Indirect-direct Band Gap Transition Research Articles

Strain engineering of electronic, mechanical, and optical properties of orthorhombic III–V group monolayers by first principles calculations

Using first principles calculations, we systematically investigated the effects of strain engineering on the electronic, mechanical, and optical properties of two-dimensional (2D) orthorhombic III–V group materials, including BN, BP, BAs, AlN, AlP, and GaN. It is shown that all the III–V orthorhombic monolayers exhibit excellent mechanical anisotropy for Young’s modulus, Shear modulus, and Poisson’s ratio, especially for the AlN and GaN monolayers. AlN, AlP, and GaN are predicted to be indirect bandgap semiconductors, with their bandgap of 0.70, 0.15, and 0.53 eV, respectively. And BN is demonstrated to be a direct bandgap semiconductor (0.63 eV). Under uniaxial tensile strains, their electronic structures have non-monotonic anisotropic variations and these monolayers can be effectively modulated from metal to semiconductor, experiencing indirect–direct bandgap transitions. In addition, all the orthorhombic III–V materials exhibit highly anisotropic light-harvesting performances and the optical absorbance can be efficiently tailored with tensile strains applied along a- and b-directions. The strong optical absorptions in the visible light regions suggested that AlN, BN, and GaN may be optically tunable 2D materials for component absorbance layers for solar cell applications. The excellent anisotropic and tunable electronic, mechanical, and optical performances indicate that the orthorhombic III–V monolayers are promising candidates for potential applications of optoelectronics and photovoltaics.

Ab-initio investigation of the structural, electronic and optical properties of CsPbBr3 perovskite under pressure using the DFT-1/2 method

Pressure-induced phase transitions in metal halide perovskites lead to significantly different material properties and offer great potential for diverse applications. The electronic, structural, and optical properties of Cesium lead bromide (CsPbBr3) crystals are investigated using density functional theory (DFT) to draw an integrated picture of orbital morphology, absorption spectra, and band gap values. Spin-orbit coupling and DFT-1/2 correction are used to gain a better insight into electronic properties and band splitting in the cubic phase. Calculations of pressure-induced phase transitions, electronic band gap engineering, and manipulation of optical absorption edges of CsPbBr3 are conducted. In the cubic and tetragonal crystal lattices, a band gap closure and opening occurs demonstrating a Dirac cone upon application and increase of pressure. Indeed, in the tetragonal lattice of CsPbBr3, a topological band occurs. Orthorhombic perovskite crystal lattice undergoes successive direct-indirect and indirect-direct band gap transitions, followed by a space group change at extreme pressures. However, under pressure, orthorhombic non-perovskite lattice experiences a sudden band gap breakdown. Phase and band gap transitions are explained by orbital hybridizations and structural symmetry evolution (e.g., bond length contraction/dilation and octahedral cage rotation/distortion). These results provide comprehensive guidelines for further experimental studies on pressure engineering, advanced photovoltaic and optoelectronic applications, discovering and understanding new topological band structures of perovskite materials.

Fundamental properties of the MgCl2 monolayer from first-principles calculations

The band gap of 2D MgCl2 can be modulated flexibly in a large range by applied strain, layer control and cutting into nanoribbons and a direct–indirect band gap transition can be obtained.

Effect and mechanism analysis of surface hydrogenation and fluorination on the electronic properties of th-GeC2.

A two-dimensional (2D) tetrahex-GeC2 nanosheet demonstrates excellent electronic properties such as a finite direct band gap and high carrier mobilities, as predicted from theoretical calculations. To further expand its potential applications, various strategies can be employed to tailor its electronic properties. These strategies include alloying, strain application, and edge and surface functionalization. This work specifically focuses on the impact of surface functionalization with hydrogen and fluorine adsorption on the 2D tetrahex-GeC2 nanostructures. It was discovered that the electronic properties of these nanostructures undergo significant alterations through surface functionalization by adjusting the adsorption sites and coverage of H/F species. The underlying mechanisms responsible for these property changes have been thoroughly analyzed and discussed in detail. Our calculations, based on density functional theory, reveal that the band gap of tetrahex-GeC2 widens as the surface coverage of H atoms increases. Conversely, the band gap narrows in the case of F adsorption. Additionally, the indirect-direct band gap transition can be triggered through surface functionalization. Such modifications in the electronic band structure are primarily due to the disappearance of the π bond when the C atom is converted from sp2 to sp3 hybridization through the adsorption of surface functionalized species. Furthermore, the results indicate that surface adsorption can regulate the effective mass of carriers, electron affinity, and work function in the 2D tetrahex-GeC2 nanostructure.

