16 research outputs found
Composition dependent electrochemical properties of earth-abundant ternary nitride anodes
Growing energy storage demands on lithium-ion batteries necessitate
exploration of new electrochemical materials as next-generation battery
electrode materials. In this work, we investigate the previously unexplored
electrochemical properties of earth-abundant and tunable Zn1-xSn1+xN2 (x = -0.4
to x = 0.4) thin films, which show high electrical conductivity and high
gravimetric capacity for Li insertion. Enhanced cycling performance is achieved
compared to previously published end-members Zn3N2 and Sn3N4, showing decreased
irreversible loss and increased total capacity and cycle stability. The average
reversible capacity observed is > 1050 mAh/g for all compositions and 1220
mAh/g for Zn-poor (x = 0.2) films. Extremely Zn-rich films (x = -0.4) show
improved adhesion; however, Zn-rich films undergo a phase transformation on the
first cycle. Zn-poor and stoichiometric films do not exhibit significant phase
transformations which often plague nitride materials and show no required
overpotential at the 0.5 V plateau. Cation composition x is explored as a
mechanism for tuning relevant mechanical and electrochemical properties, such
as capacity, overpotential, phase transformation, electrical conductivity, and
adhesion. The lithiation/delithiation experiments confirm the reversible
electrochemical reactions. Without any binding additives, the as-deposited
electrodes delaminate resulting in fast capacity degradation. We demonstrate
the mechanical nature of this degradation through decreased electrode thinning,
resulting in cells with improved cycling stability due to increased mechanical
stability. Combining composition and electrochemical analysis, this work
demonstrates for the first time composition dependent electrochemical
properties for the ternary Zn1-xSn1+xN2 and proposes earth-abundant ternary
nitride anodes for increased reversible capacity and cycling stability
NiGaO interfacial layers in NiO/GaO heterojunction diodes at high temperature
NiO/GaO heterojunction diodes have attracted attention for
high-power applications, but their high-temperature performance and reliability
remain underexplored. Here we report on the time evolution of the static
electrical properties in the widely studied
p-NiO/n-GaOheterojunction diodes and the formation of
NiGaO interfacial layers when operated at C. Results
of our thermal cycling experiment show an initial leakage current increase
which stabilizes after sustained thermal load, due to reactions at the
NiO-GaO interface. High-resolution TEM microstructure analysis of
the devices after thermal cycling indicates that the NiO-GaO
interface forms ternary compounds at high temperatures, and thermodynamic
calculations suggest the formation of the spinel NiGaO layer
between NiO and GaO. First-principles defect calculations find that
NiGaO shows low p-type intrinsic doping, and hence can also serve
to limit electric field crowding at the interface. Vertical NiO/GaO
diodes with intentionally grown 5 nm thin spinel-type NiGaO
interfacial layers show excellent device ON/OFF ratio of > 10(3 V),
V of ~1.9 V, and breakdown voltage of ~ 1.2 kV for an initial
unoptimized 300-micron diameter device. These p-n heterojunction diodes are
promising for high-voltage, high-temperature applications.Comment: 16 pages, 5 figure
Evidence of a second-order Peierls-driven metal-insulator transition in crystalline NbO2
The metal-insulator transition of NbO2 is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed NbOx that may be inherently different to crystalline NbO2. We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial NbO2 thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at ~810β°C. The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline NbO2 as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap
Epitaxy of LiNbO3: Historical Challenges and Recent Success
High-quality epitaxial growth of thin film lithium niobate (LiNbO3) is highly desirable for optical and acoustic device applications. Despite decades of research, current state-of-the-art epitaxial techniques are limited by either the material quality or growth rates needed for practical devices. In this paper, we provide a short summary of the primary challenges of lithium niobate epitaxy followed by a brief historical review of lithium niobate epitaxy for prevalent epitaxial techniques. Available figures of merit for crystalline quality and optical transmission losses are given for each growth method. The highest crystalline quality lithium niobate thin film was recently grown by halide-based molecular beam epitaxy and is comparable to bulk lithium niobate crystals. However, these high-quality crystals are grown at slow rates that limit many practical applications. Given the many challenges that lithium niobate epitaxy imposes and the wide variety of methods that have unsuccessfully attempted to surmount these barriers, new approaches to lithium niobate epitaxy are required to meet the need for simultaneously high crystalline quality and sufficient thickness for devices not currently practical by existing techniques