Transmission electron microscopy characterizations with in situ electrical manipulations on two-dimensional materials

Abstract

With nanomaterials emerged since the 1980s, nanotechnology rapidly develops. Recently, two-dimensional materials get enormous attention, which means a van der Waals materials, for example, single-layered graphite, graphene, the first two-dimensional materials. Afterwards, other layered/van der Waals materials have been successfully lowered down to monolayer structure, including h-BN, transition metal dichalcogenides, and Xene (X = Group IV elements). Most of them exhibit excellent properties compared with conventional materials. Thus, the next-generation two-dimensional-based devices are intensively reported. However, there is a lack of fundamental studies in the nano-contact with mechanical and electrical stimulations. In this thesis, transmission electron microscopy characterizations with in situ mechanical and electrical manipulations have been conducted. In the nano-contact, the size effect induced electric field is high enough for tunnelling. Field emission dominant transport is preferred, and it has been successfully demonstrated. The corresponding barrier width and height have been statistically analysed in different contact geometry and thickness. All results demonstrated thickness dependence expect for barrier width in face contact. Thickness-dependent electrical conductivity has been studied by current density versus voltage plot and the conductivity is increased with the decreasing thickness. Mechanical force induced screw dislocation has been observed and proved by video record and simulated high-resolution electron microscope image. The slope of the current-voltage plot is almost twice when the screw dislocation is formed, that implies the conductivity is almost double by screw dislocation. Moreover, if the nano-contact is intimately such as welding, the current-voltage usually behaves like thermionic field emission. In terms of mechanical test, titanium alloy ultrathin film has been bent. The results indicate the grain boundary is the weakest in the whole structure and the flexibility of the film is high. Other than metallic films, wrinkling observations in two-dimensional membranes provide a valuable shape transition and the strain release mechanism. In terms of mechanical-electrical test, stress-dependent electrical properties have been measured. The data demonstrates the tunnelling current and barrier form change with the local stress. These two-dimensional contacts are believed to apply shortly soon. Hence, the in situ studies are beneficial to not only the two-dimensional materials fundamental studies, but also the next-generation electronic devices in terms of design, processing, and fabrication. The challenges of two-dimensional devices are stability and feasibility on two-dimensional contacts. The flexibility of two-dimensional materials enhances the tolerance of the contacts. More investigations are needed for forming better two-dimensional contacts in the future.