FRACTALS OF GRAPHENE QUANTUM DOTS IN PHOTO LUMINESCENCE OF SHUNGITE
B. S. Razbirina, N. N. Rozhkovab, E. F. Shekac*, D. K. Nelsona, A. N. Starukhin
Iofje Physical- Technical Institute, Russian Academy of Sciences 194021, Saint Petersburg, Russia
b Institute of Geology, Karelian Research Centre, Russian Academy of Sciences 185910, Petrozavodsk, Russia
Peoples' Friendship University of Russia 117198, Moscow, Russia
Received August 12, 2013
Viewing shungite as loosely packed fractal nets of graphene-based (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 temperatures. As expected, the photoluminescence of the GQD aqueous dispersions is quite similar to that of synthetic 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 spectra position while the fractal structure of GQD colloids provides high broadening of the spectra due to structural in homogeneity, 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 toluene dispersions, which paves the way for a theoretical treatment of the GQD photonics.
DOI: 10.7868/S0044451014050073
1. INTRODUCTION
Originally, the term "graphono quantum dot" (GQD) appeared in theoretical research and was attributed to fragments limited in size, or domains, of a single-layer two-dimensional 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 single-layer graphene domains but
* E-mail: sheka'flicp.ac.ru
multi-layer formations containing up to 10 layers of reduced graphene oxide (rGO) of less than 30 mil in size.
Optical spectroscopy, and photoluniincsccncc (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 summarized as follows.
1. Regardless of the method of obtaining GQDs in water solution, the final product is a mixture of particles differing in size (both width and thickness).
2. Morphology of GQDs reveals that the particle size is not determined by the production process, although depending 011 the starting materials. The average linear dimension is about 10 11111, while the maximum is 60 11111. The average thickness of particles indicates multi-layer, in most cases, five-layer GQDs, although obtaining single-layer GQDs is also not uncommon (see, e.g. [13 17]).
3. Fourier-transform infrared spectroscopy and
photooloctroil spectra show that in almost all the eases studied, the chemical composition of GQDs corresponds to partially oxidized graphene.
4. The absorption spectra in the visible and UV range show a well-marked size effect that is manifested as a red shift of the spectrum with increasing the GQD size.
A detailed description of these features with the presentation of their possible explanations and links to the relevant publications is given in [12].
As seen from the synopsis, optical spectroscopy of GQDs gives a complicated picture with many features. However, in spite of this diversity, common patterns can be identified that can be the basis of the GQD spectral analysis. These general characteristics include: (1) structural inhoniogcncity of GQD solutions, which should rather be called dispersions; (2) low-concentration limit that provides surveillance of the PL spectra; (3) dependence of the GQD PL spectrum on the solvent, and (4) dependence of the GQD PL spectrum on the excitation light wavelength. It is these four circumstances that determine usual conditions under which the spectral analysis of complex polyatomic molecules is performed. The condition optimization, primarily including the choice of solvent and the experiment performance at low temperature, in many cases led to good results, based on structural PL spectra (see, e.g., the relevant research of fullerene solutions [18 21]). In this paper, we show that implementing this optimization for the spectral analysis of GQDs turns out to be quite successful.
2. GRAPHENE QUANTUM DOTS OF NATURAL ORIGIN
Synthetic GQDs described in the preceding section have recently been complemented with GQDs of natural origin [22,23]. It has been shown that GQDs present the main structural peculiarity of shungitc of Karelian deposits. Based on the detailed analysis of physical and chemical properties of graphene and its derivations, it was established in [22] that shungitc should be regarded as one of the natural carbon allotropes, possessing multistage fractal nets of rGO fragments less than 1 mil in size. The generality of the basic structural elements of shungitc and synthetic GQDs as well as the ability of the former to disperse in water provided a basis of a research project [23] aimed at establishing the generality of the spectral properties of aqueous dispersions of shungitc and synthetic GQDs and proving the structural formula of shungitc given above. The conducted
spectral studies provided the desired confirmation, at the same time exhibiting particular features of the observed spectral characteristics that allowed insights into the structural and spectral peculiarities of the GQDs dissolved in different solvents.
3. FRACTAL NATURE OF THE OBJECT UNDER STUDY
The GQD concept evidently implies a dispersed state of a number of nanosizc rGO fragments. Empirically, the state is provided by the fragment dissolution in a solvent. Once dissolved, the fragments unavoidably aggregate, forming colloidal dispersions. As mentioned above, so far only aqueous dispersions of synthetic GQDs have been studied [11,12]. In the case of shungitc GQDs, two molecular solvents, carbon tetrachloride and toluene, were used to replace water in the pristine dispersions. In each of these cases, the colloidal aggregates are the main object of study. Although there has been no direct confirmation of their fractal structure, there arc serious reasons to suppose that it is an obvious reality. Actually, first, the fragment formation occurred under conditions that unavoidably involve elements of randomness in the course of both laboratory chemical reactions and natural graphitiza-tion [22]; the latter concerns the fragment size and shape. Second, the fragment structure certainly bears the stamp of polymers, for which fractal structure of aggregates in dilute dispersions has been convincingly proved (sec [24] and the references therein).
As shown in [24], the fractal structure of colloidal aggregates is highly sensitive to the ambient solvent, the temperature of the aggregates formation, and other external actions such as mechanical stress and so forth. This fact makes the definition of quantum dots of colloidal dispersions rather vague at the structural level. In the case of GQDs of different origin, the situation is additionally complicated because the aggregation of synthetic (Sy) and shungitc (Sh) rGO fragments occurred under different external conditions. In view of this, it must be assumed that rGO-Sy and rGO-Sli aggregates of not only different but also the same solvent dispersions are quite different.
Seeking the answer to the question of whether the same term GQD can be attributed to colloidal dispersion in the above two cases, we should recall that a feature of fractal structures is that fractals are typically self-similar patterns, where "self-similar" means that they are "the same from near as from far" [25]. This means that the peculiarities, e.g., of the optical
behavior of each of the two rGO-Sy and rGO-Sh colloidal dispersions obey the same law. From this standpoint, there apparently is 110 difference which structural element of a multilevel fractal structure of their colloidal aggregates should be attributed to a quantum dot. However, the identity of both final and intermediate fractal structures of aggregates in different solvents is highly questionable and only the basic rGO structural units can be identified without a doubt. Therefore, GQDs of both rGO-Sy and rGO-Sh dispersions should associate with rGO individual fragments. This is why different fractal nets of GQDs provided by different colloidal dispersions are the object of this study. As regards the spectral behavior of the dispersions, we should expect an obvious generality provided by the common nature of GQDs, but simultaneously complicated by the difference in fractal packing of the dots in the different-solvent dispersions. The latter mainly concerns the rGO-Sh dispersions [23] that are considered in detail below.
4. rGO-Sh AQUEOUS DISPERSIONS
In full agreement with commonly used methods for the preparation of colloidal dispersions of graphene and its derivatives [26, 27], rGO-Sh aqueous dispersions were obtained by sonication of the pristine shungite powder for 15 mill with an ultrasonic disperser UZ-2M (at a frequency 22 kHz and the operating power 300 W) followed by filtration and ultracentrifugation [28]. The maximum achievable concentration of carbon is less than 0.1 mg/'ni
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