Heat is transferred by radiation between two well-separated bodies at temperatures of finite difference in vacuum. At large distances the heat transfer can be described by black body radiation, at shorter distances evanescent modes start to contribute, and at separations comparable to inter-atomic spacing the transition to heat conduction should take place. We report on quantitative measurements of the near-field mediated heat flux between a gold coated near-field scanning thermal microscope tip and a planar gold sample at nanometre distances of 0.2–7 nm. We find an extraordinary large heat flux which is more than five orders of magnitude larger than black body radiation and four orders of magnitude larger than the values predicted by conventional theory of fluctuational electrodynamics. Different theories of phonon tunnelling are not able to describe the observations in a satisfactory way. The findings demand modified or even new models of heat transfer across vacuum gaps at nanometre distances.

We report on a precise in situ procedure to calibrate the heat flux sensor of a near-field scanning thermal microscope. This sensitive thermal measurement is based on 1ω modulation technique and utilizes a hot wire method to build an accessible and controllable heat reservoir. This reservoir is coupled thermally by near-field interactions to our probe. Thus, the sensor's conversion relation V(th)(Q(GS)*) can be precisely determined. V(th) is the thermopower generated in the sensor's coaxial thermocouple and Q(GS)* is the thermal flux from reservoir through the sensor. We analyze our method with Gaussian error calculus with an error estimate on all involved quantities. The overall relative uncertainty of the calibration procedure is evaluated to be about 8% for the measured conversion constant, i.e., (2.40 ± 0.19) μV/μW. Furthermore, we determine the sensor's thermal resistance to be about 0.21 K/μW and find the thermal resistance of the near-field mediated coupling at a distance between calibration standard and sensor of about 250 pm to be 53 K/μW.

In the field of scanning probe microscopy, great attention must be paid to the state of sample and probe with respect to unintentionally adsorbed molecules. There are many techniques for cleaning tips described in literature, among them the use of accelerated electrons as an energy source. So far, all of the setups described yielded either no or only indirect information about the probe's temperature reached during the cleaning procedure. The Near-Field Scanning Thermal Microscopy probe not only serves as scanning tunneling microscope tip, but also includes a thermosensor in the vicinity of the probe's apex. Since the tip's body mainly consists of glass, which has a softening point of 1100 K, it must not be heated excessively in order to prevent its destruction. The authors use electron bombardment for cleaning these unique sensors, while the thermosensor is used as feedback for an automated device which is controlling the procedure. Our findings reveal that probe temperatures of up to 1220 K can be reached for short periods of time without causing any damage. In this article, the authors describe the device as well as experimental data concerning the relation between the energies used for cleaning and the resulting temperature of the probe. The presented data might serve as an indicator for other setups where a direct measurement of the temperature of the apex is impossible.

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Nature Communications 8: Article number: 14475 (2017); Published: 15 February 2017, Updated: 25 June 2018 The original HTML version of this Article omitted the article number; it should have been ‘14475’. This has now been corrected in the HTML version of the Article. The PDF version was correct from the time of publication.