Functionalized hexagonal boron nitride bilayers: desirable electro-optical properties for optoelectronic applications.

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.

Two Janus Ga2STe monolayers and their electronic, optical, and photocatalytic properties.

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.

Two-dimensional MX2Y4 systems: ultrahigh carrier transport and excellent hydrogen evolution reaction performances.

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.

Theoretical prediction of 2D biphenylene as a potential anchoring material for lithium-sulfur batteries.

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.

Impacts of defects on the mechanical and thermal properties of SiC and GeC monolayers.

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.

Investigation of the anchoring and electrocatalytic properties of pristine and doped borophosphene for Na-S batteries.

Designing an anchoring layer on the sulfur electrode has been considered one of the effective approaches to promoting the real application of room-temperature sodium-sulfur (RT-Na-S) batteries. In this work, based on the first-principles calculation method, the potential of pristine and doped borophosphene (BP) as anchoring materials for Na-S batteries has been investigated. The calculated adsorption energies of sodium polysulfides (NaPSs) adsorbed on pristine and doped substrates are higher than those of NaPSs adsorbed with the electrolytes (DOL&DME), indicating that the shuttle effect could be well alleviated. Meanwhile, the projected density of states (PDOS) suggests that the metallic characteristics of the adsorption systems are still well preserved, which is in favor of improving the electronic conductivity. More importantly, excellent electrocatalytic properties of the substrates are exhibited by reducing the catalytic decomposition energy barriers of Na2S, in which 0.27/0.79/1.02 eV is found on the pristine/N-doped/C-doped BP, indicating that the electrochemical processes could be improved smoothly. Therefore, it could be expected that pristine and doped BP are excellent anchoring materials for sodium-sulfur batteries.

Tuning electronic, magnetic and catalytic behaviors of biphenylene network by atomic doping.

Recently, a new two-dimensional allotrope of carbon named biphenylene has been experimentally synthesized. First-principles calculations are preformed to investigate the electronic properties of biphenylene and the doping effect is also considered to tune its electronic, magnetic, and catalytic properties. The metallic nature with an n-type Dirac cone is observed in the biphenylene. The magnetism can be induced by Fe, Cl, Cr, and Mn doping. More importantly, the doping position dependence of hydrogen evolution reaction (HER) performance of biphenylene is addressed, which can be significantly improved by atomic doping. In particular, the barrier for HER of Fe doping case is only -0.03 eV, denoting its great potential in HER catalysis.

Promising application of a SiC2/C3B heterostructure as a new platform for lithium-ion batteries.

Constructing heterostructures via the van der Waals coupling effect has provided an effective method for developing novel electrode materials. In this work, based on the first-principles calculation method, we proposed to construct a hexagonal SiC2/C3B heterostructure and confirmed its stability by analyzing its structural properties. Meanwhile, the electrochemical performances of the SiC2/C3B heterostructure as a new platform for lithium-ion batteries were evaluated. The calculated results illustrate that the pristine SiC2/C3B heterostructure is a semiconductor with a small bandgap of 0.15 eV and the lithiated heterostructure exhibits metallic properties which ensure superior electrical conductivity for fast electron transfer. Moreover, the low diffusion barriers of the heterostructure are acceptable to guarantee a high-rate performance for the batteries. Compared with the anode properties of isolated SiC2 and C3B monolayers, an enhancement of the storage capacity of Li ions on the SiC2/C3B heterostructure is observed, which could reach up to 1489.72 mA h g-1. In addition, the ab initio molecular dynamics simulations reveal that the SiC2/C3B heterostructure could maintain excellent structural stability during the lithiation processes even at a temperature of 350 K. All these encouraging results show that the SiC2/C3B heterostructure has fascinating potential to be an advanced platform for lithium-ion batteries.

Novel Janus diamane C4FCl: a stable and moderate bandgap semiconductor with a huge excitonic effect.

