J. Phys. Chem. B 2005, 109, 18889-18894
18889
Role of Molecular Conjugation in the Surface Radical Reaction of Aldehydes with H-Si(111): First Principles Study Yosuke Kanai,*,† Noboru Takeuchi,†,‡ Roberto Car,†,§ and Annabella Selloni*,† Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544, Centro de Ciencias de la Materia Condensada, UniVersidad Nacional Auto´ noma de Mexico, Apartado Postal 2681, Ensenada, Baja California 22800, Mexico, and Princeton Institute for the Science and Technology of Materials (PRISM), Princeton UniVersity, Princeton, New Jersey 08544 ReceiVed: May 25, 2005; In Final Form: August 5, 2005
Within the current effort to understand and develop the organic functionalization of silicon surfaces, recent experiments have identified the radical chain reaction of unsaturated organic molecules with H-terminated silicon surfaces as a particularly promising route for controlled formation of such functionalized surfaces. Using periodic density functional theory calculations, we theoretically study and characterize the basic steps of the radical chain reaction mechanism for different aldehyde molecules (formaldehyde, benzaldehyde, propanaldehyde, propenaldehyde) reacting with the H-Si(111) surface, under the assumption that a Si dangling bond is initially present on the surface. Molecular conjugation is found to play a crucial role in the viability of the reaction, by controlling the delocalization of the spin density at the reaction intermediate. Interesting differences between our present results for aldehydes and our previous study for the reactions of alkene/ alkyne molecules with H-Si(111) are observed and discussed (Takeuchi et al. J. Am. Chem. Soc. 2004, 126, 15890).
Introduction The organic functionalization of semiconductor surfaces, especially of silicon, has attracted considerable interest in the past decade.1-7 Combining diverse organic functionalities with conventional silicon technology may indeed lead to applications in various fields, such as chemical/bio sensing,8,9 surface passivation,10,11 and molecular electronics.12,13 For most applications, well-ordered organic monolayers are desirable. One of the most promising approaches for obtaining such ordered monolayers is via a radical chain reaction mechanism of terminally unsaturated molecules with hydrogenterminated Si surfaces. In this reaction, the organic molecule covalently binds to an isolated Si dangling bond of the otherwise saturated silicon surface, and gives rise to a surface-bound organic group with a carbon-centered radical. In a second step, the highly reactive molecular radical abstracts a hydrogen atom from a neighboring H-Si unit to produce a new silicon dangling bond, which can act as an attachment site for another molecule, thus leading to a surface chain reaction. This mechanism appears to be widely accepted for describing the molecular attachment reaction in cases where a Si dangling bond is present on the otherwise H-terminated surface. The situation is less clear for the cases in which no Si dangling bond is initially present, and especially the very first step of the reaction is still under debate.14-17 Although much work, both experimental and theoretical, has been devoted to studying the chain reactions of alkene and alkyne molecules forming C-Si covalent bonds with the silicon * To whom correspondence should be addressed. Email: ykanai@ princeton.edu;
[email protected]. † Department of Chemistry, Princeton University. ‡ Universidad Nacional Auto ´ noma de Mexico. § Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University.
surface,1,3,7,18-25 comparatively fewer studies have examined the adsorption of molecules such as aldehydes, alcohols, or carboxyls that are attached to the surface through Si-O-C bonds.14,16,18,24,26-28 In this paper, we shall mostly focus on the latter, and report periodic density functional theory (DFT) calculations of the surface radical chain reaction of a few different aldehyde molecules with a H-terminated Si(111) surface in which a Si dangling bond is initially present. Our calculations are based on large slab models for the surface and employ a recently implemented string method29,30 that efficiently locates minimum energy paths (MEP) and transition states (TS). We also present a comparison of the energetics and barriers for the aldehyde molecules with their alkene/alkyne counterparts from our earlier study.25 Our results show that molecular conjugation has a crucial role in the viability of the surface chain reaction, with quite interesting differences between the alkenes/ alkynes and the aldehydes. For the latter, molecular conjugation leads to a strong stabilization of the carbon radical intermediate, resulting in an enhancement of the H-abstraction barrier for the transition to the final state. For the alkene/alkyne molecules, on the other hand, molecular conjugation stabilizes not only the intermediate state but also the H-abstraction TS. As a consequence, the chain reaction is “easier” for conjugated molecules in the case of alkenes and alkynes, whereas it proceeds more efficiently for nonconjugated molecules in the case of aldehydes. Theoretical Methods Periodic DFT calculations have been performed within the Car-Parrinello approach,31-34 using the gradient-corrected Perdew, Burke, and Enzerholf (PBE) functional.35 Electronion interactions are described using a Troullier-Martins36 pseudopotential for Si and ultrasoft37 pseudopotentials for H, C, and O atoms. The electronic states are expanded in plane
10.1021/jp0527610 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/17/2005
18890 J. Phys. Chem. B, Vol. 109, No. 40, 2005
Kanai et al.
