Pressure-Engineered Structural and Optical Properties of Two-Dimensional (C₄H₉NH₃)₂PbI₄ Perovskite Exfoliated nm-Thin Flakes

Abstract

Resolving the structure−property relationships of twodimensional (2D) organic−inorganic hybrid perovskites is essential for the development of photovoltaic and photoelectronic devices. Here, pressure (0−10 GPa) was applied to 2D hybrid perovskite flakes mechanically exfoliated from butylammonium lead halide single crystals, (C 4 H 9 NH 3 ) 2 PbI 4 , from which we observed a series of changes of the strong excitonic emissions in the photoluminescence spectra. By correlating with in situ high-pressure X-ray diffraction results, we examine successfully the relationship between structural modifications in the inorganic PbI 4 2− layer and their excitonic properties. During the transition between Pbca (1b) phase and Pbca (1a) phase at around 0.1 GPa, the decrease in ⟨Pb−I−Pb⟩ bond angle and increase in Pb−I bond length lead to an abrupt blue shift of the excitonic bandgap. The presence of the P2 1 /a phase above 1.4 GPa increases the ⟨Pb−I−Pb⟩ bond angle and decreases the Pb−I bond length, leading to a deep red shift of the excitonic bandgap. The total band gap narrowing of ∼350 meV to 2.03 eV at 5.3 GPa before amorphization, facilitates (C 4 H 9 NH 3 ) 2 PbI 4 as a much better solar absorber. Moreover, phase transitions inevitably modify the carrier lifetime of (C 4 H 9 NH 3 ) 2 PbI 4 , where an initial 150 ps at ambient phase is prolongated to 190 ps in the Pbca (1a) phase along with enhanced photoluminescence (PL), originating from pressure-induced strong radiative recombination of trapped excitons.The onset of P2 1 /a phase shortens significantly the carrier lifetime to 53 ps along with a weak PL emission due to pressure-induced severe lattice distortion and amorphization. High-pressure study on (C 4 H 9 NH 3 ) 2 PbI 4 nm-thin flakes may provide insights into the mechanisms for synthetically designing novel 2D hybrid perovskite based photoelectronic devices and solar cells.

Accurate Control of VS2 Nanosheets for Coexisting High Photoluminescence and Photothermal Conversion Efficiency

“…[31] Theo ptical band gaps of crystal VS 2 ,2Damorphous VS 2 ,and 2D VO 2 (D) can be,r espectively ascertained to be 2.31, 2.87, and 2.73 eV by as emiconductor equation derived from UV/Vis-NIR spectroscopy (Figure 5b). In some cases,p ressure-induced small angles and longer bond lengths [32] or increased crystal field splitting between bonding and anti-bonding states [33] may account for the widen electronic structure,the phenomenon is further testified in our work. Particularly,t he corresponding implemented pressure is decreased from GPa to MPa through SC CO 2 .…”

Reversible Thermochromism and Strong Ferromagnetism in Two‐Dimensional Hybrid Perovskites

“…Such structural “softness” results in a highly reconfigurable crystal structure with relatively easy structural rearrangements to undergo a rich variety of solid‐solid phase transitions under external stimuli . We have observed pressure‐induced changes in photoluminescence and conductivity in several hybrid halide perovskites, all of which involved structural transformation . Beside optoelectronic properties, the color appearance can also change significantly during phase transitions in 3D hybrid perovskites.…”

Organic‐Inorganic Hybrid Perovskite Single Crystals: Crystallization, Molecular Structures, and Bandgap Engineering

“…Furthermore, the bulk inorganic hybrid perovskite single crystals can be exfoliated into few layers. [70][71][72][73][74][75] Therefore, the thickness of organicinorganic hybrid perovskite can be as thin as 2 nm after exfoliation. Besides the methods mentioned above, melt crystallization techniques such as Czochralski method, Bridgman method and zone-melting method can be also employed to grow large-size industrially important functional crystals.…”