Optical Science of Nano-graphene (graphene oxide and graphene quantum dot) Introduction of optical properties of nano-carbon materials

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1 Optical Science of Nano-graphene (graphene oxide and graphene quantum dot) J Kazunari Matsuda Institute of Advanced Energy, Kyoto University Introduction of optical properties of nano-carbon materials Optical spectroscopy of graphene oxide Luminescence properties of graphene quantum dots

2 Optical properties of graphene oxide (GO) 1. D. Kozawa, K. Matsuda, et al., J. Phys. Chem. Lett. 5, 1759 (2014). 2. D. Kozawa, K. Matsuda, et al., J. Phys. Chem. Lett. 4, 2035 (2013). 3. D. Kozawa, K. Matsuda, et al., Phys. Stat. Soli. (c), 10, 1600 (2013).

3 Structure of Graphene Oxide (GO) Oxidation and exfoliation of graphite in strong acid Graphene Oxide (GO) non-stoichiometric compound oxygen functional group + sp 2 cluster (sp 2 fragment) (-COOH, -OH, C=O, C-O-C) TEM image Chemical structure C O H sp 2 cluster ~1 μm D. C. Marcano et al., ACS Nano 4, 4806 (2010) E 2 nm E g K. Erickson et al., Adv. Mater. 22 (2010) 4467

4 Application of Graphene Oxide (GO) Photoluminescence (PL)/Fluorescence (FL) from GO 4 blue red C. Chien et al., Angew. Chem., Int. Ed. 51, 6662 (2012) electronic devices Bio labeling G. Eda et al., Adv. Mater., 22 (2010) X. Sun et al., Nano Res., 1 (2008) 203 PL (FL) by recombination of electron-hole pairs in sp 2 cluster Quasi-molecular PL (FL)

5 Sample of Graphene Oxide (GO) Height (nm) Graphene oxide (GO) GO from modified Hummer s method (Oxidation and exfoliation of graphite in strong acid) Supernatant centrifugation (35,000 g for 12h) ph adjustment using NaOH AFM image 8 Optical measurement

6 Absorbance Typical absorption and PL (FL) spectra PL Intensity (arb. units) absorption Absorption spectrum 300 Photoluminescence (PL)/ fluorescence PL spectra (FL) 5.4 ev exc. X P P Wavelength (nm) Emission Wavelength (nm) 600 Broad absorption spectrum 415 nm (P 1 ) and 300 nm (P 2 ) Blue Ultraviolet (UV)

7 PL (FL) imaging of GO P C P D PL peak: P C (415 nm(2.8 ev)), P D (300 nm (4.1 ev)) PLE peak: P C (3.8, 4.6, 5.2 ev), P D (4.6, 5.6 ev) PL from few aromatic rings structure

8 Integrated FL Intensity (arb. units) PH dependence of PL spectra 1 ph 2.5 ph 6.5 ph 11.1 P C P C P C 0 P D P D P D P C P D P 1 (330 nm exc.) P 2 (220 nm exc.) ph 10 ph dependence of PL intensity Quench at high ph: Oxygen functional group R-COOH R-COO - + H +

9 Model compounds of the luminescence centers Few aromatic rings (sp 2 )+ oxygen functional group PLE map P D P C c.f.) model compounds [1-3] PL: ~415 nm ~ 300 nm PLE: (S 1 -S 0 ) ~295 nm (S 1 -S 0 ) ~280 nm (S 2 -S 0 ) ~250 nm (S 2 -S 0 ) ~230 nm [1] I. B. Berlman et al., Academic Pr. (1971). [2] R. Martin et al., J. Phys. Chem. 82, (1978). [3] H. Hosoya et al., J. Mol. Spectrosc. 8, (1962). These are coincident with PL and PLE wavelength (P C, P D )

10 PL Intensity (arb. units) Time-resolved PL measurement PL Intensity (arb. units) Time-resolved PL decay curves P 1 (415 nm) P ev exc ev det. IRF ph P 2 (300 nm) P ev exc ev det. IRF ph Delay Time (ns) P 1 :PL lifetime : 8 ns Delay Time (ns) P 2 :PL lifetime : 6 ns 60 almost consistent with PL lifetimes of model compounds

11 Optical properties of graphene quantum dots (GQDs) 1. N. Fuyuno, K. Matsuda, et al., Adv. Opt. Mat. in press.

12 Graphene Quantum Dots (GQDs) GQDs (Nanoscale-size graphene flakes) High water solubility, low toxicity, luminescence stability etc. Bio-imaging Solar cell Photo-catalyst Peng et al., Nano Lett. 12, 844 (2012). Gupta et al., J. Am. Chem. Soc. 133, 9960 (2011). Zhuo et al., ACS Nano 6, 1059 (2012).

13 Preparation of GQDs Concentrated HNO 3 and H 2 SO 4 (Mixture Ratio 1:3) Strong oxidation process Pitch-based Carbon Fibers [1] Sonication Stir at 100 C for 1 hour Dilute and Neutralize [1] Peng et al., Nano lett. 12, 844 (2012). 1mL of Diluted GQDs HPLC HPLC-GQDs Collected every 1 minute

14 Absorbance (arb. units) Size separation of chromatography High performance liquid phase chromatography (HPLC) Size-exclusion chromatography short retention time long fr 4 l = 254 nm 2.0 fr 7 column fr Retention time (min) Sample collection: every 1 min Fraction (fr) 1-15

15 Frequency TEM images of size separated GQDs Frequency Histogram of GQDs size TEM image nm fr nm fr 9 0 fr Lateral Size (nm) 0 fr Lateral Size (nm) smaller size in longer retention time

16 Photoluminescence spectra of GQDs Broader luminescence spectrum from UV to NIR

17 Luminescence spectrum of size separated GQDs Separation by HPLC :GQDs with different PL properties Smaller size GQDs :shorter wavelength PL

18 Luminescence spectrum of size separated GQDs Emission wavelength (~ 600 nm = 2 ev) Band gap of pure GQDs (5 nm: ~ 0.2 ev) Ritter et al. Nat. Mater. 8, 235 (2009). Oxidation region :hybrid structure of sp 2 region (non-oxidized)+sp 3 region (oxidized) sp 2 region GQDs Emission from sp 2 fragment embedded in sp 3 region oxidized (sp 3 )

19 Luminescence spectrum of size separated GQDs Excitation light Larger GQDs Smaller GQDs Excitation light Longer emission Oxdized region Shorter emission Change of GQDs size; Population change of sp 2 fragment embedded in GQDs Discrete luminescence change 19

20 Summary Optical Science of Nano-graphene (GO, and GQDs) Optical science of carbon nanotube Stage for optical physics and optical applications Optical spectroscopy of GOs PL (FL) from small sp 2 fragment with oxygen functional group PL properties of size separated GQDs Size dependent PL properties