Increase In Band Gap Energy Research Articles

Fluorescence of ZnTe Quantum Dots Prepared Through Chemical Route

The room-temperature fluorescence spectroscopy of zinc telluride quantum dots is investigated. ZnTe quantum dots are synthesized by a chemical route in ethylene glycol with different reactant concentrations and reaction temperatures. Prepared samples are characterized by X-ray diffractometry (XRD) and high resolution transmission electron microscopy. XRD investigation shows the cubic structured ZnTe nanoparticles. The size measurement shows an approximate size of 4-12 nm quantum dots. UV-Vis absorption spectroscopy shows the size-quantization effect and the band gap energy increases by an amount 2.16 eV of the quantum dots. Furthermore as the chemical parameters change, the size of quantum dot changes systemically and the fluorescence peak is observed in 554 nm – 585 nm. Keywords: Chemical route, ZnTe, Quantum dots, Fluorescence.

Effect of substrate temperature and precursor ratio on properties of thin ZnS films sprayed by improved method

Abstract Zinc sulphide (ZnS) thin films were prepared by improved spray pyrolysis (ISP) method. The ISP parameters, such as carrier gas flow rate, solution flow rate and substrate temperature, were controlled with an accuracy of ±0.25 lpm, ±1 ml/h and ±1 °C, respectively. The solution was sprayed in a pulsed mode. The substrate temperature was optimized by analyzing substrate temperature dependent properties of thin films. The thin film deposited at a temperature of 450 °C was dense and fairly smooth with satisfactory crystallinity and very small impurity content. The effect of precursor ratio in the solution on structural, compositional and optical properties of thin ZnS films, deposited at a temperature of 450 °C, was studied. A gradual increase in band gap energy from 3.524 eV to 3.634 eV, refractive index from 2.5 to 2.9 and dielectric constant from 6.6 to 8.7 were observed with the variation of solution precursor (Zn:S) ratio from (1:2) to (1:6). The structural and compositional studies support this kind of enhancement in optical properties. The results show that the thin ZnS film prepared by ISP at the substrate temperature of 450 °C from a solution with specific precursor ratio can be used for optoelectronic and photovoltaic applications.

Effect of swift heavy ion irradiation on structural and opto-electrical properties of bi-layer CdS–Bi2S3 thin films prepared by solution growth technique at room temperature

CdS–Bi2S3 bi-layer thin films have been deposited by chemical bath deposition method on Indium Tin Oxide glass substrate at room temperature. The as-deposited thin films were annealed at 250°C in an air atmosphere for 1h. An air annealed thin film was irradiated using Au9+ ions with the energy of 120MeV at fluence 5×1012ions/cm2 using tandem pelletron accelerator. The irradiation induced modifications were studied using X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Raman spectroscopy, UV spectroscopy and I–V characteristics. XRD study reveals that the as-deposited thin films were nanocrystalline in nature. The decrease in crystallite size, increase in energy band gap and resistivity were observed after irradiation. Results are explained on the basis of energy deposited by the electronic loss after irradiation. The comparative results of as-deposited, air annealed and irradiated CdS–Bi2S3 bi-layer thin films are presented.

Amino acid mediated synthesis of luminescent SnO2 nanoparticles

Tin oxide (SnO2) nanoparticles of average sizes ∼6, ∼16 and ∼33nm were synthesized successfully by a simple chemical precipitation method using an amino acid, glycine at different calcination temperatures of 200, 400, and 600°C, respectively. This method resulted in the formation of spherical, polycrystalline SnO2 nanoparticles with a higher degree of monodispersity. The sizes of the SnO2 nanoparticles were found to be a factor of calcination temperature. The synthesized SnO2 nanoparticles have a tetragonal rutile structure. Due to three dimensional quantum confinement effect shown by the synthesized SnO2 nanoparticles in their electronic spectra, an increase in band gap energy (3.85–4.21eV) is observed with a decrease in particle size (∼33 to ∼6nm). The synthesized nanomaterials were characterized using X-ray diffraction (XRD), Transmission electron microscopy (TEM), and Fourier transformed infrared spectroscopy (FT-IR). The optical properties were investigated using UV–vis spectroscopy and photoluminescence spectroscopy (PL). Interestingly, the photoluminescence property was shown by the material calcined at 400°C. The effects of various polar and non-polar solvents on the absorption spectra of the synthesized SnO2 nanoparticles were studied. The plausible reaction mechanism for the formation of SnO2 nanoparticles was also proposed.

