Blue Shift Of Optical Band Gap Research Articles

Reduced Leakage Current and Enhanced Photovoltaic Effect in Zn-Doped BiFeO3 Thin Films

Zn-doped BiFeO3 (BFO) thin films with compositional formula BiFe1–xZnxO3 (x = 0, 0.1, and 0.2) have been epitaxially grown on SrRuO3 buffered SrTiO3 substrates by pulsed laser deposition. The high-concentration Zn doping does not suppress the ferroelectric polarization of the BFO films, and the Zn-doped BFO thin films also show reduced leakage current and an enhanced photovoltaic effect. By optical and photoelectron spectroscopy measurements, with Zn doping, the BFO thin films show more oxygen vacancies and a structural evolution, and moreover, a blue-shift of optical bandgap and an increase of work function are demonstrated. The reduction of leakage current and the enhancement of photovoltaic effect are related to the variation of interfacial Schottky junctions and oxygen vacancies with doping. This study reveals systematic insights into the effects of Zn doping on the physical properties of BFO thin films.

Photoluminescence of wurtzite-type Ce-doped ZnS nanocrystals processed by a low-temperature aqueous solution approach

Undoped and Ce-doped ZnS nanocrystals with hexagonal wurtzite structure were synthesized by a low-temperature aqueous solution method that uses l-cysteine as complexing and capping agent. The morphology analysis revealed the formation of polydisperse nanocrystals with sizes in the range of 2–4 nm, while the energy dispersive X-ray spectroscopy showed that all the samples were sulfur deficient. The presence of Ce3+ species in the Ce-doped samples was confirmed by the X-ray photoelectron spectroscopy technique. UV–vis spectroscopy indicated that all samples had a blue shift of optical bandgap compared to bulk ZnS. According to the room-temperature photoluminescence studies, the spectra of Ce-doped ZnS nanocrystals exhibited a broad band emission consisting of UV and blue components, associated with ZnS defects, and a green component due to the 5d-4f transition of Ce3+.

Blue-shift in optical bandgap of sprayed nanocrystalline Cu2ZnSnS4 thin films induced by 200 MeV Xe swift heavy ions irradiation

Blue-shift in optical bandgap of sprayed nanocrystalline Cu2ZnSnS4 thin films induced by 200 MeV Xe swift heavy ions irradiation

Biogenic ZnO nanoparticles: a study of blueshift of optical band gap and photocatalytic degradation of reactive yellow 186 dye under direct sunlight

Abstract Synthesis of nanoparticles (NPs) using plant extracts has been suggested as an environmentally friendly alternative to chemical synthesis of semiconductor NPs. In the present study, ZnO NPs were synthesized by a simple and cost-effective method using Coriandrum sativum leaf extract and zinc acetate as precursors. The as-synthesized ZnO NPs were characterized by UV-visible spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) analysis. The results confirmed the formation of ZnO NPs with a wurtzite structure, spherical shape and average particle size of 24 nm. The photocatalytic degradation of reactive yellow 186 (RY 186) dye was carried out under direct sunlight irradiation and its degradation efficiency and apparent rate constant (K’app) of reaction were calculated to be 93.38%, and 0.0019 min−1, respectively. The optical band gap value of the as-synthesized ZnO NPs was found to be 3.4 eV, which indicates the presence of blueshift. Owing to the presence of blueshift and a wide band gap of synthesized biogenic ZnO NPs, the overall absorption of sunlight irradiation will be enhanced, which leads to higher degradation efficiency of the dye. The current study thus highlights the optical band gap properties of biogenic ZnO NPs and their significance as a heterogeneous catalyst for the purification of polluted water.

High near-infrared transmittance, high intense orange luminescence in vanadium doped indium oxide (V: In2O3) thin films deposited by electron beam evaporation

The V (5wt.%) doped In2O3 (V: In2O3) thin films with the thickness of 50–150nm were deposited on to glass substrate by electron beam evaporation technique and the deposited films were annealed at 200°C in air for 2h. The annealed films were characterized by structural, electrical, optical and photoluminescence properties. X-ray diffraction (XRD) pattern shows that the film is an amorphous nature. The negative sign of the Hall coefficient indicates the n-type conductivity. Experimental results show that the resistivity and transmittance of the films are strongly influenced by film thickness and post deposition annealing. A minimum resistivity of 6.22×10−3Ωcm, a carrier concentration of 5.23×1019cm−3, a Hall mobility of 19.21cm2V−1s−1 and a figure of merit of 1.04×10−4Ω−1 are observed for the film with thickness of 150nm. After annealing the films, the average transmittance is enhanced up to 84% in the near infrared region. The blue-shift of optical band gap is found to increase from 1.62 to 2.25eV with increasing the thickness, whereas the refractive index decreases from 2.92 to 2.64. Structural disorder was observed from the Urbach’s tail. A prominent and high intense orange emission is achieved at room temperature, which may be attributed to oxygen deficiencies or intrinsic defects.

