Low-temperature synthesis of Zn2SiO4:Mn green photoluminescence phosphor

Zn₂SiO₄:Mn green phosphor having comparable photoluminescence (PL) efficiency with commercial phosphor has been synthesized at 1000 °C using solid state reactions involving ZnO, silicic acid and manganese acetate. The water of crystallization attached to SiO₂ in silicic acid whose dissociation at 1000 °C seem to promote the sintering efficiency of Zn₂SiO₄:Mn. Incremental ZnO addition and re-firing at 1000 °C promote the diffusion rate of ZnO and SiO₂. The formation of a single crystalline phase of willemite structure in the samples was confirmed by powder XRD measurements. The phosphor exhibit an intense excitation band centered around 275 nm and a relatively weak excitation centered around 380 nm while the broad band green emission peaks at 524 nm. Other parameters studied include PL spectra, grain morphology, ZnO/SiO₂ molar ratio, Mn concentration, co-dopant/flux and the effect of chemical forms of Mn dopant as well as silica on the PL efficiency.     

Sol–gel synthesis and luminescent properties of SiO2/Zn2SiO4 and SiO2/Zn2SiO4:V composite materials

Undoped and vanadium-doped Zn₂SiO₄ particles embedded in silica host matrix were prepared by a simple solid-phase reaction after the incorporation of ZnO and ZnO:V nanoparticles, respectively, in silica monolith using the sol–gel method with supercritical drying of ethyl alcohol in two steps. After supercritical drying and annealing in the temperature range between 1423 and 1473 K in an air atmosphere, the photoluminescence (PL) measurements show a band centered at about 760 nm in the case of non-doped Zn₂SiO₄ which is attributed to energy transfer from Zn₂SiO₄ particles to NBOHs interface defects. In the case of vanadium doped Zn₂SiO₄, the PL reveals a band centered at about 540 nm attributed to the vanadium in the interfaces between Zn₂SiO₄ particles and SiO₂ host matrix. Photoluminescence excitation (PLE) measurements show different origins of the emission bands. The PLE band (～240–350 nm) may be understood as an energy transfer process from O^2? to V⁵⁺ which occurs intrinsically in the vanadyl group.     

Luminescence properties of green emission of SiO2/Zn2SiO4:Mn nanocomposite prepared by sol–gel method

Green light emitting Mn²⁺ doped Zn₂SiO₄ particles embedded in SiO₂ host matrix were synthesized by a sol–gel method. After the incorporation of ZnO:Mn nanoparticles in a silica monolith using sol–gel method with supercritical drying of ethyl alcohol in two steps, it was heat treated in air at 1200 °C for 2 h in order to obtain the SiO₂/α-Zn₂SiO₄:Mn nanocomposites. The microstructure of phosphor crystals was characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). XRD results indicate that the pure phase α-Zn₂SiO₄ with rhombohedral structure was obtained after thermal treatment at 1200 °C. The SiO₂/α-Zn₂SiO₄:Mn nanocomposites with a Mn doping concentration of 1.5 at% exhibit two broadband emissions in the visible range: a strong green emission at around 525 nm and a second one in the range between 560 and 608 nm. This nanocomposite with a Mn doping concentration of 0.05 shows the highest relative emission intensity. Upon 255 nm excitation, the luminescence decay time of the green emission of Zn₂SiO₄:Mn around 525 nm is 11 ms. The luminescence spectra at 525 nm (⁴T₁–⁶A₁) and lifetime of the excited state of Mn²⁺ ions-doped Zn₂SiO₄ nanocrystals are investigated.     

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.     

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.     

