Infrared spectroscopic studies of the metallic behaviour of single nanorods

Using infrared (IR) spectroscopic microscopy we studied resonant light scattering from gold nanorods prepared by electron beam lithography (EBL) as well as from cylindrical gold nanowires prepared by the template method. Rectangular nanorods with diameter in the 100 nm range act as resonant antennae showing significant antenna-like plasmon resonances similar to those of cylindrical nanowires with a higher crystalline quality [1]. The obvious similarity of the IR spectra of both kinds of nanowires shows that differences in their crystallinity have a nearly negligible effect on their antenna behaviour. Optical properties of antenna-like nanostructures are a field of rapidly increasing interest since their plasmonic resonances are related to strong local electromagnetic field enhancement. Such an amplification, which is particularly strong for gold nanowires in the IR, can be exploited, e.g., for resonance enhanced vibrational spectroscopy and enhanced fluorescence [2]. This report focuses on IR spectroscopic microscopy studies of antenna-like plasmon resonances of single gold nanorods prepared by EBL and their comparison with highly crystalline single gold nanowires. Gold was chosen because of its high chemical stability. Like most metals it shows the Drude-type behaviour in the IR. Gold nanorods were prepared with EBL on an IR transparent substrate (ZnS) using a 30 kV Hitachi S3500N scanning electron microscope equipped with a nanometer pattern generation system (NPGS, JC Nabity Lithography Systems). Polymethylmethacrylate (PMMA) was used as a high-resolution resist. After electron beam exposure, the pattern was developed in a 1:3 solution of methylisobutylketone and isopropyl alcohol, and an appropriate amount of gold was evaporated on the sample. For the lift-off process acetone was used. As described in a former publication [3], the cylindrical Au nanowires were prepared by electrochemical deposition in polymeric ion track-etched membranes and transferred onto a ZnS substrate for the IR measurements. IR spectroscopic microscopy using synchrotron radiation was performed at the synchrotron light source ANKA of the Forschungszentrum Karlsruhe. This technique enables us to locate single nanowires in the visible light mode of the microscope (Bruker IR scope II) and to focus the IR radiation on the selected circular area (spot size 8.3 μm). After recording the IR transmittance spectrum of the selected nanowire, a reference spectrum is acquired at a minimum distance of 10 μm from the structure. At least 500 spectra were recorded with a resolution of 8 cm -1 in the spectral range between 700 and 7000 cm -1 using a FTIR spectrometer (Bruker IF66v/S) with a LN2 cooled mercury cadmium telluride detector. The relative transmittance spectra reveal strong fundamental antenna resonances for polarization parallel to the long wire axis.