The improvement of thermoelectric property of bulk ZnO via ZnS addition: Influence of intrinsic defects

As in any semiconducting solids, intrinsic defects can affect the properties of ZnO, such as the electrical and thermal conductivities. Defect engineering is usually focused on optimizing the materials’ synthesis or annealing parameters, i.e., temperature, atmosphere, etc. Here we report an approach to change the intrinsic defects of ZnO by adding a small amount of ZnS. During the sintering process, ZnS was decomposed. Apart from the formation of SO₂, the decomposed S and Zn can also be simultaneously doped onto O and Zn sites to change the intrinsic defects in ZnO. For instance, some of the S was converted to SO₂ and led to the formation of V_o (oxygen vacancy); meanwhile, Zn may move to the V_Zn (Zn vacancy) site and decrease the concentration of Zn vacancy. Due to the changes in these native defects, the carrier concentration increased and the thermal conductivity decreased when the content of ZnS was increased to x = 0.01. This sample had an optimal zT value, which was twice that of undoped ZnO. However, with further increase in ZnS, the carrier concentration was reduced. These results suggest a method to tune the intrinsic defects of ZnO via doping technology and bring potential opportunities to improve the thermoelectric performance of this oxide.     

Photophysical investigation of the formation of defect levels in P doped ZnO thin films

Zinc oxide (ZnO) offers a major disadvantage of asymmetry doping in terms of reliability, stability, and reproducibility of p-type doping, which is the main hindrance in realization of optoelectronic devices. The problem is even more complicated due to formation of various native defects in unintentionally doped n-type ZnO. The realization of p-type conductivity in doped ZnO requires an in-depth understanding of the formation of an effective shallow acceptor, as well as donor-acceptor compensation. Photophysical properties such as photoconductivity along with photoluminescence (PL) studies have unprecedentedly and effectively been utilized in this work to monitor the evolution of various in-gap defects. Phosphorus (P) doped ZnO thin films have been grown by RF magnetron sputtering under various Ar to O₂ gas ratios to investigate the effect of O₂ on the donor-acceptor compensation by comprehensive photoconductivity measurements supported by the PL studies. Initial elemental analyses indicate presence of abundant zinc vacancies (V_Zn) in O-rich ambience. The results predict that P sits in the zinc (Zn) site rather than the oxygen (O) site causing the formation of P_Zn–2V_Zn acceptor-like defects, which compensates the donor defects in P doped ZnO films. Photocurrent spectra uniquely reveal presence of more oxygen vacancies (V_O) defects states in lower O₂ flow, which gets compensated with an increase in the O₂ flow. Successive photocurrent transients indicate probable presence of more V_O in the films grown with lower O₂ flow and more V_Zn in higher O₂ flow. Overall the photosensitivity measurements clearly present that O-rich ambience expedites the formation of acceptor defects which are compensated, thereby lowering the dark current and enhancing the ultraviolet photosensitivity.     

Photoluminescence mechanism of annealed ZnO/Zn/Al2O3 sandwich structures deposited on glass substrates

ZnO/Zn/Al₂O₃ sandwich structures are grown on glass substrates by magnetron sputtering. The effect of Al₂O₃ layers on optical properties of ZnO/Zn/Al₂O₃ sandwich structures is investigated. Results indicated that as the deposition time of Al₂O₃ increases, violet peak centered at 402 nm gradually shifted to 412 nm and the intensity firstly decreases and then increases. We discuss the intensity change and shift of violet peak relating to V_Zn defects and the band alignment of ZnO/Zn/Al₂O₃ sandwich structures, respectively. We proposed that ZnO/Zn/Al₂O₃ sandwich structures can be approximately regarded as a quasiquantum-well-like structure. So the electron tunneling from Zn to Al₂O₃ layer is suppressed and the photogenerated carriers can be confined in the Zn Fermi level. In order to further understand the effect of posttreatment on optical properties of samples, samples are annealed in vacuum at 350 °C for 1 h. PL emissions are weakened with the increase of Al₂O₃ deposition time. Interestingly, at a same deposition condition, PL emissions are still improved after posttreatment. Combined Al₂O₃ layer modulation with annealing treatment, steady PL properties can be effectively improved.     

