Change In Band Gap Research Articles

Photoluminescence and quantum yield behaviour of vanadium doped Zn2SiO4 phosphors facilely synthesized by sol-gel method

Vanadium-doped Zinc Silicate (Zn2SiO4) phosphors were synthesized through the sol-gel method. Structural, morphological and optical techniques were used to investigate the effects of vanadium incorporation on their structure, morphology, and optical behaviour. We reported that vanadium incorporation in Zn2SiO4 phosphors significantly modifies their crystallinity, morphology, and photoluminescence properties. When vanadium is added to Zn2SiO4 phosphors, the energy band gap (Eg) changes from 5.29 to 2.34 eV. Vanadium dopants generate imperfections in Zn2SiO4 phosphors, the leading cause of their emissions in visible regions and quantum yield. Quantum yield is estimated at 7.06% for Zn2−xVxSiO4 (x = 0.08) phosphor. The luminescence decay lifetime of the prominent emissions of vanadium-doped Zn2SiO4 was measured using a double exponential fitting technique, and the average lifetime is 11.7 ns.

Chemically modified graphene sheets as potential sensors for organophosphate compounds(pesticide): A DFT study

Owing to the extensive use of pesticides in agriculture and the threat posed to the environment and humankind, attention is drawn toward the need for its remediation from various sources, in particular from water. The adsorption characteristics of hydrophobic graphene and its BN doped counterparts are evaluated using DFT methodology, to assess their sensing potential towards notable pesticide molecules (Organophosphate pesticides (OPs) such as chlorpyriphos, parathion, methyl-parathion, and fenitrothion). To accomplish this, various electronic properties such as band structure, DOS, Bader charge analysis, and CDD have been calculated. Interestingly, exothermic interactions (-0.1 eV to −2 eV) have been noted for all the OPs with NB doped single vacancy defect induced graphene surfaces. With an exception of FTN, all the other OPs showed significant changes in the electronic properties of surfaces as apparent from the change in band gap and nature of the band from indirect to direct. The appreciable adsorption energy, higher charge transfer, and notable variation in the band gap are the decisive factors behind the strong interaction exhibited by NB doped surfaces especially dg-(NB-2)2 (except FTN). These findings demonstrate the suitability of NB doped single vacancy defect induced graphene sheets for the detection of the studied OPs.

Bandgap Change in Short‐Period InN/AlN Superlattices Induced by Lattice Strain

III–V GaN‐based nitride semiconductors have attracted interest because their bandgaps can be tuned by constructing alloys and superlattices, and by applying lattice strain. Herein, the influence of uniaxial and equibiaxial strains on the bandgaps of short‐period InN/AlN superlattices is examined using first‐principles calculations. For the uniaxial strain, the bandgaps change in the same manner when the superlattices are strained in the a‐ and m‐axis directions; however, the bandgaps change in a different manner when the superlattices are strained in the c‐axis direction. Even when equibiaxial strain is applied, the change in bandgap depends on the direction in which the strain is applied.

Morphological and optical investigations on Gd2O3 nanostructures

Gd2O3 nanostructures were synthesized by hydrothermal method with five different reaction conditions (reaction temperatures: 130 °C, 180 °C and 230 °C and pH: 8, 10 and 13) and the effect on morphology of the Gd2O3 nanostructures is studied. Scanning electron microscopy results reveal that the morphology and the dimension of the nanostructures change with different preparation conditions and it ranges from thick ellipsoids to fine long nanorods. Fine long perfect nanorods are formed when the reaction temperature is 230 °C and pH is 8. Diffraction studies confirm that the crystal structure of the Gd2O3 samples is cubic. The change in the optical band gap of the Gd2O3 sample is consistent with the change in the dimension of the nanostructure. The quantum yield of Gd2O3 samples (2.74% to 6.84%) is found to be larger than the values reported for Gd2O3 samples with dimensions in quantum confinement range and in submicron range.

