Trend Of Band Gap Research Articles

Anisotropic optical response of Nb2SiTe4 under pressure

The optical response of a layered anisotropic crystal Nb2SiTe4 was investigated under varying pressure using synchrotron infrared spectroscopy, Raman spectroscopy, and first-principles calculation. This study revealed diverse trends in optical response and bandgap at different pressure levels. Below 5 GPa, the bandgap decreases rapidly due to the reduction of interlayer distance. The Raman and infrared optical response show significant changes at 11 and 21 GPa, suggesting structural and electronic structure transformation at these pressure points. Meanwhile, the optical responses exhibited distinct pressure effects along different crystal axes. Overall, these results provide valuable insights into the pressure-induced lattice deformation and optical transitions in Nb2SiTe4 and similar layered anisotropic materials, contributing to an insightful understanding of layered materials under pressure.

Comparative Study of the Mechanical, Electronic Structure, and Optical Properties of Cubic Lithium‐Based Perovskite LiMgX3 (X = Cl, Br, I) under Pressure Effects: First‐Principles Calculations

The mechanical, electronic structure, and optical properties of lithium‐based perovskite LiMgX3 (X = Cl, Br, I) are investigated for the first time at 0–20 GPa using density‐functional theory. The Born stability criteria reveal that the phase transition points of LiMgCl3, LiMgBr3, and LiMgI3 are 20.7, 20.9, and 23.4 GPa, respectively. At 0 GPa, studies on the electronic properties using the Heyd‐Scuseria‐Ernzerhof (HSE06) functional show that LiMgCl3 and LiMgBr3 are indirect bandgap insulators with values of 5.336 and 4.113 eV, whereas LiMgI3 is an indirect bandgap semiconductor with a value of 2.055 eV. In addition, the bandgap calculated using both the PBEsol and HSE06 functionals decreases with increasing pressure, and the bandgap trends with pressure are consistent. Both functionals are also used to study the optical properties of LiMgX3 compounds, which show that they have potential for use in vacuum ultraviolet and photovoltaic applications. The mechanical and optical characteristics of the materials are significantly enhanced under pressure.

Al-Mg co-doped TiO2 thin film as a promising ETL for perovskite solar cells: An experimental and DFT investigation

Solar cells offer a potential solution to the energy crisis, and perovskite solar cells are the latest frontrunner. However, low electrical conductivity of TiO2 ETL serves as a bottleneck to its efficiency. In this study, Al-Mg co-doped anatase thin films are prepared by one-step spin coating method and then characterized to tackle this problem. XRD results show that doping leads to smaller crystallite size and decreased interplanar spacing, but increases the lattice strain and dislocation density. AFM images reveal that Al-Mg co-doping increases surface roughness. Surface morphology of the co-doped film is good, covering the whole substrate uniformly without any pinholes. XPS results ensure that the desired composition is achieved. UV–vis spectroscopy data show that optical bandgap increases with Al-Mg co-doping, leading to the highest bandgap for Ti0.87Mg0.1Al0.03O2, which also has the highest transmittance peak, reaching 90 % in the visible region. Urbach energies are also calculated to get a better estimate of the effective bandgaps. Band edge positions are calculated using Mulliken’s electronegativity, demonstrating improved band alignment due to Al-Mg co-doping. The bandgap trend is further corroborated with DFT + U simulations, which show that calculated bandgaps match the experimentally obtained optical bandgaps. Additionally, density of states reveal that Ti 3d orbital dominates the conduction band and O 2p orbital dominates the valence band, with hybridization taking place between different orbitals. Four-point probe test demonstrates that Al-Mg co-doping leads to a consistent decrease in sheet resistance of the thin film. Better band alignment and higher visible light transmittance combined with enhanced conductivity indicate that Al-Mg co-doped anatase thin film has high potential as an ETL for perovskite solar cells.

