Layered Crystals Show Promise for Next-generation Optical Technology
These images show how the atomic layers inside the perovskite crystals are arranged, which helps explain the study’s main finding. In two materials (PA₂PbBr₄ and Tz₂PbBr₄), the layers stack in a simple repeating two-layer pattern, creating a more orderly structure. But when two different organic molecules are combined to form PATzPbBr₄, the layers follow a more complicated four-layer pattern before repeating. This more complex stacking introduces small shifts and irregularities in how the layers line up. The top-down views show how the atomic sheets are packed within each layer, revealing subtle distortions. Together, these structural differences help explain why the mixed-molecule crystal can interact with light across a broader range of wavelengths.
By Dave DeFusco
A UNC-Chapel Hill study is shedding light on how a special class of materials called perovskites could help improve future optical technologies, from advanced sensors to telecommunications devices. The research, published in Advanced Functional Materials, explores how subtle structural differences inside these materials affect how they interact with light.
Perovskites are materials known for their remarkable electronic and optical properties. They have already drawn attention for use in solar cells, LEDs and detectors, but scientists are still learning how their internal structure influences performance.
The researchers focused on a specific version called two-dimensional metal-halide perovskites, or 2D perovskites. These materials are built like layered sandwiches: thin sheets of inorganic material stacked with organic molecules in between. That layered design gives scientists many ways to tweak their properties.
“Two-dimensional perovskites are exciting because their structure is very flexible,” said Lina Quan, senior author of the paper and an assistant professor in the UNC Department of Chemistry. “By changing the molecules between the layers, we can influence how the material behaves electronically and optically.”
That flexibility, however, comes with complications. The layers do not always line up perfectly. Tiny irregularities, called structural disorder, can appear across the crystal. Until now, scientists have not fully understood how these irregularities affect how the material handles light.
To investigate, members of the Quan Group—Yixuan Dou, a postdoctoral associate, and Nicholas Nici and Sunhao Liu, both graduate students—created several perovskite crystals using different organic molecules placed between the inorganic layers. Some crystals contained just one type of organic molecule, while others used a carefully ordered mix of two types. They then used advanced X-ray analysis to examine how neatly the layers stacked.
The mixed-molecule crystals showed more long-range disorder, meaning the layers were slightly shifted or misaligned over larger distances. Scientists call this “crystal mosaicity,” a term that describes how a crystal is made up of many slightly misoriented regions rather than one perfectly aligned block.
“That disorder might sound like a flaw, but it can actually be useful,” said Quan. “We found it strongly changes how the material interacts with light.”
One key effect involves polarization—the direction in which light waves vibrate. Some optical devices need to control polarization precisely. Materials that can delay one component of light relative to another, a phenomenon called optical retardation, are essential for technologies such as imaging systems, lasers and fiber-optic communications.
The team discovered that crystals containing two different organic molecules showed unusually strong optical behavior. Because of both their internal disorder and their directional differences, known as anisotropy, these crystals could affect a broad range of light wavelengths rather than just a narrow band.
“In simple terms, the material can manipulate many colors of light at once,” said Quan. “That’s important because most conventional optical components only work well over a limited wavelength range.”
This broadband performance could allow engineers to design simpler and potentially cheaper optical devices. For example, components called waveplates, which are used to control polarization, often require complicated manufacturing or multiple stacked pieces. The mixed-cation perovskite crystals may achieve similar effects in a single material.
The study also highlights how organic molecules, which are relatively easy to modify chemically, provide a powerful design tool. By selecting molecules of different shapes and sizes, researchers can intentionally introduce specific kinds of disorder to tune optical performance.
“This gives us a new strategy for materials design,” said Quan. “Instead of trying to eliminate structural imperfections, we can engineer them to achieve useful optical functions.”
While the work is still at a fundamental research stage, the findings could help guide future development of photonic materials, which are substances designed to control light for applications ranging from computing to medical imaging. The researchers emphasize that understanding structure at multiple scales remains crucial. Small molecular changes can ripple outward to affect large-scale crystal behavior and ultimately device performance.
“Our study shows how microscopic structural details translate into macroscopic optical properties,” said Quan. “That connection is essential if we want to design the next generation of functional materials.”