Robust Interlayer Exciton Interplay in Twisted van der Waals Heterotrilayer on a Broadband Bragg Reflector up to Room Temperature

We report robust room temperature interlayer excitons in transition metal dichalcogenide heterostructures engineered via precise stacking orientation and twist-angle control. We integrate 2H-stacked MoSe${2}$/$^{1}$WSe${2}$/$^{2}$WSe$_{2}$ heterotrilayer onto a chirped distributed Bragg reflector that acts as a backside mirror. This way, we fabricate a platform that hosts distinct heterotrilayer, heterobilayer, and homobilayer regions with enhanced excitonic features at elevated temperatures. Although the heterobilayer supports temperature-tunable singlet and triplet interlayer excitons, it exhibits low emission yield at 4 K. In comparison, the heterotrilayer shows remarkable excitonic properties, including pronounced band modulation, intervalley interlayer exciton transitions, and a tenfold photoluminescence enhancement along with a sevenfold increase in exciton decay time at cryogenic temperatures compared to the heterobilayer system. Temperature-dependent studies reveal intriguing interlayer exciton dynamics in the heterotrilayer, including the emergence of valley-polarized interlayer excitons, and the ability to maintain optical stability up to room temperature. Our results establish a clear strategy for engineering excitonic states across multilayer van der Waals heterostructures from 4 K to room temperature, providing a versatile platform for excitonic optoelectronics, quantum photonics, and tunable long-lived interlayer exciton states in scalable TMD heterostructures.

💡 Research Summary

In this work the authors demonstrate a robust platform for room‑temperature (RT) interlayer excitons (IXs) in transition‑metal dichalcogenide (TMD) van‑der‑Waals heterostructures by combining precise twist‑angle engineering with a broadband chirped distributed Bragg reflector (cDBR). The heterostructure consists of a 2H‑stacked MoSe₂/1WSe₂/2WSe₂ trilayer (HTL) fabricated by deterministic dry transfer on the backside of a cDBR. The first two layers form a MoSe₂/WSe₂ heterobilayer (HBL) with a twist angle of 59° ± 1°, while the second and third layers create a twisted WSe₂ homobilayer (HoBL) with a twist of 54° ± 1°. This geometry yields three distinct regions—HBL, HoBL and HTL—within a single sample, enabling direct comparative optical studies.

The cDBR is designed from alternating SiO₂ (n ≈ 1.47) and Si₃N₄ (n ≈ 2.01) layers, providing a 600 nm stopband centered around 800 nm and comprising 15 mirror pairs to maximize reflectivity. This broadband mirror acts as a backside cavity, enhancing the out‑coupling of IX emission across the visible to near‑infrared range.

Temperature‑dependent photoluminescence (PL) measurements from 4 K to 300 K reveal that the HBL exhibits two well‑resolved IX peaks at 1.398 eV (IX₁) and 1.421 eV (IX₂) at 100 K. Polarization‑resolved PL under circularly polarized excitation shows a positive degree of circular polarization (DCP) for IX₁ and a negative DCP for IX₂, allowing assignment of IX₁ to a spin‑triplet (dark‑like) exciton and IX₂ to a spin‑singlet (bright‑like) exciton. The energy splitting between the two decreases from ~26 meV at 60 K to ~16 meV at 200 K, reflecting the conduction‑band spin‑orbit splitting in MoSe₂. Time‑resolved PL (TRPL) indicates that the triplet IX lifetime shortens from ~200 ps at 4 K to ~30 ps at room temperature, while the singlet decays even faster, evidencing increasing non‑radiative channels with temperature.

In contrast, the HTL region shows a dominant IX emission centered at 1.327 eV (934 nm) at room temperature, red‑shifted by ~13 nm relative to the HBL. Although the PL intensity of the HTL is about ten times lower than that of the HBL, its exciton lifetime is dramatically longer—approximately 1.4 ns at 4 K and remaining above 200 ps even at 300 K, a seven‑fold increase over the HBL. This enhancement is attributed to the additional WSe₂ layer, which further separates electrons and holes, suppresses non‑radiative recombination, and introduces new inter‑valley transitions (Q‑K, Q‑Γ) that become visible at elevated temperatures. The HoBL region displays bright intralayer exciton emission at 1.646 eV (753 nm) together with lower‑energy inter‑valley peaks (1.5–1.54 eV), confirming the presence of Q‑valley excitons unique to homobilayers.

The combined effect of near‑ideal twist angles (close to the H‑type 60° configuration) and the broadband cDBR enables strong IX emission at room temperature across all three regions, demonstrating that careful stacking order can preserve spin‑selection rules and valley polarization even when thermal energy would normally quench IXs. Moreover, the HTL’s prolonged lifetime and modest red‑shift indicate that trilayer engineering provides a pathway to long‑lived, tunable IXs suitable for practical optoelectronic devices.

The authors discuss the implications for excitonic devices: (i) the ability to sustain valley‑polarized IXs at room temperature opens routes to valleytronic applications; (ii) the enhanced lifetime in the HTL is advantageous for exciton‑based quantum light sources and exciton‑polariton condensates; (iii) the broadband cDBR offers a scalable method to boost emission without requiring microcavities. Future directions suggested include electrostatic gating to tune IX energy, magnetic field control of valley polarization, and integration with photonic crystal or plasmonic structures to further increase coherence and extraction efficiency.

Overall, the paper provides a clear experimental demonstration that twist‑angle control combined with a broadband reflective substrate can produce robust, long‑lived interlayer excitons from cryogenic temperatures up to room temperature, establishing a versatile platform for excitonic optoelectronics, quantum photonics, and scalable TMD heterostructure technologies.