Phosphine-free synthesis of ZnSe:Mn and ZnSe:Mn/ZnS doped quantum dots using new Se and S precursors

Abstract

ZnSe:Mn and core–shell ZnSe:Mn/ZnS doped quantum dots (d-dots) have been synthesized by using the alkylamine–H₂Se complex and thiourea as new phosphine-free Se and S precursors, respectively. Absorption spectroscopy, steady-state (solution and solid-state) and time-resolved photoluminescence spectroscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy were used to characterize the resulting d-dots. Results showed that Mn²⁺ was successfully doped and mainly distributed in the middle layer of the actual ZnSe/ZnSe:Mn/ZnSe d-dots and ZnSe/ZnSeS:Mn/ZnS d-dots which were simply referred to as ZnSe:Mn and core–shell ZnSe:Mn/ZnS d-dots with ZnSe and ZnS outmost shells, respectively. The increase in the intensity of phosphorescence from the dopant Mn²⁺ was obtained by the corresponding injection of Zn precursor or Zn/S precursors for ZnSe or ZnS shell growth. As compared to the ZnSe shell, the ZnS shell was grown more significantly on the surface of the ZnSe:Mn core, which led to larger d-dots due to a thicker ZnS shell and thus deeper doping of Mn²⁺ in the larger host. Outstanding improvement has been achieved in both phosphorescence intensity and stability after growth of the ZnS shell relative to the ZnSe shell. The corresponding phosphorescence efficiency of the resulting d-dot cyclohexane solution was enhanced from about 8% for the ZnSe:Mn d-dots to 35% for the ZnSe:Mn/ZnS d-dots.

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1. Introduction

In the past two decades, great progress has been made in the synthesis of high-quality luminescent Cd-based quantum dots (q-dots) and their application for biolabeling and imaging mostly in the biomedical field, where the Cd-based q-dots were used as excellent emitters with many advantages including a broad, strong and tunable absorption and a narrow, symmetric and tunable emission in the visible range.^1–5 In recent years, the application of these q-dots in the field of light emitting diodes (LEDs), especially white LEDs, has also attracted significant attention mainly due to the high quantum efficiency and color purity.^6–9 Many high-performance q-dot-based colourful and white LEDs have been developed in the further promotion of rapid advances in III-nitride LEDs serving as pump excitation sources.^10–12 Unfortunately, despite these apparent advantages and potential applications, the intrinsic toxicity of Cd to both the environment and human body has brought the Cd-based q-dots into a complicated and disadvantageous position in many practical applications.¹³ Doping of non-toxic Zn-based q-dots is regarded as a powerful technology for endowing them with new properties and functions.^14–16 For example, Mn-doped ZnSe q-dots, namely ZnSe:Mn doped quantum dots (ZnSe:Mn d-dots), not only integrate both photoluminescence (PL) properties and magnetic properties, but also overcome some drawbacks of q-dots such as instability in the environment, strong Forster energy transfer and re-absorption phenomena at high concentration.^13,17 As a greener alternative to Cd-based q-dot emitters, ZnSe:Mn d-dots are becoming a spotlight in the field of basic and applied research.^17–20

Many approaches have been developed to synthesize ZnSe:Mn d-dots, such as the traditional organometallic approach, hydro/solvothermal method, and the newly developed nucleation-doping and growth-doping strategies based on decoupling of the nucleation and growth process, and so on.^{14,17,19,21–29} To date, the nucleation-doping strategy is the most excellent and best developed synthetic approach to high-quality ZnSe:Mn d-dots, where the strong PL from the dopant Mn²⁺ is due to the relatively deep incorporation of Mn²⁺ into the ZnSe host.^18,19,29 In the previously reported synthesis of ZnSe:Mn d-dots, tri-n-octylphosphine (TOP) is usually used to prepare the selenium (Se) precursor (TOP-Se), which is pyrophoric, highly toxic, and expensive. Therefore, a proper phosphine-free Se precursor is desired for the greener synthesis of ZnSe:Mn d-dots. However, to our knowledge, no other phosphine-free Se precursor has been successfully used to synthesize high-quality ZnSe:Mn d-dots except dissolved elemental selenium (Se) in 1-octadecene (ODE) or in a mixture of oleylamine and sodium borohydride which has been used in the nucleation-doping strategy.^26,29,30

