Ultrafast Mapping of Strain Using Coherent X-Ray Diffraction
This colloquium lecture will discuss how nanoparticles differ in structure from the bulk and how these effects can be exploited in nanotechnology. The structure of nanoparticles is obtained by the Coherent X-ray Diffraction (CXD) method, being developed in my group. Coherent Bragg diffraction from crystalline particles allows access to strain fields which can be fully imaged in three dimensions.
Recent experiments will be described in which these patterns of strain can be influenced by changing the external chemical environment of the nanocrystals under investigation. A new development is a significant improvement in the image quality obtained by correcting for partial coherence. The method can be used with XFEL beams to probe the transient state of strain within crystals shortly after an optical excitation "pump". The resulting pattern of strain does not show the presence of a compression wave, but instead a uniform expansion. This supports a "two temperature" model in which an excited electronic state initially carries the pump energy, eventually transferring it to the phonons.
 M. Watari, R. McKendry, M. Voegtli, G. Aeppli, Y.A. Soh, X. Shi, G. Xiong, X. Huang, R. Harder and I. Robinson, “Differential stress induced by thiol adsorption on facetted nanocrystals”, Nature Materials 10, 862-866 (2011).
 J.N. Clark, X. Huang, R. Harder & I.K. Robinson, "High-resolution three-dimensional partially coherent diffraction imaging", Nature Communications 3, 993 (2012) .