表面電漿子於單根金奈米線及金奈米多孔性薄膜之遠場及近場光學特性研究

Plasmonic Properties of Single Gold Nanowires and a Nanoporous Gold Film Investigated by Far-Field and Near-Field Optical Techniques

陳香安(Hsiang-An Chen)

指導教授 ： 林鶴南

國立清華大學/工學院/材料科學工程學系/博士(2010年)

https://doi.org/10.6843/NTHU.2010.00603

摘要

In recent years, noble metal nanostructures have attracted increasing interests in the development of sensitive biosensors and novel photonic devices due to their pronounced optical properties related to surface plasmons (SPs).

We report a study of localized surface plasmon resonance (LSPR) in single Au nanowires (NWs) created sequentially by atomic force microscopy nanolithography on a thin polymer resist, metal deposition and lift-off process. The widths and thicknesses of the single NWs are less than 110 nm and 30 nm, respectively, and the lengths are of a few μm. In the far-field scattering spectrum of a single NW, two LSPR peaks at around 630 and 490 nm in wavelength are observed and the intensity is strongest when the incident electric field is parallel to the length direction of a NW. With the aid of near-field scanning optical microscopy, it is found that surface plasmon polaritons (SPPs) are generated and propagating along the length direction when the incident electric field is perpendicular, whereas localized surface plasmons (LSPs) are generated when parallel. This assertion is consistent with the far-field scattering spectra. In near-field optical images, both LSPs and SPPs have a periodic wavelength of around 480 nm. This value is in good agreement with the calculated SP wavelength of 471 nm at the air-Au interface at an incident wavelength of 532 nm.

The current results of polarization dependence from far-field and near-field measurements are in strong contrast to previous results of lithographically fabricated NWs in literatures. The polarization dependence of our NWs is due to the lack of well-defined axes in the cross-section of a NW from transmission electron microscopy analysis. The present work reveals the interesting variation of LSPR modes in a single NW due to different fabrication techniques.

The single NWs has also been applied to perform the sensitivity of the medium and the chemical sensing of an alkanethiolate self-assembled monolayer, octadecanethiol by LSPR peak shift. The present work reveals interesting variation of LSPR modes in single metal NWs that are potentially valuable for future plasmonic applications.

Furthermore, a nanoporous Au film is prepared sequentially by deposition of gold and copper, high temperature annealing, and chemical etching for the investigation of plasmonic properties on random nanoporous films which are commonly used as surface enhanced Raman scattering (SERS) substrates. The film has a thickness of 20 nm and randomly distributed pores with sizes ranging between 15 and 350 nm, which are suitable for the generation of SPP waves in the film. In far-field measurements, the nanoporous film has a different transmittance and a lower reflectance when compared with those of a 20 nm thick plain Au film in the wavelength range between 400 and 1000 nm. As a result, the absorbance of the nanoporous film is much higher and can be attributed to the conversion of incident light into SPP waves. In the dark-field scattering spectrum, a broad peak appears at around 630 nm and corresponds to the resonance peak of the aperture plasmon mode of the pores in the film.

From the near-field results, the local transmitted optical field distribution on the film is observed and reveals the generation of SPPs. Furthermore, two types of local field enhancement are observed. The first type has a small spatial distribution of around 200 nm and an enhancement factor of 4. The second type has a large spatial distribution of around 1 µm and an enhancement factor of 2. The two types of enhancement correspond to strong and weak SPP localization, respectively. The field enhancement effect on the nanoporous film can be utilized for surface enhanced Raman scattering and a clear Raman spectrum in a 10−6 M Rhodamine 6G solution has been obtained.

關鍵字

表面電漿子 ； 金奈米線 ； 金奈米多孔性薄膜

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