Study of surface plasmon resonances on assemblies of slits in thin Ag films by low-loss EFTEM imaging

Abstract

With the ongoing developments in nanotechnology, surface plasmon resonances (SPRs) have started to play a crucial role in many different areas of science. Surface Plasmon resonances are described as collective oscillations in the valence electron density at the surface of a conductor. They have especially received attention in the areas of biosensing in cancer diagnostics [1], near-field Raman spectroscopy [2] and different applications in optoelectronics. In this study, the optical response of a specially perforated thin Ag film is investigated with EFTEM. The experiments were carried out in the 200 kV FEG-TEM Sub-Electron-VoltSub-Angstrom-Microscope (Zeiss SESAM) equipped with an electrostatic monochromator and the in-column MANDOLINE filter [3]. The superior properties of this instrument enable EFTEM imaging in the ultraviolet–near-infrared domain with very high energy resolution and spatial sampling [4]. The Ag specimen was prepared as follows: Using physical vapour deposition, a Ag film with about 100 nm thickness was deposited onto a C film on a standard TEM Cu grid (the dimensions of each mesh is 100 × 100 μm). Focused ion beam (FIB) technique was used to drill different slit structures into the Ag film. The EFTEM series were acquired in the energy loss range from 0.4 eV to 5 eV by using a 0.19 eV energy slit and a step size of 0.2 eV. The EFTEM images were recorded on a 2k × 2k CCD camera with 8 times binning and an acquisition time of 30 sec / image (at each energy loss 3 images were recorded with an exposure time of 10 s and then aligned and averaged). Figure 1(a) shows a zero loss bright-field image of a double-slit structure with dimensions of 200 nm × 1 μm and a separation of 100 nm. A sample image taken from the drift-corrected EFTEM series at an energy loss of 0.6 eV is shown in figure 1(b). The intensity distribution is attributed to a localized plasmon resonance. Such resonances will be discussed and compared with numerical simulations [5].