New phases of two-dimensional transition metal dichalcogenides
Our colleagues Marin Petrović and Marko Kralj in collaboration with researchers from several institutions (Universities of Siegen, Cologne and Münster in Germany, and King Abdullah University in Saudi Arabia) published an article in the prestigious journal ACS Nano, on the controlled synthesis of two different phases of monolayer tantalum sulfide.
Two Phases of Monolayer Tantalum Sulfide on Au(111)
D. Dombrowski, A. Samad, C. Murray, M. Petrovic, P. Ewen, T. Michely, M. Kralj, U. Schwingenschlögl, C. Busse, ACS Nano 15, 13516-13525 (2021).
Interest in atomically thin transition metal dihalcogenides (TMDs) with general stoichiometry MX2 (where M denotes a metal atom and X a chalcogen atom), was initially focused on semiconductor systems having a direct energy gap, which is important for the development of applications in electronics and photonics. However, TMD monolayers are polymorphic, with the semiconducting or metallic MX2 phase being associated with the so-called 2H or 1T structural phase, respectively. In addition, non-stoichiometric derivatives, such as those enriched in a metal or depleted in a chalcogen element, have recently begun to be considered. As these are atomically thin materials, such changes in structure induce significant modifications in the properties of different TMD materials. This is currently a topic of fundamental importance, which needs to be explored for the development of future applications. Due to the similarity of the crystal lattices of the nonstoichiometric and stoichiometric phases, the possibility of developing advanced lateral heterostructures with well-defined and sharp phase boundaries arises.
In this work, by adjusting the conditions for the synthesis of single-layer tantalum sulfide on the Au(111) surface by the molecular beam epitaxy (MBE) method, primarily the amount of sulfur, stoichiometric 2H-TaS2 and non-stoichiometric sulfur-poor TaS phase were synthesized. By scanning tunneling microscopy (STM) imaging it is possible to clearly distinguish the contrast difference as well as the boundary between the two phases (Figure 1). In atomically resolved images, both phases show a hexagonal structure, however, with differences that are easily comparable to density functional theory (DFT) calculations. This enables the identification of the TaS phase as a sulfur-depleted phase which, in relation to the 2H-TaS2 phase, lacks a whole layer of sulfur, namely the one oriented towards the Au(111) surface.
Figure 1. STM topography of tantalum sulfide on the gold surface (a) TaS2 islands (brown, labeled δ) and TaS (yellow, labeled β) on Au(111) (blue). (b, c) Zoomed-in representations of TaS2 and TaS phase with atomic resolution. (d, e) DFT simulations of STM topography of TaS2 and TaS phase.
The lack of sulfur significantly alters the interaction of tantalum sulfide and the substrate, and due to the difference in the lattice constants and the presence of the moiré structure, an additional buckling of the TaS layer occurs. All these factors cause increased interaction with the substrate, which includes the related periodic potential. This potential leads to a significant shift of the Au(111) surface state to higher binding energies and induces the formation of replica bands of this surface state. These effects are clearly visible using angle resolved photoemission spectroscopy (ARPES) (Figure 2). Additional differences in the electronic structure are also visible in STM spectroscopy (STS), which distinguishes the electronic structure and states above the Fermi energy.
Figure 2. ARPES characterization of (a) TaS2 and (b) TaS on Au(111). The shifts in the surface state of gold can be discerned, as well as the appearance of new electronic bands characteristic of a particular system.
The published results are significant for future engineering efforts of various nonstoichiometric TMD phases, where the sulfur-depleted TaS phase investigated in this paper can be applied as a good contact with a metal electrode and can bind well laterally to the semiconducting 2H-TaS2 phase.