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C60 Adsorbed on Platinum Surface: A Good Mediator of Metal Wave Function M. Sogo,† Y. Sakamoto,† M. Aoki,† S. Masuda,*,† S. Yanagisawa,‡ and Y. Morikawa‡ Department of Basic Science, Graduate School of Arts and Sciences, The UniVersity of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan, and The Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ReceiVed: May 14, 2009; ReVised Manuscript ReceiVed: January 3, 2010
A combined spectroscopic and theoretical study has been performed to clarify the local electronic properties induced at the C60-Pt(111) interface. The C60 molecules are regularly bound to the Pt(111) substrate via covalent bond, forming hybridized states just below the Fermi level (EF). Such a metallic feature is originated by strong Pt 5d-C60 π couplings and is extended to the entire molecule, indicating that the adsorbed C60 molecule serves as an excellent mediator of metal wave function. Our findings correspond well to an extremely high conductivity in C60 bridged by a pair of Pt electrodes and will provide a useful guide for fabricating moleculebased devices. 1. Introduction The C60 molecule and its assembly on a metal substrate have received much attention for the basic understanding of large molecule-metal interactions1 and for the potential use in molecule-based devices.2 The C60-Pt system can be regarded as a prototype from structural and electric points of view. The C60 molecules chemisorb on Pt(111) via covalent bond to form the ordered (13 × 13)R13.9° overlayer,3 which eventually leads to nanopatterning of metal surface.4 The charge transport of single C60 molecule bridged by metal electrodes such as Au-C60-Au junction has been examined extensively.5 Very recently, it is suggested that the Pt-C60-Pt junction shows an extremely high electric conductivity (∼0.7 G0 where G0 ) 2e2/ h).6 This indicates that the transmission coefficient through the junction attains to ∼70% in the Landauer limit.7 In the present study, we aimed to clarify the local electronic properties induced at the C60-Pt(111) interface in connection with the transport phenomena. A combined spectroscopic and theoretical study shows that the hybridized states are formed just below the Fermi level (EF) of the substrate by strong couplings between the Pt 5d and C60 π orbitals. Such a metallic feature is extended to the entire molecule and responsible for resonance tunneling in charge transport. In other words, the adsorbed C60 molecule serves as an excellent mediator of metal wave function. Metastable atom electron spectroscopy (MAES) used here is based on energy analysis of electrons emitted by thermal collision of long-lived excited atoms such as He*(1s2s, 23S) with a solid surface.8,9 On a transition-metal surface [e.g., Pt(111)], in which the conduction bands lie opposite the 2s level of He*(23S), the 2s electron tunnels resonantly (resonance ionization, RI) and then the resulting He+(12S) ion is neutralized by an Auger process (Auger neutralization, AN). The AN process produces two holes in the valence bands, yielding a broad spectrum reflecting the self-convolution of the local density of states. On an insulator surface (e.g., C60 condensed film), Penning ionization (PI) takes place, where a valence electron of the surface fills the He* 1s hole and the 2s electron * To whom correspondence should be addressed. E-mail: masuda@ piesap.c.u-tokyo.ac.jp. Phone: +81-3-5454-6589. Fax: +81-3-5454-4328. † The University of Tokyo. ‡ Osaka University.
