Molecular Orientation of Individual Lu@C82 Molecules Demonstrated

Aug 13, 2010 - Yuki Sado , Shinobu Aoyagi , Noriko Izumi , Ryo Kitaura , Tim Kowalczyk , Jian Wang , Stephan Irle , Eiji ... Kazuhiro KUDO , Masatoshi...
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Molecular Orientation of Individual Lu@C82 Molecules Demonstrated by Scanning Tunneling Microscopy Masachika Iwamoto,†,‡ Daisuke Ogawa,†,‡ Yuhsuke Yasutake,†,⊥ Yasuo Azuma,†,‡ Hisashi Umemoto,§ Kazunori Ohashi,§ Noriko Izumi,§ Hisanori Shinohara,§,| and Yutaka Majima*,†,‡ Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan, CREST, Japan Science and Technology Agency (JST), Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan, Department of Chemistry, Nagoya UniVersity, Nagoya 464-8602, Japan, and Institute for AdVanced Research, Nagoya UniVersity, Nagoya 464-8602, Japan ReceiVed: March 15, 2010; ReVised Manuscript ReceiVed: June 20, 2010

We identified the orientation of individual Lu@C82 molecules on alkanethiol self-assembled monolayers (SAMs) by scanning tunneling microscopy (STM) at a molecular resolution. STM images of Lu@C82 on alkanethiol SAMs at 65 K showed a striped structure corresponding to the molecular orbitals of the Lu@C82 molecule, suggesting that thermal rotation of Lu@C82 on alkanethiol SAMs is prevented at 65 K. By comparing these molecular-resolution STM images with Kohn-Sham molecular orbitals of Lu@C82 calculated by density functional theory (DFT), we identified the molecular orientation of Lu@C82. Spatial mapping of the differential conductance on individual Lu@C82 molecules revealed that the local conductivity within a molecule became large around the Lu atom at a negative sample bias voltage. From spatial mapping of the differential conductance measurements, we also evaluated the HOMO-LUMO gap of Lu@C82 to be 0.47 eV. From the results of the spatial mapping of the differential conductance and DFT calculations, the locally high conductivity around the Lu atom was attributed to the HOMO-2 level orbital concentrated on the Lu atom and its six nearest C atoms at 0.055 eV below the HOMO level. We demonstrated changes in the molecular orientation of Lu@C82 by applying a high electric field (about 1 × 107 V/cm) with a large tunneling current (1.5 nA). 1. Introduction Single-molecule switching devices composed of functionalized molecules have been extensively studied because they are likely to be employed in nanomemory devices and logic circuits.1–4 Such molecular devices are anticipated to be candidates for replacing complementary metal oxide semiconductor (CMOS) devices. To realize single-molecule switches, molecular-resolution STM can be a powerful tool for demonstrating their function. By utilizing STM, we can understand the intermolecular patterns and architectural parameters for realizing molecular devices.1,4–10 Endohedral metallofullerenes are one of the candidate materials for creating single molecular orientation switching devices owing to their electric dipole moment based on the electron exchange between the encapsulated metal atom and the fullerene cage.11 For instance, in the case of a single-metal endohedral metallofullerene (M@C82) (isomer I), the M@C82 cage exhibits C2V symmetry and possesses an electric dipole moment that is derived from the trivalent M3+C823- electronic states. The control of the texture and the orbital interactions of surrounding M@C82 molecules on the subnanometer scale is the key to realizing the single molecular orientation switching device by using M@C82. We have been observing switching phenomena in the Tb@C82 * To whom correspondence should be addressed. E-mail: [email protected]. † Material and Structures Laboratory, Tokyo Institute of Technology. ‡ CREST-JST. § Department of Chemistry, Nagoya University. | Institute for Advanced Research, Nagoya University. ⊥ Present address: Graduate School of Arts and Sciences, University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan.

