Shape Controlled Synthesis of CdSe Nanocrystals via a Programmed

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Article

Shape Controlled Synthesis of CdSe Nanocrystals via a Programmed Microfluidic Process Junmei Wang, Haofei Zhao, Yuchen Zhu, and Yujun Song J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10901 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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The Journal of Physical Chemistry

Shape Controlled Synthesis of CdSe Nanocrystals via a Programmed Microfluidic Process Junmei Wang, Haofei Zhao, Yuchen Zhu, Yujun Song* Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, Center for Modern Physics Technology and Applied Physics Department, School of Mathematics and Physics, University of Science & Technology Beijing, Beijing 100083, China; * Corresponding author: Y.S., [email protected]

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Abstract: Semiconductor nanocrystals exhibit excellent electronic and optical properties, due to their shape, size and defect dependent quantum confinement effect. Controlling the formation kinetics for tunable size, shape and defect has become more and more important for desired optoelectronic properties, but very challenging. In our work, we extend our previous hybrid microfluidic-batch process in the size, shape and crystallinity controlled synthesis of semiconductor nanocrystals (NCs) using CdSe nanocrystals as model materials, by precisely regulating reaction temperatures from 150 °C to 300 °C at a flow rate of 0.6 mL/min. Highly crystalline CdSe nanocrystals with controlled shapes (vague angular, tripodia, near-spherical and sphere) and sizes (from 2.57 nm to 4.31 nm) were successfully synthesized, exhibiting unique ultra-violet absorbance and photoluminescence.

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1. INTRODUCTION

Semiconductor nanocrystals (or quantum dot) have been widely used in many fields including biomedicine, biosensors, optoelectronic devices and solar cells due to their unique size, shape and crystallinity dependent physical and chemical properties.1-8 9-12

For example, the narrow emission wavelength range and the wide excitation

wavelength range can realize many biomarker detections at the same time and promoting the application in biomedicine.7,8 The electrochemiluminescence (ECL) properties of semiconductor nanocrystals (NCs) made them possible to be used as ultrasensitive biosensors.13-15 The energy conversion efficiency of quantum dot solar cells designed based on the significant quantum confinement effects and discrete spectral characteristics of quantum dots can be extraordinarily increased.16,17 More unique properties and the associated applications can be explored if the NCs of different shapes and sizes can be synthesized uniformly, particularly those of ultra-small sizes (no more than 3 nm).18-21 However, it is still difficult to manipulate the synthesis process to obtain NCs with desired shapes, ultra-small sizes and narrow dispersion. So far, many methods, including liquid–solid–solution approaches22, thermal decomposition methods23, seed-mediated processes24, one-pot approaches25 and many other wet chemistry processes12,26, have been invented to synthesize semiconductor NCs of different-shapes and sizes.3,18,27 However they are lack of good control of the reaction parameters for desired size, shape and crystallinity and also face difficulties to realize large scale potential.28 Therefore, new methods are still desired for the controllable large-scale synthesis of semiconductor NCs. 3



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Microfluidic approaches have been widely used in the nanoparticles synthesis due to their precise control of thermodynamic and kinetic parameters during the formation process.29-32 The advantages of high mixing efficiency, uniform mixing temperature, lower reagent consumption and high yield over other synthesis methods make them more popular in nanocrystal synthesis.33-37 Microtubing based microfluidic processes with micrometer diameters and the spatiotemporally splitting nanocrystal formation stages along the tubing made them more convenient to synthesis NCs with different sizes, shapes and crystallinities and thus unique optoelectronic properties. In this article, a simple programmed hybrid microfluidic-batch process was used for the synthesis of CdSe NCs with different shapes, ultra-small sizes, good crystallinity and excellent narrow dispersion through manipulation of reaction temperatures. The crystal structure of the obtained CdSe NCs was mainly zinc blende phase mixed with a little wurtzite branches which was different from the wurtzite phase NCs synthesized

by

most

other

methods.19,27

The peak position of

the photoluminescence and the UV-VIS absorbance spectra of NCs synthesized at a higher reaction temperature shifted to a lower energy. In addition, this process has a yield of 0.68 g/h of CdSe NCs powder by only one channel at an injection rate of 0.6 ml/min, preserving large-scale synthesis potential.

