## 5 Applications of 3D-SNOM
### 3D Probing of Near-Fields of Nanostructures
This information can be used to achieve efficient light harvesting in devices such as solar cells by incorporating a layer of nanoparticles into their architecture at the appropriate location. A variation of 3D SNOM utilized stroboscopic measurements of different faces of the same film. This technique has been used to great effect to study symmetry breaking induced by IR laser illumination in a vanadium dioxide (VO\({}_{2}\)), thin film using IR s-SNOM. In strongly correlated electron materials, broken symmetries can result in separation of multiple phases in the course of a solid-solid phase transition, where VO\({}_{2}\) exhibits such a phase transition at a temperature close to room temperature (340 K) [56]. Figure 8 shows s-SNOM near-field images using an 18.8 mW near-IR pump laser taken at three different locations across the surface of a 250 nm VO\({}_{2}\) film surface with their corresponding AFM images. Elevating the sample temperature to 334 K induces a highly oriented stripe state that presents a herringbone pattern with two distinct orientations representing the two equivalent in-plane orientations of the monoclinic \(c\)-axis [56]. These images are indicative of a spontaneous symmetry-breaking phase separation due to the blocking of the pump laser between measurements that varies the temperature of the sample. The re-illumination at that start of each scan produces a new strained architecture in the film that is similar but uniquely different from the previous state, and can be seem in both s-SNOM and AFM images. The symmetry breaking in this film can be understood considering a Landau-like free elastic energy model; as the material goes through a transition to a distorted phase of lower symmetry, long-range elastic modes couple to allowed short-range modes to yield two energy minima with equal potentials and mirror symmetry, corresponding in this case to two different monoclinic \(c\)-axis orientations with mirror symmetry [56]. To gain additional insight into the structure of the films after a phase transition, s-SNOM measurements were also conducted in the cross-sectional face of the VO\({}_{2}\) film, as shown in Figure 9. These measurements were collected at a temperature 5 degrees below the transition temperature, to produce three adjacent regions are shown with characteristic near-field patterns. These images can be used to map the metal nucleation by the property that a larger s-SNOM intensity corresponds to a  
Figure 8: AFM and SNOM images of a VO\({}_{2}\) film undergoing 18.8 mW pump laser induced strain. The left side of (**a–c**) show SNOM (S\({}_{3}\)) images with AFM images on the right. The pump laser was switched off for 1 s between each scan. After each switch, patterns are formed due to a spontaneous structural symmetry breaking, with stripes aligning along two possible orientations of the monoclinic \(c\)-axis shown by white lines. Reproduced with permission from [56], Copyright Applied Physics Letters, 2014.  
higher IR conductivity a greater metal density [56]. Although the nucleation pattern varies somewhat across the film, within all the cross-section regions metallic nucleation occurs preferentially at the film/substrate interface where the epitaxial strain is the strongest. These metallic regions can then extend up to the VO\({}_{2}\) film surface, where a stripe like herringbone pattern emerges seen in Figure 8. Both cross-sectional and planar s-SNOM mapping enables a nanoscopic investigation of spontaneous symmetry breaking in a 3D geometry. This type of analysis can be extended to other materials, and illustrates a complimentary use of 3D SNOM to scan multiple faces of the same material.