Publication date: 15th July 2022
Two-dimensional materials, like transition metal dichalcogenides or III-VI semiconductors1,2, are being highly studied in fields like photovoltaics3 and optoelectronics thanks to their limited dimensionality and extraordinary properties. Hybrid (organic-inorganic) layered perovskites are currently some of the most promising among two-dimensional materials due to their exceptional optical brightness and enhanced excitonic effects. Recently, it has been reported an giant excitonic fine structure splitting (FSS) of 2 meV in 2D lead halide perovskites (LHP)4. However, accessing the fine structure of the exciton in conventional photoluminescence experiments requires layered perovskites with high crystal quality, to avoid the broadening of the exciton linewidth. Here, we propose an alternative route to access the fine level structure of the excitons in this layered hybrid perovskites. By mechanically exfoliating hexagonal boron nitride and transferring it upon the layered perovskite we capped 2D perovskites with monolayer hBN sheets, . We show that the hBN capping reduce the effects of spectral diffusion, revealing the exciton FSS. To quantify the narrowing of exciton linewidth caused by hBN-capping, we used a stochastic model5 that links this reduction with the population of active charge fluctuation centres present in the uppermost layer of organic spacers. Active fluctuation centres are reduced by a factor of 3.7 to 7.1 when we directly compare non hBN-capped and hBN-capped areas of samples by means of micro-photoluminescence measurements. As a possible hypothesis for this reduction, we propose that by hBN-capping the layered perovskite, van der Waals forces between the squared perovskite lattice and the hexagonal hBN lattice provide a partial clamping of the movement of the upper layer organic spacers. This effect causes the reduction of the amplitude of the overall spectral diffusion.. Our work provides an easy, low-cost solution to the problem of low crystal quality, providing a mechanical route to access important fine-structure excitonic state information, providing an explanation of the carrier dynamics present in the uppermost layer of the organic spacer that affect the quality of the optical emission from two-dimensional flakes.
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