Summer Scholar Justin Cheng explores process in Berggren group for making ordered metal nanostructures that display interesting new properties.
|Summer Scholar Justin Cheng holds an experimental sample of nanostructured gold on silicon that has potential for use in sensors and display technologies based on the selective light absorption properties of the material that arise from plasmonic excitations on the surface of the gold. Photo, Maria E. Aglietti.|
Ordered patterns of gold nanoparticles on a silicon base can be stimulated to produce collective electron waves known as plasmons that absorb only certain narrow bands of light making them promising for a wide array of sensors and display technologies in medicine, industry and science.
MPC-CMSE Summer Scholar Justin Cheng worked this summer in Professor of Electrical Engineering Karl K. Berggren’s Quantum Nanostructures and Nanofabrication Group to develop specialized techniques for forming these patterns in gold on silicon. “Ideally, we’d want to be able to get arrays of gold nanoparticles to be completely ordered,” Cheng says.
“My work deals with the fundamentals of how to write a pattern using electron-beam lithography, how to deposit the gold, and how to heat up the substrate so we can get completely regular arrays of particles,” Cheng explains.
Cheng wrote code to produce a pattern that will guide the dewetting of a thin gold film into nanoparticles, examined partially ordered grids with an electron microscope, and worked in a clean room to spin-coat polymer resist onto samples, develop the resist, and plasma clean samples. He is part of a team that includes graduate student Sarah Goodman, postdoctoral associate Mostafa Bedewy, and he was assisted by Research Specialist James Daley, who is the lab manager in MIT’s NanoStructures Laboratory, where this work was performed.
“Plasmons are collective oscillations of the free-electron density at the surface of a material, and they give metal nanostructures amazing properties that are very useful in applications like sensing, optics and various devices,” Goodman explained in a presentation to Summer Scholars in June. “Plasmonic arrays are very good for visible displays, for example, because their color can be tuned based on size and geometry.”
This multi-step fabrication process begins with spin coating hydrogen silsesquioxane [HSQ], which is a special electron-beam resist, or mask, onto a silicon substrate. Cheng worked on software used to write a pattern onto the resist through electron-beam lithography. Unlike some resists, HSQ becomes more chemically resistant as you expose it to electron beams, he says. The entire substrate is about 1 cm by 1 cm (0.39 inch x 0.39 inch), he notes, and the write area is about 100 microns (0.0039 inch) wide.
After the electron-beam lithography step, the resist is put through an aqueous [water-based] developer solution of sodium hydroxide and sodium chloride, which leaves behind an ordered array of posts on top of the silicon layer. “When we put the sample in the developer solution, all of the less chemically resistant areas of the HSQ mask come off, and only the posts remain,” Cheng says. Then, Daley deposits a gold layer on top of the posts with physical vapor deposition. Next, the sample is heat treated until the gold layer decomposes into droplets that self-assemble into nanoparticles guided by the posts.
A key underlying materials science phenomenon at work in this self-assembly, Cheng says, is known as solid-state dewetting. “Self-assembly is a process where you apply certain conditions to a material that allow it to undergo a transformation over a large area. So it’s a very efficient patterning technique,” Goodman explains.
|Summer Scholar Justin Cheng inserts a sample into a scanning electron microscope in the NanoStructures Laboratory. Cheng explored processes for making ordered metal nanostructures that display interesting new properties. Photo, Maria E. Aglietti.|
Because of repulsive interaction between the silicon and gold layers, the gold tends to form droplets, which can be coaxed into patterns around the posts. The Berggren group is working collaboratively with Carl V. Thompson, the Stavros Salapatas Professor of Materials Science and Engineering, who is an expert in solid-state dewetting. Thompson also is director of the Materials Processing Center.Using a scanning electron microscope, Cheng examines these patterns to determine their quality and consistency. “The gold naturally forms droplets because there is a driving force for it to decrease the surface area it shares with the silicon. It doesn’t look completely ordered but you can see beginnings of some order in the dewetting,” he says while showing an SEM image on a computer. “[In] other pictures you can clearly see the beginnings of patterning.”
“When we take the posts and we make them closer together, you can see that the gold likes to dewet into somewhat regular patterns. These aren’t completely regular in all cases, but for certain post sizes and spacings, we start to see regular arrays.
Our goal is to successfully fabricate a plasmonic array of ordered, monodisperse [equally sized] gold nanoparticles,” Cheng explains.
Goodman notes that the group of Carl V. Thompson, the Stavros Salapatas Professor of Materials Science and Engineering at MIT, has demonstrated exquisite control over dewetting in single crystalline films at the micron scale, but the Berggren group hopes to extend this control down to the nanoscale. “This will be a really key result if we’re able to bring this dewetting that’s beautifully controlled on the micro scale and enable that on the nanoscale,” Goodman says.
Cheng, a rising senior at Rutgers University, says that during his summer internship in Berggren’s lab, he learned to operate the scanning electron microscope and learned about nanofabrication processes. “I have learned a lot. Aside from the lab work that I’m doing, I’ve been scripting for the [LayoutEditor] CAD program that I use, and I’ve been using Matlab, too,” he says. “I actually learned a lot about image analysis because there are a lot of steps that go into image analysis. Since we have so much data and so many images to analyze, I’m doing it quantitatively and automatically to make sure I have repeatability.”
MPC and CMSE sponsor the nine-week National Science Foundation Research Experience for Undergraduates (NSF REU) internships with support from NSF’s Materials Research Science and Engineering Centers program (grant number DMR-14-19807). The program ran from June 7 through Aug. 6, 2016.
– Denis Paiste, Materials Processing Center | Aug. 22, 2016