Self-Assembled Monolayers: Effects of Surface Nanostructure on Wetting Part 2 (Nanotechnology)

Formation of Self-Assembled Monolayers on Annealed Gold

Fig. 4 shows the low-rate dynamic contact angle results of water on SAMs of octadecanethiol adsorbed onto a thermally annealed gold substrate. Similar to the experimental results in Fig. 2, as V increases at the beginning, 0 increases at constant R. This is because of the fact that such contact angles are not truly advancing angles and that it takes time for the drop front to advance. As V increases further, the three-phase contact radius R moves and the contact angle remains rather constant. Averaging the contact angle yields a mean value of 107.0°. We see that this water contact angle value (107.0°) is significantly lower than that shown in Fig. 2 (118.2°) on the evaporated (nonannealed) gold. We also note that the contact angle obtained here (107.0°) for the CH3(CH2)17S or thermally annealed gold is similar to those obtained on the hexa-triacontane (cf. Table 2) and paraffin. This result agrees with the expectation that a monolayer surface exposing predominately methyl groups should have a similar solid surface tension, and hence contact angle, as those of hexatriacontane and paraffin surfaces. Low-rate dynamic contact angle measurements for the remaining five liquids were performed and found to be also constant. These angles, together with the receding angles, are summarized in Table 3.

Fig. 4 Low-rate dynamic contact angles of water on SAMs of octadecanethiol CH3(CH2)i7SH adsorbed onto annealed gold.

Table 3 Experimental advancing and receding contact angles on SAMs of octadecanethiol CH3(CH2)17SH adsorbed onto evaporated (nonannealed) and annealed gold

It is of interest to plot these contact angles in Fig. 5 together with those on the hexatriacontane surface. We see that the apparent scatter in Fig. 3 has disappeared in Fig. 5 using the contact angles on the annealed samples.

Fig. 5 (a) The solid-liquid work of adhesion Wsl; (b) the cosine of the contact angle cos 0; and (c) the liquid vapor surface tension times the cosine of the contact angle glv cos 0 vs. the liquid-vapor surface tension glv for hexatriacontane (o) and SAMs of octadecanethiol CH3(CH2)17SH adsorbed onto thermally annealed Au (•).

The resulting Wsl, cos 0, and glv cos 0 vs. glv curves are quite smooth and similar to those shown in Fig. 1. We conclude that the experimental results in Fig. 5 are compatible with the functional relationships in Eqs. 4-6. The origin of the difference in experimental patterns between the nonannealed and annealed gold will be discussed below.

Characterizations by Ellipsometry, Fourier Transform Infrared Spectroscopy, and Atomic Force Microscopy

The ellipsometer thickness for octadecanethiol CH3 (CH2)17SH adsorbed onto annealed gold was consistently 21 A, whereas that formed on the nonannealed Au varied between 20 and 21 A. Although this difference is not statistically significant, we noticed that the experimental tan C and cos A for the nonannealed Au samples did not always match those of the theoretical curves. Nevertheless, our thicknesses are consistent with those reported in the literature.[82]

The reflectance spectra for SAMs derived from octadecanethiol on Au and annealed Au are shown in Fig. 6. In both spectra of Fig. 6, the asymmetrical methylene peaks appeared at ~ 2918 cm-1. This indicates a primarily trans-zigzag extended hydrocarbon chain with few gauche conformers. Both spectra demonstrate that SAMs of octadecanethiol adsorbed onto Au and annealed Au are highly crystalline. However, the intensities of the methylene peaks are larger on Au and smaller on the annealed Au. The difference in the peak intensity could reflect different canted orientations for the polymethylene chains on these surfaces, or different amounts of the polymethylene chains that the IR detected. Because the tilt of the chain on the Au substrate for alkanethiolate SAMs is known to be ~ 30° and this structural orientation is unlikely to be changed by annealing, we speculate that the difference in the intensity of the asymmetrical methylene peaks appears at ~ 2918 and 2850 cm-1, indicating different amounts of polymethylene chains that were detected by the IR. The spectrum for the adsorbed layer of octadecanethiol on nonannealed Au exhibits a higher dichroic ratio (~ 2) for the methylene adsorption modes [va(CH2)/vs(CH2)], and that on the annealed Au exhibits a much lower dichroic ratio (~ 1.3). We also note the lower asymmetrical methyl intensity va(CH3) at 2964 (asym) cm-1 and the higher symmetrical methyl intensity vs(CH3) at 2879 (sym) cm-1 for SAMs on annealed Au in Fig. 6, suggesting that the methyl groups are oriented more toward the surface normal. These features in the spectra provide evidence that SAMs of octadecanethiol on nonannealed Au have a structure that is not the same as that on the annealed Au. Independent AFM images shown in Fig. 7 suggest that the annealed Au has larger terraces (as much as 200 nm), whereas that of the nonannealed Au has much smaller gold steps. From the interpretation of the above IR and AFM results, we constructed a model in Fig. 8 that illustrates a possible arrangement of octadec-anethiol adsorbed onto nonannealed and annealed Au. From the schematic, it is expected that the reflectance IR would detect more methylenes per unit projected area on the nonannealed Au than that for the annealed Au. This is because of the polycrystallinity nature of nonannealed Au that causes variations of the methyl and methylene groups exposed to water. The schematic also supports the IR results for lower asymmetrical methyl intensity and higher symmetrical methyl intensity for SAMs on the annealed Au, as the methyl groups are oriented more toward the surface normal that those on the nonannealed Au.

