have been presented. [5][6][7] For many years SERS substrates were primarily restricted to noble metal (Au, Ag, and Cu) structures. Recently, it has been found that various semiconductors, such as ZnO, [8] ZnS, [9] TiO 2 , [10] Cu 2 O, [11] and CuO, [12] can also generate weak SERS activity with typical prominent enhancement factors ranging from 10 1 to 10 3 . Therefore, composites or heterostructures between semiconductors (Si, ZnO, and TiO 2 ) and noble metals (Au and Ag) have attracted attention, as a much higher SERS effect could be achieved due to the contributions from both the electromagnetic enhancement (excited by the localized surface plasmon resonance of noble metals) and the semiconductor supporting chemical enhancement (caused by the charge transfer between the noble metal and the adjacent semiconductor). [13][14][15][16][17][18][19][20] ZnO, a versatile semiconductor material with a direct band gap of 3.37 eV, has received special attention due to its excellent performance in supporting chemical enhancement of SERS substrates. [21,22] Several methods have thus been developed to fabricate this new type of ZnO/noble metal hybrid SERS substrate. For example, electroless plating was employed to assemble Ag nanoparticles (Ag-NPs) onto the surface of ZnO nanorods (ZnO-NRs) by sensitizing and activating with Sn 2+ or by irradiation with UV light. [22,23] Interestingly, the Ag-NPs could also be located on the tips of the ZnO-NRs via a photodeposition method or galvanic reduction. [24,25] Alternatively, with the help of surface modification with amino or mercapto groups, Au-NP/ZnO-particle and Au-or Ag-NPs/ZnO-multipods hybrid structures were achieved. [21,26] However, the spatial gaps between these small noble metal particles on the ZnO supporter were too large to work efficiently as SERS "hot spots" and, moreover, the complicated coupling agents for linking the metal NPs onto the ZnO surface, such as (3-aminopropyl) triethoxysilane, may bring extra interferential bands in the SERS measurements, especially if the target analytes have similar Raman spectral response with the coupling agents. To tackle this problem, physical sputtering was exploited. For example, a SERS substrate called "3D hybrid Ag-nanocluster-decorated ZnO nanowire arrays" was fabricated Arrays of Cone-Shaped ZnO Nanorods Decorated with Ag Nanoparticles as 3D Surface-Enhanced Raman Scattering Substrates for Rapid Detection of Trace Polychlorinated BiphenylsA new, highly sensitive and uniform three-dimensional (3D) hybrid surfaceenhanced Raman scattering (SERS) substrate has been achieved via simultaneously assembling small Ag nanoparticles (Ag-NPs) and large Ag spheres onto the side surface and the top ends of large-scale vertically aligned coneshaped ZnO nanorods (ZnO-NRs), respectively. This 3D hybrid substrate manifests high SERS sensitivity to rhodamine and a detection limit as low as 10 −11 m to polychlorinated biphenyl (PCB) 77-a kind of persistent organic pollutants as global environmental hazard. Three kinds of inter-Ag-NP gaps in ...

A hierarchically ordered array of Ag-nanorod bundles is achieved using an inexpensive binary-template-assisted electrodeposition technique. In every bundle, many small gaps are formed between adjacent Ag-nanorods, where "hot spots" are generated. As a result, this plasmonic nanostructure exhibits SERS enhancements of approximately eight orders of magnitude with uniform and reproducible SERS signal throughout the whole chip.

Ag-capped Au nanopillar arrays on a resin supporter (see left upper figure), with a typical adjacent pillar tip gap of 10 nm, show obviously higher surface-enhanced Raman scattering (SERS) sensitivity (right column in red) than that of the bare Au nanopillar array while using 10 nM R6G as probe molecules. The large-area Ag-capped Au nanopillar array has potential in trace detection of special chemicals.