Direct Band Gap Semiconductor Research Articles

Layer-dependent exciton polarizability and the brightening of dark excitons in few-layer black phosphorus

The evolution of excitons from 2D to 3D is of great importance in photo-physics, yet the layer-dependent exciton polarizability hasn’t been investigated in 2D semiconductors. Here, we determine the exciton polarizabilities for 3- to 11-layer black phosphorus—a direct bandgap semiconductor regardless of the thickness—through frequency-resolved photocurrent measurements on dual-gate devices and unveil the carrier screening effect in relatively thicker samples. By taking advantage of the broadband photocurrent spectra, we are also able to reveal the exciton response for higher-index subbands under the gate electrical field. Surprisingly, dark excitons are brightened with intensity even stronger than the allowed transitions above certain electrical field. Our study not only sheds light on the exciton evolution with sample thickness, but also paves a way for optoelectronic applications of few-layer BP in modulators, tunable photodetectors, emitters and lasers.

(Electronics and Photonics Division Award) Epitaxy of Group-IV Semiconductors for Nanoelectronics and Optoelectronics

Epitaxy stands for the deposition of a perfectly ordered layer on a monocrystalline substrate, which is typically, for group-IV semiconductors, bulk Si or Silicon-On-Insulator (SOI). If the layer is chemically different from the substrate and lattice parameters are not the same, epitaxy can be of two types. If the atomic columns of the substrate extend into the layer with the same in-plane lattice parameter, the layer will be pseudomorphic. The built-in strain will be compressive if the layer has a larger lattice parameter than that of the substrate or tensile if it is lower. If the layer thickness exceeds the so-called critical thickness for plastic relaxation, it will relax, with the formation of misfit dislocations to accomodate the lattice parameter mimatch.Epitaxy of group-IV semiconductors is key for numerous Si-based nanoelectronics and optoelectronics devices. Although they crystallize in the same diamond structure, C, Si, Ge and Sn bandgaps (5.47, 1.12, 0.66 and -0.41 eV), lattice parameters (3.57, 5.43, 5.66 and 6.49 Å) and fusion temperatures (> 4000°C, 1414°C, 938°C and 232°C) are very different. This can be an advantage (bandgap and mobility engineering in devices) or a difficulty (when growing stacks). Finally, although Si and Ge are miscible in all proportions, solid solubilities of C in Si and Sn in Ge are exceedingly low (less than 10-4 % and 0.8%, respectively). Low growth temperatures and high growth rates will then be mandatory in order to have high quality alloys.In that presentation, I will show how epitaxy can be put to good use in order to boost the properties of devices. Layers or stacks were grown on 200 mm and 300 mm diameter wafers in industrial Reduced-Pressure – Chemical Vapour Deposition tools with temperatures in the 300°C-900°C range and pressures in the 20-100 Torr range, typically. Precursors were mono-methysilane (for C), silane, dichlorosilane or disilane (for Si), germane or digermane (for Ge) and tin tetrachloride (for Sn). Finally, diborane and phosphine were the P-type and N-type dopant precursors, respectively.The electrical properties of p-type Metal Oxyde Semiconductor – Field Effect Transistors (MOSFETs) can be improved by the insertion of a thin pseudomorphic SiGe layer beneath the gate of transistors. The compressive strain in it will boost by a factor of ~2 the hole mobility in pMOSFETs. The use of SiGe:B in the sources and drains regions on each side of such devices will help in minimizing the contact resistivity and increasing the compressive strain in the channel. Ramping-up, at high temperatures, the Ge concentration in microns thick layers grown on Si and capping them with constant composition layers will result in SiGe virtual substrates which are almost fully relaxed (e.g. with a lattice parameter close to that of bulk SiGe), slightly defective (Threading Dislocations Density (TDD) around 105 cm-2) and with surface cross-hatches. Growing thin tensily-strained Si layers on top and transferring them onto oxydized Si will result in strained SOI substrates, with an increase by a factor of two of the electron mobility in n-type MOSFETs built on top. Using low contact resistance tensily-strained SiP sources and drains will further boost nMOSFET performances.Microns-thick Ge layers grown directly on Si thanks to a low temperature/high temperature approach can be used as templates for III-V epitaxy. The surface root mean square roughness will be less than 1 nm and the TDD around 107 cm-2 with some short duration thermal cycling. Ge can also be grown selectively in trenches at the end of Si waveguides and near infra-red photo-detectors processed. Microns-thick SiGe layers (capped with Si) and pure Ge layers (capped with SiGe) can otherwise be used to fabricate mid and long Infra-Red waveguides. Mid-IR supercontinuum light was generated when sending ultrashort light pulses in such waveguides. Finally, germanium can be transformed into a direct bandgap semiconductor thanks to high amounts of tensile strain or some alloying with more than 8% of tin. Optically pumped lasing at room temperature was recently achieved in bonded or pillared micro-disks made of thick, plastically relaxed GeSn 16-17% layers.Finally, the feasability of selectively etching SiGe versus Si can be used to fabricate stacked nanosheet MOSFETs, with superior ION/IOFF tradeoffs for a given footprint. The core structures of such devices are epitaxial Si/SiGe superlattices, with the alternance of typically ten nm thick Si and SiGe layers. SiGe layers are later on selectively etched. The resulting voids are conformally filled with gate dielectrics and metals. We recently succeeded in stacking seven Si nanosheets, with a record nMOSFET saturation current of 2.8mA/µm for a 1V drain voltage. Figure 1

