Direct Bandgap Semiconductor Material Research Articles

A novel large-cell boron nitride polymorph

Through density functional theory simulations, the structural properties, stability, mechanical properties, elastic anisotropy as well as electronic properties for boron nitride allotropes with cubic structure, Pn-3n BN, are investigated in this work. Studying the phonon spectra, molecular dynamics as well as elastic constants of Pn-3n BN, it is found that porous Pn-3n BN is stable in dynamics, thermodynamics and mechanics. The B (bulk modulus), G (shear modulus) and E (Young's modulus) of Pn-3n BN are 64 GPa, 26 GPa and 69 GPa, respectively. The B/G (the ratio of bulk modulus to shear modulus) and v (Poisson's ratio) of Pn-3n BN are 2.43 and 0.319, respectively, which exhibit ductility. The three-dimensional surface construction for Young's modulus, the maximum and minimum of shear modulus, the maximum and minimum of Poisson's ratio of Pn-3n BN are all irregular spheres, indicating that Pn-3n BN has elastic anisotropy. The Emax/Emin of Pn-3n BN displays isotropy with 1.00 in (111) plane, and is not equal to 1.00 in other principal planes, showing anisotropy. The conduction band minimum (CBM) and the valence band maximum (VBM) of Pn-3n BN are both located at point G, and the band gap width of Pn-3n BN is 5.197 eV, from which Pn-3n BN is a wide band gap and direct band gap semiconductor material.

Insights on structural, elastic, electronic, optical and thermodynamic properties and population analysis of double perovskite oxide Ba2MgXO6 (X = Se, Te)

In this paper, the elastic, optical, electronic and thermodynamic properties and population analysis of double perovskite oxide Ba2MgXO6 (X = Se, Te) are calculated based on first-principles. The results show that the phonon spectrum of Ba2MgXO6 has no virtual frequency, its binding energy and formation enthalpy are negative, which satisfies the Born-Huang stability criterion, indicating that the compound is stable. The elastic parameters of Ba2MgXO6 (X = Se, Te) indicate that they are all brittle materials. Ba2MgSeO6 and Ba2MgTeO6 are direct band gap semiconductor materials with band gap values of 2.197 eV and 2.788 eV, respectively. The charge population analysis indicates that Ba2MgXO6 is a compound with a tendency towards ionic type. The heat capacity reaches the Dulong-Petit limit when the temperature is close to 800K, and both materials show high absorption coefficients in the far ultraviolet spectrum region. Therefore, our study suggests that Ba2MgXO6 (X = Se, Te) may be a good candidate for photovoltaic materials and photoelectric devices in space.

Electronic and optical properties of Sc2C(OH)2 with phenylsulfonic groups: A first-principles study

Polystyrene sulfonic acid (PSS) polymer thin film can be applied to the surface passivation of Si-based semiconductor devices because of its low cost and the ability to be fabricated without the need for extreme temperatures or high vacuum. Here, the phenylsulfonic groups (–pSO3H, a typical part of PSS) were grafted onto the Sc2C(OH)2 the surface to form Sc2C(OH)2–pSO3H. The electronic structure and optical properties were investigated by first-principles calculations. The results show that Sc2C(OH)2–pSO3H is a direct bandgap semiconductor material with a bandgap of 0.99 eV, which is higher than that of Sc2C(OH)2. The bandgap of Sc2C(OH)2–pSO3H has a small change by replacing cation H of –pSO3H with Li, Na and K, indicating that the Sc2C(OH)2–pSO3H maintains its semiconductor properties. The reason is that the conduction band minimum (CBM) state and the valence band maximum (VBM) are mainly determined by the contribution of C 2p states and Sc 3d states of the main body Sc2C(OH)2. Versus the conventional photovoltaic material Si, the absorption coefficient of Sc2C(OH)2–pSO3H is very high, thus resulting in a maximum photoelectric conversion efficiency of up to 31 %. These results suggest promising 2D visible light absorption materials that are photovoltaics in solar cells.

