Breakthrough in transistor fabrication without semiconductors

Michigan Technological University scientists led by professor of physics Yoke Khin Yap have created a quantum tunneling device that acts like like an FET transistor and works at room temperature without using semiconducting materials.

The trick was to use boron nitride nanotubes (BNNTs) with quantum dots made from gold. When sufficient voltage is applied to the device, it switches from insulator to a conducting state. When the voltage is low or turned off, it reverts to its natural state as an insulator. There is no leakage current of electrons escaping from the gold dots into the insulating BNNTs, thus keeping the tunneling channel cool. In contrast, silicon is subject to leakage, which wastes energy in electronic devices and generates a lot of heat, limiting miniaturization of transistors.

Carpets of boron nitride nanotubes, which are insulators and highly resistant to electrical charge were grown on a substrate. Using lasers, quantum dots (QDs) of gold  are deposited as small as three nanometers across on the tops of the BNNTs, forming QDs-BNNTs. BNNTs are the perfect substrates for these quantum dots due to their small, controllable, and uniform diameters, as well as their insulating nature. BNNTs confine the size of the dots that can be deposited. Now if we applied biasing, electrons jumped very precisely from gold dot to gold dot, which is known as quantum tunneling.

Other people have made transistors that exploit quantum tunneling. However, those tunneling field effect transistors have only worked in low temperature. The gold islands have to be on the order of nanometers across to control the electrons at room temperature. If they are too big, too many electrons can flow.

For further reading
http://www.readcube.com/articles/10.1002/adma.201301339?

New material for fuel cell catalyst

Efficient, robust and economical catalyst materials hold the key to achieving a breakthrough in fuel cell technology. Scientists from Jülich and Berlin have developed a material for converting hydrogen and oxygen to water using a tenth of the typical amount of platinum that was previously required. With the aid of state-of-the-art electron microscopy, the researchers discovered that the function of the nanometre-scale catalyst particles is decisively determined by their geometric shape and atomic structure. This discovery opens up the opportunity for further improved catalysts for energy conversion and storage. 

Hydrogen-powered fuel cells are regarded as a clean alternative to conventional combustion engines, as the only substance produced during operation is water. At present, the implementation of hydrogen fuel cells is being hindered by the high material costs of platinum. Large quantities of the expensive noble metal are still required for the electrodes in the fuel cells at which the chemical conversion processes take place. Without the catalytic effect of the platinum, it is not currently possible to achieve the necessary conversion rates.

As catalysis takes place at the surface of the platinum only, material can be saved and, simultaneously, the efficiency of the electrodes improved by using platinum nanoparticles, thus increasing the ratio of platinum surface to material required. Although the tiny particles are around ten thousand times smaller than the diameter of a human hair, the surface area of a kilogram of such particles is huge.

However, more platinum can be saved by mixing it with nickel or copper. Scientist have succeeded in developing efficient metallic catalyst particles for converting hydrogen and oxygen to water using only a tenth of the typical amount of platinum that was previously required. 

The new catalyst consists of octrahedral-shaped nanoparticles of a platinum-nickel alloy. The researchers discovered that the unique manner in which the platinum and nickel atoms arrange themselves on the surfaces to accelerate the chemical reaction between hydrogen and oxygen to form water. Round or cubic particles have different atomic arrangements at the surface and are therefore less effective catalysts for the chemical reaction, which could be compensated by using increased amounts of noble metal.

Thermocrystal: an excellent idea to control the direction of heat by nanoparticle alloy

An MIT scientist has developed a technique that provides a new way of manipulating heat, allowing it to be controlled much as light waves can be manipulated by lenses and mirrors.

The approach relies on engineered materials consisting of nanostructured semiconductor alloy crystals. Heat is a vibration of matter, a vibration of the atomic lattice of a material like sound. Such vibrations can also be thought of as a stream of phonons, which is equivalent to the photons that carry light. The new approach is similar to recently developed photonic crystals that can control the passage of light.

