Growth of Acetone Molecular Lines on the Si(001)(2×1)–H Surface

Article pubs.acs.org/JPCC

Growth of Acetone Molecular Lines on the Si(001)(2×1)−H Surface: First-Principle Calculations Marco Gallo,*,† Edgar Martínez-Guerra,*,‡ and Jairo A. Rodríguez§ †

Facultad de Ciencias Químicas, UASLP, Av. Manuel Nava No. 6, Zona Universitaria San Luis Potosí, SLP 78210, México Facultad de Ciencias Físico-Matemáticas, UANL, San Nicolás de los Garza, N.L. 66451, México § Departamento de Física, Universidad Nacional de Colombia, Bogotá D.C. 5997, Colombia ‡

ABSTRACT: Recent experimental work has shown that addition of acetone molecules to hydrogen-terminated Si(001) surfaces leads to the formation of one-dimensional molecular structures through a chain reaction mechanism. These structures are observed experimentally to be parallel to dimer rows on the Si(001)(2×1)−H surface. Using periodic density functional theory calculations, we have studied the initial steps of the radical chain mechanism of these reactions, and we have determined whether (or not) perpendicular growth could be possible. Our results show that, while the calculated difference of 0.03 eV between parallel and perpendicular attachment may not be enough to exclude growth between rows on Si(001)(2×1)−H at room temperature, the growth between dimers rows is kinetically less favorable because of the additional energy barrier associated with the hydrogen diffusion to the adjacent dangling bond on the same Si dimer (intradimer hopping motion), a necessary step to avoid steric repulsion between close proximity adsorbed acetone molecules. molecules like borane11 are bonded to up-buckled dimer ends through a lone pair orbital of the surface. From an organic domain, acetone is attractive due to its high dielectric constant and its ability to separate ionic charges fairly well. This organic molecule is itself an ambiphilic reactant. However, because the Si(001) surface presents ambipolar character, many structures can be formed on it. Energetically it is not possible to induce growth of acetone molecular lines on this surface. However, chain reactions of some organic molecules initiated by a dangling bond site on the H-terminated Si(001) surfaces have emerged as one of the most promising approaches to developing a molecular line connecting two points.12 Until now, many studies have shown molecular line growth with different organic molecules (alkenes, alkynes, and aldehydes) on the Si(001)(2×1) surface through chain reactions based on a hydrosilylation process.12−17 The chemistry for this class of reactions is quite clear now. A parallel-row chain reaction starts on a dangling bond site where the organic molecule through a C−C or C−O bond binds to the surface, creating a Si−C or a Si−O bond and an organic group with a carbon center radical (metastable state). Next, the intermediate C-centered reactive radical abstracts a hydrogen atom from the nearest Si dimer in the same row to generate a more stable structure where the molecule is chemically

1. INTRODUCTION Currently, there is a great interest in the chemical modification of silicon surfaces by chemisorbing organic molecules to create a new class of electronic devices that keep the well-known properties of silicon and add the useful properties of organic molecules as light emission and absorption. When exploring this concept, the main goal is to develop a new kind of hybrid molecular devices for applications in molecular electronics and sensors. However, it is clear that a previous understanding of the silicon organic chemistry is essential for the assembling of these hybrid molecular devices in different functional patterns into complex circuits.1,2 One of the requirements for these hybrid molecular devices is the stability of the organic molecules against a range of voltages and currents, which have been found to induce fragmentation or desorption. In this regard, the strength and stability of the Si−O bond opens the possibility for molecules containing the carbonyl CO group (acetone) to be good candidates for attachment to the Si surface, in this case for use in device applications where the transport of charge through the molecule is important.3 Nowadays, it is well-known that the Si−Si dimers on the surface present nucleophilic and electrophilic reactivities with their up and down dimer ends, respectively. Thus, different reaction patterns are possible when the surface is exposed to organic molecules susceptible to these two types of reactivities. Molecules such as alcohols,4−7 amines,8,9 and phosphines10 react preferably with down-buckled silicon dimers, while © 2012 American Chemical Society

