Photoluminescence enhancement in CdS quantum dots by thermal annealing
- Jae Ik Kim†1,
- Jongmin Kim†1,
- Junhee Lee†1,
- Dae-Ryong Jung1,
- Hoechang Kim1,
- Hongsik Choi1,
- Sungjun Lee1,
- Sujin Byun1,
- Suji Kang1 and
- Byungwoo Park1Email author
© Kim et al.; licensee Springer. 2012
Received: 12 July 2012
Accepted: 18 August 2012
Published: 29 August 2012
The photoluminescence behavior of CdS quantum dots in initial growth stage was studied in connection with an annealing process. Compared to the as-synthesized CdS quantum dots (quantum efficiency ≅ 1%), the heat-treated sample showed enhanced luminescence properties (quantum efficiency ≅ 29%) with a narrow band-edge emission. The simple annealing process diminished the accumulated defect states within the nanoparticles and thereby reduced the nonradiative recombination, which was confirmed by diffraction, absorption, and time-resolved photoluminescence. Consequently, the highly luminescent and defect-free nanoparticles were obtained by a facile and straightforward process.
KeywordsCdS quantum dot photoluminescence quantum efficiency local strain relaxation
Due to the benefits of their size-tunable physical properties [1–3], nanoscale semiconductor materials have promising future applications, including the optoelectronic devices such as light-emitting diodes [4–8] and next-generation quantum dot solar cells [9–14]. Moreover, nanoscale semiconductors functionalized with biomolecules are used as molecular fluorescent probes in biological applications .
In recent years, there has been a rapid development of the growth techniques for quantum dots with high crystallinity and narrow size distribution [16–18]. The hot-injection techniques allow the affordable growth of a wide range of nanoscale materials with high quality [19–21]. On the other hand, low-temperature synthesis has not been actively studied yet. Low-temperature synthesis has higher potential than hot-injection techniques because the process is relatively simple and nontoxic . However, the size distribution and the crystallinity of nanoparticles are generally poor because of low synthetic temperature and surface defects . Recently, several papers have introduced advanced low-temperature synthesis and colloidal growth that can yield quantum dots with a sufficiently narrow size distribution [24–27].
In this regard, introducing a facile annealing process has great potential for enhancing the quantum efficiency and tuning the size of nanocrystals. However, systematic analysis of the initial growth stage of the nanoparticles has rarely been studied. In this work, a simple aqueous system and straightforward annealing process were applied to the preparation of highly luminescent CdS quantum dots. The appropriate annealing condition was well correlated with the quantum dot size, local strain (crystallinity), and radiative/nonradiative recombination rates.
The CdS quantum dots were synthesized by using a combination of the reverse-micelle method and post-growth annealing process. Cadmium chloride (CdCl2, 0.182 g) and sodium sulfide (Na2S, 0.036 g) were separately dissolved in distilled water (15 ml) and stirred to achieve their complete dissolution. Linoleic acid ((C17H31)COOH, 2.4 ml) and sodium linoleate ((C17H31)COONa, 2 g) were dissolved in ethanol (15 ml) and formed transparent solutions. After the two solutions were mixed and stirred vigorously, the color changed from transparent to opaque white, implying the formation of a microemulsion consisting of cadmium linoleate. After the addition of sodium sulfide, the color changed from white to greenish yellow. For the annealing process, the autoclave was heated at 100°C for 1 to 24 h. In order to increase the quantum dot size, post-growth annealing was also conducted at 125°C to 225°C with the same annealing time (12 h). The resultant CdS quantum dots were precipitated by using centrifugation and cleaned several times with ethanol. Finally, the CdS quantum dots were dispersed into chloroform (CHCl3, 40 ml), displaying a translucent yellow solution.
The structural properties of the quantum dots, such as crystal size and local strain, were studied using X-ray diffraction (XRD; M18XHF-SRA, MAC Science, Yokohama, Japan) with θ to 2θ curves. To analyze the optical properties, the absorbance was measured using UV/visible spectrometry, and the photoluminescence (PL) data were measured under 360-nm excitation wavelength with a spectrofluorometer (FP-6500, JASCO, Essex, UK). The binding energy of CdS quantum dots was analyzed by X-ray photoelectron spectroscopy (XPS; Sigma Probe, Thermo VG Scientific, Logan, UT, USA) using Al Kα radiation (1,486.6 eV). Time-resolved PL was measured by using a picosecond laser system (FLS920P, Edinburgh Instruments Ltd., Livingston, UK), and the nanostructures of the CdS nanoparticles were analyzed by a high-resolution transmission electron microscopy (TEM; JEM-3000 F, JEOL Ltd., Tokyo, Japan).
Results and discussion
The luminescence properties of CdS quantum dots in the initial growth stage were examined in connection with a simple annealing process. Both the accumulated defect states and nonradiative recombination rates were reduced, and these correlations were confirmed systematically by diffraction, absorption, and time-resolved photoluminescence. Consequently, the highly luminescent (quantum efficiency of 29% from the initial 1%) and defect-free nanoparticles were obtained by a facile annealing process.
This research was supported by the National Research Foundation of Korea through the World Class University (WCU, R31-2008-000-10075-0) and the Korean government (MEST:NRF, 2010–0029065).
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