Chalcogenide glasses contain S, Se, or Te alloyed with Group IV, and/or other Group V and VI elements.   The group IV elements have covalent four covalent bonds with their nearest neighbors, and group VI elements have two. Thus, a chalcogenide glass can be viewed as a covalent bond network with a degree of connectivity that is adjustable by varying the composition. That adjustability gives rise to materials with a wide variety of properties and metastable phenomena.

One consequence of the adjustable connectivity of the bond network is that chalcogenide glasses exhibit a phase transition from a floppy network to a rigid as the mean number of bonds per atom increases. A floppy network is one which supports zero-frequency vibrational modes - it will flow under a shear stress. A rigid network has no such modes. A classic theory by Thorpe and Phillips predicted a first-order phase transition between floppy and rigid networks at a mean atomic coordination of 2.4.¹ A series of very careful experiments by Punit Boolchand² has revealed that there is a third phase in between the floppy and rigid phases, dubbed the intermediate phase. Boolchand and others have suggested that this phase forms because of self-organization of the bond network at a length scale of a few nanometers.³

Many chalcogenides also exhibit a light-driven metastability called photodarkening.³ Photodarkening is a reduction in bandgap on exposure to light. It is associated with a volumetric expansion of the material, but not by any detectable rearrangement of nearest neighbor pair of atoms. It can be reversed by thermal annealing.

Chalcogenides have strong potential for applications in photonics. Photodarkening has been used to fabricate waveguides,⁵ and some chalcogenides show extremely high fast optical nonlinearities which could be used for all-optical switching,⁶ and a bandgap that can be matched to the mid-IR telecommunications wavelengths.

We are investigating the atomic structure of various chalcogenides using electron diffraction, electron spectroscopy, and fluctuation electron microscopy. Fluctuation microscopy in particular should provide new insight into the nanoscale self-organization responsible for the intermediate phase and the structural changes underlying photodarkening.

This work is a collaboration with David Drabold of Ohio University and John Abelson of the University of Illinois at Urbana-Champaign. Prof. Drabold's group will perform first-principles and reverse Monte Carlo simulations of chalcogenide glass structures and their electronic and vibrational properties. They were the first group to include FEM data in an RMC simulation.⁷ Prof. Abelson's group will grow chalcogenide thin films and characterize their electronic and optical properties.

This work is supported by the U.S. National Science Foundation under contract DMR-0605890.

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