Molecular Fluorescence in Citric Acid-Based Carbon Dots - Supporting Information

Transcription

1 Molecular Fluorescence in Citric Acid-Based Carbon Dots - Supporting Information Julian Schneider, Claas J. Reckmeier, Yuan Xiong, Maximilian von Seckendorff, Andrei S. Susha, Peter Kasák, and Andrey L. Rogach,* Department of Physics and Materials Science and Center for Functional Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong S.A.R. Center for Advanced Materials, Qatar University, PO Box 2713, Doha, Qatar *Corresponding author. andrey.rogach@cityu.edu.hk In the following, additional Figures with the relevant information included into figure captions are provided to expand the characterization of the three carbon dot species discussed in the manuscript. Figure S1: TEM images of CDs Figure S2: FT-IR spectra of CDs Figure S3: Photoluminescence decay curves of CDs Figure S4: Additional photoluminescence emission and excitation spectra of e-cds Figure S5: Additional photoluminescence emission and excitation spectra of h-cds Figure S6: Additional photoluminescence emission and excitation spectra of t-cds Figure S7: Excitation dependent emission properties of e-, h- and t-cds at different concentrations. 1

2 Figure S1 TEM images of the three carbon dot species. Agglomeration tendency is seen for e- and h-cds, while in case of the t-cds the particles are rather well separated.. Figure S2 FT-IR spectra indicate the presence of various functional groups in e-, h- and t- CDs. Alcohol (-OH) and amine (-NH 2 ) groups are present in all three CDs, as evidneced by the broad signals around 3500 cm -1. More distinct differences can be observed at lower wavenumbers. Strong absorbance in the range of cm -1 is associated with C=O stretching vibration and reveal multiple carbonyl and carboxyl functionalities, especially in e- and h-cds. The strong absorption peak at 1710 cm -1 in e- and h-cds can be ascribed to C=O stretching vibration of the α,β unsaturated carboxylic acid, which is found in citrazinic acid and its derivatives. A common feature in all three CDs can be found at 1383 cm -1, which is assigned to the CH 3 deformation vibration. At even lower wavenumbers, t-cds show no specific signals anymore, whereas both e- and h-cds show a large number of different vibrations which eventually belong to aromatic structures of molecular fluorophores present in these two samples. 2

3 Figure S3 Photoluminescence decay curves for citrazinic acid and e-, h- and t-cds at two excitation wavelengths (left column: 320 nm, right column: 405 nm). The corresponding average PL lifetimes were estimated from the multi-exponential fitting and are presented in Figure 4. 3

4 Figure S4 Photoluminescence emission spectra of e-cds at different excitation wavelengths (left column) and photoluminescence excitation (PLE) spectra at different emission wavelengths (right columns), measured at two different concentrations ( low and high ). Low concentration corresponds to the concentration range which was used for the measurements presented in Figure 2, 3 and 4, at an optical density between 0.1 and 0.3. High concentration indicates a solution which is app. 20 times higher in concentration (see inset for a respective photograph of the sample). When changing from low to high concentration of e-cds, the emission spectra show increased intensities at larger excitation wavelengths. This is also reflected by the strong signal at 450 nm in the excitation spectra at longer emission wavelengths. The emission peak shifts and intensities are further illustrated in Figure S7. 4

5 Figure S5 Photoluminescence emission spectra of h-cds at different excitation wavelengths (left column) and photoluminescence excitation (PLE) spectra at different emission wavelengths (right columns), measured at two different concentrations ( low and high ). Low concentration corresponds to the concentration range which was used for the measurements presented in Figure 2, 3 and 4, at an optical density between 0.1 and 0.3. High concentration indicates a solution which is app. 20 times higher in concentration (see inset for a respective photograph of the sample). Aside from emission red shift, the emission intensities at higher excitation wavelengths significantly increase in comparison to the low concentration samples. In agreement to these changes in the emission spectra, a strong red shift with an additional pronounced feature at 450 nm appears in the excitation spectra for increasing emission wavelengths. The emission peak shifts and intensities are further illustrated in Figure S7. 5

6 Figure S6 Photoluminescence emission spectra of t-cds at different excitation wavelengths (left column) and photoluminescence excitation (PLE) spectra at different emission wavelengths (right columns), measured at two different concentrations ( low and high ). Low concentration corresponds to the concentration range which was used for the measurements presented in Figure 2, 3 and 4, at an optical density between 0.1 and 0.3. High concentration indicates a solution which is app. 20 times higher in concentration (see inset for a respective photograph of the sample). The emission spectra exhibit a general red shift and increased emission intensities at higher excitation wavelengths. Corresponding to that, a strong red shift with an additional feature at 450 nm appears in the excitation spectra for increasing emission wavelengths. The emission peak shifts and intensities are further illustrated in Figure S7. 6

7 Figure S7 PL peak position ( ) and normalized PL intensity ( ) as a function of the excitation wavelength for all three CD species at low and high concentrations. The correspondent emission spectra are in Figures S5, S6 and S7. The peak positions show an overall red shift in all three CDs at high concentrations. Furthermore, the maximum intensity also shifts to longer wavelengths. Especially in e- and h- CDs, the main emission in samples measured at high concentrations is significantly reduced. Although t-cds show a reduced emission at short wavelengths in highly concentrated samples as well, their main emission is not as strongly reduced as for the other CDs. Several aspects have to be considered to analyse the results. Highly concentrated solutions of CDs, whose absorption is beyond the detection limit, show strong effects of reabsorption, especially for emission in the shorter wavelength range. An emission red shift with relatively higher intensities at longer wavelengths is thus expectable. Apart from that, quenching in the main emission seems to be the strongest for e- and h-cds, which could be a sign for the presence of aggregates of molecular fluorophores. 7