Supporting information for: Carbon Dots: A Unique. Fluorescent Cocktail of Polycyclic Aromatic. Hydrocarbons

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1 Supporting information for: Carbon Dots: A Unique Fluorescent Cocktail of Polycyclic Aromatic Hydrocarbons Ming Fu,, Florian Ehrat,, Yu Wang, Karolina Z. Milowska, Claas Reckmeier, Andrey L. Rogach, Jacek K. Stolarczyk, Alexander S. Urban,, and Jochen Feldmann Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Amalienstr. 54, Munich, Germany, and Department of Physics and Materials Science and Center for Functional Photonics (CFP), City University of Hong Kong, Hong Kong SAR urban@lmu.de KEYWORDS: Carbon dots, polycyclic aromatic hydrocarbons, excitation-wavelength dependent photoluminescence, self-trapped exciton, energy transfer, single-particle spectroscopy To whom correspondence should be addressed Ludwig-Maximilians-Universität München City University of Hong Kong Contributed equally to this work S1

2 Sample Fabrication and Preparation The carbon dots (CDs) used in this study were fabricated based on a previously reported method. 1 Briefly, g of citric acid and 335 µl of ethylenediamine were dissolved in 10 ml of Milli-Q water and the solution was heated hydrothermally in a Teflon-equipped stainless-steel autoclave at 200 C for 5 hours. The brown-black product was purified by dialysis to obtain colloidal solution of CDs. Then, CDs were collected as a powder by applying the freeze-drying process. For each ensemble measurement of CDs, a fresh aqueous CD solution was prepared by dissolving the CD powder in Milli-Q water. The solution was subsequently drop-casted onto indium tin oxide (ITO) coated glass substrates for the X-ray photoelectron spectral measurements. For singleparticle spectroscopy measurements, the CD powder was dispersed into an 0.1 wt% polyvinylalcohol (PVA) aqueous solution, then spin-coated onto a clean glass substrate at 1000 rpm. The anthracene, pyrene, and perylene doped PMMA films were prepared according to a previously reported method. 2 Anthracene ( g/mol ), pyrene ( g/mol), perylene ( g/mol), toluene (92.14 g/mol), and PMMA with a number-average molecular weight of , were purchased from Aldrich and were used as-received. PMMA was dispersed in toluene, with the fixed concentration of 2 wt%. Then anthracene was added to the PMMA/toluene solution at concentrations of 0.01, 0.1, 1, 10, and 100 mol% relative to the quantity of PMMA monomer units. Anthracene-doped PMMA/toluene solutions were drop-casted onto glass substrates (Roth Objektträger Art. Nr. 0656, soda-lime-silica glass). The cast films were dried at ambient temperature for 24 h. The other molecule-doped PMMA films were prepared via the same procedure. For the samples with two or three of the three species of molecules, each of these was dispersed in PMMA/toluene solution on its own and then the solutions were mixed together. Different ratios between the molecules were achieved by using differently concentrated solutions which were always mixed together in equal parts. S2

3 Experimental methods The chemical composition of the carbon dots was investigated with an X-ray photoelectron spectrometer equipped with a VSW TA10 X-ray source and a VSW HA100 hemispherical analyser. A VSW AS10 argon ion gun was used to remove the top layers of the sample. All spectra were normalized by the area of the C 1s peak. Transmission electron microscopy (TEM) and highresolution TEM images were taken on a JEOL JEM-2100F instrument operated at 200 kv. UV-Vis absorption measurements were taken with a Varian Cary 5000 UV-Vis spectrometer. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded with a Horiba Jobin Yvon Fluorolog-3 FL3-22 spectrometer equipped with water-cooled Horiba R928 photomultiplier tubes mounted at 90 angle. A monochromated Xe-lamp was used for excitation. The signals were multiplied with the correction files of the detector and divided by the corrected excitation intensity. The single-particle PL was detected using a a wide-field epifluorescence microscope comprising a long-working-distance microscope objective (numerical aperture 0.55). 3 The samples were mounted in a cold-finger helium microscope cryostat, excited at 400 nm by a frequency-doubled Ti:Sapphire 80 MHz femtosecond laser (Tsunami, Spectra-Physics), and 488 nm by an Ar-ion laser, incident on the sample at an angle of 30. The scattered excitation light was blocked using a nonfluorescent broadband interference filter. PL was detected using a Peltier cooled chargecoupled device (PCO AG, Sensicam QE) after optionally spectrally dispersing the fluorescence in a spectrometer. References (1) Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; Jin, H.; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Angew. Chem. Int. Ed. 2013, 52, (2) Ito, F.; Kogasaka, Y.; Yamamoto, K. J. Phys. Chem. B 2013, 117, (3) Becker, K.; Lupton, J. M.; Müller, J.; Rogach, A. L.; Talapin, D. V.; Weller, H.; Feldmann, J. Nat. Mater. 2006, 5, S3

