Insulator-to-metal transition in the black diamond from molecular-dynamics-Landauer method.

The critical condition and mechanism of the insulator-to-metal transition (IMT) for the black diamond were studied by the molecular-dynamics-Landauer method. The IMT will occur at sufficiently high contents of vacancies in the diamond. The critical concentration of vacancies for the IMT might be between V:C143 (0.69%) and V:C127 (0.78%). At a low concentration of vacancies (below 0.69%), the intermediate band (IB) consists of a filled band and a separate empty band, which makes the material to be an insulator. The IMT of the black diamond is due to the mergence between the two isolated IBs when the concentration of vacancies is high, and the merged IB is partially filled by electrons. The distribution of vacancies also influences the IMT of the black diamond.

Recent progress in 2D bipolar magnetic semiconductors.

Bipolar magnetic semiconductor (BMS) is a class of magnetic semiconductors, whose valence band maximum and conduction band minimum are fully spin-polarized with opposite spin directions. Due to the special energy band, half-metallicity can be easily obtained in BMS by gate voltage, and the spin polarization can be reversed between spin-up and down when the gate voltage switches from positive to negative. BMSs have great potential applications in spintronic devices, such as the field-effect spin valves, spin filters and spin transistors,etc. With the rapid progress of the two-dimensional (2D) magnetic materials, researchers have identified a series of potential intrinsic 2D BMS materials using high-throughput computational methods. Additionally, methods such as doping, application of external stress, introduction of external fields, stacking of interlayer antiferromagnetic semiconductors, and construction of Janus structures have endowed existing materials with BMS properties. This paper reviews the research progress of 2D BMS. These advancements provide crucial guidance for the design and synthesis of BMS materials and offer innovative pathways for the future development of spintronics.

Boosting photodetection performance of Cs2AgBiBr6 through A-site Rb substitution and interfacial engineering.

Bismuth-based double perovskite Cs2AgBiBr6 shows promise as a photodetection material. However, its detection performance and application are limited by high-exciton binding energy and poor carrier mobility. In this study, we address these limitations by delicately designing a solution-based method for incorporating A-site Rubidium (Rb) substitution into Cs2AgBiBr6 double perovskite films. The introduction of Rb resulted in a significant decrease in trap defect density and an improvement in film quality. The trap-filled limit voltage (VTFL) of pure and Rb-doped CABB film is determined to be 1.71 V and 0.48 V, respectively. Subsequently, by introducing an ultrathin atomic-layer-deposited (ALD) TiO2 films, the fabricated CABB photodetectors exhibit significantly improved photoresponse performance. The response speed and -3dB bandwidth are boosted from ∼93 ms to ∼350 µs and broadened from 1.4 kHz to 17 kHz, respectively. Density Functional Theory (DFT) calculations indicate Rb-substitution shortens the bond length and weaken exciton binding energy. Furthermore, we demonstrate a wireless near ultraviolet (UV) light communication system using CABB photodetectors as light receivers. Our findings provide an efficient approach to utilize A-site cation substitution as a tuning parameter for photodetection in high-exciton binding energy perovskite materials, thereby extending the potential applications of other functional perovskites.

Computational design of a reliable intermediate-band photovoltaic absorber based on diamond.

To reduce the wide bandgap of diamond and expand its applications in the photovoltaic fields, a diamond-based intermediate-band (IB) material C-Ge-V alloy was designed by first-principles calculations. By replacing some C with Ge and V in the diamond, the wide bandgap of the diamond can be reduced sharply and a reliable IB, which is mainly formed by the d states of V, can be formed in the bandgap. With the increase of Ge content, the total bandgap of the C-Ge-V alloy will be reduced and close to the optimal value of an IB material. At a relatively low atomic concentration of Ge (below 6.25%), the IB formed in the bandgap is partially filled and varies little with the concentration of Ge. When further increasing the content of Ge, the IB moves close to the conduction band and the electron filling in the IB increases. The 18.75% content of Ge might be the limitation to form an IB material, and the optimal content of Ge should be between 12.5% and 18.75%. Compared with the content of Ge, the distribution of Ge has a minor effect on the band structure of the material. The C-Ge-V alloy shows strong absorption for the sub-bandgap energy photons, and the absorption band generates a red-shift with the increase of Ge. This work will further expand the applications of diamond and be helpful to develop an appropriate IB material.

