Alloying, de-alloying and reentrant alloying in (sub-)monolayer growth of Ag on Pt(111)

摘要

An in-situ nanoscopic investigation of the prototypical surface alloyingsystem Ag/Pt(111) is reported. The morphology and the structure of theultrathin Ag-Pt film is studied using Low Energy Electron Microscopy duringgrowth at about 800 K. An amazingly rich dynamic behaviour is uncovered inwhich stress relieve plays a governing role. Initial growth leads to surfacealloying with prolonged and retarded nucleation of ad-islands. Beyond 50%coverage de-alloying proceeds, joined by partial segregation of Pt towards thecentre of large islands in violent processes. Upon coalescence the irregularlyshaped vacancy clusters are filled by segregating Pt, which then take a compactshape (black spots). As a result at around 85% coverage the strain of theinitially pseudo-morphological film is almost completely relieved andPt-segregation is at its maximum. Further deposition of Ag leads to transientre-entrant alloying and recovery of the pseudo-morphological layer. The blackspots persist even in/on several layers thick films. Ex-situ atomic forcemicroscopy data confirm that these are constituted by probably amorphousPt(-rich) structures. The (sub-)monolayer films are very much heterogeneous.