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Issue Date: 31-Mar-2017
Authors: Lo Faro, Maria Josè
Title: Silicon Nanowires the Route from Synthesis towards Applications
Abstract: This work provides an overview on the silicon nanowires synthesis scenario by introducing the most employed bottom-up and top-down strategies, exploiting their main advantages and drawbacks. The impressive structural, optical and electrical properties of these nanostructures promote their implementation in low power consumption nanodevices with improved performances. Some representative examples of Si NW applications in field-effect transistors, broadband photodetectors, low-cost solar cells with improved light absorption and for selective and ultrasensitive biological detectors are reported. The realization of Si NWs by metal assisted chemical etching (MACE) by the use of discontinuous Au layers and the relationship among structural features and growth conditions are here discussed in detail. Indeed, the NW length, diameter, crystalline structure and doping can be precisely defined by using this low cost and industrially compatible process. Si NWs with quantum confined size are realized by MACE leading to the observation of room temperature light emission from Si. According to quantum confinement theory, the emission wavelength can be red-shifted by tuning the NW diameter opening the routes towards low-cost, Si-based photonics. Moreover, the realization of innovative multiwavelength light sources operating at room temperature is investigated by embedding a carbon nanotube (CNT) dispersion inside Si nanowire arrays using a low cost and Si technology compatible technology. The NW/CNT hybrid system exhibits a tunable emission both in the visible and in the infrared which is strategic for telecommunication applications. The conditions leading to the prevalence of the visible or the IR signal have been identified and are herein discussed. The design of 2D random fractal arrays of Si nanowires is here described. Indeed, the structural arrangement of MACE synthesized Si NWs can be engineering by the deposition of a thin Au layer that superimposes its complimentary fractal arrangement onto the Si NW arrays. Si NW fractal arrays display strong self-similarities over a wide range of length scales and the correlation among the fractal parameters and the optical properties are demonstrated. In fact, the ability to control the scattering, absorption and emission properties is investigated as a function of fractal dimension and lacunarity for different designs. A strong light trapping behavior in the visible range due to the efficient in-plane multiple scattering occurring in the Si NW layer has a promising potential for both photovoltaics and photonics. Furthermore, the first experimental observation of a constructive interference effect in the backscattered Raman light from strongly diffusing Si nanowires is reported. Coherent backscattering of light (CBS) is observed when electromagnetic waves undergo multiple scattering within a disordered optical medium. CBS effect arising from the interference of inelastic scattered Raman radiation has been demonstrated in random Si NW arrays. The results are interpreted within the theoretical model of mixed Rayleigh-Raman random walks, exploiting the role of phase coherence in multiple scattering phenomena. In conclusion, the decoration of MACE-synthesized Si NWs by Ag nanoparticles (NPs) produced by pulsed laser deposition (PLD) is an appealing strategy in order to couple the huge aspect ratio of NWs to plasmonic effects leading to the realization of ultrasensitive surface enhanced Raman spectroscopy (SERS) sensors. PLD conditions have been optimized to guarantee the uniform decoration of NW sidewall along all their length without the need of any post-deposition annealing by using a low-cost and Si implementable technology. The Ag NP morphology can be precisely tuned as a function of the NW length or the number of laser pulses and the correlation among the structural and optical properties of Si NWs decorated is reported, demonstrating a great potentiality for SERS applications.
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