## 3 Results and Discussion
### Characterization of Morphology and Chemical States of Polycrystalline Diamond Films
#### 3.2.2 UPS Analysis of PCD Films  
As was shown in the previous section, there is conduction in the grain boundary even though the current flow is small. Since all the grains are interconnected, there should be some form of metallic behavior. UPS measurements can show the density of states (DoS) at the Fermi level if the material is metallic. The work function (\(\Phi\)) can be inferred from the measurements if a sufficient DC offset is placed at the stage to differentiate the cutoff energy at which electrons are accelerated to the vacuum or extracted from the material.  
Figure 9a shows the band structure of the PCD films as obtained from UPS measurements. The reference value of 21.2 eV is the energy of the He(I) photon incident on the samples. Since the reference UV energy is close to the cutoff of the instrument, 9.25 V were applied between the holder and ground to offset the measurement and better determine the cutoff energy of the PCD films. The work function was then calculated by subtracting the cutoff energy from the incident UV energy. For He(I), the cutoff is fairly broad, which resulted in an approximate value of the \(\Phi\).  
Figure 9b is a closeup of the DoS around the \(\mathrm{E_{F}}\). All the samples show the presence of electrons from the valance band to the Fermi level, which characterizes the metallic behavior. The binding energy (\(\mathrm{B_{E}}\)) can be calculated by extending the signal of the valance band to the intercept and subtracting the \(\mathrm{E_{F}}\) (0 V). See insert in Figure 9b.  
All the \(\Phi\) measurements are between 4.5 and 5.5 eV, which are in the range of reported MCD values [27] and in the range of the metal \(\Phi\) measurements [28]. The \(\Phi\) is not a localized measurement but rather an average measurement of a larger area, and the UPS technique using He(I) does not yield a precise measurement of \(\Phi\). However, the point here is to show that all the samples, regardless of grain size, have characteristic metallic behavior. This point further emphasizes the suggestion that the nature of the grain boundary is the same where the diamond surface bonds are either passivated with H atoms or form a C=C bond from the same crystal or the adjacent crystal. The calculation of the binding energy \(\mathrm{B_{E}}\), as shown in Figure 9b, is a guide to determine whether there is difference in the depth of the valance band for PCD films with higher grain boundary concentrations. There seems to be a trend, as \(\mathrm{B_{E}}\) decreases with smaller grain size (Table 4). Sample D does not follow the trend, probably because of the grain size overestimation, as previously discussed. This trend supports the argument that the higher concentration of grain boundaries contributes to the availability of conduction electrons. More careful measurements need to be taken.  
Figure 9: UPS survey spectra using (**a**) a He(I) source for all samples A–G; and (**b**) high-resolution spectra around the Fermi level for sample G. The insert is a representation of the conduction and valance band relative to the Fermi level for a metal. \(\mathrm{B_{E}}\) is the binding energy.