Self-Organisation of metal nanoparticles on ion beam produced ripple templates

Ion beam sputtering has been used for pre-structuring of the substrate. Low energy ion beam (Ar+, 500 eV) incident on the substrate surface (Si in our case) at an angle of 67o to the surface normal to produce well ordered (20-50 nm) ripple patterns [1, 2]. The periodicity and regularity of the pattern is estab-lished 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 produced in this way on very different materials including semiconductors, iso-lators, and metals, demonstrating the universality of the mechanism [2].
Depending on deposition angle, substrate temperature, beam flux, and deposition time, the metal nanostruc-tures align parallel to the ripples, eventually coalesce forming nanowires [3,4].A very high degree of align-ment not reported so far using the present technique has been achieved. Due to the alignment the nanoparti-cles exhibit strongly anisotropic optical properties. The difference in the interparticle distance along the paral-lel and perpendicular direction, respectively, leads to different plasmonic coupling in the respective direc-tions. 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 resonance of 0.2 eV to 0.7 eV have been determined for aspect ratios in the range of 2 to 5.
References:
[1]. Keller.A, Rossbach.S, Facsko.S, Nanotechnology 19(2008) 135303
[2]. Keller.A, Facsko.S,Möller.W, Jour. Phys. Cond. Matt. 21(2009) 495305
[3]. Oates.T.W.H, Keller.A, Facsko.S, A. Mücklich, Plasmonics 2(2007) 47
[4]. Oates.T.W.H, Keller.A, Noda.S, Facsko.S, Appl. Phys. Lett. 93(2008) 063106