Exciton Fine Structure of CsPbCl3 Nanocrystals: An Interplay of Electron-Hole Exchange Interaction, Crystal Structure, Shape Anisotropy, and Dielectric Mismatch.

In the semiconducting perovskite materials family, the cesium-lead-chloride compound (CsPbCl3) supports robust excitons characterized by a blue-shifted transition and the largest binding energy, thus presenting a high potential to achieve demanding solid-state room-temperature photonic or quantum devices. Here we study the fundamental emission properties of cubic-shaped colloidal CsPbCl3 nanocrystals (NCs), examining in particular individual NC responses using micro-photoluminescence in order to unveil the exciton fine structure (EFS) features. Within this work, NCs with average dimensions ⟨Lα⟩ ≈ 8 nm (α = x, y, z) are studied with a level of dispersity in their dimensions that allows disentangling the effects of size and shape anisotropy in the analysis. We find that most of the NCs exhibit an optical response under the form of a doublet with crossed polarized peaks and an average inter-bright-state splitting, ΔBB ≈ 1.53 meV, but triplets are also observed though being a minority. The origin of the EFS patterns is discussed in the frame of the electron-hole exchange model by taking into account the dielectric mismatch at the NC interface. The different features (large dispersity in the ΔBB values and occasional occurrence of triplets) are reconciled by incorporating a moderate degree of shape anisotropy, observed in the structural characterization, by preserving the relatively high degree of the NC lattice symmetry. The energy distance between the optically inactive state and the bright manifold, ΔBD, is also extracted from time-resolved photoluminescence measurements (ΔBD ≈ 10.7 meV), in good agreement with our theoretical predictions.

Unexpected Anisotropy of the Electron and Hole Landé g-Factors in Perovskite CH3NH3PbI3 Polycrystalline Films.

In this work, we studied, at low temperature, the coherent evolution of the localized electron and hole spins in a polycrystalline film of CH3NH3PbI3 (MAPI) by using a picosecond-photo-induced Faraday rotation technique in an oblique magnetic field. We observed an unexpected anisotropy for the electron and hole spin. We determined the electron and hole Landé factors when the magnetic field was applied in the plane of the film and perpendicular to the exciting light, denoted as transverse ⟂ factors, and when the magnetic field was applied perpendicular to the film and parallel to the exciting light, denoted as parallel ∥ factors. We obtained |ge,⟂|=2.600 ± 0.004, |ge,∥|=1.604 ± 0.033 for the electron and |gh,⟂|=0.406 ± 0.002, |gh,∥|=0.299 ± 0.007 for the hole. Possible origins of this anisotropy are discussed herein.

Energy Tuning of Electronic Spin Coherent Evolution in Methylammonium Lead Iodide Perovskites.

We investigated the coherent evolution of the electronic spin at low temperature in high-quality CH3NH3PbI3 polycrystalline films by picosecond-resolved photoinduced Faraday rotation. We show that this coherent evolution can be tuned by choosing the pump-probe energy within the lowest optical-absorption band, and we explain it as the result of two main contributions: the localized electron and the localized hole. Their corresponding amplitude ratios are not constant across the lowest absorption band-an observation which disqualifies a free exciton from being at the origin of the electronic spin coherent evolution. We measured a spin coherence time of localized electrons (holes) of 4.4 ns (3.7 ns) at 1.635 eV, which evolves to about 7 ns at 1.612 eV (the hole coherence time remains almost constant at lower energies). Finally, we provide a global image of the spin coherent evolution in bulk metal halide perovskite, which overcomes recent controversies.

Anisotropic shape of CsPbBr3 colloidal nanocrystals: from 1D to 2D confinement effects.

