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Transparent conductive oxides (TCOs) are a high-performance material system that could enable new wearable sensors and electronics, but traditional fabrication methods face scalability and performance challenges. In this work, we utilize liquid metal printing to produce ultrathin two-dimensional (2D) indium tin oxide (ITO) films with superior microstructural, optical, and electrical properties compared to conventional techniques. We investigate the dynamics of grain growth and its influence on conductivity and the optical properties of 2D ITO, demonstrating the tunability through annealing and multilayer deposition. Additionally, we develop Au-decorated transparent electrodes, showcasing their adhesion and flexibility, low contact impedance, and biocompatibility. Leveraging the transparency of these electrodes, we enable enhanced simultaneous multimodal biosignal acquisition by integrating biopotential-based methods, such as electrocardiogram (ECG) or bioimpedance sensing (e.g., impedance plethysmography, IPG), with optical modalities like photoplethysmography (PPG). This study establishes CLMP-fabricated flexible 2D TCOs as a versatile platform for advanced bioelectronic systems and multimodal diagnostics. 

A conductive MXene (Ti₃C₂T_x) integrated with 2D Ni₃(HITP)₂-MOFvia in situsynthesis enhances active site exposure, boosting electrocatalytic HER performance for efficient, sustainable hydrogen production. 

Abstract— In this study, we present continuous liquid metal printing (CLMP) to produce flexible and transparent indium tin oxide (ITO) layers compatible with plastic substrates with low glass transition temperatures. By leveraging the low melting point of an indium-tin (In-Sn) alloy, we achieve spontaneous two-dimensional (2D) oxide growth at low temperatures, following Cabrera-Mott (CM) oxidation kinetics. A robotically controlled roller deforms the molten alloys, depositing a thin native oxide (ITO) via van der Waals adhesion across large areas (approximately 1200 cm²) in mere seconds. The printed 2D ITO is highly crystalline with large plate-like grains with an average size of 55 nm. They demonstrate low resistivity (approximately 714 μΩ⋅cm) and transparency (>92% in visible light) with an optical bandgap of 3.71 eV. Mechanical testing reveals superior adhesion, 2X greater bendability, and 3X better scratch resistance of flexible 2D ITO. Finally, we demonstrate an application towards flexible transparent electrocardiogram electrodes based on flexible 2D ITO. 

2D native surface oxides formed on low melting temperature metals such as indium and gallium offer unique opportunities for fabricating high-performance flexible electronics and optoelectronics based on a new class of liquid metal printing (LMP). An inherent property of these Cabrera-Mott 2D oxides is their suboxide nature (e.g., In2O3−x), which leads high mobility LMP semiconductors to exhibit high electron concentrations (ne > 1019 cm−3) limiting electrostatic control. Binary alloying of the molten precursor can produce doped, ternary metal oxides such as In-X-O with enhanced electronic performance and greater bias-stress stability, though this approach demands a deeper understanding of the native oxides of alloys. This work presents an approach for hypoeutectic rapid LMP of crystalline InGaOx (IGO) at ultralow process temperatures (180 °C) beyond the state of the art to fabricate transistors with 10X steeper subthreshold slope and high mobility (≈18 cm2 Vs−1). Detailed characterization of IGO crystallinity, composition, and morphology, as well as measurements of its electronic density of states (DOS), show the impact of Ga-doping and reveal the limits of doping induced amorphization from hypoeutectic precursors. The ultralow process temperatures and compatibility with high-k Al2O3 dielectrics shown here indicate potential for 2D IGO to drive low-power flexible transparent electronics.