Research background
Perovskite structure and its almost infinitely adaptable derivative array, is regarded as one of the very important disciplines in materials science . The basic structure prototype ABX3 (A=large cation; B=small cation; Greater interest in perovskites has arisen due to rapid advances in the preparation of mixed or all-inorganic halide perovskites with ABX3 structures (where A is an organic or alkali metal ion, B is typically lead or tin, and
However, bulk halide perovskites are reactive and suffer from surface hydration, phase transitions and high defect density, reducing their performance and lifetime. Therefore, there is a need to develop dimensionally reduced halide perovskites focusing on colloidal, 2D, quantum dot, and molecular scale preparation in thin films. While perovskites formed in lower dimensions enhance some desirable properties, they also increase their tendency to degrade, although surface passivation reduces decomposition in the film. Nonetheless, size remains key in the engineering and fine-tuning of the physical properties of halide perovskites at the nanoscale due to their critical role in determining electronic structure.
Research results
Reza J. Kashtiban, Richard I. Walton and Jeremy Sloan from the University of Warwick, UK, collaborated to report the melt insertion of CsPbBr3 and lead-free CsSnI3 at about 1.2-1.6 Four isolated subnanometer (or picoscale) halide perovskite structures formed inside nanometer single-walled carbon nanotubes (SWCNTs). Three are directly related to the ABX3 perovskite prototype, while the fourth is a perovskite-like layered structure with alternating Cs4 and polyhedral Sn4Ix layers.
In a SWCNT with a diameter of about 1.4 nm, CsPbBr3 forms a Cs3PbIIBr5 nanowire of an ABX3 unit cell in the cross section, and the Pb2+ oxidation state is maintained by ordered Cs+ vacancies. Within SWCNTs about 1.2 nm in diameter, CsPbBr3 and CsSnI3 form a inorganic polymer -like bilayer structure with a cross-section 1/4 that of the ABX3 unit cell with systematically reproducible ABX3 stoichiometry. Generating these smallest halide perovskite structures at their absolute synthetic cross-section limit enables quantum confinement effects, with first-principles calculations showing a broadening of the band gap compared to the corresponding bulk structural forms.
Related research work was published in the top international journal "Advanced Materials" under the title "Picoperovskites: the Smallest Conceivable Isolated Halide Perovskite Structures Formed Within Carbon Nanotubes".
Graphic Express
Figure 1. Encapsulated single unit cell wide Cs3PbIIBr5 halide perovskite structure derived from CsPbBr3
Figure 2. Encapsulated bilayer CsPbIIBr3-like perovskite structure derived from Pm3-m-CsPbBr3
Figure 3. Layered perovskite derivatives in SWCNTs with a diameter of approximately 1.6 nm
Figure 4. Derived from Pm3-m Structure of encapsulated double-layer CsSnIII3 perovskite polymer of CsSnI3
Figure 5. Electronic structures and models of Cs3PbBr5, CsPbBr3 and CsSnI3 skin perovskites
Conclusion and Outlook
The study shows that a new generation of halide perovskite structures can be systematically created by studying their steric stability in variable diameter SWCNTs. Another focus of the research is how halide perovskite surface chemistry responds to constraints, which will profoundly affect their structure and physical properties, as seen in surface studies of lattice capped perovskites. Surface states in terminated halide perovskites are reconstructed from various mechanisms revealed by scanning tunneling microscopy , enabling imaging of halide dimers or "zigzag" patterns. When we imagine a perovskite formed by confinement, we nonetheless see consequences for the surface structure. When Cs3PbBr5 accounts for 75%, the ordered Cs vacancies cause the tilt of the local structure. These features will affect the photoelectric performance and charge carrier mobility. Similar to the reference, another possibility is the influence of the layered charge change structure in SWCNTs.Studies have shown that linear chains of Cs+ and I- ions cause fluctuations in the external electronic structure of narrow SWCNTs. We should expect similar charge modifications from layered structures in wider nanotubes.
The extensive literature on carbon nanotubes , perovskites, and halide perovskites will provide further avenues for modification and development. The SWCNTs themselves can be chirally refined to produce thin tubes with unique electronic structures and single (n,m) chirality, and will also selectively produce each individual encapsulated structure in pure composite form. Encapsulated nanotubes can also be made from other materials, including insulating boron nitride (BN) or semiconducting glycols (MoS2, WS2, etc.). The nanowire ribbon structure can be tuned through chemical substitution or doping. The various possibilities provide a roadmap for the targeted synthesis of specific 1D perovskite structures, allowing the design and synthesis of new generations of low-dimensional halide perovskite structures beyond this preliminary work. The observed properties of the growth of crystals indicate a dominant relationship between the confining nanotube diameter, its inner surface and the obtained crystal structure , enabling the formation of halide perovskites at the polymer scale for the first time in the extreme.
Literature link:
https://doi.org/10.1002/adma.202208575
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