Enhanced photo-electro catalytic CO2 conversion using transition metal doped TiO2 nanoparticles

The Fe, Co, and Ni-doped TiO2 were synthesised by sol–gel process, and the nanomaterials were coated on electrodes through a slurry with Nafion. This study aims to use prepared nanoparticles of Fe, Co, and Ni-doped TiO2 to enhance the photo-electrocatalytic conversion of CO2. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and electrochemical analyses were utilized to characterise the doped and undoped TiO2 nanoparticles. Transmission electron microscopy (TEM) studies revealed that the nanomaterials had an average size below 20 nm. Tauc plot analysis for optical absorption studies shown a reduction in band gap when doped with Fe, Co and Ni, with the most significant decrease observed for Fe-doped TiO2, following the order Fe < Co < Ni. Electrochemical CO2 conversion efficiencies were investigated through cyclic voltammetry (CV) curves, electrochemical impedance spectroscopy (ESI), and linear sweep voltammetry (LSV) studies. The results were analysed based on various scan rates and photo exposure, indicating superior performance for Fe-TiO2, aligning with the observed band gap trends. Gas chromatography (GC) analysis of the reduction products revealed the generation of hydrocarbons, with Fe-TiO2 exhibiting the most favourable outcomes. This research provides valuable insights into tailoring TiO2 nanoparticles for efficient photo-electro catalytic reduction of CO2.

Mapping the correlations between bandgap, HOMO, and LUMO trends for meta substituted Zn-MOFs.

Bandgap is a key property that determines electrical and optical properties in materials. Modulating the bandgap thus is critical in developing novel materials particularly semiconductors with improved features. This study examines the bandgap, highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO) energy level trends in a metal organic framework, metal-organic framework 5 (MOF-5), as a function of Hammett substituent effect (with the constant σm in the meta-position of the benzene ring) and solvent dielectric effect (with the constant ε). Specifically, experimental design and response surface methodologies helped to assess the significance of trends and correlations between these molecular properties with σm and ε. While the HOMO and LUMO decrease with increasing σm, the LUMO exhibits greater sensitivity to the substituent's electron withdrawing capability. The relative difference in these trends helps to explain why the bandgap tends to decrease with increasing σm.

Electronic structure of Ta2O5 polymorphs: Variation trend of band gap and the role of oxygen vacancies

We provide a systematic study on the electronic structure of a series of Ta2O5 polymorphs using standard density functional theory (DFT) calculations as well as the more accurate many-body perturbation theory within the GW approximation. For the defect-free polymorphs, the variation trend of band gap can be microscopically related to the strength of orbital hybridization, and to the macroscopic bulk formation energy. The presence of oxygen vacancies is found to have profound effects on the electronic properties near the Fermi level, notably the reduction of band gap and gap closure (insulator-to-metal transition). Furthermore, depending on the vacancy sites, the band gap of some defective system may be enlarged with comparison to its defect-free counterpart. Such an anomalous behavior originates from the unexpected formation of Ta–Ta bonds.

Band Gap Alteration of Halide Mixing in Hybrid Perovskites: A First-Principles Study with Statistical Analysis.

Despite numerous studies on the band gap of three-dimensional halide perovskites using the first-principles calculations, there are still significant discrepancies between theoretical and experimental values. Various solutions have been proposed, such as employing a system-specific hybrid functional with varying degrees of exact exchange and explicitly incorporating spin-orbit coupling effects. Our research involved a comprehensive investigation of three typical lead-containing three-dimensional perovskites MAPbI3, MAPbBr3, and MAPbCl3 (MA = CH3NH3). Through a statistical analysis comparing mean absolute error (MAE) with experimental results, we demonstrated that the nonlocal van der Waals (vdW) density functional corrections (i.e., optB86b) yielded the most approximate lattice parameters in comparison to experimental values. Furthermore, based on these lattice parameters, the HSE06 hybrid functional is the optimal estimation of the band gap among all the options. Moreover, we investigated three sets of mixed three-dimensional halide perovskites by varying the halide component. This exploration contributes to the identification of MAPb(Br0.333I0.667)3 and MAPb(Cl0.333I0.667)3 as exhibiting the smallest band gap of 1.315 (1.867) eV and 1.313 (1.885) eV for PBE (HSE06), respectively. These band gaps were determined using the HSE06 method with the optimized lattice by PBE considering the optB86b corrections. The approach employed in this work produced a band gap trend closely aligned with experimental observations, underscoring the importance of adopting a reliable and material-independent computational strategy when screening new halide perovskite materials for optoelectronic applications.

