Heterogeneous Fluorescence Intermittency in Single Layer Reduced

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Heterogeneous fluorescence intermittency in single layer reduced graphene oxide Jixin Si, Sandor Volkan-Kacso, Ahmed Eltom, Yurii Morozov, Matthew McDonald, Masaru Kuno, and Boldizsar Janko Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b00191 • Publication Date (Web): 09 Jun 2015 Downloaded from http://pubs.acs.org on June 14, 2015

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Heterogeneous fluorescence intermittency in single layer reduced graphene oxide Jixin Si1, Sándor Volkán-Kacsó3, Ahmed Eltom2, Yurii Morozov2, Matthew P. McDonald2, Masaru Kuno2 and Boldizsár Jankó1* 1. Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA 2. Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA 3. Noyes Laboratory of Chemical Physics, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA

ABSTRACT: We provide, for the first time, direct experimental evidence for heterogeneous blinking in reduced graphene oxide (rGO) during photolysis.

The spatially-resolved

intermittency originates from regions within individual rGO sheets and shows 1/f-like power spectral density. We describe the evolution of rGO blinking using the multiple recombination center (MRC) model that captures common features of nanoscale blinkin. Our results illustrate the universal nature of blinking and suggest a common microscopic origin for the effect.

Keywords: fluorescence intermittency, graphene oxide, heterogeneity, MRC model

*

To whom correspondence should be addressed. Email: [email protected] Fax: 574-631-5952

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Blinking has been reported in a wide variety of nanostructures, ranging from selfassembled/colloidal quantum dots and perylene diimide molecules to nanowires, nanorods and even nanoplatelets1-13.

All these nanoemitters show power law behavior in their intensity

trajectory power spectral density (PSD)14. During the last decade, research in graphene and related materials like GO has provided us another platform for studying the electronic properties of two dimensional (2D) systems. Here we report for the first time the observation of fluorescence intermittency in rGO. In contrast to the ‘stable’ blinking trajectories of other nanoscale emitters, where the quantum yield and its fluctuation remain constant through long observation periods that can be as long as minutes15 to hours4, the evolution of GO’s chemical structure through its photolytic reduction into rGO introduces a transient process in its blinking behavior. As shown in detail below, analyses of spatially-resolved GO blinking trajectories reveal heterogeneity stemming from sp3 to sp2 interconversion and the chemical transformation of GO/rGO from an interconnected network of aromatic domains to isolated single molecule fragments. The evolution of heterogeneous blinking, from emergence to extinction, is mapped into a spatio-temporal distribution of MRCs16,17 interacting with single molecule fragments, providing insight into blinking’s microscopic mechanism. GO is known to be emissive.

However, its exact origin has been debated18,19,20. The

emission is generally attributed to emissive, basal plane sp2 domains with relatively large size distributions (1~5 nm2) 18,21. It has also been thought to arise from quasi-molecular ligand-related states, associated with oxygen containing functionalities22. Most recently, GO’s broad emission has been attributed to heterogeneous carrier recombination kinetics19. Our spatially-resolved single layer GO emission ratio maps show that individual GO sheets emit broadly with clear blue and red components, suggesting intrasheet spectral heterogeneities.


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Unexpectedly, we find (cf. Fig. 1 and Fig. S1) that both the spectrum and emission quantum yield (QY) of single layer GO evolve during irradiation: the emission spectrum blueshifts under continued 405 nm excitation, while the QY decreases considerably, from ~0.7% to 0.08%23. Even more intriguing is the subsequent redshift, occurring along with a noticeable photobrightening of the sheet. Resulting QYs are on the order of ~14%. The photobrightening is simultaneously peppered by episodes of emission intermittency wherein regions of the sheet clearly blink. Furthermore, as GO photobrightening progresses, eventually “consuming” the entire sheet, blinking extends to different regions. Following this, photobleaching occurs and the entire sheet becomes non-emissive (see Supplementary Discussion S1 and Movie S1). Through concerted single layer GO absorption and emission experiments we have explained much of this behavior. We find23, 24 that GO’s initial emission quenching and spectral blueshift stem from its photolytic reduction to rGO. Subsequent photolytic reactions lead to rGO fragmentation and simultaneously produce an apparent photobrightening effect due to the gradual exclusion of fast non-radiative relaxation channels present in a highly interconnected sp2 network24. These isolated sp2 domains subsequently exhibit single molecule behavior, including blinking and eventual photobleaching. During reduction, the overall blinking amplitude evolves due to photolysis and thus represents an intriguing opportunity to reveal additional insight into blinking. In contrast to synchronized blinking from a single emitter, rGO blinking entails the evolution of spatially distinct blinkers, initially in strong communication (see Supplementary Discussion S5 and Fig. S6), but which through ongoing photochemistry become temporally disparate entities.


