Electrochemical Sculpting of Phosphorene Nanoribbons: Unleashing the Anisotropy of Black Phosphorous

In recent years, phosphorene, a two-dimensional (2D) form of black phosphorus (BP), has garnered significant research attention due to its exceptional properties, including high carrier mobility (2,000 cm² V^-1s^-1), thickness-dependent bandgap (0.3 – 2.0 eV), and strong in-plane anisotropy. Moreover, phosphorene nanoribbons (PNRs) exhibit even more impressive characteristics owing to their one-dimensional (1D) nanostructure, which gives rise to additional quantum confinement effects, density of states redesign, and a high density of active edge sites. While several methods for producing PNRs have been explored, scalable synthesis with narrow widths remains challenging.<br/><br/>Here, we present our recently developed method termed "electrochemical sculpting" to synthesize PNRs through an electrochemical process utilizing the highly anisotropic ion diffusion in BP along the [001] (zigzag) direction. In this method, BP flakes are nanostructured via an electrochemical process into bundles of parallel PNRs separated by narrow amorphous phosphorus channels running along the zigzag direction, as confirmed by transmission electron microscopy (TEM) and in-situ Raman spectroscopy. A subsequent ultrasonic treatment in a solvent can be used to separate the PNR bundles into individual, well-defined PNRs. Using this low-cost and scalable electrochemical method, we were able to fabricate PNRs with confined widths (&lt; 10 nm) that are significantly narrower than those produced by most previous methods. Our fabricated PNRs exhibit a highly confined structure with a suppressed B2g vibrational mode. Notably, when employed in field-effect transistors (FETs), the PNR bundles demonstrate n-type behavior, which differs from that of BP flakes.<br/><br/>In our ongoing study, we focus on understanding this process and extending it to other anisotropic 2D layered materials. So far, the obtained results indicate that electrochemical sculpting and the fabrication of nanoribbons can also be achieved in black arsenic-phosphorus (b-AsP) alloys, i.e., 2D layered materials where some of the phosphorus atoms in the BP structure are substituted by arsenic atoms. This is significant because b-AsP alloys are promising 2D materials, exhibiting unique electronic and optical properties. However, the fabrication of their nanoribbons using chemical approaches is particularly challenging due to the low solubility of arsenic in many solvents and the difficulties in controlling the crystal structure of the resulting material. While the overall mechanism responsible for electrochemical sculpting in BP and b-AsP alloys is similar, our study indicates several significant differences between these materials. These differences can be attributed to the variation in in-plane anisotropy as well as the disparity in defect formation energy between these systems.<br/><br/>Our study provides valuable insights into this novel synthesis approach for PNRs, opening up new possibilities for the development of nanoribbons using BP and other anisotropic 2D layered materials. The demonstrated electrochemical sculpting method offers improved scalability and holds promise for advancing the applications of PNRs in various fields.