Band Gap Shrinkage Research Articles

Temperature-dependent optical absorption spectroscopy of ion irradiated silicon carbide epilayers on silicon

Transmission optical absorption spectra of ion-irradiated 3C-SiC epitaxial films on a silicon substrate are measured in the visible-near infrared range from room temperature down to about 10 K. These data show strong interference fringe patterns on top of the silicon absorption edge at about 10,460 cm1 which limits the transmittance of the samples. The radiation damage by 2.3-MeV Si+ and 3.0-MeV Kr+ ions is studied by following the impact of ion irradiation on these transmission spectra as a function of ion fluence and at various temperatures. The temperature dependence of the optical gap of silicon is deduced from the evolution of the fundamental absorption edge and that of the refractive index of SiC is deduced from the evolution of fringe spacing. The low temperature measurements evidence the shrinkage of band gap of silicon which is assigned to the gradual amorphization of the ion-implanted zone of the substrate as a function of fluence. These new results bridge a gap in the data on the properties of ion-induced damage in silicon carbide and silicon.

Impacts of the Lattice Strain on Light Emission in Layered Perovskite Thin flakes

AbstractStrain engineering, as a non‐chemical tuning knob, can enhance the performance of semiconductor devices. Here, an efficient manipulation of light emission is revealed in thin‐layered 2D perovskite strongly correlated to layer numbers of [PbI6]4− octahedron (n) and [C6H5(CH2)2NH3]2(CH3NH3)n‐1PbnI3n+1 (N) by applying uniaxial strains (ɛ) via bending the flexible substrate. As <n> increases from 1 to 3, an efficient light emission redshift (ɛ from −0.97% to 0.97%) is observed from bandgap shrinkage, and the shrinkage rate increases from 1.97 to 10.38 meV/%, which is attributed to the predominant uniaxial intralayer deformation due to the anisotropy of the [PbI6]4− octahedron lattice strain. Conversely, as <N> increases from 7 to 48 for n = 3, the deformation related to bandgap shrinkage rate is more prominent in small‐N flakes (<N> ≈ 7, 15.2 meV/%) but is easily offset in large‐N flakes (<N> ≈ 48, 7.7 meV/%). This anisotropic lattice deformation, meanwhile, inevitably modulates the carrier recombination dynamics of [C6H5(CH2)2NH3]2(CH3NH3)n‐1PbnI3n+1, which is essential for the development of highly efficient photoelectronic devices.

Thermochromic Properties of 3C-, 6H- and 4H-SiC Polytypes up to 500°C

The thermochromic properties (color change with temperature) of n type doped SiC wafers of different polytypes (3C, 4H and 6H) have been investigated up to 500°C under air. It was found that 3C-SiC color passes from bright yellow at room temperature to deep orangeat 500°C leading to a color contrast (ΔE) as high as 64. The hexagonal polytypes undergo also a color change upon heating but far less pronounced, with ΔE values <20. All these semiconductors undergo band gap shrinkage upon heating which effect largely participated to the observed color change. This effect is very sensitive for 3C polytypesince its bandgap is already in the visible energy range at room temperature. The thermochromicity of 3C-SiC was found to be reversible thanks to its thermal stability and its resistance towards oxidation.

The effect of quantum confinement and the role of electron-phonon interaction on the band gap shrinkage of some II-VI semiconductors

The role of electron-phonon interaction in band gap shrinkage for some II-VI bulk and low-dimensional semiconductors is investigated in this work. The variation of the energy band gap is studied as a function of temperature using Varshni's, Vina's, and Passler's relations. It is observed that the change in the energy band gap is affected due to the quantum confinement as the dimensionality of these semiconductors is decreased.

