This year’s top five stories –

1. Graphene tackles arsenic

A composite made from reduced graphene oxide and magnetite could effectively remove arsenic from drinking water according to new work by researchers in Korea. Arsenic in drinking water is a huge problem in many areas of South Asia and the western US.

A composite made from reduced graphene oxide and magnetite could help to clean up drinking water.

Arsenic is one of the most carcinogenic elements known and is toxic above 10 ppb (the World Health Organisation’s standard). Drinking water contaminated with arsenic is a dangerous everyday reality for many people across the world and it can lead to chronic illness and death. The arsenic mainly comes from naturally occurring arsenic-rich rocks through which the water has filtered but it may occur in areas where arsenic is mined as well.

Scientists now also believe that changes in agricultural practices, such as using groundwater wells for irrigation rather than surface water sources like rivers and ponds, may also be to blame. Indeed, this might explain the elevated levels of drinking-water arsenic in countries like Bangladesh, which has seen massive epidemics of arsenic poisoning in recent years.

Arsenic can be removed from drinking water by using activated carbon or precipitating it out with iron minerals, such as iron oxides – for example magnetite (Fe₃O₄) nanocrystals. However, such particles cannot be used in rivers, or other environments where water flows, because of their small size and the fact that magnetite rapidly oxidises when exposed to the atmosphere. Researchers have recently overcome the latter problem by combining iron oxides with carbon and carbon nanotubes, and graphene-based materials such as graphene oxide.

Superparamagnetic hybrid
Building on this work, Kwang Kim, In-Cheol Hwang and colleagues at Pohang University of Science and Technology have synthesised a new type of magnetite composite based on reduced graphene oxide (RGO). The hybrid material, which is superparamagnetic at room temperature, can remove over 99.9% of arsenic in a sample, and reduce its concentration to below 1 ppb – as measured by inductively coupled plasma (ICP) techniques.

The magnetite-RGO composite can be dispersed in water. Once it has adsorbed arsenic, it can quickly be removed from a sample using a permanent hand-held magnet (with a strength of 20 mT) within a fraction of a minute.

The composite is ideal for removing arsenic (and perhaps other heavy metals) compared to bare magnetite because the presence of the graphene flakes among the magnetite particles increases the number of arsenic adsorption sites. And both As(III) and As(V) can be strongly adsorbed. “The reduced graphene oxide also increases the stability of magnetite so that it can be used in continuous-flow systems for longer periods,” Kim told nanotechweb.org.

The researchers made their composite by first synthesising graphene oxide via Hummer’s method (see figure). Next, they exfoliated the graphene oxide in water to produce a suspension of graphene oxide sheets. A mixed suspension of FeCl₃ and FeCl₂ was then added slowly to the graphene oxide solution, and ammonia quickly introduced to precipitate Fe²⁺ and Fe³⁺ ions for synthesising the magnetite nanoparticles. The graphene oxide was reduced using hydrazine hydrate and the dark black coloured solution filtered, washed with water/ethanol and dried in vacuum.

The team is now looking into other large-scale graphene synthesis methods as well as making graphene-based hybrid materials for various environmental and biological applications.

The present results were reported in ACS Nano.

2. Junctionless transistor makes its debut

Researchers in Ireland have succeeded in making the first junctionless transistor ever. The device, which resembles a structure first proposed way back in 1925 but not realized until now, has nearly “ideal” electrical properties, according to the team. It could potentially operate faster and use less power than any conventional transistor on the market today.

When a voltage is applied to the gate, the “squeezing” effect is reduced and current can flow.

Transistors are the fundamental building blocks of modern electronic devices – and all existing transistors contain semiconductor junctions. The most common type of junction is the p–n junction, which is formed by the contact between a p-type piece of silicon – doped with impurities to create an excess of holes – and an n-type piece of silicon, doped to create an excess of electrons. Other junctions include the heterojunction, which is simply a p–n junction containing two different semiconductors, and the Schottky junction between metal and semiconductor.

