Localization of Mn2+ ions in mesoporous NnS

Nanocrystalline cubic ZnS doped with 0.2% mol manganese, exhibiting a stable mesoporous structure, was synthesized at room temperature by a non toxic surfactant-assisted liquid-liquid reaction. The X-ray diffraction measurements demonstrate the formation of a sponge-like mesoporous material built from cubic ZnS nanocrystals of 1.8 nm average sizes, with a tight distribution of pores of 1.8 nm mean diameter. The transmission electron microscopy images confirm the formation of the mesoporous structure with walls of 3.1 nm mean thickness built from cubic ZnS nanocrystallites of 2.1 nm average size. The resulting tight distribution of crystallites and pores yields a well resolved Electron Paramagnetic Resonance spectrum, with the narrowest reported component lines attributed to three types of isolated Mn2+ centers, called Mn2+(I), Mn2+(II) and Mn2+(III). From the analysis of the spin Hamiltonian parameters it is shown that in the Mn2+(I) centers the paramagnetic ion is situated at substitutional Zn sites in the ZnS nanocrystals, being also subjected to a small axial distortion. The relative concentration changes under thermal treatment experiments strongly suggest that in both Mn2+(II) and Mn2+(III) centers the Mn2+ ion is localized on the surface of the ZnS nanocrystallites, being bond to an oxygen ion in the first case and to an additional water molecule in the second case.     

EPR study of ZnS: Mn2+ nanocrystals in Pyrex glasses

ZnS: Mn2+ nanocrystals embedded in Pyrex glasses were spectrally studied using EPR and photoluminescence techniques. Photoluminescence (PL) and excitation (PLE) spectra revealed that manganese impurities can be classified as two types of luminescent centers, i.e., occupying substitutional sites (Mn2+)sub or interstitial sites (Mn2+)int. Three types of manganese sites of (Mn2+)sub, (Mn2+)int, and Mn clusters were identified by the EPR spectra. An increase of the g1 factor and hyperfine structure (HFS) constant with decreasing sizes of nanocrystals was observed. The increase was attributed to a hybridization of the s-p state of ZnS and the d state of manganese ions enhanced by quantum confinement effects or surface states.     

Structure and luminescence properties of TiO2:Er3+ nanocrystals annealed at different temperatures

Erbium doped TiO₂ nanocrystals with the structures of anatase, pyrochlore Er₂Ti₂O₇, and rutile, characterized by X-ray diffraction, have been obtained at different annealing temperatures from 300 °C to 900 °C. The nanocrystalline size for anatase TiO₂ is reduced with increasing doped erbium concentration. Following ultraviolet 325 nm irradiation, the intensity of the green emission is the most intense for the TiO₂:Er³⁺ nanocrystals with a structure of pyrochlore Er₂Ti₂O₇, which evolves from the structure of anatase annealed at 800 °C. Moreover, following ultraviolet 325 nm and infrared 980 nm irradiation, the visible emission spectra for the nanocrystals annealed at 900 °C change drastically. Correspondingly, the structure of anatase disappears, while that of rutile becomes dominant, which indicates that phase transformation occurs.     

Temperature dependence of up-conversion luminescence and photoluminescence of Mn2+ in ZnS:Mn2+ nanoparticles

The photoluminescence (excited at both 300 nm and 383.5 nm) and up-conversion luminescence (excited at 767 nm) of the Mn2+ 4T1-->6A1 transition in both bulk and ZnS:Mn2+ nanoparticles have been measured as a function of temperature. The Mn2+ emission spectra shift monotonically to longer wavelengths at lower temperatures, whereas the intensity change of the luminescence is more complex. The complicated temperature behavior is explained by considering the processes of nonradiation relaxation via phonon coupling, exciton thermal dissociation (binding energy), energy transfer, carrier trapping, and the temperature change of the absorption spectra. The fact that the temperature dependence of the 767 nm excited up-conversion luminescence is the same as the 383.5 nm excited photoluminescence in both bulk and nanoparticles supports the conclusion that the up-conversion luminescence is due to two-photon absorption.     

