A research collaboration led by Pennsylvania State University (Penn State) and Université Paris-Saclay has experimentally demonstrated a new way to tune and localize light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs). By embedding monolayer molybdenum diselenide (MoSe₂) nanodots within a monolayer tungsten diselenide (WSe₂) matrix, the team achieved size-dependent control of color and emission frequency, a breakthrough that could have broad applications in nanophotonics, quantum computing, and energy-efficient display technologies.
Although theoretical studies have long suggested that zero-dimensional TMD “islands” could produce localized light, this work offers the first clear experimental validation. The researchers used cathodoluminescence in a scanning transmission electron microscope (STEM) to inspect MoSe₂ nanodots of varying sizes, all held within a WSe₂ layer. The measurements revealed a clear relationship between nanodot diameter and the character of the emitted light. Dots larger than about 85 nm showed emission dominated by MoSe₂ excitons, whereas dots smaller than 50 nm emitted primarily due to MoSe₂/WSe₂ interface excitons. Dots below 10 nm exhibited quantized electron energy levels, which resulted in a pronounced blue-shift in interface exciton emission. This progression underscores how subtle changes in a nanodot’s spatial footprint can strongly influence its excitonic properties, demonstrating the ability to systematically tune optical outputs by adjusting the dot size.
Central to this discovery is the method of cathodoluminescence, performed inside a STEM. By focusing an electron beam on specific sample locations, the team collected high-resolution structural and optical data, discerning the optical signals of individual nanodots only a few nanometers apart. This level of detail is not generally achievable with conventional photoluminescence techniques.
Because light emission from TMDs is closely tied to their band gap energies, size- and composition-based engineering holds significant commercial promise. This is particularly true in the field of high-resolution displays, where sharp color accuracy and energy efficiency are essential. Nanodots that can be finely tuned for specific colors and wavelengths pave the way for next-generation displays with improved brightness, richer color depth, and potentially lower power consumption. Beyond displays, quantum technologies could benefit from zero-dimensional nanodots that act as single-photon emitters or serve as building blocks for quantum computing hardware. Nanophotonics could harness precisely tuned exciton behavior to drive new generations of ultra-compact photonic and optoelectronic devices.
Looking ahead, the researchers plan to further investigate how atomic structure, local chemistry, and other variables influence exciton behavior. By combining MoSe₂ and WSe₂ in different ratios and spatial arrangements, and potentially incorporating other TMDs, they aim to broaden the range of achievable emission wavelengths. According to the team, these findings represent only the starting point for engineering advanced 2D-based light sources and delving deeper into quantum confinement phenomena.
Reference
Bachu, S., Habis, F., Huet, B., Woo, S. Y., Miao, L., Reifsnyder Hickey, D., Kim, G., Trainor, N., Watanabe, K., Taniguchi, T., Jariwala, D., Redwing, J. M., Wang, Y., Kociak, M., Tizei, L. H. G., & Alem, N. (2025). Quantum Confined Luminescence in Two Dimensions. ACS Photonics, 12(1), 364–374. https://doi.org/10.1021/acsphotonics.4c01739