2. EXPERIMENTAL SECTION

2.1 Materials Cadmium oxide (CdO, AR, 99%,) and oleic acid (OA, AR) were purchased from Xilong Chemical Co., Ltd. Selenium powder (Se, ≥ 99.99%) was 4


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purchased from Beijing Eastern Chemical Works. 1-octadecene (ODE, 90%, Alfa) and Tri-n-octylphosphine (TOP, 90%, Alfa) were purchased from Alfa Aesar. Toluene (C6H5CH3, AR, ≥ 99.5%) and ethanol (CH3CH2OH, AR, ≥ 99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used without further purification. 2.2 Synthesis of CdSe Nanocrystals The synthesis process was carried out in a 208 cm long micro-channel which was made up of 15 cm polytetrafluoroethylene (PTFE) tube, 153 cm stainless steel tube and another 40 cm PTFE tube. Their inner diameter is 0.75 mm. The first and the third section with PTFE tubing was connected with the syringe and the collector flexibly and the stainless steel (SS) tubing was used as the second section for its good thermal conductivity and high temperature stability. A syringe pump was used to control the injection rate and a tube furnace was used to heat the reaction solution in the SS tubing to a required temperature (150 °C - 300 °C). The residence time of the reaction solution in the heating zone was determined by the flow rate, the micro-channel diameter and length. The detailed setup of the microfluidic device was shown in Scheme 1. The Cd precursor solution was made by heating cadmium oxide (0.54 g, 4 mmol), oleic acid (11 ml, 35 mmol) and 1-octadecene (9 ml, 28 mmol) to 180 °C in a quartz cuvette under nitrogen protection with continuous magneton stirring. An hour later at this temperature, pale yellow mixture can be obtained. Then the oil bath was turned off and the solution in the oil was cooled for another 30 minutes to obtain a solution of about 120 °C. Then 20 ml Cd oleate solution was prepared. Selenium powder 5



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(0.159 g, 2 mmol), TOP (4.5 ml, 12 mmol) and 1-octadecene (14.5 ml, 45 mmol) were mixed with magneton stirring under nitrogen replenishing in a beaker in room temperature to get 20 ml Se-TOP solution. Then the Se precursor solution was inhaled into a syringe and injected into the Cd oleate solution uniformly and rapidly. After 10 minutes’ stirring under nitrogen, the CdSe precursor solution was prepared. The CdSe precursor solution was sucked into a 50 ml syringe fixed in the platform of the syringe pump. Then the precursor solution was delivered into a microfluidic channel with an injection rate of 0.6 ml/min. The reaction was performed at 150 °C, 200 °C, 250 °C and 300 °C. The final solution was collected and precipitated with anhydrous ethanol. Then the centrifugation process was performed at a speed of 13000 rpm for 10 min. The top supernatant was decanted. The precipitated NCs were re-dispersed into toluene again. The CdSe NCs were precipitated again by centrifugation at a speed of 13000 rpm for 10 min after addition of a little bit of anhydrous ethanol. The same washing process was repeated twice and the desired nanocrystal powder was obtained. The product was re-dispersed into toluene and stored for future use. 2.3 Structure and Optical Property Characterization. The size, shape and chemical composition of CdSe NCs were characterized with transmission electron microscopy (TEM; JOEL 2100F, 200KV) coupled with energy dispersive X-ray spectroscopy (EDX). The X-ray diffraction (XRD) data was collected on a D/max2500 PC diffractometer (Cu Kα radiation, λ= 1.54056 Å, Rigaku, Japan) at 40 KV and 30 mA to analyze their crystal structure. For XRD characteration, the 6


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obtained NCs dispersed in toluene were dropped on the glass sheet. The scan rate was set as 5° min-1. Room temperature UV-VIS absorption spectra were collected by a PerkinElmer Lambda 950 UV-VIS-NIR Spectrometer and the photoluminescence spectra were tested by an FLS980 Edinburch instrument that contains a xenon lamp. The excitation wavelength was set at 360 nm.

