Combinatorial Synthesis of CdSe Nanoparticles Using Microreactors

Chem. C , 0, (),. DOI: 10.1021/jp911876s@proofing. Copyright © American Chemical Society. * To whom correspondence should be addressed. (H.M.) Tel.: ...
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J. Phys. Chem. C 2010, 114, 7527–7534

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Combinatorial Synthesis of CdSe Nanoparticles Using Microreactors Ayumi Toyota,† Hiroyuki Nakamura,*,† Haruka Ozono,‡ Kenichi Yamashita,† Masato Uehara,† and Hideaki Maeda*,†,‡,§ Micro- & Nano-space Chemistry Group, Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 807-1, Shuku, Tosu, Saga 841-0052, Japan, Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu UniVersity, 6-1 Kasuga-kouen, Kasuga, Fukuoka 816-8580, Japan, and CREST, Japan Science and Technology Agency, 4-1-8, Hon-cho, Kawaguchi 332-0012, Japan ReceiVed: May 19, 2009; ReVised Manuscript ReceiVed: February 16, 2010

Several microreactors combined with an online detector were used as a combinatorial synthesis system to optimize nanoparticle synthesis for rapid and flexible development of nanoparticles to meet various needs and applications. Three reaction parametersstemperature, reaction time, and reaction additive (dodecylamine) concentrationswere combined systematically to produce synthesis condition sets (five points each, total of 125 points). The photoluminescence (PL) wavelength, PL quantum yield (PLQY), PL full width at halfmaximum (PL fwhm), particle size, and product yield (PY) of the products were determined for each condition to obtain property data sets. The average time to complete all procedures to obtain one reaction condition per particle property data set was approximately 20 min. The reaction conditions were varied to provide a series of data sets meeting three specific objectives: (1) to seek condition sets producing superior properties, (2) to assess reaction condition effects on property data sets and elucidate their underlying mechanisms, and (3) to find reaction conditions meeting practical requirements for applications and achieve a balance of criteria. These objectives were met. High reproducibility verified the system reliability. Data set maps were used to determine the reaction conditions to produce high-QY particles (56%). These maps supported systematic assessment of the reaction condition effects on product properties. The data sets show agreement with formally reported findings related to QY dependence on particle size and CdSe deposition rate enhancement with amine concentration, thereby confirming the system reliability. Results also showed that high amine concentration suppressed the deposition rate. The maximum deposition rate of 5-10% indicated that these systems can be assessed quantitatively to achieve balanced conditions. Finally, by applying a weighting function to the data sets, a point of balance among properties was determined easily. This approach is effective for determination and selection of optimum conditions for practical applications. 1. Introduction Many methods for synthesis of metal and semiconductor nanoparticles have been reported.1 Their various applications, including fluorescence tags for biological molecules and optoelectronic devices, are promising. Generally, the physical and chemical properties of nanoparticles depend on structure (e.g., defects, composite structures, and crystal phases) and morphology (e.g., particle size and shape).1,2 Thermodynamic and kinetic control of nucleation, growth, and aggregation are important to control particle morphology and structure.1-3 Therefore, experimental parameters such as the reagent type and concentration, temperature and thermal and concentration history must be optimized to produce nanoparticles with targeted properties.4-6 Researchers continue to make great efforts to optimize important experimental parameters and thereby obtain improved properties, but practical applications often demand specific requirements to balance two or more properties. Several attempts have been made to find a relation between experimental parameters and * To whom correspondence should be addressed. (H.M.) Tel.: +81-94281-3676. Fax: +81-942-81-3657. E-mail: [email protected]. (H.N.) Tel.: +81-942-81-3650. Fax: +81-942-81-3657. E-mail: nakamura-hiroyuki@ aist.go.jp. † AIST. ‡ Kyushu University. § CREST.

properties to rationalize synthesis mechanisms further.4,5 Nevertheless, acquisition of sufficient data and determination of optimum parameters using conventional chemical experiments is time-consuming, even in cases for which basic procedures have been reported and adopted. Here, combinatorial synthesis has been developed to overcome such limitations. Synthesis using combinatorial systems has been mainly developed for biological and organic chemical applications. Such systems provide enormous benefits for synthesis, especially for applications in the fields of molecular biology, pharmaceuticals,8,9 and, recently, even material sciences.10,11 The applications using combinatorial system described above emphasize chemical equilibrium. To the best of our knowledge, the application of combinatorial systems for nanoparticle synthesis has not been reported, probably because of the difficulty in controlling the particle generation kinetics. Unlike the previously described applications, which emphasize equilibrium, nanoparticle synthesis requires control of kinetics as well. Recently, controlled synthesis using microfluidic systems has attracted attention in various fields such as organic and inorganic synthesis, biology, and medicine.12-14 Microfluidic systems enable rapid mass and thermal transfer and provide an ideal medium for nanoparticle synthesis, specifically with respect to kinetic control. A special feature of microfluidic system is its ability to control one parameter independently (e.g., reaction