Characterization of Electronic Transport Properties of Narrow-Band Gap Fe(Se1-xTex)2 Alloys via the Two-Band Model

Environmentally sustainable thermoelectric technologies can be more broadly applied in industries once the performance of thermoelectric materials is improved. Several approaches have been proposed to improve the electronic transport properties of thermoelectric materials. The effects of each approach on the electronic properties can be evaluated by changes in the band parameters. The Single Parabolic Band (SPB) model has been widely used to determine the effect of different materials engineering strategies on band parameters, such as the density-of-states effective mass and deformation potential. However, when the material has a narrow band gap, the Two-Band (TB) model better describes the changes in band parameters, as it includes the bipolar conduction from the minority carrier band. Here, the band parameters of previously reported Fe(Se1-xTex)2 (x/i> = 0, 0.2, 0.6, 0.8, 1) alloys, whose band gap significantly decreases with x/i>, have been estimated using the SPB and TB models. While the x-dependent band parameters obtained via the SPB model varied abruptly with x/i>, all the band parameters estimated by the TB model changed linearly with x/i>. The abruptness observed in the band parameters of the SPB model can be attributed to artifacts reflected in the single band, which occurs when the minority carrier band and band gap change are not included. The bipolar thermal conductivity of Fe(Se1-xTex)2 alloys was also calculated using the TB model, and is understood in terms of changes in the weighted mobility ratio, Hall carrier concentration, and band gap in terms of alloy composition, x/i>.

Adsorption and gas-sensing properties of SF6 decomposition components (SO2, SOF2 and SO2F2) on Co or Cr modified GeSe monolayer: a DFT study

This paper first proposed to modify the GeSe monolayer by doping Co and Cr atoms and investigated the potential application in the monitoring SF6 insulation equipment. The adsorption mechanism of these three gases on Co/Cr-Gese monolayers is systematically studied based on density functional theory. The adsorption mechanism of three gas molecules on the new sensors (Co-GeSe and Cr-GeSe) was evaluated from the aspects of adsorption energy, charge transfer amount, energy band structure, molecular frontier orbital, and recovery time. The results show that Co and Cr modification can significantly improve surface activity of the intrinsic GeSe monolayer, which can make the intrinsic GeSe monolayer possess better absorption and sensing performance. Especially, the Cr-GeSe monolayer gets more sensitive to the target gas, indicating this monolayer has the prospect of becoming the adsorption material of the target gas (SO2, SOF2, and SO2F2). The reasonable adsorption energy and large change of bandgap indicate that Co-GeSe monolayer can be a potential candidate for SO2 and SOF2 sensors. In addition, this work provides theoretical guidance for the further study of transition metal-doped GeSe monolayer for detecting the decomposition products of SF6 in the field of electrical engineering.

Review on structure, optical and magnetic properties of cobalt doped ZnO nanoparticles

In recent years, semiconductor nanostructure gained much attention due to their application in next generation optoelectronics devices. In this article it is reviewed how cobalt doping impact zinc oxide nanoparticles structural, optical, and magnetic properties. The basic reason causing variation in structural parameter have been explored. The change in bandgap lead toward significant improvement in absorption and emission spectra. We explored how doping affects the optical property of ZnO and special attention have been paid to cobalt doping effect. The application of doped ZnO has been discussed in detail with the help of suitable table and figure. The synthesis process plays an important role to get the desired property for specific application. Therefore, the special attention has given to explore the entire synthesis route applicable to ZnO nanostructure fabrication. Investigation of the magnetic and optical characteristics of cobalt-doped ZnO nanoparticles shows its potential application in Dilute magnetic semiconductors and optoelectronic devices.

Optical band-gap evolution and local structural change in Ge2Sb2Te5 phase change material

The amorphous to crystalline phase transition in Ge2Sb2Te5 (GST) phase change material is investigated using XRD and the systematic variation in the optical band-gap (Eg ) and structural disorder (B 1/2) employing UV-Vis-NIR spectroscopy. The amorphous phase is observed to have Eg value of 0.708 eV while crystalline phase (200 °C) shows 0.442 eV. Variation in B 1/2 slope of 13.4 % is noted around the crystallization temperature (150 °C), depicting structural disorder reduction and hence structural ordering in the material forming the cubic phase. The change in the optical band-gap and local structural disorder upon crystallization occurs due to alternation in the atomic bonding configurations, which is explored using XPS technique. The findings reveal Ge-Te (~1218.35 eV binding energy) and Sb-Te (~528.8 eV) bonds in the amorphous phases. However, their bond lengths increase (hence binding energy reduces) as the annealing temperature rises, demonstrating phase switching occurs upon reaching the crystallization temperature. Insight into the optical band-gap, local structural disorder, and atomic arrangement governs many vital features of phase change material, such as fast crystallization speed, better thermal stability, high endurance, and large resistance contrast, which provide the path for non-volatile memory and nanophotonic applications.

