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carbon quantum dot composite solid films exhibiting intense and tunable blue–red emission
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Poly(ethylene glycol)/carbon quantum dot composite solid films exhibiting intense and tunable blue–red emission Yanling Hao a,b , Zhixing Gan a , Jiaqing Xu a , Xinglong Wu a,∗ , Paul K. Chu c a b c
Key Laboratory of Modern Acoustics, MOE, Institute of Acoustics, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China Department of Physics, XingYi Normal University for Nationalities, Xingyi 562400, Guizhou, People’s Republic of China Department of Physics and Materials Science City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 23 January 2014 Received in revised form 13 May 2014 Accepted 14 May 2014 Available online xxx Keywords: Carbon quantum dots Solid films Tunable emission
a b s t r a c t Although carbon quantum dots (CQDs) possess excellent luminescence properties, it is a challenge to apply water-soluble CQDs to tunable luminescent devices. Herein, quaternary CQDs are incorporated into poly(ethylene glycol) to produce poly(ethylene glycol)/CQD composite solid films which exhibit strong and tunable blue–red emission. The fluorescent quantum yield reaches 12.6% which is comparable to that of many liquid CQDs and the photoluminescence characteristics are determined to elucidate the fluorescence mechanism. The CQD solid films with tunable optical properties bode well for photoelectric devices especially displays. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Carbon quantum dots (CQDs), a new class of photoluminescence (PL) nanoparticles [1–20], possess unique advantages including low toxicity, simple preparation, chemical inertness, biocompatibility and easy functionalization and have the potential of replacing traditional semiconductor quantum dots comprising toxic and/or expensive heavy metals [1]. These superior properties thus spur new applications such as bio-imaging devices [2–6], photocatalysts [6,7], fluorescence sensors [8], light-emitting diodes [9], and temperature probes [10]. However, since fluorescence from solid state CQDs is quenched significantly, the performance of luminescent devices based on CQD films is not satisfactory and only a few examples of strong fluorescence have been reported [11–14]. Pan and coworkers [11] fabricated carbon thin films with high fluorescence efficiency but the emission was not tunable. Wang et al. [12] synthesized organosilane-functionalized CQDs which could be easily fabricated into CQDs fluorescent solid-state materials but they emit blue light primarily. Qu et al. [13] reported that lens paper printed with CQDs ink showed green to red emission, but the green–red emission was suspected to originate from the lens paper. Wang et al. [14] showed blue to orange emission from a mixture of CQDs and fluorescent dyes printed on paper but it
∗ Corresponding author. Tel.: +86 025 83686303; fax: +86 025 83595535. E-mail address: [email protected] (X. Wu).
was suspected that the green, yellow–green, and orange emission stemmed from calcein, CdTe QDs, and RhB, respectively, in lieu of the CQDs. Although CQDs in an aqueous solution exhibit excellent color-tunable PL properties, solid films composed of CQDs with tunable emission have not been reported, but in order to realize their full potential, it is crucial to obtain CQDs films with tunable emission characteristics. In addition, owing to the lack of experimental evidence, the PL mechanism of CQDs is not well understood. Based on the PL characteristics of CQDs in solutions, several PL mechanisms have been proposed [4,6,15,16]. However, since the solution environment and the structure of the CQDs are quite complex, it is difficult to determine the origins of PL from CQDs. In this case, an investigation of the PL properties of solid CQD films can provide insights to the mechanism. Diamine-terminated poly(ethylene glycol) (PEG NH2 ) has been used to passivate CQDs by amidation of the carboxyl groups in order to attain large quantum yield. The electrons and holes are generated via photo-induced charge separation in the carbon nanoparticles and surface passivation is believed to stabilize the surface sites facilitating radiative recombination due to the smaller number of nonradiative centers [15,17]. However, the passivation mechanism is still controversial. First of all, in semiconductor quantum dots, passivation can remove the surface/defect/nonradiative recombination states and so quantum confined PL can be achieved. Second, transforming some surface-bound carboxyl groups into amide groups may not restore the sp2 network to repair the defects. Third, surface passivation does not necessarily lead to a higher
