Electronic Supplementary Material
Heterogeneity in the fluorescence of graphene and graphene oxide quantum dots
Siobhan J. Bradley,1 Renee Kroon,2 Geoffry Laufersky,1 Magnus Röding,3 Renee V. Goreham,1 Tina Gschneidtner,2 Kathryn Schroeder,1 Kasper Moth-Poulsen,2 Mats Andersson2,4 and Thomas Nann1*
1The MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences,Victoria University of Wellington, PO Box 600 Wellington 6140,New Zealand
2Chalmers University of Technology, SE-412 96 Göteborg, Sweden
3SP Food & Bioscience, Structure and Material Design, Box 5401, SE-402 29, Göteborg, Sweden
4FI Institute,University of South Australia, Mawson Lakes Boulevard, Adelaide, SA 5095,Australia
*Corresponding Author, e-mail:
Table of contents
1. TEM
Figure S1: TEM of the graphitic starting material graphite nanoparticles.
Figure S2: Particle size distribution histogram.
2. AFM
Figure S3: AFM image and height histogram of GQDs and GQDs.
3. XPS
Figure S4: XPS survey spectra of GQD and GOQDs.
4. Raman experimental conditions
Figure S5: Masking of Raman signals for GO/GQDs due to inherent fluorescence.
5. NMR and size-exclusion chromatography
Figure S6: Fluorescence of size-exclusion chromatography fractions.
Figure S7: NMR of purified and unpurified GQDs indicating aromatic signals present before hexanes washing step which correspond to smaller particles than can be seen in the TEM images.
6. “Excitation wavelength-dependent emission”
Figure S8: Emission spectra for GQDs and GOQDs excited at different wavelengths.
7. Quantum Yields
Calculation of quantum yields.
8. Fluorescence lifetimes
Figure S9: Fluorescence lifetime decays fitted with lognormal model.
Figure S10: Lifetime distributions for GQD and GOQD fluorescence decay data fit with a lognormal model.
Table S2: relevant statistical values from the lognormal model fit.
1. TEM
Figure S1: TEM image of the precursor material used to make GO/GQDs - graphite nanoparticles (a) with high resolution image in (b).
2. AFM
AFM was performed of GQDs on HOPG and GOQDs on Mica. The various substrates were chosen as they performed best – i.e. inappropriate choice of substrate led to movement of particles with the rastoring of the AFM tip.
Figure S3: AFM image of GQDs and height histogram for GQDs (a,c) and GOQDs (b,d).
3. XPS
Figure S4: XPS survey spectra of a) GQDs and b) GOQDs.
Table S1: Weight of each peak in the deconvoluted C1s and O1s XPS spectra.
GQDs / C1s / O1sPeak BE (eV) / % Area / Bond / Peak BE (eV) / % Area / Bond
284.9 / 96.56 / C=C/C-C / 532.3 / 76.78 / C=O
286.9 / 2.61 / C-O-C/C-OH/C=O / 533.2 / 15.29 / C-O
289.0 / 0.83 / O=C-OH / 534.1 / 7.94 / Si-O
GOQDs / C1s / O1s
Peak BE (eV) / % Area / Bond / Peak BE (eV) / % Area / Bond
284.7 / 79.11 / C=C/C-C / 531.1 / 73.14 / C=O
285.9 / 7.73 / C-O-C/C-OH / 532.1 / 17.16 / C-O
287.7 / 3.4 / C=O / 533.5 / 6.52 / Si-O
288.6 / 7.09 / O=C-OH / 535.5 / 3.18 / Na Auger
289.5 / 2.67 / Pi-Pi* shake-up
4. Raman experimental conditions
GOQDs were measured in water and GQDs in dichloromethane using a LabRam (Horiba Jobin-Yvon) Ramanspectrometer with a 633 nm and 568 nm laser (HeNe) attenuated to 70 µW incident power, a 20x long distance objective, and a 300 lines/mm diffraction grating. All spectra were averaged over 6 integrations of 10 seconds. Note that the fluorescence intensity is a reflection of the differing concentration of quantum dots in each sample.
Figure S5: Raman signals overwhelmed by the fluorescence of GOQDs with laser excitation at either 633 nm (a) or 568 nm (b). The red line in (a) is a 6th order polynomial fit to the fluorescence spectrum, with the residual shown in panel (c) (red). No Raman line is visible in this spectrum, dominated by photon noise and fixed structure noise from the CCD detector. For comparison, the Raman spectrum of 2-bromo-2-methyl-propane acquired using the same experimental conditions is shown in blue(x20 objective 70 µW intensity at the sample, 6 acquisitions of 10s). A recently-developed method for extracting weak Raman signals in the presence of fluorescence, Continuously-Shifted Raman Spectroscopy (CSRS) was also used (50 shifts of 2 cm–1, each averaging 60 acquisitions of one second), with resulting spectrum shown in (d). A weak Raman peak of intensity ~4 cts is recovered within the 9000 counts of fluorescence, at around 1300 cm–1, but the method requires a long acquisition time to overcome photon noise and appears to provoke laser-induced changes in the sample's spectrum.
