Formation Mechanisms and Packing Structures of Alkoxyl and Alkyl

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Formation Mechanisms and Packing Structures of Alkoxyl and Alkyl Monolayers on Si(111): Theoretical Studies with Quantum Chemistry and Molecular Simulation Models Yong Pei, Jing Ma,* and Yuansheng Jiang Department of Chemistry, Institute of Theoretical and Computational Chemistry, Lab of Mesoscopic Materials Science, Nanjing University, Nanjing, 210093, People’s Republic of China Received January 23, 2003. In Final Form: June 12, 2003 Quantum chemistry and molecular mechanics calculations are performed to study formation mechanisms and packing structures of octadecanal and octadecene monolayers on Si(111). The radical chain mechanism is investigated by density functional theory with cluster models. Transition states of key steps involving abstracting a neighboring hydrogen atom from the surface are confirmed with the six-membered ring structures. Energy barriers for abstractions of the surface H to form new reactive surface Si radicals in the substitution of Si(111) via Si-O and Si-C linkages are 18.05 and 14.97 kcal/mol, respectively. Based on the radical chain mechanism, we investigate the linear and zigzag packing structures of alkoxyl chains on Si(111) with substitution percentages of 50%, 66.7%, and 75% using a series of two-dimensional repeating cells. By comparison of packing energies of octadecanoyl chains at different substitution percentages, 66.7% is predicted to be an optimal substitution percentage, which agrees with experimental observations. At this surface substitution, packing structures of the monolayer such as tilt angles and film thickness are well correlated with experimental data. The difference in packing structures between monolayers on Si(111) via Si-O and Si-C linkages is rationalized by their different van der Waals radii of surface linkage groups and tilt angles of chains.

1. Introduction The preparation of covalent attachment organic monolayers on silicon surfaces has attracted great interest in recent years1-3 because of their interesting properties and potential use in soft lithography,4 microelectromechanical systems (MEMS),5-7 and electronic and sensing devices. Traditionally, monolayers on silicon surfaces have been achieved via the use of siloxane chemistry with the presence of an insulating oxide interlayer, which may not be desirable in some applications.8 Many efforts have been devoted to exploring an alternative way to prepare densely packed monolayers through the direct attachment of various organic molecules to the Si surface without an oxide interlayer. Chidsey and co-workers9,10 demonstrated for the first time that densely packed alkyl monolayers can be formed on a silicon surface from the direct reaction of 1-alkene with hydrogen-terminated Si(111). Since then, there have been a number of reports on the direct preparation of organic monolayers on silicon surfaces via Si-C linkages (Scheme 1) by wet chemical methods such as thermally induced hydrosilylation,10,11 hydrosilylation involving a radical initiator,9,12b photochemical (UV12a,13,14 and white light15) hydrosilylation, electrochemical graft(1) Buriak, J. M. Chem. Commun. 1999, 1051-1060. (2) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudho¨lter, E. J. R. Adv. Mater. 2000, 12, 1457-1460. (3) Buriak, J. M. Chem. Rev. 2002, 102, 1271-1308. (4) Xia, Y.; Whitesides, G. M. Angew. Chem. 1998, 110, 568-594; Angew. Chem., Int. Ed. 1998, 37, 550-575. (5) Manoudian, R. Surf. Sci. Rep. 1998, 30, 207-269. (6) Jiely, J. D.; Houston, J. E.; Mulder, J. A.; Hsung, R. P.; Zhu, X. Y. Tribol. Lett. 1999, 7, 103-107. (7) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. J. Chem. Phys. 1999, 85, 213-221. (8) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (9) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631-12632. (10) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155.

ing,16 and hydrosilylation by a metal complex.17 New synthetic methods have also been developed for the formation of the densely packed monolayers on H-terminated18 or oxide19-21 surfaces via Si-O linkages (Scheme 1). Properties and applications of monolayer-modified surfaces are closely related to structures of monolayers. Various experimental techniques including FT-IR spectroscopy, ellipsometry, X-ray reflectivity, atomic force microscopy (AFM), and contact angle goniometry have been used to characterize these monolayers. Some bulk properties such as the thickness, tilt angles of chains, and wetting properties have been observed, which showed the densely packed, well-ordered monolayers packing on Si surfaces.10,11,19-21 Additional stability tests have demon(11) (a) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759-1768. (b) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1999, 15, 8288-8291. (12) (a) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688-5695. (b) Cicero, R. L.; Chidsey, C. E. D.; Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Langmuir 2002, 18, 305307. (13) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056-1058. Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. J. Appl. Phys. 1999, 85, 213-221. (14) Effenberger, F.; Go¨tz, G.; Bidingmainer, B.; Wezstein, M. Angew. Chem., Int. Ed. 1998, 37, 2462-2464. (15) (a) Stemart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. 1998, 37, 3257-3260. (b) Stewart, M. P.; Buriak, J. M. J. Am. Chem. Soc. 2001, 123, 7821-7830. (16) Gurtner, C.; Wun, A. W.; Sailor, M. Angew Chem., Int. Ed. 1999, 38, 1966-1968. (17) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831-3835. Fabre, B.; Lopinski, G. P.; Wayner, D. D. M. Chem. Commun. 2002, 23, 2904-2905. (18) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M.; Allongue, P. Langmuir 2000, 16, 7429-7434. (19) Zhu, X. Y.; Boiadjiev, V.; Mulder, J. A.; Hsung, R. P.; Major, R. C. Langmuir 2000, 16, 6766-6772. (20) Major, R. C.; Zhu, X. Y. Langmuir 2001, 17, 5576-5580. (21) Jun, Y.; Le, D.; Zhu, X. Y. Langmuir 2002, 18, 3415-3417.