http://dx.doi.org/10.1016/j.apsusc.2014.05.095 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Hao, et al., Poly(ethylene glycol)/carbon quantum dot composite solid films exhibiting intense and tunable blue–red emission, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.095
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quantum yield in many cases [7,18–20]. Therefore, it is possible that poly(ethylene glycol) (PEG) diamine (or amine linkages) does not passivate the CQDs effectively. PEG, a simple linear polymer chain made of (CH2 CH2 O)n , has the hydroxyl group on one end (referred to as PEG OH here) which can be grafted [21,22] and may produce passivation effects different from PEG NH2 . In this work, quaternized CQDs (QCQDs) are produced and passivated with PEG to produce QCQD and PEG/QCQD composite solid films. The PEG/QCQD composite solid films show intense tunable blue–red emission and the mechanism is discussed. 2. Experimental 2.1. Materials Betaine hydrochloride (99%), tris(hydroxymethyl) aminomethane (99.9%), and PEG with a molecular weight of 1500 Da was purchased from Aladdin (Shanghai, China). Other chemicals were of analytical grade and all the chemicals were used as received without purification. 2.2. Preparation of QCQD and QCQD films The QCQDs were synthesized by a bottom-up approach proposed by Giannelis et al. [23] with some modifications. The obtained syrup after extraction by isopropanol was dried in an oven at 70 ◦ C for 24 h. The white powder was calcined at 250 ◦ C in air for 2 h at a heating rate of 10 ◦ C/min in a quartz tube. After cooling to room temperature naturally, a brown solid was obtained. It was ground in an agate mortar for 20 min and the powder was dispersed in 50 ml of deionized water, ultrasonically processed for 20 min, and centrifuged at 9000 rpm/min for 15 min. The suspension was collected and further dialyzed in a dialysis bag with a molecular weight cut-off of 500 Da for 3 days to remove the residual tris and betaine hydrochloride. Finally, the suspension was dried in a vacuum oven to obtain the pure luminescent QCQD powder. The QCQD film was fabricated on a silicon wafer by conventional drop casting. After the concentrated QCQDs solution was dropped on the safer, it was dried carefully in a vacuum oven at 70 ◦ C. 2.3. Preparation of PEG/QCQD composite films One milligram of the QCQDs were dispersed in 20 ml of distilled water, with the pH adjusted to 4–5 with diluted HCl. Various amounts (0, 20, 40, 60, and 80 mg) of PEG OH were added to the solution prior to sonification for 10 min to form a homogenous suspension. The suspension was transferred to a three-necked flask, heated to 110 ◦ C, and vigorously stirred under N2 for 24 h. Afterwards, the liquid was cooled to room temperature naturally. According to the different weights of the PEG OH, the samples were referred to as PEG0 /QCQD, PEG20 /QCQD, PEG40 /QCQD, PEG60 /QCQD, and PEG80 /QCQD. In order to prepare the PEG/QCQD composite solid films, these samples were dried for one day at 70 ◦ C in a vacuum oven to obtain a sticky colloid. The conventional drop casting was employed to prepare the PEG20 /QCQD, PEG40 /QCQD and PEG60 /QCQD composite solid films. 2.4. Passivation of QCQDs with PEG OH The extent of passivation of the QCQDs can be varied by the amount of PEG OH. The amount of PEG OH that can completely passivate the QCQDs is estimated according to the atomic percent of C and O. It should be noted that the maximum emission efficiency requires an optimized density of QCQDs, otherwise fluorescence quenching results in if the concentration of QCQDs is too large, and the optimal concentration of QCQDs is about 0.05 mg/ml. According
