Tightly Bound Excitons in MonolayerWSe2

Abstract

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe 2 ) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37 eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one-and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model. This result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors. The observed large exciton binding energy will also have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals. DOI: 10.1103/PhysRevLett.113.026803 PACS numbers: 73.21.Fg, 71.35.Cc, 78.20.Ci, 78.55.Hx One of the most distinctive features of electrons in twodimensional (2D) semiconductors, such as single atomic layers of group VI transition metal dichalcogenides (TMDs) [1], is the significantly reduced dielectric screening of Coulomb interactions. An important consequence of strong Coulomb interactions is the formation of tightly bound excitons. Indeed, recent theoretical studies have predicted a large exciton binding energy between 0.5 and 1 eV in MoS 2 monolayers [2-10], a representative 2D direct gap semiconductor from the family of TMDs [11,12]. These values for the exciton binding energy are more than an order of magnitude larger than that in conventional III-V-based quasi-2D semiconductor quantum wells (QWs) [13,14]. Such tightly bound excitons are expected to not only dominate the optical response, but also to play a defining role in the optoelectronic processes, such as photoconduction and photocurrent generation in 2D semiconductors [1,15]. On the other hand, little is known about these tightly bound excitons from the experimental standpoint, except the energy of the lowest energy one-photon active exciton states [11] and an indirect evidence of large binding energies through recent studies on trions, quasiparticles of two electrons and a hole, or two holes and an electron [16][17][18]. Furthermore, a non-Rydberg series has been predicted for excitons in 2D semiconductors, arisen from the nonlocal character of screening of the Coulomb interactions [4,19]. While a Rydberg series for the exciton energy spectrum has been observed in bulk MoS 2 [20,21], similar experimental studies on monolayers of MoS 2 or other TMDs have not been reported [22].The challenge in experimental determination of the exciton binding energy in 2D TMDs by linear optical methods, commonly used for bulk semiconductors [23] or conventional semiconductor QWs [13], lies in the identification of the onset of band-to-band transitions in the optical absorption or emission spectrum. Such an onset of band-to-band transitions has not been observed in 2D TMDs presumably due to the sig...

Probing Critical Point Energies of Transition Metal Dichalcogenides: Surprising Indirect Gap of Single Layer WSe2

“…Based on the measured quasi-particle gaps and the exciton transition energies, we find a similar exciton binding energy of ~ 0.5 eV for MoSe 2 and WSe 2 .…”

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Experimental determinations of the electronic band structures in TMDs are quite non-trivial.Optical spectroscopies [1][2][3][4][5] are unsuitable to measure the quasi-particle band structures due to the existence of large exciton binding energies. Using angle resolved photoemission (ARPES), it is difficult to probe the conduction band structures 6,7 .

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Sub-bandgap Voltage Electroluminescence and Magneto-oscillations in a WSe2 Light-Emitting van der Waals Heterostructure

“…Strikingly, the onset for EL of V e ∼ ±1.7 V is significantly smaller as compared to the direct band gap of a WSe 2 monolayer which is of about 2 − 2.2 eV. 20,21 Because the base temperature of our experiment T = 4.2 K implies a thermal energy below 400 µeV and given the relative alignment of the graphene electronic bands with respect to those of hBN, the large difference can hardly be explained in terms of thermal activation of carriers or a lowering of the effective hBN barrier caused be the electric field. However, the EL onset at about V e ∼ ±1.7 V corresponds well with the emitted free exciton energy of ∼ 1.72 eV.…”

Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

“…They correspond to the optical transitions from the two spin-split valence bands to the corresponding conduction bands 1,3,20 [ Fig. 1(a)], modified by the strong e-h interactions [41][42][43][44][45] . Near !…”