Urbach Edge Research Articles

Improved pH performance of cresol red radiochromic dye for spectroscopic investigations

Optical properties of a synthetic dye cresol red (CR) were investigated at its absorption band maxima (λmax) using double beam scanning UV/VIS spectrophotometer for the sample solutions having initial optimized dye-concentration. Urbach Edge method was utilized for the investigations of absorption edge (AE= 2.35 eV) while Mott and Davis model was utilized for the investigations of band gap energy for direct (Eg,direct= 2.45 eV) and indirect (Eg,indirect= 2.23 eV) allowed transitions. The spectrophotometric-evaluations assisted most-sensitive working pH of sample solutions was optimized for a long spectrum of pH values from 1 to 12 for further investigations. Some spectrophotometric and comercial parameters e.g., molar extinction coefficient ε, stability, reproducibility and fading were figured out. The response efficacy was also observed to enhanc the sensitivity and stability of the CR solutions by incorporating a selected amount (by %vol) of a polar additive ethanol. The invariant aborbance related isosbestic wavelength for both acidic and alkaline samples remained unaffected from pH and firmly stood at 475 nm. The improved and reasonable pH values were found to be pH 4 and pH 9 and hence were opted for CR solutions and may further be applicable to the radiation-induced degradation phenomenon based on radiolytic bleaching.

Biosynthesis, Structural, Spectroscopic, Photoluminescence, and Antifungal Activity of Ni-doped CeO2 Nanoparticles.

Pedalium Murex leaf extract was used in this study to create Nickel-doped Cerium oxide (Ni-CeO2) nanoparticles at 3mol% and 5mol% molar concentrations. The biosynthesized process was applied for the fabrication of Ni-CeO2 NPs. The X-ray diffraction method was used to identify their crystal structure. The XRD measurements showed that the Ni-CeO2 NPs crystallized into the face-centred cubic system. Fourier transform infrared spectral study was applied to explore the molecular vibrations and chemical bonding. The surface texture and chemical ingredients of Ni-CeO2 NPs were studied using field-emission scanning electron microscopy and energy-dispersive X-ray analysis. The EDX mapping spectra illustrate the uniform dispersal of Ce, Ni, and O atoms over the sample's surface. X-ray photoelectron spectroscopy (XPS) was conducted to confirm the chemical state of the Ni-CeO2 NPs. UV-Vis spectrum study was performed to ascertain the photon absorption, bandgap, and Urbach edge of Ni-CeO2 NPs. Photoluminescence (PL) research has been used to study the light-emitting characteristic of Ni-CeO2 NPs. The emissive intensity transition corresponding to Ni-CeO2 NPs was found to increase with the dopant level. The CIE 1931 chromaticity map was plotted to find the aptness of the samples for optical uses.The antifungal ability of Ni-CeO2 NPs was evaluated against the fungi candida albicans and candida krusein with the agar well-diffusion process. The fungicidal activity of the 3mol% Ni doped CeO2 nanoparticles has shown a maximum zone of inhibition. The experimental findings illustrate the utility of Ni-CeO2 NPs for optical and antifungal applications.

A Full-Scale Analysis of Absorption Edges in Dye-Doped Nonlinear Optical Polymers.

A full-scale analysis of the absorption edges by modified Tauc-Lorentz models is essential in determining the optical bandgap and Urbach energy of semiconductors, transparent conductors, ionic compounds, and dielectric materials. This technique has not yet been applied to analyzing organic nonlinear optical (NLO) materials. This problem is tackled by preparing high-quality films of guest-host NLO polymers with a wide thickness range from sub-micron to 200 microns, allowing accurate measurement of full-spectral absorption coefficients of NLO materials over four orders of magnitude by the UV-VIS-NIR spectroscopy. The Tauc model and a new Monolog-Lorentz model are used to study the optical absorption edge of guest-host NLO polymers containing various push-pull chromophores and the dependence of optical bandgap and Urbach edge on the structure and composition of materials is analyzed. The results reveal the critical transition of the Urbach exponential tail to a low energy tail that overlaps with vibrational overtones of materials at the telecom wavelengths. Determining the fundamental absorption region of organic NLO films in this study provides quantitative insight into the research to harness the resonance-enhanced nonlinear coefficients of materials by operating at the wavelengths near the band edge with the control of optical loss.

