# The Effect of Adsorbed Volatile Organic Compounds on an Ultrathin Water Film Measurement
## 3 Results and Discussion
### The Effect of Adsorption of VOCs on SPR Reflectance
For a two-layer (overlayer and substrate) structure where the overlayer does not produce electrons with the binding energy of interest, i.e., the overlayer does not produce photoelectrons with the binding energy of Au 4f, the thickness (\(d_{over}\)) of the adsorbed VOCs on the gold surface can be estimated from XPS data [25]:  
\[I_{XPS}\;=\;I_{XPS,\text{ sub}}\exp(\frac{-d_{over}}{\lambda\cos\theta}) \tag{5}\]  
Figure 5: _Cont._  
For a thick substrate (\(d_{sub}>>\lambda\)), \(I_{XPS,\,sub}\) can be considered as \(I_{\infty}\), the intensity of a thick sublayer without an overlayer. This is the case for our surface as the 50 nm thick gold layer is much greater than the electron attenuation length of Au 4f photoelectrons (3 nm) through gold. \(I_{\infty}\) is usually measured directly. Normally, for a metal substrate such as gold, the surface is argon sputtered in the XPS chamber to get the signal without the overlayer. However, our samples were not argon sputtered in the XPS chamber and they had a small overlayer at the beginning. Argon sputtering was not been implemented as it would likely have affected the surface morphology and homogeneity of the surface [26] (for studying the mechanism of dropwise condensation, the surface needs to be smooth). The initial overlayer in the first measurement can be seen by the presence of C 1s in the broad range scan of samples (see Figure 3). Therefore, our thickness measurement was an estimate of the additional overlayer thickness formed after the initial measurement and not the actual overlayer thickness. By changing the order of Equation (1):  
\[d_{over}=-\ln\,(\frac{I_{XPS}}{I_{XPS,\,0}})\,\lambda\cos\theta \tag{6}\]  
where \(I_{XPS,\,0}\) is the signal associated with Au 4f generated from the substrate layer during the first measurement (t = 0 h), and the collection angle relative to the surface (\(\theta\)) is 45\({}^{\circ}\). The thickness of the additional overlayer was measured by assuming the adsorbed layer formed a uniform layer on the surface. Figure 5c presents the trend of thickness variations in one month for sample 2; the thickness exponentially increased in the first 17 h (from 0 to 0.24 nm), then it dropped slightly at 41 h (from 0.24 nm to 0.22 nm); finally, it increased from 0.22 nm to 0.68 nm at 893 h. As these results are based on a single XPS measurement, we cannot present the error of measurement. The increasing trend in the thickness of adsorbed VOCs means the initial drop in the SPR reflectance was due to a drastic decrease in the refractive index, i.e., the chemistry of the adsorbed VOCs on the plasma-cleaned sample initially changed from compounds with a higher refractive index to compounds with a lower refractive index. As these thickness measurements were estimated, we cannot precisely estimate the changes in the refractive index of the adsorbed layer. It is worth noting that a quantitative calculation of the adsorbed layer's refractive index was not the goal of this work. Therefore, we only mention this to note the trend of changes in the adsorbed VOCs for sample 2. By tracking the atomic percentage of C and O in the XPS data of sample 2, we can understand how the amount of C-O (compared to C-C (C-H) and C=O) in a high-resolution carbon spectrum reduced on the surface for the first 41 h and then  
Figure 5: SPRi results of (**a**) sample 2 and (**b**) sample 4 (the inner plots show the angle-scanning result and the outer curve shows the effect of adsorbed VOCs on the SPR signal at 44.8\({}^{\circ}\)), and (**c**) XPS result of sample 2 shows the trend of adsorbed VOCs layer thickness variation. increased. Further, XPS data indicated that the peak binding energy of O-C (in high-resolution oxygen spectrum) changed from 533.08 eV to 532.6 eV in 41 h and bounced back to 532.93 eV in the one-month period. The O-C peak shift and atomic percentage can be deciphered as adsorbed VOCs changing from aromatic hydrocarbons to aliphatic hydrocarbons in the first 41 h after exposure.