Despite the weak nature of interlayer forces in transition metaldichalcogenide (TMD) materials, their properties are highly dependent on thenumber of layers in the few-layer two-dimensional (2D) limit. Here, we presenta combined scanning tunneling microscopy/spectroscopy and GW theoretical studyof the electronic structure of high quality single- and few-layer MoSe2 grownon bilayer graphene. We find that the electronic (quasiparticle) bandgap, afundamental parameter for transport and optical phenomena, decreases by nearlyone electronvolt when going from one layer to three due to interlayer couplingand screening effects. Our results paint a clear picture of the evolution ofthe electronic wave function hybridization in the valleys of both the valenceand conduction bands as the number of layers is changed. This demonstrates theimportance of layer number and electron-electron interactions on van der Waalsheterostructures, and helps to clarify how their electronic properties might betuned in future 2D nanodevices.
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