High-quality MOVPE butt-joint integration of InP/AlGaInAs/InGaAsP-based all-active optical components

In this paper, we demonstrate the applicability of MOVPE butt-joint regrowth for integration of all-active InP/AlGaAs/InGaAsP optical components and the realization of high-functionality compact photonic devices. Planar high-quality integration of semiconductor optical amplifiers of various epi-structures with a multi-quantum well electro-absorption modulator has been successfully performed and their optical and crystalline quality was experimentally investigated. The regrown multi-quantum well material exhibits a slight bandgap blue-shift of less than 20meV, when moving away from the regrowth interface. In closest vicinity to the mask, the growth profile revealed a bent-up shape which is associated with an increase in the bandgap energy resulting from the combined effect of growth rate suppression and higher Ga concentration. This increase in bandgap energy makes the interface partially transparent (thus beneficial for unaffected light transmission) and forces carriers away from possible interfacial defects. The internal reflectivity below 2.1×10−5 ensures minimization of detrimental intracavity feedback.

Experimental and theoretical investigations of electronic structure and photoluminescence properties of β-Ag2MoO4 microcrystals.

In this paper, we investigate a correlation between theoretical calculations and experimental data to explain the electronic structure and optical properties of silver molybdate (β-Ag2MoO4) microcrystals synthesized by the microwave-assisted hydrothermal method. X-ray diffraction, Rietveld refinement, and micro-Raman spectroscopy confirmed that these microcrystals crystallize in a spinel-type cubic structure. Field-emission scanning electron microscopy images revealed that the processing temperatures influence in the final shape of microcrystals. Optical properties were analyzed by ultraviolet-visible diffuse reflectance spectroscopy; the increase in the optical band gap energy (Egap) (from 3.24 to 3.31 eV) with processing temperature is associated with the reduction of intermediary energy levels. First-principles quantum mechanical calculations based on the density functional theory at the B3LYP level were conducted. The calculated band structure revealed an indirect Egap of approximately 4.00 and 3.34 eV for the β-Ag2MoO4 without and with the formation of defects, respectively. Theoretical calculations based on density of states and electron density maps were employed to understand the polarization phenomenon induced by structural defects in the β-Ag2MoO4 crystals. Finally, photoluminescence properties at room temperature of β-Ag2MoO4 microcrystals were explained by the charge-transfer mechanism involving tetrahedral [MoO4] clusters.

Effect of Transferred Electronic Energy Density on Optical, Electrical and Structural Properties of Polyallyl-Diglycol Carbonate (CR-39) Polymer

Commercially available polyallyl-diglycol carbonate (CR-39) track detectors have attracted wide interest in many fields of science and technology. This is because of their low cost, relatively easy handling, and being more similar to human tissue than other passive detectors. After the Fukushima accident, there was a need to study the impact of the released alpha particles from radionuclides. In this study, CR-39 polymers were irradiated with α-particles with different linear energy transfers 11.11, 19.70, and 28.77 eV/Å, at fluences of 49,490/cm2 and 2,482,763/cm2. The modifications in the optical, electrical, and structural properties induced by the radiation were measured. The results showed that low values of transferred electronic energy density along the alpha particle tracks led to a slight increase in the optical band gap energy and a small reduction in the conductivity. The amorphous nature of the CR-39 samples was not affected by the changes of the transferred electronic energy density.

Influence of preparation parameters on structure and optical properties of ZnO thin films

ZnO thin films were deposited on cleaned glass substrates through spray pyrolysis technique using precursor solution of zinc acetate. A comprehensive study was carried out to understand the influence of preparation parameters such as substrate temperature and precursor solution concentration on structure and optical properties of ZnO thin films. X-ray diffraction studies showed that higher substrate temperature and lower precursor solution concentration gave good crystallinity with preferred orientation along (002) direction. The films prepared at a substrate temperature of 723 K with 0.015 M precursor solution exhibited high transparency (~95 % in visible region). The marginal increase of optical band gap energy with increase of substrate temperature was observed. However, band gap of the films decreased with the increase of solution concentration. It was observed that the grain size of the films increase with the increase of substrate temperature. The preparation conditions were optimized to obtain uniform, homogeneous and highly transparent polycrystalline ZnO thin films.

