Theoretical study of intradimer mechanism for diamond growth over diamond (100)

Abstract

The study of the energetics of the accepted intradimer diamond growth mechanism over (100) diamond surface is presented. The calculations were made in a density functional approach with the DGauss code using a DZVP2 basis set and a BLYP interchange and correlation potential. A simple 9-carbon cluster modeling the (100) diamond surface was used

its validity is discussed in relation with other calculations that used larger model clusters. The mechanism, presented in six steps, is based in the Harris and Garrison's work that considers the methyl radical as the main growth precursor agent and the breaking of the dimer surface bond with the corresponding methylene radical formation as a prior step to the formation of a CH2-bridge structure, which is a feasible step

in contrast to these molecular dynamics results, Huang and Frenklach, using semiempirical methods, consider the breaking of the dimer surface bond and the formation of a CH2-bridge structure as one step and this step as the energetically determinant of the mechanism. They also found an activation energy barrier for the interaction between a radical surface center with a H. and CH3.. The present work tries to discern between these two ideas by calculating the activation barriers and the reaction energies for each step of the Harris and Garrison's mechanism in a density functional approach and comparing them to the results of Huang and Frenklach. The energy calculations point toward the scission of the dimer bond (step 4) as the determinant step

this step is endothermic, with an energy barrier of 50.43 kcal-mol(-1). On the other hand, the formation of the CH2-bridge structure (step 5) is a feasible step with an energy barrier of 13.57 kcal-mol(-1). The adsorption of CH3. (step 2) and H. (step 6) species over radical surface sites did not involve any energy barriers, as it would be hoped. These steps were strongly exothermic and are close to the thermodynamic values for C-C and C-H bond energies. The removal of methylic hydrogen (step 3) did not show any problem because the activation barrier is only 3.68 kcal-mol(-1) less than the removal of a surface hydrogen (step 1), which has an energy barrier of 19.59 kcal-mol(-1). All steps, except number 4, were exothermic.