[Paper Review] Donor-acceptor discrete optical emission in 2D perovskites

Setatira Gorji, Marie Kreĉmarová+13 more|arXiv (Cornell University)|Apr 13, 2022

Perovskite Materials and Applications4 citations

TL;DR

This study provides direct experimental evidence for discrete donor-acceptor pair (DAP) optical transitions in 2D lead halide perovskites, achieving sub-meV linewidths (~120 μeV) and long radiative decay times (5–8 ns). The DAP states arise from methylammonium cations in thicker phases (n ≥ 2), which induce lead displacement and form localized DAP centers, enabling sharp, bright emission for use in optoelectronic and quantum photonic devices at room temperature.

ABSTRACT

Two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and optoelectronic applications in the nanoscale, due to their outstanding electrical and optical properties, differing from their bulk state. Currently, 2D perovskite belonging to this group of nanomaterials is widely studied for a wide range of optoelectronic applications. Thanks to their excitonic properties, 2D perovskites are also promising materials for photonics and nonlinear devices working at room temperature. Nevertheless, strong excitonic effects can reduce the photocurrent characteristics when using thinner perovskites phases. In this work, we present solid experimental evidence for the presence of single donor-acceptor pair optical transitions in 2D Lead Halide Perovskites, characterized by sub meV linewidths ($\simeq 120 μeV$) and long decay times (5-8 ns). Micro-photoluminescence evidence is supported by detailed Photoemission measurements, and a model simulation. The association of Phenethylammonium with Methylammonium cations, the latter molecule being only present in 2D Halide Perovskites with thicker phases $n \geq 2$, has been identified as the source of the donor-acceptor pair formation, corresponding to the displacement of lead atoms and their replacement by methylamonium. Our seminal study of discrete Donor-Acceptor Pair (DAP) sharp and bright optical transitions in 2D Lead Halide Perovskites opens new routes to implement DAP as the carrier sources for novel designs of optoelectronic devices with 2D perovskites, and will foster the development of future outstanding properties in non-linear quantum technologies.

Motivation & Objective

• To identify and characterize discrete donor-acceptor pair (DAP) optical transitions in 2D lead halide perovskites.
• To determine the origin of sharp, bright optical emission in 2D perovskites with n ≥ 2 phases.
• To establish the role of methylammonium cations in forming DAP centers via lead displacement.
• To link DAP states to enhanced photoluminescence properties for optoelectronic applications.
• To provide experimental validation of DAP transitions using micro-photoluminescence, photoemission, and simulation.

Proposed method

• Micro-photoluminescence (μ-PL) and μ-time-resolved PL (μ-TRPL) were used to measure emission spectra and decay dynamics at room temperature.
• Confocal microscopy with 405 nm, 450 nm, 532 nm, and 660 nm excitation lasers enabled spatially resolved optical characterization.
• High-resolution x-ray photoemission spectroscopy (HR-XPS) at the ALOISA beamline (Elettra) and SPECS system confirmed chemical states and surface composition.
• Micro-Raman spectroscopy with 638 nm excitation probed lattice dynamics and structural features.
• Model simulations were used to support the interpretation of experimental data and DAP transition assignment.
• Mechanical exfoliation of 2D perovskite crystals (PEA2PbI4 and PEA2MAPb4I7) on Si substrates enabled high-quality, low-defect sample preparation.

Experimental results

Research questions

• RQ1 What causes the sharp, sub-meV optical emission lines observed in 2D perovskites with n ≥ 2?
• RQ2 How do methylammonium cations contribute to the formation of donor-acceptor pair states in 2D perovskites?
• RQ3 What is the origin of the long radiative decay times (5–8 ns) observed in these materials?
• RQ4 Can DAP transitions be experimentally distinguished from excitonic or defect-related emissions in 2D perovskites?
• RQ5 To what extent do DAP states enable enhanced optical properties for optoelectronic and quantum photonic applications?

Key findings

• Sub-meV linewidths of ~120 μeV were measured for discrete donor-acceptor pair (DAP) optical transitions in 2D perovskites with n ≥ 2.
• DAP emission exhibited long radiative decay times of 5–8 ns, indicating high radiative efficiency.
• The presence of methylammonium cations in thicker 2D perovskite phases (n ≥ 2) was identified as the source of DAP formation via lead displacement.
• HR-XPS confirmed the coexistence of phenethylammonium and methylammonium cations in n ≥ 2 phases, supporting the DAP origin.
• Micro-PL and Raman measurements confirmed the structural and electronic stability of the DAP states at room temperature.
• The study establishes DAP transitions as a viable, bright, and narrow optical source for future 2D perovskite-based optoelectronic and nonlinear quantum devices.