Surface Pseudorotation in Lewis-Base-Catalyzed Atomic Layer

Article pubs.acs.org/JPCC

Surface Pseudorotation in Lewis-Base-Catalyzed Atomic Layer Deposition of SiO2: Static Transition State Search and Born− Oppenheimer Molecular Dynamics Simulation Guoyong Fang,†,§ Shuang Chen,† Aidong Li,‡ and Jing Ma*,† †

Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China ‡ National Laboratory of Solid State Microstructure, Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, P. R. China § College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, P. R. China S Supporting Information *

ABSTRACT: Atomic layer deposition (ALD) is a novel deposition technique for constructing uniform, conformal, and ultrathin films in microelectronics, catalysis, energy storage, and conversion. The possible reaction pathways for the uncatalyzed and catalyzed ALD of silicon dioxide (SiO2) using SiCl4 and H2O have been investigated by density functional theory (DFT) calculations, combining static transition state searches with Born−Oppenheimer molecular dynamics (BOMD) simulations. In stepwise pathways of the uncatalyzed SiO2 ALD reaction, the rate-determining step is the Si−O bond formation accompanied by the rotation of SiCl4 with the activation free energy of 23.8 kcal/mol. The introduction of Lewis-base catalyst, pyridine or NH3, can reduce the activation free energy to 6.8 or 2.7 kcal/mol. The low energy barrier and flexible pentacoordinated intermediate facilitate the surface pseudorotation (SPR) pathway, which is similar to Berry pseudorotation (BPR) pathway of the trigonal bipyramid (TBP) molecules, such as Fe(CO)5, SiCl5−, and PF5. The catalyzed reaction may undergo multistep pathways, including adsorption of precursor, axial addition, surface pseudorotation, axial elimination, and desorption of byproduct steps. With one ligand pivot linked to the surface, the catalyzed reaction possesses three possible rotation modes. Through the lowbarrier pseudorotation transition states, the axial angle changes from near 180° to 120° and the equatorial angle changes from 120° to near 180°, indicating the pairwise exchange of axial and equatorial ligands. The generality of Berry and surface pseudorotations with the characterized TBP topology exhibits the common fluxional behavior in pentacoordinated compounds containing main-group and metal elements. Useful information can be provided for ALD fabrication of various functional materials.

1. INTRODUCTION

applications in catalysis, sensing, energy storage and conversion, etc.13−16 Essentially, ALD belongs to CVD process, but it divides the CVD reaction into two half-reactions and separately introduces the gaseous precursors into the reaction chamber.3−10 Taking the ALD growth of SiO2 as an example, the two half-reactions, A and B, using silicon tetrachloride (SiCl4) and water (H2O) reactants, are summarized as follows:

Numerous nanomaterials, with interesting architectures, novel properties, and promising applications, have been prepared by various nanofabrication technologies, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD).1,2 Among those deposition processes, atomic layer deposition (ALD) is a powerful technique for fabricating nanomaterials and nanostructures with uniform and conformal films and coatings at the atomic level.3−10 ALD uses sequential selfterminating gas−solid reactions to deposit various materials, including metal, oxide, and metal nitride, layer by layer. In addition to the wide applications in the microelectronics industry,4−12 ALD holds the promise of building the ultrathin coatings on the existing nanostructures, ranging from 0D to 3D, with controllable thickness and tunable properties for potential © 2012 American Chemical Society

(A)

|−SiOH* + SiCl4 → |−SiOSiCl3* + HCl

(B)

|−SiCl* + H 2O → |−SiOH* + HCl

Received: October 9, 2012 Revised: November 21, 2012 Published: November 26, 2012 26436

dx.doi.org/10.1021/jp309974r | J. Phys. Chem. C 2012, 116, 26436−26448

The Journal of Physical Chemistry C

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Figure 1. Schematic illustrations of the SiCl4 half-reaction of SiO2 ALD catalyzed by Lewis-bases. The pale green, gold, red, gray, and blue balls represent Cl, Si, O, H, and N atoms, respectively.