Effect of thermal annealing on electrical degradation characteristics of Sb–Bi–Mn–Co-added ZnO varistors

The effects of thermally annealing Bi–Mn–Co–Sb₂O₃-added ZnO varistors on their electrical degradation were investigated. For the samples added with 0.01 mol% Sb₂O₃ and without Sb₂O₃, no marked difference in the nonlinearity index α of the voltage–current (V–I) characteristics was observed upon electrical degradation for the annealed and nonannealed samples. Upon increasing the amount of Sb₂O₃ added, the values of α increased after electrical degradation for the annealed samples. Moreover, the value of α after electrical degradation was proportional to the width of gauss function (width) of the X-ray diffraction peak for Zn_2.33Sb_0.67O₄-type spinel particles under various annealing conditions. The added Sb₂O₃ did not dissolve in the ZnO grains but became segregated at grain boundaries. Therefore, it is speculated that the increase in the width of the spinel particles is due to the increase in the numbers of fine spinel particles at grain boundaries and triple points. Furthermore, it is suggested that the improvement of the electrical degradation is due to the decrease in the mobility of oxide ions or Zn²⁺ ions owing to their being blocked by uniformly dispersed fine spinel particles at grain boundaries.     

Characterisation of Zn3(VO4)2 phases in V2O5-doped ZnO varistors

Zinc oxide-rich ZnO–V₂O₅ (ZV), ZnO–V₂O₅–MnO₂ (ZVM) and ZnO–V₂O₅–Sb₂O₃ (ZVS) polycrystalline ceramics have been prepared for detailed microstructural and electrical characterisation. All samples exhibit non-linear current-voltage behaviour, with non-linear coefficients ranging from 5·0 for ZV to 16·7 for ZVM. The use of X-ray powder diffraction together with microstructural examination by transmission electron microscopy and a comparison of measured interplanar spacings with those quoted in the literature for α-, β- and γ-Zn₃(VO₄)₂, has shown evidence for the formation of β-Zn₃(VO₄)₂ in ZV, γ-Zn₃(VO₄)₂ in ZVM and α-Zn₃(VO₄)₂ in ZVS. The Zn₃(VO₄)₂ phases are found to exist as smaller grains embedded in ZnO grains or residing at triple junctions. Electron diffraction suggests that β-Zn₃(VO₄)₂ has an orthorhombic A lattice, while γ-Zn₃(VO₄)₂ has a monoclinic C lattice.     

Experimental and theoretical study of visible light driven scandium (III) doped ZnO for antibacterial activity

ZnO was doped with Sc(III) ions to obtain a low-cost and environment-friendly antibacterial material with highly synergistic antimicrobial activity. The combination of experimental results and theoretical insights was used to describe the effect of Sc doping on the electronic and structural properties of ZnO. Sc(III)-doped ZnO materials with different Sc(III) contents were deposited on white carbon black (WCB) by a facile sol-gel method. The Sc(III) doped antibacterial materials were characterized by FESEM, EDX, HR-TEM, BET, XPS, XRD, ICP-OES, UV–visible spectroscopy, Fastsage and Materials Studio (MS). The antibacterial activities of Zn WCB and Zn–Sc WCB were determined by counting Escherichia coli (E. coli) and Staphylococcus aureus (S.aureus) colonies on bacterial culture plates. The results show that the specific surface area of Sc-doped Zn on WCB was increased by 31.9 m²/g compared to Zn WCB. The optimum doping ratio of Zn and Sc was determined in Zn_0.9574Sc_0.0426O cut from hexagonal wurtzite structure ZnO. Moreover, pure ZnO and Zn_0.9574Sc_0.0426O models were established by density functional theory (DFT). The experimental results and DFT calculations demonstrated that ZnO WCB possessed excellent antibacterial properties after doping with Sc. This improved antibacterial activity was due to the effects of Sc₂O₃ on the ZnO lattice, which resulted in the generation of excess reactive oxygen species (ROS).     

