(1×1) rippled relaxation of (100) transition-metal carbide surfaces

Abstract

An investigation of (100) surfaces of TaC and HfC was performed using low-energy electron diffraction (LEED) and angle-resolved ultraviolet photoemission spectroscopy with synchrotron radiation. Detailed LEED I–V analyses show that, on both surfaces, carbon atoms displace outward and metal atoms inward, giving rise to (1×1) rippled relaxations, with the magnitude of the ripple being larger on TaC(100). Photoemission spectra of metal 4f levels from these surfaces show no obvious surface core-level shifts. Using x-ray photoemission spectroscopy (XPS) and Auger electron spectroscopy (AES), changes in (bulk) core-level binding energies, valence-band structures, and local carbon environments were determined for TaCx (0.5<x<1.0) and HfCx (0.6<x<1.0). From these bulk studies, it was inferred that, as x decreases, added negative charge accumulates in the vicinity of metal atoms while some charge transfers away from remaining carbon atoms. Insight obtained from the XPS and AES investigations is used in this paper to discuss the charge distribution at the (100) surfaces of TaC and HfC. It is concluded that carbon atoms in the outermost layer move outward to allow their compressed s,p electrons to expand to a state of lower kinetic energy, and metal atoms move inward because of added charge accumulating in their vicinity. We propose that the larger ripple on TaC(100) results form a larger accumulation of charge near the metal atoms on that surface.