Atomic Resolution Observation of a Size-Dependent Change in the Ripening Modes of Mass-Selected Au Nanoclusters Involved in CO Oxidation.

Identifying the ripening modes of supported metal nanoparticles used in heterogeneous catalysis can provide important insights into the mechanisms that lead to sintering. We report the observation of a crossover from Smoluchowski to Ostwald ripening, under realistic reaction conditions, for monomodal populations of precisely defined gold particles in the nanometer size range, as a function of decreasing particle size. We study the effects of the CO oxidation reaction on the size distributions and atomic structures of mass-selected Au(561±13), Au(923±20) and Au(2057±45) clusters supported on amorphous carbon films. Under the same conditions, Au(561±13) and Au(923±20) clusters are found to exhibit Ostwald ripening, whereas Au(2057±45) ripens through cluster diffusion and coalescence only (Smoluchowski ripening). The Ostwald ripening is not activated by thermal annealing or heating in O2 alone.

Atomic structure control of size-selected gold nanoclusters during formation.

We report the ability to control the atomic structure of nanoclusters by systematically varying the gas-phase formation parameters during the generation of size-selected Au923. From aberration-corrected, scanning transmission electron microscopy (HAADF-STEM) imaging, we are able to identify the proportions of icosahedral (Ih), decahedral (Dh), and face-centered cubic (fcc) isomers within a set of populations, with each population corresponding to a specific set of formation conditions. We demonstrate that, by tuning the formation conditions, we can eliminate completely all icosahedral nanoclusters, which are commonly found under other conditions. In future, this approach may lead to the preparation of arrays or ensembles of nanoclusters containing a dominant or single isomer, thus enabling the investigation of nanocluster (or nanoparticle) properties as a function of both size and atomic configuration.

Using electron paramagnetic resonance to map N@C60 during high throughput processing.

The endohedral fullerene molecule, N@C60, is a candidate for molecular spin qubits (quantum bits) and spin probes owing to its exceptional electron spin properties. Advancements in the processing of N@C60 are key to obtaining samples of high purity on a reasonable timescale. We investigate enrichment by high throughput processing (flow rate of 18 L h(-1) and operating pressure of 1.5-2 MPa) using high performance liquid chromatography (HPLC) as a means of scaling N@C60 production. We use detection by electron paramagnetic resonance (EPR) spectroscopy to map N@C60 during processing, and through the reconstruction of the peak position in the chromatogram, we are able to determine the retention time and relative purity of N@C60 without the need for its isolation. Based on this, we establish a procedure for time-efficient, high throughput processing to isolate N@C60 in high purity.

The effects of 1-pentyne hydrogenation on the atomic structures of size-selected Au(N) and Pd(N) (N = 923 and 2057) nanoclusters.

We report an investigation into the effects of the vapour-phase hydrogenation of 1-pentyne on the atomic structures of size-selected Au and Pd nanoclusters supported on amorphous carbon films. We use aberration-corrected high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) to image populations of the nanoclusters at atomic resolution, both before and after the reaction, and we assign their atomic structures by comparison with multi-slice image simulations over a full range of cluster orientations. Gold nanoclusters consisting of 923 ± 20 and 2057 ± 45 atoms are found to be robust, exhibiting high structural stability. However, a significant portion of Pd923±26 nanoclusters that appear amorphous prior to treatment are found to exhibit high symmetry structures post-reaction, which is interpreted as the reduction of oxidised Pd nanoclusters under the reaction conditions.

A bimetallic endohedral fullerene: PrSc@C80.

The synthesis, isolation and characterization of a previously undiscovered bimetallic endohedral fullerene, PrSc@C80, are presented, and this may pave the way for a whole family of bimetallic endohedral fullerenes in useful quantities.

Chromium inhibition and size-selected Au nanocluster catalysis for the solution growth of low-density ZnO nanowires.

