Building Blocks for Molecular Devices: Organic Molecules on the MgO

Oct 3, 2007 - Sébastien Fernandez , Alexis Markovits and Christian Minot. The Journal of Physical Chemistry C 2008 112 (42), 16491-16496. Abstract | ...
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J. Phys. Chem. C 2007, 111, 15375-15381

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Building Blocks for Molecular Devices: Organic Molecules on the MgO (001) Surface T. Trevethan* and A. L. Shluger Department of Physics and Astronomy, UniVersity College London, Gower Street, London WC1E 6BT, U.K. ReceiVed: April 12, 2007; In Final Form: August 14, 2007

The interaction of hydrocarbon and basic polar organic molecules with the MgO (001) surface has been theoretically studied using an embedded cluster model and hybrid density functional. It is found that both methane and benzene molecules do not bind strongly with the perfect surface and will not be deprotonated. Polar organic groups interact more strongly with the surface, with nitromethane and pyridine providing the largest binding energies (0.37 and 0.75 eV, respectively); however, in each case the molecules are physisorbed and not strongly polarized by the surface. The interaction of benzene and the larger aromatic hydrocarbon molecules, naphthalene and pyrene, with a single oxygen vacancy (F center) in neutral, +1, and +2 charge states is investigated. The +2 charged vacancy increases the binding energy of the molecules to the surface; however, there is no charge transfer to or from the vacancy in any of the states. It is found that the charged vacancies can significantly affect the electronic energy levels of the molecule and that the +2 vacancy can lower both the HOMO and LUMO levels of the molecule by approximately 3 eV. These calculations have importance for the design and control of molecular structure and energy levels in single-molecule electronic devices.

1. Introduction Understanding the structure and properties of organic molecules adsorbed on insulating surfaces is important for the realization of single-molecule devices and technology as well as in many other areas of surface science and chemistry. It is crucial to have precise control over the atomic and electronic structure of single molecules adsorbed on a surface in order to develop a single-molecule electrical device, which may form the basis of future information technologies.1-3 The ability of scanning probe microscopes to identify and manipulate individual atoms and molecules on a variety of surfaces4,5 has meant that the experimental realization of such a device is now feasible. However, the design of this technology requires a detailed theoretical understanding of the properties of the surfacemolecule system.6 In particular, conjugated molecular “boards”, which have been synthesized to perform electronic device functions and act as a digital logic gates, should be equipped with lateral chemical groups, not contributing to the function directly but protecting the molecular electronic functionality and stabilizing the molecule on a given substrate. One may want to maintain the molecular board away from the surface to reduce the effect of the substrate or to ease the single-molecule manipulation. Other chemical groups must provide a good electronic coupling between the molecular board and the local metallic leads (see, for example, the discussion in ref 7). Another essential aspect of the architecture of a surface-based single-molecule electronic device is that the surface on which the molecule is adsorbed is an (wide band gap) insulator, in order to enable more than one electrical contact to the molecule.8 For example, alkali halide and metal oxide surfaces are structurally simple and can be probed on the atomic scale by scanning probe microscopy methods. This can be achieved either using noncontact atomic force microscopy (NC-AFM)9 on the * Corresponding author. E-mail: [email protected].

bulk insulator or scanning tunneling microscopy (STM) on a thin insulating film adsorbed on a conducting substrate.10 In particular, the MgO (001) surface could serve as an excellent model system for exploring molecular electronic architectures as it is well-characterized, is fairly inert, and has a wide band gap. The MgO (001) surface has been imaged with atomic resolution with both NC-AFM11 and STM (as a thin film), and it has been shown experimentally how it is possible to manipulate the charge state of a single oxygen vacancy with a STM tip.12 Recent theoretical studies have also predicted how it is possible to manipulate both single oxygen vacancies and single Pd adatoms on the MgO (001) surface in a controlled way using NC-AFM.13-15 The controlled manipulation of surface defects and adatoms in this way could be used to build atomic wires and then to connect an immobilized molecule to single-atom electrodes.8 The precise positioning of charged vacancies could be used to anchor molecules in desired positions and even modify the electronic structure of a molecule to change its electrical or chemical properties.16 This makes this system a good potential candidate for developing single-molecule devices and can be used as a model to investigate possible surfacebased architectures. In this paper, we investigate the interaction of various types of organic molecules with the perfect MgO (001) surface and single oxygen vacancies (F centers) in neutral, +1, and +2 charge states. The molecules investigated are considered to be the “building blocks” of larger, more complex molecules that would form a molecular device, in a philosophy similar to that developed in ref 17. By investigating the interaction of these smaller molecules with the surface, it is possible to characterize the behavior of a larger molecule that could be constructed from these smaller fragments. Typically, molecules that have so far been proposed for single-molecule electrical or computational devices are formed of a planar conjugated board, to which are attached chemical