Excited‐State Dynamics and Optical Properties of Silica Under Ultrafast Laser Irradiation

AbstractExcited by intense infrared ultrafast light pulses, a wide bandgap material undergoes nonlinear ionization, generating a high density of free electrons in conduction states. As a result, the electronic band structure is critically modified and the bandgap shrinks. This induces rapid changes in optical properties, dramatically affecting the absorption spectrum during light coupling to the dielectric surface or during nonlinear propagation inside the bulk. This study analyzes the structural behavior and the modification of the optical properties of laser‐excited silica glass at the molecular cluster level through first‐principles simulations. Employing density functional theory and the GW approximations for band structure under nonequilibrium conditions, alongside the Bethe–Salpeter equation, the dynamics of the optical properties of fused silica are comprehensively explored. The behavior of excited fused silica in a wide photon energy range (from a few to 20 eV) is thus predicted. Laser‐induced electron excitation triggers a redistribution of charges between oxygen and silicon atoms, accompanied by a significant increase in electronic pressure, local atomic structure rearrangement, and material expansion. Molecular dynamics simulations offer a temporal perspective on the excited state dynamics, unveiling the intricate interplay between electronic and atomic effects on bandgap evolution. The analysis sheds light on excitonic resonances, intraband and interband transitions in fused silica under ultrafast laser irradiation, providing valuable insights into its excited state behavior and optical properties.

Improved photocatalytic degradation of malachite green, crystal violet and antibacterial activity of mesoporous Zn1-xCoxO nanoflakes

ABSTRACT The photocatalytic ability of the Zinc Oxide nanoparticles can be significantly enhanced through transition metal doping. In this work, Zn1-xCoxO nanoparticles were synthesised by a chemical coprecipitation method at room temperature. XRD studies indicated non-monotonic decrease in the Scherrer’s crystalline size from 19.79 nm for the pristine ZnO NPs to 14.81 nm for 2.5% Co2+ doped ZnO nanocrystals. The morphology of the nanoparticles was revealed by FESEM to resemble nanoflakes, and the existence of Co, Zn and O elements was verified by EDS findings. Tauc plots demonstrated a shrinkage of the optical band gap from 3.15 eV to 2.98 eV. The mesoporous nature and noteworthy increase in specific surface area (from 22.198 m2/gm to 54.159 m2/gm) were revealed by BET analysis of the N2 adsorption of the nanoflakes. Further, the degradation experiments of cationic dyes Malachite Green (MG) and Crystal Violet (CV) were carried out in the presence of synthesised catalysts under a high-pressure mercury vapour (450W-UV) lamp. Among the doped ZnO nanocatalysts, 2.5% Co2+ doped ZnO nanoflakes showed the most efficient photocatalytic degradation against MG (97.87%) and CV (93.27%) in 20 minutes. Furthermore, antibacterial activity was investigated against pathogenic bacteria Staphylococcus aureus, Bacillus subtilis and Escherichia coli which revealed improved bactericidal effects of 2.5% Co2+ doped ZnO nanoflakes. Hence, the current finding explored 2.5% Co2+ doped ZnO nanoflakes can serve as dual role efficient and potential candidate for eradication of organic as well as microbial contaminants from wastewater.

Synergistic effects of co-doping WO3 with Al and Pt on photoelectrochromic performance

In this work, we conduct a detailed study to understand the photoelectrochromic properties of Al-Pt co-doped tungsten oxide (WO3). Similarly, we support the photoelectrochromic findings with material characterization, providing additional understanding of the photoelectrochromic performance exhibited by the prepared samples. Additionally, we discuss the effects of Al and Pt doping on photoelectrochromic performance in detail, with a focus on X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) combined with EDS, X-ray photoelectron spectroscopy (XPS), and optical transmittance. XRD peaks confirm the presence of hexagonally-structured WO3 in the prepared samples with a polycrystalline nature, and a similar observation is confirmed via TEM analysis. Surface characterizations conducted through SEM and EDS analyses unveil the morphologies and particle sizes. Optical studies show a bandgap of 1.88 eV and 1.95 eV for Al doped and Al-Pt co-doped WO3, indicating bandgap shrinkage in comparison to the optical bandgap of WO3 of 2.36 eV. The Al-Pt co-doping results in a significant enhancement in photoelectrochromic performance, attaining an optical modulation of 43.61 % at 550 nm and consequently 85 % of the initial transmittance recovered after 2 h of bleaching in the dark. This is an improvement in comparison to the Al doped sample which attains a coloration depth of 43.15 % at 550 nm with only 75 % of the original transmittance being recovered after 2 h under dark conditions.