The wet chemical synthesis of nanostructures has many crucial advantages over high-temperature methods, including simplicity, low-cost, and deposition on almost arbitrary substrates. Nevertheless, the density-controlled solution growth of nanowires still remains a challenge, especially at the low densities (e.g. 1 to 10 nanowires/100 µm(2)) required, as an example, for intracellular analyses. Here, we demonstrate the solution-growth of ZnO nanowires using a thin chromium film as a nucleation inhibitor and Au size-selected nanoclusters (SSNCs) as catalytic particles for which the density and, in contrast with previous reports, size can be accurately controlled. Our results also provide evidence that the enhanced ZnO hetero-nucleation is dominated by Au SSNCs catalysis rather than by layer adaptation. The proposed approach only uses low temperatures (≤70 °C) and is therefore suitable for any substrate, including printed circuit boards (PCBs) and the plastic substrates which are routinely used for cell cultures. As a proof-of-concept we report the density-controlled synthesis of ZnO nanowires on flexible PCBs, thus opening the way to assembling compact intracellular-analysis systems, including nanowires, electronics, and microfluidics, on a single substrate.

Using size-selected gold clusters on graphene oxide films to aid cryo-transmission electron tomography alignment.

A three-dimensional reconstruction of a nano-scale aqueous object can be achieved by taking a series of transmission electron micrographs tilted at different angles in vitreous ice: cryo-Transmission Electron Tomography. Presented here is a novel method of fine alignment for the tilt series. Size-selected gold clusters of ~2.7 nm (Au561±14 ), ~3.2 nm (Au923± 22 ), and ~4.3 nm (Au2057±45) in diameter were deposited onto separate graphene oxide films overlaying holes on amorphous carbon grids. After plunge freezing and subsequent transfer to cryo-Transmission Electron Tomography, the resulting tomograms have excellent (de-)focus and alignment properties during automatic acquisition. Fine alignment is accurate when the evenly distributed 3.2 nm gold particles are used as fiducial markers, demonstrated with a reconstruction of a tobacco mosaic virus. Using a graphene oxide film means the fiducial markers are not interfering with the ice bound sample and that automated collection is consistent. The use of pre-deposited size-selected clusters means there is no aggregation and a user defined concentration. The size-selected clusters are mono-dispersed and can be produced in a wide size range including 2-5 nm in diameter. The use of size-selected clusters on a graphene oxide films represents a significant technical advance for 3D cryo-electron microscopy.

Size-dependent propagation of Au nanoclusters through few-layer graphene.

We report the size-dependent propagation of gold nanoclusters through few-layer graphene (FLG). We employ aberration-corrected scanning transmission electron microscopy (STEM) to track the fate of Au55 and Au923 clusters that have been deposited, independently and isoenergetically, onto suspended FLG films using cluster beam deposition. We demonstrate that Au55 clusters penetrate through the FLG, whereas the monodisperse Au923 clusters reside at the surface. Our approach offers a route to the controlled incorporation of dopant nanoparticles and the generation of nanoscale defects in graphene.

Atomic scale growth dynamics of nanocrystals within carbon nanotubes.

The confined interior region of carbon nanotubes has proved to be an effective "nano-test-tube" to conduct chemical reactions in a restricted volume. It also benefits from being thin and relatively transparent to electrons, enabling structural characterization using high-resolution transmission electron microscopy. This permits real-time monitoring of chemical reactions with atomic resolution. Here, we have studied the dynamics of single Pr atoms released from Pr(2)@C(72) metallofullerenes. We show that the Pr atoms form small nanoclusters that subsequently coalesce to ordered, stable nanocrystals within the confines of a carbon nanotube. This process has been tracked in situ with atomic-resolution using low-voltage aberration-corrected high-resolution transmission electron microscopy. We reveal that nanocrystal formation within a nanotube does not generally occur by the addition of single atoms to one pre-existing cluster but rather through aggregation of several smaller clusters. These results provide some of the deepest insights into the dynamics of single-atom behavior in the solid state.

Direct imaging and chemical identification of the encapsulated metal atoms in bimetallic endofullerene peapods.

In this paper, a chemically sensitive local characterization technique is used to characterize fullerene peapods containing two metal atoms within each fullerene. By combining bright-field imaging, high-angle annular dark-field imaging, and electron energy loss spectroscopy in a scanning transmission electron microscope, unambiguous identification of the metal atoms present is possible. Key to making this possible is aberration correction, which allows atomic resolution at lower beam energies. The peapods can be imaged for several consecutive scans at 80 keV beam energy, and the combination of techniques allows the position as well as the species of the encapsulated atoms to be identified. Movements of the encapsulated atoms are monitored.