Graphene Liquid-Cell TEM Investigation of Confined Liquids

Studying liquids and wet samples poses a significant challenge for conventional transmission electron microscopy (TEM) due to stringent requirements of ultra-high vacuum design and narrow pole piece gap for sample insertion in the microscope column. Although recent developments in silicon-based microfluidics have enabled the flow of liquids between thin silicon nitride membranes for in situ TEM experiments, this technique suffers from low spatial and analytical resolutions for imaging and spectroscopy. The most recent liquid cell TEM technique, graphene liquid cell (GLC) microscopy, employs only layers of graphene to encapsulate liquid specimens. Recent efforts with GLC-TEM have demonstrated superior imaging resolution and spectroscopic analysis of beam-sensitive specimens. Herein, we aim to investigate the microscopic characteristics of liquids entrapped in graphene nano enclosures using GLC-TEM. In the first chapter we review the parameters that affect the quality of GLC imaging and analysis, including the graphene transfer onto TEM grids. Several important factors that affect the in situ TEM imaging of specimens, including the variations in GLC geometries and capillary pressure are discussed. The interaction between the electron beam and the liquid, along with the possibility for artifacts or the formation of radical ions are also highlighted in this review. The scientific discoveries enabled by GLC-TEM in the areas of nucleation and growth of crystals, corrosion, battery science, as well as high-resolution imaging of organelles and proteins are also briefly discussed. The second chapter looks into the low-loss region of electron energy-loss spectroscopy (EELS) of water encapsulated between sheets of graphene. Our results indicate a significant increase in the density of water upon graphene encasement. The energy analysis considering the effect of VdW forces, Laplace pressure and strain energy supported the EELS results where total pressures as high as 600MPa in confined water were recorded. Our analysis also reveals that graphene strain energy is the major contributor to the pressure in the graphene-encased water, followed by VdW effects and the Laplace pressure. In the third chapter, in situ TEM and core-loss EELS analysis of water encased in thin GLCs exposed to room and cryogenic temperatures is presented to examine the nanoscale arrangement of the contained water molecules. Simultaneous quantification of GLC thickness leads to the conclusion that H-bonding probability rises under increased water confinement. The present results demonstrate the feasibility of nanoscale chemical characterization of aqueous fluids trapped in GLC nanovessels, and offer new insights on water molecule arrangement under high-confinement conditions. Finally, we discuss possible future research directions of GLC-TEM and the associated challenges in chapter four. The synergistic effort to accomplish the proposed research directions has the potential to yield new discoveries in both materials and life sciences.