Phase engineering in tantalum sulfide monolayers on Au(111)

Abstract

We prepare monolayers of tantalum sulfide on Au(111) evaporation of Ta in a reactive background of H2S. Under sulfur-rich conditions monolayers of 2H-TaS2 develop, whereas under sulfur-poor conditions TaS forms a structure that can be derived from 2H-TaS2 by removal of the bottom layer. We analyse the alignment of the layers with respect to the substrate and the relation with the domains in the Au(111) herringbone reconstruction using scanning tunneling microscopy (STM). With the help of density functional theory (DFT) calculations we can determine the registry of the two phases with the substrate. We develop a growth process that allows preparation of uniquely oriented 2H-TaS2 on Au(111). 2H-TaS2 and TaS have a remarkably similar plane lattice structure and we observe the formation of lateral 2H-TaS2-TaS heterostructures with atomically well-defined and defect-free boundaries. observe mirror twin boundaries within 2H-TaS2 along the S- and Ta-edge.

Superstructures, Commensurations, and Rotation of Single-Layer TaS2 on Au(111) Induced by Cs Intercalation/Deintercalation

“…Within the precision offered by STM and electron diffraction, we can provide a first rough estimate of the epitaxial relationship between TaS 2 and Au(111), consisting of about (7 × 7) unit cells of TaS 2 on (8 × 8) unit cells of Au(111), in agreement with previous work. [24][25][26]33 This 7/8 superstructure, a moireś uperlattice, has a periodicity of about 2.3 nm. No perceptible rotation of the TaS 2 lattice with respect to the Au(111) one is detected, as illustrated in Figure 1b by the perfect alignment of the atomic rows with the moirélattice's high symmetry directions.…”

“…This substrate advantageously promotes a very well-defined crystallographic orientation of the 2D layer, which is acknowledged to be difficult to achieve for many 2D TMDs. We demonstrate intercalation and deintercalation of cesium atoms, controlling these processes with temperature and an H 2 S atmosphere that crucially allows adjusting the stoichiometry of the 2D layer . We combine in situ high-resolution synchrotron grazing incidence surface X-ray diffraction/scattering (GIXD/S) and X-ray reflectivity (XRR) with scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) to analyze fine structural changes during the cycles of intercalation/deintercalation, so that the 2D material respectively decouples from its substrate and recouples with it.…”

Nucleation Stage for the Oriented Growth of Tantalum Sulfide Monolayers on Au(111)

“…Close-packed noble-metal surfaces (foremost Au(111)) are particularly well suited for the preparation of transition-metal disulfides like MoS 2 , ,, WS 2 , and TaS 2 , due to the symmetry match and the ability to catalyze the decomposition of sulfur precursors despite an otherwise low chemical reactivity. This allows the preparation of the TMDCs by evaporating the transition metal and annealing the system in a background of H 2 S. However, the 6-fold symmetry of the (111) surface layer very often leads to the growth of two rotational domains (separated by 180°) of the 3-fold-symmetric sulfides in the 2H phase, , which is detrimental for studies using global methods like surface X-ray diffraction (SXRD) or angle-resolved photoemission spectroscopy (ARPES) because they will only measure a superposition of the two orientations. As an example, effects like spin splitting of the band structure become undetectable for an equal distribution of the phases.…”

“…For Au(111), the herringbone reconstruction, where two stackings of the first layer (fcc/hcp) are present, is an additional complication, as the first two substrate layers are themselves rotated against each other by 180° for the two stackings. However, also here suitable growth recipes can be found. ,, It suggests itself that the selection of orientation already takes place during the nucleation phase, which was, however, not studied in detail so far.…”

“…Islands of TaS strongly prefer a single orientation on Au(111), in contrast to islands of 2H-TaS 2 . We could exploit this behavior to grow a uniquely oriented layer of 2H-TaS 2 in a two-step process by first preparing well-oriented TaS and then post-sulfurizing it to 2H-TaS 2 …”