Chapter 2: Anomalous Photoluminescence Enhancement and Photoluminescence
2.3. Results and Discussion
2.3.3. Optical Analysis
2.3.3.1. UV-vis Absorption Study
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.
41 | 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
characteristic -* transition due to sp2 carbon domains as well as QC effect and an absorption tail extending up to 500 nm corresponds to n-* transition appears from the functional groups/defects in U-GQDs. The optical bandgap of the U-GQDs is estimated to be ~3.24 eV by Tauc plot, as presented in the inset of Fig. 2.8(a).13 For the case of SWCNTs, the absorption spectra with SWCNTs concentration 2 g/mL (2 SW) and 60 g/mL (60 SW) are presented in Fig. 2.8(b, c), respectively, in the UV- vis-NIR region. Fig. 2.8(b) shows a sharp absorption peak at ~273 nm in 2 SW, along with the metallic M11 band in the region ~450–670 nm and the semiconducting S22 bands at ~963 nm and 1157 nm,32, 33 which confirm the presence of both metallic and
Fig. 2.8.(a) UV absorption spectrum of U-GQDs; the inset shows the Tauc plot exhibiting a bandgap of ~3.24 eV.
(b) UV-vis-NIR absorption spectra of pristine SWCNTs solution with the concentration of 2 µg/mL, showing semiconducting peaks in the region 900–1300 nm. The inset shows the metallic peaks in the region 450–670 nm. (c) (c) The absorption spectrum of SWCNTs with concentration 60 µg/mL. The inset shows the metallic peaks in the region 400–830 nm. (d) Comparison of absorption spectra of U-GQDs and U-GQDs/SWCNTs solutions, showing the prominent redshift at a higher SWCNTs concentration with respect to U-GQDs.
semiconducting nature in the used SWCNTs, consistent with Raman analysis. On the other hand, 60 SW in Fig. 2.8(c) exhibits a broad absorption band at ~270 nm, together with a small contribution of the M11 band, as shown in the inset. The sharp absorption at ~273 nm is identified as plasmon resonance peak of SWCNTs.34 Multiple peaks corresponding to metallic and semiconducting regions are due to the presence of different diameter SWCNTs.35, 36 Note that the absorption spectrum of SWCNTs is an important tool for assessing their dispersion quality.
Bundled SWCNTs exhibit very weak absorption in the wavelength region ~200–1200 nm due to the carrier tunneling effect between the nanotubes.37 The presence of various absorption peaks in the region ~200–1200 nm for sample 2 SW in Fig. 2.8(b) reveals the good dispersion of SWCNTs in the aqueous medium without buddle effect, while the absorption of 60 SW in Fig. 2.8(c) demonstrates the presence of aggregation at a higher SWCNTs concentration. Due to the less aggregation at low concentration, the SWCNTs primarily show a metallic nature, while at a higher concentration, aggregated SWCNTs show only the semiconducting behavior.36 A comparison of the UV-vis absorption spectra of U-GQDs is presented in Fig. 2.8(d) before and after the composite formation with SWCNTs at two different concentrations, which show the enhancement and reduction of -* absorption at ~273 nm for U-GQDs/2 SW and U-GQDs/60 SW, respectively. At low concentration of SWCNTs, due to their dominant metallic nature, the plasmonic contribution of SWCNTs increases the local incident field on U-GQDs, and subsequently, -* absorption of U-GQDs increases in U-GQDs/2 SW composites. On the other hand, the reduction of -* absorption intensity in U-GQDs/60 SW, along with the redshift of the absorption peak, endorse the semiconducting nature of SWCNTs. At low concentration of SWCNTs, U-GQDs also prevent SWCNTs from agglomeration by surrounding the SWCNTs, while with the high concentration of SWCNTs, due to the insufficient amount of U-GQDs, SWCNTs agglomerate easily and then SWCNTs start to show semiconducting nature only.36 Note that the appearance of another absorption band at ~370 nm in only U-GQDs/2 SW is due to the strong metallic contribution of SWCNTs at the low concentration. Since SWCNTs contain highly delocalized -electrons, they can easily attach with electron rich U-GQDs through -
interactions.20 Additionally, the change of the absorption tail of U-GQDs in the composites indicates that the interaction between U-GQDs and SWCNTs also happens with the functional groups and defect states. Fig. 2.9 shows a schematic of the attachment of U-GQDs on the surface of SWCNT in two different pathways for U-GQDs/SWCNTs composite formation.
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Fig. 2.9.A schematic illustration of the attachment of U-GQDs on the surface of SWCNT.