The geometrical conformations of dodecanethiol (DDT) molecules grafted on small gold nanoparticles (AuNPs, 7.5 nm diameter) deposited with various densities on a substrate were detected by two-color sum-frequency generation (SFG) spectroscopy. The high level of gauche defects indicates a L-shape structure of the alkane chains calculated classically through the amplitude ratios of the resonant modes of vibration for CH2 methylene and CH3 methyl groups (d+/r+ and d−/r−). Our amplitude ratios are higher than those found in the existing literature, and such effect can be ascribed to the combined action of several phenomena highlighting the geometrical molecular complexity on metal spheres: (i) the matching of the visible wavelength of excitation with the plasmonic resonance of either isolated AuNPs (490 nm) or AuNPs multimers (600 nm), and (ii) the local field enhancement. Additionally, we apply Density Functional Theory calculations on gold clusters functionalized by DDT molecules in interaction in a 3D space. By comparison with SFG results, we evidence through condensed Fukui analysis and computed IR/Raman spectra that the CH2 methylene gauche defect in the alkane chain is present in the first one located beyond the sulfur atom giving precise insight on molecular geometry.

Yves Huttel, Lidia Martínez and José Miguel García-Martín acknowledge grant PID2021-126524NB-100 funded by MCIN/AEI/10.13039/ 501100011033 and by “ERDF A way of making Europe” .

Peer reviewed