Growth of Nanocrystalline MoSe2 Monolayers on Epitaxial Graphene from Amorphous Precursors

A new approach to the growth of MoSe2 thin films on epitaxial graphene on SiC(0001) by the use of modulated elemental reactants (MER) precursors has been reported. The synthesis applies a two-step process, where first an amorphous precursor is deposited on the substrate which self-assembles upon annealing. Films with a nominal thickness of about 1ML are successfully grown on epitaxial graphene monolayer as well as buffer layer samples. Characterization of the films is performed using XPS, LEED, AFM, and Raman spectroscopy. The films are nanocrystalline and show randomly rotated domains. This approach opens up an avenue to synthesize a number of new van-der-Waals systems on epitaxial graphene and other substrates

Structural Changes in 2D BiSe Bilayers as <i>n</i> Increases in (BiSe)_1+δ(NbSe₂)_<i>n</i> (<i>n</i> = 1–4) Heterostructures

(BiSe)_1+δ(NbSe₂)_<i>n</i> heterostructures with <i>n</i> = 1–4 were synthesized using modulated elemental reactants. The BiSe bilayer structure changed from a rectangular basal plane with <i>n</i> = 1 to a square basal plane for <i>n</i> = 2–4. The BiSe in-plane structure was also influenced by small changes in the structure of the precursor, without significantly changing the out-of-plane diffraction pattern or value of the misfit parameter, δ. Density functional theory calculations on isolated BiSe bilayers showed that its lattice is very flexible, which may explain its readiness to adjust shape and size depending on the environment. Correlated with the changes in the BiSe basal plane structure, analysis of scanning transmission electron microscope images revealed that the occurrence of antiphase boundaries, found throughout the <i>n</i> = 1 compound, is dramatically reduced for the <i>n</i> = 2–4 compounds. X-ray photoelectron spectroscopy measurements showed that the Bi 5d_3/2, 5d_5/2 doublet peaks narrowed toward higher binding energies as <i>n</i> increased from 1 to 2, also consistent with a reduction in the number of antiphase boundaries. Temperature-dependent electrical resistivity and Hall coefficient measurements of nominally stoichiometric samples in conjunction with structural refinements and XPS data suggest a constant amount of interlayer charge transfer independent of <i>n</i>. Constant interlayer charge transfer is surprising given the changes in the BiSe in-plane structure. The structural flexibility of the BiSe layer may be useful in designing multiple constituent heterostructures as an interlayer between structurally dissimilar constituents