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Bright high-colour-purity deep-blue carbon dot light-emitting diodes via efficient edge amination

Abstract

Deep-blue light-emitting diodes (LEDs) (emitting at wavelengths of less than 450 nm) are important for solid-state lighting, vivid displays and high-density information storage. Colloidal quantum dots, typically based on heavy metals such as cadmium and lead, are promising candidates for deep-blue LEDs, but these have so far had external quantum efficiencies lower than 1.7%. Here we present deep-blue light-emitting materials and devices based on carbon dots. The carbon dots produce emission with a narrow full-width at half-maximum (about 35 nm) with high photoluminescence quantum yield (70% ± 10%) and a colour coordinate (0.15, 0.05) closely approaching the standard colour Rec. 2020 (0.131, 0.046) specification. Structural and optical characterization, together with computational studies, reveal that amine-based passivation accounts for the efficient and high-colour-purity emission. Deep-blue LEDs based on these carbon dots display high performance with a maximum luminance of 5,240 cd m−2 and an external quantum efficiency of 4%, notably exceeding that of previously reported quantum-tuned solution-processed deep-blue LEDs.

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Fig. 1: Density functional theory calculations on model CDs.
Fig. 2: Synthesis and optical properties of HCP-DB-CDs.
Fig. 3: Time-resolved PL spectra, low temperature-dependent PL spectra and femtosecond transient absorption (TA) spectra of HCP-DB-CDs.
Fig. 4: LED structure, energy diagram and performance.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work is supported by Tier 1 Canada Research Chair in Organic Optoelectronics (grant number 950-220944), the Natural Sciences and Engineering Research Council of Canada (NSERC, grant number 216956-12), and the National Natural Science Foundation of China (grant number 11774304). E.H.S. and all coauthors from the Department of Electrical and Computer Engineering at the University of Toronto acknowledge financial support from the Ontario Research Fund−Research Excellence Program and from the Natural Sciences and Engineering Research Council of Canada (NSERC). M.I.S. acknowledges the support of the Banting Postdoctoral Fellowship Program, administered by the Government of Canada. Computations were performed on the Niagara supercomputer at the SciNet HPC Consortium. SciNet is funded by the Canada Foundation for Innovation; the Government of Ontario; Ontario Research Fund-Research Excellence; and the University of Toronto.

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Contributions

Z.-H.L. and E.H.S. supervised the project. F.Y. conceived the idea and designed the experiments. F.Y. prepared the samples and fabricated devices. Y.-K.W. conducted the AFM characterization and helped prepare the figure sets. O.V., G.S. and K.S. conducted the density functional theory calculations. X.Z. and O.M.B. provided useful suggestions and helped with the manuscript revision. Y.D. and G.B. measured the femtosecond TA spectra. P.L. and H.K. measured the UPS and XPS. A.J. measuerd the temperature-dependent PL. J.Z.F. measured the FITR spectra. B.C. measured the SEM image. F.Y. wrote the first draft of the manuscript. Z.-H.L., O.V. and E.H.S. provided major revisions. All authors discussed the results and contributed to the manuscript.

Corresponding authors

Correspondence to Zheng-Hong Lu or Edward H. Sargent.

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Extended data

Extended Data Fig. 1 Optical properties of C-DB-CDs.

a, b, e, f, PL spectra (a), absorption spectra (b), time-resolved PL spectra (e) and femtosecond TA spectra (f) of C-DB-CDs. c, Colour coordinates of the emission spectra of C-DB-CDs and HCP-DB-CDs. d, PL spectra of C-DB-CDs treated at 200 °C for 1 h without the addition of amination reagents. Source data

Extended Data Fig. 2 Density functional theory calculations on model CDs.

a, b, c, The geometries of COOH-CD at different time intervals, such as 700 fs (a), 1,000 fs (b) and 1,200 fs (c), respectively. d, e, The structure (d) and bandgap fluctuations (e) of model CDs functionalized with different functional groups such as OH and NH2 without COOH. Source data

Extended Data Fig. 3 Optical properties of HCP-DB-CDs.

