Transition metal oxides have attracted intense interest owing to their abundant physical and chemical properties. The controlled preparation of large-area, high-quality two-dimensional crystals is essential for revealing their inherent properties and realizing high-performance devices. However, fabricating two-dimensional (2D) transition metal oxides using a general approach still presents substantial challenges. Herein, we successfully achieve highly crystalline nickel oxide (NiO) flakes with a thickness as thin as 3.3 nm through the salt-assisted vapor–liquid–solid (VLS) growth method, which demonstrated exceptional stability under ambient conditions. To explore the great potential of the NiO crystal in this work, an artificial synapse based on the NiO-flake resistive switching (RS) layer is investigated. Short-term and long-term synaptic behaviors are obtained with external stimuli. The artificial synaptic performance provides the foundation of the neuromorphic application, including handwriting number recognition based on artificial neuron network (ANN) and the virtually unsupervised learning capability based on generative adversarial network (GAN). This pioneering work not only paves new paths for the synthesis of 2D oxides in the future but also demonstrates the substantial potential of oxides in the field of neuromorphic computing.

Nickel oxide (NiO_X) has been established as a highly efficient and stable hole-transporting layer (HTL) in perovskite solar cells (PSCs). However, existing deposition methods for NiO_X have been restricted by high-vacuum processes and fail to address the energy level mismatch at the NiO_X/perovskite interface, which has impeded the development of PSCs. Accordingly, we explored the application of NiO_X as a hybrid HTL through a sol–gel process, where a NiO_X film was pre-doped with Ag ions, forming a p/p⁺ homojunction in the NiO_X-based inverted PSCs. This innovative approach offers two synergistic advantages, including the enlargement of the built-in electric field for facilitating charge separation, optimizing energy level alignment, and charge transfer efficiency at the interface between the perovskite and HTL. Incorporating this hybrid HTL featuring the p/p⁺ homojunction in the inverted PSCs resulted in a high-power conversion efficiency (PCE) of up to 19.25%, significantly narrowing the efficiency gap compared to traditional n-i-p devices. Furthermore, this innovative strategy for the HTL enhanced the environmental stability to 30 days, maintaining 90% of the initial efficiency.