An innovative approach for the synthesis of 4Tris@Porphyrin and anderson polyoxometalate based covalent organic framework to reveal its potential for photocatalytic properties

Herein, a polyhydroxy containing metallo porphyrin linked through amide functional group with Anderson polyoxometalate (POM) was used to form TPMnA-COF. Structure and applications have been unlocked thoroughly by means of FT-IR, UV–visible, 1HNMR, PXRD, SEM and cyclic voltammetric (CV) spectral analysis. The TPMnA-COF was tested as catalyst to explore the photocurrent and photodegradation properties. ITO electrode was fabricated to study optical activity, absorption study and photocurrent generation. The adsorption study of TPMnA-COF has been studied using Langmuir and Freundlich model with calculated Langmuir constants qm, KL, RL and R2 values of about 12.42 mg/g, 0.33 M-1, 0.30 and 0.98 respectively. The 1/n, Kf and R2 values obtained from Freundlich adsorption isotherm were 0.70, 3.16 M-1 and 0.96 respectively. The band gap value evaluated via absorption edge of UV–visible spectra in DMSO solution was 2.75 eV while the band gap value calculated for ultra-thin film was 1.95 eV. The calculated real, imaginary dielectric constants, Urbach edge, refractive index and the electrical conductivity values for ultra-thin film of TPMnA-COF were 518.8, 2.9 × 10−11-8.1 × 10−11, −3.802 meV, 22.8–22.6 and 8.3 × 104−3.4 × 104 S/m), respectively. Responsivity for film was also tested using purple, blue, green, yellow, orange and red colourlights and the observed responsivity for different lower light intensities were 3.05, 2.9, 1.6, 1.55, 1.5 and 1.4 mA/W respectively. Furthermore, photodegradation activity of TPMnA-COF in Methylene-Blue (MB) and Rhodamine B(Rh–B) dyes under 3 W light. The results showed higher photocatalytic efficiency for Rh–B than that for MB with obtained degradation efficiency of 81 % and 60.4 %, respectively after 660 min. The scavengers such as ammonium oxalate (AO, h+), isopropanol (ISOP, •OH), catalase (CAT, H2O2) and superoxide dismutase (SODM, O2−●) were used to find out the effectiveness of each reactive specie to degrade the dyes solution in the mechanism of degradation. It was observed that AO (h+) and ISOP(•OH) were more active species in the photodegradation mechanism than CAT(H2O2) and SODM(O2−●).

Below the Urbach Edge: Solar Cell Loss Analysis Based on Full External Quantum Efficiency Spectra

Below the Urbach Edge: Solar Cell Loss Analysis Based on Full External Quantum Efficiency Spectra

A simplistic approach for the synthesis of Covalent Organic Frameworks(COFs) comprising of tetrafunctionalized porphyrin and polyoxometalates to uncover catalytic applications

Tetrafunctionalized porphyrins are energetic synthons to prepare MOFs(Metal organic framework) and COFs(Covalent organic framework). Herein, a new tetrafucntionalized metallo porphyrin was used and connected via amide functionality with anderson polyoxometalates(POMs) and assembled two COFs(P@Ni-AndCOF & P@Cu-AndCOF). Structures, properties and their relationships have been unlocked thoroughly by means of FT-IR, UV–visible, 1HNMR, PXRD, SEM and cyclic voltammetric(CV) spectral analysis. The catalysts were tested to uncover the photocatalytic properties such as photocurrent and photodegradation properties. P@Cu-AndCOF and P@Ni-AndCOF were used to fabricate the ITO TiO2 coated electrodes to study optical activity, adsorption study and photocurrent generation. The adsorption study of P@Cu-AndCOF and P@Ni-AndCOF has been studied by using Langmuir and Freundlich model with calculated Langmuir constants qm, KL and RL values of about [7.44(mg/g), 8.02(mg/g)], [0.79(L/mg), 0.69(L/mg)] and (0.22 & 0.23) respectively. The correlation coefficient for adsorption obtained through Langmuir analysis were 0.92, and 0.95 for P@Ni-AndCOF and P@Cu-AndCOF respectively. The 1/n, Kf and R2 values obtained from Freundlich adsorption isotherm for P@Ni-AndCOF and P@Cu-AndCOF were (0.50 & 0.52), (3.325676 mg/g &3.328971 mg/g) and (0.94 & 0.93) respectively. The bandgap values evaluated via absorption edge of UV–visible spectra for P@Ni-AndCOF and P@Cu-AndCOF in DMSO solution were 2.82eV, and 2.83eV respectively while the bandgap values calculated for ultra-thin films were 1.78eV, and 1.85eV for P@Ni-AndCOF and P@Cu-AndCOF correspondingly. The calculated real, imaginary dielectric constants, Urbach edge, refractive index values and the electrical conductivity for ultra-thin film of P@Ni-AndCOF and P@Cu-AndCOF were (1.4x104-1.41x104& 2.19x10−9-2.45x10−9), (6.6x104-7.4x104&1.7x10−10-2.47x10−10), (0.323eV & 0.362eV), (32.9–32.4 & 33–30.4) and (5.8 × 104-2.9 × 104 &6.1 × 104-4.7 × 103 S/m) respectively. Furthermore, photodegradation activity of both catalysts was checked in Methylene-Blue(MB) and Rhodamine B(Rh-B) dyes under 3 W light. They showed higher photocatalytic efficiency for MB than that for Rh-B caused degradation efficiency of 89% and 79.3% for MB and 83% and 71.3% for Rh-B respectively after 660min. The photodegradation activity was also observed in dyes solutions along with some scavenger's ammonium oxalate (AO, h+-scavenger) and isopropanol (ISOP, •OH-scavenger) and it was observed that by the addition of these scavengers photodegradation rate was reduced.

