Ion Beam Milling as a Symmetry-Breaking Control in the Synthesis of Periodic Arrays of Identically Aligned Bimetallic Janus Nanocrystals

Abstract

Bimetallic Janus nanostructures represent a highly functional class of nanomaterials due to important physicochemical properties stemming from the union of two chemically distinct metal segments where each maintains a partially exposed surface. Essential to their synthesis is the incorporation of a symmetry-breaking control that is able to induce the regioselective deposition of a secondary metal onto a preexisting nanostructure even though such depositions are, more often than not, in opposition to the innate tendencies of heterogeneous growth modes. Numerous symmetry-breaking controls have been forwarded but the ensuing Janus structure syntheses have not yet achieved anywhere near the same level of control over nanostructure size, shape, and composition as their core−shell and single-element counterparts. Herein, a collimated ion beam is demonstrated as a symmetry-breaking control that allows for the selective removal of a passivating oxide shell from one side of a metal nanostructure to create a configuration that is transformable into a substrate-bound Au−Ag Janus nanostructure. Two different modalities are demonstrated for achieving Janus structures where in one case the oxide dissolves in the growth solution while in the other it remains affixed to form a three-component system. The devised procedures distinguish themselves in their ability to realize complex Janus architectures arranged in periodic arrays where each structure has the same alignment relative to the underlying substrate. The work, hence, provides an avenue for forming precisely tailored Janus structures and, in a broader sense, advances the use of oxides as an effective means for directing nanometal syntheses.

Rapid formation of gold core–satellite nanostructures using Turkevich-synthesized satellites and dithiol linkers: the do's and don'ts for successful assembly

“…Drawbacks of using cores derived from a dewetting procedure as opposed to ones derived from colloidal synthesis include the (i) need to incorporate lithographic techniques if a narrow core size distribution is desired, (ii) use of a substrate that can withstand the high temperatures needed for dewetting to occur, and (iii) requirement that the linker shows no affinity for binding to the substrate if a bare substrate is to be preserved. Advantageous is that (i) the pristine surface of the Au core is amenable to a collection of synthetic techniques that are able to transform the shape of the substrate-bound structures to achieve geometries including nanoplates, 76 nanocubes, 77,78 nanostars, 79,80 and Janus structures, 81 (ii) lithographic techniques allow for the site-selective placement of core–satellite structures, (iii) core–satellite structures can be formed on surfaces at high densities (Fig. S5a†), and (iv) the assemblies are well-bonded to the substrate to the extent that they can withstand sonication.…”

“…69 The procedure used to form periodic arrays of polycrystalline Au discs followed by their assembly into single-crystal Au cores is described in detail elsewhere. 70,81 The process yields a hexagonal pattern of 10 8 structures over a 8 mm × 8.3 mm with a center-to-center distance of 600 nm. Apart from the aforementioned core aging experiments, the Au core structures, once assembled, remained in the owing Ar ambient until they were subjected to the core-satellite assembly process.…”

Site-Specific Chemistry on Gold Nanorods: Curvature-Guided Surface Dewetting and Supracolloidal Polymerization

“…Adding surface ligands to colloidal building blocks provides a meaningful postsynthetic way to design anisotropic metal NPs and potentially guide their self-assembly in a specific pathway. ,− A number of methods in the literature have been reported to introduce surface ligands on metal NPs with regioselectivity: e.g., surface facet recognition, − templated surface grafting, , and phase segregation of mixed ligands . Together with anisotropic NP topologies, site-specific ligand grafting can provide, or sometimes amplify, the directional interparticle interactions to drive NP assembly in a desired manner. ,− For example, gold nanorods (AuNRs) are a classic type of anisotropic building blocks having distinct LSPR bands along their transverse and longitudinal directions. , Without regioselective ligand coverage, AuNRs can pack as clusters with a local preferential direction along their sides; , however, with site-specific modification with ligands, AuNRs can be organized in an end-to-end fashion that allows their longitudinal LSPR coupling. − An early example from Kumacheva and co-workers demonstrated that thiol-terminated polystyrene (PS-SH) could preferentially bind with Au (111) facets located at the two ends in tetrahydrofuran (THF), because the grafting density of cetyltrimethylammonium bromide (CTAB) on convex (111) facets was considerably lower as compared to that on flat (110) and (100) facets as the sides of AuNRs.…”

Periodic arrays of structurally complex oxide nanoshells and their use as substrate-confined nanoreactors

“…[41][42][43][44] Sacrificial oxide layers formed over substrate-bound Au nanostructures, followed by their partial removal, have also proven effective in directing liquid-state nanometal syntheses toward the formation of periodic arrays of aligned bimetallic Janus nanocrystals. 45 Collectively, these various schemes demonstrate ALD-enabled synthesis techniques as a viable means for generating otherwise unobtainable nanomaterials and provides the impetus for pursuing related strategies.…”