Transcript Document
X-ray diffraction Antony D. Han Chem 750/7530 Feb. 21, 2006 Outline History and background information of Xray diffraction. Theory behind the equipment Challenges of applying traditional XRD to nano-technology Summary History of X-ray and XRD Wilhelm Conrad Röntgen discovered X-Rays in 1895. 1901 Nobel prize in Physics Wilhelm Conrad Röntgen (1845-1923) Bertha Röntgen’s Hand 8 Nov, 1895 A modern radiograph of a hand History of X-ray and XRD Radiographs like the ones in the last slide are simply shadowgrams. The X-rays either pass straight through or are stopped by the object. The diagram on the upper left illustrates the principle and shows a perfect shadow. In reality, a large fraction of the Xrays are not simply absorbed or transmitted by the object but are scattered. The diagram on the bottom left illustrates this effect and illustrates the fuzzy edge of the object that is produced in the image by the scattered X-rays. History of X-ray and XRD The first kind of scatter process to be recognised was discovered by Max von Laue who was awarded the Nobel prize for physics in 1914 "for his discovery of the diffraction of X-rays by crystals". His collaborators Walter Friedrich and Paul Knipping took the picture on the bottom left in 1912. It shows how a beam of X-rays is scattered into a characteristic pattern by a crystal. In this case it is copper sulphate. The X-ray diffraction pattern of a pure substance is like a fingerprint of the substance. The powder diffraction method is thus ideally suited for characterization and identification of polycrystalline phases. Max von Laue (1897-1960) Bragg’s Law Sir William Henry Bragg (1862-1942) William Lawrence Bragg (1890-1971) The father and son team of Sir William Henry and William Lawrence Bragg were awarded the Nobel prize for physics "for their services in the analysis of crystal structure by means of Xrays“ in 1915. Bragg's law was an extremely important discovery and formed the basis for the whole of what is now known as crystallography. This technique is one of the most widely used structural analysis techniques and plays a major role in fields as diverse as structural biology and materials science. Equipment Bruker D8 Analytical X-ray Systems Goniometer 2-Theta:Theta Setup Goniometer Theta:Theta Setup Fundamentals How it works? An electron in an alternating electromagnetic field will oscillate with the same frequency as the field. When an X-ray beam hits an atom, the electrons around the atom start to oscillate with the same frequency as the incoming beam. In almost all directions we will have destructive interference, that is, the combining waves are out of phase and there is no resultant energy leaving the solid sample. However the atoms in a crystal are arranged in a regular pattern, and in a very few directions we will have constructive interference. The waves will be in phase and there will be well defined X-ray beams leaving the sample at various directions. Hence, a diffracted beam may be described as a beam composed of a large number of scattered rays mutually reinforcing one another. Crystal systems, space groups, reciprocal lattice, Miller indices… Planes going through areas with high electron density will reflect strongly, planes with low electron density will give weak intensities. Sample preparation Single crystal X-ray diffraction The single crystal sample is a perfect crystal (all unit cells aligned in a perfect extended pattern) with a cross section of about 0.3 mm. The single crystal diffractometer and associated computer package is used mainly to elucidate the molecular structure of novel compounds. Powder (polycrystalline) X-ray diffraction It is important to have a sample with a smooth plane surface. If possible, we normally grind the sample down to particles of about 0.002 mm to 0.005 mm cross section. The ideal sample is homogeneous and the crystallites are randomly distributed. The sample is pressed into a sample holder so that we have a smooth flat surface. Data collection and analysis Collecting data: computer and software Analysis: ICDD database – Identification Structure refinement – GSAS Quantitative phase analysis – GSAS Novel structure – single crystal Applications Identification Polymer crystallinity Residual stress Texture analysis Challenge of applying to nanotechnology Traditional X-ray powder-diffraction techniques rely on the long-range order in crystals to produce sharp "Bragg peaks" in a diffraction pattern. By examining these Bragg peaks, which result from X-ray scattering, scientists can determine the material's atomic structure. But nanocrystals lack long-range order and often incorporate a large number of defects. As a result, their diffraction patterns are much more diffuse with few, if any, Bragg peaks. "This poses a real challenge to the traditional techniques for structure determination," -- Valeri Petkov of Michigan State. Breakthroughs for diffraction RTMS detection technology and the implementation of mono-capillary have reduced measurement times and minimum amounts of material required considerably. Many X-ray diffraction techniques are at disposal of the nanoscientist now for the structural characterization of the nanomaterials, such as high-resolution diffraction, reflectometry, smallangle X-ray scattering and line profile analysis. Summary X-ray diffraction provides a powerful tool to study the structure and composition of the materials which is a key requirement for understanding materials properties An X-ray diffraction system should not be missing in a modern laboratory for research on nano- and advanced materials. – www.panalytical.com Some useful links GSAS: http://www.ncnr.nist.gov/xtal/software/gsas.html ICDD: http://www.icdd.com/ CCP14 http://ccp14.sims.nrc.ca/ Diffraction tutorials http://www.uniwuerzburg.de/mineralogie/crystal/teaching/basic.html Paper addressed the problem Phase Transitions, 2003, Vol 76, Nos. 1-2, pp. 171-185