Synthesis of two-dimensional lead sheets by spark discharge in liquid nitrogen

Abstract

A simple method to synthesise hexagonal lead sheets, which belong to the class of two-dimensional materials, is proposed. These objects are collected on a substrate located under two lead electrodes, between which nanosecond-pulsed spark discharges are ignited in liquid nitrogen. The hexagonal sheets are single crystals produced by gas phase condensation. Once nitrogen completely evaporates, the sheets change to PbO2 by oxidation in air. The oxidation process induces stress that pleats the uppermost sheets or open cracks at the centre. The thickness of the individual objects typically varies from 4 to 20nm. When the number of discharges is more than about 2000, in addition to sheets, two types of PbO2 sticks start to form: bundles composed of nanosticks (length 5μm and diameter 50nm) and isolated sticks (length 20μm and diameter 500nm). These new nanostructures mainly form because of the way the discharge erodes the lead electrodes. Initially, anisotropic erosion driven by the orientation of the crystallographic planes of the lead crystals produces octahedra and nanosticks, and the nanosticks grow on the electrode surfaces as discharge proceeds. After about 2000 discharges, the nanosticks are sufficiently long that they can be easily broken, probably by mechanical stress, and they fall onto the underlying substrate.