SpringerOpen Newsletter

Receive periodic news and updates relating to SpringerOpen.

Open Access Nano Express

An effective oxidation approach for luminescence enhancement in CdS quantum dots by H2O2

Woojin Lee, Hoechang Kim, Dae-Ryong Jung, Jongmin Kim, Changwoo Nahm, Junhee Lee, Suji Kang, Byungho Lee and Byungwoo Park*

Author affiliations

WCU Hybrid Materials Program, Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul, 151-744, Korea

For all author emails, please log on.

Citation and License

Nanoscale Research Letters 2012, 7:672  doi:10.1186/1556-276X-7-672

The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/7/1/672


Received:23 November 2012
Accepted:28 November 2012
Published:12 December 2012

© 2012 Lee et al.; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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.

Keywords:
Photoluminescence; Surface passivation; Quantum efficiency

Background

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 [13]. 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.

Methods

The CdS nanocrystals were synthesized using a reverse micelle method previously reported by Wang et al. [16]. 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 [17]. The surface chemical states of CdS nanocrystals were analyzed by XPS (Sigma Probe, Thermo VG Scientific, Logan, UT, USA) using Al radiation (1,486.6 eV).

Results and discussion

The effects of oxidation on the size and strain of CdS nanocrystals were investigated by XRD analysis. Figure 1 shows the XRD patterns of CdS nanocrystals prepared with different oxidation steps. To qualitatively estimate the local strain and the effective size of CdS nanocrystals, the diffraction peak widths (full width at half maximum) were fitted with the scattering vector (k = (4π/λ)sinθ) using a double-peak Lorentzian function, considering the effect of 1 and 2[18-21] and the instrumental broadening effect.

thumbnailFigure 1. XRD patterns, nanocrystal size, and local strain. (a) X-ray diffraction of CdS nanocrystals and (b) the size of nanocrystals and their local strain. The peak positions and intensities of CdS (JCPDS #75-0581) are marked.

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.

The change in surface states of CdS nanocrystals after oxidation was investigated by XPS (Figure 2). The peak shift in O 1s from 532.5 to 531.7 eV indicates the change of chemical bonding from carboxyl acid bound to nanocrystals to cadmium-containing oxide [25-27]. In addition, the 168.5 eV peak from S 2p and the 531.7 eV peak from O 1s are observed only after oxidation [26], indicating to the formation of CdSO4 layers. A proposed mechanism for the surface oxidation in CdS nanocrystals is schematically described in Figure 3. During the oxidation reaction with H2O2, organic ligands dissolve into the solvent [28], and the surface modification by the amine functional group prevents the quantum dots from agglomeration [29].

thumbnailFigure 2. XPS spectra. The spectra correspond to (a) Cd 3d, (b) O 1 s, and (c) S 2p for the CdS nanocrystals with various amounts of injected H2O2 solution. The dashed lines on each spectrum are from [26] and [27].

thumbnailFigure 3. Schematic figure of the oxidation process and surface modification with n-butylamine. (a) The synthesized nanocrystal covered with linoleic acid, (b) loss of organic ligands during surface oxidation, and (c) surface modification with n-butylamine.

As the amount of H2O2 solution increases, the absorbance spectra of the samples exhibit blueshift depending on their size reduction, and the first exciton peak becomes clear with the addition of H2O2 solution over 1.6 mL (Figure 4). The exciton peak with 2.8 mL of H2O2 injection is not clearly observed, which may have resulted from the nearly complete oxidation to the core part of quantum dots due to fast oxidation by H2O2[24].

thumbnailFigure 4. The absorbance spectra of CdS nanoparticles. As the amount of H2O2 solution increased, the optical bandgap and exciton peak shifted to a higher energy.

The luminescence characteristics display broad emission ranging from 450 to 650 nm, which originates from the trap-state emission, as shown in Figure 5[11,12]. The highest emission peak intensity of CdS nanocrystals is about two orders of magnitude higher than that of the as-synthesized one. Moreover, the CdS quantum dots oxidized with the addition of H2O2 solution over 1.6 mL show a weak band-edge emission at 450 nm and spectral blueshift of the photoluminescence.

thumbnailFigure 5. Photoluminescence spectra of CdS nanoparticles with various amount of H2O2.

