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aPhys. Status Solidi C 7, No. 11–12, 2683–2687 (2010) / DOI 10.1002/pssc.200983835

Highly luminescent nanostructures of CdS and ZnS prepared by microwaves heating: effect of sulphide concentration Samuel Ortíz1, Idalia Gómez*,1, Perla Elizondo1, and José Cavazos**,2

1 Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, Av. Universidad s/n, C.P. 66450 San Nicolás de los Garza, N.L. México

2 Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Av. Universidad s/n, C.P. 66450 San Nicolás de los Garza, N.L. México

Received 17 October 2009, revised 3 February 2010, accepted 3 February 2010 Published online 9 August 2010

Keywords CdS, ZnS nanoparticles, chemical synthesis, microwave heating, structure, photoluminescence * Corresponding author: e-mail mgomez@fcq.uanl.mx, Phone: +52 8183294000 ext. 6362 ** e-mail jcavazos@gmail.com, Phone: +52 8183294000 ext. 6230

Nearly monodisperse and highly luminescent ZnS and CdS NPs were obtained by microwave irradiation. The ZnS and CdS NPs solutions were prepared by adding freshly pre-pared ZnSO4 or CdSO4 solution to a thioacetamide solution at pH 8 in the presence of sodium citrate in solution used as stabilizer. The precursors concentration were such that the sulphide ion concentrations were 3 × 10–2 M, 6 × 10–2 M and 8 × 10–2 M, for each of these [S] concentrations the [Zn] or

[Cd] content were fixed at 3 × 10–2 M. NPs were prepared under microwave irradiation for 1 min at 905 W of power. The NPs samples were taken when the temperature de-scended to ambient temperature for further analysis. Effect of concentration of Cd and Zn ions were studied in the lu-minescence property. RXD, AFM, TEM and UV-Vis were used too as analytical equipment for characterization.

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Semiconductor nanoparticles synthe-sis has attracted much interest, due to their size-dependent properties and great potential for many applications, espe-cially as nonlinear optical materials [1–4]. Nanoparticles exhibit unique properties owing to quantum size effects and the presence of a large number of unsaturated surface atoms.

Colloidal semiconductor nanocrystals (NCs) are of great interest for fundamental studies [5] and technical applica-tions such as light-emitting devices [6] lasers [7] and fluo-rescent labels [8]. Because of their size-dependent photo-luminescence tunable across the visible spectrum [9]. Be-sides the development of synthesis techniques to prepare samples with narrow size distributions [10, 11] much ex-perimental work is devoted to molecular surface modifica-tion to improve the luminescence efficiency [12] and col-loidal stability of the particles or to develop a reliable processing chemistry [14, 15].

Semiconductor nanoparticles are expected to exhibit quantum confinement when their size becomes comparable

to the 1s-exciton diameter [16-18] which results in the ap-pearance of a quantized eigenspectrum and an increase in the energy gap relative to the band gap (Eg) of the bulk solid. Quantum crystals of CdS, a II–VI semiconductor with 6 nm exciton diameter and 2.5eV band gap, have been successfully synthesized using many stabilizers in an effort led by Henglein and Brus [19, 20]. Various surface-capping agents used to stabilize II-VI semiconductor nano-particles include polyphosphate [21], trioctylphosphine/ trioctylphosphine oxide, and thiols [22].

Synthesis of nanocrystals with a size [23, 24] and shape [25-31] defined has advance in recent years, to which high temperature approaches (roughly 250–350 ºC) in organic solvents, either through organometallic schemes [10, 26, 32] or alternative approaches (or greener approaches) [33, 34] have played a key role and often been regarded as the mainstream synthetic chemistry in the field. Emphasis on synthetic chemistry of nanocrystals is a currently moving into nano-objects with complex structures and composi-

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tions [35] and formation of three-dimensional (3D) colloi-dal nanocrystals is especially underdevelopment.

Nanocrystals with complex 3D structures are interesting for solar cells, catalysis, sensing, and other surface/shape related properties and applications [36, 37]. For instance, CdTe and other semiconductor tetrapods [38, 39] are ideal structures for fabrication of high performance solar cells [40]. Such tetrapods, however, were typically formed by a traditional path, atom by atom growing from nuclei, and the intrinsic crystal structures seem to play a key role, i.e., nanolitography. Thus, is not clear how to extend the syn-thetic methods to different structures. Some reports indi-cate that nanodots and nanorods can self-assembling into different complex shaped particles [41-43]. Such complex structures, however, were often quite large, fragile, and/or polycrystalline. Some other reports indicated possibilities of formation of complex nanostructures through 3D at-tachment [44]. Thus; a general pathway to reach 3D ori-ented attachment has not yet been achieved.

The recent widespread interest in semiconductor quan-tum dots (QDs) is due largely to their distinct optical prop-erties, including broad absorption bands, narrow, size-tunable, emission bands, and excellent photostabilities [45]. Physically, the quantum properties of QDs (a size-dependent fluorescence emission) occur in electron-hole pairs (excitons) that are confined to dimensions that are smaller than the electron-hole distance (exciton diameter) [45-47]. As the result of this condition, the state of free charge carriers within a nanocrystal is quantized and the spacing of the discrete energy states (emission colors) is linked to their size of nanoparticle. The combination of small size, high photostability, and size-tunable emission properties makes quantum dots highly attractive probes for biological, biomedical, and bioanalytical imaging applica-tions [48-51].