Influence of defects on the linear and nonlinear optical properties of Cu-doped rutile TiO2 microflowers.

Defects and disorder work as controlling parameters to alter the electronic structure of nanostructures and significantly influence their electronic, magnetic, and nonlinear optical (NLO) properties. In this study, we found that defect engineering is an effective strategy for tailoring the linear and nonlinear optical properties of Cu-doped titanium oxide (TiO2) flower-shaped nanostructures. The concentration of chemical doping of Cu in the TiO2 lattice creates intermediate defect states that impact electronic bandgap reduction and tunable defect luminescence. The estimation of the bandgap from density functional theory calculation follows the same trend of bandgap narrowing with Cu doping. The XPS study reveals that oxygen defects are responsible for bandgap narrowing and quenching of the PL intensity. A single-beam Z-scan technique with open and closed aperture configurations using ultrashort pulses centered at 532 nm excitation wavelength was used to study the NLO measurements. The open aperture reveals saturable absorption, whereas the closed aperture shows self-focusing behavior. The nonlinear absorption coefficient and refractive index extracted from NLO measurements demonstrate the linear dependence on the defect concentration and bandgap. The effects of heterogeneous dopants and lattice disorder on the nonlinear absorption behavior of these nanostructures are discussed in comparison with the figure of merit, non-linear refractive index, and absorption coefficient. The tunable NLO properties achieved by controlling such dopant-induced defects boost the scope of these nanostructures as optical limiting, optical switching, and optical photodiode applications.

Electronic and Excitonic Properties of MSi2 Z4 Monolayers.

MA2 Z4 monolayers form a new class of hexagonal non-centrosymmetric materials hosting extraordinary spin-valley physics. While only two compounds (MoSi2 N4 and WSi2 N4 ) are recently synthesized, theory predicts interesting (opto)electronic properties of a whole new family of such two-dimensional(2D) materials. Here, the chemical trends of band gaps and spin-orbit splittings of bands in selected MSi2 Z4 (M = Mo, W; Z = N, P, As, Sb) compounds are studied from first-principles. Effective Bethe-Salpeter-equation-based calculations reveal high exciton binding energies. Evolution of excitonic energies under external magnetic field is predicted by providing their effective g-factors and diamagnetic coefficients, which can be directly compared to experimental values. In particular, large positive g-factors are predicted for excitons involving higher conduction bands. In view of these predictions, MSi2 Z4 monolayers yield a new platform to study excitons and are attractive for optoelectronic devices, also in the form of heterostructures. In addition, a spin-orbit induced bands inversion is observed in the heaviest studied compound, WSi2 Sb4 , a hallmark of its topologicalnature.

Investigation of physical and optical properties of ZnO doped lithium boro bismuthate glasses

Lithium Bismuth Borate glasses with ZnO addition was examined in relation to their physical, structural, and optical properties from Differential Thermal Analysis and density measurement. Infrared absorption (IR) spectra were recorded to know structural details. The optical properties of these glasses were investigated by UV–VIS absorption spectra. The optical band gap obtained from Tauc’s plots had shown an increasing trend. The rising trend in optical band gap and descendant trend in cut-off wavelength is due to a reduction in NBOs this is evidenced by IR.

Oxygen mediated defect evolution in RF sputtered Ga2O3 thin films on p-Si substrate

