Band Gap Properties Research Articles

Ni/Ga2O3 based Schottky diode temperature sensor

Temperature sensors are the integrated part of electronics used in most of the applications. Present work demonstrates the temperature sensor based on ultra-wide bandgap semiconducting material, i.e.,Ga2O3. The material is known for its wider bandgap and radiation hardness properties. Ni/Ga2O3 based Schottky barrier device is simulated for sensing the temperature in the current range of 100 µA to 0.1 µA and in temperature range from 200 K to 600 K using SILVACO Atlas software. Relevant models such as bandgap narrowing, Schokely-Read-Hall, etc. were employed to ensure accuracy in the results. The simulated current–voltage (I-V) plots showed exponential variation and followed ideal thermionic emission model. The I-V plots were used to find the thermal sensitivity of device under considerations, which showed linearity in the measured current range. The maximum sensitivity is estimated as 3.22 mV/K at measuring current level of 0.1 µA.

Monolithically Integrated Directly Pumped Time-Varying Transmission Lines

A novel directly pumped time-varying transmission line (DP-TVTL) is proposed here and implemented on gallium arsenide (GaAs) microwave monolithic integrated circuit (MMIC). Comparing to existing TVTL architectures where the pump wave is launched through the transmission line, the pump wave in the DP-TVTL is directly fed to the different parts of the transmission line through a parallel power division network. The impedance of TVTL is modulated by the pump through diode bridges that isolate the signal path from the pump path. The power division network creates a phase delay emulating that of a traveling-wave delay line yet without launching the physical pump wave on the TVTL. This avoids the loss of the pump energy propagating through the transmission line. It also adds more freedom in the design of the bandgap properties of TVTL and simplifies the combination/separation circuitry for waves at different frequencies in and out of the TVTL. The resulting MMIC TVTL offers exceptional, broadband conversion gain performance of up to 4.2 dB, a low noise figure of 3 dB, and input referred P1dB of 12 dBm when used as a parametric mixer. It may be used in an RF front end as a mixer first receiver or an RF correlator.

Effect of Cu doping on structural, electronic and thermoelectric properties of double perovskite Cs2NaVCl6

Double halide perovskite constitutes an interesting class of eco-friendly, and stable materials with significant potential as efficient energy harvesting materials. Their simple crystalline packing and tunability offers substantial advantage. Herein, the structural, electronic, and thermoelectric attributes of percentage Cu doped double perovskite (Cs2NaVCl6) is explored with the quantum espresso code within the framework of density functional theory using the generalized gradient approximation (GGA-PBE) with ultra-soft pseudopotential for the inherent atoms. Electronic and thermoelectric properties were all achieved via the GGA + U method to accurately compute the band gap and similar properties. The results express an inverse relationship between the lattice parameters and band gap. The lattice parameters increased with the increase in doping percentage. Similarly, the conductivity of the studied perovskite increased considerably with the increase in the percentage dopant. The calculated band gap was observed within the range of 1.7 eV–1.2 eV decreasing considerably with the increase in percentage doping and therefore suggesting an increase in the conductivity and other thermoelectric properties of the engineered perovskite material. However, the equilibrium lattice constants in accordance with the percentage doping reveal that the higher the percentage doping, the higher the equilibrium lattice constant. This inferred that 25% endohedral replacements of vanadium atoms by copper atoms give the best perovskite material owing to the fact that when the lattice constant increases with a corresponding decrease in the value of modulus of elasticity, there is a decrease in the stability of the perovskite material.

