Influence of low and high temperature annealing on the blue and green emission bands of ZnS crystals

After annealing at 350 °C, in the luminescence spectra of pure and Cu-doped ZnS crystals a reversible blue-green conversion has been obtained. The blue-green conversion of luminescence is correlated with analogous reversible variations in the optical transmission and thermoluminescence properties. Besides these low temperature results, in the fluorescence spectra of undoped ZnS crystals, which are annealed at 1,000 °C in sealed evacuated quartz ampoules, a reversible blue-blue conversion has been observed. All these effects, induced by distinct heat treatments, seem to be mainly controlled by changes in the structural disorder or/and by the evaporation of lattice atoms from the surface of the crystals.     

Influence of low and high temperature annealing on the blue and green emission bands of ZnS crystals

After annealing at 350 °C, in the luminescence spectra of pure and Cu-doped ZnS crystals a reversible blue-green conversion has been obtained. The blue-green conversion of luminescence is correlated with analogous reversible variations in the optical transmission and thermoluminescence properties. Besides these low temperature results, in the fluorescence spectra of undoped ZnS crystals, which are annealed at 1,000 °C in sealed evacuated quartz ampoules, a reversible blue-blue conversion has been observed. All these effects, induced by distinct heat treatments, seem to be mainly controlled by changes in the structural disorder or/and by the evaporation of lattice atoms from the surface of the crystals.Dedicated to Prof. Dr. Dr. Ing. E.h.W. Hanle on occasion of his 75^th birthdayPart of thesis, Gießen 1974 (D26)     

Effects of neutron irradiation and subsequent annealing on the optical characteristics of sapphire

In this paper, the effect of neutron irradiation on sapphire single crystal with fast neutron of 1.0×10¹⁸ and 1.0×10¹⁹ neutrons/cm² has been investigated along with the effect of annealing temperature. It is found that the colorless transparent sapphire single crystals were turned yellow after 10 MeV fast neutron irradiation at room temperature. There are peaks at 206, 230, 258, 305, 358 and 452 nm after neutron irradiation. And the intensity of optical absorption bands decrease with wavelength and annealing temperature. A new absorption peak at 452 nm was found after isothermal annealing at 400 °C for 10 min, which was ascribed to F₂⁺ color center. Because of the recombination of interstitial ions and vacancies, color centers were almost removed after annealing at 1000 °C. The TL peaks were found to shift to higher temperature after neutron irradiation. And a higher fluence of the neutron irradiation would result in deep traps revealed as the new TL peaks at 176 and 227 °C.     

Microscopic and spectroscopic characterization of chemo-mechanically polished, furnace-annealed, nitrogen-doped 4H:SiC

The effect of chemical–mechanical polishing and high-temperature furnace annealing at temperatures ranging from 1000 °C to 1600 °C on nitrogen-doped crystalline 4H:SiC was investigated. Techniques used to characterize the samples included environmental scanning electron microscopy (ESEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy. The ESEM micrographs and EDS data indicated that there were structural defects on the unannealed sample that did not propagate into the sample or vary in composition from the bulk. The sample annealed at 1000 °C showed oxygen-rich and carbon-depleted surface defects. Annealing at temperatures above 1200 °C introduced defects that grew out of the sample surface. These were carbon and oxygen rich, but depleted in silicon. This supported the XPS data, which showed an increase in the surface C bonding with annealing temperatures above 1200 °C. The XPS data also suggested that the oxycarbide content may be increasing on annealing above 1200 °C. Raman micro-probe data from the defects on the sample annealed at 1200 °C showed the maximum shift in the transverse optical phonon mode at 776 cm^-1, indicating that the beginning of carbon out-diffusion may be accompanied with structural changes. Optimal annealing temperatures are thus below 1200 °C. PACS 78.20.-e; 81.05.-t; 81.70.Fy     

Correlation between ferromagnetism and defects in MgO nanocrystals studied by positron annihilation

High purity MgO nanopowders were pressed into pellets and annealed in air from 100 to 1400 °C. Variation of the microstructures was investigated by X-ray diffraction and positron annihilation spectroscopy. Annealing induces an increase in the MgO grain size from 27 to 60 nm with temperature increasing up to 1400 °C. Positron annihilation measurements reveal vacancy defects including Mg vacancies, vacancy clusters, microvoids and large pores in the grain boundary region. Rapid recovery of Mg monovacancies and vacancy clusters was observed after annealing above 1200 °C. Room temperature ferromagnetism was observed for MgO nanocrystals annealed at 100, 700, and 1000 °C. However, after 1400 °C annealing, MgO nanocrystals turn into diamagnetic. Our results suggest that the room temperature ferromagnetism in MgO nanocrystals might originate from the interfacial defects.     

