Synthesis of Monodisperse Nanocrystals via Microreaction: Open-to-Air Synthesis with Oleylamine as a Coligand
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Microreaction provides a controllable tool to synthesize CdSe nanocrystals (NCs) in an accelerated fashion. However, the surface traps created during the fast growth usually result in low photoluminescence (PL) efficiency for the formed products. Herein, the reproducible synthesis of highly luminescent CdSe NCs directly in open air was reported, with a microreactor as the controllable reaction tool. Spectra investigation elucidated that applying OLA both in Se and Cd stock solutions could advantageously promote the diffusion between the two precursors, resulting in narrow full-width-at-half maximum (FWHM) of PL (26 nm). Meanwhile, the addition of OLA in the source solution was demonstrated helpful to improve the reactivity of Cd monomer. In this case, the focus of size distribution was accomplished during the early reaction stage. Furthermore, if the volume percentage (vol.%) of OLA in the precursors exceeded a threshold of 37.5%, the resulted CdSe NCs demonstrated long-term fixing of size distribution up to 300 s. The observed phenomena facilitated the preparation of a size series of monodisperse CdSe NCs merely by the variation of residence time. With the volume percentage of OLA as 37.5% in the source solution, a 78 nm tuning of PL spectra (from 507 to 585) was obtained through the variation of residence time from 2 s to 160 s, while maintaining narrow FMWH of PL (26–31 nm) and high QY of PL (35–55%).
KeywordsQuantum dots Microreaction Reaction kinetics Size control
Nanocrystaline semiconductors exhibit unique size-tunable electronic and optical properties distinct from their bulk counterparts. Taking advantages of this nanoscale tunability on a macroscopic length scale could provide new classes of materials for scientific research and technological applications [1, 2, 3, 4, 5]. Owning to the size-dependent PL covering the whole visible spectrum, CdSe nanocrystals (NCs) have become the most extensively investigated target among various semiconductor materials. Since the milestone created by Murray et al. in 1993 [6, 7, 8, 9, 10, 11, 12, 13, 14, 15], tremendous progresses have been made with regards to the size and shape control for CdSe NCs, and monodisperse products can be obtained with environmental-benign raw materials by fairly facile methods. Although the formed CdSe NCs can be stable in air at room temperature, but some key chemical ingredients utilized in the synthesis are air sensitive under high temperature, and the open-to-air synthesis of CdSe NCs by batch reaction usually leads to low quantum yield (QY) of PL . As a result, most of the reported bath reaction concerning the synthesis of CdSe NCs were conducted in inert atmosphere (such as Ar and N2).The difficulties associated with the oxygen-free operation lead to reduced flexibility and increased cost of the whole process, and the large-scale preparation can hardly be achieved without compromising the NC quality.
The recently introduced microfluidic methods can realize operation in closed systems, and the precise control of synthetic conditions offered by the strengthened heat and mass transfer makes the reproducible reaction achievable [17, 18, 19]. Furthermore, with several functional microdevices integrated in a miniature platform, multi-step reaction can be performed in a continuous manner [20, 21]. Recent studies have demonstrated that microfluidic reactors drastically outperform batch systems in the direct production of nanoparticles with short reaction time [22, 23, 24, 25, 26, 27, 28, 29, 30], and the open-to-air preparation of well dispersed CdSe NCs can be realized in a simple capillary microfluidic system . However, as a result of the surface defects formed during the fast growth in microenvironment, CdSe NCs prepared by microreaction exhibited generally low QY of PL, and this situation was exacerbated for the reactions performed in open air [16, 31]. Various amines are known as favorable coligand to obtain highly luminescent CdSe NCs [13, 32, 33, 34], among which octadecylamine and hexadecylamine showed impressive results to improve both the size distribution and emission efficiency for CdSe NCs. For primary amines, a long organic chain length is required to obtain a boiling point compatible with the high temperature needed for the synthesis of CdSe NCs. As a result, these ingredients are in solid state at room temperature, and their applications in microreaction would result in the clogging of microchannels. OLA was a favorable ligand for the synthesis of CdSe NCs via both batch reactions and microreactions [24, 35], and fairly high QY of PL (25–50%) was observed. But the reported methods were conducted in inertial environment, and high quality products can only be favored under long residence time, high temperature, and small microchannel (inner diameter <250 μm). Meanwhile, for CdSe NCs synthesized with OLA, the observed reaction kinetics varied from each other. The kinetics observed by Yen et al. followed the classical diffusion-controlled growth theory. While an unusually long-term fixation of particle size and size distribution has been reported by Zhong et al. . Furthermore, the qualitative study for the influence of OLA on the property of CdSe NCs was in scarcity, resulting in the experiential determination of OLA concentrations in different reports.
