Beyond turbostratic disorder: Towards epitaxial alignment in Bi2Se3/MoSe2 van-der-Waals bilayer stacks grown from amorphous elemental precursors

Bottom-up synthetic approaches to synthesize stacks of individual quasi-two-dimensional materials remain a major experimental challenge. Here, we evaluate the applicability of the little-known technique of modulated elemental reactants (MER) for the synthesis of one-unit-cell-thick <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mrow><a:msub><a:mi>Bi</a:mi><a:mn>2</a:mn></a:msub><a:msub><a:mi>Se</a:mi><a:mn>3</a:mn></a:msub></a:mrow><a:mo>/</a:mo><a:msub><a:mi>MoSe</a:mi><a:mn>2</a:mn></a:msub></a:mrow></a:math> bilayer stacks via the self-assembly of amorphous elemental Bi|Se and Mo|Se precursors. Using a two-step annealing approach, both layers can be crystallized in a single heterostructure, as evidenced by a combination of x-ray photoelectron spectroscopy, Raman spectroscopy, and x-ray diffraction. However, we observe a limited stability of the <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:msub><b:mi>Bi</b:mi><b:mn>2</b:mn></b:msub><b:msub><b:mi>Se</b:mi><b:mn>3</b:mn></b:msub></b:mrow></b:math> layer, reflected in its partial decomposition under conditions required to crystallize <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>MoSe</c:mi><c:mn>2</c:mn></c:msub></c:math>. The choice of substrate proves to be a crucial piece to control the thin film growth: We demonstrate the evolution of larger-scale epitaxial alignment within a heterostructure grown via MER by low-energy electron diffraction.