Photon Reabsorption and Nonradiative Energy-Transfer-Induced

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Photon Re-Absorption and Non-Radiative Energy Transfer-Induced Quenching of Blue Photoluminescence from Aggregated Graphene Quantum Dots Zhixing Gan, Hao Xu, and Ying Fu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10704 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

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The Journal of Physical Chemistry

Photon Re-Absorption and Non-Radiative Energy Transfer-Induced Quenching of Blue Photoluminescence from Aggregated Graphene Quantum Dots Zhixing Gan1,*, Hao Xu2, Ying Fu2,*

1. Key Laboratory of Optoelectronic Technology of Jiangsu Province, School of Physics and Technology, Nanjing Normal University, Nanjing 210023, China 2. Science for Life Laboratory, Department of Applied Physics, Royal Institute of Technology, SE-106 91 Stockholm, Sweden

ABSTRACT. A deep understanding on the photoluminescence (PL) from aggregated graphene quantum dots (GQDs) is very important for their practical applications. Here in this study, the PL spectra from GQDs solutions at different concentrations are studied. We find that the intensity of the green emission (ca. 530-560 nm) linearly relies on the concentration of GQDs, whereas the blue PL (ca. 425 nm) intensity is below linear relationship, indicating a concentration-induced

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partial quenching of blue PL. Confocal fluorescence images explicitly demonstrate the aggregation of GQDs at high concentration. The concentration-induced PL quenching is successfully interpreted by a model of photon re-absorption and non-radiative energy transfer, indicating that, at the aggregated states, the excited electrons of GQDs may non-radiatively relax to ground states through couplings with neighboring ones. Simulated fluorescence decay results show that the energy transfer between neighboring GQDs results in a prolonged dwell time of electron on high energy state and thus increases the decay time of 425 nm emission, while 550 nm emission remains unaffected, which are consistent with the experimental results. This work will contribute to a deep understanding on PL of GQDs, and also is of huge importance to extend GQDs’ applications.

1. INTRODUCTION Fluorescence intensity does not always linearly change in term with the variation of concentration of luminophore solution. Commonly, the emission from a solution of luminophores would be partially quenched along with the concentration increase, known as the concentration quenching (CQ).1 The aggregation-caused CQ (ACQ) effects frequently happen to many luminophores. Obviously, for light emission, ACQ is an adverse photophysical effect hindering many practical applications. In contrast to ACQ, aggregation-induced emission (AIE) refers to the effect that solutions of luminogenic molecules which are originally non-emissive or weakly luminescent become highly luminescent when aggregation of molecules appears in poor solvents or with the presence of solid states.1,2 For many practical applications of fluorescent nanoparticles, such as light emitting films and devices, luminescent solar concentrators (LSC),3,4


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the nanoparticles are essentially presented as aggregated states. Therefore, it is of ultimate significance to study what happens to the luminescence when the nanoparticles aggregate. In the past years, graphene quantum dots (GQDs) and carbon nanodots (CNDs) have drawn extensive interest due to their superior properties, like good biocompatibility, low cost, easy preparation, etc.5-7 GQDs have so far been applied in single electron transistor (SET) based charge sensors,8 electrode materials for fuel cells,9 super capacitors,10 photovoltaic cells,11 and electrochemical sensors.12 Besides, one most investigated property is their photoluminescence.13 Essentially, light emitting diodes,14 bioimaging,15 biosensor16 are all designed based on the fluorescence property. Moreover, upconversion fluorescence from GQDs by two-photon absorption has also been achieved by excitation of pulsed lasers,17 suggesting they are very favorable for multiphoton bioimaging techniques. However, the variations in luminescence of GQDs or CNDs solutions at different concentrations are still fiercely debated.18-21 Gao et al. reported that the luminescent carbon quantum dots showed AIE enhancement.18 They proposed that the interactions between hydrophilic groups and solvent molecules dictated the non-radiative pathways, and then the aggregation must in some way deactivate non-radiative emission which resulted in the increase in PL emission.18 Very similar transformation in emission induced by self-assembled aggregation was observed in GQDs.19 Fan et al. reported that the luminescence of colloidal carbon quantum dots redshifted as the concentration increased. And it was alternatively explained by the photon re-absorption.20 Therefore, it is urgent and important to uncover the abnormalities in PL of aggregated GQDs in order to further employ these properties in practical use. In this work, the GQDs were obtained through a common hydrothermal method. The GQDs exhibited excitation-dependent emissions. Emissions centered at 425 and 565 nm could be acquired by excitation of 320 and 480


