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2D transition metal dichalcogenides (TMDs) play increasingly significant roles in research and future optoelectronics. However, the large-scale deposition of 2D TMDs remains challenging due to sparse nucleation and substrate dependency. Liquid metals can offer effective solutions to meet these challenges due to their reactive, non-polarized, and templating properties. Here, self-deposition of 2D molybdenum sulfide is shown by introducing a molybdenum precursor onto the surface of a eutectic alloy of gallium and indium (EGaIn). EGaIn serves as an ultra-smooth template and reducing agent for the precursor to form large-scale planar molybdenum sulfides, which is transferrable to any substrate. The molybdenum sulfides form spontaneously on the surface of EGaIn, which has a sufficient potential to drive the cathodic reactions of the deposition process. A highly crystalline 2H-MoS2 is obtained after a final annealing step. This work demonstrates a fundamentally new capability for the formation of large-scale 2D TMDs.

The emergence of piezoelectricity in two-dimensional (2D) materials has represented a milestone towards employing low-dimensional structures for future technologies. 2D piezoelectric materials possess unique and unprecedented characteristics that cannot be found in other morphologies; therefore, the applications of piezoelectricity can be substantially extended. By reducing the thickness into the 2D realm, piezoelectricity might be induced in otherwise non-piezoelectric materials. The origin of the enhanced piezoelectricity in such thin planes is attributed to the loss of centrosymmetry, altered carrier concentration, and change in local polarization and can be efficiently tailored via surface modifications. Access to such materials is important from a fundamental research point of view, to observe the extraordinary interactions between free charge carriers, phonons and photons, and also with respect to device development, for which planar structures provide the required compatibility with the large-scale fabrication technologies of integrated circuits. The existence of piezoelectricity in 2D materials presents great opportunities for applications in various fields of electronics, optoelectronics, energy harvesting, sensors, actuators and biotechnology. Additionally, 2D flexible nanostructures with superior piezoelectric properties are distinctive candidates for integration into nano-scale electromechanical systems. Here we fundamentally review the state of the art of 2D piezoelectric materials from both experimental and theoretical aspects and report the recent achievements in the synthesis, characterization and applications of these materials.

Phonon-polaritons (PhPs) in layered crystals, including hexagonal boron nitride (hBN), have been investigated by combined scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared (FTIR) spectroscopy. Nevertheless, many of such s-SNOM-based FTIR spectra features remain unexplored, especially those originated from the impact of boundaries. Here we observe real-space PhP propagations in thin-layer hBN sheets either supported or suspended by s-SNOM imaging. Then with a high-power broadband IR laser source, we identify two major peaks and multiple auxiliary peaks in the near-field amplitude spectra, obtained using scattering-type near-field FTIR spectroscopy, from both supported and suspended hBN. The major PhP propagation interference peak moves toward the major in-plane phonon peak when the IR illumination moves away from the hBN edge. Specific differences between the auxiliary peaks in the near-field amplitude spectra from supported and suspended hBN sheets are investigated regarding different boundary conditions, associated with edges and substrate interfaces. The outcomes may be explored in heterostructures for advanced nanophotonic applications.

The nascent field of nanotechnology-enabled metallurgy has great potential. However, the role of eutectic alloys and the nature of alloy solidification in this field are still largely unknown. To demonstrate one of the promises of liquid metals in the field, we explore a model system of catalytically active Bi-Sn nano-alloys produced using a liquid-phase ultrasonication technique and investigate their phase separation, surface oxidation, and nucleation. The Bi-Sn ratio determines the grain boundary properties and the emergence of dislocations within the nano-alloys. The eutectic system gives rise to the smallest grain dimensions among all Bi-Sn ratios along with more pronounced dislocation formation within the nano-alloys. Using electrochemical CO2 reduction and photocatalysis, we demonstrate that the structural peculiarity of the eutectic nano-alloys offers the highest catalytic activity in comparison with their non-eutectic counterparts. The fundamentals of nano-alloy formation revealed here may establish the groundwork for creating bimetallic and multimetallic nano-alloys.

Exploring and controlling surface tension‐driven phenomena in liquid metals may lead to unprecedented possibilities for next‐generation microfluidics, electronics, catalysis, and materials synthesis. In pursuit of these goals, the impact of minor constituents within liquid alloys is largely overlooked. Herein, it is showed that the presence of a fraction of solute metals such as tin, bismuth, and zinc in liquid gallium can significantly influence their electrocapillarity and electrochemistry. The instability‐driven fractal formation of liquid alloy droplets is investigated with different solutes and reveals the formation of distinctive non‐branched droplets, unstable fractals, and stable fractal modes under controlled voltage and alkaline solution conditions. In their individually unique fractal morphology diagrams, different liquid alloys demonstrate significantly shifted voltage thresholds in transition between the three fractal modes, depending on the choice of the solute metal. Surface tension measurements, cycle voltammetry and surface compositional characterizations provide strong evidence that the minor alloy components drastically alter the surface tension, surface electrochemical oxidation, and oxide dissolution processes that govern the droplet deformation and instability dynamics. The findings that minor components are able to regulate liquid alloys' surface tensions, surface element distributions and electrochemical activities offer great promises for harnessing the tunability and functionality of liquid metals. [ABSTRACT FROM AUTHOR]

