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 Cu 3 (HHTP) 2 and Ni 3 (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.