Adrian Boatwright

Electrostatic analysis of the interactions between charged particles of dielectric materials

An understanding of the electrostatic interactions that exist between charged particles of dielectric materials has applications that span much of chemistry, physics, biology, and engineering. Areas of interest include cloud formation, ink-jet printing, and the stability of emulsions. A general solution to the problem of calculating electrostatic interactions between charged dielectric particles is presented. The solution converges very rapidly for low values of the dielectric constant and is stable up to the point where particles touch. Through applications to unspecified particles with a range of size and charge ratios, the model shows that there exist distinct regions of dielectric space where particles with the same sign of charge are strongly attracted to one another.

Preparation of Ultrathin Nanowires Using Superfluid Helium Droplets

Direct preparation of long one-dimensional (1D) nanostructures with diameters <10 nm inside superfluid helium droplets is reported. Unlike conventional chemical synthetic techniques, where stabilizers, templates, or external fields are often required to induce 1D growth, here, we exploit the use of quantized vortices to guide the formation of ultrathin nanowires. A variety of elements have been added to the droplets to demonstrate that this is a general phenomenon, including Ni, Cr, Au, and Si. Control of the length and diameter of the nanowires is also demonstrated.

Why like-charged particles of dielectric materials can be attracted to one another.

Calculations of surface charge density provide evidence of the physical effects responsible for particles of a dielectric material carrying the same sign of charge being attracted to one another. The results show that attraction requires a mutual polarisation of charge leading to regions of negative and positive surface density close to the point where the particles make contact. These results emphasise the significance of using charged particle models where the surface charge is non-stationary.

Vortex-induced aggregation in superfluid helium droplets

The formation of Ag nanoparticles by the addition of Ag atoms to helium droplets has been investigated. The resulting nanoparticles were then imaged by transmission electron microscopy after being deposited on a thin solid surface. In large helium droplets chains of Ag nanorods were observed similar to recently reported track-like deposits [Gomez et al., Phys. Rev. Lett., 2012, 108, 155302]. However, by adjusting the experimental conditions chains of spherical nanoparticles could also be seen with a nearly uniform inter-particle spacing. Given that spherical Ag nanoparticles have no intrinsic anisotropy, the only viable explanation is that these particles must be guided into position by interaction with a quantized vortex spanning the diameter of the helium droplet. Furthermore, addition of Si to the droplets immediately after Ag resulted in Si inserting between the Ag nanoparticles to form continuous nanowires. This eliminates the possibility that the segmented Ag nanostructures are the result of nanowire fragmentation when the helium droplets collide with the deposition substrate. Thus segmented Ag chains are shown to be an intrinsic feature of Ag aggregation in helium droplets in the presence of a quantized vortex.

Exchange Bias in Fe@Cr Core-Shell Nanoparticles

We have used X-ray magnetic circular dichroism and magnetometry to study isolated Fe@Cr core-shell nanoparticles with an Fe core diameter of 2.7 nm (850 atoms) and a Cr shell thickness varying between 1 and 2 monolayers. The addition of Cr shells significantly reduces the spin moment but does not change the orbital moment. At least two Cr atomic layers are required to stabilize a ferromagnetic/antiferromagnetic interface and generate the associated exchange bias and increase in coercivity.

Helium droplets: a new route to nanoparticles

Helium droplets are large helium clusters that are capable of picking up individual atoms and molecules and show promise as nano-reactors for the synthesis of unique nanoparticles. In particular, the sequential addition of materials of different types offers opportunities for the fabrication of novel core-shell nanoparticles that cannot be synthesised by other methods. To exploit this potential, here we have carried out a mass spectrometry investigation on metal clusters in order to establish how to control the doping conditions for the fabrication of nanoparticles in superfluid helium droplets, and in particular to develop a recipe to control core and shell ratios in the case of core-shell nanoparticles. Several types of metal nanoparticles, including pure Ag, Au and Ni nanoparticles, and Ag/Au and Ni/Au core-shell systems, have been synthesised and then removed from the helium droplets by deposition on substrates for ex situ investigations using high-resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The TEM imaging has been used to estimate the sizes of nanoparticles, which show a bimodel distribution under the conditions employed. We also present the first evidence that crystalline metal nanoparticles are formed by self-assembly of metal atoms in helium droplets. The XPS investigation of Ni/Au core-shell nanoparticles shows an absence of any Au 4f core-level shift that would occur on alloying of Au and Ni, which provides the first direct evidence for the successful formation of core-shell nanoparticles using superfluid helium droplets.

A systematic shift in the electronic spectra of substituted benzene molecules trapped in helium nanodroplets

Electronic spectra (S1<--S0) have been recorded from five separate substituted benzene derivatives trapped in helium nanodroplets. Each member of the series is found to exhibit a blueshift with respect to the equivalent transition in the gas phase. Taken together with previous results for benzene, the observed shifts show a remarkably good correlation with changes in electron density that occur within each of the aromatic rings as a result of electronic excitation.

Communication: Infrared spectroscopy of salt-water complexes

To explore how the ion-pair in a single salt molecule evolves with the addition of water, infrared (IR) spectra of complexes composed of NaCl and multiple water molecules have been recorded for the first time. The NaCl(H2O)n complexes were formed and probed in liquid helium nanodroplets, and IR spectra were recorded for n = 1 → 4. The spectra for n = 1, 2, and 3 are consistent with formation of the lowest energy contact-ion pair structures in which each water molecule forms a single ionic hydrogen bond to an intact Na(+)Cl(-) ion-pair. Alternative structures with hydrogen bonding between water molecules become energetically competitive for n = 4, and the IR spectrum indicates likely the coexistence of at least two isomers.

Electronic spectroscopy of toluene in helium nanodroplets : evidence for a long-lived excited state.

Optical excitation of toluene to the S1 electronic state in helium nanodroplets is found to alter the rate of production of the fragment ions C7H7(+) and C5H5(+) when the droplets are subjected to subsequent electron ionization. The optical excitation process reduces the abundance of C7H7(+) ions delivered into the gas phase, whereas C5H5(+) ions become more abundant beyond a minimum droplet size. This process contrasts with normal optical depletion spectroscopy, where the optical absorption of a molecular dopant in a helium nanodroplet shrinks the helium droplet, and thus, the electron impact cross-sections because of dissipation of the absorbed energy by evaporative loss of helium atoms. The observations here are interpreted in terms of formation of an excited state in the neutral molecule, which survives for several hundred μs. This long-lived excited state, which is assumed to be the lowest triplet electronic state, shows different cross-sections for production of C7H7(+) and C5H5(+) relative to the S0 state.