Synthesis of Black Phosphorene/P-Rich Transition Metal Phosphide NiP3 Heterostructure and Its Effect on the Stabilization of Black Phosphorene

Black phosphorus (BP), as a direct band gap semiconductor material with a two-dimensional layered structure, has a good application potential in many aspects, but the surface state of it is extremely unstable, especially that of single-layer black phosphorus. In this study, BP crystals and two-dimensional black phosphorus (2D BP) are prepared by a mechanical ball-milling–liquid-phase exfoliation method. The X-ray diffraction (XRD) spectrum and high-resolution transmission electron microscopy (HRTEM) results showed that red phosphorus (RP) successfully turned to BP by the mechanical ball-milling method. The spectrophotometric analysis has detected absorption peaks at 780 nm, 915 nm, and 1016 nm, corresponding to single, double, and three-layer BP bandgap emission. A simple solvothermal strategy is designed to synthesize in-plane BP/P-rich transition metal phosphide (TMP) heterostructures (BP/NiP3) by defect/edge-selective growth of NiP3 on the BP nanosheets. HRTEM analysis indicates that the metal ions are preferentially deposited on the defects of 2D BP such as edges and unsaturated sites, forming a 2D BP/NiP3 in-plane heterojunction.

Theoretical study on structural, electronic, transport and thermoelectric properties of Si/Ge doped graphene

First-principles simulations are used in this study to show the structural, electronic, transport, and thermoelectric features of silicon and germanium-doped monolayer graphene. We employed a supercell of 2 × 2 × 1 graphene that contains 8 C atoms. One C atom was substituted by Si/Ge at a doping concentration of 12.5 %. The results obtained show that the band gap of pure graphene may be effectively opened and converted into a direct bandgap semiconductor material by replacing the C atom with Si/Ge atoms. Phonon dispersion provides a representation of the dynamical stability of all three structures. Graphene's thermal conductivity is also decreased when heteroatoms are added. Additionally, the Seebeck coefficient, the power factor, the electrical conductivity, and the figure of merit of pure, silicon, and germanium-doped graphene were investigated. This study offers a theoretical basis for using a heteroatom-doping process to improve the performance of graphene's electronic, transport, and thermoelectric characteristics.

A DFT study of the structural, magnetic, electronic, mechanical and optical properties of M0.75Cu0.25Fe2O4 (M = Co, Ni, Mg, Mn) ferrites: Optoelectronic applications

DFT calculations were used to investigate the structural, magnetic, electronic, mechanical and optical properties of M0.75Cu0.25Fe2O4 (M = Co, Ni, Mg, Mn) ferrites by employing the CASTEP Code. The simulated XRD of M0.75Cu0.25Fe2O4 (M = Co, Ni, Mg, Mn) ferrites revealed an increase in lattice constants (8.353–8.501 Å), d-spacings (2.426–2.592 Å), the volume of the unit cell (582.811–614.342 Å3), hopping lengths (LA = 7.234–7.441 Å; LB = 5.906–6.074 Å) and a decrease in X-ray density (5.376–5.035 g/cm3). The M0.75Cu0.25Fe2O4 (M = Co, Ni, Mg, Mn) ferrites exhibited the characteristics of direct bandgap semiconductor materials, which increased from 0.931–1.176 eV The computed DOS revealed that M0.75Cu0.25Fe2O4 (M = Co, Ni, Mg, Mn) ferrites exhibited good spin polarization behavior as well, and according to Hirshfeld analyses, the magnetic moment from 3.348–1.436 μB and saturation magnetization from 79.264–34.435 emu/g were increased. The cubic crystal mechanical stability criteria of M0.75Cu0.25Fe2O4 (M = Co, Ni, Mg, Mn) ferrites indicated that all ferrites were mechanically stable as potential candidates in electrochemical applications. The values of Pugh's ratio of M0.75Cu0.25Fe2O4 (M = Co, Ni, Mg, Mn) ferrites predicted the ductile nature of all ferrites and all the values of Poisson's ratio exhibited the behavior of ionic bonds of all ferrites. The optical characteristics of M0.75Cu0.25Fe2O4 (M = Co, Ni, Mg, Mn) ferrites lie in visible regions, which proved suitable candidates in optoelectronic applications.

