Depressed Phase Transitions and Thermally Persistent Local Distortions in CsPbBr₃ Quantum Dots

Abstract

The optoelectronic properties of CsPbX3 quantum dots (where X = Cl, Br, or I) are influenced by both their local and average structures. Variable-temperature synchrotron X-ray diffraction measurements of CsPbBr3 quantum dots show that the temperatures for both the orthorhombic-to-tetragonal (50 °C < T γ‑β < 59 °C) and tetragonal-to-cubic (108 °C < T β‑α < 117 °C) phase transitions of the average structure are depressed relative to their temperatures in bulk CsPbBr3. Simultaneously, pair distribution function analysis of synchrotron X-ray total scattering measurements indicates that the local crystal structure of the quantum dots is best described as orthorhombically distorted over the temperature range of 22 °C < T < 160 °C, with only small changes in the magnitude of the distortion occurring during the observed changes in the average structure. Taken together, these results suggest that phase transitions in CsPbBr3 quantum dots are order–disorder, involving the gradual ordering of individually coherent domains that cannot be attributed to changes in the surface area or to ferroelectric phenomena.

Dual phases of crystalline and electronic structures in the nanocrystalline perovskite CsPbBr3

“…There is also a significant loss in intensity as the sample is slowly heated and again when the temperature is reduced back to 100 K. This loss is likely due to damage from the electron beam over time and the formation of Pb nanoparticles within the lattice 41 . XRD has been previously performed on CsPbBr 3 using powder diffraction 14 and HR-TEM [20][21][22][23][24][25][42][43][44] . However, from this result alone, it has been difficult to determine the difference between the orthorhombic, tetragonal and cubic structural lattices of CsPbBr 3 .…”

“…CsPbBr 3 has been shown to undergo two structural phase changes in the single-crystal form: the orthorhombic (Pbnm) to tetragonal (P4/mbm) phase transition at 361 K and the tetragonal (P4/mbm) to (Pm3m) cubic transition at 403 K, both of which occur far above room temperature 18,19 . However, these transition temperatures are lower for CsPbBr 3 quantum dots and powders 20,21 . Our understanding of both the electronic and crystalline structures of nanocrystalline CsPbBr 3 , particularly below these transition temperatures, is still lacking 17 .…”

“…An experiment based on temperature-and angle-dependent spectroscopic ellipsometry combined with high-resolution transmission electron microscopy (HR-TEM) and THz measurements as a function of temperature and supported by first-principle calculations is a powerful way to reveal the fundamental properties of nanocrystalline materials. Using this method, we are also able to determine the proportionality of the nanocrystals in each phase in a nondestructive manner, which HR-TEM and diffraction studies are unable to do alone [20][21][22][23][24][25] . This finding has significant repercussions regarding the electronic structure and therefore the band gap and efficiency of future solar cell devices that will be produced.…”

Low-energy paths for octahedral tilting in inorganic halide perovskites

“…In reality, the picture of a perfect defect free bulk that is dynamically fluctuating along particular octahedral tilting paths is very likely to simplistic. Recent studies indicate the prevalence of ferroelastic twin boundaries [16,17,[39][40][41] in halide perovskites. Bertlotti et al [16] demonstrated that the apparent tetragonal and cubic phases, as probed using x-ray diffraction, can result from domains of twinned orthorhombic a − a − c + variants.…”

“…Recently, there has been increasing evidence that the high temperature cubic phase of many perovskites is not, in fact, cubic on a local scale [12][13][14][15][16][17]. Instead, the cubic phase emerge as a result of static and/or dynamic disorder.…”