Liquid Metal Printing Research Articles

Ultrathin α-Bi2O3 Thin-Film Transistor for Cost-Effective Oxide-TFT Inverters.

Electronics is advancing toward greater diversity and sustainability by prioritizing energy efficiency and cost-effectiveness. Metal oxide thin-film transistor (TFT) represents a technology at the forefront of advancing next-generation sustainable electronics, and exploring oxide channel compositions is a crucial step in opening opportunities for developing next-generation device applications. This study presents the first development of n-channel α-Bi2O3-TFTs using a 4 nm ultrathin channel prepared by a cost-effective vacuum-free and solvent-free liquid metal printing method in ambient air. Even the pristine device exhibited a clear TFT action but required a large negative gate bias to turn off due mainly to excess carriers from oxygen vacancy in the α-Bi2O3 channel. Oxygen-containing post-annealing reduced both channel carrier and subgap defect densities, enabling the development of depletion and enhancement-type α-Bi2O3-TFTs with the saturation mobility of 2-4 cm2 V-1 s-1. Two types of oxide-TFT-based inverter circuits, zero-VGS-NMOS and CMOS inverters, were fabricated by using α-Bi2O3-TFTs, operating in a high voltage gain of over 130. This work demonstrates the potential of oxide semiconductor materials toward the development of next-generation sustainable electronics.

Embedded Hybrid-Dimensional Heterointerface for Filament Modulation in 2D Material-Based Artificial Nociceptor.

Nociceptors are key sensory receptors that transmit warning signals to the central nervous system in response to painful stimuli. This fundamental process is emulated in an electronic device by developing a novel artificial nociceptor with an ultrathin, nonstoichiometric gallium oxide (GaOx)-silicon oxide heterostructure. A large-area 2D-GaOx film is printed on a substrate through liquid metal printing to facilitate the production of conductive filaments. This nociceptive structure exhibits a unique short-term temporal response following stimulation, enabling a facile demonstration of threshold-switching physics. The developed heterointerface 2D-GaOx film enables the fabrication of fast-switching, low-energy, and compliance-free 2D-GaOx nociceptors, as confirmed through experiments. The accumulation and extrusion of Ag in the oxide matrix are significant for inducing plastic changes in artificial biological sensors. High-resolution transmission electron microscopy and electron energy loss spectroscopy demonstrate that Ag clusters in the material dispersed under electrical bias and regrouped spontaneously when the bias is removed owing to interfacial energy minimization. Moreover, 2D nociceptors are stable; thus, heterointerface engineering can enable effective control of charge transfer in 2D heterostructural devices. Furthermore, the diffusive 2D-GaOx device and its Ag dynamics enable the direct emulation of biological nociceptors, marking an advancement in the hardware implementation of artificial human sensory systems.

Hypoeutectic Liquid Metal Printing of 2D Indium Gallium Oxide Transistors.

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 >1019cm-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 (≈18cm2Vs-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.

Advances in Liquid Metal Printed 2D Oxide Electronics

Abstract2D metal oxides (2DMOs) have recently emerged as a high‐performance class of ultrathin, wide bandgap materials offering exceptional electrical and optical properties for a wide area of device applications in energy, sensing, and display technologies. Liquid metal printing represents a thermodynamically advantageous strategy for synthesizing 2DMOs by a solvent‐free and vacuum‐free scalable method. Here, recent progress in the field of liquid metal printed 2D oxides is reviewed, considering how the physics of Cabrera‐Mott oxidation gives this rapid, low‐temperature process advantages over alternatives such as sol‐gel and nanoparticle processing. The growth, composition, and crystallinity of a burgeoning set of 1–3 nm thick liquid metal printed semiconducting, conducting, and dielectric oxides are analyzed that are uniquely suited for the fabrication of high‐performance flexible electronics. The advantages and limitations of these strategies are considered, highlighting opportunities to amplify the impact of 2DMO through large‐area printing, the design of doped metal alloys, stacking of 2DMO to electrostatically engineer new oxide heterostructures, and implementation of 2D oxide devices for gas sensing, photodetection, and neuromorphic computing.