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15376 J. Phys. Chem. C, Vol. 111, No. 42, 2007 groups that are used to connect to metallic electrodes and form inputs1-3 to the device, as well as other groups or “legs” that are used to position the molecule and limit diffusion on the surface.8,18 The basic aromatic hydrocarbon, benzene, as well as naphthalene (two rings) and pyrene (four rings) are used here to characterize the interaction of larger conjugated molecules with the surface. The inputs would consist of “donor-acceptor” groups such as aldehyde and nitro groups,2,3 and here methanal, nitromethane, and pyridine molecules are chosen for these basic fragments. These groups are also potential candidates for binding or “grafting” groups, due to their polar nature, but would be separated from main board by hydrocarbon chains that can be characterized by the methane molecule. For each of these basic molecules, the strength and the character of bonding to the surface and how this changes the electronic structure of the molecule is determined. The position of highest occupied state (HOMO) and the lowest unoccupied state (LUMO) of benzene and the other larger aromatic hydrocarbons when adsorbed on or near a single oxygen vacancy is then also studied in detail. There have been previous theoretical studies that have investigated the adsorption of several of these molecules on the MgO (001) surface. In the case of methane, adsorption on the perfect surface in a periodic monolayer was studied19 using a plane-wave basis set and it was found that the strongest binding to the surface was 0.05 eV per molecule. In ref 20, a single methane molecule was adsorbed above a finite cluster, and it was found that no configuration leads to a positive binding energy; however, in these calculations the surface was not allowed to relax. The interaction of methanal with low coordinated and defect sites was studied in ref 21 within an isolated nanocluster model. The methanal was found to physadsorb above a Mg atom in the center of a small (001) facet with an energy of 0.2 eV. The plan of the remainder of this paper is as follows: in the next section, the details of the computational methods used are described. Then, in the following section the results of calculations performed on the adsorption of the above molecules on the regular MgO (001) are presented. In the fourth section, the adsorption of the aromatic hydrocarbons above oxygen vacancies and how this affects the position of electronic states within the molecules is studied. Then, in the final section a discussion and conclusion is given. 2. Computational Details The calculations presented in this study have been performed at the DFT level of theory with the B3LYP hybrid functional22,23 in an embedded cluster approach implemented in the Gaussian Used for Embedded Systems Studies (GUESS) methodology.24 The B3LYP functional is employed in order to correctly reproduce the electronic structure and band gap of the perfect and defective MgO (001) surface,25 and is well known to correctly reproduce the electronic structure of conjugated organic molecules.26 The use of an embedded cluster is appropriate here because the system is nonperiodic and we are interested in a single molecule adsorbed on the extended surface. This also results in a relatively small quantum system, allowing the use of a high-level quantum chemistry method. Additionally, it is possible to study different charged states of molecules and defects at the surface. The molecule and a cluster of MgO atoms in the surface form the quantum region, which is embedded in an array of 12 × 12 × 6 classical polarizable shell model atoms, interacting via Buckingham potentials. This region is then finally surrounded

Trevethan and Shluger

Figure 1. Illustration of the configuration of the embedded cluster system, showing the quantum cluster with a single benzene molecule adsorbed parallel to the surface.

