The authors gratefully acknowledge the financial support provided by North Carolina State University. Milad Abolhasani and Robert W. Epps gratefully acknowledge financial support from the UNC Research Opportunities Initiative (UNC-ROI) grant.

1. Reactor Assembly
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57666/57666fig1.jpg" /> 
Figure 1. Step-by-step illustration of a sample platform assembly process. The panels shows a step-by-step illustration of a sample platform assembly process detailing (i) the initial arrangement of the translation stage and optical post holders on the mounting bread broad, (ii) the mounting of the precursor t...

<ol>
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W., Yu, Y., Dou, L., Yang, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solution-phase+synthesis+of+cesium+lead+halide+perovskite+nanowires.">Solution-phase synthesis of cesium lead halide perovskite nanowires.</a> Journal of the American Chemical Society. 137 (29), 9230-9233 (2015).</li><li>Lignos, I., Stavrakis, S., Nedelcu, G., Protesescu, L., deMello, A. J., Kovalenko, M. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+of+cesium+lead+halide+perovskite+nanocrystals+in+a+droplet-based+microfluidic+platform:+fast+parametric+space+mapping.">Synthesis of cesium lead halide perovskite nanocrystals in a droplet-based microfluidic platform: fast parametric space mapping.</a> Nano Letters. 16 (3), 1869-1877 (2016).</li><li>Wei, S., Yang, Y., Kang, X., Wang, L., Huang, L., Pan, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Room-temperature+and+gram-scale+synthesis+of+CsPbX3+(X+=+Cl,+Br,+I)+perovskite+nanocrystals+with+50-85%+photoluminescence+quantum+yields.">Room-temperature and gram-scale synthesis of CsPbX3 (X = Cl, Br, I) perovskite nanocrystals with 50-85% photoluminescence quantum yields.</a> Chemical Communications. 52 (45), 7265-7268 (2016).</li><li>Protesescu, L., Yakunin, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanocrystals+of+cesium+lead+halide+perovskites+(CsPbX+3+,+X+=+Cl,+Br,+and+I):+novel+optoelectronic+materials+showing+bright+emission+with+wide+color+gamut.">Nanocrystals of cesium lead halide perovskites (CsPbX 3 , X = Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut.</a> Nano Letters. 15 (6), 3692-3696 (2015).</li><li>Epps, R. W., Felton, K. C., Coley, C. 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W., Lignos, I. G., Maceiczyk, R. M., deMello, A. J., deMello, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanocrystal+synthesis+in+microfluidic+reactors:+where+next.">Nanocrystal synthesis in microfluidic reactors: where next.</a> Lab on a Chip. 14 (17), 3172-3180 (2014).</li><li>Lignos, I., Stavrakis, S., Kilaj, A., deMello, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Millisecond-timescale+monitoring+of+PBS+nanoparticle+nucleation+and+growth+using+droplet-based+microfluidics.">Millisecond-timescale monitoring of PBS nanoparticle nucleation and growth using droplet-based microfluidics.</a> Small. 11 (32), 4009-4017 (2015).</li><li>Abolhasani, M., Coley, C. W., Xie, L., Chen, O., Bawendi, M. G., Jensen, K. 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Automated sampling system: The autonomous operation of the screening platform is carried out with a central control finite state machine. Movement between these states occurs sequentially with multiple recursive segments to allow for operation across a varying number of sampling conditions. The general system controls can be divided into 3 core stages. First, the system begins with an initialization step, which establishes communications through each USB-controlled component, automatically defines file s...

