Magneto-optical properties of α-Fe2O3@ZnO nanocomposites prepared by the high energy ball-milling technique

Magnetic–fluorescent nanocomposites (NCs) with 10 wt% of α-Fe₂O₃ in ZnO have been prepared by the high energy ball-milling. The crystallite sizes of α-Fe₂O₃ and ZnO in the NCs are found to vary from 65 nm to 20 nm and 47 nm to 15 nm respectively as milling time is increased from 2 to 30 h. XRD analysis confirms presence of α-Fe₂O₃ and ZnO in pure form in all the NCs. UV–vis study of the NCs shows a continuous blue-shift of the absorption peak and a steady increase of band gap of ZnO with increasing milling duration that are assigned to decreasing particle size of ZnO in the NCs. Photoluminescence (PL) spectra of the NCs reveal three weak emission bands in the visible region at 421, 445 and 485 nm along with the strong near band edge emission at 391 nm. These weak emission bands are attributed to different defect – related energy levels e.g. Zn-vacancy, Zn interstitial and oxygen vacancy. Dc and ac magnetization measurements show presence of weakly interacting superparamagnetic (SPM) α-Fe₂O₃ particles in the NCs. ⁵⁷Fe-Mössbauer study confirms presence of SPM hematite in the sample milled for 30 h. Positron annihilation lifetime measurements indicate presence of cation vacancies in ZnO nanostructures confirming results of PL studies.     

Growth-morphology-luminescence correlation in ZnO-containing nanostructures synthesized in different media

Zinc hydroxide particles were prepared by a two-step process employing zinc nitrate hexahydrate, urea, ethylene glycol, water and p-toluene-sulfonic acid monohydrate (p-TSA). We used different concentrations of the reactants as well as different volume ratios of the solvents. ZnO particles were obtained by thermal treatment of the reaction products at two different temperatures: 350 °C and 500 °C. The samples were characterized by scanning field emission electron microscopy (SEM), X-ray diffraction (XRD) spectroscopy, BET analysis, thermogravimetry (TG) analysis and photoluminescence (PL) spectroscopy. It was found that after the thermal treatment particles become smaller, with the p-TSA concentration strongly affecting the morphology of the particles. Luminescence properties of the samples probed by PL at 8 K and room temperature exhibited a remarkable correlation with specimens′ nanomorphology. Luminescent features at ～2.0–2.2 eV, ～2.4–2.5 eV, ～2.65 eV, ～2.9 eV, ～3.0–3.1 eV and ～3.3 eV were observed in most specimens, although their relative intensity and temperature dependence were specific to an individual group of samples vis-à-vis their growth history and morphology.     

High quality nitrogen-doped zinc oxide thin films grown on ITO by sol–gel method

Highly transparent N-doped ZnO thin films were deposited on ITO coated corning glass substrate by sol–gel method. Ammonium nitrate was used as a dopant source of N with varying the doping concentration 0, 0.5, 1.0, 2.0 and 3.0 at%. The DSC analysis of prepared NZO sols is observed a phase transition at 150 °C. X-ray diffraction pattern showed the preferred (002) peak of ZnO, which was deteriorated with increased N concentrations. The transmittance of NZO thin films was observed to be ~88%. The bandgap of NZO thin films increased from 3.28 to 3.70 eV with increased N concentration from 0 to 3 at%. The maximum carrier concentration 8.36×10¹⁷ cm⁻³ and minimum resistivity 1.64 Ω cm was observed for 3 at% N doped ZnO thin films deposited on glass substrate. These highly transparent ZnO thin films can be used as a window layer in solar cells and optoelectronic devices.     

Optical and physical characteristics of In-doped ZnO nanorods

In this paper the effect of indium dopants on structure, optical, electrical and mechanical properties of ZnO nanorods are studied. The average surface potentials and the surface currents of ZnO:In nanorods were 0.25–0.84 mV and 2.2–200 MΩ-cm, respectively. The turn-on threshold field for the vertical ZnO nanorods was around 2–16 V μm⁻¹. Emission current densities of 3.3–911.4 mA cm⁻² were obtained for an electrical field of 60–160 V μm⁻¹. The photoluminescence (PL) spectrum measured at 15–300 K showed that the intensity of the peak at 2.06 eV increased with decreasing temperature, while the peak at 2.06 eV further red shifted and the peak at 3.39 eV blue shifted.     

