Raman Spectroscopy and Mapping Analysis of Low-Dimensional Nanostructured Materials and Systems

This chapter describes the use of Raman spectroscopy and mapping analysis for the characterization of low dimensional nanostructures, including 2D sheets (graphene oxide, graphene sheets, MoS 2 , siloxene), and one-dimensional carbyne chains. The Raman mapping analysis and their application towards understanding the molecular level interactions in these low dimensional materials, nanostructured polymer composites, and nanopaints are also discussed. The stoichiometric com-position and structure of these low dimensional materials were correlated with the Raman spectral and mapping analysis. Further, Raman spectroscopy for understanding or probing the mechanism of mechanical to electrical energy harvesting properties of carbyne films via the structural transformation from cumulene to polynne networks of carbyne is demonstrated.


Introduction
Raman spectroscopy is a promising non-destructive testing of materials to understand their crystallinity, chemical bonding vibrations and effects of surface defects [1,2]. The Raman spectroscopy becomes an important technique for the characterization of nanostructured materials, especially the two-dimensional (2D) materials (such as graphene, MoS 2 , siloxene, metalenes), biomaterials, composites, and for understanding energy-conversion process in the recent years [3][4][5]. Additionally, these techniques are useful for criminological & forensic applications, biomedical applications, and as well as bio-sensors for health care sectors [6][7][8]. The basic principle of Raman spectroscopy relies on the "Raman effect", i.e., the inelastic scattering of light which can directly probe vibration and rotational-vibration states of any molecules and/or materials [9]. In 1923, Smekal et al. predicted the Raman scattering of light using molecules explained via classical quantum theory which was experimentally observed by Raman and Krishnan in 1928 [10]. Based on this principle, nearly 25 types of Raman spectroscopic techniques are available for characterization of different materials for various applications. Some of them are (i) spontaneous Raman, (ii) hyper-Raman scattering, (iii) Fourier-transform Raman scattering, (iv) Raman-induced Kerr-effect spectroscopy and (v) stimulated-coherent Raman and so on [9]. There are numerous works discussed the fundamental principles and theory of Raman spectroscopy and their working mechanism were available in literature [11,12]. In addition to these, Raman mapping is often used for identification of various information such as crystallinity, homogeneity, defect sites and molecular level bonding for materials science research [13,14]. This book chapter focus on the use of Raman spectroscopy and mapping analysis for studying the molecular level vibrations in the 2D materials, composites, solid electrolyte entrapped in piezo-polymer matrix, low-dimensional materials such as carbyne chains, and probing method for elucidating the mechanism of energy harvesting in carbyne via mechanical stimuli.

Experimental section 2.1 Preparation of graphene oxide with different oxidation levels, graphene sheets and graphene nanopaint
The modified Hummers method is used for the preparation of graphene oxide sheets using the chemical reaction between graphite powders with potassium permanganate, and sulfuric acid. The oxidation degree of graphene oxide was varied via changing the concentration of potassium permanganate by keeping the remaining parameters constant as reported in our earlier work [15]. The graphene sheets were prepared via the reduction of graphene oxide using hydrazine hydrate in presence of ultrasound irradiation [16]. The graphene based nanopaint was obtained by mechanical milling process for 12 h using appropriate amount of graphene sheets (pigment) and alkyd resin binder. The graphene paint was coated on glass substrate using brush coating [17].

Preparation of 2D molybdenum disulphide nanosheets and quantum sheets
A hydrothermal method is used for the formation of MoS 2 on the surface of Mo foil (Mo source) using thiourea (sulfur source). The hydrothermal reaction process is carried out for 24 h at a temperature of 180 °C. The detailed experimental procedure can be seen from our reported work [18].

Preparation of 2D siloxene sheets
The siloxene nanosheets was obtained via topochemical de-intercalation reaction between calcium disilicide and conc. Hydrochloric acid at a temperature of 0 °C for four days [19].

Preparation of 2D antimonene
The 2D antimonene with nanodendrites structures anchored on the surface of the Ni foam was achieved via a facile electrochemical deposition process as mentioned in literature [20].