with an array of fixed point charges (resulting in a total surface cluster of 20 × 20 × 10 atoms) to represent the Madelung potential of the extended surface (see Figure 1). The atoms in the quantum cluster and molecule are described by all-electron Gaussian-type atomic orbital basis sets. All atoms in the molecule and all oxygen atoms in the surface have polarizable 6-31G+(d) basis sets, and all-electron Mg atoms in the surface have 6-31G basis sets. All oxygen atoms at the boundary of the quantum cluster are coordinated with Mg atoms (not belonging to the quantum cluster) are described with an effective core potential (ECP) and a single contracted s-type basis (this is to prevent an unrealistic polarization of O atoms at the boundary of the quantum region). Only the electrostatic interaction between the molecule and classical ions outside the quantum cluster was included. We have checked, by increasing the size of the quantum cluster, that this has negligible effect on the adsorption of the molecule. Several sizes and configurations of quantum clusters have been used to represent the regular surface and a single oxygen vacancy to accommodate different molecules and configurations. Four of these clusters are shown in Figure 2. To find a stable position of an organic molecule above the surface, we minimized the potential energy of the system with respect to the coordinates of all atoms in the quantum region and all cores and shells in the polarizable region, using the BFGS algorithm. The adsorption energy, or binding energy, can then be determined from the difference between the total relaxed energy of the combined molecule and surface system and the sum of the relaxed energy of the isolated molecule and the isolated surface. 3. Interaction of Molecules with the Regular MgO (001) Surface The adsorption of various simple organic molecules on the regular MgO (001) surface was investigated to see how the electronic structure of the molecule is perturbed and also to identify chemical groups that provide the strongest binding to the surface. The molecules studied here are (a) methane (CH4), (b) benzene (C6H6), (c) methanal (CH2O), (d) nitromethane (CH3NO2), and (e) pyridine (C5H6N). The results of these calculations are summarized in Table 1 and Figure 3, which show the final configurations, separations, charge transfer, and total binding energy. The separation from the surface is defined as the distance from the surface plane to the lower-most atom of the molecule. We note that the small rumpling of the MgO

Building Blocks for Molecular Devices

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Figure 2. Quantum clusters used in this study: (a) Mg29O13 (b) single oxygen vacancy Mg29O12, (c) large area Mg45O17 and, (d) large area with oxygen vacancy Mg45O17.

TABLE 1: Properties of Molecules Adsorbed on the Perfect MgO (001) Surface in Various Configurations. molecule

configuration

separation (Å)

charge

energy (eV)

methane methane benzene benzene methanal methanal nitromethane nitromethane nitromethane pyridine pyridine pyridine

1 2 3 4 5 6 7 8 9 10 11 12

2.2 2.8 3.3 2.3 2.4 3.3 2.4 3.0 3.0 3.5 2.5 3.5

-0.03 -0.02 -0.03 -0.03 0.01 0.00 0.00 0.01 0.01 -0.01 -0.02 0.00

0.02 0.05 0.01 0.02 0.19 0.02 0.37 0.05 0.09 0.38 0.75 0.26

(001) surface is well reproduced by our method (see the discussion in ref 27). The initial separation before relaxation is 2 Å , and the initial molecular structure is taken as the gasphase structure. The total charge on the molecule is obtained as the sum of the effective charges calculated from a natural population analysis (NPA) of electron density in the molecule. The basis-set superposition energy (BSSE) was calculated for the adsorption of nitromethane on the MgO surface in configuration 7 using the counterpoise method.28 This resulted in a correction to the adsorption energies of less than 0.05 eV and is not expected to significantly alter the results. In each of the molecules investigated, there is very little transfer of electronic charge between the molecule and the surface and the separation between the lowest atom and the surface plane is considerably larger than typical bond distances; therefore, the interaction with the surface appears purely electrostatic. For the nonpolar hydrocarbon molecules (benzene

