Single-Molecule Vibrational Spectroscopy and Inelastic-Tunneling

The STM-IETS spectrum of formate adsorbed at the short-bridge site exhibited a broad peak at 80 mV and a sharp resonance peak at 360 mV. The former pe...
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J. Phys. Chem. C 2010, 114, 3003–3007

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Single-Molecule Vibrational Spectroscopy and Inelastic-Tunneling-Electron-Induced Diffusion of Formate Adsorbed on Ni(110) Satoshi Katano,†,‡ Yousoo Kim,† Yuma Kagata,† and Maki Kawai*,†,§ Surface Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198 Japan, Research Institute of Electrical Communication, Tohoku UniVersity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577 Japan, and Department of AdVanced Materials Science, UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8651, Japan ReceiVed: September 28, 2009; ReVised Manuscript ReceiVed: January 13, 2010

We have investigated the inelastic-tunneling-electron-induced vibrational excitation of single formate (HCOOand DCOO-) molecules adsorbed on Ni(110) using a low-temperature scanning tunneling microscope (STM). Formate molecules adsorbed on the long-bridge site can be moved laterally along [11j0] by the injection of tunneling electrons from the STM tip. Using an isotope-labeled molecule, the diffusion probability and distance were found to be enhanced significantly when the applied voltage reaches the energies of specific vibrational modes, that is, C-H bending and C-H stretching modes. Inelastic electron tunneling spectroscopy based on STM (STM-IETS) and scanning tunneling spectroscopy was used to identify the vibrational and electronic state of a single formate molecule adsorbed at different sites. The STM-IETS spectrum of formate adsorbed at the short-bridge site exhibited a broad peak at 80 mV and a sharp resonance peak at 360 mV. The former peak is assigned to the electronic states generated by the metal-molecule hybridization; the latter peak is assigned to the C-H stretching mode. The STM-IETS spectrum of formate adsorbed at the long-bridge site showed features similar to those of formate adsorbed at the short-bridge site, but the intensity of the broad peak appearing at near the Fermi level was markedly suppressed, indicating a weak metal-molecule interaction. 1. Introduction Inducing the motion and reaction of a single molecule adsorbed on a metal surface has been extensively studied using a scanning tunneling microscope (STM) with the aim of promoting the future development of nanometer-scale molecular electronics.1,2 Various types of excitation mechanism, including vibrational3-9 and electronic excitations10-18 and mechanical interactions,19-21 have been investigated for controlling the dynamic behavior of an adsorbed single molecule. In particular, vibrational excitation induced by the inelastic tunneling electrons has attracted much attention, since a reaction and motion occur when the molecular vibration coincides with the reaction coordinate.3-9 A decade ago, a pioneering study, the dissociation of an oxygen molecule on Pt(111), was demonstrated by Stipe and co-workers.4 The power-law dependence of the dissociation rate on the tunneling current indicated the bond breaking of the oxygen molecule by inelastic tunneling electrons, which proceeded upon the vibrational excitation of the internal stretching mode. Kim and co-workers investigated the dehydrogenation of butene on Pd(110) as a function of applied voltage and found that the C-H bond scission is induced by multiple excitation of the C-H stretching mode.7 It should be noted that the vibrational mode excited via an inelastic process does not always coincide with the reaction coordinate. Komeda and co-workers examined the lateral hopping of CO on Pd(110) induced by inelastic tunneling electrons and demonstrated that the excitation of the low-energy vibrational mode (i.e., that along the reaction coordinate) can * Corresponding author. Phone: +81-48-467-9405. Fax: +81-48-4624663. E-mail: [email protected]. † RIKEN. ‡ Tohoku University. § University of Tokyo.

be accomplished by anharmonic coupling with a high-energy vibrational mode.22 Such indirect dynamical behavior initiated by vibrational excitation has also been reported for the rotation of acetylene on Cu(100),5 O2 on Pt(111),3 and cis-2-butene on Pd(110);8 the lateral hopping of CH3S on Cu(111);23 and the bond bending of chloronitrobenzene on Cu(111).24 Note that not all vibrational modes are responsible for inducing motion or a reaction. Sainoo and co-workers found that inelastictunneling-electron-induced rotational motion is governed by the resonant molecular electronic state.8 Thus, it is important to clarify the inelastic-tunneling-electron-induced dynamics in terms of the electronic state of the molecule coupled with the vibrational mode. However, such a mechanism has not been well-discussed yet, and further experimental investigation is required. In this study, we have investigated the inelastic-tunnelingelectron-induced vibrational excitation of single formate molecules (HCOO- and DCOO-) adsorbed on Ni(110) using a lowtemperature STM. In our previous reports, we have clarified that a formate molecule can adsorb at the short- and long-bridge sites on Ni(110).25,26 Our temperature-controlled STM study revealed that the formate adsorbing at the long-bridge site is metastable and diffuses along the Ni row direction when the substrate temperature exceeds 15 K.26 Here, we demonstrate how we can manipulate the lateral motion of a single formate molecule using an STM tip and examine the mechanism of tunneling-electron-induced dynamics at different adsorption sites. Formate adsorbed at the short-bridge site was found to be stable under the injection of tunneling electrons from the STM tip, but that at the long-bridge site moves laterally along the Ni row. Using an isotope-labeled molecule, the diffusion probability and distance were found to be enhanced significantly when the applied voltage reaches the energies of specific

