Band-Gap Engineering of Nanowires Could Boost Batteries

The reason for replacing graphite in the electrodes of the ubiquitous lithium-ion (Li-ion) battery is clear to anyone who uses a smartphone: The batteries run out of charge in just a few hours under regular use.

One answer has been to replace the graphite with silicon. Unfortunately, the expanding and contracting that occurred as the lithium ions transported in and out of silicon electrodes quickly cracks it.

The next solution was to create “nanostructured silicon” electrodes, sometimes with the help of graphene or good old carbon nanotubes.

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Nanowires Give Off Light Under Pressure

Nanomaterials have offered the tantalizing possibility of lifelike artificial skin. Now researchers at the Georgia Institute of Technology have developed a use for zinc-oxide (ZnO) nanowires to serve as tiny LEDs whose emission intensity is dependent on the local strain put on them.

The Georgia Tech researchers believe that this work offers a new approach to imaging force and could lead to a new approach for human-machine interfaces.

“You can write with your pen and the sensor will optically detect what you write at high resolution and with a very fast response rate,” said Zhong Lin Wang, Regents’ professor and Hightower Chair in the School of Materials Science and Engineering at Georgia Tech, in a press release. “This is a new principle for imaging force that uses parallel detection and avoids many of the complications of existing pressure sensors.”

The research, which was published in the journal Nature Photonics (“High resolution electroluminescent imaging of pressure distribution using a piezoelectric nanowire-LED array”), builds on Wang’s previous work in applying ZnO nanowires to uses that can exploit their piezoelectric properties.

The ZnO nanowires used in this latest device exploit a phenomenon Wang has dubbed piezo-phototronics because they operate on the same principal as piezoelectric materials, but emit different light intensities based on the level of pressure applied to them.

“When you have a zinc oxide nanowire under strain, you create a piezoelectric charge at both ends which forms a piezoelectric potential,” Wang explained in the press release. “The presence of the potential distorts the band structure in the wire, causing electrons to remain in the p-n junction longer and enhancing the efficiency of the LED.”

Wang and his team fabricated the devices by growing the nanowires on a gallium nitride thin film substrate with the c-axis pointing upward. A polymer is then added, filling the space between the nanowires. A nickel-gold electrode is then attached to the gallium-nitride film to form an ohmic contact, and a transparent indium-tin oxide (ITO) film is deposited on top of the array to serve as a common electrode.

When pressure is applied to the device, some of the nanowires are compressed along their axial directions, resulting in a negative piezo-potential; the uncompressed nanowires have no potential.

The researchers demonstrated that when they used an ordinary pen to write on the material, light is emitted from the bottom of the material that corresponds to the letter written on the top.

“The response time is fast, and you can read a million pixels in a microsecond,” said Wang. “When the light emission is created, it can be detected immediately with the optical fiber.”

While the light turning on and off is completed in a mere 90 milliseconds, Wang believes that the spatial resolution, which currently stands at 2.7 micrometers, can be improved. He thinks the team can achieve this by reducing the diameter of the nanowires, making it possible to fit more of them into a given area.

Photo: Georgia Institute of Technology

Nanowires Grow Better on Graphene – from IEEE Spectrum

By Dexter Johnson
In an attempt to grow nanowires on a graphene substrate, researchers at the University of Illinois may have stumbled upon a new paradigm for epitaxy (the growth of crystals on a susbstrate).

Some believe that developing new manufacturing methods for nanoscale devices—like epitaxy—may be more crucial to meeting the demands of next generation chips than creating new materials, especially when feature sizes start falling below three nanometers. So, the Illinois researchers’ development of a new method of epitaxy may ultimately be more significant than creating a new material.

The research, which was published in the journal Nano Letters (“InxGa1–xAs Nanowire Growth on Graphene: van der Waals Epitaxy Induced Phase Segregation”), produced nanowires made from III-V compound semiconductors. Generally, III-V semiconductors like gallium arsenide don’t integrate well with silicon, but  recently  it was discovered that when these materials were brought down to the nanoscale that they were compatible.

Researchers have previously combined two of these semiconductors in gaseous form so that they deposit themselves on a graphene substrate (a process known as metalorganic chemical vapor deposition, or MOCVD) and self assemble into ordered crystalline form. However, the Illinois research marks the first time three of the semiconductors have been mixed together in this way.

The researchers discovered that something remarkable occurred when this third semiconductor was added to the mix. The materials began spontaneously to segregate into an indium arsenide (InAs) core with an InGaAs shell around the outside of the nanowire.

“This is unexpected,” says professor Xiuling Li, who led the research, in a press release. “A lot of devices require a core-shell architecture. Normally you grow the core in one growth condition and change conditions to grow the shell on the outside. This is spontaneous, done in one step. The other good thing is that since it’s a spontaneous segregation, it produces a perfect interface.”

This precise delineation between the core and the outside of the nanowire has to do with relationship between the atomic structure of the semiconductors and that of the graphene. The crystal structure of InAs has the same distance between its atoms as the carbon atoms in a sheet of graphene. As a result, the InAs fits in that space perfectly, leaving the gallium compound to form on the outside of that core.

The next step for the researchers will be to see if they can exploit their new manufacturing technique to create solar cells and other optoelectronic devices.

Image: Joshua D. Wood/University of Illinois at Urbana-Champaign