Comparative emission characteristics of CQDs synthesized by various research groups.

Synthesis by group	Synthesis technique	% QYa (highest)	Emission characteristics	Emission caused by type (band, surface states, etc.)	Doping/surface passivation	Prepared CQDs employed in application	Ref.
Ya-Ping Sun	Laser ablation	>10%	Blue emission @ 400 nm excitation	Surface states and quantum confinement	Surface passivation	No	5
			Excitation-dependent emission: emission intensity decreases and the emission peak red shifts as the incident wavelength increases
Ruili Liu	Pyrolysis followed by chemical treatment	14.70%	Blue emission @ 365 nm excitation	Surface states and quantum confinement	Surface passivation	Bioimaging	43
			Excitation-dependent emission: the emission intensity varies and the emission peak red shifts as the excitation wavelength increases from 320−500 nm
Hui Peng	Chemical oxidation	13%	Blue emission @ 360 nm excitation	Surface states and quantum confinement	Surface passivation	No	50
			Excitation-dependent emission: the emission intensity varies and the peak red shifts as the excitation wavelength increases beyond 360 nm
Xiangyou Li	Laser ablation	—	Blue emission @ 360 nm excitation	Surface states and quantum confinement	Surface passivation	No	102
			Excitation-dependent emission: the emission intensity varies and the peak red shifts (from 400–520 nm) as the excitation wavelength is increased from 300–480 nm
Dengyu Pan	Pyrolysis	40.6%	Blue emission @ 365 nm excitation	Large HOMO–LUMO gap of small sp2 clusters	N-Doping	No	45
			Excitation-dependent emission: the emission intensity decreases and peak red shifts (from 425–510 nm) as the excitation wavelength is increased from 320–500 nm
			Emission is also sensitive to pH, solvent and spin
Shengliang Hu	Laser ablation	Sample A: 12.2%	Blue emission @ 365 nm excitation (all 3 samples)	Quantum confinement	No	No	62
		Sample B: 06.2%	Excitation dependent emission in all the samples: the emission intensity and peak varies with the excitation wavelength
		Sample C: 01.2%
Qi Wang	Microwave irradiation	14%	Blue emission @ 365 nm excitation	Not mentioned	Not mentioned	A fluorescent probe for sensitive turn-on sensing of glutathione	37
			Emission intensity increases with pH
Mingbo Wu	Chemical oxidation + hydrothermal (for N-CQD)	CQD: 8.7%	Excitation-dependent emission in both CQDs	Quantum confinement and surface states	N-Doping and surface passivation (N-CQD)	No	51
		N-CQD: 15.8%	CQD: yellow emission @ 340 nm excitation. The emission intensity varies and peaks red-shift from 480 to 600 nm as the excitation wavelength increases from 320 to 580 nm
			N-CQD: blue emission @ 340 nm excitation. The emission peak remains at 475 nm when excitation wavelength increases from 320 nm to 400 nm. As the excitation wavelength further increases to 520 nm, the maximum emission wavelength shows a red-shift from 480 to 540 nm
Jianhui Deng	Electrochemical carbonization	CQD(6.0 V): 15.9%	Blue emission @ 365 nm excitation (by all 4-sets of CQDs synthesized under different potentials)	Quantum confinement and surface states	Undoped, surface passivation not required	Bio-imaging of Hela cells	53
		CQD(3.0 V): 4.0%	The intensity of emission is very low for CQD (3.0 V), it increases for CQD (4.5 V), and reaches a maximum for CQD (6.0 V) and then decreases for CQD (7.5 V)
		CQD(4.5 V): 9.1%	Excitation dependent emission: the emission intensity varies and emission peak red shifts as the excitation wavelength increases from 300 nm–500 nm
		CQD(6.0 V): 15.9%	The PL intensity of the CQDs were independent of pH under acidic conditions, but decreased under basic conditions
		CQD(7.5 V): 5.0%
Hui Ding	Hydrothermal	∼35%	Blue, green, yellow and red emission @ 365 nm excitation (by selected 4-sets of CQDs having different degrees of oxidation)	Surface states	N-Doping	In vivo bioimaging of mice	26
			Excitation-dependent emission: each of the 4 samples have a specific emission centre that does not change with the excitation wavelength but the intensity varies
Yubin Song	Hydrothermal	77.07% (Et-EDA CQD)	Blue emission @ 360 nm excitation (for Et-EDA CQDs)	Molecular states or molecular fluorescence and core states	N-Doping	Bio-application	108
		46.36% (Ac-EDA CQD)	Excitation-dependent emission: the emission intensity and peak shifts (from blue to green) as the excitation wavelength is increased from 300–500 nm
Xugen Han	Hydrothermal	84.80%	Blue emission @ 365 nm excitation	Surface states	N-Doping and surface passivation	Silicon-nanowire solar cells	130
			Excitation-independent emission: almost no shift in emission peak (peak appears at around 430 nm) but the emission intensity decreases as the excitation wavelength is increased from 300–390 nm
			Emission intensity is constant for pH values 4–8 but it decreases for too basic or acidic medium
Hao Wang	Pyrolysis	∼36%	Blue emission @ 365 nm excitation	Surface states	N-Doping	QDSC	47
			Excitation-dependent emission: the emission intensity varies and peak red shifts (from 425–510 nm) as the excitation wavelength is increased from 300–500 nm
Fanglong Yuan	Hydrothermal	∼75% (blue CQD)	Blue, green, yellow, orange and red emission @365 nm excitation (5 different colors of CQDs were prepared by modification in the synthesis process with the highest QY for blue CQDs)	Quantum confinement (bandgap emission)	N-Doping, surface passivation	LED	90
