Tag Archives: chirality

Gold spring-shaped coils for detecting twisted molecules

An April 3, 2017 news item on ScienceDaily describes a technique that could improve nanorobotics and more,

University of Bath scientists have used gold spring-shaped coils 5,000 times thinner than human hairs with powerful lasers to enable the detection of twisted molecules, and the applications could improve pharmaceutical design, telecommunications and nanorobotics.

An April 3, 2017 University of Bath press release (also on EurekAlert), which originated the news item, provides more detail (Note: A link has been removed),

Molecules, including many pharmaceuticals, twist in certain ways and can exist in left or right ‘handed’ forms depending on how they twist. This twisting, called chirality, is crucial to understand because it changes the way a molecule behaves, for example within our bodies.

Scientists can study chiral molecules using particular laser light, which itself twists as it travels. Such studies get especially difficult for small amounts of molecules. This is where the minuscule gold springs can be helpful. Their shape twists the light and could better fit it to the molecules, making it easier to detect minute amounts.

Using some of the smallest springs ever created, the researchers from the University of Bath Department of Physics, working with colleagues from the Max Planck Institute for Intelligent Systems, examined how effective the gold springs could be at enhancing interactions between light and chiral molecules. They based their study on a colour-conversion method for light, known as Second Harmonic Generation (SHG), whereby the better the performance of the spring, the more red laser light converts into blue laser light.

They found that the springs were indeed very promising but that how well they performed depended on the direction they were facing.

Physics PhD student David Hooper who is the first author of the study, said: “It is like using a kaleidoscope to look at a picture; the picture becomes distorted when you rotate the kaleidoscope. We need to minimise the distortion.”

In order to reduce the distortions, the team is now working on ways to optimise the springs, which are known as chiral nanostructures.

“Closely observing the chirality of molecules has lots of potential applications, for example it could help improve the design and purity of pharmaceuticals and fine chemicals, help develop motion controls for nanorobotics and miniaturise components in telecommunications,” said Dr Ventsislav Valev who led the study and the University of Bath research team.

Gold spring shaped coils help reveal information about chiral molecules. Credit Ventsi Valev.

Here’s a link to and a citation for the paper,

Strong Rotational Anisotropies Affect Nonlinear Chiral Metamaterials by David C. Hooper, Andrew G. Mark, Christian Kuppe, Joel T. Collins, Peer Fischer, Ventsislav K. Valev. Advanced Materials DOI: 10.1002/adma.201605110  View/save citation First published: 31 January 2017

This is an open access paper.

The nanotube of a thousand faces (similar nanomaterials behaving differently)

Kudos to any one who recognizes the reference to the ‘man of a thousand faces’, Lon Chaney, a silent film horror star. As for the nanotubes, there’s this Sept. 14, 2016 news item on ScienceDaily,

Nanotubes can be used for many things: electrical circuits, batteries, innovative fabrics and more. Scientists have noted, however, that nanotubes, whose structures appear similar, can actually exhibit different properties, with important consequences in their applications. Carbon nanotubes and boron nitride nanotubes, for example, while nearly indistinguishable in their structure, can be different when it comes to friction. A study conducted by SISSA/CNR-IOM and Tel Aviv University created computer models of these crystals and studied their characteristics in detail and observed differences related to the material’s chirality. …

A Sept. 14, 2016 Scuola Internazionale Superiore di Studi Avanzati (SISSA) press release (PDF), which originated the news item, describes the research in more detail,

“We began with a series of experimental observations which showed that very similar nanotubes exhibit different frictional properties, with intensities ranging up to two orders of magnitude,” says Roberto Guerra, a researcher at CNR-IOM and the International School for Advanced Studies (SISSA) in Trieste, first author of the study. “This led us to hypothesize that the chirality of the materials may play a role in this phenomenon.” The study involving also Andrea Vanossi (CNR-IOM) and Erio Tosatti (SISSA), was conducted in collaboration with the University of Tel Aviv.

For materials, such as those used in the study, chirality is linked to the three-dimensional arrangement of the weft that form the nanotube. “If we wrap a sheet of lined paper around itself to form a tube, the angle that the lines form with the axis of the tube determines its chirality,” says Guerra. “In our work we reconstructed the behavior of double-walled nanototubes, which can be imagined as two tubes of slightly different diameters, one inside the other. We observed that the difference in chirality between the inner tube and the outer tube has a remarkable effect on the three-dimensional shape of the nanotubes.”

