Sensing and optoelectronics
Scheme 3.1. Schematic representation of the synthesis steps of the CDs. The photographs of the CD solution in the presence and absence of UV light are shown in the inset
3.3. Results and discussion
3.3.4. Characteristics of CD-MWCNT nanocomposite
attachment with CDs (figure 3.14a-c) along with the PL spectra of MWCNT before and after the attachment with CDs (figure 3.14d). Figure 3.14a represents the FETEM image
Figure 3.15. (a) Raman spectra of MWCNT and CD-MWCNT nanocomposite, (b) deconvoluted Raman peaks of MWCNT, and (c) deconvoluted Raman peaks of CD- MWCNT nanocomposite.
of MWCNT when CDs were not attached with them, whereas figure 3.14b shows the attachment of CDs with the MWCNT surface. The dark cluster-like spots on the image are evidence of the CD attachment. In figure 3.14c, another image is shown where the CDs are attached to the MWCNT in an entirely segregated manner. The CDs on the MWCNT are indicated with arrows in this figure. Attachment of the CDs on the MWCNT has introduced PL property to the nanocomposite, which is shown in figure 3.14d. From
this figure, it is clear that MWCNT does not have any PL property itself (the black line).
However, when the CDs get attached with the MWCNT, the composite shows the presence of PL property in it, and this is shown in figure 3.14d (red line). The presence of PL property in the nanocomposite can be ascribed as the presence of CDs in the nanocomposite.
Raman spectra, shown in figure 3.15a, give a clearer idea about the attachment of the CDs with MWCNT. The Raman spectra of oxidized MWCNT and the CD-MWCNT nanocomposite show the presence of three bands at 1340, 1573, and 2680 cm−1, called the D−band, G−band, and the 2D−band, respectively. In the case of MWCNT, D−band specifies the existence of the defected carbon structure of the nanotubes, which is associated with the sp2 sites existing in carbon tubes in ring form [48]. These defects can be either because of defects in the sp2 carbon or distortion of the lattice due to bending or can be due to the oxidization of the nanotubes [48]. Whereas the stretching vibrations of the sp2 carbons are specified by the G−band [48], and the 2D band corresponds to the ‘D band’ overtone of the MWCNTs [49]. The ratio between the intensities of the D−band and the G−band (ID/IG) gives information about the degree of defects present in the nanotubes [50]. The inherent properties of the MWCNT have been retained in the nanocomposite as the characteristic Raman peaks of the nanotubes can be observed in the Raman spectra of the synthesized nanocomposite. The calculated value of the ID/IG ratio for the synthesized CD-MWCNT nanocomposite is 1.03, whereas this value for oxidized MWCNT is 1.17. This indicates that the attachment of the CDs on MWCNT has reduced the degree of defects of MWCNT by blocking the honeycomb structure of the surface of MWCNT [50]. We have deconvoluted the D and G peaks to obtain more detailed information about the MWCNT and the CD-MWCNT nanocomposite. These
deconvoluted peaks are shown in figure 3.15b and c, respectively. The peak at 1339.81 cm−1 in figure 3.15b is considered the characteristic peak of the D−band. The asymmetry of the G−band leads to get deconvoluted into two different peaks. One is at 1572.22 cm−1, which suggests its semiconducting nature (G− band), whereas the other is at 1603.3 cm−1, which can be assigned to the (G+ band [51-53]. All the deconvoluted peaks of the nanocomposite, shown in figure 3.15c, especially the deconvoluted G−band peaks have been redshifted, although the semiconducting behavior has remained intact.
Figure 3.16. (a) Emission peaks of CDs at an excitation wavelength of 360 nm in the presence of different concentrations of MWCNT. (b) Modified S-V plot of the CD- MWCNT solution. (c) TRPL spectra of the different CD-MWCNT solutions. The error bars in (b) represent the standard deviation values taken from three different measurements.
This little shift also confirms the attachment of CDs with the MWCNT. Moreover, it is noted that the G−band gets narrower after the attachment of the CDs with the MWCNT owing to the good dispersion of the CDs [54]. The change in PL intensity of the synthesized CDs in the presence of different concentrations of MWCNT was also observed. The PL intensity decreased with the increasing concentration of MWCNT, as shown in figure 3.16a. This figure shows no shift in the emission wavelength after attaching MWCNT to the CDs. To get more insight of the PL quenching mechanism of CDs, we have demonstrated the S−V plot in figure 3.16b. The plot shows an exponential rise curve. This rise of the curve in the S−V plot can be ascribed to either static quenching or a combination of static and dynamic quenching. Amongst which, the former was usually understood as a “sphere of action” which is basically defined by the modified S−V equation [19];
𝐹0
𝐹 = (1 + 𝐾𝐷[𝑄])𝑒([𝑄]𝑉) (3.5)
where [𝑄] represents MWCNT-concentration; 𝑉 and 𝐾𝐷 are static and dynamic quenching constants, respectively. The quencher material comes adjacent to the fluorophore during excitation, which is the reason behind the static quenching according to the sphere of action phenomenon. So, during excitation, the quencher-fluorophore pair helps to quench the PL emission immediately [19]. This pair is generally termed as dark complex. The quenching mechanism was further studied by performing TRPL analysis.
The TRPL plot is shown in figure 3.16c. In this figure, it is observed that there is a very minute but noticeable change in the PL decay curves with MWCNT concentrations. This tells us that a certain percentage of dynamic quenching is also responsible, along with the static quenching of PL of the CDs. So, the S−V characteristics follow double exponential
growth and accordingly, the static and dynamic quenching constants (𝑉, 𝐾𝐷 respectively) were calculated and their calculated values were found to be 8.5 (mg/ml)−1 and 0.08 (mg/ml)−1 respectively.