Synthesis and characterization of MoSe2 nanoscrolls via pulsed laser ablation in deep eutectic solvents.

There is ongoing interest in the rapid, reproducible production of 2-dimensional (2-D) transition metal dichalcogenides (TMD), such as molybdenum-based TMD (MoX2), where X is a chalcogen atom such as sulphur (S), selenium (Se) or tellurium (Te), driven by their unique optical and electronic properties. Once fabricated into an atomically thin layer structure, these materials have a direct-indirect bandgap transition, strong spin-orbit coupling, and favourable electronic and mechanical strain-dependent properties which are attractive for electronics. Pulsed laser ablation in liquid (PLAL) is an economic, green alternative for synthesis of TMD. It has been shown that in the case of MoX2, the chemical processes during the plasma phase of the ablation can yield the formation of multispecies, including MoOx quantum dots when oxygen-containing solvents are used. Here, we introduce the formation of MoSe2 nanoscrolls with low oxygen content synthesized via pulsed laser ablation in deep eutectic solvents (PLADES). Our results suggest that the synthesis produces a stable colloidal solution of large 2-D structures with tuneable surface charge by replacing the deep eutectic solvent (DES) with DI water. Nuclear Magnetic Resonance (NMR) results suggest that irradiating the solvent at near infrared NIR energy does not affect its chemical composition. NMR also proves that serial washing can completely remove solvent from the nanostructures. Raman shifts suggest the formation of large, thin MoSe2 nanosheets aided by the solvent confinement resulting from van der Waal forces and hydrogen bonds interactions between MoSe2 and urea. Binding energies measured by X-ray photoelectron spectroscopy (XPS) confirm MoSe2-DES preference to form 1T-MoSe2versus molybdenum oxides and 2H MoSe2 in DI-water. Raman and XPS findings were validated by transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Results of this work validate the use of PLADES for the synthesis of stable, crystalline, low-surface-oxygen-content colloidal MoSe2 nanoscrolls in scalable quantities.

Modulating electronic properties of β-Ga2O3 by strain engineering

β-Ga2O3 is a promising material for the development of next-generation power electronic and optoelectronic devices due to its exceptional properties, including ultrawide bandgap and thermodynamic stability. Strain engineering has emerged as a powerful method to modulate the physical properties of materials and has been widely employed in semiconductor devices to enhance their performance and functionality. Our study focuses on the effects of strain engineering on the electronic properties of β-Ga2O3. Using density functional theory, we calculated the band structures and electron effective masses of β-Ga2O3 under different strain states. Our investigation revealed that strain manipulation can induce an indirect-direct bandgap transition. Strain can also lead to changes in effective masses and anisotropy of electron mobility. Our calculations provide important insights into the potential of strain engineering as a powerful tool for modulating the electronic properties of β-Ga2O3, with important implications for practical device applications.

First principles study of 2D ring-Te and its electrical contact with a topological Dirac semimetal.

