Vibrational Excitation of a Single Benzene Molecule Adsorbed on Cu

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Vibrational Excitation of a Single Benzene Molecule Adsorbed on Cu(110) Studied by Scanning Tunneling Microscope Light Emission Spectroscopy Satoshi Katano,*,† Sukekatsu Ushioda,‡ and Yoichi Uehara*,† †

Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan, and National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

‡

ABSTRACT We have investigated the vibrational excitation of a single benzene molecule adsorbed on Cu(110) using scanning tunneling microscope light emission (STM-LE) spectroscopy. The STM-LE spectrum of the benzene molecule has a broad light emission peak and is similar to that of the Cu substrate, indicating that the STM-LE is radiated via the excitation of localized surface plasmons (LSP) confined in the tip-sample gap. However, careful analysis revealed that the STMLE spectrum of the benzene molecule exhibits a steplike structure appearing at an energy near the quantum cutoff. Isotope-labeled experiments revealed that the fine structure is associated with the vibrational excitation of the benzene molecule, that is, the CH out-of-plane bending mode. We concluded that the vibrational excitation of benzene observed in the present study is not caused by the tunneling electrons through the inelastic tunneling process but by the Raman process due to the strong electric field generated just under the STM tip. SECTION Surfaces, Interfaces, Catalysis

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he investigation of a single molecule has received much attention because of the use of single molecules as nanoelectronic devices.1 In particular, scanning tunneling microscopy (STM) is a powerful tool for elucidating the physical and chemical properties of materials at the singlemolecule scale.2,3 Furthermore, it is known that an atomic-size source of electrons from the STM tip can excite individual molecules electronically and vibrationally, which enables us to manipulate a single atom and a molecule and even to induce a chemical reaction.4-9 Such precise control of the molecular dynamics on the atomic scale is expected to be indispensable for realizing actual molecular nanodevices and machines.10-12 On the other hand, identification of the chemical composition of a single molecule is considered to be another central issue for the fabrication of nanomolecular devices. Above all, the recent development of inelastic electron tunneling microscopy based on STM (STM-IETS) has been paid particular attention because of the ability to examine individual molecules by vibrational spectroscopy.5 Pioneering work was performed by Stipe et al., who have succeeded in detecting the vibrational mode of a single acetylene molecule using STM-IETS.13 To date, a number of molecules have been thoroughly investigated by STM-IETS.5,14 However, a problem with STM-IETS is that not all molecules can be detected. One of the representative examples of such a molecule is benzene, which is adsorbed on Cu substrates. Lauhon and Ho examined the adsorption of benzene on Cu(001) by STM-IETS spectroscopy and showed that intact benzene gives no signal in the STM-IETS spectrum.15 Thereafter, a similar result was also

r 2010 American Chemical Society

reported by another group for the adsorption of benzene on Cu(110).16 Lorente and co-workers performed a density functional theory (DFT) calculation on the above system and revealed that the absence of electronic states of the molecule near the Fermi level leads to no excitation of the vibrational modes by the tunneling electrons.17-20 In this study, we have investigated the adsorption of a benzene molecule on Cu(110) by STM light emission (STM-LE) spectroscopy. Here, we demonstrate an alternative approach to detecting the vibrational mode of a single molecule by STM. The STM-LE spectrum of the benzene molecule has a broad light emission peak and is similar to that of the Cu substrate, indicating that the STM-LE is radiated via the excitation of localized surface plasmons (LSP) confined in the tip-sample gap. We found a characteristic steplike feature that only appeared in the STM-LE spectrum obtained from the benzene molecule. The steplike structure is caused by the vibrational excitation of the benzene molecule. Here, we show that the vibrational response of the benzene molecule observed in the present study occurs via the Raman process due to the strong enhancement of the electromagnetic field by the STM gap. Furthermore, the present study suggests that STM-LE can be utilized as a tool to identify a single molecule through the vibrational excitation and that it can be applicable in systems where STM-IETS is inoperable. Received Date: June 14, 2010 Accepted Date: August 31, 2010 Published on Web Date: September 07, 2010

