Time Domain Reflectance for Thermal Conductivity of Electronic Materials

Electronic materials represent a vast category with wide-ranging properties. Though definitionally, their electronic properties are of interest, study of their thermal properties provides additional important information. Whether to find materials with desirable thermal conductivity—high to dissipate heat or low to limit heat transport—or because the mechanisms underlying heat transport provide insight into the fundamental physics of the material, thermal characterization allows better understanding of such materials./nIn the study of the thermal properties of materials, optical pump-probe methods are a key tool. Laser-based measurements allow probing of small areas without requiring microfabrication, and the use of pulsed lasers and modulated beams allows measurement of high-speed effects and so small sample areas. Time domain thermoreflectivity (TDTR), which uses modulated pulsed laser beams, stands as a key tool for measurement of the thermal properties of both bulk and thin film samples. This work seeks to provide a detailed introduction to the technique and the mathematical analysis required for such measurements, apply TDTR to interesting materials systems, and push beyond the limits of traditional TDTR with the development of a transducerless time domain reflectance (tTDR) technique which uses the same equipment./nStudy of thermal properties can provide new insights into the fundamental properties of materials. Metallic vanadium dioxide nanobeams have a much lower thermal conductivity than would be expected given their electrical conductivity, implying that electrons are more effective at carrying charge than heat. This result is not always seen in other sample geometries, and TDTR measurements allow characterization of thin film samples, allowing further study. Unfortunately, the samples measured in this work did not provide high enough electrical conductivity to draw conclusions about electronic thermal transport, but this stands as an interesting line of investigation./nOne category of electronic materials of increasing interest is that of two-dimensional materials, whose ultra-thin nature offers new properties and applications. However, it also makes study of their thermal properties challenging and makes traditional TDTR infeasible. By contrast, tTDR is well suited to characterize their properties, and in this work initial measurements were made on suspended molybdenum disulfide. In addition to the fundamental properties of the materials, other effects can be engineered. For example, creating bilayers with twists between layers causes the development of moiré patterns which have novel properties. Their in-plane thermal conductivities are not well characterized, and tTDR provides an avenue for changing that. Preparation of such samples is very challenging, thus far preventing systematic study as a function of twist angle, but initial measurements demonstrated that tTDR is an applicable tool for such systems./nTime domain reflectance measurements, both in the form of TDTR and tTDR, are powerful tools for the characterization of a wide range of materials, including many electronic materials. This work seeks to offer insight into and expand their applications.

Advisor: Wu, Junqiao