Transition-metal dichalcogenides and the scanning tunnelling microscope: The creation and imaging of vacancy defects

This thesis is concerned with the investigation of the creation and imaging of monatomic defects on the surfaces of the transition-metal dichalcogenides with the Scanning Tunnelling Microscope (STM). The ab-initio Projector Augmented Wave (PAW) method is used to study single-atom vacancies on the MoS2 and MoTe2 surfaces. The presence of the vacancies results in localised electronic states in the surface band-gap which, according to the Tersoff-Hamann approximation, will affect the STM image. Based upon the results, the 'trimer' and monatomic hole defects previously created and observed on WSe2 and MoS2 can be explained as monatomic metal and chalcogen vacancies respectively. It is also found that, while the chalcogen vacancy systems behave in a qualitatively similar manner for both MoS2 and MoTe2, different ordering of the gap-states of the two metal vacancy systems leads to a Jahn-Teller effect in the MoTe2-metal vacancy system. The effects this has on the ground-state atomic and electronic structure are discussed. The mechanisms behind surface modification using the STM are then examined for the specific case of the MoS2 surface. The PAW method is used to simulate the MoS2 surface in the presence of a Mo tip under an electric field; the barriers against extraction of single Mo and S atoms are calculated with and without the field. The temperatures required to extract Mo and S atoms from the surface by the STM tip with an applied bias are found to be around 5100K and 1600K respectively. It therefore seems unlikely that thermal activation alone can explain the atom-transfer process and alternatives are discussed