where an asterisk designates the surface species.17,18 The SiO2 ALD reactions without catalyst are usually very slow and require high temperatures and large precursor fluxes. The lowtemperature ALD (LT-ALD) of SiO2 can be achieved by introducing the Lewis-base catalysts, such as pyridine (NC5H5) and ammonia (NH3).4,19−22 Recently, a self-catalytic ALD of SiO2 was also reported using aminosilane.23,24 To get more insight into the SiO2 ALD reaction mechanism, theoretical calculations, mainly within the framework of density functional theory (DFT), have been performed to illustrate the possible reaction pathways.25−27 The influence of alkalinity and steric hindrance of Lewis-base catalysts on the reactivity were also addressed theoretically.27 However, the low energy barriers for those Lewis-base-catalyzed ALD reactions cause the difficulty of locating the transition states (TSs) without ambiguity. In such cases, the ab initio molecular dynamics simulations can be applied to present reaction pathways instead. Through showing a sequence of snapshots, the Born−Oppenheimer molecular dynamics (BOMD) simulations on the elimination reactions of ligands in the HfO2 ALD have indicated the crucial role of solvation during the water pulse in the ALD reaction.28 But few ab initio MD simulations have been reported for the other ALD reactions yet. In the present paper, we will carry out a comprehensive study by using both static transition-state searches and BOMD simulations to investigate the possible reaction pathways for the catalyzed ALD of SiO2 (Figure 1). The existence of several possible rotation transition states (TSs) and pentacoordinated intermediates (Ims) in the SiO2 ALD reactions prompts us to recall the Berry pseudorotation (BPR) mechanism, which has been proposed to explain the rapid stereomutation and fluxional behavior of the pentacoordinated main-group element and transition-metal compounds with the trigonal bipyramidal (TBP) geometry.29−33 In the BPR reaction, a pentacoordinated system in a TBP structure undergoes a pairwise exchange between axial and equatorial ligand coordination sites (with one ligand acting as the pivot) via a transition state with the square pyramid (SP) geometry, as

illustrated in Figure 2a. The Berry pseudorotation reactions in pentacoordinated compounds with Si and P as central atoms,

Figure 2. Schematic illustrations of (a) Berry pseudorotation (BPR) and (b) surface pseudorotation (SPR).

such as SiH5−nXn− and PH5−nXn (X = F, Cl, etc.), have been investigated extensively by using both semiempirical and ab initio quantum chemistry methods.34−39 The pseudorotation reactions in the enzyme-catalyzed synthesis of ATP have also been studied.40−42 In addition, the ab initio MD simulations have been successfully applied to monitor the fluxional behavior of PF5 and the change in chemical shielding of F atom in 26437



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strategy because of the low energy barrier and flat energy landscape. To trace the possible reaction pathways, a series of adiabatic (0 K) Born−Oppenheimer molecular dynamics (BOMD) simulations for the SiCl4 half-reaction were carried out, starting from all rotation transition states.75−80 To follow three surface pseudorotation pathways, 15 independent thermostatic (298 K) BOMD trajectories were collected with the initial velocities generated randomly from a Boltzmann distribution. We compare the relative potential energy curves of the NH3catalyzed SiCl4 half-reaction during adiabatic (0 K) BOMD simulations with different initial kinetic energies (Figure 3).

SF4.43−45 The Berry pseudorotations of some metal complexes of Fe, Co, and Rh, with catalytic behavior have also been demonstrated by both experimental measurements and theoretical calculations.46−49 On the other hand, the turnstile rotation (TR) pathway was also proposed for rationalizing the stereomutations in some bridged TBP molecules, such as caged oxyphosphorane compounds.50−52 The equivalence of Berry pseudorotation and turnstile rotation, with the same stereomutation result, has been revealed on the basis of topology and graph theory analysis.31−33,53−56 Recently, chemists have successfully used Berry pseudorotation and turnstile rotation reactions to design and synthesize a series of new and complicated species of main-group elements and transition metals, including acyclic, spirocycle, and even caged structures.57−66 To the best of our knowledge, however, the surfaceconfined pseudorotation, called “surface pseudorotation (SPR)” in this work, has not yet been explored. When these pentacoordinated intermediates exist in the surface reaction, will the pseudorotation pathway also occur? Our DFT and BOMD calculation results will give an answer to this question. The low energy barrier and flexible pentacoordinated reaction intermediates in the catalyzed SiO2 ALD do facilitate the surface pseudorotation reaction (Figure 2b), which is similar to the Berry pseudorotation of the other TBP molecules. As illustrated in Figure 1, the whole catalyzed ALD reaction has a possible adsorption−addition−pseudorotation−elimination−desorption pathway. With one ligand pivot being linked to the surface, the catalyzed reaction possesses three possible rotation modes, corresponding to three different directions, Path1, Path2, and Path3, respectively, shown in Figure 2b. The stereodynamics involving with fast conformational changes will be demonstrated by the BOMD simulations of the first half-reaction of SiO2 ALD. The generality underlying the Berry and surface pseudorotation with the characterized TBP topology exhibits the common fluxional behavior in the pentacoordinated compounds and stimulates the discovery of the crucial role of stereomutation in the mechanism of catalysis. These results may also provide rich information for ALD fabrication of materials with promising applications in semiconductors, fuel cells, and photovoltaic devices, and so on.

Figure 3. Relative potential energy curves of the SiCl4 half-reaction obtained from adiabatic (0 K) BOMD simulations using different initial kinetic energies (in units of kcal/mol), starting from one surface pseudorotation TS (NH3-Path1-TS2).

