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.
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