A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals

Summary

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.

Abstract

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.

Introduction

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 "QLED" 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....

Protocol

1. Reactor Assembly

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 th....

Results

Sample spectra: 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. Whi.......

Discussion

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.......

Materials

List of materials used in this article

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