Indirect Transitions Research Articles

Investigations on optical and photoluminescence properties of Dy3+ doped zinc tellurite glass

• The structural, thermal and optical properties of the Dy 3+ doped tellurite (TZND) glass system were investigated. • Properties of samples were measured and described in the current study using XRD, SEM, and EDS methods. • The forbidden energy gap values for the direct and indirect transitions are found between 2.98 and 3.02 eV and 2.90–2.94 eV respectively. • The effects of Dy 3+ ion on absorption and photoluminescence spectra were studied. • The TZND glasses doped with Dy 3+ ion have potential to be the best photonic and light emitting host material. Zinc-sodium-tellurite glasses doped with Dy 3+ (TZND) with the chemical compositions (70-x) %TeO 2 –20 %ZnO–10 %Na 2 O-x%Dy 2 O 3 (where × is 0, 0.5 and 1 in mol%) were synthesize by traditional melt-quenching method. The samples were characterized using X-ray diffraction, SEM-EDS, optical absorption and PL spectra. Differential scanning calorimetry (DSC) was carried to determine the glass transition temperature of synthesized samples. The non-crystalline nature of TZND samples was confirmed by XRD measurements and compositions were verified by EDS. Raman spectra have been carried to characterize the architectural attributes of glass matrices. Other parameters of prepared samples as density(ρ), molar volume (V m ), polaron radius R p , oxygen packing density and intermolecular distance R i were calculated. The optical band gap, refractive index (n), cut-off wavelength, and the Urbach's energy were determined from the absorption spectra. The PL spectra for upconversion characteristics indicate two intense bands due to 4 F 9/2 → 6 H 15/2 (around 480 nm) and 4 F 9/2 → 6 H 13/2 (around 570 nm) transitions respectively.

First principles study of BAs/MoSi2N4 van der Waals heterostructure: tunable electronic and optical properties via vertical strain.

Constructing van der Waals heterostructures from layered materials can form new optoelectronic devices with superior performance to the individual monolayers. Here, we use first-principles calculations to explore the modulation of a two-dimensional BAs/MoSi2N4 van der Waals heterostructure via strain, including the structure stabilities, electronic properties, charge transfer, and optical properties. Our calculated results reveal that the BAs/MoSi2N4 heterostructure has a direct bandgap of 0.72 eV and type-I band alignment. In addition, the BAs/MoSi2N4 heterostructure exhibits enhanced light absorption in the visible light region. The electronic properties of the BAs/MoSi2N4 heterostructure are tunable by vertical strain, exhibiting a direct to indirect bandgap transition as well as a type-I to type-II band alignment transition when the vertical distance is reduced. Our research provides a comprehensive understanding of the electronic and optical properties of the BAs/MoSi2N4 heterostructure and could be helpful for their potential applications in optoelectronic devices.

First-principles study on structural, electronic and optical properties of halide double perovskite Cs2AgBX6 (B = In, Sb; X = F, Cl, Br, I).

All-inorganic halide double perovskites (HDPs) attract significant attention in the field of perovskite solar cells (PSCs) and light-emitting diodes. In this work, we present a first-principles study on structural, elastic, electronic and optical properties of all-inorganic HDPs Cs2AgBX6 (B = In, Sb; X = F, Cl, Br, I), aiming at finding the possibility of using them as photoabsorbers for PSCs. Confirming that the cubic perovskite structure can be formed safely thanks to the proper geometric factors, we find that the lattice constants are gradually increased on increasing the atomic number of the halogen atom from F to I, indicating the weakening of Ag-X and B-X interactions. Our calculations reveal that all the perovskite compounds are mechanically stable due to their elastic constants satisfying the stability criteria, whereas only the Cl-based compounds are dynamically stable in the cubic phase by observing their phonon dispersions without soft modes. The electronic band structures are calculated with the Heyd-Scuseria-Ernzerhof hybrid functional, demonstrating that the In (Sb)-based HDPs show direct (indirect) transition of electrons and the band gaps are decreased from 4.94 to 0.06 eV on going from X = F to I. Finally, we investigate the macroscopic dielectric functions, photo-absorption coefficients, reflectivity and exciton properties, predicting that the exciton binding strength becomes weaker on going from F to I.

