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Atomic Point Contact Raman Spectroscopy of a Si(111)-7 × 7 Surface
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  • Shuyi Liu
    Shuyi Liu
    Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6, Berlin 14195, Germany
    More by Shuyi Liu
  • Adnan Hammud
    Adnan Hammud
    Department of Inorganic Chemistry, Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6, Berlin 14195, Germany
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  • Martin Wolf
    Martin Wolf
    Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6, Berlin 14195, Germany
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  • Takashi Kumagai*
    Takashi Kumagai
    Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6, Berlin 14195, Germany
    Center for Mesoscopic Sciences, Institute for Molecular Science, Okazaki 444-8585, Japan
    *Email: [email protected]
    More by Takashi Kumagai
    Orcidhttp://orcid.org/0000-0001-7029-062X
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Nano Letters

Cite this: Nano Lett. 2021, 21, 9, 4057–4061
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https://doi.org/10.1021/acs.nanolett.1c00998
Published May 2, 2021

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

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Abstract

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Tip-enhanced Raman scattering (TERS) has recently demonstrated the exceptional sensitivity to observe vibrational structures on the atomic scale. However, it strongly relies on electromagnetic enhancement in plasmonic nanogaps. Here, we demonstrate that atomic point contact (APC) formation between a plasmonic tip and the surface of a bulk Si sample can lead to a dramatic enhancement of Raman scattering and consequently the phonons of the reconstructed Si(111)-7 × 7 surface can be detected. Furthermore, we demonstrate the chemical sensitivity of APC-TERS by probing local vibrations resulting from Si–O bonds on the partially oxidized Si(111)-7 × 7 surface. This approach will expand the applicability of ultrasensitive TERS, exceeding the previous measurement strategies that exploit intense gap-mode plasmons, typically requiring a plasmonic substrate.

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Plasmonic nanostructures can be used to manipulate electromagnetic fields well below the diffraction limit and to largely enhance light–matter interactions, offering manifold applications in nanoscale science and technology. (1) The plasmonic field confinement at the scale of tens of nanometers is readily achieved with various nanostructures. (2) Even more tightly confined fields can be generated in ultrathin dielectric gaps (<1 nm) between metals, (3) although the precise and reproducible control of such tiny gaps is a challenging task. The extreme confinement of plasmonic fields has attracted increasing attention in nanophotonics due to potential applications in nonlinear nanooptics, cavity optomechanics, quantum optoelectronics, ultrasensitive and high-spatial-resolution optical spectroscopy. (4) In particular, the crucial role of atomic-scale structures in the field confinement and enhancement has been highlighted in a plasmonic “picocavity”, where the ångström-scale field confinement occurs at atomic-scale protrusions. (5−7) However, in experiments, the stability of atomic-scale protrusions is a key issue to exploit atomistic plasmonic fields. (8) Cooling the system to cryogenic temperature provides exceptional thermal stability. Remarkably, low-temperature tip-enhanced Raman spectroscopy (TERS) has recently proved the unprecedented chemical sensitivity with even submolecular spatial resolution. (9−11) TERS is a promising technique in wide-ranging fields including single-molecule spectroscopy, (9−11) electrochemistry, (12,13) heterogeneous catalysis, (14,15) biomolecular identification, (16−18) and 2D materials characterization, (19−23) bearing a great potential as nanoscale chemical microscopy. In most cases, however, the underlying enhancement mechanism of the Raman scattering intensity relies largely on the extreme field enhancement in plasmonic nanogaps. (24−27) This imposes a severe limitation on measurable systems, thus typically requiring a plasmonic substrate. As an attempt to overcome this obstacle, here we demonstrate that an atomic point contact (APC) between a plasmonic tip and a nonplasmonic surface can dramatically enhance Raman scattering and that the phonon modes of the Si(111)-7 × 7 surface can be observed.

Figure 1a depicts the schematic of the experiment. The STM junction consisting of a Ag tip and the reconstructed Si(111)-7 × 7 surface at 10 K is illuminated by a 633(532) nm narrow-band laser, generating a strong plasmonic field localized on the tip. Typically, the tip has an apex diameter of several tens of nm which couples the propagating electromagnetic field through localized surface plasmon excitation. Additional confinement will occur at the atomic-scale protrusion (single atoms) existing at the end of the tip, yielding the extreme field confinement and enhancement through the atomic-scale lightning effect. (6,7,28) The Raman signal from the junction is collected in the backscattering geometry. Figure 1b shows a scanning tunneling microscopy (STM) image of the empty states of the Si(111)-7 × 7 surface scanned under illumination. We found that the illumination during scanning creates atomic defects in the topmost layer, which appear as a pair of bright and dark spots (Figure 1c). These defects occur only in faulted half unit cells (FHUCs) of the reconstructed Si(111)-7 × 7 surface. (29) The dark/bright pair is attributed to the removal and subsequent rebinding of a Si adatom. A similar illumination-induced defect formation was reported previously. (30,31) The Si(111)-7 × 7 surface exhibits a surface state near the Fermi level and undergoes a transition from metallic to insulating behavior below ∼20 K. (32) The surface state under illumination is confirmed by scanning tunneling spectroscopy (STS) as shown in Figure 1d, and similar spectra were obtained under illumination of 633 nm. Although the appearance of the surface state is consistent with observations for the nonilluminated surface, its energy is down-shifted by ∼0.5 eV in the STS under illumination. (32) This suggests that the surface is positively charged, causing band bending at the surface. (33) As the FHUCs have a larger local density of states near the Fermi level than that of unfaulted half unit cells (UHUCs), (32) the illumination may result in more charging in FHUCs, which could promote the bond rupture of the Si adatoms.

Figure 1

Figure 1. (a) Schematic of the experiment. EAS illustrates the extreme plasmonic field occurring at the atomic-scale protrusion at the tip apex. (b) STM image of Si(111)-7 × 7 under illumination (10 K, Vbias = 0.3 V, ISTM = 1 nA, λext = 633 nm, Pext = 0.7 W/cm2, scale bar = 5 nm). (c) Enlarged STM image (scale bar = 1 nm). The bright/dark pairs are indicated by the dashed ellipses. The faulted/unfaulted half unit cell is indicated by the pink/blue triangle. (d) Conductance spectrum measured under illumination (10 K, set-point: Vbias = 0.5 V, ISTM = 0.1 nA, λext = 532 nm, Pext = 0.15 mW/μm2).

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Figure 2a shows a waterfall plot of the TERS spectra recorded while vertically approaching and retracting the tip to and from a corner adatom in an UHUC. The vertical and horizontal axes correspond to the tip–surface distance (Δz) and the Raman shift, respectively, and the color scale represents the Raman intensity. At large Δz only the optical phonon mode of the bulk Si is observed at 520 cm–1 (top panel of Figure 2a). (34) No characteristic TERS signals appear as the Δz decreases in the tunneling regime, however, a sudden, dramatic change takes place when the tip contacts the surface, whereas the intensity of the bulk mode is not affected. As can be seen in the current–distance (ISTM–Δz) curve in the left panel of Figure 2a, the ISTM increases (decreases) monotonically in the tunneling regime, but it saturates when the contact between the tip apex and the surface is formed. This is a typical behavior observed for the APC formation. (35) In the following, we refer to the TERS measurement in the contact regime as APC-TERS.

Figure 2

Figure 2. (a) Water fall plot of TERS recorded during tip-approach and retraction over Si(111)-7 × 7 (10 K, Vbias = 0 V, λext = 633 nm, Pext = 0.7 mW/μm2). The left panel shows the simultaneously recorded ISTM–Δz curve. Although the Vbias is nominally set to zero, the current occurs due to the photovoltage under illumination. The red shaded region indicates the APC. The top and bottom panels display the TERS spectra in the tunneling and contact regimes, respectively. (b) TERS spectra obtained for 11 different APCs (red) recorded for different location (UHUCs) and different tip conditions. The black one shows the averaged spectrum. The spectra are normalized to its area from range of Raman shift 0–500 cm–1.

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The onset of the saturation of the ISTM coincides with the dramatic changes in TERS. In the APC regime, characteristic Raman peaks appear at 100–450 cm–1 and at 550 cm–1 which are assigned to surface phonons of the Si(111)-7 × 7 surface (as discussed below). The strong TERS signals disappear when the tip is retracted and the tip–Si bond is broken. It should be noted that, after APC-TERS measurement and retraction of the tip, a small Ag cluster always appeared on the surface (Figure S1), indicating a bond formation between the tip apex atoms and the Si surface. This causes the asymmetric behavior of the TER spectra and the ISTM–Δz curve with respect to the turning point of the sequential approach and retraction (Δz = −3 Å) as seen in Figure 2a. Although the exact peak positions and the intensities of APC-TERS depend on the tip conditions, the dramatic enhancement is highly reproducible (Figure S2). Furthermore, spectral shifts of the peaks occur as the tip is further squeezed into the surface (as well as during retraction), which potentially result from mechanical stress and/or charge transfer (36) accompanied by structural deformation of the junction.

To evaluate the APC-TERS spectra of the Si(111)-7 × 7 surface, we selected the spectrum at the moment of the APC formation from repeated measurements because at this point the surface structure should not be disturbed significantly. The reproducibility of these spectra is confirmed by repeating the measurement under different tip conditions (Figure 2b). A small difference in the peak positions can be attributed to slightly different bonding geometries of the APC. We also confirmed that an Au tip gives the similar results (Figure S3), corroborating that the observed peaks in APC-TERS can be assigned to the vibrations of the Si surface. In the averaged spectrum, peaks at 150, 210, 310, 400, and 550 cm–1 are resolved, which are in good agreement with the simulated surface phonon modes of Si(111)-7 × 7. (37) The observed Raman peaks originate from the phonons that extend over a substantial momentum space in the Brilliouin zone due to spatial localization of the confined field in the APC, leading to a large momentum uncertainty. Therefore, the surface phonons observed in APC-TERS appear broader than the bulk phonon at 520 cm–1. A similar mechanism has been proposed for near-field optical excitation of Si. (38,39) In addition, the peaks between 150 and 380 cm–1 might be further broadened by coupling to the Ag phonons of the tip, which serves as an additional damping channel. (40)

The enhancement of TERS is generally attributed to two mechanisms, namely electromagnetic (EM) (41) and charge-transfer (chemical) (42) effects. The APC formation results in hybridization between the Ag atom at the tip apex and the surface Si atom(s), which modifies the electronic structure. This may significantly affect the Raman scattering cross section through the charge-transfer mechanism. (43,44) As discussed in our previous publication, (45) the reactivity of the surface, and thus the degree of the hybridization, has a decisive impact on the enhancement caused by the ACP formation. The important role of surface reactivity is further corroborated by examining ACP-TERS for a highly oriented pyrolytic graphite (HOPG), a very inert surface, which does not show dramatic enhancement upon a contact formation, as weak hybridization will not lead to changes in the electronic structure. The EM enhancement mechanism is also operative as the intensity of APC-TERS is correlated with the plasmon response of the tip (Figure S4). In addition to these two general mechanisms, the current-driven Raman scattering process in TERS was also discussed recently, (46) which should also play a role for APC-TERS. Additionally, light–matter coupling should be further enhanced in the vicinity of the APC because of extreme (atomic scale) field confinement. We find that the Raman enhancement is significantly reduced as the tip is further squeezed into the surface by ∼2 Å from the onset of APC formation (Figure S5). This implies attenuation of the atomic-scale field caused by a change of the APC configuration (probably breakdown of the single APC).

APC-TERS measured over different Si adatoms do not exhibit significant differences in either FHUCs or UHUCs. Most probably their subtle spectral differences are overwhelmed by different tip conditions (slightly different geometries of the APC). However, clear spectral changes can be observed when APC-TERS is recorded at step edges (Figure 3a and Figure S6) where the peak positions are clearly shifted compared to those over the flat surface. Especially, more peaks appear around ∼400 cm–1, where the collective mode of the surface vibrations is located. (47) In contrast to the other modes that are more localized at the adatom (at 150, 210, 310 cm–1), the collective mode involves the substantial motion of the atoms in the underneath layers. (37) This mode could be split at the step edge because of the symmetry reduction (lateral periodicity of the surface perpendicular to the step direction is broken), leading to multiple peaks.

Figure 3

Figure 3. (a) APC-TERS at the step edge of the Si(111)-7 × 7 surface (10 K, Vbias = 0 V, λext = 633 nm, Pext = 0.7 mW/um2). The gray dashed line is the averaged spectrum over the terrace. The inset shows the STM image of the step edge. (b) APC-TERS at oxidized region of the Si(111)-7 × 7 surface (10 K, Vbias = 0 V, λext = 532 nm, Pext = 1 mW/μm2). The left panel shows the simultaneously recorded ISTM–Δz curve. The red shaded region indicates the APC. The top and bottom panels display the TERS spectra in the tunneling and contact regimes, respectively. The STM image of the partially oxidized surface is shown in top-left (scale bar is 2 nm). The position for APC-TERS measurement is indicated by the arrow.

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To test the chemical sensitivity of APC-TERS, we also investigated the partially oxidized Si(111)-7 × 7 surface. The dark areas in the inset STM image (top-left) of Figure 3b correspond to the surface oxide. (48) When APC-TERS is recorded at this dark area, three peaks are observed at higher frequency (>650 cm–1) where no Raman signals occur from the bare Si(111)-7 × 7 surface. The peak at 683 cm–1 involves the inserted O atom, denoting as O(ins), moving perpendicular to the Si–O(ins)–Si bond and the peak at 989 cm–1 results from the O atom moving along this bond. (49,50) The peak at 1108 cm–1 may be assigned to the stretching of Si–O(ad) in which the O atom is chemically adsorbed on the adatom. This mode is red-shifted in APC-TERS compared to other experiments, (50−52) possibly because the bond is softened by the contact with the Ag tip (causing charge transfer from the Si surface). We found that these spectral signatures are characteristic for the dark (oxidized) areas and that multiple peaks are observed at the range of stretching of Si–O(ins)–Si (see also Figure S7), indicating further oxidized products. Because of the destructive nature of the APC (Figure S1), Raman imaging was impossible. However, the above results demonstrate that APC-TERS will be capable of studying local chemical structures on the atomic scale by combing with STM images. It may be possible to record APC-TERS mapping for a moderately reactive tip/surface and a controlled modification of the tip apex, for example, attaching a single molecule, would be a potential approach.

APC-TERS is, in principle, applicable to nonplasmonic substrates and the exceptional sensitivity will be obtained for many other materials. The strong TERS signal only appeared for the APC configuration, suggesting that atomic-scale structures are indispensable to understand light–matter interactions in metal–semiconductor hybrid nanosystems.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.1c00998.

