High-Level Incorporation of Silver in Gold Nanoclusters: Fluorescence Redshift upon Interaction with Hydrogen Peroxide and Fluorescence Enhancement with Herbicide.

High-level incorporation of Ag in Au nanoclusters (NCs) is conveniently achieved by controlling the concentration of Ag(+) in the synthesis of bovine serum albumin (BSA)-protected Au NCs, and the resulting structure is determined to be bimetallic Ag28 Au10-BSA NCs through a series of characterizations including energy-dispersive X-ray spectroscopy, mass spectroscopy, and X-ray photoelectron spectroscopy, together with density functional theory simulations. Interestingly, the Ag28 Au10 NCs exhibit a significant fluorescence redshift rather than quenching upon interaction with hydrogen peroxide, providing a new approach to the detection of hydrogen peroxide through direct comparison of their fluorescence peaks. Furthermore, the Ag28 Au10 NCs are also used for the sensitive and selective detection of herbicide through fluorescence enhancement. The detection limit for herbicide (0.1 nm) is far below the health value established by the U.S. Environmental Protection Agency; such sensitive detection was not achieved by using AuAg NCs with low-level incorporation of Ag or by using the individual metal NCs.

Self-assembly of ordered epitaxial nanostructures on polygonal nanowires.

We study the self-assembly of ordered nanostructures, that is, nanorings (NRs) and quantum dots (QDs), epitaxially grown on a polygonal cross-section nanowire (PCS-NW) using both theoretical and phase-field modeling. Our studies show that, by increasing the PCS-NW size, transitions from ordered NRs to ordered QDs on facets and further to ordered QDs on ridges occur. The predicted morphologies and their transitions are in excellent agreement with existing experiments. Our study suggests a novel approach to fabricate ordered nanostructures on nanowires.

Dynamical approach to the spatiotemporal aspects of the Portevin-Le Chatelier effect: chaos, turbulence, and band propagation.

The analysis of experimental time series, obtained from single and polycrystals subjected to constant strain rate tests, reports an intriguing dynamical crossover from a low-dimensional chaotic state at medium strain rates to an infinite-dimensional power-law state of stress drops at high strain rates. We present the results of an extensive study of all aspects of the Portevin-Le Chatelier (PLC) effect within the context of a recent model that reproduces this crossover. We characterize the dynamics of this crossover by studying the distribution of the Lyapunov exponents as a function of the strain rate, with special attention to system size effects. The distribution of the exponents changes from a small set of positive exponents in the chaotic regime to a dense set of null exponents in the scaling regime. As the latter is similar to the result in a shell model for turbulence, we compare the results of our model with that of the shell model. Interestingly, the null exponents in our model themselves obey a power law. The study is complimented by visualizing the configuration of dislocations through the slow manifold analysis. This shows that while a large proportion of dislocations are in the pinned state in the chaotic regime, most of them are pushed to the threshold of unpinning in the scaling regime, thus providing insight into the mechanism of crossover. We also show that this model qualitatively reproduces the different types of deformation bands seen in experiments. At high strain rates, where propagating bands are seen, the model equations can be reduced to the Fisher-Kolmogorov equation for propagative fronts, which in turn shows that the velocity of the propagation of the bands varies linearly with the strain rate and inversely with the dislocation density. These results are consistent with the known experimental results. We also discuss the connection between the nature of band types and the dynamics in the respective regimes. The analysis demonstrates that this simple dynamical model captures the complex spatiotemporal features of the PLC effect.

Dynamics of crossover from a chaotic to a power-law state in jerky flow.

We study the dynamics of an intriguing crossover from a chaotic to a power-law state as a function of strain rate within the context of a recently introduced model that reproduces the crossover. While the chaotic regime has a small set of positive Lyapunov exponents, interestingly, the scaling regime has a power-law distribution of null exponents which also exhibits a power law. The slow-manifold analysis of the model shows that while a large proportion of dislocations are pinned in the chaotic regime, most of them are pushed to the threshold of unpinning in the scaling regime, thus providing an insight into the mechanism of crossover.

Destabilization of gold clusters for controlled nanosynthesis: from clusters to polyhedra.

A precisely controlled destabilization of gold thiolate clusters is demonstrated to grow 12 {110}-faceted gold dodecahedra with greatly enhanced catalytic capability, and reveal the growth mechanism by DFT simulations. This greatly advances our understanding of nanocrystal growth and opens a new window for controlling the dissociation of clusters to produce nanocrystals with specific shapes.

Convenient purification of gold clusters by co-precipitation for improved sensing of hydrogen peroxide, mercury ions and pesticides.

An effective separation process is developed to remove free protein from the protein-protected gold clusters via co-precipitation with zinc hydroxide on their surface. After dialysis, the purified clusters exhibit an enhanced fluorescence for improved sensitive detection and selective visualization.

The role of surface oxygen in the growth of large single-crystal graphene on copper.

The growth of high-quality single crystals of graphene by chemical vapor deposition on copper (Cu) has not always achieved control over domain size and morphology, and the results vary from lab to lab under presumably similar growth conditions. We discovered that oxygen (O) on the Cu surface substantially decreased the graphene nucleation density by passivating Cu surface active sites. Control of surface O enabled repeatable growth of centimeter-scale single-crystal graphene domains. Oxygen also accelerated graphene domain growth and shifted the growth kinetics from edge-attachment-limited to diffusion-limited. Correspondingly, the compact graphene domain shapes became dendritic. The electrical quality of the graphene films was equivalent to that of mechanically exfoliated graphene, in spite of being grown in the presence of O.

Multifractal burst in the spatiotemporal dynamics of jerky flow.

The collective behavior of dislocations in jerky flow is studied in Al-Mg polycrystalline samples subjected to constant strain rate tests. Complementary dynamical, statistical, and multifractal analyses are carried out on the stress-time series recorded during jerky flow to characterize the distinct spatiotemporal dynamical regimes. It is shown that the hopping type B and the propagating type A bands correspond to chaotic and self-organized critical states, respectively. The crossover between these types of bands is identified by a large spread in the multifractal spectrum. These results are interpreted on the basis of competing scales and mechanisms.