3. RESULTS AND DISCUSSION

CdSe nanocrystals were synthesized by a microfluidic approach. The Cd and Se precursor solutions were first prepared for the preparation of CdSe precursor solution.28,38 The CdSe precursor solution was heated by a tube furnace whose temperatures were controllable. Since the reaction temperature is one of the most important thermodynamic parameters in the formation of semiconductor NCs,31 CdSe NCs synthesized at reaction temperature dependent size, shape and crystallinity were mainly investigated in this article. A wide viewed TEM images of CdSe NCs synthesized at reaction temperatures of 150 °C, 200 °C, 250 °C and 300 °C were shown in Figure 1(i). All NCs present nearly narrow dispersion with upper-limit size median of 4.31 nm. Clearly, reaction temperatures have great effects on their shapes or morphologies. The three diffraction rings in the selected area electron diffraction (SAED) pattern (insert in Figure 1(i)) have lattice spaces of 3.51, 2.15 and 1.83 Å that can be indexed as the (111), (220), (311) reflections of cubic CdSe. As shown in Figure 1(ii), the morphologies changed from vague angular shapes (lots of triangles) at 150 °C (Figure 1a(ii)) to tripodia shapes at 200 °C (Figure 1b(ii)), to near-spherical 7



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at 250 °C (Figure 1c(ii)) and to sphere at 300 °C (Figure 1d(ii)). Their lattice fringes were shown clearly through the high-resolution transmission electron microscopy (HRTEM) images, suggesting that the crystallinity could be improved at elevated temperatures. The crystal lattices of the HRTEM image insert in Figure 1a (ii) can be indexed as the (111) planes (0.350 nm) of zinc blend structure CdSe for nanocrystals synthesized at 150 °C. The HRTEM images insert in Figure 1b (ii) reveals different crystal lattices of the core and different branches, which represent the (111) plane of fcc CdSe (0.348 nm and 0.350 nm) and the (101) plane of hcp CdSe (0.326 nm and 0.328 nm) for the nanocrystals synthesized at 200 °C. The HRTEM images insert in Figure 1c (ii) indicates the formation of the (111) plane of the fcc CdSe (0.349 nm and 0.357 nm) synthesized at 250 °C. While the HRTEM images insert in Figure 1d (ii) indicates that the existence of the (111) plane of fcc CdSe (0.327 nm) synthesized at 300 °C. From Figure 1(iii), it can also be drawn that the average size can be tuned from about 2.57 ± 0.48 nm (150 °C) to 4.26 ± 0.62 nm (250 °C), 4.31±0.76 nm (300 °C) and the branch length of the tripodia CdSe NCs synthesized at 200 °C is about 3.58 ± 0.58 nm by the statistics analysis of at least one hundred NCs for each sample. To understand the formation mechanism of temperature dependent morphologies and crystallinities, the crystal structure and the reaction environment should be considered. All the morphologies during the NC formation tend to realize the minimum surface energy of NCs.39,40 Surface energies of different planes in zinc blende phase CdSe NCs and the adsorption energies of capping ligands (oleic acid) on 8


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different crystal planes have been calculated by Liu et al. using (100) and (111) planes as moderns, and they proved that the high index face has a higher surface energy and a higher adsorption energy of capping ligands.39 At 150 °C, the desorption of the surfactant is difficult especially from the high energy planes (e.g., (220) or (311) planes for face-centered-cubic phase CdSe) that leads to the relative different grow rates between different planes.39 But the whole grow rate is too slow at low temperature and the formed branches grown from different planes are not so distinct, leading to the vague angular shapes. As the temperature is increased to 200 °C, which is still not enough high to cause high desorption of surfactant from all planes, the surfactant can be released from the low energy planes (e.g., (100) plane) more easily than the high energy planes (e.g., (111) plane).38 Thereby, the crystal planes (e.g., (100) without so much surfactant limitation can grow faster than those at a lower temperature (i.e., 150

