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Effects of Radical Site Location and Surface Doping on the Radical Chain-reaction on H-Si(100)-(2 × 1): A Density Functional Theory Study Yong Pei,†,‡ Jing Ma,*,‡ and Xiao Cheng Zeng*,‡ School of Chemistry and Chemical Engineering, Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing UniVersity, Nanjing, 210093, Peoples Republic of China, and Department of Chemistry and Nebraska Center for Materials and Nanoscience, UniVersity of Nebraska-Lincoln, Lincoln, Nebraska 68588 ReceiVed: March 10, 2008; ReVised Manuscript ReceiVed: August 1, 2008
The radical chain-reactions of allyl mercaptan, ethylene oxide, propylene oxide, and 1,3-butadiene molecules on the H-Si(100)-(2 × 1) are systematically investigated on the basis of a hybrid (ONIOM) model. The formation of γC-, δC-, or δS-site radical intermediate reduces the kinetic selectivity of H-abstraction reactions. The H-abstraction activation energies have the order of across dimer row < interdimer (in the same dimer row) < intradimer H-abstraction, contrasting to the previously reported tendency of βC-site radical intermediate. The steric factor greatly affects the direction-selectivity of radical chain-reaction. The discrepancy between chain-reaction of allyl mercaptan [J. Am. Chem. Soc. 2007, 129, 12304] and trimethylene sulfide molecules [J. Phys. Chem. C 2007, 111, 11965] is rationalized by the doping effect of silicon substrate. It suggests the doping of silicon substrate can alter the direction of the surface chain-reaction. Furthermore, we also theoretically predict the self-directed growth behaviors of ethylene oxide, propylene oxide, and 1,3-butadiene molecules on the H-Si(100)-(2 × 1). The predicted growth behavior of 1,3-butadiene molecules is in good agreement with recent experimental observations. 1. Introduction The preparation of ordered low-dimensional nanostructures on the semiconductor silicon surface is of great interest due to the potential usage in the design of novel electronic devices.1 The atomic terraces of various silicon surfaces offer good templates for patterning the self-assembly of ordered organic and organometallic nanostructures.2 The Si(100)-(2 × 1) is one of the most widely studied silicon surfaces, which is highly reactive due to the presence of surface dangling bonds. Many unsaturated organic molecules such as alkene, alkyne, and aldehyde, etc. can readily react with the bare Si(100)-(2 × 1) surface through the cycloaddition reaction, forming stable organic molecular nanostructures.3,4 Transition metal atoms, such as In and Bi,5-8 as well the main group element Si,9 are able to settle down between two neighboring Si-Si dimer rows, forming one-dimensional (1D) nanostructures. However, the control over patterning ordered organic nanostructures on the crystal silicon surface is rather difficult. The self-directed growth of an ordered 1D organic molecular line on the H-terminated Si(100)-(2 × 1) was first reported by Lopinski et al.10 It was shown that the reaction was initiated by the surface dangling bond, where the styrene molecules can attach to the silicon surface one-by-one. Stimulated by this discovery, a variety of ordered low-dimensional organic molecular nanostructures through reactions of acetone,11 vinyl ferrocene,12 long-chain alkenes,13 cyclopropyl methyl ketone,14 benzaldehyde,15 allyl mercaptan (ALM),16,17 and trimethylene sulfide (TMS)18,19 on the H-Si(100)-(2 × 1) are prepared. The self-directed growth of a styrene molecular line on the H-Si(100)* To whom correspondence should be sent. E-mail: (J.M.) majing@ nju.edu.cn and (X.C.Z)
[email protected]. † Nanjing University. ‡ University of Nebraska-Lincoln.
