No abstract

Heterostructures of three-dimensional (3D) halide perovskites are unstable because of facile anion interdiffusion at halide interfaces. Two-dimensional (2D) Ruddlesden–Popper halide perovskites (RPPs) show suppressed and anisotropic ion diffusion that could enable stable RPP heterostructures, yet the direct and general growth of lateral RPP heterostructures remains challenging. Here, we show that halide miscibility in RPPs decreases with perovskite layer thickness (n), enabling the formation of sharp halide lateral heterostructures from n = 1 and 2 RP lead iodide microplates via anion exchange with hydrogen bromide vapor. In contrast, RPPs with n ≥ 3 form more diffuse lateral heterojunctions, more similar to those in 3D perovskites. The anion exchange behaviors are further modulated by the spacer and A-site cations in the RPP structures. These new insights, and kinetic studies of the exchange reactions, enable the preparation of lateral heterostructures from various n = 2 RPPs that are more stable against anion interdiffusion and degradation for potential optoelectronic device applications.

The fundamental properties of electrons in the prototypical n-type oxide nanocrystal, Al 3+ -doped ZnO, have been studied at both the ensemble and single-particle levels by spectroscopic and electron force microscopic techniques. We developed and implemented a new synthetic methodology that enables the tunable incorporation of Al 3+ in the ZnO nanocrystal in an "etching−regrowth−doping" (ERD) strategy in a single-pot reaction. The ensemble-averaged properties and evolution of the Al 3+ speciation in ZnO were studied using electronic absorption spectroscopy and powder X-ray diffraction and reveal the successful substitution of Al 3+ only after implementation of the ERD strategy. Characterization of individual ZnO, surface Al 3+ -doped ZnO, and internal Al 3+doped ZnO nanocrystals using electrostatic force microscopy reveals strong responses in both the quantity of surface charges and electron polarizabilities, which are dependent on the amount of Al 3+ in the ZnO lattice. These results appear to suggest that an upper limit to the electron polarizability exists for Al 3+ -doped ZnO nanocrystals.

Halide alloying of 3D and 2D Ruddlesden–Popper lead halide perovskites (RPPs) allows their bandgaps to be fine-tuned for optoelectronic applications. Many studies of mixed-halide RPPs assume that halide mixing yields homogeneous alloys like in 3D lead halide perovskites, with halide segregation only occurring under perturbations like light or heat. Here, we carefully investigate the mixed I/Br phases in solution-grown microplates of three representative n = 1 and n = 2 RPP phases to reveal heterogeneous halide microscale domainseven in absence of photoinduced phase separation. Such halide immiscibility is revealed by X-ray diffraction, and heterogeneity is confirmed by secondary ion mass spectrometry (SIMS) imaging; however, comparison with photoluminescence (PL) imaging results show how PL information alone can falsely imply homogeneous alloy formation in heterogeneous RPPs. We then demonstrate the use of spectral imaging as an alternative high-throughput tool for accurate halide phase characterization in mixed-halide RPPs.

Quantum confinement in two-dimensional (2D) Ruddlesden−Popper (RP) perovskites leads to the formation of stable quasi-particles, including excitons and biexcitons, the latter of which may enable lasing in these materials. Due to their hybrid organic−inorganic structures and the solution phase synthesis, microcrystals of 2D RP perovskites can be quite heterogeneous, with variations in excitonic and biexcitonic properties between crystals from the same synthesis and even within individual crystals.Here, we employ one-and two-quantum two-dimensional whitelight microscopy to systematically study the spatial variations of excitons and biexcitons in microcrystals of a series of 2D RP perovskites BA 2 MA n−1 Pb n I 3n+1 (n = 2−4, BA= butylammonium, MA = methylammonium). We find that the average biexciton binding energy of around 60 meV is essentially independent of the perovskite layer thickness (n). We also resolve spatial variations of the exciton and biexciton energies on micron length scales within individual crystals. By comparing the one-quantum and twoquantum spectra at each pixel, we conclude that biexcitons are more sensitive to their environments than excitons. These results shed new light on the ways disorder can modify the energetic landscape of excitons and biexcitons in RP perovskites and how biexcitons can be used as a sensitive probe of the microscopic environment of a semiconductor.