(Tl, Au)/Si(1 1 1)7×7 2D compound: an ordered array of identical Au clusters embedded in Tl matrix

Formation of the highly-ordered 7×7-periodicity 2D compound has been detected in the (Tl, Au)/Si(1 1 1) system as a result of Au deposition onto the Tl/Si(1 1 1) surface, its composition, structure and electronic properties have been characterized using scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and density-functional-theory calculations. On the basis of these data, the structural model of the Tl-Au compound has been proposed, which adopts 12 Tl atoms and 10 Au atoms (in total, 22 atoms) per 7×7 unit cell, i.e. 1.71 ML of Tl and 1.43 ML of Au (in total, 3.14 ML). Qualitatively, the model can be visualized as consisting of truncated-pyramid-like Au clusters with a Tl atom on top, while the other Tl atoms form a double layer around the Au clusters. The (Tl, Au)/Si(1 1 1)7×7 compound has been found to exhibit pronounced metallic properties at least down to temperatures as low as 25 K, which makes it a promising object for studying electrical transport phenomena in the 2D metallic systems.