16932
J. Phys. Chem. C 2008, 112, 16932–16937
Electrophilic Reaction Mechanism for Alkyl Monolayer Formation Initiated at Isolated Dangling Bonds of the H-GaN (0001) Surface: A Periodic Density Functional Theory Study Chun-Li Hu, Jun-Qian Li,* Yong Chen, and Wen-Feng Wang Department of Chemistry, Fuzhou UniVersity, Fuzhou, Fujian 350002, China; State Key Laboratory of Structural Chemistry, Fuzhou, Fujian 350002, China ReceiVed: January 14, 2008; ReVised Manuscript ReceiVed: May 28, 2008
The addition reactions of a few different terminal alkenes to the H-terminated GaN (0001) surface, which is initiated at a Ga dangling bond, have been studied using periodic density functional theory calculations. Detailed information on the reaction pathways of 5-amino-1-pentene with the H-GaN (0001) surface is provided, which indicates that the reaction consists of two steps: an initial adsorption of the terminal CdC to the surface Ga dangling bond, forming a metastable intermediate, and the subsequent abstraction of a hydrogen atom from the neighboring Ga-H site, generating a new Ga dangling bond. On the basis of the analysis of the spin and charge populations, in particular, the mechanism underlying the reactions of alkenes with the H-GaN (0001) surface is suggested to be an electrophilic addition reaction that is different from the radical addition mechanism for the similar reaction on Si surfaces. The variation trend of the barrier height of these reactions for different terminal alkenes can be predicted by VB correlation theory and correlated with the trend of the relative electron-withdrawing or -donating capacity of the β-carbon substituents. It is found that alkene with a moderate electron-donating substituent would be favorable for the electrophilic reaction on the H-GaN (0001) surface. 1. Introduction The functionalization of traditional semiconductor surfaces with organic molecules has been inspiring increasing interest over the last 10 years, because new promising properties, such as optical, electrical, chemical, and biological activities have been obtained by such combining.1-3 GaN is an intriguing semiconductor that has been widely applied in optoelectronics and electronic devices.4,5 The modification of GaN surfaces by the attachment of organic molecules would greatly widen its applications in the field of biosensors and nano electronic devices.1,6,7 The functionalization of GaN surfaces with organic molecules has been studied extensively in experiments. Bermudez has linked some molecules bearing reactive groups (amine, thiol, etc.) to the GaN (0001) surface under untrahigh-vacuum (UHV) conditions.8-11 In an ambient environment, the functionalization of GaN with organosilanes has been achieved by Baur et al.6,12 In addition, some organic and biological materials have also been used to modify GaN surfaces by a noncovalent bonding process.13,14 Recently, an experimental report7 has shown that a hydrogenterminated GaN (0001) surface can be functionalized with an alkene compound by illumination with 254 nm ultraviolet (UV) light, and a self-assembled monolayer (SAM) of alkyl chains is formed on the surface. A similar reaction on Si surfaces has been widely studied,15-19 and the mechanism has been proposed as a radical chain reaction on the covalent semiconductor15-17 (see Figure 1). Whether it is also a radical reaction on GaN, which shows great ionic character, has not been demonstrated either experimentally or theoretically. In this paper, we present a first-principles DFT investigation of the reactions of alkenes bearing different β-carbon substit* Corresponding author. Fax: (+86) 5918 7892522. E-mail: jqli@ fzu.edu.cn.
