Effect of size and indium-composition on linear and nonlinear optical absorption of InGaN/GaN lens-shaped quantum dot

Based on the Schr ¨odinger equation for envelope function in the effective mass approximation, linear and nonlinear optical absorption coefficients in a multi-subband lens quantum dot are investigated. The effects of quantum dot size on the interband and intraband transitions energy are also analyzed. The finite element method is used to calculate the eigenvalues and eigenfunctions. Strain and In-mole-fraction effects are also studied, and the results reveal that with the decrease of the In-mole fraction, the amplitudes of linear and nonlinear absorption coefficients increase. The present computed results show that the absorption coefficients of transitions between the first excited states are stronger than those of the ground states. In addition, it has been found that the quantum dot size affects the amplitudes and peak positions of linear and nonlinear absorption coefficients while the incident optical intensity strongly affects the nonlinear absorption coefficients.     

Effects of annealing temperature on shape transformation and optical properties of germanium quantum dots

The influences of thermal annealing on the structural and optical features of radio frequency(rf) magnetron sputtered self-assembled Ge quantum dots(QDs) on Si(100) are investigated.Preferentially oriented structures of Ge along the(220) and(111) directions together with peak shift and reduced strain(4.9%to 2.7%) due to post-annealing at 650 ℃ are discerned from x-ray differaction(XRD) measurement.Atomic force microscopy(AFM) images for both pre-annealed and post-annealed(650 ℃) samples reveal pyramidal-shaped QDs(density ~ 0.26×10~(11) cm~(-2)) and dome-shape morphologies with relatively high density ~ 0.92×10~(11) cm~(-2),respectively.This shape transformation is attributed to the mechanism of inter-diffusion of Si in Ge interfacial intermixing and strain non-uniformity.The annealing temperature assisted QDs structural evolution is explained using the theory of nucleation and growth kinetics where free energy minimization plays a pivotal role.The observed red-shift ~ 0.05 eV in addition to the narrowing of the photoluminescence peaks results from thermal annealing,and is related to the effect of quantum confinement.Furthermore,the appearance of a blue-violet emission peak is ascribed to the recombination of the localized electrons in the Ge-QDs/SiO_2 or GeO_x and holes in the ground state of Ge dots.Raman spectra of both samples exhibit an intense Ge-Ge optical phonon mode which shifts towards higher frequency compared with those of the bulk counterpart.An experimental Raman profile is fitted to the models of phonon confinement and size distribution combined with phonon confinement to estimate the mean dot sizes.A correlation between thermal annealing and modifications of the structural and optical behavior of Ge QDs is established.Tunable growth of Ge QDs with superior properties suitable for optoelectronic applications is demonstrated.     

Effects of Mg substitution on the structural and magnetic properties of Co_(0.5)Ni_(0.5-x)Mg_xFe_2O_4 nanoparticle ferrites

In this study, nanocrystalline Co–Ni–Mg ferrite powders with composition Co_(0.5)Ni_(0.5-x)Mg_xFe_2O_4 are successfully synthesized by the co-precipitation method. A systematic investigation on the structural, morphological and magnetic properties of un-doped and Mg-doped Co–Ni ferrite nanoparticles is carried out. The prepared samples are characterized using x-ray diffraction(XRD) analysis, Fourier transform infrared spectroscopy(FTIR), field emission scanning electron microscopy(FESEM), and vibrating sample magnetometry(VSM). The XRD analyses of the synthesized samples confirm the formation of single-phase cubic spinel structures with crystallite sizes in a range of ~ 32 nm to ~ 36 nm. The lattice constant increases with increasing Mg content. FESEM images show that the synthesized samples are homogeneous with a uniformly distributed grain. The results of IR spectroscopy analysis indicate the formation of functional groups of spinel ferrite in the co-precipitation process. By increasing Mg2+substitution, room temperature magnetic measurement shows that maximum magnetization and coercivity increase from ~ 57.35 emu/g to~ 61.49 emu/g and ~ 603.26 Oe to~ 684.11 Oe(1 Oe = 79.5775 A·m-1), respectively. The higher values of magnetization Ms and Mr suggest that the optimum composition is Co_(0.5)N_(i0.4)Mg_(0.1)Fe_2O_4 that can be applied to high-density recording media and microwave devices.     

Preparation and characterization of crack-free sol–gel based SiO2–TiO2 hybrid nanoparticle film

Owing to the diverse potential applications of hybrid silica–titania thin films, the synthesis and characterization of these films have been carried out with a special focus on application as a medium index layer for multilayered functional coatings. For synthesis, tetraethylorthosilicate and titanium tetraisopropoxide were chosen as precursors for the formation of silica-titania hybrid thin films/nano-composites through an in situ sol–gel process. These films were sequentially obtained on Cu substrate utilizing spin coating. The hybrids were characterized by field emission scanning electron microscope, energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction, atomic force microscopy and Fourier transform infrared spectroscopy (FTIR). Field emission scanning electron microscope morphology displayed a smooth, densified and crack- free layer of silica-titania hybrid nanoparticles in the range of 20–71 nm after calcinations at low temperature of 300ºC for 1 h. X-ray diffraction pattern confirms the phases of titania with higher crystallinity and phase transformation at low temperature. The prepared films were uniform with low 8.852 nm RMS value. The stoichiometry of films was confirmed by EDX results. The FTIR spectroscopy indicated the establishment of heterogeneous chemical bonding between the Ti and Si surfaces through oxygen.     

