Table 2.

Different Photoluminescence mechanism.

S. No.	Photoluminescence Mechanism	Features	Reference
1	Bandgap transition related to conjugated π-domains	aThe creation of sp2 hybridised island dominated by p-electrons tends to isolate p-domains bBand gap calculation : c$\sqrt{Ah\nu }$versus hν, where A is the measured absorbance, h is the Planck constant, ν is the frequency, and hν is equal to 1240/wavelength in units of eV. $\sqrt{Ah\nu }$ has a linear relationship with hν with a slope of D and the optical band gap is the x-intercept dThe conduction band and valence band energy levels calculation: eE conduction band = -(E onset, ox + 4.66) eV, and E valence band = -(E onset, red + 4.66) eV, where Eonset, ox and Eonset, red are the onset of the oxidation and reduction potentials, respectively	[58]
2	Surface defects	aDepends upon the degree of surface oxidation and surface functional groups. bThe oxygen content present on the surface of CDs/NCDs causes the red-shifted emission of CDs/NCDs cAs the oxygen content increases, the number of surface defects increases. dThese defects will trap excitons, leading to the red-shifted emission	[61]
3	Quantum size effect	aThe size of the carbon core depends upon the optical band gap(π - π* transition)	[59]
4	Quantum confinement effect	aInfluenced by the crystal boundry bDepends upon different sizes of the small carbonic core cDepends upon different optical band gap dDifferent size with different optical band gap relaxes and recombines at the larger population of diverse surface defect sites result in quantum confined electron-hole pair	[59] [61]
5	Fluorescence Quenching	aDynamic Quenching	[63]
		bStatic Quenching
		cInner filter effect (IFE)
		dPhtoinduced electron transfer (PET)
		eEnergy Transfer
		fDexter energy transfer (DET),
		gSurface energy transfer (SET)
		hFoster resonance energy (FRET).