The nitridation of ZnO nanowires

ZnO nanowires (NWs) with diameters of 50 to 250 nm and lengths of several micrometres have been grown by reactive vapour transport via the reaction of Zn with oxygen on 1 nm Au/Si(001) at 550°C under an inert flow of Ar. These exhibited clear peaks in the X-ray diffraction corresponding to the hexagonal wurtzite crystal structure of ZnO and a photoluminescence spectrum with a peak at 3.3 eV corresponding to band edge emission close to 3.2 eV determined from the abrupt onset in the absorption-transmission through ZnO NWs grown on 0.5 nm Au/quartz. We find that the post growth nitridation of ZnO NWs under a steady flow of NH₃ at temperatures ≤600°C promotes the formation of a ZnO/Zn₃N₂ core-shell structure as suggested by the suppression of the peaks related to ZnO and the emergence of new ones corresponding to the cubic crystal structure of Zn₃N₂ while maintaining their integrity. Higher temperatures lead to the complete elimination of the ZnO NWs. We discuss the effect of nitridation time, flow of NH₃, ramp rate and hydrogen on the conversion and propose a mechanism for the nitridation.     

Role of vanadium ions,oxygen vacancies,and interstitial zinc in room temperature ferromagnetism on ZnO-V2O5 nanoparticles

In this work, we present the role of vanadium ions (V⁺⁵ and V⁺³), oxygen vacancies (V_O), and interstitial zinc (Zn_i) to the contribution of specific magnetization for a mixture of ZnO-V₂O₅ nanoparticles (NPs). Samples were obtained by mechanical milling of dry powders and ethanol-assisted milling for 1 h with a fixed atomic ratio V/Zn?=?5% at. For comparison, pure ZnO samples were also prepared. All samples exhibit a room temperature magnetization ranging from 1.18?×?10⁻³ to 3.5?×?10⁻³ emu/gr. Pure ZnO powders (1.34?×?10⁻³ emu/gr) milled with ethanol exhibit slight increase in magnetization attributed to formation of Zn_i, while dry milled ZnO powders exhibit a decrease of magnetization due to a reduction of V_O concentration. For the ZnO-V₂O₅ system, dry milled and thermally treated samples under reducing atmosphere exhibit a large paramagnetic component associated to the formation of V₂O₃ and secondary phases containing V⁺³ ions; at the same time, an increase of V_O is observed with an abrupt fall of magnetization to σ?~?0.7?×?10⁻³ emu/gr due to segregation of V oxides and formation of secondary phases. As mechanical milling is an aggressive synthesis method, high disorder is induced at the surface of the ZnO NPs, including V_O and Zn_i depending on the chemical environment. Thermal treatment restores partially structural order at the surface of the NPs, thus reducing the amount of Zn_i at the same time that V₂O₅ NPs segregate reducing the direct contact with the surface of ZnO NPs. Additional samples were milled for longer time up to 24 h to study the effect of milling on the magnetization; 1-h milled samples have the highest magnetizations. Structural characterization was carried out using X-ray diffraction and transmission electron microscopy. Identification of V_O and Zn_i was carried out with Raman spectra, and energy-dispersive X-ray spectroscopy was used to verify that V did not diffuse into ZnO NPs as well to quantify O/Zn ratios.     

