Journal of Physical Chemistry B, Vol.105, No.42, 10340-10347, 2001
Possible dissociative adsorption of CH3OH and CH3NH2 on Si(100)-2 x 1 surface
Possible dissociative adsorptions of organic molecules such as CH3OH (CH3NH2) to form CH3O . . .H or CH3. . . OH (CH3NH . . .H or CH3. . . NH2) on the Si(100)-2 x 1 surface are discussed by using the hybrid density functional B3LYP method. The Si9H12 cluster is used as a model of the Si(100)-2 x I surface. First, a CH3OH adsorbs molecularly on the Si(100)-2 x 1 surface with no barrier and its stabilization energy is estimated to be 14 kcal/mol. Next, the molecularly adsorbed CH3OH dissociates to CH3O . . .H or CH3. . . OH. The activation energies to form CH3O . . .H/Si9H12 and CH3. . . OH/Si9H12 from the molecularly adsorbed CH3OH/Si9H12 are calculated to be 3.8 and 27.8 kcal/mol, respectively. The transition states in these reactions lie 10.5 kcal/mol below and 14.0 kcal/mol above the energy of the initial state (isolated bare Si9H12 cluster + CH3OH molecule), respectively, and thus the former reaction has no barrier to dissociative chemisorption and will occur under milder conditions than the latter. The overall exthothermicity from initial state is calculated to be 65.3 kcal/mol in the former reaction, while 80.9 kcal/mol in the latter reaction and thus the product in the latter reaction is more stable than in the former reaction. Therefore, the former reaction would proceed under mild conditions while the latter reaction would proceed under severe conditions. Same results are obtained in the case of the dissociative adsorptions of CH3NH2 on the Si(100)-2 x 1 surface; the dissociatively adsorbed CH3NH . . .H/Si9H12 is more likely to be produced under mild conditions while the dissociatively adsorbed CH3. . . NH2/Si9H12 is more likely to be produced under severe conditions. Orbital interactions and charge transfers have been analyzed to clarify the differences of the adsorption mechanisms. Significant negative charge transfer from the silicon surface to the CH3OH and CH3NH2 molecules is the main reason these molecules dissociatively adsorb on the silicon surface in a reductive manner.