Received: March 17, 2012 Revised: August 15, 2012 Published: August 15, 2012 20292

dx.doi.org/10.1021/jp3025914 | J. Phys. Chem. C 2012, 116, 20292−20299

The Journal of Physical Chemistry C

Article

the Si surface was simulated as a slab of five (001) layers with 12 atoms per layer. On the bottom layer, all Si atoms were saturated by two H atoms, while a monolayer of H atoms was arranged as a set of SiH2 units on the upper Si layer (2×1 reconstruction). On the other hand, the adsorption of the acetone molecule at the beginning of the chain reaction was supposed to initiate on one vacancy of H on the upper Si layer as it has been shown by reports with other unsaturated molecules with hydrogenated Si surfaces23,24,31−34 and in a recent report on the system just described.22 The four topmost Si layers, as well as SiH units and the adsorbed acetone molecule, were fully relaxed, whereas the lowest Si layers and the SiH2 units on the bottom were fixed at the ideal positions to describe the bulk behavior. The residual forces acting on atoms were always smaller than 10−4 atomic units. To study the chemical reactions pathways, transition states, and their energy barriers, we have used the string method35 as it is implemented by Kanai et al.36 in the context of the Car− Parrinello molecular Dynamics. This method is suitable when the potential energy surface (PES) is smooth on the thermal energy scale, and thus metastable and transition states can be located numerically on it. Specifically, the transition states (TS) are the saddle points along the minimum energy path (MEP) that connect two possible metastable states, which correspond to local minima on the PES. Therefore, the search of TS implies the description of the minimum energy path of a proposed reaction. Thus, with A and B being two states (initialmetastable, metastable-metastable, or metastable-final), ψ (string) will be a pathway in the configuration space that connects both states. Analytically, a MEP ψ* is a path that satisfies eq 1.

saturated and a new dangling bond site that can react in turn with another molecule and repeat the reaction steps continuously. Organic molecules such as alkenes, alkynes, and aldehydes12,13,15−17 have grown as molecular lines in the parallel row direction (parallel to the Si dimer row) through this chain reaction mechanism. Previously, Hossain, Kato, and Kawai have identified a molecule (allylmercaptan ALM) that reacts like other alkenes, but the reaction propagates perpendicular to the dimer rows.16−18 Also, it has been reported that a ketone molecule (acetophenone) grows through perpendicular dimer rows.19,20 Even in this case, it is not as an exclusive path as in the allylmercaptan molecule. Thus, one cannot be completely certain that any ketone will follow exclusively one type of molecular line growth on the Si(100)(2×1) surface. Specifically, an issue that is not fully understood is whether acetone would be too far from the next dimer row to form a molecular line through a perpendicular row line. Recently, Ferguson et al.,21 using DFT calculations, studied computationally the reactivity of ALM, acetone, and styrene at radical sites formed by an ALM adsorbate on the Si(100)-2×1 surface. They also studied the chain reaction of acetone on the surface initiated by a dangling bond using a Si35H32 model cluster with DFT/B3LYP and calculated the reaction mechanism: the initial adsorption energy on the db at the surface (0.73 eV), the energy barrier for the hydrogen abstraction by the organic carbon radical (0.95 eV), and the final adsorption energy where the organic molecule becomes chemically saturated and a new dangling bond at the surface is formed (0.8 eV). In this work, because it is known that cluster models do not represent the surface chemistry completely, we use an extended surface model with DFT to answer two questions. The first question asks how possible it is to induce a perpendicular growth of acetone (inter dimer row) in comparison to inducing a paralell growth (intra dimer row) on the Si(100)(2×1)−H surface. The second question that we address involves the way that the reactivity of a second acetone molecule is affected by the presence of the first acetone molecule on both growth models (intra- and interdimer row pathways). To answer these questions, we have examined the possible reactions mechanisms for both models.