  • Methods, small Ag clusters created after APC-TERS measurement, TERS for different APCs over Si(111)-7 × 7, APC-TERS of Si(111)-7 × 7 measured with an Au tip, LSPR dependence of APC-TERS, wavelength dependence of APC-TERS and attenuation of the intensity in the contact regime, APC-TERS at step edges of Si(111)-7 × 7, APC-TERS over partially oxidized Si(111)-7 × 7 (PDF)

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Author Information

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  • Corresponding Author
    • Takashi Kumagai - Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6, Berlin 14195, Germany;  Center for Mesoscopic Sciences, Institute for Molecular Science, Okazaki 444-8585, Japan;  Orcidhttp://orcid.org/0000-0001-7029-062X;  Email: [email protected]
  • Authors
    • Shuyi Liu - Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6, Berlin 14195, Germany
    • Adnan Hammud - Department of Inorganic Chemistry, Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6, Berlin 14195, Germany
    • Martin Wolf - Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6, Berlin 14195, Germany
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors thank Robert Schlögl for supporting the production of the Ag tips using focused ion beam. T.K. acknowledges the support by JST-PRESTO (JPMJPR16S6).

References

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    The internal vibrations of mols. drive the structural transformations that underpin chem. and cellular function. While vibrational frequencies are measured by spectroscopy, the normal modes of motion are inferred through theory because their visualization would require microscopy with angstrom-scale spatial resoln.-nearly three orders of magnitude smaller than the diffraction limit in optics1. Using a metallic tip to focus light and taking advantage of the surface-enhanced Raman effect to amplify the signal from individual mols., tip-enhanced Raman spectromicroscopy (TER-SM) reaches the requisite sub-mol. spatial resoln., confirming that light can be confined in picocavities and anticipating the direct visualization of mol. vibrations. Here, by using TER-SM at the precisely controllable junction of a cryogenic ultrahigh-vacuum scanning tunnelling microscope, we show that angstrom-scale resoln. is attained at subat. sepn. between the tip atom and a mol. in the quantum tunnelling regime of plasmons. We record vibrational spectra within a single mol., obtain images of normal modes and atomically parse the intramol. charges and currents driven by vibrations. Our anal. provides a paradigm for optics in the atomistic near-field.
  10. 10
    Zhang, Y.; Yang, B.; Ghafoor, A.; Zhang, Y.; Zhang, Y.-F.; Wang, R.-P.; Yang, J.-L.; Luo, Y.; Dong, Z.-C.; Hou, J G Visually Constructing the Chemical Structure of a Single Molecule by Scanning Raman Picoscopy. Natl. Sci. Rev. 2019, 6, 1169,  DOI: 10.1093/nsr/nwz180
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    10
    Visually constructing the chemical structure of a single molecule by scanning Raman picoscopy
    Zhang, Yao; Yang, Ben; Ghafoor, Atif; Zhang, Yang; Zhang, Yu-Fan; Wang, Rui-Pu; Yang, Jin-Long; Luo, Yi; Dong, Zhen-Chao; Hou, J. G.
    National Science Review (2019), 6 (6), 1169-1175CODEN: NSRACI; ISSN:2053-714X. (Oxford University Press)
    The strong spatial confinement of a nanocavity plasmonic field has made it possible to visualize the inner structure of a single mol. and even to distinguish its vibrational modes in real space. With such ever-improved spatial resoln., it is anticipated that full vibrational imaging of a mol. could be achieved to reveal mol. structural details. Here we demonstrate full Raman images of individual vibrational modes at the angstr.ovrddot.om level for a single Mg-porphine mol., revealing distinct characteristics of each vibrational mode in real space. Furthermore, by exploiting the underlying interference effect and Raman fingerprint database, we propose a new methodol. for structural detn., which we have called 'scanning Raman picoscopy', to show how such ultrahigh-resoln. spectromicroscopic vibrational images can be used to visually assemble the chem. structure of a single mol. through a simple Lego-like building process.
  11. 11
    Jaculbia, R. B. Single-molecule resonance Raman effect in a plasmonic nanocavity. Nat. Nanotechnol. 2020, 15, 105– 110,  DOI: 10.1038/s41565-019-0614-8
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    11
    Single-molecule resonance Raman effect in a plasmonic nanocavity
    Jaculbia, Rafael B.; Imada, Hiroshi; Miwa, Kuniyuki; Iwasa, Takeshi; Takenaka, Masato; Yang, Bo; Kazuma, Emiko; Hayazawa, Norihiko; Taketsugu, Tetsuya; Kim, Yousoo
    Nature Nanotechnology (2020), 15 (2), 105-110CODEN: NNAABX; ISSN:1748-3387. (Nature Research)
    Tip-enhanced Raman spectroscopy (TERS) is a versatile tool for chem. anal. at the nanoscale. In earlier TERS expts., Raman modes with components parallel to the tip were studied based on the strong elec. field enhancement along the tip. Perpendicular modes were usually neglected. Here, we investigate an isolated copper naphthalocyanine mol. adsorbed on a triple-layer NaCl on Ag(111) using scanning tunnelling microscope TERS imaging. For flat-lying mols. on NaCl, the Raman images present different patterns depending on the symmetry of the vibrational mode. Our results reveal that components of the elec. field perpendicular to the tip should be considered aside from the parallel components. Moreover, under resonance excitation conditions, the perpendicular components can play a substantial role in the enhancement. This single-mol. study in a well-defined environment provides insights into the Raman process at the plasmonic nanocavity, which may be useful in the nanoscale metrol. of various mol. systems.
  12. 12
    Zeng, Z.-C. Electrochemical Tip-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 2015, 137 (37), 11928– 11931,  DOI: 10.1021/jacs.5b08143
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    12
    Electrochemical Tip-Enhanced Raman Spectroscopy
    Zeng, Zhi-Cong; Huang, Sheng-Chao; Wu, De-Yin; Meng, Ling-Yan; Li, Mao-Hua; Huang, Teng-Xiang; Zhong, Jin-Hui; Wang, Xiang; Yang, Zhi-Lin; Ren, Bin
    Journal of the American Chemical Society (2015), 137 (37), 11928-11931CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Interfacial properties are highly important to the performance of some energy-related systems. The in-depth understanding of the interface requires highly sensitive in situ techniques that can provide fingerprint mol. information at nanometer resoln. The authors developed an electrochem. tip-enhanced Raman spectroscopy (EC-TERS) by introduction of the light horizontally to the EC-STM cell to minimize the optical distortion and to keep the TERS measurement under a well-controlled condition. The authors obtained potential-dependent EC-TERS from the adsorbed arom. mol. on a Au(111) surface and obsd. a substantial change in the mol. configuration with potential as a result of the protonation and deprotonation of the mol. Such a change was not observable in EC-SERS (surface-enhanced), indicating EC-TERS can more faithfully reflect the fine interfacial structure than EC-SERS. This work will open a new era for using EC-TERS as an important nanospectroscopy tool for the mol. level and nanoscale anal. of some important electrochem. systems including solar cells, lithium ion batteries, fuel cells, and corrosion.
  13. 13
    Martín Sabanés, N.; Ohto, T.; Andrienko, D.; Nagata, Y.; Domke, K. F. Electrochemical TERS elucidates potential-induced molecular reorientation of adenine/Au(111). Angew. Chem., Int. Ed. 2017, 56, 9796– 9801,  DOI: 10.1002/anie.201704460
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    13
    Electrochemical TERS Elucidates Potential-Induced Molecular Reorientation of Adenine/Au(111)
    Martin Sabanes, Natalia; Ohto, Tatsuhiko; Andrienko, Denis; Nagata, Yuki; Domke, Katrin F.
    Angewandte Chemie, International Edition (2017), 56 (33), 9796-9801CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Electrochem. surface activity arises from the interaction and geometric arrangement of mols. at electrified interfaces. The authors present a novel electrochem. tip-enhanced Raman spectroscope that can access the vibrational fingerprint of <100 small, nonresonant mols. adsorbed at atomically flat Au electrodes to study their adsorption geometry and chem. reactivity as a function of the applied potential. Combining exptl. and simulation data for adenine/Au(111), protonated physisorbed adenine adopts a tilted orientation at low potentials, whereas it is vertically adsorbed around the potential of zero charge. Further potential increase induces adenine deprotonation and reorientation to a planar configuration. The extension of EC-TERS to the study of adsorbate reorientation significantly broadens the applicability of this advanced spectroelectrochem. tool for the nanoscale characterization of a full range of electrochem. interfaces.
  14. 14
    Zhong, J. Probing the electronic and catalytic properties of a bimetallic surface with 3 nm resolution. Nat. Nanotechnol. 2017, 12, 132– 136,  DOI: 10.1038/nnano.2016.241
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    14
    Probing the electronic and catalytic properties of a bimetallic surface with 3 nm resolution
    Zhong, Jin-Hui; Jin, Xi; Meng, Lingyan; Wang, Xiang; Su, Hai-Sheng; Yang, Zhi-Lin; Williams, Christopher T.; Ren, Bin
    Nature Nanotechnology (2017), 12 (2), 132-136CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    An at.- and mol.-level understanding of heterogeneous catalysis is required to characterize the nature of active sites and improve the rational design of catalysts. Achieving this level of characterization requires techniques that can correlate catalytic performances to sp. surface structures, so as to avoid averaging effects. Tip-enhanced Raman spectroscopy combines scanning probe microscopy with plasmon-enhanced Raman scattering and provides simultaneous topog. and chem. information at the nano/at. scale from ambient to ultrahigh-vacuum and electrochem. environments. Therefore, it has been used to monitor catalytic reactions and is proposed to correlate the local structure and function of heterogeneous catalysts. Bimetallic catalysts, such as Pd-Au, show superior performance in various catalytic reactions, but it has remained challenging to correlate structure and reactivity because of their structural complexity. Here, we show that TERS can chem. and spatially probe the site-specific chem. (electronic and catalytic) and phys. (plasmonic) properties of an atomically well-defined Pd(sub-monolayer)/Au(111) bimetallic model catalyst at 3 nm resoln. in real space using Ph isocyanide as a probe mol. (Fig. 1a). We observe a weakened N≃C bond and enhanced reactivity of Ph isocyanide adsorbed at the Pd step edge compared with that at the Pd terrace. D. functional theory corroborates these observations by revealing a higher d-band electronic profile for the low-coordinated Pd step edge atoms. The 3 nm spatial resoln. we demonstrate here is the result of an enhanced elec. field and distinct electronic properties at the step edges.
  15. 15
    Yin, H. Nanometre-scale spectroscopic visualization of catalytic sites during a hydrogenation reaction on a Pd/Au bimetallic catalyst. Nat. Catal. 2020, 3, 834– 842,  DOI: 10.1038/s41929-020-00511-y
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    15
    Nanometre-scale spectroscopic visualization of catalytic sites during a hydrogenation reaction on a Pd/Au bimetallic catalyst
    Yin, Hao; Zheng, Li-Qing; Fang, Wei; Lai, Yin-Hung; Porenta, Nikolaus; Goubert, Guillaume; Zhang, Hua; Su, Hai-Sheng; Ren, Bin; Richardson, Jeremy O.; Li, Jian-Feng; Zenobi, Renato
    Nature Catalysis (2020), 3 (10), 834-842CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
    Understanding the mechanism of catalytic hydrogenation at the local environment requires chem. and topog. information involving catalytic sites, active hydrogen species, and their spatial distribution. Here we used tip-enhanced Raman spectroscopy (TERS) to study the catalytic hydrogenation of chloronitrobenzenethiol on a well-defined Pd(submonolayer)/Au(111) bimetallic catalyst (pH2 = 1.5 bar, 298 K), where the surface topog. and chem. fingerprint information were simultaneously mapped with nanoscale resoln. (∼10 nm). TERS imaging of the surface after catalytic hydrogenation confirms that the reaction occurs beyond the location of Pd sites. The results demonstrate that hydrogen spillover accelerates hydrogenation at Au sites as far as 20 nm from the bimetallic Pd/Au boundary. D. functional theory was used to elucidate the thermodn. of interfacial hydrogen transfers. We demonstrate TERS to be a powerful anal. tool that provides a unique approach to spatially investigate the local structure-reactivity relationship in catalysis.
  16. 16
    Zhang, R. Distinguishing Individual DNA Bases in a Network by Non-Resonant Tip-Enhanced Raman Scattering. Angew. Chem., Int. Ed. 2017, 56 (20), 5561– 5564,  DOI: 10.1002/anie.201702263
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    16
    Distinguishing Individual DNA Bases in a Network by Non-Resonant Tip-Enhanced Raman Scattering
    Zhang, Rui; Zhang, Xianbiao; Wang, Huifang; Zhang, Yao; Jiang, Song; Hu, Chunrui; Zhang, Yang; Luo, Yi; Dong, Zhenchao
    Angewandte Chemie, International Edition (2017), 56 (20), 5561-5564CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The importance of identifying DNA bases at the single-mol. level is well recognized for many biol. applications. Although such identification can be achieved by elec. measurements using special setups, it is still not possible to identify single bases in real space by optical means owing to the diffraction limit. Herein, we demonstrate the outstanding ability of scanning tunneling microscope (STM)-controlled non-resonant tip-enhanced Raman scattering (TERS) to unambiguously distinguish two individual complementary DNA bases (adenine and thymine) with a spatial resoln. down to 0.9 nm. The distinct Raman fingerprints identified for the two mols. allow to differentiate in real space individual DNA bases in coupled base pairs. The demonstrated ability of non-resonant Raman scattering with super-high spatial resoln. will significantly extend the applicability of TERS, opening up new routes for single-mol. DNA sequencing.
  17. 17
    Lipiec, E.; Kaderli, J.; Kobierski, J.; Riek, R.; Skirlinska-Nosek, K.; Sofinska, K.; Szymonski, M.; Zenobi, R. Nanoscale Hyperspectral Imaging of Amyloid Secondary Structures in Liquid. Angew. Chem. 2021, 133, 4595,  DOI: 10.1002/ange.202010331
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    He, Z. Tip-Enhanced Raman Imaging of Single-Stranded DNA with Single Base Resolution. J. Am. Chem. Soc. 2019, 141 (2), 753– 757,  DOI: 10.1021/jacs.8b11506
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    18
    Tip-Enhanced Raman Imaging of Single-Stranded DNA with Single Base Resolution
    He, Zhe; Han, Zehua; Kizer, Megan; Linhardt, Robert J.; Wang, Xing; Sinyukov, Alexander M.; Wang, Jizhou; Deckert, Volker; Sokolov, Alexei V.; Hu, Jonathan; Scully, Marlan O.
    Journal of the American Chemical Society (2019), 141 (2), 753-757CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Tip-enhanced Raman scattering (TERS) is a promising optical and anal. technique for chem. imaging and sensing at single mol. resoln. In particular, TERS signals generated by a gap-mode configuration where a silver tip is coupled with a gold substrate can resolve a single-stranded DNA (ssDNA) mol. with a spatial resoln. below 1 nm. To demonstrate the proof of subnanometer resoln., the authors show direct nucleic acid sequencing using TERS of a phage ssDNA (M13mp18). M13mp18 provides a known sequence and, through the deposition strategy, can be stretched (uncoiled) and attached to the substrate by its phosphate groups, while exposing its nucleobases to the tip. After deposition, the authors scan the silver tip along the ssDNA and collect TERS signals with a step of 0.5 nm, comparable to the bond length between two adjacent DNA bases. By demonstrating the real-time profiling of a ssDNA configuration and furthermore, with unique TERS signals of monomeric units of other biopolymers, the authors anticipate that this technique can be extended to the high-resoln. imaging of various nanostructures as well as the direct sequencing of other important biopolymers including RNA, polysaccharides, and polypeptides.
  19. 19
    Beams, R.; Cançado, L. G.; Jorio, A.; Vamivakas, A. N.; Novotny, L. Tip-enhanced Raman mapping of local strain in graphene. Nanotechnology 2015, 26, 175702,  DOI: 10.1088/0957-4484/26/17/175702
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    19
    Tip-enhanced Raman mapping of local strain in graphene
    Beams Ryan; Cancado Luiz Gustavo; Jorio Ado; Vamivakas A Nick; Novotny Lukas
    Nanotechnology (2015), 26 (17), 175702 ISSN:.
