Solid State Reaction Epitaxy, A New Approach for Synthesizing Van der Waals heterolayers: The Case of Mn and Cr on Bi₂Se₃

Van der Waals (vdW) heterostructures that pair materials with diverse properties enable various quantum phenomena. However, the direct growth of vdW heterostructures is challenging. Modification of the surface layer of quantum materials to introduce new properties is an alternative process akin to solid state reaction. Here, vapor deposited transition metals (TMs), Cr and Mn, are reacted with Bi2Se3 with the goal to transform the surface layer to XBi2Se4 (X = Cr, Mn). Experiments and ab initio MD simulations demonstrate that the TMs have a high selenium affinity driving Se diffusion toward the TM. For monolayer Cr, the surface Bi2Se3 is reduced to Bi-2-layer and a stable (pseudo) 2D Cr1+delta Se2 layer is formed. In contrast, monolayer Mn can transform upon mild annealing into MnBi2Se4. This phase only forms for a precise amount of initial Mn deposition. Sub-monolayer amounts dissolve into the bulk, and multilayers form stable MnSe adlayers. This study highlights the delicate energy balance between adlayers and desired surface modified layers that governs the interface reactions and that the formation of stable adlayers can prevent the reaction with the substrate. The success of obtaining MnBi2Se4 points toward an approach for the engineering of other multicomponent vdW materials by surface reactions.
(TR Abstract Type: graphical; Abstract Language: en) Solid state reaction of Cr and Mn with Bi2Se3 surface is investigated. Cr reacts with Se to form a 2D Cr1+delta Se2 adlayer while reducing Bi2Se3 to Bi-2. In contrast, precisely controlled monolayer amounts of Mn transforms the surface layer to MnBi2Se4. The different interface reactions are due to varying stability of the monolayer selenides and the mixed bismuth selenide phases.