An effective oxidation approach for luminescence enhancement in CdS quantum dots by H2O2
- Woojin Lee†1,
- Hoechang Kim†1,
- Dae-Ryong Jung†1,
- Jongmin Kim1,
- Changwoo Nahm1,
- Junhee Lee1,
- Suji Kang1,
- Byungho Lee1 and
- Byungwoo Park1Email author
© Lee et al.; licensee Springer. 2012
Received: 23 November 2012
Accepted: 28 November 2012
Published: 12 December 2012
The effects of surface passivation on the photoluminescence (PL) properties of CdS nanoparticles oxidized by straightforward H2O2 injection were examined. Compared to pristine cadmium sulfide nanocrystals (quantum efficiency ≅ 0.1%), the surface-passivated CdS nanoparticles showed significantly enhanced luminescence properties (quantum efficiency ≅ 20%). The surface passivation by H2O2 injection was characterized using X-ray photoelectron spectroscopy, X-ray diffraction, and time-resolved PL. The photoluminescence enhancement is due to the two-order increase in the radiative recombination rate by the sulfate passivation layer.
Semiconductor nanocrystals or quantum dots have attracted great attention because their optical and electrical properties can be tuned by changing their sizes and surface states [1–6]. Photoluminescence (PL) characteristics of semiconductor nanocrystals are strongly dependent on their surface states since a large portion of atoms are located at or near the surface of nanoparticles, forming dangling bonds as main trap states against radiative recombination. Focused on the surface states, various strategies for the enhancement of optical properties in CdS nanocrystals have been developed by employing a core/shell structure, size-selective photoetching, and surface passivation by reducing agents [7–12].
In this regard, the artificial formation of an oxide layer on the surface of CdS nanocrystals holds great potential for surface passivation and tuning the size of nanocrystals . Despite the aforementioned advantages, the formation of an oxide layer leads to the elimination of the passivating ligands bound to the surface of quantum dots. It is difficult to synthesize surface-oxidized quantum dots with ligands by traditional methods [14, 15].
In this study, we have developed a facile and straightforward oxidation process by injection of H2O2 with subsequent ligand exchange for highly luminescent CdS quantum dots. In order to describe the mechanism of PL enhancement, the changes in the chemical states during the oxidation process are examined based on X-ray photoelectron spectroscopy (XPS) data. The correlations of the enhanced PL properties with the quantum dot size, local strain, chemical states, and radiative recombination rates are systematically investigated.
The CdS nanocrystals were synthesized using a reverse micelle method previously reported by Wang et al. . Cadmium chloride (CdCl2, 0.182 g) and sodium sulfide (Na2S, 0.078 g) were dissolved separately in distilled water (15 mL) and stirred until complete dissolution. The cadmium chloride solution was placed into an autoclave followed by the addition of sodium sulfide. Linoleic acid ((C17H31)COOH, 2.4 mL) and sodium linoleate ((C17H31)COONa, 2 g) dissolved in ethanol were added to the resulting solution. The resultant CdS nanocrystals were precipitated using centrifugation and cleaned several times with ethanol. After synthesis, the CdS nanocrystals were dispersed into chloroform (CHCl3, 40 mL), which displayed a transparent yellow color.
For the oxidation step of CdS nanocrystals, 3.0 wt. % H2O2 solution was added to the solution of nanocrystals in a dark environment, and n-butylamine ((C4H9)NH2, 10 mL) was added for the ligand exchange. The samples were oxidized with the addition of different amounts (0, 0.8, 1.2, 1.6, 2.0, 2.4, and 2.8 mL) of H2O2 solution. During injection, 0.4 mL of the H2O2 solution was repeatedly injected at 24-h time intervals.
The nanostructure of the CdS nanoparticles was analyzed by X-ray diffraction (XRD, M18XHF-SRA, MAC Science Co., Yokohama, Japan). The PL spectra were measured using a spectrofluorometer (FP-6500, JASCO, Essex, UK) with a Xe lamp, and the absorption spectra were recorded on a UV/Vis spectrophotometer (Lambda 20, PerkinElmer, Waltham, MA, USA). The quantum efficiency of colloidal CdS samples was estimated using Rhodamine 6G in ethanol (quantum efficiency of approximately 95% for an excitation wavelength of 488 nm) by comparing their absorbance in order to examine the luminescence properties quantitatively . The surface chemical states of CdS nanocrystals were analyzed by XPS (Sigma Probe, Thermo VG Scientific, Logan, UT, USA) using Al Kα radiation (1,486.6 eV).
Results and discussion
As shown in Figure 1b, the core size of CdS nanocrystals gradually decreases with the increasing amount of H2O2 solution, indicating the formation of an oxide layer. As the thickness of the oxide layers increases, the local strain of CdS nanocrystals slightly decreases. The synthesis of nanocrystals at room temperature can lead to defective shells of CdS quantum dots due to the insufficient kinetics for complete crystallization [22–24]. Therefore, the reduced local strain may be caused by the oxidation of this defective shell by H2O2.
Highly luminescent CdS QDs were obtained using a facile and straightforward H2O2 oxidation process with ligand exchange. The amount of H2O2 used in the CdS oxidation process was correlated with the quantum dot size, local strain, chemical states, and radiative/nonradiative recombination rates. The oxidized CdS nanocrystals exhibited a quantum efficiency (20%) two orders of magnitude higher than that of an as-synthesized sample (0.1%) by an effective passivation promoting radiative recombination rate.
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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