Substituent Effects in the Synthesis of Heterostructures

There have been a number of surprising reports of unexpected products when preparing heterostructures of Bi₂Se₃ with other 2D layers. These reports prompted us to explore the formation of metastable heterostructures containing Bi₂Se₃ using X-ray diffraction techniques to follow the reaction pathway. We discovered that the products formed depend on the electronic properties of the second constituent. Bi|Se layers deposited in a 2:3 ratio with enough atoms to make a single five-plane layer evolved to form thermodynamically stable Bi₂Se₃ as expected from the phase diagram. When the same Bi|Se layers were sequentially deposited with M|Se layers that form semiconductor layers (PbSe and 2H-MoSe₂), Bi₂Se₃-containing heterostructures formed. When the same Bi|Se layers were deposited with M|Se layers that form metallic layers (TiSe_2, VSe₂, and 1T-MoSe₂), BiSe-containing heterostructures formed. The amount of excess Se in the precursor controls whether [(Bi₂Se₃)_1+δ]₁[(MoSe₂)]₁ or [(BiSe)_1+γ]₁[(MoSe₂)]₁ forms. XPS data indicates that a mixture of both metallic 1T and semiconducting 2H-MoSe₂ is present in [(BiSe)_1+γ]₁[(MoSe₂)]₁, while only semiconducting 2H-MoSe₂ is present when layered with Bi₂Se₃. The electronic structure of adjacent layers impacts the formation of different structures from layers with similar local compositions. This provides an important additional parameter to consider when designing the synthesis of heterostructures, similar to substituent effects in molecular chemistry.