Semiconducting two-dimensional Janus materials have drawn increasing attention due to their novel optoelectronic properties. Here, employing first-principles calculations, we systematically explore the stability and electronic and optical properties of Janus diamane C4FCl. The energetic and dynamical stabilities of C4FCl have been verified using the cohesive energy and phonon dispersion calculations. It is predicted to possess a direct bandgap of ∼3 eV at the Γ point using the G0W0 method. Also, the optical absorption spectrum of C4FCl is dominated by the enhanced excitonic effects, in which a bright bound exciton with a large binding energy beyond 1 eV can be observed. The light absorption coefficient of C4FCl for sunlight can be as large as 8 × 104 cm-1 in the range of visible and near-ultraviolet light, suggesting its potential for optoelectronic applications. These findings enable a deep understanding of the physical properties of novel C4FCl.

Penta-BCN monolayer with high specific capacity and mobility as a compelling anode material for rechargeable batteries.

With the increasing demand for sustainable and clean energies, seeking high-capacity density electrode materials applied in rechargeable metal-ion batteries is urgent. In this work, using first-principles calculations, we evaluate the ternary pentagonal BCN monolayer as a compelling anode material for metal ion batteries. Calculations show that the penta-BCN monolayer has favorable metallic behaviors after adsorbing Li (Na) atoms. More interestingly, the saturated adsorption systems provide a large storage capacity of 2183.12 (1455.41) mA h g-1 for Li (Na) ions. A low energy barrier of 0.14 (0.16) eV for Li (Na) diffusion is observed, being smaller than the reported other two-dimensional anode materials. Also, the wrinkled structure of penta-BCN has been demonstrated to be very beneficial to improve the energy density and cycle life of batteries. The calculated low open-circuit voltage and peculiar surface area expansion together with the thermal stability of saturated intercalation structures, further indicate that the penta-BCN monolayer has great potential as the anode material for Li (Na) ion batteries.

Mechanical, electronic and optical properties of a novel B2P6 monolayer: ultrahigh carrier mobility and strong optical absorption.

Two-dimensional (2D) materials with a moderate bandgap and high carrier mobility are useful for applications in optoelectronics. In this work, we present a systematic investigation of the mechanical, electronic and optical properties of a B2P6 monolayer using first-principles calculations. Monolayer B2P6 was estimated to be an anisotropic material from direction-dependent in-plane Young's moduli and Poisson's ratios. Also, B2P6 exhibits an ultrahigh electron mobility of ∼5888 cm2 V-1 s-1, showing advantages for application in high-speed optoelectronic devices. More importantly, for the B2P6 monolayer, a desirable transformation from an indirect to direct band gap was observed at a biaxial tensile strain of ∼4%. Increasing the biaxial strain reduces the gap and preserves the suitable band edge positions for photocatalytic water splitting in the observed strain range of 1-8%. The decreased gap also enhances the visible light absorption of the B2P6 monolayer. These findings indicate that the B2P6 monolayer has promising applications in photocatalytic and photovoltaic devices.

Multidimensional B4N materials as novel anode materials for lithium ion batteries.

Based on first-principles calculations and ab initio molecular dynamics simulations, multidimensional B4N materials are investigated as anode materials for lithium ion batteries. The present results show that the monolayer B4N can reach a remarkably high specific capacity of 1874.27 mA h g-1 and possesses a low diffusion barrier (0.29 eV). Testing of bilayer B4N and bulk B4N reveals that the materials exhibit irreversible structural phase transformation. They are transformed from a layered structure to the more stable cavity-channel structure due to the adsorption of Li atoms. The volume expansions of their saturated lithiation cavity-channel structures are about 12%, which is close to that of graphite (10%). Moreover, it is found that the energy barriers of the bilayer and bulk B4N are less than 0.5 eV in the cavity-channel. The saturated adsorption of bulk B4N yields a specific capacity of 468.57 mA h g-1, which is higher than that of commercial graphite (372 mA h g-1). More importantly, all the lithiation structures in the monolayer, bilayer, and bulk B4N are verified to be thermodynamically stable at 350 K. These findings may encourage further experimental investigation in the design of multidimensional B4N materials as novel candidate anode materials for lithium ion batteries.