molecule on an octanethiol SAM using low-temperature ultrahigh-vacuum (UHV)-STM and scanning tunneling spectroscopy (STS).4 An alkanethiol SAM was introduced between Tb@C82 and the Au(111) substrate as a template for STM observation. In our previous report, we demonstrated single-molecule switching based on STS measurements, which suggested changes in molecular orientation.4 To reveal the mechanisms of molecular orientation switching, it is necessary to directly observe intermolecular structures using molecular-resolution STM, where the so-called intramolecular structures are clearly observed.12 Here, we demonstrate the molecular orientation of an individual Lu endohedral metallofullerene (Lu@C82) adsorbed on an alkanethiol SAM by comparing molecular-resolution STM images with molecular orbitals calculated by density functional theory (DFT). The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap and molecular Fermi level are evaluated by dI/dV-V characteristics obtained via the spatial mapping of the differential conductance. Local conductance within an individual Lu@C82 near the HOMO and LUMO levels is discussed. We also demonstrate changes in the molecular orientation of an individual Lu@C82 before and after application of a sample bias voltage. 2. Experimental Section Lu@C82 (isomer I) was produced by the dc arc-discharge method, followed by purification and isolation via multistage high-performance liquid chromatography.13 The Au(111) substrate was fabricated by thermal evaporation of gold onto freshly cleaved mica. Prior to gold evaporation, the mica substrates were

10.1021/jp1023394  2010 American Chemical Society Published on Web 08/13/2010

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Figure 1. Molecular-resolution 3-D STM images of Lu@C82 molecules adsorbed on an octanethiol/Au(111) surface by a W probe at 65 K. In both panels, (a) and (b), the sample bias voltage and set-point current are 2.0 V and 2.0 pA, respectively.

held in a vacuum at 773 K for 4 h. The substrate temperature was maintained at 773 K during gold evaporation, and postannealing was performed at 748 K for 1 h. The substrate was briefly flame-annealed and quenched in ethanol to form an atomically flat Au(111) surface. The Au(111) substrate was immersed in a 1 mM solution of octanethiol (Sigma Aldrich Tokyo, Japan) or heptanethiol (Wako Pure Chemical Industries, Ltd., Japan) in ethanol for 24 h, and the samples were rinsed with ethanol two times and dried in a pure N2 flow. Lu@C82 was degassed at 573 K for 12 h, and then a submonolayer of Lu@C82 was sublimated on the alkanethiol/Au(111) surface in a sublimation chamber at 723-823 K. As the sublimation chamber was attached to an UHV-STM, the sample was introduced into a modified UHV-STM system with a base pressure below 3.0 × 10-8 Pa (UNISOKU) without being exposed to air. The STM images were measured at sample bias voltages (V) in the constant-current mode at 65 K. The STM tip was an electropolished W probe and a mechanically cut PtIr probe. During STS measurements, the tip was held at a fixed position above the sample surface. The tunneling current depends on the electronic density of states (DOS) of the tip and the sample.14 The differential conductance (dI/dV) provides a direct measure of the DOS of the sample surface.14,15 3. Results and Discussion i. Molecular Orientation of Lu@C82 on Alkanethiol SAMs. The STM images of Lu@C82 adsorbed on the octanethiol SAMs are shown in Figure 1. STM images are analyzed by using WSxM.26 In Figure 1b, a well-ordered octanethiol SAM can be observed as a (3 × 3)R30° structure with a distance of 0.50 nm between octanethiol molecules. In Figure 1a,b, the bright spherical spots are Lu@C82 molecules. Lu@C82 molecules tend to be trapped along the Au(111) monatomic step edge, at etch pits that arise from the rearrangement of surface gold atoms, and at the grain boundary of the octanethiol SAM. It is difficult to observe any Lu@C82 on octanethiol and heptanethiol SAMs that cover the Au(111) terraces. Consequently, Lu@C82 molecules can diffuse on the alkanethiol SAM and stop at the step edge, the etch pit, or the grain boundary of the SAM. Furthermore, these molecular diffusions show that a weak interaction exists between the molecule and the alkanethiol SAM due to van der Waals attraction.4,8 Figure 1b shows a molecular-resolution STM image of Lu@C82 adsorbed on the octanethiol SAMs. A striped structure of the Lu@C82 molecule is observed, suggesting that thermal rotations of Lu@C82 adsorbed on octanethiol SAMs are prevented.6,7 The striped structure corresponds to the molecular