Preparation of proton conducting solid electrolyte-piezoelectric PVDF hybrids
The piezo-polymer electrolyte nanocomposite film made of phosphotungstic acid (PTA) solid electrolyte and PVDF were obtained by ultrasound irradiation Raman Spectroscopy and Mapping Analysis of Low-Dimensional Nanostructured Materials and… DOI: http://dx.doi.org /10.5772/intechopen.99775 followed by solvent casting method [16]. Appropriate amount of PVDF was dissolved in dimethylacetamide and acetone with the use of ultrasonication in which different weight ratios (5-25 wt%) of PTA electrolyte was added under mechanical stirring and ultrasound irradiation process. Then, the entire solution was transferred into a Petri dish and allowed to dry at 70 °C for complete evaporation of the solvents which led to the formation of PTA-PVDF piezo-electrolyte film via peel-off process.

Preparation of free standing carbyne-enriched carbon (CEC) films
The CEC film was prepared by immersing the free-standing PVDF film in a solution containing dehydrohalogenation mixture (potassium ethylate and tetrahydrofuran) in presence of ultrasound irradiation for 2 h [21,22]. The change in color from white (PVDF) into black (CEC) confirms the occurrence of dehydrohalogenation process. Then, the CEC film was rinsed with ethanol to removal chemical impurities and dried at 60 °C. The entire reaction was performed inside an Ar-filled glove box.

Raman spectral and mapping acquisition
The Raman spectral and mapping acquisition of the samples were carried out on a LabRam HR-Evolution Raman spectrometer (Horiba Jobin-Yvon, France). The Raman system used an Ar + ion laser operating at a laser power of 15 mW with an excitation wavelength of 514 nm. The Raman mapping of samples were performed over the desired area to obtain spectral arrays. The results are processed and analyzed using software. The spectral arrays map was processed and analyzed using classical least squares (CLS) fitting (multivariate analysis) method on LabSpec (Ver. 6.2) software.

Raman spectral studies of graphene oxide with various levels of oxidation
This section focused on the use of Raman spectroscopy as a promising tool for characterizing graphene-based materials and their system. It is well known that graphene sheets emerged as a material of this decade due to their wide-spread properties and applications in variety of sectors [23]. For preparation of gramscale graphene sheets, researchers often used graphitic oxide or graphene oxide as a starting material that is originally synthesized a century ago [24]. Graphene is a one-atom thick sheet in which hexagonal carbon chains are present laterally [25]. The structure of graphene oxide is similar to that of graphene in which the carbon atoms are bonded with different functional groups (hydroxyl, carbonyl, carboxyl, and epoxide) [26]. The formation of these groups occurred because of oxidation of graphite and removal of these groups lead to the formation of chemically derived graphenes.
In general, the Raman spectrum of graphite possess G band (first order scattering of the E 2g mode), D band cm −1 (defects), and 2D band (c-axis stacking order) at 1570, 1354, and 2700 cm −1 , respectively [1]. Figure 1(A) shows the Raman spectra of graphene oxide sheets with different oxidation degrees (S1-S6 shows the low to high oxidation content). It shows that the increase in oxidation degree results in the shift of G band position (in comparison with graphite) towards higher wavenumber and reaches to 1596 cm −1 for the heavily oxidized graphene oxide (Figure 1(B)) [15]. The full-width half maximum (FWHM) of the G band raised from 45 to 124 cm −1 with increase in oxidation level (Figure 1(B)). The shift in G band position and increase in FWHM is due to the formation of different levels of sp 3 hybridized carbons with respect to the oxidation process. Likewise, the D band in graphene oxide is broadened (increased FWHM) and highly intense in comparison with graphite, i.e., due to the formation more defects and disorders as a result of oxidation process [27]. The intensity of the 2D band reduces after oxidation process because of breaking of stacking order and new band is observed around 2950 cm −1 that is denoted as D + G band. At much higher oxidation levels, the peak broadening of 2D band is observed. Figure 1(C) shows the plot of I (D) /I (G) and I (2D) /I (G) ratio versus the oxidation degree of graphene oxide. The I (D) /I (G) ratio rises with initial increase in oxidation and decreased/saturated at higher degrees that is compensated by the higher FWHM. The variation of I (2D) /I (G) ratio (Figure 1(C)) against the oxidation degree of graphene oxide indicating that they tend to decrease up to S-3 and increases at higher oxidation levels. These changes indicated the decrease in crystallinity and increase in amorphous nature in graphene oxide with increase in oxidation ranges. Further, the observed changes in overtone bands (2D, D + G and 2G), confirms the disruption of the stacking order in graphene oxide at higher oxidation level. The average crystallite size (L a ) of the sp 2 domains present in the prepared graphene oxides was determined in accordance with the method given by Cancodo et al. [28]. The L a values are 18.2, 11.2, 9.6, 15.6, 14.0, and 13.7 nm for the different graphene oxides (from S-1 to S-6), and these values are lesser than the L a of graphite (122 nm). These results indicated that there is no linear relation to quantify the amounts of sp 3 and sp 2 domains present in graphene oxide with different oxidation degrees. Altogether, the Raman analysis showed the occurrence crystalline to amorphous transition with disruption of stacking order when graphite is oxidized into graphene oxide.

