Nickel Surface Morphology Study

Figure 6.1: AFM image and associated line profile of an untreated Ni thin film on a SiO₂/Si(100) substrate in ambient conditions.

Figure 6.1 shows an ambient AFM image of the Ni thin film surface of a sample prepared in the Edwards system, which was removed to ambient conditions and then imaged before introduction to the Nanograph STM-01 UHV system for the graphene formation procedure. All ambient AFM imaging was performed with an Asylum Research MFP-3D system. Although the peak-to-peak roughness of the sample was only several nanometres, there were no flat areas or terraces found, and the average Ni grain size was only ∼20-30 nm.

The Nanograph STM-01 system was used to image the Ni thin film surface

C for ∼12-18 hours. All STM images in this chapter have been processed using WSxM software [90].

Figure 6.2: STM image of an outgassed Ni thin film on a SiO2/Si(100) substrate, some small terraces can be seen with many step edges, as shown in the

associated line profile. Imaging parameters Vbias= 1.0 V, It= 150 pA.

STM imaging of the Ni thin film after the initial outgassing of the sample revealed a rough surface, as shown in Figure 6.2, however there was a small amount of terraces observed with lateral sizes ≤ 10 nm. The surface primarily consisted

Chapter 6. Graphene Formation on Ni Thin Films 151

of step edges nanometres in size which created many steep slopes on the surface.

Figure 6.3 shows an ambient AFM image of a 500^◦C annealed Ni thin film – the change in surface morphology due to the anneal in UHV becomes apparent when comparing Figure 6.3 to Figure 6.1.

Figure 6.3: AFM image and associated line profile of an outgassed Ni thin film on a SiO₂/Si(100) substrate in ambient conditions.

When the Ni thin film was annealed at a higher temperature of 700^◦C the terraces on the surface observed with STM became larger, as shown in Figure 6.4.

Terraces several tens of nanometres in length scale were observed but these terraces were not in abundance. However, as found before the 700^◦C anneal, there were many instances of step edges and the surface roughness was nanometre in scale overall. The Ni surface was difficult to image with STM in many instances due to the topography which caused the STM tip to frequently contact the surface.

Figure 6.5 shows the corresponding ambient AFM image of a sample annealed at 700^◦C, revealing an increase in both surface roughness and nickel grain size.

Figure 6.4: UHV-STM image and associated line profile of a Ni thin film on a SiO2/Si(100) substrate after the sample was annealed to 700^◦C, STM imaging revealed some terraced areas but with an instability due to the rough surface.

Imaging parameters Vbias= 1.0 V, It= 200 pA.

Chapter 6. Graphene Formation on Ni Thin Films 153

Many different Ni grain orientations were revealed with associated terraces that were similar to the terraces observed in Figure 6.4.

Figure 6.5: AFM image and associated line profile of a Ni thin film on a SiO2/Si(100) substrate after the sample was annealed to 700^◦C in UHV and

transferred to ambient conditions.

The Ni thin film samples were further annealed to 800^◦C which caused a radical change in the Ni surface morphology, as shown in Figure 6.6. The surface was predominately terraced, with terrace sizes of up to 100 nm. Although there were still step edges present on the surface these were less abundant and so the slopes of the surface were not as steep. A ‘patchwork’ terrace formation was also regularly observed on the surface as shown in the bottom part of Figure 6.6b, where several (in this case) hexagon adjacent terraces were imaged with STM.

The corresponding AFM image of the 800^◦C annealed surface in Figure 6.7 shows a larger scale image of the surface, where large (widths of greater than 1 µm) highly faceted islands were observed (on which the UHV-STM imaging

Figure 6.6: UHV-STM imaging of a Ni thin film on a SiO2/Si(100) substrate after annealing to 800^◦C. The topography was flat in most areas with examples of (a) large and (b) smaller terraces, a ‘patchwork’ formation of terraces was also

observed. Profiles (c) and (d) correspond to images (a) and (b) respectively.