Atomically thin, mechanically flexible, memory‐functional, and power‐generating crystals play a crucial role in the technological advancement of portable devices. However, the adoption of these crystals in such technologies is sometimes impeded by expensive and laborious synthesis methods, as well as the need for large‐scale, mechanically stable, and air‐stable materials. Here, an instant‐in‐air liquid metal printing process utilizing liquid bismuth (Bi) is presented, forming naturally occurring, air‐stable, atomically thin, mechanically flexible nanogenerators and ferroelectric oxides. Despite the centrosymmetric nature of the monoclinic P21/c system of achieved α‐Bi2O3‐δ the high kinetics of liquid metal synthesis leads to the formation of vacancies that disrupt the symmetry which is confirmed by density functional theory (DFT) calculations. The polarization switching is measured and utilized for ferroelectric nanopatterning. The exceptional attributes of these atomically thin multifunctional stable oxides, including piezoelectricity, mechanical flexibility, and polarizability, present significant opportunities for developing nano‐components that can be seamlessly integrated into a wide range of devices. [ABSTRACT FROM AUTHOR]

Liquid metals provide new dimensions for controlling and governing of reactions. The concept of freely moving metal solutes in liquid metals can be potentially used for enhancing and tuning interfacial reactions. In this work, zinc (Zn), as the solute metal is shown that, can move to the interface, enriching the interfacial area for the efficient synthesis of highly crystalline and porous zeolitic imidazolate framework‐8 (ZIF‐8). The highest rate of reaction is illustrated, and the best quality ZIF‐8, are obtained when a eutectic liquid metal system of Zn with gallium (Ga), containing 3.6 wt.% of Zn, is implemented. It is computationally shown that a combination of Ga activation and freely moving Zn leads to the highest reaction rate and better coordination with organic linkers. The comparisons with solid samples and non‐eutectic systems of Ga liquid demonstrate the advantages of eutectic mix. The work can be expanded to a variety of future interfacial reactions of zeolite imidazole frameworks for commercial scale up. [ABSTRACT FROM AUTHOR]

CO2 sensing is important in many applications ranging from air‐quality monitoring to food packaging. In this study, an amine‐functionalized copolymer, poly(N‐[3‐(dimethylamino)propyl]‐methacrylamide‐co‐2‐N‐morpholinoethyl methacrylate) (p(D‐co‐M)) is synthesized, offering moderate basicity suitable for a wide CO2 detection range. Taking advantage of this characteristic of p(D‐co‐M), this polymer is used for designing a chemiresistive, low‐cost, flexible, and reversible CO2 sensor. The p(D‐co‐M)‐based sensors show a noticeable decrease in their direct current resistance and alternating current impedance upon exposure to a wide range of CO2 concentration (1–100%) at room temperature with a response and a recovery time of 6 and 14 min, respectively. Additionally, the p(D‐co‐M)‐based sensors demonstrate a favorable selectivity to CO2 in the presence of interfering gases including methanol, ethanol, toluene, and acetone. Surface potential measurements show an increase of +6.34 V upon exposure to humidity and CO2, indicating the protonation of the polymer's amine sites, facilitating the detection of CO2 in the wet environment. This sensor is efficient for detecting CO2 concentration released during fermentation of kimchi as a food model. [ABSTRACT FROM AUTHOR]

Gallium (Ga) is a low melting point metal in the liquid state in the biological environment which presents a unique combination of fluidity, softness, and metallic electrical and thermal properties. In this work, liquid Ga is proposed as a biocompatible electrode material for cell culture by electro-stimulation since the cytotoxicity of Ga is generally considered low and some Ga compounds have been reported to exhibit anti-bacterial and anti-inflammatory activities. Complementarily, polydopamine (PDA) was coated on liquid Ga to increase the attachment capability of cells on the liquid Ga electrode and provide enhanced biocompatibility. The liquid Ga layer could be readily painted at room temperature on a solid inert substrate, followed by the formation of a nanoscale PDA coating layer resulting in a conformable and biocompatible composite electrode. The PDA layer was shown to coordinate with Ga3+, which is sourced from liquid Ga, providing electrical conductivity in the cell culture medium. The PDA–Ga3+ composite acted as a conductive substrate for advanced electro-stimulation for cell culture methods of representative animal fibroblasts. The cell proliferation was observed to increase by ∼143% as compared to a standard glass coverslip at a low potential of 0.1 V of direct coupling stimulation. This novel PDA–Ga3+ composite has potential applications in cell culture and regenerative medicine. [ABSTRACT FROM AUTHOR]