Engineering topics
Zhang, Cheng
Publications and source records attributed to Zhang, Cheng.
Human-centered automation for resilient nuclear power plant outage control
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Improving the phenotype predictions of a yeast genome‐scale metabolic model by incorporating enzymatic constraints
Abstract Genome‐scale metabolic models ( GEM s) are widely used to calculate metabolic phenotypes. They rely on defining a set of constraints, the most common of which is that the production of metabolites and/or growth are limited by the carbon source uptake rate. However, enzyme abundances and kinetics, which act as limitations on metabolic fluxes, are not taken into account. Here, we present GECKO , a method that enhances a GEM to account for enzymes as part of reactions, thereby ensuring that each metabolic flux does not exceed its maximum capacity, equal to the product of the enzyme's abundance and turnover number. We applied GECKO to a Saccharomyces cerevisiae GEM and demonstrated that the new model could correctly describe phenotypes that the previous model could not, particularly under high enzymatic pressure conditions, such as yeast growing on different carbon sources in excess, coping with stress, or overexpressing a specific pathway. GECKO also allows to directly integrate quantitative proteomics data; by doing so, we significantly reduced flux variability of the model, in over 60% of metabolic reactions. Additionally, the model gives insight into the distribution of enzyme usage between and within metabolic pathways. The developed method and model are expected to increase the use of model‐based design in metabolic engineering.
High-performance multilayer WSe 2 field-effect transistors with carrier type control
In this paper, high-performance multilayer WSe 2 field-effect transistor (FET) devices with carrier type control are demonstrated via thickness modulation and a remote oxygen plasma surface treatment. Carrier type control in multilayer WSe 2 FET devices with Cr/Au contacts is initially demonstrated by modulating the WSe 2 thickness. The carrier type evolves with increasing WSe 2 channel thickness, being p-type, ambipolar, and n-type at thicknesses <3, ~4, and >5 nm, respectively. The thickness-dependent carrier type is attributed to changes in the bandgap of WSe 2 as a function of the thickness and the carrier band offsets relative to the metal contacts. Furthermore, we present a strong hole carrier doping effect via remote oxygen plasma treatment. It non-degenerately converts n-type characteristics into p-type and enhances field-effect hole mobility by three orders of magnitude. Finally, this work demonstrates progress towards the realization of high-performance multilayer WSe 2 FETs with carrier type control, potentially extendable to other transition metal dichalcogenides, for future electronic and optoelectronic applications.
GSK3β attenuates TGF-β1 induced epithelial–mesenchymal transition and metabolic alterations in ARPE-19 cells
While TGF-β1 is known to induce epithelial–mesenchymal transition (EMT), a major factor in the pathogenesis of proliferative vitreoretinopathy (PVR), in ARPE-19 cells. The molecular pathways involved in EMT formation have not yet to be fully characterized. In this study, we have found that TGF-β1-mediated induction of EMT in ARPE-19 cells varied in a dose- and time-dependent manner. Specifically, TGF-β1 inhibited GSK-3β by accelerating phosphorylation at ser9. GSK-3β inhibitor or knockdown of GSK-3β resulted in enhanced TGF-β1-mediated EMT, migration and collagen contraction in ARPE-19 cells, which were then abrogated by GSK-3β overexpression and PI3K/AKT inhibitor. Importantly, GSK-3β also mediated metabolic reprogramming in TGF-β1-treated cells. Our results indicate that GSK-3β plays a pivotal role in TGF-β1-mediated EMT in ARPE-19 cells. - Highlights: • GSK-3β mediates epithelial-mesenchymal transition in TGF-β1 treated ARPE-19 cells. • GSK-3β regulates cell migration and collagen contraction of ARPE-19 cells. • TGF-β1 induces extracellular metabolomic changes of ARPE-19 cells via a GSK-3β-dependent mechanism.
High-Q, Low-Threshold Monolithic Perovskite Thin-Film Vertical-Cavity Lasers
A vertical-cavity surface-emitting perovskite laser is achieved using a microcavity configuration where CH 3 NH 3 PbI 3 thin solid films are embedded within a custom GaN-based high-quality (Q-factor) resonator. This single-mode perovskite laser reaches a low threshold (≈7.6 µJ cm -2 ) at room temperature and emits spatially coherent Gaussian laser beams. The devices allow direct access to the study of perovskite gain dynamics and material robustness.