As an alternative to thermally driven syntheses, micro-wave heating has been applied successfully to a variety of chemical reactions, including those that impact the fields of materials and nanoscience. For example, microwave techniques have been used to prepare a wide range of ma-terials including nanocrystalline composite solid electro-lyte like Bi2O3-HfO2-Y2O3 by microwave plasma [52] deep-red-emitting CdSe quantum dots [53] carbon-coated core shell structured copper and nickel nanoparticles in a ionic liquid [54] Bi2Se3 nanosheets [55] Au@Ag core-shell nanoparticles [56]. Overall, while microwave assisted syn-theses of quantum dots and other nanomaterials have been reported [57, 58] including ZnSe(S) quantum dots synthe-sis using microwave heating [59] in comparison to the strategies reported herein, these procedures often result in QDs of varying morphologies or aggregated crystals with comparably poor luminescent properties, such as broad emission bands. Indeed, a recent review that surveyed the latest productive routes to high-quality ZnS quantum dots did not include microwave heating. Shall et al, synthesized ZnS rods by microwave [61] heating, but they do not stud-

ied the changes in the band gap absorption according to the particle size, the ZnS nanorods produced by them are not highly luminescent, indeed, they used the same method to prepared nano semiconductor. While they used cycles of microwave heating, we use an uninterrupted process of heating.

In the study described herein, we show that microwave heating can indeed provide a very powerful strategy for the synthesis of high-quality ZnS and CdS quantum dots with highly luminescent properties and offers several advan-tages over traditional thermally driven approaches. In par-ticular, these advantages include get quickly reactions temperatures and a straightforward process control, thus making quantum dot materials with quantum yield (QYs) of 70%, accessible to an increased number of research labs.

2 Experimental procedures 2.1 Materials Thioacetamide (CH3CSNH2) was pur-

chased from MERCK Gehalt (99.0%). ZnSO4 and CdSO4 were obtained from Spectrum Quality Products, Inc. (99.9%), KOH was purchased from MERCK (99.0%). All chemicals were used without additional purification. All solutions were prepared using Milli-Q water (Millipore) as the solvent.

2.2 Preparation of QDs Nearly monodisperse ZnS and CdS NPs were obtained by microwave irradiation. The ZnS or CdS NPs solution was prepared by adding freshly prepared either ZnSO4 or CdSO4 solution to a thio-acetamide solution at pH 8 in the presence of sodium cit-rate solution used as stabilizer. The precursors concentra-tion were [S] = 3 × 10-2M, [S] = 6 × 10-2M, [S] = 8 × 10-2M and also using concentrations of precursor [Zn-Cd] = 3 × 10-2M. The NPs were prepared under microwave irradia-tion for 1 min at 905W of power. The NPs samples were taken when temperature decreases until ambient tempera-ture for further analysis.

2.3 Apparatus The microwave system used for the synthesis of NPs operates at 1150W, 2.45 GHz, working at 90% of power under continuous heating. UV-vis absorp-tion spectra were obtained using a Perkin Elmer UV-vis Lambda 12 spectrophotometer. For luminescence quantum yield measurements, a dilute solution of coumarin 1 in ethanol was used as standard. Both the nanoparticle disper-sion and the coumarin 1/ethanol solution were adjusted to have an absorbance of 0.10. A corrected luminescence in-tegrated area was used to calculate the quantum yield. Fluorescence experiments were performed using a Perkin Elmer PL Lambda 12 spectrofluorimeter using a wave-length of excitation of 250 nm. All optical measurements were performed at room temperature under ambient condi-tions. Samples were precipitated with ethanol and dried in a vacuum oven for XRD characterization. The XRD pat-terns were obtained from a Siemmens D5000 Cu Kα (λ = 1.5418 Å) diffractometer. AFM images were recorded in a Quesant Q-Scope 3500 atomic force microscope using contact mode. HRSEM was used.

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3 Results and discussion Figure 1 displays the UV-vis absorption of ZnS and CdS NPs synthesized trough microwave irradiation at 905 W. A blue-shift was observed in the absorption of ZnS and CdS NPs. This is indicative of the NPs formation and, the blue shift is due to the concen-tration decreasing of S2- ions in samples of ZnS but, a con-traries effect in samples of CdS was observed. The size of the ZnS NPs increased when the concentration of S2- also increases, which results in a gradual red-shift. This kind of absorption provides evidence of the quantum size effect and the presence of particles of nanometer size. Different effect was observed in samples of CdS the increase of S2- ions produce the blue shift, theoretical works have shown that the absorption threshold provides a reasonable estima-tion of the particle size as function of the position and spacing of the threshold absorption [60]. The blue-shift in-dicates the size of the particle can be changed by different addition of S2- ions and produce a variation of the optical properties of the ZnS and CdS NPs.