Bandgap and defects regulation of La2−xAxNi1−yByO4+δ (A = K, Sr, B = Co, Mn) Ruddlesden-Popper type perovskites for efficient photocatalytic hydrogen evolution

The doping of certain exotic metal ions in photocatalysts may produce surface defects (such as oxygen vacancies) as well as bandgap changes, which may play an important role for their photocatalytic performance. In this work, bandgap modulation by precise control of defect concentration and transition metal oxidation state mainly owning to A-site doping and the B-site cations were achieved for the Ruddlesden-Popper structure (La2NiO4). The effects of K+, Sr2+, Co2+, Mn2+ single-mental doping and co-doping on the photocatalytic activity of La2NiO4 photocatalysts were investigated. It was found that the co-doping samples had a significant synergistic effect on the optical properties and photocatalytic activity of La2NiO4 photocatalysts. All the single-metal-doped La2NiO4 photocatalysts exhibited higher photocatalytic activity than their pristine counterpart. Moreover, the Sr2+, Co2+ co-doped La2NiO4 exhibited the best optical properties, photocatalytic activity and a the hydrogen production rate of La1.9Sr0.1Ni0.8Co0.2O4+δ more than 29 times of pure La2NiO4, has been achieved. Further analysis indicates that the surface oxygen vacancies as active sites promote efficient separation and migration of photogenerated carriers. The co-doped photocatalyst not only has a narrower bandgap, but also owns a more favorable conduction band position for photoinduced electron reduction. Overall, this work provides a simple method for introducing oxygen defects and bandgap modulation in Ruddlesden-Popper structure through metal ion doping. It is also expected to enlarge the understanding of the structure-dependence of the photocatalytic performance of perovskite materials.

Ultra-sensitive one-dimensional phononic crystals temperature sensor: theoretical optimization

Abstract The present study intends to solve the problem of low-temperature sensitivity of most conventional period materials. A solid/solid one-dimensional phononic crystal design is proposed as a pragmatic thermal sensor. Here, each unit cell of the proposed phononic crystal sensor is designed from epoxy and concrete. In this structure, irregularity in layers’ arrangement results in the generation of resonant peaks at the transmission spectrum. The irregularity is introduced to the system by stacking two mirror phononic crystals to form a whole symmetric phononic crystal design. Thermal variations lead to significant changes in Young’s modulus of concrete and epoxy; consequently, transmission spectra and local resonant peaks are shifted as well. Firstly, the band gap changes, and sensor performance were investigated under different conditions such as the incident angle and the number of unit cells to get the optimum parameters. Meanwhile, promising results have been obtained, with increasing the incident angle, the sensitivity increases exponentially till it reaches 1.5 × 103 Hz/°C at an incident angle = 18°. Also, with increasing the temperature from 25 to 100 °C, the sensitivity increases dramatically from 188.8 to 362.4 Hz/°C, respectively. In addition, the largest value of Q-factor was investigated at periodicity number = 4. Moreover, in this regard, the Q-factor has the value of 3708 at 25 °C and it reaches to the value of 896.6 at 100 °C. That is to say, the obtained results may be useful in designing thermal sensors with a high sensitivity value.

Intense blue emission from one-pot synthesized quaternary CsZnxPb1-xBr3 perovskite quantum dots

Suitable ions incorporation into CsPbX3 perovskite nanocrystals can improve the luminescence efficiency, chemical stability, and more importantly modify the emission bands from semiconductor band gap change, for various applications. In this research, high content Zn2+ was introduced into CsPbBr3 quantum dots through a simple one-pot synthesis method. A new intense blue emission centered at 440 nm was observed from heavy Zn alloyed nanocrystals, for increased semiconductor band gap, along with green emission at 510 nm from CsPbBr3 quantum dots, when the ratio of ZnBr2/PbBr2 precursor was larger than unity. The appearance of Zn 3d peaks and the decreasing of bonding energy of Pb 4f peaks from X-ray photoelectron spectroscopy spectra indicated that Zn2+ ions were incorporated into CsPbBr3 quantum dots to form CsZnxPb1-xBr3 alloyed quantum dots. The elements quantitative analysis showed that partial Zn2+ ions in precursors were involved in the synthesis of the quantum dots, and the Zn alloyed perovskite nanocrystals could be described as CsPbBr3, CsZn0.43Pb0.57Br3, CsZn0.54Pb0.46Br3, CsZn0.68Pb0.32Br3 respectively when the ZnBr2/PbBr2 precursor ratios were 0:1, 1:1, 3:1 and 5:1. The X-ray diffraction patterns confirmed that the incorporation of small radius Zn2+ ions into crystals led to crystal shrinkage, instead of crystal phase change. The transmission electron microscope images of the nanocrystals showed cubic geometries and reduced average sizes from 23.3 nm to 17.6 nm with Zn2+ content increase, resulting in the blue shift of emission bands. The energy dispersive spectroscopy elemental mapping of nanocrystals also confirmed the incorporation of Zn2+ ions into the nanocrystals.