orbital of the fullerene shell in an individual Lu@C82 molecule. It is noted that molecular-resolution STM images of Lu@C82 adsorbed on the heptanethiol SAMs show the same images as on octanethiol SAMs. To demonstrate the Kohn-Sham molecular orbitals and their eigenvalues of an individual Lu@C82 molecule, we have carried out ab initio pseudopotential DFT calculations.16–20 The generalized gradient approximation of Becke’s exchange functional and the Lee-Yang-Parr correlational functional (B3LYP) were used to represent the exchange correlation.21,22 We employed a 6-31G(d) basis set for C23 and a CEP-121G basis set for Lu.24 Kohn-Sham eigenvalues are sensitive to the choices of the calculation conditions, such as changes of basis set, exchangecorrelation functional, and geometry; however, the shape of the Kohn-Sham orbital did not change significantly by the choice of basis sets, such as CRENBL and Stuttgart RSC ANO/ECP, for Lu.17 As Lu@C82 molecules were isolated in a vacuum on alkanethiol SAMs, the molecular orbital interaction between Lu@C82 and the Au(111) surface is weak. Therefore, the molecular orbitals of Lu@C82 should not be affected by the Au(111) surface. All calculations were carried out with Gaussian03.25 On the basis of the optimized geometry, the Lu atom lies along a C2V axis on the six-membered ring of the C2V-C82 isomer. The distance between the Lu atom and the six nearest C atoms is 0.24 nm. The molecular-resolution STM images of individual Lu@C82 molecules adsorbed on heptanethiol SAMs are shown in Figure 2a,b. The striped patterns of the Lu@C82 molecular orbitals are clearly observed with the topographic image of the Lu@C82 molecules. The (3 × 3)R30° structure of heptanethiol SAMs is also clearly observed. The molecular orientation of Lu@C82 is decided by comparing the striped molecular orbital pattern of the STM image of Lu@C82 molecules with Kohn-Sham orbitals calculated by DFT. The left column of panels (a)-1 and (a)-2 and (b)-1 to (b)-4 of Figure 2 are magnified STM images shown in Figure 2, panels a and b, respectively. The LUMO and HOMO states of Lu@C82 calculated by DFT are shown in the middle column of Figure 2, panels (a)-1,2 and (b)-1 to (b)4, respectively. Knotted stripes in the middle column of Figure 2 indicate the Kohn-Sham orbital. The right column of panels (a)-1,2 and (b)-1 to (b)-4 of Figure 2 shows the atomic geometries corresponding to the middle column of Figure 2. The molecular orientation is shown by the C2V axis in the right column of Figure 2. The purple sphere indicates the Lu atom. The STM-derived striped patterns of the molecular orbital in the left column of Figure 2 are in good agreement with the electron density isosurface from the DFT-calculated LUMO and HOMO states shown in the middle column of Figure 2. In Figure

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Figure 2. (a, b) 3-D STM images of Lu@C82 molecules adsorbed on the heptanethiol/Au(111) surface by a PtIr probe at 65 K. (a) scan size ) 6.25 × 6.25 nm2, V ) 1.0 V, I ) 2.6 pA. (b) scan size ) 6.25 × 6.25 nm2, V ) -0.9 V, I ) -10 pA. Panels (a)-1, (a)-2, (b)-1, (b)-2, (b)-3, and (b)-4 show magnified molecular-resolution STM images of Lu@C82 molecules from panels a and b (size ) 2.3 × 2.3 nm2) (left column), Kohn-Sham molecular orbitals of LUMO (panels (a)-1, (a)-2) and HOMO (panels (b)-1 to (b)-4) by DFT, B3LYP for Lu@C82 with an isovalue of 0.003 e/A3 in a vacuum (middle column), and corresponding atomic geometries in ball-and-stick form calculated by DFT (right column). Elevation angles between the C2V axis of the Lu@C82 molecules and the substrate are indicated to the lower right of the atomic geometry.

