CPM Seminar

The Dynamic TEM (DTEM): Ultrafast In Situ Studies of Microstructural Evolution in Various Materials

Mitra Taheri

Department of Material Science & Engineering
Drexel University

Optimization of the properties and performance of materials is a continual challenge, and necessitates control of microstructural evolution during thermo-mechanical processing. To gain a greater understanding of these processing steps, researchers often turn to in situ TEM. This technique provides insight into many aspects of mechanisms that are otherwise unclear in static experiments. Conventional in situ TEM is limited by video frame rate (30 Hz) time resolution. Ultrafast in-situ TEM (the DTEM) can potentially fill in gaps in the current understanding of various structural, chemical, electronic and magnetic properties in a myriad of materials. Pulsed photoemission of electrons is used to achieve nanosecond time resolution. A secondary laser is used to heat the sample, which is critical to any thermally activated in-situ experiment. Our current set-up achieves ~5nm spatial resolution at 10ns time resolution; with modifications, atomic spatial resolution at microsecond temporal resolution is possible. These capabilities allow for vast improvements of in-situ TEM studies limited by video rate.

Two case studies of thermally activated microstructural evolution are presented:

1. The development of Si nanowires produced by laser heating and ablation. A complete understanding of the origin of texture, morphology and defect origins during nucleation and growth of nanowires will have a great impact on the future of the use of NWs in electronic device fabrication.
2. Crystallization of amorphous Si by pulsed laser heating inside the DTEM. Optimization of polycrystalline Si for thin film transistor (TFT) applications is dependent upon grain size and defect concentration, both of which must be controlled. The DTEM allows for in situ characterization of the crystallization process, which is a major step toward achieving control in these devices.

Finally, use of DTEM for other materials, such as metallic glasses, phase change materials, is discussed. The results of these time resolved experiments are critical to the fields of grain boundary engineering, catalysts role in fuel cells for energy, and electronics, such as blue light emitting diodes and data storage.

This work was performed in part under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory and supported by the Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract DE-AC52-07NA27344.