Fig. 7: Excitons in BP.

a PLE intensity (right axis) at the emission peak of the monolayer BP as a function of excitation photon energies. The reflectance contrast spectra (left axis) are also superimposed for comparison. b Schematic of in-plane anisotropic Coulomb screening of excitonic states in BP. x, y, and z denote armchair, zigzag, and out-of-plane orientations, respectively. c Exciton wave function in real space of first bright state, i.e., the 1s state, for bilayer BP. The hole is fixed at the center. d Binding-energy spectrum of the excitonic series of the bilayer BP. Left panel: experimental results. Middle panel: GW + BSE calculation results. Right panel: calculation results of anisotropic effective-mass model. Bright states are in red and dark states are in black. e Exciton binding energy as a function of BP layer number, showing experimental measured binding energies, BSE-simulated binding energies with the calculated interlayer distance, and BSE-simulated binding energies with the experimentally extracted interlayer distance. Error bars are based on standard deviation errors. The figure was reproduced with permission from ref. ⁵³. f Experimental transition energies for the exciton states as a function of their index, n, measured monolayer, bilayer, and trilayer of BP. The black curves show fits to the data with the model described by Eq. 3. The experimental data was taken from ref. ⁵³. g Normalized PL spectra of the BP monolayer at different excitation powers. h Integrated intensity of the charged (T) and neutral (X) exciton emission versus excitation power. i Normalized PL spectra of the BP monolayer measured at different temperatures. j Emission energy of the neutral exciton as a function of the temperature. k Integrated intensity of the neutral exciton versus temperature. The dashed lines are Gaussian fits. The solid lines are fits to the models. The figure was reproduced with permission from ref. ¹³⁶.