Relative to the nucleation-doping strategy, traditional synthetic approaches seem to be less successful in the synthesis of high-quality ZnSe:Mn d-dots in terms of the performance of phosphorescence from the dopant Mn²⁺.^17,18,21,24 It was found that the required improvement of phosphorescence performance of these ZnSe:Mn d-dots could be achieved by coating with a ZnS shell to further form core–shell ZnSe:Mn/ZnS d-dots.^20,31 So, the phosphine-free growth of the ZnS shell on the surface of the ZnSe:Mn core is also desired to completely realize the environmentally friendly synthesis of ZnSe:Mn/ZnS d-dots. However, the sulfur (S) precursor stock solution used in the coating process often contains the TOP too, and to date only the dissolved S powder in ODE has been recently used as the phosphine-free S precursor for the synthesis of ZnSe:Mn/ZnS d-dots.^30,31

Herein, we report the phosphine-free synthesis of ZnSe:Mn d-dots and ZnSe:Mn/ZnS d-dots using an alkylamine–H₂Se complex (obtained by adsorbing H₂Se gas with alkylamine^32,33) and thiourea as new Se and S precursors, respectively. A similar synthetic protocol to the traditional synthetic approach and the growth-doping strategy was selected in order to avoid the fast formation of manganese oxide at high temperature in the presence of the relatively strong alkaline alkylamine.^34,35 The corresponding phosphorescence efficiency of the resulting d-dot cyclohexane solution has been significantly enhanced from about 8% for the ZnSe:Mn d-dots to 35% for the ZnSe:Mn/ZnS d-dots after growth of the ZnS shell.

2. Experimental section

2.1. Chemicals

All chemicals were used as received without further purification. Selenium powder (Se, 99.5%) and 1-octadecene (ODE, 90%) were purchased from Alfa Aesar. Zinc acetate dihydrate (ZnAc₂·2H₂O, AR), manganese acetate tetrahydrate (MnAc₂·4H₂O, AR), octylamine (OA, 99%), sodium borohydride (NaBH₄, AR) and thiourea (AR) were purchased from Beijing Chemical Plant in China. n-Nonanoic acid (NA, 98%) and oleylamine (OLA, 80–90%) were purchased from Aladdin Reagent Co. Ltd. in China. Anhydrous ethanol, acetone and cyclohexane were of analytical grade and supplied by Shantou Xilong Chemical Co. Ltd. in China.

2.2. Preparation of precursors and stock solutions

2.2.1. Zinc (Zn) and manganese (Mn) precursors. The Zn precursor, namely zinc nonanoate (Zn(NA)₂), was prepared by the reaction of ZnAc₂·2H₂O with NA according to our method reported previously.³² The Mn precursor, namely manganese nonanoate (Mn(NA)₂), was prepared by the reaction of MnAc₂·4H₂O with NA as follows: Briefly, 40 mmol of MnAc₂·4H₂O and 120 mmol of NA were first mixed and heated at 150 °C for 2 h, then refluxed at 70 °C overnight and finally cooled naturally to room temperature. 200 mL of ethanol was added to this reaction mixture and the resulting precipitation was collected, washed successively with ethanol and distilled water and isolated by filtration. The washing and filtration process was repeated twice. Finally, the obtained precipitation was dried in a 30 °C vacuum oven to give a pinkish powder which was determined to be Mn(NA)₂ by infrared spectroscopy, elemental analysis and thermal gravimetric analysis results described in the (ESI†).

2.2.2. Se stock solution. The Se stock solution was freshly prepared at room temperature according to our method reported previously.^32,33 2 mL of Se stock solution could be obtained by using a mixture of 1 mL of OLA and 1 mL of OA to adsorb theoretically 0.5 mmol of H₂Se gas, and the actual Se precursor in the 2 mL of Se stock solution was about 0.4 mmol of the alkylamine–H₂Se complex.

