Metal−Molecule Interfaces Formed by Noble-Metal−Chalcogen Bonds

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Metal-Molecule Interfaces Formed by Noble-Metal-Chalcogen Bonds for Nanoscale Molecular Devices Kazumichi Yokota,† Masateru Taniguchi,*,†,‡ and Tomoji Kawai*,† The Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Ibaraki, Osaka 567-0047, Japan, and PRESTO, Japan Science and Technology Agency, Honcho, Kawaguchi, Saitama 332-0012, Japan ReceiVed: NoVember 17, 2009; ReVised Manuscript ReceiVed: January 22, 2010

Using benzenethiol and benzeneselenol monolayers on Ag(111) and Cu(111) substrates, we systematically studied the electronic states of metal-molecule interfaces formed by noble-metal-chalcogen bonds. X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy revealed that the Ag-S, Ag-Se, and Cu-S interfaces retain their metallic nature, whereas the Cu-Se interface gets oxidized. On the basis of the results of this study and our previous study, we conclude that, among Au-S, Au-Se, Au-Te, Ag-S, Ag-Se, Cu-S, and Cu-Se interfaces, Au-Se is most suitable for nanoscale molecular devices. We found a screening parameter of 0.5 in the metal-molecule interface system, indicating that interfacial electronic states near the Fermi level are produced at the interface between the noble metal and molecule. However, we expect the density of states that originates from molecules near the Fermi level to be very small. The findings imply that the electrical conduction mechanism of metal-single-molecule-metal junctions involves tunneling via interfacial electronic states composed of the frontier orbitals of molecules and that single-molecule conductance is very small at low voltages. 1. Introduction Nanoscale molecular devices are candidates for overcoming the physical and financial limitations of Si semiconductors.1-4 To realize such devices, we need to develop a new fabrication process suitable for them and evaluate physical properties of single-molecule devices.5-7 New fabrication processes have been developed that exploit molecular and sequential self-organization using chemical reactions, thus enabling the fabrication of nanoscale molecular devices.8,9 Furthermore, it is now possible to evaluate physical properties of single-molecule devices as a result of dramatic improvements in scanning tunneling microscopy break junctions (STM-BJs),10-13 atomic force microscopy BJs (AFM-BJs),14-16 and mechanically controllable BJs (MCBJs).17-21 Both experimental and theoretical studies of single-molecule devices indicate that electronic states of the metal-molecule interface are crucial to determining the electrical properties of such devices.22-26 Therefore, understanding the electronic states of metal-molecule interfaces is important for controlling the characteristics of nanoscale molecular devices, thereby facilitating the development of these devices. Thus far, studies of the electronic states of metal-molecule interfaces have focused on monolayers on metal substrates because it is easy to evaluate the properties of these systems, owing to their well-defined structures.27-30 Information regarding electronic states of monolayers on metal substrates provides an insight into metal-single-molecule interfaces when interactions between the monolayer molecules are very weak. The electronic states of metal-single-molecule interfaces are equivalent to those of metal-single-molecule-metal junctions31 (i.e., singlemolecule junctions) at low voltages. Consequently, studies of * To whom correspondence should be addressed. E-mail: taniguti@ sanken.osaka-u.ac.jp. Tel: +81-6-6879-4289 (M.T.). Fax: +81-6-6875-2440 (M.T.). † Osaka University. ‡ Japan Science and Technology Agency.

electronic states of metal-molecule interfaces via monolayers on metal substrates provide useful information for nanoscale molecular devices.32-34 However, since the discovery of selfassembled monolayers, metal-molecule interfaces formed by Au-S bonds have attracted significant research interest. This, however, has hindered the development of metal-molecule interfaces by other means. A major cause of this hindrance is the lack of guiding principles for controlling device characteristics of nanoscale molecular devices.35,36 To address this issue, we studied Au-Se and Au-Te bonds for developing interfaces other than Au-S.37 Here, we discuss the electronic states of metal-molecule interfaces formed by Au-S, Au-Se, and Au-Te bonds. To form the interfaces, we used benzenethiol (BT), benzeneselenol (BS), and biphenyl ditelluride monolayers on Au(111) substrates. We found that Au-Se interfaces can provide larger electrical conductance than Au-S or Au-Te interfaces. Because low power consumption and stable operation are required for nanoscale molecular devices, the metal-molecule interface most suitable for a nanoscale molecular device is one that yields the maximum electric current at low voltages. Consequently, Au-Se interfaces are more appropriate for nanoscale molecular devices than Au-S or Au-Te interfaces. Furthermore, we found that the Au-Te interface is unsuitable for nanoscale molecular devices because of the oxidation of Te and that BS monolayers on Au(111) substrates formed well-defined structures.38 Further research on metal-molecule interfaces formed by metalchalcogen bonds would prove a good strategy for achieving metal-molecule interfaces suitable for nanoscale molecular devices. Noble metals are important electrode materials for electronic devices because Au is the primary material used to make Ohmic contact and Ag and Cu are used as interconnecting materials. Because the valence electrons of noble metals and chalcogen atoms are in both the s and the p orbitals, orbital interactions between noble metals and chalcogen atoms are expected to be