Effect of Aging Heat Treatment in an Al-4008 Produced by Liquid Metal Printing

In today's world, additive manufacturing (AM) is one of the most popular technologies and has the potential to revolutionize the manufacturing industry. As one of the most recent advances in this industry, liquid metal printing has a growing value in the engineering field. This study aims to evaluate the effect of two heat treatment conditions in an Al-4008 alloy produced by this technique in the microstructure and mechanical properties. It was concluded that the heat treatment (HT) enhances the Si particle coalescence and Fe-rich intermetallic compound precipitation, increasing the sample hardness significantly (50%). Density analysis showed a slight porosity decrease with HT. Tensile tests indicated heat-treated, same-directionally pulled samples exhibited brittleness compared to as-printed ones, while HT increased both yield strength (245 MPa) and ultimate tensile strength (294 MPa).

Fast and Facile Liquid Metal Printing via Projection Lithography for Highly Stretchable Electronic Circuits.

Soft electronic circuits are crucial for wearable electronics, biomedical technologies, and soft robotics, requiring soft conductive materials with high conductivity, high strain limit, and stable electrical performance under deformation. Liquid metals (LMs) have become attractive candidates with high conductivity and fluidic compliance, while effective manufacturing methods are demanded. Digital light processing (DLP)-based projection lithography is a high-resolution and high-throughput printing technique for primarily polymers and some metals. If LMs can be printed with DLP as well, the entire soft devices can be fabricated by one printer in a streamlined and highly efficient process. Herein, fast and facile DLP-based LM printing is achieved. Simply with 5-10 s of patterned ultraviolet (UV)-light exposure, a highly conductive and stretchable pattern can be printed using a photo-crosslinkable LM particle ink. The printed eutectic gallium indium traces feature high resolution (≈20 µm), conductivity (3 × 106 S m-1), stretchability (≈2500%), and excellent stability (consistent performance at different deformation). Various patterns are printed in diverse material systems for broad applications including stretchable displays, epidermal strain sensors, heaters, humidity sensors, conformal electrodes for electrography, and multi-layer actuators. The facile and scalable process, excellent performance, and diverse applications ensure its broad impact on soft electronic manufacturing.

Bi2O3 doped indium tin oxide thin films fabricated via liquid metal printing

Indium tin oxide (ITO) is a classical transparent conducting oxide (TCO), which is widely used in semiconductor industry. Reducing the cost of preparing ITO films and further improving its optoelectronic properties are urgent technological challenges to be solved. Here, we proposed to tackle this issue through doping elements in ITO films via liquid metals printing process. Conceptual experiments demonstrate the synthesis of Bi2O3 doped ITO at the centimeter scale using low-temperature fabrication techniques, which display the advantages of high uniformity, strong visible light transmission and low resistivity. The band gap decreases from 3.88 eV to 3.07 eV with increasing doping of Bi2O3 and demonstrates n-type semiconductor nature. The Schottky diodes based on doped ITO films show typical Ohmic contact. The best output current of doped ITO Schottky diodes can reach 1261.7 μA at 10 V, which is about 41 times the output current of the undoped ITO Schottky diodes. Additionally, the nanofilm resistance decreased linearly with increasing Bi mass fraction in ITO film. Moreover, Schottky diodes have good stability in the atmosphere within half a year and multilayer printing can enhance output current. The process as established here offers significant opportunities for future synthesis and fabrication processes in the electronics industries.

Multi‐Functional Atomically Thin Oxides from Bismuth Liquid Metal

AbstractAtomically 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.

Multiple-State Nonvolatile Memory Based on Ultrathin Indium Oxide Film via Liquid Metal Printing.

In this work, the ultrathin two-dimensional (2D) indium oxide (InOx) with a large area of more than 100 μm2 and a high degree of uniformity was automatically peeled off from indium by the liquid-metal printing technique. Raman and optical measurements revealed that 2D-InOx has a polycrystalline cubic structure. By altering the printing temperature which affects the crystallinity of 2D-InOx, the mechanism of the existence and disappearance of memristive characteristics was established. The tunable characteristics of the 2D-InOx memristor with reproducible one-order switching was manifest from the electrical measurements. Further adjustable multistate characteristics of the 2D-InOx memristor and its resistance switching mechanism were evaluated. A detailed examination of the memristive process demonstrated the Ca2+ mimic dynamic in 2D-InOx memristors as well as the fundamental principles underlying biological and artificial synapses. These surveys allow us to comprehend a 2D-InOx memristor using the liquid-metal printing technique and could be applied to future neuromorphic applications and in the field of revolutionary 2D material exploration.

Liquid-Metal-Printed Ultrathin Oxides for Atomically Smooth 2D Material Heterostructures.

