2, Korea Advanced Institute of Science and Technology, Daejeon, , Korea (the Republic of)
3, University of California, Irvine, Irvine, California, United States
Nanoscale thermometry is vital for the experimental characterization of sub-continuum thermal transport, such as nanoscale solid conduction [Park et al., Journal of Heat Transfer (2008)], near-field thermal radiation [Kim et al., Nature (2015)], and heat transfer in atomic junctions [Mosso et al., Nat. Nanotech. (2016)]. At such small scales, thermal transport greatly deviates from macroscale observations. Therefore, to understand the underlying physics of sub-continuum thermal transport, nanoscale thermometry should be implemented to quantify both the temperature gradient and heat transfer rate for geometric constrictions typically much smaller than 1 μm. While advancements in nanothermometry have enabled groundbreaking experimental research in sub-continuum thermal transport, no work has combined temperature feedback control with nanothermometry to actively control the local temperature of a nano-hotspot.
There are two major challenges when conducting experiments without local temperature feedback control: (1) undesirable variations in the background heat transfer to the surrounding environment during an experiment and (2) temperature varying discrepancies in the nanoscale thermal resistance. To address these challenges, we report the active control of a local nano-hotspot temperature for accurate nanoscale thermal transport measurement. To this end, we have fabricated resistive on-substrate nanoheater/thermometer (NH/T) devices that have a sensing area of ~350 nm × 300 nm, which forms a local nano-hotspot upon Joule heating. The NH/T devices operate with a 4-probe detection scheme: an electric current flows through the outer electrical leads to Joule heat the nano-patterned platinum (Pt) wire, the local electrical resistance is computed by measuring the voltage drop between the inner electrical leads. The key advantage of using resistive nanoscale thermometry is its simultaneous use of the sensing area as both a heater and thermometer.
To examine the controller’s integration with the NH/T device, feedback-controlled temporal heating and cooling experiments of the nano-hotspot reveal that the integral gain plays a dominant role in the device's response time for various temperature setpoints. The NH/T device with temperature feedback control is then applied to a local tip-induced cooling experiment, where a silicon atomic force microscope (AFM) tip is scanned over the NH/T’s nano-hotspot. The tip-induced solid conduction local cooling experiments are performed in both air and vacuum with optimized experimental parameters to separately identify the dominant modes of heat transfer: solid conduction, air conduction, water meniscus, and near-field thermal radiation. The obtained results demonstrate the precision controllability of a local nano-hotspot temperature and its application towards characterization of nanoscale thermal transport, which will provide insight to sub-contiuum heat transfer.