First-principles design of g-C3N4/HfSSe heterojunctions for optoelectronic applications.

Based on first-principles calculations, the structure, electronic and optical properties of g-C3N4/HfSSe heterojunctions have been systematically explored. We prove the stability of two heterojunctions by comparing the binding energies from six different stacking heterojunctions, which name are g-C3N4/SHfSe heterojunction and g-C3N4/SeHfS heterojunction, respectively. It is shown that both heterojunctions behave direct band gaps with type II band alignment. The charge is rearranged at the interface after the heterojunctions are formed, which results in the formation of the built-in electric field. In the ultraviolet, visible and near-infrared regions, excellent light absorption is found in g-C3N4/HfSSe heterojunctions.

The Adsorption Effect of Methane Gas Molecules on Monolayer PbSe with and without Vacancy Defects: A First-Principles Study.

In this paper, the adsorption effect of methane (CH4) gas molecular on monolayer PbSe with and without vacancy defects is studied based on first-principles calculations. The effects of the adsorption of methane molecular on monolayer PbSe and on the Se vacancy (VSe) and Pb vacancy (VPb) of monolayer PbSe are also explored. Our results show that methane molecules exhibit a good physical adsorption effect on monolayer PbSe with and without vacancy defects. Moreover, our simulations indicate that the adsorption capacity of CH4 molecules on monolayer PbSe can be enhanced by applying strain. However, for the monolayer PbSe with Vse, the adsorption capacity of CH4 molecules on the strained system decreases sharply. This indicates that applying strain can promote the dissociation of CH4 from VSe. Our results show that the strain can be used as an effective means to regulate the interaction between the substrate material and the methane gas molecules.

Amorphous Mixed-Vanadium-Tungsten Oxide Films as Optically Passive Ion Storage Materials for Solid-State Near-Infrared Electrochromic Devices.

Near infrared (NIR) electrochromic (EC) devices that selectively modulate the NIR light without affecting the daylight represent a promising window technology for saving energy consumption of buildings. Current research efforts have been focused on developing NIR-EC materials, while little attention has been directed to the optically passive ion storage materials that are crucial for balancing charges in a full NIR-EC device. Herein, we report that amorphous phase mixed-vanadium-tungsten oxide films exhibit minimum optical change with high ion storage capacity, which enables the usage of the mixed-metal oxides as optically passive counter electrode materials for NIR-EC devices. The mixed-vanadium-tungsten oxide films are synthesized by a room-temperature solution-based photodeposition method that allows us to precisely engineer the metal compositions and thicknesses of the mixed-metal oxide films, thus optimizing their optical inertness and ion storage capability. A solid-state NIR-EC device assembled with the mixed-vanadium-tungsten oxide film as an ion storage layer and the amorphous tungsten oxide hydrate as the NIR-EC layer shows fast response speed with cycling stability up to 10,000 cycles, proving the outstanding charge balancing capability of mixed-metal oxide. Our work provides an efficient strategy for developing optically passive ion storage films with high ion storage capability for high-performance EC devices.

Thermochromic Smart Windows Assisted by Photothermal Nanomaterials.

Thermochromic smart windows are optical devices that can regulate their optical properties actively in response to external temperature changes. Due to their simple structures and as they do not require other additional energy supply devices, they have great potential in building energy-saving. However, conventional thermochromic smart windows generally have problems with high response temperatures and low response rates. Owing to their great effect in photothermal conversion, photothermal materials are often used in smart windows to assist phase transition so that they can quickly achieve the dual regulation of light and heat at room temperature. Based on this, research progress on the phase transition of photothermal material-assisted thermochromic smart windows is summarized. In this paper, the phase transition mechanisms of several thermochromic materials (VO2, liquid crystals, and hydrogels) commonly used in the field of smart windows are introduced. Additionally, the applications of carbon-based nanomaterials, noble metal nanoparticles, and semiconductor (metal oxygen/sulfide) nanomaterials in thermochromic smart windows are summarized. The current challenges and solutions are further indicated and future research directions are also proposed.

A two-dimensional α-As/α-AsP van der Waals heterostructure for photovoltaic applications.

