Carbon Nanodots with Nearly Unity Fluorescent Efficiency Realized via Localized Excitons

Abstract Carbon nanodots (CDs) have emerged as an alternative option for traditional nanocrystals due to their excellent optical properties and low toxicity. Nevertheless, high emission efficiency is a long‐lasting pursuit for CDs. Herein, CDs with near‐unity emission efficiency are prepared via atomic condensation of doped pyrrolic nitrogen, which can highly localize the excited states thus lead to the formation of bound excitons and the symmetry break of the π–electron conjugation. The short radiative lifetimes (<8 ns) and diffusion lengths (<50 nm) of the CDs imply that excitons can be efficiently localized by radiative recombination centers for a defect‐insensitive emission of CDs. By incorporating the CDs into polystyrene, flexible light‐converting films with a high solid‐state quantum efficiency of 84% and good resistance to water, heating, and UV light are obtained. With the CD–polymer films as light conversion layers, CD‐based white light‐emitting diodes (WLEDs) with a luminous efficiency of 140 lm W−1 and a flat‐panel illumination system with lighting sizes of more than 100 cm2 are achieved, matching state‐of‐the‐art nanocrystal‐based LEDs. These results pave the way toward carbon‐based luminescent materials for solid‐state lighting technology.

dissolve the phloroglucinol completely. Then add the reactant to a 25-ml Teflon reactor and heat it at 200 °C for 5 hours. After the reaction, the reactant was cooled to room temperature, then CD1 was obtained. The synthesis method of CD2 and CD3 is similar to that of CD1. The raw materials of CD2 are 0.1-g phloroglucinol and 0.1-g o-phenylenediamine, and the raw material for CD3 is 0.1-g o-phenylenediamine.

Fabrication of the Composite Film with CDs In PS
To fabricate CDs@PS composite films with different mass ratio from 0.05 wt. % to 0.30 wt. %, 0.5 g of PS was added into 5-30 ml of CDs solution (0.05 mg ml -1 in toluene). The mixture was heated and continuously stirred at 60 °C until the PS was dissolved. Then, the homogeneous mixture was poured onto a piece of glass. After the evaporation of the solvent at 60 °C, CDs@PS composite films were obtained on the glass.

Fabrication of LED Devices
CDs and PS (0.10 wt. %) mixture in toluene solution was dropped on the LED chip carefully, and heated the LED chip at 45 °C to control the evaporation rate of solvent at a low level. After the evaporation of the solvent, the WLED was obtained.

Fabrication of Flat Panel Illumination System
Firstly, a light reflecting film was attached to the back surface of a light guide plate, and a light bar that consists of several blue GaN LED chips was installed on one side of the light guide plate (Epistar Corporation, China). Then, full sides of the light guide plate were covered with reflective tape. To obtain a uniform CDs@PS composite film on the top surface of the light guide plate, a blade coating was used. Certain amount of the mixture of CDs and PS was dropped on one side of the top surface of the light guide plate and then the blade was pushed forward with a low and constant speed to form a layer of wet film on the surface. After heated about 2 h at 45 °C, the solvent in the composite film could be completely moved and a uniform film generated on the top surface of the plate. Finally, a light diffuser plate was attached on the composite film and the CD-based flat panel illumination system was obtained.

Femtosecond Transient Absorption
Helios pump-probe system (Ultrafast Systems LLC) coupled with an amplified femtosecond laser system (Coherent, 35 fs, 1 kHz, 800 nm) was used. The probe pulses (from 450 to 760 nm) were generated by focusing a small portion (around 10 μJ) of the fundamental 800-nm laser pulses into a 1-mm CaF2. The 365-nm pump pulses were generated from an optical parametric amplifier (TOPAS-800-fs).

Preparation of CD-Based Electroluminescence Device
Transparent indium tin oxide (ITO) glass substrates were ultrasonically cleaned and plasma treated for 10 min in the different organic solvents (acetone, ethanol and deionized water) as a cathode. And then, ZnO nanoparticles (30 mg ml -1 ) as an electron transport layer were spin-coated onto ITO substrate at a speed of 2000 rpm with a heat treatment at 120 °C for 30 min. were deposited in the glove box. After cooling, the CDs dispersed in toluene solutions (8 mg ml -1 ) as the luminous layer were spin-coated on the ZnO layer at the speed of 2000 rpm, then annealed at 80 °C for 30 min in glovebox. And finally, 4,4′-bis(carbazol-9-yl)biphenyl (CPB) hole transport layer and MoO3/Al double-layered anode were deposited by step-by-step thermal evaporation. With a help of metal mask, the effective emission area of the device is kept at about 4 mm 2 .

