Penn State – 3D Imaging Technique Unlocks Properties of Perovskite Crystals – Applications for Perovskite Solar Cells


A reconstruction of a perovskite crystal (CaTiO3) grown on a similar perovskite substrate (NdGaO3) showing electron density and oxygen octahedral tilt. (insert) Artist’s conception of the interface between substrate and film. Credit: Yakun Yuan/Penn State

A team of materials scientists from Penn State, Cornell and Argonne National Laboratory have, for the first time, visualized the 3D atomic and electron density structure of the most complex perovskite crystal structure system decoded to date.

Perovskites are minerals that are of interest as electrical insulators, semiconductors, metals or superconductors, depending on the arrangement of their atoms and electrons.

Perovskite crystals have an unusual grouping of oxygen atoms that form an octahedron, an eight-sided polygon. This arrangement of oxygen atoms acts like a cage that can hold a large number of the elemental atoms in the periodic table. Additionally, other atoms can be fixed to the corners of a cube outside of the cage at precise locations to alter the material’s properties, for instance in changing a metal into an insulator, or a non-magnet into a ferromagnet.

In their current work, the team grew the very first discovered perovskite crystal, called calcium titanate, on top of a series of other perovskite crystal substrates with similar but slightly different oxygen cages at their surfaces. Because the thin film perovskite on top wants to conform to the structure of the thicker substrate, it contorts its cages in a process known as tilt epitaxy.

The researchers found this tilt epitaxy of calcium titanate caused a very ordinary material to take on the property of ferroelectricity, a spontaneous polarization, and to remain ferroelectric up to 900 Kelvin, around three times hotter than room temperature. They were also able to visualize the three-dimensional electron density distribution in calcium titanate thin film for the first time.

“We have been able to see atoms for quite some time, but not map them and their electron distribution in space in a crystal in three dimensions,” said Venkat Gopalan, professor of materials science and physics, Penn State. “If we can see not just where atomic nuclei are located in space, but also how their electron clouds are shared, that will tell us basically everything we need to know about the material in order to infer its properties.”

That was the challenge the team set for itself over five years ago when Gopalan gave his student and lead author of a new report in Nature Communications, Yakun Yuan, the project.

Based on a rarely used x-ray visualization technique called COBRA, for coherent Bragg rod analysis, originally developed by a group in Israel, Yuan figured out how to expand and modify the technique to analyze one of the most complicated, least symmetrical material systems studied to date: the strained three-dimensional perovskite crystal with octahedral tilts in all directions, grown on another equally complex crystal structure.

“To reveal 3D structural details at the atomic level, we had to collect extensive datasets using the most brilliant synchrotron X-ray source available at Argonne National Labs and carefully analyze them with the COBRA analysis code modified for accommodating the complexity of such low symmetry,” said Yakun Yuan.

Gopalan went on to explain that very few perovskite oxygen cages are perfectly aligned throughout the material. Some rotate counterclockwise in one layer of atoms and clockwise in the next. Some cages are squeezed out of shape or tilt in directions that are in or out of plane to the substrate surface.

From the interface of a film with the substrate it is grown on, all the way to its surface, each atomic layer may have unique changes in their structure and pattern.  All of these distortions make a difference in the material properties, which they can predict using a computational technique called density functional theory (DFT).

“The predictions from the DFT calculations provide insights that complement the experimental data and help explain the way that material properties change with the alignment or tilting of the perovskite oxygen cages,” said Prof. Susan Sinnott, whose group performed the theoretical calculations.

The team also validated their advanced COBRA technique against multiple images of their material using the powerful Titan transmission electron microscope in the Materials Research Institute at Penn State.  Since the electron microscopes image extremely thin electron transparent samples in a 2-dimensional projection, not all of the 3-dimensional image could be captured even with the best microscope available today and with multiple sample orientations.

Why Perovskite Solar Cells Are So Efficient

This is an area where 3-dimensional imaging by the COBRA technique outperformed the electron microscopy in such complex structures.

The researchers believe their COBRA technique is applicable to the study of many other three-dimensional low-symmetry atomic crystals.

