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.

Owing to direct band gap and strong spin-orbit coupling, monolayer transition-metal dichalcogenides (TMDs) exhibit rich new physics and great applicable potentials. The remarkable valley contrast and light emission promise such two-dimensional (2D) semiconductors a bright future of valleytronics and light-emitting diodes (LEDs). Though the electroluminescence (EL) has been observed in mechanically exfoliated small flakes of TMDs, considering real applications, a strategy that could offer mass-product and high compatibility is greatly demanded. Large-area and high-quality samples prepared by chemical vapor deposition (CVD) are perfect candidates toward such goal. Here, we report the first demonstration of electrically tunable chiral EL from CVD-grown monolayer WS2 by constructing a p-i-n heterojunction. The chirality contrast of the overall EL reaches as high as 81% and can be effectively modulated by forward current. The success of fabricating valley LEDs based on CVD WS2 opens up many opportunities for developing large-scale production of unconventional 2D optoelectronic devices.

The aim of valleytronics is to exploit confinement of charge carriers in local valleys of the energy bands of semiconductors as an additional degree of freedom in optoelectronic devices. Thanks to strong direct excitonic transitions in spin-coupled K valleys, monolayer molybdenum disulphide is a rapidly emerging valleytronic material, with high valley polarization in photoluminescence. Here we elucidate the excitonic physics of this material by light helicity-dependent photocurrent studies of phototransistors. We demonstrate that large photocurrent dichroism (up to 60%) can also be achieved in high-quality molybdenum disulphide monolayers grown by chemical vapour deposition, due to the circular photogalvanic effect on resonant excitations. This opens up new opportunities for valleytonic applications in which selective control of spin–valley-coupled photocurrents can be used to implement polarization-sensitive light-detection schemes or integrated spintronic devices, as well as biochemical sensors operating at visible frequencies.

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