Electrically injected GeSn laser with stairs-structure based on SiN stressor

After decades of advancement, optoelectronic technology has emerged as a pivotal player in various domains such as optical communication, optical interconnection, and optical computer However, the challenge of integrating optoelectronic devices with complementary metal-oxide-semiconductor (CMOS) technology remains unresolved. Germanium, with its unique band structure, can be transformed into a direct bandgap semiconductor through the introduction of tensile strain, enabling efficient light emission. This provides a solution for the development of integrated light sources based on Group IV materials. A GeSn laser with a stairs-structure based on SiN stressor is proposed in this paper. The combination of the high-stress SiN film and GeSn alloy technology allows for the modification of Ge material, enabling efficient light emission in the laser. The stairs-structure facilitates a higher tensile strain in the GeSn material. The laser utilizes an F–P resonator as the optical cavity. Through simulation experiments, the laser exhibits a threshold current density of 90kA/cm2 at room temperature, a laser wavelength of 2477 nm, and an electrical-to-optical conversion efficiency of 6.5%.

Electronic structures and physical properties of transition metal tetraphosphides TMP4 (TM = V, Cr, Mo)

Transition metal phosphides have promising applications in catalysis, sensors, environmental remediation, and energy storage due to their diverse crystal structures and physicochemical properties. In this study, using first principles calculations, we investigate the structural, electronic, mechanical, optical, and thermally conductive properties of transition metal tetraphosphides, TMP4 (TM = V, Cr, Mo). The mechanic properties and phonon spectra prove that TMP4 (TM = V, Cr, Mo) are mechanically and thermodynamically stable. The electronic structures calculated by HSE06 reveal that VP4 is metallic, while CrP4 and MoP4 are direct bandgap semiconductors. Both CrP4 and MoP4 have very low carrier effective masses and much high carrier mobilities in the [100] direction. Moreover, CrP4 and MoP4 exhibit relatively high optical absorption coefficients and lattice thermal conductivities, promising them potential use as high-temperature solar-light-absorbing 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.

Modulating electronic structure of two-dimensional AsP via external strain and electric field

In this paper, the density functional theory calculations show that the electronic structure of monolayer honeycomb AsP can be effectively tailored by both applied strain and electric field. By applying external strains, the band gap of monolayer honeycomb AsP can be monotonous reduced by bigger strains. More interestingly, by applying 10% tensile strains along biaxial and uniaxial zigzag directions, monolayer honeycomb AsP becomes a direct band gap semiconductor with the band gaps of 1.28 and 1.73 eV, respectively, which are in the visible light region and suggest solar energy conversion applications. This work provides a feasible way of making monolayer β-AsP a direct semiconductor and a good candidate for photocatalysts.

