Vibrational sum frequency generation spectroscopy of dodecanethiol on metal nanoparticles

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 was achieved, corresponding to as few as 105 isolated particles within the laser beam spot. A new effect arises when the nanoparticle size approaches the molecular scale: the dependence of the molecular conformation on the geometry of the substrate. Conformation of the dodecane chain of the ligand shows systematic variation with 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 behavior observed for both gold and silver particles suggests a nanoscale geometric packing effect, whence more volume is available to the chain on a curved surface than a flat surface. Drying-mediated aggregation of gold nanoparticles results in an enhancement of the SFG signal, enabling vibrational SFG spectra of single submicron size aggregates to be obtained. These are different from spectra of isolated particles, indicating a vibrational mode-selective enhancement mechanism.