Toward Strong UV-Vis-NIR Second-Harmonic Generation by Dimensionality Engineering of Zinc Thiocyanates

The precise modulation of the spatial orientations and connection modes of primitives is vital for certain critically important optical functions for nonlinear optical (NLO) materials (specifically, second-harmonic generation (SHG) and optical bandgap); however, we are yet to achieve a sufficient level of control for the designed construction of efficient broadband NLO materials. Exploiting the changes in microscopic polarization that may result from dimensional increase, we propose herein a zero-dimensional (0D)-to-three-dimensional (3D) dimensionality-increase strategy to realize strong broadband SHG responses for the first time. The novel 3D pseudo diamond-like Zn(SCN)₂ has been synthesized by removing SHG-inactive [NH₄]⁺ counter cations and H₂O molecules that are located between the adjacent discrete [Zn(SCN)₄] building blocks within the 0D (NH₄)₂Zn(SCN)₄·3H₂O. The 0D-to-3D dimensionality engineering, proceeding from (NH₄)₂Zn(SCN)₄·3H₂O to Zn(SCN)₂, results in significantly enhanced SHG responses and efficient broadband activity (8 × KH₂PO₄ @ 1064 nm, 4.18 eV bandgap for the former c.f. 2 × β-BaB₂O₄ @ 380 nm, 30 × KH₂PO₄ @ 1064 nm, 2 × KTiOPO₄ @ 2100 nm, 4.78 eV bandgap for the latter) from the UV to the NIR regions (SHG@300-1050 nm). Theoretical calculations and crystal structure analyses reveal that the coordination-bond-connected [Zn(SCN)₄] building blocks within the diamond-like structure of Zn(SCN)₂ are responsible for its giant broadband SHG responses.