The results show that the initial kinetic energy has little effect on the shape of the potential energy curves of BOMD trajectories, but significant influence on the time scale of simulations. The larger initial kinetic energy leads to faster change in structures and energies. The following discussions are mainly based on the simulation results without adding the initial kinetic energy. BOMD trajectories were collected with the step size 0.500 amu1/2 bohr. All calculations in this paper were performed with Gaussian 09 program.81

2. COMPUTATIONAL DETAILS The possible reaction pathways for the uncatalyzed and catalyzed ALD of SiO2 were investigated using both a static approach (i.e., locating the stationary points on the potential energy surface) and Born−Oppenheimer MD simulation within the DFT framework. Two kinds of density functional, B3LYP and M06-2X,67−71 were compared in DFT calculations on the uncatalyzed SiCl4 half-reaction of SiO2 ALD. In order to select the proper basis set, both 6-31G(d,p) and 6-31++G(d,p) were used in the study of the uncatalyzed reaction path. All the stationary points were tested by the frequency analysis. The energies reported here include the zero-point energy (ZPE) corrections. Starting from the transition states, intrinsic reaction coordinate (IRC) calculations were also performed to examine the possible reaction pathway connecting the studied reactant to product via transition state (TS).72−74 Gibbs free energies of all species were estimated from the partition functions, and the enthalpy and entropy terms at 298 K and 1 atm. As mentioned before, the fluxional behavior of possible pentacoordinated intermediate in the SiO2 ALD reaction is difficult to be detected by using the static transition state search

3. RESULTS AND DISCUSSION 3.1. Comparison of Computational Models. Influence of the Size of Surface Cluster Model. In order to model the surface reactions on the SiO2 bulk surface, we adopt the double-hydroxyl Si9O5H12−(OH)2 cluster model (Figure 4), called model I, which is based on the fully oxidized Si(100) surface and similar to the hydroxylated α-SiO2 (101̅0) surface.82−84 This model consists of nine silicon atoms with five oxygen atoms inserted into the Si−Si bonds at the surface and upper subsurface, and 12 hydrogen atoms are used to saturate the dangling bonds. The similar single-hydroxyl Si9O5H12−H−OH cluster model, which represents the partially hydroxylated SiO2 surface, has been applied to the study of ZrO2, HfO2, and Al2O3 ALD reactions.85−87 Another doublehydroxyl cluster model, Si9H12−(OH)2, named model II, has also been used to the study of SiO2 ALD reaction.25,26 Here, we compare the SiO2 ALD reaction barriers of two kinds of the double-hydroxyl cluster models, I and II, in Figure 4 and Figure S1 in the Supporting Information. It can be seen that the 26438



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Figure 4. Comparison of Gibbs free energy profiles for the uncatalyzed SiCl4 half-reaction on the cluster model I, Si9O5H12−(OH)2, model II, Si9H12−(OH)2, and model III, (SiH3O)3Si−OH, using B3LYP and M06-2X functionals with the 6-31G(d,p) and 6-31++G(d,p) basis sets, respectively.

Supporting Information, from which we can find that the optimized geometries from two different functionals are similar to each other. As shown in Figure 4, on the basis of cluster model I, the activation free energies obtained by using the B3LYP functional is about 10.0 kcal/mol higher than that of the M06-2X functional using 6-31G(d,p) basis set. Such a difference between these two functionals can be also seen from the binding energy, Eb, between SiCl4 reactant and the cluster model, shown in Figure S1. Herein, Eb is defined as

reaction barriers on the basis of cluster model II is about 5.0 kcal/mol lower than that on the basis of cluster model I. In fact, the difference between the cluster models I and II is distinct: the model II represents the hydroxylated Si (100) surface rather than the hydroxylated SiO2 substrate (which is mimicked by model I). Despite those differences, the two cluster models can still give similar reaction profiles. To further investigate the effect of the neighboring hydroxyl group on the energy barriers, we consider an even smaller and more flexible single-hydroxyl cluster model III, (SiH3O)3Si− OH. A comparison of the results with the single-hydroxyl (SiH3O)3Si−OH (model III) and double-hydroxyl Si9O5H12− (OH)2 clusters (model I) shows that the difference in the reaction barriers is just about 1.0 kcal/mol. Extensive theoretical calculations have already demonstrated that the cluster size also has little effect on SiO2, TiO2, and Al2O3 ALD reactions.27,88,89 Some first-principles calculations and BOMD simulations have been performed on the HfO2 ALD by using the periodic slab surface model.28 In comparison with the periodic surface model, the cluster model can give similar reaction profiles of HfO2 ALD reactions.28 The negligible influence of the neighboring hydroxyl group in both cluster model and periodic slab model is attributed to the locality property of chemical reaction. Assessment of Density Functionals and Basis sets. Two kinds of density functionals, B3LYP and M06-2X,67−71 were compared in DFT calculations on the uncatalyzed SiCl4 halfreaction of SiO2 ALD (Figure 4 and Figure S1 in the Supporting Information). The optimized geometries, characterized by some key parameters, are given in Table S1 in the

