Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are emerging as a new platform for exploring 2D semiconductor physics1,2,3,4,5,6,7,8,9. Reduced screening in two dimensions results in markedly enhanced electron–electron interactions, which have been predicted to generate giant bandgap renormalization and excitonic effects10,11,12,13. Here we present a rigorous experimental observation of extraordinarily large exciton binding energy in a 2D semiconducting TMD. We determine the single-particle electronic bandgap of single-layer MoSe2 by means of scanning tunnelling spectroscopy (STS), as well as the two-particle exciton transition energy using photoluminescence (PL) spectroscopy. These yield an exciton binding energy of 0.55 eV for monolayer MoSe2 on graphene—orders of magnitude larger than what is seen in conventional 3D semiconductors and significantly higher than what we see for MoSe2 monolayers in more highly screening environments. This finding is corroborated by our ab initio GW and Bethe–Salpeter equation calculations14,15 which include electron correlation effects. The renormalized bandgap and large exciton binding observed here will have a profound impact on electronic and optoelectronic device technologies based on single-layer semiconducting TMDs.

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Figure 1: Morphology of monolayer MoSe2 on bilayer graphene.
Figure 2: Electronic structure of monolayer MoSe2 on bilayer graphene.
Figure 3: Optical characterization of monolayer MoSe2 on bilayer graphene.
Figure 4: Comparison between ab initio excited-state calculations and single-layer MoSe2 experiment.

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Acknowledgements

Research supported by Office of Basic Energy Sciences, Department of Energy Early Career Award No. DE-SC0003949 (optical measurements), and by the sp2 Program (STM instrumentation development and operation), the Theory Program (GW–BSE calculations), and the SciDAC Program on Excited State Phenomena in Energy Materials (algorithms and codes) which are funded by the US Department of Energy, Office of Basic Energy Sciences and of Advanced Scientific Computing Research, under Contract No. DE-AC02-05CH11231. Support also provided by National Science Foundation award no. 1235361 (image analysis) and National Science Foundation award no. DMR10-1006184 (substrate screening theory and calculations). Computational resources have been provided by the NSF through XSEDE resources at NICS and DOE at NERSC. A.J.B. was supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. W. R acknowledges support from the Chinese Scholarship Council (No. 201306210202). D.Y.Q. acknowledges support from NSF Graduate Research Fellowship Grant No. DGE 1106400 and S.G.L. acknowledges support of a Simons Foundation Fellowship in Theoretical Physics. STM/STS data were analysed and rendered using WSxM software31. S-F.S. and F.W. acknowledge X. Hong and J. Kim for technical help.

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M.M.U., A.J.B., S-F.S., F.W. and M.F.C. conceived the work and designed the research strategy. M.M.U. and A.J.B. measured and analysed the STM/STS data in collaboration with W.R. for the MoSe2/HOPG experiments. S-F.S. carried out the photoluminescence and Raman experiments. F.H.d.J. and D.Y.Q. performed the theoretical calculations. Y.Z. and S-K.M. performed the MBE growth and characterization (LEED, RHEED, core-level spectroscopy) of the samples. Z.H. and Z-X.S. supervised the MBE and sample characterization. F.W. supervised the optical measurements. S.G.L. supervised the theoretical calculations. M.F.C. supervised the STM/STS experiments. M.M.U. wrote the paper with help from A.J.B. and M.F.C. M.M.U. and M.F.C. coordinated the collaboration. All authors contributed to the scientific discussion and manuscript revisions.

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Correspondence to Miguel M. Ugeda or Michael F. Crommie.

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The authors declare no competing financial interests.

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Ugeda, M., Bradley, A., Shi, S. et al. Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor. Nature Mater 13, 1091–1095 (2014). https://doi.org/10.1038/nmat4061

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