Researchers at MIT have developed a technique to precisely control the arrangement and placement of nanoparticles on a material, such as the silicon used in computer chips, without damaging or contaminating the material’s surface.
The technique, which combines chemistry and directed assembly processes with traditional manufacturing techniques, enables the efficient formation of high-resolution, nanoscale features integrated with nanoparticles for devices such as sensors, lasers, and LEDs that could boost their performance.
Transistors and other nanoscale devices are typically fabricated from the top down—materials are etched away to achieve the desired array of nanostructures. But fabricating the smallest nanostructures that can enable the highest performance and new functionalities requires expensive equipment and remains challenging at scale and with the desired resolution.
A more precise way to assemble nanoscale devices is bottom-up. In one scheme, engineers used chemistry to “grow” nanoparticles in solution, drip that solution onto a template, arrange the nanoparticles, and then transfer them to a surface. However, this technology also brings with it major challenges. First, thousands of nanoparticles must be efficiently assembled on the template. And their transfer to a surface typically requires a chemical adhesive, great pressure, or high temperatures that could damage the surfaces and the resulting device.
MIT researchers developed a new approach to overcome these limitations. They used the powerful forces that exist at the nanoscale to efficiently arrange particles into a desired pattern and then transfer them to a surface without chemicals or high pressure and at lower temperatures. Because the surface material remains pristine, these nanoscale structures can be incorporated into components for electronic and optical devices, where even tiny imperfections can affect performance.
“This approach allows you to place the nanoparticles, despite their very small size, in deterministic arrangements with single-particle resolution and on different surfaces by engineering forces to create libraries of nanoscale building blocks that can have very unique properties, be they theirs.” Light-matter interactions, electronic properties, mechanical performance, etc.,” says Farnaz Niroui, EE Landsman Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS) at MIT, a member of the MIT Research Laboratory of Electronics, and senior author a new paper describing the work. “By integrating these building blocks with other nanostructures and materials, we can then achieve devices with unique functionalities that would not be readily possible using traditional top-down fabrication strategies alone.”
The study is published today in scientific advances. Niroui’s co-authors are lead author Weikun “Spencer” Zhu, a graduate student in the Department of Chemical Engineering, and EECS graduate students Peter F. Satterthwaite, Patricia Jastrzebska-Perfect, and Roberto Brenes.
Use the powers
To begin their manufacturing process, known as nanoparticle contact printing, the researchers use chemistry to create nanoparticles of a defined size and shape in a solution. To the naked eye, this looks like a vial of colored liquid, but zooming in with an electron microscope would reveal millions of cubes, each just 50 nanometers in size. (A human hair is about 80,000 nanometers wide.)
Researchers then make a template in the form of a flexible surface covered with nanoparticle-sized guides, or traps, arranged in the desired shape for the nanoparticles to take on. After adding a drop of nanoparticle solution to the stencil, they use two nanoscale forces to force the particles into the correct position. The nanoparticles are then transferred to any surface.
At the nanoscale, other forces become dominant (just as gravity is a dominant force at the macroscale). Capillary forces are dominant when the nanoparticles are in liquid and van der Waals forces are dominant at the interface between the nanoparticles and the solid surface with which they are in contact. When the researchers add a drop of liquid and drag it across the template, capillary forces move the nanoparticles into the desired trap, placing them in exactly the right spot. Once the liquid dries, van der Waals forces hold these nanoparticles in place.
“These forces are ubiquitous and can often be detrimental when it comes to fabricating nanoscale objects, as they can cause the structures to collapse. But we’re able to find ways to control these powers very precisely, to use them to control how things are manipulated at the nanoscale,” says Zhu.
They design the template guides the right size and shape, and in just the right arrangement so the forces work together to line up the particles. The nanoparticles are then printed onto surfaces without the need for solvents, surface treatments, or high temperatures. This keeps surfaces pristine and properties intact while yields in excess of 95 percent are possible. In order to promote this transfer, the surface forces must be engineered so that the van der Waals forces are strong enough to consistently cause particles to detach from the template and adhere to the receiving surface when in contact to be brought.
Unique shapes, diverse materials, scalable processing
Using this technique, the team arranged nanoparticles into random shapes, such as letters of the alphabet, and then transferred them to silicon with very high positional accuracy. The method also works with nanoparticles that have other shapes, such as spheres, and with various types of materials. And it can effectively transfer nanoparticles to various surfaces such as gold or even flexible substrates for next-generation electrical and optical structures and devices.
Their approach is also scalable, so it can be extended to manufacture real devices.
Niroui and her colleagues are now working to use this approach to create even more complex structures and integrate it with other nanoscale materials to develop new types of electronic and optical devices.
This work was supported in part by the National Science Foundation (NSF) and the NSF Graduate Research Fellowship Program.