Mar 17 2017
An example of a gold foil peeled from single crystal silicon. Reprinted with permission from Naveen Mahenderkar et al., Science [355]:[1203] (2017) (Credit: http://www.mst.edu/)
Researchers at Missouri S&T may have made it possible to one day conform the smartphone to a person’s wrist and if that happens it may be covered in pure gold.
The researchers reported in the March 17 issue of the Science journal that they have developed a method to “grow” thin layers of gold on single crystal wafers of silicon, and then remove the gold foils and apply them as substrates on which they plan to grow other electronic materials.
This discovery could transform “flexible” or wearable technology research, significantly enhancing the versatility of such electronics going forward.
According to lead researcher Jay A. Switzer, a big chunk of research into wearable technology has been conducted using polymer substrates, or substrates composed of numerous crystals.
And then they put some typically organic semiconductor on there that ends up being flexible, but you lose the order that (silicon) has.
Jay A. Switzer, Professor, S&T
As the polymer substrates are composed of a number of crystals, they have something called grain boundaries, says Switzer. These grain boundaries can significantly restrict the performance of an electronic gadget.
“Say you’re making a solar cell or an LED,” he says. “In a semiconductor, you have electrons and you have holes, which are the opposite of electrons. They can combine at grain boundaries and give off heat. And then you end up losing the light that you get out of an LED, or the current or voltage that you might get out of a solar cell.”
A majority of electronics available commercially are composed of silicon because it is “relatively cheap, but also highly ordered,” Switzer says.
99.99 percent of electronics are made out of silicon, and there’s a reason – it works great. It’s a single crystal, and the atoms are perfectly aligned. But, when you have a single crystal like that, typically, it’s not flexible.
Jay A. Switzer, Professor, S&T
By beginning with single crystal silicon and growing gold foils on it, Switzer could sustain the high order of silicon on the foil. But since the foil is gold, it is also highly flexible and durable.
“We bent it 4,000 times, and basically the resistance didn’t change,” he says.
The gold foils are also essentially transparent as they are very thin. According to Switzer, his team has peeled foils as thin as 7 nm. Switzer says the challenge his team encountered was not in growing gold on the single crystal silicon, but in making it to peel off as such a thin layer of foil. Gold usually bonds extremely well to silicon.
“So we came up with this trick where we could photo-electrochemically oxidize the silicon,” Switzer says. “And the gold just slides off.”
Photoelectrochemical oxidation is the method by which light allows a semiconductor material, in this case silicon, to support a catalytic oxidation reaction.
Switzer says numerous gold foils - or foils of numerous other metals - can be produced from a single crystal wafer of silicon.
The discovery was a kind of a “happy accident.” Switzer says they were searching for an economical way to form single crystals when they stumbled upon this process.
This is something that I think a lot of people who are interested in working with highly ordered materials like single crystals would appreciate making really easily. Besides making flexible devices, it’s just going to open up a field for anybody who wants to work with single crystals.
Jay A. Switzer, Professor, S&T
The research team included Naveen Kumar Mahenderkar, a Ph.D. candidate in materials science and engineering; Qingzhi Chen and Ying Chau Liu, both Ph.D. candidates in chemistry; Alexander Duchild, an undergraduate chemistry student; Seth Hofheins, a student at Rolla High School; and Eric Chason, professor of engineering at Brown University.