Fractals of graphene quantum dots in photoluminescence of shungite
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OLIDS AND LIQUIDS
Fractals of Graphene Quantum Dots in Photoluminescence of Shungite1 B. S. Razbirina, N. N. Rozhkovab, E. F. Shekac,*, D. K. Nelsona, and A. N. Starukhina a
b
Ioffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia Institute of Geology, Karelian Research Centre, Russian Academy of Sciences, Petrozavodsk, 185910 Russia c Peoples’ Friendship University of Russia, Moscow, 117198 Russia *email: [email protected] Received August 12, 2013
Abstract—Viewing shungite as loosely packed fractal nets of graphenebased (reduced graphene oxide, rGO) quantum dots (GQDs), we consider photoluminescence of the latter as a convincing proof of the structural concept as well as of the GQD attribution to individual rGO fragments. We study emission from shungite GQDs for colloidal dispersions in water, carbon tetrachloride, and toluene at both room and low tempera tures. As expected, the photoluminescence of the GQD aqueous dispersions is quite similar to that of syn thetic GQDs of the rGO origin. The morphological study of shungite dispersions shows a steady trend of GQDs to form fractals and to drastically change the colloid fractal structure caused by the solvent exchange. Spectral study reveals a dual character of the emitting centers: individual GQDs are responsible for the spec tra position while the fractal structure of GQD colloids ensures high broadening of the spectra due to struc tural inhomogeneity, thus causing a peculiar dependence of the photoluminescence spectra on the excitation wavelength. For the first time, photoluminescence spectra of individual GQDs were observed in frozen tolu ene dispersions, which paves the way for a theoretical treatment of the GQD photonics. DOI: 10.1134/S1063776114050161 1
1. INTRODUCTION
Originally, the term “graphene quantum dot” (GQD) appeared in theoretical research and was attributed to fragments limited in size, or domains, of a singlelayer twodimensional graphene crystal. The subject of the investigations concerned the quantum size effects, manifested in the spin [1, 2], electronic [3] and optical [4–9] properties of the fragments. These studies significantly stimulated the interest in GQDs and their attractive applications (see, e.g., [10] and the references therein), which raised the question of their preparation. This proved to be a difficult task and the progress achieved by now has been presented in exhaustive reviews [11, 12]. On the basis of spectral studies, we have found that in almost all cases, the GQDs are not singlelayer graphene domains but multilayer formations containing up to 10 layers of reduced graphene oxide (rGO) of less than 30 nm in size. Optical spectroscopy, and photoluminescence (PL) in particular, was the primary method of studying the properties of the GQDs. Review [12] presents a complete picture of the results, which can be summa rized as follows. 1
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1. Regardless of the method of obtaining GQDs in water solution, the final product is a mixture of p
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