Structure of clean and arsenic-covered GaN(0 0 0 1) surfaces
Abstract The effect of trace arsenic on the growth and surface structure of GaN(0 0 0 1) has been studied. We find that a partial pressure of only 10 −9  Torr of arsenic during molecular beam epitaxial growth significantly modifies the growth kinetics. Such a small background pressure of arsenic leads to an arsenic-terminated surface displaying a 2×2 reconstruction during growth which is absent for the clean surface. First-principles theoretical calculations show that As-terminated surfaces are energetically more favorable than Ga-terminated surfaces for arsenic pressures of 10 −9  Torr, and structural models for the As-adatom 2×2 reconstruction are presented.
Phase engineering of MoS2 through GaN/AlN substrate coupling and electron doping.
The polymorphism of two dimensional MoS2 promises new possibilities for nanoelectronics. The realization of those possibilities necessitates techniques to enable flexible and controllable phase engineering of MoS2. In the present study, based on first-principles calculations, a new and flexible route to engineer the phase stability of MoS2 by interfacing it with a GaN or AlN substrate is reported. Depending on the surface termination of the underlying substrate, MoS2 may exhibit either the 2H or 1T' (1T'') phase. The interface coupling between MoS2 and the substrate also affects the phase transition kinetics. In addition, electron doping can act as another means to influence MoS2-substrate interactions and enable further phase engineering of MoS2. The present findings contribute to new knowledge towards phase engineering of MoS2 and the design of hybrid nanodevices comprising both 2D and 3D optoelectronic materials.
EFFECT OF SAMPLE PREPARATION ON WATER TREE RESISTANCE OF XLPE/SEBS BLENDS
The effects of an aromatic polymer, such as poly[styrene-b-(ethylene-co-butylene)b-styrene] (SEBS), and its preparation processes on water tree resistance, morphology, crystallinity, dielectric constant and dielectric loss tangent of crosslinked polyethylene (XLPE)/SEBS and XLPE/SEBS/ethylene vinyl acetate (EVA) blends were investigated. The XLPE/SEBS samples were characterized by means of differential scanning calorimetry, scanning electron microscopy, dielectric spectral analysis and the accelerated water treeing experiment. The results clearly showed that although SEBS was partially compatible with LDPE and it was dispersed evenly in XLPE matrix, the blends demonstrated excellent dielectric performances, such as dielectric constant and dissipation factor. The crystallinity of XLPE/SEBS blends was decreased with increase in SEBS content and it was further decreased with the addition of EVA. SEBS could effectively improve the water-tree resistance of XLPE and both SEBS and EVA were found to improve the water tree resistance of XLPE synergistically as expected. The differences in sample preparation processes could change the phase morphology and dielectric performance of the blends, and could also influence the dispersion of SEBS in XLPE matrix as well as the crystallinity of the blends. Different preparation processes led to different water tree resistance behaviours of the samples. The improvement of SEBS dispersion in XLPE matrix helped to improve the water tree resistance of XLPE.