## 3 Results and Discussion  
### Characterization of COOH-APTMS SAM  
An amino-terminated APTMS-SAM is the basis for carboxyl modification in the preparation of a COOH-APTMS structure. Therefore, the first step of this study was to verify whether the APTMS-SAM system was properly formed. For this purpose, the water contact angles and the XPS data of the APTMS-SAM specimens (Figure 1a) were analysed. According to the results, the water contact angle of the APTMS-SAM was 64.6\({}^{\circ}\)\(\pm\) 2\({}^{\circ}\), and the binding energies of the N1s peaks were 399.7 eV and 401.7 eV. These values were in good agreement with those reported in the literature, [28; 34; 35] which indicated the successful formation of the APTMS-SAM structure.  
Figure 2 displays the SEM images of the COOH-APTMS/SiO\({}_{2}\)/Si samples at different modification times. At the reaction times of 1 h and 2 h, the unreacted areas were observed on the sample surface (Figure 2a,b) because of the insufficient dehydration condensation reaction between the -NH\({}_{2}\) and the -COOH groups. As the reaction time was increased to 3 h, the unreacted areas disappeared, and no obvious particles existed on the surface (Figure 2c). However, after 4 h of the reaction, particles emerged on the surface of the samples (Figure 2d). Those were the partially unreacted trimesi acid residues that remained on the sample surface, while the -NH\({}_{2}\) binding sites on the surface were completely reacted. Thus, it could be concluded from the SEM images that the COOH-APTMS SAM structure was successfully assembled, and no particles were detected on the film surface after 3 h of the reaction due to the complete reaction between the APTMS and the trimesi acid.  
Figure 2: SEM images of COOH-APTMS/SiO\({}_{2}\)/Si with different modification times: (a) 1 h; (b) 2 h; (c) 3 h; (d) 4 h.  
Figure 1: XPS patterns of the samples: (a) N1s peak of APTMS/SiO\({}_{2}\)/Si; (b) C1s peak of APTMS/SiO\({}_{2}\)/Si; (c) C1s peak of COOH-APTMS/SiO\({}_{2}\)/Si.  
Figure 3 depicts the AFM images of the COOH-APTMS/SiO\({}_{2}\)/Si systems at different modification times. From these images, the surface roughness (RMS) for each COOH-APTMS SAM sample was found to be 0.647 nm, 0.907 nm, 1.211 nm, and 1.272 nm, corresponding to the modification times of 1 h, 2 h, 3 h, and 4 h, respectively. Obviously, the RMS value of the COOH-APTMS SAM increased with the increase in the reaction time. The increase in RMS was most pronounced between 1 h and 3 h of the reaction, whereas it was slower after 3 h of modification. In the first case, the remarkable increase in RMS could be explained by the introduction of benzene and carboxyl groups and the extension of the COOH-APTMS chains during the modification process [23; 32]. However, the reaction between the APTMS and the trimesic acid after more than 3 h was complete, resulting in a slight change in RMS from 1.211 nm to 1.272 nm due to the accumulation of trimesic acid molecules on the sample surface, which could be observed in the SEM image (Figure 2d).  
Figure 1 displays the XPS patterns of the samples. In Figure 1c, the O=C-N-related peak was observed at the binding energy of 287.9 eV. This peak indicated that the carboxyl groups of the trimellitic acid were dehydrated and condensed with the amino groups of the APTMS to form O=C-N bonds [32; 36]. Compared to Figure 1b, it could be proved that the trimellitic acid was successfully grafted onto the APTMS surface. In addition, the surface water contact angle of the COOH-APTMS was 35.5\({}^{\circ}\)\(\pm\) 2\({}^{\circ}\), which further confirmed the successful preparation of the COOH-APTMS structure [37].