Correlation between microstructure and optical properties of nano-crystalline TiO2 thin films prepared by sol-gel dip coating

Titanium dioxide thin films have been prepared from tetrabutyl-orthotitanate solution and methanol as a solvent by sol-gel dip coating technique. TiO₂ thin films prepared using a sol-gel process have been analyzed for different annealing temperatures. Structural properties in terms of crystal structure were investigated by Raman spectroscopy. The surface morphology and composition of the films were investigated by atomic force microscopy (AFM). The optical transmittance and reflectance spectra of TiO₂ thin films deposited on silicon substrate were also determined. Spectroscopic ellipsometry study was used to determine the annealing temperature effect on the optical properties and the optical gap of the TiO₂ thin films. The results show that the TiO₂ thin films crystallize in anatase phase between 400 and 800 °C, and into the anatase-rutile phase at 1000 °C, and further into the rutile phase at 1200 °C. We have found that the films consist of titanium dioxide nano-crystals. The AFM surface morphology results indicate that the particle size increases from 5 to 41 nm by increasing the annealing temperature. The TiO₂ thin films have high transparency in the visible range. For annealing temperatures between 1000 and 1400 °C, the transmittance of the films was reduced significantly in the wavelength range of 300-800 nm due to the change of crystallite phase and composition in the films. We have demonstrated as well the decrease of the optical band gap with the increase of the annealing temperature.     

Effects of annealing temperature on YAG:Ce synthesized by spray-drying method

In this report, YAG:Ce phosphors were synthesized by spray-drying method. The effects of annealing temperature on crystal structure, morphology and photoluminescence property (PL) of as-prepared samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and spectrofluorometer, respectively. The XRD patterns showed all the samples are in consistence with a single garnet phase, and the location of strongest peak shifts to smaller angle with increasing the annealing temperature. The SEM micrographs revealed the sample annealed at 1200 °C appears to be a spherical polycrystalline aggregate; as the samples were annealed at 1300?1400 °C, spherical grains obviously grow up; but the sample annealed at over 1400 °C forms an irregular bulk. The emission spectra of samples indicated the PL of samples annealed at 1200?1400 °C improve with increasing the annealing temperature because of the diffusion of Si⁴⁺ ions; whereas the PL of sample annealed at the temperature over 1400 °C decreases likely resulting from inflection effects of multiangular shape of grains. Therefore, the samples annealed at 1400 °C are suitable for gaining phosphor with high brightness and good morphology.     

Thermoluminescent and stuctural properties of BaAl2O4:Eu,Nd,Gdphosphors prepared by combustion method

BaAl₂O₄:Eu²⁺,Nd³⁺,Gd³⁺ phosphors were prepared by a combustion method at different initiating temperatures (400–1200 °C), using urea as a comburent. The powders were annealed at different temperatures in the range of 400–1100 °C for 3 h. X-ray diffraction data show that the crystallinity of the BaAl₂O₄ structure greatly improved with increasing annealing temperature. Blue-green photoluminescence, with persistent/long afterglow, was observed at 498 nm. This emission was attributed to the 4f⁶5d¹–4f⁷ transitions of Eu²⁺ ions. The phosphorescence decay curves were obtained by irradiating the samples with a 365 nm UV light. The glow curves of the as-prepared and the annealed samples were investigated in this study. The thermoluminescent (TL) glow peaks of the samples prepared at 600 °C and 1200 °C were both stable at ∼72 °C suggesting that the traps responsible for the bands were fixed at this position irrespective of annealing temperature. These bands are at a similar position, which suggests that the traps responsible for these bands are similar. The rate of decay of the sample annealed at 600 °C was faster than that of the sample prepared at 1200 °C.     