Structural and optical properties of Mn-doped ZnO nanocrystalline thin films with the different dopant concentrations

The transparent nanocrystalline thin films of undoped zinc oxide and Mn-doped (Zn_1−xMn_xO) have been deposited on glass substrates via the sol–gel technique using zinc acetate dehydrate and manganese chloride as precursor. The as-deposited films with the different manganese compositions in the range of 2.5–20 at% were pre-heated at 100 °C for 1 h and 200 °C for 2 h, respectively, and then crystallized in air at 560 °C for 2 h. The structural properties and morphologies of the undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/Vis spectroscopy. The analyzed results indicates that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn related phases. Room temperature photoluminescence is observed for the ZnO and Mn-doped ZnO thin films.     

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Characteristics of ZnO MSM UV photodetector with Ni contact electrodes on poly propylene carbonate (PPC) plastic substrate

In this study, we report the fabrication of ZnO metal-semiconductor-metal UV photodetector (MSM UV PD) by deposition ZnO thin film on poly propylene carbonate (PPC) plastic substrate using direct current (DC) sputtering technique, and Nickel (Ni) contact as electrodes. The structural, optical and electrical properties of the ZnO thin film were investigated by using atomic force microscopy (AFM), X-Ray diffraction (XRD) measurement, and photoluminescence (PL). The electrical characteristics of the detector were investigated using the current–voltage (I–V) measurements, the dark- and photo-currents were found to be 1.04 and 93.80 μA, respectively. Using forward dark conditions at 5 volt; the barrier height Φ_B was calculated to be 0.675 eV. Under incident wavelength of 385 nm, it was found that the maximum responsivity (R) of the Ni/ZnO/Ni MSM PD was found to be 1.59 A/W.     

Effect of Ce and Cu co-doping on the structural,morphological, and optical properties of ZnO nanocrystals and first principle investigation of their stability and magnetic properties

Ce, Cu co-doped ZnO (Zn_1−2xCe_xCu_xO: x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) nanocrystals were synthesized by a microwave combustion method. These nanocrystals were investigated by using X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Cu co-doped ZnO were probed by first principle calculations. XRD results revealed that all the compositions are single crystalline. hexagonal wurtzite structure. The optical band gap of pure ZnO was found to be 3.22 eV, and it decreased from 3.15 to 3.10 eV with an increase in the concentration of Cu and Ce content. The morphologies of Ce and Cu co-doped ZnO samples confirmed the formation of nanocrystals with an average grain size ranging from 70 to 150 nm. The magnetization measurement results affirmed the antiferro and ferromagnetic state for Ce and Cu co-doped ZnO samples and this is in agreement with the first principles theoretical calculations.     

Luminescence and physical properties of copper doped CdO derived nanostructures

In this paper, we studied the photoluminescence (PL), the morphological, electrical and optical properties of pure and copper-doped cadmium oxide. CdO films were grown by a facile sol–gel spin coating process at 1200 rpm, and doped with copper at 2 and 3%. A (1 1 1)-oriented cubic structure with a lattice parameter of a=4.69 Å was confirmed by X-ray diffraction. Copper was shown to improve the optical transmittance in the short wavelength range of the visible spectrum. The optical band gap of CdO ranged between 2.49 and 2.62 eV as a result of Cu content. At room temperature, resistance fell drastically with Cu doping levels. AFM analysis of samples exhibited nano-mounts and nanowires. Finally, PL results showed a strong blue–violet emission peak at 2.80 eV.     