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DOI: 10.1021/jz100808y |J. Phys. Chem. Lett. 2010, 1, 2763–2768

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surface followed by the injection of tunneling electrons with a sample bias of 2.3 V and a tunneling current of 2.0 nA. The spectrum consists of a broad peak, which can be characterized by the light emission induced via the excitation of the LSP.26 The observed higher-energy cutoff appearing at 2.3 eV meets the quantum cutoff condition eV0 = hνmax, where e is the elementary charge, V0 is the applied bias voltage, h is Planck's constant, and νmax is the maximum photon frequency. On the other hand, the STM-LE spectrum obtained above the center of a deuteride benzene (C6D6) molecule is shown in Figure 2b. Although the C6D6 spectrum has similar features to that of the Cu substrate spectrum, careful analysis indicated a small difference between them at around the quantum cutoff energy. The C6D6 spectrum exhibits a fine structure with a steplike shape. As indicated in Figure 2b, the energy of the step width measured from the quantum cutoff energy (2.3 V) is determined to be 62 meV. Note that the shape of the STM tip is known to affect the shape of the STM-LE spectrum as well as the intensity of the STM-LE.27 It may be claimed that the observed difference can be explained merely by the effect of the STM tip. We investigated this possibility by performing the same experiment with different STM tips. Figure 2c and d shows another set of STM-LE spectra obtained from the Cu surface and the C6D6 molecule, respectively. These spectra were acquired by the same procedure as before. The STM-LE spectra in Figure 2c and d exhibit a broad peak, similar to those in Figure 2a and b. However, the two Cu substrate spectra (Figure 2a and c) differ in shape, indicating that the shape of the STM tip affects the STM-LE spectrum. It is noteworthy that the steplike structure is also visible in the C6D6 spectrum (Figure 2d) with the same energy of the step width (62 meV). Therefore, we consider that the appearance of the step structure in the STM-LE spectrum reflects the response of the benzene molecule under the STM-LE process. The energy of the step width observed in Figure 2 is 62 meV, which is in the range of the vibrational energy of a benzene molecule. Thus, the energy width of the step appearing in the STM-LE spectrum should change when the experiment is performed using an isotope-substituted benzene molecule. Figure 3b shows the STM-LE spectrum obtained from a H-terminated benzene (C6H6) molecule adsorbed on Cu(110), and the spectrum obtained from the Cu substrate is also shown in Figure 3a for reference. As expected, the step structure only appears in the STM-LE spectrum obtained from C6H6. Note that the energy width of the step increases to 85 meV, in comparison with 62 meV for the step observed in Figure 2b, thus exhibiting the expected H-D isotope shift (νH/ νD = 1.4). Therefore, the excitation of the vibrational mode should be involved in the STM-tip-induced light emission process observed in the present study. In a previous vibrational study performed on the benzene/Cu(110) system, Lomas et al. reported that four types of vibrational modes, which are assigned to the CH stretching mode, the CC stretching mode, and the CH out-of-plane/in-plane bending modes, can be detected by high-resolution electron energy loss spectroscopy (HREELS).21 As mentioned above, we have confirmed that the energy width of the step observed in the STM-LE spectrum of benzene is 85 meV for C6H6 and 62 meV

Figure 1. STM image of benzene molecules adsorbed on Cu(110) surface obtained at a sample bias of 1.0 V and a tunneling current of 0.1 nA.

Figure 2. (a, b) STM-LE spectra obtained after fixing the STM tip above (a) the Cu substrate and (b) a deuteride benzene (C6D6) molecule. (c, d) Another set of STM-LE spectra obtained from (c) the Cu substrate and (d) C6D6 using a different STM tip. All spectra were obtained at a the sample bias of 2.3 V, a tunneling current of 2.0 nA, and an exposure time of 100 s.

