Conductance and SERS Measurement of Benzenedithiol Molecules

Jan 4, 2013 - We have measured electric conductance and surface enhanced Raman scattering (SERS) spectra for the benzenedithiol molecules bridging ...
0 downloads 0 Views 924KB Size
Download PDF
Article pubs.acs.org/JPCC

Conductance and SERS Measurement of Benzenedithiol Molecules Bridging Between Au Electrodes Ryuji Matsuhita,† Masyo Horikawa,‡ Yasuhisa Naitoh,‡ Hisao Nakamura,‡ and Manabu Kiguchi†,* †

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology 2-12-1 W4-10 Ookayama, Meguro-ku, Tokyo 152-8551, Japan, ‡ Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Central 2, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan S Supporting Information *

ABSTRACT: We have measured electric conductance and surface enhanced Raman scattering (SERS) spectra for the benzenedithiol molecules bridging between Au nano gap electrodes at room temperature to investigate the electric and optical properties of the molecular junction. The current−voltage characteristic of the molecular junction was investigated based on the single level tunneling model considering that the Fermi distribution function got smeared out for finite temperature, which provided the information on the number of molecules bridging metal electrodes, energy difference between conduction orbital and Fermi level, strength of the molecule-metal coupling. The conductance measurements confirmed the interaction between molecule and Au electrodes. Due to the interaction between molecule and metal electrodes, we could observe the SERS spectra characteristic of the molecular junction. The SERS measurement showed the disappearance of the SH modes and red shift of some internal vibration modes, which confirmed the molecular bridge between Au electrodes. For some molecular junctions, we observed the b2 mode, which was not observed in the bulk crystal. The appearance of the b2 mode could be explained by the photon-driven charge transfer from molecule to metal electrodes.

■

INTRODUCTION The molecular electronics, where molecules serve as the basic electrical component, are of fundamental scientific interest and relevant to future technologies. Various methods (e.g., nano pore, scanning tunneling microscope, atomic force microscope) have been utilized to fabricate molecular junctions, which are the basic components in molecular electronics.1−10 The electron transport properties of the molecular junctions have been investigated for number of molecules, such as benzenedithiol, porphyrin, DNA. Recently, diode, transistor, switching behaviors have been reported for the single molecular junctions.11−13 The characterization of the molecular junction is essential to understand the properties of the molecular junction. Inelastic electron tunneling spectroscopy (IETS) and point contact spectroscopy (PCS) have been widely used to obtain the structural information about the molecular junction.14−20 However, this technique requires low-temperature and vacuum conditions. Conventional optical spectroscopic techniques, such as IR and Raman spectroscopy, are the most promising ones to study the molecular junction at room temperature. On the basis of these interests, Raman spectroscopy has been utilized to the molecular junction, because surface enhanced Raman scattering (SERS) offers a single-molecule sensitivity.21−24 The strong electric field formed between the nano gap electrodes enhances the intensity of Raman signal, and thus, © 2013 American Chemical Society

Raman spectrum from the molecule in the nano gap is selectively observed.26,27 Tian et al. observed the increase in the SERS intensity of molecules in the nano gap with decreasing the gap size using mechanically controllable break junction method (MCBJ).21 Liu et al. observed the splitting of the Raman peak, and the peak energy shift by applying the bias voltage to the single molecular junction.22 Ioffe et al. succeeded in evaluating the effective temperature of the molecular junction by measuring both the Stokes and anti-Stokes components of the Raman scattering.23 Ward et al. and Konishi et al. succeeded in the simultaneous conductance and SERS measurement of the single molecular junction.24,25 They observed the appearance of the b2 modes for the molecular junctions, which were not observed in the bulk crystal. While the combination of SERS and conductance measurement of the molecular junctions is a powerful technique to characterize the molecular junction, there are several subjects. First, most previously reported experimental measurements have been limited to the conductance near zero bias, the current−voltage (I−V) characteristics of the molecular junctions must be determined in order to study the junctions in detail.28−30 The I−V characteristics provide the information about the Received: November 14, 2012 Revised: January 3, 2013 Published: January 4, 2013 1791

