Graphene based nanomaterials for strain sensor application—a review

Journal Pre-proof Graphene based nanomaterials for Strain Sensor Application A-review Ahsan Mehmood, N.M. Mubarak, Mohammad Khalid, Rashmi Walvekar, E.C. Abdullah, M.T.H Siddiqui, Humair Ahmed Baloch, Sabzoi Nizamuddin, Shaukat Mazari

PII:

S2213-3437(20)30091-9

DOI:

https://doi.org/10.1016/j.jece.2020.103743

Reference:

JECE 103743

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

29 September 2019

Revised Date:

6 January 2020

Accepted Date:

2 February 2020

Please cite this article as: Mehmood A, Mubarak NM, Khalid M, Walvekar R, Abdullah EC, Siddiqui MTH, Ahmed Baloch H, Nizamuddin S, Mazari S, Graphene based nanomaterials for Strain Sensor Application A-review, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103743

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Graphene based nanomaterials for Strain Sensor Application Areview Ahsan Mehmood1, N.M. Mubarak1*, Mohammad Khalid2, Rashmi Walvekar3, E.C. Abdullah4, M.T.H Siddiqui5, Humair Ahmed Baloch5, Sabzoi Nizamuddin5, Shaukat Mazari6 1

Department of Chemical Engineering, Faculty of Engineering and Science, Curtin University,

2

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98009 Sarawak, Malaysia

Graphene & Advanced 2D Materials Research Group (GAMRG), School of Science and

Technology, Sunway University, No. 5, Jalan Universiti, Bandar Sunway, 47500 Subang Jaya,

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School of Engineering, Taylor’s University, 47500 Subang Jaya, Selangor, Malaysia

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Selangor, Malaysia

Department of Chemical Process Engineering, Malaysia-Japan International Institute of

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Kuala Lumpur, Malaysia

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Technology (MJIIT) Universiti Teknologi Malaysia (UTM), Jalan Sultan Yahya Petra, 54100

School of Engineering, RMIT University, Melbourne, 3001, Australia

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Department of Chemical Engineering, Dawood University of Engineering and Technology,

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5

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M.A Jinnah Road, Karachi, Sindh, Pakistan

Graphical abstract

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Highlights

The article presents comprehensive review on graphene as strain sensor application

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Presents a review property on graphene properties and their connection to strain sensor

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application

Provides a review on functionalisation, synthesis, structure and its derivatives of graphene

Graphene based various nanocomposites are discussed and their application.

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Abstract Strain Sensors have rapidly pervasived in modern era due to their ability to identify, respond and exchange mechanical motion into an electrical signal that can be interpreted on the basis of electrical resistance. As a matter of fact, strain sensors applications have expanded the technology scope and enabled us to observe the changes in the surroundings in distinct ways. More recently, advanced carbon nanomaterials-based films for sensing application have been

extensively growing because of their outstanding properties. Based on nanomaterials distinctive structure, enhanced material strength can be obtained in addition to the added multifunctionality of these materials. Such remarkable feature is a main factor in the design of sensors composed of functional nanomaterial. At present carbon nanomaterials, particularly graphene is promising choice for the sensor development due to its unique thermal, electrical and mechanical strength. This chapter emphasizes on studies conducted by various researchers on graphene. Therefore, this chapter discussed on brief history, graphene synthesis, properties,

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structure, derivatives, graphene based nanocomposites and strain sensor application of graphene based nanomaterials.

Keywords: graphene synthesis and properties, derivatives of graphene, functionalization

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approaches

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*Corresponding author at: Department of Chemical Engineering, Faculty of Engineering and Science, Curtin University, 98009 Sarawak, Malaysia. Tel.: +60 85443939 Ext 2414; Fax: +60

E-mail addresses:

[email protected]; [email protected] (N.M.

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Mubarak).

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85 443837.

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1. Introduction Piezoresistive behaviour of iron and copper was first reported by Thomson in 1856, and stated that resistance of these metals changes with elongation, since than various metallic and strain

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semiconductor

sensors

based

on

piezeoresitive

behaviour

have

been

fabricated[1].However, other than geometry, the resistivity change of a material plays vital role in resistance change. The gauge factor is an important factor to measure sensitivity (piezeoresitive response) of a sensor and it is a measures of electrical shift sensitivity due to mechanical strain[2]. The relation between electrical resistance change and applied strain is defined as relative resistance change divided by mechanical strain. The change in resistance

depends on both resistivity and geometry as [3]. (∆𝑅/𝑅) = (1 + 2𝑣)𝜀 +

∆𝜌 𝜌

.However, it can

be demonstrated form the equation that the effect of geometric deformation produces a GF(Gauge Factor) from 1.4 to 2.Whereas resistivity changes in metals ∆𝜌/𝜌 is smaller (order of 0.3) as compare to semiconductor as in germanium and silicon where it is 50 to 100 times higher than geometric term [2]. Since then, the quest to further explore both features geometric deformation and resistivity change are in progress in search of material with ultrahigh piezeoresitive properties. The persistent pursuit for miniaturized and low-cost devices, the

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conventional semiconductor of silicon faces challenges, the research now is more focused on nanomaterials[4]. In nanomaterials field, one of compelling candidate is low-dimensional carbon[5]. Two most widely studied components of the family that have attracted much

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attention are carbon nanotubes and graphene, due to their exceptional electrical and mechanical

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properties responsible for piezoresistive behaviour. In comparison with carbon nanotubes the graphene offers various advantages in the form of scalable fabrication method (top down

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approaches) that is compatible with earlier fabrication technologies. Graphene based thin transparent layer strain sensor can be easily and commercially obtained in comparison to other

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material strain sensor.

In a past few decades, the carbon nanomaterials have been the source of excitement in the

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scientific community, due to its peculiar structural properties in one, two- and threedimensional arrangements and high figures of merit[6]. More recently, the addition of graphene

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to the family is regarded as extraordinary development [7, 8].Graphene is a two dimensional sheet arranged in a honey combed structure of six membered rings. Furthermore, as an essential two dimensional carbon construction, can be regarded theoretically as indeterminately prolonged, two dimensional aromatic macromolecule measured as elementary or fundamental structural unit of other dimensionalities [9], [8]such as completely rounded zero dimensional as “buckyballs” or fullerene, curved cylindrical one dimensional structure as nanotubes and

stacked sheets as three dimensional graphite [10]. Graphene is thinnest, flexible and strongest material [11]. Due to distinctive molecular structure, it has some remarkable chemical, electrical and mechanical properties which includes significant flexibility, elasticity, and capability to familiarize to various surfaces[12]. It stands first in terms of strength and hardness compared to other well-known materials whereas, mono-layer graphene has tensile strength and elastic modulus around 125 Giga Pascal, and 1.1 Tetra Pascal, respectively. Whereas, an ideal strength of this extraordinary nanomaterial is around 100 times that steel. The movement

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of π electrons in graphene travels freely with a speed at 1/1300 of light[13].The interference of electron is insignificant which inclines the electrical strength of graphene. Furthermore, comparison of electrical conductivity of graphene is 60 times higher than SWNTs[14] .

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Moreover, conductivity of graphene stays steady at wide range of temperatures, such steadiness is vital for reliability within countless applications. Likewise, electron mobility in graphene is

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also 200 times greater when compared with the electron mobility in silicon i.e. 1000 cm2/V s [11]. The exceptional electrical properties of graphene are due to long stretched range of

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𝜋 conjunction However, the surface area comparison of graphene (theoretical) with carbon nanotubes, shows as two times greater i.e. 2630 m2/g and 1315 m2/g for graphene and carbon

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nanotubes, respectively. Consequently, graphene attained significant attention due to these

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exceptional properties in various areas of investigation.

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1.1. Graphene Discovery The discovery of graphene was revolutionary; it expanded the horizon of research on this carbon allotrope and its derivatives. However, research on the existence of graphene had been continuous for nearly sixty years before its discovery in 2004 [15].Whereas, graphene was previously believed to be unstable, and forms curved-structures like nanotubes and fullerene, but lacks ability to exist freely due to thermodynamic instability [16]. Mermin and Wagner also supported the claim experimentally that melting point of graphite layers reduces drastically

with reduction in thickness [17]. Therefore, two-dimensional atomic material was believed to be part of (graphite) three-dimensional system and that have been developed on the top of nanocrystals with similar crystal lattices epitaxial [18]. Novoselov and Geim were the first to separate one layer of graphite by using scotch tape known as graphene.[19]. Hanns-Peter Bohem produced and studied graphene while supported on metal surface under electron microscope in 1962 and described it as one layer of carbon, additionally named this structure as graphene. The graphene as a word is a composed of “graphite” and a suffix -ene.

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[20]. Long afterwards, the graphene word was not used commonly. The International Union of Pure and Applied Chemistry (IUPAC) endorsed term graphene and replaced graphite because graphite is defined as 3D structure while graphene is 2D structure although building

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block of graphite [21].

1.2. Graphene Structure To better understand graphene, it is helpful to start with the basic building element of graphene known as carbon. The atomic number of carbon is 6,and has electronic configuration as 1s2, 2s2, 2px1 and 2py1 [29]. Carbon is tetravalent element (elements that have four electrons in outermost shell) and valence shell electrons participate to form covalent bond. Carbon while making bonds with other atoms of carbon moves one electron from 2s orbital into vacant 2pz orbital, as result hybrid orbitals are formed. The 2s orbitals in diamond hybridises with three 2p orbitals and creates four new sp3 orbitals of equal energy and newly formed orbitals contains

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one electron each [30]. The orientation of four sp3 orbitals is such a way that they are at maximum possible distance therefore; they face towards the corner of imaginary tetrahedron. The three-dimensional diamond structure is result of overlapping between sp3 orbitals of carbon

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atoms and high hardness of diamond is due to strong C-C bonds.

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In Graphite hybridization happens in two 2p-orbitals and forms three sp2-orbitals while one 2porbital do not participate in hybridization. The orientation of sp2 orbitals is at right angle to

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remaining 2p-orbitals, thus they are symmetrically laid in X-Y plane at angle of 120o [31].Further, sp2 atoms of carbon forms covalent bonds and effects the planar hexagonal

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“honeycomb” graphite structure . Whereas, in-line sigma σ- bonds between layers of graphene are stronger as compared to C-C bonds in diamond. Carbon atoms in lattice are attached closely

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to other three nearest neighbouring carbon atoms via a strong covalent bond. These strong

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sigma (σ) bonds are formed due to presence of electrons in valence orbitals 2px, 2py and 2s whereas, the forth valence electron resides in 2pz orbitals and is at right angle to graphene plane, thus it has no interaction with in-plane sigma bond (σ) electrons [30]. Further, the 2pz orbitals from neighbouring atoms overlaps and results in creation of delocalized π (occupied) and 𝜋 ∗ (unoccupied) bands. The most electrical and electronic conduction of graphene is due

to π bond. Graphene lattice consist of unit cell of two atoms of carbon with a distance of 1.42

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Å between them and interplane distance around 3.4 Å [31].

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Figure 1.1: Carbon Atomic Orbital Diagram. The Electrons of Spherical Shaped 2s Orbital and Dumbbell Shaped 2p-orbitals Participate in Bonding. (a) Ground State (b) sp3 Hybrid Orbitals in Diamonds (c) sp2 Hybrid Orbitals in Graphene Chemists compare graphene to benzene ring, describes as fused benzene ring. The geometric

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centre of graphene and benzene is identical and every atom of carbon has sp2 hybridization and shares single 2pz electron for pi bonding [19]. However, the difference in graphene and

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benzene is evident: the benzene ring possesses HOMO-LUMO gap, while graphene is zero gap semiconductor where conduction band and valence band intersect at single point known as

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Dirac points and liner dispersion interaction results into an effective zero mass [32].