The STM image of benzene molecules adsorbed on Cu(110) at 80 K is shown in Figure 1. Previous spectroscopic studies revealed that benzene is molecularly adsorbed on Cu(110) below room temperature.21 As can be seen in Figure 1, individual benzene molecules are resolved as roundshaped protrusions on Cu(110). Several groups have previously succeeded in imaging a single benzene molecule on Cu(110), revealing that the benzene molecule is adsorbed at the hollow site on the Cu(110) surface.16,22 Nilsson and coworkers performed near-edge X-ray absorption spectroscopy (NEXAFS) on the same system and clarified that benzene molecules are adsorbed with the molecular plane parallel to the Cu(110) substrate,23,24 which was also indicated by DFT calculation.25 We have performed the STM-LE spectroscopy on the benzene/Cu(110) system. Figure 2a shows the STM-LE spectrum obtained after the STM tip was fixed above the Cu


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sufficient σ*CH states at the Fermi level owing to the rehybridization of the electronic states between the benzene molecule and the Cu substrate, which makes it possible for a tunneling electron to excite the vibrational mode of the benzene molecule. Thus, we consider that the vibrational feature observed in the STM-LE spectra of the benzene molecule with the flat-lying configuration cannot be explained by the direct excitation of the vibrational state of the benzene molecule via the inelastic electron tunneling process. We reiterate that the STM-LE spectrum of benzene is similar to that of the Cu substrate in terms of the overall spectral shape, except for a small difference in the energy near the quantum cutoff energy (Figures 2 and 3). This indicates that the light emitted from the benzene molecule is radiated via the excitation of the LSP of the Cu substrate. A plausible explanation for the origin of the steplike structure superimposed in the benzene spectrum is shown in Figure 4. The appearance of the step near the quantum cutoff energy in the STM-LE spectrum can be explained by the convolution of the two components of the STM-LE spectra shown in Figure 4a. The quantum cutoff energy of the STM-LE spectrum obtained from the Cu substrate coincides with the applied bias energy eV0 (Figure 4a). If the benzene molecule is vibrationally excited in the STM-LE process, the radiated light is expected to show a decrease in the energy (eV0 - hνvib) corresponding to the excitation of the vibrational mode (Figure 4a). The resultant STM-LE spectrum (Figure 4b) is represented as the convolution of the two components of the STM-LE spectra with and without accompanying the excitation of the vibrational mode. We estimate the amount of the light that loses the energy due to the excitation of the molecular vibration. As illustrated in Figure 4c, the experimental STM-LE spectrum from the benzene (Figure 2b) can be reproduced when 30-50% of the emitted light was radiated through the excitation of the molecular vibration. Now let us discuss the origin of the two components of the spectra. STM light emission from metallic samples is welldescribed by a current fluctuation theory. In this theory,29 the tunneling current which fluctuates with a quantum mechanical origin first excites LSP, and then STM light is radiated by LSP.30,31 Since the tunneling current fluctuation has the quantum cutoff at eV0,29 the light emitted via this mechanism inherits this feature. We consider that the component shown in Figure 4a, which has the quantum cutoff at eV0, is excited via this mechanism. The inset in Figure 4d shows the cross-sectional view of the tip-sample gap. The z axis and x-y axes are taken perpendicular and parallel to the sample surface, respectively. The origin of the x-y-z coordinate system lies at the cross point of the sample surface and the symmetrical axis of the tip. The near-electric-field pattern radiated by the fluctuating tunneling current is analytically obtained by solving the Maxwell equations.31 Figure 4d show the x dependence of the z-component amplitude of the electric field (Ez) along the line of z = d/2. While Figure 4d was calculated for a frequency of 2 eV, the results did not show strong frequency dependence in the measured spectral range. We found that Ez has a sharp peak at x = 0. Thus, when the tip is located just over a single molecule of benzene adsorbed on Cu(110), a Raman dipole is