(3 × 1) was also investigated.19,20 The fundamental mechanism underlying these surface reactions was proposed as a radical chain-reaction, which involves an initial addition reaction of an unsaturated double or triple bond to the pre-existing surface dangling bond (step IfIII), and a key H-abstraction process (IIIfV) as shown in Scheme 1.21 Due to the anisotropic array of the first layer atoms on the H-Si(100)-(2 × 1), the chain-reaction has directional selectivity. From Scheme 2, three kinds of H-abstractions, including the across dimer row (r1 direction), the intradimer (r2 direction), and the interdimer (r3 directions) are possible. To date, most of the reported radical chain-reactions, such as the reactions of styrene,10 vinyl ferrocene,12 long-chain alkenes,13 acetaldehyde,15 and benzaldehyde15 on the H-Si(100)(2 × 1) led to the formation of double line structures restricted within a single surface dimer row. We classify these as case 1 in Scheme 2, which involves the successive H-abstraction steps along the r3 direction and irregular intradimer H-abstractions. The formation of a single molecular line on one side of the surface dimer row was observed recently with the acetone molecule,11 corresponding to case 2 (Scheme 2). A common feature in these two cases is that the key H-abstraction step involves the same kind of βC-site radical intermediate. Our recent density functional theory (DFT) calculations of Habstraction reactions of a series of 1-alkene, 1-alkyne, and aldehyde molecules on the H-Si(100)-(2 × 1) indicate the βCsite radical intermediate prefers interdimer H-abstraction (r3 direction).22 The H-abstraction barrier has the tendency of interdimer (r3 direction) < intradimer (r2 direction) , across dimer-row (r1 direction) H-abstraction.22b As a result, molecular lines formed by styrene,10 vinyl ferrocene,12 long-chain alkenes,13 acetaldehyde,15 and benzaldehyde15 are generally restricted within a single HSi-SiH dimer row.
10.1021/jp802098s CCC: $40.75 2008 American Chemical Society Published on Web 09/24/2008
Radical Chain-Reaction on H-Si(100)-(2 × 1)
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SCHEME 1: The radical chain-reaction mechanism on the H-terminated silicon surface
A new kind of perpendicular dimer-row single molecular line was recently prepared using the reaction of allyl mercaptan (ALM) on the H-Si(100)-(2 × 1) (case 3 in Scheme 2).16,17 By combing the reactions of ALM and styrene17 or ALM and benzaldehyde,19 some joint “L” shape nanostructures were successfully prepared. Moreover, the conversion of a radical chain-reaction from the cross-row direction (r1) to the parallelrow direction (r3) at any desirable point without generating new surface dangling bond was also realized.11 The across-dimer growth of the ALM molecular line was interpreted from the formation of the δS-site radical intermediate,16 which aroused arguments recently. The radical chainreaction of the trimethlyene sulfide (TMS) on the doped H-Si(100)-(2 × 1) was investigated by DiLabio et al. both theoretically and experimentally.18 The addition of the TMS to the dangling bond on the n-type doped H-Si(100)-(2 × 1) led to the formation of the same kind of δS-site radical intermediate, but the chain reaction of TMS is distinctly different from that of ALM.16 A double line along the surface dimer-row direction was observed for the TMS (case 4 in Scheme 2), in contradiction to the perpendicular dimer-row single line formed by the ALM (case 3 in Scheme 2).18 The causes for the different growth behaviors of ALM16 and TMS18 molecular lines in two independent experiments are not clear. To date, the radical chainreactions via the βC-site radical intermediate have been wellstudied both theoretically13,18,22,23 and experimentally,11-15,24,25 but those via the γ- and δ-site radical intermediates are less investigated.16,18,26 In the present work, three questions are addressed: (1) the possible mechanism of high direction-selectivity of the radical chain-reaction of allyl mercaptan on the H-Si(100)-(2 × 1); (2) the origin of different growth behaviors of ALM16 and TMS18 molecular lines, which is rationalized by the different doping status of the silicon substrate; and (3) the effects of radical site location in the (β-, γ-, or δ-centered) intermediates on the direction-selectivity of radical chain-reactions. 2. Computational Model and Details Cluster Model. All H-abstraction reactions are investigated based on a finite cluster model, Si60H51 in Figure 1. A dangling bond is initially presented. The use of a cluster model had been validated in previous theoretical work.22a The predicted activation barriers and exothermicities by the cluster model is close to those from the periodic slab model.22a
