Synthesis and optical characterization of PVP and SHMP-encapsulated Mn-doped ZnS nanocrystals

Zinc sulfide semiconductor nanocrystals doped Mn²⁺ have been synthesized via a solution-based method utilizing optimum dopant concentration (4%) and employing polyvinyl pyrrolidone (PVP) and sodium hexametapolyphosphate (SHMP) as capping agents. UV-vis absorbance spectra for all of the synthesized nanocrystals show an exitonic peak at around 310 nm. The particle size and morphology were characterized by scanning electron microscopy (SEM), FT-IR, X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence spectrum (PL). Diffraction data confirmed that the crystallite size is around 3-5 nm. Room temperature photoluminescence (PL) spectrum for the bare ZnS sample shows a strong band at ∼445 nm. The uncapped and capped(SHMP, PVP) ZnS:Mn²⁺ samples show a strong and broad band in the ∼580-585 nm range.     

ZnS:Mn2+,Sm3+粉末材料中Mn2+和Sm3+之间的能量传递

研究了ZnS粉末材料中Mn2+中心和Sm3+中心之间的相互作用.通过测量单独由Mn2+或Sm3+掺杂及Mn2+,Sm3+同时掺杂的ZnS粉末材料的发射光谱、激发光谱、发光衰减以及选择激发发光光谱,证实了Mn2+和Sm3+之间存在偶极子-偶极子相互作用的无辐射能量传递.同时还计算了能量传递几率和传递效率.     

The optical properties and energy transition process in nanocomposite of polyvinyl-pyrrolidone polymer and Mn-doped ZnS

This study has been carried out on the optical properties of polyvinyl-pyrrolidone (PVP), the energy transition process in nanocomposite of PVP capped ZnS:Mn nanocrystalline and the influence of the PVP concentration on the optical properties of the PVP capped ZnS:Mn nanocrystalline thin films synthesized by the wet chemical method. The microstructures of the samples were investigated by X-ray diffraction, the atomic absorption spectroscopy, and transmission electron microscopy. The results showed that the prepared samples belonged to the sphalerite structure with the average particle size of about 2–3 nm. The optical properties of samples are studied by measuring absorption, photoluminescence (PL) spectra and time-resolved PL spectra in the wavelength range from 200 to 700 nm at 300 K. From data of the absorption spectra, the absorption edge of PVP polymer was found about of 230 nm. The absorption edge of PVP capped ZnS:Mn nanoparticles shifted from 322 to 305 nm when the PVP concentration increases. The luminescence spectra of PVP showed a blue emission with peak maximum at 394 nm. The luminescence spectra of ZnS:Mn–PVP exhibits a blue emission with peak maximum at 437 nm and an orange–yellow emission of ion Mn²⁺ with peak maximum at 600 nm. While the PVP coating did not affect the microstructure of ZnS:Mn nanomaterial, the PL spectra of the PVP capped ZnS:Mn samples were found to be affected strongly by the PVP concentration.     

Influences of capping molecules on optical properties of nanocrystalline ZnS:Cu

ZnS:Cu nanocrystals capped with different capping molecules have been successfully synthesized by a simple aqueous method. The prepared nanocrystals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive analysis by X-rays (EDAX). The surface characterization of the nanocrystals was done by FTIR spectroscopy. The effect of capping agents on absorption and photoluminescence (PL) spectra of the ZnS:Cu nanocrystals was studied. A blue shift of the absorption peaks was observed and attributed to a quantum confinement effect, which increases the band gap energy. The photoluminescence spectra of the capped ZnS:Cu nanocrystals showed a broad peak in the range of 460–480 nm. The intensity of the PL spectra strongly depended on the capping agents.     

Enhancement of photoluminescence of ZnS:Mn nanocrystals modified by surfactants with phosphate or carboxyl groups via a reverse micelle method

A colloidal suspension of ZnS:Mn nanocrystals was prepared in sodium bis(2-ethylhexyl)suflosuccinate reverse micelles, and then modified by surfactants with phosphate or carboxyl groups. The photoluminescent intensity at 580 nm due to d-d transition of Mn²⁺ ions increases up to a factor of 6.3 and its quantum efficiency increases from 1.7% to 8.1% after modification. According to ³¹P nuclear magnetic resonance spectra, surfactants with phosphate groups adsorb on the surface of ZnS nanocrystal and ³¹P nucleus spins are relaxed rapidly by interaction with five unpaired 3d electrons of Mn²⁺, showing that phosphate groups are located in the vicinity of Mn²⁺. The excitation spectra for the emission due to phosphate or carboxyl groups are similar to those for the emission at 580 nm corresponding to the excitation of ZnS. Both excitation spectra shift in parallel with increasing the amount of surfactant to show the linear relationship. We, therefore, attribute the increase in quantum efficiency at 580 nm to additional energy transfer of ZnS→functional groups→Mn²⁺ as well as to the reduction of energy loss due to non-radiative transition by surface modification.     

