Computational Screening of Ligands for Enhanced Interactions between Lead Halide Perovskite Quantum Dots

Abstract

Ligand choice in nanoparticle systems is vital for developing efficient materials and enhancing electronic and chemical properties. Focusing on CsPbBr₃, we demonstrate a strategy for modifying the electronic properties of lead halide perovskites through a systematic computational study on ligands with varying binding motifs, sizes, bridge lengths, π-electron conjugation, and electron withdrawing and donating groups. The calculations are benchmarked against experimental data. Choosing a ligand's π-electron system and binding group, followed by tuning the ligand's properties with substituents to the π-system, allows one to introduce ligand electronic states into the perovskite system's bands, close to band edges, and inside the material's fundamental band gap. One can also design surface states by inducing local distortions at the binding site, which can be tuned by altering the binding group of the ligand. Extension of a material's frontier orbitals onto ligands and the creation of surface states make charges available for transport and chemical reactivity, while avoiding charge trapping. In contrast, midgap ligand states trap charges permanently. Large ligands with high coverages interact among themselves, influencing ligand electronic properties and binding. Carboxylate tends to bind more strongly than the ammonium group. Electronegative oxygens in the carboxylate binding group and electron withdrawing substituents bound to the π-system lower ligand orbital energies relative to perovskite states. The reported theoretical analysis guides experimental design of perovskite-ligand systems for optoelectronic, energy, and quantum information applications.

1

Ligand molecules considered. The binding motifs are illustrated in Figure , while the electronic properties are characterized in Figures , and S1–S5.

2

Ligand–LHP binding motifs illustrated through four distinct cases, exemplified by ligands II, IX, XIV, and XV, Figure . A cesium atom is removed for systems IX, XIV, and XV to better visualize their binding motifs.

3

LUMO charge densities of the representative systems shown in Figure , obtained with the PBE functional. The LUMO of II is localized on the ligand. The LUMOs of IX and XV are localized on the LHP. The LUMO of XIV is delocalized between the ligand and the LHP. A cesium atom is removed for systems IX and XIV to better visualize their binding motifs. For similar reasons, the polyhedral formation of the LHP QD is not visualized to better observe the charge densities located within the QD structure (IX, XIV, XV).

4

Densities of states (DOS) obtained with the PBE functional in the singlet state and separated into ligand and perovskite contributions.

5

(a) UV–vis and PL spectra for CsPbBr₃ quantum dots with added cinnamate ligand at varying concentration. (b) PLQY of CsPbBr₃ quantum dots with added cinnamate or oleate ligand.