Forecasting optical response in lieu of electronic properties of TlCaF3 via halogen substitution for UV filtration: a DFT perspective

ABSTRACT It is demonstrated that the substitution of Cl, B, and I at different concentrations in TlCaF3 significantly alters the phase, electronic band structure and optical, elastic, mechanical and anisotropic properties of the material. At 2.81% and 5.63% concentrations of all substituted elements, the cubic phase of TlCaF3 transforms into a pseudo-cubic tetragonal phase. With the addition of substitutional atoms, a systematic considerable narrowing of the bandgap is seen, and it can be either direct or indirect depending on the symmetrical positions. The bandgap shrinking can be addressed, in part, via the total/partial/elemental density of states (DOS) in line with structural variations. From the optical response, the refractive index rises with substitution atoms. According to the mechanical stability requirements for each substitution, the determined elastic constants for cubic and tetragonal structures match. Additionally, using elastic parameters, it is possible to estimate the unique mechanical features to evaluate the ductility and brittleness of pure and substituted compounds. Furthernore, Cl, Br and I-atoms’ substitution would make them a suitable choice for improved optimisation in ultraviolet filters (UVF) due to the existence of their absorption spectra in the ultra-violet region and the modification in structural, electrical, optical, elastic and mechanical properties.

Effective Charge Collection of Electron Transport Layers for High-Performance Quantum Dot Infrared Solar Cells.

Infrared (IR) solar cells, capable of converting low-energy IR photons to electron-hole pairs, are promising optoelectronic devices by broadening the utilization range of the solar spectrum to the short-wavelength IR region. The emerging PbS colloidal quantum dot (QD) IR solar cells attract much attention due to their tunable band gaps in the IR region, potential multiple exciton generation, and facile solution processing. In PbS QD solar cells, ZnO is commonly utilized as an electron transport layer (ETL) to establish a depleted heterostructure with a QD photoactive layer. However, band gap shrinkage of large PbS QDs makes it necessary to tailor the behaviors of the ZnO ETL for efficient carrier extraction in the devices. Herein, the characteristics of ZnO ETL are efficiently and flexibly tailored to match the QD layer by handily adjusting the postannealing process of ZnO ETL. With a suitable temperature, the well-matched energy level alignment and suppressed trap states are simultaneously achieved in the ZnO ETL, effectively reducing the nonradiative recombination and accelerating the electron injection from the QD layer to ETL. As a consequence, a high-performance PbS QD photovoltaic device with power conversion efficiencies (PCEs) of 10.09% and 1.37% is obtained under AM 1.5 and 1100 nm filtered solar illumination, demonstrating a simple and effective approach for achieving high-performance IR photoelectric devices.

Responses Toward Structural, Electrical, Optical, and Mechanical Properties of RbGeF3 Under Pressure: DFT Insights

AbstractDensity functional theory is used in the current study to thoroughly examine the physical properties of perovskites made of halide RbGeF3 under a variety of hydrostatic pressure ranges between 0 and 50 GPa. This research seeks to reduce the electronic bandgap of RbGeF3 under pressure to enhance the optical properties and evaluate the compounds' decency for usage in optoelectronic and electrical uses. The precision of the structural characteristics is relatively high, and they fit well with published earlier research. A higher interaction between atoms is also a result of the large drop in lattice characteristics and link length. Pressurization reveals the ionic and covalent characteristics of the bonds between Rb‐F and Ge‐F, respectively. Both the conductivity and the optical absorbance vary noticeably when hydrostatic pressure is applied. A zero bandgap finally arises via pressure‐induced bandgap shrinkage, improving conductivity and electromagnetic absorption. Based on their optical properties, the materials being studied could be used in a variety of visible and ultraviolet optoelectronic devices. External pressure increases the anisotropy and ductility of the aforementioned perovskites, hence influencing their mechanical behavior. This study describes the changes in physical characteristics brought on by applied stress and offers a thorough examination of those changes.

