Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities with application in very exciting areas including integrated circuits, transistors, ultra capacitors, Li-ion batteries and bio-devices. Graphene exhibits many exciting properties, like room temperature quantum hall effect, long range ballistic transport with around ten times higher electron mobility than that in silicon, availability of charge carriers that behave as mass less relativistic quasi-particles like Dirac fermions, quantum confinement resulting in finite band gap and coulomb blockade effects, which could be useful to make many novel electronic devices in the near future. However, in order to fully realize the above properties and applications, consistent, reliable and inexpensive methods of growing high quality, graphene layers in good yields are crucial, as the existence of residual defects will heavily impact their electronic properties, despite their expected “insensitivity to impurity scattering”.
Reported techniques for the synthesis of graphene such as mechanical cleavage, silicon carbide sublimation, solvothermal synthesis, chemical vapour deposition, and plasma etching suffer from limitations such as, a) poor quality and yield (<2%) of graphene ribbons, b) formation of over oxidized and defective nanoribbons, c) substrate-dependant behavior and finally, d) difficulty of controlling both layer thickness and edge smoothness in an accurate manner.
One of the more successful approaches to date for converting MWCNT (multi-walled carbon Nanotubes) to graphene is the recent longitudinal unzipping of MWCNTs, facilitating large scale preparation of graphene ribbons.
References may be made to Journal by Kosynkin et al in Nature, Volume 458, April 2009, titled “Longitudinal unzipping of carbon nanotubes to from graphene nanoribbons” describes a solution based oxidative process for producing graphene nanoribbons by lengthwise cutting and unraveling of MWCNT's side walls.
References may be made to Journal by Jiao et al in Volume 458, April 2009, titled “Narrow graphene nanoribbons from carbon nanotubes” describe the process of making graphene by unzipping multi-walled carbon nanotubes by plasma etching of nanotubes partly embedded in a polymer film. However, these methods have several problems primarily involved with the selection of strong oxidizing agents. The choice of the chemical oxidation route has issues such as over oxidation of edges that create defect sites which hamper electronic properties of graphene. In addition the use of strong reducing agents may pose difficulties in controlling the layer thickness of graphene ribbons. More significant would be the separation and disposal problem of these reagents with respect to the environment and hazardous effluent treatment.
References may be made to patent JP2009196840, wherein inventor discloses a cutting process for CNTs, comprising: Cutting process of a carbon nanotube oxidizing electrochemically contained in a carbon electrode by impressing voltage to a carbon nanotube electrode which is immersed in electrolysis solution, and cutting carbon nanotube. Further, the process results in electrochemical oxidative scission of carbon nanotubes, the oxidation being optimally carried out at a positive electromotive force for 10 hours. The document also states that if the electromotive force is beyond 4V, a cluster like graphene structure is observed.
References may be made to an article titled “Facile and controllable electrochemical reduction of graphene oxide and its applications” by YuyanShao et al. published in Journal of Materials Chemistry, Vol. 20, pg. 743-748, 2010 relates to electrochemical reduction of graphene oxide to form graphene. Accordingly, the oxygen content is significantly decreased and the amount of sp2 carbon is restored after electrochemical reduction.
References may be made to an article titled “A green approach to the synthesis of graphene nanosheets” by Guoht et al. published in ACS Nano. 2009 Sep. 22; 3 (9):2653-9. relates to the reduction of exfoliated graphene oxide to yield graphene, by applying a reducing potential of −1.5 V. The hence obtained graphene nanosheets are characterized.
References may be made to patent JP2009196840, wherein inventor discusses the electrochemical oxidation of CNTs to obtain oxidized graphene, it also teaches that if the electromotive force is beyond 4 V, a cluster like graphene structure observed. But GuoHL et al discuss electrochemical reduction of graphene oxide to graphene at −1.5 V. Also, GuoHL et al and Yuyan Shao et al start with graphite and the result of their processes is few layered graphene with uncontrolled layer thickness.
Thus it is observed that prior at documents combine processes such as chemical oxidation and reduction or chemical oxidation with electrochemical reduction to result in graphene, but with poor yields and poor characteristics. Also, in many of these cases graphite in either its exfoliated or pristine form is the starting compound.
It is surprising to note that there are no prior arts that each a process for synthesis of graphene with rich yields or that which results in graphene in various physical forms of uniform layer thickness starting with single, double or multi-walled carbon nanotubes.
Hence, an accurate control of the quality of graphene layers along with the preparation in good yields is an unresolved need in the art.