Fabrication of flexible silicon nanowires by self-assembled metal assisted chemical etching for surface enhanced Raman spectroscopy

A homogenous array of flexible gold coated silicon nanowires was fabricated by the combination of nano spheres lithography and metal assisted chemical etching to obtain highly effective Surface Enhanced Raman Spectroscopy (SERS) substrates. 3D nanostructures with different aspect ratios and well-defined geometries were produced by adjusting the fabrication parameters in order to select the best configuration for SERS analysis. The optimum flexible nanowires with an aspect ratio of 1 : 10 can self-close driven by the microcapillary force under exposure to liquid and trap the molecules at their metallic coated ``fingertips'', thus generating hot spots with ultrahigh field enhancement. The performance of these SERS substrates was evaluated using melamine as the analyte probe with various concentrations from the millimolar to the picomolar range. Flexible gold coated SiNWs demonstrated high uniformity of the Raman signal over large area with a variability of only 10% and high sensitivity with a limit of detection of 3.20 x 10(-7) mg L-1 (picomolar) which promotes its application in several fields such food safety, diagnostic and pharmaceutical. Such an approach represents a low-cost alternative to the traditional nanofabrication processes to obtain well ordered silicon nanostructures, offering multiple degrees of freedom in the design of different geometries such as inter-wire distance, density of the wires on the surface as well as their length, thus showing a great potential for the fabrication of SERS substrates

Peptide immobilisation on porous silicon surface for metal ions detection

In this work, a Glycyl-Histidyl-Glycyl-Histidine (GlyHisGlyHis) peptide is covalently anchored to the porous silicon PSi surface using a multi-step reaction scheme compatible with the mild conditions required for preserving the probe activity. In a first step, alkene precursors are grafted onto the hydrogenated PSi surface using the hydrosilylation route, allowing for the formation of a carboxyl-terminated monolayer which is activated by reaction with N-hydroxysuccinimide in the presence of a peptide-coupling carbodiimide N-ethyl-N'-(3-dimethylaminopropyl)-carbodiimide and subsequently reacted with the amino linker of the peptide to form a covalent amide bond. Infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy are used to investigate the different steps of functionalization

Electrical and optical properties of annealed plasma-modified porous silicon

A very large surface to volume ratio of nanoporous silicon (PS) produces a high density of surface states, which are responsible for uncontrolled oxidation of the PS surface. Hence it disturbs the stability of the material and also creates difficulties in the formation of a reliable electrical contact. To passivate the surface states of the nanoporous silicon, hydrocarbon films prepared by plasma enhanced chemical vapor deposition of methane (PECVD) have been deposited on porous silicon (PS). Conduction properties as well as optical characteristics of the as-deposited samples and annealed ones have been studied by current-voltage (I-V) and capacitance-voltage (C-V) measurements, spectral response, ellipsometry and Fourier transform infrared spectroscopy (FTIR) as a function of the annealing temperature, ranging from 200 to 750 °C. By high-temperature treatment, the low rectifying I-V curves become strong rectifying and C-V curves are similar to those of metal-insulator-semiconductor structures. These characteristics were explored and can be appropriately used in the fabrication of optoelectronics and sensor devices based on PS

Etude et réalisation de capteurs de gaz à base de silicium poreux

L‟intérêt croissant pour la prévention de la pollution de l‟environnement est devenu ces dernières années un facteur clé dans le développement de toute industrie, en particulier celle des matériaux largement utilisés en microélectronique. Dans ce cadre,le silicium poreux a suscité une grande attention comme matériau appliqué à la détection en raison de la haute sensibilité de sa surface interne à la présence de gaz et d‟humidité. Dans notre travail, on se propose de contrôler les variations des propriétés électriques des capteurs à base de silicium poreux, causées par des substances chimiques gazeuses. Plusieurs caractéristiques électriques de notre structure ont été étudiées afin de vérifier la règle connue des six « S » : Sensibilité (sensibility); sélectivité (selectivity), réponse rapide (speed of réponse), stabilité (stability), la taille/la forme (size/shape) et le coût ($/cost.). Les résultats montrent que les caractéristiques courant-tension et capacité-tension sont modifiées par la réactivité du gaz sur la surface de notre capteur et ceci pour différents gaz et différentes concentrations. En outre, les résultats montrent que la conduction change selon la nature du gaz.Mot clés : Silicium Poreux - Détecteur - Gaz - Caractéristiques Electrique

Caractéristiques morphologiques et optiques du silicium nanoporeux préparé par anodisation électrochimique

