Characterization of freestanding few-layer transition metal phosphorus trichalcogenides studied by low-voltage analytical transmission electron microscopy

Abstract

Transition metal phosphorous trichalcogenides (TMPTs) are layered materials and a promising class of materials due to their compelling properties as a large variety of band gaps and inherent magnetic orderings. In recent years, few-layer TMPTs have gained ample interest as intriguing properties persist down to the two-dimensional limit making them promising candidates in various novel optoelectronic devices and energy storage. However, the thickness-dependent properties and the possibility to tailor the properties of these few-layer systems are not fully understood to this point. In this work, freestanding few-layer TMPTs (namely MnPS3, FePS3, NiPS3, MnPSe3, and FePSe3) are studied by various analytical transmission electron microscopy (TEM) techniques and experiments are supplemented by complementary ab-initio calculations. These TEM techniques are aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM), (momentum-resolved) energy-electron-loss spectroscopy ((MR-)EELS), electron diffraction (ED), besides energy dispersive X-ray spectroscopy (EDX). These experiments are primarily conducted in the low voltage regime (i.e. 20 kV – 80 kV). First, the thickness-dependent signatures of pristine few-layer TMPTs in TEM are studied by AC-HRTEM and MR-EELS, and the possibility to prepare and investigate few-layer samples of good crystal quality is proven. Further, the inherent thickness-dependent properties for the plasmon dispersion and the band gap are elucidated by MR-EELS experiments and complementary calculations based on density functional theory (DFT). In addition, the interaction of the electron beam and the few-layer samples is investigated in the TEM. Here, a strong modification of the few-layer TMPTs by the primary removal of the chalcogen atom (i.e. S and Se) is observed for all investigated acceleration voltages. The emergence of new phases in the α- and γ- MnS and MnSe on the nanoscale is proven in few-layer MnPS3 and MnPSe3, respectively. DFT calculations predict that the electronic and magnetic properties of the emerging ultrathin crystals depend on the crystal type, orientation, and thickness. As an additional stimulus to tailor the properties, annealing under vacuum conditions is studied by dedicated in-situ annealing experiments in the TEM. For all studied few-layer TMPTS phase transitions in the range of 380 °C to 500 °C are observed. Moreover, the large scale transformation of MnPS3 to α-MnS and MnPSe3 to α-MnSe is proven for annealing under vacuum conditions. Eventually, these emerging phases are the identical ones as observed for the case of electron irradiation of few-layer MnPS3 and MnPSe3.