Band Gap Measurements Research Articles

Machine Learning‐Enhanced Prediction of Inorganic Semiconductor Bandgaps for Advancing Optoelectronic Technologies

AbstractA pivotal challenge in advancing inorganics optoelectronic technologies, is the precise characterization of materials' electronic attributes, with the bandgap being a critical property. Conventional approaches, heavily reliant on time‐intensive and financially demanding experimental and computational methods, such as density functional theory (DFT) calculations, face limitations due to inherent estimation errors. Machine learning methodologies are developed for the prediction of bandgaps of inorganic semiconductors but most of them are employed for datasets created by DFT calculations, hence limiting their performance. Addressing this, the study leverages machine learning methodologies, harnessing both compositional and structural features, to predict the band gaps of inorganic semiconductors with enhanced accuracy. This advancement is reinforced by the employment of an experimental bandgap dataset, which, when integrated with structural descriptors obtained from the Materials Project, significantly improves prediction capabilities. This is evidenced by the model's exceptional performance across two distinct benchmark datasets. Furthermore, the model's adeptness in predicting formation energies underscores its versatility and applicability to a broad spectrum of electronic properties. These findings suggest that the predictive accuracy of this model can be further augmented through the inclusion of additional experimental bandgap measurements and the refinement of structural descriptors. This approach offers a promising and efficient alternative to traditional methodologies, potentially accelerating the development of optoelectronic technologies.

Exploring the semiconductor characteristics of a polyoxometalate-based organic-inorganic hybrid dimer and its photocatalytic applications

Polyoxometalate-based organic-inorganic hybrid compounds have been widely used in the field of photocatalysis, photoelectrocatalysis, interfacial materials of semiconductor device. However, their intrinsic semiconductor characteristics such as carrier diffusion lengths and carrier mobility are still unclear. Herein, we first unravel the carriers diffusion length and carrier mobility of the novel POM-based dimeric compound, {(HBW12O40)[Cu(2,2′-bipy)2]2[CuO(2,2′-bipy)(H2O)]}2·2H2O (1), which is successfully synthesized through co-assembly of binuclear Cu(II)-complex and BW12 in hydrothermal reaction. Space charge-limited currents (SCLC) method shows that the holes carriers mobility (μh) of this compound reached 0.84 cm2V−1s−1, which is more 30 times than that of raw BW12 (2.6 × 10−2 cm2V−1s−1). The calculated carrier diffusion lengths of compound 1 is 82 nm, which is almost 3 time higher than that of BW12 (25 nm), indicating that photogenerated carriers are more easily separated and diffused in compound 1. Furthermore, the semiconductor properties of compound 1 have been demonstrated by the measurements of photocurrent response, VB-XPS spectrum and band gap (Eg) values. In addition, its applications of photocatalysis indicate the excellent activities of compound 1 with 71 % degradation rate for photodegradation of methylene blue (MB) and 67 % degradation rate for photocatalytic reduction of Cr(VI).

Novel supramolecular luminescent metallogels containing Tb(iii) and Eu(iii) ions with benzene-1,3,5-tricarboxylic acid gelator: advancing semiconductor applications in microelectronic devices.

A novel strategy was employed to create supramolecular metallogels incorporating Tb(iii) and Eu(iii) ions using benzene-1,3,5-tricarboxylic acid (TA) as a gelator in N,N-dimethylformamide (DMF). Rheological analysis demonstrated their mechanical robustness under varying stress levels and angular frequencies. FESEM imaging revealed a flake-like hierarchical network for Tb-TA and a rod-shaped architecture for Eu-TA. EDX analysis confirmed essential chemical constituents within the metallogels. FT-IR, PXRD, Raman spectroscopy, and thermogravimetric analysis assessed their gelation process and material properties, showing semiconducting characteristics, validated by optical band-gap measurements. Metal-semiconductor junction-based devices integrating Al metal with Tb(iii)- and Eu(iii)-metallogels exhibited non-linear charge transport akin to a Schottky diode, indicating potential for advanced electronic device development. Direct utilization of benzene-1,3,5-tricarboxylic acid and Tb(iii)/Eu(iii) sources underscores their suitability as semiconducting materials for device fabrication. This study explores the versatile applications of Tb-TA and Eu-TA metallogels, offering insights for material science researchers.