to the shell approximation [24,25], the number of C atoms in one nanocrystal (NC) is given by NC = 4d3 /3a3 , where d and a are the NC diameter and lattice constant (0.246 nm for hexagonal graphite), respectively. For QCQDs with a diameter of 3.3 nm, there are about 10,070 C atoms. After integration and correcting for the carbon and oxygen contents in a reference sample (a clean Si wafer), the C/N/O atomic ratio is 1:0.04:0.1 and hence, there are 1,007 O atoms surrounding the QCQD. It is assumed that all the oxygen atoms on the QCQD react with PEG OH and the diameter of the QCQDs is 3.3 nm. Thus, the amount of PEG OH (M) which can completely passivate the QCQDs with the most probable diameter can be calculated according to the following relation: M = Mr × 1007 × m/NA × , where Mr is the average molecular weight of PEG OH (1500), m is the mass of the QCQD in 20 ml of aqueous solution (1 mg), NA is Avogadro’s constant, is the density of graphite (2 g/cm3 ), and is the QCQD volume with a diameter of 3.3 nm. Accordingly, M is calculated to be 66 mg. 2.5. Characterization Transmission electronic microscopy (TEM) was conducted on a JOEL TEM-2100F at 200 kV and PL and PL excitation (PLE) were performed on an Edinburgh FLS920 fluorescence spectrometer with the photos taken by a Nikon digital camera. The PL quantum yields were determined on a Hamamatsu absolute PL quantum yield spectrometer. The UV–vis spectra were recorded on a Shimadza UV–vis–NIR spectrophotometer and the Fourier transform infrared spectroscopy (FTIR) spectra were acquired from a Nicolet Nexus 870 spectrometer. X-ray photoelectron spectroscopy (XPS) was performed on the K-Alpha spectroscopy (Thermo Fisher Scientific Corporation, USA) with an Al K˛ X-ray source. 3. Results and discussion The transmission electron microscopy (TEM) image (Fig. 1a) discloses that the QCQDs have a nearly spherical shape and a mean diameter of 3.3 nm ± 0.5 nm (one standard deviation) as shown in Fig. 1b. The high-resolution TEM (HR-TEM) images in Fig. 1c and d reveal the high crystallinity of QCQDs. The lattice spacings of 0.34 and 0.21 nm correspond to the (0 0 2) and (1 0 0) faces of hexagonal graphite, respectively. The data suggest that the CQDs are graphite quantum dots. Fig. 1e displays the UV–vis absorption spectrum of the QCQDs revealing the absence of distinctive peaks in the UV–vis region except the one at about 270 nm which has been observed from other UV–vis absorption results of CQDs with a graphitic structure [26,27]. Experimental and theoretical studies suggest that the ideal graphite is a semiconductor with a narrow band gap of about 40 meV [28,29] and the electronic states of nanocrystalline carbon particles are size dependent. Meanwhile, passivation (e.g. hydrogenation) of the dangling bonds is expected to influence the properties of the carbon particles [30]. Similar to that of an amorphous semiconductor, the energy gap can be calculated from the UV–vis absorption spectra according to the following relationship known as Tauc’s expression [31,32]:
˛hv = A hv − Eg
2
where ˛ is the optical absorbance coefficient, hv is the photon energy, A is a constant, and Eg is the optical √ band gap. As demonstrated in previous studies [30,33], plotting ˛hv versus the photon energy (hv) and extrapolating the linear region of the curve to the x-axis yield the optical band gap of the QCQDs to be 2.46 eV (Fig. 1e). The PL and PLE spectra of the QCQDs in an aqueous solution are displayed in Fig. 2a and b. As previously reported, the PL spectra exhibit excitation wavelength-dependent emissions but as the excitation wavelength increases, the emission intensity drops
Please cite this article in press as: Y. Hao, et al., Poly(ethylene glycol)/carbon quantum dot composite solid films exhibiting intense and tunable blue–red emission, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.095
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Fig. 1. (a) TEM and (c and d) HR-TEM images of the QCQDs with the size distribution histogram of the QCQDs √ in (b). (e) UV–vis absorption spectrum of the QCQDs and the inset showing the ˛h versus photon energy curve.
