Nanoscale worlds sometimes resemble macroscale roller-coaster style hills, placed at the tip of a series of hexagons. Surprisingly, these nanohills stem from the self-organisation of particles - the very particles that have been eroded and subsequently redeposited following the bombardment of semi-conductors with ion beams. Now, a new theoretical study constitutes the first exhaustive investigation of the redeposition effect on the evolution of the roughening and smoothing of two-dimensional surfaces bombarded by multiple ions. The results demonstrate that the redeposition can indeed act as stabilising factor during the creation of the hexagonally arranged dot patterns observed in experiments. These findings by Christian Diddens from the Eindhoven University of Technology, in the Netherlands, and Stefan Linz, from Munster University, Germany, have been published in a study published in ("Continuum modeling of particle redeposition during ion-beam erosion"). To calculate multiple simulations of redeposition within reasonable computation times, the authors have developed an elaborate new highly efficient algorithm that combines established erosion models with a redeposition model. The latter made it possible to approximate the entire microscopic redeposition dynamics as a function of the relative height and the local slope of a coarse-grained surface. This approach is also supplemented by a new numerical algorithm to calculate precisely how the matter lifted by the ion beams is subsequently redeposited. This led to the realisation that eroded particles predominantly redeposit in the vicinity of the valleys, whereas almost no particles reattach at the hilltops. Overall, they found that the redeposition mechanism can contribute towards the formation of stable hexagonal patterns. They also confirmed that the aspect ratio of the well-ordered structures resulting from numerical simulation is comparable with experimental findings. This means that the reattachment of eroded particles can play an important role in the observed nanostructures formations. At the same, they comprehensively investigated the distribution of redepositing particles on patterned surfaces.
read more "Shaping the hilly landscapes of a semi-conductor nanoworld"
Plasmonic material could bring ultrafast all-optical communications
Researchers have created a new "plasmonic oxide material" that could make possible devices for optical communications that are at least 10 times faster than conventional technologies. In optical communications, laser pulses are used to transmit information along fiber-optic cables for telephone service, the Internet and cable television. Researchers at Purdue University have shown how an optical material made of aluminum-doped zinc oxide (AZO) is able to modulate – or change – how much light is reflected by 40 percent while requiring less power than other "all-optical" semiconductor devices. 
This rendering depicts a new "plasmonic oxide material" that could make possible devices for optical communications that are at least 10 times faster than conventional technologies. (Image: Nathaniel Kinsey) "Low power is important because if you want to operate very fast - and we show the potential for up to a terahertz or more - then you need low energy dissipation," said doctoral student Nathaniel Kinsey. "Otherwise, your material would heat up and melt when you start pushing it really fast. All-optical means that unlike conventional technologies we don't use any electrical signals to control the system. Both the data stream and the control signals are optical pulses." Being able to modulate the amount of light reflected is necessary for potential industrial applications such as data transmission. "We can engineer the film to provide either a decrease or an increase in reflection, whatever is needed for the particular application," said Kinsey, working with a team of researchers led by Alexandra Boltasseva, an associate professor of electrical and computer engineering, and Vladimir M. Shalaev, scientific director of nanophotonics at Purdue's Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering. "You can use either an increase or a decrease in the reflection to encode data. It just depends on what you are trying to do. This change in the reflection also results in a change in the transmission." Findings were detailed in a research paper appearing in July in the journal ("Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths"), published by the Optical Society of America. The material has been shown to work in the near-infrared range of the spectrum, which is used in optical communications, and it is compatible with the complementary metal–oxide–semiconductor (CMOS) manufacturing process used to construct integrated circuits. Such a technology could bring devices that process high-speed optical communications. The researchers have proposed creating an "all