CoFe@[CoFe2O4/Fe(Co)Nx-C] nanoparticles prepared via an ultrasonic atomization coupling pyrolysis process and their magnetic and optical properties

Due to their special qualities, core-shell nanomaterials with two or more functionalities have drawn a lot of attention. Core-shell materials exhibit stronger combined electrical, optical, magnetic, and electronic characteristics than single-component nanomaterials thanks to the creation of the core-shell structure and the synergistic interaction between components. However, the formation of core-shell structures is often difficult due to the incompatibility between components, especially for lattice-mismatched core-shell structures. In this study, CoFe@CoFe2O4-N-C nanoparticles (∼150 nm) were successfully prepared via an ultrasonic atomization coupling pyrolysis process. This exhibits higher coercivity (108 Oe), saturation magnetization (52.3 emu/g) and Curie temperature (>400 K) than CoFe nanoparticles. At the same time, it exhibits high UV–Vis and infrared absorption characteristics and has good electronic properties as a direct band gap semiconductor material (bandgap of 2.87 eV). These characteristics suggest that CoFe@CoFe2O4-N-C NPs have a lot of potential in sectors like magneto-optics and absorbent materials.

Structural, mechanical, anisotropic and electronic properties of BC8 carbon under pressure

The structural, mechanical, mechanical anisotropy, electronic properties and X-ray diffraction (XRD) pattern of BC8 carbon were investigated utilizing density functional theory. The mechanical anisotropy calculations show that BC8 carbon has smaller anisotropy in elastic moduli. The anisotropy in the shear modulus, Young's modulus, and Poisson's ratio decreases with increasing pressure. The band structure calculations reveal that BC8 carbon is a direct band gap semiconductor material, and it maintains the direct band gap semiconductor properties up to 100 GPa. In addition, the X-ray diffraction (XRD) pattern of BC8 carbon under different pressures is also investigated in this work.

Two-dimensional Mg2Si-111: A direct bandgap semiconductor with excellent optical response properties predicted by first-principles calculations

A new two-dimensional(2D) direct bandgap semiconductor material, 2D Mg2Si-111, with a reticulated hexagonal structure and excellent photoresponse properties is predicted by using the first-principles calculation method based on density generalized function theory (DFT). 2D Mg2Si-111 has excellent properties that can be tuned with the number of layers and Mg defects. The calculation shows that the semiconductor of 2D Mg2Si-111 is stable at absolute temperature, and the bandgap value increases to a range suitable for light absorption, which is 1.17 eV. The electronic transition in 2D Mg2Si-111 is changed from the indirect transition of three-dimensional(3D) Mg2Si to direct transition, and its high-efficiency application potential in optoelectronic devices is more prominent. The electron effective mass in single-layer Mg2Si-111 ranges from 0.277 mo to 0.357 m0, which is lower than typical 2D materials, and exhibits an unconventional optical response that is not lower than the bulk parent body and has excellent absorption characteristics in the near-infrared region. Meanwhile, consider the effects of layer number modulation and defect regulation on 2D Mg2Si-111. The band gap value of 2D Mg2Si-111 can be modulated with the number of layers, from single layer to four layers, with an adjustable band gap of 0 eV - 1.17 eV that changes with the number of layers. The band gap change caused by Mg vacancy defects is only 0.89 eV - 1.1 eV, lower than the band gap of multilayer modulation, however, defect control can maintain a higher value of the band gap in 2D Mg2Si-111. Therefore, vacancy defects are a more refined means of regulation, and the influence of layer-number modulation on the characteristics of 2D Mg2Si-111 is significant. The light absorption characteristics of three-layer Mg2Si-111 have considerable light absorption responses from short-wave infrared to near-ultraviolet. Therefore, the layer number modulation has obvious changes to 2D Mg2Si-111. Compared with Mg2Si, 2D Mg2Si-111 is predicted to be a more efficient and promising optoelectronic material, and this work paves the way for the preparation, design and application of novel 2D Mg2Si-111 in 2D optoelectronic devices.

