Fig.1: (a) Diamagnetic shift of CsPbBr₃ under ultra-high magnetic fields. (b) Circularly polarized photoluminescence of 2D nanoplatelets with varying layer numbers in high magnetic fields. (c) Simulation of Rashba band splitting effects on the momentum distribution near the band edges.

Background

The advancement in spintronic devices requires semiconductor materials with spin-polarized electronic properties. Over the past few decades, spin polarization has been found in GaAs- and ZnSe-based semiconductor devices at low temperatures. However, these materials suffer from limited photoluminescence (PL) efficiency, and both spin polarizations and emission quickly diminish due to thermal fluctuations at high temperatures. In contrast, two-dimensional (2D) CsPbBr₃ nanoplatelets (NPLs) have emerged as highly promising candidates for spin-optoelectronic applications. Their exceptional properties, including a large exciton binding energy, high quantum yield, strong quantum confinement, pronounced spin-orbit coupling (SOC), and Rashba effect, position them as a compelling platform for next-generation spintronic technologies.

What we discover?

By employing thermal injection methods and adjusting the halogen ion content, we successfully synthesized CsPbBr₃ nanocrystals (NCs) with edge sizes of 20 to 30 nm, and atomic-layered CsPbBr₃ NPLs. Collaborating with Yoshimitsu Kohama's team, we conducted 150 T transmission spectroscopy measurements using single-turn coil techniques. The results revealed a high density of 2D-confined excitons within the CsPbBr₃ NPLs (Fig. 1a). Magneto-optical measurements revealed pronounced anisotropy in the spin-polarization of excitons in the NPLs. When the external magnetic field is aligned parallel to the incident light vector, the degree of circular polarization (P) of PL for the NPLs is nearly an order of magnitude higher than that of the NCs, with a significant enhancement in the exciton g-factor. Conversely, when the magnetic field is perpendicular to the light vector, the NPLs exhibit a remarkable magnetic enhanced PL intensity, with no circular polarization. Furthermore, measurements on multilayered CsPbBr₃ were conducted to investigate the dependence of the P on the different layers (n), revealing σ⁺-and σ⁻-polarized PL spectra with several discrete peaks (Fig. 1b). First-principles calculations reveal that the Rashba bands in 2D CsPbBr₃ further split in energy under a magnetic field. By calculating the circular polarization using transition matrix elements using a k×p model, theoretical results are found to be in good agreement with experimental observations (Fig. 1c). This demonstrates that the Rashba effect in 2D CsPbBr₃ plays a crucial role in enhancing excitonic spin polarization in a magnetic field.

Why is this important?

This study observed the circularly polarized excitonic emission in 2D CsPbBr₃ over a wide temperature range in the application of a magnetic field along the out-of-plane direction. The near band−edge circular polarization was predominantly influenced by the Rashba effect. Furthermore, the study provided insights into the spin-related optical properties in the presence of strong SOC effects and the fabrication of spin photonic devices based on 2D perovskites.

Who did the research?

Ruiqin Huang¹, Jingnan Hu¹, Zhuo Yang², Yoshimitsu Kohama², Xinhui Zhang³, Xixiang Zhu⁴, Huakun Zuo¹, Jinyu Zou¹, Tao Peng¹, Liang Li¹, Gang Xu^1,5,6* and Yibo Han^1*

(1) Wuhan National High Magnetic Field Center and School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China

(2) Institute for Solid State Physics, The University of Tokyo, Kashiwa 277-8581, Japan

(3) State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

(4) Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Physical Science and Engineering, Beijing Jiaotong University, Beijing, 100044 China

(5) Institute for Quantum Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China

(6) Wuhan Institute of Quantum Technology, Wuhan, 430206, China

ACS Photonics, 2024, 11(8): 3160-3166

https://pubs.acs.org/doi/10.1021/acsphotonics.4c00513

Funding

This study was supported by the National Key R&D Program of China (grant no. 2022YFA1602702) and the Natural Science Foundation of China (grant nos. 11974126 and 12274154).