Multistage Microfluidic Platform for the Continuous Synthesis of III–V Core/Shell Quantum Dots

Article abstract of DOI:10.1002/anie.201805264

We present a fully continuous chip microreactor-based multistage platform for the synthesis of quantum dots with heterostructures. The use of custom-designed chip reactors enables precise control of heating profiles and flow distribution across the microfluidic channels while conducting multistep reactions. The platform can be easily reconfigured by reconnecting the differently designed chip reactors allowing for screening of various reaction parameters during the synthesis of nanocrystals. III–V core/shell quantum dots are chosen as model reaction systems, including InP/ZnS, InP/ZnSe, InP/CdS and InAs/InP, which are prepared in flow using a maximum of six chip reactors in series.

Full text of DOI:10.1002/anie.201805264

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Multistage Microfluidic Platform for the Continuous Synthesis of
III-V Core/Shell Quantum Dots
Authors: Jinyoung Baek, Yi Shen, Ioannis Lignos, Moungi G. Bawendi,
and Klavs F. Jensen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201805264
Angew. Chem. 10.1002/ange.201805264
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http://dx.doi.org/10.1002/ange.201805264

10.1002/anie.201805264
Angewandte Chemie International Edition
Supporting Information for
Multistage Microfluidic Platform for the Continuous Synthesis of
III-V Core/Shell Quantum Dots
Jinyoung Baeka,b,‡, Yi Shena,‡, Ioannis Lignosa,c, Moungi G. Bawendic, and Klavs F. Jensen*,a
a Department of Chemical Engineering and c Department of Chemistry, Massachusetts Institute of
Technology, Cambridge, MA, USA, 02139
b The Boston Consulting Group, Seoul 04539, Korea
‡ JB and YS contributed equally.
*Corresponding authors: kfjensen@mit.edu.
Content
Page
Methods
Figure S1
Figure S2
Figure S3
Table S1
Table S2
Figure S4
Table S3
Figure S5
Figure S6
References
Experimental materials and methods
S2
S4
S5
S6
S7
S8
S9
Pictures of the multistage microfluidic platform
Pictures of the chip reactors used for the InP core synthesis
A picture of the shell-formation microreactor with ten sub-channels
Compositions of InP/ZnS QDs measured by WDS
Lattice constants of the zinc blende bulk InP, ZnS, ZnSe, CdSe
Emission spectra and representative TEM image of InP/ZnSe QDs
Experimental conditions for the synthesis of InP/CdS with various shell thicknesses S10
TEM images of representative InP/CdS QDs S11
Absorption and emission spectra, and representative TEM images of InAs/InP QDs S12
S13
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10.1002/anie.201805264
Angewandte Chemie International Edition
Experimental Section
Materials. Octane (anhydrous) and dodecane (anhydrous) were purchased from Sigma-Aldrich.
The solvents were degassed by freeze-pump-thaw method and further dried with 4 Å molecular sieves. Tri-
n-octylphosphine (TOP, Strem) was dried with 4 Å molecular. Tri-n-octylphosphine oxide (TOPO, Sigma-
Aldrich) and 1-dodecanethiol (Sigma-Aldrich) were dried under vacuum (< 70 mTorr) at 100 °C for 2 hours
before use. The following chemicals were used as received. Bis(trimethylsilyl) sulfide ((TMS)2S), myristic
acid (MA), oleic acid (OA), and 1-hexanethiol were purchased from Sigma-Aldrich.
Tris(trimethylsilyl)phosphine ((TMS)3P) and cadmium acetate (Cd(Ac)2, anhydrous) were purchased from
Strem. Zinc acetate (Zn(Ac)2, anhydrous) was obtained from Alfa Aesar, and Tris(trimethylsilyl)arsine
((TMS)3As was obtained from Nanomeps.
Precursor preparation. All the precursor preparation and transfer were performed under nitrogen
or vacuum in a Schlenk line or glove box. In(MA)3 was synthesized as previously reported[1]. The indium
precursor solution was prepared by dissolving In(MA)3 in octane with 10% TOP. Zn(OA)2 or Cd(OA)2 was
prepared by reacting Zn(Ac)2 or Cd(Ac)2, with oleic acid under vacuum (<70 mTorr) at 140 °C for 2 hours.
TOP-S were prepared by dissolving elemental sulfur in TOP (concentration 1.5 M). TOP-S was used for
CdS shell synthesis, while (TMS)2S was used for ZnS shell overcoating. (TMS)3P and (TMS)3As were
diluted to pure octane. All the shell precursors were diluted to octane containing 20% TOP. The
concentration of the core precursors was 30mM, and that of shell precursors was 20 mM if it was not
specified in the later discussion.
Ligand exchange reaction. The synthesized InP/ZnS QDs were purified by precipitation using
ethanol and acetone as the anti-solvents, and then redisperesed in chloroform. The purified QD samples
were mixed with a solution of excess 3-mercaptopropionic acid in water. The biphasic mixture was stirred
vigorously until the non-polar phase became colorless.
Microreactor fabrication and system operation. Preparation of the silicon-based microreactors
was conducted based on a previously reported precedure[2]. We first assembled 4.5 μm silicon oxide as a
hard mask onto the 6 inch silicon wafer (the thickness was 800 μm); then 0.5 μm silicon oxide layer was
grown on the silicon wafer by wet-oxidation process (MRL industries, Model 718), and 4 μm silicon oxide
layer was further grown by dielectric Plasma-enhanced chemical vapor deposition (PECVD) process
(Applied Materials, Centura 5300). Thin positive photoresist with a thickness of 1 μm was spin-coated on
the top of the silicon oxide layer. Model MA6 (Karl Suss) was used as a mask-aligner. This process allowed