dx.doi.org/10.1021/jp3112638 | J. Phys. Chem. C 2013, 117, 1791−1795

The Journal of Physical Chemistry C

Article

electronic structure of the molecular junctions, such as the energy difference between conduction orbital and Fermi level, and strength of the metal−molecule coupling. Second, the Raman signals which are characteristic of the molecular junction have not been fully understood. For some molecular junctions, b2 modes have been observed, while they are not observed in the bulk crystal.24,25 The origin of the appearance of the b2 mode is not fixed yet. In the present study, we have investigated the I−V characteristics and SERS of the 1,4-benzendithiol molecular junction using the stable fixed planar nanogap electrodes (gap size ≈ 1 nm). The I−V characteristics revealed the number of molecules bridging metal electrodes, energy difference between the conduction orbital and Fermi level, and the strength of metal-molecule coupling. The conductance measurements confirmed the molecular bridge between Au electrodes. The SERS measurement showed the disappearance of the SH modes and red shift of internal vibration modes, which confirmed the molecular bridge between Au electrodes. For some molecular junctions, we observed the b2 mode, which was not observed in the bulk crystal. The appearance of the b2 mode could be explained by the photon-driven charge transfer from molecule to metal electrodes.

Figure 2. (a) Photo image of the Au nano gap electrode, (b) Map of BDT SERS signal from the a1 symmetry mode at 1561 cm−1 (integrated from 1500 to 1620 cm−1), (c) Raman spectra of (A) underlying Si, (B) Au nanogap, and (C) Au electrode.

■

RESULTS AND DISCUSSION Figure 2(c) shows the examples of Raman spectra of underlying Si (A), the Au nanogap (B), and Au electrode (C). Strong band was observed around 520 cm−1, corresponding to the underlying Si substrate. No features were observed in the Raman spectrum on the flat Au electrode. At the nano gap, intense bands were observed at 520, 1068, 1561 cm−1, which were assigned to the Si band, C−C stretching and CC stretching modes, respectively.34 Figure 2(b) shows the map of BDT SERS signal from the a1 symmetry mode at 1561 cm−1 (integrated from 1500 to 1620 cm−1). The SERS signal was enhanced around the nanogap region. In the absence of BDT molecules, intensity of the Raman signal was under the detection limit, which proved that the observed Raman signal was solely due to the BDT molecules in the gap, and not due to the bare Au nanogap electrodes or other contaminants. Figure 3(a) shows the I−V characteristics of the molecular junction before and after the SERS measurement. The I−V

■

EXPERIMENTAL SECTION Figure 1(a) shows a schematic view of the experimental setup. The fixed planar nanogap electrodes (gap size ≈ 1 nm) were

Figure 1. (a) Schematic view of the electronic and SERS measurements. (b) Scanning electron microscope image of Au nano gap electrodes.

fabricated on Si wafer with 300 nm of thermal oxide, using oblique metal evaporation with a shadow mask, followed by an electromigration technique.31−33 The detail experimental condition is shown in the Supporting Information. Figure 1(b) and Figure 2(a) show the scanning electron microscope (SEM) and optical microscope images of the Au nanogap electrode, respectively. The Au nanogap electrodes were immersed in a 1 mM 1, 4-benzendithiol ethanol solution at room temperature for 10 min. After removal of the solution, I− V characteristics were measured before and after the SERS measurements at room temperature using an electrometer (Keithley 2612A). The bias voltage was not applied between nanogap electrodes during the SERS measurements. The SERS measurements were performed using commercial Raman microprobe spectrometer with 4 s integrations (Nanofinder30, Tokyo Instruments). Near-infrared (NIR) laser light (λex = 785 nm, 70 mW) was used as excitation light. The NIR beam was focused onto the sample using a objective lens with 100× magnification and a numerical aperture of 0.95. The estimated spot size of irradiation was about 1 μm.

