Zero-Dimensional Cesium Lead Halides: History, Properties, and Challenges

Abstract

Over the past decade, lead halide perovskites (LHPs) have emerged as new promising materials in the fields of photovoltaics and light emission due to their facile syntheses and exciting optical properties. The enthusiasm generated by LHPs has inspired research in perovskite-related materials, including the so-called "zero-dimensional cesium lead halides", which will be the focus of this Perspective. The structure of these materials is formed of disconnected lead halide octahedra that are stabilized by cesium ions. Their optical properties are dominated by optical transitions that are localized within the individual octahedra, hence the title "'zero-dimensional perovskites". Controversial results on their physical properties have recently been reported, and the true nature of their photoluminescence is still unclear. In this Perspective, we will take a close look at these materials, both as nanocrystals and as bulk crystals/thin films, discuss the contrasting opinions on their properties, propose potential applications, and provide an outlook on future experiments.

Figure 1

Properties of Cs₄PbX₆ and CsPbBr₃. (a) Crystal structures of cubic CsPbX₃ and rhombohedral Cs₄PbX₆ (shown in its primitive cubic cell). (b) Comparison of orbital overlap between the p orbitals of each Br^– anion and the s and p orbitals of Pb²⁺ cations, showing the strong orbital overlap of the [PbBr₆]^4– clusters in CsPbBr₃ and the decoupling in Cs₄PbBr₆, courtesy of Urko Petralanda. (c) Comparison of the absorption spectra of Cs₄PbX₆ and CsPbBr₃ NCs, showing the large bandgaps and strong excitonic absorption for Cs₄PbX₆, adapted from ref (13). (d) DFT density of state calculations for CsPbBr₃ and Cs₄PbBr₆, adapted from ref (13).

Figure 2

Overview of studies describing the embedment of CsPbBr₃ in Cs₄PbBr₆. (a) Cs₄PbBr₆ thin films exhibiting appreciable absorption at around 520 nm, (b) accompanied by narrow PL at 550 nm originating from the presence of CsPbBr₃. Adapted from ref (6). (c) Schematic representation of a band structure and absorption/PL spectrum of CsPbBr₃ NCs embedded in a Cs₄PbBr₆ matrix. The absorption spectrum exhibits strong excitonic absorption at around 315 nm due to the Cs₄PbBr₆ host and broad absorption up to 515 nm due to the absorption of the CsPbBr₃ NCs, which emit at the band edge (around 515 nm). (d) Theoretical model for a Cs₄PbBr₆|CsPbBr₃ composite material, with various embedded cubic CsPbBr₃ NCs in a rhombic Cs₄PbBr₆ prism matrix, adapted from ref (62). (e) TEM image and electron diffractogram of a Cs₄PbBr₆|CsPbBr₃ composite, indicating the existence of small CsPbBr₃ NCs in the Cs₄PbBr₆ host, adapted from ref (30).

Figure 3

Overview of proposed defect emission in Cs₄PbBr₆. (a) Sketch of a bromide vacancy in Cs₄PbBr₆. (b) Schematic representation of a band structure with a midbandgap defect state, causing midbandgap emission. (c) Defect-related PL emission in a layered 2D organic lead bromide (N1-methylethane-1,2-diammonium)PbBr₄, adapted from ref (98). (d) Absorption, PL, and PLE from green-emitting 26 nm Cs₄PbBr₆ NCs, showing a narrow PL fwhm and strong quenching of the PL at around 315 nm, adapted from ref (14). (e) Absorption and PL of a green-emitting Cs₄PbBr₆ single crystal, showing narrow band-edge-like PL, adapted from ref (21). (f) Absorption and PL from CsPbBr₃ NCs, exhibiting very similar PL and absorption, with narrow PL and a small Stokes shift, adapted from ref (13).

Figure 4

XRD data. (a) XRD calibration experiment indicating that the synthesized Cs₄PbBr₆ single crystals were either pure or contained less than 0.5 wt % of CsPbBr₃, adapted from ref (21). (b) Similar experiment performed with NCs, in which Cs₄PbBr₆ NCs were mixed (2% molar, 0.5 wt %) with CsPbBr₃ NCs, indicating strong green PL after mixing but no detectable CsPbBr₃ XRD diffraction peaks.

Figure 5

Overview of transformation reactions of Cs₄PbBr₆ into CsPbBr₃ and back. (a) Proposed reaction mechanisms for the exchange reaction via the extraction of CsBr or the insertion of PbBr₂. (b) TEM image showing the preservation of NC size from Cs₄PbBr₆ NCs to CsPbBr₃ NCs, adapted from ref (13). (c) First excitonic absorption peak and (d) XRD pattern of NCs after several reversible exchanges from CsPbBr₃ to Cs₄PbBr₆ and back, adapted from ref (37).