Quasi-direct Band Gap Semiconductor Research Articles

S-C3N6 monolayer by atomic doping serving as solar cells and photocatalyst

Atomic doping is an effective method to adjust the electronic and optical properties of s-C3N6. Herein, we studied the effects of atomic doping (S, Cl, Na and Mg) on the electronic, optical and redox properties of s-C3N6 monolayer by first-principles calculations. We consider the substitutional doping of non-equivalent C or N atoms and interstitial doping at the hole position. The results show that atomic doping can effectively adjust the band gap of s-C3N6 (2.59 eV) monolayer. Most notably, we found that S(Nl)-s-C3N6 is a quasi-direct band gap semiconductor with a band gap of 1.27 eV and the theoretical photovoltaic efficiency of S(Nl)-s-C3N6 is 31.84%, which indicates it has the potential to be used in thin-film solar cells. In addition, Cl6@s-C3N6 meets the conditions of photocatalytic water splitting at pH = 7, and the Mg6@s-C3N6 monolayer appears the reduction ability. Meanwhile, the atomic doped s-C3N6 monolayer enhances the absorption intensity in the visible light range. The emergence of redox capacity and the enhancement of the absorption coefficient in carbon nitride monolayer via atomic doping reveals an exciting material platform for designing novel photovoltaic cells and nanoelectronics devices.

Two-Dimensional Planar Penta-NiPN with Ultrahigh Carrier Mobility and Its Potential Application in NO and NO2 Gas Sensing.

Two-dimensional (2D) materials with novel structures and electronic properties are promising candidates for the next generation of micro- and nano-electronic devices. Herein, inspired by the recent experimental synthesis of penta-NiN2 (ACS Nano, 2021, 15, 13539-13546), we propose for the first time a novel ternary penta-NiPN monolayer with high stability by partial element substitution. Our predicted penta-NiPN monolayer is a quasi-direct bandgap (1.237 eV) semiconductor with ultrahigh carrier mobilities (103-105 cm2V-1s-1). Furthermore, we systematically studied the adsorption properties of common gas molecules (CO, CO2, CH4, H2, H2O, H2S, N2, NO, NO2, NH3, and SO2) on the penta-NiPN monolayer and its effects on electronic properties. According to the energetic, geometric, and electronic analyses, the penta-NiPN monolayer is predicted to be a promising candidate for NO and NO2 molecules. The excellent electronic properties of and the unique selectivity of the penta-NiPN monolayer for NO and NO2 adsorption suggest that it has high potential in advanced electronics and gas sensing applications.

A larger-cell tetragonal carbon allotrope tP176 carbon with superhardness

• A new larger carbon allotrope t P176 carbon with P 4/ nbm phase is first proposed. • The t P176 carbon is a potentially superhard material with hardness of 55 GPa. • t P176 carbon is a wide and quasi-direct band gap semiconductor material. A new allotrope of sp 3 hybrid superhard carbon, t P176 carbon, is proposed and investigated in this work. The t P176 carbon is dynamically, thermodynamically and mechanically stable, respectively. The bulk modulus ( B ) and shear modulus ( G ) of t P176 carbon are 242 GPa, and 192 GPa, respectively. In addition, t P176 carbon is a wide band gap semiconductor material with quasi-direct band gap, and the band gap is 2.892 eV. The hardness of t P176 carbon is 55 GPa, which implies that it is a potentially superhard material.

