Using Scanning-Probe Block Copolymer Lithography and Electron Microscopy To Track Shape Evolution in Multimetallic Nanoclusters

Growing Gold Nanostars on 3D Hydrogel Surfaces

Nanocomposites comprising hydrogels and plasmonic nanoparticles are attractive materials for tissue engineering, bioimaging, and biosensing. These materials are usually fabricated by adding colloidal nanoparticles to the uncured polymer mixture and thus require time-consuming presynthesis, purification, and ligand-exchange steps. Herein, we introduce approaches for rapid synthesis of gold nanostars (AuNSt) in situ on hydrogel substrates, including those with complex three-dimensional (3D) features. These methods enable selective AuNSt growth at the surface of the substrate, and the growth conditions can be tuned to tailor the nanoparticle size and density (coverage). We additionally demonstrate proof-of-concept applications of these nanocomposites for SERS sensing and imaging. High surface coverage with AuNSt enabled 1-2 orders of magnitude higher SERS signals compared to plasmonic hydrogels loaded with premade colloids. Importantly, AuNSt can be prepared without the addition of any potentially cytotoxic surfactants, thereby ensuring a high biocompatibility. Overall, in situ growth becomes a versatile and straightforward approach for the fabrication of plasmonic biomaterials.

Surface Lattice Plasmon Resonances by Direct In Situ Substrate Growth of Gold Nanoparticles in Ordered Arrays

Precise arrangements of plasmonic nanoparticles on substrates are important for designing optoelectronics, sensors and metamaterials with rational electronic, optical and magnetic properties. Bottom-up synthesis offers unmatched control over morphology and optical response of individual plasmonic building blocks. Usually, the incorporation of nanoparticles made by bottom-up wet chemistry starts from batch synthesis of colloids, which requires time-consuming and hard-to-scale steps like ligand exchange and self-assembly. Herein, an unconventional bottom-up wet-chemical synthetic approach for producing gold nanoparticle ordered arrays is developed. Water-processable hydroxypropyl cellulose stencils facilitate the patterning of a reductant chemical ink on which nanoparticle growth selectively occurs. Arrays exhibiting lattice plasmon resonances in the visible region and near infrared (quality factors of >20) are produced following a rapid synthetic step (<10 min), all without cleanroom fabrication, specialized equipment, or self-assembly, constituting a major step forward in establishing in situ growth approaches. Further, the technical capabilities of this method through modulation of the particle size, shape, and array spacings directly on the substrate are demonstrated. Ultimately, establishing a fundamental understanding of in situ growth has the potential to inform the fabrication of plasmonic materials; opening the door for in situ growth fabrication of waveguides, lasing platforms, and plasmonic sensors.

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.

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.

Direct Observation of Early Stages of Growth of Multilayered DNA-Templated Au-Pd-Au Core-Shell Nanoparticles in Liquid Phase

We report here on direct observation of early stages of formation of multilayered bimetallic Au-Pd core-shell nanocubes and Au-Pd-Au core-shell nanostars in liquid phase using low-dose in situ scanning transmission electron microscopy (S/TEM) with the continuous flow fluid cell. The reduction of Pd and formation of Au-Pd core-shell is achieved through the flow of the reducing agent. Initial rapid growth of Pd on Au along <111> direction is followed by a slower rearrangement of Pd shell. We propose the mechanism for the DNA-directed shape transformation of Au-Pd core-shell nanocubes to adopt a nanostar-like morphology in the presence of T30 DNA and discuss the observed nanoparticle motion in the confined volume of the fluid cell. The growth of Au shell over Au-Pd nanocube is initiated at the vertices of the nanocubes, leading to the preferential growth of the {111} facets and resulting in formation of nanostar-like particles. While the core-shell nanostructures formed in a fluid cell in situ under the low-dose imaging conditions closely resemble those obtained in solution syntheses, the reaction kinetics in the fluid cell is affected by the radiolysis of liquid reagents induced by the electron beam, altering the rate-determining reaction steps. We discuss details of the growth processes and propose the reaction mechanism in liquid phase in situ.

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.

When lithography meets self-assembly: a review of recent advances in the directed assembly of complex metal nanostructures on planar and textured surfaces

