Elsevier

Journal of Luminescence

Volume 127, Issue 2, December 2007, Pages 474-482
Journal of Luminescence

Light emitting CdS quantum dots in PMMA: Synthesis and optical studies

https://doi.org/10.1016/j.jlumin.2007.02.037Get rights and content

Abstract

Freshly prepared CdS-quantum dots (QDs) in DMF (clear pale solution) when loaded in polymethylmethacrylate (PMMA) lead to excellent optical properties. The tuning of the absorption and emission wavelengths via experimentally control parameters is considered novel and significant. The absorption band for CdS was observed at about 370 nm in polymeric matrix. The blue, green and orange light emissions from such composite solution were tuned and stabilized by simply varying the concentration of CdS, cadmium and sulphur in the final product. Photoluminescence (PL) measurement with 2% CdS loading showed band-edge emissions from the composite with only about 20–25 nm Stokes shift in emission wavelength. Observation of such optical properties indicated that the composite has narrow particle size distribution and particle diameter may well be below 10 nm. X-ray diffraction (XRD) patterns of the film with higher loading of CdS showed broad pattern for hexagonal CdS. Thermo-gravimetric analysis (TGA) of CdS/PMMA composite film revealed that it has better thermal stability than PMMA alone. Transmission electron microscopy (TEM) showed agglomerated tiny dots in nano-meter regime.

Introduction

There are numerous applications of quantum dots (QDs) of CdS and a few which are regularly eyed are their use as photocatalyst [1], non-linear optical material [2], [3], [4], in solar cells [5], X-ray detectors [6], photocatalytic solar energy stockings [7] and in display devices [8]. Thus over the years, it has been shown that QDs of cadmium sulphide and other II–VI semiconductors can be potential candidates for electronic and optoelectronic application such as QDs lasers and single electronic transistors [9], [10].

In recent years, light emitting devices (LEDs) based on polymer have been studied in much detail and have attained a great deal of maturity [1], [5], [11], [12]. Incorporation of semiconductor nano-particles in the polymer has been widely studied and has opened-up the new dimensions in nano-research. Vigorous research has been conducted on the synthesis of II–VI semiconductor nano-particles embedded and/or encapsulated in polymeric matrices due to their tunable optical properties in contrast to the polymer alone. The advantages from such materials can be two fold (i) tiny dots would enhance the life of matrix i.e. the polymer and (ii) enhanced brightness of light emission can be expected due to presence of QDs. The literature carries a large number of reports on synthesis and characterization of CdS nano-particles that are either surface passivated through use of a suitable surfactant or embedded in various matrices including polymer [1], [8], [9], [10], [13], [14], [15], [16], [17], [18], [19], [20] because one of the major requirements for applications of QDs is, their safe transport at the site of application. This means, incorporation of dots in organics or polymers would ideally be considered suitable for most electronic and opto-electronic applications. Therefore, it is appropriate to focus on the synthesis of QDs in optically transparent polymers. Efforts to optimize blue light emission from CdS QDs are considered challenging and often tuned properties of the dots have been accomplished through surface passivation of the QDs. The trapping of the dots in polymer appears more attractive as this offers commercial viability for QDs based modern technologies. The nano-particles could thus be synthesized in polymers so as to cast thin film directly thus avoiding multistage processing. One of the authors [8], has previously reported synthesis of CdS QDs in phosphine functionalized polybutadiene matrix where the CdS was typically in the phosphorus environment similar to that has been described for trioctylphosphine (TOP) capped semiconductors [20].

There are several methods for preparation of Q-particles of CdS with varied morphology and optical properties e.g. by reaction of respective ions of the constituents, by electrochemical reactions, use of gamma and UV-irradiation and by solvo-thermal reactions [21], [22], [23], [24], [25], [26]. Many methods have been applied for preparation of QDs of CdS/polymer nano-composites with variation in particle sizes and/or morphologies using various polymers such as polybutadiene, polystyrene, PVA, polymethylmethacrylate (PMMA), PVK etc. [8], [22], [25], [27], [28]. These methods describe efficient synthesis of CdS QDs either trapped in polymers directly or via first isolating CdS particles followed by their embedding into polymer for light emitting properties. Thus, range of activities on CdS/Polymer QDs such as polymerization and simultaneous sulphidation process to fabricate CdS-PVAc polyl(vinyl acetate) nano-rod composite has been described in the literature. Similarly, polystyrene microbeads doped with highly luminescent CdS QDs have also been prepared [22]. Preparation of high quality nano-crystals needs special skills of the synthetic chemists that ensures the reproducibility. In fact, QDs of CdS when prepared in N,N-dimethylformamide (DMF) show emission of bright fluorescent light in the yellow-green region of visible spectrum [26]. Despite an impressive literature on the synthesis of CdS QDs, simple synthetic routes to prepare surface passivated CdS nano-particles still continues to be an area much sought after simply because of the possibility of further enhancement in the quality of and tuning of light emission in the desired wavelength of the visible spectrum. It is understood that the efforts are mostly associated with multi-stage loading of QDs in polymeric matrices which lead to deterioration in the optical properties of the dots due to highly sensitive surface properties of the particles. The area of fabricating such Q-particles of desired size and morphology thus has still not been fully perfected and therefore, new methods keep appearing in the literature with a rapid pace. We have recently reported that CdS QDs in PVA can be synthesized by various methods and the optical properties can be suitably tuned albeit not without the drawbacks [27]. In one of our earlier work, we discovered that CdSe QDs in funtionalized polybutadiene can lead to fabrication of white laser, however, CdS QDs showed short lived blue light emission (stable for almost a week) that eventually diminished when exposed to normal environment [8]. In view of the formidable challenges that lie in the synthesis of QDs, we herein, report a simple synthesis of CdS QDs in PMMA with excellent optical characteristics.

Section snippets

General

Methyl methacrylic acid (the monomer for PMMA) and cadmium acetate [Cd(OOCH3)2] (qualigens) were purchased commercially and were used as received. N,N-dimethylformamide (DMF) was distilled under diminished pressure prior to use and hydrogen sulphide gas was generated in a Kipp's apparatus from ferrous sulphide and HCl acid. Solution UV–Visible spectra were recorded in DMF on a Hitachi 3210 UV–Visible Spectrophotometer and on JASCO V-570 model at room temperature. Photoluminescence (PL)

Results and discussion

The reaction of freshly prepared PMMA with cadmium sulphide can be considered as a very simple and straight forward method for fabrication of QDs/polymer composite. It is believed that the dots when stirred with PMMA get well encapsulated through carboxylate functional group as shown in Scheme 1. It is found that the light emission from such preparations depend on various factors viz; the initial preparation of the CdS dots, the addition of polymeric stock solution to CdS solution and also the

Conclusion

The paper describes studies of optical properties of CdS nano-particles potentially incorporated in PMMA. The air stable light emission in Blue-green and yellow-orange region of visible spectrum has been studied and the findings discussed. It is found that the CdS nano-particles are well-dispersed but sometime un-evenly distributed in the matrix. The optical behaviour is supportive of narrow size distribution despite indication of slight agglomeration as shown in TEM picture. Reasonably, sharp

Acknowledgement

PKK thanks Department of Science and Technology, Government of India, for financial assistance through Grant No. SP/S1/H-34/99/-Part II.

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