Structural analysis of the nitrogen-adsorbed NbC(111) surface.

Abstract

The structure of the nitrogen-adsorbed NbC(111) surface has been investigated in ultrahigh vacuum by impact-collision ion-scattering spectroscopy (ICISS), reflection high-energy electron diffraction (RHEED), and by computer simulation. At room temperature, nitrogen is dissociatively adsorbed on the surface, which shows a diffuse RHEED pattern of (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} structure at low coverage and (1\ifmmode\times\else\texttimes\fi{}1) structure at saturation coverage. When the nitrogen-saturated surface is annealed at 950 \ifmmode^\circ\else\textdegree\fi{}C for a few seconds, the RHEED pattern changes into a sharp (2\ifmmode\times\else\texttimes\fi{}2) pattern. It has been found that, in both cases, a nitrogen atom goes into the threefold-hollow site on the second-layer carbon atom without changing the structure of the substrate surface. The height of a nitrogen atom from the first layer is 1.14 \AA{} and the N-Nb distance is 2.15 \AA{}, which is close to the sum of each of the ionic radii. The coverage of the (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} and (2\ifmmode\times\else\texttimes\fi{}2) structure are estimated at 0.33 and 0.5 ML, respectively. Judging from the coverage, the apparent (2\ifmmode\times\else\texttimes\fi{}2) structure is considered to be the (2\ifmmode\times\else\texttimes\fi{}1) structure with three equivalent domains.