Functionalization of group V monolayers and their compounds: alloying, doping and surface modification
Author(s)
Advisor
Durgun, EnginDate
2020-11Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
There has been growing interest during the last decade in two-dimensional (2D)
materials due to their important roles in various scientific and technological applications such as detectors, lasers and light emitting diodes. In this thesis we
present a theoretical investigation of a couple of such 2D materials from group V
monolayers and their compounds. Firstly, ordered alloys of GaxAl1−xN hexagonal monolayer are studied and the effect of Al content on mechanical, electronic, thermal and optical properties are investigated. The optimized lattice constants
and band gaps change in accordance to Vegard’s Law. Low barrier energies and
favorable substitution of Ga by Al may show feasibility of fabrication. Segregation is also checked with mixing energy calculations. The dynamical stability of
alloys is shown by phonon spectrum analysis and MD simulations. GaxAl1−xN
alloys give lower in-plane stiffness than h-BN or graphene, but higher Poisson’s
ratio than most realized 2D systems. Heat capacity of alloys delivers a decrease
with Al content at low temperatures but it converges to the classical limit at high
temperatures. The absorption onset of GaxAl1−xN alloys remain in the near UV
range and prominent absorption peaks blue-shifts with increasing x in compliance
with the variation of the band gap. The considered systems, in regard to their
stability and tunable fundamental properties seem to be very promising 2D semiconductors for wide range of applications at reduced scales. Then, the interaction of alkali metal atoms (Li, Na, and K) with single layer and periodic structures
of hb-As and sw-As phases are revealed by first-principles methods. Arsenene
phases are considered to be used as electrodes (anode) for ion-batteries. Strong
alkali-electrode binding and low diffusion energy barriers gives out better cycling
stability and faster diffusion, respectively. hb-As shows better storage capacity
than sw-As. However, deviations from ordered pattern and decline of formation
energy with increasing doping level point out a possible structural transformation. By MD calculations, crystalline to amorphous phase transition is seen even for low concentrations level at ambient temperature. The average open-circuit
voltages of 0.68-0.88 V (0.65-0.98 V) with specific capacity up to 715 mAhg−1
(358 mAhg−1) are calculated for single layer (periodic) configurations. Overall,
non-crystalline phases are calculated to offer more favorable structures than crystalline configurations and they provide more coherent results when compared with experimental data. The obtained voltage profile together with low diffusion barriers and strong metal-electrode binding suggests arsenene as a promising anode material to be used in for alkali-ion battery applications. Lastly, the formation of
dumbbell (DB) geometry upon adsorption of Ga, N adatoms to GaN monolayer
is investigated. While Ga-N DBs are unstable, Ga-Ga and N-N DB geometries
are predicted to form in an exothermic and spontaneous scheme. Cohesive energy
of hexagonal GaN monolayer decreases when a DB is formed on its surface. Electronic structures for Ga-Ga DBs for 2×2, 3×3, 4×4 and 5×5 phases show spinpolarized and degenerate bands mainly contributed by p-orbitals of the atoms in
impurity zone. Degenarated bands are not observed for N-N dumbbell for HDP,
TDP, 2×2 and 3×3 phases. Upon DB formation, semiconductor GaN monolayer
become spin-polarized semiconductor with varying band gap, where this functionalization allows electronic structure to be tuned substantionally. This would be highly desired for nanoscale electronic and optical devices. These Ga-Ga and
N-N DB geometries may also be used for the synthesis of layered GaN structures.
Keywords
First principlesab initio
Density functional theory (DFT)
Nanostructures
Monolayer
Phonon
Electronic properties
Doping
2D materials
GaN
AlN
Arsenene
Dumbbell