Enhanced Near-Field Heat Flow of a Monolayer Dielectric Island

Ludwig Worbes, David Hellmann, and Achim Kittel

Phys. Rev. Lett. 110, 134302 – Published 28 March 2013

Abstract

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. We present spatially resolved measurements by near-field scanning thermal microscopy showing a distinct enhancement in heat transfer above NaCl islands compared to the bare Au(111) film. Experiments at this subnanometer scale call for a microscopic theory beyond the macroscopic fluctuational electrodynamics used to describe near-field heat transfer today. The method facilitates the possibility of developing designs of nanostructured surfaces with respect to specific requirements in heat transfer down to a single atomic layer.

Received 27 November 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.134302

Authors & Affiliations

Ludwig Worbes, David Hellmann, and Achim Kittel^*

Institut für Physik, Carl von Ossietzky Universität, D-26111 Oldenburg, Germany

^*kittel@uni-oldenburg.de

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Vol. 110, Iss. 13 — 29 March 2013

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Images

Figure 1
(a) Schematics of the heat flow in the NSThM. The probe holder serves as a room-temperature heat bath for the probe while the sample is cooled down to 135 K. The overall temperature difference $Δ T_{0}$ causes a heat flow $\dot{Q}$ across the vacuum gap between the probe and sample. This heat flow is measurable as a slight temperature drop $Δ T_{probe}$ across the probe. (b) Schematic view of the probe in its Omicron-type tip holder; the right inset is the magnified cross section of the apex. At the contact point of Au and Pt a thermocouple is formed to measure $Δ T_{probe}$ . The pointed apex serves as a tunneling probe. (c) scanning electron microscope (SEM) micrograph of the fore end of the actual sensor. (d) High-resolution SEM micrograph of the tunneling tip. Images were taken at Carl Zeiss, Oberkochen (Germany).Reuse & Permissions
Figure 2
(a) Topographic STM image of NaCl monolayer islands on an Au(111) surface with a few atomic steps (image size $500 \times 500 {nm}^{2}$ , sample bias $- 600 mV$ , set point for the tunnel current 250 pA, sample temperature 145 K). (b) Temperature change $Δ T$ of the tip’s apex, acquired simultaneously with the STM image (image filtered at power line frequency $50 / 100 Hz$ ). (c) Horizontal line scans of both topography (upper curve) and temperature change $Δ T$ (lower curve), taken along the line labeled “LS” in (a). Please note that $Δ T$ is almost unaffected by the gold step, but increases clearly above the NaCl layer. (d) A three-dimensional representation of the topographic data. The false color image of the temperature change is used to color the surface enabling a visual assignment of topographic structure and heat transfer.Reuse & Permissions
Figure 3
$Δ T_{probe}$ measured versus the probe’s distance to a noncovered Au(111) surface, and to a gold surface covered by a NaCl monolayer. With $Δ T_{probe} \propto \dot{Q}$ , the heat flow $\dot{Q}$ is enhanced by a factor of 2 by a monolayer of NaCl compared to a Au(111) surface. For both measurements, the point of zero distance is defined by a tunneling current set point of 250 pA at a sample bias of $- 600 mV$ . Each curve is averaged over 10 measurements taken in the area depicted in Fig. 2; see the Supplemental Material [26] for the raw data.Reuse & Permissions
Figure 4
(a) Topographic image of an Au(111) surface with several atomic steps, partially covered by a NaCl monolayer, as recorded with a chromium-platinum thermocouple probe (image size $400 \times 500 {nm}^{2}$ , sample bias 600 mV, set point for the tunneling current 250 pA, sample temperature 135 K). (b) Temperature change image (image filtered at power line frequency).Reuse & Permissions
Figure 5
(a) STM topographic image of a partially NaCl-covered gold surface, as in Fig. 2a. (b) Temperature change of the tip’s apex, recorded simultaneously with the topographic data while being in tunneling contact with the sample. (c) Procedure employed for scanning at distances larger than the tunneling distance: after scanning a line of the sample in constant-current mode, the scanning trajectory is replayed with a specified distance offset. (d) Temperature change of the tip’s apex recorded with a distance offset of 1 nm [image (b) and (d) filtered at power line frequency]. (e) The tunneling current at that distance, recorded simultaneously with (d) is zero; only interference patterns at power-line frequency are visible (unfiltered data).Reuse & Permissions

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