J. Phys. Chem. C 2009, 113, 14145–14150
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Facile Synthesis of Highly Luminescent UV-Blue-Emitting ZnSe/ZnS Core/Shell Nanocrystals in Aqueous Media Zheng Fang, Yan Li, Hua Zhang, Xinhua Zhong,* and Linyong Zhu* Laboratory for AdVanced Materials, Department of Chemistry, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China ReceiVed: April 25, 2009; ReVised Manuscript ReceiVed: June 26, 2009
Unlike that for cadmium based nanocrystals, little research has been found on the synthesis of the UV-blueemitting ZnSe based QDs with photoluminescence (PL) quantum yield (QY) superior to 50%. In this article, high-quality water-dispersible ZnSe/ZnS core/shell nanocrystals have been prepared in aqueous media. The epitaxial overgrowth of the ZnS shell was carried out at 90 °C in a reaction flask consisting of the shell precursor compounds (zinc acetae as zinc resource and thiourea as sulfur resource), together with the asprepared ZnSe core nanocrystals and the capping reagent glutathione. The optical features and structure of the obtained ZnSe/ZnS core/shell nanocrystals have been characterized by UV-vis, PL spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy-dispersive X-ray analysis (EDX). The influences of various experimental variables, including amounts of ligand and thiourea as well as pH value, on the growth rate and luminescent properties of the obtained core/shell nanocrystals have been systematically investigated. The PL QY of the as-prepared water-soluble ZnSe/ZnS core/shell QDs is up to 65%, which is one of the best results for the water-soluble UV-blue-emitting semiconductor nanocrystals. In comparison with the plain ZnSe nanocrystals, both the PL QY and the stability against UV irradiation and chemical oxidation of the ZnSe/ZnS core/shell QDs have been greatly improved. Introduction Colloidal luminescent semiconductor nanocrystals (NCs), also known as quantum dots (QDs), are of great interest and have been widely investigated due to their unique size-dependent properties and potential applications such as in optoelectronic devices, lasers, and biomedical tags, etc.1-5 So far, great progress has been made in the preparation of QDs, and a large number of high-quality QDs, such as CdSe, CdTe, and alloy NCs, have been successfully synthesized by the organometallic and/or the aqueous approaches; however, their efficient emissions are mostly limited in the range from the green to near-infrared spectral window.6-10 For example, relatively large CdSe QDs provide high quantum yields (QYs) above 520 nm, but low QYs and broad spectral widths from very small CdSe QDs was observed below 520 nm.6,7 In principle, ZnSe nanomaterials with bulk bandgap of 2.7 eV should be promising materials for UVblue-emitting NCs and thus are considered as the leading candidate for the fabrication of blue light-emitting diodes (LEDs) and laser diodes (LDs).11 Another important factor that determines the advantageous position of the ZnSe as UV-blue-emitting NCs is its lower toxicity compared to the wide-bandgap materials CdS. Unfortunately, little research has been found on the successful synthesis of ZnSe QDs, and the QYs superior to 50%, routinely obtained for cadmium based NCs,6,7 have not yet been achieved on ZnSe based nanomaterials. Adapting the work on CdSe NCs,6a Hines and Guyot-Sionnest12 reported the synthesis of ZnSe NCs with QYs up to 20-50% through the organometallic hotinjection approach, and then the zinc precursor (diethylzinc) was replaced by the air-stable zinc carboxylates in the following alternative routes.13-15 Since aqueous synthesis is simpler, cheaper, and less toxic, and the as-prepared samples are more * Corresponding author. Fax: +86 21 6425 0281. E-mail: zhongxh@ ecust.edu.cn (XZ);
[email protected] (L.Z).