    We demonstrate local strain measurements in graphene by using tip-enhanced Raman spectroscopy (TERS). We find that a single 5 nm particle can induce a radial strain over a lateral distance of ∼170 nm. By treating the particle as a point force on a circular membrane, we find that the strain in the radial direction (r) is .varies. r-(2 3),in agreement with force-displacement measurements conducted on suspended graphene flakes. Our results demonstrate that TERS can be used to map out static strain fields at the nanoscale, which are inaccessible using force-displacement techniques.
  20. 20
    Sheng, S. Vibrational Properties of a Monolayer Silicene Sheet Studied by Tip-Enhanced Raman Spectroscopy. Phys. Rev. Lett. 2017, 119 (19), 196803,  DOI: 10.1103/PhysRevLett.119.196803
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    20
    Vibrational properties of a monolayer silicene sheet studied by tip-enhanced Raman spectroscopy
    Sheng, Shaoxiang; Wu, Jiang-bin; Cong, Xin; Li, Wenbin; Gou, Jian; Zhong, Qing; Cheng, Peng; Tan, Ping-heng; Chen, Lan; Wu, Kehui
    Physical Review Letters (2017), 119 (19), 196803/1-196803/5CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
    Combining ultrahigh sensitivity, spatial resoln., and the capability to resolve chem. information, tip-enhanced Raman spectroscopy (TERS) is a powerful tool to study mols. or nanoscale objects. Here we show that TERS can also be a powerful tool in studying two-dimensional materials. We have achieved a 109 Raman signal enhancement and a 0.5 nm spatial resoln. using monolayer silicene on Ag(111) as a prototypical 2D material system. Because of the selective enhancement on Raman modes with vertical vibrational components in TERS, our expt. provides direct evidence of the origination of Raman modes in silicene. Furthermore, the ultrahigh sensitivity of TERS allows us to identify different vibrational properties of silicene phases, which differ only in the bucking direction of the Si-Si bonds. Local vibrational features from defects and domain boundaries in silicene can also be identified.
  21. 21
    Park, K.-D.; Khatib, O.; Kravtsov, V.; Clark, G.; Xu, X.; Raschke, M. B. Hybrid Tip-Enhanced Nanospectroscopy and Nanoimaging of Monolayer WSe2 with Local Strain Control. Nano Lett. 2016, 16 (4), 2621– 2627,  DOI: 10.1021/acs.nanolett.6b00238
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    21
    Hybrid Tip-Enhanced Nanospectroscopy and Nanoimaging of Monolayer WSe2 with Local Strain Control
    Park, Kyoung-Duck; Khatib, Omar; Kravtsov, Vasily; Clark, Genevieve; Xu, Xiaodong; Raschke, Markus B.
    Nano Letters (2016), 16 (4), 2621-2627CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Many classes of two-dimensional (2D) materials have emerged as potential platforms for novel electronic and optical devices. However, their phys. properties are strongly influenced by nanoscale heterogeneities in the form of edges, twin boundaries, and nucleation sites. Using combined tip-enhanced Raman scattering and photoluminescence (PL) nanospectroscopy and nanoimaging, we study the assocd. effects on the excitonic properties in monolayer WSe2 grown by phys. vapor deposition. With ∼15 nm spatial resoln., we resolve nanoscale correlations of PL spectral intensity and shifts with crystal edges and internal twin boundaries assocd. with the expected exciton diffusion length. Through an active at. force tip interaction we can control the crystal strain on the nanoscale and tune the local bandgap in reversible (up to 24 meV shift) and irreversible (up to 48 meV shift) fashion. This allows us to distinguish the effect of strain from the dominant influence of defects on the PL modification at the different structural heterogeneities. Hybrid nano-optical spectroscopy and imaging with nanomech. strain control thus enables the systematic study of the coupling of structural and mech. degrees of freedom to the nanoscale electronic and optical properties in layered 2D materials.
  22. 22
    Shao, F.; Dai, W.; Zhang, Y.; Zhang, W.; Schlüter, A. D.; Zenobi, R. Chemical Mapping of Nanodefects within 2D Covalent Monolayers by Tip-Enhanced Raman Spectroscopy. ACS Nano 2018, 12 (5), 5021– 5029,  DOI: 10.1021/acsnano.8b02513
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    22
    Chemical Mapping of Nanodefects within 2D Covalent Monolayers by Tip-Enhanced Raman Spectroscopy
    Shao, Feng; Dai, Wenyang; Zhang, Yao; Zhang, Wei; Schluter, A. Dieter; Zenobi, Renato
    ACS Nano (2018), 12 (5), 5021-5029CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Nanoscale defects in monolayers (MLs) of 2-dimensional (2D) materials, such as graphene, transition-metal dichalcogenides, and 2-dimensional polymers, can alter their phys., mech., optoelectronic, and chem. properties. However, detailed information about nanodefects within 2-dimensional covalent monolayers is difficult to obtain because it requires highly selective and sensitive techniques that can provide chem. information at the nanoscale. Here, the authors report a 2-dimensional imine-linked ML prepd. from 2 custom-designed building blocks by dynamic imine chem. at the air/H2O interface, in which an acetylenic moiety in one of the blocks was used as a spectroscopic reporter for nanodefects. Combined with d. functional theory calcns. that take into account surface selection rules, tip-enhanced Raman spectroscopy (TERS) imaging provides information on the chem. bonds, mol. orientation, as well as nanodefects in the resulting ML. Addnl., TERS imaging visualizes the topog. and integrity of the ML at Au(111) terrace edges, suggesting possible ductility of the ML. Also, edge-induced mol. tilting and a stronger signal enhancement were obsd. at the terrace edges, from which a spatial resoln. around 8 nm could be deduced. The present work can be used to study covalent 2-dimensional materials at the nanoscale, which are expected to be of use when engineering their properties for specific device applications.
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    Balois, M. V.; Hayazawa, N.; Yasuda, S.; Ikeda, K.; Yang, B.; Kazuma, E.; Yokota, Y.; Kim, Y.; Tanaka, T. Visualization of subnanometric phonon modes in a plasmonic nano-cavity via ambient tip-enhanced Raman spectroscopy. NPJ. 2D Mater. Appl. 2019, 3, 38,  DOI: 10.1038/s41699-019-0121-7
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    Pettinger, B.; Schambach, P.; Villagómez, C. J.; Scott, N. Tip-Enhanced Raman Spectroscopy: Near-Fields Acting on a Few Molecules. Annu. Rev. Phys. Chem. 2012, 63, 379– 399,  DOI: 10.1146/annurev-physchem-032511-143807
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    24
    Tip-enhanced Raman spectroscopy: near-fields acting on a few molecules
    Pettinger, Bruno; Schambach, Philip; Villagomez, Carlos J.; Scott, Nicola
    Annual Review of Physical Chemistry (2012), 63 (), 379-399CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews Inc.)
    A review. Tip-enhanced Raman spectroscopy (TERS) is a very powerful variant of surface-enhanced Raman spectroscopy (SERS). In a sense, TERS overcomes most of the drawbacks of SERS but keeps its advantages, such as its high sensitivity. TERS offers the addnl. advantages of high spatial resoln., much beyond the Abbe limit, and the possibility to correlate TER and other scanning probe microscope images, i.e., to correlate topog. and chem. data. TERS finds application in a no. of fields, such as surface science, material science, and biol. Single-mol. TERS has been obsd. even for TERS enhancements of "only" 106-107. In this review, TERS enhancements are discussed in some detail, including a condensed overview of measured contrasts and estd. total enhancements. Finally, recent developments for TERS under ultrahigh vacuum conditions are presented, including TERS on a C60 island with a diam. of a few tens of nanometers, deposited on a smooth Au(111) surface.
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    Pozzi, E. A. Ultrahigh-Vacuum Tip-Enhanced Raman Spectroscopy. Chem. Rev. 2017, 117 (7), 4961– 4982,  DOI: 10.1021/acs.chemrev.6b00343
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    Ultrahigh-Vacuum Tip-Enhanced Raman Spectroscopy
    Pozzi, Eric A.; Goubert, Guillaume; Chiang, Naihao; Jiang, Nan; Chapman, Craig T.; McAnally, Michael O.; Henry, Anne-Isabelle; Seideman, Tamar; Schatz, George C.; Hersam, Mark C.; Duyne, Richard P. Van
    Chemical Reviews (Washington, DC, United States) (2017), 117 (7), 4961-4982CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    Mol.-surface interactions and processes are at the heart of many technologies, including heterogeneous catalysis, org. photovoltaics, and nanoelectronics, yet they are rarely well understood at the mol. level. Given the inhomogeneous nature of surfaces, mol. properties often vary among individual surface sites, information that is lost in ensemble-averaged techniques. In order to access such site-resolved behavior, a technique must possess lateral resoln. comparable to the size of surface sites under study, anal. power capable of examg. chem. properties, and single-mol. sensitivity. Tip-enhanced Raman spectroscopy (TERS), wherein light is confined and amplified at the apex of a nanoscale plasmonic probe, meets these criteria. In ultrahigh vacuum (UHV), TERS can be performed in pristine environments, allowing for mol.-resoln. imaging, low-temp. operation, minimized tip and mol. degrdn., and improved stability in the presence of ultrafast irradn. The aim of this review is to give an overview of TERS expts. performed in UHV environments and to discuss how recent reports will guide future endeavors. The advances made in the field thus far demonstrate the utility of TERS as an approach to interrogate single-mol. properties, reactions, and dynamics with spatial resoln. below 1 nm.
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    Richard-Lacroix, M.; Zhang, Y.; Dong, Z.; Deckert, V. Mastering high resolution tip-enhanced Raman spectroscopy: towards a shift of perception. Chem. Soc. Rev. 2017, 46, 3922– 3944,  DOI: 10.1039/C7CS00203C
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    Mastering high resolution tip-enhanced Raman spectroscopy: towards a shift of perception
    Richard-Lacroix, Marie; Zhang, Yao; Dong, Zhenchao; Deckert, Volker
    Chemical Society Reviews (2017), 46 (13), 3922-3944CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Recent years have seen tremendous improvement of our understanding of high resoln. reachable in TERS expts., forcing us to re-evaluate our understanding of the intrinsic limits of this field, but also exposing several inconsistencies. On the one hand, more and more recent exptl. results have provided us with clear indications of spatial resolns. down to a few nanometers or even on the subnanometre scale. Moreover, lessons learned from recent theor. investigations clearly support such high resolns., and vice versa the obvious theor. impossibility to evade high resoln. from a purely plasmonic point of view. On the other hand, most of the published TERS results still, to date, claim a resoln. on the order of tens of nanometers that would be somehow limited by the tip apex, a statement well accepted for the past 2 decades. Overall, this now leads the field to a fundamental question: how can this divergence be justified. The answer to this question brings up an equally crit. one: how can this gap be bridged. This review aims at raising a fundamental discussion related to the resoln. limits of tip-enhanced Raman spectroscopy, at revisiting our comprehension of the factors limiting it both from a theor. and an exptl. point of view and at providing indications on how to move the field ahead. It is our belief that a much deeper understanding of the real accessible lateral resoln. in TERS and the practical factors that limit them will simultaneously help us to fully explore the potential of this technique for studying nanoscale features in org., inorg. and biol. systems, and also to improve both the reproducibility and the accuracy of routine TERS studies. A significant improvement of our comprehension of the accessible resoln. in TERS is thus crit. for a broad audience, even in certain contexts where high resoln. TERS is not the desired outcome.
  27. 27
    Shao, F.; Zenobi, R. Tip-enhanced Raman spectroscopy: principles, practice, and applications to nanospectroscopic imaging of 2D materials. Anal. Bioanal. Chem. 2019, 411, 37– 61,  DOI: 10.1007/s00216-018-1392-0
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    Tip-enhanced Raman spectroscopy: principles, practice, and applications to nanospectroscopic imaging of 2D materials
    Shao, Feng; Zenobi, Renato
    Analytical and Bioanalytical Chemistry (2019), 411 (1), 37-61CODEN: ABCNBP; ISSN:1618-2642. (Springer)
    Two-dimensional (2D) materials have been one of the most extensively studied classes of modern materials, due to their astonishing chem., optical, electronic, and mech. properties, which are different from their bulk counterparts. The edges, grain boundaries, local strain, chem. bonding, mol. orientation, and the presence of nanodefects in these 2D monolayers (MLs) will strongly affect their properties. Currently, it is still challenging to investigate such atomically thin 2D monolayers with nanoscale spatial resoln., esp. in a label-free and non-destructive way. Tip-enhanced Raman spectroscopy (TERS), which combines the merits of both scanning probe microscopy (SPM) and Raman spectroscopy, has become a powerful anal. technique for studying 2D monolayers, because it allows very high-resoln. and high-sensitivity local spectroscopic investigation and imaging and also provides rich chem. information. This review provides a summary of methods to study 2D monolayers and an overview of TERS, followed by an introduction to the current state-of-the-art and theor. understanding the spatial resoln. in TERS expts. Surface selection rules are also discussed. We then focus on the capabilities and potential of TERS for nanoscale chem. imaging of 2D materials, such as graphene, transition metal dichalcogenides (TMDCs), and 2D polymers. We predict that TERS will become widely accepted and used as a versatile imaging tool for chem. investigation of 2D materials at the nanoscale. [Figure not available: see fulltext.].
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    Trautmann, S. A classical description of subnanometer resolution by atomic features in metallic structures. Nanoscale 2017, 9, 391– 401,  DOI: 10.1039/C6NR07560F
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    A classical description of subnanometer resolution by atomic features in metallic structures
    Trautmann, S.; Aizpurua, J.; Goetz, I.; Undisz, A.; Dellith, J.; Schneidewind, H.; Rettenmayr, M.; Deckert, V.
    Nanoscale (2017), 9 (1), 391-401CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Recent expts. have evidenced sub-nanometer resoln. in plasmonic-enhanced probe spectroscopy. Such a high resoln. cannot be simply explained using the commonly considered radii of metallic nanoparticles on plasmonic probes. In this contribution, the effects of defects as small as a single atom found on spherical plasmonic particles acting as probing tips are investigated in connection with the spatial resoln. provided. The presence of abundant edge and corner sites with at. scale dimensions in cryst. metallic nanoparticles is evident from transmission electron microscopy (TEM) images. Electrodynamic calcns. based on the Finite Element Method (FEM) are implemented to reveal the impact of the presence of such at. features in probing tips on the lateral spatial resoln. and field localization. Our anal. is developed for three different configurations, and under resonant and non-resonant illumination conditions, resp. Based on this anal., the limits of field enhancement, lateral resoln. and field confinement in plasmon-enhanced spectroscopy and microscopy are inferred, reaching values below 1 nm for reasonable at. sizes.
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    Journal of Physical Chemistry Letters (2018), 9 (11), 3074-3080CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Tip-enhanced Raman spectromicroscopy (TERS) with CO-terminated plasmonic tips can probe angstrom-scale features of mols. on surfaces. The development of this technique requires understanding of how chem. environments affect the CO vibrational frequency and TERS intensity. At the scanning tunneling microscope junction of a CO-terminated Ag tip, rather than the classical vibrational Stark effect, the large bias dependence of the CO frequency shift is due to ground-state charge transfer from the Ag tip into the CO π* orbital softening the C-O bond at more pos. biases. The assocd. increase in Raman intensity is attributed to a bias-dependent chem. enhancement effect, where a pos. bias tunes a charge-transfer excited state close to resonance with the Ag plasmon. This change in Raman intensity is contrary to what would be expected based on changes in the tilt angle of the CO mol. with bias, demonstrating that the Raman intensity is dominated by electronic rather than geometric effects.
