**2. Graphene and graphene oxide**

Even if for several decades the isolation of single layer graphite seemed to be impossible two researchers from Manchester University successfully managed in 2004 to isolate monolayer graphite. The "scotch-tape" technique reported by Geim and Novoselov consisted in obtaining single layer of graphene on a silicon oxide substrate by peeling the graphite by micromechanical cleavage [10]. In 2010 the two researchers won the Nobel Prize in Physics for their pioneering study [31]. Several methods have been established for graphene production, such as micromechanical or chemical exfoliation of graphite [32], graphitization of silicon carbide [33], chemical vapor deposition (CVD) growth [34], and chemical, thermal or electrochemical reduction of graphene oxide [35].

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice [36]. Due to its 2D nature graphene exhibit a unique combination of characteristics not seen in other carbon allotropes. Graphene is the thinnest, strongest and stiffest material [37]. Graphene has extraordinary electrical properties due to the high electron mobility at room temperature [38]. In terms of mechanical properties graphene exhibit greater performances when single or few layer graphene are employed. Therefore, the superior mechanical properties of graphene and its derivatives make them the ideal candidates for incorporation into a large variety of materials in order to produce composites with enhanced properties.

Graphene also exhibits other high characteristics such as large specific surface are, high transparency and high thermal conductivity. Because of its high surface area graphene also finds potential applications as support material in catalysis field as an electrode material in electrochemical applications such as supercapacitors and batteries [39].

Due to these extraordinary properties, graphene found already a great number of important applications with potential used in touch screens displays, fuel cells, intelligent coatings, transparent conductive films and flexible electronics [40]. In addition, once functionalized with biomolecules like polysaccharides [41], proteins [42], etc. or other biological systems graphene can be integrated for developing new applications in biomedicine and bio-nanotechnology such as biosensors [43], biocatalysis [44], biofuel cells [45], etc.

between the intensity of D band and the intensity of G band (Id/Ic) indicates that some disorder of the graphene sheets is induced. As previously mentioned graphite consists of stacks of planar sheets of graphene and carbon nanotubes are basically rolled-up graphene sheets. Therefore the presence of the 2D band (also named G` band) is observed in all spectra of the investigated samples which features the arrangement and the number of graphene layers [30]. The importance of the 2D band of graphene will be better explained in section 2 of this chapter. The radial breathing mode (RBM) is particularly important for the determination of the diameter of CNT, its frequency being related to the aggregation state of SWCNTs. The RBM band is unique to SWNCTs and corresponds to the expansion and contraction of the nanotubes. Comparing the Raman spectrum of MWCNTs to that of SWCNTs some important differences can be easily noticed: the absence of RBM mode in MWCNTs spectrum and much sharper D peak in MWNCTs. The RBM band is not present in case of MWNCTs due to the outer tubes that restrict the breathing mode. The presence of a more outlined D band in case of MWCNTs is observed because of the multilayer configuration of nanotubes suggesting a more disordered structure. In addition, a sharper D+G combination peak strongly supports

**Figure 4.** The Raman spectra of a) graphite, b) single wall carbon nanotubes (SWCNTs) and c) multiwall carbon

Even if for several decades the isolation of single layer graphite seemed to be impossible two researchers from Manchester University successfully managed in 2004 to isolate monolayer graphite. The "scotch-tape" technique reported by Geim and Novoselov consisted in obtaining

the presence of higher disorder in the MWCNTs, compared to SWCNTs.

**2. Graphene and graphene oxide**

nanotubes (MWCNTs).

184 Raman Spectroscopy

It is worth noting that the electronic properties of graphene drastically depend on the number of graphene layers. For that reason, the graphene community distinguishes between monolayer graphene, bilayer or few-layer graphene. A structure composed of more than 10 graphene layers exhibits the electronic properties of graphite and therefore is considered as a thin film of graphite. Being transparent as well as a good conductor, graphene may replace the electrodes in the indium used in touchscreens [46].