°C

), and thus the tripodia structure with some phase changed

branches will be formed.18,41 With the temperature continuously increasing to 250 °C, the surfactant effects begin to decrease and the structure effects of the high energy planes grow faster than low energy planes start to take effect and the near-sphere crystals formed.40 When the temperature increased to 300 °C, the energy of the whole system was improved. The growth probability is basically same for each plane. The points, edges and angles couldn’t exist stably at such a high energy system and thus the sphere shape formed. The XRD patterns of CdSe synthesized at different temperatures shown in Figure 2 present three peaks of 25.4o, 42.0o and 49.7o that correspond to (111), (220), (311) 9



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planes,

respectively,

confirming

that

they

are

face-centered-cubic

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phase

(PDF#19-0191) CdSe NCs. The XRD patterns are consistent with the electron diffraction patterns. The broad peaks demonstrate the formation of CdSe crystals with small sizes. The trend of the main diffraction peak FWHM becoming narrower from 5.35o to 3.94o, to 3.53o and then to 3.35o with the temperature increasing indicates that the sizes become larger which is consistent with the TEM image observation. The average grain sizes calculated from the XRD spectra with Gaussian fitting and Scherrer equation are 1.61 ± 0.10 nm (150 °C), 1.97 ± 0.09 nm (200 °C), 2.20± 0.08 nm (250 °C) and 2.45± 0.04 nm (300 °C) as shown in Table 1. The size difference between the results of XRD and the statistics from TEM images for the CdSe NCs synthesized at 150 °C and 200 °C come from the effects of angular and tripodia shapes that are composed of some small grains. While the decreased sizes calculated from XRD spectra for the CdSe NCs synthesized at 250 °C and 300 °C mainly due to the existence of twin crystals and stacking faults which can be seen from the HRTEM images.18 The additional peaks at 23.5o, 30.2o, 36.5o and 43.7o for the NCs synthesized at 150 °C come from the (310), (031), (-321) and (025) planes of monoclinic phase Se (PDF#24-0714; PDF#24-1202) possibly due to the incomplete reaction at such a low reaction temperature.27,42 During the reaction, the solution color changed with the reaction temperature from a pale yellow at the inlets to an orange-yellow at 150 °C (Figure 3a(i)), to bright red at 200 °C (Figure 3a(ii)), to wine red at 250 °C (Figure 3a(iii)) and to brownish red at 300 °C (Figure 3a(iv)) at the outlets of microchannels. The solution color changes as 10


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collected at different reaction temperatures further confirm the formation of CdSe nanocrystals of different sizes, shapes or crystallinities since the solution colors of semiconductor NCs are size, shape and crystallinity sensitive, which is consistent with the observation of TEM. Figure 3b depicts the normalized UV-VIS absorption spectra at room temperature. With the reaction temperature increasing, the absorption shoulders were red-shifted. As a kind of direct band-gap semiconductors, the bandgap can be calculated as 2.32 eV at 150 °C, 2.14 eV at 200 °C, 1.99 eV at 250 °C and 1.94 𝐴

eV at 300 °C (Table 2) through the Tauc method of α = ℏ𝜔 (ℏ𝜔 − 𝐸ℊ )1⁄2 based on the relationship between the absorption edge and the band. According to TEM and XRD analysis, these CdSe NCs are all smaller than the exciton Bohr radius of CdSe semiconductors (5.6 nm).43 This will cause the quantum confined effect and make the lowest energy of exciton move to a higher direction (blue-shift) when comparing with the bulk materials. In theory, semiconductor valence bond hybridization of the s and p state (sp3 hybridization) will appear as the atoms bond to crystals and lead to the changes of band gap as shown in scheme 2. The band gap changes with size changing at different temperatures can also be deduced from the effective mass approximation theory equation of 𝐸𝑔 = 𝐸𝑔,𝑏𝑢𝑙𝑘 +