Figure 3. (a) Example of I−V characteristic of BDT molecular junction before (black) and after (green) the SERS measurement, (b) I−V characteristic of BDT molecular junction together with fitted results (red line) based on the single level tunneling model, and (c) the distribution of ε0 for BDT molecular junction obtained by 26 samples.

characteristics did not change with time, showing that the molecular junction had a high stability. The conductance of the molecular junction was obtained by numerical differentiation of the current with a voltage value of around zero in the voltage regime. The conductance was 9.6 × 10−3 G0 for the molecular junction shown in Figure 3(a,b). The conductance of the single BDT molecular junction could be obtained by constructing the conductance histogram with break junction technique.35,36 By 1792

dx.doi.org/10.1021/jp3112638 | J. Phys. Chem. C 2013, 117, 1791−1795

The Journal of Physical Chemistry C

Article

ε0 were −0.59 and 0.11 eV, respectively. The energy difference between conduction orbital and Fermi level was 0.6 (±0.1) eV for the BDT molecular junction in the present study. On the other hand, the thermoelectricity measurment has determined the energy difference between conduction orbital (HOMO) and Fermi level to be 1.2 eV,39 which was close to the our experimental results. The close agreement supports the formation of the BDT molecular junction. The finite strength of the coupling (∼10 meV on average) suggested the binding of the BDT molecule to Au electrodes and interaction between them. In the present study, we took into account that the Fermi distribution function got smeared out for finite temperature. The temperature correction term was about several % of the first term at 300 K. Here, we briefly comment on the inelastic effects on the I−V characteristics. In the present study, the I−V characteristics were measured at room temperature. Generally, inelastic effect by electron−phonon scattering becomes large with increasing temperature. We checked the inelastic effect up to 3000 K by the lowest order expansion (LOE) formalism with using the parameters derived by the IETS calculations.40,41 The details are given in the Supporting Information. In short, the inelastic current is only 4−5% of the ballistic current. Therefore, we investigated I−V characteristics based on the Landauer approach, which assumed that the electric current was ballistic and inelastic effects by electron−phonon scatterings was sufficiently small. The conductance measurement confirmed the bridging of BDT molecule between Au electrodes. We then investigated the Raman spectra of the molecular junction. Figure 4 shows

comparing the conductance value with that of the single molecular junction, the number of molecules bridging the Au electrodes was determined to be one for the molecular junction shown in Figure 3(a,b). The average number of molecules bridging Au electrodes was around 30 by evaluating 36 samples. Here, it is noteworthy that the conductance of the molecular junction is affected by the surrounding molecules. Previously reported theoretical study showed that the conductance of the single molecule, where the molecule was surrounded by molecules, was up to five times higher than that of the isolated single molecule junction.37 The conductance of the single molecule junction also varies with the metal−molecule contact configuration, bias voltage, environment, and other conditions.38 We roughly evaluated the number of molecules bridging Au electrodes in the present study. The I−V characteristic was investigated based on the single level tunneling model.1,3,5 At 0 K, I-V characteristic of the molecular junction is represented by ⎧ ⎛ ΓR ⎞ 8e ΓLΓR ⎪ −1⎜ ΓL + ΓR eV − ε0 ⎟ ⎨tan ⎜ I (V ) = ⎜ ΓL + ΓR ⎟⎟ h ΓL + ΓR ⎪ ⎝ ⎠ ⎩ ⎫ + ε0 ⎞⎪ ⎟⎬ ΓL + ΓR ⎟⎟⎪ ⎠⎭

⎛ ΓL eV −1⎜ ΓL + ΓR

+ tan ⎜ ⎜ ⎝

(1)

where ε0, ΓL, and ΓR are the energy of the conduction orbital, and the strength of the coupling between molecule and left and right electrodes, respectively. At finite temperature, we have to take into account that the Fermi distribution function gets smeared out. I−V characteristic of the molecular junction at finite temperature T can be represented by the following: ⎧ ⎛ ΓR ⎞ 8e ΓLΓR ⎪ −1⎜ ΓL + ΓR eV − ε0 ⎟ ⎨tan ⎜ I (V ) = ⎜ ΓL + ΓR ⎟⎟ h ΓL + ΓR ⎪ ⎝ ⎠ ⎩ ⎫ + ε0 ⎞⎪ 8π 2 ⎟⎬ e(kT )2 ΓL − 3h ΓL + ΓR ⎟⎟⎪ ⎠⎭

