Exciton Dynamics in Two-Dimensional PTCDA/WS₂ Heterojunctions

Understanding exciton dynamics in two-dimensional (2D) heterojunctions is essential for future applications such as photocatalysis, photosynthesis, and transistors. 2D organic crystals are an ideal model system for studying excitonic behaviors under extreme confinement and reduced dielectric screening. In this work, we report on a confocal transient absorption spectroscopy setup optimized for 2D inorganic and organic semiconductors and interlayer excitonic behavior in their heterojunctions. For a higher sensitivity and flexibility on sample requirements, the change in absorption was obtained in the reflectance mode. The differential reflectance of 140-fs supercontinuum white-light probe beam was measured as a function of the time delay with respect to 100-fs pump pulses (350 ~ 1320 nm). The instrument response function determined by sum frequency generation was ~350 fs, mostly limited by the refractive type microscope objective. For reference, monolayer MoS₂ supported on quartz substrates, transient reflectance signals at 660 nm showed a triexponential decay with lifetimes of 0.5, 1.8, and 8 ps, which respectively originate from exciton formation[1], trapping by defect, and carrier scattering[2]. Preliminary data on 2D PTCDA (perylenetetracarboxylic dianhydride) crystals-monolayer WS₂ will also be presented.

[1] H. Zhao, Phys. Rev. B, 102, 035414 (2020)

[2] L. Huang, ACS Nano, 7, 2, 1072 (2013)