10.1021/jp909394q  2010 American Chemical Society Published on Web 02/02/2010

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Figure 2. Action spectra for diffusion of (a) HCOO- and (b) DCOOadsorbed at long-bridge site as a function of the energy of the applied voltage pulses. The tunneling current used in the measurements is 3.0 nA, except in the case of DCOO- below 180 meV (9 nA).

Figure 1. (a) STM image of formate (HCOO-) adsorbed on Ni(110) surface. S and L indicate formate molecules adsorbed at short- and long-bridge sites, respectively. (b) STM image obtained after applying voltage pulse (500 mV, 3 nA, 1 s) to the L-formate indicated by the horizontal arrow in panel a. The initial position of the L-formate is marked by “×” in panel b. All STM images were obtained at a sample bias of 20 mV and a tunneling current of 1.0 nA. (c) Schematic model of the STM tip-induced lateral diffusion of L-formate on Ni(110). (d) The average diffusion distance of L-formate as a function of applied bias voltage.

vibrational modes. In addition, inelastic electron tunneling spectroscopy based on STM (STM-IETS) and scanning tunneling spectroscopy (STS) was used to identify the vibrational and electronic state of a single formate molecule adsorbed at different sites. The vibrational modes observed in the STMIETS spectra are compared with those observed in the action spectrum for formate diffusion. 2. Experimental Section All experiments were performed in an ultrahigh vacuum (UHV) chamber under a base pressure of 4 × 10-11 Torr. The Ni(110) surface was cleaned by repeated Ar ion sputtering and annealing at 1000 K until the clean surface was confirmed by STM imaging. The clean Ni(110) surface was exposed to formic acid at about 50 K through a dosing tube located near the substrate. At this temperature, HCOOH dissociated to form formate (HCOO-) on Ni(110), while DCOOD was adsorbed intact. This is because of the kinetic isotope effect observed for the dissociation of a hydroxyl group.27 Thus, we prepared formate from DCOOD by applying a bias voltage of 500 mV to dissociate the O-D bond. All experiments were carried out using a low-temperature STM system (LT-STM, Omicron GmbH). The STM images shown here were obtained in constant-current mode. STM-IETS and STS measurements were carried out using a lock-in amplifier with a modulation voltage of 5 mV and a frequency at 797 Hz. 3. Results and Discussion Figure 1a shows an STM image of formate (HCOO-) adsorbed on Ni(110) obtained after exposing the Ni(110) surface to formic acid (HCOOH) at 50 K, followed by subsequent cooling to 4.7 K. Each formate appears as a protrusion with

