No abstract

Chemical ordering in face-centered-cubic-like PdPt nanoparticles consisting of 38-201 atoms is studied via density-functional calculations combined with a symmetry orbit approach. It is found that for larger particles in the Pd-rich regime, Pt atoms can segregate at the center of the nanoparticle (111) surface facets, in contrast with extended systems in which Pd is known to segregate at the surface of alloy planar surfaces. In a range of compositions around 1:1, a novel multishell chemical ordering pattern was favored, in which each shell is a patchwork of islands of atoms of the two elements, but the order of the patchwork is reversed in the alternating shells. These findings are rationalized in terms of coordination-dependent bond-energy variations in the metal-metal interactions, and their implications in terms of properties and applications of nanoscale alloy particles are discussed.

The equilibrium state of a reaction mixture comprised of a small number of molecules is modeled for three different nanoconfined systems. The issue is relevant to several advanced routes for the synthesis of encapsulated organic molecules, metallic or inorganic nanoclusters, and other nanoscale structures. Canonical-ensemble based formulations and computations predict for the equilibrated closed small systems significant deviations from the (macroscopic) thermodynamic limit. The effects include the enhancement/suppression of the equilibrium extent of the exothermic/endothermic model reactions, associated mainly with reduced numbers of mixed reactant-product microstates in the closed system. Fluctuations in the nanochemical reaction extent, which are found to be closely related to the stoichiometric coefficients, become more dominant for smaller systems and modify considerably the temperature dependence of the equilibrium constant.

Recently we predicted that when a reaction involving a small number of molecules occurs in a nanometric-scale domain entirely segregated from the surrounding media, the nanoconfinement can shift the position of equilibrium toward products via reactant-product reduced mixing. In this Letter, we demonstrate how most-recently reported single-molecule fluorescence measurements of partial hybridization of ssDNA confined within nanofabricated chambers provide the first experimental confirmation of this entropic nanoconfinement effect. Thus, focusing separately on each occupancy-specific equilibrium constant, quantitatively reveals extra stabilization of the product upon decreasing the chamber occupancy or size. Namely, the DNA hybridization under nanoconfined conditions is significantly favored over the identical reaction occurring in bulk media with the same reactant concentrations. This effect, now directly verified for DNA, can be relevant to actual biological processes, as well as to diverse reactions occurring within molecular capsules, nanotubes, and other functional nanospaces.

Chemical order in platinum-iridium truncated-octahedron nanoparticles as a model system was studied using coordination-dependent bond-energy variations (CBEV) and the statistical-mechanical free-energy concentration expansion method (FCEM) adapted for handling axially symmetric structures. Pt-Ir side-separated ("Quasi-Janus", QJ) configurations are found to be stabilized at low temperatures mainly due to CBEV-related preferential strengthening of Pt-surface-Ir-subsurface bonds, and the greatly reduced number of hetero-atomic bonds. In comparison, the roles of local strain (by only ~2% atomic mismatch), short-range-order and vibrational entropy are minor. At higher temperatures, the QJ configuration is transformed into a partially disordered central-symmetric onion-like structure, and the sharp transition is accompanied by extensive pre-transition atomic exchange processes, reflected in a lambda-type heat capacity curve. The nanoparticle composition and size dependent transition temperatures, which are well below the bulk miscibility gap, furnish the first Pt-Ir nanophase diagram, which is likely to represent a distinct class of asymmetrically phase-separated nanoalloys having negligible mismatch but large preferential bond strengthening at the near-surface region.