Effect of Fe doping on the structural and gas sensing properties of ZnO porous microspheres

Fe-doped ZnO porous microspheres composed of nanosheets were prepared by a simple hydrothermal method combined with post-annealing, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller N₂ adsorption–desorption measurements and photoluminescence (PL) spectra. In this paper we report Fe doping induced modifications in the structural, photoluminescence and gas sensing behavior of ZnO porous microspheres. Our results show that the crystallite size decreases and specific surface area increases with the increase of Fe doping concentration. The PL spectra indicate that the 4 mol% Fe-doped ZnO has higher ratio of donor (V_O and Zn_i) to acceptor (V_Zn) than undoped ZnO. The 4 mol% Fe-doped ZnO sample shows the highest response value to ppb-level n-butanol at 300 °C, and the detected limit of n-butanol is below 10 ppb. In addition, the 4 mol% Fe -doped ZnO sample exhibits good selectivity to n-butanol. The superior sensing properties of the Fe-doped porous ZnO microspheres are contributed to higher donor defects contents combined with larger specific surface area.     

First-principles study on electronic states of SrWO4 crystals containing F-type color centers

The electronic structures of the SrWO₄ crystals containing F-type color centers are studied within the framework of the fully relativistic self-consistent Dirac–Slater theory using a numerically discrete variational (DV-X^α) method. The calculations indicate that either F or F⁺ center has donor energy level within the forbidden band. The electronic transition energies from the two donor levels to the bottom of the conduction band are 1.82 eV and 2.28 eV corresponding to the 685 nm and 545 nm absorption bands, respectively. It is, therefore, concluded that the 545–685 nm absorption bands are originated from the F and F⁺ center in SrWO₄ crystal respectively.     

Comparative study on the effects of different annealing conditions on the surface morphology,crystallinity, and optical properties of ZnO micro/nanoparticle-based discs

The effects of annealing parameters on the surface morphology, crystallinity, and optical properties of ZnO disc were investigated. Variations in the annealing temperatures and gas flow rates were found to have a profound impact; grain growth was enhanced even at the low annealing temperature of only 400 °C. SEM and AFM revealed smooth and uniform grain growth after annealing treatment, especially at 800 °C. A unique secondary growth of ZnO nanoparticles and multilayer grain growth that have not been reported elsewhere were also observed. The annealing treatment was also found to improve grain crystallinity as illustrated by the lowering of intrinsic compressive stress based on the XRD lattice constant and FWHM data. The PL spectra of the M-Disc showed a huge band edge emission at 371–376 nm. In contrast, the N-Disc exhibited a dominant and broad visible PL emission in the green band with peaks at 519–533 nm. These peaks were attributed to a very high concentration of structural defects (oxygen vacancies and zinc/oxygen interstitials). The annealing conditions had a significant effect on the properties of ZnO. Increased percentage of oxygen in the O/Ar from 50% to 100% did not change the M-Disc spectra. However, the XRD pattern of the N-Disc revealed that the (0 0 2) peak intensity decreased, the position of the (1 0 1) peak slightly shifted toward a higher angle, and the FWHM of the (1 0 1) peak improved. The experimental results showed that thermal annealing could enhance the different properties of ZnO discs.     

Photoluminescence of AgGaS2 and CuGaS2 doped with rare-earth impurities

Photoluminescence (PL) related to rare-earth (RE) impurities (Ho, Er and Eu) in AgGaS₂ and CuGaS₂ crystals has been studied. In Ho-doped AgGaS₂ and CuGaS₂, two series of PL lines are observed in 1.86–1.92 eV region and 2.24 eV region, and they are assigned to ⁵F₃–⁵I₇ and ⁵S₂–⁵I₈ transitions of the Ho³⁺ ion, respectively. Similarly, in Er-doped AgGaS₂ and CuGaS₂, Er³⁺-related two PL series are observed: 1.83–1.88 eV region (⁴F_9/2–⁴I_15/2) and 2.22–2.26 eV region (⁴S_3/2–⁴I_15/2). For both Ho and Er impurities, the profile of the PL spectrum in AgGaS₂ is complex, and PL exhibited large number of lines compared with that in CuGaS₂. The differences in PL spectra between this two compounds are related to the crystal field at the cation site and the local atomic arrangement of the RE impurities. This work also refers to the PL band at 2.28 eV observed for the Eu-doped AgGaS₂ crystal.     

