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Cite this: DOI: 10.1039/c4cp04906c

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Surface enhanced Raman scattering of a single molecular junction Ryuji Matsushita and Manabu Kiguchi* The characterization of a single molecular junction is essential to investigate and utilize the single molecular junction in single molecular devices. Vibration spectroscopy is a promising technique for characterizing the atomic structure of the single molecular junction. In this review paper, we describe the surface-enhanced

Received 27th October 2014, Accepted 15th January 2015

Raman scattering (SERS) as a vibration spectroscopy of a single molecular junction. A strong electric field is

DOI: 10.1039/c4cp04906c

The Raman signal from a single molecule in the nanogap is selectively observed thanks to the strong

formed in the nanogaps in the single molecular junction, which enhances the intensity of the Raman signal. electric field. Simultaneous SERS and conductance measurements provide information of the geometric

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structure of the single molecular junction, which can clarify the single molecular dynamics.

Introduction The single molecular junction, where a single molecule is bridged between metal electrodes, has attracted attention owing to its potential for application in molecular electronics.1–3 Currently, single molecular diodes, transistors, switches, and other interesting single molecular devices have been reported.4–6 The basic electron transport properties have been also clarified at the single molecule level using the single molecular measurement technique.7,8 The single molecular junction is a low-dimensional nanomaterial having two metal–molecular interfaces. The properties of a molecule in the single molecular junction can thus be different from those of an isolated molecule and bulk crystal. We can expect the presence of novel functionalities that are not observed in other phases. Based on these interests, the single molecular junction is one of the hot topics in materials science.3,9,10 Characterization of the atomic and electronic structures of the single molecular junction is essential to investigate and utilize single molecular junctions as single molecular devices. Vibration spectroscopy is a promising technique to characterize the atomic configuration of the single molecular junction. Various techniques for performing vibration spectroscopy of a single molecular junction, including inelastic electron tunneling spectroscopy (IETS),11–17 point contact spectroscopy (PCS),18,19 and surface enhanced Raman scattering (SERS),20,21 have been developed to characterize the single molecular junction. While IETS and PCS are powerful techniques to characterize the single molecular junctions, they should be measured at low temperatures. In terms of the application, vibration spectroscopy at room Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan. E-mail: [email protected]

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temperature is important for the single molecular devices. SERS is the most promising technique as a vibration spectroscopy of the single molecular junction at room temperature. A single molecule is trapped in the nanogap in the single molecular junction. A strong electric field is formed between the nanogap electrodes, which enhances the intensity of the Raman signal. The Raman spectrum from a single molecule in the nanogap is selectively observed, which provides information about the atomic configuration of the single molecular junction.

Fabrication of single molecular junctions Single molecular junctions can be fabricated using break junction (BJ) techniques including STM-BJ and mechanically controllable break junctions (MCBJs).22–24 Fig. 1 shows the schematic view of the STM and MCBJ, together with the formation process of the single molecular junction. Single molecular junctions are fabricated by breaking metal contacts in the presence of target molecules. After breaking the metal contact, several molecules can be bridged between metal electrodes. The number of molecules bridging the metal electrodes decreases one by one. Finally, the single molecular junction is formed just before break-down of the molecular junction. Fig. 1(a) shows the conductance change of the contact during the breaking process, together with the schematic image of the contact. The 1 G0 (2e2/h) plateau corresponds to the Au atomic contact. Just before break-down of the molecular junction, the conductance decreases in a stepwise fashion, with each step occurring at an integral multiple of a certain value (Gm in Fig. 1(a)), corresponding to the single molecule conductance.

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Fig. 1 (a) Schematic view of the formation process and conductance of the single molecular junction during the stretching of the metal contact. (b) STM system to investigate the single molecule junction fabricated in solution. (c) MCBJ system to investigate the single molecular junction.3

SERS of a molecular junction Tian et al. reported the first SERS measurement of a molecular junction.25 The BDT molecular junction was fabricated using the experimental setup shown in Fig. 2. A strong SERS signal is observed from the gap, and the intensity of the SERS signal depends on the direction of the incident light. Fig. 3 shows the SERS when the electrodes are parallel and perpendicular to the polarization direction of the light. The SERS intensity is large for the parallel configuration. Since the SERS from molecules

