Band Gap Size Research Articles

The effects of MS2 (M = Mo or W) substrates on electronic properties under electric fields in germanene-based field-effect transistors

Germanene has attracted significant attention due to its novel electronic properties and strong spin-coupling effect. However, the tiny band gap of the germanene dramatically limits its application in field-effect transistors (FETs). Inspired by the utilization of the substrates and electric fields to adjust the band gaps of two-dimensional materials, we investigated the fundamental mechanism of electric fields on the atomic structures and electronic properties of germanene supported by MS2 (M = Mo or W) substrates through first-principles calculation. The results show that the substrates can induce a symmetry breaking in the germanene sublattice via van der Waals interaction, leading to a sizable band gap at the Dirac point. In addition, the band gaps of the germanene/MS2 heterostructures can be effectively modulated by applying an external electric field. Under suitable electric fields, the considerable band gap values of CMo germanene/MoS2 and TGeL-W germanene/WS2 configurations can open the maximum band gaps with 263 and 247 meV, which satisfy the requirements of FETs at room temperature. Meanwhile, the evolutions of charge transfers under electric fields were explored to illustrate how electric fields and substrates promote the electronic properties of germanene. More interestingly, a Schottky–Ohmic transition can occur when a specific electric field is imposed on the germanene/MS2 heterostructures. Note that the hole and electron carrier mobilities of germanene/MS2 heterostructures are still significantly preserved, showing some superior electronic performances than some heterostructures. The results provide a critical theoretical guide for improving the electronic properties of germanene, and demonstrate the designed germanene/MS2 heterostructures with the tunable band gaps and higher carrier mobilities as germanene-based FETs.

Strain-induced photoresponsivity in gallium-doped ZnO thin film based UV photodetectors

The effect of varying Ga-doping on structural, optical, and UV detection properties of ZnO/p-Si based photodetectors is reported in this paper. Herein, the pure and gallium (Ga) (1 at%, 2 at%, and 3 at%) doped ZnO thin films are deposited on p-Si substrates by the sol–gel spin-coating method. On the basis of characteristic investigations, it is found that the varying Ga-doping affects not only the structural and optical properties of ZnO films, but also their electrical characteristics and hence significantly changes the performance of the devices employing ZnO films. Moreover, it is observed that the band gap size of the films is linearly dependent on strain values. As for the photodetector performance, it is found that the photodetector with 2 at% Ga doped ZnO demonstrates an excellent device performance, yielding a RR of 7908, PDCR of 171.7, PR of 8.15 (A/W) with D* of 5.4 × 1012 Jones, EQE of 2769.5%, and G of 27.69. The results suggest that the resulting device has a high potential for UV sensor applications.

Higher-Order Harmonics in Hexagonal Graphene Quantum Dots

We have considered the high-order harmonic generation in plane graphene quantum dots of hexagonal shape by the independent quasiparticle approximation-tight binding model. We have investigated how such a nonlinear effect is affected by a strong optical wave field, quantum dot typical band gap and lateral size, and dephasing processes. The equation of motion for the density matrix is solved by performing the time integration with the eight-order Runge–Kutta algorithm. If the optical wave frequency is much less than the quantum dot intrinsic band gap, the main aspects of multiphoton high harmonic emission in quantum dots are revealed. In such case dependence of the cutoff photon energy on the strength of the optical pump wave is almost linear. But when the wave frequency is comparable to the bandgap of the quantum dot, the cutoff photon energy shows saturation behavior with an increase in the wave field strength.

Study of Chemical Polymerization of Polypyrrole with SDS Soft Template: Physical, Chemical, and Electrical Properties.