Figure 3. STM-LE spectra obtained after fixing the STM tip above (a) the Cu substrate and (b) a benzene (C6H6) molecule. All spectra were obtained at a sample bias of 2.3 V, a tunneling current of 2.0 nA, and an exposure time of 100 s.

for C6D6, which correspond to the vibrational energies of the out-of-plane CH bending mode. The vibrational excitation of a single benzene molecule induced by the tunneling electrons from a STM tip was studied by several groups from the viewpoints of experimental measurement and theoretical calculation. A decade ago, pioneering experimental work using STM-IETS was reported by Lauhon et al.15 They clarified that a benzene molecule adsorbed on Cu(100) with the flat-lying configuration exhibits no conductance (second derivative of the tunneling current) change in the STM-IETS spectrum, indicating no vibrational response by the tunneling electrons. However, a different result was obtained when STM-IETS measurement was performed on a chemically transformed benzene molecule. The injection of tunneling electrons from the STM tip at a sample bias voltage of 3 V resulted in a structural transformation, from the flatlying to upright configuration, accompanied by the dehydrogenation of benzene. The STM-IETS spectrum acquired for the upright benzene exhibits a peak at 378 (282) meV, corresponding to the vibrational excitation of the CH (CD) stretching mode of the benzene molecule induced by the tunneling electrons. Komeda et al. performed a similar experiment on the Cu(110) surface and reported a comprehensive examination of the vibrational response of a benzene molecule against tunneling electrons.16 They showed that the capture of a tunneling electron in the antibonding resonance of the CH bond (σ*CH) is important for inducing the vibrational excitation of the benzene molecule. Lorente et al. performed a DFT calculation for the above-described system.17-20 In the case of flat-lying benzene, they claimed that the absence of the σ*CH orbital at the Fermi level leads to no excitation of the CH stretching modes by the tunneling electrons. However, the upright benzene molecule transformed by the STM tip has


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may expect that the component radiated by the Raman dipole becomes comparable in intensity to that radiated by the LSP, as shown in Figure 4c, which has been confirmed by numerical calculations for a moderate size of Raman polarizability. Note that the configuration of our experimental system is similar to that of tip-enhanced Raman spectroscopy (TERS).32,33 However, it is widely recognized that the enhancement factor for TERS is not large, that is, ∼104. Instead, we consider that our experimental system rather resembles surface-enhanced Raman spectroscopy (SERS) using noble metal colloids in the configuration of the excitation of the Raman dipole.34 The hot spot existing between the gap of the aggregated metal colloids plays a crucial function for the enhancement of the electric field. SERS from metal colloids exhibits the significant large Raman cross section, which is comparable to the cross section of the fluorescence.35 Therefore, it would be reasonable to interpret that large enhancement of the electric field generated by the tunneling electrons at the STM gap is responsible for the fact that 30-50% of the light was radiated through the excitation of the molecular vibration. Moreover, the out-of-plane bending mode observed in the STM-LE spectra of benzene (Figures 2 and 3) is the only vibrational mode that oscillates parallel to the surface-normal direction. It is interesting to note that the direction of this vibrational mode coincides with the direction of the electromagnetic field Ez. This suggests that the spatial alignment between the vibrational motion and the electromagnetic field generated just under the STM tip is also important for the excitation of the vibrational mode in the STM light emission process. In summary, STM-LE spectroscopy was used to investigate the vibrational excitation of a single benzene molecule adsorbed on Cu(110). The STM-LE spectra taken above the benzene molecule and the Cu substrate exhibit a broad peak. This indicates that light emission occurs through the excitation of the LSP, which is generated by the injection of a tunneling electron from the STM tip. However, careful analysis revealed that a steplike structure appeared near the quantum cutoff energy when STM-LE was performed on the benzene molecule. We found that the energy widths of the step observed in the STM-LE spectra of C6H6 and C6D6 were 85 and 62 meV, respectively, in agreement with the expected isotope shift. Referring to the previous vibrational spectroscopy study, we assigned the observed vibrational mode to the CH out-of-plane bending mode of the benzene molecule. The vibrational excitation by a tunneling electron through the inelastic tunneling process is not an appropriate explanation in the present case since the electronic state near the Fermi level consists of fewer σ*CH states of the benzene molecule. We concluded that the vibrational excitation of benzene in the STM-LE is caused by the Raman process due to the strong electric field generated just under the STM tip.

