B.M. Foley, A.H. Jones, P.E. Hopkins, J.T. Gaskins
Laser Thermal,
United States
Keywords: scanning probe microscopy, nanoscale thermal transport, 2D materials
Summary:
As with most other functional properties of 2D materials, thermal energy transport via phonons can be impacted quite dramatically through their scattering by a variety of material defects. These phonon scattering events in 2D systems arise from the plethora of defects and interfaces that arise from both the growth parameters and post-processing steps that are often required 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 a few to 10’s of nanometers, are difficult to isolate and measure individually with the thermal measurement techniques available previously. For example, optical based techniques for measuring thermal properties of 2D materials (e.g., Raman, TDTR) are ultimately diffraction limited and thus restricted in-practice 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 approach called Nanoscale Thermoreflectance Microscopy (NTM), capable of characterizing the thermal properties of 2D materials with < 50 nm areal spatial resolution. Thermal maps of CVD and MBE-grown molybdenum disulfide (MoS2) are presented, quantifying the impact of the chosen growth method on material quality through direct visualization of how the thermal resistance increases near defects such as wrinkles/boundaries, adlayer nucleation sites, etc. These local increases in resistance are attributed to the impact of the defect in question on phonon transport. As a result, this new capability enables an estimation of the length scales over which various defect structures exert influence over phonon transport in these 2D materials, providing important thermal insight to guide future synthesis, processing and device-integration efforts using this important family of materials.