Computations are done using the hybrid (ONIOM) model implemented in the Gaussian03 program.27 As shown in Figure 1, the cluster model is composed of two parts: reactive part 1 (in the ball-and-stick form, containing 23 Si atoms and 13 H atoms) and substrate part 2 (in the stick form, containing 37 Si atoms and 38 H atoms). For the reactive part, the density function theory (DFT)28 with the three-parameter hybrid exchange functional of the Becker and the Lee, Yang, and Parr correlation functional (B3LYP)29 is employed. All the reactants, products, and transition states are optimized by the unrestricted DFT method with the all-electron basis set of 6-31G(d,p). Part 2 is treated by the semiempirical AM1 method.30 In all calculations, no geometry constraint is applied on either the surface atoms or the reactants. We note the present cluster model give a good description of the local H-silicon (100) surface. As shown in Figure 1, the optimized dimer Si-Si bond length (2.40 Å), the interdimer distance (3.87 Å), and the interdimerrow distance (5.10 Å), using the ONIOM model, are in good agreement with the experimental data.31 Periodic Model. The periodic surface model is adopted to simulate the scanning tunneling microscopy (STM) profile of surface nanostructure. The STM profile of the molecular line is simulated using the CASTEP (Cambridge Sequential Total Energy Package) at Nebraska.31 A (2 × 3) slab model (7.73 × 11.60 × 20.00 Å) containing six layers of surface silicon atoms is used. The bottom two layers of silicon atoms and hydrogen atoms are fixed during geometrical optimizations. The dangling bonds of cleaved surface are passivated by H atoms. The Si, C, S, and H atoms are described by the ultrasoft pseudopotential.32 The exchange-correlation functional of Perdew, Burke, and Enzerholf (PBE) is used in calculations.33 The plane-wave basis set has the cutoff at 310 eV, and the k space integration is done with the mesh of 3 × 2 × 1. The STM profiles of the ALM molecular line at the bias voltages of 2.0 and -2.0 V are both simulated on the basis of the optimized structures. The SCF calculation has the convergence criteria of 1.0-6 eV/atom. The tolerances of energy, maximum force, and maximum displacement for the geometry optimization are set as 1.0-5 eV/atom, 0.03 eV/Å, and 0.001 Å, respectively. 3. Results and Discussion 3.1. Understanding of High Direction-selectivity of the Radical Chain-reaction of ALM. The radical chain-reaction of ALM on the H-Si(100)-(2 × 1) exhibited very high
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SCHEME 2: Possible manners of radical chain-reaction on the H-Si(100)-(2 × 1), resulted in different kinds of radical intermediates (Im)
direction-selectivity,16 but the underlying mechanism is not yet well established. Three kinds of H-abstraction mechanisms based on various radical intermediates, including (i) the δS-site radical intermediate via the Si-RC linkage (originally proposed by Hossian et al.16); (ii) the βC-site radical intermediate; and (iii)
Figure 1. Side views of the hybrid ONIOM surface model, Si60H51. The optimized bond lengths (in unit of angstrom) are compared with the experimental data (in the parentheses31).
the δS-site radical intermediate via the Si-βC linkage, are investigated. Mechanism i: δS-Site Radical Intermediate Wia the Si-rC Linkage. As illustrated in Scheme 3a, mechanism (i) is composed of three steps: (1) the formation of a δS-site radical intermediate by the initial addition and subsequent intramolecular H-transfer reactions (I(δS-ALM)fIV(δS-ALM)); (2) the across HSi-SiH dimer-row H-abstraction reaction by the δS-site radical intermediate (IV(δS-ALM)fVII(δS-ALM)), and (3) the diffusion of the surface dangling bond results in an unsheltered surface radical site after the across dimer-row H-abstraction (VII(δS-ALM)-r1fVIII(δS-ALM)). The energy curve of mechanism (i) is given in Figure 2. The initial addition of the allyl mercaptan molecule via the RC to the silicon surface is exothermetic, with an energy release of 12.22 kcal/mol. A barrier of 20.11 kcal/mol is obtained for the subsequent intramolecular H-transfer reactions (II(δSALM)fIII(δS-ALM)). The value of the H-transfer barrier is