A polygonal tube

“If we continue with the paper metaphor, the difference in orientation between the lattice on the inner tube and the outer tube determine to what extent, and, in what way, planar regions (faces) along the tube will form,” says Guerra. To better understand what is meant by “faces,” imagine a cross section of the tube, which is polygonal rather than perfectly circular. “The smaller the difference in chirality, the clearer and more obvious the faces,” concludes Guerra. If, however, the difference in chirality becomes too large, the faces disappear and the nanotubes take on the classic cylindrical shape.

The faces appear spontaneously depending on the characteristics of the material. Double-walled carbon nanotubes tend to form with a greater difference in internal and external chirality compared to boron nitride. Therefore, the former usually maintains a cylindrical shape that allows for less friction. In further studies, Guerra and colleagues intend to work directly on measuring the level of friction between nanotubes.

Here’s a link to and a citation for the paper,

Multiwalled nanotube faceting unravelled by Itai Leven, Roberto Guerra, Andrea Vanossi, Erio Tosatti, & Oded Hod. Nature Nanotechnology (2016) doi:10.1038/nnano.2016.151 Published online 22 August 2016

This paper is behind a paywall.

Hologram with nanostructures could improve fraud protection

This research on holograms comes from Harvard University according to a May 13, 2016 news item on ScienceDaily,

Holograms are a ubiquitous part of our lives. They are in our wallets — protecting credit cards, cash and driver’s licenses from fraud — in grocery store scanners and biomedical devices.

Even though holographic technology has been around for decades, researchers still struggle to make compact holograms more efficient, complex and secure.

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences have programmed polarization into compact holograms. These holograms use nanostructures that are sensitive to polarization (the direction in which light vibrates) to produce different images depending on the polarization of incident light. This advancement, which works across the spectrum of light, improves anti-fraud holograms as well as those used in entertainment displays.

A May 13, 2016 Harvard University press release (also on EurekAlert) by Leah Burrows, which originated the news item, provides more detail,

“The novelty in this research is that by using nanotechnology, we’ve made holograms that are highly efficient, meaning that very little light is lost to create the image,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering and senior author of the paper. “By using incident polarized light, you can see far a crisper image and can store and retrieve more images. Polarization adds another dimension to holograms that can be used to protect against counterfeiting and in applications like displays.”

Harvard’s Office of Technology Development has filed patents on this and related technologies and is actively pursuing commercial opportunities.

Holograms, like digital photographs, capture a field of light around an object and encode it on a chip. However, photographs only record the intensity of light while holograms also capture the phase of light, which is why holograms appear three-dimensional.

“Our holograms work like any other but the image produced depends on the polarization state of the illuminating light, providing an extra degree of freedom in design for versatile applications,” said Mohammadreza Khorasaninejad, postdoctoral fellow in the Capasso Lab and first author of the paper.

There are several states of polarization. In linearly polarized light the direction of vibration remains constant while in circularly polarized light it rotates clockwise or counterclockwise. The direction of rotation is the chirality.

The team built silicon nanostructured patterns on a glass substrate, which act as superpixels. Each superpixel responds to a certain polarization state of the incident light. Even more information can be encoded in the hologram by designing and arranging the nanofins to respond differently to the chirality of the polarized incident light.

“Being able to encode chirality can have important applications in information security such as anti-counterfeiting,” said Antonio Ambrosio, a research scientist in the Capasso Lab and co-first author. “For example, chiral holograms can be made to display a sequence of certain images only when illuminated with light of specific polarization not known to the forger.”

“By using different nanofin designs in the future, one could store and retrieve far more images by employing light with many states of polarization,” said Capasso.

Because this system is compact, it has application in portable projectors, 3D movies and wearable optics.

“Modern polarization imaging systems require cascading several optical components such as beam splitters, polarizers and wave plates,” said Ambrosio. “Our metasurface can distinguish between incident polarization using a single layer dielectric surface.”

“We have also incorporated in some of the holograms a lens function that has allowed us to produce images at large angles,” said Khorasaninejad. “This functionality combined with the small footprint and lightweight, has significant potential for wearable optics applications.”