In order to integrate graphene into various functional structures or other materials for making performant nanodevices one preliminary condition is required: graphene sheets have to be exfoliated into individual or few-layer sheets and stabilized. Also, unwanted by-products and structural damage can be produced while synthesizing graphene. A quick and precise method for determining the number of layers of graphene sheets is essential for speeding up the research and exploration of graphene. In sp2-bonded carbon species, as a highly sensitive and non-destructive technique, Raman spectroscopy can be used to investigate the number of layers, the type and relative quantity of defects, mechanical strain, and any further functionalization. Therefore Raman spectroscopy is one of the most powerful tools available for analysing graphene.

In specific case of graphene, Raman spectroscopy can evaluate not only the number of graphene layers, but also can provide a quick and non-destructive means to distinguish monolayer, bilayer and few layer graphene. The most prominent Raman features of graphene are the so-called G band and 2D as easily seen in **Figure 5**, which depicts a typical Raman spectrum for graphite and graphene respectively obtained using a 514 nm excitation laser.

Raman spectrum of graphite in which the 2D band is formed from two elements, namely 2D1

exhibits a single sharp 2D signal, approximately four times more intense than G band [47]. When more graphene layers are present, the 2D peak is shifted to higher frequencies due to

Many studies showed that Raman spectroscopy can be used as an indicator for single or multi-layer graphene [48]. As one can see the 2D peak evolves as the number of graphene layers increases to about ten layers upon its profile resembles with that of graphite. As the number of graphene layers increases an important reduction of the relatively intensity for the

mode is noticed. Therefore graphene stacks that have more than five layers are more dif-

It is worth pointing out that this technique for identifying the number of graphene sheets is precisely established only for graphene with AB Bernal stacking [49]. Graphene samples that exhibit AB Bernal stacking features are graphene layers where half of their atoms lie directly over the center of a hexagon in the lower graphene sheet, and half of the atoms lie over an atom. Bernal stacked bilayer graphene exhibit much interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical electrical field [50]. Such type of samples are obtained from highly oriented pyrolytic graphite (HOPG) produced by mechanical exfoliation. Also chemical vapor deposition (CVD) or thermal deposition of SiC can be used to synthetize bilayer graphene but these procedures do not

Since the graphene flakes have small dimensions it is important to select a Raman instrument with high microscopy performances. Consequently for more accurate results most Raman measurements are performed using an optical microscope which allows a better localization

**Figure 6.** (a) and (b) The evolution of G band by increasing the number of layers, (c) and (d) The evolution of the 2D band by increasing the number of graphene layers using 514 and 633 nm excitation laser (Ferrari A (2007) copyrights).

ficult to discriminate from graphite by Raman spectroscopy (**Figure 6**).

[45], which are roughly ¼ and ½ of the height of the G peak, respectively. Graphene

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and 2D2

2D1

the interactions between the graphene layers.

lead to homogeneously AB stacking layers.

Graphite consists of sp2 bonded planar graphene sheets stacked through Van der Waals intermolecular forces. When comparing the Raman spectra of graphite and graphene one can observe a tremendous similarity. The reason for that is the fact that graphite consists in multiple stacked graphene layers. As in the case of graphite, the G mode from graphene occurs around 1580 cm−1 and the 2D signal is situated around 2700 cm−1. Two further signals can be noticed: the D band that is observed at ~1350 cm−1 and 2D` band at ~3250 cm−1.

In terms of vibrational behavior, the G band originates from in-plane stretching vibrations of sp2 carbon atoms in both rings and chains. Even at low intensity the D mode can be observed in the Raman spectrum of graphene which occurs due to the breathing modes of sp2 carbon atoms rings. Generally the D mode is associated with the presence of graphene structural defects. When the D band is higher it means that the sp2 bonds are broken and new sp<sup>3</sup> bonds are created. Consequently the increase of the ratio between the intensity of D band and the intensity of G band (Id/Ic) demonstrates that new defects are created during the modification of pristine graphene.

The D mode is almost absent in well-ordered structure of graphene and graphite. Despite the similarities, there are some significant differences as one can notice in **Figure 5**. In case of pure graphene the 2D band situated at ~2700 cm−1 is much sharper. The 2D band originates from a two-phonon double resonance process and it is interrelated to the band structure of graphene layers. **Figure 5(b)** indicates a significant change in the shape and intensity of the 2D band of graphene compared to graphite. It can be easily observed that in case of graphene the 2D peak is much narrower and its position is down-shifted. Another difference is observed in the

**Figure 5.** (a) The Raman spectra of graphene and graphite measured at 514.5 nm. (b) 2D peaks in graphene and graphite (Ferrari A (2007) copyrights).

