Fig. 10: Strain engineering in BP.

a A highly anisotropic inverse funnel effect that pushes excitons away from the area of high tensile strain has been predicted in BP. The figure was reproduced with permission from ref. ¹²⁹. b Extinction spectra (1−T/T₀) of a 6L BP sample under varying tensile strains, with strain applied along the AC (red) and ZZ (blue) directions. c E₁₁ and E₂₂ peak energies as a function of tensile strains, the strain direction is along the AC (red) and ZZ (blue) directions, respectively. The figure was reproduced with permission from ref. ¹³⁰. d Evolution of absorption spectra with varying strains induced in AC (left) and ZZ (right) directions in 20 nm thick BP. e Bandgap plotted as a function of strain measured for two 20 nm thick BP samples. The figure was reproduced with permission from ref. ¹³¹. f Transmission-mode optical image of ripples in a 10 nm thick BP flake. Below the optical image, an atomic force microscopy topography image acquired in the region highlighted with the dashed rectangle in f is shown. g Optical absorption spectra acquired on three ripple summits, three valleys, and three flat regions, indicated with colored circles in f. The figure was reproduced with permission from ref. ³⁷. h Averaged biaxial strain-induced shift rates of E₁₁, E₂₂, and E₃₃ peaks as a function of the layer number in 2–10L BP. i Illustration of 2L BP showing that when biaxial in-plane tensile stain is applied, the average distance between two layers (D + d) decreases due to the Poisson effect, while the gap between two layers (D) increases accompanied by a stronger decrease of the height of an individual layer d. j Schematic illustration of the band structure evolution of a bilayer BP under tensile and compressive strain. The orange dashed curves are the bands for a monolayer BP. The figure was reproduced with permission from ref. ¹³².