High resolution and analytical transmission electron microscopy in a liquid flow cell via gas purging

Abstract text (incl. figure legends and references)

Liquid phase electron microscopy (LPEM) based on sandwiched MEMS sample carriers provides the means to

observe time-resolved dynamics in a liquid state. Until now, LPEM has been widely used in materials science, energy and life science, providing fundamental insights into nucleation and growth, the dynamical evolution of key materials in batteries and fuel cells, as well as the 3D imaging of biomolecules [1]. Compared to liquid cells without a flowing function (such as static graphene pocket cells), liquid flow cells have obvious advantages. This includes the control of the liquid environment, the modulation of the effect of electron beam irradiation [2] and the integration of functional electrodes for heating or/and biasing. Due to the pressure difference between the TEM column (~ 0 bar) and the enclosed liquid cell (~1 bar), the two membranes (silicon nitride with a typical thickness of ~50 nm) bulge outwards, resulting in a thick liquid layer, which can reach more than 1 micrometer in the cell center region. Therefore, performing high resolution and analytical electron microscopy studies in a liquid flow cell comes with a multitude of challenges.

Several strategies have been proposed to solve this issue, including (1) decreasing the membrane thickness or replacing it with ultrathin materials e.g. graphene, h-BN, MoS2, etc. [3], (2) developing novel cell configurations, namely hole array patterns [4] and nanochannel [5], to avoid or decrease the bulging, (3) generating a gas bubble via electron beam irradiation [6,7], (4) generating a gas bubble via electrochemical water splitting [8] and (5) mitigating the window´s bulging by changing the pressure difference between the cell and TEM column, either via an external pressure controller [9,10] or via the internal Laplace pressure [10]. Those methods have been proven useful in high resolution and analytical electron microscopy studies in LPEM, however, there are also intrinsic limitations in each method.

In this work, we propose a general and robust method to perform high resolution and analytical electron microscopy studies in a flow cell (the Stream Nano-Cell), which can be implemented during liquid heating or liquid biasing experiments. Thanks to the on-chip flow channel of the Stream Nano-Cell [11], the liquid in the field of view can be removed by flowing gas (including inert gases to avoid problems with air sensitivity), which is termed "purging". This purging method enables the acquisition of high-resolution TEM images, chemical composition and valence analysis through energy-dispersive X-ray spectroscopy (EDX) mapping and Electron Energy-Loss Spectroscopy (EELS), respectively. In addition, the purging approach is both reversible and reproducible, which therefore enables the alternation between a full cell and a thin liquid configuration to study liquid-thickness-dependent physical and chemical phenomena.

References

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