Nanocavity-in-multiple nanogap plasmonic coupling effects from vertical sandwich-like Au@Al2O3@Au arrays for surface-enhanced raman scattering

The development of ideal three-dimensional (3D) tailorable surface-enhanced Raman scattering (SERS) substrates with the properties of timesaving, large area, high throughput, single or few molecules detection, reproducibility, reusable ability, and high density of "hot spots" has been the mainstream challenge and the robust task. Here, we construct perpendicular sandwich-like Au@Al2O3@Au hybrid nanosheets (PSHNs) on the Al foil as a 3D flexible substrate for SERS. The design of 3D PSHNs incorporates several advantageous aspects for SERS to enhance the performance of plasmonic diamers via bifunctions of vertical Al2O3 nanosheets (NSs) including the nanoscaffold and nanobaffle plate effects. As a nanoscaffold, it increases the space utilization of Au-Au diamers, whereas as a nanobaffle, it forms densely homogeneous Au@Al2O3@Au nanojunctions by sub-4 nm thickness of Al2O3 NSs as the dielectric isolated layer for the double-sided exposure of slitlike surface plasmon resonance. The optimized PSHN substrate exhibits a fascinating SERS sensitivity with an experimental enhancement factor of 1012 and is able to detect rhodamine B at an extremely low concentration up to the limit of single or few molecules (10-18 M), as well as can be recycled without the loss of SERS enhancement via the simple impregnation process. These advantages will greatly facilitate the wider use of SERS in many fields.