Synthesis and analysis of silicon nanowire below Si-Au eutectic temperatures using very high frequency plasma enhanced chemical vapor deposition

Silicon nanowires (SiNWs) were synthesized from pure silane precursor gas and Au nanoparticles catalyst at below Au-Si eutectic temperature. The SiNWs were grown onto Si (1 1 1) substrates using very high frequency plasma enhanced chemical vapor deposition via a vapor-solid-solid mechanism at temperatures ranging from 363 to 230 °C. The morphology of the synthesized SiNWs was characterized by means of field emission scanning electron microscope equipped with energy dispersive X-ray, high resolution transmission electron microscopy, X-ray diffraction technique and Raman spectroscope. Results demonstrated that the SiNWs can be grown at the temperature as low as 250 °C. In addition, it was revealed that the grown wires were silicon-crystallized.     

Spectral features and antibacterial properties of Cu-doped ZnO nanoparticles prepared by sol-gel method

Zn_(1-x)Cu_x O(x=0.00, 0.01, 0.03, and 0.05) nanoparticles are synthesized via the sol-gel technique using gelatin and nitrate precursors. The impact of copper concentration on the structural, optical, and antibacterial properties of these nanoparticles is demonstrated. Powder x-ray diffraction investigations have illustrated the organized Cu doping into ZnO nanoparticles up to Cu concentration of 5%(x = 0.05). However, the peak corresponding to CuO for x= 0.01 is not distinguishable. The images of field emission scanning electron microscopy demonstrate the existence of a nearly spherical shape with a size in the range of 30–52 nm. Doping Cu creates the Cu–O–Zn on the surface and results in a decrease in the crystallite size. Photoluminescence and absorption spectra display that doping Cu causes an increment in the energy band gap. The antibacterial activities of the nanoparticles are examined against Escherichia coli(Gram negative bacteria)cultures using optical density at 600 nm and a comparison of the size of inhibition zone diameter. It is found that both pure and doped ZnO nanoparticles indicate appropriate antibacterial activity which rises with Cu doping.     

Effect of electron beam irradiation on morphology and sieving characteristics of nylon-66 membranes

Nylon-66 is a typical semicrystalline polymer that can be crosslinked through electron beam (EB) irradiation. Crosslinking can dramatically change polymer properties. The objective of this research was to observe how EB irradiation affects morphology and sieving characteristics of nylon-66 membranes. EB irradiation was carried out in air at 60, 70 and 80 kGy doses. Scanning electron microscopy (SEM), swelling and gel content studies evidenced the morphological changes and crosslinking of nylon-66 membranes. Sieving characteristics were also measured using pure water permeation, and rejection of raffinose, vitamin B12, and mono- and divalent salts. These results show that nylon-66 membrane surface and permeation characteristics changed with different irradiation doses. The nylon-66 surface became denser and the gel content increased with increasing irradiation dose. Furthermore, pure water permeation decreased and small molecules were increasingly rejected with greater irradiation doses. The amount of rejection was between 33% and 88.4% for vitamin B12 and between 16% and 83% for raffinose. The highest vitamin B12 and raffinose rejection was seen with a N-80 membrane, and no rejection was measured with N-0 or N-60 membranes. Salt rejection, however, was very low, especially for NaCl, with only 10.51-46% rejected. Based on flux, permeability and uncharged solute (vitamin B12 and raffinose) rejection data, nylon-66 type N-70 and N-80 membranes were estimated to be in the nanofiltration (NF) range.     

Germanium nanoislands grown by radio frequency magnetron sputtering： Annealing time dependent surface morphology and photoluminescence

Structural and optical properties of ～ 20 nm Ge nanoislands grown on Si(100) by radio frequency (rf) magnetron sputtering under varying annealing conditions are reported. Rapid thermal annealing at a temperature of 600℃ for 30 s, 90 s, and 120 s are performed to examine the influence of annealing time on the surface morphology and photoluminescence properties. X-ray diffraction spectra reveal prominent Ge and GeO 2 peaks highly sensitive to the annealing time. Atomic force microscope micrographs of the as-grown sample show pyramidal nanoislands with relatively high-density (～ 10 11 cm-2 ). The nanoislands become dome-shaped upon annealing through a coarsening process mediated by Oswald ripening. The room temperature photoluminescence peaks for both as-grown (～ 3.29 eV) and annealed (～ 3.19 eV) samples consist of high intensity and broad emission, attributed to the effect of quantum confinement. The red shift (～ 0.10 eV) of the emission peak is attributed to the change in the size of the Ge nanoislands caused by annealing. Our easy fabrication method may contribute to the development of Ge nanostructure-based optoelectronics.     

Optical behavior of self-assembled high-density Ge nanoislands embedded in SiO2

The radio frequency magnetron sputtering method is used to prepare well-dispersed pyramidal-shaped Ge nanoislands embedded in amorphous SiO2 sublayers of various thicknesses. The estimated size and number density of Ge nanoislands in SiO2 sublayer thicknesses beyond 30 nm are approximately 15 nm and 1011 cm-2, respectively. Atomic force microscopy （AFM） reveals root mean square （RMS） roughness sensitivity as the SiO2 sublayer thickness varies from 30 to 40 nm. The formation of nanoislands with high aspect ratios is attributed to the higher rate of surface reactions between Ge adatoms and nucleated Ge islands than reactions associated with SiO2 and Ge. The Ge nanoisland polyorientation on SiO2 （50-nm thickness） is revealed by X-ray diffraction （XRD） patterns. Photoluminescence （PL） peaks of 2.9 and 1.65 eV observed at room temperature （RT） are attributed to the radiative recombination of electrons and holes from the Ge nanoislands/SiO2 and Si02/Si interfaces, respectively. The mean island sizes are determined by fitting the experimental Raman profile to two models, namely, the phonon confinement model and the size distribution combined with phonon confinement model. The latter model yields the best fit to the experimental data. We confirm that SiO2 matrix thickness variations play a significant role in the formation of Ge nanoislands mediated via the minimization of interfacial and strain energies. OCIS codes： 250.5230, 170.5660.