Modifying the Interfacial Properties of Gallium-Based Liquid Metals using Carbon Nanotubes for Sensors and Interconnects

Abstract

Gallium-based liquid metal alloys are promising for soft and stretchable electronics due to their unique properties such as high electrical conductivity, self-healing capabilities, and fluidic behavior at room temperature. Upon exposure to air, these gallium-based liquid metals instantaneously form a skin of gallium oxide, Ga2O3, with a thickness of a few nanometers. The oxide skin helps the liquid metal retain its 3D geometry, enabling applications as a 3D component in electronics, such as high-resolution interconnects between electronic devices. Beyond providing structural stability, this oxide skin can serve as a 2D planar interface in the fabrication of 2D metal oxides. The oxide layer can be readily separated due to its weak interfacial attachment to the liquid metal and manually transferred onto desired substrates using techniques such as contact printing, pressing, and squeezing. However, achieving controlled exfoliation, consistent separation, and minimal damage to the oxide layer remains a significant challenge. The incorporation of liquid metal into electronic devices is limited by several concerns that arise when liquid metal contacts other solid-state metallic components. At the liquid-solid interface between the two metals, gallium in the liquid metal penetrates the grain boundaries of the solid metal, weakening the bonds between solid metal atoms. This process, known as liquid metal embrittlement, leads to device failure. The phenomenon can also be described as dissolutive or reactive wetting, as a liquid metal droplet spreads on the solid metal while forming an alloy or intermetallic compound with it. Many studies have explored the interfacial wetting mechanism of liquid metals. However, a precise understanding of the spreading and suppression mechanisms remains unclear and requires further investigation based on similarities to existing wetting mechanisms. Furthermore, there is a need to develop effective approaches to mitigate liquid metal spreading, which causes metal surface degradation. Therefore, this PhD thesis aims to elucidate the mechanism of liquid metal wetting and spreading on a metal substrate and develops an approach for suppression it by demonstrating integration into electronic devices. The primary objectives of this research were to: (1) exploring a novel approach for extracting ultrathin skin of gallium oxide layer from liquid metal for further applications. (2.1) investigate the interfacial wetting behavior of low melting point liquid metal alloy known commercially as eutectic gallium-indium alloy (eGaIn) at room-temperature on platinum surfaces. (2.2) develop the liquid metal composites through carbon nanotube integration into liquid metal to suppress spreading of liquid metal The first objective of this thesis is to explore the exfoliation surface oxide skin of gallium-based liquid metal, offering a simple way to create 2D metal oxides at room temperature. This work introduced a novel approach to controlled exfoliation of an ultrathin and highly uniform gallium oxide film from a liquid metal balloon. By injecting air into a liquid metal droplet, the droplet was inflated and transformed into a liquid metal balloon, during which the oxide layer was separated from the bulk liquid to form a translucent film. The balloon was then transferred onto a silicon substrate by simple contact- printing, which resulted in a gallium oxide layer with uniform thickness of 4 nm and smooth surfaces. Furthermore, the gallium oxide film prepared in this manner demonstrated the selective binding with carbon nanotubes (CNTs) dispersed in an aqueous solution. The mechanism of selective CNT binding to gallium oxide surfaces was elucidated through the Cabrera-Mott oxidation model, highlighting the critical role of surface charge in the deposition process. By patterning electrodes on the percolating network of CNTs, the chemiresistive gas sensors were fabricated for the detection of chemical warfare agents (CWAs) simulants. The resulting CNT/Ga2O3-based chemiresistors achieved detection limits for chemical warfare agent simulants: 13 ppb for diisopropyl methylphosphonate (DIMP), 28 ppb for dimethyl methylphosphonate (DMMP), and 53 ppb for triethyl phosphate (TEP). The second phase of this thesis was to characterize the intermetallic wetting behavior and spreading of liquid metal on solid metal film. An eGaIn droplet was placed on Pt sputtered thin films, where it spontaneously alloyed with the Pt metal at room temperature, resulting in periodic pattern formation of metallic compound. The GaIn-Pt dissolutive wetting and spreading system was studied using energy dispersive X-ray spectroscopy and elemental mapping over time. The resulting images revealed Pt element migration toward the ripples during spreading, with depletion occurring near the ripples. Furthermore, alloy formation in the eGaIn-Pt system was confirmed through XRD, HRTEM, XPS analysis. The research followed by addressing the suppression spreading and penetration of liquid metal on a solid metal film, by incorporating CNTs into the liquid metal. CNTs are decorated with Pt nanoparticles into eutectic gallium-indium (eGaIn) to form a CNT/eGaIn composite with minimal CNT aggregation. A droplet of the CNT/eGaIn, when placed on a Pt film, showed no sign of spreading for up to 30 days. Pristine eGaIn, however, underwent dissolutive spreading on the Pt layer, which permanently damaged the metal substrate. The suppression mechanism is achieved by reinforcing gallium oxide by CNTs, making the oxide skin less prone to disruption and acting as a diffusion barrier for gallium. The stability of the electrical connection between two Pt electrodes, using either pristine eGaIn or CNT/eGaIn as the interconnect were monitored. A CNT/eGaIn interconnect bridging two Pt electrodes maintained robust ohmic contact, whereas an eGaIn interconnect caused significant damage to the electrodes, by the spreading eGaIn. As a result, the resistance (R) between the electrodes, which was initially 209 Ω, rapidly increased by 70.4% (R = 357 Ω) at 24 h and by 96.4% (R = 411 Ω) at 240 h. In contrast to the pristine eGaIn, the CNT/eGaIn interconnect did not show any spreading and demonstrated stable for 10 and 30 days (R = 238 Ω at 24 h and R = 264 Ω at 240 h). Overall, the author of this PhD thesis believes that the outcomes presented herein will contribute to the advancement in liquid metal-based electronic interconnect technology, offering a promising approach of the CNT/eGaIn composite for mitigating liquid metal spreading. The exfoliation of the gallium oxide layer from a liquid metal balloon and the subsequent deposition of CNTs on the oxide can serve as a unique approach for potential applications in various electronic devices.