Amanda S. Barnard

Observation and control of blinking nitrogen-vacancy centres in discrete nanodiamonds

Nitrogen-vacancy colour centres in diamond can undergo strong, spin-sensitive optical transitions under ambient conditions, which makes them attractive for applications in quantum optics, nanoscale magnetometry and biolabelling. Although nitrogen-vacancy centres have been observed in aggregated detonation nanodiamonds and milled nanodiamonds, they have not been observed in very small isolated nanodiamonds. Here, we report the first direct observation of nitrogen-vacancy centres in discrete 5-nm nanodiamonds at room temperature, including evidence for intermittency in the luminescence (blinking) from the nanodiamonds. We also show that it is possible to control this blinking by modifying the surface of the nanodiamonds.

A model for the phase stability of arbitrary nanoparticles as a function of size and shape

A thermodynamic model describing relative stability of different shapes for nanoparticles as a function of their size was developed for arbitrary crystalline solids and applied to group IV semiconductors. The model makes use of various surface, edge and corner energies, and takes into account surface tension. Approximations and importance of each term of the model were analyzed. The predictions for clean and hydrogenated diamond nanoparticles are compared to explicitly calculated density functional results. It is shown that diamond nanocrystal morphology is markedly different from silicon and germanium.

Modeling the Morphology and Phase Stability of TiO2 Nanocrystals in Water

The potential of titanium dioxide nanoparticles for advanced photochemical applications has prompted a number of studies to analyze the size, phase, and morphology dependent properties. Previously we have used a thermodynamic model of nanoparticles as a function of size and shape to predict the phase stability of titanium dioxide nanoparticles, with particular attention given to the crossover of stability between the anatase and rutile phases. This work has now been extended to titanium dioxide nanoparticles in water, to examine the effects of various adsorption configurations on the equilibrium shape and the phase transition. Density functional calculations have been used to accurately determine surface energies and surface tension of low index hydrated stoichiometric surfaces of anatase and rutile, which are presented along with a brief outline of the surface structure. We have shown that morphology of TiO2 nanocrystals is affected by the presence of water, resulting in variations in the size of the (001) and (001̄) truncation facets in anatase, and a reduction in the aspect ratio of rutile nanocrystals. Our results also highlight that the consideration of hydrated nanocrystal surfaces is necessary to accurately predict the correct size dependence of the anatase to rutile phase transition.

Nanogold: A Quantitative Phase Map

The development of the next generation of nanotechnologies requires precise control of the size, shape, and structure of individual components in a variety of chemical and engineering environments. This includes synthesis, storage, operational environments and, since these products will ultimately be discarded, their interaction with natural ecosystems. Much of the important information that determines these properties is contained within nanoscale phase diagrams, but quantitative phase maps that include surface effects and critical diameter (along with temperature and pressure) remain elusive. Here we present the first quantitative equilibrium phase map for gold nanoparticles together with experimental verification, based on relativistic ab initio thermodynamics and in situ high-resolution electron microscopy at elevated temperatures.

Crystallinity and surface electrostatics of diamond nanocrystals

Colloidal diamond nanoparticles are currently among the most synthesized nanomaterials on the market, and new emerging applications for nanodiamonds include bio-nano and polymer-based composites. The reliability and reproducibility of these composite materials will be strongly linked to the size, shape and stability of the individual nanodiamonds, which has an important impact on the strength and uniformity of the bonding between the particles and the polymers or bio-molecules. Although some attention has been given previously to the structure of nanodiamond surfaces, little attention has been given to the electrostatic potential at the surface, or to the structure of the cores. In the present study we use density functional tight binding to systematically examine the core structure of diamond nanoparticles of various shapes between ∼1–3.3 nm in diameter. In addition to this, we present results of the surface electrostatic potential that indicate a preferred orientation for particle–particle interactions in agglomerates, and the assembly of nanodiamond with polymers or bio-molecules.

Modelling of nanoparticles: approaches to morphology and evolution

As we learn more about the physics, chemistry and engineering of materials at the nanoscale, we find that the development of a complete understanding is not (in general) possible using one technique alone. Computer simulations provide a very valuable addition to our scientific repertoire, but it is not immediately intuitive which of the many methods available are right for a given problem. In this paper, various computational approaches are described as they apply to the study of the structure and formation of discrete inorganic nanoparticles. To illustrate how these methods are best used, results of studies from many research groups are reviewed, and informal case studies are constructed on carbon, titania and gold nanoparticles.

Self-assembly in nanodiamond agglutinates

The stable suspension of single-nano diamond particles has eluded researchers due to persistent agglomeration during purification. Presented here are results of simulations revealing that the aggregation behavior observed in samples of colloidal nanodiamond is due to strong Coulombic interactions, that depend upon the crystallographic index and spatial orientation of the surface facets, resulting in a type of self-assembly.

Naturally occurring iron oxide nanoparticles: morphology, surface chemistry and environmental stability

The widespread nanostructures of iron oxides and oxyhydroxides are important reagents in many biogeochemical processes in many parts of our planet and ecosystem. Their functions in various aspects are closely related to their shapes, sizes, and thermodynamic surroundings, and there is much that we can learn from these natural relationships. This review covers these subjects of several phases (ferrihydrite, goethite, hematite, magnetite, maghemite, lepidocrocite, akaganeite and schwertmannite) commonly found in water, soils and sediments. Due to surface passivation by ubiquitous water in aquatic and most terrestrial environments, the difference in formation energies of bulk phases can decrease substantially or change signs at the nanoscale because of the disproportionate surface effects. Phase transformations and the relative abundance are sensitive to changes in environmental conditions. Each of these phases (except maghemite) displays characteristic morphologies, while maghemite appears frequently to inherit the precursors morphology. We will see how an understanding of naturally occurring iron oxide nanostructures can provide useful insight for the production of synthetic iron oxide nanoparticles in technological settings.

Resolving the Structure of Active Sites on Platinum Catalytic Nanoparticles

Accurate understanding of the structure of active sites is fundamentally important in predicting catalytic properties of heterogeneous nanocatalysts. We present an accurate determination of both experimental and theoretical atomic structures of surface monatomic steps on industrial platinum nanoparticles. This comparison reveals that the edges of nanoparticles can significantly alter the atomic positions of monatomic steps in their proximity, which can lead to substantial deviations in the catalytic properties compared with the extended surfaces.

An environmentally sensitive phase map of titania nanocrystals.

The incorporation of more experimentally relevant parameters into theoretical descriptions of nanomaterials is important for our understanding of the stability of nanostructures in different chemical environments. Using a size-, shape-, and temperature-dependent thermodynamic model we have generated the first phase map for anatase and rutile nanocrystals, that includes both the equilibrium shape and the affects of surface chemistry. The calculated phase map indicates that the equilibrium boundary between anatase and rutile nanocrystals is surface charge chemistry dependent, which relates to both their formation and postsynthesis environments.