Imaging and understanding metal organic frameworks using cryogenic focused ion beam scanning electron microscopy

Metal organic frameworks (MOFs) are a structurally tuneable class of hierarchical porous materials with a wide range of host-guest chemistry. Their design acts as a platform for advanced functional materials resulting in properties ranging from charge conductivity, catalytic metal centers, high surface area and organic capacitance. As a result, MOFs have a wide range of applications and can be employed in catalysis, health care, batteries, supercapacitors, and carbon capture.

Due to their organic and porous nature, MOFs are incredibly difficult to structurally characterize using scanning electron microscopy. Beam stability, along with
non-conductive nature and porous framework result in a combination of problematic issues for nanoimaging and structural milling.

An example of the degradation MOFs suffer from is electron beam interaction causing surface damage as well as difficulty in imaging of nanoscale structures due to charging artifacts. Additionally, the amorphous nature of adsorbing MOFs results in severe sensitivity to electron beam current resulting in material loss and degradation. The preparation of MOF composite TEM lamellae by FIB milling can prove time consuming and laborious.

Herein we demonstrate a novel technique to imaging, 3D volumetric chemical analysis and TEM lamellae preparation using MOF-74 type analogue for carbon capture and mixed membrane composite CPO-27-Ni. Using imaging strategies that include high resolution variable pressure microscopy with optimised beam path lengths using "NanoVP" charge reduction, we demonstrate superior imaging at a low vacuum, improving imaging quality and eliminating sample charging and low accelerating voltage to reduce material degradation. Alongside this we employ a cryogenically cooled in situ stage to undertake 3D volumetric
analysis of a MOF composite membrane in conjunction with energy dispersive x-ray spectroscopy. In doing so we show a new methodology for TEM lamellae preparation, 3D volumetric analysis of MOF composites and best practice imaging using low pressure, low kV scanning electron microscopy.