10.1021/jp911876s  2010 American Chemical Society Published on Web 04/13/2010

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time, temperature, and concentration) in a precise and reproducible manner. As demonstrated by previous studies, higher reproducibility of kinetic control is achievable via microreactor synthesis.15-18 Furthermore, the development of micro total analysis systems (µ-TASs) enables mounting of small online analysis chips19 for optical, electrochemical, and magnetic analyses. Recently, Krishanadasa et al. reported application of a microreactor as an autonomous “black-box” system for controlled synthesis, where two reaction parameters were controlled precisely while monitoring photoluminescence (PL) spectra to maximize the CdSe nanocrystal PL intensity.7 Nanoparticles embedded in materials have various applications. For instance, semiconductor nanoparticles (e.g., CdSe) are useful as optical light sources, as fluorescence tags for biological study and diagnostics, as light-harvesting device materials, and as building blocks for nanostructured semiconductors. Such applications benefit from multiple properties of nanoparticles such as band gap energy, band edge energy, photoluminescence, and conductivity. Furthermore, specific examination of a single property such as photoluminescence may reveal that it can be used for several purposes and that applications can be assigned different priority levels. For example, application as tags in biological systems demands a narrow FWHM but such narrow FWHM is not necessarily required when used as optical light source. Such examples underscore that nanoparticles have several properties. They can be perceived in terms of applications depending on those properties. Moreover, each property is perceived differently by users. In fact, users choose and prioritize properties depending on the application. It is therefore natural that balance among properties be examined. A system offering a means of determining such balance among properties is important. Additionally, a pantoscopic assessment of the effects of parameters will help determine the trends of the effects of reaction conditions and elucidate the underlying mechanisms. These objectives require application of a practical approach based on microfluidic combinatorial synthesis. We therefore developed a simple prototype for a combinatorial synthesis system using multiple-microreactors intended for multifaceted and pantoscopic assessment of reaction conditions in relation to multiple properties, and designed for determination of balance among these properties. Numerous studies of CdSe nanoparticle synthesis have been reported.5-7,16-33 The present study of combinatorial synthesis system uses CdSe nanoparticles as a model to provide a suitable comparison with past reports, to verify the validity of the proposed microfluidic combinatorial approach, and to present some applied examples of the approach. Three reaction condition parametersstemperature, reaction time, and reaction additive (dodecylamine) concentrationswere combined systematically to produce synthesis condition sets (five points each, total of 125 points). Then the properties of the products, including photoluminescence (PL) wavelength, PLQY, PL full width at half-maximum (PL fwhm), particle size, and product yield (PY) were determined for each condition to obtain property data sets. The data sets were subsequently mapped to elucidate the relation between the reaction parameters and the nanoparticle properties and to demonstrate the three uses of the maps: (1) determining conditions producing the best data, (2) assessing the effects of reaction conditions and elucidating the synthesis mechanisms, and (3) determining specific reaction conditions meeting the practical requirements and criteria to select conditions to obtain the desired particle properties.

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Figure 1. Schematic representation of the combinatorial synthesis system for CdSe nanoparticles.

