Force-induced acoustic phonon transport across single-digit nanometre vacuum gaps

Heat transfer between bodies separated by nanoscale vacuum gap distances has been extensively studied for potential applications in thermal management, energy conversion and data storage. For vacuum gap distances down to 20 nm, state-of-the-art experiments demonstrated that heat transport is mediated by near-field thermal radiation, which can exceed Planck's blackbody limit due to the tunneling of evanescent electromagnetic waves. However, at sub-10-nm vacuum gap distances, current measurements are in disagreement on the mechanisms driving thermal transport. While it has been hypothesized that acoustic phonon transport across single-digit nanometre vacuum gaps (or acoustic phonon tunneling) can dominate heat transfer, the underlying physics of this phenomenon and its experimental demonstration are still unexplored. Here, we use a custom-built high-vacuum shear force microscope (HV-SFM) to measure heat transfer between a silicon (Si) tip and a feedback-controlled platinum (Pt) nanoheater in the near-contact, asperity-contact, and bulk-contact regimes. We demonstrate that in the near-contact regime (i.e., single-digit nanometre or smaller vacuum gaps before making asperity contact), heat transfer between Si and Pt surfaces is dominated by force-induced acoustic phonon transport that exceeds near-field thermal radiation predictions by up to three orders of magnitude. The measured thermal conductance shows a gap dependence of $d^{-5.7\pm1.1}$ in the near-contact regime, which is consistent with acoustic phonon transport modelling based on the atomistic Green's function (AGF) framework. Our work suggests the possibility of engineering heat transfer across single-digit nanometre vacuum gaps with external force stimuli, which can make transformative impacts to the development of emerging thermal management technologies.Comment: 9 pages with 4 figures (Main text), 13 pages with 7 figures (Methods), and 13 pages with 6 figures and 1 table (Supplementary Information

Doctor of Philosophy

dissertationThe investigation of thermal transport across extremely small vacuum gap distances is of both practical and fundamental significance. Previous theoretical studies have predicted that in the near-field, or when the emitter-receiver separation is less than the thermal wavelength defined byWien's Law, thermal radiation can exceed Planck's blackbody limit by up to several orders of magnitude due to radiation tunneling of evanescent electromagnetic waves. This enhancement has been verified quantitatively by experimental efforts providing exciting opportunities for novel advancements of thermophotovoltaic energy conversion, passive radiative cooling, and nanoscale thermal management. While experimental findings for gap distances above 20 nm have consistently observed good agreement with near-field thermal radiation theory based on the fluctuational electrodynamics framework, the underlying physics of thermal transport at sub-10 nm gaps (i.e., extreme near-field regime) is still in significant debate. Furthermore, minimal experimental effort has investigated thermal transport between the gap and contact regimes to elucidate the radiation-to-conduction transition mechanism. To address these fundamental knowledge barriers, this dissertation is divided into four research objectives: (1) the theoretical evaluation of extreme near-field thermal radiation between a tip and planar surface, (2) development of a measurement platform enabling sub-10-nm tip-plane gap control, (3) quantitative measurement of nanoscale thermal transport using a feedback-controlled nanoheater, and (4) measurement of thermal transport between a tip and planar surface separated by single-digit nanometer vacuum gap distances. The dissertation begins with a literature survey and a general overview of the dissertation organization. Then, extreme near-field thermal radiation is evaluated theoretically using the finite dipole model based on the fluctuational electrodynamics framework. To experimentally evaluate nanoscale thermal transport at sub-10-nm gap distances, a measurement platform to precisely control the tip-plane separation and measure tip-scattered near-field radiation is developed and tested. Afterwards, the capability to quantify tipinduced thermal transport is verified through the combination of on-substrate platinum nanoheaters with temperature feedback control. Lastly, thermal transport between a silicon tip and platinum nanoheater is measured in the near-contact, asperity-contact, and bulk-contact regimes. The outcomes of this dissertation shed light on the fundamental physics of thermal transport across nanogaps as well as providing new avenues for its control

Hydrogel tip attached quartz tuning fork for shear force microscopy

Abstract This paper reports the first demonstration of hydrogel conical tip attachment onto quartz tuning fork (QTF) by using an elastomeric tip mold that is soft-lithographically replicated from an electrochemically etched tungsten wire. The tungsten tip of 10–100 nm radius obtained by time-controlled electrochemical etching is replicated with h-polydimethylsiloxane (h-PDMS) to make negative conical tip molds large enough to be used for QTFs. By approaching a QTF to the negative h-PDMS tip mold filled with polyethylene glycol-diacrylate (PEGDA), a PEGDA tip is attached to the QTF without using an adhesive. Then, the PEGDA tip attached QTF is employed for shear force microscopy for calibration grating and atomic layers of hexagonal silicon carbide and also compared with a silicon tip attached QTF. Exclusively for the PEGDA tip attached QTF, we demonstrate that the imaging tip could be regenerated multiple times to address issues associated with tip wear. In a stark contrast with conventional QTF probes in attachment of electrochemically etched metallic wires or microfabricated AFM cantilevers, photocuring of liquid phase prepolymer within a tip mold demonstrated herein allows adhesive-free and exclusive attachment of the imaging tip onto a QTF. The relatively large PEGDA tip enables facile operation during approach and engagement. Moreover, the organic and inorganic combination of imaging tip and resonating body offers regeneration of the imaging tip upon its degradation