(b) SEM image of CsPbBr 3 NCs, revealing their typical shapes and sizes. The 3D diffraction pattern is obtained by recording intensities at the detector plane for a set of incidence angles δθ in the vicinity of (002)/(110) Bragg conditions. Coherent x-rays fully illuminate the nanocrystal and produce the Bragg peak shown in red isosurface. įigure 1. (a) Scheme of Bragg coherent diffraction imaging experiment in the sample reference frame. The NCs of CsPbBr 3 are stabilized in an orthorhombic phase with lattice constants: a = 8.207 Å, b = 8.255 Å, and c = 11.759 Å (space group Pbnm), just like the bulk material. The orientation of the particles in the plane of the window is random, which allows measurement of isolated Bragg reflections from single crystallites. In the case of macroscopic scale single crystals of CsPbBr 3, (002)/(110) domains with facets parallel to the silicon nitride window as a substrate was revealed (see appendix B for details). Recent studies only indirectly inferred the presence of sub-domains in CsPbBr 3 NCs with the size of several nanometers, which was done by evaluating powder diffraction data and constructing a model of coherent nanotwins. However, the information on internal structure and strain remains limited. Recently, it was demonstrated that the emission characteristics of lead halide NCs also depend on their size and shape. Additionally, there is a strong debate whether MHPs can exhibit ferroelectricity similar to other well-known perovskites such as BaTiO 3 and PbTiO 3, and which role strain and defects play in this case. Consequently, incompatibility of the two domains at the surface may lead to large strains. Surface relaxation may also affect the behavior of twin walls as they intersect surfaces in three-dimensional (3D) nanostructures like nanoparticles or nanowires. Similarly to semiconductors, strain engineering approaches can alter the properties of MHPs, including phase transition diagrams, via thermal, mechanical, and light-induced stress. One important question is how the twin substructures modify transport and chemical properties in the vicinity of the domain boundaries relative to the bulk. If two interfacing domains have different lattice spacing, the domains will generally tilt due to the lattice mismatch at the domain boundary. This process is possible due to coordination octahedra tilting, causing fragmentation of the crystals into domains with twin boundaries. ![]() Particularly, lead halide CsPbX 3 single crystals transition through cubic, tetragonal, and orthorhombic structures and the formation of twin domains is induced. They demonstrate high efficiency as light emitters through photoluminescence (PL) across the entire visible spectral range (410–700 nm), where the formation of Cl/Br and Br/I solid solutions enables a highly accurate adjustment of their band gap energies.Īn important aspect of MHPs is the structural phase transitions at moderate temperatures, changing from higher to lower symmetries upon cooling. All-inorganic lead halide CsPbX 3 nanocrystals (NCs) are one of the promising candidates for MHP-based novel optoelectronic devices. ![]() The crystal structure of MHPs with a common formula of ABX 3 allows for flexible chemical composition, which is an efficient way to tune their properties. Metal halide perovskites (MHPs) have been attracting significant attention over the past few years due to their outstanding performance in photovoltaic, light-emitting, energy storage, and scintillation applications. Moreover, this work paves the way towards real-time imaging of the domain dynamics in ferroic systems. These results provide important insights both for the fundamental understanding of ferroelastic nanoscale materials and for the performance improvement of perovskite-based devices. The domains exhibit a difference in lattice tilt of 0.59 degrees, in excellent agreement with calculations of the lattice mismatch at the (112) twin boundary. The particles exhibit similar scattering volume ratios of 0.12 ± 0.026 between twin phases, suggesting the possibility of a deterministic formation process. A preferential double-domain structure is revealed in four identical particles, with one domain oriented along the and the other along the direction. In this work, twinned ferroelastic domains in single 500 nm CsPbBr 3 particles are studied with 3D Bragg coherent x-ray diffraction imaging. However, the gap in understanding the relationship between their nanoscale structure and properties limits their application towards novel devices. Metal halide perovskites attract significant interest due to their remarkable performance in optoelectronic devices.
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