Carrier thermalization and zero-point bandgap renormalization in halide perovskites from the Urbach tails of the emission spectrum

We develop techniques to study the temperature dependent localization, thermalization, and the effects of phonon scattering on the excitons in halide perovskites from the analysis of the emission spectra. The excitonic Urbach edge, when inferred from the low energy tails of the temperature dependent luminescence spectra, is shown to be sensitive to the electron distribution and thermalization. A method to observe the Urbach focus is devised for halide perovskites where the temperature dependence of the excitonic gap is anomalous. The value of the zero-point bandgap renormalization is inferred to be about 33 meV. This small value of the bandgap renormalization rules out the formation of small polarons and points to weak electron–phonon coupling. The experiments are performed on the nanosheets of the archetypal halide perovskite, CsPbBr3.

Impact of Y2O3 on the structural, optical, radiation shielding, and ligand field parameters of transparent borate glass containing constant CrO3 and high Na2O contents

This study aims to investigate the impact of the rare earth element yttrium on the optical properties of chromium (Cr)-doped borate glasses. A series of borate glasses (labeled as BNaCrY glasses) were prepared and studied, each with a constant amount of chromium trioxide (CrO3) but doped with varying amounts of yttrium oxide (Y2O3). Infrared analysis revealed four distinct spectral areas in the internal structure of all BNaCrY glasses. Optical absorption spectra were used to investigate the impact of Y2O3 doping on the optical transitions of BNaCrY glasses. The presence of Cr cations caused the appearance of an absorption band at 379–383 nm, which overlapped with the Urbach edge that is commonly given an edge-like at 430 nm. The Urbach edge has been manifested through a full deconvolution attempt. Cr cations in the Cr3+ state were confirmed by visible absorption bands and are nestled in the octahedral positions. The band at 574–593 nm was used to determine the crystal field splitting (10Dq) for all BNaCrY glasses. Due to the increased number of O anions around the Cr cation, the values of 10Dq increased with Y2O3 doping. The remarkable increase in 10Dq was attributed to increased interaction between Cr cations and their surroundings. In contrast, the shielding parameters were taken into account when evaluating the ability of these transparent glasses to protect against nuclear radiation. We found that the sample with the highest Y2O3 concentration had the highest linear attenuation coefficient and the lowest half-value layer. Thus, BNaCrY glasses are more efficient as attenuators than commercial glasses (such as the Rider and Osmania glasses), especially at low photon energies.

Recovery of damage in electron-irradiated ceria

The damage recovery of an electron-irradiated CeO2 single crystal was studied by in situ UV–visible absorption spectroscopy during isochronal thermal annealing up to 873 K. The spectrum of the as-irradiated crystal shows band tailing below the fundamental absorption edge for photon energy near 3.2 eV. Two broad Gaussian bands centered at about 0.9 and 1.4 eV are also recorded. The edge region is treated by including the Urbach edge contribution. The variation of bandgap energy and Urbach energy is followed as a function of the annealing temperature. The absorption edge was red-shifted with annealing temperature then returned to the starting value after cooling back to room temperature. The disorder contribution to the Urbach edge is deduced by subtracting the temperature contribution. It shows a decay of the disorder contribution following a first-order kinetics process with an onset of recovery at 373 K and activation energy of about 0.15 eV. Full recovery is achieved at 873 K, whereas a partial recovery of the two absorption bands (at 0.9 and 1.4 eV) is found. The low activation energy is interpreted in reference to the available literature data on the energy barriers for point defect migration, and assigned to the migration of oxygen interstitials associated to electronic levels near the top of the valence band edge. A recombination mechanism is suggested, involving both electron hopping and interstitial oxygen diffusion.