Resistive switching characteristics of W/TiO2/ITO devices

Materials with resistance-switching properties are proving to be promising for the novel non-volatile memories-resistive random-access memory (ReRAM). Despite extensive research on electric field-induced resistance switching, there are diverse opinions about its reliability and working mechanism. In this work, we analysed the impact of SET and RESET voltages on the switching characteristics and the possible current conduction mechanism of W/TiO2/ITO devices. By simple solution processing and spin coating, titanium dioxide thin films were grown on ITO, glass, silicon, and quartz substrates. The precursor and solvent used for the synthesis of the active layer were titanium isopropoxide and ethanol, respectively. Polycrystalline films with an anatase structure were formed after annealing at 400 °C with a 20 nm average crystallite size, as evidenced by X-ray diffraction patterns. The crystalline nature of the fabricated film has been found to improve on annealing. Using Raman spectroscopy, the chemical bonding states of the film were studied. A UV–visible spectrometer study confirms that anatase TiO2 possesses an indirect band gap transition with a 3.42 eV band gap. The device shows reversible, reproducible, stable, and bipolar resistance-switching behaviour as indicated by current–voltage (I-V) measurements, and it was found that there exists a minimum voltage (∼-4 V) to set the device into an ON state. Compared to crystalline TiO2, theamorphous active layer offers much better switching characteristics, such as good cyclic endurance with a Roff-Ronratio ∼ 10.The current–voltage characteristics were fitted to various carrier transport mechanismssuch as Ohmic conduction, Schottky emission (SE), Space-charge-limited conduction (SCLC), and Poole-Frenkel emission (PF). Among these, the ohmic and SE mechanisms were detected at low and high fields, respectively. Schottky barrier modulation at the metal-oxide interface has been identified as the motive for switching.

Intrinsic defects formation and subsequent direct and indirect transitions due to ammonia in rGO – ZnO nanocomposites

Intrinsic defects formation and subsequent direct and indirect transitions due to ammonia in rGO – ZnO nanocomposites

Effect of Titanium Nitrate Nanoparticles on Optical Properties of PVA/PEG Blend for Optoelectronics Detectors

In this work, casting method was used to fabricate the polyvinyl alcohol (PVA)/ poly ethylene glycol (PEG) nanocomposite with different content of titanium nitrate nanoparticle (TiN NPs). This nanocomposite was analyzed to use a Spectrophotometry with a spectral region of 200-800 nm. The absorption enhances when the added TiN NPs to the polymeric matrix which may be used for various applications such as optoelectronic detector. The optical energy gap for indirect transition (allowed and forbidden) decreased as TiN NPs content increases. Moreover, all optical constants have been determined.

Improved photocatalytic performance of Nb4TeO12 particles by the surface depositing α-Bi2O3

Mixed-metal oxides composed of Nb5+(d0) and Te4+(p-) cations generally exhibit excellent optoelectronic properties due to the stereoactive lone pair of electrons in the structure. In this work, a mixed-metal niobate tellurite Nb4TeO12 is prepared using solid state reaction method. The p-n junctions α-Bi2O3/Nb4TeO12 are prepared via soaking the as-prepared particles and post annealing treatments. Nb4TeO12 has indirect electronic transition characteristic with a band energy of 3.01 eV. The bottom of conduction band is − 0.55 eV and the top of valance band is + 2.46 eV. Surface deposits of α-Bi2O3 induce the recrystallization on the particle surface. The microstructure, electrical impedance, and photocurrent properties are analyzed. The p-n junction can significantly delay the recombination between photo-created h+/e− pairs. As a consequence, significant improvement of photocatalysis in p-n junction α-Bi2O3/Nb4TeO12 was realized. The rate constants of as-prepared Nb4TeO12 and p-n junction α-Bi2O3/Nb4TeO12 (30/100) are 4.21 × 10−3 min−3 and 1.82 × 10−2 min−1, respectively. The experimental results show that deposits of α-Bi2O3 on the surface is effective to improve the surface light absorption and photochemical properties of the Nb5+−Te4+ oxide semiconductors.