Spin glass behavior of Mn2+ ions in zeolite-Y at very low temperature

Magnetic properties of manganese ion spin systems in zeolite-Y have been measured. These ions act as almost free ions in the zeolite. Low frequency ac-susceptibility can be described with the Curie-Weiss law down to 50 mK. The susceptibility has a maximum around 30 mK showing a magnetic anomaly. Characteristic relaxation times deduced from frequency dependence of the maxima are studied. They diverge towardsT _c = 0 K according to the following formula, = ₀exp{(b/T)⁺¹} where ₀ = 3 x 10_–10 sec, b = 0.28K, and = 0.4, respectively. It is expected from ESR measurements that the dominant interaction between the manganese ions is dipole-dipole like. This divergence is interpreted as that of a dipolar spin glass.     

Influence of organic polymers as capping agent on structural and optical properties of ZnS:Mn2+ and CdS nanocrystals

We have synthesized the luminescent ZnS:Mn2+ and bare CdS nanocrystals by employing polyvinyl alcohol (PVA), citric acid (CA) and biotin as organic capping agents by chemical co-precipitation route. The synthesized materials were characterized by X-ray diffraction, small angle X-ray scattering and transmission electron microscopy for the structural analysis, while UV-Visible spectroscopy and photoluminescence (PL) for optical properties. The results show that all the three organic polymers have the same effect and are capable of controlling the growth of the nanoclusters. From UV-Visible absorption spectra, it was observed that different capping agents do not affect the band gap of ZnS:Mn2+ system as well, while capping effect clearly observed on PL properties of ZnS:Mn2+ system. We have observed a novel result that capping with biotin show excellent photoluminescence as compared to capping of PVA and CA on ZnS:Mn2+ -systems. On the other hand, the annealing of these systems leads to degradation of luminescence intensity.     

Luminescence properties of ZnS:Mn nanocrystals embedded in SiO2 by ion implantation

Sequential multi-energy implantations of zinc and sulphur ions have been performed in a 250-nm thick SiO₂ layer thermally grown on 1 1 1 silicon. Energies and doses have been chosen to produce 10 at.% constant concentration profiles overlapping over about 100 nm. Manganese is subsequently introduced at various levels by the same way. Thermal treatments (from 700 to 1100 °C) lead to the formation of nanometric precipitates of the luminescent compound ZnS:Mn. A bimodal size distribution is observed, with a quasi-single layer of large particles (40 nm) in the end-of-range region and much smaller precipitates between this layer and the surface. The orange emission is maximal when the Mn concentration is close to 3%. Several hours at 900 °C is the best thermal budget for maximal luminescence intensity at room temperature. A shift of the excitation spectrum related to size variations, shows that the particles of smaller size are mainly responsible for the observed luminescence. In agreement with other authors, the luminescence lifetime is found in the ms range and increases with the nanocrystal diameter, tending to the lifetime of bulk ZnS. The luminescence of ZnS:Mn nanoparticles embedded in SiO₂ by ion implantation is also shown to be very stable during long UV light irradiation.     

Optical spectroscopy of Eu3+ ions in tetragonal ZrO2 nanocrystals

A facile solvothermal method was introduced to incorporate Eu3+ ions into the monodisperse tetragonal ZrO2 nanocrystals (NCs) with small size of approximately 4 nm. The optical properties for Eu3+ doped ZrO2 NCs were investigated in detail by using the photoluminescence (PL) spectroscopy at room and low temperatures. Intense red emissions from Eu3+ ions could be achieved via the host sensitization, which was found to be much more efficient than the direct excitation of lanthanide ions. Moreover, multiple sites of Eu3+ as well as the host-to-Eu3+ energy transfer were also revealed based on the PL analyses.     

