X-Ray Diffraction Lab

(Person in Charge: Yishay Feldman)

The aim of the X-ray diffraction laboratory is the structural characterization of natural and synthetic solids including powders, thin films, single- and poly-crystals. X-ray diffractometry is a powerful, non-destructive technique capable of determining a number of parameters which characterize the structure of the irradiated material. The volume of a sample which can be studied at a given time depends on the dimensions of the X-ray beam (up to 15 x 15 mm) and on the penetration depth of the X-rays used. In the case of our generators (Cu target, ?av=0.154nm) the penetration depth reaches several tens of micrometers in inorganic materials and millimeters in organic materials. High intensity X-ray sources and high quality X-ray optics allow us to obtain a sufficiently strong diffraction signal even when the X-ray beam dimension is less than 200 µm.

Our lab is equipped with three X-ray generators manufactured by Rigaku Ltd. (Japan). The instrumentation includes a wide angle X-ray scattering (WAXS) camera affixed to a RU200 rotating anode generator (12kW) with an imaging plate for data acquisition and two theta-theta vertical diffractometers: a sealed tube generator-based ULTIMA III (2kW) and a rotating anode generator-based TTRAXS III (18 kW). Data acquisition for the latter two is computer controlled and data analysis is performed on separate platforms with Jade9.1 software. Search/match protocols use the Powder DIffraction File (PDF-4+) of the ICDD (International Center for Diffraction Data) on CD-Rom.

The two theta-theta diffractometers have a combination of modern XRD software and hardware which together permit almost all the structural characterization of solids that can be realized under laboratory conditions. These include:

• Qualitative and quantitative phase analysis (including evaluation of degree of crystallinity);
• Lattice parameter evaluation;
• Determination of microstrain and the evaluation of crystallite size;
• Crystal structure refinement by the Rietveld method;
• Texture determination using pole figure mapping;
• Residual stress measurement;
• Thin film reflectivity;
• High resolution measurements using a Ge (220) channel-cut monochromator;
• Capillary sample holders for use with materials that require a controlled environment and/or materials with severe preferential orientation on flat plate sample holders;
• Measurements in the low and medium temperature range (from -180° to 300° C); optionally, in an inert environment

It should be noted that the TTRAX III is the only diffractometer of its kind manufactured today – i.e. a rotating-anode theta-theta diffractometer- which makes possible the characterization of a horizontally mounted sample with 18 kW of X-radiation. Our instrument is the only one of its kind in Israel and there are only a few in Europe and in the US. Its high intensity X-ray beam and high quality X-ray optics allow us to obtain good powder diffraction patterns from gold films with thickness as small as 2 nm. Another example of the extraordinary capabilities of this instrument is the high quality XRD patterns obtained in transmission geometry from inorganic membranes with thickness of half a micron and lateral size of 200 microns.

Recently a polycapillary optic has been acquired which, when installed on the RU200 generator, will allow us to produce an X-ray microbeam (about 20 µm) with very high intensity (the manufacturers compare the flux density in the focal spot to that of a regular synchrotron beam) for WAXS measurements. Possible applications include characterizing the two-diimensional heterogeneity of both biological and inorganic samples as well as determining crystallite size in the range up to 1 micron. Another planned addition is a high speed, high sensitivity, high resolution solid state X-ray detector which will be used in conjunction with the existing variable temperature attachment in order to avoid long data acquisition times at low and high temperatures which may cause sample damage. It also will improve the angular resolution of measured XRD profiles up to 0.02° in 2-theta and will markedly reduce the turn-around time for heavy use of the Ultima (or TTRAX) diffractometer.

Light Scattering

Dynamic light scattering (DLS), also called quasi-elastic light scattering (QELS) or photon correlation spectroscopy (PCS), is a widely used, rapid and nondestructive technique to determine the hydrodynamic size and aggregational state of particles dispersed in solution. The particles may be biological macromolecules such as proteins or oligonucleotides, surfactant aggregates (micelles or vesicles), or inorganic nanoparticles. Both organic and aqueous solvents may be used. Information on translational diffusion of the particles is obtained from intensity fluctuations measured on the microsecond to millisecond time scale. The diffusion constant may be related to a hydrodynamic radius via the Stokes - Einstein relationship. When the dispersion contains a mixture of sizes, mathematical algorithms are used to extract a weighted distribution.

The department DLS instrument (Viscotek, model 802 DLS) is characterized by high sensitivity, fast and accurate temperature control ( 0-90oC), and low sample volume. It uses a 50mW fiber coupled laser diode light source, with wavelength 830nm, in order to avoid strong absorptions in the visible. The suitable size range of particles is 0.5nm - 0.1 micron in hydrodynamic radius.