Narrow Results

Physical Vapor Deposition (PVD) is a coating technique that relies on the creation of a vapor phase, under vacuum conditions, that condenses on a substrate to form a coating. PVD coatings of titanium nitride are commonly used in functional applications to promote faster cutting speeds and to prolong tool life, leading to operational cost savings and improved productivity. Some of the limitations of a PVD coating for functional applications are based on the coating thickness, where a lower coating thickness reduces the wear volume available on a contacting surface. Also of issue is the presence of globules or "macros" in the coating resulting in a non-homogeneous, rougher surface. The formation of macros in a PVD coating is particularly associated with the cathodic arc PVD system. This study investigated the effects of chamber pressure, substrate bias voltage and arc current and their interaction, on physical parameters of titanium nitride coatings deposited in a commercial cathodic arc PVD system. Scanning Electron Microscopy (SEM) was used to provide a measure of the consistency of the coating topography and an indication of the number of macros in the coatings. Atomic Force Microscopy (AFM) was used to provide numerical values for the roughness of the coatings. The information from these two instruments was combined to provide the optimum processing conditions for the reduction of macros.

An experimental investigation was conducted to examine the fretting fatigue behavior of physical vapor deposition-coated CrN film deposited on Ti-6Al-4V specimens contacted on both sides with pads of the same material. Behavior against the CrN-coated specimens was characterized through the determination of fretting fatigue strength up to 10⁷ cycles. Fretting damage of specimen surface was characterized by SEM and surface profilometer. The experimental results from the S–N tests indicate that the CrN coating is effective to improve the fretting fatigue strength until about 10⁶ cycles, but over 10⁶ cycles, the strength is lower than that of uncoated specimens. The enhanced fretting fatigue resistance can be attributed to the improved hardness of the CrN film due to change of bias voltage during the film deposition. It has also been concluded that below and about 10⁶ cycles, there is smaller influence of bias voltage on fatigue strength, whereas, over 10⁶ cycles, fatigue strength is clearly changed by bias voltage as well as contact pressure.

Spin-dependent transport properties of Fe₃O₄ spheres with diameters from 200 nm to 900 nm have been investigated and polyethylene glycol (PEG) exists on the surface of Fe₃O₄ particles. The nonlinear I–V curve became obvious with the increase of Fe₃O₄ diameter, which indicated the tunneling barrier height decreases with the increasing diameter. The magnetoresistance (MR) can reach −13% with an applied low field of 0.2 T at room temperature. With the diameter increase, the MR decreases and the required applied field increases. Moreover, the decrease of MR with the bias voltage increase can be attributed to the spin-dependent tunneling effect through the insulating surface layer of Fe₃O₄ and PEG.

Through the silicon modulation-doping (MD) growth method, the electrical performance of AlGaN-based deep ultraviolet light-emitting diodes (DUV-LEDs) is improved by replacing the commonly uniform-doped (UD) method of n-AlGaN layer. The electroluminescence characterisic measurements demonstrate the MD growth method could effectively enhance the light emission intensity. Both the forward voltage and reverse leakage current of the MD samples are obviously reduced compared to those of the UD sample. Due to the existence of periodic Si-MD superlattices in n-AlGaN layers, which may behave like a series of capacitors, the built-in electric fields are formed. Both the measured capacitance–voltage (C–V) characteristics, and related photoluminescence (PL) intensity with the Si-MD growth method are enhanced. In detail, the effects of these capacitors can enhance the peak internal capacitance up to 370 pF in the MD sample, whereas the UD sample is only 180 pF. The results also mean that with better current spreading ability in the MD sample, the MD processes can effectively enhance the efficiency and reliability of DUV-LEDs. Thus, the investigations of the Si-MD growth methods may be useful for improving the electrical performance of DUV-LEDs in future works. Meanwhile, this investigation may partly suggest the minor crystalline quality improvements in the epi-layers succeeding the MD n-AlGaN layer.

This paper simulated the optimum solar cell power conversion efficiency-based planar solar surface morphology with various solar cell absorber structures. The spectral light intensity, reflected, absorbed, and transmission coefficients versus wavelength band spectrum varied from 300nm to 1300nm through the visible to near-infrared regions. The optimum solar cell performance parameters or key parameters are studied with different solar cell structure designs based on different surface contact and back contact layers. Cumulative generation current, cumulative electron–hole pair generation rate and electron–hole pair generation rate are clarified versus substrate depth for various solar cell structure designs. The absorbed current in substrate is optimized for various solar cell structure designs. Maximum power voltage and current are also optimized for the proposed solar cell structure design. The effective solar cell power efficiency is also demonstrated based on the optimized maximum power current and maximum power voltage.

Polycrystalline Vanadium dioxide (VO₂) thin film can be fabricated on glass substrates by high power impulse magnetron sputtering at a relative high temperature. In order to apply an effective bias voltage on substrate and control the energy of the ions impinged to the substrate, conductive indium-tin oxide (ITO) glass was used as the substrate. UV-visible-near IR transmittance spectra and X-ray diffraction (XRD) patterns of the as-deposited films exhibited that M-VO₂ thin film with a metal–insulator transition temperature of 37${}^{\circ }$C was fabricated successfully at 300${}^{\circ }$C with a bias voltage of $-$200V, and the calculated average crystalline size of this film was about 12nm. XRD patterns at varied temperatures showed that the structural change of MIT of the VO₂ thin film was suppressed during the phase transition process, and a pure Mott transition was obtained.

Since the successful fabrication of graphene, two-dimensional hexagonal lattice structures have become a research hotspot in condensed matter physics. In this short review, we theoretically focus on discussing the possible realization of a topological insulator (TI) phase in systems of graphene bilayer (GBL) and boron nitride bilayer (BNBL), whose band structures can be experimentally modulated by an interlayer bias voltage. Under the bias, a band gap can be opened in AB-stacked GBL but is still closed in AA-stacked GBL and significantly reduced in AA- or AB-stacked BNBL. In the presence of spin–orbit couplings (SOCs), further demonstrations indicate whether the topological quantum phase transition can be realized strongly depends on the stacking orders and symmetries of structures. It is observed that a bulk band gap can be first closed and then reopened when the Rashba SOC increases for gated AB-stacked GBL or when the intrinsic SOC increases for gated AA-stacked BNBL. This gives a distinct signal for a topological quantum phase transition, which is further characterized by a jump of the ℤ₂ topological invariant. At fixed SOCs, the TI phase can be well switched by the interlayer bias and the phase boundaries are precisely determined. For AA-stacked GBL and AB-stacked BNBL, no strong TI phase exists, regardless of the strength of the intrinsic or Rashba SOCs. At last, a brief overview is given on other two-dimensional hexagonal materials including silicene and molybdenum disulfide bilayers.