Broad intrinsic luminescence properties in Nb4TeO12 mixed-metal niobate tellurite

Mixed-metal oxides composed of Nb5+(d0) and Te4+(p-) cations generally exhibit excellent optoelectronic properties due to the stereoactive lone pair of electrons in the structure. In this work, a mixed-metal niobate tellurite Nb4TeO12 was prepared using solid state reaction method. The polycrystalline samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), optical absorption, photoluminescence (PL) and dynamic properties. This sample shows characteristic indirect electronic transitions with an interband energy of 3.01 eV. The bottom of conduction band and the top of valence band are +0.37 eV and +3.38 eV, respectively. Using a Nd:YAG pulsed laser as the excitation light source, the intrinsic luminescence of Nb4TeO12 was investigated over a wide temperature range of T = 10–300 K. The broad band emission peaking at 513 nm corresponds to the self-activating luminescence of the niobate octahedral group [NbO6]7−. The luminescence decay curve did not depend on the laser excitation energy, and it showed a non-exponential characteristic with two decay lifetime components. The experiments also preliminarily studied the performance of the luminescence application of Nb4TeO12.

Epitaxial n+-Ge/p+-Si(0 0 1) heterostructures with ultra sharp doping profiles for light emitting diode applications

The prototype light emitting diodes (LEDs) were fabricated from the n+-Ge/p+-Si(001) heterostructures grown by low-temperature Hot Wire Chemical Vapor Deposition. The n+-Ge epitaxial layers were heavily doped with P from a sublimation solid-phase GaP source. Abrupt profiles of the donor impurity concentrations at the n+-Ge/p+-Si heterojunctions were obtained. Negative differential resistance was observed in the current–voltage curves of the LEDs at forward bias. Electroluminescence (EL) related to the indirect (phonon-assisted) interband radiative optical transitions in the n+-Ge layers (from the l-valleys of the conduction band into the Г-point in the valence band) was observed at room temperature. The EL intensity increased with increasing P concentration in the n+-Ge layers.

Optimization of the structural and optical properties of ALD grown ZnO thin films for photocatalytic applications: thickness dependence

Thin films of ZnO with different thicknesses (ranging from 8 to 40 nm) have been prepared by plasma-enhanced atomic layer deposition. Grazing incidence x-ray diffraction shows the nano-crystalline structure of the films with high degree of disorder. The films have also lattice oxygen and non-lattice oxygen where the film with 20 nm thickness has the highest percentage of the non-lattice oxygen. These films have indirect optical transitions. The energy gap increases slightly with decreasing the film thickness (2.96, 3.03 and 3.16 eV for the thicknesses 40, 20 and 8 nm, respectively). These films have strong photocatalytic activity to treat the water from the organic dyes such as Levafix Brilliant Red. The film with thickness 20 nm has the optimum photocatalytic activity and the lowest contact angle with water. The photoinduced super-hydrophilic nature of ZnO film (20 nm) renders this film suitable for antifogging application. The high photocatalytic activity and super-hydrophilicity are due to the low recombination rate of charge carriers accompanied to the excess of oxygen vacancies and the high degree of structural disorder.

Manganese(II) Bromide Compound with Diprotonated 1-Hydroxy-2-(pyridin-2-yl)-4,5,6,7-tetrahydrobenzimidazole: Dual Emission and the Effect of Proton Transfers

An organic–inorganic cation–anion manganese(II) tetrabromide compound with diprotonated 1-hydroxy-2-(pyridin-2-yl)-4,5,6,7-tetrahydrobenzimidazole, [H3L][MnBr4][H2O], has been synthesized and investigated. The compound has a few possible pathways for proton transfers, which play an important role in the observed luminescence, optical, and magnetic properties. The proton transfers result in the appearance of two-band luminescence. One band is caused by the Mn(II) d-d transitions. The other band is caused by the transition from the triplet state of organic cation and the d-d transition of manganese(II) coupled through {[H3L]}-{[MnBr4]}-{[H2O]} vibrations. The optical absorption spectra of [H3L][MnBr4][H2O] indicate the presence of two direct and one indirect band transitions. The reason for the two-band luminescence and complex optical absorption in [H3L][MnBr4][H2O] were additionally considered using the DFT calculations.

