Mesoscale Metallic Pyramids with Nanoscale Tips (25026)
A facile top-down process for fabricating anisotropic mesoscale (100-300 nm) metallic pyramids, with nanoscale tips (radii of curvature < 2 nm), has been devised by Northwestern researchers. Mesoscale holes in a chromium film are used as an etch mask to fabricate pyramidal pits in a silicon substrate and then as a deposition mask to form the metallic pyramids. Two- and three-layered pyramids of different metals and chemical functionality can be created. The orientation-dependent plasmon resonance modes of the anisotropic metallic nanoparticles can be generated when aligned inside a uniform dielectric environment. The broad nanoparticle optical tunability from ultraviolet to near-infrared wavelengths can be exploited in nanoscale AFM, photonics, chemical and biological sensing, and photovoltaic applications.
ADVANTAGES: A process affording well defined monodisperse, mesoscale, multilayer, metallic pyramids of uniform shape and thickness, in contrast to solution-based methods, which typically yield other particle shapes and sizes in addition to the desired product. The method produces ~109 pyramids / in2 which can be readily scaled to increase mesostructures density.
SUMMARY: A combination of phase-shifting photolithography (PSP), wet-chemical etching, and electron (e)-beam deposition is used to create metallic pyramids within the etched pits of a Si (100) substrate (figure 1). The simple procedure employs sub-250 nm holes in a chromium film as both an etch mask and deposition mask. The technique affords multi-layered, pyramidal structures utilizing the layer-by-layer capabilities of e-beam deposition. Free-standing mesoscale (100-300 nm) pyramids with nanoscale (1-10 nm) tips are produced. The top-down nanofabrication produces anisotropic structures that are monodisperse (>95%), highly uniform in shape and size, and multi-functional. Ni pyramids with radius of curvature (r < 10 nm), Au pyramids (r < 8 nm), and both two Au/Ni and three-layer Au/Ni/Au (r < 2 nm) pyramids have been fabricated (figure 2).
The orientation-dependent optical properties of two-dimensional arrays of the anisotropic metallic nanoparticles was demonstrated by simple encapsulation and alignment of the three- dimensional particles inside a uniform dielectric environment. Dark field microscopy and scattering spectroscopy of the plasmon resonances of 250-nm Au pyramidal shells, imbedded in a poly(dimethylsiloxane) matrix, exhibit strong dependence of the particle array scattering spectra on the direction and polarization of the incident white light relative to the orientation of the pyramidal shells. The ability to align and manipulate arrays of pyramidal nanoparticles within a uniform dielectric environment allows the correlation of nanoparticle orientation and anisotropic shape with specific multipolar plasmon resonances. This understanding of relatively large plasmonic particles, promises enhanced capabilities of plasmon-based applications, including nanoscale photonics, chemical and biological sensing, and photovoltaic devices.
Teri Odom, Joel Henzie, Eun-Soo Kwak
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