10.1021/jp9109139  2010 American Chemical Society Published on Web 02/12/2010

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Figure 1. Metal-molecule interfaces formed by noble-metal-chalcogen bonds.

similar near the Fermi level. Electronic states of noblemetal-chalcogen interfaces are, therefore, affected mainly by properties specific to metal and chalcogen atoms, such as work function, density of states, and ionization potential. This enables us to understand metal-molecule interfaces formed by noblemetal-chalcogen bonds systematically, based on specific properties of participating metals and molecules. In the present research, we deposited BT and BS monolayers on Ag(111) and Cu(111) substrates and investigated the electronic states of metal-molecule interfaces using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) to understand metal-π-molecule interfaces systematically formed by Ag-S, Ag-Se, Cu-S, and Cu-Se bonds (Figure 1). Metal-Te interfaces were excluded because of Te oxidation. We determined that Ag-S, Ag-Se, and Cu-S interfaces retained their metallic nature, whereas Cu-Se interfaces get oxidized. Considering this and our previous research,37 we conclude that the Au-Se interface is most suitable for nanoscale molecular devices with noblemetal-chalcogen interfaces. We obtained a screening parameter39 of 0.5 for the noble-metal-molecule interface system, indicating that interfacial electronic states exist near the Fermi level. This finding implies that the electrical conduction mechanism of single-molecule junctions involves tunneling via interfacial electronic states composed of the frontier orbitals of the molecules. 2. Experimental Section 2.1. Materials. We used BT (C6H5SH, 98%, Tokyo Chemical Industry, Japan) and BS (C6H5SeH, 95%, Tokyo Chemical Industry) without purification. 2.2. Preparation of Metal Substrates. Ag (5 N %) and Cu (5 N %) obtained from Nilaco were thermally evaporated from a tungsten basket onto cleaved mica that was heated to 300 and 450 °C, respectively. The base and operating pressures were ∼10-7 and ∼10-5 Pa, respectively, and the deposition rate was approximately 20 nm/min. The substrates were characterized by XRD (2θ-θ), which confirmed 〈111〉 orientation of the mica substrate and the absence of impurities, such as oxidized elements [see Figure S1 in the Supporting Information (SI)]. 2.3. X-ray Diffraction. XRD patterns were acquired using a Philips Xpert Pro MRD. The incident beam was Cu KR1 radiation (λ ) 1.54 Å) conditioned by a hybrid monochromator. The 2θ-θ scans were performed from 2θ ) 5° to 105° in 0.01° steps. 2.4. X-ray Photoelectron Spectroscopy. XPS spectra were acquired using a 100 mm hemispherical analyzer (CLAM2) with an Al KR (1486.6 eV) X-ray source (XR3E2). The binding energies (Eb) were calibrated from the peak of the Ag 3d5/2 (368.26 eV) and Cu 2p3/2 (932.67 eV) bands,40 and we defined the kinetic energy (Ek) of electrons as Ek ) 1486.6 - Eb. The full width at half-maximum (fwhm) of the Ag 3d3/2 peak was 1.21 eV with a pass energy of 50 eV and a spin-orbit coupling between Ag 3d3/2 and 3d1/2 of 6.00 eV. The spectra were fit using a pseudo-Voigt function with a Shirley background. The energy step for wide-range scans was 1.0 eV and that for narrow

Figure 2. XPS spectra of BT and BS monolayers on the Ag(111) substrate. (a) Spectra of S 2p and (b) spectra of Se 3p core levels and Auger spectra of Se L3MM.

