Catriona O'Sullivan

Spontaneous room temperature elongation of CdS and Ag2S nanorods via oriented attachment.

Spontaneous elongation from nanorod to nanowire in the presence of an amine is reported for nanocrystals of cadmium sulfide and silver sulfide (cation exchanged from CdS). Elongation occurs instantaneously where the final aspect ratio is a controllable multiple of the original nanorod length. Transmission electron microscopy (TEM) analysis reveals the influential factors on the attachment process are the concentration of amine, duration and temperature of the reaction. The elongated nanorods are further characterized by X-ray diffraction (XRD), photoluminescence (PL), ultraviolet-visible spectroscopy (UV-vis) and X-ray photoelectron spectroscopy (XPS). A mechanism of oriented attachment is evidenced by the doubling in length of asymmetrically gold tipped CdS nanorods with the corresponding absence of elongation in symmetrically tipped nanorods.

Size controlled gold tip growth onto II–VI nanorods

Gold tip size and multiplicity are controlled in hybrid gold–semiconductor nanorods (CdS–Au, CdSe–Au and CdTe–Au) in fast reaction times of less than 2 minutes by optimising precursor type, concentration and temperature. Controllable gold tips up to as large as 40 nm on a rod diameter of 7 nm are reported with the tip size shown to be directly related to the redox potentials of the dangling atoms on the nanorod and the gold chloride precursors (mono- or trivalent). The preference for symmetric (both ends) over asymmetric (single end) is achieved by simply elevating the reaction temperature to 80 °C without changing the reaction time. TEM and XRD analyses were preformed to verify both the gold nanoparticle size and crystallinity of the hybrid nanostructures. Increased quenching of nanorod emission is observed as the size of the gold tip increases suggesting optimisation of charge transfer between the semiconductor and the metal.

Gold tip formation on perpendicularly aligned semiconductor nanorod assemblies

A facile spin-casting process is used for the instantaneous asymmetric formation of gold tips on perpendicularly aligned CdS nanorod superlattices. Tip size varies as a function of precursor solution concentration and growth time. A single uniform tip occurs on the end facets of each nanorod in the array when an optimised gold chloride solution is used, with multiple tipping occurring with variations in precursor concentration. HRTEM shows that the gold tip growth does not always occur centrosymmetrically on the nanorods, with growth occurring on the (101) or the (001) facet of the wurtzite nanocrystal depending on the rod shape. X-Ray diffraction confirms that the gold tips are crystalline with a 60% lattice mismatch with the wurtzite CdS nanocrystal, suggesting strain relief may be a factor in tip formation. The gold-tipped nanorods are further characterised by photoluminescence spectroscopy.

Metal surface nucleated supercritical fluid–solid–solid growth of Si and Ge/SiOx core–shell nanowires

High yields of both single-crystalline Si and Ge/SiOx core–shell nanowires were nucleated and grown in metal reactor cells under high-pressure supercritical fluid conditions, without the addition of catalyst particle seeds or a porous template. Nanowire growth was only achieved when the fluid medium of supercritical CO2 and the organometallic precursors were used in conjunction with a coordinating solvent, trioctylphosphine. The diameter and length of the nanowires are found to be in the ranges of 30 to 60 nm and 1 to 10 µm, respectively. The correlation of nanowire growth with the eutectic binary phase diagrams of the semiconductor–metal and the presence of metal impurities at the base of the synthesized nanowires suggest a supercritical fluid–solid–solid growth mechanism occurring from the reaction cell walls. The nanowires are characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. The electrical characteristics for individually picked nanowires are also investigated by means of mechanical nanoprobing.

Protein immobilisation on perpendicularly aligned gold tipped nanorod assemblies

A multi component assembly consisting of the redox protein cytochrome c (cyt c) immobilised onto vertically aligned gold tipped semiconductor nanorods is described. Cyt c was successfully immobilised using a thiol linker. A faradaic response demonstrated that the protein is electroactive in this ultra high density array.

Silver Tip Formation on Colloidal CdSe Nanorods by a Facile Phase Transfer Protocol

A facile phase transfer procedure is described for the formation of uniform silver metal tips on II–VI semiconductor nanorods. Judicious choice of a functional ligand dimethyl phenol (DMP) which binds to the semiconductor rod in the organic phase enables the transfer of metal ions from the aqueous phase and their reduction onto the nanorod. The nanorod hybrids can be assembled into perpendicularly aligned arrays by simple solvent evaporation.

Complete synthesis of germanium nanocrystal encrusted carbon colloids in supercritical CO2 and their superhydrophobic properties

Colloidal carbon spheres were synthesized by the carbonization of squalane, a nonvolatile hydrocarbon solvent, in supercritical carbon dioxide. Precise pressure modulation of the fluid medium led to size controlled growth of carbon spheres ranging from 300 to 1500 nm in diameter. This unique synthetic approach of carbonizing a hydrocarbon suspension in supercritical fluid is found to suppress any particle aggregation, resulting in excellent sphere monodispersity. Core-shell hybrid structures of C-Ge were subsequently formed by inducing the growth of 10-40 nm sized germanium nanocrystals from the spheres in a hierarchical bottom-up approach. Extensive characterization of the spheres and nanocrystals was conducted using transmission and scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman, and thermogravametric analysis. Assemblies of nanocrystal modified carbon colloids impart outstanding superhydrophobic properties due to the combined nano- and microstructuring of the particle arrays.