Impact of oxygen concentration in governing the structural and optical properties of RF sputter Ga 2 O 3 thin film deposited on p-Si at 500 o C by controlling the oxygen flow rate (OFR) has been demonstrated in this work. To maintain the oxygen rich and oxygen poor condition, the argon (Ar) to oxygen (O 2 ) ratio systematically varied from 1:1, 1:2, to 1:0, 2:1 and, 3:1 respectively. Such OFR reliant change in crystalline quality of the grown Ga 2 O 3 thin film is investigated by X-ray Diffraction (XRD) study. The atomic force microscopy results further illustrate an increase in film roughness owing to the inevitable introducing of defects/ oxygen vacancy ( V O ) during deposition with higher OFR. The modification in material stoichiometry via generation of V O and defects in surplus of oxygen is further supported with XPS analysis. Moreover, a rising trend of optical bandgap (Eg) due to transformation from oxygen rich to oxygen poor condition was revealed by spectroscopic ellipsometery study and further correlated with the improved crystallinity of the film. A maximum Eg of 4.44 eV was obtained for the Ga 2 O 3 thin film deposited with 3:1 Ar to O 2 ratio. Finally, the photoluminescence (PL) investigation confirms a strong correlation between defects/ V O generation with OFR variation. Where, the decrease in blue band contribution in lower oxygen concentration was correlated with reduced Vo concentration via its active participation in gallium vacancy V Ga and Vo pair (V Ga +Vo) formation. Finally, the impact of OFR on the Ga 2 O 3 /Si heterojunction device property is also investigated by utilizing the current–voltage (I-V) measurements, which shows a superior device performance for film grown in oxygen poor condition owing to the presence of lower defect concentration, surface roughness, and interface states. • Oxygen mediated structural, chemical and optical transformation of Ga 2 O 3 thin film deposited on p-Si substrate at 500 o C temperature by RF sputtering. • Impact of oxygen extremes on the defect evolution of Ga 2 O 3 thin film by photoluminescence study. • The heterojunction properties of β-Ga 2 O 3 /Si are studied electrically and such heterojunctions exhibit relatively superior rectification properties. • 3:1 is the optimum Ar to O 2 ratio to achieve high quality, defect-free Ga 2 O 3 film on Si.

Realization of Ultrahigh Strain Modulation in Two‐Dimensional β‐InSe Layers

Abstract2D materials are regarded as ideal candidates for fabricating flexible devices in electronics, due to their intrinsic clean surface and malleability. However, due to the weak interaction between 2D materials and the substrates underneath, bending or stretching will inevitably cause severe slippage, which degrades the device's performance or even leads to failure. The realization of no slippage between 2D materials and substrates under ultrahigh strain has become a key topic in the field of flexible electronics. Here, a strategy to overcome this limitation, by which strain can be effectively transferred to 2D materials is demonstrated. By applying this improved method to few‐layer β‐InSe, it is found that the loaded strain reaches as high as 7.2% without any slippage, along with an apparent redshift of ≈4.18 cm−1 in Raman scattering signals. The evolution trend of bandgap observed in the luminous properties of β‐InSe is consistent with the author's density functional theory (DFT) calculations. This convenient method can be intensively expanded to other van der Waals (vdW) layered materials and sheds light on flexible electronic applications.

The multi-dimensional characterization of Al-doped CuO thin films by laser-induced breakdown spectroscopy (LIBS) with nanosecond and picosecond lasers

Glasses coated with various optical thin film materials are widely used in people's life and scientific research. In this study, a series of aluminum-doped (Al-doped) copper oxide (CuO) thin films with different sputtering pressures and powers were deposited on soda-lime glass substrates by RF-magnetron sputtering to explore the effect of doping element on the optical properties of the thin films. However, laser-induced breakdown spectroscopy (LIBS) with nanosecond and picosecond lasers was employed to achieve the multi-dimensional analysis of the Al element. With ns-LIBS, the rapid quantitative analysis was performed by drawing the calibration curves of LIBS intensity ratio of Al/Cu versus concentration ratio under different sputtering pressures and powers. Furthermore, the 2D chemical mappings were drawn to provide a more complete and detailed relative spatial distribution of the Al element according to the LIBS intensity ratio of Al/Cu. Meanwhile, the thin film thickness was determined by the picosecond laser pulse number since the depth ablated by a single laser pulse was assessed as nearly 46.75 ± 4.25 nm. Moreover, the variation trends of optical band gaps with sputtering parameters were similar to that of LIBS intensity ratios with sputtering parameters, as suggested by analyzing the transmission spectra of thin films. The results demonstrated that the multi-dimensional characterization (quantitative analysis, 2D chemical mapping, thin-film thickness estimation, and optical properties analysis) of Al-doped CuO thin films prepared by RF-magnetron sputtering can be effectively realized by LIBS.