scans was 0.1 eV. The pressure of the measurement chamber was 4.0 × 10-7 Pa. 2.5. Ultraviolet Photoelectron Spectroscopy. UPS spectra were obtained with a 150 mm hemispherical analyzer (PHOIBOS150) using He I excitation (UVS300). When the measurement was performed with a constant retarding ratio of 10, we were able to fit the resulting Fermi edge of Au with a FermiDirac function at 300 K convoluted with a Gaussian with an fwhm of 0.14 eV. The pressure of the measurement chamber was 2.0 × 10-7 Pa and that of the preparation chamber was 2 × 10-6 Pa. To measure the work function of the metal substrate, the metal surface was cleaned in a preparation chamber by annealing at 400 °C for 5 min using a ceramic heater. The measurement was performed in the measurement chamber without exposing the sample to air and with a sample bias voltage of -5.0 V. 2.6. Theoretical Calculations. BT and BS molecules were geometrically optimized using the hybrid Hartree-Fock/density functional theory (HF/DF) scheme (B3LYP), which combines Becke’s three-parameter nonlocal exchange functional with the nonlocal correlation functional of Parr and co-workers, and by employing Gaussian basis sets [6-31G+(d, p)] for each atom. The ionization potentials of BT and BS were calculated from the energies of neutral and cationic molecules and corrected by the zero-point vibration energy derived from vibrational analyses. All calculations were performed using Gaussian03 program.41 3. Results and Discussion 3.1. BT and BS Monolayers on Ag(111). The Ag(111) substrate was soaked in 10 mM toluene solutions of BT and BS for over 12 h each, followed by rinsing with toluene. Figure 2 shows the XPS spectra of BT and BS monolayers on Ag(111) substrates. In the S 2p area of the spectrum, S 2p3/2 and S 2p1/2

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Figure 3. UPS spectra of the Ag(111) substrate (gray), BT monolayers on the Ag(111) substrate (red), and BS monolayers on the Ag(111) substrate (green) near the Fermi level. EF shows the Fermi level of Ag.

peaks appear at Eb ) 162.3 and 163.5 eV, respectively, indicating the formation of the Ag-S bond.42,43 In the Se 3p and Se L3MM areas of the spectrum, Se 3p3/2, Se 3p1/2, and Se L3MM peaks were observed at Eb ) 161.0 eV, Eb ) 166.3 eV, and Ek ) 1307.0 eV, respectively, indicating the formation of the Ag-Se bond.44,45 We, therefore, conclude that monolayers of BT and BS form on Ag(111). We investigated the spectra in the O 1s peak region but found no peaks indicating the presence of oxidized chemical species. We used UPS of the BT and BS monolayers to investigate electronic states of the Ag-molecule interfaces. UPS spectra near the Fermi level of the two monolayers on Ag(111) are indicated in Figure 3. In the BT monolayer spectra, the density of states increased from around -1.5 eV (measured from the Fermi level of Ag), whereas in the BS monolayer, the density of states increased from around -1.3 eV. The energy at which the density of states increases was determined using the peak energy of the first derivatives of the UPS spectra of monolayers. Because no peaks were observed in the first derivative of the UPS spectrum of Ag(111), molecular orbitals contribute to the density of states observed around -1.5 and -1.3 eV. The increase in the density of states was also observed in BT and BS monolayers on Au(111).37 We note that density of states appears at the Fermi level, indicating that, like the Au-S and Au-Se bonds, the metal-molecule interfaces formed by Ag-S and Ag-Se bonds are metallic in nature. 3.2. BT and BS Monolayers on Cu(111). We performed careful experiments with Cu-BT and Cu-BS interfaces because it is well-known that Cu gets oxidized more easily46-48 than Au and Ag.49 XPS was performed on the (A) BT/Cu(111) substrate, (B) Cu(111) substrate that had been exposed to air after rinsing with toluene for 1 h, and (C) Cu(111) substrate that had been exposed to air without any treatment after film formation (Figure 4). Sample A was prepared by soaking the Cu(111) substrate in 10 mM toluene solution of BT for over 12 h, followed by rinsing with toluene. In the spectral region of S 2p, peaks were observed only for A but not for B or C. The binding energy of the peak for A was 163.4 eV, indicating that this peak energy did not originate from thiol oxides of disulfide50 or sulfonate51 but from S 2p electrons bound to Cu forming thiolate. We considered that a monolayer of BT formed on Cu(111). Because Cu2+ 2p charge-transfer satellite peaks were not observed for any of the samples, the chemical state of Cu in A, B, and C was Cu0 or Cu+. Although it is difficult to distinguish Cu0 and Cu+ on the basis of the chemical shift of the Cu 2p orbital electron, they can be distinguished using Cu

