Bond softening in monolayer graphite formed on transition-metal carbide surfaces

Full phonon-dispersion curves of the graphitic layer on some transition-metal carbides were measured by using high-resolution electron-energy-loss spectroscopy. The graphitic layer on (111) surfaces of TaC, HfC, and TiC showed anomalous softening within the layer, whereas the graphite layer on TaC(001) is similar to bulk graphite. The measured phonon dispersion has been analyzed with a force-constant model, and it has been revealed that the force constants for vertical angle bending and for bond twisting are much weaker in the former case. This is ascribed to the charge transfer into the overlayer from the substrate, resulting in the weakening of the bond in graphite. The clear contrast in variations of phonons between the graphitic layer on TaC(111) and that on TaC(001) indicates that the microscopic structure at the interface determines the charge transfer.