Self-Organisation of metal nanoparticles and nanowires grown on ripple templates

Nobel metal nanoparticles exhibit distinct optical properties due to their localized surface plas-mon resonance. Potential applications of these structures can be found in solar cells, nonlinear optical devices, or sensors. Especially for nanoscale optics aligned equidistant chains of metal nanoparticles are favored.
Ion beam sputtered surfaces featuring self-organized ripple patterns have proven to be excellent templates for the alignment of these metal nanoparticles [1]. In the continuous sputtering proc-ess, induced by the bombardment with low-energy ions (100 – 2000 eV), periodic surface pat-terns appear in form of ripples or arrays of hexagonally ordered mounds, which show a very high degree of regularity [2]. The dimension of the pattern is related to the size of the typical collision cascade of a single ion event and lies in the range of ten to tens of nanometers. The periodicity and regularity of the pattern is established by the effective filtering of a narrow band of spatial frequencies on the surface, which results from the interplay between a surface instability caused by the sputtering and surface diffusion processes. Regular ripple morphologies have been pro-duced in this way on very different materials including semiconductors, isolators, and metals, demonstrating the universality of the mechanism [3].
Depending on deposition angle, substrate temperature, beam flux, and deposition time, the metal nanostructures align parallel to the ripples, eventually coalesce forming nanowires [4]. Due to the alignment the nanoparticles exhibit strongly anisotropic optical properties. The difference in the interparticle distance along the parallel and perpendicular direction, respectively, leads to different plasmonic coupling in the respective directions. Therefore a red shift of the plasmon-polariton resonance is observed for light polarized parallel to the ripple direction. In addition, the resonance shifts with the aspect ratio of the nanoparticles. Energy shifts of the plasmon reso-nance of 0.2 eV to 0.7 eV have been determined for aspect ratios in the range of 2 to 5.
This work is partly supported by DFG FOR 845.
REFERENCES
1. T.W.H. Oates, A. Keller, S. Facsko , A. Mücklich , Plasmonics 2, 47 (2007).
2. A. Keller, S. Rossbach, S. Facsko, et al., Nanotechnology 19, 135303 (2008).
3. A. Keller, S. Facsko, and A. Moller, Jour. Phys. Cond. Matt. 21, 495305 (2009).
4. T.W. H.Oates, A.Keller, S.Noda, S.Facsko , Appl. Phys. Lett. 93, 063106 (2008