Fabrication of perfect silicon nanostructures

Polymer engineers have succeeded in developing a template for nanostructures that allows the production of liquid silicon from an organic polymer material. The research findings could lead to the development of three-dimensional and perfect nanostructures in the form of single crystals.

[Translation by Dr. Nachmani Moshe]
Polymer engineers have succeeded in developing a template for nanostructures that allows the production of liquid silicon from an organic polymer material. The research findings could lead to the development of three-dimensional and perfect nanostructures in the form of single crystals.

The use of templates to design defined shapes is as old as humanity itself. In the Bronze Age, for example, a copper-tin alloy was melted and then cast into weapons in ceramic molds. Today, injection and extrusion methods make it possible to shape shapes from hot liquids into almost any imaginable object, from car parts to toys. In order for these methods to work properly, it is necessary for the mold itself to be stable while the molten liquid material hardens into the required final shape. In a breakthrough revealed by polymer engineers from Cornell University, they were able to develop a template for nanostructures capable of forming liquid silicon from a material made of an organic polymer. The findings could pave the way for the development of perfect XNUMXD nanostructures for the production of single crystals.

The progress in this area comes from the laboratory of researcher Uli Wiesner, a professor of engineering in the Department of Materials Science and Engineering at Cornell University, the same laboratory that previously developed innovative materials made from organic polymers. With the right chemistry, organic polymers self-organize, and the researchers used this unique ability of the polymers to develop a pattern consisting of nano-nozzles of extremely precise shapes and sizes.

Normally, the melting of amorphous silicon, which has a melting point of about 2350 degrees Celsius, will lead to the destruction of the fragile polymer mold, which decomposes already at a temperature of 600 degrees Celsius. However, the scientists circumvented this problem by using extremely short melting times based on the use of a laser. The researchers discovered that the polymer pattern remains stable if the silicon is heated using laser pulses with durations of only nanoseconds. In such short periods of time, the silicon can be heated and turned into a liquid, but the time it is in the form of a fuse is so short that the polymer (the template) does not have enough time to oxidize and break down. In fact, they "tricked" the polymer template so that it would keep its shape even at a temperature above its decomposition point. After the mold was removed, the researchers showed that the silicone molded perfectly into the shapes inside the mold. Perfection of the method could lead to the production of perfect nanostructures of silicon in a single crystal configuration. The research findings were published in the prestigious scientific journal Science. The lead researcher called the breakthrough "beautiful" and claimed that it may be a fundamental insight into the field of nanomaterials research. In the field of materials science, the goal is always to obtain defined structures—especially those that can be studied without fearing the interference of structural defects in the material.

Most self-organized nanostructures are currently obtained in amorphous or polycrystalline form - that is, they include more than one single type of material configuration. Because of this, it is very difficult to determine if the properties of these materials arise from the nanostructure itself or if they are at all controlled by the defects within the material. The discovery of monocrystalline silicon - the semiconductor found today in every integrated circuit - brought about a revolution in the field of electronics. It was only after the single crystals were sliced ​​into thin slices that the semiconducting property of silicon was properly understood. Today, nanotechnological methods enable detailed and precise notching in silicon wafers on a nanometric scale, up to the level of 10 nanometers. At the same time, methods for nanoscale production, for example photo-lithography, in which a polymeric material shapes the shape of the silicon, encounter pitfalls when it comes to XNUMXD structures.

Semiconductor materials, such as silicon, do not self-organize into perfectly ordered structures, as occurs with polymers. It is almost impossible to obtain a XNUMXD single crystal from a semiconductor material. In order to obtain nanostructures of single crystals, there are two options: performing a large number of incisions or designing within a template. This research group has now found a way to use the template. The way they made the template was a breakthrough of its own - previously they had learned how to cause the independent organization of specially ordered porous nanomaterials, using molecules known as block copolymers. In the first step, they used a carbon dioxide laser to embed the nanoporous materials inside the silicon wafer. On top of the surface of this slice, block co-polymers were incorporated that directed the self-assembly of polymer resin. In the next step, the block copolymers that shaped the nanostructures were removed from the substrate creating a porous nanostructure that serves as a template. In the next step, the liquid silicon was introduced into these nanostructures, and after cooling it turned into crystalline silicon. "The ability to shape the basic material in the field of electronics, silicon, into any shape we want, is unprecedented," explains one of the researchers.

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