Nitrogen-doped carbon quantum dots are synthesized by a one-step atmospheric pressure microplasma process. The origin of the observed photoluminescence emission and its relationship with nitrogen doping is studied using a range of optical and chemical measurements along with verification by theoretical calculations. Nitrogen doping into the core and functionalization of surface states with nitrogen and oxygen groups gives rise to a hybrid structure which is responsible for the luminescence with quantum yields up to 33%. Carrier multiplication is observed as a step-like enhancement in the quantum yield. The analysis of visible-light emission suggests that the emission originates for the most part from surface states and not due to recombination within the quantum dot core. The role of surface functional groups is dominant over quantum confinement in determining the optical properties.
Bibliographical noteFunding Information:
The authors thank Ulster University , National Institute of Advanced Industrial Science and Technology (AIST) and Engineering and Physical Sciences Research Council (EPSRC) ( EP/M024938/1 , EP/K022237/1 , EP/R008841/1 ) for the financial support. S.D.D acknowledges the financial support from Department for Economy NI (PhD Studentship and USI-146). M.B. and V.S acknowledge the support by Kakenhi ( 20H02579 ) by the Japanese Society for the Promotion of Science . A.M. acknowledges funding from Invest Northern Ireland ( RD0713920 ) and the European Union's INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB). We would like to acknowledge the contribution of Dr Darragh Carolan who has carried out initial experiments that motivated the work reported in this publication.
The photoluminescence (PL) measurements (240 nm excitation) for the N-CQDs synthesized with varying processing currents result in maximum emission intensities at wavelength 410 nm, 414 nm, 417 nm for 4 mA, 6 mA and 8 mA samples as shown in the normalized PL spectra in Fig. 7a. The data has been normalized to the emission maxima. The small red-shift of the emission wavelength could be linked to the small reduction in the bandgap for the N-CQD synthesized at higher current, however this is super-imposed onto surface-state emission (see below) . Colored-coded maps of the photoemission are given in Fig. 7b?d with emission wavelength in the x-axis, excitation in y-axis and the colour scale giving the PL intensity, (the full set of PL emission spectra for each excitation can be found in the Supporting Information, Figs. S9 and S10). For all the samples, the excitation-emission figures show similar patterns with three different regions excited at different wavelengths, labelled as ?A?, ?B? and ?C?. Bandgap emission is expected to be affected by size distribution for each sample and therefore would appear as an elongated pattern in Fig. 7 . This corresponds for instance to the emission that peaks in the range 400?450 nm and excited by ?320 nm (A). This is also sufficiently close to the indirect bandgap (2.38?2.46 eV) for all samples and we can therefore attribute it to bandgap emission from the core of the N-CQDs and originating from above-bandgap energy levels. This core bandgap emission is however relatively weak, and we can observe that lower excitation wavelengths offer different emission pathways. The strongest emission for all samples is for instance in the same region (400?450 nm) but excited at ?240 nm (B). This appears to be different than emission from the core bandgap transitions and it is specifically excited only with a narrow range of wavelengths; it therefore appears as a confined and non-elongated spot (B) in Fig. 7. This emission originates from UV absorption and excitons may have sufficient energy to recombine in different states from where they have been produced. It is therefore likely that this emission is associated with surface states which may include transitions due to N, associated with pyrrolic and amine groups, or O surface states, edge states and molecular states . Finally, Fig. 7b, corresponding to the sample produced at 4 mA, also shows a noticeable emission at 300?325 nm, excited with ?270 nm (C); this is progressively weaker for increasing current in Fig. 7b?c. The emission originates from EDA precursor (see Supporting Information) and suggests that this is consumed more effectively at higher current, possibly due to increasing surface doping over a greater total surface area. This is again in agreement with our previous absorbance results that indicated a higher number of N-CQDs synthesized with increasing current. Overall Fig. 7 shows that emission is dominated by surface states (B) with weak emission from bandgap transitions (A). This is also in agreement with previous reports [91,92]. Therefore, to manipulate the emission of CQDs, varying the nature and density of surface functional groups is the most direct way to produce considerable modification of PL whereas quantum confinement effect or core doping in CQDs is far less effective. This is also confirmed by Deng et al. while studying the optical properties of separated fragments of carbon dots with high colloidal stability by gradient-based centrifugation method . We have observed that the emission properties of these N-CQDs in colloidal form are stable over time and also under given operational conditions (e.g. under prolonged illumination, see section G in the Supporting Information); however, the impact of film formation steps from the colloid, including annealing has shown some changes in the emission patterns (section F of the Supporting Information) that will need to be investigated fully in order to effectively integrate N-CQDs in application devices.The authors thank Ulster University, National Institute of Advanced Industrial Science and Technology (AIST) and Engineering and Physical Sciences Research Council (EPSRC) (EP/M024938/1, EP/K022237/1, EP/R008841/1) for the financial support. S.D.D acknowledges the financial support from Department for Economy NI (PhD Studentship and USI-146). M.B. and V.S acknowledge the support by Kakenhi (20H02579) by the Japanese Society for the Promotion of Science. A.M. acknowledges funding from Invest Northern Ireland (RD0713920) and the European Union's INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB). We would like to acknowledge the contribution of Dr Darragh Carolan who has carried out initial experiments that motivated the work reported in this publication.
© 2021 The Authors
- Nitrogen doping
- Quantum yield
- Optical properties