Characterization of hydroxyl groups on water-reacted $\mathrm{Si}\left(001\right)\text{−}2\times 1$ using synchrotron radiation O $1s$ core-level spectroscopies and core-excited state density-functional calculations

The system constituted by the $\mathrm{Si}\left(001\right)\text{−}2\times 1$ surface exposed to water molecules at room temperature has been chosen to single out the electron structure of the hydroxyl SiOH, a surface species playing an important role in many technologically relevant processes. We confront here original core-electron spectroscopy density functional theory (DFT) calculations to synchrotron radiation O $1s$ x-ray photoelectron spectroscopy (XPS) and (polarization-dependent) near-edge x-ray absorption spectroscopy (NEXAFS) data. Using clusters to mimic the silicon surface, various SiOH environments (unpaired and paired hydroxyls) have been examined. The impact of the hydrogen bond on the calculated core-ionized and/or neutral core-excited states is examined, and compared to the limit case of the water dimer. The theoretical approach enables us to label the main experimental NEXAFS transitions and to interpret their polarization-dependent dichroism. As water dissociation on the surface can go beyond the formation of hydroxyls, the DFT electron structure of bridging oxygens (SiOSi) is calculated. It is predicted that the XPS line associated to the latter species is shifted by $0.5–1.0\mathrm{eV}$ to lower binding energy with respect to that of the hydroxyls. Then, on this basis, the reconstruction of the experimental O $1s$ XPS spectrum can provide the relative distribution of hydroxyls and of the bridging oxygens (a minority species). The present work paves the way for future spectroscopic works examining the bonding and reactiveness of hydroxyls with other molecular species.

Figure 1

Experimental Si $2p_{3∕2}$ photoemission spectra of the two-domain Si(001) surface before and after exposure to water at room temperature $\left(1.5\mathrm{L}\right)$. The photon energy $h\nu$ is $145\mathrm{eV}$. ODA is the acronym for “outer dimer atom.”

Figure 2

Experimental valence band spectrum of the two-domain Si(001) surface before and after exposure to water at room temperature $\left(1.5\mathrm{L}\right)$. The photon energy $h\nu$ is $145\mathrm{eV}$. Note the quenching of the “surface states” (at a binding energy of $0.9\mathrm{eV}$) after water dosing.

Figure 3

Experimental O $1s$ XPS spectrum (dots) of the two-domain Si(001) exposed to water $\left(1.5\mathrm{L}\right)$ at room temperature. The photon energy $h\nu$ is $660\mathrm{eV}$. The best fit (solid line) is also given.

Figure 4

Normalized experimental NEXAFS spectra of the two-domain Si(001) surface exposed to water $\left(1.5\mathrm{L}\right)$ at room temperature (measurement at room temperature). The electric field $\mathbf{E}$ is oriented either nearly normal (dotted line) or parallel (solid line) to the two-domain silicon surface. Inset: the measurements are performed while the sample temperature is kept at $150\mathrm{K}$.

Figure 5

(Color online) The unpaired hydroxyl [single-dimer cluster $\left({\mathrm{Si}}_9{\mathrm{H}}_{12}\right)$ plus (H,OH)]. Two minimum energy conformers are calculated, gauche $({\alpha }_{\mathrm{Si}\text{−}\mathrm{Si}\text{−}\mathrm{O}\text{−}\mathrm{H}}=60°)$ and trans $({\alpha }_{\mathrm{Si}\text{−}\mathrm{Si}\text{−}\mathrm{O}\text{−}\mathrm{H}}=180°)$.

