All about Graphene

a.      Fabrication and Characteristics

Graphene is two-dimensional crystalline form of carbon: a single layer of carbon atoms arranged in hexagons, like a honeycomb, with sp² bonding, unlike diamond and amorphous carbon materials as having sp³ bonding. Chemical functionalization of the main graphene sheet (not the edges) is achieved by either covalent or non-covalent methods. Covalent functionalization requires the breaking of sp² bonds and can be achieved using a wide range of reactions. Non-covalent functionalization relies on van der Waals forces often due to  pi-pi stacking between aromatic molecules and the graphene lattice. Regarding its electronic properties, a good approximation to the band structure of mono-layer graphene can be obtained from a simple nearest neighbor tight-binding calculation. Inspection of this band structure immediately reveals three electronic properties of mono-layer graphene which have excited such interest in this material: the vanishing carrier density at the Dirac points, the existence of pseudo-spin and the relativistic nature of carriers [1].  In case of magnetic properties of graphene, weak paramagnetism at low temperatures is reported for relatively defect free graphene, and the use of ion irradiation to add vacancies can increase the paramagnetism [2].

Manufacturing process of graphene can be done by several methods in which each method shows different characteristics of the obtained graphene including crystallite size, sample size, and charge carrier mobility.  As reported by Novoselov et al [3], the summary of graphene`s fabrication methods is shown as following table.

Table 1 | Properties of graphene obtained by different methods

Method	Description	Characteristics
		Crystallite Size (μm)	Sample Size (mm)	Charge Carrier Mobility (at ambient temperature) (cm2 V-1 s-1)
Mechanical Exfoliation	The weathering of a graphite structure by freeze/thaw cycles or temperature differentials between the graphite surface and interior, then physically scrubbed.	>1000	>1	>2.10^5 and >10^6 at low temperature
Liquid-Phase Chemical Exfoliation	Sonication-aided exposure of the materials to a non-aqueous solvent (or aqueous solutions with surfactant) with a surface tension that favors an increase in the total area of graphite crystallites [4]	<=0.1	Infinite as a layer of overlapping flakes	100 (for a layer of overlapping flakes)
Chemical Exfoliation via Graphene Oxide	Oxidizing graphite pellets and then ultrasonically exfoliated in an aqueous solution [5]	around 100	Infinite as a layer of overlapping flakes	1 (for a layer of overlapping flakes)
CVD	Large-area uniform polycrystalline graphene films are grown on copper foils and films by deposition [6]	1000	around 1000	10000
SiC	Graphitic layers can be grown either on the silicon or carbon faces of a SiC wafer at high temperature (1000) by sublimating Si atoms, thus leaving a graphitized surface [7]	50	100	10000

b.      Application and Current Situation (Issues and Challenges)

Due to its unique characteristic, graphene has possibility to be implemented in various fields, especially in electronics devices. Semiconductor industry may rely on silicon technology such for CMOS fabrication, but silicon devices have a physical limit such quantum tunneling of electrons across the insulator into the gate which increases the power consumption. The promise of graphene as replacement of semiconductor in CMOS may beat this silicon limit. Here, the summary of other possible application of graphene, followed with current challenges and issues are presented in below table [3].

 Table 2 | Applications of graphene

Field	Application	Drivers and Challenges	Issues
Photonics	Tunable fiber mode-locked laser	Wide spectral range of graphene	The requirement of a cost-effective graphene-transferring technology
	Solid-state mode-locked laser	Graphene-saturable absorber would be cheaper and easy to integrate into laser system
	Photo-detector	Graphene can provide bandwidth per wavelength of 640 GHz for chip to chip or inter-chip communication in which IV or III-IV detectors are not compatible)	The necessary to increase responsivity which might require a new structure and/or doping control in which the modulator bandwidth must also follow suit.
	Polarization Controller	Current polarization controlling devices are difficult to integrate, but graphene is easy integrate with silicon	The necessary to gain full control of parameters of high-quality graphene
	Optical Modulator	Graphene could increase operating speed where Si operation bandwidth is limited to about 50 GHz, thus avoiding the use of complicated III-IV epitaxial growth or bonding on SI	The requirement of high-quality graphene with low sheet resistance to increase bandwidth to over 100 GHz
	Isolator	Graphene can provide both compact and integrated isolators on Si substrate, dramatically aiding miniaturization.	The necessity of magnetic field strength decrement and process architecture optimization
	Passively mode-locked semiconductor laser	Core-to-core and core-to-memory bandwidth increase requires a dense wavelength-division-multiplexing optical interconnect with over 50 wavelength in which not achievable with a laser array, but a graphene-saturable absorber can provide.	The necessity of low power consumption of interconnect architecture using graphene
Electronics	Touch screen	Graphene has better endurance	Requirement of better control od contact resistance and low sheet resistance
	E-paper	High transmittance of monolayer graphene could provide visibility
	Foldable OLED	High-quality graphene provides bendability of below 5 mm. atomically flat-surface of graphene avoid electrical short and leakage current.	The necessary of low sheet resistance and conformal coverage of 3D structure
	High-frequency transistor	No producible solution for InP high-electron mobility transistor after 2021.	Need to achieve current saturation, 850 GHz of cut-off frequency and 1200 GHz of maximum oscillating frequency
	Logic transistor	High mobility	The necessary of new structure to resolve band gap-mobility trade off and an on/off ratio larger than 106
Energy Generation and Storage	Transparent electrodes of solar cell	Uniform absorption over a broad spectrum but low intrinsic optical absorption.	Requirement of complex interferometry or plasmonic enhance structure
	Electrodes for Lithium Batteries	High thermal and electrical conductivity	Mass production technology
	Supercapacitor	Combination of a high-surface-area activated carbon material and a nanoscopic charge separation at the electrode–electrolyte interface.	Necessary to reduce high irreversible capacitance of graphene-based supercapacitor
	Support material for platinum catalysts for fuel cells	Strong interaction between the platinum atoms and graphene decreases the platinum particle size to under  nanometer order	Applicable on only direct methanol fuel cells
Composite Materials, coating and paints	Reinforcement component	High Young`s modulus of graphene	Chemical modification of graphene to provide adhesion properties is necessary
	Coatings and Paints	Graphene is highly inert, and so can act as a corrosion barrier against water and oxygen diffusion	Durability assessment is necessary
Sensor	Strain-gauge transducer	Graphene is the only crystal which can be stretched by 20%,	Good electrical and/or optical readouts of sensor
	Gas sensor	Extremely high sensitivity	Low selectivity and poisoning by water are main issues
Bio-application	Drug  delivery vehicles	Large  surface area and delocalized p electrons and lipophilic of graphene	Necessary to understand the bio-distribution, biocompatibility and acute and chronic toxicity  of graphene under conditions that are relevant to exposure during manufacture and subsequent use