Epitaxial growth of α-(AlxGa1−x)2O3 by suboxide molecular-beam epitaxy at 1 µm/h

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow α-(AlxGa1−x)2O3 films on (110) sapphire substrates over the 0 < x < 0.95 range of aluminum content. In S-MBE, 99.98% of the gallium-containing molecular beam arrives at the substrate in a preoxidized form as gallium suboxide (Ga2O). This bypasses the rate-limiting step of conventional MBE for the growth of gallium oxide (Ga2O3) from a gallium molecular beam and allows us to grow fully epitaxial α-(AlxGa1−x)2O3 films at growth rates exceeding 1 µm/h and relatively low substrate temperature (Tsub = 605 ± 15 °C). The ability to grow α-(AlxGa1−x)2O3 over the nominally full composition range is confirmed by Vegard’s law applied to the x-ray diffraction data and by optical bandgap measurements with ultraviolet–visible spectroscopy. We show that S-MBE allows straightforward composition control and bandgap selection for α-(AlxGa1−x)2O3 films as the aluminum incorporation x in the film is linear with the relative flux ratio of aluminum to Ga2O. The films are characterized by atomic-force microscopy, x-ray diffraction, and scanning transmission electron microscopy (STEM). These α-(AlxGa1−x)2O3 films grown by S-MBE at record growth rates exhibit a rocking curve full width at half maximum of ≊ 12 arc secs, rms roughness <1 nm, and are fully commensurate for x ≥ 0.5 for 20–50 nm thick films. STEM imaging of the x = 0.78 sample reveals high structural quality and uniform composition. Despite the high structural quality of the films, our attempts at doping with silicon result in highly insulating films.

Effect of silica addition on the photocatalytic activity of MO2@SiO2 (M=Ti, Zr, Sn, Ce) nanocomposites prepared by sol-gel

AbstractPhotocatalysis uses wide band gap ceramic semiconductors (anatase, zirconia, ceria, cassiterite…) that have the ability to promote electron-hole pairs by UV light irradiation producing both reducing and oxidants ions and radicals. In general, oxidizing species are more efficient, using photocatalysis as an advanced oxidation process (AOP) for the biodegradation of organic substances refractory to conventional treatments, such as azo dyes, or atmospheric pollutants as NOx. In this communication, the photocatalytic activity of xSiO2@(1-x)MO2 (M = TiO2, ZrO2, SnO2, CeO2, x = 0, 0.1, 0.3 and 0.5) composites on the Orange II azo dye in solution, as well as on NOx in air, was analysed. The composites were obtained by Sol-Gel methods based on the hydrolysis-condensation of alcoholic solutions of silicon tetraethoxide (TEOS) and titanium n-butoxide, zirconium iso-propoxide, SnCl2.2H2O and Ce(NO3)3.6H2O as precursors of MO2 oxides. The resulting xerogels were studied by XRD, bandgap measurements from UV-Vis-NIR diffuse reflectance spectroscopy, photocatalytic activity against Orange II in dissolution and NOx in air (efficiency in nitrate abatement), and SEM-EDX microstructural analysis. In the case of non-stabilized samples, charring samples at 300 °C/1 h and fired samples at 500 °C/1 h were also studied. The cassiterite-silica composite stands out for its photocatalytic activity against Orange II in solution and the cerianite-silica composite against NOx in air. Silica increases the photocatalytic activity on NOx in zirconia, tin oxide and ceria composites and also on Orange II in zirconia and tin oxide. Graphical Abstract

Photocatalytic Degradation of Organic Pollutants-Nile Blue, Methylene Blue, and Bentazon Herbicide-Using NiO-ZnO Nanocomposite.

Water pollution poses a significant threat to both human health and ecosystem integrity. Chemical pollutants such as dyes and pesticides affect the water quality and endanger aquatic life. Among the methods for water purification from organic pollutants, photodegradation is certainly a valid technique to decrease such contaminants. In this work, pristine NiO, ZnO, and NiO-ZnO photocatalysts were synthesized by the homogeneous co-precipitation method. X-ray diffraction confirms the formation of a photocatalyst consisting of ZnO (Hexagonal) and NiO (Cubic) structures. The crystalline size was calculated by the Scherrer formula, which is 19 nm for the NiO-ZnO photocatalyst. The band gap measurements of the prepared samples were obtained using the Tauc Plot, equation which is 2.93 eV, 3.35 eV and 2.63 eV for NiO, ZnO, and NiO-ZnO photocatalysts, respectively. The photocatalytic performance of NiO-ZnO nanocomposite was evaluated through the degradation of Methylene Blue and Nile Blue dyes under sunlight, and Bentazon herbicide under a UV light. Photocatalyst degradation efficiency was 95% and 97% for Methylene Blue and Nile Blue in 220 min under sunlight while a degradation of 70% for Bentazon after 100 min under UV light source was found.