Enhancement of saturation magnetization in Cr-ion implanted silicon by high temperature annealing

Magnetic properties and microstructure of Cr-implanted Si have been investigated by alternating gradient magnetometer (AGM), superconducting quantum interference device (SQUID) magnetometer, and transmission electron microscopy (TEM). p-Type (1 0 0) Si wafers were implanted at 200 keV at room temperature with a dosage of 1 × 10¹⁶ cm⁻² Cr ions and then annealed at 600-900 °C for 5 min. The effect of annealing on the structure and magnetic properties of Cr-implanted Si is studied. The as-implanted sample shows a square M-H loop at low temperature. Magnetic signal becomes weaker after short time annealing of the as-implanted sample at 600 °C, 700 °C, and 800 °C. However, the 900 °C annealed sample exhibits large saturation magnetization at room temperature. TEM images reveal that the implanting process caused amorphization of Si, while annealing at 900 °C led to partial recovery of the crystal. The enhancement of saturation magnetization can be explained by the redistribution and accumulation of Cr atoms in the vacancy-rich region of Si during annealing.     

Effect of thermal annealing on the structure and magnetism of Fe-doped ZnO nanocrystals synthesized by solid state reaction

High purity Fe₂O₃/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe₂O₃/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe₂O₄. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe₂O₃/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects.     

Optical properties of annealed Mn-doped ZnS nanoparticles

Cysteine stabilized ZnS and Mn²⁺-doped ZnS nanoparticles were synthesized by a wet chemical route. Using the ZnS:Mn²⁺ nanoparticles as seeds, silica-coated ZnS (ZnS@Si) and ZnS:Mn²⁺ (ZnS:Mn²⁺@Si) nanocomposites were formed in water by hydrolysis and condensation of tetramethoxyorthosilicate (TMOS). The influence of annealing in air, formier gas, and argon at 200-1000 °C on the chemical stability of ZnS@Si and ZnS:Mn²⁺@Si nanoparticles with and without silica shell was examined. Silica-coated nanoparticles showed an improved thermal stability over uncoated particles, which underwent a thermal combustion at 400 °C. The emission of the ZnS@Si and ZnS:Mn²⁺@Si passed through a minimum in photoluminescence intensity when annealed at 600 °C. Upon annealing at higher temperatures, ZnS@Si conserved the typical emission centered at 450 nm (blue). ZnS:Mn²⁺@Si yielded different high intensity emissions when heated to 800 °C depending on the gas employed. Emissions due to the Mn²⁺ at 530 nm (green; Zn₂SiO₄:Mn²⁺), 580 nm (orange; ZnS:Mn²⁺@Si), and 630 nm (red; ZnS:Mn²⁺@Si) were obtained. Therefore, with a single starting product a set of different colors was produced by adjusting the atmosphere wherein the powder is heated.     

Fabrication and analysis of the structural phase transition of ZrO2 nanoparticles using modified facile sol–gel route

The synthesis of zirconia nanoparticles is achieved through a modified facile sol–gel route. The as-prepared gel is analyzed thermally using TGA and DTA techniques to spot the crystallization process of zirconia nanoparticles. The prepared gel is then annealed at different temperatures and the structure was found to change between tetragonal and monoclinic crystal systems. The first stable tetragonal phase is achieved after annealing for 2?h at 400°C. The annealed powders between 600°C and 800°C demonstrate mixed tetragonal/monoclinic phases. Annealing at 1000°C and higher temperatures up to 1200°C resulted in pure monoclinic phase. Cubic phase was not detected within the annealing temperature range in this study. The elemental analysis of the annealed powder confirmed the formation of zirconia nanoparticles with the chemical formula ZrO₂. The FTIR spectra of the annealed samples introduced a variation in the vibrational bands especially around the phase transition temperature. HR-TEM images reported the formation of nano-zirconia crystals with apparently large particle sizes. The optical energy gap of zirconia nanoparticles is investigated and determined.     

