Supporting Information

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1 Supporting Information Efficient Room-Temperature Phosphorescence from Nitrogen-Doped Carbon Dots in Composite Matrices Qijun Li, Ming Zhou, *,, Qingfeng Yang, Qian Wu, Jing Shi, Aihua Gong and Mingyang Yang State Key Laboratory of Tribology, School of Mechanical Engineering, Tsinghua University, Tsinghua University, Beijing , China Department of Industrial Engineering, Purdue University,225 South University Street, West Lafayette, Indiana 47907, United States School of Medicine, Jiangsu University, Zhenjiang, Jiangsu , China *Address correspondence to 1

2 Materials. Unless otherwise noted, all reagents were purchased from commercial suppliers and were used without additional purification. Biuret and urea were of analytical-reagent grade and purchased from Aladdin Industrial Corporation. Folic acid was obtained from Capital Bio Corporation. Commercial phosphors (LMS-4949-B) were purchased from Dalian Luming Technology Group Co., Ltd. UV LED chips (peak wavelength: 365 nm) were purchased from Zhuhai Tianhui Electronics Co., Ltd. Deionized water (18.2 MΩ.cm at 25 C) prepared by a Milli-Q (MQ) water system was used throughout all experiments. Synthesis of Fluorescent NCDs. HN-CDs were synthesized similar to the previous method. 1 Briefly, folic acid (FA) (1 g) was dissolved DI water (100 ml). After stirring for mixing, the solution was transferred to a poly (tetrafluoroethylene) (Teflon)-lined autoclave (200 ml) and heated at 260 C for 2 h. After the reaction, the reactors were cooled to room temperature naturally. The obtained dark brown solution was centrifuged under at high speed (10000 rad min -1 ) for 20 min to remove large or agglomerated particles. Purely luminescent HN-CDs were obtained via freeze drying. The obtained HN-CDs were then used to make stock solution (5 mg/ml) for further use. MN-CDs and LN-CDs were prepared under the same method, of which one was prepared at 260 for 6 h, while another was prepared at 260 for 6 h by adding concentrated HCl (12 ml) to the precursor. Preparation of NCD-based RTP Materials. Firstly, different volume (0 ml,0.06 ml, 0.1 ml, 0.2 ml, 0.5 ml, 1 ml, 2 ml, 4 ml) of HN-CD aqueous (5 mg/ml) were mixed respectively with urea (6 g) to obtain HN-CD/urea solutions (1 g/ml for urea) in beakers by shaking for 10 min to completely dissolve the urea. Then the beakers were put into oven at 155 C for 6 h to get different HN-CDs contents in solid bulks (0 mg, 0.3 mg, 0.5 mg, 1 mg, 2.5 mg, 5 mg, 10 mg, 20 mg). After the reaction, corresponding HN-powders with different HN-CDs contents (0 mg, 0.3 mg, 0.5 mg, 1 mg, 2.5 mg, 5 mg, 10 mg, 20 mg) were then obtained by grinding dried blocks in agate mortar and screening out by mesh sieve. MN-powders and LN-powders were prepared based on MN-CDs and LN-CDs, respectively, under the same method as HN- 2

3 powders. Unless stated otherwise, HN-powders in this article is on behalf of HN-powders (0.5 mg). Preparation of HN-starch. The HN-starch phosphors were prepared by simply mixing the HN-CDs (500 ul) and starch (1.5 g) in water under constant stirring for 24 h. The reaction mixtures were filtered to remove the unabsorbed HN-CDs, and the remaining solid blocks on filter paper were freeze-dried in a vacuum freeze dryer. The HN-starch phosphors were then obtained by grinding the dried blocks in agate mortar. Preparation of LED. Different types of emitting powders were blended with a two component silicone on top of a UV (365 nm) LED chips. The mixture was hardened for 2 h at 80 C, forming a homogeneous color-conversion layer. Characterization. TEM images were taken by a H-7650 instrument (Hitachi, Japan). HRTEM images were obtained on a JEM2010 high-resolution field-emission transmission electron microscope at 200 KV (JEOL, Japan). XPS was investigated by using an ESCALAB 250 XI system (Thermo Electron Corporation, USA). Elemental analysis was performed on an Vario EL III elemental analyzer. FTIR spectra were recorded on a Bruker Tensor 27 spectrophotometer (Germany). The absorbance spectra were performed by a Shimadzu UV spectrometer. Steady-state PL and phosphorescence spectra were collected on a Hitachi F-7000 fluorescence spectrophotometer. Time-resolved luminescence spectra were obtained using an Edinburgh FLS920 fluorescence spectrophotometer with the following settings: total decay time, 9 ms; delay time, 0.1 ms; gate time, 0.05 ms. The absolute quantum efficiency was obtained on an Edinburgh FLS920 spectrophotometer equipped with an integrating sphere using BaSO4 as the reflectance standard. Lifetime decay profiles were measured using an Edinburgh FLS920 fluorescence spectrophotometer equipped with a xenon arc lamp (Xe900), a nanosecond hydrogen flash-lamp (nf920) and a microsecond flash-lamp (µf900), respectively. For the temperature-dependent experiment, the sample was placed in an Optistat DN-V liquid nitrogen cryostat with temperatures controlled between 77 and 301 K. The 3

