Walter J. Salcedo

Photoluminescence quenching effect on porous silicon films for gas sensors application.

Porous silicon (PS) films were investigated by Raman, and photoluminescence (PL) spectroscopies using different laser excitations: 488.0, 514.5, 632.8, and 782.0 nm. The analysis of the first-order and second-order Raman spectra have shown that the band gaps of the PS films are indirect as in the bulk c-Si. The Raman phonon and the PL spectra as well as the spectral distribution of the linear polarisation degree (LPD) of PS layers have shown to be dependent on the laser excitation energy. This dependence cannot be explained within the quantum confinement model. A mechanism for the PL emission in PS layers is presented in which the radiative recombination of electron-hole pairs occurs in localised centres (the Si-O-SiR moieties) at the pore/crystallite interface. These quasi-molecular centres are Jahn-Teller active, i.e. the radiative recombination is a phonon-assisted phenomena. The adsorption of gas molecules on the porous silicon surface was studied throughout photoluminescence quenching effect. The adsorption experiments were performed at 10(-6) bar of pressure using gas molecules of organic solvents. In all these cases, the PL intensity was recovered after gas desorption. The PL quenching effect was explained in the sense of electron transfer mechanism (ET).

Porous silicon optical cavity structure applied to high sensitivity organic solvent sensor

The present work reports the thermal annealing process, the number of layer and electrochemical process effect in the optical response quality of Bragg and microcavity devices that were applied as organic solvent sensors. These devices have been obtained by using porous silicon (PS) technology. The optical characterization of the Bragg reflector, before annealing, showed a broad photonic band-gap structure with blue shifted and narrowed after annealing process. The electrochemical process used to obtain the PS-based device imposes the limit in the number of layers because of the chemical dissolution effect. The interface roughness minimizations in the devices have been achieved by using the double electrochemical cell setup. The microcavity devices showed to have a good sensibility for organic solvent detection. The thermal annealed device showed better sensibility feature and this result was attributed to passivation of the surface devices.

Surface-Enhanced Resonance Raman Scattering (SERRS) Using Au Nanohole Arrays on Optical Fiber Tips

AbstractCircular and bow tie-shaped Au nanoholes arrays were fabricated on gold films deposited on the tips of single-mode optical fibers. The nanostructures were milled using focused ion beam with a high quality control of their shapes and sizes. The optical fiber devices were used for surface-enhanced resonance Raman scattering (SERRS) measurements in both back- and forward-scattering geometries, yielding promising performance in both detection arrangements. The effect of the hole shape on the SERRS performance was explored with the bow tie nanostructures presenting a better SERRS performance than the circular holes arrays. The results present here are another step towards the development of optical fiber tips modified with plasmonic nanostructures for SERRS applications. FigureCircular and bow tie-shaped nanohole arrays were milled on gold films deposited on the tips of single-mode optical fibers. The arrays were fabricated by focused ion beam milling, which allowed good control over the sizes and the shapes of the nanostructures. The optical fiber devices were used for surface-enhanced resonance Raman scattering (SERRS) measurements in both back- and forward-scattering geometries. This work represents another step towards the development of optical fiber tips modified with plasmonic nanostructures for SERRS applications

Anodic porous alumina structural characteristics study based on SEM image processing and analysis

The present work reports the porous alumina structures fabrication and their quantitative structural characteristics study based on mathematical morphology analysis by using the SEM images. The algorithm used in this work was implemented in 6.2 MATLAB software. Using the algorithm it was possible to obtain the distribution of maximum, minimum and average radius of the pores in porous alumina structures. Additionally, with the calculus of the area occupied by the pores, it was possible to obtain the porosity of the structures. The quantitative results could be obtained and related to the process fabrication characteristics, showing to be reliable and promising to be used to control the pores formation process. Then, this technique could provide a more accurate determination of pore sizes and pores distribution.