E b = Ecomplex −

∑ Esubsystem

(1)

where Ecomplex and Esubsytem represent the energies of complex and subsystem, respectively. The results show that the binding energy (−8.0 kcal/mol) using M06-2X functional is larger than that (−1.4 kcal/mol) using B3LYP functional. The M06-2X method predicts shorter Si···O distances (3.24 Å) than that (3.75 Å) obtained from B3LYP between SiCl4 and surface hydroxyl (−OH*) group. For another cluster model model II, the binding energy (−8.3 kcal/mol) obtained by M06-2X functional is also larger than that (−1.3 kcal/mol) using B3LYP functional. The M06-2X calculation also gives shorter Si···O distances (3.14 Å) than that (3.64 Å) obtained using B3LYP, indicating that M06-2X presents a picture of stronger physisorption of SiCl4 on the hydroxylated surface than B3LYP. Since the van der Waals interactions play an important role in the reaction path, especially for the physical adsorption, M06-2X may give more reliable results than B3LYP. Two kinds of basis sets, 6-31G(d,p) and 6-31++G(d,p), were also tested. As shown in Figure 4 and Figure S1 in the 26439



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Figure 5. (a) Adiabatic BOMD trajectory, starting from TS1uncat, and (b) IRC path, starting from TS2uncat in the uncatalyzed SiCl4 half-reaction on the cluster model I, Si9O5H12−(OH)2.

1.61 and 1.28 Å, respectively, indicating the formation of new Si−O and H−Cl bonds. At the same time, the cleavage of O−H and Si−Cl bonds is shown by the increase in distances of the O−H from 0.96 to 3.05 Å and Si−Cl bond from 2.03 to 5.42 Å. In fact, the pentacoordinated silicon intermediate (Im2uncat) is nearly energetically degenerate to the transition state TS1uncat with the energy difference of less than 2.0 kcal/mol. The structure of TS1uncat is also very close to that of Im2uncat. It is quite difficult to search the stationary points on such a flat energy surface. Starting from these two transition states, the possible reaction paths were traced, as shown in Figure 5. It should be mentioned that the negligible energy difference between TS1uncat and Im2uncat causes huge difficulty in obtaining the IRC connecting TS1uncat to intermediates. Instead, the adiabatic BOMD simulation is carried out to demonstrate the possible pathway from TS1uncat to Im2uncat. The present stepwise reaction mode differs from the previous concert pathway in the existence of two transition states, TS1uncat and TS2uncat, whose energies are close to each and they are connected by an intermediate, Im2uncat. The small energy difference in the stepwise path between between TS1uncat and Im2uncat (Figure 4) as well as the flat energy profile in BOMD trajectory starting from from TS1uncat to Im2uncat (Figure 5a) suggests that both concerted and stepwise pathways are possible in the uncatalyzed reaction. The rate-determining activation energy barriers of both reaction modes are comparable to each other. 3.3. Catalyzed SiCl4 Half-Reaction: Static TS Search and BOMD Simulation. 3.3.1. Adsorption−Addition− Pseudorotation−Elimination−Desorption Pathway. The reduction in the activation energy of SiO2 ALD by introducing the Lewis-base catalysts has been demonstrated in several theoretical works.25,27 On the basis of Si9H12−(OH)2 cluster model, the activation energy of the NH3-catalyzed SiCl4 halfreaction was obtained from the second-order Møller−Plesset perturbation (MP2) single-point calculations based on the Hartree−Fock-optimized geometries.25 The decrease in activation energy barriers to about 6.7−8.9 kcal/mol in the Lewis-base-catalyzed SiCl4 half-reactions was also shown in the previous calculations at the B3LYP/6-31G(d,p) level.27 In our

Supporting Information, the reaction energies with diffuse basis sets are about 5.0 kcal/mol lower than those without the diffuse basis set. Significant effects of the diffusion basis sets on the heat of reaction have also been shown for the reaction between CF4 and SiCl4.90 It was also addressed that a tighter d function is needed for Si and Cl atoms,90 but the formidable computational cost of the aug-cc-pv(Q+d)z basis set prohibits its applications to the ALD SiO2 reaction on surface. Since the energy profiles and the geometric parameters predicted by the 6-31++G(d,p) basis set are very close to those at 6-31G(d,p) level, all calculation results reported below were obtained at the M06-2X/6-31G(d,p) level to gain a compromise between accuracy and computational cost. 3.2. Uncatalyzed SiCl4 Half-Reaction: Stepwise Pathway. The previous theoretical works mainly devoted to understanding the SiO2 ALD reactions from a viewpoint of the concerted pathway.26,27 The uncatalyzed SiCl4 half-reaction between the precursor SiCl4 and surface hydroxyl was proposed to undergo through a four-membered ring (4MR) transition state, forming the Si−O and H−Cl bonds and breaking the Si− Cl and O−H bonds simultaneously. However, there is another possible stepwise pathway, especially when the interaction between SiCl4 and surface hydroxyl is physisorption, instead of chemisorption. In such a stepwise pathway, the ratedetermining step is the Si−O bond formation accompanied by the rotation of SiCl4 with the activation free energy barrier of 23.8 kcal/mol. In addition to the double-hydroxyl Si9O5H12− (OH)2 cluster model I, we consider another two cluster models, model II and model III, respectively. From free energy profiles (Figure 4) and potential energy profiles (Figure S1), it can be seen that the uncatalyzed SiCl4 half-reaction profiles on the basis of three cluster models are similar to each other and the difference in the reaction energies is about 3.0 kcal/mol. In Figure 4, we show the possible stepwise pathway, via two transition states, TS1uncat and TS2uncat, respectively. The TS1uncat with an imaginary frequency of 42i cm−1 represents the formation of Si−O bond accompanied by the rotation of SiCl4. The TS2uncat with an imaginary frequency of 491i cm−1 shows the trend of Si−Cl and O−H bond cleavages and H−Cl bond formation. As shown in Table S1, the distances of Si···O and Cl···H bonds gradually decrease from 3.24 and 3.32 Å to 26440



