Using first-principles GW Bethe-Salpeter equation calculations and the k·p theory, we unambiguously show that for two-dimensional (2D) semiconductors, there exists a robust linear scaling law ...between the quasiparticle band gap (E_{g}) and the exciton binding energy (E_{b}), namely, E_{b}≈E_{g}/4, regardless of their lattice configuration, bonding characteristic, as well as the topological property. Such a parameter-free universality is never observed in their three-dimensional counterparts. By deriving a simple expression for the 2D polarizability merely with respect to E_{g}, and adopting the screened hydrogen model for E_{b}, the linear scaling law can be deduced analytically. This work provides an opportunity to better understand the fantastic consequence of the 2D nature for materials, and thus offers valuable guidance for their property modulation and performance control.
The subject of thermal transport at the mesoscopic scale and in low‐dimensional systems is interesting for both fundamental research and practical applications. As the first example of truly ...two‐dimensional materials, graphene has exceptionally high thermal conductivity, and thus provides an ideal platform for the research. Here we review recent studies on thermal and thermoelectric properties of graphene, with an emphasis on experimental progresses. A general physical picture based on the Landauer transport formalism is introduced to understand underlying mechanisms. We show that the superior thermal conductivity of graphene is contributed not only by large ballistic thermal conductance but also by very long phonon mean free path (MFP). The long phonon MFP, explained by the low‐dimensional nature and high sample purity of graphene, results in important isotope effects and size effects on thermal conduction. In terms of various scattering mechanisms in graphene, several approaches are suggested to control thermal conductivity. Among them, introducing rough boundaries and weakly‐coupled interfaces are promising ways to suppress thermal conduction effectively. We also discuss the Seebeck effect of graphene. Graphene itself might not be a good thermoelectric material. However, the concepts developed by graphene research might be applied to improve thermoelectric performance of other materials.
Thermal conductivity as a function of temperature for graphene‐based systems. The tunable thermal conductivity over three orders of magnitude is achieved by different phonon scattering mechanisms and offers great opportunity for thermal management and themoelectric applications.
Topological crystalline insulators (TCIs) are new states of matter whose topological distinction relies on the crystal symmetry of periodic solids. The first material realization of TCIs has recently ...been predicted and observed in IV-VI semiconductor SnTe and related alloys. By combining k times p theory and band structure calculation, we present a unified approach to study topological surface states on various crystal surfaces of these TCI materials based on the electronic structure of the bulk. Depending on the surface orientation, we find two types of surface states with qualitatively different properties. In particular, the (111) surface states consist of four Dirac cones centered at Gamma and M, while Dirac cones on (001) and (110) surfaces are located at non-time-reversal-invariant momenta. The latter types of surface states exhibit a Lifshitz transition as a function of Fermi energy, which is accompanied by a Van Hove singularity in the density of states arising from saddle points in the band structure.
•The crumb waste tire rubber (WTR) was modified by plasma polymerization of ethanol.•Hydrophilic groups were introduced onto WTR surface and improved its hydrophilia.•The functionalized crumb WTR was ...applied in oil-well cement.•The mechanical properties of modified oil-well cement were intensively enhanced.
Crumb waste tire rubber (WTR) was pretreated by oxygen low temperature plasma (LTP) and modified by LTP polymerization process of ethanol monomer to improve the adhesion property with oil-well cement matrix and the mechanical properties of cement. The surface properties of modified crumb WTR and the mechanical properties and structures of modified oil-well cement were investigated by means of contact angle measurement, dispersion test, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), mechanics performance tests, permeability test and scanning electron microscopy (SEM). It was demonstrated that LTP treatment changed both the surface composition and roughness. The contact angle of pretreated crumb WTR dramatically fell from 122° to 34°, and sample with ethanol LPT polymer film decreased even further to 11°. The ATR-FTIR and XPS analysis results demonstrated that hydrophilic groups, such as –COOH, C–OH, and –CHO, were introduced on the WTR surface. The oxygen atomic percent increased from 8.11% to 14.50% and 24.83%. The mechanical properties, porosity and permeability of raw cement were compared to samples modified by untreated crumb WTR, pretreated crumb WTR and ethanol LTP polymerization treated crumb WTR. It was found that after 28 days, the compressive strength of the samples with the untreated crumb WTR decreased to 80% with respect to raw cement. The tensile strength and flexural strength also had a slight reduction compared with the raw cement. On the contrary, after 28 days, the tensile strength of cement modified by LTP polymerization treated WTR increased 11.03% and 13.36%, and the flexural strength increased 9.65% and 7.31%, respectively. A decrease in the compressive strength also occurred but was inconspicuous. A tight interface bonding for ethanol LTP polymerization treated WTR with cement matrix was observed via an SEM image.