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The electronic properties depend on the thickness of graphene layers. Bilayer graphene (BLG) and single layer graphene (SLG) are zero-gap semiconductors with single type of holes and electrons. However, three or more-layers graphene have very complicated spectra and charge carriers are noticed within three or more layers. However, the ability of valence bands and conduction starts to overlap in multi-layer graphene. Thus, consideration of these features,

makes it easy to differentiate between single, double and more layer of graphene as three different type of 2-dimensional crystal [15]. Graphene has numerous applications due to exceptional electrical, optical, thermal and mechanical properties and weight to large surface area ratio (e.g. 1-gram graphene due to large surface area can cover many football fields). Remarkable and substantial strength of graphene is mainly due to extended pi-pi (π-π) conjugation [33].Additionally, graphene contains several surface-active functional groups for example ketonic (C=C) and quinonic. The groups, for

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instance ketonic and carboxylic are very reactive in nature and linked easily with several molecules, thus influence the functionalization of graphene for sensing applications. Research also testified that graphene as well as the modified graphene exhibit excellent properties for

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energy storage devices [34], polymer composites [35], mechanical resonators[36], paper like

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materials[37] ,liquid crystal devices[38] and strain sensors.

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Figure 1.2: (a) Carbon atomic structure (b) Energy levels of valence electrons in carbon atom (c) The sp2 hybrids formation (d) The crystal lattice of graphene (e) pi and sigma bonds formed by sp2hybridization

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1.3. Properties of Graphene 1.3.1. Mechanical Properties Graphene like other carbon-based materials have not disappointed in terms of mechanical

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properties. Diamond is known to be toughest naturally occurring material while carbon CNTs have been known for high tensile strength.Lee, Wei, Kysar and Hone [39] measured

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mechanical properties of single layer graphene and claimed to be strongest material ever measured by nanoindentation using an AFM and reported tensile strength of 130Gpa. In addition, also demonstrated experimentally that monolayer graphene exhibits fracture strength 200 times more than steel and 1TPa Young’s modulus. Furthermore, the strength of graphene highly depends on purity of graphene. The secret behind these outstanding mechanical properties lies in the stability of sp2 bonds that makes honey comb like lattice crystals and

resists various in line deformations.Frank, Tanenbaum, van der Zande and McEuen [40] reported Young modulus value of 0.5 TPa and while in literature some other values also had been reported. The difference in values of mechanical properties is believed to be due to inevitable and inherent wrinkling of graphene directed to out of plane direction of single layer. Wrinkling could either emerge from static crumpling or out of plane flexural phonons, that is caused due to uneven stress at graphene boundary. Other reasons for crumpling could be evenly distributed point defects such as stone Wales defects [41].Graphene as materials with such high

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sustainable tensions proves to be viable option for strain sensor application as its prime function is to convert strain into sensible and readable form. 1.3.2. Optical Properties

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Silvery-Blackish Graphite powder or crystal becomes transport when reduced to single layer

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graphene. This promising behaviour along with conductance makes worthwhile for various applications. The graphene shows linear behaviour of decreasing transparency with an increase

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in thickness. The transmittance for 2nm thick layer is 95% while for 10nm thickness it decreases to 70% [42]. Furthermore, graphene exhibits flat optical spectrum between range of

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500 and 3000nm and absorption is significant below 400nm [43].Consequently, the absorbance of graphene increases linearly with an increase in thickness. One atom thickness layer of

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graphene absorbs 2.3% white light while this absorbance goes up to 4.6% for bilayer graphene with insignificant reflection (<0.1%) [44]. The linear transmittance behaviour of graphene with

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thickness is intimately related to its zero-gap structure. Graphene transmittance depends on fine structure constant which can be demonstrated by 𝑎 =

𝜋𝑒 2 ℎ𝑐

=

1 37

(where c, represents speed of

light), that explains the coupling between relativistic electrons and light, and typically related to quantum electrodynamics phenomena against material science [45]. For identification purpose, optical microscopy can be used on Si/SiO2 substrate layer due to phenomena of interference [46].

1.3.3. Thermal Properties Graphene is anisotropic material with hundred folds of heat flow between out of plane and inplane directions. High in-plane graphene thermal conductivity is result of strong sp2 covalent bonding of carbon atoms, while out of plane thermal conductivity is limited due to weak vander waals interaction between layers of graphene [47].Furthermore, phonon transport decides thermal conductivity such as ballistic and diffusive conduction at low and high temperatures respectively. Whereas, electronic thermal conductivity of non-doped graphene is limited due

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to low carrier density. At room temperature suspended graphene sheets exhibits natural thermal conductivity around 2000-6000 W/mK [48]whereas this value decreases to 600 W/mK when suspended on SiO2 substrate [49]. Thermal conductivity is highly dependent on the defects in graphene such as sample fabrication residues, isotopic doping and edge scattering that causes

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cleavage method shows high thermal conductivity.

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localization and phonon scattering. Therefore, graphene sheets developed by mechanical

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1.3.4. Electronic Properties The study of electronic and electrical properties led the graphene revolution. It has been observed that properties of graphene vary largely with the number of layers. The properties

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shown by single, double and tri-layer completely contradict each other and suddenly appears to be different materials. Whereas, earlier studies by Novoselov, Jiang, Schedin, Booth,

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Khotkevich, Morozov and Geim [50] demonstrated its potential candidature in transistors as it allows to vary charge carriers from holes to electrons. This phenomenon happens to be true for

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single layer graphene, while with the increase in layers this electron hole dependence starts to weaken due to electron field scattering from other layers. Graphene has shown quantum hall effect phenomena for both electron and hole carriers due to tremendous electron mobility at several temperatures and under the effect of magnetic field [33]. However, at room temperature electron mobility has been observed to be as high as 2000

cm2V-1s-1 for graphene produced mechanically. Following paragraphs gives insight to electronic properties and its relation to structure of graphene. Classically, the typical integer QHE occurs at

4𝑒 2 ℎ

where, h is plank’s constant and 𝑒 is electron

charge while for graphene it happens only half integers. This phenomenon has been believed to happen due to unique band structure [46]. It had been proven experimentally that electron mobility of graphene largely depends on the material it is been supported like Bolotin, Sikes,

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Jiang, Klima, Fudenberg, Hone, Kim and Stormer [51]reported electron mobility of 200,000 cm2V-1s-1 while it was supported on Si/SiO2.Graphene is a zero-gap semiconductor and its quasiparticles are at low energy formally defined by the Dirac-like equation, Hamiltonian illustrated as 𝐻 = −𝑖ℏ𝑣𝐹 𝜎∇, where 𝜎 = (𝜎𝑥 , 𝜎𝑦 ) are known as Pauli matrices while 𝑣𝐹 ≈

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106 𝑚/𝑠 is defined as Fermi velocity. By neglecting various effect of body, this has been

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proven accurate theoretically and experimentally by the measurement of cyclotron mass in graphene that depends on energy and furthermore, by clear observation of relativistic analogue

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of integer Quantum hall effect [52]. In fact, as mentioned earlier Dirac like equation describes charge carriers in graphene rather than Schrö dinger equation, that can be seen as result of

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crystal structure of graphene which constitutes of two equivalent sublattices of carbon namely A and B. Resultant, quantum mechanical hopping among sublattices creates two energy bands

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while their intersection near Brillouin zones edges results in formation of conical energy spectrum close to Dirac points 𝐾́ and 𝐾 , as a consequence, quasiparticles exhibits liner energy

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dispersion relationship in graphene. 𝐸 = ℏ𝑘𝑣𝐹 , where as if they were zero mass relativistic particles, Fermi velocity plays role of speed of light 𝑣𝐹 ≈

𝑐

. However, due linear energy

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spectrum, it can be expected that quasiparticles of graphene behaves unlikely metals and semiconductors that exhibits parabolic dispersion relation [52].

1.3.5. Electrochemical Properties 1.3.5.1. Piezoresistive Properties Piezoresistive response of graphene is a result of interplay between its electrical and mechanical characteristics. The term gauge factor is usually used to define piezoresistive properties. Further, gauge factor (GF) is a measure of ratio between change in electric resistance of material to applied strain [53]. In strain sensors, the sensitivity is defined by following relationship,

Where 𝜌𝑜 and

(1)

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∆𝜌 𝑅 − 𝑅𝑜 ∆𝑅 𝑅𝑜 𝑅 𝜌 𝐺𝐹 = ( ) = ( 𝑜 ) = 1 + 2𝑣 + ( 𝑜 ) 𝜀 𝜀 𝜀

𝑅𝑜 are resistivity and resistance of material without any applied strain

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respectively, 𝑅 represents resistance under applied strain, 𝜀 represents strain, 𝑣 represents Poisson’s ratio, ∆𝑅 is change in resistance under strain, ∆𝜌 represents change in resistivity

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under strain.

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The experimental observation reveals that the sensitivity of graphene is indirectly proportional to thickness [54]. The reason for decrease in sensitivity could be due to weak van der walls

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forces between layers of graphene.Graphene is a material which demonstrates ability to restrict flow through it except for hydrogen atoms and can endure differential pressure of greater than 1 bar [55]. This ability makes it viable to be used as pressure sensor which is itself an

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application as strain sensor. Its sensitivity as pressure sensor is defined as [56]: ∆𝑅 ∆𝑉 ∆𝑉 𝑅𝑜 𝑆=( )=( 𝑉 )= 𝑃 𝑃 𝑉𝑃

(2)

Whereas, 𝑉 and 𝑅𝑜 are voltage and resistance respectively without any applied pressure and 𝑅 is the resistance of the membrane when differential pressure 𝑃 is applied and ∆𝑉 is resultant voltage difference. 1.3.5.2. Piezoelectric properties Piezoelectric or piezoelectric materials generate electricality upon application of mechanical stress (stretching or squeezing). Applied pressure, produces small voltage due changing charges as result of moving electrons and distortion of material shape [57]. The intrinsic

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graphene exits in 2D form naturally and possess perfect physical symmetry therefore, does not show any piezoelectric behaviour. However, there had been several experimental demonstrations and theoretical predictions that have shown confidence in modification or

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engineering of graphene to achieve piezo-response [58]. Wang, Tian, Xie, Shu, Mi, Mohammad, Xie, Yang, Xu and Ren [53] measured in-plain direct piezoelectric effect when

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in-plain biaxial strain is applied by using AFM-tip on graphene membranes across the supported/suspended graphene boundary, band bending happens due to biaxial strain. The

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mismatch due to corresponding work function separates charges and gathers in a space charge region. Further, experimentally showed the piezoelectric coefficient as 12.5µmV-1 and direct

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piezoelectric constants as 37nCN-1 [53].

1.4. Derivatives of Graphene 1.4.1. Graphene Oxide and Reduced Graphene Oxide Graphene oxide (GO) and reduced Graphene oxide(rGO) are the most researched and studied intensively as a replacement to graphene as method of production is simple and has potential to be scaled up easily. Graphene oxide has graphene layers containing functional groups that are hydrophilically oxygenated on basal planes and edges. Graphene and graphene oxide both have identical structure except several functional groups are attached to the surface, such as

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carboxylic acid (-COOH), alkoxy (C-O-C), hydroxyl (-OH), carbonyl (C=O), and other functional groups. The synthesis of GO can be accomplished by oxidation process which adds oxygen lattice into structure as a result carbon lattice is distorted [62]. This distortion is stated

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as defects. Further, oxygen functionalities present on surface of graphene improves

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hydrophilicity and enhances separation. Thus, it helps GO to disperse easily in organic solvents and aqueous solvents. Addition of oxygenated lattice increases graphene thickness form 0.345

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to 1.1 nm [61].

In an effort to achieve properties close to pristine graphene an extensive effort is made for the

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removal of oxygen lattice from graphene oxide by using reduction reagents as a result reduced graphene oxide is produced. Furthermore, graphene oxide reduction can be achieved by several

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methods, like electrochemical, thermal, chemical and each method leads to different electrical properties, morphology etc [63]. The key design objectives for reduced graphene oxide rely in

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C/O ratio of product, removal of desired oxygen group, healing of damaged surface of Graphene oxide and maintaining the desired properties of reduced graphene oxide [64].

