Tunable Van Der Waals Heterostructures Align Electronic Levels and Reveal Exciton Diversity in 2D Materials

The creation of novel materials with tailored electronic and optical properties drives innovation in modern technology, and recent research focuses on combining two-dimensional materials in van der Waals heterostructures. Aurélie Champagne (Institut de Chimie de la Matière Condensée de Bordeaux, CNRS and Lawrence Berkeley National Laboratory), Olugbenga Adeniran (Wayne State University), and Jonah B. Haber, alongside Antonios M. Alvertis, Zhen-Fei Liu, and Jeffrey B. Neaton, investigate the potential of combining organic and inorganic materials in these structures. Their work predicts that layering atomically thin molecular crystals with traditional two-dimensional materials, such as molybdenum and tungsten disulfides, creates unique properties not found in either material alone. Specifically, the team demonstrates substantial control over the energy levels within the combined structure and the ability to generate a diverse range of excitons, including those with long lifetimes and strong interactions, establishing these organic-inorganic heterostructures as promising candidates for advanced optoelectronic devices and fundamental studies of excitonic phenomena.

Exciton Dynamics at Organic-2D Material Interfaces

This body of work explores hybrid materials created by combining organic molecules with two-dimensional materials, focusing on how these materials interact with light and electricity. Scientists investigate the behavior of excitons, bound pairs of electrons and holes, at the interface between organic semiconductors and atomically thin materials like molybdenum disulfide and black phosphorus. The research employs sophisticated computational methods to accurately model these complex systems and understand their electronic and optical properties. A central focus lies on understanding changes in electronic structure at the interface, including charge transfer and hybridization of electronic states.

Researchers meticulously calculate exciton energies, confinement, and the impact of vibrations on exciton behavior, relying on many-body perturbation theory, specifically the GW approximation, to achieve accurate results. Through these calculations, scientists are uncovering insights into phenomena like gap renormalization and screening effects, and exploring more exotic phenomena like exciton condensation and exciton-polaritons. The research highlights the importance of understanding interface dipoles, charge separation, and the hybridization of electronic states between the organic and inorganic components. This research program, driven by leading computational materials scientists, aims to design and optimize these hybrid materials for applications in optoelectronics, including solar cells, transistors, and light-emitting diodes. This work focuses on bilayers combining molecular crystals with transition metal dichalcogenides, utilizing advanced computational techniques to model their behavior. This approach accurately captures dielectric screening effects while minimizing computational demands, enabling detailed analysis of large-scale structures.

The team calculated the band structures for four distinct heterobilayers, combining PDI and PTCDA molecular crystals with both MoS2 and WS2, revealing significant renormalization of the molecular crystal band gaps due to polarization induced by the TMD layers. To gain deeper insight into excited states, the team developed efficient methods for exciton decomposition and visualization, allowing them to map the localization and charge-transfer character of excitons within the bilayers. Analysis of real-space wave functions revealed hybridization between TMD and molecular monolayer states, evidenced by features in the band structures. Notably, the PDI-WS2 bilayer exhibited the formation of charge-transfer excitons with high binding energies, small exciton sizes, and long radiative lifetimes, characteristics advantageous for exploring quantum phenomena. This work reveals a powerful method for engineering material interfaces with tailored functionality and opens new avenues for optoelectronic devices. The team meticulously calculated the electronic band structures of these hybrid materials using advanced computational techniques, revealing significant renormalization of the molecular crystal band gap due to interactions with the TMD layer. Results show that the band gap of the PTCDA monolayer decreases significantly when interfaced with MoS2 and WS2, demonstrating a substantial shift in electronic properties.

Importantly, the TMD band gap remains largely unaffected by the molecular layer, indicating that the molecular component dominates the observed changes. Calculations reveal that the PDI-WS2 bilayer exhibits a unique type-II band alignment, where the gap lies between the molecular LUMO band and the TMD valence band maximum, a characteristic not previously reported in similar hybrid structures. Further analysis revealed that the PDI-WS2 bilayer exhibits substantial bandwidth broadening and lifting of the degeneracy of the LUMO band, reflecting strong intermolecular interactions and enhanced hybridization. Scientists successfully investigated bilayers formed by stacking atomically thin molecular crystals with monolayer transition metal dichalcogenides, specifically molybdenum disulfide and tungsten disulfide. Through detailed theoretical calculations, they reveal that these hybrid structures exhibit properties not found in either material alone, opening new avenues for materials design. The team discovered substantial tuning of the electronic properties and exciton behavior within these bilayers, controlled by the choice of transition metal dichalcogenide.

Notably, bilayers incorporating tungsten disulfide exhibit a unique, strongly bound exciton with high binding energy and a long lifetime. This exciton displays characteristics that suggest potential for efficient charge separation in solar cells and reduced energy loss due to minimal interaction with heat-producing vibrations. Furthermore, the researchers observed hybrid excitons with anisotropic optical responses, indicating rich light-matter interactions within these systems. These findings establish molecularly engineered van der Waals heterostructures as a highly tunable platform for studying exciton physics and developing advanced quantum and optoelectronic technologies. While the study relies on theoretical modelling, future work will likely focus on synthesizing and characterizing these heterostructures to confirm the predicted properties and explore their performance in practical devices.