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    In this work, our first-principles calcns. reveal that direct wave-vector excitation (i.e., interband transitions between different wavenumbers without phonon assistance) can occur in the indirect-band-gap semiconductor silicon. The wave-vector excitation is successfully induced by irradn. of a silicon thin film with an optical near-field (ONF). As a result, the absorption-band-edge energy Eedge shifts to a lower photon energy of 1.6 eV for ONF excitation from Eedge of 2.1 eV for the conventional excitation by propagating far-field light. The direct wave-vector excitation is caused by the sufficiently large components of wave vectors inherent in the ONF, and thus does not require phonon assistance. For a realistic silicon system, it is clarified that the wave-vector excitations are detd. by the energy difference between the valence and conduction bands and occur irresp. of the initial and final wave vectors.
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    First-principle calcns. based on d. functional and non-equil. Green's functions are used to compute the power emitted in conducting mol. systems due to scattering with localized vibrations. The balance between the rate of phonons emitted and dissipated into the contacts allows the computation of the steady-state distribution of phonon quanta localized in the junction, from which we ext. the local temp. reached by the mol. The model includes two crit. quantities; (i) the rate of phonon emitted in the junction due to electron-phonon scattering and (ii) a microscopic approach for the computation of the phonon decay rate, accounting for the dynamical coupling between the vibrational modes localized on the mol. and the contact phonons. The method is applied to the discussion of several limiting conditions and trends, depending on electron-phonon coupling, incoherent transmission and phonon dissipation rates, using both anal. results and numerical calcns.
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    Surface-enhanced Raman spectroscopy (SERS) and related spectroscopies are powered primarily by the concn. of the electromagnetic (EM) fields assocd. with light in or near appropriately nanostructured elec.-conducting materials, most prominently, but not exclusively high-cond. metals such as silver and gold. This field concn. takes place on account of the excitation of surface-plasmon (SP) resonances in the nanostructured conductor. Optimizing nanostructures for SERS, therefore, implies optimizing the ability of plasmonic nanostructures to conc. EM optical fields at locations where mols. of interest reside, and to enhance the radiation efficiency of the oscillating dipoles assocd. with these mols. and nanostructures. This review summarizes the development of theories over the past four decades pertinent to SERS, esp. those contributing to our current understanding of SP-related SERS. Special emphasis is given to the salient strategies and theor. approaches for optimizing nanostructures with hotspots as efficient EM near-field concg. and far-field radiating substrates for SERS. A simple model is described in terms of which the upper limit of the SERS enhancement can be estd. Several exptl. strategies that may allow one to approach, or possibly exceed this limit, such as cascading the enhancement of the local and radiated EM field by the multiscale EM coupling of hierarchical structures, and generating hotspots by hybridizing an antenna mode with a plasmonic waveguide cavity mode, which would result in an increased local field enhancement, are discussed. Aiming to significantly broaden the application of SERS to other fields, and esp. to material science, we consider hybrid structures of plasmonic nanostructures and other material phases and strategies for producing strong local EM fields at desired locations in such hybrid structures. In this vein, we consider some of the numerical strategies for simulating the optical properties and consequential SERS performance of particle-on-substrate systems that might guide the design of SERS-active systems. Finally, some current theor. attempts are briefly discussed for unifying EM and non-EM contribution to SERS.
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    Xia, L.; Chen, M.; Zhao, X.; Zhang, Z.; Xia, J.; Xu, H.; Sun, M. Visualized method of chemical enhancement mechanism on SERS and TERS. J. Raman Spectrosc. 2014, 45 (7), 533– 540,  DOI: 10.1002/jrs.4504
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    Journal of Raman Spectroscopy (2014), 45 (7), 533-540CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)
    We review our developed visualization method of charge transfer (CT) for chem. enhancement mechanism on surface-enhanced Raman scattering (SERS) and tip-enhanced Raman spectroscopy (TERS). Firstly, we describe our visualization method of charge difference d., which provides direct visual evidence for photoinduced CT. And then, using the visualization method of CT, we interpreted the mechanism of SERS and TERS. Photoinduced charge transfer in the processes of SERS and TERS can be clearly seen. Our visualization method provides a visual and easy understanding way for the mechanism of SERS and TERS. Copyright © 2014 John Wiley & Sons, Ltd.
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    Oren, M.; Galperin, M.; Nitzan, A. Raman scattering from molecular conduction junctions: Charge transfer mechanism. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 115435,  DOI: 10.1103/PhysRevB.85.115435
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    We present a model for the charge transfer contribution to surface-enhanced Raman spectroscopy (SERS) in a mol. junction. The model is a generalization of the equil. scheme for SERS of a mol. adsorbed on a metal surface. We extend the same phys. consideration to a nonequil. situation in a biased mol. junction and to nonzero temps. Two approaches are considered and compared: a semiclassical approach appropriate for nonresonance Raman scattering, and a quantum approach based on the nonequil. Green's function method. Nonequil. effects on this contribution to SERS are demonstrated with numerical examples. It is shown that the semiclassical approach provides an excellent approxn. to the full quantum calcn. as long as the mol. electronic state is outside the Fermi window, i.e., as long as the field-induced charge transfer is small.
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    Liu, S. Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation. Nano Lett. 2020, 20 (8), 5879– 5884,  DOI: 10.1021/acs.nanolett.0c01791
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    Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation
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    Nano Letters (2020), 20 (8), 5879-5884CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Tip-enhanced Raman scattering (TERS) in angstr.ovrddot.om-scale plasmonic cavities has drawn increasing attention. However, Raman scattering at vanishing cavity distances remains unexplored. Here, we show the evolution of TERS in transition from the tunneling regime to at. point contact (APC). A stable APC is reversibly formed in the junction between an Ag tip and ultrathin ZnO or NaCl films on the Ag(111) surface at 10 K. An abrupt increase of the TERS intensity occurs upon APC formation for ZnO, but not for NaCl. This remarkable observation is rationalized by a difference in hybridization between the Ag tip and these films, which dets. the contribution of charge transfer enhancement in the fused plasmonic junction. The strong hybridization between the Ag tip and ZnO is corroborated by the appearance of a new vibrational mode upon APC formation, whereas it is not obsd. for the chem. inert NaCl.
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    Crampton, K. T.; Lee, J.; Apkarian, V. A. Ion-Selective, Atom-Resolved Imaging of a 2D Cu2N Insulator: Field and Current Driven Tip-Enhanced Raman Spectromicroscopy Using a Molecule-Terminated Tip. ACS Nano 2019, 13 (6), 6363– 6371,  DOI: 10.1021/acsnano.9b02744
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    Ion-Selective, Atom-Resolved Imaging of a 2D Cu2N Insulator: Field and Current Driven Tip-Enhanced Raman Spectromicroscopy Using a Molecule-Terminated Tip
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    ACS Nano (2019), 13 (6), 6363-6371CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Tip-enhanced Raman scattering (TERS) with a Co tetraphenylporphyrin (CoTPP)-terminated Ag tip is used to obtain ion-selective, atomically resolved images of an insulating Cu2N monolayer grown on Cu(100). Ion selective images are obtained through vibrational frequency shift maps using CoTPP vibrations with oppositely signed Stark tuning rates (STR). The images allow a quant. anal. of the electrostatic field of the ionic lattice using in situ calibrated STRs. Both intensity and Stark shift maps yield atomically resolved images in the tunneling regime of plasmons. The CoTPP is bonded to the Ag tip through its central Co atom, whereby TERS taps into intramol. currents and polarizations. The bias dependence of vibrational line intensities shows diode-like response with opposite polarity for current carrying modes of opposite polarization phase. The phase sensitive detection of vibrational lines and their voltage gating is explained in terms of distinct field- and phototunneling current-driven Raman, offering an alternate paradigm for the long-sought optoelectronic rectifier in mol. electronics.
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    Liebhaber, M. Surface phonons of the Si(111)-(7 × 7) reconstruction observed by Raman spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89 (4), 045313,  DOI: 10.1103/PhysRevB.89.045313
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    Surface phonons of the Si(111)-(7 × 7) reconstruction observed by Raman spectroscopy
    Liebhaber, M.; Bass, U.; Bayersdorfer, P.; Geurts, J.; Speiser, E.; Raethel, J.; Baumann, A.; Chandola, S.; Esser, N.
    Physical Review B: Condensed Matter and Materials Physics (2014), 89 (4), 045313/1-045313/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We have studied the surface phonon modes of the reconstructed Si(111)-(7 × 7) surface by polarized Raman spectroscopy. Six surface vibration modes are obsd. in the frequency range between 62.5-420.0 cm-1. The mode frequencies agree very well with reported calcn. results. This enables their attribution to calcd. eigenmodes, whose elongation patterns are dominated by specific at. sites: the two most characteristic novel fingerprints of the (7 × 7) reconstruction are sharp Raman peaks from localized adatom vibrations, located at 250.9 cm-1, and collective vibrations of the adatoms and 1st- and 2nd-layer atoms, located at 420.0 cm-1. While the sharp localized adatom vibration peak is a substantial refinement of an earlier broad spectral structure from electron energy-loss spectroscopy, no spectroscopic features were reported before in the collective-vibration frequency region. Furthermore, we observe in-plane wagging vibrations in the range from 110-140 cm-1, and finally the backfolded acoustic Rayleigh wave at 62.5 cm-1, which coincides with He atom scattering data. Moreover, the Raman peak intensities of the surface phonons show a mode-specific dependence on the polarization directions of incident and scattered light. From this polarization dependence the relevant symmetry components in the Raman scattering process (A1 and/or E symmetry) are deduced for each mode.
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    Kaya, D.; Cobley, R. J.; Palmer, R. E. Combining scanningtunneling microscope imaging and local manipulation to probe thehigh dose oxidation structure of the Si(111)-7 × 7 surface. Nano Res. 2020, 13, 145– 150,  DOI: 10.1007/s12274-019-2587-1
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    Combining scanning tunneling microscope (STM) imaging and local manipulation to probe the high dose oxidation structure of the Si(111)-7×7 surface
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    Nano Research (2020), 13 (1), 145-150CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
    Understanding the atomistic formation of oxide layers on semiconductors is important for thin film fabrication, scaling down conventional devices and for the integration of emerging research materials. Here, the initial oxidn. of Si(111) is studied using the scanning tunneling microscope. Prior to the complete satn. of the silicon surface with oxygen, we are able to probe the at. nature of the oxide layer formation. We establish the threshold for local manipulation of inserted oxygen sites to be +3.8 V. Only by combining imaging with local at. manipulation are we able to det. whether inserted oxygen exists beneath surface-bonded oxygen sites and differentiate between sites that have one and more than one oxygen atom inserted beneath the surface. Prior to the creation of the thin oxide film we observe a flip in the manipulation rates of inserted oxygen sites consistent with more oxygen inserting beneath the silicon surface. [Figure not available: see fulltext.].
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    Electronic and vibrational properties of initial-stage oxidation products on Si(111)-(7×7)
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    Chemisorption of O2 mols. on the adatom site of Si(111)-(7×7) has been studied by d.-functional theory calcns. for all possible dissocn. configurations. Structures possessing an oxygen atom on top of the Si adatom are all found to be metastable and account well for the metastable electronic and vibrational spectra obsd. in previous expts., while structures with only oxygen atoms inserted into the adatom back bonds appear quite stable. The present structural models therefore are all compatible with either the metastable or the stable O2 reaction products found in this system. The calcd. decay pathways of the metastable structures provide addnl. informations useful for identifying the exptl. metastable structures.
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    The oxidn. products on Si(111)-(7×7) were studied at 82 K by high-resoln. EELS. The isotope-labeled vibrational spectra with O216, O218, and O16 O18 show that, in the initial stage of the oxidn., an O2 mol. dissocs. to form a metastable product with an O atom bonding on top of the Si adatom and the other inserted into the backbond. The metastable product is obsd. as a dark site in the topog. scanning tunneling microscopy (STM) image and can be transformed to a stable product by the STM manipulation. The authors' results are in good agreement with recent theor. calcns.
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    Vibrational study of the initial stages of the oxidation of silicon (111) and silicon (100) surfaces
    Ibach, H.; Bruchmann, H. D.; Wagner, H.
    Applied Physics A: Solids and Surfaces (1982), A29 (3), 113-24CODEN: APSFDB; ISSN:0721-7250.
    By using high-resoln. electron energy loss spectroscopy, the vibrations of Si(111) and Si(100) surfaces in the early stags of oxidn. were investigated. Three different stages of oxidn., the last being the formation of a thin layer of vitreous SiO2 were identified when the surfaces were held at 700 K during exposure to O2. The 1st 2 stages involve at. O in bridging positions between Si atoms. Small exposures at low temps. (100 K) produce vibrational features of a different, possibly mol., species. For higher exposures at the same temp., the spectrum again develops the characteristics of at. O and the mol. species eventually disappears. Exposure at room temp. leads to a mixt. of O and O2 for smaller exposures and to purely at. O for exposures greater than ∼102 lambert.
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    Okuyama, H.; Ohtsuka, Y.; Aruga, T. Secondary oxidation product on Si(111)-(7 × 7) characterized by isotope-labeled vibrational spectroscopy. J. Chem. Phys. 2005, 122, 234709,  DOI: 10.1063/1.1937394
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    Secondary oxidation product on Si(111)-(7×7) characterized by isotope-labeled vibrational spectroscopy
    Okuyama, H.; Ohtsuka, Y.; Aruga, T.
    Journal of Chemical Physics (2005), 122 (23), 234709/1-234709/5CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The reaction of O2 with Si(111)-(7×7) has been studied by electron energy-loss spectroscopy at 82 K. In addn. to the losses due to Si-O-Si configurations, we obsd. two Si-O stretch modes depending on the coverage. A 146-meV peak appears at the initial reaction stage and was ascribed to a metastable product with one oxygen atom bonding on top of Si adatom and the other inserted into the back-bond. The initial product is further oxidized to produce the second Si-O stretch peak at 150 meV. The secondary product was partially substituted with isotopes and analyzed with a simple model of coupled oscillators. The vibrational spectra reflect dynamical couplings between the isotopes, which is consistent with those predicted from the tetrahedral SiO4 structure with one on top and three inserted oxygen atoms.

