Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties

Abstract

Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we will review the fundamental optical properties of halide perovskite nanocrystals by focusing on their linear optical properties, on the effects of quantum confinement, and on the current knowledge of their exciton binding energies. We will also discuss the emergence of nonlinear phenomena such as multiphoton absorption, biexcitons, and carrier multiplication. Finally, we will discuss open questions and possible future directions.

Figure 1

Schematic representations of (a) an ideal 3D cubic structure, as observed in α-FAPbI₃; (b) an orthorhombically distorted 3D structure, typically reported for CsPbBr₃; (c) a one-dimensional (1D) hexagonal lattice, found in the yellow phase of FAPbI₃; and (d) a 1D orthorhombic structure, found in the yellow phase of CsPbI₃. (e) Reported 3D and 1D structures of different all-inorganic and hybrid organic–inorganic ABX₃ MHP compounds. The light blue squared area represents the region in which stable compounds are located. The tolerance and octahedral factors were mainly taken from the report of Travis et al. All panels are reproduced from ref (67). Copyright 2017 American Chemical Society.

Figure 2

Schematic representation of different metal halide structures: (a) cubic-phase ABX₃ (3D); (b) pseudocubic ABX₃ (3D); (c) A₄BX₆ (0D); (d) AB₂X₅ (2D); (e) A₂BX₄ (2D); (f) A₂BX₆ (0D); (g) A₂B⁺B³⁺X₆ (3D); and (h and i) A₃B₂X₉ (2D).

Scheme 1. Outline of the Various Methods Employed in the Synthesis of MHP Nanocrystals

Polar solvent controlled ionization.

Figure 3

(a) Sketch of the HI method used for the synthesis of colloidal MHP NCs. (b) A typical TEM image of CsPbBr₃ NCs obtained using the hot injection (HI) strategy. (c) Colloidal perovskite CsPbX₃ (X = Cl, Br, I) NC dispersions (in each vial, the NCs have a different halide composition) in toluene under an ultraviolet (UV) lamp (λ = 365 nm). Panels (b) and (c) are adapted from ref (26). Copyright 2015 American Chemical Society.

Scheme 2. Shape and Size Control of CsPbBr₃ NCs in the HI Approach

Reproduced from ref (120). Copyright 2016 American Chemical Society.

Figure 4

(a) Scanning electron microscopy (SEM) and (b) low-magnification transmission electron microscopy (TEM) images of CsPbBr₃ NSs. (c) Low- and (d) high-resolution TEM micrographs of CsPbBr₃ NWs. (a) is reproduced with permission from ref (121). Copyright 2016 Wiley-VCH. Panel b is reproduced from ref (122). Copyright 2016 American Chemical Society. (c) is reproduced from ref (123). Copyright 2016 American Chemical Society. (d) is reproduced from ref (124). Copyright 2016 American Chemical Society.

Figure 5

(a) Variation in the size of CsPbBr₃ nanocubes, depending on the concentration of oleylamine (OLA), OA, and the reaction temperature (vertical bars represent the size distributions). (b) Illustration of the different CsPbBr₃ nanostructures that were obtained using OLA and OA as ligands, together with their corresponding: (c) absorbance (black lines) and photoluminescence (blue dashed line) spectra; (d) X-ray diffraction (XRD) patterns; and (e–k) TEM images. All panels are reproduced from ref (105). Copyright 2016 American Chemical society.

Figure 6

Single step ultrasonication method: (a) schematic illustration of a CsPbX₃ NC synthesis; (b) colloidal dispersions of CsPbX₃ NCs with different halide compositions in hexane under room light (top) and UV light (bottom, λ_ex = 367 nm); (c) photograph (under UV light) of CsPbBr₃ and CsPbI₃ NCs solutions obtained by scaling up the reaction; and (d) PL decay curves of the samples shown in panel (b). All panels are reproduced with permission from ref (117). Copyright 2016 Wiley-VCH.

Figure 7

Illustrations of the HI approaches used for the preparation of (a) Mn-doped CsPbCl₃ and (c) Mn-doped CsPbBr₃ NCs. (b) Photograph of Mn doped CsPbCl₃ NCs with different Mn-contents, illuminated by a UV lamp (365 nm). (a) is reproduced from ref (150). Copyright 2016 American Chemical Society. (b) is reproduced from ref (149). Copyright 2016 American Chemical Society. (c) is reproduced from ref (152). Copyright 2018 American Chemical Society.

