XPS Surface Study

X-ray photoelectron spectroscopy (XPS) is a quantitative technique per-formed in UHV that measures the elemental composition, and chemical and elec-tronic states of the atoms within the surface of a sample. XPS spectra are acquired by irradiating the sample surface with a beam of X-rays and simultaneously mea-suring the number of electrons that escape from the surface versus the kinetic energy of the electrons – typically to a depth of 1-10 nm.

Two separate XPS systems (operated by S. Gangopadhyay and I. Villar-Garcia) were used, allowing both in-situ and ex-situ investigation of the Ni thin films. Sam-ples could be prepared in the Edwards system or Nanograph STM-01 UHV system and then transported through ambient conditions to a high resolution XPS sys-tem. A Kratos Axis Ultra spectrometer employed a monochromated Al Kα X-ray source and hemispherical analyser which achieved a resolution of ∼0.2 eV (for further details see Reference [110]). Untreated or processed Ni thin film samples could also be introduced to a UHV system with a lower resolution XPS system;

an Omicron DAR400 Mg Kα X-ray source and VG Scienta R3000 hemispherical analyser with ∼0.7 eV resolution that had the facilities to anneal the sample to 800^◦C.

To investigate the oxidation of the nickel surface a Ni thin film prepared in the Edwards evaporator was introduced to the Omicron XPS UHV system (via ambient conditions) and in-situ XPS was performed on the sample before and after a 15 min 500^◦C anneal. This annealing process replicated the outgassing phase of the sample preparation. Figure 6.11 shows Ni-2p XPS spectra for the sample before and after the anneal. All XPS results in this thesis were analysed using CasaXPS software.

From Figure 6.11, the 15 min in-situ 500^◦C anneal sharpened the Ni-2p1

(869.9 eV [111]) and Ni-2p3 (852.6 eV [111]) peaks dramatically, correspond-ing to an increase in elemental nickel in the surface. The nickel oxide peak at 856.1 eV [112] also disappeared after annealing the Ni thin film. The disappearance of the nickel oxide after the 15 min 500^◦C anneal was confirmed in the O-1s XPS spectra shown in Figure 6.12. Three oxygen peaks were found for the untreated sample, relating to carbon contaminants (531.2 eV [113] and 532.7 eV [114]) and nickel oxide (529.6 eV [111] and 531.3 eV [115]). However, after the low tempera-ture outgassing of the sample all three of these peaks also disappeared, therefore the nickel oxide and associated carbon contaminants had been removed from the surface.

Figure 6.11: XPS Ni-2p spectra for a Ni thin film sample both before and after a 500^◦C anneal (labelled ‘Low Anneal’). The sample was prepared in the Edwards

evaporator system before it was transferred to a UHV system (via ambient conditions) for XPS analysis, but was annealed in-situ. XPS performed by

Subhashis Gangopadhyay.

Ni thin films produced in the Edwards evaporator were also annealed and then dosed with propylene in the Nanograph STM-01 UHV system and then transferred to the Kratos Axis Ultra XPS system via ambient conditions. Three thin films were investigated with ex-situ XPS (after they had been transferred through ambient conditions): untreated (transferred directly from Edwards evaporator via ambient conditions), annealed at 600^◦C and then dosed with propylene, and annealed at 800^◦C and then dosed with propylene. The resulting O-1s peaks are shown in Figure 6.13.

Chapter 6. Graphene Formation on Ni Thin Films 161

Figure 6.12: XPS O-1s spectra for a Ni thin film sample before and after a 500^◦C (‘Low’) anneal. The sample was prepared in the Edwards evaporator system before it was transferred to a UHV system (via ambient conditions) for XPS analysis, but was annealed in-situ. XPS performed by Subhashis Gangopadhyay.

XPS O-1s spectra for the untreated Ni thin film (no anneal or propylene dose) were unsurprisingly similar to the XPS spectra for the untreated Ni thin film in Figure 6.12 – both samples had been produced in the Edwards evaporator and then transferred through ambient conditions to the two different XPS systems.

However, when comparing the XPS spectra for the two ‘Low Anneal’ samples in Figures 6.12 and 6.13, a large oxygen peak was found for the 600^◦C (‘Low’) anneal sample in Figure 6.13 whilst no oxygen peaks were observed for the 500^◦C (‘Low’) anneal sample in Figure 6.12. Although the 600^◦C anneal sample had been dosed with propylene and the 500^◦C anneal sample had not, it was found

Figure 6.13: XPS O-1s spectra comparing the effect of annealing temperature for three separate samples. The samples investigated were (from top spectra to bottom) an untreated Ni thin film sample, a Ni thin film sample annealed to

600^◦C (‘Low Anneal’) and dosed with propylene in UHV, and a Ni thin film sample annealed to 800^◦C (‘High Anneal’) and dosed with propylene in UHV.

All the Ni thin film samples were produced in the Edwards evaporator system and prepared in a different UHV system other than the ex-situ XPS UHV

system. XPS performed by Ignacio Villar-Garcia.

that the appearance of the oxygen peaks was actually due to exposure to ambient conditions before the ex-situ XPS was performed (discussed later in this section).

The 800^◦C (‘High’) anneal sample revealed vastly reduced oxygen peaks when compared to the 600^◦C anneal sample, however there were still very small oxygen peaks observed for the ‘High Anneal’ sample. After any 500-800^◦C anneal of the surface the NiO peak at 529.6 eV was completely removed and did not reappear

Chapter 6. Graphene Formation on Ni Thin Films 163

Figure 6.14: XPS Ni-2p spectra comparing the effect of annealing temperature for three separate samples. The samples investigated were (from top spectra to

bottom) a Ni thin film sample annealed to 800^◦C (‘High’) and dosed with propylene in UHV, a Ni thin film sample annealed to 600^◦C (‘Low’) and dosed with propylene in UHV, and an untreated Ni thin film sample. All the Ni thin film samples were produced in the Edwards evaporator system and prepared in a

different UHV system other than the ex-situ XPS UHV system. XPS performed by Ignacio Villar-Garcia.

after exposure to ambient conditions for the Ni thin film samples.