Raman spectral and mapping studies on graphene-based nanopaint
This section describes the use of Raman mapping for the identification of pigment dispersion in alkyd resin-based paint coating which utilizes graphene sheets as pigment and/or conductive agent. These electrically conductive paints are of high significance with applications ranging from electromagnetic interference shielding, static charge dissipation, and space [29,30]. The comparative Raman spectrum of graphene paint and alkyd resin is given in Figure 2(A). The spectrum of graphene paint coatings indicating the presence of G band (1585 cm −1 ), and D band (1350 cm −1 ) which confirms the presence of graphene sheets dispersed well in the alkyd resin binder [31,32]. There were no bands related to the alkyd resin were observed in the Raman spectrum of graphene paint coating since the vibrations arise from graphene sheets overwhelms the vibrations of alkyd resin. The peak position and intensity ratios of finger imprint modes were used to study the spatial distribution of graphene sheets in the paint matrix. Figure 2(B)-(D) presents the peak position maps of graphene's finger imprint bands such as D band (1350-1370 cm −1 ), G band (1584-1590 cm −1 ) and 2D band (2710 to 2730 cm −1 ), respectively. In comparison with the Raman spectrum of bare graphene sheets alone, the G band is red shifted as a result of molecular level bonding between the graphene sheets with the functional groups of alkyd resin [17]. Additionally, G and 2D bands of graphene sheets were seen over the entire mapped regime of the paint coatings that is responsible for the observed electrical conductivity. Figure 2(E) shows the intensity ratio map of I D /I G band in the range from 0.20 to 0.55 (blue to yellow) revealed the interconnection between of sp 2 domains of graphene with alkyd resin counterparts in the prepared paint coating. The I 2D /I G ratio map (Figure 2(F)) is from of 0.25 to 0.65 (blue to yellow) indicating the restacking of graphene sheets (c-axis) occurred in the paint coating. This uniform distribution of restacked graphene sheets inside the alkyd resin matrix provides enough conductive channels to facilitate the electrical transport in the graphene paint [17].