The spacing of tiny gaps in these materials is tuned to match the wavelength of the heat phonons. It’s a completely new way to manipulate heat. Heat differs from sound in the frequency of its vibrations: Sound waves consist of lower frequencies (up to the kilohertz range, or thousands of vibrations per second), while heat arises from higher frequencies (in the terahertz range, or trillions of vibrations per second).

In order to apply the techniques already developed to manipulate sound, first step was to reduce the frequency of the heat phonons, bringing it closer to the sound range. Phonons for sound can travel for kilometres, but phonons of heat only travel for nanometers. That’s why we couldn’t hear heat even with ears.
Heat also spans a wide range of frequencies, while sound spans a single frequency. To get rid of the problem, the first thing to do is to reduce the number of frequencies of hea, bringing these frequencies down into the boundary zone between heat and sound. Making alloys of silicon that incorporate nanoparticles of germanium in a particular size range accomplished this lowering of frequency, scientist found.

Reducing the range of frequencies was also accomplished by making a series of thin films of the material, so that scattering of phonons would take place at the boundaries. This ends up concentrating most of the heat phonons within a relatively narrow window of frequencies.

Following the application of these techniques, more than 40 percent of the total heat flow is concentrated within a hypersonic range and most of the phonons align in a narrow beam, instead of moving in every direction.
As a result, this beam of narrow-frequency phonons can be manipulated using phononic crystals similar to those developed to control sound phonons. Because these crystals are now being used to control heat instead, these are referred to as thermocrystals, a new category of materials.

These thermocrystals might have a wide range of applications, including in improved thermoelectric devices, which convert differences of temperature into electricity. Such devices transmit electricity freely while strictly controlling the flow of heat.

Most conventional materials allow heat to travel in all directions, like ripples expanding outward from a pebble dropped in a pond; thermocrystals could instead produce the equivalent of those ripples only moving out in a single direction. The crystals could also be used to create thermal diodes; materials in which heat can pass in one direction, but not in the reverse direction. Such a one-way heat flow could be useful in energy-efficient buildings in hot and cold climates.

Other variations of the material could be used to focus heat to concentrate it in a small area. Another intriguing possibility is thermal cloaking, materials that prevent detection of heatto shield objects from detection by visible light or microwaves.

For further reading: http://prl.aps.org/pdf/PRL/v110/i2/e025902

Gold nanoparticles to observe the interaction of molecules in liquid

Thanks to a new device that is the size of a human hair, it is now possible to detect molecules in a liquid solution and observe their interactions. This is of major interest for the scientific community, as there is currently no reliable way of examining both the behavior and the chemical structure of molecules in a liquid in real time.

This process could potentially make a whole new class of measurements possible by bringing together infrared detection techniques and gold nanoparticles, which would be a critical step in understanding basic biological functions as well as key aspects of disease progression and treatment. This could also prove useful for studying the behaviour of proteins, medicines and cells in the blood or pollutants in water.

The device is based on infrared absorption spectroscopy. Infrared light can already be used to detect elements: The beam excites the molecules, which start to vibrate in different ways depending on their size, composition and other properties. It's like a guitar string vibrating differently depending on its length. The unique vibration of each type of molecule acts as a signature for that molecule.

This technique works very well in dry environments but not at all well in aqueous environments. A large number of molecules need to be present for them to be detected. It's also more difficult to detect molecules in water, as when the beam goes through the solution, the water molecules vibrate as well and interfere with the target molecule's. To get rid of this problem, the researchers have developed a system capable of isolating the target molecules and eliminating interferences.

The device is made up of miniature fluidic chambers with nano scale gold particles on one side of its cover. Now to catch a particular molecule gold nanoparticle is attached with specific antibodies. Once the target element is introduced in to the small chamber, nanoparticles get attached to the target element. This technique makes it possible to isolate the target molecules from the rest of the liquid. But this is not the only role the nanoparticles play. They are also capable of concentrating light in nanometer-size volumes around their surface as a result of plasmonic resonance.

