5 research outputs found
Electron–Phonon-Mediated Temperature-Dependent Optical Bandgap of MAPbCl<sub><i>x</i></sub>Br<sub>3–<i>x</i></sub> Single Crystals
Methylammonium-lead-halide compounds have emerged as
promising
bandgap engineering materials due to their ability to fine-tune the
energy gap through halogen element mixing. We present a comprehensive
investigation of the temperature-dependent photoluminescence (PL)
transition characteristics exhibited by single crystals of chlorine-
and bromine-based methylammonium lead halides. MAPbCl3 and
MAPbBr3 crystals exhibit a distinct, sharp, free exciton
transition with an abrupt transition behavior associated with the
structural phase transition as the temperature varies. However, when
the two halogen elements are mixed within the crystals, no structural
phase transition is observed. This study explores the temperature-dependent
variations in integrated PL intensity, full width at half-maximum,
and peak transition energy of the crystals. The obtained results discuss
the intricate interplay between temperature, crystal structure, and
composition, providing valuable insights into the optical properties
and potential applications of organic–inorganic hybrid methylammonium
lead halide single crystals as tunable energy gap semiconductor materials
In Situ Analyses of Carbon Dissolution into Ni-YSZ Anode Materials
A combination of in situ analyses, including measurement
of both
electrical resistance and volumetric expansion, and thermogravimetric
analysis (TGA) was employed to elucidate the deactivation process
of a nickel-yttria-stabilized zirconia (Ni-YSZ) cermet (60 wt % NiO-YSZ)
upon exposure to methane at 750 °C. In conjunction with the aforementioned
in situ techniques, a number of ex situ analyses, including scanning
electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray
diffraction (XRD), and Raman spectroscopy, revealed that carbon deposition
initially occurred at the Ni centers, followed by carbon dissolution
into the Ni-YSZ cermet after an induction period of 200 min, which
then led to three-dimensional expansion. The structural change of
the Ni-based cermet induced increases in electrical resistance of
the material. The increased electrical resistance likely originated
from the breakage of the Ni–Ni conducting network as well as
from the formation of microscopic cracks within the Ni-YSZ material,
resulting from the observed process of carbon dissolution. Moreover,
a combination of TGA involving measurements of electrical resistance
was demonstrated to be useful for determining amounts of carbon deposits
critical for carbon dissolution. These results strongly suggest that
changes in electrical resistance can be utilized to monitor the extent
of carbon dissolution into the Ni-YSZ catalysts in situ, which would
be helpful for the development of an efficient curing system for solid
oxide fuel cells (SOFCs)
Effect of Activating a Nickel–Molybdenum Catalyst in an Anion Exchange Membrane Water Electrolyzer
Water electrolysis using anion exchange membranes is
promising
for hydrogen production, and Ni–Mo catalysts have shown high
activity for alkaline hydrogen evolution reaction (HER). However,
their performance has been mostly tested in a half-cell setup and
rarely studied in a single-cell setup with a membrane electrode assembly
(MEA) structure, which is used for practical applications. With Ni3Mo as the cathode, a single cell was fabricated using non-noble
metal catalysts exclusively. Interestingly, the activation procedure
significantly affected the cell performance. The single cell performed
better than that with the Pt/C catalyst when the Ni3Mo
catalyst was mildly activated. The distribution of Mo in electrodes,
membrane, and electrolytes was estimated, confirming Mo dissolution
from the cathode. Once the cell was activated, the cell performance
was stable without degradation in long-term chronopotentiometry operation,
but the performance was degraded by sudden voltage change such as
imposing open circuit voltage (OCV). The surface structure and reaction
mechanism were studied with density functional theory: the Mo-dissolved
Ni3Mo(101) surface could promote H2O dissociation,
while MoO3 stably adsorbed on the surface weakened H* adsorption,
promoting HER. This study provides important insights into the development
of efficient catalysts for large-scale hydrogen production
Relating Electronic and Geometric Structure of Atomic Layer Deposited BaTiO<sub>3</sub> to its Electrical Properties
Atomic layer deposition allows the
fabrication of BaTiO<sub>3</sub> (BTO) ultrathin films with tunable
dielectric properties, which
is a promising material for electronic and optical technology. Industrial
applicability necessitates a better understanding of their atomic
structure and corresponding properties. Through the use of element-specific
X-ray absorption near edge structure (XANES) analysis, O K-edge of
BTO as a function of cation composition and underlying substrate (RuO<sub>2</sub> and SiO<sub>2</sub>) is revealed. By employing density functional
theory and multiple scattering simulations, we analyze the distortions
in BTO’s bonding environment captured by the XANES spectra.
The spectral weight shifts to lower energy with increasing Ti content
and provides an atomic scale (microscopic) explanation for the increase
in leakage current density. Differences in film morphologies in the
first few layers near substrate–film interfaces reveal BTO’s
homogeneous growth on RuO<sub>2</sub> and its distorted growth on
SiO<sub>2</sub>. This work links structural changes to BTO thin-film
properties and provides insight necessary for optimizing future BTO
and other ternary metal oxide-based thin-film devices
Plasma-Enhanced Atomic Layer Deposition of SiN–AlN Composites for Ultra Low Wet Etch Rates in Hydrofluoric Acid
The continued scaling in transistors
and memory elements has necessitated the development of atomic layer
deposited (ALD) of hydrofluoric acid (HF) etch resistant and electrically
insulating films for sidewall spacer processing. Silicon nitride (SiN)
has been the prototypical material for this need and extensive work
has been conducted into realizing sufficiently lower wet etch rates
(WERs) as well as leakage currents to meet industry needs. In this
work, we report on the development of plasma-enhanced atomic layer
deposition (PEALD) composites of SiN and AlN to minimize WER and leakage
current density. In particular, the role of aluminum and the optimum
amount of Al contained in the composite structures have been explored.
Films with near zero WER in dilute HF and leakage currents density
similar to pure PEALD SiN films could be simultaneously realized through
composites which incorporate ≥13 at. % Al, with a maximum thermal
budget of 350 °C