M A Paesler | PubMed Health

PubMed2006

Application of bond constraint theory to the switchable optical memory material Ge2Sb2Te5.

A new extended x-ray-absorption fine structure spectroscopy study of local bonding identifies for the first time significant concentrations of Ge-Ge bonds in amorphous Ge2Sb2Te5. The study provides a new understanding of the local molecular structure of this phase-change material. Application of bond constraint theory indicates that the amorphous phase is an ideal network structure in which the average number of constraints per atom equals the network dimensionality. Analysis within this framework imparts new and significant insights concerning the nature of the reversible optically driven amorphous-crystalline phase transition of Ge2Sb2Te5.

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PubMed2001

Fundamental differences between micro- and nano-Raman spectroscopy.

Electric field polarization orientations and gradients close to near-field scanning optical microscope (NSOM) probes render nano-Raman fundamentally different from micro-Raman spectroscopy. With x-polarized light incident through an NSOM aperture, transmitted light has x, y and z components allowing nano-Raman investigators to probe a variety of polarization configurations. In addition, the strong field gradients in the near-field of a NSOM probe lead to a breakdown of the assumption of micro-Raman spectroscopy that the field is constant over molecular dimensions. Thus, for nano-Raman spectroscopy with an NSOM, selection rules allow for the detection of active modes with intensity dependent on the field gradient. These modes can have similar activity as infra-red absorption modes. The mechanism can also explain the origin and intensity of some Raman modes observed in surface enhanced Raman spectroscopy.

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