UV Induced Photocatalytic Degradation of Caffeine Using TiO2–H-Beta Zeolite Composite

An anatase phase of the TiO2-H-beta (THB) zeolite composite photocatalyst is used for the photocatalytic degradation of caffeine, a persistent organic pollutant (POP). It is synthesized by a simple two-step sol-gel method. Phase formation, morphology, bandgap, and photocatalytic properties were analyzed using powder X-ray diffraction, scanning electron microscopy, and UV-Vis diffuse reflectance spectroscopy, respectively. The THB and the anatase TiO2 samples were then tested for the photocatalytic activity of the degradation of caffeine. Photocatalytic studies reveal that the as-prepared THB composite showed excellent activity for the degradation of 10 ppm caffeine solution. The chemical oxygen demand (COD) analysis found caffeine to have degraded with an efficiency of 96%. Scavenging experiments indicated that the hydroxide radical played an important role in the degradation of caffeine. The results highlight the role of the H-beta zeolite as an effective support to TiO2 and improved the photocatalytic activity. The study demonstrates that the THB composite could be effectively applied for the photocatalytic degradation of other POPs largely present in active pharmaceutical ingredients.

Excellent NO2 gas sensor based on the oxygen inhibiting effect of Ni3+-doped WO3

Due to the intrinsic properties of wide band gap and the 5d shell being vacant, WO3 still has the disadvantages of low response dynamics, slow response/recovery speed and high detection limit for real-time gas monitoring. To address such issues, the Ni3+ doped WO3 hierarchical hollow structure composed of irregular nanosheets was achieved and the content of adsorbed oxygen was adjusted by Ni3+ doping to further improve the gas sensing performance of WO3. In comparison to pure WO3, the 3 wt%Ni-WO3 sensor exhibited remarkable response value (396), short response/recovery speed (13 s/6 s) and low detection limit (510 ppt) at 160 ℃. The impressive gas properties could be attributed to the reduced band gap and oxygen inhibiting effect of Ni3+ doping, which provides more NO2 adsorption sites and making it easier for NO2 to capture more electrons in the conduction band by restraining the oxygen ions chemisorbed on the surface. The unique sensing characteristics of the Ni3+ doped WO3 suggest its wonderful potential applications in environmental pollution gas detection. The mechanism of oxygen inhibiting effect induced by Ni3+ doping was also discussed in detail

Density-functional quantum computations on bandgap engineering and tuning of optoelectronic properties of MgH2 via Mo doping: Prospects and potential for clean energy hydrogen-storage fuel and optoelectronic applications

With the increasing depletion of conventional energy sources and their detrimental environmental hazards, it is imperative to search for sustainable alternative clean energy sources. In the recent decades, hydrogen has emerged as potential source of clean energy. One of the potential alternatives to achieve the objective is the designing and characterization of materials for hydrogen-storage energy applications. In this regard, metal-bearing hydrides are the most promising candidates. For instance, magnesium-bearing hydrides are the focus of current research work owing to high hydrogen capacity of 7.6 wt%. In this paper, we first time report density functional-based quantum theoretical analysis to explore the potential of Mo-doped magnesium hydrides MgH2:Mo for optoelectronic and hydrogen-storage applications. For the quantum computations of the required optoelectronics and energy storage properties, we employed all-electron methods within generalized gradient approximation (GGA). Besides applying GGA approximation to account for the electronic correlated effects, we employed the Hubbard potential U (= 4 eV) for onsite repulsive Coulomb force. We predict that 10% doping by weight of Mo into MgH2 suppresses its insulating band gap of 4.9 eV to semiconducting band gap of order 3.15 eV for spin up and 0.15 eV for spin down. As such the doping of Mo can tune the the bandgap, structural, electronic and optoelectronic properties of MgH2 considerably for potential applications.

Topological optimization of a composite square lattice structure for bandgap property based on an improved multi-parameter genetic algorithm

This paper proposed a two-dimensional composite square lattice structure containing two kinds of inclusions (polymethylmethacrylate and T2 copper). To maximize the relative widths of the gaps between the adjacent energy bands of the phononic crystals (PnCs), an improved multi-parameter genetic algorithm was adopted in this paper. The material distribution and ligament sizes were considered simultaneously by ternary encoding and binary encoding. The propagation wave behaviors of the composite lattice structures were studied by the finite element method. The effects of different lattice shapes and other relevant influencing parameters on the bandgaps were discussed. The results showed that the lattice shape, ligament width, and material density affect the width and the location of the bandgaps, and the effectiveness of the proposed method was demonstrated by a transmission spectrum experiment.