and methane), there is very little polarization by the surface and negligible binding to the surface in any configuration (similar to what was found in refs 19 and 20). For the polar molecules, methanal (aldehyde group), pyridine, and nitromethane (nitro group), the binding energy is significantly increased. The most-favorable configurations for these molecules are with the negatively charged pole (O or N) above a Mg ion in the surface (or straddling two neighboring Mg ions in the case of nitromethane), and there is a small polarization of the C-O, N-O, or C-N bond (but less than 0.1e in each case from a NPA). This small degree of binding and polarization is due to the fact that the electrostatic field from the ions in the MgO (001) surface decays very rapidly. The electronic structure of the adsorbed molecules are relatively unperturbed from that in isolation, and this was analyzed in detail for the case of benzene parallel to the surface plane above an O in the surface. The position of the HOMO level of the isolated (gas phase) benzene molecule is at -6.69 eV and the LUMO level at 0.04 eV, resulting in a gap of 6.73 eV. When the molecule is relaxed on the surface, in configuration 1 in Figure 3, the HOMO of the molecule moves to -6.81 eV and the LUMO to -0.15 eV, reducing the gap slightly to 6.66 eV. We have checked that these prototype molecules will not adsorb dissociatively. In particular, for benzene this was done by calculating the relaxed energy of the system with a H+ ion adsorbed above an O ion in the surface, and the deprotonated molecule (a negative ion) adsorbed above a Mg ion three interatomic distances away, as shown in Figure 4a. The calculations on the terrace were performed with the Mg45O17 (c) quantum cluster from Figure 2. Proton extraction from the benzene molecule and methane at the monolayer step edge was also considered, where the H+ ion is adsorbed above an O ion on the step edge and the deprotonated molecule ion adsorbed above a Mg ion three interatomic distances away further along the step edge. For the benzene and methane molecules on the (001) terrace, the dissociated state is 2.23 and 2.27 eV, respectively, less energetically favorable than the isolated molecule and surface state. With the benzene and methane molecules in the disassociated state on the monolayer step-edge, the energies are 2.11 and 2.38 eV less energetically favorable, respectively. A larger molecule, composed of the smaller “fragments” discussed above, should interact with the surface in generally the same way as the smaller molecules individually because the interaction with the surface is purely electrostatic. As discussed in the first section, a molecule of the type considered as a potential molecular electronic device would consist of an aromatic base with “anchoring” groups attached to bind the molecule to the surface. As an example, we considered a naphthalene molecule with two nitromethane groups attached at each end. This was then adsorbed on the MgO (001) surface in a configuration where the nitromethane groups would bind to the surface in a configuration similar to structure 7 in Figure 3. The relaxed configuration of this molecule on the surface is shown in Figure 4b. As in the case of the smaller molecules, there is negligible charge transfer between the surface and the molecule and small polarization of polar bonds closest to the surface. In this configuration, the naphthalene base is raised further from the surface plane by the anchoring groups (to a height of 4.3 Å) and the distance could be increased by substituting the nitromethane groups for nitroethane groups. In this way, the perturbation to the molecule due to its interaction with the surface can be reduced or eliminated.

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Figure 3. Lowest energy configurations of the molecules adsorbed on the perfect surface (quantum cluster a) as listed in Table 1.

Figure 4. (a) Configuration of benzene dissociatively adsorbed on the MgO (001) surface. (b) Relaxed configuration of a larger organic molecule adsorbed on the MgO (001) surface. The molecule consists of a naphthalene “base” with two nitromethane groups added at opposite ends of the molecule to bind the molecule to the surface.

4. Interaction of Aromatic Hydrocarbons with Oxygen Vacancies A neutral oxygen vacancy (F center) can be created in a (001) surface cluster (as is the case in quantum clusters b and d) by removing a single O atom but leaving the full oxygen basis set (6-31G(d)) in the center of the vacancy. The total relaxed energy of the neutral vacancy is 0.13 eV lower than the total energy if the oxygen basis was not included in the center. The NPA charge on this center is -0.12e with the rest of the additional charge localized on the four Mg atoms surrounding the vacancy (with an average charge of 1.32e). The energy required to remove a

