Two-dimensional (2D) atomic crystals have emerged as a class of materials that may impact our future electronics technology [1]. A key issue is controlling their electronic state to overcome the limit of natural properties. In this regard, there are rapidly growing interests in black phosphorus, a layered material consisting of 2D phosphorene layers. The low-energy band structure of black phosphorus has been widely predicted to be tunable by external perturbations, such as applied strain and electric field [2]. In this talk, I will introduce our recent angle-resolved photoemission spectroscopy studies on the tunable band gap of black phosphorus. By the in-situdeposition of alkali-metal atoms on the surface of black phosphorus, we found that the vertical electric field from dopants modulates the band gap, and changes the material from a narrow-gap semiconductor to a band-overlapped semimetal [3]. At the critical density of this semiconductor-semimetal transition, the material becomes a 2D Dirac semimetal, whose band dispersion is highly anisotropic, linear in armchair and quadratic in zigzag directions [3].

[1] E. S. Reich, Nature 506, 19 (2014).

[2] H. O. H. Churchill and P. Jarillo-Herrero, Nature Nanotech. 9, 330 (2014).

[3] J. Kim, S. S. Baek, S. H. Ryu, Y. Sohn, S. Park, B. G. Park, J. D. Denlinger, Y. Yi, H. J. Choi, and K. S. Kim, Science 349, 723 (2015).