In situ reversible tuning of photoluminescence of Mn2+-doped ZnS quantum dots by redox chemistry

Herein we report the development of a new method for in situ reversible tuning of photoluminescence properties of quantum dots (Qdots) by partial oxidation of population of the emitting species and subsequent chemical reduction of the oxidized form. The concept has been demonstrated using Mn(2+)-doped ZnS Qdots stabilized by acetyl acetonate. Treatment of an aqueous solution of the Qdots (with Mn(OAc)(2) being the source of Mn used for the synthesis of the Qdots) by potassium peroxodisulfate (KPS) led to reduction of intensity of emission due to Mn(2+) ((4)T(1)-(6)A(1)). Subsequent treatment of the solution containing KPS-treated Qdots with NaBH(4) led to regaining of intensity, thus providing reversibility to the tuning, which was possible for more than one cycle. Electron spin resonance (ESR) spectroscopic investigations revealed reduction of the population of Mn(2+) upon treatment with KPS, whereas it went back up upon further treatment with NaBH(4). Interestingly, a mixed population of oxidation states of Mn was indicated to be present in the Qdots prepared using KMnO(4) as the source of Mn. The fluorescence intensity of the Qdots so prepared increased upon treatment with NaBH(4) following synthesis, which was not possible when the source of Mn was Mn(OAc)(2). Transmission electron microscopic and X-ray diffraction studies indicated that oxidation and reduction did not change the sizes of Qdots significantly.     

'Old' clusters with new function: oxidation catalysis by high oxidation state manganese and cerium/manganese clusters using O2 gas

The family of polynuclear manganese clusters of formula [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] (R = Et, Ph, etc.) has been investigated in great detail over the years for their ability to function as single-molecule magnets (SMMs), but they have not been employed as oxidation catalysts. In the present report, the ability is described of these clusters to act as catalysts in the selective oxidation of benzyl alcohol to benzaldehyde using molecular O(2) as the primary oxidant and the nitroxyl radical TEMPO as a cocatalyst. A systematic investigation of Mn clusters varied in their R group, oxidation state, and size was conducted in order to realize the electronic requirements that will lead to the best catalytic activity. The best reactivity (>99%) was obtained when the catalyst was the mixed-metal cluster [CeMn(6)O(9)(O(2)CMe)(9)(NO(3))(H(2)O)(2)], which contains Ce(4+)Mn(4+)(6) ions; in this case, lower loadings of catalysts (cluster and TEMPO) are required and the reaction can proceed even without a solvent. In addition, it has been demonstrated that the high efficiency can be only achieved when both high oxidation Ce(4+) and Mn(4+) ions are present within the same cluster.     

Water-based route to colloidal Mn-doped ZnSe and core/shell ZnSe/ZnS quantum dots

Relatively monodisperse and highly luminescent Mn(2+)-doped zinc blende ZnSe nanocrystals were synthesized in aqueous solution at 100 °C using the nucleation-doping strategy. The effects of the experimental conditions and of the ligand on the synthesis of nanocrystals were investigated systematically. It was found that there were significant effects of molar ratio of precursors and heating time on the optical properties of ZnSe:Mn nanocrystals. Using 3-mercaptopropionic acid as capping ligand afforded 3.1 nm wide ZnSe:Mn quantum dots (QDs) with very low surface defect density and which exhibited the Mn(2+)-related orange luminescence. The post-preparative introduction of a ZnS shell at the surface of the Mn(2+)-doped ZnSe QDs improved their photoluminescence properties, resulting in stronger emission. A 2.5-fold increase in photoluminescence quantum yield (from 3.5 to 9%) and of Mn(2+) ion emission lifetime (from 0.62 to 1.39 ms) have been observed after surface passivation. The size and the structure of these QDs were also corroborated by using transmission electron microscopy, energy dispersive spectroscopy, and X-ray powder diffraction.     