⎛ ΓL eV −1⎜ ΓL + ΓR

+ tan ⎜ ⎜ ⎝ ⎡ ⎢ ⎢ ΓR ⎢ ⎢⎧ ⎨ ⎢⎣ ⎩

ΓR eV ΓL + ΓR

(

ΓR eV ΓL + ΓR

2

− ε0

)

ΓL eV ΓL + ΓR

− ε0 ⎫2 + (ΓL + ΓR )2 ⎬ ⎭

⎤ ⎥ + ε0 ⎥ 2⎥ ⎫ + (ΓL + ΓR )2 ⎬ ⎥ ⎭ ⎥⎦

(2)

Figure 4. Example of SERS of BDT molecular junctions. The inset shows the conductance of the molecular junction. The vertical lines indicate the vibration modes of the bulk crystal assigned to each mode with the calculation result.34

The first term is the same as eq 1, and the second term is the correction term caused by the finite temperature. The detail discussion is shown in the Supporting Information. By fitting the experimental result with eq 2, the ΓL, ΓR, and ε0 were determined to be 0.030 eV, 0.033 eV, and −0.63 eV, respectively. In order to accurately determine ε0, 26 samples were investigated, resulting in the ε0 histogram presented in Figure 3(c). The histogram shows a well-defined peak at -0.61 eV with 0.08 eV width. The average and standard deviation of

the typical SERS spectra of BDT molecular junction. In most of spectra, C−C and CC stretching modes were observed. Sometimes, we observed modes around 1400 cm−1 (upper 2 spectra) for 8 distinct samples. Here, it is noteworthy that the SH modes (908, 2556 cm−1) were not observed in the BDT molecular junction.34 The absence of the SH modes indicated that the S−H bonds were broken and BDT molecule bound to both Au electrodes. The SH modes would be observed when

+

⎧ ⎨ ⎩

(

ΓL eV ΓL + ΓR

2

+ ε0

)

1793

dx.doi.org/10.1021/jp3112638 | J. Phys. Chem. C 2013, 117, 1791−1795

The Journal of Physical Chemistry C

Article

modes for p-aminothiophenol on Ag surface.46−48 Further investigation is required to fix the origin of the appearance of the b2 modes.