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1.1 nm

Addition of oxygen lattice

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Figure 1.3: Molecular Structure of Graphene Oxide

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1.4.2. Porous Graphene and Porous Graphene Oxide Porous graphene and porous graphene oxide are graphene derivatives and the name given due to formation of numerous holes in graphene nanostructure. These holes are formed due to

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crumbling of graphene sheets by hydrothermal method which results in creation of voids among layers of graphene [65]. Alteration of the pore structures can be done by insertion of

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impurities, for example zinc oxide among graphene layers and followed by their removal which results in formation of voids [66]. The availability of these pores in nanostructure of graphene

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makes it sponge like material. Sponge structure of porous graphene and porous graphene oxide

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is observed by HRTM images and field emission scanning electron microscope (FESEM) [67].

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Figure 1.4: Formation of Pores by Intercalation of Zno and its removal

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1.4.3. Graphene Quantum Dots Graphene Quantum Dots is one of the smallest derivatives of graphene with a size mostly lesser

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than 10 nanometres. The ultrafine graphene is very useful due to its extra ordinary properties like stable chemical stability ,photoluminescence, quantum confinement effect and low toxicity Furthermore the quantum- induced band gap in QOD structure was revealed by

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[68].

Ponomarenko and cowerkers [66]. However, GQD has very efficient properties of hole

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transport and the mobility of graphene quantum dots is due to very tiny size and it disperses well in several solvents, thus, permitting solution processes and many organic reactions. The

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outstanding property of graphene quantum dots can be examined by photoluminescence (PL)

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spectroscopy[69] .

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0.345 nm

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Figure 1.5: Molecular Structure of Graphene Quantum Dots

1.5. Synthesis of Graphene At present, synthesis of graphene is under in-detail studies and an essential matter. The research to find methodology that enables to produce not only high quality graphene sheets but also significant surface area thus production is always the primary issue to be concerned by the researchers [11]. Since discovery of graphene, various techniques have been developed for graphene synthesis. Among all, chemical exfoliation, thermal chemical vapor deposition, chemical synthesis, and mechanical cleaving are most widely used [75].In past, synthesized

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graphene was supported on substrates like metals and silicon carbide (SiC) but such graphene lacks ability to form 2-D structure. Graphene as freestanding was not believed until the remarkable discovery by Kostya Novoselvo and Andre Geim in which they suggested micromechanical cleavage (Scotch tape strategy) for separation of graphene into monolayer

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from graphite [76].

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In general, there are three phases method considered as primary approaches to manufacture graphene that includes liquid, solid and gas phase. In solid phase approach, carbon sources

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(mechanical exfoliation, epitaxial growth etc.) are provided. Graphene produced from solid phase method is considered of high quality but of lower yield compared with other two-phase

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methods. In liquid phase process, the manufacturing process like redox, direct synthesis and expansion usually take place in solvents. The production rate is comparatively higher but of

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low quality. Processes like, plasma enhanced, chemical vapour deposition (CVD) etc. falls

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under the category of gas phase method. This method suggested as the most appropriate for the synthesis of graphene film quantity and ease to handle the structure of graphene till some level [77]

For better understanding the approaches mentioned above for the synthesis of graphene are further categorized as Bottom-up and Top-down method. The approaches fall under the category of Top-down strategy include mechanical exfoliation, chemical exfoliation and

chemical fabrication, and bottom-up methods include pyrolysis, epitaxial growth, (CVD) chemical vapor deposition, and few other approaches too. Top-Down approach mainly based on an attack on powdered raw graphite. The attack generally helps to disperse its layers to form sheets of graphene. While, Bottom-Up approach refers to the consumption of carbonaceous gas to produce graphene [75]. 1.5.1. Mechanical Exfoliation Mechanical exfoliation was the technique used by Novoselov, Gem and co-workers to discover

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mono-layer graphene and the particular mechanical method used was scotch tape method[78]. Graphite is formed by stacking of graphene stacked onto each other and held in palace by Vander Waals forces. The in-line carbon- carbon bonds are stronger in contrast to weak Vander

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Waals forces between layers of graphene. The interlayer bond energy and distance value are 2 eV/nm2 and 3.34Ȧ respectively and to cleave graphite to monolayer graphene mechanically

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require external forces around 300nN/µm2 [79].For mechanical exfoliation through scotch technique, graphite flake is sandwiched between scotch tapes and then graphite flakes are

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cleaved repeatedly by scotch tape. Finally, scotch tape with attached graphene is rubbed onto SiO2/Si substrate (or other favoured substrates) to detach graphene layer[80].This is repeated

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until a mono or few layer graphene is obtained.

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1.5.2. Chemical Exfoliation Chemical exfoliation involves graphite exfoliation in a solution. It is a scalable technique to produce graphene suspension and most suited among other top down approaches. Graphene is

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naturally hydrophobic but attachment of oxygen containing functional groups like epoxide, hydroxyl, carbonyl and carboxy alters the surface of graphene to hydrophilic. The oxidized graphite can be dispersed in water and monolayer of graphene oxide can be obtained by centrifugation and sonication in water or other suitable solvent [80]. The choice of solvent is critical as it can help to enhance the solubility of graphene [43].Hernandez, Nicolosi, Lotya, Blighe, Sun, De, McGovern, Holland, Byrne and Gun'Ko [43] suggested that the solvent with

surface tension between 25 to 69 mJ/m2 are suitable for chemical exfoliation [43].Chemical exfoliation usually involves two step, firstly space between layers of graphene is increased by reducing van der waals forces. Second step involves sonication or fast heating to separate or exfoliate graphene into mono or few layer graphene[79]. 1.5.3. Chemical Reduction of Graphite Oxide Graphite Oxide reduction is another top-down approach to synthesise graphene and this technique produces large amount of graphene. Graphite is always oxidized to produce graphite

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oxide. The oxidation of graphite is done by oxidants like concentric sulfuric acid, potassium permanganate, and nitric acid [79]. While, reduction of monolayer graphite oxide films could be either done by thermally or chemically. Chemical reduction is done by hydrazine is known

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chemically reduced graphene oxide (CRGO) while thermal reduction is done by annealing in hydrogen/argon environment is known as thermally reduced graphene oxide (TRGO) [81]. The

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quality of graphene obtained by both methods is of lower quality as compared to scotch tape method [82]. Other method to produce graphene is use convergence of solar radiation which

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produce high temperature rise around 100 oC/s and pressure applies between layers of Graphite oxide and over comes van der vaals forces between them as a result reduced graphene is

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produced. While, Chemical reduction involves dispersion of graphite oxide in water with the application of ultrasonic exfoliation and followed by reduction of dispersed graphene oxide by

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the introduction of hydrazine [83].

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1.5.4. Epitaxial growth This approach falls under the category of Bottom-Up. It is one of the methods in which graphene is synthesised on the surfaces. Furthermore, this has been surfaces to produce graphene with thickness less than 10nm. This technique involves heating and cooling of Silicon carbide crystals. In this process as first step SiC crystals are decomposed by high temperature annealing. Heating desorbs Silicon from the surface and creates carbon surface layer due to accumulation of carbon atoms. Temperature or heating rate and pressure are quite critical in

this process as at constant pressure and different temperature monolayer, bilayer, trilayer and more layer graphene can be produced[80]. 1.5.5. Chemical Vapour Deposition Chemical vapor deposition is a promising technique to synthesize graphene at large scale. In this method, graphene is produced by chemical decomposition of materials(precursors) like methanol, methane, ethanol and acetylene on catalytic substrates like copper and nickel [84]. The most common used transition metal catalytic substrate is nickel. CVD involves four steps,

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in the first step precursors with carbon content diffuses and gets absorbed on the surface of substrate. In second step molecules of precursor pyrolyse and carbon ad-atoms are released on the substrate surface. And in next step carbon ad-atoms gets dissolved on the surface of

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substrate. Then the temperature is lowered to saturate the solution and atoms of carbons precipitate out of solution as a result graphene films formed [85]. Nickel has although proved

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to be excellent substrate material in case of CNTs but for graphene there are still major issues

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that needs to be addressed. Nickel forms grains with multilayer graphene at the grain edges/boundaries. Furthermore, due to significant solubility of carbon in nickel, poses an issue

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of multilayer growth of graphene on the surface of nickel during precipitation [86]. 1.5.6. Pyrolysis The popular method reported in literature for graphene synthesis by pyrolysis is Solvo thermal

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method. The ethanol and sodium were reacted in ratio of 1:1 thermally in a closed container.

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With the help of Sonication sheets of graphene were detached smoothly by pyrolization of graphene ethoxide. Graphene sheets up to 10 µm were produced. The benefits of this process include high purity graphene with easy way of fabrication, functionalization of graphene at low temperature and low cost of manufacturing. Yet, graphene produced is of low quality due to presence of defects on the surface of graphene [75].

Comparison between both Techniques In bottom-up approach, natural and normal thickness and size of graphene sheet are produced. Graphene produced is of low quality and purity but offers better control over process of production hence merely suitable for industrial scale. Table 2.4 displays an overview of different methods of bottom-up with its advantages and drawbacks. According to top-down methods, graphene/ unchanged graphene sheets are generated through separation or graphite exfoliation/ graphene derivatives (graphite fluoride and graphite oxide). This technique

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produces graphene of high quality but lower control of the of process, so it is suitable for laboratory scale production. Table 2.5 summarized the advantages and drawbacks of different

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methods of top-down approach.

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Graphene Synthesis Techniques

Plasma

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Therma

Pyrolysis

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Expitaxial growth

CVD

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Bottom-up approach

Other approaches

Chemical exfoliation

Top-down approach

Mechanical exfoliation

Chemical synthesis

Reduced graphene oxide

Hammer's method

Scotch tape

Figure 1.6: Techniques for Synthesis of Graphene

Atomic force microscopy (AFM) tips

Sonication

1.6. Functionalization of Graphene 1.6.1. Covalent Functionalization of Graphene Reduced graphene oxide (r-GO) and graphene oxide (GO) takes part in a covalent functionalization due to their structural defects which acts as sites for reaction. Graphene is a chemically inert substance that has been used as insulator from ( 𝐻2 𝑂2 ) Hydrogen peroxide and for the protection of metals from oxidation at temperature higher than 200oC [95]. Heating at high temperatures like 350 oC forms strain in sp2 bonds in curve structures like fullerene and carbon nanotubes while has no effect on flat surface of graphene [96].

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Unlikely, other graphene family member, carbon nanotubes and fullerene, that have misaligned π-orbitals and curved structure, graphene has π-electrons delocalized over the whole 2D network. This structural arrangement makes graphene inert and it has been

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revealed that graphene is less reactive than fullerene and carbon nanotubes. The functionalization of graphene involves mostly reaction with highly reactive species which

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forms covalent adducts to sp2 structures of carbon in graphene [97].The alteration in the

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structure of graphene can happen at the end of sheet or on the structure. Covalent functionalization changes sp2 hybridization to sp3 and preferably this structure

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tends to stay in tetrahedral geometry with longer bonds. The transformation not only affects sp3 directly by reaction but also creates distortions at multiple lattice positions on the

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structure of graphene [98].Famously there are different techniques to functionalize graphene covalently namely, condensation reaction, electrophilic Addition, , nucleophilic substitution

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and addition reaction[99] [100]. 1.6.1.1. Electrophilic Substitution Reaction In an electrophilic substitution reaction of graphene, hydrogen attached on the surface of graphene is displaced by an electrophile. Various electrophile had been mentioned in the literature. One example of electrophilic substitution reaction is grafting of aryl diazonium to graphene surface.Zhu, Higginbotham and Tour [101] developed a organosoluble graphene

by aryl diazonium salt electrophilic substitution graphene surface wrapped by surfactant [101]. 1.6.1.2. Addition Reaction Organic addition reaction involves combination or addition of two or more molecule to form one large molecule. Epitaxial graphene is functionalized by Cycloaddition of azidotrimethylsilane is based on similar concept. The graphene modified organically is dispersed easily and therefore, allows their easy manipulation in several process such as

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dispersion, mixing or blending.Hsiao, Liao, Yen, Liu, Pu, Wang and Ma [102] functionalized graphene by usage of remaining oxygen functional group present on the structure of Reduced graphene oxide[102].

