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Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 11 Nanomaterials 2020, 10, 830 5 of 11 Figure 2a,b show in-plane and tilted SEM images of a supported alumina membrane with pore depth around 100 nm (sample named h = 100 nm) and ~18 nm pore diameter. The transference of a single layer graphene on top of the membrane strongly modifies its AFM topographic image (Figure (Figure 2c,d). The analysis of the SEM and AFM images and the AFM profiles allow to estimate the 2c,d). The analysis of the SEM and AFM images and the AFM profiles allow to estimate the pore to pore to pore distance to be around 35 nm. An accurate determination of the pore diameter from pore distance to be around 35 nm. An accurate determination of the pore diameter from the AFM the AFM images is challenging, since the tip size (20–30 nm) limits the estimation of pore lateral images is challenging, since the tip size (20–30 nm) limits the estimation of pore lateral dimensions, dimensions, however, obtaining pore to pore distance is quite precise. For the samples with transferred however, obtaining pore to pore distance is quite precise. For the samples with transferred graphene, graphene, the profiles (and the height statistics) evidence how graphene mimics the membrane surface the profiles (and the height statistics) evidence how graphene mimics the membrane surface but but strongly limits the oscillations to around ± 3 nm, much smaller than the pore depth (100 nm). strongly limits the oscillations to around ± 3 nm, much smaller than the pore depth (100 nm). The The chosen small pore diameter, d < 20 nm, is therefore adequate to get small fluctuations of the overall chosen small pore diameter, d < 20 nm, is therefore adequate to get small fluctuations of the overall dielectric thickness defined by the graphene top layer (the molecules to be sensed are deposited on top dielectric thickness defined by the graphene top layer (the molecules to be sensed are deposited on of the graphene). top of the graphene). Figure 2. (a,b) SEM images of a membrane with pore diameter d ≈ 18 nm and h ≈ 100 nm, (c,d) atomic Figure 2. (a) and (b) SEM images of a membrane with pore diameter d ≈ 18 nm and h ≈ 100 nm, (c) force microscopy (AFM) topographic images and height profiles of the pristine membrane and after and (d) atomic force microscopy (AFM) topographic images and height profiles of the pristine graphene transfer, respectively. membrane and after graphene transfer, respectively. The graphene transfer process and final graphene quality are first checked by optical microscopy. The graphene transfer process and final graphene quality are first checked by optical Figure 3a corresponds to the alumina membrane after fishing the graphene/PMMA film. Once the microscopy. Figure 3a corresponds to the alumina membrane after fishing the graphene/PMMA film. process to eliminate PMMA is concluded, typical micron sized regions of bi/tri-layer graphene can be Once the process to eliminate PMMA is concluded, typical micron sized regions of bi/tri-layer easily seen (darker micron-sized spots in the optical images) as well as graphene wrinkles (Figure 3b,c). graphene can be easily seen (darker micron-sized spots in the optical images) as well as graphene The interference process can also increase the contrast of optical images, as it occurs here and it is wrinkles (Figures 3b,c). The interference process can also increase the contrast of optical images, as it the first indication that the system will provide Raman amplification. In Figure 3c the edge of the occurs here and it is the first indication that the system will provide Raman amplification. In Figure transferred single-layer graphene is easily seen showing its high quality up to the very edge. 3c the edge of the transferred single-layer graphene is easily seen showing its high quality up to the The quality of the transferred graphene is checked analyzing the Raman spectra. In Figure 3d very edge. the spectra of the h = 100 nm membrane at distant points are presented showing the characteristic D The quality of the transferred graphene is checked analyzing the Raman spectra. In Figure 3d (≈1580 cm−1) and 2D (≈2700 cm−1) peaks with an intensity ratio I2D/IG > 2 indicating the single layer the spectra of the h = 100 nm membrane at distant points are presented showing the characteristic D character of the transferred graphene. A defect peak (D at ≈1350 cm−1) is detected with very small (≈1580 cm−1) and 2D (≈2700 cm−1) peaks with an intensity ratio I2D/IG > 2 indicating the single layer intensities. The black line in Figure 3d corresponds to a dark point in Figure 3b and clearly signals to character of the transferred graphene. A defect peak (D at ≈1350 cm−1) is detected with very small a graphene bi-layer with the characteristic almost identical intensities of G and 2D peaks (IG ≈ I2D), intensities. The black line in Figure 3d corresponds to a dark point in Figure 3b and clearly signals to being IG two times that of the single layer spectra and a higher intensity of the defect peak D. a graphene bi-layer with the characteristic almost identical intensities of G and 2D peaks (IG ≈ I2D), Membranes with h = 60 nm were fabricated to optimize the E.F., however, for such small pore being IG two times that of the single layer spectra and a higher intensity of the defect peak D. depth the quality of the membranes is compromised in terms of the order of the pores as well as in the height uniformity. The AFM topographic images show an increased disorder of the pores for the h = 60 nm membrane compared to the h = 100 nm one (Figure 4a,b), also, the height distribution of the AFM image in the case of the 60 nm sample is increased significantly, especially in comparison withPDF Image | Supported Ultra-Thin Alumina Membranes with Graphene
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