[∇V (ψ *]⊥ = 0

(1)

∇V⊥ is the component of the potential energy gradient orthogonal to ψ*. This scheme allows us to find the most probable pathway to occur from the metastable states proposed as reaction steps. In our case, we explored the pathway through a parametric representation in terms of a scalar variable α defined along the path ψ = ψ(α). Particularly, we choose α to be the normalized arch length along the path 0 ≤ a ≤ 1, where A and B corresponded to α = 0 and α = 1, respectively. Finally, to optimize the MEP, we evoluted ψ using a steepest descent dynamics:

2. COMPUTATIONAL METHODOLOGY Density Functional Theory calculations (DFT) have been performed within the Car−Parrinello approach25,26 using the Quantum-Espresso code.27 The exchange-correlation effects among electrons were simulated at the Generalized Gradient Approximation (GGA) using the gradient-corrected Perdew, Burke, and Ernzerholf (PBE) functional.28 The electron−ion interactions were implemented using a Troullier−Martins pseudopotential29 for Si and ultrasoft pseudopotentials30 for C, O, and H. The electronic states were expanded in plane waves with kinetic energy cutoffs of 25 and 200 Ry for the wave function and charge density, respectively. As we have supposed that reaction initiates on one unsaturated Si dangling bond, we performed spin-unrestricted calculations. Furthermore, we showed in previous reports23,31,32 that spin-polarized calculations were necessary to describe well the energetics of reactions when molecular radicals are involved. To study the basics steps of the reaction mechanism of the acetone molecule with the Si(001)(2×1)−H surface, we performed a periodic supercell approach. From this scheme,

ψt = [∇V (ψ [α ; t ])]⊥ + λ(α ; t )t (̂ α ; t )

(2)

where t labels string configurations during the dynamics of ψt, t ̂ = (dψ/da)/(|dψ/da) is a unit vector tangent to the string, and λ (α;t) is a Lagrange multiplier that was introduced to satisfy the restriction that the parametrization of ψ(α) was kept. This constraint is expressed by (d/dα)|ψ(α)|, which states that the infinitesimal change in the arc length is constant along the string, thus ensuring that the local elastic stretching energy is distributed uniformly along the string. The continuous string was described as a discrete set of replicas ψ(n) with integers n = 0,1,2,...P. In our case, we used 10 replicas even in the initial step. These replicas were increased to precise the transition states. The nuclear and electronic degrees of freedom were simultaneously optimized using Car−Parrinello damped dynamic equation of motion, and the potential energy was obtained on the fly from the DFT energy functional. A more detailed explanation of this method can be found in ref 36. 20293



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3. RESULTS In our calculations, we first relaxed the hydrogen-passivated Si(100)(2×1)−H surface. This reconstruction is induced when the clean surface Si(001) is exposed to atomic hydrogen with the surface temperature of ∼625 K as in the experiments of Hossain et al.22 Figure 1a,b shows the schematic side and top

