In Situ Transmission Electron Microscopy Measurements of Ge Nanowire Synthesis with Liquid Metal Nanodroplets in Water

Formation and characterization of Group IV semiconductor nanowires

To enable the application to next-generation devices of semiconductor nanowires (NWs), it is important to control their formation and tune their functionality by doping and the use of heterojunctions. In this paper, we introduce formation and the characterization methods of nanowires, focusing on our research results. We describe a top-down method of controlling the size and alignment of nanowires that shows advantages over bottom-up growth methods. The latter technique causes damage to the nanowire surfaces, requiring defect removal after the NW formation process. We show various methods of evaluating the bonding state and electrical activity of impurities in NWs. If an impurity is doped in a NW, mobility decreases due to the scattering that it causes. As a strategy for solving this problem, we describe research into core-shell nanowires, in which Si and Ge heterojunctions are formed in the diameter direction inside the NW. This structure can separate the impurity-doped region from the carrier transport region, promising as a channel for the new ultimate high-mobility transistor.

Upper critical solution temperature polymer assemblies via variable temperature liquid phase transmission electron microscopy and liquid resonant soft X-ray scattering

Here, we study the upper critical solution temperature triggered phase transition of thermally responsive poly(ethylene glycol)-block-poly(ethylene glycol) methyl ether acrylate-co-poly(ethylene glycol) phenyl ether acrylate-block-polystyrene nanoassemblies in isopropanol. To gain mechanistic insight into the organic solution-phase dynamics of the upper critical solution temperature polymer, we leverage variable temperature liquid-cell transmission electron microscopy correlated with variable temperature liquid resonant soft X-ray scattering. Heating above the upper critical solution temperature triggers a reduction in particle size and a morphological transition from a spherical core shell particle with a complex, multiphase core to a micelle with a uniform core and Gaussian polymer chains attached to the surface. These correlated solution phase methods, coupled with mass spectral validation and modeling, provide unique insight into these thermoresponsive materials. Moreover, we detail a generalizable workflow for studying complex, solution-phase nanomaterials via correlative methods.

Understanding and Expanding the Prospects for Electrosynthesis with Liquid Metal Electrodes

ConspectusThis Account describes and summarizes the latest work from our laboratory on developing and maturing strategies based on low-temperature liquid metals as reaction environments for materials synthesis. The electrochemical liquid-liquid-solid (ec-LLS) crystal growth concept is a hybrid method that combines electrodeposition and melt crystal growth. Using liquid metals as both electrodes and solvents for the purpose of producing inorganic crystals and materials, a simple and environmentally friendly process is possible. The impetus is to address the key deficiency in the inorganic crystalline materials that are the basis of modern optoelectronics and renewable energy capture/conversion systems. Specifically, existing methods for synthesizing crystalline inorganic materials for these purposes are largely energy- and resource-intensive, with a substantial impact on the environment when scaled. A long-term goal of our work with ec-LLS is to realize a materials synthetic process that is matured without requiring intensive resources or negatively impacting the environment. To this end, the factors that both limit and govern ec-LLS processes must be identified and understood. To date, questions regarding the factors that affect crystal nucleation and growth, form factors, and overall composition remain.Previous work established concretely ec-LLS as a versatile method for synthesizing and producing crystalline semiconductors at low temperatures as either particles, nanowires, or microwires. Subsequent experiments have focused on two tiers. First, the microscopic details of the liquid metal and its interfaces that dictate materials synthesis and crystal growth must be identified. Second, strategies that widen the attainable material form factors to facilitate device architectures must be realized. Hence, this Account describes results aimed at answering three questions: (1) What are the consequences of reaching supersaturation by an electrochemical rather than a thermal driving force for crystal growth in ec-LLS? (2) Can the location of nucleation and subsequent crystal growth be controlled? (3) Does the atomic structure of the liquid metal affect product formation in ec-LLS? The science described herein illustrates the value of in situ methods spanning transmission electron microscopy, X-ray diffraction, and X-ray reflectance for revealing the role that liquid metal composition and structure can play in ec-LLS. Additionally, we summarize work that shows for the first time that it is possible to produce both single-crystalline epitaxial films and complex intermetallic compounds through ec-LLS by tuning the cell design, electrochemical excitation waveform, and composition of the liquid metal electrodes.The cumulative findings described here substantially enrich our understanding of the ec-LLS concept while simultaneously motivating further questions moving forward. Is it possible to attain complete control over the crystalline quality and composition of ec-LLS products? Can the materials produced by ec-LLS provide tailored functional properties for targeted applications? Can the ec-LLS strategy be further refined to allow material synthesis and deposition at precise locations with deterministically chosen form factors? What synthetic pathways are accessible when even more sophisticated electrochemical waveforms and cell designs are used? Our hope is that this Account will spur additional researchers to help answer such questions.

