Synthesis of Efficiently Green Luminescent CdSe/ZnS Nanocrystals Via Microfluidic Reaction
© to the authors 2008
Received: 30 January 2008
Accepted: 21 February 2008
Published: 18 March 2008
Quantum dots with emission in the spectral region from 525 to 535 nm are of special interest for their application in green LEDs and white-light generation, where CdSe/ZnS core-shell structured nanocrystals (NCs) are among promising candidates. In this study, triple-ligand system (trioctylphosphine oxide–oleic acid–oleylamine) was designed to improve the stability of CdSe NCs during the early reaction stage. With the precisely controlled reaction temperature (285 °C) and residence time (10 s) by the recently introduced microfluidic reaction technology, green luminescent CdSe NCs (λ = 522 nm) exhibiting narrow FWHM of PL (30 nm) was reproducibly obtained. After that, CdSe/ZnS core-shell NCs were achieved with efficient luminescence in the pure green spectral region, which demonstrated high PL QY up to 70% and narrow PL FWHM as 30 nm. The strengthened mass and heat transfer in the microchannel allowed the formation of highly luminescent CdSe/ZnS NCs under low reaction temperature and short residence time (T = 120 °C,t = 10 s). The successful formation of ZnS layer was evidence of the substantial improvement of PL intensity, being further confirmed by XRD, HRTEM, and EDS study.
Colloidal luminescent semiconductor nanocrystals (NCs), also known as quantum dots (QDs), have attracted considerable attention as potential candidates for LED and displays, photoluminescent and chemiluminescent biological labels, and so on . QDs with emission in the spectral range from 525 to 535 nm are of special interest for the preparation of white-light and QDs-based green LEDs [2, 3]. To date, pure green luminescence has been realized by binary CdSe NCs and pseudobinary (AB x C1−x ) semiconductor alloy NCs, such as CdSe x S1−x , Zn x Cd1−x Se, etc. [2–6]. The synthesis of these pseudobinary NCs usually involves high temperature and multi-step reaction, and the control over their size, shape, and composition was far from ideal as compared with CdSe NCs. While bare CdSe NCs tend to be oxidized, the reduced photoluminescence (PL) quantum yield (QY) is generally observed during the post processing for special purposes. Overcoating bare QDs with a higher-band-gap material as a shell can result in the improved QY of PL. One possible configuration is that both the valence and the conduction band edges of the core material are located in the band gap of the shell material. This makes carriers be strongly confined to the core material, enhancing their probability of radiative recombination. Typical examples are CdSe/ZnS, CdSe/ZnSe, and CdSe/CdS [7–9], among which ZnS capped CdSe NCs exhibit low cytotoxicity and excellent stability [10, 11]. Using CdSe/ZnS core-shell NCs to achieve λ = 525 nm emission highly requires small CdSe cores (about 2.5 nm in diameter). Such small NCs with narrow size distribution and high QY of PL are difficult to be synthesized in batch reactions, which involve low reaction temperature (<200 °C) and extremely short reaction time (<10 s) (b)]. Nevertheless, the elongated nucleation period under low reaction temperature usually leads to polydisperse size for the products, while the realization of short reaction time is challenging due to the difficulties associated with quenching the reaction in a short period by batch methods (b)]. Moreover, it is difficult to overcoat such small NCs with higher-band-gap inorganic semiconductor, which is indispensable to realize their high quantum efficiency, good stability, as well as reduced cytotoxicity.
The enhanced transfer properties of mass and heat in a micro environment facilitate the chemical synthesis with improved efficiency and reproducibility. The superiority of microfluidic reaction with regard to precise synthetic-condition control, on-line sample characterization, as well as parallel operation has been demonstrated based on the study of CdSe, CdS, and so on .
In this study, a triple-ligand system (trioctylphosphine oxide–oleic acid–oleylamine) was designed to synthesize small sized CdSe NCs, and a capillary microreactor was set up to realize the controllable residence time. For the as-formed CdSe NCs, zinc diethyl dithiocarbamate was utilized as an environmental-benign ZnS source to synthesize CdSe/ZnS core-shell structures, and UV–Vis, PL, EDS, XRD, as well as HRTEM were utilized to characterize the formed NCs.
Cadmium oxide (CdO, SCR, 99.9%), selenium (Se, SCR, 99.5%), trioctylphosphine (TOP, Fluka, 90%), trioctylphosphine oxide (TOPO, Fluka, 98%), 1-octadecene (ODE, Fisher, 90%), oleic acid (OA, SCR, 90%), oleylamine (OLA, Fluka, 70%,), zinc diethyl dithiocarbamate (ZDC, Shanghai Dunhuang Chemical Plant, 99%) analytic grade methanol, and chloroform (SCR) were used directly without further processing.
UV–vis absorption spectra were measured at room temperature with a Cary 100 UV–vis spectrometer (Varian). PL spectra were acquired at room temperature with a Cary Eclipse spectrofluorometer on colloidal solutions with an optical density of less than 0.2 at the excitation wavelength (430 nm). PL quantum efficiency measurements were performed as described in Ref. , utilizing Rhodamine 6 G as a reference. Powder XRD measurements were performed on a D/max2550 X-ray diffraction system (Rigaku). Samples for XRD measurements were prepared by dropping a colloidal suspension of NCs in chloroform on a standard single crystal Si wafer and evaporating the solvent. A JEM-2100F high resolution transmission electron microscope (HRTEM) was used to evaluate the microstructures of the prepared NCs, and the sample was prepared by dipping an amorphous carbon–copper grid in a dilute chloroform dispersed NC solution, then the sample was left to evaporate at room temperature. Energy-dispersive spectrum (EDS) was acquired usinga scanning electron microscope (JSM-6360LV, JEOL) equipped with EDX (FALCON, EDAX, America).
Synthesis of CdSe NCs
In a typical synthesis, a Se stock solution was prepared by dissolving 79 mg of Se powder in 2 mL TOP. The obtained solution was further diluted with 2 mL ODE. Meanwhile, a suspension of 12.85 mg CdO, 0.25 mL OA, 2 mL OLA, and 1.75 mL ODE washeated at 150 °C with stirring to prepare a clear yellow cadmium precursor solution. Before being drawn into the syringes, the two stock solutions were thoroughly degassed. Details for the experiments can be found elsewhere .
Synthesis of CdSe/ZnS NCs
Results and Discussions
Microfluidic reaction offers a convenient method to conduct chemical synthesis in a totally continuous fashion . However, the continuous synthesis of CdSe/ZnS NCs via microreaction can be challenging, owing to the side reactions involved in the two-step reaction with multi-component precursors existing in the same solution. In this case, the temperature for the coating process shows significant importance: at higher temperatures, the CdSe cores begin to grow via Ostwald ripening, and deteriorate their size distribution, finally lead to broader spectral line widths; while the lower temperature will result in incomplete decomposition of the precursors and the reduced crystallinity of the ZnS shell.
In conclusion, a facile method was developed to prepare small sized CdSe NCs, and an environmental-benign precursor for S and Zn was utilized to synthesize core-shell structured CdSe/ZnS NCs with pure green luminescence. With the strengthened mass and heat transfer in the microchannel, highly luminescent CdSe/ZnS NCs were obtained under short residence time and low reaction temperature (t = 10 s,T = 120 °C). Homogenous coating of ZnS was achieved with fairly wide operation parameters. With the presented low temperature overcoating process, the purification process of CdSe NCs can be eliminated, which offered the feasibility to synthesize CdSe/ZnS NCs in a totally continuous fashion.
This work was financially supported by the National Natural Science Fund of China ofcontract number 50772036.
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