Hyper-Rayleigh Scattering (HRS) from Nanoparticles

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Motivation: Mechanical, optical and electrical properties of nanoparticles are inherently correlated to particle size, atomic structure and surface morphology. Controlling these parameters during synthesis and particle growth is therefore highly desirable, but often not obtainable due to the lack of analytical tools that can probe these intriguing properties in-situ and in real time. Therefore, in most cases time-consuming ex-situ characterizations and subsequent optimization of synthesis conditions have to be applied. A deeper understanding of nucleation and growth mechanisms is, however, needed in order to predict possible synthesis results. Therefore on-line measurement techniques are needed, which allow a simultaneous observation of different particle properties. Hyper-Rayleigh Scattering (HRS) is an incoherent nonlinear scattering process and is sensitive to particle size and number density, but also provides information on changes in crystal structure, particle shape and morphology. Thus as a non-invasive optical technique HRS is a promising tool for in-situ nucleation and growth studies.



Experimental Setup and Applications of HRS: To study the HRS signal from nanoparticles in solution, ultra-short laser pulses (80 fs) are focused into the sample. The scattered HRS light is detected with a photon counting unit. The experimental setup for HRS is shown in Fig.1.
To demonstrate the sensitivity of HRS to conformational changes, measurements on amphiphilic invertable polyester micelles were carried out. Depending on the solvent polarity changes in micelle size and hyperpolarizability can be observed as shown in Fig.2 [1]. In addition, HRS was used to monitor different stages of ZnO nucleation and growth (see Fig.2). Here, time-resolved measurements have allowed us to distinguish between nucleation, growth and ripening. The precipitation kinetics were determined from growth and ripening rates from HRS data [2].

Shematic SHG setup
Fig. 1: Experimental setup for HRS measurements.
Scattering profile 0,6µm PS
Fig. 2: Left: HRS signal and DLS size of polyester micelles as a function of solute polarity [1].
Right: Time-resolved HRS measurements during ZnO precipitation for different time resolutions [2].

HRS Sensor Design: Beside the application of HRS to study particle growth we are currently also developing a HRS sensor that allows for HRS measurements in chemical reactors. The design of such an optical sensor is part of ongoing work.

Publications:

1.

L. Martinez Tomalino, A. Voronov, A. Kohut, W. Peukert
Study of Amphiphilic Polyester Micelles by Hyper-Rayleigh Scattering: Invertibility and Phase Transfer
J. Phys. Chem. B, Vol. 112, No. 20, 6338-6343, 2008

2.

D. Segets, L. Martinez Tomalino, J. Gradl, W.Peukert
Real-Time Monitoring of the Nucleation and Growth of ZnO Nanoparticles Using an Optical Hyper-Rayleigh Scattering Method
J. Phys. Chem. C, Vol. 113, No. 28, 11995-12001, 2009

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