Structural, elastic, and electronic properties of BC12 carbon under pressure

The crystal structure, mechanical, elastic anisotropy in shear modulus, Young’s modulus and Poisson’s ratio, electronic band structures and X-ray diffraction (XRD) pattern of BC12 carbon are investigated by density functional theory. The elastic anisotropy calculations show that BC12 carbon has smaller anisotropy in Young's modulus, and shear modulus than that of Poisson's ratio. The elastic anisotropy in the shear modulus, Young's modulus, and Poisson's ratio is almost unchanged. The band structure calculations reveal that BC12 carbon is a direct band gap semiconductor material at ambient pressure, and it maintains the direct band gap semiconductor property up to 100 GPa. In addition, it is found that the diffraction angles of the same diffraction peak slightly increase with increasing pressure through the X-ray diffraction (XRD) pattern.

Theoretical prediction of valley spin splitting in two-dimensional Janus MSiGeZ4 (M = Cr and W; Z = N, P, and As).

With the exploration of valleytronic materials in MA2Z4 structures, larger valley spin splitting has become a hot topic of research. Based on first-principles calculations, we predicted six valleytronic 2D (two-dimensional) Janus MSiGeZ4 (M = Cr and W; Z = N, P, and As) materials. The valley spin splitting value of WSiGeZ4 (Z = N, P, and As) can reach more than 400 meV, which is favorable for the practical application of valleytronics. Two-dimensional WSiGeZ4 (Z = N, P, and As) materials are dynamically and mechanically stable and have an abundance of electronic properties. The two-dimensional Janus WSiGeZ4 (Z = N, P, and As) structures comprise both direct and indirect bandgap semiconductor materials. Among them, WSiGeN4 is an indirect bandgap semiconductor material with a bandgap of 1.654 eV and WSiGeP4 is a direct bandgap semiconductor material. The valley spin splitting originates from the symmetry breaking of the material structure and the spin-orbit coupling effect of the transition metal, which is manifested as the Berry curvature. In particular, the Berry curvature of 2D Janus WSiGeP4 and WSiGeAs4 is as high as 300 Bohr2, which is quite large. The W atom has more d-orbital electrons than the Cr atom, and the SOC (spin-orbit coupling) effect is stronger; thus, the valley spin splitting value CrSiGeZ4 of WSiGeZ4 is more than 300 meV, which is quite large. In addition, the bandgap and valley spin splitting of WSiGeZ4 (Z = N, P, and As) can be significantly modulated by applying a biaxial strain. Our study shows that WSiGeZ4 (Z = N, P, and As) has great potential for valleytronic applications.

Investigation of the optoelectronic properties of cubic metal halide perovskite compound RbSiCl3

Lead free halide metal perovskite compounds have been exploring more since last decades due to their wide potential applications as well as their environmental friendly nature. Presented work agreements with the density functional investigation of the material properties (optoelectronic) of lead free halide metal perovskite compound RbSiCl3 by using WIEN2k code that is rely on FP-LAPW: full potential-linearized augmented plane wave method. The optoelectronic response are well explored by the Perdew Burke Ernzergof GGA exchange correlation functional. The computed energy band gap of this compound is 0.96 eV and results reveal that it is direct band gap semiconductor material. In addition, the integrated absorption coefficient also computed through absorption spectra, which gives the value 123.39 (×104 eV/cm). All computation proves the utility of the compound for photovoltaic and optoelectronic applications.

Ball milling synthesis and optical properties of Na and Bi ions Co-doped double perovskite Cs2AgInCl6

