Ag nanoparticles synthesized on n and p-type Si were shown to exhibit charge-selective surface-enhanced Raman scattering and fluorescence quenching. As revealed by electric force microscopy, the polarity and magnitude of the nanoparticle charge is controllable with the metal-semiconductor Fermi level difference and nanoparticle size. It is inferred that the Fermi level alignment is dominantly contributed by the charge-induced nanoparticle voltage. Nanoparticle charging also accounts for self-inhibition of coalescence during chemical reduction, allowing strong plasmon hybridization.

Photoactive yellow protein (PYP) is a small cytosolic photoreceptor that actuates the negative phototactic response in its host organism Halorhodospira halophila. It has an optical absorption maximum at 446 nm (blue light). We report an initial study of the photocycle of PYP at the single molecule level using “high enhancement factor” surface-enhanced Raman scattering (SERS)-active nanostructures with 514 nm laser excitation. The SERS-active “nanometal-on-semiconductor” structures are prepared employing a redox technique on thin germanium films, coated on glass slides. Single molecule spectra are observed in terms of sudden appearance of discernable Raman peaks with spectral fluctuations. The single molecule spectra capture protonation, photo-isomerization, and H-bond breaking - the steps that are instrumental in the photocycle of PYP. This is indicative of single PYP molecules diffusing to high-enhancement-factor SERS sites, and undergoing photo-cycle under 514 nm excitation.

The present work demonstrates selective charging of metal nanoparticles on semiconductor substrates, exploiting the Fermi level difference. Monolayers of surfactant-free silver nanoparticles (AgNP) were prepared by electroless reduction as well as vapor deposition on high conductivity n-and p-type Si substrates. The selective charging of the AgNP was investigated using electric force microscopy (EFM). As deduced from EFM, polarity and magnitude of the nanoparticle charge can be controlled by varying the Fermi level difference. The present study also reveals the impact of charging on the growth and coalescence of AgNP during chemical reduction. We also demonstrate that the monolayers of charged AgNP can be employed as charge-selective Raman scattering (i.e., surface-enhanced Raman scattering) substrates.