for precise channel development. For the shell growth chip reactors, the width and depth of the main channel
were 400 μm and 420 μm and corresponding dimensions of the side channels were 80 μm and 100 μm
respectively. Each side channel had 40 - 60 times higher flow resistance than the main channel to prevent
any backflows. This high flow resistance also helped to maintain uniform flow distribution when the
flowrates of each injection (main, and two side injections) changes. After patterning the wafer, the channels
were sealed by the covalent bonding of a Pyrex wafer through the anodic bonding process. The reaction
platform is illustrated in Figure S1, and the picture of differently designed reactors are shown in Figure S2
and S3. The growth of nanocrystals was monitored by a custom designed in-line UV-vis measurement
device, which was connected with Ocean Optics DH-2000 light source and SD2000 fiber-optics
spectrometer. Stainless steel tubes with an inner diameter of 0.02 inch were used for connections between
reactors.
S2
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10.1002/anie.201805264
Angewandte Chemie International Edition
Characterization. All the solutions were diluted in hexane for the bench-top optical
measurements. Absorbances were measured by Hewlett Packard 8452 diode-array spectrometer. Emissions
spectra were recorded by an SD2000 fiber-optics spectrometer. TEM images were acquired by JEOL
200CX transmission electron microscope. XRD data were measured using PANalytical X’pert Pro
Multipurpose Diffractometer and Rigaku H3R. Quantum yields were measured by comparing the integrated
emission of the QDs in hexane to that of the dyes (rhodamine or indotrycarbocyanine) in methanol and
ethanol. The QY was calculated based on the integrated intensities of the emission spectra, the absorption
at the excitation wavelength and the refraction index of the solvent using the equation:
푅푒푓푟푎푐푡푖표푛 푖푛푑푒푥푡2표푙푢푒푛푒
푅푒푓푟푎푐푡푖표푛 푖푛푑푒푥푒2푡ℎ푎푛표푙
퐴푏푠표푟푏푎푛푐푒푑푦푒 퐸푚푖푠푠푖표푛 푖푛푡푒푔푟푎푙푄퐷푠
푄푌
= 푄푌
∗
∗
∗
푄퐷푠
푑푦푒
퐴푏푠표푟푏푎푛푐푒푄퐷푠 퐸푚푖푠푠푖표푛 푖푛푡푒푔푟푎푙푑푦푒
S3
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10.1002/anie.201805264
Angewandte Chemie International Edition
Shell Formation
Mixing
Seq-Growth
Aging
Figure S1. A picture of the multistage set-up under ambient light (top) and UV light (bottom). The
pictures were taken after cooling down the system to room temperature.
S4
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10.1002/anie.201805264
Angewandte Chemie International Edition
Figure S2. Differently designed chip reactors used for the multistep synthesis of core/shell nanocrystals.
These reactors were used for mixing, aging and sequential growth steps (from left to right).
S5
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10.1002/anie.201805264
Angewandte Chemie International Edition
Figure S3. Shell-formation microreactor with ten sub-channels
S6
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10.1002/anie.201805264
Angewandte Chemie International Edition
Table S1. Compositions of InP / ZnS core-shell QDs measured with wavelength dispersive spectroscopy
(WDS).
Photoluminescence
Emission Peak (nm)
In
P
Zn
S
(atomic %)
(atomic %)
(atomic %)
(atomic %)
554
599
605
630
20.4
21.4
21.0
18.2
16.5
18.1
18.9
17.6
26.5
25.6
27.5
28.0
36.6
34.9
32.6
36.1
S7
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10.1002/anie.201805264
Angewandte Chemie International Edition
Table S2. Lattice constants of the bulk materials in zinc blende structure at 300K
Material
InP
ZnS
5.82
ZnSe
5.41
CdS
5.67
Lattice Constant (Å)
5.87
S8
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10.1002/anie.201805264
Angewandte Chemie International Edition
Figure S4-1 Emission spectra of InP/ZnSe QDs
Figure S4-2 TEM images of InP / ZnSe core-shell QDs emitting at 582 nm. The scale bar corresponds to
20 nm.
S9
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10.1002/anie.201805264
Angewandte Chemie International Edition
Table S3. Experimental conditions for the synthesis of InP/CdS QDs and the corresponding emission
properties
Temperature at the Temperature at the
Photoluminescence Total CdS precursor
FWHM
(nm)
sequential injection
annealing stage
Emission Peak (nm)
Flow (μl / min)
stage (°C)
(°C)
608
648
683
702
721
768
10
20
20
50
70
90
280
280
290
290
290
290
280
300
320
320
330
330
75
74
74
81
84
102
S10
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10.1002/anie.201805264
Angewandte Chemie International Edition
Figure S5. TEM images of InP / CdS core-shell QDs emitting at 721 nm (left) and 768 nm (right)
respectively. The scale bar corresponds to 20 nm.
S11
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10.1002/anie.201805264
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Figure S6-1 The absorption (left) and emission (right) spectra of InAs/InP QDs. The arrow indicates an
increasing thickness of the InP shell on the InAs core.
Figure S6-2 TEM images of InAs/InP core-shell QDs emitting at 744 nm (A) and 783 nm (B) respectively.
The scale bar corresponds to 20 nm.
S12
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10.1002/anie.201805264
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References
[1] Baek Jinyoung, Allen Peter M., Bawendi Moungi G., Jensen Klavs F., Angew. Chem. Int. Ed. 2010,
50, 627–630.
[2] S. Marre, A. Adamo, S. Basak, C. Aymonier, K. F. Jensen, Ind. Eng. Chem. Res. 2010, 49, 11310–
11320.
S13
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