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Structural, electronic, and optical properties of functionalized hexagonal boron nitride (h-BN) bilayer were deeply explored by carrying out the PBE + G0W0 + BSE calculations. Hydrogenation/hydrofluorination/fluorination can cause the planar h-BN bilayer to form a novel diamane-like monolayer by interfacial sp3 atom bonding. These functionalized h-BN bilayers are estimated to be stable dynamically due to their phonon dispersions. The functionalization on h-BN bilayer can induce its electronic nature to be transformed from an indirect wide-gap insulator to direct narrow-gap semiconductor, which is desirable for its application in optoelectronics. In particular, hydrogenated and hydrofluorinated h-BN bilayers have strong absorbance coefficients for the near-infrared and visible part of the incident sunlight (larger than 105 cm−1). More interestingly, the binding energy of the observed first bright exciton can achieve a value beyond 1 eV, which can effectively reduce the recombination of photogenerated electron–hole pairs. These results are potentially important for extending the applications of the h-BN bilayer in optoelectronic devices. [ABSTRACT FROM AUTHOR]

Here, we construct a novel Ga2TeS monolayer with the diamondene-like structure. Based on DFT + G0W0 + BSE calculations, we deeply explore the monolayer (stability, transport, and electro-optical properties) and biaxial strain effects. The Ga2TeS monolayer shows dynamical and thermal stabilities, a high electron mobility (larger than 2000 cm2∙V−1∙s−1), a moderate direct bandgap, and a superior light absorption in a broad region, respectively. More interestingly, its optical spectrum is ruled by the excitons, in which the exciton binding energy can gain a desirable value. Under the applied biaxial strains, a tunable bandgap, an improved electron mobility, and a significant enhancement for the visible-light absorption are observed in the Ga2TeS monolayer. These results suggest the Ga2TeS monolayer can be considered as a potential optoelectronic material. [ABSTRACT FROM AUTHOR]

Defect engineering has been considered as an effective way for controlling the heat transport properties of two-dimensional materials. In this work, the effects of point vacancies and grain boundaries on the mechanical and thermal performances of SiC and GeC monolayers are investigated systematically by molecular dynamics calculations. The failure strength in SiC and GeC is decreased by introducing vacancies at room temperature, and the stress–strain relationship can be tuned significantly by different kinds of vacancies. When the grain boundary of 21.78° is applied, the maximal fracture strengths can be as large as 27.56% for SiC and 23.56% for GeC. Also, the thermal properties of the two monolayers show a remarkable dependence on the vacancies and grain boundaries. The high vacancy density in SiC and GeC can induce disordered heat flow and the C/Ge point defect is crucial for thermal conductivity regulation for the Si/GeC monolayer. More importantly, the SiC and GeC monolayers with a grain boundary of 5.09° show excellent interfacial thermal conductance. Our findings are of great importance in understanding SiC and GeC monolayers and seeking their potential applications. [ABSTRACT FROM AUTHOR]

Two-dimensional Janus structures provide a platform for the manufacture of high-performance optoelectronic devices by breaking vertical structural symmetry. Herein, by employing the first-principles calculations, Janus Ga2STe bilayers with eight stacking orders are investigated. In particular, the electronic and optical properties of the stacking configurations (AB1 and AB4) are explored in detail due to their relatively high stabilities. The AB1 and AB4 configurations are estimated to be semiconductors with moderate direct gaps. They also exhibit strong absorptions of near-infrared and visible light as light absorbance coefficients larger than 106 cm−1 were observed in the two configurations. More interestingly, a type-II band alignment and a dominant intrinsic electric field are formed in the AB1 configuration, which can greatly promote the effective separation of photogenerated carriers. A carrier mobility of higher than 103 cm−2 V−1 s−1 can be acquired in the AB1 configuration. In addition, the AB1-stacked Ga2STe bilayer is proposed to act as a potential excitonic solar cell with a high power conversion efficiency (PCE) of larger than 20%; this value can be further increased by strain engineering. These extraordinary properties make the Janus Ga2STe bilayer particularly promising for use in photovoltaic devices. [ABSTRACT FROM AUTHOR]

