Scientists have been making nanoparticles for more than two decades in two-dimensional sheets, three-dimensional crystals and random clusters. But they have never been able to get a sheet of nanoparticles to curve or fold into a complex three-dimensional structure. Now researchers from the University of Chicago, the University of Missouri and the U.S. Department of Energy's Argonne National Laboratory have found a simple way to do exactly that. 
This highly magnified image of a folded gold nanoparticle scroll shows that even though researchers can fold the membrane, the internal structure remains intact. (Image: Xiao-Min Lin et. al, taken using a scanning electron microscope at the University of Chicago) The findings open the way for scientists to design membranes with tunable electrical, magnetic and mechanical properties that could be used in electronics and may even have implications for understanding biological systems. Working at the Center for Nanoscale Materials (CNM) and the Advanced Photon Source (APS), two DOE Office of Science User Facilities located at Argonne, the team got membranes of gold nanoparticles coated with organic molecules to curl into tubes when hit with an electron beam. Equally importantly, they have discovered how and why it happens. The scientists coat gold nanoparticles of a few thousand atoms each with an oil-like organic molecule that holds the gold particles together. When floated on water the particles form a sheet; when the water evaporates, it leaves the sheet suspended over a hole. “It’s almost like a drumhead,” says Xiao-Min Lin, the staff scientist at the Center for Nanoscale Materials who led the project. “But it’s a very thin membrane made of a single layer of nanoparticles.” To their surprise, when the scientists put the membrane into the beam of a scanning electron microscope, it folded. It folded every time, and always in the same direction. “That got our curiosity up,” said Lin. “Why is it bending in one direction?” The answer lay in the organic surface molecules. They are hydrophobic: when floated on water they try to avoid contact with it, so they end up distributing themselves in a non-uniform way across the top and bottom layers of the nanoparticle sheet. When the electron beam hits the molecules on the surface it causes them to form an additional bond with their neighbors, creating an asymmetrical stress that makes the membranes fold. 
Argonne researchers are able to fold gold nanoparticle membranes in a specific direction using an electron beam because two sides of the membrane are different. (Image: Xiao-Min Lin et. al, taken at Argonne’s Electron Microscopy Center). (click on image to enlarge) Zhang Jiang and Jin Wang, X-ray staff at the APS, came up with an ingenious way to measure the molecular asymmetry, which at only six angstroms, or about six atoms thick, is so tiny it would not normally be measurable. Subramanian Sankaranarayanan and Sanket Deshmukh at CNM used the high-performance computing resources at DOE’s National Energy Research Scientific Computing Center and the Argonne Leadership Computing Facility (ALCF), both DOE Office of Science User Facilities, to analyze the surface of the nanoparticles. They discovered that the amount of surface covered by the organic molecules and the molecules’ mobility on the surface both have an important influence on the degree of asymmetry in the membrane. “These are fascinating results,” said Fernando Bresme, professor of chemical physics at the Imperial College in London and a leading theorist on soft matter physics. “They advance significantly our ability to make new nano-structures with controlled shapes.” In principle, scientists could use this method to induce folding in any nanoparticle membrane that has an asymmetrical distribution of surface molecules. Said Lin, “You use one type of molecule that hates water and rely on the water surfaces to drive the molecules to distribute non-uniformly, or you could use two different kinds of molecules. The key is that the molecules have to distribute non-uniformly.” The next step for Lin and his colleagues is to explore how they can control the molecular distribution on the surface and therefore the folding behavior. They envision zapping only a small part of the structure with the electron beam, designing the stresses to achieve particular bending patterns. “You can maybe fold these things into origami structures and all sorts of interesting geometries,” Lin said. “It opens the possibilities.”
