NEUBRECH ET AL . VOL. 6 ’ NO. 8 ’ 7326–7332 ’ 2012 www.acsnano.org 7326 July 18, 2012 C 2012 American Chemical Society Infrared Optical Properties of Nanoantenna Dimers with Photochemically Narrowed Gaps in the 5 nm Regime Frank Neubrech, †, * Daniel Weber, † Julia Katzmann, ‡ Christian Huck, † Andrea Toma, § Enzo Di Fabrizio, § Annemarie Pucci, † and Thomas Ha ¨ rtling ‡ † Kirchhoff Institute for Physics, University of Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany, ‡ Fraunhofer Institute for Nondestructive Testing, Maria-Reiche-Straße 2, 01109 Dresden, Germany, and § Nanostructures, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy P lasmonic nanostructures separated by gaps with sub-nanometer dimensions are of great interest in physics. For example, as a result of the sub-nanometer distance, nanogaps are well suited to serve as a model system for nonlocality 1 or quan- tum mechanical effects in plasmonics, such as tunneling. 2,3 Furthermore, nanoscale gaps of a few nanometers offer the possibil- ity to enhance nonlinear signals 4 and carry great potential for the enhancement of spectroscopic signals of molecular species due to the strong confinement of electro- magnetic fields. The application of such “hot spots” induced by localized surface plasmon polaritons was reported for surface-enhanced fluorescence, 5,6 surface-enhanced infrared absorption (SEIRA), 7,8 and surface-enhanced Raman spectroscopy (SERS) 9,10 in many dif- ferent geometries. More recently, the exten- sion of the experiments to nanoantenna- assisted SEIRA with nanostructures resonantly tuned to molecular infrared (IR) vibration bands was initiated, and attomolar sensitivity was achieved. 11À13 In comparison to conven- tional IR techniques, signal enhancements up to 500 000 accompanied by Fano-type line shapes (as a result of the electromagnetic coupling between the molecular and plas- monic excitation) were observed. 14 In agree- ment with theoretical simulations, 15,16 which predict an increased near-field and thus a tremendous increase in the IR signal, 17 it was found that nanoantennas separated by gaps of about 20 nm feature an increased molecular IR signal in comparison to indivi- dual structures. 18 For all these aspects the challenge of large-scale and reproducible fabrication of nanostructures with suitable hot spots remains. This is especially demanding in the case of nanoantenna-assisted SEIRA or ter- ahertz applications, as structures in the micrometer range and hence resonant in the IR 19 or terahertz spectral range 20 have to be fabricated with gaps in the 10 nm regime. In a previous contribution we reported on the exploitation of photochemical metal deposi- tion 21 to narrow the gaps sizes in IR nano- antennas on a flat substrate. 22 The spectral tuning of the antennas as well as the fabrica- tion of interparticle gap sizes below 10 nm was successfully demonstrated with this ap- proach. In that earlier work, the arrangement * Address correspondence to neubrech@kip.uni-heidelberg.de. Received for review June 1, 2012 and accepted July 17, 2012. Published online 10.1021/nn302429g ABSTRACT In this paper, we report on the manipulation of the near-field coupling in individual gold nanoantenna dimers resonant in the infrared (IR) spectral range. Photoche- mical metal deposition onto lithographically fabricated nanoantennas is used to decrease the gap between the antenna arms down to below 4 nm, as confirmed by finite-difference time-domain simulations. The increased plasmonic coupling in the gap region leads to a shift of the surface plasmon resonances to lower energies as well as to the appearance of hybridized plasmonic modes. All of the occurring electron oscillation modes can be explained by the plasmon hybridization model. Besides the bonding combination of the fundamental resonances of individual antennas, also the antibonding combination is observed in the IR transmittance at normal incidence. Its appearance is due to both structural defects and the small gaps between the antennas. The detailed analysis of individual IR antennas presented here allows a profound understanding of the spectral features occurring during the photochemical manipulation process and therefore paves the way to a full optical process monitoring of sub-nanometer scale gaps, which may serve as model systems for experimental studies of quantum mechanical effects in plasmonics. KEYWORDS: plasmonics . nanoantennas . preparation of nanogaps . plasmon hybridization . infrared spectroscopy ARTICLE