Jul 6 2015
Polymer engineers from Cornell University have created a novel mold for nanostructures which is capable of forming liquid silicon from organic polymer materials. The study, published in the July issue of Science, is a major breakthrough in nanoscience and could help develop perfect, single crystal 3D nanostructures.
Scanning electron microscopy micrographs show a periodically ordered mesoporous gyroidal resin template (A and B) and the resulting laser-induced crystalline silicon nanostructure after template removal (C and D).
Molds have long been used to shape things. During the Bronze Age, the alloy of tin and copper was first melted and then cast into weapons in ceramic-based molds. Present-day injection and extrusion molding processes mould hot liquids into a wide range of products, ranging from toys to car components. In order to make this work, it is important to make sure that the mold remains stable as the hot liquid material solidifies into shape.
The latest breakthrough came from the lab of Uli Wiesner, the Spencer T. Olin Professor of Engineering in the Department of Materials Science and Engineering, whose laboratory had earlier pioneered the development of innovative materials made of organic polymers. Organic polymers have the ability to self-organize when the right chemistry is used. The scientists leveraged this unique feature of polymers to create a mold dotted with nano-pores of an accurate shape and size.
Amorphous silicon has a melting temperature of approximately 2350 ºC. If this silicon is melted, the fragile polymer mold would be completely destroyed as it decomposes at about 600 °C.The researchers collaborated with Michael Thompson, associate professor of materials science and engineering, and resolved this problem using laser-induced short melt periods. They observed that if the silicon is heated by nanosecond laser pulses, the polymer mold appears to hold up. At this short duration, silicon can be heated and changed into a liquid, but since the duration of the melt is extremely short, the polymer does not get the time to oxidize and decompose. The research team effectively made the polymer mold to retain its shape at temperatures well over its decomposition point. Once the mold was completely etched away, the silicon was found to have been flawlessly shaped. Although this has been done at the theoretical level, the study, published in Science, shows that this is indeed possible. Wiesner and colleagues used an oxide mold to demonstrate the pathway for this process. Results of this study were published in 2010. Generally, in materials science, the main aim is achieve well-defined structures which can be examined without any interference from other material defects.
Today, a large number of self-assembled nanostructures are either polycrystalline or amorphous. As a result, it is difficult to predict whether they are governed by defects in the material or whether their characteristics are the result of the nanostructure itself. The discovery of single-crystal silicon transformed the electronics realm, and conversion of single crystals into wafers provided a better understanding of the semiconducting properties of silicon.
At present, nanotechnology enables comprehensive nanoscale etching, as low as 10nm on a single silicon wafer. However, nanofabrication methods, such as photolithography, reach their saturation limit when it comes to 3D structures. In photolithography, a polymeric material is written with a structure which is etched into the silicon.
Unlike polymers, silicon does not self-organize into perfectly ordered structures. Hence, creating a single crystal 3D structured semiconductor is a major achievement. Single crystal nanostructures can be made by using either multiple molding or etching. Wiesner’s team has created the mold, which itself marks a significant breakthrough.
The researchers already knew how to self-organize porous and highly ordered nanomaterials by means of uniquely structured molecules known as block copolymers. First, they utilized a carbon dioxide laser in Thompson’s laboratory to write the nanoporous materials onto a silicon wafer. A film that was spin-coated on the wafer included a block copolymer that directed the network of a polymer resin. When a laser was used to write the lines in the film, the block copolymer started to decompose, and functioned as a positive-tone resist. The negative-tone resin which was left behind formed the porous nanostructure, which ultimately became the mold.
"We demonstrated that we can use organic templates with structures as complicated as a gyroid, a periodically ordered cubic network structure, and ‘imprint’ it onto molten silicon, which then transforms into crystalline silicon," said Wiesner.
"Having the ability to mold the workhorse of all electronics, silicon, into intricate shapes is unprecedented. This beautiful work shows how it could be done by taking advantage of the unique design properties offered by polymeric materials," added Andy Lovinger, a program director in the materials research division at the National Science Foundation.
The paper is titled “Transient Laser Heating-Induced Hierarchical Porous Structures from Block Copolymer Directed Self-Assembly.” Kwan Wee Tan, a former graduate student in the Wiesner Lab, is the first author of the study.
The National Science Foundation funded the study, which used the research facilities at the Cornell NanoScale Science and Technology Facility and the Cornell Center for Materials Research.