GRAPHENE NEWS

The SI system of measures consists of many units, of which seven are considered base units. These base units are meter, kilogram, second, kelvin, mole, candela and ampere. Most of these units are well defined and constant, with the kilogram being an exception (kilogram is still defined as the mass of the International Kilogram Prototype).

Ampere is well-defined as well, but its definition is a bit clumsy for practical use. The ampere is defined as the constant current which, if maintained in two parallel conductors of infinite length, 1 meter apart and straight, would produce a force between these two conductors equal to 2x10^-7

Graphene could come in handy in the re-definition of the ampere. Scientists have managed to create an electron pump which can emit individual electrons at command using graphene. These graphene electron pumps could be used to produce a current of exactly one ampere. Since the charge of a single electron is well-known, the ampere could be prototyped on site as needed as a flow of one couloumb of charge per second using one single electron pump, or many individual graphene electron pumps.

Aside from this metrology use of the graphene single-electron pump, there are many potential uses in experimental quantum physics. These uses could lead to some interesting experiments and proofs. We find it exciting that graphene could offer to change our world in such a fundamental way as changing the definition of one of the base units of the SI system.

Graphene's chemical stability combined with properties of other free two-dimensional crystals allows scientists to stack these crystalline materials and create novel devices using advanced functional materials. Scientists have managed to induce a tunneling effect which produces a negative differential conductance at room temperature using a layer of boron nitride a few atoms thick, sandwiched between graphene electrodes.

The tunnel effect has been known for almost half a century, however conventional tunneling devices were tens of nanometers thick, while these new graphene-boron nitride tunneling transistors are merely a few atomic layers thick. Their miniscule thickness allows these transistors to achieve ultra-fast transit times. In theory, these devices could be applied to high-frequency logic circuits.

This device can be controlled by applying a variable gate voltage. The current through the transistor rises with the applied gate voltage increase until it reaches a maximum value, after which it decreases steadily. This area of output current decrease as gate voltage increases can be used similarly to the way tunneling diodes are used, only at much higher frequencies.

A review of studies focused on enhancing the thermal conductivity of phase change materials (PCM) for thermal energy storage upon introduction of nanostructures is presented. These emerging materials have only been studied since 2005 and represent a clear departure from previous/existing practices of utilizing fixed, stationary high-conductivity inserts/structures into PCM. Carbon-based nanostructures (nanofibers, nanoplatelets and graphene flakes), carbon nanotubes, both metallic (Ag, Al, C/Cu and Cu) and metal oxide (Al2O3, CuO, MgO and TiO2) nanoparticles and silver nanowires have been explored as the materials of the thermal conductivity promoters. Emphasis of the work so far has been placed on the dependence of the enhanced thermal conductivity on mass fraction of the nanostructures and temperature for both liquid and solid phases, however issues related to modifications of the degree of supercooling, melting temperature, viscosity, heat of fusion, etc. are also reported.

In general, carbon-based nanostructures and carbon nanotubes exhibit far greater enhancement of thermal conductivity in comparison to metallic/metal oxide nanoparticles due to the high aspect-ratio of these nanofillers. Utilizing a figure of merit for the observed thermal conductivity enhancement, the majority of 340+ measured data points in both liquid and solid phases are summarized.

23.4.2013. Growing better nanowires on graphene

Researchers at the University of Illinois produced III-VI compound semiconductors. This was previously not feasible using silicon, and researchers used a graphene substrate instead of the old-fashioned silicon. They used a process called MOCVD (MetalOrganic Chemical Vapour Deposition) to deposit In-Ga-As (Indium-Gallium-Arsenic) semiconductors onto the surface of graphene. The semiconductor atoms self-assembled into a nanostructure resembling a crystalline form. This is the first time that these three semiconductors were merged in this fashion.

This discovery will allow scientists to grow coaxial core-shell structures in a single step instead of a two-step process. A single step process ensures a better interface between the nanostructure and the substrate, since the growth process is spontaneous. This is because the distance between the atoms in a InAs crystal structure is the same as the distance between carbon atoms in graphene. Therefore, the InAs fits into the space perfectly, like the final piece in a puzzle, allowing gallium to form a shell around the InAs core.

Original research text: In_xGa_1-xAs Nanowire Growth on Graphene: van der Waals Epitaxy Induced Phase Segregation