One sentence summary: Experiments reveal that small clusters of hard spheres with short-range attractions favor equilibrium structures determined by geometrical rules.The study of clusters has provided the most tangible link between local geometry and bulk condensed matter. But experiments have not yet systematically explored the thermodynamics of even the smallest clusters. Here we present experimental measurements of the structures and free energies of colloidal clusters in which the particles act as hard spheres with short-range attractions. We find that highly symmetric clusters are strongly suppressed by rotational entropy, while the most stable clusters have anharmonic vibrational modes or extra bonds. Many of these are subsets of closepacked lattices. As the number of particles increases from 6 to 10 we observe the emergence of a complex free energy landscape with a small number of ground states and many local minima.An isolated system of 10 interacting atoms or molecules will in general adopt a structure that differs in symmetry and average energy from that of a bulk liquid, solid, or even a system containing 100 particles. Yet the study of such small clusters has shed light on a wide variety of phenomena in condensed matter physics and physical chemistry. Since Frank first predicted (1) that icosahedral short-range order * This is the authors version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published

One sentence summary: A constant background Gaussian curvature alters crystal growth and favors the formation of anisotropic, ribbon-like domains.Although the effects of kinetics on crystal growth are well-understood, the role of substrate curvature is not yet established. We study rigid, two-dimensional colloidal crystals growing on spherical droplets to understand how the elastic stress induced by Gaussian curvature affects the growth pathway. In contrast to crystals grown on flat surfaces or compliant crystals on droplets, these crystals form branched, ribbon-like domains with large voids and no topological defects. We show that this morphology minimizes the curvature-induced elastic energy. Our results illustrate the dramatic effects of curvature on the ubiquitous process of crystallization, with practical implications for nanoscale disorder-order transitions on curved manifolds, including the assembly of viral capsids, phase separation on vesicles, and crystallization of tetrahedra in three dimensions.Since Nicolaus Steno's pioneering work on crystal growth in the 17th century (1), it has been established that the shape of a crystal is a vestige of its growth pathway. Near equilibrium, crystals grown from the melt form compact, faceted * This is the authors' version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution.

Many industrial catalysts are composed of metal particles supported on metal oxides (MMO). It is known that the catalytic activity of MMO materials is governed by metal and metal oxide interactions (MMOI), but how to optimize MMO systems via manipulation of MMOI remains unclear, due primarily to the ambiguous nature of MMOI. Herein, we develop a Pt/NbO/C system with tunable structural and electronic properties via a modified arc plasma deposition method. We unravel the nature of MMOI by characterizing this system under reactive conditions utilizing combined electrochemical, microscopy, and in situ spectroscopy. We show that Pt interacts with the Nb in unsaturated NbO owing to the oxygen deficiency in the MMO interface, whereas Pt interacts with the O in nearly saturated NbO, and further interacts with Nb when the oxygen atoms penetrate into the Pt cluster at elevated potentials. While the Pt-Nb interactions do not benefit the inherent activity of Pt toward oxygen reduction reaction (ORR), the Pt-O interactions improve the ORR activity by shortening the Pt-Pt bond distance. Pt donates electrons to NbO in both Pt-Nb and Pt-O cases. The resultant electron eficiency stabilizes low-coordinated Pt sites, hereby stabilizing small Pt particles. This determines the two characteristic features of MMO systems: dispersion of small metal particles and high catalytic durability. These findings contribute to our understandings of MMO catalytic systems.

The energetics and assembly pathways of small clusters may yield insights into processes occurring at the earliest stages of nucleation. We use a model system consisting of micrometer-sized, spherical colloidal particles to study the structure and dynamics of small clusters, where the number of particles is small (N ≤ 10). The particles interact through a short-range depletion attraction with a depth of a few k B T . We describe two methods to form colloidal clusters, one based on isolating the particles in microwells and another based on directly assembling clusters in the gas phase using optical tweezers. We use the first technique to obtain ensemble-averaged probabilities of cluster structures as a function of N. These experiments show that clusters with symmetries compatible with crystalline order are rarely formed under equilibrium conditions. We use the second technique to study the dynamics of the clusters, and in particular how they transition between free-energy minima. To monitor the clusters we use a fast three-dimensional imaging technique, digital holographic microscopy, that can resolve the positions of each particle in the cluster with 30-45 nm precision on millisecond timescales. The real-space measurements allow us to obtain estimates for the lifetimes of the energy minima and the transition states. It is not yet clear whether the observed dynamics are relevant for small nuclei, which may not have sufficient time to transition between states before other particles or clusters attach to them. However, the measurements do provide some glimpses into how systems containing a small number of particles traverse their free-energy landscape.

High stability, availability of multiple oxidation states, and accessibility within a wide electrochemical window are the prime features of Nb that make it a favorable candidate for electrocatalysis, especially when it is combined with Pt. However, Nb has been used as a support in the form of oxides in all previously reported Pt−Nb electrocatalysts, and no Pt−Nb alloying phase has been demonstrated hitherto. Herein, we report a multifunctional Pt−Nb composite (PtNb/NbO x -C) where Nb exists both as an alloying component with Pt and as an oxide support and is synthesized by means of a simple wet chemical method. In this work, the Pt−Nb alloy phase has been firmly verified with the help of multiple spectroscopic methods. This allows for the experimental evidence of the theoretical prediction that Pt−Nb alloy interactions improve the oxygen reduction reaction (ORR) activity of Pt. In addition, such a combination of multiphase Nb brings up myriad features encompassing increased ORR durability, immunity to phosphate anion poisoning, enhanced hydrogen oxidation reaction (HOR) activity, and oxidative carbon monoxide (CO) stripping, making this electrocatalyst useful in multiple fuel cell systems.