Synthesis and characterization of nanostructural CuS–ZnS binary compound thin films prepared by spray pyrolysis

We have investigated the effect of zinc concentration ([Zn]/[Cu]=0–100 at%) on nanostructural, optical and electrical properties of CuS–ZnS binary thin films grown on glass substrate by the spray pyrolysis technique. X-ray diffraction analysis showed that the films were crystallized with mixed structures of CuS hexagonal and ZnS cubic structure. UV–vis optical measurements analysis showed that these binary films have a relatively high absorption coefficient (～10⁵ cm^?1) in the visible spectrum with a direct band gap in the range of 2.57–2.45 eV in agreement with the corresponding room temperature PL spectra. The electrical studies showed that all these samples have a p-type conductivity and the free hole density decreases with increasing [Zn]/[Cu] molar ratio, in agreement with the reflectance spectra of the layers, originating from plasma oscillations.     

Acceptors related to group I elements in ZnO ceramics

ZnO ceramics doped with Li, Na or K were sintered in air for 4 h at 1000 °C. Electrical conductivity as well as photoluminescence (PL), PL excitation and photoconductivity spectra were measured and compared with those in undoped samples. The influence of both fast and slow cooling of the samples from 1000 °C on measured characteristics was investigated. The yellow–orange PL bands associated with the deep acceptors Li_Zn, Na_Zn and K_Zn were observed and the corresponding PL excitation spectra were determined. These acceptors were found to form some complexes with other lattice defects.     

Synthesis of nanocrystalline ZnO nanobelts via pyrolytic decomposition of zinc acetate nanobelts and their gas sensing behavior

The pyrolytic decomposition of layered basic zinc acetate (LBZA) nanobelts (NBs) into nanocrystalline ZnO NBs is investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). We also report on the gas sensing response of the resulting ZnO nanomaterial to CO. The LBZA NBs are grown at 65 °C in an aqueous solution of zinc acetate dihydrate. AFM and SEM results show as-grown products possess the characteristic layered structure of the LBZA crystals. XRD and XPS results show that annealing as-grown products at 210 °C in air causes a transformation from zinc acetate to nanocrystalline ZnO NBs via thermal decomposition. The ZnO crystalline domain size increases with temperature from 9.2 nm at 200 °C to 94 nm at 1000 °C, as measured from XRD. SEM shows evidence of sintering at 600 °C. The thickness of the NBs, determined via AFM, ranges from 10 to 50 nm and remains approximately constant with annealing temperature. XPS confirmed the chemical transformation from zinc acetate to ZnO and showed a significant remaining zinc hydroxide component for the ZnO NBs consistent with published results. PL measurements at room temperature show a blue shift in peak emission as the nanobelts change from LBZA to ZnO at 200 °C. Above this transition temperature, the ZnO nanobelts possess strong band edge emission at 390 nm and little broad band emission in the visible region. The AFM and SEM images reveal that the crystallites within the nanobelts orientate in rows along the long axis during annealing. This structure provides a high surface area to volume ratio of aligned nanoparticles which is beneficial for gas sensing applications. Gas sensors fabricated from 400 °C annealed nanobelts showed a response of 1.62 when exposed to 200 ppm of CO in dry air at 400 °C, as defined by the ratio of resistance before and during exposure. This indicates that ZnO nanostructures obtained by thermal decomposition of LBZA NBs could provide a cost effective route to high sensitivity gas sensors.     

Comparative study on structural and optical properties of ZnO thin films prepared by PLD using ZnO powder target and ceramic target

Zinc oxide thin films have been obtained in O₂ ambient at a pressure of 1.3 Pa by pulsed laser deposition (PLD) using ZnO powder target and ceramic target. The effect of temperature on structural and optical properties of ZnO thin films was investigated systematically by XRD, SEM, FTIR and PL spectra. The results show that the best structural and optical properties can be achieved for ZnO thin film fabricated at 700 °C using powder target and at 400 °C using ceramic target, respectively. The PL spectrum reveals that the efficiency of UV emission of ZnO thin film fabricated by using powder target is low, and the defect emission of ZnO thin film derived from Zn_i and O_i is high.     

