Luminescence is a strongly temperature-dependant effect. Usually, light emission diminishes when T increases. We have developed a probe that uses a small fluorescent particle as a temperature sensor. This new Scanning Thermal Microscope (SThM) allows to obtain a very good lateral resolution in the order of the particle size.
The materials we use (Er/Yb codoped particules) have a very interesting property. Two of their emission lines (at 520 and 550nm) come from levels that are in thermal equilibrium. The fluorescence intensity ratio is directly linked to the temperature of the particle by a low of the form :
See for instance:
Therefore, the knowledge of the fluorescence intensity ratio allows to determine the temperature.
We are currently using the technique to observe the heating of electronic devices and nanoheaters, and to study the heat transfer mechanisms at the sub-micron scale. As examples, the images of Fig. 2 show the topography and the temperature of a 100nm wide Ti nanowire run by an electrical current. Similarly, images of Fig.3 show the topography and the temperature map of a titanium microstripe in which small constriction have been made. Hot spots are visible at every constriction where the current density is high.