New observations on the luminescence decay lifetime of Mn2+ in Zns: Mn2+ nanoparticles

A fast decay emission peaking at 645 nm with a decay lifetime within the experimental resolution of 0.14 micros is observed in ZnS:Mn2+ nanoparticles. This short-lived signal is also observed in pure ZnS and MgS: Eu3+ nanoparticles, which has nothing to do with Mn(2+)-doped ions but is from the deep trap states of the host materials. The short-lived component decreases in intensity relative to the Mn2+ emission at higher excitation powers, while it increases in intensity at low temperatures and shifts to longer wavelengths at longer time delays. Our observations demonstrated further that the emission of Mn2+ in ZnS: Mn2+ nanoparticles behaves basically the same as in bulk ZnS: Mn2+; the fast decay component is actually from the intrinsic and defect-related emission in sulfide compounds.     

(Zn, Mg)2GeO4:Mn2+ submicrorods as promising green phosphors for field emission displays: hydrothermal synthesis and luminescence properties

(Zn(1-x-y)Mg(y))(2)GeO(4): xMn(2+) (y = 0-0.30; x = 0-0.035) phosphors with uniform submicrorod morphology were synthesized through a facile hydrothermal process. X-Ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photoluminescence (PL), and cathodoluminescence (CL) spectroscopy were utilized to characterize the samples. SEM and TEM images indicate that Zn(2)GeO(4):Mn(2+) samples consist of submicrorods with lengths around 1-2 μm and diameters around 200-250 nm, respectively. The possible formation mechanism for Zn(2)GeO(4) submicrorods has been presented. PL and CL spectroscopic characterizations show that pure Zn(2)GeO(4) sample shows a blue emission due to defects, while Zn(2)GeO(4):Mn(2+) phosphors exhibit a green emission corresponding to the characteristic transition of Mn(2+) ((4)T(1)→(6)A(1)) under the excitation of UV and low-voltage electron beam. Compared with Zn(2)GeO(4):Mn(2+) sample prepared by solid-state reaction, Zn(2)GeO(4):Mn(2+) phosphors obtained by hydrothermal process followed by high temperature annealing show better luminescence properties. In addition, codoping Mg(2+) ions into the lattice to substitute for Zn(2+) ions can enhance both the PL and CL intensity of Zn(2)GeO(4):Mn(2+) phosphors. Furthermore, Zn(2)GeO(4):Mn(2+) phosphors exhibit more saturated green emission than the commercial FEDs phosphor ZnO:Zn, and it is expected that these phosphors are promising for application in field-emission displays.     

Growth kinetic on the optical properties of the Pb(1-x)Mn(x)Se nanocrystals embedded in a glass matrix: thermal annealing and Mn2+ concentration

Semimagnetic Pb(1-x)Mn(x)Se nanocrystals were synthesized by a fusion method in a glass matrix and characterized by optical absorption (OA), atomic/magnetic force microscopy (AFM/MFM), and photoluminescence techniques. MFM images strongly indicated the formation of Pb(1-x)Mn(x)Se magnetic phases in the glass system. Quantum dot size was manipulated by tuning annealing time. It was shown that Mn(2+) impurity affects nucleation, where Mn(2+)-doped samples present a redshift of the OA peak after a short annealing time and a blueshift after long annealing time compared to undoped PbSe NCs. This behavior was linked to the dependence of band-gap energy and the absorption selection rule on Mn(2+) concentration. Photoluminescence in the Pb(1-x)Mn(x)Se nanocrystals increases as the temperature rises up to a point and then decreases at higher temperatures. Anomalous increases in emission efficiency were analyzed by considering temperature induced carrier-transfer in semimagnetic Pb(1-x)Mn(x)Se quantum dots nanocrystals of different sizes.     

Selective detection of mercury (II) ion using nonlinear optical properties of gold nanoparticles

Contamination of the environment with heavy metal ions has been an important concern throughout the world for decades. Driven by the need to detect trace amounts of mercury in environmental samples, this article demonstrates for the first time that nonlinear optical (NLO) properties of MPA-HCys-PDCA-modified gold nanoparticles can be used for rapid, easy and reliable screening of Hg(II) ions in aqueous solution, with high sensitivity (5 ppb) and selectivity over competing analytes. The hyper Rayleigh scattering (HRS) intensity increases 10 times after the addition of 20 ppm Hg(2+) ions to modified gold nanoparticle solution. The mechanism for HRS intensity change has been discussed in detail using particle size-dependent NLO properties as well as a two-state model. Our results show that the HRS assay for monitoring Hg(II) ions using MPA-HCys-PDCA-modified gold nanoparticles has excellent selectivity over alkali, alkaline earth (Li(+), Na(+), K(+), Mg(2+), Ca(2+)), and transition heavy metal ions (Pb(2+), Pb(+), Mn(2+), Fe(2+), Cu(2+), Ni(2+), Zn(2+), Cd(2+)).     