玻璃中的ZnS:Mn2+超微粒发光的量子尺寸效应

自从Bhargava等^[1]报道了化学反应合成的ZnS:Mn²⁺纳米微粒的光学性质,掺杂半导体纳米微粒发光性质的研究受到了极大的重视。掺杂纳米微粒有可能成为新的一类发光材料.本文报导用熔融法制备的ZnS:Mn²⁺玻璃在光学性质上的量子尺寸效应。     

Temperature dependence of photoluminescence and magnetic properties in Yb-doped ZnS nanocrystals

Zinc sul?de semiconductor nanocrystals doped with Yb³⁺ ions have been prepared through a chemical precipitation method using PVP (polyvinylpyrrolidone) as a capping agent. The structure of the nanoparticles has been analyzed by X-ray diffraction (XRD). The average size of the nanoparticles is found to be 3.7±0.4 nm. Photoluminescence spectra were recorded for doped ZnS nanoparticles as a function of temperature between 9 and 300 K. The results suggest that two emission bands have been observed at different temperatures. The width of 1.269 eV peak increases as temperature is raised. A shift of lower energy emission band has been observed with the change of temperature. Moreover, the magnetic measurement showed that the sample exhibits paramagnetic behavior.     

Strong green luminescence of Ni2+-doped ZnS nanocrystals

ZnS nanoparticles doped with Ni²⁺ have been obtained by chemical co-precipitation from homogeneous solutions of zinc and nickel salt compounds, with S^2- as precipitating anion, formed by decomposition of thioacetamide (TAA). The average size of particles doped with different mole ratios, estimated from the Debye–Scherrer formula, is about 2–2.5 nm. The nanoparticles could be doped with nickel during synthesis without altering the X-ray diffraction pattern. A Hitachi M-850 fluorescence spectrophotometer reveals the emission spectra of samples. The absorption spectra show that the excitation spectra of Ni-doped ZnS nanocrystallites are almost the same as those of pure ZnS nanocrystallites (λ_ex=308–310 nm). Because a Ni²⁺ luminescent center is formed in ZnS nanocrystallites, the photoluminescence intensity increases with the amount of ZnS nanoparticles doped with Ni²⁺. Stronger and stable green-light emission (520 nm) (its intensity is about two times that of pure ZnS nanoparticles) has been observed from ZnS nanoparticles doped with Ni²⁺. Received: 18 December 2000 / Accepted: 17 March 2001 / Published online: 20 June 2001     

Synthesis and photoluminescence characteristics of doped ZnS nanoparticles

Free-standing powders of doped ZnS nanoparticles have been synthesized by using a chemical co-precipitation of Zn²⁺, Mn²⁺, Cu²⁺ and Cd²⁺ with sulfur ions in aqueous solution. X-ray diffraction analysis shows that the diameter of the particles is ∼2–3 nm. The unique luminescence properties, such as the strength (its intensity is about 12 times that of ZnS nanoparticles) and stability of the visible-light emission, were observed from ZnS nanoparticles co-doped with Cu²⁺ and Mn²⁺. The nanoparticles could be doped with copper and manganese during the synthesis without altering the X-ray diffraction pattern. However, doping shifts the luminescence to 520–540 nm in the case of co-doping with Cu²⁺ and Mn²⁺. Doping also results in a blue shift on the excitation wavelength. In Cd²⁺-doped ZnS nanometer-scale particles, the fluorescence spectra show a red shift in the emission wavelength (ranging from 450 nm to 620 nm). Also a relatively broad emission (ranging from blue to yellow) has been observed. The results strongly suggest that doped ZnS nanocrystals, especially two kinds of transition metal-activated ZnS nanoparticles, form a new class of luminescent materials. Received: 16 October 2000 / Accepted: 17 October 2000 / Published online: 23 May 2001     

Effects of doping on the surface energies of nanocrystals and evidence from studies at high pressure

Zn_1−XMn_XS (X=0.85% and 1.26%) nanoparticles have been synthesized using a specially designed equipment and we have studied the influence of doping Mn²⁺ on the surface energy of ZnS. The high pressure behaviors of ZnS nanocrystals with different dopant contents have been investigated using angle-dispersive synchrotron X-ray powder diffraction up to 45.1 GPa. Theoretical calculations show that doping with Mn²⁺ increases the surface energy of the nanocrystals. The theoretical result has been further corroborated by our experimental observation of an increase in the phase transition pressure of Mn²⁺ doped ZnS nanocrystals in diamond-anvil-cell studies.     

