Abstract
This work focuses on investigating the structural and electronic prop-erties of ReS2, a layered semiconducting member of the transition
metal dichalcogenide (TMD) family, which is distinguished by its high
in-plane anisotropy.
The structural and electronic properties of layered materials ReS2
have been probed at the nanometre scale by employing low temper-
ature ultra high vacuum (LT-UHV) scanning probe techniques in-
cluding: Non contact atomic force microscopy (NC-AFM), scanning
tunneling microscopy (STM), and scanning tunneling spectroscopy
(STS).
By performing both STM and NC-AFM, we were able to distinguish
the non-equivalent chalcogen atoms as well as identify the b-axis (or
Re-chain direction); and it was possible to assess the structural and/or
electronic contribution of each of the S and Re sublattices to the
features in images recorded using both techniques as they result from
different phenomena. Analysis shows that, as opposed to STM, the
NC-AFM images show a clear transition with the imaging parameters
that allows the Re-sublattice contribution to be measured or ”picked
up” more readily. We believe that this is the first NC-AFM study
performed on ReS2. The ability of the NC-AFM technique to reveal
information from subsurface atomic layers can be exploited further,
in the imaging of subsurface defects.
The STM/STS measurements reveal the effect of defects in ReS2 on
the electronic structure of the material. Localized defects, associated
with S monovacancies, as well as local density of states (LDOS) disturbances extending over several nm in lateral size and associated with subsurface, intercalated dopants, have been identified, as well as their signature density of states spectra, leading to localized, in-gap
states. These in-gap states significantly reduce the energy gap in the
vicinity of these defects, leading to electronic perturbations across the ReS2 surface.
Finally, a separate study was done to probe the anisotropic trans-
port in few-layer ReS2 by localised current injection and conductance
measurements using mobile scanning probe microscopy (SPM) probes.
The angle-dependent conductance measurements on a 4-layer mechan-
ically exfoliated ReS2 flake show strong angle-dependence over a nar-
row angle range around the b-axis of the material: this is consistent
with the existence of localised structural anisotropy. To reach this
conclusion, finite element analysis of transport through ReS2 films of
different geometries was performed; and a detailed understanding of
the current-voltage characteristics acquired via two-local probe mea-
surements and the nature of the contact junctions achieved with this
method was necessary. This study proposes methods to improve the
large in-built transport anisotropy in multi-layer ReS2 films by intro-
ducing structural inhomogeneity into the system.
| Date of Award | 22 Jun 2022 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Sponsors | University of Jeddah & The Saudi Arabian Cultural Bureau (SACB) |
| Supervisor | Adelina Ilie (Supervisor) & Marcin Mucha-Kruczynski (Supervisor) |