Radical Chain-Reaction on H-Si(100)-(2 × 1)
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SCHEME 3: Radical chain-reaction mechanisms of allyl mercaptan: (a) mechamism (i) via δS-site radical intermediate with the Si-rC linkage; (b) mechanism (iii) via δS-site radical intermediate with the Si-βC surface linkage
further validated via the higher level calculations based on model molecules (Figure S1 in Supporting Information). Note the calculated barrier of intramolecular H-transfer is much higher (7.89 kcal/mol) than the initial adsorption energy, but the reaction is still possible because it is an intramolecular process. Following the intramolecular H-transfer step, the H-abstractions are induced by the formed δS-site radical intermediate (IV(δSALM)). Interestingly, in mechanism i, the H-abstractions in three pathways (IV(δS-ALM)fV(δS-ALM)-r1, -r2, and -r3, corresponding to three directions, respectively) have nearly equal barriers and reaction exothermicities. We note these results are in good agreement with previous theoretical results of TMS.18 Mechanism ii: Wia βC-Site Radical Intermediate. The H-abstraction may also proceed through the βC-site radical intermediate (II(δS-ALM)), called mechanism ii. The interdimer H-abstraction barrier is calculated as 14.04 kcal/mol, a little higher than the adsorption energy (12.22 kcal/mol) by about
2.00 kcal/mol. We do not consider the other two βC Habstractions ways because the across dimer-row and intradimer H-abstractions generally have higher barriers than that of interdimer H-abstraction.22a Mechanism iii: Wia δS-Site Radical Intermediate with SiβC Linkage. In mechanisms (i) and ii, the ALM molecule initially attacks the surface dangling bond via the RC, which leads to the formation of a βC-site radical intermediate (linked to the silicon surface via the Si-RC bond). Mechanism (iii) is composed of the attack of molecules on the surface radical site via the βC atom (Scheme 3). In this mechanism, an intramolecular H-transfer reaction from the δS-H group to the RC-radical gives rise to a new type of δSsite radical intermediate (labeled as IV(δS-ALMnew)), which is able to abstract a neighboring H atom to further propagate the radical chain reaction. In Figure 2, the energy curve (in gray) of mechanism (iii) is compared with that (in black) of mechanism i. The initial
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Figure 2. The energy curves of mechanisms (i) (black) and (iii) (gray) of H-abstraction reactions of ALM molecules on the H-Si(100)-(2 × 1). The energy unit is kcal/mol. Snapshots of reactants, intermediates, transition states, and products are available in the Supporting Information.
TABLE 1: Torsion Angle Changes from the Intermediates to the Transition State within H-Abstraction Reactions of Ally Mercaptan, Ethylene Oxide, Propylene Oxide, and 1,3-Butadiene Transition States and Intermediatesa ALM, mechanism (i) (Scheme 3) torsion angles (°)
IV(δS-ALM)
V(δS-ALM)-r1
V(δS-ALM)-r2
V(δS-ALM)-r3
RC-βC-γC-δS Si(2)-RC-βC-γC Si(1)-Si(2)-RC-βC
179.7 179.4 62.3
177.0 167.7 170.8
53.0 50.1 63.5
158.9 72.1 95.8
ethene oxide (Figure 4) Si(2)-O-βC-γC Si(1)-Si(2)-O-βC
III(γC)
IV(γC)-r1
IV(γC)-r2
IV(γC)-r3
168.3 56.7
138.6 106.9
75.6 38.4
92.2 48.1
III(δC)
IV(δC)-r1
IV(δC)-r2
IV(δC)-r3
176.9 171.1 57.7
174.4 113.6 161.1
90.1 91.1 49.7
151.8 83.5 103.1
propylene oxide (Figure 5) O-βC-γC-δC Si(2)-O-βC-γC Si(1)-Si(2)-O-βC
1,3-butadiene (Figure 6) RC-βC-γC-δC Si(2)-RC-βC-γC Si(1)-Si(2)-RC-βC
II(δC-butadiene)
III(δC-butadiene)-r1
III(δC-butadiene)-r3
179.4 113.9 56.3
174.5 107.9 164.1
158.4 97.6 69.8
a The definitions of atoms such as Si(1), Si(2), RC, βC, etc. refer to Scheme 3, Figures 4-6, and also the structures shown in the Supporting Information.