Polypyrrole (PPy) is a conductive polymer known for its biocompatibility and ease of synthesis. Chemically polymerized PPy was synthesized in the presence of sodium dodecyl sulfate (SDS), showing correlations among chemical properties, physical morphology, and electrical properties. Focused synthesis parameters included the pyrrole (Py) concentration, SDS concentration, and ammonium persulfate (APS)/Py ratio. The addition of SDS during chemical polymerization influenced the physical morphology of PPy by altering the self-assembling process via micelle formation, yielding sheet-like morphologies. However, the phenomenon also relied heavily on other synthesis parameters. Varying SDS concentrations within the 0.01 to 0.30 M window produced PPy sheets with no significant difference in optical band gap or physical size. While using 0.10 M SDS, an increase in Py concentration from 0.10 to 0.30 M yielded a larger size of PPy as the morphology changed from sheet-like to irregular shape. The band gap dropped from 2.35 to 1.10 eV, and the conductivity rose from 6.80 × 10-1 to 9.40 × 10-1 S/m. With an increase in the APS/Py ratio, the PPy product changed from a random to a sheet-like form. The product provided a larger average size, a decreased band gap, and increased electrical conductivity. Py polymerization in the absence of SDS revealed no significant change in shape or size as the Py concentration increased from 0.10 to 0.30 M; only a sphere-like form was observed, with a large band gap and small conductivity. Results from Raman spectral analysis indicated a correlation between optical band gap, physical morphology, and bipolaron/polaron ratio, mainly at the wavelengths associated with C-C stretching and C-H deformation. The increase in average size was associated with a decrease in band gap and resistance as well as an increase in the bipolaron/polaron ratio. This work indicates a strong correlation between size, morphology, electrical properties, and the bipolaron/polaron ratio of PPy in the presence of SDS.

Theoretical Investigation of Delafossite-Cu2ZnSnO4 as a Promising Photovoltaic Absorber.

In the quest for efficient and cost-effective photovoltaic absorber materials beyond silicon, considerable attention has been directed toward exploring alternatives. One such material, zincblende-derived Cu2ZnSnS4 (CZTS), has shown promise due to its ideal band gap size and high absorption coefficient. However, challenges such as structural defects and secondary phase formation have hindered its development. In this study, we examine the potential of another compound, Cu2ZnSnO4 (CZTO), with a similar composition to CZTS as a promising alternative. Employing ab initio density function theory (DFT) calculations in combination with an evolutionary structure prediction algorithm, we identify that the crystalline phase of delafossite structure is the most stable among the 900 (meta)stable CZTO. Its thermodynamic stability at room temperature is also confirmed by the molecular dynamics study. Excitingly, this new phase of CZTO displays a direct band gap where the dipole-allowed transition occurs, making it a strong candidate for efficient light absorptions. Furthermore, the estimation of spectroscopic limited maximum efficiency (SLME) directly demonstrates the high potential of delafossite-CZTO as a photovoltaic absorber. Our numerical results suggest that delafossite-CZTO holds promise for future photovoltaic applications.

First-principles prediction of a two-dimensional semiconductor ferromagnetism in Cr2X2Se6 (X = Al, Ga, In) materials

Based on first-principles calculations, I have predicted the existence of a new class of two-dimensional (2D) transition-metal tri-chalcogenides (TMTCs) materials with the chemical formula Cr2X2Se6 (X = Al, Ga, In). The molecular dynamics and phonon dispersion calculations showed that these materials are thermally and dynamically stable, suggesting that they can be synthesized in the laboratory. Cr2X2Se6 structures are all semiconductors with an indirect bandgap, and the bandgap sizes decrease as the X atom changes from Al to Ga to In. For instance, the bandgaps of Cr2Al2Se6, Cr2Ga2Se6, and Cr2In2Se6 are 1.29, 0.91, and 0.86 eV, respectively. The Cr2X2Se6 monolayer structures exhibit ferromagnetic order and possess an out-of-plane magnetic anisotropy energy (MAE), which increases in magnitude from Al to In along the periodic table. The Curie temperature (Tc) of Cr2X2Se6 monolayers was determined using Monte Carlo simulations based on Heisenberg's model. The Tc exhibited the expected dependence on X atoms. The Cr2X2Se6 monolayers exhibit strong absorption and reflection coefficients in the visible region, with an absorption coefficient of about 105 cm−1. Overall, the discovery and development of new 2D materials with interesting electronic, magnetic, and optical properties offer many opportunities for developing new spintronic and magneto-optical devices that can be used in many different applications.

