Noble metal nanocrystals: plasmon electron transfer photochemistry and single-molecule Raman spectroscopy

The excited electronic states of noble metal Au and Ag nanocrystals are very different than those of molecules. Ag and Au nanocrystal optical transitions (plasmons) in the visible can be so intense that they significantly modify the local electromagnetic field. Also, coherent elastic Rayleigh light scattering is stronger than normal electronic absorption of photons for larger nanocrystals. These two facts make Au and Ag nanocrystals ideal nanoantennas, in that they focus incident light into the local neighborhood of subwavelength size. Surface-enhanced Raman scattering (SERS), in which the Raman scattering rate of nearby molecules increases by many orders of magnitude, is a consequence of this nanoantenna effect. Metallic nanocrystals also have no band gap; this makes them extraordinarily polarizable. Their electronic transitions sense the local environment. An extreme case is the interaction of two 30 nm Ag nanocrystals separated by a 1 nm gap. Their mutual polarization completely transforms the nature of the metallic excited electronic state. Single particles have an excited state uniformly distributed throughout the interior, while the nanocrystal dimer has its excited state localized on the metal surface in the junction. This creates an electromagnetic "hot spot" in the junction, enabling the observation of single-molecule SERS. The fact that surface molecules are typically chemisorbed and exchange electrons with the metal has interesting chemical consequences. First, the enhanced Raman intensities are controlled by quantum mechanical coupling of the molecular lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) with the optically excited electrons in the metal. Second, charge-transfer photochemistry can result from metal plasmon excitation. In crystalline Ag nanocrystals the photochemistry quantum yield can be high because the nanocrystal surface dominates plasmon nonradiative relaxation. Colloidal Ag nanocrystals stabilized by sodium citrate build up a photovoltage under visible excitation, caused by irreversible "hot hole" photo-oxidation of adsorbed citrate anion. This creates a driving force for photochemical transformation of round 8 nm Ag seeds into 70 nm single-crystal disk prisms under room lights, in a novel type of light-driven Ostwald ripening.