Figure 8

Scheme showing the synthesis of perovskite Cs₂SnI₆ NCs, with corresponding photographs of the as-prepared Cs₂SnI₆ samples under UV light and TEM images of Cs₂SnI₆ NCs with different shapes. Reproduced from ref (158). Copyright 2017 American Chemical Society.

Figure 9

(a) The structure of a double perovskite crystal (e.g., Cs₂AgBiBr₆). (b) Scheme of the HI synthesis of Cs₂AgBiX₆ NCs and (c) their postsynthesis anion-exchange reactions using trimethylsilyl halide reagents. TEM images of (d) Cs₂AgBiCl₆ and (e) Cs₂AgBiBr₆ NCs. All panels are reproduced from ref (115). Copyright 2018 American Chemical Society.

Scheme 3. LARP Synthesis Approach

Scheme 4. Two Different Synthetic Routes, Both Employing NMF as the Source of MA⁺ Ions, To Produce Either Perovskite NCs or Bulk Crystals

Reproduced from ref (178). Copyright 2016 American Chemical Society.

Figure 10

(a) Schematic illustration of a LARP synthesis of MAPbBr₃ nanostructures using oleic acid (OA), oleylamine (OLA), and pyridine as ligands. Representative TEM images of MAPbBr₃ NCs synthesized (b–d) without and (e–g) with pyridine at different precipitation temperatures: (b,e) 0 °C, (c,f) 25 °C, and (d,g) 60 °C, respectively. All panels are reproduced from ref (183). Copyright 2017 American Chemical Society.

Figure 11

(a) Photograph of FAPbX₃ NC dispersions under UV-light and (b) the corresponding PL emission curves. (c) Theoretical effective mass approximation and experimental band gaps of FAPbBr₃ NPLs as a function of their thickness. (d) TEM characterization of vertically stacked FAPbBr₃ NPLs. All panels are reproduced with permission from ref (188). Copyright 2017 American Chemical Society.

Figure 12

Sketch of the mechanism proposed for the formation of (a) NPLs (cyan) and nanobars (green) in ethyl acetate and (b) larger nanocubes, nanorods, and NWs from smaller nanocubes in toluene. (a–b) are reproduced with permission from ref (192). Copyright 2016 Macmillan Publishers Limited. (c–e) TEM images of CsPbBr₃ NWs at (c) 0 days, (d) 1 day, and (e) 7 days after leaving the colloidal toluene-based solution at RT without stirring. (c–e) are reproduced with permission from ref (193). Copyright 2017 Royal Society of Chemistry.

Figure 13

LARP synthesis of CsPb₂Br₅ NWs and NSs. Reproduced with permission from ref (201). Copyright 2017 American Chemical Society.

Figure 14

Illustration of the LARP approaches used for the synthesis of (a) Cs₃Bi₂Br₉ and (c) Cs₃Sb₂Br₉NCs. Photographs of vials containing colloidal (b) Cs₃Bi₂Br₉ and (d) Cs₃Sb₂Br₉ NCs dispersions with and without UV light excitation. (b) Absorption and PL spectra of Cs₃Bi₂Br₉ NC solutions. (a) and (b) are reproduced with permission from ref (80). Copyright 2018 Wiley-VCH. (c) and (d) are reproduced with permission from ref (76). Copyright 2017 American Chemical Society.

Figure 15

Effects of the solvents on the crystal structure of CH₃NH₃PbI₃ NCs: the use of coordinated solvents (top) leads to the formation NC defects, which are prone to degradation under humidity; noncoordinated solvents (bottom) allow for the formation of “defect-free” and stable NCs. Reproduced with permission from ref (205). Copyright 2017 American Chemical Society.

Scheme 5. Emulsion LARP Synthesis

(i) Formation of the emulsion, (ii) demulsion by adding a demulsifier with the concomitant formation of perovskite NCs, and (iii) purification of the NCs. Reproduced from ref (209). Copyright 2015 American Chemical Society.

Figure 16

Differently shaped CsPbB₃ NCs, which can be achieved via an emulsion LARP approach at RT by varying the amount of organic acid and amine ligands. Reproduced from ref (210). Copyright 2016 American Chemical Society.

Figure 17

Schematic representation of the emulsion LARP synthesis of shape-controlled MAPbI₃ perovskites by employing hexadecylamonium (HA) and varying the DMF volume, together with (b–d) the corresponding SEM pictures. All panels are reproduced with permission from ref (212). Copyright 2017 Elsevier.

