Perovskites, known for their distinct crystalline structure, are valued for their optical and electronic properties. LHPs are composed of thin sheets of perovskite semiconductor material separated by organic spacer layers, forming multi-sheet thin films. These materials are of great interest due to their ability to efficiently convert electrical charge into light, making them ideal for next-generation photonic devices.
Despite their promise, understanding how to manipulate LHPs to control performance characteristics has remained a challenge. The team's research sheds new light on this issue by examining quantum wells - thin layers of semiconductor material embedded between spacer layers.
"We knew quantum wells were forming in LHPs - they're the layers," explained Aram Amassian, professor of materials science and engineering at NC State and corresponding author of the study.
The size of quantum wells is crucial because energy moves from high-energy structures to lower-energy ones. "A quantum well that is two atoms thick has higher energy than a quantum well that is five atoms thick," added Kenan Gundogdu, co-author of the paper and professor of physics at NC State. "In order to get energy to flow efficiently, you want a gradual slope between these layers."
Previously, researchers encountered a puzzling inconsistency between X-ray diffraction and optical spectroscopy when analyzing quantum wells. The diffraction results suggested quantum wells of specific thickness, while optical measurements showed a broader range of well sizes.
The breakthrough came when researchers identified nanoplatelets - individual sheets of perovskite material that form on the solution's surface during LHP production - as a key factor. "These nanoplatelets act as templates for the formation of quantum wells," Amassian explained. Over time, as nanoplatelets grow thicker, the wells they form also change in size, eventually leading to the formation of three-dimensional crystals.
This discovery resolved the discrepancy between X-ray and optical techniques, with diffraction detecting stacked sheets while optical spectroscopy identified isolated nanoplatelets.
"We can now control the growth of nanoplatelets to fine-tune the size and distribution of quantum wells in LHP films," said Amassian. This level of control enables highly efficient energy transfer, which is critical for lasers and LED applications.
Beyond LHPs, the researchers also explored the role of nanoplatelets in other perovskite materials, such as those used in solar cells. "Nanoplatelets play a similar role in these materials, and we can use them to improve photovoltaic performance and stability," noted Milad Abolhasani, co-author and professor of chemical and biomolecular engineering at NC State.
Research Report:Cationic Ligation Guides Quantum Well Formation in Layered Hybrid Perovskites
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