Advances in electron microscopy have enabled the imaging of dynamics at atomic-level resolution. Nanocrystals present well-defined systems to study fundamental transient phenomena which are well defined in bulk materials. Cadmium chalcogenides (CdS, CdSe, and CdTe) are one of the most extensively studied and understood nanocrystals systems. These materials have been applied to a variety of optical and optoelectronic applications such as display phosphors, biological markers, and photovoltaics. Many first order phase transitions such as melting point and pressure-induced structural changes for these materials have been investigated through indirect spectroscopic methods; direct observation of phase changes in anisotropic structures have not been reported. In this work, we present the first direct observation of sublimation in CdSe nanorods. We observe the dynamics of sublimation in bright field TEM with atomic-level resolution at 10-7 Torr at four different temperatures. We found that vaporization occurs exclusively from the (001) and (00-1) planes. At a temperature just below the sublimation point, the electron beam heats the rod inducing vaporization from both ends in process which is limited by nucleation rather than vaporization. At temperatures much higher than the sublimation point, vaporization proceeds rapidly from only one end of the rod and does not appear to be nucleation limited. We believe that at these elevated temperatures, the sublimation behavior may be strongly influenced by a roughening transition.

High-resolution TEM of CdSe nanorod sublimation

247th National Meeting (2014)

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