Figure 4. XPS spectra of BT monolayers on the Cu(111) substrate. (a) Spectra of S 2p and (b) spectra of O 1s core levels and Auger spectra of Cu LMM. Spectra A, B, and C were obtained from the BT/Cu(111) substrate, Cu(111) substrate exposed to air after rinsing with toluene for 1 h, and Cu(111) substrate exposed to air without any treatment, respectively. The inset shows the normalized intensity of the O 1s spectra originating from O2 and O2-.

L3MM Auger spectra.52,53 For Cu, the peak appears at a kinetic energy of Ek ) 918.1 eV, whereas for Cu+, it appears at Ek ) 916.2 eV. In the XPS spectrum of Cu L3MM (Figure 4b), the Auger electron peak originating from Cu0 is easily observed for A, whereas the Auger peak originating from Cu+ becomes larger for B and C. These changes in the Auger spectrum were accompanied by changes in the O 1s spectrum originating from O2- and O2. The O2 peak observed at kinetic energy Eb ) 533.6 eV is the main peak for A, and the O2- peak observed at Eb ) 531.2 eV increases for B and C. The intensity ratio of these peaks normalized by the Cu 2p3/2 intensity is shown in the inset of Figure 4b. For A, almost 90% of the total O 1s peak intensity originated from O2. The total O 1s peak intensity of B and C is greater than that for A by a factor of 6.4 and 4.6, respectively. An even more notable characteristic was the extremely significant increase in the O2- peak by a factor of 36 and 27 for B and C, respectively, which is an order of magnitude larger than the increase in the O2 peak (by a factor of 2.1 and 1.3, respectively). These results indicate that, for A, the Cu surface is protected against oxidation because O2 is terminated by benzenethiolate, whereas in the unterminated samples B and C, the Cu surface oxidizes to Cu2O to form Cu2O/Cu(111). It is surprising to find that the interface formed by Cu-S bonds maintained a metallic state, as Cu(111) is oxidized in air and in solutions. To study the metallic interface in detail, electronic states near the Fermi level were evaluated by UPS. In the UPS spectrum (Figure 5), the density of states at the Fermi level was observed even after exposure to air for 1 h, indicating that the metallic state endured. This result indicates

Noble-Metal-Molecule Interfaces via Chalcogen Atoms

Figure 5. Air exposure time dependence of UPS spectra of BT monolayers on the Cu(111) substrate near the Fermi level. UPS spectra of the Cu(111) substrate after annealing at 400 °C under vacuum. The broken line indicates the Fermi level of Cu.

Figure 6. Time dependence of XPS spectra of the BS monolayer on the Cu(111) substrate. Spectra of O 1s and Se 3p core levels and Auger spectra of Se L3MM measured after the monolayers were set in vacuum for 0, 24, and 31 h.

that BT termination suppresses Cu surface oxidation. We also note that the density of states increases from -1.4 eV [measured from the Fermi level of Cu(111)] obtained from the peak energy of the first derivative of the UPS spectrum of the monolayers. As with the UPS spectra of the Ag-S, Ag-Se, Au-S, and Au-Se interfaces, because no peaks were observed in the first derivative of the UPS spectrum of Cu(111), molecular orbitals contributing to the density of states were observed around -1.4 eV. We next attempted to deposit a BS monolayer on the Cu(111) surface. The Cu(111) substrate was soaked in 10 mM toluene solution of BS for 1 h in an Ar atmosphere. In the spectral region of O 1s, the peak intensity of O2- (Eb ) 531.0 eV) decreased drastically with increasing measurement time, whereas the peak intensity of O2 (Eb ) 533.7 eV) was independent of measurement time (Figure 6). In the spectral area of Se 3p, the peak value of Se 3p (Eb ) 161.5 eV for 3p3/2 and Eb ) 166.9 eV for 3p1/2) is independent of measurement time. In the Auger spectral region of Se L3MM, the intensity of the peak observed at Ek ) 1305.6 eV decreased and that peak at Ek ) 1307.3 eV increased with increasing measurement time. The peaks appearing at Ek ) 1305.6 eV and Ek ) 1307.3 eV are due to oxidized Se (Seδ+) and reduced Se (Seδ-). As noted above, Cu is more easily oxidized than Au and Ag. The SeH group oxidizes more easily than the SH group, and it has been reported that C6H5SeH oxidizes to C6H5SeO2- because of atmospheric oxygen.54 Considering the spectra obtained and taking into account the oxidation properties of noble metals and the SeH group, we