In recent years, researchers have manifested their interest in two-dimensional (2D) mono-elemental materials of group-VI elements because of their excellent optoelectronic, photovoltaic and thermoelectric properties. Despite the intensive recent research efforts, there is still a possibility of novel 2D allotropes of these elements due to their multivalency nature. Here, we have predicted a novel 2D allotrope of tellurium (ring-Te) using density functional theory. Its stability is confirmed by phonon and ab initio molecular dynamics simulations. Ring-Te has an indirect band gap of 0.69 eV (1.16 eV) at the PBE (HSE06) level of theories and undergoes an indirect-direct band gap transition under tensile strain. The higher carrier mobility of holes (∼103 cm2 V-1 s-1), good UV-visible light absorption ability and low exciton binding (∼0.35 eV) of ring-Te give rise to its potential applications in optoelectronic devices. Furthermore, the electrical contact of ring-Te with a topological Dirac semimetal (sq-Te) under the influence of an electric field shows that the Schottky barriers and contact types can undergo transition from p-type to n-type Schottky contact and then to ohmic contact at a higher electric field. Our study provides an insight into the physics of designing high-performance electrical coupled devices composed of 2D semiconductors and topological semimetals.

Effect of strain and electric field on electronic structure and optical properties of Ga<sub>2</sub>SeTe/In<sub>2</sub>Se<sub>3</sub> heterojunction

Stacking two-dimensional materials into heterogeneous structures is an effective strategy to regulate their physical properties and enrich their applications in modern nanoelectronics. The electronic structure and optical properties of a new two-dimensional Janus Ga<sub>2</sub>SeTe/In<sub>2</sub>Se<sub>3</sub> heterojunction with four stacked configurations are investigated by first principles calculations. The heterojunction of the four configurations is an indirect band-gap semiconductor with a type-II band structure, and the photoelectron donor and acceptor materials are determined by the polarization direction of two-dimensional In<sub>2</sub>Se<sub>3</sub>. The light absorption rises to 25% in the visible region, which is conducive to the effective utilization of the solar visible light. The biaxial strain can induce direct-indirect bandgap transition, and the applied electric field can effectively regulate the bandgap of heterogeneous structure. The bandgap of AA2 configuration increases monotonically from 0.195 eV to 0.714 eV, but that of AB2 configuration decreases monotonically from 0.859 eV to 0.058 eV. The band of the heterojunction always maintains the type-II structure under the two kinds of configurations. The heterojunctions under compressive strain show better light absorption capability in the visible region with shorter wavelength. These results reveal the regulatory mechanism of the Janus Ga<sub>2</sub>SeTe/In<sub>2</sub>Se<sub>3</sub> van der Waals heterojunction electronic structure and provide theoretical guidance in designing novel optoelectronic devices.

Photoluminescence characterization of GeSn prepared by rapid melting growth method

In this work, single crystalline GeSn on insulator (GSOI) were grown by rapid melting growth (RMG). The Sn content distribution along the strip was investigated, reaching to 7.8% near the end of the strip. The GeSn strip had high quality with no threading dislocations, as revealed by cross-sectional transmission electron microscopy (XTEM). Photoluminescence (PL) measurements along the strip were performed to check the indirect-direct bandgap transition Sn content of GeSn alloys. It was found the integrated PL intensity increased about 6.6 times for Sn content 0.86%–7.8%. For GeSn with Sn content 6.1%, the temperature-dependent PL shows that the energy difference between direct Γ-Γ valley and indirect L-Γ valley decreases due to the bandgap narrowing (BGN) effect, and the direct band transition gradually dominates the PL spectra as temperature increases. The temperature-dependent PL spectra have only one peak for GeSn with Sn content 7.8%, suggesting the direct bandgap property. These results indicate that GeSn grown by RMG is very promising for fabricating efficient Si based light sources.

DFT investigations of pressure effect on lead-free double perovskites Cs2ZrCl6 and Cs2ZrBr6: Structural stability and band gap evolution

Perovskites have piqued the interest of many people due to their exceptional properties and promising applications. In this paper, the effect of pressure on lead-free double perovskites Cs 2 ZrCl 6 is investigated using first-principles calculations. It demonstrates that Cs 2 ZrCl 6 is stable up to 75 GPa, and that the alternation of the charge density distribution on the conduction bands caused by external pressure can force Cs 2 ZrCl 6 to transform from an indirect band gap semiconductor to a direct band gap semiconductor, which is applicable for isostructural Cs 2 ZrBr 6 . This suggests that Cs 2 ZrCl 6 and Cs 2 ZrBr 6 have promising underlying applications in high-pressure environments. This work is also useful for designing novel perovskites with superior performance. • Pressure effect on double perovskites Cs 2 ZrX 6 (X=Cl, Br) were explored. • The structure of Cs 2 ZrX 6 can be stable up to ~75 GPa. • Pressure-induced indirect-direct band-gap transition was observed.