Ca(9)Lu(PO(4))(7):Eu(2+),Mn(2+): a potential single-phased white-light-emitting phosphor suitable for white-light-emitting diodes

A novel white-light-emitting phosphor Ca(9)Lu(PO(4))(7):Eu(2+),Mn(2+) has been prepared by solid-state reaction. The photoluminescence properties indicate that there is an efficient energy transfer from the Eu(2+) to Mn(2+) ions via a dipole-quadrupole reaction. The obtained phosphor exhibits a strong excitation band between 250 and 430 nm, matching well with the dominant emission band of a UV light-emitting-diode (LED) chip. Upon excitation of UV light, white light is realized by combining a broad blue-green emission band at 480 nm and a red emission band at 645 nm attributed to the Eu(2+) and Mn(2+) ions. The energy-transfer efficiency and critical distance were also calculated. Furthermore, the phosphors can generate lights from blue-green through white and eventually to red by properly tuning the relative ratio of the Eu(2+) to Mn(2+) ions through the principle of energy transfer. Preliminary studies showed that the phosphor might be promising as a single-phased white-light-emitting phosphor for a UV white-light LED.     

Aggregation of FeCl2 clusters in supercritical water investigated by molecular dynamics simulations

We have carried out molecular dynamics (MD) simulations of the aggregation of FeCl 2 clusters in supercritical water. The particle formation in systems of 2048 water molecules (rigid SPC/E-model) and 120 Fe (2+) ions and 240 Cl (-) ions has been investigated for 250 ps at five different state points at temperatures from 798 to 873 K and system densities from 0.18 g/cm (3) to 0.13 g/cm (3). We describe the particle growth by means of properties of the largest cluster in a system as well as cluster size averaged and time averaged observables. From preexisting or immediately formed units of Fe (2+)-Cl (-), Fe (2+)-Cl (-) 2, Fe (2+)-Cl (-) 3 etc., the further growth of clusters is dominated by aggregation of such small building blocks. Clusters up to 10 ions in size with large charge imbalances are found during the growth process while a balanced positive to negative charge ratio is found on the average with time and cluster size development. Water molecules are found within the FeCl 2 clusters during the whole time interval covered by the simulations, which is in agreement with the existence of crystal water in FeCl 2 crystals grown from aqueous solutions. The radial distribution functions obtained from the simulation data are in good agreement with experimental results of slightly distorted FeCl 2.4H 2O crystals.     

A tunable single-component warm white-light Sr3Y(PO4)3:Eu2+,Mn2+ phosphor for white-light emitting diodes

The photoluminescence properties and energy transfer of the Eu(2+) and Mn(2+) co-doped Sr(3)Y(PO(4))(3) phosphors are investigated in detail. Two main emission bands attributed to the Eu(2+) and Mn(2+) ions are observed under UV light excitation via an efficient energy transfer process. When the Eu(2+) doping content is fixed, the emission chromaticity can be varied by simply adjusting the content of Mn(2+). The study of the behavior as a function of doping concentration indicates that the warm white-light can be obtained in a single host lattice. Furthermore, the analysis of the fluorescence decay curves based on the Inokuti-Hirayama theoretical model reveals that the dipole-quadrupole interaction is mainly responsible for the energy transfer mechanism from the Eu(2+) to Mn(2+) ions in the Sr(3)Y(PO(4))(3) phosphor. The developed phosphor exhibits a strong absorption in UV spectral region and white-light emission which may find utility as a single-component white-light-emitting UV-convertible phosphor in white LED devices.     