Here’s a link to and a citation for the paper,

Broadband and chiral binary dielectric meta-holograms by Mohammadreza Khorasaninejad, Antonio Ambrosio, Pritpal Kanhaiya, and Federico Capasso. Science Advances  13 May 2016: Vol. 2, no. 5, e1501258 DOI: 10.1126/sciadv.1501258

This paper is open access.

Nanocellulose and an intensity of structural colour

I love the topic of structural colour (or color, depending on your spelling preferences) and have covered it many times and in many ways. One of the best pieces I’ve encountered about structural colour (an article by Christina Luiggi for The Scientist provided an overview of structural colour as it’s found in plants and animals) was featured in my Feb. 7, 2013 posting. If you go to my posting, you’ll find a link to Luiggi’s article which I recommend reading in its entirety if you have the time.

As for this latest nanocellulose story, a June 13, 2014 news item on Nanowerk describes University of Cambridge (UK) research into films and structural colour,

Brightly-coloured, iridescent films, made from the same wood pulp that is used to make paper, could potentially substitute traditional toxic pigments in the textile and security industries. The films use the same principle as can be seen in some of the most vivid colours in nature, resulting in colours which do not fade, even after a century.

Some of the brightest and most colourful materials in nature – such as peacock feathers, butterfly wings and opals – get their colour not from pigments, but from their internal structure alone.

Researchers from the University of Cambridge have recreated a similar structure in the lab, resulting in brightly-coloured films which could be used for textile or security applications.

A June 13, 2014 University of Cambridge news release, which originated the news item, describe the phenomenon of structural colour as it applies to cellulose materials,

In plants such as Pollia condensata, striking iridescent and metallic colours are the result of cellulose fibres arranged in spiral stacks, which reflect light at specific wavelengths. [emphasis mine]

Cellulose is made up of long chains of sugar molecules, and is the most abundant biomass material in nature. It can be found in the cells of every plant and is the main compound that gives cell walls their strength.

The news release goes on to provide a brief description of the research,

The researchers used wood pulp, the same material that is used for producing paper, as their starting material. Through manipulating the structure of the cellulose contained in the wood pulp, the researchers were able to fabricate iridescent colour films without using pigments.

To make the films, the researchers extracted cellulose nanocrystals from the wood pulp. When suspended in water, the rod-like nanocrystals spontaneously assemble into nanostructured layers that selectively reflect light of a specific colour. The colour reflected depends on the dimensions of the layers. By varying humidity conditions during the film fabrication, the researchers were able to change the reflected colour and capture the different phases of the colour formation.

Cellulose nanocrystals (CNC) are also known as nanocrystalline cellulose (NCC).

Here’s a link to and a citation for  the paper,

Controlled, Bio-inspired Self-Assembly of Cellulose-Based Chiral Reflectors by Ahu Gumrah Dumanli, Gen Kamita, Jasper Landman, Hanne van der Kooij, Beverley J. Glover, Jeremy J. Baumberg, Ullrich Steiner, and Silvia Vignolini. Optical Materials Article first published online: 30 MAY 2014 DOI: 10.1002/adom.201400112

© 2014 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

While the researchers have supplied an image of the Pollia condensata, I prefer this one, which is also featured in my Feb. 7, 2013 posting,

AGELESS BRILLIANCE: Although the pigment-derived leaf color of this decades-old specimen of the African perennial Pollia condensata has faded, the fruit still maintains its intense metallic-blue iridescence.COURTESY OF P.J. RUDALL [downloaded from http://www.the-scientist.com/?articles.view/articleNo/34200/title/Color-from-Structure/]

AGELESS BRILLIANCE: Although the pigment-derived leaf color of this decades-old specimen of the African perennial Pollia condensata has faded, the fruit still maintains its intense metallic-blue iridescence.COURTESY OF P.J. RUDALL [downloaded from http://www.the-scientist.com/?articles.view/articleNo/34200/title/Color-from-Structure/]

Stunning, non?

Control the chirality, control your carbon nanotube

A Feb. 18, 2014 news item on ScienceDaily features a story not about a breakthrough but about a discovery that* could lead to one,

A single-walled carbon nanotube grows from the round cap down, so it’s logical to think the cap’s formation determines what follows. But according to researchers at Rice University, that’s not entirely so.