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  1. Imran Chaudhry, Gaohe Hu, Hepeng Ye, Lasse Jensen. Toward Modeling the Complexity of the Chemical Mechanism in SERS. ACS Nano 2024, 18 (32) , 20835-20850. https://doi.org/10.1021/acsnano.4c07198
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Cite this: Nano Lett. 2021, 21, 9, 4057–4061
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  • Abstract

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    Figure 1

    Figure 1. (a) Schematic of the experiment. EAS illustrates the extreme plasmonic field occurring at the atomic-scale protrusion at the tip apex. (b) STM image of Si(111)-7 × 7 under illumination (10 K, Vbias = 0.3 V, ISTM = 1 nA, λext = 633 nm, Pext = 0.7 W/cm2, scale bar = 5 nm). (c) Enlarged STM image (scale bar = 1 nm). The bright/dark pairs are indicated by the dashed ellipses. The faulted/unfaulted half unit cell is indicated by the pink/blue triangle. (d) Conductance spectrum measured under illumination (10 K, set-point: Vbias = 0.5 V, ISTM = 0.1 nA, λext = 532 nm, Pext = 0.15 mW/μm2).

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    Figure 2

    Figure 2. (a) Water fall plot of TERS recorded during tip-approach and retraction over Si(111)-7 × 7 (10 K, Vbias = 0 V, λext = 633 nm, Pext = 0.7 mW/μm2). The left panel shows the simultaneously recorded ISTM–Δz curve. Although the Vbias is nominally set to zero, the current occurs due to the photovoltage under illumination. The red shaded region indicates the APC. The top and bottom panels display the TERS spectra in the tunneling and contact regimes, respectively. (b) TERS spectra obtained for 11 different APCs (red) recorded for different location (UHUCs) and different tip conditions. The black one shows the averaged spectrum. The spectra are normalized to its area from range of Raman shift 0–500 cm–1.

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    Figure 3

    Figure 3. (a) APC-TERS at the step edge of the Si(111)-7 × 7 surface (10 K, Vbias = 0 V, λext = 633 nm, Pext = 0.7 mW/um2). The gray dashed line is the averaged spectrum over the terrace. The inset shows the STM image of the step edge. (b) APC-TERS at oxidized region of the Si(111)-7 × 7 surface (10 K, Vbias = 0 V, λext = 532 nm, Pext = 1 mW/μm2). The left panel shows the simultaneously recorded ISTM–Δz curve. The red shaded region indicates the APC. The top and bottom panels display the TERS spectra in the tunneling and contact regimes, respectively. The STM image of the partially oxidized surface is shown in top-left (scale bar is 2 nm). The position for APC-TERS measurement is indicated by the arrow.

