Halide perovskites are a group of substances that have drawn attention due to their outstanding optoelectronic characteristics and their potential use in devices like high-performance solar cells and light-emitting diodes and lasers.
They have been largely integrated into micron-sized or thin-film devices. Integrating these materials precisely on a nanoscale may allow for even more impressive applications like on-chip photodetectors, light sources, and memristors. However, the process of integrating these materials is a challenge because the delicate material is susceptible to being damaged by traditional fabrication and patterning methods.
To conquer this challenge, MIT researchers created a method that permits individual nanocrystals of halide to be produced on-site, in the exact locations required, with precise control of their location and size, with a precision of just 50 nm. (A sheet of paper is 100,000 nanometers in thickness.) The size of nanocrystals can be controlled precisely by this method, which is vital since it affects their properties. Since the material is produced locally to the desired characteristics, The traditional lithographic patterns that can cause damage are not required.
This technique can be scalable, flexible, and compatible with standard fabrication techniques, allowing the nanocrystals to be used in functional nanoscale devices. Researchers employed this technique to create arrays of nanoscale light-emitting diodes (nanoLEDs), tiny crystals that emit light when activated. These arrays may have applications in optical communications and computing lenses, lensless microscopes, novel kinds of quantum sources for light, as well as high-density, high-resolution displays that can be used for virtual and augmented reality.
“As our research demonstrates that it is crucial to design new engineering frameworks to facilitate the incorporation of nanomaterials into functional nanodevices. By extending the traditional limits of nanofabrication, engineering of materials and design of devices These techniques will allow researchers to alter matter in most extreme nanoscales, assisting us develop novel device platforms that are crucial to address current technological requirements,” says Farnaz Niroui of one of the EE Landsman Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS) and part of the Research Laboratory of Electronics (RLE) and the lead author of a paper detailing the research.
Niroui’s co-authors include the lead author Patricia Jastrzebska -Perfect Patricia Jastrzebska-Perfect, the EECS master’s student. Weikun “Spencer” Zhu is a graduate student in the Department of Chemical Engineering; Mayuran Saravanapavanantham Sarah Spector, Roberto Brenes as well as Peter Satterthwaite, all EECS graduate students; Zheng Li, an RLE postdoc and Rajeev Ram, a professor in electrical engineering. The study was published in Nature Communications.
Tiny crystals, enormous challenges
Integrating halide perovskites in on-chip nanoscale devices is very difficult with conventional nanoscale fabrication methods. One approach is that thin films of delicate perovskites can be patterned with the lithographic process, which requires solvents that could harm the perovskite. In another method, smaller crystals are created in the solution, then retrieved and arranged from the solution to form the desired pattern.
“In both cases there is a lack of control, resolution, and integration capability, which limits how the material can be extended to nanodevices,” Niroui states.
Instead, the team devised a way to “grow” halide perovskite crystals precisely on the desired surface. The nanodevice is then fabricated.
Their method’s primary goal is pinpointing the solution used for the growth of nanocrystals. To achieve this, they design the nanoscale template using small wells that hold the chemical process by which crystals form. They alter both the surfaces of the templates and those inside the wells, regulating the property called “wettability” so a solution composed of perovskite does not gather on the surface of the template and remains inside the wells.
“Now, you have these very small and deterministic reactors within which the material can grow,” she explains.
This is precisely what happens. A mixture of the growth material halide perovskite is applied to the template. When the solvent evaporates, the growth material expands, creating a tiny crystal within each well.
A versatile and flexible technique
The study found that the form of the wells is crucial in determining the nanocrystal’s positioning. When square wells are utilized because of the impact of nanoscale effects, these crystals will have the same possibility of being placed at each of the four corners. In some cases, this may be enough; however, for other applications, it is essential to attain greater precision for the nanocrystal placement.
By altering its shape, researchers created these nanoscale forces so that crystals are placed in the exact location.
When the solvent evaporates within the well, the crystal is subjected to the pressure gradient, which creates the force in a direction that is precisely determined by the shape of the well’s asymmetrical.
“This allows us to have very high precision, not only in growth, but also in the placement of these nanocrystals,” Niroui states.
They also discovered they can regulate how big the crystal that develops within a well. The size of wells to allow for more or less growth solutions produces crystals with larger or smaller sizes.
They have demonstrated the effectiveness of their method by creating precisely designed arrays of nanoLEDs. This method makes each nanocrystal transform into a nanopixel, which emits light. NanoLED arrays with high density could be used to enable optical communication on chips and for computing quantum light sources microscopy, as well as high-resolution displays for virtual and augmented real-world applications.