single electron from the vacancy (the first ionization potential) is 2.71 eV for cluster a and 2.69 eV for cluster d. In this, the +1 charge state (F+ center), the Mg atoms directly surrounding the vacancy are displaced away from the vacancy in the surface plane by approximately 0.1 Å. The spin density in this case is approximately evenly shared between the central basis and the four surrounding Mg atoms. An additional 4.58 eV is needed to remove the second electron for cluster a, and 4.50 eV for cluster d. In this state, the structure is significantly distorted with the four Mg atoms surrounding the vacancy displaced away from the vacancy by 0.2 Å and the total charge on the central basis is now -0.01e. For each of the charge states of the vacancy, a benzene molecule is located directly above the vacancy parallel to the surface plane, at an initial separation of 2 Å , and the total energy is minimized. In the neutral (see Figure 5 a) and +1 charge states, there is no binding of the molecule to the vacancy. However, in the +2 charge state the binding energy is increased to 0.20 eV. The separation of the ring from the surface plane is approximately 4.2 Å for the neutral vacancy, approximately 3.7 Å for the +1 charge vacancy, and approximately 3.2 Å for the +2 charge vacancy. The total charge on the benzene molecule from an NPA is 0.00 for neutral vacancy, 0.00 for +1 charged vacancy, and -0.02e for +2 charged vacancy, indicating negligible charge transfer. The binding energy of the benzene ring to the vacancy perpendicular to the surface plane was also calculated; however. this resulted in no binding in either of the three charge states. The effect of the vacancy on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states of the benzene molecule is shown in Figure 5b. For the isolated benzene molecule, the HOMO level is located at -6.69 eV, and the LUMO level at 0.04 eV (relative to the vacuum level). Above the neutral vacancy, the levels are virtually unperturbed at -6.55 eV for the HOMO and 0.08 for

Building Blocks for Molecular Devices

Figure 5. (a) Configuration of the benzene molecule directly above the neutral oxygen vacancy. (b) Illustration of the energy levels of the benzene adsorbed directly above the neutral, +1, and +2 charged oxygen vacancy. The HOMO and LUMO levels of the benzene molecule are shown in each state, along with the highest occupied level of the vacancy in the neutral and +1 charge states. The position of the valence and conduction bands of the extended MgO surface are also shown.

the LUMO. However, if the vacancy is charged then the levels of the molecule are significantly shifted downward. In the case of the +1 vacancy, the energy of the highest occupied level is shifted down to -8.13 eV (by 1.44 eV), and the lowest unoccupied level to -1.63 eV (by 1.67 eV). Here, the lowest unoccupied level of the molecule is still 2.92 eV above the energy level of the electron in the vacancy, at -4.55 eV. With the benzene molecule above the +2 charged vacancy, the HOMO of the molecule is shifted down further to -9.63 eV. and the LUMO to -3.42 eV. With the benzene molecule adsorbed above the +2 charged vacancy of the larger cluster (d), the effect that the position of the molecule relative to the vacancy has on the HOMO and LUMO levels of the molecule was investigated. With the molecule directly over the vacancy in this cluster, in the same position as shown above, the binding energy is 0.19 eV (as opposed to 0.20 eV with cluster b) and the relaxed separation from the surface plane is the same as that with cluster b. The HOMO level of the molecule is lowered to -9.80 eV, and the LUMO to -3.30 eV, in this position. The molecule displaced laterally by 5 Å from this position and relaxed. The relaxation does not significantly affect the lateral position of the molecule, and the binding energy is now 0.05 eV. In this configuration,

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Figure 6. Configurations of (a) naphthalene and (b) pyrene adsorbed directly above the oxygen vacancy. (c) LUMO and HOMO levels of the three aromatic molecules (benzene, naphthalene, and pyrene) adsorbed above the +2 vacancy of quantum cluster d. The position of the valence and conduction bands of the extended MgO surface are also shown.

the HOMO level of the molecule is at -9.54 eV and the LUMO level is at -2.85 eV, which is 0.26 and 0.45 eV, respectively, higher than that with the molecule directly over the vacancy. As the size of a conjugated aromatic system increases, the level of the HOMO increases and level of the LUMO decreases. To investigate how the levels of larger aromatic systems would be shifted by the +2 charged vacancy, we considered two additional molecules, naphthalene (C10H8) and pyrene (C16H10). These molecules could potentially form the base of a molecular device. The HOMO-LUMO gap of benzene is 6.73 eV, of naphthalene 4.55 eV, and of pyrene 3.57 eV. Figure 6a and b shows the configuration of these two molecules adsorbed directly above the vacancy, parallel to the surface. The HOMO and LUMO energy levels of the molecules in these systems, relative to the surface valence and conduction bands, are illustrated in Figure 6c (the benzene molecule is included for comparison, also above the +2 vacancy). In the case of the benzene molecule, the HOMO state is lowered from -6.69 to -9.80 eV (3.11 eV), and the LUMO from 0.04 eV to -3.30 eV (3.34 eV). In the case of the naphthalene molecule,

