Richard E. Palmer

High-resolution detection of Au catalyst atoms in Si nanowires

The potential for the metal nanocatalyst to contaminate vapour-liquid-solid grown semiconductor nanowires has been a long-standing concern, because the most common catalyst material, Au, is highly detrimental to the performance of minority carrier electronic devices. We have detected single Au atoms in Si nanowires grown using Au nanocatalyst particles in a vapour-liquid-solid process. Using high-angle annular dark-field scanning transmission electron microscopy, Au atoms were observed in higher numbers than expected from a simple extrapolation of the bulk solubility to the low growth temperature. Direct measurements of the minority carrier diffusion length versus nanowire diameter, however, demonstrate that surface recombination controls minority carrier transport in as-grown n-type nanowires; the influence of Au is negligible. These results advance the quantitative correlation of atomic-scale structure with the properties of nanomaterials and can provide essential guidance to the development of nanowire-based device technologies.

Three-dimensional atomic-scale structure of size-selected gold nanoclusters

An unambiguous determination of the three-dimensional structure of nanoparticles is challenging. Electron tomography requires a series of images taken for many different specimen orientations. This approach is ideal for stable and stationary structures. But ultrasmall nanoparticles are intrinsically structurally unstable and may interact with the incident electron beam, constraining the electron beam density that can be used and the duration of the observation. Here we use aberration-corrected scanning transmission electron microscopy, coupled with simple imaging simulation, to determine with atomic resolution the size, three-dimensional shape, orientation and atomic arrangement of size-selected gold nanoclusters that are preformed in the gas phase and soft-landed on an amorphous carbon substrate. The structures of gold nanoclusters containing 309±6 atoms can be identified with either Ino-decahedral, cuboctahedral or icosahedral geometries. Comparison with theoretical modelling of the system suggests that the structures are consistent with energetic considerations. The discovery that nanoscale gold particles function as active and selective catalysts for a variety of important chemical reactions has provoked much research interest in recent years. We believe that the detailed structure information we provide will help to unravel the role of these nanoclusters in size- and structure-specific catalytic reactions. We note that the technique will be of use in investigations of other supported ultrasmall metal cluster systems.

Size-selected cluster beam source based on radio frequency magnetron plasma sputtering and gas condensation

We report on a source for producing size-selected nanoclusters based on the combination of radio frequency magnetron plasma sputtering and gas condensation. The use of plasma sputtering to vaporize a target is applicable to a large range of materials; Ag, Au, Cu, and Si have been attempted to date. The source, combined with a time-of-flight mass filter, can produce clusters in the size range from 2 up to at least 70 000 atoms, depending on the target material, with a constant mass (M) resolution (M∕ΔM∼25) at an intensity that produces atomic monolayer coverage in as little as a few minutes. The source is also attached to an ultrahigh vacuum analysis chamber, which allows in situ surface chemical and structural analysis. Examples of cluster deposition experiments with the source are also presented.

Two-electron dissociation of single molecules by atomic manipulation at room temperature

Using the tip of a scanning tunnelling microscope (STM) to mechanically manipulate individual atoms and molecules on a surface is now a well established procedure. Similarly, selective vibrational excitation of adsorbed molecules with an STM tip to induce motion or dissociation has been widely demonstrated. Such experiments are usually performed on weakly bound atoms that need to be stabilized by operating at cryogenic temperatures. Analogous experiments at room temperature are more difficult, because they require relatively strongly bound species that are not perturbed by random thermal fluctuations. But manipulation can still be achieved through electronic excitation of the atom or molecule by the electron current tunnelling between STM tip and surface at relatively high bias voltages, typically 1–5 V. Here we use this approach to selectively dissociate chlorine atoms from individual oriented chlorobenzene molecules adsorbed on a Si(111)-7 × 7 surface. We map out the final destination of the chlorine daughter atoms, finding that their radial and angular distributions depend on the tunnelling current and hence excitation rate. In our system, one tunnelling electron has nominally sufficient energy to induce dissociation, yet the process requires two electrons. We explain these observations by a two-electron mechanism that couples vibrational excitation and dissociative electron attachment steps.

A new high transmission infinite range mass selector for cluster and nanoparticle beams

A new mass selection technique has been developed, which allows one to size-select charged particles from atoms to nanoparticles of almost unlimited size. It provides a mass resolution of m/Δm=20–50 and a transmission of about 50% for the selected size, both independent of mass. The technique is based on the time-of-flight principle, but differs fundamentally from time-of-flight mass selection normally used. The basic idea is to use time-limited high voltage pulses to displace laterally a preaccelerated ion beam, without changing its direction or shape. As the movement of the ions perpendicular to their original beam direction is independent of their forward velocity, mass resolution and calibration does not depend on the ion beam energy. A mass selector of this type has been implemented successfully into a cluster deposition experiment and has proven to be reliable and simple to operate.

Gas condensation source for production and deposition of size-selected metal clusters

We describe the construction and performance of a gas condensation cluster source. The source was designed for deposition of mass-selected metal clusters with controlled landing energy. We have produced clusters of Pbn (n=2–∼300) and Agn (n=∼20–∼300) with sufficient intensity to deposit size-selected clusters to a density of 1012 clusters/cm2 in 10 min. The landing energy of the clusters can be controlled from ∼25 to 800 eV.

Amorphous structures of Cu, Ag, and Au nanoclusters from first principles calculations

We have carried out first-principles density functional calculations for clusters of the coinage metals containing thirteen atoms (M13, where M=Cu, Ag, or Au). We find that for this geometric “magic number” the low energy isomers are actually disordered, forming almost a continuous distribution as a function of energy.

Pinning of size-selected Ag clusters on graphite surfaces

We have explored the deposition of size-selected AgN+ clusters (N=50–200) onto the graphite surface (at room temperature) over the impact energy range of 250–2500 eV, via a combination of scanning tunneling microscopy experiments and molecular dynamics simulations. We show that the clusters are pinned to the surface when the impact energy exceeds a critical value, which is proportional to the cluster size, N, via the formation of a point defect at the impact site. This prevents lateral diffusion of the clusters even at room temperature.

Fabrication of silicon cones and pillars using rough metal films as plasma etching masks

We have developed a simple fabrication process which allows the production of nanoscale silicon structures. Rough silver films are used as an etching mask for reactive ion etching at 10 °C. Variation of the etching parameters, such as the rf power, allows control over the shape of the features; the production of both pillars and cones is possible. The density and diameter of these features are controlled by the etching time. Pillars with diameters as small as 5 nm are reported.

Structures and optical properties of 4-5 nm bimetallic AgAu nanoparticles

Three types of bimetallic AgAu nanoparticles, with mean size of 4-5 nm, Ag(core)Au(shell), Au(core)Ag(shell) and alloyed AgAu, have been synthesized using an inverse micelle method. To image these small size nanoparticles, quantitative high angle annular dark field imaging using scanning transmission electron microscopy was successfully applied. Our results show that good control of nanoparticle size dispersion and composition modulation was achieved. Optical properties of the nanoparticles are correlated with direct internal structure analysis. The structural stability is discussed, based on thermodynamic considerations.