Figure 18

Shape control in the reverse microemulsion synthesis of colloidal MAPbBr₃ NCs. By controlling the nucleation and growth parameters (solvents, surface ligands, and temperature), the shape of the NCs can be systematically changed. Oleic acid is present in each synthesis. (a–d) TEM images of the corresponding nanostructures are also reported. The scale bars are 50 nm. All panels are reproduced from ref (219). Copyright 2017 American Chemical Society.

Figure 19

Reverse microemulsion synthesis of Cs₄PbBr₆ microcrystals. The inset shows the products prepared on a large scale without and with UV (365 nm) irradiation. Adapted with permission from ref (220). Copyright 2016 Royal Society of Chemistry.

Figure 20

Illustration of the micelle structure that is formed in the reverse microemulsion process, which comprises an “oil” phase with n-hexane, and an “aqueous” phase with DMF. TEM images of Cs₄PbBr₆, CsPb₂Br₅, and CsPbBr₃ perovskite NCs formed with the different Cs:Pb:Br feed ratios. Reproduced from ref (222). Copyright 2017 American Chemical Society.

Scheme 6. States Which Characterize the Polar Solvent Controlled Ionization and LARP Approaches

Reproduced with permission from ref (223). Copyright 2018 Wiley-VCH.

Figure 21

(a–c) TEM images of CsBr NCs of three representative sizes (top panels) and the resulting CsPbBr₃ NCs (bottom panels) that were obtained after the addition of Pb-oleate. Scale bars correspond to 50 nm in all panels. All panels are reproduced from ref (225). Copyright 2018 American Chemical Society.

Figure 22

(a) Influence of ligands on the reduction of Au(III) at the surface CsPbBr₃ NCs to form Au-CsPbBr₃ hybrid structures. Reproduced from ref (238). Copyright 2017 American Chemical Society. (b) PL spectra of CsPbBr₃ NCs (a) before and after the sequential addition of (b and d) AuBr₃ and (c and e) 1-dodecanethiol. (c) Photographs depicting the modulation cycles of the system under room light and UV light, respectively. Reproduced with permission from ref (240). Copyright 2018 The Royal Society of Chemistry.

Figure 23

TEM images of the CsPbBr₃/SiO₂ Janus NCs obtained at different reaction times: (a) 0, (b) 0.5, (c) 2, and (d) 12 h. (e) Schematic illustration of the formation process. Reproduced from ref (245). Copyright 2017 American Chemical Society.

Figure 24

(a) Sketch of the fabrication process employed in the production of CsPbBr₃/TiO₂ core/shell NCs. TEM images of (b) CsPbBr₃ NCs and (c) CsPbBr₃/TiO₂ core/shell NCs after calcination at 300 °C for 5 h. Reproduced with permission from ref (249). Copyright 2018 Wiley-VCH.

Figure 25

(a) Scheme of the AE reaction involving CsPbX₃ NCs. (b) XRD patterns and (c) PL spectra of CsPbX₃ (X = Br, Cl, or I) NCs prepared by AE from CsPbBr₃ NCs. Reproduced from ref (21). Copyright 2015 American Chemical Society.

Scheme 7. Proposed Mechanism for the Photoinduced AE Process of Perovskite NCs in the Presence of Dihalomethane as the Solvent

Reproduced from ref (269). Copyright 2017 American Chemical Society.

Scheme 8. Different Possible AE Pathways for a CsPbBr₃ NC and Iodide Anions

Paths A + C describe the CsPbBr₃ → CsPbI₃ conversion having as intermediate CsPb(Br+I)₃ alloyed structures. In the transformation that is depicted in paths B + D1 + D2, core@graded-shell heterostructures form as intermediate steps, and CsPbI₃ is the final product. Reproduced from ref (274). Copyright 2018 American Chemical Society.

Figure 26

(a) Confocal image of a three-color heterojunction NW. Blue, green, and red represent the PL emissions at 410–450, 500–550, and 580–640 nm, respectively. Reproduced from ref (277). Copyright 2017 National Academy of Sciences. (b) Band edge energies of CsPbX₃ NCs extracted from cyclic voltammetry data. Reproduced from ref (279). Copyright 2016 American Chemical Society.