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•The nutritive value and function of vinegar products.•Relationship between nutrients and bioactive components of vinegar and health.•Novel functional compounds are present in special ...vinegars.•The mechanisms of healthy effects of vinegar require future investigation.
Vinegar has been widely used as acidic condiment worldwide for thousands of years. Vinegar contains various nutrients and bioactive components, which are brewed by liquid-state and solid-state fermentation techniques. This review highlights the nutrients and bioactive components in different types of vinegars and their functional properties. Nutrients in vinegar include amino acids, sugars, vitamins, and minerals. The functions of these nutrients were providence energy, regulation of cell metabolism regulation, immunoregulation, antioxidation, anticoagulation and improvement of brain development. In addition, the bioactive components in vinegar include organic acids, polyphenols, melanoidins, and tetramethylpyrazine, which have the functions of antioxidative activity, regulation of lipid metabolism, liver protection, blood pressure and glucose control, anti-fatigue and anti-tumor. However, further studies are needed to explore the novel functional compounds in vinegars and their molecular mechanisms on health benefits in future.
Abstract
Topological semimetals have recently attracted extensive research interests as host materials to condensed matter physics counterparts of Dirac and Weyl fermions originally proposed in high ...energy physics. Although Lorentz invariance is required in high energy physics, it is not necessarily obeyed in condensed matter physics, and thus Lorentz-violating type-II Weyl/Dirac fermions could be realized in topological semimetals. The recent realization of type-II Weyl fermions raises the question whether their spin-degenerate counterpart—type-II Dirac fermions—can be experimentally realized too. Here, we report the experimental evidence of type-II Dirac fermions in bulk stoichiometric PtTe
2
single crystal. Angle-resolved photoemission spectroscopy measurements and first-principles calculations reveal a pair of strongly tilted Dirac cones along the Γ-A direction, confirming PtTe
2
as a type-II Dirac semimetal. Our results provide opportunities for investigating novel quantum phenomena (e.g., anisotropic magneto-transport) and topological phase transition.
The nitrogenated porous two-dimensional (2D) material C2N has been successfully synthesized using a simple wet-chemical reaction, which provides a high-performance way to produce such 2D materials ...with novel electronic and optical properties. In this work, density functional theory (DFT) calculations were performed to investigate the structural, electronic, and optical properties of the layered C2N/MoS2 van der Waals (vdW) heterojunction. The C2N/MoS2 heterojunction was found to have a direct band gap of 1.30 eV and to present the typical type-II heterojunction feature, facilitating the effective separation of photogenerated electrons and holes. The calculated band alignment and enhanced optical absorption suggest that the C2N/MoS2 heterojunction should exhibit good light-harvesting properties. The vertical strain can effectively tune the electronic properties and optical absorption of the C2N/MoS2 heterojunction by changing the interaction between the p z orbital of C2N and the d z 2 orbital of MoS2. The moderate band gap, well-separated photogenerated electrons and holes, and enhanced visible-light absorption indicate that the C2N/MoS2 heterojunction is a potential photovoltaic structure for solar energy.
Topological crystalline insulators are new states of matter in which the topological nature of electronic structures arises from crystal symmetries. Here we predict the first material realization of ...topological crystalline insulator in the semiconductor SnTe by identifying its non-zero topological index. We predict that as a manifestation of this non-trivial topology, SnTe has metallic surface states with an even number of Dirac cones on high-symmetry crystal surfaces such as {001}, {110} and {111}. These surface states form a new type of high-mobility chiral electron gas, which is robust against disorder and topologically protected by reflection symmetry of the crystal with respect to {110} mirror plane. Breaking this mirror symmetry via elastic strain engineering or applying an in-plane magnetic field can open up a continuously tunable band gap on the surface, which may lead to wide-ranging applications in thermoelectrics, infra-red detection and tunable electronics. Closely related semiconductors PbTe and PbSe also become topological crystalline insulators after band inversion by pressure, strain and alloying.
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