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1.6.1.3. Condensation Reaction It is a type of reaction in which two molecules (more precisely functional groups) joins to

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form a single molecule at expense of entropy. For the case of graphene, the condensation

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reaction happens with diisocyanate, amine and isocyanate compounds by the formation of carbamate and amide ester linkages.Stankovich, Dikin, Dommett, Kohlhaas, Zimney, Stach,

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Piner, Nguyen and Ruoff [103] modified surface of graphene oxide by using various isocyanate [103]. In addition to organic isocyanate, other form organic diisocyanate, is also

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used in cross linking and functionalize graphene oxide.

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1.6.1.4. Nucleophilic substitution reaction Epoxy groups of graphene oxide are main reaction sites for nucleophilic substitution reaction. Epoxy group of graphene oxide is attacked by the amine functionality that belongs to organic modifiers and possess lone electron pair. This technique of nucleophilic substitution happens easily in comparison to other techniques and takes place even at room temperature in an aqueous medium. Thus, this approach is widely employed for large scale manufacturing

of functionalized production

Aromatic

acids, amine

terminated

biomolecules, amino acids, low MW polymers, aliphatic of all types, silane compounds and ionic liquids have been used for production of functionalized graphene [99]. 1.6.2. Non-Covalent Functionalization Non-covalent functionalization of graphene is basically governed by van der Waals, hydrophobic and electrostatic forces. In addition, molecules physically adsorb at the surface of graphene and notability it is well acknowledged technique for modification of surface of carbon based nanomaterials. [99]. The graphene layers in graphite are binded together

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due to van der Waals forces rising from π-π bond and stacking of aromatic ring on each other. The small molecules which possesses aromatic rings bends to single layer graphene could be due to strong Van der Waals forces as a result provides basis of functionalization

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of graphene without disturbing covalent bonding [104].This approach serves a advantage that sp2 bonding network is not disrupted while the binding strength of van der Waal forces

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is weaker than covalent bonding strength . [15].

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Graphene can be believed as an enormous aromatic molecule with an ability to attach numerous molecules with any help of coupling agents through non-covalent interactions

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such as cation-π interaction, hydrophobic interactions, π-π interactions and anion- π interactions etc. The properties of original constituents are not altered in these interactions. The non-covalent interactions have lower than covalent bonds. Furthermore, the non-

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covalent interactions have lower energy of disassociation and can be reversed or kinetically

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liable [105]. Few interactions are stated below: (1) Non-polar gas-π interaction: These interactions are formed between non polar molecule or an inert gas and lewis acid or polar molecule. Dispersion energies and electrostatic energies govern these interactions [106]. (2) H-π interactions: This type of Hydrogen bonded interactions exits between a π cloud and Hydrogen atom of cation molecule. The strength and geometry of the interactions is

governed by polarizability of π- cloud. These interactions are commonly found in graphene-organo metallic complexes. The strength of interactions is enhanced by increase in participation of orbitals [107]. (3) π-π interactions: These interactions exist throughout graphene and can be altered to functionalize graphene. Other molecules can be incorporated with the help of these interactions [108]. (4) Cation-π interactions: These interaction forms supramolecular interactions between π

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cloud and a cation. Furthermore, are considered as pathway for self-assembly and anion sensing. An example such bonding is interaction between graphene and ammonium salt[109].

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2.0 Graphene Nanocomposites

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2.1 Graphene polymer nanocomposite

Electrochemical or in situ chemical polymerization of monomers in graphene presence are

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well-known techniques to synthesize graphene-conducting polymer nanocomposites. Graphene nanocomposites polymers that are vastly investigated are prepared by the

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combination of graphene with conducting polymer of pi-conjugated system [110]. Infiltration of graphene into polymer matrix have numerous applications such as energy

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storage devices, drug delivery, biosensors and photo catalyst. However, various methods of GO and GNP dispersion into polymer matrix have been reported. Fabrication of graphene

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polymer composites is greatly facilitated by ultrasonication through better dispersion within the matrix, though the size of and weight of nanomaterials is carefully controlled [111]. Most graphene/nanocomposites investigated are synthesized using chemically reduced graphene oxide, graphene oxide and thermally reduced graphene oxide. Three-dimensional graphenebased nanocomposites (3D-GONCs) are greatly acknowledged and considered to be the material of new generation. Furthermore, the well-known approaches to fabricate 3D-

GNPCs includes organic molecule cross linked graphene, three dimensional graphene based template and polymer particle/foam template [112] 2.1.1 Fabrication Process of Graphene /Polymer Nanocomposites Dispersion is an important factor to achieve maximized reinforced surface area and affects neighbouring polymer chains consequently the properties of whole matrix. Thus, an immense effort had been invested to achieve well dispersed and homogeneous system

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through covalent and noncovalent functionalization of filler material. Currently, the main processes to prepare graphene/polymer nanocomposite includes in-situ polymerization, solution blending and melt blending. In situ polymerization and solution blending achieves good dispersion at the expense of high solvent consumption and environment contamination.

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Contrarily, melt blending is environment friendly and solvent free while aggregation

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2.1.1.1 In-situ Polymerization

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happens at high graphene loading that limits the properties of the nanocomposite.

In this process, first graphene and its derivatives are absorbed into liquid monomer than the

Various

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polymerization is initiated with the help of suitable initiator either radiation or heat[113]. nanocomposites

prepared

through

this

technique

includes

polyamide/

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graphene[114], polyurethane/graphene[115], Poly (vinyl chloride)/RGO[116], polyimide/fgraphene nanosheets [117] etc. By using this technique graphene and its derivatives are well

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dispersed within polymer matrix due to strong interfacial interaction of filler material and enhanced interfacial compatibility, that enables to equally transfer stress and increases overall performance. In addition, the presence of functional groups provides number of reactive sites that helps to modify polymer matrix or nanofiller. There are few drawbacks of the process, such that the addition of graphene and its derivatives effects the rate of

polymerization, thus effecting the molecular weight of final product and makes process difficult to control [118]. 2.1.1.2 Solution Blending In this process, graphene and polymer matrix are initially dissolved into solvent, and chains of polymer matrix intercalate into graphene sheets by external forces such as ultrasound and mechanical stirring and enables equal dispersion of graphene into polymer matrix. Solvent

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is removed to obtain polymer/graphene nanocomposite[103]. Solution blending is an easy technique and ensures better dispersion of graphene in polymer matrix. The polymer matrix obtained

through

solution

blending

includes

poly

(vinyl

alcohol)/GO

[119],

polystyrene/graphene, polypropylene/exfoliated graphene [120], polylactic acid/GO[121],

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epoxy/RGO[122], poly (vinyl chloride)/graphene [123] etc. The grafting polymer segment

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chains or small molecules on graphene surface, interfacial compatibility and interfacial interaction between polymer matrix and graphene can be greatly strengthened, and enables

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better dispersion of graphene in the matrix[124] . Thus the solvent removal is a critical issue and makes this technique environmental non-friendly, difficult to scalable and high

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cost[125].

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2.1.1.3 Melt Blending

This technique involves, the preparation of graphene/polymer nanocomposite in melt state

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of polymer matrix using methods like injection molding, extrusion molding, compression molding etc. The polymer viscosity in this technique is so high that it makes graphene and its derivatives to agglomerate and difficult to exfoliate under strong shearing forces. According to one investigation, on solution blending and in-situ polymerization method concludes that dispersion of graphene in the PU matrix is poor in melt blending technique[115]. On the other hand, melt blending has various advantages such as low-cost

and high efficiency that proves to better technique for large scale production. Various polymer/nanocomposites prepared though this

method includes poly (ethylene

oxide)/graphene[126], poly (vinylidene fluoride)/GO[127], ultra-high molecular weight

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polyethylene/graphene[128], polyethylene terephthalate/graphene[129].

2.2 Graphene/metal oxide nanocomposite Metal oxides are used in capacitors as pseudocapacitive electrodes due to their characteristic of high energy density. Graphene/metal oxides have been able to gain much attention as anode material for Sodium-ion batteries due to its high kWh/cost and outstanding performance as electrode for electrochemical capacitors. The previous methods employed for composites preparation and its constituents were done separated and then mixed or

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composite was prepared by external aid that limits the production at industrial scale.

Othmen, Hamdi, Addad, Sieber, Elhouichet, Szunerits and Boukherroub [156] fabricated nanocomposite by hybridizing (FE-doped SnO2 NPs) iron oxide tin oxide nanoparticles with

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variant iron concentrations and reduced graphene oxide by three step elaboration method in

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order to develop Fe-doped SnO2/rGO nanocomposites. Furthermore, the nanocomposites were studied by TEM, Rahman spectroscopy, XRD and TEM.

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2.3 Graphene/fibres nanocomposites

Direct covalent bonding is well known approach used by researchers to deposit graphene

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onto carbon fibres to cast a Graphene/carbon fibre reinforcing structure.

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Davoodi, Mazinani, Sharif and Ranaei-Siadat [157] used electrospinning method to prepare substrate form graphene oxide and poly lactic acid. The study mainly focused on placing of

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graphene nano-sheets into electro spun fibre. The research was characterised by various techniques that includes AFM micrographs, ATR-FTIR and SEM. Furthermore, the study was also conducted on nanofiber synthesising in terms of topology, mechanical properties and surface chemical structure [157].Wan, Li, Yang, Ao, Xiong and Luo [158] fabricated a nanocomposite material namely bacterial cellulose/graphene/polyaniline (BC/GE/PANI) by using facile two step strategy. In first step Bacterial cellulose/graphene is prepared by in situ

membrane liquid interface method, in which graphene is introduced into bacterial cellulose that improves mechanical properties due to uniform dispersion of graphene into BC matrix. Further, polyaniline was deposited that improved connectivity within the matrix resultantly electrical conductance was also improved. This nanocomposite BC/GE/PANI showed electrical conductivity 1.7± 0.1 Scm-1.This sensor showed a great potential for flexible electrodes and electromagnetic shielding [158]. 2.4 Graphene/metal nanocomposites

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Various metals such as Ce, Au, Cu, Fe etc are infiltrated into graphene to synthesize composites suitable for various applications. The success of graphene nanocomposites led the research further in counterpart nanoclusters. Nanocomposites at ambient temperature

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offers ultra-low resistivity in contrast to well-known conductors such copper which are also

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considered as next generation conductor. Nanocomposites have not yet reached its maturity level as there are still lot of scientist and technical issues in their fabrication.

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Xuan, Kim, Hui, Das, Yoon and Park [159] developed three-dimensional porous laser induced silver graphene nanocomposites. The composite electrodes demonstrated steady

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and high electrical conductance even subjected to mechanical deformations. Moreover, addition of gold and platinum nanocomposites into laser induced graphene further improved

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electrical performance significantly for glucose sensors. Sensors demonstrated a low

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detection limit of 5𝜇m and a satisfactory detection range between o to 1.1 mM [159]. 2.5 Graphene/activated carbon nanocomposite Activated carbon is used commercially for variety of applications due to its wide adaptability, good performance and low cost that are result of its beneficial pore structure and mighty surface area. Adsorption properties of activated carbon are well known similar trend is observed in graphene/activated carbon nanocomposite materials to metal ions.