Two-dimensional (2D) semiconductors are promising channel materials for continued downscaling of complementary metal-oxide-semiconductor (CMOS) logic circuits. However, their full potential continues to be limited by a lack of scalable high-k dielectrics that can achieve atomically smooth interfaces, small equivalent oxide thicknesses (EOTs), excellent gate control, and low leakage currents. Here, large-area liquid-metal-printed ultrathin Ga2O3 dielectrics for 2D electronics and optoelectronics are reported. The atomically smooth Ga2O3/WS2 interfaces enabled by the conformal nature of liquid metal printing are directly visualized. Atomic layer deposition compatibility with high-k Ga2O3/HfO2 top-gate dielectric stacks on a chemical-vapor-deposition-grown monolayer WS2 is demonstrated, achieving EOTs of ∼1 nm and subthreshold swings down to 84.9 mV/dec. Gate leakage currents are well within requirements for ultrascaled low-power logic circuits. These results show that liquid-metal-printed oxides can bridge a crucial gap in dielectric integration of 2D materials for next-generation nanoelectronics.

Multimodal health monitoring via a hierarchical and ultrastretchable all-in-one electronic textile

Wearable electronics with conductive yet stretchable bio-interfaces, multimodal health regulating capabilities, and strong weather resistance are especially expected by athletes and outdoor workers, yet conventional rigid, impermeable, and monofunctional wearables can hardly meet these requirements simultaneously. Here, a hierarchical and ultrastretchable all-in-one health-regulating electronic textile (AIO E-textile) with robust superhydrophobic and antibacterial surfaces is developed through electrospin-assisted layer-by-layer assembly and liquid metal (LM) printing. Skin-like and bio-compatible styrene-ethylene-butylene-styrene (SEBS) nonwoven textile is employed as a breathable matrix for the layered E-textile, in which an electrophysiological recording circuit, a strain-sensing circuit, and a Joule heating circuit are successively patterned using EGaIn. To ensure human skin health and weather resistance of the device, antibacterial and waterproof surfaces are respectively installed on both sides of the electronic textile via anchoring functionalized nanoparticles. Thus, this ultrastretchable (1141 %) AIO E-textile shows favorable moisture permeability (1300 g m−2 day), exceptional water repellency, remarkable antibacterial efficacy (>99.99 %) against E-coli, and low-watt Joule heating ability, which even exhibits accurate under-water motion detection capability, and high-definition acquiring of bioelectrical signals (electrocardiogram (ECG) and surface electromyography (sEMG)) when used as flexible epidermal bioelectrode. This work opens up a new avenue for daily applicable and multifunctional on-skin electronics.

Heterojunction Transistors Printed via Instantaneous Oxidation of Liquid Metals

Semiconducting transparent metal oxides are critical high mobility materials for flexible optoelectronic devices such as displays. We introduce the continuous liquid metal printing (CLMP) technique to enable rapid roll-to-roll compatible deposition of semiconducting two-dimensional (2D) metal oxide heterostructures. We leverage CLMP to deposit 10 cm2-scale nanosheets of InOx and GaOx in seconds at a low process temperature (T < 200 °C) in air, fabricating heterojunction thin film transistors with 100× greater Ion/Ioff, 4× steeper subthreshold slope, and a 50% increase in mobility over pure InOx channels. Detailed nanoscale characterization of the heterointerface by X-ray photoelectron spectroscopy, UV-vis, and Kelvin probe elucidates the origins of enhanced electronic transport in these 2D heterojunctions. This combination of CLMP with the electrostatic control induced by the heterostructure architecture leads to high performance (μlin up to 22.6 cm2/(V s)) while reducing the process time for metal oxide transistors by greater than 100× compared with sol-gels and vacuum deposition methods.

Recent progress in liquid metal printing and its applications.

This paper focuses on the latest research printing technology and broad application for flexible liquid metal (LM) materials. Through the newest template printing method, centrifugal force assisted method, pen lithography technology, and laser method, the precision of liquid metal printing on the devices was improved to 10 nm. The development of novel liquid metal inks, such as PVA-LM ink and ethanol/PDMS/LM double emulsion ink, have further enhanced the recovery, rapid printing, high conductivity, and strain resistance. At the same time, liquid metals also show promise in the application of biochemical sensors, photocatalysts, composite materials, driving machines, and electrode materials. Liquid metals have been applied to biomedical, pressure/gas, and electrochemical sensors. The sensitivity, biostability, and electrochemical performance of these LM sensors were improved rapidly. They could continue to be used in healthy respiratory, heartbeat monitoring, and dopamine detection. Meanwhile, the applications of liquid metal droplets in catalytic-assisted MoS2 deposition, catalytic growth of two-dimensional (2D) lamellar, catalytic free radical polymerization, catalytic hydrogen absorption/dehydrogenation, photo/electrocatalysis, and other fields were also summarized. Through improving liquid metal composites, magnetic, thermal, electrical, and tensile enhancement alloys, and shape memory alloys with excellent properties could also be prepared. Finally, the applications of liquid metal in micro-motors, intelligent robot feet, nanorobots, self-actuation, and electrode materials were also summarized. This paper comprehensively summarizes the practical application of liquid metals in different fields, which helps understand LMs development trends, and lays a foundation for subsequent research.