Emission of Cu-related complexes in ZnO:Cu nanocrystals

Photoluminescence (PL), its temperature dependence, scanning electronic microscopy (SEM) and X ray diffraction (XRD) have been applied for the comparative study of varying the emission, morphology and crystal structure of ZnO and ZnO:Cu nanocrystals (NCs) versus technological routines, as well as the dependence of ZnO:Cu NC parameters on the Cu concentration. A set of ZnO and ZnO Cu NCs was prepared by the electrochemical (anodization) method at a permanent voltage and different etching durations with follows thermal annealing at 400 °C for 2 h in ambient air. The size of ZnO NCs decreases from 300 nm×540 nm down to 200 nm×320 nm with etching duration increasing. XRD study has confirmed that thermal annealing stimulates the ZnO oxidation and crystallization with the formation of wurtzite ZnO crystal lattice. XRD method has been used for monitoring the lattice parameters and for confirming the Cu doping of ZnO Cu NCs. In ZnO Cu NCs four defect related PL bands are detected with the PL peaks at 1.95–2.00 eV (A), 2.15-2.23  eV (B), 2.43–2.50 eV (C) and 2.61–2.69 eV (D). Highest PL intensities of orange, yellow and green emissions have been obtained in ZnO Cu NCs with the Cu concentration of 2.28 at%. At Cu concentration increasing (≥2.28 at%) the PL intensities of the bands A, B, C decrease and the new PL band peaked at 2.61–2.69 eV at 10 K appears in the PL spectrum. The variation of PL intensities for all PL bands versus temperature has been studied and the corresponding activation energies of PL thermal decay have been estimated. The type of Cu-related complexes is discussed using the correlation between the PL spectrum transformation and the variation of XRD parameters in ZnO Cu NCs.     

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.     

Effect of heat treatments on the luminescence properties of Zn2SiO4:Mn2+ phosphors prepared by glycothermal methods

Manganese-doped Zn₂SiO₄ phosphors with different crystal structures and morphologies were synthesized by glycothermal reactions of zinc acetate dihydrate and manganese(II) acetate tetrahydrate with tetraethyl orthosilicate in various glycols at 315 °C. The reactions in 1,3-propanediol and 1,4-butanediol yielded α-Zn₂SiO₄:Mn²⁺, whereas the reactions in ethylene glycol and 1,5-pentanediol yielded β-Zn₂SiO₄:Mn²⁺ and ZnO, respectively. The samples obtained in 1,4-butanediol and 1,3-propanediol emitted green light (522 nm), and the sample prepared in 1,4-butanediol showed a higher emission intensity. The photoluminescence intensity of the Zn_1.96Mn_0.04SiO₄ phosphor prepared by a glycothermal reaction in 1,4-butanediol and subsequently calcined at 1100 °C was twice as high as that of the sample synthesized by a conventional solid-state reaction. The high emission efficiency was obtained because the highly homogeneous distribution of Mn²⁺ in the α-Zn₂SiO₄ host synthesized by the glycothermal reaction was maintained during calcination treatment in air.     

Zinc silicates with tunable morphology by surfactant assisted sonochemical route suitable for NUV excitable white light emitting diodes

The cationic surfactants assisted ultrasound route was used to prepare Dy³⁺ doped Zn₂SiO₄ nanophosphors. The final products were characterized by powder X-ray diffraction (PXRD), ultraviolet visible spectroscopy, scanning electron microscopy, transmission electron microscopy and photoluminescence. Orthorhombic phase of Zn₂SiO₄:Dy³⁺ (JCPDS card No. 35-1485) was confirmed from PXRD. It was evident that the morphology of spherical and broom like structures were obtained with epigallocatechin gallate (EGCG) and cetyltrimethylammonium bromide (CTAB) surfactants respectively. Further the size and agglomeration of the products were varied with surfactants concentration, sonication time, pH and sonication power. The probable formation mechanisms to obtain various micro/nano superstructures were discussed. The characteristic PL peaks were observed at 484, 574 and 666 nm due to the electronic transitions ⁴F_9/2 → ⁶H_j (j = 15/2, 13/2, 11/2) of Dy³⁺ ions upon excited at NUV pumping wavelength of 350 nm [⁶H_15/2 → ⁶P_7/2 (⁴M_15/2)]. The Judd–Ofelt intensity parameters and radiative properties were estimated by using PL emission data. The photometric studies indicated that the obtained phosphors could be promising materials in white light emitting diodes (wLED’s). The present synthesis route was rapid, environmentally benign, cost-effective and useful for industrial applications such as solid state lighting and display devices.     

Structural and optical properties of Co-doped ZnO nanocrystallites prepared by a one-step solution route

Zinc oxide (ZnO) nanocrystallites with different Co-doping levels were successfully synthesized by a simple one-step solution route at low temperature (95 °C) in this study. The structure and morphology of the samples thus obtained were characterized by XRD, EDS, XPS and FESEM. Results show that cobalt ions, in the oxidation state of Co²⁺, replace Zn²⁺ ions in the ZnO lattice without changing its wurtzite structure. The dopant content varies from 0.59% to 5.39%, based on Co-doping levels. The pure ZnO particles exhibit well-defined 3D flower-like morphology with an average size of 550 nm, while the particles obtained after Co-doping are mostly cauliflower-like nanoclusters with an average size of 120 nm. Both the flower-like pure ZnO and the cauliflower-like Co:ZnO nanoclusters are composed of densely arrayed nanorods. The optical properties of the ZnO nanocrystallites following Co-doping were also investigated by UV–Visible absorption and Photoluminescence spectra. Our results indicate that Co-doping can change the energy-band structure and effectively adjust the luminescence properties of ZnO nanocrystallites.     

