Role of Bilayer Graphene Microstructure on the Nucleation of WSe2 Overlayers

Over the past few years, graphenegrown by chemical vapordeposition(CVD) has gained prominence as a template to grow transition metaldichalcogenide (TMD) overlayers. The resulting two-dimensional (2D)TMD/graphene vertical heterostructures are attractive for optoelectronicand energy applications. However, the effects of the microstructuralheterogeneities of graphene grown by CVD on the growth of the TMDoverlayers are relatively unknown. Here, we present a detailed investigationof how the stacking order and twist angle of CVD graphene influencethe nucleation of WSe2 triangular crystals. Through thecombination of experiments and theory, we correlate the presence ofinterlayer dislocations in bilayer graphene with how WSe2 nucleates, in agreement with the observation of a higher nucleationdensity of WSe2 on top of Bernal-stacked bilayer grapheneversus twisted bilayer graphene. Scanning/transmission electron microscopy(S/TEM) data show that interlayer dislocations are present only inBernal-stacked bilayer graphene but not in twisted bilayer graphene.Atomistic ReaxFF reactive force field molecular dynamics simulationsreveal that strain relaxation promotes the formation of these interlayerdislocations with localized buckling in Bernal-stacked bilayer graphene,whereas the strain becomes distributed in twisted bilayer graphene.Furthermore, these localized buckles in graphene are predicted toserve as thermodynamically favorable sites for binding WSe x molecules, leading to the higher nucleation densityof WSe2 on Bernal-stacked graphene. Overall, this studyexplores synthesis-structure correlations in the WSe2/graphene vertical heterostructure system toward the site-selectivesynthesis of TMDs by controlling the structural attributes of thegraphene substrate.

Automated summary

By combining experiments and theory, we investigated the influence of stacking order and twist angle of CVD graphene on the nucleation of WSe2 crystals. We found that interlayer dislocations are present only in Bernal-stacked bilayer graphene, not in twisted bilayer graphene. The localized buckles in Bernal-stacked graphene serve as thermodynamically favorable sites for binding WSe x molecules, resulting in a higher nucleation density of WSe2.