Photoluminescence from doped ZnS nanostructures

Photoluminescence (PL) properties of differently doped nanocrystalline ZnS encapsulated by ZnO (ZnS/ZnO) are reported. It is found that in all cases aluminium as an extra/additional dopant leads to PL enhancement. In comparison to reported blue emitting bulk ZnS:Ag, or green emitting bulk ZnS:Cu, our nanocrystalline samples show a different PL emission profile. This observation is attributed to nanogranule formation, different dopant levels and ZnO capping related energy level modifications.     

Study of energy transfer from capping agents to intrinsic vacancies/defects in passivated ZnS nanoparticles

The study of energy transfer mechanism from different capping agents to intrinsic luminescent vacancy centres of zinc sulphide (ZnS) has been reported in the present work. Nanoparticles of capped and uncapped ZnS are prepared by co-precipitation reaction. These nanoparticles are sterically stabilized using organic polymers—poly vinyl pyrrolidone, 2-mercaptoethanol and thioglycerol. Monodispersed nanoparticles were observed under TEM for both capped and uncapped ZnS nanopowders. However, for uncapped ZnS nanopowders, tendency for formation of nanorod like structure exists. Size of ZnS crystallites was calculated from X-ray diffraction pattern. The primary crystallite size estimated from X-ray diffraction pattern is 1.95–2.20 nm for capped nanostructures and 2.2 nm for uncapped nanostructures. FTIR spectra were conducted to confirm capping. Zeta potential measurements have been done to check the stability of dispersed nanoparticles. Band gap measurement was done by UV–visible spectrophotometer. Excitation and emission spectra are also performed in order to compare optical properties in various samples. Increase in emission intensity and band gap has been observed by adding different capping agents in comparison to uncapped ZnS nanoparticles. The results show that in capped ZnS nanoparticles the mechanism of energy transfer from capping layer to photoluminescent vacancy centres is more pronounced.     

双功能配体在ZnSTb乙醇溶胶合成中的应用

以双功能团配体SCN^-链接ZnS和稀土离子Tb³⁺,制备了ZnSTb乙醇溶胶,以紫外吸收和光致发射光谱研究了其光学性质.该溶胶经紫外激发得到了Tb³⁺的四个特征发射,分别对应于Tb³⁺的⁵D₄→⁷F_J(J=6,5,4,3)跃迁;Tb³⁺的特征发射可能来自ZnS到Tb³⁺的能量传递,即ZnS纳米微粒吸收能量,并将其通过SCN^-桥传递给稀土离子Tb³⁺,从而导致Tb³⁺的特征发射.文中还讨论了对双功能团桥联配体的一般要求.     

Strong violet luminescence from ZnO nanocrystals grown by the low-temperature chemical solution deposition

The luminescence properties of zinc oxide (ZnO) nanocrystals grown from solution are reported. The ZnO nanocrystals were characterized by scanning electron microscopy, X-ray diffraction, cathodo- and photoluminescence (PL) spectroscopy. The ZnO nanocrystals have the same regular cone form with the average sizes of 100-500 nm. Apart from the near-band-edge emission around 381 nm and a weak yellow-orange band around 560-580 nm at 300 K, the PL spectra of the as-prepared ZnO nanocrystals under high-power laser excitation also showed a strong defect-induced violet emission peak in the range of 400 nm. The violet band intensity exhibits superlinear excitation power dependence while the UV emission intensity is saturated at high excitation laser power. With temperature raising the violet peak redshifts and its intensity increases displaying unconventional negative thermal quenching behavior, whereas intensity of the UV and yellow-orange bands decreases. The origin of the observed emission bands is discussed.     

