Type-I Energy Level Alignment at the PTCDA-Monolayer MoS2 Interface Promotes Resonance Energy Transfer and Luminescence Enhancement.

Soohyung Park,Norbert Koch,Sergey A Kovalenko,Sylke Blumstengel,Lain‐Jong Li,Thorsten Schultz,Patrick Amsalem,Niklas Mutz,Xiaomin Xu,Julia Stähler,Emil J W List‐Kratochvil,Areej Aljarb,Vincent Tung,Dongguen Shin

doi:10.1002/advs.202100215

Abstract

Van der Waals heterostructures consisting of 2D semiconductors and conjugated molecules are of increasing interest because of the prospect of a synergistic enhancement of (opto)electronic properties. In particular, perylenetetracarboxylic dianhydride (PTCDA) on monolayer (ML)‐MoS2 has been identified as promising candidate and a staggered type‐II energy level alignment and excited state interfacial charge transfer have been proposed. In contrast, it is here found with inverse and direct angle resolved photoelectron spectroscopy that PTCDA/ML‐MoS2 supported by insulating sapphire exhibits a straddling type‐I level alignment, with PTCDA having the wider energy gap. Photoluminescence (PL) and sub‐picosecond transient absorption measurements reveal that resonance energy transfer, i.e., electron–hole pair (exciton) transfer, from PTCDA to ML‐MoS2 occurs on a sub‐picosecond time scale. This gives rise to an enhanced PL yield from ML‐MoS2 in the heterostructure and an according overall modulation of the photoresponse. These results underpin the importance of a precise knowledge of the interfacial electronic structure in order to understand excited state dynamics and to devise reliable design strategies for optimized optoelectronic functionality in van der Waals heterostructures.

Highlights

All spectra were recorded at the Γ point of the Brillouin zone (BZ) to obtain reliable secondary electron cutoff (SECO) spectra and for improved signal-to-noise-ratio
Since our further experimental results point toward a van der Waals type interaction at the interface, we assign this small Φ change not to interfacial charge transfer but to a surface electron push-back effect, in analogy to what has been observed for the deposition of molecules onto various inert metal oxide surfaces, which is characteristic for interfaces formed by weakly interacting compounds.[22,23,24,25]
We demonstrate by direct and inverse photoelectron spectroscopy that the energy level alignment at the interface of perylenetetracarboxylic dianhydride (PTCDA) and monolayer MoS2 (ML-MoS2) corresponds to a type-I heterojunction, where the PTCDA frontier levels straddle those of ML-MoS2 by well over 100 meV

Summary

Introduction

Molecular layers can enhance and spectrally tune light absorption and emission of such heterostructures, often of the van der Waals type.[6] Investigations of the photoexcitation dynamics in organic molecule/TMDC heterostructures have mostly focused on optically induced charge-transfer (CT) processes. When these structures exhibit a staggered type-II energy level alignment, excitons are dissociated and electrons and holes are spatially separated in the two material components. RET in type-I TMDC/molecule heterostructures can provide an effective route toward improving the PL characteristics of ML-TMDCs

Results and Discussion

Conclusions

Experimental Section

Conflict of Interest

Data Availability Statement