10.1021/la0341198 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/30/2003

Theoretical Studies of Alkoxy and Alkyl Monolayers

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Scheme 1. Formation of Monolayers on the Hydrogen-Terminated Silicon Surfaces via (a) Si-C and (b) Si-O Linkages

Scheme 2. Mechanisms for Radical-Based Hydrosilylationsa

a After the surface activation by cleaving the Si-H bond, (a) the 1-alkene directly reacts with the surface radical to form a Si-C bond and (b) the aldehyde reacts with the surface radical to form a Si-O bond.

strated that these monolayers also have remarkable chemical stability.10,11,12,18,20 Properties of monolayers on Si(111) via Si-C linkages have been extensively studied. For example, the direct reaction between 1-octadecene and H-terminated Si(111) led to the formation of a monolayer with approximately 50% Si-H groups of the surface reacted with 1-octadecene.10,11a,14 Tilt angles of alkyl chains were around 2836° detected by IR spectroscopy10 and 28 ( 1° by X-ray reflectivity.11b However, for monolayers on a Si surface via Si-O linkages, the surface morphologies were found to be different from those of monolayers via Si-C linkages. Monolayers with Si-O linkages have been observed to have tilt angles of around 5-15°,19 which are smaller than those with the Si-C linkages. Using a two-step strategy, Zhu et al.20 showed a Si-OR (R ) -C18H37) monolayer on Si(111) with the highest surface coverage corresponding to a film thickness of 24 Å, from which a densely packed monolayer with alkoxyl chains almost normal to the surface is expected. Effenberger et al.14 proposed a photoactivation route to prepare monolayers with 1-alkene and aldehyde on H-terminated Si(111) surfaces. From their comparative study under the same reaction conditions, the surface coverage of the octadecanal monolayer (with linkages of Si-O) was always higher than that of the octadecene (with Si-C linkages) on Si(111). Even the surprisingly high surface coverage of 97% was observed for the octadecanal monolayer with Si-O linkages on

Si(111).14 These differences in tilt angles and surface coverages between the Si-C and Si-O linked monolayers are puzzling, because the molecular shapes of the octadecanal and the octadecene are similar except the difference in their terminal groups of -CdC and -CdO. Then, a question is naturally raised of why monolayers linked by Si-C and Si-O behave so differently. In this work, we aim to answer this question from a systematic study. Although a lot of experiments have been performed to characterize monolayers on Si surfaces with Si-C and Si-O linkages, there is little information available about their structures at the molecular or atomic level yet. Especially, the detailed reaction mechanism of the formation of a monolayer is not clear. Mechanisms of the surface radical chain reactions, as shown in Scheme 2a, have been frequently proposed in the formation of monolayers with Si-C linkages under radical conditions.2,3 When monolayers formed by the reaction of aldehydes (R-CHO) with the H-terminated silicon surface, the radical chain mechanism was also proposed with the key step involving the abstraction of a neighboring surface H atom to form a new Si radical on surface (Scheme 2b).18 In both radical chain mechanisms proposed for monolayer formation with Si-O and Si-C linkages, transition states with sixmembered ring structures were postulated.12a,18 In the present work, we perform a quantum chemistry calculation to study these radical chain mechanisms (Scheme 2)

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on the basis of cluster models. Employing the density functional theory (DFT), transition states with sixmembered rings are located for the key step of radical chain reactions. Active energies of those key steps in these mechanisms are thereby predicted. To further obtain detailed information on packing structures of octadecanal monolayers on Si surfaces, molecular modeling with molecular mechanics calculations is desired. These simulations have been successfully used to characterize packing structures of the selfassembled monolayers on Au(111).22,23 Recently, Sieval et al.,24 Zhang et al.,25 and Yuan et al.26 carried out molecular mechanics (MM) or molecular dynamics (MD) simulations to study packing structures of alkyl monolayers on Si(111). By investigating a variety of surface models with different substitution percentages, Sieval et al.24b predicted that the lowest energy packing pattern is zigzag-like with a surface substitution of 50% for the octadecene monolayer on Si(111). Further considering initial orientations of alkyl chains and temperature effects, Zhang et al.25 showed another optimal zigzag packing pattern with surface substitution at 50%. Both of their results showed that the zigzag arrangement of alkyl chains on the Si surface is more favorable than the linear arrangement of the chains. Those simulation results were in good agreement with the available experimental data. To the best of our knowledge, however, there is no theoretical simulation on monolayers attached to Si surfaces via Si-O linkages to date. Here, we carry out molecular mechanics optimizations on a series of surface patterns for the Si-O linked monolayers. We focus our studies on substitution patterns with high surface substitution percentages (50%, 66.7%, 75%, and 100%) for the octadecanal monolayer on Si(111) to correlate with experimental facts.14 By investigating these surface patterns, it is possible to obtain the optimal packing structure of the monolayer on the Si surface via Si-O linkages and its optimal and possibly maximum substitution percentage. Consequently, differences in packing structures of monolayers between Si-O and Si-C linkages can be shown. The organization of this article is as follows. At first, computational details of quantum chemical calculations with the cluster model and molecular simulations by molecular mechanics and molecular dynamics on a large surface with the two-dimension period boundary condition are briefly introduced. We also implement extensive validations on force fields in molecular mechanics calculations by available experimental data and quantum chemistry models. Then the proposed surface radical chain reaction mechanisms for formation of alkyl and alkoxyl monolayers with Si-O and Si-C linkages are investigated by the quantum chemistry model. The optimal packing structure of the monolayer on the Si surface via Si-O linkages and its maximum substitution percentage are explored by molecular modeling. Finally, the role of linkages with Si-O and Si-C in packing structures of monolayers is discussed. (22) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389-9401. (23) (a) Li, T. W.; Chao, I.; Tao, Y. T. J. Phys. Chem. B 1998, 102, 2935-2946. (b) Pertsin, A. J.; Grunze, M. Langmuir 1994, 10, 36683674. (c) Ulman, A. Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 11471152. (24) (a) Sieval, A. B.; van der Hout, B.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 2000, 16, 2987-2990. (b) Sieval, A. B.; van der Hout, B.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 2001, 17, 2172-2181. (25) Zhang, L.; Wesley, K.; Jiang, S. Langmuir 2001, 17, 6275-6281. (26) Yuan, S.; Cai, Z.; Jiang, Y. New J. Chem. 2003, 27, 626-633.

