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Zhang, Yu

Publications and source records attributed to Zhang, Yu.

At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Conductivity based on selective etch for GaN devices and applications thereof

This invention relates to methods of generating NP gallium nitride (GaN) across large areas (>1 cm2) with controlled pore diameters, pore density, and porosity. Also disclosed are methods of generating novel optoelectronic devices based on porous GaN. Additionally a layer transfer scheme to separate and create free-standing crystalline GaN thin layers is disclosed that enables a new device manufacturing paradigm involving substrate recycling. Other disclosed embodiments of this invention relate to fabrication of GaN based nanocrystals and the use of NP GaN electrodes for electrolysis, water splitting, or photosynthetic process applications.

Zhang, Yu↗
DOE OSTI2019

Non-adiabatic molecular dynamics of molecules in the presence of strong light-matter interactions

When the interaction between a molecular system and confined light modes in an optical or plasmonic cavity is strong enough to overcome the dissipative process, hybrid light-matter states (polaritons) become the fundamental excitations in the system. The mixing between the light and matter characters modifies the photophysical and photochemical properties. Notably, it was reported that these polaritons can be employed to control photochemical reactions, charge and energy transfer, and other processes. In addition, according to recent studies, vibrational strong coupling can be employed to resonantly enhance the thermally-activated chemical reactions. In this work, a theoretical model and an efficient numerical method for studying the dynamics of molecules strongly interacting with quantum light are developed based on nonadiabatic excited-state molecular dynamics. The methodology was employed to study the cis-trans photoisomerization of a realistic molecule in a cavity. Numerical simulations demonstrate that the photochemical reactions can be controlled by tuning the properties of the cavity. In the calculated example, the isomerization is suppressed when polaritonic states develop a local minimum on the lower polaritonic state. Moreover, the observed reduction of isomerization is tunable via the photon energy and light-molecule coupling strength. However, the fluctuation in the transition dipole screens the effect of light-matter, which makes it harder to tune the photochemical properties via the coupling strength. These insights gained from this study suggest quantum control of photochemical reactions is possible by specially designed photonic or plasmonic cavities.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Double core hole valence-to-core x-ray emission spectroscopy: A theoretical exploration using time-dependent density functional theory

With the help of newly developed X-ray free-electron laser (XFEL) sources, creating double core holes (DCHs) simultaneously at the same or different atomic sites in a molecule has now become possible. DCH X-ray emission is a new form of X-ray nonlinear spectroscopy that can be studied with a XFEL. Here in this paper, we computationally explore the metal K-edge valence-to-core (VtC) X-ray emission spectroscopy (XES) of metal/metal and metal/ligand DCH states in a series of transition metal complexes with time-dependent density functional theory. The simulated DCH VtC-XES signals are compared with conventional single core hole (SCH) XES signals. The energy shifts and intensity changes of the DCH emission lines with respect to the corresponding SCH-XES features are fingerprints of the coupling between the second core hole and the occupied orbitals around the DCHs that contain important chemical bonding information of the complex. The difference between delocalized/localized core hole models on DCH VtC-XES is also briefly discussed. We theoretically demonstrate that DCH XES provides subtle information on the local electronic structure around metal centers in transition metal complexes beyond conventional linear XES. In conclusion, our predicted changes from calculations between SCH-XES and DCH-XES features should be detectable with modern XFEL sources.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Ground state in the novel dimer iridate Ba 13 Ir 6 O 30 with Ir 6+ (5$d^3$) ions

We have synthesized and studied an iridate, Ba 13 Ir 6 O 30 , with unusual Ir oxidation states: 2/3 Ir 6+ (5$d^3$) ions and 1/3 Ir 5+ (5$d^4$) ions. Its crystal structure features dimers of face-sharing IrO 6 octahedra, and IrO 6 monomers that are linked via long, zigzag Ir-O-Ba-O-Ir pathways. Nevertheless, Ba 13 Ir 6 O 30 exhibits two transitions at $T_{\text{N}1}$ = 4.7K and $T_{\text{N}2}$ = 1.6 K. This magnetic order is accompanied by a huge Sommerfeld coefficient 200 mJ/mole K 2 below $T_{\text{N}2}$, signaling a coexisting frustrated/disordered state persisting down to at least 0.05 K. This iridate hosts unusually large $J_{\text{eff}}$ = 3/2 degrees of freedom, which is enabled by strong spin-orbit interactions (SOI) in the monomers with Ir 6+ ions and a joint effect of molecular orbitals and SOI in the dimers occupied by Ir 5+ and Ir 6+ ions. Features displayed by the magnetization and heat capacity suggest that the combination of covalency, SOI, and large effective spins leads to highly frustrated ferrimagnetic ordering, an interesting feature of this high-spin iridate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Time-resolved dual-comb measurement of number density and temperature in a laser-induced plasma

