Innovation in optoelectronic nanotechnology: MIT grows precise nano-LED arrays

A new technique can produce perovskite nanocrystals exactly where they are needed, so this extremely delicate material can be integrated into nanoscale devices. MIT researchers have developed a breakthrough method to precisely grow halide perovskite nanocrystals, eliminating the need for disruptive manufacturing techniques. The technique could help develop nanoLEDs and other functional nanoscale devices with the potential for advances in optical communications, computing, and high-resolution display technologies.

A new platform at MIT enables researchers to "grow" halide peritectic nanocrystals and precisely control the position and size of each crystal to integrate them into nanoscale light-emitting diodes. The picture shows the light-emitting effect of the nanocrystal array. Image credit: Sampson Wilcox, RLE

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

These materials have been primarily used in thin-film or micrometer-sized device applications. Precise integration of these materials at the nanoscale could open up more extraordinary applications, such as on-chip light sources, photodetectors, and memristors. However, achieving such integration remains challenging because such delicate materials can be damaged by conventional fabrication and patterning techniques.

To overcome this obstacle, MIT researchers have invented a technique that can grow individual halide perovskite nanocrystals on-site, exactly where they are needed, with precise control of location and size to within 50 nanometers. (A sheet of paper is 100,000 nanometers thick) The size of the nanocrystals can also be precisely controlled with this technique, which is important because size affects their properties. Because the material is grown locally with the desired features, there is no need for traditional photolithography patterning steps that can cause damage.

Nan OLED arrays (shown here) could have applications in optical communications and computing, lens-free microscopy, new quantum light sources, and high-density, high-resolution displays for augmented and virtual reality. Image credit: Provided by researchers

The technique is also scalable, versatile, and compatible with conventional manufacturing steps, so it can enable the integration of nanocrystals into functional nanoscale devices. The researchers used it to create arrays of nanoscale light-emitting diodes (nanoLEDs), tiny crystals that emit light when electrically activated. Such arrays could have applications in optical communications and computing, lens-free microscopy, new 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 to integrate nanomaterials into functional nanodevices. By pushing beyond the traditional boundaries of nanofabrication, materials engineering, and device design, these techniques can allow us to manipulate matter at the extreme nanoscale, helping us realize unconventional device platforms that are important for addressing emerging technology needs,” said Landsman, Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS), a 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, a graduate student in EECS; Weikun Zhu, a graduate student in chemical engineering; Mayuran Saravanapavanantham, Sarah Spector, Roberto Brenes and Peter Satterthwaite, all graduate students in EECS; Li Zheng, a postdoc in RLE; and Rajeev Ram, a professor of electrical engineering. The study was published July 6 in the journal Nature Communications.

Tiny crystals, big challenges

Integrating halide perovskites into on-chip nanoscale devices is extremely difficult using conventional nanoscale manufacturing techniques. In one approach, fragile perovskite films can be patterned using a photolithography process that requires solvents that can damage the material. In another approach, smaller crystals are first formed in a solution and then picked up from the solution and placed in the desired pattern.

"Both of these cases suffer from a lack of control, resolution and integration capabilities, which limits the way the material can be scaled up to nanoscale devices," Niroui said. Instead, she and her team developed a method to "grow" halide perovskite crystals directly onto a desired surface at precise locations and then fabricate nanodevices on that surface.

The core of their process is to localize the solutions used in the growth of nanocrystals. To do this, they create a nanoscale template with tiny holes that contain the chemistry for the crystal growth. They modify the surface of the template and the interior of the holes, controlling a property called "wettability" so that the solution containing the perovskite material will not accumulate on the template surface and will be confined within the holes.

“Now you have these very small, deterministic reactors where materials can grow,” she said. They apply a solution containing the halide perovskite growth material to the template, and as the solvent evaporates, the material grows and forms tiny crystals in each hole.

A versatile and adaptable technology

The researchers found that the shape of the hole plays a key role in controlling the placement of the nanocrystal. If a square hole is used, the crystal has an equal chance of being placed in the four corners of the hole due to the influence of nanoscale forces. For some applications, this may be sufficient, but for others, the placement of the nanocrystal requires greater precision.

By varying the shape of the pores, the researchers were able to engineer these nanoscale forces to preferentially place the crystals in desired locations. As the solvent evaporates within the pores, the nanocrystals experience pressure gradients that create directional forces, with the exact direction determined by the asymmetric shape of the pores. "This allows us to have very high precision not only in the growth, but also in the placement of these nanocrystals," Niroui said.

They also found that they could control the size of the crystals that formed inside the wells. Changing the size of the holes to allow more or less growth solution inside produced larger or smaller crystals. They demonstrated the effectiveness of their technique by making precise nanoLED arrays. In this approach, each nanocrystal was made into a light-emitting nanopixel. These high-density nanoLED arrays could be used for on-chip optical communications and computing, quantum light sources, microscopy, and high-resolution displays for augmented and virtual reality applications.

In the future, the researchers hope to explore more potential applications for these tiny light sources. They also want to test the limits of these devices and work to efficiently integrate them into quantum systems. In addition to nanoscale light sources, this process opens up other opportunities for developing on-chip nanodevices based on halide perovskites.

Their technique also provides researchers with an easier way to study materials at the level of individual nanocrystals, which they hope will inspire others to do more research on these and other unique materials.

Jastrzebska-Perfect added: "Studying materials at the nanoscale with high-throughput methods often requires that the materials be precisely positioned and designed at that scale. By providing local control, our technique could improve the way researchers study and tune material properties for different applications."

“The team has developed a very clever method to deterministically synthesize individual perovskite nanocrystals on a substrate. They can control the precise placement of the nanocrystals at an unprecedented scale, providing a platform for making highly efficient nanoscale LEDs based on single nanocrystals,” said Ali Javey, a professor of electrical engineering and computer science at the University of California, Berkeley, who was not involved in the research. “This is exciting work because it overcomes fundamental challenges in the field.”