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
3.3.1. Optical properties of CDs
UV–Vis and PL spectroscopy have been carried out to understand the optical properties of the synthesized CDs. The UV–vis spectrum and PL spectra have been shown in figure 3.2. In figure 3.2a, the black line corresponds to the UV–vis spectrum of the CDs ranging from 220 nm to 600 nm, whereas the blue line depicts the PL spectrum of the CDs at an excitation wavelength of 360 nm. A little hump-like peak at 260 nm in the UV–vis spectrum is observed. This peak at 260 nm can be ascribed as the overlapping of spectra
caused due to the n−π* and π−π* electronic transitions of conjugated C=O and C=C bonds [21-23]. Moreover, the broadening of the peak can be due to the presence of 𝜋- 𝜋∗ transition of C=C along with the n-𝜋∗ transition of C=O . In this plot, it is also observed
that the UV–vis spectrum is extended till 600 nm as a long tail, which also confirms the formation of CDs, and this accords with some of the previous reports [24]. The PL property of the CDs was also examined. The blue line in figure 3.2a shows the emission maxima of the CDs at 457 nm wavelength when excited at 360 nm. Thus, it confirms the presence of the PL property in the synthesized CDs. The PL property of the CDs has been further tested to obtain the optimum excitation wavelength. For that, the CD solution was excited at different wavelengths ranging from 280 nm to 440 nm. The plots are shown in figure 3.2b, and it is observed from the figure that the maximum PL intensity was obtained at an excitation wavelength of 360 nm. In the case of PL spectra, the emission wavelength remained almost the same at 457 nm when excited with a wavelength in the range of 280–380 nm, and also, the emission maxima were observed at an excitation wavelength in the UV spectral region. This indicates that the PL property of the CDs was dominating in the UV domain. However, the PL intensity got decreased and red-shifted when the excitation wavelength was varying from 400 nm to 440 nm. Various sorts of double bond conjugation of different degrees in the CDs can be the probable reason behind this PL behavior [25]. The PL intensity can be modified using a surface passivator like EDA. The change in the PL maxima of the N-doped CDs in the presence of different dosage amounts of EDA during the synthesis is shown in a plot between normalized PL intensity and emission wavelength in figure 3.2c. It is observed in this figure that the emission maxima get redshifted with the addition of EDA. This can be ascribed as the
existence of multi emission centres [26] due to the influence of the additional doping agents. To get a clearer
Figure 3.2. (a) UV–vis absorption peak (black line) and PL emission spectrum (blue line) of aqueous CDs solution. (b) Various excitation wavelength-based PL spectra of the CDs.
(c) Normalized peaks of PL intensities of the CDs with different EDA dosage (λex = 360 nm).
picture, we have deconvoluted the normalized PL spectra and represented in figure 3.3.
The peaks present in the range of ~410 nm can be attributed to the presence of a zigzag edge state in the synthesized CDs [27]. The peaks in the range of ~440–460 nm are observed due to the N-doping in the CDs [26]. Interestingly, the zigzag edge state gets a little prominent when EDA is added to the precursor material, as observed in figure 3.3b and c. However, this zigzag effect gets minimized when further EDA is added (figure
3.3d). Moreover, based on our XPS investigation (discussed later), the presence of peaks in the range of ~450 nm and ~460 nm are due to the recombination of the electron and hole at C−N [26] and the peaks at ~490 nm can be attributed to the presence of C−OH [27]. Therefore, the increment in the QY with the increase in the dosage of EDA can also be attributed to the addition of functional groups. The gap energy and highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy
Figure 3.3. Deconvoluted peaks of normalized PL intensities of the CDs with different dosages of EDA ((a) 0 µl, (b) 50 µl, (c) 150 µl, and (d) 300 µl).
levels of the synthesized CDs were also calculated. UV–vis spectrum (figure 3.2a) was used to get the Tauc’s plot (figure 3.4a,b) for gap energy calculation, whereas the HOMO- LUMO energy levels were estimated using cyclic voltammetry (CV) measurement (figure
3.4c). In the CV curve, one reduction peak is observed at − 0.241 V. For calculations of the HOMO-LUMO levels, the following equations have been utilized [28, 29]:
𝐸𝐿𝑈𝑀𝑂 = −(𝐸𝑟𝑒𝑑+ 4.4 + 0.34) eV (3.1)
Figure 3.4. (a) (𝛼ℎ𝜗)2 vs. ℎ𝜗 and (b) (𝛼ℎ𝜗)1⁄2 vs ℎ𝜗 plot of the synthesized CDs, (c) CV curve of 0.5 mg/mL CD solution in 0.1 M KCl solution (vs. Ag/AgCl).
𝐸𝐻𝑂𝑀𝑂= (𝐸𝐿𝑈𝑀𝑂− 𝐸𝑔) eV (3.2)
where, 𝐸𝐿𝑈𝑀𝑂 and 𝐸𝐻𝑂𝑀𝑂 denote the energy levels of LUMO and HOMO, respectively.
𝐸𝑟𝑒𝑑 implies reduction potential obtained against a saturated calomel electrode (SCE) and 𝐸𝑔 corresponds to the energy gap of the CDs, which was calculated by using Tauc’s equation as given below [30],
(𝛼ℎ𝜗)1⁄𝑛 = 𝐴(ℎ𝜗 − 𝐸𝑔) (3.3) where, α is the absorption coefficient, hϑ is the incident photon energy, whereas A is a constant that depends on the transition probability. 𝑛 is dependent on the type of electronic transition. When the value of 𝑛 is ½, the transition is a direct allowed transition, and for the case of indirect allowed transition, the value of 𝑛 becomes 2. Figure 3.4a,b represent the plots of (𝛼ℎ𝜗)1⁄𝑛 vs ℎ𝜗 for 𝑛 = 1/2 and 𝑛 = 2 respectively. In these plots, the linear regions were extrapolated to the x-axis to get the gap energy values. From the figure, the obtained gap energy values were 3.3 eV and 2.1 eV for direct and indirect bandgap, respectively; the HOMO and LUMO levels were also calculated with the help of equations 3.1 and 3.2. The calculated LUMO energy level is -4.499 eV, whereas the HOMO energy level is calculated to be -7.749 eV for the direct gap and -6.549 eV for the indirect gap.