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Printing has come a long way since the days of Johannes Gutenberg. Now, researchers have developed a new method that uses plasma to print nanomaterials onto a 3-D object or flexible surface, such as paper or cloth. The technique could make it easier and cheaper to build devices like wearable chemical and biological sensors, flexible memory devices and batteries, and integrated circuits.

One of the most common methods to deposit nanomaterials–such as a layer of nanoparticles or nanotubes–onto a surface is with an inkjet printer similar to an ordinary printer found in an office. Although they use well-established technology and are relatively cheap, inkjet printers have limitations. They can’t print on textiles or other flexible materials, let alone 3-D objects. They also must print liquid ink, and not all materials are easily made into a liquid.

Some nanomaterials can be printed using aerosol printing techniques. But the material must be heated several hundreds of degrees to consolidate into a thin and smooth film. The extra step is impossible for printing on cloth or other materials that can burn, and means higher cost for the materials that can take the heat.

The plasma method skips this heating step and works at temperatures not much warmer than 40 degrees Celsius. “You can use it to deposit things on paper, plastic, cotton, or any kind of textile,” said Meyya Meyyappan of NASA Ames Research Center. “It’s ideal for soft substrates.” It also doesn’t require the printing material to be liquid.

The researchers, from NASA Ames and SLAC National Accelerator Laboratory, describe their work in Applied Physics Letters, from AIP Publishing.

They demonstrated their technique by printing a layer of carbon nanotubes on paper. They mixed the nanotubes into a plasma of helium ions, which they then blasted through a nozzle and onto paper. The plasma focuses the nanoparticles onto the paper surface, forming a consolidated layer without any need for additional heating.

The team printed two simple chemical and biological sensors. The presence of certain molecules can change the electrical resistance of the carbon nanotubes. By measuring this change, the device can identify and determine the concentration of the molecule. The researchers made a chemical sensor that detects ammonia gas and a biological sensor that detects dopamine, a molecule linked to disorders like Parkinson’s disease and epilepsy.

But these were just simple proofs-of-principle, Meyyappan said. “There’s a wide range of biosensing applications.” For example, you can make sensors that monitor health biomarkers like cholesterol, or food-borne pathogens like E. coli and Salmonella.

The nozzle firing a jet of carbon nanotubes with helium plasma off and on. When the plasma is off, the density of carbon nanotubes is small. The plasma focuses the nanotubes onto the substrate with high density and good adhesion. Credit: NASA Ames Research Center

The nozzle firing a jet of carbon nanotubes with helium plasma off and on. When the plasma is off, the density of carbon nanotubes is small. The plasma focuses the nanotubes onto the substrate with high density and good adhesion. Credit: NASA Ames Research Center

Because the method uses a simple nozzle, it’s versatile and can be easily scaled up. For example, a system could have many nozzles like a showerhead, allowing it to print on large areas. Or, the nozzle could act like a hose, free to spray nanomaterials on the surfaces of 3-D objects.

“It can do things inkjet printing cannot do,” Meyyappan said. “But anything inkjet printing can do, it can be pretty competitive.”

The method is ready for commercialization, Meyyappan said, and should be relatively inexpensive and straightforward to develop. Right now, the researchers are designing the technique to print other kinds of materials such as copper. They can then print materials used for batteries onto thin sheets of metal such as aluminum. The sheet can then be rolled into tiny batteries for cellphones or other devices.

Crumple a piece of paper and it’s probably destined for the trash can, but new research shows that repeatedly crumpling sheets of the nanomaterial graphene can actually enhance some of its properties. In some cases, the more crumpled the better.

The research by engineers from Brown University shows that graphene, wrinkled and crumpled in a multi-step process, becomes significantly better at repelling water–a property that could be useful in making self-cleaning surfaces. Crumpled graphene also has enhanced electrochemical properties, which could make it more useful as electrodes in batteries and fuel cells.

The results are published in the journal Advanced Materials.