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other than the molecular junction is not polarized, the observed polarization dependence shows that the SERS signal originates from the molecular junction. They also measured the dependence of the SERS on the gap width. The SERS intensity increases with the decrease in the gap width caused by the increase in the strength of the electric field in the nanogap. El-Khoury et al. reported intensity spikes in Raman scattering for dithiolated biphenyl molecules adsorbed on a thin silver substrate.26 They measured the Raman spectra for the monothiol and dithiol of biphenyl using a gold AFM tip. Spikes in Raman scattering, which occur only for the dithiol, are caused by making and breaking S-metal bonds between the AFM tip and the molecules. The shortening of the plasmon through molecular conductive bridges leads to the change in the SERS intensities. Hess’s group also reported time- and frequency- dependent variations in the sequences of the Raman spectra for dithiol molecules adsorbed on a silver substrate using a silver AFM tip.27 The variations are explained by molecular reorientation. Ward et al. reported the first simultaneous SERS and conductance measurements of p-mercaptoaniline (pMA) and fluorinated oligophenylyne ethynylene (FOPE) molecular junctions using the fixed nanogap electrodes.28,29 Fig. 4 shows a waterfall plot of the Raman spectrum and the conductance measurements. The sudden changes in the Raman spectrum are observed at the boundary shown by the dotted line, and the change in the Raman spectrum correlates with changes in the conductance. The intensity of the anti-Stokes Raman spectrum provides information about the vibrational occupation of the mode. Natelson et al. examined the evolution of the Stokes and anti-Stokes Raman spectra as a function of dc bias for the C60 molecular junctions.30 As the bias is increased, anti-Stokes peaks appear and become stronger. The count rates for some peaks (B1147 and 1264 cm1) do not rise

Fig. 2 Schematic view of the experimental setup for SERS measurement of molecular junction using MCBJ.25

Fig. 3 SERS of 1,4-benzendithiol. The polarization of the incident light is (a) parallel and (b) perpendicular to the junction axis.25

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Fig. 4 Waterfall plot of Raman spectrum, Raman intensity and conductance measurements for a (A) pMA and (B) FOPE samples. Vertical red lines indicate points of rapidly changing Raman intensity and conduction.29

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noticeably above the background until the dc bias exceeds a certain threshold (B150 mV). The energy scale is close to the criterion eV = hn, which suggests current-driven vibrational pumping. Ioffe et al. and Ward et al. evaluated the effective temperature of the molecular junction by measuring both the Stokes and anti-Stokes components of the Raman scattering signal using fixed nanogap electrodes.31,32 The effective temperature (T eff n ) can be evaluated using the following equation    ðoL þ ov Þ4 IvAS ¼ Av exp hov kB Tveff S 4 Iv ðoL  ov Þ

(1)

where I Sn and IAS n are the intensity of the Stokes and anti-stokes  L and o  n are the incident laser frequency, and frequency Raman, o of Raman scattering, and An is a correction factor. In the case of the oligophenylene vinylene (OPV) molecular junction, the effective temperature increases linearly with the bias voltage. It can increase by more than 100 K at a bias voltage of 400 mV.31 On the other hand, the effective temperature does not significantly increase with the bias voltage for the 4,4-biphenyldithiol molecular junction (up to 40 K at a bias voltage of 500 mV).32 Chen et al. investigated the local heating in the single molecular junction using first principle calculations. The local temperature of the single BDT molecular junction increases by up to 20 K at the bias voltage of 1 V.33 Further investigation is needed to solve the disagreement over the local heating. While detailed studies have been performed for the multiple molecular junction, there are several subjects in the research of SERS of multiple molecular junction. As for the multiple molecular junction, each molecule is in a different environment. The signal from each molecule overlaps, leading to a broad SERS signal. On the other hand, the SERS signal from a single molecule is observed as a sharp one. In addition, we have to consider the intermolecular interaction for the multiple molecular junctions.34

