Plasmonics study suggests how to maximize production of 'hot electrons'

New research from Rice University could make it easier for engineers to harness the power of light-capturing nanomaterials to boost the efficiency and reduce the costs of photovoltaic solar cells. 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 ("Distinguishing between plasmon-induced and photoexcited carriers in a device geometry"), 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. Diagram analyzing a hot electron Rice researchers selectively filtered high-energy hot electrons from their less-energetic counterparts using a Schottky barrier (left) created with a gold nanowire on a titanium dioxide semiconductor. A second setup (right), which did not filter electrons based on energy level, included a thin layer of titanium between the gold and the titanium dioxide. CREDIT: B. Zheng/Rice University 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.”
read more "Plasmonics study suggests how to maximize production of 'hot electrons'"

Make mine a decaf: breakthrough in knowledge of how nanoparticles grow

A team of researchers from the University of Leicester and France’s G2ELab-CNRS in Grenoble have for the first time observed the growth of free nanoparticles in helium gas in a process similar to the decaffeination of coffee, providing new insights into the structure of nanoparticles. Nanoparticles have a very large surface area compared with their volume and are often able to react very quickly. This makes them useful as catalysts in chemical reactions and they are often used in sports equipment, clothing and sunscreens. In a paper published by the ("Formation of Positively Charged Liquid Helium Clusters in Supercritical Helium and their Solidification upon Compression") and funded by the Royal Society, The Leverhulme Trust, the British Council and CONACYT, the teams from the University of Leicester’s Department of Physics and Astronomy and the CNRS in Grenoble measured how helium ions cluster with neutral helium atoms and grow into nanoparticles. Researchers observe how nanoparticles grow when exposed to helium Researchers observe how nanoparticles grow when exposed to helium. (© American Chemical Society) During the study they examined how helium ions drift through a cell filled with helium atoms. When the pressure of helium was increased the researchers observed a decrease in the mobility of the ions. Dr Klaus von Haeften from the University of Leicester’s Department of Physics and Astronomy, who has received a Visiting Professorship from the University Joseph Fourier, said: “We concluded that the increased pressure forced more and more helium atoms to bind to the ions gradually, until the clusters grew to nanometre-sized particles. This process continued until the nanoparticles reached the maximum size possible which also depended on the temperature. “Further increase of the pressure was found to reduce the size, which we interpreted as compression. These size changes could then be followed in great detail. For low and moderate pressures the size changed rather rapidly whereas in the high pressure region the changes were slow.” By analysing how quickly the particle volume changed with pressure the researchers were able to investigate the structure of the nanoparticles. Nelly Bonifaci from the G2ELab-CNRS said: "At low and moderate pressure the nanoparticles were much softer than solid helium and we concluded that they must be liquid. At high pressures they became progressively harder and eventually solid." Dr von Haeften added: “By choosing helium we were able to study a system of greatest possible purity and our results are therefore very precise. Similar processes occur in the decaffeination of coffee in high pressure carbon dioxide, in dry cleaning and in chemical manufacturing. In all these processes nanoparticles grow. By knowing their size we can much better understand these processes and improve them." This is the first time that researchers have been able to observe the growth of free nanoparticles in a large range of pressure in gaseous helium. Frédéric Aitken from the G2ELab-CNRS added: "Our work is an important benchmark for the research on the formation and size of nanoparticles.”
read more "Make mine a decaf: breakthrough in knowledge of how nanoparticles grow"

Artificial moth eyes enhance the performance of silicon solar cells

Mimicking the texture found on the highly antireflective surfaces of the compound eyes of moths, researchers at Brookhaven National Laboratory use block copolymer self assembly to produce precise and tunable nanotextured designs in the range of ~20 nm across macroscopic silicon solar cells ("Sub-50-nm Self-Assembled Nanotextures for Enhanced Broadband Antireflection in Silicon Solar Cells"). Moth eyes are highly antireflective due to their surface nanostructure Moth eyes are highly antireflective due to their surface nanostructure. This nanoscale texturing imparts broadband antireflection properties and significantly enhances performance compared with typical antireflection coatings. Proper design of an antireflection coating involves managing the refractive index mismatch at an abrupt optical interface. The most straightforward approach introduces a single layer of an intermediate optical index atop of a surface to create a system that engenders destructive interference in reflected light. This usually provides full antireflection at only a single wavelength. An image of a silicon moth eye, fabricated by polymer self-assembly An image of a silicon moth eye, fabricated by polymer self-assembly. Increasingly broadband coverage, for application in transparent window coatings, military camouflage, or solar cells, is possible using multilayered thin-film schemes. An alternative to thin-film coating strategies, nanoscale patterns applied to the surface of a material, can create an effective medium between the substrate and air. Such structures provide broadband antireflection over a wide range of incident light angles when nanoscale, sub-wavelength textures are sufficiently tall and closely spaced. In this work, the team enhances the broadband antireflection properties of a nanofabricated moth eye structure through simultaneous control of both the geometry and optical properties, using block copolymer self assembly to design nanotextures that are sufficiently small to take advantage of a beneficial material surface layer that is only a few nanometers thick. A polished, highly reflective silicon solar cell (right) turns completely black (left) after the application of surface nanotexture A polished, highly reflective silicon solar cell (right) turns completely black (left) after the application of surface nanotexture. Why does this matter? Self-assembly based approaches to produce texturing reduce reflections from silicon solar cell surfaces to less than 1% across entire visible and near infrared spectrum and across a wide range of incident light angles. Furhter, block-copolymer based approaches to material design are scalable for the manufacture of large-area photovoltaic devices, with potential for implementation in silicon, silicon nitride, and glass, among others.
read more "Artificial moth eyes enhance the performance of silicon solar cells"