the BDT molecule binds to one of the Au electrode and other SH anchoring group is free (See Supporting Information). The present SERS result supported the formation of the molecular junction. We then discuss the peak shift of the vibration mode of the molecular junction. On average, the CC (C−C) stretching mode was observed at 1562 cm−1 (1063 cm−1) for the molecular junction, while it was observed at 1572 cm−1 (1094 cm−1) in the bulk crystal. The CC and C−C stretching modes showed the red-shift. When the molecule adsorbs on metal surface, the bonding orbitals are formed by the interaction between HOMO of molecule and unoccupied state of metal, and between LUMO and occupied state of metal. Electron transfers from HOMO (bonding orbital) to metal state, and electron transfers from metal to LUMO (antibonding).42 Therefore, the interaction between molecule and metal leads to the decrease in the bond strength of the molecule. The red-shift of CC and C−C stretching modes is originated from the interaction between BDT molecule and metal electrode, which confirms the bridging of BDT molecule between Au electrodes. We then discuss the appearance of 1400 cm−1 peak (b2 mode) based on the Lombardi’s model.43−45Lombardi et al. extended the resonance Raman theory to fit the metaladsorbate system. According to this model, Raman tensor elements are represented as α = A + B + C. Term A represents a Franck−Condon contribution, and B and C terms represent Herzberg−Teller contributions. Terms B and C correspond to molecule-to-metal and metal-to-molecule charge transfer transitions, respectively. Totally symmetric modes can be enhanced by all three terms, while nontotally symmetric modes can be enhanced via the terms B and C. The previously reported thermoelectricity measurement of BDT molecular junction, showed that the HOMO level and the LUMO level of BDT were 1.2 eV below and 2.5 eV above the Fermi level of metal electrodes, respectively.39 The wavelength of the incident laser used in this SERS study was 785 nm (∼ 1.6 eV). Therefore, only B term (molecule-to-metal charge transfer transition) could contribute to the Raman scattering. In order for the term B to be nonvanishing, (1) There is at least one allowed transition from the molecular ground state to the excited state. (2) Transitions from the molecule to the metal is allowed. (3) The direct product of ΓM × ΓK × ΓQ contains the totally symmetric representation, where ΓM, ΓK and ΓQ are the irreducible representation of charge transfer sate, excited state and the vibraional mode, respectively. HOMO of BDT is b1 symmetry and the LUMO is a2 symmetry, assuming the C2V symmetry. Therefore, the transition from the molecular ground state to the excited state (b1 × a2 = B2) is allowed. At room temperature, molecular orientation fluctuates with time in the molecule junction. In some instances, the molecular axis could orient normal to the surface. In this configuration, charge transfer state is a1 symmetry.45 ΓM × ΓK × ΓQ contains the totally symmetric representation for b2 mode. Under these conditions, b2 mode is enhanced by the B term. Here again, the interaction between molecule and metal electrodes leads to appearance of the b2 modes. Since molecular orientation fluctuated at room temperature, b2 modes did not always appear, which agreed with the experimental results. Several possibilities have been considered to explain the appearance of b2 modes in SERS. A charge transfer (CT) process as discussed in the present study, and chemical reaction induced by the laser irradiation have been discussed to explain the appearance of b2

■

CONCLUSIONS We have measured electric conductance and SERS spectra for the benzenedithiol molecules bridging between Au nano gap electrodes at room temperature. The number of the molecules and electronic structure of the benzenedithiol molecular junction were determined by the I−V characteristics. The energy difference between conduction orbital and Fermi level was determined to be 0.6 eV based on the single level tunneling model considering that the Fermi distribution function got smeared out for finite temperature. The finite strength of the metal-molecule coupling (∼10 meV) indicated the interaction between benzenedithiol molecule and Au electrodes. The SERS spectra were different from that of the bulk crystal, due to this interaction. The SH modes were not observed for the molecular junction, because the S−H bonds were broken and BDT molecule bound to both Au electrodes. The CC (C− C) stretching mode was observed at 1562 cm−1 (1063 cm−1) for the molecular junction, while it was observed at 1572 cm−1 (1094 cm−1) in the bulk crystal. The redshift of the CC and C−C stretching modes could be explained by the interaction between benzenedithiol molecule and Au. We observed the apperance of the b2 modes for some molecular junctions, which were not observed in the bulk crystal. The appearance of the b2 modes could be explained by the charge transfer between benzenedithiol molecule and Au. We could investigate the interaction between benzenedithiol molecule and Au by measurments on conductance and SERS.

■

ASSOCIATED CONTENT

S Supporting Information *

The fabrication of planar nano gap electrodes, molecular orbital of benzenedithiol, current−voltage of molecular junction at finite temperature, and inelastic effects on I−V characteristics. The material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by Grants-in-Aids for Scientific Research in Innovative Areas (No. 23111706) and Grant-in-Aid for Scientific Research (A) and (B) (No. 21340074 and No. 24245027) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Sumitomo and Asahi Glass foundation.