Wrinkles and crumples, introduced by placing graphene on shrinky polymers, can enhance graphene's properties. Credit: Hurt and Wong Labs / Brown Unviversity

Wrinkles and crumples, introduced by placing graphene on shrinky polymers, can enhance graphene’s properties. Credit: Hurt and Wong Labs / Brown Unviversity

Generations of wrinkles

This new research builds on previous work done by Robert Hurt and Ian Wong, from Brown’s School of Engineering. The team had previously showed that by introducing wrinkles into graphene, they could make substrates for culturing cells that were more similar to the complex environments in which cells grow in the body. For this latest work, the researchers led by Po-Yen Chen, a Hibbit postdoctoral fellow, wanted to build more complex architectures incorporating both wrinkles and crumples. “I wanted to see if there was a way to create higher-generational structures,” Chen said.

To do that, the researchers deposited layers of graphene oxide onto shrink films–polymer membranes that shrink when heated (kids may know these as Shrinky Dinks). As the films shrink, the graphene on top is compressed, causing it to wrinkle and crumple. To see what kind of structures they could create, the researchers compressed same graphene sheets multiple times. After the first shrink, the film was dissolved away, and the graphene was placed in a new film to be shrunk again.

The researchers experimented with different configurations in the successive generations of shrinking. For example, sometimes they clamped opposite ends of the films, which caused them to shrink only along one axis. Clamped films yielded graphene sheets with periodic, basically parallel wrinkles across its surface. Unclamped films shrank in two dimensions, both length- and width-wise, creating a graphene surface that was crumpled in random shapes.

The team experimented with those different modes of shrinking over three successive generations. For example, they might shrink the same graphene sheet on a clamped film, then an unclamped film, then clamped again; or unclamped, clamped, unclamped. They also rotated the graphene in different configurations between shrinkings, sometimes placing the sheet perpendicular to its original orientation.

The team found that the multi-generational approach could substantially compress the graphene sheets, making them as small as one-fortieth their original size. They also showed that successive generations could create interesting patterns along the surface–wrinkles and crumples that were superimposed onto each other, for example.

“As you go deeper into the generations you tend to get larger wavelength structures with the original, smaller wavelength structure from earlier generations built into them,” said Robert Hurt, a professor of engineering at Brown and one of the paper’s corresponding authors.

A sheet that was shrunk clamped, unclamped, and then clamped looked different from ones that were unclamped, clamped, unclamped, for example.

“The sequence matters,” said Wong, also a corresponding author on the paper. “It’s not like multiplication where 2 times 3 is the same as 3 times 2. The material has a ‘memory’ and we get different results when we wrinkle or crumple in a different order.”

The researchers generated a kind of taxonomy of structures born from different shrinking configurations. They then tested several of those structures to see how they altered the properties of the graphene sheets.

Enhanced properties

They showed that a highly crumpled graphene surface becomes superhydrophobic–able to resist wetting by water. When water touches a hydrophobic surface, it beads up and rolls off. When the contact angle of those water beads with an underlying surface exceeds 160 degrees–meaning very little of the water bead’s surface touches the material–the material is said to be superhydrophobic. The researchers showed that they could make superhydrophobic graphene with three unclamped shrinks.

The team also showed that crumpling could enhance the electrochemical behaviors of graphene, which could be useful in next-generation energy storage and generation. The research showed that crumpled graphene used as a battery electrode had as much as a 400 percent increase in electrochemical current density over flat graphene sheets. That increase in current density could make for vastly more efficient batteries.

“You don’t need a new material to do it,” Chen said. “You just need to crumple the graphene.”

In additional to batteries and water resistant coatings, graphene compressed in this manner might also be useful in stretchable electronics–a wearable sensor, for example.

The group plans to continue experimenting with different ways of generating structures on graphene and other nanomaterials.

“There are many new two-dimensional nanomaterials that have interesting properties, not just graphene,” Wong said. “So other materials or combinations of materials may also organize into interesting structures with unexpected functionalities.”

Material scientists at Lawrence Livermore National Laboratory have found certain metal oxides increase capacity and improve cycling performance in lithium-ion batteries.

The team synthesized and compared the electrochemical performance of three graphene metal oxide nanocomposites and found that two of them greatly improved reversible lithium storage capacity.

The research appears on the cover of the March 21 edition of the Journal of Materials Chemistry A.

Graphene-metal oxide (GMO) nanocomposites have become renowned for their potential in energy storage and conversion, including capacitors, lithium-ion batteries, catalysis (for fuel cells, water splitting and air cleaning) and sensors.

