48 research outputs found
Electronic and spectral properties of Ge1-xSnx quantum dots: an atomistic study
In this paper, we study theoretically the electron and spectral properties of
Ge1-xSnx systems, including alloys, cubic- and spherical quantum dots. The
single-particle electron and hole states are calculated within the sp3d5s*
tight-binding approach and used in further modeling of the optical properties.
We systematically study the interplay of Sn-driven indirect-direct band-gap
transition and the quantum confinement effect in systems of reduced
dimensionality. We demonstrate the regime of sizes and composition, where the
ground state in Ge1-xSnx quantum dot is optically active. Finally, we calculate
absorbance spectra in experimentally-relevant colloidal quantum dots and
demonstrate a satisfactory agreement with experimental data
Electron Beam-Induced Reduction of Cuprite
Cu-based materials are used in various industries, such as electronics, power generation, and catalysis. In particular, monolayered cuprous oxide (Cu2O) has potential applications in solar cells owing to its favorable electronic and magnetic properties. Atomically thin Cu2O samples derived from bulk cuprite were characterized by high-resolution transmission electron microscopy (HRTEM). Two voltages, 80 kV and 300 kV, were explored for in situ observations of the samples. The optimum electron beam parameters (300 kV, low-current beam) were used to prevent beam damage. The growth of novel crystal structures, identified as Cu, was observed in the samples exposed to isopropanol (IPA) and high temperatures. It is proposed that the exposure of the copper (I) oxide samples to IPA and temperature causes material nucleation, whereas the consequent exposure via e-beams generated from the electron beam promotes the growth of the nanosized Cu crystals
Strong substrate strain effects in multilayered WS2 revealed by high-pressure optical measurements
The optical properties of two-dimensional materials can be effectively tuned
by strain induced from a deformable substrate. In the present work we combine
first-principles calculations based on density functional theory and the
effective Bethe-Salpeter equation with high-pressure optical measurements in
order to thoroughly describe the effect of strain and dielectric environment
onto the electronic band structure and optical properties of a few-layered
transition metal dichalcogenide. Our results show that WS2 remains fully
adhered to the substrate at least up to a -0.6% in-plane compressive strain for
a wide range of substrate materials. We provide a useful model to describe
effect of strain on the optical gap energy. The corresponding
experimentally-determined out-of-plane and in-plane stress gauge factors for
WS2 monolayers are -8 and 24 meV/GPa, respectively. The exceptionally large
in-plane gauge factor confirm transition metal dichalcogenides as very
promising candidates for flexible functionalities. Finally, we discuss the
pressure evolution of an optical transition closely-lying to the A exciton for
bulk WS2 as well as the direct-to-indirect transition of the monolayer upon
compression.Comment: 39 pages, 13 figure
High-pressure Raman scattering in bulk HfS2: comparison of density functional theory methods in layered MS2 compounds (M = Hf, Mo) under compression
We report high-pressure Raman-scattering measurements on the transition-metal dichalcogenide (TMDC) compound HfS2. The aim of this work is twofold: (i) to investigate the high-pressure behavior of the zone-center optical phonon modes of HfS2 and experimentally determine the linear pressure coefficients and mode Grüneisen parameters of this material; (ii) to test the validity of different density functional theory (DFT) approaches in order to predict the lattice-dynamical properties of HfS2 under pressure. For this purpose, the experimental results are compared with the results of DFT calculations performed with different functionals, with and without Van der Waals (vdW) interaction. We find that DFT calculations within the generalized gradient approximation (GGA) properly describe the high-pressure lattice dynamics of HfS2 when vdW interactions are taken into account. In contrast, we show that DFT within the local density approximation (LDA), which is widely used to predict structural and vibrational properties at ambient conditions in 2D compounds, fails to reproduce the behavior of HfS2 under compression. Similar conclusions are reached in the case of MoS2. This suggests that large errors may be introduced if the compressibility and Grüneisen parameters of bulk TMDCs are calculated with bare DFT-LDA. Therefore, the validity of different approaches to calculate the structural and vibrational properties of bulk and few-layered vdW materials under compression should be carefully assessed
Carrier dynamics in thin Germanium–Tin Epilayers
