For some years now, PTB has been using monochromatised synchrotron radiation in the X-ray range to be able to determine the dimensions of nanoobjects. In contrast to X-ray tubes, the wavelength can thereby be optimally adapted to the nanosystem to be examined. To determine the thickness of oxide layers on silicon samples via X-ray reflectometry, radiation in the range of the oxygen absorption edge at 2.3 nm is used for contrast enhancement, for example, as is the case for the silicon spheres of the Avogadro project.
To determine the diameter of na-noparticles in suspension via small-angle X-ray scattering, wavelengths below 0.5 nm are selected. Such radiation can easily penetrate the thin-walled sample cavity containing the suspension. Thereby, the exact wavelength is measured by means of back reflection of the radiation from a silicon crystal. The scattering angle can be determined by means of length measurements carried out with incremental displacement indicators. The scatter diagram developing in forward direction under small angles – of just a few degrees – is recorded by means of a large-area
X-ray detector. For spherical nanoparticles with not too broad a distribution of the diameter, concentric rings result. The radial integration yields an oscillating curve for the scattering intensity as a function of the momentum transfer, to which modelled scattering curves are fitted. From this, not only the mean diameter of the particles is obtained, but also the distribution width, as – contrary to microscopic procedures – the mean is taken via a large number of particles.
If small-angle X-ray scattering is used for surfaces under grazing incidence – as is the case for a number of EMRP projects which are just being launched in the industry call – also nanoparticles or nanostructured surfaces can be characterised.
michael.krumrey