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As such it appears that the initial work function of the photocathode effects the amount that the PEI effects the work function. This is most likely due to a decrease in the electronegativity at the surface and the addition of PEI causes less of a dipole at the surface.

Figure 4.22:∆WF of ZnO films spin coated with PEI as a function of initial WF

4.6

Summary

The standard deviation in WF measurements across a surface were found to be about 0.03eV which is smaller than the error on the WF on the tip of the KP 0.07eV, meaning we can get more accurate determination of relative changes compared to the actual WF values.

It was found that polishing a copper surface with Brasso instead of the standard solvent cleaning method resulted in a much larger signal under illumination. Despite this, on average, polished copper surfaces showed only slightly lower WF (about 0.08±0.04eV). Also, polishing samples already measured showed little to no change in work function, therefore this is unlikely to be the reason for the increased signal seen under illumination.

From the XPS data, the polished copper surfaces showed a marked reduction in the satellite peaks at 940eV and above, which is attributed to the Cu2+components such as CuO and CuOH. The reduction in this oxide layer was further evidenced by a larger metallic contribution in the LMM Auger spectrum as well. Therefore we can say that polishing samples removes the top oxide layer exposing the metallic layer underneath which causes the increase in signal seen un- der illumination.

4.6. SUMMARY

Samples that were cleaned in a solution of Micro-90 and distilled water showed much lower work function of about 4.01±0.11eV. The large change in work function when Micro-90 was used as cleaning solution could be explained by the introduction of a Na 1s region in the XPS spectra. Micro-90 cleaning solution contains 10 to 25% of ethylene diamine tetraacetic acid tetrasodium (EDTA-Na4) which could explain the introduction of Na to the XPS spectrum. The Na 1s on the surface could cause a surface dipole lowering the work function measured. Copper shows remarkable stability over time with almost no change in work function over a period of 20 days. There does appear to be a small increase in the WF measured after exposure to ambient air over a couple of days which is more pronounced for the polished samples. This is most likely due to oxidation or addition of water to the surface after being exposed to ambient conditions. After this initial increase in WF, however, the samples showed no change in work function.

For the deposited films of MgO and ZnO it was found that the work function is dependent on the thickness of the overlayer coated on the copper substrate. With thinner layers having work functions somewhere between the saturated overlayer value and the substrate layer.

Table 4.3 shows the average WF measured for MgO and ZnO films grown via PVD under two

Material Vacuum WF (eV) Ambient WF (eV)

MgO 3.64±0.04 4.15±0.17

ZnO 4.11±0.06 4.35±0.12

Table 4.3: Average WF for MgO and ZnO under different oxidation methods

different oxidation conditions: letting oxygen leak in during growth under vacuum and letting the sample oxidise naturally in ambient air by opening the chamber straight to air. Both MgO and ZnO show an increase in WF for samples oxidised under ambient conditions compared to samples oxidised under vacuum. MgO shows a much larger change in work function for samples oxidised in ambient compared to samples oxidised under vacuum conditions, which are consis- tently around a value which is similar to that found for clean polycrystalline magnesium [106]. It was found from the O1s region that the sample oxidised under vacuum conditions had a larger Mg(OH)2component, whilst the sample allowed to oxidise naturally under ambient conditions had a larger MgCO3component.

Unlike the standard copper photocathodes, the deposited Mg films were found to actually de- crease in work function during exposure to ambient air. From the XPS of samples left in ambient for a long period ( 6 months) the XPS shows a larger metallic component in the Mg 2p region suggesting a thinner oxide layer. There does not appear to be any relation to the amount of metallic component present to the work function, instead the O 1s region is still the best indi- cator of work function (with increasing Mg(OH)2showing lower work function). There is also a much larger MgO component present in the XPS spectra of the sample left in ambient, which could be the reason for the decrease in WF over time.

4.6. SUMMARY

annealed under vacuum [136], where the average work function for ZnO films oxidised in am- bient conditions is more similar to the freshly cleaved ZnO single crystals in Moorman et al. [136]. ZnO films that were deposited using electrochemical deposition were found to have a mean work function of 4.4±0.2 quite similar but a little higher than ZnO films oxidised in air. Almost all the ZnO samples left in ambient conditions all appear to tend to and settle at the slightly higher work function of 4.35±0.11eV, which is closest to the value of ZnO films grown via electrochemical deposition.

It was seen from the LMM spectra for ZnO films that samples oxidised in vacuum showed a much larger metallic component as shown by a much larger peak at 992.85eV and 996.2eV, with a secondary peak at 988.1eV which almost perfectly fits the LMM value for ZnO from Dake et al. [139]. Samples oxidised in ambient showed a small metallic component but much less than samples oxidised in vacuum, also the secondary peak was shifted to a lower K.E. at around 987.1eV, which again fits perfectly the LMM value for hydrazincate from Dake et al. [139] Further evidence for an increased Zn(OH)2/ZnCO3or hydrazincite component in ambient oxi- dised and electrochemically deposited films is shown in the O 1s and Zn2p regions. Whereas, vacuum oxidised samples show largest ZnO component along with a large metallic contribu- tion, ambient oxidised samples show a large Zn(OH)2but small ZnCO3and electrochemically deposited samples show almost no ZnO but the largest ZnCO3 and the smallest ZnCO3contri- bution. Therefore it can be summarised that the formation of a mixture of Zn(OH)2/ZnCO3or hydrazincite raises the WF of the film and this will form naturally over time if left out in ambient conditions.

Finally PEI coatings on various photocathodes were investigated in attempt to lower the work function of our photocathodes. It was found that at lower speeds (<2000rpm) the∆WF is larger. However at very low speeds the film was visibly patchy so as a standard the speed of 2000rpm was used.

For speeds<2000rpm the average∆WF for copper was found to be−0.52±0.07eV.

The reduction in work function was found to decrease with exposure to ambient conditions, with only a day reducing the∆W Fby 0.2eV and eventually going back to the original WF. However, the WF reduction appeared to remain reasonably constant when left in nitrogen. This suggests that although care will have to be taken with depositing and storing the devices, PEI could be used in a gaseous environment to reduce the work function of a photocathode with higher work function such as copper.

It was also found that if ZnO had low work functions (4eV and lower) the∆WF was very small or negligible, whereas ZnO films with higher WF (4.2 and above) had a∆WF of−0.39±0.04 on average. MgO films were found to actually increase after being spin coated with PEI with a∆WF of 0.17±0.06 on average. Therefore it appears as if the∆WF from coating materials in PEI actually depends on its initial WF. This is likely due to the fact there is already a strong surface dipole on the MgO and ZnO films with low WF, therefore adding PEI does not help with a contribution to the surface dipole.

4.6. SUMMARY

However this does not match exactly the results found for Zhou et al. [143] as they found the largest work function reduction for Au, which has a higher WF than the other materials tested.