Theoretical Investigations of a BN Polymorph with sp2 + sp3 Hybridizations

The crystal structure, mechanical anisotropy, elastic properties and electronic characteristics, as well as the stability, of P4/m BN are predicted by means of density functional theory. In this work, BN in the P4/m phase demonstrates mechanical and dynamical stability. Compared with the values of bulk B, E and G in the P4/m phase, the B of BN in the P4/m phase is greater than that of dz4 BN, while the G and E of P4/m BN are greater than those of Pnc2 BN and dz4 BN. The ratio of the bulk-to-shear modulus for P4/m BN is less than 1.75 and dz4 BN, dz2 BN and lzlz2 BN, indicating that P4/m BN is more brittle than dz4 BN, dz2 BN and lzlz2 BN. P4/m BN exhibits stronger mechanical anisotropy in G and E than Pbca BN, P42/mnm BN and Pm-3m BN but much weaker mechanical anisotropy than P4/mbm BN, B7N7, B11N11 and B15N15. In addition, P4/m BN is a quasi-direct bandgap semiconductor, and the difference between the direct and the indirect bandgap is 0.008 eV. In order to obtain further characteristics of P4/m BN for future synthetic verification, the X-ray diffraction (XRD) patterns for P4/m BN are also calculated. Given its properties, P4/m BN is a good candidate for photoelectric devices.

Physical properties of group 14 elements in P2/m phase

Physical properties of J-carbon, J-silicon, and J-germanium (space group: P2/m) were studied by density functional theory calculations. J-germanium was found to be dynamically, mechanically and thermodynamically stable under ambient pressure. The bulk modulus B and the shear modulus G of J-germanium are 55 ​GPa and 41 ​GPa, respectively, slightly greater than those of P42/mnm Ge, hP12 Ge and oC12 Ge. J-carbon, J-silicon, and J-germanium exhibit a larger anisotropy in Young's modulus than do carbon, silicon, and germanium in the diamond phase and also exhibit larger anisotropy in bulk modulus and shear modulus than do the corresponding diamond phase allotropes. J-germanium is a quasi-direct band gap semiconductor material with the band gap of 0.943 ​eV that may be a candidate material for use in photovoltaic devices. The band gap and the minimum thermal conductivity κmin of J-carbon are both greater than those of diamond, and therefore, J-carbon may be a promising candidate material for deep ultraviolet photodetection.

Direct and quasi-direct band gap of novel Si-Ge alloys in P-3m1 phase

This work investigates the crystal structure, stability, mechanical properties, electronic properties, effective masses, and optical properties of Si-Ge alloys in the P-3m1 phase. The elastic constants and phonon spectra proven that the Si-Ge alloys in the P-3m1 phase have mechanical and dynamic stability. The bulk modulus, shear modulus and Young’s modulus of Si-Ge alloys in the P-3m1 phase decrease with the increase of Ge composition, and the three-dimensional diagram of Young's modulus and effective mass show that the mechanical and transport properties have anisotropy. The Si12Ge12 in the hP24 phase is a quasi-direct band gap semiconductor material with a band gap of 1.081 eV, while the Si30−x Ge x alloy (x = 6, 12, 18, 24) in the hP30 phase are all direct band gap semiconductor materials with the band gaps of 0.541 eV, 0.430 eV, 0.561 eV, and 0.387 eV, respectively. The hP30-Si6Ge24 has a very small effective electron mass. The hP24-Si12Ge12 show excellent absorptive capacity in the visible and infrared region region. Based on this work, Si-Ge alloys in the P-3m1 phase are promising materials for photovoltaic applications.

Mechanical anisotropy and electronic properties of X2N2(XH2): first-principles calculations**Project supported by the National Natural Science Foundation of China (grant numbers 61864004 and 61564005).

The mechanical anisotropy, structural properties, electronic band structures and thermal properties of C2N2(CH2), Si2N2(SiH2) and Ge2N2(GeH2) are detailed and investigated in this work. The novel silicon nitride phase Si2N2(SiH2) and germanium nitride phase Ge2N2(GeH2) in the Cmc21 structure are proposed in this work. The novel proposed Si2N2(SiH2) and Ge2N2(GeH2) are both mechanically and dynamically stable. The electronic band calculation of the HSE06 hybrid functional shows that C2N2(CH2), Si2N2(SiH2) and Ge2N2(GeH2) are all wide band gap semiconductor materials, and C2N2(CH2) and Si2N2(SiH2) are direct band gap semiconductor materials, while Ge2N2(GeH2) is a quasi-direct band gap semiconductor material, the band gap of C2N2(CH2), Si2N2(SiH2) and Ge2N2(GeH2) are 5.634 eV, 3.013 eV, and 2.377 eV, respectively. The three-dimensional and plane distributions of Young’s modulus, shear modulus and Poisson’s ratio of C2N2(CH2), Si2N2(SiH2) and Ge2N2(GeH2) show that these materials have different degrees of mechanical anisotropy. The order of Young’s modulus of Si2N2(SiH2) and Ge2N2(GeH2) in different directions is different from that of C2N2(CH2). When the tensile axis is in a particular direction, the order of the Young’s modulus of Si2N2(SiH2): E[110] < E[120] < E[111] < E[101] < E[010] = E[100] < E[011] < E[001], and the order of the Young’s modulus of Ge2N2(GeH2): E[110] < E[111] < E[101] < E[120] < E[100] < E[010] < E[011] < E[001].