However, various method for synthesis have been developed to prepare nanosheets composites of graphene/activated carbon as electrode material of high performance for supercapacitors. Xin developed a nanocomposite material by combining graphene with activated carbon for electrode (oxygen) to be used in Li-ion batteries. In graphene/activated carbon, threedimensional network is formed by graphene with excellent electrical conductivity and good mechanical properties while activated carbon forms a layer over it with several meso/micro

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pores (less than nanometre size) which acts as active sites for discharge reaction. Further, the discharged product Li2O2 particle size is reduced to smaller (10nm) and are distributed monogenetically in graphene/activated carbon matrix and particles of large size around 100-

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200nm are at pristine graphene cathode. The charge voltage gradually increases

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approximately to 3V-4V for graphene/activated carbon cathode while the charge voltage for

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graphene cathode also increases to 4.2-4.3V and eventually crosses 4.5V [160]

3.0

Strain Sensors

In recent times flexible sensors have attracted exceptional attention due to considerable compatibility to surfaces arbitrarily curved and satisfactory developments in the field of healthcare, electronics, robotics, communication, environment, energy, wearable electronics and so many others. [161] In particular, the focus of flexible strain sensors has been on structural health monitoring, human health monitoring. Conventional strain sensors manufactured from metals and ceramics suffer from brittleness, intrinsic hardness and

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fabricated from complex processes of manufacturing and cannot satisfy the needs and demands of rapidly growing engineering application [162]. The strain sensors of nanocomposite material exhibit outstanding properties for example high flexibility ,low

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cost, and light weight, can be synthesized easily and sensitivity can also be further improved by optimization of synthesis technique and infiltration with suitable Nano filler [163]. The

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expeditious advancement in the fields of nanotechnology and nanoscience has benefited the

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development of highly sensitive materials for strain sensors in last decade. Currently, number of materials such as graphene and its derivatives, carbon black particles, metal

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nanostructures and polymer nanofibers have been established to illustrate desired piezoelectric properties [164]. The strain sensor of optimized performance needs to be made

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of material that possesses substantial piezoelectric characteristics and high flexibility. In addition, the manufacturing of strain sensors needs several factors to be considered mainly

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stretch ability, flexibility and sensitivity. In addition to manufacturing cost, ease to wear and the reliability of strain sensor should also be considered [165].

3.1

Applications of Graphene Based Sensors

3.1.1 Human Health Monitoring Healthcare costs results in major share of the United States GDP. According to an estimate by OECD, it constitutes 17% of the gross domestic product. While other developed countries have around 7 -11 % of Gross domestic product. Chronic illness costs seventy percent of the total United States healthcare spending [182]. Furthermore, ninety five percent of the total $1.4 trillion is spent on direct medical services while only five percent is only spent on preventive

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and health promotion [183].The current healthcare system defined as reactive and not preventive or predictive. In several cases, as result large sum is spent at stage where much cannot be done. One solution to the problem is to have personnel heath care but it cost a lot

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which makes it impractical for large portion of the population. The other more feasible solution is to use technology for the purpose. Technology focuses on providing health monitoring by

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carefully monitoring critical health parameters such blood pressure, motion detection etc.

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around the clock. Graphene based strain sensors due to high flexibility, sensitivity and durability poses a good viable option for both as wearable health monitoring and structural

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health monitoring. Human motion detection is an important prospect of modern research to assist human being regarding health by monitoring activities like steps walked, blood pressure,

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exercise duration and many others.

3.1.2 Structural Health Monitoring Complex infrastructures for example dams, bridges and buildings, are regularly subjected to abnormal loads and extreme environmental conditions such as high humidity, strong winds, temperature variations, and heavy rains that are difficult to be anticipated in design phase. These events result in structural deterioration that are often not identified by visual inspection. Furthermore, catastrophic events like floods, earthquakes and hurricanes can adversely affect structures [203].There are around 33,000 steel railway bridges and approximately 600,000

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bridges on highways in the United States; more than 30% of them are functionally obsolete or structurally deficient. Mostly visual inspection is performed to detect cracks and it is estimated that such inspection can only detect up to 3.9 % crack propagation with respect to last

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inspection. Several bridges were constructed fifty years ago and traffic loads we have today were not considered in design phase [204]. Structural health monitoring is an important aspect

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of strain sensor application to monitor structure integrity. There is need of intelligent

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technologies and sensor that have potential to take diverse data and show picture of structural health and further help to detect early damage to structures from natural disasters and other

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hazards[205].

Structural health monitoring system is a systemic technology in decision ,identification,

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detection and quantification related to state of health of various structures e,g. civil structures and aviation vehicles [206]. In past various traditional sensors, include optical fibre sensors,

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eddy current sensors[207], strain gauges based on metal foil, piezoelectric sensors [208], metal oxide or semiconductors thick or thin films, magnetostrictive sensors had been considered for the purpose. In recent times, various carbon nanomaterials were explored for developing noninvasive, conformable, scalable, multifunctional, light weight and embeddable piezoresistive sensors applicable for structural health monitoring. Carbon nanotube as a sensing medium various carbon nanotube based piezoelectric sensors e.g. chemical vapor deposition developed

fuzzy fibre, buckypaper, Carbon nanotube/polymer composites and carbon nanotube yarn were developed for detection of strain, failure and damage[209] . Graphene based strain sensor have been explored extensively in recent past as multifunctional and smart sensors for applications of structural health monitoring.

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3.1.3. Pressure Sensor Pressure sensor is an important instrument of engineering and industrial applications. Pressure sensors helps to monitor strain as a pressure impact on sensor surface. The unique electrochemical properties of nanomaterials have made them viable option for pressure sensing applications. The following table compares important feature of various notable graphene

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based nanomaterial pressure sensor.

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4.0 Future Challenges

Production of graphene at industrial scale is still a challenge. There are various rising methods but none of them is capable enough to produce at

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large scale. The other challenge includes, acceptability and adaptability for various applications for which it is capable enough. Development of

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bandgap so that it can be used for fabrication of digital logic and replace silicon. The characteristics of graphene vary verily with different methods of synthesis, though mechanical exfoliation produces graphene with better characteristics than by chemical vapour deposition, but mechanical

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synthesis technique is tedious and not capable enough to meet industrial demands. Graphene as single layer shows promising characteristics, but these properties deteriorates with the increase in thickness such as electrical and optical conductance that limits the range of its applications. The

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dispersion of graphene is difficult which limits the homogeneity consequently communication between the layers of the graphene is affected that effects the performance of strain sensor.

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5.0 Conclusion Since the discovery of graphene, this 2D structure has proved to be very useful material due to its potential properties and wide range of application in electrical, mechanical, electromechanical and optical fields. Graphene is enveloped into round (0D) dimensional structure known as fullerene, rolled into curved cylindrical (1D) structure known as carbon nanotube and stacked upon each other to form graphite. Graphene is thinnest material as thin as nanometres and strongest material ever measured. Various synthesis techniques have been developed to synthesis graphene commercially but most widely used technique for synthesis is chemical vapour deposition. Various applications of graphene-based strain sensors prove graphene to be important prospect for different other applications. Graphene based nanomaterials are also discussed, with emphasis on polymer-based

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nanomaterials due to their significance for their application as strain sensor Furthermore, graphene nanomaterials allows to transform graphene

monitoring) of graphene has been reported in detail in this work.

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Declaration of interests

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for specialist jobs. Thus, three important commercial and industrial application (pressure sensor, human health monitoring and structural health

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work

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reported in this paper.

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References

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Table 1.1: Occurrence of Graphene Contribution

Year

1986 1997 1999 2004 2010 2010 2011 2010 2012 2013 2016

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1969 1970 1975

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1968

Ruoff and his associates produced multiple layers of graphene by micro-mechanically shed graphite into thin layers like lamellae. With the help of micro-mechanical exfoliation graphene was prepared by Geim and his associates Noble Prize awarded to Geim and Novoselov in Physics for “ground-breaking experiments regarding the two-dimensional material GN” Initial trials of commercial industrial Research & Development GN flakes in aqueous media was produced by electrochemically oxidizing, intercalating and exfoliating graphite. to Further advancement in electrochemical production of GN flakes (e.g. GN composite products, non-aqueous media, reduction of GO etc) to Appearance of Gimmick consumer

Jo ur

1962

Methods include Hummers, Schaflhaeutl, Brodie, Staudenmaier etc. were used to prepared graphite oxide (GO) Reduced graphene oxide (r-GO) was prepared by Boehm and his associates. The preparation was done by thermal and chemical reduction process of graphite oxide. LEED patterns were generated by minute molecule adsorption on Pt (100). Morgan and Somorjai accomplished these patterns. Monolayer of graphite was present on the surface of Pt after analysing the Morgan and Somorjai works Monolayer graphite on the Ni (100) surface by segregating carbon was obtained by Blakely and his associates. Monolayer of graphite was prepared by subliming Si from SiC, accomplished by van Bommel and his associates. Boehm and his associates suggested name Graphene to monolayer graphite. Definition of graphene was formalized by The International Union of Pure and Applied Chemistry (IUPAC)

pr

1840-1958

Reference [22] [23] [11] [11] [24] [11] [11]

[11] [25] [26] [27] [28] [28] [27]

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The big motor company Ford puts graphene enhanced into vehicle production

Table 1.2: Properties of Graphene

Pr

e-

pr

2019

Value 1,100 GPa 125 GPa 5,000 W m-1 K-1

Remarks [15] [59] [60]

Charge carrier’s mobolity Specific surface area Phenomena of transport Absorption of visible light up to Minimum Hall conductivity at zero concentration Second Order Elastic Stiffness (thickness 0.335 nm) Intrinsic Strength Thickness of monolayer graphene Poisson’s ratio Bandgap

250,000cm2V-2 s-1 2,630 m2 g-1 Excellent 2.3% 4 e2/h

[60] [60] [15] [15] [59]

340 ±50 N/m

[60]

130 Gpa 0.345nm 0.16 Zero (eV)

[60] [61] [59] [59]

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Property Young’s modulus Fracture strength Thermal conductivity

[27]

f

Disadvantages

References

oo

Surface area is high Conductivity is high

pr

One and two-layer thickness approx. 0.345 nm sp2 bonds are high in number

Oxygenated lattice is added to Good insulation properties graphene and it is thicker than Well dispersed in aqueous media, graphene organic solvent and several medias

Pr

Graphene oxide

Features

Exhibits Poor dispersion in [70] aqueous state Insulation is poor Conductivity is poor

e-

Type of Carbon Graphene

Table 1.3: Derivatives of Graphene Advantages

[63] [71]

Exhibits poor dispersion for [72] porous graphene [73] Conductivity is poor for porous graphene oxide

Graphene quantum dots

Easily agglomerated

na l

Porous Pores are available in large Surfaces areas increment graphene and number porous graphene dots

Jo ur

Smallest graphene derivative Shows photoluminescence with respect to size Property

[74] [74]

f

Thickness surety

More than Significant (cm) mono-layer

Relatively large in size; quality Limited production good Expensive.

na l

Jo ur

~10

Drawbacks

Defects do occur.

pr

Generate graphene

Reference [87] rate; [88]

of Limited production rate; [89] Contains contaminations like carbonaceous. Generate significantly larger area Insufficient scale. [90] of uncontaminated graphene

Pr

Mono-layer, From 100nm up to Bi-layers and few μm few layers Epitaxial growth Few layers Size up to cm on silicon carbide (SiC) Unzipping of Several layers few μm long nano carbon ribbons nanotubes (CNTs) Reduction of Several layers Sub-μm carbon monoxide (CO)

oo

Dimensions Width Lateral Mono-layer Up to 100 nm

gram/hour

e-

Synthesis Method Confined selfassemble Chemical vapor depositions (CVD) Arc- Discharge

Table 1.4: Comparison of Bottom Approach Advantages

Size can be controlled by the choice of preliminary nanotubes; another method to manufactured ribbons. Generate un-oxidized graphene sheets

Higher initial material cost; [88] oxidized graphene Impurities with α-Al2O3 and [88] α-Al2S.

