Engineering Exciton Dynamics in Two-dimensional Organic Semiconductors

Abstract

Two-dimensional (2D) organic semiconductors have emerged as a rising star due to rich physics. Compared with bulk materials, these 2D materials provide an ideal platform to investigate the strong many-body interactions. These enhanced many-body interactions lead to the formation of robust quasi-particles called excitons, which enable exotic properties in 2D molecular crystals. Among them, superradiance (SR), the spontaneous coherent emission from bright excitons, has sparked considerable interest in quantum-information applications. In addition, optically forbidden states (dark excitons) have the potential both to achieve Bose-Einstein condensation and modulate exciton dynamics. However, gaining insights into both SR and dark excitons at room temperature poses a major challenge. Here, we report a unique series of dark excitonic states in highly crystalline 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) organic monolayers (MLs) via two-photon excitation spectroscopy. These dark excitons convert to the emissive states that undergo room temperature SR. By developing a vibronic exciton model, we identify these dark states as mixed FE-CTE states with the majority intralayer CTE character (>99.9%) and weak coupling to the emissive FE states. We observe significantly higher photochemical stability of MLs under two-photon excitation, which we attribute to the suppression of exciton-exciton annihilation.

Secondly, the emission of light from organic materials is greatly influenced by the quality of samples and the type of excitation used. Although the light-induced luminescence of 2D materials has been extensively studied, but there is potential for luminescence induced by the injection of free electrons, resulting in new applications for these materials. Here, we report the observation of robust emission from organic single molecules via cathodoluminescence (CL) spectroscopy. The emission originates from PTCDI organic molecules at room temperature. In the PL spectrum, the single molecules have weak emission, whereas in CL the emission is immensely enhanced. The CL emission has a dependency on the thickness of the organic layers and the quantum efficiency of the monolayer is much higher than the other layers due to the more active states leading to more CL emission. The single molecules and injected free electrons have a linear relation, with a slope of ~9.1 and the quantum efficiency ~18 times higher than the free counterparts. The CL enhancement depends on the beam interaction volume in the sample, and the Monte Carlo simulation has been performed to predict the penetration depth.

Finally, it's necessary to analyze the molecular coupling, vibrational modes, and how they evolve with the number of layers are important for mechanical and the electrical properties. Here, we report the observation of strong out-of-plane vibrational modes in organic 2D molecular crystals via nano-FTIR spectroscopy. The PTCDI organic crystal exhibit strong out-of-plane modes at 1590 cm-1 and 1690 cm-1, whereas these modes are undetectable in isolated molecules. Thickness-dependent analyses showed the IR signal and absorption increased and red-shifted with the increase of layer number. The DFT simulations support our hypothesis of interlayer molecular coupling in a organic system by showing the simulated out-of-plane modes closely matched with the experimental results. The s-SNOM imaging with 20-nm resolution shows strong absorption in trilayer areas compared to the monolayer areas at 1690 cm-1 (out-of-plane mode) and weak absorption almost incomparable between trilayer and monolayer at 1800 cm-1 (in-plane mode).

Overall, our results open new avenues to better understand and systematic studies for exploring phenomenon's in 2D material (i.e., enhanced light-matter interactions, carrier transports, molecular coupling, chemical compositions, phases, defects, etc) and may help to develop potential controllable optoelectronic devices.