Orientation of fluorophenols on Si(111) by near edge x-ray absorption fine structure spectroscopy

The adsorption of phenol, 4-fluorophenol, and 2,3,4-trifluorophenol on the $\mathrm{Si}\left(111\right)7\times 7$ surface is investigated by near edge x-ray absorption fine structure spectroscopy (NEXAFS). A strong polarization dependence of the ${\pi }^{*}$ transitions is observed for fluorinated phenols, while phenol itself is more isotropic. A quantitative method is developed to convert polarization-dependent NEXAFS data into orientational information for such a situation. The model includes two angular degrees of freedom, the tilt angle $\gamma$ of the end group and its rotation angle $\beta$ around the phenyl-to-oxygen bond. The tilt is fixed by the geometry of the free molecule, while the rotation is characterized by an average angle ${\beta }_0$ with a Gaussian distribution $\sigma$ due to thermal fluctuations. The model is applicable to a variety of self-assembled monolayers with tailored end groups that are used in molecular electronics and biosensors.

Figure 1

Adsorption geometry of phenol and fluorophenols on Si(111) and definition of the angles involved in modeling it. $\beta$ is the angle of rotation of the aromatic ring around the phenyl ether bond $\mathrm{O}\mathrm{C}$. $\gamma$ is the angle of the $\mathrm{O}\mathrm{C}$ bond from the surface normal. $\theta$ is the angle of the electric field vector $\mathbf{E}$ with respect to the surface normal, i.e., the angle of incidence from grazing for the $p$-polarized geometry used here. $\phi$ is the azimuthal angle that defines the rotation of molecules around the surface normal. Our model assumes a fixed value of $\gamma$ (taken from the bond angle of a free molecule) and calculates the $\theta$ dependence of the NEXAFS signal for a Gaussian distribution of $\beta$ values around ${\beta }_0$, which is also taken from the literature. This model is applicable to a more general class of self-assembled monolayers with aromatic end groups (Ref. 8).

Figure 2

C $1s$ NEXAFS spectra of saturation coverage of phenol, 4-fluorophenol, and 2,3,4-trifluorophenol on $\mathrm{Si}\left(111\right)7\times 7$ at normal incidence $(\theta =90°)$. The structures correspond to transitions from the C $1s$ core level to empty ${\pi }^{*}$ and ${\sigma }^{*}$ orbitals. Peaks 1 and 2 are assigned to the ${\pi }_1^{*}$ orbitals of aromatic carbon in $\mathrm{C}\mathrm{H}$ and $\mathrm{C}\mathrm{O}$, $\mathrm{C}\mathrm{F}$ groups, peaks 3 and 4 to the ${\pi }_2^{*}$ orbitals of the same groups.

Figure 3

C $1s$ NEXAFS intensity of peak 1 in Fig. 2 versus exposure for 4-fluorophenol on $\mathrm{Si}\left(111\right)7\times 7$. The line represents an approximate model with a constant initial sticking coefficient for each open adsorption site. The sticking coefficient decreases when approaching monolayer coverage.

Figure 4

Polarization dependence of the C $1s$ NEXAFS spectra of phenol and 2,3,4-trifluorophenol on $\mathrm{Si}\left(111\right)7\times 7$ at saturation coverage. Fluorophenols exhibit strong anisotropy of the ${\pi }^{*}$ orbitals, while phenol is more isotropic. This is explained by electrostatic forces leading to an orientation of the F-induced dipoles, as well as by different orientation angles ${\beta }_0$. The spectra are modeled in Fig. 6.

Figure 5

Effect of the parameters ${\beta }_0$ and $\sigma$ on the polarization dependence of the NEXAFS intensity, obtained from Eq. (13). Simulated NEXAFS spectra series for (a) ${\beta }_0=0°$ and several $\sigma$, (b) ${\beta }_0=90°$ and several $\sigma$, (c) $\sigma =0°$ and several ${\beta }_0$. (d) shows the Gaussian distribution of $\beta$ for ${\beta }_0=0°$ and the $\sigma$ values used in (a)–(c).

Figure 6

Polarization dependence of the C $1s$ to ${\pi }_1^{*}$ transitions for phenol and 2,3,4-trifluorophenol (peaks 2 in Fig. 2). $\theta$ is the angle of the electric field vector from the surface normal (i.e., the angle of incidence from grazing). The data points are modeled using Eqs. (11, 12, 13), using an average rotation angle ${\beta }_0=0°$ for phenol (Refs. 29, 31) and ${\beta }_0=30°$ for 2,3,4-trifluorophenol (Ref. 32). The width $\sigma$ of the $\beta$ distribution is used as fit parameter. The value of $\gamma$ is fixed at 70° for phenol and 78.1° for 2,3,4-trifluorophenol using the bond angle of the free molecules. The resulting standard deviation $\sigma$ is 27° for phenol and 24° for 2,3,4-trifluorophenol, corresponding to a full width at half maximum (FWHM) of 64° and 55°.