Roles of Nitrogen Functionalities in Enhancing the. Excitation-Independent Green-Color Photoluminescence of

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1 Electronic Supplementary Material (ESI) for Nanoscale. This journal is The Royal Society of Chemistry 2017 Electronic Supplementary Information Roles of Nitrogen Functionalities in Enhancing the Excitation-Independent Green-Color Photoluminescence of Graphene Oxide Dots Chiao-Yi Teng, a Ba-Son Nguyen, a Te-Fu Yeh, a Yuh-Lang Lee, a Shean-Jen Chen, b and Hsisheng Teng* a a Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan b College of Photonics, National Chiao Tung University, Tainan 71150, Taiwan *To whom correspondenec should be addressed. hteng@mail.ncku.edu.tw, Tel: , Fax: Supporting Information for: (1) Color and quantum yield of graphene oxide dots (2) Color and quantum yield of carbon dots (3) TEM images of Ds (4) XPS analysis of NDs and Ds (5) Raman spectra of the -based dots (6) FTIR spectra of the -based dots (7) PL quantum yield measurements (8) The Mott-Schottky equation for the conductivity-type determination 1

2 (9) UPS analysis for valence band maxima of the -based dots (10) Application of A-Ds as a phosphor for white-light emission 2

3 1. Color and quantum yield of graphene oxide dots Table S1 Summary of the color/wavelength (λ) and quantum yield (QY) of photoluminescence emissions from graphene oxide dots synthesized through top-down and bottom-up routes. color/λ (nm) QY (%) ultraviolet/ heteroatom (source) top-down (poly(ethylene glycol)) violet/407* 3.4 violet/ (ammonia) violet/ violet/ violet/ (ammonia) (ammonia) violet/ violet/ (dimethylformamide) blue/ blue/ blue/ blue/ cyan/490* 13 cyan/ (poly(ethylene glycol) diamine) (dimethylformamide) (poly(ethylenimine)) boron (borax) (dimethylformamide) cyan/ green/ green/ green/ green/ (dimethylformamide) (dimethylformamide) (dimethylformamide) (insulin) 3 precursor (synthesis) (ultrasonic) graphite flask, CNT (electrochemical) (solvothermal) (microwave) (chemical-oxidation) graphite nano-particles (solvothermal) graphite rod (electrochemical) (solvothermal) (microwave) (solvothermal) (solvothermal) (solvothermal) carbon black (acidic-refluxing) Ref

4 green/ (dimethylformamide) green/540* 14 yellow/ (dimethylformamide) yellow/ red/ violet/402* 84 violet/425* 40 violet/435* 78 violet/436* 79 (dimethylformamide) bottom-up (melamine) (ammonia) /sulfur (urea/thiourea) (melamine) blue/ blue/450* 94 blue/ blue/ (ethylene diamine) (dicyandiamide) (hydrazine) blue/ blue/ cyan/ cyan/495* 76 (hydrazine) (melamine) cyan/ green/524* 44 green/ green/535* 12 green/560* 71 yellow/ yellow/ /chlorine (ethylenediamine/hcl) (hydrazine) sulfur (Na2S) (precursor) (hydrazine) (ammonia) 4 graphite nano-particles (solvothermal) graphite rod (electrochemical) graphite nano-particles (solvothermal) carbon black (acidic-refluxing) graphite nano-particles (solvothermal) melamine (pyrolysis) citric acid citric acid melamine (pyrolysis) L-glutamic acid (pyrolysis) citric acid citric acid pyrene glucose (microwave) glucose (microwave) pyrene melamine (pyrolysis) hexabenzocoronene (pyrolysis) glucose pyrene pyrene o-phenylenediamine (electrochemical) pyrene glucose (basic-mixing)

