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Localized surface plasmons in metal nanoparticles engineered by electron beam lithography

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2009
Güler, Urcan
In this study, optical behavior of metal nanoparticles having dimensions smaller than the wavelength of visible light is studied experimentally and numerically. Gold (Au) and silver (Ag) nanoparticles are studied due to their superior optical properties when compared to other metals. A compact code based on Discrete Dipole Approximation (DDA) is developed to compute extinction efficiencies of nanoparticles with various different properties such as material, dimension and geometry. To obtain self consistent nanoparticle arrays with well defined geometries and dimensions, Electron Beam Lithography (EBL) technique is mainly used as the manufacturing method. Dose parameters required to produce nanoparticles with dimensions down to 50 nm over substrates with different electrical conductivities are determined. Beam current is found to affect the doseV size relation. The use of thin Au films as antistatic layer for e-beam patterning over insulating substrates is considered and production steps, involving instabilities due to contaminants introduced to the system during additional removal steps, are clarified. 4 nm thick Au layer is found to provide sufficient conductivity for e-beam patterning over insulating substrates. An optical setup capable of performing transmittance and reflectance measurements of samples having small areas patterned with EBL is designed. Sizes of the metal nanoparticles are determined by scanning electron microscope (SEM) and spectral data obtained using the optical setup is analyzed to find out the parameters affecting the localized surface plasmon resonances (LSPR). Arrays of particles with diameters between 50 – 200 nm are produced and optically analyzed. Size and shape of the nanoparticles are found to affect the resonance behavior. Furthermore, lattice constants of the particle arrays and surrounding medium are also shown to influence the reflectance spectra. Axes with different lengths in ellipsoidal nanoparticles are observed to cause distinguishable resonance peaks when illuminated with polarized light. Peak intensities obtained from both polarizations are observed to decrease under unpolarized illumination. Binary systems consisting of nanosized particles and holes provided better contrast for transmitted light.