  Advances in analytical scanning transmission electron microscopy (STEM) and
microelectronic mechanical systems (MEMS) based microheaters have enabled
in-situ materials characterization at the nanometer scale at elevated
temperature. In addition to resolving the structural information at elevated
temperatures, detailed knowledge of the local temperature distribution inside
the sample is essential to reveal thermally induced phenomena and processes.
Here, we investigate the accuracy of plasmon energy expansion thermometry
(PEET) as a method to map the local temperature in a tungsten (W) lamella in a
range between room temperature and 700 degC. In particular, we address the
influence of sample thickness in the range of a typical electron-transparent
TEM sample (from 30 nm to 70 nm) on the temperature-dependent plasmon energy.
The shift in plasmon energy, used to determine the local sample temperature, is
not only temperature-dependent, but in case of W also thickness-dependent in
sample thicknesses below approximately 60 nm. The results highlight the
importance of considering sample thickness (and especially thickness
variations) when analyzing the local bulk plasmon energy for temperature
measurement using PEET. However, in case of W, an increasing beam broadening
(FWHM) of the bulk plasmon peak with decreasing sample thickness can be used to
improve the accuracy of PEET in TEM lamellae with varying sample thickness.