2. Experimental Section 2.1. Materials. Reagent grade Cd(CH3COO)2 · 2H2O (SigmaAldrich Co., Ltd.), oleic acid (Wako Pure Chemical Industries, Ltd.), 1-octadecene (ODE; Wako Pure Chemical Industries, Ltd.), Se powder (Soekawa Chemical Co., Ltd.), trioctylphosphine (TOP; Tokyo Chemical Industry Co., Ltd.), and dodecylamine (DDA; Sigma-Aldrich Corp.) were used. Oleic acid, ODE, and DDA were used after vacuum distillation in argon atmosphere; other reagents were used as purchased. 2.2. Preparation of Raw Material Solutions. Three raw material solutions (Cd, Se, and DDA solutions) were prepared under a dry nitrogen atmosphere. The Cd solution consisted of 0.480 g of Cd(CH3COO)2 · 2H2O dissolved in a solution mixture of 2.89 g of oleic acid and 76.6 g of ODE at 240 °C under Ar flow for 20-25 min. The solution was then degassed at 100 °C for 20-45 min. The Se solution consisted of 0.711 g of Se powder dissolved in 33.4 g of TOP, which was then diluted with 47.2 g of ODE. The molar composition ratio of Cd to Se was 1:5. The DDA solution was prepared from dried DDA diluted to 0, 2, 5, 10, and 20 wt % with ODE (0 wt % DDA solution is pure ODE). The DDA concentrations described herein refer to concentrations after mixing the three material solutions in the same volume. Each glass syringe was filled with the three material solutions. 2.3. Methods. Figure 1 presents a schematic of the experimental setup for CdSe nanoparticle formation using the combinatorial system. The system comprises four sections: a solution supply section equipped with three syringes on a syringe pump, a mixing section, a heated reaction section, and the spectroscopic measurement section. The cadmium and selenium sources, and the DDA solution were transported by syringe pumps set at a particular pumping rate (31.4 mL min-1 for each), mixed using a micromixer (a split-and-recombine type micromixer), then passed through a silica glass capillary tube (ID 200 µm, OD 360 µm) that was partly immersed in an oil bath that had been preheated to a specified reaction temperature.16-18 After the

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TABLE 1: Reaction Conditions for CdSe Nanoparticle Synthesis Cd concentration oleic acid concentration Se concentration TOP concentration DDA concentration linear velocity reaction temperature reaction time capillary length

mM mM mM mM wt % cm s-1 °C s cm

0

2

195 3 15

210 7 35

6 30 30 300 5 5 240 15 75

(const.) (const.) (const.) (const.) 10 (const.) 270 30 150

20 300 60 300

reaction, the product solution flowed through an online UV-vis spectrometer. It was collected into a sample tube for offline measurements of UV-vis and photoluminescence (PL). The combinatorial synthesis of CdSe nanocrystals was examined using a parallel operating system of five sets. The experimental parameters assessed in relation to the CdSe nanoparticle properties were reaction time, temperature, and amine concentration. Table 1 presents the reaction conditions that were used. These experiments were designed carefully to ensure accuracy of experimental parameters. Technical problems that might have arisen from unbalanced pumping performance were avoided. Control of the concentration by controlling the ratio of pumping rates of the three raw material solutions is possible. However, in this experimental setup, the three raw materials described above were placed on one multisyringe pump and transported simultaneously to free them from any fluctuation of the mixing ratio (i.e., reagent concentration) that might occur from minor pumping performance fluctuations. Moreover, for this experiment, a fixed total pumping rate (94.2 µL min-1 corresponding to a linear velocity of 5 cm s-1) was applied to avoid undesired variations in reaction kinetics during synthesis experiments. The flow rate also might affect the heating rate and mixing efficiency, and consequently alter the nucleation and growth kinetics. Because a uniform flow rate was used, the capillary length was adjusted from 15 to 300 cm so that the reaction solution’s residence time in the reactor heating section was controlled from 3 to 60 s. Additionally, the duration before heating after mixing was kept constant. The reaction temperature (195-300 °C) was controlled using an oil bath. Finally, approximately 1 mL of the product was collected for offline analysis at the capillary outlet for 10 min, during which time the temperature was kept stable. 2.4. Characterization of Products. For optical analyses, the collected solutions were diluted with toluene for measurement of the absorption spectra using a UV-vis spectrophotometer (UV-570; Jasco Corp., Japan), and for measurement of PL spectra using a spectrofluorometer (FP-6600; Jasco Corp., Japan). The spectra were measured 30 min after collection, and the optical density (OD) for UV-vis sample was adjusted to 0.1-0.3 at 365 nm by diluting the product solution with toluene. Overly high OD can cause underestimation of quantum yield; lower OD can cause cluster peak generation, which can increase errors. In the experiment, OD of the sample for PL spectra was further adjusted to 0.05 at 365 nm to suppress self-absorption. Moreover, the absorption spectra of CdSe nanoparticles were monitored online using a spectrometer (QE65000; Ocean Optics Inc., USA) attached to a glass tube (ID 1.02 mm, corresponding to the light path length) and connected to a capillary outlet. The reaction progress was monitored continuously according to the time dependence of online (in situ) or off-line (ex situ) spectra, as in other reported studies.7,19-23 It was possible to measure

Figure 2. Characterization of products using (a) UV-vis and (b) PL emission spectroscopy.

