Synthesis of anisotropic Au, Pd/Au and Pd NPs via oblique angle deposition on glass: Plasmon resonance and sensing applications
Synthèse de NP anisotropes Au, Pd / Au et Pd par évaporation oblique sur du verre: applications de détection et de résonance Plasmon
Résumé
Gold metal nanoparticles (NPs) display localised surface plasmon resonances (LSPR) in the visible to close infra-red range. The wavelength position of the LSPR depends strongly on the shape of the NPs. Typically, elongated NPs such as nanorods display a high (low) wavelength LSPR for polarisation of light along the large (short) axis. This property opens the path to elaborate dichroic films based on plasmonic metal NPs.
In order to obtain such dichroic samples, multiple methods could be used. Should it be a bottom-up method through the chemical synthesis of nanoparticles [1], which allows one to greatly control their shape and, consequently, their LSPR, though makes it difficult to organise them on a surface or in a matrix, as it has been shown in our group [2]. An alternative is the top-down method using primarily lithographic techniques, which allow one to get a very controlled pattern, though limiting the minimum size of a feature to around 100nm [3,4]; or using evaporation onto a prior nanostructured substrate [5,6]
In this poster, another method was chosen known as oblique angle deposition. It consists to evaporate metals at a grazing angle to a non-structured substrate. As long as the metal does not wet the substrate e.g. gold on glass, anisotropic nano-islands form with a preferential orientation, which leads to a global optical anisotropy. Although this method does not produce as defined features as the methods mentioned above, it has the advantage of being very simple and easy to put in place. Furthermore, in this way very small particles can be synthesised, which can have applications for chemical reactivity or catalysis issues. The shape of the NPs and the optical anisotropy can be tuned by varying the angle of deposition as well as the amount evaporated.
The morphology of the NPs was analysed using scanning tunnelling microscopy (SEM) and compared to results obtained by atomic force microscopy (AFM) which is limited due to the effect of convolution of the tip. The samples obtained in such way display a strong dichroism, which can be tuned by varying the elaboration parameters. It was measured by transmission spectroscopy and by reflectance anisotropy spectroscopy (RAS). This is illustrated in Fig.1, where the absorption of a gold film is drawn for two perpendicular polarisations of light.
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