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Chen, Hong

Publications and source records attributed to Chen, Hong.

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Steep-slope field-effect transistors with AlGaN/GaN HEMT and oxide-based threshold switching device

We report the demonstration of a steep-slope field-effect transistor with AlGaN/GaN MIS-HEMTs employing SiO 2 -based threshold switching devices in series with the source. The SiO 2 -based threshold switching devices exhibited steep slope when changing resistance states. The integrated steep-slope transistor showed a low subthreshold swing of sub-5 mV/dec with a transition range of over 10 5 in the transfer characteristics in both sweep directions at room temperature, as well as the low leakage current (10 -5 μA μm -1 ) and a high I ON /I OFF ratio (>10 7 ). Moreover, with the SiO 2 -based threshold switching devices we also observed a positive shift of threshold voltages of the integrated device. Results from more than 50 transfer characteristics measurements also indicate the good repeatability and practicability of such a steep-switching device, where the average steep slopes are below 10 mV/decade. In conclusion, this steep-slope transistor with oxide-based threshold switching devices can be further extended to various transistor platforms like Si and III–V and are of potential interest for the development of power switching and high frequency devices.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Surface Solar Irradiance in Continental Shallow Cumulus Fields: Observations and Large-Eddy Simulation

This study examines shallow cumulus cloud fields and their surface shortwave radiative effects using large-eddy simulation (LES) along with observations across multiple days at the Atmospheric Radiation Measurement Southern Great Plains atmospheric observatory. Pronounced differences are found between probability density functions (PDFs) of downwelling surface solar irradiance derived from observations and LES one-dimensional (1D) online radiation calculations. The shape of the observed PDF is bimodal, which is only reproduced by offline three-dimensional (3D) radiative transfer calculations, demonstrating PDF bimodality as a 3D radiative signature of continental shallow cumuli. Local differences between 3D and 1D radiative transfer calculations of downwelling surface solar irradiance are, on average, larger than 150 W m -2 on one afternoon. The differences are substantially reduced when spatially averaged over the LES domain and temporally averaged over the diurnal cycle, but systematic 3D biases ranging from 2 to 8 W m -2 persist across different days. Covariations between the domain-averaged surface irradiance, framed as a surface cloud radiative effect, and the simulated cloud fraction are found to follow a consistent diurnal relationship, often exhibiting hysteresis. In contrast, observations show highly variable behavior. By subsampling the LES domain, it is shown that this is due to the limited sampling density of inherently 3D observations. These findings help to define observational requirements for detecting such relationships, provide valuable insight for evaluating weather and climate models against surface observations as they push to ever higher resolutions, and have important implications for future assessments of solar renewable energy potential.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Threshold Switching and Memory Behaviors of Epitaxially Regrown GaN-on-GaN Vertical $p-n$ Diodes With High Temperature Stability

This letter reports the observation of threshold switching and memory behaviors of epitaxially regrown GaN-on-GaN vertical p-n diodes. This mechanism was ascribed to the conductive path formed by traps in the insulating layer at the regrowth interface after soft breakdown. The device can reliably switch more than 1000 cycles at both room temperature and 300 °C with small fluctuation on the set and reset voltage. The set voltage increased with the increasing temperature due to the enhanced thermal detrapping effect that made it harder to form conductive path at high temperatures. Besides, the device showed memory behaviors when the reset voltage was higher than 4.4 V. This letter can serve as an important reference to further developing GaN-based memory devices and integrated circuits.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Direct Observation of Structural Evolution of Metal Chalcogenide in Electrocatalytic Water Oxidation

As one of the most remarkable oxygen evolution reaction (OER) electrocatalysts, metal chalcogenides have been intensively reported during the past few decades because of their high OER activities. It has been reported that electron-chemical conversion of metal chalcogenides into oxides/hydroxides would take place after the OER. However, the transition mechanism of such unstable structures, as well as the real active sites and catalytic activity during the OER for these electrocatalysts, has not been understood yet; therefore a direct observation for the electrocatalytic water oxidation process, especially at nano or even angstrom scale, is urgently needed. In this work, by employing advanced Cs-corrected transmission electron microscopy (TEM), a step by step oxidational evolution of amorphous electrocatalyst CoS x into crystallized CoOOH in the OER has been in situ captured: irreversible conversion of CoS x to crystallized CoOOH is initiated on the surface of the electrocatalysts with a morphology change via Co(OH) 2 intermediate during the OER measurement, where CoOOH is confirmed as the real active species. Besides, this transition process has also been confirmed by multiple applications of X-ray photoelectron spectroscopy (XPS), in situ Fourier-transform infrared spectroscopy (FTIR), and other ex situ technologies. Moreover, on the basis of this discovery, a high-efficiency electrocatalyst of a nitrogen-doped graphene foam (NGF) coated by CoS x has been explored through a thorough structure transformation of CoOOH. In conclusion, we believe this in situ and in-depth observation of structural evolution in the OER measurement can provide insights into the fundamental understanding of the mechanism for the OER catalysts, thus enabling the more rational design of low-cost and high-efficient electrocatalysts for water splitting.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Investigation of GaN-on-GaN vertical p - n diode with regrown p -GaN by metalorganic chemical vapor deposition

