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.

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.

Rechargeable aqueous Zn-ion batteries (ZIBs) are very promising for large-scale grid energy storage applications owing to their low cost, environmentally benign constituents, excellent safety, and relatively high energy density. Their usage, however, is largely hampered by the fast capacity fade. The complexity of the reactions has resulted in long-standing ambiguities of the chemical pathways of Zn/MnO 2 system. In this study, we find that both H + /Zn 2+ intercalation and conversion reactions occur at different voltages and that the rapid capacity fading can clearly be ascribed to the rate-limiting and irreversible conversion reactions at a lower voltage. By limiting the irreversible conversion reactions at â1.26 V, we successfully demonstrate ultrahigh power and long life that are superior to most of the reported ZIBs or even some lithium-ion batteries.

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.