**4. Conclusions**

We investigated characteristics of the flexible and stretchable Galinstan electrodes under various conditions including sub-zero temperature (i.e., <0 ◦C) and successfully demonstrated solar-blind photodetection via the spontaneous oxidation of Galinstan. In this work, a simple and rapid method was introduced for fabricating the flexible and stretchable Galinstan electrodes with precise patterns and exfoliating the surface oxide layers to complete the device structure enabling solar-blind photodetection. A suspension consisting of Galinstan microdroplets was prepared by sonication. Thin Galinstan films with thickness less than 1 μm were uniformly deposited on flexible PDMS substrates by compression of the dried suspension of the microdroplets. The Galinstan films, deposited on a large area (50 mm × 50 mm), were sequentially patterned using a fiber laser marking machine (λ~1064 nm), and accurate and desirable features with a high resolution of 20 μm were fabricated. Although the electrical conductivity of the fabricated films was lower than that of pure Galinstan, they still possessed electrical conductivity high enough to be used as flexible and stretchable electrodes even below 0 ◦C. For the photoactive components, thin Ga2O3 layers, spontaneously formed on the Galinstan surfaces, were exfoliated using elastomeric PDMS stamps and successfully transferred onto the patterned Galinstan electrodes to complete the device structure for solar-blind photodetection. The solar-blind photodetectors demonstrated a distinct increase of up to ~15.1% in the 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. These results strongly sugges<sup>t</sup> that Galinstan can be used for flexible and stretchable electrodes working under extreme conditions, and the combination with its surface oxide layer also shows grea<sup>t</sup> potential for sensitive solar-blind photodetectors that possess outstanding advantages, such as low-cost and easy processability under ambient conditions. We anticipate that these results will contribute to the development of flexible and stretchable electronic devices based on liquid metals, which can lead to further application of sensors under extreme conditions.

**Author Contributions:** P.X.: conceptualization, methodology, formal analysis, investigation, data curation, visualization, and writing—original draft preparation; J.-H.K.: investigation, data curation, visualization, writing—original draft preparation, review and editing, and supervision; S.S.: conceptualization, formal analysis, visualization, writing—review and editing, supervision, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2021R1F1A1047036 and NRF-2018R1D1A1B07041253).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.
