David Hellmann

Giant heat transfer in the crossover regime between conduction and radiation

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.

Dancing the tight rope on the nanoscale--Calibrating a heat flux sensor of a scanning thermal microscope.

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 sensors conversion relation V(th)(Q(GS)*) can be precisely determined. V(th) is the thermopower generated in the sensors 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 sensors 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.

Compact device for cleaning scanner-mounted scanning tunneling microscope tips using electron bombardment

Most scanning probe techniques rely on the assumption that both sample and tip are free from adsorbates, residues, and oxide not deposited intentionally. Getting a clean sample surface can be readily accomplished by applying ion sputtering and subsequent annealing, whereas finding an adequate treatment for tips is much more complicated. The method of choice would effectively desorb undesired compounds without reducing the sharpness or the general geometry of the tip. Several devices which employ accelerated electrons to achieve this are described in the literature. To minimize both the effort to implement this technique in a UHV chamber and the overall duration of the cleaning procedure, we constructed a compact electron source fitted into a sample holder, which can be operated in a standard Omicron variable-temperature (VT)-STM while the tip stays in place. This way a maximum of compatibility with existing systems is achieved and short turnaround times are possible for tip cleaning.

Publisher Correction: Giant heat transfer in the crossover regime between conduction and radiation

In this Letter, we report on quantitative measurements of the absolute near-field mediated heat flux between a gold coated near-field scanning thermal microscope (NSThM) tip and a planar gold sample at nanometer distances of 0.2 nm- 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. Additionally, we compare our data with different theories of phonon tunneling which might explain a drastically increased heat flux, but are found not to be able to reproduce the distance dependence observed in our experiment. The findings demand modified or even new models of heat transfer across vacuum gaps at nanometer distances.This corrects the article DOI: 10.1038/ncomms14475.

Giant heat transfer and its material dependence at the nanometer scale

Summary form only given. We report on quantitative measurements of the absolute heat flux between a gold coated near-field scanning thermal microscope (NSThM) [2] tip and a planar gold sample at nanometer distances of 0.2 nm-7 nm. After a precise calibration procedure we are able to measure the local heat transfer in a quantitative way [1]. We find an extraordinary large heat flux which is more than five orders of magnitude larger than black-body radiation and three orders of magnitude larger than the values predicted by conventional theory of fluctuational electrodynamics [3-10]. Furthermore, we have investigated the influence of thin films of a dielectric material on the near-field mediated heat transfer at the fundamental limit of single monolayer islands on a metallic substrate. Spatially resolved measurements by Near-Field Scanning Thermal Microscopy are presented which are showing a distinct enhancement in heat transfer above NaCl islands compared to the bare Au(111) film [11]. Experiments at this sub-nanometer scale call for a microscopic theory beyond the macroscopic fluctuational electrodynamics used to describe near-field heat transfer today. The method facilitates the possibility to develop designs of nanostructured surfaces with respect to specific requirements in heat transfer down to a single atomic layer. These findings open up the possibility for a local surface modification by means of local heating, e.g., chemical modification and heat assisted magnetic recording, on a scale of a few nanometers.

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

Summary form only given. We report on a precise in situ procedure to calibrate the heat flux sensor of a near-field scanning thermal microscope. 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 sensors conversion relation can be precisely determined which is the thermopower, generated in the sensors 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.

Theoretical description of a near-field scanning thermal microscope

In the last 10 years a near-field scanning thermal microscope (NSThM) has been developed in the group of Achim Kittel [1-3] at Oldenburg University. The NSThM allows for measuring near-field radiative heat fluxes in the ultrasmall distance regime of a few nanometer between a cold sample and the hot tip of the NSThM under ultrahigh vacuum conditions. In our presentation we will discuss the theoretical modelling of the radiative heat flux measured by the NSThM. We will show how we have applied Rytovs fluctuational electrodynamics [4] to model the signal of the NSThM for planar, structured or rough surfaces [5-8]. Our numerical results show a good qualitative agreement with the measurements of surface profiles [6]. However, a comparison of approximative and exact numerical theoretical results using proximity approximation [9] and a surface current approach [10, 11] with new measurements shows that there is a huge quantitative difference between the experimental data and the theoretical prediction [12]. This discrepancy indicates that Rytovs theory as it is commonly used is lacking an important heat flux channel or mechanism.

Investigation of the time evolution of STM-tip temperature during electron bombardment

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 probes 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 probes apex. Since the tips 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 reach...