two depressions on both sides in the STM image, and the two depressions are always aligned parallel to the [11j0] or [001] direction.25-27 These species are represented by S and L in Figure 1a, which are assigned to formate molecules adsorbed at short- and long-bridge sites, respectively.25 Because L-formate is a metastable adsorption state,25 lateral diffusion was thermally activated and was observed during the STM measurement when the temperature of the Ni(110) substrate exceeded 15 K.26 We found that S-formate is stable under the injection of tunneling electrons, whereas L-formate moves laterally upon the injection of electrons from the STM tip. The STM image shown in Figure 1b was obtained after applying a voltage pulse from the STM tip. The STM tip was fixed precisely above the center of the molecule indicated by the horizontal arrow in Figure 1a, which was followed by the application of a voltage pulse of 500 mV with a tunneling current of 3 nA for 1 s. Figure 1c is a schematic diagram showing the lateral diffusion of an L-formate along the Ni row direction. Lateral diffusion was anisotropic and was never observed across the Ni row, similar to the behavior observed in a thermal diffusion process.26 Note that the probability of the STM tip-induced diffusion depends strongly on the applied bias voltage and tunneling current. Figure 2 shows action spectra for L-formate diffusion, which are defined by the number of diffusion events per electron as a function of applied bias voltage.8 The action spectra exhibit a significant increase in diffusion yield at certain bias voltages. Clear thresholds appeared at 140 and 360 mV in the spectrum for HCOO- (Figure 2a), and both threshold voltages were shifted to a lower voltage (110 and 270 mV) for DCOO- (Figure 2b). Previous high-resolution electron energy loss spectroscopy and infrared reflection absorption spectroscopy measurements showed that the peaks appearing at 134 (104) and 355 (271) meV are due to the vibrational excitation of out-of-plane C-H (C-D) bending and C-H (C-D) stretching modes, respectively.28,29 Therefore, we can conclude that the significant increase in diffusion yield of the L-formate corresponds to the excitation of the C-H bending and C-H stretching modes. The shifts of the threshold voltage observed for the deuterated formate can be reasonably explained in terms of the isotope shift of corresponding vibration modes. The average diffusion distance as a function of applied bias voltage is shown in Figure 1d. L-Formate is apt to diffuse a longer distance when a higher bias voltage is applied. In addition, careful examination of Figure 1d reveals a steep increase in the diffusion distance when the voltage exceeded the vibrational energy of the C-H (360 mV)

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Figure 3. Diffusion rates of (a) HCOO- and (b) DCOO- adsorbed at long-bridge site as a function of tunneling current for various applied bias voltages. The solid lines are least-squares fits to the data and correspond to power laws.

stretching mode for HCOO- and the C-D (270 mV) stretching mode for DCOO-. The dependences of the diffusion rate of L-formate on the tunneling current obtained at several voltages are shown in Figure 3. Linear dependence on the number of dosed tunneling electrons was observed at bias voltages fixed at 140, 200, and 400 mV when HCOO- was examined (Figure 3a). The same measurements were performed for DCOO- at 110, 200, and 400 mV, which also exhibited linear current dependence (Figure 3b). These results clearly demonstrated that L-formate diffusion is induced via a single-electron process rather than a multipleelectron process. In a previous report, we studied the thermal diffusion of L-formate and found the activation barrier for L-formate diffusion along the Ni row to be 37 meV.26 Owing to this small value, a single excitation of the C-H bending mode or C-H stretching mode is sufficient to overcome the diffusion barrier. Considering the reaction coordinate for diffusion, the momentum of the C-H bending mode coincides with the diffusion direction, and that of the C-H stretching mode does not. The internal energy transfer via anharmonic coupling between the C-H stretching mode and C-H bending mode, which is directly related to the diffusion coordinate, probably induces diffusion, as pointed out in previous papers.5,7,22,30 The STM-IETS spectra were obtained in relation to the action spectra for a single formate diffusion. The changes in tunneling conductance measured by STM-IETS can provide a direct indication of the fraction of inelastically tunneled electrons that excite the vibrational modes.5 Figure 4 shows the STM-IETS spectra of S-formate adsorbed on Ni(110). Each STM-IETS spectrum was obtained after fixing the STM tip above the center of the S-formate, which corresponds to the center of a protrusion observed in the STM image, with a set voltage of 0.1 V and a set tunneling current of 0.4 nA, followed by turning off the feedback loop. The background spectrum taken over the bare Ni(110) substrate was subtracted from each spectrum. The spectrum of HCOO- is characterized by the appearance of broad peaks near the Fermi level, and sharp resonance at 360 mV is observed for both positive and negative bias voltages. The peaks at (360 mV are readily assigned to the excitation of the C-H stretching vibrational mode, since corresponding peaks appeared at (270 mV for DCOO-. The shift of the peak position can be well-explained in view of the isotope effect. In contrast, we assigned the broad peaks appearing at near the Fermi level to

Figure 4. STM-IETS spectra of HCOO- and DCOO- adsorbed at short-bridge site of Ni(110). The background (substrate) spectrum taken over the Ni(110) substrate has been subtracted from each spectrum. All spectra were measured at the center of the formate with a set voltage of 100 mV and a set current of 0.4 nA.

