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Nanomaterials 2020, 10, 830 3 of 11 With this selection we manage to extraordinarily slow down the speed of the anodization process and fabricate extremely thin layers with controlled thickness. Finally, the barrier layer formed at the bottom of the pores, inherent to the electrochemically obtained alumina, needed to be eliminated to increase the E.F. of the designed platforms. A simple and rapid method for its elimination was developed for these extremely small pores that consisted of the steady decrease of the anodization current followed by a pore-widening treatment performed by wet chemical etching in H3PO4 5 wt % at 35 ◦C for 4.5 min. PMMA (polymethilmetacrylate)/Graphene on copper foil from Graphenea company (San Sebastián, Spain) was transferred onto the porous alumina. Cu was first eliminated in a 2.1 M FeCl3 and 1.3 M HCl solution for 15 min. The PMMA/Graphene stack was rinsed in a deionized water bath twice, immerged in a 10% HCl solution and then again into deionized water three times with the last bath for 20 h to complete the removal of Cu. Subsequently, the Gr/PMMA film was fished onto the porous alumina samples and baked on a hot plate at 90 ◦C. PMMA was then removed by immersing in warm acetone at 50 ◦C followed by further vacuum thermal treatment at 250 ◦C. Silver nanoparticles were deposited at room temperature with the gas aggregation technique [4,33] using a magnetron sputtering source (Nanogen50, from Mantis Ltd., Manchester, United Kingdom) and an Ag target 99.95% purity. The ejection of atoms and nucleation of clusters is assisted by a mixture of Ar/He gas, carried along the aggregation chamber through an orifice and reaching the substrate. The base pressure was 5 × 10−9 mbar and the work pressures were 2.5 × 10−1 mbar inside the aggregation chamber and 2 × 10−3 mbar in the deposition chamber, with an Ar/He ratio of 1:2.4. These parameters give rise to spherical single crystalline nanoparticles with average diameter ~4 nm [34]. The Ag NPs were simultaneously deposited on the membrane/graphene and on the fused silica/graphene reference sample. Rhodamine 6G (R6G) films were then deposited by spin coating from a methanol solution (10−3 M) on the two previous samples (membrane/graphene/AgNPs and fused silica/graphene/AgNPs). Micro-Raman experiments were performed at room temperature with the 488 nm line of an Ar+ laser, incident power in the 0.1–8 mW range, an Olympus microscope (×100 and ×20 objectives) and a “super-notch-plus” filter from Kaiser. The scattered light was analyzed with a Horiba monochromator coupled to a Peltier cooled Synapse CCD. The estimated Raman spatial resolution is around 0.7 μm at 488 nm for the high NA (0.95) ×100 objective. The Raman signal of graphene transferred on fused silica is used as the reference to calculate the enhancement factors. Raman signal of single layer graphene is very regular over any standard substrate so the average of three measurements was used. The morphology and roughness of the samples were examined using atomic force microscopy (AFM) (equipment and software from NanotecTM, Madrid, Spain) [35]. Topographic characterization was carried out in the tapping mode, using commercial Si tips (Nanosensors PPP-NCH-w) with a cantilever resonance frequency f0 ≈ 270 kHz and k ~ 30 Nm−1. Several regions were probed to confirm homogeneity of the surfaces to the micrometer scale. An estimation of the roughness is obtained from the full width at half-maximum (FWHM) of the height distribution of the analyzed topographic images. The simulations of the Raman signal amplification in the membranes were performed calculating the propagation of light through multilayered media using the matrix transfer method. In this method, the amplitude of the electromagnetic waves at two different depths inside the structure are related by a complex matrix—the transfer matrix is constructed taking into account the geometry and the refractive indexes of each layer, the effect on the electromagnetic (EM) field amplitude when traveling through different layers is calculated by matrix multiplication. The method provides a solution to Maxwell equations that considers the interference of the infinite number of multiple reflections occurring for light propagating through multilayered media. In the particular case of Raman scattering, multiple interference impacts in two ways, firstly the amplitude of the light arriving at a particular location in the structure, where the scattering process takes place should be calculated, secondly the scattered light should travel to the detector outside the structure, the calculation of the intensity involves again the transfer matrix method. Both aspects are taken into account in the calculations carried out previously for different metals and dielectrics [25].PDF Image | Supported Ultra-Thin Alumina Membranes with Graphene
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