S T R U C T U R E O F M A T E R I A L S

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

1 5 8 H I G H L I G H T S 2 0 2 1 I

Synthesis of a new Janus SPtSe 2D material by substitution of Se by S, monitored by X-ray scattering

A new two-sided Janus SPtSe 2D material, with potential applications in magnetic memories, has been synthetised by exposing a PtSe2 single layer to a partial pressure of H2S gas, thus substituting the top Se atoms by S. The process was monitored in situ by grazing incidence X-ray scattering.

PtSe2 is a two-dimensional (2D) material of the transition metal dichalcogenides (TMDCs) family, in which a honeycomb layer of metal atoms is sandwiched between two honeycomb layers of chalcogens (S, Se or Te). TMDCs are of interest for their electronic, photonic, catalytic and magnetic properties, which are linked to the specific features of their 2D structure and symmetries.

The ultimate goal of this study was to intensify the Rashba spin-orbit coupling in TMDC, which generates a spin accumulation from a simple charge current flowing in the material. This is a relativistic effect, in which an electric field is felt as a magnetic field due to the spin-orbit coupling and the asymmetry of the crystal, allowing the generation of a spin current from a charge current. In a sense, it allows to sort the spins dynamically (up and down) in the material. The TMDC of interest, PtSe2 has a strong spin- orbit interaction thanks to the heavy platinum atoms, but its inversion symmetry limits the Rashba coupling.

This obstacle can be overcome by chemical means by substituting the Se atoms in the top layer with another chalcogen species (Figure 141, bottom right), thus creating a chemical vertical asymmetry. This was achieved by means of controlled sulfurisation (under H2S atmosphere) of a pristine epitaxial PtSe2 grown by selenisation of a single crystal Pt(111) surface, to make an asymmetric two-dimensional SPtSe. In SPtSe, the symmetry-breaking creates an electric field that generates a strong Rashba spin-orbit coupling. Note that SPtSe is called Janus (after the two-faced Roman God) because the metal layer is sandwiched between two external layers ( faces ) of different chalcogen species.

In order to form the Janus SPtSe (Figure 141), the pristine PtSe2 material was first heated to 370°C to generate vacancies in the top layer. It was then exposed to an H2S gas atmosphere in order to fill these vacancies with S atoms and obtain the substitution of Se by S in the whole top layer. Upon cooling, the new 2D compound reordered. These steps were monitored in situ by grazing incidence X-ray diffraction/scattering (GIXD/S) at beamline BM32. The different atomic composition of the two sides of the Janus compounds was confirmed ex situ by angle-resolved X-ray photoemission spectroscopy.

The GIXS measurements are summarised in Figure 142. Figure 142a shows a reciprocal space map, parallel to the crystal surface, of the PtSe2 layer on Pt(111). It shows the moiré pattern resulting from the perfect coincidence (after a small, 0.7% compressive strain of PtSe2) between three units of PtSe2 sitting on top of four surface units of Pt(111), yielding a Pt(111)-4x4/PtSe2(0001)-3x3 superlattice.

Figure 142b shows a radial scan along the in-plane direction h (expressed in superlattice reciprocal lattice units, s.r.l.u.). In both figures, the intense Bragg peaks of Pt, the weaker rods from the PtSe2 layer and the superlattice peaks are clearly visible. The Se-by-S substitution was monitored by scanning along rods and in-plane directions. Figure 142c shows a small map around the (110) PtSe2 Bragg peak (indexed here (330)) and the (2-20) Pt one (indexed here (440)) after substitution, which was found to be very similar (except from strain effects) to the PtSe2 one (see inset of Figure 142a).

Figure 142d shows a radial scan along the in plane direction h (expressed in s.r.l.u.) before and after the substitution of the top Se layer. All show a remarkable similarity of the scattered intensity before and after Se-by-S substitution, proving that the strong bonding at the interface between the 2D layer and the substrate

Fig. 141: Pictorial scheme for the transformation of PtSe2 into SPtSe

(Pt: blue; Se: orange; S: yellow).