Bonggeun Shong

Finite-Size Effects in O and CO Adsorption for the Late Transition Metals

Gold is known to become significantly more catalytically active as its particle size is reduced, and other catalysts are also known to exhibit finite-size effects. To understand the trends related to finite-size effects, we have used density functional theory to study adsorption of representative adsorbates, CO and O, on the late transition metals Co, Ni, Cu, Ir, Pd, Ag, Rh, Pt and Au. We studied adsorption energies and geometries on 13-atom clusters and compared them to the fcc(111) and fcc(211) crystal facets. In all cases, adsorbates were found to bind significantly more strongly to the 13-atom clusters than to the extended surfaces. The binding strength of both adsorbates were found to correlate very strongly with the average coordination number of the metal atoms to which the adsorbate binds, indicating that the finite-size effects in bonding are not specific to gold.

In Vacuo Photoemission Studies of Platinum Atomic Layer Deposition Using Synchrotron Radiation.

The mechanism of platinum atomic layer deposition using (methylcyclopentadienyl)trimethylplatinum and oxygen is investigated with in vacuo photoemission spectroscopy at the Stanford Synchrotron Radiation Lightsource. With this surface-sensitive technique, the surface species following the Pt precursor half cycle and the oxygen counter-reactant half cycle can be directly measured. We observed significant amounts of carbonaceous species following the Pt precursor pulse, consistent with dehydrogenation of the precursor ligands. Significantly more carbon is observed when deposition is carried out in the thermal decomposition temperature region. The carbonaceous layer is removed during the oxygen counter reactant pulse, and the photoemission spectrum shows that a layer of adsorbed oxygen remains on the surface as previously predicted.

Unidirectional Adsorption of Bifunctional 1,4-Phenylene Diisocyanide on the Ge(100)-2 × 1 Surface.

Adsorption of bifunctional organic molecules on semiconductor surfaces is important for surface modification; however, most bifunctional molecules previously studied have yielded mixtures of singly and dually tethered adsorbates. Here we report the adsorption of bifunctional 1,4-phenylene diisocyanide (PDI) on the Ge(100)-2 × 1 surface, in which singly bound adsorbates are selectively produced. As shown by polarized multiple internal reflection infrared spectroscopy experiments and density functional theory calculations, PDI adsorbates form a single C-dative bonding configuration through one of the isocyanide functionalities, retaining one unreacted isocyanide moiety per adsorbate. The angle of the molecular axis is ∼30° from the surface normal. The delocalized π* molecular orbital of the free molecule is also preserved upon adsorption. These results demonstrate the potential usefulness of isocyanide adsorbates as a means toward selective organic functionalization of semiconductor surfaces.

Strong Carbon-Surface Dative Bond Formation by tert-Butyl Isocyanide on the Ge(100)-2 × 1 Surface

Carbon dative bond formation between an organic molecule and a semiconductor surface is reported here for the first time. Our studies show that the adsorption of tert-butyl isocyanide on the (100) surface of germanium, measured using Fourier transform infrared spectroscopy, temperature-programmed desorption, and density functional theory calculations, occurs via formation of a dative bond to the surface through the isocyanide carbon. The experimentally observed adsorption energy of 26.8 kcal/mol is the largest among any organic molecule dative bonded on the Ge(100)-2 × 1 surface studied to date. The dative-bonded adsorbate is characterized by a N≡C stretching frequency significantly blue-shifted from that of the free molecule. Moreover, the adsorbate N≡C vibrational frequency red-shifts back toward that of the free molecule upon increasing coverage. These spectroscopic effects are attributed to σ-donation of the isocyanide lone pair electrons to the surface.