The electron in Graphene has 100 times the speed of electrons in silicon

Physicists at the University of Maryland have recently demonstrated that the limit of mobility in graphene material (the criterion for determining how good a conductive material is) is higher than that of any material. At room temperature. Graphene material, made of a graphite slab, has only one atomic layer. It is a new material that combines the properties of semiconductors and metals.

Their results are published online in Nature Nanotechnology. Research shows that graphene can replace conventional semiconductor materials such as silicon in many applications, from producing high-speed microprocessors to biochemical sensors.

The team is led by physics professor Michael S. Fuhrer of the Center for Advanced Materials and Nano Physics - University of Maryland. The Maryland nano center suggests that the team's findings provide the first calculations of the effect of thermal fluctuations on the conductivity of electrons in graphene materials; At the same time, the study also showed that thermal fluctuations have a small and unusual effect on electrons in graphene.

In other materials, energy is related to the temperature of the material that causes the atoms of the material to vibrate in place. When electrons move in the material, they can bounce off these vibrating atoms to increase resistance. This resistor is characteristic of the material: it cannot be removed unless the material is absolutely cooled to 0 ⁰ . Therefore, the resistance that makes the upper limit allows a material to conduct electricity well.

Optical microscope image of single and dual graphene materials. (Photo: University of Maryland)

For graphene, atoms vibrating at room temperature produce a resistivity of about 1.0 microOhm-cm (the resistivity is a specific unit of resistance, the resistance of a piece of material is equal to the cross-sectional area. of metal pieces divided by the product of resistivity with its length). The resistivity of graphene is less than 35% of the resistivity of copper and is the lowest resistivity known at room temperature.

Fuhrer explains: 'Currently external sources in unsaturated graphene samples increase the resistivity of graphene. Therefore, the average resistivity of graphene is not as small as the resistivity of copper at room temperature. However, graphene has very few electrons compared to copper, so in graphene current is transported by a few electrons that have velocities much faster than copper's electrons'.

For semiconductor materials, mobility standards are used to determine how fast electrons move. Limiting the mobility of electrons in graphene is determined by the thermal vibration of the atom and is about 200,000 cm ² / Vs at room temperature. While silicon is 1,400 cm ² / Vs, at indium antimonide is 77,000 cm ² / Vs. Graphene's electron has the highest mobility compared to conventional semiconductors.

Fuhrer said: 'Interestingly, in semiconductor carbon nanotubes, which are thought to be rolled-up graphene materials into cylinders, we have demonstrated the flexibility at room temperature above 100,000 cm ² /. Vs' (T. Dürkop, SA Getty, Enrique Cobas, and MS Fuhrer, Nano Letters 4, 35 (2004)).

Flexibility determines the speed at which electronic devices (such as a field-effect transistor, which forms the basis of computer chips) can turn on or off. With very high mobility, graphene is very promising in many applications in which transistors require extremely fast toggles such as in the processing of extremely high frequency signals.

Mobility can also be considered as the electrical conductivity of metals through charged electrons. Therefore high mobility is also beneficial for biochemical applications in which an electrical signal from a molecule collected on the device is converted to an electronic signal by changing the conductivity. Power of the device.

That's why graphene is a promising material in chemical and biochemical sensors. Low resistivity along with graphene's ultra-thin characteristics can be used in making transparent, conductive, thin and flexible films. Such films are extremely necessary in a variety of electronic applications from touch screens to photovoltaic cells.

Fuhrer and his colleagues have demonstrated that although limiting the mobility of graphene at room temperature as high as 200,000 cm ² / Vs, current specimens have smaller mobility - about 10,000 cm ² / Vs and need to make great efforts to improve. Since graphene is composed of only one atomic layer, the samples must now be placed in a silicon dioxide substrate.

The charge stored in the silicon dioxide substrate (a type of atomic scale) can affect electrons in graphene, reducing mobility. The vibrations of silicon dioxide atoms themselves could also have an effect on even greater graphene than the effect of its own atomic vibrations. The so-called 'dispersion phonon remote interface' effect is a relatively small bonding wire for mobility in a silicon transistor. But because the phonons in the graphene material itself are not effective in dispersing electrons, this effect becomes very important in graphene.

Fuhrer said: 'We believe that this study has shown the importance of external efficiencies, thereby outlining the direction for finding better substrates for future graphene devices. in order to reduce the impact of dispersing charged impurities as well as dispersing phonon of remote interface '

'Limits of internal and external activity of SiO ₂ ' substrate graphene devices by JH Chen, C. Jang, S. Xiao, M. Ishigami, MS Fuhrer, have been published online in Nature Nanotechnology on 23rd. March 3, 2008.