Figure 4. (a, b) Schematic model of the STM light emission spectrum observed in the present study (benzene on Cu(110)). (a) The STM-LE spectrum exhibits a broad peak whose cutoff energy coincides with the applied bias energy (eV0) in the case that vibrational excitation does not occur in the STM-LE process. The cutoff energy is shifted to a lower energy (eV0 - hνvib) upon the vibrational excitation of the molecule in the STM-LE process. (b) The experimentally obtained STM-LE spectrum is represented by the convolution of two components of the spectra shown in (a). (c) The simulation of the STM-LE spectrum obtained from the benzene molecule. The black curve represents the experimental STMLE spectrum of the benzene taken from Figure 2b. The red curve represents the calculated STM-LE spectrum of the Cu substrate using the dielectric theory.28 The intensity of the red curve is normalized to that of the experimental spectrum. The orange curve represents the calculated STM-LE spectrum whose cutoff energy is shifted to lower energy corresponding to the energy of the vibrational mode (62 meV). The steplike structure appearing in the benzene spectrum can be represented by mixing the red curve with the orange curve in various proportions. The mixing ratios (the intensity of STM-LE accompanying the vibrational excitation divided by the total intensity of the STM-LE) are shown in the inset of (c). The experimental STM-LE spectrum (black curve) can be represented when 30-50% of the light was radiated through the excitation of the molecular vibration. (d) The x dependence of the z-component amplitude of the electric field (Ez) along the line of z = d/2. The inset shows the cross-sectional view of the tipsample gap. The z axis and x-y axes are taken perpendicular and parallel to the sample surface, respectively. The origin of the x-y-z coordinate system lies at the cross point of the sample surface and the symmetrical axis of the STM tip.

induced by the strong electric field Ez at x = 0 (Figure 4d). Since the strong electric field Ez is localized just beneath the STM tip, the light emitted from the excitation of the Raman dipole must be radiated from an extremely small area that is comparable to the atomic scale. It should be noted that the quantum cutoff of the Raman dipole is eV0 - hνvib. We consider that the radiation from the Raman dipole, which is electromagnetically enhanced by the nanometer scale gap before coupling to free electrons, is the origin of the component shown in Figure 4a. Since the strength of Ez for exciting the Raman dipole is much stronger than those which characterize LSP with the lateral size of 2(ad)1/2 (Figure 4d), one


EXPERIMENTAL METHODS All experiments were performed in an ultrahigh vacuum (UHV) chamber under the base pressure of 1 10-10 Torr. The Cu(110) surface was cleaned by repeated Ar ion sputtering

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and annealing until a clean surface was confirmed by STM imaging. The clean Cu(110) surface was exposed to benzene (C6H6 and C6D6) at 80 K through a dosing tube located near the substrate. STM experiments were performed using a lowtemperature STM at 80 K with an electrochemically etched tungsten tip. The STM images shown here were obtained in constant-current mode. The light emitted from the STM junction was measured with an image-intensified optical multichannel detector after passing through a spectrograph.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. E-mail: skatano@ riec.tohoku.ac.jp (S.K.); [email protected] (Y.U.). Tel: þ8122-217-5498. Fax: þ81-22-217-5500.