4 lifetimes (τ) of the luminescence were obtained by fitting the decay curve with a multiexponential decay function of I(t) = A i i where A i and τ i represent the amplitudes and lifetimes, respectively, of the individual components for multi-exponential decay profiles. The CIE chromaticity coordinates and correlated color temperatures (CCT) of the two types phosphor-based LEDs were measured by LED Photometric, chromatic Analysis System (Hangzhou Electronic Information Co., Ltd.) at room temperature. AFM measurements were performed on a SA400HV with a Seiko SPI3800N controller. e t τ i 4

5 Scheme S1. Reaction scheme of FA towards formation of NCDs. Figure S1. a-c) TEM images of HN-CDs, MN-CDs and LN-CDs, repetively (insert: HRTEM images. Scale bar, 5 nm), Scale bar, 50 nm. d-f) Size distribution. 5

6 Figure S2. a-c) AFM images and size distributions of HN-CDs, MN-CDs, and LN-CDs. Height profiles are given for the marked red line in the AFM images. Table S1. Element composition percentage of three nitrogen doped carbon dots Element analysis and XPS Deconvoluted N 1s Composition percentage (%) Integral area (%) C N O H Pyridine N Pyrrolic N Graphite N HN-CDs 46.15/ / / MN-CDs 50.26/ / / LN-CDs 59.31/ / / Note: Red character stand for element composition percentage of XPS. 6

7 Figure S3. a-c) Deconvolution of high-resolution C 1s XPS spectra. d-f) Deconvolution of high-resolution O 1s XPS spectra. g-i) Steady-state PL spectra. Table S2. Fit parameters of the fluorescence decay curves of HN-CDs, MN-CDs and LN-CDs respectively. τ 1 (ns) A 1 (%) τ 2 (ns) A 2 (%) τ 3 (ns) A 3 (%) τave (ms) χ 2 HN-CDs MN-CDs no no LN-CDs no no Note: Determined from the fitting function of I(t) = A 1 e x t 1 according to the fluorescence decay curves. χ 2 is the value fit. + A 2 e x t 2 + A 3 e x t 3 For sample 1: the 6 g urea was dissolved in 6 ml solution in a beaker and then heated the 155 C for the 6 h in air blowing oven, at last about 4 g products were obtained. After redissolving, crystallization, filtered and dried, 2.8 g white powders were acquired (labeled as 7

8 sample 1), which indicates the presence of 1.2 g unreacted urea. According to the FT-IR spectra (Figure S4c) sample 1 was confirmed as biuret. For sample 2: the HN-powders (2.5 mg) were re-dissolved in solution and after redissolving, nature crystallization, filtered and dried, white powders were acquired (labeled as sample 2). After a series of treatment, unreacted urea of sample 1 and sample 2 was removed. Compared FTIR spectra of sample 1 with sample 2 (Figure S4c), the peaks at 3234 (N-H stretching vibrations) and 1606 cm-1 (N-H in-plane bending vibrations) diminished in the FTIR spectrum of sample 2, which indicates a possible amidation reaction between HN-CDs and urea or biuret. For sample 3: the prepared HN-powders were re-dissolved in water, after drying and grinding, finally white solid powders were obtained (labeled as sample 3). For sample 4: the HN-CDs, urea and biuret were dissolved in water together under the same proportion as that of the HN-powders, after drying and grinding, white solid powders were obtained (labeled as sample 4). 8

9 Figure S4. a) Fluorescence excitation spectra of HN-CDs. b) Normalized steady-state PL (black) spectrum and phosphorescence (red) spectrum of HN-powders excited at 280 nm. c) FTIR spectra of sample 1 and sample 2. d) Steady-state PL spectra of sample 3, sample 4 and HN-powders excited at 280 nm, respectively. 9

