Crop circles are curious shapes found out in fields, which seem to have been made by some alien force, when really it was just someone pulling a prank. On the nanoscale though, there is no one to pull pranks, so scientists have been somewhat confused as to why circles were forming with squares in the center during some experiments. Finally researchers at Berkeley Lab have found the answer, and it may influence the world (of nanoscale materials and nanotechnology).
The original discovery of the circles was during an experiment in which gold was heated on a silicon substrate and allowed to cool. It was during the cooling that the circles and squares appeared and it has taken three years for their formation to be explained. Part of the explanation is the formation of a eutectic, a special kind of alloy which has a much lower melting temperature than its constituent metals, gold and silicon in this case.
The researchers started with a substrate of pure silicon that had developed a layer of silicon dioxide on its surface. This layer prevented the layers of gold placed on top from touching the silicon. When the entire sample was heated to 600ºC though, defects in the silicon dioxide opened up, allowing the gold and silicon to touch, and creating the eutectic alloy. As the melting temperature of the alloy is only 363ºC, it existed as a liquid.
The liquid metal expands in a circle, between the silicon dioxide and gold. Eventually it expands too much and pops, causing the molten metal to fly to the edges of the circle, leaving behind clean silicon dioxide and the curious square. A closer look reveals the square is actually the base of an inverted pyramid of silicon and gold. When the circle pops, not all of the alloy escapes to the edges; some is left at the center where the defect in the silicon dioxide was. The alloy cools there, and as it does, the gold and silicon separate. The reason for the pyramid shape is because of the crystal lattice of the pure silicon substrate.
With the mystery solved, the researchers hope their findings will enable new means of creating and working with nanoscale materials.