## 4 Discussion  
In this study, four kinds of self-assembled phosphonic acid solutions were used to coat the SAMs of phosphonate on the TiO layers. However, it was clear that there was the presence of larger aggregated island-like structures on the surfaces of the HDPA-SAM/TiO, NDPA-SAM/TiO, and PHA-SAM/TiO samples, as shown in Figure 2. The amount of the aggregated island-like structures in the PHA-SAM/TiO sample was more than that of the HDPA-SAM/TiO sample and NDPA-SAM/TiO sample. The formation of this feature could be attributed to the higher molecular weights in HDPA, NDPA, and PHA, which induced more organophosphorus compounds to bond with the O atoms. Typically, there are two organophosphorus compounds C-PO (OH)\({}_{2}\) or C-PO(OA)\({}_{2}\) groups (where A = alkyl, aryl) contained in the phosphonic acids [29; 30]. They easily bond with O in the oxide layer to form the SAMs via Ti-O-P linkages [26]. Therefore, the SAMs-coated samples with a higher molecular weight caused the formation of non-uniform layered interfaces. Similar topography characteristics can also be found in a report by [PERSON] and [PERSON] [31]. On the contrary, the PUA-SAM/TiO sample had the great potential to form a uniform SAM surface. Moreover, chemical bonding states of the control and SAMs-coated samples were further analyzed using XPS. Based on the XPS results, the formation of a homogenous nanostructured TiO layer on the substrate surface can be fabricated using an oxygen plasma treatment [13; 32]. An exothermic plasma reaction generates some of O\({}_{2}\), O\({}^{*}\), and O\({}^{2-}\) species or radicals to fix at lattice or interstitial sites [13]. This reaction offers a key driving force for the transport of O species to form the TiO/Ti or TiO\({}_{2}\)/Ti [33]. There was no detectable P at its corresponding binding energy, which demonstrated that there was no organic compound contaminants on the surface of the control TiO sample. After being coated with different SAMs, the binding energy of O 1s indicated that the component characteristics of P=O (\(\approx\)532 eV), P-Ti-O (\(\approx\)530 eV), and O-Ti (\(\approx\)529 eV) were bonded in the surface layer. The three O 1s signals confirmed the formation of the bisphosphonate monolayer on the SAMs-coated samples [34; 35]. Meanwhile the use of the C 1s signal for quantification purposes was not considered because it is difficult to accurately determine the uncertainty of C on the Ti surface [36]. Furthermore, [PERSON] et al. [37] indicated that the hydrophilic-hydrophobic balance of the investigated surface covered by various substances depends on the orientation of the surface molecules and density of the surface layer. For surface modification of the TiO surface, phosphonic acids of different chain lengths and terminal functional groups were chosen: (i) CH\({}_{3}\) as a  
Figure 7: The surface contact stiffness results of the investigated samples after nano-indentation (\({}^{*}\)\(p<0.05\)). hydrophobic and non-reactive group (HDPA-SAM/TiO and NDPA-SAM/TiO samples) and (ii) COOH as a hydrophilic and reactive group (PUA-SAM/TiO and PHA-SAM/TiO samples). Thus, their terminal functional groups (CH\({}_{3}\)/COOH) will determine the surface properties of the modified substrates. According to the results (Figure 6), surfaces with CH\({}_{3}\)-terminated SAMs were slightly hydrophobic and surfaces with COOH-terminated SAMs were more hydrophilic compared with the TiO surface. The results for these investigated SAMs were quite reasonable. However, surfaces with CH\({}_{3}\)-terminated SAMs should be more hydrophobic than the TiO surface if their SAMs are compact and homogeneous. In the case of stable, compact SAM where alkyl chains are connected via van der Waals interactions, only a terminal functional group is in direct contact with the solvent drop. Therefore, the presence of a COOH-terminated group on the compact and homogeneous SAM surface (PUA-SAM/TiO sample), which contributes to the reactive group of hydrophilic, thus enhances the surface wettability [26].