Figure 1. Chemical structures of (a) formaldehyde, (b) benzaldehyde, (c) propanaldehyde, and (d) propenaldehyde.
waves with kinetic energy up to 25 and 200 Ry for the wave function and charge density, respectively. As the systems of interest have a spin-unpaired electron (we assume that one unsaturated Si dangling bond is initially present on the surface), spin-unrestricted calculations were performed. Our previous theoretical work25 showed that polarized calculations are necessary to well describe the energetics of molecular radicals. In our periodic supercell approach, the surface is modeled as a slab of six (111) layers with eight silicon atoms per layer, forming a periodically repeated (4x3) surface cell in the (111) plane. Finite size effects were checked using a larger slab model with 12 Si atoms per layer. A vacuum region of 10.2 Å is taken between the slabs in the direction normal to the surface, and the convergence was also checked with a vacuum region of 16.4 Å. Molecules are adsorbed on the upper surface of the slab, where a monolayer of H atoms is also present to saturate part of the Si dangling bonds (DB). On the slab’s bottom surface, all Si DBs are saturated by H atoms. The four topmost Si layers of the slab as well as the H monolayer and the adsorbed molecules are fully relaxed, with residual forces smaller than 0.01 eV/Å, whereas the two lowest Si layers and the saturating H atoms are held fixed at the equilibrium positions in order to simulate a bulklike environment (the H-Si distance was previously optimized). To study reaction pathways, we used the string method as implemented by Kanai et al., in the context of the CarParrinello approach.29,30 A short description of the method can be found in our recent paper,25 and more details on the method and implementation can be found in ref 30. Minimum energy pathways (MEP) and transition states (TS) were obtained using 10 replicas to represent a path. In some cases, a series of string calculations was carried out in order to precisely locate the saddle point. At this point, all residual forces on mobile atoms are smaller than 0.03 eV/Å. Reaction Pathways Aldehydes form a class of organic molecules that contain the carbonyl group (CdO) in which lone-pair electrons reside on the oxygen atom. In this section, we discuss the surface radical chain reaction for a few nonconjugated and conjugated aldehyde molecules, namely, formaldehyde, benzaldehyde, propanaldehyde, and propenaldehyde. Formaldehyde. Formaldehyde (OdCH2) is the simplest of the aldehydes, as the carbon atom of the carbonyl group is saturated with two hydrogen atoms. Upon adsorption at the Si DB site, the OdC double bond (1.22 Å) is reduced to form a Si-O-CH2 radical, with an O-C bond length of 1.37 Å. For this adsorption process, our string calculation yields a negligibly small barrier, approximately 0.03 eV. The adsorbed radical may assume two different conformations, which we denote B and V (Figure 2). The energy difference between B and V is only 0.03 eV, and the energy barrier separating them is similarly
Figure 2. Vertical [V] and Bent [B] radical conformations at the intermediate metastable states. ∆E and ∆Eq denote the energy difference and energy barrier between these two states, respectively.