Industrially-friendly rapid printing of stretchable liquid metal circuits for patch-mode wearable healthcare devices

Sponge-roller assisted liquid metal printing (SALP) technology provides an industrially-friendly method for achieving rapid and economical liquid metal electrode or circuit printing.

Controlling the oxidation and wettability of liquid metal via femtosecond laser for high-resolution flexible electronics.

Liquid metal-based electronic devices are attracting increasing attention owing to their excellent flexibility and high conductivity. However, a simple way to realize liquid metal electronics on a microscale without photolithography is still challenging. Herein, the wettability and adhesion of liquid metal are controlled by combining the stirring method, femtosecond laser microfabrication, and sacrificial layer assistant. The adhesive force of liquid metal is dramatically enhanced by adjusting its oxidation. The wetting area is limited to a micro-pattern by a femtosecond laser and sacrificial layer. On this basis, a high-resolution liquid metal printing method is proposed. The printing resolution can be controlled even less than 50 μm. The resultant liquid metal pattern is applied to electronic skin, which shows uniformity, flexibility, and stability. It is anticipated that this liquid metal printing method will hold great promise in the fields of flexible electronics.

Liquid metal printing opening the way for energy conservation in semiconductor manufacturing industry

Liquid metal printing opening the way for energy conservation in semiconductor manufacturing industry

A Miniature Soft Sensor with Origami-Inspired Self-Folding Parallel Mechanism.

Miniature soft sensors are crucial for the perception of soft robots. Although centimeter-scale sensors have been well developed, very few works addressed millimeter-scale, three-dimensional-shaped soft sensors capable of measuring multi-axis forces. In this work, we developed a millimeter-scale (overall size of 6 mm × 11 mm × 11 mm) soft sensor based on liquid metal printing technology and self-folding origami parallel mechanism. The origami design of the sensor enables the soft sensor to be manufactured within the plane and then fold into a three-dimensional shape. Furthermore, the parallel mechanism allows the sensor to rotate along two orthogonal axes. We showed that the soft sensor can be self-folded (took 17 s) using a shape-memory polymer and magnets. The results also showed that the sensor prototype can reach a deformation of up to 20 mm at the tip. The sensor can realize a measurement of external loads in six directions. We also showed that the soft sensor enables underwater sensing with a minimum sensitivity of 20 mm/s water flow. This work may provide a new manufacturing method and insight into future millimeter-scale soft sensors for bio-inspired robots.