Strain-regulated electronic and optical properties of InSe/WS2 heterostructure from first-principle calculations

InSe/WS2 heterostructure (HS) was constructed in this study to combine the excellent properties of these two monolayer semiconductors. The calculation of enthalpy of formation, phonon spectrum, and elastic constants of heterostructure based on first-principle proves that InSe/WS2 HS has good stability and mechanical property. The study of electronic properties illustrates that InSe/WS2 HS is a type II direct band gap semiconductor with photogenerated electrons (holes) located mainly on the InSe (WS2)-layer, the electron mobility of heterostructure increased a lot (4357.65 cm2V−1s−1), and the hole mobility enhanced a little after the combination of the heterostructure. In addition, the band gap is greatly reduced, providing it with a stronger light absorption capability. Likewise, optical properties (absorption coefficient, reflectivity, loss function, refractive index, dielectric function, and photoconductivity) can be modulated under applied biaxial strain. More importantly, the band gap can be significantly tuned under applied strain (direct ⇆ indirect, 0.02 eV ∼ 1.5eV) to meet the different requirements of devices for semiconductor devices.

Highly efficient and stable lead-free cesium copper halide perovskites for optoelectronic applications: A DFT based study

Recently synthesized industrially significant perovskites Cs3Cu2X5 (X=Cl,Br,I) are subjected to a density functional theory (DFT) investigation utilizing the CASTEP code. This study explores various physical features, including structural, optical, thermodynamic, elastic, mechanical, and electronic properties. There is a strong correlation between the optimized structure parameters and the existing experimental data, which demonstrates the reliability of our DFT-based computations. The band structure and density of states (TDOS and PDOS) analysis revealed that all the studied perovskites are direct band gap semiconductors, and Cs3Cu2Br5 has the smallest band gap (2.092 eV). We also discussed the mechanical and cell stability using the Born stability criterion and formation energy, respectively. The mechanical and dynamic stability of each phase is confirmed by the analysis of the elastic constants. According to the computed values of Pugh's and Poisson's ratios as well as Cauchy's pressure, all of the studied compounds are ductile in nature. The study of density of states, total charge density, and Mulliken atomic populations reveal that all the compounds have complex bonding with both ionic and covalent properties. Finally, utilizing the elastic constant data, the Debye temperatures of Cs3Cu2Cl5, Cs3Cu2Br5, and Cs3Cu2I5 have been determined as 82.90 K, 100.00 K, and 80.70 K, respectively. The analysis of thermodynamics (relatively low values of both ΘD and Kmin) as well as optical properties indicate that all the investigated materials have the potential to serve as thermal barrier coating (TBC) materials.

High Carrier Mobility and Controllable Electronic Property of the h-BN/SnSe2 Heterostructure.

Building two-dimensional (2D) vertical van der Waals heterostructures (vdWHs) is one of the effective methods to regulate the properties of single 2D materials. In this paper, we stack the hexagonal boron nitride (h-BN) monolayer (ML) on the SnSe2 ML to construct the stable h-BN/SnSe2 vdWH, of which the crystal and electronic structures, together with the optical properties, are also analyzed by the first-principles calculations. The results show that the h-BN/SnSe2 vdWH belongs to a type-I heterostructure with an indirect bandgap of 1.33 eV, in which the valence band maximum and conduction band minimum are both determined by the component SnSe2 ML. Interestingly, the h-BN/SnSe2 vdWH under the tensile strain or electric field undergoes the transitions both from type-I to type-II heterostructure and from the indirect to direct bandgap semiconductor. In addition, the carrier mobility of the h-BN/SnSe2 heterostructure has a significant enhancement relative to that of the SnSe2 ML, up to 104 cm2 V-1 s-1. Meanwhile, the h-BN/SnSe2 heterostructure presents the superb optical absorption and unique type-II hyperbolic property. Our findings will broaden the potential applications of SnSe2 ML and provide theoretical guidance for the related experimental studies.

