ZnS Host Research Articles

Hydrothermal Synthesis and Luminescent Properties of Microtubes Constructed by Fluffy Zns:Mn2+ with Nanostructures

Tubular micrometer-sized ZnS:Mn2+ constructed by fluffy nanostructures were fabricated in the mixed solutions of water and ethanol in a fixed volume ratio with the aid of ethylenediamine. In the X-ray diffraction pattern, the products obtained in the presence and absence of ethylenediamine show the wurtzite and sphalerite phases, respectively. Field-emission scanning electron microscopic images reveal the evolution process from nanowires to fluffy ZnS:Mn2+ to microtubes with the reaction times of 2, 4, and 8 hours at 100 degrees C, and the basal nanowires are below 10 nm in diameter. Photoluminescence and photoluminescence excitation spectra were investigated. The results suggest that the wurtzite phase, instead of the sphalerite phase ZnS:Mn2+ is luminescence-active for the 4T1 -6A1 transition of the Mn2+ in the ZnS host. The excitation spectra monitored at orange emission bands exhibit sharp peaks at 320, 326 and 327 nm with increasing reaction times of 2, 4, and 8 hours, respectively, indicating the energy transfer from ZnS host to Mn2+ ions, and the blue-shifts compared with the band gap absorption of the bulk counterpart (344 nm) are also observed due to the quantum confinement effects. The formation mechanism of the wurtzite one-dimensional nanostructures at such a low temperature is proposed based on a molecular template mechanism involving the bidentate coordinating ligand, ethylenediamine, and the possible formation mechanism of novel tubular structure are also discussed.

Photophysical and photoluminescence properties of co-activated ZnS:Cu,Mn phosphors

ZnS:Cu,Mn phosphors were prepared by conventional solid state reaction with the aid of NaCl–MgCl 2 flux at 900 °C. The samples were characterized by X-ray powder diffraction, UV–vis absorbance spectra and photoluminescence spectra. All samples possess cubic structure. Cu has a much stronger effect on the absorption property of ZnS than Mn. Incorporation of Mn into ZnS host only slightly enhances the light absorption, while addition of Cu remarkably increases the ability of absorption due to ground state Cu + absorption. The emission spectra of the ZnS:Cu,Mn phosphors consist of three bands centered at about 452, 520 and 580 nm, respectively. Introduction of Mn significantly quenches the green luminescence of ZnS:Cu. The excitation energy absorbed by Cu is efficiently transferred to Mn activators non-radiatively and the Mn luminescence can be sensitized by Cu behaving as a sensitizer (energy donor).

Effect of Precursor Molar Ratio of [S2−]/[Zn2+] onParticle Size and Photoluminescence of ZnS:Mn2+ Nanocrystals

We investigate the influence of precursor molar ratio of[S2−]/[Zn2+] on particle size and photoluminescence (PL)of ZnS:Mn2+ nanocrystals. By changing the[S2−]/[Zn2+] ratio from 0.6 (Zn-rich) to 2.0 (S-rich),the particle size increases from nearly 2.7 nm to about 4.0 nm. Theincrease in the ratio of [S2−]/[Zn2+] causes a decreaseof PL emission intensity of ZnS host while a distinct increase ofMn2+ emission. The maximum intensity for the luminescence ofMn2+ emission is observed at the ratio of[S2−]/[Zn2+]≈1.5. The possible mechanism for the resultsis discussed by filling of S2− vacancies and the increase ofMn2+ ions incorporated into ZnS lattices.

The optical excitation mechanism in ZnS: Sm 3+ grown by molecular-beam epitaxy

We fabricated a thin film of Sm 3+ doped ZnS by molecular-beam epitaxy (MBE) and investigated its photoluminescence properties. The excitation spectrum of the Sm 3+ luminescence at 10 K is dominated by broad bands peaked at the free exciton energies, in contrast to many earlier reports on rare-earth-doped semiconductors. This result shows that the free exciton contributes significantly to the excitation of Sm 3+ in our MBE sample, probably because the concentration of the trap centers which capture free excitons but do not transfer energy to Sm 3+ is sufficiently low in our sample. The thermal quenching characteristic of the Sm 3+ luminescence under interband excitation of the ZnS host is fitted well to a formula expressed by taking into account two deactivation channels. The origins of these deactivation channels are argued on the basis of previously proposed excitation mechanisms of the rare-earth ions in semiconductors.

