**1. Introduction**

Liquid metal alloys based on gallium are a group of metallic alloys, liquid at room temperature, that could possibly replace toxic materials such as mercury or alloys based on lead. The most commonly used alloy based on gallium is the eutectic Ga–In–Sn alloy Galinstan, which has potential to be used in many fields including electrical engineering, computer engineering [1] and medicine [2,3]. Eutectic Ga–In–Sn alloy is currently being studied intensively as an effective micro-cooling agen<sup>t</sup> with superior power-handling properties [4–6]. Other alloys have been proposed as a cheaper alternative to Galinstan, based on gallium, tin and zinc. These include Ga–Sn–Zn eutectic alloy [7]. Liquid metal alloys have a range of advantages and could find uses in many different applications. Those alloys exhibit excellent thermal and electrical conductivity, are safe to handle due to almost no vapour pressure [8], and exhibit a high supercooling effect [7]. Moreover, the surface of liquid gallium and its alloys oxidises easily, forming an oxide skin on the alloy [9–11]. The oxide skin on pure gallium has been investigated [12,13], but there are only limited data on oxides forming on alloys containing gallium as the main component. In the case of pure gallium, according to reference [12], the thickness of the solid oxide film is about 0.5 nm. For ambient oxygen pressure and temperatures, it was proposed that isotropic diffusion

aggregation was the process by which the gallium oxide grows and expands on the surface of liquid gallium [13]. The oxide first grows in a fractal-like pattern, then interfacial smoothening occurs, which may be caused by insoluble impurities in the oxide [13].

This study focuses on gallium-based alloys with tin and zinc additions. The Ga–Sn–Zn ternary system is formed from three simple eutectic systems [14–16]. The Ga–Sn–Zn eutectic alloy, with a composition of 90.15 of Ga, 6.64 of Sn, and 3.21 of Zn (atom %) corresponding to 86.3, 10.8, and 2.9 (wt %), has a melting point of 288 K and a solidification point of 266.5 K [7]. It has been found that adding a fourth element to the ternary eutectic system can increase the difference between the melting and solidification temperature [17,18]. By adding 0.5 atom % aluminum to Ga–Sn–Zn, the difference between the two temperatures increases from 20.6 K to 65.8 K, with a melting point of 301.3 K and a solidification point of 235.5 [17]. Similarly, indium additions cause the solidification temperature to drop even further to 231 K in the case of 14.7 atom % In, with a low melting point of 289 K [18].

As gallium alloys remain liquid over a large temperature range, touch-printing can be used in order to obtain nanometric films from the surface of the alloys [19]. The method developed in reference [19] allows the transfer of oxidised material on the liquid metal surface to virtually any substrate. The method has been used to obtain 2D Ga2O3, HfO2, Gd2O3, and Al2O3 [19]. Due to the low costs of fabrication of these nanometric films, the materials could be used in many important fields of science and industry, including as ultra-thin insulator dielectrics in field-effect transistors, for energy storage, and in gas sensing [20]. Using gallium alloys with additions of different elements could lower the synthesis temperature of chosen oxides; however, it is crucial to understand how those additions will affect the final composition of the oxide layer and which elements will be incorporated into the obtained material. In this work, we aimed to obtain 2D layers using the touch-printing technique on silicon, glass and quartz, in order to characterise the oxide layer formed on the surface of liquid eutectic Ga–Sn–Zn alloy using atomic force microscopy, X-ray photon spectroscopy, and optical and transmission electron microscopy.