The number of transistors on a single silicon microchip has been increasing exponentially since the early 1970s, and has gone up from a few hundred to over several billion today. As a result, transistors are becoming so tiny that it is becoming increasingly difficult to create high-quality junctions. In particular, it is very difficult to change the doping concentration of a material over distances shorter than about 10 nm. Junctionless transistors could therefore help chipmakers continue to make smaller and smaller devices.

Patented in 1925
Now, Jean-Pierre Colinge and colleagues at the Tyndall National Institute of University College Cork have dispensed with the very idea of a junction and instead have turned to a concept first proposed in 1925 by Austrian-Hungarian physicist Julius Edgar Lilienfield. Patented under the title “Device for controlling electric current”, it is a simple resistor and contains a gate that controls the density of electrons and holes, and thus current flow.

The team’s version of the device consists of a silicon nanowire in which current flow is perfectly controlled by a silicon gate that is separated from the nanowire by a thin insulating layer. The structure itself is very simple, looking a bit like a telephone cable that is fixed to a surface by a plastic clip (see figure). Crucially, there is no need to alter the doping over very short distances. Instead, the entire silicon nanowire is heavily n-doped, making it an excellent conductor. However, the gate is p-doped and its presence has the effect of depleting the number of electrons in the region of the nanowire under the gate.

If a voltage is simply applied along the nanowire, current cannot flow through this depleted region. According to Colinge, this region “squeezes” the current in the nanowire in the same way as the flow of water in a hose is stopped by squeezing it. However, if a voltage is applied to the gate, the squeezing effect is reduced and current can flow. The team also made a similar device with a p-type nanowire and n-type gate.

The most perfect of transistors
The structure is simple to build, even at the nanoscale, which means reduced costs compared with conventional junction fabrication technologies, which are becoming more and more complex. The device also has near-ideal electrical properties, adds Colinge, and behaves like the most perfect of transistors. This means that it hardly suffers at all from current leakage – the bane of conventional devices – and so could potentially operate faster and using less energy.

The Tyndall team says that it is now talking to some of the world’s leading semiconductor companies to further develop and possibly license its technology.

“Although the idea of a transistor without junctions may seem quite unorthodox, the word “transistor” does not imply the presence of junctions, per se,” write the researchers in Nature Nanotechnology, where the work was published. “A transistor is a solid-state device that controls current flow and the word transistor is a contraction of ‘trans-resistor’.”

3. Graphene supercapacitor breaks storage record

Researchers in the US have made a graphene-based supercapacitor that can store as much energy as nickel metal hydride batteries but which can be charged or discharged in just seconds or minutes. The new device provides a specific energy density of 85.6 Wh/kg by electrode weight at room temperature and 136 Wh/kg at 80 °C. These are the highest ever energy values reported for “electric double layer” supercapacitors based on nano-carbon materials.

Structures like this may be able to fast-charge mobile electronics.

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.

The new device made by Bor Jang of Nanotek Instruments Inc. in Ohio and colleagues has electrodes made of graphene mixed with 5wt% Super P (an acetylene black that acts as a conductive additive) and 10wt% PTFE binder. The researchers coat the resulting slurry onto the surface of a current collector and assemble coin-sized capacitors in a glove box. The electrolyte-electrode interface is made of “Celguard-3501” and the electrolyte is a chemical called EMIMBF4.

Fast charging
The energy density values of the supercapacitor are comparable to that of nickel metal hydride batteries. “This new technology makes for an energy storage device that stores nearly as much energy as in a battery but which can be recharged in seconds or minutes,” Jang told nanotechweb.org. “We believe that this is truly a breakthrough in energy technology.”

The fast charging feature means that the device might be used to recharge mobile phones, digital cameras and micro-EVs, he adds.

The team, which includes scientists from Angstron Materials Inc. in Dayton, Ohio, and Dalian University of Technology in China, are now working hard to further improve the energy density of the device. “Our goal is to make a supercapacitor that stores as much energy as the best lithium-ion batteries (for the same weight) but which can still be recharged in less than two minutes,” said Jang.