adsorption energy in the step of I(δS-ALMnew)fII(δS-ALMnew) is 4.71 kcal/mol, much smaller than that (-12.22 kcal/mol) in mechanism i. The subsequent intramolecular H-transfer barrier is 10.07 kcal/mol (II(δS-ALMnew)fIV(δS-ALMnew)). The across dimer-row and interdimer H-abstractions (IV(δSALMnew)fV(δS-ALMnew)-r1 and IV(δS-ALMnew)fV(δSALMnew)-r3) have barriers of 5.51 and 5.42 kcal/mol, respectively. Comparisons Among Three Possible Mechanisms. Reviewing the energy curves in mechanisms i-iii, mechanism (ii) through the βC-site radical intermediate can be ruled out because of the relatively higher H-abstraction barrier. Indeed, the βC H-abstraction was not observed in previous STM studies.16 Mechanism (i) is prior to (ii) on the initial adsorption energy. However, the energy curve of mechanism (i) does not support the high selectivity of H-abstractions observed in experiments.16 The geometries of intermediates and transition states in mechanism (i) are further analyzed. The geometrical analysis in Table 1 indicates the alkane “arm” in the transition state
V(δS-ALM)-r1 undergoes the smallest conformational change from the radical intermediate (IV(δS-ALM)). The IV(δS-ALM) can easily reach the V(δS-ALM)-r1 just by rotating the torsion angle Si(1)-Si(2)-RC-βC. However, relatively larger changes of torsion angles Si(1)-Si(2)-RC-βC, Si(2)-RC-βC-γC, and RC-βC-γC-δS are required to reach two other transition states V(δS-ALM)-r2, and V(δS-ALM)-r3 (cf. Table 1). We conclude that the specific orientation of the adsorbed ALM molecule facilitates the across dimer-row H-abstraction. On the other hand, the simulated STM image (Figure 3) of the ALM molecular line provides auxiliary evidence for mechanism (i). The protrusions are clearly observed in the simulated STM images, in good agreement with experimental observations.16 n-Type Doped Wersu Undoped H-Si(100)-(2 × 1) Surfaces. The remaining question is why the ALM and TMS demonstrate different growth behaviors on the H-Si(100)-(2 × 1)16,18,19 even though they form the same kind of δS-site radical intermediate
Radical Chain-Reaction on H-Si(100)-(2 × 1)
Figure 3. Simulated STM profiles of molecular lines formed by the ALM molecule, under bias voltages at 2.0 and -2.0 V, respectively. The isosurface is constructed from the density due only to orbitals corresponding to the bias. The color corresponds to the electron density. Note the color is not shown in experiment.
(IV(δS-ALM)). We attribute such discrepancy to the different kinds of silicon substrate used in two experiments: the ALM molecular line was prepared on the undoped silicon substrate;16 whereas the n-type doped H-Si(100)-(2 × 1) was used in the growth of TMS molecular lines.18 The charge density of radical sites on the undoped and the n-type doped H-Si(100)-(2 × 1) is analyzed (see Figure S7 in the Supporting Information34-36). Radical sites on the n-type doped and undoped H-Si(100)-(2 × 1) exhibit distinctly different charge distributions. From Figure 3, after the across dimer-row H-abstraction, the -SH group (having lone-pair electrons on the S atom) in the H-abstraction product can either electrostatically attracted the positive charge (Scheme 4, on the undoped substrate16) or repell the negative charge (Scheme 4, on the n-type doped silicon substrate18) at the radical site. In the case of ALM on the undoped surface,16 the alkane arm may shield the generated surface radical, thereby blocking the attachment of a new molecule to the surface radical site. An intradimer radical diffusion thus propagates the chain-reaction.16 However, the electrastatic repulsions on the n-type doped silicon surface may push the adsorbed TMS molecule away from the radical site (Scheme 4b). The new TMS molecule therefore attacks the radical site easily prior to the occurrence of intradimer Hdiffusion. Becuase the intradimer H-abstraction (r2 direction) is sterically unfavorable, the double line structure results from a series of zigzag-across-dimer-row and interdimer H-abstractions as observed by experiments.18 3.2. H-Abstraction Reactions via the γ- and δ-site Radical Intermediates. The γ- and δ-site radical intermediates involved in radical chain reactions are less studied, both theoretically and experimentally.16,18,19,26 Herein the radical chain-reaction of ethylene oxide, propylene oxide, as well as the 1,3-butadiene are investigated. We note that during the review of our manuscript, a joint theoretical and experimental study on the radical chain-reaction of 1,3-butadiene was published.26