Theoretical physicist Boris Yakobson and his Rice colleagues found through exhaustive analysis that those who wish to control the chirality of nanotubes — the characteristic that determines their electrical properties — would be wise to look at other aspects of their growth.

The scientists have provided this image to illustrate chirality (‘twisting’) in carbon nanotubes,

Carbon nanotube caps are forced into shape by six pentagons among the array of hexagons in the single-atom-thick tube. Rice University researchers took a census of thousands of possible caps and found the energies dedicated to their formation have no bearing on the tube's ultimate chirality. Credit: Evgeni Penev/Rice University

Carbon nanotube caps are forced into shape by six pentagons among the array of hexagons in the single-atom-thick tube. Rice University researchers took a census of thousands of possible caps and found the energies dedicated to their formation have no bearing on the tube’s ultimate chirality.
Credit: Evgeni Penev/Rice University

The Feb. 17, 2014 Rice University news release (also on EurekAlert), which originated the news item, describe the process the scientists used to research chirality in carbon nanotubes,

To get a clear picture of how caps are related to nanotube chirality, the Rice group embarked upon a detailed, two-year census of the 4,500 possible cap formations for nanotubes of just two diameters, 0.8 and 1 nanometer, across 21 chiralities.

The cap of every nanotube has six pentagons – none of which may touch each other — among an array of hexagons, Penev said. They pull the cap and force it to curve, but their positions are not always the same from cap to cap.

But because a given chirality can have hundreds of possible caps, the determining factor for chirality must lie elsewhere, the researchers found. “The contribution of the cap is the elastic curvature energy, and then you just forget it,” Penev said.

“There are different factors that may be in play,” Yakobson said. “One is the energy portion dictated by the catalyst; another one may be the energy of the caps per se. So to get the big picture, we address the energy of the caps and basically rule it out as a factor in determining chirality.”

A nanotube is an atom-thick sheet of carbon atoms arranged in hexagons and rolled into a tube. Chirality refers to the hexagons’ orientation, and that angle controls how well the nanotube will conduct electricity.

A perfect conducting metallic nanotube would have the atoms arranged in “armchairs,” so-called because cutting the nanotube in half would make the top look like a series of wells with atoms for armrests. Turn the hexagons 30 degrees, though, will make a semiconducting “zigzag” nanotube.  Nanotubes can be one or the other, or the chiral angle can be anything in between, with a shifting range of electrical properties.

Getting control of these properties has been a struggle. Ideally, scientists could grow the specific kinds of nanotubes they need for an application, but in reality, they grow as a random assortment that must then be separated with a centrifuge or by other means.

Yakobson suspects the answer lies in tuning the interaction between the catalyst and the nanotube edge. “This study showed the energy involved in configuring the cap is reasonably flat,” he said. “That’s important to know because it allows us to continue to work on other factors.

Here’s a  link to and a citation for the paper,

Extensive Energy Landscape Sampling of Nanotube End-Caps Reveals No Chiral-Angle Bias for Their Nucleation by Evgeni S. Penev, Vasilii I. Artyukhov, and Boris I. Yakobson. ACS Nano, Article ASAP DOI: 10.1021/nn406462e Publication Date (Web): January 23, 2014
Copyright © 2014 American Chemical Society

This article is behind a paywall.

One final comment, it took these scientists two years of painstaking work to establish that caps are not the determining factor for chirality. It’s this type of story I find as fascinating, if not more so, as the big breakthroughs because it illustrates the  extraordinary drive it takes to extract even the smallest piece of information. I wish more attention was given to these incremental efforts.

* March 7, 2014 changed ‘while’ to ‘that’.

Distinguishing between left-handed and right-handed molecules with nanocubes

Learning to distinguish your left from your right isn’t all that easy for children. It’s also remarkably easy to lose the ability (temporarily) to make that distinction if you start experimenting with certain kinds of brain repatterning. However, the distinctions are important not only in daily life but in biology too according to a June 26, 2013 news item on Nanowerk,

In chemical reactions, left and right can make a big difference. A “left-handed” molecule of a particular chemical composition could be an effective drug, while its mirror-image “right-handed” counterpart could be completely inactive. That’s because, in biology, “left” and “right” molecular designs are crucial: Living organisms are made only from left-handed amino acids. So telling the two apart is important—but difficult.