the spectra either in situ or ex situ with no intrinsic differences (a few ex situ spectra of 20 wt % DDA showed a small peak around 407 nm caused by cluster development34 when the collected sample was left for a long time (ca. 30 min) after collection, although no such peak was observed for in situ spectra). The average particle diameter was estimated using the empirical relation between particle size and absorption peak wavelength introduced by Yu et al.24,25 (Figure 2a). The PL emission spectra were measured at an excitation wavelength of 365 nm. The synthetic yield of CdSe was determined from the absorbance intensity and fwhm of the PL spectrum according to the method reported by Yu et al.24,25 (It should be noted that Protasenko26 made a correction (εCdSe ) 21145d2.0226) to the original study conducted by Yu et al.) Automatic computer-aided analyses of PL and absorption spectra were performed to obtain PL peak wavelengths, fwhm (used as a particle size distribution indicator), and peak area (used to determine the PLQY) (Figure 2b). To determine the PLQYs, sample absorption (ex ) 365 nm) and emissions were compared with that of rhodamine B. They were then corrected by the relation between absolute PLQY (absolute quantum PL determination system; Hamamatsu Photonics K.K., Shizuoka, Japan) and PLQYRB determined using rhodamine B (see Figure S6), described as

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PLQY ) PLQYRBXY Y ) (1.7316 × 10-7)X3 - (2.2561 × 10-4)X2 + .010178X - 15.706 where PLQY is the corrected PLQY by the above correction coefficient, and where PLQYRB stands for PLQY determined by rhodamine B, where Y is the correction coefficient and where X is the PL wavelength. 3. Results and Discussion 3.1. Assessment of Reproducibility of the Current Combinatorial Synthesis System. A combinatorial system runs multiple conditions of an experiment; upon use of experimental condition and results sets, it is possible to find optimum conditions that might fit to obtain the user’s aim or even discover new findings. Aside from rapidity in performing an experiment, suitable reliability and reproducibility of the condition sets and data sets are equally necessary for obtaining the user’s objective. Experimental reliability will be proven by the following discussion, which compares our current data with reported results. Therefore, we determine the reproducibility of the current combinatorial system. An experimental condition corresponding to 5 wt % DDA solution with a residence time of 15 s at 195-300 °C was conducted for six replicates to assess the reproducibility of the current reaction system. Raw material solutions for each experiment were prepared individually. The particle size was determined from the absorption peak wavelength. Figure 3 shows that the particle size is reproducible. Therefore, the PL peak wavelength, which depends on particle size, is reproducible. A comparison of six individual experiments reveals that the particle growth kinetics (Figure 3a) and the PL peak wavelengths (Figure 3b) show good reproducibility. The standard deviations obtained for six replicates for respective properties were reported as follows: 0.05 nm for particle diameter (Figure 3a), 7.3% for CdSe synthetic yield, 11% for QY, 1.3 nm for fwhm, and 4.3 nm for PL peak wavelength (Figure 3b). Reproducibility was much higher when we conducted similar experiments using identical raw material solutions (see, e.g., ref 18). Therefore, the deviation of these values was thought to have arisen because of small fluctuations (e.g., uncontrollable contamination such as water or oxygen) of raw material solutions. These results demonstrate the reproducibility of CdSe nanoparticle formation using a microreaction technique with the combinatorial synthesis system. 3.2. Effects of Reaction Conditions on CdSe Formation and Properties. 3.2.1. Effects of Reaction Temperature and Amine Concentration on Product Yields. The nucleation and growth kinetics of nanoparticles affect particle size, size distribution, morphology and other properties. In terms of application, the product yield (PY), QY, and PL fwhm of CdSe nanoparticles are important properties. Semiconductor nanoparticle properties reportedly depend on the average particle diameter,5,6 as determined by nucleation and growth kinetics, and determines the average band gap energy and consequently, the PL wavelength, as described in the experimental section. Therefore, CdSe PY, QY, and fwhm were evaluated based on the particle diameter. Figure 4 portrays the product yield dependence on particle diameters prepared under various reaction temperature (a) and

Figure 3. Reproducibility of (a) average particle diameter and (b) PL peak wavelength of the CdSe nanoparticles synthesized with 5 wt % DDA concentration at 15 s residence time at temperatures of 195-300 °C.

Figure 4. CdSe yield as a function of particle diameter under (a) various reaction temperatures and (b) DDA concentrations.