First-principles investigation of structural, electronic and thermoelectric properties of SmMg2X2 (X = P, As, Sb, Bi) zintl compounds

Structure complexity, adequate electronic character and congenital lower thermal conductivity of zintl phases result in designing thermoelectrics of tremendous efficiency. Such qualities of zintl phases embolden us to present a detailed study of SmMg2X2 (X = P, As, Sb, Bi) zintl compounds by first principles. The structural characteristics fit well with the existing data. Electronic properties of titled compounds are scrutinized by adopting PBE-GGA, TB-mBJ and hybrid (YS-PBE0) potentials. The band structure calculations through TB-mBJ and hybrid (YS-PBE0) proclaim the semiconducting behavior of compounds and there is a shrinkage of band gap by changing X anion from P to Bi. The density of states (DOS) calculation reveals that there is a major contribution of Sm-f states and Mg atom in valence band while in conduction band, the prominent role is offered by Sm-d states and p states of X anion. Similarly, the temperature-dependent thermoelectric properties are scrutinized by using BoltzTraP2 code embedded within WIEN2k software for with and without spin–orbit coupling (SOC) and also through hybrid (YS-PBE0) functional. SOC, but even more hybrid functional, has been shown to have a significant effect on the transport behavior. Optimized values of carriers’ concentration are achieved which subsidize thermoelectric parameters like power factor (PF), Seebeck coefficient ([Formula: see text] and thermoelectric figure of merit (ZT) at elevated temperature. The compounds show excellent thermoelectric performance in the studied temperature range.

Unraveling the strain tuning mechanism of interlayer excitons in WSe2/MoSe2 heterostructure

Atomically thin transition metal dichalcogenides (TMDs) exhibit rich excitonic physics, due to reduced dielectric screening and strong Coulomb interactions. Especially, some attractive topics in modern condensed matter physics, such as correlated insulator, superconductivity, topological excitons bands, are recently reported in stacking two monolayer (ML) TMDs. Here, we clearly reveal the tuning mechanism of tensile strain on interlayer excitons (IEXs) and intralayer excitons (IAXs) in WSe2/MoSe2 heterostructure (HS) at low temperature. We utilize the cryogenic tensile strain platform to stretch the HS, and measure by micro-photoluminescence (μ-PL). The PL peaks redshifts of IEXs and IAXs in WSe2/MoSe2 HS under tensile strain are well observed. The first-principles calculations by using density functional theory reveals the PL peaks redshifts of IEXs and IAXs origin from bandgap shrinkage. The calculation results also show the Mo-4d states dominating conduction band minimum shifts of the ML MoSe2 plays a dominant role in the redshifts of IEXs. This work provides new insights into understanding the tuning mechanism of tensile strain on IEXs and IAXs in two-dimensional (2D) HS, and paves a way to the development of flexible optoelectronic devices based on 2D materials.

Effect of zirconium oxide nanoparticles on thermal, optical, and radiation shielding properties of Bi2O3-B2O3-MnO2 glasses

This study has explored the DSC, UV–Vis absorption spectroscopy, gamma ray and neutron shielding properties of Bi2O3-B2O3-MnO2: ZrO2 glasses. It demonstrates a unique approach to photon shielding analysis using JENDL/PD-2016 photonuclear data and employs a validated spherical neutron model for neutron shielding. Five transparent glasses were prepared with the chemical composition (in mol%) of 29Bi2O3-70B2O3-(1-x)MnO2: xZrO2, and labeled as MZ0.00 (for x = 0), MZ0.25 (for x = 0.25), MZ0.50 (for x = 0.5), MZ0.75 (for x = 0.75) and MZ1.00 (for x = 1). The glass ceramic nature of the samples has been characterized by DTA thermograms. The glass forming ability parameters (Kgl, S & H) were found to be highest for the sample MZ1.00. The UV–Visible optical absorption spectra have been interpreted, and hence the cut-off wavelength (λcut-off) and optical band gap (Eo) were evaluated. The absorption spectra have revealed the co-existence of manganese ions in three stable valence states Mn4+, Mn3+ and Mn2+ in the samples. When ZrO2 nanoparticles were added in the composition up to x = 0.50 mol%, the red shift in the cut-off wavelength (λcut-off) with gradual shrinkage in optical band gap (Eo) has been observed. Also, the linear and non-linear optical parameters viz., refractive index (no), non-linear refractive index (n2), linear optical susceptibility (χ(1)) and non-linear optical susceptibility (χ(3)) have been evaluated. These parameters showcased that B-O, Bi-O, Mn-O, Zr-O, etc. bonds could be strengthened by subsequent reduction of polarization of the trivalent ions (B3+ ions, Bi3+ ions and Mn3+ ions) in the glass system at higher concentrations of ZrO2. Photoatomic and photonuclear attenuation studies portrayed that the sample MZ0.50 has the lowest photon shielding capability. The fast neutron effective removal cross section (ΣR) was observed to be the highest for the sample MZ1.00. Thus, these glasses can be used to design the thermally stable transparent glasses, tunable optical elements, and radiation shielding materials.