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      12
      Electrochemical Tip-Enhanced Raman Spectroscopy
      Zeng, Zhi-Cong; Huang, Sheng-Chao; Wu, De-Yin; Meng, Ling-Yan; Li, Mao-Hua; Huang, Teng-Xiang; Zhong, Jin-Hui; Wang, Xiang; Yang, Zhi-Lin; Ren, Bin
      Journal of the American Chemical Society (2015), 137 (37), 11928-11931CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Interfacial properties are highly important to the performance of some energy-related systems. The in-depth understanding of the interface requires highly sensitive in situ techniques that can provide fingerprint mol. information at nanometer resoln. The authors developed an electrochem. tip-enhanced Raman spectroscopy (EC-TERS) by introduction of the light horizontally to the EC-STM cell to minimize the optical distortion and to keep the TERS measurement under a well-controlled condition. The authors obtained potential-dependent EC-TERS from the adsorbed arom. mol. on a Au(111) surface and obsd. a substantial change in the mol. configuration with potential as a result of the protonation and deprotonation of the mol. Such a change was not observable in EC-SERS (surface-enhanced), indicating EC-TERS can more faithfully reflect the fine interfacial structure than EC-SERS. This work will open a new era for using EC-TERS as an important nanospectroscopy tool for the mol. level and nanoscale anal. of some important electrochem. systems including solar cells, lithium ion batteries, fuel cells, and corrosion.
    13. 13
      Martín Sabanés, N.; Ohto, T.; Andrienko, D.; Nagata, Y.; Domke, K. F. Electrochemical TERS elucidates potential-induced molecular reorientation of adenine/Au(111). Angew. Chem., Int. Ed. 2017, 56, 9796– 9801,  DOI: 10.1002/anie.201704460
      13
      Electrochemical TERS Elucidates Potential-Induced Molecular Reorientation of Adenine/Au(111)
      Martin Sabanes, Natalia; Ohto, Tatsuhiko; Andrienko, Denis; Nagata, Yuki; Domke, Katrin F.
      Angewandte Chemie, International Edition (2017), 56 (33), 9796-9801CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Electrochem. surface activity arises from the interaction and geometric arrangement of mols. at electrified interfaces. The authors present a novel electrochem. tip-enhanced Raman spectroscope that can access the vibrational fingerprint of <100 small, nonresonant mols. adsorbed at atomically flat Au electrodes to study their adsorption geometry and chem. reactivity as a function of the applied potential. Combining exptl. and simulation data for adenine/Au(111), protonated physisorbed adenine adopts a tilted orientation at low potentials, whereas it is vertically adsorbed around the potential of zero charge. Further potential increase induces adenine deprotonation and reorientation to a planar configuration. The extension of EC-TERS to the study of adsorbate reorientation significantly broadens the applicability of this advanced spectroelectrochem. tool for the nanoscale characterization of a full range of electrochem. interfaces.
    14. 14
      Zhong, J. Probing the electronic and catalytic properties of a bimetallic surface with 3 nm resolution. Nat. Nanotechnol. 2017, 12, 132– 136,  DOI: 10.1038/nnano.2016.241
      14
      Probing the electronic and catalytic properties of a bimetallic surface with 3 nm resolution
      Zhong, Jin-Hui; Jin, Xi; Meng, Lingyan; Wang, Xiang; Su, Hai-Sheng; Yang, Zhi-Lin; Williams, Christopher T.; Ren, Bin
      Nature Nanotechnology (2017), 12 (2), 132-136CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      An at.- and mol.-level understanding of heterogeneous catalysis is required to characterize the nature of active sites and improve the rational design of catalysts. Achieving this level of characterization requires techniques that can correlate catalytic performances to sp. surface structures, so as to avoid averaging effects. Tip-enhanced Raman spectroscopy combines scanning probe microscopy with plasmon-enhanced Raman scattering and provides simultaneous topog. and chem. information at the nano/at. scale from ambient to ultrahigh-vacuum and electrochem. environments. Therefore, it has been used to monitor catalytic reactions and is proposed to correlate the local structure and function of heterogeneous catalysts. Bimetallic catalysts, such as Pd-Au, show superior performance in various catalytic reactions, but it has remained challenging to correlate structure and reactivity because of their structural complexity. Here, we show that TERS can chem. and spatially probe the site-specific chem. (electronic and catalytic) and phys. (plasmonic) properties of an atomically well-defined Pd(sub-monolayer)/Au(111) bimetallic model catalyst at 3 nm resoln. in real space using Ph isocyanide as a probe mol. (Fig. 1a). We observe a weakened N≃C bond and enhanced reactivity of Ph isocyanide adsorbed at the Pd step edge compared with that at the Pd terrace. D. functional theory corroborates these observations by revealing a higher d-band electronic profile for the low-coordinated Pd step edge atoms. The 3 nm spatial resoln. we demonstrate here is the result of an enhanced elec. field and distinct electronic properties at the step edges.
    15. 15
      Yin, H. Nanometre-scale spectroscopic visualization of catalytic sites during a hydrogenation reaction on a Pd/Au bimetallic catalyst. Nat. Catal. 2020, 3, 834– 842,  DOI: 10.1038/s41929-020-00511-y
      15
      Nanometre-scale spectroscopic visualization of catalytic sites during a hydrogenation reaction on a Pd/Au bimetallic catalyst
      Yin, Hao; Zheng, Li-Qing; Fang, Wei; Lai, Yin-Hung; Porenta, Nikolaus; Goubert, Guillaume; Zhang, Hua; Su, Hai-Sheng; Ren, Bin; Richardson, Jeremy O.; Li, Jian-Feng; Zenobi, Renato
      Nature Catalysis (2020), 3 (10), 834-842CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
      Understanding the mechanism of catalytic hydrogenation at the local environment requires chem. and topog. information involving catalytic sites, active hydrogen species, and their spatial distribution. Here we used tip-enhanced Raman spectroscopy (TERS) to study the catalytic hydrogenation of chloronitrobenzenethiol on a well-defined Pd(submonolayer)/Au(111) bimetallic catalyst (pH2 = 1.5 bar, 298 K), where the surface topog. and chem. fingerprint information were simultaneously mapped with nanoscale resoln. (∼10 nm). TERS imaging of the surface after catalytic hydrogenation confirms that the reaction occurs beyond the location of Pd sites. The results demonstrate that hydrogen spillover accelerates hydrogenation at Au sites as far as 20 nm from the bimetallic Pd/Au boundary. D. functional theory was used to elucidate the thermodn. of interfacial hydrogen transfers. We demonstrate TERS to be a powerful anal. tool that provides a unique approach to spatially investigate the local structure-reactivity relationship in catalysis.
    16. 16
      Zhang, R. Distinguishing Individual DNA Bases in a Network by Non-Resonant Tip-Enhanced Raman Scattering. Angew. Chem., Int. Ed. 2017, 56 (20), 5561– 5564,  DOI: 10.1002/anie.201702263
      16
      Distinguishing Individual DNA Bases in a Network by Non-Resonant Tip-Enhanced Raman Scattering
      Zhang, Rui; Zhang, Xianbiao; Wang, Huifang; Zhang, Yao; Jiang, Song; Hu, Chunrui; Zhang, Yang; Luo, Yi; Dong, Zhenchao
      Angewandte Chemie, International Edition (2017), 56 (20), 5561-5564CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The importance of identifying DNA bases at the single-mol. level is well recognized for many biol. applications. Although such identification can be achieved by elec. measurements using special setups, it is still not possible to identify single bases in real space by optical means owing to the diffraction limit. Herein, we demonstrate the outstanding ability of scanning tunneling microscope (STM)-controlled non-resonant tip-enhanced Raman scattering (TERS) to unambiguously distinguish two individual complementary DNA bases (adenine and thymine) with a spatial resoln. down to 0.9 nm. The distinct Raman fingerprints identified for the two mols. allow to differentiate in real space individual DNA bases in coupled base pairs. The demonstrated ability of non-resonant Raman scattering with super-high spatial resoln. will significantly extend the applicability of TERS, opening up new routes for single-mol. DNA sequencing.
    17. 17
      Lipiec, E.; Kaderli, J.; Kobierski, J.; Riek, R.; Skirlinska-Nosek, K.; Sofinska, K.; Szymonski, M.; Zenobi, R. Nanoscale Hyperspectral Imaging of Amyloid Secondary Structures in Liquid. Angew. Chem. 2021, 133, 4595,  DOI: 10.1002/ange.202010331
      There is no corresponding record for this reference.
    18. 18
      He, Z. Tip-Enhanced Raman Imaging of Single-Stranded DNA with Single Base Resolution. J. Am. Chem. Soc. 2019, 141 (2), 753– 757,  DOI: 10.1021/jacs.8b11506
      18
      Tip-Enhanced Raman Imaging of Single-Stranded DNA with Single Base Resolution
      He, Zhe; Han, Zehua; Kizer, Megan; Linhardt, Robert J.; Wang, Xing; Sinyukov, Alexander M.; Wang, Jizhou; Deckert, Volker; Sokolov, Alexei V.; Hu, Jonathan; Scully, Marlan O.
      Journal of the American Chemical Society (2019), 141 (2), 753-757CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Tip-enhanced Raman scattering (TERS) is a promising optical and anal. technique for chem. imaging and sensing at single mol. resoln. In particular, TERS signals generated by a gap-mode configuration where a silver tip is coupled with a gold substrate can resolve a single-stranded DNA (ssDNA) mol. with a spatial resoln. below 1 nm. To demonstrate the proof of subnanometer resoln., the authors show direct nucleic acid sequencing using TERS of a phage ssDNA (M13mp18). M13mp18 provides a known sequence and, through the deposition strategy, can be stretched (uncoiled) and attached to the substrate by its phosphate groups, while exposing its nucleobases to the tip. After deposition, the authors scan the silver tip along the ssDNA and collect TERS signals with a step of 0.5 nm, comparable to the bond length between two adjacent DNA bases. By demonstrating the real-time profiling of a ssDNA configuration and furthermore, with unique TERS signals of monomeric units of other biopolymers, the authors anticipate that this technique can be extended to the high-resoln. imaging of various nanostructures as well as the direct sequencing of other important biopolymers including RNA, polysaccharides, and polypeptides.
    19. 19
      Beams, R.; Cançado, L. G.; Jorio, A.; Vamivakas, A. N.; Novotny, L. Tip-enhanced Raman mapping of local strain in graphene. Nanotechnology 2015, 26, 175702,  DOI: 10.1088/0957-4484/26/17/175702
      19
      Tip-enhanced Raman mapping of local strain in graphene
      Beams Ryan; Cancado Luiz Gustavo; Jorio Ado; Vamivakas A Nick; Novotny Lukas
      Nanotechnology (2015), 26 (17), 175702 ISSN:.
      We demonstrate local strain measurements in graphene by using tip-enhanced Raman spectroscopy (TERS). We find that a single 5 nm particle can induce a radial strain over a lateral distance of ∼170 nm. By treating the particle as a point force on a circular membrane, we find that the strain in the radial direction (r) is .varies. r-(2 3),in agreement with force-displacement measurements conducted on suspended graphene flakes. Our results demonstrate that TERS can be used to map out static strain fields at the nanoscale, which are inaccessible using force-displacement techniques.
    20. 20
      Sheng, S. Vibrational Properties of a Monolayer Silicene Sheet Studied by Tip-Enhanced Raman Spectroscopy. Phys. Rev. Lett. 2017, 119 (19), 196803,  DOI: 10.1103/PhysRevLett.119.196803
      20
      Vibrational properties of a monolayer silicene sheet studied by tip-enhanced Raman spectroscopy
      Sheng, Shaoxiang; Wu, Jiang-bin; Cong, Xin; Li, Wenbin; Gou, Jian; Zhong, Qing; Cheng, Peng; Tan, Ping-heng; Chen, Lan; Wu, Kehui
      Physical Review Letters (2017), 119 (19), 196803/1-196803/5CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
      Combining ultrahigh sensitivity, spatial resoln., and the capability to resolve chem. information, tip-enhanced Raman spectroscopy (TERS) is a powerful tool to study mols. or nanoscale objects. Here we show that TERS can also be a powerful tool in studying two-dimensional materials. We have achieved a 109 Raman signal enhancement and a 0.5 nm spatial resoln. using monolayer silicene on Ag(111) as a prototypical 2D material system. Because of the selective enhancement on Raman modes with vertical vibrational components in TERS, our expt. provides direct evidence of the origination of Raman modes in silicene. Furthermore, the ultrahigh sensitivity of TERS allows us to identify different vibrational properties of silicene phases, which differ only in the bucking direction of the Si-Si bonds. Local vibrational features from defects and domain boundaries in silicene can also be identified.
    21. 21
      Park, K.-D.; Khatib, O.; Kravtsov, V.; Clark, G.; Xu, X.; Raschke, M. B. Hybrid Tip-Enhanced Nanospectroscopy and Nanoimaging of Monolayer WSe2 with Local Strain Control. Nano Lett. 2016, 16 (4), 2621– 2627,  DOI: 10.1021/acs.nanolett.6b00238
      21
      Hybrid Tip-Enhanced Nanospectroscopy and Nanoimaging of Monolayer WSe2 with Local Strain Control
      Park, Kyoung-Duck; Khatib, Omar; Kravtsov, Vasily; Clark, Genevieve; Xu, Xiaodong; Raschke, Markus B.
      Nano Letters (2016), 16 (4), 2621-2627CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Many classes of two-dimensional (2D) materials have emerged as potential platforms for novel electronic and optical devices. However, their phys. properties are strongly influenced by nanoscale heterogeneities in the form of edges, twin boundaries, and nucleation sites. Using combined tip-enhanced Raman scattering and photoluminescence (PL) nanospectroscopy and nanoimaging, we study the assocd. effects on the excitonic properties in monolayer WSe2 grown by phys. vapor deposition. With ∼15 nm spatial resoln., we resolve nanoscale correlations of PL spectral intensity and shifts with crystal edges and internal twin boundaries assocd. with the expected exciton diffusion length. Through an active at. force tip interaction we can control the crystal strain on the nanoscale and tune the local bandgap in reversible (up to 24 meV shift) and irreversible (up to 48 meV shift) fashion. This allows us to distinguish the effect of strain from the dominant influence of defects on the PL modification at the different structural heterogeneities. Hybrid nano-optical spectroscopy and imaging with nanomech. strain control thus enables the systematic study of the coupling of structural and mech. degrees of freedom to the nanoscale electronic and optical properties in layered 2D materials.
    22. 22
      Shao, F.; Dai, W.; Zhang, Y.; Zhang, W.; Schlüter, A. D.; Zenobi, R. Chemical Mapping of Nanodefects within 2D Covalent Monolayers by Tip-Enhanced Raman Spectroscopy. ACS Nano 2018, 12 (5), 5021– 5029,  DOI: 10.1021/acsnano.8b02513
      22
      Chemical Mapping of Nanodefects within 2D Covalent Monolayers by Tip-Enhanced Raman Spectroscopy
      Shao, Feng; Dai, Wenyang; Zhang, Yao; Zhang, Wei; Schluter, A. Dieter; Zenobi, Renato
      ACS Nano (2018), 12 (5), 5021-5029CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Nanoscale defects in monolayers (MLs) of 2-dimensional (2D) materials, such as graphene, transition-metal dichalcogenides, and 2-dimensional polymers, can alter their phys., mech., optoelectronic, and chem. properties. However, detailed information about nanodefects within 2-dimensional covalent monolayers is difficult to obtain because it requires highly selective and sensitive techniques that can provide chem. information at the nanoscale. Here, the authors report a 2-dimensional imine-linked ML prepd. from 2 custom-designed building blocks by dynamic imine chem. at the air/H2O interface, in which an acetylenic moiety in one of the blocks was used as a spectroscopic reporter for nanodefects. Combined with d. functional theory calcns. that take into account surface selection rules, tip-enhanced Raman spectroscopy (TERS) imaging provides information on the chem. bonds, mol. orientation, as well as nanodefects in the resulting ML. Addnl., TERS imaging visualizes the topog. and integrity of the ML at Au(111) terrace edges, suggesting possible ductility of the ML. Also, edge-induced mol. tilting and a stronger signal enhancement were obsd. at the terrace edges, from which a spatial resoln. around 8 nm could be deduced. The present work can be used to study covalent 2-dimensional materials at the nanoscale, which are expected to be of use when engineering their properties for specific device applications.
    23. 23
      Balois, M. V.; Hayazawa, N.; Yasuda, S.; Ikeda, K.; Yang, B.; Kazuma, E.; Yokota, Y.; Kim, Y.; Tanaka, T. Visualization of subnanometric phonon modes in a plasmonic nano-cavity via ambient tip-enhanced Raman spectroscopy. NPJ. 2D Mater. Appl. 2019, 3, 38,  DOI: 10.1038/s41699-019-0121-7
      There is no corresponding record for this reference.
    24. 24
      Pettinger, B.; Schambach, P.; Villagómez, C. J.; Scott, N. Tip-Enhanced Raman Spectroscopy: Near-Fields Acting on a Few Molecules. Annu. Rev. Phys. Chem. 2012, 63, 379– 399,  DOI: 10.1146/annurev-physchem-032511-143807
      24
      Tip-enhanced Raman spectroscopy: near-fields acting on a few molecules
      Pettinger, Bruno; Schambach, Philip; Villagomez, Carlos J.; Scott, Nicola
      Annual Review of Physical Chemistry (2012), 63 (), 379-399CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews Inc.)
      A review. Tip-enhanced Raman spectroscopy (TERS) is a very powerful variant of surface-enhanced Raman spectroscopy (SERS). In a sense, TERS overcomes most of the drawbacks of SERS but keeps its advantages, such as its high sensitivity. TERS offers the addnl. advantages of high spatial resoln., much beyond the Abbe limit, and the possibility to correlate TER and other scanning probe microscope images, i.e., to correlate topog. and chem. data. TERS finds application in a no. of fields, such as surface science, material science, and biol. Single-mol. TERS has been obsd. even for TERS enhancements of "only" 106-107. In this review, TERS enhancements are discussed in some detail, including a condensed overview of measured contrasts and estd. total enhancements. Finally, recent developments for TERS under ultrahigh vacuum conditions are presented, including TERS on a C60 island with a diam. of a few tens of nanometers, deposited on a smooth Au(111) surface.