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Figure 6. Gibbs free energy profiles for the SiCl4 half-reaction with (a) NH3 or (b) pyridine catalyst on the cluster model I, Si9O5H12−(OH)2.

energy (Eb) of −4.3 (−8.5) kcal/mol. But, these TBP complexes easily result in the loss of the reactive site of SiCl4 and deactivation of Lewis-base catalyst. Pyridine (NH3) prefers to interact with surface −OH* and form hydrogen bond complex with the binding energy (Eb) of −13.6 (−15.1) kcal/ mol. Those have been verified using in situ transmission Fourier transform infrared (FTIR) spectroscopy. The experimental results show that the hydroxyl stretching vibration frequency shows a red shift from 3800 cm−1 to about 3000 cm−1 after adsorption of Lewis-base catalyst,91,92 indicating the strong interaction between Lewis-base and surface −OH*. Simultaneously, SiCl4 is attached to the hydroxylated surface through physisorption. The Si−O distance under pyridine (NH3) catalyst is 3.07 (3.05) Å, which is slightly shorter than that (3.24 Å) without catalyst. The coadsorption of pyridine (NH3) and SiCl4 leads to the formation of the complex, Im1, with the binding energy of −19.9 (−21.5) kcal/mol. Due to the strong hydrogen-bonding interaction between Lewis-base catalyst and surface −OH* group, a pentacoordinated silicon intermediate, Im2, is formed through a concerted transition state, TS1, of the formation of Si−O bond and the elongation of O−H bond simultaneously, with an imaginary frequency of 102i (105i) cm−1 for pyridine (NH3) catalyst. At the same time, pyridine (NH3) is protonated to form HNC5H5+ (NH4+) cation. The pentacoordinated silicon intermediate (|−OSiCl4−) is a negatively charged compound with a TBP geometry, in which the O atom of original −OH* group acts as an axial ligand. The activation free energy of the Si−O bond formation for pyridine (NH3) catalyst is 6.8 (2.7) kcal/mol, which is much lower than that (23.8 kcal/mol) of the Si−O formation in the uncatalyzed reaction. Such a process of Si−O bond formation is similar to the axial addition, which was proposed in nucleophilic substitution reactions at silicon and phosphorus.93−98 It is possible for the pentacoordinated silicon intermediate (Im2) to change into another intermediate (Im3) with TBP structure through pseudorotation transition states (TS2). As

previous work, we discussed about the possible reaction pathways through three main steps: adsorption of precursor, ligand exchange, and desorption of byproduct.27 Here, we add more detailed information of the pseudorotation into the multistep pathway, i.e., adsorption−addition−pseudorotation− elimination−desorption pathway (Figure 1). To represent the stationary points along the catalyzed pathways in a concise way, we simply use denotations of intermediates Im1, Im2, Im3, and Im4 and transition states TS1, TS2, TS3 without the superscript (Figure 6 and Figure S2 in the Supporting Information). During the SiCl4 half-reaction, a steady state of Lewis-base catalyst is usually maintained in the deposition reactor at room temperature.4,19−21 As shown in Figure 7, pyridine (NH3) can interact with SiCl4, forming the TBP complex with the binding

Figure 7. Schematic illustrations of the interaction between (a) NH3 or (b) pyridine catalyst and SiCl4 or surface hydroxyl. Eb represents the binding energy, d represents the distance of N atom of Lewis-base catalyst and Si atom of SiCl4 or H atom of surface hydroxyl, or Si atom of SiCl4 and O atom of surface hydroxyl. 26441



