Strong optical response and light emission from a monolayer molecular crystal

Huijuan Zhao ,Yue Ma ,Biying Song ,Xiang Zhang ,Der Hsien Lien ,Xiaoqing Chen ,Peng Wang ,Yi Shi ,Wei Lv ,Zhangting Wu ,Ming Zhou ,Jian Xu ,Junqing Zhang ,Alessandro Troisi ,Jun Xiao ,Yingbo Zhao ,Matin Amani ,Yinxuan Song ,Zhenhua Ni ,Hao Ma ,Hyungjin Kim ,Ali Javey ,Ying Wang ,Tao Liu ,Yuzhang Feng ,Xinran Wang

doi:10.1038/s41467-019-13581-9

Abstract

Excitons in two-dimensional (2D) materials are tightly bound and exhibit rich physics. So far, the optical excitations in 2D semiconductors are dominated by Wannier-Mott excitons, but molecular systems can host Frenkel excitons (FE) with unique properties. Here, we report a strong optical response in a class of monolayer molecular J-aggregates. The exciton exhibits giant oscillator strength and absorption (over 30% for monolayer) at resonance, as well as photoluminescence quantum yield in the range of 60–100%. We observe evidence of superradiance (including increased oscillator strength, bathochromic shift, reduced linewidth and lifetime) at room-temperature and more progressively towards low temperature. These unique properties only exist in monolayer owing to the large unscreened dipole interactions and suppression of charge-transfer processes. Finally, we demonstrate light-emitting devices with the monolayer J-aggregate. The intrinsic device speed could be beyond 30 GHz, which is promising for next-generation ultrafast on-chip optical communications.

Highlights

Excitons in two-dimensional (2D) materials are tightly bound and exhibit rich physics
Compared to popular 2D semiconductors such as transition metal dichalcogenides (TMDs), the dipole-dipole interaction is even stronger in molecular systems given the localized nature of excitation (e.g., Frenkel excitons (FE) is localized on a single molecule) and low dielectric constant
We study the optical properties of monolayer (ML) perylene derivatives, namely dimethyl-3,4,9,10-perylenetetracarboxilic diimide (Me-PTCDI), 3,4,9,10-perylene-tetracarboxylic diimide (PTCDI) and 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA), where the molecular packing ensures strong dipole interaction but weak electronic coupling, an ideal condition for long-range J-aggregation

Summary

30 K 120 K

Of metal-sulfides to near unity, it rolls off below 1% at high excitation density that is most relevant to devices[22,26]. Further modeling on ΔR/R of multi-layer Me-PTCDI showed that the oscillator strength of the FE was about one order of magnitude smaller than ML (Supplementary Fig. 7, Supplementary Table 2) This is not surprising because the FE in multi-layers is heavily mixed with interlayer CT, leading to spatial separation of electrons and holes[28,32]. We note that near-unity PLQY at room temperature is expected to maintain at low temperature, because the increased coherent length would lead to even shorter radiative lifetime To validate this hypothesis, we plot integrated PL and absorption at 532/514 nm as a function of temperature (Supplementary Fig. 5c, d). The EL peak shows ~50 meV red shift and reduced linewidth by a factor of 2 compared to singlemolecule 0-0 Frenkel transition (at 2.274 eV) This suggests that the excitonic state can be excited electrically, which is an important step towards coherence-enabled device functions[13]. The transient lightemitting devices are very preliminary, these results clearly demonstrate the potential of ML perylene derivatives for efficient and high-speed optoelectronic devices

Discussion

Findings

Methods