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Nat Nanotechnol. 2014 Feb;9(2):111-5. doi: 10.1038/nnano.2013.277. Epub 2013 Dec 22.

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

Author information

1
1] Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA [2] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
2
Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan.
3
1] Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA [3] Department of Physics and Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK.
4
1] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA.
5
Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA.
6
1] Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan [2] Institute of Physics, Academia Sinica, Taipei 11529, Taiwan.
7
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.

Abstract

Quantum systems in confined geometries are host to novel physical phenomena. Examples include quantum Hall systems in semiconductors and Dirac electrons in graphene. Interest in such systems has also been intensified by the recent discovery of a large enhancement in photoluminescence quantum efficiency and a potential route to valleytronics 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 monolayers. 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.

PMID:
24362235
DOI:
10.1038/nnano.2013.277

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