Doping-induced band-gap shrinkage to modify the electronic structure of MoS2 for organic wastewater management.

MoS2, with its high specific surface area and tunable electronic structure, has received much interest in the fields of sensing and environmental remediation. Nevertheless, pure MoS2 has the disadvantages of easy aggregation and high electron-hole pair complexity, which affect its SERS and photocatalytic performance. In this work, a band-gap shrinkage strategy was used to improve MoS2 performance for SERS and photocatalytic applications. It exhibited high SERS activity (enhancement factor (EF) = 3.61 × 108), great stability (4 mth), broad applicability (CV, CR and R6G), and excellent reusability (with a recovery of 95% after 5 cycles). In addition, the interfacial dipole-dipole interaction and charge transfer (CT) process caused by doping Ru together enhanced the SERS sensitivity, reducing the limit of detection of CV to 1011 M. The degradation rate of 10-5 M CV was up to 99% after 60 min of Ru-MoS2 photocatalytic degradation under visible light. This study investigated the effect of doping-induced bandgap shrinkage on charge transfer (CT), providing new insights into improving the sensitivity of semiconductor SERS substrates for efficient low-concentration SERS detection and low-cost sustainable wastewater remediation.

Morphological evolution of individual microrods to self-assembled 3D hierarchical flower architectures of CuBixIn1−xSe2 for photo response applications

CuBixIn1−xSe2 3D hierarchical microrod-based flowers were prepared by microwave-mediated method. The morphology tuning for different Bi/In content shows optical and electrical properties variation with better white light photo response applications.

Fabrication of high-performance RT-NH3 gas sensor based on Cu and La co-doped ZnO films through a facile drop-casting method

In this work, undoped ZnO, 2 % Cu-doped ZnO, 2 % La-doped ZnO and 2 % (Cu:La) co-doped ZnO thin films are deposited on glass substrates using the facile drop-casting method, and their physical characteristics were investigated for the gas sensing application. The X-ray diffraction (XRD) patterns confirmed the hexagonal wurtzite structure and increase in crystallinity for the doped samples. Field Emission Scanning Electron Microscopy (FESEM) images revealed a significant change in morphology from the granular particles to densely packed nanoplates-like morphology for (Cu:La) co-doped ZnO. The doped samples exhibited a red shift in absorption edge and shrinkage of the optical bandgap. Multiple emission peaks in photoluminescence (PL) spectra indicated the presence of defect level in the material. It was discovered that Cu-doped and La-doped ZnO thin films had a considerable NH3 sensing response at ambient temperature. The gas response was remarkably enhanced to 341 % for (Cu:La) co-doped ZnO film and a quicker response/recovery time of 80/10 s for 250 ppm NH3. The (Cu:La) co-doped ZnO can serve as a feasible alternative for pure ZnO gas sensors.

Experimental determination of intrinsic carrier density in 4H-SiC based on electron diffusion current in an npn bipolar junction transistor

The intrinsic carrier density of 4H-SiC at temperatures ranging from 294 to 595 K was derived by analyzing a collector current in an npn-type SiC bipolar junction transistor, the structure of which was designed based on a device simulation. The obtained intrinsic carrier density was in good agreement with the value calculated from the bandgap and effective densities of states taking multiple and non-parabolic SiC bands into account. The coincidence of the intrinsic carrier density obtained by these two different approaches indicates the usefulness of the proposed method and the validity of the evaluated value of intrinsic carrier density. The temperature dependence of the bandgap was also estimated from the deduced intrinsic carrier density and compared with an empirical formula. The derived bandgap agreed well with the empirical formula showing bandgap shrinkage at high temperatures. The errors in evaluating the intrinsic carrier density and the bandgap caused by the estimation of the hole density and electron mobility in the base layer are also discussed for the proposed method.

