Article pubs.acs.org/JPCC
Temperature-Dependent Fluorescence in Carbon Dots Pyng Yu,* Xiaoming Wen, Yon-Rui Toh, and Jau Tang* Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan
ABSTRACT: Carbon dots are cost-effective, environmental friendly, and biocompatible nanoparticles with many potential applications in optoelectronics and biophotonics. Their dual fluorescence bands were observed and could be attributed to core and surface state emission. We also conduct temperature-dependent fluorescence measurements from cryogenic to room temperatures. The dual emission bands exhibit similar temperature dependence. The strong electron−electron interactions and weak electron−phonon interactions could account for the very broad photoluminescence (PL) band even at 77 K. Our experimental results also suggest that carbon dots exhibit similar temperature behavior as metallic quantum dots (nanoclusters) but are different from inorganic semiconductor quantum dots. Here, for the first time, we present the temperature-dependent spectroscopic results to shed some light on the presently unclear fluorescence mechanism. although they differ in their carbon skeleton.4,24 Few-layered graphene samples have also been shown to exhibit PL behavior quite similar to that observed in carbon quantum dots (CQDs). Hence, controlling the layers of graphene can realize the transition from GQDs (below tens of layers) to CDs. So far, explanation of λex-dependent25,26 or independent21,27,28 PL of CDs and the origin of fluorescence in CDs and GQDs can be generally classified into two main categories: (a) core related29−31 and (b) surface state related emission.2,4 More recently, Krysmann et al. demonstrated that PL of CDs exhibits dual emission, arising from both carbogenic core and surface molecular fluorophores.32 The optical properties of CDs have been extensively investigated by steady-state and time-resolved PL techniques.2,10,33 Up to now, most of the experimental work has been focused on their synthesis and the improvement of quantum yields. The mechanism of fluorescence is presently still a matter of strong debate, varying from case to case. Temperature-dependent PL can provide unique insight into the understanding of the mechanism. To date, temperaturedependent PL has not been studied for CDs. In this study, temperature-dependent experiments reveal dual PL bands in CDs, which were ascribed to core and surface states of the
1. INTRODUCTION Development of nanoparticles free of cadmium (Cd), lead (Pb), and mercury (Hg) is important and urgent due to the environmental hazard for biological and photovoltaic applications for such toxic materials. In recent years, carbon dots (CDs) have been demonstrated to possess a high quantum yields (QY, ∼60%), nontoxicity, nonblinking, low photobleaching, high photostability, and large two-photon crosssection.1−4 They are competitive in performance to the commercially available CdSe/ZnS quantum dot (QD).1−4 CDs have also been demonstrated to be good electron donors, as well as electron acceptors.5−7 Therefore, CDs offer great potential for a broad range of applications in light-emitting diodes (LEDs),8−10 sensors,11,12 surface-enhanced Raman scattering (SERS),13 and biomedical imaging.14−17 Some photoluminescence (PL) characteristics of CDs, such as λex-dependent emission2,18 and the up-conversion property,5,19−21 are very different from those of the conventional QDs. In addition, their PL spectrum usually has a full width at half-maximum (fwhm) of more than 100 nm. In general, luminescence enhancement of CDs could be achieved through surface passivation and doping by organic polymers, diamine, and a zinc shell, or coupling to metal-based nanostructures.20,22,23 It is interesting to note that similar PL behavior was observed in other members of the fluorescent carbon family: shortened carbon nanotubes, amorphous and crystalline CDs, hollow structures, and graphene quantum dots (GQDs), © 2012 American Chemical Society
Received: July 24, 2012 Revised: November 16, 2012 Published: November 17, 2012 25552
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Figure 1. (a) Absorption spectrum of CDs in solution. (b) Fluorescent excitation (dashed line) and emission (solid line) spectra of CDs in solution. PLE were recorded at 500, 550, and 600 nm, respectively. (c) TEM image of the CDs. (d) The histogram of the size for the CDs.
recorded by a cooled CCD (SynapseTM CCD). The spectral resolution of the system is around 0.5 nm. The sample was installed in a cryostat (ST500) with a controllable temperature between 77 and 300 K using liquid nitrogen cooling.
green CDs, respectively. The experimental data also indicate relatively strong electron−electron scattering and weak electron−phonon interactions in CDs, which could be attributed to large amounts of mobile π-electron carriers in CDs. Therefore, CDs exhibit similar optical properties to metallic QDs (nanoclusters), rather than inorganic semiconductor QDs. To the best of our knowledge, the results presented here are unique in the context of determining the fluorescent mechanism of CDs from the temperature-dependent viewpoint.
