**2. Experimental**

The Ga–Sn–Zn eutectic alloy was prepared in IMMS PAS using pure Ga (99.99999% PPM Pure Metals), Sn and Zn (99.999% Alfa Aeaser) in a glovebox under a protective atmosphere of high-purity argon (99.9999% Air Products). The oxygen, nitrogen and water vapour concentrations were lower than 0.1 ppm in order to avoid the oxidation of the prepared liquid alloys. The elements were melted together and homogenised in a graphite crucible in an electrical furnace placed in the glovebox.

The thin films were obtained at room temperature in ambient air by placing a drop of liquid metal on a glass substrate. In the next step, the drop was touched with the chosen substrate. The oxide skin attached itself onto the substrate with some residual liquid metal. The liquid metal was then cleaned off the substrate by dipping the material in boiling water and wiping it with Kimwipes. The material thus cleaned and dried was analysed. Various substrates were tested, including glass, quartz and silicon. The substrates were cleaned twice before the process, in isopropanol, ethanol and water.

In order to determine the substrate on which the deposition would be easiest, and, consequently, on which the best quality of films could be obtained, the wettability of the Ga–Sn–Zn eutectic alloy on different substrates was measured at room temperature in ambient air, outside the glovebox, in order to imitate the same conditions in which the layers are deposited on the substrates. The contact angles were measured using the static sessile drop method in IMMS PAS.

Moreover, the obtained thin films were studied at RMIT University by atomic force microscopy (Dimension Icon-Bruker with scan-assist software), and the thickness of the layers was determined. Due to the thickness of the layers, X-ray photoelectron spectroscopy (Thermo Scientific K Alpha XPS) was used to analyse the chemical composition, as this is a suitable method in the case of nanometric materials. The X-ray source was a monochromated Al K α source. The samples on 3 mm TEM copper grids were also studied by transmission electron microscopy using a Tecnai G2 F20 operating at 200 keV in IMMS PAS. The chemical composition was also assessed using an energy-dispersive X-ray (EDX) microanalyser in TEM.

#### **3. Results and discussion**

The contact angle between the Ga–Sn–Zn eutectic and the substrates, namely, silicon, glass and quartz, was analysed first. For each material, the contact angles were measured three times. Representative images of the contact angles are presented in Figure 1.

**Figure 1.** Representative contact angles for (**a**) glass, (**b**) quartz and (**c**) silicon.

The obtained contact angles were as follows: 132 ± 1◦ for glass, 118 ± 2◦ for quartz and 134 ± 1◦ in the case of silicon. From the conducted measurements, it can be concluded that the wetting behaviour of Ga–Sn–Zn eutectic alloy is similar in the cases of glass and silicon. The obtained values in the case of silicon are similar to the value obtained in reference [21] for Galinstan on silicon (139◦ at 303 K). However, it was found in reference [22] that Galinstan was found to be non-wetting on different substrates, including glass, when not oxidised. Furthermore, it was determined that the moment the oxide skin forms on the surface of Galinstan, the alloy sticks on surfaces and gives the illusion that Galinstan wets the substrates [22].

In relation to 2D materials, our investigations are focused on obtained layers which were analysed using optical microscopy, scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. An optical micrograph of the nanolayers obtained on silicon is presented in Figure 2.

**Figure 2.** Optical microphotograph of thin film obtained from Ga–Sn–Zn eutectic alloy on glass (the layer has been circled).

As can be seen from Figure 2, the oxide layer obtained by the touch-printing method is continuous, although there are some traces of metal left on the surface of the sample. The same microstructure is observed in the cases of all different substrates. In the next step, the layers were further analysed using atomic force microscopy. For example, the layers obtained from the Ga–Sn–Zn eutectic alloy on the silicon substrate are presented in Figure 3.

**Figure 3.** Image from AFM of nanolayers obtained from Ga–Sn–Zn eutectic alloy on silicon.

Using AFM, the layer appears to be continuous. The measured thickness of the layers obtained from Ga–Sn–Zn eutectic alloy is about 1.5 nm. Due to the low thickness of the sample, comparable to background noise, the line depth profile is not shown. The layers obtained on glass and quartz were of similar thicknesses, ranging from 1.5 to 3 nm in the thinnest parts. The thickness of the thin films is 3 times the thickness of the layer of the oxide skin on pure liquid gallium reported in reference [12].