energy corresponds to the noninteracting molecule plus surface system.23,24 Configuration 2a(b) (intra dimer) is more stable than the noninteracting molecule and surface system by 0.52 eV, while the corresponding adsorption energy for configuration 2c(d) (inter dimer) is 0.49 eV. Thus, it is slightly more favorable by 0.03 eV to adsorb an acetone molecule via an intra dimer row pathway than using the inter dimer row pathway. The value of the CO bond length for the unadsorbed acetone molecule is 1.22 Å; the value of the C−C−C angle is 116.35°, and the values for the O−C−C angles are 121.85°. As the acetone molecule gets adsorbed in the surface, these values change in Figure 2a(b) to 1.38 Å for the C−O bond, 121.42° for the C−C−C angle, and 114.56° and 116.87° for the O−C− C angles, and in Figure 2c(d) to 1.393 Å for the C−O bond, 120.33° for the C−C−C angle, and 114.17° and 114.14° for the O−C−C angles. From these values, we can observe that the configuration in Figure 2c(d) is more symmetrical than the configuration in Figure 2a(b), and the C−O bond increases from 1.22 to 1.38 Å as the acetone molecule attaches to the Si surface. In the chain reaction mechanism, the next step corresponds to the hydrogen abstraction from the Si dimer on the surface by the carbon radical in the adsorbed acetone molecule (metastable state), creating a new dangling bond in the surface to repeat the adsorption process of a new acetone molecule and producing a more stable structure where the organic molecule is chemically saturated. Figure 3a−d is equivalent to Figure 2a− d, except that a new Si db is formed on a neighboring site as a result of the hydrogen abstraction from the Si dimer on the surface by the carbon radical. Similar to Figure 2a(b), in configuration 3a(b) we observe that the plane of the acetone molecule is nearly parallel to the Si dimers rows and the hydrogen abstraction occurs on the same Si dimer row. In Figure 3c(d), similar to configurations 2c(d), the plane of the acetone molecule is perpendicular to the Si dimer rows where it is attached. As shown in Table 1, geometries corresponding to Figure 3a,b are energetically more stable than the system formed by the noninteracting molecule with the surface by 1.16 eV. Geometries 3c,d resulted in an adsorption energy of 1.13 eV. Therefore, it is slightly more favorable to remove a hydrogen atom via an intradimer pathway. Even though the adsorption energy for configuration 3a(b) (intra dimer) is more stable than 3c(d) (inter dimer), the energy difference between these conformations is quite small (0.03 eV), suggesting that at room temperature the acetone molecule easily could rotate between configurations 3a(b) or 3c(d). The geometric values for the acetone molecule in the final step of the chain reaction mechanism where the hydrogen abstraction from the Si dimer on the surface by an intermediate carbon radical from the organic group takes place as shown in Figure 3a(b) are 1.45 Å for the C−O bond length, 112.86° for the value of the C−C−C angle, and 108.22° and 108.41° for values of the O−C−C angles, and in Figure 3c(d) the geometric parameters are 1.453 Å for the C−O bond length, 112.52° for the value of the C−C−C angle, and 108.81° and 108.76° for the values of the O−C−C angles. From these values, we can observe that the geometric values for the acetone molecule in both configurations are very similar, how the angles decrease with respect to the previous step in the chain reaction mechanism, and the lengthening of the CO

Figure 1. Schematic structure of the Si(100)(2×1)−H surface. (a) Side and (b) top views. The big yellow circles correspond to Si atoms, and the small gray circles represent H atoms.

views of the hydrogenated surface, respectively. After atomic and electronic relaxation, the Si(001)(2×1)−H surface resulted in monohydride silicon dimers rows running parallel to each other, where the interdimer distance across rows (7.68 Å) was twice that of the intradimer distance along rows (3.84 Å). We now visualize the geometric structures of an acetone molecule adsorbed on an isolated dangling bond (db) in a Si dimer of the Si(001)(2×1)−H surface and distinguish between structures where all Si dangling bonds are saturated and structures that are formed after hydrogen abstraction where a new Si db appears on the surface. Figure 2a−d corresponds to an acetone initially attached to the surface, and all Si db’s are saturated. It is important to notice that the carbon atom in the CO bond in acetone forms a carbon radical, because it has an unsaturated electron once the CO double bond is broken. In Figure 2a,b, we can observe (side and top views) that acetone is oriented perpendicular to the surface, and the plane formed by the Si−CO and the CH3 atoms is parallel to the dimer rows (intra dimer). Figure 2c,d (side and top views) is similar to Figure 2a,b, except that the acetone molecule is rotated by 90° along the axis passing through the Si−CO bond (inter dimer). Table 1 displays the calculated energies considering that the zero 20294



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Figure 2. Intermediate adsorption structures of (CH3)2−CO• acetone molecule containing a carbon radical on the Si(001)(2×1)−H surface with all Si dangling bonds saturated. (a) Side and (b) top views of acetone oriented perpendicular to the surface and parallel to the dimer rows (intra dimer model). (c) Side and (d) top views of acetone oriented perpendicular to the surface and to the dimer rows (inter dimer model).

