5 research outputs found
Phase Stability and Transport Mechanisms in Antiperovskite Li<sub>3</sub>OCl and Li<sub>3</sub>OBr Superionic Conductors
We
investigate phase stability and ionic transport mechanisms in
two recently discovered superionic conductors, Li<sub>3</sub>OX (X
= Cl, Br), from first principles. These compounds, which have an antiperovskite
crystal structure, have potential applications as solid electrolytes
in Li-ion batteries. We identify a low-barrier three-atom hop mechanism
involving Li interstitial dumbbells. This hop mechanism is facile
within the (001) crystallographic planes of the perovskite crystal
structure and is evidence for the occurrence of concerted motion,
similar to ionic transport in other solid electrolytes. Our first-principles
analysis of phase stability predicts that antiperovskite Li<sub>3</sub>OCl (Li<sub>3</sub>OBr) is metastable relative to Li<sub>2</sub>O
and LiCl (LiBr) at room temperature. We also find that although the
band gap of Li<sub>3</sub>OCl exceeds 5 eV, the metastable antiperovskite
becomes susceptible to decomposition into Li<sub>2</sub>O<sub>2</sub>, LiCl and LiClO<sub>4</sub> above an applied voltage of 2.5 V, suggesting
that these compounds are most suited for low-voltage Li batteries
provided the formation of Li<sub>2</sub>O can be suppressed
Tuning Ionic Transport in Memristive Devices by Graphene with Engineered Nanopores
Memristors, based on inherent memory
effects in simple two-terminal
structures, have attracted tremendous interest recently for applications
ranging from nonvolatile data storage to neuromorphic computing based
on non-von Neumann architectures. In a memristor, the ability to modulate
and retain the state of an internal variable leads to experimentally
observed resistive switching (RS) effects. Such phenomena originate
from internal, microscopic ionic migration and associated electrochemical
processes that modify the materials’ electrical and other physical
properties. To optimize the device performance for practical applications
with large-size arrays, controlling the internal ionic transport and
redox reaction processes thus becomes a necessity, ideally at the
atomic scale. Here we show that the RS characteristics in tantalum-oxide-based
memristors can be systematically tuned by inserting a graphene film
with engineered nanopores. Graphene, with its atomic thickness and
excellent impermeability and chemical stability, can be effectively
integrated into the device stack and can offer unprecedented capabilities
for the control of ionic dynamics at the nanoscale. In this device
structure, the graphene film effectively blocks ionic transport and
redox reactions; thereby the oxygen vacancies required during the
RS process are allowed to transport only through the engineered nanosized
openings in the graphene layer, leading to effective modulation of
the device performance by controlling the nanopore size in graphene.
The roles of graphene as an ion-blocking layer in the device structure
were further supported by transmission electron microscopy, energy-dispersive
X-ray spectroscopy, and atomistic simulations based on first-principles
calculations
Electronic and Optical Properties of Two-Dimensional GaN from First-Principles
Gallium nitride (GaN)
is an important commercial semiconductor
for solid-state lighting applications. Atomically thin GaN, a recently
synthesized two-dimensional material, is of particular interest because
the extreme quantum confinement enables additional control of its
light-emitting properties. We performed first-principles calculations
based on density functional and many-body perturbation theory to investigate
the electronic, optical, and excitonic properties of monolayer and
bilayer two-dimensional (2D) GaN as a function of strain. Our results
demonstrate that light emission from monolayer 2D GaN is blueshifted
into the deep ultraviolet range, which is promising for sterilization
and water-purification applications. Light emission from bilayer 2D
GaN occurs at a similar wavelength to its bulk counterpart due to
the cancellation of the effect of quantum confinement on the optical
gap by the quantum-confined Stark shift. Polarized light emission
at room temperature is possible via uniaxial in-plane strain, which
is desirable for energy-efficient display applications. We compare
the electronic and optical properties of freestanding two-dimensional
GaN to atomically thin GaN wells embedded within AlN barriers in order
to understand how the functional properties are influenced by the
presence of barriers. Our results provide microscopic understanding
of the electronic and optical characteristics of GaN at the few-layer
regime
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub>: A Lillianite Homologue with Promising Thermoelectric Properties
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3)
Ã…, <i>b</i> = 4.262(2) Ã…, <i>c</i> =
23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional
structure, two NaCl-type layers A and B with respective thicknesses <i>N</i><sub>1</sub> = 5 and <i>N</i><sub>2</sub> = 4
[<i>N</i> = number of edge-sharing (Pb/Bi)ÂSe<sub>6</sub> octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal
prisms, PbSe<sub>7</sub>, are located on a pseudomirror plane parallel
to (001). This complex atomic-scale structure results in a remarkably
low thermal conductivity (∼0.33 W m<sup>–1</sup> K<sup>–1</sup> at 300 K). Electronic structure calculations and
diffuse-reflectance measurements indicate that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a narrow-gap semiconductor with an indirect
band gap of 0.23 eV. Multiple peaks and valleys were observed near
the band edges, suggesting that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a promising compound for both n- and p-type doping. Electronic-transport
data on the as-grown material reveal an n-type degenerate semiconducting
behavior with a large thermopower (∼−160 μV K<sup>–1</sup> at 300 K) and a relatively low electrical resistivity.
The inherently low thermal conductivity of Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> and its tunable electronic properties point to a
high thermoelectric figure of merit for properly optimized samples
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub>: A Lillianite Homologue with Promising Thermoelectric Properties
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3)
Ã…, <i>b</i> = 4.262(2) Ã…, <i>c</i> =
23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional
structure, two NaCl-type layers A and B with respective thicknesses <i>N</i><sub>1</sub> = 5 and <i>N</i><sub>2</sub> = 4
[<i>N</i> = number of edge-sharing (Pb/Bi)ÂSe<sub>6</sub> octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal
prisms, PbSe<sub>7</sub>, are located on a pseudomirror plane parallel
to (001). This complex atomic-scale structure results in a remarkably
low thermal conductivity (∼0.33 W m<sup>–1</sup> K<sup>–1</sup> at 300 K). Electronic structure calculations and
diffuse-reflectance measurements indicate that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a narrow-gap semiconductor with an indirect
band gap of 0.23 eV. Multiple peaks and valleys were observed near
the band edges, suggesting that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a promising compound for both n- and p-type doping. Electronic-transport
data on the as-grown material reveal an n-type degenerate semiconducting
behavior with a large thermopower (∼−160 μV K<sup>–1</sup> at 300 K) and a relatively low electrical resistivity.
The inherently low thermal conductivity of Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> and its tunable electronic properties point to a
high thermoelectric figure of merit for properly optimized samples