## 5 Applications of 3D-SNOM
### Optimization of Nano-Optical Devices
The light scattering properties of copper nanoparticles have also been extended to evanescent waveguide technology. It has been shown recently that arrays of copper nanoparticles can be thermally nucleated from a thin layer of copper deposited on few layer graphene sheets [28]. Under controlled conditions of annealing temperature and atmosphere, the nanoparticles can be encouraged to form ordered arrays with interesting optoelectronic properties [28]. Figure 14 presents scanning electron microscopy (SEM) images of copper nanoparticle arrays nucleated on graphene flakes at different annealing temperatures. Superlattices of copper nanoparticles are found to be formed at intermediate annealing temperatures (\(\sim\)350 \({}^{\circ}\)C) where nanoparticles nucleate preferentially along graphene armchair edges. At the extremes, nucleation is either limited by low temperature, or promotes the appearance of disordered arrays at higher temperatures. Figure 13: Intensity profiles of scattered light above the copper nanoparticles are shown in panel (**a**) for two cases of (**b**) bare nanoparticles and (**c**) nanoparticles modified with an SiO\({}_{2}\) spacer. Intensity enhancement of scattered light is observed when the spacer is in place. These films are subsequently incorporated into solar cell architectures of (**d**) the reference solar cell containing bare copper nanoparticles without a spacer and (**e**) the enhanced cell including a 200 nm SiO\({}_{2}\) spacer. I–V characteristics of the solar cells with a Cu-nanoparticle plasmonic layer (**f**) without and (**g**) with the SiO\({}_{2}\) spacer between copper nanoparticles and the ITO-P3 HT/PCBM solar cell active layer. Reproduced with permission from [48], Copyright Nanoscale, 2015. C-SNOM (\(x\), \(z\)) scans were used to characterize the copper nanoparticle arrays and determine the electric field intensity perpendicular to the plane of the substrate during excitation by a laser source. Figure 15f,g schematically show how the laser light is applied to the copper nanoparticle superlattices, where an ordered lattice can exhibit waveguiding properties. Modes are generated by the laser source, with an evanescent near-field propagating along the sample surface which can be collected through the SNOM tip aperture. Figure 15a-c show the AFM, c-SNOM, and SEM images, respectively, of the same copper nanoparticle super lattice. Figure 15d shows c-SNOM scans in \(z\)-direction for the two line sections, A-A' and B-B' representing superlattice and disordered areas. The light fields displayed indicate that section A-A' demonstrates evanescent waveguiding properties by the strong intensity observed near the surface with an absence of destructive interference. Another scan along the section B-B' with disordered nanoparticles exhibits the expected intensity oscillation along the \(z\)-axis that indicates incoherent Mie scattering from randomly arrayed nanoparticles [60]. The integrated light intensity profiles extracted from Figure 15d are shown in panel e relative to the distance away from the surface, \(z\). Figure 15e shows an exponential decay of light intensity is observed in the proximity of ordered nanoparticle arrays (region A-A') moving away from the surface, in agreement with the evanescent wave decay profile. By contrast the scattered light intensity from the disordered section B-B' shows multiple oscillations. This effect arises from a superposition of multipole and incoherent Mie scattering [60]. Figure 14: SEM images of copper nanoparticle decorated graphene thin films annealed for 8 h at (**a**) 200 \({}^{\circ}\)C (**b**) 360 \({}^{\circ}\)C, (**c**) 440 \({}^{\circ}\)C. (**d**) At lower temperatures copper nanoparticles are formed in an uneven layer, which are formed which transitions to assemblies of particles that become spatially separated at the highest temperature. Superlattices formed of lines of nanoparticles are formed at the intermediate temperature of 360 \({}^{\circ}\)C. Reproduced with permission from [28], Copyright RSC Advances, 2015.