Crystallization in supersaturated solution plays a fundamental role in a variety of natural and industrial processes. However, a thorough understanding of crystallization phenomena in supersaturated solution is still difficult because the real-time visualization of crystallization processes under supersaturated condition is a great challenge. Herein, an electron beam-induced crystallization method is carried out in in situ liquid cell TEM to visualize the crystallization of NaCl under supersaturated condition in real time. Crucial steps and behaviors in the crystallization of NaCl are captured and clarified, including the growth of NaCl nanocrystals with different morphologies, the formation of initial crystalline seeds from amorphous ion clusters, and the non-equilibrium growth behaviors caused by uneven distribution of precursor ions. This study provides the real-time visualization of detailed nucleation and growth behaviors in NaCl crystallization and brings an ideal strategy for investigating crystallization phenomena under supersaturated condition.

In pursuit of miniaturization in the semiconductor industry, two-dimensional (2D) materials are used to fabricate new electronic devices. The topological insulator (TI) material bismuth telluride (Bi₂Te₃), as an emerging 2D material, has potential applications in electronic and spintronic devices due to its unique electrical properties. It is well known that the surface-to-volume ratio increases as the thickness of the material decreases, resulting in a more prominent edge effect. Therefore, for a single-layer Bi₂Te₃, the atomic structure of the edge plays a crucial role in its electrical properties. Here, combining first-principles calculations and in situ transmission electron microscopy (TEM) experimental studies, we report that there are two types of edge structures in single-layer Bi₂Te₃: semiconducting flat edges and metallic zigzag edges. The dynamic evolution process of the edge structure with atomic resolution shows that the proportions of these two edges change with continuous electron beam irradiation. Our findings demonstrate the viability to use electron beam as an effective tool to precisely tailor the edge of Bi₂Te₃ with desired properties, which paves the way for implementation of single-layer Bi₂Te₃ in electronics and spintronics.

Two-dimensional (2D) crystals are attractive due to their intriguing structures and properties which are strongly dependent on the synthesis conditions. To achieve their superior properties, it is of critical importance to fully understand the growth processes and mechanisms for tailored design and controlled growth of 2D crystals. Due to the high spatiotemporal resolution and the capability to mimic the realistic growth conditions, in situ transmission electron microscopy (TEM) becomes an effective way to monitor the growth process in real-time at the atomic scale, which is expected to provide atomic-scale insights into the nucleation and growth of 2D crystals. Here we review the recent in situ TEM works on the formation of 2D crystals under electron irradiation, thermal excitation as well as voltage bias. The underlying mechanisms are also elucidated in detail, providing key insights into the nucleation and formation of 2D crystals.

Two-dimensional (2D) transition metal chalcogenides (TMCs) are known to be susceptible to the atmosphere, which greatly obscures the intrinsic physical and chemical properties. The quantitative origin of the instability on the atomic scale has not been well investigated due to the lack of environmentally stable TMCs sample. Here, we find the stability of the grown TMCs is strongly relevant to their initial element ratios, and thus the stoichiometric bonded TMCs have favorable stability, benefitted from the TMCs with controllable chalcogenisation. In this study, the degree of structural degradation has been quantitatively defined by the reduced element ratio of chalcogen to metal through the time-dependent characterizations, and the non-stoichiometric ratios in TMCs reveal the atomic lattices with point defects like additive bonds or vacancies inside. This study not only provides a potential view to fabricate environmentally stable TMCs based devices, but also will bring an effective feasibility of stacking stable vertical heterostructures.

Solid state phase transformations have drawn great attention because they can be effectively exploited to control the microstructure and property of materials. Understanding the physics of such phase transformation processes is critical to designing materials with controlled structure and with desired properties. However, in traditional ex situ experiments, it is hard to achieve position controlled phase transformations or obtain desirable crystal phase on nanometer scale. Meanwhile the underlying mechanisms of the reaction processes are not fully understood due to the lack of direct and real-time observation. In this paper, we observe phase transformation from body-centered tetragonal PX-PbTiO₃ to monoclinic TiO₂(B) on the atomic scale by in situ electron irradiation during heat treatment in transmission electron microscope, at pre-defined locations on the sample. We demonstrate that by controlling the location of the incident electron beam, a porous TiO₂(B) crystal structure can be formed at the desired area on the nanowire, which is difficult to achieve by traditional synthesis methods. Upon in situ heating, the Pb atoms in the crystal migrate out of the pristine nanowire through inelastic scattering under incident electrons while high temperature(> 400 °C) provides energy for the crystallization of TiO₂(B) and the volatilization of a substantial number of Pb atoms, which makes the resultingTiO₂(B) nanowires to be porous. In contrast, at temperatures < 400 °C, the segregated Pb atoms form Pb particles and the TiO_x nanowires remain in the amorphous state. This work not only provides in situ visualization of the phase transition from the PX-PbTiO₃ to monoclinic TiO₂(B), but also suggests a crystallography engineering strategy to obtain the desired crystal phase at controlled locations on the nanometer scale.

Because of its high compatibility with conventional microfabrication processing technology, epitaxial graphene (EG) grown on SiC shows exceptional promise for graphene-based electronics. However, to date, a detailed understanding of the transformation from three-layer SiC to monolayer graphene is still lacking. Here, we demonstrate the direct atomic-scale observation of EG growth on a SiC (1100) surface at 1, 000 ℃ by in situ transmission electron microscopy in combination with ab initio molecular dynamics (AIMD) simulations. Our detailed analysis of the growth dynamics of monolayer graphene reveals that three SiC (1100) layers decompose successively to form one graphene layer. Sublimation of the first layer causes the formation of carbon clusters containing short chains and hexagonal rings, which can be considered as the nuclei for graphene growth. Decomposition of the second layer results in the appearance of new chains connecting to the as-formed clusters and the formation of a network with large pores. Finally, the carbon atoms released from the third layer lead to the disappearance of the chains and large pores in the network, resulting in a whole graphene layer. Our study presents a clear picture of the epitaxial growth of the monolayer graphene from SiC and provides valuable information forfuture developments in SiC-derived EG technology.