For the carrier dynamics in oxidized CdS nanocrystals, each decay time (τ = ktotal-1) was acquired from a single-exponential fitting at the initial stage of the time-resolved PL (Figure 6). The quantum efficiency (η) is

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/672/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/672/mathml/M1">View MathML</a>

(1)

where ktotal, krad, knonrad, and τ are the total, radiative, and nonradiative recombination rates, and the decay time (krad + knonrad)−1, respectively. Figure 7 shows the quantum efficiency and radiative/nonradiative recombination rates of oxidized nanocrystals with different amounts of injected H2O2 solution. The quantum efficiency enhancement in CdS is mainly caused by the increased radiative recombination rate, while the nonradiative recombination rate remains constant, even though our previous papers reported reduced nonradiative recombination by the formation of a passivation layer on quantum dots [30,31].

thumbnailFigure 6. Decay curves of CdS nanoparticles as a function of H2O2solution with corresponding decay time (τ).

thumbnailFigure 7. Quantum efficiency, radiative recombination rate, and nonradiative recombination rate as a function of H2O2solution.

Conclusions

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.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

WL and DRJ drafted and revised the manuscript. HK carried out the synthetic experiments and characterizations. JK, CN, JL, SK, and BL participated in the scientific flow. BP conceived the study and participated in its design and coordination. All authors read and approved the final manuscript.

Acknowledgments

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).

References

  1. Alivisatos AP: Semiconductor cluster, nanocrystals, and quantum dots.

    Science 1996, 271:933-937. Publisher Full Text OpenURL

  2. Alivisatos AP: Perspectives on the physical chemistry of semiconductor nanocrystals.

    J Phys Chem 1996, 100:13226-13239. Publisher Full Text OpenURL

  3. Murray CB, Norris DJ, Bawendi MG: Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites.

    J Am Chem Soc 1993, 115:8706-8715. Publisher Full Text OpenURL

  4. Qi H, Alexson D, Glembocki O, Prokes SM: Plasmonic coupling on dielectric nanowire core-metal sheath composites.

    Nanotechnology 2010, 21:085705. Publisher Full Text OpenURL

  5. Lee SM, Choi KC, Kim DH, Jeon DY: Localized surface plasmon enhanced cathodoluminescence from Eu3+-doped phosphor near the nanoscaled silver particles.

    Opt Express 2011, 19:13209-13217. PubMed Abstract | Publisher Full Text OpenURL

  6. Hu M-S, Chen H-L, Shen C-H, Hong L-S, Huang B-R, Chen K-H, Chen L-C: Photosensitive gold-nanoparticle-embedded dielectric nanowires.

    Nat Mater 2006, 5:102-106. PubMed Abstract | Publisher Full Text OpenURL

  7. Yu WW, Peng XG: Formation of high-quality CdS and other II-VI semiconductor nanocrystals in noncoordinating solvents: tunable reactivity of monomers.

    Angew Chem Int Ed 2002, 41:2368-2371. Publisher Full Text OpenURL

  8. Steckel JS, Zimmer JP, Coe-Sullivan S, Stott NE, Bulovic V, Bawendi MG: Blue luminescence from (CdS) ZnS core-shell nanocrystals.

    Angew Chem Int Ed 2004, 43:2154-2158. Publisher Full Text OpenURL

  9. Torimoto T, Kontani H, Shibutani Y, Kuwabata S, Sakata T, Mori H, Yoneyama H: Characterization of ultrasmall CdS nanoparticles prepared by the size-selective photoetching technique.

    J Phys Chem B 2001, 105:6838-6845. Publisher Full Text OpenURL

  10. Jang E, Jun S, Chung Y, Pu L: Surface treatment to enhance the quantum efficiency of semiconductor nanocrystals.

    J Phys Chem B 2004, 108:4597-4600. Publisher Full Text OpenURL

  11. Saunders AE, Ghezelbash A, Sood P, Korgel BA: Synthesis of high aspect ratio quantum-size CdS nanorods and their surface-dependent photoluminescence.

    Langmuir 2008, 24:9043-9049. PubMed Abstract | Publisher Full Text OpenURL

  12. Joo J, Na HB, Yu T, Yu JH, Kim YW, Wu FX, Zhang JZ, Hyeon T: Generalized and facile synthesis of semiconducting metal sulfide nanocrystals.

    J Am Chem Soc 2003, 125:11100-11105. PubMed Abstract | Publisher Full Text OpenURL

  13. Sato K, Kojima S, Hattori S, Chiba T, Ueda-Sarson K, Torimoto T, Tachibana Y, Kuwabata S: Controlling surface reactions of CdS nanocrystals: photoluminescence activation, photoetching and photostability under light irradiation.

