Nanoscale Imaging of Hot-Electrons Using Electron-Scanning Thermal Microscopy

Hot-carrier generation and transport under highly non-equilibrium conditions are pivotal for the operation of advanced clean energy technologies such as plasmonic photocatalysis and photovoltaics, as well as operation of high-frequency electronic and optoelectronic devices. This study aims to probe the spatial distribution of hot carrier generation, transport, and dissipation in nanoscale systems. We demonstrate a novel Electron-Scanning Thermal Microscopy (E-SThM) technique capable of mapping the electron temperature field with nanometric spatial resolution and sub-kelvin temperature resolution on conductive or semiconductor surfaces. The principle of E-SThM is as follows. A custom-fabricated scanning thermal probe is employed, where the probe tip is coated with a thin platinum (Pt) electrode, followed by deposition of an ultrathin (<5 nm-thickness) layer of aluminum oxide (Al₂O₃), to form a fixed tunneling barrier. Upon contact with a conductive surface, the E-SThM tip establishes a metal-insulator-metal tunneling junction with a fixed insulator thickness. When a bias voltage is applied across the tip-sample junction, it induces a tunneling current through the thin insulating film. The tip tunneling current exhibits an energy distribution that is primarily concentrated near the Fermi levels of both the sample and the probe. This concentration results in a characteristic double peak in the energy distribution of the current. An increase in the electron temperature within the sample leads to a broadening of the Fermi-Dirac distribution, consequently increasing the tunneling current. A calibration involving lattice (phonon) temperature measurements allows us to extract the electron temperature map on the surface of the sample. To demonstrate this technique, we measured the hot-electron temperature distribution on plasmonic Ag nanoparticles under resonant optical excitation and show that localized hot-spots with high local electron temperature exist on the nanoparticles. The proposed technique can provide microscopic insights on the magnitude and spatial distribution of non-equilibrium electrons in nanoscale systems and will be specifically applied to address key fundamental questions in plasmonic photocatalysis.