J. Phys. Chem. C, Vol. 114, No. 9, 2010 4047 speculate that the adsorption mechanism of BS on the Cu(111) surface involves rapid formation of C6H5Se-O-Cu after BS is adsorbed on the Cu(111) surface by the formation of Cu-Se bonds. This oxidation mechanism is supported by the fact that the strong peak at Ek ) 1305.6 eV appears as soon as the measurements start. Subsequently, C6H5Se-O-Cu changes to C6H5-Se-Cu because Se atoms are reduced by measurement effects,55 including radicals produced by X-ray radiation,56 electrons from the electron beam,57 and local heating by thermal electrons.58 The reduction mechanism is supported by the time dependence of the intensity of the peak appearing at Ek ) 1307.3 eV. Therefore, BS on Cu(111) in vacuum changes to C6H5-Se-Cu, C6H5Se-O-Cu, and C6H5-Se-Cu, in that order. Consequently, we obtained a BS monolayer on Cu(111) after the sample was irradiated by X-rays in vacuum for a sufficient time. Thus, we conclude that the Cu-Se interface is unsuitable for molecular devices. 3.3. Interfacial Electronic States of Metal-Molecule Interfaces near the Fermi Level. We now discuss the electronic states of the metal-molecule interfaces that retain their metallic nature in air. For BT and BS monolayers on Ag(111) substrates, the density of states increased from -1.5 and -1.3 eV, respectively, measured from the Fermi level of Ag. For the BT monolayer on the Cu(111) substrate, the density of states increased from -1.4 eV measured from the Fermi level of Cu. Because the density of states at the binding energy was different from that of isolated molecules in the gas phase,59,60 it must have originated from interfacial electronic states generated by the formation of a metal-molecule interface. We then focused on the interfacial electronic states of metal-molecule interfaces near the Fermi level. Such states at the interface between metal electrodes and organic semiconducting layers can be discussed within the framework of the screening parameter (S parameter),39 which is defined by

S)

dφBP dφm

where φm and φBP denote the work function of the metal and the barrier height for carrier injection at the interface for holes, respectively. For S ) 0, Fermi level pinning occurs (Bardeen limit), and organic semiconducting layers have many carriers. However, for S ) 1, an ideal Schottky barrier is created at the interface (Schottky limit), and organic semiconducting layers have few carriers.61,62 For metal-molecule interfaces formed by chemical bonds, it is reasonable to interpret the Bardeen limit as the situation where strong orbital interactions between frontier orbitals of metals and the highest occupied molecular orbital (HOMO) are created. The Schottky limit is interpreted as the situations where weak orbital interactions between the frontier orbitals of metals and the HOMO are created, when molecules have large energy gaps between the HOMO and the lowest unoccupied molecular orbital (LUMO). In a previous study of the electronic structure of metalmolecule interfaces formed by Au-Se bonds, we demonstrated that the increasing density of states from -0.9 eV (measured from the Fermi level of Au) originated from the HOMO of BS.37 Since orbital interactions in noble metals and chalcogen atoms are expected to be similar near the Fermi level because the valence orbitals of noble metals and chalcogen atoms are s and p orbitals, the increasing density of states from -1.5, -1.3, -1.4, and -1.2 eV originates from the HOMO of BT or BS molecules. Therefore, it is reasonable to define φPB as the binding

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Figure 7. Interfacial electronic states near the Fermi level. (a) Dependence of the energies relative to the Fermi energy of metals (φPB) on the work function of substrate metals (φm). φBP is defined as the energy from which the density of states for monolayers on substrates increases from the Fermi level of the metal. φBP corresponds to the binding energy obtained from UPS spectra shown in Figures 3 and 5. The φBP of monolayers on Au(111) is taken from our previous study. (b) Schematic of interfacial electronic states of metal-molecule interfaces formed by noble-metal-chalcogen bonds.