Enhanced photovoltaic properties of halide perovskites due to multi-centered X–B–X bonding and p–p orbital coupling

Halide perovskites ABX3 are valuable photovoltaic and light-emitting materials due to their favorable optical properties of a direct bandgap and strong optical absorption just above the bandgap. Here, we show that these properties arise mainly from multi-centered X-B-X bonding and B-p X-p coupling. A bonding analysis finds that B–X bonds have shared electrons intermediate between that of ionic and covalent bonding. The B cation site p orbital interacts with left and right lobes of X-p orbitals, forming a special three-centered, two-electron X–B–X bond. This bonding mechanism is shown to give rise to unusually large optical matrix elements, especially in the visible spectrum, with the normalized matrix elements twice that of typical two-center bonds and transition probabilities four times larger. These cause a very sharp increase in optical absorption just above the gap. Experimental values of Urbach energies are found to be small, even if perovskites have disordered structures. The broad valence and conduction bands of the linearly aligned p orbitals cause dispersive band extrema, with small effective masses of both carriers at the R point, indicating a high carrier mobility.

A solar-blind photodetector with ultrahigh rectification ratio and photoresponsivity based on the MoTe2/Ta:β-Ga2O3 pn junction

High-performance solar-blind photodetectors have attracted increasing attention due to their important implications in military, medical, and industrial applications. Among a large number of wide-bandgap semiconductors, the quasi-two-dimensional β-Ga2O3 material is considered as a novel candidate for the solar-blind region (200–280 nm) owing to its suitable bandgap and superior optoelectronic properties. Herein, we report an outstanding solar-blind photodetector based on the MoTe2/Ta:β-Ga2O3 pn junction, which exhibits excellent optoelectrical properties with a superhigh rectification ratio ∼1.0 × 107, an ultrahigh responsivity (R ∼358.9 A/W), a large detectivity (D* ∼3.1 × 1012 Jones), an excellent external quantum efficiency (EQE ∼1.76 × 105%), a fast photoresponse (21.1/84.5 ms), and a high rejection ratio (∼1.1 × 103) under forward bias (1.5 V) and deep-ultraviolet (UV) illumination. These superior photoresponses result from the rapid separation of photoexcited electron-hole pairs driven by the synergistic effect of the forward bias and built-in electric field of the pn junction. Moreover, the MoTe2/Ta:β-Ga2O3 pn junction photodetector also demonstrates competitive optoelectronic characteristics in self-powered mode with an ultralow dark current of 8.5 fA and a fairly high R of 9.2 mA/W. As a result, this work provides an alternative solution for designing and fabricating high-performance solar-blind photodetectors with potential applications in UV imaging, environmental monitoring, and insecure communication.

Strongly Correlated Electronic Properties of FeO Studied by the SCAN+U Functional

The recently developed strongly constrained and appropriately normed (SCAN) meta-GGA exchange-correlation functional has been demonstrated to be able to give an accurate description of strongly correlated cuprates. However, it remains to be seen if SCAN calculations can predict the physics of compounds involving partially filled 3d orbitals. In this work, we present the first-principles electronic structure studies about the phase transition of FeO using the SCAN functional. Compared with the exchange-correlation functionals widely used in density functional theory, such as the local-density approximation (LDA) or the generalized gradient approximation (GGA), SCAN correctly predicts an antiferromagnetic insulating ground state. However, it still significantly underestimates the band gap for compounds involving partially filled 3d orbitals and, hence, predicts an incorrect phase transition pressure of FeO. Interestingly, the electronic structure and band gap properties of FeO from the SCAN calculations are similar to those from the GGA+U calculations with U0 = 0.7 eV. An on-site interelectronic interaction U is therefore still needed in order to correctly describe the phase transition physics of FeO for the SCAN calculations, predominantly as a simplified self-interaction-error reduction term that enhances the spatial compactness of 3d orbitals. The required value of U in SCAN+U is less than that required in GGA+U, and their difference is rightly equal to the value of U0. Most importantly, in comparison with GGA+U, the SCAN+U method no longer needs to use the prior-known knowledge to break orbital symmetry avoiding the metastability issue, benefiting from the spontaneous symmetry breaking it allows. The calculated properties of FeO from SCAN+U agree well with experiments.

