Graphene or one-atom-thick sheets of carbon atoms has been the material of talk for the future technology visioners since its first isolation by two researchers, Prof. Andre Geim and Prof. Kostya Novoselov at the University of Manchester [1,2].
In very simple terms, graphene can be considered as the thinnest material which can be obtained by either repeated breaking of graphite or repeated peeling off layers of graphite by even our office sticky tape.
It’s very surprising that the material in pencil tips which children often break during various school activities is now the potential material of choice for future electronics and various other applications.
There are two routes by which this awe-amazing material can be obtained: the bottom-up approach and the top-down approach.
The bottom-up approach deals with combining atoms of carbon to produce two-dimensional (2-D) graphene nanosheets. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods [3,4] mark their importance as the techniques very commonly used for graphene production by the bottom-up approach. PVD is characterized by a process in which the material goes from a condensed phase to a vapor phase and then back to a thin film condensed phase whereas, in CVD, a wafer (substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit.
The top-down approach on the contrary deals with breaking of bulk thick graphite into single carbon nanosheets. Mechanical exfoliation and chemical or liquid-phase exfoliation techniques are popular players [5,6] when it comes to graphene production by the top-down approach. Mechanical exfoliation refers to repeated peeling of bulk graphite using thin transparent scotch tape to produce graphene nanosheets. Chemical exfoliation, on the other hand, refers to breaking layers of graphite by inserting bulk graphite in a suitable solvent having a surface energy in the close vicinity of graphite and providing external energy.
Unique properties of graphene
Graphene is so far the thinnest material known with a reported theoretical thickness of 0.335 nm. However, AFM results in the reported literature have been slightly inaccurate demonstrating a wide range of measured values for graphene monolayers from 0.4 to 1.7 nm . Each carbon atom in graphene binds to nearest neighbour carbon atoms with the help of three electrons forming strong covalent bonds.
It is due to this strong covalent bonding that graphene is visioned 5 times stronger than steel. The fourth electron of every carbon atom is delocalised on the whole graphene layer imparting high electron mobility of 200,000 cm2/V-s or higher at room temperature. It also has a high thermal conductivity of approximately 5000 W/ m-K which marks its importance in high-temperature applications. It is completely impermeable, stretchable and transparent which makes it the material of interest for flexible device fabrication .
Graphene in electronics
Graphene has been a subject of various research studies in electronics due to its unique zero band gap feature between the conduction and valence bands. In electronics, the valence band and the conduction band are of high significance when it comes to the selection of materials for various purposes.
The valence band is the outermost filled electron band, and the conduction band is the band responsible for any electronic transport in the material if it has electrons present in it. In conductors like most metals, valence band and conduction band overlap making them electrically conducting. In semiconductors and insulators, there is a gap between the two bands. This gap is called band gap. Semiconductors have relatively small band gaps when compared to insulators, and thus they may be made to conduct by applying a small amount of energy sufficient enough to make electrons in the semiconductor jump from the valence band to the conduction band.
Graphene is a zero-gap semiconductor, meaning that the valence and the conduction band are neither overlapping, nor there is energy difference between them. Thus, it is sometimes also referred to as semi-metal. This opens a window to tune the electrical conductivity of graphene as per the desired purpose which was not possible in conventional conductors. Thus, it is explored as the material of interest for conductive contacts.
Current standpoint of graphene in photodetector industry
Photodetectors or photosensors are devices that are used for sensing light and electromagnetic radiation. They find applications in communications, environmental sensing, process control, defense, medical imaging, remote sensing, consumer electronics, safety, and security.
The basic setup of a photodetector consists of a semiconducting channel and conductive contacts. When light or other electromagnetic radiation falls on the semiconducting channel, the photons imparts energy to electrons in the valence band of semiconducting material to jump to conduction band resulting in an increase in the flow of current.
This increase in current when the light of suitable wavelength falls on a semiconducting material is called photocurrent. The amount of photocurrent, the time required to reach that photocurrent value, responsivity and sensitivity to incident radiation are the key figures-of-merit to determine the application of a particular device as a photodetector. It usually varies based on semiconducting channel material and conductive contacts used for device fabrication.
Various researchers have made graphene-based photodetectors which are uniquely fast and super sensitive to incident radiations and showcased graphene’s talent as the next-generation conductive contacts for high-speed photodetection. However, to date, large-scale graphene production with uniform properties and long-term stability remains a question to be answered in the near future.
Scope in the future
The current roadblock for graphene that is preventing it to commercialize and make it to actual mass device fabrication is the lack of repeatable and sustainable approach for producing stable single layers of graphene at large scale. In the near future, chemical exfoliation technique has enormous potential to generate different avenues to overcome this hindrance.
A small amount of graphite powder mixed with good surfactants in appropriate solvents and a reasonable amount of energy for exfoliation may just do the trick.
 Novoselov, Kostya S., et al., Electric field effect in atomically thin carbon films, Science, 666-669, 2004.
 Novoselov, K. S., et al., Two-dimensional atomic crystals, Proceedings of the National Academy of Sciences, 10451-10453, 2005.
 Narula, Udit, Tan Cher Ming, and Lai Chao Sung, Growth Mechanism for Low-Temperature PVD Graphene Synthesis on Copper Using Amorphous Carbon, Scientific reports, 44112, 2017.
 Kim, Keun Soo, et al., Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature, 706, 2009.
 Yi, Min, and Zhigang Shen., A review on mechanical exfoliation for the scalable production of graphene, Journal of Materials Chemistry A, 11700-11715, 2015.
 Hernandez, Yenny, et al., High-yield production of graphene by liquid-phase exfoliation of graphite, Nature Nanotechnology, 563, 2008.
 Shearer, Cameron J., et al., Accurate thickness measurement of graphene, Nanotechnology, 125704, 2016.
 Kaul, Anupama B., Two-dimensional layered materials: Structure, properties, and prospects for device applications, Journal of Materials Research, 348-361, 201.
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