Blue Shift In Band Gap Energy Research Articles

Investigations on the electronic properties and effect of chitosan capping on the structural and optical properties of zinc aluminate quantum dots

Quantum confined uncapped and chitosan capped nanoparticles of ZnAl2O4 synthesized by microwave-assisted sol–gel method were investigated by structural analysis and optical techniques. X-ray diffraction studies confirmed the formation of cubic spinels with crystallite size 4.5 nm and 3.4 nm, respectively for uncapped and chitosan capped ZnAl2O4 nanoparticles. Chitosan capping produced a blue shift in bandgap energy (3.84 to 3.94 eV) agreeing with size effects according to the Brus equation. A blue shift in emission peak and enhancement in photoluminescence intensity was also observed upon chitosan capping. The electronic band structure and the density of states of the bulk spinel were also calculated using density functional theory. The effective masses of electrons and holes estimated based on the band structure were used to extract the excitonic Bohr radius.

Investigation of the layer-dependent optical properties of ultrathin BiOI by spectroscopic ellipsometry

Layered semiconductor BiOI has gained considerable interests in the field of optoelectronics, but the basic knowledge of its optical and dielectric properties is still lack of a full investigation. Here, by using spectroscopic ellipsometry (SE), the complex refractive indexes and dielectric constants of ultrathin BiOI films over a wide spectral range are measured for the first time. The ellipsometric angle Δ is found to be linear with the layer number of BiOI, suggesting that SE could be a facile way to estimate the thickness of 2D BiOI. The optical properties of BiOI show a distinct layer-dependent feature, and the bandgap values of 2D BiOI increase from 1.99 to 2.33 eV with decreasing thickness. Further, the results from absorption spectrometer also show a blue shift of bandgap energy with decreasing thickness. Our work provides valuable data about the optical properties of ultrathin BiOI, and can shed a light on the exploration of original applications based on 2D BiOI.

Defects and oxygen vacancies tailored structural, optical and electronic structure properties of Co-doped ZnO nanoparticle samples probed using soft X-ray absorption spectroscopy

The structural, optical and electronic structure properties of all the Zn1-xCoxO nanoparticles [x = 0.01, 0.03, and 0.05], synthesized using co-precipitation method have been studied using X-ray diffraction (XRD), Surface Enhanced Raman Spectroscopy (SERS), UV–Vis–NIR spectroscopy and X-ray Absorption Spectroscopy measurements. The XRD and SERS analysis have confirmed the formation of hexagonal wurtzite structure (space group C46v). UV–Vis–NIR spectra indicate the blue shifting of the absorbance edge and the band gap is found to decrease with doping. The electronic structure of undoped and Co-doped ZnO nanoparticles have been investigated using site selective and element sensitive X-ray absorption spectroscopy. From the XAS spectra at Zn L3,2, Co L3,2 and O K-edges we have found that Zn ions are in +2 valence state in undoped and Co-doped ZnO nanoparticles. From the XAS analysis it can be seen that proper incorporation of Co-ion in the host lattice take place in t2geg state Co+2 (d7) under tetrahedral symmetry and maintain the crystal symmetry with lattice distortion. The Co-ion doping in the ZnO nano-matrix leads to creation of various defects & oxygen vacancies, which results in the blue shift in band-gap energy makes them useful for various applications e.g. photocatalytic water splitting, hydrogen generation, spintronics and optoelectronics.

Structural, optical, and electrical modification of hydrothermally grown ZnO nanorods by tin-doping

ZnO nanorod has been a material of interest in numerous electronic applications. Here, ZnO nanorods were grown on a fluorine doped tin oxide (FTO) coated glass substrate with a spray-pyrolysis deposited seed layer via a low-temperature hydrothermal method, and the effects of tin-doping on structural formation, optical and electrical properties were explored. The XRD result confirmed the formation of ZnO wurtzite phase without secondary phases; however, the different preferential growth orientations were observed as tin dopants were added. The result was consistent with the surface morphology examined by SEM showing that the ZnO nanorod structure was likely to become a plate-like structure when Sn content increased. Moreover, increasing Sn doping content also resulted in a blue shift in bandgap energy and electrical conductivity enhancement. The results suggest that the incorporation of Sn ion in the ZnO structure can tailor its preferential growth orientation and morphology potentially leading to the performance improvement of electronic devices.

