Thursday, September 1, 2011

Carbon nanotube structures changed by ‘attack’ from within, researchers discover

 A team of researchers involving scientists from The University of Nottingham has shown for the first time that chemical reactions at the nano-level which change the structure of carbon nanotubes can be sparked by an 'attack' from within.

The discovery challenges previous scientific thinking that the internal surface of the hollow nanostructures is chemically unreactive, largely restricting their use to that of an inert container or a 'nano-reactor' inside which other chemical reactions can take place.

Their research, published in the journal Nature Chemistry, shows that carbon nanotubes that have had their structures changed are exciting new materials that could be useful in the development of new technologies for gas storage devices, chemical sensors and parts of electronic devices such as transistors.

Dr Andrei Khlobystov, of the University's School of Chemistry, who led the work at Nottingham, said: "It has universally been accepted for some time now that the internal surface of carbon nanotubes -- or the concave side -- is chemically unreactive, and indeed we have been successfully using carbon nanotubes as nano-reactors.

"However, in the course of this new research we made the serendipitous discovery that in the presence of catalytically active transition metals inside the nanotube cavity, the nanotube itself can be involved in unexpected chemical reactions."

Carbon nanotubes are remarkable nanostructures with a typical diameter of 1-2 nanometres, which is 80,000 times smaller than the thickness of a human hair. Dr Khlobystov and his research associates were recently involved in the discovery -- published in Nature Materials -- that nanotubes can be used as a catalyst for the production of nanoribbon, atomically thin strips of carbon created from carbon and sulphur atoms. These nanoribbons could potentially be used as new materials for the next generation of computers and data storage devices that are faster, smaller and more powerful.

In this latest research, the scientists found that an individual atom of Rhenium metal (Re) sets off a chemical reaction leading to the transformation of the inner wall of the nanotube. Initially, the attack by the Rhenium creates a small defect in the nanotube wall which then gradually develops into a nano-sized protrusion by 'eating' additional carbon atoms.

The protrusion then rapidly increases in size and seals itself off, forming a unique carbon structure dubbed a NanoBud, so called because the protrusion on the carbon nanotube resembles a bud on a stem.

Previously, NanoBuds were believed to be formed outside the nanotube through reactions on the outer surface with carbon molecules called fullerenes.

The new study demonstrates for the first time that they can be formed from within, provided that a transition metal atom with suitable catalytic activity is present within the nanotube.

In collaboration with the Electron Microscopy of Materials Science group at Ulm University in Germany, the scientists have even been able to capture 'on camera' the chemical reaction of the transition metal atom with the nanotube in real time at the atomic level using the latest Aberration-Corrected High Resolution Transmission Electron Microscopy (AC-HRTEM). Their videos show nanotubes with a diameter of around 1.5 nanometers, while the NanoBuds are just 1 nanometer across.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University of Nottingham.

Journal Reference:

Thomas W. Chamberlain, Jannik C. Meyer, Johannes Biskupek, Jens Leschner, Adriano Santana, Nicholas A. Besley, Elena Bichoutskaia, Ute Kaiser, Andrei N. Khlobystov. Reactions of the inner surface of carbon nanotubes and nanoprotrusion processes imaged at the atomic scale. Nature Chemistry, 2011; DOI: 10.1038/nchem.1115

Future of inks, paints and coatings takes shape: Researchers determine that particle shape affects the 'coffee ring effect'

 If you've ever spilled a drop of coffee on a surface, you might have noticed the curious way the color concentrates at the edges when the coffee dries. This is known as the "coffee ring effect," and recently, researchers have determined that the shape of the particles in the liquid is an important factor in creating this pattern. The research results could eventually translate into new techniques or formulations for product coatings, or better inks and paints.

This work, published in the August 18 issue of the journal Nature was performed by Arjun Yodh and colleagues at the University of Pennsylvania.

"We found that if you change the shape of the particles in the solution, the coffee ring effect goes away, and you end up with a uniform coating," said Peter Yunker, a graduate student in Yodh's lab.

