Molecular dynamics with combined quantum and empirical potentials: C2H2 adsorption on Si(100)

Abstract

Classical trajectory calculations were employed to study the reaction of acetylene with dimer sites on the Si(100) surface at 105 K. Two types of potential energy functions were combined to describe interactions for different regions of the model surface. A quantum mechanical potential based on the semiempirical AM1 Hamiltonian was used to describe interactions between C2H2 and a portion of the silicon surface, while an empirically parametrized potential was developed to extend the size of the surface and simulate the dynamics of the surrounding silicon atoms. Reactions of acetylene approaching different sites were investigated, directly above a surface dimer, and between atoms from separate dimers. In all cases, the outcome of C2H2 surface collisions was controlled by the amount of translational energy possessed by the incoming molecule. Acetylene molecules with high translational energy reacted with silicon dimers to form surface species with either one or two Si–C bonds. Those molecules with low translational energy either rebounded away from the surface or became trapped in a physisorbed state as evidenced by their bouncing motion above the surface. The reaction of C2H2 to form a bridge between dimers within the same dimer row was found to occur, while bridging between adjacent dimer rows appeared to be unlikely, the C2H2 molecule preferring to migrate to either of the dimers for direct reaction. A mechanism is proposed for chemisorption in which C2H2 first bonds to a dimer site in a mono-σ structure, subsequently attaining the more stable di-σ bonded state through radical–radical recombination. The simulations are consistent with C2H2 adsorption on Si(100) occurring through a mobile precursor mechanism.

Acetylene adsorption on Si(100): A study of the role of surface steps

“…Furthermore this size also allows a correct description of the bulk cohesive energy, of the first-and second-nearest neighbour distances, of the intradimer distance and of the dimer formation energy. These results are consistent with literature data which indicate some tens atoms as the size needed for an accurate description, in a cluster representation, of adsorption of atoms and small molecules [9,10,13]. The following calculations are obtained with a cluster size N between 100 and 200.…”

“…The simulation conditions adopted in the time-dependent calculations (TD) are similar to the ones used in [9,13,16]. Accordingly, the transient is generated by depositing the molecule at a variable distance above the surface with its CC axis parallel or perpendicular to the dimer rows.…”

“…However further experiments (see [3][4][5][6] and references therein) suggested the existence of different adsorption sites, though the assignment of the observed configurations to those sites is still hotly contended. Furthermore the dependence on temperature and coverage of the population of those sites indicates that the early stages of adsorption are kinematically, rather than thermodynamically, driven so that the motions of the molecule have to be carefully considered (earlier and recent references are [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], respectively. These studies offer a representative list of theoretical and experimental works up to date).…”

Real-space adsorption studies of cyclooctyne on Si(001)

“…The first geometry is well known from the adsorption of other organic molecules, e.g., C 2 H 2 or C 2 H 4 on Si(0 0 1) [14][15][16][17][18][19][20][21][22][23][24][25]11,26]. The latter geometry was discussed controversially in case of C 2 H 2 / Si(0 0 1) [27,28,14,15,29,17]. Because of the experimental results at low cyclooctyne coverage, the tetra-r geometry, if it exists, should be much more favorable than the adsorption in the di-r geometry which then should only occur next to an already adsorbed molecule.…”

Multiple cluster model (MCM) for surface reaction systems

“…w x Various hybrid models 14᎐25 integrating the quantum mechanical methods for the chemically active part and classical force-field methods for the surrounding part have been applied to large molecules such as biomolecules, organometallic complexes, and solute᎐solvent systems. The application of the hybrid method to surface chemisorpw x tion was reported by Carmer et al 26 . Morokuma and co-workers have developed not only the integrated molecular orbital q molecular mechanics Ž .…”