carbon radical to obtain a stable structure wherein the organic molecule is chemically saturated. In the third step, because of strong steric repulsions between the acetone molecules when the acetone molecules are in very close proximity, a hydrogen diffusion step toward the dangling bond of the first abstraction is necessary. This induces a new dangling bond in the adjacent hydrogen atom in the same Si dimer, where the chain reaction mechanism now can be restarted to keep a molecular line growth via an inter dimer row pathway. This avoids the strong repulsions between the adsorbed acetone molecules, as their separation distance now exceeds 5 Å. The minimum energy path (MEP) for the chain reaction mechanism step, where the hydrogen abstraction from the surface by the carbon radical in the adsorbed molecule occurs via an intra dimer row pathway, was implemented with one string containing 19 replicas, as shown in Figure 4. The initial state corresponds to Figure 2a(b) and the final state to Figure 3a(b). The reaction, which corresponds to the case of a parallel line growth, shows an activation barrier for the hydrogen abstraction of 0.53 eV. The energy of the corresponding transition state (TS1) is near zero (0.009 eV). The energies of the metastable state and stable state along the intra dimer row pathway are summarized in Table 1. The MEP for the adsorption of acetone molecules to the Si(001)(2×1)−H surface through inter dimer rows (Figure 5) was determined by performing two separate string calculations. The first calculation corresponds to the hydrogen abstraction from the surface by the carbon radical in the adsorbed acetone molecule. This step was implemented with 15 replicas from the initial state Figure 2c(d) to the intermediate state Figure 3c(d) as shown in Figure 5a. The second calculation was

Table 1. Local Minima and Transition State Energies in the Addition Reaction of Acetone to the Si(001)(2×1)−H Surfacea geometry

energy (eV)

geometry

energy (eV)

geometry

energy (eV)

2a(b) 2c(d)

−0.52 −0.49

TS1 TS2 TS3

0.009 0.109 0.087

3a(b) 3c(d)

−1.16 −1.13

6a 6b

−2.30 −2.27

a

The values of the two last rows correspond to the addition of a second acetone molecule to the surface.

value from 1.22 Å in the unadsorbed molecule to 1.45 Å in the final adsorption step. In this work, as a result of the energy calculations presented in last section, we propose two different reaction paths for the formation of acetone molecular lines: the intra and inter dimer pathways. These reaction paths are schematically shown in Figures 4 and 5. For the intra dimer row reaction pathway (Figure 4), the acetone molecule is attached to a Si db as shown in Figure 2a(b). After its adsorption, a hydrogen atom is abstracted from the same Si dimer row as shown in Figure 3a(b) to stabilize the carbon radical in the acetone molecule where it creates a new dangling bond for the attachment of a second acetone molecule, and so on, completing a molecular line growth. In contrast, for the inter dimer pathway as shown in Figure 5, the acetone molecule is attached to a Si db as shown in Figure 2c(d). The next step is an abstraction of a hydrogen atom from the closest Si dimer row as shown in Figure 3c(d) by the 20295



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Figure 3. Adsorption structures of (CH3)2−CHO (acetone) on the Si(001)(2×1)−H surface with a single Si dangling bond. (a) Side and (b) top views of acetone oriented perpendicular to the surface and nearly parallel to the dimer rows (intra dimer model). (c) Side and (d) top views of acetone oriented perpendicular to the surface and to the dimer rows (inter dimer model).