Here, to mimic selective-area doping, p -GaN was regrown on an etched GaN surface on GaN substrates by metalorganic chemical vapor deposition. Vertical GaN-on-GaN p - n diodes were fabricated to investigate the effects of the etch-then-regrowth process on device performance. The crystal quality of the sample after each epitaxial step was characterized by X-ray diffraction, where the etch-then-regrowth process led to a very slight increase in edge dislocations. A regrowth interfacial layer was clearly shown by transmission electron microscopy. Strong electroluminescence was observed with three emission peaks at 2.2 eV, 2.8 eV and 3.0 eV, respectively. The forward current density increased slightly with the increasing temperature, while the reverse current density was almost temperature independent indicating tunneling as the reverse transport mechanism. This result is very similar to the reported Zener tunnel diode comprising a high doping profile at the junction interface. High levels of silicon and oxygen concentrations were observed at the regrowth interface with a distribution width of ~100 nm. This work provides valuable information of p -GaN regrowth and regrown GaN p - n diodes, which can serve as an important reference for developing selective doping for advanced GaN power electronics for high voltage and high power applications.

42 ENGINEERING↗
DOE OSTI2018

Full spectrum light driven photocatalytic in-situ epitaxy of one-unit-cell Bi 2 O 2 CO 3 layers on Bi 2 O 4 nanocrystals for highly efficient photocatalysis and mechanism unveiling

In-situ epitaxial growth is a fascinating strategy to nicely couple two low dimensional semiconductors as highly efficient composite photocatalysts. Meanwhile, organic contaminants in the photocatalytic process are usually decomposed into greenhouse gas (CO 2 ) that can’t be reused. Herein, we reported a green full spectrum light (UV, visible and NIR lights) induced epitaxial growth strategy to synthesize highly efficient Bi 2 O 4 /Bi 2 O 2 CO 3 heterostructure photocatalyst by reusing waste carbon source, in which one-unit-cell Bi 2 O 2 CO 3 layers (1.0 nm) in-situ grew on the surface of Bi 2 O 4 nanocrystals during the photocatalytic degradation of rhodamine B (RhB) or phenol. More importantly, 13 C nuclear magnetic resonance (NMR) spectroscopy confirmed that the carbon element in Bi 2 O 2 CO 3 was from the photocatalytic degradation of organic contaminations. Furthermore, density functional theory (DFT) calculations confirm that the Bi 2 O 4 nanocrystals with exposed {-101} facets have the larger percentage of undercoordinated Bi atoms, which provided favorable conditions for the in-situ epitaxy of Bi 2 O 2 CO 3 during the photocatalytic reaction. Additionally, the increased charge density near the Fermi level resulted in improved photoresponsivity of Bi 2 O 4 /Bi 2 O 2 CO 3 composite and the coalescence of Bi 2 O 4 and Bi 2 O 2 CO 3 could favor the travel of photogenerated carriers from one to another owing to the close work functions for Bi 2 O 4 (4.295 eV) and Bi 2 O 2 CO 3 (4.410 eV). As we expected, the Bi 2 O 4 /Bi 2 O 2 CO 3 composite presented higher photocatalytic activity for phenol and ciprofloxacin (CIP) degradation than pure Bi 2 O 4 nanocrystals. The possible degradation pathway of CIP in aqueous solution and photocatalytic mechanism of Bi 2 O 4 /Bi 2 O 2 CO 3 composite were also proposed based on liquid chromatography mass spectrometer (LC–MS) analysis and experimental results. Furthermore, this work provides a green strategy for designing highly efficient photocatalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Coupled Kinetics of Ferrihydrite Transformation and As(V) Sequestration under the Effect of Humic Acids: A Mechanistic and Quantitative Study

