Introduction

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 do** 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 [8], samarium [9], europium [

Experimental Details

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

In order to make ZnO shells coating on ZnS:Mn nanocrystals, the reaction between Zn(NO3)2 and NaOH was employed. In a typical process, 0.2 g bare ZnS:Mn nanocrystals powder was put into 200 mL deionized water and ultrasonically dispersed for 1 h, followed by slow drop** of appropriate amount of 0.05 M Zn(NO3)2 aqueous solution into the ZnS:Mn suspension under vigorous stirring. Drops of 0.1 M NaOH aqueous solution were then slowly added until the pH value arrived at ~10 [29], at which point ZnO precipitated at the surface of the ZnS:Mn nanocrystals. The nucleation and growth of a separate ZnO particle were suppressed by the very low concentration of NaOH in the ZnS:Mn suspension in the drop** process. The resulting precipitates were dealt with in the same procedure as that for ZnS:Mn nanocrystals. The thickness of ZnO shell was controlled by varying the dosage of Zn(NO3)2, as indicated in Table 1.

Table 1 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)
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Characterization

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

Structural Characterization

The X-ray diffraction results are plotted in Fig. 1. Figure 1a shows the XRD patterns of the uncoated ZnS:Mn nanocrystals. The four diffraction peaks can be indexed as (111), (200), (220), and (311) planes of cubic zinc blende structure, which match well with the standard card (JCPDS NO. 05-0566). The XRD patterns of some representative core/shell structural ZnS:Mn/ZnO nanoparticles are presented in Fig. 1b–e. From Fig. 1b, it can be found that there are almost no XRD peaks from crystalline ZnO, indicating that the amount of ZnO is very low in this sample. Compared with the bare ZnS:Mn core, some new diffraction peaks appear in Fig. 1c–e, which centered at about 31.7°, 34.4°, 36.2°, 62.8°, 67.9°, and 69.0°. These peaks correspond to (100), (002), (101), (103), (112) and (201) diffraction peaks from the standard card (JCPDS NO. 80-0074), respectively, indicating that the ZnO shells possess a wurtzite structure. From the XRD results, it can be clearly observed that the intensity of the diffraction peaks from ZnO becomes stronger progressively, providing evidence for the gradual increase in the amount of ZnO shell as planed.

Figure 1
figure 1

XRD patterns of ZnS:Mn nanoparticlesa without andb–e with ZnO shells of different thicknesses

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Figure 2a, b shows two typical TEM images of ZnS:Mn nanoparticles without and with ZnO shell. From Fig. 2a, we can see that the as-synthesized ZnS:Mn nanoparticles are spherical, with an average diameter of about 14 nm. In comparison with Fig. 2a, it can be seen in Fig. 2b that the ZnS:Mn nanoparticles coated with a ZnO shell (shell/core = 1) are almost kept unchanged in average diameter. This is possibly because the thickness of ZnO shell is too thin to induce an obvious change in particle size. Figure 2c shows the high-resolution TEM (HRTEM) image of a single ZnS:Mn nanoparticle. From the figure, the lattice spacing was calculated to be around 0.32 nm, which corresponds to the (111) lattice plane spacing of ZnS in cubic phase. HRTEM investigations of core/shell ZnS:Mn/ZnO(1) (Fig. 2d) show the interface between ZnO shell and ZnS:Mn core. The two measured distances between adjacent lattice fringes are 0.26 and 0.32 nm, which correspond to the (002) lattice plane of a hexagonal ZnO crystal and the (111) lattice plane of a cubic ZnS crystal, respectively.

Figure 2
figure 2

TEM images of ZnS:Mn and ZnS:Mn/ZnO nanoparticles.a ZnS:Mn.b ZnS:Mn/ZnO(1).c HRTEM iamge of a single ZnS:Mn nanoparticle.d HRTEM image of ZnS:Mn/ZnO(1) nanoparticle showing the interface between shell and core

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The surface sensitivity of XPS can be used to indicate whether or not the ZnO covers the outer surface of ZnS:Mn nanocrystal [30]. XPS survey spectrum of sample ZnS:Mn/ZnO(0.5) is plotted in Fig. 3a. The peaks located at 1,021.6, 532.1, and 162.2 eV correspond to Zn2p3/2, O1s, and S2p, respectively. Displayed in Fig. 3b, c and d are the finely scanned Zn2p3/2, O1s, and S2p peaks of ZnS:Mn/ZnO(0.5), respectively.

Figure 3
figure 3

XPS spectra of sample ZnS:Mn/ZnO(0.5).a Survey spectrum.b Zn 2p3/2 spectrum;c O1s spectrum.d S2p spectrum

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In order to show further evidence that ZnO covers on the outer surface, XPS measurements were carried out on ZnS:Mn/ZnO(0.5) with and without sputtering (Fig. 4). The atomic ratio of oxygen to sulfur can be calculated by dividing the area of the XPS peaks by their respective sensitivity factors (2.93 for O and 1.68 for S) [31]. As shown in Fig. 4, the oxygen to sulfur atomic ratio first decreases gradually and remains almost invariant thereafter with increasing sputtering time, which confirms the presence of ZnO shell on the outer surface of ZnS:Mn cores [23].

Figure 4
figure 4

Representative XPS spectrum of sample ZnS:Mn/ZnO(0.5) showing relative change in O/S atomic ratio with sputtering time

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Photoluminescence Properties

The room temperature photoluminescence emission (PL) and excitation (PLE) spectra of all samples are plotted in Fig. 5 and Fig. 6. In Fig. 5, the PL spectra of all samples present two emission bands. The orange emission, with a peak wavelength of ~580 nm, is attributed to the transition from 4T1 (excited) to 6A1 (ground) of Mn2+ ions [34]. The photoluminescence quantum yield (PLQY) was measured against rhodamine as standard, and the values for bare ZnS:Mn, ZnS:Mn/ZnO(0.1), and ZnS:Mn/ZnO(1) are 0.042, 0.19, and 0.034%, respectively.

Figure 5
figure 5

PL spectra excited at 350 nm of ZnS:Mn nanoparticlesa without andb–g with ZnO shells of different thickness

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Figure 6
figure 6

PLE spectra monitored at 580 nm of ZnS:Mn nanoparticlesa without andb–g with ZnO shells of different thicknesses

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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 [20]. 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 [28], 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 [27]. 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.

Conclusion

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.