    25. 25
      Pozzi, E. A. Ultrahigh-Vacuum Tip-Enhanced Raman Spectroscopy. Chem. Rev. 2017, 117 (7), 4961– 4982,  DOI: 10.1021/acs.chemrev.6b00343
      25
      Ultrahigh-Vacuum Tip-Enhanced Raman Spectroscopy
      Pozzi, Eric A.; Goubert, Guillaume; Chiang, Naihao; Jiang, Nan; Chapman, Craig T.; McAnally, Michael O.; Henry, Anne-Isabelle; Seideman, Tamar; Schatz, George C.; Hersam, Mark C.; Duyne, Richard P. Van
      Chemical Reviews (Washington, DC, United States) (2017), 117 (7), 4961-4982CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      Mol.-surface interactions and processes are at the heart of many technologies, including heterogeneous catalysis, org. photovoltaics, and nanoelectronics, yet they are rarely well understood at the mol. level. Given the inhomogeneous nature of surfaces, mol. properties often vary among individual surface sites, information that is lost in ensemble-averaged techniques. In order to access such site-resolved behavior, a technique must possess lateral resoln. comparable to the size of surface sites under study, anal. power capable of examg. chem. properties, and single-mol. sensitivity. Tip-enhanced Raman spectroscopy (TERS), wherein light is confined and amplified at the apex of a nanoscale plasmonic probe, meets these criteria. In ultrahigh vacuum (UHV), TERS can be performed in pristine environments, allowing for mol.-resoln. imaging, low-temp. operation, minimized tip and mol. degrdn., and improved stability in the presence of ultrafast irradn. The aim of this review is to give an overview of TERS expts. performed in UHV environments and to discuss how recent reports will guide future endeavors. The advances made in the field thus far demonstrate the utility of TERS as an approach to interrogate single-mol. properties, reactions, and dynamics with spatial resoln. below 1 nm.
    26. 26
      Richard-Lacroix, M.; Zhang, Y.; Dong, Z.; Deckert, V. Mastering high resolution tip-enhanced Raman spectroscopy: towards a shift of perception. Chem. Soc. Rev. 2017, 46, 3922– 3944,  DOI: 10.1039/C7CS00203C
      26
      Mastering high resolution tip-enhanced Raman spectroscopy: towards a shift of perception
      Richard-Lacroix, Marie; Zhang, Yao; Dong, Zhenchao; Deckert, Volker
      Chemical Society Reviews (2017), 46 (13), 3922-3944CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Recent years have seen tremendous improvement of our understanding of high resoln. reachable in TERS expts., forcing us to re-evaluate our understanding of the intrinsic limits of this field, but also exposing several inconsistencies. On the one hand, more and more recent exptl. results have provided us with clear indications of spatial resolns. down to a few nanometers or even on the subnanometre scale. Moreover, lessons learned from recent theor. investigations clearly support such high resolns., and vice versa the obvious theor. impossibility to evade high resoln. from a purely plasmonic point of view. On the other hand, most of the published TERS results still, to date, claim a resoln. on the order of tens of nanometers that would be somehow limited by the tip apex, a statement well accepted for the past 2 decades. Overall, this now leads the field to a fundamental question: how can this divergence be justified. The answer to this question brings up an equally crit. one: how can this gap be bridged. This review aims at raising a fundamental discussion related to the resoln. limits of tip-enhanced Raman spectroscopy, at revisiting our comprehension of the factors limiting it both from a theor. and an exptl. point of view and at providing indications on how to move the field ahead. It is our belief that a much deeper understanding of the real accessible lateral resoln. in TERS and the practical factors that limit them will simultaneously help us to fully explore the potential of this technique for studying nanoscale features in org., inorg. and biol. systems, and also to improve both the reproducibility and the accuracy of routine TERS studies. A significant improvement of our comprehension of the accessible resoln. in TERS is thus crit. for a broad audience, even in certain contexts where high resoln. TERS is not the desired outcome.
    27. 27
      Shao, F.; Zenobi, R. Tip-enhanced Raman spectroscopy: principles, practice, and applications to nanospectroscopic imaging of 2D materials. Anal. Bioanal. Chem. 2019, 411, 37– 61,  DOI: 10.1007/s00216-018-1392-0
      27
      Tip-enhanced Raman spectroscopy: principles, practice, and applications to nanospectroscopic imaging of 2D materials
      Shao, Feng; Zenobi, Renato
      Analytical and Bioanalytical Chemistry (2019), 411 (1), 37-61CODEN: ABCNBP; ISSN:1618-2642. (Springer)
      Two-dimensional (2D) materials have been one of the most extensively studied classes of modern materials, due to their astonishing chem., optical, electronic, and mech. properties, which are different from their bulk counterparts. The edges, grain boundaries, local strain, chem. bonding, mol. orientation, and the presence of nanodefects in these 2D monolayers (MLs) will strongly affect their properties. Currently, it is still challenging to investigate such atomically thin 2D monolayers with nanoscale spatial resoln., esp. in a label-free and non-destructive way. Tip-enhanced Raman spectroscopy (TERS), which combines the merits of both scanning probe microscopy (SPM) and Raman spectroscopy, has become a powerful anal. technique for studying 2D monolayers, because it allows very high-resoln. and high-sensitivity local spectroscopic investigation and imaging and also provides rich chem. information. This review provides a summary of methods to study 2D monolayers and an overview of TERS, followed by an introduction to the current state-of-the-art and theor. understanding the spatial resoln. in TERS expts. Surface selection rules are also discussed. We then focus on the capabilities and potential of TERS for nanoscale chem. imaging of 2D materials, such as graphene, transition metal dichalcogenides (TMDCs), and 2D polymers. We predict that TERS will become widely accepted and used as a versatile imaging tool for chem. investigation of 2D materials at the nanoscale. [Figure not available: see fulltext.].
    28. 28
      Trautmann, S. A classical description of subnanometer resolution by atomic features in metallic structures. Nanoscale 2017, 9, 391– 401,  DOI: 10.1039/C6NR07560F
      28
      A classical description of subnanometer resolution by atomic features in metallic structures
      Trautmann, S.; Aizpurua, J.; Goetz, I.; Undisz, A.; Dellith, J.; Schneidewind, H.; Rettenmayr, M.; Deckert, V.
      Nanoscale (2017), 9 (1), 391-401CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Recent expts. have evidenced sub-nanometer resoln. in plasmonic-enhanced probe spectroscopy. Such a high resoln. cannot be simply explained using the commonly considered radii of metallic nanoparticles on plasmonic probes. In this contribution, the effects of defects as small as a single atom found on spherical plasmonic particles acting as probing tips are investigated in connection with the spatial resoln. provided. The presence of abundant edge and corner sites with at. scale dimensions in cryst. metallic nanoparticles is evident from transmission electron microscopy (TEM) images. Electrodynamic calcns. based on the Finite Element Method (FEM) are implemented to reveal the impact of the presence of such at. features in probing tips on the lateral spatial resoln. and field localization. Our anal. is developed for three different configurations, and under resonant and non-resonant illumination conditions, resp. Based on this anal., the limits of field enhancement, lateral resoln. and field confinement in plasmon-enhanced spectroscopy and microscopy are inferred, reaching values below 1 nm for reasonable at. sizes.
    29. 29
      Takayanagi, K.; Tanishiro, Y.; Takahashi, S.; Takahashi, M. Structure analysis of Si(111)-7 × 7 reconstructed surface by transmission electron diffraction. Surf. Sci. 1985, 164, 367– 392,  DOI: 10.1016/0039-6028(85)90753-8
      29
      Structure analysis of silicon(111)-7 × 7 reconstructed surface by transmission electron diffraction
      Takayanagi, Kunio; Tanishiro, Yasumasa; Takahashi, Shigeki; Takahashi, Masaetsu
      Surface Science (1985), 164 (2-3), 367-92CODEN: SUSCAS; ISSN:0039-6028.
      The at. structure of the 7 × 7 reconstructed Si(111) surface was analyzed by ultra-high vacuum (UHV) transmission electron diffraction (TED). A possible projected structure of the surface is deduced from the intensity distribution in TED patterns of normal electron incidence and from Patterson and Fourier syntheses of the intensities. A new 3-dimensional structure model, the DAS model, is proposed: the model consists of 12 adatoms arranged locally in the 2 × 2 structure, a stacking fault layer and a layer with a vacancy at the corner and 9 dimers on the sides of each of the 2 triangular subcells of the 7 × 7 unit cell. The Si layers in 1 subcell are stacked with the normal sequence, CcAa/C + adatoms, while those in the other subcell are stacked with a faulted sequence, CcAa/C + adatoms. The model has only 19 dangling bonds, the smallest no. among models so far proposed. Previously proposed models are tested quant. by the TED intensity. Advantages and limits of the TED anal. are discussed.
    30. 30
      Kanasaki, J.; Ishida, T.; Ishikawa, K.; Tanimura, K. Laser-induced electronic bond breaking and desorption of adatoms on Si (111)-(7× 7). Phys. Rev. Lett. 1998, 80 (18), 4080– 4083,  DOI: 10.1103/PhysRevLett.80.4080
      30
      Laser-Induced Electronic Bond Breaking and Desorption of Adatoms on Si(111)-(7×7)
      Kanasaki, J.; Ishida, T.; Ishikawa, K.; Tanimura, K.
      Physical Review Letters (1998), 80 (18), 4080-4083CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Laser-induced structural changes on the Si(111)-(7×7) surface were studied for laser fluences below thresholds of melting and ablation. The adatoms of the reconstructed structure are removed selectively by an electronic process, and Si atoms in the electronic ground state are ejected with a peak translational energy of 0.15 eV. The electronic process of this bond breaking of adatoms exhibits the site-sensitive efficiency which shows a resonant wavelength dependence and is highly superlinear with respect to the excitation intensity.
    31. 31
      Tanimura, K.; Kanasaki, J. Excitation-induced structural instability of semiconductor surfaces. J. Phys.: Condens. Matter 2006, 18, S1479– S1516,  DOI: 10.1088/0953-8984/18/30/S07
      31
      Excitation-induced structural instability of semiconductor surfaces
      Tanimura, Katsumi; Kanasaki, Jun'ichi
      Journal of Physics: Condensed Matter (2006), 18 (30), S1479-S1516CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
      A review of laser-induced electronic processes of structural instability on covalent semiconductor surfaces.
    32. 32
      Modesti, S. Low-temperature insulating phase of the Si(111)–7 × 7 surface. Phys. Rev. B: Condens. Matter Mater. Phys. 2020, 102 (3), 035429,  DOI: 10.1103/PhysRevB.102.035429
      There is no corresponding record for this reference.
    33. 33
      Odobescu, A. B.; Zaitsev-Zotov, S. V. Energy gap revealed by low-temperature scanning-tunnelling spectroscopy of the Si(111)- 7 × 7 surface in illuminated slightly doped crystals. J. Phys.: Condens. Matter 2012, 24, 395003,  DOI: 10.1088/0953-8984/24/39/395003
      33
      Energy gap revealed by low-temperature scanning-tunnelling spectroscopy of the Si(111)-7 × 7 surface in illuminated slightly doped crystals
      Odobescu, A. B.; Zaitsev-Zotov, S. V.
      Journal of Physics: Condensed Matter (2012), 24 (39), 395003/1-395003/5CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
      Phys. properties of the Si(111)-7 × 7 surface of low-doped n- and p-type Si samples are studied in the liq. He temp. region by scanning-tunneling microscopy and spectroscopy. Conduction required for the study is provided by illumination of the surface. Application of illumination completely removes the band bending near the surface and restores the initial population of the surface states. Our results indicate the existence of the energy gap 2Δ = 40 ± 10 meV in the intrinsically populated Si(111)-7 × 7 surface.
    34. 34
      Parker, J. H.; Feldman, D. W.; Ashkin, M. Raman Scattering by Silicon and Germanium. Phys. Rev. 1967, 155 (3), 712– 714,  DOI: 10.1103/PhysRev.155.712
      34
      Raman scattering by silicon and germanium
      Parker, James Henry, Jr.; Feldman, Donald W.; Ashkin, Martin
      Physical Review (1967), 155 (3), 712-14CODEN: PHRVAO; ISSN:0031-899X.
      Raman scattering from single-crystal Si and Ge at 300°K. was measured by using an Ar laser as the exciting source. The first-order Raman spectrum yields energies for the k ≈ 0 optical modes of 520.2 ± 0.5 cm.-1 for Si and 300.7 ± cm.-1 for Ge. These values are in reasonable agreement with other detns. The full widths at half-intensity were 4.6 cm.-1 for Si and 5.3 cm.-1 for Ge. These values are compared with theoretical predictions. A Raman band was observed in Si at 950 cm.-1 which is attributed to second-order scattering and is compared with theoretical predictions.
    35. 35
      Kröger, J.; Ńeel, N.; Limot, L. Contact to single atoms and molecules with the tip of a scanning tunnelling microscope. J. Phys.: Condens. Matter 2008, 20, 223001,  DOI: 10.1088/0953-8984/20/22/223001
      There is no corresponding record for this reference.
    36. 36
      Gieseking, R. L. M.; Lee, J.; Tallarida, N.; Apkarian, V. A.; Schatz, G. C. Bias-Dependent Chemical Enhancement and Nonclassical Stark Effect in Tip-Enhanced Raman Spectromicroscopy of CO-Terminated Ag Tips. J. Phys. Chem. Lett. 2018, 9 (11), 3074– 3080,  DOI: 10.1021/acs.jpclett.8b01343
      36
      Bias-Dependent Chemical Enhancement and Nonclassical Stark Effect in Tip-Enhanced Raman Spectromicroscopy of CO-Terminated Ag Tips
      Gieseking, Rebecca L. M.; Lee, Joonhee; Tallarida, Nicholas; Apkarian, Vartkess Ara; Schatz, George C.
      Journal of Physical Chemistry Letters (2018), 9 (11), 3074-3080CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Tip-enhanced Raman spectromicroscopy (TERS) with CO-terminated plasmonic tips can probe angstrom-scale features of mols. on surfaces. The development of this technique requires understanding of how chem. environments affect the CO vibrational frequency and TERS intensity. At the scanning tunneling microscope junction of a CO-terminated Ag tip, rather than the classical vibrational Stark effect, the large bias dependence of the CO frequency shift is due to ground-state charge transfer from the Ag tip into the CO π* orbital softening the C-O bond at more pos. biases. The assocd. increase in Raman intensity is attributed to a bias-dependent chem. enhancement effect, where a pos. bias tunes a charge-transfer excited state close to resonance with the Ag plasmon. This change in Raman intensity is contrary to what would be expected based on changes in the tilt angle of the CO mol. with bias, demonstrating that the Raman intensity is dominated by electronic rather than geometric effects.
    37. 37
      Kim, J.; Yeh, M.-L.; Khan, F. S.; Wilkins, J. W. Surface phonons of the Si (111)-7 × 7 reconstructed surface. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 52, 14709,  DOI: 10.1103/PhysRevB.52.14709
      There is no corresponding record for this reference.
    38. 38
      Noda, M.; Iida, K.; Yamaguchi, M.; Yatsui, T.; Nobusada, K. Direct Wave-Vector Excitationin an Indirect-Band-Gap Semiconductor of Silicon with an Optical Near-field. Phys. Rev. Appl. 2019, 11 (4), 044053,  DOI: 10.1103/PhysRevApplied.11.044053
      38
      Direct Wave-Vector Excitation in an Indirect-Band-Gap Semiconductor of Silicon with an Optical Near-field
      Noda, Masashi; Iida, Kenji; Yamaguchi, Maiku; Yatsui, Takashi; Nobusada, Katsuyuki
      Physical Review Applied (2019), 11 (4), 044053CODEN: PRAHB2; ISSN:2331-7019. (American Physical Society)
      In this work, our first-principles calcns. reveal that direct wave-vector excitation (i.e., interband transitions between different wavenumbers without phonon assistance) can occur in the indirect-band-gap semiconductor silicon. The wave-vector excitation is successfully induced by irradn. of a silicon thin film with an optical near-field (ONF). As a result, the absorption-band-edge energy Eedge shifts to a lower photon energy of 1.6 eV for ONF excitation from Eedge of 2.1 eV for the conventional excitation by propagating far-field light. The direct wave-vector excitation is caused by the sufficiently large components of wave vectors inherent in the ONF, and thus does not require phonon assistance. For a realistic silicon system, it is clarified that the wave-vector excitations are detd. by the energy difference between the valence and conduction bands and occur irresp. of the initial and final wave vectors.
    39. 39
      Yatsui, T.; Okada, S.; Takemori, T.; Sato, T.; Saichi, K.; Ogamoto, T.; Chiashi, S.; Maruyama, S.; Noda, M.; Yabana, K.; Iida, K.; Nobusada, K. Enhanced photo-sensitivity in a Si photodetector using a near-field assisted excitation. Commun. Phys. 2019, 2, 62,  DOI: 10.1038/s42005-019-0173-1
      There is no corresponding record for this reference.
    40. 40
      Pecchia, A.; Romano, G.; Di Carlo, A.; Gagliardi, A.; Frauenheim, T. Joule heating in molecular tunnel junctions: application to C60. J. Comput. Electron. 2008, 7, 384– 389,  DOI: 10.1007/s10825-008-0219-1
      40
      Joule heating in molecular tunnel junctions: application to C60
      Pecchia, Alessandro; Romano, Giuseppe; Di Carlo, Aldo; Gagliardi, Alessio; Frauenheim, Thomas
      Journal of Computational Electronics (2008), 7 (3), 384-389CODEN: JCEOA7; ISSN:1569-8025. (Springer)
