AN STM STUDY OF SURFACE-STRUCTURES ON WO3(001)

Structure on the (001) surface of monoclinic WO3 has been studied by s canning tunnelling microscopy. Electrostatic instability at the polar surface of WO3(001) is avoided by termination of the outer WO2 layer w ith half a monolayer of oxygen ions to produce a (root 2 x root 2)R45 degrees arrangement of on-top oxygen. An additional periodicity is int roduced in some terrace areas by tilting of WO6 octahedra to produce a lternately long and short on-top O-O separations along the [110] direc tion. This produces a (2 x 2) superstructure approximating to the spac e group c2mm. Prominent defect troughs on oxygen annealed surfaces are attributed to missing oxygen ions. Topographically resolved I-V curve s show that significant filled state electron density resides in the t rough bottoms. Argon ion bombardment and subsequent annealing leads to the appearance of defects in which on-top oxygen is removed from adja cent W ions to generate linear features three unit cells wide running along the [100] direction. These new defects aggregate with the trough s to produce characteristic branched structures. Prolonged bombardment and annealing leads eventually to large terrace areas displaying a ne w (2 x 2) superstructure in which only a 1/4 of the W ions carry on-to p oxygen.