Wiring up Liquid Metal: Stable and Robust Electrical Contacts Enabled by Printable Graphene Inks

Abstract

electronic materials. By leveraging these unique characteristics, researchers have recently demonstrated advances in reconfigurable, responsive, and stretchable electronic devices. [5][6][7][8] Select applications of liquid metals include soft electronic skins, [9,10] dynamic and flexible antennas, [11][12][13][14] and self-healing and elastic electronics. [15,16] Moreover, the fluid nature of GaLMAs such as eutectic gallium-indium (eGaIn) enables broad process compatibility with additive printing methods such as direct write, inkjet, transfer, and 3D printing. [17][18][19][20][21][22][23] As such, research toward the control and integration of GaLMAs for printed, stretchable, and reconfigurable electronics has attracted broad scientific and practical interest.Despite their promise, the development of liquid metal electronics must overcome several challenges for widespread application. In particular, stable electrical contacts have been identified as a critical challenge for the integration of GaLMAs in electronic circuits and systems. [4] Since gallium alloys rapidly with most metals, GaLMAs lead to unstable or mechanically sensitive interfaces when combined with metal electrodes or interconnects, thereby preventing the reliable integration of eGaIn functionality with conventional electronics. In a recent dramatic demonstration of this effect, Dickey and coworkers exploited the aggressive alloying of eGaIn with silver to direct the motion of liquid metal droplets across a surface, completely removing the silver in the process. [24] Therefore, to enable broader application of eGaIn with conventional circuits, interfacial alloying needs to be suppressed without compromising electrical or mechanical properties.Here, we demonstrate printed graphene as a reliable and high-performance interfacial layer to enable electrical connections to eGaIn. In contrast to conventional metals, sp 2 -bonded carbon materials are stable to alloy formation with liquid metals. [25,26] To leverage this property in a platform that is suitable for printed GaLMAs, graphene inks based on cellulose derivatives are used for robust contacts to liquid metal. This class of graphene inks has shown broad process compatibility with excellent electrical conductivity, mechanical durability, and environmental stability. [27][28][29][30] A thin film (≈100 nm) of fewlayer graphene flakes printed between conventional silver leads and eGaIn acts as a physical barrier, effectively passivating the surface against alloying while retaining the ability to conduct current across the interface. Moreover, graphene interfacial Gallium-based liquid metal alloys (GaLMAs) are a unique class of advanced materials with the potential to offer unprecedented opportunities in stretchable and reconfigurable electronics. Despite their promise, the development of liquid metal electronics must overcome several challenges for widespread application. In particular, stable electrical contacts have been identified as a critical challenge for the integration of GaLMAs in electronic...

2D SnO/In2O3 van der Waals Heterostructure Photodetector Based on Printed Oxide Skin of Liquid Metals

“…Liquid metals are emerging materials which can be used in microfluidics components, sensors, electrodes, phototransistors, flexible and stretchable devices, disease treatment, biomedical field, and in synthesis of low‐dimensional materials . Liquid metals have been shown to form a naturally occurring atomically thin layer of oxide at their interface with air, and using liquid metal as a reaction solvent can give access to a sizable portion of oxide elements including oxides which are intrinsically nonlayered crystals .…”

Liquid Metal Direct Write and 3D Printing: A Review

“…This reality underscores the importance of staying below the critical yield stress (i.e., keeping the left hand side of the equation equal to the right). Liquid metal direct writing has been used to make electrical contacts with graphene, silver, carbon nanotubes, copper, and other electrode materials [57][58][59][60] At its simplest, direct writing enables liquid metal features to be patterned directly on a surface in an arbitrarily complex 2D geometry. Two general approaches can be used to drive the direct writing process, i) shear and ii) volumetric control.…”

Shear‐Driven Direct‐Write Printing of Room‐Temperature Gallium‐Based Liquid Metal Alloys

“…[35,55] It is difficult to characterize the fundamental interfacial properties between liquid metal and various surfaces due to the presence of the oxide. [35,55] It is difficult to characterize the fundamental interfacial properties between liquid metal and various surfaces due to the presence of the oxide.…”

“…Although GaLMAs exhibit high surface tension (624 mN m À1 for eutectic gallium indium (EGaIn)), [21] the spontaneous formation of a passivating solid-state surface oxide skin dominates the surface properties and enables patterning of the metal into shapes that would normally not be stable. [35] While the ability to deposit liquid metal may appear to be straight forward, several comments within these publications-such as concerns about print reproducibility and substrate effects-indicate that there are opportunities to identify the most important variables that dictate printability. Additive manufacturing of GaLMAs has been explored recently, utilizing techniques such as inkjet printing, [23,24] direct-writing, [25][26][27][28][29][30] aerosol printing, [31] and freeze casting.…”