**3. Results**

LM based on gallium alloys (EGaIn) was used for the device. In Figure 1a, the preparation process of EGaIn microdroplets is shown. A suspension of LM microdroplets was formed by ultrasonication in ethanol. The inner core of the droplet is EGaIn and is surrounded by gallium oxide with an outer carbon shell [40–42]. In Figure 1b, a schematic illustration of the entire process is shown. The solution was dropped on a flat, fully-cured PDMS substrate and dried at room temperature for 24 h to avoid the formation of cracks by rapid solvent evaporation during the drying process.

LM droplets by ultrasonication were distributed on the PDMS substrate after solvent evaporation, as shown in Figure 2a. EGaIn microdroplets synthesized by ultrasonication were distributed uniformly on the PDMS substrate, and the size of the droplets was smaller than 3 μm, as shown in Figure 2b. Round droplets were observed, and the droplets were wrapped with oxidized gallium material. The film with stacked LM droplets itself is not conductive because it is di fficult for an electron to move a long distance through the nonconductive oxidized layers from one droplet to another. The advantage of using the droplets formed by ultrasonication is that it is possible to fabricate a thinner and more uniform LM film when the droplets are used. With bulk EGaIn, it is di fficult to create a thin film because the high surface tension of a bulk LM makes it di fficult to manage the LM.

**Figure 2.** (**a**) Liquid metal microdroplets distributed on a PDMS substrate; (**b**) SEM image of the liquid metal (LM) microdroplets; (**c**) Image of a thin conductive LM film after peeling o ff process; (**d**) SEM image of the continuous conductive LM film on a PDMS substrate; (**e**) The transparent gallium oxide film attached on a PDMS after peeling o ff from the LM film; (**f**) Magnified optical image; (**g**) SEM image of the transparent film of (**e**); (**h**) a piece of the thin tore transparent film on a PDMS substrate; (**i**) The thickness and (**j**) the roughness of transparent gallium oxide film (boxed area) by AFM; (**k**) XRD pattern of the exfoliated film on a PDMS substrate.

Subsequently, another flat, fully-cured PDMS substrate was placed onto the dried microdroplet film, which was then pressed by a hydraulic press at a pressure of 15 MPa. The droplets were squashed out and connected to each other by pressure after breaking the oxidized layers of the droplets. The oxidized parts remained in the film inside. However, the connected droplets formed a large conductive thin film, as shown in Figure 2c,d. The amount of oxidized material is much lower than that of the conductive part, and the film is su fficiently conductive for use in electric devices. The top PDMS substrate on the sandwiched LM film was then peeled o ff. After that, half of the LM remained on the bottom substrate, and another half was transferred onto the top substrate. As a result of the peeling process, two conductive LM sheets were obtained on the top and bottom PDMS substrates. The LM film was formed on the flat PDMS substrate over the entire area. It is di fficult to fabricate a very thin LM film from bulk LM because of its high surface tension. However, this di fficulty has been overcome by using LM microdroplets. In this work, one of the LM-coated sheets (bottom) was used as a conductive electrode, and another (top) was used for utilizing a photo-responsive layer.

Moreover, an extremely thin oxidized layer could be separated from the LM films by peeling o ff with a sticky PDMS film. The incompletely-cured PDMS film with strong adhesion was designed to peel o ff the oxidized layer from the surface of the LM thin film for large-area fabrication. The incompletely-cured PDMS film was placed and covered on the surface of the LM thin film, and it was brought into contact with the oxidized layer of the LM, forming a conformal contact. Hereafter, the transparent oxidized layer was sliced out from the LM film, at the time of peeling o ff the uncured PDMS film. The optical image of the separated gallium oxide film from the LM film is shown in Figure 2e. The film is very thin and transparent. As shown in Figure 2f,g, small LM spots (<10 μm) remained on the transparent film. However, all the spots were surrounded by gallium oxide film and isolated from each other. Thus, the separated gallium oxide film is laterally nonconductive. The transparent nonconductive film was analyzed by grazing incidence X-ray di ffraction (GIXRD). The oxidized sample was prepared on a PDMS substrate, and the graph obtained by GIXRD is shown in Figure 2k. The baseline was similar to reported GIXRD data of bare PDMS [43], and a broad peak was observed around 35◦. It is known that GIXRD peaks are also observed at 34◦ and 36◦ for a gallium oxide material. Thus, it is concluded that the transparent film is gallium oxide, since the peaks of gallium oxide appear on the graph, and the material was also responsive to UV light in this work. Finally, the separated transparent film was used for fabricating a photodetector because the oxidized thin film is a material mainly based on gallium oxide and it shows high photo-detective property, as reported previously [37,38]. The separated oxidized film on a PDMS substrate could be easily cut using a scissor. Then, it was placed on the substrate between the cathode and anode to fabricate a photodetector. After placing the gallium oxide on the PDMS film between the electrodes, the photodetector with transparent film and flexible LM electrodes was completed, as shown in Figure 1b.