In this study, OLA was utilized as coligand to realize the open-to-air preparation of highly luminescent CdSe NCs via microreaction, and qualitative investigation of the influence of OLA on the PL property as well as size distribution was carried out to optimize the synthesis. The influence of OLA on the reaction kinetics was evidenced by the collected absorption spectra. The high quality of the obtained CdSe NCs has been confirmed by PL spectroscopy, powder X-ray diffraction (P-XRD) and transmission electron microscopy (TEM).
Cadmium oxide (CdO, SCR, 99.9%), selenium powder (Se, SCR, 99.5%), trioctylphosphine (TOP, Fluka, 90%), 1-octadecene (ODE, Fisher, 90%), oleic acid (OA, SCR, 90%), oleylamine (OLA, Fluka, ≥75%), analytic grade methanol, and chloroform (SCR) were used directly without further treatment.
Synthesis of CdSe NCs
The CdSe NCs were prepared using a modification of previously reported method . Typically, a Se stock solution was prepared by dissolving 39.50 mg of Se powder in TOP (1.00 mL) and OLA (1.50 mL). The obtained solution was further diluted with 0.50 mL ODE. Meanwhile, a suspension of 12.85 mg CdO, 0.12 mL OA, 1.50 mL OLA, and 1.38 mL ODE were heated 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. During the optimizing process for OLA concentration, various amounts of OLA were replaced with ODE, while maintaining the constant solution volume. The whole process was performed in open air.
UV–Vis. spectra were acquired using a Varian Cary 100 UV–VIS spectrometer, and PL spectra were recorded with Varian Cary Eclipse. QY of PL was obtained by comparing the integrated PL intensities of the NCs and the organic dye (Rhodamine 6G) . All measurements were performed on original QD samples without any purifying process.
To obtain samples for TEM and XRD characterization, the formed CdSe NCs were precipitated by adding methanol into the chloroform solution and were further isolated and purified by repeated centrifugation and decantation. HRTEM images were taken on a JEM-2100F microscope operated at 200 kV, and the sample was prepared by dipping an amorphous carbon–copper grid in a dilute chloroform dispersed NCs solution, then sample was left to evaporate at room temperature. XRD measurements were performed on a Rigaku D/max2550 operating with Cu Kα (λ = 0.154056 nm), samples were deposited as a thin layer on a Si wafer.
Results and Discussion
Obtained results of CdSe NCs prepared with OLA being added in different source (vol.OLA% = 37.5%,T = 280 °C,t = 10 s)
FWHM of PL (nm)
CdSe concentration (10−6 M)
In order to investigate the influence of OLA systematically, OLA was partly replaced by ODE, and the temporal evolution of UV–Vis. and PL spectra were recorded to reflect the influence of OLA amount on the mean particle size and size distribution of CdSe NCs. The kinetic data with regards to CdSe molar concentration and mean particle diameter were obtained by the methods reported by Yu and his colleagues . To establish a baseline for comparison of the different reaction conditions studied, initial experiment using OA as the sole ligand was also conducted.