For applications in lithium-ion batteries, nanosized metal oxide (MO) particles and highly conductive graphene are considered beneficial for shortening lithium diffusion pathways and reducing polarization in the electrode, leading to enhanced performance.

In the experiments, the team dipped prefabricated graphene aerogel electrodes in metal ion solutions where all metal oxide nanoparticles appear to be anchored on the surface of graphene and are fully accessible to the electrolyte (i.e., open pore space).

“In essence, our approach helps to optimize the system-level performance by ensuring that most metal oxides are active,” said LLNL material scientist Morris Wang and corresponding author of the paper.

The method can deposit most types of MOs onto the same prefabricated 3D graphene structure, allowing for direct comparison of electrochemical performance of a wide range of GMOs.

“We found that the experiments showed large reversible lithium storage capacities of graphene sheets, enabled by the unheralded roles of metal oxides,” Wang said. “Surprisingly we saw the magnitude of capacity contributions from graphene is mainly determined by active materials and the type of MO bound onto the graphene surface.”

Specifically, the lithium storage mechanisms of MOs and their loading ratio versus graphene play key roles in determining graphene capacity contributions.

In an article published in Nature today, researchers at Lund University in Sweden show how different arrangements of atoms can be combined into nanowires as they grow. Researchers learning to control the properties of materials this way can lead the way to more efficient electronic devices.

Nanowires are believed to be important elements in several different areas, such as in future generations of transistors, energy efficient light emitting diodes (LEDs) and solar cells.

The fact that it is possible to affect how nanowires are formed and grow has been known for a long time. What researchers have now been able to show is what needs to be done to give the nanowires a particular structure.

The gound-breaking discovery includes showing how nanowires grow, and affect the formation of different atomic layers, by using a powerful microscope and theoretical analysis.

“We now have on tape the events that take place, and what is required to be able to control the nanowire growth”, says Daniel Jacobsson, former doctoral student at the Lund University Faculty of Engineering, and currently a research engineer at the Lund University Centre for Chemistry and Chemical Engineering.

The team wanted to understand how nanowires grow, and chose to film them though an electron microscope. The article in Nature is about these films, which show nanowires made from gallium arsenide and composed of different crystal structures.

“The nanowires grow through a self-assembly process which is spontaneous and hard to control. But if we can understand how the nanowires grow, we can control the structures that are formed in a more precise way, and thereby create new types of structures for new fields of application”, says Daniel Jacobsson.

At the Centre for Chemistry and Chemical Engineering in Lund, a world-leading “super microscope” is under construction, which will be able to show, in high resolution, how atoms are joined together when nanostructures are formed.

“In our Nature article, we show how dynamic the growth of nanowires really is. Once the new microscope is in place, we hope to be able to provide even more details and expand the scope of materials studied.

Both the current results, and hopefully those to come, are important for an even more exact formation of nanowires for various applications”, says Professor Kimberly Dick Thelander.

Study about nanowires

Nanotechnology could be seen as engineering of functional systems at the atomic scale, which illustrates the growth of nanowires, where different atomic layers are stacked on top of each other. In the study Interface Dynamics and Crystal Phase Switching in GaAs Nanowires, the researchers were able to monitor in real time where each new atomic layer is placed in a growing nanowire, and explain why they place themselves where they do. The study shows that it is possible to control the position of each new atomic layer, and was conducted in collaboration with researchers at the IBM T. J. Watson Research Center, USA, and Cambridge University, UK.

Nanowires

A nanowire is an extremely thin wire with a diameter equal to one thousandth of a human hair. They are made out of many different materials, for example metals such as silver and nickel, semiconductor materials such as silicon and gallium arsenide, and insulating material such as silicon oxide.

Nanowires are useful because they enable the formation of complex structures with many chemical compounds, and sometimes different atomic arrangements. Nanowires are usually made out of single crystals, and the specific atomic arrangement is what determines the structure of the crystal.

Every new type of complicated structure – whether it be a combination of different materials or a new way of joining atoms together – involve new properties and thereby different applications in areas such as electronics and lighting.

