Nanoscale Heat Flow and Thermometry in Laser-Heated Resonant Silicon Mie Nanospheres Probed with Spatially Resolved Cathodoluminescence Spectroscopy

Many nanoscale technologies depend critically on precise knowledge and control of local temperature and heat flow, making robust nanothermometry essential for designing, optimizing, and ensuring the reliability of next-generation devices. In this work, we introduce a correlative method that combines laser excitation with scanning electron microscopy-based cathodoluminescence (SEM-CL) to probe photothermal effects <i>in situ</i> with nanoscale spatial resolution. We analyze the spatially resolved CL (30 keV) of resonant Mie modes in single silicon nanoparticles under continuous-wave laser irradiation (λ = 442 nm). The 235-250-nm-diameter crystalline nanospheres, placed on a Si₃N₄ membrane, show a strong electric quadrupole CL resonance of which the peak wavelength reversibly red-shifts upon laser-induced heating. A temperature of up to 585 ± 12 °C is derived from the spectral shifts for the highest laser power used (9.6 mW, ∼1 × 10⁶ W/cm² at the substrate). Numerical heat flow simulations show that the measured steady-state temperatures are consistent with a geometry in which heat flow occurs through a contact area of up to 100 nm², depending on laser power, between the Si nanosphere and the Si₃N₄ membrane. We postulate that this contact forms by reshaping of the particle-membrane geometry as it heats up in the initial phase of the laser irradiation, leading to an equilibrium geometry that results in the measured steady-state temperature. This work shows that CL of resonant nanostructures in combination with simulations can serve as sensitive probes of temperature and thermal conductivity. Spatially resolved CL nanothermometry in a SEM enables studies of nanoscale thermal properties of a wide range of device geometries such as electronic integrated circuits, surface catalysts, photovoltaic devices, and more.