For CdSe NCs synthesized with small amount of OLA below 37.5 vol.%, there was clearly an optimal time for FWHM of PL, a point after which the growth slowed down while the size distribution of the NCs widened. Increasing the volume percentage of OLA to 37.5% resulted in a long term (up to 5 min) fixing of FWHM, and the narrowest FWHM as 27 nm was obtained at a short residence time as 20 s, which was significantly shorter than the reported value in ref. . The observed phenomenon resulted primarily from the kinetic issues. For steady-state, diffusion-controlled growth of spherical particles, there exist a critical size (rcr), and the focus of size distribution occurs when all the NCs sizes (r) are larger than rcr. The high monomer concentration realized under elevated OLA amount is favorable to achieve large rcr [39, 40]. In this way, the focusing of size distribution took place in the early reaction stage, and the shortened reaction time for monodisperse CdSe NCs was achieved. For CdSe prepared with higher volume percentage of OLA over 37.5%, high quality samples (FWHM 31 nm) were obtained under quite short residence time as 3 s. While in the subsequent heating phase, the FWHM of PL continuously narrowed down, and the narrowest FWHM (27 nm) was obtained at 20 s. The further increase of residence time resulted in little variation of PL wavelength and FWHM of PL. During the synthesis of NCs, the ligands are dynamically bonded onto the surface of NCs. In this case, the ligand needs to be able to exchange on and off the growing NCs, so that regions of the NCs surface are transiently accessible for growth, yet entire crystals are, on average, monolayer-protected to block aggregation . In the early reaction stage, the chemical potential of the NCs was low, and the high monomer concentration in the precursor would promote the dynamical bonding of ligand, resulting in the growth of NCs. However, the prolonged growth led to significant reduction in monomer concentration and the decreased growth rate was observed. Furthermore, increase of the total OLA concentration in the solution should increase the number of ligands on the surface of each NC. This should also decrease the growth rate of the NCs. When the OLA was applied in large excess (more than 37.5 vol.% in this experiment), the increased coverage of ligand and the large amount of free amine in the precursor would hinder the “off” process of OLA. As a result, the new monomers were inaccessible for the NCs and the long-term fixing of size as well as size distribution NCs were thus made possible.
The PL efficiency of NCs prepared using vol.OLA >25.0% was maintained at a high level during the evolution of residence time. In our experiment, QYs of PL were continuously decreased with the proceeding of reaction, and the “bright point” (corresponding to the crossover of QY) observed in batch reactions was not detected , this may be due to the lack of slow annealing process for NCs in microenvironment. From Fig. 7a and b, it was clear that CdSe NCs prepared with 37.5 vol.% and 50 vol.% OLA showed little difference. For the economical concern, 37.5 vol.% OLA were applied for the subsequent experiments.
In conclusion, closed environment realized by microreactor was utilized to eliminate the requirement of inert atmosphere for the synthesis CdSe, and OLA was applied as a coligand to improve the size uniformity and PL efficiency of the products. The addition of OLA both in Se and Cd side was demonstrated advantageous to increase the reactivity of Cd monomer and promote the diffusion of the two precursors, which resulted in CdSe NCs with quite narrow FWHM of PL. The addition of OLA in the source solution resulted in significant improvement of both size distribution and QY of CdSe NCs, and high OLA concentration is favored to prepare highly luminescent CdSe NCs with small size as well as narrow size distribution. When the applied volume percentage of OLA exceeded 37.5%, long-term fixing of FWHM was observed, and high QY of PL was maintained over a long period. The obtained results facilitate an easy tuning of PL range by the variation of residence time. With 3 mL OLA in the source solution, a size series (2.3–3.6 nm) of highly luminescent CdSe NCs (QY 35–55%) with narrow FWHM of PL (26–31 nm) were obtained under varied residence time from 2 s to 160 s.
Authors appreciated the financial supports from the National Natural Science Fund of China (50772036) and the State Key Laboratory of Chemical Engineering at East China University of Science and Technology.
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