f

oo

pr

μ m or Sub-μm Cheaper and graphene sheets

e-

Mono-layer as well as multiplelayers Mono-layer as well as multiplelayers Typically, mono-layer

unmodified Limited yield; separation

From 500- 700 Single step exfoliation and nm functionalization; good electrical conductivity of the functionalized graphene From 300-900 Produce graphene unmodified but nm can be measured

Jo ur

na l

Functionalization/ electrochemical exfoliation of graphene Super acid dissolution of graphite

Dimensions Width Lateral Few layers From μ m to Generate graphene sheets of Insufficient yield cm significant size and unmodified

Pr

Synthesis Method Micromechanical exfoliation Direct sonication of graphite

Table 1.5: Comparison of Top-Down Approach Advantages Drawbacks

Reference [91]

[92]

Sufficient cost needed for [93] ionic liquids

Consume of dangerous acid [94] i.e. HSO3Cl; sufficient cost for acid removal

Table 1.6: Graphene Nanocomposites Nanocomposites Reduced graphene oxide Graphene

Fabrication Au chip --

Graphene oxide

Gold SPR chip

Graphene oxide- AuNRs

Ab

Silver-Graphene oxide

Silver-Graphene oxide chips

Technique of Sensitivity Enhancement Analyte rGO has higher binding ability with dyes TPA and Dye Cy5 Graphene based film with an ultra-sensing depth Paclitaxel of 2 micrometre Graphene oxide-based signal amplification Alpha-Fetoprotein

Detection Limit 10-8 and 10-6 M 2.6 × 10-8 mV/RIU

Sensitivity -1.2 × 108 mV/RIU

--

AuNR for high sensitivity and higher loading Transferrin capacity of graphene oxide Increased surface adsorption of graphene oxide Dopamine

0.0375 𝜇g/mL

Between 0 ng/mL to 100 pg/mL [133] 0.0375 - 40 𝜇g/mL

49nM

---

[134]

Dopamine

10-13

10-8 to 10-13

[135]

ssDNA

--

72-76 deg/RIU

[136]

ssDNA cDNA GM food

--12 pM

87.8 deg/RIU 125o/RIU 0.5 to 500 nM

[137] [138] [139]

miRNA-141 ssDNA IgG

1 fM 1.6 𝜇g/mL 0.76 nM

1pM 10-6 to 10-8 0.019-40

[140] [141] [142]

IgG in clenbuterol IgG

6.57 pg mL-1 10 fM

0.01-10 ng mL-1 0.4985 nm (𝜇g/mL)

[143] [144]

IgG

--

0.15 to 40 𝜇g/mL

[13]

BSA

--

--

[145]

BSA GM food Methylene blue Carbohydratelection Concanavalin A

-12 pM -1𝜇mL-1 0.39𝜇g/mL

-0.5 to 500 nM 0.01 to 10𝜇M --1 to 20 𝜇g/mL

[146] [147] [148] [149] [150]

A gas sensor PB (ІІ) Ammonia gas CH4 gas

1 ppm 1.112 ppm -10 ppm

1 to 10000 ppm 5 ppm 10 to 100 ppm 10 to 100 ppm

[151] [152] [153] [154]

Copper ion

0.01 ppm

0.01 ppm to 0.5 ppm

[155]

Tilted fibre brag grafting Amplification of surface refractive index (TFBG) modulation by NC over the surface of fibre Graphene/ Zinc oxide Au SPR chips Sensitivity of sensogram affected by number of Bilayer sheets Graphene Au-MOS2 High surface to volume ratio of graphene Phosphorous nanoparticles Coating of graphene cDNA detection based on desorption Reduced graphene oxide- -Hairpin catalysis AuNPs Graphene -Graphene based layer Graphene oxide/Silver p-Au Indirect competitive inhibition assay (ICIA) PDA-Ag Fe3O4/rGO Gold doped reduced Improved sandwich assay graphene oxide COOH graphene ODA SAM layer COOH graphene and SAM layer sensitivity Graphene oxide /AuNPs Silver coated polymer silicon Electric field enhancement due to graphene oxide fibre Graphene oxide Gold pyramids Antibody scaffold enhance due to SPA for sensing GO-CHIT,PDDA-GO, Covalent functionalization Direct quantification of protein by graphene oxide rGO-CHIT activity GO-COOH/Au NCs-Au chip Adsorption of bio-molecules via hydrophilicity AuNPs-rGO Au-S-ssDNA Hairpin catalysis Graphene -MB-Br electrostatic attraction Graphene/Au Gold based SPR Enhanced adsorption Dextran and Graphene Capped auNPs Sandwich assay oxide Graphene –Au monolayer Gold SPR chip Graphene oxide reactivity CS-Graphene oxide Gold SPR chip Graphene oxide- CS covalent bonding rGO/PMMA Gold SPR chip Enhanced sensitivity of nanocomposites GCNT – Graphene carbon Optical fibre cladding Graphene carbon nanotube-based methane carbon nanotubes adsorption-based interaction Graphene oxide composite Hexadecyltrimethylammoni Increased sensitivity of thin film nanocomposites thin film/cellulose um bromide

Jo

ur

na

lP

re

-p

ro of

DNA Fn Graphene gold

References [130] [131] [132]

Fabrication Technique Chemical Vapor Deposition

Drop casting

Graphene/paper

Ink-jet printing

Graphene/rubber

Template induced assembly Procedure

graphene

Layer by layer self-assembly

Adhesion

PDMS/Carbon black

Jo

Spandex/CNT

Flame treatment/Droplet coating

ur

bandage/natural

na

PDMS/CNT mesh

Cotton rubber/GO

re

Functionalized nanoplatelets/PDMS

Spray coating

lP

Reduced graphene oxide/Titanium oxide composite

-p

Graphene/PET

Knitting

Spin coating

ro of

Material Graphene/PDMS

Table 3.1: Various Strain Sensors Gauge Factor Max Strain (%) Remarks 151 5 1. Resistance decreased initially until 2.47% due to relaxation of wrinkles already existing in the structure. 2. Resistance increased between strain range 2.4% to 4.5%. 3. An irreversible resistance change happened with an increment of strain above 5% which shows complete structure destruction of nanocomposite, thus defines the working range of sensor below 5%. 0.11 7.5 1. High quality graphene stacks are obtained by Light-Scribe DVD burner 2. Graphene based strain sensor demonstrated multi cycle operation and linear response thus demonstrates its longevity and preciseness as a sensor. 125 1.25 1. Inkjet jet process of fabrication allows fast fabrication, direct on the surface with higher sensing area. 2. Inkjet allows to vary printing parameters and allows combination of various 2D materials. 3. Drop casting 99 printing layers, whereas 20µm is optimum conditions to synthesize highly sensitive sensor. 82.5 100 1. Double layered interconnected GE network increased electrical properties and decreased percolation threshold 2. A very low threshold of 0.3 volume % was observed. 3. Sensor showed good stretch ability for 300 cycles even at 100% strain 12 -23 5 1. A linear stain response was observed for range of 5% 2. 25% weight percent showed a gauge factor of 23, which shows optimum concentration for highly sensitive sensor. 1037 2 1. Graphene based strain sensor showed high degree of sensitivity of 1037 (GF) at an expense of 2% strain. 2. Marangoni effect helped to scaled up graphene films rapidly at liquid/air interface. 4 30 1. Macroscopic grid and spider web like microscopic structure was prepared by the depositions on carbon nanotubes by nickel gauze. 2. Sensor demonstrated good recoverable response for 1000 cycles. 3. Sensor showed an increase in gauge factor with an increment in strain. 416 7.5 1. The flexible strain sensor by GO woven fabrics through ethanolflame reduced and cotton bandage template process. 2. The quick response time 20 ms was recorded. 0.4 80 1. Textile look like sensor was prepared by combining SPX fibres and carbon nanotubes in a three-dimensional knitted structure 2. A linear resistance change behaviour was observed up to a strain of 80 % with very little indication of hysteresis. 12 30 1. The sensor demonstrated stable and high sensitivities high strain range. 2. A sharp change in resistance was observed at high strain of 55 to 60%. 3. Conduction paths destroyed at strain higher than 35%.

References [166]

[167]

[168]

[169]

[170]

[171]

[172]

[173]

[174]

[175]

Compression moulding

2

100

Screen printing

100

52

By embedding silver nanoparticles into PDMS material

268.4

110

PVA-Ca2+ mortar material mixed with GO Nano sheets and silver nanowires

infiltrated

-

30

-

21-25

-

5-7.75

0-127,127-200

lP

re

Silver NM/silk protein

-p

Silver nanoparticles with PDMS

ro of

Ecoflex /Carbon black

By deposition of Ag nanoparticles into nylon yarn

na

Ag/nylon yarn

Coating Technique

Jo

ur

Wool yarn/conductive ink

1. Sensors analysed in this study are composed of thin microchannel of different commercially available silicone and gap between channels are filled with two different conducive liquids. 2. The fabrication methods used involves laser engraving, 3D printing and plasma bonding. 3. The sensor output is linear to the applied strain and gauge factor is either not dependent on applied strain nor the base material or conductive liquid. 1. Sensor exhibits ultrahigh sensitivity, outstanding re-healing capacity, large stretch-ability and high healing efficiency. 2. The healing properties of sensor are resultant of coordination bonds, chain movement of poly vinyl alcohol and hydrogen bonds 3. In a strain range of 38-52% sensor demonstrates ultrahigh gauge factor of over 3500. 1. AgNps concentration in a PDMS was varied in a range of 0.10,0.15,0.20,0.25 and 0.30 wt.%, while first concentration demonstrated highest gauge factor of 238.04 with strain of 110% (linear after 15%) contrarily, last concentration showed a low gauge factor of 109.4 with strain of 130% (linear). 2. The sensors were tested for 10 cycles with crosshead speed of 1 mm/min 1. The sensor demonstrated practical extraction of biomechanical energy from human skin via electrical and biological compatibility of silk. 2. The higher power density of around 2 mWm-2 was harvested, that is 4 times higher than contact resistance of glass and enough to light up LEDs. 3. The phenomena was facilitated by high surface area of periodic AgNW/silk pattern of surface. 1. The sensor was prepared by depositing silver onto nylon yarn. 2. SEM image should show uniform and continuous film of silver metal onto yarn. 3. Mechanical testing showed a reproducibly in results. 1. The active material’s geothermal effect was tested by changing shape of conductive wool yarn within the elastomer. 2. The sensor were tested for 5 cycles up to 200% strain range followed by various electromechanical tests. 3. It is concluded that low cost abundantly available yarn wool increases strain sensing performance and gives way forward for green electronics.

