04 Sep 2015
The technology, which is described online in the American Chemical Society journal Nano Letters, relies on a configuration of light-activated gold nanoparticles that harvest sunlight and transfer solar energy to highly excited electrons, which scientists sometimes refer to as “hot electrons.”
“Hot electrons have the potential to drive very useful chemical reactions, but they decay very rapidly, and people have struggled to harness their energy,” said lead researcher Isabell Thomann, assistant professor of electrical and computer engineering and of chemistry and materials science and nanoengineering at Rice. “For example, most of the energy losses in today’s best photovoltaic solar panels are the result of hot electrons that cool within a few trillionths of a second and release their energy as wasted heat.”
Capturing these high-energy electrons before they cool could allow solar-energy providers to significantly increase their solar-to-electric power-conversion efficiencies and meet a national goal of reducing the cost of solar electricity.
In the light-activated nanoparticles studied by Thomann and colleagues at Rice’s Laboratory for Nanophotonics (LANP), light is captured and converted into plasmons, waves of electrons that flow like a fluid across the metal surface of the nanoparticles. Plasmons are high-energy states that are short-lived, but researchers at Rice and elsewhere have found ways to capture plasmonic energy and convert it into useful heat or light. Plasmonic nanoparticles also offer one of the most promising means of harnessing the power of hot electrons, and LANP researchers have made progress toward that goal in several recent studies.
Rice University researchers have demonstrated an efficient new way to capture the energy from sunlight and convert it into clean, renewable energy by splitting water molecules. Credit: I. Thomann/Rice University
Thomann and her team, graduate students Hossein Robatjazi, Shah Mohammad Bahauddin and Chloe Doiron, created a system that uses the energy from hot electrons to split molecules of water into oxygen and hydrogen. That’s important because oxygen and hydrogen are the feedstocks for fuel cells, electrochemical devices that produce electricity cleanly and efficiently.
To use the hot electrons, Thomann’s team first had to find a way to separate them from their corresponding “electron holes,” the low-energy states that the hot electrons vacated when they received their plasmonic jolt of energy. One reason hot electrons are so short-lived is that they have a strong tendency to release their newfound energy and revert to their low-energy state. The only way to avoid this is to engineer a system where the hot electrons and electron holes are rapidly separated from one another. The standard way for electrical engineers to do this is to drive the hot electrons over an energy barrier that acts like a one-way valve. Thomann said this approach has inherent inefficiencies, but it is attractive to engineers because it uses well-understood technology called Schottky barriers, a tried-and-true component of electrical engineering.
Although the domestic solar-energy industry grew by 34 percent in 2014, fundamental technical breakthroughs are needed if the U.S. is to meet its national goal of reducing the cost of solar electricity to 6 cents per kilowatt-hour.
In a study published July 13 in Nature Communications, scientists from Rice’s Laboratory for Nanophotonics (LANP) describe a new method that solar-panel designers could use to incorporate light-capturing nanomaterials into future designs. By applying an innovative theoretical analysis to observations from a first-of-its-kind experimental setup, LANP graduate student Bob Zheng and postdoctoral research associate Alejandro Manjavacas created a methodology that solar engineers can use to determine the electricity-producing potential for any arrangement of metallic nanoparticles.
LANP researchers study light-capturing nanomaterials, including metallic nanoparticles that convert light into plasmons, waves of electrons that flow like a fluid across the particles’ surface. For example, recent LANP plasmonic research has led to breakthroughs in color-display technology, solar-powered steam production and color sensors that mimic the eye.
“One of the interesting phenomena that occurs when you shine light on a metallic nanoparticle or nanostructure is that you can excite some subset of electrons in the metal to a much higher energy level,” said Zheng, who works with LANP Director and study co-author Naomi Halas. “Scientists call these ‘hot carriers’ or ‘hot electrons.'”