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of the Si−Cl bond cleavage, Path1-TS3 and Path2-TS3, with imaginary frequencies at 84i and 111i cm−1, respectively. The chloride and surface silicon compound (|−OSiCl3) in Im4 are subsequently formed. The NH3-catalyzed reaction passes through similar transition states of the cleavage of Si−Cl bond with imaginary frequencies at 117i (Path1-TS3) and 85i (Path2-TS3) cm−1, respectively. Although the Path3 is too flat to locate the transition state of the Si−Cl bond cleavage, it has the similar BOMD trajectory as Path1 and Path2 pathways (cf. subsection 3.3.2). Such a process of Cl atom dissociation, Si−Cl bond cleavage and chloride formation is similar to the axial elimination in silicon and phosphorus stereochemistry.93−98 The byproducts are the chloride (ammonium chloride or pyridinium chloride) and the complex between HCl and Lewis bases. They can desorb from SiO2 surface under a certain temperature and nitrogen pulse.4,19−21,91,92 To summarize, the whole catalyzed SiCl4 half-reactions may undergo adsorption of precursor, axial addition, surface pseudorotation, axial elimination, and desorption of byproduct steps. As occurring in other nucleophilic substitution reactions at silicon and phosphorus,93−98 the axial addition and elimination will be more favorable than equatorial addition and elimination. As shown in Table 1, the fluxional behavior of the pentacoordinated TBP molecules results from the low

shown in Figure 2b, since O ligand is tied to the surface and served as the pivot, the catalyzed reaction possesses three possible rotation modes, called Path1, Path2, and Path3, respectively. Path1 rotation represents the movement of ligand 1 (Cl atom) and 4 (Cl atom) in the same direction, Path2 rotation represents the movement of ligand 2 (Cl atom) and 4 (Cl atom) in the same direction, and Path3 rotation represents the movement of ligand 3 (Cl atom) and 4 (Cl atom) in the same direction. After these pseudorotations, the axial Cl and O atoms change to equatorial atoms, and at the same time, two equatorial Cl atoms change to axial atoms. In the pyridinecatalyzed SiCl4 half-reaction, three rotation transition states, Path1-TS2, Path2-TS2, and Path3-TS2, are located to have imaginary frequencies at 50i, 50i, and 39i cm−1, respectively, as shown in Figure 8. The low activation free energies of 8.1

Table 1. Activation Energy Barriers (Ea) of the Pseudorotation Reaction of Some Pentacoordinated Molecules compound SiH5− SiHF4− SiHCl4− SiF5− SiCl5− PH5 PF5 PF4H PF4CH3 Fe(CO)5 |−OSiCl4−NH4+ Path1 Path2 Path3 |−OSiCl4−HNC5H5+ Path1 Path2 Path3

Figure 8. Displacement vectors of surface pseudorotation transition states: (a) NH3-TS2 and (b) pyridine-TS2.

(Path1), 7.3 (Path2), and 5.8 (Path3) kcal/mol, respectively, indicate a facial pseudorotation on surface. The NH3-catalyzed SiCl4 half-reaction exhibits a similarity to the pyridine-catalyzed reaction in the very low free energy barriers of 7.7 (Path1), 6.0 (Path2), and 3.1 (Path3) kcal/mol, respectively. It can been seen from the vibration modes of imaginary frequency (Figure 8) that the displacement vectors of four Cl atoms of the pentacoordinated silicon intermediate (|−OSiCl4−) are the most significant and hence the protonated Lewis-base cation (HNC5H5+ or NH4+) has to move away or rotate in order to accommodate the pentacoordinated silicon intermediate anion during surface pseudorotation. From the activation barriers, the pseudorotation of the Path3 pathway is more favorable than other two. Due to the steric hindrance of Lewis-base cation, the pseudorotations of the Path1 and Path2 pathways, in which the axial Cl atom moves to the Lewis-base cation, have relatively higher activation energy barriers than that of Path3. With the presence of the protonated Lewis-base cation (HNC5H5+ or NH4+), the axial Cl atom of the pentacoordinated silicon intermediate (|−OSiCl4−) is further pulled away from the central silicon atom. For the pyridine catalyst, the dissociation of axial Cl atom goes through two transition states

Ea (kcal/mol) 2.5−3.0 4.0−5.0 2.5−6.7 ∼3.0 ∼3.0 1.5−2.0 4.2−4.8 5.7−6.9 3.9−4.5 2.1

ref 35−37 37 37 37 37 38, 39 38 38 38 46

6.4 4.4 2.0

this work this work this work

6.3 5.3 4.4

this work this work this work

activation energy barriers (less than 10.0 kcal/mol) of the pseudorotation reaction. The pentacoordinated silicon species having pseudorotation or fluxional behavior are usually anions. Under the Lewis-base catalyst, the H atom of surface −OH* can be taken away by catalyst during axial addition reaction, resulting in the formation of surface pentacoordinated TBP intermediate anion and Lewis-base cation. Similar to Berry pseudorotation, surface pseudorotation is a low energy barrier process and easily occur in the negatively charged pentacoordinated species on the surface. Similar to the uncatalyzed SiCl4 half-reaction, the rate-determining step is the formation of Si− O bond in the catalyzed ALD reactions. The activation free energy of the rate-determining step declines significantly from 26442



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Figure 9. Relative potential energy curves during adiabatic (0 K) BOMD simulations, starting from three surface pseudorotation TSs: (a) NH3-TS2 and (b) pyridine-TS2.