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nm respectively. The intensity of 565 nm linearly depended on the concentration, while the 425 nm PL gradually got partially quenched at high concentrations. Systematic spectral analyses combining with time-resolved PL (TRPL) and absorption spectra revealed that non-radiative couplings between neighboring GQDs could effectively account for the aggregation-induced quenching of blue PL. Similar effect was also observed in fluorescent CNDs additionally, hinting our model might be widely adopted to interpret those phenomena. 2. EXPERIMENTAL SECTION The preparation of GQDs has been described elsewhere.17 Graphene oxide (GO) was prepared by the Hummers method.22 The GQDs were fabricated by a hydrothermal method using the GO as precursor. In details, 40 mg GO was dispersed in 40 mL deionized water to form a 1 mg/mL GO suspension and the pH was adjusted to be neutral by 1 M NaOH. The suspension was then transferred to a Teflon-lined autoclave and heated at 180 °C for 5 h. Afterwards the obtained suspension was ultra-sonicated for 1 h and the supernatant contained GQDs was obtained by centrifugation. The supernatant was then concentrated to c0 (~ 0.05 mg/mL) in a vacuum drying oven. And GQDs solution of c0/2, c0/4 and c0/8 for experiments were sequentially obtained by dilution. In the preparation of CNDs, 3.96 g glucose was dissolved in 40 mL water to form a clear solution in a 50 mL Teflon-sealed autoclave and maintained at 160 °C for 16 h. After reactions, the precipitates were collected and washed thoroughly with distilled water and ethanol thrice by repeated centrifugations. The CNDs were acquired from the supernatant from the last centrifugation step.


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The structural characterization of the GQDs and CNDs can be found in previous work.17 PL measurements including steady-state PL (SSPL), time-resolved PL (TRPL) and quantum yields (QYs) were performed by an Edinburgh FLS-920 PL spectrometer. 375 and 405 nm picosecond pulse lasers were used as prompt light for the measurements of TRPL. The absorption spectra were monitored by a Shimadzu UV-3600 UV-Vis-NIR spectrophotometer. Images of GQDs dispersed on a microscope slide with concentration of C0 and C0/10 were acquired by a confocal microscope LSM 780 (Carl Zeiss) through lambda mode with a spectral resolution of ca. 3 nm (32-channel GaAsP detector). The images shown in this report were colored by the operation system of the microscope-ZEN imaging software, reflecting the fluorescence spectra of GQDs. 3. RESULTS AND DISCUSSION Figure 1 shows the PL spectra acquired from GQDs at various concentrations under different excitations. As shown in Figure 1a, the PL peak position increases monotonically from 420 to 600 nm as the excitation changes from 320 to 540 nm, and the PL intensity drops as the spectrum shifts towards long wavelength. These PL characters are consistent with many other reports.5-7,13,23-27 To date, the intrinsic mechanism for the excitation dependent PL is still quite controversial.5-7,13,23-27 Many different models, such as size effects, surface functional groups model, giant red-edge effect, and edge states have been proposed to explain this emission characteristic.13 As revealed previously,28 the ca. 425 nm band remains fixed under excitation within proper range while the 480-565 nm bands are movable under different excitations. Secondly the PL spectral profiles are asymmetric and can be divided into two Gaussian bands. Thirdly the PL lifetime of 425 nm emission is evidently shorter than that of 500-565 nm emission. Therefore, it is no doubt that there are intrinsically two different emissive centers fluorescing with blue and green emissions. One possible mechanism suggests that carbon