Raman analysis of 2D materials directly grown on conductive substrate
In this section, we discuss about the use of Raman spectroscopy as a prominent tool understanding the crystallinity and layer numbers of 2D materials that are randomly or vertically oriented on the conductive substrates. Generally, binder-free electrodes neglecting the inclusion of insulating polymers are of great significance in electrochemical energy conversion and storage devices since they offer enhanced electrochemical active sites [33,34]. Usually, atomic force microscope (AFM) is used for the understanding the thickness of the 2D sheets whereas this technique is suitable only for laterally oriented sheets [35]. The AFM technique is not suitable for measurements for samples such as randomly or vertically aligned sheets on conductive substrates due to the structural issues [18]. Herein, Raman spectroscopy and mapping analysis are promising for these types of binder-free electrodes that are mainly used in electrochemical energy devices.
In recent years, MoS 2 sheets are considered as structural analogue to graphene due to their structural integrity in which the covalently bonded S-Mo-S layers were separated by Van der Waals forces [36]. These layered structure and redox properties of Mo transition metal led to superior electrochemical charge-storage properties (supercapacitors) which is extensively studied during this decade [37]. Additionally, the presence of band gap in MoS 2 that make them as an ideal candidate for optoelectronic, field effect transistor, and photoelectrochemical cells [38]. The specific capacitance of MoS 2 electrode fabricated using conventional slurry coating method is in the range of 80 to 120 F g −1 [39]. To boost the specific capacitance, binder-free MoS 2 electrodes were fabricated via growing MoS 2 directly on conductive substrates such as Mo foil, and Ni foam. Interestingly, the specific capacitance of MoS 2 /Mo foil increases upto 192 F g −1 that is higher compared to many of the reported MoS 2 based planar supercapacitor electrodes and these electrodes possess better Columbic efficiency than others [18]. To understand the structural properties of MoS 2 /Mo binder-free electrodes, Raman mapping analysis were performed. Figure 3(A) shows the Raman spectrum of MoS 2 sheets randomly aligned on Mo foil. The presence of Raman fingerprint bands of MoS 2 at 381 cm −1 (E 1 2g mode) and 405 cm −1 (A 1g mode) is observed in Figure 3(A). The intensity ratio between these bands, and their band positions can be directly correlated to their thickness [40]. Figure 3(B) shows the integral intensity ratio map of E 1 2g /A 1g modes shows that their ratio varies from 0.1 to 0.7. Bulk MoS 2 possess an intensity ratio of 0.7 whereas tri-, bi-, and mono-layered MoS 2 have an intensity ratio of 0.6, 0.3 and 0.1 based on previous works [41]. Therefore, Figure 3(B) confirms the presence of minor fractions with singlelayer MoS 2 (blue color), major fractions with bi-layered MoS 2 (green color), few layered (n < 6) MoS 2 (red color) and some bulk counterparts (yellow color) present in the MoS 2 /Mo foil [18]. Figure 3(C) and (D) shows the peak position maps of A 1g and E 1 2g modes which shows their variation from 403 to 410 cm −1 (bulk to single-layer) and from 380 to 386 cm −1 (bulk to single-layer), respectively [18]. The presence of bulk and few-layered MoS 2 is due to the initial phase of the reaction due to Kirkendall effect and secondary phase resulting in the randomly oriented sheets. Altogether, the Raman mapping analysis confirmed the presence of few-layered MoS 2 sheets (n < 5) and some bulk counterparts were grown on the Mo foil.
Likewise, in our recent study, Raman spectroscopy coupled with mapping is effectively used to quantify the thickness of the antimonene nanodendrites grown on the surface of nickel foam via electrochemical deposition technique [20]. Antimonene is one of the important materials from the family of 2D metalenes (P, Sb, As, and Bi) due to their semi-metallic properties [42], high carrier mobility [43], oxidation resistant nature, and tuneable band gap that make them as alternative candidate for application in solar cells [44], CO 2 reduction [45], biological applications, supercapacitors, and batteries [20,46]. Usually, antimonene sheets can be prepared via exfoliation methods (mechanical or liquid phase) like other 2D materials, whereas the yield is low. Recently, chemical/physical vapor deposition techniques were used to grow antimonene on conductive substrates [47]. In our recent work, we demonstrated the use of electrosynthesis route for the preparation of antimonene nanodendrites grown on nickel foam and these electrodes showed excellent charge-storage properties with a high specific capacity (1618 mA g −1 ) than of the reported binder-free electrodes [20]. Herein, Raman mapping analysis is used for the identification of layer numbers in antimonene. Based on the E g /A 1g intensity ratio map of antimonene nanodendrites that showed a ratio values vary from 0.41 to 0.49, the presence of few-layered sheets in the prepared antimonene/ Ni foam binder-free electrodes were confirmed [48].