Biaxial strain induced tunable electronic properties study of ZnO nanoparticles via first-principles density functional theory

Strain engineering can be used to manipulate electronic and optical properties of ZnO suitable for various applications. In the present work, we have characterized the hydrothermal prepared different wurtzite-ZnO nanoparticles using X-ray diffraction, microscopy and spectroscopy techniques. The lattice parameters of prepared ZnO nanoparticles, obtained from Rietveld refinement of XRD data, have been used for in-depth analysis of the phonon and electronic properties of ZnO. The calculated bandgap Eg using Density functional theory with GGA + U (Generalized Gradient Approximation with Hubbard correction) approach is found to be ∼ 3.48 eV with a strong Zn-3d O-2p hybridization and matches our experimental bandgap value (∼ 3.25 eV). Further, the effect of biaxial strain on the electronic band structure of ZnO has also been studied. A strong non-linear behavior is found in both direct (Γ-Γ) and indirect transitions (Γ-A). Strain application on ZnO may be useful for straintronics and other nano-devices such as NEMS and NOMS.

Exploring the structural and electrochemical sensing of wide bandgap calcium phosphate/CuxFe3-xO4 core-shell nanoceramics for H2O2 detection

Calcium phosphate/Cu x- Fe 2-x O 4 core-shell nanoceramics have been synthesized using the organic sol-gel process and calcined at 600 o C. The nanoceramics were explored by X-ray powder diffractometry (XRD), transmission electron microscopy (TEM)), Fourier transforms infrared spectroscopy (FT-IR), UV–visible diffuse reflectance spectra (DRS), and electrochemical techniques including cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The FTIR results confirmed the presence of calcium phosphate (CP) and CP coated with Cu-ferrite (0-30 mol% Cu:Fe 3 O 4 ) core-shell nanoceramics. TEM analysis displayed nanospheres of calcium phosphate shielded with a porous spheres-like morphology with internal nanoparticles of about 33 nm. The reflectance spectra increase for all samples with increasing wavelength. Both direct and indirect gap transitions were detected with lower values of the direct case than the indirect type. The dielectric constant, refractive index, and optical electronegativity of all samples were calculated. When the ferricyanide was applied as the standard redox probe for the electrochemical characterization, CV and EIS results revealed a remarkable enhancement in the generated electrochemical signals. Moreover, modified screen-printed electrodes with the 20 Cu-CPF produced the highest redox current (0.198 mA) towards the direct oxidation of hydrogen peroxide. The obtained interesting features nominate the newly developed core-shell calcium phosphate/Cu x Fe 2-x O 4 nanoceramics as promising candidates for talented magneto-optical and peroxide sensor applications.

Linking optical and electronic properties to photoresponse of heterojunctions based on titania nanotubes and chromium, molybdenum, and tungsten oxides

The development of photosensitization strategies for titanium dioxide is necessary for the enhancement of its optical and electronic properties towards its application potential in solar photoelectrochemistry. In this work, significant differences in the photosensitizing capability of the 6th group transition metal oxides applied on the surface of titania nanotubes are reported. For the first time, correlations between the experimentally determined Tauc coefficients, sample photoresponse, and ab-initio simulated properties of the heterojunctions are established. Experimental results show undoubtedly that the decoration of TiO2 nanotubes with chromium oxides leads to the enhanced photoresponse, which originates from the interplay of mid-gap states and both direct and indirect nature of the transitions contributing to the optical absorption. The opposite tendency and decrease of photocurrent were found for molybdenum and tungsten oxides which exhibited forbidden nature of dominating transition. Although computations report intraband states in all interfaces, experimentally only chromium oxides contribute to the photocurrent. The uniqueness of this interface lies in the highest density of states in the vicinity of the conduction band and the low energy difference between the direct and indirect transitions of the innate chromium oxide. The obtained results demonstrate that the determination of the Tauc exponent and the nature of optical transition are more reliable experimental predictors of the photoactivity enhancement in the heterojunctions than the value of the band gap.

