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. 2019 Mar 26;31(6):2182-2190.
doi: 10.1021/acs.chemmater.9b00489. Epub 2019 Mar 4.

Fully Inorganic Ruddlesden-Popper Double Cl-I and Triple Cl-Br-I Lead Halide Perovskite Nanocrystals

Quinten A Akkerman  1   2 , Eva Bladt  3 , Urko Petralanda  1 , Zhiya Dang  1 , Emanuela Sartori  2 , Dmitry Baranov  1 , Ahmed L Abdelhady  1 , Ivan Infante  1   4 , Sara Bals  3 , Liberato Manna  1
Affiliations

Fully Inorganic Ruddlesden-Popper Double Cl-I and Triple Cl-Br-I Lead Halide Perovskite Nanocrystals

Quinten A Akkerman et al. Chem Mater. .

Abstract

The vast majority of lead halide perovskite (LHP) nanocrystals (NCs) are currently based on either a single halide composition (CsPbCl3, CsPbBr3, and CsPbI3) or an alloyed mixture of bromide with either Cl- or I- [i.e., CsPb(Br:Cl)3 or CsPb(Br:I)3]. In this work, we present the synthesis as well as a detailed optical and structural study of two halide alloying cases that have not previously been reported for LHP NCs: Cs2PbI2Cl2 NCs and triple halide CsPb(Cl:Br:I)3 NCs. In the case of Cs2PbI2Cl2, we observe for the first time NCs with a fully inorganic Ruddlesden-Popper phase (RPP) crystal structure. Unlike the well-explored organic-inorganic RPP, here, the RPP formation is triggered by the size difference between the halide ions. These NCs exhibit a strong excitonic absorption, albeit with a weak photoluminescence quantum yield (PLQY). In the case of the triple halide CsPb(Cl:Br:I)3 composition, the NCs comprise a CsPbBr2Cl perovskite crystal lattice with only a small amount of incorporated iodide, which segregates at RPP planes' interfaces within the CsPb(Cl:Br:I)3 NCs. Supported by density functional theory calculations and postsynthetic surface treatments to enhance the PLQY, we show that the combination of iodide segregation and defective RPP interfaces are most likely linked to the strong PL quenching observed in these nanostructures. In summary, this work demonstrates the limits of halide alloying in LHP NCs because a mixture that contains halide ions of very different sizes leads to the formation of defective RPP interfaces and a severe quenching of LHP NC's optical properties.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Optical and structural data of Cs2PbI2Cl2 NCs. (a) TEM image of monodisperse Cs2PbI2Cl2 NCs. (b) XRD pattern of Cs2PbI2Cl2 NCs matching that of bulk Cs2PbI2Cl2. (c) Crystal structure of Cs2PbI2Cl2 RPP. (d) Strong excitonic absorption from Cs2PbI2Cl2 and broad PL of Cs2PbI2Cl2 NCs. PLE spectra of Cs2PbI2Cl2 NCs (red line, recorded at 411 nm) overlapping with the absorption spectrum, indicating that the PL originates from Cs2PbI2Cl2. The asterisk marks the instrumental artifact. XRD reference patterns correspond to ref (20).
Figure 2
Figure 2
HAADF-STEM analysis of Cs2PbI2Cl2 NCs. (a) Top view of a Cs2PbI2Cl2 NC that is parallel to the substrate, within the inset, a highlight of the atomic arrangement. (b) Side view of a Cs2PbI2Cl2 NC, which clearly matches the alternating layers of the RPP. (c) Volume of the fitted Gaussian peaks of the halide columns of the NC as shown in Figure 2b. The significant increase in the intensity values measured at the axial positions matches the excepted iodide positions. (d) Cs2PbI2Cl2 NC observed perpendicular to the substrate overlaid with the crystal structure of Cs2PbCl2I2 (Cs+ = purple, Pb2+ = black, Cl– = blue, I– = red, and [PbI2Cl4]4− octahedra = gray), indicating a single layer of Cs2PbI2Cl2 in between two bilayers of Cs3Pb2I2Cl5. For additional HAADF-STEM data, see Figures S3 and S4.
Figure 3
Figure 3
Overview of structural and optical data of CsPb(Cl:Br:I)3 NCs. (a) TEM image of CsPb(Cl:Br:I)3 NCs. (b) XRD pattern of CsPb(Cl:Br:I)3 NCs that matches that of CsPbBr2Cl NCs. (c) Optical properties of CsPb(Cl:Br:I)3 NCs, evidencing an absorption edge at around 460 nm and a weak but narrow emission around 470 nm, matching the same band gap of CsPbBr2Cl NCs. (d) Absorption-corrected PL of CsPb(Cl:Br:I)3 NCs compared to brightly emitting CsPbBr2Cl NCs, with a photo of CsPbBr2Cl and CsPb(Cl:Br:I)3 NCs under UV excitation, showing no visible PL for the CsPb(Cl:Br:I)3 NCs. CsPbCl3 (cubic) and CsPbBr3 (orthorhombic) XRD reference patterns correspond to 98-002-9076 and 96-451-0746.
Figure 4
Figure 4
HAADF-STEM analysis of Cs(Cl:Br:I)3 NCs. HAADF-STEM images of Cs(Cl:Br:I)3 NCs, showing (a) the perovskite lattice and (b,c) NCs with several plane shifts. (d) Volume of the fitted Gaussian peaks of the halide columns of the NC as shown in (c) indicates increased intensity values of the halide columns around the RPP planes, confirming an increased concentration of iodide ions at these positions. (e) RPP plane shift model (Cs+ = purple, Pb2+ = black, Cl–/Br– = blue, I– = red, and PbX6 octahedra = gray) overlapping an HAADF-STEM image of a CsPb(Cl:Br:I)3 NC. For additional HAADF-STEM data, see Figures S7–S9.
Figure 5
Figure 5
DFT studies performed on the CsPbBr2Cl and CsPb(Cl:Br:I)3 NCs. (a) Relaxed CsPbBr2Cl structures (left) and CsPb(Cl:Br:I)3 structures (right) shown from different directions and computed at the DFT/PBE levels of theory. (b) Electronic structure near the conduction and valence band regions. Each molecular orbital is decomposed in terms of atomic type contributions and is highlighted in a different color. (c) Absorption spectrum of each system computed using the simplified time-dependent (TDDFT/PBE) methodology.
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