Article pubs.acs.org/crystal
Understanding the Occurrence of the Maximum Band-Edge Photoluminescence of TGA-Capped CdS QDs via Growth Kinetic Study Xiaogang Xue, Zanyong Zhuang, Feng Huang, and Zhang Lin* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *
ABSTRACT: The mechanism by which the maximum band-edge emission of quantum dots (QDs) occurs remains unclear. In this work, systematic kinetic studies revealed that the growths of thioglycolic acid (TGA)-capped CdS QDs at three concentrations (7.0, 2.3, and 0.8 mM) undergo a two-stage process: an initial oriented attachment (OA) dominant stage and a subsequent Ostwald Ripening (OR) stage. At the transition point from the OA dominant to the OR stage, the band-edge PL peaked at 450−470 nm and reached its “maximum”, with the narrowest peak width about 28 nm. Investigation on particle size distribution (PSD) showed size “focusing” in the OA dominant stage. Nevertheless, the ideally narrowest PSD occurs far earlier than the maximum band-edge emission. Furthermore, its PL emission was found broadened by competitive defect-related emissions, reasonably assigned to lattice defects generated by the initial OA growth. Annealing of defects at the later OA stage can be responsible for enhanced PL band-edge emission to a certain maximum value, which will decrease in the subsequent OR growth due to the broadening PSD. The role of surface-capping was also discussed and proved to be the key factor to achieve the maximum band-edge emission of QDs by tuning the crystal growth kinetics.
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sensitive to the synthetic condition.16−19 A few more in-depth works reveal that it usually occurs in the specific growth period of QDs, characterized by nearly zero growth rate20 or the narrowest size distribution.19,21 Nevertheless, it is still considered a challenge to predict and control the “bright point”. Currently, due to the lack of direct connection between the crystal growth kinetics and the corresponding PL characteristics of QDs, the underlying mechanism governing the occurrence of high-quality PL QDs cannot be welladdressed. Research results indicate complex growth mechanisms of QDs. Generally, the growth of QDs is regarded as following the classical Ostwald Ripening (OR) mechanism.22 Theoretically, the crystal growth via OR is inclined to generate QDs with relatively complete internal lattices, making the surface defects
INTRODUCTION Owing to their size-dependent optical properties, II−VI and III−V semiconductor luminescent quantum dots (QDs) have great potential applications in biomarkers, sensors, and lightemitting devices.1−4 To achieve high-quality photoluminescence (PL) (i.e., narrow spectrum and high brightness), it requires the fabricated QDs to have a narrow size distribution, passivated surface state, and high crystallinity.4−8 Up to now, there have been various liquid-phase synthetic methods of QDs,9 in which the experimental conditions including pH, the ratio of precursors, temperature, reaction time, and the selection of capping agents10−14 were empirically optimized to obtain high-quality QDs. It has been discovered in the synthetic process, the PL brightness (measured by quantum yields or PL intensity at a certain concentration) of II−VI and III−V QDs can sometimes reach a maximum value, which is denoted as the PL “bright point”.15 Increasing evidence suggests that the occurrence and characteristics (wavelength, PL peak intensity, and peak width) of the “bright point” are © 2013 American Chemical Society
Received: June 23, 2013 Revised: September 26, 2013 Published: October 22, 2013 5220
dx.doi.org/10.1021/cg400935w | Cryst. Growth Des. 2013, 13, 5220−5228
Crystal Growth & Design
Article
the main factor influencing the PL properties.23,24 Efforts, hence, are focused on the passivation of surface dangling bonds via organic and inorganic coating.7,25 However, the presence of strong capping agents may induce a growth mode named “oriented attachment” (OA) mechanism in nanomaterials,26 which is frequently neglected. A strongly capping agent can even result in OA dominant growth.27 So far, it remains a strong need for kinetic analysis of QDs to help in disclosing the intrinsic growth mechanism of nanomaterials.28 Recently, an ideally pure OA or OR growth of CdS QDs was achieved by tuning CdS concentration up to 20 mM or down to 0.1 mM, respectively. Thereby, the dependence of the PL properties on the pure OA or OR growth modes was first studied by Zheng et. al.29 It revealed that different from the PL characteristics corresponding to the pure OR kinetics, the PL corresponding to the pure OA kinetics is characterized by enhanced defectrelated emissions in the initial OA growth stage, which is ascribed to the generation of internal lattice defects induced by the OA mechanism. In some cases, it revealed that the concentration of nanocrystals is an important influencing factor to determine the OA growth mode. Some other reports suggest that the pure OA growth kinetics could be achieved only under extremely dense or strong capping conditions.29 The characteristics of the growth kinetics of a material are not