is emitted simultaneously. In this case, a single hole is produced in the valence band, as in the case of ultraviolet photoemission spectroscopy (UPS). Since the metastable atom cannot penetrate into the bulk, an orbital exposed outside the surface gives more effective overlap than an orbital localized on the surface, yielding a stronger band in the spectrum. This feature enables us to obtain selective information on the valence states of the topmost surface layer. 2. Experimental Section The experiments were performed using an ultrahigh vacuum spectrometer (base pressure, 1 × 10-10 Torr).10 The condensed films were prepared by vacuum deposition of C60 on the Pt(111) substrate at room temperature. The layer thickness was monitored by a quartz-crystal microbalance and given by the units of monolayer (ML). The ordered monolayer was obtained by heating the condensed film up to 600 K, showing the (13 × 13)R13.9° pattern in low-energy electron diffraction (LEED). 3. Computational Details The first-principles calculations based on a generalized gradient approximation in density functional theory were performed for the C60 monolayer on Pt(111) using a program package “STATE”.11 To examine the electronic structure at the C60-Pt(111) interface and the chemical bond formation, we calculated the layer-resolved density of states (LDOS) and crystal orbital overlap population (COOP),12 respectively. A repeated slab model was used with each slab composed of three atomic layers, separated by a vacuum region of 13.8 Å. C60 molecules were introduced on one side of the slab. The ordered (13 × 13)R13.9° overlayer on a defect-free Pt(111) surface was assumed, and the removal of the topmost Pt atom faced to C60 as proposed in literature3 was not taken into account. For the structural optimization, the molecules and the topmost Pt atoms were allowed to relax. 4. Results and Discussion Figure 1 shows the typical He I UPS and He*(23S) MAES spectra of C60 on the Pt(111) substrate measured at room temperature, together with the (13 × 13)R13.9° LEED
10.1021/jp9044836 2010 American Chemical Society Published on Web 02/09/2010
C60 Adsorbed on Platinum Surface
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Figure 2. Crystal orbital overlap population (COOP) for the Pt(111)(13 × 13)R13.9°-C60 structure calculated by density functional theory.
Figure 1. (A) He I UPS spectra of C60 adsorbed on the Pt(111) surface at room temperature. (B) The corresponding He*(23S) MAES spectra. (C) Schematic view of Penning ionization on the C60 overlayer. (D) The (13 × 13)R13.9° LEED pattern of the C60 monolayer on Pt(111) taken with the primary energy of 18.4 eV.
pattern observed at 1 ML. The binding energy (EB) in the spectra is referred to EF of the substrate. In the condensed film (70 ML), photoemission bands at 2.2, 3.5, and 5.7 eV have been assigned to the hu (highest occupied molecular orbital, HOMO), gg + hg, and gu + t2u orbitals, respectively.13 In the ordered monolayer, the corresponding bands appear at 1.9, 3.2, and 5.1 eV, indicating that the C60 molecules are bound intact to the substrate without polymerization and decomposition.14 However, due to heavy overlap with the Pt 5d bands, it is not clear whether the hybridized state (or chemisorption-induced state) emerges near EF or not. The MAES spectra are apparently different from the UPS spectra and their characteristics can be summarized as follows: (i) On the bare substrate, the He*(23S) atoms deexcite via RI + AN to yield a broadband reflecting the self-convolution of the local density of Pt 5d states.15 (ii) On the condensed film, the He*(23S) atoms decay dominantly via PI,16 yielding the C60-derived bands as observed in the UPS spectrum. The absence of emission near EF indicates that the electronic properties of C60 without direct contact to the substrate are insulating (or semiconducting) in nature. Judging from the leading edge of the HOMO band, the HOMO-LUMO (lowest unoccupied molecular orbital) gap is more than 1.7 eV. (iii) On the ordered monolayer, PI partly takes place as a competing process of dominant RI + AN. The former process produces the HOMO-derived band and a weak tail extending
to EF, whereas the latter process produces a broad background. The tail structure is attributed to the chemisorption-induced states, because the corresponding emission is missing in the spectra of the bare substrate and the condensed film. Since the tail structure reveals the Fermi edge, the C60 molecules with direct contact to the substrate are metallic in character. Furthermore, it is clear that the HOMO-derived state and chemisorption-induced states are exposed further outside the surface compared to the higher-lying states, which results in the preferential interaction with the incident He* atoms. Finally, the dominant operation of RI + AN indicates that the C60derived conduction states are induced at 0-2 eV above EF (which corresponds to the 2s level of He* near the metal surface).8 To examine the spectral features, we performed first-principles calculations using density functional theory for the (13 × 13)R13.9° structure. In this structure, the nearest-neighbor C60-C60 distance is 10.0 Å,3,4 which corresponds to 10.03 Å in the face-centered cubic (fcc) solid.17 The total energy calculations show that the fcc hollow site on Pt(111) is more stable than the terminal and bridge sites, and the C6 ring in C60 is faced to the substrate. The molecular structure is slightly distorted to the lower symmetry and the topmost Pt atoms are displaced from the original positions. The calculated C-Pt bond length is 2.20 Å [which is slightly longer than the sum of the covalent radii of Pt (rPt ) 1.28 Å) and C (rC ) 0.77 Å)] and the calculated heat of formation is -123.2 kJ mol-1, indicating the covalent bond formation between C60 and Pt(111). Figure 2 shows the crystal orbital overlap population (COOP)12 between the C60 MOs (quintuply degenerate HOMO, triply degenerate LUMO, and triply degenerate LUMO + 1) and the substrate Pt wave functions, where the positive and negative parts stand for the bonding and antibonding couplings, respectively. The overlap populations are widely distributed below and above EF. It turns out from Figure 2 that the covalent bond is originated mainly from the LUMO, because the overlap populations below EF are positive. The HOMO shows the positive and negative populations below EF so that the net contributions to the covalent bond are minor.