2, the elevation angles between the C2V axis of the Lu@C82 molecule and the substrate are evaluated. The Lu atom exists in the northern hemisphere when the elevation angle of the C2V axis is positive. The average elevation angle between the C2V axis of a Lu@C82 molecule and the substrate is evaluated to be 28° for 10 individual Lu@C82 molecules. Therefore, the orientation of the C2V axis of the Lu@C82 molecule is tilted slightly upward from the substrate. As Lu@C82 (isomer I) has a prolate spheroid geometry with C2V as the major axis, the orientation of the C2V axis should be parallel rather than perpendicular to the substrate. The dipole moment of a Lu@C82 molecule is calculated to be 1.72 D by DFT, and the direction of the dipole moment is parallel to the C2V axis where the side of the Lu atom is positively charged. The methyl end group of alkanethiol SAMs has a positive dipole moment of 0.35 D.27 Consequently, the positive value of the average elevation angle (28°) suggests a dipole-dipole interaction between Lu@C82 and the alkanethiol SAM. ii. STS of Lu@C82 on Octanethiol SAMs. Figure 3a shows the tunneling current-voltage (I-V) characteristic of the Lu@C82 molecule on octanethiol SAMs. The I-V characteristic is obtained from averages over 31 I-V characteristics measured on the individual Lu@C82 molecule with the same set-point current at separate positions indicated in the inset of Figure 3a. Spatial mapping of the differential conductance dI/dV (dI/ dV map) is obtained on the double-barrier tunnel junction (DBTJ) system consisting of STM tip/vacuum/Lu@C82/octanethiol SAM/Au(111) at the 31 separate positions in the inset of Figure 3a.28,29 According to the standard theory,30,31 adding (+) [removing (-)] an electron to (from) a quantum dot through

Figure 3. (a) Averaged I-V characteristic measured on the Lu@C82 molecule on an octanethiol SAM/Au(111) surface. The set-point current is 4.0 pA at 2.0 V. The inset shows an STM image (3 × 3 nm2) of Lu@C82 on an octanethiol SAM by a W probe. The I-V characteristic shown in panel a is obtained from averaging I-V characteristics observed at the 31 points indicated as white circles. (b) dI/dV characteristics derived from STS spectra.

one junction of the DBTJ system requires work EC((n) against the electric field of n excess electrons already residing on the quantum dot

EC((n) )

( 21 ( n) C

1

e2 + C2

(1)

where C1 is the capacitance of the vacuum layer between the STM tip and Lu@C82 and C2 is the capacitance between Lu@C82 and the Au(111) substrate, that is, the octanethiol SAM. When C1 is smaller than C2, the threshold voltages (V() at which tunneling will occur are indicated by eqs 2 and 3.

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Figure 4. Local differential conductivity of Lu@C82 obtained by spatial mapping of the differential conductance dI/dV (dI/dV map) measurements at the sample bias voltages of (a) -0.5 and (b) 1.1 V by a W probe. These results are obtained from the Lu@C82 molecule shown in Figure 3a.

V+ )

C1 + C2 + + (EC (0) + Edis ) eC2

V- ) -

C1 + C2 (EC (0) + Edis ) eC2

(2)

(3)

+ Here, Edis and Edis express the energy differences between the LUMO and Fermi level of the Lu@C82 molecule (EF) and between the Fermi level and HOMO level, respectively.29 Using an electric image method that assumes mirror charges,32 C1 and C2 are estimated as 0.092 aF and 0.19 aF, respectively, where the relative permittivity of the alkanethiol SAM is 2.6,9,33 the distance between the STM tip and Lu@C82 is 0.30 nm,4 and the height of the octanethiol SAM from the Au(111) surface is 1.24 nm.34 From eqs 2 and 3, an electron tunnels through the LUMO and HOMO levels in positive and negative sample bias voltages, respectively. In Figure 3a, a zero-current region is observed in the sample bias voltage range of -0.16 to 0.55 V, attributed to the Coulomb

gap and the HOMO-LUMO gap. This gap voltage region is shifted toward positive sample bias voltages. This onset voltage shift indicates a Fermi level shift of [email protected] Figure 3b shows differential conductance-voltage (dI/dV-V) curves obtained from the I-V curves of Figure 3a. In Figure 3b, two peaks are observed at negative and positive sample bias voltages of -0.47 and 1.05 V, respectively, next to the Coulomb gap. From these + two peaks, Edis and Edis are evaluated to be 0.42 and 0.033 eV, respectively, by eqs 2 and 3. Therefore, the HOMO-LUMO gap of Lu@C82 becomes 0.47 eV, and the HOMO is located 33 meV lower than the Fermi level of the Lu@C82 molecule. In this manner, by taking into account partial voltages and charging energy, the Coulomb gap observed from STS characteristics is wider than the HOMO-LUMO gap of the molecule. Panels a and b of Figure 4 show results of spatial mapping of the differential conductance dI/dV measured at 64 points on and around the Lu@C82 molecule, which show the dI/dV images at the sample bias voltages of -0.50 and 1.1 V, respectively.

