An Optical Thermoreflectance Technique for Accurately Measuring Thermal Energy Storage of Nanoscale Materials

Current techniques for experimentally measuring heat capacity and latent heats, such as differential scanning calorimetry, are not suited for precise characterization of thin films. The majority of the difficulties in applying current techniques to nanoscale material systems is due to the large dimensionality of the heat flux source and monitor systems; the ‘depth’ resolution of thermal-based techniques is directly related to the width of these heat flux components (e.g., a 100 nm film requires thermometers on the order of hundreds of nanometers for precise measurements of thermophysical properties). However, the thermophysical properties of nanoscale materials can be drastically different than their bulk counterparts, increasing the necessity for accurate determination of their thermal storage properties and thus the need for new techniques. Further, due to the cost and time required to produce bulk quantities of novel material systems, the ability to measure the specific heat of materials at the nanoscale is of increasing interest to industries spanning from energy to pharmaceuticals.<br/><br/>In this work, we introduce a novel optical pump-probe technique for simultaneous measurement of numerous thermal parameters including thermal conductivity, volumetric heat capacity, and the latent heat of thin films on the order of tens of nanometers thick. More specifically, we implement a nanosecond-resolved moving boxcar averaging detection scheme to allow for monitoring of the transient temperature rise and decay of the sample surface with a focused laser “probe” beam (e.g., detector) as it is periodically heated by a high power “pump” beam (e.g., source). By measuring the laser-induced heating over multiple modulation frequencies, we are able to decouple the material’s thermal properties, and calculate its specific heat capacity, thermal conductivity, and thermal boundary conductance. We demonstrate the application of this new measurement technique by analyzing a variety of thin films including a collection of phase change materials.