Structural modifications of quasi-one-dimensional submonolayers by impurity doping

Abstract

In this study, carried out by analysis of intensities and spot profiles in low energy electron diffraction, we investigated the influence of electronegative oxygen atoms as dopants on the long range order, the thermal stability, and the type of phase transitions in the strongly anisotropic chain system Sr on Mo(112) in the low coverage regime at constant Sr concentrations up to 0.2 monolayers (ML), where the pure Sr layers show $p(8\ifmmode\times\else\texttimes\fi{}1)$ and $p(5\ifmmode\times\else\texttimes\fi{}1)$ structures, respectively. A single oxygen atom always influences many unit cells of Sr, as expected from a model of coupling the Sr chains by Friedel oscillations. Whereas concentrations of only $0.02\phantom{\rule{0.3em}{0ex}}\text{ML}$ of oxygen transform the $p(8\ifmmode\times\else\texttimes\fi{}1)$ into a $p(9\ifmmode\times\else\texttimes\fi{}1)$ structure, small concentrations of oxygen atoms stabilize, but do not alter the $p(9\ifmmode\times\else\texttimes\fi{}1)$ structure over a wide range of oxygen concentrations. Oxygen induced disappearance of depinning transitions was found already for concentrations above $0.01\phantom{\rule{0.3em}{0ex}}\text{ML}$. For these concentrations only melting transitions are still observed. In a concentration range of oxygen and Sr atoms between 1:2 and 1:1 incommensurate structures are formed yielding finally islands of a $(3.3\ifmmode\times\else\texttimes\fi{}1)$ structure in all cases, interpreted as a sign of beginning formation of mixed oxides. Implications for adsorbate induced structuring of templates are discussed.