Synthesis of water-dispersible fluorescent, radio-opaque, and paramagnetic CdS:Mn/ZnS quantum dots: a multifunctional probe for bioimaging

Ultra-small (3.1 nm) multifunctional CdS:Mn/ZnS core-shell semiconductor quantum dots (Qdots), which possess fluorescent, radio-opacity, and paramagnetic properties, have been shown here. To demonstrate in vivo bioimaging capability, a rat was administered endovascularly with Qdots conjugated with a TAT peptide. The labeling efficacy of these Qdots was demonstrated on the basis of the histological analysis of the microtome sliced brain tissue, clearly showing that TAT-conjugated Qdots stained brain blood vessels.     

Spectral characterization of a newly synthesized fluorescent semicarbazone derivative and its usage as a selective fiber optic sensor for copper(II)

In this work photoluminescent properties of highly Cu(2+) selective organic fluoroionophore, semicarbazone derivative; bis(naphtho[2,1-b]furan-2-yl)methanone semicarbazone (BNF) was investigated in different solvents (dichloromethane, tetrahydrofuran, toluene and ethanol) and in polymer matrices of polyvinylchloride (PVC) and ethyl cellulose (EC) by absorption and emission spectrometry. The BNF derivative displayed enhanced fluorescence emission quantum yield, Q(f)=6.1 x 10(-2) and molar extinction coefficient, epsilon=29,000+/-65 cm(-1)M(-1) in immobilized PVC matrix, compared to 2.6 x 10(-3) and 24,573+/-115 in ethanol solution. The offered sensor exhibited remarkable fluorescence intensity quenching upon exposure to Cu(2+) ions at pH 4.0 in the concentration range of 1.0 x 10(-9) to 3.0 x 10(-4)M [Cu(2+)] while the effects of the responding ions (Ca(2+), Hg(+), Pb(2+), Al(3+), Cr(3+), Mn(2+), Mg(2+), Sn(2+), Cd(2+), Co(2+) and Ni(2+)) were less pronounced.     

Synthesis and spectroscopic characterization of water-soluble Mn-doped ZnO(x)S(1-x) quantum dots

A non-cadmium and water-soluble Mn-doped ZnO(x)S(1-x) QDs was synthesized with denatured bovine serum albumin (dBSA) as stabilizer under nitrogen atmosphere, and the as-prepared products were characterized by X-ray powder diffraction (XRD), UV-vis absorption spectroscopy, fluorescence (FL) emission spectroscopy, high resolution transmission electronmicroscopy (HRTEM) and Raman spectrum. XRD patterns indicate that the Mn-doped ZnO(x)S(1-x) QDs have a zinc-blende structure, and that manganese emerges in the form of divalent manganese (Mn(2+)) and trivalent manganese (Mn(3+)) (the intermediate of the reaction). The size of Mn-doped ZnO(x)S(1-x) QDs is about 3.2±0.7 nm according to HRTEM imaging. The FL spectra reveal that the Mn-doped ZnO(x)S(1-x) QDs have two distinct emission bands: the defect-related emission and the Mn(2+)-related emission, which exhibit a competing process. A good FL signal of the transition of Mn(2+) ((4)T(1)-(6)A(1)) is observed when the doping amounts are 1.0% and 20% respectively, and the as-prepared solutions are stable for more than 6 months at 4°C. This method has the advantages of good stability and environment-friendly stabilizer, for involving no heavy metal ions or toxic reagents.     

To dope Mn2+ in a semiconducting nanocrystal

It has been an outstanding problem that a semiconducting host in the bulk form can be doped to a large extent, while the same host in the nanocrystal form is found to resist any appreciable level of doping rather stubbornly, this problem being more acute in the wurtzite form compared to the zinc blende one. In contrast, our results based on the lattice parameter tuning in a Zn(x)Cd(1-x)S alloy nanocrystal system achieves approximately 7.5% Mn(2+) doping in a wurtzite nanocrystal, such a concentration being substantially higher compared to earlier reports even for nanocrystal hosts with the "favorable" zinc-blende structure. These results prove a consequence of local strains due to a size mismatch between the dopant and the host that can be avoided by optimizing the composition of the alloyed host. Additionally, the present approach opens up a new route to dope such nanocrystals to a macroscopic extent as required for many applications. Photophysical studies show that the quantum efficiency per Mn(2+) ion decreases exponentially with the average number of Mn(2+) ions per nanocrystal; en route, a high quantum efficiency of approximately 25% is achieved for a range of compositions.     