Electronic structure, optical properties and defect induced half-metallic ferromagnetism in kagome Cs2Ni3S4

Recent research focuses on electronic structure of kagome materials due to their fascinating properties such as topological insulators, Dirac semimetals, and topological superconductors. Materials with sizable electronic band gap are found to play vital role in device applications. Here, by means of density functional theory calculations, we study the electronic and optical properties of ternary transition metal sulphide Cs2Ni3S4 by using the full potential local orbital code. Standard generalized gradient approximation (GGA) has been employed to consider the electron exchange and correlation effect, and modified Becke–Johnson (mBJ) potential has been used to obtain the accurate band gap of the material. From our electronic structure calculations Cs2Ni3S4 is found to be nonmagnetic semiconductor with an indirect band gap of ∼1.4 eV within GGA + mBJ calculations. The structural analysis demonstrates that Ni atoms form a kagome lattice in a two-dimensional plane, resulting in the presence of a dispersionless flat band located below the Fermi energy. From the optical calculations, analyzing the dielectric function, loss function, and optical conductivity, Cs2Ni3S4 is found to be optically active in the visible as well as lower ultraviolet energy ranges. This suggests that Cs2Ni3S4 may be a suitable candidate for the opto-electronic devices. Additionally, this work may provides a foundation for the development of opto-electronic device and a framework for experimental work. We additionally investigated the effect of vacancy defects in Cs2Ni3S4 to see it’s influence on the electronic and magnetic properties. Interestingly, the Cs-vacancy defect give rise to half-metallic ferromagnetism with an effective magnetic moment of per unit cell.

Widely tunable magnetorheological metamaterials with nonlinear amplification mechanism

This paper proposes the design of an active nonlinear metamaterial for broadband and tunable low-frequency wave attenuation governed by a lever-based nonlinear amplification mechanism and a varying stiffness of a magnetorheological elastomer (MRE) driven by an external magnetic field. The metamaterial is formed by periodical spring-lever-MRE resonators connected to a base beam. Its strongly nonlinear behavior is described by a nonlinear amplification factor that depends on a magnetic flux density, a lever ratio, and an excitation amplitude. We use the Galerkin method to theoretically study the tunability of a low-frequency bandgap in terms of the MRE properties and the nonlinear amplification conditions. The obtained results are validated in numerical simulations. We also explore the mechanisms for broadband low-frequency wave attenuation governed by the strongly nonlinear dynamics of the proposed metabeam. Our results show that the metabeam with a large lever ratio can achieve substantial shifts of the bandgap frequencies for reasonably small variations of excitation amplitudes, while the bandgap size can be actively changed by varying the magnetic flux density. Our metamaterial is thus a promising solution to realize adaptive and actively tunable wave-attenuating metastructures that could be beneficial for practical applications.

MX6C2H monolayers: The novel two-dimensional tunable Dirac cone materials family and quantum spin Hall insulators

Two-dimensional (2D) Dirac cone materials have many interesting physical properties. Therefore, people have been trying to find 2D materials with Dirac cones. Here, based on first principle calculations, we studied a new 2D Dirac cone material family MX6C2H monolayers (M = N,P,As and Sb, X = C,Si,Ge and Sn). The calculation of the cohesive energy indicated that the MX6C2H sheets have excellent energy stability. And the phonon spectrum showed their excellent dynamical stability. The electronic calculation revealed that the 2D MX6C2H monolayers have graphene-like band structures and ultra-high Fermi velocities comparable to graphene. The spin–orbit–coupling effect can open a sizable topological band gap in SbSn6C2H monolayer so that it can be effectively classified as quantum spin Hall insulators. In addition, the Dirac cone of PC6C2H monolayer can be adjusted by applying in-plane uniaxial strain, which is much easier than that of graphene. These interesting properties indicate that the novel 2D MX6C2H system has potential application value in nanoelectronics.