10 Figure S5. a) and b) Steady-state PL and phosphorescence spectra of HN-powders in different reaction time excited at 280 nm, respectively. c) Steady-state PL spectra of the HNpowders at different HN-CDs contents under 280 nm excitation. d) Phosphorescence spectra of the HN-powders at different HN-CDs contents under 280 nm excitation. e) Steady-state PL spectra of HN-powders excited at 280 nm in air, nitrogen, respectively. f) Time-resolved fluorescence spectra of HN-powders excited at 280 nm in air, nitrogen, respectively (total decay time, 9 ms; delay time, 0.1 ms; gate time, 0.05 ms). Note: In figure S5 c and d, with the increases of the concentration of HN-CDs, the emission peak of HN-powders shifted toward longer wavelengths, which is ascribed to self-absorption of HN-CDs. 2 Table S3. Photoluminescence lifetimes (τ) of NCD-powders Photoluminescence τ 1 (ms) A 1 (%) τ 2 (ms) A 2 (%) τ 3 (ms) A 3 (%) τ ave (ms) HN-powders MN-powders LN- powders / / Sample Sample Note: the excitation wavelength was 280 nm, and lifetime was monitored by the emission wavelength at 490 nm. 10

11 The multiple phosphorescent lifetimes (Table S3-6) imply various electronic transition processes, which may be due to a wide range of chemical environments on the surface of NCDs for C=N and C=O bonds. Combining the structural characteristic and composition changes of three kinds of NCDs, lifetime components loss from the HN-powders to the LNpowders further proves that C=N bonds are responsible for the phosphorescence (Table S3). The average lifetimes were calculated using the equation: τ = α i τ i 2 / α i τ i, where A i and τ i represent the amplitudes and lifetimes, respectively, of the individual components for multi-exponential decay profiles. Table S4. Photoluminescence lifetimes (τ) of HN-powders at different reaction time Time (h) Photoluminescence τ 1 (ms) A 1 (%) τ 2 (ms) A 2 (%) τ 3 (ms) A 3 (%) τave (ms) Note: the excitation wavelength was 280 nm, and lifetime was monitored by the emission wavelength at 490 nm. Table S5. Photoluminescence lifetimes (τ) of HN-powders at different HN-CDs contents under ambient conditions Content (mg) Photoluminescence τ 1 (ms) A 1 (%) τ 2 (ms) A 2 (%) τ 3 (ms) A 3 (%) τave (ms) Note: the excitation wavelength was 280 nm, and lifetime was monitored by the emission wavelength at 490 nm. 11

12 Table S6. Photoluminescence lifetimes (τ) of HN-powders at different temperatures from 78 K to 301 K Temperature (K) Photoluminescence τ 1 (ms) A 1 (%) τ 2 (ms) A 2 (%) τ 3 (ms) A 3 (%) τave (ms) Note: the excitation wavelength was 280 nm, and lifetime was monitored by the emission wavelength at 490 nm. Figure S6. a) Steady-state PL spectra of HN-powders measured at temperatures from K. b) Lifetime decay profile of the emission band at 490 nm of HN-powders excited at 360 nm c) Normalized steady-state PL (black) spectrum of HN-powders and commercial phosphors (LMS-4949-B) excited at 360 nm. d) The emission spectrum of assembled LEDs 12

13 by mixing HN-powders and commercial phosphors. The insets are optical image of LEDs. Our HN-powders have a high total absolute QY of 36% under 360 nm excitation and display a wide full width at half maximum (fwhm) of 84 nm (Figure S6c) due to a dualemissive property. One approach to generate white light is to utilize tri-component RGB phosphors excited by UV, but the reabsorption of emitted light in different phosphors give rise to the decrease of the luminous efficiency. In order to reduce self-absorption, we only mixed two types of phosphors (HN-powders/commercial phosphors (LMS-4949-B)) by 365 nm UV chips to fabricate the WLEDs. The mixtures have high PL quantum yields of 38% according to their optimized mass ratio of 20:1. A white light with the CIE coordinates of (0.338, 0.363) was obtained, which is very close to ideal white coordinates (0.333, 0.333). The correlated color temperature (CCT) and color rendering index (CRI) are 5281 K and 67, respectively. Supplementary videos The videos demonstrate that when excited by excited with a 365 nm hand-held UV lamp, the HN-complexes could emit bright cool white light. After the switch off of the UV lamp, an emission color change from cool white to green light was clearly observed by the naked eye. This process was repeated for two times and the same phenomenon was observed. References (1) Shen, C.; Sun, Y. P; Wang, J.; Lu, Y. Facile Route to Highly Photoluminescent Carbon Nanodots for Ion Detection, ph Sensors and Bioimaging. Nanoscale. 2014, 6, (2) Sun, M. Y.; Qu, S. N.; Hao, Z. D.; Ji, W. Y.; Jing, P. T.; Zhang, H.; Zhang, L. G.; Zhao, J. L.; Shen, D. S. Towards Efficient Solid-State Photoluminescence Based on Carbon- Nanodots and Starch Composites. Nanoscale. 2014, 6,