The chemical composition was analysed using X-ray photoelectron spectroscopy. The XPS spectra are presented in Figure 4. In the case of the gallium, the peak at 20.2 eV corresponds to Ga2O3, while the peak at around 25 eV is the O2s peak, which is in the same range as Ga. The peak observed at 18.2 eV is elemental Ga, which may indicate some inclusions of gallium in the obtained material. In the case of the zinc spectrum, the analysis is more complicated due to the fact that the difference between the binding energies of elemental zinc and zinc oxide is less than 1 eV. However, the main zinc peak is located at 1022 eV, which is closer to the ZnO oxide binding energy. In the case of tin, the peak around 485 eV can be identified as corresponding to metallic tin, while the peak at 486.7 eV corresponds to tin oxide SnO2.

**Figure 4.** XPS spectra of Ga, Zn, Sn and O in oxide layers obtained from Ga–Sn–Zn eutectic alloy.

It was determined that the chemical composition of the touch-printed layers is 88.1 atom % of gallium, 6.9 atom % of tin and 5.0 atom % of zinc. When oxygen is taken into account, the composition of the layer is as follows: 19.7 atom % of gallium, 1.6 atom % of tin, 1.1 atom % of zinc and 77.6 atom % of oxygen. It is crucial to note that, according to simple thermodynamics, the Ga2O3 (ΔGf = −998.3 kJ/mol) phase is more likely to form than SnO2 or ZnO (ΔGf = −515.8 and −350.5 kJ/mol, respectively) [23]. However, the presented results indicate the presence of not only gallium oxide but also tin and zinc oxides. It is possible that the tin and zinc substitute for gallium atoms in the Ga2O3 oxide lattice, or that some form of segregation occurs on the surface of the liquid alloy. It should also be taken into account that some tin and zinc oxide could be formed after the transfer occurs, and the metal left on the substrate before cleaning oxides.

The TEM images are shown in Figure 5. The presented image shows a piece of the layer of approximately 1.2 × 0.8 μm. The oxide appears to be nanocrystalline. The material obtained on the TEM grid is much smaller than the material that can be obtained on non-porous surfaces, i.e., glass, silicon or quartz. Even though TEM grids with carbon film were used, the material does not transfer well onto the grids and does not appear in the form of clean sheets, but rather as folded pieces of oxide skin. EDS analysis was conducted, and the results are shown in Figure 6.

**Figure 5.** (**a**) TEM bright field image and (**b**) the corresponding selected area diffraction pattern.

**Figure 6.** EDS point analysis of the chemical composition of the material. Insert: dark field image.

Although the XPS and EDS results of the chemical composition investigation vary slightly, it can be concluded that they are consistent and indicate that the oxide layer is formed mainly of gallium oxide.

## **4. Conclusions**

It is possible to obtain materials by transferring the oxide formed on liquid Ga–Sn–Zn alloy at room temperature onto different substrates, namely, quartz, glass and silicon. The contact angle between the liquid metal and the studied substrate (being in the range between 117◦ and 134◦) does not affect the conducted measurements, although in this respect it could be interesting to study more hydrophobic and more hydrophilic surfaces. It is difficult to obtain macroscopic clear and smooth layers, due to folding of the oxide layer and oxidation of the metal on the analysed substrate. However, as can be seen from the atomic force image, it is possible to obtain smooth nanometric layers of a few micrometres in length. The XPS and EDS results sugges<sup>t</sup> that the thin films obtained from eutectic Ga–Sn–Zn alloy contain not only gallium oxide, which should form according to thermodynamics, but also smaller quantities of tin and zinc oxides. The selective area diffraction pattern shows the presence of Ga2O3. It has not been determined whether the tin and zinc are substituting for gallium in the oxide or forming small patches of tin and zinc oxide in the main phase of gallium oxide. The study of liquid metal alloy surfaces should be undertaken as an important step in future research concerning alloys with low melting points, as it may prove crucial in many studied applications.

**Author Contributions:** Conceptualisation, K.K.-Z. and T.G.; methodology, W.M.; validation, W.M.; investigation, A.D., A.W., A.Z. and B.Y.Z.; resources, T.G. and K.K.-Z.; writing—original draft preparation, A.D.; writing—review and editing, A.D., T.G. and B.Y.Z.; visualisation, A.D.; supervision, T.G. and T.D.; project administration, T.D.; funding acquisition, A.D. and T.G.

**Funding:** This work was financed by the National Science Centre Poland gran<sup>t</sup> 2016/21/B/ST8/00324 "Design and physicochemical, thermal properties of low temperature metal alloys based on gallium" in 2017-2019. The research was co-financed by the European Union from resources of the European Social Fund (Project No.WND-POWR.03.02.00-00-I043/16.

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