The post-graphene materials termed also as 2D-Xenes (X = Si, Ge, Sn, Pb, As, Sb or Bi) are atomic-layer sheets comprised of single-element atoms arranged in a honeycomb lattice on a suitable substrate. In the present report, we introduce a new member to the current 2D-Xene family, thallene built of atoms of the Group-III element, thallium (Tl). It is formed when 2/3 monolayer of mobile Tl atoms on a single-layer NiSi2 atop Si(111) substrate crystallizes upon cooling below ∼ 150 K into a 3 × 3 -R30° superstructure with a honeycomb geometry. As compared to the hypothetical free-standing thallene layer, the thallene on the NiSi2/Si(111) substrate experiences a strong tensile strain. It was recognized that though the substrate can virtually tune the thallene into the topological phase, in reality the intrinsic electronic properties of thallene are suppressed due to the hybridization with electrons of the NiSi2/Si(111) substrate.

Two-dimensional (2D) topological insulator is a promising quantum phase for achieving dissipationless transport due to the robustness of the gapless edge states resided in the insulating gap providing realization of the quantum spin Hall effect. Searching for two-dimensional realistic materials that are able to provide the quantum spin Hall effect and possessing the feasibility of their experimental preparation is a growing field. Here we report on the two-dimensional (In, Sb)2[Formula: see text]2[Formula: see text] compound synthesized on Si(111) substrate and its comprehensive experimental and theoretical investigations based on an atomic-scale characterization by using scanning tunneling microscopy and angle-resolved photoelectron spectroscopy as well as ab initio density functional theory calculations identifying the synthesized 2D compound as a suitable system for realization of the quantum spin Hall effect without additional functionalization like chemical adsorption, applying strain, or gating.

Formation of the highly-ordered [Formula: see text]-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 [Formula: see text] 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)[Formula: see text] 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.