optical plasmonic modulator using CMOS-compatible materials," or an optical transistor. In electronics, silicon-based transistors are critical building blocks that switch power and amplify signals. An optical transistor could perform a similar role for light instead of electricity, bringing far faster systems than now possible. The paper, featured on the cover of the journal, was authored by Kinsey, graduate students Clayton DeVault and Jongbum Kim; visiting scholar Marcello Ferrera from Heriot-Watt University in Edinburgh, Scotland; Shalaev and Boltasseva. Exposing the material to a pulsing laser light causes electrons to move from one energy level called the valence band to a higher energy level called the conduction band. As the electrons move to the conduction band they leave behind "holes" in the valance band, and eventually the electrons recombine with these holes. The switching speed of transistors is limited by how fast it takes conventional semiconductors such as silicon to complete this cycle of light to be absorbed, excite electrons, produce holes and then recombine. "So what we would like to do is drastically speed this up," Kinsey said. This cycle takes about 350 femtoseconds to complete in the new AZO films, which is roughly 5,000 times faster than crystalline silicon and so fleeting that light travels only about 100 microns, or roughly the thickness of a sheet of paper, in that time. "We were surprised that it was this fast," Kinsey said. The increase in speed could translate into devices at least 10 times faster than conventional silicon-based electronics. The AZO films are said to be "Epsilon-near-zero," meaning the refractive index is near zero, a quality found normally in metals and new "metamaterials," which contain features, patterns or elements that enable unprecedented control of light by harnessing clouds of electrons called surface plasmons. Unlike natural materials, metamaterials are able to reduce the index of refraction to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material. The pulsing laser light changes the AZO's index of refraction, which, in turn, modulates the amount of reflection and could make higher performance possible. "If you are operating in the range where your refractive index is low then you can have an enhanced effect, so enhanced reflection change and enhanced transmission change," he said. The researchers "doped" zinc oxide with aluminum, meaning the zinc oxide is impregnated with aluminum atoms to alter the material's optical properties. Doping the zinc oxide causes it to behave like a metal at certain wavelengths and like a dielectric at other wavelengths. A new low-temperature fabrication process is critical to the material's properties and for its CMOS compatibility. "For industrial applications you can't go to really high fabrication temperatures because that damages underlying material on the chip or device," Kinsey said. "An interesting thing about these materials is that by changing factors like the processing temperature you can drastically change the properties of the films. They can be metallic or they can be very much dielectric." The AZO also makes it possible to "tune" the optical properties of metamaterials, an advance that could hasten their commercialization, Boltasseva said.
read more "Plasmonic material could bring ultrafast all-optical communications"
This rendering depicts a new "plasmonic oxide material" that could make possible devices for optical communications that are at least 10 times faster than conventional technologies. (Image: Nathaniel Kinsey) "Low power is important because if you want to operate very fast - and we show the potential for up to a terahertz or more - then you need low energy dissipation," said doctoral student Nathaniel Kinsey. "Otherwise, your material would heat up and melt when you start pushing it really fast. All-optical means that unlike conventional technologies we don't use any electrical signals to control the system. Both the data stream and the control signals are optical pulses." Being able to modulate the amount of light reflected is necessary for potential industrial applications such as data transmission. "We can engineer the film to provide either a decrease or an increase in reflection, whatever is needed for the particular application," said Kinsey, working with a team of researchers led by Alexandra Boltasseva, an associate professor of electrical and computer engineering, and Vladimir M. Shalaev, scientific director of nanophotonics at Purdue's Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering. "You can use either an increase or a decrease in the reflection to encode data. It just depends on what you are trying to do. This change in the reflection also results in a change in the transmission." Findings were detailed in a research paper appearing in July in the journal ("Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths"), published