NEWS FROM THE WORLD OF MATERIALS

Materials in Focus

Hydrogen pressure may control graphene growth
Oak Ridge National Laboratory (ORNL). See also the press release by Ron Walli of ORNL.)
Image credit: ORNL. Click image to enlarge.

Image caption: Graphene grains come in several different shapes. Hydrogen gas controls the grains' appearance. 

In one currently popular way of making graphene, methane (the carbon source) is mixed with molecular hydrogen, and the two gases flow over a copper substrate at a temperature of approximately 1000 °C. The role of hydrogen in this process has been difficult to assess because its concentration has varied from essentially zero to several thousand times the amount of methane in different experiments. Now researchers from Oak Ridge National Laboratory (ORNL) and New Mexico State University, by carefully controlling the hydrogen/methane ratio, have determined that hydrogen performs two roles in the graphene formation process: (1) it acts as a co-catalyst with the copper, and (2) it acts as an etchant to control graphene grain size and shape. The results were reported recently in ACS Nano. 

The group, led by Ivan Vlassiouk at ORNL, observed the growth of graphene over copper at a constant methane concentration of 30 ppm while the pressure of H₂ was varied from zero to 19 Torr. They discovered that below a hydrogen pressure of 2 Torr, no graphene formed. Between approximately 2 and 9 Torr, irregular graphene grains were formed. At hydrogen pressures of 10 Torr and above the researchers observed perfect hexagons of graphene. Kinetically, the rate of graphene formation peaked near 11 torr. The grains reached a maximum size of approximately 12 microns (edge-to-edge) at 19 Torr, at which point further grain growth ceased.

Mechanistically, below 2 Torr of hydrogen the methane has to chemisorb on the copper surface—a thermodynamically unfavorable condition. Above 2 Torr, molecular hydrogen readily dissociates on copper and promotes activation of physisorbed methane, producing surface bound CH₃ radicals.    Dehydrogenation of this species eventually leads to a single layer of graphene on the copper surface.  Remaining chemisorbed hydrogen etches weak carbon-carbon bonds, thereby limiting the further growth of graphene grains. [ACS Nano]

Nano Focus

Remove lens, insert algorithm
(University of California San Diego. See also the press release by Kim McDonald of the UC San Diego News Center.)
Image credit: UC San Diego. Click image to enlarge.

Image caption: Magnetic domains appear like the repeating swirls of fingerprint ridges. As the spaces between the domains get smaller, computer engineers can store more data. 

By replacing the lenses of an x-ray microscope with a computer algorithm, researchers led by Oleg Shpyrko of the University of California San Diego have taken advantage of 100% efficiency in x-ray photon collection to achieve 50-nm resolution images of magnetic domains of a gadolinium/iron film. As reported recently in the Proceedings of the National Academy of Sciences, such precise imaging could prove to be invaluable to scientists and engineers who are trying to form ever smaller magnetic bits to increase the storage capacity of memory devices.  

Lenses that focus x-rays are difficult to fabricate, and they generally do not have very high resolution. To avoid these problems, Shpyrko and his colleagues simply allow the incident x-ray beam to impinge on the sample and diffract, creating a Fourier transform of the sample. In the process the phase of the x-ray beam is lost. Ashish Tripathi, a graduate student in Shpyrko’s lab, did the hard work of developing an algorithm to correct for this loss. “The algorithm allows us to recover the phase that was lost in the measurement,” Shpyrko says, “and invert the image in the computer to get a real-space image. It does this in a way that does not produce any aberrations in the image, and it’s highly scalable with the new types of x-ray sources that will come on line soon.”
This technology uses every single photon that strikes the sample, according to Shpyrko, so no photons are wasted. Because the resolution is limited only by the number of photons collected, he says, “you can actually get all the way down, in principle, to the diffraction limit for x-rays, which would be atomic level resolution. We’re not there yet, but in principle it just requires more photons.”    

The imaging of magnetic domains was just a first step in demonstrating the capabilities of this new x-ray microscope. The researchers are currently using it to investigate orbital ordering and charge ordering, and can envision using it to image defects in crystals and for other materials science challenges. Because it uses elemental x-ray edge photons, the microscope can also identify where specific elements are located in a sample, revealing information about the sample’s chemistry. [Proceedings of the National Academy of Sciences] 

Bio Focus

Filamentary serpentine layout is key to epidermal electronic "smart skin"
(University of Illinois at Urbana-Champaign. See also the press release by Liz Ahlberg of the University of Illinois News Bureau.)
Image credit: John A. Rogers, University of Illinois. Click image to enlarge.

Image caption: University of Illinois researchers can mount electronic devices on an ultrathin, skin-like platform that mounts directly onto the skin with the ease, flexibility and comfort of a temporary tattoo. 

“Narrow, wavy, and thin”—that’s how John Rogers of the University of Illinois at Urbana-Champaign describes the new “epidermal electronics” that he and his co-workers have developed for both monitoring electrical signals from the heart, brain, and muscles, and for stimulating muscles by supplying electrical signals. As reported recently in Science, they have fabricated elastomeric patches containing open, spider–web layouts of electrical circuits that have modulus and bending properties very close to that of human skin, making them easy to wear and potentially useful in sleep studies, neonatal care, and rehabilitation applications, among others. 

