CsPbBr₃ (caesium lead bromide): structure, nanocrystals and uses

CsPbBr₃ has the perovskite ABX₃ structure: lead atoms sit at the nodes of a (pseudo)cubic lattice, each surrounded by an octahedron of six bromide ions, the octahedra share corners in three dimensions, and the caesium ions sit in the cavities between them (Bertolotti et al. 2017). For the general picture of the ABX₃ lattice and how it relates to oxides such as SrTiO₃ and BaTiO₃, see the perovskite structure page.

The ideal, untilted framework is the cubic phase, the high-temperature form of all three CsPbX₃ compounds (Protesescu et al. 2015). On cooling, the octahedra tilt: tilts in one plane give a tetragonal structure, tilts about two directions an orthorhombic one (Bertolotti et al. 2017). Bulk CsPbBr₃ goes from cubic to tetragonal at about 130 °C and to orthorhombic at about 88 °C, and the orthorhombic phase is the stable one at room temperature (He et al. 2018). Caesium is slightly too small for the cavity: the Goldschmidt tolerance factors of CsPbX₃ are about 0.85–0.87 (Bertolotti et al. 2017).

In nanocrystals the phase is harder to pin down. The first CsPbX₃ nanocrystals were assigned the cubic structure (Protesescu et al. 2015); total-scattering studies later showed that CsPbBr₃ nanocrystals are orthorhombic at room temperature, made of orthorhombic subdomains joined by twin boundaries that can make the average pattern look tetragonal or cubic (Bertolotti et al. 2017).

1. Dissolve the lead halide. PbBr₂ is dried in octadecene, and oleylamine and oleic acid (1:1) are added to dissolve it and later to stabilize the nanocrystals (Protesescu et al. 2015).
2. Inject caesium oleate. At 140–200 °C, a solution of caesium oleate is injected quickly. Nucleation and growth are very fast: most growth happens in the first 1–3 s (Protesescu et al. 2015). In a typical CsPbBr₃ run the injection is at 180 °C and the flask is cooled in an ice-water bath 5 s later (Bertolotti et al. 2017).
3. Tune size with temperature. Because growth is so fast, the size of the cubes (4–15 nm) is set by the reaction temperature rather than by the growth time (Protesescu et al. 2015).
4. Tune colour with the halide. Mixing PbCl₂ and PbBr₂, or PbBr₂ and PbI₂, gives the mixed-halide nanocrystals CsPb(Cl/Br)₃ and CsPb(Br/I)₃; Cl/I mixtures cannot be made because the ionic radii differ too much (Protesescu et al. 2015).

Bulk CsPbBr₃ crystals for detectors are instead grown from the melt by the Bridgman method, from CsBr and PbBr₂ (He et al. 2018).

• The whole visible range. By changing the halide (Cl → Br → I) and the size, the band gap and emission of CsPbX₃ nanocrystals cover 410–700 nm (Protesescu et al. 2015); CsPbBr₃ itself is the green emitter (Akkerman et al. 2015).
• Narrow and bright. Emission line widths are 12–42 nm and quantum yields 50–90%, a combination earlier reached only with core–shell chalcogenide quantum dots such as CdSe/CdS (Protesescu et al. 2015).
• Fast. Radiative lifetimes are 1–29 ns, faster for wider-gap nanocrystals (Protesescu et al. 2015).
• Saturated colours. A triangle of red, green and blue CsPbX₃ nanocrystals covers 140% of the NTSC colour standard (Protesescu et al. 2015).

A nanocrystal behaves as a quantum dot when it is about as small as the exciton Bohr diameter or smaller. Effective-mass estimates give Bohr diameters and exciton binding energies of about 5 nm and 75 meV for CsPbCl₃, 7 nm and 40 meV for CsPbBr₃ and 12 nm and 20 meV for CsPbI₃, so nanocrystals in the 4–15 nm range show size-dependent band gaps: 5–12 nm CsPbBr₃ nanocrystals absorb and emit further to the blue the smaller they are (Protesescu et al. 2015). In practice, though, the colour of CsPbX₃ nanocrystals is set mainly by the halide composition rather than by the size (Bertolotti et al. 2017).

Because the halide ions are mobile, the colour of ready-made nanocrystals can be changed by exchanging the anions. Adding halide sources to a colloid of CsPbX₃ nanocrystals exchanges the halide quickly at low temperature, partially or completely, tuning the emission over 410–700 nm while keeping quantum yields of 20–80% and line widths of 10–40 nm (Nedelcu et al. 2015). Starting from green CsPbBr₃, exchange with chloride or iodide precursors gives emitters across the visible, and adding bromide brings them back (Akkerman et al. 2015).

The exchange also runs between nanocrystals: simply mixing CsPbCl₃ and CsPbBr₃, or CsPbBr₃ and CsPbI₃, nanocrystals gives uniform mixed-halide particles that emit at an intermediate colour (Nedelcu et al. 2015; Akkerman et al. 2015).

The nanocrystals are capped by oleylamine and oleic acid, and this ligand shell is highly dynamic: the ligands come off easily, so they are lost during isolation and purification. Adding a small amount of both oleic acid and oleylamine lets the nanocrystals be purified while keeping their optical, colloidal and material integrity, and a high amine content in the shell raises the quantum yield (De Roo et al. 2016).

CsPbX₃ nanocrystals were also put forward as chemically robust in the blue and green, where chalcogenide quantum dots photodegrade, and they can be embedded in PMMA while staying brightly emissive (Protesescu et al. 2015). Of the three halides, CsPbI₃ is the fragile one: its emissive perovskite phase is metastable and turns slowly into a non-emissive yellow phase (Protesescu et al. 2015; Bertolotti et al. 2017). As a solid, CsPbBr₃ contains no organic molecules, which gives it far greater long-term stability than hybrid perovskites (He et al. 2018).

• Light-emitting diodes. Quantum-dot LEDs with CsPbX₃ nanocrystals as the emitter were reported in 2015 (Song et al. 2015).
• Displays and colour conversion. Narrow emission and a wide colour gamut make CsPbX₃ nanocrystals candidates for display colour conversion; they cover up to 140% of the NTSC standard (Protesescu et al. 2015), and displays are among the proposed uses of CsPbX₃ quantum dots (Song et al. 2015).
• Photodetectors, lasers and solar cells. Proposed applications of the nanocrystals (Song et al. 2015).
• Gamma-ray detectors. Melt-grown CsPbBr₃ single crystals resolve gamma rays at room temperature, with an energy resolution of 3.9% at 122 keV and 3.8% at 662 keV, helped by a hole lifetime of more than 25 μs (He et al. 2018).

For a paper, slide or poster, draw CsPbBr₃ as corner-sharing PbBr₆ octahedra with a Cs⁺ ion in each cavity; show the octahedra tilted if you mean the room-temperature orthorhombic phase, or untilted for the cubic high-temperature phase. For nanocrystals, a cube of a few unit cells with ligands on its surface and a coloured glow tells the story at a glance. In the LabCanvas figure editor, the Library has a “Perovskite QD” object (a CsPbX₃ nanocube with corner-sharing octahedra, ligands and an adjustable emission colour) and a “Crystal” object that draws the perovskite lattice in 3D with or without octahedra; both export as high-resolution SVG and can be recoloured and resized.