# Formation of Micro- and Nano-Trenches on Epitaxial Graphene
## 3 Results and Discussions
The trenches with the direction highlighted by the green arrow in Figure 2b deviates from the white dashed line by an angle of ~67\({}^{\circ}\), other trenches have an angle of 116\({}^{\circ}\)-118\({}^{\circ}\) with the green arrow. Meanwhile, some trenches are formed with an angle of ~35\({}^{\circ}\) using the green arrow. Figure 2c shows the enlarged image of the paralleling and straight channels, in which the clean substrate after sliding can be observed. As shown in Figure 2d, the sliding can be also swerved at some defects. Figure 1e displays the surface area without trenches,  
Figure 1: **(a,b)** macro view of Ag trenches after UHV annealing. **(c)** swerved and branched particle due to the defective point. **(d)** the details of Ag trench heads. **(e)** surface morphology of Ag thin film on EG without sliding trenches. **(f)** AFM image of trench after the sliding of Ag particles. **(g)** corresponding 3D image. The scale bars are marked in each image. only showing the morphology of In particles on EG. Because the annealing temperature (~570 K) is higher than the melting point (430 K) of In, which is enough to melt In islands on EG, the migration of In particles is extremely easy, and they will be solidified into ball-shaped morphology after annealing. As a result, the sliding of In particles is much easier and a few randomly orientated trenches will be observed as marked by the dashed cycles in Figure 2b. Moreover, the morphology of the In trench head is greatly different from those of Ag trenches. Many small In particles instead of one large particle are scattered around the trench heads. The bright and small particles shown in Figure 2e are the starting sites for particle sliding for the formation of In trenches. Figure 1f,g show the AFM morphology and corresponding 3D image, from which the trench as well as In partilces are clearly observed. As exhibited by the green height profile, the depth and width of In trench are is ~10 nm and ~2.0 \(\upmu\)m. It indicates that vacuum annealing is enough to move the In particles to form macro-trenches. At first sight, the micro-trenches should be surface scratches. Actually, they are produced through the sliding of metal particles which happens only though high vacuum annealing. In previous reports, nano-particles induced etching of graphene was commonly under catalytic gas. For example, particle cutting happens in H\({}_{2}\) gas after annealing, and the etching can be controlled along zigzag or armchair directions [37], but most of the cutting direction is uncontrollable and randomly distributed, especially under O\({}_{2}\) gas, owing to the anisotropic interactions [36]. However, the formation of micro-trenches in this research is only through high vacuum annealing without any catalytic gas. It is extremely simple for the sliding of metal particles regardless of graphene pits or SiC steps, and most of the trenches are highly orientated and parallel. As shown by the image of Figure 3a tested through optical microscopy (OM), the trenches are stable even exposed under atmosphere for many days. Figure 3b displays the Raman mapping of 2D-band measured from the dashed black rectangle region  
Figure 2: (**a,b**) Macro view of In trenches. (**c**–**e**) Magnified image showing more details about In trenches. (**f**) AFM image of one trench after the sliding of In particles. (**g**) Corresponding 3D image. The scale bars are marked in each image. in Figure 3a. For the Raman mapping tested inside the trench, the intensity of 2D-band is much weaker than of the flat area. It is indicated that graphene sheets are damaged by the friction and a similar etching effect is achieved. Figure 4 shows more details about the two types of trenches by enlarged SEM images and corresponding Raman spectra. As shown in Figure 4a, there are three different regions, inside trench, Ag film without sliding and the trench head, which are marked as A\({}_{1}\), B\({}_{1}\) and C\({}_{1}\), respectively.