      First-principle calcns. based on d. functional and non-equil. Green's functions are used to compute the power emitted in conducting mol. systems due to scattering with localized vibrations. The balance between the rate of phonons emitted and dissipated into the contacts allows the computation of the steady-state distribution of phonon quanta localized in the junction, from which we ext. the local temp. reached by the mol. The model includes two crit. quantities; (i) the rate of phonon emitted in the junction due to electron-phonon scattering and (ii) a microscopic approach for the computation of the phonon decay rate, accounting for the dynamical coupling between the vibrational modes localized on the mol. and the contact phonons. The method is applied to the discussion of several limiting conditions and trends, depending on electron-phonon coupling, incoherent transmission and phonon dissipation rates, using both anal. results and numerical calcns.
    41. 41
      Ding, S.-Y.; You, E.-M.; Tian, Z.-Q.; Moskovits, M. Electromagnetic theories of surface-enhanced Raman spectroscopy. Chem. Soc. Rev. 2017, 46, 4042– 4076,  DOI: 10.1039/C7CS00238F
      41
      Electromagnetic theories of surface-enhanced Raman spectroscopy
      Ding, Song-Yuan; You, En-Ming; Tian, Zhong-Qun; Moskovits, Martin
      Chemical Society Reviews (2017), 46 (13), 4042-4076CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Surface-enhanced Raman spectroscopy (SERS) and related spectroscopies are powered primarily by the concn. of the electromagnetic (EM) fields assocd. with light in or near appropriately nanostructured elec.-conducting materials, most prominently, but not exclusively high-cond. metals such as silver and gold. This field concn. takes place on account of the excitation of surface-plasmon (SP) resonances in the nanostructured conductor. Optimizing nanostructures for SERS, therefore, implies optimizing the ability of plasmonic nanostructures to conc. EM optical fields at locations where mols. of interest reside, and to enhance the radiation efficiency of the oscillating dipoles assocd. with these mols. and nanostructures. This review summarizes the development of theories over the past four decades pertinent to SERS, esp. those contributing to our current understanding of SP-related SERS. Special emphasis is given to the salient strategies and theor. approaches for optimizing nanostructures with hotspots as efficient EM near-field concg. and far-field radiating substrates for SERS. A simple model is described in terms of which the upper limit of the SERS enhancement can be estd. Several exptl. strategies that may allow one to approach, or possibly exceed this limit, such as cascading the enhancement of the local and radiated EM field by the multiscale EM coupling of hierarchical structures, and generating hotspots by hybridizing an antenna mode with a plasmonic waveguide cavity mode, which would result in an increased local field enhancement, are discussed. Aiming to significantly broaden the application of SERS to other fields, and esp. to material science, we consider hybrid structures of plasmonic nanostructures and other material phases and strategies for producing strong local EM fields at desired locations in such hybrid structures. In this vein, we consider some of the numerical strategies for simulating the optical properties and consequential SERS performance of particle-on-substrate systems that might guide the design of SERS-active systems. Finally, some current theor. attempts are briefly discussed for unifying EM and non-EM contribution to SERS.
    42. 42
      Xia, L.; Chen, M.; Zhao, X.; Zhang, Z.; Xia, J.; Xu, H.; Sun, M. Visualized method of chemical enhancement mechanism on SERS and TERS. J. Raman Spectrosc. 2014, 45 (7), 533– 540,  DOI: 10.1002/jrs.4504
      42
      Visualized method of chemical enhancement mechanism on SERS and TERS
      Xia, Lixin; Chen, Maodu; Zhao, Xuming; Zhang, Zhenglong; Xia, Jiarui; Xu, Hongxing; Sun, Mengtao
      Journal of Raman Spectroscopy (2014), 45 (7), 533-540CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)
      We review our developed visualization method of charge transfer (CT) for chem. enhancement mechanism on surface-enhanced Raman scattering (SERS) and tip-enhanced Raman spectroscopy (TERS). Firstly, we describe our visualization method of charge difference d., which provides direct visual evidence for photoinduced CT. And then, using the visualization method of CT, we interpreted the mechanism of SERS and TERS. Photoinduced charge transfer in the processes of SERS and TERS can be clearly seen. Our visualization method provides a visual and easy understanding way for the mechanism of SERS and TERS. Copyright © 2014 John Wiley & Sons, Ltd.
    43. 43
      Persson, B. N. J. On the theory of surface-enhanced Raman scattering. Chem. Phys. Lett. 1981, 82, 561– 565,  DOI: 10.1016/0009-2614(81)85441-3
      43
      On the theory of surface-enhanced Raman scattering
      Persson, B. N. J.
      Chemical Physics Letters (1981), 82 (3), 561-5CODEN: CHPLBC; ISSN:0009-2614.
      A model for Raman scattering from mols. chemisorbed on surfaces is proposed. Part of the enhancement may be due to charge-transfer excitations between the metal and the adsorbed mols.
    44. 44
      Oren, M.; Galperin, M.; Nitzan, A. Raman scattering from molecular conduction junctions: Charge transfer mechanism. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 115435,  DOI: 10.1103/PhysRevB.85.115435
      44
      Raman scattering from molecular conduction junctions: charge transfer mechanism
      Oren, Michal; Galperin, Michael; Nitzan, Abraham
      Physical Review B: Condensed Matter and Materials Physics (2012), 85 (11), 115435/1-115435/12CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We present a model for the charge transfer contribution to surface-enhanced Raman spectroscopy (SERS) in a mol. junction. The model is a generalization of the equil. scheme for SERS of a mol. adsorbed on a metal surface. We extend the same phys. consideration to a nonequil. situation in a biased mol. junction and to nonzero temps. Two approaches are considered and compared: a semiclassical approach appropriate for nonresonance Raman scattering, and a quantum approach based on the nonequil. Green's function method. Nonequil. effects on this contribution to SERS are demonstrated with numerical examples. It is shown that the semiclassical approach provides an excellent approxn. to the full quantum calcn. as long as the mol. electronic state is outside the Fermi window, i.e., as long as the field-induced charge transfer is small.
    45. 45
      Liu, S. Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation. Nano Lett. 2020, 20 (8), 5879– 5884,  DOI: 10.1021/acs.nanolett.0c01791
      45
      Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation
      Liu, Shuyi; Cirera, Borja; Sun, Yang; Hamada, Ikutaro; Mueller, Melanie; Hammud, Adnan; Wolf, Martin; Kumagai, Takashi
      Nano Letters (2020), 20 (8), 5879-5884CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Tip-enhanced Raman scattering (TERS) in angstr.ovrddot.om-scale plasmonic cavities has drawn increasing attention. However, Raman scattering at vanishing cavity distances remains unexplored. Here, we show the evolution of TERS in transition from the tunneling regime to at. point contact (APC). A stable APC is reversibly formed in the junction between an Ag tip and ultrathin ZnO or NaCl films on the Ag(111) surface at 10 K. An abrupt increase of the TERS intensity occurs upon APC formation for ZnO, but not for NaCl. This remarkable observation is rationalized by a difference in hybridization between the Ag tip and these films, which dets. the contribution of charge transfer enhancement in the fused plasmonic junction. The strong hybridization between the Ag tip and ZnO is corroborated by the appearance of a new vibrational mode upon APC formation, whereas it is not obsd. for the chem. inert NaCl.
    46. 46
      Crampton, K. T.; Lee, J.; Apkarian, V. A. Ion-Selective, Atom-Resolved Imaging of a 2D Cu2N Insulator: Field and Current Driven Tip-Enhanced Raman Spectromicroscopy Using a Molecule-Terminated Tip. ACS Nano 2019, 13 (6), 6363– 6371,  DOI: 10.1021/acsnano.9b02744
      46
      Ion-Selective, Atom-Resolved Imaging of a 2D Cu2N Insulator: Field and Current Driven Tip-Enhanced Raman Spectromicroscopy Using a Molecule-Terminated Tip
      Crampton, Kevin T.; Lee, Joonhee; Apkarian, V. Ara
      ACS Nano (2019), 13 (6), 6363-6371CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Tip-enhanced Raman scattering (TERS) with a Co tetraphenylporphyrin (CoTPP)-terminated Ag tip is used to obtain ion-selective, atomically resolved images of an insulating Cu2N monolayer grown on Cu(100). Ion selective images are obtained through vibrational frequency shift maps using CoTPP vibrations with oppositely signed Stark tuning rates (STR). The images allow a quant. anal. of the electrostatic field of the ionic lattice using in situ calibrated STRs. Both intensity and Stark shift maps yield atomically resolved images in the tunneling regime of plasmons. The CoTPP is bonded to the Ag tip through its central Co atom, whereby TERS taps into intramol. currents and polarizations. The bias dependence of vibrational line intensities shows diode-like response with opposite polarity for current carrying modes of opposite polarization phase. The phase sensitive detection of vibrational lines and their voltage gating is explained in terms of distinct field- and phototunneling current-driven Raman, offering an alternate paradigm for the long-sought optoelectronic rectifier in mol. electronics.
    47. 47
      Liebhaber, M. Surface phonons of the Si(111)-(7 × 7) reconstruction observed by Raman spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89 (4), 045313,  DOI: 10.1103/PhysRevB.89.045313
      47
      Surface phonons of the Si(111)-(7 × 7) reconstruction observed by Raman spectroscopy
      Liebhaber, M.; Bass, U.; Bayersdorfer, P.; Geurts, J.; Speiser, E.; Raethel, J.; Baumann, A.; Chandola, S.; Esser, N.
      Physical Review B: Condensed Matter and Materials Physics (2014), 89 (4), 045313/1-045313/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We have studied the surface phonon modes of the reconstructed Si(111)-(7 × 7) surface by polarized Raman spectroscopy. Six surface vibration modes are obsd. in the frequency range between 62.5-420.0 cm-1. The mode frequencies agree very well with reported calcn. results. This enables their attribution to calcd. eigenmodes, whose elongation patterns are dominated by specific at. sites: the two most characteristic novel fingerprints of the (7 × 7) reconstruction are sharp Raman peaks from localized adatom vibrations, located at 250.9 cm-1, and collective vibrations of the adatoms and 1st- and 2nd-layer atoms, located at 420.0 cm-1. While the sharp localized adatom vibration peak is a substantial refinement of an earlier broad spectral structure from electron energy-loss spectroscopy, no spectroscopic features were reported before in the collective-vibration frequency region. Furthermore, we observe in-plane wagging vibrations in the range from 110-140 cm-1, and finally the backfolded acoustic Rayleigh wave at 62.5 cm-1, which coincides with He atom scattering data. Moreover, the Raman peak intensities of the surface phonons show a mode-specific dependence on the polarization directions of incident and scattered light. From this polarization dependence the relevant symmetry components in the Raman scattering process (A1 and/or E symmetry) are deduced for each mode.
    48. 48
      Kaya, D.; Cobley, R. J.; Palmer, R. E. Combining scanningtunneling microscope imaging and local manipulation to probe thehigh dose oxidation structure of the Si(111)-7 × 7 surface. Nano Res. 2020, 13, 145– 150,  DOI: 10.1007/s12274-019-2587-1
      48
      Combining scanning tunneling microscope (STM) imaging and local manipulation to probe the high dose oxidation structure of the Si(111)-7×7 surface
      Kaya, Dogan; Cobley, Richard J.; Palmer, Richard E.
      Nano Research (2020), 13 (1), 145-150CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
      Understanding the atomistic formation of oxide layers on semiconductors is important for thin film fabrication, scaling down conventional devices and for the integration of emerging research materials. Here, the initial oxidn. of Si(111) is studied using the scanning tunneling microscope. Prior to the complete satn. of the silicon surface with oxygen, we are able to probe the at. nature of the oxide layer formation. We establish the threshold for local manipulation of inserted oxygen sites to be +3.8 V. Only by combining imaging with local at. manipulation are we able to det. whether inserted oxygen exists beneath surface-bonded oxygen sites and differentiate between sites that have one and more than one oxygen atom inserted beneath the surface. Prior to the creation of the thin oxide film we observe a flip in the manipulation rates of inserted oxygen sites consistent with more oxygen inserting beneath the silicon surface. [Figure not available: see fulltext.].
    49. 49
      Lee, S.-H.; Kang, M.-H. Electronic and vibrational properties of initial-stage oxidation products on Si (111)–(7 × 7). Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61 (12), 8250– 8255,  DOI: 10.1103/PhysRevB.61.8250
      49
      Electronic and vibrational properties of initial-stage oxidation products on Si(111)-(7×7)
      Lee, Sung-Hoon; Kang, Myung-Ho
      Physical Review B: Condensed Matter and Materials Physics (2000), 61 (12), 8250-8255CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Chemisorption of O2 mols. on the adatom site of Si(111)-(7×7) has been studied by d.-functional theory calcns. for all possible dissocn. configurations. Structures possessing an oxygen atom on top of the Si adatom are all found to be metastable and account well for the metastable electronic and vibrational spectra obsd. in previous expts., while structures with only oxygen atoms inserted into the adatom back bonds appear quite stable. The present structural models therefore are all compatible with either the metastable or the stable O2 reaction products found in this system. The calcd. decay pathways of the metastable structures provide addnl. informations useful for identifying the exptl. metastable structures.
    50. 50
      Okuyama, H.; Aruga, T.; Nishijima, M. Vibrational Characterization of the Oxidation Products on Si(111)–(7 × 7). Phys. Rev. Lett. 2003, 91 (25), 256102,  DOI: 10.1103/PhysRevLett.91.256102
      50
      Vibrational Characterization of the Oxidation Products on Si(111)-(7×7)
      Okuyama, H.; Aruga, T.; Nishijima, M.
      Physical Review Letters (2003), 91 (25), 256102/1-256102/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      The oxidn. products on Si(111)-(7×7) were studied at 82 K by high-resoln. EELS. The isotope-labeled vibrational spectra with O216, O218, and O16 O18 show that, in the initial stage of the oxidn., an O2 mol. dissocs. to form a metastable product with an O atom bonding on top of the Si adatom and the other inserted into the backbond. The metastable product is obsd. as a dark site in the topog. scanning tunneling microscopy (STM) image and can be transformed to a stable product by the STM manipulation. The authors' results are in good agreement with recent theor. calcns.
    51. 51
      Ibach, H.; Bruchmann, H. D.; Wagner, H. Vibrational study of the initial stages of the oxidation of Si(111) and Si(100) surfaces. Appl. Phys. A: Solids Surf. 1982, 29, 113– 124,  DOI: 10.1007/BF00617767
      51
      Vibrational study of the initial stages of the oxidation of silicon (111) and silicon (100) surfaces
      Ibach, H.; Bruchmann, H. D.; Wagner, H.
      Applied Physics A: Solids and Surfaces (1982), A29 (3), 113-24CODEN: APSFDB; ISSN:0721-7250.
      By using high-resoln. electron energy loss spectroscopy, the vibrations of Si(111) and Si(100) surfaces in the early stags of oxidn. were investigated. Three different stages of oxidn., the last being the formation of a thin layer of vitreous SiO2 were identified when the surfaces were held at 700 K during exposure to O2. The 1st 2 stages involve at. O in bridging positions between Si atoms. Small exposures at low temps. (100 K) produce vibrational features of a different, possibly mol., species. For higher exposures at the same temp., the spectrum again develops the characteristics of at. O and the mol. species eventually disappears. Exposure at room temp. leads to a mixt. of O and O2 for smaller exposures and to purely at. O for exposures greater than ∼102 lambert.
    52. 52
      Okuyama, H.; Ohtsuka, Y.; Aruga, T. Secondary oxidation product on Si(111)-(7 × 7) characterized by isotope-labeled vibrational spectroscopy. J. Chem. Phys. 2005, 122, 234709,  DOI: 10.1063/1.1937394
      52
      Secondary oxidation product on Si(111)-(7×7) characterized by isotope-labeled vibrational spectroscopy
      Okuyama, H.; Ohtsuka, Y.; Aruga, T.
      Journal of Chemical Physics (2005), 122 (23), 234709/1-234709/5CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The reaction of O2 with Si(111)-(7×7) has been studied by electron energy-loss spectroscopy at 82 K. In addn. to the losses due to Si-O-Si configurations, we obsd. two Si-O stretch modes depending on the coverage. A 146-meV peak appears at the initial reaction stage and was ascribed to a metastable product with one oxygen atom bonding on top of Si adatom and the other inserted into the back-bond. The initial product is further oxidized to produce the second Si-O stretch peak at 150 meV. The secondary product was partially substituted with isotopes and analyzed with a simple model of coupled oscillators. The vibrational spectra reflect dynamical couplings between the isotopes, which is consistent with those predicted from the tetrahedral SiO4 structure with one on top and three inserted oxygen atoms.

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.1c00998.

    • Methods, small Ag clusters created after APC-TERS measurement, TERS for different APCs over Si(111)-7 × 7, APC-TERS of Si(111)-7 × 7 measured with an Au tip, LSPR dependence of APC-TERS, wavelength dependence of APC-TERS and attenuation of the intensity in the contact regime, APC-TERS at step edges of Si(111)-7 × 7, APC-TERS over partially oxidized Si(111)-7 × 7 (PDF)


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