UPS is a powerful technique for measuring the valence band maximum (VBM) and work function (WF) of ultra-thin film; in addition, it gives the most directive figure about the energy level alignment at the interface between two layers [76]. Figures 4.3(a) and 4.3(b) show the UPS spectra obtained from the as-received and ALD-deposited TiO electrodes. The
VBM relative to the Fermi-level (EF) was determined by the values of cutting points in the VB region of UPS assuming that EF is 0 eV, and the resulting values are listed in table 4-2.
Following the ALD process, the VBM was shifted by 0.9 eV from 3.7 eV to 4.9 eV, which is larger than the shifts in core levels, and it is believed that the VBM shifted toward the reported value of 5.6 eV for pure Al2O3 thin film [85]. On the other hand, the work function (WF) was measured from the values of cutting points in the secondary electron (SE) region of the UPS, as can be seen in Fig. 4.3(b). It should be noted that the WF of ultra-thin Al2O3
overlayers increased from 4.7 eV to 5.1 eV at the surface in the first ALD cycle process, which indicates that the band is bending downward across the surface to the interface, and the difference of WF, which induced a 0.4 eV built-in potential (Vbi), accelerated the electron transport from Al2O3 over-layers to the TiO2 electrode. The WF of ultra-thin Al2O3 overlayers decreased with subsequent 2-10 cycle ALD deposition and shifted to 4.4 eV after the 10-cycle ALD deposition process. A similar trend was found in the reference [86], where the interfacial dipole layers were regarded as the main factor that determined the work function of ultra-thin Al2O3 films on a Cu-9%Al surface. The thickness dependence of the WF for the Al2O3
overlayers revealed the interfacial charge stability of the dipole layers, and this is shown in Fig. 4.4.
Table 4-2 Thickness, energy levels of the ALD Al2O3 overlayers of the DSSCs.
Deposition
The REELS spectrum of as-received TiO2 in Fig. 4.3(c) shows that the onset of electron
signals located at 3.9 eV lies within the band gap value reported in the literature for nano-crystalline TiO2, which indicated the blue-shift or quantum confinement compared to 3.2 eV for a large grain size of TiO2 [87, 88]. By considering the depth sensitivity in the REELS spectra, we calculated the depth of electron penetration (X) from the following equation [89]:
X (m) =0.1E1.5/d (4-2)
where E is the e-beam energy (KeV), and d is the sample density (g/cm3) in the REELS spectra. Substituting the experimental e-beam energy value (17 eV), ALD Al2O3 density value (2.9 g/cm3) [68], and TiO2 density value (3.893 g/cm3, JCPDS-ICDD No:31-1272) into the equation (4-2), the resulting depths of electron penetration in our REELS analysis were determined to be about 0.07 nm for Al2O3 and about 0.05 nm for TiO2 materials. This result may guarantee that our REELS spectra are depth-sensitive enough to the Al2O3 over-layers for measuring their band gap. After the deposition of the Al2O3 overlayers, the onset of electron signals shifted from 4.0 eV to 5.9 eV as the thicknesses of the Al2O3 over-layers increased as shown in Fig. 4.3(c). The trend are similar to the earlier reports that used REELS to measure the band gap of Al2O3ultra-thin films from 2.5 eV for the surface to 8.7 eV for the bulk state, and the lower band gap value is mainly due to structural surface states, such as defects, Al spz dangling bond [90], and the amorphous structure [91]. The present results are consistent with earlier findings in REELS studies that suggested that the band gap of the Al2O3 overlayer is positively correlated with its thickness at the sub-nanometer scale due to the fact that the ratio of reflective electrons from structural surface-states is a major portion of the REELS spectra in ultra-thin Al2O3 films.
Figure 4.3 (a) UPS valence band region and (b) UPS secondary electron region of the as-received and ALD Al2O3 deposited TiO2 electrode. (c) REELS spectra of the as-received and ALD deposited TiO2 electrode.
The work function (WF) changes as a function of the thickness of the Al2O3 over-layers (Fig. 4.4), indicate that the magnitude and the direction of the core level shifts are in excellent agreement with the decrease in WF. The measured WF of the pure, nanoporous TiO2
electrode was 4.7 eV, which is close to the WF of 4.8 eV of the pure TiO2(110) surface [92].
Following the deposition of the Al2O3 over-layers, the shifts of Ti 2p3/2 and Al 2p XPS peaks showed a similar trend to that of the WF. Significant changes of WF and Ti 2p3/2 core level by 0.4 eV at about a 0.2-nm thickness of Al2O3 deposition indicate the occurrence of an interfacial reaction between the Al2O3 over-layer and the TiO2 electrode. This could be due in part to the formation of Ti-O-Al(OH)2, as previously mentioned when we discussed Fig. 4.2.
The reduction of WF, when the thickness of the Al2O3 over-layers exceed 0.2 nm, was associated with the increase of binding energy of Ti 2p3/2 and Al 2p core levels. These results
were consistent with previous studies [77, 86], and they could be attributed to the interfacial Ti-O-Al(OH)2 and the formation of dipole layers, which stabilized after the formation of two Al2O3layers.
Figure 4.4 Changes of the work function, Ti2p3/2 XPS peak and Al2p XPS peak with various thicknesses of Al2O3layers.
We have repeated the UPS/XPS and REELS measurements several times, indicating the data’s error less than 0.1 eV. The accuracy or resolution of our data was calibrated before every spectral experiment by following analysis: The “resolution” of our electron spectroscopy determined as the energy between 80% to 20% intensity of pure Ag Fermi-edge was ~98 meV. But the real ability to resolve signals was determined as <20 meV using the same spectral condition in this study and tested a pure Ag surface from sample bias in the range of -5.00 eV to -5.04 eV by increments of 0.02 eV. The UPS deviations were smaller than 0.005 eV, and it could be assured that the resolving power was < 0.02 eV. Therefore, the
larger data’s error was mainly resulted from the deviations of samples preparation.