Raman spectrum of graphite in which the 2D band is formed from two elements, namely 2D1 and 2D2 [45], which are roughly ¼ and ½ of the height of the G peak, respectively. Graphene exhibits a single sharp 2D signal, approximately four times more intense than G band [47]. When more graphene layers are present, the 2D peak is shifted to higher frequencies due to the interactions between the graphene layers.

In specific case of graphene, Raman spectroscopy can evaluate not only the number of graphene layers, but also can provide a quick and non-destructive means to distinguish monolayer, bilayer and few layer graphene. The most prominent Raman features of graphene are the so-called G band and 2D as easily seen in **Figure 5**, which depicts a typical Raman spectrum for graphite and graphene respectively obtained using a 514 nm excitation laser.

molecular forces. When comparing the Raman spectra of graphite and graphene one can observe a tremendous similarity. The reason for that is the fact that graphite consists in multiple stacked graphene layers. As in the case of graphite, the G mode from graphene occurs around 1580 cm−1 and the 2D signal is situated around 2700 cm−1. Two further signals can be

In terms of vibrational behavior, the G band originates from in-plane stretching vibrations of

atoms rings. Generally the D mode is associated with the presence of graphene structural

are created. Consequently the increase of the ratio between the intensity of D band and the intensity of G band (Id/Ic) demonstrates that new defects are created during the modification

The D mode is almost absent in well-ordered structure of graphene and graphite. Despite the similarities, there are some significant differences as one can notice in **Figure 5**. In case of pure graphene the 2D band situated at ~2700 cm−1 is much sharper. The 2D band originates from a two-phonon double resonance process and it is interrelated to the band structure of graphene layers. **Figure 5(b)** indicates a significant change in the shape and intensity of the 2D band of graphene compared to graphite. It can be easily observed that in case of graphene the 2D peak is much narrower and its position is down-shifted. Another difference is observed in the

**Figure 5.** (a) The Raman spectra of graphene and graphite measured at 514.5 nm. (b) 2D peaks in graphene and graphite

in the Raman spectrum of graphene which occurs due to the breathing modes of sp2

carbon atoms in both rings and chains. Even at low intensity the D mode can be observed

noticed: the D band that is observed at ~1350 cm−1 and 2D` band at ~3250 cm−1.

defects. When the D band is higher it means that the sp2

bonded planar graphene sheets stacked through Van der Waals inter-

bonds are broken and new sp<sup>3</sup>

carbon

bonds

Graphite consists of sp2

186 Raman Spectroscopy

of pristine graphene.

(Ferrari A (2007) copyrights).

sp2

Many studies showed that Raman spectroscopy can be used as an indicator for single or multi-layer graphene [48]. As one can see the 2D peak evolves as the number of graphene layers increases to about ten layers upon its profile resembles with that of graphite. As the number of graphene layers increases an important reduction of the relatively intensity for the 2D1 mode is noticed. Therefore graphene stacks that have more than five layers are more difficult to discriminate from graphite by Raman spectroscopy (**Figure 6**).

It is worth pointing out that this technique for identifying the number of graphene sheets is precisely established only for graphene with AB Bernal stacking [49]. Graphene samples that exhibit AB Bernal stacking features are graphene layers where half of their atoms lie directly over the center of a hexagon in the lower graphene sheet, and half of the atoms lie over an atom. Bernal stacked bilayer graphene exhibit much interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical electrical field [50]. Such type of samples are obtained from highly oriented pyrolytic graphite (HOPG) produced by mechanical exfoliation. Also chemical vapor deposition (CVD) or thermal deposition of SiC can be used to synthetize bilayer graphene but these procedures do not lead to homogeneously AB stacking layers.

