Chapter 2: Anomalous Photoluminescence Enhancement and Photoluminescence
2.3. Results and Discussion
2.3.2. Structural Analysis: XRD and Raman Analysis
To confirm the crystalline structure of U-GQDs, the powder XRD pattern of U-GQDs is presented in Fig. 2.6. The diffraction peak at 2θ ~26.1° is attributed to (002) plane of sp2 hybridized carbon
Fig. 2.6. XRD pattern of U-GQDs. The inset presents the enlarged view of the XRD pattern in the range 2θ ~40–50°.
atoms for the highly ordered and crystalline graphitic structure. The inset of Fig. 2.6 shows the magnified view of the XRD pattern of U-GQDs in the range 2 ~40–50, which shows the weak reflections from (100) and (101) planes of U-GQDs.
For further structural analysis of U-GQDs, the Raman spectrum of pristine U-GQDs is presented in Fig. 2.7(a). In U-GQDs, the characteristic D and G bands arise at ~1355 cm−1 and 1580 cm-1, respectively. The presence of another band at ~1614 cm-1 (D') is attributed to the vacancies and/or
Fig. 2.7. (a) Raman spectrum of pristine U-GQDs. (b) Comparison of the Raman spectra of pristine SWCNTs, U- GQDs, and U-GQDs/SWCNTs composites at two different concentrations of SWCNTs keeping the U-GQDs concentration unchanged. The corresponding inset shows the presence of different chirality in SWCNTs related to the metallic and semiconducting nature of SWCNTs. (c) Deconvoluted Raman spectrum of SWCNTs with one BWF and five Lorentz line shape. (d) Comparison of normalized Raman spectra of G peak for different composite samples, showing the redshift with respect to the pristine SWCNTs spectrum.
pentagonal and octagonal defects, usually referred to as zigzag defects.26 The Raman band in U- GQDs at ~2723 cm-1, known as the 2D band, originates from the in-plane breathing-like mode of
39 | T u n i n g o f P L I n t e n s i t y o f U n d o p e d - G Q D s
the carbon rings.27 The D + D′ band at ~3243 cm-1 in U-GQDs is a combined effect of D and D.27 In order to understand the change in the structural features of U-GQDs after the formation of the composites with SWCNTs, we performed a comparative study of the Raman spectra of bare SWCNTs, U-GQDs as well as their composites at different SWCNTs concentrations, as shown in Fig. 2.7(b). For SWCNTs, the Raman spectrum shows the clear signature of radial breathing modes (RBM) lying in the region ~92–374 cm−1. The inset of Fig. 2.7(b) shows the enlarged view of the RBM in the region ~50–350 cm-1 with four prominent peaks at ~154.7 cm−1, 187.3 cm−1, 248.0 cm−1, and 263.8 cm−1. From these RBM, the diameters of SWCNTs are calculated using the empirical formula:28
RBM = C/dt (2.1)
where, C =248 for isolated SWCNTs on a SiO2 substrate, dt (nm) is the diameter of SWCNT, RBM
is in cm-1. The value of dt and the corresponding chirality are presented in Table 2.1. The calculated value of
Table 2.1. Chirality (n, m) of pristine SWCNTs calculated from the RBM. S and M denote the semiconducting and metallic nature of the SWCNTs.
the diameter of SWCNTs using eqn. (2.1) consistent with the TEM analysis (see Fig. 2.3(d)).
Additionally, the chirality analysis reveals the metallic nature of SWCNTs along with their semiconducting nature. Chirality (16,7) and (12,0) correspond to the metallic SWCNTs with diameter ~1.60 nm and 0.94 nm, respectively, while, (13,6) and (11,3) are attributed to the semiconducting nature of SWCNTs with diameter ~1.32 nm and 1.00 nm, respectively.29, 30 The presence of both metallic and semiconducting nature of the pristine SWCNTs are also observed by the deconvolution of Raman spectrums of SWCNTs with five Lorentz peaks and one Breit–
Wigner–Fano (BWF) line shape in the range ~1200–1850 cm-1, as shown in Fig. 2.7(c). The Lorentz peaks at ~1339 cm−1 and 1593 cm-1 correspond to the characteristic D and G bands of SWCNTs, respectively. The BWF line shape fitted peak at ~1531 cm-1 (peak 2) corresponds to
RBM (cm-1) dt =248/RBM(nm) Chirality
S (n,m) M (n,m)
154.7 1.60 - (16,7)
187.3 1.32 (13,6) -
248.0 1.00 (11,3) -
263.8 0.94 - (12,0)
metallic SWCNTs28, and the Lorentzian peak at ~1570 cm-1 (peak 3) reveals the presence of semiconducting nature of SWCNTs.28 Other two weak Raman bands at ~1720 and 1760 cm-1 attribute to the presence of M- and M+ bands in SWCNTs.28 The semiconducting and metallic nature of the SWCNTs is further confirmed from the Raman frequency (ω) calculation using the equation:28
ω = ω0+dt2 (2.2)
where −45.7 cm−1 nm2 for semiconducting nature and −79.5cm−1 nm2 for metallic nature.
In the present case, dt of SWCNT is =1.5 nm, and ω0 is the Raman frequency of G band =1593 cm−1 from the fitted graph. The calculated values of ‘ω’ nicely match with the fitted band frequencies of the metallic and semiconducting nature of SWCNTs, i.e., with the value of peak 2 and peak 3, respectively. Also, the first overtone of the D band of SWCNTs appears at ~2661 cm-
1 (2D band) 31 along with D + D′ band at ~3138 cm-1. Interestingly, after the composite formation with U-GQDs, RBM features of SWCNTs are observed to be disappeared completely from the Raman spectra (see Fig. 2.7(b)), confirming the strong attachment U-GQDs with SWCNTs. Fig.
2.7(d) represents the normalized Raman spectra of the G band of pristine SWCNTs, U-GQDs, and their composites. The formation of composites of SWCNTs with U-GQDs shows a redshift of the G peak (see Fig. 2.7(d)) and the blueshift of D, 2D, and D + D' peaks with respect to SWCNTs (see Fig. 2.7(b)). The shifts of defect-related peaks are shown in Fig. 2.7(b) with the vertical dashed lines. The shift in the Raman modes confirms the bonding of U-GQDs with SWCNTs. It is clear from Fig. 2.7(d) that Raman peak at ~1617 cm-1 (D') in U-GQDs is redshifted after the addition of 2 µg/mL SWCNTs, while this band disappears with 60 µg/mL SWCNTs. Similarly, the intensity of D and D' band reduces at a higher concentration of SWCNTs, which is due to a stronger bonding between U-GQDs and SWCNTs in the defects sites. The redshift of G mode in U-GQDs/2 SW (~10 cm-1) is less than that of U-GQDs/60 SW (~12.2 cm-1) with respect to SWCNTs (see Fig. 2.7(d)), which indicates that tensile strain is increasing with the SWCNTs concentration and binding becomes stronger.