ℏ2 𝜋 2 1 ( ) (𝑚∗ 2 𝑅̅ 𝑒

1

1.8𝑒 2

+ 𝑚∗ ) − 4𝜋𝜀 ℎ

̅ 𝑟 𝜀0 𝑅

+ 𝑠𝑚𝑎𝑙𝑙𝑒𝑟 𝑡𝑒𝑟𝑚𝑠.43

The material parameters44,45 used in our calculation and the calculated results are shown in Table 3. From the results, we can see that as the sizes (𝑅̅ ) become bigger at a higher temperature, the band gaps become narrower, which is different from other kinds of materials. However, the results calculated based on the TEM sizes are smaller than the results calculated by UV-VIS absorption spectra. This is mainly due 11



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to that the absorption spectra are tested in solution and the CdSe nanocrystals are wrapped with one or more layers of oleic acid molecules in the solution. The calculation assuming that nanocrystals are wrapped with one layer of oleic acid molecules are also shown in Table 3, which is more close to the results calculated by the UV-VIS absorption spectra. The

solution

colors

under

UV exposure

in

Figure

4a

show

distinct

photoluminescence (PL). The solution color changed from orange-yellow to bright green PL (Figure 4a(i)) for NCs synthesized at 150 °C, from bright red to lemon yellow PL (Figure 4a(ii)) for NCs synthesized at 200 °C, from wine red to bright red PL (Figure 4a(iii)) for NCs synthesized at 250 °C and from brownish red to red PL(Figure 4a(iv)) for CdSe NCs synthesized at 300 °C under UV exposure. Their emission peaks of CdSe NC solutions (Figure 4b) changed from 533 nm to 585 nm, to 642 nm, and then to 653 nm as the temperature changed from 150 °C to 200 °C, to 250 °C and then to 300 °C. But they are all blue-shifted as comparing with the bulk materials due to the strong quantum confinement effects at nanoscale.46,47 The weak and wide emission peak at 620 nm of 150 °C is mainly due to the surface-defect luminescence for the low temperature or the precursor phase coexisting with the nanocrystalline phase soon after the reaction.47 With the temperature increase from 150 °C to 200

°C

, to 250

°C

and then to 300 °C, the full width at half maximum

(FWHM) changed from 35 nm to 33 nm, to 37 nm and then to 41 nm, respectively, according to the Gaussian fitting. Even the widest FWHM (41 nm) is still narrower than those of inorganic phosphors (FWHM = 50~100 nm), making QDs outstanding 12


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sources of nearly pure color emission.46 Besides, the narrow FWHM demonstrates the narrow dispersion and crystallinity of CdSe NCs synthesized by the method in this article.48

CONCLUSIONS

In conclusion, a simple microfluidic process is successfully used for the synthesis of CdSe NCs with controlled size, shape and crystallinity. Face-centered cubic phase CdSe NCs of small sizes and vague angular, tripodia, near-spherical and sphere shapes with good crystallinity and uniform dispersion were synthesized. Photoluminescence and UV-VIS absorbance characterization indicate that they show distinct

size,

shape

and

crystallinity

dependent

optical

properties.

The

photoluminescence from bright green to red can be obtained by simply changing the reaction temperatures.

Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (FRF-BR-15-027A & FRF-BR-14-001B) and NSFC (Grant No. 51371018 & 50971010).