4 O -C = O O -C -O s p 3 s p 2 U n s p u tte re d S p u tte re d C o u n ts B in d in g E n e rg y (e V ) Figure S1: C 1s X-ray photoelectron spectra (XPS) of unsputtered (red) and argon ion sputtered (blue) CD samples. N o rm. P L In te n s ity 1.0 E x c ita tio n w a v e le n g th in c re a s e n m n m n m n m n m n m n m n m n m n m Figure S2: Normalized PL spectra of CDs in aqueous solution excited at different wavelengths. S4

5 P L In te n s ity (a.u.) T im e (s ) Figure S3: Temporal evolution of PL intensity of a single CD, measured using single-particle PL spectroscopy. 1.0 C D n m e x n m e x. N o rm. P L In te n s ity 1.0 C D n m e x n m e x Figure S4: Normalized PL spectra of single CDs, excited at 400 nm (blue curves) and 488 nm (cyan curves). S5

6 (a ) (b ) 1.0 P y 1 m o l% n m e x. 1.0 P y 1 m o l% E x c ita tio n W a v e le n g th In c re a s e n m e x. P y 1 m o l% n m e x. P y 1 m o l% 1.0 P y 0.1 m o l% 1.0 P y 0.1 m o l% P y 0.1 m o l% P y 0.1 m o l% C o n c e n tra tio n In c re a s e N o r m. A b s o r b a n c e 1.0 P y 1 m o l% N o r m. P L In te n s ity 1.0 P y 1 m o l% P y 1 m o l% P y 1 m o l% 1.0 P y 1 0 m o l% 1.0 P y 1 0 m o l% C a rb o n d o ts P y 1 0 m o l% P y 1 0 m o l% Figure S5: (a) Normalized absorption and (b) PL spectra of pyrene in PMMA films with different concentrations excited at different wavelengths, as well as of CDs (black dashed lines) in aqueous solution. S6

7 (a ) (b ) 1.0 A n 1 m o l% n m e x. 1.0 A n 1 m o l% E x c ita tio n W a v e le n g th In c re a s e n m e x. A n 1 m o l% n m e x. A n 1 m o l% 1.0 A n 0.1 m o l% 1.0 A n 0.1 m o l% A n 0.1 m o l% A n 0.1 m o l% C o n c e n tra tio n In c re a s e N o r m. A b s o r b a n c e 1.0 A n 1 m o l% N o r m. P L In te n s ity 1.0 A n 1 m o l% A n 1 m o l% A n 1 m o l% 1.0 A n 1 0 m o l% 1.0 A n 1 0 m o l% C a rb o n d o ts A n 1 0 m o l% A n 1 0 m o l% Figure S6: (a) Normalized absorption and (b) PL spectra of anthracene in PMMA films with different concentrations excited at different wavelengths, as well as of CDs (black dashed lines) in aqueous solution. S7

8 (a ) (b ) 1.0 P e 1 m o l% n m e x. 1.0 P e 1 m o l% E x c ita tio n W a v e le n g th In c re a s e n m e x. P e 1 m o l% n m e x. P e 1 m o l% C o n c e n tra tio n In c re a s e N o r m. A b s o r b a n c e 1.0 P e 0.1 m o l% 1.0 P e 1 m o l% N o r m. P L In te n s ity 1.0 P e 0.1 m o l% 1.0 P e 1 m o l% P e 0.1 m o l% P e 1 m o l% P e 0.1 m o l% P e 1 m o l% 1.0 P e 1 0 m o l% 1.0 P e 1 0 m o l% P e 1 0 m o l% P e 1 0 m o l% Figure S7: (a) Normalized absorption and (b) PL spectra of perylene in PMMA films with different concentrations excited at different wavelengths, as well as of CDs (black dashed lines) in aqueous solution. S8

9 1.0 A n 1 (a ) 0.5 M ix tu r e S u p e r p o s itio n P y 1 P e 1 m o l% N o rm. P L In te n s ity 1.0 (b ) A n (c ) A n M ix tu r e S u p e r p o s itio n M ix tu r e P y 0.1 P e 0.1 m o l% P y 1 P e 1 m o l% S u p e r p o s itio n (d ) M ix tu r e A n 1 0 P y 1 0 P e 1 0 m o l% S u p e r p o s itio n Figure S8: Normalized PL spectra of mixed anthracene/pyrene/perylene/pmma films (red curves), and the calculated superposition of the measured spectra of anthracene/pmma, pyrene/pmma, and perylene/pmma films (blue curves), with different concentrations. The excitation wavelength is 337 nm. S9