A systematic investigation of structural and optical properties of Li ion implanted MgTiO3 thin films

Effects of lithium (Li) ions implantation on structural, optical, and luminescent properties of MgTiO3 (MTO) thin films were investigated in the present report. The films were initially deposited at a substrate temperature of 200 °C, which were then annealed at 700 °C for crystallization. Crystalline MTO films were implanted with 30 keV Li ions at various fluences ranging from 1 × 1014 to 1 × 1015 ions/cm2. The crystallinity of films decreases upon Li ions implantation at a fluence of 1 × 1014 ions/cm2. In contrast, a further increase in fluence to 1 × 1015 ions/cm2 results in an improvement in crystallinity. Annealed and implanted films show lesser transmittance than the as-deposited films. As-deposited film exhibits a bandgap of 3.79 eV, which increases to 4.28 eV upon annealing the films. Subsequently, with an increase in implantation fluence, the shrinkage in optical bandgap from 4.26 to 4.09 eV is observed. Moreover, the annealed films exhibit a refractive index of ∼2.2 which is higher as compared to as-deposited and implanted MTO samples. X-ray photoelectron spectroscopy was carried out to investigate the surface states related to Mg, Ti, and O in the samples and to analyze the variation of defects’ concentration with implantation. MTO thin films show luminescent centres in near ultraviolet and visible region. The annealing results in the enhancement of photoluminescence intensity which decreases on implantation, and a drastic quenching of intensity was observed at a fluence of 1 × 1015 ions/cm2. The average decay lifetime of as-deposited film is 9.1 ns. Annealing results in an increase in the average decay lifetime, while implantation leads to the reduction of average decay lifetime of MTO films.

Formula omitted] thin-films as efficient optical limiters: A study on impact of annealing

We present a detailed investigation on the impact of thermal annealing on the third-order nonlinear optical (NLO) properties of sputtered WSe2 thin films. The process of annealing results in an appreciable change in morphology and texture of WSe2 films that is governed by the annealing temperature and duration of annealing. These modifications are apparent from the electron micrographs as well as X-ray diffraction and Raman spectrum. The visible absorption spectroscopy point towards a gradual shrinkage of electronic bandgap which motivated us to explore the impact of NLO properties of the annealed WSe2 films. The NLO measurements were carried out using a single-beam Z-scan technique using ultrashort pulses centered at 1030 nm excitation wavelength. The investigations reveal a distinct two-photon absorption (TPA) signature for the annealed WSe2 films along with a self-focusing effect. The observations indicate the strengthening of optical limiting behavior exhibited by WSe2 films when they are annealed for a longer duration. The outcomes suggest that the process of annealing in WSe2 films allows controlling the NLO properties and renders them more suitable candidates as optical limiters for high-power applications.

Effect of Bi doping on the opto-electronic properties of ZnO nanoparticles for photodetector applications

Bi (0, 1, 3 and 5 wt%) doped ZnO nanopowders were generated in this study using a simple and inexpensive wet chemical method. X-ray diffraction, Fourier-transform infrared spectroscopy, Scanning electron microscopy, photoluminescence spectroscopy, UV–Vis-DRS, studies are used to evaluate the effect of dopants in ZnO sample. Crystallite size and grain size of the samples were reduced and enhanced the surface to volume ratio due to Bi doping. Oxygen vacancies and shrinkage of bandgap plays a significant role in enhancement of photosensing property. Bi doped ZnO nanopowders had better photosensing activity than pristine ZnO nanopowders. Also, the doped metal ions improved the photosensing efficiency by increasing electromagnetic wave absorption and inhibiting the recombination rates of charge carriers resulting raise of hole-electron pair production. The significant photosensing parameters responsivity (R), detectivity (D*) and external quantum efficiency (EQE) of the 3 % of Bi doped ZnO sample exhibited maximum values such as 0.163 AW−1, 1.26 × 1010 Jones and 53% respectively. The findings of this present study show that the3 % Bi doped ZnO can be an excellent photosensing material.