Cation substitution induced blue-shift of optical band gap in nanocrystalline $$\hbox {Zn}_{(1-x)} \hbox {Ca}_{x}\hbox {O}$$ Zn ( 1 - x ) Ca x O thin films deposited by sol–gel dip coating technique

Transparent nanocrystalline \(\hbox {Zn}_{(1-x)}\hbox {Ca}_{x}\hbox {O }(0 \le x \le 0.20)\) thin films were deposited on glass substrates by sol–gel dip coating method. The X-ray diffraction (XRD) pattern revealed the polycrystalline nature of the films with hexagonal wurtzite structure and confirmed the non-existence of the secondary phase corresponding to CaO indicating the monophasic nature of the deposited films. The crystallinity of the films deteriorated with higher dopant concentration due to the segregation or separation of dopant ions in grain boundaries. The lattice parameters and the unit cell volume increased to a higher Ca-dopant concentration. This was due to the successful incorporation of \(\hbox {Ca}^{2+}\) ions with larger ionic radius in the host zinc oxide (ZnO) lattice. The optical transmittance spectra of the samples showed transmittances above 60% in the visible spectral range and the absorption edge in the near ultra-violet region got blue-shifted with cation substitution. The estimated optical energy gaps confirmed the band gap widening with increase in Ca-dopant concentration. The calculated values increased from 3.30 eV for undoped ZnO to 3.73 eV for \(\hbox {Zn}_{0.8}\hbox {Ca}_{0.2}\hbox {O}\) thin films giving 13.03% enhancement in the energy gap value due to the electronic perturbation caused by cation substitution as well as deterioration in crystallinity.

Synthesis and enhanced photocatalytic property of feather-like Cd-doped CuO nanostructures by hydrothermal method

Feather-like Cd-doped CuO nanostructures were fabricated by a one-step hydrothermal method. X-ray diffraction pattern (XRD) and field emission scanning electron microscopy (FESEM) demonstrated that Cd2+ entered the crystal lattice of CuO and substituted Cu2+ without destroying crystal structures to form feather-like CuO nanostructures. The optical property of Cd-doped CuO was investigated by using UV–vis spectrophotometer. A slight blue-shift of optical band gap was observed because of quantum confinement effect. The doped samples exhibited obviously higher absorbance in UV light region and better photocatalytic activity for the photodegradation of methyl blue than the pure CuO nanosheets.

Study of blueshift of optical band gap in stannic oxide nanosheets by an uncharged surfactant mediated alcohothermal process

Two-dimensional (2-D) nanostructures have attracted great attention in recent years. Herein, we report the synthesis of 2-D nanosheets of stannic oxide (SnO2) via an uncharged surfactant mediated alcohothermal process, by using stannic tetrachloride pentahydrate, alkali base pellets (caustic soda), ethyl alcohol as a solvent, and poly (oxy) ethylene glycol-200 as a surfactant. The powder X-ray diffraction (XRD) study shows the formation of the tetragonal rutile phase SnO2. The Fourier transform infrared (FTIR) study confirms the formation of Sn–O bond. The morphology and sizes of the SnO2 powder have been studied by the scanning electron microscopy (SEM) analysis. A strong absorption peak was observed in the ultraviolet region at 335nm from the optical absorption spectrum.

Microstructure and blueshift in optical band gap of nanocrystalline AlxZn1−xO thin films

In this paper, we report the structural and optical properties of Al doped ZnO (AZO) thin films grown on glass substrates using the sol–gel process. To understand the effect of Al doping on the structural and optical response of ZnO nanoparticles thin films, the prepared samples have been characterized using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), photoluminescence (PL), UV–vis absorption and Raman spectroscopy. X-ray diffraction results show that Al doped ZnO nanoparticles have hexagonal phase similar to ZnO nanoparticles. TEM images as well as XRD data exhibit the estimated size of nanoparticles to be in the range 35–45nm. The optical band gap has been determined from optical absorption spectra. The band gap varied from 3.27eV for undoped ZnO film to 3.87eV for AZO film having 3atwt% Al. The blue shift in energy band gap mainly related to carrier concentration induced by Al-donor doping, and to the degree of crystalline order. Photoluminescence study further confirms the blue shift in UV emission when Al doping concentration is increased, as a consequence of extension in band gap.