反胶束法合成Cd1-xMnxS量子点及其发光性能研究

应用反胶束法制备了稀磁半导体Cd1-xMnxS量子点.量子点的大小可通过改变ωo值(wo=[水]/[表面活性剂])来控制.高分辨透射电镜的分析结果表明,量子点呈单分散性,是几乎没有缺陷的单晶体.量子点的大小约为4.8～6nm,随wo值增大而增大.电子能谱(EDS)测定结果表明,Mn2+离子在量子点中的摩尔分数为1.5%.由电子自旋共振(ESR)分析确定一部分Mn2+离子取代Cd2+离子位置而位于晶格,另一部分Mn2+离子位于Cd1-xMnxS的表面或间隙位置.吸收光谱显示,随着量子点变小,吸收带边发生蓝移,显示明显的量子尺寸效应.光致荧光光谱分析表明,发光峰属于Mn2+的4T1-6A1跃迁,而且随着ωo和粒径的增大,发光峰从2.26,2.10,2.05eV红移到1.88eV;其发光峰偏离2.12eV,主要是由于Mn2+离子位于扭曲的四面体晶体场所致.     

ZrO_2表面上Zr~(3+)中心上的O_2吸附平衡

以ESR为主要研究手段,研究了一种最可几粒径为9.8 nm的ZrO_2样品在室温不 同气体（He,H_2,O_2,空气,温空气）中及经不同温度处理后ZrO_2中Zr~(3+)中 心信号的变化。根据ZrO_2样品中Zr~(3+)浓度在室温下对于介质气体中O_2分压改 变的响应极快及Zr~(3+)浓度在室温下对于介质气体变化可逆性等特点提出, ZrO_2中Zr~(3+)中心绝大多数存在于样品的表面上,Zr~(3+)是ZrO_2晶体表面上那 些O~(2-)配位不饱和的Zr原子点位。ZrO_2表面上ESR活性Zr~(3+)中心的数目与其 上氧吸附平衡有关。由ESR法对ZrO_2样品中Zr~(3+)中心的定量和对ZrO_2晶体的粒 度分析,推测ZrO_2表面上Zr~(3+)中心是那些位于ZrO_2微晶的晶角处O~(2-)配位 不饱和的Zr原子。     

Adsorption and redox reactions of heavy metals on Fe-Mn nodules from Chinese soils

Adsorption of heavy metals and redox reactions of Cr(W) ions on Fe-Mn nodules from five soils of China were investigated by chemical analysis, equilibrium adsorption/redox, and X-ray photoelectron spectroscopy (XPS). Results show that Mn is mainly present as Mn(3+) and Mn(4+) forms in Fe-Mn nodules. The maximum adsorption amounts for different heavy metal ions follow the order Pb(2+) approximately Cu(2+)>Zn(2+)>Co(2+)>Ni(2+)>Cd(2+). The adsorption capacity for heavy metals by Fe-Mn nodules from calciaquert in Shandong province (N5-1) is the highest, while that from hapludalf in Shandong province (N6-1) is the lowest. About 44-100% of the heavy metals adsorbed on Fe-Mn nodules were dissolved in 0.1 mol/L hydroxylamine hydrochloride (HAHC). The maximum amounts of Cr(VI) production by Fe-Mn nodules follow the order of N1-1 (69 mmol/kg)>N4-1 (57 mmol/kg)>N2-1 (52 mmol/kg)>N5-1 (44 mmol/kg). Based on the content of MnO(2) in Fe-Mn nodules dissolved in HAHC, the amount of Cr(VI) production by Mn oxides in N1-1, N2-1, N4-1, and N5-1 is 326, 624, 726, and 482 mmol/kg (MnO(2)), respectively. We propose that the amounts of Cr(VI) production through oxidation Cr(III) by Mn oxides are related to the types of Mn oxides in Fe-Mn nodules.     

