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New Method to Assemble Arrays of Quantum Rods

Flat-screen TVs that integrate quantum dots are commercially available. However, making arrays of their extended cousins, quantum rods, has been highly challenging for commercial devices.

New Way to Assemble Arrays of Quantum Rods

MIT engineers have used DNA origami scaffolds to create precisely structured arrays of quantum rods, which could be incorporated into LEDs for televisions or virtual reality devices. Image Credit: Dr. Xin Luo, Bathe BioNanoLab

Quantum rods have the potential to regulate both the polarization and color of light to produce 3D images available for virtual reality devices.

Utilizing scaffolds comprised of folded DNA, Massachusetts Institute of Technology (MIT) engineers have developed a new approach to accurately assemble arrays of quantum rods.

By depositing quantum rods onto a DNA scaffold in a highly regulated manner, scientists could control their orientation. This is a main factor in identifying the polarization of light emitted by the array. This makes it simpler to add dimensionality and depth to a virtual scene.

One of the challenges with quantum rods is: How do you align them all at the nanoscale so they’re all pointing in the same direction?. When they’re all pointing in the same direction on a 2D surface, then they all have the same properties of how they interact with light and control its polarization.

Mark Bathe, Study Senior Author and Professor, Biological Engineering, Massachusetts Institute of Technology

The study’s lead authors are MIT postdocs Chi Chen and Xin Luo, and the study is published in Science Advances. Robert Macfarlane, an associate professor of materials science and engineering; Alexander Kaplan Ph.D., and Moungi Bawendi, the Lester Wolfe Professor of Chemistry, are also authors of the study.

Nanoscale Structures

For the past 15 years, Bathe and others have headed the fabrication and design of nanoscale structures made of DNA, also called DNA origami.

DNA is a highly stable and programmable molecule and is also a perfect building material for small structures that can be utilized for a range of applications, such as delivering drugs, serving as biosensors, or developing scaffolds for light-harvesting materials.

Bathe’s laboratory has come up with computational techniques that enable scientists to just enter a target nanoscale shape they wish to make, and the program will assess the sequences of DNA that will self-assemble into the correct shape. Also, they came up with scalable fabrication techniques that integrate quantum dots into such DNA-based materials.

In a 2022 study utilizing scalable biological fabrication Bathe and Chen displayed that they can utilize DNA to scaffold quantum dots in accurate positions. Constructing on that work, they paired with Macfarlane’s lab to handle the difficulty of organizing quantum rods into 2D arrays, which is highly challenging as the rods require to be lined up in the same direction.

The present method that is responsible for making aligned arrays of quantum rods utilizing mechanical rubbing with an electric field or a fabric to sweep the rods into a single direction has just had limited success.

This is because high-efficiency light-emission needs the rods to be kept at a minimum of 10 nm from each other so that they would not “quench,” or repress, their neighbors’ light-emitting activity.

For that to be obtained, the scientists devised a method to fix quantum rods to diamond-shaped DNA origami structures. This could be constructed at the correct size to retain that distance. Furthermore, such DNA structures are fixed to a surface, where they fit collectively like puzzle pieces.

The quantum rods sit on the origami in the same direction, so now you have patterned all these quantum rods through self-assembly on 2D surfaces, and you can do that over the micron scale needed for different applications like microLEDs.

Mark Bathe, Study Senior Author and Professor of Biological Engineering, Massachusetts Institute of Technology

Bathe added, “You can orient them in specific directions that are controllable and keep them well-separated because the origamis are packed and naturally fit together, as puzzle pieces would.”

Assembling the Puzzle

As the first step in getting this method to work, the scientists developed a method to fix DNA strands to the quantum rods. For that to be performed, Chen came up with a process that includes emulsifying DNA into a mixture with the quantum rods, then quickly dehydrating the mixture, thereby enabling the DNA molecules to develop a dense layer on the rods’ surface.

This process just takes a few minutes, much faster compared to any current technique for fixing DNA to nanoscale particles, which might be the main to allowing commercial applications.

Chen stated, “The unique aspect of this method lies in its near-universal applicability to any water-loving ligand with affinity to the nanoparticle surface, allowing them to be instantly pushed onto the surface of the nanoscale particles. By harnessing this method, we achieved a significant reduction in manufacturing time from several days to just a few minutes.”

Such DNA strands then act like Velcro, assisting the quantum rods to adhere to the DNA origami template, which develops a thin film that coats a silicate surface. Initially, this thin film of DNA is formed through self-assembly by joining neighboring DNA templates collectively through overhanging strands of DNA together at their edges.

Currently, scientists believe to make wafer-scale surfaces with etched patterns, which could enable them to scale their design to device-scale arrangements of quantum rods for several applications, beyond just micro LEDs or virtual reality or augmented reality.

The method that we describe in this paper is great because it provides good spatial and orientational control of how the quantum rods are positioned. The next steps are going to be making arrays that are more hierarchical, with programmed structure at many different length scales. The ability to control the sizes, shapes, and placement of these quantum rod arrays is a gateway to all sorts of different electronics applications,” stated Macfarlane.

DNA is particularly attractive as a manufacturing material because it can be biologically produced, which is both scalable and sustainable, in line with the emerging U.S. bioeconomy. Translating this work toward commercial devices by solving several remaining bottlenecks, including switching to environmentally safe quantum rods, is what we’re focused on next.

Mark Bathe, Study Senior Author and Professor of Biological Engineering, Massachusetts Institute of Technology

The study was financially supported by the Office of Naval Research, the National Science Foundation, the Army Research Office, the Department of Energy, and the National Institute of Environmental Health Sciences.

Journal Reference

Chen, C., et al. (2023) Ultrafast dense DNA functionalization of quantum dots and rods for scalable 2D array fabrication with nanoscale precision. Science Advances.

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