Low-energy electron microscopy investigation of germanene segregation on Ag(111) thin films

We used low-energy electron microscopy to investigate the formation processes of various surface structures on Ag(111) thin films grown on Ge(111) during Ge segregation. During annealing the Ag(111) samples cleaned by Ar+ sputtering, the Ag2Ge surface alloy first appeared along the Ag grain boundaries, signifying preferential diffusion of Ge atoms through these boundaries. Subsequent annealing at higher temperatures induced the phase transformation from the surface alloy to germanene. However, further higher temperature annealing caused back-diffusion of Ge atoms into the Ag thin films, reverting to the surface alloy. Formation of the ordered 77x77R +/- 19.1 degrees germanene required cooling from high temperatures around 500 degrees C. However, high-temperature annealing occasionally resulted in the formation of three-dimensional (3D) Ge islands on the surface. During cooling, the 3D islands absorbed Ge atoms from the surrounding germanene, triggering phase transformation to the surface alloy in the vicinity of the islands.