Modulation Doped 2D InOx/GaOx Heterostructure Tfts Via Liquid Metal Printing

Indium gallium zinc oxide (IGZO) and similar wide bandgap metal oxides are among the most widely used channel materials for drive transistors in displays due to their excellent electronic mobility and their ultra-high transparency1. However, industry-standard processing involves expensive vacuum deposition and elevated activation temperatures to produce semiconducting thin films. Liquid metal printing (LMP) is an emerging technique for oxide semiconductor fabrication poised to overcome these drawbacks via scalable vacuum-free transfer of the native oxide layers formed by spontaneous surface oxidation of molten metals2–4. Heterostructures of these 1-4 nm 2D oxide layers provide unprecedented opportunities for engineering electrostatic control of multilayers in thin film transistors, leading to improved mobility, Ion/Ioff ratios, and faster switching capabilities. Likewise, the backchannel is of high importance to these devices, as selection of an appropriate capping layer can enhance performance via remote doping while also mitigating bias stress effects. Herein, we compare the results of heterostructure InOx/GaOx with pure InOx TFTs, which demonstrates the mobility enhancement provided by GaOx modulation doping.Bottom gate thin film transistors (TFTs) (Figures 1a and 1b) were fabricated on Si substrates with 100 nm of thermally grown SiO2. 4 nm thick InOx and GaOx were deposited at 240 ˚C and 180 ˚C, respectively, using a linear printing speed of 8 cm/s. InOx and GaOx were printed in less than 10 s, with no post annealing necessary. Figure 1c illustrates the proposed mechanism for electron donation from GaOx at the heterointerface with InOx. The conduction band offset between these materials results in band bending at the interface and an increased carrier concentration in the InOx layer. The substoichiometric, defective GaOx is expected to further enhance this effect. Figure 1d demonstrates the transfer characteristics of heterostructure InOx/GaOx in comparison with pure InOx. The improved mobility for the heterostructure (7.8 cm2/Vs) vs pure InOx (3.8 cm2/Vs) channels can be attributed to modulation doping provided by GaOx and can be analyzed by extracting the electronic density of states (eDOS). These results illustrate a unique capability of LMP, which is to engineer the electronic structure of highly conductive 2D oxides while maintaining electrostatic control.This work also investigates the material properties of these 2D oxide heterostructures by UV-vis, XRD and XPS characterization. UV-Vis analysis revealed that the GaOx capping layer induces band gap widening and enhanced transparency, which can be explained by the Burstein-Moss effect from modulation doping. Unique to this LMP process is also the low temperature crystallization of the InOx films. XRD showed that even with low deposition temperatures (200 – 240 ˚C), these InOx films are highly crystalline with grain sizes substantially larger than the film thickness. Finally, XPS analysis of the O1s peak was utilized to understand the stoichiometry and interactions between the InOx and GaOx layers.This work demonstrates an effective pathway to enhance electronic transport in semiconducting metal oxides through liquid metal printed 2D heterostructures. The ultrathin films produced by LMP are well suited for thin film devices requiring nm-scale electrostatic control for effective gating. Combining this 2D nature of LMP InOx with a 2D GaOx backchannel capping layer is shown to yield high-performance printed transistors. This approach demonstrates a rapid, open-air compatible and low temperature manufacturing method, elucidating the broad impact of this technology in display fabrication, low-cost and flexible electronics. H. Hosono, Nat Electron, 1, 428–428 (2018).K. A. Messalea et al., ACS Nano, 15, 16067–16075 (2021).R. S. Datta et al., Nat Electron, 3, 51–58 (2020).A. Jannat et al., ACS Nano, 15, 4045–4053 (2021).A. Goff et al., Dalton Transactions, 50, 7513–7526 (2021).C.-H. Choi, Y.-W. Su, L.-Y. Lin, C.-C. Cheng, and C. Chang, RSC Advances, 5, 93779–93785 (2015). Figure 1

Hydrogen-Terminated Diamond MOSFETs Using Ultrathin Glassy Ga2O3 Dielectric Formed by Low-Temperature Liquid Metal Printing Method

The p-type surface conductivity of hydrogen-terminated diamond (H-diamond) provides a viable approach toward diamond-based wide-bandgap metal-oxide-semiconductor field-effect transistors (MOSFETs) for high-power and high-frequency electronics. A facile, low-cost, and low-temperature method to form gate dielectrics on diamond that also preserves the integrity of hydrogen-termination is highly desirable for high-performance diamond surface electronics with process flexibility and high yield. In this work, we demonstrate a p-channel diamond MOSFET with an ultrathin glassy Ga2O3 dielectric layer derived from liquid metal. A liquid metal printing method was employed to transfer an amorphous Ga2O3 layer over the desired active p-channel region of H-diamond at low temperature, allowing the protection and preservation the hydrogen-terminated surface while also forming an efficient gate dielectric. The results of this work suggest that the liquid metal method can provide an efficient, low-cost, and high-yield pathway to form high-quality dielectrics for diamond-based transistors.

Liquid Phase 3D Printing: How This New Technology Can Help Bring 3D Printing to the Operating Room.

The purpose of this review is to discuss the state of technology in liquid phase three-dimensional (3D) metal printing, how this has affected the field of orthopedic surgery, and changes that we can expect in the future with the rise of this printing technology. We will also discuss how liquid phase metal printing can possibly bring three-dimensional printing to the operating room. The use of liquid phase 3D metal printing may become commonplace for manufacturing orthopedic implants and devices. Traditional metal printing involved powder-based metals and high-energy beam technologies that are expensive, time-consuming, and potentially wasteful. This unfortunately leaves them out of reach for most end consumers such as orthopedic surgeons. Liquid phase metal printing is less expensive and faster. However, there is still major work required to bring this technology to the operating room and benefit patients. While major strides have been made in the field of liquid phase metal three-dimensional printing, there are still significant developments in the pipeline. These could lead to future production of personalized orthopedic implants and devices with optimal material properties for patients.