    Nanotechnology 2007, 18:465702. PubMed Abstract | Publisher Full Text OpenURL

  14. Dubois F, Mahler B, Dubertret B, Doris E, Mioskowski C: A versatile strategy for quantum dot ligand exchange.

    J Am Chem Soc 2007, 129:482-483. PubMed Abstract | Publisher Full Text OpenURL

  15. Querner C, Benedetto A, Demadrille R, Rannou P, Reiss P: Carbodithioate-containing oligo- and polythiophenes for nanocrystals surface functionalization.

    Chem Mater 2006, 18:4817-4826. Publisher Full Text OpenURL

  16. Wang X, Zhuang J, Peng Q, Li YD: Synthesis and characterization of sulfide and selenide colloidal semiconductor nanocrystals.

    Langmuir 2006, 22:7364-7368. PubMed Abstract | Publisher Full Text OpenURL

  17. Fischer M, Georges J: Fluorescence quantum yield of rhodamine 6 G in ethanol as a function of concentration using thermal lens spectrometry.

    Chem Phys Lett 1996, 260:115-118. Publisher Full Text OpenURL

  18. Kim T, Oh J, Park B, Hong KS: Correlation between strain and dielectric properties in ZrTiO4 thin films.

    Appl Phys Lett 2000, 76:3043. Publisher Full Text OpenURL

  19. Kim Y, Oh J, Kim TG, Park B: Effect of microstructures on the microwave dielectric properties of ZrTiO4 thin films.

    Appl Phys Lett 2001, 78:2363. Publisher Full Text OpenURL

  20. Moon T, Hwang S-T, Jung D-R, Son D, Kim C, Kim J, Kang M, Park B: Hydroxyl-quenching effects on the photoluminescence properties of SnO2:Eu3+ nanoparticles.

    J Phys Chem C 2007, 111:4164-4167. Publisher Full Text OpenURL

  21. Warren BE: X-Ray Diffraction. Dover, New York; 1990:257-262. OpenURL

  22. Berrettini MG, Braun G, Hu JG, Strouse GF: NMR analysis of surface and interfaces in 2-nm CdSe.

    J Am Chem Soc 2004, 126:7063-7070. PubMed Abstract | Publisher Full Text OpenURL

  23. Chen X, Samia AC, Lou Y, Burda C: Investigation of the crystallization process in 2 nm CdSe quantum dots.

    J Am Chem Soc 2005, 127:4372-4375. PubMed Abstract | Publisher Full Text OpenURL

  24. Liu L, Peng Q, Li Y: An effective oxidation route to blue emission CdSe quantum dots.

    Inorg Chem 2008, 47:3182-3187. PubMed Abstract | Publisher Full Text OpenURL

  25. Rengaraj S, Venkataraj S, Jee SH, Kim Y, Tai C, Repo E, Koistinen A, Ferancova A, Sillanpää M: Cauliflower-like CdS microspheres composed of nanocrystals and their physicochemical properties.

    Langmuir 2011, 27:352-358. PubMed Abstract | Publisher Full Text OpenURL

  26. Jin ZS, Li QL, Xi CJ, Jian ZC, Chen ZS: Effect of high-temperature treatment in air ambience on the surface composition and structure of CdS.

    Appl Surf Sci 1988, 32:218-232. Publisher Full Text OpenURL

  27. Kundu S, Wang YM, Xia W, Muhler M: Thermal stability and reducibility of oxygen-containing functional groups on multiwalled carbon nanotube surfaces: a quantitative high-resolution XPS and TPD/TPR study.

    J Phys Chem C 2008, 112:16869-16878. Publisher Full Text OpenURL

  28. Sykora M, Koposov AY, McGuire JA, Schulze RK, Tretiak O, Pietryga JM, Klimov VI: Effect of air exposure on surface properties, electronic structure, and carrier relaxation in PbSe nanocrystals.

    ACS Nano 2010, 4:2021-2034. PubMed Abstract | Publisher Full Text OpenURL

  29. Zillner EF, Fengler S, Niyamakom P, Rauscher F, Kohler K, Dittruch T: Role of ligand exchange at CdSe quantum dot layers for charge separation.

    J Phys Chem C 2012, 116:16747-16754. Publisher Full Text OpenURL

  30. Jung D-R, Son D, Kim J, Kim C, Park B: Highly-luminescent surface-passivated ZnS:Mn nanoparticles by a simple one-step synthesis.

    Appl Phys Lett 2008, 93:163118. Publisher Full Text OpenURL

  31. Jung D-R, Kim J, Park B: Surface-passivation effects on the photoluminescence enhancement in ZnS:Mn nanoparticles by ultraviolet irradiation with oxygen bubbling.

    Appl Phys Lett 2010, 96:211908. Publisher Full Text OpenURL