In natural environments, kinetics of As(V) sequestration/release is usually coupled with the dynamic iron mineral transformation, which is further influenced by the presence of the natural organic matter (NOM). However, both quantitative and mechanistic understanding of this process is still lacking. In this study, we investigated the effect of humic acids (HA), a major component of NOM, on the coupled kinetics of ferrihydrite transformation into hematite/goethite and sequestration of As(V) in iron minerals. Time-resolved As(V) and HA interactions with iron minerals were studied in stirred-flow kinetic experiments, using aberration-corrected scanning transmission electron microscopy (STEM), and X-ray adsorption spectroscopy. Based on the experimental results, we developed a mechanistic kinetics model for As(V) reactions with iron minerals with varying iron mineral compositions and solution chemistry. Our results showed that HA slowed ferrihydrite transformation and the rates of As(V) speciation changes within iron minerals, while HA did not incorporate into iron minerals. Our kinetics model is able to account for the variations of iron mineral compositions, solution chemistry, and As(V) speciation, which has significant environmental implications on predicting the As(V) behavior in the environment.

ferrihydrite, humic acid↗
DOE OSTI2018

Nonpolar vertical GaN-on-GaN p–n diodes grown on free-standing $(10\bar{1}0)$ m -plane GaN substrates

We report nonpolar vertical GaN-on-GaN p–n diodes grown on m-plane free-standing substrates. Cathodoluminescence measurements showed that the nonpolar p-GaN had a high quality with very few deep-level states. The device exhibited good rectifying behaviors with a turn-on voltage of 4.0 V, on-resistance of 2.3 mΩ∙cm 2 , and high on/off ratio of 10 10 . The reverse current leakage was described by a trap-assisted space-charge- limited current conduction mechanism. The critical electrical field was calculated to be 2.0 MV/cm without field plates or edge termination. Finally, these results pave the way for development of novel nonpolar power electronics and polarization-engineering-related advanced power devices.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Synthesis of Silver Nanowires with Reduced Diameters Using Benzoin-Derived Radicals to Make Transparent Conductors with High Transparency and Low Haze

Reducing the diameter of silver nanowires has been proven to be an effective way to improve their optoelectronic performance by lessening light attenuation. The state-of-the-art silver nanowires are typically around 20 nm in diameter. Here in this paper we report a modified polyol synthesis of silver nanowires with average diameters as thin as 13 nm and aspect ratios up to 3000. The success of this synthesis is based on the employment of benzoin-derived radicals in the polyol approach and does not require high-pressure conditions. The strong reducing power of radicals allows the reduction of silver precursors to occur at relatively low temperatures, wherein the lateral growth of silver nanowires is restrained because of efficient surface passivation. The optoelectronic performance of as-prepared 13 nm silver nanowires presents a sheet resistance of 28 Ω sq -1 at a transmittance of 95% with a haze factor of ~1.2%, comparable to that of commercial indium tin oxide (ITO).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

High Performance Vertical GaN-on-GaN p-n Power Diodes With Hydrogen-Plasma-Based Edge Termination

This letter reports the first implementation of a hydrogen-plasma-based edge termination technique (HPET) in vertical GaN p-n power diodes grown on bulk GaN substrates using metalorganic chemical vapor deposition. The device with a 9-μm-thick drift layer exhibited a high breakdown voltage (V bd ) of 1.57 kV, a low ON-resistance (R ON ) of 0.45 mQ · cm 2 (or 0.70 mQ · cm 2 with current spreading considered) and a high Baliga's figure-of-merit (V $2\atop{bd}$/R ON ) of 5.5 GW/cm 2 (or 3.6 GW/cm 2 ) without passivation or field plate, which are close to the theoretical limit of GaN. This technique enabled a significant reduction in leakage current (~10 6 times at -300 V) and a huge enhancement in V bd (from ~300 V to 1.57 kV). Furthermore, the device showed good forward characteristics with a turn-ON voltage of 3.5 V, an ON-current of ~2 kA/cm 2 (or 1.3 kA/cm 2 ), an ON/OFF ratio of -10 9 , and an ideality factor of 1.4. This work shows the HPET can serve as an effective, low cost, and easy-to-implement edge termination technique for high voltage and high power GaN p-n power diodes.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Dendritic core-shell nickel-iron-copper metal/metal oxide electrode for efficient electrocatalytic water oxidation

