Chapter 3: Formation Mechanism of Graphene Quantum Dots During
3.8. Photoluminescence Analysis
Room temperature steady state PL measurements were performed to understand the origin of visible PL emission from GQDs. Fig. 3.7shows the PL spectra excited with 355 nm laser for various GQDs thin films before and after annealing. Fig. 3.7(a-c)shows the PL spectra before and after annealing and the Gaussian peak fitting for S12, S12F and S24, respectively. All the samples clearly show the PL emission in the blue and green regions. It is clear from the asymmetry and shift in the PL spectra (after annealing) that multiple peaks contribute to the strong PL emission from the GQD thin films. A proper understanding of the various PL peaks is explored by the Gaussian peak fitting of each spectrum. Note that each PL spectrum is fitted with four Gaussians peaks (P1, P2, P3 and P4) and different samples show different intensities of these peaks based on the growth and annealing conditions. Typically, the blue and green PL emissions from GQDs have been assigned to edge defects and functional groups of GQDs, respectively 7, 21. However, the nature of edge states has not been distinguished properly through PL analysis and it was found necessary to monitor the evolution of different PL peaks after functionalization and annealing to identify the exact origin of each peak. Interestingly, we found an intriguing correlation between the evolution of the PL peaks with annealing and the new Raman bands discussed in the section 3.6. Note that there is a drastic change in the relative intensities of peaks P1 (at ~407 nm) and peak P2 (at ~440 nm) after annealing in both H2 and O2 environment. The relative intensities of peak P1 is reduced significantly after annealing, while that of peak P2 is strongly increased after annealing, and there is a one to one correspondence between the intensities of the two peaks. At high temperatures the zigzag edges convert to the zigzag 5-7 and armchair edges 27, the armchair edges are the most prevalent edge structure among various edge configurations in graphene 55. The high temperature annealing of GQDs results in the reduction in the zigzag edges and increase the armchair edges in GQDs through the inter-conversion of edges. Thus, the
drastic reduction in the intensity of peak P1 and the concomitant increase in the intensity of peak P2 after annealing clearly signify the conversion of edge states from zigzag to armchair and it allows us to unambiguously distinguish the PL peaks related to zigzag and armchair states.
In order to quantify the relative changes in the density of edge states and functional groups with annealing, we monitored the changes in integrated intensity of PL peak with annealing. Fig. 3.8(a-c) shows the relative percentage of each PL band before and after annealing. Importantly, we monitor the conversion of edge states due to the thermal annealing, while the edge functional groups are controlled by the annealing environment (H2 and O2). The decrease in the intensity of peak P1 and the concomitant increase in the intensity of peak P2 follow a trend similar to that of the changes in the
Figure 3.7: PL spectra for various GQDs thin films before and after (H2 and O2) annealing. (a-c) Comparison of the PL spectra of with Gaussian fitting before and after annealing of S12, S12F and S24, respectively. Note that the peaks P1 and P2 are significant for the zigzag and armchair edge states, while the P3 and P4 are significant for the COOH/C-OH and C=O/C-O edge functional groups, respectively. All the peak positions are in nm units. The experimental data are shown with symbols and the fitted data are shown with red line in each case.
Raman D bands of GQDs. Since annealing at 620C enables to convert the zigzag edges to armchair edges, the decrease in intensity of P1 is due to the decrease in the density/percentage of zigzag edges. Similarly, the strong increase in the intensity of P2 after annealing implies the enrichment of armchair edges in the GQDs. Thus, peaks P1 and P2 at ~407 nm and ~440 nm, respectively, are assigned to the zig-zag (carbene-like, with a triplet ground state being most common) and free armchair sites (carbyne-like, with a singlet ground state most common) in GQDs. At the same time, during the annealing the density of oxygen functional groups is altered, based on the annealing environment. Two green emissions P3 and P4 are attributed to the COOH/C-OH and C=O groups, respectively.
Note that the overall PL intensity is reduced in annealed samples that might be due to the creation of additional defects, e.g., vacancies, pentagon/heptagon defects, and some of these defects may act as a nonradiative recombination centers. These results are consistent with the Raman analysis that showed increased ID'/IG ratio in the annealed
Figure 3.8: Quantitative analysis of edge states (zigzag and armchair) and oxygenated functional groups before and after annealing, as derived from the relative integrated intensities of PL. Relative percentage of (a) zigzag edges, (b) armchair edges, (c) C=O/C-O edge functional groups before and after annealing. (d) Schematic of the band diagram of GQDs showing the edge states and functional groups related transitions giving rise to two blue and two green PL emission bands.
samples. Fig. 3.8(a) shows change in the relative percentage of zigzag edges in pristine and annealed samples, while Fig. 3.8(b) shows change in the relative percentage of armchair edges in pristine and annealed samples. Fig. 3.8(c) represents that the concentration of C=O/C-O bonds is increased after O2 annealing, as expected, and it again decreases upon H2 annealing at 620 C. We find from the PL analysis that the as- grown samples contain mostly zigzag edges, while the annealed samples contain mostly the armchair edges without any significant presence of zigzag states. The slight increment in the intensity of P3 may be due to the C-H bond formation. Based on these results, we provide a schematic of the energy band diagram of GQDs, as shown Fig. 3.8(d). It depicts the edge states and functional groups related transitions giving rise to two blue (P1, P2) and two green (P3, P4) PL emission bands.