Searching for good anchoring materials that can suppress the shuttle effect is critical to large-scale commercialization of lithium–sulfur (Li–S) batteries. In this work, the adsorption behavior of lithium polysulfides (LiPSs, such as S8 and Li2Sn, n = 1, 2, 4, 6, and 8), the sulfur reduction reaction (SRR), the decomposition processes of Li2S and the diffusion behavior of Li atoms on intrinsic and doped 2D biphenylene (BIP) are systematically investigated by employing the first-principles calculation method. Calculations show that the adsorption energies of LiPSs on the electrolyte (DOL and DME) are smaller than those on the intrinsic/B doped BIP. The moderate anchoring strength (0.8–2.0 eV) between LiPSs and the BIP can effectively suppress the shuttle effect. Moreover, the Gibbs free energy barrier for SRR is 0.72/0.64 eV on intrinsic/B doped BIP. The dissociation energy barrier of Li2S on intrinsic/B doped BIP is 1.35 eV, while the diffusion energy barrier of Li atoms on intrinsic/B doped BIP is 0.18 eV/0.30 eV. Lower energy barriers are conducive to enhancing the discharging and charging efficiency. Therefore, intrinsic and B doped BIP are predicted as good anchoring materials for Li–S batteries. [ABSTRACT FROM AUTHOR]

Recently, two-dimensional Janus materials have attracted increasing interest due to their unique structure and novel properties. Based on density-functional and many-body perturbation theories (i.e. DFT + G0W0 + BSE methods), the electronic, optical, and photocatalytic properties of Janus Ga2STe monolayers with two configurations are explored systematically. It is found that the two Janus Ga2STe monolayers exhibit high dynamical and thermal stabilities and have desirable direct gaps of about 2 eV at the G0W0 level. Their optical absorption spectra are dominated by the enhanced excitonic effects, in which bright bound excitons possess moderate binding energies of about 0.6 eV. Most interestingly, Janus Ga2STe monolayers show high light absorption coefficients (larger than 106 cm−1) in the visible light region, effective spatial separation of photoexcited carriers, and suitable band edge positions, which make them potential candidates for photoelectronic and photocatalytic devices. These observed findings enrich the deep understanding of the properties of Janus Ga2STe monolayers. [ABSTRACT FROM AUTHOR]

Very recently, two-dimensional MoSi2N4 has been synthetized (Y.-L. Hong, Z. Liu, L. Wang, T. Zhou, W. Ma, C. Xu, S. Feng, L. Chen, M.-L. Chen and D.-M. Sun, Chemical vapor deposition of layered two-dimensional MoSi2N4 materials, Science, 2020, 369, 670–674.). In this work, we systematically explore the mechanical, electronic, and catalytic properties of the MX2Y4 (M = Cr, Hf, Mo, Ti, W, Zr; X = Si, Ge; Y = N, P, As) monolayers by first-principles calculations. These observed monolayers exhibit an isotropic Young's moduli of 165–514 N m−1 and a Poisson's ratio of 0.26–0.33. The calculated band structures indicate that their bandgaps are in the range of 0.49–2.05 eV at the HSE06 level. In particular, a high electron mobility of about 1.04 × 104 cm2 V−1 s−1 is observed in TiSi2N4 monolayers, which shows potential for high-speed electronic devices. MX2Y4 monolayers also reveal decent performances in the hydrogen evolution reaction. More importantly, the Gibbs free energy change of the TiSi2N4 (ZrSi2N4) monolayer is as small as 0.078 eV (−0.035 eV), even being comparable with that of Pt (−0.09 eV). This investigation suggests that the MoSi2N4 family monolayers have potential advanced applications such as photocatalytic, electrocatalytic, and photovoltaic devices. [ABSTRACT FROM AUTHOR]