Dichroism of Thin Films Containing Aligned Ag Nanoparticles Prepared by Multi-Source Dynamic Oblique Deposition.
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Dichroism of Thin Films Containing Aligned Ag Nanoparticles Prepared by Multi-Source Dynamic Oblique Deposition.
Wataru Maekita, Motofumi Suzuki, Kazuaki Kishimoto, Kaoru Nakajima, and Kenji Kimura Department of Engineering Physics and Mechanics, Kyoto University, Kyoto 606-8501, Japan
ABSTRACT We have demonstrated the direct formation of elongated Ag particles with a quasi-parallel major axis on SiO2 template layer by using dynamic oblique deposition (DOD). The peculiar nanomorphology of the particles and template is physically self-organized owing to the self-shadowing in the oblique vacuum deposition. The resulting films exhibit anisotropic optical absorption due to plasma resonance sensitive to the shape of the Ag particles on the template. Since our method can be applied to any combination of thin film materials, it is useful for enabling the plasmon-mediated optical phenomena and to apply them for the development of various photonic devices such as thin film polarizers. INTRODUCTION Research on the optical properties of nanoparticles has been conducted for over 150 years1 and is still active today.2–5 In particular, noble metal nanoparticles in various matrices are attracting considerable attention toward surface plasmon-mediated optical phenomena such as resonance absorption,4 surface-enhanced Raman scattering,5 nonlinear2 and nearfield optics.3 Since the behavior of the surface plasmon and the local field between particles is sensitive to the depolarization factor, the plasmon-mediated optical phenomena will be significantly modified by designing the shape, size and orientation of the nanoparticles. To date, the polarization-selective transmittance6 of commercial grade and second-harmonic generation enhancement2 have been reported for stretching glass containing elongated Ag particles with a parallel major axis. The process to align the particle orientation requires a large deformation at high temperature ( ≈ 600 ˚C), so that the matrices are limited to deformable materials. Thus this process is not suitable for direct formation on various photonic devices. Obliquely deposited metal films including nanocolumns show properties similar to the glass containing elongated particles.7 Unfortunately, the matrix for these nanocolumns is limited to the voids between them. Therefore the plasma resonance frequency cannot be tuned flexibly by changing the matrix. The recently popularized DOD,8 in which the deposition angle and/or inplane direction of a substrate are varied during vacuum evaporation, is a promising candidate for a technique to tailor the particle shape and size. Thin films with highly controlled isolated columns such as helix8 and zigzag9 have been developed by DOD. Furthermore, integration of these morphologies has been achieved.10 If we use these ‘hypercolumnar’ structures as a template and deposit a small amount of metal on them, metallic islands are expected to develop in a shape reflecting the surface morphology. Since the DOD provides a self-organization process primarily governed by a g
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