We are continuously developing new sub-micron sized and very high resolution sensors, to improve the measurements of near field heat transfer and quantized thermal conduction. The principle of operation of our sensors is currently based on the thermo-electrical effect (Seebeck Effect).
If a metal is heated at one end, electrons (on average) move from the hot to the cold end due to diffusion processes. The accumulating electrons build up an opposing electric field, which reduces the thermal diffusion current to zero at a material-dependent voltage (related to the Seebeck coefficient). Connecting two metals with different Seebeck coefficients at one end and keeping the temperature at the "loose ends" at a constant value allows for the measurement of the relative voltage difference between the two electric fields when a temperature change is applied to the connected end(s) of the metals. This voltage difference is what we refer to as a thermo-voltage. Through special preparation techniques, such a thermocouple is embedded in a scanning tunneling microscope tip and allows for the simultaneous measurement of heat transfer (by thermo-voltage) and the sample topography (via tunneling current)
Development of PASA Tips
One enhancement of our pervious coaxial thermocouple tips are the so called PASA tips. Onto the existing thermocouple (platinum-gold), an insulating layer of silicon dioxide is evaporated, and then another layer of gold (PASA is an acronym of this layer structure). The tunneling current then only runs through the outer gold layer. This decouples the tunneling current from the thermovoltage measurement. Another advantage is that the layered structure protects the thermocouple from interfering signals, similar to the design of a coaxial cable.
Development of Sphere Probes
The so-called Derjaguin-Approximation, sometimes referred to as the Proximity-Force Approximation, is an approximation which can be used to describe spherical tips analytically in the theoretical calculation of the heat transfer between a sphere and a plane. In such a geometry, the heat transfer scales with the square of the sphere radius. A sensor with a larger radius therefore has a reduced spatial resolution, but its increased interaction crosssection results in higher sensitivity when measuring the distance-dependence of the heat transfer (which can also be measured to greater distances). To avoid any losses in heat transfer resolution, we have glued micrometer-sized spheres to our coaxial thermocouple sensors, which sounds a whole lot easier than it turned out to be.
Development of Superconducting Tips
We also develop sensors using superconducting materials, which promise further significant improvement of the heat transfer resolution at low temperatures. Instead of a gold outer layer, which also provides the tunneling contact, a superconducting film is used. This film can then be operated at the transition temperature between normal- and superconductivity, which means that a very small temperature change will cause a significant change of the electrical resistance of the sensor.