The key to the flexibility and stretchability of the design is the “wavy” nature of the electronic circuits, known more technically as a “filamentary serpentine” layout, which consists of components with many large loops instead of shorter, linear circuit paths. “If you look at the designs that best match the properties of skin in our work, they involve the entire circuitconsisting of this filamentary serpentine shape,” Rogers says. “So not only the interconnect wires but the devices themselves—the silicon itself, including transistors and the other device components, have this serpentine geometry.” Quantitative mechanics modeling was used to determine the optimal thickness of the filaments and the loop geometry for the best skin matching.

The result is an elastomeric patch less than 7 microns thick containing an antenna LED, a wireless power coil, radio frequency coils and diodes, a temperature sensor, and electroencephalogram, electrocardiogram, and electromyogram sensors to monitor the brain, heart, and muscle signals, respectively. The circuit is attached to the skin by van der Waals forces only, so no adhesive is needed; the van der Waals forces are sufficient to maintain conformal contact with the skin, withstanding normal body movements over periods of hours without cracking or delamination. The researchers have also experimented with commercially available temporary transfer tattoos that could conceal the circuitry and provide greater adhesion if necessary.

This technology is an outgrowth of the macro-scale stretchable electronics that Rogers’ group and others have been investigating.  Earlier versions were just too thick (a few mm to a cm), with elastic moduli a few orders of magnitude too high to match the skin. “We’ve extended some of those design concepts that we and others have been exploring in stretchable electronics to an extreme, in terms of design, filamentary shape, thinness, and modulus-matched substrate to enable this epidermal format,” Rogers says. “We view it as a different class of technology for that reason, but it has historical origins in flexible and, more recently, stretchable forms of electronics.” [Science]

Energy Focus

High energy density demonstrated in Li-air batteries
(Massachussetts Institute of Technology. See also the press release by David L. Chandler, MIT News Office.)
Image credit: Mitchell, Gallant, and Shao-Horn. Click image to enlarge.

Photo caption: Ions of lithium combine with oxygen from the air to form particles of lithium oxides, which attach themselves to carbon fibers on the electrode as the battery is being used. During recharging, the lithium oxides separate again into lithium and oxygen and the process can begin again.

Lithium-air (also known as Li-O₂) batteries may one day compete with lithium ion (Li-ion) batteries as the next generation energy storage device, but researchers are still in the early stages of understanding how they operate and how to improve their performance.  A big step in both these directions was taken by researchers Robert Mitchell, Betar Gallant, and their colleagues at MIT recently, with their description in Energy & Environmental Science (a publication of the Royal Society of Chemistry) of a new oxygen electrode made of binder-free, all-carbon hollow nanofibers. They have achieved among the highest energy densities by weight reported to date for Li-air batteries, according to Mitchell, and in the process have observed the growth and disappearance of Li₂O₂ in the electrode during the battery’s discharging and charging stages, respectively.
The projected energy density for Li-air batteries is approximately 3 to 5 times that of Li-ion batteries.  “One of the reasons is the way that Li-air batteries operate and the way that they’re designed,” Gallant says. “The heavy cathode material in a Li-ion battery is replaced with a very lightweight porous carbon support, which stores energy by reacting molecular oxygen from the air with Li ions to form a new solid phase that grows inside the carbon framework. This new phase is also significantly lighter compared to Li-ion cathodes, resulting in a gravimetric energy density that is several times higher.”
This new phase, which is typically lithium peroxide (Li₂O₂), forms when the battery is discharging, and needs a conductive scaffold in which to grow. Using chemical vapor deposition to grow nanofibers directly on a porous substrate produced a “carpet” of hollow carbon nanofibers with diameters on the order of 30 nm and a void volume greater than 90%. “Typically the electrode structure in these types of batteries is composed of nanosized carbon particles with void volumes on the order of 60%,” Mitchell says. The additional 30% of void volume “allows you to store a lot of Li₂O₂ in the electrode. We’re trying to see how much carbon is really necessary to support the reversible formation and storage of Li₂O₂.”
In the process, the researchers used ex situ SEM to observe the formation of Li₂O₂ on the sidewalls of the carbon nanofibers during discharge, starting as spheres that transformed to toroids before coalescing into monolithic Li₂O₂. During charging, they were able to see the Li₂O₂ slowly break down and disappear. “For increasing the discharge capacity, which is directly related to the amount of energy that can be stored in the battery,” Gallant says, “we’ve shown that modifying the carbon structure is one very promising approach.” [Energy & Environmental Science]

Image in Focus

Eleven Masks

Color-enhanced scanning electron micrograph of a lanthanum strontium manganite (LSM) thin layer that is deposited on a silicon substrate via spray pyrolysis process. The bizarre faces are created by rapid expansion of the film when the precursor solution droplets reach the hot surface of the substrate to decompose into the LSM oxide. Eleven strange masks can be recognized when eyes are artificially added to the original image.

Credit: Hoda Amani Hamedani, Georgia Institute of Technology 
(Click image to enlarge.)

Posted in: