Work Function Modifiers

(a) (b)

(c)

Figure 4.19: Zn2p3/2 region for a)sample oxidised in vacuum b)left in ambient and

c)electrochemically deposited

4.5

Work Function Modifiers

Also of interest is the possibility of tuning or modifying the work function of our photocathodes using post-deposition treatment. One particular way of doing so is to coat the photocathodes in an ultrathin layer that is chemically or physically absorbed onto the surface which creates a strong interface, molecular dipole or contribution of both which in turn will induce a shift in the Fermi level of the conductor and lower the work function.

Zhou et al. [143] showed that polymers with simple aliphatic amine groups, polyethylenimine ethoxylated (PEIE) and branched polyethylenimine (PEI), could reduce the work function of a variety of different conductors. Chemisorption of tetrakis(dimethylamino)ethylene (TDAE) has also been demonstrated to cause reductions in work function for Au [144]. However, due to TDAE’s instability in air it has found limited use, whereas PEI & PEIE can be easily processed in air from dilute solutions of water or alcohol, which remain stable for more than a year in ambient conditions.

4.5. WORK FUNCTION MODIFIERS

Zhou et al. [143] also showed, by washing away islands of thicker PEIE without the WF being affected, that only a ultra-thin layer is needed to produce the WF changes seen. Whilst extra PEI or PEIE seemed to not affect the work function, it was found to hinder device performance with increasing PEI/PEIE thickness due to its insulating nature.

Zhou et al. [143] gave KP measurements of the work function changes in air, with a 0.98eV, 1.2eV, 0.9eV & 0.65eV∆WF for ZnO, Au, Ag & Al respectively after depositing a thin layer (10nm) of PEIE on the surface and 1.16eV, 1.16eV, 1eV for ZnO, Au & Ag with a deposition of a thin layer (10nm) of PEI.

In this section we will coat layers of PEI and PEIE of varying thickness on our various photo- cathodes and measure the WF reduction using the Kelvin probe.

Figure 4.20 shows the change in work function from copper films coated in PEI at different

Figure 4.20: Work function changes from spin coating thin films of PEI on copper at varying spin speeds

spin speeds. It can be seen that at very high speeds (>4000rpm) there is a lower change in work function. This suggests that at thinner PEI films the work function reduction is not as strong. This seems to be at odds with the results in Zhou et al. [143], where even ultra-thin films resulted in large work function changes.

Low spin speeds (<2000rpm) resulted in films that were patchy and not fully coated. As such 2000rpm is the standard speed used for coated films of PEI as it results in large∆WF but main- tains a visually uniform film. The average∆WF for speeds less than 2000rpm is−0.52±0.07eV, similar reduction to that found for aluminium [143].

4.5. WORK FUNCTION MODIFIERS

period of several days. The results are shown in figure 4.21. Here it can be seen that for PEI left in ambient the∆WF drops rapidly to much smaller values even over a period of a day, whereas the PEI coatings left in a nitrogen flow are relatively stable for about a week with little change in the∆WF. However eventually all samples return to standard copper WF values. Samples stored in Nitrogen are naturally exposed to air during measurement which could be the primary cause for their decay.

Although this does not take into account the fact that the WF of the bare copper also changes with time. However from figure 4.2 this was found to be negligible in comparison, especially for solvent cleaned copper, therefore the drop in work function must be due to a removal of the PEI from the surface.

As such PEI-coated photocathodes may be stable in a gaseous environment such as that present in ThGEMs, but without measurements of WF under this gaseous environment it is impossible to say for certain as even a small amount of exposure to ambient conditions could effect the reduction caused by the PEI.

Figure 4.22 shows the∆WF of PEI coated on ZnO films, a spin speed of 2000rpm and solution

Figure 4.21: KP measurements of PEI coated Cu surface over a period of days

concentration of 0.7% by weight of PEI is used. The figure is plotted as a function of the WF measured for the sample before coating with PEI, it can be seen that samples with higher WF initially show a larger∆WF than samples that already had a lower WF. However even the largest ∆WF of−0.39±0.04eV is well below the change in WF for ZnO from Zhou et al. [143] It was found that PEI coated on MgO films appeared to make little difference to the WF only appearing to actually increase it by 0.17±0.06eV on average.