Franklinite-Zincochromite-Gahnite Solid Solutions for Cool Red Ceramic Pigments with Visible Light Photocatalysis

Franklinite-zincochromite-gahnite solid solutions were prepared using ceramic or coprecipitation methods, and their pigmenting capacity as cool ceramic pigments in different glazes (double and single firing frits and porcelain frit) was studied. XRD, UV–Vis–NIR diffuse reflectance, CIEL*a*b* colour analysis, band gap measurements, and the photocatalytic degradation of Orange II were carried out to characterise the samples. The following criteria for high red colouring capacity and high NIR reflectance at the minimum Cr amount were found to be the optimal compositions for an intense reddish cool pigment: Zn(Fe1.8Cr0.2), Zn(Al1.5Cr0.5) and Zn(Al1.3Cr0.5Fe0.2)O4. All the powders showed a direct semiconductor behaviour, with a band gap of approximately 2 eV, which fell in the visible range (620 nm); the visible light photocatalysis of Orange II was moderate, but franklinite-zincochromite Zn(Fe1.8Cr0.2) stood out compared with silver orthophosphate.

Optical Investigation of Reduced Graphene Oxide / Titanium Dioxide Nanocomposite Thin Films Synthesized by Hydrothermal Method

The growing need for unique optical properties in the manufacturing of electronic devices has led the world to the field of hybrid materials and their composites. In this study, a simple physical technique was used to successfully manufacture hybrid nanocomposites containing nanoparticles of titanium dioxide (TiO2) and reduced graphene oxide (rGO) from tetrabutyl titanate (TBT) and graphene oxide (GO) powder using the hydrothermal method. For two hydrothermal treatment times (12h and 24h), various samples were created: TiO2, rGO, and TiO2/rGO nanocomposites thin films. Fourier transformer infrared (FTIR) spectra of the samples gave clear evidence for the reduction of GO and the engagement of Ti with reduced graphene by the formation of a Ti-O-C bond. The measurement of the energy band gap obtained by the photoluminescence spectrometer shows a decrease in the energy band gap for all samples after the hydrothermal reaction time increases, for the composite, the band gap decreases to 2.65 eV and 2.64 eV.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis.

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Momentum resolved band gap measurement by high energy resolution electron energy loss spectroscopy

Momentum resolved band gap measurement by high energy resolution electron energy loss spectroscopy

Structural Investigation and Optical Properties of Dysprosium (Dy3+) Ions Doped Oxyfluoro Antimony Borate Glasses for Photonics Applications.

Dysprosium oxide-doped glasses with a composition of 60B2O3-10Sb2O3-10Al2O3-10NaF-(10-x) LiF-xDy2O3 (x = 0.1,0.5, 1.0,1.5,2.0,2.5 mol%) were prepared using a conventional melt-quenching technique. The glasses were characterized through various analytical investigations, including X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, refractive index, density, optical absorption, excitation, photoluminescence (PL) studies, decay measurements and radiation shielding parameters. The XRD and FT-IR confirms the glassy nature and functional groups present in the titled glass. The absorption spectra were used to determine the oscillator strength of the Dy3+ absorption transitions as well as the bond created with the O-2 ion in the titled glass network. The degree of the suitability of developed glasses for lasing applications was demonstrated by radiative parameters determined using Judd-Ofelt theory. In the prepared glass samples, the optical bandgap measurements indicate the presence of non-bridging oxygen (NBOs), localization of charges and donor centers in the titled glasses. Due to the de-excitation of 4F9/2 to the corresponding 6H15/2,6H13/2 and 6H11/2 states, the PL emission spectrum shows two main strong emissions at blue(480nm), yellow (575nm) and one less emission at red (663nm). The CIE coordinates determined using PL emission spectra reveal the coordinates that are falling within the white light region. Various shielding parameters such as mass attenuation coefficient, mean free path, effective atomic number were estimated to understand the radiative shielding nature of the titled glasses. Within the addition of Dy2O3, it was found that the shielding parameters values of the titled glass samples are increasing. The Mass Attenuation Coefficient, Half Value Layer and Mean Free Path of the as prepared glasses has been compared with different types of concretes to understand the shielding effectiveness of prepared glass.

