In Situ Observation of Field-Induced Nanoprotrusion Growth on a Carbon-Coated Tungsten Nanotip.

Nanoprotrusion (NP) on metal surface and its inevitable contamination layer under high electric field is often considered as the primary precursor that leads to vacuum breakdown, which plays an extremely detrimental effect for high energy physics equipment and many other devices. Yet, the NP growth has never been experimentally observed. Here, we conduct field emission (FE) measurements along with in situ transmission electron microscopy (TEM) imaging of an amorphous-carbon (a-C) coated tungsten nanotip at various nanoscale vacuum gap distances. We find that under certain conditions, the FE current-voltage (I-V) curves switch abruptly into an enhanced-current state, implying the growth of an NP. We then run field emission simulations, demonstrating that the temporary enhanced-current I-V is perfectly consistent with the hypothesis that a NP has grown at the apex of the tip. This hypothesis is also confirmed by the repeatable in situ observation of such a nanoprotrusion and its continued growth during successive FE measurements in TEM. We tentatively attribute this phenomenon to field-induced biased diffusion of surface a-C atoms, after performing a finite element analysis that excludes the alternative possibility of field-induced plastic deformation.

Polarizable Nonvolatile Ferroelectric Gating in Monolayer MoS2 Phototransistors.

Given the requirements for power and dimension scaling, modulating channel transport properties using high gate bias is unfavorable due to the introduction of severe leakages and large power dissipation. Hence, this work presents an ultrathin phototransistor with chemical-vapor-deposition-grown monolayer MoS2 as the channel and a 10.2 nm thick Al:HfO2 ferroelectric film as the dielectric. The proposed device is meticulously modulated utilizing an Al:HfO2 nanofilm, which passivates traps and suppresses charge Coulomb scattering with Al doping, efficiently improving carrier transport and inhibiting leakage current. Furthermore, a bipolar pulses excitable polarization method is developed to induce a nonvolatile electrostatic field. The MoS2 channel is fully depleted by the switchable and stable floating gate originating from remanent polarization, leading to a high detectivity of 2.05 × 1011 Jones per nanometer of gating layer (Jones nm-1) and photocurrent on/off ratio >104 nm-1, which are superior to the state-of-the-art phototransistors based on two-dimensional (2D) materials and ferroelectrics. The proposed polarizable nonvolatile ferroelectric gating in a monolayer MoS2 phototransistor promises a potential route toward ultrasensitive photodetectors with low power consumption that boast of high levels of integration.

Tuneable effects of pyrrolic N and pyridinic N on the enhanced field emission properties of nitrogen-doped graphene.

Graphene is one of the most potential field emission cathode materials and a lot of work has been carried out to demonstrate the effectiveness of nitrogen doping (N doping) for the enhancement of field emission properties of graphene. However, the effect of N doping on graphene field emission is lacking systematic and thorough understanding. In this study, undoped graphene and N-doped graphene were prepared and characterized for measurements, and the field emission property dependence of the doping content was investigated and the tuneable effect was discussed. For the undoped graphene, the turn-on field was 7.95 V µm-1 and the current density was 7.3 µA cm-2, and for the 10 mg, 20 mg, and 30 mg N-doped graphene samples, the turn-on fields declined to 7.50 V µm-1, 6.38 V µm-1, and 7.28 V µm-1, and current densities increased to 21.0 µA cm-2, 42.6 µA cm-2, and 13.2 µA cm-2, respectively. Density functional theory (DFT) calculations revealed that N doping could bring about additional charge and then cause charge aggregation around the N atom. At the same time, it also lowered the work function, which further enhanced the field emission. The doping effect was determined by the content of the pyrrolic-type N and pyridinic-type N. Pyridinic-type N is more favourable for field emission because of its smaller work function, which is in good agreement with the experimental results. This study would be of great benefit to the understanding of N doping modulation for superior field emission properties.

Synthesis of violet phosphorus with large lateral sizes to facilitate nano-device fabrications.

Violet phosphorus has been proven to be the most stable phosphorus allotrope and has attracted much attention recently. The growth of violet phosphorus with large lateral sizes is crucial to obtain good quality violet phosphorene for nanodevice fabrication. Herein, a large number of violet phosphorus plates have been produced from molten lead using an optimized method to achieve red bronze luster. The crystal structure of the as-produced violet phosphorus was determined by single-crystal X-ray diffraction to be monoclinic with the space group P2/n (13) (CSD-2160375), identical to the one from the chemical vapor transport method (CSD-1935087). The as-produced violet phosphorus plates were found to have lateral sizes of 1.30 ± 0.41 mm2. The violet phosphorus plates were easily exfoliated and directly transferred to silicon substrates to facilitate building of a back-gate field-effect transistor. A hole mobility of 2.308 cm2 V-1 s-1 was obtained from a violet phosphorus nanosheet with a thickness of 52 nm under ambient conditions. The absolute responsivity of 130 mA W-1 with a fast response time of 27 ms was also obtained under the irradiation of a 530 nm laser.

