Two-Dimensional Violet Phosphorus P 11 : A Large Band Gap Phosphorus Allotrope.

The discovery of novel large band gap two-dimensional (2D) materials with good stability and high carrier mobility will innovate the next generation of electronics and optoelectronics. A new allotrope of 2D violet phosphorus P 11 was synthesized via a salt flux method in the presence of bismuth. Millimeter-sized crystals of violet-P 11 were collected after removing the salt flux with DI water. From single-crystal X-ray diffraction, the crystal structure of violet-P 11 was determined to be in the monoclinic space group C 2/ c (no. 15) with unit cell parameters of a = 9.166(6) Å, b = 9.121(6) Å, c = 21.803(14)Å, β = 97.638(17)°, and a unit cell volume of 1807(2) Å 3 . The structure differences between violet-P 11 , violet-P 21 , and fibrous-P 21 are discussed. The violet-P 11 crystals can be mechanically exfoliated down to a few layers (∼6 nm). Photoluminescence and Raman measurements reveal the thickness-dependent nature of violet-P 11 , and exfoliated violet-P 11 flakes were stable in ambient air for at least 1 h, exhibiting moderate ambient stability. The bulk violet-P 11 crystals exhibit excellent stability, being stable in ambient air for many days. The optical band gap of violet-P 11 bulk crystals is 2.0(1) eV measured by UV-Vis and electron energy-loss spectroscopy measurements, in agreement with density functional theory calculations which predict that violet-P 11 is a direct band gap semiconductor with band gaps of 1.8 and 1.9 eV for bulk and monolayer, respectively, and with a high carrier mobility. This band gap is the largest among the known single-element 2D layered bulk crystals and thus attractive for various optoelectronic devices.