Developing Imaging and Spectroscopy Capabilities in Liquid-Phase Transmission Electron Microscopy

UoM administered thesis: Phd

  • Authors:
  • Daniel Kelly

Abstract

The ability to characterise the structure and chemical composition of specimens in a liquid environment has been enabled by the emergence of liquid-phase scanning transmission electron microscopy (LP-STEM). This involves hermetically sealing a thin liquid layer between electron transparent windows to form a liquid cell (LC). Graphene liquid cells (GLC) have been used to overcome the spatial resolution limitations of SiNx, though they cannot be fabricated consistently and offer no route to further technical advances. This work demonstrates a novel platform for LP-STEM consisting of graphene windows and an array of liquid microwells in a hexagonal boron nitride spacer. Characterisation of these engineered GLCs (EGLC) shows they are robust to vacuum cycling and contain liquid layers of tens of nanometres. These LCs enable successful imaging of platinum nanoparticles in solution with atomic resolution, as well as allowing analysis of the nanoscale diffusion characteristics of ultrasmall tungsten crystals. The utility of energy dispersive X-ray spectroscopy (EDS) for solution-phase materials is demonstrated in these EGLCs by characterisation the distribution of elements in bimetallic nanoparticles, which are indistinguishable through imaging alone, with an unprecedented nanometre spatial resolution. A key motivation in the development of EGLCs is the requirement for a design conducive to more complex functionalities, such as flow or mixing capabilities, that are found in SiNx-based liquid cell holders. This is demonstrated here in the form of graphene mixing cells (GMC) which have been fabricated based on the design of the EGLC with MoS2 separation membrane incorporated to keep the precursor solutions apart. The mechanism used to controllably mix two encapsulated solutions in situ is based on beam-induced stress cracking of the separation layer. This method allows the user to instigate mixing through the application of high electron flux to a small region, causing crack propagation through the membrane. As a result the liquids are mixed outside of the irradiated area and thus beam damage of the mixing product is precluded. Liquid-phase EDS and EELS show pristine solutions prior to mixing and post-mixing EDS and dark field (DF) STEM imaging reveals precipitates of CaCO3 successfully formed from the reaction of precursor solutions. These results show the power of analytical spectroscopies for the characterisation of multimetallic systems in GLCs. However this utility does not extend to conventional SiNx liquid cells where thick liquid layers and shadowing of X-rays significantly hamper the performance of EELS and EDS respectively. This thesis contains experimental measurements of these limitations and shows that with optimised acquisition parameters, quantitative EDS analysis of liquid-phase specimens is possible. New strategies for thickness measurements in liquid cells using both EELS and EDS are also presented and applied to analyse the degree of bowing in LCs that is a frequent issue for LP-STEM experiments due to its impact on the limits of imaging and spectroscopic spatial resolution.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
Award date1 Aug 2020