**3. Results**

The structural simplicity of graphene is also exhibited in its Raman spectrum in contrast to its other fullerene relatives [11]. The two prominent bands located at 1580 and 2700 cm<sup>−</sup><sup>1</sup> are customarily called the G and G′-bands, respectively [11]. The high energy first order G-band has been identified with the intra-planar stretching modes of the strongly connected σ-bonded carbons [5]. The G′-band at 2700 cm<sup>−</sup><sup>1</sup> is attributed to a second order Raman scattering event with the phonon wave vector q ≠ 0 [5]. **Figure 7** shows both bands obtained from a graphene sample on a nickel substrate.

Discerning the two-dimensional nature of graphene can be accomplished by contrasting the G′-band features of graphite and the former material [11]. First, the relative intensities between the G and G′ bands are different for graphene and its macroscopic relative graphite. In the case of graphene, the G′-band has a greater intensity than the G-band, which is the case for G-bands illustrated in **Figures 7** and **8**. The G′-band of graphite is also shifted to a higher frequency compared to that of graphene [11]. Thirdly, the overall shape of the G′-band is usually more uniform compared to that of graphite, usually requiring a single Lorentzian to be fitted [11]. This last effect especially arises due to interactions among the multiple layers of graphite [11]. The Raman spectrum of the graphene sample was recently collected on an aged sample, and the degradation and contamination of this extremely thin material over time may be responsible for our Raman spectra of graphene and graphite only satisfying the first of these three criteria convincingly.

Not only the dimensionality, or number of layers present can be obtained via the Raman bands of graphene or graphite, but the average lateral characteristic size can of the graphene layers in the beam spot can also be determined. This was initially

#### **Figure 7.**

*(Top) Raman spectrum of CVD graphene on nickel substrate collected using 514 nm laser excitation. (Bottom left) G-band and Lorentzian (1582.4 cm<sup>−</sup><sup>1</sup> , height: 11,655.9). (Bottom right) G′-band and Lorentzians (2709.6 cm<sup>−</sup><sup>1</sup> , 2759.6 cm<sup>−</sup><sup>1</sup> , heights: 19,669.9, 1856.6).*

**13**

**Figure 8.**

**Figure 9.**

*Plot of (ID/IG)\*(EL)4*

*(1579.7 cm<sup>−</sup><sup>1</sup>*

*Lorentzians (2611.8 cm<sup>−</sup><sup>1</sup>*

*, 2651 cm<sup>−</sup><sup>1</sup>*

*Raman Spectroscopy of Graphene, Graphite and Graphene Nanoplatelets*

discovered by Tuinstra and Koenig, who correctly deduced that the intensity ratio of the D and G-bands varies directly with the characteristic size La of the planar graphite crystallites [12]. Further work done by Cancado et al. [13], expanded on Tuinstra and Koenig's work, by demonstrating the excitation energy dependence of the proportionality factor in the original relation as shown in **Figure 9** and as expressed in Eq. (5).

*intensities, respectively, and La is the characteristic lateral size of the graphene layer. Adapted from [5].*

 *vs. 1/La. EL is the laser excitation energy in eV, ID and IG are the D and G band* 

*(Top) Raman spectrum of HOPG graphite at excitation of 780 nm. (Bottom left) G-band and Lorentzian* 

*, heights: 56.1, 123.1).*

*, height: 128.9); (Bottom middle) SEM image of HOPG sample; (Bottom right) G′-band and* 

For the graphene sample in **Figure 7** with a D-band intensity of 2719.7 and the graphite sample in **Figure 8** with a D-band intensity of 8.6, the respective La values

( \_\_ *ID IG*) −1

(5)

*La* = (2.4 × 10−10) λ<sup>4</sup>

are 71.8 nm and 1.3 μm according to Eq. (5) (**Table 1**).

*DOI: http://dx.doi.org/10.5772/intechopen.84527*

*Raman Spectroscopy of Graphene, Graphite and Graphene Nanoplatelets DOI: http://dx.doi.org/10.5772/intechopen.84527*

#### **Figure 8.**

*2D Materials*

**3. Results**

and 2700 cm<sup>−</sup><sup>1</sup>

display sample structure; SEM magnification ranges between 25 and 1,000,000×. The modular software program Gwyddion was used to generate three-dimensional

The structural simplicity of graphene is also exhibited in its Raman spectrum in contrast to its other fullerene relatives [11]. The two prominent bands located at 1580

energy first order G-band has been identified with the intra-planar stretching modes

uted to a second order Raman scattering event with the phonon wave vector q ≠ 0 [5]. **Figure 7** shows both bands obtained from a graphene sample on a nickel substrate. Discerning the two-dimensional nature of graphene can be accomplished by contrasting the G′-band features of graphite and the former material [11]. First, the relative intensities between the G and G′ bands are different for graphene and its macroscopic relative graphite. In the case of graphene, the G′-band has a greater intensity than the G-band, which is the case for G-bands illustrated in **Figures 7** and **8**. The G′-band of graphite is also shifted to a higher frequency compared to that of graphene [11]. Thirdly, the overall shape of the G′-band is usually more uniform compared to that of graphite, usually requiring a single Lorentzian to be fitted [11]. This last effect especially arises due to interactions among the multiple layers of graphite [11]. The Raman spectrum of the graphene sample was recently collected on an aged sample, and the degradation and contamination of this extremely thin material over time may be responsible for our Raman spectra of graphene and

of the strongly connected σ-bonded carbons [5]. The G′-band at 2700 cm<sup>−</sup><sup>1</sup>

graphite only satisfying the first of these three criteria convincingly.

