Different arrays of Ag-nanoparticles grown on anodic alumina nanochannels with precisely tunable gaps (5~25 nm) are
exploited for surface-enhanced Raman spectroscopy (SERS). The enhancement becomes significant for gaps below 10
nm and turns dramatically large when gaps reach an unprecedented value of 5 nm. The results are quantitatively
consistent with theories based on collectively coupled surface plasmon. Such nanofabricated substrates with consistently
uniform and large dynamic range have many chemical/biological sensing applications.