A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals

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.