A positron annihilation spectroscopic investigation of europium-doped cerium oxide nanoparticles

Doping in ceria (CeO2) nanoparticles with europium (Eu) of varying concentrations (0, 0.1, 0.5, …, 50 atom%) is studied using complementary experimental techniques and novel observations were made during the investigation. The immediate observable effect was a distinct reduction in particle sizes with increasing Eu concentration attributed to the relaxation of strain introduced due to the replacement of Ce(4+) ions by Eu(3+) ions of larger radius. However, this general trend was reversed in the doping concentration range of 0.1-1 atom% due to the reduction of Ce(4+) to Ce(3+) and the formation of anion vacancies. Quantum confinement effects became evident with the increase of band gap energy when the particle sizes reduced below 7-8 nm. Positron annihilation studies indicated the presence of vacancy type defects in the form of vacancy clusters within the nanoparticles. Some positron annihilation was also seen on the surface of crystallites as a result of diffusion of thermalized positrons before annihilation. Coincidence Doppler broadening measurements indicated the annihilation of positrons with electrons of different species of atoms and the characteristic S-W plot showed a kink-like feature at the particle sizes where quantum confinement effects began.

Electronic and Magnetic Properties of Graphene M?bius Strips: Density Functional Theory Approach

Electronic and magnetic properties of graphene Mobius strips with different widths are studied using density functional theory. It is shown that the multiplicity of the Mobius strip, the cohesive energy, and the band gap energy increase with increasing the width of Mobius strip. We show that the magnetic moment of Mobius strip decreases with increasing the curvature and strain. Then the effects of an external electric field applied in the direction of the Mobius strip axis are studied and it is found that the Mobius strip keeps its metallic surface (edge) states even in the presence of the electric field. For sufficiently high applied electric field, the spin-flipping can take place in the Mobius strip. In addition, in contrast with the graphene nanoribbons, the graphene Mobius strips show half-semiconducting properties when an external electric field is applied.

Acidic and neutral caesium salts of 12-molybdophosphoric acid supported on SBA-15 mesoporous silica. The influence of Cs concentration and surface coverage on textural and structural properties

The Cs salts of molybdophosphoric acid Cs1H2PMo12O40 (Cs1PM) and Cs3PMo12O40 (Cs3PM), were supported on SBA-15 in the concentration of 20, 30 and 40wt.% loadings. Because Cs salts are insoluble, the SBA-15 supported Cs-acid salts were prepared by two-step sequential impregnation and in situ reaction on the support. The structure and texture of these CsPM/SBA-15 composites were studied by XRD, SEM-EDS, FT-IR and micro-Raman spectroscopy, UV–vis-DRS and N2 adsorption. Thermal stability was investigated by thermo gravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The acidity of composites was studied by the adsorption of ammonia and its temperature programmed desorption – TPD using thermogravimetry. The evolved gases during the adsorption–desorption of ammonia on CsPM/SBA-15 composites were identified by online mass spectrometry coupled with thermal gravimetry technique. FT-IR and Raman studies demonstrated that CsPM maintained its Keggin structure after deposition on mesoporous SBA-15, regardless of the active phase concentration. For both Cs1H2PMo12O40 and Cs3PMo12O40-SBA-15 composites could be observed an important increase of band gap energy in comparison with pure salts. The values of specific surface area of pure CsPM were increased by deposition on mesoporous silica support. The immobilization of CsPM on mesoporous SBA-15 obviously increases the thermal stability and the total acidity of the strong acidic sites of the Keggin structures in comparison with their parent bulk Cs salts.