Electroluminescent properties of device based on ZnS:Tb/CdS core-shell nanocrystals

ZnS:Tb/CdS core/shell nanocrystals (NCs) were synthesized by reverse micelle method, and with a core crystal diameter of 3.0 and a 0.5 nm thick CdS shell and used as an electroluminescent material. Electroluminescent (EL) devices having a hybrid organic/inorganic multilayer structure were fabricated. Electron–hole recombination was confined at the ZnS:Tb/CdS NCs layer. Three emissions peaked at 440 nm form ZnS host, 546 nm ( 5D 4– 7F 5) and 577 nm ( 5D 4– 7F 4) of Tb 3+ center were observed when the hybrid EL device at a bias of 13 V. The maximum luminance of the ZnS:Tb/CdS NCs-based reached 19 cd/m 2 at 25 V.

On the energy transfer from nanocrystalline ZnS to Tb 3+ ions confined in reverse micelles

ZnS nanoparticles with different sizes are synthesized using a reverse micelles method in the presence of Tb 3+ ions. The photoluminescence, excitation and absorption spectra of ZnS/Tb 3+ NPs are studied to elucidate remarkable energy transfer from both ZnS host and the surfactant, i.e., AOT, to Tb 3+ ions. When Tb 3+ ions are introduced merely on the outside of ZnS nanoparticles in reverse micelles, obvious energy transfer from ZnS to the Tb 3+ ions is also observed, indicating the important role of spatial confinement on the performance of energy transfer between host and luminescent centers which are even not doped into the host lattices.

Photoluminescence of doped ZnS nanoparticles under hydrostatic pressure

AbstractThe pressure dependence of the photoluminescence from ZnS:Mn2+, ZnS:Cu2+, and ZnS:Eu2+ nanoparticles were investigated under hydrostatic pressure up to 6 GPa at room temperature. Both the orange emission from the 4T1 – 6A1 transition of Mn2+ ions and the blue emission from the DA pair transition in the ZnS host were observed in the Mn‐doped samples. The measured pressure coefficients are –34.3(8) meV/GPa for the Mn‐related emission and –3(3) meV/GPa for the DA band, respectively. The emission corresponding to the 4f65d1 – 4f7 transition of Eu2+ ions and the emission related to the transition from the conduction band of ZnS to the t2 level of Cu2+ ions were observed in the Eu‐ and Cu‐doped samples, respectively. The pressure coefficient of the Eu‐related emission was found to be 24.1(5) meV/GPa, while that of the Cu‐related emission is 63.2(9) meV/GPa. The size dependence of the pressure coefficients for the Mn‐related emission was also investigated. The Mn emission shifts to lower energies with increasing pressure and the shift rate (the absolute value of the pressure coefficient) is larger in the ZnS:Mn2+ nanoparticles than in bulk. Moreover, the absolute pressure coefficient increases with the decrease of the particle size. The pressure coefficients calculated based on the crystal field theory are in agreement with the experimental results. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

ＺｎＳ：Ｍｎ発光層の発光中心に対するスパッタ雰囲気中Ｈ２ガスの影響

A very small amount of H2 gas was introduced in the sputtering environment to get high quality ZnS : Mn active layer for thin-film electroluminescent device. The effect of H2 gas concentration on the ZnS host material and the luminescent center in the active layer was examined.Electron spin resonance (ESR) investigations were performed to examine the Mn2+ cohesive state. The peak intensity of isolated single Mn2+ increased when the partial pressure of H2 was increased up to about 2.7 × 10-6Pa. And when the amount of H2 was increased further, isolated single Mn2+ signal decreased and crystallinity decreased because of the decrease in S of the ZnS host material.

