Inert Liquid Exfoliation\nand Langmuir-Type Thin Film\nDeposition of Semimetallic Metal Diborides

Graphite is one of only a few layered materials that\ncan be exfoliated\ninto nanosheets with semimetallic properties, which limits the applications\nof nanosheet-based electrodes to material combinations compatible\nwith the work function of graphene. It is therefore important to identify\nadditional metallic or semimetallic two-dimensional (2D) nanomaterials\nthat can be processed in solution for scalable fabrication of printed\nelectronic devices. Metal diborides represent a family of layered\nnon-van der Waals crystals with semimetallic properties for all nanosheet\nthicknesses. While previous reports show that the exfoliated nanomaterial\nis prone to oxidation, we demonstrate a readily accessible inert exfoliation\nprocess to produce quasi-2D nanoplatelets with intrinsic material\nproperties. For this purpose, we demonstrate the exfoliation of three\nrepresentative metal diborides (MgB<sub>2</sub>, CrB<sub>2</sub>,\nand ZrB<sub>2</sub>) under inert conditions. Nanomaterial is characterized\nusing a combination of transmission electron microscopy, scanning\nelectron microscopy, atomic force microscopy, IR, and UV–vis\nmeasurements, with only minimal oxidation indicated postprocessing.\nBy depositing the pristine metal diboride nanoplatelets as thin films\nusing a Langmuir-type deposition technique, the ohmic behavior of\nthe networks is validated. Furthermore, the material decomposition\nis studied by using a combination of electrical and optical measurements\nafter controlled exposure to ambient conditions. Finally, we report\nan efficient, low-cost approach for sample encapsulation to protect\nthe nanomaterials from oxidation. This is used to demonstrate low-gauge\nfactor strain sensors, confirming metal diboride nanosheets as a suitable\nalternative to graphene for electrode materials in printed electronics.