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

TL;DR

This study investigates the complex dynamic processes of alloying, de-alloying, and reentrant alloying in ultrathin Ag films on Pt(111) surfaces at high temperature, revealing stress-driven morphological changes and Pt segregation.

Contribution

It provides the first in-situ nanoscopic analysis of the dynamic alloying behaviors and stress effects in Ag/Pt(111) ultrathin films during growth.

Findings

Stress relief governs alloying and de-alloying dynamics.

Pt segregation occurs during de-alloying and coalescence.

Reentrant alloying transiently restores the pseudomorphic layer.

Abstract

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