by the Optical Society of America. The material has been shown to work in the near-infrared range of the spectrum, which is used in optical communications, and it is compatible with the complementary metal–oxide–semiconductor (CMOS) manufacturing process used to construct integrated circuits. Such a technology could bring devices that process high-speed optical communications. The researchers have proposed creating an "all optical plasmonic modulator using CMOS-compatible materials," or an optical transistor. In electronics, silicon-based transistors are critical building blocks that switch power and amplify signals. An optical transistor could perform a similar role for light instead of electricity, bringing far faster systems than now possible. The paper, featured on the cover of the journal, was authored by Kinsey, graduate students Clayton DeVault and Jongbum Kim; visiting scholar Marcello Ferrera from Heriot-Watt University in Edinburgh, Scotland; Shalaev and Boltasseva. Exposing the material to a pulsing laser light causes electrons to move from one energy level called the valence band to a higher energy level called the conduction band. As the electrons move to the conduction band they leave behind "holes" in the valance band, and eventually the electrons recombine with these holes. The switching speed of transistors is limited by how fast it takes conventional semiconductors such as silicon to complete this cycle of light to be absorbed, excite electrons, produce holes and then recombine. "So what we would like to do is drastically speed this up," Kinsey said. This cycle takes about 350 femtoseconds to complete in the new AZO films, which is roughly 5,000 times faster than crystalline silicon and so fleeting that light travels only about 100 microns, or roughly the thickness of a sheet of paper, in that time. "We were surprised that it was this fast," Kinsey said. The increase in speed could translate into devices at least 10 times faster than conventional silicon-based electronics. The AZO films are said to be "Epsilon-near-zero," meaning the refractive index is near zero, a quality found normally in metals and new "metamaterials," which contain features, patterns or elements that enable unprecedented control of light by harnessing clouds of electrons called surface plasmons. Unlike natural materials, metamaterials are able to reduce the index of refraction to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material. The pulsing laser light changes the AZO's index of refraction, which, in turn, modulates the amount of reflection and could make higher performance possible. "If you are operating in the range where your refractive index is low then you can have an enhanced effect, so enhanced reflection change and enhanced transmission change," he said. The researchers "doped" zinc oxide with aluminum, meaning the zinc oxide is impregnated with aluminum atoms to alter the material's optical properties. Doping the zinc oxide causes it to behave like a metal at certain wavelengths and like a dielectric at other wavelengths. A new low-temperature fabrication process is critical to the material's properties and for its CMOS compatibility. "For industrial applications you can't go to really high fabrication temperatures because that damages underlying material on the chip or device," Kinsey said. "An interesting thing about these materials is that by changing factors like the processing temperature you can drastically change the properties of the films. They can be metallic or they can be very much dielectric." The AZO also makes it possible to "tune" the optical properties of metamaterials, an advance that could hasten their commercialization, Boltasseva said.
How to look for a few good catalysts
Two key physical phenomena take place at the surfaces of materials: catalysis and wetting. A catalyst enhances the rate of chemical reactions; wetting refers to how liquids spread across a surface. Now researchers at MIT and other institutions have found that these two processes, which had been considered unrelated, are in fact closely linked. The discovery could make it easier to find new catalysts for particular applications, among other potential benefits. “What’s really exciting is that we’ve been able to connect atomic-level interactions of water and oxides on the surface to macroscopic measurements of wetting, whether a surface is hydrophobic or hydrophilic, and connect that directly with catalytic properties,” says Yang Shao-Horn, the W.M. Keck Professor of Energy at MIT and a senior author of a paper describing the findings in the ("Reactivity of Perovskites with Water: Role of Hydroxylation in Wetting and Implications for Oxygen Electrocatalysis"). The research focused on a class of oxides called perovskites that are of interest for applications such as gas sensing, water purification, batteries, and fuel cells. 
