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Monolayer transition metal dichalcogenides (ML‐TMDCs) possess degenerate levels with antiparallel spins in K and K′ valleys, providing the intrinsic valley polarization, which attracts great interest for potential applications on quantum information technology and on‐chip nanophotonics. Unfortunately, it is difficult to distinguish the degree of valley polarization (DoP) near room temperature due to the intensive phonon‐assisted intervalley scattering and the long‐range electron‐hole exchange interaction in ML‐TMDCs, limiting their practical applications. In this study, a novel design is proposed for great promotion of DoP in ML‐WSe2 with a microsphere cavity array, introducing Purcell effect and nanofocusing effect into the system. The radiative decay rate is dramatically enhanced owing to Purcell effect in microcavity in weak coupling regime, thus locking more polarized excitons in the corresponding valley under certain circularly‐polarized pumping. In addition, the nanofocusing effect contributes to increasing the number of charged excitons by suppressing the bright to dark exciton conversion. The present work achieves a great DoP of ML‐WSe2 with a simple configuration and promises broad applications from valleytronic devices to chiral optics in the future. [ABSTRACT FROM AUTHOR]

The emergence of atomically thin valleytronic semiconductors and 2D ferromagnetic materials is opening up new technological avenues for future information storage and processing. A key fundamental challenge is to identify physical knobs that may effectively manipulate the spin‐valley polarization, preferably in the device context. Here, a novel spin functional device that exhibits both electrical and magnetic tunability is fabricated, by contacting a monolayer MoSe2 with a 2D ferromagnetic semiconductor Cr2Ge2Te6. Remarkably, the valley‐polarization of MoSe2 is found to be controlled by a back‐gate voltage with an appreciably enlarged valley splitting rate. At fixed gate voltages, the valley‐polarization exhibits magnetic‐field and temperature dependence that corroborates well with the intrinsic magnetic properties of Cr2Ge2Te6, pointing to the impact of magnetic exchange interactions. Due to the interfacial arrangement, the charge‐carrying trion photoemission predominates in the devices, which may be exploited to enable drift‐based spin‐optoelectronic devices. These results provide new insights into valley‐polarization manipulation in transition metal dichalcogenides by means of ferromagnetic semiconductor proximitizing and represent an important step forward in devising field‐controlled 2D magneto‐optoelectronic devices. [ABSTRACT FROM AUTHOR]

Twist angle provides a new degree of freedom for 2D material modifications. In principle, the intrinsic properties of twisted multilayers can be regulated by twist angle between each adjacent layer and thus have greater tunability than widely studied bilayer structures. Considering its complexity, it is important to first investigate the simplest twisted multilayers with only one interface twisted. In this work, multilayer Moiré superlattices with only one twisted interface via paraffin‐assisted folding of non‐twisted stacked (highly symmetrically stacked) multilayer MoS2 are successfully fabricated, and their twist‐angle dependent optical properties are systematically studied. Compared to non‐twisted stacked multilayer MoS2, the one‐interface‐twisted multilayers show a 2–3.5 times higher PL intensity, and their interlayer coupling, indirect bandgap, and degree of circular polarization (DOCP) are tunable by twist angle. Notably, the DOCP for the one‐interface‐twisted four‐layer (folded bilayer) can reach 86%, which is the highest value ever reported for transition metal dichalcogenide homostructures above liquid nitrogen temperature. This work provides a solid base for understanding twist‐angle dependent properties of twisted multilayer 2D‐materials. [ABSTRACT FROM AUTHOR]

The valley polarization dynamics of excitonic states (excitons and trions) in monolayer (1L)‐MoSe2 under strong magnetic field are investigated. The experimentally observed magnetic‐field‐induced valley polarizations of trions (excitons) are analyzed by the rate equation model. It is found that the magnetic‐field‐induced valley polarizations are attributed to the asymmetric valley scattering of trions from K+ to K− valley and vice versa due to the degeneracy lifting of K+ and K− valleys by valley Zeeman effect under magnetic field. Moreover, the feeding of the valley‐polarized excitons for trions formation and longer inter‐valley scattering time reaching to a few nanoseconds at 10 K contribute to the larger valley polarization of trions than that of excitons in 1L‐MoSe2. [ABSTRACT FROM AUTHOR]

Enhancement and continuous control of the excitonic valley polarization in electrostatically doped monolayer WSe[sub 2] are demonstrated. Under excitation with circularly polarized light, 20% valley polarization of excitons around the charge neutrality condition at 70 K is increased to 40% by modulating the electron/hole density up to 2 x 1012 cm-2. This increase originates from slow valley relaxation for neutral exciton between the K and -K valleys owing to screening of long-range e-h exchange interactions by doped carriers. The gate-dependences of the exciton valley polarization at various temperatures are reproduced by theoretical calculations, which holds potential for next-generation valleytronic devices continuously controlled by an applied bias voltage. [ABSTRACT FROM AUTHOR]