very small, indicating that the adsorbed molecule frequently flips between the two conformations at room temperature. With respect to the initial state in which the molecule and the surface are uncoupled, the intermediate states B and V lie 0.78 and 0.81 eV lower in energy, respectively. Although very close in energy, the B and V intermediates lead to different pathways for H-abstraction from a neighboring Si-H surface unit, the B pathway having a barrier to the final state at least 0.1 eV lower than the MEP from the V state (see Table 1). The main reason for such a difference appears to come from the different orientation of the radical carbon atom with respect to the H atom that is abstracted. The adsorbed aldehyde radical has a spin-unpaired electron density facing the H atom that is abstracted at the intermediate state B, whereas a C-H bond is pointing toward this H atom at the intermediate state V (Figure 2). The final state, after H-abstraction, is 1.63 eV more stable with respect to the initial state, and the O-C bond length is increased to 1.44 Å. The evolution of the spin density (σ(r) ) Fv(r) - FV(r)) along the MEP is also shown in Figure 3. Initially, there is a highly localized spin density at the surface DB site. In the intermediate state, the spin density is not fully localized on the radical carbon atom, as some density is also present on the electronegative oxygen atom. At the TS, we see a spin density extending over both the adsorbed radical and the surface. The spin density again becomes highly localized at a DB in the final state, enabling a further surface radical reaction to occur. Benzaldehyde. Since the rate-limiting step in the surface chain reaction is the H-abstraction by the molecular radical intermediate state, from here on we shall focus on this step without considering the initial reaction between the molecule and the Si DB. As in the case of formaldehyde, for benzaldehyde adsorption there are two intermediate metastable states (see Figure 2). The B intermediate, in which the adsorbed radical is tilted, is about 0.1 eV lower in energy than the V intermediate, in which the radical is vertical. The relative stability of the B and V states is opposite to that of the formaldehyde case. This is probably due to the electrostatic interaction between the phenyl ring and the surface. Using the string method, the B f V energy barrier is found to be 0.11 eV. From Table 1, we can see that the H atom abstraction barrier along the MEP from the intermediate state B is 0.79 eV, and this is at least 0.3 eV lower than the barrier for the V pathway. The final state energy is
Molecular Conjugation in Surface Radical Reactions
J. Phys. Chem. B, Vol. 109, No. 40, 2005 18891 TABLE 2: Energiesa of the V Intermediate and Final States for Various Aldehyde Molecules Interacting with either H-Si(111) or Model Systemsb H-Si(111) Emid Efin SiH3 + SiH4 Emid Efin H + H2 Emid Efin
propanaldehyde
propenaldehyde
benzaldehyde
-0.61 -1.31
-1.05 (-0.44)c -1.01 (+0.30)
-0.85 (-0.24) -1.03 (+0.28)
-1.26 -0.86
-1.73 (-0.47) -0.58 (+0.28)
-1.59 (-0.34) -0.61 (+0.24)
-1.39 -0.97
-1.88 (-0.48) -0.71 (+0.26)
-1.73 (-0.33) -0.73 (+0.24)
a Energies given in eV. b Systems are shown in Figure 5. c Values are referred to uncoupled molecule and surface/clusters, whereas values in parentheses are differences with respect to propanaldehyde.
Figure 3. Potential energy profile along the MEP for H-abstraction: formaldehyde (top), benzaldehyde (bottom). On the abscissa, the normalized arc length is the parameter used to describe the MEP.29,30 Full and dashed lines refer to H-abstraction from the B and V intermediates, respectively. The energy reference (E ) 0) corresponds to the potential energy for the noninteracting surface and molecule. The star indicates the saddle point where the reported energy barrier is evaluated. The spin density, σ(r) ) Fv(r) - FV(r), where Fv,V is the charge density of spin up/down electrons, is also shown (isosurface at 0.01 au), along with molecular rearrangements. Si atoms are in blue, O atoms are in red, C atoms are in green, and H atoms are in white. The upperleft inset shows the H-Si(111) surface with a dangling bond before molecular adsorption.
