Abstract:
This thesis investigated the formation of compositional gradients across 0.5 – 1 cm of
porous silicon layers which had thicknesses of 2 – 10 μm. These compositional gradients
were then characterised, and their potential use as vapour sensors was probed. Surface
composition gradients have been reported on flat surfaces, but this is the first time that
they have been reported on a three-dimensional material with controlled pore geometry.
Chemical gradients have been generated across the surface of porous silicon by
performing electrochemical attachment of organohalides with an asymmetric electrode
arrangement, and by chemical hydrosilylation of alkenes in the presence of a diffusion
gradient of diazonium salts across the porous silicon surface. Samples with
electrochemical gradients of methyl, pentyl acetate, and decyl and using chemical
hydrosilylation with gradients of undecanoic acid and decyl groups. The latter four
gradient-modified porous silicon types have been ‘endcapped’ with methyl groups to give
improved stability and greater hydrophobicity. The pentyl acetate and undecanoic groups
have been converted into pentanol and undecanoate groups respectively to increase the
hydrophilicity of these porous silicon surfaces. The gradients have been characterised
using two-dimensional FTIR microspectrophotometry and water contact angle
measurements.
The interaction of these gradient porous silicon samples with ethanol, heptane, toluene
and 2-hexanol vapours have been monitored either by UV-Vis reflectance spectroscopy
at selected points across the surface or more globally using a digital camera. The
undecanoate gradient porous silicon sample showed a large difference in optical response
between the undecanoate end and the methyl end of the gradient when exposed to water
vapour, showing that imposition of a chemical gradient can alter the sensing character of
porous silicon in a controllable manner.