Study of Ce3+-substitution induced defects restructuring in BiFeO3 by positron annihilation and other supportive methods

The results of Ce3+-substitution in place of Bi3+ in BiFeO3 nanocrystallites are reported in this work. Although Bi1-xCexFeO3 (x = 0, 0.025, …, 0.175) was formed with distinct crystallographic planes of X-ray diffraction, no structural transformation was observed unlike in the case of other lanthanide ion substitution of Bi3+ ions as reported earlier. The nanocrystallite sizes and the lattice parameters decreased during the increase of substitution. Transmission electron microscopic images also revealed the nanostructure formation and the decrease of the particle sizes. Energy dispersive X-ray analysis revealed appreciable incorporation of Ce into the BiFeO3 lattice. The measured band gap energies decreased owing to the creation of acceptor levels by Ce3+ ions. Distinct peaks in the X-ray photoelectron spectra at the expected binding energies indicated the presence of the two oxidation states Ce3+ and Ce4+. Positron annihilation studies proved the formation of interfacial defects with increase in substitution. Majority of the positrons diffused to the surfaces of the nanocrystallites and the mean lifetime of positrons increased. Coincidence Doppler broadened spectra, however, indicated positron trapping in the interfacial defects of the oxides of Ce and Bi overriding the annihilations of positrons at the surfaces in the range x = 0.075 to 0.125.

C(NH2)3Cd(C2O4)Cl(H2O)·H2O and BaCd(C2O4)1.5Cl(H2O)2: Two Oxalate Chlorides Obtained by Chemical Scissors Strategy Exhibiting Low-Dimensional Structural Networks and Balanced Overall Optical Properties.

Low-dimensional crystalline materials have attracted much attention due to their special physical and chemical properties. Herein, two new oxalate chlorides, C(NH2)3Cd(C2O4)Cl(H2O)·H2O and BaCd(C2O4)1.5Cl(H2O)2, were synthesized. C(NH2)3Cd(C2O4)Cl(H2O)·H2O presents the unique {[Cd(C2O4)Cl(H2O)]-}∞ zigzag chain, while BaCd(C2O4)1.5Cl(H2O)2 shows a novel {[Cd(C2O4)1.5Cl]2-}∞ layer. They showed large measured band gaps, which were 3.76 and 4.53 eV, respectively, and the latter was the largest band gap in the A-M-C2O4-X (A = Monovalent cationic or alkaline earth metals, X = F, Cl, Br, I) family. They exhibit a large calculated birefringence of 0.075 and 0.096 at 1064 nm, respectively. This study promotes the exploration of synthesizing low-dimensional crystalline materials with balanced overall optical performances by a chemical scissors strategy.

Structure property correlation study of energetic solid XeO3

In the realm of material science, xenon trioxide (XeO3) is characterized by an significant degree of shock sensitivity, resembling characteristics associated with primary explosives. To determine the underlying principles and quantify such behaviour we present a comprehensive computational study addressing the structural, vibrational, electronic and optical behaviour of energetic XeO3 by employing DFT calculations (Density Functional Theory). Our study demonstrates a good agreement between our computed structural parameters at equilibrium and experimental findings, particularly after incorporating vdW (van der Waals) interactions. Also, the mechanical stability of XeO3 is assessed by computing the Cij (elastic constants) and the B (bulk modulus). The evaluation of elastic constants in various directions indicates that (C33 > C22 > C11) XeO3 is highly shock sensitive along crystallographic a-axis. The computed (B) bulk modulus is determined to be 12.98 GPa, aligning closely with the values observed in other primary explosives. The phonon dispersion spectra computed within the harmonic approximation confirms the dynamical stability of XeO3 in P212121 structure. The spectra also reveal a notable interaction between the acoustic and optical branch, resulting in low thermal conductivity which is a trait for the energetic materials. The precise measurement of the energy band gap is crucial for establishing potential correlations with the photodecomposition process of energetic materials. To support this, we have conducted band structure calculations using the TB-mBJ (Tran-Blaha modified Becke-Johnson) potential, alongside the conventional GGA/LDA methods, generally known to underestimate band gaps. Our TB-mBJ based calculations yielded an energy band gap around 2.5 eV, which can also be compared with other energetic compounds. The rapid fluctuations of the optical constants suggest photodecomposition nature of XeO3 in the near UV region. The insights gained from our study would be of high interest for the study of other xenon (Xe) compounds and their oxides, delivering direction for potential applications across diverse fields.