energy at which the density of states increases because the difference between the ionization potential of a molecule and the Fermi level of a metal is equal to the hole injection barrier. The φm dependence of φBP in this scenario is shown in Figure 7a. From UPS measurements, we determined that the work functions of Au(111), Ag(111), and Cu(111) substrates63 were 5.3, 4.7, and 5.0 eV, respectively (SI, Figure 2S). The data clearly show that φBP is proportional to φm with a slope of 0.5 for both the BT and the BS monolayer systems and that S ) 0.5. We expect that a monolayer on a metal substrate has few carriers because the bandwidth formed by the HOMO is small due to the very weak π-π interactions between molecules. Therefore, the value of S ) 0.5 is reasonable and indicates that interfacial electronic states were generated at the metal-molecule interface formed by noble-metal-chalcogen bonds (Figure 7b) and that moderate orbital interactions exist between a metal electrode and a molecule in a metal-molecule interface formed by chemical bonds. Although interfacial electronic states generally have complex origins,64-67 including charge neutrality levels68,69 and metalinduced gap states,70,71 they primarily originate from effective orbital interactions between metal and molecules in the noblemetal-molecule monolayer system. In fact, theoretical calculations support the interpretation that interfacial electronic states form effective orbital interactions between the frontier orbitals of Au and BS.38 What causes the difference between electronic states formed by noble-metal-S bonds and those formed by noble-metal-Se

Yokota et al. bonds? Experimental results show that φBP of the BS monolayer is smaller than that of the BT monolayer and is independent of metal work functions. A simple explanation is that the ionization potential of BS is smaller than that of BT because φBP is given as φBP) IP - φm, where IP denotes the ionization potential of a molecule. The calculated ionization potentials of BS and BT were 7.94 and 8.09 eV, respectively. Because the difference between the calculated ionization potentials (0.15 eV) is close to the experimental values (0.2 and 0.3 eV), we conclude that the difference in φBP is due to the difference in the ionization potentials of the two molecules. Recently, the electrical conductance of single-molecule junctions formed by Au-S bonds has been studied using break junctions.1-35 The conductance is small, ranging from 100 to 1 mG0, where G0 ) 2e2/h is the conductance quantum (e and h have their usual meanings).5-10,17,35 The ideal conduction mechanism of single-molecule junctions for realizing highconductance molecular devices is interpreted as coherent tunneling in which electrons pass through molecular orbitals such as HOMO and LUMO. However, the molecules measured have large HOMO-LUMO gaps, such as Eg∼5 eV for benzenedithiol, resulting in large carrier injection gaps; thus, we can expect that the electron conduction path is not the HOMO or LUMO at low voltages (i.e., below 2.5 eV). Therefore, the conduction mechanism of single-molecule junctions depends on the carrier injection gaps and voltage applied between electrodes. In the present situation, our findings of φBP and S ) 0.5 in metal-molecule interfaces are significant for understanding the conduction mechanism of single-molecule junctions. The large φBP of more than 0.9 eV is equal to the carrier injection gap, assuming p-type carriers in single-molecule junctions. Therefore, among metal-molecule interfaces formed by noblemetal-chalcogen bonds, that formed by Au-Se bonds is the most suitable for nanoscale molecular devices because it has the smallest carrier injection gap, 0.9 eV. On the other hand, the conduction mechanism of single-molecule junctions is tunneling at low voltage below φPB, indicating that the conduction path is not the HOMO. So, what is the conduction path? The value of S ) 0.5 indicates formation of interfacial electronic states, which are composed of hybrid orbitals between the frontier orbitals of metals and molecules as a result of moderate orbital interactions between the frontier orbitals. The contribution of the frontier orbitals of molecules to the density of states near the Fermi level is supported by theoretical calculations and STM observations of BS monolayers on a Au(111) surface.38 To estimate the density of states D(EF) of interfacial electronic states, we use S ) (1 + e2δD(EF)/ε)-1,39,72-74 where e, δ, and ε denote the elementary charge, width of the metal-molecule interface, and dielectric constant, respectively. Taking δ and ε to be 5.7 Å and 4.38,75 respectively, D(EF) is estimated to be 4.2 × 1013 states/(cm2 · eV), which is less than 20% of the density of states of noble metals at the Fermi level.76 Because the singlemolecule conductance is proportional to the density of states when the conduction mechanism is tunnelling, a small density of states leads to a small single-molecule conductance. In fact, it has been reported that the single-molecule conductance of Au-BDT-Au junctions is 11 mG0.77-82 Although the electrical conductance is very small, a finite density of states originates from the frontier orbitals of a molecule near the Fermi level, indicating that electrons pass through hybrid orbitals composed of frontier orbitals of the metal and molecule. Therefore, our results imply that the conduction mechanism of single-molecule junctions having metal-molecule interfaces of Au-S, Au-Se,

