Abstract
Thin Au films (~5 nm) are known to form island-like structures with small gaps between the islands, which produce intense electric field "hot spots" under visible illumination. In this work, we perform finite difference time domain (FDTD) simulations based on experimentally observed high resolution transmission electron microscope (HRTEM) images of these films in order to study the nature of the "hot spots" in more detail. Specifically, we study the dependence of the electric field intensity in the hot spots on the surrounding film environment and on the size of the nanogaps. From our simulations, we show that the surrounding film contributes significantly to the electric field intensity at the hot spot by focusing energy to it. Widening of the gap size causes a decrease in the intensity at the hot spot. However, these island-like nanoparticle hot spots are far less sensitive to gap size than nanoparticle dimer geometries, studied previously. In fact, the main factor in determining the hot spot intensity is the focusing effect of the surrounding nano-islands. We show that these random Au island films outperform more sophisticated geometries of spherical nanoparticle clusters that have been optimized using an iterative optimization algorithm.
Highlights
It is well known that thin films (~ 5nm) of noble metals such as Au and Ag exhibit a strong plasmonic response under visible illumination due to their discontinuous nature
We performed finite difference time domain (FDTD) simulations [20] of several hot spots based on experimentally measured high resolution transmission electron microscope (HRTEM) images of 5nm Au films and systematically varied the nano-island and gap size
The electric field intensity integrated over a small volume provides a more reliable value than the intensity at a point, by minimizing the effects that may arise due to sharp features artificially created by the spatial discretization
Summary
It is well known that thin films (~ 5nm) of noble metals such as Au and Ag exhibit a strong plasmonic response under visible illumination due to their discontinuous nature. Royer et al studied the structure of 5nm thick Au and Ag films using scanning electron microscope (SEM) and approximated the shape of the film islands using oblate spheroids in order to perform theoretical calculations [10]. These island-like structures create localized “hot spot” regions, in which the local electric field intensity can be enhanced by several orders of magnitude [11]. The general consensus is that these hot spots tend to form in regions where the neighboring islands are nearly touching, allowing for maximum plasmonic coupling This has been observed in the case of nanoparticle dimers as well [16,17,18,19]. We performed finite difference time domain (FDTD) simulations [20] of several hot spots based on experimentally measured high resolution transmission electron microscope (HRTEM) images of 5nm Au films and systematically varied the nano-island and gap size
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