Gallium-doped zinc oxide semiconductor nanoparticles for plasmonic applications: a combined experimental and...

In this study, gallium-doped zinc oxide (GZO) nanoparticles were synthesized via a sol-gel approach followed by controlled thermal treatment, yielding nanocrystalline semiconductors with tunable Ga concentrations for advanced plasmonic applications. Structural and morphological analyses using transmission electron microscopy (TEM) revealed uniform grain distribution with particle sizes of ∼60-80 nm and the preservation of the wurtzite crystal framework. It further confirmed the successful substitution of Zn²⁺ by Ga³⁺ ions (2.5% doping), demonstrating effective doping without disrupting the lattice integrity. The analysis of the complex dielectric function, including the real (ε ₁) and imaginary (ε ₂) components, exhibited a crossover of ε ₁ from negative to positive values and a corresponding peak in ε ₂ within the near-infrared region, indicative of strong plasmonic resonance. Complementary electron energy loss spectroscopy (EELS) revealed a sharp, intense peak near ∼0 eV, confirming the presence of collective free-carrier oscillations. To rationalize these observations, first-principles density functional theory (DFT) calculations were performed, revealing an energy gap of 3.1 eV. We have observed an upward shift of the Fermi level toward the conduction band, consistent with enhanced free-carrier density due to Ga incorporation. The emergence of partially occupied conduction states, spanning -1.3 to -1.7 eV for GZO2.5 and GZO6.25, promotes intraband transitions, leading to a pronounced low-energy optical response and a robust epsilon-near-zero (ENZ) effect. Collectively, these results highlight the coexistence of semiconducting and plasmonic behavior in GZO nanoparticles, underscoring their potential for tunable optoelectronic devices, low-loss infrared plasmonics, and ENZ-enabled photonic applications.