Effect of hydrogenation on the thermal conductivity of 2D gallium nitride.

The indirect bandgap of two-dimensional GaN hinders its application in the optical field. Hydrogenation can convert the bandgap type of the GaN monolayer from an indirect to a direct one and also tune the bandgap size. The thermal transport, an important property in the application of two-dimensional materials, is also influenced by hydrogenation. By performing first-principles calculations and solving the phonon Boltzmann equation, we investigate the effect of hydrogenation on the thermal conductivity of the GaN monolayer. The results show that hydrogenation will slightly increase the thermal conductivity of the GaN monolayer from 70.62 Wm-1 K-1 to 76.23 Wm-1 K-1 at 300 K. The little effect of hydrogenation on thermal conductivity is mainly dominated by two competing factors: (1) the reduction of ZA mode lifetime due to the breaking of reflection symmetry after hydrogenation and (2) the increased contribution from TA and LA modes due to the reduction of anharmonic scattering caused by the enlarged phonon bandgap after hydrogenation. The results are compared with other two-dimensional materials with hexagonal monolayer structures.

Structure and electronic bandgap tunability of m-plane GaN multilayers.

Two-dimensional (2D) gallium nitride (GaN) has attracted a lot of attention due to its promising applications in photoelectric nano-devices. Most previous research studies have focused on polar c-plane 2D structures. Here, by employing first principles calculations, we systematically investigate the structural and electronic properties of non-polar m-plane GaN with different numbers of atomic layers. The results show a layer-dependent structure transition and electronic band variation for m-plane GaN. It is found that the monolayer keeps a planar hexagonal structure due to sp2 hybridization, whereas the multilayers are formed by stacking of buckled hexagonal monolayers with unsaturated coordination number at the surface sublayer and bulk-like inner layers. These discrepancies in the structure further induce an indirect to direct transition of the band gap type when the layer number reaches twelve. By carefully examining the relationship between the structure and electronic bandgap, we find that the indirect bandgap comes from the unsaturated surface with a planar like structure. On surface modification, saturation of the surface dangling bonds results in an indirect to direct band gap transition.

Layered Inorganic Silicate Aerogel Pillared by Nanoclusters for High Temperature Thermal Insulation.

Layered inorganic material, with large-area interlayer surface and interface, provides an essential material platform for constructing new configuration of functional materials. Herein, a layered material pillared with nanoclusters realizing high temperature thermal insulation performance is demonstrated for the first time. Specifically, systematic synchrotron radiation spectroscopy and finite element calculation analysis show that ZrOx nanoclusters served as "pillars" to effectively produce porous structures with enough boundary defect while maintaining the layered structure, thereby significantly reducing solid state thermal conductivity (≈0.32 W m-1  K-1 , 298-573 K). Moreover, the layered inorganic silicate material assembled aerogel also exhibits superior thermal insulation performance from room temperature (0.034 W m-1  K-1 , 298 K, air conditions) to high temperature (0.187 W m-1  K-1 , 1073 K, air conditions) and largely enhanced compressive strength (42 kPa at 80% compression), which is the best layered material-based aerogel that has achieved synergistic improvement in thermal and mechanical performance so far. Layered inorganic silicate aerogel pillared by nanoclusters will pave a new avenue for the design of advanced thermal insulation materials under extreme conditions.

Strain-induced phase transition and giant piezoelectricity in monolayer tellurene.

We report the transition of a strain-induced centrosymmetric ß-phase to a non-centrosymmetric α-phase for monolayer tellurene based on density functional theory calculations. The phase transition is represented by the displacement of the middle-layer Te atoms from the center of the unit cell in the y-direction. The critical point for the phase transition is found to be at 0.5% biaxial tensile strain. By analyzing the bond variation and the phonon spectra, we attribute the phase transition to the decrease of the bonding strength at the tensile strain and the atom migration corresponding to the phonon vibration mode along the distorted direction. The transition to the α-phase under strain is further confirmed from the calculated electronic band structure where the spin-orbit coupling (SOC) induces a large Rashba splitting due to symmetry breaking, which may enable the control of spin via the electric field. Two-dimensional ferroelectrics can be formed upon transition of the strain-induced ß-phase to the α-phase, and a high polarization of about 90 µC cm-2 can be achieved via a tensile strain, giving rise to a giant piezoelectric coefficient that is two orders of magnitude higher than that of the MoS2 monolayer.

Antioxidant Activity of Water-soluble Polysaccharides from Brasenia schreberi.

OBJECTIVE: In order to investigate the antioxidant activities of polysaccharides (BPL-1 and BPL-2), one of the most important functional constituents in Brasenia schreberi was isolated from the external mucilage of B. schreberi (BPL-1) and the plant in vivo (BPL-2). This paper examines the relationship between the content of sulfuric radicals and uronic acid in BPL and the antioxidant activity of BPL. MATERIALS AND METHODS: The free radicals, 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS+) and 1,1-diphnyl-2-picrylhydrazyl (DPPH-), were used to determine the antioxidant activity of BPL. The Fourier-transform infrared spectroscopy of BPL-1 and BPL-2 revealed typical characteristics of polysaccharides. RESULTS: The two sample types had different contents. This was proved by their different adsorption peak intensities. The IC50 values of BPL-1 (31.189 mg/ml) and BPL-2 (1.863 mg/ml) showed significant DPPH radical scavenging activity. Based on the quantification of ABTS radical scavenging, the IC50 value of BPL-1 (5.460 mg/ml) was higher than that of BPL-2 (0.239 mg/ml). Therefore, in terms of the reducing power, the IC50 value of BPL-1 was too high to determine, and the IC50 value of BPL-2 was found to be 50.557 mg/ml. Hence, the antioxidant activity and total reducing power were high, and they were greater in BPL-2 than in BPL-1. In addition, BPL-2 was found to have more sulfuric radicals and uronic acid than BPL-1. CONCLUSION: The contents of sulfuric radicals and uronic acid are significantly correlated to the antioxidant activity and reducing power of BPL; the more sulfuric radicals and uronic acid, the more antioxidant activity and reducing power BPL has. SUMMARY: The water-soluble crude polysaccharides obtained from the external mucilage and the Brasenia schreberi plant in vivo were confirmed to have high contents of sulfuric radicals and uronic acidBoth BPL-1 and BPL-2 exhibited antioxidative activity and reducing power, and their antioxidative activity gradually improved with increasing concentrationsThe content of sulfuric radicals and uronic acid in BPL-1 and BPL-2 might explain their high antioxidant activity. Abbreviations used: BPL-1:Polysaccharide were isolated from the external mucilage of B. Schreberi; BPL-2: Polysaccharide were isolated from the plant in vivo of B. schreberi; BPL:Polysaccharide were isolated from B. Schreberi.