C. Binns

The construction of a gas aggregation source for the preparation of size-selected nanoscale transition metal clusters

The design and operation of a gas aggregation source is described. The source combines the attributes of high-temperature operation (enabling preparation of transition metal clusters), mass selection, ultrahigh vacuum compatibility, and transportability. This makes it ideally suited to in situ studies such as scanning tunneling microscope or synchrotron radiation experiments. Data are presented to illustrate the performance of the source; recent results obtained in synchrotron radiation studies are highlighted.

Growth, electronic, magnetic and spectroscopic properties of transition metals on graphite

Abstract This report presents a combined theoretical and experimental study of the electronic structure of nanostructures of transition-metals on graphite. In the last few years a variety of techniques have been used to prepare mesoscopic metal islands on this substrate and novel effects associated with the electronic structure in nanoscale systems have been reported. These include magnetism in the bulk paramagnetic metals V and Ru. In the present report, following the introduction in Section 1, Section 2 is devoted to review the growth and spectroscopic properties of 3d and 4d metals on graphite. A description of the experimental techniques and conditions used by the authors is presented. The morphology of deposited MBE films of Cr, V, Mn and Fe and pre-formed nanoscale clusters of Mn and Fe, determined by XUV reflectivity, SEM and STM is discussed. Results from XPS and other photoemission-based techniques, used to examine the electronic and magnetic properties of the films, are shown and several novel effects expected in nanoscale systems are demonstrated. These include enhanced magnetic moments in Mn nanoclusters, size-dependent screening effects in islanded Mc, Cr and V films and evidence for increased magnetic disorder in Cr and Fe particles. Section 3 is dedicated to the theoretical investigation of magnetism for hexagonal 3d and 4d metal monolayers, epitaxially adsorbed on a (0001) graphite surface, by spin-polarized electronic structuer calculations using semi-empirical and ab-initio methods. It is shown that, when they exist, the magnetic moments of the adsorbed monolayers are substantially reduced from their unsupported monolayer values. A comparison with one and two adatoms is discussed. Also in this chapter, we present calculations of the 3s-XPS of 3d metal atoms chemisorbed on graphite, using a cluster model that takes into account intra-atomic dd and d-core electron exchange and Coulomb interactions as well as hybridization between 3d metal orbitals and graphite p states. The ability of the system to undergo a low-to-high spin transition is discussed along the 3d series with respect to the metal atom-graphite distance and is clearly evidenced in the evolution of the 3s-core-XPS line shape. All the transition metal films on graphite presented in this report grow as islands with a thickness of a few nanometers. However, the monolayer growth of 3d or 4d metals on graphite might be achieved in certain cases, even if this corresponds to thermodynamically metastable phases. In this way our report presents interesting and systematic results for new epitaxial systems but still remains a challenge for both experimentalists and theoreticians.

Enhanced oxidation of nanoparticles through strain-mediated ionic transport

Geometry and confinement effects at the nanoscale can result in substantial modifications to a materials properties with significant consequences in terms of chemical reactivity, biocompatibility and toxicity. Although benefiting applications across a diverse array of environmental and technological settings, the long-term effects of these changes, for example in the reaction of metallic nanoparticles under atmospheric conditions, are not well understood. Here, we use the unprecedented resolution attainable with aberration-corrected scanning transmission electron microscopy to study the oxidation of cuboid Fe nanoparticles. Performing strain analysis at the atomic level, we reveal that strain gradients induced in the confined oxide shell by the nanoparticle geometry enhance the transport of diffusing species, ultimately driving oxide domain formation and the shape evolution of the particle. We conjecture that such a strain-gradient-enhanced mass transport mechanism may prove essential for understanding the reaction of nanoparticles with gases in general, and for providing deeper insight into ionic conductivity in strained nanostructures.

The construction of a gas aggregation source for the preparation of mass-selected ultrasmall metal particles

The design and operation of a high temperature gas aggregation source, capable of in situ deposition of mass-selected atomic clusters of transition metals onto a substrate in an ultrahigh vacuum chamber, is described. Mass-selection is achieved by an ultrahigh mass quadrupole filter operating at masses up to 3×104 amu.