His team first discovered that graphene could be used as a supercapacitor electrode material in 2006. Since then, scientists around the world have made great strides in improving the specific capacitance of these electrodes but the devices still fall short of the theoretical values of 550 F/g.

“Despite the theoretically high specific surface area of single-layer graphene (which can reach up to 2.675 m²/g), a supercapacitance of 550 F/g has not been reached in a real device because the graphene sheets tend to re-stack together,” explained Jang. “We are trying to overcome this problem by developing a strategy that prevents the graphene sheets from sticking to each other face-to-face. This can be achieved if curved graphene sheets are used instead of flat ones.”

The work was reported in Nano Letters.

4. Laser welding boosts efficiency of TiO2 solar cells

Dye-sensitized solar cells (DSSCs) have excellent charge collection capabilities, high open-circuit voltages and good fill-factors. However, they do not completely absorb all of the photons from visible and near-infrared ranges and consequently have lower short-circuit photocurrent densities than inorganic photovoltaics. Increasing the short-circuit current density of DSSCs is a key factor in improving the efficiency of these devices.

Current flow can be greatly improved by welding the inter-electrode interface with a laser.

Options include the development of new dyes that absorb photons from a wider solar spectral range and tailoring the TiO₂ nanostructures to offer more efficient charge transport. A number of different schemes regarding these two factors have been suggested and shown to enhance the efficiency. Nevertheless, DSSC efficiencies are still low compared with inorganic devices. One of the main reasons is that the developed methods are only self-effective and can be difficult to combine with other schemes to cumulatively improve the efficiency of the device.

Simple, fast and additive

Researchers from the Department of Materials Science and Engineering at Yonsei University, Korea, have recently demonstrated that the inter-electrode contact resistance arising from poor interfacial adhesion is responsible for a considerable portion of the total resistance in the DSSC. The group has shown that the current flow can be greatly improved by welding the interface with a laser.

TiO₂ films formed on transparent conducting oxide (TCO)-coated glass substrates are irradiated with a pulsed UV laser beam at 355 nm, which transmits through TCO and glass, but is strongly absorbed by TiO₂. It has been found that a thin continuous TiO₂ layer is formed at the interface as a result of the local melting of TiO₂ nanoparticles. This layer completely bridges the gap between the two electrodes and improves current flow by reducing the contact resistance.

Using the process, the team could improve the efficiency of devices by 35–65%. For example, DSSC cells fabricated with and without the interface welding exhibited efficiencies of 11.2% and 8.2%, respectively. The laser welding technique is simple, fast and, more importantly, additive to any other efficiency-enhancing schemes.

The researchers presented their results in the journal Nanotechnology.

5. Rapid synthesis of highly stable gold nanoparticles

Researchers at the Norwegian University of Science and Technology (NTNU) and Chengde Cancer Hospital China have recently developed a new environmentally friendly route for synthesizing gold nanoparticles (AuNPs) in one step at room temperature. The product is highly stable, non-toxic, biocompatible, water-soluble, monodispersed and size-controllable.

Researchers in Norway and China have developed a new environmentally friendly route for synthesizing gold nanoparticles in one step at room temperature. The team plans to use the materials for bio-applications.

The gold particles are made in just a few minutes by simply adding sodium hydroxide to the reaction mixture. The chemical acts as an initiator for the reduction of HAuCl₄ in aqueous solution in the presence of polyvinylpyrrolidone without the use of any reducing agent. To control the size of the AuNPs, the researchers simply adjust the PVP/HAuCl₄ ratio.

The PVP-stabilized AuNPs demonstrate remarkable in vitro stability in a wide range of ionic strength (0–30 M), temperature (4–100 °C), pH (4.4–13.5), various buffer solutions and physiological conditions.

The generation of PVP-stabilized AuNPs has been found to be non-toxic as assessed through MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assays. Also, the production strategy features several green concepts: the choice of friendly solvent, the selection of friendly benign and non-toxic reducing and capping agents, the one-step reaction at room temperature and a reaction time of just a few minutes.

The researchers presented their work in the journal Nanotechnology.