Obeticholic acid protects against carbon tetrachloride-induced acute liver injury and inflammation
The farnesoid X receptor (FXR) is a ligand-activated transcription factor that plays important roles in regulating bile acid homeostasis. The aim of the present study was to investigate the effects of obeticholic acid (OCA), a novel synthetic FXR agonist, carbon tetrachloride (CCl{sub 4})-induced acute liver injury. Mice were intraperitoneally injected with CCl{sub 4} (0.15 ml/kg). In CCl{sub 4} + OCA group, mice were orally with OCA (5 mg/kg) 48, 24 and 1 h before CCl{sub 4}. As expected, hepatic FXR was activated by OCA. Interestingly, OCA pretreatment alleviated CCl{sub 4}-induced elevation of serum ALT and hepatic necrosis. Moreover, OCA pretreatment inhibited CCl{sub 4}-induced hepatocyte apoptosis. Additional experiment showed that OCA inhibits CCl{sub 4}-induced hepatic chemokine gene Mcp-1, Mip-2 and Kc. Moreover, OCA inhibits CCl{sub 4}-induced hepatic pro-inflammatory gene Tnf-α and Il-1β. By contrast, OCA pretreatment elevated hepatic anti-inflammatory gene Il-4. Further analysis showed that OCA pretreatment inhibited hepatic IκB phosphorylation and blocked nuclear translocation of NF-κB p65 and p50 subunits during CCl{sub 4}-induced acute liver injury. In addition, OCA pretreatment inhibited hepatic Akt, ERK and p38 phosphorylation in CCl{sub 4}-induced acute liver injury. These results suggest that OCA protects against CCl{sub 4}-induced acute liver injury and inflammation. Synthetic FXR agonists may be effective antidotes for hepatic inflammation during acute liver injury. - Highlights: • OCA pretreatment activates hepatic FXR. • FXR activation protects against CCl{sub 4}-induced acute liver injury. • FXR activation inhibits hepatocyte apoptosis during CCl{sub 4}-induced liver injury. • FXR activation differentially regulates hepatic inflammatory genes. • Synthetic FXR agonists are effective antidotes for acute liver injury.
Doubling the critical current density in superconducting FeSe 0.5 Te 0.5 thin films by low temperature oxygen annealing
Iron chalcogenide superconducting thin films and coated conductors are attractive for potential high field applications at liquid helium temperature for their high critical current densities J c , low anisotropies, and relatively strong grain couplings. Embedding flux pinning defects is a general approach to increase the in-field performance of superconductors. However, many effective pinning defects can adversely affect the zero field or self-field J c , particularly in cuprate high temperature superconductors. Here, we report the doubling of the self-field J c in FeSe 0.5 Te 0.5 films by low temperature oxygen annealing, reaching ~3 MA/cm 2 . In-field performance is also dramatically enhanced. In conclusion, our results demonstrate that low temperature oxygen annealing is a simple and cost-efficient post-treatment technique which can greatly help to accelerate the potential high field applications of the iron-based superconductors.
A route for a strong increase of critical current in nanostrained iron-based superconductors
The critical temperature T c and the critical current density J c determine the limits to large-scale superconductor applications. Superconductivity emerges at T c . The practical current-carrying capability, measured by J c , is the ability of defects in superconductors to pin the magnetic vortices, and that may reduce T c . Simultaneous increase of T c and J c in superconductors is desirable but very difficult to realize. Here we demonstrate a route to raise both T c and J c together in iron-based superconductors. By using low-energy proton irradiation, we create cascade defects in FeSe 0.5 Te 0.5 films. Tc is enhanced due to the nanoscale compressive strain and proximity effect, whereas J c is doubled under zero field at 4.2 K through strong vortex pinning by the cascade defects and surrounding nanoscale strain. At 12 K and above 15 T, one order of magnitude of J c enhancement is achieved in both parallel and perpendicular magnetic fields to the film surface.
Pursuing High-Mobility n-Type Organic Semiconductors by Combination of “Molecule-Framework” and “Side-Chain” Engineering
Abstract not provided
Discovery of superconductivity in hard hexagonal ε-NbN
Since the discovery of superconductivity in boron-doped diamond with a critical temperature (T C ) near 4 K, great interest has been attracted in hard superconductors such as transition-metal nitrides and carbides. Here we report the new discovery of superconductivity in polycrystalline hexagonal ε-NbN synthesized at high pressure and high temperature. Direct magnetization and electrical resistivity measurements demonstrate that the superconductivity in bulk polycrystalline hexagonal ε-NbN is below ~11.6 K, which is significantly higher than that for boron-doped diamond. The nature of superconductivity in hexagonal ε-NbN and the physical mechanism for the relatively lower T C have been addressed by the weaker bonding in the Nb-N network, the co-planarity of Nb-N layer as well as its relatively weaker electron-phonon coupling, as compared with the cubic δ-NbN counterpart. Moreover, the newly discovered ε-NbN superconductor remains stable at pressures up to ~20 GPa and is significantly harder than cubic δ-NbN; it is as hard as sapphire, ultra-incompressible and has a high shear rigidity of 201 GPa to rival hard/superhard material γ-B (~227 GPa). Furthermore, this exploration opens a new class of highly desirable materials combining the outstanding mechanical/elastic properties with superconductivity, which may be particularly attractive for its technological and engineering applications in extreme environments.
