Xuezhe Zhou 2
, Julio Rodriguez Manzo 1
, Matthew Lim 2
, Norman Salmon 1
, Peter Pauzauskie 2 2
, University of Washington , Seattle, Washington, United States, 1
, Hummingbird Scientific, Lacey, Washington, United States
Bulk techniques that analyze thermal transformations in materials, such as calorimetry, give an averaged response of the complete transformation, missing distinct events in aggregates or inhomogeneous materials. Capturing different transformation paths is particularly important when studying nanoparticles or devices that have at least one nanometer-sized functional dimension, because it is known that transition temperatures are size dependent [1,2]. In-situ TEM heating holders provide a platform to observe thermal transformations with high spatial resolution—even atomic resolution. With this technique, distinct localized microstructural changes can be detected within a global transformation in a particle by particle basis.
The so-called in-situ bulk heating TEM holders comprise a small furnace compatible with 3mm TEM grids, but a newer approach uses microfabricated micron-sized heating elements to heat the sample under observation. This latter option has advantages: the power required to heat a sample is about 3 orders of magnitude lower; faster heating and cooling rates; and natural compatibility with microfabricated devices.
Here we present in-situ TEM observations of the heat-induced cubic to hexagonal phase transformation of NaYF4 nanocrystals (< 50nm wide) at temperature > 300°C. This phase transformation is accompanied by a depletion of material from the center of the nanocrystals. We discussed the observed hollowing effect by highlighting the importance of tracking individual particles during phase transformations. This is made possible using an in-situ heating TEM holder with microfabricated heating elements. The heating elements are patterned into a removable chip containing an electron-transparent window where the sample is deposited. The holder contains 9 electrodes that control the temperature and provide optional biasing capabilities to bias a sample (or device) during heating, if needed. Our observations were performed on a TEM operated at 200kV (JEOL JEM 2100) and equipped with a direct electron detection camera (Direct Electron DE-12) with 24 frames per second rate at full frame (4k × 3k pixels), which allows us to capture structural changes in discrete nanocrystals within 1 second.
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