A rendering of the emission from a nanocrystal array produced by the new technique. Image: Sampson?Wilcox, MIT RLE.
A rendering of the emission from a nanocrystal array produced by the new technique. Image: Sampson?Wilcox, MIT RLE.

Halide perovskites are a family of materials that have attracted attention for their superior optoelectronic properties and potential applications in devices such as high-performance solar cells, light-emitting diodes and lasers.

So far, these materials have largely been implemented for thin-film or micron-sized device applications. Precisely integrating these materials at the nanoscale could open up even more remarkable applications, like on-chip light sources, photodetectors and memristors. However, achieving this integration has remained challenging because this delicate material can be damaged by conventional nanofabrication and patterning techniques.

To overcome this hurdle, researchers at Massachusetts Institute of Technology (MIT) have created a technique that allows individual halide perovskite nanocrystals to be grown on-site exactly where they are needed, to within less than 50nm. The size of the nanocrystals can also be precisely controlled through this technique, which is important because their size affects their characteristics. Since the material is grown locally with the desired features, conventional lithographic patterning steps that could introduce damage are not needed.

The technique is also scalable, versatile and compatible with conventional fabrication steps, allowing the nanocrystals to be integrated into functional nanoscale devices. The researchers used the technique to fabricate arrays of nanoscale light-emitting diodes (nanoLEDs) — tiny crystals that emit light when electrically activated. Such arrays could have applications in optical communication and computing, lensless microscopes, new types of quantum light sources, and high-density, high-resolution displays for augmented and virtual reality.

“As our work shows, it is critical to develop new engineering frameworks for integration of nanomaterials into functional nanodevices,” says Farnaz Niroui, an assistant professor of electrical engineering and computer science (EECS) at MIT, a member of the Research Laboratory of Electronics (RLE), and senior author of a paper on this work in Nature Communications. “By moving past the traditional boundaries of nanofabrication, materials engineering and device design, these techniques can allow us to manipulate matter at the extreme nanoscale dimensions, helping us realize unconventional device platforms important to addressing emerging technological needs.”

Integrating halide perovskites into on-chip nanoscale devices is extremely difficult using conventional nanoscale fabrication techniques. In one approach, a thin film of fragile perovskites may be patterned using lithographic processes, but this requires solvents that can damage the material. In another approach, smaller crystals are first formed in solution and then extracted from the solution and placed in 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 says.

Instead, she and her team developed an approach for ‘growing’ halide perovskite crystals in precise locations directly onto the desired surface where the nanodevice will then be fabricated.

Central to their process is localizing the solution used for nanocrystal growth. To do this, they create a nanoscale template with small wells that house the chemical process through which crystals grow. They modify the surface of the template and the inside of the wells to control a property known as ‘wettability’. This ensures that a solution containing the perovskite material won’t pool on the template surface and will be confined inside the wells.

“Now, you have these very small and deterministic reactors within which the material can grow,” Niroui explains.

And that is exactly what happens. The researchers apply a solution containing halide perovskite growth material to the template and, as the solvent evaporates, the material grows and forms a tiny crystal in each well.

The researchers found that the shape of the wells plays a critical role in controlling the nanocrystal positioning. If square wells are used, the crystals have an equal chance of being placed in each of the well’s four corners, due to the influence of nanoscale forces. For some applications, that might be good enough, but for others it is necessary to have a higher precision in the nanocrystal placement.

By changing the shape of the well, the researchers were able to engineer the nanoscale forces in such a way that a crystal would preferentially grow at the desired location. As the solvent evaporates inside the well, the nanocrystal experiences a pressure gradient that creates a directional force, with the exact direction being determined by the well’s asymmetric shape.

“This allows us to have very high precision, not only in growth, but also in the placement of these nanocrystals,” Niroui says.

The researchers also found they could control the size of the crystal that forms inside a well. Changing the size of the wells to allow more or less growth solution inside generates larger or smaller crystals.

They demonstrated the effectiveness of their technique by fabricating precise arrays of nanoLEDs. In this approach, each nanocrystal is made into a nanopixel that emits light. These high-density nanoLED arrays could be used for on-chip optical communication and computing, quantum light sources, microscopy, and high-resolution displays for augmented and virtual reality applications.

In the future, the researchers want to explore more potential applications for these tiny light sources. They also want to test the limits of how small these devices can be, and work to effectively incorporate them into quantum systems. Beyond nanoscale light sources, this process also opens up other opportunities for developing halide-perovskite-based on-chip nanodevices.

Their technique also provides an easier way for researchers to study materials at the individual nanocrystal level, which they hope will inspire others to conduct additional studies on these and other unique materials.

“Studying nanoscale materials through high-throughput methods often requires that the materials are precisely localized and engineered at that scale,” adds Patricia Jastrzebska-Perfect, an EECS graduate student and lead author of the paper. “By providing that localized control, our technique can improve how researchers investigate and tune the properties of materials for diverse applications.”

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.