Quasi-periodic one-dimensional photonic crystal as a perspective structures for nanophotonics: Analysis of transmittivity spectra

The characteristic matrix approach is used to evaluate optical transmission spectra for a 1D multilayered photonic system. The characteristics of transmittance are analyzed for different incident angles, layer thicknesses (from a few to hundred nanometers), and electron densities for the light wave propagating through both periodic and quasi-periodic photonic structures. The shifting trend of photonic band gap in the infrared wavelength regimes is observed for different types of quasi-sequence. Increased electron density, on the other hand, makes the band gap more robust, although collision frequency has a minor effect on the band gap. Moreover, the physical entities play a key role in the appearance of multiple band gaps in the greater wavelength regimes. For example, such nanophotonic devices as optical bandpass filters in the infrared regime can be designed by using the concept of the proposed structure. We further show that the suggested quasi-periodic multilayered structures can function as resilient adjustable photonic lattices with the above-mentioned properties that can be tuned.

Structural, Third Order Nonlinear and magnetic properties of pristine and Ni-doped CuO nanoparticles: Diluted magnetic semiconductors

Ni-doped Copper Oxide nanostructures have been synthesized using the modified auto-combustion method. The structural, morphological, vibrational, linear, and nonlinear optical properties were examined. XRD pattern revealed that the samples exhibited a monoclinic structure having a space group C2/c. Origin of adsorbed oxygen and bivalent state of Cu and Ni were revealed from XPS analysis. HRTEM and FEG-SEM images with EDAX spectra confirmed the formation of spherical particles with smooth surfaces and also validated the purity of synthesized nanostructures. The magnetic measurements confirmed the nanoscale doping of Ni specifically from the superparamagnetic behaviors. The characteristic vibrational modes of CuO were identified from the vibrational analysis. The red-shifted Raman mode with broadening indicates the formation of disorders and size effects. The UV-Visible spectrum showed a decreasing trend in bandgap with an increment in nickel concentration, corresponding to the presence of intra-band defect/imperfection states. Emission peaks obtained from room temperature photoluminescent spectrum indicated the presence of defects in the nanostructures. Third-order nonlinear optical properties, measured via, the Z-scan method with the help of a continuous wave (CW) laser and an enhancement in the nonlinear optical parameters were observed on doping. Hence it confirmed that the prepared nanostructures can be used for various optoelectronic applications.

Bandgap engineering and optoelectronic properties of all-inorganic lead-free Pd-based double perovskites

The various physical properties of lead-free double perovskites A 2 PdX 6 (A = K, Rb, Cs; X = Cl, Br, I) are revealed for the first time. The calculated structural parameters of these Pd-based compounds are consistent with the experimental data. It is likely to possess a tetragonal structure for K 2 PdI 6 at room temperature. Cs 2 PdBr 6 is dynamically stable when the pressure is in the range of 0–6.95 GPa. The mechanical properties are analysed and all the compounds are mechanically stable. The band gap trend of the A 2 PdX 6 compound is identified when the A-site cation and halide anion are varied. Three A 2 PdBr 6 compounds exhibit suitable band gaps for photovoltaic applications. An optimum band gap can be achieved for Cs 2 PdBr 6 when the moderate pressure is applied. In addition, the electron shows better mobility than that of the hole for three A 2 PdBr 6 compounds. The optical absorption coefficient of the A 2 PdBr 6 compound is improved when the A-site cation changes from Cs to Rb to K. Applying pressure is beneficial to enhance light absorption capacity of Cs 2 PdBr 6 . The findings of this work can provide guidance for the design of potential A 2 PdBr 6 compounds for photovoltaic applications.

