Correlation spectroscopy with photons and electrons - insight into medium-range order in amorphous materials

Condensed Matter lunchtime seminar

Correlation spectroscopy with photons and electrons - insight into medium-range order in amorphous materials

  • Event time: 1:00pm
  • Event date: 3rd October 2005
  • Speaker: J. Murray Gibson (Argonne National Laboratory)
  • Location: Room 2511,

Event details

Coherent radiation scattered from small sample volumes gives rise to ''speckle''. The time dependence of speckle can be used to extract spatio-temporal correlations, such as diffusion coefficients, from complex disordered systems. Recent applications of x-ray photon correlation spectroscopy (XPCS) has extended the capabilities of light-optical techniques (PCS) to smaller length scales,e g. measurement of polymer viscocity in ultra-thin films. In our work, we carry out a speckle correlation measurement which is the spatial analog of PCS. Fluctuation microscopy, as we call it, reveals higher-order spatial correlation functions. For example, whereas simple diffraction meaures the two-body correlation function, the speckle variance connects to the 4-body correlation function. Experiments with trannsmission electron microscopy of amorphous materials has revealed the sensitivity of such measurements to medium-range order (~5d < L < ~50d, where d is the pair spacing and L is the correlation length). Our studies have revealed that amorphous silicon is typically grown in a very topologically ordered state (''paracrystalline'') but relaxes on gentle annealing to a random network state. Both structures have essentially the same diffraction patterns, but their structure and properties are very different.

Using x-rays instead of electrons for fluctuation microscopy is ideally suited to studying strutucres on the nano-scale, such as co-polymers. We have set-up such a capability at the APS, and I will describe preliminary results and future applications to nanotechnology and soft materials.

This is a weekly series of informal talks given primarily by members of the soft condensed matter and statistical mechanics groups, but is also open to members of other groups and external visitors. The aim of the series is to promote discussion and learning of various topics at a level suitable to the broad background of the group. Everyone is welcome to attend..

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