Enhancement of Structural and Optical Properties of CMC/PAA Blend by Addition of Zirconium Carbide Nanoparticles for Optics and Photonics Applications

Nanocomposites of (CMC-PAA-ZrC) made with different nano zirconium carbide percentages by casting method (0, 1.5, 3, 4.5, and 6) wt%. The results showed that FTIR spectra shift in peak position and change in shape and intensity, compared with pure (CMC-PAA) blend. Microscopic photographs show a clear difference in the samples when increasing proportions of zirconium carbide nanoparticles, when the concentration of zirconium carbide NP reached 6% wt, the nanoparticles make up a continuous network inside (CMC-PAA) blend. Structural and optical characteristics have investigated the findings showed that the absorption of (CMC-PAA-ZrC) nanocomposites increases with increasing of ZrC NPs, while transmission decrease. The absorption coefficient, extinction coefficient, refractive index, real and imaginary parts of dielectric and optical conductivity are increasing with rises concentration of ZrC. Also optical energy gap decreased from 4.9 eV to 4.05 eV and from 4.5 eV to 3.65 eV for allowed and forbidden indirect transition respectively with increasing ZrC NPs. The results indicate that the (CMC-PAA-ZrC) nanostructures can be considered as promising materials for optoelectronics applications.

Chemical functionalization of SnAs monolayer: a first-principles study of SnAsX (X = Cl, Br, and I) monolayers

Chemical functionalization is one of the effective methods to tune the electronic properties of two-dimensional (2D) nanostructures. In this paper, we study the structural, electronic properties, and carrier mobilities of 2D SnAs monolayer functionalized by chlorine, bromine, and iodine, namely SnAsX (X = Cl, Br, and I), by means of density functional theory. The obtained results show that the 2D SnAsX are energetically, dynamically, and mechanically stable. While the 2D pristine SnAs monolayer exhibits metallic characteristics, SnAsX monolayers are direct semiconductors with direct band gaps. The influence of spin–orbit coupling on the electronic characteristics of SnAsX is significant, especially in the case of SnAsI monolayer. The direct–indirect gap transitions are found in all three SnAsX monolayers when the biaxial strain is applied. Meanwhile, the effects of external electric fields on the electronic characteristics of SnAsX are insignificant. Our calculated results indicate that SnAsX monolayers have very high electron mobility and their transport characteristics are directionally isotropic along the investigated transport directions.

Study of the optical properties of BiFeO3 under different heat treatment temperatures

It is known that the optical absorption property of a material is linked to its crystal structure. Therefore, it can be modified or displaced when the material is subjected to thermal treatments, generating displacements of the unit cell, atomic positions and bond angles of its atoms. In this work, the analysis of three samples of BiFeO3 powders subjected to different heat treatment temperatures was carried out to study their optical properties by analyzing the crystal structure, diffuse reflectance, Raman scattering spectroscopy, density of state and band structure. In the results, the pure phase and the hexagonal structure of the samples were identified, showing an increase in the angles between the Fe-O-Fe atoms when the material is thermally treated. The band gap (Egexperimental) of the BiFeO3 samples without and with treatment temperature showed direct and indirect transitions according to the Tauc equation, these values were 2.36, 2.11 and 2.19 eV for direct band gaps and 2.10, 1.58 and 1.85 eV for indirect band gaps. In the Raman scattering spectroscopy analysis, overtones were observed in the samples when a green laser with a wavelength of 532nm (2.33 eV) was applied. In the density of states, the maximum valence band formed by the O(2p) state and the minimum of the conduction band formed by the hybridization of the Fe(3d) and O(2p) states were identified. In the electronic band structure, indirect band gap values (Egtheoretical) of 2.274, 2.216 (500 °C) and 2.217 eV (600 °C) were found for the samples without and with heat treatment. It is concluded that BiFeO3 absorbs visible light and that its band gap can decrease by increasing the bond angle of Fe-O-Fe atoms when the sample is subjected to heat treatment. These results demonstrate the semiconductor property of BiFeO3 and can be applied to solar cells, batteries, hydrogen production, among other applications.

