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

Sacrificial templates present an effective pathway for gaining high-level control over nanoscale reaction products. Atomic layer deposition (ALD) is ideally suited for such approaches due to its ability to replicate the surface topography of a template material through the deposition of an ultrathin conformal layer. Herein, metal nanostructures are demonstrated as sacrificial templates for the formation of architecturally complex and deterministically positioned oxide nanoshells, open-topped nanobowls, vertically standing half-shells, and nanorings. The three-step process sees metal nanocrystals formed in periodic arrays, coated with an ALD-deposited oxide, and hollowed out with a selective etch through nanopores formed in the oxide shell. The procedure is further augmented through the use of a directional ion beam that is used to sculpt the oxide shells into bowl- and ring-like configurations. The functionality of the so-formed materials is demonstrated through their use as substrate-confined nanoreactors able to promote the growth and confinement of nanomaterials. Taken together, the work expands the design space for substrate-based nanomaterials, creates a platform for advancing functional surfaces and devices and, from a broader perspective, advances the use of ALD in forming complex nanomaterials.

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

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.

Synergistic roles of vapor- and liquid-phase epitaxy in the seed-mediated synthesis of substrate-based noble metal nanostructures.

Colloidal growth modes reliant on the replication of the crystalline character of a preexisting seed through homoepitaxial or heteroepitaxial depositions have enriched both the architectural diversity and functionality of noble metal nanostructures. Equivalent syntheses, when practiced on seeds formed on a crystalline substrate, must reconcile with the fact that the substrate enters the syntheses as a chemically distinct bulk-scale component that has the potential to impose its own epitaxial influences. Herein, we provide an understanding of the formation of epitaxial interfaces within the context of a hybrid growth mode that sees substrate-based seeds fabricated at high temperatures in the vapor phase on single-crystal oxide substrates and then exposed to a low-temperature liquid-phase synthesis yielding highly faceted nanostructures with a single-crystal character. Using two representative syntheses in which gold nanoplates and silver-platinum core-shell structures are formed, it is shown that the hybrid system behaves unconventionally in terms of epitaxy in that the substrate imposes an epitaxial relationship on the seed but remains relatively inactive as the metal seed imposes an epitaxial relationship on the growing nanostructure. With epitaxy transduced from substrate to seed to nanostructure through what is, in essence, a relay system, all of the nanostructures formed in a given synthesis end up with the same crystallographic orientation relative to the underlying substrate. This work advances the use of substrate-induced epitaxy as a synthetic control in the fabrication of on-chip devices reliant on the collective response of identically aligned nanostructures.

Sequential Symmetry-Breaking Events as a Synthetic Pathway for Chiral Gold Nanostructures with Spiral Geometries.

Symmetry-breaking synthetic controls allow for nanostructure geometries that are counter to the underlying crystal symmetry of a material. If suitably applied, such controls provide the means to drive an isotropic metal along a growth pathway yielding a three-dimensional chiral geometry. Herein, we present a light-driven solution-based synthesis yielding helical gold spirals from substrate-bound seeds. The devised growth mode relies on three separate symmetry-breaking events ushered in by seeds lined with planar defects, a capping agent that severely frustrates early stage growth, and the Coulombic repulsion that occurs when identically charged growth fronts collide. Together they combine to advance a growth pathway in which planar growth emanates from one side of the seed, advances to encircle the seed from both clockwise and counterclockwise directions, and then, upon collision of the two growth fronts, sees one front rise above the other to realize a self-perpetuating three-dimensional spiral structure.

Plasmon-Mediated Synthesis of Periodic Arrays of Gold Nanoplates Using Substrate-Immobilized Seeds Lined with Planar Defects.

The seed-mediated growth of noble metal nanostructures with planar geometries requires the use of seeds lined with parallel stacking faults so as to provide a break in symmetry in an otherwise isotropic metal. Although such seeds are now routinely synthesized using colloidal pathways, equivalent pathways have not yet been reported for the fabrication of substrate-based seeds with the same internal defect structures. The challenge is not merely to form seeds with planar defects but to do so in a deterministic manner so as to have stacking faults that only run parallel to the substrate surface while still allowing for the lithographic processes needed to regulate the placement of seeds. Here, we demonstrate substrate-imposed epitaxy as a viable synthetic control able to induce planar defects in Au seeds while simultaneously dictating nanostructure in-plane alignment and crystallographic orientation. The seeds, which are formed in periodic arrays using nanoimprint lithography in combination with a vapor-phase assembly process, are subjected to a liquid-phase plasmon-mediated synthesis that uses light as an external stimuli to drive a reaction yielding periodic arrays of hexagonal Au nanoplates. These achievements not only represent the first of their kind demonstrations but also advance the possibility of integrating wafer-based technologies with a rich and exciting nanoplate colloidal chemistry.

Arrays of highly complex noble metal nanostructures using nanoimprint lithography in combination with liquid-phase epitaxy.

Current best-practice lithographic techniques are unable to meet the functional requirements needed to enable on-chip plasmonic devices capable of fully exploiting nanostructure properties reliant on a tailored nanostructure size, composition, architecture, crystallinity, and placement. As a consequence, numerous nanofabrication methods have emerged that address various weaknesses, but none have, as of yet, demonstrated a large-area processing route capable of defining organized surfaces of nanostructures with the architectural diversity and complexity that is routinely displayed in colloidal syntheses. Here, a hybrid fabrication strategy is demonstrated in which nanoimprint lithography is combined with templated dewetting and liquid-phase syntheses that is able to realize periodic arrays of complex noble metal nanostructures over square centimeter areas. The process is inexpensive, can be carried out on a benchtop, and requires modest levels of instrumentation. Demonstrated are three fabrication schemes yielding arrays of core-shell, core-void-shell, and core-void-nanoframe structures using liquid-phase syntheses involving heteroepitaxial deposition, galvanic replacement, and dealloying. With the field of nanotechnology being increasingly reliant on the engineering of desirable physicochemical responses through architectural control, the fabrication strategy provides a platform for advancing devices reliant on addressable arrays or the collective response from an ensemble of identical nanostructures.

A Wulff in a Cage: The Confinement of Substrate-Based Structures in Plasmonic Nanoshells, Nanocages, and Nanoframes Using Galvanic Replacement.

Galvanic replacement reactions carried out on solid core-shell structures typically yield a noble metal nanorattle geometry in which a mobile core is contained within a hollowed shell. Here, we adapt this colloidal synthesis to substrate-based structures to obtain a fundamentally altered product in which an immobilized core is separated from the shell by a well-defined gap, an architecture unobtainable using colloidal techniques and that offers unique advantages in terms of generating plasmonic near-field effects within the confines of a single structure. In the devised route, Wulff-shaped templates of Au, Pt, or Pd, formed through the dewetting of ultrathin films, are first transformed into core-shell structures through the reduction of Ag(+) ions onto their surface and then further transformed through the galvanic replacement of Ag with Au. Through suitable adjustments to the shell geometry, the epitaxial relationship with the substrate, and the extent to which the shell is replaced, it is possible to generate an entire family of nanostructures in which a Wulff-shaped core is confined within a nanoshell, nanocage, or nanoframe, where, in all cases, bonds formed between the structure and the substrate preclude motion. With the potential to tune the gap width, the geometry of the confining structure, and the composition of the core, shell, and substrate, these structures could find application as catalytic nanoreactors able to drive both single-step and cascade reactions or as plasmon-based sensing elements for biological and chemical detection.