Two-dimensional electronic devices could inch closer to their ultimate promise of low power, high efficiency and mechanical flexibility with a processing technique developed at the Department of Energy’s Oak Ridge National Laboratory.

A team led by Olga Ovchinnikova of ORNL’s Center for Nanophase Materials Sciences Division used a helium ion microscope, an atomic-scale “sandblaster,” on a layered ferroelectric surface of a bulk copper indium thiophosphate. The result, detailed in the journal ACS Applied Materials and Interfaces, is a surprising discovery of a material with tailored properties potentially useful for phones, photovoltaics, flexible electronics and screens.

This diagram illustrates the effect of helium ions on the mechanical and electrical properties of the layered ferroelectric: a.) Disappearance domains in the exposed area; as the mound forms yellow regions (ferroelectricity) gradually disappear; b.) Mechanical properties of the material; warmer colors indicate hard areas, cool colors indicate soft areas; c.) Conductivity enhancement; warmer colors show insulating areas, cooler colors show more conductive areas. Credit: ORNL

This diagram illustrates the effect of helium ions on the mechanical and electrical properties of the layered ferroelectric: a.) Disappearance domains in the exposed area; as the mound forms yellow regions (ferroelectricity) gradually disappear; b.) Mechanical properties of the material; warmer colors indicate hard areas, cool colors indicate soft areas; c.) Conductivity enhancement; warmer colors show insulating areas, cooler colors show more conductive areas. Credit: ORNL

“Our method opens pathways to direct-write and edit circuitry on 2-D material without the complicated current state-of-the-art multi-step lithographic processes,” Ovchinnikova said.

She and colleague Alex Belianinov noted that while the helium ion microscope is typically used to cut and shape matter, they demonstrated that it can also be used to control ferroelectric domain distribution, enhance conductivity and grow nanostructures. Their work could establish a path to replace silicon as the choice for semiconductors in some applications.

“Everyone is looking for the next material – the thing that will replace silicon for transistors,” said Belianinov, the lead author. “2-D devices stand out as having low power consumption and being easier and less expensive to fabricate without requiring harsh chemicals that are potentially harmful to the environment.”

Reducing power consumption by using 2-D-based devices could be as significant as improving battery performance. “Imagine having a phone that you don’t have to recharge but once a month,” Ovchinnikova said.

With new technology getting smaller and smaller, requiring lower power, University of Cincinnati physics research points to new robust electronic technologies using quantum nanowire structures.

The semiconductor nanowires may lead to advances in sensitive electronic technology including heat detecting optical infrared sensors and biomedical testing, all of which can fit inside small electrical devices.

Supported by multiple National Science Foundation grants, the UC research team is working with a collaborative team of physicists, electronic materials engineers and doctoral students from around the world — all to perfect the growth and development of crystalline nanowires that could form the backbone of new nanotechnologies.

But to fully apply this technology to modern devices, UC researchers are first looking closely — on a fundamental level — at how energy is distributed and measured along thin-strand nanowires so small that thousands of them could theoretically fit inside a human hair.

“Now that we know the technology can be developed, we need to understand exactly how the electrical processes work inside the nanowire cores,” say Howard Jackson and Leigh Smith, professors of physics at the University of Cincinnati. “After finally perfecting a standardized process for growing and developing crystalline nanowire fibers with our partners at the Australian National University in Canberra, we have been able to take it one step further.

“Using a combination of materials like indium gallium arsenide and indium phosphide, we can develop thin nanowire cores with protective outer shells.”

It turns out that these unique nanowire materials have unusually large spin orbit interactions, which the researchers find can conduct electricity really well and may allow the use of spin to enable new computing paradigms.

Jackson and Smith are presenting these findings at the American Physical Society Conference, in Baltimore, March 16, titled, “Exploring Dynamics and Band Structure in Mid Infrared GaAsSb and GaAsSb/InP Nanowire Heterostructures.”

Small yet mighty

The researchers claim the secret to the success of this multi-collaborative effort is in the combination of materials used to create the nanowires. Initially grown at the Australian National University in Canberra, the nanowires are sprouted from a combination of beads of molten gold scattered across a particular surface.

As the process is heated inside a chamber using indium gallium arsenide gases, long microscopically thin core fibers sprout up from between the controlled surface environment.