Raman analysis of siloxene sheets
Siloxene sheets are one of the emerging materials from the 2D silicon family that can be prepared via topochemical reaction (given in Section 2.3) between CaSi 2 (Figure 4(A)) with conc. HCl that results in the dissolution of calcium ions and oxidation of Si sheets (Figure 4(B)) [19,49]. The structure of siloxene consists of Si 6 rings interconnected through Si-O-Si bridges with the addition of surfaceterminated hydroxyl groups as seen in Figure 4(C) [19]. Siloxene sheets can be explained as an oxidized form of silicene sheets, and the latter is known for their excellent conductivity comparable to that of graphene [50]. Further, the 2D siloxene or silicene sheets are highly useful for micro-electronic devices since existing technology is established based on silicon [19]. Recent studies on siloxene sheets shows that they are promising candidate for applications in water splitting, Li-ion batteries, supercapacitors, electrochemical sensors, and biomedical fields [51][52][53][54]. However, the structure of siloxene is quite complicated and there are different models (such as Weiss structure, chain-like structure, and Kautsky structure) were proposed till date [54]. The Raman spectrum of siloxene sheets (Figure 4(D)) showed the presence of two bands viz. (i) Si-O-Si (495 cm −1 ) and Si-Si ((520 cm −1 ) which showed that Kautsky structure of siloxene [52]. The minor bands present at 375 cm −1 is due to ν (Si-Si) and others located at 640 and 740 cm −1 are originated from ν (Si-H) . The peak position maps of Si-O-Si and Si-Si (Figure 4(E) and (F)) vibrations shows that they vary from 495 to 505 cm −1 and from 518 to 526 cm −1 , respectively. Here, the Si-Si and Si-O-Si bonds relates to the crystalline and amorphous domains of the siloxene sheets [19], and therefore, their intensity ratio map was constructed as shown in Figure 4(G). The intensity ratio map of Si-O-Si/Si-Si bands revealed that they vary from 0.3 to 2.0 over the entire mapped region. This highlighted the heterogenous distribution of Si-O-Si bridged over the Si 6 rings in the structure of siloxene and confirmed the presence of Kautsky structure [55].

Raman analysis of solid electrolyte-piezopolymer nanocomposite
Polymer electrolyte composites plays a key role in a variety of electrochemical energy conversion-, storage, and delivery devices [56]. Recently, solid electrolyte entrapped piezo polymeric separators were developed by few researchers for direct applications in self-charging power (supercapacitor or battery) cells [57]. Herein, Raman spectral and mapping analysis are used to study the distribution of electrolyte ions inside the polymer matrix and also to understand the role of electrolyte ions on the self-poling properties to yield the electroactive phase in the PVDF [58]. The piezo-polymer electrolyte nanocomposite film was prepared via the method given in Section 2.5 which contains PTA electrolyte distributed in the PVDF matrix. Initial studies on the Raman spectrum of bare and electrolyte incorporated films showed the presence of α-phase (765 & 880 cm −1 ) and electroactive β-phase (842 cm −1 ) in the bare PVDF whereas the presence of band due to W-O stretching (1011 cm −1 ) were found in the composite films [16]. The Raman spectral analysis of the piezoelectrolyte film shows that there were no changes found in the electroactive phases of PVDF due to the addition of PTA. Additionally, Raman mapping analysis is performed over an area of −1 × 1 µm 2 with acquisition of 100 spectral arrays to study the distribution of the PTA in the PVDF polymer. Figure 5(A) depicts the 3D array map of PTA-PVDF revealing the presence of γ (811 cm −1 ) and β (842 cm −1 ) phase of PVDF and W-O vibrations (1011 cm −1 ) of PTA electrolyte [16]. The peak position map of β-phase (Figure 5(B)) shows that their positions vary from 838 to 839 cm −1 (blue to red regime) whereas the position of the W-O vibration (Figure 5(C)) ranged from 1006.6 to 1007.5 (blue to red regime). Figure 5(D) and (E) presents the Raman intensity maps of β-phase PVDF (842 cm −1 ) and (W-O 1011 cm −1 ) respectively. The intensity ratio maps of I W-O /I β (given in in Figure 5(F)) shows that these values are in the range from 1.198 (blue) to 1.202 (red) indicating a high order of homogeneity [16]. There is no drastic variation in the intensity ratio from which it can be concluded that PTA electrolyte is homogeneously dispersed well in the PVDF matrix [16].

