Optical probing and control of heat propagation at the nanoscale
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
The transport of heat on the nanometer scale is of central importance for the continued miniaturization of electronic and optoelectronic components. The physical description of the various transport mechanisms and their complex interaction represents a major challenge. In particular, the efficiency of radiative heat transport and its range is difficult to grasp due to the broadband nature of heat radiation. At room temperature, the wavelength of the maximum spectral radiation density is approximately 10 µm according to Wien’s law. On shorter length scales, radiative heat transport therefore only takes place via evanescent fields in the near-field of the bodies involved, the decay lengths of which are in turn strongly dependent on wavelength. The goal of this project, conducted in collaboration with our partners at the University of Torun, Poland, was to demonstrate the optical probing of heat propagation and distribution at the nanoscale with a spatial resolution significantly smaller than the wavelength of light. Our approach was based on photoluminescence (PL) nanothermometry utilizing NaYF4:Er3+/Yb3+ core-shell nanocrystals (rare-earth ion nanocrystals, REINCs) as local temperature probes either deposited on the sample substrate or attached to a scanning tip. The detection principle exploits the pronounced temperature dependence of the REINCs PL spectra. In our project we successfully employed REINCs for nanoscale temperature measurements after careful calibration using an electrically heated microscope coverslip. We then probed the temperature at different distances to an electrically heated silver nanowire revealing longranged heat transport presumably mediated by the glass substrate. By positioning a sharp gold tip at different distances on top of a REINC we observed a surprisingly efficient radiative cooling by the tip with short- (< 50 nm) and long-ranged contributions. The temperature of REINIC directly attached to gold tips was also found to be strongly affected by goldmediated cooling, which can be attributed to efficient heat transfer and gold’s high thermal conductivity. For local temperature mapping we thus employed glass tips for which the substantially lower thermal conductivity prevents tip-induced cooling and with this the determination of false temperature values. Using a REINC attached to a glass tip, we achieved thermal microscopy on an optically heated 2D semiconducting WSe₂ monolayer with a spatial resolution of better than 150 nm and a temperature resolution of approximately 1 K. Unlike gold tips, attaching REINCs to glass tips proved challenging due to the difficulty in optimizing the tip-REINC interaction strength. As a result, we were unable to attach smaller RE- INCs, which could potentially provide even higher resolution. Notably, the short-range heat transfer contributions observed in our studies highlight the potential of this approach for temperature mapping with spatial resolutions below 50 nm.
Publications
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Single up-conversion nanocrystal as a local temperature probe of electrically heated silver nanowire. Nanoscale, 15(25), 10614-10622.
Wiwatowski, K.; Sulowska, K.; Houssaini, R.; Pilch-Wróbel, A.; Bednarkiewicz, A.; Hartschuh, A.; Maćkowski, S. & Piątkowski, D.
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„Near-Field Cooling and Local Temperature Sensing”, Invited presentation at conference TERS9, Belo Horizonte, Brazil, 06.11.2024
Achim Hartschuh
