We have successfully demonstrated a new design of engineered graphene liquid cell with the use of hexagonal boron nitride (BN) as a spacer layer (Kelly et al, Nano Letters 2018). This makes possible atomic resolution TEM imaging and elemental analysis which is limited only by the capabilities of the microscope; previous cell designs have had reduced imaging capabilities due to the presence of the liquid environment and cell itself. Our new graphene liquid cell design is substantially more robust than previous designs – surviving many rounds of vacuum cycling and imaging at a high accelerating voltage without losing liquid. This provided the experimental platform for development of more advanced graphene-enabled environmental cell designs, particularly liquid-liquid mixing (Kelly et al, Advanced Materials 2021). We have also demonstrated the first imaging of single adatom dynamic motion at a solid liquid interface using TEM (Clark et al, Nature 2023). In this work we also studied how the observation of preferred adatom lattice sites for the hydrogen evolution reaction catalyst Pt on MoS2, depends whether the sample is imaged in liquid or in the TEM vacuum, with the former providing a good match to complementary density functional theory (DFT) calculations (performed by collaborators in Cambridge).
Developing our 2D heterostructure liquid cell platform required us to understand the mechanical (bend/fold) properties of 2D materials (Rooney et al Nature Comms 2018), as well as optimal conditions for exfoliation (Zheling et al ACS Nano 2020) and scanning electron microscopy (SEM) imaging of these materials (Tillotson et al, ACS Nano 2024). We have used our developed platform to explore the synthesis and degradation of a wide range of materials systems. For example, studying electron beam induced patterning in 2D black phosphorus (Clark et al, Nano Letters, 2018) and graphene-BN stacks (Clark et al, 2D materials, 2019). We have also investigated 2D monochalcogenide crystals, InSe and GaSe, and have reported their novel optical properties (Terry et al, 2D materials, 2018), and the evolution of point and extended defects (Hopkinson et al, ACS Nano, 2019). We have studied a range of atmospherically sensitive magnetic materials, and published the first imaging of defects in atomically thin CrBr3 (Hamer et al Nano Letters 2020). We have investigated oxidation to improve superconductivity performance in TaS2 (Bekaert et al Nano Letters, 2020). Larger area, cleaner 2D heterostructures are achievable with recent advances in UHV nanofabrication, which has advantages for the cleanliness of our graphene liquid cells (Wang et al Nature Electronics, 2023).
We have supported investigations of unusual flow properties in few layer silicates (Mogg et al Nature Nanotechnology 2019) and performed the first studies of ion exchange in these materials as a function of layer thickness (Zou et al, Nature Materials 2021). This pioneering study uncovered that the few-layer vermiculite clay minerals, ion exchange in liquids four orders of magnitude more quickly than bulk crystals.
We have further supported studies of ultra-thin metal layers (Su et al, Nano Letters, 2019), few layer metal thiophosphate synthesis (Synnatschke et al, 2D Materials, 2023), HgTe quantum dots (Mirzi et al, Nanoscale Advances, 2021), Mo6S8 Chevrel phase materials for lithium battery electrodes (Elgendy et al Nanoscale 2022), MXenes for sodium ion batteries (Maughan et al, Langmuir 2020) and novel synthesis of Germanane and Silicane (Georgantas et al. Small Methods, 2024).
Beam damage to the sample is a key challenge for in situ TEM. We have studied of damage behaviour in MFM-300 metal organic frameworks (MOFs) (Tien et al J Materials Chemistry A, 2023). We also used the single particle reconstruction (SPR) approach to reduce the required electron dose. We applied SPR to energy dispersive X-ray spectroscopy TEM data for the first time, providing 3D analysis of surface chemistry information at the nanoscale (Wang et al Nano Letters, 2018) and providing a better understanding of their performance for the oxygen evolution reaction (OER) in fuel cells (Leteba et al, Nano Letters, 2021). The benefit of SPR over conventional tomography is that it does not require tilting so is compatible with environmental cells. We have shown that in situ TEM 3D elemental mapping is therefore feasible; revealing elemental and structural evolution of nanoparticle catalysts during heating (Wang et al Small 2023).
We are now exploring the potential to improve environmental cell gas phase TEM investigations. We have tested the limitations of environmental TEM for high-speed imaging of the mechanism of oxidation of aluminium (Nguyen et al ASC Applied Materials and Interfaces, 2018) but such instruments are limited in the gas pressure that can be applied. Experiments regarding the fundamentals of gas flow in 2D nanochannels have yielded highly promising results (Keerthi et al Nature 2019). Our gas phase environmental TEM imaging with commercial Protochips holders have demonstrated that elemental mapping can benefit the development of catalytic nanoparticles (Govender et al, Materials Characterisation, 2019, Stewart et al, ACS Catalysis 2019, Lindley et al, ACS Catalysis, 2024). However, we believe our new graphene-based technology can significantly improve TEM imaging and analysis capabilities in gases. We have been awarded an ERC Proof of Concept award (2025-2026) to further this technology after the end of this project.
The results have been published by the PI in high impact scientific journals and been disseminated through regular invited and plenary presentations at national and international meetings.