A first principles investigation of ternary and quaternary II–VI zincblende semiconductor alloys

One of the most common ways of tuning the stability, electronic structure, and optical behavior of semiconductors is via composition engineering. By mixing multiple isovalent elements at any cation or anion site, new compositions may be generated with markedly different properties than end-point compositions, and not always lying within a predictable trend. In this work, we explore the trends in lattice constant, electronic band gap, formation and mixing energy, and optical absorption behavior in a series of II–VI zincblende semiconductors with Cd/Zn at the cation site and S/Se/Te at the anion site, using multiple levels of density functional theory approximations. We find that while the GGA-PBE functional reproduces all trends correctly, full geometry optimization with the HSE06 functional predicts band gaps with much higher experimental accuracy. We find that all mixed S–Se and mixed Cd–Zn compounds show linear trends in band gap, rising from Se to S and Cd to Zn, respectively, whereas all Se–Te mixed compounds exhibit band gap bowing. All mixing energy curves, calculated based on decomposition to end point compositions, show inverted bowing behavior but with small positive mixing energy values <50 meV per formula unit, indicating robust stability of all solid solutions. Formation energies, calculated based on decomposition to elemental species, always show linear trends and remain sufficiently negative for all binaries, ternaries and quaternaries, whereas lattice constants show expected linear trends. We further report trends in optical absorption spectra and relationships between PBE and HSE computed properties, which reveal equations that can be used to accurately predict higher fidelity data. This work lays out systematic trends in the stability and optoelectronic characteristics of Cd–Zn–S–Se–Te alloys and enables the selection of optimal compositions for desired applications.

Morphology of ZnO nanorods and Au–ZnO heterostructures on different seed layers and their influence on the optical behavior

Zinc oxide nanorods were synthesized by low cost and low temperature chemical synthesis path by taking both ZnO and metal as the seed layer on the silicon substrate. The morphological study depicts seed layer as one of the key parameter in the growth process. ZnO NR/Au nanoparticles heterostructures were also fabricated. The enhancement of the visible emissions of the ZnO nanostructures is seen after the incorporation of gold nanoparticles at the surface. The visible (defect) emission corresponds to the yellow emission caused by oxygen interstitials. Luminescence vary considerably with position on the nanorod. The optical band gap obtained for Au–ZnO heterostructure to be 2.63 eV, which is lower than that of ZnO NRs. This changing trend of optical band gap for prepared Au/ZnO sample originated from the variety of defects. Tuning of the band gap energy of such wide band gap semiconductor is highly important for room temperature optical devices.

Nonlinear variable band gap of reduced graphene oxide examined by in-situ absorption spectrum and light response

The band gap of reduced graphene oxide, playing a key role in many application fields, could be controllably tuned by oxygen group. Otherwise, up to now, it is unclear for the relation between these two. Herein, it was addressed by analyzing the thermogravimetric analysis, fourier infrared spectroscopy, and in-situ infrared spectroscopy, a nonlinear variable band gap was confirmed by fitting Tauc plot and also by light responses of RGO detective devices. The results showed that the reduction of oxygen group strongly relies on the annealing temperature. Different oxygen groups were lost at different temperatures. The hydroxyl group and carboxyl groop was lost before 200 °C, the epoxy group and carbonyl group was lost between 200 and 400 °C, the C-OH was lost at 400–600 °C, which exactly led to nonlinear variable band gap from 1.9 eV to 0.9 eV. In addition, the nonlinear photoelectric response of RGO light detective device further proved that trend of band gap. Our work pave a way to deeply apply RGO into electronic fields.

Tunable optoelectronic properties of two-dimensional PbSe by strain: First-principles study

Based on first-principles calculations, we studied the band gap, work function and effective mass of two dimensional (2D) PbSe by using applied external in-planed strain. Our results show a linearly decrease trend of band gap under both compressive and tensile axial strain. However, the typical character of direct band gap of 2D PbSe has been changed to indirect with a minimum value about 0.3 eV. In contrast, under biaxial compression, the original type of direct band gap is kept. Under biaxial compression strain, we predict that the optical absorption coefficient of 2D PbSe in the ultraviolet region has significant enhancement. Moreover, we found that the work function of 2D PbSe is sensitive and can be tuned between 3.60 eV and 5.31 eV by applying biaxial strain. In addition, we predict that the decrease of the effective mass of electrons or holes along with the compressive strain can promote the carrier mobility in PbSe monolayer. These results imply that 2D PbSe has potential applications in optoelectronic devices.