Effects of the Template Composition and Coating on the Photoluminescence Properties of ZnS:Mn Nanoparticles
© The Author(s) 2010
Received: 16 December 2009
Accepted: 26 February 2010
Published: 16 March 2010
Mn-doped ZnS nanocrystals based on low dopant concentrations (0–2%) and coated with a shell of Zn(OH)2 have been prepared via soft template and precipitation reaction. The results indicate that the ZnS:Mn nanocrystal is cubic zinc blende structure and its diameter is 3.02 nm as demonstrated by XRD. Measured by TEM, the morphology of nanocrystals is a spherical shape, and their particle size (3–5 nm) is similar to that of XRD results. Photoluminescence spectra under ultraviolet region shows that the volume ratio of alcohol to water in the template has a great effect on the luminescence properties of ZnS:Mn particles. Compared with unpassivated ZnS:Mn nanocrystals, ZnS:Mn/Zn(OH)2 core/shell nanocrystal exhibits much improved luminescence and higher absolute quantum efficiency. Meanwhile, we simply explore the formation mechanism of ZnS:Mn nanocrystals in alcohol and water system and analyze the reason why alcohol and water cluster structures can affect the luminescent properties of nanoparticle.
The preparation and characterization of II–VI nanoscale semiconductor compounds have attracted much attention over the past few years due to their fundamental properties  and applications, mostly as tunable emitters for biomedical labeling , light emitting diodes (LED), lasers, and sensors [3–5]. The intrinsic toxicity of cadmium has cast a doubtful future in this promising field. However, wide band gap semiconductor nanocrystals, such as zinc chalcogenide ones doped with transition metal ions [6, 7, 9–11], may overcome this concern and yet maintain the advantages of the nanocrystal emitters. In 1994, Bhargava et al.  initiatively proposed luminescence properties of Mn-doped ZnS nanocrystals that were prepared by a room temperature chemical process. Then, nanocrystalline ZnS has been widely and deeply investigated  since nanosized ZnS can be easily prepared. As a typical II–VI semiconductor, ZnS, especially doped with divalent manganese ions, has been commercially used as a phosphor as well as in thin film electroluminescent devices. In the bulk form, ZnS is an interesting II–VI semiconductor with a large band gap 3.68 eV (340 nm). At the nanometer scale, ZnS has attracted more attention due to its interesting optical, electric properties, and large quantum efficiencies depending on its size. For instance, it can be used as a higher band gap material to passivate other quantum semiconductor heterostructure to increase their quantum yields . The properties of the photoluminescent and electroluminescent materials could be greatly affected by doping concentration of Mn. The Mn ion, used as a dopant in many luminescent materials, has a d 5 configuration that exhibits a broad emission peak, and its position strongly depends on the host lattice, which lies on the change in strength of crystal field with host. The emission color can vary from green to deep red, corresponding to a 4T1 6A1 transition. To obtain nanometer-sized particles, a variety of methods have been proposed, including microemulsion method , sol–gel processing , competitive reaction chemistry method , and aqueous chemical method . Since Dixit et al.  observed molecular segregation in a concentrated alcohol–water(A/W) solution, there have been few reports on generating of nano-crystallined CeO2 and silicon in A/W mixed solvents  or in ethylalcohol liquid bridges . In this paper, we focus on preparation of stabilized Mn-doped ZnS nanocrystals by soft template method and coated Zn(OH)2 shells through precipitation reaction. Photoenhanced luminescence and higher quantum efficiency have been observed in ZnS:Mn/Zn(OH)2 nanoparticle.
Zn(CH3COO)2·2H2O(A.R.), Na2S·9H2O(A.R.), and Mn(CH3COO)2·4H2O(A.R.) were employed as raw materials and C2H5OH and H2O mixed solution as soft template. First, Na2S and dopant precursor Mn(CH3COO)2 were ground in mortar to form MnS crystal nucleus, and C2H5OH and H2O solutions were added as soft template in grinding process. Then, Zn(CH3COO)2 was added and ground in soft template for the regrowth of ZnS on the surface of MnS layer. The concentration of the dopant in the nuclei can be regulated varying the precursor ratio. The products were collected by centrifugal sedimentation and first washed by ionized water then by alcohol. Finally, the products were ultrasonically dispersed in alcohol and dried at 40°C in vacuum.
Zn(OH)2 shell, coating on ZnS:Mn nanocrystals can be produced as follows: ZnS:Mn nanocrystal was put into deionized water and ultrasonically dispersed for 2 h. Appropriate amount of Zn(CH3COO)2 aqueous solution was slowly dropped into the ZnS:Mn suspension under the conditions of vigorous stirring. Ten minutes later, appropriate amount of NaOH aqueous solution was dropped into the suspension to form stoichiometric Zn(OH)2. After 1 h of continuous stirring, the resulting precipitates were dealt in the same way as the ZnS:Mn nanocrystals.