First, a little fluid dynamics: As the liquid in a droplet evaporates the edges remain fixed, so as the volume decreases fluid flows outward from the middle of the droplet to its edges. This flow carries particles to the edges, and round particles at the edge will pack closely. By the time all of the liquid in the droplet evaporates, most of the particles will be at the edge, producing the coffee ring effect.

Both the shape that liquid droplets take, and the way the shape changes as the droplets evaporate, is greatly influenced by surface tension at the air-liquid interface. This tension is a property of the interface, based on how the molecules in the liquid interact with one another versus the air. For example, liquids with a high surface tension, like water, may form a raised droplet, because the molecules are very attracted to one another and not so attracted to the air. In contrast, liquids with lower surface tension, like alcohols, are more likely to form flat spots instead of curved droplets.

The Yodh group found that elongated particles in a liquid behave differently than round ones because of the way they are affected by the surface tension of the air-liquid interface. The forces at work are even observable in a common breakfast cereal.

"If you make the particles elongated or ellipsoidal, they deform the air-water interface, which causes the particles to strongly attract one another. You can observe this effect in a bowl of cheerios-if there are only a few left they clump together in the middle of the bowl, due to the surface tension of the milk," explained Yunker.

This clumping changes the way the particles distribute themselves within the droplet. Even if the clumped ellipsoidal particles reach the edge of the droplet, they do not pack as closely as round particles. The loosely packed clumps eventually spread to cover the entire surface, filling it so an even coating of particles is deposited when evaporation is complete.

"This work gives us a new idea about how to make a uniform coating, relatively simply. If you change the particle shape, you can change the way a particle is deposited. You can also make mixtures. In some cases, even just a small amount of ellipsoids can change the way the particles deposit when they dry," said Yodh.

In future studies, the research team will explore drying and deposition of different types of fluids. They will also investigate different particle sizes and shapes, and the interplay of particle mixtures.

"This is an exciting scientific result with potential commercial applications, which was in part enabled by support of the Materials Research Science and Engineering Center at the University of Pennsylvania," said Mary Galvin, program director for the division of materials research at the National Science Foundation, which partially funded the research. The centers program, recently renamed Materials Research Centers and Teams, provides support for interdisciplinary materials research and education while addressing fundamental problems in science and engineering.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by National Science Foundation.

Journal Reference:

Peter J. Yunker, Tim Still, Matthew A. Lohr, A. G. Yodh. Suppression of the coffee-ring effect by shape-dependent capillary interactions. Nature, 2011; 476 (7360): 308 DOI: 10.1038/nature10344

Researchers find way to align gold nanorods on a large scale

Researchers from North Carolina State University have developed a simple, scalable way to align gold nanorods, particles with optical properties that could be used for emerging biomedical imaging technologies.

Aligning gold nanorods is important because they respond to light differently, depending on the direction in which the nanorods are pointed. To control the optical response of the nanorods, researchers want to ensure that all of the nanorods are aligned.

The NC State researchers developed a way to align the gold nanorods using electrospun polymer "nano/microfibers." Electrospinning is a way of producing fibers, with a liquid polymer being discharged from a needle and then solidifying. The researchers produced fibers as thin as 40 nanometers (nm) in diameter and as thick as three microns in diameter -- thus, nano/microfibers.

The researchers mixed the gold nanorods into the polymer solution, causing them to be incorporated directly into the polymer. The nanorods align when the fibers form. The force experienced by the liquid polymer as it is emitted from the electrospinning needle creates "streamlines" in the polymer solution.

"The nanorods are forced into alignment with these streamlines, like logs in a river that swing into alignment with the current," says Dr. Joe Tracy, an assistant professor of materials science and engineering at NC State and co-author of a paper describing the study. "And as the polymer solidifies, the aligned nanorods are locked into place."

"Electrospinning efforts at NC State are world-class and have yielded a wide range of novel and functional materials," adds Dr. Rich Spontak, a professor of chemical and biomolecular engineering and materials science and engineering at NC State and paper co-author. "What makes this result truly exciting is that the alignment is multiscale, or simultaneously achieved at different length scales. The nanorods are aligned at nanoscale dimensions, whereas the fibers are aligned at larger length scales."