Influence of PbO content on the glass forming ability, crystallization, electrical, UV–Vis. and photoluminescence spectroscopy properties of LiF.PbO system

New glass samples were synthesized from the binary system LiF. PbO with different ratiosby the traditional melt-quenching technique. The corresponding glass-ceramics were formed by heat treating the prepared glasses. Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and Transmission electron microscope (TEM) were employed to study the structure of the samples; while characterization of the glass and glass-ceramics were studied using photoluminescence (PL) spectral analysis, electrical, UV–Visible absorption, optical band gap and hardness measurements. The ability to form glass was increased with increase PbO content (1LiF:≥5PbO). Tetragonal and orthorhombic PbO, orthorhombic and monoclinic Pb2O3, α-PbF2 and traces of both Li4PbO4, PbO1.37 were crystallized under specific heat-treatment circumstances as identified by XRD analysis. The crystal size decreased with increasing the PbO content; it reaches ∼15 nm in case of 10 Pb. PL revealed concentration quenching in glass with 10 Pb, a red shift and enhancement of bands intensity were observed with decrease λex, increase heat-treatment temperature and increase PbO concentration. The UV–Visible absorption elucidated that the cutoff wavelength (λ) was shifted toward lower wavelength (blue shift) by increasing PbO content and the values of optical band gap energies recorded 2.69, 2.66 and 2.72 eV for LiF·5PbO, LiF·8PbO, and LiF·10PbO, respectively. The electrical properties recorded conductivity ∼1.2 × 10−6 S m−1 and dielectric constant ∼ 46.8. These characteristics suggests that, the prepared materials exhibited semiconducting and photoluminescence behavior nominated it to be utilized in solar cell applications.

Doping of large amount tetravalent Ge ions into Fe2O3 structure and experimental results on modified structural, optical and electronic properties

We report the doping of high concentration of tetravalent Ge4+ ions (5 mol % for x = 0.05–30 mol % for x = 0.30) at the Fe3+ sites of Fe2-xGexO3 system by chemical coprecipitation route. The charge state of Fe and Ge ions has been modified into lower values, in addition to their normal +3 and + 4 states, to stabilize the rhombohedral phase of hematite (α-Fe2O3) structure. X-ray photoelectron spectra and optical band gap measurements indicated a combination of ionic and covalence character of metal-oxygen bonds as an effect of Ge doping in the hematite structure. The Ge doped hematite samples have exhibited semiconductor properties with band gap of 1.50–4.70 eV and remarkably enhanced electrical conductivity (σ ∼ 10−4 S/m) in comparison to α-Fe2O3 (10−11 S/m). The results confirmed the strategy of enhancing electrical conductivity in hematite structure by doping of tetravalent cations. The thermo-conductivity measurements using warming and cooling modes showed highly irreversible feature at higher temperatures. Some of the samples indicated metal-like state at lower temperature, in addition to semiconductor state at higher temperatures. It has been understood that combination of ionic and covalence character of the metal-oxygen bonds has played an important role in modifying the semiconductor properties in Ge doped Fe2O3 system.

Optical, magnetic, and electrochemical properties of a fluorite-perovskite Ce0.9Gd0.1O2-∂ - La0.6Sr0.4FeO3-∂ nanocomposite synthesized by one pot sol-gel auto-combustion route

Gadolinium-doped cerium oxide (Ce0.9Gd0.1O2-∂; CGO) and strontium-doped lanthanum ferrite (La0.6Sr0.4FeO3; LSF) are well-known compounds for their application in solid oxide fuel cells, dense oxygen permeable membrane, and high-temperature ceramic reactor. The superior electro-catalytic performance of these two individual oxides or their composites is well established as it originates from multivalent cations (Ce3+/4+ and Fe2+/3+/4+) and doping-induced defects like electrons, holes, and oxygen vacancies. This work focuses on synthesizing and characterizing composite taking CGO and LSF in different weight ratios, i.e., 10:90, 30:70, 50:50, 70:30, and 90:10 by sol-gel auto-combustion process. End members CGO and LSF were also synthesized for comparison purposes. All synthesized nanopowders (single phases and their composites) were characterized with X-ray diffraction (XRD) for phase identification and Scanning electron microscopy (SEM) for elemental composition analysis. XRD and SEM results established the formation of the required phase and targeted composition. The optical properties of the materials were evaluated with the help of ultraviolet–visible Diffuse Reflectance Spectroscopy (UV–Vis DRS). Tauc's approximation measured band gap energies, and all the composites showed a very high band gap in the 4.85–5 eV range. Magnetic properties such as saturation magnetization (Ms), remanence (Mr), and coercive force (Hc) were calculated from a magnetic hysteresis loop obtained from the Vibrating-sample magnetometer (VSM) experiment at room temperature. Furthermore, the law of approach to saturation (LAS) model was applied to more precisely calculate Ms and other essential parameters. In magnetic characterization, the LSF phase fraction controls the overall magnetic property of the composites. Electrochemical behavior, such as the composites' charge storage ability and stability were tested by cyclic voltammetry and chronopotentiometry method. All the samples exhibited good stability, and the highest specific capacitance of 26 F/gm was obtained for the composite sample (CL91) when only 10 % LSF phase was incorporated in the nanocomposite.