DDA concentration (b) conditions. The experimental errors of product yields reported in this method fall under the SD ((10-15%) reported by Yu et al.24 The resulting pattern presented in Figure 4a shows that higher temperatures have higher product yields of larger particles. The CdSe deposition rate and particle growth rate are higher at high temperatures; higher yield and particle size are obtainable in a limited time (60 s). However, Figure 4(b) shows that the product yield is

Synthesis of CdSe Nanoparticles more dependent on the amine concentration than on temperature. The product-yield dependence on particle diameter increased with lower amine concentration (2-5 wt %), but decreased with higher amine concentration (10-20 wt %). The slope of the curve approximately indicates the particle number relative to the amine concentration: the particle number increased with amine concentration from 0% to 5%, but decreased in a higher concentration range (10-20 wt %). A closer investigation provides additional information. As in Figure S2 (see the Supporting Information) shows, at different amine concentrations, the curve slope is almost independent of temperature when the amine concentration is 0-5 wt %, indicating almost equal particle concentrations. However, the curve slope is enhanced with increasing temperature when the amine concentration is high (10-20 wt %), which indicates a temperature-dependent nucleation rate. Several reasons that might explain these phenomena, including the possibility of nucleation rate suppression and enhancement by amine and temperature, are explained below. The results reveal the general increasing tendency of particle number with respect to the amine concentration in the following order: 10-20 wt % < 0 wt % < 2-5 wt % amine. Generally, amines play multiple roles in nanoparticle synthesis: as stabilizing reagents for monomers, as reaction activating and inhibiting reagents for monomers, as capping reagents for nanoparticles, and as reagents for improving the nanoparticle PL characteristics.5,6,28,29 The effects of increasing the concentration of the capping reagent (i.e., oleic acid) are reported as nucleation rate suppression, which gives fewer nuclei and a larger particle size of the resultant product.30-32 A lower nucleation rate is said to give a smaller nuclei number. Here, amine can also coordinate to the precursor Cd ion. Therefore, in our system, we consider that the amine concentration can affect the nucleation stage as well as the growth stage of nanoparticles. Results show that such stages are sensitive to amine concentration: lower amine concentration (2-5 wt %) enhances nucleation and yields higher particle concentrations (and smaller final particle size), whereas higher amine concentration (10-20 wt %) suppresses the nucleation rate and yields lower particle concentration (and larger final particle size). The appearance of temperature effects on particle numbersfound only for high amine concentrations (Figure S2)smight indicate that the representative time scale of the nucleation process lengthens at higher amine concentrations. Such suppression of the nucleation process is inferred to occur as follows. In the microreactor, the reaction solution temperature rises to the reaction temperature within a few hundred milliseconds.18 A higher temperature gives a higher heating rate. If the characteristic time scale for nucleation resembles that of heating, the heating rate variation is expected to affect the nucleation kinetics and give a larger number of nuclei for a higher nucleation rate (i.e., a higher heating rate), as might be apparent at high temperature at high amine concentrations in this case. However, if the nucleation time scale is much shorter than the heating time scale, then the heating rate cannot affect the nuclei number, as might be apparent for a low amine concentration case. Although these hypotheses must be confirmed in future experiments, these results suggest the complex commitment of amine to the kinetics of CdSe formation, which can be controlled according to the balance between nucleation enhancement and suppression. In this system, the reversion point for particle concentration from the effect of amine concentration was about 5-10 wt % amine.

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Figure 5. Quantum yield as a function of particle diameter under (a) various reaction temperatures and (b) DDA concentrations.

3.2.2. Effects of Reaction Temperature and Amine Concentration on QY. Figure 5 shows dependence of the photoluminescence QY on the diameter of particles prepared at (a) various reaction temperatures and (b) DDA concentrations. Considering the general results, it was noted that QY increased with the particle diameter and then gradually decreased. The QY probably decreases because of surface defects (e.g., unsaturated bonds, ion vacancies, or disorder) when the particle diameter becomes smaller. It decreases for larger particles because of quantum confinement reduction. These results are congruent with those reported for other studies.5,6 Figure 5(a) shows that higher temperature (270-300 °C) caused higher QY. High temperature is considered effective for atomic rearrangement, and consequently, for reduction of surface trapping states for photogenerated charges that can cause nonradiative transition, as reported by Donega´ et al.5 and Qu et al.6 Figure 5b shows that the QY was also markedly dependent on the amine concentration. The QYs of the CdSe nanoparticles prepared without amine were less than 15%, although the QYs were improved greatly by the introduction of amine into the reaction system. The highest QY value was given under that condition. In fact, amine behaves as a capping agent on the nanoparticle surface,5,6,28,29 and electron donation from the amine to the surface atom of the CdSe nanoparticle passivates surface defects. Generally, QY improves with increasing capping density,6,28 which depends on the amine concentration. However, as demonstrated by graphs of different amine concentrations shown in Figure S3 (see the Supporting Information) for low concentrations of 2-5% amine, 5% amine gives higher QY for all temperatures studied (195-300 °C). Higher amine concentrations (10-20 wt %) show lower QY at e240 °C. In contrast, at g270 °C, no significant change in QY was observed at various amine concentrations. This phenomenon might be attributable to the balance between the two effects of amines controlling nucleation and growth rate, and passivating the surface defects (the capping density and the electron donation ability of amine are important)sbecause QY depends on CdSe nanoparticle surface structures and morphology.5,6,26 Particle growth might retain defects if atomic rearrangement is slower