Halas, Rice’s Stanley C. Moore Professor of Electrical and Computer Engineering and professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering, said hot electrons are particularly interesting for solar-energy applications because they can be used to create devices that produce direct current or to drive chemical reactions on otherwise inert metal surfaces.
Today’s most efficient photovoltaic cells use a combination of semiconductors that are made from rare and expensive elements like gallium and indium. Halas said one way to lower manufacturing costs would be to incorporate high-efficiency light-gathering plasmonic nanostructures with low-cost semiconductors like metal oxides. In addition to being less expensive to make, the plasmonic nanostructures have optical properties that can be precisely controlled by modifying their shape.
“We can tune plasmonic structures to capture light across the entire solar spectrum,” Halas said. “The efficiency of semiconductor-based solar cells can never be extended in this way because of the inherent optical properties of the semiconductors.”
The plasmonic approach has been tried before but with little success.
Zheng said, “Plasmonic-based photovoltaics have typically had low efficiencies, and it hasn’t been entirely clear whether those arose from fundamental physical limitations or from less-than-optimal designs.”
He and Halas said Manjavacas, a theoretical physicist in the group of LANP researcher Peter Nordlander, conducted work in the new study that offers a fundamental insight into the underlying physics of hot-electron-production in plasmonic-based devices.
Manjavacas said, “To make use of the photon’s energy, it must be absorbed rather than scattered back out. For this reason, much previous theoretical work had focused on understanding the total absorption of the plasmonic system.”
He said a recent example of such work comes from a pioneering experiment by another Rice graduate student, Ali Sobhani, where the absorption was concentrated near a metal semiconductor interface.
“From this perspective, one can determine the total number of electrons produced, but it provides no way of determining how many of those electrons are actually useful, high-energy, hot electrons,” Manjavacas said.
He said Zheng’s data allowed a deeper analysis because his experimental setup selectively filtered high-energy hot electrons from their less-energetic counterparts. To accomplish this, Zheng created two types of plasmonic devices. Each consisted of a plasmonic gold nanowire atop a semiconducting layer of titanium dioxide. In the first setup, the gold sat directly on the semiconductor, and in the second, a thin layer of pure titanium was placed between the gold and the titanium dioxide. The first setup created a microelectronic structure called a Schottky barrier and allowed only hot electrons to pass from the gold to the semiconductor. The second setup allowed all electrons to pass.
“The experiment clearly showed that some electrons are hotter than others, and it allowed us to correlate those with certain properties of the system,” Manjavacas said. “In particular, we found that hot electrons were not correlated with total absorption. They were driven by a different, plasmonic mechanism known as field-intensity enhancement.”
LANP researchers and others have spent years developing techniques to bolster the field-intensity enhancement of photonic structures for single-molecule sensing and other applications. Zheng and Manjavacas said they are conducting further tests to modify their system to optimize the output of hot electrons.
Halas said, “This is an important step toward the realization of plasmonic technologies for solar photovoltaics. This research provides a route to increasing the efficiency of plasmonic hot-carrier devices and shows that they can be useful for converting sunlight into usable electricity.”
Additional co-authors include Hangqi Zhao and Michael McClain, both of Rice. The research was supported by the Welch Foundation, the Office of Naval Research and the Air Force Office of Science and Research.
A copy of the Nature Communications paper is available at: http://www.
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17 Jul 2015
Rice researchers Rouzbeh Shahsavari and Navid Sakhavand have completed the first theoretical analysis of how 3-D boron nitride might be used as a tunable material to control heat flow in such devices.
Their work appears in Applied Materials and Interfaces.
In its 2-D form, hexagonal boron nitride (h-BN), aka white graphene, looks just like the atom-thick form of carbon known as graphene. One well-studied difference is that h-BN is a natural insulator, where perfect graphene presents no barrier to electricity.
But like graphene, h-BN is a good conductor of heat, which can be quantified in the form of phonons. (Technically, a phonon is one part—a “quasiparticle”—in a collective excitation of atoms.) Using boron nitride to control heat flow seemed worthy of a closer look, Shahsavari said.
