Nanogaps for Nanoantenna-Assisted Infrared Spectroscopy

Abstract

Lithograhically prepared gold nanorods, showing antenna-like plasmon resonances in the infrared (IR) spectral range, are able to strongly enhance local electromagnetic fields. This can be exploited for nanoantenna-assisted IR spectroscopy (NAIRS), a sensing application allowing detection of very small amounts of molecules (attomol). Since near-field coupling between nanorods over small gaps considerably increases the field enhancement, the main focus of the thesis was the characterization and preparation of gaps in the sub-10nm range. Therefore, two approaches were applied: the narrowing of lithographically prepared gaps (20 – 50 nm) by chemically induced metal deposition, on the one hand, and the milling of long rods by focused ion beams (FIB) on the other hand. IR optical investigations of the grown or milled nanorods allowed non-destructive assessment of the fabrication quality. In regard to the chemical method, gaps in the range of 10nm were presumably prepared, whereas FIB milling resulted in gaps down to 20 nm. In addition, near-field and far-field coupling effects in nanorod arrays were analyzed to determine the optimal rod arrangement for NAIRS. Finally, the probe molecule mercaptoundecanoic acid was spectroscopically investigated with the help of gold nanoantennas and enhancement factors up to 48 000 were estimated.