-Chieh; Yang, Sen; Zhu, Enbo; He, Qiyuan; Ding, Mengning; Li, Dehui; Guo, Jian; Weiss, Nathan O.; Huang, Yu; Duan, Xiangfeng
      Nano Letters (2015), 15 (5), 3030-3034CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) have attracted tremendous interest as a new class of electronic materials. However, there are considerable challenges in making reliable contacts to these atomically thin materials. Here the authors present a new strategy by using graphene as the back electrodes to achieve ohmic contact to MoS2. With a finite d. of states, the Fermi level of graphene can be readily tuned by a gate potential to enable a nearly perfect band alignment with MoS2. The authors demonstrate for the 1st time a transparent contact to MoS2 with zero contact barrier and linear output behavior at cryogenic temps. (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, a metal-insulator transition can be obsd. in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barriers found in conventional metal-contacted MoS2 devices. With further passivation by boron nitride (BN) encapsulation, the authors demonstrate a record-high extrinsic (two-terminal) field effect mobility up to 1300 cm2/(V s) in MoS2 at low temp.
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    10. 10
      Shi, Y.; Zhou, W.; Lu, A.-Y.; Fang, W.; Lee, Y.-H.; Hsu, A. L.; Kim, S. M.; Kim, K. K.; Yang, H. Y.; Li, L.-J.; Idrobo, J.-C.; Kong, J. van der Waals Epitaxy of MoS2 Layers Using Graphene As Growth Templates. Nano Lett. 2012, 12, 2784– 2791,  DOI: 10.1021/nl204562j
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      10
      van der Waals Epitaxy of MoS2 Layers Using Graphene As Growth Templates
      Shi, Yumeng; Zhou, Wu; Lu, Ang-Yu; Fang, Wenjing; Lee, Yi-Hsien; Hsu, Allen Long; Kim, Soo Min; Kim, Ki Kang; Yang, Hui Ying; Li, Lain-Jong; Idrobo, Juan-Carlos; Kong, Jing
      Nano Letters (2012), 12 (6), 2784-2791CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The authors present a method for synthesizing MoS2/Graphene hybrid heterostructures with a growth template of graphene-covered Cu foil. Compared to other recent reports, a much lower growth temp. of 400 °C is required for this procedure. The chem. vapor deposition of MoS2 on the graphene surface gives rise to single cryst. hexagonal flakes with a typical lateral size ranging from several hundred nanometers to several micrometers. The precursor (ammonium thiomolybdate) together with solvent was transported to graphene surface by a carrier gas at room temp., which was then followed by post annealing. At an elevated temp., the precursor self-assembles to form MoS2 flakes epitaxially on the graphene surface via thermal decompn. With higher amt. of precursor delivered onto the graphene surface, a continuous MoS2 film on graphene can be obtained. This simple chem. vapor deposition method provides a unique approach for the synthesis of graphene heterostructures and surface functionalization of graphene. The synthesized two-dimensional MoS2/Graphene hybrids possess great potential toward the development of new optical and electronic devices as well as a wide variety of newly synthesizable compds. for catalysts.
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    11. 11
      Wang, H. W.; Skeldon, P.; Thompson, G. E.; Wood, G. C. Synthesis and characterization of molybdenum disulphide formed from ammonium tetrathiomolybdate. J. Mater. Sci. 1997, 32, 497– 502,  DOI: 10.1023/a:1018538424373
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      11
      Synthesis and characterization of molybdenum disulfide formed from ammonium tetrathiomolybdate
      Wang, H. W.; Skeldon, P.; Thompson, G. E.; Wood, G. C.
      Journal of Materials Science (1997), 32 (2), 497-502CODEN: JMTSAS; ISSN:0022-2461. (Chapman & Hall)
      An investigation was carried out on the possibility of in-situ formation of MoS2 within porous anodic films on aluminum, to improve subsequent tribol. behavior, by re-anodization in thiomolybdate electrolyte. Acidification of thiomolybdate was used to simulate the conditions for formation of the sulfide at the anodic film-electrolyte interface, followed by appropriate vacuum heat treatments to study possible temp. effects on the sulfide due to either friction or Joule heating during anodizing. The products of both acidification and heat treatment, characterized by x-ray powder diffraction and SEM, were compared with those formed by direct thermal decompn. of ammonium tetrathiomolybdate crystals. The ppt. formed by acidification was mainly amorphous molybdenum trisulfide (MoS3), which on heat treatment at 450° and 850° yielded 3R-MoS2 (identified by x-ray diffraction). 3R-MoS2 also formed by the thermal decompn. of thiomolybdate crystals. Thermogravimetric and differential thermal analyses showed that the decompn. of MoS3 to MoS2 occurred at 220-370° and revealed the sequence of reaction steps. Mainly amorphous MoS3 is formed as a consequence of pH changes in the film-electrolyte interface during re-anodizing but the product is relatively easily transformed to cryst. MoS2 upon moderate heating which may occur during wear processes.
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    12. 12
      Sygellou, L. An in-situ photoelectron spectroscopy study of the thermal processing of ammonium tetrathiomolybdate, (NH4)2MoS4, precursor. Appl. Surf. Sci. 2019, 476, 1079– 1085,  DOI: 10.1016/j.apsusc.2019.01.193
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      12
      An in-situ photoelectron spectroscopy study of the thermal processing of ammonium tetrathiomolybdate, (NH4)2MoS4, precursor
      Sygellou, Labrini
      Applied Surface Science (2019), 476 (), 1079-1085CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Nano scale molybdenum disulfide (MoS2) is a very promising material for the next generation electronics and energy storage devices. The active defect sites located at the periphery of the lattice may influence certain redox reactions, thus classifying it as an excellent non-noble metal catalyst. One effective approach to synthesize large scale MoS2 films is through simple thermolysis of (NH4)2MoS4 precursor. Herein, a combined in situ X-ray and Ultra-Violet photoelectron spectroscopies (XPS/UPS) study has been carried out to follow the evolution of at. compn., the surface chem. species and the electronic properties (Work Function and Ionization Potential) during the thermal decompn. of (NH4)2MoS4 salt in Ultra High Vacuum conditions. It was found that, up to 400C, thermal conversion of (NH4)2MoS4 salt to transient intermediates, across amorphous MoSx phase have been performed towards to MoS2 out of significant percent of sulfur active sites. The active sites have been decreased upon heating at higher temps. Assignment of the thermally evolved active species at defect sites and the correlation of surface at. concn. combined with either the Ionization Potential or the Work Function of annealed MoSx nanoflakes makes this study a knowledgeable factor of awareness through electrocatalytic properties of MoS2-based nanostructures. Acknowledgement: We acknowledge support of this work by the project "National Infrastructure in Nanotechnol., Advanced Materials and Micro-/ Nanoelectronics" (MIS 5002772) which is implemented under the Action "Reinforcement of the Research and Innovation Infrastructure", funded by the Operational Program "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).