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.     

100 MeV Si8+ ion induced luminescence and thermoluminescence of nanocrystalline Mg2SiO4:Eu3+

Nanoparticles of Mg₂SiO₄:Eu³⁺ have been prepared by the solution combustion technique and the grain size estimated by PXRD is found to be in the range 40–50 nm. Ionoluminescence (IL) studies of Mg₂SiO₄:Eu³⁺ pellets bombarded with 100 MeV Si⁸⁺ ions with fluences in the range 1.124–22.48×10¹² ions cm^?2 are carried out at IUAC, New Delhi, India. Five prominent IL bands with peaks at 580 nm, 590 nm, 612 nm, 655 nm and 705 nm are recorded. These characteristic emissions are attributed to the luminescence centers activated by Eu³⁺ cations. It is found that IL intensity decreases rapidly in the beginning. Later on, the intensity decreases slowly with further increase of ion fluence. The reduction in the ionoluminescence intensity with increase of ion fluence might be attributed to degradation of Si–O (ν₃) and Si–O (2ν₃) bonds present on the surface of the sample. The red emission with peak at 612 nm is due to characteristic emission of ⁵D₀→⁷F₂ of the Eu³⁺ cations. Thermoluminescence (TL) studies of Mg₂SiO₄:Eu³⁺ pellets bombarded with 100 MeV Si⁸⁺ cations with fluences in the range 5×10¹¹ ions cm^?2 to 5×10¹³ ions cm^?2 are made at RT. Two prominent and well resolved TL glows with peaks at ～220 °C and ～370 °C are observed. It is observed that TL intensity increases with increase of ion fluence. This might be due to creation of new traps during swift heavy ion irradiation.     

Optical and electrical properties of Ge-implanted SiO2 layers on n-Si and p-Si

Ge ions of 100 keV were implanted into a 120 nm-thick SiO₂ layer on n-Si at room temperature while those of 80 keV were into the same SiO₂ layer on p-Si. Samples were, subsequently, annealed at 500°C for 2 h to effectively induce radiative defects in the SiO₂. Maximum intensities of sharp violet photoluminescence (PL) from the SiO₂/n-Si and the SiO₂/p-Si samples were observed when the samples have been implanted with doses of 1×10¹⁶ and 5×10¹⁵ cm⁻², respectively. According to current–voltage (I–V) characteristics, the defect-related samples exhibit large leakage currents with electroluminescence (EL) at only reverse bias region regardless of the type of substrate. Nanocrystal-related samples obtained by an annealing at 1100°C for 4 h show the leakage at both the reverse and the forward region.     

Photoluminescence properties of Er-doped β-FeSi2 grown by ion implantation

Photoluminescence (PL) properties of Er-doped β-FeSi₂ (β-FeSi₂:Er) and Er-doped Si (Si:Er) grown by ion implantation were investigated. In PL measurements at 4.2 K, the β-FeSi₂:Er showed the 1.54 μm PL due to the intra-4f shell transition of ⁴I_13/2→⁴I_15/2 in Er³⁺ ions without a defect-related PL observed in Si:Er. In the dependence of the PL intensity on excitation photon flux density, the obtained optical excitation cross-section σ in β-FeSi₂:Er (σ=7×10⁻¹⁷ cm²) is smaller than that in Si:Er (σ=1×10^-15 cm²). In the time-resolved PL and the temperature dependence of the PL intensity, the 1.54 μm PL in β-FeSi₂:Er showed a longer lifetime and larger activation energies for non-radiative recombination (NR) processes than Si:Er. These results revealed that NR centers induced by ion implantation damage were suppressed in β-FeSi₂:Er, but the energy back transfer from Er³⁺ to β-FeSi₂ was larger than Si:Er.     

