"Basically it's just like a tree," said Wang, who along with Sun published an article about the nanotrees in the journal Nanoscale.
The nanotrees were several years in the making. Wang had worked on branched nanostructures before but they didn't resemble trees, and they weren't intended to harvest light. In August 2009, Sun approached Wang with the idea to engineer branched structures for solar cells.
"By that time we already knew that the vertical nanowire really would be very good for light harvesting," Wang said. "We aimed for something that would be practical."
Each nanotree is grown in a liquid. It starts when scientists put a silicon nanowire into a solution containing zinc. Silicon and zinc are abundant in the Earth and zinc oxide is best known as an ingredient in sunscreen, Wang said. In the solution, the wire undergoes a chemical reaction, growing zinc oxide branches. Altogether, a single nanotree's length can range from a few hundred nanometers to a couple microns. That means roughly 10,000 nanotrees could fit on the cross-section of a human hair.