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    13. 13
      Yang, J.; Gu, Y.; Lee, E.; Lee, H.; Park, S. H.; Cho, M.-H.; Kim, Y. H.; Kim, Y.-H.; Kim, H. Wafer-scale synthesis of thickness-controllable MoS2 films via solution-processing using a dimethylformamide/n-butylamine/2-aminoethanol solvent system. Nanoscale 2015, 7, 9311– 9319,  DOI: 10.1039/c5nr01486g
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      13
      Wafer-scale synthesis of thickness-controllable MoS2 films via solution-processing using a dimethylformamide/n-butylamine/2-aminoethanol solvent system
      Yang, Jaehyun; Gu, Yeahyun; Lee, Eunha; Lee, Hyangsook; Park, Sang Han; Cho, Mann-Ho; Kim, Yong Ho; Kim, Yong-Hoon; Kim, Hyoungsub
      Nanoscale (2015), 7 (20), 9311-9319CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      The wafer-scale synthesis of two-dimensional molybdenum disulfide (MoS2) films, with high layer-controllability and uniformity, remains a significant challenge in the fields of nano and optoelectronics. Here, we report the highly thickness controllable growth of uniform MoS2 thin films on the wafer-scale via a spin-coating route. Formulation of a dimethylformamide-based MoS2 precursor soln. mixed with addnl. amine- and amino alc.-based solvents (n-butylamine and 2-aminoethanol) allowed for the formation of a uniform coating of MoS2 thin films over a 2 in. wafer-scale SiO2/Si substrate. In addn., facile control of the av. no. of stacking layers is demonstrated by simply manipulating the concn. of the precursor soln. Various characterization results reveal that the synthesized MoS2 film has wafer-scale homogeneity with excellent cryst. quality and a stoichiometric chem. compn. To further demonstrate possible device applications, a mostly penta-layered MoS2 thin film was integrated into a top-gated field-effect transistor as the channel layer and we also successfully transferred our films onto transparent/flexible substrates.
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    14. 14
      Yang, H.; Giri, A.; Moon, S.; Shin, S.; Myoung, J.-M.; Jeong, U. Highly scalable synthesis of MoS2 thin films with precise thickness control via polymer-assisted deposition. Chem. Mater. 2017, 29, 5772– 5776,  DOI: 10.1021/acs.chemmater.7b01605
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      14
      Highly Scalable Synthesis of MoS2 Thin Films with Precise Thickness Control via Polymer-Assisted Deposition
      Yang, Heeseung; Giri, Anupam; Moon, Sungmin; Shin, Sangbae; Myoung, Jae-Min; Jeong, Unyong
      Chemistry of Materials (2017), 29 (14), 5772-5776CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The versatile applications of MoS2 thin films demand large-scale synthesis with high uniformity in thickness and compn. Among several synthetic approaches, the two-step process including soln.-based precursor coating followed by thermolysis is advantageous in terms of cost and scalability. In this study, we developed a polymer-assisted deposition process to prep. a highly scalable polymer-precursor complex thin film and apply thermolysis at a relatively low temp. without sulfurization. This process enables fine control of the precursor film, which leads to precise thickness control of the resulting MoS2 film (≥2 nm). In addn., we demonstrate the fabrication of a MoS2-based photodetector with a broad spectral response and excellent performance, as indicated by its fast response time (≤1.0 ms) and high on-off ratio (104).
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    15. 15
      Zhu, Y.; Yuan, D.; Zhang, H.; Xu, T.; Sun, L. Atomic-scale insights into the formation of 2D crystals from in situ transmission electron microscopy. Nano Res. 2021, 14, 1650– 1658,  DOI: 10.1007/s12274-020-3034-z
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      Mishra, R.; Ishikawa, R.; Lupini, A. R.; Pennycook, S. J. Single-atom dynamics in scanning transmission electron microscopy. MRS Bull. 2017, 42, 644– 652,  DOI: 10.1557/mrs.2017.187
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      16
      Single-atom dynamics in scanning transmission electron microscopy
      Mishra, Rohan; Ishikawa, Ryo; Lupini, Andrew R.; Pennycook, Stephen J.
      MRS Bulletin (2017), 42 (9), 644-652CODEN: MRSBEA; ISSN:0883-7694. (Cambridge University Press)
      The correction of aberrations in the scanning transmission electron microscope (STEM) has simultaneously improved both spatial and temporal resoln., making it possible to capture the dynamics of single atoms inside materials, and resulting in new insights into the dynamic behavior of materials. In this article, we describe the different beam-matter interactions that lead to at. excitations by transferring energy and momentum. We review recent examples of sequential STEM imaging to demonstrate the dynamic behavior of single atoms both within materials, at dislocations, at grain and interface boundaries, and on surfaces. We also discuss the effects of such dynamic behavior on material properties. We end with a summary of ongoing instrumental and algorithm developments that we anticipate will improve the temporal resoln. significantly, allowing unprecedented insights into the dynamic behavior of materials at the at. scale.
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    17. 17
      Su, C.; Tripathi, M.; Yan, Q.-B.; Wang, Z.; Zhang, Z.; Hofer, C.; Wang, H.; Basile, L.; Su, G.; Dong, M.; Meyer, J. C.; Kotakoski, J.; Kong, J.; Idrobo, J.-C.; Susi, T.; Li, J. Engineering single-atom dynamics with electron irradiation. Sci. Adv. 2019, 5, eaav2252  DOI: 10.1126/sciadv.aav2252
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      There is no corresponding record for this reference.
    18. 18
      Fei, L.; Lei, S.; Zhang, W.-B.; Lu, W.; Lin, Z.; Lam, C. H.; Chai, Y.; Wang, Y. Direct TEM observations of growth mechanisms of two-dimensional MoS2 flakes. Nat. Commun. 2016, 7, 12206,  DOI: 10.1038/ncomms12206
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      Direct TEM observations of growth mechanisms of two-dimensional MoS2 flakes
      Fei, Linfeng; Lei, Shuijin; Zhang, Wei-Bing; Lu, Wei; Lin, Ziyuan; Lam, Chi Hang; Chai, Yang; Wang, Yu
      Nature Communications (2016), 7 (), 12206CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      A microscopic understanding of the growth mechanism of two-dimensional materials is of particular importance for controllable synthesis of functional nanostructures. Because of the lack of direct and insightful observations, how to control the orientation and the size of two-dimensional material grains is still under debate. Here we discern distinct formation stages for MoS2 flakes from the thermolysis of ammonium thiomolybdates using in situ transmission electron microscopy. In the initial stage (400 °C), vertically aligned MoS2 structures grow in a layer-by-layer mode. With the increasing temp. of up to 780 °C, the orientation of MoS2 structures becomes horizontal. When the growth temp. reaches 850 °C, the cryst. size of MoS2 increases by merging adjacent flakes. Our study shows direct observations of MoS2 growth as the temp. evolves, and sheds light on the controllable orientation and grain size of two-dimensional materials.