Figure 6

Simulation of the O 1s NEXAFS spectra for the unpaired hydroxyl [$\left({\mathrm{Si}}_9{\mathrm{H}}_{12}\right)$ cluster] using the ${\left(1s\right)}^1{\left(L1\right)}^{1∕2}{\left(L2\right)}^{1∕2}$ FCH-UMOs potential (the spectra are rescaled energetically using the $\Delta \mathrm{KS}$ NEXAFS energies given in Table ). Curve (a) corresponds to the ${\alpha }_{\mathrm{Si}\text{−}\mathrm{Si}\text{−}\mathrm{O}\text{−}\mathrm{H}}=60°$ conformer, curve (b) to the ${\alpha }_{\mathrm{Si}\text{−}\mathrm{Si}\text{−}\mathrm{O}\text{−}\mathrm{H}}=180°$ conformer. Curve (c) is the weighted sum of the (a) and (b) curves assuming a gauche:trans distribution of 60:40 (see text). The fourfold symmetry of the two-domain silicon surface is taken into account. The thick (thin) line corresponds to $\mathbf{E}$ parallel (perpendicular) to the silicon surface.

Figure 7

(Color online) Three-dimer cluster $\left({\mathrm{Si}}_{21}{\mathrm{H}}_{20}\right)$ plus 3(H,OH). The oxygens are labeled $D$ (“donor”), $A$ (“acceptor”), and $F$ (“free”). A H bond is established between $\mathrm{O}\left(D\right)$ and $\mathrm{O}\left(A\right)$. On the other hand, no H bond forms between $\mathrm{O}\left(A\right)$ and $\mathrm{O}\left(F\right)$. The H bond length [$\mathrm{H}\left(D\right)\text{−}\mathrm{O}\left(A\right)$ distance] and $\mathrm{O}\left(D\right)\text{−}\mathrm{O}\left(A\right)$ distance depend on the fact that the basis sets are supplemented by diffuse functions or not (see Table ).

Figure 8

(Color online) Core-excited donor OH $\left(D^{*}\right)$. Contour plots of KS molecular orbitals $(L1,L2,L3)$ to which the electron is promoted.

Figure 9

(Color online) Core-excited acceptor OH $\left(A^{*}\right)$. Contour plots of KS molecular orbitals $(L1,L2,L3)$ to which the electron is promoted.

Figure 10

Simulation of the O $1s$ NEXAFS spectra for the donor $\left(D\right)$ species [curve (a)] and acceptor $\left(A\right)$ species [curve (b)] using the optimized geometry of the “${\mathrm{Si}}_{21}{\mathrm{H}}_{20}$ plus 3(H,OH)” cluster. Curve (c) corresponds to the $AD$ pair, that is the average of curve (a) and (b). The ${\left(1s\right)}^1{\left(L1\right)}^{1∕2}{\left(L2\right)}^{1∕2}$ FCH-UMOs potential is used. The fourfold symmetry of a two-domain silicon surface is taken into account. The thick (thin) line corresponds to $\mathbf{E}$ parallel (perpendicular) to the silicon surface.

Figure 11

(Color online) Three dimer cluster ${\mathrm{Si}}_{21}{\mathrm{H}}_{20}$ plus O,2H. The oxygen atom is inserted in the central silicon dimer. Note that the nonreacted dimers remain buckled.

Figure 12

(Color online) Core-excited inserted oxygen (H-Si-O-Si-H) calculated with a ${\mathrm{Si}}_{21}{\mathrm{H}}_{20}$ cluster. Contour plots of KS molecular orbitals $(L{1}^{\prime },L{2}^{\prime })$ to which the electron is promoted. Note that $L{1}^{\prime }$ is the LUMO.

Figure 13

Simulation of the O $1s$ NEXAFS spectra for the inserted oxygen species (H-Si-O-Si-H) using the optimized geometry of the “${\mathrm{Si}}_{21}{\mathrm{H}}_{20}$ plus (2H, O)” cluster. The ${\left(1s\right)}^1{\left(L{1}^{\prime }\right)}^{1∕}{\left(L{2}^{\prime }\right)}^{1∕2}$ FCH-UMOs potential is used. The fourfold symmetry of a two-domain silicon surface is taken into account. The thick (thin) line corresponds to $\mathbf{E}$ parallel (perpendicular) to the silicon surface.