uents (CH2dCH-R) with a hydrogenated GaN (0001) surface. Detailed information on the reaction process of alkene with a H-GaN (0001) surface is provided, and a mechanism that distinguishes from the radical reaction of Si surfaces is proposed by analyzing the spin and charge populations. Furthermore, the effects of different β-carbon substituents on this type of reaction are discussed. 2. Computational Method and Models Periodic density functional theory calculations are performed using the double numerical plus polarization basis set implemented in the DMol3 package.20-22 The effective core potential is employed for Ga atoms and the all-electron calculation for other atoms such as C, N, and H. The 3d electrons of Ga are treated as valence electrons. A spin-unrestricted calculation in the generalized-gradient approximation with the Perdew-Wang scheme22 is used to obtain all the results presented in this work. The popular nudged elastic band method is chosen to determine the minimum energy pathway (MEP) of the reaction. To test the reliability of the method above, the reaction process of C2H4 with a H-terminated Si (111) surface is calculated. The results are very close to those in ref 17 for the same systems (see the Supporting Information), indicating that our method can give reasonable results. The surface is modeled by (4 × 4) supercell geometries for the long-chain alkene of 5-amino-1-pentene, or (3 × 3) for other short-chain alkenes, with six layers of GaN slab and at least a 10 Å vacuum region. A monolayer of H atoms is used to saturate most of the Ga dangling bonds on the top side (one Ga dangling bond is initially present for initiation of the reaction that is generated using 254 nm UV to illuminate the H-terminated GaN (0001) surface in the experiment report of ref 7), and the bottom side of the slab is also passivated by hydrogen (see Figure 2). Among these layers, the bottom three layers are fixed at the
10.1021/jp800325f CCC: $40.75 2008 American Chemical Society Published on Web 10/04/2008
Alkyl Monolayer Formation at (0001) H-GaN Surface
J. Phys. Chem. C, Vol. 112, No. 43, 2008 16933
Figure 1. Radical chain reaction mechanism for the addition of alkenes to H-terminated Si surfaces.
Figure 2. Schematic side and top view of H-saturated GaN (0001) surface with one Ga dangling bond. IM is the most stable configuration of intermediate obtained by an energy scanning (see the Supporting Information). r1 and r2 denote two different routes for IM to capture the neighboring hydrogen atoms.
ideal positions to simulate a bulk environment (the N-H distance has been previously optimized), and others are relaxed. Alkenes are adsorbed on the upper surface of the slab. The size of the supercells ensures that the adsorbates are isolated from each other and that the molecule-molecule interactions can be ignored. The binding energy of the adsorbate with the H-GaN (0001) surface is defined as Eb ) Etotal[surface + adsorbate] Etotal[surface] - Etotal[adsorbate], where Etotal is the total energies of systems per supercell. 3. Results and Discussions 3.1 Reaction Process for 5-Amino-1-pentene with H-GaN (0001) Surface. First, we take 5-amino-1-pentene (CH2d CH-(CH2)3-NH2, or written as C5H11N) as a representative of the long-chain alkene compound in the experimental study of ref 7 to study the reaction process of alkene with the H-terminated GaN (0001) surface. The potential energy profile and the key configurations for the reaction of C5H11N with H-GaN (0001) surface are shown in Figures 3 and 4, respectively. Similar to that on H-terminated Si surface,17,18 the reaction of alkene on the H-GaN (0001) surface also consists of two steps separated by a metastable intermediate: an initial adsorption of the terminal CdC to the surface Ga dangling bond and the subsequent abstraction of a hydrogen atom from the neighboring Ga-H site, generating a new Ga dangling bond. The initial state (IS) is a structure optimized by placing the alkene vertically with its CdC end close to the surface and at a distance of 6 Å above it. The binding energy of IS is -5.027 kcal/mol, belonging to weak physisorption. In the intermediate, the alkene adsorbs at the Ga dangling bond of the surface by the C1 end. The most stable configuration of the intermediate, IM, with the CdC bond being coplanar with the Ga1-Ga2 linking line and C3 locating on the hollow site, is obtained by an energy scanning (see Supporting Information). The binding energy of IM is -15.843 kcal/mol. For the subsequent Habstraction process, two different routes are considered, marked as r1 and r2 in Figure 2 (r1, directly abstracting the H of the same direction with the CdC bond; r2, rotating by 60° to abstract the H of different direction). The energy barrier of the H-abstraction process along r1 (7.933 kcal/mol) is much lower
Figure 3. The potential energy profile of C5H11N adsorption at the Ga dangling bond and the subsequent H-abstraction process along r1 and r2 on the H-GaN (0001) surface.