Monte Carlo simulation of gamma-ray response of BaF2 and CaF2

We have employed a Monte Carlo (MC) method to study intrinsic properties of two alkaline-earth halides, namely, BaF2 and CaF2, relevant to their use as radiation detector materials. The MC method follows the fate of individual electron-hole (e-h) pairs and thus allows for a detailed description of the microscopic structure of ionization tracks created by incident γ-ray radiation. The properties of interest include the mean energy required to create an e-h pair, W, Fano factor, F, the maximum theoretical light yield, and the spatial distribution of e-h pairs resulting from γ-ray excitation. Although W and F vary with incident photon energy at low energies, they tend to constant values at energies higher than 1 keV. W is determined to be 18.9 and 19.8 eV for BaF2 and CaF2, respectively, in agreement with published data. The e-h pair spatial distributions exhibit a linear distribution along the fast electron tracks with high e-h pair densities at the end of the tracks. Most e-h pairs are created by interband transition and plasmon excitation in both scintillators, but the e-h pairs along fast electron tracks in BaF2 are slightly clustered, forming nanoscale domains and resulting in the higher e-h pair densities than in CaF2. Combining the maximum theoretical light yields calculated for BaF2 and CaF2 with those obtained for CsI and NaI shows that the theoretical light yield decreases linearly with increasing band gap energy.

Microwave synthesis, optical properties and surface area studies of NiO nanoparticles

We report here the synthesis of nickel oxide (NiO) nanoparticles by microwave-assisted method, using a common precipitating agent followed by calcination at 400°C. The effect of the microwave and pH on the crystallite size, morphology, structure, energy band gap and surface area of NiO have been investigated by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet visible spectroscopy (UV–vis) and BET surface area studies. It was observed that on increasing the volume of NaOH, the optical band gap energy (2.85–2.95eV) and specific surface area (33.1–39.8m2/g) increases, however the average particles size decreases (16.5–14nm).

CaCO3/TiO2 Nanoparticles Based Dye Sensitized Solar Cell

In this communication, the synthesis and structural, morphological, optical, and photo-electrochemical properties of TiO2 and CaCO3/TiO2 nanoparticles as well as their applications in dye sensitized solar cells (DSSCs), have been reported. In an X-ray diffraction pattern of CaCO3/TiO2 nanoparticles, the peak at 29.41° of CaCO3 has been detected, demonstrating its coating on the surface of TiO2, which is further verified using high resolution-transmission electron microscopy, energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The strong quenching in photoluminescence emission, in the case of CaCO3/TiO2 nanoparticles, has been attributed to the decrease in recombination rate of photo-generated electron–hole pairs. In the case of UV–visible reflectance spectra, the absorption edge for CaCO3/TiO2 nanoparticles has slightly been found to be blue-shifted as compared to bare TiO2 nanoparticles, which corresponds to an increase in energy band gap of the former. The dye desorption studies reveal that CaCO3/TiO2 electrodes adsorbed more dye than the bare TiO2 electrode. CaCO3/TiO2 based DSSC show improved photo-electrochemical properties compared to the bare TiO2 based DSSC as CaCO3 coating on TiO2 forms an energy barrier, and, consequently suppressing the charge recombination, and, thus, improving the overall energy conversion efficiency (η) from 0.46% to 1.44% under the illumination of simulated light of 100 mW/cm2.

Optimized design of Si-cap layer in strained-SiGe channel p-MOSFETs based on computational and experimental approaches

In this paper, we study the hole transport properties in strained-SiGe channel p-MOSFETs (sSG pMOSFETs) with a Si-cap layer, which is introduced to avoid degradation of interface quality between gate oxide and channel. By using device simulation considering Ge diffusion, quantum confinement effects, surface roughness scattering and Coulomb scattering due to interface charges, and also experimental measurement, we clarify the roles of a Si-cap layer in sSG pMOSFETs, and furthermore propose its optimized design to obtain a higher device performance. We also demonstrate that the insertion of a Si-cap layer is effective to reduce an OFF-state leakage current owing to an increased band gap energy in the Si-cap layer.

Quantum Confinement Stark Effect of Different Gainnas Quantum Well Structures

The quantum confinement Stark effect of three types of GaInNAs quantum wells, namely single square quantum well, stepped quantum wells and coupled quantum wells, is investigated using the band anti-crossing model. The comparison between experimental observation and modeling result validate the modeling process. The effects of the external electric field and localized N states on the quantized energy shifts of these three structures are compared and analyzed. The external electric field applied to the QW not only changes the potential profile but also modulates the localized N states, which causes band gap energy shifts and increase of electron effective mass.

