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Chemistry Optional Notes On – Graphene Synthesis – For W.B.C.S. Examination.
Graphene is an allotrope of carbon that exists as a two-dimensional planar sheet. One way to think of graphene is as a single atomic graphite layer.Continue Reading Chemistry Optional Notes On – Graphene Synthesis – For W.B.C.S. Examination.
Graphene is technically a non-metal but is often referred to as a quasi-metal due to its properties being like that of a semi-conducting metal. As such, it has many unique properties that you don’t find with other non-metallic materials.
Each carbon atom is covalently bonded (sp2 hybridized) to three other carbon atoms in a hexagonal array, leaving one free electron per each carbon atom.
This free electron exists in a p-orbital that sits above the plane of the material. Each hexagon in the graphene sheet exhibits two pi-electrons, which are delocalized, allowing for an efficient conduction of electricity.
The holes in the structure also allow phonons to pass through unimpeded, which gives rise to a high thermal conductivity.
Graphene has many unique properties, making it an ideal material for use in electronic applications when compared to conventional materials.
Electrical conductivity the most prevalent and important property of graphene. Graphene doesn’t have an electronic band-gap (meaning that it can’t be switched on or off) as the valence and conduction bands have a small overlap and the electrons act as massless relativistic particles.
At room temperature, graphene can exhibit a concentration of charge carriers up to 1013 cm-2, with a mobility of 1 X 104 cm2 V-1 s-1. At low temperatures, this can increase to 2 X 105 cm2 V-1 s-1.
Because the charge carriers act as quasi-particles, otherwise known as massless Dirac Fermions, graphene also exhibits a half-integer Quantum Hall Effect (QHE).
The QHE is the relationship of the charge, density and velocity of the charge carriers. It occurs when a magnetic field is applied along the axis perpendicular to the plane of the conducting material.
Under these conditions, the path of the carriers becomes curved, leading to an accumulation of opposite charges at either end of the material. Due to the two-dimensional nature of graphene, the electron confinement produces discrete band levels known as Landau levels, which are filled by the charge carriers.
Unlike other materials, the charge carriers in graphene only half-fill these levels, leading to a quantization of the Landau levels, and in effect the energy levels of graphene.
Graphene also has great optical, thermal and mechanical properties. Single sheet graphene is a highly transparent material but each layer in thickness absorbs up to 2.3% of white light, with less than 0.1% reflectance.
There is also a linear absorbance increase with respect to the number of layers stacked on top of each other. A suspended graphene sheet can exhibit a thermal conductivity of 3000-5000 W m-1 K-1 at room temperature. However, this can drop to as low as 600 W m-1 K-1 when it is attached to another substrate.
The drop is caused by a scattering of phonons at the interface which impedes their movement, whereas in free standing graphene the phonon path is uninterrupted. Even at this lower conductivity, the thermal conductivity is still twice as high as copper.
Graphene is also known to be one of the strongest materials ever made and a single-layer graphene sheet can withstand up to 42 N m-1 of stress, with a Young’s modulus of 1.0 Tpa.
There are many types of graphene. True Graphene is only one atomic layer thick (often called a monolayer) and it typically exists as a film but it can be floated off the substrate and can be redeposited onto another substrate or used in it’s isolated form.
There are, however, several types of graphene containing powder form materials such as graphene oxide, graphene nanoplatelets, graphene nanoribbons, and graphene quantum dots as well as graphene enabled products such as graphene ink or graphene masterbatches.
There are 3 main ways to synthesize graphene, they are:
- Chemical Vapor Deposition
- Chemical or Plasma Exfoliation from natural Graphite
- Mechanical cleavage from natural Graphite
Graphene can also be fully synthetic but those methods haven’t proven to be commercially viable.
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