Since the graphene flakes have small dimensions it is important to select a Raman instrument with high microscopy performances. Consequently for more accurate results most Raman measurements are performed using an optical microscope which allows a better localization

**Figure 6.** (a) and (b) The evolution of G band by increasing the number of layers, (c) and (d) The evolution of the 2D band by increasing the number of graphene layers using 514 and 633 nm excitation laser (Ferrari A (2007) copyrights).

of the graphene layers. Raman microscopy couples a Raman spectrometer to a standard optical microscope, allowing high magnification visualization of graphene and Raman analysis with a microscopic laser spot.

and ultrasonic cleavage graphite oxide is obtained, at the end presenting on its surface a significant number of functional groups such as hydroxyl, carboxyl and epoxy [57]. The introduction of oxygenated functionalities not only expands the layer separation, but also makes the material hydrophilic and relatively easy to disperse in aqueous media or other polar solvents [58]. This property enables graphite oxide to exfoliate in hydrophilic medium under sonication and finally to produce single or few layer graphene oxide (GO). Consequently the

In the last decade GO attracted the researchers attention due to its many important properties which can be used to tailor novel applications. Graphene oxide sheets exhibit high design flexibility. GO is decorated with a significant number of carboxyl groups (-COOH) and most of them are located at the GO edges. These carboxyl groups are extremely useful as they can easily react for attachment of various functionalities. Thus, reactions may be established with (a) amines and various organic molecules or polymers which exhibit in their composition amino groups, by forming an amide linkage, (b) alcohols, phenols or epoxy groups to form ester bond, (c) various other organic reactive macromolecules, resulting in the functionalization of GO.

Functionalization of GO can fundamentally change graphene oxide's properties and consequently, graphene oxide's applications. Graphene oxide flakes can be used to remove radioactive ions from water for disposal [59]. Also, graphene oxide can be used to develop sensors that can detect tumorous cells by attaching to GO surfaces molecules that contain antibodies that are further linked to the cancer cells. The cancer cells are then tagged with fluorescent molecules to make the cancer cells stand out in a microscope [60]. Withal graphene oxide is used to obtain anodes for rechargeable lithium-ion batteries. The graphene oxide is thermally treated in order to extract the oxygen form the film and driven to cause pores in the film which are rapidly filled with lithium ions, resulting in quicker charge-discharge process for batteries [61].

With respect to electrical conductivity, graphene oxide behaves as an electrical insulator,

oxide so as to restore the honeycomb hexagonal lattice of graphene, in order to recover electrical conductivity. The product of this reduction reaction has been named in different ways, including: reduced graphene oxide (rGO), chemically-reduced graphene oxide (CRGO), and graphene. For the purposes of clarity, we will refer to the product as reduced graphene oxide (rGO). Chemical reduction of graphene oxide is mostly employed in the presence of hydra-

production. Usually chemical reduction agents are classified as toxic or corrosive. The electrochemical method to reduce graphene oxide in order to produce large rGO films is a greener, safer and more convenient procedure for reducing graphene oxide films. Also the thermal expansion of graphite oxide can be used for exfoliating graphite layers and finally to produce functionalized graphene sheets. Temperatures around 550°C or higher can break the Van der Waals forces that stack the graphene layers together and exfoliation occurs. After the reduction of graphene oxide defect sites within the lattice are produced which provide new routes for chemical functionalization. Chemical modification of the graphene oxide by functionalization with different other molecules or polymers opens new routes for the incorporation of

) where the majority of the oxidized groups of GO are reduced. But the use of

demands a dry environment which creates difficulties for the large-scale

bonding networks. It is important to reduce the graphene

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because of the disruption of its sp2

H4

zine (N2

H4

anhydrous N2

main difference between graphite oxide and graphene oxide is the number of layers.

However, the addition of a microscope to a Raman spectrometer does not provide full 3D spatial resolution. More recently, confocal Raman microscopy (CRM) was used in order to investigate graphene layers [51]. As the name suggests confocal Raman microscopy refers to the ability to spatially filter the analysis volume of the sample, in the XY (lateral) and Z (depth) axes. A confocal microscope practically designs clear images of a sample by removing most of the light from the investigated sample [52]. Apart from allowing better observation of fine details of the sample CRM gives rich information concerning the distribution of individual chemical components, and variation in other effects such as phase, polymorphism, stress/strain, and crystallinity. Based on thousands of Raman spectra acquired from different positions on the sample Raman spectral mapping can be created generating detailed chemical images.

