Probing colloidal interfaces with second harmonic light scattering

Suche




Surfaces and interfaces are of great importance for fundamental research and for industry since many processes are controlled by interfacial properties. Accessing buried interfaces is, however, particularly challenging since many techniques are not interfacial specific or cannot be applied under technical relevant conditions.

Optical technologies are a crucial part of current research in this field, since light matter interactions can be used to obtain interfacial information and allow for investigations at short time scales. In particular, technology that is based on second-harmonic generation (SHG), hyper-Rayleigh scattering (HRS), sum-frequency generation (SFG) as nonlinear optical spectroscopies are well-established in the Institute of Particle Technologies (LFG).

Our research activities address colloidal particle interfaces where particle-particle interaction and interactions of the particle surface with molecules (Fig. 1) and ions (Fig. 2) are controlled by the molecular structure of the particle surface as well as by the accumulation of charge at the interface. The latter give rise to distinct electric double-layer structures and influences the stability of a colloidal solution significantly. Here, inherently surface sensitive SHG combined with nonlinear Mie theory can reveal polar ordering of molecules and interfacial charging which results in considerable changes in the scattered second-harmonic (SHS) light. As a research highlight in this field it should be pointed out that we succeeded to perform the first SHS studies of nanoparticles which have not been previously modified with a nonlinear active dye (Fig. 3 & Fig. 4) [1]. This was made possible due to considerable efforts not only for the application of optical technologies as an analytical tool, but also for method development within the LFG. Besides the above mentioned interactions of colloidal suspensions, effects of particle shape [2] and size [3-5] are also important aspects in particle research and technology and are, therefore, subject to current research.

Shematic SHG setup Scattering profile 0,6µm PS
Fig. 1: Adsorption of SHG active malachite green molecules on polystyrene nanoparticles. Fig. 2: Screening of surface charges by sodium ions leads to a substantial changes in SHG intensities

Here, SHS experiments of polystyrene particles show that the SH scattering profiles exhibit distinct angular features that are strongly dependent on the particle size (Fig. 4) and the molecules and their orientation at the particle interface [6]. Furthermore, second-order surface susceptibility contains the information about species, number density, order and orientation of the molecules at the particle interface.

Shematic SHG setup
Fig. 3: Goniometer setup for angle-resolved SHG measurements [1]
Scattering profile 0,6µm PS
Fig. 4: Second harmonic scattering profiles of polystyrene particles of different size

Publications:

[1]

Schürer, B.; Wunderlich, S.; Sauerbeck, C.; Peschel, U.; Peukert, W. (2010): Probing colloidal interfaces by angle-resolved second harmonic light scattering. In: Phys. Rev. B 82 (24), S. 241404.

[2]

Schürer, B.; Elser, M. J.; Sternig, A.; Peukert, W.; Diwald, O. (2011): Delamination and Dissolution of Titanate Nanowires: A Combined Structure and in Situ Second Harmonic Generation Study. In: J. Phys. Chem. C 115 (25), S. 12381–12387.

[3]

Schneider, L.; Schmid, H. -J; Peukert, W. (2007): Influence of particle size and concentration on the second harmonic signal generated at colloidal surfaces. In: Appl. Phys. B 87 (2), S. 333–339.

[4]

Schürer, B.; Peukert, W. (2010): In situ surface characterization of polydisperse colloidal particles by second harmonic generation. In: Particul. Sci. Technol. 28 (5), S. 458–471.

[5]

Schürer, B.; Hoffmann, M.; Wunderlich, S.; Harnau, L.; Peschel, U.; Ballauff, M.; Peukert, W. (2011): Second harmonic light scattering from spherical polyelectrolyte brushes. In: J. Phys. Chem. C 115 (37), S. 18302–18309.

[6]

Wunderlich, S.; Schürer, B.; Sauerbeck, C.; Peukert, W.; Peschel, U. (2011): Molecular Mie model for second harmonic generation and sum frequency generation. In: Phys. Rev. B 84 (23), S. 235403.

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