Pressure dependence of Mn2+ fluorescence in MnS/ZnS core-shell quantum dots

The high pressure photoluminescence spectra of MnS/ZnS core-shells quantum dots were measured using a diamond anvil cell up to 9.4 GPa. Orange emission at 590 nm from the 4T1 --> 6A1 transition of Mn2+ ions was observed. The Mn2+ emission shifted to red with increasing pressure. The experimental pressure coefficient was -48.3 meV/GPa, which is agreement with the calculated value based on the crystal field theory. The redshift is attributed to the increase of crystal field strength and decrease of Racah parameters during compression.     

Synthesis and characterization of sol-gel derived ZnS : Mn2+ nanocrystallites embedded in a silica matrix

Synthesis and characterization of undoped and Mn²⁺ doped ZnS nanocrystallites (radius 2–3 nm) embedded in a partially densified silica gel matrix are presented. Optical transmittance, photoluminescence (PL), ellipsometric and electron spin resonance measurements revealed manifestation of quantum size effect. PL spectra recorded at room temperature revealed broad blue emission signal centred at ∼ 420 nm and Mn²⁺ related yellow-orange band centred at ∼ 590 nm while ESR indicated that Mn in ZnS was present as dispersed impurity rather than Mn cluster.     

The structure of epitaxial films of ZnS evaporated onto NaCl in a vacuum

The defect structures of epitaxial films of ZnS evaporated onto NaCl in vacuum were studied by transmission electron microscopy. Included grains of doubly-positioned wurtzite-structure material were found in many of the monocrystalline sphalerite-structure films. No twins or wurtzite grains in other orientations were ever observed.Numerous {111} planar defects were present in the films that were grown free of included grains. Interaction between doubly diffracted electron beams, arising from the 111 streaks due to the planar defects and the direct beam produced characteristic systems of fine-scale fringes and dots which dominated the appearance of the micrographs. Annealing in vacuum and in H₂S failed to eliminate the planar defects.     

Crystal structure,energy gap and photoluminescence investigation of Mn2+/Cr3+-doped ZnS nanostructures by precipitation method

Mn added ZnS (Zn_0.97Mn_0.03S) and Mn–Cr-doped ZnS (Zn_0.95Mn_0.03Cr_0.02S) nanostructures were synthesized by co-precipitation process. XRD pattern confirmed the cubic phase with highest intensity along (111) orientation. The shrinkage of crystallite size from 36 Å (Zn_0.97Mn_0.03S) to 26 Å (Zn_0.95Mn_0.03Cr_0.02S) and the influence of Cr/Mn on microstructural, optical and photoluminescence properties in ZnS were investigated. The substitution of Cr in Zn_0.97Mn_0.03S lattice not only diminished the crystallite size and also produced more defect-associated luminescent activation centres. The elevated micro-strain from 9.71?×?10^–3 (Zn_0.97Mn_0.03S) to 13.11?×?10^–3 (Zn_0.95Mn_0.03Cr_0.02S) by Cr substitution is due to the decrease of size and the higher micro-strain at Cr?=?2% is owing to the drop off of activation energy which is originated from higher electro-negativity of Cr ions than Zn²⁺ ions. The enhanced lattice parameters by Cr doping may be due to the coexistence of both Cr³⁺ ions and Cr²⁺ ions where the existence of Cr²⁺ ions is higher than Cr³⁺ ions and substitute Zn²⁺ basic ions with the ionic radius of 0.74 Å in the Zn–Mn–S host lattice. The presence of Zn²⁺, Mn²⁺ and Cr³⁺ ions in Zn–Mn–Cr–S lattice was confirmed by XPS spectra. SEM/TEM micrographs explored the microstructure and confirmed the sized reduction by Cr doping. The elevation in band gap from 3.50 eV (Zn_0.97Mn_0.03S) to 3.63 eV (Zn_0.95Mn_0.03Cr_0.02S, ?E_g?~?0.13 eV) by Cr addition was explained by Burstein–Moss effect and reduced crystallite size. The tuning of band gap and crystallite size of basic ZnS nanostructure by Mn/Cr substitution encourages these materials for modern electronic applications. FTIR spectra established the occurrence of Mn/Cr in Zn–S lattice by their characteristic bondings. The elevated yellowish-orange emission at 594 nm in Mn/Cr substituted ZnS is due to the exchange communication among the s–p electron states of Cr³⁺, Mn²⁺ and Zn²⁺ ions in Zn–S lattice. The inclusion of Mn /Cr provides an efficient control over modification of various emissions which suggests their applications in organic LED materials.

