M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

8 6 H I G H L I G H T S 2 0 2 3 I

Observing charge density waves in a strictly two-dimensional crystal with X-ray diffraction

In-situ grazing-incidence X-ray diffraction and X-ray reflectivity were used to investigate the structure of a two-dimensional single layer of tantalum sulfide grown on a gold surface and its evolution during intercalation by caesium atoms, and de- intercalation, which decouples and recouples the two materials, respectively.

Dimensionality influences the electronic properties of two-dimensional (2D) materials in different ways. In crystals thinned to atomic scales, the range of electronic phenomena can be enriched by modulating the structure along planar directions. Transition metal dichalcogenides (TMDs) constitute an important family of 2D materials that comprises metallic compounds having specific d-band filling or metal atom coordination. Some are subject to electronic instabilities, mediated by electron electron and electron phonon interactions. Charge- ordered states, so-called charge density waves (CDWs), have been observed in 3D tantalum-based TMDs. The objective of this work was to explore if CDW that exist in bulk layered dichalcogenide structures could also develop in the limit of a single-layer prepared in a state in which it is strictly 2D, i.e., free from interaction with other layers or substrates.

A tantalum-sulfide (TaS2) single layer exhibiting a pronounced nanometre-scale moiré [1] was prepared on a gold (Au(111)) substrate. The single layer was then decoupled from its substrate by intercalating alkali caesium (Cs) atoms [2], and the resulting structural changes in reciprocal space were explored using grazing- incidence X-ray diffraction at beamline BM32. This was done in situ, during the intercalation process, which was controlled by two kinds of treatments that modify the TaS2 stoichiometry, namely thermal annealing and exposure to a sulfur source, H2S gas.

Figure 66 summarises the evolution during successive cycles of Cs intercalation (decoupling) and de-intercalation (recoupling). Along radial scans of reciprocal space, the coupled state is signalled by strong diffraction peaks stemming from the super-periodic coincidence lattice of the moiré (Figure 66a, halfway between the peaks of the substrate and of the single layer). Conversely, disappearance of the moiré peak (Figure 66b) proves a complete decoupling by intercalation: the process is reversible.

The intercalation/de-intercalation cycles have another important side effect: they alter the single layer s lattice parameter, as shown by a progressive shift of the layer s diffraction peak position towards higher momentum transfer (Figure 66c). It was established that the interaction step and the required moderate thermal annealing cause S departure, hence creating a S-deficient single layer. This effect can be counteracted by providing S via H2S during the de-intercalation step, and using this process repeatedly eventually yields a highly- stoichiometric TaS2 single layer. The final coupled TaS2 single layer is of better quality with respect to the starting one, with larger domain size (Figure 66d) and a perfectly commensurable moiré, featuring exactly seven TaS2 unit cells in perfect coincidence with eight Au ones.

Fig. 66: Structural changes during successive cycles of intercalation

and de-intercalation of the 2D layer with Cs. a) Radial scans around the (200) TaSx and Au

reflections, for the de-intercalated states and (b) for the intercalated

states. c) Evolution of the 2D layer lattice parameter, for the different states (intercalated in red and de- intercalated in black). d) Evolution of the average crystalline domain

sizes upon cycling.