Near-Isotropic Local Attosecond Charge Transfer within the Anisotropic Puckered Layers of Black Phosphorus

Black phosphorus possesses useful two-dimensional (2D) characteristics of van der Waals coupled materials but additionally features an in-plane anisotropic puckered layer structure that deviates from common 2D materials. Three distinct directions exist within the lattice of black phosphorus: the in-plane armchair and zigzag directions and the out-of-plane direction, with each distinct phosphorus 3p partial density of states. This structural anisotropy is imprinted onto various collective long-range properties, while the extent to which local electronic processes are governed by this directionality is unclear. At the P L₁ edge, the directional selectivity of the core-hole clock method was used to probe the local charge transfer dynamics of electrons excited into the 3p-derived conduction band on an attosecond time scale. Here we show that the surprisingly small anisotropy of 3p electron transfer times reflects the similarly small differences in the 3p-derived unoccupied density of states caused by the underlying phosphorus bonding angles within the puckered layers.