Effect of Zn migration on the thermoelectric properties of Zn4Sb3 material

β-Zn₄Sb₃ is interesting as thermoelectric material at moderate temperature due to the extreme low thermal conductivity. Recent success in energy band engineering or nano-engineering led to a significant improvement in the thermoelectric properties of β-Zn₄Sb₃. In this work, we utilize the direct current to drive the migration of Zn by designing of sintering mould. Obvious Zn migration under the direct current applied in the plasma activated sintering (PAS) process is found in Zn₄Sb₃ compounds, and Zn exhibits significantly heterogeneous gradient composition distribution. At the top of sample, the single-phase Zn₄Sb₃ decomposes into ZnSb phase because of the loss of Zn, while Zn originated from lattice and interstitial sites in Zn₄Sb₃ is abundant in the bottom. The temperature-dependent transport measurements are also carried out at 323–673 K. Zn migration has a huge influence on the thermoelectric properties because of the sensitivity of Zn₄Sb₃. The maximum power factor can reach ~ 1.44 mW m⁻¹ K⁻² at 673 K due to the high Seebeck coefficient and low resistivity, which is one of the highest values in the reported results. The resulting peak ZT value of ~ 1.2 at 673 K is obtained. To control the Zn distribution by tuning the current is a feasible approach to improve the thermoelectric properties of Zn₄Sb₃ material.     

Co-doping effect of Zn and Sb in SnO2: Valence stabilization of Sb and expanded solubility limit

In this study, the co-doping effect of ZnO and Sb₂O₅ on the solubility limit in SnO₂ was investigated. When ZnO was added to SnO₂, its solubility limit was around 3 at%, while that of Sb₂O₅ could not be evaluated due to the severe evaporation of Sb₂O₅ during sintering. For the co-doping of ZnO and Sb₂O₅, the ZnSb₂O₆ phase was used for the source of Zn and Sb dopants to prevent the evaporation of Sb₂O₅. When ZnO and Sb₂O₅ were co-doped by ZnSb₂O₆, the solubility limit expanded to 60 at%. XPS analysis of the Sb revealed that Sb⁵⁺ is stable when Zn is co-doped. The extended solubility limit is explained by electrostatic and strain energy minimization in the lattice.     

Integrated experimental phase equilibria study and thermodynamic modelling of the binary ZnO–Al2O3, ZnO–SiO2, Al2O3–SiO2 and ternary ZnO–Al2O3–SiO2 systems

Phase equilibria of the ZnO–SiO₂, Al₂O₃–SiO₂ and ZnO–Al₂O₃–SiO₂ systems at liquidus were characterized at 1340–1740 °C in air. The ZnO–Al₂O₃ subsolidus phase equilibria were derived from the experiments with the SiO₂- and CaO + SiO₂-containing slags. High-temperature equilibration on silica or platinum substrates, followed by quenching and direct measurement of Zn, Al, Si and Ca concentrations in the phases with the electron probe X-ray microanalysis (EPMA) was used to accurately characterize the system. Special attention was given to zincite phase that was shown to consist of two separate ranges of compositions: round-shaped low-Al zincite (<2 mol.% AlO_1.5) and platy high-Al zincite (4–11 mol.% AlO_1.5). A technique was developed for more accurate measurement of the ZnO solubility in the low-ZnO phases (corundum, mullite, tridymite and cristobalite) surrounded by the ZnO-containing slag, using l-line for Zn instead of K-line, avoiding the interference of secondary X-ray fluorescence. Solubility of ZnO was found to be below 0.03 mol.% in corundum and cristobalite, and below 0.3 mol.% in mullite. Present experimental data were used to obtain a self-consistent set of parameters of the thermodynamic models for all phases in this system using FactSage computer package. The modified quasichemical model with two sublattices (Zn²⁺, Al³⁺, Si⁴⁺) (O^2?) was used for the liquid slag phase; the compound energy formalism was used for the spinel (Zn²⁺,Al³⁺)[Zn²⁺,Al³⁺,Va]₂O^2-₄ and mullite Al³⁺₂(Al³⁺,Si⁴⁺) (O^2?,Va)₅ phases; the Bragg-Williams formalism was used for the zincite (ZnO, Al₂O₃); other solid phases (tridymite and cristobalite SiO₂, corundum Al₂O₃, and willemite Zn₂SiO₄) were described as stoichiometric. Present study is a part of the research program on the characterization of the multicomponent Pb–Zn–Cu–Fe–Ca–Si–O–S–Al–Mg–Cr–As–Sn–Sb–Bi–Ag–Au–Ni system.     