This approach has been used in the past to align other kinds of nanorods, but this is the first time it has been done with gold nanorods. "To the best of our knowledge, this is also the first time nanorods of this size have been aligned in electrospun fibers," Tracy says, referring to the fact that the study focused on relatively short nanorods.

Specifically, the researchers used nanorods with an aspect ratio of 3.1. For example, that means that a nanorod measuring 10 nm wide would be 31 nm long. The nanorods in the study were approximately 49 nm long.

This aspect ratio is important, because it affects the way the nanorods interact with light -- and, therefore, their optical properties.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by North Carolina State University.

Journal Reference:

Kristen E. Roskov, Krystian A. Kozek, Wei-Chen Wu, Raghav K. Chhetri, Amy L. Oldenburg, Richard J. Spontak, Joseph B. Tracy. Long-Range Alignment of Gold Nanorods in Electrospun Polymer Nano/Microfibers. Langmuir, 2011; : 110811133320019 DOI: 10.1021/la2021066

Tiny gold particles boost organic solar cell efficiency: Plasmonic technique helps enhance power conversion by up to 20 percent

 In the world of solar energy, organic photovoltaic solar cells have a wide range of potential applications, but they are still considered an upstart. While these carbon-based cells, which use organic polymers or small molecules as semiconductors, are much thinner and less expensive to produce than conventional solar cells made with inorganic silicon wafers, they still lag behind in their ability to efficiently convert sunlight into electricity.

Now, UCLA researchers and their colleagues from China and Japan have shown that by incorporating gold nanoparticles into these organic photovoltaics -- taking advantage of the plasmonic effect, by which metal helps to enhance the absorption of sunlight -- they can significantly improve the cells' power conversion.

In a paper recently published in ACS Nano, the team of researchers, led by Yang Yang, a professor of materials science and engineering at the UCLA Henry Samueli School of Engineering and Applied Science and director of the Nano Renewable Energy Center at UCLA's California NanoSystems Institute, demonstrate how they sandwiched a layer of gold nanoparticles between two light-absorbing subcells in a tandem polymer solar cell in order to harvest a greater fraction of the solar spectrum.

They found that by employing the interconnecting gold-nanoparticle layer, they were able to enhance power conversion by as much as 20 percent. The gold nanoparticles create a strong electromagnetic field inside the thin organic photovoltaic layers by a plasmonic effect, which concentrates light so that much more of it can be absorbed by the subcells.

The team is the first to report a plasmonic-enhanced polymer tandem solar cell, having overcome the difficulties involved in incorporating metal nanostructures into the overall device structure.

"We have successfully demonstrated a highly efficient plasmonic polymer tandem solar cell by simply incorporating gold nanoparticles layer between two subcells," Yang said. "The plasmonic effect happening in the middle of the interconnecting layer can enhance both the top and bottom subcells simultaneously -- a 'sweet spot' -- leading to an improvement in the power conversion efficiency of the tandem solar cell from 5.22 percent to 6.24 percent. The enhancement ratio is as high as 20 percent."

The research team included Xing Wang Zhang from the Key Lab of Semiconductor Materials Science at the Institute of Semiconductors at Beijing's Chinese Academy of Science and Ziruo Hong from the Graduate School of Science and Engineering at Japan's Yamagata University.

Experimental and theoretical results demonstrate that the enhancement effect was attained from local near-field enhancement of the gold nanoparticles. The results show that the plasmonic effect has great potential for the future development of polymer solar cells. The team's proposed interlayer structures as an open platform can be applied to various polymer materials, opening up opportunities for highly efficient, multi-stacked tandem solar cells.

The research was financially supported by grants from the U.S. Office of Naval Research and the National Science Foundation.

The team also included Jun Yang, Jingbi You, Chun-Chao Chen, and Wan-Ching Hsu of the UCLA Department of Materials Science and Engineering and the California NanoSystems Institute.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University of California - Los Angeles. The original article was written by Jennifer Marcus.

