Nanoscale Investigation of Tungsten Ditelluride and Zinc Oxide Thin Films via Scanning Probe Microscopy

Abstract

Scanning probe microscopy (SPM) is a powerful technique to investigate the surface properties at the nanoscale such as topography, ferroelectricity, piezoelectricity, and conductivity. In this thesis, these properties are studied in bulk WTe2, La-doped ZnO (ZnO:La) thin films, Highly Oriented Pyrolytic Graphite (HOPG) and Nb-doped SrTiO3. HOPG is firstly studied as the standard calibration material for scanning tunnelling microscopy (STM) and observed Moire patterns of twisted layers with a rotation angle of 5.4 degrees. To utilizing cross-sectional STM (XSTM), the fractured Nb-doped SrTiO3 (001) reveals different termination terraces of TiO2 and SrO on cleaved surfaces. Surface oxidation of WTe2 surfaces in different environments reveals non-uniform surface oxide dynamics, which upon saturation after several hours of atmospheric exposure yields a self-limiting approximately 2.5 nm-thick amorphous surface layer. Also, in a controlled environment involving a continuous flow of nitrogen, the surface oxide layer forms dendritic surface texture and is considerably impeded. WTe2 is a semimetal in which ferroelectricity was observed for the first time. SPM allows resolving spontaneously formed domains as well as so far un-detected stripe and ripple structures in bulk WTe2. The stripes are pushed or dragged by the electric field generated by the STM tip, and they can be bent or pinned by a non-uniform external field and defects on the surface. The ripples present merging and splitting of lines and, to some degree, are crystallographically oriented and have a concentration of point defects at the valleys of the ripples. Both defects and intrinsic strains of the material likely drive the formation of the ripple structure. Another material system investigated in this thesis is ZnO, which is highly promising for electronic and photonic devices. SPM study of spray pyrolysis grown ZnO:La thin films reveal that as-grown in-plane piezoelectric domains independent of topographic properties. At the same time, conductance correlates to topography and piezoelectric domains.