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Below we describe key features of rGO blinking and illustrate its spatio-temporal evolution as sp2 domains become isolated. We have analyzed rGO blinking via PSD estimation25, a technique successfully applied to several different classes of blinking fluorophores14. The PSD of the time-dependent emission intensity x(t) is the Fourier transform of the auto-covariance function:

= 〈 − 〉 . Throughout the GO/rGO specimen the PSD follows a power law (Fig. 1d) ∝

(1)

,

where the exponent α ranges between 0.2-1.2. This feature is identical to those of other nanoscale blinkers14. However by dividing the trajectory of each CCD camera pixel into smaller temporal sections (Fig. 1c and Supplementary Discussion S3) we observe a dramatic evolution of the GO/rGO PSD as blinking emerges and eventually disappears. Such transient behavior has not been observed before. Moreover, in contrast to all other nanoscale blinkers, rGO is a twodimensional system, in which blinking shows spatial heterogeneity (Fig. 1b). We now discuss the various stages of rGO blinking. Initially, the PSD has a low amplitude and α is smaller than 0.3 (Fig. 2a, section1). The emission fluctuation in this stage is practically identical to featureless background noise (α ~ 0.2). When the emission grows to a maximum, blinking emerges as PSD amplitudes increase drastically, and α approaches 0.9 across most of the rGO sheet. An overall decrease in QY follows, yet blinking remains intense for minutes. Finally, when blinking diminishes, both the PSD amplitude and α become similar to their preblinking values. Due to its parallel evolution with blinking power, α can be used as a quantitative indicator of blinking: when α < 0.3, the intensity fluctuation is mainly caused by noise, while α > 0.3 correlates with the presence of blinking. The value of α is extracted by ordinary least square


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linear regression of PSD versus frequency in log-log scale. The standard deviations of α estimated for all sections of every pixel are consistently smaller than 0.1 (see Fig. S12).

Figure 1: Heterogeneity and temporal evolution of both emission intensity and PSD of single GO sheet. a. A snapshot of the fluorescence video of the rGO sheet during the photolytic reduction. b. Contour plot of the slope power law exponent α throughout the whole sheet in one section (section 6). There is clear spatial heterogeneity of the exponent which ranges from 0.3 to 0.9. c. The intensity trajectory as a function of time for a single CCD pixel (32, 32). The area is marked by a black square in a. Fluorescence intermittency is apparent after about 500s. d. The corresponding PSD at different stages of the pixel (32, 32), which shows the emission undergoes process from no blinking (section 1) to blinking (sections 6 - 16), and finally, extinction (sections 16 - 21). The log-log scale reveals the power law nature of the PSD. Scale bars: 5 µm.


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As shown in Fig. 1b, α shows clear spatial heterogeneity ranging from 0.3 to 0.9. Both the emergence of blinking and persistence of emission varies from one point to another within the sample (Fig. 2).

During section 1 of the trajectories, the entire sample shows emission

fluctuations with a vanishing α value; by section 5 blinking emerges from several areas with α>0.7. At this point, blinking spreads across the entire rGO sheet, reminiscent of quenching and brightening rate heterogeneities observed during GO photoreduction. We find a strong correlation between the evolution of the baseline intensity and the emergence of blinking. Indeed, the time evolution of the background intensity across the sample is near-identical with the frequency-integrated PSD (see Fig. S2-S4); it is only ahead by 500s. This means that background intensity changes foreshadow blinking changes. Such behavior is observed in all GO samples studied, and is supported by correlations between pairs of intermittency parameters (see Fig. S5). To explain these correlations and to establish a qualitative mechanism for the evolution of GO/rGO blinking, we turn to the MRC14 as a phenomenological theoretical framework. Within this model, recombination centers (RCs) randomly switch between active and inactive states. Compared to an inactive state, active RCs introduce a higher rate for non-radiative recombination of excitations. Changes in non-radiative rates subsequently lead to temporal fluctuations in the emission QY, ! , of single fluorophores, ! = "

"# ( # "$ ∑&)* "& '& +

(2)

where t is time, N is the total number of RCs, kr, k0 are radiative and background non-radiative rates, with ki the non-radiative rate introduced by an RC in an active (, = 1) state.


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Here we extend the framework to multiple blinkers by summing the contribution of individual fluorescent sp2 domains24 imaged within each CCD camera pixel. RC switching rates are assumed to follow the MRC framework distribution used to model near universal power law PSDs14. Assuming little emission and relaxation rate spatial and temporal variations, GO/rGO photobrightening, blinking, and bleaching are attributed to the light-induced changes in both domain and RC populations (see Supplementary Discussion S2). We propose that fluorescent species created during photolysis do not initially blink. RCs appear after an average lag of ~500s with respect to background emission, suggesting that the RCs are initiated by the same light-induced mechanism that produces photobrightening24. A fraction of emissive sp2 domains interact with these RCs and undergo blinking. Using the MRC framework, we extract the minimum number of time-dependent nonradiative rates necessary to explain the PSD (see Supplementary Discussion S4). This provides a rough estimate for the number of RCs active within a CCD pixel. Variations of this number across the sample support our interpretation of inhomogeneous rGO blinking. Indeed, before the emergence of blinking (Fig. 2b, section 1), hardly any area of the GO/rGO sheet exhibits a power law PSD, indicating that the RCs have not yet appeared. As blinking emerges, PSDs in most areas can be reproduced by the MRC model (Fig. 2b, section 5). Next, as α evolves and blinking increases, the number of RCs climbs to 3-4/pixel. As blinking disappears (Fig. 2b, section 25), the channel number drops back to two. Finally, with the photo-induced destruction of rGO24 (section 20-25) the RCs disappear accordingly.


