Plasmon spectroscopy: Robust metallicity of Au wires on Si(557) upon oxidation

We investigated initial steps of oxidation of the Si(557)-Au system by plasmon spectroscopy and first-principles calculations. The measurements, performed using an electron energy loss instrument with simultaneous high resolution in energy and momentum, reveal that metallicity is preserved under all oxidation conditions that are experimentally accessible in UHV. Corresponding simulations, performed within density functional theory, confirm this finding: Only the oxidation of the Si environment of the Au chains turned out to be strongly exothermic, with similar binding energy for adsorption on different structural elements. While large and site specific changes of the band structure were observed, the upper edge of the excitation spectrum of electron-hole pairs, to which plasmon dispersion is most sensitive, remains almost unchanged during the various steps of oxidation, due to the opposite and largely compensating contributions of different adsorption configurations. This investigation not only proves the robustness of metallicity of the gold chains upon oxidation of the surrounding environment of Si atoms, but also demonstrates the usefulness of plasmon spectroscopy in characterizing the electronic excitation spectrum of quasi-one-dimensional systems and unoccupied band structure.

Figure 1

SPA-LEED patterns of Si(557)-Au surface before (left) and after (right) adsorption of 15 L of oxygen (1 L = $1\times {10}^{-6}$ mbar s): The overlayer curves depict the 1D-line scans along the [$1\overline{1}0$] direction.

Figure 2

EEL spectra of the Si(557)-Au system for $k$ vectors parallel to wire direction. Left-hand side: Clean surfaces. Right-hand side: After adsorption of 7.5 L of ${\mathrm{O}}_2$. In both cases, a dispersing loss is only seen in the $\left[1\overline{1}0\right]$ direction parallel to the steps. The black lines in the second spectra from bottom are fits according to the procedure described in Ref. [30].

Figure 3

Plasmon dispersion of the clean Si(557)-Au system ($\diamond$) and after various doses of oxygen, as indicated. The green dashed line corresponds to the fit of Ref. [25] with harmonic confinement (width 4 Å) of a 2D nearly free electron gas. Solid lines are guides to the eye with the assumption that the curves have to start at $E=0$ in the long wavelength limit.

Figure 4

Geometrical models of the oxidized Si(557)-Au system. (a)–(c) Oxygen adsorbed at the Si-adatom, adatom and honeycomb chain, and between honeycomb and Au chain, respectively. Yellow: gold chain, gray: topmost Si atom layer, dark gray: Si honeycomb chain (upper edge corresponds to step edge), blue: Si adatom row, green: Si rest atom row, red: oxygen atoms.

Figure 5

Band structures calculated in an extended zone scheme along $\overline{\mathrm{\Gamma }Y}$ ($\overline{Y}$ at 0.82 Å${}^{-1}$) according to the structural models represented in Figs. 4–4, respectively. The black dashed lines below the Fermi level ($E_F$, energy zero) correspond to the assumed $\hslash {\omega }_{-}$. Above $E_F \hslash {\omega }_{+}$ is plotted (dashed-dotted line, green) which was calculated from the experimental plasmon dispersion and $\hslash {\omega }_{-}$ according to Eq. (1), as described in the text. The black dashed line in (a) indicates the analogous result for the clean system.

Figure 6

Modifications of plasmon dispersion, as predicted from band structure calculations and the theory according to Eq. (1) after local adsorption of atomic oxygen at the various sites indicated in the legend. The dashed line represents the pristine Si(557)-Au surface.