4. Raman Spectroscopy of van der Waals Heterostructures

The research of two-dimensional (2D) atomic crystals has progressed rapidly since the isolation of graphene in 2004 [1, 2]. The family of 2D crystals now include many different types of materials, including metals (e.g. graphene, NbSe₂), semiconductors (e.g. phosphorene, MoS₂, WSe₂), insulators (e.g. BN), superconductors and charge-density-wave materials (e.g. NbSe₂ and TiSe₂). Alongside with the rapid development of individual 2D materials, the research frontier has also advanced to explore their hybrid systems [3, 4]. In particular, the flat and inert surfaces of 2D materials enable the construction of heterogeneous stacks of different 2D crystals with atomically sharp interfaces, coupled vertically only by van der Waals forces. These van der Waals heterostructures exhibit many unique properties that cannot be realized in individual 2D crystals [3, 4]. For instance, graphene on hexagonal boron nitride (BN) can exhibit the Hofstadter’s butterfly phenomenon because of the nanoscale periodic interaction between the graphene and BN lattices [5–7]. Transition metal dichalcogenide (TMD) heterostructures can host long-lived interlayer excitons due to the staggered band alignment between different TMD layers [8]. Electronic and optoelectronic devices made from van der Waals heterostructures can exhibit performance superior to that of traditional devices with lateral 2D junctions [9–11]. More generally, the 2D building blocks can be combined to form more complex structures. By incorporating the unique properties of each class of 2D crystal (e.g., semiconducting TMDs, insulating BN and metallic graphene), integrated circuits can in principle be constructed entirely with 2D materials. Such 2D systems of electronics, once realized, could open a route to post-silicon technology.