Templated fabrication of metal half-shells for surface-enhanced Raman scattering.

Here we report a scalable colloidal templating approach for producing metal half-shells as efficient surface-enhanced Raman scattering (SERS) substrates. Nonclose-packed monolayer colloidal crystals created by a spin-coating technology are used as structural template to fabricate both water-dispersed half-shells and disordered arrays of half-shells with preferential upright orientation. The sharp edges of the half-shells and the small gaps between adjacent shells can significantly enhance the local electromagnetic field, resulting in high SERS enhancement factor (up to 10(10)) which is nearly 2 orders of magnitude higher than those of periodic substrates produced by other colloidal templating approaches. We have also demonstrated that the thickness of the half-shells determines the surface plasmon resonance and the resulting SERS enhancement. Counterintuitively, the disordered arrays of oriented half-shells show reproducible enhancement with a standard deviation of less than 20%. This new bottom-up approach enables the large-scale production of SERS substrates that are at least 2 orders of magnitude larger in area than those made by other colloidal lithography technologies. The resulting substrates with high and reproducible SERS enhancement are promising for ultrasensitive chemical and biological sensing.