L'anodisation du silicium dans des solutions d’acide hydrofluorique HF est une méthode acquise dans la préparation du silicium poreux avec des applications potentielles dans les domaines tels l’électronique, l’optique ainsi que la technologie chimique et celle des capteurs de gaz et des biocapteurs. Le contrôle de la densité de courant et du temps d’anodisation permet de modifier l’épaisseur et le taux de porosité de la couche du silicium poreux ainsi formé. Les échantillons du silicium nanoporeux sont préparés par anodisation électrochimique à partir du silicium monocristallin de type P, d’orientation (1,0,0) avec une faible résistivité de l’ordre de 1 Ωcm et d’épaisseur voisine de 400 μm. Les caractéristiques morphologiques, optiques ainsi que la composition chimique de nos échantillons sont étudiées et analysées pour différentes conditions d’anodisation. Dans notre travail on a utilisé plusieurs techniques de caractérisations .Parmi ces techniques l’ellipsométrie. Pour comprendre le comportement optique de ce matériau, on utilise un modèle théorique basé sur l’approximation des milieux effectifs (modèle de Bruggeman) afin de déterminer la relation entre l’indice de réfraction du film nanoporeux, sa porosité et son degré d’oxydation.Mots clés: silicium nanoporeux; modèle de Bruggeman; indice de réfraction; degrés d’oxydation. The silicon anodisation in hydrofluoric acid solutions HF is an acquired method in the preparation of porous silicon, with potential applications in many fields such as electronics, optics as well as chemical technology and that of gas sensors and biosensors. The control of the density of current and the time of anodisation make it possible to modify the thickness and the proportion of porosity of the layer of porous silicon thus formed. Samples of nanoporous silicon are prepared by electrochemical anodization starting from the single-crystal silicon of type P, orientation (1,0,0) with a low resistivity of about 1 Ωcm and a thickness close to 400 μm. The morphological, optical characteristics as well as the chemical composition of our samples are studied and analyzed for various conditions of anodization. We have used several techniques of characterizations among them ellipsometry. To understand the optical behavior of this material, we have used a theoretical model based on the approximation of the effective mediums (model of Bruggeman) in order to determine the relation between the index of refraction of nanoporous film, its porosity and its oxidation degrees.Keywords: nanoporous silicon; model of Bruggeman; index of refraction; oxidation degrees. 

Experimental study of macropore formation in p-type silicon in a fluoride solution and the transition between macropore formation and electropolishing

Anodic dissolution of p-Si is studied in diluted fluoride solution (HF 0.05M+NH4F 0.05 M, pH 3), with special focus on the physico-chemical parameters which govern the morphology of pore formation (crystallographic orientation, applied potential, and etching time). The effect of potential has been investigated in the transition region between macropore formation and electropolishing.Upon increasing the anodization potential, the pore cross-section changes from circular to square shape, and the bottom of the pores changes from a rounded to a V-shaped profile. Prolonged etching of the contour of (1 1 0) p-Si disks in the regime of porous silicon formation allows for a comparison of the etching characteristics of the (1¯1x) orientations. SEM observation indicates indeed different morphologies as a function of the crystal orientation, and the formation of fractal-like structures is obtained for some orientations. In the same geometry and at a potential just above the onset of the electropolishing regime, prolonged anodization allows for a direct measurement of the Si thickness removed as a function of the crystallographic orientation. We clearly observe the etching anisotropy, with etch depth (111) < (110) < (1 0 0). This sequence, similar to that observed for current density in more concentrated HF, differs from that observed for the chemical etching of Si in an alkaline solutio

Photoluminescence from undoped silicon after chemical etching combined with metal plating

Photoluminescent porous layers were formed on highly resistive p-type silicon by metal-assisted chemical etching using Na2S2O8 as an oxidizing agent. A thin layer of Ag was deposited on the (100) Si surface prior to immersion in a solution of HF and Na2S2O8. The morphology of the porous silicon (PS) layer formed by this method as a function of etching time was investigated by scanning electron microscopy (SEM). It shows that the surface is porous and the thickness of PS layer increases with etching time and is not limited as observed with the electrochemical method. Energy-dispersive X-ray (EDX) was used to analyse the chemical composition of PS layers. The EDX spectra show that the metal is not present on the PS surface after etching. Photoluminescence (PL) from metal-assisted chemically etched layers was measured using a He-Cd laser as excitation source. It was found that the PL intensity increases with increasing etching time. However, it was shown that after an etching time of 30min, the fit of the PL spectrum using Gaussian functions exhibits two peaks centred at 617 nm and 646 nm. This behaviour was attributed to an increase of the silicon nanostructure density

Wehrspohn, “Quantitative Analysis of the Morphology of Macropores on Low-Doped p-Si

The formation of macropores by anodization of low-doped p-Si in HF electrolyte has been investigated quantitatively. As anodization proceeds, structures of increasing characteristic size are formed, then a steady state is reached, where macropores grow parallel. The intermediate regime is well understood on the basis of a linear stability approach, incorporating the known physics and chemistry of the Si/electrolyte interface: semiconductor space charge and interface reaction velocity. The characteristic size of the macropores and their dependence on Si doping and electrolyte resistivity and composition are quantitatively accounted for after realizing that parallel growth is strongly favored by the channeling of the current in the macropores. Below a critical resistivity, no macropores are observed. It is shown, through a numerical simulation, that this change of behavior results from a loss of the insulating character of the walls, due to effects of disorder in a depletion layer when doping increases