Revealing Giant Exciton Fine-Structure Splitting in Two-Dimensional Perovskites Using van der Waals Passivation.

Organic-inorganic layered perovskites are currently some of the most promising 2D van der Waals materials. Low crystal quality usually broadens the exciton line width, obscuring the fine structure of the exciton in conventional photoluminescence experiments. Here, we propose a mechanical approach to reducing the effect of spectral diffusion by means of hBN capping on layered perovskites, revealing the exciton fine structure. We used a stochastic model to link the reduction of the spectral line width with the population of charge fluctuation centers present in the organic spacer. van der Waals forces between both lattices cause the partial clamping of the perovskite organic spacer molecules, and hence the amplitude of the overall spectral diffusion effect is reduced. Our work provides a 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.

The effect of quantum size confinement on the optical properties of PbSe nanocrystals under exposure to heat and hydrostatic pressure.

A study based on photoluminescence and absorption measurements as a function of temperature and pressure for PbSe nanocrystals with sizes in the range 3-13 nm reveals the influence of size quantum confinement on the observed variation. In the case of the temperature variation, the effective bandgap changes from showing a positive rate of change to showing a negative one (for a quantum dot 3 nm in diameter), which can be accounted for by incorporating a linear variation of the carrier effective masses into a simple calculation of the exciton ground state in the quantum dot. In the case of the pressure variation, we observe a clear inverse correlation between the absolute value of the pressure coefficient and the nanocrystal size, a signature of quantum size confinement, with values changing from -76 to -41 meV GPa⁻¹ for quantum dots ranging from 13 to 3 nm in diameter, respectively, clearly smaller in absolute value than the rate for bulk PbSe (-84 meV GPa⁻¹). We used again the hypothesis of a linear variation of the carrier effective masses with pressure in order to fit this experimental variation quantitatively.