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Testing optical properties of metal nano-particles in large arrays, fabricated by
Electron Beam Lithography (EBL)
Martin M. Greve1,2, Thomas Reisinger2, Thomas Håvardstun2, Phil Denby1, Bodil Holst2
1
EnSol AS, Apeltunvegen 2, 5222 Nestun, Norway
2
University of Bergen, Dep. of Physics and Technology, Allégaten 55, 5007 Bergen, Norway
[email protected]
Metal in bulk has its metallic reflective properties due to the, loosely bound, valence electrons
shared by the metal ions. When moving into the nano-regime, the optical properties of metals
change significantly. If the bulk metal is split into particles smaller than the wavelength of the
incoming light the wavelength will move the valence electrons in a collective motion, and thus
polarise the whole particle. This effect is known as surface plasmon resonance of metal nanoparticles. The excitation of plasmons in the nano-particles leads to absorption of some frequencies
of the incoming electro-magnetic wave depending on the size of the nano-particle.
We have developed a method to make two dimensional arrays in the order of several millimetres
with periodically spaced nano-particles using Electron Beam Lithography (EBL). The nanoparticles can be adjusted and calibrated in size with high precision in all three dimensions to the
desired specifications. The particle spacing in the array can also be chosen freely in both
dimensions. The method is based on the pioneering work of W. Rechberger et al.1. The new tools
available in the NanoStructures Laboratory at the University of Bergen, will enable much larger
arrays and smaller nano-particles to be prepared for investigation.
The optical properties can be tested by exciting surface plasmons with a white light source and
analysing the transmitted spectrum using a commercial spectroscope attached with standard optical
fibres. We have used cover glass slips coated with a conducting transparent oxide (Indium Tin
Oxide) as substrates since a the EBL exposure requires a conducting layer to prevent charging when
patterning.
The tunable absorption spectra of the nano-particles is of particular interest in solar cell applications,
but can also be of interest in sensing applications.
1
W. Rechberger et al. / Optics Communications 220 (2003) 137–141