Raman Markers from Silver Nanowire Crossbars

One dimensional metal nanostructures with the capability of guiding electromagnetic energy are attractive for nanoscale optical devices and circuits. These nanostructures can be also excellent candidates for Raman markers that exploit surfaceenhanced Raman scattering (SERS) phenomena. Onedimensional nanostructures show promising capabilities for guiding and confining the exciting light in nanogaps (“hot spots”), providing dramatically enhanced local electromagnetic field and coupling of localized surface plasmon resonances. To explore this important phenomenon, a number of different noble metal nanostructures have been fabricated, assembled, and synthesized, and some have been suggested as prospective Raman markers for chemical and biological detection. Several recent examples of label-free Raman markers from metallic/bimetallic nanostructures include silver nanowires and their bundles, silver nanocubes and nanoplates, rods with fabricated gaps, nanodumbbells, gold nanowire-gold nanoparticles, crossed gold nanowires, ZnO nanowiresilver nanoparticle hybrid nanotrees, and silver-gold nanocobs. Efficient SERS phenomena have been observed for crossed silver nanowires and for prefabricated bundles with localized nanojunctions. Moreover, intriguing polarizationdependent SERS phenomenon that can be utilized for waveguiding and controlled longitudinal surface plasmon localization have been demonstrated for silver nanowires decorated with multiple nanoparticles and their clusters. However, examples of single-nanoparticle-based hot spots with SERS appearance tuned by the polarization-dependent excitation conditions are rarely demonstrated. Here, we report on remarkable properties of crossbars of silver nanowires partially decorated with spherical silver nanoparticles associated with SERS hot spots with optical turning on/off mediated by polarization conditions at a nanojunction. Massive fabrication can be achieved via a double-step transversal capillary transfer microprinting approach. Such addressable on/off behavior is controlled by the rotation of the polarization plane which excites transversal plasmon resonances and initiates selective excitation of SERS hot spots at different locations. Moreover, we demonstrated the possibility of massive fabrication of such junctions by simple, double-step capillary transfer lithography, which might open a path for large-scale assay applications.