Simultaneously enhanced electrical stability and nonlinearity in ZnO varistor ceramics: Role of Si-stabilized δ-Bi2O3 phase

In the present study, simultaneously enhanced electrical stability (low degradation rate of 8.0 × 10^?3 mA? h^1/2) and high nonlinear coefficient of 56 were obtained in ZnO varistors by doping SiO₂. To clarify the mechanism of enhanced properties, comprehensive microscopic analyses were studied. Particularly, the intrinsic point defects were quantitatively characterized for the first time. Results showed that the densities of zinc interstitials (Zn_i) and oxygen vacancies (V_o) were dramatically decreased, resulting in enhanced stability. Besides, reduced Zn_i and V_o decreased the total donor density, contributing to the improved barrier height and thus leading to enhanced nonlinearity. Combined with XRD and SEM results, it is deduced that such reduced Zn_i and V_o are attributed to the Si-stabilized high oxygen conducting δ-Bi₂O₃ phase. Furthermore, this elucidated mechanism, which has been long neglected in Si-doped varistors, may provide valuable insights into further developing high-performance ZnO varistors.     

Role of defects in tailoring optical,electronic, and luminescent properties of Cu-doped ZnO films

We present a detailed characterization study on copper-doped ZnO films to correlate the films' electronic and optical properties with the existing native defects in the lattice. In addition, we describe the variation in the concentration of these defects with Cu dopant and temperature. The results of XRD confirmed the single-phase würtzite-structure of the synthesized films. The SEM images showed a homogeneous and dense grain morphology with a granular form and a signature for a hexagonal-like shape. The EDX, XPS, and UV–Vis spectra showed the proper doping of Cu ions into the lattice. The XPS analysis indicated mixed electronic states of both Cu²⁺ and Cu¹⁺ and showed a clear increase in the Cu²⁺ intensity relative to Cu¹⁺, with Cu dopant. The transmittance spectra exhibited an average value above 80% in all doped films in the visible and infrared regions. The overall results indicated a clear link between the films’ optical and electronic responses and the level of the intrinsic defects in the lattice. By increasing the Cu dopant, we find a slight reduction in the energy bandgap (E_g). This is correlated with a clear reduction in the blue emission luminescence band associated with the V_Zn and in the yellow emission band associated with the O_i. On the other hand, we observed a clear enhancement in the green emission band originating from the V_O, and in the emission band related to possible transitions from Zn_i levels to O_i levels. The slight reduction in the E_g signals a weak sp-d hybridization between the ZnO conduction band electrons and the Cu²⁺ ions, which is mediated by the intrinsic defects. With reducing the temperature, the photoluminescence temperature profiles indicated a slight increase in the E_g values and a negligible effect on the distribution of the native defects.     

Growth and Characterization of ZnO, SnO2 and ZnO/SnO2 Nanostructures from the Vapor Phase

Zinc oxide (ZnO), tin dioxide (SnO₂) and compounds ZnO/SnO₂ (ZTO) nanostructures have been synthesized successfully from the vapor phase without a catalyst using three different approaches. XRD analyses showed that ZnO with a wurtzite crystal structure, SnO₂ with a rutile crystal structure and zinc stannate (ZnSnO₃) and/or dizinc stannate (Zn₂SnO₄) were condensed from the vapor phase when Zn and/or Sn metal powders or their oxides individually or mixed were used as the starting materials. The formation of either zinc or dizinc stannate was controlled by the Zn/Sn ratio and growth technique. SEM and TEM investigations showed that ZnO grew mainly in the form of wires, rods and belts. These are believed to be originated from the common tetrapod structure of ZnO. While SnO₂ grew in the form of tetragonal rods with rectangle-like cross section and nanoparticles, ZTO grew in the form of nanobelts. The final length, width and thickness were as low as 40, 10 and 5 nm, respectively. The driving forces for growth of nanowires, nanorods, nanobelts, and nanoparticles were found to be vapor density or supersaturation, temperature, pressure and location of deposition from the source materials. The optical absorbance and photoluminescence spectra of all samples showed excitonic character at room temperature implying good crystal quality, and high photocurrent properties suggesting possible applications in nanoscaled functional devices such as optoelectronics and gas sensors.     