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    19. 19
      Sang, X.; Li, X.; Puretzky, A. A.; Geohegan, D. B.; Xiao, K.; Unocic, R. R. Atomic Insight into Thermolysis-Driven Growth of 2D MoS2. Adv. Funct. Mater. 2019, 29, 1902149,  DOI: 10.1002/adfm.201902149
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      19
      Atomic Insight into Thermolysis-Driven Growth of 2D MoS2
      Sang, Xiahan; Li, Xufan; Puretzky, Alexander A.; Geohegan, David B.; Xiao, Kai; Unocic, Raymond R.
      Advanced Functional Materials (2019), 29 (52), 1902149CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Understanding and controlling the transformations of transition metal dichalcogenides (TMDs) from amorphous precursors into 2-dimensional (2D) materials is important for guiding synthesis, directing fabrication, and tailoring functional properties. Here, the combined effects of thermal energy and electron beam irradn. are explored on the structural evolution of 2-dimensional MoS2 flakes through the thermal decompn. of a (NH4)2MoS4 precursor inside an ultrahigh vacuum (10-9 Torr) scanning transmission electron microscope (STEM). The influence of reaction temp., growth substrate, and the initial precursor morphol. on the resulting 2-dimensional MoS2 flake morphol., edge structures, and point defects are explored. Although thermal decompn. occurs extremely fast at elevated temps. and is difficult to capture using current STEM techniques, electron beam irradn. can induce local transformations at lower temps., enabling direct observation and interpretation of crit. growth steps including oriented attachment and transition from single- to multilayer structures at at. resoln. An increase in the no. of layers of the MoS2 flakes from island growth is studied using electron beam irradn. These findings provide insight into the growth mechanisms and factors that control the synthesis of few-layer MoS2 flakes through thermolysis and toward the prospect of atomically precise control and growth of 2-dimensional TMDs.
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    20. 20
      Kondekar, N.; Boebinger, M. G.; Tian, M.; Kirmani, M. H.; McDowell, M. T. The Effect of Nickel on MoS2 Growth Revealed with in Situ Transmission Electron Microscopy. ACS Nano 2019, 13, 7117– 7126,  DOI: 10.1021/acsnano.9b02528
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      The Effect of Nickel on MoS2 Growth Revealed with in Situ Transmission Electron Microscopy
      Kondekar, Neha; Boebinger, Matthew G.; Tian, Mengkun; Kirmani, Mohammad Hamza; McDowell, Matthew T.
      ACS Nano (2019), 13 (6), 7117-7126CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      MoS2 has important applications in (electro)catalysis and as a semiconductor for electronic devices. Other chem. species are commonly added to MoS2 to increase catalytic activity or to alter electronic properties through substitutional or adsorption-based doping. While ground breaking work has been devoted to detg. the at.-scale structure of MoS2 and other layered transition-metal dichalcogenides (TMDCs), there is a lack of understanding of the dynamic processes that govern the evolution of these materials during synthesis. Here, in situ transmission electron microscopy (TEM) heating, in combination with larger length scale ex situ expts., is used to investigate the effects of added Ni on the growth of MoS2 during the thermolysis of the solid-state (NH4)2MoS4 precursor. Low concns. of Ni are obsd. to cause significant differences in the MoS2 crystn. and growth process, leading to an increase in MoS2 crystal size. This is likely a result of the altered mobility of interfaces between crystals during growth. These findings demonstrate the important role of addnl. elements in controlling the evolution of TMDCs during synthesis, which should be considered when designing these materials for a variety of applications.
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      Morgan, A. J.; Martin, A. V.; D’Alfonso, A. J.; Putkunz, C. T.; Allen, L. J. Direct exit-wave reconstruction from a single defocused image. Ultramicroscopy 2011, 111, 1455– 1460,  DOI: 10.1016/j.ultramic.2011.07.005
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      Direct exit-wave reconstruction from a single defocused image
      Morgan, A. J.; Martin, A. V.; D'Alfonso, A. J.; Putkunz, C. T.; Allen, L. J.
      Ultramicroscopy (2011), 111 (9-10), 1455-1460CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)
      We propose a direct, non-iterative method for the exact recovery of the complex wave in the exit-surface plane of a coherently illuminated object from a single defocused image. The method is applicable for a wide range of illumination conditions. The defocus range is subject to certain conditions, which if satisfied allow the complex exit-surface wave to be directly recovered by solving a set of linear equations. These linear equations, whose coeffs. depend on the incident illumination, are obtained by analyzing the autocorrelation function of an auxiliary wave which is related to the exit-surface wave in a simple way. This autocorrelation is constructed by taking the inverse Fourier transform of the defocused image. We present an exptl. proof of concept by recovering the exit-surface wave of a microfiber illuminated by a plane wave formed using a HeNe laser.
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      Kirkland, E. J. Improved high resolution image processing of bright field electron micrographs: I. Theory. Ultramicroscopy 1984, 15, 151– 172,  DOI: 10.1016/0304-3991(84)90037-8
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      Nellist, P. D.; Pennycook, S. J. Accurate structure determination from image reconstruction in ADF STEM. J. Microsc. 1998, 190, 159– 170,  DOI: 10.1046/j.1365-2818.1998.3260881.x
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      Accurate structure determination from image reconstruction in ADF STEM
      Nellist, P. D.; Pennycook, S. J.
      Journal of Microscopy (Oxford) (1998), 190 (1/2), 159-170CODEN: JMICAR; ISSN:0022-2720. (Blackwell Science Ltd.)
      Annular dark-field (ADF) imaging in a scanning transmission electron microscope results in direct structure images of the at. configuration of the specimen. Since such images are almost perfectly incoherent they can be treated as a convolution between a point-spread function, which is simply the intensity of the illuminating electron probe, and a sharply peaked object function that represents the projected structure of the specimen. Knowledge of the object function for an image region of perfect crystal allows the point-spread function to be directly detd. for that image. We examine how the object function for an image can then be reconstructed using a Wiener filter, the CLEAN algorithm and a max. entropy reconstruction. Prior information is required to perform a reconstruction, and we discuss what nature of prior information is suitable for ADF imaging.