Oil/water separation and functionality of smart carbon nanotube–titania nanotube composite

Water treatment applications are in high demand recently. In this work, the titania nanotube (TNT) was successfully grown onto the outer surface of the carbon nanotubes (CNTs) via the hydrothermal method. The resultant prepared composite was doped with different ratios of nitrogen. The structural and morphological merits of the composites displayed the successful composition of the matrices, as well as the particle size of the composite within the nanoscale. The optical specifications of the composites demonstrate successful direct and indirect transitions with a high energy gap (> 3 eV). The testing of different oils in the water/oil separation exhibited a high rate of success to split oil and water (> 95%). In this regard, CNT-TNT 1.0 sample reflects the highest efficiency. Compared to other researchers that demonstrate the highest efficiency of their proposed structure, our membrane offers a decent separation efficiency. The proposed composite might provide a feasible and cost-effective method for water/oil separation application in the nanotechnological fields.Graphical abstract

Nano-crystalline precursor formation, stability, and transformation to mullite-type visible-light photocatalysts

A new precursor for the formation of mullite-type visible-light active photocatalyst Bi2Al4O9 has been identified. The crystal structure of the organic–inorganic hybrid perovskite can be described using the hexagonal setting of the rhombohedral unit cell with lattice parameters a = 1.1342(2) nm, c = 2.746(1) nm, and V = 3.059(2) nm3. The presence of di-nitro-glycerin as organic component, which is centered together with two bismuth atoms at the A-sites of the ABX3-type perovskite, suggests for doubling of the a- and c-lattice parameters compared to isostructural BiAlO3 perovskite. The nano-crystalline precursor with the chemical composition [Bi2(C3H5N2O7)]Al4[O9(□1-x(H2O)x)3] (□: vacancies) decomposes at 540(10) K to a quantum-crystalline phase with an average crystallite size of 1.4(1) nm, refined from X-ray powder data Bragg reflections and confirmed by atomic pair distribution function data analysis. Further heating enables a controlled formation of quantum- or nano-crystalline mullite-type phases, depending on temperature and time. The same precursor structure could also be obtained as iron-containing phase and for Al/Fe solid-solution samples. UV/Vis diffuse reflectance spectroscopy suggests an indirect band-gap transition energy of 3.50(3) eV calculated by the Reflectance-Absorption-Tauc-DASF (RATD) method. Temperature-dependent UV/Vis allows to follow the change of band-gap energy across all associated phase transformations. The long- and short-range appearance of each phase has been presented using X-ray Bragg scattering and total scattering data analyses. This is supported by Raman and infrared spectroscopic investigations complemented by density functional theory (DFT) calculations. Moreover, the theoretical calculation confirms the incorporated di-nitro-glycerin. Thermal stabilities of the phases are investigated by using thermal analysis and temperature-dependent X-ray diffraction.

Optical transmission, dispersion, and transition behavior of ZnGa2O4transparent ceramic

AbstractAs an emerging transparent ceramic with extremely wide transmission range, the unique optical properties of ZnGa2O4are of particular interest. In this work, ZnGa2O4transparent ceramic with high optical quality was prepared by pressureless sintering and hot isostatic pressing. Its optical behavior was then comprehensively studied. The critical optical data, including refractive index, extinction coefficient, Abbé number, absorption coefficient in infrared region, and phonon frequency, were investigated. The indirect band gap and the phonon energy assisting indirect transition were determined as 4.10 and 0.17 eV, respectively. The modified Sellmeier equation and single‐oscillator dispersion relation were derived to describe the optical dispersion. Benefitting from the weaker tetrahedral Zn–O bond and heavier cationic masses, the infrared transmission range of ZnGa2O4transparent ceramic was broadened significantly. Oxygen vacancies were suggested to induce the absorption in lower photon energy region of ZnGa2O4transparent ceramic, which may enrich its functionalization. This work is meaningful for gaining overall insight into optical properties of transparent materials.