van der Waals Gap Engineering of Emergent Two-Dimensional Materials

ConspectusLayered materials bound by weak van der Waals (vdW) interactions offer a rich platform for exploring intriguing fundamental science in the two-dimensional (2D) limit and advancing technological innovations. Transition from bulk to 2D geometry results in profound alterations in electronic structures and crystallographic symmetries, giving rise to a plethora of novel physical effects and functionalities. Due to their atomic-scale thinness, 2D materials with a high specific surface area enable post-processing chemical modification of their basal planes to further regulate their intrinsic physical properties. Moreover, the interfacial effects induced by surface modifications can modulate properties without altering the original lattice, facilitating the emergence of novel electronic phases and exotic quantum phenomena. Consequently, extensive research is delving into surface modifications of 2D materials, paving the way to further expand the research fields of 2D materials.Notably, layered materials also feature a subnanometer-sized vdW gap between adjacent layers, enabling the incorporation of guest species and evoking a new type of surface modification called vdW gap engineering, without the need for pre-exfoliation into 2D structures. Unlike postprocessing surface modifications, direct vdW gap engineering protects guest species within the layers from environmental degradation, fostering stable guest–host structures with enhanced environmental stability. Additionally, the confined vdW gap engineering prompts electronic interactions between guest species and host materials, resulting in new physics and functionalities that cannot be achieved through traditional surface modifications. Furthermore, vdW gap engineering also enables the creation of a new class of hybrid vdW superlattices with highly adaptable structural motifs, harnessing the synergistic effects of guest species and host materials.This Account highlights recent advancements in vdW gap engineering of 2D materials from our group and other researchers. We focus on three key aspects of vdW gap engineering including the design and synthesis of low-dimensional materials, modulation of phase transitions, and fabrication of hybrid superlattices. Specifically, we provide a comprehensive overview of current vdW gap engineering methodologies such as intercalation, interlayer growth, and direct chemical growth. Various forms of host–guest interactions and their underlying mechanisms are introduced along with the exciting physical properties and functional applications. Finally, we outline the present challenges and future prospects for vdW gap engineering of 2D materials. We emphasize the crucial role of in situ characterization techniques and machine learning in advancing vdW gap engineering studies as well as potential new research directions that could open new frontiers in creating artificial vdW materials for technological innovations.