Alternative transducer and optical pumping scheme for nanoscale thermal metrology and imaging

Heat dissipation is intrinsic to technologies ranging from micro- and power-electronics to energy-storage devices and thermoelectrics. The ability to characterize and understand thermal transport processes can help regulate the flow of heat and improve device functionality, efficiency, and stability, especially for those made of materials with poor thermal conductivities. Nonetheless, measuring thermal transport in low thermal-conductivity materials at a high spatial resolution is still challenging. A new approach is proposed here, in which the heat flow will be directly imaged by a high-speed camera with spatial resolution down to hundreds of nanometers. The proposed approach is laser-based, hence non-contact, and will take advantage of organic semiconducting materials, which are abundant and ubiquitously used in organic light-emitting diodes and displays. The organic semiconductors will serve as local temperature reporters of materials underneath, whose thermal conductivity can be assessed by observing how quickly the temperature of the organic semiconductor varies in time. The project will provide extensive training opportunities to graduate and undergraduate students in the construction and modification of comprehensive optical measurement systems with high spatial and temporal resolution. K-12 students from local public high schools will gain understanding and experience in state-of-the-art thermal metrology experiments. Defects, grain boundaries, among other types of inhomogeneities are characteristics of materials that are not single crystals or epitaxial films. To understand the influence of such imperfections on heat transport in emerging energy-related materials, the team will explore organic semiconductors as an alternative type of transducer for optical pump-probe thermoreflectance measurements. The morphology, optical properties, and thermal transport properties of several prototypical organic semiconductors will be thoroughly investigated over a wide temperature range. Meanwhile, a time-resolved, wide-field optical imaging setup will be constructed, which will permit efficient thermal excitation of organic transducers and their accurate temperature monitoring with nanosecond time resolution. The team will perform proof-of-concept experiments to spatiotemporally imaging the flow of heat in heterogeneous energy materials and in prototypical two-dimensional ferromagnetic materials. It is expected that the alternative transducer and imaging technique proposed here can complement the existing thermoreflectance techniques for the investigation of thermal transport properties of a wide range of technologically important materials used for energy conversion and storage, optoelectronics, and beyond. The ability to remove the transducer after their use is a useful feature for measuring unreproducible or high-value samples, and for failure analysis and quality control of semiconductor chips. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.