Researchers in Austria, Germany and the Netherlands have succeeded in creating a light-hole exciton ground state by applying tensile elastic stress to a semiconductor quantum dot. The light-hole exciton, which is a quasiparticle formed from a single electron bound to a single light-hole, is very different to the more commonly known heavy-hole exciton and could come in handy for quantum information science and technology applications. Quantum dots are tiny pieces of semiconductor, each containing an electron or hole in a certain quantum spin state. Light-hole states have been little studied in the past, because most quantum dots made to date had a valence-band ground state with a predominantly heavy-hole character, explains team leader Armando Rastelli of the Institute of Semiconductor and Solid State Physics at Johannes Kepler University in Linz. “My colleague Yongheng Huo at the Institute for Integrative Nanosciences in IFW Dresden has now managed to make high-quality quantum dots with a valence band ground state that have almost pure light-hole character, something that will allow us to investigate the properties of light-hole excitons and light-hole spins. Light-hole spins may have several advantages over their heavy-hole counterparts when it come to spin manipulation,” he told nanotechweb.org. Normally, and in the absence of any external strain, the edges

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The so-called nitrogen vacancy (NV) centre in nanodiamond could be ideal for use in a host of future quantum technologies, including quantum computing and nanoscale sensing. However, most nanodiamonds available today contain a high density of paramagnetic impurities that make the electron spins in NV centres extremely fragile – they cannot hold their spin direction for very long (microseconds at most), which means that they are unable to store quantum information for any practical length of time. A team of researchers in the US is now reporting on record spin coherence times of more than 200 µs in NVs in highly pure nanodiamond fabricated using a new mask and ion etching process – a feat that could allow for real-world applications using these defects.   Atomic impurities, or defects, in natural diamond lead to the colour seen in pink, blue and yellow diamonds. One such defect, the NV, occurs when two neighbouring carbon atoms in diamond are replaced by a nitrogen atom and an empty lattice site. NVs in nanodiamonds could be ideal as biological probes because they are non-toxic, photostable and can easily be inserted into living cells. They are also capable of detecting the very weak magnetic fields that

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Researchers at the Oak Ridge National Lab and the University of South Carolina in the US say that they have observed chaotic behaviour in a ferroelectric material – an unexpected discovery that they claim could lead to the development of computers that resemble the human brain. Anton Ievlev and colleagues used the tip of a scanning probe microscope (SPM) to draw patterns on the surface of a ferroelectric material. Ferroelectrics have a spontaneous electric polarization, the direction of which can be reversed by applying an electric field. By applying a voltage between the SPM tip and the electrically polarized surface of lithium niobate, the researchers were able to draw dots where the polarization is reversed. The presence of the dots could then be read back by passing the tip surface over the surface in force-microscopy mode. This allowed the team to store binary information on the surface, with a small dot representing a “0″ and a large dot a “1″, for example. This worked very well when the dots were separated by about 500 nm, but when Ielev and colleagues tried to reduce this distance, something unexpected happened. “When we reduced the distance between domains, we started to see things that

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Researchers at Columbia University in New York and the Lawrence Berkeley National Lab have succeeded in tuning the behaviour of rectifiers and diodes at the single molecule level for the first time. The feat is an important step forward for single-molecule electronics, says the team.   Molecular electronics has advanced in leaps and bounds since the first molecular rectifiers were put forward by Aviram and Ratner back in 1974. These devices worked thanks to donor−σ−acceptor molecules connected symmetrically to two metal termini. Such donor-acceptor diodes, as they are known, are very sensitive to how the molecular energy levels are aligned with respect to each other, and with any connecting electrodes, which makes them very difficult to control. More importantly, a σ bridge is required to make these devices act as rectifiers – something that adds a large tunnel barrier to the device structure backbone and, which in turn, produces a very large junction resistance. The end result is that most functional single-molecule diodes available today all have resistances greater than 10 MΩ, which is far too big for practical device applications. Although researchers have been busy looking for alternatives to donor−acceptor type diodes in the past few years, most of