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Figure 1. Dissociation energy of a Si-H bond calculated by DFT/B3LYP.

2. Computational Methods 2.1. Quantum Chemical Calculations. Surface Cluster Model. A Si13H22 cluster is adopted as a model of the Si(111) surface in quantum chemical calculations (cf. Figure 1). This Si13H22 cluster is cut from a bulk H-Si(111) surface with the first two layers of silicon atoms represented. It consequently has seven and six Si atoms in the first and second layers, respectively. The seven Si atoms in the first layer are vertically capped with seven hydrogen atoms, which can be replaced by alkoxyl and alkyl chains or abstracted by the radical intermediate to form a new Si radical on the surface. The dangling bonds, resulted from the truncation of the bulk Si-Si bonds to form this cluster, are saturated with hydrogen atoms to maintain the sp3 hybridization of the terminated Si atoms. Instead of the octadecanal (C18H36O) and 1-octadecene (C18H36), the butanal (C4H8O) and 1-butene (C4H8) molecules are selected as short-chain models in our quantum chemical calculations on mechanisms of radical chain reactions in the formation of alkoxyl and alkyl monolayers on H-Si(111). The calculated results from these models can be reasonably extended to understand formation mechanisms of octadecanal and octadecene monolayers on Si(111), because the studied radical chain reaction mainly involves the reactions between the terminal unsaturated group -CdC or -CdO and the H-terminated Si surface. Computational Details. All the quantum chemical (QC) calculations are carried out on the SGI 3800 and SGI 3000 workstations of Nanjing University with the Gaussian98 package.27 DFT is employed with the three-parameter hybrid exchange functional of the Becker and the Lee, Yang and Parr correlation functional (B3LYP). For Si (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998.


Langmuir, Vol. 19, No. 18, 2003 7655 Table 1. Bond Lengths, Bond Angles, and Vibration Frequencies of the Symmetrical Stretching of Si-C and Si-O Bonds in Model Molecules A and B, Calculated by the DFT/B3LYP at Different Basis Sets and MM with the PCFF and UFF optimized geometries

Figure 2. The optimized structures of cluster models A (with Si-C linkage) and B (with Si-O linkage) by DFT/B3LYP.

atoms, the relativistic effective core potential (ECP) of LANL2DZ is employed in all B3LYP calculations. For other atoms such as C, O, and H, the all-electron basis sets with polarization functions, 3-21g(d), 6-31g(d), and 6-311g(d), are used to test the influence of basis sets. All reactants, products, and transition states of reactions are determined through the partial optimizations with all Si atoms fixed28 by the unrestricted DFT method, since the radicals are involved in the reactions. Frequency calculation is then implemented at the optimized geometry of each stationary point. Transition states are confirmed by having one and only one imaginary frequency. All energies reported in this work take the zero-point energy (ZPE) corrections into account. 2.2. MM and MD Simulations. All MM and MD calculations are performed on the SGI Origin200 workstation with Cerius2 (version 3.5) from Molecular Simulation Inc.29 Optimizations by molecular mechanics are completed with “high convergence criteria” using the “smart minimizer” routine in the “minimizer” module. Molecular dynamic calculations are carried out in an NVT ensemble. The polymer consistent force field (PCFF)30 and universal force field (UFF)31 are applied in our calculations. 2.2.1. Validations of Theoretical Calculations. To validate our force field calculations, we perform both DFT and MM calculations on model molecules A and B, as shown in Figure 2. The optimized geometries of these two molecules by DFT, PCFF, and UFF are listed in Table 1. From Table 1, we can see that PCFF and UFF provide good predictions of geometries for the cluster model A, agreeing with the DFT/B3LYP and available experimental results. For the cluster model B, geometries obtained by UFF somewhat stray away from the DFT/B3LYP and PCFF results, especially in the description of the Si-O-C angle: the UFF gives a result of 110.7°, which is about 12° less than the DFT/B3LYP result. Furthermore, DFT and PCFF calculations show an increase in the bending angle by 5-6° going from model A with the Si-C linkage to model B with the Si-O linkage, yielding a smaller tilt angle of the short alkoxyl chain (B) than that of the alkyl chain (A) (cf. Figure 2). This difference in tilt angles still exists in packing structures of long alkoxyl and alkyl chains on Si(111) as addressed in the next section, which (28) The Si-Si bond is fixed with the length of 2.346 Å corresponding to the bond length in the bulk Si crystal. To obtain a local minimum, the fixed Si atoms in the products 4 and 8 are relaxed for the further optimizations. (29) Cerius2, version 3.5; Molecular Simulation Inc., 1997. (30) (a) Sun, H. Macromolecules 1995, 28, 701-712. (b) Sun, H.; Mumby, S. J.; Maple, J. R.; Hagler, A. T. J. Phys. Chem. 1995, 99, 5873-5882. (c) Hill, J. R.; Sauer, J. J. Phys. Chem. 1994, 98, 12381244. (d) Hwang, M. J.; Stochfisch, T. P.; Hagler, A. T. J. Am. Chem. Soc. 1994, 116, 2515-2525. (31) (a) Rappe´, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024-10035. (b) Castongauy, L. A.; Rappe´, A. K. J. Am. Chem. Soc. 1992, 114, 5832-5842. (c) Rappe´, A. K.; Colwell, K. S.; Casewit, C. J. Inorg. Chem. 1993, 32, 3438-3450.