Investigation of exciton states of ReS2 by temperature- and polarization-dependent photoluminescence and oxygen plasma treatment

Rhenium disulfide (ReS2) is a 1T′ phase, direct band-gap layered semiconductor with in-plane anisotropy of optical and electrical properties. It has been researched extensively for polarization-sensitive optoelectronic applications in the energy range of visible light. However, the comprehensive excitonic states of ReS2, including the emission induced by incidental defects such as sulfur vacancies, have not been investigated fully. We performed temperature- and polarization-dependent photoluminescence (PL) measurements to study the exciton states of ReS2 with thicknesses varying from monolayer to multilayer. Notably, we firstly identified the PL peak at ∼1.4 eV (X0). It showed a gradual enhancement by the oxygen plasma treatment. The X0 peak showed an isotropic polarization of the PL. This was distinguished from the linearly polarized emission of the other PL peaks of ReS2. Density functunal theory (DFT) calculations predicted the narrowing of the bandgap by the formation of ReO bonds at the sulfur vacancy sites. This is consistent with the observed energy of X0. Our comprehensive investigation of the four excitonic transitions of ReS2 including X0 impurity state, expands the utility of anisotropic ReS2 as a practical optoelectronic device.

Idealizing Tauc Plot for Accurate Bandgap Determination of Semiconductor with Ultraviolet-Visible Spectroscopy: A Case Study for Cubic Boron Arsenide.

The Tauc plot is widely used to determine the bandgap of semiconductors, but the actual plot often exhibits significant baseline absorption below the expected bandgap, leading to bandgap discrepancies from two different extrapolations. In this work, we first discuss the origin of baseline absorption and show that both extrapolation methods can produce significant errors by simulating Tauc plots with varying levels of baseline absorption. We then propose and experimentally verify a new method that idealizes the absorption spectrum by removing its baseline before constructing the Tauc plot. Finally, we apply this new method to cubic boron arsenide (c-BAs), resolve its bandgap discrepancies, and obtain a converging bandgap of 1.835 eV based on both previous and new transmission spectra. The method is applicable to both indirect and direct bandgap semiconductors with absorption spectrum measured via transmission or diffuse reflectance, which will become essential to obtain accurate values of their bandgaps.

Precise Design of Molecular Ferroelectrics with High TC and Tunable Band Gap by Molecular Modification.

Molecular ferroelectric materials are widely applied in piezoelectric converters, non-volatile memorizers, and photovoltaic devices due to their advantages of adjustable structure, lightweight, easy processing, and environmental friendliness. However, designing multifunctional molecular ferroelectrics with excellent properties has always been a great challenge. Herein, a multiaxial molecular ferroelectric is successfully designed by modifying the quasi-spherical cation dabco with CuBr2 to obtain halogenated [Bretdabco]CuBr4 (Bretdabco = N-bromoethyl-N'-diazabicyclo [2.2.2]octane), which crystallizes in polar point groups (C6). Typical ferroelectric behaviors featured by the P-E hysteresis loop and switched ferroelectric domain are exhibited. Notably, the molecular ferroelectric shows a high TC of 460 K, which is rare in the field and could greatly expand the application range of this material. In addition, the band gap is adjustable through the regulation of halogen. Both the UV absorption spectra and theoretical calculations indicate that the molecular ferroelectrics belong to a direct band gap (2.14 eV) semiconductor. This tunable and narrow band gap semiconductor molecular ferroelectric material with high TC can be utilized more effectively in the study of optoelectronics and sensors, including piezoelectric energy harvesters. This research may provide a promising approach for the development of multiaxial molecular ferroelectrics with a tiny band gap and high TC.

