In thin film devices such as light-emitting diodes, photovoltaic cells and field-effect transistors, the processes of charge injection, charge transport, charge recombination, separation and collection are critical to performance. Most of these processes are relevant to nanoscale metal and metal oxide electrode-organic material interfacial phenomena. In this report we present a unique method for creating tailored onedimensional nanostructured silver, tin and/or zinc substituted indium oxide electrode structures over large areas. The method allows production of high aspect ratio nanoscale structures with feature sizes below 100 nm and a large range of dimensional tunability. We observed that both the electronic and optical properties of these electrodes are closely correlated to the nanostructure dimensions and can be easily tuned by control of the feature size. Surface area enhancement accurately describes the conductivity studies, while nanostructure dependent optical properties highlight the quasi-plasmonic nature of the electrodes. Optimization of the nanostructured electrode transparency and conductivity for specific opto-electronic systems is expected to provide improvement in device performance.
The effective medium approximation is used to determine the optical constants of novel silver (Ag)/indium-tin oxide (ITO) multilayer nanopillar structures within the 300-800 nm wavelength range. The structures are modeled as inclusions in air with the pillar volume fraction at 42.4%, agreeing with SEM images of the sample. The simulated reflection intensity of the nanopillars is much less than that of the planar reference sample and is a result of the small difference between the refractive index of the top effective medium layer and that of air. Furthermore, the minimum in the reflection at around 450 nm in the nanostructured sample is evidence of surface plasmon enhancement, indicating suitability for plasmonic applications. The simulated Brewster angle decreases in the pillar region, which is an indication of smaller effective refractive index.
Plasmonic Ag nanopillars
have been fabricated and used as a surface-enhanced Raman scattering
(SERS) substrate. The effective surface area of the sample is determined
using underpotential deposition (UPD) of thallium and agrees well
with a geometrical calculation using ImageJ analysis of SEM images.
In order to find the SERS enhancement factor (SEF), a similar sample
is coated with Pt, which shows no plasmon response at the excitation
wavelength of 532 nm. SEF values on the order of 105 are
obtained for Ag nanopillar substrates. Several monolayers of C60 were deposited on these Ag nanopillars, and the Raman spectral
results indicate charge delocalization at the interface between C60 and Ag. FDTD simulation of the electric field confirms the
experimental results; on the basis of these simulations, the electric
field modulates with increasing diameter of the pillars, while the
pitch (center-to-center distance) is fixed at 200 nm.
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