A study of the optical properties and adhesion of zinc sulfide anti-reflection thin film coated on a germanium substrate

To conduct this study, zinc sulfide (ZnS) thin films deposited on germanium (Ge) substrates were prepared by an evaporation method. The effects of deposition rate and annealing on the optical properties and adhesion of the ZnS thin films were investigated. The transmission intensity and the X-ray diffraction (XRD) pattern of the samples showed that the transmittance of the samples decreases by increasing the evaporation rates. However, with the increase of the annealing temperature, crystallinity of the thin films improves which, in turn, results in the enhancement of the transmission intensity in a far infrared region. The maximum grain size was obtained at the annealing temperature of 225 °C. Our experimental results also show that evaporation rate and annealing influences the adhesion of ZnS thin films to Ge substrates.     

Gas-phase Crystallization of Titanium Dioxide Nanoparticles

We have investigated the development of crystal morphology and phase in ultrafine titanium dioxide particles. The particles were produced by a droplet-to-particle method starting from propanolic titanium tetraisopropoxide solution, and calcined in a vertical aerosol reactor in air. Mobility size classified 40-nm diameter particles were conveyed to the aerosol reactor to investigate particle size changes at 20–1200°C with 5–1-s residence time. In addition, polydisperse particles were used to study morphology and phase formation by electron microscopy. According to differential mobility analysis, the particle diameter was reduced to 21–23-nm at 600°C and above. Precursor decomposition occurred between 20°C and 500°C. The increased mobility particle size at 700°C and above was observed to coincide with irregular particles at 700°C and 800°C and faceted particles between 900°C and 1200°C, according to transmission electron microscopy. The faceted anatase particles were observed to approach a minimized surface energy by forming {101} and {001} crystallographic surfaces. Anatase phase was observed at 500–1200°C and above 600°C the particles were single crystals. Indications of minor rutile formation were observed at 1200°C. The relatively stable anatase phase vs. temperature is attributed to the defect free structure of the observed particles and a lack of crystal–crystal attachment points.     

Structural changes of MoS2 nano-powder in dependence on the annealing temperature

To search for structural changes of molybdenum disulphide (MoS₂) nano-powder under thermal treatment, the annealing of the powder was carried out in vacuum or in argon. MoS₂ powder with a grain size of 3–5 nm was synthesized by a chemical method. The temperature of annealing was varied in the range of 380–1000 °C. The time of annealing was varied in the range from 4 h (380 °C) to 5 min (1000 °C). X-ray diffraction and transmission electron microscopy analyses were made to see the character of the change of the crystallization process and the curvature of (002) MoS₂ planes. The increase of crystalline phase and the decrease of amorphous phase in the powder appears as a result of the rise of the annealing temperature. The sample annealed at T_ann=380 °C and 500 °C has a small curvature of (002) planes and more enlarged grains in comparison with non-annealed MoS₂ powder. The increase of the annealing temperature to T_ann=700 °C leads to a strong curvature of the (002) planes. The character of the curved planes varies from quite long planes with a large radius of curvature to right-angle-form and U-form structures. The increase of the annealing temperature to T_ann=1000 °C leads to a strong crystallization of the powder and a reduction of the amorphous phase. PACS 81.05.Tp; 81.07.Wx; 81.20.Ka; 61.10.-i; 68.37.Lp     

Effect of isochronal annealing on photoluminescence properties of Mn-implanted GaN

Mn ions were implanted into metal organic chemical vapour deposition (MOCVD)-grown GaN with dose ranging from 10¹⁴ to 5×10¹⁶ cm⁻². Isochronal annealing at 800 and 850 °C has been carried out after implantation of the samples. Photoluminescence measurements were carried out on the implanted samples before and after annealing. A peak found at 3.34 eV in the spectra of implanted samples after annealing at 850 °C is attributed to the stacking faults. Blue and green luminescence bands have been observed suppressed and an oxygen-related peak appeared at 3.44 eV in the PL spectra. The suppression of blue and green luminescence bands has been assigned to dissociation of V_GaO_N complex. Near-band-edge (NBE) peak exhibited a blue shift after 800 °C anneal and then red shift to restore its original energy position when annealed at 850 °C.     

Growth, thermal and spectroscopic characterizations of BaNd2(MoO4)4 crystal

A new crystal, BaNd₂(MoO₄)₄, has been grown from the flux melt based on Li₂Mo₃O₁₀ by a spontaneous nucleation method. The phase structure of the obtained crystals was determined by X-ray powder diffraction. The result shows that the as-grown crystals are well crystallized and indexed in a monoclinic crystal system with space group B2/b. The specific heat of BaNd₂(MoO₄)₄ crystal at 20 °C is 0.485 J/g K. Absorption and fluorescence spectra were also measured at room temperature. There are several strong and broad absorption peaks from 200 to 1200 nm and three emission transition bands located at 890, 1060, and 1334 nm are detected.     

