Graphene Nanoelectronics

 

(Taken from Mechanical Engineering vol.132/No. 3 March 2010 pg 27)

Nowadays, transistor and semiconductor technologies are looking for ways to make devices of even smaller size. When the size of these devices approach the size of molecules, quantum effects become important as the flow of electrons is limited to the pathway delineated by the molecular structures. These small devices will have a wide range of applications. In the medical field, for example, advances in the treatment of cancer by Mauro Ferrari in an article titled "Infernal Mechanism" published in the ASME's (American Society of Mechanical Engineers) magazine, proposes the manufacture of a vessel of nanoscopical dimensions, that when transported by blood, is programmed to kill cancerous cells by releasing toxins through a selective analysis of the proteins found in the cancerous cells. This vessel or nanoscopic system will have sensors able to distinguish cancerous cells from healthy ones through an amplification of electrical signals found in molecules(15). Graphene could replace a transistor in the circuit. In this sense, new materials that would maximize the efficiencies of molecular interactions and reduce the pathways for electrons to travel, become very important.

Typical graphene layer (taken from wikipedia)

A newly discoverd material, graphene, has very promising applications if used as a transistor. Graphene is a simple material which consists of a single layer of carbon atoms in a hexagonal array; one can think of graphene as a honeycomb structure, with carbon atoms at each joint. This configuration makes graphene very tough, even tougher than diamond. In one experiment, Ponomarenko et al., etched graphene transistors as small as 30nm, which have quantum-confined states, and control the flow of single electrons (1). In a typical semiconductor, electrons and holes(positively charged electrons), need an input of finite energy , called "energy gap," to be able to move between different molecular valence orbitals of conductance. In graphene, this is not the case. It was found that, in graphene, holes and electrons move at constant velocity, free of kinetic effects (energy gap), much like photons do. Photons always move at the speed of light c. In graphene the speed of electrons and holes is slower than the speed of light by a factor of 300 (1). The quantum behavior of such structure is new for solids, and since there is no energy gap, it allows for electrons and holes to coexist in the same relativistic quantum space. The conductance of such structure can be made less or more positive as a voltage gate opens or closes. As the density of positive charge is decreased, for example, it allows the flow of electrons to decrease also. The conductance of the material would never reach zero. Further research should then be conducted for transistor application, or find new ways to take advantage of this property.

There is another unusual phenomenon in graphene layers, and this has to do with the Klein paradox. The Klein paradox allows relativistic particles to pass freely through a tall barrier of great width (10), whereas an ordinary particle would bounce backwards, like a baseball after it hits a wall. Essentially, as an electron approaches the barrier, it stops, and instead of bouncing off the wall, it transforms into a hole, and moves through the barrier freely. As the particle leaves the barrier, it resumes its inertial frame and flips its charge turning back into an electron! (as if the particle had memory). Graphene layers follow this behavior almost ideally. Some research in this property propose the construction of nanoscale transistors, which could revolutionize the world of electronics in unimaginable ways.

The steps taken towards the goal of manufacturing nanoscopic graphene transistors depend on the creation of quantum dots. Quantum dots act as single-electron transistors depending on its size, the smaller the better, and are able to achieve operation at room temperature (typical quantum experiments are conducted at -452F such as the Hall effect) (13). Some researchers like Ponomarenko et al., Novoselov et al., Berry and Mondragon (7), have constructed quantum dots in the 100nm (1) scale and obtained satisfactory results. The transistor is considered one of the greatest inventions of the 20th century; in the 21st century the nanotransistor could reshape our future.

R.M. Westervelt. Science 320, 324 (2008); DOI: 10.1126/science.1156936
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