Chapter 2: Anomalous Photoluminescence Enhancement and Photoluminescence
2.3. Results and Discussion
2.3.3. Optical Analysis
2.3.3.2. Photoluminescence Study
43 | 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
Fig. 2.9.A schematic illustration of the attachment of U-GQDs on the surface of SWCNT.
nm. The first peak corresponds to band to band excitonic transition, which is consistent with the absorption spectrum. To understand the origin of the other two peaks, we measured the PL spectrum with 355 nm laser excitation, as shown in Fig. 2.10(b). The deconvoluted PL peak at
~427 nm is due to the contribution of the zigzag edge states, and the other two peaks at ~475 nm and 530 nm are assigned to the COOH and C=O functional groups in U-GQDs.26 Moreover, the relative PL quantum yield (QY) () of the as-synthesized U-GQDs was also measured with 0.05 M quinine sulfate (QS) as a reference fluorophore. The relative PL QY is defined as:
=Q𝑟𝐴𝐼𝑟𝐼𝑠𝜂2𝑠
𝑟𝐴𝑠𝜂2𝑟 (2.3)
where Qr is the absolute QY of reference (QS). As and Ar are the absorbance of the sample and reference, respectively. Is and Ir are integrated PL intensity of sample and reference, respectively.
ηs and ηr are the refractive index of sample and reference, respectively. The value of Qr of QS is
~0.5238, and the refractive index of water is 1.33. Here, for aqueous medium, ηs = ηr= 1.33 is used.
For U-GQDs, value is calculated as ~2.4%. In the purpose of tuning of the PL intensity, U-GQDs is further functionalized with SWCNTs. With different concentrations of SWCNTs, the PL intensity of U-GQDs is monitored at ~440 nm under 355 nm laser excitation, as shown in Fig.
2.11. The PL spectra of U-GQDs before and after the addition of SWCNTs with concentrations of 2, 4, 6, and 8 µg/mL are presented in Fig. 2.11(a). The nature of the relative change in the PL intensity at low concentration region is observed to follow a linear equation:
𝐼0
𝐼 = 𝐴 + 𝐵𝑄 (2.4)
where I0 and I are the PL intensities of the U-GQDs before and after the addition of SWCNTs, ‘Q’
is the concentration of SWCNTs, ‘A’ and ‘B’ are the constants. The linear fitting of the experimental data gives the values of A =0.51 and B =0.01 mL/g, as shown in Fig. 2.11(b). In Fig. 2.11(a), a redshift of ~5 nm in the PL maxima (~440 nm) of U-GQDs is observed after the addition of SWCNTs, indicating the formation of a composite of U-GQDs with SWCNTs at low concentration of SWCNTs. The enhancement in PL intensity at low concentration region is due to the high absorption at ~370 nm in U-GQDs/2 SW, as observed from Fig. 2.8(d). In addition to the metallic contribution of the SWCNTs, the PL enhancement is also partly due to the improved dispersion of U-GQDs in the aqueous medium with the addition of SWCNTs at low concentration,
45 | 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
despite the extremely low absorbance of the SWCNTs (see Fig. 2.8(b)). Note that the value of is calculated as ~5.5% for U-GQDs/2 SW. Fig. 2.11(c) represents the PL spectra of U-GQDs before and after the addition of SWCNTs with a concentration of 10–50 µg/mL, i.e., in the high concentration region. In this high concentration region, PL intensity of U-GQDs is observed to be quenched systematically with increasing concentration of SWCNTs and the PL intensity of
Fig. 2.11. (a) PL spectra of 1.0 mg/ mL concentration of U-GQDs at the low concentration region of SWCNTs (2, 4, 6, and 8 µg/mL) and (b) the corresponding relative change of the PL intensity with SWCNTs concentration. (c) The change of the PL intensity of U-GQDs with the high concentration of SWCNTs (10–50 µg/mL) and (d) the corresponding non-linear Stern–Volmer plot.