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Figure 6. Full width at half-maximum (fwhm) of photoluminescence spectra as a function of the particle diameter under (a) various reaction temperatures and (b) DDA concentrations.

than the growth rate at low temperatures. For that reason, low QY can be expected. Figure S4 shows that higher DDA concentration engenders a higher particle growth rate. 3.2.3. Effects of Reaction Temperature and Amine Concentration on fwhm. The fwhm of PL spectra usually correspond to the CdSe nanoparticle size distribution. Figure 6 shows the dependence of fwhm of the PL spectra on diameter of particles prepared at (a) various reaction temperatures and (b) DDA concentrations. Overall, larger particles have narrower PL fwhm. The results show that the particle size becomes uniform during particle growth. Generally, diffusion-limited growth reduces the fwhm; Ostwald ripening increases the fwhm,6,27,33 which explains the formation of a minimum fwhm. The fwhm also depends on amine concentration, as depicted in Figure 6b; 2-5 wt % amine reduces the fwhm of the PL spectra. According to the LaMer model for synthesis of monodispersed particles, a narrower particle size distribution is expected for a short nucleation step followed by diffusional growth of the nuclei.35 Reportedly, amine can enhance the nucleation rate of CdSe nanoparticles.29 Generally, a high nucleation rate yields a high nucleus concentration.30-32 Amine concentration of 2-5 wt % increases the concentration of CdSe nanoparticles, as portrayed in Figure 4b, suggesting a higher nucleation rate. Under an identical precipitation rate, a higher particle concentration also implies a slower particle growth rate (see Figure S4, Supporting Information). Accordingly, a lower amine concentration (2-5 wt %) gives narrower particle size distribution (Figure 6b). Conversely, a slower nucleation rate with higher amine concentration (10-20 wt %) engenders a lower particle concentration (Figure 4b) and a higher growth rate (Figure S4), and therefore broader fwhm (Figure 6b). Effects of temperature on fwhm were also observed for high amine concentration at 10-20 wt % (see Figure S5 in the Supporting Information). Figure S2 shows a lower particle concentration at high temperature with 10-20 wt % amine, which suggests the effect of nucleation rate on the particle size distribution. These results show that amine can enhance the nucleation rate and thereby shorten the nucleation step and decrease the growth rate, which results in narrowing of the size distribution over time. However, at high amine concentration, the suppression

