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Nanomaterials 2020, 10, 830 9 of 11 M solution of R6G was deposited by spin coating on both substrate which correspond finally to the equivalent concentration of one R6G monolayer on the substrates [4]. The spectra present the characteristic Raman peaks of R6G. In both cases the same SERS and CM amplifications are occurring so that the intensity increase in the membrane spectra is due to the interference process demonstrating the cooperative amplification of SERS and interference effects. 4. Conclusions Supported alumina membranes where specifically designed and fabricated to be used as amplification platforms to enhance the Raman signal of analytes by interference processes of the incoming and scattered light beams. The metallic aluminum support is used as the reflecting medium and the dielectric layer is the combination of the air of the pores and the alumina of the walls. Pore diameters in the 10–20 nm range are adequate to transfer a CVD single-layer graphene to serve as the substrate to deposit the analyte to be detected. Graphene mimics the membrane surface but presents a flat surface with small height fluctuations ~3 nm, which is found to be adequate for interference efficiency. The theoretical optimum pore depth depends on the pore fraction of the membrane, it is around 60 nm for the first interference order and around 200 nm for the second order for 20% pore fraction. Platforms based on membranes with pores height around 60, 100 and 200 nm were prepared, however, the control of the membrane quality in the 60 nm range is not enough. The elimination of the alumina barrier layer (at the bottom of the pores) is crucial according to calculations and the employed process for its elimination is found to be totally efficient. The Raman signals of Rhodamine 6G, spin-coated on graphene and graphene itself were used to test the platforms. E.F. up to 400 is obtained for membranes with ~100 nm pore depth, ~18 nm pore diameter and the complete elimination of the Al2O3 bottom barrier layer. The most limiting parameter is the pore fraction in the membrane, which reaches around 20% for 18 nm pore diameter. Further pore widening, which is favorable to increase E.F. in principle, produces larger in-plane disorder and surface roughness (height distribution). We demonstrated the possibility to further enhance the Raman signal of R6G by depositing ultra-small (4 nm diameter) silver nanoparticles on the graphene layer prior to spin-coating the analyte. Combined SERS and IERS processes is observed. This new approach to amplify the Raman signal of analytes by means of interference is cheap and robust with useful enhancement factors (~400). It allows the combination of plasmonic and interference amplifications, however, these platforms are also appropriate to amplify Raman signals in the cases where the use of nanoparticles is to be avoided. Author Contributions: Conceptualization, A.d.A.; investigation, M.A.-P., R.R.-J., E.X.-P., L.F.M., S.C.-C., J.B.; writing—original draft preparation, A.d.A., L.F.M.; writing—review and editing, A.d.A., L.F.M., J.B., S.C.-C.; funding acquisition, A.d.A. and L.F.M. All authors have read and agreed to the published version of the manuscript. Funding: The research leading to these results has received funding from Ministerio de Ciencia, Innovación y Universidades (RTI2018-096918-B-C41) and RTI2018-094040-B-I00) and by the Agency for Management of University and Research Grants (AGAUR) 2017-SGR-1527. S.C. acknowledges the grant BES-2016-076440 from MINECO. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript or in the decision to publish the results. References 1. Stiles, P.L.; Dieringer, J.A.; Shah, N.C.; Van Duyne, R.P. Surface-Enhanced Raman Spectroscopy. Annu. Rev. Anal. Chem. (Palo Alto. Calif.) 2008, 1, 601–626. [CrossRef] 2. Sharma, B.; Frontiera, R.R.; Henry, A.-I.; Ringe, E.; Van Duyne, R.P. SERS: Materials, Applications, and the Future. Mater. Today 2012, 15, 16–25. [CrossRef] 3. Xie, W.; Schlücker, S. Rationally Designed Multifunctional Plasmonic Nanostructures for Surface-Enhanced Raman Spectroscopy: A Review. Rep. Prog. Phys. 2014, 77, 116502. [CrossRef]PDF Image | Supported Ultra-Thin Alumina Membranes with Graphene
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