Figure 6.14 shows the Ni-2p peaks found for the same three samples shown in Figure 6.13. The untreated sample spectra revealed smaller Ni-2p1 and Ni-2p3

peaks compared to the two annealed samples which showed a drastically increased magnitude and sharpness of the peaks, similar to the ex-situ XPS spectra in Fig-ure 6.11. The sharpness of the nickel 2p1 and 2p3 peaks for the 800^◦C anneal

Figure 6.15: XPS C-1s spectra comparing the effect of annealing temperature for three separate samples. The samples investigated were (from top spectra to

bottom) a Ni thin film sample annealed to 800^◦C (‘High’) and dosed with propylene in UHV, a Ni thin film sample annealed to 600^◦C (‘Low’) and dosed with propylene in UHV, and an unannealed Ni thin film sample. All the Ni thin film samples were produced in the Edwards evaporator system and prepared in a different UHV system other than the ex-situ XPS UHV system. XPS performed

by Ignacio Villar-Garcia.

Chapter 6. Graphene Formation on Ni Thin Films 165

Figure 6.15 shows the corresponding C-1s XPS spectra for the three samples.

The untreated sample spectra showed the convolution of several peaks with the most prominent C-1s peak at 285.0 eV – which corresponds to C-C bonding – the peak then became sharper as the sample was annealed. Other peaks were also observed for the untreated sample which corresponded to carbon contaminants, these peaks were still present in the lower anneal sample – due to readsorption in ambient conditions – although not to the extent of the untreated sample. There was minimal carbon contamination shown in the XPS spectra for the higher an-neal sample which would coincide with the very small O-1s peaks in Figure 6.13.

However, the ester (COOR) peak at 288.9 eV was absent for the higher anneal sample and still observed for the lower anneal sample.

Figures 6.11-6.15 suggest that although only a 500^◦C anneal was required to remove the nickel oxide and carbon contaminants from the Ni thin film samples, upon exposure to ambient conditions nickel oxide and carbon contaminants were re-adsorbed onto the bare Ni surface. However, after an 800^◦C anneal the surface was almost completely protected from the re-adsorption of both oxygen and carbon contaminants when exposed to ambient conditions. This leads to the hypothesis that graphene layers, formed on the Ni thin film surface after an 800^◦C anneal, protected the Ni surface from any oxygen or carbon contaminants and thus only a very small amount of contaminants were re-adsorbed, presumably in areas that contained defects in the graphene. This hypothesis was supported by the fact the C-C bonding peak sharpened as the annealing temperature increased.

However, the affect of the propylene dosing of the samples has not yet been considered. To this end an 800^◦C annealed and propylene dosed sample and a 800^◦C annealed sample were investigated using XPS, as shown in Figures 6.16 and 6.17. From Figures 6.16 and 6.17, there were no significant differences between propylene dosed and annealed samples that had both been annealed at 800^◦C.

From the XPS data in Figures 6.11-6.17, the overall graphene formation process can be understood. Upon annealing the Ni thin film in UHV at 500^◦C or above all the oxygen peaks were removed, the Ni-2p peaks were sharper and associated oxide peaks were removed. For the C-1s spectra the COOR peaks were removed and the C-C bonding peak increased and became sharper. Contamination was revealed for the 800^◦C anneal samples using ex-situ XPS due to the exposure to ambient conditions, but the intensity of these peaks was drastically reduced compared to lower temperature annealed samples. The annealed and the dosed samples produced approximately the same C:Ni:O ratio of 45:53:2 (found from XPS data shown in Figures 6.16 and 6.17) corresponding to the atoms present in

Figure 6.16: XPS O-1s spectra as a comparison between annealing and dosing and solely annealing the Ni thin films. The samples investigated were (from top

spectra to bottom) an untreated Ni thin film sample, a Ni thin film sample annealed to 800^◦C in UHV and subsequently dosed with propylene (‘Dosed Sample’), and a Ni thin film sample annealed to 800^◦C (‘Annealed Sample’).

XPS performed by Ignacio Villar-Garcia.

the top few nanometres of the surface of the sample. The protection of the Ni surface observed in the XPS data shows that the annealing cycles are critical for the formation of graphene films, with the majority of the surface covered with graphene when anneal temperatures in the range 700-800^◦C were used.

Increasing the anneal temperature above 800^◦C produced a high abundance of areas where there was no Ni thin film coverage of the SiO₂ substrate, as discussed in Section 6.1.1. XPS spectra of areas that appeared purple to the naked eye were

Chapter 6. Graphene Formation on Ni Thin Films 167

Figure 6.17: XPS C-1s spectra as a comparison between annealing and dosing and solely annealing the Ni thin films. The samples investigated were (from top

spectra to bottom) a Ni thin film sample annealed to 800^◦C (‘Annealed Sample’), a Ni thin film sample annealed to 800^◦C in UHV and subsequently dosed with propylene (‘Dosed Sample’), and an untreated Ni thin film sample.

XPS performed by Ignacio Villar-Garcia.

found to have small Si and SiO2 contributions to the Si-2p peaks as well as an SiO2

contribution to the O-1s peaks. However, the remainder of the sample produced the same XPS spectra previously observed for an 800^◦C anneal sample, where no silicon contributions were revealed.