Composition, structural and optical properties of Golden grass
Résumé
Golden grass is a vegetal fiber used in arts and crafts for its peculiar optical properties, namely, its shiny golden-like reflectivity. Since vegetal fiber structure is usually heterogeneous, complementary techniques have been used to understand the electronic properties of the external and core regions of golden grass fibers. The composition and dielectric function of the few nanometer-thick external surfaces has been investigated using X-ray photoelectron spectroscopy and energy loss spectroscopy (XPS-PEELS), while a comparison of vibrational and optical properties of the few-micron-thick external and fiber core regions was derived from Raman, optical, photoluminescence (PL) and photoacoustic (PAS) spectroscopies. Raman signature and valence-band distribution indicate that the internal and external parts of the fibers are made of some oxygen-depleted lignin-like carbonaceous material. Besides a plasmon peak at 21 eV, two main absorption bands at 3.7 eV and 8.2 eV have been identified as π−π* and σ−σ* electronic transitions at aromatic moieties. PL excitation and emission behavior confirm heterogeneous molecular structures at the core and external regions. Specular reflectance spectra calculated from the complex refractive index (derived from XPS-PEELS) do not explain the shiny golden-like aspect of golden grass, which may arise from diffuse reflectance within a collection of heterogeneous hollow fibers.
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