SERS of a single molecular junction Konishi et al. reported the simultaneous SERS and conductance measurements for a single 4,4 0 -bipyridine molecular junction fabricated using MCBJ.35 Fig. 5(a) shows the experimental setup. The Al2O3 film is used as an insulating film, because the oxide reduces the background of the Raman signal compared to the organic insulating films, which is normally used for nanofabricated MCBJ samples. The Au electrode is patterned on a substrate covered with a thin polyimide layer using electron beam lithography and a lift-off technique. The polyimide underneath the Au bridge and uncovered polyimide are completely removed by reactive ion etching. Fig. 5(b) shows the SEM image of the free-standing Au nanobridge in the MCBJ sample. Fig. 5(c) shows the map of the 4,4 0 -bipyridine SERS signal from the Au nanobridge from the ring breathing vibrational mode at 1015 cm1. Intense Raman scattering is observed at the gap. The intensity of this scattering is very sensitive to the polarization direction of the laser light. Polarization parallel to the Au nanogap results in the most intense scattering signal at the nanogap, which indicates that the 4,4 0 -bipyridine signal is

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Fig. 5 (a) Schematic view of the simultaneous SERS and conductance measurement using MCBJ (b) SEM image of an Au nanobridge in the MCBJ sample. (c) Map of the 4,4 0 -bipyridine SERS signal from the Au nanobridge from the ring breathing vibrational mode at 1015 cm1.35

due largely to the localized surface plasmon excitation of the Au nanoelectrodes. Fig. 6 shows the typical time course of the conductance and the SERS spectra of the junction. In Fig. 6a, a plateau appears around 1 G0, which corresponds to the Au atomic contact (Region A). Another plateau appears at 0.01 G0, which corresponds to the single 4,40 -bipyridine molecular junction (Region B). The conductance drops below 0.001 G0, which indicates the breakdown of the single molecular junction (Region C). Before (Region A) and after (Region C) the formation of the single molecule junction, SERS bands are observed at 1015 cm1, 1074 cm1, 1230 cm1, and 1298 cm1, which are normal Raman modes of 4,40 -bipyridine. During the formation of the single molecule junction (Region B), additional intense bands are observed at 990, 1020, 1065 and 1200 cm1, which are not observed for bulk 4,4 0 -bipyridine. Fig. 6(b)–(d) depict close-ups of the conductivity time course and coincident SERS spectra for other molecule junctions. The characteristic Raman modes which are not observed for bulk 4,4 0 -bipyridine are observed in the formation of the single molecular junction. Fig. 7 shows the typical three types of SERS spectra. The bottom of Fig. 7(a) shows bands at 1025 cm1, 1062 cm1, 1217 cm1 and 1296 cm1, which are assigned to totally symmetric a modes.

Fig. 6 (a) Time course of the conductivity and SERS spectra during self-breaking of the junction. (b–d) Time course of the conductivity and SERS spectra for different molecule junctions at the single molecule conductance regime.35

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Fig. 7 (a) Three types of SERS spectra of a single 4,4 0 -bipyridine molecule junction. Assignment of the totally symmetric a mode (red), and the nontotally symmetric b1 mode (blue) and b2 mode (green) are shown as dotted lines. (b) SERS intensity dependence of a (red), b1 (blue) and b2 (green) modes on the conductance of the junction.35

The middle of Fig. 7(a) shows relatively intense bands at 840, 998, 1026 and 1205 cm1, which are assigned to non-totally symmetric b1 modes. The top of Fig. 7(a) shows the bands at 975, 1072, 1221 and 1299 cm1, which are assigned to nontotally symmetric b2 modes. The intensities of the non-totally symmetric b1 and b2 modes are significantly higher relative to totally symmetric a modes. Fig. 7(b) shows the intensity of the SERS signal of the totally and non-totally symmetric modes as a function of the conductance of the junction. The SERS signals are strongly enhanced around 0.01 G0, which corresponds to the single 4,4 0 -bipyridine molecular junction. This indicates conclusively that the SERS spectra are induced by the formation of a single molecular junction. The simultaneous SERS and conductance measurements of the single molecular junction are also reported by Liu et al. using ‘‘fishing mode’’ tip-enhanced Raman spectroscopy.36 The distance between the tip and the substrate is controlled in the fishing mode measurements, while the proportional gain and the integral gain are decreased to around 0.03% of their normal values. Thermal motion of a molecule on the surface leads to the formation and breaking of the molecular junctions. The 1609 cm1 peak is reversibly changed to a doublet with increase in the bias voltage. This splitting of the peak is explained by the asymmetric metal–molecule coupling induced by application of the bias voltage to the single molecular junction.