Structure and photoluminescence studies on ZnS:Mn nanoparticles

ZnS:Mn was produced in nanocrystalline form by a chemical method using polyvinylpyroledone as a chemical capping agent. Mn was stoichiometrically substituted for Zn in ZnS. The manganese (Mn) concentration was varied over its whole solid solution limit in ZnS, i.e., from 0 to 40%. In the high concentration regime this material formed may be thus written as nanocrystalline (Zn, Mn)S. The material formed is thus a wide gap diluted magnetic semiconductor. The characterized material was in powder form. X-ray diffraction was used to estimate the crystallite size and to confirm formation of the material in single phase. The average crystallite size obtained was about 2 nm. The material remained cubic over the whole Mn solid solution range. The room temperature photoluminescence (PL) when deconvoluted using a Gaussian fit showed two extra peaks in nanocrystalline ZnS:Mn when compared to pure nanocrystalline ZnS, which had only two peaks. Mn incorporation significantly enhanced the PL intensity in nanocrystalline ZnS:Mn (400–850 nm range) thereby suggesting Mn2+ induced PL. The red shift of the two new peaks with increase in Mn2+ concentration can be attributed to the change in band structure due to the formation of ZnS:Mn alloy. These extra peaks were due to (a) various Mn2+ transitions in the ZnS host, (b) related to S as the nearest neighbor of Mn2+ ion in the nanocrystallite (due to the high concentration of Mn2+), or (c) Mn–Mn interactions at high Mn concentrations. However, our prepared pure MnS samples did not show any photoluminescence at room temperature. So it is concluded that the observed PL is Mn2+ induced in the nanocrystalline ZnS host.

Energy transfer from organic surface adsorbate-polyvinyl pyrrolidone molecules to luminescent centers in ZnS nanocrystals

Multi-colour emitting doped ZnS nanocrystals surface capped with pyridine (P-ZnS) or polyvinyl pyrrolidone (PVP-ZnS) have been synthesized by wet chemical methods. The photoluminescence studies show that the dopant related emission from P-ZnS nanocrystals are caused by the energy transfer from band-to-band excitation of the host lattice. However, in the case of PVP capped ZnS, considerable enhancement in the emission intensity was observed and the corresponding excitation spectra appeared dramatically broadened due to the presence of multiple excitation bands with peak maxima at 235, 253, 260, 275, and 310 nm. The bands from 235 to 275 nm are assigned to the electronic transitions in the chemisorbed PVP molecules whereas the excitation maximum around 310 nm corresponds to the band-to-band transition within the nanocrystalline ZnS host. The presence of PVP related energy bands in the excitation spectrum indicates the process of energy transfer from the surface adsorbed PVP molecules to dopant centers in ZnS nanocrystals. This study brings out a heterogeneous sensitizer-activator relation between organic surface adsorbate and doped semiconducting nanocrystals.

Blue-emitting ZnS:Ag,Al phosphors with low defect density for high-voltage field-emission displays

In recent years, deterioration of luminescence efficiency of blue-emitting ZnS:Ag,Al powder phosphors has been investigated for high-voltage (e.g., 6–10 kV) field-emission displays (FEDs). It has been found by cross-sectional transmission electron microscopy that the degree of crystallinity of ZnS host crystal within the electron penetration depth is one of the important factors in order to obtain ZnS:Ag,Al phosphors having a longer lifetime of luminescence efficiency. ZnS:Ag,Al phosphors with lower defect density N0 as compared with a P22B ZnS:Ag,Al phosphor for color-TV use have been developed by using a suitable firing method with subsequent annealing and surface treatments. As a result, ZnS:Ag,Al phosphors with lower N0 were successfully prepared, and also FED tubes having a longer lifetime in blue luminescence were made by using ZnS:Ag,Al phosphors with lower N0. In addition, it was revealed that both the initial luminescence efficiency and the thermal quenching point of ZnS:Ag,Al phosphors with lower N0 are considerably higher than those of a P22B ZnS:Ag,Al phosphor.