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Figure 2: Evolution of PSD power law exponent and corresponding minimum number of switching channels. a. Contour plot of the power law exponent throughout the whole sheet for different trajectory sections. Sections 1 and 5 have length of 300s for a more precise view of blinking emergence. Sections 10 – 25 all have 800s length. b. The corresponding evolution of the minimum number of switching channels from MRC modelling. Blank areas are points at which the experimental PSD cannot be well reproduced by the MRC model with exponents in the range mentioned in the main text. Scale bar: 5 µm.


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We believe that functionalized sp2 domains in GO/rGO behave as single molecules22 and propose that functional groups act as dynamic RCs. In-silico studies have revealed useful information about the structure and energetics of such groups, and show great promise towards elucidating their dynamics with atomistic detail26. A recent report of carbon nanodot blinking27 also attributes the behavior to the sp2 core and/or oxygen-containing functional groups on the surface. In conclusion, we present for the first time direct experimental evidence of heterogeneous fluorescence intermittency in a 2D system. Furthermore, we provide extensive numerical analysis and theoretical modeling of GO/rGO blinking. Spatio-temporal maps of PSD power law exponents provide the first quantitative characterization of 2D blinking, demonstrate long-range time correlations and document the dynamical evolution of intra-sheet heterogeneity.

The

analysis yields surprising results. First, despite the strong 2D nature of GO/rGO, their blinking trajectories and 1/f PSDs strongly resemble those seen in colloidal QDs and fluorescent molecules. The 2D character of GO/rGO is nevertheless present, as our data shows multiple blinkers contributing to the signal registered within one pixel. Also new and unique to GO/rGO blinking is our detection of direct, time-delayed correlations between changes in the background intensity and the emergence/disappearance of blinking. Given the detailed and suggestive results of the PSD analysis and the phenomenological MRC modeling, it is tempting to speculate about the microscopic mechanism behind GO/rGO blinking. Indeed, some of the RC switching rates obtained from the MRC model are similar to the reduction rates extracted from our previous studies. Consequently, blinking frequencies involved in blinking may be related to functional


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group migration/dissociation. These molecular processes are one of many candidates for the microscopic mechanism behind GO/rGO blinking. We will address the various microscopic scenarios in a future publication. Methods Graphene oxide sheets were prepared using a modified Hummer’s synthesis. Ensembles were characterized using X-ray photoelectron spectroscopy.

Samples for optical microscopy

measurements were prepared by making dilute ethanol solutions of GO, followed by dropcasting onto fused silica coverslips. Individual GO sheets were probed using a homebuilt single molecule imaging system. For blinking measurements, single layer GO sheets were irradiated with 405 nm light (Iexc~380 W/cm2). Subsequent, spatially-resolved, emission from the sheets was acquired with an EMCCD camera. Additional details about the sample preparation as well as information regarding the optical microscopy can be found in Ref. 23 and 24. Acknowledgments M. K. thanks the American Chemical Society Petroleum Research Fund, the Army Research Office (W911NF-12-1-0578) for support. B.J. was supported in part by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract W-31-109-Eng-38. We thank Anthony Ruth for valuable discussions in structure and properties of graphene oxide. Author Contributions A. E., Y. M., and M. P. M performed the experiments. Subsequent data analysis was done under the supervision of M.K.

J.S. and S.V.-K. performed the numerical analysis and the

phenomenological modeling of the data under the supervision and guidance of B. J. All authors contributed to the interpretation of the results and the writing of the manuscript.


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Supporting Information Available: Further detailed discussion of reduction mechanism of GO, correlation between evolution of background fluorescence intensity and blinking power, extracting of minimum number of RC switching rates, correlation between intermittency parameters can be found in the SI. The single layer GO/rGO emission spectrum, sectioning of fluorescence of trajectory and calculation of spatial correlation are also presented in detail. This material is available free of charge via the Internet at http://pubs.acs.org. Competing financial interests: The authors declare no competing financial interests.

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Heterogeneous Single-Layer Graphene Oxide Reduction Kinetics. Nano Lett. 2013, 13, 5777– 5784. 25 Pelton, M.; Grier D. G.; Guyot-Sionnest, P. Characterizing quantum-dot blinking using noise power spectra. Appl. Phys. Lett. 2004, 85, 819-821. 26 Paci, J. T.; Belytschko, T.; Schatz, G. C. Computational Studies of the Structure, Behavior upon Heating, and Mechanical Properties of Graphite Oxide. J. Phys. Chem. C 2007, 111, 18099−18111. 27 Das, S. K., Liu; Y., Yeom, S.; Kim, D. Y.; Richards, C. I. Single-Particle Fluorescence Intensity Fluctuations of Carbon Nanodots. Nano Lett. 2014, 14, 620-625.

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Heterogeneous Fluorescence Intermittency in Single Layer Reduced

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