THESIS
2020
iv, 152 pages : illustrations (some color) ; 30 cm
Abstract
Two-dimensional (2D) materials have attracted sustained intense interest in
the condensed matter physics community for many years because they exhibit a
variety of unique and interesting physical properties. The growth of 2D materials
on the surfaces of single crystalline metal and semiconductor substrates is an
important approach to fabricate these intriguing materials for fundamental study
of their structures and properties. This approach is also under intense development
as a scalable method for the production of 2D materials with large area and high
quality suitable for practical applications. The work described in this thesis
addresses several outstanding issues in the growth and structure of 2D materials
using powerful low energy electron microscopy and diffraction method...[
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Two-dimensional (2D) materials have attracted sustained intense interest in
the condensed matter physics community for many years because they exhibit a
variety of unique and interesting physical properties. The growth of 2D materials
on the surfaces of single crystalline metal and semiconductor substrates is an
important approach to fabricate these intriguing materials for fundamental study
of their structures and properties. This approach is also under intense development
as a scalable method for the production of 2D materials with large area and high
quality suitable for practical applications. The work described in this thesis
addresses several outstanding issues in the growth and structure of 2D materials
using powerful low energy electron microscopy and diffraction methods.
We first performed a comparative study of the growth and structure of iron
oxide monolayers formed by oxidizing Fe on Pt(111) using molecular oxygen and,
for the first time, atomic oxygen. A new (9x9) coincidence superstructure of the
FeO(111) monolayer formed using atomic oxygen was identified. Oxidation with
atomic oxygen also drives the formation of a new oxygen-enriched monolayer
that exhibits a (√403x√403) R22.8° higher order coincidence superstructure. This
novel iron oxide was observed to facilitate CO oxidation at elevated temperature.
The biphase surface structure of a α-Fe
2O
3(0001)/Pt(111) thin film was also
revealed to be composed of a FeO(111) monolayer on top of α-Fe
2O
3(0001) film
with (19x19) coincidence superstructure. These outcomes expand knowledge of
reactive enriched iron oxide monolayers and resolve the controversial nature of
the biphase structure.
Our investigations also addressed the structure of the well-known and
decades-long puzzling (5.55x5.55) incommensurate structure of a Cu
2Si
monolayer on the Si(111) surface. This monolayer attracts new interest as a
possible 2D Dirac material. The key observation in our work was the coexistence
of mirror reflection symmetry and broken mirror symmetry of superstructure peak
intensities in the LEED pattern. We proposed that Cu
2Si monolayer forms a
rotated (11/√103 x 11/√103)R9.83° structure. This structure generates a (11x11)
R60° coincidence superstructure that explain the perplexing LEED pattern
symmetry properties. Our proposal is confirmed by observations of domains with
oppositely rotated structure that were made with tilted dark field LEEM data. We
also discovered a new “2D distillation” growth mechanism that can be exploited
to scalably produce high quality hexagonal boron nitride (h-BN). 2D distillation
growth transformed a R24.3° misaligned h-BN monolayer into an aligned h-BN
layer with well-known (12x12) coincidence superstructure and improved quality.
The novel 2D distillation mechanism is understood to be an unconventional
coarsening process that is driven by counter-intuitive concentration gradients.
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