Preparation, characterisation and photocurrent study of sol-gel-derived Al, Mg-doped ZnO transparent thin films

In this work, sol-gel spin-coating was employed to prepare ZnO, Al-doped ZnO (AZO) and Mg-doped ZnO (MZO) thin films onto glass substrates. Structural properties and surface morphologies of as-prepared films were characterised by X-ray diffraction (XRD) and scanning electron microscope (SEM) while their optical properties were scrutinised from their transmission spectra measured by UV-VIS spectrophotometer. Their optical responses and sensitivity of the films were investigated by photocurrent spectroscopy. The XRD and SEM results show that the crystallinity and physical structure of as-prepared films are highly affected by Al and Mg doping. UV-VIS spectrophotometer results suggest that Mg and Al dopants could not only improve their optical transparency but also significantly cause the blue-shift in optical band gap of ZnO films. In addition, the enhancement in photosensitivity of ZnO films can be attained by specific Al and Mg doping content.

UV Photodetector based on Al-doped ZnO Nanocrystalline Sol-gel Derived Thin Fims

Aluminum doped zinc oxide (AZO) thin films were studied as an UV photodetector devices. The AZO thin films were prepared on FTO substrates by spin coating technique. All samples exhibit prominent optical absorption edge with high optical transparency in visible range and significant blue-shift in optical band gap with increasing Al composition. The photoilluminated current increases linearly with increasing bias voltage reflecting ohmic contact behavior. The detectors have excellent ultraviolet response in wavelength region of 250-380nm suggesting that tunable wavelength response can be obtained by increasing Al composition.

Study of blueshift of optical band gap in zinc oxide (ZnO) nanoparticles prepared by low-temperature wet chemical method

We report synthesis of zinc oxide (ZnO) nanoparticles via wet chemical method using molar solutions of zinc nitrate hexahydrate [Zn(NO3)2·6H2O] and ammonium hydroxide (NH4OH) with polyvinylpyrollidone (PVP) as capping agent. The synthesized ZnO nanoparticles are characterized by X-ray diffraction (XRD) technique and electron microscopy (TEM and SEM with EDX) for compositional analysis and surface morphology. The average crystallite size calculated from XRD pattern has been found to be on the order of 8.5nm. In this work, the hyperbolic band model (HBM) and UV–vis absorption spectra have been utilized to calculate the band gap and nanoparticle size which was proposed by Meulenkamp. It was Meulenkamp who correlated the particle sizes with the wavelength at which the absorption becomes half of that at the shoulder (λ1/2). The optical band gap value of the prepared ZnO nanoparticle is found to be on the order of 3.63eV which indicates the presence of blueshift. In order to compare the size and distribution of ZnO nanoparticles, TEM has been utilized.

Blue shift of optical band-gap in LiNbO3 thin films deposited by sol–gel technique

Lithium niobate (LN) thin films were deposited on quartz substrates by sol–gel technique. The measured absorption and photoluminescence spectra show that the band structure of LN thin films is direct unlike indirect band-gap in bulk LN and the optical band-gap of these LN thin films was measured to be 4.7eV which is ~1eV greater than that for stoichiometric bulk LN. The dependence of the blue shift of band-gap on several parameters like quantum confinement, composition (Li:Nb ratios of LN thin films) and strain was also investigated. The results obtained show that the large blue shift in band-gap of LN thin films is primarily due to strain in the film.

Effect of substrate temperature on the properties of copper nitride thin films deposited by reactive magnetron sputtering

Copper nitride (Cu3N) thin films were deposited on glass substrates by reactive magnetron sputtering with changing various substrate temperatures. The effects of substrate temperature on the structural, morphological, optical, and electrical properties of the Cu3N thin films were investigated. The results showed that the substrate temperature has a significant effect on the properties of the Cu3N thin films. The preferential orientation of the films was transformed from Cu3N (100) to Cu (111) and the grain size was gradually increased with increasing the substrate temperature. The optical transmittance begins to appear at 800 nm for the film deposited at 25 °C, where the highest value of the transmittance was obtained. The blueshift of optical band gap on carrier concentration is well described by the Burstein-Moss effect.

Effect of high temperature annealing on strain and band gap of GaN nanoparticles

Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200 °C for 30 min. XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200 °C, and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions. If the as-synthesised GaN nanoparticles at 950 °C are regarded as standard, the thermal expansion changes nonlinearly with temperature and is anisotropic; the expansion below 1000 °C is smaller than that above 1000 °C. This study provides an experimental demonstration for selecting the proper annealing temperature of GaN. In addition, a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200 °C is observed, which can be ascribed to the dominant transitions from the C(Γ7) with the peak energy at 3.532 eV.