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defects, e.g., carbon vacancies and irregular carbon rings, are responsible for the 425 nm blue emission.28-31 And the excitation wavelength-dependent long-wavelength emissions (500-565 nm) stem from the radiative recombination of electron–hole pairs in the sp2 carbon clusters due to quantum confinement effects. This mechanism well interprets the PL spectra from GQDs and other graphene oxide derivatives28,29 and has been further supported by the PL from CNDs.30,31 The obtained GQDs solutions with concentrations of c0 were then diluted to c0/2 and c0/4, and the corresponding PL spectra are shown in Figures 1c and e respectively. Obviously from the curves, the intensity of 565 nm emission drops rapidly when the concentration decreases. Meanwhile the intensity ratio between 425 and 565 nm emissions notably rises as the concentration changes from c0 to c0/4. Figures 1b, d, and f are PL excitation-emission mappings corresponding to Figures 1a, c, and e. The blue and yellow arrows point out the 425 and 565 nm emission excited at 320 and 480 nm respectively. The contour figures clearly show that the intensity of blue emission increases slower than the green emission when the concentration increases. The green emission (565 nm) seems enhanced at high concentration, which is likely consistent with the reported AIE effect.18,19 Also the blue emission looks partially quenched at high concentrations , which could be truly the reason for this observation. More results are required to clarify the ambiguous issue. Figure 2a presents the PL spectra excited at 320 and 480 nm (selected from Figures 1a, c, and e). The averaged values of PL-peak intensities of the two groups from three independent experiments are plotted in Figure 2b. It unambiguously shows that the intensities of emissions centering at around 565 nm linearly change with the concentrations. While the intensity of the blue emission at about 425 nm severely deviates from linear relationship. Especially at concentration of c0, the intensity ratio between 425 and 565 nm emissions drops conspicuously,


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implying that the 425 nm emission is partially quenched at such a concentration. Note that, practically, it is difficult to precisely ensure the identity of c0 from independent sample preparations, thence the standard deviations shown by the errors are rather big. In fact, each individual measurement shows the same effect (Figure S1, Supporting Information). Moreover, PL QYs of different concentrations shown in Figure S2, Supporting Information further confirm the partially quenching of blue emission at high concentration. Besides, the normalized PL spectra of these two groups are presented in Figures 2c and d, demonstrating highly identical profiles. To gain more insight into this aggregation induced partially quenching of blue PL, the TRPL of the two groups of emissions were measured. Figure 3a shows the PL decay curves of the 430 nm emissions. Notably, the PL decays faster as GQDs concentration increases. However, the lifetime of the green emission remains stable despite GQDs concentration (Figure 3b). Figure 4 shows the absorption spectra of GQDs solutions at different concentrations. As the concentration changes from c0 to c0/8, the absorbance drops gradually. The absorbance at 320 nm and tenfold absorbance at 480 nm versus the concentration are plotted in Figure 4b. It clearly shows that the absorbance is linearly related with the concentration. The absorption spectra confirm that there is no shielding effect at concentration of c0, hinting that the abnormal variation in PL intensity cannot be attributed to absorption process. Obviously, a most direct prediction of concentration increase is the approaching between different GQDs. Specifically, when the concentration is very low, the interactions between different quantum dots are weak and negligible. The PL of the solution can be simply regarded as PL of ensemble GQDs collectively contributed by every individual GQDs. However, when the concentration increases, interactions between different GQDs cannot be neglected. Effects


