Doctor of Philosophy (PhD)
In this work, various chemical and electrochemical methods were demonstrated to attach application-specific organic monolayers to crystalline silicon substrates. In one study, alkyl monolayers were anodically electrografted or thermally grafted onto planar (100) silicon substrates using Grignard precursors. The results show electrografted methyl monolayers provide a stable Si-C termination, resisting oxidation on (100) surfaces for approximately 55 days in air. The alkyl termination could provide a potential alternative to defective native oxides and kinetically unstable hydride surfaces. A mechanism involving two electron transfers per grafting event was established for both the thermal and electrochemical routes. In another study, unsaturated organic functional groups (phenylacetylene, 5-hexynoic acid) were cathodically electrografted onto planar (100) silicon substrates. Although cathodic grafting mechanism is considerably different, its voltammetric behavior (hysteresis, onset potential shifts) appears similar to anodic grafting process. Experimental results show both anodic and cathodic grafting methods may be applied to pattern the silicon surfaces in situ. Dielectric templates such as polystyrene microspheres or polydimethylsiloxane stamps were used to obtain high throughput, nanoscale monolayer patterns on silicon surfaces. The patterned monolayers may be further used to immobilize biological enzymes via 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) chemistry or to direct selective copper electrodeposition or etching on silicon. Established grafting mechanisms were applied to functionalize nanoscale silicon anodes in lithium batteries to improve capacity retention with charge/discharge cycling. Silicon anodes present a safe, high-capacity alternative to conventional carbonaceous anodes; however, a significant capacity loss (>20% initial value) is observed within the first few cycles, primarily due to 300% volume expansion upon lithiation. Silicon nanowires with atleast one dimension <300 nm may withstand the volume expansion effects but may not completely eliminate the capacity fade. This residual fade is mainly a result of protective solid electrolyte interphase (SEI) layer formed on the anode surface due to electrolyte dissociation. Various ex situ and in situ functionalized silicon surfaces were investigated to establish engineered silicon-SEI interface with improved chemical, mechanical and electrical aspects. The work shows silicon lithiation is a function of surface chemistry and in situ methyl siloxane functionalization offers improved capacity retention with nanoscale silicon anodes in lithium batteries.
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Vegunta, Sri Sai, "Surface functionalization of crystalline silicon substrates" (2011). LSU Doctoral Dissertations. 2205.
Flake, John C.