131wileyonlinelibrary.com COMMUNICATION www.MaterialsViews.com www.advopticalmat. de Being entangled in controlling the electronic properties of graphene for next-generation electronics, [ 1,2 ] monolayer transition metal dichalcogenides such as MS 2 (M = Mo, W) are attracting great interest as 2D semiconductors with a native direct-energy gap in the visible frequency range. [ 3,4 ] Monolayers of other layered materials such as h -BN, GaS, GaSe, TaSe 2 , and so on, have also attracted much attention because of their unique properties when scaled down to monolayers. [5][6][7][8] There are comprehensive and intensive studies on monolayer MoS 2 , including its optical and electronic properties, [9][10][11][12][13][14][15][16][17] valleytronics, [18][19][20][21] strain effects, [22][23][24] thermal effects, [ 25 ] and so on. However, investigations of WS 2 have just started. Similar to 2H-MoS 2 , monolayer 2H-WS 2 can be constructed by sandwiching two atomic layers of S and one atomic layer of W through covalent W-S bonds, where W locates at the body center of a trigonal-prismatic case formed by six S atoms. Confi nement of charge carriers inside the horizontal atomic plane gradually enlarges energy gaps when thinning WS 2 layers. [ 26 ] Instead of an indirect energy gap for multiple layers, a direct energy gap of ∼ 2 eV at the corners (K and K' points) of the Brillouin Zone could be formed in monolayer WS 2 as clearly demonstrated by both theoretical and experimental studies. [ 9,[27][28][29] The immediate consequence, also a benefi t of the existence of such direct bandgap, is the signifi cant enhancement of visible light emission. In WS 2 monolayers, breaking inversion symmetry leads to the strong spin-orbit coupling and the splitting of valence bands at K/K' points with a sub-gap of around 0.4 eV. [ 30 ] Furthermore, the split spins at the time-reversed K and K' valleys have the opposite signs. Thus, such spin-valley coupling offers an extra degree of freedom to charge carriers in WS 2 monolayers. Though it has not been reported in monolayer WS 2 , theory predicts and experiments have observed in monolayer MoS 2 a non-equilibrium charge carrier imbalance at two valleys, revealed by the remarkable difference of absorption of left-( σ -) and right-handed ( σ +) circular polarized lights at the two valleys. [ 9,[18][19][20][21]31 ] All these interesting and important properties, plus the newly revealed potential in the fl exible heterostructures of graphene-WS 2 stacks [ 32,33 ] guarantee a promising future of WS 2 as the candidate of nextgeneration nanoelectronics, spintronics, valleytronics, and optoelectronics. [ 34 ] However, compared to graphene, it is very diffi cult to prepare MS 2 monolayers, and atomically thin MS 2 fl akes made by mechanical exfoliation are much smaller, in fact too small to be well characterized and processed for devices. Most recently, chemical vapor deposition (CVD) has been used to successfully grow large-area single crystals of monolayer MoS 2 . [ 11,[35][36][37][38] However, the c...

Two-dimensional (2D) semiconductors, such as transition-metal dichalcogenide monolayers (TMD 1Ls), have attracted increasing attention owing to the underlying fundamental physics (e.g., many body effects) and the promising optoelectronic applications such as light-emitting diodes. Though much progress has been made, intrinsic excitonic states of TMD 1Ls are still highly debated in theory, which thirsts for direct experimental determination. Here, we report unconventional emission and excitonic fine structure in 1L WS2 revealed by electrical doping and photoexcitation, which reflects the interplay of exciton, trion, and other excitonic states. Tunable excitonic emission has been realized in a controllable manner via electrical and/or optical injection of charge carriers. Remarkably enough, the superlinear (i.e., quadratic) emission is unambiguously observed which is attributed to biexciton states, indicating the strong Coulomb interactions in such a 2D material. In a nearly neutral 1L WS2, trions and biexcitons possess large binding energies of ∼ 10-15 and 45 meV, respectively. Moreover, our finding of electrically induced robust emission opens up a possibility to boost the luminous efficiency of emerging 1L TMD light emitting diodes.

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2D semiconductor tungsten disulfide (WS2) attracts significant interest in both fundamental physics and many promising applications such as light emitters, photodetectors/sensors, valleytronics, and flexible nanoelectronics, due to its fascinating optical, electronic, and mechanical properties. Herein, basic exciton properties of monolayer WS2 are reviewed including neutral excitons, charged excitons, bounded excitons, biexcitons, and the effects of electrostatic gating, chemical doping, strain, magnetic field, circular polarized light, and substrate on these excitonic structures. Besides basic excitonic emission, single‐photon emission, exciton–polaritons, and stimulated emission in monolayer WS2 are also discussed. The understanding of these optical phenomena is critical for the development of potential optical applications in electronic and optoelectronic devices. Finally, a summary and future prospective of the challengers and developments regarding 2D semiconductor WS2 is presented.