Si-C (Å) ∠Si-C-C (deg) Si-C sym str (cm-1)c tilt angle (deg) Si-O (Å) ∠Si-O-C (deg) Si-O sym str (cm-1)c tilt angle (deg) a

molecular mechanics

DFT/B3LYP

3-21g(d) 6-31g(d) 6-311g(d) PCFF UFF

expt

1.93 115.8 716.3

Model Aa 1.92 1.92 116.9 116.9 706.5 707.3

1.92 1.88 1.87b 115.0 112.4 600d

34.4

36.1

34.7

1.72 124.7 756.9

Model Ba 1.71 1.73 122.5 123.4 751.6 734.0

1.67 1.74 128.2 110.7

5.3

7.5

6.7

36.6

6.1

38.4

28-36°

800e 29.3

5-15°

b

Model molecules A and B are shown in Figure 2. Reference 10. c sym ) symmetry; str ) stretch. d Reference 33. e Reference 34.

is consistent with macroscopic measurements19 that monolayers with Si-O linkages have smaller tilt angles than those with Si-C linkages. However, in comparison with DFT and PCFF results, the UFF produces a larger tilt angle of 29.3° for model B. Thus, PCFF is employed to study structures of monolayers on the Si surface with Si-O linkages. The stretching frequencies of Si-C and Si-O bonds are also computed by DFT with the B3LYP functional at various levels of basis sets. The Si-C stretching frequency obtained by DFT/6-31G(d) is 706.5 cm-1, which falls into the range of the calculated values between 671 and 717 cm-1 for a series of smaller model molecules at the MP2/ 6-311++G(d,p) level.32 Despite the difficulty in the assignment of experimental FT-IR spectra with certainty, our result of the Si-C stretching mode is roughly corresponding to the 602 cm-1 band, which has been assigned to Si-C vibrations in some experiments.33 A good agreement of the Si-O stretching frequency between the DFT/6-31G(d) result (751 cm-1) and the experimental spectra (800 cm-1)34 is also obtained. In addition, our DFT calculations with different basis sets yield almost identical results, showing the little influence of basis sets in the optimized geometries and vibration frequencies. Therefore, the basis set of 6-31G(d) is employed in our DFT calculations to study the radical chain reaction of monolayer formation on Si(111). 2.2.2. Construction of Starting Structures. Surface substitution and substitution pattern are two important factors to be considered in the construction of initial monolayer structures on a surface. Molecular substitution determines the percentage of the terminated hydrogen atoms on Si(111) being replaced by alkoxyl chains in searching for an optimal packing structure. At a certain surface coverage, the substitution pattern determines which terminated hydrogen atoms are replaced by chains. Given a certain substitution percentage and substitution pattern, a corresponding packing structure of the monolayer on the surface can be built. We build the starting structures of surface models for MM studies by a unit cell, which contains four silicon (32) Bateman, J. E.; Eagling, R. D.; Horrocks, B. R.; Houlton, A. J. Phys. Chem. B 2000, 104, 5557-5565. (33) Bean, A. R.; Newman, R. C. J. Phys. Chem. Solids 1971, 32, 1211. (34) Warnttjes, M.; Viellard, C.; Ozanam, F.; Chazalviel, J. N. J. Electrochem. Soc. 1995, 142, 4138.

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atoms that represent the atoms in the first four layers of the Si surface. This structure is obtained by cleaving the Si crystal structure along the (111) plane. The top Si atom is initially terminated by a H atom. It has dimensions of 3.846 Å × 3.846 Å. This unit cell is then copied in one or two dimensions by as many times as necessary to generate the required larger simulation cells.24,25 With these larger cells, the desired surface substitution patterns are then obtained by replacing the surface hydrogen atoms at certain positions with the preoptimized all-trans octadecanoyl (C18H36O-) chains. The octadecanoyl chains in initial structures are normal to the surface. 2.2.3. Molecular Dynamics Simulation. The canonical ensemble (NVT) molecular dynamics is performed based on the optimal packing structure from the MM optimizations. The time step in the MD simulation is 1 fs. The simulation temperature is controlled at 298 K with the Nose´-Hoover35 method. 2.2.4. Characterization of Packing Structures. Initial structures are optimized with two bottom Si layers fixed. The two-dimension period boundary condition (PBC) is applied in simulations. The resulting structures and MD trajectory are characterized by a variety of properties such as the monolayer thickness, molecular and system tilt angle,36 and gauche/trans distribution.37 The average packing energy per alkoxyl chain (Eave) is determined by removing the substrate Si atoms and surface terminated H atoms. By comparing the Eave of different surface substitution patterns, we can then pick out the optimal substitution pattern for the octadecanal monolayer on Si(111). 3. Results and Discussion 3.1. Radical Chain Mechanism. Up to now, most studies on formation mechanisms of monolayers on Si surfaces focus on reactions involving the olefin addition to the clean Si(100)-(2 × 1) prepared in ultrahigh vacuum (UHV).38,39 Using quantum chemical calculations, Kang et al.40 investigated the self-directed growth mechanism of molecular nanowires on the Si(100)-(2 × 1) surface from the molecular styrene and propylene, providing useful information to experimentalists. For hydrocarbon chain monolayers forming on H-Si(111) via Si-C or Si-O linkages, the radical chain mechanism (Scheme 2) has been proposed by many authors.3 Recently, Cicero and co-workers12b reported experimental evidence for a surface radical chain reaction of the styrene with H-Si(111). The reaction is initiated at the isolated dangling bond on the H-Si(111) surface. They found the key radical chain propagation step of hydrogen abstraction was able to readily occur under radical conditions. Here we use cluster models described in section 2.1 to study radical chain mechanisms for monolayer formation on H-Si(111) with Si-C and Si-O linkages. According to the proposed radical (35) Nose´, S. Mol. Phys. 1984, 52, 255-268. Hoover, W. G. Phys. Rev. A 1985, 31, 1695-1697. (36) The molecular tilt angle is defined as the angle between the surface normal vector and the vector from the oxygen atom to the last carbon atom in a molecular chain. The system tilt angle used here is the average of the molecular tilt angles. (37) The gauche defects in molecular chains were determined by analyzing the torsion angles of all alkoxyl chains. If a torsion angle in the alkoxyl chains differs by more than (10° from that of the all-trans conformation ((180°), it is defined as a gauche defect. The gauche ratio is a ratio of gauche defects to the total number of torsion angles. (38) Liu, H.; Hamers, R. J. J. Am. Chem. Soc. 1997, 119, 75937594. (39) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D. M.; Wolkow, R. A. J. Am. Chem. Soc. 2000, 122, 3548-3549. (40) Kang, K. J.; Musgrave, C. B. J. Chem. Phys. 2002, 16, 99079913.

