(327e) Spectroscopic Characterization of Excitons in Two-Dimensional Semiconductors and Their Heterostructures

2D semiconductors refer to a family of semiconductors that are atomically thin. These materials possess bandgaps that cover a wide range of the electromagnetic wave spectrum, and they exhibit various novel optoelectronic properties such as strong light-matter interaction, quantum degree of freedom, and ultrafast carrier transfer, making them ideal candidates for various optoelectronic applications such as ultrafast photodetection and solar-energy harvesting. Because of the reduced screening in atomically thin materials, the optical properties of 2D semiconductors are largely determined by excitons, which are electron-hole pairs bound by Coulomb interaction created upon optical excitation. Thus, understanding the properties of excitons is critical to the development of novel optoelectronic devices based on 2D semiconductors.

In order to provide a more sensitive probe into the optical absorption of excitons in 2D materials, we have developed and applied the technique of photocurrent spectroscopy to study the absorption spectra of various 2D semiconductors. Firstly, we demonstrated the high spatial selectivity of this approach by measuring the absorption spectra of TiS₃ nanoribbons, which are difficult to obtain due to their sub-micrometer size. We further used this technique to characterize the excitonic absorption of monolayer WSe₂ in an in-plane electric field and revealed the Stark shift of Rydberg excitons, providing more fundamental insights into how to manipulate excitons with electric fields.

By controlling the twist angle and the lattice mismatch of a 2D semiconductor homo-structure or hetero-structure, one can create a new crystal lattice called the moiré superlattices, which provides a new approach to engineer the electronic and optoelectronic properties of 2D semiconductors. It has been discovered that 2D Moiré superlattices can exhibit many novel physics phenomena such as locally trapped moiré excitons and correlated insulating states such as generalized Wigner crystals and Mott insulators, which provide new opportunities in various applications, including single-quantum emitters and unconventional superconductivity. While previous studies on 2D moiré superlattices have mainly focused on homobilayers and heterobilayers, the properties of multilayer moiré superlattices remain uninvestigated. Moreover, by adding extra layers to Moiré superlattices, one can potentially achieve manipulation of carrier occupation and correlation strength in these heterostructures.

In order to study how the layer degree of freedom affects the properties of moiré superlattices, we have performed low-temperature reflectance contrast measurements on angle-aligned heterostructures formed between WSe₂ and single-layer WS₂ with varying WSe₂ layer numbers. From the reflectance spectra as functions of carrier density, we identified three moiré exciton peaks in all the heterostructures. However, compared with the monolayer WSe₂/ monolayer WS₂ (1L/1L WSe₂/WS₂) region, bilayer WSe₂/ monolayer WS₂ (2L/1L WSe₂/WS₂) and trilayer WSe₂/ monolayer WS₂ (3L/1L WSe₂/WS₂) show an additional peak which is close to the original A-exciton peak of WSe₂, indicating the excitons in the second and third layer of WSe₂ are not affected by Moiré coupling. The carrier densities to reach correlated insulating states were also found to be independent of the layer number of WSe₂, indicating that these correlated states are also interfacial phenomena.

We further explored the tunability of correlated insulating states using an out of plane electric field in dual-gated 2L/1L WSe₂/WS₂ devices. By analyzing the reflectance contrast spectra taken at different electric fields, we found an external electric field can drive the holes in WSe₂ away from the interface towards the second layer while the carrier density of the insulating state remains the same. We conclude that 2L/1L WSe₂/WS₂ hosts an excitonic insulator phase, making it a possible candidate for realizing exciton condensation and superfluidity.

To summarize, we have systematically investigated the excitonic behaviors in various 2D semiconductors and their heterostructures, with various advanced optical spectroscopy techniques, including photocurrent spectroscopy. The excitonic behaviors and correlated insulating states in multilayer moiré superlattices have been studied by reflectance contrast. These results have advanced the fundamental physics understanding of 2D semiconductors and may lay the foundation for future quantum optoelectronic applications based on these materials.