Fig. 6 Grazing incidence polarized infrared spectra for SAMs of octadecanethiol CH3(CH2)i7SH adsorbed onto evaporated (nonannealed) and annealed gold. The approximate positions of the methylene modes are 2918 (asym) and 2850 (sym) cm- \ and those for the methyl modes are 2964 (asym), 2935 (sym, Fermi resonance), and 2879 (sym) cm- \ The spectra have been offset vertically for clarity.

Fig. 7 Atomic force microscopy images of (a) annealed Au and (b) nonannealed Au for a scan size of 1 mm

Fig. 8 Schematic illustration of SAM assembly on two different Au substrates. The upper figure demonstrates the SAM assembly of octadecanethiol adsorbed onto nonannealed Au with smaller gold steps. The lower figure illustrates the SAM assembly of octadecanethiol adsorbed onto annealed Au with larger terraces.

Wetting Interpretation in Terms of Solid Surface Tensions

Contact angle interpretation requires extreme experimental care to ensure that all of the commonly accepted assumptions are not violated. It is apparent in Table 3 that, in general, the contact angle hysteresis H = 0a — 0r decreases for the annealed gold substrate, suggesting better surface quality. The slightly larger errors for the contact angle data of octadecanethiol adsorbed onto annealed Au were because of the variation of our annealing procedures. From the AFM results above, we conclude that the nonannealed surface consists of smaller gold steps, whereas that of the annealed Au has larger terraces and less defects, as illustrated schematically in Fig. 8. In the case of the nonannealed samples, water could ”see” deeper of the surface layer (the methylenes) under the methyl groups because of the relatively less packed monolayers as a result of smaller gold steps or defects. This allows the formation of additional intermolecular interactions between water and the methylenes in the solid-liquid interface. In this case, the surface no longer consists of predominantly methyl groups, but a mixture of methyl and methylene groups in the solid-liquid interfa-cial region. These molecular interactions with methylenes would result in a higher, additional, interfacial tension between the solid-liquid interface Agsl, and is illustrated schematically in Fig. 9. This additional effect is believed to have less influence on the solid-vapor interface because of the presence of the relatively less dense water vapor on the solid-vapor interface. Assuming that gsv is roughly constant for both annealed and nonannealed surfaces, an increase in gsl for the latter would cause the contact angle to increase from that of a surface where water can only ”see” most of the outer methyl groups and less methylene groups. Thus it appears that the change in the contact angle is a result of additional and unexpected formation of intermolecular interactions in the solid-liquid interface for less packed monolayers. The magnitude would depend, of course, on the intermolecular strength, polarity of the liquid, and structures of the monolayers. In this case, even though we apparently fix our substrate, the intermolecular interactions would not be the same for a given pair of liquid and solid, depending on the surface defect, structure, polycrystallinity, and arrangement of monolayers. In Table 3, we also note that the advancing contact angles for the annealed and nonannealed surfaces are nearly identical and did not change very much for both ethylene glycol and hexadecane. However, the molecular interactions between hexadecane/CH3(CH2)17SH and ethylene glycol/CH3(CH2)i7SH on two different Au surfaces are not clearly known. We conclude that, whatever these interactions are, they appear to be insensitive to the structural changes and surface defects of the monolayers. If the intermolecular interactions change drastically as we change the substrate’s surface preparation for a given liquid, this violates our expectation of constant solid properties along the curves shown in Figs. 1, 3, and 5. Interpretation of such angles on the nonannealed ones would be difficult.[90-92] Because the annealed surfaces contain less defects, additional variation of the solid-liquid interfacial interactions Agsl would be minimal. We conclude that the surface structures of the underlying gold on which the monolayers of octadecanethiol are formed can affect directly the interpretation of contact angles. Only when a fundamental framework (such as that shown in Fig. 1 or Fig. 5) has been established can the variation of the interfacial interactions from liquid to liquid as an additional effect be studied more systematically. With the above stipulation, the variation of gsl as a result of surface structural change can be estimated if we assume gsv for the nonannealed and annealed Au to be the same. Therefore a Agsl of 12.9 mJ/m2 can be estimated by taking the difference in the water advancing angles, Agsl = [72.7(cos 107 — cos 118)], using Young’s equation. This value represents an increase in gsl because of the variation of surface structures of octadecanethiol SAMs adsorbed onto Au when interacting with water. However, conventional thinking would have interpreted the water contact angle of 118° for octadecanethiol SAMs on Au to have a much lower solid surface tension similar to that of fluorocarbon (gsv = ~ 12 mJ/m2), rather than a gsv of 1920 mJ/m2 for methyl-terminated surfaces.

Fig. 9 Schematic illustration of how the variation of surface structures affects the solid-liquid interfacial tension gsl for SAMs of octadecanethiol on nonannealed and annealed Au. Agsl in the upper figure represents the increase in the solid-liquid interfacial tension from that of an annealed Au. Given that the liquid vapor interfacial tension glv and the solid-vapor interfacial tension gsv remain unchanged, an increase in Ags results in a higher contract angle 0j than that of the annealed Au, 02.

CONCLUSION

We conclude that the surface defect, structure, and crystallinity of SAMs of octadecanethiol adsorbed onto gold can affect the interpretation of contact angles in terms of solid surface tensions. The contact angle and adhesion patterns of various liquids for SAMs of octa-decanethiol adsorbed onto annealed gold are consistent with recent experimental findings on the relatively thick polymer-coated surfaces. We also found that interpretation of solid surface tensions using contact angle data on less well-prepared polycrystalline gold surfaces can be misleading. The variation of surface structures in terms of surface energetics can be estimated only when a fundamental understanding of contact angles and surface tensions is achieved.