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chain propagation mechanism for the monolayer formation on Si(111), the initial step is the activation of the surface (Figure 1). The surface Si-H bond is first dissociated by UV illumination12a or by a scanning tunneling microscope tip12b to form a dangling bond on the surface. The bond strength of the Si-H bond on the Si(111) surface is calculated to be 80.42 kcal/mol by the present cluster model, agreeing well with the experimental observation of 79-84 kcal/mol.41 Reactions of the H-Si(111) cluster with the selected model molecules, 1-butene (C4H8) (and butanal (C4H8O)) with terminal unsaturated groups of -CdC (and -CdO), are investigated by our DFT calculations with all reactants, 1 (and 5), reaction intermediates, 2 (and 6), transition states, 3 (and 7), and products, 4 (and 8), shown in Figure 3. The corresponding reaction paths a and b for attachment of 1-butene and butanal on H-Si(111) clusters are depicted in Figure 4. The creation of a surface dangling bond permits the addition of the terminal unsaturated bond CdC (or CdO) of 1-butene (or butanal) to the surface Si radical to form a Si-C (or Si-O) bond and an intermediate carbon radical, 2 (or 6), as shown in Figure 3. Adsorption energies are predicted to be exothermic by 3.26 kcal/mol (in path a) and 3.11 kcal/mol (in path b) for the surface bond formation of Si-C and Si-O, respectively. Subsequently, a neighboring surface H atom is abstracted by the resulting carbon radical, 2 (or 6), to generate a new dangling bond on the surface, which can then propagate the reaction as depicted in Scheme 2. This is the key step in the surface radical chain mechanism. According to our calculations, transition states 3 and 7 are obtained with the barriers of hydrogen abstraction from the neighboring site being 14.97 and 18.05 kcal/mol for 1-alkene and alkanal, respectively (cf. Figure 4). Our prediction on the energy barrier of the formation of the Si-C linkage is close to that (around 15.50 kcal/mol) reported by Kang et al.40 for a propylene radical abstracting a H atom from the Si(100)-(2 × 1) surface. Until now, there is no theoretical study reported for the formation of a monolayer with Si-O linkages. The energy barrier of regenerating a new Si radical by abstracting a neighboring terminated hydrogen with the Si-O linkage in path b is comparable to that of the Si-C linkage in path a, implying that model molecules 1-C4H8 and C4H8O with different terminal unsaturated groups have similar reactivities in such radical chain reactions. Their relatively low energy barriers for abstractions of surface H to form new reactive silicon radicals in the substitution of Si(111) with Si-O and Si-C linkages indicate that the proposed surface radical chain mechanism is possible in the monolayer formation on Si(111). Transition states 3 and 7 exhibit six-membered ring structures, which are consistent with the previous postulation from the steric factor.12a Taking the transition state 7 as an example, we find that in order to form a sterically favored six-membered ring structure, both Si-O and O-C bonds are slightly lengthened by 0.02 Å, the carbon radical center further approaches the Si surface with the nonbonded C‚‚‚H distance of 1.651 Å, and the neighboring Si-H bond is nearly broken with a big separation of 1.737 Å (cf. Figure 3). Subsequently, the reaction path b in Figure 4 leads to the product 8, in which the silicon radical is formed with the hydrogen atom migrating to the alkoxyl chain and the Si-O bond is shortened to 1.710 Å. The similar structure with the sixmembered ring of transition state 3 appears in the radical chain reaction with Si-C attachment as depicted in Figure (41) Laarhoven, L. J. J.; Mulder, P.; Wayner, D. D. M. Acc. Chem. Res. 1999, 32, 342-349.


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Figure 4. Reaction paths a and b for the key radical chain propagation steps of monolayer formation on the silicon surface with Si-C and Si-O linkages, respectively.

Figure 3. Intermediates and transition states along reaction paths.

3. The high strengths of Si-O and Si-C bonds make reactions essentially irreversible. The formation of the six-membered ring transition state structure may explain the observed more favorable hydrogen-atom abstraction than carbon-carbon bond formation in the solution conditions.12a From the above discussions, we can find that key steps involving the abstraction of a neighbor H atom to form a new surface Si radical in radical chain mechanisms for formation of alkoxyl and alkyl monolayers on H-Si(111) are kinetically possible and mechanisms for these two kinds of monolayers are similar. However, some geometrical differences in the surface-attached chains between Si-O and Si-C linkages can still be noticed (cf. Figure 3). An obvious difference is that the tilt angle of the alkoxyl chain is smaller than that of the alkyl chain, which is mainly caused by the larger Si-O-C angle (122.9°) for the alkoxyl chain on the surface. To gain a