Effect of band gap and particle size on photocatalytic degradation of NiSnO3 nanopowder for some conventional organic dyes

The present article deals with the study of degradation of organic dyes: congo red (CR), methylene blue (MB) and rhodamin B (RB), using nickel stannate (NiSnO3) nanopowder as photocatalyst. Nanocrystals of NiSnO3 have been synthesized by co-precipitation method. The powder X-ray diffraction (XRD) data taken at room temperature confirms the formation of crystalline phase in NiSnO3. The average particle size, estimated from transmission electron microscopy (TEM) increases from ∼5.05 to ∼8.05 nm with an increase in sintering temperature from 250 to 400 °C. The band gap of the photocatalyst measured (de-ionized water used as eluent) employing Ultraviolet–Visible (UV–Vis) spectroscopy reduces from ∼3.38 to ∼2.90 eV for an identical rise in sintering temperature. The percentage degradation is maximum (∼94%) for CR dye. Among our studied samples, NiSnO3 photocatalyst sintered at 400 °C has the lowest band gap, and the largest average particle size. This shows the best photocatalytic performance. Degradation efficiency of CR dye in various pH ranges (the optimized pH value is ∼5.0) and cyclic stability of the catalyst were also examined. CR dye degradation is ∼92.3% after fifth cycle which is very close to ∼94% degradation of first cycle.

Studying the structural and optical properties of Er3+ doped TiO2 powders synthesized by the sol-gel process

A series of TiO2:xEr powders (with x from 0% to 1% at.) was synthesised by a simple sol–gel process. The structural and optical properties were studied in detail using diffusion reflection spectroscopy, x-ray diffraction, Raman spectroscopy, high-resolution transmission electron microscopy, and energy-dispersive x-ray spectroscopy, to systematically determine the band edge, morphology, and structural properties of both undoped and Er-doped TiO2 samples. Notably, the introduction of Er dopants in the TiO2 matrix results in a decrease in both the band gap and crystallite size as well as the anatase phase of host matrix TiO2 remained in the anatase phase even at 800 °C. The heat treatment temperature has an insignificant effect on the green/red emission ratio of Er ions. However, this ratio strongly depends on excitation wavelengths corresponding to down-conversion and up-conversion processes. The colour emission of TiO2:Er is easily tunable from green to yellow under different excitations. The emission mechanism of TiO2:Er is proposed based on spectroscopy techniques. These findings reveal that the TiO2:Er powders are potential materials for labelling and photocatalyst applications.

Tunable electronic structure and magnetic characteristics of ZnO monolayer via vacancy defects, and domain/atomic doping

Two-dimensional ZnO monolayers have attracted great attention due to the potential applications in nano-optoelectronic and spintronic devices. However, tuning a sizeable band gap or magnetic properties is a significant challenge to the actual applications. Herein, the effects of monovacancy, divacancy, and voids defects, as well as domain/atomic doping on the electronic properties and magnetic characteristics of ZnO monolayer are investigated by spin-polarized density functional theory. The results demonstrate that single oxygen vacancy could realize the tunablity of bandgap and Zn vacancy could lead the transition between the nonmagnetic state and magnetic state. Remarkably, ZnO monolayers with different divacancies demonstrate magnetic or nonmagnetic characteristic. Zn divacancy produces the magnetic states due to the unpaired O atom, while O divacancy and Zn-O pair vacancy lead no spin polarization due to the occupied states. Void defects also can modify the magnetic properties of ZnO monolayer with the maximum magnetic moment of 1.258 μB. VI-IIA domain doping induces the nonmagnetic states, and widens or narrows the energy gap of ZnO monolayer. Additionally, ZnO monolayers with the doping of the selected atomic Rh and Ru exhibit the magnetic behavior with the total magnetic moment of 2.770 μB, and 3.678 μB, respectively. The total moments are mainly contributed by the dopants and their nearest O atoms. Our study demonstrates that the electronic properties and magnetic properties of ZnO monolayer could be effectively controlled by vacancy, divacancy, and voids as well as domain/atomic doping, being suitable for applications in optoelectronic and spintronic devices.