water-soluble and biocompatible in comparison with nonaqueous routes, water-based synthesis of QDs with thiols as the capping agent has been developed as an interesting alternative. Watersoluble ZnSe QDs capped with various thiols (such as thioglycerol, thioglycolic acid, 3-mercaptopropionic acid, etc.) have been synthesized through the aqueous route, but they usually exhibited broad emissions and low QYs.16-21 Recently, Ren presented a new aqueous procedure for rapid preparation of high quality ZnSe QDs with QY up to 17% through the use of microwave irradiation.22 Ying reported the aqueous synthesis of glutathione-capped ZnSe QDs with a maximum QY of 22%.23 These results represented the best optical quality of the UV-blue fluorescent QDs in aqueous solutions.22,23 The aqueous approach was further modified by Muscat and co-workers with the introduction of hydrazine, which allowed the reaction to be carried out under ambient atmosphere.24 A high emission efficiency and a high stability are the prerequisites of QDs in numerous technical applications. Epitaxially overgrowing a semiconductor material with higher bandgap around the QDs, resulting in the so-called type-I core/ shell systems, has proven to be a key procedure for improving the PL efficiency and stability against chemical degradation of NCs.25,26 Thus, ZnSe/ZnS core/shell nanostructures have been the subject of intensive investigation in recent times.14,19-22 Nikesh et al. reported the synthesis of ZnSe QDs and the ZnSe/ ZnS core/shell system and have shown that this core/shell system yielded a remarkable enhancement of PL QY.19 Lomascolo et al. investigated the ZnSe/ZnS system by means of time-resolved PL spectroscopy and have shown that these NCs were stable against photo-oxidation and had PL QYs around 15%.27 Ali et al. reported the synthesis of both ZnSe/ZnS and reverse-type ZnS/ZnSe core/shell nanostructures.28 Their study revealed that when ZnSe NCs were passivated with higher band gap ZnS,
10.1021/jp903806b CCC: $40.75 2009 American Chemical Society Published on Web 07/09/2009
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Figure 1. (A) Temporal evolution of normalized UV-vis absorption (solid lines) and PL emission spectra (dashed lines, λex ) 300 nm) of ZnSe/ ZnS core/shell NCs with identical sample concentration together with the initial ZnSe core NCs. Inset: photograph of emission color of the ZnSe/ ZnS sample with growth time of 2.5 h under the irradiation of a UV lamp. (B) PL QYs and PL peak positions of the ZnSe/ZnS core/shell QDs at different growth times.
then the integrated PL intensity increased 2.6-fold that of the ZnSe core. The recently reported aqueous synthesis of ZnSe NCs with a sulfur-enriched (ZnS) shell achieved through postillumination is one of the successful examples in this way.20,21 The same phenomenon has been observed when the ZnSe QDs were treated under microwave irradiation.22 Nevertheless, QYs superior to 50%, routinely obtained for cadmium based NCs, have not yet been achieved on ZnSe based NCs. Furthermore, the chemical and photo stability of those core/ shell structures were not studied in detail. Herein, we demonstrate a new facile route to synthesize highly luminescent ZnSe/ ZnS core/shell nanostructures in aqueous media. The ZnS shell was epitaxially overgrown around the ZnSe core NCs through the introduction of additional Zn(OAc)2, the capping agent GSH, and thiourea as sulfur source to the original glutathione capped ZnSe reaction solution. For the as-prepared ZnSe/ZnS samples, the PL QY can be up to 65%, and the stability to UV irradiation and chemical oxidation is also improved significantly in comparison with that of the ZnSe core NCs. Experimental Section Chemicals. Zinc acetate (Zn(OAc)2,99.0+%), sodium borohydride (NaBH4, 99%), selenium powder (-100 mesh, 99.999%), thiourea (99+%), and L-glutathione (GSH, 98+%) were purchased from Aldrich and used as received. For all operations carried out in aqueous media, deionized water was used throughout. Synthesis of GSH-Capped ZnSe Core NCs. A literature method was used to synthesize GSH-capped ZnSe core NCs.23 In a typical procedure, 36.7 mg (0.2 mmol) of Zn(OAc)2 and 73.7 mg (0.24 mmol) of GSH were dissolved in 20.0 mL of deionized water, and the pH value of the solution was adjusted to 11.5 by dropwise addition of 2.0 M NaOH solution with stirring. The air in the system was then pumped off and replaced with N2, then 1.0 mL of fresh NaHSe solution (0.08 M), which was prepared from sellurium powder through a reduction reaction in NaBH4 solution,10d was added through a syringe into the Zn precursor solution at room temperature. The molar ratio Zn:GSH:NaHSe was set at 1:1.2:0.4. The system was heated to 90 °C under N2 atmosphere, and the growth of ZnSe NCs was kept at this temperature for 60 min before the heating was
stopped to terminate the reaction. By this approach, one can obtain ZnSe QDs with an average size of 2.7 nm and an emission wavelength at λ ) 372 nm. Growth of ZnSe/ZnS Core/Shell QDs. Typically, 10.0 mL of the as-prepared ZnSe core NC crude reaction solution (containing 0.04 mmol ZnSe) was loaded in a 50 mL threeneck flask, followed by the addition of 5.0 mL of mixture solution containing 0.06 mmol of Zn(OAc)2, 0.09 mmol of GSH, and 0.06 mmol thiourea, then the final pH value of the reaction mixture was adjusted to 10.2 with the addition of NaOH solution. The air in the system was pumped off and replaced with N2. Subsequently, the reaction mixture was heated to 90 °C under N2 atmosphere and timing started at this temperature. Aliquots of the sample were taken at different time intervals and used to record their UV-vis absorption and PL emission spectra. The reaction was terminated by allowing the reaction mixture to cool down to room temperature. The obtained ZnSe/ ZnS core/shell QDs was purified by centrifugation and decantation with the addition of 2-propanol. The excess ligand and unreacted precursors were removed by extensive purification prior to transmission electron microscopy, powder X-ray diffraction, and energy-dispersive X-ray analyses. Characterization of ZnSe and ZnS/ZnSe Core/Shell NCs. The pH value of a solution was measured by a PHS-3C pH meter. UV-vis and PL spectra were obtained on a Shimadzu UV-2450 UV-vis spectrophotometer and a Cary Eclipse (Varian) fluorescence spectrophotometer, respectively. The room-temperature PL QYs were determined by comparing the integrated emission of the QD samples in water with that of a fluorescent dye (such as 2-aminopyridine in 0.1 M H2SO4, QY ) 60%) with identical optical density.8a,b To conduct investigations in transmission electron microscopy (TEM), the NCs were deposited from dilute aqueous solutions onto copper grids with carbon support by slowly evaporating the solvent in air at room temperature. TEM images and energy-dispersive X-ray analysis (EDX) were acquired using a JEOL JEM-1400 transmission electron microscope equipped with a EDAX Falcon EDS system operating at an acceleration voltage of 120 kV. Powder X-ray diffraction (XRD) was obtained by wide-angle X-ray scattering, using a Siemens D5005 X-ray powder diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178
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Figure 3. EDX spectra of the ZnSe and ZnSe/ZnS NCs samples.