Luminescence properties of Mn(2+)-doped Li2ZnGeO4 as an efficient green phosphor for field-emission displays with high color purity

Green emitting Li(2)ZnGeO(4):Mn(2+) phosphors were synthesized through a high temperature solid-state reaction process. X-Ray diffraction, field emission scanning electron microscopy, photoluminescence (PL) and cathodoluminescence (CL) spectra were utilized to characterize the synthesized samples. Under UV and electron-beam excitation, the pure Li(2)ZnGeO(4) sample shows a blue emission due to defects, while the Li(2)ZnGeO(4):Mn(2+) sample exhibits a green emission corresponding to the characteristic transition of Mn(2+) ((4)T(1)→(6)A(1)). In particular, the CL intensity (brightness) of Li(2)ZnGeO(4):Mn(2+) is higher than that of commercial green phosphor ZnO:Zn. In addition, the CL properties of Li(2)ZnGeO(4):Mn(2+) phosphor, the dependence of CL intensity on accelerating voltage and filament current, the decay behavior of CL intensity under electron bombardment, and the stability of CIE chromaticity coordinates, have been investigated in detail. The results indicate that the as-prepared Li(2)ZnGeO(4):Mn(2+) phosphor has a good CL intensity and CIE coordinate stability with green emission under low-voltage electron beam excitation. Therefore, Li(2)ZnGeO(4):Mn(2+) is a promising green phosphor for application in full-color field-emission displays.     

Mn~(2+)掺杂NaBiF_4∶Yb/Er体系的可调控上转换发光

采用溶剂热法制备了不同Mn~(2+)掺杂量的NaBiF_4∶Yb/Er/Mn上转换发光体系,研究了其形貌、晶相、上转换发光性能随Mn~(2+)掺杂量的变化,并探讨了该体系的能量传递机理.实验结果表明,Mn~(2+)的掺杂不会引起NaBiF_4从六方相转变为立方相,但会增大其尺寸;同时在NaBiF_4体系中,Mn~(2+)可以与Er~(3+)进行能量传递,使红光发射得到增强,并且随着Mn~(2+)浓度的增加,红/绿光发射强度比也会随之增大.此外,还考察了NaBiF_4∶Yb/Er/Mn体系的变温发射光谱,发现当温度升高时,红/绿光强度比以及520 nm绿光与540 nm绿光发射强度比都总体上呈增大趋势.     

EPR and photoluminescence studies of ZnO:Mn nanophosphors prepared by solution combustion route

Nanocrystalline ZnO:Mn (0.1 mol%) phosphors have been successfully prepared by self propagating, gas producing solution combustion method. The powder X-ray diffraction of as-formed ZnO:Mn sample shows, hexagonal wurtzite phase with particle size of ～40 nm. For Mn doped ZnO, the lattice parameters and volume of unit cell (a=3.23065 ?, c=5.27563 ? and V=47.684 (?)(3)) are found to be greater than that of undoped ZnO (a=3.19993 ?, c=5.22546 ? and V=46.336 (?)(3)). The SEM micrographs reveal that besides the spherical crystals, the powders also contained several voids and pores. The TEM photograph also shows the particles are approximately spherical in nature. The FTIR spectrum shows two peaks at ～3428 and 1598 cm(-1) which are attributed to O-H stretching and H-O-H bending vibration. The PL spectra of ZnO:Mn indicate a strong green emission peak at 526 nm and a weak red emission at 636 nm corresponding to (4)T(1)→(6)A(1) transition of Mn(2+) ions. The EPR spectrum exhibits fine structure transition which will be split into six hyperfine components due to (55)Mn hyperfine coupling giving rise to all 30 allowed transitions. From EPR spectra the spin-Hamiltonian parameters have been evaluated and discussed. The magnitude of the hyperfine splitting (A) constant indicates that there exists a moderately covalent bonding between the Mn(2+) ions and the surrounding ligands. The number of spins participating in resonance (N), its paramagnetic susceptibility (χ) have been evaluated.