only dependent on its composition or crystal structure, for example, chalcognide or metal oxide, but also closely related with aggregation or dispersity, which is generally determined by types of capping agents. For instance, the growth of ZnS NCs capped by inorganic alkali or organic ligands follows different rules, which can be described by different OA models. However, under normal capping states, the OA growth of QDs can be found frequently as coexisted OR,27,30 potentially involving complex microscopic processes generally associated with the contribution of the two mechanisms. Generally, besides generating absolutely different PL emission characteristics, the OR normally leads to the broadened particle size distribution (PSD) of QDs, while the OA has been discussed in favor of a size-focusing process.31 Hence, the coexistence of OA and OR, as well as different contribution and transition tendency of OA and OR, could induce additional complexity, too. Difficulties thus arise for the determination of the material characteristics at different growing periods and the prediction of the occurrence of high-quality PL QDs. In this work, the growth kinetics and PL emission properties of TGA-capped CdS at three selected concentrations (0.8, 2.3, and 7.0 mM) were investigated. Both the information of the PL emission of QDs during the growths and the kinetic rules of QDs (mechanism, size, morphology, and microstructure) were systematically obtained. In this way, for the first time, the correlation between the PL evolution rule and the evolution of crystal growth mechanisms could be established, and the subtle tuning mechanism of surface-capping agent to the crystal growth mode (as well as the PL properties) could also be deeply disclosed. This study provides important insight into how to acquire high-quality PL properties of QDs via tuning the crystal growth kinetics. The fundamental understanding in this work may also be extended to other QD systems such as CdSe, CdTe, and ZnSe.
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chemicals were purchased from Aldrich and used without further purification. Preparation of TGA-Capped CdS QDs. TGA-capped CdS QDs used in this study were synthesized using a reported method with slight modifications.32 First dissolved was 20 mmol of CdCl2 in 200 mL of deionized water, followed by an injection of 60 mmol TGA under continuous stirring. The pH was adjusted to 7.0 with NaOH. After stirring for 30 min, 100 mL of 0.2 M Na2S was slowly injected. The reaction proceeded for 10 h under N2 protection and constant stirring. TGA-capped CdS QDs could then be harvested by ethanol precipitation. Ethanol precipitation was repeated three times to ensure removal of most excess TGA molecules and other impurities such as Cl− and Na+. With appropriate dilution by distilled water, a stock solution containing 21.0 mM CdS QDs was prepared. Samples containing 7.0, 2.3, and 0.8 mM CdS QDs were prepared from this stock solution accordingly. Growth Kinetics of TGA-Capped CdS QDs. Growth experiments were carried out at three CdS concentration levels (7.0, 2.3, and 0.8 mM) at three temperatures of 355, 347, and 339 K, respectively. For each CdS concentration series, 60 mL QD solution was added into a 100 mL three-neck flask and refluxed under N2 protection at 355, 347, and 339 K, respectively. At appropriate time intervals, a certain amount of samples were extracted and cooled down to room temperature for the following characterizations. Characterizations of TGA-Capped CdS QDs. UV−vis Spectra. The UV−vis absorbance data were recorded using a Shimadza UV2550 double monochromator UV visible spectrophotometer at room temperature (298 K). Samples from 7.0, 2.3, and 0.8 mM CdS QD solutions were all diluted to 0.26 mM before the UV−vis measurement. The average size of the CdS QDs during growth could be calculated from the absorption edge by using the effective mass model:33 ER * = Eg +
ℏ2π 2 ⎛ 1 1 ⎞ 1.8e 2 + ⎟− 2 ⎜ m0mh ⎠ 4πεε0r 2r ⎝ m0me
(1)
where ER* is the band gap of the nanoparticles (ER* = hc/λ), Eg is the band gap of the corresponding bulk material, r is the particle radius, m0 is the mass of a free electron, me is the effective mass of the electrons, mh is the effective mass of the holes, ε is the relative permittivity, ε0 is the permittivity of free space, ℏ is Planck’s constant, and e is the charge on the electron. Since the relationship between the particle radius and the band gap can be expressed in eq 1 by substituting ER* with a function of r, the relationship between absorbance (A) and band gap (ER*) can be transformed into relationship between A and particle radius (r). Then an expression relating the size distribution n(r) to the local slope of the absorbance spectrum can be obtained:34 n(r ) ∝
dA /dr 4 3 πr 3
(2)
The PSD curve for each time point was calculated using eq 2. PL Spectra. The PL spectra were recorded on a Hitachi F-7000 spectrofluorometer with a xenon lamp as the excitation source, with the excitation wavelength of 350 nm at room temperature (298 K). All extracted samples were directly diluted into 6.5 μM before PL measurement to ensure an appropriate absorbance (
