| Scope and Method of Study. An interesting attribute of monolayers of metal nanoparticles (NP) synthesized on semiconductor substrates is charging of NP induced by Fermi level difference. In this study, silver nanoparticles (AgNP) were synthesized on high conductivity Si wafers (p- and n-Si) employing the electroless reduction of AgNO3 by Si, as well as vapor deposition. Electric force microscopy (EFM) confirms the presence of NP charge and its expected variation (polarity and magnitude) with the Fermi level difference. Furthermore, Atomic force microscopy (AFM), optical absorption, and Mie scattering was performed to study the impact of this charging phenomenon on self-inhibiting growth and coalescence of AgNP during chemical reduction. The present work demonstrates that this selective charging of the AgNP can be exploited in charge-selective Raman scattering and fluorescence quenching of ionic fluorophores (fluorescein (-), rhodamine-6G (+) and acridine orange (+)).;Findings and Conclusions. Owing to Fermi level differences, AgNP synthesized on p-Si acquire a positive charge, while AgNP synthesized on n-Si acquire a negative charge. The polarity and magnitude of the acquired charge can be controlled by varying the Fermi level difference. Owing to the selective charging, during electroless deposition, positively charged AgNP display self-inhibiting growth and do not coalesce despite a few nm interparticle spacing, leading to strong electromagnetic interactions between the AgNP. As a result, well resolved hybrid plasmon modes develop, enabling the use of positively charged AgNP as SPR sensors / SERS substrates. In contrast, self-inhibition is not observed for negatively charged AgNP, due to which the nanoparticles coalesce, resulting in the loss of LSPR. Charged AgNP can be utilized as charge-selective SERS substrates for ionic analytes. Due to Coulombic interactions between the charged AgNP and ionic fluorophores, SERS is only observed when the analyte and the AgNP are oppositely charged. Charged metal NP also display charge-selective fluorescence quenching of ionic fluorophores. Again, owing to Coulombic interactions, fluorescence quenching is only observed for oppositely charged metal NP and fluorophores. |