The most striking features are as follows: first one all the sodium citrate stabilizer particles show a well-developed behavior of classical semiconductor at nanome-ter scale, and a well-developed curve near the onset of ab-sorption which is ascribed to the first excitonic (1s - 1s) transition. In some cases even higher energy transitions are observed. Second one with decreasing particle size the transition energy shift to higher values as a consequence of the size quantization effect.

It has been reported that ZnS and CdS are direct band gap semiconductors and therefore plots of (Ahv)2 versus hv should be straight lines with intercepts on the energy axis giving the band gaps of the NPs [61]. The band gap of the ZnS nanoparticles using the three different concentrations of [S] was found to be 4.32 eV, but for CdS the values for Eg were 2.8, 2.74 and 2.61eV according to concentration of [S] 3x10-2, 6x10-2 and 8x10-2 M respectively, these are according to colour effect observed to CdS nanoparticles. Figure 2 shows the room temperature photoluminescence spectrum of samples of NPs. The line width on emission is shifted to the red with the reduction of the particle size. This shift is the result of a combination of relaxation into shallow trap states and the size distribution. No deep trap emission features suggest highly monodisperse samples; this was observed principally for ZnS samples. High quan-tum yields and narrow emission line widths indicate growth of NPs with few electronic defect sites. The sharp luminescence is a dramatic example of the efficiency of the capping stabilizer in electronically passivity the crystallites from chemical degradation yielding robust systems. Sam-ples stored in the original growth solution still show strong, sharp emission after storage for more than a month. A luminescence quantum yield of 70% and 60% was measured for the ZnS and CdS NPs respectively using a di-lute solution of Coumarin 1 in ethanol as standard.

(a)

(b) Figure 1 UV–Vis spectra showing a blue-shift at different wave-length for (a) ZnS and (b) CdS at 3 × 10-2, 6 × 10-2 and 8 × 10-2 sulphide concentration.

These room temperature optical experiments, carried out

on common laboratory equipment, demonstrate the bene-fits of high quality samples and point to the potential of more sophisticated optical studies on these samples. Under UV light the NPs shows highly luminescent as is shown in Fig. 3. To see this effect each sample must be irradiated under UV light using a wavelength with an energy higher than the band gap found for each sample.

The as-prepared chalcogenide were characterized by X-ray powder diffraction, which showed a match with the diffraction pattern published in the literature. All the dif-fraction peaks can be indexed to the hexagonally structured for ZnS. The broad nature of the ZnS and CdS XRD peaks shows that the sizes of the quantum dots are very small. The broad diffraction peak at 28.6º (2θ scale) arising from (111) reflections from ZnS is assigned to the cubic sphalerite form (JCPDS card, file no. 50566). The obvi-ously weaker diffractions at 47.50, 56.30 (220) and (311) reflections are of the cubic sphalerite ZnS form, respec-tively. The CdS synthesized is present in wurtzite form.

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Figure 2 Room temperature PL spectra for ZnS and CdS at 3 × 10-2, 6 × 10-2 and 8 × 10-2 sulphide concentration. In CdS samples can be seen major effect over red-shift according diminishing the sulphide concentration (minor particle sizes).

In AFM analysis was observed the formation of islands,

in some of this we observe the formation of centres in the middle of the islands with a height about 20-40 nm, this kind of morphology is not present in all cases, closer to this islands is possible to observe particles of minor size. HRSEM analysis was observed the formation of singular agglomerates of nanoparticles with sizes minor to 5 nm, Fig. 4 shows two images representative to this phenomena. TEM analyses showed the obtention of nanoparticles al-ready of 6 nm.

4 Conclusions UV-vis spectra of the nanoparticles synthesized show a blue-shift due to the quantum confine-ment and the reduction of the particle size. This effect is due the increase of the concentration of metal ion. The band gap of the semiconductors increases when the particle size decreases. The crystal structure of the ZnS synthesized is cubic sphalerite form and for CdS was cubic type zinc blend. The morphology of the prepared ZnS shows islands with a nanocenter as well as nanoparticles of about 100 nm. The ZnS NPs obtained shown high monodispersity ac-cording to PL analysis. The high luminescence is present when the NPs are irradiated with UV light using energy higher than the band gap value found for each sample, this

property is due to the reduction of the particle at nanometer scale. HRSEM and TEM analyses showed nanoparticles of 5 nm and nanostructures of agglomerates with sizes al-ready of 10-20 nm. Synthesis by microwave heating pro-vides a very powerful option to prepare ZnS and CdS nanoparticles with highly luminescent properties.

Figure 3 Photos for ZnS and CdS under UV radiation. CdS pre-sent the major effect over color in relation of sulphide concentra-tion in the samples.

Figure 4 TEM images for ZnS and CdS at 6x10-2 sulphide con-centration. CdS sample presents characteristics of agglomeration of nanoparticles. The sizes of ZnS are already of 6 nm.

Acknowledgements The authors express their gratitude to

CONACYT and PAICYT for financial support.

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