Two-dimensional overlayers of Au and Ag on the Ge(111) surface: insights from first-principles calculations

Experimental observations using low-energy electron microscopy (LEEM) of Au overlayers on the Ge(111) surface have revealed two-dimensional Ge(111)-Au(√3×√3)R30° structure as the only ordered structure being irrespective of Au coverage and sample temperature [1] while Ag overlayers form a number of structures on the same range of coverages and temperatures [2]. The charge density distribution analysis is performed using the density functional theory to see the variation in binding of Ag and Au on the Ge surface taking the thermodynamically stable √3 structures which shows that the Au atoms form strong covalent bond with Ge atoms (height = 0.56 Å) than bond between Ge atoms whereas the Ag atoms does not share charge with the Ge atoms indicating that they are trapped at height 0.73 Å inside the cage of the strong Ge-Ge bond. We will compare the electronic and vibrational structures of additional overlayer structures and strain on those systems to trace the reason behind such a difference in observed structures.
[1]. Jason Allen Giacomo (2009, Ph.D. Thesis, UC, Davis).
[2]. S. R. Acharya, C. H. Mullet, J. A. Giacomo, D. Le, S. Chiang, and T. S. Rahman, arXiv preprint 1908.00180 (2019)