Robert Gunning

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.

Controlled semiconductor nanorod assembly from solution: influence of concentration, charge and solvent nature

Spontaneous supercrystal organisation of semiconductor nanorods (CdS and CdSe) of different aspect ratios into ordered superstructures was obtained by controlled evaporation of a nanorod solution. The rods either align into two dimensional close packed perpendicular arrays or into one dimensional rail tracks depending on the total interaction energy between the rods in solution. A detailed study has identified critical factors that affect this interaction energy such as nanorod concentration, surface charge, dipole moment and solvent nature (polarity and volatility), thereby allowing a general approach to control the nature of nanorod assembly (1D or 2D). Molecular dynamics (MD) of small charged nanorods showed that opposite dipolar alignment (antiferromagnetic) was the preferred rod orientation during self-assembly.

A panchromatic anthracene-fused porphyrin sensitizer for dye-sensitized solar cells

The development of ruthenium-free sensitizers which absorb light over a broad range of the solar spectrum is important for improving the power conversion efficiency of dye-sensitized solar cells. Here we study three chemically tailored porphyrin-based dyes. We show that by fusing the porphyrin core to an anthracene unit, we can extend the conjugation length and lower the optical gap, shifting the absorption spectrum into the near-infrared (NIR). All three dyes were tested in dye-sensitized solar cells, using both titanium dioxide and tin dioxide as the electron-transport material. Solar cells incorporating the anthracene-fused porphyrin dye exhibit photocurrent collection at wavelengths up to about 1100 nm, which is the longest reported for a porphyrin-based system. Despite extending the photon absorption bandwidth, device efficiency is found to be low, which is a common property of cells based on porphyrin dyes with NIR absorption. We show that in the present case the efficiency is reduced by inefficient electron injection into the oxide, as opposed to dye regeneration, and highlight some important design considerations for panchromatic sensitizers.

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.

Block copolymer mediated stabilization of sub-5 nm superparamagnetic nickel nanoparticles in an aqueous medium

This paper presents a facile method for decreasing the size of water dispersible Ni nanoparticles from 30 to 3 nm by the incorporation of a passivating surfactant combination of pluronic triblock copolymer and oleic acid into a wet chemical reduction synthesis. A detailed study revealed that the size of the Ni nanoparticles is not only critically governed by the concentration of the triblock copolymers but also dependent on the hydrophobic nature of the micelle core formed. The synthesized Ni nanoparticles were thoroughly characterized by means of transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy and temperature and field dependent magnetic measurements, along with a comprehensive Fourier transform infrared spectroscopy analysis, in order to predict a possible mechanism of formation.

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.

The influence of 1D, meso- and crystal structures on charge transport and recombination in solid-state dye-sensitized solar cells

We have prepared single crystalline SnO2 and ZnO nanowires and polycrystalline TiO2 nanotubes (1D networks) as well as nanoparticle-based films (3D networks) from the same materials to be used as photoanodes for solid-state dye-sensitized solar cells. In general, superior photovoltaic performance can be achieved from devices based on 3-dimensional networks, mostly due to their higher short circuit currents. To further characterize the fabricated devices, the electronic properties of the different networks were measured via the transient photocurrent and photovoltage decay techniques. Nanowire-based devices exhibit extremely high, light independent electron transport rates while recombination dynamics remain unchanged. This indicates, contrary to expectations, a decoupling of transport and recombination dynamics. For typical nanoparticle-based photoanodes, the devices are usually considered electron-limited due to the poor electron transport through nanocrystalline titania networks. In the case of the nanowire-based devices, the system becomes limited by the organic hole transporter used. In the case of polycrystalline TiO2 nanotube-based devices, we observe lower transport rates and higher recombination dynamics than their nanoparticle-based counterparts, suggesting that in order to improve the electron transport properties of solid-state dye-sensitized solar cells, single crystalline structures should be used. These findings should aid future design of photoanodes based on nanowires or porous semiconductors with extended crystallinity to be used in dye-sensitized solar cells.

Magnetite nanocrystals from a single source metallorganic precursor: metallorganic chemistry vs. biogeneric bacteriaElectronic supplementary information (ESI) available: Mssbauer spectra of nanocrystals at room temperature (RT), 150 K and 19 K. See http://www.rsc.org/suppdata/jm/b3/b316906e/

Magnetite nanocrystals, which are normally formed by magnetogeneric bacteria, have been prepared using a single source metallorganic precursor.