Moreover, the substrate on which the graphene samples are deposited and the Raman equipment performances has a significant role for graphene investigation. Monolayer graphene can be observed on many types of substrates like sapphire, single crystal quartz, glass, metal alloys like NiFe, or polymers like polytetrafluoroethylene (PTFE), but the most popular substrate surface used to discriminate the monolayer graphene is the silicon wafer with a silicon dioxide layer (SiO2 /Si). The Si/SiO2 substrate usually with 300 nm thickness was reported in many studies as the most appropriate for visual detection of single layer graphene (SLG) [53]. Additionally, it is important to study the interaction between monolayer graphene and the substrate because possible interaction may appear due to defects or surface changes between the two interfaces. In case of monolayer graphene obtained by epitaxial growth from SiC substrates a strong interaction between graphene and SiC substrate appears by strong blueshifting the G and 2D signals (~11 cm−1 and ~34 cm−1, respectively) due to covalent bonding of the two interfaces [54]. On the other hand, the interactions between graphene produced by micro-cleaving and standard SiO2 /Si substrate do not influence the physical structure of graphene because only weak Van der Waals forces could appear [55].

It was found that the Raman spectrum depends not only on the substrate, but also on the wavelength of the excitation laser. Regarding the laser excitation energy for graphene investigation usually the visible lights laser are employed (from 633 to 473 nm). The near infrared (NIR) or ultraviolet (UV) sources are not frequently used. When analyzing graphene layers placed on silicon wafers with silicon dioxide (SiO2 /Si) strong fluorescence signals are observed using a NIR laser (780 or 785 nm). Also, graphene layers are difficult to investigate using a UV excitation laser (from 244 to 364 nm) due to the fact that the obtained Raman spectrum exhibits differences concerning relative intensities of the characteristic graphene signals. Not least the selection of the excitation source is important. In order to avoid sample damaging usually powers between 0.04 to 4 mW are employed. At higher laser power it was observed that the laser may burn the graphene sample leading to graphitization and therefore to spectral variations. Also, if lower laser power is used the ratio between the Raman signal and noise is very poor [56].

Graphene oxide (GO) is another graphene material that can be intense characterized by Raman spectroscopy. By chemical oxidation of graphite in the presence of strong oxidizing agents and ultrasonic cleavage graphite oxide is obtained, at the end presenting on its surface a significant number of functional groups such as hydroxyl, carboxyl and epoxy [57]. The introduction of oxygenated functionalities not only expands the layer separation, but also makes the material hydrophilic and relatively easy to disperse in aqueous media or other polar solvents [58]. This property enables graphite oxide to exfoliate in hydrophilic medium under sonication and finally to produce single or few layer graphene oxide (GO). Consequently the main difference between graphite oxide and graphene oxide is the number of layers.

of the graphene layers. Raman microscopy couples a Raman spectrometer to a standard optical microscope, allowing high magnification visualization of graphene and Raman analysis

However, the addition of a microscope to a Raman spectrometer does not provide full 3D spatial resolution. More recently, confocal Raman microscopy (CRM) was used in order to investigate graphene layers [51]. As the name suggests confocal Raman microscopy refers to the ability to spatially filter the analysis volume of the sample, in the XY (lateral) and Z (depth) axes. A confocal microscope practically designs clear images of a sample by removing most of the light from the investigated sample [52]. Apart from allowing better observation of fine details of the sample CRM gives rich information concerning the distribution of individual chemical components, and variation in other effects such as phase, polymorphism, stress/strain, and crystallinity. Based on thousands of Raman spectra acquired from different positions on the sample

Moreover, the substrate on which the graphene samples are deposited and the Raman equipment performances has a significant role for graphene investigation. Monolayer graphene can be observed on many types of substrates like sapphire, single crystal quartz, glass, metal alloys like NiFe, or polymers like polytetrafluoroethylene (PTFE), but the most popular substrate surface used to discriminate the monolayer graphene is the silicon wafer with a silicon

many studies as the most appropriate for visual detection of single layer graphene (SLG) [53]. Additionally, it is important to study the interaction between monolayer graphene and the substrate because possible interaction may appear due to defects or surface changes between the two interfaces. In case of monolayer graphene obtained by epitaxial growth from SiC substrates a strong interaction between graphene and SiC substrate appears by strong blueshifting the G and 2D signals (~11 cm−1 and ~34 cm−1, respectively) due to covalent bonding of the two interfaces [54]. On the other hand, the interactions between graphene produced