Raman spectroscopy as a tool to probe the mechanical to electrical energy transduction process in CEC film
The digital photographs of the bare PVDF and CEC film (Figure 6(A)) shows the transformation of white to black-colored films as a result of dehydrohalogenation process [59]. The predicted structures of linear carbyne chain present in the CEC film in horizontal and vertical orientation is provided in (Figure 6(B) and (C). The 3D Raman array map of CEC film (Figure 6(D)) shows the presence of β-carbyne (cumulene) and carbonoid (sp, sp 2 , and sp 3 hybridized carbon) networks at 1140 and 1540 cm −1 , respectively [60]. The formation of sp 2 and sp 3 hybridized carbon can be explained via cross-linking reactions of adjacent sp-hybridized carbon in the carbyne chains during chemical reaction [22,61]. The position map of the cumulene (Figure 6(E)) shows the width of about 40 cm −1 (blue to red-yellow color) as an indication of uniform distribution of ((=C=C=) n ) chains in the entire region of the CEC film. The carbonoid position map (Figure 6(F)) lies between 1525 and 1560 cm −1 (blue to red color) revealing the existence of sp-carbon in the mapped area. Figure 6(G) and (H) shows the intensity maps of cumulene and carbonoid bands of CEC. Figure 6(I) depicts the intensity-ratio map of cumulenic/ carbonoid type carbon present in the CEC is in the range of 0.20 to 0.75, thus, confirmed the presence of uniformly distributed cumulene chains randomly oriented in the carbonoid species of the CEC [22].    subjected to an external applied force of 0.2 N [22]. The electrical output ranges were increased linearly with respect to an increase in forces, demonstrating their ideal electromechanical stability [62]. The observation of mechanical to electrical energy harvesting properties in carbon-based materials are not new. For instance, these types of properties were observed earlier in graphene sheets, graphene oxides, lithiated carbon fibers, carbon nanotubes (CNT), twisting CNT yarn, and graphene nitrides very recently [63][64][65][66]. However, the mechanism of energy harvesting in these carbons are different to each other. Very recently, Raman spectroscopy is used to probe the presence of alkene/alkyne transition as a cause for the electrochemical actuation process involved in graphdiyne [67]. Considering the physical properties of carbynes that can be tuned upon subjected to any form of external stimuli, and the recent theoretical studies, it is possible that chemical bonding/ structural transformation can be occurred in CEC when subjected to mechanical deformation. Therefore, ex-situ Raman spectral analysis of CEC (in deformed state) as shown in Figure 7(C) is analyzed to understand their detailed mechanism of energy transduction process. In comparison to the Raman spectrum of CEC film recorded at normal states, the ex-situ Raman analysis obtained at deformed state showed interesting observations as follows: (i) The cumulenic band is retained in the CEC even at deformed condition, (ii) the origin of D band (~1370 cm −1 ) as a  [68,69], (iii) occurrence of band splitting of the carbonoid band into G − (1540 cm −1 ) and G + (1590 cm −1 ) bands due to origin of semiconducting behavior [68,70], and (iv) the appearance of new bands over the region 1750 to 2300 cm −1 corresponding to the polynne type sp-carbon [71,72]. These findings demonstrated the structural transformation of carbonoid type carbon in the CEC into semiconducting-type and polynne-type carbons via mechanical deformation which is the reason for the observed mechanical to electrical energy conversion in CEC. Moreover, these experimental results support the theoretical analysis on "electronic flexoelectricity" in low-dimensional carbon nanostructures subjected to mechanical deformation [73,74].

Conclusions
This chapter describes the use of Raman spectroscopy and mapping analysis for the characterization for low-dimensional nanostructured materials, paint coatings, and solid electrolyte-piezo-polymer composites. Further, utilization of Raman spectroscopy to monitor the molecular level changes occurred in carbyne-enriched carbyne film based mechanical energy harvester when they are subjected to applied force is also described. Overall, this chapter provides a novel insight for the use of Raman spectroscopy and mapping as an important tool for characterization of lowdimensional nanostructures.