In the chamber, the beam doesn't need to pass through the whole solution. Instead, it is sent straight to the nanoparticle, which concentrates the light. Caught in the trap, the target molecules are the only ones that are so intensely exposed to the photons. The reaction between the molecules and the infrared photons is extremely strong, which means they can be detected and observed very precisely. This technique enables to observe molecules in-situ as they react with elements in their natural environment. This could prove extremely useful for both medicine and biology.

Electrodes made by Carbon Nanotubes for Recording Neuron Activity

Scientists have been studying how neuron communication in the brain relies on glass and metal electrodes to detect the tiny, high speed synaptic potentials. Yet, nanotechnology offers the possibility of scaling down the electrodes so that individual neurons could be tapped in living, moving animals. Researchers at Duke University are now claiming that they developed electrodes made out of self-entangled carbon nanotubes that are a millimeter long and feature a sub-micron tip. Carbon nanotubes have excellent electrical properties and are incredibly strong for their size. The new needles are small enough to penetrate individual cells and record intracellular electrical activity. The Duke team used the new electrodes to make such recordings in live animals and on brain slices and envision using the new electrodes to record neuronal activity for extended periods in freely moving animals.

The computational complexity of the brain depends in part on a neuron’s capacity to integrate electrochemical information from vast numbers of synaptic inputs. The measurements of synaptic activity that are crucial for mechanistic understanding of brain function are also challenging, because they require intracellular recording methods to detect and resolve millivolt scale synaptic potentials. Although glass electrodes are widely used for intracellular recordings, novel electrodes with superior mechanical and electrical properties are desirable, because they could extend intracellular recording methods to challenging environments, including long term recordings in freely behaving animals. Carbon nanotubes (CNTs) can theoretically deliver this advance, but the difficulty of assembling CNTs has limited their application to a coating layer or assembly on a planar substrate, resulting in electrodes that are more suitable for in vivo extracellular recording or extracellular recording from isolated cells. Here a novel, yet remarkably simple, millimeter-long electrode with a sub-micron tip, fabricated from self-entangled pure CNTs can be used to obtain intracellular and extracellular recordings from vertebrate neurons in vitro and in vivo. This fabrication technology provides a new method for assembling intracellular electrodes from CNTs, affording a promising opportunity to harness nanotechnology for neuroscience applications.

New Graphene Sensor for more sensitive camera

Recently scientists at Nanyang Technological University invented a new camera sensor which could revolutionize current camera market by its ability to take clear photos in dim conditions.

The new sensor made from graphene, is believed to be the first to be able to detect broad spectrum light, from the visible to mid-infrared, with high photoresponse or sensitivity. This means it is suitable for use in all types of cameras, including infrared cameras, traffic speed cameras, satellite imaging and more.

Not only is the graphene sensor 1,000 times more sensitive to light than current imaging sensors found in today's cameras, it also uses 10 times less energy as it operates at lower voltages. When mass produced, graphene sensors are estimated to cost at least five times cheaper due to its high electrical conductivity among other properties such as durability and flexibility.

This sensor could have great impact not only on the consumer imaging industry, but also in satellite imaging and communication industries, as well as the mid-infrared applications, While designing this sensor, current manufacturing practices have been kept in mind. This means the industry can in principle continue producing camera sensors using the CMOS (complementary metal-oxide-semiconductor) process, which is the prevailing technology used by the majority of factories in the electronics industry. Therefore manufacturers can easily replace the current base material of photo sensors with our new nano-structured graphene material. Cost of manufacturing imaging sensors will fall, which eventually leads to cheaper cameras with longer battery life, if this is adopted by industry.