A comparison between spectra of runaway electron beams in SF{sub 6} and air
Runaway electron (RAE) with extremely high-energy plays important role on the avalanche propagation, streamer formation, and ionization waves in nanosecond-pulse discharges. In this paper, the generation of a supershort avalanche electron beam (SAEB) in SF{sub 6} and air in an inhomogeneous electric field is investigated. A VPG-30-200 generator with a pulse rise time of ∼1.6 ns and a full width at half maximum of 3–5 ns is used to produce RAE beams. The SAEBs in SF{sub 6} and air are measured by using aluminum foils with different thicknesses. Furthermore, the SAEB spectra in SF{sub 6} and air at pressures of 7.5 Torr, 75 Torr, and 750 Torr are compared. The results showed that amplitude of RAE beam current generated at the breakdown in SF{sub 6} was approximately an order of magnitude less than that in air. The energy of SAEB in air was not smaller than that in SF{sub 6} in nanosecond-pulse discharges under otherwise equal conditions. Moreover, the difference between the maximum energy of the electron distributions in air and SF{sub 6} decreased when the rise time of the voltage pulse increased. It was because the difference between the breakdown voltages in air and SF{sub 6} decreased when the rise time of the voltage pulse increased.
HfO{sub 2} dielectric thickness dependence of electrical properties in graphene field effect transistors with double conductance minima
We investigate the electrical properties in back-gated graphene field effect transistors (GFETs) with SiO{sub 2} dielectric and different thickness of high-k HfO{sub 2} dielectric. The results show that transform characteristic (I{sub ds}–V{sub gs}) curves of GFETs are uniquely W-shaped with two charge neutrality point (left and right) in both SiO{sub 2} and HfO{sub 2} dielectric (SiO{sub 2}-GFETs and HfO{sub 2}-GFETs). The gate voltage reduces drastically in HfO{sub 2}-GFETs compared with that in SiO{sub 2}-GFETs, and it becomes much smaller with the decline of HfO{sub 2} thickness. The left charge neutrality point in I{sub d}–V{sub g} curves of all HfO{sub 2}-GFETs is negative, compared to the positive ones in SiO{sub 2}-GFETs, which means that there exists n-doping in graphene with HfO{sub 2} as bottom dielectric. We speculate that this n-doping comes from the HfO{sub 2} layer, which brings fixed charged impurities in close proximity to graphene. The carrier mobility is also researched, demonstrating a decreasing trend of hole mobility in HfO{sub 2}-GFETs contrast to that in SiO{sub 2}-GFETs. In a series of HfO{sub 2}-GFETs with different HfO{sub 2} dielectric thickness, the hole mobility shows a tendency of rise when the thickness decreases to 7 nm. The possible reason might be due to the introduced impurities into HfO{sub 2} film from atomic layer deposition process, the concentration of which varies from the thickness of HfO{sub 2} layer.
Band structure engineering and thermoelectric properties of charge-compensated filled skutterudites
Thermoelectric properties of semiconductors are intimately related to their electronic band structure, which can be engineered via chemical doping. Dopant Ga in the cage-structured skutterudite Co 4 Sb 12 substitutes Sb sites while occupying the void sites. Combining quantitative scanning transmission electron microscopy and first-principles calculations, we show that Ga dual-site occupancy breaks the symmetry of the Sb-Sb network, splits the deep triply-degenerate conduction bands, and drives them downward to the band edge. The charge-compensating nature of the dual occupancy Ga increases overall filling fraction limit. By imparting this unique band structure feature, and judiciously doping the materials by increasing the Yb content, we promote the Fermi level to a point where carriers are in energetic proximity to these features. Increased participation of these heavier bands in electronic transport leads to increased thermopower and effective mass. Further, the localized distortion from Ga/Sb substitution enhances the phonon scattering to reduce the thermal conductivity effectively.
