Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening

Abstract

Organic-inorganic hybrid halide perovskites are promising semiconductors with tailorable optical and electronic properties. The choice of A-site cation to support a three-dimensional (3D) perovskite structure AMX 3 (where M is a metal, and X is a halide) is limited by the geometric Goldschmidt tolerance factor. However, this geometric constraint can be relaxed in twodimensional (2D) perovskites, providing us an opportunity to understand how various the A-site cations modulate the structural properties and thereby the optoelectronic properties. Here, we report the synthesis and structures of single-crystals (BA) 2 (A)Pb 2 I 7 where BA = butylammonium, and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA) or guanidinium (GA), a series of A-site cation varied in size. Single-crystal X-ray diffraction reveals that the MA, FA, and GA structures crystallize in the same Cmcm space group, while the DMA imposes the Ccmb space group. We observe that as the A-site cation becomes larger, the Pb−I bond continuously elongates, expanding the volume of the perovskite cage, equivalent to exerting "negative pressure" on the perovskite structures. Optical studies and DFT calculations show the Pb−I bond length elongation reduces overlap of the Pb s-and I p-orbitals and increases the optical bandgap, while Pb−I−Pb angles play a secondary role. Raman spectra show lattice softening with

Two‐Dimensional Guanidine‐Based Hybrid Perovskites with Strong Dichroism for Multiwavelength Polarization‐Sensitive Detection

“…The thermal stability of (BA) 2 (GA)Pb 2 I 7 was confirmed by TGA measurement, showing a notable thermal stability up to 535 K (Figure S8, Supporting Information). Structure analyses reveal that (BA) 2 (GA)Pb 2 I 7 crystallizes in the orthorhombic space group Cmcm at room temperature (Table S1), and it adopts 2D Ruddlesden–Popper hybrid perovskite architecture that can be described as the sandwich‐like structure (Figure 1a) [39] . Its basic motif consists of the infinite perovskite bilayers of corner‐sharing PbI 6 octahedra with GA + cations locating in cavities and spacing layers of organic BA + cations, which link to inorganic frameworks with strong N−H⋅⋅⋅I hydrogen bonds (Figure S2, Supporting Information).…”

“…Structure analyses reveal that (BA) 2 (GA)Pb 2 I 7 crystallizes in the orthorhombic space group Cmcm at room temperature (Table S1), and it adopts 2D Ruddlesden-Popper hybrid perovskite architecture that can be described as the sandwich-like structure (Figure 1a). [39] Its basic motif consists of the infinite perovskite bilayers of cornersharing PbI 6 octahedra with GA + cations locating in cavities and spacing layers of organic BA + cations, which link to inorganic frameworks with strong NÀ H•••I hydrogen bonds (Figure S2, Supporting Information). It is worth noting that such alternating arrangement of bilayers organic BA + cations and inorganic frameworks in (BA) 2 (GA)Pb 2 I 7 forms the 2D quantum-well architecture.…”

Rational alloying of secondary and aromatic ammonium cations in a metal-halide perovskite toward crystal-array photodetection

“…The length and coordination preference should fulfil the strict limitation of the Goldschmidt's tolerance factor concept [20]. Notably, some 2D Ruddlesden-Popper materials containing large-radius cations in metal-halide perovskite cavities were recently obtained, such as guanidine (~278 pm) and dimethylamine (DMA,~272 pm) [21][22][23]. Despite the attempts to explore new candidates, studies on their physical attributes and potential optoelectronic applications remain quite scarce.…”

Additive Engineering by Bifunctional Guanidine Sulfamate for Highly Efficient and Stable Perovskites Solar Cells

“…The compositions of perovskite (PVK) are represented by ABX 3 , where A usually represents methylammonium (CH 3 NH 3 + , MA), formamidinium (CH(NH 2 ) 2 + , FA), and Cs + , B is indicated as Pb 2+ , Ge 2+ , or Sn 2+ , and X is expressed as Cl − , Br − , or I − . [ 14–16 ] The perfect combination of these elements provides brilliant photoelectric performance of PSCs.…”