The advent of semiconductor nanocrystals, particularly quantum dots, has driven significant advancements in electronic materials research and manufacturing. For example, quantum dot LEDs1 have already been implemented in commercially available &#34;QLED&#34; displays. More recently among this class of semiconductors, perovskites have sparked substantial interest and research towards high-efficiency and low-cost PV technologies. Since the first demonstration of a perovskite-based PV in 2009,2 the lab-scale power conversion efficiency of perovskite-based solar cells has increased at a rate unparalleled by any PV technology in history.3,4 In addition to the driving interest in perovskite-based PVs, a variety of recent methods describing the facile colloidal synthesis of perovskite nanocrystals have created the opportunity for low-cost, solution-phase processing of perovskite QDs in commercial electronics.5,6,7,8,9,10,11,12,13,14
In the effort towards large-scale nanomanufacturing of colloidal perovskite QDs, a better fundamental understanding of the nanocrystal growth pathways and an effective control of the reaction conditions must first be developed. However, existing studies of these processes have traditionally relied on flask-based approaches. Batch synthesis strategies present a variety of inherent limitations in terms of material characterization and production, but most significantly, flask-based techniques are highly inefficient in screening time and precursor consumption, and demonstrate flask size-dependent mass transfer properties, which inhibit the synthesis consistency.15 To effectively study the growth pathways of colloidal semiconductor nanocrystals across the large variety of reported syntheses procedures and within the broad relevant sample space, a more efficient screening technique is required. Over the past two decades, a range of microfluidic strategies have been developed for studies of colloidal nanocrystals leveraging the substantially lower chemical consumption, the accessibility of high-throughput screening methods, and the potential for a process control implementation in continuous synthesis systems.12,16,17,18,19,20
In this work, we report the design and development of an automated microfluidic platform for the high-throughput in situ studies of colloidal semiconductor nanocrystals. A novel translating flow cell, a highly modular design, and the integration of off-the-shelf tubular reactors and fluidic connections form a unique and adaptable reconfigurable platform with direct applications in the discovery, screening, and optimization of colloidal nanocrystals. Capitalizing on the translational capability of our detection technique (i.e., a three-port flow cell), for the first time, we demonstrate the systematic decoupling of mixing and reaction timescales, while simultaneously improving the sampling efficiency and collection rates over traditional stationary flow cell approaches. The utilization of this platform enables the high-throughput and precise band-gap engineering of colloidal nanocrystal syntheses towards continuous nanomanufacturing strategies.

<table border="0" cellpadding="0" cellspacing="0" style="width:888px;" width="888">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Toluene</td>
			<td style="width:191px;">Fisher Scientific</td>
			<td style="width:168px;">AC364410010</td>
			<td style="width:257px;">99.85% extra over molecular sieves</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Oleic acid</td>
			<td style="width:191px;">Sigma Aldrich</td>
			<td style="width:168px;">364525&#160;ALDRICH</td>
			<td>technical grade 90%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Cesium hydroxide (50 wt% in water)</td>
			<td style="width:191px;">Sigma Aldrich</td>
			<td style="width:168px;">232041 ALDRICH</td>
			<td>50 wt% in water &#62; 99.9% trace metals</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Lead(II) oxide</td>
			<td style="width:191px;">Sigma Aldrich</td>
			<td style="width:168px;">211907 SIGMA-ALDRICH</td>
			<td>&#62; 99.9% trace metals basis</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Tetraoctylammonium bromide</td>
			<td style="width:191px;">Sigma Aldrich</td>
			<td style="width:168px;">294136 ALDRICH</td>
			<td>98%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">1/16&#34; OD, 0.04&#34; ID FEP tubing</td>
			<td style="width:191px;">MicroSolv</td>
			<td style="width:168px;">48410-40</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">1/16&#34; OD, 0.02&#34; ID ETFE tubing</td>
			<td style="width:191px;">MicroSolv</td>
			<td style="width:168px;">48510-20</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">0.02&#34; thru hole PEEK Tee</td>
			<td style="width:191px;">IDEX Health &#38; Science</td>
			<td style="width:168px;">P-712</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">1/4-28 ETFE flangeless ferrule for 1/16&#34;</td>
			<td style="width:191px;">IDEX Health &#38; Science</td>
			<td style="width:168px;">P-200N</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">1/4-28 PEEK flangeless nut for 1/16&#34;</td>
			<td style="width:191px;">IDEX Health &#38; Science</td>
			<td style="width:168px;">P-230</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">4-way PEEK L-valve</td>
			<td style="width:191px;">IDEX Health &#38; Science</td>
			<td style="width:168px;">V-100L</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Syringe pump</td>
			<td style="width:191px;">Harvard Apparatus</td>
			<td style="width:168px;">70-3007</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">8 mL stainless steel syringe</td>
			<td style="width:191px;">Harvard Apparatus</td>
			<td style="width:168px;">70-2267</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">25 mL glass syringe</td>
			<td style="width:191px;">Scientific Glass Engineering</td>
			<td style="width:168px;">25MDF-LL-GT</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Optical breadboard</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">MB1224</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">300 mm translation stage</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">LTS300</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Optical post</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">TR2-4</td>
			<td>TR2, TR3, or TR4</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Optical post holder</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">PH4-6</td>
			<td>PH4 or PH6</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">365 nm LED</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">M365LP1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">LED driver</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">LEDD1B</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">600 micron patch cord</td>
			<td style="width:191px;">Ocean Optics</td>
			<td style="width:168px;">QP600-1-SR</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Deuterium-halogen light source</td>
			<td style="width:191px;">Ocean Optics</td>
			<td style="width:168px;">DH-2000-BAL</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Miniature spectrometer</td>
			<td style="width:191px;">Ocean Optics</td>
			<td style="width:168px;">FLAME-S-XR1-ES</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Multifuction I/O device (DAQ)</td>
			<td style="width:191px;">National Instruments</td>
			<td style="width:168px;">USB-6001</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Virtual Instrument Software</td>
			<td style="width:191px;">National Instruments</td>
			<td style="width:168px;">LabVIEW 2015 SP1</td>
			<td></td>
		</tr>
	</tbody>
</table>

a modular microfluidic technology for systematic studies of colloidal semiconductor nanocrystals

