(637h) Flow Reactors for Quantum Dot Synthesis: Single Nanocrystal Spectroscopy in Flow

Flow Reactors for
Quantum Dot Synthesis: Single Nanocrystal Spectroscopy in Flow

Ioannis
Lignos,^a,b Hendrik Utzat,^b Yiming Mo,^a Moungi
G. Bawendi^b* and Klavs F. Jensen^a*

^aDepartment of Chemical Engineering, Massachusetts Institute of
Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A.

^bDepartment of Chemistry, Massachusetts Institute of Technology, 77
Massachusetts Avenue, Cambridge, MA 02139, U.S.A.

Semiconductor nanocrystals (NCs)
(or quantum dots) have received enormous attention as luminescent materials for
biological and optoelectronic applications.¹ This is
mainly driven by their unique and synthetically tunable optical and electronic
properties. However, most of the NC-based applications have been hindered by
the lack of quantitative information regarding the mechanisms governing
particle inhomogeneities in a solution. This is due to a loss of spectral
information in the ensemble studies due to the environmental inhomogeneity.² Typically,
systems at the nanoscale often exhibit both significant heterogeneity between
individual objects as well as time-dependent fluctuations on their properties.³ Recently,
a single molecule technique based on solution photon correlation Fourier
spectroscopy (S-PCFS) has been proposed.⁴ Using
low intensity excitation it is possible to interrogate single NCs freely
diffusing in a solution. Such methodology provides single and ensemble NC
spectral information, while overcoming issues related to user selection bias in
traditional single NC techniques due to their instability. Single-NC spectroscopy
and interferometric analysis can uncover the basic processes governing spectral
linewidths, however linking such spectroscopic tools with more sophisticated
synthetic configurations would potentially accelerate the optimized synthesis
of NC populations. Recently, flow reactor systems with integrated detection
modalities are able to reassess standard synthetic approaches and to rapidly
optimize reaction conditions, while providing a unique environment for
elucidating reaction kinetics.⁵

Herein, we present one-stage and
multi-stage flow-reactor configurations (capillary microfluidic reactor and a
series of miniature continuous stir tanked reactors - μCSTRs), for the
quality-assured production of binary and core-shell semiconductor NCs. Most
importantly, we demonstrate for the first-time a flow-based system coupled with
an interferometry setup for a detailed investigation of single NC properties
using photon correlation Fourier spectroscopy in flow (F-PCFS). Such flow
configurations with single-NC based spectroscopic capabilities could provide
unprecedented precision in the synthesis of existing and novel NC compositions
while shifting the current paradigm of reaction optimization from ensemble to
single-dot level. The F-PCFS setup allows for a real-time investigation of the
effect of reaction parameters such as (reaction time, temperature, molar ratios
of precursors) on ensemble and single dot emission spectra of the synthesized NCs
in their native solution. We explore the potential of the proposed technique
for the in-situ determination of single and ensemble emission spectra of a
model reaction system, CdSe NCs, while varying the temperature (240 – 270 °C) and molar ratios of Cd and Se precursors. Combining the F-PCFS setup
with a fast-reaction feedback control algorithm and extending the proposed
methodology to quantum confined multinary and core-shell semiconductor NCs will
allow for a detailed understanding of the underlying photophysics of such
complex materials.

References

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