Nanotip Formation from Liquid Metals for Soft Electronic Junctions

Mechanism behind the Controlled Generation of Liquid Metal Nanoparticles by Mechanical Agitation

The size-controlled synthesis of liquid metal nanoparticles is necessary in a variety of applications. Sonication is a common method for breaking down bulk liquid metals into small particles, yet the influence of critical factors such as liquid metal composition has remained elusive. Our study employs high-speed imaging to unravel the mechanism of liquid metal particle formation during mechanical agitation. Gallium-based liquid metals, with and without secondary metals of bismuth, indium, and tin, are analyzed to observe the effect of cavitation and surface eruption during sonication and particle release. The impact of the secondary metal inclusion is investigated on liquid metals' surface tension, solution turbidity, and size distribution of the generated particles. Our work evidences that there is an inverse relationship between the surface tension and the ability of liquid metals to be broken down by sonication. We show that even for 0.22 at. % of bismuth in gallium, the surface tension is significantly decreased from 558 to 417 mN/m (measured in Milli-Q water), resulting in an enhanced particle generation rate: 3.6 times increase in turbidity and ∼43% reduction in the size of particles for bismuth in gallium liquid alloy compared to liquid gallium for the same sonication duration. The effect of particles' size on the photocatalysis of the annealed particles is also presented to show the applicability of the process in a proof-of-concept demonstration. This work contributes to a broader understanding of the synthesis of nanoparticles, with controlled size and characteristics, via mechanical agitation of liquid metals for diverse applications.

Galinstan Liquid Metal Electrical Contacts for Monolayer-Modified Silicon Surfaces

Galinstan is the brand name for a low-melting gallium-based alloy, which is a promising nontoxic alternative to mercury, the only elemental metal found in the liquid state at room temperature. Liquid alloys such as Galinstan have found applications as electromechanical actuators, sensors, and soft contacts for molecular electronics. In this work, we validate the scope of Galinstan top contacts to probe the electrical characteristics of Schottky junctions made on Si(111) and Si(211) crystals modified with Si-C-bound organic monolayers. We show that the surface-to-volume ratio of the Galinstan drop used as a macroscopic contact defines the junction stability. Further, we explore chemical strategies to increase Galinstan surface tension to obtain control over the junction area, hence improving the repeatability and reproducibility of current-voltage (I-V) measurements. We explore Galinstan top contacts as a means to monitor changes in rectification ratios caused by surface reactions and use these data, most notably the static junction leakage, toward making qualitative predictions on the DC outputs recorded when these semiconductor systems are incorporated in Schottky-based triboelectric nanogenerators. We found that the introduction of iron particles leads to poor data repeatability for capacitance-voltage (C-V) measurements but has only a small negative impact in a dynamic current measurement (I-V).

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.

Flexible-to-Stretchable Mechanical and Electrical Interconnects

Stretchable electronic devices that maintain electrical function when subjected to stress or strain are useful for enabling new applications for electronics, such as wearable devices, human-machine interfaces, and components for soft robotics. Powering and communicating with these devices is a challenge. NFC (near-field communication) coils solve this challenge but only work efficiently when they are in close proximity to the device. Alternatively, electrical signals and power can arrive via physical connections between the stretchable device and an external source, such as a battery. The ability to create a robust physical and electrical connection between mechanically disparate components may enable new types of hybrid devices in which at least a portion is stretchable or deformable, such as hinges. This paper presents a simple method to make mechanical and electrical connections between elastomeric conductors and flexible (or rigid) conductors. The adhesion at the interface between these disparate materials arises from surface chemistry that forms strong covalent bonds. The utilization of liquid metals as the conductor provides stretchable interconnects between stretchable and non-stretchable electrical traces. The liquid metal can be printed or injected into vias to create interconnects. We characterized the mechanical and electrical properties of these hybrid devices to demonstrate the concept and identify geometric design criteria to maximize mechanical strength. The work here provides a simple and general strategy for creating mechanical and electrical connections that may find use in a variety of stretchable and soft electronic devices.