Computational study of Be doped LaAlO3 perovskite

DFT based calculations are used to study the structural and elastic properties of pure and doped LaAlO3. By inclusion Be along La-site and Al-site (La1-xBexAlO3 and LaAl1-x BexO3, x = 0%, 20%, 40%, 60%, 80% and 100%) LaAlO3 structure are investigated. The effect of Be doping along with La-site and Al-sites was studied using supercell calculation. The structural properties show a change in the lattice parameter because of Be doping. Our results show that after doping Be the nature and magnitude of band structure are affected significantly. Be doping along La-sites the band structure diagram shows valence band and conduction band are overlapped indicating metallic conductivity of doped material. While for Be doping along with Al-sites the band gap increases from 3.01 eV to 3.31 eV, also, the nature of the band gap changes from indirect to direct band gap. By studying the elastic constant, bulk moduli, shear moduli, Young's moduli, Poisson's ratio, Pugh's ratio and Cauchy pressure we concluded that by Be doping, the structure is mechanically stable and more ductile. The study of mean sound velocity and Debye temperature shows that the under-discussion material has weaker bonding and lower thermal conductivity than the pure LaAlO3. The inclusion of Be atoms along La-sites and Al-sites of LaAlO3 structure significantly shakes up the properties of the selected material and make it compatible material for a vast range of applications. We also considered the pressure induced structural properties of pure LaAlO3 and Be doped LaAlO3.

8-band k ⋅ p modeling of strained InxGa(1−x)As/InP heterostructure nanowires

An 8-band k⋅p theory is implemented for studying the electronic properties of near-surface lateral InxGa(1−x)As/InP heterostructure nanowires in the (100) direction. The change in bandgap and effective mass due to inhomogeneous strain are compared to unstrained scenario nanowires, and the lattice mismatch is varied from −3.2% to 1%. The nanowires' height is H = 5, 13 nm, and the width varies from the smallest possible width for a given height to 100 nm. The change in the bandgap exhibited a nonlinear trait with strain for all sizes of the nanowire. The tensile strain reduces the bandgap irrespective of the width of the nanowire for a given height, while the effect of compressive strain on change in the bandgap becomes width dependent. Minima and maxima in the change in effective mass with respect to the nanowire width are observed in compressively and tensile strained nanowires, respectively, due to the interplay of quantum confinement and strain. The electrical performance of a single nanowire In0.85Ga0.15As/InP MOSFET in quantum capacitance limit is discussed for various nanowire sizes. The implemented 8-band k⋅p method is verified with the available experimental work and demonstrated that the developed model can be extended to study electronic parameters of arbitrarily shaped core–shell structures over a wide range of strain.

Adsorption and gas-sensing performances of C2H2, C2H4, CO, H2 in transformer oil on Pt-doped MoTe2 monolayer: A DFT study

In this paper, the DFT method is employed to analyze the sensing and adsorption performance intrinsic MoTe 2 and Pt doped MoTe 2 to the characteristic oil dissolved gases (C 2 H 2 , C 2 H 4 , CO, and H 2 ). The value of binding energy demonstrates Pt atom prefer to be trapped at T Mo site among T H , T Mo , T Te , and T Br sites. Pt–MoTe 2 depicts the excellent adsorption property to the four gases compared with MoTe 2 . Based on the change of band gap, the conductivity and sensitivity of Pt–MoTe 2 decreases and increases after adsorption of four gases. The moderate recovery time reveals the higher gas-sensing and response of Pt–MoTe 2 to C 2 H 2 and C 2 H 4 than to CO and H 2 . This paper provides theoretical guidance of potential application of a novel and high-performance gas sensor for online-monitoring the characteristic dissolved gases in transformer.

Temperature Dependent Band Gap Correction Model Using Tight-Binding Approach for UTB Device Simulations

In order to accurately determine the electrostatics of Ultra-Thin-Body (UTB) devices, the semi-empirical tight-binding (TB) approach is widely used for calculating the channel thickness dependent band structure of any material at those temperatures where TB parameters are available (generally defined at 0 K and 300 K). In this work, we analyze the variation of band structure for Si, Ge, and GaAs over different channel thicknesses at 0 K and 300 K, and show that the band curvature at the band minima remains unchanged with temperature, while the band gap changes significantly and affects the channel electrostatics. Based on this finding, we propose an approach to simulate the electrostatics of UTB devices, at any temperature between 0 K and 300 K, using the band structure obtained at 0 K, along with a suitable channel thickness and temperature-dependent band gap correction. From the results obtained for the channel charge density, we show good agreement with band structure based simulation results, at 300 K, over a wide range of channel thicknesses, for Si, Ge, and GaAs, while also showing good agreement with TCAD simulation results, at a typical intermediate temperature of 150 K, thus highlighting the accuracy, simplicity and wide applicability of the proposed approach.

