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The weak magnetic anisotropy and low Curie temperature of two-dimensional Cr2Ge2Te6 greatly restrict its application in spintronics. In this work, a simple but effective strategy has been proposed to promote the perpendicular magnetic anisotropy and Curie temperature of monolayer Cr2Ge2Te6, via stacking into van der Waals heterostructure with monolayer PtTe2. Based on first-principles calculations, the Cr2Ge2Te6/PtTe2 heterostructures are demonstrated to significantly enhance the magnetic anisotropy, which is improved from −0.06 meV Cr−1 for the monolayer into 0.24 meV Cr−1 for the heterostructure. By analyzing the orbit-resolved magnetic anisotropy and the projected density of states, it is revealed that the enhancement of magnetic anisotropy energy is closely related to interface Te atoms of monolayer Cr2Ge2Te6. The Curie temperature of the heterostructure is estimated to be 90 K by Monte Carlo simulation, increased by 300% compared with the value of 30 K for monolayer Cr2Ge2Te6. These results suggest the magnetic performance of Cr2Ge2Te6 can be significantly improved by van der Waals engineering. Compared with other methods, such as doping atoms or applying strain, this stacking-strategy is more convenient and controllable. Although there are still important challenges toward the room-temperature ferromagnetism of two-dimensional Cr2Ge2Te6 and other semiconductors, our work has proved a new approach to solve this problem and is of reference value for the design of novel magnetic heterostructures.

Streamer-to-spark transition and hydrodynamic expansion in a three-electrode pulsed surface dielectric barrier discharge are studied under atmospheric-pressure air. Three sequential discharge processes of the primary streamer, transitional streamer and spark phase during a single pulse are observed from the time-resolved plasma morphologies. The primary streamer and transitional streamer phases, with a rising voltage and low current, followed by a spark phase with a rapidly falling pulse and ascending current are characterized. Images of the discharge development show that the transitional streamer is maintained in the ionization channel after the primary streamer bridges the high-voltage electrode and the second grounded electrode. When the transitional streamer develops to a certain level, the streamer discharge transfers into the spark discharge. As a result, two shock waves are induced in the two exposed electrode domains, and then merge into a single ellipse during the process of hydrodynamic expansion. Boltzmann plots indicate that the electron temperature is 4.815 eV in the initial phase of spark discharge and gradually decreases in the spark phase. Stark broadening of the O atomic line shows that the electron density is 7.06 × 1017 cm−3 during the spark phase.

In the present work, a magnetic-assisted DC superimposed nanosecond-pulsed streamer discharge (DC-NPSD) with a 0.4 T parallel magnetic field is developed that achieves good performance for ozone production and toluene degradation. The influence of the assisted parallel magnetic field on the electrical characteristics, streamer propagation behavior, reactive species generation and plasma chemical properties of the DC-NPSD are systematically investigated. The experimental results indicate that better impedance matching of a nanosecond pulsed power supply and a discharge reactor can be realized by superimposing DC voltage (U DC), which facilitates reactive species production and toluene degradation. The discharge current, input energy and reactive species production can be further enhanced by the application of a parallel magnetic field under different pulse and DC voltage conditions. There are two distinct streamer phases in the DC-NPSD: a primary streamer (PS) with longer propagation distance and higher propagation velocity and a secondary streamer (SS) with shorter propagation distance and lower propagation velocity. The propagation velocities of both the PS and the SS increase with increasing U DC. Only PS propagation velocity is accelerated by a parallel magnetic field; however, that of the SS remains almost constant with or without a magnetic field. Both ozone generation and toluene degradation performance are improved by a magnetic field, which is attributed to the lengthened electron motion path under the action of Lorentz force and the constraint effect on energetic electrons in the presence of a parallel magnetic field.

The degradation of toluene by DC discharge has been evaluated in a multi-wire-plate reactor with MgO/NiO/Ni (MNN) cathode and compared with a normal discharge with Ni cathode. The effects of various operating parameters, including the position distribution of wire electrodes, discharge gap, gas flow rate and toluene initial concentration have been systematically investigated. Morphological characterizations demonstrate that in addition to the normal discharge around the wire electrodes, extra discharge also occurs on the MNN cathode surface which is mainly concentrated in the vertical corresponding region of the wire electrodes and diffuses to both sides. The position distribution of two wires influences the total discharge current. It is because that it affects not only the discharge around the wire electrode, but also the discharge on MNN cathode surface. Beside, a wire-wire spacing of 25 mm in discharge reactor with MNN cathode is optimized due to a relative large discharge current. A small discharge gap between wire electrode and cathode contributes to the improvement of toluene degradation and mineralization and it is optimized of 15.3 mm. The effects of discharge power, gas flow rate and toluene initial concentration and the interactions between these parameters on toluene removal performance by discharge with MNN cathode are investigated using Box-Behnken design (BBD). The Analysis of Variance (ANOVA) is employed to evaluate the significance of the independent variables and their interactions. The results show that the initial concentration plays the most important role in determining the toluene removal efficiency, while discharge power is the most significant factor affecting the energy efficiency in this discharge process.

