In-Plane Mechanical Properties of Two-Dimensional Hybrid Organic–Inorganic Perovskite Nanosheets: Structure–Property Relationships

Abstract

In-plane strains are commonly found in two-dimensional (2D) metal halide organic–inorganic perovskites (HOIPs). The in-plane mechanical properties of 2D HOIPs are vital for mitigating the strain-induced stability issues of 2D HOIPs, yet their structure and mechanical property relationship largely remains unknown. Here, we employed atomic force microscope indentation to systematically investigate the in-plane Young’s moduli E ∥ of 2D lead halide Ruddlesden–Popper HOIPs with a general formula of (R-NH3)2PbX4, where the spacer molecules R-NH3 + are linear alkylammonium cations (C m H2m+1-NH3 +, m = 4, 6, 8, or 12) and X = I, Br, or Cl. Fixing the spacer molecule to butylammonium, we discovered that the E ∥ of 2D HOIPs generally follows the trend of Pb–X bond strength, different from the tendency found in the out-of-plane moduli E ⊥, showing more prominent effects of the metal halide inorganic framework on E ∥ than E ⊥. E ∥ exhibits nonmonotonic dependence on the chain length of the linear alkyl spacer molecules, which would first decrease and plateau but then increase again. This is likely due to the competition of the bond strength and structural distortion in the inorganic layer, the relative fraction of the soft organic spacers, and the interfacial mechanical coupling associated with the interdigitation of the alkyl chains. The mechanical anisotropy of 2D HOIPs, marked by E ∥/E ⊥, shows wide tunability based on structural composition, particularly for iodide-based 2D HOIPs. Our results provide valuable insights into the structure–property relationships regarding the mechanical anisotropy and in-plane mechanical behaviors of 2D HOIPs, which can guide the materials design and device optimization to achieve required mechanical performance in 2D HOIP-based applications.

2D Materials for Wearable Energy Harvesting

“…Metal halide hybrid organic-inorganic perovskite (HOIPs), having the chemical formula AMX 3 , where A, M, and X are small organic cations, divalent Group 14 elements, and halogen, respectively, has been widely employed in solar cells due to its excellent defect tolerance and charge transport. [380][381][382][383][384][385][386][387][388][389][390] By reducing 3D HOIPs crystals to a single-layer thickness along the (100) direction, 2D HOIPs are structural analogs to the Ruddlesden-Popper phase with the expression A 2 A' n-1 M n X 3n+1 and exhibit greater ambient stability than their 3D counterparts while retaining excellent optoelectronic properties. [381,[391][392][393][394][395][396][397][398] Thus, 2D HOIPs can be thought of as layers of [MX 6 ] 4− octahedral material sandwiched between two layers of organic spacer cations (Figure 2h).…”

“…[380][381][382][383][384][385][386][387][388][389][390] By reducing 3D HOIPs crystals to a single-layer thickness along the (100) direction, 2D HOIPs are structural analogs to the Ruddlesden-Popper phase with the expression A 2 A' n-1 M n X 3n+1 and exhibit greater ambient stability than their 3D counterparts while retaining excellent optoelectronic properties. [381,[391][392][393][394][395][396][397][398] Thus, 2D HOIPs can be thought of as layers of [MX 6 ] 4− octahedral material sandwiched between two layers of organic spacer cations (Figure 2h). [398,399] The mechanical properties of 2D perovskite are thought to be dependent on the number of [MX 6 ] 4− octahedral layers, the length of organic spacer molecules, and the strength of the MX bond.…”

“…[398,399] The mechanical properties of 2D perovskite are thought to be dependent on the number of [MX 6 ] 4− octahedral layers, the length of organic spacer molecules, and the strength of the MX bond. [381,[400][401][402] While 2D HOIPs are mechanically weaker than their 3D counterparts, Tu et al discovered that the out-of-plane mechanical hardness and Young's modulus of 5-layered 2D HOIPs are comparable to those of their 3D counterparts using nanoindentation. [400] On the other hand, when the chain length of the organic spacer molecules increases, the out-of-plane Young's modulus declines initially before plateauing, showing that the softening effect in long-chain spacer cations has been saturated.…”

“…[401] Later, Kim et al examined the in-plane Young's modulus of single-layered 2D HOIPs (R-NH 3 ) 2 PbX 4 , where R and X represent alkyl and halogen groups, respectively. [381] The bond strength of PbX was discovered to be a significant factor in determining the in-plane Young's modulus and had a minor effect on the out-of-plane Young's modulus, owing to the stronger PbX bonding resulting in a larger in-plane Young's modulus. [381] Interestingly, the chain length of organic spacers can also affect the in-plane mechanical stiffness by distorting the [PbX 6 ] 4− octahedral structure and altering the length of the PbX bond.…”

“…[381] The bond strength of PbX was discovered to be a significant factor in determining the in-plane Young's modulus and had a minor effect on the out-of-plane Young's modulus, owing to the stronger PbX bonding resulting in a larger in-plane Young's modulus. [381] Interestingly, the chain length of organic spacers can also affect the in-plane mechanical stiffness by distorting the [PbX 6 ] 4− octahedral structure and altering the length of the PbX bond. [381] Another study conducted by Yin et al investigated the phase transition and optical characteristics of 2D (C 4 H 9 NH 3 ) 2 PbI 4 flakes at various applied pressures.…”