Imaging parameters (a) and (b) Vbias= 1.0 V, It= 200 pA.

Chapter 6. Graphene Formation on Ni Thin Films 155

revealed terraces). The 800^◦C annealed Ni thin films were investigated using X-ray diffraction (XRD) which revealed peaks in a θ-2θ scan consistent with the formation of (111) oriented Ni crystallites [109]. There was a significant change in morphology of the Ni surface after the 800^◦C anneal, however no difference in morphology was found when the samples were subsequently dosed with propylene after the 800^◦C anneal.

Figure 6.7: Ambient AFM image and line profile of a Ni thin film on a SiO2/Si(100) substrate after the sample was annealed to 800^◦C in UHV.

Patchwork terraces were also observed in the STM images shown in Fig-ures 6.8a and 6.8c, but were rectangular and aligned in rows rather than hexagons, as described previously. The rectangular terraces were ∼10 nm in width but their length varied from 10 nm to 40 nm with a median value of ∼15 nm, as shown in Figure 6.8c. The formation mechanism for this patchwork of terraces is not understood. However, terraces that did not exhibit the patchwork formation but were still aligned to the nearby patchwork terraces were observed, as shown in

Figure 6.8: UHV-STM imaging of a Ni thin film on a SiO₂/Si(100) substrate after annealing to 800^◦C, where (b) is a smaller scan of an area in (a). There were many flat terraces over the surface (a), and instances of terraces with laterally rectangular step edges (b), in other areas ‘patchwork’ terrace formation

was observed (c). Profiles (d) and (e) correspond to images (b) and (c) respectively, showing nickel step heights of ≥ 2 ˚A. Imaging parameters (a) and

(b) V_bias= -1.0 V, I_t= 200 pA, (c) V_bias= -1.0 V, I_t= 150 pA.

Chapter 6. Graphene Formation on Ni Thin Films 157

Figure 6.8b. Whether these larger terraces would have also produced further patchwork terraces after further annealing is unknown, but these patchwork ter-races were only observed when the Ni surface was substantially flatter due to the 800^◦C anneal.

Figure 6.9 shows two examples of STM images where the grain boundary of a Ni domain was observed. The grain boundaries were easier to distinguish after an 800^◦ anneal than pre-anneal as the surface domains were much smaller for an untreated sample.

Figure 6.9: UHV-STM images of a Ni thin film on a SiO₂/Si(100) surface after the sample was annealed to 800^◦C. Grain boundaries formed in the Ni thin film are shown as red lines. Imaging parameters (a) and (b) Vbias= 1.0 V, It= 200 pA.

The Ni thin film morphology was not investigated after annealing at temper-atures higher than 800^◦C due to degradation of the thin films. SPM imaging performed after the 800^◦C anneal of the Ni thin film implied that there was a sig-nificant diffusion of Ni atoms in the surface to produce the observed large, flat Ni grains. This had a detrimental effect to other areas of the sample as bare areas of SiO₂ were revealed, as observed in an ambient AFM image shown in Figure 6.10.

Figure 6.10a shows a 5 µm AFM image where large Ni islands were produced through annealing at 800^◦C, while Figure 6.10b shows a 10 times larger scan size AFM image of a different area of the surface. Holes in the Ni thin film with widths of several microns and depths of several tens of nanometres are observed in Figure 6.10b, depths approximately equal to the thickness of the Ni thin film.

Thus the gaps in the Ni thin film revealed the SiO2 substrate – confirmed by XPS results in Section 6.1.2.

Figure 6.10: AFM images and line profiles of a Ni thin film on a SiO₂/Si(100) substrate after the sample was annealed to 800^◦C and transferred to ambient

conditions. Although the smaller scan size image in (a) shows large, flat Ni grains, the larger scan size image in (b) shows areas of the SiO₂ surface were

exposed after the annealing process.

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