Ferroelectric control of band alignments and magnetic properties in the two-dimensional multiferroic VSe2/In2Se3.

Our first-principles evidence shows that the two-dimensional (2D) multiferroic VSe2/In2Se3experiences continuous change of electronic structures, i.e. with the change of the ferroelectric (FE) polarization of In2Se3, the heterostructure can possess type-I, -II, and -III band alignments. When the FE polarization points from In2Se3to VSe2, the heterostructure has a type-III band alignment, and the charge transfer from In2Se3into VSe2induces half-metallicity. With reversal of the FE polarization, the heterostructure enters the type-I band alignment, and the spin-polarized current is turned off. When the In2Se3is depolarized, the heterostructure has a type-II band alignment. In addition, influence of the FE polarization on magnetism and magnetic anisotropy energy of VSe2was also analyzed, through which we reveal the interfacial magnetoelectric coupling effects. Our investigation about VSe2/In2Se3predicts its wide applications in the fields of both 2D spintronics and multiferroics.

Electric control of nearly free electron states and ferromagnetism in the transition-metal dichalcogenides monolayers.

Using the first-principles calculations, we explore the nearly free electron (NFE) states in the transition-metal dichalcogenidesMX2(M= Mo, W;X= S, Se, Te) monolayers. It is found that both the external electric field and electron (not hole) injection can flexibly tune the energy levels of the NFE states, which can shift down to the Fermi level and result in novel transport properties. In addition, we find that the valley polarization can be induced by both electron and hole doping in MoTe2monolayer due to the ferromagnetism induced by the charge injection, which, however, is not observed in other five kinds ofMX2monolayers. We carefully check band structures of all theMX2monolayers, and find that the exchange splitting in the top of the valence band and the bottom of conduction band plays the key role in the ferromagnetism. Our researches enrich the electronic, spintronic, and valleytronic properties ofMX2monolayers.

VS2nanosheet as a promising candidate of recycle and reuse NO2gas sensor and capturer: a DFT study.

We describe the utilization of VS2nanosheet as high sensing response, reuse, and thermodynamic stability at room temperature NO2and NO gas sensors by using the density functional theory method. We focus on the electronic structures and adsorption energy toward a variety of gaseous molecules (such as O2, CO, H2O, NH3, NO, and NO2) adsorbed on the VS2nanosheet. The results show that chemical interactions existed between NO/NO2molecules and VS2nanosheet due to sizable adsorption energy and strong covalent (S-N) bonds. In particular, the adsorption energies, charge transfer and electronic properties between NO2adsorbed system is significantly changed compared with the other gas molecules (CO, NO, H2O, NH3, and O2) adsorbed systems under biaxial strains, which is effective to achieve the capture or reversible release of NO2for cycling capability. Our analysis indicates that VS2nanosheet is promising as electrical devices candidate for NO2high-performance gas sensor or capturer.

Deep-level impurities hyperdoped diamond: a first-principles calculations.

A hyperdoped diamond material is engineered by first-principles calculations in this work. Several deep-level elements, such as S, Se, Te, Co, Au, V, Ni, are chosen as dopants in the diamond. The formation energy results show that the substitutional configuration of the dopants is more stable than the interstitial ones. The substitutional configurations of chalcogen dopants (S, Se, Te) can introduce a nearly filled intermediate band (IB) in the upper half of the bandgap of the diamond. The substitutional configurations of several transition metals, such as Co, Au, V, Ni, and Cu, can form partially filled IB(s) near the center of the bandgap, which is more appropriate than that formed by the chalcogens. The dielectric function results indicate that all of these deep-level elements can lead to the sub-bandgap absorption and the absorption range and intensity vary dramatically with different dopants. Among these dopants, Co, Au, and Cu exhibit a special strong sub-bandgap absorption in a longer wavelength range, which would make the material to be an excellent photoelectric device. With reducing the concentration of the transition metal dopants, the IBs in the bandgap are narrower and tend to separate from each other and the sub-bandgap absorptions reduce sharply. Our conclusions imply that the photoelectric properties of the novel diamond material would be modulated by changing the dopant types and concentrations.