As unique crystal which combines many superior properties from electronic to mechanical, graphene and its full ability will only can be realized in very new application designed specially with this material rather than replacing other materials in existing applications. Recent development in printable and flexible electronics, flexible solar cells, and supercapacitor gives such an opportunity in very soon. Hopefully, for next few years, graphene-based devices will come to the end-users, the customers

c.       Graphene on Devices

Relating to the previous section about the possible applications of graphene, three different applications of graphene are pointed out according its electrical, chemical and optical properties. High conductivity, chemical responsivity, and photo-sensitivity of graphene are used to develop new OLED, biosensor and photodetector, respectively. Here, table 3 summarizes some ideas to apply graphene in those three applications.

Table 3 | Idea to apply graphene in electronic devices

Device	Schematic Design	Description
OLED [8]		High work function (WF) and low sheet resistance-modified graphene has possibility to be used as anode for OLED. It is reported that the efficiency was beyond Indium tin oxide-based OLED both for fluorescence and phosphorescence. The method that was used to increase WF and reduce sheet resistance was by using conducting polymer to modify the surface thus creating a WF gradient from graphene to the over lying organic layer, and by doping of p-dopant.
Field Effect Transistor -based Bio-sensor [9]		High sensitivity and excellent selectivity of graphene toward biomolecules become the reason to combine graphene with CMOS platform to provide direct, label-free and real-time electrical detection of biomolecules. The ability of graphene-FETs biosensor to detect single molecule or single cell-level has been reported, but yet to be mass produced due to the difficulty of controlling layer number and surface cleanness of graphene. System integration also becomes a challenge since it was reported in most of studies that sample delivery system was used.
Photo-detector [10]		Owing to unique band structure, graphene shows remarkable response in photocurrent generation near metal/graphene interfaces. This was followed by utilizing graphene as photo-detectors which could be operated at hundreds GHz level. It was reported that  in graphene-FET photodetector, the internal electric fields which are responsible for separation of photo-generated carriers near electrode/graphene interfaces leads to band bending, while the absence of strong electric in  the bulk graphene sheet allows carrier recombination. By using interdigitated metal finger, large, high electric field-light detection area could be created.

References

[1]      J. M. Warner, F. Schäffel, A. Bachmatiuk, M. H. Rümmeli, Graphene: Fundamentals and Emergent Applications 1^st edition,  ISBN 978-0-12-394593-8

[2]      R. R. Nair, M. Sepioni, I-Ling Tsai, O. Lehtinen, J. Keinonen, A. V. Krasheninnikov, T. Thomson, A. K. Geim and I. V. Grigorieva, Nat. Phys. 8 (2012) 199–202

[3]      K. S. Novoselov, V. I. Fal`ko, L. Colombo, P. R. Gellert, M. G. Schwab & K. Kim, Nature 490 (2012) 192–200

[4]      Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, T. McGovern, B. Holland, M. Byrne, Y. K. Gun`ko, J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari, J. N. Coleman, Nature Nanotechnology. 3 (2008), 563–568

[5]      X. S. Li, W. W. Cai, J. H. An, S. Y. Kim, J. H. Nah, D. X. Yang, R. Piner, A. Vemalakanni, I. H. Jung, E. Tutuc, S. K. Banarjee, L. Colombo, R. S. Ruoff, Science 324 (2009) 1312–1314

[6]      D. R., Dreyer, R. S. Ruoff, & C. W. Bielawski, Angew. Chem. Int. Ed. 49 (2010) 9336–9344.

[7]      I. Fourbeaux, J. M. Themlin, J. M. Debever, Phys. Rev. B 58 (1998) 16396-16406

[8]      T. H. Han, Y. Lee, M. R. Choi, S. H. Woo, S. H. Bae, B. H. Hong, J. H. Ahn and T. W. Lee, Nature Photonics 6 (2012) 105-110

[9]      S. Liu, X. Guo, NPG Asia Materials 4 (2012), e23;

[10]  T. Mueller, F. Xia, P. Avouris, Nature Photonics 4 (2010) 297 - 301