Fig. 4. Atomic structure analysis and associated spontaneous SHG.

a The polarization-resolved SHG response with a 1064 nm excitation laser. b Calculated SHG response derived from the AB′ stacking atomic model containing 0.17 Å interlayer gliding along the [120] direction. c Cross-sectional ADF-STEM image exhibiting a long-range order AB′-stacked SnSe₂ along the [100] zone axis. d Enlarged ADF-STEM image marked by the white box in (c) and corresponding Sn atom displacement map (e). f Simulated ADF-STEM image of pristine AB′-stacked SnSe₂ and corresponding Sn atom displacement map (g). h DFT calculated energy landscape gliding from the AC′ stacking order to the AB′ stacking order using a bilayer SnSe₂ model along the [120] direction. i The nonlinear susceptibility tensor element ${\mathrm{\chi }}^{\left(2\right)}$ of AB′-stacked SnSe₂ with minor sliding (orange) and pure AB′ stacking sequence (pink). j The ${\mathrm{\chi }}_{\mathrm{xxx}}^{\left(2\right)}$ component as a function of interlayer gliding along the [120] direction. Scale bars: c 2 nm; d, f 0.5 nm.