Figure 5. (a) Kohn-Sham eigenvalues of a free Lu@C82 molecule by DFT. The zero-energy level is shifted to the HOMO level. (b-g) Kohn-Sham molecular orbitals of Lu@C82 with an isovalue of 0.02 e/A3 at LUMO+1 (2.26 eV), LUMO (0.83 eV), HOMO (0.00 eV), HOMO-1 (-0.01 eV), HOMO-2 (-0.09 eV), and HOMO-3 (-0.23 eV) levels, respectively.

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We note that the differential conductance of the HOMO level at -0.50 V is especially large at position 9. On the contrary, the differential conductance of the LUMO level at 1.1 V is large at position 37. The Kohn-Sham eigenvalues of Lu@C82 calculated by DFT are shown in Figure 5a. Molecular orbitals of Lu@C82 at the LUMO+1, LUMO, HOMO, HOMO-1, HOMO-2, and HOMO-3 levels are shown in Figure 5, panels b-g, respectively. In Figure 5a, the HOMO-LUMO gap of Lu@C82 according to DFT is 0.8 eV. DFT Kohn-Sham eigenvalues are well known not to be good estimators of one-particle energies in molecules. The DFT HOMO-LUMO gap (0.8 eV) is 0.3 eV larger than the experimental value. This result is consistent with the previous paper in which DFT HOMO-LUMO gaps were systematically compared with experimental values.35 Ton-That et al. also reported the HOMO-LUMO gap estimated by STS and ultraviolet photoelectron spectroscopy (UPS).36 The HOMOLUMO gap evaluated from STS tends to be smaller than that evaluated from UPS due to perturbation of the molecular electric structure by the strong electric field. From Figure 5a, the energy differences between HOMO and HOMO-1 and that between HOMO and HOMO-2 are 0.006 and 0.093 eV, respectively. These values are smaller than that between LUMO and LUMO+1 (1.44 eV). If we convert the value of the DFT-calculated HOMO-LUMO gap (0.8 eV) to the experimental value (0.47 eV), the HOMO-HOMO-1, HOMO-HOMO-2, and LUMO-LUMO+1 gaps are 0.004, 0.055, and 0.85 eV, respectively. These energy gaps, in turn, correspond to sample bias voltage differences of 0.006, 0.081, and 1.26 V, respectively, using eqs 2 and 3. Consequently, the normalized differential conductance peak around -0.5 V corresponds to the HOMO, HOMO-1, and HOMO-2 levels, whereas the normalized peak around 1.1 V corresponds to the LUMO level. As shown in Figure 5b-g, the orbital population analysis around the HOMO and LUMO levels shows that the LUMO+1 level is metal-dominated, the HOMO and HOMO-1 levels are cage-dominated, and the LUMO and HOMO-2 levels are metal-cage hybrid orbitals. The HOMO-2 orbital, in particular, concentrates around the encapsulated Lu atom and the six nearest C atoms. As a result, the existing probability of electrons becomes large around the Lu atom in the HOMO-2 level. Wang et al. measured dI/dV maps of the Dy@C82 molecule and compared these results with DFT calculations. They observed a dI/dV peak from metal-cage hybrid orbitals.37 Metal-cage hybrid orbitals are detected by STS, rather than local density of states (LDOS) of metal-dominated orbitals.37 Therefore, the dI/dV peak at -0.5 V (position 9 in Figure 4a) is attributed to HOMO-2 orbitals around the Lu atom. On the other hand, the dI/dV peaks at +1.1 V in Figure 4b are largely at the center of the C82 cage because the LUMO level metal-cage hybrid orbitals around the Lu atom are smaller than the HOMO-2 orbitals around the Lu atom (Figure 5c). Therefore, the differential conductance became large near the Lu atom due to metal-cage hybrid HOMO-2 orbitals. iii. Orientation Switching by External Electric Fields. To realize a molecular orientation switching device by using endohedral metallofullerene, the switching mechanism must be clarified. We compared molecular-resolution STM images of individual Lu@C82 molecules on octanethiol SAMs with molecular orbitals calculated by DFT before and after application of the sample bias voltage. To change the molecular orientation, the STM tip is first placed and held atop the Lu@C82 molecule. Subsequently, a single-cycle, asymmetric triangular wave of