Other material combinations are then introduced to form an outer shell acting as a sheath around each core, resulting in quantum nanowire semiconducting heterostructures all uniform in size, shape and behavior.

After the fibers are shipped across the globe to Cincinnati, Jackson, Smith and their team of doctoral students are then able to use sophisticated equipment to measure the electrical and photovoltaic potentials of each fiber along its surface.

In earlier research, the collaborative team found extrinsic and intrinsic problems when the fiber cores did not have the outer sheath-like shells.

“If we don’t have this outer sheath, the nanowires have a very short energy lifetime, says Jackson. “When we surround the core with this sheath, the energy lifetime can go up by an order or two orders of magnitude.”

And while gallium arsenide alone is a very common semiconductor, its energy gap is large and in the visible range, which absorbs light. To achieve success in detecting optical heat or infrared, the team says using indium gallium arsenide fibers have smaller energy gaps that can be used successfully in optical detector devices. doctoral student in physics lab with laser lights

“The goal for one of our research equipment grants is to work with the local L3 Cincinnati Electronics Company, which makes infrared (small gap) detectors for night-vision imaging for military applications,” says Smith. “Future direct applications for this type of technology also include medical devices that detect body heat, as well as remote sensors installed in iphones that can be used for environmental purposes that detect and measure heat loss in houses.”

The researchers say this new nanowire technology is unique because it can turn different wavelengths of light into an electrical signal, and in this case it means turning an infrared light into an electric signal that can be measured.

Smith explains that with the geometry of the nanowires you can have a long axis running the length of the wire, which gives you lots of possibilities for absorption as the light comes down, but then you also have this very small diameter.

“When contacts are interspersed along either side, essentially then the electrons in the holes don’t have to travel very far before they are collected,” says Smith. “So in principle it can become a more effective detector as well as a more effective solar cell.”

Small dimension nanowires

“When you get to very small dimensions in nanowires that are small in diameter, but are a few microns long, those properties then change and can show quantum properties and become almost one-dimensional,” says Jackson. “The physics then changes as you change those sizes.”

Jackson and Smith found that the nanowire’s ultra-thin outer shells functioned best at widths of four to eight nanometers, which is 25,00 and 12,500 times smaller respectively, than the diameter of a human hair.

When looking at the overarching benefits of working with microscopic nanostructures the researchers see tremendous potential for its ability to pack much more high-energy efficiency into small devices with finite space. It’s getting closer to a win-win for everyone, they’re saying, especially when this research enters the next stage, bringing it closer to functioning inside electronic and optical sensor devices.

“Our fundamental investigation is still a step away from a direct optical device application,” says Jackson. “But you can clearly see over time that this collaborative research has made an impact.”

Light and electrons interact in a complex dance within fiber optic devices. A new study by University of Illinois engineers found that in the transistor laser, a device for next-generation high-speed computing, the light and electrons spur one another on to faster switching speeds than any devices available.

Milton Feng, the Nick Holonyak Jr. Emeritus Chair in electrical and computer engineering, found the speed-stimulating effects with graduate students Junyi Qiu and Curtis Wang and Holonyak, the Bardeen Emeritus Chair in electrical and computer engineering and physics. The team published its results in the Journal of Applied Physics.

As big data become bigger and cloud computing becomes more commonplace, the infrastructure for transferring the ever-increasing amounts of data needs to speed up, Feng said. Traditional technologies used for fiber optic cables and high-speed data transmission, such as diode lasers, are reaching the upper end of their switching speeds, Feng said.

“You can compute all you want in a data center. However, you need to take that data in and out of the system for the user to use,” Feng said. “You need to transfer the information for it to be useful, and that goes through these fiber optic interconnects. But there is a fundamental switching limitation of the diode laser used. This technology, the transistor laser, is the next-generation technology, and could be a hundred times faster.”

Diode lasers have two ports: an electrical input and a light output. By contrast, the transistor laser has three ports: an electrical input, and both electrical and light outputs.

The three-port design allows the researchers to harness the intricate physics between electrons and light. For example, the fastest way for current to switch in a semiconductor material is for the electrons to jump between bands in the material in a process called tunneling. Light photons help shuttle the electrons across, a process called photon-assisted tunneling, making the device much faster.

