Transition metal dichalcogenide MoSe2 nanostructures

Transition metal dichalcogenides (TMDs) are a family of van der Waals (vdW) layered materials exhibiting unique electronic, optical, magnetic, and transport properties. Their technological potentials hinge critically on the ability to achieve controlled fabrication of desirable nanostructures. Here I present three kinds of nanostructures of semiconducting TMD MoSe₂, created by molecular beam epitaxy (MBE) and characterized by scanning tunneling microscopy and spectroscopy (STM/STS). The three kinds of nanostructures are two-dimensional (2D) nanoislands, quasi one-dimensional (1D) nanoribbons, and heterostructures. The successful growth of 2D nanoislands lays the foundation for the preparation of the other two structures. By properly controlling the substrate temperature and Se over-pressure, the MoSe₂ atomic layers undergo a dramatic three-stage shape transformation: from fractal to compact 2D nanoislands, and eventually to nanoribbons, in stark contrast to the traditional two-stage growth behaviour involving only the transformation from the fractal to compact regime. Experimentally, it is found that the Se:Mo flux ratio during MBE growth plays a central role in controlling the nanoribbon formation. Theoretically, first-principles calculations show that the abundance/deficiency of extra Se atoms at different island edges significantly modifies the relative step energies between zigzag and armchair edges, which in turn impacts the island shape evolution during nonequilibrium growth. The successful preparation of MoSe2/hBN/Ru(0001) heterostructure is a demonstration that MBE technique is suitable for fabricating vdW heterostructures. Surprisingly, we found that the quasi-particle gap of the MoSe₂ on hBN/Ru is about 0.25 eV smaller than those on graphene or graphite substrates. We attribute this result to the strong interaction between hBN/Ru which causes residual metallic screening from the substrate. The surface of MoSe₂ exhibits Moiré pattern that replicates the Moiré pattern of hBN/Ru. In addition, the electronic structure and the work function of MoSe₂ are modulated electrostatically with an amplitude of ~ 0.13 eV. Most interestingly, this electrostatic modulation is spatially in phase with the Moiré pattern of hBN on Ru(0001) whose surface also exhibits a work function modulation of the same amplitude