Red gold (Au-Cu) as a material for use in plasmonic devices
K.S.B de Silva1, A. Gentle1, M. Arnold1, A. Dowd1, V. Keast2, M.B. Cortie1
1Institute for Nanoscale Technology, University of Technology Sydney,
Sydney, 2007, Australia
2School of Mathematical and Physical Sciences, University of Newcastle, Callaghan
NSW 2308, Australia
e: sujeewa.desilva@uts.edu.au
The dielectric functions of the coinage metals (Cu, Ag and Au) make them
individually suitable for applications in which a localised surface plasmon resonance
is desired. Au is the most widely used material in such applications due to its
resistance to oxidation, however Cu and Ag might be better choices if oxidation can
be eliminated.
Here we consider whether (Au,Cu) solid solutions and intermetallic compounds could
be an alternative material for plasmonic devices. Samples of Au-Cu alloys were
produced by melting of the elements, or by co-deposition using a magnetron
sputtering system. There are significant discrepancies in the literature [1-3] in regard
to whether the crystal structure (face-centred cubic solid solution vs. ordered
intermetallic compounds) of these alloys has an effect on their optical properties. The
sensitivity of the optical properties due to surface effects (roughness, oxidation) can
have a significant effect on the measured optical properties, and may have
contributed to much of the discrepancies in the literature.
Therefore, we carried out heat-treatments to convert samples into either the ordered
intermetallic compound AuCu or to retain the metastable face centred cubic (Au,Cu)
phase. X-ray diffraction and microstructural examination were used to verify that the
desired structures had been achieved. Optical properties of the samples were then
determined using ellipsometry followed by fitting of a optical model that ensured that
the necessary Kramers-Kronig consistency was maintained. The optical properties of
samples were also investigated using density functional theory calculations
performed using the Wien2K code. We found that Au-Cu is a viable material for
nanoscale plasmonic applications at wavelengths longer than about 550 nm
although its performance cannot match that of pure Au or pure Cu.
References
[1] W.R. Scott, and L. Muldawer Phys. Rev. B 14, 4426 (1976)
[2] A. Henkel, A. Jakab, G. Brunklaus and C. Sonnichsen, J. Phys. Chem. C 113,
2200 (2009).
[3] J. Rivory, J de physique 35 Colloque C4, 51 (1974)