Quasi-particle interference (QPI) from defects combined with Fourier-transform scanning tunnelling spectroscopy (FT-STS) proved to be a powerful probe of the electronic structure, most notably in a complex 1H-WSe2/Au(111) heterostructure system. Combined with angle-resolved photoemission spectroscopy (ARPES) and theory, the electronic states could be mapped; revealing that the R25° 1H-WSe2/Au(111) could be described as a weakly interacting Umklapp system. QPI patterns showed the signature band-folding induced by the moiré super-lattice, and were used to experimentally determine the location of the shifted Au(111) Shockley state. Both FT-STS and ARPES identified a non-dispersive intragap state, which was explained by simulations as a hybridised continuum of states between the W and Se gap edge states and Au bulk bands; now indicating that the system was of metallic nature. To our knowledge this the most extensive analysis involving QPI over such a wide range of energies in a complex system. In addition, FT-STS was used to investigate defects in the transition metal dichalcogenide (TMD) WSe2; where the presence of spin-flip scattering processes determined a magnetic defect. One such defect with a structure corresponding to a column defect of which is predicted to be magnetic, induced these spin-flip channels, whereas other defects only displayed spin-conserving processes. The structures of other intrinsic defects were also explored with scanning tunnelling microscopy (STM), and two types were identified; the Se monovacancy and the Se, or O interstitial defect. A separate study was done on the decoration of Pd atoms on the WSe2 crystal surface. Pd dopants were found to locally and globally influence the material; introducing edge states into the electronic structure and a resulting p-type doping of the material. High-temperature annealing then resulted in a large-scale reconstruction of the surface, mediated by the Pd adatoms. Regions were observed with a 1T’-WSe2 structural phase indicating a phase transition.
|Date of Award
|24 Jun 2020
|Adelina Ilie (Supervisor) & Simon Crampin (Supervisor)