[176]

[177]

[178]

[179]

[180]

[181]

Table 3.2: Human Health Monitoring Strain Sensors Materials

Fabrication Technique

Gauge Factor

Graphene/PDMS

Atmospheric pressure vapour deposition

GNP/PVA/Yarn

Layer by layer

rGO/Elastic Tape

Stretching and releasing composite 16.2 to 150 films

82

rGO/PES

Simple fabrication technique

0.2 to 0.35

GnPs/silicon rubber(PDMS)/Medical Tape

Mechanical pressing

Ultra-thin graphene films/PDMS

Self-Assembly

GWFs/PDMS/Medical Tape

-

chemical 500

110.4

20

1037

2

-

30

re

lP

na

ur

Jo

150

-p

--

Strain Remarks 1.The sensor demonstrates a good trade-off between stretch ability and sensitivity with a high gauge factor in a low strain range. 2. The ultrahigh sensitivity of sensor is attributed to micro woven fabric geomatical structure of graphene, which makes controllable microscale, zig zag cracks locally oriented close to crossover locations and interlaced ribbons interfacial resistance both responsible for synergistic effect on sensitivity improvement. 3. The sensor also application for sound signal acquisition, human motion detection and monitoring of external stress. 1. The sensor helped to monitor motions of human body. It effectively monitored smallest movements at chest and throat and big movement of arm and hands. 1. The sensor demonstrated the successful detection of both bending and stretching deformations with good linearity. 2. The sensor showed fast and stable response to finger bending movement and clear measurement of pulse from wrest. 3. The sensor also demonstrated measurement of muscle movement over the throat. 1. The sensor showed high degree of repeatability for 1000 mechanical deformation cycles. 2. The sensor has ultra-sensitivity to a low strain of even 0.02%. 1. The sensor has response time less than 50 ms and durable for 1000 cycles. 2. The sensor demonstrated detection of finger movement, music from mobile speakers and movement of cricket with superior reproducibility and stability and fast response. 3. It also demonstrated application of wearable LED under reachability of 10V for strain range of 20%. 1. The sensor showed it applicability to measure pulse, and indicated three various pulse peaks (diastolic, percussion and tidal) for monitoring of healthcare. 2. The sensor also demonstrated measurement of time dependent mechanical movement of loudspeaker due to sound waves. 3. The sensor demonstrated good stability and recoverability to various applied frequencies and strains. 1. The senor was used to monitor slight movements such as hands from clench to stretch, facial expression change, pulse and phonation. 2. The sensor also showed detection of human words, and by producing specific resistance change and resistance change curve was repeatable for each word. 1. Graphene inclusion into CNTS network provided essential strength that was preciously missioning and results in failure under mechanical strain. 2. The CEG sensor stretched during bending of index finger while with the unbending sensor returned to base level. 3. The sensor also demonstrated the successful measurement of strain at different positions of the finger. 1. The sensor exhibited good repeatability for 10000 cycles at stretch ability of 70%

ro of

--

Max (%) 2

CNTs/Graphene/PDMS

CVD/Catalytic Growth/Pouring

-

20

FGF/PDMS

3D Percolation

15 to 29

70

References [184]

[185] [186]

[187]

[188]

[171]

[189]

[190]

[191]

Direct Laser Pattern

457

35

rGO/PDMS/Polystyrene nanoparticles

Doping

250

1.05

GnPs/MWCNTS

Spray/ Vacuum Filtration Method

181.36

GF/PDMS

-

7.5

24

10

18.5

40.6

282.28

0.25 to 0.75

Scalable Dipping-Reduce Method

Jo

Graphene/Spandex/Nylon Fabric

ur

na

lP

re

-p

ro of

GO/THF/Ecoflex

rGO/TPP(NH2)4,ZnTPP,CuTPP,and CoTPP

2. The sensor shows promising function as health monitoring device by detecting various body motions such as finger and elbow bending in addition to pulse measurement at radial artery. 3. It also offers opportunity to synthesize stretchable touch sensor by combining FGF/PDMS with µ-LED composite that could be used as artificial skin. 1. The sensor even demonstrated a strain range of 100% with a gauge factor of 268. 2. Graphene flakes separation by random separation is regarded for superior performance through theoretical analysis. 3. The sensor demonstrated the successful measurement of breath rate, an important health indicator, for normal breathing it is 19 breath per minute, that was quite accurately measured by the LPG strain sensor, along with it also demonstrated other human body motions, such as throat, finger bending, and subtle motions. 1. The nanoparticles acts as insulators and separates stacked rGO fragments and thus creates partially conducting channel therefore signifying the strain to resistance change behaviour. 2. The linear deformation range demonstrated a gauge factor reading of 250 while this value reaches 725 in nonlinear range. 3. The strain sensor demonstrated successful monitoring of human body motions such as lower back posture, swallowing process and pulse on neck. 1. The sensor fabricated with 30wt% loading of GnPS shows an electrical conductivity of 104 S/m, tensile strength of 5.4MPa, Young’s modulus of 9.8MPa and strain sensing linearity of 99.545 % with great reproducibility for 5000 cycles. 2. The sensor successfully demonstrated monitoring of bending and relaxing of elbow with an accurate demonstration of angle elbow makes during the movement. 3. Along with it also demonstrated measurement of angles casted by finger and wrist bending. 1. The dual function composite strain sensor and switch possess outstanding durability and mechanical strength. 2. The skin conforming sensors were connected to circuit to real time monitor, process and wireless transmission of human physiological parameters that includes pulse, respiration, acoustics and finger motion. 3. The integrated monitoring system was capable to collect data and transmit to smartphones through Bluetooth communication. 4. Additionally a smartphone app was developed that analyses the data and comments through voice commands. 1. The sensor was tested for two finger movements first slight bending and forceful bending, the bending movement was repeated every 10 seconds, by dividing time into two sections first 60seconds slight bending followed by 60seconds of forceful bending. 2. In slight bending movement the relative resistance change stayed for 280% and through forceful bending strain stayed between 460% and 540%. 3. The treatment of fabrics with graphene loses its stretch ability, but contrarily increases recovery rate with steady state values. 1. The sensor was tested for 1000 cycles in a strain range 0.25% with strain change almost identical for each cycle.

[192]

[193]

[194]

[195]

[196]

[197]

Spraying Method

1054

rGOFF/PDMS

-

1668.45

Laser Induced Graphene

One Step Laser Scribing Strategy

316.3

ZnO/GNP Nanocomposites

Solvothermal Method

Graphene Foam/Natural Rubber latex

-

-p

ro of

Graphene/PDMS

ur

na

lP

re

8.8 to 12.8

210

Thermal Expansion Pressing Forming 9.78, 47.6 Process

Jo

Graphene Film/Nylon Fiber

2. The sensor demonstrated high response to VOC vapours, sensors successfully detected eight different types of VOC biomarkers with clear distinction between them as pattern recognition approach 3. The sensor also monitored various human body motion such as respiration rates and pulse. Furthermore, the detection of nephrotic and diabetic breath of exhaled samples of individuals by clear distinction between healthy individuals. 26 1.The results of sensor performance testing shows a stable response for 500 loading and unloading cycles. 2. The sensor demonstrated excellent detection of human pulse of 78 beats per minute including sensitivity to tidal wave (T), percussion wave (P) and diastolic wave (D). 0.24 to 70 1. Under compression of 66% sensor showed a GF of 1686.48 at an expense of 0.24 to 70% strain with quick response of 30ms. 2. The sensor demonstrated a reproducible response for 1200 continuous cycles, whereas excellent durability and stability can be attributed to fascinating porous 3D structure and a continuous conducting network. 3. The sensor demonstrated an excellent linear response to motion of finger, elbow and hand. 11 1. The sensor demonstrated durability within a strain range of 3% for 1000 cycles. 2. Three different pattern were fabricated namely square, shutter and zigzag, in comparison shutter presented high gauge factor with good strain range. 3. The shutter patterned electronic skin sensor also acts as audible alarm device due its thermoacoustic effect and shows broad spectrum frequency in a range of 200 to 20kHz and relatively high SPL. 0 to 44 1. The demonstrated highly fascinating response with high mechanical characteristics (elongation at break of 90% and fracture strength of 0.6MPa) with a good reproducibility for 1700 cycles. 2. The coupling interaction among GNP and ZnO NPs that collectively showed steady deformation of sensing materials during releasing/stretching paying a way for perfect linearity of strain sensor. 3. The sensor not only demonstrated detection of large motions such as wrist motion, waving badminton racket and elbow rotation but also subtle human motions like phonation, swallowing, pulse and coughing. 10 to 40 1. The sensor is developed by unique, scalable convenient, cost effective and ultrafast strategy. 2. The sensor a wise trade-off between stretch ability and sensitivity and demonstrated a monitoring of various human motion such as finger bending, movement steps and strain types of various motions with quick response time of 0.37s. 0 to 12 and 17 1. A simple and cost-effective process namely thermal expansion is proposed for to 19 the fabrication of GnPs, and the GnPs produced exhibits good plasticity. 2. The graphene films exhibits a excellent conductivity of 90900 S/m, due to intrinsic properties of graphene. 3. The sensor demonstrated a to human motion detection for more 1000 cycles with low hysteresis.

[198]

[199]

[200]

[201]

[105]

[202]

Table 3.3: Structural Health Monitoring Strain Sensors Fabrication Technique Two Step Dispersion Method

Gauge Factor Max Strain (%) 752±28 -

SIC Film/Graphene

Mixing/Deposition

358

-

Graphene/LCP Film

Filtration

375 to 473

0.1 to 1.4

re

-p -

>100 to 125

22.45

-

223

3

Graphene Fabric/PDMS

GO/silver

Jo

ur

GnPs/Epoxy

Method/Spin >100

lP

Yarns/Silicon Dip Coating Coating Process

na

GNP/Spx Rubber

Woven

Hybrid Computer-Controlled 450 to 0.6 Laser Scribing/Spray Coating/Ink Dispensing Method

Remarks 1. The sensor with 2,3 and 5wt% of GNPs showed electrical resistance of 40MΩ, 150kΩ and 50kΩ respectively and electrical conductivity of (5.5±5).10-5,(3.7±0.7).10-3 and 2.5±0.8.103 S/m respectively. 2. The sensing mechanism in sensor was based on changes in tunnel resistance due to variance of distance between nano-platelets and makes exponential electrical response. 3. The sensors in monitoring shows clear difference in flexural and tensile strain modes as different behaviour during tests. Flexural strain makes new electrically conductive paths that means a decrease in sensitivity and strain in tensile test breaks contact and tunnel based conducive paths. 1. The superionic film posses a conductivity between 1.54 x 10-2 and 1.72 x 10-2 s/m and activation energy of 0.156 eV, these properties suggest ion hopping to be the main mechanism of conduction. 2. The sensor not only demonstrated sensitivity for strain but also for temperature of 21.5 kΩoC-1 3. The sensor has ability to be retrofitted into structures such as building, wind turbines, pipelines and bridges for integrity monitoring. 1. The sensor demonstrated outstanding performance under various tensile strain. Furthermore, it demonstrated 40% relative resistive change to tension of 0.1%. 2. The lengthy test cycles around 1000 within a strain range of 0.16%, the tests results show the relative resistance changes in a range of 75 % from the measurements results various cycles. 3. The sensor showed good stability and demonstrated potential for structural health monitoring such as buildings, state of bridges and various other structures subjected to natural disasters. 1. The sensors allows tuneable and adjustable sensitivity and sensitivity range by adjustment of fabrication factors that includes the concentration of GnP in the solution and immersion time. 2. The embedment of sensor into rubber sheath makes it rugged and usable for harsh conditions such as structural health monitoring. 3. The sensor showed no response to change in temperature and humidity that enhances its applicability in various conditions and sensitive to only strain. 1. The sensor demonstrated low percolation threshold of 0.76 vol.%. 2. The sensor exhibited various sensing stages such as (1-0.2%), (0.2-0.6%) and (0.6-1.2%) and high gauge factor 22.54,15.25 and 11.81 for 0.84 vol.% , 11.46, 8.13 and 3.63 for 1.05 vol% and lastly 4.69,3.77 and 2.53 for 1.58vol%. 3. The composition 1.05 vol% exhibited higher reversibility than 0.84vol% under loading and unloading cycles. 1. The interconnected network of graphene tubes serves as an electrically conductive network and demonstrates change in resistance to the applied strain. 2. The orientation and quality of Graphene Woven films in a GWF/PDMS composite defines reversibility and sensibility to a bending and tensile load. 1. In this study, effect of laser irradiation on chemical composition, piezo-resistivity and morphology to explore structure property relationship. 2. It had been concluded in the study that laser irradiation time is a most important aspect to decide structure and performance of LSG sensor. 3. The laser irradiation time in the arrange of 3 to 6 gives the best results with gauge factor higher than 100 and resistivity less than 10kΩ/sq.

ro of

Materials GNP/DGEBA

-

References [210]

[211]

[212]

[213]

[214]

[215]

[131]

Intercalation-ExpansionStripping

6.5 to 11.6

GnP/Polystyrene

-

4.6

(0 to 0.5%) (0.5 1. The paper reported an improvement in dispersion of GnPs into epoxy matrix through an [216] to 1.67%) optimized ultrasonic time and ball mill mixing process. 2. The mixture of GnPs and epoxy with 1 vol% GnPs was tested for various characterisation studies. The sensor exhibited two linear sensing stages 0 − 5000𝜇𝜀 and 5000 − 16700𝜇𝜀 and demonstrated high gauge factors. 1. The sensor consisting of graphene nanoplatelets dispersed in polystyrene with a wt% of 6 [217] exhibited a gauge factor than traditional gauges. 2. The strain gauges allows ease in their manufacturing that they can be produced in various sizes and configurations. 3. Sensor allows flexibility to be pasted or printed on various strategic locations of structure to monitor online in situ SHM.