Synergy of Substrate Chemical Environments and Single-Atom Catalysts Promotes Catalytic Performance: Nitrogen Reduction on Chiral and Defected Carbon Nanotubes.

The catalytic activities of single-atom catalysts (SACs) are strongly influenced by the local chemical environments of their substrates, by which the electronic structures of the SACs can be effectively tuned. Together with the freedom of available reactive metallic centers, it would be feasible to maximize the catalytic performance by means of a synergetic optimization in the chemical space spanned by the features of both the substrate and the catalytic center. In this work, using first-principles calculations, we systematically assessed the synergetic effect between the substrate geometric/electronic structures and the catalytic centers on the electrocatalytic nitrogen reduction reaction (NRR). Carbon nanotubes with different chirality, defects, and chemical functionalization were used to support 15 transition metal atoms. Three SACs, TiN4CNT(3,3), TiN4CNT(5,5), and VN4CNT(3,3), simultaneously possess high NRR selectivities (w.r.t hydrogen evolution) and low overpotentials of 0.35, 0.35, and 0.37 V, respectively. Electronic structure analysis elucidated that larger metal atoms anchored on CNTs with higher curvature and doped by N atoms facilitate the rupture of the N-N bond in *NH2NH2 to lower the overpotentials. The synergy of substrate chemical environments and single atomic catalysis is a promising strategy to optimize the catalytic performance.

Unlocking the Potential of Vanadium Oxide for Ultrafast and Stable Zn2+ Storage Through Optimized Stress Distribution: From Engineering Simulation to Elaborate Structure Design.

Compared with lithium-ion batteries (LIBs), aqueous zinc batteries (AZIBs) have received extensive attention due to their safety and cost advantages in recent years. The cathode determines the electrochemical performance of AZIBs to a large extent. Vanadium-based materials exhibit excellent capacity when used as AZIB cathodes. However, unexpected structural stress is inevitably induced during cycling and high current densities, which can gradually lead to structural deterioration and capacity decay. In fact, the stress/strain distribution in nanomaterials is crucial for electrochemical performance. In this work, the optimized stress distribution of the hierarchical hollow structure is verified by the finite element simulation of COMSOL software firstly. Guided by this model, a simple solvothermal method to synthesize hierarchical hollow vanadium oxide nanospheres (VO-NSs), consisting of ≈10 nm ultrathin nanosheets and ≈500 nm hollow inner cavities, is employed. And a highly disordered structure is introduced to the VO-NSs by in situ electrochemical oxidation, which can also weaken the structural stress during Zn2+ insertion and extraction. Benefiting from this unique structure, VO-NSs exhibit high-rate and stable Zn2+ storage capability. The strategy of engineering-driven material design provides new insights into the development of AZIB cathodes.

Vapor-Liquid-Solid Growth of Morphology-Tailorable WS2 toward P-Type Monolayer Field-Effect Transistors.

Although substantial efforts have been made, controllable synthesis of p-type WS2 remains a challenge. In this work, we employ NaCl as a seeding promoter to realize vapor-liquid-solid (VLS) growth of p-type WS2. Morphological evolution, including a one-dimensional (1D) nanowire to two-dimensional (2D) planar domain and 2D shape transition of WS2 domains, can be well-controlled by the growth temperature and sulfur introduction time. A high growth temperature is required to enable planar growth of 2D WS2, and a sulfur-rich environment is found to facilitate the growth of high-quality WS2. Raman and photoluminescence (PL) mappings demonstrate uniform crystallinity and high quantum efficiency of VLS-grown WS2. Moreover, monolayer WS2-based field-effect transistors (FETs) are fabricated, showing p-type conducting behavior, which is different from previous reported n-type FETs from WS2 grown by other methods. First-principles calculations show that the p-type behavior originates from the substitution of Na at the W site, which will form an additional acceptor level above the valence band maximum (VBM). This facile VLS growth method opens the avenue to realize the p-n WS2 homojunctions and p/n-WS2-based heterojunctions for monolayer wearable electronic, photonic, optoelectronic, and biosensing devices and should also be a great benefit to the development of 2D complementary metal-oxide-semiconductor (CMOS) circuit applications.

Sandwich-Structured h-BN/PVDF/h-BN Film With High Dielectric Strength and Energy Storage Density.