Scheme 9. (a) Partial Pb²⁺→ M²⁺ (M= Sn, Cd, or Zn) CE in CsPbBr₃ NCs and the (b) Corresponding Proposed Reaction Mechanism

Reproduced from ref (283). Copyright 2017 American Chemical Society.

Figure 27

(a) Temporal evolution of the PL emission of CsPbBr₃ NCs after the addition of MnCl₂, together with (inset) the corresponding photographs taken under an UV lamp (365 nm). (b) Schematic representation of the ion exchange process from pure CsPbBr₃ NCs to Mn-doped CsPb(Cl/Br)₃ NCs obtained by the addition of MnCl₂. Reproduced with permission from ref (289). Copyright 2017 Wiley-VCH.

Scheme 10. Schematic Illustration of CsPbX₃ NCs That Have Been (a) CsX or (b) PbX₂ Surface Terminated; (c) the Dynamic Surface Stabilization of a CsPbBr₃ NC by Oleylammonium Bromide (OA is Not Part of the Ligand Shell); and (d) LHP NC That Has Been Passivated by OLA⁺ Ions, Which Replace Surface Cs⁺ Ions

(c) Reproduced with permission from ref (301). Copyright 2016 American Chemical Society.

Scheme 11. Transformation of Cubic CsPbBr₃ Perovskite NCs into Tetragonal CsPb₂Br₅ NSs Which Takes Place by the Progressive Exfoliation of the Former

Reproduced from ref (314). Copyright 2018 American Chemical Society.

Scheme 12. Copolymerization of Chemically Addressable NCs (Methacrylic Acid-Capped) with POSS-Appended Methacrylate Monomer (MA-POSS) and/or Methyl Methacrylate (MMA) to Produce Polymer Composites

Reproduced from ref (343). Copyright 2018 American Chemical Society.

Scheme 13. One-Pot Strategy Used to Prepare Perovskite-Polymer Composites (CsPbBr₃-Polymer or MAPbBr₃-Polymer)

(a) Formation of perovskite crystals in liquid monomers. The digital picture under room light illustrates a dispersion of CsPbBr₃ crystals in styrene; (b) the subsequent UV- or thermal-initiated polymerization leads to the formation of perovskite-polymer composites. Representative disks (under room and UV light) are shown in the photos. Reproduced from ref (338). Copyright 2018 American Chemical Society.

Scheme 14. Encapsulation of MAPbX₃ NCs in a Silica Matrix Using a LARP-Based Approach Together with Triethoxysilane (APTES)

Reproduced from ref (377). Copyright 2018 American Chemical Society.

Scheme 15. Sol–Gel Route (Moisture-Induced Hydrolysis), Based On the Use of Liquid PSZ, Employed to Protect the Surface of LHP with Either SiN_x, SiN_xO_y, or SiO_y

Reproduced from ref (349). Copyright 2018 American Chemical Society.

Figure 28

(a) Schematic representation of the conversion of a Pb-MOF into a luminescent MAPbBr₃/Pb-MOF composite. MAX is a halide salt (CH₃NH₃X, X = Cl, Br, or I), and the green spheres in the matrix are MAPbBr₃ NCs. The two black boxes show the crystal structures of the Pb-MOF (left) and the MAPbBr₃ (right). (b and c) Optical images of MAPbBr₃ NCs@Pb-MOF powder under (b) ambient light and (c) a 365 nm UV lamp. (d) Illustration of an inkjet setup based on Pb-MOF, which can be used for information encryption and decryption. Reproduced with permission from ref (379). Copyright 2017 Macmillan Publishers Limited.

Figure 29

Bandgap tuning of CsPbX₃ NCs as a function of halide content, as demonstrated by UV–vis and PL spectra. Reproduced from ref (26). Copyright 2015 American Chemical Society.

Scheme 16. Formation of Energetic Bands in a Lead Iodide Perovskite Material through the Hybridization of Lead and Iodide Orbitals

Figure 30

(a) Broadening of the 1s exciton level (Γ_total, fwhm of the absorption edge) of bulklike MAPbI₃ NPLs as a function of the temperature, together with the contribution of homogeneous and inhomogeneous broadening. Adapted from ref (391). Copyright 2018 American Chemical Society. (b) Effective spectral line width (ESL) of single nanocubes (PNC) as a function of their respective first absorption peaks. Adapted from ref (392). Copyright 2017 American Chemical Society.

