Abstract
Despite the weak nature of interlayer forces in transition metal dichalcogenide (TMD) materials, their properties are highly dependent on the number of layers in the few-layer two-dimensional (2D) limit. Here, we present a combined scanning tunneling microscopy/spectroscopy and GW theoretical study of the electronic structure of high quality single- and few-layer MoSe2 grown on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a fundamental parameter for transport and optical phenomena, decreases by nearly one electronvolt when going from one layer to three due to interlayer coupling and screening effects. Our results paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. This demonstrates the importance of layer number and electron–electron interactions on van der Waals heterostructures and helps to clarify how their electronic properties might be tuned in future 2D nanodevices.
Highlights
O wing to their inherently 2D nature, few-layer semiconducting transition metal dichalcogenide (TMD) exhibit a number of unique physical attributes that are extremely sensitive to the number of layers.[1−6] This provides new opportunities for creating van der Waals heterostructures with tailored properties and designed functionalities
Low temperature (5 K) STM/STS experiments were carried out on high quality MoSe2 grown on bilayer graphene (BLG) on 6H-SiC(0001) substrates via molecular beam epitaxy.[14]
Variations in the electronic structure between ML, BL, and TL MoSe2 films on BLG were experimentally determined via STS using standard lock-in techniques[6]
Summary
Letter (bilayer (BL)), and 3 (trilayer (TL)). These measurements compare favorably with ab initio GW calculations, revealing the important influence of interlayer coupling and Coulomb interactions on these properties, as well as the relative contributions from different parts of the Brillouin zone. The final calculated quasiparticle band structure (including screening contributions from the BLG substrate) for ML, BL, and TL MoSe2 is plotted in the right panels of Figure 3 This electronic structure was used to compute the LDOS above the MoSe2 surface which gives a measure of the STM differential conductance (dI/dV) within the Tersoff-Hamann approximation[26] with no adjustable parameters (see Supporting Information for technical details). We find that the addition of layers in the 2-dimensional regime causes the electronic bandgap to significantly shrink in size while simultaneously creating new features in both the valence and conduction bands These experimental results are explained with theoretical GW calculations that take into account stacking geometry, wave function hybridization, electron−electron interactions, and substrate screening, providing new insight into how different electronic structure features arise from Bloch state properties within the Brillouin zone. Present Addresses ¶Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstr. 8-10/134, 1040 Wien, Austria. #Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
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