Energy storage film is one of the most important energy storage materials, while the performance of commercial energy storage films currently cannot meet the growing industrial requirements. Hence, this work presents a h-BN/PVDF/h-BN sandwich composite structure film prepared by laminating a large area of ultrathin hexagonal boron nitride (h-BN) and polyvinylidene fluoride (PVDF), the existence of which was confirmed by using an optical microscope and elemental composition analysis based on scanning electron microscopy and X-ray diffraction. This film has an ultrahigh dielectric strength of 464.7 kV/mm and a discharged energy density of up to 19.256 J/cm3, which is much larger than the commercial energy storage film biaxially oriented polypropylene (BOPP) (<2.5 J/cm3). Although the thickness of the h-BN film is only 70 nm compared with that of PVDF (about 12 µm), the dielectric strength of the sandwich-structured film presents a great increase. It is because of the excellent insulation performance of the h-BN film that helps to resist the electron injection and migration under high electric field, and then suppress the formation and growth of the breakdown path, leading to an improvement of the charge-discharge efficiency.

Efficient pollutants removal and microbial flexibility under high-salt gradient of an oilfield wastewater treatment system.

The treatment of hypersaline oilfield wastewater (HSOW) is a challenge due to its complex composition and low biodegradability, especially in coastal areas. In this study, an HSOW treatment system of gas flotation and biochemistry technology combined with constructed wetland (CW) was investigated. The combined treatment system could efficiently remove COD, NH4+-N and oil under high salinity (1.36-2.21 × 104 mg/L), with average removal rates of 98.5%, 99.9% and 96%, respectively. Meanwhile, different salinity shaped particular community structures and functions. The abundance of Marivita, Parvibaculum, etc. was highly correlated with salinity. Co-occurrence network resulted that the microorganisms were highly interconnected, and formed a functional group of petroleum degrading. Pseudomonas, Rosevarius, Alternaria, etc. were the key genera. Moreover, functional prospected revealed that high salinity reduced the energy metabolism activity. This study will optimize the combined process and provide the basis for further extraction of high-efficiency degradation strains for HSOW enhanced treatment.

Preparation and Characterization of Narrow Size Distribution PMSQ Microspheres for High-Frequency Electronic Packaging.

Polymethylsilsesquioxane (PMSQ) has become a kind of widely studied filler used in the electronic circuit board substrates due to its organic-inorganic hybrid structure, low dielectric constant, and good thermal stability, among other factors. Herein, the PMSQ microspheres were prepared by a two-step acid-base-catalyzed sol-gel method; the influences of reaction conditions including the ratio of water/methyltrimethoxysilane (MTMS), reaction temperature, concentration of the catalyst, and stirring time were systematically investigated; and the optimized reaction condition was then obtained towards a narrow particle size distribution and good sphericity. The microstructure of PMSQ microspheres was analyzed by the infrared spectrum and X-ray diffraction (XRD), which indicated that the as-prepared PMSQ had a ladder-dominant structure. The thermogravimetric analysis (TGA) demonstrated an excellent thermal stability of as-prepared PMSQ microspheres. More specifically, the dielectric constants at high frequency (1~20 GHz) of as-prepared PMSQ microspheres were measured to be about 3.7, which turned out a lower dielectric constant compared to SiO2 powder (≈4.0). This study paves the way to further improve the performance of the electronic circuit board substrates for the application of high-frequency electronic packaging.

Simultaneously enhanced dielectric properties and through-plane thermal conductivity of epoxy composites with alumina and boron nitride nanosheets.

Dielectric materials with good thermal transport performance and desirable dielectric properties have significant potential to address the critical challenges of heat dissipation for microelectronic devices and power equipment under high electric field. This work reported the role of synergistic effect and interface on through-plane thermal conductivity and dielectric properties by intercalating the hybrid fillers of the alumina and boron nitride nanosheets (BNNs) into epoxy resin. For instance, epoxy composite with hybrid fillers at a relatively low loading shows an increase of around 3 times in through-plane thermal conductivity and maintains a close dielectric breakdown strength compared to pure epoxy. Meanwhile, the epoxy composite shows extremely low dielectric loss of 0.0024 at room temperature and 0.022 at 100 ℃ and 10-1 Hz. And covalent bonding and hydrogen-bond interaction models were presented for analyzing the thermal conductivity and dielectric properties.

A high-throughput assessment of the adsorption capacity and Li-ion diffusion dynamics in Mo-based ordered double-transition-metal MXenes as anode materials for fast-charging LIBs.