Structural studies of xLi2O–(40 − x)Bi2O3–20CdO–40B2O3 glasses

Raman and infrared spectroscopies have been employed to investigate the xLi2O–(40−x)Bi2O3–20CdO–40B2O3 glasses in order to obtain information about the competitive role of Bi2O3 and B2O3 in the formation of glass network. IR and Raman spectra show that these glasses are made up of [BO3], [BO4] and [BiO3] pyramidal and [BiO6] octahedral units. The formation of CdO4 in tetrahedral co-ordination was not observed. The optical absorption studies revealed that the cut-off wavelength decreases while optical band gap energy (Eopt) increases. The average electronic polarizability of the oxide ion (αO2-), optical basicity Λ(Eopt), and Yamashita–Kurosawa’s interaction parameter A(Eopt) were also examined.

Effect of tin content on the electrical and optical properties of sprayed silver sulfide semiconductor thin films

Silver sulfide (Ag2S) thin films have been deposited on glass substrates by t spray pyrolysis using an aqueous solution which contains silver acetate and thiourea as precursors. The depositions were carried out at a substrate temperature of 250°C. Structural studies by means of X-ray diffraction show that all tin (Sn)-doped Ag2S thin films crystallized in a monoclinic space group with noticeable changes in the crystallites' orientation. The discussion of some structural calculated constants has been made with Sn doping in terms of microhardness measurements. Moreover, the optical analysis via the transmittance, reflectance as well as the photocurrent reveals that the direct band gap energy (Egd) decreases (Egd varies from 2.34 to 2.16eV) and the indirect band gap energy (Egi) increases (Egi varies from 0.98 to 1.09eV) slightly as a function of Sn content. Electrical study shows that Sn doping changes the electrical conductivity and proves the thermal activation of electrical conduction.

High intense violet luminescence in fluorine doped zinc oxide (FZO) thin films deposited by aerosol assisted CVD

Abstract Fluorine doped nanocrystalline zinc oxide (F: ZnO–FZO) thin films with varying fluorine dopant level (0–5%) were prepared on corning glass substrates by aerosol assisted chemical vapor deposition. The films exhibited polycrystalline hexagonal wurtzite structure with preferred orientation along c-axis and crystallinity deteriorated with increasing fluorine. As prepared films were found to be stressed in the range from 1.2 to 1.7 GPa and the amount of stress depends strongly on growth conditions. Undoped ZnO films were in a state of less tensile stress, whereas in FZO, the tensile stress increases with the increasing F. Films exhibit hexagonal platelets morphology analyzed using Field emission scanning electron microscope (FESEM). UV–Vis-NIR transmission studies indicated a increase of energy bandgap and increase of optical transmission with the increase in fluorine. Urbach energy (EU) of films have also been determined. Raman spectroscopy and photoluminescence (PL) analyses identified the origin of stress due to defects introduced by fluorine. A special emphasis was devoted to E2 high phonon mode analysis of Raman spectra. High intense violet PL has been achieved at room temperature and intensity improved vigorously with the dopant concentration due to a radiative transition of electrons from zinc vacancies level to the conduction band.

Optical dispersion and interband transition in Na0.5Bi0.5TiO3-x%BaTiO3 lead-free relaxor ferroelectric single crystals

Optical properties of A-site disordered Na0.5Bi0.5TiO3-x%BaTiO3 (NBT-xBT) single crystals with three typical compositions were studied systematically. Refractive index as a function of wavelength was measured by Spectroscopic ellipsometry, and modified Sellmeier dispersion equations were obtained by the least square fitting. High optical transmittance (65%) over a wide transparent region (from 400 to 6000 nm) has been found in NBT-7.5BT, much higher than that of NBT and NBT-5BT. Optical band gap energies were calculated using the absorption coefficient through Tauc equation. With increasing BT content, refractive index and absorption coefficient decrease, while transmission and band gap energy increase.