Chemists in China have precisely grown arrays of ultrathin flakes of bismuth selenide and bismuth telluride on a surface (J. Am. Chem. Soc., DOI: 10.1021/ja3021395). The bismuth compounds belong to a recently discovered class of materials called topological insulators, which researchers think promise a new realm of fast, energy-efficient electronic devices and computers. Making well-defined nanoparticle arrays such as these flakes is a key step toward such devices, says Hailin Peng, of Peking University, who led the new study with colleague Zhongfan Liu.
Topological insulators conduct electrons only along their surfaces, not through their insides. Theoretical physicists predicted their existence in 2006 and experimentalists demonstrated the first such material in 2008 (Nature, DOI: 10.1038/nature06843). Bismuth compounds have become researchers’ topological insulators of choice because they are simple and cheap to make.
Engineers find a few traits of topological insulator especially exciting: One is that the electrons move in a direction determined by their spin, a quantum-mechanical property that forms the basis of magnetic data storage. Engineers hope to exploit the spin-motion connection to make superfast hard drives. The materials could also find use in “spintronic” devices, which would perform computer logic using spin rather than electron charge to represent information, leading to faster, more energy-efficient computers.
However, creating high-quality topological insulator materials is a challenge. Since the useful properties occur on the surface, nanoscale ribbons and plates would be ideal to work with because of their large surface area, says Yi Cui of Stanford University, who was not involved in the new work. So some groups have used sticky tape to peel off thin layers of material from larger crystals. Meanwhile, Cui’s group has grown nanoribbons and nanoplates on silicon substrates using physical vapor deposition (Nano Lett., DOI: 10.1021/nl101260j).
But those methods don’t allow precise control over the flakes’ thickness, their locations on the substrate, or how they lie on the surface, says Peng, all of which are crucial if the materials are to be integrated into devices. To achieve that control, Peng’s team chose mica, a smooth, chemically inert surface, as a substrate. They covered it with a mask patterned with triangular or circular patches and exposed it to oxygen plasma. The plasma etched the uncovered mica surface outside the patches, leaving behind an array of unetched triangular or circular spots. Then the researchers introduced vapors of bismuth selenide or bismuth telluride. Van der Waals forces acting between the unetched spots and the bismuth compounds deposited the compounds at those spots, forming thin plates.
The researchers used scanning electron microscopy and atomic force microscopy to confirm that the plates lay flat on the substrate, were 3 to 8 nm thick, and all had about the same area. They also used a technique called angle-resolved photoemission spectroscopy to confirm that the nanoplates had conducting surfaces and insulating interiors.
“This is quite fascinating,” says Cui, who served as Peng’s postdoctoral advisor. Controlling how topological insulator nanoplates grow at specific locations has been difficult to do, he says. “This will open up opportunities in making devices,” he says, because the plates’ array pattern should allow engineers to make multiple devices at once.
CORRECTION: This story was updated on April 11, 2012, to add the name of the scientist who jointly led the research, Zhongfan Liu.