Type II Homo-Type Bi2O2Se Nanosheet/InSe Nanoflake Heterostructures for Self-Driven Broadband Visible–Near-Infrared Photodetectors

Bi2O2Se nanosheets, an emerging ternary non-van der Waals two-dimensional (2D) material, have garnered considerable research attention in recent years owing to their robust air stability, narrow indirect bandgap, high mobility, and diverse intriguing properties. However, most of them show high dark current and relatively low light on/off ratio and slow response speed because of the large charge carrier concentration and bolometric effect, hindering their further application in low-energy-consuming optoelectronics. Herein, a homotype van der Waals heterostructure based on exfoliated n-InSe integrated with chemical vapor deposition (CVD)-grown n-Bi2O2Se nanosheets that have type II band alignment was fabricated. The efficient interfacial charge separation, strong interlayer coupling, and effective built-in electric field across the heterointerface demonstrated excellent, stable, and broadband self-driven photodetection in the range 400–1064 nm. Specifically, a high responsivity (R) of 75.2 mA·W–1 and a high specific detectivity (D*) of 1.08 × 1012 jones were achieved under 405 nm illumination. Additionally, a high R of 13.3 mA·W–1 and a high D* of 2.06 × 1011 jones were achieved under 980 nm illumination. Meanwhile, an ultrahigh Ilight/Idark ratio over 105 and a fast response time of 5.8/15 ms under 405 nm illumination confirmed the excellent photosensitivity and fast response behavior. Furthermore, R could be enhanced to 13.6 and 791 mA·W–1 under 405 and 980 nm illumination at a drain–source voltage (Vds) of 1 V, respectively, originating from a lower potential barrier. This study suggested that the Bi2O2Se nanosheet/InSe nanoflake homotype heterojunction can offer potential applications in next-generation broadband photodetectors that consume low energy and exhibit high performance.

Temperature effects on wave attenuation properties of metamaterials: Bandgap drift

The bandgap properties of phononic crystals and acoustic metamaterials have been investigated far and wide due to their potential application in vibration and noise reduction devices. The factors affecting the bandgaps including the material parameters and structural parameters were widely discussed, while there are few papers about the effect of temperature on the bandgap properties. In this paper, the influence of temperature on bandgap characteristics of metamaterial devices is studied theoretically and numerically. Since the elastic constants of some materials are affected by temperature, the bandgap drift phenomenon caused by temperature is displayed visually in the one-dimensional spring-mass system. Based on one-dimensional spring-mass metamaterials, the temperature-dependent theoretical formulations for calculating the dispersion relationships are derived and the effective mass equations related to temperature are also deduced to illustrate the negative effective mass density properties. Finally, the vibration attenuation properties and the directional transmission characteristics of two metastructures at different temperatures are performed to manipulate elastic wave field by temperature. This study could provide some insight into the use of acoustic metamaterials in extreme environments and provides a new approach for elastic wave field manipulation.

