Stable Metal/Metal Hydroxide Multilayers with Controlled Nanoscale Thickness Prepared from Liquid Metal Droplets with Oxide Skin

Abstract

The practical applicability of ultrathin films, which offer interesting and novel functionalities, is often hampered by difficulties in large-area deposition while maintaining homogeneous film properties. Here, we induce a breakup after forced wetting to produce the ultrathin film [Runde, S. Adv. Mater. Interfaces 2018, 5(16), 1800323] and apply this deposition method to selected liquid metals and alloys to produce electrically conductive films at ambient conditions on wafer-scaled areas. In addition to ultrathin monolayers, vertically stacked and heterostructured multilayers of metal and metal hydroxide can be built by repeating the deposition method. Structural analysis using X-ray reflectometry shows that the multilayer thickness is proportional to the number of deposition cycles, yielding a single layer thickness between 2.9 and 5.2 nm, depending on the material used. Every single layer consists of a complex heterostructure composed of a nanometer-thin metallic core surrounded by stabilizing metal (hydr)oxide skin layers. The crystallinity of the layers within the films was investigated with grazing incidence X-ray diffraction; X-ray amorphous materials were Ga, GaIn(1:1), and GaInSn(7:2:1), which also showed low optical absorbance and low electrical resistivity. Films made from InSn(1:1)- and Bi-containing alloys showed weak diffraction peaks, indicating partial crystallization. The electrical conductivity of all multilayers increases with the number of deposition cycles, allowing to fine-tune the sheet resistance. The preparation of ultrathin multilayers of metallic materials at the centimeter scale is attributed to the low melting temperature combined with the high surface tension and wettability of the liquid metals.