5 orange/587* 72 (precursor ) melamine (pyrolysis) 21 *excitation independent 5

6 2. Color and quantum yield of carbon dots Table S2 Summary of the color/wavelength (λ) and quantum yield (QY) of photoluminescence emissions from carbon dots synthesized from the bottom-up route. color/λ (nm) QY (%) violet/ violet/ violet/ violet/ blue/ blue/ blue/450* 80 blue/450* 85 yellow/568* 33 yellow/573* 38 red/620* 47 heteroatom (source) bottom-up (melamine) /sulfur (L-cysteine) (2-aminoethanol) /phosphorous (ammonia/phosphoric acid) /magnesium (ethylenediamine/mg(oh)2) (ethylenediamine) /boron (ethylenediamine/boric acid) (ethylenediamine) (precursor) (precursor) /sulfur/copper (L-cysteine/CuCl) precursor (synthesis) citric acid citric acid citric acid (microwave) glucose citric acid poly(saccharide) citric acid citric acid triaminobenzene (solvotherml) o-phenylenediamine (microwave) HEDTA a (microwave) Ref *Excitation independent a N-(hydroxyethyl)-ethylenediaminetriacetic acid 6

7 3. TEM images of Ds Fig. S1 Morphology and crystal structure of Ds. (a) TEM image with the inset showing the histogram of size distribution. (b) HRTEM image of a D particle, showing graphene {1100 } lattice planes with a d-spacing of nm. 7

8 4. XPS analysis of NDs and Ds Fig. S2 XPS spectra of NDs and Ds. (a) Full-range spectrum of NDs. (b) Full-range spectrum of Ds. (c) C 1s spectrum of NDs. (d) C 1s spectrum of Ds. (e) N 1s spectrum of NDs. C 1s and N 1s spectra are decomposed into several peaks (indicated by dashed lines) and fitted using a Gaussian function. 8

9 5. Raman spectra of the -based dots Fig. S3 Raman spectra of Ds, NDs, A-Ds, and A-NDs. The dash lines indicate the positions of the D, G, D, and 2D bands of Ds. 9

10 6. FTIR spectra of the -based dots Fig. S4 FTIR spectra of Ds, NDs, A-Ds, and A-NDs. 10

11 7. PL quantum yield measurements A highly reliable method for evaluating the PL quantum yield (QY) is the comparative method of Williams et al., 48 which involves the use of well characterized standard samples with known QY values. Plot a graph of integrated fluorescence intensity from fluorescence spectrum vs. absorbance from UV-vis absorbance spectrum. Repeat above steps for five solutions with increasing concentrations of the standards and samples. The PL QY of the samples were calculated in accordance with the following cross-calibration equation, 49 QY x Grad η 2 x x = QY 2 st Grad st η st (3) where Grad is the gradient of the plot of fluorescence intensity vs. absorbance and η is the reflective index of the solvent. Note that x refers to the sample and subscript st refers to a standard with known quantum yield (the values of QYst here for a fluorescein-ethanol solution and a rhodamine 6G-ethanol solution are 0.79 and 0.95, respectively 50,51 ). In order to minimize re-absorption, absorbance in the mm fluorescence cuvette was kept below 0.1 at an excitation wavelength 470 nm. For each test sample, two QYx values were obtained, one relative to standard A, the other to standard B. The QY of the sample was then taken as the average of the two values. Fig. S5 presents the typical fluorescence and absorbance measurements for an A-D dispersion and a fluorescein-ethanol standard solution. Fig. S6 shows the determination of the gradients in the plots of fluorescence intensity vs. absorbance for the two standards and the A-Ds. Table S3 presents the gradients and PL QY of the -based dots determined using the data of Figs. S5 and S6 in compliance with Eq. (3). 11

12 Fig. S5 PL spectra of (a) the A-Ds dispersion (0.1 mg ml -1 ) and (b) the fluorescein-ethanol standard solution (0.03 mg ml -1 ). Absorption spectra of (c) the A-Ds dispersion and (d) the fluorescein-ethanol standard solution. 12

13 Fig. S6 Plots of PL intensity vs. absorbance for Rhodamine 6G (black line), fluorescene (red line), and A-Ds (blue line). Note that the gradient of the lines is proportional to the QY of the corresponding sample. 13

14 Table S3 The gradients and PL QY of the -based dots determined using the data of Figs. S5 and S6 in compliance with Eq. (3). Gradx QYFL QYR6G QYaverage Ds NDs A-Ds A-NDs The parameters used in Eq (3): GradFL = GradR6G = ηwater = 1.33 ηethanol = 1.36 QYFL = 0.79 QYR6G =

