Thermal Control of Size Distribution and Optical Properties in Gallium Nanoparticles
Gallium nanoparticles (Ga-NPs) exhibit promising plasmonic properties spanning ultraviolet to infrared spectral regions, making them suitable for diverse nanophotonic applications. However, the synthesis of uniform and ordered Ga-NP arrays remains challenging due to size heterogeneity arising from coarsening during physical deposition. Here, we systematically investigate the influence of substrate temperature on the nucleation, growth, and homogenization dynamics of Ga-NPs formed via Joule-effect thermal evaporation on GaAs substrates. Atomic force microscopy and scanning electron microscopy reveal temperature-dependent transitions from broad, bimodal size distributions to narrow dispersed arrays within an optimal substrate temperature range of 300-350{\deg}C. Above this window, increased diffusion and desorption induce size increase, decreased density, and morphological relaxation manifested as NP shape flattening. Optical reflectance measurements identify distinct localized surface plasmon resonance (LSPR) modes whose energies are closely correlated with NP dimensions and aspect ratios. A figure of merit combining NP density and size uniformity quantifies optimal conditions at intermediate substrate temperatures, consistent with enhanced plasmonic quality factors derived from spectral LSPRs. In-situ post-deposition annealing experiments confirm Ostwald ripening as the dominant coarsening mechanism at elevated temperatures and highlight the necessity of rapid cooling and oxide shell formation to stabilize homogeneous arrays. Cross-sectional transmission electron microscopy with electron energy-loss spectroscopy validates the core-shell structure of individual Ga-NPs, quantifies temperature-dependent oxide shell thickening, and confirms the liquid metal character of the Ga within the core...