Xinyu Shen

Abstract:

All-inorganic CsPbI₃ perovskite nanocrystals (PNCs) have emerged as promising emitters for optoelectronic applications owing to their high color purity, tunable bandgap, and high photoluminescence quantum yield (PLQY). However, dynamic surface ligands, abundant trap states, and rapid halide ion migration limit their emission efficiency and operational stability. Here, methylammonium (MA⁺), formamidinium (FA⁺), and guanidinium (GA⁺) were systematically investigated as surface modulators for PNCs via an anti-solvent-assisted post-treatment strategy. Spectroscopic analyses and density functional theory calculations reveal that the passivation efficacy is determined by the strength and denticity of hydrogen-bonding interactions between the organic cations and the PNC surface. Among the three candidates, GA⁺ exhibits the highest efficacy owing to its trifunctional N─H groups, which enable robust multidentate surface interactions that suppress trap formation and halide ion migration. As a result, GA-treated PNCs achieve a PLQY of 99.3% and retain ∼80% of their initial PLQY after 9 days under ambient conditions. The resulting perovskite light-emitting diodes deliver a maximum external quantum efficiency of 10.2% and a maximum luminance of 473 cd m^-2, representing a significant improvement over pristine devices. These findings identify molecular hydrogen-bonding geometry as a key design parameter for surface engineering of efficient and stable PNC-based optoelectronic devices.

Abstract:

Strongly-confined perovskite nanoplatelets (PeNPLs) offer opportunities not found in conventional isotropic nanocubes, especially in producing linearly polarized light, as well as enhancing outcoupling through control over the transition dipole moment. But this requires ultrathin nanoplatelets with three or fewer monolayers of PbI₆ octahedra across the thickness, which are challenging to synthesise uniformly, and their luminescence is strongly affected by surface defects. Together, these limit the performance of ultrathin PeNPLs in light-emitting diodes (LEDs). Here, we address these challenges with an ancillary ligand engineering strategy. We demonstrate that ligands with phosphoryl functional groups strongly bind to the perovskite surface, while having an organic backbone that is not sterically bulky ensures high ligand density. By modulating nucleation and growth, these ancillary ligands lead to monodisperse PeNPLs that stack more uniformly when self-assembled into superlattices, with suppressed agglomeration. As a result, from edge-up PeNPL superlattices, we achieve an enhanced degree of polarization, while from face-down PeNPL superlattices, we achieve enhanced outcoupling that results in LEDs with 13.1% external quantum efficiency, the highest reported for ultrathin PeNPL LEDs. This work establishes ancillary ligand-induced synthesis as a decisive route to achieve uniform nanoplatelets with robust orientation control, enabling full utilization of the multifunctionality of anisotropic PeNPLs.

Abstract:

Lead-halide perovskite nanocrystals (NCs) have gained attention for optoelectronics, but careful selection of the antisolvent used for purification is essential to achieve high monodispersity and yield while minimizing surface damage. Current understanding indicates that this requires lowering the relative polarity of the antisolvent, yet high-polarity antisolvents are widely used for purification, as we confirm through data mining. We show that polarity alone is insufficient for antisolvent selection by comparing ethyl acetate and acetonitrile for CsPbI3 NC purification. Despite its higher polarity, acetonitrile yields improved colloidal stability compared to ethyl acetate. Using 1H NMR, FTIR, and XPS measurements, alongside DFT calculations, we demonstrate that acetonitrile acts as a stronger Lewis base, binding to and passivating the NC surface. Coordination of acetonitrile to the perovskite NC surface enhances stability and improves their performance in light-emitting diodes. These findings establish a mechanistic framework for antisolvent selection to realize bright and stable halide perovskite NCs.

Abstract:

The anisotropy of perovskite nanoplatelets (PeNPLs) opens up many opportunities in optoelectronics, including enabling the emission of linearly polarized light. But the limited stability of PeNPLs is a pressing challenge, especially for red-emitting CsPbI3. Herein, we address this limitation by alloying formamidinium (FA) into the perovskite cuboctahedral site. Unlike Cs/FA alloying in bulk thin films or nanocubes, FA incorporation in nanoplatelets requires meticulous control over the reaction conditions, given that nanoplatelets are obtained in kinetically-driven growth regimes instead of thermodynamically-driven conditions. Through in-situ photoluminescence (PL) measurements, we find that excess FA leads to uncontrolled growth, where phase impurities and nanoplatelets of multiple thicknesses co-exist. Restricting the FA content to up to 25% Cs substitution enables monodisperse PeNPLs, and increases the PL quantum yield (from 53% to 61%), exciton lifetime (from 18 ns to 27 ns), and stability in ambient air (from ~2 days to >7 days) compared to CsPbI3. This arises due to hydrogen bonding between FA and the oleate and oleylammonium ligands, anchoring them to the surface to improve optoelectronic properties and stability. The reduction in non-radiative recombination, improvement in the nanoplatelet aspect ratio, and higher ligand density lead to FA-containing PeNPLs more effectively forming edge-up superlattices, enhancing the PL degree of linear polarization from 5.1% (CsPbI3) to 9.4% (Cs0.75FA0.25PbI3). These fundamental insights show how the stability limitations of PeNPLs could be addressed, and these materials grown more precisely to improve their performance as polarized light emitters, critical for utilizing them in next-generation display, bioimaging, and communications applications.

Abstract:

All-inorganic halide perovskite materials have attracted significant interest for display applications because of their narrow bandwidths and high photoluminescence quantum yields. However, the development of blue-light-emitting perovskite materials has been slower than that of green- and red-light-emitting perovskites. In this study, we successfully produced single-halide CsPbBr3 nanoplatelets with a quantum confinement effect using ligand-assisted reprecipitation techniques. NaBr, an inorganic ligand with strong binding affinity (adsorption energy, -2.13 eV) and low steric hindrance, was used to prevent the continued growth of nanoplatelets. A series of experimental results demonstrate that CsPbBr3 nanoplatelets with a dense Na+ shell on the surface exhibit stable deep blue emission. UV chip-based light-emitting devices activated by these nanoplatelets demonstrated remarkable spectral stability as the current injection increased. Moreover, the synthesis process is conducted under simple conditions at room temperature, demonstrating potential for batch production.