Research on the protection of black phosphorus (BP) by the monolayer 7,7,8,8-tetracyanoquinodimethane (TCNQ)

Abstract

Black phosphorus (BP) has attracted widespread attention in multiple research fields due to its excellent optoelectronic properties, such as adjustable direct bandgap, optical and electrical anisotropy. However, the poor air stability of BP limits its practical application. To protect BP through physical isolation and chemical passivation, a strong n-type organic semiconductor, 7,7,8,8-tetracyanoquinodimethane (TCNQ) was deposited on BP. The protective effect and the underlying mechanisms have been comprehensively investigated through various interface characterization techniques and density functional theory (DFT) calculations. It is found that the high adsorption energy (AE) between TCNQ and BP leads to the desorption of TCNQ from BP surface and TCNQ forms a dense monolayer (ML) film on BP in a stable lying configuration, which can protect BP from air exposure to some extent. Due to the higher work function (WF) of TCNQ compared to BP, the electron of BP at the interface are transferred to TCNQ through the quinone ring, resulting in a redistribution of electrons near the quinone ring of TCNQ and a shift of the C 1 s peak as well as the Raman peaks of C-C and C-C-H. The electron transfer from BP to TCNQ not only leads to an upward bending of the conduction band (CB) of BP at the interface, increasing the electron transfer barrier (ETB) between BP and O₂, but also depletes the lone pair electrons of BP at the interface, protecting the shallow layer BP from oxidation and degradation. The ML of TCNQ and the shallow layer of BP significantly delays the contact between inner layer of BP and air, prolonging the stability of BP in air for more than 10 days. In addition, it is also confirmed that ML TCNQ has the minimal impact on the intrinsic structure and properties of BP. This work provides a new strategy for the protection and application of BP.