Identification and Manipulation of Defects in Black Phosphorus

Black phosphorus and black phosphorene are attractive for device applications because of their thickness-dependent band gap. We identify and manipulate commonly occurring defects in black phosphorus, combining density functional theory (DFT) calculations with STM experiments. The nature of a ubiquitous defect, imaged at negative bias as a bright dumbbell extending over several nanometers, has been the subject of much debate. By comparing simulated and experimental STM images at both negative and positive bias, this feature is shown to arise from a substitutional Sn impurity in the second P sublayer. Similarly, another frequently observed defect type is identified as arising from an interstitial Sn atom; this defect can be switched to a more stable configuration consisting of a Sn substitutional defect + P adatom, by application of an electrical pulse via the STM tip. DFT calculations show that this pulse-induced structural transition switches the system from a non-magnetic configuration to a magnetic one. We show that an unambiguous identification of defect features requires a comparison of simulated and experimental STM images; identifications based on electronic charge densities alone can be misleading. Moreover, these comparisons should be carried out at multiple bias voltages. We introduce States Projected Onto Individual Layers (SPOIL) quantities which provide information about atom-wise and orbital-wise contributions to bias-dependent features observed in STM images.