TABLE 1: Minimum and Transition State Energies for the Hydrogen Abstraction Process bent [B] formaldehyde benzaldehyde propanaldehyde propenaldehyde
vertical [V]
Emida
ETS
Emid
ETS
Efin
-0.78 -0.94 -0.69 -1.11
-0.43 -0.15 -0.22 -0.20
-0.81 -0.85 -0.61 -1.05
> -0.3 > 0.1
-1.63 -1.03 -1.31 -1.01
a Emid, ETS, and Efin are the potential energies at the intermediate, transition, and final states, respectively, and are given in eV. The energies are referred to the initial state where the molecule and the surface are uncoupled.
-1.03 eV with respect to the initial state, with a gain of stability of less than 0.1 eV with respect to the B intermediate. In Figure 3, the evolution of the spin density along the MEP is also shown. At the metastable state, the spin density is delocalized within the phenyl ring as well as on the radical carbon atom and the oxygen atom. The highly conjugated phenyl ring allows for the delocalization of the spin-unpaired electron, thus stabilizing the system, as found also for alkenes.22,25 The qualitative features of the delocalized spin density within the phenyl group (Figure 3) can be most conveniently rationalized in terms of valence bond (VB) theory. The kinetic energy
lowering associated with the electron delocalization can be interpreted as resonance stabilization among valence bond structures. At the TS, we observe the spin density extending over the adsorbed molecular radical and the surface, as in the formaldehyde case. The spin density again becomes highly localized as a DB in the final state, enabling a further radical chain reaction at the surface. Propanaldehyde and Propenaldehyde. It appears from Figure 3 that the potential energy profiles for formaldehyde and benzaldehyde are very different, despite the similarity of their reaction mechanisms. To verify whether this difference is due to the difference in conjugation, we have also studied the reactions of unconjugated propanaldehyde (C3H6O) and conjugated propenaldehyde (C3H4O). As shown in Figure 1, these two aldehydes are similar in chemical structures but quite different in molecular conjugation (electronic structures). We considered only the pathways from the B intermediates (the intermediate state V is about 0.05 eV higher in energy, see Table 1). The potential energy profiles along the MEPs for propanaldehyde and propenaldehyde are shown in Figure 4. These two molecules have considerably different spin densities in the intermediate state, as expected from VB theory. The spin density in the intermediate state is delocalized for C3H4O but highly localized at the radical carbon atom for C3H6O. Correspondingly, the intermediate state is much more stable for the conjugated C3H4O than for the unconjugated C3H6O molecule. Actually, for propenaldehyde the intermediate is even more stable than the state resulting from H-abstraction, indicating that such a process is thermodynamically unfavorable. Our calculated reaction energy profile for C3H6O is qualitatively at variance with the one in ref 24 for butanal (C4H8O). A possible reason for these discrepancies may be the use of a rather small cluster model in ref 24. We remark also that the intermediate, transition, and final state energies for alkene in ref 24 are much higher than those of other theoretical predictions.22,23,25 Molecular Conjugation and Reaction Viability For the radical chain reaction to be most effective, there are a few criteria to be satisfied. The intermediate state must be stable enough and the barrier to the final state small enough so that the hydrogen abstraction process is preferred over the molecular radical desorption. At the same time, this intermediate state should be sufficiently unstable so that the final state is thermodynamically favorable. For achieving the optimal reaction viability, the intermediate state should satisfy both these kinetic and thermodynamic criteria. We have already noticed the qualitative difference in the reaction potential energy profiles for formaldehyde and benz-
18892 J. Phys. Chem. B, Vol. 109, No. 40, 2005
Kanai et al.
Figure 5. Schematics of systems used to assess the generality of the trend for molecular conjugation effects. (a) Noninteracting SiH3 + SiH4. (b) Noninteracting H + H2.