Tuning band gap, structural and optical properties of tin selenide nanoparticles by alkali metal doping

The layered chalcogenide semiconductor, Tin monoselenide is an extraordinary candidate among chalcogenide materials because of their novel physical and chemical properties and tremendous applications in thermoelectrics, solar cells, energy storage devices, i.e. metal–ion batteries, supercapacitors etc. In this work, we synthesized polycrystalline SnSe by a simple hydrothermal method using sodium borohydride as a reducing agent for the single phase formation of SnSe nanostructures. X–ray diffraction spectroscopy has been done to confirm the orthorhombic structure and single phase formation. Fourier transform infrared characterization has been done to observe the linkage of groups among the precursors and solvents used. Further, Photoluminescence characterization has been done to estimate the band gap values and radiative recombination mechanism of the material. Ultraviolet–Visible (UV–Vis) spectroscopy has been used to study the optical properties of the material. Moreover, extrinsic heteroatom doping of sodium and potassium have been done to form NaxSnx-1Se and KxSnx-1Se respectively, which further decreases the band gap of the host material. The absorbance spectra, percentage of transmittance, extinction coefficient, direct band gap values depend upon photonic interactions and has been determined by UV–Vis spectroscopy. Elemental composition of each sample has been confirmed by EDX spectroscopy. All these properties have made the material a potential candidate for opto–electronic and photovoltaic applications.

Density dependence of elastic properties of graphynes

Graphyne is a two-dimensional carbon allotrope of graphene. Its structure is composed of aromatic rings and/or carbon-carbon bonds connected by one or more acetylene chains. As some graphynes present the most of the excellent properties of graphene and non-null bandgap, they have been extensively studied. Recently, Kanegae and Fonseca reported calculations of four elastic properties of 70 graphynes, ten members of the seven families of graphynes [Carbon Trends 7, 100152 (2022)]. They showed that the acetylene chain length dependence of these properties can be simply modelled by a serial association of springs. Here, based on those results, we present the density dependence of these properties and show that the elastic moduli, $E$, of graphyne are less dependent on density, $\rho$, than porous cellular materials with an exponent of $E \sim \rho^{n}$, smaller than 2. We discuss the results in terms of the shape of the pores of the graphyne structures.

Role of anchoring groups on the light harvesting and optoelectronic properties of triphenylamine derivatives: insights from theory

In the present work, DFT and time-dependent DFT calculations were performed to investigate the role of anchoring groups on the photophysical properties and reveal structure-property correlations of triphenylamine (TPA) derivatives. The selected anchoring groups are tetrazole, acrylamide, hydantoin, and rhodanine. Our results show that the different anchoring groups employed alter the planarity, intramolecular charge transfer properties, and HOMO-LUMO gap and hence influence the optoelectronic properties of the dyes. Although all molecules fulfill the basic requirements with suitable energy levels, band gap, absorption, and charge transfer properties, the dye with rhodanine acceptor (TPA4) was the most promising candidate due to its lowest HOMO-LUMO gap, red-shifted highestλmaxabsorption value, better ICT pattern, low total reorganization energy, and good electron injection properties. Overall, it is anticipated that the results of this investigation will point to new avenues for the experimental fabrication of remarkably effective metal-free organic dyes for solar cell applications.

Numerical and experimental analysis of the stiffness and band-gap properties of shell structures with periodically variable cross sections

This paper describes one-dimensional periodic shell structures that have variable cross sections, a new type of periodic shell structures made from photopolymer. This paper will discuss the stiffness of periodic sub-cells that have variable cross sections and the band gaps of Bragg scattering shell structures based on numerical analysis and a series of experiments. This paper uses the Bloch theorem and lumped-mass method to create a band gap model for periodic shell structures. In this paper, an equivalent stiffness model for sub-cells is also created based on the principle of superposition and validated by experiments. Numerical studies and experiments are conducted to examine the effects of geometrical parameters, number of sub-cells, and stiffness of sub-cells on band gaps of one-dimensional periodic shell structures and to test the effectiveness of the models. The findings in this paper prove that by varying the stiffness of sub-cells under a fixed lattice constant, band gaps of one-dimensional periodic shell structures can be decreased. The findings also confirmed that the initial band gap of one-dimensional periodic shell structures can be lowered by increasing the number of sub-cells in a period. Unlike other types of Bragg scattering periodic structures, one-dimensional periodic shell structures allow their longitudinal band gaps to be adjusted under a fixed lattice constant. Those findings serve as a theoretical foundation for the application of Bragg scattering periodic shell structures in low-frequency vibration.