read more "Bend me, shape me, any way you want me: Scientists curve nanoparticle sheets into complex forms"
This highly magnified image of a folded gold nanoparticle scroll shows that even though researchers can fold the membrane, the internal structure remains intact. (Image: Xiao-Min Lin et. al, taken using a scanning electron microscope at the University of Chicago) The findings open the way for scientists to design membranes with tunable electrical, magnetic and mechanical properties that could be used in electronics and may even have implications for understanding biological systems. Working at the Center for Nanoscale Materials (CNM) and the Advanced Photon Source (APS), two DOE Office of Science User Facilities located at Argonne, the team got membranes of gold nanoparticles coated with organic molecules to curl into tubes when hit with an electron beam. Equally importantly, they have discovered how and why it happens. The scientists coat gold nanoparticles of a few thousand atoms each with an oil-like organic molecule that holds the gold particles together. When floated on water the particles form a sheet; when the water evaporates, it leaves the sheet suspended over a hole. “It’s almost like a drumhead,” says Xiao-Min Lin, the staff scientist at the Center for Nanoscale Materials who led the project. “But it’s a very thin membrane made of a single layer of nanoparticles.” To their surprise, when the scientists put the membrane into the beam of a scanning electron microscope, it folded. It folded every time, and always in the same direction. “That got our curiosity up,” said Lin. “Why is it bending in one direction?” The answer lay in the organic surface molecules. They are hydrophobic: when floated on water they try to avoid contact with it, so they end up distributing themselves in a non-uniform way across the top and bottom layers of the nanoparticle sheet. When the electron beam hits the molecules on the surface it causes them to form an additional bond with their neighbors, creating an asymmetrical stress that makes the membranes fold. Argonne researchers are able to fold gold nanoparticle membranes in a specific direction using an electron beam because two sides of the membrane are different. (Image: Xiao-Min Lin et. al, taken at Argonne’s Electron Microscopy Center). (click on image to enlarge) Zhang Jiang and Jin Wang, X-ray staff at the APS, came up with an ingenious way to measure the molecular asymmetry, which at only six angstroms, or about six atoms thick, is so tiny it would not normally be measurable. Subramanian Sankaranarayanan and Sanket Deshmukh at CNM used the high-performance computing resources at DOE’s National Energy Research Scientific Computing Center and the Argonne Leadership Computing Facility (ALCF), both DOE Office of Science User Facilities, to analyze the surface of the nanoparticles. They discovered that the amount of surface covered by the organic molecules and the molecules’ mobility on the surface both have an important influence on the degree of asymmetry in the membrane. “These are fascinating results,” said Fernando Bresme, professor of chemical physics at the Imperial College in London and a leading theorist on soft matter physics. “They advance significantly our ability to make new nano-structures with controlled shapes.” In principle, scientists could use this method to induce folding in any nanoparticle membrane that has an asymmetrical distribution of surface molecules. Said Lin, “You use one type of molecule that hates water and rely on the water surfaces to drive the molecules to distribute non-uniformly, or you could use two different kinds of molecules. The key is that the molecules have to distribute non-uniformly.” The next step for Lin and his colleagues is to explore how they can control the molecular distribution on the surface and therefore the folding behavior. They envision zapping only a small part of the structure with the electron beam, designing the stresses to achieve particular bending patterns. “You can maybe fold these things into origami structures and all sorts of interesting geometries,” Lin said. “It opens the possibilities.”
Physics professor Yoke Khin Yap says the chemical structures of graphene (gray) and boron nitride nanotubes (pink and purple) are key in creating a digital switch. “The question is: How do you fuse these two materials together?” Yap says. The key is in maximizing their existing chemical structures and exploiting their mismatched features. Nanoscale Tweaks Graphene is a molecule-thick sheet of carbon atoms; the nanotubes are like straws made of boron and nitrogen. Yap and his team exfoliate graphene and modify the material’s surface with tiny pinholes. Then they can grow the nanotubes up and through the pinholes. Meshed together like this, the material looks like a flake of bark sprouting erratic, thin hairs. “When we put these two aliens together, we create something better,” Yap says, explaining that it’s important that the materials have lopsided band gaps, or differences in how much energy it takes to excite an electron in the material. “When we put them together, you form a band gap mismatch—that creates a so-called ‘potential barrier’ that stops electrons.” 