“Typically in all electronics, it is highly desired to get heat out of the system as quickly and efficiently as possible,” he said. “One of the drawbacks in electronics, especially when you have layered materials on a substrate, is that heat moves very quickly in one direction, along a conductive plane, but not so good from layer to layer. Multiple stacked graphene layers is a good example of this.”
Heat moves ballistically across flat planes of boron nitride, too, but the Rice simulations showed that 3-D structures of h-BN planes connected by boron nitride nanotubes would be able to move phonons in all directions, whether in-plane or across planes, Shahsavari said.
A 3-D structure of hexagonal boron nitride sheets and boron nitride nanotubes could be a tunable material to control heat in electronics, according to researchers at Rice Univ. Image: The Shahsavari Group
The researchers calculated how phonons would flow across four such structures with nanotubes of various lengths and densities. They found the junctions of pillars and planes acted like yellow traffic lights, not stopping but significantly slowing the flow of phonons from layer to layer, Shahsavari said. Both the length and density of the pillars had an effect on the heat flow: more and/or shorter pillars slowed conduction, while longer pillars presented fewer barriers and thus sped things along.
While researchers have already made graphene/carbon nanotube junctions, Shahsavari believed such junctions for boron nitride materials could be just as promising. “Given the insulating properties of boron nitride, they can enable and complement the creation of 3-D, graphene-based nanoelectronics.
“This type of 3-D thermal-management system can open up opportunities for thermal switches, or thermal rectifiers, where the heat flowing in one direction can be different than the reverse direction,” Shahsavari said. “This can be done by changing the shape of the material, or changing its mass—say one side is heavier than the other—to create a switch. The heat would always prefer to go one way, but in the reverse direction it would be slower.”
Source: Rice Univ.
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This transmission electron microscope image shows a graphene quantum dot with zigzag edges. The quantum dots can be created in bulk from carbon fiber through a chemical process discovered at Rice University.
A Rice University laboratory has found a way to turn common carbon fiber into graphene quantum dots, tiny specks of matter with properties expected to prove useful in electronic, optical and biomedical applications.
The Rice lab of materials scientist Pulickel Ajayan, in collaboration with colleagues in China, India, Japan and the Texas Medical Center, discovered a one-step chemical process that is markedly simpler than established techniques for making graphene quantum dots. The results were published online this month in the American Chemical Society’s journal Nano Letters.
“There have been several attempts to make graphene-based quantum dots with specific electronic and luminescent properties using chemical breakdown or e-beam lithography of graphene layers,” said Ajayan, Rice’s Benjamin M. and Mary Greenwood Anderson Professor of Mechanical Engineering and Materials Science and of chemistry. “We thought that as these nanodomains of graphitized carbons already exist in carbon fibers, which are cheap and plenty, why not use them as the precursor?”
Quantum dots, discovered in the 1980s, are semiconductors that contain a size- and shape-dependent band gap. These have been promising structures for applications that range from computers, LEDs, solar cells and lasers to medical imaging devices. The sub-5 nanometer carbon-based quantum dots produced in bulk through the wet chemical process discovered at Rice are highly soluble, and their size can be controlled via the temperature at which they’re created.
The Rice researchers were attempting another experiment when they came across the technique. “We tried to selectively oxidize carbon fiber, and we found that was really hard,” said Wei Gao, a Rice graduate student who worked on the project with lead author Juan Peng, a visiting student from
Green-fluorescing graphene quantum dots created at Rice University surround a blue-stained nucleus in a human breast cancer cell. Cells were placed in a solution with the quantum dots for four hours. The dots, each smaller than 5 nanometers, easily passed through the cell membranes, showing their potential value for bio-imaging.
Nanjing University who studied in Ajayan’s lab last year. “We ended up with a solution and decided to look at a few drops with a transmission electron microscope.”