Temperature Dependence of the Fluorescence Emission Intensity of Eu/TiO2 Nanocrystals

The samples of europium ions doped titanium dioxide （Eu^3＋/TiO2） nanocrystals are synthesized by a modified sol-gel method with hydrothermal treatment. The x-ray diffraction and scanning electron microscopy are used to characterize the sample. The temperature-dependent fluorescence emission effect of Eu^3＋-doped samples is investigated. It is found that under the excitation of 514.5nm light, the emission intensity of Eu^3＋ reaches a maximum value at 450K among various Eu^3＋ dopant concentrations in Eu^3＋ /TiO2 nanocrystals. The variation of the emission intensity may be attributed to the photon-assist absorption and the temperature-quenching effect.     

场致发光薄膜ZnS:Mn,Er内Mn中心和Er中心的相互作用

首次提出了场致发光中Mn及稀土离子的能量传递问题.研究了同时掺有Mn和Er的ZnS:Mn,Er薄膜在脉冲激发下场致发光的衰减常数与杂质浓度的关系.证实了由Mn到Er的能量传递.估计了传递系数及传递效率.     

Highly luminescent (ZnSe)ZnS core-shell quantum dots for blue to UV emission: synthesis and characterization

We synthesized nearly monodisperse bare ZnSe nanocrystallites having luminescence which ranges in wavelength from 340 to 430 nm via nucleation due to supersaturation and growth followed by size selective precipitation. Bare ZnSe dots' outermost surface is passivated with organic HDA/TOP. In order to enhance the radiative emission from the semiconductor nanocrystals, we capped the bare ZnSe quantum dots with ZnS semiconductor materials of a wider band gap and 5% of lattice mismatch and produced highly luminescent core-shell (ZnSe)ZnS quantum dots. The core-shell (ZnSe)ZnS nanocrystals show 20 times or more as greatly enhanced luminescence quantum yields as those of bare ZnSe nanocrystals. The ZnSe bare dots and the (ZnSe)ZnS core-shell dots have cubic zinc blende structures as expected from the bulk structure. The observed shapes of bare ZnSe and core-shell (ZnSe)ZnS dots are nearly spherical or ellipsoidal with the aspect ratios of 1.2 and 1.4, respectively. They are not faceted.     

Mn2+, Pb2+共掺杂ZnS纳米材料制备及光致发光

采用聚乙烯基吡咯烷酮(PVP)为表面包覆剂,在室温大气条件下的水溶液中制备了ZnS:Mn,Pb纳米晶。讨论了Mn²⁺和Pb²⁺掺杂量对ZnS纳米发光材料光致发光强度的影响,确定了Mn²⁺和Pb²⁺掺杂量相对于Zn²⁺的最佳的量的比,并对其发光机理进行了初步的探讨。     

Dopant induced morphology changes in ZnO nanocrystals

Zinc oxide (ZnO) nanocrystals doped with different groups of impurities, e.g., Li, Na, Cu, Pr and Mg synthesized by solid-state reaction method under similar conditions exhibit different morphology. XRD showed monophasic wurtzite structure but change in lattice parameters and Zn-O bond length indicates incorporation of dopant ion in ZnO lattice. The morphology of ZnO nanocrystals exhibited striking dependence on type of dopant ion with the shape changing from nanorods, spherical to petal like particles. Photoluminescence (PL) shows pronounced UV emission and negligible visible emission for Li, Na and Cu doped ZnO nanocrystals with peak positions coinciding with that of undoped ZnO. Whereas signature emission of Pr³⁺ ion as well as broad visible emission from Mg doped ZnO revealed the role of intra gap metastable states formed by the dopant ion in the emission process.     

Ba2MgSi2O7:RE荧光粉发光性能的研究

采用高温固相法合成了Ba2MgSi2O7:RE(RE=Eu2 ,Ce3 ,Tb3 )系列荧光粉,研究了其结构和光谱特性.样品的XRD衍射图表明稀土离子的掺入没有引起基质晶格结构的变化.样品Ba2MgSi2O7:Ce3 在紫外光激发下呈蓝紫色发射,而样品Ba2MgSi2O7:Eu2 在紫外光激发下呈绿色发射.对样品Ba2MgSi2O7:Eu2 的光谱分析表明Eu2 在焦硅酸钡体系中可能占据两个不同格位.此外还分析了Ce3 ,Eu2 和Ce3 ,Tb3 共激活焦硅酸钡盐在紫外光激发下的光谱特性,并对其中存在的能量传递机理进行了讨论.