Precise Phase Control of Large-Scale Inorganic Perovskites via Vapor-Phase Anion-Exchange Strategy.

Anion exchange offers great flexibility and high precision in phase control, compositional engineering, and optoelectronic property tuning. Different from previous successful anion exchange process in liquid solution, herein, a vapor-phase anion-exchange strategy is developed to realize the precise phase and bandgap control of large-scale inorganic perovskites by using gas injection cycle, producing some perovskites such as CsPbCl3 which has never been reported in thin film morphology. Ab initio calculations also provide the insightful mechanism to understand the impact of anion exchange on tuning the electronic properties and optimizing the structural stability. Furthermore, because of precise control of specific atomic concentrations, intriguing tunable photoluminescence is observed and photodetectors with tunable photoresponse edge from green to ultraviolet light can be realized accurately with an ultrahigh spectral resolution of 1 nm. Therefore, a new, universal vapor-phase anion exchange method is offered for inorganic perovskite with fine-tunable optoelectronic properties.

Silicene/boron nitride heterostructure for the design of highly efficient anode materials in lithium-ion battery.

The performance of silicene/boron nitride heterostructure as anode material in lithium-ion batteries (LIBs) has been investigated by first-principles calculations. From the interfacial synergy effect, an enhanced adsorption of Li ions on BN is found in the resulted heterostructure compared with pristine BN system. Also, lowered diffusion barriers are found in the BN/Li/silicene and BN/silicene/Li systems compared with pristine silicene system. In addition, silicene/BN system can achieve high Li storage capacity with a maximum value reaching 1015 mA h g-1. The junction shows a volume change of only 1.3% between the charged and uncharged states. It means the highly enhanced thermodynamic stability compared with the pristine silicene sheet, which is promising as a good potential anode material in LIBs.

MoB2: a new multifunctional transition metal diboride monolayer.

Several layered transition metal borides can now be realized by a simple and general fabrication method (Fokwa et al 2018 Adv. Mater. 30 1704181), inspiring our interest to transition metal borides monolayer. Herein, we predict a new two-dimensional (2D) transition metal diboride MoB2 monolayer (ML) and study its intrinsic mechanical, thermal, electronic, and transport properties. The MoB2 ML has isotropic mechanic properties along the zigzag and armchair directions with a large Young's stiffness, and has an ultralow room-temperature thermal conductivity. The Mo atoms dominate the metallic nature of MoB2 ML. It shows an obvious electrical anisotropy and a current-limiting behavior. Our findings suggest that MoB2 ML is a promising multifunctional material used in ultrathin high-strength mechanical materials, heat insulating materials, electrical-anisotropy-based materials, and current limiters. It is helpful for the experimentalists to further prepare and utilize the transition metal diboride 2D materials.

Br doped porous bismuth oxychloride micro-sheets with rich oxygen vacancies and dominating {0 0 1} facets for enhanced nitrogen photo-fixation performances.

