Passivation of (100) silicon surfaces using alkyl Grignard reagents is explored via electrochemical and thermal grafting methods. The electrochemical behavior of silicon in methyl or ethyl Grignard reagents in tetrahydrofuran is investigated using cyclic voltammetry. Surface morphology and chemistry are investigated using atomic force microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS). Results show that electrochemical pathways provide an efficient and more uniform passivation method relative to thermal methods, and XPS results demonstrate that electrografted terminations are effective at limiting native oxide formation for more than 55 days in ambient conditions. A two-electron per silicon mechanism is proposed for electrografting a single (1:1) alkyl group per (100) silicon atom. The mechanism includes oxidation of two Grignard species and subsequent hydrogen abstraction and alkylation reaction resulting in a covalent attachment of alkyl groups with silicon.
Nanoscale patterns are created on silicon ͑100͒ surfaces via cathodic electrografting of alkyls. Self-assembled polystyrene microspheres on silicon surfaces can serve as dielectric masks. Cathodic potentials are applied to masked substrates in liquid hexynoic acid or phenylacetylene leading to the formation of nanoscale islands with monolayers of covalently bound functional groups on silicon. Silicon surfaces functionalized with islands of hexynoic acid are demonstrated using a biotin reaction. Cyclic voltammetry, scanning electron microscopy, and atomic force microscopy are used to characterize the patterned surfaces. The method provides a facile and high throughput process for spatially defining the local reactions on silicon with nanometer-scale resolution.Organic-semiconductor interfaces are interesting for applications in biological 1 or chemical 2 integrated devices and photoelectrochemical cells. 3-6 Hydrogen-terminated silicon surfaces are thermodynamically unstable at ambient conditions and typically react with water or oxygen within a few hours after HF treatment. If the hydrogen termination is replaced or modified with a covalently bonded organic species, a relatively stable surface with controllable functionalization and desirable electronic 7 properties may be created. In contrast with siloxane-coupling reactions involving intermediate surface oxides and suboxides, direct ͑Si-C͒ functionalization allows close proximity between silicon and target molecules and limits other physical and electrical 8 defects associated with poor-quality native oxides. 9 Monolayers consisting of -functionalized reactive groups such as acids, aldehydes, 10 or esters 1,11,12 may be used to mediate the attachment of complementary molecular species. 13 Likewise, surfaces treated with relatively unreactive groups may improve the passivation and increase the stability in wet or humid environments allowing electron-transfer reactions between silicon surfaces and aqueous solutions. Several organic functionalization methods 14 have been demonstrated for the direct attachment of organic moieties onto silicon hydride surfaces including catalytic, 15,16 thermal, 17 and photonic 11,18 initiated hydrosilylation reactions or cathodic and anodic electrografting. 19 Electrografting allows the efficient formation of molecular monolayers at metal or semiconductor surfaces. 20 As proposed in the work by Robins et al., cathodic electrografting ͑CEG͒ of alkylfunctional molecules on silicon ͑111͒ proceeds by the reduction of surface Si-H bonds to generate silyl anion intermediates at the surface. 19 The weakly acidic alkyl groups are deprotonated in situ by the surface silyl anion to produce solution carbanions. Subsequently, a nucleophilic attack by the alkyl carbanion leads to the covalent grafting of the alkyne across Si-Si back bonds and the regeneration of a surface silyl anion.Electrografting provides passivating or reactive surfaces for interfacing with other organics such as proteins or molecular wires. 21 In this respect, patterned fi...
The solid electrolyte interphase (SEI) plays a significant role in maintaining reversible capacities by protecting silicon-based lithium insertion anodes from deleterious chemical reactions with electrolyte and mechanical failures. In this work, silicon nanowires were functionalized with s organic species and used as anode to study the effects of surface functionalization. The results from cyclicvoltammetry, charge/discharge and SEI characterizations (XPS, FTIR, and AFM) show capacity is a strong function of surface chemistry.
Direct Si-C functionalization of silicon surfaces via electrografting is used to create passive or reactive monolayers on the surface. Cathodic electrochemical grafting of phenyl acetylene (passive) and hexynoic acid (reactive) monolayers on (100) silicon is previously reported. The surface chemistry of the grafted monolayers is analyzed by atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS) techniques. Versatile applications of the grafted monolayers in the fields of resists for selective copper electrodeposition, protein immobilization and impedance based biosensors are demonstrated.
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