Mesoporous silicon in drug delivery applications.

Silicon surface chemistry is of fundamental importance because of the ubiquitous role of silicon in modern technology and yet is only just beginning to be investigated.1 Virtually all microprocessor chips in electronic products are based upon crystalline silicon wafers.2 Control of silicon surface chemistry is crucial to allow access to technologically interesting thin films for fabrication of new electronic devices.3 Canham and co-workers showed in 1990 that silicon wafers could be etched to produce a microns-thick porous layer (termed porous silicon) which exhibits photoluminescence upon exposure to UV light.4 Potential applications for porous silicon include use as chemical sensors,5 optoelectronic devices6 such as electroluminescent displays,7 photodetectors,8 and as a matrix for photopumped tunable lasers.9 As a result, modification and characterization of photoluminescent porous silicon surfaces has become an area of intense interest.10 In this work we report a mild and general approach for covalent modification of the surface of porous silicon through EtAlCl2 mediated hydrosilylation of readily available alkynes and alkenes which yields surface bound vinyl and alkyl groups, respectively, as outlined in eqs 1 and 2.11

Lewis acid mediated functionalization of porous silicon with substituted alkenes and alkynes

Organometallic chemistry on silicon surfaces: formation of functional monolayers bound through Si–C bonds

Surface chemistry of Luminescent Silicon Nanocrystallites

A novel white light-promoted reaction using photoluminescent nanocrystalline silicon enables the hydrosilylation of alkenes and alkynes, providing stabilization of the porous silicon without significant loss of the photoemissive qualities of the material. Photopatterning and lithographic fabrication of isolated porous silicon structures are made possible. Experiments and observations are presented which indicate that the light promoted hydrosilylation reaction is unique to photoluminescent silicon, and does not function on nonemissive material. Hydrosilylation using a reactive center generated from a surface-localized exciton is proposed based upon experimental evidence, explaining the photoluminescence requirement. Indirect excitons formed by light absorption mediate the formation of localized electrophilic surface states which are attacked by incoming alkene or alkyne nucleophiles. Supra-band gap charge carriers have sufficient energy to react with nucleophilic alkenes and alkynes, thereupon causing Si−...

Exciton-mediated hydrosilylation on photoluminescent nanocrystalline silicon.

Light-induced reactions of Si surfaces with carboxylic acids to generate Si ester-modified surfaces are studied. The reaction proceeds by photoelectrochemical oxidation of porous or (100)-oriented single-crystal n-type Si in formic, acetic, or trifluoroacetic acid electrolyte solutions. The reaction products at the porous Si surface are identified by Fourier-transform infrared (FTIR) spectroscopy. Derivatization with esters reduces the photoluminescence intensity of porous Si. X-ray photoelectron spectroscopy (XPS) of derivatized single-crystal Si is used to confirm the compositional and bonding information and to demonstrate that the same chemistry occurs at a single-crystal Si surface. A mechanism is proposed in which illumination of reverse-biased Si removes electron density from the Si surface, rendering Si−Si bonds susceptible to nucleophilic attack by carboxylic acid. The reaction has a marked dependence on light intensity and the Si surface can be photopatterned by illumination through a mask durin...

Light-Induced Reactions of Porous and Single-Crystal Si Surfaces with Carboxylic Acids

Visible light emission from porous silicon can be stimulated by applying a positive bias to a formic acid/sodium formate liquid junction cell. The emission lasts for 45 min at +2.75 V applied potential (<5 mA/cm2, power conversion efficiency ≳10−2%) and is reliably generated from n‐ or p‐type porous silicon. An applied voltage as low as 1.3 V is capable of generating the red (720 nm) emission, indicating that current‐induced chemical reactions aid in the generation of charge carriers. A mechanism involving oxidation of formic acid followed by electron injection from a CO2− radical is proposed. Infrared spectra of the porous silicon surface taken after anodization show formation of a stable silyl‐ester species that is thought to be responsible for the increase in radiative recombination efficiency through passivation of surface defects.

Electrochemiluminescence from porous silicon in formic acid liquid‐junction cells

Light induced surface reactions of reverse‐biased n‐type porous Si with H 2O, CH3OH or CF3COOH are studied by diffuse reflectance and transmission Fourier transform infrared spectroscopy. The goal of this work is to identify reagents which will react with the irradiated, electron deficient surface but not the non‐irradiated, neutral surface of reverse‐biased porous Si. The porous Si is the anode in an electrochemical cell with the desired reagent present as the electrolyte solution. H2O/0.5M NaCl is found to react with the reverse‐biased Si in both the light and the dark to form the native oxide. CH 3OH/0.1M NaBF4 generates a small amount of methoxide‐modified Si in both the light and the dark. CF3COOH/1M CF3COONa reacts with the surface when the porous Si is optically excited to create a surface‐bound trifluoroacetic ester species. CF 3COOH/1M CF3COONa does not react with reverse biased Si in the dark. The Si surface can be photolithographically patterned with the trifluoroacetic ester species.

Theodore W. Bitner

Papers

Light-Induced Reactions of Porous and Single-Crystal Si Surfaces with Carboxylic Acids

Electrochemiluminescence from porous silicon in formic acid liquid‐junction cells

Photoinduced surface reactions of reverse‐biased n‐type porous Si