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Shape-Controlled Wurtzite CdSe Nanocrystals: Platelets, Cubes, and Rods. J. Am. Chem. Soc. 2013, 135, 6669-6676. (25) Qu, L. H.; Peng, Z. A.; Peng, X. G. Alternative routes toward high quality CdSe nanocrystals. Nano Lett. 2001, 1, 333-337. (26) Bawendi, C. B. M. D. J. N. M. G. Synthesis and Characterization of Nearly Monodisperse CdE(E = S, Se, Te) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706-8715. (27) Rice, K. P.; Saunders, A. E.; Stoykovich, M. P. Seed-mediated growth of shape-controlled wurtzite CdSe nanocrystals: platelets, cubes, and rods. J. Am. Chem. Soc. 2013, 135, 6669-6676. (28) Nightingale, A. M.; Bannock, J. H.; Krishnadasan, S. H.; O'Mahony, F. T. F.; Haque, S. A.; Sloan, J.; Drury, C.; McIntyre, R.; deMello, J. C. Large-scale synthesis of nanocrystals in a multichannel droplet reactor. J. Mater. Chem. A 2013, 1, 4067. (29) Wang, J. M.; Zhao, K.; Shen, X. M.; Zhang, W. W.; Ji, S. X.; Song, Y. J.; Zhang, X. D.; Rong, R.; Wang, X. Y. Microfluidic synthesis of ultra-small magnetic nanohybrids for enhanced magnetic resonance imaging. J. Mater. Chem. C 2015, 3, 12418-12429. (30) Wang, R. M.; Yang, W. T.; Song, Y. J.; Shen, X. M.; Wang, J. M.; Zhong, X. D.; Li, S. A.; Song, Y. J. A General Strategy for Nanohybrids Synthesis via Coupled Competitive Reactions Controlled in a Hybrid Process. Scientific Reports 2015, 5. (31) Li, S.; Zhong, X. D.; Song, Y. J.; Shen, X. M.; Sun, J. G.; Song, Y. J.; Wang, R. M.; Zhu, M.; Zhong, H. Z.; Zheng, A. G. Controlled hybridization of Sn-SnO2 17



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nanoparticles via simple-programmed microfluidic processes for tunable ultraviolet and blue emissions. J. Mater. Chem. C 2014, 2, 7687-7694. (32) Pedro, S. G.; Martinez-Cisneros, C. S.; Puyol, M.; Alonso-Chamarro, J. Microreactor with integrated temperature control for the synthesis of CdSe nanocrystals. Lab Chip 2012, 12, 1979-1986. (33) Nightingale, A. M.; de Mello, J. C. Microscale synthesis of quantum dots. J. Mater. Chem. 2010, 20, 8454. (34) Harrell, S. M.; McBride, J. R.; Rosenthal, S. J. Synthesis of Ultrasmall and Magic-Sized CdSe Nanocrystals. Chem. Mater. 2013, 25, 1199-1210. (35) Zhao, Y.; Shum, H. C.; Chen, H.; Adams, L. L.; Gu, Z.; Weitz, D. A. Microfluidic generation of multifunctional quantum dot barcode particles. J. Am. Chem. Soc. 2011, 133, 8790-8793. (36) Yen, B. K.; Gunther, A.; Schmidt, M. A.; Jensen, K. F.; Bawendi, M. G. A microfabricated gas-liquid segmented flow reactor for high-temperature synthesis: the case of CdSe quantum dots. Angew. Chem. 2005, 44, 5447-5451. (37) Chan, E. M.; Alivisatos, A. P.; Mathies, R. A. High-temperature microfluidic synthesis of CdSe nanocrystals in nanoliter droplets. J. Am. Chem. Soc. 2005, 127, 13854-13861. (38) Li, Z.; Yao, W.; Kong, L.; Zhao, Y.; Li, L. General Method for the Synthesis of Ultrastable Core/Shell Quantum Dots by Aluminum Doping. J. Am. Chem. Soc. 2015, 137, 12430-12433. (39) Liu, L. P.; Zhuang, Z. B.; Xie, T.; Wang, Y. G.; Li, J.; Peng, Q.; Li, Y. D. 18


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Y. Continuous in situ synthesis of ZnSe/ZnS core/shell quantum dots in a microfluidic reaction system and its application for light-emitting diodes. Small 2012, 8, 3257-3262. (48) Tian, Z.-H.; Xu, J.-H.; Wang, Y.-J.; Luo, G.-S. Microfluidic synthesis of monodispersed CdSe quantum dots nanocrystals by using mixed fatty amines as ligands. Chemical Engineering Journal 2016, 285, 20-26.