Noble-Metal-Molecule Interfaces via Chalcogen Atoms Ag-S, Ag-Se, and Cu-S bonds is tunnelling via the interfacial electronic states near the Fermi level. 4. Conclusions We systematically investigated the electronic states of monolayers of BT and BS on Ag and Cu substrates. Photoelectron spectroscopy measurements revealed that BT and BS monolayers on Ag substrates retain their metallic nature. We expect that the Ag-Se interface has a larger electrical conductance than the Ag-S interface. Photoelectron spectroscopy measurements revealed that BT monolayers on Cu substrates retain their metallic nature, whereas the Cu-BS interface gets oxidized. We estimated that Cu coated with a BT monolayer has better oxidation resistance than bare Cu. The carrier injection gaps φBP in Au-S, Au-Se, Ag-S, Ag-Se, and Cu-S interfaces indicate that the metal-molecule interface formed by Au-Se bonds is most suitable for nanoscale molecular devices among metal-molecule interfaces formed by noble-metal-chalcogen bonds because it has the smallest carrier injection gap of 0.9 eV. The screening parameter S ) 0.5 in noble-metal-molecule interfaces indicates that interfacial electronic states are created near the Fermi level, though their density of states is very small. The findings imply that tunneling via the interfacial electronic states is the electrical conduction mechanism of the singlemolecule junctions and that the single-molecule conductance is very small at low voltages. Acknowledgment. Financial support for this work was provided, in part, by a Grant-in-Aid for Scientific Research on Priority Area “Electron transport through a linked molecule at nano-scale” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Supporting Information Available: XRD and UPS spectra of Ag(111) and Cu(111) substrates and a complete author list for ref 41. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Aviram, A.; Ratner, M. A. Molecular Electronics: Science and Technology; New York Academy of Sciences: New York, 1998. (2) Tour, J. M. Molecular Electronics: Commercial Insights, Chemistry, DeVices, Architecture and Programming; World Scientific: Singapore, 2003. (3) James, D. K.; Tour, J. M. Chem. Mater. 2004, 16, 4423–4435. (4) Selzer, Y.; Allara, D. L. Annu. ReV. Phys. Chem. 2006, 57, 593– 623. (5) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384–1389. (6) McCreery, R. L. Chem. Mater. 2004, 16, 4477–4496. (7) Tao, N. J. Nat. Nanotechnol. 2006, 1, 173–181. (8) Taniguchi, M.; Nojima, Y.; Yokota, K.; Terao, J.; Sato, K.; Kambe, N.; Kawai, T. J. Am. Chem. Soc. 2006, 128, 15062–15063. (9) Chen, X.; Braunschweig, A. B.; Wiester, M. J.; Yeganeh, S.; Ratner, M. A.; Mirkin, C. A. Angew. Chem., Int. Ed. 2009, 48, 5178–5181. (10) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571–574. (11) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2001, 123, 5549– 5556. (12) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nature 2006, 442, 904–907. (13) Huang, Z.; Chen, F.; D’Agosta, R.; Bennett, P. A.; Di Ventra, M.; Tao, N. J. Nat. Nanotechnol. 2007, 2, 698–703. (14) Xu, B.; Tao, N. J. Science 2003, 301, 1221–1223. (15) Huang, Z. F.; Xu, B. Q.; Chen, Y. C.; Di Ventra, M.; Tao, N. J. Nano Lett. 2006, 6, 1240–1244. (16) Jang, S.-Y.; Reddy, P.; Majumdar, A.; Segalman, R. A. Nano Lett. 2006, 6, 2362–2367. (17) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252–254. (18) Smit, R. H. M.; Noat, Y.; Untiedt, C.; Lang, N. D.; van Hemert, M. C.; van Ruitenbeek, J. M. Nature 2002, 419, 906–909.

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