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Register a new userExciton-phonon coupling strength in single-layer MoSe2 at room temperature
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Single-layer transition metal dichalcogenides are at the center of an ever increasing research effort both in terms of fundamental physics and applications. Exciton-phonon coupling plays a key role in determining the (opto)electronic properties of these materials. However, the exciton-phonon coupling strength has not been measured at room temperature. Here, we develop two-dimensional micro-spectroscopy to determine exciton-phonon coupling of single-layer MoSe2. We detect beating signals as a function of waiting time T, induced by the coupling between the A exciton and the A1 optical phonon. Analysis of two-dimensional beating maps combined with simulations provides the exciton-phonon coupling. The Huang-Rhys factor of ~1 is larger than in most other inorganic semiconductor nanostructures. Our technique offers a unique tool to measure exciton-phonon coupling also in other heterogeneous semiconducting systems with a spatial resolution ~260 nm, and will provide design-relevant parameters for the development of optoelectronic devices.
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Disorder-induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in the two-dimensional (2D) materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single-layer regime. Here, we report a room-temperature colossal MR of up to 5,000% at 9 T in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO2 terminated SrTiO3, we demonstrate a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices. Strikingly, a colossal MR of >1,000% was also achieved in the terraced graphene even at a high carrier density of ~1012 cm-2. Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. Our results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.
We investigate the excitonic dynamics in MoSe2 monolayer and bulk samples by femtosecond transient absorption microscopy. Excitons are resonantly injected by a 750-nm and 100-fs laser pulse, and are detected by a probe pulse tuned in the range of 790 - 820 nm. We observe a strong density-dependent initial decay of the exciton population in monolayers, which can be well described by the exciton-exciton annihilation. Such a feature is not observed in the bulk under comparable conditions. We also observe the saturated absorption induced by exciton phase-space filling in both monolayers and the bulk, which indicates their potential applications as saturable absorbers.
We study experimentally and theoretically the exciton-phonon interaction in MoSe2 monolayers encapsulated in hexagonal BN, which has an important impact on both optical absorption and emission processes. The exciton transition linewidth down to 1 meV at low temperatures makes it possible to observe high energy tails in absorption and emission extending over several meV, not masked by inhomogeneous broadening. We develop an analytical theory of the exciton-phonon interaction accounting for the deformation potential induced by the longitudinal acoustic phonons, which plays an important role in exciton formation. The theory allows fitting absorption and emission spectra and permits estimating the deformation potential in MoSe2 monolayers. We underline the reasons why exciton-phonon coupling is much stronger in two-dimensional transition metal dichalcodenides as compared to conventional quantum well structures. The importance of exciton-phonon interactions is further highlighted by the observation of a multitude of Raman features in the photoluminescence excitation experiments.
In monolayer transition metal dichalcogenides, quantum emitters are associated with localized strain that can be deterministically applied to create designer nano-arrays of single photon sources. Despite an overwhelming empirical correlation with local strain, the nanoscale interplay between strain, excitons, defects and local crystalline structure that gives rise to these quantum emitters is poorly understood. Here, we combine room-temperature nano-optical imaging and spectroscopy of excitons in nanobubbles of localized strain in monolayer WSe2 with atomistic structural models to elucidate how strain induces nanoscale confinement potentials that give rise to highly localized exciton states in 2D semiconductors. Nano-optical imaging of nanobubbles in low-defect monolayers reveal localized excitons on length scales of approximately 10 nm at multiple sites along the periphery of individual nanobubbles, which is in stark contrast to predictions of continuum models of strain. These results agree with theoretical confinement potentials that are atomistically derived from measured topographies of existing nanobubbles. Our results provide one-of-a-kind experimental and theoretical insight of how strain-induced confinement - without crystalline defects - can efficiently localize excitons on length scales commensurate with exciton size, providing key nanoscale structure-property information for quantum emitter phenomena in monolayer WSe2.
Monolayer transition metal dichalcogenide semiconductors, with versatile experimentally accessible exciton species, offer an interesting platform for investigating the interaction between excitons and a Fermi sea of charges. Using hexagonal boron nitride encapsulated monolayer MoSe2, we study the impact of charge density tuning on the ground and excited Rydberg states in the atomic layer. Consistent excitonpolaron behavior is revealed in both photoluminescence and reflection spectra of the A exciton 1s (A:1s) Rydberg state, in contrast to previous studies. The A:2s Rydberg state provides an opportunity to understand such interactions with greatly reduced exciton binding energy. We found that the impact of the Fermi sea becomes much more dramatic. With a photoluminescence upconversion technique, we further verify the 2s polaron-like behavior for the repulsive branch of B:2s exciton whose energy is well above the bare bandgap. Our studies show that the polaron-like interaction features are quite generic and highly robust, offering key insights into the dressed manybody state in a Fermi sea.
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