a, PL spectra of HCP-DB-CDs excited at different wavelengths. b, PL excitation spectra of HCP-DB-CDs with emission at 433 nm. c, PL spectra of HCP-DB-CDs measured in the fresh state and after storing for six months in ambient conditions. d, e, Time evolution of femtosecond TA spectra in the time range from 0.2 ps to 2,027 ps (d) and from 0.2 ps to 2.15 ps (e), excited at 355 nm. f, Two-dimensional pseudocolour map of TA spectra of HCP-DB-CDs expressed in ΔOD (the change of the absorption intensity of the sample after excitation) as a function of both delay time (from 0 to 20 ps) and probe wavelength excited at 355 nm. Source data

Extended Data Fig. 4 UPS data of HCP-DB-CDs.

a, b, Valence band spectrum of HCP-DB-CDs in the binding energy range from –3 eV to –16 eV (a) and from −10 eV to −16 eV (b). c, Secondary electron spectrum of HCP-DB-CDs. Source data

Extended Data Fig. 5 Structural characterization of HCP-DB-CDs and C-DB-CDs.

a, b, TEM image (a) (inset is the corresponding high-resolution TEM image), and the size distribution (b) of HCP-DB-CDs. c, d, e, f, Comparison of X-ray powder diffraction patterns (c), Raman spectra (d), Fourier transform infrared spectra (e) and XPS full spectra (f) of HCP-DB-CDs and C-DB-CDs. Source data

Extended Data Fig. 6 High-resolution C1s and N1s XPS spectra of HCP-DB-CDs and C-DB-CDs.

a, b, High-resolution C 1s XPS spectra of C-DB-CDs (a) and HCP-C-DB-CDs (b). c, d, High-resolution N 1s XPS spectra of C-DB-CDs (c) and HCP-C-DB-CDs (d). Source data

Extended Data Fig. 7 Cross-sectional TEM image of LEDs and film properties of the active emission layer.

a, Cross-sectional TEM image of the HCP-DB-CD-based LEDs. b, c, SEM image (b) and AFM height image (c) of the active emission layer of PVK:HCP-DB-CDs film. d, PL spectra of PVK and PVK:HCP-DB-CD films. (Insets are the photographs of two films under ultraviolet light). e, Comparison of the PL spectra of PVK:HCP-DB-CD films (HCP-DB-CDs are dispersed in the polymer PVK with the concentration in the range 1.5–4 mg ml–1) and HCP-DB-CDs solution. f, Time-resolved PL spectra of PVK:HCP-DB-CDs film. Source data

Extended Data Fig. 8 EL properties and current density curves as a function of applied voltage for the HCP-DB-CD-based LEDs.

a, b, EL spectra (a) and normalized EL spectra (b) of HCP-DB-CD-based LEDs at different operation voltage. (Inset is the operation photograph of the HCP-DB-CD-based LEDs operated at 7 V). c, Current density curves as a function of applied voltage for the HCP-DB-CD-based LEDs prepared with different concentrations of PVK:HCP-DB-CDs. d, Normalized EL spectra before and after operation for three hours. Source data

Extended Data Fig. 9 Luminance-voltage characteristic and driving voltages for the HCP-DB-CD-based LEDs versus operation time.

a, Luminance–voltage characteristic of HCP-DB-CD-based deep-blue LEDs with maximum brightness of 5,240 cd m–2 (The concentration of TFB was 3.5 mg ml–1 for spin-coating to form a hole-transport layer). b, Luminance–voltage characteristic of HCP-DB-CD-based deep-blue LEDs with maximum brightness of 954 cd m–2 (without the TFB hole-transport layer). c, Driving voltages for the HCP-DB-CD-based LEDs versus operation time under ambient conditions with constant driving current density of 40 mA cm–2. Source data

Extended Data Fig. 10

Comparison of the device structure and performance of CD-based LEDs for previously reported works relative to the present work.

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Supplementary Figs. 1–7 and Table 1

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Yuan, F., Wang, YK., Sharma, G. et al. Bright high-colour-purity deep-blue carbon dot light-emitting diodes via efficient edge amination. Nat. Photonics 14, 171–176 (2020). https://doi.org/10.1038/s41566-019-0557-5

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