Electron-beam-induced modifications in two-dimensional materials

Abstract

Electron-beam-induced structural modifications were studied in high-resolution transmission electron microscopy (TEM) images of the two-dimensional (2D) transition metal dichalcogenides (TMDs) MoS2, MoSe2 and MoTe2 as well as graphene liquid cells embedding thin NaCl and CaSO4 crystals. Studies on MoS2, MoSe2 and graphene liquid cells were performed using a Cs-corrected TEM operated at an electron acceleration voltage of 80 kV. Further experiments on MoS2 and MoTe2 were performed using the newly developed chromatic (Cc) and spherical (Cs) aberration-corrected Sub-Ångström Low-Voltage Electron microscope (SALVE) operated at voltages from 80 kV down to 20 kV. Voltage-dependent damage rates in MoS2 and MoSe2 below the knock-on threshold revealed that atomic defect creation can only be explained by a two-step process, starting with electronic excitations and followed by ballistic collisions. Additionally, structural determination of produced defects in MoTe2 - combined with density functional theory - exhibited local change of electronic properties. Within this work it was also shown that graphene encapsulations significantly increase the stability of TMDs against electron-beam irradiation. Therefore, graphene was also used as encapsulating material to study ionic crystals such as CaSO4 and NaCl in liquid cells. Due to the crystallization of CaSO4, pressure and temperature conditions within the graphene liquid cell were estimated via a superposition of p-T phase diagrams combined with thickness-dependent transition temperatures. Moreover, the decomposition of NaCl crystals in graphene liquid cells was followed. This revealed that the encapsulated material is only stable as long as the protecting graphene is undamaged and that a first defect in graphene initiate already an immediate decomposition of the NaCl crystals.