Researchers have developed a new technique to manipulate a type of material known as layered hybrid perovskites (LHPs) at the atomic level. This advancement enables precise control over how these materials convert electrical charge into light. This breakthrough paves the way for creating materials specifically designed for next-generation printed LEDs and lasers, as well as potential applications in photovoltaic devices.
Researchers have developed a new technique to manipulate a type of material known as layered hybrid perovskites (LHPs) at the atomic level. This advancement enables precise control over how these materials convert electrical charge into light. This breakthrough paves the way for creating materials specifically designed for next-generation printed LEDs and lasers, as well as potential applications in photovoltaic devices.
Perovskites are materials characterized by their unique crystalline structure, offering excellent optical, electronic, and quantum properties. LHPs consist of very fine layers of perovskite semiconductor separated by thin organic “spacer” layers. They can be organized into thin films made up of multiple layers, which make them effective in converting electrical energy into light. This quality makes them ideal candidates for future LEDs, lasers, and photonic integrated circuits.
Despite the interest in LHPs, researchers previously struggled to understand how to alter these materials to control their performance.
To grasp the discoveries made by the researchers, it’s essential to start with the concept of quantum wells, which are semiconductor layers placed between spacer layers.
“We identified that quantum wells naturally form within LHPs, as they represent the layered structure,” explains Aram Amassian, the lead author of the research paper and a materials science and engineering professor at North Carolina State University.
Understanding the distribution of these quantum wells is crucial because energy transfers from higher-energy structures to lower-energy ones at the atomic level.
“A quantum well of two atoms thickness has a higher energy level compared to one that is five atoms thick,” states Kenan Gundogdu, co-author of the study and physics professor at NC State. “To promote efficient energy transfer, it’s beneficial to have quantum wells that are three to four atoms thick nestled between those of two and five atoms thick. This creates a gentle slope for the energy to move downwards.”
“Researchers were perplexed by an anomaly: the quantum well size observed through X-ray diffraction did not match that identified through optical spectroscopy,” Amassian says.
“For instance, diffraction may indicate quantum wells are two atoms thick and also identify a three-dimensional bulk crystal. In contrast, spectroscopy could suggest the presence of quantum wells at two, three, and four atoms thickness alongside the 3D bulk phase,” Amassian adds.
“Thus, our initial inquiry was: why were we observing this discrepancy between X-ray diffraction and optical spectroscopy? Our second question was: how can we regulate the size and distribution of quantum wells in LHPs?”
Through a series of experiments, researchers found that nanoplatelets were pivotal in addressing both of these questions.
“Nanoplatelets are single sheets of the perovskite material that form on the surface of the solution used to create LHPs,” Amassian shares. “We discovered that these nanoplatelets act as molds for the layered materials that develop underneath them, meaning if a nanoplatelet is two atoms thick, the LHP that forms below it consists of two-atom-thick quantum wells.”
“However, nanoplatelets are not stable like the rest of the LHP material. Over time, the thickness of the nanoplatelets continues to increase, leading to the development of additional atomic layers. Thus, a nanoplatelet that is three atoms thick results in three-atom quantum wells, and this process continues until the nanoplatelet becomes a three-dimensional crystal,” Amassian explains.
This discovery also clarified the longstanding issue regarding the differing results from X-ray diffraction and optical spectroscopy: diffraction examines the stacking of layers and does not recognize nanoplatelets, while optical spectroscopy identifies isolated sheets.
“What’s exciting is that we found a method to control the growth of nanoplatelets, allowing us to adjust the size and distribution of quantum wells in LHP films,” Amassian states. “By managing the size and arrangement of the quantum wells, we can create efficient energy cascades, ensuring the material is exceptionally effective at transferring charges and energy for laser and LED applications.”
Realizing how crucial nanoplatelets were in creating perovskite layers within LHPs inspired researchers to explore their potential in engineering the structure and properties of other perovskite materials, such as those utilized in solar cells and photovoltaic technologies.
“We discovered that nanoplatelets have a similar effect on other perovskite materials and can be leveraged to enhance their structures, improving stability and photovoltaic performance,” says Milad Abolhasani, co-author of the paper and ALCOA Professor of Chemical and Biomolecular Engineering at NC State.
This research received funding from the National Science Foundation under grant 1936527 and the Office of Naval Research under grant N00014-20-1-2573.
