Exotic material shows promise as flexible, transparent electrode

The result could open this class of unusual materials, called topological insulators, to its first practical applications: flexible, transparent electrodes for solar cells, sensors and optical communications devices.

“It’s rare for a good conductor to be both transparent and durable as well,” said Zhi-Xun Shen of SLAC and Stanford’s Institute for Materials and Energy Sciences (SIMES).

Researchers led by Shen, Zhongfan Liu and Hailin Peng of Peking University in China, and Yulin Chen of Oxford University in England published their results last week in Nature Chemistry. Until recently, Peng and Chen were graduate students and postdoctoral researchers at Stanford and SIMES. They have continued to collaborate with Shen’s research team after being named professors at their current universities.

The basic structural unit for bismuth selenide is a five-layer sandwich made up of alternating single-atom sheets of selenium (orange) and bismuth (purple). Units are stacked on top of each other as thicker samples are made. The selenium-selenium bonds between the units are weak, allowing the overall material to flex durably without being damaged, unlike conventional electronic circuits. Credit: Hailin Peng, Peking University

The researchers made and tested samples of a compound in which sheets of bismuth and selenium, each just one atom thick, alternate to form five-layer units. The bonds between the units are weak, allowing the overall material to flex while retaining its durability. And as a topological insulator – a new state of quantum matter – the material conducts electricity only on its surface while its interior remains insulating, an unexpected property with unknown potential for fundamental research and practical applications.

Since surface atoms dominate the structure of bismuth selenide, it is an exceptionally good electrical conductor – as good as gold. Unlike gold, however, bismuth selenide is transparent to infrared light, which we know as heat. While about half the solar energy that hits the Earth comes in the form of  infrared light, few of today’s solar cells are able to collect it. The transparent electrodes on the surfaces of most cells are either too fragile or not transparent or conducting enough.  The new material could get around that problem and allow cells to harvest more of the sun’s spectrum of wavelengths.

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This informal in-lab video shows several different samples of bismuth selenide formed atop thin mica insulator substrates as they are tested for conductivity before and after flexing. In the first segment, the sample is placed on a piece of paper showing the Chinese characters for ?topological insulator.? In the second segment, the material?s conductivity is essentially unchanged when flexed. In the third segment, some 60 cycles of deep flexing causes only a very small change in the material?s conductivity. The researchers also used a mechanical rig (not shown) to bend the material 1,000 times reproducibly, after which the material still showed minimal deterioration in conductivity. Credit: Hailin Peng, Peking University.

The researchers’ experiments also showed that bismuth selenide does not degrade significantly in humid environments or when exposed to oxygen treatments that are common in manufacturing.

“In addition to being a scientific success,” Chen said, “this demonstration should alert engineers and companies that topological insulators can also be important commercially.”

Peng added, “Infrared light pulses carry phone calls and data through optical fiber networks, so bismuth selenide may be useful in communications devices. This material could also improve infrared sensors common in scientific equipment and aerospace systems.”

Peng and colleagues made the bismuth selenide samples and conducted the flexing, conductivity and transparency tests in China. The researchers confirmed that the samples were topological insulators at the Stanford Synchrotron Radiation Lightsource’s Beam Line 5-4 at SLAC.

Theorists first proposed topological insulators in 2004, and experimentalists made the first examples, using mercury telluride at very low temperatures, two years later. Guided by theory, Chen, Shen and colleagues proved in 2009 that cheaper, more abundant and easier-to-handle bismuth telluride and similar compounds containing antimony and selenium are topological insulators at room temperature. Also in 2009, Peng, Shen and colleagues discovered important electrical conduction behavior in bismuth selenide nanoribbons.

Provided by SLAC National Accelerator Laboratory (news : web)