Theoretical study on the photocatalytic potential of BSe nanotubes for water splitting under visible light

Photocatalytic splitting of water has been a useful tool in recent years for solving the world's energy and environmental problems. However, the wide band gap and low carrier mobility of traditional photocatalysts are the reasons that limit their wide application. In this work, the structure, and optical properties of BSe nanotubes are studied for the first time based on density functional theory.The results show that single-walled BSe nanotubes (SW BSe NTs) not only have a stable structure but can also extend the ultraviolet light absorption range of the BSe monolayer to the visible light range. Significantly, the SW BSe NT has excellent photoreduction ability, low electron-hole recombination rate, and high hole carrier mobility (∼103 cm2V−1s−1). Under uniaxial strain, the SW BSe NT exhibits a direct-indirect band gap transition. Furthermore, the electronic structure of double-walled BSe nanotubes facilitates charge separation and has a wider range of light absorption. Unluckily, its lower band edge position prevents it from having an overall photocatalytic decomposition capability. Our results show that SW BSe NTs have potential applications in visible light-driven photocatalytic water splitting.

The electronic and optical properties of Cs2 Ti1-xBxI6(B=Sn, Te, Se) with first principle method

Lead-free halide perovskite is regarded as the most promising light-harvesting material for solar cells recently. In this work, the properties of CsTiBI (B=Sn, Te, Se) were investigated with first principle method based on density functional theory. The results show that CsTiSnI is an excellent photoelectric material in the sunlight. When the doping concentration was over , CsTiSeI and CsTiTeI have direct–indirect band gap transition. The values of energy gap, density of states and optical absorption coefficients in the visible light region are well matched to each other. Optical results show that absorption peaks locate near 330 nm and have strong ultraviolet absorption capacity. Doping Sn results in the spectrum redshifts and the absorption peak drops. While the peak becomes greater with the increase of tellurium, and the spectrum has obvious blueshift. The spectrum trends of CsTiSeI are more complicated. This work provides theoretical supports for the development of novel optoelectronic materials by means of partial substitution doping.

Pressure-Induced Indirect-Direct Bandgap Transition of CsPbBr3 Single Crystal and Its Effect on Photoluminescence Quantum Yield.

Despite extensive study, the bandgap characteristics of lead halide perovskites are not well understood. Usually, these materials are considered as direct bandgap semiconductors, while their photoluminescence quantum yield (PLQY) is very low in the solid state or single crystal (SC) state. Some researchers have noted a weak indirect bandgap below the direct bandgap transition in these perovskites. Herein, application of pressure to a CsPbBr3 SC and first‐principles calculations reveal that the nature of the bandgap becomes more direct at a relatively low pressure due to decreased dynamic Rashba splitting. This effect results in a dramatic PLQY improvement, improved more than 90 times, which overturns the traditional concept that the PLQY of lead halide perovskite SC cannot exceed 10%. Application of higher pressure transformed the CsPbBr3 SC into a pure indirect bandgap phase, which can be maintained at near‐ambient pressure. It is thus proved that lead halide perovskites can induce a phase transition between direct and indirect bandgaps. In addition, distinct piezochromism is observed for a perovskite SC for the first time. This work provides a novel framework to understand the optoelectronic properties of these important materials.

Strain Effects on the Electronic and Optical Properties of Blue Phosphorene.