Fabrication of PVA-MWCNT nanocomposite films for UV-shielding applications

Polyvinyl Alcohol (PVA) polymer nanocomposites are incorporated with Multiwall Carbon Nanotubes (MWCNT) of 20 nm diameter by solution casting. The SEM results show uniform dispersion of MWCNTs in the PVA matrix. The glass transition temperature (Tg) and the thermal decomposition temperature of PVA slightly increases with the incorporation of the MWCNTs. The DC electrical conductivity of PVA increases systematically from 4.1 × 10-13 S/cm to 8.1 × 10-8 S/cm for the highest 0.9 wt% MWCNTs. The direct band gap changes from 6.13 to 5.57 eV while the indirect band gap from 5.88 to 5.17 eV, also Urbach energy changes from 0.223 to 0.282 eV as the 0.9 wt% of MWCNTs reinforced in PVA.

Synthesis and characterizations of nickel doped Co-Zn-Y ferrites

The Nickel substituted Cu-Co-Zn-Ce nano ferrites, Zn0.15Co0.45Cu0.40-xNix Fe1.85Ce0.15O4 with x=0, 0.1, 0.20, 0.30, 0.40, were synthesized using the coprecipitation technique. The sample were sintered at 900 ℃ for 5h. The structural, electrical, dielectric and magnetic properties of all the prepared samples were characterized by XRD, SMU2401, UV-Vis and FTIR. The powder X-ray diffraction patterns of all the prepared samples confirmed the formation of single-phase cubic spinel structures. These samples further characterized for the electrical properties by using two-probe type method. The DC resistivity of all the ferrite composition decreased as the temperature increased, showing their semiconductor nature. UV-Vis and FTIR confirmed the substitution of nickel in Cu-Co-Zn-Ce ferrites. From UV-Vis it is observed that the optical band gap changes from 3.9 eV to 5.3 eV with addition of Ni. FTIR analysis revealed that a strong variation on tetrahedral absorption frequency band is present which is due to the replacement of Ni on Cu. All these results suggested that these materials can be used for wastewater treatment.

Arsenene as a promising sensor for the detection of H2S: a first-principles study.

To explore the feasibility of arsenene in detecting H2S gas, we employ the density-functional theory to investigate the geometry, electronic structure and magnetic properties of defected and doped arsenene. Point defects do not appreciably improve the sensing performance of arsenene due to small adsorption energies and charge transfer. The doping of transition metals (Ti, V, Cr, Mn, Co and Ni) introduces magnetic moments and narrows the band gap of arsenene. Transition metal (TM) dopants can enhance the interaction between H2S and a modified arsenene substrate. Adsorption energies and charge transfers increase significantly, and the adsorption converts to chemisorption. After adsorption, the Ti and Cr-doped system's band gap change is twice that of the pristine and defective arsenene. The adsorption of H2S changes the system properties of two TM-doped arsenenes: Ti-doped arsenene transforms from semiconductor to half-metal, and Ni-doped arsenene transforms from half-metal to conductor. Electrical signals can be observed in this process to detect H2S molecules. Our calculations show that doping improves the detecting performance of arsenene to H2S molecules more efficiently than defects. Our results indicate that arsenene has a promising future in developing H2S gas sensors.

Quantifying interfacial energetics of 2D semiconductor electrodes using in situ spectroelectrochemistry and many-body theory

In situ optical measurements can reveal bandgap changes in monolayer semiconductor electrodes.

Photo-induced effects in Ge-Ga-Se films

We investigated photo-induced effects in Ge-Ga-Se films. It was found that, thermal annealing causes the increase of the bandgap in the as-prepared films, and all three annealed films show photodarkening (PD) behavior with light illumination but the change of optical bandgap is minimum in Ge25 film. Such PD behavior can be recovered by reannealing of the film. The kinetic process of Tf/Ti exhibits part recovering of PD during the cycle of laser on and off. Moreover, no obvious structural change can be observed in the Raman spectra of the films at different states, especially in that of Ge25 film.