In this paper, an improved corona discharge in air at atmospheric pressure is ignited by a MgO/NiO/Ni sandwich cathode with high secondary electron emission and is then employed for volatile organic compound degradation. Current–voltage characteristics, discharge images, and spectroscopic investigations are compared for two corona discharges with a MgO/NiO/Ni cathode and a Ni cathode, respectively. The discharge current with the MgO/NiO/Ni cathode is about 2.5–3.3 times larger than the one with the Ni cathode at various applied voltages from 12 to 19 kV. Besides, discharge images show that, as well as the expected glow discharge around the needle tips, luminous points also occur on the surface of the MgO/NiO/Ni cathode simultaneously, indicating that ionization and excitation processes take place around the cathode surface. This is attributed to the fact that an internal field within the oxide layer calculated by the measured surface potential, which is induced by positive charges accumulated on the oxide layers, is sufficient to induce field emission and further cause the improved corona discharge generation. Such improved corona discharge with the MgO/NiO/Ni cathode is then applied to toluene degradation, and the toluene degradation efficiency and energy yield are much higher than those with the Ni cathode, respectively.

We present a dynamically tunable metamaterial graphene absorber (MGA) in the terahertz regime. The unit cell of the proposed MGA consists of metal wire and graphene sheet over the grounded dielectric absorber. The MGA achieves frequency tunable characteristics via changing the chemical potential. In order to understand the absorption mechanism of this absorber, a simple equivalent circuit method has been proposed. Because the coupling between wire-based metamaterial and graphene is complicated and cannot be neglected an equivalent surface impedance was introduced and extracted for simplification. In addition to the chemical potential of graphene, the constitutive parameters of metal wire are also discussed in detail to completely understand how these factors affect the absorption properties. It is believed that this study may be useful for providing valuable guidance in the development of more advanced MGAs.

In this work, a nanosecond bipolar pulsed voltage coupled with a negative DC component is employed to generate sliding dielectric barrier discharge (DBD) plasma in a three-electrode geometry reactor and improve volatile organic compound (VOC) degradation at room temperature. The effects of the bipolar pulsed voltage (U ±pulse) and negative DC voltage (U −DC) on the discharge characteristic, optical characteristic, plasma gas temperature (T gas), and vibrational temperature (T vib) are discussed. The horizontal distribution characteristics of the N2(C3Πu → B3Πg) emission intensity, T gas, and T vib are also investigated to understand the propagation mechanism of sliding DBD along the dielectric surface. The experimental results reveal that a negative DC component applied to a third electrode can extend the plasma extension region, indicating that the gas ionization is ignited by the nanosecond high-voltage pulse, while charge drift is forced by the surface potential difference caused by the negative high-voltage DC. The T gas is measured by optical emission spectroscopy related to the rotational bands of N2(C3 Πu → B3Πg), and is approximately 375 ± 5 K under the condition of U ±pulse = 20 kV and U −DC = −20 kV. Compared with typical surface DBD plasma, sliding DBD plasma is quasi-diffusive and distributed more uniformly within the whole discharge gap. Furthermore, both surface DBD and sliding DBD are used for removing toluene from flowing air. It is found that sliding DBD has higher toluene degradation efficiency and energy yield than surface DBD when they are excited by the positive pulsed voltage (U +pulse).

The purpose of this paper is to develop a high-efficiency air-cleaning system for volatile organic compounds (VOCs) existing in the workshop of a chemical factory. A novel parallel surface/packed-bed discharge (PSPBD) reactor, which utilized a combination of surface discharge (SD) plasma with packed-bed discharge (PBD) plasma, was designed and employed for VOCs removal in a closed vessel. In order to optimize the structure of the PSPBD reactor, the discharge characteristic, benzene removal efficiency, and energy yield were compared for different discharge lengths, quartz tube diameters, shapes of external high-voltage electrode, packed-bed discharge gaps, and packing pellet sizes, respectively. In the circulation test, 52.8% of benzene was removed and the energy yield achieved 0.79 mg kJ−1 after a 210 min discharge treatment in the PSPBD reactor, which was 10.3% and 0.18 mg kJ−1 higher, respectively, than in the SD reactor, 21.8% and 0.34 mg kJ−1 higher, respectively, than in the PBD reactor at 53 J l−1. The improved performance in benzene removal and energy yield can be attributed to the plasma chemistry effect of the sequential processing in the PSPBD reactor. The VOCs mineralization and organic intermediates generated during discharge treatment were followed by CO x selectivity and FT-IR analyses. The experimental results indicate that the PSPBD plasma process is an effective and energy-efficient approach for VOCs removal in an indoor environment.

With a probe of atomic dimensions, the aberration-corrected scanning transmission electron microscope (STEM) has become one of the most advanced analytical tools for materials research. However, electron-beam damage limits its applications to materials that are insensitive to damage, and these are a small fraction of the materials that are important to the future of material science. One important group of materials which are susceptible to the electron beam is electrically insulating inorganic materials, including both crystals and glasses. In this paper, damage phenomena for these materials under the illumination of a finely focused electron beam in STEM are summarized. The leading mechanism for damage is massive atom displacement driven by an electric field, which is induced by the excitations and ionizations of atomic electrons. The electric field induced by the STEM probe, and thus the damage, is spatially delocalized and dependent on beam current density (dose rate) and exposure time but approximately independent of specimen thickness. This mechanism causes either phase separation, including precipitation and formation of gas bubbles, or phase transformation. From a microscopic analysis point of view, higher beam currents and longer exposures should therefore be avoided if the induced electric field is the main cause for damage. Methods of minimizing damage are reviewed.