Enhancing electronic and optical properties of monolayer MoSe2via a MoSe2/blue phosphorene heterobilayer.

Type-II heterostructures are appealing for application in optoelectronics due to their effective separation of photogenerated charge carriers. Based on density functional and many-body perturbation theories, we investigate the MoSe2/blue phosphorene (MoSe2/Blue-P) heterobilayer with three representative stacking configurations. Our calculations indicate that the AA-stacking structure has more thermodynamic and dynamic stability. And it possesses a type-II band alignment with significant band offsets. The band offsets together with an interlayer polarized field will efficiently separate the photogenerated holes and electrons. More interestingly, compared with the MoSe2 monolayer, the MoSe2/Blue-P heterobilayer exhibits a significant enhancement of optical absorption in the range of near-ultraviolet and visible light. Also, the observed interlayer exciton has an impressive binding energy (∼670 meV), suggesting that the radiative recombination can be suppressed by the formation of an interlayer exciton. The predicted maximum energy conversion efficiency of MoSe2/Blue-P can achieve a value as large as 14.3%. These prominent electronic and optical properties provide the MoSe2/Blue-P heterobilayer with great potential in optoelectronics.

Novel electronic and optical properties of ultrathin silicene/arsenene heterostructures and electric field effects.

In zero-gap semimetallic silicene, introducing a sizable band gap without degrading its high carrier mobility is vital to its application in optoelectronic devices. Herein, we design a novel atomically thin system based on silicene and arsenene nanocomposites (Si/As heterostructure), which could open a direct band gap of about 125 meV at the K point in silicene. Moreover, its band gap is linearly controllable over a wide range even with a semiconductor-metal transition by the external electric field (E⊥), with an impressive band gap of up to 328 meV at E⊥ = -0.9 V Å-1. Additionally, the Si/As heterostructure can exhibit pronounced optical absorption in the far infrared range. The binding energy of the first bright exciton is as large as 623 meV, which can be significantly increased with an enhanced E⊥. The tunable bandgap together with a superior optical absorption makes the Si/As heterostructure a potential candidate for nanoelectronic and optoelectronic applications.

The stacking dependent electronic structure and optical properties of bilayer black phosphorus.

By employing density-functional theory, the G0W0 method and Bethe-Salpter equation, we explore quasi-particle energy bands, optical responses and excitons of bilayer black phosphorus (BBP) with four different stacking patterns. All the structures are direct band gap semiconductors and the band gap is highly dependent on the stacking pattern, with a maximum of 1.31 eV for AB-stacking and a minimum of 0.92 eV for AD-stacking. Such dependence can be well understood by the tight-binding model in terms of the interlayer hopping. More interestingly, stacking sensitive optical absorption and exciton binding energy are observed in BBPs. For x-polarized light, more red-shift of optical adsorption appears in AA-stacking and the strong exciton binding energy in the AA-stacking bilayer can be as large as 0.82 eV, that is ∼1.7 times larger than that of AD-stacking bilayer.

Quasi-particle energies and optical excitations of hydrogenated and fluorinated germanene.

Using density functional theory, the G0W0 method and Bethe-Salpeter equation calculations, we systematically explore the structural, electronic and optical properties of hydrogenated and fluorinated germanene. The hydrogenated/fluorinated germanene tends to form chair and zigzag-line configurations and its electronic and optical properties show close geometry dependence. The chair hydrogenated/fluorinated and zigzag-line fluorinated germanene are direct band-gap semiconductors, while the zigzag-line hydrogenated germanene owns an indirect band-gap. Moreover, the quasi-particle corrections are significant and strong excitonic effects with large exciton binding energies are observed. Moreover, the zigzag-line hydrogenated/fluorinated germanene shows highly anisotropic optical responses, which may be used as a good optical linear polarizer.