(CH3)2−COH compound with an energy of 1.13 eV. Also, the energy barrier for the hydrogen abstraction via inter dimer rows resulted to be 0.07 eV higher than the energy barrier for the hydrogen abstraction via the intra dimer row pathway. These results, along with the additional energy barrier for the hydrogen diffusion step, help in explaining why the acetone molecules are experimentally observed to grow parallel to dimer rows on the Si(001)(2×1)−H at 300 K as reported by Hossain et al.22 Also, a critical step for the growth of acetone perpendicular to the dimer rows is the diffusion of dangling bonds. From the calculated minimum energy pathway, we obtained an energy barrier of 1.22 eV for the hydrogen intradimer diffusion (TS3). In Figure 5a, it is shown that it would be energetically most probable to desorb the molecule than to carry this hydrogen diffusion mechanism shown in Figure 5b. From these results, it is clear that this hydrogen diffusion mechanism represents the rate-limiting step for the growth of acetone lines in the direction perpendicular to the dimer rows. Finally, we calculated the adsorption energies and structural properties of two adsorbed acetone molecules on intra and inter models. In the model proposed for the acetone molecules adsorbed on the same dimer row (intra dimer), Figure 6a, the computed binding energy after the second molecular adsorption is 2.30 eV. In contrast, in Figure 6b (inter dimer), both acetone molecules are very far from each other, so that a negligible interaction between them is expected. From our calculations, the binding energy of these two acetone molecules is 2.27 eV. Thus, it is slightly less favorable than the intra dimer reaction path. Apparently, in the case of two acetone molecules adsorbed along a dimer row, because of steric effects, the

Figure 4. Potential-energy profile along minimum energy path (MEP) of the reaction of acetone with the Si(001)(2×1)−H surface through an intra dimer pathway. The zero of energy corresponds to the non interacting acetone−surface system.

implemented from the intermediate state Figure 3c(d) to the final state, which represents the hydrogen diffusion via intradimer using 10 replicas as shown in Figure 5b. The first part of the reaction, which corresponds to the case of a perpendicular line growth, shows an energy barrier for the hydrogen abstraction of 0.60 eV, and the energy of the corresponding transition state (TS2) is 0.109 eV. This result shows that the process of hydrogen abstraction with an energy barrier of 0.60 eV is kinetically less favorable than desorption of the intermediate state with an energy barrier of 0.49 eV (this energy barrier is equal to minus the energy of the intermediate state, assuming a negligible energy barrier for the adsorption step). Nevertheless, if this adsorption occurs, the adsorbed carbon radical should transform into a more stable Si− 20296



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interdimer growth. Our main objectives have been to understand why acetone molecular lines grow parallel to dimer rows on this surface and to elucidate the structure and energetics of the intermediates and transition states involved in these surface reactions, information that is very difficult to obtain experimentally. Our calculations indicate that, after a negligible energy barrier for the initial adsorption step of acetone on a dangling bond, acetone has two different reaction scenarios: parallel (intra) and perpendicular (inter) to dimer rows. At this point, the molecule is slightly more probable to be attached parallel to dimer rows, because the adsorption energy for this pathway is more stable (0.52 eV) than for the perpendicular route (0.49 eV). Therefore, once the molecule is attached, the probability of desorption is larger for both routes, and an energy barrier must be overcome for the proceeding chain reaction step. This energy barrier is related to the breaking of a Si−H bond on the surface; however, this energy barrier 0.53 eV is lower when the bond breaking is on the same dimer row than when it is on a neighbor dimer row 0.6 eV. Also, intra dimer pathway is a process kinetically more favorable because, after hydrogen abstraction, a new vacancy is formed and a second molecule is ready to be attached. In contrast, the inter dimer path requires an extra hydrogen diffusion process with an energy barrier of 1.22 eV. It is important to stress that the energetic barriers in this work were calculated using transition state theory.37 In TST we are interested in the rate of passage of trajectories between reactants (A) and products (B) through a dividing transition state surface (dynamic bottleneck). If all trajectories from A to B or from B to A cross the dividing surface without ever returning to the dividing surface, then the rates calculated from TST are exact. However, when we observe recrossing effects and not all of the trajectories started from reactants proceed completely to products, then TST transition rate constants overestimate the actual rate constants, and corrections to TST are necessary.38 A possibility to correct TST is to look for the dividing surface that minimizes the rate of reaction, that is, the “variational TST dividing surface”. Also, the dynamical corrections to TST are generally stated in terms of the transmission coefficient of the dividing surface. The transmission coefficient is a number between 0 and 1, which gives the ratio between the actual rate

