When you shop for apples in a grocery store, you often find that they pack into towers on the shelf, following one closest packing symmetry of spheres. What if you are looking at the packing of non-spherical, anisotropic objects? What if the objects are ultra-tiny, so tiny that their individual self has quantum-confined properties, which will then crosstalk depending on their packing symmetry? Researchers at the University of Illinois at Urbana-Champaign found that by modulating the interactions governing the packing, they obtained polymorphic, instead of a single type, antenna arrays with distinct “hot” spot patterns out of the same, tiny triangular nanoprism building blocks.
The research team led by Materials Science and Engineering assistant professor Qian Chen assembled gold nanoprism building blocks in water, automatedly with a special “glue” named “depletion attraction.” “This is like playing with Legos,” Chen said, “depletion attraction also relies on shape complementarity to attach building blocks together. This attraction is highly tunable. Getting it to work for non-spherical nanotriangles generates different packing geometries all from the same elementary units at high precision.”
Dr. Juyeong Kim, a post-doctoral research associate in Chen’s group, said that conventionally, people think triangles pack into honeycomb lattices, side-by-side within the same plane. “The prisms we used are beveled, with a slight facet enclosing detail at the sides”, Kim said, “we are able to amplify this nanometer facet detail and induce assembled structures of new packing symmetries up to hundreds of micrometers in size, which has not been obtained before. This ‘amplification’ concept can generalize to other systems.”
The polymorphic assemblies sitting on wafers have distinct “hot” spot patterns that concentrate electromagnetic fields locally. They are great candidates for applications in near-field optical imaging, meta-materials or diagnostic-related sensing. For example, one of the unexpected packing greatly boosts the device performance for detecting organic molecules. “There is a new interlocking geometry which maximizes prism contacting and electromagnetic coupling,” said Dr. Xiaohui Song in the Chen group, “this allows five-fold more sensitive detecting of organic molecules than conventional nanoantenna arrays.”
Kim also brought attention to Dr. Nicole F. Ice’s involvement in this project. Ice is a high school mathematics teacher; not a position you’d normally associate with engineering nanoprism building blocks. She came to learn nanoscience in our group through the nano@illinois Research Experiences for Teachers, or RET project, supported by National Science Foundation, last summer and has been instrumental in this research,” Kim said. “She is part of a very special STEM school in Georgia and plans to incorporate the creation of nanoparticles into her coursework for high school students.”
Read “Polymorphic Assembly from Beveled Gold Triangular Nanoprisms” published in Nano Letters.
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