In the latest study, Feng’s group found that not only does photon-assisted tunneling occur in the transistor laser, but that it in turn stimulates the photon absorption process within the laser cavity, making the optical switching in the device even faster and allowing for ultra-high-speed signal modulation.

“The collector can absorb the photon from the laser for very quick tunneling, so that becomes a direct-voltage-modulation scheme, much faster than using current modulation,” Feng said. “We also proved that the stimulated photon-assisted tunneling process is much faster than regular photon-assisted tunneling. Previous engineers could not find this because they did not have the transistor laser. With just a diode laser, you cannot discover this.

“This is not only proving the scientific point, but it’s very useful for high-speed device modulation. We can directly modulate the laser into the femtosecond range. That allows a tremendous amount of energy-efficient data transfer,” Feng said.

The researchers plan to continue to develop the transistor laser and explore its unique physics while also forming industry partnerships to commercialize the technology for energy-efficient big data transfer.

A group of researchers from the UK, including academics from Cardiff University, has demonstrated the first practical laser that has been grown directly on a silicon substrate.

It is believed the breakthrough could lead to ultra-fast communication between computer chips and electronic systems and therefore transform a wide variety of sectors, from communications and healthcare to energy generation.

The EPSRC-funded UK group, led by Cardiff University and including researchers from UCL and the University of Sheffield, have presented their findings in the journal Nature Photonics.

Silicon is the most widely used material for the fabrication of electronic devices and is used to fabricate semiconductors, which are embedded into nearly every device and piece of technology that we use in our everyday lives, from smartphones and computers to satellite communications and GPS.

Electronic devices have continued to get quicker, more efficient and more complex, and have therefore placed an added demand on the underlining technology.

Researchers have found it increasingly difficult to meet these demands using conventional electrical interconnects between computer chips and systems, and have therefore turned to light as a potential ultra-fast connector.

Whilst it has been difficult to combine a semiconductor laser – the ideal source of light – with silicon, the UK group have now overcome these difficulties and successfully integrated a laser directly grown onto a silicon substrate for the very first time.

Professor Huiyun Liu, who led the growth activity, explained that the 1300nm wavelength laser has been shown to operate at temperatures of up to 120°C and for up to 100,000 hours.

Professor Peter Smowton, from Cardiff University’s School of Physics and Astronomy, said: “Realising electrically-pumped lasers based on Si substrates is a fundamental step towards silicon photonics.

“The precise outcomes of such a step are impossible to predict in their entirety, but it will clearly transform computing and the digital economy, revolutionise healthcare through patient monitoring, and provide a step-change in energy efficiency.

“Our breakthrough is perfectly timed as it forms the basis of one of the major strands of activity in Cardiff University’s Institute for Compound Semiconductors and the University’s joint venture with compound semiconductor specialists IQE.”

Professor Alwyn Seeds, Head of the Photonics Group at University College London, said: “The techniques that we have developed permit us to realise the Holy Grail of silicon photonics – an efficient and reliable electrically driven semiconductor laser directly integrated on a silicon substrate. Our future work will be aimed at integrating these lasers with waveguides and drive electronics leading to a comprehensive technology for the integration of photonics with silicon electronics.”

For most of human history, the discovery of new materials has been a crapshoot. But now, UConn researchers have systematized the search with machine learning that can scan millions of theoretical compounds for qualities that would make better solar cells, fibers, and computer chips. The search for new materials may never be the same.

No one knows why an early metallurgist decided to smelt a hunk of tin into some copper, but the resulting bronze alloy was harder and more durable than any material previously known. Most materials experimentation over the ensuing 7,000 years has been similarly random, guided largely by philosophy and chemical intuition.

But in a world that contains at least 95 stable elements – the basic building blocks of matter – the number of possible combinations is enormous, and experimentation is an awfully inefficient way to find what you’re looking for.

Enter UConn materials scientist Ramamurthy ‘Rampi’ Ramprasad. Instead of randomly mixing chemicals to see what they do, Ramprasad designs them rationally, using machine learning to figure out which atomic configurations make a polymer a good electrical conductor or insulator.