A new cubic superhard large-cell carbon allotrope: c-C200

A new cubic superhard large-cell carbon allotrope with 200 carbon atoms per unit cell (named c-C200) is theoretically predicted by the unbiased swarm structure searching techniques combined with the first-principles calculations. The phonon and elastic constants calculations show that c-C200 is dynamically and mechanically stable. The calculated hardness of c-C200 is 47 GPa, which means that c-C200 is a potential superhard material. The calculated electronic band structure of c-C200 shows that it is a quasi-direct band gap semiconductor with a gap of 2.781 eV.

Erratum to: 2D Hexagonal SnTe monolayer: a quasi direct band gap semiconductor with strain sensitive electronic and optical properties

Erratum to: 2D Hexagonal SnTe monolayer: a quasi direct band gap semiconductor with strain sensitive electronic and optical properties

2D Hexagonal SnTe monolayer: a quasi direct band gap semiconductor with strain sensitive electronic and optical properties

The stability and electronic and optical properties of two-dimensional (2D) SnTe monolayer has been systematically studied by using first-principles calculations based on density functional theory. Our computations demonstrate that the predicted 2D SnTe monolayer is a stable quasi-direct semiconductor. Also, analysis of its electronic property shows that the ground state of this monolayer is a quasi-direct semiconductor with a band gap of ~2.00. This band gap can be effectively modulated by external strains. Investigation of optical properties shows that monolayer SnTe exhibits significant absorption and reflectivity in the ultraviolet region of the electromagnetic spectrum.

Correction to: Strain-modulated mechanical, electronic, and thermal transport properties of two-dimensional PdS2 from first-principles investigations

Strains can effectively modulate the electronic structure and thermal conductivity of materials. In this work, the electronic structure and thermal conductivity properties of PdS2 under different strains were investigated using first-principles calculation combined with iterative method for solving Boltzmann transport equation theory. Through phonon spectrum calculations, it is found that PdS2 is thermodynamically stable in the strain range from 0 to 10%. Interestingly, the strained single-layer PdS2 transforms from an indirect bandgap semiconductor to a quasi-direct bandgap semiconductor, and the bandgap of PdS2 decreases to 1.41 eV (decreased by 20.8%). Due to the softening of the phonons and the decreasing of phonons group velocity, the thermal conductivity is reduced with the applied biaxial area strains. Thermal transport investigations reveal that the in-plane thermal conductivity of unstrained PdS2 is 32.32 Wm− 1 K− 1. When the area strain reaches 10%, the thermal conductivity of PdS2 is reduced by nearly twice the ratio of strain. The sensitive strain dependence of bandgap and thermal conductivity indicates that PdS2 can flexibly select the substrate and match the substrate, and the thermoelectric coefficient can be effectively adjusted, indicating that PdS2 has good application prospects in thermoelectric, photoelectric, and catalytic materials.