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REFERENCES (1)

(2)

(3) (4)

(5) (6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

Joachim, C.; Gimzewski, J. K.; Aviram, A. Electronics Using Hybrid-molecular and Mono-Molecular Devices. Nature 2000, 408, 541–548. Katano, S.; Kato, H. S.; Domen, K.; Kawai, M. Self-Activated Catalyst Layer for the Partial Hydrogenation of 1,3-Butadiene on a Hydrogen-Precovered Pd(110) Surface. J. Phys. Chem. C 2009, 113, 14872–14878. Bartels, L. Tailoring Molecular Layers at Metal Surfaces. Nat. Chem. 2010, 2, 87–95. Moresco, F. Manipulation of Large Molecules by LowTemperature STM: Model Systems for Molecular Electronics. Phys. Rep. 2004, 399, 175–225. Ho, W. Single-Molecule Chemistry. J. Chem. Phys. 2002, 117, 11033–11061. Grill, L.; Moresco, F. Contacting Single Molecules to Metallic Electrodes by Scanning Tunneling Microscope Manipulation: Model Systems for Molecular Electronics. J. Phys.: Condens. Matter 2006, 18, S1887–S1908. Katano, S.; Kim, Y.; Hori, M.; Trenary, M.; Kawai, M. Reversible Control of Hydrogenation of a Single Molecule. Science 2007, 316, 1883–1886. Katano, S.; Kim, Y.; Kagata, Y.; Kawai, M. Vibration-assisted Rotation and Deprotonation of a Single Formic Acid Adsorbed on Ni(110) Studied by Scanning Tunneling Microscopy. J. Phys. Chem. C 2009, 113, 19277–19280. Katano, S.; Kim, Y.; Kagata, Y.; Kawai, M. Single Molecule Vibrational Spectroscopy and Inelastic Tunneling ElectronInduced Diffusion of Formate Adsorbed on Ni(110). J. Phys. Chem. C 2010, 114, 3003–3007. Mayne, A. J.; Dujardin, G.; Comtet, G.; Riedel, D. Electronic Control of Single-Molecule Dynamics. Chem. Rev. 2006, 106, 4355–4378. Chiaravalloti, F.; Gross, L.; Rieder, K. H.; Stojkovic, S. M.; Gourdon, A.; Joachim, C.; Moresco, F. A Rack-and-pinion Device at the Molecular Scale. Nat. Mater. 2007, 6, 30–33. Manzano, C.; Soe, W. H.; Wong, H. S.; Ample, F.; Gourdon, A.; Chandrasekhar, N.; Joachim, C. Step-by-Step Rotation of a Molecule-Gear Mounted on an Atomic-Scale Axis. Nat. Mater. 2009, 8, 576–579. Stipe, B. C.; Rezaei, M. A.; Ho, W. Single-Molecule Vibrational Spectroscopy and Microscopy. Science 1998, 280, 1732– 1735. Komeda, T. Chemical Identification and Manipulation of Molecules by Vibrational Excitation via Inelastic Tunneling


(21)

(22)

(23)

(24)

(25)

(26)

(27)

(28)

(29)

(30)

(31)

(32) (33)