Double perovskite Cs2AgInCl6 is a new direct band gap semiconductor material. Compared with traditional perovskite CsPbX3 (X = Cl, Br, I), double perovskite Cs2AgInCl6 has good stability and wide fluorescence half peak width. This study reports the mechanochemical synthesis, characterization and application of a lead-free double perovskite Cs2AgInCl6 doped with Na and Bi ions. It was found that through a facile room temperature mechanical ball milling process, the double perovskite structure can be obtained within 30 min. Then, Na and Bi ions were used to dope to optimize its luminescence properties. By this synthesis method, the sample with the best doping ratio obtained in the experiment is Cs2Ag0.7Na0.3In0.9Bi0.1Cl6 with the photoluminescence quantum yield (PLQY) of 32.7%. The X-ray diffraction (XRD) and photoluminescence (PL) results show that Na and Bi ions can promote the crystallization and optimize the luminescence properties of Cs2AgInCl6. The theoretical calculation results show that both conduction band minimum (CBM) and valance band maximum (VBM) of Cs2Ag0.7Na0.3In0.9Bi0.1Cl6 are flat, which is favorable for the generation of self-trapped excitons. Packaging Cs2Ag0.7Na0.3In0.9Bi0.1Cl6 sample on an ultraviolet (UV) LED chip, and a warm white LED with good performance can be obtained, which shows a bright potential application in the field of white LED.

The structural, mechanical and electrical properties of 2D SiC with C-related point defects and substitution of C by foreign atoms

As a wide and direct bandgap semiconductor material, two-dimensional (2D) graphene-like SiC has attracted extensive research attention recently. Native defects and foreign impurities often have remarkable effects on the properties of semiconductors. In this paper, the structural, mechanical and electrical properties of 2D SiC with carbon antisite (CSi), vacancy (VC), interstitial (Ci) and substitution of C by N (NC) and B (BC) have been studied systematically using first-principles calculations. It is found that defects do not cause structural reconstruction for 2D SiC. The formation energies of CSi, VC, NC and BC are not sensitive to the uniaxial strain. CSi and Ci are likely to be formed spontaneously due to the negative formation energy values. In addition, the phonon spectra and elastic constant calculations show that the properties of 2D SiC with VC and Ci are unstable. Furthermore, from the calculations of the band structures and density of states, we observed that the bandgap of 2D SiC is slightly reduced by CSi, VC, Ci, and NC, however is increased by BC. Prominent additional charge states are generated in the bandgap of 2D SiC except for NC. Moreover, the overall carrier mobility of 2D SiC could be significantly reduced by the defects.

Perovskite micro-nano lasers and on-chip integration

<p indent="0mm">With the rapid development of photonic technologies and micro-nano fabrication technologies, optoelectronic devices tend to be miniaturized and integrated. Photonic integrated circuits (PICs) can adress the bottleneck problems of electronic chips, such as high-power consumption and slow computing speed. Micro-nano lasers are an important part in PICs, and they are the key device to generate optical signals. Perovskites are direct band gap semiconductor materials with high exciton binding energy and low defect densities. Therefore, they have high photoluminescence quantum yield and can be used as gain materials for micro-nano lasers. In addition, perovskite materials also have advantages of low non-radiation recombination rate, low manufacturing cost, tunable bandgap width, solution treatment, and so on. Because of these advantages, perovskite materials have been widely used in micro-nano lasers and other fields. At present, researchers have developed a variety of micro-nano fabrication methods to fabricate perovskite micro-nano structures. A variety of perovskite micro-nano lasers have been obtained by using perovskite micro-nano structures under the pump of pulses or continuous lasers. In PICs, it is necessary to use optical waveguides to couple laser sources to other photonic devices to form a complete photonic circuit. This paper first summarizes the synthesis methods of perovskite materials, including spin coating methods, cooling crystallization methods, inversion crystallization methods, antisolvent crystallization methods, space limited methods, and gas phase epitaxial growth methods. Among these synthesis methods, spin coating methods are simple, but only polycrystalline perovskite materials can be prepared. Single crystal perovskites can be prepared by cooling crystallization methods, inversion crystallization methods, antisolvent crystallization methods, space limited methods, and gas phase epitaxial growth methods. Next, fabrication methods of perovskite micro-nano structures are introduced, including space limited synthetic growth methods, nano imprinting methods, focused ion beam etching methods, and electron beam exposure methods. Space limited synthetic growth methods and nano imprinting methods have no damage to perovskite materials. Electron beam exposure methods have small damage to perovskite materials, and focused ion beam etching methods cause great damage to perovskite materials. Then, perovskite micro-nano lasers, including out-plane emission lasers and in-plane emission lasers, are introduced. Out-plane emission lasers emit the laser towards the free space, and they are not favorite for on-chip integration. In-plane emission lasers emit the laser towards the in-plane space, and they are easy to be integrated with the waveguide. In addition, the combination of perovskites and metal structures is expected to reduce the size of the laser to subwavelength scales based on plasmonic effects. We also introduce the on-chip integration of perovskite micro-nano lasers, including tip manipulation methods, overlay alignment methods, and transfer printing methods. Tip manipulation and overlay alignment methods can damage perovskite materials. Transfer printing methods do not cause damage to perovskite materials. Finally, the development trends of perovskite materials and micro-nano lasers are discussed. Although perovskite micro-nano lasers have made great progress, there are still some problems to be addressed. For example, lead in perovskite materials is toxic, so it is necessary to develop lead-free perovskite materials. Perovskite materials have poor stability, and their stability should be improved. Moreover, the realization of electrically pumped micro-nano lasers and on-chip integration is also a great challenge.