Tunable electronic and optical properties of monolayer silicane under tensile strain: a many-body study.

The electronic structure and optical response of silicane to strain are investigated by employing first-principles calculations based on many-body perturbation theory. The bandgap can be efficiently engineered in a broad range and an indirect to direct bandgap transition is observed under a strain of 2.74%; the semiconducting silicane can even be turned into a metal under a very large strain. The transitions derive from the persistent downward shift of the lowest conduction band at the Γ-point upon an increasing strain. The quasi-particle bandgaps of silicane are sizable due to the weak dielectric screening and the low dimension; they are rapidly reduced as strain increases while the exciton bound energy is not that sensitive. Moreover, the optical absorption edge of the strained silicane significantly shifts towards a low photon energy region and falls into the visible light range, which might serve as a promising candidate for optoelectronic devices.

Photoabsorption Tolerance of Intrinsic Point Defects and Oxidation in Black Phosphorus Quantum Dots.

Black phosphorus quantum dots (BPQDs) exhibit excellent optical and photothermal properties and promising applications in optoelectronics and biomedicine. However, various intrinsic structural defects and oxidation are nearly unavoidable in preparation of BPQDs and how they affect the electronic and optical properties remains unclear. Here, by employing time-dependent density functional theory, we reveal that there are two types of photoabsorption in BPQDs for both point defects and oxidation. A close structure-absorption relation is unraveled: BPQDs are defect-tolerant and show excellent photoabsorption as long as the coordination number (CN) of defective P atoms is 3. By contrast, the unsaturated or oversaturated P atoms with CN ≠ 3 create in-gap-states (IGSs) and completely quench the optical absorption. An effective way to eliminate the IGSs and repair the photoabsorption of defective BPQDs via sufficient hydrogen passivation is further proposed.

Arsenene-Based Heterostructures: Highly Efficient Bifunctional Materials for Photovoltaics and Photocatalytics.

Constructing suitable type II heterostructures is a reliable solution for high-efficient photovoltaic and photocatalytic materials. Arsenene, as a rising member of monoelemental two-dimensional materials, shows great potential as a building block of heterostructures because of its suitable band gap, high carrier mobility, and good optical properties. On the basis of accurate band structure calculations by combining the many-body perturbation GW method with an extrapolation technique, we demonstrate that arsenene-based heterostructures paired with molybdenum disulfide, tetracyano-quinodimethane, or tetracyanonaphtho-quinodimethane can form type II band alignments. These arsenene-based heterostructures cannot only satisfy all the requirements as photocatalysts for photocatalytic water splitting but can also show an excellent power conversion efficiency of ∼20% as potential photovoltaics.

Revealing the underlying absorption and emission mechanism of nitrogen doped graphene quantum dots.

Nitrogen-doped graphene quantum dots (N-GQDs) hold promising application in electronics and optoelectronics because of their excellent photo-stability, tunable photoluminescence and high quantum yield. However, the absorption and emission mechanisms have been debated for years. Here, by employing time-dependent density functional theory, we demonstrate that the different N-doping types and positions give rise to different absorption and emission behaviors, which successfully addresses the inconsistency observed in different experiments. Specifically, center doping creates mid-states, rendering non-fluorescence, while edge N-doping modulates the energy levels of excited states and increases the radiation transition probability, thus enhancing fluorescence strength. More importantly, the even hybridization of frontier orbitals between edge N atoms and GQDs leads to a blue-shift of both absorption and emission spectra, while the uneven hybridization of frontier orbitals induces a red-shift. Solvent effects on N-GQDs are further explored by the conductor-like screening model and it is found that strong polarity of the solvent can cause a red-shift and enhance the intensity of both absorption and emission spectra.