MRSEC X-Ray Facility

Sekar Thirunavukkarasu
577-0529
A-529

Main description

X rays - discovered by W.C. Rontgen in 1895 - have become established as an invaluable probe of the structure of matter. Progress in understanding the interaction between x rays and matter and of exploiting X rays experimentally has been increasing steadily since its discovery. In more recent times this has lead to unraveling the structure and morphology of compounds of increasing complexity like self-assembling molecular systems, including functional units of bio-organisms like DNA and proteins. MRSEC has funded within the Department of Polymer Science and Engineering an X-Ray Facility with several modern instruments for x-ray scattering (small, intermediate and wide angles), powder x-ray diffraction, and x-ray reflectivity studies of thin films.

Instruments

Small and wide-angle x-ray scattering (SAXS-WAXS)

For SAXS and WAXS a versatile instrument is available from Molecular Metrology Inc. (presently sold as Rigaku S-Max3000). It uses a high-brilliance microsource of x rays matched to an optical system of two parabolic graded multilayers (Osmic Maxflux). This leads to a low-divergence beam of monochromatic CuKalpha radiation with a size of about 0.4 mm diameter at the sample position. The sample chamber is equipped with a sample changer and a variable temperature holder. The SAXS intensity is collected by a two-dimensional gas-filled wire array detector at a distance of about 1500 mm from the sample. A beamstop of 4 mm diameter in front of the detector has in its center a photodiode allowing monitoring the intensity of the direct beam. WAXS is performed using an image plate positioned in the sample chamber at the desired distance. The image plate (maximum resolution about 50 micrometers) has a hole in its center allowing measuring SAXS and WAXS simultaneously. The whole system is evacuated. The accessible angular range for SAXS corresponds to dimensions between about 4 and 100 nm.

Setup for x-ray scattering at intermediate angles

An in-house-designed setup is available for scattering at intermediate angles, which is increasingly important for modern self-assembling systems. The x-ray beam is shaped by a set of graded multilayers (Osmic Maxflux) connected to a Rigaku 1.2 kW finefocus rotating anode generator. The optical system focuses the x-ray beam at the position of the image plate detector, leading to high-quality narrow x-ray peaks. The sample position can be varied from close to the detector (100 mm) to a larger distance of 435 mm. This provides an angular range corresponding to dimensions between about 0.3 nm and 17 nm, overlapping with SAXS as well as WAXS.

Powder x-ray diffraction

For powder x-ray diffraction a PANalytical X'PertPert Material Research Diffractometer is available. The essential feature of the powder method is a narrow beam of monochromatic x-ray radiation impinging upon a randomly oriented powder in which all possible crystallographic planes are available for Bragg reflection. In the present system the x rays are emitted from a narrow line focus, perpendicular to the vertical scattering plane. The 'line' is positioned at the focal point of a single parabolic graded multilayer mirror. This leads to an x-ray beam with a height of 1.2 mm and low-divergence (<= 50 mdeg) in the scattering plane, illuminating the sample in reflection geometry. The system allows for automatic changes of multiple samples and a 2Theta-range of 0.5-150 degrees. A solid-state multiple-strip detector (X'celerator) is used, which has a resolution of 80 mdeg, mainly determined by the pixel size.

X-ray reflectivity of thin films

The PANalytical system mentioned above can be modified with a different sample stage and a Xe-filled proportional counter to allow for x-ray reflectivity measurements. The incoming beam is limited in height to 75 micrometers. In that situation the footprint of the beam along the sample is for an incident angle of 0.2 deg. about 22 mm. The instrument is routinely used to determine film thicknesses from the spacing between maxima (Kiessig fringes) that originate from constructive interference of x-rays reflected at the two film interfaces. A full analysis of the reflectivity curve provides detailed information about the density profile projected along the normal to the film.