Additional authors on “Three-dimensional atomic scale electron density reconstruction of octahedral tilt epitaxy in functionals perovskites” are Yanfu Lu, a Ph.D. student in Sinnott’s group, Greg Stone, Gopalan’s former postdoctoral scholar, Ke Wang, a staff scientist in Penn State’s Materials Research Institute, Darrell Schlom and his Ph.D. student Charles Brooks, Cornell University, and Hua Zhou, staff scientist, Argonne National Laboratory.

img_0885-1Penn State University

Funding was provided by the National Science Foundation with additional support provided through the Department of Energy and the Penn State 2D Crystal Consortium, a NSF Materials Innovation Platform, and the Penn State institute for CyberScience.

Contact Venkat Gopalan at vxg8@psu.edu or Hua Zhou at hzhou@aps.anl.gov.

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Toward a smart graphene membrane to desalinate water: Penn State University


Graphene H2O towardasmartA scalable graphene-based membrane for producing clean water Credit: Aaron Morelos-Gomez. Credit: Pennsylvania State University

An international team of researchers, including scientists from Shinshu University (Japan) and the director of Penn State’s ATOMIC Center, has developed a graphene-based coating for desalination membranes that is more robust and scalable than current nanofiltration membrane technologies. The result could be a sturdy and practical membrane for clean water solutions as well as protein separation, wastewater treatment and pharmaceutical and food industry applications.

“Our dream is to create a smart  that combines high flow rates, high efficiency, long lifetime, self-healing and eliminates bio and inorganic fouling in order to provide clean water solutions for the many parts of the world where clean water is scarce,” says Mauricio Terrones, professor of physics, chemistry and materials science and engineering, Penn State. “This work is taking us in that direction.”

The hybrid membrane the team developed uses a simple spray-on technology to coat a mixture of graphene oxide and few-layered graphene in solution onto a backbone support membrane of polysulfone modified with polyvinyl alcohol. The support membrane increased the robustness of the hybrid membrane, which was able to stand up to intense cross-flow, high pressure and chlorine exposure. Even in early stages of development, the membrane rejects 85 percent of salt, adequate for agricultural purposes though not for drinking, and 96 percent of dye molecules. Highly polluting dyes from textile manufacturing is commonly discharged into rivers in some areas of the world.

Chlorine is generally used to mitigate biofouling in membranes, but chlorine rapidly degrades the performance of current polymer membranes. The addition of few-layer graphene makes the new membrane highly resistant to chlorine.

Graphene is known to have high mechanical strength, and porous graphene is predicted to have 100 percent salt rejection, making it a potentially ideal material for desalination membranes. However, there are many challenges with scaling up graphene to industrial quantities including controlling defects and the need for complex transfer techniques required to handle the two-dimensional material. The current work attempts to overcome the scalability issues and provide an inexpensive, high quality membrane at manufacturing scale.

The work was performed in the Global Aqua Innovation Center and the Institute of Carbon Science and Technology at Shinshu University, Nagano, Japan, where Terrones is also a Distinguished Invited Professor. The team includes researchers Aaron Morelos-Gomez, Josue Ortiz-Medina and Rodolfo Cruz-Silva, former Ph.D. students of Terrones. Morelos-Gomez is lead author on a paper published online on August 28 in Nature Nanotechnology describing their work titled “Effective NaCL and dye rejection of hybrid graphene oxide/graphene layered membranes.” The Japanese researchers, Hiroyuki Muramatsu, Takumi Araki, Tomoyuki Fukuyo, Syogo Tejima, Kenji Takeuchi, and Takuya Hayashi, were also led by Professor Morinobu Endo.

First author Aaron Morelos-Gomez says, “Our membrane overcomes the water solubility of graphene oxide by using polyvinyl alcohol as an adhesive making it resistant against strong water flow and high pressures. By mixing  with  we could also improve significantly its chemical resistance.”

Professor Morinobu Endo concludes that “this is the first step towards more effective and smart membranes that could self-adapt depending on their environment.”

 Explore further: Graphene sieve turns seawater into drinking water

More information: Aaron Morelos-Gomez et al. Effective NaCl and dye rejection of hybrid graphene oxide/graphene layered membranes, Nature Nanotechnology (2017). DOI: 10.1038/nnano.2017.160

Read more at: https://phys.org/news/2017-09-smart-graphene-membrane-desalinate.html#jCp

Read more at: https://phys.org/news/2017-09-smart-graphene-membrane-desalinate.html#jCp

Transparent Optogenetic Brain Implants: Amazing Use for Graphene!