Figure 4. Potential energy profile along the MEP for the hydrogen abstraction process from intermediate state B for unconjugated propanaldehyde (top) and conjugated propenaldehyde (bottom). The energy reference (E ) 0) corresponds to the potential energy for the noninteracting surface and molecule. The star indicates the saddle point where the energy barrier is calculated. The spin density, σ(r) ) Fv(r) - FV(r), where Fv,V is the charge density of spin up/down electrons, is also shown (isosurface at 0.01 au), along with molecular rearrangements. Details are as in Figure 3.
aldehyde. For the latter, the highly stable intermediate state is a consequence of the spin-unpaired electron being delocalized. Indeed, the spin density of the benzaldehyde radical extends over the phenyl ring as well as the electronegative oxygen atom. This causes a lowering of the kinetic energy. On the other hand, for formaldehyde, the final state is much more stable relative to the intermediate state than that for benzaldehyde. The stability of the benzaldehyde intermediate also contributes to the high energy barrier to the final state, resulting in unfavorable kinetics. Thus, in terms of the reaction viability criteria enunciated above, the reaction with formaldehyde appears to be more viable than that with benzaldehyde. The terminal conjugation directly influences the stability at the intermediate state by determining the extent of delocalization for the spin-unpaired electron. The stability at the final state is also largely determined by the terminal conjugation, because the reduction of conjugated double bonds is less exothermic than that of unconjugated ones. The results in Figure 4 for the unconjugated propanaldehyde and conjugated propenaldehyde illustrate the importance of such an effect. The spin density for conjugated C3H4O at the intermediate state reflects the fact that the terminal conjugation allows the delocalization of the spinunpaired electron, stabilizing the state substantially. The intermediate state for unconjugated C3H6O is not as stable, due to the highly localized spin-unpaired electron. By contrast, the final state of C3H6O is more stable than that of conjugated C3H4O.
Having the characteristics of both the intermediate and final states, the TS is less influenced by the conjugation. To assess the generality of the trends associated with molecular conjugation, we also considered the model systems shown in Figure 5. Instead of the H-Si(111) surface with a DB site, the aldehydes react either with a system composed of two noninteracting SiH3 and SiH4 clusters or, even more simply, with a noninteracting H atom and H2 molecule. We considered the intermediate state V rather than the intermediate state B because these small clusters are not likely to show the latter as a minimum. Energetic comparisons of the corresponding reactions are summarized for aldehydes in Table 2. Values reported in parentheses are the potential energy differences from the unconjugated C3H6O case. For each case, the stability associated with molecular conjugation has quantitatively the same trend as in the case of the surface, indicating the validity of our analysis. In summary, the molecular conjugation influences the reaction viability of the surface chain reaction in two ways. First, the terminal conjugation allows the spin-unpaired electron to delocalize at the intermediate state, where the molecular radical is formed, stabilizing the system. Second, the exothermicity from the initial to the final state is decreased by the terminal conjugation because the reduction of the conjugated carbonyl double bond is energetically less favorable than that of the unconjugated one. Comparison with Alkenes and Alkynes We now compare the results from the Reaction Pathways section for formaldehyde and benzaldehyde with our findings for alkenes and alkynes in ref 25. As for the aldehydes, the alkenes and alkynes have two different intermediate states. For the alkene/alkyne molecules, however, the V metastable states (and corresponding H atom abstraction barriers) are considerably higher in energy, and for this reason they were not considered in our previous work.25 The energies and geometries of the intermediate, transition, and final states in the reaction of H-Si(111) with the different alkene, alkyne, and aldehyde molecules are compared in Figures 6 and 7. The differences between aldehydes and alkenes/alkynes do not only reflect the stronger O-Si bond; the exothermicity in reducing the CdO and CdC/C≡C bonds is equally important. The intermediate state, with respect to the final state, tends to be slightly more stable for aldehydes than for alkenes. This is due to the more delocalized spin-unpaired electron that extends to the electronegative oxygen atom (Figures 6 and 7). A more important difference between the aldehydes and alkenes/alkynes is observed in the relative stability of the TS for the unconjugated and conjugated molecules. For aldehydes, the formaldehyde TS is more stable than the benzaldehyde one, whereas the alkene/alkyne counterparts show the opposite behavior: the conjugated molecules have a more stable TS than the unconjugated ones. This difference is likely due to the fact that alkene/alkyne molecules have earlier transition states compared to aldehydes, such that the TS has more of the character of the intermediate state than of the final state. This
Molecular Conjugation in Surface Radical Reactions
J. Phys. Chem. B, Vol. 109, No. 40, 2005 18893 completely localized on the molecular radical, whereas in the TS for the aldehydes a significant fraction of the spin density is already on the surface. Because a large portion of spin density still resides on the molecular radical at the TS, conjugation stabilizes the TS of conjugated alkenes/alkynes with respect to the unconjugated ones. Summary and Conclusions
Figure 6. Structures and energies of the intermediate states (top), TS (middle), and final states (bottom) for (A) ethene, (B) ethyne, and (C) formaldehyde. Corresponding spin densities (isosurface at 0.01 au) are also shown. Energies are referred to the uncoupled molecule and surface.
