An international team led by Pennsylvania State University (Penn State) and Université Paris-Saclay achieved localized light emission from 2D monolayer molybdenum diselenide (MoSe2) nanodots embedded in a monolayer tungsten diselenide (WSe2) matrix. By changing the size of the nanodots, the researchers were able to control the color and frequency of the light the dots emitted. The ability to fine-tune light emission from spatially confined 2D nanodots could assist in the advancement of quantum computing and high-resolution displays. “If you have the opportunity to have localized light emission from these materials that are relevant in quantum technologies and electronics, it’s very exciting,” professor Nasim Alem said. The researchers used cathodoluminescence, performed BY using scanning transmission electron microscopy (STEM), to investigate how the nanodots of varying sizes emitted light when they were embedded in WSe2, a 2D transition metal dichalcogenide (TMD) material. “By combining a light detection tool with a transmission electron microscope, which is a powerful microscope that uses electrons to image samples, you can see much finer details than with other techniques,” researcher Saiphaneendra Bachu said. “Electrons have tiny wavelengths, so the resolution is incredibly high, letting you detect light from one tiny dot separately from another nearby dot." 2 nanodots, represented by red triangles, are embedded in WSe2 and encapsulated by hexagonal boron nitride (hBN) on top and bottom. A focused electron beam, shown in green, in a scanning transmission electron microscope (STEM) is aimed at the structure. The emitted light is collected to generate an intensity map. On the upper right is a dark-field STEM image of the MoSe2 nanodot embedded inside WSe2. The contour of the nanodot is marked by dotted green lines. On the lower right is an artificially colored light emission intensity map of the same region, with the localized emission from the nanodot clearly visible. Courtesy of S. Bachu et al." style="width: 450px; height: 337px; float: left; margin-top: 7px; margin-right: 10px; margin-bottom: 7px;" /> On the left is an illustration of the experimental setup from this study. MoSe2 nanodots, represented by red triangles, are embedded in WSe2 and encapsulated by hexagonal boron nitride (hBN) on top and bottom. A focused electron beam, shown in green, in a STEM is aimed at the structure. The emitted light is collected to generate an intensity map. On the upper right is a dark-field STEM image of the MoSe2 nanodot embedded inside WSe2. The contour of the nanodot is marked by dotted green lines. On the lower right is an artificially colored light emission intensity map of the same region, with the localized emission from the nanodot clearly visible. Courtesy of S. Bachu et al. The researchers observed a strong correlation between light emission and nanodot size. The nanodot emission was dominated by MoSe2 excitons in dots larger than 85 nm and by MoSe2/WSe2 interface excitons in nanodots smaller than 50 nm. At extremely small dot sizes of less than 10 nm, the electron energy levels in the nanodot became quantized, which was demonstrated by a striking blue-shift in interface exciton emission. The quantized nanodots induced quantum confined luminescence. The quantization of the nanodots could result in discrete characteristics that could unlock new properties, including new electronic and optical capabilities. Excitons, which can transport energy but do not carry a net charge, can influence the performance of semiconductor materials. By precisely controlling the excitons in materials, it is possible to manipulate the light they emit more effectively. Control comes from adjusting the band gap of the semiconductor material. Materials with lower dimensions, like a single layer of 2D WSe2, can have a direct band gap, which is more efficient at emitting light than thicker materials with indirect band gaps. “Think about how OLED displays work,” Bachu said. “Each pixel has its own tiny light source behind it so you can control the exact color or brightness of each one. This lets the screen show true black and accurate colors like red, green, and blue. If you improve this process, you make the picture much sharper and more vibrant.” Emission efficiency and other electronic and optical properties vary among the different materials that comprise the TMD class of 2D materials, because each material has different band gap energies. “By mixing them — like combining molybdenum diselenide and tungsten diselenide in specific ratios — you can fine-tune the band gap to emit light at a specific color,” Bachu said. “This process, called band gap engineering, is possible because of the wide variety of materials in this family, making them an excellent platform for studying and creating these light sources.” The capability to control light emission from spatially confined 2D nanodots could lead to faster, more secure quantum systems and energy-efficient, high-resolution displays. The researchers’ findings potentially could be generalized to other 2D systems for future nanophotonic applications. “Envision getting light from a zero-dimensional point in your field, like a dot in space, and not only that, but you can also control it,” Alem said. “You can control the frequency. You can also control the wavelength where it comes from.” The researchers said they are planning to build on their work. “This is just the tip of the iceberg,” Alem said. “By exploring the role of atomic structure, chemistry, and other factors in controlling light emission while expanding on lessons learned in this study, we can move this research to the next level and develop practical applications.” The research was published in ACS Photonics (www.doi.org/10.1021/acsphotonics.4c01739).