Nanoscale Heat Transfer Across a Point Constriction

Since its invention in 1986, atomic force microscopy (AFM) has become a major nanotechnology instrument for imaging and modifying structural, physical, and even chemical features of the surface with a sub-nanometer spatial resolution. Moreover, local heating of the cantilever tip, either through self-heating or laser-induced heating, has been intensively implemented in a number of nanomanufacturing and imaging applications that make use of local heating: see Fig. 1 for details. While these tip-based thermal applications have realized unprecedented quality of nanoscale imaging and manufacturing, they have also created strong demands on the fundamental understanding of nanoscale thermal transport across a point constriction with a nanometer scale gap or in contact. The spatial resolution of the tip-based thermal technologies is determined by the tip-induced localized temperature distribution. However, direct measurement of nanoscale thermal transport and local temperature distribution on the substrate has not been successful, mainly due to (1) poor spatial resolution and sensitivity of currently available thermometers; (2) incapability to precisely control the tip-substrate distance below 10 nm; and (3) difficulty in the precise temperature control of a highly local source and accurate temperature measurement. Moreover, very few measurement and modeling have considered more than one heat transfer mechanisms together in in-situ environments where real applications take place. This research aims to fundamentally understand nanoscale thermal energy transport across a point constriction and resultant non-uniform heated zone of the substrate. To this end, we will fabricate and characterize nanothermometers that have a spatial resolution as small as the tip radius, i.e., 10−50 nm. The local substrate temperature will be measured with the developed nanothermometer while the cantilever is precisely controlled in its temperature and tip position. Figure 2 illustrates the fabrication process of the nanothermometer and the experimental setup. To measure and compare various thermal transport mechanisms, we will perform the measurement in various operational and environmental conditions, including different vacuum conditions and various tip temperatures. A multiscale model that includes sub-continuum air conduction, solid conduction at the contact, and near-field radiation will be developed to understand the physics in tip-based thermal applications. The success of this project will provide, for the first time, the quantitative measurement of (1) nanoscale thermal transport between a heated tip and substrate across a sub-10 nm air gap and in contact; (2) temperature distribution of extremely localized heated zone near the tip; and (3) tip-to-substrate near- field thermal radiation and its spatial distribution. Proposed nanothermometers will provide a sensing probe size smaller than 50 nm, which will be the smallest thermometer ever made. Its fabrication and characterization will be well recognized in the thermal science and engineering community. Systematic approaches on the sub-10 nm gap control of heated cantilevers will be readily applicable to other AFM-based metrologies and technologies, such as SThM and nanomanufacturing. In addition, numerical modeling of coupled nanoscale thermal transport between the tip and substrate will advance the fundamental understanding of nanoscale heat transfer across a point constriction. This research is being supported by the National Science Foundation (CBET-1067441).

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