SYNCHROTRON-RELATED METHODS OF INVESTIGATION OF NANOSTRUCTURES

Václav Holý,

Department of Condensed Matter Physics, Charles University in Prague, Czech Republic[1]

    Technologically relevant parameters of nanostructures (nano-crystals, quantum dots, nano-rods, etc.) substantially depend on their structural parameters, especially on sizes and their local chemical composition. Since the nanostructures grown by the “bottom-up” methods exhibit statistical fluctuations of their parameters, the claim on a reliable structural characterization is even more important.

    The characterization of nanostructures by x-ray based methods is a demanding task, due to the small interaction volume (and therefore very weak sample response). New experimental possibilities appeared with the advent of dedicated synchrotron sources two decades ago and the x-ray based methods belong now to almost routine characterization tools. In a standard x-ray scattering technique one measures the reciprocal-space distribution of x-ray intensity scattered from a large ensemble of nano-objects. Due to lack of phase information, the connection of the measured data with relevant structure parameters (electron density for instance) is only indirect. This fact requires a time consuming fitting procedure to an a-priori known structure model.

    The application of synchrotron radiation made it possible to develop further this standard measuring concept, namely (i) by measuring the energy-dependence of the scattered intensity, (ii) by limiting the size of the primary x-ray beam so that a single nano-object can be investigated and directly depicted, and (iii) by using various sample environments during the measurement, enabling to perform in-situ studies. From the energy-dependence of the scattered intensity it is possible to study the local chemical composition in the nano-objects directly, using the anomalous scattering effect, which gives the missing phase information. The fine structure of this dependence above the absorption edge of a chosen element represents a very promising combination of x-ray scattering and absorption spectroscopy. Using a very narrow incident beam it is possible to irradiate a single object by a fully coherent primary wave, which enables to analyze fully the structure of this object by numerical reconstruction of the phase of the scattered wave (phase recovery algorithm). In-situ x-ray scattering studies are used for the investigation of the growth kinetics on nanostructures and for the phase-transition investigations.In the talk, an overview will be given of basic and advanced x-ray related methods with several application examples and including a discussion of advantages and weak points of the methods.