comprehensive understanding of the octadecanal packing structures on Si(111) and the difference in the packing structures between the alkoxyl and alkyl monolayers, we resort to molecular mechanics simulation on more realistic surface models. 3.2. Packing Structures of the Alkoxyl Chain on Si(111). Since there was no predefined chemical adsorption structure of the octadecanal monolayer on Si(111), we try to explore most possible surface substitution patterns by our MM calculations analogous to those done on octadecene monolayers.24,25 Through comparing the average packing energy per alkoxyl chain (Eave) of various surface patterns, we can then gain an insight into the optimal packing structure of monolayer formation with octadecanal on Si(111) and its possibly maximum surface substitution. 3.2.1. Simulations with One-Dimensional Cells. To have a primary knowledge of the relation between the surface substitution percentage and the packing energy per alkoxyl chain, different one-dimensional cells are designed by repeating the unit cell described in section 2.2.2 in one direction. These cells, in which some surface H atoms are substituted by octadecanoyl (C18H36O-) chains, represent the surface substitution percentages varying from 20% to 100% (Figure 5a). Figure 5b shows the average packing energy per chain, Eave, calculated by both the PCFF and UFF as a function of substitution percentages. PCFF and UFF give roughly consistent results that the lowest packing energy per alkoxyl chain (Eave) corresponds to high substitution percentages of 75% and 66.7%, respectively. These results are in agreement with the experimental surface coverage of about 60%70% for the octadecanal monolayer formation on H-Si(111) reported by Effenberger et al.14 Therefore, in our following discussions, we focus on surfaces with high substitution percentages of 50%, 66.7%, 75%, and 100%. 100% Substitution. To simplify our discussions, we restrict our simulations to the unreconstructed Si(111) surfaces, where the unit area per surface Si atom is about 12.8 Å2/atom. This unit area is much smaller than the unit area per alkyl chain in crystalline n-alkanes of 18.518.6 Å2/chain.42 If the Si(111) surface has 100% substitution of C18H36O- chains, the crowded alkoxyl chains on the surface will strongly interpenetrate into their van der Waals radii. The resulting surface is thus thermodynamically unstable in such a situation. Our MM calculation (42) Craievich, A. F.; Denicolo, I.; Doucet, J. Phys. Rev. B 1984, 30, 4782-4787.

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Figure 5. Simulations with (a) one-dimensional cells with various substitution percentages and simulation results of (b) average packing energy per chain, Eave, at different substitution percentages calculated with UFF and PCFF. The letters O and H in the cells represent surface Si atoms terminated by C18H36O- and H, respectively.

also shows an extremely high packing energy in this case as shown in Figure 5b, indicating the unrealistic 100% surface substitution. 3.2.2. Simulations with Two-Dimensional Cells. Substitution Patterns. To find the optimal packing pattern of the octadecanal monolayer on Si(111), two kinds of surface substitution patterns at different surface coverages are considered in the present study: one is the linear pattern, and another is the regular zigzag one. These patterns are two extreme situations with highly ordered alignments for octadecanyl (C18H36O-) on Si(111). Real packing patterns may mediate between these two extremes. Such selection scheme of packing patterns was applied by Sieval et al.24 and Zhang et al.25 to investigate packing patterns of the octadecene monolayer on Si(111). Here the same strategy is adopted, since formations of octadecanal and octadecene monolayers on the Si surface are demonstrated by our DFT calculations to follow similar radical chain reactions. As seen in Figure 6, a series of surface patterns with substitution percentages of 50%, 66.7%, and 75% are investigated in this study. Among them, unit cells of 50-a, 50-b, and 50-j (with 50% substitution), 67-a (with 66.7% substitution), and 75-a (with 75% substitution) are of linear patterns, and 50c-50-i, 67-b and 67-c, and 75-b are zigzag ones. Then a series of two-dimensional cells are designed by extending the unit cell (described in section 2.2.2) in two dimensions, and we substitute the surface H with C18H36O- chains at certain positions. Tables 2 and 3 collect all simulation results of packing energies and structures for various

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substitution patterns and cell sizes with 50%, 66.7%, and 75% substitutions. 50% Substitution. For the surface with 50% substitution, Eave values of patterns 50-a-50-j calculated by PCFF and UFF are listed in Table 2. By comparing packing energies and structures of these patterns, the linear pattern 50-a and zigzag patterns 50-c and 50-g with relatively lower packing energies and neat arrangements of alkoxyl chains are selected for a more comprehensive study on packing structures of the C18H36O- monolayer on Si(111) with 50% substitution (Table 3). Optimal orientations of chains for patterns 50-a, 50-c, and 50-g are denoted by arrows in Figure 6a. As seen from Tables 2 and 3, both PCFF and UFF produce the same prediction that the zigzag pattern 50-c has the lowest packing energy (-66.24 kcal/mol by PCFF; -27.64 kcal/mol by UFF). To save our computational time, only PCFF is used in our subsequent calculations. 66.7% Substitution. For the linear pattern 67-a and zigzag patterns 67-b and 67-c (cf. Figure 6b), the optimized packing structures are energetically more favorable with lower Eave than those of 50% and 75% substitutions either in the linear or zigzag patterns as shown in Table 3. The packing pattern 67-c has the lowest packing energy Eave of -74.10 kcal/mol. From Table 3, one can also find that packing structures of 67-c (tilt angles of around 19° and film thickness of 23.5 Å) at this substitution are in better agreement with experimental results (tilt angles of 5-15° and film thickness of 24 Å)19,20 than those of pattern 50-c with 50% substitution (the tilt angles and the thickness of the monolayer are around 40° and 19 Å, respectively). Moreover, in all patterns, C18H36O- chains on the surface are mainly in trans conformations with negligible gauche ratios and well-ordered packing structures on the Si(111) surface, which agrees well with experimental observations.14,19,20 75% Substitution. There is little theoretical study on the monolayer with such a high substitution percentage of 75%. With this high substitution percentage, the surface is so crowded that almost all alkoxyl chains on the surface interpenetrate into the van der Waals radii of their neighboring chains. The packing energy of the zigzag pattern 75-b (-71.81 kcal/mol) is much lower than that of 50% substitution and slightly higher than that of 66.7% substitution. But significant deformations of surface Si-O bonds are found in its packing structures. So Si-O bonds are no longer nearly perpendicular to the surface but have large tilt angles of more than 20°. The strong interchain van der Waals interactions are then anticipated in the monolayer. Moreover, these “ill” conformations imply a thermodynamically unstable monolayer on the surface with 75% substitution. Thus, the surface with such a high substitution percentage may be less possible for the monolayer formation with octadecanal on Si(111). To summarize, the optimal packing structures of the monolayer on Si(111) with the substitution percentage of 66.7% are predicted, which is in agreement with experimental results (60-70%).14 At this substitution, the zigzag pattern 67-c is suggested to be the optimal surface pattern with the lowest Eave among all surface patterns (Table 3). It is worthwhile to compare the optimal surface patterns between the monolayers on Si(111) with Si-O and Si-C linkages. As mentioned in the Introduction, the regular zigzag pattern with 50% substitution is also found to be an optimal substitution pattern.24,25 In Figure 7, we display the optimal packing structure of 67-c[(3 × 2)] with 72 chains. It is interesting to observe that the packing structure of the pattern 67-c[(3 × 2)] can be taken as a composition of six independent units of 67-c[(1 × 1)] (which


