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