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Since the discovery of monolayer graphene in 2004 many other layered materials have been reduced to monolayer thickness. Strong confinement and reduced screening in two-dimensional materials result in novel electronic properties, different from which of their 3D counterpart.
Recently, transition metal dichalcogenides (TMDs) have moved under the spotlight. Group VI TMDs are indirect bandgap semiconductors. In the monolayer limit the band gap becomes direct with giant exciton binding energies due to strong electronic correlations. Also, monolayer group VI TMDs exhibit broken inversion symmetry that, in combination with strong spin-orbit interaction, leads to a sizable spin-splitting of the band structure of up to several hundreds of meV.
Stacking different 2D materials to form van der Waals (vdW) heterostructures with new functionalities is a long-sought goal that now starts to turn into reality. Such heterostructures can be used to tailor screening for bandgap engineering as well as charge and spin transfer across the layers.
We have synthesized WS2/graphene vdW heterostructures via chemical vapor deposition (CVD) [1] on epitaxial graphene on SiC(0001) [2] and characterized the electronic properties with high-resolution angle-resolved photoemission spectroscopy (ARPES) [3]. These measurements demonstrate perfect epitaxial alignment of WS2 on graphene and provide evidence for significant static charge transfer between the two layers. Further, we excited these heterostructures with femtosecond laser pulses resonant to the exciton in WS2 and probed the resulting carrier dynamics with time-resolved ARPES, providing evidence for ultrafast charge transfer between WS2 and graphene.
[1] Avsar et al., Nat. Comm. 5, 4875 (2014)
[2] Rossi et al., 2D Mat 3, 031013 (2016)
[3] Forti et al., Nanoscale 9, 16412 (2017)
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