| Abstract
| - We report studies of metal nanoparticle surfaces using vibrational sum frequency generation (SFG)spectroscopy. SFG is demonstrated as a sensitive and information-rich probe of nanostructured surfaces.Detection of SFG spectra from a few percent of a monolayer of nanoparticles on a transparent substrate wasachieved, corresponding to as few as 105 isolated particles within the laser beam spot. A new effect ariseswhen the nanoparticle size approaches the molecular scale: the dependence of the molecular conformationon the geometry of the substrate. Conformation of the dodecane chain of the ligand shows systematic variationwith the particle diameter in the 1.8−25 nm range. More gauche defects are observed on smaller particles,judged from the relative intensities of the CH2 and CH3 stretch transitions in SFG spectra. Similar behaviorobserved for both gold and silver particles suggests a nanoscale geometric packing effect, whence more volumeis available to the chain on a curved surface than a flat surface. Drying-mediated aggregation of goldnanoparticles results in an enhancement of the SFG signal, enabling vibrational SFG spectra of single submicronsize aggregates to be obtained. These are different from spectra of isolated particles, indicating a vibrationalmode-selective enhancement mechanism.
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