Andrew T. Heitsch

Solution−Liquid−Solid (SLS) Growth of Silicon Nanowires

Here we report the solution-liquid-solid (SLS) synthesis of silicon (Si) nanowires. Nanowires are grown by trisilane (Si3H8) decomposition in a high boiling solvent, octacosane (C28H58) or squalane (C30H62), in the presence of either Au or Bi nanocrystals. To our knowledge, this is the first report of a colloidal synthetic route carried out in a solvent at atmospheric pressure that provides crystalline Si nanowires in large quantities.

Colloidal Silicon Nanorod Synthesis

The colloidal synthesis of crystalline silicon (Si) nanorods with diameters of 5 to 10 nm and lengths of 15 to 75 nm is demonstrated. Trisilane was decomposed in a hot solvent in the presence of dodecylamine and gold (Au) nanocrystals. Nanorods form by Au-seeded solution-liquid-solid growth with dodecylamine serving as capping ligands that stabilize the nanorod dispersion. Post-synthesis etching of the Au seeds from the nanorod tips is also demonstrated.

Stacking of Hexagonal Nanocrystal Layers during Langmuir–Blodgett Deposition

Hexagonally ordered close-packed monolayers of sterically stabilized FePt nanocrystals were deposited on substrates using the Langmuir-Blodgett technique. Monolayers of nanocrystals were also stacked by sequential Langmuir-Blodgett transfer. The structures of the nanocrystal monolayers and multilayer stacks were examined with scanning electron microscopy (SEM) and grazing-incidence small-angle scattering (GISAXS). An analytical model derived from the quasikinematic approximation provides a convenient description of the GISAXS data of the stacked layers. The transferred monolayers showed good in-plane hexagonal order, even for trilayers. Bilayers exhibited spatial registry with the top layer positioned above the 3-fold coordinated sites of the bottom layer. Trilayers, on the other hand, exhibited significant disorder.

Gold seed removal from the tips of silicon nanorods.

A chemical method was developed to remove the gold (Au) seed particles from the tips of solution-liquid-solid (SLS) grown silicon (Si) nanorods. The nanorods are capped with hydrophobic ligands during the synthesis, which made it necessary to perform the Au etching in an aqua regia and chloroform emulsion. Preliminary etching experiments revealed that a thin Si shell coated the Au seeds and prevented Au removal. Therefore, a rapid thermal quench of the reaction mixture was needed to crack this shell and provide etchant access to the Au seed. More than 95% of the Au seeds could be removed from the tips of thermally quenched samples without damaging the crystalline Si nanorods.

Printed magnetic FePt nanocrystal films

Patterned monolayers and multilayers of FePt nanocrystals were printed onto substrates by first assembling nanocrystals on a Langmuir-Blodgett (LB) trough and then lifting them onto prepatterned polydimethylsiloxane (PDMS) stamps, followed by transfer printing onto the substrate. Patterned features, including micrometer-size circles, lines, and squares, could be printed using this approach. The magnetic properties of the printed nanocrystal films were also measured using magnetic force microscopy (MFM). Room-temperature MFM could detect a remanent (permanent) magnetization from multilayer (>3 nanocrystals thick) films of chemically ordered L1(0) FePt nanocrystals.

Colloidal magnetic nanocrystals: synthesis, properties and applications

There has been a tremendous research effort in the past few years in colloidal magnetic nanocrystals. New synthetic methods have been developed that enable a wide variety of magnetic materials to be synthesized in nanocrystal form, including ferromagnetic transition metals, intermetallics and metal oxides. These nanocrystals can be obtained with controlled size, shape and composition. Nanocrystal heterostructures, such as core/shell particles and heterodimers can be synthesized. New materials such as doped magnetic semiconductor quantum dots and nanowires have been made. The magnetic properties of these unique nanostructures have been measured and their applications in medicine, information storage and processing and sensing are being developed. This review attempts to provide a summary and a flavor for this exciting research area as it has evolved during the past few years.