An experimental and theoretical study of the structural, optical, electrical, and dielectric properties of PrAsO4

This study reports the synthesis (by the co-precipitation method) and characterization of the PrAsO 4 polycrystals. The crystal structure has been determined using the Rietveld method. Two validation methods were used to confirm the structural model: the bond valence sum (BVS) and the charge distribution (CHARDI). A constructive description of the structure was made, starting with the connection modes of the coordination polyhedron. The electronic structure of the compound, determined by the DFT method, shows a direct band gap semiconductor material with a calculated energy of 2.21 eV. This compares favorably with the experimental band gap energy value of about 2.52 eV. The microstructural study showed grain and crystalline sizes of about 500 and 17 nm, respectively. TGA analysis was used to study the thermal stability of the compound, which showed a plateau in the 550 − 650 °C range due to the oxidation of Pr 3+ to Pr 4+ . This oxidation was confirmed by the emission spectra of Pr 3+ obtained from the photoluminescence study. The electrical study in the 440–600 °C temperature range shows that the PrAsO 4 is a bad ionic conductor with a value of about 10 −8 Scm −1 . The dielectric study showed a value of 1400 for the dielectric constant ε, and the dipolar polarization dominated the polarization phenomenon. About 10 −6 s was obtained for the relaxation time (τ r ) • The crystal structure of PrAsO4 was determined Rietveld refinement and confirmed by two validation models (BVS) and CHARDI. • The electronic structure was determined by DFT method which shows a direct band gap with energy of 2.21 eV. • The experimental band gap energy is of about 2.52 eV. • The thermal analysis shows the oxidation of Pr3 + to P4 + in the 550–650 °C temperature range. • The electrical measurement shows that PrAsO4 has a conductivity of about 10 −8 Scm −1

Evaluation of structural, morphological, optical, and electrical properties of zinc oxide semiconductor nanoparticles with microwave plasma treatment for electronic device applications

ZnO is an important II-IV n-type direct bandgap semiconductor material that has exhibited wide consideration due to its applications in optoelectronic devices. In the present investigation, untreated and plasma-treated ZnO NPs were fabricated by a sol-gel route and analyzed with XRD, Raman, UV-Vis spectra, and I–V characteristic techniques. To be used in electronic applications, the ZnO nanoparticles were subjected to a plasma treatment process to tune their optical, vibrational mode, structural and electrical properties. XRD confirmed the pure Wurtzite tetragonal phase crystalline nature of produced samples. The average crystalline size, lattice constant, unit cell volume, and porosity were found in the range of 3.81–4.68 nm, a = 3.217–3.323 Å and c = 2.032–2.178 Å, 5.662–11.274 Å3 and 7.09–5.03%, respectively. The Raman spectra revealed that the intensity of Raman bands was increased due to the increase in the vibrational amplitude and increase in force constants with the enhancement in grain size, these spectra results are in good agreement with XRD and reveal the purity of samples. The bandgap found reduced from 3.56 eV to 3.39 eV by plasma treatment. The electrical studies revealed that plasma treatment improves the electrical properties of ZnO NPs without producing any structural distortions. I–V characteristic curves confirmed that the conductivity was increased from 8.3 X 10−6 to 5.5 X 10−4 ℧ cm−1 on plasma treatment which caused an increase in leakage current. The improved structural, vibrational mode, optical and electrical properties of prepared ZnO NPs make them a suitable candidate for application in electronic devices.