than r2 (14.644 kcal/mol), and the corresponding final states (FS-r1 and FS-r2) are very similar in structures and binding energies (-31.224 versus -30.924 kcal/mol). So the Habstraction process along r1 is more effective than r2. From the potential energy curve (Figure 3), it is clear that C5H11N hurdles an energy barrier of 8.786 kcal/mol to link to the surface and reaches IM. The barrier is related to breaking of the π bond of CdC and changing of the hybridization style of C1, from sp2 to sp3, that is clearly found in the configuration of TS1. In IM, the bond length of C1-C2 is 1.369 Å, a value between the standard single and double bond. The dihedral angel of ∠C1C3H(1)C2 is equivalent to 0.156°, indicating the hybridization of C2 is still sp2. The activation barrier of H-abstraction along r1 is 7.933 kcal/mol, much lower than the desorption barrier of 19.602 kcal/mol, indicating that the H-abstraction process is more likely than the desorption of alkene from the surface. For the final state, with a stabilization energy of ∼15 kcal/mol with respect to IM, the bond lengths of the C1-C2 bond and the formed C2-H(2) bond are 1.505 and 1.140 Å, respectively, very close to those of a standard C-C single bond and a C-H bond. It is clear that a stable
16934 J. Phys. Chem. C, Vol. 112, No. 43, 2008
Hu et al.
Figure 4. The key structures in the reaction of C5H11N with the H-GaN (0001) surface. The detailed structural parameters are listed in the Supporting Information.
TABLE 1: The Spin Population and Charge Population of the Involved Atoms of IS, IM and FS in the Reaction Processes of C5H11N on H-GaN (0001) and H-Si (111) surfacesa systems
spin population
atoms/adsorbate GaN (0001) Si (111) a
IS IM FS IS IM FS
charge population
Ga1/Si1
Ga2/Si2
RC (C1)
βC (C2)
Ga1/Si1
Ga2/Si2
RC (C1)
βC (C2)
adsorbate
0.025 0.021 0.002 0.424 0.076 0.011
0.000 0.011 0.006 0.000 -0.046 0.415
0.000 0.011 0.006 0.000 -0.046 0.000
0.000 0.107 0.002 0.000 0.577 0.008
0.983 0.878 0.842 0.095 0.307 0.357
0.547 0.594 0.892 0.197 0.178 0.073
-0.049 -0.056 -0.122 -0.049 -0.161 -0.205
0.045 0.176 0.039 0.045 0.102 0.068
0.000 0.209 0.034 0.000 0.018 -0.108
The charge population of C atoms is obtained by summing hydrogens into heavy atoms.
adsorbed species of Ga-(CH2)5-NH2 is formed, and another Ga dangling bond is created in the final state. 3.2 The Mechanism Analysis for the Reaction of Alkene with the H-GaN (0001) Surface. To understand the mechanism underlying the addition reaction of alkene with H-GaN (0001), as a comparison, the reaction of C5H11N on the H-Si (111) surface is also calculated with the same method. The spin population and charge population of the involved atoms in the reaction processes of the alkene with H-GaN (0001) and H-Si (111) surfaces are listed in Table 1. The spin population [n(R) - n(β)] is usually regarded as a tag of a radical reaction. From Table 1, it is clear that on the Si (111) surface, the spin population is mostly localized at Si1 for IS, βC for IM, and Si2 for FS, being 0.424, 0.577, and 0.415e, respectively. However, on the GaN (0001) surface, the spin is delocalized. The spin population of the involved atoms in the reaction, Ga1 for IS, βC for IM, and Ga2 for FS, is as small as 0.025, 0.107, and 0.018e, respectively. Compared with Si, the spin population of GaN systems is too small to prove a radical character. From the charge population displayed in Table 1, it is obvious that the Ga1 atom that is not saturated by hydrogen in IS has a net charge of +0.983|e|, which is approximately a monovalent cation. In IM, the π bond is broken and Ga1 is bonded to RC, Ga1 obtains 0.105e, and βC loses 0.131e. In FS, after the neighbor H is abstracted by βC, βC obtains 0.137e and Ga2 loses 0.298e. It is clear that the positive charge is transferred during the reaction process, from Ga1 to βC, then to Ga2, in agreement with the character of an electrophilic addition reaction. Moreover, on the GaN (0001) surface, the adsorbate in IM has a net charge of +0.209|e|, most of which is focused