Monolayer blue phosphorene (BlueP) systems were investigated under biaxial strain range from −10% to +10%. All these systems exhibit excellent stability, accompanying changes in the electronic and optical properties. BlueP becomes metallic at −10% strain and transforms into a direct semiconductor at 10% strain while maintaining indirect semiconductor behaviors at −8% to +8% strain. The bandgap of BlueP decreases linearly with strain, and tensile strain exhibits a more moderate bandgap modulation than compressive strain. The real part of the dielectric function of BlueP is enhanced under compressive strain, while the optical absorption in the visible and the infrared light regions increases significantly under tensile strain. The maximum absorption coefficient of 0.52 ×105/cm occurs at 530 nm with the 10% strain. Our analysis indicates that the semiconductor–metal transition and the indirect–direct bandgap transition are the competition results of the energy states near the Fermi level under a massive strain. The potent compressive strain leads the p y orbitals of the conduction band to move downward and pass through the Fermi level at the K point. The robust tensile strain guides the energy states at the Γ point to approach the Fermi level and become the band edges. Our results suggest that the energy storage capacity of BlueP can be significantly improved by compressive strain, while the visible light photocatalytic performance is enhanced by tensile strains of less than 8%. Our works provide a reference for the practical applications of BlueP in photocatalyst, photovoltaic cells, and electronic devices.

First-principles study on the vertical heterostructure of the BSe and AlN monolayers

In this investigation, we employed density functional theory simulations to explore the structural, optical, mechanical, and electronic properties of the vertical heterostructure based on AlN and BSe monolayers, named AlN@BSe. Our results suggest that the AlN@BSe is mechanically stable. Compared with the pure BSe and AlN monolayer, its in-plane stiffness is greatly improved. From the absorption coefficient analysis, it is found that the AlN@BSe has a strong infrared light absorption. Furthermore, indirect-direct band gap transition can be found by adjusting interlayer distance (d). In addition, under the biaxial strain and the external electric field (E-field), the band alignment of AlN@BSe undergoes a transition from type-I to type-II, as well as direct to indirect band gap transition. The diverse electronic properties endow the AlN@BSe with high potential for novel nano-electronic applications.

Study on Zr, Sn, Pb, Si and Pt doped TiO2 photoanode for dye-sensitized solar cells: The first-principles calculations

Density functional theory (DFT) calculations have been performed to investigate the doping of Zr, Sn, Pb, Si and Pt atoms into an Ti-vacancy of TiO2. The results show that the band gap of TiO2 is changed from 2.081 eV to 2.044, 2.032, 1.811, 1.718 eV and 0.207 eV upon interaction with Zr, Sn, Pb, Si and Pt atom respectively. such as indirect–direct band gap transition, by the analysis of the optical properties, the absorption coefficient in 1.0 × 105 cm−1, demonstrates thin crystal structure can achieve strong absorption of light energy. Such a reasonable match satisfies the photoelectric conversion efficiency.

First-principles study on the strain-modulated structure and electronic properties of janus tin oxide selenide monolayer

Using first-principles calculations, the structure and electronic properties of a Janus tin oxide selenide (SnOSe) monolayer under the influence of uniaxial and biaxial strain were investigated. The results revealed that in the absence of strain, the Janus SnOSe monolayer displayed semiconducting properties with a direct band gap of 0.32 eV, as calculated by the HSE06 method. Uniaxial strain along the x-direction led to an indirect-direct band gap transition, while under the impact of uniaxial strain along the y-direction, the material changed from a metal to a direct band gap semiconductor. When the biaxial strain was applied to monolayer SnOSe, a similar transition from a metal to a semiconductor occurred, and the tunable range of the direct band gap significantly increased and could be adjusted from 0 eV to 1.30 eV. It has also been found that under the influence of uniaxial or biaxial tensile strain, monolayer SnOSe remained as a direct band gap semiconductor whose band gap increased upon the increment in tensile strain. Our results also demonstrated that the work function of monolayer SnOSe increased rapidly as the strain changed from −10% to 10%. These findings prove that strain engineering can be employed to tune the electronic properties of the Janus SnOSe monolayer.

Engineering Exciton Recombination Pathways in Bilayer WSe2 for Bright Luminescence.

