Controlled nanometer liquid layer thicknesses for the study of nanomaterials and biospecimens in liquid-cell transmission electron microscopy

Abstract

Liquid-cell transmission electron microscopy (LC-TEM) is an in-situ microscopy technique which aims to image nanomaterials and biospecimens in their native liquid phase environment. To accomplish this task, miniature vessels known as nanofluidic cells (NFCs) are utilized to enclose the liquid sample droplet and protect it from evaporating in the high or ultra-high vacuum of the electron microscope. These NFCs must conform to strict requirements so that a small volume of liquid sample is hermetically sealed between two ultrathin windows. Although the technique has seen significant growth in recent years with the utilization of hybrid material window membranes for improved imaging resolution, the incorporation of machine learning techniques for image processing, and advancements in bio-imaging in the liquid phase, the establishment of uniform thin liquid layers in the absence of membrane bulging has remained a challenge. The bulging of window membranes causes inhomogeneity across the viewing area of the assembled NFC. Since conventional NFCs are structurally sensitive to handling and are assembled in the atmosphere of the laboratory environment before being inserted into the electron microscope for imaging, bulging of the thin window membranes occurs. Researchers thus have been led to collect their LC-TEM data along the edges of the windows where bulging is minimized and therefore resolution is maximized. In this thesis a complete suite of instrumentation is presented, comprising: i) proprietary shape-engineered NFCs, ii) LC-TEM holders, and iii) a proprietary sample loading method/station that allows the enclosure of liquid in the absence of air. This combined technology enables the formation of uniform thin liquid layers for LC-TEM measurements in the absence of window membrane bulging. The capabilities of this method to achieve uniform thin liquid layers in assembled NFCs are demonstrated through the high-resolution imaging of gold (Au) nanorods, polystyrene (PS) nanospheres, and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes as model sample specimens. In addition to these high-resolution images, electron energy loss spectroscopy (EELS) measurements were conducted within the electron microscope as a means to quantify the liquid layer thickness. The developed instrumentation and methodology resolves a long-standing issue in the LC-TEM community, conferring high-throughput imaging and establishing uniform thin liquid layers across the complete viewing area of the assembled NFC, enabling lattice resolution for crystalline materials and high-contrast for biospecimens.