Two-dimensional (2D) materials are atomically thick and without out-of-plane dangling bonds. As a result, they could break the confinement of lattice matching, and thus can be freely mixed and matched together to construct vertical van der Waals heterostructures. Here, we demonstrated an asymmetrical vertical structure of graphene/hexagonal boron nitride (h-BN)/tungsten disulfide (WS2)/graphene using all chemical vapor deposition grown 2D materials. Three building blocks are utilized in this construction: conductive graphene as a good alternative for the metal electrode due to its tunable Fermi level and ultrathin nature, semiconducting transition-metal dichalcogenides (TMDs) as an ultrathin photoactive material, and insulating h-BNas a tunneling barrier. Such an asymmetrical vertical structure exhibits a much stronger photovoltaic effect than the symmetrical vertical one without h-BN. By changing the sequence of h-BN in the vertical stack, we could even control the electron flow direction. Also, improvement has been further made by increasing the thickness of h-BN. The photovoltaic effect is attributed to different possibilities of excited electrons on TMDs to migrate to top and bottom graphene electrodes, which is caused by potential differences introduced by an insulating h-BN layer. This study shows that h-BN could be effectively used as a tunneling barrier in the asymmetrical vertical heterostructure to improve photovoltaic effect and control the electron flow direction, which is crucial for the design of other 2D vertical heterostructures to meet various needs of electronic and optoelectronic devices.
Keywords: 2D materials; CVD; WS2; asymmetrical vertical heterostructure; graphene; h-BN; photovoltaic.