15380 J. Phys. Chem. C, Vol. 111, No. 42, 2007 the HOMO state is lowered from -5.61 to -8.56 eV (2.95 eV), and the LUMO from -1.06 to -3.99 eV (2.93 eV). In the case of the pyrene molecule, the HOMO state is lowered from -5.13 to -8.16 eV (3.03 eV), and the LUMO state from -1.56 eV to -4.37 eV (2.81 eV). For each of the molecules here, the effect of the charged vacancy appears to be to reduce the levels of the molecule by approximately 3 eV, with the gap being relatively unchanged. 5. Discussion and Conclusion. The interaction of various hydrocarbon and polar organic molecules, building blocks of larger potential molecular devices, with the MgO (001) surface has been studied. In the case of the hydrocarbon molecules (benzene and methane), there is virtually no binding to the surface at all, and we have shown that the molecule will not deprotonate on the surface. It is to be expected that dispersion interactions will contribute to the binding of these molecules to the surface, and given the small binding energies of the hydrocarbon molecules, the dispersion is likely to dominate in these cases. The computational method we employed (B3LYP hybrid-DFT) will underestimate these dispersion interactions, and more-sophisticated and expensive quantum chemistry methods would be required to evaluate them accurately. However, the additional binding would not significantly affect the mobility of the molecules on the surface. Polar molecules bind more strongly because of the electrostatic interaction with surface ions; however, the molecules are not strongly polarized by the interaction with the surface. These polar molecules, specifically nitromethane and pyridine, which provide the largest binding energies, could form the basis of groups attached to a larger hydrocarbon molecule in order to anchor or “graft” a larger molecule to the surface and limit the diffusion. This would be essential if the MgO (001) surface were used as a substrate for a molecular electronic device, where it would be necessary to immobilize a large molecule, but not significantly perturb its electronic structure. The interaction of benzene with oxygen vacancies in different charge states was studied, and it was found that a single oxygen vacancy in the +2 charge state can significantly increase the binding energy of a single benzene molecule to the surface. In any charge state of the vacancy, there is no transfer of charge between the surface and molecule. With the vacancy in the neutral state, the levels of the molecule are relatively unperturbed, as is the case when the benzene molecule is adsorbed above the undefected surface. In the +1 charge state, both the HOMO and LUMO levels of the molecule are reduced by about 1.5 eV, and in the +2 charge state by about 3.5 eV, due to the electric field created by the vacancy. With larger aromatic molecules adsorbed above the +2 charged vacancy, the effect on the molecular energy levels was very similar, with both the HOMO and LUMO levels being reduced by approximately 3 eV. Moving the benzene molecule further away from the +2 charged vacancy by 5 Å reduces the amount by which the levels are affected, but not very significantly. The strong dependency of the molecular levels on the position of the charged vacancy with respect to the adsorbed molecule could be used to control the electron current through the molecule attached to electrodes. As mentioned in the introduction, it has been shown that it should be possible to manipulate the lateral position of a bare vacancy in +2 charge state in the MgO (001) surface with an atomic force microscope tip.13 The charge state of the vacancy on a bulk surface can be controlled through electron deposition from a tip.29 In the case of thin MgO films grown on metal substrates, such as Ag or Mo, the vacancy