2.2.3. Zn stock solution for ZnSe shell. 0.5 mmol (189 mg) of Zn(NA)₂, 0.2 mL of OLA and 5 mL of ODE were mixed by stirring for 10 min, and the resulting mixture became a clear and transparent solution which was used as the Zn stock solution for growing the ZnSe shell on the surface of the ZnSe:Mn core.

2.2.4. Zn/S stock solution for ZnS shell. The Zn/S stock solution for growing the ZnS shell on the surface of the ZnSe:Mn core d-dots was also freshly prepared. First, 0.3 mmol of thiourea, 0.6 mL of OLA, and 0.6 mL of OA were mixed by stirring for 30 min at 60 °C, then a mixture composed of 0.5 mmol (189 mg) of Zn(NA)₂, 0.2 mL of OLA and 5 mL of ODE was added upon cooling to room temperature, and finally the resulting mixture was continuously stirred for another 30 min to produce the desired Zn/S stock solution.

2.3. Synthesis of ZnSe:Mn and ZnSe:Mn/ZnS d-dots

2.3.1. Synthesis of ZnSe:Mn d-dots. At room temperature, 0.1 mmol (38 mg) of Zn(NA)₂, 0.03 mmol (11 mg) of Mn(NA)₂ and 5 mL of ODE were mixed under argon gas flow for 10 min, which further formed a colorless and transparent solution when heated up to 240 °C. The reaction mixture was maintained at 240 °C during the synthesis reaction. After 5 min, 2 mL of the freshly prepared Se stock solution was rapidly injected under vigorous stirring and a 0.2 mL aliquot of reaction solution (sample a, #a) was extracted after 40 min; then 2.5 mL of the Zn stock solution was rapidly injected and three samples were taken at the next 5 min (#b), 15 min (#c), and 40 min (#d), respectively; finally the remaining Zn stock solution was also injected rapidly and another three samples were taken at the next 5 min (#e), 15 min (#f), and 40 min (#g) after the second injection of Zn stock solution. Also, before further characterizing the obtained samples, a purification treatment was carried out as follows. Each sample taken at different reaction times was injected into a five-fold volume of acetone for flocculation, and further isolated by centrifugation at 4000 rpm min⁻¹ for 10 min. After the supernatant was discarded, the obtained precipitate was successively re-dispersed and re-precipitated by adding a small amount of cyclohexane and acetone, respectively.

2.3.2. Synthesis of ZnSe:Mn/ZnS d-dots. At room temperature, 0.1 mmol (38 mg) of Zn(NA)₂, 0.03 mmol (11 mg) of Mn(NA)₂ and 5 mL of ODE were mixed under argon gas flow for 10 min, which formed a colorless and transparent solution when heated up to 240 °C. When the reaction mixture was held at 240 °C for 5 min, 0.8 mL of the freshly prepared Se stock solution was rapidly injected under vigorous stirring. 0.2 mL of reaction solution (#A) was first taken after 40 min, then 3 mL of the Zn/S stock solution was rapidly injected and 0.5 mL of reaction solution (#B) was taken after 40 min, finally the remaining Zn/S stock solution (approximately 3 mL) was also injected rapidly and 0.5 mL of reaction solution (#C) was taken after the next 40 min. Also, a purification treatment of the ZnSe:Mn/ZnS d-dot samples was carried out, similar to that of the ZnSe:Mn d-dots.

2.4. Characterization

Ultraviolet-visible (UV-Vis) absorption and PL spectra of the purified d-dots dispersed in cyclohexane were measured on a Varian Cary 100 UV/vis spectrophotometer and a Shimadzu RF-5301PC fluorescence spectrophotometer equipped with a 150 W Xe-lamp, respectively. These PL data were recorded in the range of 420–750 nm, with the same absorbance (0.1) at the excitation wavelength of 400 nm. The quantum yield (QY) of d-dots was determined by comparing the integrated PL of the d-dots dispersed in cyclohexane with that of rhodamine B ethanol solution (QY, 89%) considering the different refractive index of the used solvent.³⁶ Solid-state PL spectroscopy and PL lifetime measurements were performed on a French Horiba Jobin Yvon FL3-TCSPC time-resolved fluorescence spectrometer. Transmission electron microscopy (TEM) images were acquired using a Philips EM 430 transmission electron microscope operating at an acceleration voltage of 100 keV and a magnification of 100 k. The TEM samples were prepared by depositing a drop of dilute d-dot cyclohexane solutions onto 300-mesh copper grids with a carbon support. Energy-dispersive X-ray spectroscopy (EDS) was performed on a Philips FEI Quanta 200 FEG field emission scanning electron microscope configured OXFORD spectrum analyzer.