Now, a team of scientists at the U.S. Department of Energy’s Brookhaven National Laboratory and Ohio University has developed a new, simpler way to discern molecular handedness, known as chirality.

The June 26, 2013 Brookhaven National Laboratory news release, which originated the news item, describes the new technique for distinguishing left- from right-handed molecules,

They used gold-and-silver cubic nanoparticles to amplify the difference in left- and right-handed molecules’ response to a particular kind of light. The study, described in the journal NanoLetters, provides the basis for a new way to probe the effects of handedness in molecular interactions with unprecedented sensitivity.

The scientists knew that left- and right-handed chiral molecules would interact differently with “circularly polarized” light—where the direction of the electrical field rotates around the axis of the beam. This idea is similar to the way polarized sunglasses filter out reflected glare unlike ordinary lenses.

Other scientists have detected this difference, called “circular dichroism,” in organic molecules’ spectroscopic “fingerprints”—detailed maps of the wavelengths of light absorbed or reflected by the sample. But for most chiral biomolecules and many organic molecules, this “CD” signal is in the ultraviolet range of the electromagnetic spectrum, and the signal is often weak. The tests thus require significant amounts of material at impractically high concentrations.

The team was encouraged they might find a way to enhance the signal by recent experiments showing that coupling certain molecules with metallic nanoparticles could greatly increase their response to light. Theoretical work even suggested that these so-called plasmonic particles—which induce a collective oscillation of the material’s conductive electrons, leading to stronger absorption of a particular wavelength—could bump the signal into the visible light portion of the spectroscopic fingerprint, where it would be easier to measure.

The group experimented with different shapes and compositions of nanoparticles, and found that cubes with a gold center surrounded by a silver shell are not only able to show a chiral optical signal in the near-visible range, but even more striking, were effective signal amplifiers. For their test biomolecule, they used synthetic strands of DNA—a molecule they were familiar with using as “glue” for sticking nanoparticles together.

When DNA was attached to the silver-coated nanocubes, the signal was approximately 100 times stronger than it was for free DNA in the solution. That is, the cubic nanoparticles allowed the scientists to detect the optical signal from the chiral molecules (making them “visible”) at 100 times lower concentrations.

The observed amplification of the circular dichroism signal is a consequence of the interaction between the plasmonic particles and the “exciton,” or energy absorbing, electrons within the DNA-nanocube complex, the scientists explained.

“This research could serve as a promising platform for ultrasensitive sensing of chiral molecules and their transformations in synthetic, biomedical, and pharmaceutical applications,” Lu [Fang Lu, the first author on the paper] said.

“In addition,” said Gang [Oleg Gang, a researcher at Brookhaven’s Center for Functional Nanomaterials and lead author on the paper], “our approach offers a way to fabricate, via self-assembly, discrete plasmonic nano-objects with a chiral optical response from structurally non-chiral nano-components. These chiral plasmonic objects could greatly enhance the design of metamaterials and nano-optics for applications in energy harvesting and optical telecommunications.”

I last mentioned chirality in the context of work being done with controlling the chirality of carbon nanotubes at Finland’s Aalto University in an April 30 , 2013 posting.

Here’s a link to and a citation for the paper published by the Brookhaven National Laboratory and Ohio University,

Discrete Nanocubes as Plasmonic Reporters of Molecular Chirality by Fang Lu, Ye Tian, Mingzhao Liu, Dong Su, Hui Zhang, Alexander O. Govorov, and Oleg Gang. Nano Lett., Article ASAP
DOI: 10.1021/nl401107g Publication Date (Web): June 18, 2013
Copyright © 2013 American Chemical Society

This paper is behind a paywall.

“Control my chirality, please,” said the carbon nanotube to the researchers

A combined Finnish, Russian, and Danish team have found a way to control the chirality of single-walled carbon nanotubes according to an Apr. 30, 2013 news item on Azonano,

An ultimate goal in the field of carbon nanotube research is to synthesise single-walled carbon nanotubes (SWNTs) with controlled chiralities. Twenty years after the discovery of SWNTs, scientists from Aalto University in Finland, A.M. Prokhorov General Physics Institute RAS in Russia and the Center for Electron Nanoscopy of Technical University of Denmark (DTU) have managed to control chirality in carbon nanotubes during their chemical vapor deposition synthesis.