Lead-free halide double perovskites Rb2InSbX6 (X = F, Cl, Br, I): A first-principles study of structural and optoelectrical properties

First-principles calculation employing PBE and HSE06 hybrid functionals have been used to investigated the structural optoelectrical properties of lead-free halide double perovskite Rb2InSbX6 (X = F, Cl, Br, and I). The optical bandgaps are 2.48 eV for Rb2InSbF6, 1.18 eV for Rb2InSbCl6, 0.60 eV for Rb2InSbBr6, and 0.23 eV for Rb2InSbI6, which are all direct bandgap semiconductors. Electronic structure calculations reveal that the valence band is made up of the In-5 s and X-p orbitals, whereas conduction band is mostly made up of hybridization between the Sb-5p and X-p orbitals. According to optical properties, these four compounds have strong optical absorption in multi regions. When the number of halogen atoms increased, the absorption spectra shift from the ultraviolet region to the visible region. According to the examination of various optical parameters, Rb2InSbBr6 and Rb2InSbI6 exhibit greater light absorption than Rb2InSbF6 and Rb2InSbCl6 in the visible light range, making them suitable candidates for testing the high-power conversion efficiency and lead-free nature of solar cells.

Strain Modulation of Electronic Properties in Monolayer SnP2S6 and GeP2S6

In recent years, two-dimensional (2D) materials have attracted significant attention due to their distinctive properties, including exceptional mechanical flexibility and tunable electronic properties. Via the first-principles calculation, we investigate the effect of strain on the electronic properties of monolayer SnP2S6 and GeP2S6. We find that monolayer SnP2S6 is an indirect bandgap semiconductor, while monolayer GeP2S6 is a direct bandgap semiconductor. Notably, under uniform biaxial strains, SnP2S6 undergoes an indirect-to-direct bandgap transition at 4.0% biaxial compressive strains, while GeP2S6 exhibits a direct-to-indirect transition at 2.0% biaxial tensile strain. The changes in the conduction band edge can be attributed to the high-symmetry point Γ being more sensitive to strain than K. Thus, the relocation of the conduction band and valence band edges in monolayer SnP2S6 and GeP2S6 induces a direct-to-indirect and indirect-to-direct bandgap transition, respectively. Consequently, the strain is an effective band engineering scheme which is crucial for the design and development of next-generation nanoelectronic and optoelectronic devices.

C2O Nanotubes with Negative Strain Energies and Improvements of Thermoelectric Properties via N-Doping Predicted from First-Principle Calculations

In this study, the geometric structure, strain energy, and electronic properties of armchair C2O nanotubes (A_C2ONTs(n,n)) and zigzag C2O nanotubes (Z_C2ONTs(n,0)) are studied in detail. The results show that these nanotubes behave as special shapes; the section of Z_C2ONTs(n,0) along the axial direction is an n-edge type, and the section tends to be round with an increase in n while the boundary of section along the axial direction for A_C2ONTs(n,n) fluctuates. With an increase in n, the fluctuation disappears gradually, and the section also tends to be round. Compared with C2O nanosheets (g_C2O), C2O nanotubes have similar or even smaller strain energy, when the tube diameter is greater than or equal to 15 Å, the strain energy begins to show a negative value, and the negative value tends to be stable as the pipe diameter increases. Z_C2ONTs start to show negative strain energies from n = 8 and A_C2ONTs from n = 12, indicating their higher stabilities relative to the g_C2O sheets. The calculation of the electronic band structure shows that Z_C2ONTs are an indirect band gap semiconductor, and A_C2ONT is a direct band gap semiconductor. The band gap value of Z_C2ONTs first increases and then stabilizes with the diameters of the nanotubes, and the band gap value of A_C2ONTs decreases with the increase in the nanotubes and tends to the band gap value of the g_C2O sheet. In addition, the electronic properties and thermoelectric properties of C2ONTs(n=4,6,8) before and after N-doping were also studied. We found that N-doping changed the electronic and thermoelectric properties of C2ONTs. It reduced the nanotube band gap value and significantly improved the thermoelectric figures of merit of Z_C2ONTs(n=4,6,8) and A_C2ONT(4,4), which comprised an effective strategy to improve the thermoelectric figure of merit of nanotubes. The results showed that the C2ONTs had potential as thermoelectric materials after N-doping, which provided important guidance for designing low-dimensional g_C2O nanostructures.

