This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We propose to image the structure of individual silver nanowires (AgNWs) using coherent x-ray diffraction and to make time-resolved measurements of changes in this structure following excitation of the wires with ultrafast laser pulses. The measurements will be made at BioCARS / Sector 14 taking advantage of their laser-pump x-ray-probe capabilities. Thermodynamic processes in nanoscale objects are of significant scientific and technological importance. Processes governing the dissipation of thermal energy at the nanoscale are known to differ significantly from those on macroscopic length scales and managing these processes is essential for example for heat management in integrated circuits optoelectronic devices (such as photovoltaics and solid-state lighting) and energy-generation systems. Melting temperatures in particular have been known for some time to drop significantly as the dimensions of objects are reduced from the bulk to the nanometer scale. At temperatures even below these depressed nanoscale melting points disordering or premelting processes are believed to occur but little is known about these structural changes. By heating a nanostructure with a short laser pulse and using time-resolved x-ray diffraction to observe subsequent changes in its structure it will be possible to gain mechanistic information about these nanoscale melting processes that has not previously been accessible. We will use chemically synthesized silver nanowires as model systems and make measurements on individual isolated nanowires in order to eliminate the effects of inhomogeneities in nanowire size and structure. The first goal of the experiments will be to demonstrate coherent x-ray diffraction from individual silver nanowires. The quantitative information obtained about single-nanoparticle diffraction will be essential for the progress of our larger research project which is aimed at measuring x-ray diffraction from single metal nanoparticles that are trapped and aligned in a laser-tweezers apparatus. Once we have demonstrated diffraction from a single AgNW we will measure changes in the diffraction pattern after the nanowire has been excited with an ultrafast laser pulse. Calculations will be used to correlate changes in the diffraction pattern to structural changes in the wire. This will be the first time-resolved melting measurement of a single nanoparticle and will provide key information about the structural changes following laser heating that cannot be obtained otherwise.
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