Exciton-exciton annihilation (EEA) in counterdoped monolayer transition metal dichalcogenides (TMDCs) can be suppressed by favorably changing the band structure with strain. The photoluminescence (PL) quantum yield (QY) monotonically approaches unity with strain at all generation rates. In contrast, here in bilayers (2L) of tungsten diselenide (WSe2) we observe a nonmonotonic change in EEA rate at high generation rates accompanied by a drastic enhancement in their PL QY at low generation rates. EEA is suppressed at both 0% and 1% strain, but activated at intermediate strains. We explain our observation through the indirect to direct transition in 2L WSe2 under uniaxial tensile strain. By strain and electrostatic counterdoping, we attain ∼50% PL QY at all generation rates in 2L WSe2, originally an indirect semiconductor. We demonstrate transient electroluminescence from 2L WSe2 with ∼1.5% internal quantum efficiency for a broad range of carrier densities by applying strain, which is ∼50 times higher than without strain. The present results elucidate the complete optoelectronic photophysics where indirect and direct excitons are simultaneously present and expedite exciton engineering in a TMDC multilayer beyond indirect-direct bandgap transition.

Electronic and optical properties and quantum tuning effects of As/Hfs<sub>2</sub> van der Waals heterostructure

Stacking two or more monolayer materials to form van der Waals heterostructures is an effective strategy to realize ideal electronic and optoelectronic devices. In this work, we use As and HfS<sub>2</sub> monolayers to construct As/Hfs<sub>2</sub> heterostructures by six stacking manners, and from among them the most stable structure is selected to study its electronic and optic-electronic properties and quantum regulation effects by hybrid functional HSE06 systematically. It is found that the As/Hfs<sub>2</sub> intrinsic heterostructure is a II-type band aligned semiconductor, and its band gap can be significantly reduced (~ 0.84 eV) in comparison with two monolayers (band gap > 2.0 eV), especially the valence band offset and conduction band offset can increase up to 1.48 eV and 1.31 eV, respectively, which is very favorable for developing high-performance optoelectronic devices and solar cells. The vertical strain can effectively adjust the band structure of heterostructure. The band gap increases by tensile strain, accompanied with an indirect-direct band gap transition. However, by compressive strain, the band gap decreases rapidly until the metal phase occurs. The applied external electric field can flexibly adjust the band gap and band alignment mode of heterostructure, so that the heterostructure can realize the transformation between I-, II-, and III-type band alignments. In addition, intrinsic As/Hfs<sub>2</sub> heterostructure has ability to strongly absorb light in the visible light region, and can be further enhanced by external electric field and vertical strain. These results suggest that the intrinsic As/Hfs<sub>2</sub> heterostructure promises to have potential applications in the fields of electronic, optoelectronic devices and photovoltaic cells.

Stable nanotube construction conditions and electronic properties of possible Si double-walled nanotubes (nin,min)@(6,mout) (nin =3, 4) by SCC-DFTB calculations

The geometric structural optimization and electronic properties of double-walled silicon nanotubes (DWSiNTs) (nin,min)@(6,mout) (nin = 3, 4, min = 0 to nin, mout = 0 to 6) are studied in terms of the self-consistent charge density functional tight binding (SCC-DFTB) method. A suitable spacing range of initial model that can form a stable DWSiNT is explored. Most of the nanotubes present circular-like shapes of their cross-sectional configurations. When a DWSiNT has the same chiral angle, 0 or 30 in degree, in its inner and outer walls, a regular periodic structure develops. The bond length, inner and outer wall diameters, and wall spacing are also significantly affected by changes in the chiral index. The stability of the nanotube is affected by three factors: the configuration, the diameter, and the regularity of the atomic arrangements. The electrons in the inner wall are transferred to the outer wall, and the electron distribution of the atoms in the outer wall tube has the regional distribution of the interval between the electron-obtaining and the electron-losing regions. Among the seven tubes with fully commensurate inner and outer walls, DWSiNTs (3,2)@(6,4) exhibit semi-metallic characteristics; the others exhibit semiconductor-like properties with a narrow band gaps. The band gap of DWSiNTs is changed by tuning the chirality index, resulting in the transition between direct and indirect band gaps of nanotubes, which can be adjusted to match the performance needs of different devices, inducing a metal-semiconductor transition and direct-indirect band gap transition. These theoretical studies provide references for the preparation, and future application, of DWSiNTs.