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are a rapidly growing class of materials in commercial electronics, such as light emitting diodes (LEDs) and photovoltaics (PVs). Among this material group, inorganic/organic perovskites have demonstrated significant improvement and potential towards high-efficiency, low-cost PV fabrication due to their high charge carrier mobilities and lifetimes. Despite the opportunities for perovskite QDs in large-scale PV and LED applications, the lack of fundamental and comprehensive understanding of their growth pathways has inhibited their adaptation within continuous nanomanufacturing strategies. Traditional flask-based screening approaches are generally expensive, labor-intensive, and imprecise for effectively characterizing the broad parameter space and synthesis variety relevant to colloidal QD reactions. In this work, a fully autonomous microfluidic platform is developed to systematically study the large parameter space associated with the colloidal synthesis of nanocrystals in a continuous flow format. Through the application of a novel translating three-port flow cell and modular reactor extension units, the system may rapidly collect fluorescence and absorption spectra across reactor lengths ranging 3 - 196 cm. The adjustable reactor length not only decouples the residence time from the velocity-dependent mass transfer, it also substantially improves the sampling rates and chemical consumption due to the characterization of 40 unique spectra within a single equilibrated system. Sample rates may reach up to 30,000 unique spectra per day, and the conditions cover 4 orders of magnitude in residence times ranging 100 ms - 17 min. Further applications of this system would substantially improve the rate and precision of the material discovery and screening in future studies. Detailed within this report are the system materials and assembly protocols with a general description of the automated sampling software and offline data processing.

Sample spectra:&#160;Utilizing the discussed microfluidic platform, the nucleation and growth stages of colloidal semiconductor nanocrystals at the synthesis temperature can be directly studied by monitoring the time-evolution of the absorption and fluorescence spectra of the formed nanocrystals under uniform mixing conditions. Figure 5A shows an example set of spectra obtained within a single pass of the three-port flow cell...

Watch this Scientific Journal Video about A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals at JoVE.com

A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals

Detailed herein are the operation and assembly protocols of a modular microfluidic screening platform for the systematic characterization of colloidal semiconductor nanocrystal syntheses. Through fully adjustable system arrangements, highly efficient spectra collection may be carried out across 4 orders of magnitude reaction time scales within a mass transfer-controlled sampling space.

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are a rapidly growing class of materials in commercial electronics, such as light ...