3. RESULTS AND DISCUSSIONS Figure 1 shows the absorption, fluorescence emission, and excitation spectra measured from the CDs in aqueous solution. The absorption spectrum exhibits two peaks around 250 and 300 nm and extends to 600 nm without noticeable structures. According to previous studies, the peak at 250 nm is ascribed to π−π* transition of aromatic CC bonds, while a shoulder at 300 nm attributes to n−π* transition of CO bonds.34 Carboxyl group may originate from the surface of CDs, while CC from the core. Strong green fluorescence was observed with a peak at 510 nm when the excitation wavelength is between 360 and 440 nm. It should be noted that the fluorescence maxima do not evidently shift with increasing excitation wavelength, as shown in Figure 1b. These results are consistent with the observation by Fang et al. However, further increasing excitation wavelength from 440 to 500 nm, the fluorescence peak obviously red shifts. In film, the fluorescence exhibits a 30 nm red shift with respect to that of solution. The transmission electron microscope (TEM) image of CDs was shown in Figure 1c, and the size of the CDs was found to be 5.5 ± 1.4 nm, as shown in Figure 1d for the histogram. Shown in Figure 2 are the observed temperature-dependent PL lifetimes, excited at 400 nm and monitored with a 550 nm bandpass filter by using the TCSPC technique. The PL relaxation dynamics becomes faster at higher temperatures, which could be attributed to the occurrence of nonradiative decay processes. In general, the time evolution of fluorescence with multiexponential components was observed,16,35 which
2. EXPERIMENTAL SECTION The CDs used in this study were synthesized according to a simple bottom-up approach developed by Fang et al.27 Briefly, a mixture solution of 1 mL of glacial acetic acid and 80 μLof H2O was quickly added to 2.5 g of P2O5 without additional heat. The carbon dots were collected by dispersing into 5 mL of DI water for further optical measurements. The film samples used for cryogenic experiment were fabricated by drop casting on the glass substrate that was carefully ultrasonic cleaned and hydrophilic treated. The absorption and fluorescence spectra of CDs were recorded using a JASCO UV/visible (V-670) and fluorescent (FP-6300) spectrophotometer, respectively. The temperaturedependent PL lifetimes were measured by the time-correlated single photon counting (TCSPC) technique (PicoHarp 300, Picoquant). The excitation source is frequency-doubled from a Ti:Sapphire oscillator laser (Tsunami, Spectra Physics) by a 1 mm BBO crystal at 400 nm with 80 MHz repetition rate, coupled with an ADP. For temperature-dependent fluorescence spectra measurements, a 406 nm CW laser was used as an excitation source. Fluorescence was collected into a spectrometer (HORIBA Jobin Yvon) with a 1200 l/mm grating and 25553
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Figure 4. Fluorescence spectrum can be well fitted by a two-Gaussian function, suggesting that the spectrum is composed of two different emission mechanisms.
Figure 2. Time-resolved PL measurements at 550 nm as a function of temperature. A clear decreased lifetime was observed with an increasing temperature.
suggests the photoexcited carriers following the complicated relaxation processes. It is noteworthy that the PL relaxation dynamics of CDs reveals multiexponential from room temperature to cryogenic temperature, which is different with the conventional semiconductor QDs. In QDs, the decay trace is usually monoexponential at low temperatures because the radiative recombination dominates the dynamic process.36,37 To gain detailed insight into the underlying decay process, we measured the fluorescence spectra of the CD film as a function of temperature from cryogenic to room temperature, as shown in Figure 3. With an increasing temperature, the
high energy band has a peak at 530 nm (referred to band-I hereafter) with a small bandwidth of 43.6 nm. In contrast, the low energy band has a peak at 563 nm (referred to band-II hereafter) with a much larger bandwidth of 84.1 nm at 300 K (shown in Figure 4). We extracted the fitting of the energy gap of PL emission as a function of temperature. It is interesting to note that these two bands exhibit a small red shift upon increasing temperature, band-I with 15 meV shift and band-II with 6 meV shift only. The temperature dependence of electronic states in semiconductors has been studied extensively. Two mechanisms that are responsible for the temperature-dependent energy gap are the renormalization of band energies by electron−phonon interactions and thermal expansion of QDs.36 In semiconductor bulk and quantum dots, the fluorescence peak shifts toward the lower energy dominantly due to exciton−phonon interactions, while thermal expansion typically has a negligible effect. A large red shift of the energy gap, up to 100 meV, was observed in semiconductor quantum dots due to strong phonon scattering when temperature was increased from 77 K to room temperature.36,39,41 By contrast, a small red shift of 11 meV was observed previously in Au25 nanoclusters by our group due to a weak electron−phonon coupling with the same temperature variation. The temperature dependence of the band gap is often explained using an expression proposed by O’Donnell and Chen for quantum dots based on an analysis of the exciton−phonon coupling mechanism.42,43 Following the theory that phonon scattering induced energy gap renormalization, we obtained a fit of the energy variation in CDs using
Figure 3. Fluorescence as a function of temperature between 77 and 300 K. It is evident that the intensity decreases upon an increase in temperature.