the electronic states originating from the metal-molecule contact, which will be discussed in the following. STM-IETS spectra of formate measured at different adsorption sites are shown in Figure 5a. The background spectrum for the bare Ni substrate is also depicted at the bottom of Figure 5a. As we compare the spectrum of L-formate with that of S-formate, the C-D stretching peak is a common feature, although there is a difference in the peak energy (273 mV for L-formate). A notable difference between the spectra of S- and L-formate is the appearance of a broad peak near the Fermi level. As shown in Figure 4, the broad peak centered at around 80 mV in the S-formate spectrum becomes small in the L-formate spectrum. This characteristic difference can be interpreted as evidence for the hybridization of the electronic states between the formate and the Ni substrate. Details of the electronic states of formate/Ni(110) were examined using STS. STS spectra of S-formate, L-formate, and Ni substrate are shown in Figure 5b. It is well-known that a substantial part of the 3d band dominates the electronic state of Ni(110) near the Fermi level.31 The STS spectrum of the Ni substrate exhibits peaks at -165, -24, and 111 mV, which arise from the 3d band of the Ni substrate. Because the effect of the STM tip generally appeared on the STS spectrum, we would not like to discuss the fine structure of signals in the Ni spectrum and therefore argue only the difference in characteristics between the STS spectra obtained above the molecule and the metal

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Figure 5. (a) STM-IETS and (b) STS spectra of DCOO- adsorbed at different sites on the Ni(110) surface. The spectra indicated by red and blue curves were obtained from formate adsorbed at short-bridge and long-bridge sites, respectively. The spectrum indicated by the black curve was obtained from the Ni substrate. All spectra were measured with a set voltage of 100 mV and a set current of 0.4 nA.

substrate. For the spectrum of S-formate, the peaks observed at -136 and 160 mV can be assigned to the electronic states generated by the adsorption of formate. Chemisorption of a molecule onto the metal surface gives rise to the significant hybridization of its molecular orbital (MO) with metal d electrons around the Fermi level.32-35 According to previous reports on the adsorption of formate on metal surfaces, the highest occupied molecular orbital (HOMO) appears at 5 eV below the Fermi level, and the lowest unoccupied molecular orbital (LUMO) appears at 2 eV above the Fermi level.36,37 Thus, the peaks observed in the S-formate spectrum should be assigned to the fraction of the hybridized electronic states generated by the interaction between the 3d bands of Ni and the frontier orbitals of the formate. Note that hybridization peaks can also be found at -131 and 156 mV in the L-formate spectrum, but their peak intensities are markedly suppressed in comparison with those in the S-formate spectrum. The peak appearing above the Fermi level is shifted slightly to higher energy when a molecule is strongly chemisorbed onto the metal surface,32 indicating that this peak can be assigned to the fraction of the antibonding state generated by the hybridization between the d-state of Ni and the HOMO of formate. Likewise, the peak appearing at below the Fermi level can be assigned to the bonding state generated by the hybridization between the d-state of Ni and the LUMO of formate. This indicates that the electronic interaction between Ni and L-formate is very weak, which agrees with the fact that L-formate forms a metastable adsorption state.26 Furthermore, we expect that the hybridization of electronic states will also affect the inelastic-tunneling-electron-induced diffusion process. The negative ion state is temporarily formed when tunneling electrons are injected from the STM tip to the molecule. Vibrational damping through the creation of electronhole pairs occurs effectively when a large number of density of states exist near the Fermi level. Indeed, the C-H stretching mode of S-formate was excited by inelastic tunneling electrons, as shown in Figure 4, but the excess energy formed temporarily in the higher-energy vibrational mode does not dissipate to the lower-energy vibrational mode efficiently, owing to the existence of the large number of density of states near the Fermi level. This is probably the reason that we were unable to observe the diffusion of S-formate induced by inelastic tunneling electrons.

