Tightly bound trions in monolayer MoS2

Abstract

Two-dimensional (2D) atomic crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable physical properties 1 . In contrast to graphene, monolayer MoS 2 is a non-centrosymmetric material with a direct energy gap 2-5 . Strong photoluminescence 2,3 , a current on-off ratio exceeding 10 8 in field-effect transistors 6 , and efficient valley and spin control by optical helicity 7-9 have recently been demonstrated in this material. Here we report the spectroscopic identification in doped monolayer MoS 2 of tightly bound negative trions, a quasi-particle composed of two electrons and a hole. These quasiparticles, which can be created with valley and spin polarized holes, have no analogue in other materials. They also possess a large binding energy (~ 20 meV), rendering them significant even at room temperature. Our results open up new avenues 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 MoS 2 is nearly an order of magnitude larger than that found in conventional quasi-2D systems, such as semiconductor quantum wells (QWs) [10][11][12][13] . This is a consequence of the greatly enhanced Coulomb interactions in monolayer MoS 2 , arising from reduced dielectric screening in gapped 2D crystals and the relatively heavy carrier band masses associated with the Mo d-manifolds 4,5,14 . For an electron density as high as n = 10 11 cm -2 , for instance, the dimensionless interaction parameter r s is ~60 in monolayer MoS 2 (Supplementary Information S1). This value is significantly larger than that for carriers in QWs even at very low doping levels 15 . Monolayer MoS 2 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 crystallization 16 .The atomic structure of MoS 2 consists of hexagonal planes of S and Mo atoms in a trigonal prismatic structure (Fig. 1a) 17 . The two sublattices of the hexagonal MoS 2 structure are occupied, respectively, by one Mo and two S atoms (Fig. 1b). Monolayer MoS 2 is a direct gap semiconductor with energy gaps located at the K and K' points of the Brillouin zone (Fig. 1c). Both the highest valence bands and the lowest conduction bands are formed primarily from the Mo d-orbitals 4,17 . The large spin-orbit interaction 2 splits the highest valence bands at the K (K') point by ~ 160 meV 2,3,7,14,18 . The valley and spin (VS) degrees are coupled because of the lack of inversion symmetry in monolayer MoS 2 . As has been recently shown experimentally, this allows optical pumping of a single valley (and spin) with circularly polarized light 7-9 .Here we investigate the optical response of monolayer MoS 2 as a function of carrier density by means of absorption and photoluminescence (PL) spectroscopy. In our investigations we have made use of MoS 2 monolayers p...

Spectroscopic Signatures of AA′ and AB Stacking of Chemical Vapor Deposited Bilayer MoS2

“…The exciton and trion luminescence peak intensities are known to depend sensitively and differently on the local carrier density. 35 This we believe is the reason for the variation observed in the A À /A peak ratio.…”

Helicity-Resolved Raman Scattering of MoS2, MoSe2, WS2, and WSe2 Atomic Layers

“…[2][3][4][5] A prominent example is the semiconducting transition metal dichalcogenides (TMDCs) that exhibit rich physical phenomena, including indirect to direct bandgap transition, 6, 7 large exciton and trion binding energy, [8][9][10][11] strong photoluminescence and electroluminescence, 7, 12-14 superior transistor performance with large on-off ratio [15][16][17] and reasonably high mobility, 5, 18, 19 and perhaps most strikingly, the capability to address the valley degree of freedom. [20][21][22][23][24] Manipulation of valley polarized carriers excited by circularly polarized light has led to recent observation of the valley Hall effect 25 that opens up potential for applications in 'valleytronics' envisioned before in graphene.…”

Gate-induced superconductivity in atomically thin MoS2 crystals

“…Depending on the specific system, experimental conditions, and physical processes investigated, however, interaction effects can also play an important role. Examples are excitonic effects 17 18 19 and gap renormalization in semiconducting TMDs 20 , or the transport properties of few-layer, suspended graphene very close to charge neutrality 21 22 23 . As compared to properties described by a single-particle picture, exploring the thickness dependence of phenomena in which interactions play a key role is considerably more complex 24 , and only limited work has been done.…”