Method to Reduce the Contact Resistivity between Galinstan and a Copper Electrode for Electrical Connection in Flexible Devices

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).

Passivation-Free, Liquid-Metal-Based Electrosynthesis of Aluminum Metal-Organic Frameworks Mediated by Light Metal Activation

The production of aluminum (Al) metal-organic frameworks (MOFs) by electrosynthesis using solid-state Al electrodes always faces significant challenges due to the formation of a passivating aluminum oxide layer in the process. Here, we developed a liquid-metal-based method to electrosynthesize an aluminum Al-MOF (MIL-53). This method uses a liquid-state gallium (Ga) anode as a reservoir and activator for a light metal, Al, in the form of Al-Ga alloys that releases Al3+ for the electrosynthesis of Al-MOFs. Introducing Ga into the system inhibits the formation of aluminum oxide passivation layer and promotes the electrochemical reaction for Al-MOF synthesis. The electrosynthesis using liquid Al-Ga alloy is conducted at ambient temperatures for long durations without requiring pretreatment for aluminum oxide removal. We show that the Al-MOF products synthesized from 0.40 wt % Al in liquid Ga lead to the highest crystallinity and possess a specific surface area greater than 800 m2 g-1 and a low capacity for CO2 adsorption that can be used as a potential matrix for CO2/N2 separation. This work provides evidence that employing liquid-metal electrodes offers a viable pathway to circumvent surface passivation effects that inevitably occur when using conventional solid metals. It also introduces an efficient electrosynthesis method based on liquid metals for producing atomically porous materials.

Voltage-Driven Chemical Reactions Enable the Synthesis of Tunable Liquid Ga-Metal Nanoparticles

Nanosized particles of liquid metals are emerging materials that hold promise for applications spanning from microelectronics to catalysis. Yet, knowledge of their chemical reactivity is largely unknown. Here, we study the reactivity of liquid Ga and Cu nanoparticles under the application of a cathodic voltage. We discover that the applied voltage and the spatial proximity of these two particle precursors dictate the reaction outcome. In particular, we find that a gradual voltage ramp is crucial to reduce the native oxide skin of gallium and enable reactive wetting between the Ga and Cu nanoparticles; instead, a voltage step causes dewetting between the two. We determine that the use of liquid Ga/Cu nanodimer precursors, which consist of an oxide-covered Ga domain interfaced with a metallic Cu domain, provides a more uniform mixing and results in more homogeneous reaction products compared to a physical mixture of Ga and Cu NPs. Having learned this, we obtain CuGa2 alloys or solid@liquid CuGa2@Ga core@shell nanoparticles by tuning the stoichiometry of Ga and Cu in the nanodimer precursors. These products reveal an interesting complementarity of thermal and voltage-driven syntheses to expand the compositional range of bimetallic NPs. Finally, we extend the voltage-driven synthesis to the combination of Ga with other elements (Ag, Sn, Co, and W). By rationalizing the impact of the native skin reduction rate, the wetting properties, and the chemical reactivity between Ga and other metals on the results of such voltage-driven chemical manipulation, we define the criteria to predict the outcome of this reaction and set the ground for future studies targeting various applications for multielement nanomaterials based on liquid Ga.

High-Accuracy Contact Resistance Measurement Method for Liquid Metal by Considering Current-Density Distribution in Transfer Length Method Measurement

Liquid metals (LMs) are used as stretchable conductors in various stretchable electronic devices. Moreover, such devices using Ga-based LMs have attracted considerable attention. Herein, we propose a method for accurately determining the contact resistance (Rc) between galinstan and Cu electrodes by considering the current-density distribution in transfer length method (TLM) measurement. Conventional TLM measurements assume that the sheet resistance of the metal electrode (Rshe) is negligible compared with that of the object (Rsho), such as Si. However, this assumption may be problematic because the Rsho of Ga-based liquid metals (LMs) is close to the Rshe. Therefore, we developed a method of applying current to each measuring electrode and compared it with the conventional method of applying current to the outer electrodes. Simulation results indicated that Rshe cannot be ignored for galinstan, and the measured resistance in the contact area (RcTotal) included <10% of the Rc component when current was applied to the outer electrodes. In contrast, RcTotal included the entire Rc component when current was applied to each electrode. Furthermore, we found that the volume resistances of the object and electrode included in RcTotal cannot be ignored. Therefore, for accurate measurement, current must be applied to each electrode, and Rc must be determined from the intersections of the measured and simulated RcTotal. The obtained contact resistivity (ρc), i.e., the contact resistance per unit contact area, was 0.115 mΩ·mm2. The maximum error was 0.085 mΩ·mm2, which was lower than the ρc of the solders (≥10-1 mΩ·mm2) with the lowest ρc among the electrical interface materials between the electronic components and wiring. This study provides valuable insight into the Rc measurement of LMs, along with new opportunities for the development of stretchable electronics using LMs.

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.

Correlated Study of Material Interaction Between Capillary Printed Eutectic Gallium Alloys and Gold Electrodes

Liquid metals (LMs) play a growing role in flexible electronics and connected applications. Here, LMs come into direct contact with metal electrodes thus allowing for corrosion and additional alloying, potentially compromising device stability. Nevertheless, comprehensive studies on the interfacial interaction of the materials are still sparse. Therefore, a correlated material interaction study of capillary-printed Galinstan (eutetic alloy of Ga/In/Sn) with gold surfaces and electrodes is conducted. Comprehensive application of optical microscopy, vertical scanning interferometry, scanning electron microscopy/spectroscopy, x-ray photon spectroscopy, and atomic force microscopy allow for an in depth characterization of the spreading process of LM lines on gold films, revealing the differential spread of the different LM components and the formation of intermetallic nanostructures on the surface of the surrounding gold film. A model for the growth process based on the penetration of LM along the gold film grain boundaries is proposed based on the obtained time-dependent characterization. The distribution of gold, Galinstan, and intermetallic phases in a gold wire dipped into LM is observed using X-ray nano tomography as a complementary view on the internal nanostructure. Finally, resistance measurements on LM lines connecting gold electrodes over time allow to estimate the influence on the material interaction on electronic applications.

Are Liquid Metals Bulk Conductors?

Stretchable electronics have potential in wide-reaching applications including wearables, personal health monitoring, and soft robotics. Many recent advances in stretchable electronics leverage liquid metals, particularly eutectic gallium-indium (EGaIn). A variety of EGaIn electromechanical behaviors have been reported, ranging from bulk conductor responses to effectively strain-insensitive responses. However, numerous measurement techniques have been used throughout the literature, making it difficult to directly compare the various proposed formulations. Here, the electromechanical responses of EGaIn found in the literature is reviewed and pure EGaIn is investigated using three electrical resistance measurement techniques: four point probe, two point probe, and Wheatstone bridge. The results indicate substantial differences in measured electromechanical behavior between the three methods, which can largely be accounted for by correcting for a fixed offset corresponding to the resistances of various parts of the measurement circuits. Yet, even accounting for several of these sources of experimental error, the average relative change in resistance of EGaIn is found to be lower than that predicted by the commonly used bulk conductor assumption, referred to as Pouillet's law. Building upon recent theories proposed in the literature, possible explanations for the discrepancies are discussed. Finally, suggestions are provided on experimental design to enable reproducible and interpretable research.