Figure 31

(a) Absorption and PL emission of Mn-doped CsPbCl₃ NCs: (b) the large Stokes shifts in these systems make them an ideal candidate material for solar concentrators. Adapted from ref (31). Copyright 2017 American Chemical Society. (c) PL curves of CsPbCl₃ NCs doped with different lanthanide ions. Adapted from ref (143). Copyright 2017 American Chemical Society.

Figure 32

(a) PL and absorption curves of CsPbBr₃ thin films, cubic NCs, and NPLs with different thicknesses. As the thickness is reduced, pronounced quantum confinement effects can be seen in the absorption and photoluminescence spectra. (b) TEM picture of 3 ML thick NPLs. (c) Postsynthesis AE reactions performed on NPLs lead to a wide tuning range of their PL, while retaining their size and shape. Adapted from ref (19). Copyright 2016 American Chemical Society.

Figure 33

(a) TEM and HRTEM images of 10 nm and ultrathin (∼2.2 nm, 4 ML) CsPbBr₃ NWs, and (b) their PL and absorption spectra. Similar to 2D NPLs, pronounced quantum confinement effects can be seen in these spectra. Adapted from ref (16). Copyright 2016 American Chemical Society.

Figure 34

(a) PL and absorption spectra of CsPbBr₃ nanocubes of different sizes together with (b) the corresponding calculated bandgaps. Adapted from ref (26). Copyright 2015 American Chemical Society. (c) Depiction of corrugated 0D metal halide perovskite structures. (d) Corresponding excitation and PL spectra of these structures exhibiting a strong Stokes shift and a significant broadening of the PL, which are both induced by self-trapping excitons. (e) Energy diagram depicting the formation and recombination of self-trapped excitons. Adapted from ref (411) with permission from the Royal Society of Chemistry.

Figure 35

Variation in the PL emission as a function of the size of different CsPbBr₃ nanostructures: 2D, 1D, and 0D. Data extracted from refs (, , , , and 398).

Figure 36

Determining the exciton binding energy in 2D LHP NPLs. (a) Optical density of 2 ML NPLs dispersed in toluene: the 1s exciton absorption peak (green ○) and continuum onset (black □) are determined by applying the Elliot’s formula. The exciton binding energy is given by the energy difference between these two levels as a function of NPL thickness. Adapted from ref (398). Copyright 2018 American Chemical Society. (c) Scheme depicting that a reduced screening of the Coulombic electron–hole interaction in the thin NPLs strongly contributes to a large value in the hundreds of meV of the exciton binding energy. Adapted from ref (50). Copyright 2017 American Chemical Society.

Figure 37

Biexctionic decay (lifetime τ_2X) in CsPbI₃ nanocubes for high pump intensities in time-resolved PL measurements. The late-time tail of the decay curve corresponds to a single exciton recombination (lifetime τ_X), while M and B correspond to the intensities of the multiexciton and single exciton signals, respectively. Adapted from ref (421). Copyright 2016 American Chemical Society.

Figure 38

Biexciton Auger lifetimes of small LHP NCs and their dependence of the NCs size. A deviation from the universal volume scaling law can be observed for NCs when their diameter exceeds the exciton Bohr radius. Adapted from ref (423). Copyright 2016 American Chemical Society.

Figure 39

Time-dependent PL spectra of a single CsPbI₃ NC excited with an average of 1.5 excitons at 4K, revealing not only a single exciton recombination (X) but also singly charged (X^–) and doubly charged (X^2–) single excitons, biexcitons (XX), and charged biexcitons (XX^–). Adapted with permission from ref (430). Copyright 2017 American Physical Society.

Figure 40

(a) Angular-resolved single NC PL measurements revealed a fine structure, which can be reproduced through simulations (b). (c) Energy scheme depicting the splitting of energetic states into cubic NCs. The Rashba effect is introduced to explain the experimental findings. This results in the lowest energetic state being optically active in contrast to common semiconducting NCs. Adapted with permission from ref (428). Copyright 2018 Macmillan Publishers Limited.

Figure 41

(a) Interesting resonances appear in wavelength-dependent below bandgap excitation spectra of LHP NC films. (b) This occurs whenever an integer multiple of the exciting photon energy equals an integer multiple of the exciton energy. This is explained with a multiphoton absorption, leading to multiple excitons being generated. Adapted with permission from ref (437). Copyright 2018 Macmillan Publishers Limited.