Interfacial π-Electron Cloud Extension and Charge Transfer Between Preferable Single-Crystalline Conjugated MOFs and Graphene for Ultrafast Pulse Generation.

Abstract

2D conjugated metal-organic frameworks (MOFs) have attracted significant attention in various fields due to their outstanding characteristics. However, due to the strong interlayer π-π stacking interactions, the preparation of high-quality and atomic-scale single-crystalline conjugated MOF structures continues to pose a significant challenge. The investigation of its nonlinear optical (NLO) property and application for ultrafast photonics is still rare. Herein, the ultrathin Cu₃(HHTP)₂ and Ni₃(HHTP)₂ (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) nanosheets (CuHHTPNs and NiHHTPNs) with single-crystalline characteristic are prepared by surfactant-assisted solution synthesis strategy. Moreover, the π-π stacked CuHHTPNs(NiHHTPNs)/graphene van der Waals heterostructures (CuNsG-VHS and NiNsG-VHS) are achieved by ultrasound-assisted method. According to characterization analyses and theoretical simulations, this preferable stacking ultrathin van der Waals heterostructures exhibits superior π-conjugated electron cloud extension, charge transfer, and NLO properties. Noticeably, the third-order NLO polarizability of CuNsG-VHS keeps in a relatively high level compared with the reported 2D saturable absorber materials in the near-infrared wavelength range. Based on these outstanding properties, CuNsG-VHS can serve as an excellent saturable absorber to achieve fundamental mode-locking with femtosecond pulse duration, and high-order harmonic mode-locking with GHz repetition frequency. These demonstrations provide a valuable strategy for the development of promising conjugated MOFs for ultrafast photonics and advanced optoelectronic devices.