Materials that have good wetting properties, as illustrated on the left, where droplets spread out flat, tend to have hydroxyl groups attached to the surface, which inhibits catalytic activity. Materials that repel water, as shown at right, where droplets form sharp, steep boundaries, are more conducive to catalytic activity, as shown by the reactions among small orange molecules. Since determining a surface’s wettability is “trivially easy,” says senior author Kripa Varanasi, an associate professor of mechanical engineering, that determination can now be used to predict a material’s suitability as a catalyst. Since researchers tend to specialize in either wettability or catalysis, this produces a framework for researchers in both fields to work together to advance understanding, says Varanasi, whose research focuses primarily on wettability; Shao-Horn is an expert on catalytic reactions. “We show how wetting and catalysis, which are both surface phenomena, are related,” Varanasi says, “and how electronic structure forms a link between both.” While both effects are important in a variety of industrial processes and have been the subject of much empirical research, “at the molecular level, we understand very little about what’s happening at the interface,” Shao-Horn says. “This is a step forward, providing a molecular-level understanding.” “It’s primarily an experimental technique” that made the new understanding possible, explains Kelsey Stoerzinger, an MIT graduate student and the paper’s lead author. While most attempts to study such surface science use instruments requiring a vacuum, this team used a device that could study the reactions in humid air, at room temperature, and with varying degrees of water vapor present. Experiments using this system, called ambient pressure X-ray photoelectron spectroscopy, revealed that the reactivity with water is key to the whole process, she says. The water molecules break apart to form hydroxyl groups — an atom of oxygen bound to an atom of hydrogen — bonded to the material’s surface. These reactive compounds, in turn, are responsible for increasing the wetting properties of the surface, while simultaneously inhibiting its ability to catalyze chemical reactions. Therefore, for applications requiring high catalytic activity, the team found, a key requirement is that the surface be hydrophobic, or non-wetting. “Ideally, this understanding helps us design new catalysts,” Stoerzinger says. If a given material “has a lower affinity for water, it has a higher affinity for catalytic activity.” Shao-Horn notes that this is an initial finding, and that “extension of these trends to broader classes of materials and ranges of hydroxyl affinity requires further investigation.” The team has already begun further exploration of these areas. This research, she says, “opens up the space of materials and surfaces we might think about” for both catalysis and wetting.
read more "How to look for a few good catalysts"
Materials that have good wetting properties, as illustrated on the left, where droplets spread out flat, tend to have hydroxyl groups attached to the surface, which inhibits catalytic activity. Materials that repel water, as shown at right, where droplets form sharp, steep boundaries, are more conducive to catalytic activity, as shown by the reactions among small orange molecules. Since determining a surface’s wettability is “trivially easy,” says senior author Kripa Varanasi, an associate professor of mechanical engineering, that determination can now be used to predict a material’s suitability as a catalyst. Since researchers tend to specialize in either wettability or catalysis, this produces a framework for researchers in both fields to work together to advance understanding, says Varanasi, whose research focuses primarily on wettability; Shao-Horn is an expert on catalytic reactions. “We show how wetting and catalysis, which are both surface phenomena, are related,” Varanasi says, “and how electronic structure forms a link between both.” While both effects are important in a variety of industrial processes and have been the subject of much empirical research, “at the molecular level, we understand very little about what’s happening at the interface,” Shao-Horn says. “This is a step forward, providing a molecular-level understanding.” “It’s primarily an experimental technique” that made the new understanding possible, explains Kelsey Stoerzinger, an MIT graduate student and the paper’s lead author. While most attempts to study such surface science use instruments requiring a vacuum, this team used a device that could study the reactions in humid air, at room temperature, and with varying degrees of water vapor present. Experiments using this system, called ambient pressure X-ray photoelectron spectroscopy, revealed that the reactivity with water is key to the whole process, she says. The water molecules break apart to form hydroxyl groups — an atom of oxygen bound to an atom of hydrogen — bonded to the material’s surface. These reactive compounds, in turn, are responsible for increasing the wetting properties of the surface, while simultaneously inhibiting its ability to catalyze chemical reactions. Therefore, for applications requiring high catalytic activity, the team found, a key requirement is that the surface be hydrophobic, or non-wetting. “Ideally, this understanding helps us design new catalysts,” Stoerzinger says. If a given material “has a lower affinity for water, it has a higher affinity for catalytic activity.” Shao-Horn notes that this is an initial finding, and that “extension of these trends to broader classes of materials and ranges of hydroxyl affinity requires further investigation.” The team has already begun further exploration of these areas. This research, she says, “opens up the space of materials and surfaces we might think about” for both catalysis and wetting.
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