We demonstrate time-resolved dual-comb spectroscopy of a laser-induced plasma and characterize the temporal evolution of the absorption excitation temperature and population number density for multiple Fe transitions. The spectrum of three excited-state transitions of Fe around 533 nm are simultaneously detected at different time delays following laser ablation of a stainless steel sample. This work probes late-time behavior of laser-induced ablation plumes during plasma cooling, significant for revealing important information on molecule and particle formation. The high spectral resolution and broadband capability of the dual-comb technique, combined with time-resolved measurement capability, will aid in characterization of plasma properties, species identification, and higher resolution spectroscopy including isotopic analysis.

(140.3440) Laser-induced breakdown, (280.5395) Pla↗
DOE OSTI2019

A Generic Modeling and Development Approach for WECC Composite Load Model

The composite load model (CLM) proposed by Western Electricity Coordinating Council (WECC) gains increasing traction in the industry, particularly in North America. At the same time, it is recognized that further improvements in the structure, initialization and aggregation methods are needed. However, lack of an open implementation of WECC CLM becomes a roadblock for many researchers to work on improving it. This paper presents the first open reference implementation of WECC CLM. Individual load components and the CLM are first developed and tested in Matlab, then translated to the high performance computing (HPC), parallel simulation platform GridPACK. Main contributions of the paper include: 1) presenting some important yet undocumented details of modeling and initializing the CLM, particularly for a parallel simulation platform like GridPACK; 2) implementation details of the load components such as the single-phase air conditioner motor; 3) implementing the CLM in a modular and extensible manner. The implementation has been tested at both the component as well as system levels and bench-marked against commercial simulation programs.

Huang, Qiuhua↗
DOE OSTI2019

Preferred states of open electronic systems

System-environment interaction may introduce dynamic destruction of quantum coherence, resulting in a special representation named as pointer states. In this work, pointer states of an open electronic system are studied. The decoherence effect is taken into account through two different ways which are Büttiker's virtual probe model and strong electron-phonon interaction in the polaron picture. The pointer states of the system with different coupling strength are investigated. Here, the pointer states are identified by tracking the eigenstates of the density matrix in real-time propagation. It is found that the pointer states can emerge for arbitrary coupling strength. And the pointer states deform to the eigenstates of the system in the strong coupling limit, which indicates the vanish of quantumness in the strong coupling limit.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Dual Ca 2+ -dependent gates in human Bestrophin1 underlie disease-causing mechanisms of gain-of-function mutations

Mutations of human BEST1, encoding a Ca 2+ -activated Cl - channel (hBest1), cause macular degenerative disorders. Best1 homolog structures reveal an evolutionarily conserved channel architecture highlighted by two landmark restrictions (named the “neck” and “aperture”, respectively) in the ion conducting pathway, suggesting a unique dual-switch gating mechanism, which, however, has not been characterized well. Using patch clamp and crystallography, we demonstrate that both the neck and aperture in hBest1 are Ca 2+ -dependent gates essential for preventing channel leakage resulting from Ca 2+ -independent, spontaneous gate opening. Importantly, three patient-derived mutations (D203A, I205T and Y236C) lead to Ca 2+ -independent leakage and elevated Ca 2+ -dependent anion currents due to enhanced opening of the gates. Moreover, we identify a network of residues critically involved in gate operation. Together, our results suggest an indispensable role of the neck and aperture of hBest1 for channel gating, and uncover disease-causing mechanisms of hBest1 gain-of-function mutations.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

Polypeptides having cellulolytic enhancing activity and polynucleotides encoding same

The present invention provides isolated polypeptides having cellulolytic enhancing activity and polynucleotides encoding the polypeptides, catalytic domains, cellulose binding domains and polynucleotides encoding the polypeptides, catalytic domains or cellulose binding domains. The invention also provides nucleic acid constructs, vectors, and host cells comprising the polynucleotides as well as methods of producing and using the polypeptides, catalytic domains or cellulose binding domains.

