We report the observation and gate manipulation of intrinsic dark trions in monolayer WSe2. By using ultraclean WSe2 devices encapsulated by boron nitride, we directly resolve the weak photoluminescence of dark trions. The dark trions can be tuned continuously between negative and positive charged trions with electrostatic gating. We also reveal their spin triplet configuration and distinct valley optical emission by their characteristic Zeeman splitting under magnetic field. The dark trions exhibit large binding energy (14 -16 meV). Their lifetime (~1.3 ns) is two orders of magnitude longer than the bright trion lifetime (~10 ps) and can be tuned between 0.4 to 1.3 ns by electrostatic gating. Such robust, optically detectable, and gate tunable dark trions provide a new path to realize electrically controllable trion transport in two-dimensional materials.Monolayer transition metal dichalcogenides (TMDs), such as MoS2 and WSe2, are remarkable two-dimensional (2D) semiconductors with strong Coulomb interactions [1, 2]. Their optical properties are dominated by tightly bound electron-hole pairs (excitons) at two time-reversal valleys (K, K') in the momentum space [3]. The strong spin-orbit coupling splits both the conduction and valence bands into two subbands with opposite spins [Fig. 1] [4-7]. The spin configuration governs an exciton's optical properties. If the electron and hole come from bands with the same electron spin, their recombination can efficiently emit light. These bright excitons have short lifetime (<10 ps) [8], in-plane dipole moment and out-of-plane light emission [Fig. 1] [9]. But if the electron and hole come from bands with opposite electron spins, the spin mismatch strongly suppresses their radiative recombination. They form dark excitons with long lifetime (>100 ps), out-ofplane dipole moment, and in-plane light emission [Fig. 1] [10-18]. Compared to bright excitons, the long-lived dark excitons are much better candidates for the studies of exciton transport and Bose-Einstein condensate [19-21], but their optical inactivity poses a significant challenge for experiment.Monolayer WSe2 is an exceptional material to explore dark excitons. Unlike other semiconductors (e.g. MoSe2) with bright excitons in the lowest energy level, monolayer WSe2 hosts dark excitons well below the bright exciton level [ Fig. 1] [6, 13, 22]. The dark excitons can thus accumulate a sufficiently large population to achieve observable light emission, as reported by prior research [16,[23][24][25][26]. Moreover, as the dark excitons lie at the lowest energy level, they play a crucial role in the carrier dynamics of monolayer WSe2. It is therefore important to understand their diverse properties.Similar to bright excitons/trions, dark excitons can capture an extra charge to form dark trions [ Fig. 1]. The dark trions are fascinating entities for excitonic transport, because their finite net charge, together with their long lifetime, would enable effective control of exciton dynamics by electric field. However, detection an...

We report the observation of QΓ intervalley exciton in bilayer WSe2 devices encapsulated by boron nitride. The QΓ exciton resides at ∼18 meV below the QK exciton. The QΓ and QK excitons exhibit different Stark shifts under an out-of-plane electric field due to their different interlayer dipole moments. By controlling the electric field, we can switch their energy ordering and control which exciton dominates the luminescence of bilayer WSe2. Remarkably, both QΓ and QK excitons exhibit unusually strong two-phonon replicas, which are comparable to or even stronger than the one-phonon replicas. By detailed theoretical simulation, we reveal the existence of numerous (≥14) two-phonon scattering paths involving (nearly) resonant exciton–phonon scattering in bilayer WSe2. To our knowledge, such electric-field-switchable intervalley excitons with strong two-phonon replicas have not been found in any other two-dimensional semiconductors. These make bilayer WSe2 a distinctive valleytronic material with potential novel applications.

Ultrathin layers of hexagonal boron nitride (h-BN) are used to fully encapsulate single perylene microcrystals. The morphology and chemical stability for samples prepared using different encapsulation methods are characterized using electron, optical, and atomic force microscopies. Only multilayer MBE-grown h-BN could fully protect the organic crystals from dissolution and sublimation. To determine the interaction of the two-dimensional material with the underlying organic chromophores, a polymer film with Lumogen Red dye molecules that act as energy donors was used to characterize the fluorescence quenching ability of the encapsulation layer. Encapsulation using wet-transfer method leads to h-BN layers that have an effective Forster quenching radius of 2.9 nm, as compared to 14.6 nm for graphene. Fluorescence quenching by h-BN can be completely avoided by using dry-transfer methods, suggesting that wet transfer leads to structural defects that act as energy acceptors. Both the type of h-BN and its method of transfer determine its ability to act as an inert coating and avoid fluorescence quenching. Encapsulation of organic molecular crystals using multilayer h-BN is feasible, but attention must be paid to preparation conditions and the nature of the h-BN sample.