Issue Date: April 16, 2012
Nanoparticle Arrays For Faster Electronics
Chemists in China have grown precise 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.
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, a bismuth compound, in 2008 (Nature, DOI: 10.1038/nature06843).
In topological insulators, electrons move in a direction determined by their spin, a quantum mechanical property that forms the basis of magnetic data storage. Engineers find the spin-motion connection especially exciting and hope to exploit it to make superfast hard drives. Topological insulators 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.
Creating high-quality topological insulator materials is a challenge, however. 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. 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, all of which are crucial if the materials are to be integrated into devices, Peng says. To achieve that control, Peng and his team chose mica, a smooth, chemically inert surface, as a substrate. They covered it with a mask patterned with circular or triangular patches and exposed it to oxygen plasma. The plasma etched the uncovered mica surface, 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 each plate lay flat on the substrate, was 3 to 8 nm thick, and had about the same area. This uniformity will be key for making consistent devices. The researchers 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 adviser. “This will open up opportunities in making devices,” he says, because the plates’ array pattern should allow engineers to make multiple devices at once.
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