Effect of physical and chemical activation methods on the structure, optical absorbance, band gap and urbach energy of porous activated carbon

In this study, activated carbon was synthesized using the almond shell and palm kernel by physical activation with water vapor and chemical activation with phosphoric acid (H3PO4) methods. Then, the structural and optical properties of the activated carbons were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and UV–Vis spectroscopy. The SEM images showed that in the raw sample of hard almond shell and palm kernel, there is no porosity and pores, but in the activated carbon samples, porosity and structural defects were clearly observed. The XRD patterns showed that porous and amorphous structure was formed in all samples synthesized with physical and chemical activation. The results of FTIR spectra of activated carbons showed that there are carbon functional groups in all samples. The optical absorption coefficient (α) of the activated carbon with physical and chemical activation methods was obtained in order of 105–106. The band gap measurement of porous nanostructures showed that the activated carbon synthesized with chemical and physical activation methods have energy gap (Eg) in region = 2.80 to 3.15 eV and urbach energy (EU) in region = 120 to 210 meV.

Defect analysis and microstructural characterization of zinc-substituted cadmium oxide nanocrystallites by positron annihilation and supplementary methods

Pure and zinc-doped cadmium oxide nanocrystallites of sizes in the range 25 nm to 16 nm are synthesized by adopting a chemical precipitation method and by varying the doping concentration from 0.0 to 0.25. The decrease in nanocrystallite sizes with increasing substitution is expected from the smaller ionic radii of Zn2+. But more revealing is the interfacial defects formation at higher concentration of doping, which is attributed to the dissimilar crystalline structure of ZnO and CdO. X-ray diffraction patterns show well defined peaks and additional characterisation is done through transmission electron microscopy. The optical band gap measurements indicate the dominance of substitution-induced disorder over the confinement of excitons, leading to a decrease in the band gap energies. The results of positron annihilation studies confirm the cancellation of the existing vacancy type defects in the initial stage, followed by the substitution. Photoluminescence spectra reveal the distinct peaks of optical plasmonic excitations and the defect population in the bandgap and the intensity variations agreed with that of the defect related positron annihilation lifetime intensity. The segregation of ZnO phase leading to the formation of interfacial boundaries is found as a strong deterrent against the success of continued substitution.

Luminescence properties of ZrO2: Dy3+ phosphor for solid-state lighting applications

Herein, we report Dy3+-activated ZrO2 phosphors synthesized via the sol-gel technique. The structural, morphological, and optical properties of the synthesized powder samples annealed at 900 °C were investigated. The monoclinic phase of the synthesized phosphor was confirmed by recording the X-ray powder diffraction (XRPD) pattern. Ultraviolet–visible analysis was performed for the reflectance and bandgap measurements of the phosphors. Field emission scanning electron microscope and X-ray energy dispersive spectroscopy analyses were used to study the morphology and elemental composition of the phosphors, respectively. The distribution of ions in the undoped and Dy3+-doped samples was confirmed using time-of-flight secondary ion mass spectroscopy. The photoluminescence (PL) behavior of the synthesized phosphors was investigated by recording the excitation and emission spectra. When the synthesized phosphors were exposed to UV light at 353 nm, we observed characteristic blue and yellow Dy3+ PL emissions, corresponding to the transition 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2, respectively. Additionally, we observed low-intensity red and brown-red emission peaks corresponding to 4F9/2 → 6H11/2 and 4F9/2 → 6H9/2. These observations indicate that the non-aqueous sol-gel route, coupled with the suitable post-synthesis heat treatment, resulted in the development of a monophase (monoclinic) zirconia. This makes it a strong candidate for phosphor applications due to its high room temperature stability and efficient characteristic features of the monoclinic phase. These findings strongly suggest that Dy3+-doped ZrO2 can serve as a superior phosphor material for white light-emitting diode applications.