Bismuth oxychloride micro-sheets with rich oxygen vacancies (BiOCl-OV) are firstly prepared through a surfactant assisted solvothermal method. Due to the selective surfactant adsorption, the as-prepared BiOCl-OV exposes high percentage {0 0 1} facets. Moreover, the ion-exchange process not only introduces Br atoms but also creates cavities in crystal structure of the Br doped BiOCl-OV (Br-BiOCl-OV). When used as photocatalyst for N2 photo-fixation, the optimized Br-BiOCl-OV demonstrates an ammonia producing rate of 6.3 µmol h-1 under visible light irradiation, which is greatly enhanced than that of pristine BiOCl-OV (4.1 µmol h-1). The density functional theory (DFT) calculation suggests that because of the introduced Br atoms and the oxygen vacancies, the adsorbed N2 on Br-BiOCl-OV exhibits greater NN bond length than that adsorbed on pristine BiOCl-OV, attributing to the effective activation of the inert NN bonds. Photocurrent response, state/transient PL spectra and electrochemical impedance spectroscopy indicate that the Br-BiOCl-OV possesses promoted charge separation and suppressed charge recombination, which finally leads to the high N2 photo-fixation performances.

Modulation of the electronic properties and spin polarization of 2H VS2 nanoribbons by tuning ribbon widths and edge decoration.

The band structures and spin-polarization characteristics of armchair and zigzag VS2 nanoribbons with different terminated edges are investigated based on density functional theory (DFT) calculations with a spin polarized meta-GGA. The results reveal that zigzag 2H VS2 nanoribbons exhibit metal, half-metal, or semiconductor electrical characteristics with different edge decorations or ribbon widths. And the spin polarized ratio can achieve 100% self-polarization for the zigzag VS2 nanoribbons with V atom edges. The Curie temperatures (TC) estimated by mean field approximation simulations for the zigzag 2H VS2 nanoribbons with terminated edges of V systems are 276 K. These preliminary findings offer an effective treatment option for controllable and adjustable spintronic devices.

Multifunctional 2D CuSe monolayer nanodevice.

In a very recent experimental work (Gao et al 2018 Adv. Mater. 30 1707055), a graphene-like CuSe monolayer (ML) was realized. Motivated by this success, we performed first-principles calculations to investigate its electronic transport and photoelectronic properties. We find that the CuSe ML shows a strong electrical anisotropy, and its current-voltage (I-V) curves along the zigzag and armchair directions are noticeably different. The CuSe ML also displays a useful negative differential resistance (NDR) effect along the both directions when the bias is beyond 1.0 V. Moreover, it has a large photon absorption to orange light. Our study suggests that CuSe ML is a multifunctional material and has various potential applications in electrical-anisotropy-based, NDR-based, and even optical nanodevices.

Negative differential conductance effect and electrical anisotropy of 2D ZrB2 monolayers.

Two-dimensional (2D) metal-diboride ZrB2 monolayers was predicted theoretically as a stable new electronic material (Lopez-Bezanilla 2018 Phys. Rev. Mater. 2 011002). Here, we investigate its electronic transport properties along the zigzag (z-ZrB2) and armchair (a-ZrB2) directions, using the density functional theory and non-equilibrium Green's function methods. Under low biases, the 2D ZrB2 shows a similar electrical transport along zigzag and armchair directions as electric current propagates mostly via the metallic Zr-Zr bonds. However, it shows an electrical anistropy under high biases, and its I-V curves along zigzag and armchair directions diverge as the bias voltage is higher than 1.4 V, as more directional B-B transmission channels are opened. Importantly, both z-ZrB2 and a-ZrB2 show a pronounced negative differential conductance (NDC) effect and hence they can be promising for the use in NDC-based nanodevices.

How does the electric current propagate through fully-hydrogenated borophene?

We study the electronic transport properties of two-dimensional (2D) fully-hydrogenated borophene (namely, borophane), using density functional theory and non-equilibrium Green's function approaches. Borophane shows a perfect electrical transport anisotropy and is promising for applications. Along the peak- or equivalently the valley-parallel direction, 2D borophane exhibits a metallic characteristic and its current-voltage (I-V) curve shows a linear behavior, corresponding to the ON state in borophane-based nano-switches. In this case, electrons mainly propagate via the B-B bonds along the linear boron chains. In contrast, electron transmission is almost forbidden along the perpendicular buckled direction (i.e., the OFF state), due to its semi-conductor property. Our work demonstrates that 2D borophane could combine metal and semiconductor features and may be a promising candidate for nano-switching materials with a stable structure and high ON/OFF ratio.