Not only the dimensionality, or number of layers present can be obtained via the Raman bands of graphene or graphite, but the average lateral characteristic size can of the graphene layers in the beam spot can also be determined. This was initially

*(Top) Raman spectrum of CVD graphene on nickel substrate collected using 514 nm laser excitation. (Bottom left)* 

*, height: 11,655.9). (Bottom right) G′-band and Lorentzians (2709.6 cm<sup>−</sup><sup>1</sup>*

are customarily called the G and G′-bands, respectively [11]. The high

is attrib-

*,* 

visualization of the nanoplatelet aggregate structures.

**12**

**Figure 7.**

*2759.6 cm<sup>−</sup><sup>1</sup>*

*G-band and Lorentzian (1582.4 cm<sup>−</sup><sup>1</sup>*

*, heights: 19,669.9, 1856.6).*

*(Top) Raman spectrum of HOPG graphite at excitation of 780 nm. (Bottom left) G-band and Lorentzian (1579.7 cm<sup>−</sup><sup>1</sup> , height: 128.9); (Bottom middle) SEM image of HOPG sample; (Bottom right) G′-band and Lorentzians (2611.8 cm<sup>−</sup><sup>1</sup> , 2651 cm<sup>−</sup><sup>1</sup> , heights: 56.1, 123.1).*

**Figure 9.** *Plot of (ID/IG)\*(EL)4 vs. 1/La. EL is the laser excitation energy in eV, ID and IG are the D and G band intensities, respectively, and La is the characteristic lateral size of the graphene layer. Adapted from [5].*

discovered by Tuinstra and Koenig, who correctly deduced that the intensity ratio of the D and G-bands varies directly with the characteristic size La of the planar graphite crystallites [12]. Further work done by Cancado et al. [13], expanded on Tuinstra and Koenig's work, by demonstrating the excitation energy dependence of the proportionality factor in the original relation as shown in **Figure 9** and as expressed in Eq. (5).

$$L\_a = \left\{2.4 \times 10^{-10}\right\} \lambda^4 \left(\frac{I\_D}{I\_G}\right)^{-1} \tag{5}$$

For the graphene sample in **Figure 7** with a D-band intensity of 2719.7 and the graphite sample in **Figure 8** with a D-band intensity of 8.6, the respective La values are 71.8 nm and 1.3 μm according to Eq. (5) (**Table 1**).

#### *2D Materials*


**Table 1.**

*Average x, y, z axis measurements of functionalized graphene nanoplatelet aggregates.*

#### **Figure 10.**

*3D view of SEM data of functionalized graphene nanoplatelet aggregates doped with argon (A), carboxyl (B), oxygen (C), ammonia (D), fluorocarbon (E), and nitrogen (F), respectively, using Gwyddion software.*

#### **Figure 11.**

*(a) D-band on left (Intensity: 33.5, Center: 1312.4 cm<sup>−</sup><sup>1</sup> ), (b) G-band on right (Intensity: 62.5, Center: 1580.2 cm<sup>−</sup><sup>1</sup> ) for graphene nanoplatelets (ammonia) with 780 nm excitation and using fityk peak fitting software [14].*

The observed sub-micron size of the platelets obtained mentioned above, and imaged in **Figure 10**, was also verified via Raman spectroscopy, based on the use of Eq. (3) for the graphene nanoplatelets functionalized with Ammonia, whose D and G bands are shown in **Figure 11**.

**15**

provided the original work is properly cited.

\*Address all correspondence to: pmisra@howard.edu

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Daniel Casimir, Hawazin Alghamdi, Iman Y. Ahmed, Raul Garcia-Sanchez

Laser Spectroscopy Laboratory, Howard University, Washington, DC, USA

*Raman Spectroscopy of Graphene, Graphite and Graphene Nanoplatelets*

For an excitation wavelength of 780 nm, the characteristic size La, for this sample calculates to a value of 0.17 μm. This characteristic sheet size corresponds with the dimensions for the aggregate samples shown in **Figure 10**. This value is also closer in magnitude to the calculated La value for graphite, than that for graphene, due to the greater chance for multiple stacked sheets among the graphene nano-

To recap, in this chapter we have discussed the ability to discern whether certain graphitic nanomaterials are primarily 2 or 3 dimensional in character, based on features of their Raman bands. For all three materials (namely graphene, graphite, and functionalized graphene nanoplatelets), we have made use of Tuinstra and Koenig's relationship between the intensities of the D and G Raman bands to characterize the nanomaterials. In addition to the analysis based on Raman spectroscopy, SEM visualization/dimensional analysis was also performed on the graphene nanoplatelet samples. To conclude, the bulk macroscopic 3D character of graphite was clearly apparent compared to the 2D nature of graphene. However, based on the results for the graphene nanoplatelets, both 2D and 3D characteristics/behaviors were present

Financial support from the National Science Foundation (Award# PHY-1358727

*DOI: http://dx.doi.org/10.5772/intechopen.84527*

platelets to be responsive to the measurements.

for them, without one dimension dominating the other.

and PHY-1659224) is gratefully acknowledged.

**4. Conclusion**

**Acknowledgements**

**Conflict of interest**

**Author details**

and Prabhakar Misra\*

No conflict of interest.

*Raman Spectroscopy of Graphene, Graphite and Graphene Nanoplatelets DOI: http://dx.doi.org/10.5772/intechopen.84527*

For an excitation wavelength of 780 nm, the characteristic size La, for this sample calculates to a value of 0.17 μm. This characteristic sheet size corresponds with the dimensions for the aggregate samples shown in **Figure 10**. This value is also closer in magnitude to the calculated La value for graphite, than that for graphene, due to the greater chance for multiple stacked sheets among the graphene nanoplatelets to be responsive to the measurements.
