XPS analyses

The XPS is used to further investigate the changes of chemical bonds on ion-beam treated SE-130B films which possess the typical chemical structure of PI drawn in Fig.

2.4.7 [10-12]. A series of conditions including different ion energies, incidence angles, and bombarding times are chosen for comparison in the ion-beam bombardments. We will begin with the issue of ion beam energy.

Four kinds of samples treated by ion beam bombardments with various V_b, bombarding time of 5 min, incidence angle of 60°, and current density of 255 μA/cm² are scanned in survey mode with the step size of 1 eV and the analyzer pass energy of 117.4 eV. The survey spectra shown in Fig. 2.4.8 reveal that a significant amount of iron appears on the treated surface with ion beam energy higher than 560 V. The Fe 2p and Fe 3p core-level signals are located in the ranges of 705-735 eV and 53-60 eV, respectively [13]. We speculate that this iron contamination comes from the electrodes of ion beam system. More discussions on these unexpected results will be given later on.

In addition, the Si2s signal probably comes from the glass substrate. Figure 2.4.9 shows the core-level 1s signals of carbon (C), oxygen (O), and nitrogen (N) elements scanned in multiplex mode with the step size of 0.2 eV and the analyzer pass energy of 23.5 eV.

As we can see, the signals of C1s and N1s are dramatically reduced by increasing the ion beam energy in the treatments. However, the peak position of O1s signal moves to a lower binding energy. And the increase of peak intensity indicates that the extra bonds have been newly formed. The deconvolutions are further carried out for C1s and O1s

signals and their fitting results are plotted in Fig. 2.4.10.

For core-level C1s signal, it is composed of five main signals, C-C/C-H (284.56 eV), C-N (285.68 eV), C-O-C (286.29 eV), N-C=O (288.6 eV) and O-C=O (287.4 eV), and a signal contributed from the shakeup satellite at BE=291.30 eV described in [10].

The shakeup satellites are usually shown on the higher BE side of the core-level spectra

of aromatic and unsaturated polymer [14,15]. As shown in Fig. 2.4.10(a), a significant decrease of signal from C-C/C-H, C-N, and C-O-C bonds are found in the high-energy ion beam treatments. The intensity of shakeup satellite also disappears after ion beam treatment with energy of 1120 V. This indicates that the aromatic components like benzene ring and pyrrolidine are destroyed by ion beam bombardments. Similar trend of variation for carbonyl group is also found in O1s spectra as shown in Fig. 2.4.10(b). It should be noticed that two bonds, C-O-Fe (531.1 eV) and Fe2O3 (529.77 eV), related to the iron element are newly formed on the surfaces after high energy ion-beam treatments. Their appearances cause the movement of envelope of spectrum to a lower BE side.

To quantitate the variation of signals, the intensity of each chemical bond convoluted to a core-level spectrum is obtained by the integration of its fitted curve, i.e.

the mentioned Gaussian-Lorentzian sum function in Sec. 2.3.3, and summarized in Table 2.4.1 and Table 2.4.2. Besides, the integrated intensity is further divided by the overall intensity of the corresponding spectrum to obtain the composition ratio. Figure 2.4.10 shows the composition of each bond as a function of ion beam energy. We can find out that the shakeup satellite in Fig. 2.4.11(a) is gradually reduced with the raise of ion beam energy. In addition, the iron-related bonds appear to become larger as the ion energy increases, as shown in Fig. 2.4.11(b). In Table 2.4.1 and Table 2.4.2, another chemical reaction should be noticed is that the re-oxidization of the dangling bonds occurs after the ion beam bombardments. It causes the increase of oxygen content.

However, most contribution is made from the formation of iron oxide on the treated surface for the cases with the ion energy higher than 560 V.

Furthermore, the impact of different bombarding times of the ion beam treatments with ion energy of 560 V, incidence angle of 60°, and current density of 255 μA/cm² on the chemical bonding of treated surface is discussed as well. Figure 2.4.12 shows the survey spectra of PI surfaces treated for different bombarding times, 0 min, 2 min, and 8 min. It is not surprising that the iron content is raised by an increase in length of bombarding time. Figure 2.4.13 shows the raw spectra of C1s, O1s, and N1s scanned in the multiplex mode. The deconvoluted signals of C-O-C and C-C/C-H bonds are obviously attenuated for 8 min treatment, as seen in Fig. 2.4.14(a). No significant

reduction but a movement to a lower BE of signal is found for shakeup satellites. The same behavior as remarked before is also observed in the O1s spectrum, as shown in Fig.

2.4.14(b). The intensities of chemical bonds convoluted to the C1s and O1s spectra are also organized in Table 2.4.3 and Table 2.4.4. The decrease of intensity of C-N bond means that a part of the backbones of PI are broken into pieces; meanwhile, the dangling bonds react with oxygen to form the carbonyl groups, N-C=O and O-C=O.