Table 1. Average Sizes of the CdSe Nanocrystals Calculated from their XRD Spectra with Gaussian Fitting and Scherrer Equation Sizes Average grain Sizes by TEM 2θ FWHM CdSe nanocrystals from sizes from XRD (degree) (degree) (nm) XRD (nm) 25.4 5.35 1.51 Synthesized at 150 °C 1.61 ± 0.10 2.57 ± 0.48 (nm) 42.0 5.20 1.62 49.7 5.06 1.71 25.4 3.94 2.04 Synthesized at 200 °C 1.97 ± 0.09 3.58 ± 0.58 42.0 4.5 1.87 49.7 4.32 2.00 25.4 3.53 2.28 42.0 3.95 2.13 Synthesized at 250 °C 2.20 ± 0.08 4.26 ± 0.62 49.7 3.96 2.19 25.4 3.35 2.40 Synthesized at 300 °C 4.31± 0.76 42.0 3.41 2.47 2.45 ± 0.04 49.7 3.50 2.47 FWHM: Full Width at Half Maximum

Table 2. Emission Peak and Calculated Band Gap of CdSe Nanocrystals at Different Reaction Temperatures Temperature (°C) 150 °C 200 °C 250 °C 300 °C Emission peak (nm) 533 585 642 653 Band gap (eV) 2.32 2.14 1.99 1.94

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Table 3. Band Gap of CdSe Calculated through the Effective Mass Approximation Theory Equation Average 𝑅̅ in solution Temperature Average sizes Average 𝑅̅ Band gap Band gap (Assuming wrapped with a (°C) by TEM (nm) by TEM (eV) (eV) layer of oleic acid molecules) 150 °C 2.57 1.29 4.27 2.41 2.5 200 °C 3.58 1.79 3.09 2.91 2.27 250 °C 4.26 2.13 2.71 3.25 2.17 300 °C 4.31 2.16 2.69 3.28 2.16 * * The diameter of oleic acid molecules: 1.12 nm; me =0.13m0; mh =0.45m0; Ɛr =5.8; m0 =9.109E-31 Schemes

Scheme 1 The experiment setup of simple programmed microfluidic batch process.

ε Conduction band

ε ε ε

ε εc

εc

εv

εv

Valence band

k Bulk materials

k Quantum dots

k Smaller quantum dots

Scheme 2 The band structure change mechanism of semiconductor materials with size changing. 21



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FIGURES

a (i)

(ii)

0.350 nm

(iii)

111 220 311

b (i)

(ii)

0.328 nm 0.350 nm

(iii)

111 220 311

c (i)

0.326nm

(ii)

0.349 nm

(iii)

111

0.357 nm

220 311

d (i)

(ii)

0.349 nm

(iii)

111 220 311

22


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Figure 1 Wide viewed TEM images (i) and the selected area electron diffraction pattern (insert in i); high resolution TEM images (ii) and the corresponding one single HRTEM image (insert in ii); the histogram of size distribution (iii) of CdSe nanocrystals synthesized at 150 °C (a), 200 °C (b), 250 °C (c) and 300 °C (d). The scale bar=50 nm for all the wide viewed TEM images and the scale bar=10 nm for all the HRTEM images.

(i) (ii) (iii) (iv)

Figure 2 XRD patterns of CdSe nanocrystals synthesized at different reaction temperatures.

(a)

(b)


(i) (ii) (iii) (iv)

23


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Figure 3 The optical colors of CdSe nanocrystal solution (a) and the normalized absorbance spectra of CdSe nanocrystals (b) synthesized at different reaction temperatures of (i) 150 °C; (ii) 200 °C; (iii) 250 °C and (iv) 300 °C.

(a)

(b)

(i)

(ii)

(iii)

(i) (ii) (iii) (iv)

(iv)

Figure 4 the solution color under ultraviolet illumination of CdSe nanocrystals solution (a) and the normalized photoluminescence spectra of CdSe nanocrystals (b), synthesized at different reaction temperatures of (i) 150 °C; (ii) 200 °C; (iii) 250 °C and (iv) 300 °C.

24


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TOC Graphic

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Shape Controlled Synthesis of CdSe Nanocrystals via a Programmed

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