The overall purpose of this procedure is to assemble and use a microfluidic high-throughput screening platform for systematic inline studies of the reaction pathways of colloidal semiconductor nanocrystals. This platform provides researchers with access to a full absorption and emission spectra within a parameter space that was previously inaccessible. Beyond the extended parameter range, the high sampling rate and low chemical consumption allows many more conditions to be tested at a fraction of the cost compared to flask-based screening.Further implementation of this system will improve the pace of research and, therefore, bring us closer to commercial scale production of low-cost, high-efficiency quantum dot based photovoltaic cells. To begin assembling the microfluidic platform, fix a linear translation stage lengthwise on an aluminum optical breadboard. Fix four optical post holders on the board around the track and post two holders on the stage platform.Connect an optical post to each of the four corners of the junction stage then place the optical posts into the four post holders, mount. Connect the flow cell to the optical posts on the translation stage platform. Then, cut a length of FEP tubing as the reactor line and three lengths of ETFE tubing as the precursor feed lines.Fit each line with a flangeless ferrule and nut at one end. Fit the other end to the precursor lines with gas-tight syringe fittings and flow valves as needed for the syringe configuration to be used. Connect the reactor and precursor feed lines to a custom-built, four-way cross junction so that the reactor line will be next to the flow cell.Place the cross junction in the junction mounting stage. Feed the precursor lines through the channels of the junction stage. Then, thread the reactor line through a sampling port.Fit the sampling port through the flow cell being careful not to stretch or crimp the reactor line as the sampling port is moved along the line. Connect the port to the junction stage. Fasten the precursor line cover onto the junction stage to secure the tubing and sampling port in place.Connect the desired number of sampling ports and extension units to the assembly keeping the modules as straight and level as possible to avoid distorting or damaging the tubing. Connect a support bracket to the outlet of the last sampling port so that the bracket is under the reactor tubing outlet. Secure the support bracket on the remaining two optical posts.Guided by a carpenter's level, adjust the outlet support structure until the reactor assembly is straight and level. Then, use fiber optic patch cords to connect a spectrometer and LED in a deuterium halogen light source to the flow cell ports. Test the translation stage to ensure that the cables do not restrict the flow cell movement.To begin preparing the precursors, combine 109 milligrams of tetraoctylammonium bromide, one milliliter of oleic acid and 14 milliliters of toluene in a 20-milliliter vial equipped with a stir bar. Seal the vial and stir the mixture vigorously at room temperature until clear and colorless to form the bromide precursor solution. Next, place 0.6 millimoles of cesium hydroxide, 0.6 millimoles of lead two oxide and three milliliters of oleic acid in an eight-milliliter vial equipped with a stir bar.Seal the vial with a septum cap. Pierce the septum with a needle as a vent. Vigorously stir the mixture at 160 degrees Celsius until clear and colorless.Then, with the vent needle still in place, heat the mixture in an oven at 120 degrees Celsius for one hour. After that, remove the venting needle and allow the cesium lead mixture to cool to room temperature in open air. Combine 0.5 milliliters of the concentrated cesium lead mixture with 47.5 milliliters of toluene in a sealed 50-milliliter vial equipped with a stir bar.Vigorously stir the mixture until homogenous to obtain the dilute cesium lead precursor solution. Load the bromide and cesium lead precursors into 25-milliliter glass syringes. Fill an eight-milliliter stainless steel syringe with nitrogen gas from a gas cylinder.Connect the liquid precursor syringes and the nitrogen gas syringe to the precursor lines. If absorption reference spectra will be collected using a blank solution, connect a syringe filled with the blank solution to one of the liquid feed lines. Mount the syringes on computer controlled syringe pumps then thread the reactor line through the septum of a 50-milliliter vial.Pressurize the vial with nitrogen gas via a two-stage gas regulator to complete the setup. Once ready to begin the experiment, open the automated operation software and set the path to the folder in which the data should be saved. Select the USB connection address for the spectrometer.Set the integration time, the number of spectra to average and the number of spectra to save for both absorption and fluorescence. If multiphase flow will be characterized, click the multiphase button, set the minimum sample length so that approximately two complete gas liquid oscillations will pass the sampling point. Set the number of samples to be taken within that window.Then, set the communication addresses for the syringe pumps and fill in the syringe inner diameters for the syringes in use. Leave the diameters of the extraneous syringes at the default values. If absorption reference spectra are to be collected, set the flow rate of the syringe containing the reference solution, or precursor, to 300 microliters per minute.Then, either select a previously optimized set of stage locations or choose an appropriate reference file and a stage position window size. Ensure that the stage increment is 0.05 millimeters and the startup passes value is eight. Fill in the volume in microliters of the reactor tubing from the center of the junction to the final sampling port as the system volume.Ensure that the minimum equilibration time is set to 10 seconds. Double check all values and then click run. Set up to 30 flow rate configurations to test leaving unused syringe inputs blank.Choose whether to save reference spectra if applicable. The system will run through the selected conditions and will automatically shut down when finished. A series of fluorescence and absorbance spectra were collected in a single pass of a multiphase cesium lead bromide perovskite nanocrystal system with an average slug velocity of approximately 0.2 centimeters per second.Similar sets of spectra were collected at other flow rates and reactor lengths. Plotting the peak fluorescence wavelength as a function of residence time revealed the trend of higher peak fluorescent wavelengths at lower fluid velocities. A notable difference in peak fluorescence wavelength was observed when the slug velocity was increased from 75 millimeters per second to 130 millimeters per second while retaining a residence time of 0.9 seconds.Once assembled, this system has the capability to collect up to 30, 000 unique optical spectra within a single day all within a mass transfer controlled sampling space. By applying this platform to other colloidal semiconductor synthesis, researchers will gain access to a broad range of nanocrystal growth information with far greater accuracy and precision than through conventional flask-based strategies at a fraction of the cost and time.

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Research

JoVE Journal

Engineering

9.5K visualizações.  North Carolina State University. The overall purpose of this procedure is to assemble and use a microfluidic high-throughput screening platform for systematic inline studies of the reaction pathways of colloidal semiconductor nanocrystals. This platform provides researchers with access to a full absorption and emission spectra within a parameter space that was previously inaccessible. Beyond the extended parameter range, the high sampling rate and low chemical consumption allows many more conditions to be tested at a fractio...