The Applications of Van der Waals Heterostructures in TEM in-situ Imaging Researches

UoM administered thesis: Phd

  • Authors:
  • Mingwei Zhou

Abstract

In this work, three projects related to the applications of two-dimensional (2D) materials heterostructures in in-situ imaging with transmission electron microscope (TEM) are discussed in detail, they are adhesion improved MoS2 TEM support, a new design of engineered graphene liquid cell (EGLC) and a graphene mixing cell based on the engineered graphene liquid cell. In the first project, a new design of TEM support is demonstrated based on existing silicon nitride (SiNx) TEM supports. A TEM support is required to hold the samples in a TEM due to its high vacuum chamber. The main novelty of this new design is a thin layer of deposited metal, and an additional layer of MoS2 on top of the SiNx surface. The new TEM support is demonstrated to have improved adhesion by achieving much higher (close to 100%) success rate of transfers even with relatively small crystals’ flakes (with a typical size of 20 μm in diameter). The improved adhesion enables a direct peel off at the last step of dry transfers and this offers better sample quality and lower amount of contamination in general. This new design potentially can be developed into a manufacturable product and many TEM related research can be benefited. In the second project, a new design of engineered graphene liquid cell which is a micro￾container used to hold liquid samples in TEM is demonstrated, it is a sandwich structure consists of three layers, two on the top and bottom are 2-5 layer graphene sheets that are used as windows, and the middle layer is hexagonal boron nitride (hBN) spacer layer of thickness between 20-50nm that is lithographically patterned to have pockets to hold the liquid. The new design is proved to have high quality hermetic sealing, with the ability to withstand high pressure in TEM without much change in the internal pressure in the pockets. The new geometry also demonstrated compatibility with electron energy loss spectroscopy (EELs) and energy dispersive X-ray spectroscopy (EDXs), nanometre resolution elemental mapping is achieved using this liquid cell. The third project, the graphene mixing cell, is an extension of the second project. The structure of the mixing cell involves 5 layers, of which the two on the top and bottom are still graphene window layers, the second and fourth layers are hBN spacer layers, and in the middle is a piece of MoS2 less than 3 atomic layers. This graphene mixing cell keeps two solutions separated before the sample is sent into TEM, and the mixing can be initiated by incidence of the beam, therefore enabling in-situ imaging of chemical reactions or other processes between two distinct substances in liquid environment. This is demonstrated by studying a reaction that results in precipitation of CaCO3 (solid). This mixing cell inherits most advantages from the EGLC, in addition, it exhibits possible development based on the EGLC. From these three projects, it can be shown that 2D materials and their heterostructures can play an important role in TEM in-situ imaging by offering great potential to improve the qualities and compatibilities of current nano-devices or nanostructures involved.

Details

Original languageEnglish
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Award date31 Dec 2020