Structural, optoelectronic and photocatalytic properties of MX (M=Ga, In; X=S, Se, Te) contact with novel janus GaInS3 monolayers

Using DFT calculations, we have fabricated the MX-GaInS3 (M = Ga, In; X = S, Se, Te) van dar Waals Heterostructures (vdWHs) and systematically calculated the structural, electronic, optical and photocatalytic properties. The calculated binding energies and AIMD simulations confirm the robust evidence for the stability of MX-GaInS3 vdWHs. Despite of the indirect bandgap nature of MX (M = Ga, In; X = S, Se, Te) monolayer, interestingally MX-GaInS3 vdWHs are direct bandgap semiconductors with type-II band alignments. Charge density difference, bader charges and the average electrostatic potential are calculated to investigate the transfer of charges amoung the layers at the interface of MX-GaInS3 vdWHs. Using dispersion along high symmetry path by fitting the band edges, we have also calculated the effective mass of carriers in MX-GaInS3 vdWHs. Smaller effective mass with higher mobility of carrier, suggest MX-GaInS3 vdWHs best candidate for optoelectronic devices. Absorption spectra in term of ε2(ω) of MX-GaInS3 (M = Ga, In; X = S, Se, Te) vdWHs are also calculated and investigated in detail. Both the conduction and valence band edges in GaS-GaInS3 vdWHs straddle the standard redox band edge potentials, hence have the ability to perform both reduction and oxidation, confirming the potentiality for full water splitting at pH = 0.

Quantum control of substitutional effects on the S-involved optoelectronic properties of BeSxSe1-X ternary alloys

The research on the quantum control of substitutional effects in BeSxSe1-x ternary alloys opens up exciting opportunities in semiconductor device engineering, photovoltaic, optoelectronic integration, and quantum information processing. By understanding and manipulating the optoelectronic properties of these alloys, researchers can pave the way for advanced technologies with enhanced performance, efficiency, and functionality in various fields. We are investigating how S alloying affects the structural, electronic and optical properties of tin chalcogenide materials. We are assessing the energy band structure and optical properties of BeSxSe1-x (x = 0%, 25%, and 50%) materials using density functional theory (DFT) within a full potential linearized augmented plane wave method (FP-LAPW) framework. The theoretically expected band energy gaps of BeSxSe1-x compounds indicate that S alloying improves their electrical and optical performances, based on the assumptions used in our calculations. The majority of the bonds in the BeSxSe1-x materials (x = 0%, 25%, and 50%) are covalent. All ternary alloys and binary compounds with (x = 0%, 25%, and 50%) exhibit direct (R-Γ) band gap semiconductor behavior, with a band gap energy (Eg). Each alloy system shows a nonlinear increase in the band gap (Eg) with increasing x. As BeSxSe1-x compound belongs to strongly correlated systems, therefore modified Becke–Johnson potential is used to calculate formation enthalpy (ΔHf), and optical properties of the compounds. The intense peaks in the imaginary part of the dielectric function εi(ω) in each spectrum are contributed by chalcogen Se, Be, and S optical excitations. Similar to the trend of band gap variation (Eg) with x, the zero-frequency limit of each imaginary dielectric function as well as the extinction coefficient, refractive index, and real dielectric function, exhibit a corresponding trend of change.