Single molecular dynamics Fig. 8(a) shows the time courses of the conductance and the SERS intensities of a, b1, and b2 modes. Here, the symmetry of the 4,4-bipyridine molecule is assumed to be C2v. The b1 mode is observed up to 3 s, and the conductance of the junction is higher than 0.01 G0. At 4 s, the b1 mode disappears together with a drop of conductance below 0.01 G0. At 7 s, the b2 mode appears and the conductance increases above 0.01 G0. The theoretical calculation of the Raman tensor shows that the b2 mode is observed when a molecule vertically bridges the gap, while the b1 mode is observed when a molecule is tilted. Therefore, the molecule initially bridges the gap with its molecular

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Fig. 8 (a) Time course of the Raman intensity of the a mode (red), b1 mode (blue), and b2 mode (green) together with the conductance of the molecular junction. (b) Time course of the energy of the b1 mode of the ring breathing around 1050 cm1 and the conductance of the single molecule junction. Schematics of the relevant molecular orientation are shown as insets.35

long axis inclined to the junction axis, then the molecular junction breaks (B3 s), and finally the molecule bridges the gap with its molecular long axis parallel to the junction axis (B7 s). Fig. 8(b) shows the time-course of the conductance and the energy of the ring breathing mode around 1050 cm1 in the single molecular junction regime. The energy of the ring breathing mode becomes lower (higher) as the conductance becomes larger (smaller). When the molecule is adsorbed on the metal surface, electrons transfer from the HOMO (bonding orbital) to the metal state, and electrons transfer from the metal to the LUMO (anti-bonding). Both electron transfers weaken the chemical bond in the molecule, while they increase the interaction between the molecule and the metal substrate. The decrease in the bond strength of the molecule (energy of the vibration mode) depends on the metal–molecule interaction. Therefore, an increase in the strength of the metal–molecule interaction leads to a decrease in the energy of the vibration mode of the molecule. In the single-level tunneling model, the conductance of the single molecular junction is represented by G¼

2e2 4G2 h D2 þ 4G2

(2)

where D and G are the energy difference between the conduction orbital and the Fermi level, and the coupling between the molecule and the electrode, respectively.1,17 The coupling between the molecule and the electrode correlates with the strength of the metal–molecule interaction. The conductance of the single molecular junction, thus, increases with the strength of the metal–molecule interaction. The increase in the strength of the metal–molecule interaction leads to the increase in the conductance and decrease in the energy of vibration mode of molecule. The fluctuation of the vibrational energy and conductance

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observed in Fig. 8(b) reflects the dynamic motion of the single molecule in the single molecular junction.

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SERS active mode in a molecular junction In the single 4,4 0 -bipyridine molecular junction, non-totally symmetric b1 and b2 modes are observed. These non-totally symmetric modes are Raman inactive at visible excitations (lex = 785 nm), while the totally symmetric modes (a1 modes) are Raman active. The appearance of the non-totally symmetric mode is also observed for other molecular junctions. Fig. 9 shows the typical SERS spectra of a molecular junction of benzenedithiol (BDT: C2v symmetry) fabricated using the fixed nanogap electrodes. The non-totally symmetric b2 modes are observed around 1400 cm1 (indicated by * in the upper two spectra). The appearance of the b2 mode is explained by the charge transfer process. The resonance Raman theory is extended to fit the metal–adsorbate system reported by Lombardi et al. In this model, Raman tensor elements are represented by a = A + B + C.37 Term A is a Frank–Condon contribution, and the B and C terms are Herzberg–Teller contributions. The non-totally symmetric modes can be enhanced by terms B and C. Since the energy difference between the HOMO and the Fermi level is close to the wavelength of the incident laser (785 nm), the B term (the molecule-to-metal charge transfer transition) mainly contributes to the Raman scattering process.38 Three conditions are required for term B to be non-vanishing: (1) transition from the molecular ground state to the excited state must be allowed; (2) transitions from the molecule to the metal must be allowed; and (3) direct product of GM  GK  GQ must contain the totally symmetric representation. Here, GM, GK, and GQ are the irreducible representations of the charge transfer state, the excited state, and the vibrational mode, respectively. The symmetry of the HOMO and LUMO are b1 and a2 for BDT. The transition from the molecular ground state to the excited state (b1  a2 = b2) is allowed. The molecule fluctuates with time in the molecular junction at

Fig. 9 Example of SERS of BDT molecular junctions fabricated using the fixed nanogap electrodes. The inset shows the conductance of the molecular junction. The vertical lines indicate the vibration modes of the bulk crystal.39 The non-totally symmetric b2 modes are indicated by * in the upper two spectra.