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      Nellist, P. D.; Pennycook, S. J. Probe and object function reconstruction in incoherent scanning transmission electron microscope imaging. Scanning Microsc. 1997, 11, 81– 90
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      Kazubek, M. Wavelet domain image denoising by thresholding and Wiener filtering. IEEE Signal Process. Lett. 2003, 10, 324– 326,  DOI: 10.1109/lsp.2003.818225
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      Ede, J. M. Deep learning in electron microscopy. Mach. Learn.: Sci. Technol. 2021, 2, 011004,  DOI: 10.1088/2632-2153/abd614
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      Madsen, J.; Liu, P.; Kling, J.; Wagner, J. B.; Hansen, T. W.; Winther, O.; Schiøtz, J. A Deep Learning Approach to Identify Local Structures in Atomic-Resolution Transmission Electron Microscopy Images. Adv. Theory Simul. 2018, 1, 1800037,  DOI: 10.1002/adts.201800037
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      Lee, J.; Lee, Y.; Kim, J.; Lee, Z. Contrast Transfer Function-Based Exit-Wave Reconstruction and Denoising of Atomic-Resolution Transmission Electron Microscopy Images of Graphene and Cu Single Atom Substitutions by Deep Learning Framework. Nanomaterials 2020, 10, 1977,  DOI: 10.3390/nano10101977
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      Contrast transfer function-based exit-wave reconstruction and denoising of atomic-resolution transmission electron microscopy images of graphene and Cu single atom substitutions by deep learning framework
      Lee, Jongyeong; Lee, Yeongdong; Kim, Jaemin; Lee, Zonghoon
      Nanomaterials (2020), 10 (10), 1977CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
      The exit wave is the state of a uniform plane incident electron wave exiting immediately after passing through a specimen and before the at.-resoln. transmission electron microscopy (ARTEM) image is modified by the aberration of the optical system and the incoherence effect of the electron. Although exit-wave reconstruction has been developed to prevent the misinterpretation of ARTEM images, there have been limitations in the use of conventional exit-wave reconstruction in ARTEM studies of the structure and dynamics of two-dimensional materials. In this study, we propose a framework that consists of the convolutional dual-decoder autoencoder to reconstruct the exit wave and denoise ARTEM images. We calcd. the contrast transfer function (CTF) for real ARTEM and assigned the output of each decoder to the CTF as the amplitude and phase of the exit wave. We present exit-wave reconstruction expts. with ARTEM images of monolayer graphene and compare the findings with those of a simulated exit wave. Cu single atom substitution in monolayer graphene was, for the first time, directly identified through exit-wave reconstruction expts. Our exit-wave reconstruction expts. show that the performance of the denoising task is improved when compared to the Wiener filter in terms of the signal-to-noise ratio, peak signal-to-noise ratio, and structural similarity index map metrics.
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      Nellist, P. D.; Pennycook, S. J. The principles and interpretation of annular dark-field Z-contrast imaging. Adv. Imaging Electron Phys. 2000, 113, 147– 203,  DOI: 10.1016/s1076-5670(00)80013-0
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      Nonlinear principal component analysis using autoassociative neural networks
      Kramer, Mark A.
      AIChE Journal (1991), 37 (2), 233-43CODEN: AICEAC; ISSN:0001-1541.
      Nonlinear-principal-component anal. (NLPCA), is a novel technique for multivariate data anal., similar to the method of principal-component anal. (PCA). NLPCA like PCA, is used to identify and remove correlations among problem variables as an aid to dimensionality redn., visualization, and exploratory data anal. While PCA identifies only linear correlations between variables, NLPCA uncovers both linear and nonlinear correlations, without restriction on the character of the nonlinearities present in the data. NLPCA operates by training a feedforward neural network to perform the identity mapping, where the network inputs are reproduced at the output layer. The network contains an internal bottleneck layer (contg. fewer nodes than input or output layers), which forces the network to develop a compact representation of the input data and 2 addnl. hidden layers. The NLPCA method is demonstrated by using time-dependent, simulated batch-reaction data. NLPCA can reduce dimensionality and produce a feature space map resembling the actual distribution of the underlying system parameters.
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      Zemlin, F.; Weiss, K.; Schiske, P.; Kunath, W.; Herrmann, K.-H. Coma-free alignment of high resolution electron microscopes with the aid of optical diffractograms. Ultramicroscopy 1978, 3, 49– 60,  DOI: 10.1016/s0304-3991(78)80006-0
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      Kingma, D. P.; Ba, J. Adam: A Method for Stochastic Optimization. arXiv.org, e-Print Arch., Comput. Sci.. 2014, arXiv:1412.6980.
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      Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 6, 183– 191,  DOI: 10.1038/nmat1849
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      The rise of graphene
      Geim, A. K.; Novoselov, K. S.
      Nature Materials (2007), 6 (3), 183-191CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      A review. Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when com. products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top expts. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.
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    34. 34
      Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312– 1314,  DOI: 10.1126/science.1171245
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      Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils
      Li, Xuesong; Cai, Weiwei; An, Jinho; Kim, Seyoung; Nah, Junghyo; Yang, Dongxing; Piner, Richard; Velamakanni, Aruna; Jung, Inhwa; Tutuc, Emanuel; Banerjee, Sanjay K.; Colombo, Luigi; Ruoff, Rodney S.
      Science (Washington, DC, United States) (2009), 324 (5932), 1312-1314CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Graphene was attracting great interest because of its distinctive band structure and phys. properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. The authors grew large-area graphene films of the order of centimeters on Cu substrates by CVD using methane. The films are predominantly single-layer graphene, with a small percentage (<5%) of the area having few layers, and are continuous across Cu surface steps and grain boundaries. The low soly. of C in Cu appears to help make this growth process self-limiting. The authors also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities ≤4050 cm2/V-s at room temp.
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    35. 35
      van der Zande, A. M.; Huang, P. Y.; Chenet, D. A.; Berkelbach, T. C.; You, Y.; Lee, G.-H.; Heinz, T. F.; Reichman, D. R.; Muller, D. A.; Hone, J. C. Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide. Nat. Mater. 2013, 12, 554– 561,  DOI: 10.1038/nmat3633
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      Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide
      van der Zande, Arend M.; Huang, Pinshane Y.; Chenet, Daniel A.; Berkelbach, Timothy C.; You, YuMeng; Lee, Gwan-Hyoung; Heinz, Tony F.; Reichman, David R.; Muller, David A.; Hone, James C.
      Nature Materials (2013), 12 (6), 554-561CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Recent progress in large-area synthesis of monolayer MoS2, a new 2D direct-band gap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chem. vapor deposition synthesis to grow highly cryst. islands of monolayer MoS2 up to 120 μm in size with optical and elec. properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphol. and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror twin boundaries that are stitched together by lines of 8- and 4-membered rings. D. functional theory reveals localized mid-gap states arising from these 8-4 defects. Mirror twin boundaries cause strong photoluminescence quenching whereas tilt boundaries cause strong enhancement. Meanwhile, mirror twin boundaries slightly increase the measured in-plane elec. cond., whereas tilt boundaries slightly decrease the cond.
