Porous silicon is a nano-to macroporous high surface area material that can be fabricated from n-and p-type silicon by chemical or electrochemical etching processes involving hydrofluoric acid. Originally it was studied for its application as an insulating material in microelectronics. [1] Later on it was discovered that many forms of porous silicon show intense photoluminescence under UV excitation, [2] which opened the possibility for applications in optoelectronics, display technologies, [3] and chemical sensing. [4] Most recently, its passive optical properties have been exploited for antireflective coatings, [5] wave guides, [6] interference filters, [7] and biosensors. [8] Pristine porous silicon has a hydride-terminated surface that is prone to oxidation and corrosion. In recent years there has been much progress in the surface modification of porous silicon and hydride-terminated crystalline silicon with organic moieties. Covalently attached (Si À C) stable organic monolayers have been formed by cleavage of Si À Si bonds with Grignard [9] and alkyllithium reagents. [10] Similar results were obtained by a series of thermal, [11] photochemical, [12] and Lewis acid catalyzed [13] hydrosilylation reactions. Electrochemical oxidation of methyl Grignard reagents [9b] on porous silicon and electrochemical reduction of phenyldiazonium salts [14] on single-crystal silicon have been shown to yield dense monolayers of methyl and phenyl groups, respectively. In many cases the organic functionalization dramatically improves the chemical stability of porous silicon, which has spurred renewed interest in this material for sensor and display applications.Herein we report on a fast and highly efficient electrochemical method for surface functionalization of n-and p-type porous silicon based on reductive electrolysis of alkyl iodide, alkyl bromide, and benzyl bromide species. This new method provides high surface coverage, while requiring only very short (`2 min) reaction times. Widely available monoand bifunctional organo halide reagents with low to moderate reactivity make this method convenient to use and generally applicable.The reductions are performed in 0.2 m to 0.4 m solutions of the organo halides in dry, deoxygenated acetonitrile or mixtures of acetonitrile and tetrahydrofuran containing 0.2 m LiBF 4 . Alkyl bromides, which do not react as well as alkyl iodides or benzyl bromides, can be converted into the iodides in situ by replacing LiBF 4 with LiI as the electrolyte. Passing a cathodic current (2 ± 10 mA cm À2 ) for short periods of time (30 ± 120 s) leads to high coverage of the porous silicon surface with organic species, as determined by FT-IR (Figure 1). The Figure 1. Transmission FT-IR spectra of p-type porous silicon a) before and after derivatization with: b) methyl iodide, c) dodecyl iodide, and d) ethyl 6-bromohexanoate. Absorbance intensities A are normalized to the n Ä(SiH) band at 2116 cm À1 measured before derivatization. Thin film optical interference effects that contribute to a sinusoidal b...