15 8. The Mott-Schottky equation for the conductivity-type determination The Ds were deposited on a glassy carbon substrate. The conductivity types and Fermi level (EF) potentials of the D films were then determined via electrochemical impedance spectroscopic analysis based on the Mott-Schottky equation, 46,47 i.e. 1 C 2 1 C 2 2 kt = ( E EF ) for n-type conductivity e εε N e 0 D 2 = eεε N 0 A ( E + E F + kt e ) for p-type conductivity where C represents the capacitance of the space charge region, ε0 is the vacuum permittivity, ε is dielectric constant of semiconductors, e is the electron charge, E is applied potential, EF is the Fermi level potential, k is the Boltzmann constant, T is the absolute temperature, and NA (ND) is the acceptor (donor) density. Nota that the temperature term is generally small and can be neglected. The capacitance values of the space charge region were obtained at various applied potentials. According to the Mott-Schottky equation, 1/C 2 and E are linearly related, with a negative slope indicating p-type conductivity and a positive slope indicating n-type conductivity. Fig. S7 presents the variation of the capacitance in the space-charge region of the Ds and A-Ds with the applied potential in compliance with the Mott Schottky equation. 15

16 Fig. S7 Variation of capacitance (C) with applied potential in 2 M H2SO4 presented in the Mott-Shottky relationship for electrodes deposited with (a) Ds and (b) A-Ds. The capacitance was determined by electrochemical impedance spectroscopy and the negative and positive slopes correspond to p- and n-type conductivities, respectively. 16

17 9. UPS analysis for valence band maxima of the -based dots To identify the valence band maxima (i.e., the n-state, denoted as Ev), the Ds were deposited on the silicon substrate and the value of Ev was determined using UPS with He I light (21.2 ev) irradiation. The UPS analysis was performed in accordance with: EB + Ek + ϕ = 21.2 (1) where EB is the binding energy measured from the Fermi level, Ek is the kinetic energy of electrons, ϕ is the work function of the Ds, and 21.2 ev is the excitation energy of the He I light. Ev was then calculated as: Ev = 21.2 (EB2 EB1) (2) where EB2 is the secondary cutoff binding energy in the UPS spectra, in which the Ek of the excited electrons is equal to 0 and the EB1 is the difference between the Fermi edges and the valence band edges. Fig. S8 shows the UPS spectra of the Ds and A-Ds. Note that EB1 can be determined from the intercepts of the extrapolated straight lines on the abscissa at low binding energy. Similarly, EB2 can be estimated using the secondary cutoff values (Ek = 0 ev) in the UPS spectra, which are obtained from the intercepts of the extrapolated straight lines on the abscissa at high binding energy. The UPS widths is obtained directly as the difference between EB2 and EB1. Finally, Ev is obtained by subtracting these UPS widths from the excitation energy (21.2 ev). 17

18 Fig. S8 UPS spectra of the samples: (a) Ds and (b) A-Ds. The VBM levels with respect to the Fermi levels were determined from the intercepts of the extrapolated straight lines (blue dashed line) on the abscissa at low binding energy. The intersections of the tangent (red dashed line) with the abscissa at high binding energy give the secondary electron onset binding energy. The UPS widths (black lines) can be determined by these two intercept binding energies, and the VBM levels can be calculated by subtracting these widths from the excitation energy (21.2 ev). 18

19 10. Application of A-Ds as a phosphor for white-light emission We mixed aqueous solutions of A-D (0.5 g L -1 ) and poly(vinyl alcohol) (PVA) (10 wt%) to form the precursor mixture of the A-Ds-embedded PVA film. For the fabrication of white light emitting diode (LED), the mixture was dispensed on a violet (365 nm)-led chip and thermally dried at 60 C for 24 h. The combination of the A-Ds-embedded PVA film and violet-led chip provides white light emissions, which are tunable through adjusting the concentration of A-Ds in the PVA film. The device was characterized in a N2-filled glove box with oxygen and water contents less than 1 ppm. The Commission International d Eclairage color coordinate of the light emission from the device was measured using a Keithley 2400 source meter and a PR650 colorimeter. Fig. S9 A-Ds as a phosphor for white-light emission. (a) A device consisting of an A-Ds-embedded PVA film covering a violet (365 nm)-light emitting diode (LED) chip. (b) White light emission from the device when the LED turned on. (c) The Commission International d Eclairage color coordinate for the white light emission shown in panel (b). 19

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