Spectroscopic Real-Time Monitoring of Plasmonic Gold Nanoparticle Formation in ZnO Thin Films via Pulsed Laser Annealing

We demonstrate that pulsed laser annealing induces the formation of plasmonic gold nanoparticles in ZnO thin films, with the process monitored in real-time through pulse-by-pulse spectroscopy. Gold nanoparticles (smaller than 5 nm) were initially embedded in sputtered ZnO films on fused silica substrates. ZnO was selected as the matrix material for its inherent antimicrobial properties, which complement the plas-monic sensing functionality. Using 532 nm pulses with energies of 20 mJ, and a diame-ter of 2 mm from a Q-switched Nd:YAG laser, we induced annealing while simultane-ously monitoring transmission spectra via in-situ broadband spectroscopy. A plas-monic resonance dip emerged after approximately 100 laser pulses in the 530–550 nm region, progressively deepening and broadening with continued exposure. The process stabilized after roughly 800 shots, producing larger nanoparticles (40–200 nm diame-ter) with a significant fraction protruding from the ZnO surface. SEM analysis con-firmed substantial particle coarsening. Theoretical modeling supports these observa-tions, correlating spectral evolution with particle size and embedding depth. The pro-truding gold nanoparticles can be functionalized to detect specific biomolecules, of-fering significant advantages for biosensing applications. This approach offers the po-tential to optimize more uniform nanoparticle distributions with pronounced plas-monic resonances while providing superior spatial selectivity and real-time process monitoring compared to conventional thermal annealing.