Enhanced Anisotropic Second Harmonic Generation in\nType-II van der Waals Heterostructures of g‑C<sub>3</sub>N<sub>4</sub>/BiVO<sub>4</sub>

Due\nto the remarkably strong second-harmonic generation (SHG),\ntwo-dimensional materials have excellent potential applications in\nnonlinear optics (NLOs). However, their SHG efficiency remains limited\ndue to the short light–matter interaction length. Through van\nder Waals heterostructure (vdWHS) engineering, it is possible to elongate\nlight–matter interacting length and control structural symmetries\nand anisotropies. Here, vdWHSs were built by noncentrosymmetric monolayer\ng-C<sub>3</sub>N<sub>4</sub> and centrosymmetric BiVO<sub>4</sub> slabs\nwith varying layers. Their structural stabilities, electronic band\nstructures, band alignments, and SHG susceptibilities have been systematically\nsimulated by DFT calculations. Interestingly, BiVO<sub>4</sub> slabs\nwith two or more layers form stable type-II heterostructures with\ng-C<sub>3</sub>N<sub>4</sub>, exhibiting the interlayer separation\nof photogenerated carriers. Besides, vdWHSs broke the centrosymmetry\nof BiVO<sub>4</sub>, resulting in remarkable and complex SHG responses.\nThe in-plane SHG susceptibility of g-C<sub>3</sub>N<sub>4</sub>/BiVO<sub>4</sub> vdWHSs exceeds 100 pm/V in the 700–800 nm range and\ngradually decreases as the thickness increases. Significant polarized\nout-of-plane SHG was introduced by vdWHS. The χ<sub>zxx</sub><sup>(2)</sup>, χ<sub>zxy</sub><sup>(2)</sup>, and χ<sub>zyy</sub><sup>(2)</sup> components\nshow multiple high peaks at 400–780 and 780–1240 nm,\nwith intensities 3 times larger than that of LiNbO<sub>3</sub>. These\nindicate that such vdWHS composites hold considerable potential for\nNLO in both visible and infrared light regions, which are important\nfor advanced optical communication and photonic computing systems.