Microscopic image of graphene-BNNT heterojunctions obtained by (a,b) scanning electron microscopy (SEM). The band gap mismatch results from the materials’ structure: graphene’s flat sheet conducts electricity quickly, and the atomic structure in the nanotubes halts electric currents. This disparity creates a barrier, caused by the difference in electron movement as currents move next to and past the hair-like boron nitride nanotubes. These points of contact between the materials—called heterojunctions—are what make the digital on/off switch possible. “Imagine the electrons are like cars driving across a smooth track,” Yap says. “They circle around and around, but then they come to a staircase and are forced to stop.” Yap and his research team have also shown that because the materials are respectively so effective at conducting or stopping electricity, the resulting switching ratio is high. In other words, how fast the materials can turn on and off is several orders of magnitude greater than current graphene switches. In turn, this speed could eventually quicken the pace of electronics and computing. Solving the Semiconductor Dilemma To get to faster and smaller computers one day, Yap says this study is a continuation of past research into making transistors without semiconductors. The problem with semiconductors like silicon is that they can only get so small, and they give off a lot of heat; the use of graphene and nanotubes bypasses those problems. In addition, the graphene and boron nitride nanotubes have the same atomic arrangement pattern, or lattice matching. With their aligned atoms, the graphene-nanotube digital switches could avoid the issues of electron scattering. “You want to control the direction of the electrons,” Yap explains, comparing the challenge to a pinball machine that traps, slows down and redirects electrons. “This is difficult in high speed environments, and the electron scattering reduces the number and speed of electrons.” Much like an arcade enthusiast, Yap says he and his team will continue trying to find ways to outsmart or change the pinball set-up of graphene to minimize electron scattering. And one day, all their tweaks could make for faster computers—and digital pinball games—for the rest of us.
Doctoral students Billy McCulloch (left) and Mingzhe Yu (right) of The Ohio State University with a prototype of the new aqueous solar flow battery. The 20 percent comes from sunlight, which is captured by a unique solar panel on top of the battery, explained Yiying Wu, professor of chemistry and biochemistry at Ohio State. The solar panel is now a solid sheet, rather than a mesh as in the previous design. Another key difference comes from the use of a water-based electrolyte inside the battery. Because water circulates inside it, the new design belongs to an emerging class of batteries called aqueous flow batteries. "The truly important innovation here is that we've successfully demonstrated aqueous flow inside our solar battery," Wu said. As such, it is the first aqueous flow battery with solar capability. Or, as Wu and his team have dubbed it, the first "aqueous solar flow battery." "It's also totally compatible with current battery technology, very easy to integrate with existing technology, environmentally friendly and easy to maintain," he added. Researchers around the world are working to develop aqueous flow batteries because they could theoretically provide affordable power grid-level energy storage someday. The solar flow battery could thus bridge a gap between today's energy grid and sources of renewable energy. "This solar flow battery design can potentially be applied for grid-scale solar energy conversion and storage, as well as producing 'electrolyte fuels' that might be used to power future electric vehicles," said Mingzhe Yu, lead author of the paper and a doctoral student at Ohio State. Previously, Yu designed the solar panel out of titanium mesh, so that air could pass through to the battery. But the new aqueous flow battery doesn't need air to function, so the solar panel is now a solid sheet. The solar panel is called a dye-sensitized solar cell, because the researchers use a red dye to tune the wavelength of light it captures and converts to electrons. Those electrons then supplement the voltage stored in the lithium-anode portion of the solar battery. Something has to carry electrons from the solar cell into the battery, however, and that's where the electrolyte comes in. A liquid electrolyte is typically part salt, part solvent; previously, the researchers used the salt lithium perchlorate mixed with the organic solvent dimethyl sulfoxide. Now they are using lithium iodide as the salt, and water as the solvent. (Water is an inorganic solvent, and an eco-friendly one. And lithium iodide offers a high-energy storage capacity with low cost.) In tests, the researchers compared the solar flow battery's performance to that of a typical lithium-iodine battery. They charged and discharged the batteries 25 times. Each time, both batteries discharged around 3.3 volts. The difference was that the solar flow battery could produce the same output with less charging. The typical battery had to be charged to 3.6 volts to discharge 3.3 volts. The solar flow battery was charged to only 2.9 volts, because the solar panel made up the difference. That's an energy savings of nearly 20 percent. The project is still ongoing, and the solar flow design will undoubtedly evolve again as the researchers try to make the battery more efficient. Doctoral student and study co-author Billy McCulloch said that there are many different directions the research could take. "We hope to motivate the research community to further develop this technology into a practical renewable energy solution," he added. The team's ultimate goal is to boost the solar cell's contribution to the battery past its current 20 percent--maybe even to 100 percent. "That's our next step," Wu said, "to really achieve a fully solar-chargeable battery."