intensity of fluorescence exhibits a monotonous decrease. Basically, possible processes resulting in photoexcited electron relaxation in the CDs include radiative relaxation, Auger nonradiative scattering, and thermally activated trapping in surface and/or defect/impurity states. In our experiments, the excitation density was very low, and thus, Auger scattering could be ruled out. Therefore, the nonradiative relaxation is most likely the contributing factor due to the thermal activation of nonradiative trapping, which are often observed in semiconductor bulk, quantum dots, and core−shell structured quantum dots.36,38−40 At each temperature, the PL spectrum exhibits asymmetric peaks, suggesting multifluorophores or luminescent species. We try to fit the spectrum using a multi-Gaussian function. It is found that the PL spectrum at each temperature can be well fitted by a two-Gaussian function, as shown in Figure 4. The
Eg (T ) = Eg (0) −
2S⟨hω⟩ exp(⟨hω⟩/KBT ) − 1
(1)
where S is the Huang−Rhys factor that represents the coupling strength of exciton−phonon, ⟨hω⟩ is the average phonon energy, and KB is the Boltzmann constant. We extracted the fit parameters Eg(0) = 2.347 ± 0.001 eV, ⟨hω⟩= 15.2 ± 7.5 meV, and S = (3.98 ± 0.37) × 10−4 for band-I and Eg(0) = 2.197 ± 0.001 eV, ⟨hω⟩ = 24.3 ± 15.9 meV, and S = (1.68 ± 0.47) × 10−4 for band-II. The relatively small S represents the weaker electron−phonon coupling, which is consistent with the results in the following bandwidth analysis. It is noteworthy that a small S = (5.91 ± 1.20) × 10−4 was also obtained in Au25@BSA nanoclusters.44 In comparison, S = 1.95 and S = 1.57 were found in InP/ZnS quantum dots45 and CdSe/CdS dot-inrods,40 respectively. An evidently smaller S represents a weaker 25554
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The PL intensity evidently decreases upon increasing temperature, it is ascribed to thermally activated nonradiative trapping. At low temperatures, the nonradiative channel is not thermally activated; therefore, the excited electrons can radiatively emit photons. Once the temperature is increased, the nonradiative channels become thermally activated, such as trapping by surface/defect/ionized impurity states, as expressed by τNR = τ0 exp(Ea/KBT),47 where Ea is the activation energy. The quantum efficiency can be expressed as η = (1 + τR/τNR)−1. The nonradiative lifetime decreases with increasing temperature, resulting in a decrease in the quantum efficiency and fluorescence intensity, which could be expressed as
interaction of phonon scattering in CDs, this is consistent with our observation of a small red shift (Figure 5).
I(T ) = I0/[1 + (τR /τ0) × exp( −Ea /KBT )]
(2)
Figure 7 shows an Arrhenius plot with the fits for band-I and band-II of the fluorescence intensity. The activation energy was Figure 5. Energy gap as a function of temperature in CDs.
The bandwidth of fluorescence was also investigated as a function of temperatures. As shown in Figure 6, the fwhm of
Figure 7. Arrhenius plot of the fluorescence intensity for band-I and band-II and the fitted curves.
extracted with 24.0 ± 3.2 and 42.7 ± 3.9 meV for band-I and band-II, respectively. The relatively lower activation energy than those from many core−shell semiconductor QDs suggests a higher density of defect states due to the lack of surface protection. It has been reported that the PL emission can be enhanced by PEG passivation.48 Recently, the deep UV (DUV) graphene quantum dots (∼5 layers) were prepared by a microwave-assisted hydrothermal method.10 After 266 nm excitation, the two DUV emissions at 3.90 and 2.92 eV originated from the core, and the surface of GQDs were demonstrated by Tang and co-workers. The bandwidth of the core emission is evidently narrower than that of surface states. The GQDs consist of a cyclic aromatic hydrocarbon structure in the core and the O- and H-containing functional groups on the surface, which is similar to our green fluorescent CDs. The two emission peaks of the GQDs exhibit a small red shift from 10 to 300 K (i.e., ∼30 meV for core emission and ∼20 meV for surface emission). On the basis of Tang’s results, we assign the narrow band-I (2.34 eV) to the sp2 cluster emission in the CD core and the broad band-II (2.20 eV) to the surface states, which would comprise the oxygencontained carboxyl groups in this experiment. Eda et al. had demonstrated the PL of graphene oxide (GO) dependence on the reduction degree of GO since that PL originates from the recombination of electron−hole pairs, localized within small sp2 clusters embedded within an sp3 matrix. However, the surface includes various energy levels, which may result in a series of emissive sites. Therefore, as the excitation wavelength changes, different surface state emissive sites will become domi-
Figure 6. Bandwidth of band-I and band-II does not exhibit a significant increase between 77 and 300 K.
