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Single layer MoS2 is an ideal material for the emerging field of "valleytronics" in which charge carrier momentum can be finely controlled by optical excitation. This system is also known to exhibit strong many-body interactions as observed by tightly bound excitons and trions. Here we report direct measurements of valley relaxation dynamics in single layer MoS2, by using ultrafast transient absorption spectroscopy. Our results show that strong Coulomb interactions significantly impact valley population dynamics. Initial excitation by circularly polarized light creates electron-hole pairs within the K-valley. These excitons coherently couple to dark intervalley excitonic states, which facilitate fast electron valley depolarization. Hole valley relaxation is delayed up to about 10 ps due to nondegeneracy of the valence band spin states. Intervalley biexciton formation reveals the hole valley relaxation dynamics. We observe that biexcitons form with more than an order of magnitude larger binding energy compared to conventional semiconductors. These measurements provide significant insight into valley specific processes in 2D semiconductors. Hence they could be used to suggest routes to design semiconducting materials that enable control of valley polarization.

The motions of electrons in solids may be highly correlated by strong, long-range Coulomb interactions. Correlated electron-hole pairs (excitons) are accessed spectroscopically through their allowed single-quantum transitions, but higher-order correlations that may strongly influence electronic and optical properties have been far more elusive to study. Here we report direct observation of bound exciton pairs (biexcitons) that provide incisive signatures of four-body correlations among electrons and holes in gallium arsenide (GaAs) quantum wells. Four distinct, mutually coherent, ultrashort optical pulses were used to create coherent exciton states, transform these successively into coherent biexciton states and then new radiative exciton states, and finally to read out the radiated signals, yielding biexciton binding energies through a technique closely analogous to multiple-quantum two-dimensional Fourier transform (2D FT) nuclear magnetic resonance spectroscopy. A measured variation of the biexciton dephasing rate indicated still higher-order correlations.

Fullerene-free organic solar cells show over 11% power conversion efficiency, processed by low toxic solvents. The applied donor and acceptor in the bulk heterojunction exhibit almost the same highest occupied molecular orbital level, yet exhibit very efficient charge creation.

Three-dimensional (3D) hybrid organic−inorganic lead halide perovskites (HOIPs) feature remarkable optoelectronic properties for solar energy conversion but suffer from longstanding issues of environmental stability and lead toxicity. Associated two-dimensional (2D) analogues are garnering increasing interest due to superior chemical stability, structural diversity, and broader property tunability. Toward lead-free 2D HOIPs, double perovskites (DPs) with mixed-valent dual metals are attractive. Translation of mixed-metal DPs to iodides, with their prospectively lower bandgaps, represents an important target for semiconducting halide perovskites, but has so far proven inaccessible using traditional spacer cations due to either intrinsic instability or formation of competing non-perovskite phases. Here, we demonstrate the first example of a 2D Ag−Bi iodide DP with a direct bandgap of 2.00(2) eV, templated by a layer of bifunctionalized oligothiophene cations, i.e., (bis-aminoethyl)bithiophene, through a collective influence of aromatic interactions, hydrogen bonding, bidentate tethering, and structural rigidity. Hybrid density functional theory calculations for the new material reveal a direct bandgap, consistent with the experimental value, and relatively flat band edges derived principally from Ag-d/I-p (valence band) and Bi-p/I-p (conduction band) states. This work opens up new avenues for exploring specifically designed organic cations to stabilize otherwise inaccessible 2D HOIPs with potential applications for optoelectronics.

We quantitatively illustrate the fundamental limit that exciton-exciton annihilation (EEA) may impose to the light emission of monolayer transition metal dichalcogenide (TMDC) materials.The EEA in TMDC monolayers shows dependence on the interaction with substrates as its rate increases from 0.1 cm 2 /s (0.05 cm 2 /s) to 0.3 cm 2 /s (0.1 cm 2 /s) with the substrates removed for WS 2 (MoS 2 ) monolayers. It turns to be the major pathway of exciton decay and dominates the luminescence efficiency when the exciton density is beyond 10 10 cm -2 in suspended monolayers or 10 11 cm -2 in supported monolayers. This sets an upper limit on the density of injected charges in light emission devices for the realization of optimal luminescence efficiency. The strong EEA rate also dictates the pumping threshold for population inversion in the monolayers to be 12-18 MW/cm 2 (optically) or 2.5-4×10 5 A/cm 2 (electrically).

4733wileyonlinelibrary.com the infl uence of substrates. [ 2 ] It has been reported that substrates may affect the luminescence effi ciency of the monolayers by inducing strain, doping, or dielectric screening. [ 2 a, e-g, 3 ] However, despite the recent progress, many important questions about the substrate effect have remained to be answered. For instance, while it is known that substrates could affect the luminescence effi ciency through multiple ways, there is no quantitative understanding for the effect of each mechanism and no knowledge on which mechanism could be dominant. More importantly, it is not clear how the effect of substrates might depend on the nature of the substrate and the physical features of the monolayers. Answers to these questions would provide useful guidance for the realization of optimal luminescence effi ciency through engineering the substrate effects. Here we quantitatively evaluate the effect of substrates on the luminescence effi ciency of monolayers MoS 2 , WS 2 , and WSe 2 and demonstrate strategies of substrate engineering to improve the effi ciency by orders of magnitude. We fi nd that the main effects of the substrate lie in doping the monolayers and facilitating defect-assisted nonradiative exciton recombinations. The doping may be from substrate-borne water moisture and the substrate itself, the former of which is much stronger than the latter for WS 2 and MoS 2 but negligible for WSe 2 . Using proper substrates can substantially mitigate the doping effect on the photoluminescence (PL), such as mica for WS 2 and MoS 2 and hexagonal boron nitride (h-BN) or polystyrene (PS) for WSe 2 . The defect-assisted recombination depends on the interaction of the defects in the monolayer such as sulfur vacancies with the substrate and may be substantially suppressed by either removing the substrate or lowering the number of defects. In this work we largely ignore the optical resonance effects associated with the substrate's geometrical features. [ 4 ] Results and DiscussionWe start with comparing the PL of suspended MoS 2 , WS 2 , and WSe 2 monolayers to those of as-grown counterparts. The monolayers were synthesized on sapphire substrates using chemical vapor deposition (CVD) processes as described previously, [ 5 ] and the suspended monolayers were prepared by manually It is demonstrated that the luminescence effi ciency of monolayers composed of MoS 2 , WS 2 , and WSe 2 is signifi cantly limited by the substrate and can be improved by orders of magnitude through substrate engineering. The substrate affects the effi ciency mainly through doping the monolayers and facilitating defect-assisted nonradiative exciton recombinations, while the other substrate effects including straining and dielectric screening play minor roles. The doping may come from the substrate and substrate-borne water moisture, the latter of which is much stronger than the former for MoS 2 and WS 2 but negligible for WSe 2 . Using proper substrates such as mica or hexagonal boron nitride can substantially mitigat...

We measured the lifetime of optically created valley polarization in single layer WS 2 using transient absorption spectroscopy. The electron valley relaxation is very short (< 1ps). However the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single layer WS 2 . One process involves direct scattering of excitons from K to K valleys with a spin flip-flop interaction. The other mechanism involves scattering through spin degenerate Γ valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between Γ and K valleys.