In Situ Kinetic Observations on Crystal Nucleation and Growth

Nucleation and growth are critical steps in crystallization, which plays an important role in determining crystal structure, size, morphology, and purity. Therefore, understanding the mechanisms of nucleation and growth is crucial to realize the controllable fabrication of crystalline products with desired and reproducible properties. Based on classical models, the initial crystal nucleus is formed by the spontaneous aggregation of ions, atoms, or molecules, and crystal growth is dependent on the monomer's diffusion and the surface reaction. Recently, numerous in situ investigations on crystallization dynamics have uncovered the existence of nonclassical mechanisms. This review provides a summary and highlights the in situ studies of crystal nucleation and growth, with a particular emphasis on the state-of-the-art research progress since the year 2016, and includes technological advances, atomic-scale observations, substrate- and temperature-dependent nucleation and growth, and the progress achieved in the various materials: metals, alloys, metallic compounds, colloids, and proteins. Finally, the forthcoming opportunities and challenges in this fascinating field are discussed.

Role of chemisorbing species in growth at liquid metal-electrolyte interfaces revealed by in situ X-ray scattering

Liquid-liquid interfaces offer intriguing possibilities for nanomaterials growth. Here, fundamental interface-related mechanisms that control the growth behavior in these systems are studied for Pb halide formation at the interface between NaX + PbX₂ (X = F, Cl, Br) and liquid Hg electrodes using in situ X-ray scattering and complementary electrochemical and microscopy measurements. These studies reveal a decisive role of the halide species in nucleation and growth of these compounds. In Cl- and Br-containing solution, deposition starts by rapid formation of well-defined ultrathin (∼7 Å) precursor adlayers, which provide a structural template for the subsequent quasi-epitaxial growth of c-axis oriented Pb(OH)X bulk crystals. In contrast, growth in F-containing solution proceeds by slow formation of a more disordered deposit, resulting in random bulk crystal orientations on the Hg surface. These differences can be assigned to the interface chemistry, specifically halide chemisorption, which steers the formation of these highly textured deposits at the liquid-liquid interface.

Growth at liquid-liquid interfaces differ inherently from that on solids, making it attractive for nanomaterial formation. Here, the authors use X-ray scattering to derive a detailed microscopic picture of lead-halide growth on liquid mercury that reveals the key importance of anion adsorption.

The Native Oxide Skin of Liquid Metal Ga Nanoparticles Prevents Their Rapid Coalescence during Electrocatalysis

Liquid metals (LMs) have been used in electrochemistry since the 19th century, but it is only recently that they have emerged as electrocatalysts with unique properties, such as inherent resistance to coke poisoning, which derives from the dynamic nature of their surface. The use of LM nanoparticles (NPs) as electrocatalysts is highly desirable to enhance any surface-related phenomena. However, LM NPs are expected to rapidly coalesce, similarly to liquid drops, which makes their implementation in electrocatalysis hard to envision. Herein, we demonstrate that liquid Ga NPs (18 nm, 26 nm, 39 nm) drive the electrochemical CO₂ reduction reaction (CO₂RR) while remaining well-separated from each other. CO is generated with a maximum faradaic efficiency of around 30% at -0.7 V_RHE, which is similar to that of bulk Ga. The combination of electrochemical, microscopic, and spectroscopic techniques, including operando X-ray absorption, indicates that the native oxide skin of the Ga NPs is still present during CO₂RR and provides a barrier to coalescence during operation. This discovery provides an avenue for future development of Ga-based LM NPs as a new class of electrocatalysts.

Versatile Printing of Substantial Liquid Cells for Efficiently Imaging In Situ Liquid-Phase Dynamics

Through its ability to image liquid-phase dynamics at nano/atomic-scale resolution, liquid-cell electron microscopy is essential for a wide range of applications, including wet-chemical synthesis, catalysis, and nanoparticle tracking, for which involved structural features are critical. However, statistical investigations by usual techniques remain challenging because of the difficulty in fabricating substantial liquid cells with appreciable efficiency. Here, we report a general approach for efficiently printing huge numbers of ready-to-use liquid cells (∼9000) within 30 s by electrospinning, with the unique feature of statistical liquid-phase studies requiring only one experimental time slot. Our solution efficiently resolves a complete transition picture of bubble evolution and also the induced nanoparticle motion. We statistically quantify the effect of the electron dose rate on the bubble variation and conclude that the bubble-driven nanoparticle motion is a ballistic-like behavior insignificant to morphological asymmetries. The versatile approach here is critical for statistical research, offering great opportunities in liquid-phase-associated dynamic studies.

Reversible Underwater Adhesion for Soft Robotic Feet by Leveraging Electrochemically Tunable Liquid Metal Interfaces

Soft crawling robots have potential applications for surveillance, rescue, and detection in complex environments. Despite this, most existing soft crawling robots either use nonadjustable feet to passively induce asymmetry in friction to actuate or are only capable of moving on surfaces with specific designs. Thus, robots often lack the ability to move along arbitrary directions in a two-dimensional (2D) plane or in unpredictable environments such as wet surfaces. Here, leveraging the electrochemically tunable interfaces of liquid metal, we report the development of liquid metal smart feet (LMSF) that enable electrical control of friction for achieving versatile actuation of prismatic crawling robots on wet slippery surfaces. The functionality of the LMSF is examined on crawling robots with soft or rigid actuators. Parameters that affect the performance of the LMSF are investigated. The robots with the LMSF prove capable of actuating across different surfaces in various solutions. Demonstration of 2D locomotion of crawling robots along arbitrary directions validates the versatility and reliability of the LMSF, suggesting broad utility in the development of advanced soft robotic systems.