band-I and band-II almost remain unchanged with increasing temperature. Previous studies on the temperature dependence of fluorescence fwhm in semiconductor quantum dots and nanocrystals have shown that the broadening of the PL band can be separated into inhomogeneous and homogeneous components.39,41 Exciton−phonon interactions and ionized impurity scattering can result in bandwidth broadening, which is temperature dependent. The total bandwidth can be described by a temperatureindependent intrinsic term (Γ0, dominantly electron−electron scattering), and temperature-dependent electron−phonon and surface/defect scattering, expressed as Γ(T) = Γ0 + σT + ΓLO(exp(ELO/KBT) − 1)−1 + α exp(ES/KBT),40,46 where σ is the electron-acoustic phonon coupling coefficient, Γ LO represents the strength of exciton−LO phonon coupling, ELO is the LO phonon energy, α is the bandwidth due to fully ionized impurity/defect scattering, and ES is the ionized energy for ionization of impurity and defect states. The bandwidth of band-I and band-II are temperature-independent, which indicates that electron−electron scattering dominates in these CDs. Also, the small variation of bandwidth in band-I and band-II suggests that the phonon coupling is very small, consistent to the observation of the energy gap. 25555
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nant.2,10,29,49,50 Furthermore, taking into account band-I and band-II dominantly originating from the core and surface, the temperature-independent ratio of band-I to band-II confirms that the energy transfer between core and surface state is ineffective (data not shown). This assignment could also help to explain the shifting PL of CDs, which usually occurs at long wavelength excitation in many studies. For example, the shifting PL of the green CDs was observed only at longer than 440 nm excitation wavelength in this experiment. Essentially, the spectral broadening of the fluorescence can be determined from the electron relaxation time, as expressed Γ = 1/2πc × 1/T2 = 1/2πc(1/2T1 + 1/T2*), where T1 is the population relaxation time (radiative and nonradiative process), T2 represents the total dephasing time, and T2* is the pure dephasing time. In the green CDs, the bandwidth of PL basically is temperature independent. It is well-known that size inhomogeneity commonly leads to broad bandwidth of emission in QDs.18 Recently, Lai and co-workers experimentally confirmed that the large size inhomogeneity only resulted in a slight decrease of bandwidth in
[email protected] Therefore, the weak temperature effect in CDs is consistent with the fact that the dominant interaction mechanism involves electron− electron interactions rather than electron−phonon coupling. Ultrafast experiments confirmed that a rapid intraband carrier equilibration within 30 fs by electron−electron scattering in graphite and graphene, resulting in separate electron and hole distribution in the conduction and valence band.51−54 Therefore, to exclude the fluctuations in size, shape, and composition of CDs,16 a very broad PL band (>100 nm) usually observed even at very low temperature (77 K) is attributed to strong electron−electron interaction. This result is similar to metallic nanoclusters and different from semiconductor QDs. Hence, we speculate that the π-electrons in CDs can act similarly as free-electrons in the metallic nanoclusters.
ACKNOWLEDGMENTS We acknowledge Instrumental Center of National Taiwan University for the use of TEM. We also acknowledge the financial support by Academia Sinica and National Science Council (NSC) of Taiwan under the program 99-2221-E-001002-MY3 and No. 99-2113-M-001-023-MY3.
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4. CONCLUSIONS We have conducted temperature-dependent fluorescent measurements in highly fluorescent CDs between 77 and 300 K. The dual fluorescence bands were observed in green CDs, which are attributed to the emission from the core and the surface states. The dual bands exhibit a similar temperature effect. It is found that the intensity decreases upon increasing temperature. The emission energy is nicely fitted by the O’Donnell and Chen equation based on an analysis of the exciton−phonon coupling mechanism. However, the observed smaller Huang−Rye factor means a weak exciton−phonon coupling compared to semiconductor QDs, similar to gold nanoclusters. The weaker electron−phonon interactions, in turn, result in a temperature-dependent bandwidth. This indicates that CDs exhibit a metallic-nanocluster-like temperature behavior. Moreover, the temperature-dependent lifetime measurements are consistent with the steady-state PL results. We speculate that the π-electrons in CDs can act similarly as free electrons in the metallic nanoclusters.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (P.Y.); jautang@gate. sinica.edu.tw (J.T.). Notes
The authors declare no competing financial interest. 25556
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