Influence of high temperature annealing on the structure, hardness and tribological properties of diamond-like carbon and TiAlSiCN nanocomposite coatings

Diamond-like carbon (DLC) and TiAlSiCN nanocomposite coatings were synthesized and annealed at different temperatures in a vacuum environment. The microstructure, hardness and tribological properties of as-deposited and annealed DLC-TiAlSiCN nanocomposite coatings were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectroscopy, nano-indentation and friction tests. The TEM results reveal that the as-deposited DLC-TiAlSiCN coating has a unique nanocomposite structure consisting of TiCN nanocrystals embedded in an amorphous matrix consisting of a-Si₃N₄, a-SiC, a-CN and DLC, and the structure changed little after annealing at 800 °C. However, XPS and Raman results show that an obvious graphitization of the DLC phase occurred during the annealing process and it worsened with annealing temperature. Because of the graphitization, the hardness of the DLC-TiAlSiCN coating after annealing at 800 °C decreased from 45 to 36 GPa. In addition, the DLC-TiAlSiCN coating after annealing at 800 °C has a similar friction coefficient to the as-deposited coating.     

Structural,magnetic and Mössbauer spectral studies of nanosize aluminum substituted nickel zinc ferrites

Nanosize aluminum substituted nickel zinc ferrites were prepared through aerosol route and characterized using TEM, XRD, magnetic measurements and Mössbauer spectroscopy. The particle size of as obtained samples was found to be ∼10 nm which increases up to ∼85 nm upon annealing at 1200 °C. The unit cell parameter ‘a’ decreases linearly with concentration of aluminum due to the small ionic radius of aluminum. The saturation magnetization for all the samples after annealing at 1200 °C lies in the range 12.9–72.6 emu/g and decreases linearly with concentration of aluminum. Room temperature Mössbauer spectra of all as obtained samples of ferrite compositions exhibited a broad doublet suggesting super paramagnetic nature. This doublet is further resolved into two doublets and assigned to the surface region and internal region atoms of the particles. The samples annealed at 1200 °C show broad sextets, which were fitted with five sextets, indicating different local environment of both tetrahedrally and octahedrally coordinated Fe cation.     

Growth, structural and optical transport properties of nanocrystal Zn1−xCdS thin films deposited by solution growth technique (SGT) for photosensor applications

Solution Growth Technique (SGT) has been used for deposition of Zn_1−xCdS nanocrystalline thin films. Various parameters such as solution pH (10.4), deposition time, concentration of ions, composition and deposition and annealing temperatures have been optimized for the development of device grade thin film. In order to achieve uniformity and adhesiveness of thin film on glass substrate, 5 ml triethanolamine (TEA) have been added in deposition solution. The as-deposited films have been annealed in Rapid Thermal Annealing (RTA) system at various temperature ranges from 100 to 500 °C in air. The changes in structural formation and optical transport phenomena have been studied with annealing temperatures and composition value (x). As-deposited films have two phases of ZnS and CdS, which were confirmed by X-ray diffraction studies; further the X-ray analysis of annealed (380 °C) films indicates that the films have nanocrystalline size (150 nm) and crystal structure depends on the films stoichiometry and annealing temperatures. The Zn_0.4CdS films annealed at 380 °C in air for 5 min have hexagonal structure where as films annealed at 500 °C have represented the oxide phase with hexagonal structure. Optical properties of the films were studied in the wavelength range 350-1000 nm. The optical band gap (E_g=2.94-2.30 eV) decreases with the composition (x) value. The effect of air rapid annealing on the photoresponse has also been observed on Zn_1−xCdS nanocrystal thin films. The Zn_1−xCdS thin film has higher photosensitivity at higher annealing temperatures (380-500 °C), and films also have mixed Zn_1−xCdS/Zn_1−xCdSO phase with larger grain size than the as-deposited and films annealed up to 380 °C. The present results are well agreed with the results of other studies.