On the basis of first-principles calculations, we constructed a two-dimensional (2D) α-As/α-AsP van der Waals heterostructure (vdWH) to study the structural stability, and electronic and optical properties. Our results show that the 2D α-As/α-AsP vdWH displays indirect semiconductor characteristics with a band gap of 0.86 eV. Furthermore, it exhibits anisotropic properties and a high light absorption coefficient of up to 106 cm-1 in the ultraviolet region. Our predicted photoelectric conversion efficiency (PCE) of the 2D α-As/α-AsP vdWH reaches 21.30%, which is higher than those of blue-AsP/CdSe (13%), GeSe/AsP (16%), and As/tetracyanonaphtho-quinodimethane (TCNNQ) (20%). By applying an in-plane strain in the range from -6% to 6%, the band structure of the 2D α-As/α-AsP vdWH can be effectively tuned. In particular, under a uniaxial strain of -2% along the x-axis (2% along the y-axis), a transition from indirect to direct in the band structure occurs, while when applying a biaxial strain of -4% a transition from a semiconductor to a metal can be realized. These results indicate that the 2D α-As/α-AsP vdWH has potential applications in optoelectronic devices and solar cells.

Computational Design of α-AsP/γ-AsP Vertical Two-Dimensional Homojunction for Photovoltaic Applications.

Based on first-principles calculations, we design a α-AsP/γ-AsP homojunction with minimum lattice distortion. It is found that the α-AsP/γ-AsP homojunction has an indirect bandgap with an intrinsic type-II band alignment. The proposed α-AsP/γ-AsP homojunction exhibits high optical absorption of 1.6×106 cm-1 along the zigzag direction. A high power conversion efficiency (PCE) of 21.08% is achieved in the designed α-AsP/γ-AsP homojunction, which implies it has potential applications in solar cells. Under 4% in-plane axial strain along the zigzag direction, a transition from indirect band gap to direct band gap is found in the α-AsP/γ-AsP homojunction. Moreover, the intrinsic type-II band alignment can be tuned to type-I band alignment under in-plane strain, which is crucial for its potential application in optical devices.

Research Progress of Photo-/Electro-Driven Thermochromic Smart Windows.

Thermochromic smart windows can automatically control solar radiation according to the ambient temperature. Compared with photochromic and electrochromic smart windows, they have a stronger applicability and lower energy consumption, and have a wide range of application prospects in the field of building energy efficiency. At present, aiming at the challenge of the high transition temperature of thermochromic smart windows, a large amount of innovative research has been carried out via the principle that thermochromic materials can be driven to change their optical performance by photothermal or electrothermal effects at room temperature. Based on this, the research progress of photo- and electro-driven thermochromic smart windows is summarized from VO2-based composites, hydrogels and liquid crystals, and it is pointed out that there are two main development trends of photo-/electro-driven thermochromic smart windows. One is exploring the diversified combination methods of photothermal materials and thermochromic materials, and the other is developing low-cost large-area heating electrodes.

1D/2D van der Waals Heterojunctions Composed of Carbon Nanotubes and a GeSe Monolayer.

Based on first-principles calculations, we propose van der Waals (vdW) heterojunctions composed of one-dimensional carbon nanotubes (CNTs) and two-dimensional GeSe. Our calculations show that (n,0)CNT/GeSe (n = 5-11) heterojunctions are stable through weak vdW interactions. Among these heterojunctions, (n,0)CNT/GeSe (n = 5-7) exhibit metallic properties, while (n,0)CNT/GeSe (n = 8-11) have a small bandgap, lower than 0.8 eV. The absorption coefficient of (n,0)CNT/GeSe (n = 8-11) in the ultraviolet and infrared regions is around 105 cm-1. Specifically, we found that (11,0)CNT/GeSe exhibits type-II band alignment and has a high photoelectric conversion efficiency of 17.29%, which suggests prospective applications in photoelectronics.

Tunable Broadband THz Waveband Absorbers Based On Graphene for Digital Coding.

