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Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2

Abstract

Quantum systems in confined geometries are host to novel physical phenomena. Examples include quantum Hall systems in semiconductors1 and Dirac electrons in graphene2. Interest in such systems has also been intensified by the recent discovery of a large enhancement in photoluminescence quantum efficiency3,4,5,6,7 and a potential route to valleytronics6,7,8 in atomically thin layers of transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se, Te), which are closely related to the indirect-to-direct bandgap transition in monolayers9,10,11,12. Here, we report the first direct observation of the transition from indirect to direct bandgap in monolayer samples by using angle-resolved photoemission spectroscopy on high-quality thin films of MoSe2 with variable thickness, grown by molecular beam epitaxy. The band structure measured experimentally indicates a stronger tendency of monolayer MoSe2 towards a direct bandgap, as well as a larger gap size, than theoretically predicted. Moreover, our finding of a significant spin-splitting of 180 meV at the valence band maximum of a monolayer MoSe2 film could expand its possible application to spintronic devices.

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Figure 1: Crystal structure, RHEED patterns and overall ARPES spectra of MoSe2 thin film.
Figure 2: Band evolution with increasing thickness of MoSe2 thin films.
Figure 3: Direct bandgap in monolayer and indirect bandgap in 8 ML MoSe2 thin films.

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Acknowledgements

The work at the ALS is supported by the US Department of Energy (DoE) Office of Basic Energy Science contract no. DE-AC02-05CH11231. The work at the Stanford Institute for Materials and Energy Sciences and Stanford University is supported by the US DoE Office of Basic Energy Science under contract no. DE-AC02-76SF00515. The work at Oxford University is supported from a Defense Advanced Research Projects Agency MesoDynamic Architectures (DARPA MESO) project (no. 187 N66001-11-1-4105). The work at Northeastern University is supported by the US DoE Office of Basic Energy Sciences under contract no. DE-FG02-07ER46352 and benefited from Northeastern University's Advanced Scientific Computation Center (ASCC), theory support at the Advanced Light Source, Berkeley, and the allocation of time at the National Energy Research Scientific Computing Center (NERSC) supercomputing centre through DoE grant no. DE-AC02-05CH11231. T.R.C. and H.T.J. are supported by the National Science Council, Taiwan. H.T.J. also thanks National Center for High-Performance Computing (NCHC), Computer and Information Network Center (CINC) – National Taiwan University (NTU) and National Center for Theoretical Sciences (NCTS), Taiwan, for technical support.

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Contributions

Y.Z. led the thin-film growth effort with F.S., J.L., R.M. and S.K.M., performed ARPES measurements with B.Z., Z.L. and S.K.M., and analysed the data. Y.Z., H.L. and S.K.M. wrote the paper with suggestions and comments by A.B. and Z.X.S. Y.T.C. and Y.H. characterized samples with Raman spectroscopy and AFM. T.R.C., H.L., H.T.J. and A.B. provided theoretical support. S.K.M., Y.L.C., Z.H., A.B. and Z.X.S. were responsible for project direction, planning and infrastructure.

Corresponding authors

Correspondence to Sung-Kwan Mo or Zhi-Xun Shen.

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The authors declare no competing financial interests.

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Zhang, Y., Chang, TR., Zhou, B. et al. Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2. Nature Nanotech 9, 111–115 (2014). https://doi.org/10.1038/nnano.2013.277

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