Unusual Janus Bi2TeSe2 Topological Insulators Displaying Second-Harmonic Generation, Linear-in-Temperature Resistivity, and Weak Antilocalization.

Well-established knowledge about inversion-symmetric Bi2TexSe3-x topological insulators characterizes the promising new-generation quantum device. Noticeably, the inversion asymmetric phase containing different surface electronic structures may create an extra topological phenomenon pointing to a new device paradigm. Herein, Janus Bi2TeSe2 single-crystal nanosheets with an unconventional stacking sequence of Se-Bi-Se-Bi-Te are realized via chemical vapor deposition growth, which is clarified by atomically resolved AC-STEM and elemental mapping. An obvious polarization-dependent second-harmonic generation with a representative 6-fold rotational symmetry is detected due to the broken out-of-plane mirror symmetry in this system. Low-temperature transport measurements display a strange metal-like linear-in-temperature resistivity. Anomalous conductance peaks under low magnetic fields induced by the weak antilocalization effect of topological surface states and the two-dimensional transport-dominated anisotropic magnetoresistance are revealed. These findings correlate the Janus Bi2TeSe2 phase with emerging physics topics, which would inspire fresh thoughts in well-developed Bi3TexSe3-x topological insulators and open up opportunities for exploring hybrid nonlinear optoelectronic topological devices.

Ambient-Pressure Recoverable Polynitrogen Solids Assembled by Pentazolate Rings with High Energy Density.

Crystal structure predictions and first-principles calculations were used to predict three polynitrogen solids (aP8-N, aP12-N, and oP24-N) that possess competitive enthalpies as compared to the synthesized open-chain N8 phase at pressures in the range of 0-60 GPa. aP8-N, aP12-N, and oP24-N contain edge-shared, N2-linked, and N-bridged pentazolate rings and form molecular N8, molecular N12, and quasi-one-dimensional N∞ ribbons, respectively. The calculations of formation enthalpies show that the three polynitrogen solids can be synthesized by compressing cyclo-N5 salts in hydrogen-saturated environments. Molecular simulations suggest that the three polynitrogen solids have the ability of quench recoverability under ambient conditions once being synthesized at high pressure. With estimated energy densities in the range of 5.6-6.5 kJ/g, these three polynitrogen phases show notable promise for applications as high-energy-density materials.

Two-dimensional Si2S with a negative Poisson's ratio and promising optoelectronic properties.

Two-dimensional materials with a negative Poisson's ratio, known as auxetic materials, are of great interest owing to their improved mechanical properties, which enable plenty of advanced nanomechanical devices. Here, by first-principles swarm-intelligence structural search methods, we predict a thermodynamically stable Si2S monolayer, which has a puckered 2D lattice in which the S atoms are adsorbed on the top of a distorted tetragonal silicene layer. The puckered 2D lattice makes the Si2S monolayer exhibit in-plane negative Poisson's ratios of -0.05 and -0.069 along the x and y directions, respectively. Moreover, electronic structure calculations reveal that the Si2S monolayer is a semiconductor with a quasi-direct band gap of 1.81 eV, which can be converted into a direct gap semiconductor of 1.43 eV by applying a low tensile strain (∼2%). The Si2S monolayer has a large visible light absorption coefficient of 105 cm-1. The hole (electron) mobility is 200 (81) cm2 V-1 s-1 along the y direction, 3.4 (1.5) times that along the x direction, comparable to MoS2. Moreover, the Si2S monolayer has the good ability of oxidation resistance. We provide a possible route to experimentally grow a Si2S monolayer on a suitable substrate such as the Cu(100) surface. The versatile properties render the Si2S monolayer potential for advanced application in nanodevices.

A B2N monolayer: a direct band gap semiconductor with high and highly anisotropic carrier mobility.

Two-dimensional materials with a planar lattice, suitable direct band gap, and high and highly anisotropic carrier mobility are desirable for the development of advanced field-effect transistors. Here we predict three thermodynamically stable B-rich 2D B-N compounds with the stoichiometries of B2N, B3N, and B4N using a combination of crystal structure searches and first-principles calculations. Among them, B2N has an ultraflat surface and consists of eight-membered B6N2 and pentagonal B3N2 rings. The eight-membered B6N2 rings are linked to each other through both edge-sharing (in the y direction) and bridging B3N2 pentagons (in the x direction). B2N is a semiconductor with a direct band gap of 1.96 eV, and the nature of the direct band gap is well preserved in bilayer B2N. The hole mobility of B2N is as high as 0.6 × 103 cm2 V-1 s-1 along the y direction, 7.5 times that in the x direction. These combined novel properties render the B2N monolayer as a natural example in the field of two-dimensional functional materials with broad application potential for use in field-effect transistors.