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.
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 Kα 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 Kα1 and Kα2[18–21] and the instrumental broadening effect.
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].
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].
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.
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
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].
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.
References
Alivisatos AP: Semiconductor cluster, nanocrystals, and quantum dots. Science 1996, 271: 933–937. 10.1126/science.271.5251.933
Alivisatos AP: Perspectives on the physical chemistry of semiconductor nanocrystals. J Phys Chem 1996, 100: 13226–13239. 10.1021/jp9535506
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. 10.1021/ja00072a025
Qi H, Alexson D, Glembocki O, Prokes SM: Plasmonic coupling on dielectric nanowire core-metal sheath composites. Nanotechnology 2010, 21: 085705. 10.1088/0957-4484/21/8/085705
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. 10.1364/OE.19.013209
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. 10.1038/nmat1564
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. 10.1002/1521-3773(20020703)41:13<2368::AID-ANIE2368>3.0.CO;2-G
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. 10.1002/anie.200453728
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. 10.1021/jp0109271
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. 10.1021/jp049475t
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. 10.1021/la800964s
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. 10.1021/ja0357902
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. 10.1088/0957-4484/18/46/465702
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. 10.1021/ja067742y
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. 10.1021/cm061105p
Wang X, Zhuang J, Peng Q, Li YD: Synthesis and characterization of sulfide and selenide colloidal semiconductor nanocrystals. Langmuir 2006, 22: 7364–7368. 10.1021/la060023c
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. 10.1016/0009-2614(96)00838-X
Kim T, Oh J, Park B, Hong KS: Correlation between strain and dielectric properties in ZrTiO4 thin films. Appl Phys Lett 2000, 76: 3043. 10.1063/1.126573
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. 10.1063/1.1366359
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. 10.1021/jp067217l
Warren BE: X-Ray Diffraction. Dover, New York; 1990:257–262.
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. 10.1021/ja037228h
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. 10.1021/ja0458219
Liu L, Peng Q, Li Y: An effective oxidation route to blue emission CdSe quantum dots. Inorg Chem 2008, 47: 3182–3187. 10.1021/ic702203c
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. 10.1021/la1032288
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. 10.1016/0169-4332(88)90082-7
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. 10.1021/jp804413a
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. 10.1021/nn100131w
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. 10.1021/jp303766d
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. 10.1063/1.3007980
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. 10.1063/1.3431267
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).
Author information
Authors and Affiliations
Corresponding author
Additional information
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.
Woojin Lee, Hoechang Kim, Dae-Ryong Jung contributed equally to this work.
Authors’ original submitted files for images
Below are the links to the authors’ original submitted files for images.
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License ( https://creativecommons.org/licenses/by/2.0 ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
About this article
Cite this article
Lee, W., Kim, H., Jung, DR. et al. An effective oxidation approach for luminescence enhancement in CdS quantum dots by H2O2. Nanoscale Res Lett 7, 672 (2012). https://doi.org/10.1186/1556-276X-7-672
Received:
Accepted:
Published:
DOI: https://doi.org/10.1186/1556-276X-7-672