A DFT based prediction of a new 2D zirconium disulfide Pmmm-ZrS2 monolayer: A quasi direct band gap semiconductor

Abstract A new 2D stable crystal of zirconium-sulfur compound namely Pmmn-ZrS2 monolayer by using density functional theory is predicted. Our computations confirm that our proposed ZrS2 monolayer indicates good energetic, kinetic, and thermal stabilities as proved by the crystal energy analysis, phonon modes computation, and the molecular dynamic assessment respectively. Investigation in its electronic properties shows the new Pmmn-ZrS2 monolayer is a quasi direct semiconductor with a band gap of about 1.53 eV obtained by HSE06 hybrid functional method can be modulated by external in-plane strain effects. These results suggest this monolayer as a useful material for application in nano-electronic devices.

Theoretical investigations of Ge1−xSn x alloys (x = 0, 0.333, 0.667, 1) in P42/ncm phase

The structural, mechanical, anisotropic and electronic properties of Ge, Sn and their alloys (Ge0.667Sn0.333 and Ge0.333Sn0.667) in P42/ncm phase were studied in the framework of the density functional theory. In the present work, the lattice parameters increase with the increased tin concentration, whereas the elastic constants and elastic modulus of Ge1−xSn x alloys (x = 0, 0.333, 0.667, 1) in P42/ncm phase decrease with the increased tin concentration. A novel Sn allotrope with remarkable stability (the energy is higher than Sn in diamond phase only 24.4 meV per atom) in P42/ncm phase is first proposed in the present work, and it is a quasi-direct band gap semiconductor material with bang gap of 1.02 eV. The dynamically, mechanically and thermodynamically stable forms of P42/ncm-Sn and the Ge–Sn alloys in P42/ncm phase are proved by phonon spectra, elastic constants, formation enthalpy and cohesive energies. The universal elastic anisotropy index AU illustrates that the anisotropy of Ge1−xSn x alloys (x = 0, 0.333, 0.667, 1) in P42/ncm phase increases with the increasing percentage of the tin composition. To obtain the details of the anisotropy, the Young’s modulus, bulk modulus, shear modulus and Poisson’s ratio are also systematically investigated. The electronic band structures of Ge1−xSn x alloys (x = 0, 0.333) in P42/ncm phase with HSE06 hybrid functional show that both of them are indirect band gap semiconductors, while Ge0.333Sn0.667 and Sn in the P42/ncm phase show that they are quasi-direct band gap semiconductor materials.

A reinvestigation of a superhard phase Cm-carbon

Cm-carbon, reported in this manuscript, was first reported by Zhang et al. [40] as Cm-40 carbon. Cm phase carbon was investigated in this work using first-principles calculations. The mechanical, dynamic, and thermal stabilities are confirmed by carrying out calculations of elastic constants, phonon spectra and enthalpy. Cm-carbon is a quasidirect band gap semiconductor material with a band gap of 3.65 eV. It is predicted that the band gap of Cm-carbon increases with increasing pressure. The elastic anisotropy calculations show that Cm-carbon exhibits a smaller elastic anisotropy than C2/m-20 carbon, while it exhibits a larger elastic anisotropy than C2/m-16 carbon and M-carbon in Young’s modulus. C2/m-20 carbon exhibits a greater elastic anisotropy than the other monoclinic symmetry phases with regard to the universal elastic anisotropy index AU. The hardness of Cm-carbon is 63.6 GPa for the model of Lyakhov and Oganov and 87.2 GPa for the Ma’s model, revealing that Cm-carbon is a potential superhard material.

Stability, elastic anisotropy, and electronic properties of Ca2C3**Project supported by the National Natural Science Foundation of China (Grant Nos. 11204007 and 61474089), the Natural Science Basic Research Plan in Shaanxi Province, China (Grant Nos. 2016JM1026 and 2016JM1016), and the Natural Science Foundation from Education Committee of Shaanxi Province, China (Grant Nos. 16JK1049 and 16JK1016).

The systematic investigations of the mechanical, elastic, and electronic properties, and stability of the newly synthesized monoclinic C2/m-Ca2C3 are performed, based on the first-principles calculations. Ca2C3 is found to be mechanically and dynamically stable only from 0 GPa to 24 GPa. The elastic anisotropy studies show that Ca2C3 exhibits the elastic anisotropy increasing with the augment of pressure. Furthermore, using the HSE06 hybrid functional, the electronic properties of Ca2C3 under pressure are calculated. The structure can be regarded as a quasi-direct band gap semiconductor, and the pressure-induced direct-indirect band gap transition is studied in detail.

