Table 1 Different characterization techniques for identifying pure crystalline CNDs
Techniques | Crystalline CNDs | Amorphous CNDs | Crystalline quasi CNDs | Aggregated fluorophores |
|---|---|---|---|---|
HR-TEM | a. Size 2–5 nm, b. Lattice fringes c. Interlayer spacing of 0.34 nm for 002 plane d. Interlayer spacing of 0.21 nm for 001 plane | a. Size 2–5 nm, b. No lattice fringes c. No interlayer spacing | a. Size 2–5 nm, b. Lattice fringes c. Interlayer spacing of 0.34 nm for 002 plane d. Interlayer spacing of 0.21 nm for 001 plane | Almost similar to CNDs. A little variation may be observed based on the fluorophore structure |
XRD | Sharp peak at ~24˚ for 002 plane | Broad peaks as compared to CND/Quasi CND | Sharp peak at ~24˚ for 002 plane | Sharp peak at ~24˚ for 002 plane |
AFM | The height profile will confirm the size of 2–5 nm, topology and number of graphene layers in CNDs | The height profile will confirm the size of 2–5 nm and topology | The height profile will confirm the size of 3–5 nm, topology and number of graphene layers in CNDs | The height profile will confirm the size of 2–5 nm and topology |
Raman Spectroscopy | a. Sharp G band at ~1580 cm−1, b. 2D band at ~2680 cm−1 c. less intense D band ~1360 cm−1 due to edge defect by surface functional groups | Predominant broad D bands or high D/G ratio | Same as CNDs as it contains the same crystalline core | Almost similar to CNDs. A little variation may be observed based on the fluorophore structure |
TGA | No mass loss and mostly stable up to 800 °C | No mass loss and mostly stable up to 800 °C | Due to less thermal stability of molecular fluorophore extensive mass loss within 300 °C | Due to less thermal stability of molecular fluorophore extensive mass loss within 300 °C |
Confocal Raman PL | Due to high thermal stability of carbon core no change in PL intensity at lower temperature ~300 °C | Due to high thermal stability of carbon core no change in PL intensity at lower temperature ~300 °C | Due to less thermal stability of molecular fluorophore huge reduction in PL intensity within 300 °C | Due to less thermal stability of molecular fluorophore complete reduction in PL intensity within 300 °C |
