Julien Romann

Surface enhanced Raman spectroscopy of organic molecules deposited on gold sputtered substrates

Aggregates of Au nanoparticles have been extremely easily obtained on glass substrates by physical sputtering under primary vacuum. With such a protocol, we demonstrate that it is possible to control the surface plasmon band absorption. Surface enhanced Raman spectroscopy (SERS) experiments were performed with methylene blue, zinc octacarboxyphthalocyanine, 4-aminothiophenol and cysteamine. The correlation between the absorption band and the wavelength giving the highest SERS intensity is clearly observed for methylene blue, in accordance with the electromagnetic enhancement theory. For the other molecules, effects of the chemical enhancement are also observed. In addition, we noticed a strong influence of the nature of the adsorbed molecule on the enhancement factor for a given wavelength. The origin of this feature is discussed in terms of resonant effects or multipolar surface plasmon modes.

Wavelength and orientation dependent capture of light by diatom frustule nanostructures

The ecological success of diatoms is emphasized by regular blooms of many different species in all aquatic systems, but the reason behind their success is not fully understood. A special feature of the diatom cell is the frustule, a nano-patterned cell encasement made of amorphous biosilica. The optical properties of a cleaned single valve (one half of a frustule) from the diatom Coscinodiscus centralis were studied using confocal micro-spectroscopy. A photonic crystal function in the frustule was observed, and analysis of the hyperspectral mapping revealed an enhancement of transmitted light around 636 and 663 nm. These wavelengths match the absorption maxima of chlorophyll a and c, respectively. Additionally, we demonstrate that a highly efficient light trapping mechanism occurred, resulting from strong asymmetry between the cribrum and foramen pseudo-periodic structures. This effect may prevent transmitted light from being backscattered and in turn enhance the light absorption. Based on our results, we hypothesize that the multi-scaled layered structure of the frustule improves photosynthetic efficiency by these three mechanisms. The optical properties of the frustule described here may contribute to the ecological success of diatoms in both lentic and marine ecosystems, and should be studies further in vivo.

Optical properties of single diatom frustules revealed by confocal microspectroscopy.

Optical properties of single diatom frustule valves from two different Coscinodiscus species (C. wailesii and C. centralis) are studied by transmission confocal hyperspectral imaging and numerical calculations. Light convergence, concentration, and trapping effects are observed and depend on both the wavelength and the valve orientation. These effects seem to occur independently of the incident light angle. From our results, a wavelength-dependent multifocal lens behavior can be explained by light diffraction related to the radial symmetry of the multiscaled 3D nanostructure.

Diatom frustules as a biomaterial: effects of chemical treatment on organic material removal and mechanical properties in cleaned frustules from two Coscinodiscus species

AbstractThe three-dimensional structure of silica diatom frustules offers a great potential as nanoporous material for several nanotechnological applications, but the starting point for these applications is the ability to obtain clean frustules with sufficient mechanical strength and intact structure. Here, frustules from the diatoms Coscinodiscus centralis Ehrenberg and Coscinodiscus wailesii Gran et Angst are characterized with respect to their structural integrity, content of residual organic biomaterial and their mechanical properties after two cleaning methods using either hydrogen peroxide as oxidizing agent or a combination of a surfactant (sodium dodecyl sulphate) and a complexing agent. Fluorescence microscopy and energy dispersive spectroscopy (SEM/EDS) analysis revealed clear differences regarding the amount of organic residual within the frustules depending on the cleaning process, with little organic material left after the oxidizing method. This method, however, induced a partial cracking of the frustules suggesting an embrittlement due to the cleaning. Nanoindentation confirmed this and showed that the oxidizing method resulted in more brittle frustules compared to the surfactant/complexing method. More efficient cleaning of organic biomaterial may result in more fragile frustules, and the choice of cleaning method must be based on the planned application.

3D-hyperspectral mapping of light propagation through diatom frustule silica nanostructures

Biological structures harvesting sunlight have evolved for several millions of years in different ways to optimize conversion efficiency. One of the most noteworthy examples of optical bio-nanostructures is the complex silica nanoporous frustule surrounding diatoms (unicellular algae). Diatoms have recently attracted significant attention for their potential in photonic applications. Coscinodiscus wailesii diatom frustules have been shown to combine photonic band gap and waveguide properties resulting in a wavelength dependent light focusing effect. Our experimental and computational results show the dependence between the spatial distribution of light transmitted through a single Coscinodiscus wailesii frustule and the wavelength. We used hyperspectral transmission mapping measurements to study the optical properties of single frustules of Coscinodiscus centralis (CC) and Coscinodiscus wailesii (CW) diatoms. Our aim is to provide a new insight on these properties using direct visualization of the light distribution after transmission through diatom frustules with different structures.

Synthesis and properties of silicon/magnesium silicon nitride diatom frustule replicas

The elaboration of nanostructures including both functional materials and functional morphology is a crucial scientific and technological challenge. In this work diatom frustule replicas mainly consisting of magnesium silicon nitride and nanocrystalline silicon have been obtained by simultaneous metallothermic reduction and nitriding of silica diatom frustules at 800 °C. The frustule replicas retained most of the complex nanoporous structures from the original frustules in the conversion. The optical scattering, transmittance and luminescence properties of the replicas have been investigated. Luminescence was observed and attributed to the presence of silicon nanocrystals. Wavelength dependent diffraction of light was observed in bio-silica frustules but not in frustule replicas, this was attributed to surface coarsening of the replicas during reaction. Light transmittance was found to be lower in frustule replicas and was consistent with absorption of light by Si nanocrystals.

Self-Assembly and Raman Spectroscopy of Additive Coated Nanocrystals

Copper oxalate 1 μm-sized nanocrystalline assemblies with several shapes (cushions, lenses, drilled cushions, and square rods) have been obtained by aqueous precipitation without additive, with glycerol, with PEG and with HPMC. Respective influences of these additives on the nanocrystals self-assembly are suggested from the obtained particles morphologies to provide a better understanding of this assembly process. Raman spectroscopy is used to highlight fluorescence occurring on the copper oxalate samples synthesized with additives. This additive induced fluorescence is suggested to result from specific interactions between the nanocrystals and the adsorbed additives.