Elsevier

Journal of Luminescence

Volume 132, Issue 7, July 2012, Pages 1740-1743
Journal of Luminescence

A new and straightforward synthesis route for preparing Cds quantum Dots

https://doi.org/10.1016/j.jlumin.2012.02.015Get rights and content

Abstract

In this work, highly stable, soluble and luminescent CdS quantum dots (QDots) with a narrow size distribution were synthesized in ethylene glycol using the polyol process and the solvothermal technique. In this case instead of using a conventional highly toxic sulfur source like H2S, we use elemental sulfur dissolved in ethylene glycol to perform the reaction. When the solvent reaches its boiling point inside the autoclave, sulfur is reduced to S−2 and reacts with Cd+2 ions to form CdS nanocrystals. Analysis of the spectroscopic and TEM measurements showed that 3 nm monodispersed CdS QDots were synthesized and exhibited high photoluminescence (PL) in the blue green region of the spectra when excited with 355 nm.

Highlights

► New and straightforward route to synthesize stable, soluble and highly luminescent CdS (QDots). ► Solvothermal and the polyol technique were used to obtain highly luminescent CdS QDots. ► Quantum dots (QDots) with 3 nm of diameter and a narrow size distribution has been synthesized.

Introduction

The synthesis and properties of II–VI semiconductor quantum dots (QDots) have been extensively investigated over the last 30 years. The reason for this seems to be their special optical and electronic properties which arise from the quantum confinement of electrons and the large surface area [1], [2], [3]. In particular, CdS have been extensively studied due to their potential applications in several technological areas such as solar photovoltaic cells, nano bar codes, field effect transistors, light emitting diodes, photocatalysis and in vivo biomedical detection fluorescent tags in biology and the development of chemical and biological sensors [3], [4], [5], [6].

Due to the high potential of possible applications displayed by this semiconductor material, a wide range of synthetic routes has been developed in order to obtain monodisperse and more efficient quantum dots. These methodologies allow the size, morphology, size distribution and stabilization of nanoparticles to be controlled.

Among these methodologies, one of the most common routes for preparing CdX QDots is the colloidal chemical one by rapid injection of pyrophoric organometallic reagents into hot coordinating solvents at high temperatures (180–350 °C). However, the starting reagents used in these routes are extremely toxic, pyrophoric, explosive and expensive. These drawbacks prompted several groups to use an alternative route to overcome these problems, such as the preparation of monodisperse CdE (E=S, Se and Te) based on the corresponding organometallic precursors [7], or using a single source precursor to prepare CdS nanocrystals of a small size diameter [8], [9]. Other preparatory techniques include sol–gel processing [10], microwave heating [11], photoetching [12] reverse micelle [13], reduction route using NaBH4 [14] solvothermal routers [15], [16], [17], [18] and photochemical [19].

In this study, highly luminescent CdS quantum dots (QDots) with a narrow size distribution were synthesized in ethylene glycol using the solvothermal technique, where instead of using a conventional, highly toxic sulfur source like H2S, or organometallic precursors which are toxic, air and moisture sensitive, we have used elemental sulfur dissolved in organic solvent to react with Cd ions in a Teflon-lined autoclave reactor by a procedure slightly modified to that employed by Rao et al., to synthesize CdS nanocrystals [15], where instead of using tetralin in our method, we use the reducing power of ethylene glycol at boiling point to convert sulfur to S−2 which in turn reacts with cadmium ions to yield highly crystalline and soluble CdS nanoparticles. It is worth mentioning here that no capping agent is used to stabilize the particles in solution.

Section snippets

Experimental

All the chemicals used were of analytical grade and were used without further purification; cadmium nitrate [Cd(NO3)2] from the Aldrich company as the Cd source; sulfur from Aldrich dissolved in tetrahydrofuran (THF) and ethylene glycol from Aldrich as well. The schematic illustration of a typical synthesis process of a luminescent CdS QD, is shown in Fig. 1, where different molar ratios of analytical grade [Cd(NO3)2] and sulfur dissolved in THF were added to a Teflon-lined 70 mL capacity

Results and discussion

The reactions of Cd+2 ions with sulfur dissolved in THF in the presence of ethylene glycol within the autoclave reactor at 180 °C resulted in highly luminescent and stable CdS colloids. Fig. 1 shows the blue luminescence when the sample is excited with UV light at 365 nm and the excitation spectra for the samples synthesized as a function of the reaction time. From the excitation spectra we observe a red shift on the absorption peaks from 363nm to 381 nm as the reaction time increases from 24 to 72

Summary

To summarize, we have synthesized highly luminescent CdS QDots with a narrow size distribution using a new synthesis route and a less hazardous cadmium source without any capping agent by employing the solvothermal technique co-assisted by the polyol method at temperatures as low as 180 °C. The as-synthesized CdS QDots are monodisperse and exhibit strong PL in the blue spectral region. The mean particle size of the CdS QDots measured using TEM is 3 nm and the SAED measurement indicates that

Acknowledgment

The authors would like to thank CETENE for the TEM measurements, Marcela Bianca for comments and suggestions on the manuscript, and gratefully acknowledge financial support received during the development of this work from CNPQ contract nos. 479543/2011-9 and nos. 473.144/03-4, FACEPE and inct-INAMI.

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