Electrochemical Method of Producing Ultra-Narrow Phosphorene Nanoribbons

In recent years, phosphorene, a two-dimensional (2D) form of black phosphorous (BP), has been the subject of intense research due to its unique properties, including high carrier mobility (2,000 cm² V^-1 s^-1), thickness-dependent bandgap (0.3 – 2.0 eV), and strong in-plane anisotropy. Phosphorene nanoribbons (PNRs) display even more unique properties due to their one-dimensional (1D) morphology and the resulting additional quantum confinement effects, redesigning of the density of states, and high density of active edge sites. Starting in 2016, initial attempts of producing PNRs, such as etching electro-beam sculpting and electro-beam lithography have been explored. Only recently, however, more efficient and cost-effective top-down exfoliation approaches have been developed. Despite these recent advances, the scalable production of PNRs with narrow widths remains a challenge. To this end, here, we report a facile and straightforward method to synthesize PNRs via an electrochemical process that utilizes the highly anisotropic Na-ions diffusion in BP along the [001] (i.e., zigzag) direction. Furthermore, we validate this hypothesis and report a low-cost and feasible scalability two-step electrochemical method for synthesizing PNRs with confined width (10 nm), significantly narrower than PNRs in most of the previous methods. In the first step, BP flakes are nanostructured through an electrochemical discharge process into bundles of parallel PNRs separated from each other by regions of highly disordered phosphorous, as shown by a combination of transmission electron microscopy (TEM) and in-situ Raman spectroscopy studies. In the second step, the PNR bundles are subject to an ultrasonic treatment in a solvent to separate them into individual and well-isolated PNRs. The produced PNRs show a significantly confined structure with the suppressed B_2g vibrational mode. More interestingly, when used in field-effect transistors (FETs), the synthesized PNRs exhibit the n-type behavior, which is dramatically different from bulk BP flakes. Our work provides insights into a new synthesis approach of PNRs with confined width, paving the way towards the development of nanoribbons of phosphorene and other highly anisotropic layered materials.