Synthesis and Luminescence Properties of Core/Shell ZnS:Mn/ZnO Nanoparticles
© to the authors 2008
Received: 18 September 2008
Accepted: 30 October 2008
Published: 26 November 2008
In this paper the influence of ZnO shell thickness on the luminescence properties of Mn-doped ZnS nanoparticles is studied. Transmission electron microscopy (TEM) images showed that the average diameter of ZnS:Mn nanoparticles is around 14 nm. The formation of ZnO shells on the surface of ZnS:Mn nanoparticles was confirmed by X-ray diffraction (XRD) patterns, high-resolution TEM (HRTEM) images, and X-ray photoelectron spectroscopy (XPS) measurements. A strong increase followed by a gradual decline was observed in the room temperature photoluminescence (PL) spectra with the thickening of the ZnO shell. The photoluminescence excitation (PLE) spectra exhibited a blue shift in ZnO-coated ZnS:Mn nanoparticles compared with the uncoated ones. It is shown that the PL enhancement and the blue shift of optimum excitation wavelength are led by the ZnO-induced surface passivation and compressive stress on the ZnS:Mn cores.
KeywordsZnS:Mn/ZnO Nanoparticles Core/shell structure Luminescence II-VI Semiconductor
In recent years, a great deal of attention has been devoted to doped semiconductor nanomaterials mainly due to their unique luminescence properties arising from quantum confinement effects. The doping ions act as recombination centers for the excited electron–hole pairs and play an influential role in determining optical properties, resulting in strong luminescence. Among the semiconductor nanomaterials, zinc sulfide is particularly suitable for use as luminescent host materials for a variety of dopants because of its wide band gap energy at room temperature. There have been numerous reports about the structural and luminescence properties of doped ZnS nanocrystals, such as ZnS nanocrystals doped with manganese [1–4], copper [5–7], silver , samarium , europium [10, 11], and terbium . It has been extensively studied for a variety of commercial devices, such as electro-optic modulators, photoconductors, optical coatings, field effect transistors, infrared windows, electroluminescent display [13–17]. Unfortunately, serious drawbacks exist in these nanomaterials, such as their instability under high temperature treatment , degradation during the operation and dispersal into vacuum, which will contaminate the field emitter and thus hinder the electron emission , and high surface-to-volume ratio because of small particle size, which will result in low luminescence efficiency. In order to conquer such serious shortcomings, accordingly, core/shell structural nanomaterials have been developed and have shown dramatically enhanced properties. Enhanced luminescence and/or stability has been observed in ZnS:Mn/SiO2 nanoparticles , ZnS:Mn/ZnS nanocrystals , ZnS:Mn/Zn(OH)2 nanoparticles , ZnS:Mn/ZnO nanocrystals , and CdSe/ZnO nanoparticles . In addition, the ZnO particles on the ZnS:Ag particulates have demonstrated a high effectiveness to prevent the degradation of ZnS:Ag particulates from electron bombardment . Compared with the uncoated nanocrystals, the photoluminescence of the nanocrystals with a core/shell structure is enhanced. This is usually interpreted as being due to the surface passivation that inhibits the nonradiative recombination, thus improving the photoluminescence properties. At the same time, a higher stability will be obtained, coming from the protection effect of the surrounding matrix .
ZnO-coated ZnS:Mn nanoparticles have been prepared by different groups [23, 27]; however, the ZnO shells reported in these literatures are of rather poor crystallinity, which is indicated by their X-ray diffraction (XRD) results. Usually, the ZnO shell with higher crystallinity will produce stress on the ZnS:Mn because of the lattice mismatch, and bring efficient luminescent transitions inducing enhanced luminescence . In this paper, a convenient route to prepare higher crystallinity core/shell structural ZnS:Mn/ZnO nanoparticles is reported, with the emphasis on the improved luminescence properties brought by the ZnO capping on the ZnS:Mn nanoparticles. The shell material, ZnO, is a wide band gap semiconductor with excellent chemical and thermal stability. Therefore, surface modification of ZnS:Mn nanoparticles by ZnO shell is expected to have a passivating effect on the surface states of ZnS:Mn, which would result in enhanced luminescence.
All the reactants and solvents used in this work were of analytical grade and used without any further purification.
