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PUBLISHED ONLINE: 2 DECEMBER 2012 | DOI: http://www.nature.com/doifinder/10.1038/nmat3505
Web End =10.1038/NMAT3505
Tightly bound trions in monolayer MoS2
Kin Fai Mak1, Keliang He2, Changgu Lee3, Gwan Hyoung Lee4, James Hone4, Tony F. Heinz1
and Jie Shan2*
Two-dimensional (2D) atomic crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable physical properties1. In contrast to graphene, monolayer MoS2 is a non-centrosymmetric material with a direct energy gap25. Strong photoluminescence2,3, a current on/off ratio exceeding 108 in eld-effect transistors6, and efcient valley and spin control by optical helicity79 have
recently been demonstrated in this material. Here we report the spectroscopic identication in a monolayer MoS2 eld-effect transistor of tightly bound negative trions, a quasiparticle composed of two electrons and a hole. These quasiparticles, which can be optically created with valley and spin polarized holes, have no analogue in conventional semiconductors. They also possess a large binding energy (20 meV), rendering
them signicant even at room temperature. Our results open up possibilities both for fundamental studies of many-body interactions and for optoelectronic and valleytronic applications in 2D atomic crystals.
The trion binding energy that we observe in monolayer MoS2 is
nearly an order of magnitude larger than that found in conventional quasi-2D systems, such as semiconductor quantum wells1013. This
is a consequence of the greatly enhanced Coulomb interactions in monolayer MoS2, arising from reduced dielectric screening in gapped 2D crystals and the relatively heavy particle band masses associated with the Mo d-manifolds4,5,14. For an electron
density as high as n = 1011 cm2, for instance, the dimensionless
interaction parameter rs is 60 in monolayer MoS2 (Supplementary
Section S1). This value is significantly larger than that for carriers in quantum wells even at very low doping levels15. Monolayer MoS2 is a strongly interacting system even in the presence of relatively high carrier densities; it thus presents an ideal laboratory for exploring many-body phenomena, such as carrier multiplication and Wigner crystallization16.
The atomic structure of MoS2 consists of hexagonal planes of S and Mo atoms in a trigonal prismatic structure (Fig. 1a; ref. 17). The two sublattices of the hexagonal MoS2 structure are occupied, respectively, by one Mo and two S atoms (Fig. 1b). Monolayer MoS2 is a direct gap semiconductor with energy...