Site-selective adsorption of C_2H_5OH and NO depending on the local structure or local electron density on the Si(111)-7 X 7 surface

摘要

The sticking probability of ethanol (C_2H_5OH) on a Si(111)-7 X 7 surface is entirely coverage independent, and the adsorption is suddenly stopped when three adatoms (half of the adatoms) are changed to Si-OC_2H_5 in every half unit cell. This Si(111)-7 X 7 surface saturated with C_2H_5OH, however, is active for the adsorption of NO. A more surprising fact is that the sticking probability of NO on this surface is equal to that on the virgin Si(111)-7 X 7 surface. That is, the sticking probability of C_2H_5OH and NO on a Si(111)-7 X 7 surface has no influence of the coverage of Si-adatoms with Si-OC_2H_5. From these results, we deduced that the dissociation of NO and C_2H_5OH takes place via the precursor state adsorption. The coverage of adatoms gives no effect on the sticking probability of NO and C_2H_5OH, the adsorption of NO on the faulted halves prefers that on unfaulted halves (F/U = 2.2～2.4), although no such preference was observed in the adsorption of C_2H_5OH (F/U = 1). N and O atoms formed by the dissociation of NO migrate over the surface and are finally trapped by Si-adatoms. On the other hand, adsorption of C_2H_5OH occurs by dissociating the precursor state molecule on an adatom-rest atom site, which is the final step of the adsorption. Therefore, the scanning tunneling microscopy image of C_2H_5OH reflects the structure of the sites, while that of NO reflects the electron density of sites. The physical meaning of the sticking probability, preferential adsorption on the faulted or unfaulted halves, and site selectivity of the center to corner adatoms in a half unit cell are discussed.