Impact of bismuth oxide on the structure, optical features and ligand field parameters of borosilicate glasses doped with nickel oxide

Understanding the impact of bismuth cations on the optical properties of borosilicate glass is significant for manipulating borate glass applications. In this paper, the influence of bismuth cations on both structural and optical properties of borosilicate glass doped with NiO was investigated. Different glass samples, containing different amounts of Bi2O3 and a constant amount of NiO, were prepared and studied. Infrared (IR) analysis was carried out to study the internal structure within the investigated glass samples. Optical absorption studies were performed to investigate the impact of Bi2O3 content on optical properties of the BiBaNiB-glasses. Astonishingly, with Bi2O3 addition, an absorption band at 380 nm has appeared. Moreover, this band is overlapped with the Urbach edge; which regularly produced an artificial edge-like feature at ~450 nm. A detailed deconvolution protocol has been implemented for an appropriate understanding of these spectra and unraveling the hidden Urbach edge. Optical band gap energy, linear and nonlinear refractive index for each BiBaNiB sample were calculated. Furthermore, the metallization criterion was calculated to examine the metallic or insulating nature of the BiBaNiB-glasses. The values of the nonlinear third-order susceptibility and nonlinear refractive index were increased with Bi2O3 doping. The BiBaNiB-glasses exhibited outstanding stability and optical band gap than the pristine glass sample, which makes it possible for practical applications.

The tungsten oxide within phosphate glasses to investigate the structural, optical, and shielding properties variations

We prepared a series of sodium phosphate glasses by changing WO3/P2O5 content and investigated structure optical and radiation shielding features as a function of the glass composition. The average density ( $$\rho_{{{\text{exp}}}}$$ )was found to increase gradually from 2.49 to 3.07 g/cm3, while the average molar volume values reduced from 47.37 to 44.28 cm3/mol with WO3 addition. Also, the average field strength was also computed and found to increase with increasing WO3. The study of optical absorption spectra reveals that the absorption peaks in the visible region become higher compared to the peaks In the Ultraviolet region. This observation is accompanied by a color transformation of glasses from light to dark blue color, with more WO3 adding. The existence of a pentavalent tungsten state (W5+) is identified by this blue color; with WO3 addition an absorption band at around 350–390 nm is appeared. Moreover, this band is overlapped with the Urbach edge, which regularly produces an artificial edge-like feature at ~ 400 nm. A detailed deconvolution protocol is required for an appropriate understanding of these spectra and unraveling the hidden Urbach edge. Our analysis shows that, with increasing WO3/P2O5 content, the optical band gap decreases. This behavior can be elucidated in terms of a lower band gap of WO3 (2.7 eV) than that of P2O5 (8.5 eV) and the high polarizing power tungsten. Further, the radiation shielding parameters were investigated for the prepared glasses. WO3 addition improves these shielding parameters against radiation. Besides, upon the increase of WO3 concentration, the linear attenuation coefficient of glass material increases, which leads to decrease in have value layer values. Then it is deducible that the amount of WO3 in this glass sample has an important impact on the shielding capability at lower energy values and has a slight impact at higher energy values.

Disorder induced in silicon carbide by heavy-ion irradiation

ABSTRACT The decrease of crystal phonon peak intensities in Raman spectra of silicon carbide after heavy-ion irradiation is analysed in relation to band-gap shrinkage and Urbach edge slope increase arising from accumulation of lattice disorder. The discrepancy on amorphous fractions deduced from Raman spectroscopy and Rutherford backscattering-channelling spectroscopy is addressed by taking into account point-defect formation and amorphization by displacement damage. A new analysis of Raman data is provided on the basis of self-absorption of scattered light associated with damage build-up.

Temperature dependence of the optical absorption edge of doped gallium arsenide

The temperature dependences of the optical absorption edges of Zn doped GaAs semiconductor crystals have been measured from 300 to 560 K. The temperature dependence of the optical absorption in the Urbach edges is adequately reproduced by a Bose-Einstein model. Analysis of experimental results gave us the opportunity to offer an explicit function of two arguments (photon energy and temperature) for the absorption coefficient of doped crystals in the Urbach edge region.

