Vinyl-terminated SAMs

As previously stated, the initial strategy focused on the deposition of two alkylsilane based SAMs with a vinyl group as a functional handle and assessed their viability as precursors for further surface chemistry. Vinyl-terminated monolayers have shown promise as model systems for study of the interactions of atmospheric oxidants with boundary layer surfaces.[127] _{In the present study}

two different SAMs were deposited on Si/SiO2 surfaces from the solution phase and assessed.

The self-assembling molecules both contain the same vinyl terminal group and -SiCl3 reactive

head group but varied in alkyl chain length (Figure 43).

Figure 43: Deposition of vinyl terminated SAMs from precursor molecules 76 and 86. 2.4.1 X-Ray Photoelectron Spectroscopy

The XPS spectra of each of the vinyl terminated SAMs in this study showed the presence of Si (2s 151.6 eV, 2p 99.6 eV), C (1s 284.6 eV) and O (1s 532.7 eV) as expected. A typical survey scan of vinyl terminated SAM 96 is shown in Figure 44. There were no unexpected elements detected from XPS scans of the surface such as unreacted silane (no Cl signal was detected). The data obtained for C 1s, Si 2s and Si 2p was consistent with monolayer films in the literature.

35

Figure 44: XPS survey scan of vinyl terminated SAM 96.

High resolution scans of each element were taken to reveal their elemental composition. An example single region scan of the C 1s of SAMs 97 is shown in Figure 45. As photoelectrons are lost during the photoemission process a positive charge builds up on the sample, known as sample charging. As this happens, the kinetic energy of the emitted photoelectrons decrease in energy resulting in a shift to a higher binding energy. All analysed samples were shifted and corrected for sample charging. The main C 1s aliphatic peak in SAM 97 appears at a binding energy of 284.6 eV. On the majority of samples a shoulder peak was fitted at a higher binding energy (286.4 eV) to the main C 1s peak. Based on the polarity of the double bond it is possible that this is the more electropositive carbon in the olefin moiety which shifts its overall binding energy higher in the XPS spectrum. It is difficult to definitively detect the different chemical states of carbon in alkene moieties using XPS and as such the shoulder peak could not be conclusively assigned.[128- 130]

36

Figure 45: XPS C 1s high resolution scan of vinyl terminated SAM 97. 2.4.2 Contact Angle

The vinyl terminated SAMs 96 and 97 gave similar water contact angle values within several degrees of each other which is expected given the idetical surface termination. Experimental and literature values are shown in Table 1. Reported values were measured directly after placing drops of water onto modified silicon samples. These results are consistent with values reported for similar films.[131]

Water Contact Angle (°)

Surfactant Liquid deposition Literature

C11-vinyl 96 97±1.5 96±1.4

C15-vinyl 97 101±1 101±1.0 [115, 132]

Table 1: Comparison of experimental and literature values

2.4.3 Atomic Force Microscopy

Atomic Force Microscopy was used to determine the quality of the deposited SAMs as well as the overall topography of the surface. AFM is also useful for determining whether any aggregations from the self-assembly process are present on the surface. A typical AFM image of a vinyl terminated SAM 97 is shown in Figure 46. The AFM image shows a relatively smooth surface with a root mean squared (RMS) value of ~96 pm averaged over a 1 µm ×1 µm area for the solution phase deposited SAM of 97. In this case the RMS value gives an estimate of how homogeneous the resulting surface is after SAM deposition. The majority of vinyl terminated

37 surfaces imaged gave smooth surfaces with RMS roughness values in the range of 48-250 pm which agrees with the literature. For comparison, a clean Si/SiO2 surface has an RMS of ~50 pm.

These results, coupled with results from the other surface analysis techniques, suggest the presence of a dense, well-ordered and homogeneous SAM on the surface.

Figure 46: AFM image of 1 µm ×1 µm area of C15-vinyl SAM 97.

2.4.4 Reactivity of vinyl terminated SAM

Pd-mediated cross-coupling reactions are a valuable method for the construction of C-C bonds. In particular the Mizoroki-Heck reaction[133] _{is a useful method for the construction of substituted}

alkenes and has widespread use in organic synthesis[134] _{as well as on solid supports.}[135] _Mizoroki-

Heck reactions have been reported on a variety of functionalised silicon surfaces[98][136-137] _{and as}

such seemed a suitable candidate to test the reactivity of the vinyl group on the surface. Vinyl terminated SAM 97 was subjected to Heck conditions using p-bromophenylacetic acid 98 as a coupling partner (Figure 47). Reaction at 125 °C for 2 h gave the corresponding phenylacetic acid on the surface as confirmed by XPS and contact angle.

38 High resolution scans of the C 1s region of the XPS spectrum show the appearance of a peak at 288.6 eV and this is attributed to the C in the C=O of the newly attached phenylacetic acid component(Figure 48)

Figure 48: High resolution XPS scans of C 1s region of phenylacetic acid terminated SAM 99.

The contact angle of the sample was also measured, with a significant decrease, from 97 to 55 °, attributed to the change in hydrophilicity of the monolayer. This, coupled with XPS results, suggested successful attachment of the phenylacetic acid unit thus confirming the reactivity of the vinyl group in the monolayer.