Preparation and photoluminescence properties of the Sr1.56Ba0.4SiO4:0.04Eu2+ phosphor

The Sr_1.56Ba_0.4SiO₄:0.04Eu²⁺ phosphors were prepared via a combustion reaction and following the calcination method at low temperature. The influences of the amount of the uncommonly used SrCl₂ flux, different calcination temperatures and time on the structure and the photoluminescence (PL) properties of the phosphors were investigated. Under the excitation of 450 nm blue light, the phosphor shows the intense broad emission band from 490 nm to 650 nm, and the emission peak is centered at 553 nm. The luminescence intensity of Sr_1.56Ba_0.4SiO₄:0.04Eu²⁺ was very sensitive to the crystallinity and morphology characteristics of the phosphor. The phosphor calcined at 950 °C for 3 h in 20%H₂/80%Ar atmosphere exhibits improved PL properties due to its high crystallinity and excellent morphology characteristics. The use of the SrCl₂ flux provides a novel way to improve the crystallinity of the silicates phosphors at low preparation temperature.     

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.     

Interface Mn nanoclusters in YMnO3/Si ferroelectric gate structures revealed by electron magnetic resonance

In this work, systematic X-band electron magnetic resonance (EMR) studies for YMnO₃/Si ferroelectric gate structures were performed to trace a variation of interface characteristics as different sputtering condition of O₂/(Ar + O₂) ratio. Our result showed that the EMR signal intensities were increased with increasing O₂/(Ar + O₂) ratio. In addition, it was suggested from detailed analyses that the observed EMR signals could be originated from Mn nanoclusters existing in both the polycrystalline Y₂O₃ layer and the amorphous Si-enriched Y–Si interface layer in YMnO₃/Si thin film structure. And also, a correlation between the decrease of crystallinity in YMnO₃/Si film and the content of Mn nanoclusters within the polycrystalline Y₂O₃ layer and/or the amorphous Y–Si layer was discussed.     

Attenuation lengths of high energy photoelectrons in compact and mesoporous SiO2 films

We have experimentally evaluated attenuation lengths (AL) of photoelectrons traveling in compact and micro and mesoporous (~ 45% voids) SiO₂ thin films with high (8.2–13.2 keV) kinetic energies. The films were grown on polished Si(100) wafers. ALs were deduced from the intensity ratio of the Si 1s signal from the SiO₂ film and Si substrate using the two-peaks overlayer method. We obtain ALs of 15–22 nm and 23–32 nm for the compact and porous SiO₂ films for the range of kinetic energies considered. The observed AL values follow a power law dependence on the kinetic energy of the electrons where the exponent takes the values 0.81 ± 0.13 and 0.72 ± 0.12 for compact and porous materials, respectively.     

Synthesis and photoluminescence of core–shell structured spherical SiO2@YPO4:Tb3+ phosphors via sol–gel process

Monodispersed SiO₂@YPO₄:Tb³⁺ core–shell submicrospheres were prepared through a simply homogeneous sol–gel method. The resulted SiO₂@YPO₄:Tb³⁺ core–shell particles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence spectra (PL) and kinetic decays. The XRD results demonstrate that the YPO₄:Tb³⁺ layers begin to crystallize on the SiO₂ spheres after annealing at 500 °C and the crystallinity increases with raising the annealing temperature. The FTIR spectra show that the YPO₄:Tb³⁺ shell has linked to the silica surface through forming a Si–O–Y bond. SEM and TEM analysis indicate that SiO₂@YPO₄:Tb³⁺ core–shell submicrospheres have the regular microstructures and uniform size distributions. The emission spectra of the obtained submicrospheres are dominated by ⁵D₄–⁷F₅ transition of Tb³⁺ (545 nm, green), and the emission intensities of Tb³⁺ increase with increasing the annealing temperatures and the number of coating cycles. The optimum concentration for Tb³⁺ was determined to be 5 mol % of Y³⁺ in YPO₄ host.     

In-situ formation of SiC nanocrystals by high temperature annealing of SiO2/Si under CO: A photoemission study

We have studied CO interaction with SiO₂/Si system at high temperature (~ 1100 °C) and 350 mbar by core-level photoemission. Even for short annealing time (5 min) the signal from Si2p and C1s core levels shows a clear change upon CO treatment. Shifted components are attributed to formation of SiC. This is confirmed by TEM imaging which further shows that the silicon carbide is in the form of nano-crystals of the 3C polytype. Photoemission spectroscopy moreover reveals the formation of silicon oxicarbide which could not be evidenced by other methods. Combining these results with previous Nuclear Resonance Profiling study gives a deeper insight into the mechanisms involved in the nanocrystals growth and especially for the reaction equation leading to SiC formation. We show that CO diffuses as a molecule through the silica layer and reacts with the silicon substrate according the following reaction: 4 CO + 4 Si → SiO₂ + 2SiC + SiO₂C₂.