Layer-engineered interlayer excitons

Abstract

Photoluminescence (PL) from excitons serves as a powerful tool to characterize the optoelectronic property and band structure of semiconductors, especially for atomically thin two-dimensional transition metal dichalcogenide (TMD) materials. However, PL quenches quickly when the thickness of TMD materials increases from monolayer to a 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 a direct type. We report emission from layer-engineered interlayer excitons from these multilayer heterostructures. Moreover, as desired for valleytronics devices, the lifetime, valley polarization, and valley lifetime of the generated interlayer excitons can all be substantially 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.

Fig. 1. Observation of IXs in mL-WSe₂/1L-MoS₂ and 1L-WSe₂/mL-MoS₂ heterostructures.

(A and B) A schematic of the IXs in mL-WSe₂/1L-MoS₂ and 1L-WSe₂/mL-MoS₂ heterostructures (HS), respectively. The inset shows a type II band alignment of WSe₂/MoS₂ heterostructure [intralayer exciton (X); interlayer exciton (IX)]. For IX, the electrons and holes are separated in MoS₂ and WSe₂ layers, respectively. (C and D) The optical microscope image of the mL-WSe₂/1L MoS₂ (labeled as mL/1L; m = 1 to 3) sample (S1) and 1L-WSe₂/mL-MoS₂ (labeled as 1L/mL; m = 1, 3, and 4) sample (S2). WSe₂ and MoS₂ with different layers are marked with dashed lines of different colors. The heterostructure regions are marked with solid white lines. Scale bars, 10 μm. (E) The PL spectra of intralayer excitons in monolayer WSe₂ and MoS₂ and IXs in 1L-WSe₂/1L-MoS₂ heterostructure sample at room temperature (RT). (F and G) The PL spectra of IXs in mL-WSe₂/1L-MoS₂ heterostructure and 1L-WSe₂/mL-MoS₂ heterostructure at low temperature. au, arbitrary units.

Fig. 2. Temperature dependence of IXs in mL-WSe₂/1L-MoS₂ and 1L-WSe₂/mL-MoS₂ heterostructures.

(A and B) The PL intensity map of the IXs in mL-WSe₂/1L-MoS₂ (mL/1L; m = 1 to 3) HS at 4.3 K and room temperature, respectively. (C) The PL intensity of IXs in mL-WSe₂/1L-MoS₂ as a function of temperature. The PL intensity is the average values of multiple positions of each mL-WSe₂/1L-MoS₂ region. (D and E) The PL intensity map of the IXs in 1L-WSe₂/mL-MoS₂ (mL/1L; m = 1, 3, and 4) HS at 4.3 K and room temperature, respectively. A 1064-nm long pass was used to ensure that only the IX signal can be detected. (F) The PL intensity of IXs in 1L-WSe₂/mL-MoS₂ as a function of temperature. The PL intensity is the average values of multiple positions of each 1L-WSe₂/mL-MoS₂ region. (G to J) The calculated electronic energy structure of 1L-WSe₂/1L-MoS₂, 2L-WSe₂/1L-MoS₂, 3L-WSe₂/1L-MoS₂, and 1L-WSe₂/4L-MoS₂ heterostructures, respectively.

Fig. 3. Circularly polarized PL spectra of IXs in 1L-WSe₂/mL-MoS₂ heterostructures.

(A) A simplified schematic of valley optical selection rule for IXs in K and K′ valley. The electrons and holes are located at K (K′) point of the MoS₂ conduction band and WSe₂ valence band, respectively. (B) The valley DOP mapping of IXs in the multilayer HS calculated from the polarized PL intensity mapping measurement results. (C and D) The circularly polarized PL spectra of IXs from 1L-WSe₂/1L-MoS₂ and 1L-WSe₂/4L-MoS₂ heterostructure regions, respectively. σⁱσ^j represents excitation with σⁱ and detection with σ^j circular polarization.

Fig. 4. Layer-engineered depolarization lifetime of IX in 1L-WSe₂/mL-MoS₂ heterostructures.

(A to C) Time-resolved circularly polarized PL of IXs from 1L-WSe₂/1L-MoS₂, 1L-WSe₂/3L-MoS₂, and 1L-WSe₂/4L-MoS₂ HS regions, respectively. The IX lifetime of a few hundred nanoseconds was observed. The temperature for PL decay measurements is 4.3 K. (D to F) The time-resolved valley DOP of IXs obtained from the measured time-resolved circularly polarized PL in (A) to (C). (G) The valley DOP lifetimes under left and right circularly polarized light excitation.

Fig. 5. Suppression of valley relaxation under magnetic field for IXs in 1L-WSe₂/mL-MoS₂ heterostructures.

(A) Valley polarization for IXs in 1L-WSe₂/4L-MoS₂ under three magnetic fields: −1, 0, and 1 T. An enhanced valley polarization under magnetic field has been observed. (B) Magnetic dependence of DOP. Valley mixing has been suppressed for all m = 1, 3, and 4 cases. (C) Time-resolved DOP for IXs in 1L-WSe₂/4L-MoS₂ at −1, 0, and 1 T (0.125-MHz repetition rate is used here). An enhanced valley polarization lifetime has been observed under magnetic field. (D) The magnetic field dependence of valley DOP lifetime under left and right circularly polarized light excitation, respectively. (E) Time-resolved photon emission at different magnetic fields. (F) The magnetic field suppressed valley relaxation characterization. The temperature for all measurements under magnetic fields is around 4.3 K.