‘Nanodot’ control could fine-tune light for sharper displays and quant…

Newly achieved precise control over light emitted from incredibly tiny sources, a few nanometers in size, embedded in two-dimensional (2D) materials could lead to remarkably high-resolution monitors and advances in ultra-fast quantum computing, according to an international team led by researchers at Penn State and Université Paris-Saclay.

In a recent study, published in ACS Photonics, scientists worked together to show how the light emitted from 2D materials can be modulated by embedding a second 2D material inside them—like a tiny island of a few nanometers in size—called a nanodot. The team described how they achieved the confinement of nanodots in two dimensions and demonstrated that, by controlling the nanodot size, they could change the color and frequency of the emitted light.

“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,” said Nasim Alem, Penn State associate professor of materials science and engineering and co-corresponding author on the study. “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. You can control the frequency. You can also control the wavelength where it comes from.”

The researchers embedded nanodots made of a 2D material called molybdenum diselenide inside another 2D material, tungsten diselenide, and then aimed a beam of electrons at the structure to make it emit light. This technique, called cathodoluminescence, allowed the team to study how individual nanodots in the material emit light at high resolution.

“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,” said Saiphaneendra Bachu, first author who served as the primary doctoral student on the study before earning a doctorate from Penn State in 2023 and is now a TEM analysis engineer at Samsung Austin Semiconductor. “Electrons have tiny wavelengths, so the resolution is incredibly high, letting you detect light from one tiny dot separately from another nearby dot.”

They found that larger dots give off one type of glow, while smaller dots produce another. When the dots are extremely tiny—less than 10 nanometers wide, which is about the size of 11 hydrogen atoms arranged in a line—they behave in a unique way, trapping energy and emitting light with higher frequency, which equates to a smaller wavelength.

According to Alem, this phenomenon is called quantum confinement. It occurs when the dots are contained in a space so small that their energy becomes quantized, meaning it becomes a discrete characteristic that enables new properties, including novel electronic and optical capabilities. In this case, the researchers confirmed that nanodots confined fundamental particle pairs known as excitons at the interface of molybdenum diselenide and tungsten diselenide.

Excitons can transport energy but do not carry a net charge, and they can influence how semiconductors—the chips underpinning smartphones, computers and more—behave. By precisely controlling the excitons in materials, scientists can manipulate the light they emit more effectively, which they said could lead to faster and more secure quantum systems, as well as other customizable, energy-saving devices like higher resolution screen displays.

“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.”

The control comes from adjusting the band gap—essentially the energy threshold electrons must cross to make a material emit light—of a semiconductor material. Materials with lower dimensions, like a single layer of 2D tungsten diselenide, can have a direct band gap, which is more efficient at emitting light compared to its thicker, indirect bandgap counterpart, Alem said.

But light emission efficiency and other electronic and optical properties vary even among a family of related 2D materials—like molybdenum disulfide, tungsten disulfide, molybdenum diselenide and tungsten diselenide—because they each have 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 researchers said they are now planning to build on this 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.”