Identification of Lattice Oxygen in Few-Layer Black Phosphorous Exfoliated in Ultrahigh Vacuum and Largely Improved Ambipolar Field-Effect Mobilities by Hydrogenation and Phosphorization

Black phosphorus (BP) has recently attracted considerable attention due to its unique structure and fascinating optical and electronic properties as well as possible applications in photothermal agents. However, its main drawback is rapid degradation in ambient environments of H₂O and O₂, which has led to much research on the improvement of its stability. Unfortunately, this research has not shown great improvement in carrier mobilities. Here, we perform scanning tunneling microscopy observations of few-layer BP (FLBP) sheets exfoliated in ultrahigh vacuum and reveal, for the first time, the existence of lattice oxygen introduced during crystal growth. As a proof-of-concept application, hydrogenation is conducted to remove the lattice oxygen atoms followed by phosphorization, which repairs the phosphorous vacancies caused by mechanical exfoliation and hydrogenation. The resulting FLBP sheets show high ambipolar field-effect mobilities of 1374 cm² V^-1 s^-1 for holes and 607 cm² V^-1 s^-1 for electrons at 2 K. After storage in air for 3 days, the hole and electron mobilities only decrease to 1181 and 518 cm² V^-1 s^-1, respectively, and no structural degradation is observed. This work suggests an effective means to improve both the mobility and stability of BP sheets rendering practical application of FLBP sheets possible.