Anomalous packing in thin nanoparticle supercrystals

We report a structural study of gold nanoparticle supercrystals formed by a few layers and of micrometer size, where the long-range order has been stimulated by a soft thermal annealing. We have clearly identified an expansion of the first layer lying on amorphous carbon substrate and an ordered second layer sitting on twofold saddle points. The third nanoparticle layer recovers the conventional close-packed stacking. This anomalous packing has been explained by means of a simple theoretical model based on dispersional forces.82265277528

Estimating nanoparticle size from diffraction measurements

Nanometre-sized particles are of considerable current interest because of their special size-dependent physical properties. Debye-Scherrer diffraction patterns are often used to characterize samples, as well as to probe the structure of nanoparticles. Unfortunately, the well known 'Scherrer formula' is unreliable at estimating particle size, because the assumption of an underlying crystal structure (translational symmetry) is often invalid. A simple approach is presented here which takes the Fourier transform of a Debye-Scherrer diffraction pattern. The method works well on noisy data and when only a narrow range of scattering angles is available.3361335134

Chirality in Bare and Passivated Gold Nanoclusters

Chiral structures have been found as the lowest-energy isomers of bare (Au$_{28}$ and Au$_{55}) and thiol-passivated (Au$_{28}(SCH$_{3})$_{16}$and Au$_{38}(SCH_{3})_{24}) gold nanoclusters. The degree of chirality existing in the chiral clusters was calculated using the Hausdorff chirality measure. We found that the index of chirality is higher in the passivated clusters and decreases with the cluster size. These results are consistent with the observed chiroptical activity recently reported for glutahione-passivated gold nanoclusters, and provide theoretical support for the existence of chirality in these novel compounds.Comment: 5 pages, 1 figure. Submitted to PR

Towards a Tetravalent Chemistry of Colloids

We propose coating spherical particles or droplets with anisotropic nano-sized objects to allow micron-scale colloids to link or functionalize with a four-fold valence, similar to the sp3 hybridized chemical bonds associated with, e.g., carbon, silicon and germanium. Candidates for such coatings include triblock copolymers, gemini lipids, metallic or semiconducting nanorods and conventional liquid crystal compounds. We estimate the size of the relevant nematic Frank constants, discuss how to obtain other valences and analyze the thermal distortions of ground state configurations of defects on the sphere.Comment: Replaced to improve figures. 4 figures Nano Letter

Size dependence on the ordering process in colloidal FePt nanoparticles

An alternative method to study the effects of annealing process on colloidal FePt nanoparticles (2-4 nm) has been achieved. Annealing experiments at temperatures between 773 and 1073 K under inert atmosphere flux were performed in powder samples with excess of surfactant molecules on nanoparticle surface. Transmission electron microscopy, x-ray diffraction and magnetic measurements were performed to evidence the evolution of the chemically disordered fcc to chemically ordered face-centered tetragonal phase transformation. Magnetization measurements under zero-field-cooling and field-cooling (M-ZFC-M-FC) conditions, and hysteresis loops are extremely sensitive to the particle size distribution and were strongly affected by the annealing treatment. (c) 2007 American Institute of Physics.101

Size effect and surface tension measurements in Ni and Co nanowires

The effect of reduced size on the mechanical and elastic properties measured on 35 nm diameter Ni and Co nanowires is presented and discussed. The stresses induced in the nanowires due to the different thermal expansion constants of the metal and alumina change the magnetic properties of the nanowires, allowing one to measure the effective Young's modulus and the surface tension of the nanowires by means of simple magnetometry. The Young's modulus of the longer nanowire is higher than that of the shorter one that is comparable to its bulk value. This effect is successfully attributed to surface tension effects.762

Magnetic and structural properties of fcc/hcp bi-crystalline multilayer Co nanowire arrays prepared by controlled electroplating

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)We report on the structural and magnetic properties of crystalline bi-phase Co nanowires, electrodeposited into the pores of anodized alumina membranes, as a function of their length. Co nanowires present two different coexistent crystalline structures (fcc and hcp) that can be controlled by the time of pulsed electrodeposition. The fcc crystalline phase grows at the early stage and is present at the bottom of all the nanowires, strongly influencing their magnetic behavior. Both structural and magnetic characterizations indicate that the length of the fcc phase is constant at around 260-270 nm. X-ray diffraction measurements revealed a strong preferential orientation (texture) in the (1 0-1 0) direction for the hcp phase, which increases the nanowire length as well as crystalline grain size, degree of orientation, and volume fraction of oriented material. The first-order reversal curve (FORC) method was used to infer both qualitatively and quantitatively the complex magnetization reversal of the nanowires. Under the application of a magnetic field parallel to the wires, the magnetization reversal of each region is clearly distinguishable; the fcc phase creates a high coercive contribution without an interaction field, while the hcp phase presents a smaller coercivity and undergoes a strong antiparallel interaction field from neighboring wires. (C) 2011 American Institute of Physics. [doi:10.1063/1.3553865]1098Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Guggenheim FellowshipFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Molecular dynamics simulations of lead clusters

Molecular dynamics simulations of nanometer-sized lead clusters have been performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf 269/270}, 1109 (1992)). The binding energies of clusters forming crystalline (fcc), decahedron and icosahedron structures are compared, showing that fcc cuboctahedra are the most energetically favoured of these polyhedral model structures. However, simulations of the freezing of liquid droplets produced a characteristic form of ``shaved'' icosahedron, in which atoms are absent at the edges and apexes of the polyhedron. This arrangement is energetically favoured for 600-4000 atom clusters. Larger clusters favour crystalline structures. Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect fcc Wulff particle, containing a number of parallel stacking faults. The effects of temperature on the preferred structure of crystalline clusters below the melting point have been considered. The implications of these results for the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other minor changes for publicatio

Magnetic behavior of Ni nanoparticles with high disordered atomic structure

This report concerns the magnetic properties of colloidal Ni nanoparticles (NPs) obtained by chemical reduction of Ni(II) salt in an organic solvent. The NPs present a complex and disordered atomic structure, where small clusters of a few Ni atoms appear to coexist within each NP. These NPs exhibit interesting magnetic properties, with a low temperature ferromagnetic order followed by a transition from ferromagnetic to a 'spin-glass-like' state as the temperature decreases. The results are discussed considering the role of the atomic ordering of the NPs on the corresponding magnetic behavior. (C) 2008 American Institute of Physics.921