Figure 10. Evolutions of the angles, θCl−Si−O and θCl−Si−Cl, and bond distances, dSi−Cl and dH−Cl, during adiabatic (0 K) BOMD simulations, starting from three surface pseudorotation TSs: (a) NH3-TS2 and (b) pyridine-TS2.

pivot ligand, O atom, is confined by the surface, so that SPR has three different rotation directions (Figure 2b). Due to the limitations of static search for transition states in the low energy barrier or even barrierless reactions, we have carried out adiabatic (0 K) BOMD simulations and sampled possible surface pseudorotation pathways to trace the flat potential

23.8 kcal/mol to 6.8 (2.7) kcal/mol for pyridine (NH3) catalyst, which is in agreement with experimental results.4,19−22,91,92 3.3.2. BOMD Simulations on Surface Pseudorotation. As mentioned above, the difference between the Berry pseudorotation and surface pseudorotation reactions lies in that the 26443



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Figure 11. (a) Topology parameter (TP) and evolutions of TP during (b) adiabatic (0 K) and (c) thermostatic (298 K, first trajectory) BOMD simulations, starting from three surface pseudorotation TSs: NH3-TS2 and pyridine-TS2.

simultaneously with the pseudorotation-like movement. Thus, the surface pseudorotation found in the present system is special. Pseudorotation which occurs before surface reactions may be found in other systems in future. The evolutions of structural parameters concerning of the reactive sites are shown in Figure 10. The surface-confined pseudorotation pathways are similar to the general Berry pseudorotation of the TBP molecules. The two equatorial ligands exchange with two axial ligands, resulting in that the angle between two axial ligands decreases from near 180° to 120° and the angle between two equatorial ligands increases from 120° to near 180°. However, different from the Berry pseudorotation, the surface pseudorotation reactions can result in the dissociation of the axial Cl atom in the TPB structure and the cleavage of Si−Cl bond via an axial elimination reaction with the presence of the protonated Lewis-base cation (HNC5H5+ or NH4+). There is no distinct difference of the geometries between the NH3- and pyridine-catalyzed pathways. During the pseudorotation of silicon TBP intermediate, the axial angle, θCl−Si−O, of (axial) Cl−(central) Si−(axial) O in the original TBP structure of Im2 changes from near 180° to 120°, and the equatorial

energy surfaces of the catalyzed SiO2 ALD reactions. These BOMD simulations, starting from surface pseudorotation TSs, Path1-TS2, Path2-TS2, and Path3-TS2, respectively, could go forward to the products or go backward to the reactants. We only discuss the forward ones in the following parts. The potential energy profiles of the catalyzed SiCl4 halfreaction during the adiabatic BOMD simulations are shown in Figure 9. For the NH3-catalyzed reaction, three rotation pathways, Path1, Path2, and Path3, have similar trajectories. At the beginning, the potential energy surfaces of surface pseudorotations are very flat. Once the products form, the potential energy dramatically drops. Meanwhile, it is shown that the Path2 rotation reaction spends a longer time duration of above 1500 fs to yield the product than the other Path1 and Path3 pathways because of the geometric adjustment of the NH4+ cation to accommodate the pentacoordinated silicon intermediate anion. The trajectory of Path3 rotation reaction shows a nearly barrierless pathway of the Si−Cl bond cleavage within a very short time period of about 500 fs. The results of BOMD simulations on pyridine-catalyzed SiCl4 half-reaction are similar to those of NH3-catalyzed reaction. In fact, in the lowest Path3, the dissociation of Si−Cl bond occurs nearly 26444



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Figure 12. Evolutions of the angles, θCl−Si−O and θCl−Si−Cl, and bond distances, dSi−Cl and dH−Cl, during thermostatic (298 K) BOMD simulations, starting from three surface pseudorotation TSs: (a) NH3-TS2 and (b) pyridine-TS2.

angle, θCl−Si−Cl, of two equatorial Cl and central Si atom changes from 120° to near 180° in the trajectories between 400 and 600 fs, indicating the exchange of axial and equatorial ligands in pseudorotation reaction. The topology parameter (TP) defined by Couzijn et al.,55 as shown in eq 2 TP =