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The optical and plasmonic properties of 2D layered nanomaterials can be tuned using well established intercalation techniques, according to new work by researchers at Stanford University in the US. Intercalation is a reversible process and involves inserting atoms, ions or molecules into the spaces between crystal layers. Such intercalated 2D nanoplates might find use in optoelectronics applications, says the team.   “Intercalating with specific molecules of our choice allows us to turn 2D nanostructures into functional nanomaterials whose properties can be tuned,” team leader Yi Cui toldnanotechweb.org. “In this work, we focused our attention on tuning the optical and plasmonic properties of metal dichalcogenides, such as bismuth selenide (Bi2Se3) nanoplates and related compounds.” The compounds studied are layered materials in which one layer is made up of five atomic planes (Se-Bi-Se-Bi-Se) bonded strongly together. These layers, each around 1 nm thick, are loosely stacked to form individual crystals. The Stanford researchers looked at how photons and collective oscillations of electrons (or plasmons) travel within the material. These 2D structures are ideal for studying intercalation chemistry, because we can insert a variety of different molecules, atoms and ions into the gaps between the crystal layers, explains Cui. This allows us to control the optical

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Researchers at the Integrated Nanotechnology Lab at KAUST in Saudi Arabia are the first to have made a thermoelectric generator on flexible silicon. The device, which is capable of generating 30 times more power than previous such generators, might find use in a host of application areas – including mobile phones, laptops, biomedical sensors and other portable devices. Thermoelectric generators (TEGs) convert heat directly into electricity. The devices are good at conducting electricity but poor at conducting heat, and they have a large thermopower (the ratio of the voltage to temperature difference across the device to its temperature difference). The researchers, led by Muhammad Mustafa Hussain, began by fabricating their TEGs from the 2D layered materials bismuth telluride and antimony telluride on low-cost bulk mono-crystalline silicon. Next, they transformed the devices and the host silicon into flexible and transparent systems using state-of-the-art CMOS-compatible processes. The silicon layer is just 18 µm thick and contains 63 thermopiles. “The TEGs we made generate 0.15 µW of power, which is 30 times more than previously-made devices of this kind,” Hussain told nanotechweb.org. “The thin silicon contains trenches that serve to minimize heat loss from the hot end of the device to the cold end. The power generated

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Single-walled carbon nanotube films are not only mechanically flexible but they also conduct heat extremely well. This finding, from researchers at Stanford University in the US, means that the films might be used in a variety of innovative surface technologies – ranging from solar cells to waste heat recovery systems in automobiles and even smart phones and tablets. Materials that are both mechanically flexible and that conduct heat well do not exist in Nature. However, this unusual combination of properties is crucial for making devices that are able to transfer heat between different surfaces – a metal heat sink, for example. Previous attempts to make such structures produced systems that degraded or failed over time because the interfaces between the different materials employed cracked or delaminated during thermal cycling. A “compliant” thermal conductor would solve these problems, explains team leader Kenneth Goodson. “We now report here on the first in-plane mechanical compliance data for SWCNT films, which we found also conduct heat as well as many metals.” The researchers have pinpointed the exact physical mechanisms responsible for the mechanical modulus they measured in their SWCNT films, including van der Waals forces and nanotube interactions such as zipping/unzipping and entanglement. “We measured

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Surface plasmon polaritons travel shorter distances in metallic nanowires with a five-point star-shaped cross section than in nanowires with a pentagonal-shaped cross section. This new result, from researchers at Rice University in Houston, will be important for making improved nano-optics devices from these nanostructures in the future. One-dimensional metallic structures are ideal as basic components in nanoscale optical devices that overcome the diffraction limit of light. This is because they can confine light to subwavelength dimensions in the form of surface plasmon polaritons (SPPS). SPPs are collective oscillations of conduction band electrons at the surface of metals. Metallic nanowires and stripes that are less than 200 nm in diameter are particularly interesting for subwavelength waveguiding and for various plasmonics applications, like optical interconnects and routers in plasmonic circuits, as plasmonic logic gates and as Fabry-Perot resonators. The problem, however, is that optical frequency SPPs do not travel very far – at best several tens of microns – in metallic nanowires because they are scattered or lost as wasted heat. Researchers would like to better understand these losses and quantify them to develop low-loss nanoscale devices based on these nanostructures. “Collective modes” depend on symmetry of nanowire cross-sectional shape A team led