■

REFERENCES

(1) Cuevas, J. C.; Scheer, E. Molecular Electronics: An Introduction to Theory and Experiment; World Scientific: Singapore, 2010. (2) Kiguchi, M.; Kaneko, S. ChemPhysChem 2012, 13, 1116−1126. (3) Kiguchi, M.; Kaneko, S. Phys. Chem. Chem. Phys. DOI: 10.1039/ c2cp43960c (4) Agraït, N.; Yeyati, A. L.; van Ruitenbeek, J. M. Phys. Rep. 2003, 377, 81−279. 1794

dx.doi.org/10.1021/jp3112638 | J. Phys. Chem. C 2013, 117, 1791−1795

The Journal of Physical Chemistry C

Article

(5) Cuniberti, G.; Fagas, G.; Richter, K. Introducing Molecular Electronics; Springer-Verlag: Berlin, Heidelberg, 2005. (6) Tao, N. J. Nat. Nanotechnol. 2006, 1, 173−181. (7) Akkerman, H. B.; de Boer, B. J. Phys.: Condensed Matter 2008, 20, 013001/1−013001/20. (8) Ulgut, B.; Abruna, H. D. Chem. Rev. 2008, 108, 2721−2736. (9) McCreery, R. L.; Bergren, A. J. Adv. Mater. 2009, 21, 4303−4322. (10) Song, H.; Reed, M. A.; Lee, T. Adv. Mater. 2011, 23, 1583− 1608. (11) Song, H.; Kim, Y.; Jang, Y. H.; Jeong, H.; Reed, M. A.; Lee, T. Nature 2009, 462, 1039−1043. (12) Díez-Pérez, I.; Hihath, J.; Lee, Y.; Yu, L.; Adamska, L.; Kozhushner, M. A.; Oleynik, I. I.; Tao, N. Nat. Chem. 2009, 1, 635− 641. (13) Quek, S. Y.; Kamenetska, M.; Steigerwald, M. L.; Choi, H. J.; Louie, S. G.; Hybertsen, M. S.; Neaton, J. B.; Venkataraman, L. Nat. Nanotechnol. 2009, 4, 230−234. (14) Smit, R. H. M.; Noat, Y.; Untiedt, C.; Lang, N. D.; van Hemert, M. C.; van Ruitenbeek, J. M. Nature 2002, 419, 906−909. (15) Kiguchi, M.; Tal, O.; Wohlthat, S.; Pauly, F.; Krieger, M.; Djukic, D.; Cuevas, J. C.; van Ruitenbeek, J. M. Phys. Rev. Lett. 2008, 101, 046801/1−046801/4. (16) Hihath, J.; Arroyo, C. R.; Rubio-Bollinger, G.; Tao, N.; Agraït, N. Nano Lett. 2008, 8, 1673−1678. (17) Kim, Y.; Pietsch, T.; Erbe, A.; Belzig, W.; Scheer, E. Nano Lett. 2011, 11, 3734−3738. (18) Kiguchi, M.; Nakazumi, T.; Hashimoto, K.; Murakoshi, K. Phys. Rev. B 2010, 81, 045420/1−045420/4. (19) Matsushita, R.; Kaneko, S.; Nakazumi, T.; Kiguchi, M. Phys. Rev. B 2011, 84, 245412/1−245412/5. (20) Kiguchi, M.; Stadler, R.; Kristensen, I. S.; Djukic, D.; van Ruitenbeek, J. M. Phys. Rev. Lett. 2007, 98, 146802/1−146802/4. (21) Tian, J. H.; Liu, B.; Li, X.; Yang, Z. L.; Ren, B.; Wu, S. T.; Tao, N.; Tian, Z. Q. J. Am. Chem. Soc. 2006, 128, 14748−14749. (22) Liu, Z.; Ding, S. Y.; Chen, Z.-B.; Wang, X.; Tian, J. H.; Anema, J. R.; Zhou, X. S.; Wu, D. Y.; Mao, B. W.; Xu, X.; Ren, B.; Tian, Z. Q. Nat. Commun. 2011, 2, 1310/1−1310/6. (23) Ioffe, Z.; Shamai, T.; Ophir, A.; Noy, G.; Yutsis, I.; Kfir, K.; Cheshnovsky, O.; Selzer, Y. Nat. Nanotechnol. 2008, 3, 727−732. (24) Ward, D. R.; Halas, N. J.; Ciszek, J. W.; Tour, J. M.; Wu, Y.; Nordlander, P.; Natelson, D. Nano Lett 2008, 8, 919−924. (25) Konishi, T.; Kiguchi, M.; Takase, M.; Nagasawa, F.; Nabika, H.; Ikeda, K.; Uosaki, K.; Ueno, K.; Misawa, H.; Murakoshi, K. J. Am. Chem. Soc. DOI: 10.1021/ja307821u. (26) Sawai, Y.; Takimoto, B.; Nabika, H.; Ajito, K.; Murakoshi, K. J. Am. Chem. Soc. 2007, 129, 1658−1662. (27) Lombardi, J. R.; Birke, R. L. Acc. Chem. Res. 2009, 42, 734−742. (28) Guo, S.; Hihath, J.; Díez-Pérez, I.; Tao, T. J. Am. Chem. Soc. 2011, 133, 19189−19197. (29) Guedon, C. M.; Valkenier, H.; Markussen, T.; Thygesen, K. S.; Hummelen, J. C.; van der Molen, S. J. Nat. Nanotech. 2012, 7, 305− 309. (30) Huisman, E. H.; Guédon, C. M.; van Wees, B. J.; van der Molen, S. J. Nano Lett. 2009, 9, 3909−3913. (31) Naitoh, Y.; Liang, T.-T.; Azehara, H.; Mizutani, W. Jpn. J. Appl. Phys. 2005, 44, L472−L474. (32) Liang, T.; Naitoh, Y.; Horikawa, M.; Ishida, T.; Mizutani, W. J. Am. Chem. Soc. 2006, 128, 13720−13726. (33) Takahashi, Y.; Kiguchi, M.; Horikawa, M.; Naitoh, Y.; Tsutsui, K.; Takagi, H.; Morita, M.; Tokuda, M.; Ito, Y.; Wada, Y. Jpn. J. Appl. Phys. 2012, 51, 075201/1−075201/4. (34) Sun, M.; Xia, L.; Chen, M. Spectrochim. Acta, Part A 2009, 74, 509−514. (35) Xiao, X.; Xu, B.; Tao, N. Nano Lett. 2004, 8, 267−271. (36) Taniguchi, M.; Tsutsui, M.; Yokota, K.; Kawai, T. Nanotechnology 2009, 20, 434008/1−434008/8. (37) Dahlke, R.; Schollwöck, U. Phys. Rev. B 2008, 69, 085324/1− 085324/10.