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Sogo et al. mediator of metal wave function. In these systems, the chemisorption-induced states are formed just below EF, but are highly localized at the Pt-S bond.18 As a consequence, resonance tunneling through the junctions such as Pt-benzenedithiolate-Pt is strongly suppressed, leading to the lower conductance. 5. Conclusion In summary, we have identified the local electronic states of C60 on Pt(111) using MAES and first principles calculations. Upon the formation of the C60-Pt covalent bond, the hybridized states are emerged just below EF and distributed to the entire molecule. Our data show that the C60 molecule bound to Pt(111) serves an excellent mediator of metal wave function, which is responsible for resonance tunneling in charge transport. Acknowledgment. This study is financially supported through Special Coordination Funds of the Ministry of Education, Culture, Sports, and Science and Technology of the Japanese Government.
Figure 3. Layer-resolved density of states (LDOS) for the Pt(111)(13 × 13)R13.9°-C60 structure calculated by density functional theory.
Figure 3 shows the calculated layer-resolved density of states (LDOS) divided into nine layers (L1-L9) with 1.5 Å thickness. In the bottom layer (L1) composed of the Pt atoms, the prominent structure distributed below and above EF is due to the Pt 5d bands, whereas the weak structure extending above EF is due to the Pt sp bands. In the layers (L2-L7) including C60, the discrete levels in free C60 are broadened by strong mixings with the Pt 5d bands. Three prominent peaks below EF are attributed to the C60-derived bands in the UPS spectrum (Figure 1). The LDOS at EF is damped from the Pt substrate to C60, but substantially present even at the outermost C atoms in C60. In other words, the C60 molecule on Pt(111) is wholly metallic in nature in contrast to the cases of alkanethiolate18a and benzenethiolate18b on Pt(111) and serves as a good mediator of metal wave function. In the frontier region of C60 (L8-L9), the LDOS of the HOMO-derived state is still higher than those of the other filled states, as observed in the MAES spectrum. Also, the empty states are exposed toward the vacuum, causing the operation of the RI + AN process of the incident He*. Finally, we briefly address the relationship between the local electronic states and transport properties in C60 bridged by metal electrodes. According to the Landauer equation,7 the conductance in the molecular junction is proportional to the transmission function, which is determined by the spatial overlap of wave functions between the metal electrodes. Therefore, the asymptotic behavior of wave functions exposed outside the metal surface plays a crucial role in the transport characteristics. In the present study, we found that the wave functions at EF are sufficiently extent from the Pt substrate to the inside (and even to the outside) of C60, owing to the strong couplings between the Pt 5d and C60 π states. Such an exposure of metal wave function would occur also in the case of C60 linked by two Pt electrodes. This causes resonance tunneling through the junction, yielding the high conductivity. To the contrary, alkanethiolate and benzenethiolate formed on Pt(111) are regarded as a poor
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