First Principles Studies On Several Surface Problems Concerning Adsorption

Using first principles method within the framework of density functional theory,we systematically investigated several surface problems all concerning adsorption.We first studied the structural properties and characters (i.e., the patterns and lat-tice constants) of the adsorptions of As, Sb and Bi atoms on the 2×1 reconstructed Si(001) surface at 1/2 monolayer coverage, and their potential usages for further growingnanostructures. It was found that the surfactant atoms were dimerized on the Si surface.For Sb or Bi, the stable adsorption pattern was a surfactant: 2×2 reconstruction one(i.e., the square pattern); while for As, a surfactant: 2×4 reconstruction pattern (i.e.,the row pattern) was superior to the square pattern. This di?erence might be attributedto di?erent atomic radius and property of the surfactant with respect to Si. The stressinduced by partial As-adsorption made the caves of exposed Si (001) surface morematchable with the self-assembly growth of Ge-dimer rows. The energy gap of the sys-tem was appreciably decreased. The top of the valence bands were mainly contributedby the Ge-dimer row. This meant it could be termed as a nanowire for electronic ap-plications. This also implied a control with molecular precision in nanoscience andnanotechnology, and an attempt to integrate semiconductor nanowires into traditionalSi industry processing.We then systematically studied the adsorption of O2 molecules on Pb films withthickness ranging from 3 to 11 monolayers. It was found that the O2 molecule prefersto adsorb on the Hcp/Fcc hollow sites, and the most stable ones are the ones on theHcp hollow sites with adsorption energy in the range of several hundred meV per Oatom. No covalent bonding was observed. There exists, instead, charge transfer at theinterface between the Pb film and O2 molecule due to orbital hybridization similar tothat of molecular hydrogen binding on metal atoms. The charge transfer at the interfaceand the adsorption energy show bilayer oscillation with thickness. Our study providesthe first theoretical evidence for quantum modulation of surface reactivity in agreement with experiment.Finally, we systematically investigated the in?uences of Pb adatoms on the ad-sorption and dissociation of O2 molecules on the Pb (111) surface. It is found that theadsorption of O2 around a Pb adatom is barrierlessly dissociative, with the dissociatedO atoms bonding with both the Pb adatom and surface Pb atoms. Comparing to themolecular adsorption of O2 on clean Pb surfaces, the adsorption energy of O2 moleculedramatically increases by 1.48 eV per oxygen atom. Moreover, for an adsorbed O2molecule on a clean surface, its interaction with a di?using Pb adatom could make iteasier to dissociate. Our results suggest that the existence of the Pb adatoms may playan important role in the oxidation of the Pb (111) surface.