One of the foremost challenges in nanofabrication is the establishment of a processing science that integrates wafer-based materials, techniques, and devices with the extraordinary physicochemical properties accessible when materials are reduced to nanoscale dimensions. Such a merger would allow for exacting controls on nanostructure positioning, promote cooperative phenomenon between adjacent nanostructures and/or substrate materials, and allow for electrical contact to individual or groups of nanostructures. With neither self-assembly nor top-down lithographic processes being able to adequately meet this challenge, advancements have often relied on a hybrid strategy that utilizes lithographically-defined features to direct the assembly of nanostructures into organized patterns. While these so-called directed assembly techniques have proven viable, much of this effort has focused on the assembly of periodic arrays of spherical or near-spherical nanostructures comprised of a single element. Work directed toward the fabrication of more complex nanostructures, while still at a nascent stage, has nevertheless demonstrated the possibility of forming arrays of nanocubes, nanorods, nanoprisms, nanoshells, nanocages, nanoframes, core-shell structures, Janus structures, and various alloys on the substrate surface. In this topical review, we describe the progress made in the directed assembly of periodic arrays of these complex metal nanostructures on planar and textured substrates. The review is divided into three broad strategies reliant on: (i) the deterministic positioning of colloidal structures, (ii) the reorganization of deposited metal films at elevated temperatures, and (iii) liquid-phase chemistry practiced directly on the substrate surface. These strategies collectively utilize a broad range of techniques including capillary assembly, microcontact printing, chemical surface modulation, templated dewetting, nanoimprint lithography, and dip-pen nanolithography and employ a wide scope of chemical processes including redox reactions, alloying, dealloying, phase separation, galvanic replacement, preferential etching, template-mediated reactions, and facet-selective capping agents. Taken together, they highlight the diverse toolset available when fabricating organized surfaces of substrate-supported nanostructures.

Noble Metal Nanostructure Synthesis at the Liquid-Substrate Interface: New Structures, New Insights, and New Possibilities

Modern technologically driven societies could not exist in their current form if it were not for a great many synthetic achievements reliant on solution-based chemistry and substrate-based processing techniques. It is, hence, not surprising that these same materials preparation techniques have given rise to an impressive list of functional nanomaterials including those derived from noble metals, a class of materials renowned for their extraordinary optical and catalytic properties. Acting as the foundation for substrate-based processing is a collection of techniques such as physical and chemical vapor deposition, epitaxy, self- and directed assembly, and a host of lithographic methods. These techniques allow for precise control over nanostructure placement, but where the fabrication of sophisticated architectures and sub-50 nm feature sizes are often unattainable or reliant on the use of technically demanding cost-prohibitive routes. In contrast, solution-based chemistry allows for the formation of complex nanostructures while maintaining synthetic ease, cost-effectiveness, and exacting control over monodispersity, size, shape, composition, and crystallinity. While many methods exist for the dispersal of colloids onto substrates, few are capable of achieving nanostructure ensembles where nanostructure placement allows for true long-range order as well as control over the crystallographic alignment of the nanostructures relative to each other and the underlying substrate. A more exhaustive comparison of these two approaches reveals that, more often than not, a weakness of substrate-based processing is a strength of colloidal synthesis and vice versa. In this Account, we describe a synthetic strategy devised and validated by the Neretina laboratory that integrates the competencies of substrate-based techniques with colloidal chemistry and, in doing so, brings this rich and exciting chemistry and its associated functionalities to the substrate surface. The strategy takes advantage of an impressive collection of seed-mediated solution-based protocols in which dispersed seeds direct noble metal nanostructure formation along orderly reaction pathways. It, however, replaces the seed colloid with substrate-immobilized templates formed in periodic arrays where the crystallographic orientation of the templates is defined by an epitaxial relationship with the substrate. Demonstrated are syntheses at the liquid-substrate interface in which organized surfaces of crystalline templates formed through templated dewetting are subjected to galvanic replacement, preferential etching, and/or heterogeneous deposition facilitated by redox reactions in both the presence and absence of capping agents. While the protocols utilized are adapted from some of the most well-studied colloidal syntheses, in no case do they yield reaction products that are identical since the substrate inflicts asymmetries onto the growth mode. We believe that the strategy described herein not only demonstrates a family of nanostructures unobtainable through other means but also establishes a synthetic foundation that offers unprecedented flexibility, expands the palette of accessible template materials, provides a new vantage point from which complex reactions occurring in liquid media can be examined, and has the potential to underpin photovoltaic, catalytic, and sensing applications reliant on substrate-based noble metal nanostructures.

Citrate-Induced Nanocubes: A Re-Examination of the Role of Citrate as a Shape-Directing Capping Agent for Ag-Based Nanostructures

Seed-mediated syntheses utilizing facet-selective surface passivation provide the necessary chemical controls to direct noble metal nanostructure formation to a predetermined geometry. The foremost protocol for the synthesis of (111)-faceted Ag octahedra involves the reduction of metal ions onto pre-existing seeds in the presence of citrate and ascorbic acid. It is generally accepted that the capping of (111) facets with citrate dictates the shape while ascorbic acid acts solely as the reducing agent. Herein, a citrate-based synthesis is demonstrated in which the presence or absence of ascorbic acid is the shape-determining factor. Reactions are carried out in which Ag(+) ions are reduced onto substrate-immobilized Ag, Au, Pd, and Pt seeds. Syntheses lacking ascorbic acid, in which citrate acts as both the capping and the reducing agent, result in a robust nanocube growth mode able to withstand wide variations in the concentration of reactants, reaction rates, seed material, seed orientation and faceting, pH, and substrate material. If, however, ascorbic acid is included in these syntheses, then the growth mode reverts to one that advances the octahedral geometry. The implication of these results is that citrate, or one of its oxidation products, selectively caps (100) facets, but where this capability is compromised by ascorbic acid.

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.