Two-dimensional 4f magnetic EuSn2X2 (X = P, As) monolayers: A first-principles study

Two-dimensional (2D) ferromagnetic semiconductors (FMSs) hold exciting and promising potential for application in spintronic devices at the nanoscale. Currently, most 2D FMSs are based on 3d electrons; 4f electrons can provide nontrivial magnetism but have been much less studied to date. This paper presents a theoretical study, via first-principles calculations, of EuSn2X2 (X = P, As) monolayers based on rare-earth cations with f-electrons. The results show that EuSn2X2 monolayers possess a large magnetization (7 μB/Eu), a controllable magnetic anisotropy energy, and a unique d-electron-mediated f–f exchange mechanism. Both types of EuSn2X2 (X = P, As) monolayers are FMSs with indirect bandgaps of 1.00 and 0.99 eV, respectively, based on the Heyd–Scuseria–Ernzerhof (HSE06) method, which can be transform to direct bandgap semiconductors under biaxial strain. Interestingly, under the latter, spin–orbit coupling interaction gradually replaces the dipole–dipole interaction in the dominant position of magnetic anisotropy, resulting in the magnetic easy axis changing from in-plane to the more desirable out-of-plane. Considering their excellent dynamic, thermal, and mechanical stabilities and small cleavage energy, these EuSn2X2 monolayers can be exfoliated from their synthesized bulk. Our study not only helps to understand the properties of 2D 4f rare-earth magnets but also signposts a route toward improving the performance of EuSn2X2 monolayers in nano-electronic devices.

Elastic, piezoelectric, and electronic properties of K1−xMxNbO3 (M = Li, Na): A first-principles study

The structural, elastic, piezoelectric, and electronic properties of K1−xMxNbO3 (M = Li, Na) were investigated by first-principles calculations. The results show that the modifications have effects on the elastic properties of KNbO3 and strengthen the anisotropy. Na and Li doping can slightly improve the piezoelectric stress tensor (e33) and piezoelectric strain tensor (d33) of KNbO3. With the increase in the x value from 0 to 0.5 in K1−xMxNbO3, the d33 value is improved from 24.4 to 36.3 pC/N (M = Na) and 41.5 pC/N (M = Li), and the piezoelectric property is improved by 1.49 and 1.70 times with the modifications of Na and Li, respectively. This conclusion is consistent with the result of Born effective charge and band structure results. The Born effective charge average values (Z*̄) for K and Nb atoms in K0.5M0.5NbO3 (M = Li, Na) are slightly increased compared to those in KNbO3. KNbO3 has an indirect bandgap, while Na and Li doping makes K0.5Na0.5NbO3 or K0.5Li0.5NbO3 a direct bandgap semiconductor. This study may provide a theoretical insight into Na and Li modified KNbO3 for their piezoelectricity.

Investigation of structural, electronic and optical properties of two-dimensional MoS2-doped-V2O5 composites for photocatalytic application: a density functional theory study.

In the present research, the structural, electronic and optical properties of transition metal dichalcogenide-doped transition metal oxides MoS2-doped-V2O5 with various doping concentrations (x = 1-3%) of MoS2 atoms are studied by using first principles calculation. The generalized gradient approximation Perdew-Burke-Ernzerhof simulation approach is used to investigate the energy bandgap (Eg) of orthorhombic structures. We examined the energy bandgap (Eg) decrement from 2.76 to 1.30 eV with various doping (x = 1-3%) of molybdenum disulfide (MoS2) atoms. The bandgap nature shows that the material is a well-known direct bandgap semiconductor. MoS2 doping (x = 1-3%) atoms in pentoxide (V2O5) creates the extra gamma active states which contribute to the formation of conduction and valance bands. MoS2-doped-V2O5 composite is a proficient photocatalyst, has a large surface area for absorption of light, decreases the electron-hole pairs recombination rate and increases the charge transport. A comprehensive study of optical conductivity reveals that strong peaks of MoS2-doped-V2O5 increase in ultraviolet spectrum region with small shifts at larger energy bands through increment doping x = 1-3% atoms of MoS2. A significant decrement was found in the reflectivity due to the decrement in the bandgap with doping. The optical properties significantly increased by the decrement of bandgap (Eg). Two-dimensional MoS2-doped-V2O5 composite has high energy absorption, optical conductivity and refractive index, and is an appropriate material for photocatalytic applications.