Ultrafast Carrier Dynamics and Efficient Triplet Generation in Black Phosphorus Quantum Dots

The two-dimensional few-layer black phosphorus (BP) has been proposed as a new type of semiconductor for optoelectronic applications. Beyond optoelectronics, the BP nanostructures have recently shown great potential in producing singlet oxygen for photodynamic therapy, but its underlying mechanism remains elusive. Here, we report the observation of efficient triplet formation in BP quantum dots, which might be responsible for the singlet oxygen generation. In addition to the common dynamic behaviors of semiconductors, a long-lived photoinduced bleaching signal with a lifetime of 26 μs has been explicitly identified in BP quantum dots, which can be ascribed to the intersystem crossing from singlet to triplet states as confirmed by oxygen quenching test. The observation of highly efficient intersystem crossing in BPQDs well explains its superior performance in photodynamic therapy and indicates its exotic spintronic and magneto-optical properties.