A method of coding patterns is proposed to achieve flexible control of absorption response at terahertz frequencies. The designed absorber consists of an Au-graphene pattern layer, a SiO2 layer and a metal reflective layer. Among them, we use concentrical circle structure to achieve broadband absorption, and adjust graphene's Fermi level to achieve tunable absorption. In addition, we propose an encoding method that can achieve flexible control of the absorption response at the terahertz frequency based on the external voltage applied on the graphene membrane, thereby having a programmable function. We also use COMSOL to simulate the electric field distribution diagram to explain the underlying physical mechanism. The programmable broadband adjustable absorber proposed in this paper has potential application prospects in the fields of optical equipment, information transmission, digital coding and artificial intelligence (AI).

Fast and Broadband Photoresponse of a Few-Layer GeSe Field-Effect Transistor with Direct Band Gaps.

Few-to-monolayer germanium selenide, a new IV-VI group layered material recently fabricated by mechanical exfoliation and subsequent laser thinning, is promising in very fast and broadband optoelectronic applications for its excellent stability, complicated band structures, inert surface properties, and being a natural p-type semiconductor. However, large-scale production of such few-layer GeSe devices with superior performance is still in early stages. In this study, field-effect transistors made of few-layer GeSe with direct band gaps are fabricated. Transistor performance with Schottky contact characteristics is measured at room temperature. A field-effect mobility of 4 cm2/(V s) and drain currents modulated both by holes and electrons are measured. Photoresponses as a function of illumination wavelength, power, and frequency are characterized. The few-layer GeSe transistor shows photoresponse to the illumination wavelengths from visible up to 1400 nm and a photoresponse rise (fall) time of 13 µs (19 µs), demonstrating very broadband and fast detection. The ambipolar behavior and the photoresponse characteristics demonstrate great potential of few-layer GeSe for applications in highly stable, very fast, and very broadband optoelectronic devices.

Enhancement of photoluminescence efficiency in GeSe ultrathin slab by thermal treatment and annealing: experiment and first-principles molecular dynamics simulations.

The effect of thermal treatment and annealing under different temperatures from 100 °C to 250 °C on the photoluminescence spectroscopy of the GeSe ultrathin slab is reported. After the thermal treatment and annealing under 200 °C, we found that the photoluminescence intensity of A exciton and B exciton in GeSe ultrathin slab is increased to twice as much as that in untreated case, while is increased by ~84% in the photoluminescence intensity of C exciton. Combined by our experimental work and theoretical simulations, our study confirms the significant role of thermal treatments and annealing in reducing surface roughness and removing the Se vacancy to form more compact and smoother regions in GeSe ultrathin slab. Our findings imply that the improved quality of GeSe surface after thermal treatments is an important factor for the photoluminescence enhancement.

Functionalization of the electronic and magnetic properties of silicene by halogen atoms unilateral adsorption: a first-principles study.

Based on first-principles calculations, the structure, electronic and magnetic properties of unilateral halogenated silicene Si2X1 (X = F, Cl, Br, I) are investigated. The formation energies of all the configurations of studied Si2X1 (X = F, Cl, Br, I) are found to be lower than that in pristine silicene, which indicates the strong stability. The band structure of half-fluorinated configuration Si2F1 presents metallic property, while other unilateral halogenated silicene Si2X1 (X = Cl, Br, I) exhibits half-metallic properties. In unilateral halogenated silicene Si2X1 (X = Cl, Br, I), the unpaired electrons in unsaturated silicon atom produce the localized magnetic moment. However, due to the strong electronegativity in F atom, the half-fluorinated silicene Si2F1 is almost non-magnetic. The metallic property of Si2F1 configuration can be tuned to half-metallic by applying biaxial tensile strain from 11.95% to 13.51%. Furthermore, applying biaxial tensile strains can tune the half-metallic property of unilateral halogenated silicene Si2X1 (X = Cl, Br, I) to a semiconductor. This half-metallic property in unilateral halogenated silicene Si2X1 (X = Cl, Br, I) can be recovered and can even be tuned to metallic if continually increasing the biaxial tensile strains.

Effect of stacking order and in-plane strain on the electronic properties of bilayer GeSe.

Germanium selenide as a new layered material is promising for nanoelectronic applications due to its unique optoelectronic properties and tunable band gap. In this study, based on density functional theory, we systematically investigated the structure, stability, and electronic properties of bilayer germanium selenide with four different stacking orders (namely AA-, AB-, AC- and AD-stacking). The obtained results indicated that the band gap is dependent on the stacking order with an indirect band gap for AA- and AC-stacking and direct band gap for AB- and AD-stacking. In addition, we also found that the band gap of the GeSe bilayer with different stacking orders can be tuned by in-plane strain. The transition between the direct to indirect band gap or semiconductor to metal is tunable. In particular, the direct band gap of the AB-stacking germanium selenide bilayer can be tuned in a wide energy range under applied strain along the armchair direction.