Mn ion concentration dependence of the photoacoustic and photoluminescence spectra of ZnS:Mn nanocrystals (abstract)

ZnS nanocrystals doped with Mn2+ ions (ZnS:Mn) have attracted much attention in the past few years due to a change in optical properties in accordance with quantum confinement effects. ZnS:Mn nanocrystals exhibit an orange luminescence with a high quantum efficiency (∼18%) under the interband excitation of the host crystals. This spectacular result suggests that the ZnS:Mn nanocrystal system forms a new class of luminescent materials with many applications. To verify the mechanism of high quantum efficiency in ZnS:Mn nanocrystals, it is important to establish the optical absorption characters as an initial step. Few studies have been conducted on light absorption of ZnS:Mn nanocrystals directly because of the difficulty in using of conventional methods caused by strong scattering in the finely powdered structures. The optical absorption spectra of them can be obtained by applying the photoacoustic (PA) spectroscopy, which is a powerful technique for detecting small amounts of strongly scattered materials. We will show the PA difference spectra between ZnS:Mn and pure ZnS nanocrystals with different Mn ion concentrations. The peak position corresponds to the transition energy between A16 state (ground state) and E4 state, and it increases with the decrease of Mn2+ ion concentration, indicating a possibility of hybridization of the s–p states of the ZnS host and the d state of the Mn2+ ion. The intensity of optical absorption between those states increases linearly with the increase of Mn ion concentration, indicating that the optical absorption of Mn ion impurities is proportional to the concentration. We have obtained photoluminescence (PL) spectra with different Mn ion concentration. The PL peak of the Mn2+ transition in ZnS nanocrystals (the transition from T14 to A16 states) is 2.08 eV with a half width of 0.24 eV, which is slightly shifted to lower energy region when compared to the peak in the bulk sample, and is independent of the Mn ion concentration. The peak intensity of the PL spectrum shows a maximum, and decreases with higher Mn ion concentration region.

Comparison of electronic structures of doped ZnS and ZnO calculated by a first-principle pseudopotential method

Electronic band calculations of doped and undoped ZnO and ZnS have been done using density functional theory under the local density approximation so as to clarify the reason of the difference in behaviors of doped ZnO and ZnS. The reason why the electrical conductivity of ZnS is difficult to be increased by doping was discussed. In the case of doped ZnS, an impurity level was generated at deep position below the bottom of the conduction band of the host ZnS lattice and Fermi level was located at this impurity level. On the contrary, the shape of the density of states curve and the band structures of doped and undoped ZnO are alike with each other and the donor band is hybridized with the conduction band of the ZnO host material. This seems to result in contribution of doped electrons to electrical current in the case of doped ZnO.

Temperature behaviour of the orange and blue emissions in ZnS:Mn nanoparticles

The temperature dependences of the orange and blue emissions in 10, 4.5, and 3nm ZnS:Mn nanoparticles were investigated. The orange emission is from the4T1–6A1 transition of Mn2+ ions and the blue emission is related to thedonor–acceptor recombination in the ZnS host. With increasing temperature, theblue emission has a red-shift. On the other hand, the peak energy of the orangeemission is only weakly dependent on temperature. The luminescence intensity ofthe orange emission decreases rapidly from 110 to 300 K for the 10 nm sample butincreases obviously for the 3 nm sample, whereas the emission intensity is nearlyindependent of temperature for the 4.5 nm sample. A thermally activatedcarrier-transfer model has been proposed to explain the observed abnormaltemperature behaviour of the orange emission in ZnS:Mn nanoparticles.

Photoluminescent properties of ZnS:Mn nanocrystals prepared in inhomogeneous system

ZnS:Mn nanocrystalline colloids and powders were prepared by chemical method with H2S gas in an inhomogeneous system of gas–liquid phase. The best reactive conditions were determined. The structure and the properties of the materials were examined by X-ray diffraction (XRD) and infrared (IR) spectroscopy. From the PLE and PL spectra of ZnS:Mn nanocrystals, it is shown that the Mn2+ ions are incorporated into the ZnS host structure. The position of the characteristic peaks in the PLE spectra of ZnS:Mn nanocrystal sol or powders shift to higher energy under UV exposure, and are different from those of ZnS:Mn bulk materials. The emission sites of ZnS:Mn nanocrystal sol and powders are different from the bulk materials, but the emission sites of nanocrystal powders are the same as those of nanocrystal sol.