Monolayer black phosphorus: Tunable band gap and optical properties

Black phosphorus, with its tunable optical band gap, has gained significant interest due to its potential applications in optoelectronics. In this study, a first-principles approach based on density functional theory is used to examine the effects of Sc, Mn, and Ni doping on the electronic structure and optical properties of black phosphorus. The results indicate that the addition of impurity atoms with varying radii leads to a shift in the position of atoms surrounding the doping site. The band gaps of the doped systems are all reduced, with the most significant reduction observed in the Ni-doped system. Compared to intrinsic black phosphorus, the doped systems exhibit higher values of the imaginary part of dielectric function, optical absorbance, refractive index, and extinction coefficient in the visible and infrared regions. These findings suggest that Sc, Mn, and Ni-doped black phosphorus are suitable for optoelectronic devices, offering improved performance in the visible and infrared spectrum.

Band gap and magnetic properties modulation by substitution of cobalt in ZnO nanostructures

Band gap and magnetic properties modulation by substitution of cobalt in ZnO nanostructures

Bandgap properties and multi-objective optimization of double-cone pentamode metamaterials with curved side

Pentamode metamaterials (PM) have a promising application in noise reduction fields. In this paper, in order to improve the acoustic modulation capability of PMs, several novel curve PMs are proposed by replacing the straight sides of conventional PMs with curves. At first, the elliptic PMs (EPMs) with various unit cell arrangements (i.e., triangular, square, and hexagonal) are designed, respectively, and their bandgap properties are studied numerically in detail. The EPM with hexagonal unit cell arrangement (EPMH) presents better comprehensive bandgap properties in the EPMs. Subsequently, sinusoidal and power curves are introduced into the EPMH respectively to explore the influences of curve types on bandgap properties. The results show that the bandgap properties improvement of EPMH is higher in comparison with introducing other curves, and the reasons behind these improvements are carefully disclosed in combination with the spring-mass system. Finally, to further improve the bandgap properties of EPMH, a high accuracy Kriging model is constructed according to both the Latin hypercube design and double-point infilling. The Pareto optimal solution sets are determined using a non-dominated sorting genetic algorithm (NSGA-II), and the final compromise solution is gained by employing a fitness function. The bandwidths of phononic bandgap and single mode bandgap, and the total bandwidth of optimized EPMH are increased respectively by about 114.5, 4.3, and 7.7 times than those of the conventional straight side PMs. This investigation provides a fresh strategy for designing PMs with excellent bandgap properties.

Design and numerical characterization of high-performance all-perovskite multi-junction solar cells

Tunable bandgap and excellent optoelectronic properties can make perovskite solar cells (PSC) achieve high power conversion efficiencies in single junction as well as tandem architecture. In this work, 3 types of perovskite solar cell architectures i.e., single-junction, all-perovskite tandem and all-perovskite triple-junction, are modeled in SunSolve. The optimized layer thickness, reflection/parasitic absorptions, quantum efficiency, IV characteristics and power conversion efficiencies (PCE) of each cell are analyzed in detail. The single-junction solar cell of CH3NH3PbI3 with bandgap of 1.55 eV produces the PCE of 21.4%. The all-perovskite tandem solar cell of (CH3NH3)0.9Cs0.1Pb(I0.6Br0.4)3 (1.82 eV) as top sub-cell and CH3NH3Pb0.5Sn0.5I3 (1.22 eV) as bottom sub-cell exhibits a PCE of 26.06%. While the all-perovskite triple-junction solar cell of FA0.83Cs0.17Pb(I0.7Br0.3)3 (1.94 eV) as top sub-cell, CH3NH3PbI3 (1.55 eV) as middle sub-cell and CH3NH3Pb0.5Sn0.5I3 (1.22 eV) as bottom sub-cell has the remarkable PCE potential of 28.38%. This work guides towards multi-junction solar cells based on perovskite materials for the low-cost utilization of solar energy.