Structural and optical characterization of pulsed laser‐ablated potassium lithium niobate thin films

AbstractThin films of potassium lithium niobate (K3Li2Nb5O15: KLN) have been prepared on glass substrate, as a function of substrate temperature, using a pulsed laser‐deposition (PLD) technique for the first time. Grazing‐incidence X‐ray diffraction (GIXRD) analysis suggests that KLN films can be grown successfully at a substrate temperature as low as 300 K. The anomalous behavior of the decline of crystalline structure with increase in substrate temperature is explained. The atomic force microscopic (AFM) and scanning electron microscopic (SEM) images show an agglomerated growth mode for the films deposited at a substrate temperature of 300 K and a decrease in grain size with increase in substrate temperature. The films deposited at higher substrate temperatures show ring‐like structures. The AFM analysis shows that the rms surface roughness of the film decrease with increase in substrate temperature. The UV–Vis transmission spectra suggest that the nature of the transition in the films is directly allowed. A blue shift in optical bandgap is observed for the films compared to bulk material. The changes in the optical bandgap with substrate temperature are also discussed.

Blueshift in optical band gap in nanocrystalline Zn1−xCaxO films deposited by sol-gel method

A blueshift in the optical band gap of nanocrystalline Zn1−xCaxO thin films has been obtained. A 12.72% enhancement in the band gap of ZnO thin films has been obtained using Ca dopant for the first time. The band gap widens from 3.38 to 3.81 eV as the Ca concentration increases from x=0 to x=0.15. The films are deposited by sol-gel method and have a hexagonal wurtzite phase with no indication of calcium. Grain size lies in the range of 12–92 nm. Atomic force micrographs indicate much smaller rms surface roughness showing significantly smooth surfaces.

Blueshift of optical band gap in c-axis oriented and conducting Al-doped ZnO thin films

We have investigated the structural, optical, and electrical properties of alumina-doped zinc oxide (AZO) thin films, grown by pulsed laser deposition. The optical transmittance of the films is over 80% in the visible region, and the absorption edge shifts from about 380 nm of the undoped sample to 320 nm of the AZO film. The calculated optical band gap (Eg) of 2 wt % AZO films shows a widening up to 3.82 eV with respect to the undoped film (3.28 eV). Higher doping concentration (6 wt %) leads to films with larger Eg (4.1 eV), but also epitaxial properties are affected. A further widening of the gap occurs when the AZO films are deposited by lowering the substrate temperature (Ts) from 450 to 250 °C. These blueshifts are respectively attributed to the increase in carrier concentration, induced by Al-donor doping, and also a lower degree of crystalline order. AZO films with doping concentration of 2 wt % show resistivity values of about 10−3 Ω cm and the local I-V curves, measured by scanning tunneling spectroscopy, show higher tunneling current than ZnO film. The Al-doping route proved to be effective in tailoring the optical and electrical properties without essentially affecting the crystalline structure of the films.

Influence of Fe doping on the optical property of ZnO films

Polycrystalline Zn 1− x Fe x O films were fabricated by the sol–gel method, and the morphology, structure and optical property were investigated. With the Fe content increasing, the blue-shift in optical band gap (OBG) happened from 3.30 eV to 3.334 eV. Photoluminescence (PL) measurement showed green ( x = 0.000), green and orange ( x = 0.030, 0.051) and blue emission ( x = 0.072, 0.094) for the Zn 1− x Fe x O films. The possible mechanisms of these emissions were studied in detail, and the regular change of PL in the visible area may be important for ZnO in the optical devices.

Nanostructured tungsten oxide thin films by the reactive pulsed laser deposition technique

Preparation of nanostructured tungsten oxide thin films using the reactive pulsed laser ablation technique is reported. The structural, morphological, optical and electrical properties of deposited films are systematically studied by changing the ambient oxygen pressure (pO2). Structural dependence of tungsten oxide films on ambient oxygen pressure is discussed using grazing incidence X-ray diffraction (GIXRD) and micro-Raman spectra. The section analysis using atomic force microscopy exposed the smooth surface features of the deposited films. The blue shift in optical bandgap with an increase in ambient oxygen pressure is expounded in terms of electronic band structure of tungsten oxide. The influence of oxygen pressure on optical constants like extinction coefficient, band edge sharpness, refractive index and optical bandgap is also conveyed. The temperature variation of electrical resistance for films deposited at 0.12 mbar furnishes evidence for its semiconducting nature.