Electron energy loss spectra from silica glass optical fibers

AbstractTo investigate the possible structural differences between silica glass fibers and bulk silica glasses, electron energy loss spectroscopy (EELS) has been used to study the short‐range and medium‐range structures of both forms of silica glasses. The short‐range structure of silica glass, such as the coordination and symmetry, was investigated by the energy loss near edge structure (ELNES) of Si L2,3‐edges. The ordering structure in the medium‐range was analyzed by the exponential optical absorption edge also known as the Urbach edge of the glasses. The optical absorption data were obtained from the low energy loss spectrum of EELS through Kramers‐Kronig analysis. The results show that silica fiber has the same short‐range structure as the bulk specimen, but is significantly more disordered than the bulk glasses.

Interrelation between the photo-induced shift of the optical band gap and Urbach/exponential edge slope in a- Se

A simple correlation has been found between photo-induced changes in the optical band gap and slope of both the Tauc edge and Urbach edge. The kinetics of all the photo-induced changes observed follow a stretched exponential.

N+ Implantation-Induced Surface Hardening of Poly (Allyl Diglycol Carbonate) Polymer

ABSTRACTThe effect of 100 keV N+ ions implantation on the surface structure and hardness of poly (allyl diglycol carbonate) (CR-39) polymer was studied. The surface hardness of virgin and implanted CR-39 specimens was determined using a Knoop microhardness test. The surface hardness was found to be enhanced after implantation, e.g., becoming eight times higher at a load of 9.8 mN, for a dose of 2 × 1016 ions cm−2. The change in bonding and surface structure of the CR-39 polymer due to implantation was studied using the specular reflectance Fourier transform infrared (FTIR) technique. The disordering produced in the implanted matrix was estimated using the Urbach edge method from the UV-Visible absorption spectra. The relationship of surface hardening with the chemical and structural changes was explored

Correlation between photo-induced red shift of the optical band gap and the slope of Urbach edge in amorphous and glassy As2S3

Photo-induced changes in the Tauc gap and the slope of Urbach edge in As2S3 were found to be interrelated via simple linear relation. It is suggested that this correlation indicates a certain similarity between by disorder affected electronic transitions associated with the Tauc gap and the Urbach edge.

Spectroscopic and sub optical band gap properties of e-beam irradiated ultra-high molecular weight polyethylene

Abstract Muller matrix spectro-polarimeter has been used to study the absorption behavior of pristine and e-beam irradiated (30, 65,100 kGy) ultra-high molecular weight polyethylene (UHMWPE) over the visible spectral range i.e. 400–800 nm. As a result, significant changes occur in the absorption behavior of irradiated samples due to radiation induced physical and chemical changes. To analyze these (radiation induced) changes in polymer matrix, Urbach edge method is employed for the calculation of optical activation energy. In addition to this, direct and indirect energy band gaps along the number of carbon atoms in C=C unsaturation have been determined by using modified Urbach formula and Tauc's equation, respectively. The results obtained during study reveal that Urbach energy decreases with radiation treatment and has a lower value for 100 kGy sample i.e. E u =71.63 meV. The values of direct and indirect energy band gaps are also following the decreasing trend with e-beam irradiation. Moreover, indirect energy gaps are found to have lower values as compared to direct energy gaps. The number of carbon atoms in clusters (as estimated from modified Tauc’s equation) has been found to vary from ∼6 to 8 for direct energy band gaps and from ∼9 to 11 for indirect energy band gaps.

Photoconducting Urbach edge in amorphous Se

Photoconductivity spectra in amorphous Se have been analyzed on the basis of the geminate-pair model. It predicts that around the Urbach edge the photoconductivity spectrum exhibits also an exponential variation~exp(hν/EUp) with the steepness parameter given as EUp≈kBT, which may be universally held in other materials. In amorphous Se films, however, the exponential variation tends to be masked by below-gap responses, the origin being speculative. We also discuss the photoconductivity spectra in super-gap regions.

The complete absorption spectra of InAs0.87Sb0.13 films grown by liquid phase epitaxy

We design, fabricate and characterize the InAs0.87Sb0.13 film with cutoff wavelength of 4.6μm on InAs (100) substrate, which is widely used as active layer of the multilayer device architecture for mid-infrared device used for the monitoring of carbon monoxide at 4.6μm. In order to accommodate the large lattice mismatch between the InAs0.87Sb0.13 epilayer and the InAs substrate, the InAs0.92Sb0.08 buffer layer was introduced. We acquired the complete absorption spectra and subsequently derived the basic parameters of InAs0.87Sb0.13 film and InAs0.92Sb0.08 buffer layer about absorption coefficient, optical energy band gap and the characteristic energy of Urbach edge from the Fourier transform infrared (FTIR) transmission spectra, which are very useful for modeling and simulation in the devices design.