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Nanomaterials 2020, 10, 830 2 of 11 and strategies [3,5,6]. Nevertheless, reproducibility, stability, complexity and reusability are still issues [7]. Another crucial challenge is obtaining homogeneous enhancement across the platforms to allow reliable quantitative detection [8]. Recently, the fabrication of hybrid platforms that combine nano- or micro-structured semiconductors (nanopillars, nanorods, etc.) with metallic nanoparticles (NPs) has been proposed to further enhance amplification [9,10]; however, homogeneity is achieved by employing expensive lithographic methods. Another method proposed to increase the specific area is by the formation of nanoporous metals [11].The combination of metallic NPs or nanostructures with graphene is an interesting approach since graphene may also provide an extra enhancement of chemical nature, the chemical mechanism (CM) [9,12]. A related but different approach consists in the deposition of nanostructured metallic arrays coupled with metallic films separated by a very thin dielectric spacer layer [13] as the case of gold nanopyramid arrays coupled to a gold film separated by a silica layer, which led to strong light absorption confined in the space layer with E.F. up to 233 [14]. The high enhancement of plasmon intensity at the gap is of interest because of its applications in metamaterials, energy transfer, sensors and solar energy harvesting [15]. In this context, the amplification of the electromagnetic signal based on interference processes has been scarcely explored. The interference of light occurring at the interfaces of multilayered heterostructures that combine materials with very different refractive indices (n) can be tuned depending the application. The interference enhanced Raman scattering (IERS) was applied in the 1980s to detect the phonons of ultrathin films [16,17] obtaining enhancement factors (E.F.) of around 20. It is now commonly used to increase the Raman signal of graphene and other 2D materials typically by using a SiO2 dielectric layer on silicon single crystals with gains up to around 40 [18–21]. E.F. of up to 70 was reported for graphene bubbles on copper [22]. Some examples can be found on the combination of interference and SERS mechanisms [23,24] with low amplification factors, and, recently the design of optimized amplification platforms using aluminum as reflecting surface and Al2O3 as dielectric layer demonstrated efficient IERS and combined IERS and SERS amplifications [25]. Here we propose a new concept for an interference amplification platform which is robust and versatile. It is based on supported porous alumina membranes where the dielectric layer is the alumina membrane and the air of its pores and the reflecting layer is the metallic aluminum foil at the base of the membrane. Graphene is transferred on top of the porous alumina membrane and serves as the support where the analyte is deposited. Graphene is an excellent bio-compatible platform [26–28] with interesting characteristics since it can by-pass metal-biomaterial interactions occurring in SERS and quenches molecular fluorescence, highly inopportune for Raman spectroscopy. We have studied the different parameters that control the amplification factor such as the depth and density of the pores or the presence of an alumina barrier at the bottom of them. Interference is extremely sensitive to the thickness of the dielectric layer, which in this case is the depth of the pores but still we obtained highly efficient IERS with factors up to 400 and homogeneous amplification over the sample surface. 2. Materials and Methods Porous alumina was obtained by a two-step electrochemical anodization process of aluminum to obtain a porous alumina layers with highly ordered pore distribution [29]. Before any anodization, pure aluminum substrates (99.999% purity) were cleaned with acetone, water and ethanol and electropolished in a mixture of ethanol (EtOH) and perchloric acid (HClO4) 4:1 (v/v) at 20 V for 5 min in order to eliminate the surface roughness of the commercial aluminum substrates [30]. Subsequently, the first electrochemical anodization step took place in an aqueous solution of sulphuric acid (H2SO4) 0.3 M at 10 V and 3 ◦C. After 20 h, an alumina layer with disordered pores was obtained. It was dissolved by wet chemical etching in a mixture of 0.4 M phosphoric acid (H3PO4) and 0.2 M chromic acid (H2CrO7) at 70 ◦C for 3 h [31]. The second anodization step was performed under the same conditions as the previous one. The anodization time of this step determined the pore depth of the final ordered pore alumina layer [32]. The above mentioned combination of sulphuric acid, anodization voltage and electrolyte temperature has been specially selected for obtaining ultrathin alumina layers.PDF Image | Supported Ultra-Thin Alumina Membranes with Graphene
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