(Nanowerk News) Perovskite Halides are a family of materials that have attracted attention due to their superior optoelectronic properties and potential applications in devices such as high-performance solar cells, light-emitting diodes and lasers.
These materials have largely been implemented into micron- or thin-film device applications. Precisely integrating these materials at the nanoscale could open up even more extraordinary applications, such as in-chip light sources, photo detectors and memristors. However, achieving this integration remains challenging because these delicate materials can be damaged by conventional fabrication techniques and patterns.
To overcome this hurdle, MIT researchers created a technique that allows individual perovskite halide nanocrystals to be grown in place when needed with precise location control, down to less than 50 nanometers. (A sheet of paper is 100,000 nanometers thick.) The size of the nanocrystals can also be precisely controlled through this technique, which is important because size affects their characteristics. Since the material is grown locally with the desired features, the damaging steps of conventional lithographic patterns are unnecessary.
This technique is also scalable, versatile and compatible with conventional fabrication steps, thereby enabling nanocrystals to be integrated into nanoscale functional devices. The researchers used this to create nanoscale light emitting diode (nanoLED) arrays – tiny crystals that emit light when electrically activated. Such arrays could have applications in communications and optical 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 the integration of nanomaterials into functional nanodevices. By moving beyond the traditional boundaries of nanofabrication, materials engineering, and device design, these techniques allow us to manipulate materials at extreme nanoscale dimensions, helping us realize unconventional device platforms that are important to meet emerging technological needs,” said Farnaz Niroui , EE Assistant Career Development Landsman Professor of Electrical Engineering and Computer Science (EECS), member of the Research Laboratory of Electronics (RLE), and senior author of a new paper describing the work.
Niroui’s co-authors include lead author Patricia Jastrzebska-Perfect, an EECS graduate student; Weikun “Spencer” Zhu, a graduate student in the Department of Chemical Engineering; Mayuran Saravanapavanantham, Sarah Spector, Roberto Brenes, and Peter Satterthwaite, all EECS graduate students; Zheng Li, RLE postdoc; and Rajeev Ram, professor of electrical engineering.
Research published in Nature Communications (“Growth of on-site perovskite nanocrystal arrays for integrated nanodevices”).
Small crystals, big challenges
Integrating halide perovskites into on-chip nanoscale devices is extremely difficult using conventional nanoscale fabrication techniques. In one approach, thin layers of brittle perovskite can be patterned using the lithographic process, which requires solvents that can damage the material. In another approach, smaller crystals are first formed in solution and then removed and placed from the solution in the desired pattern.
“In both cases there is a lack of control, resolution and integration capabilities, which limit how materials can be extended to nanodevices,” said Niroui.
Instead, he and his team developed an approach for “growing” perovskite halide crystals in precise locations directly onto the desired surface where the nanodevices would then be fabricated.
The essence of their process is to localize the solutions used in growing the nanocrystals. To do so, they fabricated a nanoscale template with tiny wells containing the chemical process by which the crystals grow. They modified the surface of the template and the interior of the well, controlling a property known as “wettability” so that solutions containing perovskite materials would not pool on the surface of the template and would be confined to the well.
“Now, you have a very small, deterministic reactor where matter can grow,” he said.
And that’s what happened. They applied a solution containing perovskite halide growth material to the template and, as the solvent evaporated, the material grew and formed tiny crystals in each well.
Versatile and melodious technique
The researchers found that the shape of the wells plays an important role in controlling the position of the nanocrystals. If a square well is used, due to the influence of nanoscale forces, crystals have an equal chance of being placed in each of the four corners of the well. For some applications, that may be good enough, but for others, greater precision in nanocrystal placement is required.
By changing the shape of the wells, the researchers were able to engineer nanoscale forces in such a way that the crystals were preferentially placed in the desired locations.
As the solvent evaporates in the well, the nanocrystals experience a pressure gradient that creates a directional force, with the exact direction determined using the asymmetric shape of the well.
“This allowed us to have very high precision, not only in the growth, but also in the placement of these nanocrystals,” said Niroui.
They also discovered that they could control the size of the crystals that formed in the well. Changing the size of the well to allow more or less growth solution in it results in larger or smaller crystals.
They demonstrated the effectiveness of their technique by fabricating precise nanoLED arrays. In this approach, each nanocrystal is made into a light-emitting nanopixel. These high-density nanoLED arrays can be used for on-chip optical communications and computing, quantum light sources, microscopy, and high-resolution displays for augmented reality 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 perovskite halide-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 via high throughput methods often requires that the materials be precisely localized and engineered at the scale,” adds Jastrzebska-Perfect. “By providing such local control, our technique can improve the way researchers investigate and adapt materials properties for various applications.”