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Figure 6. Simulation cells of different substitution patterns with 50%, 66.7%, and 75% surface substitution percentages for octadecanal monolayers on Si(111). The arrows in patterns 50-a, 50-c, and 50-g represent optimal orientations of alkoxyl chains. Table 2. Average Packing Energies per Alkoxyl Chain, Eave, for Different Patterns at 50% Substitution from PCFF and UFF Calculations Eave (kcal/mol)

Eave (kcal/mol)

pattern

PCFF

UFF

pattern

PCFF

UFF

50-a 50-b 50-c 50-d 50-e

-60.39 -56.71 -66.24 -64.96 -64.58

-23.70 -23.52 -27.64 -24.45 -21.68

50-f 50-g 50-h 50-i 50-j

-62.46 -65.42 -51.43 -49.51 -62.98

-23.10 -26.63 -14.63 -15.41 -22.81

is labeled by the deeper color in Figure 7b) with the little size-dependence. Packing energy as well as properties such as tilt angle and film thickness of 67-c[(3 × 2)] are identical to those of the 67-c[(1 × 1)] unit. Such a situation also happens in other substitution patterns (such as 50-a, 50g, 67-a, 67-b, 75-a, and 75-b) provided that sufficient local minima are surveyed by various initial configurations and the lowest packing structure is found. This size-

independence of the packing energy and structure has been demonstrated by Zhang et al.25 3.2.3. Molecular Dynamics Simulation on the Optimal Substitution Pattern. Temperature has effects on packing structures of monolayers.25,43 Based on the lowest energy pattern 67-c with a system size of (2 × 2), we perform 100 ps molecular dynamics simulations at 298 K. The trajectory is collected after equilibration (>20 ps). The time average of the trajectory gives rise to the tilt angle of 16.1° and film thickness of 23.65 Å, which are correlated well with experimental results of 5-15° and 24 Å, respectively.19,20 3.3. Comparison of Monolayers on Si(111) with Si-O and Si-C Linkages. As discussed above, both our MM simulations and experiments found that the monolayer on Si(111) with Si-O linkages prefers a higher surface coverage of 66.7% than that of 50-55% for the (43) Hautman, J.; Klein, M. L. J. Chem. Phys. 1990, 93, 7483-7492.

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Table 3. Properties of Alkoxyl Monolayers with Various Surface Substitution Patterns at 50%, 66.7%, and 75% Coverages bond packing tilt gauche angle energy angle thickness ratio Si-O-C (kcal/mol) (deg) (Å) (%) (deg)

pattern

size

50-a

(1× 1) (2× 2) (3× 3) (4× 4) (5× 5) (6× 6) (1× 1) (2× 2) (3× 3) (4× 4) (5× 5) (1× 1) (2× 2) (3× 3) (4× 4)

50% Substitution -60.39 18.6 23.33 -60.37 18.6 23.33 -66.12 15.2 23.61 -63.08 21.7 22.74 -63.26 18.2 23.20 -66.12 15.2 23.61 -66.24 40.9 18.91 -66.24 40.9 18.90 -67.33 40.5 18.94 -67.46 41.1 18.78 -67.17 40.8 18.84 -65.42 39.3 19.33 -66.58 39.9 19.57 -66.74 39.3 19.29 -66.65 39.6 19.41

0.031 0.031 0.045 0.086 0.076 0.045 0.083 0.083 0.081 0.068 0.078 0.088 0.084 0.083 0.082

130.8 130.8 129.4 128.4 128.2 129.4 126.3 126.3 126.3 126.7 126.5 127.6 127.1 127.5 127.5

(1× 1) (1× 2) (2× 2) (3× 3) (4× 4) (1× 1) (2× 1) (2× 2) (2× 3) (3× 3) (4× 4) (1× 1) (1× 2) (2× 2) (2× 3) (3× 3)

66.7% Substitution -61.67 15.2 23.82 -69.34 18.9 23.13 -69.34 19.0 23.11 -67.79 8.4 24.11 -69.34 19.0 23.13 -52.56 3.4 24.46 -71.63 11.9 23.81 -71.63 11.9 23.81 -71.66 11.9 23.81 -65.28 12.0 23.78 -71.74 12.1 23.79 -74.10 19.5 23.16 -74.10 19.4 23.19 -74.10 19.3 23.20 -74.10 19.3 23.20 -74.10 19.4 23.19

0.020 0.073 0.073 0.086 0.073 0.052 0.065 0.065 0.065 0.081 0.063 0.031 0.031 0.031 0.030 0.031

128.4 129.7 129.7 129.4 129.7 129.6 129.6 129.6 129.6 129.5 129.5 129.1 129.1 129.1 129.1 129.1

(1× 1) (1× 2) (2× 2) (2× 3) (1× 1) (1× 2) (2× 2)

75% Substitution -64.56 19.5 23.24 -64.59 19.3 23.26 -64.59 19.3 23.26 -64.59 19.3 23.26 -71.81 6.0 24.31 -71.81 6.0 24.31 -71.81 6.0 24.31

