title: Spatial extent of plasmonic enhancement of vibrational signals in the infrared.
abstract: Infrared vibrations of molecular species can be enhanced by several orders of magnitude with plasmonic nanoantennas. Based on the confined electromagnetic near-fields of resonantly excited metal nanoparticles, this antenna-assisted surface-enhanced infrared spectroscopy enables the detection of minute amounts of analytes localized in the nanometer-scale vicinity of the structure. Among other important parameters, the distance of the vibrational oscillator of the analyte to the nanoantenna surface determines the signal enhancement. For sensing applications, this is a particularly important issue since the vibrating dipoles of interest may be located far away from the antenna surface because of functional layers and the large size of biomolecules, proteins, or bacteria. The relation between distance and signal enhancement is thus of paramount importance and measured here with in situ infrared spectroscopy during the growth of a probe layer. Our results indicate a diminishing signal enhancement and the effective saturation of the plasmonic resonance shift beyond 100 nm. The experiments carried out under ultra-high-vacuum conditions are supported by numerical calculations.
authors.affiliation: 4th Physics Institute and Research Center SCoPE, University of Stuttgart , Pfaffenwaldring 57, 70569 Stuttgart, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Neubrech
authors.forename: Sebastian
authors.initials: S
authors.lastname: Beck
authors.forename: Tobias
authors.initials: T
authors.lastname: Glaser
authors.forename: Mario
authors.initials: M
authors.lastname: Hentschel
authors.forename: Harald
authors.initials: H
authors.lastname: Giessen
authors.forename: Annemarie
authors.initials: A
authors.lastname: Pucci
mesh_terms: D000327:Adsorption; D015374:Biosensing Techniques; D002124:Calcium Fluoride / Q000737:chemistry; D003198:Computer Simulation; D004574:Electromagnetic Fields; D004583:Electrons; D053768:Metal Nanoparticles / Q000737:chemistry; D036103:Nanotechnology / Q000379:methods*; D009930:Organic Chemicals / Q000737:chemistry; D013055:Spectrophotometry, Infrared; D020349:Surface Plasmon Resonance; D013499:Surface Properties; D013696:Temperature; D014732:Vibration
chemical_list: D009930:Organic Chemicals; D002124:Calcium Fluoride