AREAS OF INTEREST:

• Properties of Colloids and Colloid Assemblies
• Nonlinear and Ultrafast Properties of Nanoparticles

RESEARCH:

    Sarah came to Rice in 1995 with a B. Sc. Eng. in Engineering Physics from Queen's University in Kingston, Ontario, Canada.  She received a Master's degree in Applied Physics at Rice in May, 1998.  Her master's thesis focussed on assembling gold nanoparticles using silica nanoparticles as substrates.  Other groups have assembled nanoparticles on flat, planar substrates.  However, by using silica nanoparticles as substrates, there were hundreds of cm² of surface area available in every mL of solution instead of just the one monolayer that is on a planar substrate.  Concentrated solutions of assemblies with long path lengths were made for optical measurements.

    The first type of assembly used molecules to link gold nanoparticles to each other.  Silica nanoparticles of nominally 125 nm diameter were made using the Stober method.  The silica nanoparticles were functionalized with 3-aminopropyltrime thoxysilane, which resulted in amine-terminated silica nanoparticles.  The amine groups reacted with gold nanoparticles and bound them on the silica nanoparticle surface, as can be seen in the TEM image in Figure 1(a) (both scale bars are 50 nm.  The optical absorption spectrum of these structures, shown in Figure 1(b) is dominated by the plasmon resonance absorption of the gold nanoparticles at 520 nm.  Next a bifunctional molecule, 4-aminobenzenethiol, was added to the solution.  One of the functional groups reacted with the bound gold nanoparticles, leaving the other functional group exposed.  When more gold nanoparticles were added, those gold nanoparticles bound to the exposed functional groups.  Gold nanoparticles linked to each other can be seen on the silica nanoparticle substrate in Figure 1(c).  The UV/visible absorption spectrum of these nanostructures shows a longer wavelength component due to interactions between the plasmon resonances of the linked gold nanoparticles.  More details about this work can be found in a Chemical Physics Letters paper which is scheduled to be published in February, 1999.

    Certain small gold nanoparticles self assemble into clusters >and attach to functionalized silica nanoparticle surfaces in the correct solvent. The small gold nanoparticles were made by reducing gold chloride with tetrakishydroxymethylphosphonium chloride (Langmuir 1993, 9, 2301).  When the aqueous gold nanoparticle solution was diluted with ethanol, the gold nanoparticles associated more closely.  When functionalized silica nanoparticles were added to this mixture, gold nanoparticle clusters attached to the silica nanoparticles, as seen in the TEM image of Figure 2.  The most regular cluster shapes were attached to silica nanoparticles functionalized with a mixture of a small amount of aminopropyltrimethoxysilane and a larger amount of propyltrimethoxy silane (which is inert to the gold nanoparticles).  This work has been published in Langmuir (Langmuir 1998,14, 5396).

    Recently, Sarah has been studying the ultrafast optical properties of metal nanoshells.  A cavity-dumped Ti:sapphire laser that provides 60 femtosecond pulses at ~850 nm is used for these measurements.  By directing an intense pump beam on a solution or film of nanoshells, the electrons in the metal shell are excited.  A less intense probe beam is transmitted through the nanoshells to measure the pump-induced changes in the transmission. Richard Averitt began this work by studying transient bleaching and transient absorption from metal nanoshells.  He successfully explained the observed sign and magnitude of the signals by calculating the change in the dielectric function of the gold shell.  The current studies focus on how the medium surrounding the nanoshells affects the cooling of the hot electrons in the gold shell.

Sarah is most easily reached via electronic mail: westcott@rice.edu