Similar results can also be concluded in Fig. 2.4.15(a). Moreover, a dramatic increase of the content of Fe2O3 and C-O-Fe bonds after ion beam bombardment is reconfirmed in Fig. 2.4.15(b).

Next, ion beam conditions of ion energy of 840 V, current density of 458 μA/cm², bombarding time of 5 min and different angles of incidence are discussed. The survey spectra of surfaces treated with incidence angle of 40°, 60°, and 80° are shown in Fig.

2.4.16. All the bombarded surfaces have the iron contamination without exception. The relative intensities of the fine scanned spectra shown in Fig. 2.4.17 suggest that a smaller angle of incidence offers a better etching ability. Deconvolutions of C1s and O1s

spectra have been accomplished further and shown in Fig. 2.4.18 and Fig. 2.4.19. The fitted intensity of each chemical bond is listed in Table 2.4.5 and Table 2.4.6. We can find out that the best etching effect is given by incidence angle of 60°; on the other hand, more iron contaminations are also found. A similar remark can be concluded according to the calculated composition ratio as shown in Fig. 2.4.20.

Now, we turn to look deeply into how the iron element leads to the homeotropic alignment of liquid crystal. As it has been noticed above that the iron element could be found in the sample if treated by ion beam with energy higher than 560 eV. Figure 2.4.21 shows two survey spectra scanned by using the Mg Kα and Al Kα lines as the x-ray monochromatic sources for the PI surface treated by beam energy of 1120 V, incidence angle of 60°, beam current density of 255 μA/cm², and treating time of 5 min.

It should be mentioned that the BE of auger signals remain the same no matter what the x-ray source is. The survey spectrum by Al Kα line has been shifted about +230 eV for comparison with that by Mg Kα line. In other words, a shift of BE about −230 eV for primary signals of the O1s and Fe2p3 levels when x-ray source is changed from Mg Kα to Al Kα line reveals that these unexpected signals are the auger signals of iron, Fe

LMM [16]. For a detail study of the Fe peaks, the multiplex mode scanning of Fe 2p region on the high energy IB-treated surface is carried out. The spectrum is shown in Fig. 2.4.22. According to the literatures [17,18], the shake-up satellite line at 718.2 eV is characteristic of Fe³⁺ in Fe2O3. Further, the narrow peak at 710.4 eV of the Fe 2p spectrum indicates that no Fe²⁺ iron oxidation state exists. In other words, the possibility of forming Fe3O4 (magnetite) in the film can be excluded. The film should be composed of the Fe³⁺ oxides, Fe2O3 only. The spectrum of the IB-etched ITO film reveals the same profile. Structurally, however, there are four possible types of Fe2O3. Two of them, α-Fe2O3 (hematite) and γ-Fe2O3 (maghemite) are common and widespread in soils [19].

Of the two, only the γ-Fe2O3 has permanent magnetic moments.

An interesting result that the homeotropic alignment is still achieved even on a clean substrate without PI coating after the high-energy IB treatment was discovered in addition. To further identify the coated films, the XPS Fe 2p3/2 signals of IB-etched PI and ITO films are analyzed in detail as shown in Fig. 2.4.23(a) and Fig. 2.4.23(b), respectively. The measured XPS data are smoothed and the Shirley background is subtracted. Then a deconvolution is done by fitting the spectra to multiple Gaussian peaks. The Fe 2p3/2 envelope of the measured spectra is well fitted by using peaks constrained to the multiplets calculated for the iron compound γ-Fe2O3 by Gupta and Sen [20]. The splitting of four Fe³⁺ 2p3/2 multiplet peaks with binding energies at 709.8, 710.8, 711.8, and 713.0 eV in Fig. 2.4.23 are due to the inclusion of electrostatic interactions and spin-orbit coupling in theoretical calculation [21]. The presence of satellite peak has been ascribed to the shake-up processes [22]. We conclude, therefore, that the treated substrate is coated with an iron oxide of γ-Fe2O3.

It is well known that the LC molecules can be reorientated by electric and magnetic field due to their anisotropic electrical permittivity and magnetic susceptibility [23]. The mechanism of homeotropic LC alignment might be ascribed to the magnetic field induced by the γ-Fe2O3 or/and the intermolecular interactions at the liquid-crystalline-maghemite interface.

Finally, another issue concerned in this work is that the bond-breaking effects of PI films to the LC alignment induced by ion beams here are similar to that induced by polarized ultraviolet (UV) light irradiation [24,25]. They are both attributed to the

intermolecular interactions between the liquid crystals and the unbroken polyimide chains. To confirm that the alignment in this work is indeed induced by ion beam rather than the UV irradiation in the ion beam chamber, we have formed a cell with substrates partially covered by a fused silica plate while performing the ion-beam treatment. At the covered area the UV light can transmit through the silica plate while the ion beam is blocked. The pictures of this cell between crossed-polarizers are shown in Fig. 2.4.24, only the uncovered area (right hand side) shows good alignment. Since no alignment effect appears in the untreated area (left hand side), we can conclude that the alignment effects are not caused by the UV light from plasma discharge.