**1. Introduction**

Recently, liquid metals based on gallium (Ga) alloys have received increasing attention, owing to their outstanding electrical and mechanical properties [1–3]. The Ga-based metal alloys that exist as virtually non-toxic liquids at room temperature show not only excellent stretchability and deformability but also environmental friendliness and recyclability. In this context, extensive research efforts have been devoted to the development of various applications using Ga-based metal alloys, such as sensors [4], reconfigurable antennas [5], and soft electrodes [6]. These fluidic metal alloys also show grea<sup>t</sup> potential for electronic skins [7,8] and wearable electronics [9–11]. Among various Ga-based metal alloys, Galinstan (68.5% Ga, 21.5% indium (In), and 10% tin (Sn)) has been notably studied in recent years due to its remarkably low toxicity and melting point (~−19 ◦C) [12], which is also suitable for flexible and stretchable devices operating below 0 ◦C compared to other eutectic gallium indium (EGaIn).

Although Galinstan shows outstanding properties including flexibility and stretchability even under cold conditions, its high surface tension and rapid oxidation rate hinder the fabrication of desirable patterns for electronic devices and circuits in comparison with other functional materials [13–15]. Various methods for Galinstan patterning thus have been developed and enhanced, including microfluidic injection [16–21], photolithography [22,23],

**Citation:** Xiao, P.; Kim, J.-H.; Seo, S. Flexible and Stretchable Liquid Metal Electrodes Working at Sub-Zero Temperature and Their Applications. *Materials* **2021**, *14*, 4313. https:// doi.org/10.3390/ma14154313

Academic Editor: Ricardo Alcántara

Received: 18 June 2021 Accepted: 30 July 2021 Published: 2 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

stencil lithography [24–27], imprint lithography [28], microcontact printing [29,30], and composite material synthesis [31]. Each of these methods has individual advantages (i.e., high processability, high resolution limits, high stability, or cost-effective fabrication); however, integrating all the advantageous elements is still a challenge. For instance, the demanding process conditions for delaying surface oxidation or preventing leakage of the liquid metal or the low patterning resolution limits still need to be improved depending on the method. Laser ablation is one of the patterning methods with high processability, enabling the rapid fabrication of electrodes with complex features [32,33]. Although the pattern resolution is limited by the beam spot of laser, which is normally in the range of tens to hundreds of micrometers, this method can be directly employed for various applications without laborious pre- and/or post-treatments.

In addition, it is worth noting that the spontaneous oxidation of Galinstan in air leads to the formation of thin Ga oxide (Ga2O3) films on Galinstan surfaces [3,34,35]. Ga2O3 with a wide bandgap (~4.9 eV), which is rapidly formed on Galinstan surfaces in less than one second during the patterning processes in air [2], is transparent in the visible light region and exhibits high light-absorption coefficients in the deep ultraviolet (UV) region [35–38]. The surface oxide layers normally degrade the metallic properties of Galinstan; however, these layers are also expected to be utilized for solar-blind photodetection (i.e., deep UV detection insensitive to solar radiation) if they can be neatly separated from the bulk material [39].

Herein, we investigated characteristics of the Galinstan electrodes to verify flexibility and stretchability under various conditions including sub-zero temperature (i.e., <0 ◦C). For this study, a simple and rapid method was employed to fabricate the Galinstan electrodes with precise patterns. Thin Galinstan films with high electrical conductivity were uniformly deposited on flexible polydimethylsiloxane (PDMS) substrates by the compression of Galinstan microdroplets and sequentially patterned using a fiber laser marking machine. The transparent PDMS substrates were found to be undamaged by a laser with a wavelength of 1064 nm, and only the Galinstan layers were ablated according to the designed electrode shapes. In addition, the surface oxide (i.e., Ga2O3) layers of the Galinstan electrodes were also examined to confirm their potential for solar-blind photodetection. For the photoactive components, the thin Ga2O3 films, spontaneously formed on the Galinstan surfaces, were exfoliated using elastomeric PDMS stamps [39,40] and then transferred onto the patterned Galinstan electrodes to complete the device structure for solar-blind photodetection. By combining Galinstan and Ga2O3 films, sensitive solar-blind photodetectors were successfully fabricated on flexible substrates. The photodetectors showed a distinct increase of up to ~15.1% in output current under deep UV irradiation (254 nm wavelength) with an extremely low light intensity of 0.1 mW cm<sup>−</sup>2, whereas no significant change was observed under visible light irradiation.