Smart Eutectic Gallium-Indium: From Properties to Applications

Eutectic gallium-indium (EGaIn), a liquid metal with a melting point close to or below room temperature, has attracted extensive attention in recent years due to its excellent properties such as fluidity, high conductivity, thermal conductivity, stretchability, self-healing capability, biocompatibility, and recyclability. These features of EGaIn can be adjusted by changing the experimental condition, and various composite materials with extended properties can be further obtained by mixing EGaIn with other materials. In this review, not only the are unique properties of EGaIn introduced, but also the working principles for the EGaIn-based devices are illustrated and the developments of EGaIn-related techniques are summarized. The applications of EGaIn in various fields, such as flexible electronics (sensors, antennas, electronic circuits), molecular electronics (molecular memory, opto-electronic switches, or reconfigurable junctions), energy catalysis (heat management, motors, generators, batteries), biomedical science (drug delivery, tumor therapy, bioimaging and neural interfaces) are reviewed. Finally, a critical discussion of the main challenges for the development of EGaIn-based techniques are discussed, and the potential applications in new fields are prospected.

Liquid Metal-Templated Tin-Doped Tellurium Films for Flexible Asymmetric Pseudocapacitors

Liquid metals can be surface activated to generate a controlled galvanic potential by immersing them in aqueous solutions. This creates energized liquid-liquid interfaces that can promote interfacial chemical reactions. Here we utilize this interfacial phenomenon of liquid metals to deposit thin films of tin-doped tellurium onto rigid and flexible substrates. This is accomplished by exposing liquid metals to a precursor solution of Sn²⁺ and HTeO₂⁺ ions. The ability to paint liquid metals onto substrates enables us to fabricate supercapacitor electrodes of liquid metal films with an intimately connected surface layer of tin-doped tellurium. The tin-doped tellurium exhibits a pseudocapacitive behavior in 1.0 M Na₂SO₄ electrolyte and records a specific capacitance of 184.06 F·g^-1 (5.74 mF·cm^-2) at a scan rate of 10 mV·s^-1. Flexible supercapacitor electrodes are also fabricated by painting liquid metals onto polypropylene sheets and subsequently depositing tin-doped tellurium thin films. These flexible electrodes show outstanding mechanical stability even when experiencing a complete 180° bend as well as exhibit high power and energy densities of 160 W·cm^-3 and 31 mWh·cm^-3, respectively. Overall, this study demonstrates the attractive features of liquid metals in creating energy storage devices and exemplifies their use as media for synthesizing electrochemically active materials.

Scalable Strategy to Directly Prepare 2D and 3D Liquid Metal Circuits Based on Laser-Induced Selective Metallization

Selective wetting of a gallium-based liquid metal on copper circuits is one of the ways to prepare liquid metal circuits. However, the complex fabrication processes of an adhesion layer between copper circuits (or patterns) and substrates were still inevitable, limiting scalable applications. Our work developed a facile way to directly prepare 2D and 3D liquid metal circuits by combining laser-induced selective metallization and selective wetting for the first time. The copper template was obtained on elastomers using laser-induced selective metallization, and high-resolution liquid metal circuits were fabricated by brushing Galinstan on the copper template in the alkali solution. The distribution of Cu element not only was on the top surface but also extended to the interior of the elastomer substrate. This revealed that the Cu layer prepared by laser-induced selective metallization is born to firmly embed into the substrate, which endowed the circuits with strong adhesion, reaching the highest 5B level. Moreover, the prepared liquid metal circuits (or patterns) had a typical layered structure. The liquid metal circuits exhibit good flexibility, stretchability, self-healing ability, and acid-alkaline resistance. Compared with the traditional methods of patterning liquid metals, fabricating liquid metal circuits based on laser-induced selective metallization has irreplaceable advantages, such as strong adhesion between circuits and substrate, fabricating 3D circuits, good acid-alkaline resistance, cost-effectiveness, maskless use, time savings, arbitrary design of patterns, and convenient operation, which endow this method with great application prospect.

Interfacial Electrochemical Polymerization for Spinning Liquid Metals into Core-Shell Wires

Metal wires are of great significance in applications such as three-dimensional (3D) printing, soft electronics, optics, and metamaterials. Ga-based liquid metals (e.g., EGaIn), though uniquely combining metallic conductivity, fluidity, and biocompatibility, remain challenging to be spun due to their low viscosity, high surface tension, and Rayleigh-Plateau instability. In this work, we showed that EGaIn as a working electrode could induce the oxidization of EGaIn and interfacial electrochemical polymerization of electroactive monomers (e.g., acrylic acid, dopamine, and pyrrole), thus spinning itself from an opening of a blunt needle. During the spinning process, the high surface tension of EGaIn was reduced by electrowetting and electrocapillarity and stabilized by polymer shells (tunable thickness of ∼0.6-30 μm on wires with a diameter of 90-300 μm), which were chelated with metal ions. The polymeric shells offered EGaIn wires with an enhanced endurance to mechanical force and acidity. By further encapsulating into elastomers through a facile impregnation process, the resultant elastic EGaIn wires showed a combination of high stretchability (up to 800%) and metallic conductivity (1.5 × 10⁶ S m^-1). When serving as wearable sensors, they were capable of sensing facial expressions, body movements, voice recognition, and spatial pressure distributions with high sensitivity, good repeatability, and satisfactory durability. Machine-learning algorithms further assisted to detect gestures with high accuracy.

