Effects of photoinduced charge-transfer on nanoscale crystal morphologies due to the resulting structural rearrangements can be directly visualized in space and time with ultrafast transmission electron microscopy. This is accomplished by operating an appropriately-modified transmission electron microscope in a pump-probe fashion analogous to all-optical ultrafast spectroscopic experiments. In this way, reversible structural motions occurring within a few hundred femtoseconds can be directly visualized with high spatial resolution. For structural dynamics that are irreversible on experimental time scales, the single-shot mode of operation is used. Here, I will discuss how transient structures were imaged following photoinduced electron transfer in single crystals of the kinetic phase (i.e., Phase I) of copper-tetracyanoquinodimethane [Cu(TCNQ)], and how the morphological changes in the low and high laser-fluence regimes were elucidated using both the stroboscopic and single-pulse modes of the microscope. Two distinct but mechanistically-linked dynamic processes having different rates are observed. The first is a fast, initial structural rearrangement due to photoinduced charge-transfer resulting in some Cu+ sites being reduced to neutral Cu. This is followed by a second slower step comprised of the growth of metallic Cu nanocrystals driven by minimization of surface energy. The overall mechanism involves photoinduced electron transfer from TCNQ anion-radicals to Cu+, subsequent nanoscale morphological changes, and thermally-driven growth of discrete Cu nanocrystals dispersed in an amorphous organic matrix.

Photoinduced structural rearrangements of kinetic phase Cu(TCNQ) crystals visualized with ultrafast electron microscopy

247th National Meeting (2014)

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