3. Results and discussion

3.1. Synthesis of ZnSe:Mn d-dots

In the newly developed nucleation-doping strategy, usually the Mn precursor and Se precursor were first mixed to produce very small MnSe nanoclusters (usually 1–2 nm), then the Zn precursor was swiftly injected several times to grow the ZnSe shell and thus a core–shell MnSe/ZnSe would be formed.^17,18,26 However, the present synthetic protocol is similar to the traditional synthetic approach and growth-doping strategy rather than the nucleation-doping strategy,^21,37 because the Zn and Mn precursors were pre-mixed together. After the Se precursor was injected into the mixture of Zn and Mn precursors, three categories of reaction product, including the alloyed Zn_xMn_1−xSe, core–shell ZnSe/MnSe, or the mixture of separate ZnSe and MnSe, were formed depending on the reaction route and reaction conditions.^18,19,26 It can be expected that in the present case there is a very strong tendency to mainly form core–shell ZnSe/MnSe because the solubility product constant (Ksp) of the ZnSe is significantly smaller than that of MnSe and thus for Mn²⁺ the ability to combine chalcogen elements was obviously weaker than that of Zn²⁺.¹⁸

UV-vis absorption spectroscopy is usually believed to be an effective tool for judging roughly which category of product is obtained in a reaction.^38–40 Generally, for core–shell ZnSe/MnSe with a narrow size distribution, the characteristic exciton absorption peak will be relatively sharp and the absorption band edge will red-shift remarkably when the particle size of the ZnSe core increases, whereas usually the alloyed Zn_xMn_1−xSe has no obvious exciton absorption peak and its band edge is adjusted mainly by composition rather than particle size. Whereas, the mixture of separate ZnSe and MnSe q-dots usually produces several mutational exciton absorption bands or peaks.^38–40Fig. 1a shows the UV-vis absorption spectra of the cyclohexane solution of ZnSe:Mn d-dots which were extracted at different reaction times. An obvious exciton absorption peak at 385 nm was observed for the first sample (#a), indicating the formation of core–shell ZnSe/MnSe. That is, at first the ZnSe q-dots formed easily, and then most of the Mn precursor adsorbed onto the surface of the ZnSe q-dots and further reacted with the Se precursor to produce the MnSe layer.^41,42 After two injections of Zn stock solution, the exciton absorption peak red-shifted to 404 nm, indicating a slight increase in particle size due to the formation of the new ZnSe shell on the surface of the ZnSe/MnSe.^17,18,29 In the present system, the ZnSe/ZnSe:Mn/ZnSe d-dots, referred to as ZnSe:Mn d-dots with ZnSe and Mn²⁺ respectively as the host and the dopant, should indeed be obtained due to the further interdiffusion of partial Zn, Mn, and Se compositions into the middle interface.^41,43 The particle size of the resulting ZnSe:Mn d-dots (#g) was about 3.8 nm based on the relationship between exciton absorption peak and the particle size of the pure ZnSe q-dots,²¹ which was consistent with the corresponding TEM result. As shown in the inset of Fig. 1a, the ZnSe:Mn d-dots were spherical, with a narrow size distribution and a diameter of 3–4 nm.

Fig. 1 (a) Absorption and (b) PL spectra of cyclohexane solutions of ZnSe:Mn d-dots obtained at different reaction times. Inset in (a): TEM image of ZnSe:Mn d-dots collected at 40 min after two injections of Zn stock solution (#g).

