B.M. Foley, A.H. Jones, J.T. Gaskins, P.E. Hopkins
Laser Thermal Inc,
Keywords: thermal transport, nanoscale, 2D materials, phonons
Summary:2D materials offer unprecedented, often record setting thermal properties with seemingly robust potential to structurally and chemically manipulate phonon scattering and thermal transport. These phonon scattering events in 2D systems arise from the plethora of defects and interfaces that arise from both growth and post processing that also are routinely used to manipulate the 2D materials functionalities. The thermal transport properties of 2D materials at and around these defect phonon scattering sites, which often have length scales and spacings on the order of nanometers to 10’s of nanometers, are difficult to isolate and measure individually with thermal measurement techniques. For example, optical based techniques for measuring thermal properties of 2D materials (e.g., Raman, TDTR) are ultimately diffraction limited and thus restricted to areal spatial resolution on the order of single micrometers. Techniques using lasers coupled with AFM-tips (e.g., Nano-FTIR) have shown promise in achieving sub-diffraction limited areal resolution to qualitatively interrogate optically excited surfaces, but lack the opto-thermal transduction power afforded by thermoreflectance-based methods to ensure accurate measurement of local temperature and thermal wave modulation. Here, we introduce a novel platform by Laser Thermal, Inc., capable of characterizing the thermal properties of 2D materials with ~10 nm areal spatial resolution. Thermal maps of CVD-grown molybdenum disulfide (MoS2) and exfoliated hexagonal boron nitride (hBN) flakes (both on SiO2/Si supporting substrates) are presented, highlighting both (a) the higher in-plane thermal conductivity of the hBN compared to MoS2, as expected per the literature, but more importantly (b) the direct visualization of how the thermal resistance increases near wrinkle defects, adlayer nucleation sites, and flake boundaries. These local increases in resistance are attributed to the impact of the defect on phonon transport. As a result, this new capability enables the direct visualization and estimation of the length scales over which various defect structures exert influence over phonon transport in these 2D materials.