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    36. 36
      Xu, W.; Li, S.; Zhou, S.; Lee, J. K.; Wang, S.; Sarwat, S. G.; Wang, X.; Bhaskaran, H.; Pasta, M.; Warner, J. H. Large dendritic monolayer MoS2 grown by atmospheric pressure chemical vapor deposition for electrocatalysis. ACS Appl. Mater. Interfaces 2018, 10, 4630– 4639,  DOI: 10.1021/acsami.7b14861
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      Large Dendritic Monolayer MoS2 Grown by Atmospheric Pressure Chemical Vapor Deposition for Electrocatalysis
      Xu, Wenshuo; Li, Sha; Zhou, Si; Lee, Ja Kyung; Wang, Shanshan; Sarwat, Syed Ghazi; Wang, Xiaochen; Bhaskaran, Harish; Pasta, Mauro; Warner, Jamie H.
      ACS Applied Materials & Interfaces (2018), 10 (5), 4630-4639CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The edge sites of MoS2 are catalytically active for the H evolution reaction (HER), and growing monolayer structures that are edge-rich is desirable. Here, the authors show the prodn. of large-area highly branched MoS2 dendrites on amorphous SiO2/Si substrates using an atm. pressure CVD and explore their use in electrocatalysis. By tailoring the substrate construction, the monolayer MoS2 evolves from triangular to dendritic morphol. because of the change of growth conditions. The rough edges endow dendritic MoS2 with a fractal dimension down to 1.54. The highly cryst. basal plane and the edge of the dendrites are visualized at at. resoln. using an annular dark field scanning transmission electron microscope. The monolayer dendrites exhibit strong photoluminescence, which is indicative of the direct band gap emission, which is preserved after being transferred. Post-transfer S annealing restores the structural defects and decreases the n-type doping in MoS2 monolayers. The annealed MoS2 dendrites show good and highly durable HER performance on the glassy C with a large exchange c.d. of 32 μA cmgeo-2, demonstrating its viability as an efficient HER catalyst.
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      Schweiger, H.; Raybaud, P.; Kresse, G.; Toulhoat, H. Shape and edge sites modifications of MoS2 catalytic nanoparticles induced by working conditions: A theoretical study. J. Catal. 2002, 207, 76– 87,  DOI: 10.1006/jcat.2002.3508
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      Shape and Edge Sites Modifications of MoS2 Catalytic Nanoparticles Induced by Working Conditions: A Theoretical Study
      Schweiger, Hannes; Raybaud, Pascal; Kresse, Georg; Toulhoat, Herve
      Journal of Catalysis (2002), 207 (1), 76-87CODEN: JCTLA5; ISSN:0021-9517. (Academic Press)
      Detn. of the morphol. and electronic properties of single-layer nanosize MoS2 particles is of considerable interest for a better understanding of the active phase of hydrotreating catalysts. We propose an original approach, based on d. functional calcns. applied to various types of MoS2 clusters contg. up to 200 atoms, to evaluate accurately the surface energies of the Mo-edge and S-edge terminated surfaces. The results are expressed as a function of the chem. potential of sulfur, which is in turn controlled by the temp. and the ratio of partial pressures of H2S and H2. Gibbs-Curie-Wulff equil. morphologies reveal that a high chem. potential of sulfur leads to triangular-shaped particles terminated by the Mo-edge surface, giving an interpretation of the observations made recently by scanning tunneling microscopy (STM). Simulations of the STM images for the most stable clusters reveal a qual. good agreement with expts. Moreover, our approach predicts that varying the potential of sulfur modifies the local Mo-edge structure and the shape of the nanoparticles. Finally, the thermodn. diagram for a MoS2 particle with a realistic size shows that the creation of one sulfur vacancy at the corner and on the edge is possible under HDS working conditions. (c) 2002 Academic Press.
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      Zhu, D.; Shu, H.; Jiang, F.; Lv, D.; Asokan, V.; Omar, O.; Yuan, J.; Zhang, Z.; Jin, C. Capture the growth kinetics of CVD growth of two-dimensional MoS2. npj 2D Mater. Appl. 2017, 1, 8,  DOI: 10.1038/s41699-017-0010-x
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      Pan, H.; Zhang, Y.-W. Edge-dependent structural, electronic and magnetic properties of MoS2 nanoribbons. J. Mater. Chem. 2012, 22, 7280– 7290,  DOI: 10.1039/c2jm15906f
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      Edge-dependent structural, electronic and magnetic properties of MoS2 nanoribbons
      Pan, Hui; Zhang, Yong-Wei
      Journal of Materials Chemistry (2012), 22 (15), 7280-7290CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      Two-dimensional materials have various applications in next-generation nanodevices because of their easy fabrication and particular properties. In this work, we studied the effects of edge structures on the edge stability, and electronic and magnetic properties of MoS2 nanoribbons by first-principles calcns. We predicted that S-terminated zigzag nanoribbons are the most stable even without hydrogen satn. because of their low and neg. edge energies, although hydrogen satn. of the edge states can stabilize other nanoribbons with different edge structures. MoS2 zigzag nanoribbons are metallic and ferromagnetic. Importantly, their cond. may be semiconducting (n- or p-type) or half metallic by controlling the edge structures satd. with H atoms. The magnetic states of the MoS2 zigzag nanoribbons are enhanced by H-satn. and are much stronger than those of graphene zigzag nanoribbons. The armchair nanoribbons are semiconducting, with bandgaps increased by the hydrogen satn. of their edge states, and are nonmagnetic. These MoS2 nanoribbons with versatile functions may have applications in spintronics, nanodevices, and energy harvesting.
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      Wang, L.; Zhou, X.; Ma, T.; Liu, D.; Gao, L.; Li, X.; Zhang, J.; Hu, Y.; Wang, H.; Dai, Y.; Luo, J. Superlubricity of a graphene/MoS2 heterostructure: a combined experimental and DFT study. Nanoscale 2017, 9, 10846– 10853,  DOI: 10.1039/c7nr01451a
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      Superlubricity of a graphene/MoS2 heterostructure: a combined experimental and DFT study
      Wang, Linfeng; Zhou, Xiang; Ma, Tianbao; Liu, Dameng; Gao, Lei; Li, Xin; Zhang, Jun; Hu, Yuanzhong; Wang, Hui; Dai, Yadong; Luo, Jianbin
      Nanoscale (2017), 9 (30), 10846-10853CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Graphene and other two-dimensional materials have been proved to be able to offer low friction. Here we assembled van der Waals heterostructures with graphene and molybdenum disulfide monolayers. The Raman spectrum together with a modified linear chain model indicate a two-orders-of-magnitude decrease in the interlayer lateral force const., as compared with their homogeneous bilayers, indicating a possible routine to achieve superlubricity. The decrease in the interlayer lateral force const. is consistent with the ultrasmall potential energy corrugation during sliding, which is derived from d. functional theory calcns. The potential energy corrugation is found to be detd. by the sliding-induced interfacial charge d. fluctuation, suggesting a new perspective to understand the phys. origin of the at. scale friction of two-dimensional materials.