J. Phys. Chem. C, Vol. 112, No. 41, 2008 16083 H-Abstraction Wia γC-Site Radical Intermediates. The nucleophilic attack of ethylene oxide to the surface dangling bond produces the Si-O bond linkage, γC-centered radical intermediate, III(γC) (Figure 4). Our theoretical calculations reveal that the H-abstraction reactions via the γC-centered radical intermediate are favorable at room temperature, with the Habstraction barriers as low as 5.98 (III(γC)fIV(γC)-r1), 8.49 (III(γC)fIV(γC)-r2), and 7.14 (III(γC)fIV(γC)-r3) kcal/mol. The energy releases of these processes are -63.22 (I(γC)fV(γC)r1), -63.49 (I(γC)fV(γC)-r2), and -63.62 (I(γC)fV(γC)r3) kcal/mol. H-Abstraction Wia the δC-Site Radical Intermediate. The ring-opening reaction (I(δC)fIII(δC)) of the propylene oxide produces a δC-site radical intermediate. As shown in Figure 5, the H-abstractions from the δC-site radical intermediate (III(δC)) also have low energy barriers and large exothermicities, similar to the ethylene oxide. Competition between βC-Site and δC-Site H-Abstractions. The radical intermediate formed by the 1,3-butadiene differs from all those discussed above. It has two radical centers (both βC-site and δC-site), caused by the resonance effect of the allyl radical. Here, the H-abstraction reactions via the δC-site radical intermediate are investigated. Because of the rigidity of the butadiene molecule, the intradimer H-abstraction (r2 direction) through the δC-site radical center is rather difficult. From Figure 6, the across dimer-row (r1) and interdimer (r3) H-abstractions have energy barriers of 12.93 and 15.00 kcal/mol, respectively, which are much lower than that through the βC-site radical center (19.91 kcal/mol).22a We can thus qualitatively predict the possible manners of chain-reactions of ethylene oxide, propylene oxide, and the 1,3butadiene on the H-Si(100)-(2 × 1). Note that the H-abstraction barrier is not the sole factor controlling the direction-selectivity of chain-reaction, according to the discussions in Section 3.1 on ALM and TMS molecules. For the ethylene oxide, its nearly equal activation barriers and torsion angle changes (c.f. Table 1) in all three kinds of H-abstractions suggest the irregular chain-reaction, in other words, H-abstractions along r1, r2, and r3 directions are all possible. The chain-reaction of propylene oxide is expected to lead to double line structures like the case of TMS,18 because the adsorbed propylene oxide in the H-abstraction products, V(δC)-r1, orients preferably toward the surface dimer-row direction (r3 direction). The exposed radical site on the neighboring dimer-row can easily propagate the chain-reaction in a zigzag mode like the TMS18 (case 4 in Scheme 2). In case the of 1,3-butadiene, both the intradimer and interdimerrow H-abstractions are possible. However, we note the radical sites generated by two kinds of H-abstractions are shielded by the adsorbed butadiene molecules. So, the intradimer H-diffusion may occur to continue the chain-reaction. In fact, the recent STM study found the surface nanostructure formed by butadiene covered many surface dimer-rows, indicating the chain-reaction diffused to different dimer-rows on the surface.26 Because the intradimer H-abstraction may be prohibited by the steric factor, the intradimer H-diffusion well explains the multidimer-rows diffusion of surface chain reaction of butadiene.26 4. Conclusion On the basis of the hybrid (UB3LYP:AM1) ONIOM computational model, we theoretically investigated H-abstraction reactions of ALM, ethylene oxide, propylene oxide, and 1,3butadiene molecules on the H-Si(100)-(2 × 1). The results indicate the γC-, δC-, and δS-site radical intermediates decrease
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Figure 4. The energy curves of H-abstraction reaction of ethylene oxide on the H-Si(100)-(2 × 1) (kcal/mol). Snapshots of reactants, intermediates, transition states, and products are available in the Supporting Information.
Figure 5. The energy curves of the H-abstraction reaction of propylene oxide on the H-Si(100)-(2 × 1) (values in units of kcal/mol). Snapshots of reactants, intermediates, transition states, and products are available in the Supporting Information.
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Figure 6. The energy curves of H-abstraction reaction of 1,3-butadiene on the H-Si(100)-(2 × 1) (kcal/mol). Snapshots of reactants, intermediates, transition states, and products are available in the Supporting Information.
SCHEME 4: Origin of different growth manners of ALM and TMS molecular lines on the undoped16 and n-type doped H-Si(100)-(2 × 1)18
the kinetic selectivity of the H-abstraction reactions. The steric factor plays an important role in γC-, δC-, and δS-site radical intermediates involved in the radical chain-reaction. The different manners of chain-reactions of ALM16 and TMS18 are explained from the electrostatic effects of silicon substrates, suggesting the doping of substrate could be used to alter the direction of the radical chain-reaction. On the basis of the calculated energies and structures of intermediates and transition states, we predict the radical chain-
reactions of propylene oxide on the H-Si(100)-(2 × 1) will lead to double line structure like the TMS molecule.18 However, the reactions of ethylene oxide and 1,3-butadiene molecules on the H-Si(100)-(2 × 1) may induce irregular surface nanostructures covering multidimer rows. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grants No. 20433020 and 20573050), the Chinese Ministry of Education (No. NCET-
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