Temperature calibration of TEM specimen heating holders by isothermal sublimation of silver nanocubes.

Micro electro mechanical systems (MEMS) based TEM specimen heating holders exhibit excellent thermal stability and minimal specimen drift, which allows thermally-activated processes to be studied dynamically at high spatial resolution. The advantages of MEMS-based devices arise from the very small thermal masses of the sample studied, but this poses particular challenges for the precise measurement of specimen temperature. Previously, it has been proposed that the size-dependent sublimation behavior of Ag nanoparticles could be used to measure the specimen temperature by applying the Kelvin equation, but the effects of the capping ligands used in the nanoparticle synthesis and of electron beam heating have limited the application of such approaches. Here it is shown that for an appropriate choice of experimental parameters (nanoparticle size, loading, intermediate holding temperature, and illumination conditions) the sublimation of Ag nano-cubes can be used to measure the specimen temperature to an accuracy of ±5 °C, over the range 700-850 °C. The measurements are reproducible from area to area on the same MEMS chip, and from chip to chip of the same type. The values of specimen temperature obtained are consistently lower than the calibrated MEMS heater plate temperatures, and it is shown that this cannot be explained on the basis of random errors in the experimental measurements or systematic errors in the materials parameters used for the Kelvin equation analysis. It is proposed that this is instead due to the low thermal conductivity of the electron-transparent amorphous silicon nitride support membrane on the chip. As further evidence for this, it is shown that for a thicker crystalline Si support with a higher thermal conductivity, the magnitude of the difference is smaller. This approach could be extended to other temperature ranges by using nanoparticles of other metals with different vapor pressures and sublimation temperatures.