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      Batzill, M. Mirror twin grain boundaries in molybdenum dichalcogenides. J. Phys.: Condens. Matter 2018, 30, 493001,  DOI: 10.1088/1361-648x/aae9cf
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      Mirror twin grain boundaries in molybdenum dichalcogenides
      Batzill, Matthias
      Journal of Physics: Condensed Matter (2018), 30 (49), 493001/1-493001/15CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
      A review. Mirror twin grain boundaries (MTBs) exist at the interface between 2 grains of 60° rotated hexagonal transition metal dichalcogenides (TMDC). These grain boundaries form a regular at. structure that extends in one dimension and thus may be described as a one-dimensional (1D) lattice embedded in the 2-dimensional TMDC. In this review, the different at. structures and compns. of these MTBs are discussed. The obvious formation of MTBs is by coalescence of 2 twinned grains. However, in MoSe2 and MoTe2 a different formation mechanism was revealed for the formation of Mo-rich MTBs. Excess Mo can be incorporated into the TMDC lattices. These excess Mo atoms can then reorganize into closed, triangular MTB-loops that can grow in size by adding more Mo atoms to them. This mechanism gave dense MTB networks in MoSe2 and MoTe2. Such MTB networks were obsd. in samples grown by MBE and consequently their presence needs to be considered in understanding the properties of MBE grown MoSe2 and MoTe2. D. functional theory as well as photoemission spectroscopy of MTB networks showed that MTBs exhibit dispersing 1-dimensional-bands that intersect the Fermi-level, thus suggesting that these are 1-dimensional electron systems. Consequently, exptl. data were interpreted to reveal a charge d. wave (or Peierls) instability, as well as a Tomonaga-Luttinger liq. behavior for electrons confined in 1D. These observations and the controversies that remain in the interpretation of some data are discussed. The metallic properties of the MTBs and their formation in dense networks also sparked the potential use of such crystal modifications for making metallic contacts to MoTe2 or MoSe2. Also, these crystal modifications may also boost the catalytic properties of these materials.
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      Kim, J. H.; Kim, S. Y.; Park, S. O.; Jung, G. Y.; Song, S.; Sohn, A.; Kim, S. W.; Kwak, S. K.; Kwon, S. Y.; Lee, Z. Antiphase Boundaries as Faceted Metallic Wires in 2D Transition Metal Dichalcogenides. Adv. Sci. 2020, 7, 2000788,  DOI: 10.1002/advs.202000788
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      Antiphase Boundaries as Faceted Metallic Wires in 2D Transition Metal Dichalcogenides
      Kim, Jung Hwa; Kim, Se-Yang; Park, Sung O.; Jung, Gwan Yeong; Song, Seunguk; Sohn, Ahrum; Kim, Sang-Woo; Kwak, Sang Kyu; Kwon, Soon-Yong; Lee, Zonghoon
      Advanced Science (Weinheim, Germany) (2020), 7 (15), 2000788CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Antiphase boundaries (APBs) in 2D transition metal dichalcogenides have attracted wide interest as 1D metallic wires embedded in a semiconducting matrix, which could be exploited in fully 2D-integrated circuits. Here, the anisotropic morphologies of APBs (i.e., linear and saw-toothed APBs) in the nanoscale are investigated. The exptl. and computational results show that despite their anisotropic nanoscale morphologies, all APBs adopt a predominantly chalcogen-oriented dense structure to maintain the energetically most stable at. configuration. Moreover, the effect of the nanoscale morphol. of an APB on electron transport from two-probe field effect transistor measurements is investigated. A saw-toothed APB has a considerably lower electron mobility than a linear APB, indicating that kinks between facets are the main factors of scattering. The observations contribute to the systematical understanding of the faceted APBs and its impact on elec. transport behavior and it could potentially extend the applications of 2D materials through defect engineering to achieve the desired properties.
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    43. 43
      Zhou, S.; Wang, S.; Shi, Z.; Sawada, H.; Kirkland, A. I.; Li, J.; Warner, J. H. Atomically sharp interlayer stacking shifts at anti-phase grain boundaries in overlapping MoS2 secondary layers. Nanoscale 2018, 10, 16692– 16702,  DOI: 10.1039/c8nr04486d
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      43
      Atomically sharp interlayer stacking shifts at anti-phase grain boundaries in overlapping MoS2 secondary layers
      Zhou, Si; Wang, Shanshan; Shi, Zhe; Sawada, Hidetaka; Kirkland, Angus I.; Li, Ju; Warner, Jamie H.
      Nanoscale (2018), 10 (35), 16692-16702CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      When secondary domains nucleate and grow on the surface of monolayer MoS2, they can extend across grain boundaries in the underlying monolayer MoS2 and form overlapping sections. We present an at. level study of overlapping antiphase grain boundaries (GBs) in MoS2 monolayer-bilayers using aberration-cor. annular dark field scanning transmission electron microscopy. In particular we focus on the antiphase GB within a monolayer and track its propagation through an overlapping bilayer domain. We show that this leads to an atomically sharp interface between 2H and 3R interlayer stacking in the bilayer region. We have studied the micro-nanoscale "meandering" of the antiphase GB in MoS2, which shows a directional dependence on the d. of 4 and 8 member ring defects, as well as sharp turning angles 90°-100° that are mediated by a special 8-member ring defect. D. functional theory has been used to explore the overlapping interlayer stacking around the antiphase GBs, confirming our exptl. findings. These results show that overlapping secondary bilayer MoS2 domains cause at. structure modification to underlying anti-phase GB sites to accommodate the van der Waals interactions.