The Aalto University Apr. 29, 2013 news release, which originated the news item, goes on to explain,

 Over the years, substantial progress has been made to develop various structure-controlled synthesis methods. However, precise control over the chiral structure of SWNTs has been largely hindered by a lack of practical means to direct the formation of the metal nanoparticle catalysts and their catalytic dynamics during tube growth.

– We achieved an epitaxial formation of Co nanoparticles by reducing a well-developed solid solution in CO, reveals Maoshuai He, a postdoctoral researcher at Aalto University School of Chemical Technology.

– For the first time, the new catalyst was employed for selective growth of SWNTs, adds senior staff scientist Hua Jiang from Aalto University School of Science.

By introducing the new catalysts into a conventional CVD reactor, the research team demonstrated preferential growth of semiconducting SWNTs (~90%) with an exceptionally high population of (6,5) tubes (53%) at 500 °C. Furthermore, they also showed a shift of the chiral preference from (6,5) tubes at 500 °C  to (7, 6) and (9, 4) nanotubes at 400 °C.

– These findings open new perspectives both for structural control of SWNTs and for elucidating their growth mechanisms, thus are important for the fundamental understanding of science behind nanotube growth, comments Professor Juha Lehtonen from Aalto University.

For anyone like me who needs a description of chirality, there’s this from Wikipedia,

Chirality (pron.: /kaɪˈrælɪtiː/) is a property of asymmetry important in several branches of science. The word chirality is derived from the Greek, χειρ (kheir), “hand”, a familiar chiral object.

An object or a system is chiral if it is not identical to its mirror image, that is, it cannot be superposed onto it. A chiral object and its mirror image are called enantiomorphs (Greek opposite forms) or, when referring to molecules, enantiomers. A non-chiral object is called achiral (sometimes also amphichiral) and can be superposed on its mirror image.

Human hands are perhaps the most universally recognized example of chirality: The left hand is a non-superimposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands to coincide.[2] This difference in symmetry becomes obvious if someone attempts to shake the right hand of a person using his left hand, or if a left-handed glove is placed on a right hand. In mathematics chirality is the property of a figure that is not identical to its mirror image.

One of the researchers notes why they, or anyone else, would want to control the chirality of carbon nanotubes, from the news release,

– Chirality defines the optical and electronic properties of carbon nanotubes, so controlling it is a key to exploiting their practical applications, says Professor Esko I. Kauppinen, the leader of the Nanomaterials Group in Aalto University School of Science.

ETA Apr. 30, 2013 at 4:20 pm PDT: Here’s a link to and a citation for the team’s published paper,

Chiral-Selective Growth of Single-Walled Carbon Nanotubes on Lattice-Mismatched Epitaxial Cobalt Nanoparticles by Maoshuai He, Hua Jiang, Bilu Liu, Pavel V. Fedotov, Alexander I. Chernov, Elena D. Obraztsova, Filippo Cavalca, Jakob B. Wagner, Thomas W. Hansen, Ilya V. Anoshkin, Ekaterina A. Obraztsova, Alexey V. Belkin, Emma Sairanen, Albert G. Nasibulin,  Juha Lehtonen, & Esko I. Kauppinen. Scientific Reports 3, Article number 1460  doi:10.1038/srep01460 Published15 March 2013

This article is open access.

Clone your carbon nanotubes

The Nov. 14, 2012 news release on EurekAlert highlights some work on a former nanomaterial superstar, carbon nanotubes,

Scientists and industry experts have long speculated that carbon nanotube transistors would one day replace their silicon predecessors. In 1998, Delft University built the world’s first carbon nanotube transistors – carbon nanotubes have the potential to be far smaller, faster, and consume less power than silicon transistors.

A key reason carbon nanotubes are not in your computer right now is that they are difficult to manufacture in a predictable way. Scientists have had a difficult time controlling the manufacture of nanotubes to the correct diameter, type and ultimately chirality, factors that control nanotubes’ electrical and mechanical properties.

