Pentacene Two-Dimensional Crystals with Layer-Dependent Morphologies and Mobilities Grown via Confined Molecular Flow Deposition

Pentacene has gained significant attention as a channel material in organic field-effect transistors (OFETs). In most cases, the mobility, a key performance metric of the device, is evaluated for the entire film. However, there is limited understanding of its layer-dependent electric transport properties in the two-dimensional (2D) limit, particularly when compared to inorganic 2D materials that have been more thoroughly investigated. Furthermore, in OFETs, the growth of molecules is especially crucial at the initial stage, as the electrical conduction predominantly occurs in the few molecular layers adjacent to the dielectric surface. Herein, we successfully grew pentacene 2D crystals with 1, 2, and 3 molecular layers on thermally oxidized wafer surfaces using the confined molecular flow deposition method, under conditions of both high deposition rates and elevated substrate temperatures. The electric transport properties of these crystals were measured, and the results aligned well with the theoretical calculations. In addition, the growth mode of these pentacene 2D crystals differs from the classical models: layer-by-layer, layer-plus-island, and island growth modes. This deviation arises because the nucleation of pentacene under these conditions does not follow the classical nucleation process. Instead, it develops through two distinct steps. Initially, pentacene molecules nucleate as liquid clusters with the molecules adopting a lying-down orientation. Subsequently, these liquid nuclei transition to a solid crystalline state, where the molecules flip upright. The final number of molecular layers in the pentacene 2D crystals corresponds to the layer number in their solid nucleus, with each layer within the crystal growing synchronously. Additionally, the crystals with different numbers of layers exhibit distinct shapes: ellipses, rounded hexagons, and hexagons for one, two, and three layers, respectively. This phenomenon can be explained by phase field modeling. We believe our work not only verifies the electric transport properties of 2D pentacene crystals with different molecular layers, but also may pave the way for exploring new physics using small molecules.