(38) Chen, F.; Li, X.; Hihath, J.; Huang, Z.; Tao, N. J. Am. Chem. Soc. 2006, 128, 15874−15881. (39) Reddy, P.; Jang, S.; Segalman, R. A.; Majumdar, A. Science 2007, 315, 1568−1571. (40) Nakamura, H.; Yamashita, K.; Rocha, A. R.; Sanvito, S. Phys. Rev. B 2008, 78, 235420/1−235420/18. (41) Nakamura, H. J. Phys. Chem. C 2010, 114, 12280−12289. (42) Zangwill, A. Physics at Surfaces; Cambridge University Press: England, 1988. (43) Lombardi, J. R.; Brike, R. L.; Lu, T.; Xu, J. J. Chem. Phys. 1986, 84, 4174−4180. (44) Zhao, L.; Huang, R.; Huang, Y.; Wu, D.; Ren, B.; Tian, Z. J. Chem. Phys. 2011, 135, 134707/1−134707/11. (45) Osawa, M.; Matsuda, N.; Yoshii, K.; Uchida, I. J. Phys. Chem. 1994, 98, 12702−12707. (46) Ikeda, K.; Suzuki, S.; Uosaki, K. Nano Lett. 2011, 11, 1716− 1722. (47) Huang, Y.; Zhu, H.; Liu, G.; Wu, D.; Wu, D.; Ren, B.; Tian, Z. J. Am. Chem. Soc. 2010, 132, 9244−9246. (48) Kim, K.; Kim, K. L; Lee, H. B.; Shin, K. S. . J. Phys. Chem. C 2012, 116, 11635−11642.

1795

dx.doi.org/10.1021/jp3112638 | J. Phys. Chem. C 2013, 117, 1791−1795