on βC, indicating the presence of a carbonium. While on the Si (111) surface, the adsorbed molecule of IM remains near chargeneutral, which is the important feature of a radical intermediate. On the basis of the above discussions, it can be concluded that alkene reacting with the H-GaN (0001) surface is not a radical addition reaction, but an electrophilic addition reaction. The proposed reaction mechanism is shown in Figure 5 and is described by a two-stage VB correlation diagram that was developed by Shaik23,24 in Figure 6. The first step of the reaction, from initial state to the intermediate, is an elementary electrophilic attack process of addition reaction. It refers to a vertical charge transfer, resulting in a carbonium in the βC, different from a radical attack that involves the triplet excitation of the π bond. The second step, from the intermediate to the final state, is an electrophilic exchange process. It is related to the transfer of an electron from the Ga2-H(2) bond to the p empty orbital of the βC, and another Ga dangling bond is formed, also distinguishing it from the singlet-triplet excitation in a radical exchange process. It is further inferred that the abstraction of hydrogen and the formation of a new dangling bond that can act as an attachment site for another alkene molecule would lead to a SAM of alkyl by its propagation for many cycles. 3.3 The Effects of β-Carbon Substituents. To further demonstrate the electrophilic addition mechanism proposed above, the reaction processes of a few alkenes bearing different β-carbon substituent (CH2dCH-R, R ) CF3, H, CH3, NH2) with the H-GaN (0001) surface are also calculated. The H-abstraction process along r1 is considered for these reactions. The spin population and charge population of the involved atoms during these reaction processes is shown in Table 2, and the binding energies of the key configurations are shown in Table
Alkyl Monolayer Formation at (0001) H-GaN Surface
J. Phys. Chem. C, Vol. 112, No. 43, 2008 16935
Figure 5. Proposed mechanism for the addition reaction of alkene to a H-terminated GaN (0001) surface.
Figure 6. The VB correlation diagram for the addition reaction of alkene to a H-terminated GaN (0001) surface. I and II denote the configurations of ground states (below the crossing point) and excited states (above the crossing point), respectively. They are separated by an energy gap, G1, in IS and G2 in IM.
3. The correlation among the binding energy of intermediate, the energy barrier, the exothermicity of the H-abstraction process, and the relative electron-withdrawing or -donating capacity of the βC substituents is shown in Figure 7. As shown in Table 2, the spin population and charge population of different terminal alkenes reacting with the H-GaN (0001) surface exhibit a characteristic similar to that of C5H11N: the spin population of the involved atoms is small, and the positive charge is transferred from the Ga1, to the βC, then to the Ga2 during these reaction processes, also indicating
that the alkene’s reacting with the H-GaN (0001) surface is not a radical reaction, but an electrophilic addition reaction. From Table 3 and Figure 7, it is found that with the increase in the electron-donating capability of the β-carbon substituent, from 1 to 4, the corresponding intermediate is more and more stable (C2H3CF3 < C2H4 < C2H3CH3 < C2H3NH2). The fact further confirms that the reaction of alkenes with the Hterminated GaN (0001) surface is not a radical reaction, but an electrophilic addition reaction. It is well-known that the carbonium is a system that is electron-deficient. The electron-
16936 J. Phys. Chem. C, Vol. 112, No. 43, 2008
Hu et al.
TABLE 2: The Spin Population and Charge Population of the Involved Atoms of IS, IM and FS in the Reaction Processes of C2H3CF3, C2H4, C2H3CH3 and C2H3NH2 with the H-GaN (0001) surfacea systems
spin population
atoms/adsorbates C2H3CF3 C2H4 C2H3CH3 C2H3NH2
a
IS IM FS IS IM FS IS IM FS IS IM FS
charge population
Ga1
Ga2
RC
βC
Ga1
Ga2
RC
βC
adsorbate
0.025 0.038 0.001 0.025 0.023 0.002 0.025 0.019 0.002 0.025 0.005 0.001
0.000 0.003 0.025 0.000 0.004 0.015 0.000 0.003 0.014 0.000 0.001 0.017
0.000 -0.012 0.009 0.000 -0.011 0.010 0.000 -0.012 0.009 0.000 -0.001 0.004
0.000 0.202 0.000 0.000 0.113 0.000 0.000 0.109 0.001 0.000 0.033 0.007
0.983 0.891 0.847 0.983 0.897 0.847 0.983 0.890 0.846 0.983 0.876 0.872
0.547 0.607 0.949 0.547 0.559 0.987 0.547 0.587 0.931 0.547 0.602 0.870
0.060 -0.008 -0.099 0.000 -0.022 -0.137 -0.046 -0.054 -0.118 -0.137 -0.127 -0.130
0.001 0.070 -0.044 0.000 0.171 0.064 0.027 0.144 0.014 0.163 0.283 0.116
0.000 0.049 -0.168 0.000 0.149 -0.073 0.000 0.171 -0.047 0.000 0.322 0.027
The charge population of C atoms is obtained by summing hydrogens into heavy atoms.