Temporal and spatial profiles of emission intensities in atmospheric pressure helium plasma jet driven by microsecond pulse: Experiment and simulation
A needle-circular electrode structure helium plasma jet driven by microsecond pulsed power is studied. Spatially resolved emission results show that the emission intensity of He(3{sup 3}S{sub 1}) line decreases monotonically along the axial direction, while those of N{sub 2}(C{sup 3}Π{sub u}), N{sub 2}{sup +}(B{sup 2}∑{sup +}{sub u}), and O(3p{sup 5}P) reach their maxima at 3 cm, 2.6 cm, and 1.4 cm, respectively. The plasma plume of the four species shows different characteristics: The N{sub 2} emission plume travels at a fast speed along the entire plasma jet; the N{sub 2}{sup +} emission plume is composed of a bright head and relatively weak tail and travels a shorter distance than the N{sub 2} emission plume; the He emission plume travels at a slower speed for only a very short distance; propagation of the O emission plume is not observed. Results of calculation of radiation fluxes emitted by positive streamers propagating along helium plasma jets are presented. It is shown, in agreement with the results of the present experiment and with other available experimental data, that the intensities of radiation of N{sub 2}(C{sup 3}Π{sub u}) molecules and He(3{sup 3}S{sub 1}) atoms vary with time (along the plasma jet) quite differently. The factors resulting in this difference are discussed.
Superconducting thin films of (100) and (111) oriented indium doped topological crystalline insulator SnTe
Recent discovery of the topological crystalline insulator SnTe has triggered a search for topological superconductors, which have potential application to topological quantum computing. The present work reports on the superconducting properties of indium doped SnTe thin films. The (100) and (111) oriented thin films were epitaxially grown by pulsed-laser deposition on (100) and (111) BaF2 crystalline substrates respectively. The onset superconducting transition temperatures are about 3.8 K for (100) and 3.6 K for (111) orientations, slightly lower than that of the bulk. Magneto-resistive measurements indicate that these thin films may have upper critical fields higher than that of the bulk. With large surface-to-bulk ratio, superconducting indium doped SnTe thin films provide a rich platform for the study of topological superconductivity and potential device applications based on topological superconductors.
Enhancement in magnetic properties of magnesium substituted bismuth ferrite nanoparticles
We report a potential way to effectively improve the magnetic properties of BiFeO{sub 3} (BFO) nanoparticles through Mg{sup 2+} ion substitution at the Fe-sites of BFO lattice. The high purity and structural changes induced by Mg doping are confirmed by X-ray powder diffractometer and Raman spectra. Enhanced magnetic properties are observed in Mg substituted samples, which simultaneously exhibit ferromagnetic and superparamagnetic properties at room temperature. A physical model is proposed to support the observed ferromagnetism of Mg doped samples, and the superparamagnetic properties are revealed by the temperature dependent magnetization measurements. The improved magnetic properties and soft nature obtained by Mg doping in BFO nanoparticles demonstrate the possibility of BFO nanoparticles to practical applications.
Energy transfer dynamics in trimers and aggregates of light-harvesting complex II probed by 2D electronic spectroscopy
The pathways and dynamics of excitation energy transfer between the chlorophyll (Chl) domains in solubilized trimeric and aggregated light-harvesting complex II (LHCII) are examined using two-dimensional electronic spectroscopy (2DES). The LHCII trimers and aggregates exhibit the unquenched and quenched excitonic states of Chl a, respectively. 2DES allows direct correlation of excitation and emission energies of coupled states over population time delays, hence enabling mapping of the energy flow between Chls. By the excitation of the entire Chl b Q{sub y} band, energy transfer from Chl b to Chl a states is monitored in the LHCII trimers and aggregates. Global analysis of the two-dimensional (2D) spectra reveals that energy transfer from Chl b to Chl a occurs on fast and slow time scales of 240–270 fs and 2.8 ps for both forms of LHCII. 2D decay-associated spectra resulting from the global analysis identify the correlation between Chl states involved in the energy transfer and decay at a given lifetime. The contribution of singlet–singlet annihilation on the kinetics of Chl energy transfer and decay is also modelled and discussed. The results show a marked change in the energy transfer kinetics in the time range of a few picoseconds. Owing to slow energy equilibration processes, long-lived intermediate Chl a states are present in solubilized trimers, while in aggregates, the population decay of these excited states is significantly accelerated, suggesting that, overall, the energy transfer within the LHCII complexes is faster in the aggregated state.