Flexible Tactile Sensing Microfibers Based On Liquid Metals

High-performance and intelligent fibers are indispensable parts of wearable electronics in the future. This work mainly demonstrates the preparation of flexible intelligent liquid metal (LM) fibers with three core-sheath structures. An ultra-thin (10-50 μm), conductive, and highly flexible LM was deposited on the fiber core [carbon/polyethylene terephthalate (C/PET)--150-500 μm] along the fiber direction and then deposited on a polymer-protective layer [polyvinyl alcohol/epoxy resin (PVA/EP)--10 μm]. Four kinds of LM intelligent fibers were manufactured, including the C-LM-PVA fiber, C-LM-EP fiber, PET-LM-PVA fiber, and PET-LM-EP fiber. These LM intelligent fibers (diameter, 150-600 μm) were demonstrated with a high conductivity of 7.839 × 104 S·m-1. The changes in resistance in different torsion directions were measured, and these smart LM fibers could also be used as electrical heaters or thermoelectric generators, which released heat (36-36.9 °C/1-1.5 V) into the environment. Then, these multifunctional LM fibers were applied as high-performance strain sensors and bending sensors. These flexible LM conductive fibers could be successfully utilized in intelligent wearable fabrics and were expected to be widely utilized in artificial muscle and sensor fields.

Liquid Metal Microgels for Three-Dimensional Printing of Smart Electronic Clothes

Gallium-based liquid metals (LMs), with the combination of liquid fluidity and metallic conductivity, are considered ideal conductive components for flexible electronics. However, huge surface tension and poor wettability seriously hinder the patterning of LMs and their wider applications. Herein, a recyclable liquid-metal-microgel (LMM) ink composed of LM droplets encapsulated into alginate microgel shells is proposed. During the mechanical stirring process, the released Ga³⁺ can cross-link with sodium alginate to form microgels covering the surface of LM droplets, which exhibits shear-thinning performance due to the formation and rupture of hydrogen bonds under different stress conditions, making the LMM ink possess excellent printability and superior adhesion to various substrates. Although patterns printed with the LMM ink are not initially conductive, they can be activated to recover conductivity by microstrain (<5%), pressing, and freezing. Additionally, the activated LMM circuit exhibits superior Joule heating behaviors and electrical performance in further investigation, including excellent conductivity, significant resistance response to strain with small hysteresis, great durability to nonplanar forces, and so forth. Furthermore, smart electronic clothes were fabricated and investigated by directly printing functional circuits on commercial clothes with the LMM ink, which integrate multiple functions, including tactile sensing, motion monitoring, human-computer interaction, and thermal management.

Applications of liquid metals in nanotechnology

Post-transition liquid metals (LMs) offer new opportunities for accessing exciting dynamics for nanomaterials. As entities with free electrons and ions as well as fluidity, LM-based nanomaterials are fundamentally different from their solid counterparts. The low melting points of most post-transition metals (less than 330 °C) allow for the formation of nanodroplets from bulk metal melts under mild mechanical and chemical conditions. At the nanoscale, these liquid state nanodroplets simultaneously offer high electrical and thermal conductivities, tunable reactivities and useful physicochemical properties. They also offer specific alloying and dealloying conditions for the formation of multi-elemental liquid based nanoalloys or the synthesis of engineered solid nanomaterials. To date, while only a few nanosized LM materials have been investigated, extraordinary properties have been observed for such systems. Multi-elemental nanoalloys have shown controllable homogeneous or heterogeneous core and surface compositions with interfacial ordering at the nanoscale. The interactions and synergies of nanosized LMs with polymeric, inorganic and bio-materials have also resulted in new compounds. This review highlights recent progress and future directions for the synthesis and applications of post-transition LMs and their alloys. The review presents the unique properties of these LM nanodroplets for developing functional materials for electronics, sensors, catalysts, energy systems, and nanomedicine and biomedical applications, as well as other functional systems engineered at the nanoscale.