TABLE 3: The Binding Energies (Eb) of the IS, TS1, IM, TS2 and FS in the Addition Reaction of Different Alkenes with the H-GaN (0001) Surface (Kcal/mol) 1 2 3 4
C2H3CF3 C2H4 C2H3CH3 C2H3NH2
Eb (IS)
Eb (TS1)
Eb (IM)
Eb (TS2)
Eb (FS)
-4.658 -2.283 -2.698 -2.698
-0.277
-4.105 -11.945 -12.660 -30.532
1.153 -6.065 -5.811 -19.325
-32.400 -31.362 -28.826 -27.580
4.013 0.922
donating functional group could disperse the positive charge from a carbonium center and make it stable, whereas the electron-withdrawing functional group could weaken the stability of the system. On the other hand, for a radical intermediate, it is well-established that the π conjugation and electronwithdrawing functional group could improve its stability. A recent theoretical reference18 has shown that for the radical reaction of alkenes with the Si surface, the intermediate with an electron-withdrawing βC substituent (C2H3CF3, C2H3Cl) is more stable than that of an electron-donating functional group (C2H3CH3). It is obviously different from our results. The relative electron-withdrawing or -donating capacity of the β-carbon substituent also influences the activity of the reaction regularly. The effect is to be predicted qualitatively by the VB correlation theory.23,24 The initial adsorption of the alkenes to the surface, according to the discussions above, is an elementary electrophilic attack process of addition reaction. The barrier (Ea1) is qualitatively formulized as Ea1 ∝ G1 ) I*(CdC) - A*(Ga+). I*(CdC) denotes the vertical ionization energy of the CdC bond, which is variable with the different electron-donating capacity of the βC substituent. A*(Ga+) denotes the vertical electron affinity of Ga+; here, it is nearly a constant. The stronger the electron-donating capacity of substituent, the more negative the charge of the RC and the lower the ionization energy of the CdC bond; thus, the lower the G1 and the adsorption barrier. In fact, Ea1 of C2H3NH2 is found to be the lowest among these alkenes. The subsequent Habstraction is an electrophilic exchange process. The barrier Ea2 ∝ G2 ) I*(Ga-H) - A*(R′-βC+) (R′ is the rest of the adsorbate), I*(Ga-H) can be regarded as a constant, but A* (R′-βC+) is variable with the different electron-donating capacity of the βC substituent. The stronger electron-donating capacity of a substituent would lower the electron affinity of the corresponding system, then result in a higher H-abstraction barrier. Indeed, as shown in Figure 7, our calculations confirm this prediction and suggest an increasing trend in Ea2 for C2H3CF3 < C2H4 < C2H3CH3 < C2H3NH2. Furthermore, the increase of G also makes the reaction less exothermic.25 The
Figure 7. Correlation among the binding energy of intermediate [Eb (IM)], the H-abstraction barrier (Ea2), the exothermicity of the H-abstraction process (Ex), and the relative electron-withdrawing or -donating capacity of the βC substituents.