The electronic transport properties of zigzag phosphorene-like MX (M = Ge/Sn, X = S/Se) nanostructures.

Single-layer phosphorene-like MX sheets have aroused new interest and could become a family of nanomaterials in physics and materials science. Using a first-principles method combined with non-equilibrium Green's function (NEGF) theory, we study the electronic transport properties of the zigzag phosphorene-like MX (M = Ge/Sn, X = S/Se) nanostructures. The results demonstrate that GeS and GeSe nanoribbons display very similar electronic transport properties. Their current-voltage (I-V) curves exhibit an interesting negative differential resistive (NDR) effect and are insensitive to their ribbon widths due to their similar band structures. However, for SnS and SnSe nanoribbons, their electronic transport properties are obviously dependent on their ribbon widths due to their different band structures. Most of the SnS nanoribbons display the current-limited effect. SnSe nanoribbons could also present a NDR effect, which appeared at a lower applied bias. The currents mainly propagate through the phosphorene-like MX nanoribbons along the metal-termination, while little along the S/Se-termination. Moreover, their two-dimensional monolayers present an obvious difference from their one-dimensional structures. These phosphorene-like MX nanostructures have potential applications in nanoelectronics, and could become candidates for nanodevices, such as NDR devices.

A theoretical simulation of small-molecules sensing on an S-vacancy SnS2 monolayer.

Using first-principle atomistic simulations, we focused on the electronic structures of small gas molecules (CO, H2O, NH3, NO, and NO2) adsorbed on the S-vacancy SnS2 monolayer. The results show that H2O and CO molecules were physisorbed on the S-vacancy SnS2 monolayer, whereas NH3, NO, and NO2 molecules were chemisorbed on the S-vacancy SnS2 monolayer via strong covalent bonds. Moreover, our calculations show that H2O and NH3 act as charge donors, whereas CO, NO, and NO2 gas molecules act as acceptors. Different adsorption behaviors of common gas molecules on the S-vacancy SnS2 monolayer provide a feasible way to exploit chemical gas sensors and electrical devices. In particular, our results also show that under applied biaxial strains, the adsorption energy and charge transfer of gas molecules on the S-vacancy SnS2 monolayer dramatically changed, which indicates that external factors on the S-vacancy SnS2 monolayer are highly preferred.

The rectifying and negative differential resistance effects in graphene/h-BN nanoribbon heterojunctions.

We investigate the electronic transport properties of four types of lateral graphene/h-BN nanoribbon heterojunctions using the non-equilibrium Green's function method in combination with the density functional theory. The results show that the heterojunction displays an interesting rectifying effect when the interface has a left-right type structure, while a pronounced negative differential resistance (NDR) effect when the interface has an up-down type structure. Moreover, when the interface of the heterojunction has a left-bank or right-bank type structure, it presents the rectifying (with a larger rectification ratio) and NDR effects. This work is helpful to further construct and prepare a nanodevice based on the graphene/h-BN heterojunction materials according to the proposed structures.

The magnetism and spin-dependent electronic transport properties of boron nitride atomic chains.

Very recently, boron nitride atomic chains were successively prepared and observed in experiments [O. Cretu et al., ACS Nano 8, 11950 (2015)]. Herein, using a first-principles technique, we study the magnetism and spin-dependent electronic transport properties of three types of BN atomic chains whose magnetic moment is 1 µB for BnNn-1, 2 µB for BnNn, and 3 µB for BnNn+1 type atomic chains, respectively. The spin-dependent electronic transport results demonstrate that the short BnNn+1 chain presents an obvious spin-filtering effect with high spin polarization ratio (>90%) under low bias voltages. Yet, this spin-filtering effect does not occur for long BnNn+1 chains under high bias voltages and other types of BN atomic chains (BnNn-1 and BnNn). The proposed short BnNn+1 chain is predicted to be an effective low-bias spin filters. Moreover, the length-conductance relationships of these BN atomic chains were also studied.