0.042 0.042 0.042 0.042 0.021 0.021 0.021

128.6 128.6 128.6 128.6 129.8 129.8 129.8

50-c

50-g

66-a

66-b

66-c

75-a

75-b

Si-C linked one. This can be understood by differences in van der Waals radii of linkage groups and tilt angles of chains. Difference in van der Waals Radii. For formation of monolayers on Si(111) with octadecene and octadecanal, both Si-C and Si-O linkages are strong covalent chemical bonds, which restrict motions of surface linkage groups -CH2- (or -O-) in the surface-bonded alkyl (or alkoxyl) chains. When the surface substitution arrives at 66.7%, short interchain distances result in much stronger van der Waals interactions of alkyl (or alkoxyl) chains. As demonstrated by Sieval et al.,24b an increment of the surface substitution from 50% to 66.7% leads to an increase in unfavorable conformations of alkyl chains, which cancels the increased van der Waals interactions. This increment in the number of energetically unfavorable conformations is mainly reflected by significant deformations of surface Si-C bonds (which have large tilt angles of >20° with respect to the surface normal).24b But in the octadecanal monolayer, similar deformations of Si-O bonds are not found when the surface substitution arrives at 66.7% (Figure 7). This is partially ascribed to the relatively smaller van der Waals radii for the surface linkage group, -O-, of the alkoxyl chain than the -CH2of the alkyl chain. When monolayers achieve high sub-

Figure 7. The (a) side and (b) top views of the packing structure of the extended system with 72 chains for pattern 67-c with 66.7% substitution. The unit structure of 67-c[(1 × 1)] is labeled in (b) by deeper color.

stitution, the surface linkage groups -CH2- of alkyl chains would strongly interpenetrate into the van der Waals surface of their neighboring -CH2- groups, inducing strong repulsions among terminal -CH2- groups in alkyl chains. Deformations of the surface Si-C bond may inevitably appear to reduce these repulsions. However, for alkoxyl chains attached to the Si(111) with -Ogroups, repulsions among the neighboring headgroups are weaker than those among -CH2- groups of alkyl chains due to the relatively smaller van der Waals radii for -Ogroups. On the other hand, strong repulsions among alkoxyl chains are partially canceled by the distortion of torsion angles at the bottom of alkoxyl chains. It can be observed in both 50% and 66.7% substitutions that gauche defects are mainly concentrated at the bottom of the alkoxyl chains. Thus, in contrast with the unfavorable 66.7% substitution of the alkyl-modified Si(111) surface, the octadecanal on Si(111) can form a thermodynamically stable monolayer with a coverage of 66.7%. Moreover, alkoxyl chains on the surface at this substitution percentage are packing so densely that there is almost no space between alkoxyl chains and the further attachment of alkoxyl chains on the surface is very difficult, implying that the percentage of 66.7% may be also the maximal substitution for the octadecanal monolayer on Si(111). In fact, the coverage of 66.7-69% was deduced to be a limit of maximum surface substitution for a monolayer with hydrocarbon chains on Si(111) from simulations of the octadecene monolayer by Sieval et al.24b Difference in Tilt Angles. Difference in tilt angles between monolayers with Si-O and Si-C linkages may also give a clue to understand the optimal 66.7% substitution. In previous studies of octadecene monolayers, the


relatively large tilt angle of around 25-39° was observed for the monolayer with 50% substitution both in linear and zigzag patterns.24b,25 For the alkoxyl monolayers of linear patterns, for example, 50-a, the smaller tilt angle of around 15° is found. The larger bond angle of Si-O-C (about 129°) in the octadecanal monolayer leads to smaller tilt angles of the alkoxyl chains than those of octadecene chains with a smaller bond angle of Si-C-C (about 113°). These differences can also be seen in DFT/B3LYP results with model molecules A and B (Table 1). When drawing the van der Waals surface of substitution chains, we find some defects existing in the alkoxyl monolayer. In another word, some holes exist in the monolayer, in which the surface H atom is exposed fully. According to the surface radical chain mechanism, the first step is the surface activation of the Si-H bond. So in such a surface, defects in the alkoxyl monolayer suggest the exposed sites on the H-Si(111) surface remain reactive to some extent, which may be the driving force for the octadecanal monolayer on the Si(111) surface to go from 50% coverage to a higher coverage. But for the octadecene monolayer, the larger tilt angles of the alkyl chains make the unsubstituted H atoms covered by chains, so that the further attachment of the octadecene molecule on the surface is more difficult. This in turn accounts for the higher coverage of the alkoxyl monolayer on the Si(111) surface than that of the alkyl monolayer. 4. Conclusions Quantum mechanics and molecular mechanics calculations are performed to study radical chain mechanisms and packing structures of octadecanal and octadecene monolayers on Si(111). The proposed radical chain mechanisms for formation of alkyl and alkoxyl monolayers on Si(111) are investigated

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by our DFT/B3LYP calculations with cluster models. The DFT energy barriers for abstractions of surface H to form new reactive Si radicals in substitutions of the H-Si(111) with Si-O and Si-C linkages are 18.05 and 14.97 kcal/ mol, respectively, indicating that the proposed surface radical chain mechanisms are possible in monolayer formations on Si(111). The DFT calculations also show that transition states of key steps in radical chain mechanisms have six-membered ring structures, which are consistent with the previous postulation from the steric factor. With MM optimizations, packing structures of octadecanal monolayers on Si(111) with different percentages have been investigated with a series of two-dimensional surface models. Results from molecular mechanics calculations show that the optimal packing structure occurs at the surface with the 66.7% substitution. At this surface substitution percentage, simulation results are in agreement with experimental measurements. The optimal packing pattern of the octadecanal monolayer is predicted to be the pattern 67-c, which has a zigzag arrangement of alkoxyl chains on the Si(111). MD simulations are then performed on this optimal packing structure, which describes a monolayer with tilt angles of 16.1° and the film thickness of 23.65 Å. Finally, the difference in optimal substitution percentages between monolayers on the Si(111) with Si-O and Si-C linkages is understood by different van der Waals radii of their linkage groups and tilt angles of chains. Acknowledgment. The authors thank three reviewers for their constructive and pertinent comments. This work is supported by the National Natural Science Foundation of China (Grant No. 20103004). LA0341198

Formation Mechanisms and Packing Structures of Alkoxyl and Alkyl

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