Ultra-smooth, highly spherical monocrystalline gold particles were prepared by a cyclic process of slow growth followed by slow chemical etching, which selectively removes edges and vertices. The etching process effectively makes the surface tension isotropic, so that spheres are favored under quasi-static conditions. It is scalable up to particle sizes of 200 nm or more. The resulting spherical crystals display uniform scattering spectra and consistent optical coupling at small separations, even showing Fano-like resonances in small clusters. The high monodispersity of the particles we demonstrate should facilitate the self-assembly of nanoparticle clusters with uniform optical resonances, which could in turn be used to fabricate optical metafluids. Narrow size distributions are required not only to control the spectral features, but also the morphology and yield of clusters in certain assembly schemes. KEYWORDS:Gold nanospheres, plasmonics, monodisperse, chemical etching, Fano-like resonance. TOC Graphic 3Accepted version of ACS Nano 7 (12): 11064 (2013) In equilibrium, a nanoscale crystal adopts a polyhedral morphology to minimize its surface free energy. As a result, metallic nanoparticles grown near equilibrium form facets. [1][2][3] Although the plasmon resonances and ease of functionalization 4 make such particles promising 1 for bottom-up assembly of optical resonators 5,6 and isotropic metamaterials, 7 it is difficult to make structures whose optical properties are reproducible from one particle or one multiple-particle cluster to the next since the resonances are often sensitive to features such as sharp corners, 8 number of facets, 9 roughness, 10 and overall size and shape. 11 Moreover, the interparticle optical coupling varies significantly with nanometer-scale changes in gap distance 12 and orientation. 13 The ideal particle for self-assembly of plasmonic structures is therefore not a polyhedron, but a spherical crystal without facets or grain boundaries. However, producing such particles is a materials challenge, since spherical crystals are not stable under any growth conditions. Here we show that a cyclic process of slow growth followed by slow chemical etching, which selectively removes edges and vertices, results in ultra-smooth, highly spherical monocrystalline gold particles. The etching process, which is functionally similar to (but chemically different from) that used to make monocrystalline silver nanospheres for surface-enhanced Raman spectroscopy, 14,15 effectively makes the surface tension isotropic, so that spheres are favored under quasi-static conditions. The resulting spherical crystals display uniform scattering spectra and consistent optical coupling at small separations, even showing Fano-like resonances 16 in small assemblies. The cyclic process we demonstrate could be extended to other metals and, because it is scalable up to particle sizes of 200 nm or more, might be used to create strongly scattering particles for sensors, 17,18 electromagnetic resonators, 7 and oth...

Single adatoms are expected to participate in many processes occurring at solid surfaces, such as the growth of graphene on metals. We demonstrate, both experimentally and theoretically, the catalytic role played by single metal adatoms during the technologically relevant process of graphene growth on nickel (Ni). The catalytic action of individual Ni atoms at the edges of a growing graphene flake was directly captured by scanning tunneling microscopy imaging at the millisecond time scale, while force field molecular dynamics and density functional theory calculations rationalize the experimental observations. Our results unveil the mechanism governing the activity of a single-atom catalyst at work.

Interaction with the substrate strongly affects the electronic/chemical properties of supported graphene. So far, graphene deposited by chemical vapor deposition (CVD) on catalytic single crystal transition metal surfaces -mostly 3-fold close-packed -has mainly been studied. Herein, we investigated CVD graphene on a polycrystalline nickel (Ni) substrate, focusing in particular on (100) micrograins and comparing the observed behavior with that on single crystal Ni(100) substrate. The symmetry-mismatch leads to moiré superstructures with stripe-like or rhombic-network morphology, which were characterized by atomically-resolved scanning tunneling microscopy (STM). Density functional theory (DFT) simulations shed light on spatial corrugation and interfacial interactions: depending on the misorientation angle, graphene is either alternately physi-and chemisorbed or uniformly chemisorbed, the interaction being modulated by the (sub)nanometer-sized moiré superstructures. Ni(100) micrograins appear to be a promising substrate to finely tailor the electronic properties of graphene at the nanoscale, with relevant perspective applications in electronics and catalysis. of the graphene lattice, which leads to alternate strongly-and weakly-interacting regions across the moiré supercells [13][14][15][16][17][18]. In contrast, the weak coupling between graphene and other transition metals (such as copper (Cu), iridium (Ir) and platinum) results in large interfacial spacing out of the range of chemisorption, smaller spatial corrugation of moirés with respect to strongly-coupled systems, and limited rotational alignment between graphene and the substrate [19][20][21][22][23]. From an electronic point of view, the band structures for chemisorbed graphene (such as that on Ni(111) or Ru(0001)) are fragmented or disrupted due to the hybridization of the graphene π state and the metal d orbital, while physisorbed graphene typically shows Dirac cones similar to its pristine form [24][25][26]. Therefore, the magnitude of energy gap opening, interface charge polycrystalline Ni foils are also explored, thereby bridging the material gap from single crystal to realistic, non-ideal surfaces for STM measurements. Indeed, the (100) facet is one of the most common orientations present in polycrystalline Ni foils or thin films, as reported in literature [29,37] and further corroborated in this work. In addition, nickel is among the class of most-utilized metallic catalysts for CVD growth of graphene [5][6]38]. This work is therefore of potential interest for the scalable production and applications of graphene. Our results indicate that graphene structures observed on both single-and poly-crystalline substrates are highly nanometer scale. Generally, graphene moiré originates from lattice mismatch and/or angular misorientation in two isosymmetric overlapping periodic lattices; herein the situation is further complicated by the symmetry mismatch of the two interface lattices. In figures 2(a-c), from left to right, we show three STM images with incr...