Transmission Electron Microscopy Study on the Process of Gold Nanoporous Film Formation on AAO Substrate by Thermal Treatment

Gold (Au) nanoporous film supported on an anodized aluminum oxide (AAO) substrate has been studied in recent years due to its promising technological application as a biosensor based on surface-enhanced Raman scattering (SERS) [1]. Unveiling the process formation of Au nanoporous film can drive to take control of the nanopore size and distribution in this kind of substrates, which is directly related to an increase of enhancement factor, and thus, a gain of power in the limit of biomarkers detection [2]. In this work, the AAO substrate was obtained by the known two-step anodization methodology. An aluminum foil was employed and phase transition to alumina was promoted into a potentiostat according to the previous experience of the research group [3]. Afterward, an Au film was sputtered and the Au-AAO system was thermally treated at 450 °C for 15 min. The formation process of Au nanoporous film on the AAO substrate was imaged by scanning transmission electron microscope (STEM) and field-emission scanning electron microscope (FE-SEM). Previously to STEM imaging (JEM-ARM200F), samples were prepared by focus ion beam (FIB) in JIB-4500 MultiBeam with Gallium (Ga) liquid metal ion source. To understand the formation mechanism of Au nanoporous film, the AAO with the precursor layer obtained from Au sputtering of 30 s was isothermally treated under nitrogen flux. To study the evolution of nanoporous Au film across time, the sample was removed from the furnace at one-minute intervals. Thus, fifteen samples were synthesized from a time equal to zero, till complete the whole range process (reaching 15 min). Figure 1 shows the FE-SEM images of the samples thermally treated for a) 0, c) 3, e) 8, and g) 15 min. The samples were cut by FIB to exhibit a lateral view and then recorded by the backscattering electrons (BSE) detector. As BSE were sensed, gold nanoparticles can be easily identified in the corresponding images due to the well-known contrast difference—because of the variance of the atomic number between Au and alumina in samples—for this reason, Au in FE-SEM images (depicted in Figure 1a, 1c, 1e, and 1g) can be seen brighter than the AAO substrate. To study in detail the behaviour of Au on AAO across the entire synthesis process, a selected area of samples (perpendicular to normal vector on the surface) was cut and welt in a grid by FIB, and then, imaged by STEM (Figure 1b, 1d, 1f, and 1h). Findings in the first stage of synthesis, show that gold constitutes a homogeneous layer on the surface when the sputtered Au is mainly deposited on AAO, this film covers completely the surface of the sample (see figure 1a). A detailed frontal view of this stage imaged by TEM shows that the gold layer is not geometrically well-defined and looks like a typical sputtered film (figure 1b). Even though the alumina has porous with an average diameter of up to 100 nm, the gold layer seems not to remain on the porous inner wall; it is after 3 min of thermal treatment that regular and spherical Au nanoparticles start to appear on the surface, and pore walls of AAO. Therefore, when thermal treatment is performed for 3 min, nanoparticles migration starts to be observed along the pore in alumina —Note that arrows point to Au nanoparticles placed in the darker vertical columns (corresponding to AAO pores imaged by BSE)—. The frontal view (Figure 1d) shows that exposition time under 450 °C allows the formation of spherical nanoparticles under 20 nm. Once the sample reached 8 min under thermal treatment, the nanoparticles migration evolved to build clusters between the Au spheres as can be appreciated in Figure 1e —Note that Au nanoparticles (pointed by arrows) are still present in alumina pores—. In the frontal view (Figure 1f) the nanoparticles are overlapped (areas surrounded by orange circles). This is evidence that nanoparticles keep migrating till they acquire the overlapped configuration that could be the base for building the clusters. In Figure 1g, the final stage of the synthesis for Au porous film is shown, where the FE-SEM image proves that gold structures are achieved with no ordered worm-like pores unobstructed at the end. Insert in this image shows a close view of the porous film, which shows detailed of the agglomerated Au particles, where most of them have lost the spherical shape, allowing the formation of pores in all the gold material with a diameter under 10 nm (the biggest surrounded by orange circles). The frontal view image in Figure 1h show that Au atoms have migration from smaller toward the biggest nanoparticles (pointed by a circle). This behaviour is a well-known phenomenon and has been widely studied in nucleation and growing theory [4]. Note the loss of spherical-like shape in nanoparticles as can be seen in figure 1h. The observations above exposed suggest that thermal treatment is an accessible method for developing modification on Au film to promote nanoporous formation [5]. FE-SEM images (BSE) of Au layer on AAO with Au precursor layer (lateral view) processed at 450 °C for (a) 0, (c) 3, (e) 8 and (g) 15 min (Scale bar is equal to 200 nm) and (b), (d), (f), and (h) shows the frontal view of samples imaged by STEM, respectively.