Correlation-based multi-shape granulometry with application in porous silicon nanomaterial characterization

Image-based granulometry measures the size distribution of objects in an image of granular material. Usually, algorithms based on mathematical morphology or edge detection are used for this task. We propose an entirely new approach, using cross correlations with kernels of different shapes and sizes. We use pyramidal structure to accelerate the multi-scale searching. The local maxima of cross correlations are the primary candidates for the centers of the objects. These candidate objects are filtered using criteria based on their correlations and intersection areas with other objects. Our technique spatially localizes each object with its shape, size and rotation angle. This allows us to measure many different statistics (besides the traditional objects size distribution) e.g. the shape and spatial distribution of the objects. Experiments show that the new algorithm is greatly robust to noise and can detect even very faint and noisy objects. We use the new algorithm to extract quantitative structural characteristics of Scanning Electron Microscopy (SEM) images of porous silicon layer. The new algorithm computes the size, shape and spatial distribution of the pores. We relate these quantitative results to the fabrication process and discuss the rectangle porous silicon formation mechanism. The new algorithm is a reliable tool for the SEM image processing.

Influence of laser excitation on raman and photoluminescence spectra and FTIR study of porous silicon layers

A porous silicon lm (PS) was investigated by FTIR, Raman and photoluminescence (PL) spectroscopies. The Raman and PL spectra were obtained using four different laser excitations: 488, 514, 633 and 782 nm. The analysis of the first order and second order Raman scattering lines permits to identify the band energy structure of the crystallites inside the PS film. The analysis of PL spectra shows that the intensity and full width at half-maximum values of PL emission depends on intensity and energy of laser excitation. The linear polarization degree (LPD) of the PL spectra also presents a dependence of laser excitation. The observed dependence of Raman and PL spectra due to laser excitation energy cannot be explained within the quantum confinement alone. We propose a mechanism for PL emission in PS layers, in which the radiative recombination occurs in localized centers at pore/crystallite interface. These quasi-molecular centers are Jahn-Teller active.

Self-assembled systems obtained by chemical and electrochemical techniques for photonic crystal fabrication

Abstract The present work reports the formation of self-assembled systems by chemical and electrochemical techniques for photonic crystal structures fabrication. The chemical technique was used to the fabrication of self-assembled polystyrene micro-spheres structures. The influence of temperature setup and monodispersion concentration in the chemical self-assemble mechanism was studied. The electrochemical technique also was used to fabricate the self-assembled porous alumina structures. The influence of electrolyte type and anodic voltage in the electrochemical self-assemble process was studied. The self-assembled structures were analyzed to the possibility of application as photonic crystal structures

Backside contact effect on the morphological and optical features of porous silicon photonic crystals

The present work reports on the effect of the type of backside contact used in the electrochemical process and their relation with the structural features and optical responses of the one-dimensional photonic crystal (PC) anodized in simple and double electrochemical cell. The PC, obtained in the single cell, showed to have thicker layers than of the PC obtained in double electrochemical cell. Additionally, the PC obtained in double cell showed highest reflectance in the band gap region than of the PCs obtained in single cell. These results suggest that the interface roughness between adjacent layers in the PC devices obtained in double electrochemical cell is minimized.

A New Correlation-Based Granulometry Algorithm with Application in Characterizing Porous Silicon Nanomaterials

Granulometry is the process of measuring the size distribution of objects in an image of granular material. Usually, algorithms based on mathematical morphology or edge detection are used for this task. We propose a entirely new approach for the granulometry using the cross correlations with circles of different sizes. This technique is primarily adequate for detecting circular-shaped objects, but it can be extended to other shapes using other correlation kernels. Experiments show that the new algorithm is greatly robust to noise and can detect even faint objects. This paper also reports the quantitative structural characteristics of the porous silicon layer based on the proposed algorithm applied to Scanning Electron Microscopy (SEM) images. The new algorithm computes the size distribution of pores and classifies the pores in circular or square ones. We relate these quantitative results to the fabrication process and discuss the square porous silicon formation mechanism. The new algorithm shows to be reliable in SEM images processing and is a promising tool to control the pores formation process.

Fractal Brownian motion for feature extraction in noisy signals from gas sensors

The present work reports on a new pattern recognition method applied to the electrical response of a gas sensor. The sensor is a tin oxide device used as an electronic nose. It yields noisy responses when submitted to organic solvents. Signals were analyzed by the epsilon-blanket fractal dimension associated with the fractal Brownian motion. The classification of features shows close to 100% recognition rate for five different gases.