We demonstrate a new design of graphene liquid cell consisting of a thin lithographically patterned hexagonal boron nitride crystal encapsulated from both sides with graphene windows [Kelly et al Nano Letters, in press 2018]. The resulting ultrathin window liquid cells are robust to vacuum cycling and can be produced with precisely controlled volumes and thicknesses as required for a specific experiment. The high stability of such cells allows us to demonstrate an order of magnitude improvement in the element mapping capabilities compared to previous cells, with 1 nm spatial resolution elemental mapping achievable using energy dispersive X-ray spectroscopy (EDXS). We apply this ability to observe the beam induced core shell structure of FePt nanoparticles. The presence of water was confirmed using electron energy loss spectroscopy (EELS) via the detection of the oxygen K-edge and measuring the thickness of full and empty cells.
We further demonstrate the atomic resolution imaging capabilities of these liquid cells by tracking the dynamic motion and interactions of small metal nanoparticles with diameters of 0.5−5 nm. A statistical analysis of ∼5000 measured displacements for individual particles found diffusivities of D = 3.25 × 10−3 nm2 s−1 for larger particles (with a mean size of 2.84 nm2 and standard deviation 0.44 nm2) and D = 6.18 × 10−3 nm2 s−1 for smaller particles (with a mean size 1.26 nm2, and standard deviation 0.55 nm2). These values are consistent with a previous observation of particles within graphene liquid cells,[Yuk et al Science 2012] but 10−100 times lower than those usually observed for SiN windowed liquid cells and over 106 times smaller than expected values for bulk water.
This technology enables new opportunities for the application of graphene liquid cells to a host of in situ transmission electron microscope studies, providing a reliable platform for high resolution TEM imaging and spectral mapping.