Plasmon-Induced\nVibrational Circular Dichroism Bands\nof Achiral Molecules on Gold Nanostructures with Tunable Extrinsic\nChiroptical Responses

Vibrational circular dichroism (VCD) is an important\nmethod used\nto assign absolute configuration to chiral molecules, but inherently\nweak signals necessitate high concentrations and long acquisition\ntimes for analysis. Plasmonic nanostructures are of interest for enhancement\nof VCD signals. However, understanding the influence of plasmonic\nstructures on molecular VCD spectral features is important for assessing\nthe information content of the spectra. Here, we present a study of\nplasmonic gold nanobowl hole (NBH) structures and the influence on\nthe VCD spectra of achiral molecules. VCD bands for the symmetric\nand antisymmetric methylene stretching modes of 1-octadecanethiol\n(ODT) on NBHs were observed. Measurements of ODT on CaF<sub>2</sub> substrates without plasmonic structures confirmed that the bands\nwere not an artifact of tilting the substrate. The handedness of the\nVCD bands showed an inverse dependence on the handedness of the extrinsic\nCD response of the NBHs, producing a relative handedness of the opposite\nsign. By controlling the size of the NBHs, the localized surface plasmon\nresonance (LSPR) and CD bands were tuned relative to the spectral\nposition of the ODT VCD bands. The amplitude and line shape of the\nmethylene stretch VCD bands showed a strong dependence on overlap\nwith the plasmonic CD band in the same spectral region. To further\ninvestigate the origin of the molecular VCD bands, nanobowls without\nholes (NBs) and nanodisks (NDs) were fabricated. The LSPR behavior\nof NBs and NDs was similar to NBHs, but the structures did not exhibit\nthe extrinsic CD responses of NBHs, evidenced by the lack of defined\nCD bands and handedness conversion. Although the NBs and NDs exhibited\nsmall CD signals, VCD bands were not observed for ODT on the NBs and\nNDs. Stearic acid (SA) on NBHs also produced VCD bands, demonstrating\nthe influence of the plasmonic nanostructures on molecules with different\nsurface interactions and organization. Asymmetric line shapes of the\nVCD bands of ODT appeared similar to Fano resonance line shapes observed\nin surface-enhanced infrared absorption (SEIRA) spectroscopy; however,\nthe NBH CD response was shown to be a key factor in producing the\nVCD bands, rather than the primary LSPR that influences the line shapes\nand enhancements in SEIRA. The studies of VCD spectra of achiral molecules\non plasmonic structures highlight the importance of understanding\nhow the plasmonic and chiroptical behaviors of the substrates impact\nthe spectral features used to identify enantiomers in VCD spectroscopy,\nespecially for configuration analysis and comparison with quantum\ncalculations.