Figure 2. (A) Wide-field TEM image of the as-prepared ZnSe cores with a size of 2.7 ( 0.2 nm, (B) ZnSe/ZnS core/shell QDs (size of 3.6 ( 0.3 nm) with growth time of 2.5 h, and (C) XRD pattern corresponding to samples in A and B. Line XRD patterns correspond to bulk cubic zinc blende ZnSe (bottom) and ZnS (top).
Å´). XRD samples were prepared by depositing NC powder on a piece of Si(100) wafer. Results and Discussion Optical Properties and Structural Characterization. As found in the original report by Ying and co-workers,23 because the superior ligand GSH was used in the synthesis of ZnSe core NCs, the resulting ZnSe QDs possessed a high PL QY up to 20% and a relatively narrow size distribution. It should be noted that the PL of the water-soluble ZnSe NCs capped with other thiol ligands usually is negligible and mainly shows a broad trap emission in the range of 400-600 nm.16-20 Obtaining the high-quality ZnSe core NCs paves a way for the achievement of high-quality ZnSe/ZnS core/shell nanostructures. On the basis of our previous experience,29 thiourea can serve as an excellent sulfur source in the formation of metal sulfide NCs in aqueous synthesis since thiourea can decompose gradually at high temperature in alkaline media and thus supply a steady sulfur to react with the metal precursors. With the introduction of additional Zn(OAc)2, capping reagent GSH, and sulfur source thiourea into the as-prepared ZnSe NCs reaction solution, the ZnS shell formed gradually and deposited around the ZnSe core template to form the ZnSe/ZnS core/shell nanostructures. Figure 1 shows the temporal evolution of the UV-vis absorption and PL emission spectra of the ZnSe/ZnS core/shell NCs together with the initial ZnSe core NCs. On prolonging the heating time at 90 °C, both the excitonic absorption onset in the absorption spectra and emission peak in the PL spectra of the ZnSe/ZnS samples shifted systematically to longer wavelength compared to that of the initial ZnSe core NCs, which demonstrated the growth of the ZnS shell around the ZnSe cores. The PL peak position could approach 382 nm in a period of 3 h growing time from the original 372 nm corresponding to the ZnSe core NCs. All of the samples showed a sharp wellresolved first excitonic absorption onset, which should be attributed to the electronic transition of 1S(e)-1S3/2(h), indicating
a narrow size distribution and high crystallinity of the obtained ZnSe/ZnS samples. This was further confirmed by the TEM and XRD measurements discussed below. Almost no observable absorption tail at the low energy side indicates no scattered light from colloidal dispersion, and thus, no particle aggregation took place. The small Stokes shift (∼18 nm) between the emission peak and the corresponding first excitonic absorption onset indicates the dominant band-edge luminescence from the core/ shell structures without the appearance of deep trap emission at the long-wavelength side. With the deposition of the ZnS shell around the ZnSe core, the PL QYs of the ZnSe/ZnS samples enhanced steadily and reached the maximum value of ∼65% from the original 20% corresponding to the ZnSe cores when the emission wavelength approached 380 nm with a growth time of 2.5 h (Figure 1B). With the overextension of the growth time, the PL QY decreased gradually, and the PL peak position red-shifted slowly (Figure 1B). With the deposition of ZnS shell, the PL peak width (full width at half-maximum, fwhm) of the resulting ZnSe/ZnS QDs narrowed further and kept at a low value (