Jo

ur

na

lP

re

-p

ro of

GnPs/Epoxy

Table 3.4: Pressure Strain Sensor Fabrication Technique

Sensitivity (kPa-1)

rGO/P(VDF-TrFe)

Electrospinning Technology

15.6

Pressure Range (kPa) -

rGO/Tisse Paper

Soaking/Thermal Reduction

17.2

0 to 20

GO/DVD Disc

Hummers Method/Drop 0.96 Casting/Laser Scribing Technology

rGO/Abrasive Paper/PDMS

Thermal Reduction Method

PTNWs/Graphene

Hydrothermal Process/Spin 9.4 x 10-3 Coating/CVD Process

GPN/PDMS-Nickel

CVD/Thermal Heating/Soaking

GO/PET/rGO

Spray Coating

CVD/Coating

-p

0 to 50

re

lP

na

ur

Jo

Graphene/PDMS

25.1

0.09

0 to 2.6

-

1000

0.8

-6.534

Remarks

References

1. The sensor demonstrated a great stability over 100,000 cycles, on expense of low voltage around 1 V with a response time of 5ms. 2. The low detection limit of 1.2Pa can detect rice grain and even a feather. 3. The higher detection limit and high area integration further increases the application further sensor can be used to monitor torsion and bending, blood pressure and spatial pressure distribution. 1. The tests carried out in under various pressures namely 400,600,2500 and 20000Pa, shows a good stable response and whereas change in relative resistance is significant to a small pressure change. 2. The two type of sensors were compared namely single and multilayer, in terms of sensitivity, the multilayer performed better but exhibited higher hysteresis, mainly due to air gaps between graphene paper. 3. The sensor when applied to throat of tester speaking words “hello” “sensor” and “graphene” showed distinct response to each word, hence demonstrated high degree of sensitivity. 1. The sensor demonstrated very sharp 0f 0.4ms to an applied high pressure. 2. The Laser Scribing Technology allows the fabrication of LSG at rapid, large scale and with a low cost. 3. The sensor shows a good linear relationship between resistance change with an applied pressure. 1. Sensor has spinosum structure with random height distribution as compared to previous sensors. 2. In terms of practical application, sensor demonstrated high degree of motion detection with no discomfort to tester body when attached at arm skin for two days. 3. The sensor also showed high degree of repeatability and stability for 3000 cycles at loading and unloading pressure of 1.5kPa. 1. The sensor demonstrates successful measurement of motion at radial artery. 2. The human pulse pressure of 7 kPa was recorded and response shows pulse of 63 beats/min that is cute accurate for normal adult human being. 3. The sensor demonstrated a very low time lag of 5 to 7ms. 1. The sensor showed successful detection of motion on loading and unloading pressure and stretching cycles at pressure of 1666 kPa and strain of 25%. 2. The sensor demonstrated a quick response to external loading with dropping and rising time of 80 and 100 ms respectively. 1. The sensor exhibited excellent sensitivity and detected a very low pressure of 0.24Pa in fast response time of 100ms 2. The sensor demonstrated a quick relaxation time (<< 1 s), good durability (more than 1000 flexing cycles and also more than 1000 loading and unloading cycles) and spatial resolution in pressure detection. 1. The conformal graphene shows an excellent optical and electrical properties with transmittance greater than 80% and resistance of less than 2000 Ωsq-1. 2. Furthermore, it also exhibited ultrafast response of less than 4ms and repeatability for more than 5000 cycles. 3. The sensor demonstrated detection of various wind pattern calm wind to storm and super-strong hurricane.

[218]

ro of

Materials

0 to 0.2

[219]

[220]

[221]

[222]

[223]

[224]

[225]

Freeze Drying/Stirring

229.8

0 to 0.1

WGF/PVA

Hummers Method/Poured & Dried

28.34

-

Graphene/Copper/PMMA

CVD/Spin Coating

110

75 kPa

MXene/rGO

Ice-Template Freezing technique

22.56

Graphene/PDMS/Nylon

CVD

0.33

re

-p

0 to 4.9

lP

Glycol Mille-Feuille

na

rGO/Triethylene Amine

0.4 to 1

0.82

0 to 0.6

44.5

-

0 to 40

CVD/Annealing

Graphene/PDMS

Self-Assembled

1875.53

2D shrinkage/1D Compression

1.37,1.30 and 0.98 -

Jo

ur

a-PAN/Graphene

rGO/VHB Elastomer

1. The sensor demonstrated high elasti-ability even at strain of 95% with an ultrafast response recovery of 1085 mms-1. 2. Sparkling graphene block is filled with air bubbles and gives it a fluffy look with low density of 3.7 mgcm-3 3. In terms of application sensor can detect tiny disturbance such as feather, hair and breeze. 1. The sensor has porous structure with bubbles and uneven surface, and PVA serves as filler material. 2. Under an application of pressure, “point to face”, “face to face” and point to point” works well and provides short transmission paths. 3. The device showed a good sensitivity even for a rice grain of 22.4 mg and exhibits good repeat-ability for 6000 cycles. 1. The sensor showed excellent results with low detection limit of 0.2 Pa and a fast response of less than 30 ms and excellent repeatability and stability for 10,000 loading/unloading cycles. 2. The sensor was fabricated by placing two 3D grown graphene films on PDMS face to face into a flexible sensing structure. 3. The sensor demonstrated a successful detection of pf subtle human motions like phonation and wrist pulses and sounds. 1. The sensor demonstrated a high sensitivity along with quick response of less than 200ms and good stability more than 10,000 cycles along with low detection limit of 10Pa. 2. The three-dimensional aerogel was fabricated through green, large scale and lowcost method. 1. The sensor owns a sandwich like structure with a low detection limit of 3.3 Pa and stability for more than 1000 cycles along with quick response of less than 20 ms. 2. The sensor along with quick response and sensitivity allows to detect dynamically changing pressures such as momentum transformation of water droplet. 1. The sensor shows excellent sensibility with a short response of 24 ms along with low detection limit of 7 Pa and high durability for 2000 times. 2. The sensor consumes a low power of 0.2V, with a large scalable fabrication process, low cost raw material and application for personal heath monitoring. 3. The sensor exhibited detection of subtle vibrations and muscle movements 1. The sensor demonstrated high sensitivity of 44.5 kPa-1 for 1.2 kPa at a low voltage (0.01 to 0.5V) with an outstanding stability for 5500 loading and unloading cycles. 2. The sensor exhibits high transparency of 94%, the Youngs modulus measured through nanoindentation is 1.04 ±0.08 TPa. 3. The conductive paths within the sensor remained intact under pressure and sensor demonstrated detection of pulse valve and acoustic vibrations. 1. The sensor demonstrated a good trade-off between linearity and sensitivity in a detection range of 1 to 40 kPa. 2. The sensor exhibited a high peak signal noise ratio of 78 dB with good stability and resultant a universal wearable and high accuracy wearable sensor was built. 3. Under three loads 8.5,15 and 40 kPa sensor demonstrated a good stability for 15,000 loading and unloading cycles with low detection limit of 1.8 Pa and quick response of 2ms. 1. The sensor exhibited a stable resistance less than 3% up to stain of 100% and stability under the influence of repeated relaxing/stretching cycles. 2. The sensor exhibited detection of both hard and soft impacts for sensitivity of 1.37, 1.30 and 0.98 for a strain range of 0,30 to 50%.

ro of

GO/TWEEN 80

[226]

[227]

[135]

[147]

[228]

[229]

[230]

[231]

480

rGO/PET

-

10.39

Graphene/PDMS

Sandwich

7.68

rGO/PDMS

Hummers Method/Coating

23.41 and 35.37

rGO/PDMS

Hummers Method/Coating

re

Graphene/PDMS

-

ro of

Direct Laser Scribing

[232]

[233]

[234]

[235]

-p

Graphene/ PDMS

3. The sensor also demonstrated the ability to detect unpredictable collisions and did not give any false alarms during TORS tests. 1. The sensor demonstrated relaxation time/fast response of (3µs/2µs) with an excellent stability for above 4000 loading/unloading cycles for pressure less than 28Pa. 2. The sensor has working voltage of 5V with low power consumption of 160µW. 3. The sensor demonstrated potential to detect spatial pressure distribution and epidermal electronics. 0 to 200 1. The sensor exhibits good sensitivity in a range of 0 to 2kPa with an ultrawide range of 200 kPa with high stability for 1100 cycles. 2. The sensor demonstrate a quick response of 11.6ms along with good working frequency of 6Hz. 3. The sensor showed potential for bionic skins and biomedical field. 1. The sensor demonstrated a sensitivity of 3.19 kPa-1 with an ultralow detection limit of 1 mg and quick response of 30 ms. 2. It is evident from the experiment that electrodes of MGrE as both top and bottom electrodes can enhance the sensitivity of sensor further sensitivity can be controlled by controllable micro conformal structure. 3. The finite element analysis shows that microstructure compressibility and contact area determines piezoresistive response. 0 to 16 and 16 The sensing demonstrated a good sensitivity for both bending and compression as 35.37 to 40 and 23.41 kPa-1 respectively with recovery time of 240 ms and short response of 120 ms and resolution of 21Pa under bending and 18Pa under compression. 2. The sensor exhibited a high conductivity of 0.29 S/m under thermal reduction. 3. The senor has very low 0.8% hysteresis and successfully demonstrated for various physiological signals that includes elbow flexing, breathing, blood pulse, running and speaking 0 to 1.5 and 1. The sensor demonstrated a good sensitivity to detect a pressure of 150 Pa with a 1.5 to 30 recovery time and fast response of 16 and 10 ms respectively with a repeatability of 10,000 cycles. 2. The sensor application includes bending, minute pressure detection and torsional forces. 3. The interpenetration of conductive graphene into porous interlayer and transforms non-conductive to conductive state. 4. The sensitivity of graphene decreases with the increase in thickness and it is observed that thinner the structure more sensitive it is. 0 to 10 1. The sensor composite with 20% concentration of NH4HCO3 and 2 % graphene as a dielectric layer shows a quick response of 7 ms long with low detection limit of 5Pa and tested for 5000 unloading/loading cyles. 2. The sensor shows excellent demonstration of muscle movements for frown and smile along with wireless transmission through electromagnetic coupling. 3. The sensor also shows various sensitivities for other pressure ranges like 230 kHz/kPa for 10 to 100 kPa and 37.5 kHz/kPa for 100 to 500 kPa. 0.09 to 30 1. The sensitivity of sensors is divided into various sections depending on material of construction like rGO foam, rGO/PU foam and SFrGO/PU foam shows sensitivity of 15.22, 0.22 and 46.67 kPa-1 for pressure range of 0 to 0.3, 0.09 to 30 and 48 to 72 kPa respectively. 2. The sensor act a multi-meter like switchable pressure sensor with high repeatability for 1000 times along with ultrahigh resistance change.

2.2

Jo

ur

na

lP

0.67

rGO/Polyurethane/Selenium

Hummers Method/Freeze Drying/

0.22

[236]

[237]

[238]

ro of

-p

re

lP

na

ur

Jo