It was found that the Raman spectrum depends not only on the substrate, but also on the wavelength of the excitation laser. Regarding the laser excitation energy for graphene investigation usually the visible lights laser are employed (from 633 to 473 nm). The near infrared (NIR) or ultraviolet (UV) sources are not frequently used. When analyzing graphene layers placed on

NIR laser (780 or 785 nm). Also, graphene layers are difficult to investigate using a UV excitation laser (from 244 to 364 nm) due to the fact that the obtained Raman spectrum exhibits differences concerning relative intensities of the characteristic graphene signals. Not least the selection of the excitation source is important. In order to avoid sample damaging usually powers between 0.04 to 4 mW are employed. At higher laser power it was observed that the laser may burn the graphene sample leading to graphitization and therefore to spectral variations. Also, if lower

Graphene oxide (GO) is another graphene material that can be intense characterized by Raman spectroscopy. By chemical oxidation of graphite in the presence of strong oxidizing agents

laser power is used the ratio between the Raman signal and noise is very poor [56].

substrate usually with 300 nm thickness was reported in

/Si substrate do not influence the physical structure of

/Si) strong fluorescence signals are observed using a

Raman spectral mapping can be created generating detailed chemical images.

/Si). The Si/SiO2

graphene because only weak Van der Waals forces could appear [55].

by micro-cleaving and standard SiO2

silicon wafers with silicon dioxide (SiO2

with a microscopic laser spot.

188 Raman Spectroscopy

dioxide layer (SiO2

In the last decade GO attracted the researchers attention due to its many important properties which can be used to tailor novel applications. Graphene oxide sheets exhibit high design flexibility. GO is decorated with a significant number of carboxyl groups (-COOH) and most of them are located at the GO edges. These carboxyl groups are extremely useful as they can easily react for attachment of various functionalities. Thus, reactions may be established with (a) amines and various organic molecules or polymers which exhibit in their composition amino groups, by forming an amide linkage, (b) alcohols, phenols or epoxy groups to form ester bond, (c) various other organic reactive macromolecules, resulting in the functionalization of GO.

Functionalization of GO can fundamentally change graphene oxide's properties and consequently, graphene oxide's applications. Graphene oxide flakes can be used to remove radioactive ions from water for disposal [59]. Also, graphene oxide can be used to develop sensors that can detect tumorous cells by attaching to GO surfaces molecules that contain antibodies that are further linked to the cancer cells. The cancer cells are then tagged with fluorescent molecules to make the cancer cells stand out in a microscope [60]. Withal graphene oxide is used to obtain anodes for rechargeable lithium-ion batteries. The graphene oxide is thermally treated in order to extract the oxygen form the film and driven to cause pores in the film which are rapidly filled with lithium ions, resulting in quicker charge-discharge process for batteries [61].

With respect to electrical conductivity, graphene oxide behaves as an electrical insulator, because of the disruption of its sp2 bonding networks. It is important to reduce the graphene oxide so as to restore the honeycomb hexagonal lattice of graphene, in order to recover electrical conductivity. The product of this reduction reaction has been named in different ways, including: reduced graphene oxide (rGO), chemically-reduced graphene oxide (CRGO), and graphene. For the purposes of clarity, we will refer to the product as reduced graphene oxide (rGO). Chemical reduction of graphene oxide is mostly employed in the presence of hydrazine (N2 H4 ) where the majority of the oxidized groups of GO are reduced. But the use of anhydrous N2 H4 demands a dry environment which creates difficulties for the large-scale production. Usually chemical reduction agents are classified as toxic or corrosive. The electrochemical method to reduce graphene oxide in order to produce large rGO films is a greener, safer and more convenient procedure for reducing graphene oxide films. Also the thermal expansion of graphite oxide can be used for exfoliating graphite layers and finally to produce functionalized graphene sheets. Temperatures around 550°C or higher can break the Van der Waals forces that stack the graphene layers together and exfoliation occurs. After the reduction of graphene oxide defect sites within the lattice are produced which provide new routes for chemical functionalization. Chemical modification of the graphene oxide by functionalization with different other molecules or polymers opens new routes for the incorporation of