Journal Reference:

Jun Yang, Jingbi You, Chun-Chao Chen, Wan-Ching Hsu, Hai-ren Tan, Xing Wang Zhang, Ziruo Hong, Yang Yang. Plasmonic Polymer Tandem Solar Cell. ACS Nano, 2011; 110718133857056 DOI: 10.1021/nn202144b

Physicists uncover new data on adenine, a crucial building block of life

Early Earth's atmosphere provided little shielding for ultraviolet light from space, so many prebiotic molecules, bombarded by it and light of other wavelengths, had a hard time surviving at all. But some molecules became photostable-able to withstand the assault and thrive as building blocks of life.

Five of the many molecules that survived the bombardment from UV light were the nucleic acid bases adenine, cytosine, guanine, thymine and uracil. Now, in just published research, a University of Georgia physicist and a collaborator in Germany have shown that one of these building blocks of DNA and RNA, adenine, has an unexpectedly variable range of ionization energies along its reaction pathways.

This means that understanding experimental data on how adenine survives exposure to UV light is much more complicated than previously thought. It also has far-reaching implications for spectroscopic measurements of heterocyclic compounds-those with atoms of at least two different elements in their rings.

"Photoprotection relies on the conversion of potentially harmful UV radiation into heat and has to operate on ultrafast time scales to compete over pathways that lead to the destruction of the biomolecule," said Susanne Ullrich, assistant professor in physics in the UGA department of physics and astronomy, part of the Franklin College of Arts and Sciences. "Disentangling these pathways and their time scales is challenging and requires a very close collaboration between experimentalists and theorists."

The research is in the online journal Physical Chemistry Chemical Physics. Co-author of the paper is Mario Barbatti, a theorist at the Max-Planck Institute in Mulheim, Germany.

The quantum-chemical calculations create for the first time a new baseline on how time-resolved spectroscopic techniques based on photoionization can be most reliably used to study this class of molecules.

"Photostable organic molecules participated in the complex molecular evolution that led to the formation of life," said Ullrich. "Because of the significance of nucleic acid bases as the genetic coding material, the photophysics of nucleobases has received considerable theoretical and experimental attention. This new work can help clarify inconsistencies researchers have always found in studying photoionization and photoelectron spectra of adenine."

Ullrich and her team used a technique called time-resolved photoionization with femtosecond (a quadrillionth of a second) resolution to unravel the mechanisms that protect adenine against UV damage. For the spectroscopic measurements, they employ a state-of-the-art femtosecond laser and custom-built photoelectron and photoion spectrometer.

Adenine is vaporized and transported into the spectrometer in a supersonic jet expansion. A pump pulse excites the sample of molecules, and finally a probe pulse is used to examine the sample after an adjustable delay time.

This examination is based on the process of photoionization that removes an electron from the molecule. The kinetic energy of the released photoelectron is measured in the spectrometer and provides the spectroscopic information needed to establish the photoprotection mechanism of adenine. Interpretation, however, heavily relies on the knowledge of ionization potentials (IP) along the relaxation pathways. (Ionization potential is the energy needed to remove an electron from the molecule.)

There has been a longstanding divergence between theoretical and experimental results when it comes to studying the IP of adenine and understanding on which surface adenine "relaxes" after it is excited with UV light. Understanding it more clearly could give new insights into how this important building block of life has continued to exist with stability in a world with millions of genetic threats.

"To our surprise, we found there were significant variations in the ionization energy between two different regions on this pathway," said Barbatti. "Due to the general character of the three pathways we studied, we believe the IPs computed along them can be used as a general guide for helping with setup and analysis for further experiments, not only with adenine but other related compounds."

Before this work, little has been known about the behavior of ionization potentials along the main reaction pathways for gaseous adenine in an excited state. Calling that a "knowledge gap," Barbatti and Ullrich say the new findings have "implications for experimental setup and data interpretation."

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University of Georgia. The original article was written by Philip Williams.

Journal Reference:

Mario Barbatti, Susanne Ullrich. Ionization potentials of adenine along the internal conversion pathways. Physical Chemistry Chemical Physics, 2011; DOI: 10.1039/C1CP21350D