Calibration of a nano-scaled near field sensor for the imaging of the local heat transfer quantitatively

We report on a precise in situ procedure to calibrate the heat flux sensor of a near-field scanning thermal microscope [1, 2]. This microscope is based on a scanning tunneling microscope which is equipped with a sub-micro sized, coaxial thermocouple with a tip apex radius of about 30 nm. The microscope is able to measure lateral changes in heat transfer with a resolution of about 7nm [3, 4]. The calibration procedure presented is based on modulation technique and utilizes a hot wire method to build a well-defined heat reservoir. This reservoir is coupled thermally via near-field interactions to our probe exactly in the same way as it is coupled in an actual measurement. This means at tunnel distance in ultra-high vacuum. The heat flux leaving the reservoir is determined by measuring the mean temperature of the hot wire and modeling the actual temperature distribution in the wire by the one dimensional heat diffusion equation. Thus the sensor's conversion relation can be precisely determined which is the thermopower, generated in the sensor's coaxial thermocouple, in dependence on the thermal flux leaving the reservoir through the sensor. The achieved accuracy is about 8% including the different precisions of each part of equipment used in the calibration procedure. With such a calibrated sensor we perform quantitative measurements of the heat flux between a metal coated near-field scanning thermal microscope tip and planar samples at nanometer distances across a vacuum gap.