1-Brain Transparentgraphene1-640x353Transparency is the key to many technologies. Thin conductive films, like those made from ITO (indium tin oxide) for example, can carry currents or create electric fields critical for displays or solar panels without blocking all the light.

The most powerful brain implants being built today have exactly this same requirement. Namely, they need to record fast electric signals with conductive arrays while permitting light to pass out through them for high-resolution imaging — and just to take it up a notch — let light pass in to permit optogenetic control directly under the implant for the icing on the cake.

Unfortunately, ITO is generally too stiff and too brittle for brain implants. Even if it could be made flexible, the high temperatures required to process it are incompatible with many of the materials (like parylene) that are used in the implants. Furthermore the transparency bandwidth of ITO is insufficient to fully exploit the wide spectrum of new UV and IR capable optogenetic proteins that have researchers fairly excited. The solution, now emerging from multiple labs throughout the universe is to build flexible, transparent electrode arrays from graphene. Two studies in the latest issue of Nature Communications, one from the University of Wisconsin-Madison and the other from Penn, describe how to build these devices.

1-Brain Transparentgraphene1-640x353

The University of Wisconsin researchers are either a little bit smarter or just a little bit richer, because they published their work open access. It’s a no-brainer then that we will focus on their methods first, and also in more detail. To make the arrays, these guys first deposited the parylene (polymer) substrate on a silicon wafer, metalized it with gold, and then patterned it with an electron beam to create small contact pads. The magic was to then apply four stacked single-atom-thick graphene layers using a wet transfer technique. These layers were then protected with a silicon dioxide layer, another parylene layer, and finally molded into brain signal recording goodness with reactive ion etching.

PennTransparentelectrodeThe researchers went with four graphene layers because that provided optimal mechanical integrity and conductivity while maintaining sufficient transparency. They tested the device in opto-enhanced mice whose neurons expressed proteins that react to blue light. When they hit the neurons with a laser fired in through the implant, the protein channels opened and fired the cell beneath. The masterstroke that remained was then to successfully record the electrical signals from this firing, sit back, and wait for the Nobel prize office to call.

Read: MIT successfully implants false memories with optogenetics, may explain why we remember things that didn’t happen

The Penn State group used a similar 16-spot electrode array (pictured above right), and proceeded — we presume — in much the same fashion. Their angle was to perform high-resolution optical imaging, in particular calcium imaging, right out through the transparent electrode arrays which simultaneously recorded in high-temporal-resolution signals. They did this in slices of the hippocampus where they could bring to bear the complex and multifarious hardware needed to perform confocal and two-photon microscopy.

These latter techniques provide a boost in spatial resolution by zeroing in over narrow planes inside the specimen, and limiting the background by the requirement of two photons to generate an optical signal. We should mention that there are voltage sensitive dyes available, in addition to standard calcium dyes, which can almost record the fastest single spikes, but electrical recording still reigns supreme for speed.

What a mouse looks like with an optogenetics system plugged in

One concern of both groups in making these kinds of simultaneous electro-optic measurements was the generation of light-induced artifacts in the electrical recordings. This potential complication, called the Becqueral photovoltaic effect, has been known to exist since it was first demonstrated back in 1839.

When light hits a conventional metal electrode, a photoelectrochemical (or more simply, a photovoltaic) effect occurs. If present in these recordings, the different signals could be highly disambiguatable. The Penn researchers reported that they saw no significant artifact, while the Wisconsin researchers saw some small effects with their device. In particular, when compared with platinum electrodes put into the opposite side cortical hemisphere, the Wisconsin researchers found that the artifact from graphene was similar to that obtained from platinum electrodes.

At this point both groups are busy characterizing the performance of their new devices in exacting detail. If workable as more permanent brain implants they may offer a nice compliment to other new approaches we have recently seen — flexible materials like silk for example. Where silk may offer biodegradability and reversibility, graphene may offer biocompatible permanence and reliability. The significant hype regarding optogenetics, well-founded in our opinion, seems to have died down for the moment. New advances like those just described may help refocus general attention on the huge potential benefit optogenetics holds for humans.

Now read: The wonderful world of wonder materials (such as graphene)