Molecular Dynamic Structure Investigations of the Surface Stability and Adsorption of H, H 2 , CH 3 , C 2 H 2 : (100) Di
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MOLECULAR DYNAMIC STRUCTURE INVESTIGATIONS OF THE SURFACE STABILITY AND ADSORPTION OF H, H 2 , CH 3, C 2H 2 : (100) DIAMOND Th. Frauenheim, P. Blaudeck and D. Porezag Technical University, Physics Department, PSF 964, 0-9010 Chemnitz, Germany
ABSTRACT Surface properties - stability and reconstruction - of clean and hydrogenated diamond (100) have been studied by real temperature molecular dynarnic (MD) simulations using an approximate density functional (DF) theory expanding the total electronic wave function in a minimal basis of localized atomic valence electron orbitals (LCAO - ansatz). The clean surface is highly unstable against a spontaneous dimerization resulting in a 2x1 reconstruction. Atomic hydrogen in the gas phase above the top surface at all temperatures and 112 molecules approaching the center of the dimer bond at room temperature are reactive in breaking the dimer ir-bonds forming a monohydrogenated surface which maintains a stable 2x1 structure but with elongated surface C-C dimer bonds remaining stable against continuing hydrogen supply. The dihydrogenated surface taking a lx1 structure, because of steric overcrowding dynamically becomes unstable against forming a lxl (alternating) di-, monohydrogenated surface. As first elementary reaction processes which may be discussed in relation to diamond growth we studied the thermal adsorption of CH 3 and C 2 H 2 onto a clean 2xl reconstructed (100) diamond surface.
INTRODUCTION Within the last decade there has been a growing interest in a fundametal understanding of the low-temperature, low-pressure gas phase synthesis of diamond [1-4]. To control the conditions for homo- or (may be) heteroepitaxial diamond growth there is a need to achieve a detailed understanding of elementary reaction mechanisms occuring in nucleation and adsorption processes on crystalline substrates at the molecular level of chemical bonding. First of all one has to answer the question of the surface stability under different conditions (preparation and saturation). Then one must search for active (radical or 7r-bonded) surface
sites which may act as nucleation centres for adsorption of hydrocarbon molecules, radicals or fragments forming seed particles from which the growth will propagate. At this step one has to look from the stability point of view for the energetic and geometric most favourable hydrocarbon fragments supporting crystal growth and one has to search for the reaction pathways giving rise to a growth mode for an epitaxial monolayer formation and for optimal conditions stabilizing these pathways. The only realistic way to do this has to be based on quantum mechanical origin, solving the
electron problem which through the action of interatomic forces causes the dynamic structure formation. Though we are still far away from a realistic MD modelling of CVD - processes on crystalline surfaces because of computational problems, we can start to learn more about the principal physico - chemical processes at surfaces from representative model systems. All work in this fie
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