Understanding the thermal evolution of defects in carbon-implanted ZnO single crystal

Defects and impurities play a major role in controlling the electrical and optical properties of semiconductor materials. Herein, hydrothermally grown ZnO single crystals have been implanted with carbon (C) dopants at room temperature and then annealed in argon atmosphere at various temperatures between 400 and 800 °C. The thermal evolution of C-related defects and their effects on the structural, optical and electrical properties of ZnO single crystals were systematically characterized and discussed. The results show ion implantation induces serious lattice disorder, and post-implantation annealing could promote the lattice renormalization, accompanied by an increase in crystal quality and average visible transmittance. Furthermore, it is found that the diffusion of octahedral carbon interstitial (C_i) along parallel to c-axis facilitates the growth of carbon sp² clusters due to its low migration barrier during annealing, which energetically contribute to the decrease of the resistivity. Meanwhile, abundant C_i will be able to enter into V_Zn to form C_Zn or combine with lattice O to form (C_O)_O donor defects upon annealing, dominating the increase of electron carrier concentration and enhancing the anomalous Raman mode at 510-525 cm⁻¹. These findings strengthen the fundamental understanding of the donor behavior of C impurities in ZnO.     

Vulcanization of chlorobutyl rubber. II. A revised cationic mechanism of ZnO/ZnCl2 initiated crosslinking

Data relating to the ZnO/ZnCl₂‐accelerated vulcanization of chlorinated poly(isoprene‐coisobutylene) (CIIR or chloro‐butyl) is examined. ZnCl₂ and conjugated diene butyl units on the polymer chain are both precursors to crosslinking, and a revised cationic mechanism is proposed to account for crosslinking, taking into account the involvement of conjugated diene butyl in the process. It is demonstrated that Zn₂OCl₂ will catalyze dehydrohalogenation, and the formation of catalytic amounts of Zn₂OCl₂ by the reaction of ZnCl with ZnO, followed by H⁺ abstraction to give Zn₂OCl₂ and HCl, is essential in the overall crosslinking reaction sequence. The HCl is trapped by ZnO as ZnCl₂. It is proposed that the abstraction by Zn₂OCl₂ of HCl in a concerted reaction leads to Zn(OH)Cl and ZnCl₂. Zn(OH)Cl remains in the polymer as an unextractable salt, while 50% of the chlorine in the rubber is extracted as ZnCl₂ when compounds reach their equilibrium crosslink density. ZnCl₂ initiates crosslinking by the abstraction of chlorine from the chain, but a crosslink will only result when a carbocation on a dechlorinated isoprenoid unit is close to a conjugated diene butyl on an adjacent chain; if not, dehydrohalogenation will result in the formation of a further conjugated diene butyl unit at that point in the chain. The maximum crosslink density achieved is only 1/4 that theoretically possible, as crosslinking restricts chain movement and limits the number of chance meetings between carbocations on the polymer and conjugated diene butyl units. Zinc stearate promotes dehydrohalogenation, ZnCl₂ being the only chloro‐zinc salt formed. Reversion occurs in compounds where there is insufficient ZnO to trap all of the chlorine present in the rubber. HCl per se does not attack the polymer, but promotes reversion only in the presence of carbocations on the chain, i.e., during the crosslinking process. Trapping of HCl by ZnO prevents reversion. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2302–2310, 2000     