			Excitation independent emission: the emission peak intensity varies but the position remains the same, irrespective of the excitation wavelength for all the CQDs
Julian Schneider	Hydrothermal	53% (ethylenediamine-CQDs)	Blue emission @ 320 nm excitation (by all 3 types of CQDs)	t-CQDs: quantum confinement (core bandgap emission) and surface states e-CQDs	N-Doping	No	104
			Excitation dependent emission: the emission peak intensity and peak position changes as the excitation wavelength varies from 300–480 nm	h-CQDs: molecular fluorescence and surface states
Gancheng Zuo	Hydrothermal	F-CQD: 31%	Excitation-dependent emission in both CQDs	Surface states	F-CQD: F-doping, N-doping	Red cell imaging and sensitive intracellular Ag + detection	75
		Undoped CQD: 28%	F-CQD: yellow emission @ 360 nm excitation
			Emission varies from 550–600 nm as the excitation varies from 360−580 nm
			Undoped-CQD: green emission @ 360 nm excitation
			Emission varies from 480–550 nm as the excitation wavelength varies from 360–500 nm
Hinterberger Vanessaa	Microwave irradiation	5.4%	Blue emission @ 400 nm excitation	Molecular fluorescence	Surface passivation	White LED	41
			The emission spectra is also pH sensitive
Zexi Liu	Hydrothermal	79.1% (blue)	Blue to infrared @ 365 nm excitation (by different sets of CQD)	Quantum confinement and surface states	N-Doping	No	109
			Out of 25 sets of CQD solutions, some sets exhibited excitation-independent emission and some exhibited excitation-dependent emission
Akansha Dager	Pyrolysis	9.5%	Blue emission @ 365 nm excitation	Surface states	Undoped, surface passivation not required	No	11
			Excitation-independent emission: no shift in the PL peak position as the excitation wavelength varies from 240–340 nm only the peak intensity decreases
			Changing the pH from acidic to basic resulted in a gradual increase in the PL intensity of CQD
Yushuang Zhao	Hydrothermal	16.6%	Blue-green emission @ 365 nm excitation	Surface states and carbon core states	Undoped, non-passivated	Fluorescent Cu2+ nanoprobe	34
			Excitation-dependent emission: the emission intensity varies and peak gradually red shift (from 443–489 nm) as the excitation wavelength increases from 320–400 nm. The peak also red shifted (342–379 nm) as the excitation wavelength increased from 400–540 nm. These CQDs exhibited the upconversion emission (anti-Stokes type emission) as the excitation wavelength varied from 760–940 nm
Takashi Ogi	Hydrothermal	39.7%	Blue emission @ 365 nm excitation	Not specified	N-Doping, surface passivation not done	Polyvinyl alcohol (PVA) nanofibers	33
			Emission intensity varies with the heating time/temperature and initial precursor concentration
			Emission is affected the most by the temperature, and the emission intensity first increases and then decreases with the temperature
Rabia Riaz	Hydrothermal	70%	Green emission @ 360 nm and 400 nm	Quantum confinement (bandgap emission) and surface states	N-Doping	DSSC	131
			Excitation-independent emission: almost no shift in the emission peak (peak appears at around 520 nm) but the emission intensity decreases as the excitation wavelength increases from 360–400 nm
Hang Yang	Hydrothermal	1.8%	Yellow-green emission @ 360 nm excitation	Surface states	—	No	35
			Excitation-dependent emission: the emission intensity varies and the peak shifts as the excitation wavelength varies from 300–450 nm
Qiming Yang	Hydrothermal	Not mentioned	Blue emission @ 365 nm excitation	Surface states	N-Doping	DSSC	140
			Excitation-dependent emission: the emission intensity varies and peak shifts as the excitation wavelength increases from 310–550 nm
			Also, as the excitation wavelength changes from 700 to 1000 nm, the PL emission peaks are located in the range from 450 to 540 nm (up-conversion transition or a multi-photon absorption process)
Mumtaz Ali	Hydrothermal	61% (red)	Red emission @ 360 nm excitation	Quantum confinement and surface states	N-Doping	Crystalline silicon solar cells	133
			Excitation-independent emission is attributed to the single-transition mode PL of the NR-CQDs, which resulted from the conjugated structure
Lili Tong	Hydrothermal	90.49% (green)	Green emission @ 400–520 nm excitation	Dehalogenation crosslinking and structural reorganization of reactants	N-Doping, surface passivation	Lysosome imaging	152
			Excitation-independent emission: no shift in the emission peak (peak appears at around 530 nm) but the emission intensity increases and then decreases as the excitation wavelength increases from 400–520 nm
Aysel Başoğlu	Microwave irradiation	1.8%	Blue emission @ 265 nm excitation	Not specified	Undoped, surface passivation not required	Determination of Fe3+ ions	42
			Excitation-dependent emission: the emission intensity varies and peak shifts as the excitation wavelength increases from 300–400 nm
Haitao Lin	Microwave irradiation	85%	Blue emission @ 355 nm excitation	Surface states	Surface functionalization while synthesis	Dopamine fluorescence probe and cellular imaging	153
			Excitation-dependent emission: the emission intensity varies and peak red shifts as the excitation wavelength increases from 300–400 nm
Lei Tian	Chemical oxidation	0.43%	Blue emission @ 310 nm excitation	Surface states	Surface functionalization	No	154

^aQY is irrespective of the excitation wavelength.