Raúl J. Martín-Palma

Biomimetization of butterfly wings by the conformal-evaporated-film-by-rotation technique for photonics

Mimetization of biological structures aims to take advantage of their spatial features for the development of devices of tailored functionality. In this work, we replicated the wing of a butterfly at the micro- and nanoscales by implementing the conformal-evaporated-film-by-rotation (CEFR) technique. Chalcogenide glasses were used due to their good optical and mechanical properties. Morphological characterization and optical measurements indicate high-fidelity replication of the original biotemplate; furthermore, the optical properties of the butterfly wings have a structural origin. The CEFR technique might be useful for the fabrication of highly efficient, biomimetic optical devices.

Direct determination of grain sizes, lattice parameters, and mismatch of porous silicon

High-resolution transmission electron microscopy and digital image processing were used to investigate in detail the structure of porous silicon (PS). It was found that PS is composed of rounded Si nanocrystals with characteristic sizes between 21 and 80 A, embedded into an amorphous matrix and with no preferential orientation. We have determined that the size distribution of the nanocrystals can be fitted to a Gaussian distribution centered at 45.89 A. Furthermore, the structure of the individual Si grains was studied, which allowed us to determine that the interplanar distance varies from 3.17 to 3.41 A, with Gaussian distribution centered at 3.24 A. Finally, the lattice parameter of the individual Si grains that compose PS was also measured, showing a Gaussian distribution centered at 5.61 A. The origin of the structure of PS is also discussed.

Optical biosensors based on semiconductor nanostructures.

The increasing availability of semiconductor-based nanostructures with novel and unique properties has sparked widespread interest in their use in the field of biosensing. The precise control over the size, shape and composition of these nanostructures leads to the accurate control of their physico-chemical properties and overall behavior. Furthermore, modifications can be made to the nanostructures to better suit their integration with biological systems, leading to such interesting properties as enhanced aqueous solubility, biocompatibility or bio-recognition. In the present work, the most significant applications of semiconductor nanostructures in the field of optical biosensing will be reviewed. In particular, the use of quantum dots as fluorescent bioprobes, which is the most widely used application, will be discussed. In addition, the use of some other nanometric structures in the field of biosensing, including porous semiconductors and photonic crystals, will be presented.

Biomedical applications of nanostructured porous silicon: a review

Current fabrication and characterization techniques allow the development of nanostructured systems with controlled size, shape and composition. Additionally, modifications can be made to these nanosystems to better suit their integration with biological systems, leading to such interesting properties as enhanced aqueous solubility, biocompatibility or bio-recognition. The particular morphology and overall properties of nanostructured porous silicon allow the use of this material in the fields of drug delivery, eye diseases, tumor imaging, and tissue engineering.

Silver-based low-emissivity coatings for architectural windows: Optical and structural properties

Abstract Spectrally selective coatings are, nowadays, deposited onto architectural windows to be employed in commercial and residential buildings for the purpose of saving energy. In this work, the fundamental optical and structural properties of two types of low-emissivity silver-based coatings deposited onto glass, Ag and SnO 2 /Ni-Cr/Ag/Ni-Cr/SnO 2 (Vitrage a couches avec une forte reflexion des rayonnements thermiques, European Patent EP 0 506 507 B1) are analyzed. It has been demonstrated that the silver-based multilayer coatings give an efficient heat isolation due to their low emissivity values, thus not deteriorating the optical properties in the visible range given by the glass substrate. From Atomic Force Microscopy measurements it has been determined that the Ag layer has a more homogeneous grain size when is deposited onto the Ni-Cr layer than when is deposited onto glass.

Mass fabrication technique for polymeric replicas of arrays of insect corneas

Motivated to develop a technique for producing many high-fidelity replicas for the sacrifice of a single biotemplate, we combined a modified version of the conformal-evaporated-film-by-rotation technique and electroforming to produce a master negative made of nickel from a composite biotemplate comprising several corneas of common blowflies. This master negative can function as either a mold for casting multiple replicas or a die for stamping multiple replicas. An approximately 250 nm thick nickel film was thermally deposited on an array of blowfly corneas to capture the surface features with high fidelity and then a roughly 60 microm thick structural layer of nickel was electroformed onto the thin layer to give it the structural integrity needed for casting or stamping. The master negative concurrently captured the spatial features of the biotemplate at length scales ranging from 200 nm to a few millimeters. Polymer replicas produced thereafter by casting did faithfully reproduce features of a few micrometers and larger in dimension.

RBS characterization of porous silicon multilayer interference filters

Porous silicon (PS) has great potential in optical applications due to its tunable refractive index. In particular, multilayer structures consisting of alternating PS layers with different refractive indexes can be used as interference filters for applications in optoelectronics. In the present work, Rutherford backscattering spectroscopy (RBS) measurements and optical characterization have been carried out on PS multilayer stacks consisting of alternate low-porosity/high-porosity layers to determine their compositional profile, homogeneity, and overall optical behavior. In addition, RBS has been used for the first time to determine the porosity profile of this kind of structures. The experimental results show a constant indepth composition among alternate layers, revealing the good homogeneity of the multilayer structures. Neither porosity nor oxidation degree gradient were observed.

Morphological, optical and electrical characterization of antireflective porous silicon coatings for solar cells

Porous silicon (PS) layers are formed on multicrystalline silicon solar cells by chemically etching the emitter of these devices. Stain etched PS provides the simultaneous formation of a selective emitter and an antireflective layer. The optical behavior of the antireflective coating over the solar spectrum is determined, resulting in very low values of the reflectance. The variation of the topography, as well as the grain size of the PS and the metallic contacts before and after PS formation, is analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Furthermore, the electrical properties of both the emitter and the front metallic contacts are investigated. Finally, the effect of rapid thermal processing in nitrogen and oxygen atmospheres on the antireflective properties of the PS is studied.

Depth-resolved microspectroscopy of porous silicon multilayers

We have measured depth-resolved microphotoluminescence (PL) and micro-Raman spectra on the cross section of porous silicon multilayers to sample different layer depths. The PL emission band gets stronger, blueshifts, and narrows at the high porosity layers. On the contrary, the Raman band weakens and broadens. This band is fitted to the phonon confinement model. With the bulk silicon phonon frequency and its linewidth as free parameters, we obtain crystallite size, temperature, and stress as a function of depth. Sizes are larger than those estimated from PL. Laser power was reduced to eliminate heating effects. Compressive stresses in excess of 10 kbar are found in the deepest layer due to the lattice mismatch with the substrate.

Gold Nanostructures for Surface-Enhanced Raman Spectroscopy, Prepared by Electrodeposition in Porous Silicon

Electrodeposition of gold into porous silicon was investigated. In the present study, porous silicon with ~100 nm in pore diameter, so-called medium-sized pores, was used as template electrode for gold electrodeposition. The growth behavior of gold deposits was studied by scanning electron microscope observation of the gold deposited porous silicon. Gold nanorod arrays with different rod lengths were prepared, and their surface-enhanced Raman scattering properties were investigated. We found that the absorption peak due to the surface plasmon resonance can be tuned by changing the length of the nanorods. The optimum length of the gold nanorods was ~600 nm for surface-enhanced Raman spectroscopy using a He–Ne laser. The reason why the optimum length of the gold nanorods was 600 nm was discussed by considering the relationship between the absorption peak of surface plasmon resonance and the wavelength of the incident laser for Raman scattering.