exothermicity of the H-abstraction process (Ex) obtained by us is in full agreement with the rule (Figure 7). It is noticed that for C2H3NH2, with a strong electron-donating substituent, the intermediate is very stable, but the H-abstraction process is endothermic, indicating that C2H3NH2 is not a good candidate for the reaction. For C2H3CF3 bearing a strong electron-withdrawing functional group, although the H-abstraction barrier is low, the intermediate is so unstable that the adsorbed molecule is likely to desorb from the surface and should also be excluded from the candidates for the reaction. However, the alkenes bearing the moderate electron-donating functional group, such as C2H3CH3 and the long-chain alkene of 5-amino-1-pentene, or C2H4, which have relatively stable intermediates, moderate H-abstraction barriers and large Habstraction exothermicities, may effectively facilitate this type of reaction on the H-GaN (0001) surface. It needs further demonstration in experiment. 3.4 Reasons for the Choice of the Surface Model. It is wellknown that on the clean GaN (0001) surface, each Ga has a dangling bond with the equal electron, 0.75 |e|, and these Ga atoms are equivalent. Under the experimental conditions of ref 7 (“the samples were exposed to a weak 13.56 MHz inductively coupled hydrogen plasma (20 Torr) for 10 min at room temperature to hydrogen-terminate the surfaces”), there are two possible ways for the surface terminated by H: (1) According to the electron-counting rule, for a (2 × 2) unit cell, three of four surface Ga atoms are bonded to H atoms to form three
Alkyl Monolayer Formation at (0001) H-GaN Surface two-electron Ga-H bonds, and the fourth Ga dangling bond is empty.26-28 (2) Under the environment of hydrogen plasma, those equivalent surface Ga atoms are all bonded to the H species with electrons needed according to the electron-counting rule. However, the former is based on the viewpoint of pure electron localization, and such a system can hardly be formed under the experimental conditions. In fact, the surface Ga atoms have an equal chance to be bonded to H, and the electrons in the GaN solid are nonlocalized according to the molecular orbital theory, so the surface Ga atoms will be fully terminated by H, and the Ga-H bonds will become uniform under the experimental conditions. It is understandable that whichever H atom on the upper surface is removed, the corresponding Ga has a high positive charge. Generally, the bottom side of the theoretical surface model is terminated by H to simulate the bulk. For the GaN (0001) surface, the bottom side can be terminated either by H atom with one electron29 or by pseudohydrogen with 0.75e.30,31 To compare the H-adsorbed GaN (0001) systems with the bottom side terminated by the two methods mentioned above (a, by H atom with 1e; b, by pseudohydrogen with 0.75e), the charge populations of the two systems are calculated and are given in Figure S4 of the Supporting Information. From Figure S4, it is clear that the charge populations of the corresponding atoms in the top six layers of the two systems are very close to each other, especially the topmost Ga layer (the former is +1.228|e|, and the latter is +1.215|e|), so the system (a) should naturally be the same as system b for the surface reaction. On the basis of the discussion above, it is believed that our surface model is reasonable and feasible. For the reaction properties focused by us, it also can, to the most extent, represent the experimental system. Therefore, the reaction mechanism of alkene with s H-GaN (0001) surface would not be influenced by the selected surface model. 