     

Effects of calcination temperature and Li+ ions doping on structure and upconversion luminescence properties of TiO2:Ho3+-Yb3+ nanocrystals

Ho³⁺-Yb³⁺ co-doped and Ho³⁺-Yb³⁺-Li⁺ tri-doped TiO₂ nanocrystals were prepared using the sol-gel method. Effects of the calcination temperature and Li⁺ ions doping on the structure and upconversion luminescence properties of Ho³⁺-Yb³⁺ co-doped TiO₂ nanocrystals were investigated. The upconversion luminescence of nanocrystals was enhanced with the reduction of the crystal lattice symmetry and the crystallinity improvement of the matrix, which were facilitated by the calcination temperature change and Li⁺ ions doping. The lowest lattice symmetry and the best crystallinity of the matrix resulted in the maximum luminescence intensity.     

Emission from Mn4+ ions in gadolinium gallium garnet at high laser pumping intensities

The ⁴T₂→⁴A₂ transition in Mn⁴⁺ ion is observed for the first time in the luminescence spectrum of a manganese-doped gadolinium gallium garnet Gd₃Ga₅O₁₂:Mn⁴⁺ (GGG:Mn⁴⁺) crystal under intensive laser pumping conditions. It is shown that an increase in intensity of the ⁴T₂→⁴A₂ transition as compared to that of the ²E→⁴A₂ transition in the luminescence spectra of GGG:Mn⁴⁺ becomes possible because an increase in the pumping power leads to increasing contribution of the induced transitions. This process is more pronounced in the region of maximum overlap of the bands of ²E→⁴A₂ and ⁴T₂→⁴A₂ transitions, which results in enhancement of the phononless line of the latter transition with a peak at 694 nm. It is suggested that GGG:Mn⁴⁺ can be used as a working element in continuously tunable lasers.     

Electronic absorption spectrum of Mn2+ ions doped in ammonium perchlorate single crystal

The absorption spectrum of Mn²⁺ ions doped in ammonium perchlorate has been studied. The observed bands are assigned as transitions from the ⁶A_lg(S) ground state to various excited quartet states of Mn²⁺ ion in octahedral symmetry. The observed band positions are fitted with four parameters B, C, Dq and α and the best fit is obtained with B = 800 cm⁻¹, C = 3150 cm⁻¹, Dq = 800 cm⁻¹ and α = 76 cm⁻¹.     

Investigation of the vaterite-calcite transformation by ESR spectroscopy using Mn2+ ions as a tracer

Vaterite (CaCO₃) containing 50 ppm Mn²⁺ as an electron spin resonance (ESR) tracer has been subjected to various heat treatments at temperatures up to 500° C in order to monitor the transformation to calcite. Samples have been examined both by X-ray powder diffraction and ESR spectroscopy and the results from the two techniques are correlated. In contrast to the smeared ESR signal in vaterite, the sharp ESR spectrum of Mn-doped calcite enables its crystallization to be followed, and it is shown that the transformation occurs progressively. There is no sharp transition temperature, although the rate becomes very rapid at 400° C. The ESR spectum of polycrystalline calcite prepared from vaterite shows differences of detail from that of calcite precipitated at room temperature, notably in respect of a strong signal at=0° and a weaker response from transitions other thanM=1/2M=–1/2, These features are attributed to variations in the axial field parameterD for the paramagnetic Mn ions in the sample.On leave from the Institute of General and Inorganic Chemistry, Faculty of Pharmacy, University of Turin, Italy.     