The specks they saw were bits of graphene or, more precisely, oxidized nanodomains of graphene extracted via chemical treatment of carbon fiber. “That was a complete surprise,” Gao said. “We call them quantum dots, but they’re two-dimensional, so what we really have here are graphene quantum discs.”
Gao said other techniques are expensive and take weeks to make small batches of graphene quantum dots. “Our starting material is cheap, commercially available carbon fiber. In a one-step treatment, we get a large amount of quantum dots. I think that’s the biggest advantage of our work,” she said.Further experimentation revealed interesting bits of information: The size of the dots, and thus their photoluminescent properties, could be controlled through processing at relatively low temperatures, from 80 to 120 degrees Celsius. “At 120, 100 and 80 degrees, we got blue, green and yellow luminescing dots,” she said.
They also found the dots’ edges tended to prefer the form known as zigzag. The edge of a sheet of graphene — the single-atom-thick form of carbon — determines its electrical characteristics, and zigzags are semiconducting.
Their luminescent properties give graphene quantum dots potential for imaging, protein analysis, cell tracking and other biomedical applications, Gao said. Tests at Houston’s MD Anderson Cancer Center and Baylor College of Medicine on two human breast cancer lines showed the dots easily found their way into the cytoplasm and did not interfere with the cells’ proliferation.
“The green quantum dots yielded a very good image,” said co-author Rebeca Romero Aburto, a graduate student in the Ajayan lab who also studies at MD Anderson. “The advantage of graphene dots over fluorophores is that their fluorescence is more stable and they don’t photobleach. They don’t lose their fluorescence as easily. They have a depth limit, so they may be good for in vitro and in vivo (small animal) studies, but perhaps not optimal for deep tissues in humans.
Dark spots on a transmission electron microscope grid are graphene quantum dots made through a wet chemical process at Rice University. The inset is a close-up of one dot. Graphene quantum dots may find use in electronic, optical and biomedical applications.
“But everything has to start in the lab, and these could be an interesting approach to further explore for bioimaging,” Romero Alburto said. “In the future, these graphene quantum dots could have high impact because they can be conjugated with other entities for sensing applications, too.”
Co-authors include Angel Martí, a professor of chemistry and bioengineering, postdoctoral research associates Zheng Liu and Liehui Ge, senior research scientist Lawrence Alemany and graduate student Xiaobo Zhan, all of Rice; Rice alumnus Li Song of Shinshu University, Japan; Bipin Kumar Gupta of the National Physical Laboratory, New Delhi, who worked at the Ajayan lab on an Indo-US Science and Technology Forum fellowship; Guanhui Gao of the Ocean University of China; Sajna Antony Vithayathil, a research technician, and Benny Abraham Kaipparettu, a postdoctoral researcher, both at Baylor College of Medicine; Takuya Hayashi, an associate professor of engineering at Shinshu University, Japan; and Jun-Jie Zhu, a professor of chemistry at Nanjing University.
The research was supported by Nanoholdings, the Office of Naval Research MURI program on graphene, the Natural Science Foundation of China, the National Basic Research Program of China, the Indo-US Science and Technology Forum and the Welch Foundation.
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Houston, TX | Posted on July 10th, 2014
Rice University’s breakthrough silicon oxide technology for high-density, next-generation computer memory is one step closer to mass production, thanks to a refinement that will allow manufacturers to fabricate devices at room temperature with conventional production methods.
First discovered five years ago, Rice’s silicon oxide memories are a type of two-terminal, “resistive random-access memory” (RRAM) technology. In a new paper available online in the American Chemical Society journal Nano Letters, a Rice team led by chemist James Tour compared its RRAM technology to more than a dozen competing versions.