The PL spectra of these obtained ZnSe:Mn d-dot samples with different synthesis reaction times are shown in Fig. 1b. As can be seen, all samples had broad PL bands with an almost identical intensity below about 525 nm, which probably originated from the overlap of exciton and surface-state emission of the pure ZnSe q-dots rather than the ZnSe:Mn d-dots. According to the literature, the ZnSe:Mn d-dots should exhibit typical PL properties relating to the dopant Mn²⁺, with a PL peak at about 580 nm which is usually thought to be due to a radiative transition from the ⁴T₁ state of the dopant Mn²⁺ to the ⁶A₁ state.^17,18,21,23 However, no PL band related to the dopant Mn²⁺ was observed at the beginning (#a), which might be because for sample #a, Mn²⁺ has not been incorporated into the ZnSe q-dots and has mainly adhered to the surface of the ZnSe q-dots to form core–shell ZnSe/MnSe, and the excess Se precursor adhered to the surface of ZnSe:Mn d-dots would also quench the PL of the dopant Mn²⁺.⁴⁴ With an increase in reaction time and an added amount of Zn stock solution, the Se precursor in the reaction system was consumed gradually, and thus the newly generated ZnSe grew epitaxially and further coated the surface of the ZnSe:Mn core, which resulted in the appearance and gradual enhancement of the dopant Mn²⁺ PL. As shown in Fig. 1b, the dopant Mn²⁺ PL peak appeared for the ZnSe:Mn d-dots taken at 5 min after the first injection of Zn stock solution (#b). The PL intensity enhanced gradually with the increase of reaction time (#c and #d) and/or after another injection of Zn stock solution was carried out (#e, #f and #g), and about 8% of quantum efficiency was achieved for #g with the Mn²⁺-related PL peak at 576 nm in the range of 525–700 nm. However, it was noticed that an excessively long reaction time such as two hours reduced the quantum efficiency of the dopant Mn²⁺ PL, and 40 min of reaction time after each injection of Zn stock precursor was found to be suitable for achieving a relatively high quantum efficiency.

The solid-state PL spectrum of the resulting ZnSe:Mn d-dots taken at 40 min after two injections of Zn stock solution (#g) was also determined. As observed from Fig. 2a, a PL peak appeared at 584 nm, with a full width at half maximum (fwhm) of 54 nm which is in agreement with the result (around 52(±5) nm) reported by Pradhan and Peng.¹⁸ The steady-state PL peak of the ZnSe:Mn d-dot solid powder red-shifted by 8 nm as compared to that of ZnSe:Mn d-dots dispersed in cyclohexane, which might be due to a more symmetrical crystal field of the central Mn²⁺ in the solid-state ZnSe:Mn d-dots.^17,31,45,46Fig. 2b shows the PL decay curve and the corresponding fitted curve of the solid-state ZnSe:Mn d-dots (#g) determined at 584 nm. The PL decay curve could be fitted with a double exponential function, indicating the presence of two different emission centers and two lifetimes.⁴⁷ The PL lifetime (τ) was calculated to be approximately 0.41 milliseconds (ms) and such a long PL lifetime implies that the PL peak at 584 nm could be attributed to phosphorescence originating from the dopant Mn²⁺ in the ZnSe:Mn d-dots, which is consistent with results reported elsewhere for ZnSe:Mn d-dots.^17,31 In addition, the fact that Mn²⁺ was incorporated in the ZnSe host could be also demonstrated by the EDS analysis shown in Fig. 3. For #g, the doping content of Mn was approximately 0.041 based on two metal compositions (Zn and Mn) in the ZnSe:Mn d-dots, which was close to the Mn molar content in the added Mn and Zn precursors (0.050). This result implies that the added Mn composition has been substantially incorporated into the ZnSe host.

Fig. 2 (a) Solid-state PL spectrum and (b) the corresponding PL decay and fitted curves of ZnSe:Mn d-dots obtained after two injections of Zn stock solution (#g).

Fig. 3 EDS of ZnSe:Mn d-dot powder (#g).