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such as restriction of intramolecular motions,18,19 photon reabsorption,20 Förster resonance energy transfer32 may affect this collective PL. Though the origins of the two emissions are still under debate,13,23-27 no matter what the perfect model is, the two emissions are always commonly regarded as electronic transitions from two excited states. Herein, we proposed a coupling model to interpret fluorescence intensity change with consideration of the concentration-dependent PL lifetime, as shown in Figures 5a and b. Illustratively, at low concentrations, GQDs are well separated from each other and thus aggregation is negligible in the solution, while at high concentrations, energy transfer at a rate 1 occurs between neighboring GQDs, as schematically shown in Figures 5a and b. In the energy diagram of single GQD in Figure 5a, En, E1, E0 are used to describe the simplified fluorescence decay process for emission of ca. 425 nm. and denote the occupation of exciton states of En and E1 respectively, and refers to the occupation of ground state E0. While similarly, en and e1 represent the exciton states of ca. 565 nm with ground state of E0 as the same. To distinguish two different GQD particles, we employ Fn, F1, F0 to illustrate the identical energy states in a neighboring GQD. Considering fluorescence decay experimental procedure, the GQD is initially at its vacuum state. One pulsed excitation excites the GQD at t=0 so that an=1 and a1=a0=0. Then the decay processes regarding the energy states can be mathematically described by the following rate equations: d 1 − − =− − dt d 1 − 1 − = − dt


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=

+

(1)

The basic steps are similar to Ref.33 except the term

which describes the energy transfer

from En to Fn in the neighboring GQD. Explanatively, an electron occupying En transits to E0 to transfer the energy to the electron initially occupying F0 to transit to Fn. Moreover, the emission of 550 nm could not be excited by short wavelength illumination, the electron at En has no means to transit to en and e1 states when under short wavelength excitation. en and e1 in the GQD of Figure 5b are populated by absorbing long wavelength photons when under short excitation. In the other word, under this circumstance, emission of 425 nm could be re-absorbed to excite long wavelength emission of the GQD, i.e., 550 nm. As shown in Figure 5a, the initial occupations of Fn, F1 and F0 in the GQD are bn=b1=0, b0=1, and cn=c1=0 for en and e1. The corresponding rate equations are d 1 − − =− + dt d 1 − 1 − = − dt d 1 − 1 − 1 − = − dt ′ d 1 − 1 − = − ′ dt ′

=

+

!

−

(2)

The first term of the right side of the second equation is the optical excitation of en by absorbing the photon emitted from radiation E1–E0 in Figure 5a (photon re-absorption). Simulations based on this model are further conducted. Typically, let = ′=2 ns, =4 ns, =3 ns, =1 ns and ′=1


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ns (′ is shorter than because of the small photon energy), the simulated decays of 425 nm and 550 nm emissions are presented in Figures 5c and d. The principal physics is that at high concentration, there can be energy transfer between neighboring GQDs to a certain degree, which will result in a long dwell time of electron occupying high energy state, thereafter a long decay time. Note that our experimental decay measurements showed that the decay at 530 nm detection is indeed slower than that at 430 nm detection (Figure 3), which is in good agreement with these simulated results. Furthermore, the parameters fitted from the PL decay curves are explained by our model, see details in Figures S3 and S4 of Supporting Information. As well known, the energy transfer efficiency is extremely sensitive to small changes in distance, which in principle is inversely proportional to the sixth power of the distance between donor and acceptor.34 As the concentration increases, distances between the GQDs decrease, resulting in the enhanced energy transfer and thus the blue PL partially quenches (black stars in Figure 2b). Confocal fluorescence images confirmed the presence of GQDs agglomeration at high concentration. Specifically, the solution with concentration of c0 was dropped on a coverslip. And microscopic observation was performed after the evaporation of the water. From Figures 6a and b, the agglomeration of GQDs is clearly shown. As a comparison, the bright spots are much smaller in Figures 6c and d, indicating the GQDs were well dispersed in the diluted solution. Moreover, comparison between Figures 6c and d shows the emissive sites are well overlapped. And even when the GQDs was diluted to ultralow concentration (c0/100) and dispersed by ultrasonication, all the spots, probably fluorescence of individual GQDs, contain both emissions at different wavelengths (Figure S5, Supporting Information.). It suggests that a single GQD contains multiple emissive centers rather than different GQDs emits at different wavelengths.35 The multiple emissive centers within a single QD can be classified to groups:28-31