All the samples were characterized by powder X-ray diffraction (XRD) using a Rigaku diffractometer with Ni-filtered CuKa radiation at room temperature. The transmission electron microscopy (TEM) images of the nanoparticles were obtained by using TEM (JEM-2100F Electron Microscope/JEOL Co. 200 kV). The optical properties of finely ground samples were collected by an UV–vis spectrophotometer (PE lambda950) using BaSO4 as a reference in the range of 200–700 nm. The photoluminescence (PL) spectra in the UV region were obtained using a FLS-920T fluorescence spectrophotometer equipped with Xe 900 (450 W xenon arc lamp) as the light source with spectral slits width of 1 nm. The quantum yield of the samples excited at 300 nm was recorded with HORIBA JOBIN-YVON Fluorlog-3 spectrofluorometer system. All the spectra were recorded at room temperature.
Result and Discussion
The Volume Ratio of A/W is 1:1
where ε 0 is the permittivity in vacuum and ε is the dielectric constant in a given solution. The symbols r + and r − represent the radii of ions charged z + and z −, respectively, and e represents the elementary charge (e = 1.602 × 10−19 C).
For mixed solvents of homologous alcohol and water system, values of A and B can be regarded as constants. Therefore, it is clear that the change in the dielectric constant of mixed solvent can remarkably affect the nucleation rate and the particle size. In addition, for single water system, due to the small particle size there is a big specific surface area, so the products easily aggregate. Because alcohol and water can dissolve each other unlimitedly, most of water molecules adsorbing on the particle surface can be replaced by alcohol molecules, which diminish the particle surface tension and surface energy; thus, the trend of reunion between the particles can be effectively declined. Besides, the probability of collisions between particles can be reduced by the steric effect of organic system (A/W), which contributes to the formation of nanoparticles with uniformity and good dispersion.
The doped manganese in the precursors may be introduced into inside or outside of ZnS nanocrystals. Sooklal et al.  studied the effect of the location of Mn on the photophysics of ZnS nanocrystals. They found that whether Mn incorporates into the ZnS lattice will lead to the orange emission, while ZnS with Mn on the surface-bound will yield the ultraviolet emission. Narayan Pradhan studied the possible nucleation and growth process, they found that the successful doping and decoupling of doping must fulfill following conditions: with the increase in doped ions, the steady increase in the PL intensity from the doping centers at about 605 nm, the fixed PL positions of the host ZnS nanocrystals and the doping centers, and the gradual decrease in the PL intensity of the host ZnS nanocrystals . Inset of Fig. 3 demonstrates the functional relation between characteristic emission intensity of Mn and doping amount. According to inset of Fig. 3, with the increase in Mn, the emission intensity also increases and reaches its highest intensity at the Mn content of 2%. At the same time, emission intensity of the host ZnS nanocrystals decreased. Compared with Zn, Mn is a harder Lewis acid. Therefore, the Mn precursor would be much less than Zn if they both have the same carboxylate ligand . From above all, it can be concluded that the Mn ions in our samples are indeed introduced into the host ZnS nanocrystals. From the Fig. 3, the highest doping amount is 2%.
Effect of Volume Ratio of A/W on Luminescence Behavior
According to the results, we draw a conclusion from the process of preparing nanocrystals that the function of A/W mixed system is as follows: at that moment, the hydrogen bonds, hydration, and ethanol–water molecules are no longer separate water molecules or ethanol molecules but bridged molecular clusters formed by a number of ethanol molecules and water molecules. It is deduced that there are three possible cluster structures : five ethanol molecules and six water molecules connect with each other to form a ring or chain structure through the hydrogen bond; an ethanol molecule and two water molecules form the chain structure with hydrogen bond; and an ethanol molecule and five water molecules link mutually to form a new molecular structure. When the solution has a 60% (vol) of ethanol, the concentration of the first component cluster arrives at the maximum, the second and third types of the component cluster are, respectively, correspond to the alcohol concentration of 40 and 80%. From experiment results, it could be inferred that in the course of obtaining integrated particle crystallization, the second cluster structure is the best template, because our sample with best emission intensity was produced in the condition of the closest approach to the second type molecular structure of alcohol (40%).The optimal volume of A/W is 1:2.
R was calculated ranges from 1.5 to 2.5 nm (a diameter from 3 to 5 nm), in reasonable agreement with the XRD results.
Quantum efficiency results
Luminescent decay results
In summary, ZnS:Mn and ZnS:Mn/Zn(OH)2 nanocrystals were synthesized by soft template and precipitation reaction method. As the Mn concentration increased, the emission intensity increases and reaches highest with the Mn content at 2%. Meanwhile, the emission intensity varies with the ratio of alcohol. When alcohol and water volume ratio is 1:2, the strongest emission intensity and shortest lifetime decay is achieved, which is attributed to the integrity of better particle getting from structure of alcohol/water template. When ZnS:Mn is coated with Zn(OH)2, the emission intensity of Mn enhances by 30%, and its absolute quantum efficiency increases by 2.3%.
This work was supported by the National Natural Science Foundation of China (10874061) and Research Fund for the Doctoral Program of Higher Education (200807300010).
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