θCl − Si − O − θCl − Si − Cl 60°

of H−Cl bond, are similar to the pathways from adiabatic BOMD simulations at 0 K. However, the time scale of thermostatic BOMD is much shorter than that of adiabatic BOMD. The surface pseudorotation has almost finished within 300 fs for the thermostatic BOMD. The dissociation of the axial Cl atom, the cleavage of Si−Cl bond, and the formation of chloride salt and complex between HCl and Lewis base have been accomplished within the time period of 500 fs. 3.4. Berry Pseudorotation versus Surface Pseudorotation. To summarize, the Berry pseudorotation (BPR) and the surface pseudorotation (SPR) reactions have some common features and usually occur in the pentacoordinated main-group element and transition-metal compounds with the trigonal bipyramidal (TBP) geometry. During the pseudorotation, the TBP geometry changes to another TBP geometry through via a transition state with the square pyramid (SP) geometry. The axial angle changes from 180° to 120° and the equatorial angle changes from 120° to 180°. The topology parameter (TP) value changes from 1.0 in an ideal TBP geometry to 0 in an ideal SP geometry, and then from 0 to −1.0 in another ideal TBP geometry, which is in an opposite orientation to the original one. Pseudorotation reactions, with or without the substrate support, are both easy to occur due to the low activation energy barriers (less than 10.0 kcal/mol, cf. Table 1) . As mentioned above, the difference between Berry and surface pseudorotation reactions lies in the status of the pivot

(2)

is very useful to identify the type of stereomutation along the pathway. The TP value during surface pseudorotation process changes from 1.0 (representing an ideal TBP geometry) in Im2 to 0 (representing an ideal SP geometry) in TS2, and then from 0 to −1.0 (representing another ideal TBP geometry in an inversion mode, resulting from the pairwise exchange of original axial and equatorial ligands) in Im3 (Figure 11a,b). At the same time, the Si−Cl distance, dSi−Cl, continually increases, indicating the cleavage of Si−Cl bond. The gradually decreased H−Cl distance, dH−Cl, indicates the formation of the complex between HCl and Lewis-base. To observe the effect of temperature on the surface pseudorotation reactions, we have carried out the thermostatic MD simulations at 298 K, which is the experimental temperature of the Lewis-base-catalyzed SiO2 ALD. As shown in Figures 11c and 12, all trajectories show that the processes of surface pseudorotation, the topology parameter, the departure of axial Cl atom, the cleavage of Si−Cl bond, and the formation 26445



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Figure 13. Illustration of similarities and differences between (a) Berry pseudorotation of SiCl5− and PF5, and (b) surface pseudorotation of −| OSiCl4−NH4+.

and sheds insight into the rational design of nonamaterials from ALD technology.

ligand. In Berry pseudorotation, the pivot atom is usually free of any spatial confinement, which is in contrast with the surface stuck pivot ligand in surface pseudorotation reaction. Thus, the surface pseudorotation is anisotropic and may have different rotation modes (Figures 2 and 8). The Berry pseudorotation results in the rapid stereomutation of TBP molecules through the pairwise exchange of axial and equatorial ligands and the time evolution of relative energy and geometry parameters is periodic, as illustrated by the Berry pseudorotation of SiCl5− anion and PF5 (Figure 13a). However, different from the Berry pseudorotation, the surface pseudorotation can result in the surface reactions with the other molecules, so the time evolution of relative energy is “reactive” and finally leads to the bond breaking/formation of some chemical bonds (Figure 13b). Surface pseudorotation may be facial to take place in some surface reactions containing the pentacoordinated maingroup element or transition-metal compounds with the trigonal bipyramidal geometry.

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ASSOCIATED CONTENT

S Supporting Information *

Potential energy profiles (Figures S1 and S2), structural parameters (Tables S1 and S2), and complete citation for ref 81. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program (No. 2011CB808604 and 2011CB922104), the National Natural Science Foundation of China (No. 20825312 and 21273102), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20100091110024). We are grateful to the High Performance Computing Center of Nanjing University for providing the computing resources.

4. CONCLUSIONS Static transition-state searches and Born−Oppenheimer molecular dynamics simulations have been used to investigate the reaction pathways for the uncatalyzed and catalyzed ALD of SiO2. The uncatalyzed SiO2 ALD reaction undergoes a stepwise pathway. The rate-determining step is the Si−O bond formation accompanied by the rotation of SiCl4 and has a relatively high activation free energy barrier (23.8 kcal/mol). The introduction of the Lewis-base catalyst can reduce the activation free energy of the Si−O bond formation to 6.8 (2.7) kcal/mol for pyridine (NH3) catalyst in the catalyzed ALD reactions. The low energy barrier and flexible pentacoordinated silicon intermediate facilitate the surface pseudorotation pathway. The whole catatlyzed SiCl4 half-reactions may undergo the multistep pathways, including adsorption of precursor, axial addition, surface pseudorotation, axial elimination, and desorption of byproduct steps. With one ligand pivot being linked to the surface, the surface pseudorotation possesses three possible rotation modes. During the surface pseudorotation, the axial angle changes from near 180° to 120° and the equatorial angle changes from 120° to near 180°, indicating the exchange of axial and equatorial ligands. The surface pseudorotation indicates the common fluxional behavior in pentacoordinated compounds with TBP geometry

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REFERENCES

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Surface Pseudorotation in Lewis-Base-Catalyzed Atomic Layer

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