Revealing giant exciton fine-structure splitting in 2D perovskites using van der Waals passivation

TL;DR

This paper demonstrates that hBN-capping of 2D perovskites significantly reduces spectral diffusion, enabling the observation of exciton fine structure and providing insights into carrier dynamics affecting optical emission quality.

Contribution

It introduces a simple, low-cost mechanical method using hBN-capping to reveal exciton fine structure in 2D perovskites by reducing spectral diffusion.

Findings

Active fluctuation centres are reduced by a factor of 3.7 to 7.1 with hBN-capping.

Van der Waals forces partially clamp organic spacer molecules, reducing Stark shift.

The method enables access to excitonic fine structure information.

Abstract

The study of two-dimensional (2D) van der Waals materials has been an active field of research in the development of new optoelectronics and photonic applications over the last decade. Organic-inorganic layered perovskites are currently some of the most promising 2D van der Waals materials, due to their exceptional optical brightness and enhanced excitonic effects. However, low crystal quality and spectral diffusion usually broaden the exciton linewidth, obscuring the fine structure of the exciton in conventional photoluminescence experiments. Here, we propose a mechanical approach for reducing the effect of spectral diffusion by means of hBN-capping on layered perovskites with different thicknesses, revealing the exciton fine structure. We used a stochastic model to link the reduction of the spectral linewidth with the population of active charge fluctuation centres present in the organic spacer taking part in the dynamical Stark shift. Active fluctuation centres are reduced by a factor of 3.7 to 7.1 when we include hBN-capping according to our direct spectral measurements. This rate is in good agreement with the analysis of the overlap between the squared perovskite lattice and the hexagonal hBN lattice. Van der Waals forces between both lattices cause the partial clamping of the perovskite organic spacer molecules, and hence, the amplitude of the dynamical Stark shift characteristic of the spectral diffusion effect is reduced. Our work provides an easy and low-cost solution to the problem of accessing important fine-structure excitonic state information, along with an explanation of the important carrier dynamics present in the organic spacer that affect the quality of the optical emission.