Silica-coated S(2-)-enriched manganese-doped ZnS quantum dots as a photoluminescence probe for imaging intracellular Zn2+ ions

Detection of intracellular Zn(2+) has gained great attention because of its biological significances. Here we show the fabrication of silica-coated S(2-)-enriched Mn-doped ZnS quantum dots (SiO(2)-S-Mn-ZnS QDs) by enriching S(2-) with a silica shell on the surface of Mn-doped ZnS QDs via a sol-gel process for imaging intracellular Zn(2+) ions. The developed probe gave a good linearity for the calibration plot (the recovered PL intensity of the SiO(2)-S-Mn-ZnS QDs against the concentration of Zn(2+) from 0.3 to 15.0 μM), excellent reproducibility (1.2% relative standard deviation for 11 replicate measurements of Zn(2+) at 3 μM), and low detection limit (3s; 80 nM Zn(2+)). The SiO(2)-S-Mn-ZnS QDs showed negligible cytotoxicity, good sensitivity, and selectivity for Zn(2+) in a photoluminescence turn-on mode, being a promising probe for photoluminescence imaging of intracellular Zn(2+).     

Effects of Ni2+ concentration and vacuum annealing on structure,morphology and optical properties of Ni doped ZnS nanopowders synthesized by hydrothermal method

Ni doped ZnS nanopowders were synthesized by hydrothermal method. The as-synthesized ZnS nanopowders possessed zinc blende structure with the particle sizes of ～20?nm, and they display a broad emission band from 400?nm to 700?nm centered at ～540?nm. The influence of Ni²⁺ doping and vacuum annealing on the structure, morphology and optical properties was investigated systematically. The results revealed that the emission peaks are blue shifted with Ni²⁺ doping, and the emission peaks of 0.2, 0.4, 0.6, 0.8 and 1.0?mol% Ni²⁺ doped ZnS nanopowders are centered at ～520, ～ 510, 500, 490 and 470?nm, respectively. While, the zinc blende phase transformed to wurtzite phase when annealing at 600?°C or higher, and the annealed samples exhibited three emission peaks positioned at ～490, ～547 and ～580?nm. The results indicated that the as prepared Ni²⁺-doped ZnS nanopowders have a potential application in LED as light conversion layer.     

Upconversion luminescence enhancement in Yb3+/Tm3+-codoped Lu2O3 nanocrystals induced by doping with Li+ ions

Lutetium oxide (Lu2O3) nanocrystals doped with 2%Yb3+, 0.5%Tm3+, and various doping concentrations of Li+ (0, 3, 5, 7, 10, 12, and 15 mol%) were prepared by the sol-gel method. The dependence on different doping concentrations of Li+ ions of the structure, morphology, and the upconversion emission intensity of the Lu2O3:2%Yb3+, 0.5%Tm3+ nanocrystals was investigated. The obtained Lu2O3 nanocrystals were systematically characterized by X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), Fourier transformed infrared (FT-IR) spectra, Raman spectra, and upconversion spectra measurements. It was found that all the nanocrystals can be readily indexed to pure cubic phase of Lu2O3, indicating good crystallinity. The experimental results show that Li+ doping in Lu2O3:2%Yb3+, 0.5%Tm3+ nanocrystals can greatly enhance the upconversion emission intensity. The strong blue (490 nm) and the weak red (653 nm) emissions from the prepared nanocrystals were observed under 980 nm laser excitation, and attributed to the 1G4 --> 3H6 and 1G4 --> 3F4 transitions of Tm3+ ions, respectively. An simple analysis based on steady-state rate equations and a power-dependent study both indicate that the 1G4 levels can be populated by three-step energy transfer (ET) processes. The enhancement of the upconversion luminescence was suggested to be the consequence of the modification of the local field symmetry around the Tm3+ ion, reduced number of OH- groups, and the enlarged nanocrystal size induced by the Li+ ions.