Tailor-Made Inorganic Nanopeapods: Structural Design of Linear Noble Metal Nanoparticle Chains

Abstract

Linear noble-metal nanoparticle (NP) chains have been demonstrated both theoretically and experimentally to be promising candidates for applications in one-dimensional nano-optical devices (e.g. plasmonic waveguides, plasmonic printing). The ability of metal NP chains to transport electromagnetic energy below the diffraction limit has major advantages for scaling down the size of optical devices and components to the nanometer scale. Precise control of particle size, shape and separation in metal NP chains is an important issue for constructing nano-optical devices. Conventional electron-beam lithographic techniques and scanning-probe manipulation have enabled excellent control over the size and position of metal NPs but are time-consuming and costly. Self-assembly routes enable metal NPs to be incorporated into preformed grooves but can control neither the separation between NPs nor their densities. Other approaches to the fabrication of metal NP chains, including wet-chemistry etching and exploitation of the Rayleigh instability of metal nanowires (NWs), suffer from low throughput and limited controllability. Herein, we show a facile and controllable route to the fabrication of linear noble-metal NP chains, in which the size and the separation of optically interesting NPs can be controlled easily. As an example, Pt@CoAl2O4 inorganic peapod nanostructures, which consist of well-defined Pt NPs encapsulated in continuous CoAl2O4 nanoshells, were realized by electrodeposition of cobalt/platinum multilayered (ML) NWs into nanoporous anodic aluminum oxide (AAO) membranes and subsequent solid-state reaction at high temperature. Fabrication of Pt@CoAl2O4 inorganic nanopeapods is schematically illustrated in Figure 1a. First, Co/Pt ML NWs were electrodeposited into the nanoporous AAO template by pulsed potential electrodeposition. As-prepared Co/Pt ML NWs with alternately distributed Co and Pt segments served as the precursors and the backbones for subsequent generation of Pt@CoAl2O4 inorganic nanopeapods (see Figure S1 in the Supporting Information). The size (diameter, DPt) and separation (center-to-center distance, Dc-c) of Pt “peas” in peapod nanostructures were also defined in this step by the lengths of Pt (LPt) and Co segments (LCo), respectively. After electrodeposition, the Co/Pt ML NWs/AAO composite membrane was annealed at 700 8C in an ambient atmosphere for 1–5 h. During heat treatment, Co segments reacted with alumina to form continuous CoAl2O4 nanoshells, that is, the “pods”, while most Pt segments agglomerated into a spherical shape to minimize their surface energy, forming the peas. Thus, the peapod nanostructures were obtained. As-prepared Pt@CoAl2O4 inorganic nanopeapods can either be kept in the porous AAO template or be easily released from the template by selectively removing the surrounding alumina (see Figure S2a in the Supporting Information). Recently, exploitation of the Rayleigh instability of metal NWs has been considered as an effective “bottom-up” approach to create linear metal NP chains. However, this method only allows very limited control of the separation between metal NPs. Its controlling capability is strictly confined by the maximum perturbation wavelength lmax, which is directly proportional to the radius R0 of the metal NWs used, as shown in Figure 1c. The proportionality Figure 1. Fabrication of peapod nanostructures. a,b) Fabrication of Pt@CoAl2O4 inorganic nanopeapods (Pt nanoparticle chains encapsulated in CoAl2O4 nanoshells) by template-based pulsed electrodeposition and high-temperature solid-state reaction. The separation between Pt nanoparticles (Dc-c) is proportional to the pulse duration for Co electrodeposition, tCo, and can be changed at will. c) Metal nanoparticle chains fabricated on the basis of the Rayleigh instability of metal nanowires.