Carbon nanotubes are typically grown using a chemical vapor deposition (CVD) system in which a chemical-laced gas is pumped into a chamber containing substrates with metal catalyst nanoparticles, upon which the nanotubes grow. It is generally believed that the diameters of the nanotubes are determined by the size of the catalytic metal nanoparticles. However, attempts to control the catalysts in hopes of achieving chirality-controlled nanotube growth have not been successful.

The USC [University of Southern California] team’s innovation was to jettison the catalyst and instead plant pieces of carbon nanotubes that have been separated and pre-selected based on chirality, using a nanotube separation technique developed and perfected by Zheng [Ming Zheng] and his coworkers at NIST [US National Institute of Standards and Technology]. Using those pieces as seeds, the team used chemical vapor deposition to extend the seeds to get much longer nanotubes, which were shown to have the same chirality as the seeds..

The process is referred to as “nanotube cloning.” The next steps in the research will be to carefully study the mechanism of the nanotube growth in this system, to scale up the cloning process to get large quantities of chirality-controlled nanotubes, and to use those nanotubes for electronic applications

H/T to ScienceDaily’s Nov. 14, 2012 news item for the full journal reference,

Jia Liu, Chuan Wang, Xiaomin Tu, Bilu Liu, Liang Chen, Ming Zheng, Chongwu Zhou. Chirality-controlled synthesis of single-wall carbon nanotubes using vapour-phase epitaxy. Nat. Commun., 13 Nov, 2012 DOI: 10.1038/ncomms2205

The article is behind a paywall.

Flipping chirality at the Lawrence Berkeley National Laboratory

First, it might be a good idea to define chirality. From the Lawrence Berkeley National Laboratory (Berkeley Lab) July 10, 2012 news release by LynnYarris,

Chirality is the distinct left/right orientation or “handedness” of some types of molecules, meaning the molecule can take one of two mirror image forms. The right-handed and left-handed forms of such molecules, called “enantiomers,” can exhibit strikingly different properties. For example, one enantiomer of the chiral molecule limonene smells of lemon, the other smells of orange. The ability to observe or even switch the chirality of molecules using terahertz (trillion-cycles-per-second) electromagnetic radiation is a much coveted asset in the world of high technology.

As for why anyone would want  to flip molecules back and forth between left- and right-handedness (from the news release),

A multi-institutional team of researchers that included scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has created the first artificial molecules whose chirality can be rapidly switched from a right-handed to a left-handed orientation with a  beam of light. This holds potentially important possibilities for the application of terahertz technologies across a wide range of fields, including reduced energy use for data-processing, homeland security and ultrahigh-speed communications.

Here’s how the technique works, from the July 10, 2012 news item on physorg.com,

Working with terahertz (THz) metamaterials engineered from nanometer-sized gold strips with air as the dielectric – Zhang [Xiang Zhang, one of the leaders of this research and a principal investigator with Berkeley Lab’s Materials Sciences Division] and his colleagues fashioned a delicate artificial chiral molecule which they then incorporated with a photoactive silicon medium. Through photoexcitation of their metamolecules with an external beam of light, the researchers observed handedness flipping in the form of circularly polarized emitted THz light. Furthermore, the photoexcitation enabled this chirality flipping and the circular polarization of THz light to be dynamically controlled.

“In contrast to previous demonstrations where chirality was merely switched on or off in metamaterials using photoelectric stimulation, we used an optical switch to actually reverse the chirality of our THz metamolecules,” Zhang says.

The researchers describe in more detail the potential for this new technique,

“The observed giant switchable chirality we can engineer into our metamaterials provides a viable approach towards creating high performance polarimetric devices that are largely not available at terahertz frequencies,” says corresponding author Antoinette Taylor. “This frequency range is particularly interesting because it uniquely reveals information about physical phenomena such as the interactions between or within biologically relevant molecules, and may enable control of electronic states in novel material systems, such as cyclotron resonances in graphene and topological insulators.”

Taylor and her co-authors say that the general design principle of their optically switchable chiral THz metamolecules is not limited to handedness switching but could also be applied to the dynamic reversing of other electromagnetic properties.

From what I understand metamaterials are very expensive and difficult to produce which means this exciting advance is likely to remain in the laboratory of at least 10 years.