Effect of high pressure on structural, electrical, and optical properties of graphene-like zinc oxide (g-ZnO) structure

One of the most efficient strategies to facilitate the zero-bandgap challenge in graphene is to discover graphene-like materials with intrinsic electronic bandgap and tunable properties under and beyond ambient conditions. Fortunately, zinc oxide (ZnO) is an example of such materials that offer a graphene-like structure with naturally existing electronic bandgap as well as excellent properties even in its pristine format. Inspired by this motivation, this study comprehensively surveys the structural, electronic, and enhanced optical properties of graphene-like zinc oxide (g-ZnO) structure at ambient and high-pressure conditions. As a result, it is found that the g-ZnO is a direct bandgap semiconductor material, and its bandgap values can be tuned to higher ranges under the effect of hydrostatic pressure (0–50 GPa). Additionally, the pressure-induced optical parameters are observed in good accordance with experimental measurements. Finally, this study motivates the bandgap tunability in g-ZnO to meet bandgap-related problems and new opportunities for this material in optoelectronic and photovoltaic applications.

Theoretical exploration of mechanical, electronic structure and optical properties of aluminium based double halide perovskite.

The mechanical, electronic structure and optical properties of aluminium based double halide perovskite were calculated by density functional theory. The formation energy and elastic constant confirm the stability of the cubic perovskite materials. The materials are all ductile and suitable for flexible photovoltaic and optoelectronic devices. The band gap values vary from 0.773 eV to 3.430 eV, exactly corresponding to the range of ideal band gap values for good photoresponse. The band structure analysis shows that all the materials possess small effective mass, which indicates a good transport of carriers. And these materials have a broad energy range of optical absorption for utilization and a detector of photons. Moreover, less expensive K2AgAlBr6 were investigated for comparison with materials containing a cesium element, and according to the results, is also a candidate for photoelectronic devices due to the similar properties.

First-principles study of influence of electric field on electronic structure and optical properties of GaN/g-C&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt; heterojunction

In this paper, the stability, electronic structure, optical properties, and work function of GaN/g-C&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt; heterojunction are studied by using the first-principles plane wave ultra-soft pseudopotential method based on density functional theory. The electric field effect is also considered. The results show that the total energy for each of the three stacking modes changes little for using the two different dispersion correction methods, i.e. Tkatchenko-Scheffler and Grimme, and the total energy of mode II is the lowest, indicating that the structure of mode II is the most stable. The lattice mismatch ratio and lattice mismatch energy of GaN/g-C&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt; van der Waals heterojunction are very low, indicating that the heterojunction has good stability. The heterojunction retains the basic electronic properties of GaN and g-C&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt; to a great extent and can be used as a direct bandgap semiconductor material. It can be known from the work function and differential charge diagram that the charge on the heterojunction interface is transferred from GaN to g-C&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt;, and a built-in electric field orientating g-C&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt; from GaN is formed at the interface. The built-in electric field of the heterojunction can effectively separate the photogenerated electron-hole pairs, which is conducive to improving the photocatalytic capability of the system. Further analysis shows that the applied electric field reduces the bandgap of GaN/g-C&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt; heterostructure to varying degrees. It makes it easier for electrons to transit from valence band to conduction band, which is conducive to improving the photocatalytic activity of the system. In addition, when the applied electric field is –0.6 V/Å and 0.5 V/Å separately, the semiconductor metal phase transition occurs in the heterojunction. When the applied electric field is higher than 0.3 V/Å and lower than –0.4 V/Å, in the energy band arrangement of the heterojunction there occurs the transition from type I to type II. This can better realize the separation of photogenerated electron-hole pairs and further improve the photocatalytic capactivity of the system. Therefore, the construction of heterojunction and application of external electric field proposed in this work constitute an effective means to improve the photocatalytic activity of the system.