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Register a new userInterlayer excitons in bilayer MoS2 with strong oscillator strength up to room temperature
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Coulomb bound electron-hole pairs, excitons, govern the optical properties of semi-conducting transition metal dichalcogenides like MoS$_2$ and WSe$_2$. We study optical transitions at the K-point for 2H homobilayer MoS$_2$ in Density Functional Theory (DFT) including excitonic effects and compare with reflectivity measurements in high quality samples encapsulated in hexagonal BN. In both calculated and measured spectra we find a strong interlayer exciton transition in energy between A and B intralayer excitons, observable for T$=4 -300$ K, whereas no such transition is observed for the monolayer in the same structure in this energy range. The interlayer excitons consist of an electron localized in one layer and a hole state delocalized over the bilayer, which results in the unusual combination of high oscillator strength and a static dipole moment. We also find signatures of interlayer excitons involving the second highest valence band (B) and compare absorption calculations for different bilayer stackings. For homotrilayer MoS$_2$ we also observe interlayer excitons and an energy splitting between different intralayer A-excitons originating from the middle and outer layers, respectively.
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Photoluminescence (PL) from excitons serves as a powerful tool to characterize the optoelectronic property and band structure of semiconductors, especially for atomically thin 2D transition metal chalcogenide (TMD) materials. However, PL quenches quickly when the thickness of TMD material increases from monolayer to few-layers, due to the change from direct to indirect band transition. Here we show that PL can be recovered by engineering multilayer heterostructures, with the band transition reserved to be direct type. We report emission from layer engineered interlayer excitons from these multilayer heterostructures. Moreover, as desired for valleytronic devices, the lifetime, valley polarization, and the valley lifetime of the generated interlayer excitons can all be significantly improved as compared with that in the monolayer-monolayer heterostructure. Our results pave the way for controlling the properties of interlayer excitons by layer engineering.
Atomically thin layered two dimensional (2D) material has provided a rich library for both fundamental research and device applications. One of the special advantages is that, bandgap engineering and controlled material response can be achieved by stacking different 2D materials. Recently several types of excitonic lasers have been reported based on Transition metal dichalcogenide (TMDC) monolayers, however, the emission is still the intrinsic energy bandgap of the monolayers and lasers harnessing the flexibility of Van der Waals heterostructures have not been demonstrated yet. Here, we report for the first time a room temperature interlayer exciton laser with MoS2/WSe2 heterostructures. The onset of lasing action was identified by a combination of distinct kink in the L-L curve and the noticeable collapse of spectral linewidth. Different from visible emission of intralayer excitons for both MoS2 and WSe2, our interlayer exciton laser works in the infrared range, which is fully compatible with the well-established technologies in silicon photonics. Thanks to the long lifetime of interlayer excitons, the requirement of the cavity quality factor is relaxed by orders of magnitude. The demonstration of room temperature interlayer exciton laser might open new perspectives for the development of coherent light source with tailored optical properties on silicon photonics platform.
Interlayer valley excitons in bilayer MoS2 feature concurrently large oscillator strength and long lifetime, and hence represent an advantageous scenario for valleytronic applications. However, control of valley pseudospin of interlayer excitons in pristine bilayer MoS2, which lies at the heart of valleytronics, has remained elusive. Here we report the observation of highly circularly polarized photoluminescence from interlayer excitons of bilayer MoS2 with both optical and magnetic addressability. Under excitation of circularly polarized light near exciton resonance, interlayer excitons of bilayer MoS2 show a near-unity, but negative circular polarization. Significantly, by breaking time-reversal symmetry with an out-of-plane magnetic field, a record level of spontaneous valley polarization (7.7%/Tesla) is identified for interlayer excitons in bilayer MoS2. The giant valley polarization of the interlayer excitons in bilayer MoS2, together with the feasibility of electrical/optical/magnetic control and strong oscillator strength, provides a firm basis for the development of next-generation electronic and optoelectronic applications.
Second harmonic generation (SHG) is a non-linear optical process, where two photons coherently combine into one photon of twice their energy. Efficient SHG occurs for crystals with broken inversion symmetry, such as transition metal dichalcogenide monolayers. Here we show tuning of non-linear optical processes in an inversion symmetric crystal. This tunability is based on the unique properties of bilayer MoS2, that shows strong optical oscillator strength for the intra- but also inter-layer exciton resonances. As we tune the SHG signal onto these resonances by varying the laser energy, the SHG amplitude is enhanced by several orders of magnitude. In the resonant case the bilayer SHG signal reaches amplitudes comparable to the off-resonant signal from a monolayer. In applied electric fields the interlayer exciton energies can be tuned due to their in-built electric dipole via the Stark effect. As a result the interlayer exciton degeneracy is lifted and the bilayer SHG response is further enhanced by an additional two orders of magnitude, well reproduced by our model calculations.
Diverse interlayer tunability of physical properties of two-dimensional layers mostly lies in the covalent-like quasi-bonding that is significant in electronic structures but rather weak for energetics. Such characteristics result in various stacking orders that are energetically comparable but may significantly differ in terms of electronic structures, e.g. magnetism. Inspired by several recent experiments showing interlayer anti-ferromagnetically coupled CrI3 bilayers, we carried out first-principles calculations for CrI3 bilayers. We found that the anti-ferromagnetic coupling results from a new stacking order with the C2/m space group symmetry, rather than the graphene-like one with R3 as previously believed. Moreover, we demonstrated that the intra- and inter-layer couplings in CrI3 bilayer are governed by two different mechanisms, namely ferromagnetic super-exchange and direct-exchange interactions, which are largely decoupled because of their significant difference in strength at the strong- and weak-interaction limits. This allows the much weaker interlayer magnetic coupling to be more feasibly tuned by stacking orders solely. Given the fact that interlayer magnetic properties can be altered by changing crystal structure with different stacking orders, our work opens a new paradigm for tuning interlayer magnetic properties with the freedom of stacking order in two dimensional layered materials.
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