Measurement of Heat Flow on Nanometer Length Scales

Modern progress in microengineering and nanofabrication have prompted renewed interest in accurate measurement of nanoscale thermal properties, such as thermal conductivity, thermal boundary resistance, and the influence of size and defect effects. Microscale thermal measurements, such as pump-probe thermoreflectance techniques, are typically used for the determination of these nanoscale properties. These techniques operate by focusing lasers to micrometer-sized spots on the sample surface, which results in a fundamental limit in spatial resolution. Questions also remain, such as the nature of the phonon scattering phenomena and the distribution of temperature gradients near interfaces, which would make a truly-nanoscale measurement technique valuable. Ultrafast Transmission Electron Microscopy (UTEM) techniques pair a pulsed laser with a transmission electron microscope (TEM) and have been used to visualize phonon propagation, however no studies have directly used these nanoscale observations as a measure of thermal properties.<br/>We present the development of a modulated laser-pumped electron-probe thermal scanning transmission electron microscopy imaging technique. This does not require the complicated photo-excitation of a field-emission gun as is used in UTEM. In this technique, a continuous-wave laser is used to thermally excite a sample inside a scanning transmission electron microscope (STEM), with the electron beam probing the localized temperature with atomic-scale spatial resolution. The laser is modulated, enabling lock-in signal acquisition with the high-angle annular dark field detector, allowing the observation of otherwise-undetectable thermally-induced changes.<br/>Several additional experimental considerations are also included, including mitigation of pump-induced defocus artifacts. We also perform extensive modeling to understand the mechanisms behind the acquired signal, such as localized strain and the distances of atomic vibration (Debye-Waller factors). We also use modeling to understand the measurement sensitivity under varying conditions, such as the presence of sample tilt, defocus, or aberrations.<br/><br/><br/>Electron microscopy supported by the U.S. Department of Energy, Office of Basic Energy Sciences (DOE-BES), Division of Materials Sciences and Engineering, and were conducted at the Center for Nanophase Materials Sciences, (CNMS), which is a DOE Office of Science User Facility.