The functionality of well-tailored nanomaterials can only be retained if they are robust to the environmental factors in which they operate. The inability of Cu to withstand such factors is largely responsible for its current status as a second-tier plasmonic nanomaterial. Herein, it is demonstrated that atomic layer deposition can be used as a pliable technique for the application of oxide coatings to substrate-based Cu nanostructures where suitably protected structures become robust to oxidation, high temperatures, and aqueous, acidic, and alkaline solutions without unduly influencing important plasmonic properties. Moreover, strategies are presented for maximizing plasmonic near-fields and allowing for the transport of hot electrons while maintaining coating integrity. The findings demonstrate that there does not exist a one-solution-fits-all approach but that coating design must follow an application-specific methodology. Within the scope of the investigation, alumina, hafnia, titania, and combinations thereof were all shown to be effective under certain conditions, but where hafnia shows the greatest durability in extreme pH environments and alumina-hafnia multilayers provide Cu with protection from oxidation to temperatures as high as 600 °C. The work advances the use of Cu nanostructures as durable plasmonic materials and provides broad-based strategies for protecting other vulnerable nanomaterials from harsh environments.

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Noble metal nanoplates are a unique class of two-dimensional (2D) nanomaterials whose planar geometry serves as one of the most important nanoscale building blocks. Referred to by names such as...

Even though performance metrics position silver as the preeminent plasmonic material in the visible and near-infrared regions of the electromagnetic spectrum, it remains underutilized in applications because its properties irreversibly degrade in the environments it must operate. The emergence of shell-isolated plasmonic nanostructures as a distinct class of nanomaterials has, however, created new opportunities for the utilization of silver because its vulnerable surfaces can be encapsulated in a chemically robust transparent shell while maintaining important plasmonic properties. To fully capitalize on this opportunity requires that shell−nanostructure combinations be rationally designed where consideration is given to a parameter space encompassing nanostructure stability–property relationships. Herein, we demonstrate the layer-by-layer deposition capabilities of the atomic layer deposition (ALD) technique as a means to design shell-isolated silver nanostructures where the confined structure acts as a built-in plasmonic sensor for spectroscopically evaluating durability in air, water, and chemically aggressive environments. For all cases, appropriately designed oxide shells are shown to provide long-term stability, but where their own surface chemistry and structural integrity become limiting factors in bolstering and preserving plasmonic properties. The work, therefore, forwards the use of ALD-deposited layers for the realization of shell-isolated plasmonic nanostructures that exploit the remarkable properties of silver.

The subwavelength confinement of light energy in the nanogaps formed between adjacent plasmonic nanostructures provides the foundational basis for nanophotonic applications. Within this realm, air-filled nanogaps are of central importance because they present a cavity where application-specific nanoscale objects can reside. When forming such configurations on substrate surfaces, there is an inherent difficulty in that the most technologically relevant nanogap widths require closely spaced nanostructures separated by distances that are inaccessible through standard electron-beam lithography techniques. Herein, we demonstrate an assembly route for the fabrication of aligned plasmonic gold trimers with air-filled vertical nanogaps having widths that are defined with spatial controls that exceed those of lithographic processes. The devised procedure uses a sacrificial oxide layer to define the nanogap, a glancing angle deposition to impose a directionality on trimer formation, and a sacrificial antimony layer whose sublimation regulates the gold assembly process. By further implementing a benchtop nanoimprint lithography process and a glancing angle ion milling procedure as additional controls over the assembly, it is possible to deterministically position trimers in periodic arrays and extend the assembly process to dimer formation. The optical response of the structures, which is characterized using polarization-dependent spectroscopy, surface-enhanced Raman scattering, and refractive index sensitivity measurements, shows properties that are consistent with simulation. This work, hence, forwards the wafer-based processing techniques needed to form air-filled nanogaps and place plasmonic energy at site-specific locations.

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Micro- and nanoscale ZnO tetrapods provide an attractive support for metallic nanostructures since they can be inexpensively produced using the flame transport method and nanoparticle synthesis schemes can take advantage of a coupled response facilitated by the formation of a semiconductor-metal interface. Here, we present a light-mediated solution-based growth mode capable of decorating the surface of ZnO tetrapods with nanostructures of gold, silver, copper, platinum, palladium, ruthenium, iridium, and rhodium. It involves two coupled reactions that are driven by the optical excitation of electron-hole pairs in the ZnO semiconductor by ultraviolet photons where the excited electrons are used to reduce aqueous metal ions onto the ZnO tetrapod as excited holes are scavenged from the surface. For the most part, the growth mode gives rise to nanoparticles with a roundish morphology that are uniformly distributed on the tetrapod surface. Larger structures with irregular shapes are, however, obtained for syntheses utilizing aqueous metal nitrates as opposed to chlorides, a result that suggests that the anion plays a role in shape determination. It is also demonstrated that changes to the molarity of the metal ion can influence the nanostructure nucleation rate. The catalytic activity of tetrapods decorated with each of the eight metals is assessed using the reduction of 4-nitrophenol by borohydride as a model reaction where it is shown that those decorated with Pd, Ag, and Rh are the most active.

metals proving extraordinarily successful, [2] there has recently been a concerted effort to adapt the same synthetic techniques to Rh-based nanostructures. [1a] By establishing control over the size, shape, and faceting of the catalyst and through the synthesis of Rh-based bimetallic architectures, it is proving possible to realize greater catalytic activity and selectivity, and hence, provides a more effective utilization of a finite precious resource.Exerting shape control over Rh nanostructures initially proved to be a formidable challenge due to its exceedingly high surface energy, a characteristic that made it difficult to substantially alter the order of the surface energies of the various facets through the use of capping agents. [1a,3] This synthetic challenge has now, to a large degree, been met through the ingenuity of many researchers who have demonstrated synthetic protocols able to realize well-recognized nanostructure architectures such as Rh nanocubes, [4] nanowires, [5] tetrahedrons, [6] icosahedra, [4b,7] nanoframes, [8] and nanosheets, [6,9] as well as more complex geometries offering convex and/or concave features. [3,6,10] The synthesis of Rh-based bimetallic nanostructures has also shown significant progress, but where the field is at a more nascent stage due to the much larger parameter space associated with binary systems. Bimetallic nanostructures, however, present far greater opportunities from the standpoint of tuning nanostructure properties through alloying or the heterogeneous deposition of one metal on another. [2b] The use of high melting point metals, such as Rh, have the added advantage of enhancing the shape stability of nanostructures at elevated temperatures. [11] With prominent work now appearing for the CuRh, [12] NiRh, [13] PdRh, [8a,11,14] PtRh, [10d,15] and AuRh, [8b,16] systems, it is quite evident that Rhbased bimetallic nanostructures present unique opportunities in catalysis.Galvanic replacement reactions have been widely used to synthesize hollow metal nanostructures and the mechanisms guiding these reactions have been well-documented. [17] Such reactions proceed by exposing a sacrificial metal template to ions of a second metal with a higher electrochemical potential, the result of which is the oxidation and dissolution of the template material as the metal ions are reduced onto its surface. The reaction often leads to bimetallic nanoshells caused by alloying Galvanic replacement reactions are widely used in the synthesis of bimetallic nanoshells. Essential to these syntheses is the design of template materials with electrochemical potentials that are low enough to facilitate the replacement of a wide variety of metals. While Cu is an attractive template from this standpoint, it has only rarely been used due to its propensity for oxidation and the associated difficulties in achieving chemically stable colloids. Here, a synthetic scheme is demonstrated for the design of supported Cu templates and their subsequent replacement with Rh where the detrimenta...