Transmission electron microscopy offers structural and compositional information with atomic resolution, but its use is restricted to thin, solid samples. Liquid samples, particularly those involving ...water, have been challenging because of the need to form a thin liquid layer that is stable within the microscope vacuum. Liquid cell electron microscopy is a developing technique that allows us to apply the powerful capabilities of the electron microscope to imaging and analysis of liquid specimens. We describe its impact in materials science and biology. We discuss how its applications have expanded via improvements in equipment and experimental techniques, enabling new capabilities and stimuli for samples in liquids, and offering the potential to solve grand challenge problems.
Imaging samples in liquids with electron microscopy can provide unique insights into biological systems, such as cells containing labelled proteins, and into processes of importance in materials ...science, such as nanoparticle synthesis and electrochemical deposition. Here we review recent progress in the use of electron microscopy in liquids and its applications. We examine the experimental challenges involved and the resolution that can be achieved with different forms of the technique. We conclude by assessing the potential role that electron microscopy of liquid samples can play in areas such as energy storage and bioimaging.
Liquid cell electron microscopy enables direct in situ imaging of processes in liquids and objects suspended in liquids with nanoscale resolution. However, the irradiating electrons affect the ...chemistry of the suspending medium, typically an aqueous solution, producing molecular and radical products such as hydrogen, oxygen, and hydrated (solvated) electrons. These may impact the imaged structures and phenomena. A good understanding of the interactions between the electrons and the irradiated medium is necessary to correctly interpret experiments, minimize artifacts, and take advantage of the irradiation. We predict the composition of water subjected to electron irradiation in the electron microscope. We reinterpret available experimental data, such as beam-induced variations in pH and colloid aggregation, in light of our predictions and show new observations of crystallization and etching as functions of dose rate, resolving conflicting reports in the scientific literature. We make our computer code available to readers. Our predictive model is useful for designing experiments that minimize unwanted beam effects, extending liquid cell microscopy to new applications, taking advantage of beam effects for nanomanufacturing such as the patterning of nanostructures, and correctly interpreting experimental observations. Additionally, our results indicate that liquid cells provide a new tool to study radiolysis effects on materials and processes.
Liquid cell electron microscopy has emerged as a powerful technique for in situ studies of nanoscale processes in liquids. An accurate understanding of the interactions between the electron beam and ...the liquid medium is essential to account for, suppress, and exploit beam effects. We quantify the interactions of high energy electrons with water, finding that radiolysis plays an important role, while heating is typically insignificant. For typical imaging conditions, we find that radiolysis products such as hydrogen and hydrated electrons achieve equilibrium concentrations within seconds. At sufficiently high dose-rate, the gaseous products form bubbles. We image bubble nucleation, growth, and migration. We develop a simplified reaction-diffusion model for the temporally and spatially varying concentrations of radiolysis species and predict the conditions for bubble formation by H2. We discuss the conditions under which hydrated electrons cause precipitation of cations from solution and show that the electron beam can be used to “write” structures directly, such as nanowires and other complex patterns, without the need for a mask.
The atomic structure at the interface between two-dimensional (2D) and three-dimensional (3D) materials influences properties such as contact resistance, photo-response, and high-frequency electrical ...performance. Moiré engineering is yet to be utilized for tailoring this 2D/3D interface, despite its success in enabling correlated physics at 2D/2D interfaces. Using epitaxially aligned MoS
/Au{111} as a model system, we demonstrate the use of advanced scanning transmission electron microscopy (STEM) combined with a geometric convolution technique in imaging the crystallographic 32 Å moiré pattern at the 2D/3D interface. This moiré period is often hidden in conventional electron microscopy, where the Au structure is seen in projection. We show, via ab initio electronic structure calculations, that charge density is modulated according to the moiré period, illustrating the potential for (opto-)electronic moiré engineering at the 2D/3D interface. Our work presents a general pathway to directly image periodic modulation at interfaces using this combination of emerging microscopy techniques.
Abstract
Extraordinary optoelectronic properties of van der Waals (vdW) heterostructures can be tuned via strain caused by mechanical deformation. Here, we demonstrate strong and localized ...luminescence in the ultraviolet region from interface bubbles between stacked multilayers of hexagonal boron nitride (hBN). Compared to bubbles in stacked monolayers, bubbles formed by stacking vdW multilayers show distinct mechanical behavior. We use this behavior to elucidate radius- and thickness-dependent bubble geometry and the resulting strain across the bubble, from which we establish the thickness-dependent bending rigidity of hBN multilayers. We then utilize the polymeric material confined within the bubbles to modify the bubble geometry under electron beam irradiation, resulting in strong luminescence and formation of optical standing waves. Our results open a route to design and modulate microscopic-scale optical cavities via strain engineering in vdW materials, which we suggest will be relevant to both fundamental mechanical studies and optoelectronic applications.
Liquid cell transmission electron microscopy has become a powerful and increasingly accessible technique for in situ studies of nanoscale processes in liquid and solution phase. Exploring reaction ...mechanisms in electrochemical or crystal growth processes requires precise control over experimental conditions, with temperature being one of the most critical factors. Here we carry out a series of crystal growth experiments and simulations at different temperatures in the well-studied system of Ag nanocrystal growth driven by the changes in redox environment caused by the electron beam. Liquid cell experiments show strong changes in both morphology and growth rate with temperature. We develop a kinetic model to predict the temperature-dependent solution composition, and we discuss how the combined effect of temperature-dependent chemistry, diffusion, and the balance between nucleation and growth rates affect the morphology. We discuss how this work may provide guidance in interpreting liquid cell TEM and potentially larger-scale synthesis experiments for systems controlled by temperature.
Control of interfacial morphology in electrochemical processes is essential for applications ranging from nanomanufacturing to batteries. Here, we quantify the evolution of an electrochemical growth ...front, using liquid cell electron microscopy to access unexplored length and time scales. During galvanostatic deposition of copper from an acidic electrolyte, we find that the growth front initially evolves consistent with kinetic roughening theory. Subsequently, it roughens more rapidly, consistent with diffusion-limited growth physics. However, the onset of roughening is strongly delayed compared to expectations, suggesting the importance of lateral diffusion of ions. Based on these growth regimes, we discuss morphological control and demonstrate the effects of two strategies, pulse plating and the use of electrolyte additives.
Epitaxial growth is a fundamental step required to create devices for the semiconductor industry, enabling different materials to be combined in layers with precise control of strain and defect ...structure. Patterning the growth substrate with a mask before performing epitaxial growth offers additional degrees of freedom to engineer the structure and hence function of the semiconductor device. Here, we demonstrate that conditions exist where such epitaxial lateral overgrowth can produce complex, three-dimensional structures that incorporate cavities of deterministic size. We grow germanium on silicon substrates patterned with a dielectric mask and show that fully-enclosed cavities can be created through an unexpected self-assembly process that is controlled by surface diffusion and surface energy minimization. The result is confined cavities enclosed by single crystalline Ge, with size and position tunable through the initial mask pattern. We present a model to account for the observed cavity symmetry, pinch-off and subsequent evolution, reflecting the dominant role of surface energy. Since dielectric mask patterning and epitaxial growth are compatible with conventional device processing steps, we suggest that this mechanism provides a strategy for developing electronic and photonic functionalities.
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