Aluminum-induced surface clock reconstruction of Pd(001) and the effect of oxygen adsorption

Abstract

The structure of the $\mathrm{Pd}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}2)p4g\ensuremath{-}\mathrm{Al}$ surface has been investigated using low-energy ion scattering, low-energy electron diffraction, and three-dimensional classical scattering simulations. The thermal treatment of the Al films $({\ensuremath{\theta}}_{\mathrm{Al}}>~0.5\mathrm{M}\mathrm{L})$ triggered the Al diffusion and reaction to form a clock rotated (001) Pd layer above an ordered $c(2\ifmmode\times\else\texttimes\fi{}2)$ Al-Pd underlayer, with a stoichiometry of the top two layers independent of the initial Al coverage. By using a reliability $R$-factor analysis to compare the experimental and simulated azimuthal scans, the lateral clockwise-counterclockwise displacement of the surface Pd atoms was determined to be $\ensuremath{\Delta}x=0.5\ifmmode\pm\else\textpm\fi{}0.1$ \AA{}. The driving force for this clock reconstruction is proposed to be the Al-induced interfacial strain. We have also studied the oxygen-induced lifting of the $\mathrm{Pd}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}2)p4g\ensuremath{-}\mathrm{Al}$ reconstruction. Adsorption of oxygen on the $(2\ifmmode\times\else\texttimes\fi{}2)p4g$ surface at room temperature induces Al segregation and lifts the reconstruction to yield the (1\ifmmode\times\else\texttimes\fi{}1) phase. Oxygen removal from the (1\ifmmode\times\else\texttimes\fi{}1) surface by higher temperature annealing (\ensuremath{\sim}900 K) was accompanied by depletion of Al from the surface, recovering the reconstruction. The mechanism of a reversible conversion, $(2\ifmmode\times\else\texttimes\fi{}2)p4g\ensuremath{\rightarrow}(1\ifmmode\times\else\texttimes\fi{}1)$ by O adsorption and $(1\ifmmode\times\else\texttimes\fi{}1)\ensuremath{\rightarrow}(2\ifmmode\times\else\texttimes\fi{}2)p4g$ by anneal, is discussed.