This is a schematic diagram of machine learning for materials discovery. Credit: Chiho Kim, Ramprasad Lab, UConn

This is a schematic diagram of machine learning for materials discovery. Credit:
Chiho Kim, Ramprasad Lab, UConn

A polymer is a large molecule made of many repeating building blocks. Polymers are very common in both living and man-made materials. Probably the most familiar example is plastics, and the wide variation in plastics – which can be hard, soft, stretchy, brittle, spongy, clear, opaque or translucent – gives an inkling of how diverse polymers in general can be.

Polymers can also have diverse electronic properties. For example, they can be very good insulators – preventing electrons, and thus electric current, from traveling through them – or good conductors, allowing electricity to pass through them freely. And what controls all these properties is mainly how the atoms in the polymer connect to each other. But until recently, no one had systematically related properties to atomic configurations.

So Ramprasad and his colleagues decided to do just that. First, they would analyze known polymers, using laborious but accurate quantum mechanics-based calculations to figure out which arrangements of atoms confer which properties, and quantify those atomic-level relationships via a string of numbers that fingerprint each polymer. Once they had those, they could have a computer search through any number of theoretical polymers to figure out which ones might have which properties. Then anyone looking for a polymer with a certain property could quickly scan the list and decide which theoretical polymers might be worth trying.

Many polymers are made of building blocks containing just a few atoms. For instance, polyurea, a common plastic, has as the basic structure a repeating sequence of nitrogen (N), hydrogen (H) and oxygen (O): NH-O-NH-O. Most polymers look like that, made of carbon (C), H, N and O, with a few other elements thrown in occasionally.

For their project, Ramprasad’s group looked at polymers made of just seven building blocks: CH2, C6H4, CO, O, NH, CS, and C4H2S (the S is sulfur). These are found in common plastics such as polyethylene, polyesters, and polyureas. An enormous variety of polymers could theoretically be constructed using just these building blocks; Ramprasad’s group decided at first to analyze just 283, each composed of a repeated four-block unit.

They started from basic quantum mechanics, and calculated the three-dimensional atomic and electronic structures of each of those 283 four-block polymers. This is not trivial: calculating the position of every electron and atom in a molecule with more than two atoms takes a powerful computer a significant chunk of time, which is why they did it for only 283 molecules.

Once they had the three-dimensional structures, they could calculate what they really wanted to know: each polymer’s properties. They calculated the band gap, which is the amount of energy it takes for an electron in the polymer to break free of its home atom and travel around the material, and the dielectric constant, which is a measure of the effect an electric field can have on the polymer. These properties translate to how much electric energy each polymer can store in itself. The researchers used established techniques that have long been known. They take a prohibitive amount of computing time, which is why it’s so hard to evaluate materials this way.

Ramprasad’s group then went one step further. They wanted a shorthand system that a computer could use to look at the building blocks of a polymer and how they connect to each other, and make educated guesses about its properties.

Computers deal with numbers, so first they had to define each polymer as a string of numbers, a sort of numerical fingerprint. Since there are seven possible building blocks, there are seven possible numbers, each indicating how many of each block type are contained in that polymer. But a simple number string like that doesn’t give enough information about the polymer’s structure, so they added a second string of numbers that tell how many pairs there are of each combination of building blocks, such as NH-O or C6H4-CS. Still not quite enough information, so they added a third string that described how many triples, like NH-O-CH2, there were. They arranged these strings as a three-dimensional matrix, which is a convenient way to describe such strings of numbers in a computer.

Then they let the computer go to work. Using the library of 283 polymers they had laboriously calculated using quantum mechanics, the machine compared each polymer’s numerical fingerprint to its band gap and dielectric constant, and gradually ‘learned’ which building block combinations were associated with which properties. It could even map those properties onto a two-dimensional matrix of the polymer building blocks.

Once the machine learned which atomic building block combinations gave which properties, it no longer needed the quantum mechanics calculations of atomic structure. It could accurately evaluate the band gap and dielectric constant for any polymer made of any combination of those seven building blocks, using just the numerical fingerprint of its structure.