Bandgap modulation of MoS2 monolayer by thermal annealing and quick cooling.

We developed a non-mechanical straining method to simultaneously modulate the bandgap and photoluminescence (PL) quantum efficiency of a synthesized molybdenum disulfide (MoS2) monolayer on SiO2, by vacuum annealing and subsequent quick cooling in ethanol. Influences of the thermal treatments at different temperatures from 100 °C to 600 °C on the PL and Raman spectra of the MoS2 monolayers are reported. A maximum PL peak intensity, twice that of the untreated counterparts under the same measurement conditions, was observed at the treating temperature of 200 °C. At the same time, approximately permanent tensile strains were induced, due to the quick cooling from high temperatures, which led to a red-shift of the direct optical bandgap. Modulation of the bandgap was achieved by changing the treating temperatures; nearly linear PL and Raman frequency shifts of ∼3.82 meV per 100 °C and ∼-0.28 cm-1/100 °C for A exciton photoluminescence and Raman E12g mode frequency were observed, respectively. The proposed thermal modulation promises a wide range of applications in functional 2D nanodevices and semiconductors. To our knowledge, our findings constitute the first demonstration of thermal engineering by combinational manipulation of annealing and quick cooling of the 2D transition-metal dichalcogenides.

Density functional calculation of transition metal adatom adsorption on graphene.

The functionalization of graphene (a single graphite layer) by the addition of transition metal atoms of Mn, Fe and Co to its surface has been investigated computationally using density functional theory. In the calculation, the graphene surface supercell was constructed from a single layer of graphite (0001) surface separated by vertical vacuum layers 2 nm thick. We found that the center of the hexagonal ring formed by carbon from graphene is the most stable site for Mn, Fe, Co to stay after optimization. The calculated spin-polarized band structures of the graphene encapsulating the Mn adatom indicate that the conduction bands are modified and move down due to the coupling between the Mn atom and graphene. For Fe adsorbed on the graphene surface, it is semi-half-metallic, and the spin polarization P is found to be 100%. The system of Co adatom on graphene exhibits metallic electronic structure due to the density of states (DOS) peak at the band center with both majority and minority spins. Local density of states analyses indicate a larger promotion of 4s electrons into the 3d state in Fe and Co, resulting in lower local moments compared to an Mn adatom on the graphite surface.

Structural, electronic and magnetic properties of manganese doping in the upper layer of bilayer graphene.

First-principles spin-polarized calculations have been conducted to investigate the structural, electronic and magnetic properties of 3d transition metal Mn doping into two typical sites in the upper layer of bilayer graphene with the AB Bernal structure. One of the doping sites is above the center of a carbon hexagon of the lower graphene layer (called the H site) and the other is directly on top of a carbon atom of the lower graphene layer (called the T site). We found that Mn doping enlarges the interlayer distance in bilayer graphene. Charge density distribution indicates that the region between the upper and lower graphene layer has apparent covalent-bonding characters due to the Mn doping. In the spin-polarized band structure of H site doping, the π and π(*) bands separate from each other at the Dirac point both in majority spin and minority spin. In the band structure of T site doping, the Fermi level is located above the Dirac point and moves to the conduction bands in majority spin and minority spin, making the bilayer graphene n doped. A high spin polarization of 95% is achieved due to the H site doping. The local moment of Mn for H and T site doping is reduced to 1.76 µ(B) and 1.88 µ(B), respectively, which are smaller than the value (5 µ(B)) in the free state.

A first-principle study of one-dimensional carbon atomic chain inserted single-wall carbon nanotubes.

Using first principles calculations, we investigate the atomic and electronic structure of carbon nanowires (CNWs) as the carbon chain inserted into single wall carbon nanotubes (SWCNTs). It indicates that the (5,5) CNW system exhibits metallic character, however, the insertion of carbon chain can transit a semi-conducting (9,0) SWCNT into a metallic.