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    44. 44
      Du, L.; Yu, H.; Liao, M.; Wang, S.; Xie, L.; Lu, X.; Zhu, J.; Li, N.; Shen, C.; Chen, P.; Yang, R.; Shi, D.; Zhang, G. Modulating PL and electronic structures of MoS2/graphene heterostructures via interlayer twisting angle. Appl. Phys. Lett. 2017, 111, 263106,  DOI: 10.1063/1.5011120
      Google Scholar
      44
      Modulating PL and electronic structures of MoS2/graphene heterostructures via interlayer twisting angle
      Du, Luojun; Yu, Hua; Liao, Mengzhou; Wang, Shuopei; Xie, Li; Lu, Xiaobo; Zhu, Jianqi; Li, Na; Shen, Cheng; Chen, Peng; Yang, Rong; Shi, Dongxia; Zhang, Guangyu
      Applied Physics Letters (2017), 111 (26), 263106/1-263106/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Stacking two-dimensional materials into van der Waals heterostructures with distinct interlayer twisting angles opens up new strategies for electronic structure and phys. property engineering. Here, we investigate how the interlayer twisting angles affect the photoluminescence (PL) and Raman spectra of the MoS2/graphene heterostructures. Based on a series of heterostructure samples with different interlayer twisting angles, we found that the PL and Raman spectra of the monolayer MoS2 in these heterostructures are strongly twisting angle dependent. When the interlayer twisting angle evolves from 0° to 30°, both the PL intensity and emission energy increase, while the splitting of the E2g Raman mode decreases gradually. The obsd. phenomena are attributed to the twisting angle dependent interlayer interaction and misorientation-induced lattice strain between MoS2 and graphene. (c) 2017 American Institute of Physics.
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    45. 45
      Liao, M.; Wu, Z.-W.; Du, L.; Zhang, T.; Wei, Z.; Zhu, J.; Yu, H.; Tang, J.; Gu, L.; Xing, Y.; Yang, R.; Shi, D.; Yao, Y.; Zhang, G. Twist angle-dependent conductivities across MoS2/graphene heterojunctions. Nat. Commun. 2018, 9, 4068,  DOI: 10.1038/s41467-018-06555-w
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      45
      Twist angle-dependent conductivities across MoS2/graphene heterojunctions
      Liao Mengzhou; Du Luojun; Zhang Tingting; Wei Zheng; Zhu Jianqi; Yu Hua; Tang Jian; Gu Lin; Yang Rong; Shi Dongxia; Zhang Guangyu; Liao Mengzhou; Du Luojun; Zhang Tingting; Wei Zheng; Zhu Jianqi; Yu Hua; Tang Jian; Gu Lin; Yang Rong; Shi Dongxia; Zhang Guangyu; Wu Ze-Wen; Zhang Tingting; Xing Yanxia; Yao Yugui; Yang Rong; Shi Dongxia; Zhang Guangyu; Zhang Guangyu
      Nature communications (2018), 9 (1), 4068 ISSN:.
      Van der Waals heterostructures stacked from different two-dimensional materials offer a unique platform for addressing many fundamental physics and construction of advanced devices. Twist angle between the two individual layers plays a crucial role in tuning the heterostructure properties. Here we report the experimental investigation of the twist angle-dependent conductivities in MoS2/graphene van der Waals heterojunctions. We found that the vertical conductivity of the heterojunction can be tuned by ∼5 times under different twist configurations, and the highest/lowest conductivity occurs at a twist angle of 0°/30°. Density functional theory simulations suggest that this conductivity change originates from the transmission coefficient difference in the heterojunctions with different twist angles. Our work provides a guidance in using the MoS2/graphene heterojunction for electronics, especially on reducing the contact resistance in MoS2 devices as well as other TMDCs devices contacted by graphene.
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    46. 46
      Büch, H.; Rossi, A.; Forti, S.; Convertino, D.; Tozzini, V.; Coletti, C. Superlubricity of epitaxial monolayer WS2 on graphene. Nano Res. 2018, 11, 5946– 5956,  DOI: 10.1007/s12274-018-2108-7
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      There is no corresponding record for this reference.
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      Azizi, A.; Eichfeld, S.; Geschwind, G.; Zhang, K.; Jiang, B.; Mukherjee, D.; Hossain, L.; Piasecki, A. F.; Kabius, B.; Robinson, J. A.; Alem, N. Freestanding van der Waals Heterostructures of Graphene and Transition Metal Dichalcogenides. ACS Nano 2015, 9, 4882– 4890,  DOI: 10.1021/acsnano.5b01677
      Google Scholar
      47
      Freestanding van der Waals Heterostructures of Graphene and Transition Metal Dichalcogenides
      Azizi, Amin; Eichfeld, Sarah; Geschwind, Gayle; Zhang, Kehao; Jiang, Bin; Mukherjee, Debangshu; Hossain, Lorraine; Piasecki, Aleksander F.; Kabius, Bernd; Robinson, Joshua A.; Alem, Nasim
      ACS Nano (2015), 9 (5), 4882-4890CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Vertical stacking of two-dimensional (2D) crystals has recently attracted substantial interest due to unique properties and potential applications they can introduce. However, little is known about their microstructure because fabrication of the 2-dimensional heterostructures on a rigid substrate limits one's ability to directly study their at. and chem. structures using electron microscopy. This study demonstrates a unique approach to create atomically thin freestanding van der Waals heterostructures-WSe2/graphene and MoS2/graphene-as ideal model systems to study the nucleation and growth mechanisms in heterostructures. The authors use TEM imaging and diffraction to show epitaxial growth of the freestanding WSe2/graphene heterostructure, while no epitaxy is maintained in the MoS2/graphene heterostructure. Ultra-high-resoln. aberration-cor. scanning TEM (STEM) shows growth of monolayer WSe2 and MoS2 triangles on graphene membranes and reveals their edge morphol. and crystallinity. Photoluminescence measurements indicate a significant quenching of the photoluminescence response for the transition metal dichalcogenides on freestanding graphene, compared to those on a rigid substrate, such as sapphire and epitaxial graphene. Using a combination of (S)TEM imaging and electron diffraction anal., this study also reveals the significant role of defects on the heterostructure growth. The direct growth technique applied here enables the authors to study the heterostructure nucleation and growth mechanisms at the at. level without sample handling and transfer. Importantly, this approach can be used to study a wide spectrum of van der Waals heterostructures.
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    48. 48
      Shi, J.; Liu, M.; Wen, J.; Ren, X.; Zhou, X.; Ji, Q.; Ma, D.; Zhang, Y.; Jin, C.; Chen, H.; Deng, S.; Xu, N.; Liu, Z.; Zhang, Y. All Chemical Vapor Deposition Synthesis and Intrinsic Bandgap Observation of MoS2/Graphene Heterostructures. Adv. Mater. 2015, 27, 7086– 7092,  DOI: 10.1002/adma.201503342
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      48
      All Chemical Vapor Deposition Synthesis and Intrinsic Bandgap Observation of MoS2/Graphene Heterostructures
      Shi, Jianping; Liu, Mengxi; Wen, Jinxiu; Ren, Xibiao; Zhou, Xiebo; Ji, Qingqing; Ma, Donglin; Zhang, Yu; Jin, Chuanhong; Chen, Huanjun; Deng, Shaozhi; Xu, Ningsheng; Liu, Zhongfan; Zhang, Yanfeng
      Advanced Materials (Weinheim, Germany) (2015), 27 (44), 7086-7092CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      By exploiting an all-CVD approach, the authors have successfully fabricated large area, thickness uniform, and high quality MoS2 /graphene vertical heterostructures with clean interfaces on conventional Au foil substrates. Spectroscopic characterizations reveal that the in-between graphene weakens the interface interaction between MoS2 and Au. This to some extend provide us a unique pathway for constructing a nearly freestanding MoS2 monolayer, which is evidenced by its very weak n-doping level and its intrinsic band feature according to STM/STS characterizations. Moreover, the MoS2/Graphene heterostructure is transferable onto arbitrary substrates, which will stimulate further in-depth investigations on the intrinsic electronic properties of monolayer MoS2 , and facilitate the development of ground breaking optoelectronic devices.