Near-infrared photoluminescence and thermally stimulated current in Cu3Ga5Se9 layered crystals: A comparative study

Near-infrared photoluminescence (PL) and thermally stimulated current (TSC) spectra of Cu₃Ga₅Se₉ layered crystals grown by Bridgman method have been studied in the photon energy region of 1.35–1.46 eV and the temperature range of 15–115 K (PL) and 10–170 K (TSC). An infrared PL band centered at 1.42 eV was revealed at T = 15 K. Radiative transitions from shallow donor level placed at 20 meV to moderately deep acceptor level at 310 meV were suggested to be the reason of the observed PL band. TSC curve of Cu₃Ga₅Se₉ crystal exhibited one broad peak at nearly 88 K. The thermal activation energy of traps was found to be 22 meV. An energy level diagram demonstrating the transitions in the crystal band gap was plotted taking account of results of PL and TSC experiments conducted below room temperature.     

Influence of Cu doping on the microstructure,optical properties and photoluminescence features of Cd0.9Zn0.1S nanoparticles

Cd_0.9−xZn_0.1Cu_xS (0≤x≤0.06) nanoparticles were successfully synthesized by a conventional chemical co-precipitation method at room temperature. Crystalline phases and optical absorption of the nanoparticles have been studied by X-ray diffraction (XRD) and UV–visible spectrophotometer. XRD confirms the phase singularity of the synthesized material, which also confirmed the formation of Cd–Zn–Cu–S alloy nanocrystals rather than separate nucleation or phase formation. Elemental composition was examined by the energy dispersive X-ray analysis and the microstructure was examined by scanning electron microscope. The blue shift of absorption edge below Cu=2% is responsible for dominance of Cu⁺ while at higher Cu concentration dominated Cu²⁺, d–d transition may exist. It is suggested that the addition of third metal ion (Cu²⁺/Cu⁺) is an effective way to improve the optical property and stability of the Cd_0.9Zn_0.1S solid solutions. When Cu is introduced, stretching of Cd–Zn–Cu–S bond is shifted lower wave number side from 678 cm⁻¹ (Cu=0%) to 671 cm⁻¹ (Cu=6%) due to the presence of Cu in Cd–Zn–S lattice and also the size effect. The variation in blue band emission peak from 456 nm (∼2.72 eV) to 482 nm (∼2.58 eV) by Cu-doping is corresponding to the inter-band radiation combination of photo-generated electrons and holes. Intensity of red band emission centered at 656 nm significantly increased up to Cu=4%; beyond 4% it is decreased due to the quenching of Cu concentration.     

Photoluminescence studies of ZnO thin films on R-plane sapphire substrates grown by sol–gel method

Zinc oxide (ZnO) thin films on R-plane sapphire substrates were grown by the sol–gel spin-coating method. The optical properties of the ZnO thin films were investigated using photoluminescence. In the UV range, the asymmetric near-band-edge emission was observed at 300 K, which consisted of two emissions at 3.338 and 3.279 eV. Eight peaks at 3.418, 3.402, 3.360, 3.288, 3.216, 3.145, 3.074, and 3.004 eV, which respectively correspond to the free exciton (FX), bound exciton, transverse optical (TO) phonon replica of FX recombination, and first-order longitudinal optical phonon replica of FX and the TO (1LO+TO), 2LO+TO, 3LO+TO, 4LO+TO, and 5LO+TO, were obtained at 12 K. From the temperature-dependent PL, it was found that the emission peaks at 3.338 and 3.279 eV corresponded to the FX and TO, respectively. The activation energy of the FX and TO emission peaks was found to be about 39.3 and 28.9 meV, respectively. The values of the fitting parameters of Varshni's empirical equation were α=4×10^?3 eV/K and β=4.9×10³ K, and the S factor of the ZnO thin films was 0.658. With increasing temperature, the exciton radiative lifetime of the FX and TO emissions increased. The temperature-dependent variation of the exciton radiative lifetime for the TO emission was slightly higher than that for the FX emission.