Researcher came up with an innovative idea to create nanostructures on graphene which will trap light-generated electron particles for a much longer time, resulting in a much stronger electric signal. Such electric signals can then be processed into an image, such as a photograph captured by a digital camera. The trapped electrons is the key to achieving high photoresponse in graphene, which makes it far more effective than the normal CMOS or CCD (charge-coupled device) image sensors. Essentially, the stronger the electric signals generated, the clearer and sharper the photos. The performance of graphene sensor can be further improved through nanostructure engineering of graphene.

Myths about toxic Nanosilver busted

According to Finnish-Estonian joint research with data obtained on two crustacean species, there is apparently no reason to consider silver nanoparticles more dangerous for aquatic ecosystems than silver ions. The results were reported in the journal Environmental Science and Pollution Research late last year.

Part of the magic of nano-science is that on the scale of a billionth of a metre, matter and materials behave in ways that are not yet known. It is also not very clear what types of effects the nano version of the parent matter will have on its environment.

Due to the fact that silver in nanoparticle form is bactericidal and also fungicidal and also prevents the reproduction of those organisms, it is now used in various consumer goods ranging from wound dressing products to sportswear, says Jukka Niskanen from the Laboratory of Polymer Chemistry at the University of Helsinki, Finland.

While the usefulness of silver has been established, the debate over the toxicity mechanisms of its various forms to microorganisms, but also to non-target species continues. Anne Kahru, Head of the Laboratory of Environmental Toxicology at the National Institute of Chemical Physics and Biophysics, Estonia, highlights on a new field: nanoecotoxicology.

So far, little is known about the environmental effects of silver nanoparticles and their toxicity to aquatic organisms. A joint study from the University of Helsinki and the National Institute of Chemical Physics and Biophysics, Estonia of two types of silver nanoparticles to aquatic crustaceans Daphnia magna and Thamnocephalus platyurus , shows that silver nanoparticles are apparently no more hazardous to aquatic ecosystems than a water-soluble silver salt. The study compared the ecotoxicity of silver nanoparticles and a water-soluble silver salt.

The conclusion was that the environmental risks caused by silver nanoparticles are seemingly not higher than those caused by a silver salt. However, more research is required to reach a clear understanding of the safety of silver-containing particles.

Indeed, silver nanoparticles were found to be ten times less toxic than the soluble silver nitrate -- a soluble silver salt used for the comparison.

To explain this phenomenon, the researchers refer to the variance in the bioavailability of silver to crustaceans in different tested media.

It has been observed that the inorganic and organic compounds dissolved in natural waters (such as humus), water hardness and sulfides have a definite impact on the bioavailability of silver. Due to this, the toxicity of both types of tested nanoparticles and the silver nitrate measured in the course of the study was lower in natural water than in artificial fresh water.

The toxicity of silver nanoparticles and silver ions was studied using two aquatic crustaceans, a water flea (Daphnia magna) and a fairy shrimp (Thamnocephalus platyurus). Commercially available protein-stabilised particles and particles coated with a water-soluble, non-toxic polymer, specifically synthesised for the purpose, were used in the study. First, the polymers were produced utilising a controlled radical polymerisation method. Synthetic polymer-grafted silver particles were then produced by attaching the water-soluble polymer to the surface of the silver with a sulfur bond.

It was previously known from other studies and research results that silver changes the functioning of proteins and enzymes. It has also been shown that silver ions can prevent the replication of DNA. Concerning silver nanoparticles, tests conducted on various species of bacteria and fungi have indicated that their toxicity varies. For example, gram-negative bacteria such as Escherichia coli are more sensitive to silver nanoparticles than gram-positive ones (such as Staphylococcus aureus). The difference in sensitivity is caused by the structural differences of the cell membranes of the bacteria. The cellular toxicity of silver nanoparticles in mammals has been studied as well. It has been suggested that silver nanoparticles enter cells via endocytosis and then function in the same manner as in bacterial cells, damaging DNA and hindering cell respiration. Electron microscope studies have shown that human skin is permeable to silver nanoparticles and that the permeability of damaged skin is up to four times higher than that of healthy skin.