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one is fixed for blue emission (level E1, F1, ca.425 nm) and the other one depends on the size (level e1, 480-565nm),35-37 consistent with the diagram in Figure 5a. The complex electronic structure makes the GQDs as both of donors and acceptors in this energy transfer system. Very similar concentration-dependent PL also can be observed in carbon nanodots. Figure 7 shows the PL excitation-emission mappings acquired from CNDs solutions at different concentrations. The strongest peak locates at 540 nm when the concentration is c0 but it blue shifts to 460 nm when the concentration is diluted to c0/4. Intrinsically, this spectral shift is because the blue peak quenches at concentration of c0 due to nonradiative energy transfer between neighboring nanodots. For individual CND, the blue emission is stronger than the green emission as indicated by the diluted solutions. When concentration increases, the intensity of green emission linearly increases, while the blue emission increases below the linear relationship, similar as the phenomena for GQDs in this report. When the concentration is high enough, the green emission becomes stronger than the blue emission, thus the strongest peak red shifts along with increasing concentration. 4. CONCLUSION The PL spectra from GQDs solutions at different concentrations are studied. The intensity ratio between 425 and 565 nm emissions remarkably dropped as the concentration increased. It is indeed the 425 nm emission partially quenched with the concentration increase, whereas the PL peak positions and shapes were concentration independent. It is successfully interpreted by a model of coupling between neighboring GQDs. When the concentration was high enough, agglomeration happened. The stacking of GQDs substantially resulted in photon re-absorption and non-radiative energy transfer.


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Figure 1. (a-c) PL spectra acquired from GQDs at different concentrations under excitation of different wavelengths (320-540 nm), (a) c0, (b) c0/2 and (c) c0/4. (d-f) PL excitation-emission mappings corresponding to Figs. 1 (a-c).


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Figure 2. (a) PL spectra under excitation of 320 and 480 nm with different concentrations. (b) The peak intensity of emissions excited at 320 and 480 nm. Error bars are standard deviations from three independent measurements. (c,d) The normalized PL spectra under excitation of 320 and 480 nm with different concentrations, respectively.


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Figure 3. PL lifetime of GQDs solutions at different concentrations, (a) excitation wavelength: 375 nm and monitoring wavelength: 430 nm, (b) excitation wavelength: 405 nm and monitoring wavelength: 530 nm.


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Figure 4. (a) Absorption spectra of GQDs solutions at different concentrations. (b) the absorbance at 320 nm and tenfold absorbance at 480 nm are plotted versus the concentration. They are fitted by linear relationships.


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Figure 5. (a,b) Schematics of energy relaxation and radiative recombination of an exciton in single GQD (a) and energy transfer between GQDs. an, a1, a0, bn, b1, b0, cn c1, are populations for energy levels of En, E1, E0 (a) and Fn, F1, F0, en, e1 (b). En, E1 or Fn, F1 are energy levels responsible for 425 nm emission while en, e1 (dash lines) are corresponding to 550 nm emission. α, γ, β, τ, β’, τ’ are lifetime of corresponding electronic transitions. (c,d) Simulated time-resolved fluorescence of 425 nm (c) and 550 nm (d) from neighboring GQDs without energy transfer 1 = 0 and with energy transfer.


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Figure 6. Confocal fluorescence images of GQDs at concentrations of c0 (lambda mode: color from stacking frames at different emission windows) (a,b) and c0/10 (c,d) under excitation of 405 and 488 nm. Scale bar=25 µm.


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Figure 7. PL excitation-emission mappings acquired from CNDs at different concentrations.


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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: PL intensities versus concentration from three independent measurements, PL quantum yields, numerical fitting of the decay curves, and confocal fluorescence images of GQDs at ultralow concentrations.

AUTHOR INFORMATION Corresponding Author *Email: [email protected], Phone: 0086-25-85891796, Fax: 0086-25-85891301 (Z.X. Gan) , [email protected] (Y. Fu) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was jointly supported by National Natural Science Foundation of China (No. 11604155), China Postdoctoral Science Foundation (No. 2016M600428) and Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1601023A).

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Table of Contents Graphic


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Photon Reabsorption and Nonradiative Energy-Transfer-Induced

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