Katano et al. The above results demonstrate that the vibrational excitation of the C-H stretching mode is visible in both the action spectra and the STM-IETS spectra of L-formate (Figures 2 and 5a) and that of the C-H bending mode is not visible in the STM-IETS spectrum. Lorente and Persson investigated the inelastic tunneling process via vibrational excitation at the STM junction of acetylene on Cu(100) using a density functional theory (DFT) calculation.38 The total conductance change observed in STMIETS was shown to have contributions from inelastic and elastic tunneling electrons. Note that an increase in conductance via an inelastic process is occasionally accompanied by a reduction in the elastic tunneling process. Similarly, we consider that the C-H stretching mode increased the inelastic tunneling current that gave a dominant IET signal. From the action spectrum, an increase in the inelastic tunneling current was also expected upon the excitation of the C-H bending mode. However, the total change in conductance was negligible, which should be due to the reduction of elastic tunneling current. 4. Conclusion We investigated the vibrational excitation of a single formate molecule adsorbed on Ni(110) using a STM. The lateral motion of L-formate can be induced along the Ni row by the injection of tunneling electrons from the STM tip. We found that the diffusion probability and distance were markedly increased when the applied energy reached the energy of the C-H bending or C-H stretching vibrational modes. STM-IETS experiments revealed that S-formate exhibited a broad peak near the Fermi level and a sharp resonance peak at 360 mV, which were assigned to the hybridized electronic states and the C-H stretching mode, respectively. On the other hand, although the STM-IETS spectrum of L-formate exhibited similar features, the intensity of the broad peak was suppressed. We found that the broad peak originates from the electronic hybridization between the metal and the molecule, which was indicated by the STS spectrum. This shows that the electronic interaction between L-formate and the Ni substrate is very weak, which agrees with the fact that L-formate forms a metastable adsorption state. Acknowledgment. The present work was supported in part by a Grant-in-Aid for Scientific Research on Young Scientists B (No. 16750022); in part by a Grant-in-Aid for Scientific Research on Priority Areas, “Electron transport through a linked molecule in nano-scale”; and also in part by the Global COE Program, “The Physical Sciences Frontier”, from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References and Notes (1) Ho, W. J. Chem. Phys. 2002, 117, 11033. (2) Komeda, T. Prog. Surf. Sci. 2005, 78, 41. (3) Stipe, B. C.; Rezaei, M. A.; Ho, W. Science 1998, 279, 1907. (4) Stipe, B. C.; Rezaei, M. A.; Ho, W.; Gao, S.; Persson, M.; Lundqvist, B. I. Phys. ReV. Lett. 1997, 78, 4410. (5) Stipe, B. C.; Rezaei, M. A.; Ho, W. Phys. ReV. Lett. 1998, 81, 1263. (6) Stipe, B. C.; Rezaei, M. A.; Ho, W. Science 1998, 280, 1732. (7) Kim, Y.; Komeda, T.; Kawai, M. Phys. ReV. Lett. 2002, 89, 126104. (8) Sainoo, Y.; Kim, Y.; Okawa, T.; Komeda, T.; Shigekawa, H.; Kawai, M. Phys. ReV. Lett. 2005, 95, 246102. (9) Kumagai, T.; Kaizu, M.; Hatta, S.; Okuyama, H.; Aruga, T.; Hamada, I.; Morikawa, Y. Phys. ReV. Lett. 2008, 100, 166101. (10) Bartels, L.; Meyer, G.; Rieder, K. H.; Velic, D.; Knoesel, E.; Hotzel, A.; Wolf, M.; Ertl, G. Phys. ReV. Lett. 1998, 80, 2004. (11) Gaudioso, J.; Lee, H. J.; Ho, W. J. Am. Chem. Soc. 1999, 121, 8479. (12) Lauhon, L. J.; Ho, W. J. Phys. Chem. A 2000, 104, 2463. (13) Lauhon, L. J.; Ho, W. Phys. ReV. Lett. 2000, 84, 1527.

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J. Phys. Chem. C, Vol. 114, No. 7, 2010 3007 (27) Katano, S.; Kim, Y.; Kagata, Y.; Kawai, M. J. Phys. Chem. C 2009, 113, 19277. (28) Jones, T. S.; Ashton, M. R.; Richardson, N. V. J. Chem. Phys. 1989, 90, 7564. (29) Haq, S.; Love, J. G.; Sanders, H. E.; King, D. A. Surf. Sci. 1995, 325, 230. (30) Pascual, J. I.; Lorente, N.; Song, Z.; Conrad, H.; Rust, H. P. Nature 2003, 423, 525. (31) Pollak, P.; Courths, R.; Witzel, S. Surf. Sci. 1991, 255, L523. (32) Hammer, B.; Norskov, J. K. AdV. Catal. 2000, 45, 71. (33) Hoffmann, R. Solid and Surfaces: A Chemist’s View of Bonding in Extended Structures; Wiley-VCH: Weinheim, Germany, 1989. (34) Yoshinobu, J.; Kawai, M.; Imamura, I.; Marumo, F.; Suzuki, R.; Ozaki, H.; Aoki, M.; Masuda, S.; Aida, M. Phys. ReV. Lett. 1997, 79, 3942. (35) Ohara, M.; Kim, Y.; Yanagisawa, S.; Morikawa, Y.; Kawai, M. Phys. ReV. Lett. 2008, 100, 136104. (36) Hasselstrom, J.; Karis, O.; Weinelt, M.; Wassdahl, N.; Nilsson, A.; Nyberg, M.; Pettersson, L. G. M.; Samant, M. G.; Stohr, J. Surf. Sci. 1998, 407, 221. (37) Solymosi, F.; Kovacs, I. Surf. Sci. 1991, 259, 95. (38) Lorente, N.; Persson, M. Phys. ReV. Lett. 2000, 85, 2997.

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