SURFACE MODIFICATION OF HYDROTHERMALLY GROWN ZnO NANOSTRUCTURES WITH PROCESS PARAMETERS

ZnO nanorods (NRs) were hydrothermally synthesized by using equimolar zinc nitrate hydrate (Zn(NO₃)₂ [sdot] 6H₂O) and hexamethylenetetramine (C₆H₁₂N₄) solutions. The shape of the nanostructures, obtained by aqueous method, was greatly influenced by the growth temperature and the molar concentrations. NRs grown at higher temperature (90°C) have rounded tips, whereas nanostructures of hexagonal flat-end shape were obtained at 75°C. Hardly any nanostructures were observed by further reducing the temperature to 60°C. In addition, solutions with higher molarity favored the appearance of nanoflowers. Scattered ZnO NRs were observed on silicon substrate, whereas aligned ZnO nanowires (NWs) 50–70 nm in diameter were obtained at 75°C by introducing sputtered ZnO film as a seed layer. High-resolution transmission electron microscopy (HRTEM) confirmed the growth of ZnO nanowires along [001] direction. A band-edge luminescence along with a broad visible spectrum was observed for the ZnO nanowires.     

Pyridine adsorption reveals high-coordinated cationic centres at the surface of microcrystalline ZnO

The adsorption of pyridine at 300 K on microcrystalline ZnO has been investigated by FTIR spectroscopy. Besides an interaction by H-bonding with surface OH groups, evidence is found for the Lewis coordination of pyridine onto coordinatively unsaturated surface Zn²⁺ centres possessing a regular tetrahedral coordination, as well as onto some other surface Zn²⁺ centres, only slightly uncoordinated. The creation at the surface of the ZnO of cationic centres with an anomalous coordination higher than 4 (quasi-octahedral) is thus postulated. No such high-coordinated Zn²⁺ centres seem to form to an appreciable extent when ZnO is dispersed (at least at up to a 3% weight level) at the surface of other microcrystalline oxides such as, for instance, TiO₂.     

Photoluminescence of (ZnO)X-Z(SiO2)Y:(MnO)Z green phosphors prepared by direct thermal synthesis: The effect of ZnO/SiO2 ratio and Mn2+ concentration on luminescence

Green light emitting Zn₂SiO₄:Mn²⁺ phosphors have been synthetised by the solid-state reaction in ambient atmosphere at 1300 °C for 2 h, with ZnO, SiO₂ and MnO₂ as the reagents. The ZnO/SiO₂ molar ratio varied from 2 to 0.5. The doping level was in a lower concentration range (0.01≤x≤0.05). The effect of both the Mn²⁺ concentration and ZnO/SiO₂ molar ratio on luminescence intensity and decay was investigated in detail. The microstructure and phase composition of prepared phosphors were characterised by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). XRD results indicate that the pure α-Zn₂SiO₄ phase with rhombohedral structure was obtained after heat treatment. The prepared phosphors exhibit a strong green emission centred at 525 nm from the ⁴T₁→⁶A₁ forbidden transition. The highest emission intensity was observed for phosphors with ZnO/SiO₂ molar ratio equal to 1.0, and the Mn²⁺ concentration x=0.03 (ZSMn3). The emission intensity of the ZSMn3 phosphor is comparable with the commercial Zn₂SiO₄:Mn²⁺ phosphor. The decay curves can be characterised by double exponential function. After fitting a fast component τ₁∼2 ms and a slow component τ₂∼10 ms were obtained. The decay times decrease significantly with increasing Mn²⁺ concentration. The decay time and luminescence mechanism depend on the excitation light wavelength. Temperature dependent luminescence of the ZSMn3 phosphor in the temperature range of 25–200 °C was studied.     

Photoluminescence of hydrothermally epitaxied ZnO films

In this study, epitaxial ZnO films were grown hydrothermally on (1 1 1)-oriented single crystal MgAl₂O₄ substrates at 150 °C from aqueous precursor solutions. It was observed that the film morphology varied with the pH value of the precursor solution, giving pitted films at higher pH and smooth films at lower pH. The photoluminescence spectra of these ZnO films showed a strong near band-edge ultraviolet emission together with deep level emission bands comprised of green and orange-red luminescence. The green band centred around 500 nm was attributed to the presence of Zn vacancies, whereas the orange-red band centred around 650 nm could be related to the presence of oxygen interstitials.