Producing thin, flexible semiconductors can be an expensive business, thanks to the pricey materials and procedures required for making an ordered substrate on which to grow the crystalline layers that provide desirable electrical properties. Now, researchers have developed a cost-effective and efficient method by which to produce a free-standing, inert gold foil which could serve as a basis for flexible electronics such as light-emitting diodes (LEDs) or solar cells.

Most of today’s electronic devices are based on silicon wafers, which are single crystalline, but brittle. As an alternative, nanometer-thin metal foils are garnering interest as substrates for thin-film semiconductors such as ZnO, Cu₂O, and CdTe due to their optical transmittance, flexibility, and conductivity. But growing metal foils through the expensive, energy-intensive traditional methods of sputtering or vacuum deposition leads to a polycrystalline or textured deposit. Semiconductors deposited on these metal foils suffer from poor electrical properties due to electron–hole recombination at grain boundaries. Jay Switzer, a professor of chemistry at the Missouri University of Science and Technology, sought a better way to produce highly ordered foils.

Currently, a square centimeter of bulk single crystal gold with approximately a half millimeter thickness costs about $1000. “Nobody wants to buy [gold] single crystals. But they’re chemically inert, so they can be used for electrodeposition,” Switzer says. Typically, metal ions in solution are reduced to gold films on crystalline silicon, at room temperature, while applying an electrical current. “We knew how to grow the epitaxial films, but the real challenge was producing a free-standing single-crystal foil,” he says.   

Switzer and his colleagues experimented with electrochemical deposition to grow an epitaxial gold foil deposited on top of a crystalline substrate. After depositing the gold layer on a single crystalline silicon substrate, they irradiated the surface with light to catalyze a photoelectrochemical reaction to oxidize the silicon.  The gold foil maintained its ordered structure while a layer of SiO_x grew between the gold and silicon.

The researchers then applied a polymer glue on top of the Au film for support, while etching the SiO_x with dilute hydrofluoric acid.  This enabled them to “effortlessly” peel a stand-alone sheet of gold foil from the silicon wafer. The 50 mm-diameter, 28-nm thick foil sheet contains about a penny’s worth of gold.

“We were actually putting the gold on silicon to protect it while making a photoelectrochemical cell,” Switzer said, referring to a solar cell that electrolyzes water to produce hydrogen and oxygen by irradiating the silicon-based anode. The gold’s ability to protect appeared to dwindle with time due to the formation of an interfacial SiO_x layer, inspiring him to pursue the concept of a removable sheet.

On the newly made foils, he then grew epitaxial films of cuprous oxide to form a diode; and zinc oxide nanowires to use as a wide bandgap semiconductor in LEDs and piezoelectric devices.  These and other applications exploit the transmittance and flexibility of the gold foil.

An ongoing challenge is how to maintain flexibility in inorganic semiconductor materials grown on the gold: while the foil substrate is flexible, the rigid semiconductor layers are prone to cracking. A possible solution is to lithographically apply patterns of semiconductor onto the flexible gold foil such that flexing occurs only in the gold in between patterns of semiconductor. Next steps include investigating how different metals such as silver could be used to produce foils with enhanced conductivity and flexibility, and conducting rigorous studies of how these epitaxial foil-based semiconductors improve relative to polycrystalline films.

Reginald Penner, a professor of chemistry at the University of California, Irvine, who was not involved in this work, says that these ultrathin gold foils are likely to find applications for the fabrication of flexible Schottky photovoltaics where any single crystalline n-type semiconductor is grown epitaxially onto the gold. Referring to the metal-semiconductor junctions that provide a low forward voltage drop and rapid switching action, valuable for efficiency, he observes that “multi-junction Schottky devices of this type could be prepared, in analogy to existing versions based upon a series of n-p semiconducting junctions."     

There will also be opportunities for exploiting the plasmonic properties of these foils which will allow any epitaxial material deposited on the foil to be probed by back-side illumination, or exciting surface electrons through unconventional geometries. “One can imagine flexible chemical sensors and biosensors prepared using this approach,” Penner says.

Read the abstract in Science.