New technique allows low-cost creation of 3-D nanostructures

Researchers from North Carolina State University have developed a new lithography technique that uses nanoscale spheres to create three-dimensional (3-D) structures with biomedical, electronic and photonic applications. The new technique is significantly less expensive than conventional methods and does not rely on stacking two-dimensional (2-D) patterns to create 3-D structures.

"Our approach reduces the cost of nanolithography to the point where it could be done in your garage," says Dr. Chih-Hao Chang, an assistant professor of mechanical and aerospace engineering at NC State and senior author of a paper on the work.

Most conventional lithography uses a variety of techniques to focus light on a photosensitive film to create 2-D patterns. These techniques rely on specialized lenses, electron beams or lasers -- all of which are extremely expensive. Other conventional techniques use mechanical probes, which are also costly. To create 3-D structures, the 2-D patterns are essentially printed on top of each other.

The NC State researchers took a different approach, placing nanoscale polystyrene spheres on the surface of the photosensitive film.

The nanospheres are transparent, but bend and scatter the light that passes through them in predictable ways according to the angle that the light takes when it hits the nanosphere. The researchers control the nanolithography by altering the size of the nanosphere, the duration of light exposures, and the angle, wavelength and polarization of light. The researchers can also use one beam of light, or multiple beams of light, allowing them to create a wide variety of nanostructure designs.

"We are using the nanosphere to shape the pattern of light, which gives us the ability to shape the resulting nanostructure in three dimensions without using the expensive equipment required by conventional techniques," Chang says. "And it allows us to create 3-D structures all at once, without having to make layer after layer of 2-D patterns."

The researchers have also shown that they can get the nanospheres to self-assemble in a regularly-spaced array, which in turn can be used to create a uniform pattern of 3-D nanostructures.

"This could be used to create an array of nanoneedles for use in drug delivery or other applications," says Xu Zhang, a Ph.D. student in Chang's lab and lead author of the paper.

The new technique could also be used to create nanoscale "inkjet printers" for printing electronics or biological cells, or to create antennas or photonic components.

"For this work, we focused on creating nanostructures using photosensitive polymers, which are commonly used in lithography," Zhang says. "But the technique could also be used to create templates for 3-D structures using other materials."

The researchers are currently looking at several additional ways to manipulate the technique to control the shape of resulting structures.

"We're exploring the use of nanosphere materials other than polystyrene, as well as nanoparticle shapes other than spheres," Chang says. "And ultimately we want to look at ways of controlling the placement of particles on the photosensitive film in patterns other than uniform arrays."

Selengkapnya »»  

Unusual electronic state found in new class of unconventional superconductors

A team of scientists from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory, Columbia Engineering, Columbia Physics and Kyoto University has discovered an unusual form of electronic order in a new family of unconventional superconductors. The finding, described in the journal Nature Communications, establishes an unexpected connection between this new group of titanium-oxypnictide superconductors and the more familiar cuprates and iron-pnictides, providing scientists with a whole new family of materials from which they can gain deeper insights into the mysteries of high-temperature superconductivity.

"Finding this new material is a bit like an archeologist finding a new Egyptian pharaoh's tomb," said Simon Billinge, a physicist at Brookhaven Lab and Columbia University's School of Engineering and Applied Science, who led the research team. "As we try and solve the mysteries behind unconventional superconductivity, we need to discover different but related systems to give us a more complete picture of what is going on-just as a new tomb will turn up treasures not found before, giving a more complete picture of ancient Egyptian society."

Harnessing the power of superconductivity, or the ability of certain materials to conduct electricity with zero energy loss, is one of the most exciting possibilities for creating a more energy-efficient future. But because most superconductors only work at very low temperatures-just a few degrees above absolute zero, or -273 degrees Celsius-they are not yet useful for everyday life. The discovery in the 1980s of "high-temperature" superconductors that work at warmer temperatures (though still not room temperature) was a giant step forward, offering scientists the hope that a complete understanding of what enables these materials to carry loss-free current would help them design new materials for everyday applications. Each new discovery of a common theme among these materials is helping scientists unlock pieces of the puzzle.

One of the greatest mysteries is seeking to understand how the electrons in high-temperature superconductors interact, sometimes trying to avoid each other and at other times pairing up-the crucial characteristic enabling them to carry current with no resistance. Scientists studying these materials at Brookhaven and elsewhere have discovered special types of electronic states, such as "charge density waves," where charges huddle to form stripes, and checkerboard patterns of charge. Both of these break the "translational symmetry" of the material-the repetition of sameness as you move across the surface (e.g., moving across a checkerboard you move from white squares to black squares).

Another pattern scientists have observed in the two most famous classes of high-temperature superconductors is broken rotational symmetry without a change in translational symmetry. In this case, called nematic order, every space on the checkerboard is white, but the shapes of the spaces are distorted from a square to a rectangle; as you turn round and round on one space, your neighboring space is nearer or farther depending on the direction you are facing. Having observed this unexpected state in the cuprates and iron-pnictides, scientists were eager to see whether this unusual electronic order would also be observed in a new class of titanium-oxypnictide high-temperature superconductors discovered in 2013.

"These titanium-oxypnictide compounds are structurally similar to the other exotic superconductor systems, and they had all the telltale signs of a broken symmetry, such as anomalies in resistivity and thermodynamic measurements. But there was no sign of any kind of charge density wave in any previous measurement. It was a mystery," said Emil Bozin, whose group at Brookhaven specializes in searching for hidden local broken symmetries. "It was a natural for us to jump on this problem."

The team searched for the broken rotational symmetry effect, a research question that had been raised by Tomo Uemura of Columbia, using samples provided by his collaborators in the group of Hiroshi Kageyama at Kyoto University. They conducted two kinds of diffraction studies: neutron scattering experiments at the Los Alamos Neutron Science Center (LANSCE) at DOE's Los Alamos National Laboratory, and electron diffraction experiments using a transmission electron microscope at Brookhaven Lab.

"We used these techniques to observe the pattern formed by beams of particles shot through powder samples of the superconductors under a range of temperatures and other conditions to see if there's a structural change that corresponds to the formation of this special type of nematic state," said Ben Frandsen, a graduate student in physics at Columbia and first author on the paper.

The experiments revealed a telltale symmetry breaking distortion at low temperature. A collaborative effort among experimentalists and theorists established the particular nematic nature of the order.

"Critical in this study was the fact that we could rapidly bring to bear multiple complementary experimental methods, together with crucial theoretical insights-something made easy by having most of the expertise in residence at Brookhaven Lab and wonderfully strong collaborations with colleagues at Columbia and beyond," Billinge said.

The discovery of nematicity in titanium-oxypnictides, together with the fact that their structural and chemical properties bridge those of the cuprate and iron-pnictide high-temperature superconductors, render these materials an important new system to help understand the role of electronic symmetry breaking in superconductivity.

As Billinge noted, "This new pharaoh's tomb indeed contained a treasure: nematicity."

This work was supported by the DOE Office of Science, the U.S. National Science Foundation (NSF, OISE-0968226), the Japan Society of the Promotion of Science, the Japan Atomic Energy Agency, and the Friends of Todai Inc.

Selengkapnya »»