“This memory is superior to all other two-terminal unipolar resistive memories by almost every metric,” Tour said. “And because our devices use silicon oxide — the most studied material on Earth — the underlying physics are both well-understood and easy to implement in existing fabrication facilities.” Tour is Rice’s T.T. and W.F. Chao Chair in Chemistry and professor of mechanical engineering and nanoengineering and of computer science.
Tour and colleagues began work on their breakthrough RRAM technology more than five years ago. The basic concept behind resistive memory devices is the insertion of a dielectric material — one that won’t normally conduct electricity — between two wires. When a sufficiently high voltage is applied across the wires, a narrow conduction path can be formed through the dielectric material.
The presence or absence of these conduction pathways can be used to represent the binary 1s and 0s of digital data. Research with a number of dielectric materials over the past decade has shown that such conduction pathways can be formed, broken and reformed thousands of times, which means RRAM can be used as the basis of rewritable random-access memory.
RRAM is under development worldwide and expected to supplant flash memory technology in the marketplace within a few years because it is faster than flash and can pack far more information into less space. For example, manufacturers have announced plans for RRAM prototype chips that will be capable of storing about one terabyte of data on a device the size of a postage stamp — more than 50 times the data density of current flash memory technology.
The key ingredient of Rice’s RRAM is its dielectric component, silicon oxide. Silicon is the most abundant element on Earth and the basic ingredient in conventional microchips. Microelectronics fabrication technologies based on silicon are widespread and easily understood, but until the 2010 discovery of conductive filament pathways in silicon oxide in Tour’s lab, the material wasn’t considered an option for RRAM.
Since then, Tour’s team has raced to further develop its RRAM and even used it for exotic new devices like transparent flexible memory chips. At the same time, the researchers also conducted countless tests to compare the performance of silicon oxide memories with competing dielectric RRAM technologies.
“Our technology is the only one that satisfies every market requirement, both from a production and a performance standpoint, for nonvolatile memory,” Tour said. “It can be manufactured at room temperature, has an extremely low forming voltage, high on-off ratio, low power consumption, nine-bit capacity per cell, exceptional switching speeds and excellent cycling endurance.”
This scanning electron microscope image and schematic show the design and composition of new RRAM memory devices based on porous silicon oxide that were created at Rice University.
Credit: Tour Group/Rice University
In the latest study, a team headed by lead author and Rice postdoctoral researcher Gunuk Wang showed that using a porous version of silicon oxide could dramatically improve Rice’s RRAM in several ways. First, the porous material reduced the forming voltage — the power needed to form conduction pathways — to less than two volts, a 13-fold improvement over the team’s previous best and a number that stacks up against competing RRAM technologies. In addition, the porous silicon oxide also allowed Tour’s team to eliminate the need for a “device edge structure.”
“That means we can take a sheet of porous silicon oxide and just drop down electrodes without having to fabricate edges,” Tour said. “When we made our initial announcement about silicon oxide in 2010, one of the first questions I got from industry was whether we could do this without fabricating edges. At the time we could not, but the change to porous silicon oxide finally allows us to do that.”
Wang said, “We also demonstrated that the porous silicon oxide material increased the endurance cycles more than 100 times as compared with previous nonporous silicon oxide memories. Finally, the porous silicon oxide material has a capacity of up to nine bits per cell that is highest number among oxide-based memories, and the multiple capacity is unaffected by high temperatures.”
Tour said the latest developments with porous silicon oxide — reduced forming voltage, elimination of need for edge fabrication, excellent endurance cycling and multi-bit capacity — are extremely appealing to memory companies.
“This is a major accomplishment, and we’ve already been approached by companies interested in licensing this new technology,” he said.
Study co-authors — all from Rice — include postdoctoral researcher Yang Yang; research scientist Jae-Hwang Lee; graduate students Vera Abramova, Huilong Fei and Gedeng Ruan; and Edwin Thomas, the William and Stephanie Sick Dean of Rice’s George R. Brown School of Engineering, professor in mechanical engineering and materials science and in chemical and biomolecular engineering.