Figure 7. Structures and energies of the intermediate states (top), TS (middle), and final states (bottom) for: (A) styrene, (B) phenyl acetylene, and (C) benzaldehyde. Corresponding spin densities (isosurface at 0.01 au) are also shown. Energies are referred to the uncoupled molecule and surface.
is also related to the difference in steric effects in the transition. The adsorbed molecule stands vertically in the final state, whereas the molecular radical most frequently prefers a “bent” conformation in the intermediate state. It turns out that the B f V rotation is energetically much less costly for aldehydes,38 so that steric effects play a less important important role in the transition (and in the TS) for these molecules than for alkene/ alkyne molecules. From Figures 6 and 7, we can see that in the TS for the alkene/alkyne cases, the spin density is almost
Using periodic DFT calculations, we have studied the surface chain reaction mechanism for different aldehyde molecules interacting with a hydrogenated Si(111) surface where one H vacancy is present. More specifically, we compared unconjugated formaldehyde and propanaldehyde to conjugated benzaldehyde and propenaldehyde, to understand how the terminal conjugation influences the reaction viability. We found that for conjugated aldehydes the intermediate state tends to be extremely stable so that there is a very small or vanishing thermodynamic driving force toward the final state. This stable intermediate also makes the H-abstraction barrier very high, since the TS is less influenced by the conjugation. We also compared our present results for aldehydes with those for alkene/ alkyne molecules that we have previously studied.25 The reactions of H-Si(111) with alkene/alkyne molecules appear to have an earlier TS compared to that of the reactions with aldehydes. The TS for alkene/alkyne retains most of the spin density in the molecular radical rather than at the surface, unlike the TS for aldehydes. For alkene/alkynes, this leads to a greater stability of the TS when conjugation is present, whereas the TS for aldehydes is not influenced much by the conjugation. In summary, our DFT study indicates that the surface chain reaction is a viable mechanism for alkoxyl monolayer formation in the case of terminally unconjugated aldehydes, whereas this mechanism appears to be unfavorable for conjugated aldehydes. To our best knowledge, no experimental observation of alkoxyl monolayers using conjugated molecules has been reported so far. On the other hand, several studies have reported the formation of alkoxyl monolayers from long unconjugated aldehydes on Si(111).14,16,26 For these systems, our results indicate that the radical chain reaction can be operative when dangling bonds are present on the surface, as, for example, in ref 26. We also deduce from our present and previous studies25 that in the case of unconjugated molecules, aldehydes should be more effective than alkenes in the surface chain reaction. This finding is consistent with the experimental result showing a significantly higher coverage for aldehyde (octadecanal) than for alkene (octadecene) monolayers, when prepared under similar conditions on H-Si(111).26 In cases where the presence of dangling bonds is not obvious, other possible reaction mechanisms should also be examined.14,16,17 Acknowledgment. This work was partially supported by NSF Award CHE-0121432. Calculations were performed at the W. M. Keck Computational Materials Science Computing Center of PRISM at Princeton University. References and Notes (1) Lopinsky, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48-51. (2) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2002, 2, 23-34. (3) Buriak, J. M. Chem. ReV. 2002, 102, 1271-1308. (4) Bent, S. F. Surf. Sci. 2002, 500, 879-903. (5) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Jovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russel, J. N., Jr. Acc. Chem. Res. 2000, 33, 617. (6) Wolkow, R. A. Annu. ReV. Phys. Chem. 1999, 50, 413-441.