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Process with Scanning Tunneling Microscopy. Prog. Surf. Sci. 2005, 78, 41–85. Lauhon, L. J.; Ho, W. Single-Molecule Chemistry and Vibrational Spectroscopy: Pyridine and Benzene on Cu(001). J. Phys. Chem. A 2000, 104, 2463–2467. Komeda, T.; Kim, Y.; Fujita, Y.; Sainoo, Y.; Kawai, M. Local Chemical Reaction of Benzene on Cu(110) Via STM-Induced Excitation. J. Chem. Phys. 2004, 120, 5347–5352. Lorente, N.; Hedouin, M. F. G.; Palmer, R. E.; Persson, M. Chemisorption of Benzene and STM Dehydrogenation Products on Cu(100). Phys. Rev. B 2003, 68, 155401. Bocquet, M. L.; Lesnard, H.; Lorente, N. Inelastic Spectroscopy Identification of STM-Induced Benzene Dehydrogenation. Phys. Rev. Lett. 2006, 96, 096101. Lesnard, H.; Bocquet, M. L.; Lorente, N. Dehydrogenation of Aromatic Molecules Under a Scanning Tunneling Microscope: Pathways and Inelastic Spectroscopy Simulations. J. Am. Chem. Soc. 2007, 129, 4298–4305. Lesnard, H.; Lorente, N.; Bocquet, M. L. Theoretical Study of Benzene and Pyridine STM-Induced Reactions on Copper Surfaces. J. Phys.: Condens. Matter 2008, 20, 224012. Lomas, J. R.; Baddeley, C. J.; Tikhov, M. S.; Lambert, R. M. On the Orientation of Benzene Adsorbed on Cu(110). Chem. Phys. Lett. 1996, 263, 591–596. Doering, M.; Rust, H. P.; Briner, B. G.; Bradshaw, A. M. Imaging Benzene on Nickel and Copper (110) Surfaces with Low Temperature STM: The Adsorption Site. Surf. Sci. 1998, 410, L736–L740. Nilsson, A.; Wassdahl, N.; Weinelt, M.; Karis, O.; Wiell, T.; Bennich, P.; Hasselstrom, J.; Fohlisch, A.; Stohr, J.; Samant, M. Local Probing of the Surface Chemical Bond Using X-ray Emission Spectroscopy. Appl. Phys. A 1997, 65, 147– 154. Weinelt, M.; Wassdahl, N.; Wiell, T.; Karis, O.; Hasselstrom, J.; Bennich, P.; Nilsson, A.; Stohr, J.; Samant, M. Electronic Structure of Benzene on Ni(100) and Cu(110): An X-raySpectroscopy Study. Phys. Rev. B 1998, 58, 7351–7360. Pettersson, L. G. M.; Agren, H.; Luo, Y.; Triguero, L. Benzene Adsorbed on Cu(110): Theoretical X-ray Absorption, Emission and Shake Calculations. Surf. Sci. 1998, 408, 1–20. Uehara, Y.; Matsumoto, T.; Ushioda, S. Identification of O Atoms on a Cu(110) Surface by Scanning Tunneling Microscope Light Emission Spectra. Phys. Rev. B 2002, 66, 075413. Uehara, Y.; Suda, Y.; Ushioda, S.; Takeuchi, K. Tip Shape Dependence of the Light Emission Efficiency for the Scanning Tunneling Microscope. Appl. Phys. Lett. 2001, 79, 1718– 1720. Uehara, Y.; Kimura, Y.; Ushioda, S.; Takeuchi, K. Theory of Visible-Light Emission from Scanning Tunneling Microscope. Jpn. J. Appl. Phys. 1992, 31, 2465–2469. Hone, D.; Muhlschlegel, B.; Scalapino, D. J. Theory of LightEmission from Small Particle Tunnel-Junctions. Appl. Phys. Lett. 1978, 33, 203–204. Rendell, R. W.; Scalapino, D. J.; Muhlschlegel, B. Role of Local Plasmon Modes in Light-Emission From Small-particle TunnelJunctions. Phys. Rev. Lett. 1978, 41, 1746–1750. Rendell, R. W.; Scalapino, D. J. Surface-Plasmons Confined by Microstructures on Tunnel-Junctions. Phys. Rev. B 1981, 24, 3276–3294. Pettinger, B. Tip-Enhanced Raman Spectroscopy (TERS). Top. Appl. Phys. 2006, 103, 217–240. Pettinger, B.; Picardi, G.; Schuster, R.; Ertl, G. SurfaceEnhanced and STM-Tip-Enhanced Raman Spectroscopy at Metal Surfaces. Single Mol. 2002, 3, 285–294.


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(34)

(35)

Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Surface-Enhanced Non-Linear Raman Scattering at the Single-Molecule Level. Chem. Phys. 1999, 247, 155–162. Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S. Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667–1670.


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Vibrational Excitation of a Single Benzene Molecule Adsorbed on Cu

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