Oscillation of Electronic-Band-Gap Size Induced by Crystalline Symmetry Change in Ultrathin PbTe Films.

For the semiconductors of atomic length scales, even one atom layer difference could modify crystal symmetry and lead to a significant change in electronic structure, which is essential for modern electronics. However, the experimental exploration of such effect has not been achieved due to challenges in sample fabrication and characterization with atomic-scale precision. Here, we report the discovery of crystal symmetry alternation induced band-gap oscillation in atomically thin PbTe films by scanning tunneling microscopy. As the thickness of PbTe films is reduced from an 18- to 2-atomic layer, the band-gap size not only expands from 0.19eV to 1.06eV by 5.6 fold, but also exhibits an even-odd-layer oscillation, which is attributed to the alternating crystal symmetries between P4/mmm and P4/nmm. Our work sheds new light on electronic structure engineering of semiconductors at atomic scale for next-generation nanoelectronics.

Development of size and shape dependent model for bandgap of semiconductor nanomaterials

A simple theoretical model is developed to study the effect of size and shape on the bandgap of semiconductor nanomaterials. It is found that bandgap increases by decreasing the size, which depends on the shape considered. The results obtained are compared with the available experimental data as well as with those based on earlier models. Different models predict similar trend of variations. However, such an excellent agreement with experimental data particularly in low size range using a simple model is never seen earlier. The model reduces the number of input parameters and includes more shapes as compared with earlier studies. This demonstrates the simplicity and wide applicability of the present model due to which it can be used to study the size and shape dependence of bandgap of different semiconductor nanomaterials of current interest in science and technology.

Exploring the suitability of narrow-bandgap compounds for thermoelectric application

Different opinion exists regarding the role of bandgap size on thermoelectric variables. Motivated by the observation that the band shape can in a definite way affect thermoelectric properties, two compounds (BaSiSr and IrBiZr) have been evaluated for optimum thermoelectric application. The phonon spectrum, band structure, mechanical and thermoelectric properties of the two compounds were investigated using the density functional theory. The calculated bandgap size of the compounds are comparable (IrBiZr = 0.223 eV; BaSiSr = 0.220 eV). The valence energy edge peaked at the Fermi level for both compounds. The highest valence energy edge occurred at a single high symmetry point (Γ for IrBiZr) but at two symmetry points (X and W for BaSiSr). IrBiZr gave an overall superior performance over BaSiSr in the calculated elastic, thermodynamic and thermoelectric parameters. A lesser role for bandgap size can therefore be indicated in the design and optimization of novel thermoelectric materials in view of better results in favor of IrBiZr than BaSiSr.

Nonlinearity enhanced wave bandgaps in metamaterial honeycombs embedding spider web-like resonators

Wave propagation in metamaterial honeycombs endowed with periodically distributed nonlinear resonators is addressed. The linear and nonlinear dispersion properties of the metamaterial are investigated. The nonlinear wave propagation equations obtained via a projection method and the Floquet–Bloch theorem are attacked by the method of multiple scales to obtain in closed form the nonlinear manifolds parametrized by the amplitudes, the frequency, and the wave numbers. The effects of the nonlinearity on the frequency bandgaps are thoroughly investigated and the optimization problem of the resonators nonlinearity towards increased bandgap size is tackled to provide a significant practical framework for the design of nonlinear metamaterials.