Characterization
FEI Talos F200 transmission electron microscopy was used to obtain high-resolution TEM images of the CDs. The X-ray diffraction (XRD) patterns of the CDs were recorded by using Bruker D8 Discover (Germany) X-ray diffractometer. The X-ray photoelectron energy spectra (XPS) were collected using a Kratox AXIS HIS 165 spectrometer with a monochromatized Al KR X-ray source. Fourier transform infrared (FT-IR) spectra were measured by a Thermo Nicolet iz 10 spectrometer. To collect the PL spectra of the CDs solution, a Hitachi F-7000 spectrometer was used.
The UV-vis absorption and transmittance spectra were recorded by Hitachi UH-4150 spectrometer. Fluorescence lifetimes and low-temperature fluorescence were measured using Horiba FL-322 using a 405-nm NanoLED as the excitation source.

Calculation of optical phonon energy (ℏ )
ℏ ℎ can be obtained by fitting the temperature-dependent full-width at half-maxima (FWHM) of PL peaks using the following relation (1): where S is Huang-Rhys factor and is Boltzmann constant [1].

Lifetime of radiative and nonradiative recombination
The photoluminescence decay spectra of the CDs with monitored emissions ranging from 500 to 600 nm all can be fitted by a single-exponential function with a variable PL lifetime ( ). The radiative lifetimes ( ) and effective non-radiative lifetimes ( ) for the localized exciton emission in CDs are derived from the experimental results of and internal quantum yield ( ) using the following equations (2 and 3): Where for the CDs is approximated as (300 K)/ (75 K) [2].

Theoretical calculations
Full geometry optimizations of all considered models were performed using density functional theory ((DFT) with the Becket three-parameter hybrid density functional ((B3LYP) [3] with the D3 empirical dispersion correction by Grimme et al. [4] and the 6-311G(d) basis set [5]. All the optical properties of all considered models were calculated using time-dependent density functional theory method (TDDFT) with B3LYP function and 6-311G(d) basis set. The excited state was optimized in vacuum to calculate the emission energy (wavelength) which is the energy difference between the ground and the excited state. The Gaussian09 software package has been used throughout this work [6].

Simulation of white LED
The calculation is based on the reference [7]. For a down-conversion white LED, the spectral overlap between the absorption of down-conversion materials and the electroluminescence of the LED chip results in photoluminescence of the down-conversion materials. Then the total output power spectral density is generated by combination of photoluminescence of the down-conversion materials and transmitted incoming photon flux. Therefore, the output power spectral density can be calculated by following equations (4-6): In the equations, ( ) is the output power spectral density and ( ) is the input power spectral density. ( ) is the normalized PL spectrum and Q is the photoluminescent quantum yield. ( ) is the absorption coefficient and D is optical path length. W is corresponded to the emission strength, when only emission (radiative recombination) is considered. C(λ) is the spectral multiplication factor to the overall emission due to reabsorption and inter-absorption, which is the simplified form of the sum of infinitely many reabsorption and inter-absorption cycles.
From the equations above, output power spectral density ( ( )) of the WLED can be obtained. Then, the luminous efficiency (LE) of the WLED can be calculated by the following equation (7): Where the ( ) is luminosity function and is the input power. Fig. S1 a XPS spectrum of CDs. b High-resolution XPS spectra of C 1s and (c) O 1s.    Table S1.

Note: In the work "Yellow-Emissive Carbon Dots with High Solid-State
Photoluminescence" (e.g.10.1002/adfm.202110393), the authors reported heat treatment of citric acid and urea in toluene solvents, yielding CDs with PL QY of 92%.
The high fluorescence of CDs came from the functionalized conjugated sp 2 carbon domains with edge groups (fused aromatic rings), and the luminescence position was affected by the size of the fused rings. However, the QY of the crude product is only 53%. Cumbersome column chromatography separation means must be used in order to obtain such an efficient yellow CDs, which is very detrimental to their mass production. In our work, CDs with near-unity emission efficiency have been prepared via atomic condensation of doped pyrrolic nitrogen, which can highly localize the excited states thus lead to the formation of bound excitons and the symmetry break of the π-electron conjugation. These yellow-emitting CDs were be directly prepared with a heteroatom-doping solvothermal method with Methyl red as carbon sources and o-phenylenediamine as doping sources. The synthesis strategy is quite different, and the preparation method is relatively simple compared with the literature work.
As you mentioned about stability against UV irradiation for the CDs@PS film in our work, we have compared some previous reports with our results on the stability of CDs (                    Note: We have carried out bending test on the composite film (Fig. S23). The CDs@PS composite films can be bent more than 1000 times without any breakage.
After 1000-time bends, the composite film still maintained about 60% in luminous intensity, demonstrating good fluorescence emission stability. CD solution still has good stability in variable temperature environment more than 120 hours, keeping larger than 70% of the original PL intensity even the solution temperature reaches to 80 °C. Table S1 PL QY of the reported yellow-emitting CDs.

TABLES
YEAR PLQY REF.