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Figure 6. (a, b) Molecular-resolution STM images of the Lu@C82 molecule shown in Figure 4 before (a) and after (b) the application of a high electric field (about 1 × 107 V/cm) with a large tunneling current of 1.5 nA. (c-f) LUMO level molecular orbital and ball-and-stick form of Lu@C82 (isovalue ) 0.003 e/A3) before (c, e) and after (d, f) the application of a high electric field. The STM image before applying the high electric field is the same as that shown in Figure 2a. The elevation angles of an individual Lu@C82 molecule before and after the application of the electric field are estimated to be 51° and -10°, respectively.

sample bias voltage (initial voltage range, 2.0 V; minimum voltage, -3.5 V; final voltage, 2.0 V at 0.025 Hz) was applied by STS measurement. Panels a and b of Figure 6 show molecular-resolution STM images of the same individual Lu@C82 molecule (shown in Figure 4) on octanethiol SAMs before and after application of the sample bias voltage, respectively. The initial tunneling current at 2.0 V was 10 pA; the maximum current was -1.5 nA at -3.5 V, which is 150 times larger than the initial current. We note that the STM image did not change during the STM observations of Figure 6a,b at a small set-point current of 4 pA. These STM images at a sample bias voltage of 0.3 V show LUMO states, as described in the previous section. Panels a and b in Figure 6 indicate that the striped structure of Lu@C82 was changed by applying the sample bias voltage. Because both STM images were measured at the same sample bias voltage (0.3 V) and the structure of the neighboring octanethiol SAM did not change, this change in the striped structure corresponds to a change in the molecular

Molecular Orientation of Individual Lu@C82 Molecules orientation of individual Lu@C82 molecules. Panels c and d in Figure 6 show molecular orbitals of LUMO states with their corresponding atomic geometries in ball-and-stick form calculated by DFT. The molecular orientation of the DFT images was identified by using the same procedure. In Figure 6e,f, the elevation angles of individual Lu@C82 molecules before and after the application of a negative sample bias voltage are estimated to be 51° and -10°, respectively. This change in orientation agrees with the direction of the high electric field (about 1 × 107 V/cm) from the STM tip to the substrate with a negative sample bias voltage. Therefore, the molecular orientation was changed from up to down by the interaction between its electric dipole moment and an external electric field with a negative sample bias voltage with a large current. We note that, when Lu@C82 molecules on octanethiol SAMs were subjected to a larger pulse voltage (sample bias voltage, 4.0 V; tunneling current, 10 pA; application time, 50 s), the change in molecular orientation was not observed. These results imply that the change in orientation is mainly the result of applying a large voltage with a large tunneling current. By combining the locally high conductance around the Lu atom of the HOMO-2 orbital and the change in molecular orientation due to the application of an electric field with a high current, molecular orientation switching of individual Lu@C82 molecules should become possible. 4. Conclusions We have identified the molecular orientation of individual Lu@C82 molecules on octanethiol and heptanethiol self-assembled monolayers (SAMs) by combining molecular-resolution scanning tunneling microscopy (STM) images of Lu@C82 on octanethiol and heptanethiol SAMs with molecular orbitals of Lu@C82 calculated by density functional theory (DFT). Spatial mapping of differential conductance dI/dV measurements performed on individual Lu@C82 molecules shows that the local conductivity within the Lu@C82 molecule becomes large around the Lu atom at a negative sample bias voltage, an effect attributed to the HOMO-2 level orbital concentrated on the Lu atom and its six nearest C atoms. Changes in the molecular orientation of Lu@C82 are demonstrated by applying a high electric field (about 1 × 107 V/cm) with a large tunneling current of 1.5 nA. These changes in local molecular conductance and orientation have obvious relevance for single-molecule switching of endohedral metallofullerenes. Acknowledgment. This work is partially supported by the Grant-in-Aid for Scientific Research on Innovative Areas (No. 20108011, π-space) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, the Collaborative Research Project of Materials and Structures Laboratory, Tokyo Institute of Technology, and the World Class University (WCU) Program at Sunchon National University, Korea. References and Notes (1) Liljeroth, P.; Repp, J.; Meyer, G. Science 2007, 317, 1203.

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