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Graphene Nanoribbons

During the last two decades, carbon allotrope forms have revolutionized the modern material science and industry. The discovery of a series of carbons allotropes such as graphene, carbon nanotubes and fullerenes have opened new pathways for scientists to create novel nano materials with unique and unprecedented properties. These nanomaterials discoveries joined with immense advances in theoretical chemistry, have pushed the boundaries toward a high paced evolution of novel nanomaterials. Advances in computational chemistry have offered researchers tools that never existed and allowed for investigation of material properties at atomic scale; proof coming from such theoretical works led to continuous advances in synthetic nano chemistry and especially in modifications of various nanomaterials for properties tailoring purposes.

Some of the most revolutionary nano materials of our times are scratching the surface in technologies such as solar energy applications, electronics [graphene, carbon nanotubes, fullerene], adsorption and storage of hydrogen [graphene, nanotubes], opto electronics, transparent electronics, electro chemical detectors, novel conductive polymers and composites [graphene, nanotubes], energy storage, electrochromic and thermo chromic devices and many more. The base of most carbon allotropes is graphene. Graphene is practically a single sheet structure [one atom thick] that comprises honeycomb architecture. Its unique electronic and chemical properties are a result of its unique structure. Carbon nanotubes can be seen as ‘wrapped’ graphene sheets and their properties are different compared to graphene’s due to the increased dispersion and electrostatic interactions in their interior and their exterior [in bundles].

Nanoribbon Fabrication Methods

The most novel member of the carbon allotropes family is termed carbon nanoribbon. Comprised of a single strip of a graphene sheet, carbon nanoribbons exhibit unique quantum properties in arm chain nanoribbons result to significant band gaps that correlate to their boundary chemical structure. Nanoribbons with zig zag edges on the other hand exhibit spin polarized electronic states leading to applications in various fields. Their fabrication methodologies include three options:

  1. 1) cutting existing graphene sheets using lithography
  2. 2) synthesizing nanoribbons using a bottom- up approach
  3. 3) unwrapping of carbon nanotubes.

The three different approaches demonstrate advantages and shortcomings and are used on a desired properties basis.

The first, lithography, is mainly used for production of nanoribbons when their edge properties are not of the highest importance [edge properties dictate the electronic properties and character of the nanoribbon]. Also, both graphene and the resulting nanoribbon have to be worked on an appropriate substrate. The major advantage of lithography approach is found in its ability to produce ultra-thin strips, in the range of a few atoms. As recent studies have shown, such narrow structures exhibit metallic character and can be used in solar energy applications among others, as quantum dots.

The second process, bottom up synthesis, is based on a multi steps organic syntheses approach, usually comprising a substrate that leads to a controlled edges structure and defined thickness of the strips. Most of the progress based on this field depends on parallel progress in theoretical approaches, as the possibilities of precursors and reagents are practically unlimited. Compared to the lithography approach this approach offers the potential of tailoring edge architecture and thus electronic properties and thus nanoribbons manufactured via this approach can be used in every electronic application.

Unwrapping [or unzipping] of multi walled nanotubes is the third manufacturing approach of nanoribbons, and is reason behind its increased popularity is mainly the low cost and its ability to provide large scale production of nanoribbons [in the kg scale] in comparison to the mini scale of bottom up synthesis [in the mg scale]. Low cost operation is another significant attribute of this process. The fabrication methodology usually comprises a solution based pathway that involves distinct steps: an initial insertion of potassium atoms into the multi wall nanotube, an opening stage of the nanotubes that produces nanoribbons with active edges, an in situ functionalization and a final separation stage of the multiple graphene nanoribbons. The properties of the multiple [before separation] nanoribbons resemble those of graphitic structures, providing evidence that the separation distance is found in the range close to that of graphite [3.4 Å]. Almost all nanoribbons in the market today are produced via the unzipping process mainly because the unzipping of carbons offers low cost and large quantities, but the bottom up synthesis offers detailed synthesis at atomic scale. Once as scale we estimate that bottom up synthesis will lead the charge in manufacturing of graphene nanoribbons in the near future based on the future needs for application specific graphene nanoribbons.

Potential applications of nanoribbons include:

  •      • Composites modification
  •      • Electronics
  •      • Energy storage
  •      • Solar energy [and quantum dots]
  •      • Electrode materials in electrochemical cells and batteries
  •      • Conductive polymers
  •      • Permeability modification [membranes]

 

Quantum Dots nanocrystals owe their band gap to their sizes [properties in bulk are inappropriate for solar applications] and can be substituted by graphene nanoribbons [zig zag edged]. Electrons and holes coexist in a confined space in three dimensions, resulting to quantization of the electron and hole energy levels. This is termed as the ‘zero dimensional confinement effect’. The electronic properties of these materials are found in the range between that of a bulk semiconductor and discrete molecules.

Utilization of nanoribbons in dielectric materials leads to very low loss that greatly increases the value of the polymer for various related applications [antennas manufacturing]. for supercapacitors and cells based on composites comprising metal oxides, graphene and graphene nanoribbons. This approach led to significant stability of the electrochemical cell and is currently the topic of further investigations.

Another very important application is that of modification of permeability of membranes towards various gaseous species;that very thin [5 carbon atoms width] nanoribbons could replace copper wires in microprocessors very soon. Their results suggest that such structures exhibit superior conductance and resistance to electro migration. This work was based on ab initio protein structure predictions that could not be validated until recently; however this work proved that when the nanoribbons are longer than 5nm the semiconducting character is transformed to metallic, offering a huge breakthrough for nano electronics. Synthesis of varying width nanoribbons was also investigated and confirmed based on different precursors.

Another very recent work has suggested and confirmed the use of nanoribbons in biomedical applications and specifically in the detection of nucleotides [DNA sequencing]. This work utilized the nanoribbons’ [and graphene’s] ability to transform anisotropic lattice strains into electrical current changes. The described sensor was successful in various experimental environments, outperforming currently used nucleotides sensors.

Oil and gas industry has identified a problem solver in graphene nanoribbons as well; proved that the use of nanoribbons in well stabilizing technology. It is a common issue of drilling that fractures arise that put the integrity of the wells at risk. These fractures have been repaired with micro particles of various species [including mica] with low efficiency. Nanoribbon utilizations has resulted to a stable composite formation that does not allow for appearance of related fractures.

As discussed above, numerous other applications of nanoribbons exist. It is expected that the potential applications will increase based on their unique electronic properties and the advances in the manufacturing and modification methodologies. Computational chemistry approaches are expected to have the largest impact for both of them since the properties of nanoribbons are dependent on the atomic and electronic scale.

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