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    49. 49
      Miwa, J. A.; Dendzik, M.; Grønborg, S. S.; Bianchi, M.; Lauritsen, J. V.; Hofmann, P.; Ulstrup, S. Van der Waals Epitaxy of Two-Dimensional MoS2–Graphene Heterostructures in Ultrahigh Vacuum. ACS Nano 2015, 9, 6502– 6510,  DOI: 10.1021/acsnano.5b02345
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      49
      Van der Waals Epitaxy of Two-Dimensional MoS2-Graphene Heterostructures in Ultrahigh Vacuum
      Miwa, Jill A.; Dendzik, Maciej; Groenborg, Signe S.; Bianchi, Marco; Lauritsen, Jeppe V.; Hofmann, Philip; Ulstrup, Soeren
      ACS Nano (2015), 9 (6), 6502-6510CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      In this work, we demonstrate direct van der Waals epitaxy of MoS2-graphene heterostructures on a semiconducting silicon carbide (SiC) substrate under ultrahigh vacuum conditions. Angle-resolved photoemission spectroscopy (ARPES) measurements show that the electronic structure of free-standing single-layer (SL) MoS2 is retained in these heterostructures due to the weak van der Waals interaction between adjacent materials. The MoS2 synthesis is based on a reactive phys. vapor deposition technique involving Mo evapn. and sulfurization in a H2S atm. on a template consisting of epitaxially grown graphene on SiC. Using scanning tunneling microscopy, we study the seeding of Mo on this substrate and the evolution from nanoscale MoS2 islands to SL and bilayer (BL) MoS2 sheets during H2S exposure. Our ARPES measurements of SL and BL MoS2 on graphene reveal the coexistence of the Dirac states of graphene and the expected valence band of MoS2 with the band max. shifted to the corner of the Brillouin zone at ‾K in the SL limit. We confirm the 2D character of these electronic states via a lack of dispersion with photon energy. The growth of epitaxial MoS2-graphene heterostructures on SiC opens new opportunities for further in situ studies of the fundamental properties of these complex materials, as well as perspectives for implementing them in various device schemes to exploit their many promising electronic and optical properties.
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    50. 50
      Ago, H.; Endo, H.; Solís-Fernández, P.; Takizawa, R.; Ohta, Y.; Fujita, Y.; Yamamoto, K.; Tsuji, M. Controlled van der Waals Epitaxy of Monolayer MoS2 Triangular Domains on Graphene. ACS Appl. Mater. Interfaces 2015, 7, 5265– 5273,  DOI: 10.1021/am508569m
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      50
      Controlled van der Waals Epitaxy of Monolayer MoS2 Triangular Domains on Graphene
      Ago, Hiroki; Endo, Hiroko; Solis-Fernandez, Pablo; Takizawa, Rina; Ohta, Yujiro; Fujita, Yusuke; Yamamoto, Kazuhiro; Tsuji, Masaharu
      ACS Applied Materials & Interfaces (2015), 7 (9), 5265-5273CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Multilayered heterostructures of two-dimensional materials have recently attracted increased interest because of their unique electronic and optical properties. Here, the authors present CVD growth of triangular crystals of monolayer MoS2 on single-cryst. hexagonal graphene domains which are also grown by CVD. MoS2 grows selectively on the graphene domains rather than on the bare supporting SiO2 surface. Reflecting the heteroepitaxy of the growth process, the MoS2 domains grown on graphene present two preferred equiv. orientations. The interaction between the MoS2 and the graphene induced an upshift of the Raman G and 2-dimensional bands of the graphene, while significant photoluminescence quenching was obsd. for the monolayer MoS2. Also, photoinduced current modulation along with an optical memory effect was demonstrated for the MoS2-graphene heterostructure. The authors' work highlights that heterostructures synthesized by CVD offer an effective interlayer van der Waals interaction which can be developed for large-area multilayer electronic and photonic devices.
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    51. 51
      Liu, X.; Balla, I.; Bergeron, H.; Campbell, G. P.; Bedzyk, M. J.; Hersam, M. C. Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene. ACS Nano 2016, 10, 1067– 1075,  DOI: 10.1021/acsnano.5b06398
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      51
      Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene
      Liu, Xiaolong; Balla, Itamar; Bergeron, Hadallia; Campbell, Gavin P.; Bedzyk, Michael J.; Hersam, Mark C.
      ACS Nano (2016), 10 (1), 1067-1075CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Atomically thin MoS2/graphene heterostructures are promising candidates for nanoelectronic and optoelectronic technologies. Among different graphene substrates, epitaxial graphene (EG) on SiC provides several potential advantages for such heterostructures, including high electronic quality, tunable substrate coupling, wafer-scale processability, and cryst. ordering that can template commensurate growth. Exploiting these attributes, the authors demonstrate here the thickness-controlled van der Waals epitaxial growth of MoS2 on EG via CVD, giving rise to transfer-free synthesis of a two-dimensional heterostructure with registry between its constituent materials. The rotational commensurability obsd. between the MoS2 and EG is driven by the energetically favorable alignment of their resp. lattices and results in nearly strain-free MoS2, as evidenced by synchrotron x-ray scattering and at.-resoln. scanning tunneling microscopy (STM). The electronic nature of the MoS2/EG heterostructure is elucidated with STM and scanning tunneling spectroscopy, which reveals bias-dependent apparent thickness, band bending, and a reduced band gap of ∼0.4 eV at the monolayer MoS2 edges.
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  • Supporting Information

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03002.

    • Fabrication of suspended graphene and drop casting of the MoS2 precursor, in situ STEM and MoS2 orientation analysis, training data, images of the in situ heating holder and chip with suspended graphene, time-elapsed HAADF-STEM images of mobile Mo atoms on graphene at 500 °C, time-elapsed HAADF-STEM images of the roto-translational motion of MoS2 on graphene at 500 °C, atomic-resolution TEM image of MoS2 clusters on graphene and histogram of misorientation angles, HAADF-STEM image of MoS2 clusters, architecture of the DeepSTEM framework and Appendix A: autoencoder (PDF)


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