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Carbon nanotubes (CNTs) can be directly grown on commercially available carbon “cloth” with a three-dimensional (3D) network architecture, according to new work by researchers at Central China Normal University. The resulting highly conductive electrodes can be used to construct solid-state flexible supercapacitors (AFSC) that work under high mechanical pressures and over a wide temperature window. Capacitors are devices that store electric charge. Supercapacitors, more accurately known as electric double-layer capacitors or electrochemical capacitors, can store much more charge thanks to the double layer formed at an electrolyte-electrode interface when voltage is applied. Thanks to their large surface area, excellent electrical conductivity, electrochemical stability and mechanical flexibility, CNTs are promising advanced electrode materials for flexible supercapacitors. Existing techniques to fabricate flexible CNT electrodes are generally indirect, however, and involve processes such as slurry casting, ink-jet printing, vacuum filtering or electrophoretic deposition, to name a few. What is more, the electrodes produced using such technqiues typically consist of densely packaged CNT films with limited available surface area. Entangled CNTs Now, researchers at Central China Normal University have put forward a new, straightforward method to synthesize flexible CNT electrodes that involves directly growing CNT films on flexible carbon cloth through a chemical vapour

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Monolayer hexagonal boron nitride (h-BN), a single atomic layer consisting of sp2 hybridized boron and nitrogen arranged in a honeycomb lattice, is often referred to as “white graphene” because it has a similar structure to the carbon material. Compared to graphene, however, h-BN boasts a significantly higher thermal and chemical stability, is insulating and completely transparent to visible light. These properties mean that monolayer h-BN is promising as a unique coating material. Fabricating large area, high-quality monolayer h-BN remains a challenge though and its performance as a coating material has never been demonstrated experimentally. Characterizing the optical properties of h-BN is also difficult. Researchers at the Nanjing University of Aeronautics and Astronautics in China have now successfully grown high-quality h-BN monolayer over large areas using chemical vapour deposition on copper foil with ammonia borane as the source in a two-heat zone system under low pressure. The team showed that monolayer h-BN can serve as a perfect coating layer to significantly improve the friction, oxidation, and electric resistance of surfaces. The researchers also found that the exceptional low friction and insulating properties of monolayer h-BN can help in the characterization of its optical properties using atomic force microscopy technqiues. As shown in

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Nanocarriers, such as those made of polymer nanoparticles, are promising drug delivery vehicles. Thanks to a process called emulsion free radical polymerization, researchers in Italy have now succeeded in quantifying the amount of nanoparticles that enter the cell cytoplasm. The findings from the new study will be important for developing more efficient anti-cancer therapies. The team, led by Massimo Morbidelli of the ETH Zurich in Switzerland and Davide Moscatelli of the Politecnico di Milano in Italy, together with colleagues at the Mario Negri Institute for Pharmacological Research in Milano looked at how emulsion free radical polymerization – which is routinely employed to synthesize polymer nanoparticles – can be successfully applied to the biomedical field. The researchers succeeded in optimizing a novel procedure that allowed them to quantitatively assess the amount of nanoparticles taken up by the cell cytoplasm. The technique relies on flow cytometry and plate fluorimetry, and also measures the number of cells. The method exploits polymer nanoparticles functionalized with a fluorescent dye (Rhodamine B). PhD student Raffaele Ferrari at the Politecnico di Milano synthesized biocompatible polymethylmethacrylate-based nanoparticles covalently bound to RhB to avoid dye diffusion from the nanocarriers. By changing the process parameters, different carriers with tuneable size, charge, and amount of dye were

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The ion exchange properties of the surface structure of mica have been known for many years, but it is difficult to determine how ions are spatially distributed on the nanoscale in this material. A team at Leeds University in the UK has now employed atomic force microscopy (AFM) to image how different length fragments of double-stranded DNA bind to mica ion-exchanged with nickel. The researchers used polymer chain statistics to quantify whether the DNA was in one of two conformations as determined by the binding mechanisms. Long fragments on nickel-mica behaved as expected but shorter ones (below 800 base pairs) behaved in a way that shows that nickel ions are distributed on the mica on characteristic length scales of around 100 nm. Long and short DNA fragments on mica surface.     Mica is a naturally abundant crystal and its layered structure means that it is strongly insulating. Mica consists of 1 nm thick layers of silicate ionically bonded by a group I cation (usually K+). Cleaving open a fresh surface requires that half the K+ ions stay on one surface and half on the other. These ions can be exchanged with other ions by immersing the surface in salt solutions but exchange

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