Topological insulating states in 2D transition metal dichalcogenides induced by defects and strain

First-principles calculations and extensive analyses reveal that the H phase of two-dimensional (2D) transition metal dichalcogenides (TMDs) can be tuned to topological insulators by introducing square-octagon (4-8) defects and by applying equi-biaxial tensile strain simultaneously. The 2D structure composed of hexagonal rings with 4-8 defects, named sho-TMD, is dynamically and thermally stable. The critical equi-biaxial tensile strain for the topological phase transition is 4%, 6%, and 4% for sho-MoS2, sho-MoSe2 and sho-WS2, respectively, and the corresponding nontrivial band gap induced by the spin-orbit coupling is 2, 8, and 22 meV, implying the possibility of observing the helical conducting edge states that are free of backscattering in experiment. It is equally interesting that the size of the energy band gap of the H-phase can be flexibly tuned by changing the concentration of 4-8 defects while the feature of the quasi-direct band gap semiconductor remains. These findings add additional traits to the TMD family, and provide a new strategy for engineering the electronic structure and the band topology of 2D TMDs for applications in nanoelectronics and spintronics.

A new superhard carbon allotrope: tetragonal C64

A novel superhard carbon allotrope C64 is predicted which is composed of C28 cages. It is porous and exhibits distinct topologies including zigzag 5, 6, 8, 10 and 12-fold carbon rings. The elastic constants and phonon calculations reveal that C64 is mechanically and dynamically stable at ambient pressure. The hardness of C64 is 60.2 GPa. The tensile and shear strength calculations indicate that the lowest tensile and shear strengths have the almost same value of 48.1 GPa. For the electronic properties, the band structure calculations show that C64 is a quasi-direct band gap semiconductor with a band gap of 1.32 eV.

The Mechanical and Electronic Properties of Carbon-Rich Silicon Carbide.

A systematic investigation of structural, mechanical, anisotropic, and electronic properties of SiC2 and SiC4 at ambient pressure using the density functional theory with generalized gradient approximation is reported in this work. Mechanical properties, i.e., the elastic constants and elastic modulus, have been successfully obtained. The anisotropy calculations show that SiC2 and SiC4 are both anisotropic materials. The features in the electronic band structures of SiC2 and SiC4 are analyzed in detail. The biggest difference between SiC2 and SiC4 lies in the universal elastic anisotropy index and band gap. SiC2 has a small universal elastic anisotropy index value of 0.07, while SiC2 has a much larger universal elastic anisotropy index value of 0.21, indicating its considerable anisotropy compared with SiC2. Electronic structures of SiC2 and SiC4 are calculated by using hybrid functional HSE06. The calculated results show that SiC2 is an indirect band gap semiconductor, while SiC4 is a quasi-direct band gap semiconductor.

Fabry-Perot Microcavity Modes in Single GaP/GaNP Core/Shell Nanowires.

Semiconductor nanowires (NWs) are attracting increasing interest as nanobuilding blocks for optoelectronics and photonics. A novel material system that is highly suitable for these applications are GaNP NWs. In this article, we show that individual GaP/GaNP core/shell nanowires (NWs) grown by molecular beam epitaxy on Si substrates can act as Fabry-Perot (FP) microcavities. This conclusion is based on results of microphotoluminescence (μ-PL) measurements performed on individual NWs, which reveal periodic undulations of the PL intensity that follow an expected pattern of FP cavity modes. The cavity is concluded to be formed along the NW axis with the end facets acting as reflecting mirrors. The formation of the FP modes is shown to be facilitated by an increasing index contrast with the surrounding media. Spectral dependence of the group refractive index is also determined for the studied NWs. The observation of the FP microcavity modes in the GaP/GaNP core/shell NWs can be considered as a first step toward achieving lasing in this quasidirect bandgap semiconductor in the NW geometry.