COMBINED STM AND RBS INVESTIGATION OF THE NUCLEATION AND GROWTH OF AUON NI(110) - ALLOYING AND DEALLOYING

We have performed in-situ Scanning Tunneling Microscopy (STM) and Ruth erford Backscattering Spectroscopy (RES) investigations of the room-te mperature (RT) growth of Au on Ni(110). At low coverages, we observe t he formation of a Au/Ni surface alloy, even though Au is immiscible wi th bulk Ni at RT. At a critical Au coverage of theta(Au) = 0.4 ML (mon olayer), we observe a ''dealloying'' phase transition where 0.16 ML of Au is ''popping'' out of the alloyed surface layer and nucleates into [001]-directed dimer/trimer Au chains. Increasing the Au coverage fur ther causes the nucleation and growth of more [001]-directed chains, i .e. their mutual distance in the [1 (1) over bar 0] direction decrease s until the chain structure finally saturates in a p(5 x 3) structure at theta(Au) = 0.93 ML. The formation of a surface alloy at low covera ge is supported by theoretical calculations within the Effective-Mediu m Theory (EMT), and an atomic model for the [001]-directed Au chains i s developed from a detailed interplay between experiments and theoreti cal calculations.