Toyota et al. of nucleation occurs, resulting in a broader particle size distribution. The point of the amine effect reversion on the fwhm is identified as 5-10 wt % amine. Overall, the CdSe product yield (PY), QY, and fwhm depends on the amine concentration, reaction temperature, and time. The following patterns were inferred from the results: (1) Particle Size. Higher temperature, longer reaction time, and higher DDA concentration are effective for providing larger particle size. (2) Product Yield. Generally, larger particle size gives a higher product yield. Under the same particle diameter, DDA concentrations of 2-5 wt % are effective for higher product yield. At high DDA concentrations (10-20 wt %), high temperatures give a higher product yield of CdSe. (3) PL fwhm (Higher Monochromaticity). Fundamentally, under an unsaturated product yield, larger particle size gives narrower fwhm. Under the same particle size, a DDA concentration between 2-5 wt % is effective. At high DDA concentrations (10-20 wt %), high temperature gives a narrower fwhm. (4) PL Quantum Yield (PLQY). Particles of about 2.5 nm diameter gave higher PLQY. It is better to use higher DDA concentrations at higher temperatures for larger particles. Use of lower DDA concentrations is effective for smaller particles. Reaction parameters such as temperature, concentration and raw material and additives determine the reactivity of synthetic precursors. Thereby, they control the nucleation and growth of the nanocrystals.36,37 According to results reported by Pradhan et al., the complex effects of amine on the growth stage of CdSe nanocrystals are dependent on temperature. Increasing amine concentration in the growth stage increases the growth rate at temperatures above the boiling point of amine and decreases the growth rate with temperatures below the boiling point.29 However, our present results show that nucleation is affected along with the effect of amine on the growth processes. Nucleation, growth rate, and resultant particle concentration, which are controlled by amine concentration and reaction temperature, affect nanoparticle properties such as diameter, yield, QY, and fwhm. Furthermore, nanoparticle properties (such as band gap energy, PL wavelength, and QY) depend on structure (i.e., defects) and morphology (i.e., particle size), which are consequently controlled not only by amine concentration, but also by reaction parameters such as reaction time and temperature. Therefore, the combination and balance of reaction parameters is important. Complex effects such as amine concentration, reaction time, and temperature are involved in nanoparticle synthesis. A combinatorial synthesis system has been found to be useful for the assessment of effects of reaction conditions on nanoparticle properties. 3.3. Choice of Reaction Parameter Sets to Obtain Particles with Desired Properties. Various requirements exist for nanoparticle properties that are dependent on their various applications. However, as the CdSe model presented above shows, nanoparticles with various properties were obtained through various combinations of reaction time, temperature, and amine concentration. In particular, the balance of two or more properties must often be considered. The nanoparticle properties are dependent upon the reaction parameter sets. Therefore, we attempted to find the optimum combination of reaction parameters for the desired particles while considering a balance among these parameters.

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TABLE 2: Reaction Parameters and Properties of CdSe Nanoparticlesa weighting target of λp(nm) 480-510 blue-green 510-540 green 540-570 yellow-green 570-600 orange

reaction parameter

focus

R

β

γ

general QY fwhm PY general QY fwhm PY general QY fwhm PY general QY fwhm PY

0.33 0.8 0.1 0.1 0.33 0.8 0.1 0.1 0.33 0.8 0.1 0.1 0.33 0.8 0.1 0.1

0.33 0.1 0.8 0.1 0.33 0.1 0.8 0.1 0.33 0.1 0.8 0.1 0.33 0.1 0.8 0.1

0.33 0.1 0.1 0.8 0.33 0.1 0.1 0.8 0.33 0.1 0.1 0.8 0.33 0.1 0.1 0.8

λp (nm) d (nm) QY (%) fwhm (nm) product yield (%) temp. (°C) time (s) [DDA] (wt %) 507 500 496 507 539 539 539 539 557 544 557 557 594 574 592 603

2.26 2.22 2.19 2.26 2.67 2.67 2.67 2.67 2.99 2.76 2.99 2.99 3.97 3.38 3.94 4.30

32.6 39.3 29.6 32.6 50.1 50.1 50.1 42.4 47.4 54.1 47.4 47.4 38.9 51.8 40.5 37.3

42.1 44.4 39.9 42.1 32.6 32.6 32.6 33.8 33.1 33.3 33.1 33.1 32.5 33.5 30.8 34.0

18.4 6.27 12.8 18.4 44.8 44.8 44.8 45.3 70.3 42.4 70.3 70.3 81.5 55.8 71.2 83.5

270 240 240 270 270 270 270 240 270 300 270 270 300 270 300 300

3 7 7 3 15 15 15 30 30 7 30 30 60 60 60 60

5 10 5 5 5 5 5 5 5 5 5 5 5 10 10 20

a λp, PL peak wavelength; d, particle diameter determined by UV peak wavelength; QY, photoluminescence quantum yield; fwhm, full width at half maximum; PY, product yield of CdSe nanoparticles; R, β, γ, weighting coefficients for controlling, respectively, the relative importance of QY, fwhm, and product yield.

The following weighting function expression was used:

Y ) R{(y1 - y1′)/a}2 + β{(y2 - y2′)/b}2 + γ{(y3 - y3′)/c}2

(1)

