Tuning the structural, electronic and dynamical properties of Janus M4X3Y3 (M = Pd, Ni and Co; X,Y = S, Se and Te) monolayers: a DFT study.

Based on density functional theory, the structural, electronic and vibrational properties of two-dimensional transition metal chalcogenides M2X3 and their Janus type M4X3Y3, where M = Pd, Co and Ni and X = Se, S and Te, are investigated. Motivated by the successful synthesis of a 2D Pd2Se3 monolayer and the proof of the dynamical stability of Ni2Se3 and Co2Se3 monolayers, in terms of the phonon band dispersions, we have systemically studied the fundamental physical properties of Janus transition metal chalcogenides, such as their structural, phonon and thermodynamic stability and their electronic and mechanical properties. Our results show that Janus structures of M4X3Y3 are energetically favorable and dynamically stable. The ab initio molecular dynamic simulations (AIMD) results clearly prove that they kept their thermal stability at room temperature. We have demonstrated their structural, electronic and vibrational properties and Raman spectra. The electronic band dispersions show that monolayer Co2Se3 shows half-metal properties with a moderate band gap (1.01 eV), Pd2Se3 has a 1.42 eV direct band gap, while Ni2Se3 has a 1.38 eV indirect band gap. Pd4Se3S3, Pd4Se3Te3 and Pd4S3Te3 are indirect band gap semiconductors with band gaps of 1.22 eV, 1.05 eV and 0.61 eV, respectively. Ni4Se3S3, Ni4Se3Te3 and Ni4S3Te3 are indirect band gap semiconductors with band gaps of 1.61 eV, 0.77 eV and 0.49 eV, respectively. While pristine Co2Se3 is shown to have half-metallicity (HM), the HM behaviour of the Janus Co4Se3Te3 and Co4S3Te3 monolayers disappear and Co4Se3S3 remains a HM with a moderate band gap of 0.85 eV. In addition, the Raman spectra of these Janus materials are shown to exhibit totally distinctive features as compared to those of the pristine materials. This work reveals the important material properties of Janus type M4X3Y3 monolayers, where M = Pd, Co and Ni and X = Se, S and Te, which could have wide applications in new functional devices.

First-principles investigation of electronic, mechanical and thermoelectric properties of graphene-like XBi (X = Si, Ge, Sn) monolayers.

Research progress on single layer group III monochalcogenides has been increasing rapidly owing to their interesting physics. Herein, we investigate the dynamically stable single layer forms of XBi (X = Ge, Si or Sn) using density functional theory calculations. Phonon band dispersion calculations and ab initio molecular dynamics simulations reveal the dynamical and thermal stability of the considered monolayers. Raman spectra calculations indicate the existence of 5 Raman active phonon modes, 3 of which are prominent and can be observed in possible Raman measurements. The electronic band structures of the XBi single layers were investigated with and without the effects of spin-orbit coupling (SOC). Our results show that XBi single layers show semiconducting properties with narrow band gap values without SOC. However, only single layer SiBi is an indirect band gap semiconductor, while GeBi and SnBi exhibit metallic behaviors when adding spin-orbit coupling effects. In addition, the calculated linear elastic parameters indicate the soft nature of the predicted monolayers. Moreover, our predictions for the thermoelectric properties of single layer XBi reveal that SiBi is a good thermoelectric material with increasing temperature. Overall, it is proposed that single layer XBi structures can be alternative, stable 2D single layers with varying electronic and thermoelectric properties.

Aluminum and lithium sulfur batteries: a review of recent progress and future directions.

Advanced materials with various micro-/nanostructures have attracted plenty of attention for decades in energy storage devices such as rechargeable batteries (ion- or sulfur based batteries) and supercapacitors. To improve the electrochemical performance of batteries, it is uttermost important to develop advanced electrode materials. Moreover, the cathode material is also important that it restricts the efficiency and practical application of aluminum-ion batteries. Among the potential cathode materials, sulfur has become an important candidate material for aluminum-ion batteries cause of its considerable specific capacity. Two-dimensional materials are currently potential candidates as electrodes from lab-scale experiments to possible pragmatic theoretical studies. In this review, the fundamental principles, historical progress, latest developments, and major problems in Li-S and Al-S batteries are reviewed. Finally, future directions in terms of the experimental and theoretical applications have prospected.

Embedding of atoms into the nanopore sites of the C6N6 and C6N8 porous carbon nitride monolayers with tunable electronic properties.

Using first-principles calculations, we study the effect of embedding various atoms into the nanopore sites of both C6N6 and C6N8 monolayers. Our results indicate that the embedded atoms significantly affect the electronic and magnetic properties of C6N6 and C6N8 monolayers and lead to extraordinary and multifarious electronic properties, such as metallic, half-metallic, spin-glass semiconductor and dilute-magnetic semiconductor behaviour. Our results reveal that the H atom concentration dramatically affects the C6N6 monolayer. On increasing the H coverage, the impurity states also increase due to H atoms around the Fermi-level. C6N6 shows metallic character when the H atom concentration reaches 6.25%. Moreover, the effect of charge on the electronic properties of both Cr@C6N6 and C@C6N8 is also studied. Cr@C6N6 is a ferromagnetic metal with a magnetic moment of 2.40 µB, and when 0.2 electrons are added and removed, it remains a ferromagnetic metal with a magnetic moment of 2.57 and 2.77 µB, respectively. Interestingly, one can observe a semi-metal, in which the VBM and CBM in both spin channels touch each other near the Fermi-level. C@C6N8 is a semiconductor with a nontrivial band gap. When 0.2 electrons are removed, it remains metallic, and under excess electronic charge, it exhibits half-metallic behaviour.

Control of C3N4 and C4N3 carbon nitride nanosheets' electronic and magnetic properties through embedded atoms.

In the present work, the effect of various embedded atom impurities on tuning electronic and magnetic properties of C3N4 and C4N3 nanosheets have been studied using first-principles calculations. Our calculations show that C3N4 is a semiconductor and it exhibits extraordinary electronic properties such as dilute-magnetic semiconductor (with H, F, Cl, Be, V, Fe and Co); metal (with N, P, Mg and Ca), half-metal (with Li, Na, K, Al, Sc, Cr, Mn, and Cu) and semiconductor (with O, S, B, C, Si, Ti, Ni and Zn) with the band gaps in the range of 0.3-2.0 eV depending on the species of embedded atom. The calculated electronic properties reveal that C4N3 is a half-metal and it retains half-metallic character with embedded H, O, S, F, B, N, P, Be, Mg, Al, Sc, V, Fe, Ni and Zn atoms. The substitution of Cl, C, Cr and Mn atoms create ferromagnetic-metal character in the C4N3 nanosheet, embedded Co and Cu atoms exhibit a dilute-magnetic semiconductor nature, and embedded Ti atoms result in the system becoming a semiconductor. Therefore, our results reveal the fact that the band gap and magnetism can be modified or induced by various atom impurities, thus, offering effective possibilities to tune the electronic and magnetic properties of C3N4 and C4N3 nanosheets.

Modified Li chains as atomic switches.

We present electronic structure and transport calculations for hydrogen and lithium chains, using density functional theory and scattering theory on the Green's function level, to systematically study impurity effects on the transmission coefficient. To this end we address various impurity configurations. Tight-binding results allow us to interpret our the findings. We analyze under which circumstances impurities lead to level splitting and/or can be used to switch between metallic and insulating states. We also address the effects of strongly electronegative impurities.