We synthesized strongly anisotropic CsPbBr3 nanocrystals with very narrow emission and absorption lines associated to confinement effects along one or two dimensions, called respectively nanoplatelets (NPLs) and nanosticks (NSTs). Transmission Electron Microscopy (TEM) images, absorption and photoluminescence (PL) spectra taken at low temperature are very precise tools to determine which kind of confinement has to be considered and to deduce the shape, the size and the thickness of nanocrystals under focus. We show that the energy of the band-edge absorption and PL peaks versus the inverse of the square of the NPL thickness has a linear behaviour from 11 monolayers (MLs) i.e. a thickness of 6.38 nm, until 4 MLs (2.32 nm) showing that self-energy correction compensates the increase of the exciton binding energy in thin NPLs as already observed in Cadmium chalcogenides-based NPLs. We also show that slight changes in the morphology of NSTs leads to a very drastic modification of their absorption spectra. Time-resolved PL of NSTs has a non-monotonous behaviour with temperature. At 5 K, a quasi-single exponential with a lifetime of 80 ps is obtained; at intermediate temperature, the decay is bi-exponential and at 150 K, a quasi-single exponential decay is recovered (≈0.4 ns). For NSTs, the exciton interaction with LO phonons governs the broadening of the absorption and PL peaks at room temperature and is stronger than in chalcogenides quantum dots and NPLs.

Fine Structure and Spin Dynamics of Linearly Polarized Indirect Excitons in Two-Dimensional CdSe/CdTe Colloidal Heterostructures.

Heterostructured two-dimensional colloidal nanoplatelets are a class of material that has attracted great interest for optoelectronic applications due to their high photoluminescence yield, atomically tunable thickness, and ultralow lasing thresholds. Of particular interest are laterally heterostructured core-crown nanoplatelets with a type-II band alignment, where the in-plane spatial separation of carriers leads to indirect (or charge transfer) excitons with long lifetimes and bright, highly Stokes shifted emission. Despite this, little is known about the nature of the lowest energy exciton states responsible for emission in these materials. Here, using polarization-controlled, steady-state, and time-resolved photoluminescence measurements, at temperatures down to 1.6 K and magnetic fields up to 30 T, we study the exciton fine structure and spin dynamics of archetypal type-II CdSe/CdTe core-crown nanoplatelets. Complemented by theoretical modeling and zero-field quantum beat measurements, we find the bright-exciton fine structure consists of two linearly polarized states with a fine structure splitting ∼50 µeV and an indirect exciton Landé g-factor of 0.7. In addition, we show the exciton spin lifetime to be in the microsecond range with an unusual B-3 magnetic field dependence. The discovery of linearly polarized exciton states and emission highlights the potential for use of such materials in display and imaging applications without polarization filters. Furthermore, the small exciton fine structure splitting and a long spin lifetime are fundamental advantages when envisaging CdSe/CdTe nanoplatelets as elementary bricks for the next generation of quantum devices, particularly given their ease of fabrication.

Fine structure of excitons and electron-hole exchange energy in polymorphic CsPbBr3 single nanocrystals.

All inorganic CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) belong to the novel class of confined metal-halide perovskites which are currently arousing enthusiasm and stimulating huge activity across several fields of optoelectronics due to outstanding properties. A deep knowledge of the band-edge excitonic properties of these materials is thus crucial to further optimize their performances. Here, high-resolution photoluminescence (PL) spectroscopy of single bromide-based NCs reveals the exciton fine structure in the form of sharp peaks that are linearly polarized and grouped in doublets or triplets, which directly mirror the adopted crystalline structure, tetragonal (D4h symmetry) or orthorhombic (D2h symmetry). Intelligible equations are found that show how the fundamental parameters (spin-orbit coupling, ΔSO, crystal field term, T, and electron-hole exchange energy, J) rule the energy spacings in doublets and triplets. From experimental data, fine estimations of each parameter are obtained. The analysis of the absorption spectra of an ensemble of NCs with a "quasi-bulk" behavior leads to ΔSO = 1.20 ± 0.06 eV and T = -0.34 ± 0.05 eV in CsPbBr3. The study of individual luminescence responses of NCs having sizes comparable to the exciton Bohr diameter, 7 nm, allows us to estimate the value of J to be around ≈3 meV in both tetragonal and orthorhombic phases. This value is already enhanced by confinement.