Phases of multielement nanoparticles: Thermodynamically stable metal nanoparticles composed of multiple elements could, in principle, exhibit several different phases that form multiple interfaces. Chen et al. explored the structure and composition of palladium-tin alloy nanoparticles formed with up to five other elements after high-temperature annealing. Triphase nanoparticles possessed two or three interface architectures, and tetraphase nanoparticles exhibited up to six interfaces. Theoretical and experimental studies revealed how the balance between surface and interfacial energies influences the observed phases and interface structure. Science , this issue p. 959

Mobile particles in colloidal crystals: The crystallization of nanoparticles can be controlled by functionalizing them with DNA strands that direct assembly through hybridization. The design rules for interactions between pairs of particles resemble those for ionic compounds. Inspired by molecular dynamics simulations, Girard et al. show that larger particles (∼10 nanometers in diameter) that have mutual repulsive interactions can form a stable lattice only if much smaller conjugate particles (∼1.5 nanometers in diameter) are present. These smaller particles are mobile and diffuse through the lattice, so the bonding interaction resembles the classical picture of electrons in metals. Science , this issue p. 1174

Significance: Discovering new materials is traditionally a slow process, as the materials genome encompasses a massive parameter space, particularly on the nanoscale where synthesis methods are less universal. To overcome this challenge, a cantilever-free lithography method has been used to deposit millions of discrete nanoreactors on a substrate, which can be transformed into millions of nanoparticles spatially encoded in terms of composition and size. These compositions were screened for their ability to catalytically grow carbon nanotubes, and a composition (Au ₃ Cu) was identified as optimum for this application. More generally, this combinatorial method of synthesizing nanoparticle megalibraries offers researchers a platform for identifying structures with desired properties at a rate not previously possible.