Chemically-modified Porous Silicon and its applications as sensor and in optical devices

In 1990, L.T. Canham discovered RT tunable photoluminescence properties of Porous Silicon (PS). At that time, this material was not the subject of a strong scientific interest and only used as insulator in SOI (Semiconductor On Insulator) and FIPOS (Full Insulation Porous Oxidized Silicon) techniques, and as a sacrificial layer in the micromachining technology. Since the 90’s, PS became one of the main topics in the semiconductor science, one additional reason being exploiting the processing properties already developed for crystalline Silicon. Important aspects of PS are the ease of synthesis and the cheapness of technologies employed: it is produced by means of electrochemical etching of crystalline Silicon in a HF-based solution and its porosity, morphology and specific surface area can be tailored changing the etching parameters (HF concentration, current density and etching time). SECTION I of this thesis concerns the description of main parameters to be taken into account for the preparation of the desired type of sample. Potential applications of PS are in many different fields: Sensors (change of conductivity and capacitance, quenching of the photoluminescence), Optics and Photonics (optical filters such as Bragg Reflector and Fabry-Perot microcavity), Optoelectronics (Light Emitting Diode, waveguides, Photo Detectors) and, more recently, Biomedical Technology. SECTION II describes all those properties and applications of PS related to this thesis. A detailed description of some of them is necessary to understand the main goals of the works presented. Photoluminescence of porous silicon depends not only upon morphology but also on the presence of chemicals inside its voids. For instance, conductivity and capacitance (and the related changes) are associated with the composition of the gas phase present in the pores and, not less important, to the way by which these species interact with PS surface. PS surface is highly reactive and its state controls luminescence and electrical properties. Applications of PS depend also upon the possibility to handle and control surface properties in a predictable way. As a consequence, the study of the stability of PS became an important topic for many groups that, with this purpose, tried to derivatize the surface with organic groups. One of the main attempts was the covalent bonding of saturated hydrocarbon chains which render the surface hydrophobic, so avoiding oxidation. Lewis acid mediated functionalisation using alkenes and alkynes substituted with halides, hydrosilylation of C-C double bond and cleavage of Si-Si bond with Grignard and alkyllithium reagent are only some examples. More recent studies concerning the modification of PS surface were oriented to the specificity and selectivity against target molecules. The large surface area, the possibility to obtain macropores and the biocompatibility were the impulse for the developing of novel biochemical sensors. Our interest for this material is related to its applications as sensor, where PS can be used in various fields: biomedical (e.g., for DNA derivatisation and immunoglobulines, bacteria, etc. detection), detection of pollutants (e.g., NO2), warfare agents (Sarin gas) and explosives such as DNT and TNT. 1. Concerning the sensing behaviour based on the electrical detection of the response, the experimental work of this thesis has a twofold aim: A. To provide a “chemical” interpretation of the mechanism leading to the passivation of charge free carriers (holes) in mesoporous p+-type PS. The comprehension of this behaviour might also help in explaining how, in presence of suitable gaseous phases (or liquids), the electrical conductivity is modified. Experiments and discussions on this topic are reported in Chapters 1-4 of SECTION III. B. To study the reactivity of PS surface towards proper organic molecules and try to describe the mechanisms of the reactions. The purpose is both to protect and to impart a “specificity” which may raise the number of potential applications of PS. Since the study of the reaction mechanism at the surface is not a trivial task, at first, very simply molecules have been used, because the mechanisms of their interactions would be more readily understandable. However, the actual target is to bond at the surface molecules which give rise to charge transfer processes with PS. The target is to permanently reactivate PS electrical conductivity, i.e. to produce a real “porous semiconductor”. Chapter 5 of SECTION III is devoted to these objectives. 2. Chapter 6 of SECTION III concerns the preparation of a hybrid sensor based on the optical detection of the signal: waveguides made of oxidised PS are used to host Congo-Red, a dye normally used in aqueous solution as pH indicator.