Solution-Processed Free-Standing Ultrathin Two-Dimensional PbS Nanocrystals with Efficient and Highly Stable Dielectric Properties

Two-dimensional (2D) materials with downscaled thicknesses are the quest of the electronics industry because of their immense potential in modern microelectronics. Despite the discovery of several novel 2D materials, the flexible design of highperformance free-standing ultrathin 2D dielectric nanocrystals (NCs) with a large planar morphology remains the most challenging task. We develop a method for synthesizing high-quality free-standing ultrathin 2D NCs of PbS with a well-defined large rectangular morphology with a thickness of similar to 2 nm. The lateral size can be tuned up to a few hundred nanometers by changing only the reaction annealing time. Microscopic and spectroscopic analyses at different stages of the reaction reveal formation of 2D NCs by a continuous growth mechanism. The 2D NCs exhibit a nearly temperature and frequency independent high dielectric constant (>13.4) with a small dielectric loss (0.0006 at 20 K and <0.06 at 350 K for 100 kHz) over broad temperature and frequency ranges. Low-frequency dispersion from 125 Hz to 1 MHz, frequency stability with a small dielectric loss (<0.03 at 100 kHz), and a stable temperature coefficient of the dielectric constant outline the merits of 2D NCs as a potential material. Complex impedance analyses demonstrate dominant intrinsic effects contributed by polarons in covalent NCs. Equal activation energies for the conduction and relaxation processes offer uniform energy barriers for the charges in NCs leading to high-performance dielectric behavior. This work opens up promising features of non-oxide binary semiconductors as dielectric alternatives for miniaturized electronics using flexible solution processing routes.