Switching Casimir forces with phase-change materials

We demonstrate here a controllable variation in the Casimir force. Changes in the force of up to 20% at separations of similar to 100 nm between Au and Ag-In-Sb-Te (AIST) surfaces were achieved on crystallization of an amorphous sample of AIST. This material is well known for its structural transformation, which produces a significant change in the optical properties and is exploited in optical data storage systems. The finding paves the way to the control of forces in nanosystems, such as micro- or nanoswitches, by stimulating the phase-change transition via localized heat sources.

Deposition of Fe clusters on Si surfaces

We have used an ultrahigh vacuum scanning tunneling microscope to investigate the deposition of mesoscopic iron clusters from a gas aggregation source. The size of the clusters was found to be in the range 1–7 nm. The effect of exposure of the iron clusters to the atmosphere results in a significant increase in cluster size, which we believe is consistent with total oxidation of the clusters. A specially designed quadrupole mass filter is incorporated inside the cluster source. We have investigated the sizes of the clusters deposited using different quadrupole settings and find that it is possible to size select the clusters prior to deposition. Finally we have studied the effect of sample surface reactivity and annealing on cluster distribution.

Enhancements in magnetic moments of exposed and Co-coated Fe nanoclusters as a function of cluster size

Abstract X-ray magnetic circular dichroism has been used to measure the magnetic moments of exposed and Co-coated Fe nanoclusters in the size range 180–690 atoms. The clusters were deposited in situ onto highly oriented pyrolytic graphite substrates using a ultra-high vacuum-compatible gas aggregation cluster source. Enhancements in both the spin and orbital magnetic moments, m S and m L , respectively, are observed for the exposed clusters. As the cluster size is reduced, m S gradually increases from the bulk value, and for the 180-atom clusters, it is enhanced by around 10%. The degree of enhancement in m L also increases as the cluster size decreases, although m L starts to decrease again for the 180-atom clusters; the maximum value observed for m L corresponds to an increase of ∼75% relative to bulk Fe. Coating the Fe clusters with Co further raises their magnetic moments, as a result of the increases in m S across the size range. For the 180-atom clusters, m S is 20% greater than in the bulk.

Superparamagnetic relaxation in iron nanoclusters measured by low energy muon spin rotation

Low energy (16 keV) muons were used to probe the dynamic magnetic behaviour of iron nanoclusters embedded in a silver thin film matrix. The silver film was 500 nm thick and contained a volume fraction of 0.1% iron. Measurements were made in a field of 25 mT, applied normal to the plane of the film, in the temperature range 4.7 K to 300 K. At temperatures above 20 K thermal activation of the cluster moments was seen as a narrowing of the field distribution sensed by the implanted muons. An intrinsic cluster relaxation time of τ0 = 12 ± 4 ns and an activation energy of 51 ± 9 K were deduced from fits to the data. SQUID magnetometry of thicker (1.5 µm) but otherwise identical films on graphite substrates showed the clusters to have a volume of the order of 10-26 m3, from which a cubic anisotropy constant of K = 2.3 ± 0.4 × 105 J m-3 was calculated. Remanence measurements showed no evidence of a preferred orientation for the magnetization of the cluster assembly.

Exchange bias in a superspin glass system of Co particles in Mn matrix

The magnetic properties of 1.8 nm Co particles dispersed (4.7% volume fraction) in a Mn matrix have been investigated. The results show that the antiferromagnetic matrix provides a strong interface exchange coupling with the ferromagnetic particles and, through it, an effective long range interparticle correlation. This gives rise to a superspin glass type freezing (Tg = 62 K), exchange bias and a remarkable enhancement of thermal stability with respect to the same particles in a Ag matrix, as shown by a shift of the distribution of energy barriers and switching fields to much larger values.

Magnetism of exposed and Co-capped Fe nanoparticles

Abstract The effect of capping a dilute assembly of nanoscale mass-selected Fe clusters with a Co thin film has been studied using X-ray magnetic circular dichroism (XMCD). The clusters, containing around 400 atoms, were deposited in situ from a gas-aggregation source onto highly oriented pyrolytic graphite. The exposed clusters possess magnetic moments that are enhanced compared to the bulk, by around 4% for m spin and around 75% for m orb . In addition, a surface core level shifted component is observed in the L 3,2 XMCD spectrum. Upon adding the Co layer, the surface component disappears, m orb is decreased for the Fe clusters, and m spin increases. The exposed clusters are magnetically isotropic but a strong in-plane anisotropy is observed after depositing the Co overlayer. We attribute this to the shape of the Co islands in which the Fe clusters are embedded.