4. Conclusions The addition reactions of a few different terminal alkenes to the H-terminated GaN (0001) surface with a Ga dangling bond have been studied employing periodic density functional theory calculations. Detailed information on the reaction pathways of 5-amino-1-pentene with the H-GaN (0001) surface is provided in our results, which indicate that similar to alkenes on H-Si surfaces, the reaction on the H-GaN (0001) surface also contains two steps: an initial adsorption of the terminal CdC to the surface Ga dangling bond, forming a metastable intermediate, and the subsequent abstraction of a hydrogen atom from the neighboring Ga-H site, generating a new Ga dangling bond. On the basis of the analysis of the spin and charge populations, in particular, the mechanism of alkenes reacting with the H-GaN (0001) surface is suggested as an electrophilic addition reaction that is different from the similar reaction of Si surfaces, on which a radical addition reaction has been proposed. The reaction mechanism is well-interpreted by a twostage VB correlation diagram. By comparing charge, spin, and the stability of intermediates of alkenes bearing a different β-carbon substituent reacting with the H-GaN(0001) surface, the electrophilic addition mechanism is further demonstrated. The variation trend of the barrier height of these reactions for different terminal alkenes can be predicted by VB correlation theory and correlated with the trend of the relative electronwithdrawing or -donating capacity of the substituents. It is found
J. Phys. Chem. C, Vol. 112, No. 43, 2008 16937 that an alkene with a moderate electron-donating substituent would be favorable for the electrophilic reaction on the H-GaN (0001) surface. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grants no. 20673019), the Doctoral Degree Programme Foundation of Chinese Ministry of Education (Grants no. 20050386003), and the Important Special Foundation of Fujian Province (Grants no. 2005HE012-6). Supporting Information Available: The energy and structure comparison of the reaction process of C2H4 with the H-Si (111) surface between our results and ref 17; the scanned energy curve for intermediates; the main geometrical parameters of the initial, intermediate, transition, and final states along the MEP for the reaction of C5H11N with the H-GaN (0001) surface; the charge population for two models of the H-terminated GaN (0001) surface. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sirbuly, D. J.; Law, M.; Yan, H. Q.; Yang, P. D. J. Phys. Chem. B 2005, 109, 15190–15213. (2) Wolkow, R. A. Annu. ReV. Phys. Chem. 1999, 50, 413–441. (3) Buriak, J. M. Chem. ReV. 2002, 102, 1272–1308. (4) Johnson, H. M.; Nurmikko, A. V.; DenBaars, S. P. Phys. Today 2000, 53, 31. (5) Jain, S. C.; Willander, M.; Narayan, J.; Van Overstraeten, R. J. Appl. Phys. 2000, 87, 965. (6) Kang, B. S.; Ren, F.; Wang, L.; Lofton, C.; Tan, W. W.; Pearton, S. J.; Dabiran, A.; Osinsky, A.; Chow, P. P. Appl. Phys. Lett. 2005, 87, 023508. (7) Kim, H.; Colavita, P. E.; Metz, K. M.; Nichols, B. M.; Sun, B.; Uhlrich, J.; Wang, X.; Kuech, T. F.; Hamers, R. J. Langmuir 2006, 22, 8121–8126. (8) Bermudez, V. M. Langmuir 2003, 19, 6813–6819. (9) Bermudez, V. M. Surf. Sci. 2002, 499, 109–123. (10) Bermudez, V. M. Surf. Sci. 2002, 499, 124–134. (11) Bermudez, V. M. Surf. Sci. 2002, 519, 173–184. (12) Baur, B.; Steinhoff, G.; Hernando, J.; Purrucker, O.; Tanaka, M.; Nickel, B.; Stutzmann, M.; Eickhoff, M. Appl. Phys. Lett. 2005, 87, 263901. (13) Uhlricha, J.; Garciab, M.; Wolterb, S.; Brown, A. S.; Kuech, T. F. J. Cryst. Growth 2006, 11, 035. (14) Neogi, A.; Li, J.; Neogi, P. B.; Sarkar, A.; Morkoc, H. IEEE Electron. Lett. 2004, 40, 1605–1606. (15) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631–12632. (16) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145–3155. (17) Takeuchi, N.; Kanai, Y.; Selloni, A. J. Am. Chem. Soc. 2004, 126, 15890–15896. (18) Pei, Y.; Ma, J. J. Phys. Chem. C 2007, 111, 5486–5492. (19) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. 1998, 37, 3257–3260. (20) Delley, B. J. Chem. Phys. 2003, 113, 7756. DMol3 is available from Accelrys. (21) Delley, B. J. Chem. Phys. 1990, 92, 508. (22) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (23) Shaik, S. J. Am. Chem. Soc. 1981, 103, 3692. (24) Shaik, S.; Shurki, A. Angew. Chem., Int. Ed. 1999, 38, 586. (25) Shaik, S. Prog. Phys. Org. Chem. 1985, 15, 197. (26) Bermudez, V. M. Surf. Sci. 2004, 565, 89–102. (27) Van de Walle, C. G.; Neugebauer, J. Phys. ReV. Lett. 2002, 88, 066103. (28) Rapcewicz, K.; Nardelli, M. B.; Bernholc, J. Phys. ReV. B 1997, 56, R12725. (29) Timon, V.; Brand, S.; Clark, S. J.; Gibson, M. C.; Abram, R. A. Phys. ReV. B 2005, 72, 035327. (30) Shiraishi, K. J. Phys. Soc. Jpn. 1990, 59, 3455. (31) Northrup, J. E.; Neugebauer, J.; Feenstra, R. M.; Smith, A. R. Phys. ReV. B 2000, 61, 9932.
JP800325F