3.2. Synthesis of ZnSe:Mn/ZnS d-dots

To obtain high-quality ZnSe:Mn d-dots, it is necessary to suppress the nucleation and growth of pure ZnSe q-dots and to enhance the epitaxial growth of the ZnSe shell on the surface of the ZnSe:Mn core.¹⁷ However, it is very difficult to form a perfect ZnSe shell for coating the ZnSe:Mn core because magic-sized ZnSe q-dots are formed at very low temperatures when using an alkylamine–H₂Se complex as Se precursor.^32,33 Thus, in the present case, not all the ZnSe materials formed after injecting the Zn stock solution were grown epitaxially on the surface of ZnSe:Mn to give actual core–shell ZnSe:Mn/ZnSe d-dots. Many reports suggest that a larger ZnSe host facilitates more efficient doping of Mn²⁺ and the dopant Mn²⁺ can be maintained in a deeper position in the ZnSe host, which favors emission of a stronger phosphorescence from the dopant Mn²⁺.^{14,17–19,26,41,42} However, to produce a larger ZnSe host usually requires a relatively high reaction temperature, which at the same time also facilitates the nucleation and growth of separate ZnSe and MnSe q-dots in the present phosphine-free synthesis of ZnSe:Mn d-dots. It is well known that ZnS is an excellent shell material and has been frequently selected as the outmost shell to prepare most core–shell q-dots such as CdS/ZnS, CdSe/ZnS, and ZnSe/ZnS.^48–51 This is not only because ZnS can effectively passivate these q-dots by removing their surface defects, but also because the superior antioxidant ability of ZnS can effectively reduce the toxicity from these very toxic cores such as CdS and CdSe.^52,53

Fig. 4a shows the UV-vis absorption spectra of three samples taken in the synthesis of ZnSe:Mn/ZnS d-dots. For #A, the core–shell ZnSe/MnSe was at first formed when the Zn, Mn and Se precursors were mixed, which was determined by the notable exciton absorption peak as described above. After further injecting the Zn/S stock solution, a certain degree of red shift of the corresponding absorption peak due to the exciton leakage could be found, which suggested that the newly generated ZnS was further grown epitaxially on the surface of ZnSe/MnSe to produce the ZnS outermost shell.⁵⁰ Similarly, these compositions including Zn, Mn, Se and S in the middle interface also interdiffused at the high reaction temperature of 240 °C. Thus, the resulting d-dots can be recognized as ZnSe/ZnSeS:Mn/ZnS d-dots from the above analysis, referred to as ZnSe:Mn/ZnS core–shell d-dots based on the presence of the ZnS shell as the outermost coating layer. The epitaxial growth of ZnS on the surface of the ZnSe:Mn d-dot core was also demonstrated by TEM. As seen from the TEM image in the inset of Fig. 4a, the diameter of the ZnSe:Mn/ZnS d-dots obtained at 40 min after the second injection of Zn/S stock solution (#C) was in the range of 6–8 nm. This result indicated a more significant increase in particle size when using the ZnS shell relative to the ZnSe shell, that is, the ZnS material was easier to be epitaxially grown on the ZnSe:Mn core compared with the ZnSe material.

Fig. 4 (a) Absorption and (b) PL spectra of samples taken in the synthesis of the ZnSe:Mn/ZnS d-dots. Inset: TEM image in (a) and PL digital photo in (b) of ZnSe:Mn/ZnS d-dots collected at 40 min after the second injection of Zn/S stock solution (#C).

