Synthesis of nanoparticles

An automated remote controllable microwave-based synthesis setup for colloidal nanoparticles with integrated absorption and photoluminescence online analytics

One major challenge for the synthesis of nanomaterials and nanoparticles is the reproducibility of the synthesis in terms of controlling the size, morphology, surface constitution and crystallinity of the resulting particles which often determine specific chemical and physical properties.

By even using the same synthesis protocol, two different people often achieve different results depending on their individual interpretation of the protocols and differences in physical and chemical parameters influencing the reaction which are usually not described and well documented such as, heating rate, stirring velocity, size of the reaction vessel, chemical impurities etc [1]. The FMF- nanolab provides a novel synthesis setup based on a commercially available microwave synthesis reactor (Fig.1a) with integrated absorption or photoluminescence (PL) spectroscopic detection systems for the synthesis of nanoparticles.

For the integration of absorption and PL online analytics all-quartz immersion probes and a dedicated glass vessel were constructed (Fig.1b). The fiber optical sensors were coupled to an absorption spectrometer and a fluorescence spectrometer respectively (Fig.1c). The immersion probes are limited to a temperature of up to 160°C and low pressure.
Alternatively, the PL is measured in reflection mode by a fiber bundle consisting out of microwave-tested materials. This probe is directed through the modified pressure release chamber of the microwave into the cavity and screens the reaction solution in a non-contact mode across the glass vials (Fig.1d).

By this non-contact mode commercial standard vials and an auto sampler unit can be used in combination with the pressure controlling system that handles up to 17 bar.

Fig.1): a) Commercially available computer controlled laboratory microwave system with integrated video camera and autosampler. b) Dedicated reaction vials allow the integration of an absorption or fluorescence immersion probe head to perform syntheses under inert nitrogen atmosphere. c) Schematics of the remote controllable microwave synthesis reactor with integrated fiber optical based online absorption and fluorescence detection possibilities. d) External fluorescence probe for reflective online fluorescence measurements integrated into the microwave reactor chamber (top view left and side view right (video camera image)). The fiber optical probe (marked with arrows) is installed nearby the reaction vessel.

Synthesis of CdSe quantum dots
Cadmium selenide quantum dots (CdSe QD) are used as a model system for evaluating and optimizing the synthesis setup. The reaction progress was monitored by online absorption and/or PL spectroscopy shown in Fig.2 and Fig.3, respectively.

Fig.2): Absorption spectra recorded during a CdSe quantum dot synthesis: An overlay of transmission spectra recorded 10 times per second during a synthesis at 160°C shows the generation of CdSe quantum dots. Over the time of one hour absorption signal characteristic for the growth of CdSe nanoparticles arise (following the direction described by the arrow).

Fig.3): PL spectra recorded during a CdSe QD-Synthesis. An overlay of PL spectra recorded every 15 seconds over a total time of 15 min. The first spectrum taken is highlighted with a dashed line. The arrow indicates the temporal evolution of the spectra to maxima at higher wavelengths.

The high reproducibility of syntheses performed with this system as well as the possibility to monitor the reaction by online spectroscopy with high time resolution enables a variety of fundamental investigations on growth processes occurring for nanocrystals. Furthermore, closely defined protocols can be published and reproduced easily. The synthesis set-up is also aimed to enhance the development of new synthesis approaches for novel nanomaterials.

Contact: Dr. Michael Krüger, Simon Einwächter, FMF-Nanolab at the University Freiburg [Link]

Literature:Ying Yuan, Frank-Stefan Riehle, Haoshuang Gu, Ralf Thomann, Gerald Urban, and Michael Krüger "Critical Parameters for the Scale-Up Synthesis of Quantum Dots" J. Nanosci. Nanotechnol. 10, 6041-6045 (2010) [Link]

Further reading: Simon Einwächter, Michael Krüger, "A fully automated remote controllable microwave-based synthesis setup for colloidal nanoparticles with an integrated absorption and photoluminescence online analytics", MRS Fall Meeting Proceedings 2011, 1284, mrsf10-1284-c08-01-uu8-01 doi:10.1557/opl.2011.648 [Link]

Videos: "Automated synthesis of nanomaterials at Freiburg Materials Research Center" [Link]
"BW-eLabs: Automated Synthesis of Nanoparticles" [Link]