is quenched by ~74% with 50 µg/mL SWCNTs. In this region, the nature of PL quenching of U- GQDs shows a non-linear behavior following the equation:20
𝐼0
𝐼 = exp(𝑏[𝑄])𝑎 (2.5)
where ‘b’ and ‘a’ are the constants. The fitted values of the constants are b =0.02 mL/g and a
=2.81 (see Fig. 2.11(d)). Such a positive deviation from the usual non-linear Stern–Volmer equation is observed when the extent of quenching is large.20Due to the strong composite
Fig. 2.12. (a) PL spectra of 0.5 mg/ mL concentration of U-GQDs at low concentrations of SWCNTs (2, 4, 6, and 8 µg/mL) and (b) the corresponding relative change of the PL intensity with the SWCNTs concentration. (c) The change of the PL intensity of U-GQDs with the high concentration of SWCNTs (10–50 µg/mL). (d) Nonlinear Stern–Volmer plot at high concentration of SWCNTs.
formation with SWCNTs through the interaction and the functional groups/defects sites, the quenching efficiency is unusually high, and it follows a faster than exponential growth behavior with the combined contributions of dynamic and static quenching. At a high concentration of SWCNTs (≥10 µg/mL), the semiconducting nature of SWCNTs insists on the excited-state charge
47 | 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
transfer from U-GQDs to SWCNTs after the composite formation and thus PL intensity of U- GQDs is quenched due to the reduced recombination. The band position of U-GQDs with the particle size of ~2–7 nm is ~3 eV13, and that of SWCNTs is ~5 eV39 with respect to the vacuum level also implies the possibility of charge transfer from U-GQDs to SWCNTs. To test the reproducibility of the above results, the PL spectral analysis is carried out with different concentrations of U-GQDs (0.25, 0.5, and 2 mg/mL). In Fig. 2.12, with 0.5 mg/mL of U-GQDs, the tuning of PL intensity is observed with different concentration of SWCNTs by following the similar nature as discussed for 1.0 mg/mL of U-GQDs. Fig. 2.12(a) shows the enhancement of PL intensity of U-GQDs with the low concentrations of SWCNTs (<10 g/mL) and the corresponding relative change of the PL intensity with SWCNTs concentration is observed to follow the linear nature, with the intercept and slope as 0.26 and 0.04 mL/g, as shown in Fig. 2.12(b). The change in the slope and intercept with the variation of U-GQDs concentration is attributed to the difference in the interaction possibilities. In the high concentration region of SWCNTs (>10 g/mL), the systematic quenching of the PL intensity of U-GQDs and the corresponding change of the relative PL intensity with SWCNTs are shown in Fig. 2.12(c, d). Further, the effect of concentration of U- GQDs in the tuning of the PL intensity is analyzed in both low and high concentration regions of SWCNTs, as shown in Fig. 2.13. With 2 g/mL SWCNTs, we have observed a systematic increase
Fig. 2.13. Concentration effect of U-GQDs in the tuning of PL intensity. Normalized PL spectra of different concentrations of U-GQDs with (a) 2 SW and (b) 50 SW. The dashed curve indicates the normalized PL spectrum of bare U-GQDs.
in the PL intensity with the reduction of U-GQDs concentration, and the PL intensity is highest for a U-GQDs concentration of 0.25 mg/mL (see Fig. 2.13(a)). Note that each PL spectrum is normalized with its absorption spectrum to account for the concentration effect on the PL intensity of U-GQDs. At a low concentration of U-GQDs, SWCNTs get more open surfaces to interact with the well-separated U-GQDs, and thus, the interaction of U-GQDs and SWCNTs can efficiently increase the local fields on U-GQDs and as a consequence, the PL intensity of U-GQDs increases strongly. On the other hand, 1.0 mg/mL U-GQDs exhibit maximum PL quenching with 50 g/mL SWCNTs, as presented in Fig. 2.13(b). At a high concentration of U-GQDs, the agglomeration effect in U-GQDs affects the composite formation with SWCNTs due to the limited availability of sites for interaction. Thus, 1 mg/mL concentration of U-GQDs is considered as the optimized concentration of U-GQDs for efficient quenching of the PL intensity.