Utilizing the latest SCAN-rVV10 density functional, we thoroughly assess the electrochemical properties of 35 Mo-based ordered double transition metal MXenes, including clean Mo2MC2 (M = Sc, Ti, V, Zr, Nb, Hf, Ta) and surface functionalized structures Mo2MC2T2 (T = H, O, F and OH), for the potential use as anode materials in lithium ion batteries (LIBs). The first principles molecular dynamics simulations in combination with the calculations of the site adsorption preferences for Li atoms on all investigated MXenes reveal that both Li-saturated adsorption structures and theoretical capacities of Mo-based MXenes are fundamentally influenced by the surface terminations. We find that the adsorption of Li atoms on either -OH or -F functionalized MXenes is chemically unstable. In particular, the F-groups prefer to form a separate fluoride layer with Li atoms, detaching from the Mo2MC2 substrates. The Li atoms could form a stable single adsorption layer on the -H, -O and intrinsic MXenes surface, exhibiting theoretical capacities in the range from 121 mA h g-1 to 195 mA h g-1. Besides -F and -OH terminations, the remaining Mo-based MXenes also possess superior flat open circuit voltage (OCV) profiles with the most reversible storage capacity below 1.0 V during the charging/discharging cycles. We further predict the low barrier heights of Li-ion diffusion, at a range of 0.03-0.06 eV for most Mo-based MXenes except -O and -H terminations, exceeding that of graphene or Ti3C2. Furthermore, combining the Vineyard transition state theory (TST) with the phonon spectra obtained from density functional perturbation theory (DFPT), the mean planar diffusion coefficient is calculated to be 2 × 10-8 m2 s-1 at 300 K for intrinsic Mo2MC2 monolayers. Although the overall specific capacity is fundamentally restricted with the relatively heavy molecular mass of MXenes, we conclude that Mo-based structures, especially the intrinsic Mo2MC2 (M = Sc, Ti, V) monolayers, might be promising anode materials from the aspect of fast charging/discharging application for LIBs.

Simulation-guided nanofabrication of high-quality practical tungsten probes.

Micro/nanoscale tungsten probes are widely utilized in the fields of surface analysis, biological engineering, etc. amongst several others. This work performs comprehensive dynamic simulations on the influences of electric field distribution, surface tension and the bubbling situation on electrochemical etching behaviors, and then the tip dimension. Results show that the etching rate is reliant on the electric field distribution determined by the cathode dimension. The necking position lies in the meniscus rather than at the bottom of the meniscus. A bubble-free condition is mandatory to stabilize the distribution of OH- and WO4 2- ions for a smooth tungsten probe surface. Such simulation-guidance enables the nanofabrication of probes with a high aspect ratio (10 : 1), ultra-sharp tip apex (40 nm) and ultra-smooth surface. These probes have been successfully developed for high-performance application with Scanning Tunneling Microscopy (STM). The acquired decent atomic resolution images of epitaxial bilayer graphene robustly verify the feasibility of the practical level application of these nanoscale probes. Therefore, these nanoscale probes would be of great benefit to the development of advanced analytical science and nano-to-atomic scale experimental science and technology.

The Transition to Paschen's Law for Microscale Gas Breakdown at Subatmospheric Pressure.

The decrease in electronic device size necessitates greater understanding of gas breakdown and electron emission at microscale to optimize performance. While traditional breakdown theory using Paschen's law (PL), driven by Townsend avalanche, fails for gap distance d [Formula: see text] 15 µm, recent studies have derived analytic equations for breakdown voltage when field emission and Townsend avalanche drive breakdown. This study derives a new analytic equation that predicts breakdown voltage VB within 4% of the exact numerical results of a previously derived theory and new experimental results at subatmospheric pressure for gap distances from 1-25 µm. At atmospheric pressure, VB transitions to PL near the product of pressure and gap distance, pd, corresponding to the Paschen minimum; at lower pressures, the transition to PL occurs to the left of the minimum. We further show that the work function plays a major role in determining which side of the Paschen minimum VB transitions to PL as pressure approaches atmospheric pressure while field enhancement and the secondary emission coefficient play smaller roles. These results indicate that appropriate combinations of these parameters cause VB to transition to PL to the left of the Paschen minimum, which would yield an extended plateau similar to some microscale gas breakdown experimental observations.

Two-dimensional mapping of the electric field distribution inside vacuum microgaps observed in a scanning electron microscope.

In this paper, we present an in-situ measurement method to directly observe the distribution of the local electric field between vacuum microgaps. The measurement was performed in-situ inside a high resolution scanning electron microscope (SEM), and the nature of the local electric field was characterized through secondary electron contrast images with the aid of Rutherford scattering theory. Based on the regular fringes in these contrast images, the distribution of the local electric field could be extracted from the contour lines of the fringes while the magnitude of the local electric field could be evaluated qualitatively by the gradient of the contour lines. The finite element method (FEM) simulation and the three-electrodes imaging experiment were also conducted, and the obtained two-dimensional electric field distribution agreed well with the FEM simulation, suggesting that the in-situ visualization technique could be useful for determining the local field enhancement behavior for various geometrical configurations and microscale structures. A physical mechanism for the local electric field mapping is suggested. This study demonstrates the potential of SEM imaging for obtaining information about the local electric field within microelectronic structures and devices.