Synthesis of ZnS:Mn Cores
Typically, 0.01 mol zinc acetate [Zn(CH3COO)2 · 2H2O], 0.005 mol thioacetamide [CH3CSNH2], and 5 × 10−5 mol manganese acetate [Mn(CH3COO)2 · 4H2O] were put into a Teflon-lined stainless steel autoclave of 72 mL capacity, and then the autoclave was filled with a mixture solvent of ethylenediamine and deionized water (in 1:1 volume ratio) to 80% of its total volume. Then the reactants and solvents were stirred for 20 min. After being sealed, the autoclave was maintained at 200 °C for 6 h and then cooled down to room temperature naturally. The final precipitates were separated by centrifugation at 5000 rpm and washed with deionized water and absolute ethanol three times to remove excessive ethylenediamine and by-products. The samples were then dried in vacuum at 50 °C for 5 h and collected for further characterization and treatment.
Surface Synthesis of ZnO Shells on ZnS:Mn Cores
Dosage of Zn(NO3)2aqueous solution (0.05 M) used in forming ZnO shells. (In the parentheses, 0, 0.1, 0.2, 0.3, 0.4, 0.5 and 1 are set to be the mole ratios of Zn2+ions in shells and cores)
Volume of Zn(NO3)2aqueous solution (mL)
The X-ray diffraction patterns of as-synthesized ZnS:Mn/ZnO nanoparticles were collected on a D8 ADVANVCE X-Ray diffractometer. A JEM-2100 transmission electron microscope (TEM) was used to study the morphologies of the samples. The room temperature photoluminescence (PL) spectra were achieved by a Flurolog-3p fluorescence spectrophotometer. The X-ray photoelectron spectroscopy (XPS) measurements made on the samples with and without sputtering were performed on ESCA MK II.
Results and Discussion
In Fig. 6, the excitation spectra monitoring the characteristic emission of Mn2+ ions show a broad band with a peak at ~350 nm, which is the characteristic peak of ZnS absorption. It demonstrates that the emission of transition from 4T1 to 6A1 of Mn2+ ions takes place via the energy transfer from the excited ZnS host lattice to Mn2+ ions. Compared with the uncoated ZnS:Mn nanoparticles, a blue shift shown in the PLE spectra of ZnS:Mn/ZnO(0.1) and ZnS:Mn/ZnO(0.2) is slight (Fig. 5b, c), which is dramatic for ZnS:Mn/ZnO(0.3), ZnS:Mn/ZnO(0.4), ZnS:Mn/ZnO(0.5), and ZnS:Mn/ZnO(1) (Fig. 5d, e, f, and g). Such a blue shift has also been observed in the ZnS:Mn nanocrystals after surface passivation by silica . In addition, it has been reported that the strain between core and shell in such core/shell semiconductor nanocomposites will broaden the band gap of the core, resulting in a blue shift of the emitted energy [36, 37]. Hence, a possible mechanism that can explain the observed blue shift in this excitation spectra is as follows: The stress caused by the lattice mismatch between ZnO and ZnS can alter the ZnS lattice parameters slightly , and further influence the energy level structure of ZnS host, resulting in the shift of excitation spectra to shorter wavelength. This differs from the previous report where ZnS:Mn nanocrystals coated by ZnO shell showed a red shift in comparison with the uncoated ones in PLE spectra . The distinction is presumably due to the higher crystallinity of the ZnO shells reported here, which will result in lattice mismatch with ZnS cores producing compressive stress as discussed above, and the stress should be responsible for the blue shift in the PLE spectra. However, details of this phenomenon are not very clear at this stage.
Core/shell ZnS:Mn/ZnO nanoparticles were successfully synthesized, and their structural and luminescence properties were investigated. XRD, HRTEM, and XPS results reinforced the claim that ZnO shells were coated on the surface of ZnS:Mn cores. Compared with the uncoated ZnS:Mn nanoparticles, the Mn emission intensity of ZnO-coated ZnS:Mn showed a strong increase followed by a gradual decline, which was led by the opposite effects of ZnO shells, i.e., surface passivation inducing luminescence enhancement and decrease of the luminescent centers (Mn2+ions) inducing luminescence weakening. The obvious blue shift observed in the PLE spectra was caused by the ZnO-induced compressive stress on the ZnS:Mn cores.
The authors thank the National Science Foundation of China (NSFC 50672089) and the Encouraging Foundation for the Scientific Research of the Excellent Young and Middle-aged Scientists in Shandong Province (2006BS04034) for financial support.
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