The phosphorescence characteristics of the ZnSe:Mn/ZnS d-dot cyclohexane solution was similar to that of the ZnSe:Mn d-dot cyclohexane solution, apart from a 4 nm red-shift of the phosphorescence peak (from 576 nm to 580 nm) (Fig. 4b). However, the phosphorescence intensity of the ZnSe:Mn d-dots improved significantly after two injections of the Zn/S stock solution. Strong yellow PL (excited by UV light of 365 nm) was observed in the inset of Fig. 4b for #C with the dopant Mn²⁺ with a PL quantum efficiency as high as 35%, which was much higher than that of the ZnSe:Mn d-dots. Obviously, a higher quantum efficiency of ZnSe:Mn/ZnS d-dots was related to deeper doping of Mn²⁺ in the relatively large host.^14,29 In addition, below 525 nm, the PL band was also observed and increased with more injections of the Zn/S precursor. This fact suggested that as well as the core–shell ZnSe/MnSe q-dots, some separate ZnSe q-dots were also formed when the Se precursor was injected into the mixture of Mn and Zn precursors and these further turned into core–shell ZnSe/ZnS q-dots after injection of the Zn/S precursors, which thus led to an increase in the PL band below 525 nm. The above results not only indicate that the present synthetic system is not comparable with the nucleation-doping strategy in removing the nucleation and growth of separate ZnSe q-dot as reported previously, but also that thiourea is an excellent phosphine-free S precursor for ZnS shell growth on the surface of either the ZnSe:Mn d-dot core or the separate ZnSe q-dot core. It should be noted that such a dual emission phenomenon due to the coexistence of q-dots and d-dots has been reported recently and can be potentially applied in ratiometric temperature sensing and fabricating white LEDs by properly tuning the ratio of blue band and yellow band of the PL.^54,55

In addition, compared to that of the ZnSe:Mn d-dots, the stability of the ZnSe:Mn/ZnS d-dot cyclohexane solution (#C) was greatly enhanced in terms of antioxidant ability, which could be judged by the basically unchanged phosphorescence intensity during one month.^51,56 Meanwhile, the phosphorescence intensities of the resulting ZnSe:Mn d-dots (#g) exposed in the air were quenched by approximately 20% after three days and 90% after one week, possibly due to the very fine ZnSe outmost shell. These results indicate that in the present system the resulting ZnSe:Mn d-dots were easily oxidized in the air, and the growth of the ZnS shell tended to slow the oxidation thus maintaining the dopant Mn²⁺ phosphorescence. So, compared with the ZnSe shell, the ZnS shell should be a better choice for coating ZnSe:Mn d-dots core in the present system.

4. Conclusions

ZnSe:Mn d-dots were prepared by using Zn(NA)₂, Mn(NA)₂, and an alkylamine–H₂Se complex as Zn, Mn, and Se precursors, respectively. In the present system, it is difficult to obtain high-quality ZnSe:Mn d-dots with an outermost ZnSe shell because the nucleation and growth temperature of separate ZnSe q-dots using the alkylamine–H₂Se complex as Se precursor is very low. When Zn(NA)₂ and thiourea were used as Zn and S precursors to give the ZnS shell, both the phosphorescence intensity and stability of the resulting ZnSe:Mn/ZnS d-dots improved, which is related to the thicker ZnS shell grown on the surface of the ZnSe:Mn core and deeper doping of Mn²⁺ in the host. The quantum efficiencies of 8% and 35% were achieved for the resulting ZnSe:Mn and ZnSe:Mn/ZnS d-dots, respectively. The reported alkylamine–H₂Se complex, and especially thiourea, have been herein demonstrated to be new and feasible Se and S precursors for the phosphine-free synthesis of ZnSe:Mn and ZnSe:Mn/ZnS d-dots, and can also potentially be applied in the environmentally-friendly synthesis of other metal selenide q-dots or d-dots and their corresponding coating with ZnS shells.

Acknowledgements

This work was supported by the Scientific Research Foundation of Jiangxi Provincial Department of Education (No. GJJ13711), the Natural Science Foundation of China (Nos. 21161003, 21364002), Guangxi Natural Science Foundation of China (Nos. 2013GXNSFGA019001, 2012GXNSFDA053007, 2011GXNSFA018044), Program for New Century Excellent Talents in University, and the New Century National Hundred, Thousand and Ten Thousand Talent Project of Guangxi, and State Key Laboratory Cultivation Base for the Chemistry and Molecular Engineering of Medicinal Resources (No. CMEMR2012-A12).

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Footnote

† Electronic supplementary information (ESI) available: Infrared spectrum, elemental analysis and thermal gravimetric analysis of the Mn precursor. See DOI: 10.1039/c3nj00998j

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