Figure 5. Potential-energy profile along minimum energy path (MEP) of the reaction of acetone with the Si(001)(2×1)−H surface through an inter dimer pathway following two steps: (a) hydrogen abstraction from the surface, and (b) diffusion of dangling bonds. The zero of energy corresponds to the non interacting acetone−surface system.

acetone molecules prefer to have the CH3 groups nearly parallel to the dimer rows. We have calculated the reaction mechanism for the molecular line growth of acetone on the (2×1) hydrogenated Si(001) surface by two possible mechanisms through intradimer or

Figure 6. Proposed mechanisms of the reaction of a second acetone molecule with Si(001)(2×1)−H. (a) Intra dimer and (b) inter dimer reaction path models. 20297



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constant and the rate constant obtained by TST.38 VandenEijnden et al.38 developed a procedure to compute the dynamical corrections upon TST that provided an a priori error estimate on the transmission coefficient. They also derived a systematic way to identify the variational TST dividing surface. Kanai et al.34 studied the molecular line growth mechanism of styrene on the hydrogenated Si(00)2×1 surface. They calculated the energetics of the radical chain reaction mechanism for the intradimer and intradimer row paths by means of diffusion quantum Monte Carlo (QMC) and density functional theory (DFT) calculations. Their QMC calculations validated the DFT prediction of the lower kinetic barrier for the intra-dimer-row over the inter-dimer-row paths, although the values of the DFT barriers are underestimated as compared to their QMC values. Kanai et al.34 concluded that, for surface reactions, QMC calculations are an important high-level method for evaluating the DFT results, results that may not be accurate for the prediction of energetic barriers. Variational TST, dynamical corrections to TST, or QMC calculations are beyond our present possibility to study. However, the information provided by string method using DFT can be used as an approximation for more accurate rate calculations.

133022. M.G. and E.M.-G. are researchers from the Consejo Nacional de Ciencia y Tecnologia in Mexico.

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4. CONCLUSIONS The main objective of this work has been to understand why acetone molecular lines grow only parallel to Si dimer rows on this surface and not perpendicular to the Si dimer rows, based on the information provided by the structure and energetics of the intermediates and transition states involved in these surface reactions, information obtained by first-principle calculations, which is very difficult to obtain experimentally. While the calculated energy difference of 0.03 eV for the molecular acetone attachment to the surface between parallel and perpendicular Si dimer rows may not be enough to exclude molecular growth between Si dimer rows on Si(001)(2×1)−H at room temperature, our results confirmed that molecular line growth between Si dimers rows (interdimer pathway) is kinetically less favorable because of the additional energy barrier associated with the hydrogen diffusion to the adjacent dangling bond on the same Si dimer, a necessary step to avoid the strong steric repulsion between close proximity adsorbed acetone molecules. In conclusion, our DFT calculations showed that the molecular line growth parallel to the Si dimer rows is preferred over the molecular line growth perpendicular to the Si dimer rows, as was indeed observed experimentally.22

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REFERENCES

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*Telephone: 52-444-826-2440 (M.G.). Fax: 52-444-826-2440 (M.G.); 52 (81) 8329-4030 (E.M.-G.). E-mail: marco.gallo@ uaslp.mx (M.G.); [email protected] (E.M-G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Professor Noboru Takeuchi for helpful discussions, and we also thank UNAM for the use of the Supercomputer Center DGSCS-UNAM where the calculations were performed. This work was partially financially supported by projects PROMEP/103.5/10/3889 and Conacyt no. 20298



Article

(37) Glasstone, S.; Laidler, K.; Eyring, H. The Theory of Rate Processes; McGraw-Hill: New York, 1941. (38) Vanden-Eijnden, E.; Tal, F. A. J. Chem. Phys. 2005, 123, 184103−184103-10.

20299


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