Many of the predictions of quantum mechanics and the machine learning scheme have been validated by Ramprasad’s UConn collaborators, chemistry professor Greg Sotzing and electrical engineering professor Yang Cao. Sotzing actually made several of the novel polymers, and Cao tested their properties; they came out just as Ramprasad’s computations had predicted.

“What’s most surprising is the level of accuracy with which we can make predictions of the dielectric constant and band gap of a material using machine learning. These properties are generally computed using quantum mechanical methods such as density functional theory, which are six to eight orders of magnitude slower,” says Ramprasad. The group published a paper on their polymer work in Scientific Reports on Feb. 15; and another paper that utilizes machine learning in a different manner, namely, to discover laws that govern dielectric breakdown of insulators, will be published in a forthcoming issue of Chemistry of Materials.

But even if you don’t have access to those academic journals, you can see the predicted properties of every polymer Ramprasad’s group has evaluated in their online data vault, Khazana (khazana.uconn.edu), which also provides their machine learning apps to predict polymer properties on the fly. They are also uploading data and the machine learning tools from their Chemistry of Materials work, and from an additional recent article published in Scientific Reports on Jan. 19 on predicting the band gap of perovskites, inorganic compounds used in solar cells, lasers, and light-emitting diodes.

As a theoretical materials scientist, what Ramprasad wants to know is why materials behave the way they do. What about a polymer makes its dielectric constant just so? Or what makes an insulator withstand enormous electric fields without breaking down? But he also wants this understanding to be put to work to design new useful materials rationally. So he makes the results of his calculations freely available in the hope that someone else might look through them, see one, and go, “Wow. I’m looking for a material with exactly those properties!” and then make it. If it works as predicted, they’re both happy.

His work is aligned with a larger U.S. White House initiative called the Materials Genome Initiative. Much of Ramprasad’s work described here was funded by grants from the Office of Naval Research, as well as from the U.S. Department of Energy.

Graphene is a wonder material saddled with great expectations.

Discovered in 2004, it is 1 million times thinner than a human hair, 300 times stronger than steel and it’s the best known conductor of heat and electricity. These qualities could, among other things, make computers faster, batteries more powerful and solar panels more efficient.

But the material is tough to manipulate beyond its two-dimensional form.

Recently, scientists poured graphene oxide suspension, a gel-like form of the material, into freezing molds to create 3-D objects. The process works, but only with simple structures that have limited commercial applications.

Another option is to use a 3-D printer. In this scenario, scientists typically mix graphene with a polymer or other thickening agent. This helps keep the structure from falling apart. But when the polymer is removed via thermal process, it damages the delicate structure.

A research team – comprised of engineers from the University at Buffalo, Kansas State University and the Harbin Institute of Technology in China – may have solved that problem.

A study published Feb. 10 in the journal Small describes how the team used a modified 3-D printer and frozen water to create lattice-shaped cubes and a three-dimensional truss with overhangs using graphene oxide. The structures could be an important step toward making graphene commercially viable in electronics, medical diagnostic devices and other industries.

“Graphene is notoriously difficult to manipulate, but the structures we built show that it’s possible to control its shape in three-dimensional forms,” said Chi Zhou, assistant professor of industrial and systems engineering at UB’s School of Engineering and Applied Sciences, and a corresponding author of the study.

Zhou is a member of the Sustainable Manufacturing and Advanced Robotic Technologies (SMART), a UB Community of Excellence launched in 2015; he also is a member of UB’s New York State Center of Excellence in Materials Informatics.

In their experiments, the research team mixed the graphene oxide with water. They then printed the lattice framework on a surface of -25°C. The graphene is sandwiched between the layers of frozen ice, which act as a structural support.

After the process is completed, the lattice is dipped in liquid nitrogen, which helps form even stronger hydrogen bonds. The lattice is then placed in a freeze dryer, where the ice is changed into gas and removed. The end result is a complex, three-dimensional structure made of graphene aerogel that retains its shape at room temperature.

“By keeping the graphene in a cold environment, we were able to ensure that it retained the shape we designed. This is an important step toward making graphene a commercially viable material,” said Dong Lin, assistant professor of industrial and manufacturing systems engineering at Kansas State University, and the study’s other corresponding author.

The researchers plan to build on their findings by investigating how to create aerogel structures formed of multiple materials.