CuAlO2/CuBi2O4 heterojunction photocathodes with improved charge collection for efficient solar water splitting

P-type heterojunction photoelectrode made of CuAlO2 and CuBi2O4 has been developed for the first time by using solution-based deposition method. Its advantageous bandgap size and favourable band positions make it a highly prospective option for photoelectrochemical (PEC) water splitting. The photoelectrode was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis absorption spectra, time-resolved photoluminescence (TRPL) and X-ray photoelectron spectroscopy (XPS). Compared to CuBi2O4 alone, the heterojunction photoelectrode showed improved photoelectrochemical activity with the highest photocurrent density of − 0.62 mA/cm2 at 0.6 V vs. RHE. The improvement is attributed to decreased recombination of photo-generated electrons and holes and improved charge separation and transport in the heterostructure. This research offers an efficient method to enhance the visible-light-driven activity of CuBi2O4 and a new strategy for fabricating heterogeneous nanostructure photoelectrodes for solar-driven water splitting.

Systematic strain-induced bandgap tuning in binary III–V semiconductors from density functional theory

The modification of the nature and size of bandgaps for III-V semiconductors is of strong interest for optoelectronic applications. Strain can be used to systematically tune the bandgap over a wide range of values and induce indirect-to-direct transition (IDT), direct-to-indirect transition (DIT), and other changes in bandgap nature. Here, we establish a predictive first-principles approach, based on density functional theory, to analyze the effect of uniaxial, biaxial, and isotropic strain on the bandgap. We show that systematic variation is possible. For GaAs, DITs are observed at 1.56% isotropic compressive strain and 3.52% biaxial tensile strain, while for GaP an IDT is found at 2.63% isotropic tensile strain. We additionally propose a strategy for the realization of direct-to-indirect transition by combining biaxial strain with uniaxial strain. Further transition points are identified for strained GaSb, InP, InAs, and InSb and compared to the elemental semiconductor silicon. Our analyses thus provide a systematic and predictive approach to strain-induced bandgap tuning in binary III-V semiconductors.

Plant mediated green synthesis of zinc oxide nanoparticles using Citrus jambhiri lushi leaves extract for photodegradation of methylene blue dye

Due to the discharge of organic and inorganic contaminants into the water, the textile industry is the main cause of water pollution. Nanoparticles have been synthesized in recent years to filter out such textile pollutants from water. In the current study, an efficient and facile method, i.e., the green method, has been used to prepare zinc oxide nanoparticles (ZnO NPs) by utilizing Citrus jambhiri lushi leaves extract. The morphological, optical, and structural properties were investigated by scanning electron microscopy, UV–Vis spectroscopy and x-ray diffraction technique. Results affirmed that ZnO NPs possessed the bandgap and particle size in the range of 3.00–3.05 eV and 10–35 nm, correspondingly. Furthermore, ZnO NPs demonstrated effective photodegradation of methylene blue dye in just 120 min, making them an excellent photocatalyst for removing textile pollutants from water.

Room temperature sensing of primary alcohols via polyaniline/zirconium disulphide

Among the primary alcohols, ethanol is referred to as a heavy chemical due to its many applications in a variety of industries. Detection of primary alcohols can be deployed as a non-invasive method in medical diagnosis and safety measures in food processing companies. Zirconium disulphide is a novel 2D layered material with exotic features when in mono or few layers, which include fast electron transport, high carrier mobility and sizeable band gap. ZrS2 and PANI were fabricated using liquid exfoliation and chemical polymerization methods respectively. Functionalization of the conducting polyaniline with ZrS2 was done using facile sonication process. The sensor showed good sensitivities (43%, 58% and 104%) which were estimated from slopes of the linear fitted plots with fast response-recovery times of 8 s and 27 s (111 ppm); 12 s and 130 s (77 ppm); and 58 s and 88 s (58 ppm). Good reproducibility at three repeated measurements (111 ppm, 77 ppm and 58 ppm) was also observed for methanol, ethanol, and isopropanol vapours respectively. Meanwhile, the sensor displayed more linearity and sensitivity towards isopropanol compared to methanol and ethanol. The sensor showed good performance even at RH values close to 100% making it a potential alcohol breath analyser.