Selective Functionalization of the Internal and the External Surfaces of Mesoporous Silicon by Liquid Masking

Abstract

A general approach for selective, differential functionalization of the interior and exterior surfaces of mesoporous Si is reported. The method employs two immiscible liquids, one inert and the other chemically reactive with the porous Si nanostructure. First, a porous Si sample is prepared by electrochemical etch and then it is mildly oxidized, which places a thin layer of silicon oxide at the surface. The inner pore walls of the partially oxidized porous Si film are then infiltrated with an inert liquid (octane). The sample is then immersed in aqueous solution containing hydrogen fluoride (HF), which serves as the reactive liquid. The hydrophobic phase is retained in the interior of the porous nanostructure, and HF(aq) attacks only the exposed surfaces of the oxidized porous Si sample, generating a hydrophobic, hydrogen-terminated (Si-H) outer layer. The reaction is self-limiting due to the immiscibility of octane and water, and the extent of penetration of the Si-H surface into the porous layer is dependent on the time of exposure to HF(aq). The Si-H surface can then be modified by thermal hydrosilylation (1-dodecene or 10-bromo-1-decene) in a subsequent step, resulting in a bifunctional porous Si film containing hydrophobic pore entrances to hydrophilic inner pores. The hydrophobic dodecyl species at the mouths of the pores is found to form a barrier for molecular transport; it decreases the rate of leaching (into water) of a rhodamine test molecule that is preloaded into the sample by >8 fold.

Porous Silicon Nanostructures as Effective Faradaic Electrochemical Sensing Platforms

“…THCpSi and TCpSi almost retain the same Fabry–Pérot fringe pattern as pSi, although with lower intensity. Porosity was determined via the spectroscopic liquid infiltration method (SLIM), using the Bruggeman effective medium approximation to fit the data . Both THCpSi (69 ± 1%) and TCpSi (66 ± 1%) showed similar porosity values to that calculated for freshly etched pSi (73 ± 1%).…”

Spatially Controlled Surface Modification of Porous Silicon for Sustained Drug Delivery Applications

“…In contrast, using organic solvents, different reactive solutes were attached to the inner pSi surfaces. Similarly, Wu and Sailor used an inert liquid as a “mask” to protect the inner pores from being attacked by hydrofluoric acid solution while selectively functionalizing the external pSi surface 34 . More recently, a three step functionalization procedure consisting of two hydrosilylation reactions separated by the selective etching of the external surface was described.…”

Hydrogen‐Terminated Si Nanowires as Label‐Free Colorimetric Sensors in the Ultrasensitive and Highly Selective Detection of Fluoride Anions in Pure Water Phase

“…SiNWs may aggregate into coils in deionized water because their H-terminated surfaces are strongly hydrophobic. [ 37 ] Consequently, SiNWs remain as a small cluster when suspended in water even if they are vigorously agitated or treated with ultrasonic (Figure 2 A). The long-time stability in color and strongly hydrophobic surface of SiNWs also indicate the removal of residual surface ions, such as F − .…”