5. NMR and Size-exclusion chromatography
5.1 Analysis of small impurities
The fluorescence spectra of as-synthesized GQDs had a peak at very high energy (375 nm) for particles of 3 nm. This peak was at higher energy than for the chloronaphthalene solvent alone (400 nm). We therefore performed size-exclusion chromatography (SEC) of the crude product as shown in Figure 3 (a) with corresponding fluorescence spectra in (b). Two peaks in the emission are evident, one centred at 375 nm, the other at 520 nm. The peak at 375 nm becomes more intense in the later (smaller size) fractions of the SEC column.
a)
Figure S6: Image of size exclusion chromatography fractions excited with UV light (a). From L to R; fastest eluting (largest) fractions to slowest (smallest). Corresponding emission spectra of samples excited at 320 nm are given in (b). A dominating peak at higher energy features in the smaller (slower eluting) fractions. A broader, lower energy peak dominates the larger size fractions.
We performed NMR to investigate these smaller-sized impurities and found aromatic proton signals. These can be removed by precipitating the GQDs with hexane (S5). It appears this top-down method, as well as producing the particles evident in the TEM micrographs, also produce smaller fragments beyond the limit of resolution of most TEMs. It has been shown with GQDs synthesized using a bottom-up method that structures in the 2 nm size range and above do not show peaks in the 1HNMR due to the size of their rigid cores and aggregation behaviour in solution.[1] The peaks we see in the unpurified GQD spectrum must therefore be caused by smaller conjugated structures than the particles seen in the TEM micrographs which could originate from the breakdown of the graphite nanoparticles or the chloronaphthalene. We suspect that many top-down procedures could produce fragments undetectable by the TEM but contributing to the measured fluorescence.
Figure S7: NMR of graphene quantum dots purified by washing with hexanes (a) and the impurities in the supernatant (b).
6. “Excitation wavelength-dependent” emission
Our GQDs and GOQDs did not exhibit excitation wavelength-dependent emission as the emission tail does not shift as shown in Figure S8. However the emission does change as the excitation energy decreases. As this is a very heterogeneous sample, it is very likely that different subpopulations of particles are excited with different energy excitation and this causes the PL shape to change. What exactly these sub populations could be attributed to (sizes, electronic transitions) remains unclear from our data and further experiments are required in order to elucidate the origin of this change in fluorescence.
Figure S8: Emission spectra for GQDs (a) and GOQDs (b) excited at different wavelengths as indicated. GQDs show broader emission. GOQDs, in contrast, show narrower emission and at higher energy. Note that the lower energy emission tail does not shift with different excitation wavelength.
7. Calculation of quantum yields
Absolute quantum yields QYtrue were obtained through the use of an integrating sphere whereby the difference in the total scattered light is compared to that of the emitted light for the sample and a solvent blank to obtain QYobs. Spectra were corrected for self-absorption effects by tail matching a spectrum of the sample obtained with a standard sample holder to that of the integrating sphere spectrum, with the difference in areas of the spectra providing the self-absorption correction, a, as utilised by the equation below:[2, 3]
QYtrue=QYobs1-a+a∙QYobs
Absorbance values at the excitation wavelength – 380 nm – were kept below 0.05 OD to avoid concentration effects.
8. Fluorescence Lifetime measurements
The lifetimes were fit using four different models as per an earlier study.[4] The best fit for both particles was achieved with a lognormal model.
Figure S9: Fluorescence lifetime decays fitted with lognormal model for GQDs (a) and GOQDs (b).
Figure S10: Distributions of fluorescence lifetimes from lognormal model fit to GQD and GOQD fluorescence decay data excited with a pulsed LED laser (380 nm).
Table S2: Fluorescence lifetimes of GQDs and GOQDs excited at 320 nm or 380 nm and associated statistical values.
Sample (excitation wavelength in nm) / Mean / Standard Deviation / Spread / LoglikelihoodGQD (380) / 4.93 / 3.59 / 0.73 / -1.7M
GOQD (380) / 3.664 / 3.05 / 1.20 / -1.2M
References:
1. Yan X, Cui X, Li B, Li L (2010) Large, Solution-Processable Graphene Quantum Dots as Light Absorbers for Photovoltaics. Nano Lett 10:1869–1873. doi: 10.1021/nl101060h
2. Ahn T-S, Al-Kaysi RO, Müller AM, et al (2007) Self-absorption correction for solid-state photoluminescence quantum yields obtained from integrating sphere measurements. Rev Sci Instrum 78:086105. doi: 10.1063/1.2768926
3. Lakowicz JR (2006) Principles of Fluorescence Spectroscopy, 3rd ed. Springer, New York
4. Röding M, Bradley SJ, Nydén M, Nann T (2014) Fluorescence Lifetime Analysis of Graphene Quantum Dots. J Phys Chem C 118:30282–30290. doi: 10.1021/jp510436r