Effect of nitrogen doping on structural and optical properties of ZnO nanoparticles

Influence of nitrogen doping on structural and optical properties of ZnO nanoparticles has been studied. Undoped and N doped ZnO nanoparticles were synthesized via chemical precipitation approach. The prepared samples were characterized through X-ray diffraction (XRD), Transmission electron microscopy (TEM) equipped with Energy dispersive X-ray (EDAX) spectroscopy, UV–visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy and micro-Raman spectroscopy (µRS). Wurtzite phase of undoped as well as 0.5–10% N doped ZnO nanoparticles was confirmed through characteristic XRD patterns. The particle size expansion due to N incorporation in ZnO was further revealed by TEM and EDAX analysis where 11nm size undoped and 18–22nm size 0.5–10% N doped ZnO (N:ZnO) nanoparticles without any impurity were ascertained. Slight blue-shift in band gap energy, as observed in our case, symbolized weak quantum confinement of the prepared nanoparticles. The alterations in vibrational modes of ZnO due to N incorporation, remarkably H substituting at O site and subsequently causing the passivation in N:ZnO nanoparticles, were detected through FTIR analysis. Finally, the effect of the nano-size of crystallite and gradual prominence of N into ZnO lattice due to increase of N doping concentration in prepared nanoparticles was meticulously expatiated though µRS analysis.

Optical properties of GaInNAs/GaAs prepared by molecular beam epitaxy

Optical properties of GaInNAs are studied with emphasis on photoluminescence (PL) and photoreflectance (PR) using GaInNAs single quantum wells (GaInNAs-SQW) and thick GaInNAs layers lattice-matched to GaAs (LM-GaInNAs) grown by solid-source molecular-beam epitaxy. First, a blue-shift of band-gap energy as a result of a rapid thermal annealing (RTA) has been confirmed by both PL and PR. Temperature-dependent PL and PR spectra were measured. At low temperature, GaInNAs-SQWs exhibited a PL due to the localized exciton (LE), and the localization was strongly reduced by RTA. A very high-quality GaInNAs-SQW exhibited very intense PL with small half-width (10–20 meV for 8–300 K) without LE emission. In LM-GaInNAs, low-temperature PL exhibited both the LE emission and a deep PL band. Temperature-dependent PL showed that LM-GaInNAs contains large number of non-radiative centers. PR spectra of LM-GaInNAs were composed of more than two transitions, and their relative intensity depended on temperature and annealing.

What is so exciting about non-linear viscous flow in glass, molecular dynamics simulations of brittle fracture and semiconductor–glass quantum composites

Results of recent research are reported in the areas of: (1) non-linear viscous flow in glass, (2) molecular dynamics of brittle fracture and (3) optical properties of semiconductor–glass quantum composites. In all three areas, a fundamental understanding of the underlying behavior of materials has emerged from key experiments or computer simulations. These are described here: (1) The non-linear viscosity studies have established the behavior of viscosity at high strain rates in silicate glasses. Results of measurements show marked shear thinning associated with changes in the glass structure at high shear rates. A strain-rate-dependent (SRD) viscosity equation is presented which describes well the behavior of silicate glasses. (2) Molecular dynamics computer simulations of amorphous and crystalline silica have examined brittle fracture under applied uniaxial strain. Their results begin to reveal the underlying atomic processes that take part in free surface formation and fracture. For example, an examination of the atomic dynamics at the crack tip reveals that cracks form by the coalescence of pre-existing voids in the glass structure and that fracture surfaces are formed as silicon ions are shielded from the surface by oxygen ions resulting in a predominance of oxygen ions on the fracture surface. The results also suggest a mechanism for the formation of mirror, mist and hackle regions in the fractography of fractured surfaces. (3) Experimental studies of the optical properties of semiconductors formed in a glass matrix were conducted to examine the effect of size confinement on the electronic bandgap of the semiconductors. The results reveal differences in behavior between direct-gap semiconductors (CdS, CdSe and CdTe) and indirect-gap semiconductors (Si and Ge). An analysis of the data shows that the expected blue shift in bandgap energy of direct-gap semiconductors that results from reduced crystal sizes saturates at very small sizes due to the mixing of states between conduction band side valleys and the central valley. This effect does not appear to dominate indirect-gap semiconductors allowing their size-induced bandgap blue shift to cover the entire range of visible wavelengths. This effect suggests the use of quantum confined indirect-gap materials such as Si and Ge in stacks of films with different bandgaps to produce solar cells with increased theoretical efficiency.

Impurity-induced disordering of AIGalnAs quantum wells by low temperature Zn diffusion

We investigated impurity-induced disordering (IID) in AIGalnAs multi-quantum wells (MQWs) on InP substrate by Zn diffusion under low temperature conditions. Blue-shift of band-gap energy of lattice-matched AIGalnAs MQW on InP strongly depended on the temperature of Zn diffusion. The lattice-matched MQW was not completely disordered below 500°C. On the other hand, photoluminescence spectra from compressively strained AIGalnAs MQW, after disordering was independent of the temperature of Zn diffusion. Considerable disordering was observed in the strained MQW, which was saturated even at the low temperature of 400°C. The measured hole concentration of the Zn diffused layer at 400°C was as low as 3 × l018cm−3 The IID lasers were also fabricated and characterized. No significant increase in the optical loss due to the Zn diffusion was observed in these lasers.