13 research outputs found
High pressure effect on structure, electronic structure and thermoelectric properties of MoS
We systematically study the effect of high pressure on the structure,
electronic structure and transport properties of 2H-MoS, based on
first-principles density functional calculations and the Boltzmann transport
theory. Our calculation shows a vanishing anisotropy in the rate of structural
change at around 25 GPa, in agreement with the experimental data. A conversion
from van der Waals(vdW) to covalent-like bonding is seen. Concurrently, a
transition from semiconductor to metal occurs at 25 GPa from band structure
calculation. Our transport calculations also find pressure-enhanced electrical
conductivities and significant values of the thermoelectric figure of merit
over a wide temperature range. Our study supplies a new route to improve the
thermoelectric performance of MoS and of other transition metal
dichalcogenides by applying hydrostatic pressure.Comment: 6 pages, 6 figures; published in JOURNAL OF APPLIED PHYSICS 113, xxxx
(2013
Double resonance Raman modes in monolayer and few-layer MoTe[subscript 2]
We study the second-order Raman process of mono- and few-layer MoTe[subscript 2], by combining ab initio density functional perturbation calculations with experimental Raman spectroscopy using 532, 633, and 785 nm excitation lasers. The calculated electronic band structure and the density of states show that the resonance Raman process occurs at the M point in the Brillouin zone, where a strong optical absorption occurs due to a logarithmic Van Hove singularity of the electronic density of states. The double resonance Raman process with intervalley electron-phonon coupling connects two of the three inequivalent M points in the Brillouin zone, giving rise to second-order Raman peaks due to the M-point phonons. The calculated vibrational frequencies of the second-order Raman spectra agree with the observed laser-energy-dependent Raman shifts in the experiment.National Science Foundation (U.S.). Division of Materials Research (Grant 1004147
Control of Surface and Edge Oxidation on Phosphorene
Phosphorene is emerging
as an important two-dimensional semiconductor, but controlling the
surface chemistry of phosphorene remains a significant challenge.
Here, we show that controlled oxidation of phosphorene determines
the composition and spatial distribution of the resulting oxide. We
used X-ray photoemission spectroscopy to measure the binding energy
shifts that accompany oxidation. We interpreted these spectra by calculating
the binding energy shift for 24 likely bonding configurations, including
phosphorus oxides and hydroxides located on the basal surface or edges
of flakes. After brief exposure to high-purity oxygen or high-purity
water vapor at room temperature, we observed phosphorus in the +1
and +2 oxidation states; longer exposures led to a large population
of phosphorus in the +3 oxidation state. To provide insight into the
spatial distribution of the oxide, transmission electron microscopy
was performed at several stages during the oxidation. We found crucial
differences between oxygen and water oxidants: while pure oxygen produced
an oxide layer on the van der Waals surface, water oxidized the material
at pre-existing defects such as edges or steps. We propose a mechanism
based on the thermodynamics of electron transfer to interpret these
observations. This work opens a route to functionalize the basal surface
or edges of two-dimensional (2D) black phosphorus through site-selective
chemical reactions and presents the opportunity to explore the synthesis
of 2D phosphorene oxide by oxidation
Sensitive Phonon-Based Probe for Structure Identification of 1TⲠMoTe<sub>2</sub>
In
this work, by combining transmission electron microscopy and
polarized Raman spectroscopy for the 1TⲠMoTe<sub>2</sub> flakes
with different thicknesses, we found that the polarization dependence
of Raman intensity is given as a function of excitation laser wavelength,
phonon symmetry, and phonon frequency, but has weak dependence on
the flake thickness from few-layer to multilayer. In addition, the
frequency of Raman peaks and the relative Raman intensity are sensitive
to flake thickness, which manifests Raman spectroscopy as an effective
probe for thickness of 1TⲠMoTe<sub>2</sub>. Our work demonstrates
that polarized Raman spectroscopy is a powerful and nondestructive
method to quickly identify the crystal structure and thickness of
1TⲠMoTe<sub>2</sub> simultaneously, which opens up opportunities
for the in situ probe of anisotropic properties and broad applications
of this novel material
In-Plane Optical Anisotropy of Layered Gallium Telluride
Layered
gallium telluride (GaTe) has attracted much attention recently,
due to its extremely high photoresponsivity, short response time,
and promising thermoelectric performance. Different from most commonly
studied two-dimensional (2D) materials, GaTe has in-plane anisotropy
and a low symmetry with the <i>C</i><sub><i>2h</i></sub><sup>3</sup> space group. Investigating the in-plane optical
anisotropy, including the electronâphoton and electronâphonon
interactions of GaTe is essential in realizing its applications in
optoelectronics and thermoelectrics. In this work, the anisotropic
light-matter interactions in the low-symmetry material GaTe are studied
using anisotropic optical extinction and Raman spectroscopies as probes.
Our polarized optical extinction spectroscopy reveals the weak anisotropy
in optical extinction spectra for visible light of multilayer GaTe.
Polarized Raman spectroscopy proves to be sensitive to the crystalline
orientation of GaTe, and shows the intricate dependences of Raman
anisotropy on flake thickness, photon and phonon energies. Such intricate
dependences can be explained by theoretical analyses employing first-principles
calculations and group theory. These studies are a crucial step toward
the applications of GaTe especially in optoelectronics and thermoelectrics,
and provide a general methodology for the study of the anisotropy
of light-matter interactions in 2D layered materials with in-plane
anisotropy