Application of Grapehene

Single molecule gas detection
Graphene makes an excellent sensor due to its 2D structure. The fact that its entire volume is exposed to its surrounding makes it very efficient to detect adsorbed molecules. Molecule detection is indirect: as a gas molecule adsorbs to the surface of graphene, the location of adsorption experiences a local change in electrical resistance. While this effect occurs in other materials, graphene is superior due to its high electrical conductivity (even when few carriers are present) and low noise which makes this change in resistance detectable.

Graphene nanoribbons

Graphene nanoribbons (GNRs) are essentially single layers of graphene that are cut in a particular pattern to give it certain electrical properties. Depending on how the un-bonded edges are configured, they can either be in a zigzag or armchair configuration. Calculations based on tight binding predict that zigzag GNRs are always metallic while armchairs can be either metallic or semiconducting, depending on their width. However, recent density functional theory calculations show that armchair nanoribbons are semiconducting with an energy gap scaling with the inverse of the GNR width. Indeed, experimental results show that the energy gaps do increase with decreasing GNR width. However, as of February 2008, no experimental results have measured the energy gap of a GNR and identified the exact edge structure. Zigzag nanoribbons are also semiconducting and present spin polarized edges. Their 2D structure, high electrical and thermal conductivity, and low noise also make GNRs a possible alternative to copper for integrated circuit interconnects. Some research is also being done to create quantum dots by changing the width of GNRs at select points along the ribbon, creating quantum confinement.

Graphene transistors

Due to its high electronic quality, graphene has also attracted the interest of technologists who see them as a way of constructing ballistic transistors. Graphene exhibits a pronounced response to perpendicular external electric fields, allowing one to build FETs (field-effect transistors). In their 2004 paper, the Manchester group demonstrated FETs with a “rather modest” on-off ratio of ~30 at room temperature. In 2006, Georgia Tech researchers announced that they had successfully built an all-graphene planar FET with side gates. Their devices showed changes of 2% at cryogenic temperatures. The first top-gated FET (on-off ratio of <2) was demonstrated by researchers of AMICA and RWTH Aachen University in 2007. Graphene nanoribbons may prove generally capable of replacing silicon as a semiconductor in modern technology.

Facing the fact that current graphene transistors show a very poor on-off ratio, researchers are trying to find ways for improvement. In 2008 researchers of AMICA and University of Manchester demonstrated a new switching effect in graphene field-effect devices. This switching effect is based on a reversible chemical modification of the graphene layer and gives an on-off ratio of greater than six orders of magnitude. These reversible switches could potentially be applied to nonvolatile memories.

In 2009 researchers at the Politecnico di Milano demonstrated four different types of logic gates, each composed of a single graphene transistor. In the same year, the Massachusetts Institute of Technology researchers built an experimental graphene chip known as a frequency multiplier. It is capable of taking an incoming electrical signal of a certain frequency and producing an output signal that is a multiple of that frequency. Although these graphene chips open up a range of new applications, their practical use is limited by a very small voltage gain (typically, the amplitude of the output signal is about 40 times less than that of the input signal). Moreover, none of these circuits was demonstrated to operate at frequencies higher than 25 kHz.

In February 2010, researchers at IBM reported that they have been able to create graphene transistors with an on and off rate of 100 gigahertz, far exceeding the rates of previous attempts, and exceeding the speed of silicon. The graphene transistors made at IBM were made using extant silicon-manufacturing equipment, meaning that for the first time graphene transistors are a conceivable—though still fanciful—replacement for silicon.

Integrated circuits

Graphene has the ideal properties to be an excellent component of integrated circuits. Graphene has a high carrier mobility, as well as low noise, allowing it to be used as the channel in a FET. The issue is that single sheets of graphene are hard to produce, and even harder to make on top of an appropriate substrate. Researchers are looking into methods of transferring single graphene sheets from their source of origin (mechanical exfoliation on SiO2 / Si or thermal graphitization of a SiC surface) onto a target substrate of interest. In 2008, the smallest transistor so far, one atom thick, 10 atoms wide was made of graphene. IBM announced in December 2008 that they have fabricated and characterized graphene transistors operating at GHz frequencies. In May 2009 a team from Stanford University, University of Florida and Lawrence Livermore National Laboratory announced that they have created an n-type transistor, which means that both and n and p-type transistors have now been created with graphene. At the same time, the researchers at the Politecnico di Milano demonstrated the first functional graphene integrated circuit – a complementary inverter consisting of one p- and one n-type graphene transistor. However, this inverter also suffered from a very low voltage gain. According to a January 2010 report from the UK’s National Physical Laboratory, a joint group of European research groups (including Politecnico di Milano (Italy), Chalmers University of Technology (Sweden), Linköping University (Sweden) and Lancaster University (UK), as well as the NPL) epitaxially grew layers on silicon carbide, in a quantity and with quality suitable for mass production of integrated circuits. At high temperatures, the Quantum Hall Effect was accurately measured in these samples by the Quantum Detection Group, a division of the NPL.

Transparent conducting electrodes

Graphene’s high electrical conductivity and high optical transparency make it a candidate for transparent conducting electrodes, required for such applications as touchscreens, liquid crystal displays, organic photovoltaic cells, and organic light-emitting diodes. In particular, graphene’s mechanical strength and flexibility are advantageous compared to indium tin oxide, which is brittle, and graphene films may be deposited from solution over large areas.

Large-area, continuous, transparent, and highly conducting few-layered graphene films were produced by chemical vapor deposition and used as anodes for application in photovoltaic devices. A power conversion efficiency (PCE) up to 1.71% was demonstrated, which is 55.2% of the PCE of a control device based on indium-tin-oxide.

Reference material for characterizing electroconductive and transparent materials

One layer of graphene absorbs 2.3 % of white light. This property was used to define the Conductivity of Transparency that combines the sheet resistance and the transparency. This parameter was used to compare different materials without the use of two independent parameters.

Ultracapacitors

Due to the incredibly high surface area to mass ratio of graphene, one potential application is in the conductive plates of ultracapacitors. It is believed that graphene could be used to produce ultracapacitors with a greater energy storage density than is currently available.

Graphene biodevices

Graphene’s modifiable chemistry, large surface area, atomic thickness and molecularly-gatable structure make antibody-functionalized graphene sheets excellent candidates for mammalian and microbial detection and diagnosis devices.
Energy of the electrons with wavenumber k in graphene, calculated in the Tight Binding-approximation. The unoccupied rsp. occupied states, colored in blue-red rsp. yellow-green, touch each other without energy gap exactly at the above-mentioned six k-vectors.The most ambitious biological application of graphene is for rapid, inexpensive electronic DNA sequencing. Affordable and rapid genome sequencing is widely regarded as the next great frontier for science and will eventually revolutionize personalized medicine and custom medical treatment, enabling doctors to determine genetic susceptibility to a host of diseases and tailor therapies to an individual’s genome. Integration of graphene (thickness of 0.34 nm) layers as nanoelectrodes into a nanopore can solve one of the bottleneck issues of nanopore-based single-molecule DNA sequencing.

Anti-bacterial

The Chinese Academy of Sciences has found sheets of graphene oxide are highly effective at killing bacteria such as Escherichia coli. This means graphene could be useful in applications such as hygiene products or packaging that will help keep food fresh for longer.