Optical Properties of Silver and Gold Tetrahedral Nanopyramid Arrays Prepared by Nanosphere Lithography

Abstract

Tetrahedral nanopyramids made of silver and gold over ITO/glass surfaces are fabricated. Our protocol is based on nanosphere lithography (NSL) with the deposition of thicker metal layers. After removing the microspheres used in the NSL process, an array of metallic tetrahedral nanostructures of ∼350–400 nm height is formed. The reported procedure avoids the use of any stabilizing surfactant molecules that are generally necessary to segregate the individual particles onto surfaces. We focus here on the optical and the physical properties of these plasmonic surfaces using near-field spectroscopy in conjunction with finite difference time domain (FDTD) modeling of the electric field. Remarkably, FDTD shows that the localized surface plasmon resonance is confined in the plane formed by the edges of two facing pyramids that is parallel to the polarization of the impinging excitation laser. The variable gap between the edges of two adjacent pyramids shows a broader localized surface plasmon and a larger specific surface as opposed to the usual nanotriangle array. Localized enhancement of the electric field is experimentally investigated by coating the plasmonic surface with a thin film of photosensitive azopolymer onto the surface of the nanopyramids. Upon irradiation, the deformation of the surface topography is visualized by atomic force microscopy and suggests the potentiality of these 3D nanopyramids for near-field enhancement. This last feature is clearly confirmed by surface-enhanced Raman scattering measurement with 4-nitrothiophenol molecules deposited on the pyramid platforms. The potentiality of such 3D nanostructures in plasmonics and surface spectroscopy is thus clearly demonstrated.

On-chip surface-enhanced Raman spectroscopy using nanosphere-lithography patterned antennas on silicon nitride waveguides

“…All SERS spectra were measured with a laser power of 250 µW at the entrance facet of the microscope objective, and the polarization of the pump beam was set to TE using a half-wave plate. Note that a low irradiance is necessary to avoid the photoinduced reduction of p-NTP into dimercaptoazobenzene [23]. We observed this from a change in relative strength of the 1339 cm −1 and 1080 cm −1 peaks when increasing the power above 500 µW, both for waveguide-and free-space coupling.…”

“…SERS data show that the SSEF of this pattern is 2.5 × 10 5 , and that excitation and collection of SERS spectra through the waveguide is at least as efficient as using the best possible air objective. Although the enhancement factor of the nanotriangles presented in this manuscript is not among the highest reported in literature, nanosphere-lithography has also been used to fabricate high-performance SERS substrates, at least under free-space excitation [23,28,29]. Variations of these can possibly be combined with photonic waveguides.…”

“…Spectral properties of the localized surface plasmon resonance The nanotriangles and waveguides are designed for a Raman pump laser of 785 nm and Stokes emission at 877 nm, corresponding to the 1339 cm −1 symmetric stretching mode (ν s ) of NO 2 in the gold-binding molecule para-nitrothiophenol (NTP) [23]. A maximal overlap of the LSPR with the wavelength region of the pump laser and Stokes emission is necessary to achieve a high SERS enhancement factor.…”

Bottom-Up Assembly and Applications of Photonic Materials

“…These include, but are not limited to, nanotriangles [61,62], nanodots [61,68,72], nanopillars [63,65,69,[73][74][75][76], nanorods [68], nanopyramids [64], nanorings [68,77], nanowells/ bowls [67,69] and inverted nanocones [78]. Vogel et al described a simple process of using oxgen plasma etching to modify a close packed monolayer of polystyrene beads into a non-closed packed monolayer ( Figure 5) [45].…”

“…deposition of a material into the interstitial spaces between the spherical particles [62] or the etching of the substrate using the template as a protective mask [63]. Infiltration of a desired material into the template can be done with techniques such as electron beam evaporation [64], electrodeposition [65], sol-gel [66,67], metal evaporation [68,69], and magnetron sputtering [48,70,71]. Either way, at the end of the process when the template is removed, an array of nanoparticles/nanostructures will be produced on the substrate surface.…”