Structural transformations in transition metal phosphorus trichalcogenides studied by analytical transmission electron microscopy

Abstract text (incl. figure legends and references)

Few-layer transition metal phosphorous trichalcogenides (TMPTs) is a promising class of materials due to their inherent interesting properties [1], making these materials ideal candidates to study 2D magnetism [2], as well as promising candidates for future energy storage related applications [3].

In this work, we study structural modifications of few-layer TMPTs caused by electron-irradiation, as well as thermal annealing in transmission electron microscopy (TEM), and experiments are rationalized using ab-initio calculations.

Thin TMPT flakes were prepared with the help of a newly developed polymer-based preparation method [4], which significantly enhances the sample quality. The Cc/Cs-corrected Sub-Ångström Low Voltage Electron Microscope (SALVE) at 80kV was used to study structural modifications caused by the probing electrons. Furthermore, in-situ annealing experiments were conducted with a dedicated FEI NanoEx-i/v TEM specimen heating holder. Electronic, and magnetic properties of newly emerging phases were predicted using spin-polarized density functional theory (DFT) as implemented in VASP [5].

Continuous irradiation of 4-layer NiPS₃ showed a degradation of individual layers, thus single- and double-layer regions within the layer system emerge. Fig. 1 (a) presents a sulphur vacancy in double-layer NiPS₃. By using the McKinley-Feshbach formalism [6] estimated atom displacement cross-sections were calculated predicting the preferential removal of sulphur (see Fig. 1 (b)).

Further, the emergence of different MnS phases (e.g. Fig. 2 (a)) in MnPS₃, and MnSe phases in MnPSe₃ were observed due to structural modifications caused by the impinging electrons, and the growth of these ultrathin phases could be controlled by the illuminated area, and applied total electron dose. In-situ thermal annealing induced structural modifications showed the possibility of engineering various phases depending on the observed TMPT (e.g. MnPSe₃, and NiPS₃, transforms to MnSe (see Fig. 2 (b)), and NiP, respectively). Further, the phase transition temperatures of freestanding TMPTs in vacuum were determined. Eventually, ab-initio calculations predict distinct properties of the new phases, which strongly depend on the orientation and the thickness of the transformed facets.

In conclusion, displacement cross-sections predict that more S should be removed than P due to elastic interaction in the acceleration voltage range of 50-300kV. Further, our experiments show that structural modififations can attributed to the growth of specific phases in MnPS₃ and MnPSe₃, caused by electron-irradiation or thermal annealing.

Fig. 1. (a) 80kV Cc/Cs-corrected HRTEM image of a double-layer NiPS₃. A line-scan shows the position of a missing S atom. (b) Displacement cross-sections of sulphur and phosphorous in XPS₃ (X=Fe, Mn, Ni).

Fig. 2. (a) 80kV Cc/Cs-corrected HRTEM image of a -MnS crystal. A magnified area of the patch with overlaid structure model is shown beside. The structure emerged after irradiation of a MnPS₃ flake in TEM. (b) HRTEM image of an -MnSe crystal, which formed after annealing a MnPSe₃ in-situ to 600°C.

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[6] Meyer, et al., Phys. Rev. Lett., 108 (19), 196102, (2012).