The Si-based mid-infrared photonics is an emerging technology in which group-IV germanium–tin (Ge1–xSnx) binary alloys can play a fundamental role in the development of a Si-compatible photonic components including monolithically integrated coherent light sources and detectors, on the same Si or SOI substrate. Although the Ge1–xSnx-on-Si lasers, at low temperatures, have already been demonstrated, the knowledge of the material properties necessary for such device optimization and real-life usage is very limited. In particular, carrier relaxation kinetics, relaxation pathways, and accompanied physical mechanisms, important for the laser’s dynamics, have not been subjected to in-depth research and understanding. In this work, we present detailed spectroscopic studies on photoinjected carrier dynamics in Ge1–xSnx epilayers, as a function of Sn content (6–12%) and temperature (20–300 K), by utilizing time-resolved differential reflectivity and photoluminescence. The latter technique allowed us to track separated electron and hole dynamics with a femtosecond time resolution, while the former experiment exploited a joined electron–hole recombination. This experimental approach allowed us to identify (i) two initial electron relaxation processes after photoexcitation; (ii) radiative electron–hole recombination on below-band gap states; (iii) nonradiative carrier recombination involving the Shockley–Read–Hall mechanism; and (iv) nonradiative recombination through the surface states. The research results significantly expand the knowledge on the initial carrier relaxation dynamics in the Ge1–xSnx epitaxial material. It provides unknown up-to-date kinetic parameters of the initial stage of electron relaxation and further carrier recombination dynamics, unveils the critical role of band gap inhomogeneity for the relaxation dynamics, and highlights the role of below-band gap states that can participate in the light generation process in Ge1–xSnx epilayers
The Defects Genome of 2D Janus Transition Metal Dichalcogenides
Two-dimensional (2D) Janus Transition Metal Dichalcogenides (TMDs) have
attracted much interest due to their exciting quantum properties arising from
their unique two-faced structure, broken-mirror symmetry, and consequent
colossal polarisation field within the monolayer. While efforts have been made
to achieve high-quality Janus monolayers, the existing methods rely on highly
energetic processes that introduce unwanted grain-boundary and point defects
with still unexplored effects on the material's structural and excitonic
properties Through High-resolution scanning transmission electron microscopy
(HRSTEM), density functional theory (DFT), and optical spectroscopy
measurements; this work introduces the most encountered and energetically
stable point defects. It establishes their impact on the material's optical
properties. HRSTEM studies show that the most energetically stable point
defects are single (Vs and Vse) and double chalcogen vacancy (Vs-Vse),
interstitial defects (Mi), and metal impurities (MW) and establish their
structural characteristics. DFT further establishes their formation energies
and related localized bands within the forbidden band. Cryogenic excitonic
studies on h-BN-encapsulated Janus monolayers offer a clear correlation between
these structural defects and observed emission features, which closely align
with the results of the theory. The overall results introduce the defect genome
of Janus TMDs as an essential guideline for assessing their structural quality
and device properties
Contactless electroreflectance study of the surface potential barrier in n-type and p-type InAlAs van Hoof structures lattice matched to InP
N-type and p-type In0.52Al0.48As van Hoof structures with various thicknesses of undoped In0.52Al0.48As layer (30, 60, 90, and 120 nm) were grown by metal-organic vapor phase epitaxy on InP substrates and studied by contactless electroreflectance (CER) at room temperature. The InAlAs bandgap related CER resonance followed by a strong Franz-Keldysh oscillation (FKO) of various periods was observed clearly for the two structures. This period was decreased with the decrease of thickness of undoped In0.52Al0.48As layer and was slightly narrower for p-type structures. The FKO period analysis indicates that the Fermi level is pinned 0.730.02 eV below the conduction band at In0.52Al0.48As surface. This pinning was attributed to the surface reconstruction combined with the adsorption of oxygen and carbon atoms (consequence of air exposure) which were detected on the In0.52Al0.48As surface by X-ray photoelectron spectroscopy. Also, CER measurements repeated one year after the sample growth shows that the process of InAlAs oxidation in laboratory ambient is negligible and therefore this alloy can be used as a protective cap layer in InP-based heterostructures
High Bi content GaSbBi alloys
The epitaxial growth, structural, and optical properties of GaSb 1– x Bi x alloys have been investigated. The Bi incorporation into GaSb is varied in the range 0 < x ≤ 9.6% by varying the growth rate (0.31–1.33 μm h−1) at two growth temperatures (250 and 275 °C). The Bi content is inversely proportional to the growth rate, but with higher Bi contents achieved at 250 than at 275 °C. A maximum Bi content of x = 9.6% is achieved with the Bi greater than 99% substitutional. Extrapolating the linear variation of lattice parameter with Bi content in the GaSbBi films enabled a zinc blende GaBi lattice parameter to be estimated of 6.272 Å. The band gap at 300 K of the GaSbBi epitaxial layers decreases linearly with increasing Bi content down to 410 ± 40 meV (3 μm) for x = 9.6%, corresponding to a reduction of ∼35 meV/%Bi. Photoluminescence indicates a band gap of 490 ± 5 meV at 15 K for x = 9.6%