In the Raman spectra of the studied graphene oxide structures one can observe that the G band is much broader than in case of graphene and also blue-shifted to ~1590 cm−1 (**Figure 8**). The D band from graphene oxide Raman spectra is also modified exhibiting a much higher

also due to the attachment of hydroxyl and epoxide groups on the planar carbon structure. Depending on the functionalization degree of graphene oxide, the Raman spectrum may exhibit sometimes even stronger D band than G band. Regarding the 2D band its intensity is very small compared to the D and G peaks, but may be enhanced by reducing the number of graphene oxide layers. The D+G combination peak is also observed which strongly supports

Polymeric composites are biphasic materials consisting essentially of a continuous phase, commonly referred to as polymer matrix, and a reinforcing or filler agent, which is the discrete phase. The purpose of this association is to obtain materials with enhanced properties, superior to those of individual components, capable of replacing natural materials (wood, rocks, etc.), aluminum and its alloys, and other metallic materials. Polymeric composites are obtained from a wide range of matrices (epoxy resins, polyester resins, phenol-formaldehyde resins, vinyl polymers, elastomers, polyimides, etc.) with reinforcing materials (boron fibers,

Nanocomposites represent a new class of composites, characterized by the coexistence of two distinct phases (an organic one which is the polymer as the continuous phase and an inorganic phase dispersed in the continuous phase, the latter exhibiting nanometric dimensions). The advantages of these structures consist in global properties superior to the individual components such as improved optical clarity, high mechanical resistance, better conductivity, leading to important uses in electronics, optics, constructions, etc. In order to obtain nanocomposites, two important aspects should be considered: firtsly, the nanoparticle must be compatible with the polymer and to show satisfactory interfacial interaction; secondly, the most convenient way to uniformly disperse the nanoparticles in the polymer matrix should be chosen. In most cases polymeric nanomaterials exhibit multifunctionality by combining

When the polymer is unable to intercalate between the graphene layers, a phase separation (two distinct phases) is obtained, the properties of which resemble the microcompounds. In addition to this class, two other types of composites can be prepared: intercalated structures where most of the time a single polymer chain is interposed between layers of graphene, resulting in a multilayered structure in which the polymer-graphene layers alternate and exfoliated structures in which the graphene layers are completely dispersed in the continuous

structure induced after the oxidation of graphite and

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intensity due to the disorder in the sp2

**nanocomposites**

more than one properties.

polymer matrix (**Figure 9**).

the presence of a higher disorder structure for graphene oxide.

**3. Raman investigation of graphene and graphene oxide** 

glass fibers, or filler materials (wood flour, starch, silica, talc, asbestos, etc.).

**Figure 7.** Chemical structures of graphene oxide investigated by Raman spectroscopy.

**Figure 8.** Raman spectra of various graphene oxides.

graphene in other matrices and surfaces to enhance its applicability, that which would otherwise be more difficult using pristine graphene.

Being a derivative of graphene, the graphene oxide structure and functionalized graphene oxide can be also successfully characterized by Raman spectroscopy. For this study different structures of commercial graphene oxide (**Figure 7**) were investigated using a Raman spectrometer equipped with confocal microscope.

In the Raman spectra of the studied graphene oxide structures one can observe that the G band is much broader than in case of graphene and also blue-shifted to ~1590 cm−1 (**Figure 8**). The D band from graphene oxide Raman spectra is also modified exhibiting a much higher intensity due to the disorder in the sp2 structure induced after the oxidation of graphite and also due to the attachment of hydroxyl and epoxide groups on the planar carbon structure. Depending on the functionalization degree of graphene oxide, the Raman spectrum may exhibit sometimes even stronger D band than G band. Regarding the 2D band its intensity is very small compared to the D and G peaks, but may be enhanced by reducing the number of graphene oxide layers. The D+G combination peak is also observed which strongly supports the presence of a higher disorder structure for graphene oxide.
