Room temperature near unity spin polarization in 2D Van der Waals heterostructures

Abstract

The generation and manipulation of spin polarization at room temperature are essential for 2D van der Waals (vdW) materials-based spin-photonic and spintronic applications. However, most of the high degree polarization is achieved at cryogenic temperatures, where the spin-valley polarization lifetime is increased. Here, we report on room temperature high-spin polarization in 2D layers by reducing its carrier lifetime via the construction of vdW heterostructures. A near unity degree of polarization is observed in PbI₂ layers with the formation of type-I and type-II band aligned vdW heterostructures with monolayer WS₂ and WSe₂. We demonstrate that the spin polarization is related to the carrier lifetime and can be manipulated by the layer thickness, temperature, and excitation wavelength. We further elucidate the carrier dynamics and measure the polarization lifetime in these heterostructures. Our work provides a promising approach to achieve room temperature high-spin polarizations, which contribute to spin-photonics applications.

Fig. 1. Spin polarization mechanism and spectrum of PbI₂/WS₂ heterostructures.

a Schematic of the type-I heterostructure for the study of the carrier interlayer transportation. Both electrons and holes transfer from PbI₂ to WS₂. The images on the right are optical and AFM micrographs of a PbI₂/WS₂ heterostructure with a thickness of 9.2 nm. b Band structure for the PbI₂ thin film with a thickness of 20 layers, which is spin degenerate. c Schematic illustration of the polarized optical transitions in PbI₂ thin films. d Circularly polarized PL spectra from pure PbI₂ at room temperature. e Corresponding degree of polarization ρ of pure PbI₂ PL calculated from the PL spectra shown in figure (d). f Circularly polarized PL spectra of a representative PbI₂/WS₂ heterostructure at room temperature. g Corresponding degree of polarization ρ calculated from the PL spectra shown in figure (f). All length of the scale bar is 10 µm.

Fig. 2. Thickness-dependent spin polarization and the underlying mechanism in PbI₂/WS₂ heterostructures.

a, b Circularly polarized PL spectra of PbI₂ and WS₂ in PbI₂/WS₂ heterostructures with different thicknesses. c Degree of polarization ρ of PbI₂ as a function of thickness of the heterostructures. The solid line represents the numerical simulation result. d, e Schematic illustrations of the resulting spin polarization in the thin and thick PbI₂/WS₂ heterostructures. The curled curves represent the incident light and the PL emissions. The red balls represent spin-down electrons, and the blue balls represent spin-up electrons.

Fig. 3. Spin polarization dynamics of PbI₂/WS₂ heterostructures.

a–c Circularly polarized PL spectra (a) and TRPL data from PbI₂ (b), and WS₂ (c) of PbI₂/WS₂ heterostructures. d–f Corresponding degree of polarization ρ as a function of wavelength and time calculated from the PL and TRPL spectra. For excitation, a 441 nm σ+ polarized fs-pulsed laser beam was used.

Fig. 4. Temperature-dependent degree of polarization from PbI₂/WS₂ heterostructures.

a Circularly polarized PL spectra of PbI₂/WS₂ heterostructure at different temperatures for excitation with a 488 nm σ− polarized CW laser beam. b Degree of polarization ρ of PL from PbI₂ as a function of temperature, showing a decreasing tendency at lower temperatures.

Fig. 5. Degree of spin polarization and the dynamics in PbI₂/WSe₂ heterostructures.

a Schematic of the type-II band alignment and the corresponding photogenerated carrier behaviors in PbI₂/WSe₂ heterostructures. b Circularly polarized PL spectra from a 9.5 nm thick PbI₂/WSe₂ heterostructure, and the corresponding calculated ρ as a function of wavelength. The excitation is 488 nm σ− polarized CW laser. c–e Circularly polarized PL spectra (c) and TRPL data from PbI₂ (d) and WSe₂ (e) in PbI₂/WSe₂ heterostructure, and the corresponding degree of polarization ρ calculated from PL and TRPL spectra. Excitation with 441 nm σ+ polarization femtosecond laser pulses.