Trevethan and Shluger charge state can be controlled by voltage application to a metallic substrate.12 This can provide an element of local control of molecular levels in a molecular electronic device, relative to the Fermi level of metal electrical contacts. As shown in ref 30, the transparency of a molecule to electron flow across it is very sensitive to the exact position on the LUMO level. Thus, by controlling the spatial position of a vacancy (or other stable surface defect with similar properties) relative to an adsorbed molecule and its charge state it should be possible to control the molecular conductivity. Atomic electrode atoms close to the vacancy will have also have their levels shifted down by a certain amount, but this effect will decrease the further away from the vacancy the atoms are. How this phenomena will actually affect the electronic transport properties of the molecule will need to be investigated using a more-sophisticated approach such as the Green function method.31 Finally, we note that these results are qualitatively applicable to other surfaces, such as LiF, NaCl, CaO, or NiO having the same structure, similar lattice constants, and similar properties of anion vacancies. It would be interesting to explore the effects of different band gaps and magnetic properties of these materials on the properties of molecular devices. The results concerning the character of interaction of molecules with the surface are qualitatively similar to those obtained earlier for the perfect TiO2 (110) surface.17 However, the properties of vacancies at the TiO2 surface and other easily reduced surfaces, such as CeO2, are different and would be more difficult to control. Calculations such as these may aid the design and atomic-scale control of potential molecular devices and electronics. Acknowledgment. This work has been supported by the FP6 IP Pico-Inside project. We thank M. Watkins, M. Sushko, and P. Sushko for helpful discussions and advice. References and Notes (1) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541. (2) Fiurasek, J.; Cerf, N. J.; Duchemin, I. Physica E 2004, 24, 161. (3) Duchemin, I.; Joachim, C. Chem. Phys. Lett. 2005, 406, 167. (4) Eigler, D. M. Nature 1990, 344, 524. (5) Sugimoto, Y.; Abe, M.; Hirayama, S.; Oyabu, N.; Custance, O.; Morita, S. Nat. Mater. 2005, 4, S156. (6) Xue, Y. Q.; Ratner, M. A. Int. J. Quantum Chem. 2005, 102, 911. (7) Joachim, C.; Ratner, M. A. PNAS 2005, 102, 8801. (8) Grill, F.; Moresco, L. J. Phys. Condens. Matter 2006, 18, 1887. (9) Giessibl, F. J. ReV. Mod. Phys. 2003, 75, 957. (10) Schintke, S.; Schneider, W. D. J. Phys. Condens. Matter 2004, 16, R49. (11) Barth, C.; Henry, C. R. Phys. ReV. Lett. 2003, 91, 196102. (12) Sterrer, M.; Heyde, M.; Novicki, M.; Nilius, N.; Risse, T.; Rust, H. P.; Pacchioni, G.; Freund, H. J. J. Phys. Chem. B 2006, 110, 46. (13) Watkins, M.; Shluger, A. L. Phys. ReV. B 2006, 73, 245435. (14) Trevethan, T.; Watkins, M.; Kantorovich, L.; Shluger, A. L. Phys. ReV. Lett. 2007, 98, 028101. (15) Trevethan, T.; Kantorovich, L.; Shluger, A. L. Phys. ReV. B 2007, 76, 085414. (16) Piva, P. G.; DiLabio, G. A.; Pitters, J. L.; Zikovsky, J.; Rezeq, M.; Dogel, S.; Hofer, W. A.; Wolkow, R. A. Nature 2005, 435, 658. (17) Sushko, M. L.; Gal, A. L.; Shluger, A. L. Phys. ReV. B 2006, 73, 014101. (18) Rosei, F.; Schunack, M.; Jiang, P.; Gourdon, A.; Lagsgaard, E.; Stensgaard, I.; Joachim, C.; Besenbacher, F. Science 2002, 296, 328. (19) Drummond, M. L.; Sumpter, B. G.; Shelton, W. A.; Larse, J. Z. Phys. ReV. B 2006, 73, 195313. (20) Ferrari, A. M.; Huber, S.; Knozinger, H.; Neyman, K. M.; Rosch, N. J. Phys. Chem. B 1998, 102, 4548.

Building Blocks for Molecular Devices (21) Kakkar, R.; Kapoor, P. N.; Klabunde, K. J. J. Phys. Chem. B 2004, 108, 18140. (22) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (23) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (24) Sushko, P. V.; Shluger, A. L.; Catlow, R. A. Surf. Sci. 2000, 450, 153. (25) Sushko, P. V.; Gavartin, J. L.; Shluger, A. L. J. Phys. Chem. B 2002, 106, 2269. (26) Tretiak, S.; Mukamel, S. Chem. ReV. 2002, 102, 3137.

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15381 (27) Markmann, A.; Gavartin, J. L.; Shluger, A. L. Phys. Chem. Chem. Phys. 2006, 8, 4359. (28) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (29) Bussmann, E.; Zheng, N.; Williams, C. C. Appl. Phys. Lett. 2005, 86, 163109. (30) Stadler, R.; Jacobsen, K. W. Phys. ReV. B 2005, 74, 161405. (31) Ke, S.; Baranger, H. U.; Yang, W. J. Am. Chem. Soc. 2004, 126, 15897.