18894 J. Phys. Chem. B, Vol. 109, No. 40, 2005 (7) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631-12632. (8) Strother, T.; Cai, W.; Zhao, X. S.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205-1209. (9) Lasseter, T. L.; Clare, B. H.; Abbott, N. L.; Hamers, R. J. J. Am. Chem. Soc. 2004, 126, 10220-10221. (10) Webb, L.; Lewis, N. S. J. Phys. Chem. B 2003, 107, 5404-5412. (11) Boulas, C.; Davidovits, J. V.; Rondelez, F.; Villaume, D. Phys. ReV. Lett. 1996, 76, 4797-4800. (12) Yates, J. T. Science 1998, 279, 335-336. (13) Hersam, M. C.; Reifenberger, R. G. MRS Bull. 2004, 29, 385. (14) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. M. Langmuir 2000, 16, 7429-7434. (15) Eves, B. J.; Sun, Q.; Lopinski, G. P.; Zuilhof, H. J. Am. Chem. Soc. 2004, 126, 14318-14319. (16) Hacker, C. A.; Anderson, K. A.; Richter, L. J.; Richter, C. A. Langmuir 2005, 21, 882-889. (17) Sun, Q.; de Smet, L. C. P. M.; van Lagen, B.; Giesbers, M.; Thu¨ne, P. C.; van Engelenburg, J.; de Wolf, F. A.; Zuilhof, H.; Sudho¨lter, E. J. R. J. Am. Chem. Soc. 2005, 127, 2514-2423. (18) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (19) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688-5695. (20) Cicero, R. L.; Chidsey, C. E. D.; Lopinsky, G. P.; Wayner, D. D. M.; Wolkow, R. A. Langmuir 2002, 18, 305-307. (21) Sieval, A. B.; Opriz, R.; Mass, H. P. A.; Shoeman, M. G.; Meijer, G.; Vergeldt, F. J.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 2000, 16, 10359-10368. (22) DiLabio, G. A.; Piva, P. G.; Kruse, P.; Wolkow, R. A. J. Am. Chem. Soc. 2004, 126, 16048-16050.
Kanai et al. (23) Cho, J.-H.; Oh, D.-H.; Kleinman, L. Phys. ReV. B 2002, 65, 081310. (24) Pei, Y.; Ma, J.; Jiang, Y. Langmuir 2003, 19, 7652-7661. (25) Takeuchi, N.; Kanai, Y.; Selloni, A. J. Am. Chem. Soc. 2004, 126, 15890. (26) Effenberger, F.; Go¨tz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. 1993, 32, 2462-2464. (27) Zhu, X.-Y.; Boiadejiev, V.; Mulder, J. A.; Hsung, R. P.; Major, R. C. Langmuir 2000, 16, 6766-6772. (28) Cucinotta, C. S.; Ruini, A.; Caldas, M. J.; Molinari, E. J. Phys. Chem. B 2004, 108, 17278-17280. (29) Weinan, E.; Ren, W.; Vanden-Eijnden, E. Phys. ReV. B 2002, 66, 1-4. (30) Kanai, Y.; Tilocca, A.; Selloni, A.; Car, R. J. Chem. Phys. 2004, 121, 3359-3367. (31) Car, R.; Parrinello, M. Phys. ReV. Lett. 1985, 55, 2471-2474. (32) Laasonen, K.; Pasquarello, A.; Car, R.; Lee, C.; Vanderbilt, D. Phys. ReV. B 1993, 47, 10142-10153. (33) Tossone, F.; Mauri, F.; Car, R. Phys. ReV. B 1994, 50, 1056110573. (34) Giannozzi, P.; de Angelis, F.; Car, R. J. Chem. Phys. 2004 120, 5903. (35) Perdew, J. P.; Burke, K.; Ernzerholf, M. Phys. ReV. Lett. 1996, 77, 3865-3868. (36) Troullier, N.; Martins, J. L. Phys. ReV. B 1991, 43, 1993-2006. (37) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892-7895. (38) For example, the energy barriers for the B f V conversion are at least 0.2 and 0.3 eV for styrene and phenylacetylene, respectively, whereas the barrier is about 0.1 eV (Figure 1) for benzaldehyde.