Electrochemical water splitting requires efficient water oxidation catalysts to accelerate the sluggish kinetics of water oxidation reaction. Here in this paper, we report a promisingly dendritic core-shell nickel-iron-copper metal/metal oxide electrode, prepared via dealloying with an electrodeposited nickel-iron-copper alloy as a precursor, as the catalyst for water oxidation. The as-prepared core-shell nickel-iron-copper electrode is characterized with porous oxide shells and metallic cores. This tri-metal-based core-shell nickel-iron-copper electrode exhibits a remarkable activity toward water oxidation in alkaline medium with an overpotential of only 180 mV at a current density of 10 mA cm -2 . The core-shell NiFeCu electrode exhibits pH-dependent oxygen evolution reaction activity on the reversible hydrogen electrode scale, suggesting that non-concerted proton-electron transfers participate in catalyzing the oxygen evolution reaction. To the best of our knowledge, the as-fabricated core-shell nickel-iron-copper is one of the most promising oxygen evolution catalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Thermochromic halide perovskite solar cells

Smart photovoltaic windows represent a promising green technology featuring tunable transparency and electrical power generation under external stimuli to control the light transmission and manage the solar energy. Here, we demonstrate a thermochromic solar cell for smart photovoltaic window applications utilizing the structural phase transitions in inorganic halide perovskite caesium lead iodide/bromide. The solar cells undergo thermally-driven, moisture-mediated reversible transitions between a transparent non-perovskite phase (81.7% visible transparency) with low power output and a deeply coloured perovskite phase (35.4% visible transparency) with high power output. The inorganic perovskites exhibit tunable colours and transparencies, a peak device efficiency above 7%, and a phase transition temperature as low as 105 °C. We demonstrate excellent device stability over repeated phase transition cycles without colour fade or performance degradation. The photovoltaic windows showing both photoactivity and thermochromic features represent key stepping-stones for integration with buildings, automobiles, information displays, and potentially many other technologies.

14 SOLAR ENERGY↗
DOE OSTI2018

Endothelial microparticles-mediated transfer of microRNA-19b promotes atherosclerosis via activating perivascular adipose tissue inflammation in apoE−/− mice

Highlights: • EMP{sup miR19b} promotes atherosclerosis progression. • EMP{sup miR19b} increases inflammation in perivascular adipose tissue. • EMP can transfer microRNA-19b into perivascular adipose tissue. • SOCS3 is a direct target of microRNA-19b. Microparticles(MPs) are the major carriers of circulating microRNAs. Our previous study has shown that microRNA (miR)-19b in endothelial cell-derived microparticles (EMPs) is significantly increased in patients with unstable angina. However, little is known about the relationship between miR-19b in EMPs and the progression of atherosclerosis. The aim of the present study was to define the role and potential mechanism of miR-19b incorporated in EMPs in the development of atherosclerosis. Western-diet-fed apoE{sup −/−} mice were injected with phosphate buffered solution(PBS), EMP carrying microRNA control(EMP{sup control}) or miR-19b mimic (EMP{sup miR19b}) intravenously. Systemic treatment with EMP{sup miR19b} significantly accelerated carotid artery atherosclerosis progression by increasing lipid, macrophages and smooth muscle cells and decreasing collagen content in atherosclerotic plaque. Fluorescence-labelled EMP{sup miR19b} injection proved that miR-19b could be transported into perivascular adipose tissue(PVAT) by EMPs. EMP{sup miR19b} treatment also promoted inflammatory cytokines secretion and macrophages infiltration in PVAT. In further experiment, apoE{sup −/−} mice were divided into 3 groups: EMP{sup control}PVAT(+), EMP{sup miR19b}PVAT(+) and EMP{sup miR19b}PVAT(-), based on removing or keeping pericarotid adipose tissue and injected with EMP{sup control} or EMP{sup miR19b}. Loss of PVAT attenuated EMP{sup miR19b}-mediated effects on increasing carotid atherosclerosis formation and inflammatory cytokines level in plaque. EMP{sup miR19b} inhibited suppressor of cytokine signaling 3 (SOCS3) expression in PVAT. Our findings demonstrate that miR-19b in EMPs exaggerates atherosclerosis progression by augmenting PVAT-specific inflammation proceeded by downregulating SOCS3 expression.

60 APPLIED LIFE SCIENCES↗