6 research outputs found
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Investigation of MoS2 with Ambient Pressure X-ray Photoelectron Spectroscopy
Molybdenum disulfide (MoS2) has potential applications as a low-cost catalyst for the hydrogen evolution reaction (HER). Defects on MoS2, such as edge sites and sulfur vacancies, are known to be the major active sites for HER. Controlling the formation of these defects allows for the enhancement in reactivity of MoS2 and other 2D materials. In this study, we have characterized the surface reactivity of bulk MoS2 samples using ambient pressure X-ray photoelectron spectroscopy (APXPS). Samples were exposed to 1 mbar of H2O vapor at temperatures ranging from 300-600 K. The APXPS Mo 3d, S 2p, and O 1s core levels for the as exfoliated surface showed no significant changes under all reaction temperatures due to the inert nature of the pristine MoS2 surface. To improve surface reactivity and activate the basal plane of MoS2 we have utilized Ar+ sputtering to form well controlled defect densities at the surface. The APXPS Mo 3d, S 2p, and O 1s core levels for the defective MoSx (x = 1.6, 1.2) surface showed the formation of MoO3 and MoOS at temperatures of 400 K and above. We have found that surface reactivity of MoS2 is strongly dependent on temperature and defect densities
First Principles Assessment of CdTe as a Tunnel Barrier at the -Sn/InSb Interface
Majorana zero modes, with prospective applications in topological quantum
computing, are expected to arise in superconductor/semiconductor interfaces,
such as -Sn and InSb. However, proximity to the superconductor may also
adversely affect the semiconductor's local properties. A tunnel barrier
inserted at the interface could resolve this issue. We assess the wide band gap
semiconductor, CdTe, as a candidate material to mediate the coupling at the
lattice-matched interface between -Sn and InSb. To this end, we use
density functional theory (DFT) with Hubbard U corrections, whose values are
machine-learned via Bayesian optimization (BO) [npj Computational Materials 6,
180 (2020)]. The results of DFT+U(BO) are validated against angle resolved
photoemission spectroscopy (ARPES) experiments for -Sn and CdTe. For
CdTe, the z-unfolding method [Advanced Quantum Technologies, 5, 2100033 (2022)]
is used to resolve the contributions of different values to the ARPES. We
then study the band offsets and the penetration depth of metal-induced gap
states (MIGS) in bilayer interfaces of InSb/-Sn, InSb/CdTe, and
CdTe/-Sn, as well as in tri-layer interfaces of InSb/CdTe/-Sn
with increasing thickness of CdTe. We find that 16 atomic layers (3.5 nm) of
CdTe can serve as a tunnel barrier, effectively shielding the InSb from MIGS
from the -Sn. This may guide the choice of dimensions of the CdTe
barrier to mediate the coupling in semiconductor-superconductor devices in
future Majorana zero modes experiments
Scale-dependent optimized homoepitaxy of InAs(111)A
We combined in-situ scanning tunneling microscopy (STM) with the conventional
growth characterization methods of atomic force microscopy (AFM) and reflection
high energy electron diffraction (RHEED) to simultaneously assess atomic-scale
impurities and the larger-scale surface morphology of molecular beam epitaxy
(MBE) grown homoepitaxial InAs(111)A. By keeping a constant substrate
temperature and indium flux while increasing the As flux, we find two
differing MBE growth parameter regions for optimized surface roughness on the
macro and atomic scale. In particular, we show that a pure step-flow regime
with strong suppression of hillock formation can be achieved, even on
substrates without intentional offcut. On the other hand, an indium adatom
deficient, low atomic defect surface can be observed for a high hillock
density. We identify the main remaining point defect on the latter surface by
comparison to STM simulations. Furthermore, we provide a method for extracting
root-mean-square surface roughness values and discuss their use for surface
quality optimization by comparison to scale-dependent, technologically relevant
surface metrics. Finally, mapping the separately optimized regions of the
growth parameter space should provide a guide for future device engineering
involving epitaxial InAs(111)A growth
First Principles Assessment of CdTe as a Tunnel Barrier at the -Sn/InSb Interface
Majorana zero modes, with prospective applications in topological quantum computing, are expected to arise in superconductor/semiconductor interfaces, such as -Sn and InSb. However, proximity to the superconductor may also adversely affect the semiconductor's local properties. A tunnel barrier inserted at the interface could resolve this issue. We assess the wide band gap semiconductor, CdTe, as a candidate material to mediate the coupling at the lattice-matched interface between -Sn and InSb. To this end, we use density functional theory (DFT) with Hubbard U corrections, whose values are machine-learned via Bayesian optimization (BO) [npj Computational Materials 6, 180 (2020)]. The results of DFT+U(BO) are validated against angle resolved photoemission spectroscopy (ARPES) experiments for -Sn and CdTe. For CdTe, the z-unfolding method [Advanced Quantum Technologies, 5, 2100033 (2022)] is used to resolve the contributions of different values to the ARPES. We then study the band offsets and the penetration depth of metal-induced gap states (MIGS) in bilayer interfaces of InSb/-Sn, InSb/CdTe, and CdTe/-Sn, as well as in tri-layer interfaces of InSb/CdTe/-Sn with increasing thickness of CdTe. We find that 16 atomic layers (3.5 nm) of CdTe can serve as a tunnel barrier, effectively shielding the InSb from MIGS from the -Sn. This may guide the choice of dimensions of the CdTe barrier to mediate the coupling in semiconductor-superconductor devices in future Majorana zero modes experiments
First-Principles Assessment of CdTe as a Tunnel Barrier at the α‑Sn/InSb Interface
Majorana zero modes, with prospective applications in
topological
quantum computing, are expected to arise in superconductor/semiconductor
interfaces, such as β-Sn and InSb. However, proximity to the
superconductor may also adversely affect the semiconductor’s
local properties. A tunnel barrier inserted at the interface could
resolve this issue. We assess the wide band gap semiconductor, CdTe,
as a candidate material to mediate the coupling at the lattice-matched
interface between α-Sn and InSb. To this end, we use density
functional theory (DFT) with Hubbard U corrections, whose values are
machine-learned via Bayesian optimization (BO) [npj Computational Materials 2020, 6, 180]. The results of DFT+U(BO) are validated against angle resolved
photoemission spectroscopy (ARPES) experiments for α-Sn and
CdTe. For CdTe, the z-unfolding method [Advanced Quantum Technologies 2022, 5, 2100033] is used to resolve the contributions of different kz values to the ARPES. We then study
the band offsets and the penetration depth of metal-induced gap states
(MIGS) in bilayer interfaces of InSb/α-Sn, InSb/CdTe, and CdTe/α-Sn,
as well as in trilayer interfaces of InSb/CdTe/α-Sn with increasing
thickness of CdTe. We find that 16 atomic layers (3.5 nm) of CdTe
can serve as a tunnel barrier, effectively shielding the InSb from
MIGS from the α-Sn. This may guide the choice of dimensions
of the CdTe barrier to mediate the coupling in semiconductor–superconductor
devices in future Majorana zero modes experiments