In addition, to fabricate a desirable pattern for a flexible and stretchable device, a fiber laser marker (λ ~ 1064 nm) was used for designing the patterns on a conductive LM film. The fabricated conductive films are shown in Figure 3a. A laser engraving method is an e fficient way to establish flexible electrodes and patterns for devices. The SEM images in Figure 3b show the pattern with various sizes based on EGaIn by a fiber laser marker. The advantages of using a fiber laser marker for the patterning process are fine pattern resolution and less damage to transparent substrates, such as PDMS or glass, during the patterning process. In fact, buckling of the PDMS substrate due to heat was observed when LM was blazed out. The method can establish a sub-100 μm pattern, and the minimum line was approximately ~20 μm in the experiment. Furthermore, better resolution can be achieved using the laser blazing method if highly qualified equipment is used for patterning [44]. In a previous report, a CO2 laser, not a fiber laser, was used for cutting the LM electrode inside the PDMS substrate [31]. Actually, the CO2 laser blazed out the PDMS, and not the LM, in the experiment. In this work, di fferent mechanisms were used. It is known that metal substrates absorb only a small amount of energy from the CO2 laser, and most of the energy from the CO2 laser is reflected [45]. In contrast, a metal can absorb the energy of a fiber laser. Thus, the fiber laser is suitable for patterning thin LM

films. It could easily remove a thin LM film (<1 μm) quickly. Complex patterns with a resolution below 100 μm (~20 μm) could be made using a fiber laser in this work.

**Figure 3.** (**a**) Images of a conductive EGaIn film patterned by a fiber laser marker; (**b**) SEM images of conductive EGaIn film patterned by a fiber laser marker, showing a maximum resolution of the pattern of approximately 20 μm; (**c**) Resistance of the EGaln electrode under bending and stretching.

After peeling off the top PDMS substrate, half of the LM remained on the bottom substrate and another half was transferred to the top substrate, as shown in Figure 1b. As a result of the peeling process, two conductive LM sheets were obtained. Resistance measurement during the stretching test was also performed with an LM electrode (5 mm long and 80 μm wide). It was measured by a semiconductor parameter analyzer, and the characteristic performances of the flexible electrodes are shown in Figure 3c. Resistance of the EGaIn film is between 19.7 Ω and 41.7 Ω; this increases slightly when the film is stretched to 170% of its initial length. The electrical conductivity of the EGaIn film in this work was 2.2 × 10<sup>6</sup> S/m. The value is approximately two-thirds of its known value (pure EGaIn, ~3.4 × 10<sup>6</sup> S/m) and was measured using a resistivity meter (Loresta-GX MCP-T700, Mitsubishi Chemical Analytech, Yamato, Japan) with a four-pin probe to overcome the effect of contact

resistance. According to the results, the film is sufficiently conductive to be used as an electrode in the circuit. Here, one of the LM sheets was used as a conductive electrode and the other was used for a photo-responsive layer. The thickness of the LM films measured by AFM was approximately 600 nm. It is known that it is difficult to fabricate a very thin LM film because of its high surface tension. However, we could overcome this difficulty by using LM droplets and making a thin (< 1 μm) LM film on the substrate.
