Substituent Effects in the Synthesis of Heterostructures

Abstract

There have been a number of surprising reports of unexpected products when preparing heterostructures of Bi 2 Se 3 with other 2D layers. These reports prompted us to explore the formation of metastable heterostructures containing Bi 2 Se 3 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 Bi 2 Se 3 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-MoSe 2 ), Bi 2 Se 3 -containing heterostructures formed. When the same Bi|Se layers were deposited with M|Se layers that form metallic layers (TiSe 2, VSe 2 , and 1T-MoSe 2 ), BiSe-containing heterostructures formed. The amount of excess Se in the precursor controls whether [(Bi 2 Se 3 ) 1+δ ] 1 [(MoSe 2 )] 1 or [(BiSe) 1+γ ] 1 [(MoSe 2 )] 1 forms. XPS data indicates that a mixture of both metallic 1T and semiconducting 2H-MoSe 2 is present in [(BiSe) 1+γ ] 1 [(MoSe 2 )] 1 , while only semiconducting 2H-MoSe 2 is present when layered with Bi 2 Se 3 . 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.

Challenges in synthesis of heterostructures

“…For example, a charge donating layer next to MoSe2 can cause a 1T polytype to form. [37][38][39] Since layer properties vary with thickness and identity of adjacent layers, the building-block approach of stacking 2D layers in specific sequences to form van der Waal's heterostructures results in an inexhaustible number of different target materials/heterostructures. [40][41][42][43] For example, consider an 8 layer heterostructure constructed from 4 layers of 1T-TiSe 2 and 4 layers of PbSe (total thickness of B5 nm).…”

Thin-Film Paradigm to Probe Interfacial Diffusion during Solid-State Metathesis Reactions

“…The thickness, density, and surface and interfacial roughness of thin film layers can all be independently modeled as a function of reaction time and temperature. XRR measurements have been used to observe interdiffusion and formation kinetics of thin-film bilayers as well as to probe reaction mechanisms in the formation of metastable materials using modulated elemental reactants. − Most studies focus on the synthesis of a single-composition thin film, where precursor films react to form one product, such as the reaction of two metal oxides to form a spinel material, which can be coupled to form more complex layered thin-film structures. − …”