Silicon Nanochemistry

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The focus of the research centers on new classes of silicon nanocrystals. The inspiration stems from our newfound ability to separate ensembles of poly-dispersed silicon nanocrystals into mono-dispersed fractions in the range of 1-5 nm, which has enabled the first recorded measurement and evaluation of the size-dependent chemical, physical and biological properties of silicon nanocrystals. Our proposed future research will expand and enrich upon these significant advances through four main themes. The first focuses on the synthesis of a novel class of silicon nanocrystals with redox active functional groups chemically tethered to the nanocrystal surface. This will enable the surface charge and hence electrical and optical properties of the silicon nanocrystals to be fine-tuned by chemical and/or electrochemical means. The archetype ncSi:R will be comprised of a redox active functional group R covalently attached to the surface of a hydride-terminated nanocrystalline silicon precursor ncSi:H via hydrosilation with an alkene or alkyne. Through chemical or electrochemical control of the number and extent of oxidation (p,q) of the capping R groups on the surface of the ncSi core the capped silicon nanocrystals can be transformed to ncSi:(1-p)R.pRq, where R will be chosen from metallocenes Cp2M, M = V, Cr, Mn, Fe, Co, Ni, coordination compounds RuBipy, aromatics Ar and fullerenes C60. The primary goal of this project is to demonstrate redox tuning of the electrical and optical properties of silicon nanocrystals. The second focuses on the synthesis of `naked' silicon nanocrystals, devoid of surface organics, with controlled surface charge and colloid-stability, which will provide unparalleled access to the chemical and physical properties of the silicon core unimpeded by surface organics. In this project the archetypes [ncSi+]]X-] will be synthesized from the hydride or chloride capped precursors ncSi:H or ncSi:Cl using reagents, such as HBF4.Et2O, BH3.THF and AgPF6 to form charge-stabilized [ncSi+][BF4-], [ncSi+][BH4-] and [ncSi+][PF6-], respectively. The third focuses on the synthesis and characterization of doped forms of nanosilicon using radio-frequency plasma excitation of SiH4-PH3 and SiH4-B2H6 gaseous mixtures as well as HF/H2O/FeCl3 etching of n- and p-doped Si wafers, the goal of the project being determination of the dopant concentration and spatial location of B and P dopants in the ncSi, measurement of charge transport, optical and optoelectronic properties of n- and p-doped ncSi films as well as films with ncSi p-n junctions, potentially enabling new generation ncSi devices. The fourth is founded upon the envisioned utility of colloidally-stable nanocrystal `silicon inks' for the fabrication of printable multicolor optoelectronic devices. The objective is to learn how to tailor the sizes and surfaces of silicon nanocrystals to control their solubility in polar and non-polar solvents, enhance their electrical charge transport properties, fine-tune their wavelength, and maximize their photoluminescence and electroluminescence efficiency across the visible spectral range, to optimize the stability and performance of multicolor silicon light emitting diodes, SiLEDs.