Temperature and pressure dependences of the Mn2+ and donor–acceptor emissions in ZnS:Mn2+ nanoparticles

Temperature and pressure dependent measurements have been performed on 3.5 nm ZnS:Mn2+ nanoparticles. As temperature increases, the donor–acceptor (DA) emission of ZnS:Mn2+ nanoparticles at 440 nm shifts to longer wavelengths while the Mn2+ emission (4T1–6A1) shifts to shorter wavelengths. Both the DA and Mn2+ emission intensities decrease with temperature with the intensity decrease of the DA emission being much more pronounced. The intensity decreases are fit well with the theory of thermal quenching. As pressure increases, the Mn2+ emission shifts to longer wavelengths while the DA emission wavelength remains almost constant. The pressure coefficient of the DA emission in ZnS:Mn2+ nanoparticles is approximately −3.2 meV/GPa, which is significantly smaller than that measured for bulk materials. The relatively weak pressure dependence of the DA emission is attributed to the increase of the binding energies and the localization of the defect wave functions in nanoparticles. The pressure coefficient of Mn2+ emission in ZnS:Mn2+ nanoparticles is roughly −34.3 meV/GPa, consistent with crystal field theory. The results indicate that the energy transfer from the ZnS host to Mn2+ ions is mainly from the recombination of carriers localized at Mn2+ ions.

Formation and Distinctive Decay Times of Surface- and Lattice-Bound Mn2+ Impurity Luminescence in ZnS Nanoparticles

The Mn2+ impurity luminescence decay of Mn2+-doped ZnS nanoparticles has been clearly distinguished from ZnS host emission decay by measuring transient absorption and emission kinetic profiles in t...

Luminescence of nanocrystalline ZnS:Pb2+

Nanocrystalline ZnS:Pb2+ is synthesised ia a precipitation method. The influence of the size of the nanocrystals and the sulfide concentration used in the synthesis on the luminescence properties is investigated. Nanocrystalline ZnS:Pb2+ shows a white emission under UV excitation with a rather high quantum efficiency (∽5%). At least two luminescence centres are involved. One centre is identified as a Pb2+ ion located on a regular Zn2+ site and gives a red emission under 480 nm excitation. The luminescence properties of this emission are characteristic of 3P0 → 1S0 (A-band) or charge transfer (D-band) transitions on Pb2+ ions. The other centres are not as well defined and give a broad green emission band under 380 nm excitation and also show luminescence properties typically observed for Pb2+. The green emission probably originates from a charge-transfer like D-band emission of Pb2+ in ZnS close to a defect (e.g. an S2− vacancy or an O2− ion on an S2− site). A relation between the temperature quenching of the emissions and the band gap is observed and indicates that photoionisation occurs. The higher quenching temperature for the Pb2+ luminescence in smaller particles can be explained by widening of the band gap as a result of quantum size effects in the ZnS host.

Pressure Tuning Spectroscopy of Mn2+ in Bulk and Nanocrystalline Sulfide Semiconductors

ABSTRACTWe report the results of high pressure luminescence studies of the emission of Mn2+ in bulk Zn0.55Ga0.30S up to ∼214 kbar, bulk ZnS up to ∼184 kbar, and nanocrystalline ZnS (∼8 nm particle sizes) up to ∼300 kbar. We observed a strong redshift with pressure (-30(3) cm−1/kbar) for the 4T1→6A1 emission transition of Mn2+ in all three systems. We also observed emission quenching at high pressure in all three systems and attribute the quenching to a pressure induced phase change of the ZnS host lattice to a rocksalt (NaCl) phase.

Luminescence enhancement of ZnS:Mn nanoclusters in zeolite

The luminescence intensity of Mn2+ in the ZnS:Mn nanoclusters formed in an ultrastable zeolite-Y is seven orders of magnitude stronger than that of other nanoparticles deposited out of solutions. This remarkable effect is attributed to a strong quantum size confinement, the location of Mn2+ ions at the near-surface sites, and good surface passivation. The lowering of the energy loss in the E4(D4 or/and T24(D4) states during the energy transfer from the ZnS host to the emitting state T14(G4) of Mn2+ may also contribute to the observed fluorescence enhancement.