Nanotechnology: the material of the future? by Jenn Zhao

Imagine a cassette that played for weeks rather than hours, a device that specifically collected antibodies, or a surface that could be used to precisely build polymers. These are visions shared by the Cornell nanofabrication research team lead by Professor Stephen Sass (Dept. of Material Sciences and Engineering) and Associate Professor Melissa Hines (Dept. of Chemistry and Chemical Biology).

Figure 1: The process is based on the production of a "twist-bonded bicrystal" which is formed by bonding a very thin silicon crystal (represented in blue) to a thick silicon crystal (represented in grey.) In this process, the two crystals are purposefully misoriented by an angle theta. Stephen Sass and Melissa Hines, Cornell. © Cornell University
The research team developed a method to create periodic nano-structures called "nanobumps" on silicon wafers. Currently, nano-structures are made by a process called lithography in which visible light plays an important role. Because of the relatively long wavelength of light, the size of the structures that lithography can produce is limited. The smallest lateral feature produced by lithography available today is about 150 nm, but Cornell's research team currently nanobumps with spacings less than 38 nm - far beyond the ability of lithography.
These nanobumps are created by selective etching of twist bonded bicrystals. A thin wafer of silicon is placed on a thicker wafer of the same size as in Figure 1.

The thinner one is then twisted slightly to create a Moire pattern. Because of the crystalline structure of silicon, each atom is aligned and evenly spaced from the others. In Fig. 2 each dot represents a silicon atom and the pattern formed by the dots corresponds to that of the silicon atoms. It can be seen that after the twisted overlaying, silicon atoms are periodically strained away from their positions in a perfect crystal. Also, as the twist angle increases, the spacing between strained areas decreases.
The areas where atoms are highly strained are more likely to react with an acid. After two wafers are bonded at the desired angles, a weak acid is used to disintegrate strained areas of the wafer while the unstrained areas remain unchanged. The result is a surface covered with nanobumps as seen in Fig. 3.
Since 10 nm is a typically important distance on biological molecules, such as antibodies, the Cornell research team is striving to attain an average distance between bumps of 10 nm, in order to be able to collect these molecules. Other applications of these nano-structures include controlled polymer assembly and high density magnetic information storage. Because of the periodicity of the nanobumps, this surface could be a perfect tool for building polymers from scratch. Also, by placing nano-magnets on each bump, this surface can be used to store an enormous amount of information using extremely small space. In the words of Professor Sass, "We would hope to develop properties and applications we can't even imagine today."

Figure 3: After etching, the entire surface of the silicon bicrystal is covered by an array of "nanobumps." Stephen Sass and Melissa Hines, Cornell. © Cornell University

Figure 2: Figure 2 If we were able to look down on the interface between the two crystals, the atoms would resemble the sketch above. The misaligned areas form a square grid, or moirÈ pattern. Stephen Sass and Melissa Hines, Cornell. © Cornell University