textTwo distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))
The
stacking order of 2D materials can result in fascinating physical
properties. Two different stacking orders (AA and AB) were recently
identified in ReS2, a rising star in the transition metal
dichalcogenides family with a unique anisotropic property. Their optical
and vibrational properties show drastically different features. With
intensity-scan spectrometry, the nonlinear optical absorption of both
AA and AB stackings in ReS2 was investigated. Saturable
absorption (SA) only occurs in AB stacking at certain polarization
angles, whereas it is absent in AA stacking. This saturation behavior
is attributed to strong exciton–exciton annihilation (EEA).
Excited-state absorption (ESA) is prominent in both stacking orders,
but more anisotropic with polarization in AB stacking. Our results
provide new insights in using ReS2 as a nonlinear optical
material in forefront optoelectronics.
Stacking
two or more two-dimensional materials to form a heterostructure
is becoming the most effective way to generate new functionalities
for specific applications. Herein, using GW and Bethe–Salpeter
equation simulations, we demonstrate the generation of linearly polarized,
anisotropic intra- and interlayer excitonic bound states in the transition
metal monochalcogenide (TMC) GeSe/SnS van der Waals heterostructure.
The puckered structure of TMC results in the directional anisotropy
in band structure and in the excitonic bound state. Upon the application
of compressive/tensile biaxial strain dramatic variation (∓3%)
in excitonic energies, the indirect-to-direct semiconductor transition
and the red/blue shift of the optical absorption spectrum are observed.
The variations in excitonic energies and optical band gap have been
attributed to the change in effective dielectric constant and band
dispersion upon the application of biaxial strain. The generation
and control over the interlayer excitonic energies can find applications
in optoelectronics and optical quantum computers and as a gain medium
in lasers.
Excitonic properties in 2D heterobilayers are closely governed by charge transfer (CT) and excitonic energy transfer (ET) at van der Waals interfaces. Various means have been employed to modulate the interlayer CT and ET, including electrical gating and modifying interlayer spacing, but with limited extent in their controllability. Here, we report a novel method to modulate these transfers in the MoS 2 /WS 2 heterobilayer by applying compressive strain under hydrostatic pressure. Raman and photoluminescence measurements, combined with density functional theory calculations, show pressure-enhanced interlayer interaction of the heterobilayer. Heterobilayer-to-monolayer photoluminescence intensity ratio (η) of WS 2 decreases by five times up to ≈4 GPa, suggesting enhanced ET, whereas it increases by an order of magnitude at higher pressures and reaches almost unity. Theoretical calculations show that orbital switching and charge transfers in the heterobilayer's hybridized conduction band are responsible for the non-monotonic modulation of the transfers. Our findings provide a compelling approach toward effective mechanical control of CT and ET in 2D excitonic devices.
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