Tang, Lan↗
DOE OSTI2019

Controllable Single-Molecule Light Emission by Selective Charge Injection in Scanning Tunneling Microscopy

Tunneling electron-induced light emission in scanning tunneling microscopy (STM) has recently been explored as a novel light source with tunable properties. In this work, using first-principles-based atomistic simulations, we study the STM-induced luminescence of a single free-base phthalocyanine (H 2 Pc) molecule and present a way to control the emission frequency of the STM molecular junction by precisely controlling the charge injection at different molecular positions. The luminescence is found to be directly related to the spatial distribution of the molecular states and thus depends sensitively on the tip position. Through careful refinement of the chemical structures, devices with desirable frequencies at different parts of the emitting unit can be designed. Our report has profound implications for applications in nanoscale optoelectronics and future hybrid electronic-photonic circuits.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Polypeptides having catalase activity and polynucleotides encoding same

The present invention relates to isolated polypeptides having catalase activity and polynucleotides encoding the polypeptides. The invention also relates to nucleic acid constructs, vectors, and host cells comprising the polynucleotides as well as methods of producing and using the polypeptides.

Liu, Ye↗
DOE OSTI2019

Characterization of Particulate Matter Emissions from Heavy-Duty Partially Premixed Compression Ignition with Gasoline-Range Fuels

In this study, the compression ratio of a commercial 15L heavy-duty diesel engine was lowered and a split injection strategy was developed to promote partially premixed compression ignition (PPCI) combustion. Various low reactivity gasoline-range fuels were compared with ultra-low-sulfur diesel fuel (ULSD) for steady-state engine performance and emissions. Specially, particulate matter (PM) emissions were examined for their mass, size and number concentrations, and further characterized by organic/elemental carbon analysis, chemical speciation and thermogravimetric analysis. As more fuel-efficient PPCI combustion was promoted, a slight reduction in fuel consumption was observed for all gasoline-range fuels, which also had higher heating values than ULSD. Since mixing-controlled combustion dominated the latter part of the combustion process, hydrocarbon (HC) and carbon monoxide (CO) emissions were only slightly increased with the gasoline-range fuels. In contrast, soot emissions were significantly reduced with the gasoline-range fuels, including a ~70% reduction in micro soot sensor measurements and a >50% reduction in smoke meter measurements. All gasoline-range fuel PM samples were also found to contain higher amount of volatile species and organic carbon fractions compared to ULSD PM samples as measured by thermogravimetric and EC-OC analyses. Various partially oxidized HC species and nitrophenolic compounds were also detected by TDP-GC-MS and CE-MS techniques, which implied that more pronounced PPCI combustion occurred with the gasoline-range fuels. As a whole similar PM oxidation behavior was observed despite the differences in reactivity and chemical properties of the fuels, even though there may be some significant impacts under certain operating conditions.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2019

Microstructure and Wear Performance of Cobalt-Containing Iron-Based Slag-Free Self-shielded Flux-Cored Wire

A new type of cobalt-containing iron-based slag-free self-shielded flux-cored wire was developed for hardfacing. The influence of cobalt additions on the microstructure and wear performance of the hardfacing was investigated. The cobalt-containing iron-based hardfacing alloy has a typical hypereutectic microstructure consisting of primary M{sub 7}(C, B){sub 3} carbide, and a eutectic of M{sub 3}(C, B) carbide and austenite (part of which subsequently transformed into martensite). Cobalt promoted the formation of austenite and changed the morphology of the primary carbide and eutectic colonies. The morphology of primary carbide became fragmented with increasing cobalt due to the possible impeding effect of cobalt to the movement of the M{sub 7}(C, B){sub 3}/liquid interface. Different structures of the eutectic carbides were observed—the chrysanthemum structure for 1.42 wt.% cobalt, the skeleton form for 2.88 wt.% cobalt, the radial stripe 4.37 wt.% cobalt and the discrete stripe for 5.75 wt.% cobalt. Wear loss of the hardfacing alloy containing 2.88 wt.% cobalt was the smallest among all the alloys, owing to the large primary large carbide and skeleton-shaped eutectic carbides, the proper amount of austenite phase separating the M{sub 7}(C, B){sub 3} and M{sub 3}(C, B) carbides, the solid solution strengthening by cobalt in the matrix, and the high hardness of the alloy. Moreover, the wear resistance has a tendency to decrease with the increase in velocity as well as load.

36 MATERIALS SCIENCE↗