In that equation, y1, y2, and y3 respectively signify the experimental values of QY, fwhm, and product yield (PY); y1′, y2′, and y3′ respectively denote the target values of QY, fwhm, and PY. In this case, the best values are used: a maximum QY of 85.6, a minimum fwhm of 30.8, and a maximum PY of 83.5. Furthermore, a, b, and c are the constants that adjust the gap separating the experimental and target values (yn - yn′; n ) 1-3) among the three parameters to equalize the average value of {(y1 - y1′)/a}2, {(y2 - y2′)/b}2, and {(y3 - y3′)/c}2. In addition, R, β, and γ are the weighting coefficients used to control the relative importance of QY, fwhm, and product yield (PY). Herein, R + β + γ is equivalent to 1. Table 2 presents examples of the selected property sets (PL wavelength (λp), particle diameter (d), quantum yield (QY), fwhm, and product yield (PY)), and the corresponding experimental condition sets (temperature, time, and DDA concentration). Figure 7 shows the location of selected properties in the “property map” for QY (Figure 7a), fwhm (Figure 7b), and PY (Figure 7c). The sets of weighting functions used for the property sets selection are also presented in Table 2. They were adjusted to choose four directed balance of properties: QY intended (R ) 0.8, β ) 0.1, γ ) 0.1; shown as Q in Figure 7), fwhm intended (R ) 0.1, β ) 0.8, γ ) 0.1; shown as F in Figure 7), PY intended (R ) 0.1, β ) 0.1, γ ) 0.8; shown as Y in Figure 7), and comprehensive (R ) β ) γ ) 0.33; shown as G in Figure 7). Figure 7 shows that the fluorescence wavelengths represented by different colors were categorized at 480-510, 510-540, 540-570, and 570-610 nm (which correspond respectively to blue-green, green, yellow-green, and orange color of fluorescence). In each category, the property sets of directed balance were selected. Overall, the “property map” was useful for assessing the location of selected properties among all sets of obtained data through combinatorial experiments. In Figure 7, for quantum yield, the selected property data sets occupy locations indicating high quantum yield. Simultaneously, the

Figure 7. Summary of the plots selected using four weighting coefficient sets at a given PL wavelength region for (a) QY, (b) fwhm, and (c) product yield. Symbols G, Q, F, and Y represent different weighting coefficient sets: R ) β ) γ ) 0.33 (G), R ) 0.8, β ) γ ) 0.1 (Q), R ) γ ) 0.1, β ) 0.8 (F), and R ) β ) 0.1, γ ) 0.8 (Y). See details in Table 2 and the text.

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location of these property data sets in other property maps, such as fwhm and CdSe yield are acceptable. That fact indicates that the proper choice of weighting function sets can produce the proper balance of properties. After the selection of targeted property data sets, as presented in Table 2, it is possible to obtain the reaction condition sets for each directed property data set. Using them, it would be possible to obtain reaction conditions of nanoparticles having virtually any desired balance of properties. This map concept, presented clearly in Table 2 and Figure 7, is useful for obtaining the desired data sets for a particular application. 4. Conclusions A combinatorial synthesis system was designed for CdSe nanoparticles using parallel operation of microreactors. A multifaceted and pantoscopic assessment of reaction parameters was made for the control of thermodynamics and kinetics based upon properties such as diameter, product yield (PY), size distribution, and photoluminescence QY of CdSe nanoparticles. Overall, the results showed good agreement with those reported from previous studies. The effect of amine concentration on the nucleation stage was confirmed for the first time, and the point of reversion for amine effects was identified as 5-10 wt % amine. This investigation demonstrated that a combinatorial synthesis for nanoparticles is particularly useful for obtaining higher reproducibility of kinetic control. Two important aspects were discussed. The complex effect of amine concentration from the viewpoint of kinetics effects, which involves the particle concentration, nucleation, and growth rate, was described. In addition, the capping effect relates to surface defect passivation. According to the results, a balance of the reaction parameters is important to control particle properties. Second, a multifaceted and pantoscopic assessment method was presented using ‘maps’ showing optimum sets of reaction parameters to produce desired particles with multiple properties. Based on these results, high-performance CdSe nanoparticles were obtained, as demonstrated in particular by the high QY obtained (>50%). Finally, by application of a weighting function to the data sets, the point of balance among properties can be determined easily. This is expected to be effective for determining and selecting the optimum conditions for practical applications. The run time for one synthesis set (including procedures such as preparation of raw material solutions, system setup, reaction, measurement of products, data analysis, and cleaning of apparatus) was approximately 20 min. The total number of syntheses was 228. An efficient combinatorial synthesis was achieved using this parallel operation of microreactors. A new strategy for nanoparticle development was demonstrated. The broad use of such a combinatorial nanoparticle synthesis system can provide important benefits, not only for rapid manipulation of reaction systems, but also for the development of nanoparticle science and technology. Acknowledgment. Part of this study was supported by Japan Science and Technology Agency (JST), CREST. Supporting Information Available: Details showing the dependence of particle properties (PY, QY, and fwhm) on

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