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room temperature. The molecular axis can be oriented normal to the surface of the electrode. In this configuration, the symmetry of the charge transfer state is a1. GM  GK  GQ contains the totally symmetric representation for the b2 mode. The b2 mode is enhanced by the B term under these conditions.

Combination of conductance measurements The conductance measurement is critical to clarify the formation of the single molecular junction. In addition, the conductance measurement provides information about the electronic structure of the single molecular junction. Fig. 10(a) and (b) show the I–V characteristics of the BDT molecular junction fabricated using the fixed nanogap electrodes. In the single level tunneling model, the I– V characteristic of the molecular junction can be represented by 8e GL GR h GL þ GR 8 0 0 1 19 GR GL > > > > eV  e eV þ e < 0C 0 C= B BGL þ GR 1 BGL þ GR 1 C þ tan B C  tan @ @ A A> > GL þ GR GL þ GR > > : ;

IðVÞ ¼

2 GR 6 eV  e0 6 8p2 GL þ GR 6 2 eðkTÞ GL GR 6(  )2 2 6 3h GR 4 2 eV  e0 þðGL þ GR Þ GL þ GR

(3)

3 þ (

GL 7 eV þ e0 7 GL þ GR 7 ) 27 2 7 5 eV þ e0 þðGL þ GR Þ2

GL GL þ GR

where e0, GL, GR, k and T are the energy of the conduction orbital, the strength of the coupling between the molecule and the left and right electrodes, Boltzmann constant and temperature, respectively. By fitting the experimental results with eqn (3), GL, GR, and e0 are

Fig. 10 (a) Example of I–V characteristic of BDT molecular junction before (black) and after (green) the SERS measurement. The molecular junction is fabricated using the fixed nanogap electrodes (b) I–V characteristic of BDT molecular junction together with fitted results (red line) based on the single level tunneling model, (c) the distribution of e0 for BDT molecular junction obtained by 26 samples.39

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determined to be 0.030 eV, 0.033 eV, and 0.63 eV, respectively. The number of molecules bridging between metal electrodes is not clear in this molecular junction, so the contribution of the number of the molecules is included in GL, GR, assuming that GL, GR are much smaller than eV  e0. Fig. 10(c) shows the distribution of e0 obtained from 26 samples, which indicates that the energy difference between conduction orbital and Fermi level is 0.6 (0.1) eV for the BDT molecular junction.38 At 300 K, the temperature correction term of eqn (3) is about several % of the first term. So, we can roughly discuss the IV characteristic without considering the temperature correction term.

Conclusion We have discussed the SERS of a single molecular junction. Simultaneous SERS and conductance measurements provide information about the atomic configuration and dynamic motion of the single molecular junction. Raman-inactive vibration modes in bulk and a shifting of the vibration energy are observed in the SERS of the single molecular junction, which originates from the interaction between molecule and metal electrodes. A detailed analysis of the conductance and SERS measurements can also give us information about the effective temperature and electronic structure of the molecular junction. Recently, the spatial resolution of tip-enhanced Raman scattering (TERS) has been drastically improved using STM and AFM.36,40–42 Zhang et al. succeeded in showing the Raman imaging resolving the inner structure for a single porphyrin related molecule on the Ag(111) surface using TERS.42 The sub-nm spatial resolution is achieved by matching the resonance of the plasmon to the molecular vibronic transition. Although TERS has been applied to a limited number of single molecular junctions, at present, TERS would be a powerful technique to study single molecular junctions by combining TERS with some techniques (e.g. fishing mode TERS36) in the near future. A single molecular junction is a low dimensional nanomaterial having two molecule–metal interfaces, which can exhibit interesting properties that are not observed in other phases. The unique properties lead to high-performance electronic devices and new topics in material science. In investigations into these unique properties and single molecular devices, the importance of the characterization technique, including SERS, should increase.

Acknowledgements We would like to thank Prof. K. Murakoshi, and Dr T. Konishi for useful discussions. We gratefully acknowledge the financial support of the Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan.

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Surface enhanced Raman scattering of a single molecular junction.

The characterization of a single molecular junction is essential to investigate and utilize the single molecular junction in single molecular devices...
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