rapidly. It should be mentioned that when excited by 300–340 nm, the position of the PL bands are almost fixed in the blue region and have an asymmetric shape. However, when the excitation wavelength is changed from 360 to 500 nm, the long wavelength emission bands shift evenly. The PL intensity maxima for different excitation wavelengths are shown in Fig. 3a. This phenomenon has been observed from other experiments in spite of the different sample preparation methods [3,5,34,35]. Based on this PL feature, it can be concluded that the origins of fixed blue and shifted long wavelength emissions from the QCQDs aqueous liquid are different. In the PLE spectra of the QCQDs aqueous liquid, except for the fixed band at about 285 nm which may be attributed to the −* transition of aromatic C C bonds, there is another fixed band at about 370 nm. A gradually broadened and variable band (400–500 nm) is observed when the monitoring emission wavelength is varied. In general, the redshift excitation process (400–500 nm) corresponds to long wavelength emissions which also redshift with increasing excitation wavelengths. The results are consistent with the quantum confinement effect [36,37] and hence, the shift in the long wavelength emission with increasing excitation wavelength observed from the QCQDs aqueous liquid can be attributed to quantum confinement on the QCQDs [3,6,38]. The energy gaps of the QCQDs are determined by the size of the QCQDs. The smaller the size, the wider is the energy gap. The distribution of the energy gaps
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based on the size distribution results in emission wavelengths in the range of 450–560 nm. Dual fluorescence bands (blue and green) have recently been observed from CQDs. The blue emission is assigned to amide-containing fluorophores [39]. According to the formation mechanism, if the CQDs are prepared by a bottom-up approach, organic molecules undergo polymerization first to form intermediate polymers prior to carbonization. The organic fluorophores may account for the blue emission. However, it should be mentioned that blue emission can always be observed regardless of sample preparation [6,9,10,16,34,35]. Therefore, the blue emission band ascribed to the organic fluorophores is incorrect. It is easy to understand when the bottom-up method is adopted. The irregular carbon rings or topological defects (non-hexagonal rings) in the carbon network must be present. Although the top-down approaches do not have a polymerization step, non-hexagonal rings cannot be avoided due to the severe synthetic conditions for example, laser ablation, reflux in concentrated acid, electrochemical etching. That is, both the top-down and bottom-up methods produce topological defects within the carbon network inevitably. It has been reported that the strong blue emission (about 440 nm) from reduced graphene oxide (rGO) stems from carbon defect states [40] and so we ascribe the fixed blue emission from the QCQDs to the optical transition in the topological defects in the carbon network and the fixed PLE band in Fig. 2b (at 370 nm) to defect state excitation. When the QCQDs are in the form of a thin film on a Si wafer, the PL peak is pinned at about 650 nm and there is another weak fixed peak at 460 nm (Fig. 2c in the red circle). This indicates that the effect of the quantum confinement disappears, but the defect states still exist because the topological defects are stable and common in CQDs. To identify the origin of the 650 nm PL emission band, the PLE spectra of the QCQD film (Fig. 2d) are examined in details. All the PLE spectra are pinned at 365 nm showing no dependence on the monitoring emission wavelength. The intensity of the fixed PLE band is much smaller than that of the QCQD aqueous liquid, indicating that the fixed PLE band at 365 nm in Fig. 2d is different from the 370 nm band in Fig. 2b. The fixed PLE feature has also been observed from porous Si [41] indicating that the photo-excited carriers for the 650 nm radiation are not from the carbon nanocrystal core [42] but rather from some possible oxygen-related states [43]. Hence, it is supposed that the red emission from the QCQD film is associated with oxygen-related structures, similar to the red emission from graphene oxide (GO) assigned to oxygen-related structures [44]. The FT-IR spectra impart more information about the changes in the structure and surface functionalities between the as-prepared QCQD and QCQD film on the Si wafer (Fig. 3b). The as-prepared QCQDs show a prominent absorption band between 3100 and 3500 cm−1 for the hydroxyl ( OH) groups due to the presence of moisture. The intense and broad peak at 1650 cm−1 is attributed to amide I and C C stretching mode, whereas the shoulder at 1720 cm−1 is assigned to carbonyl (C O) bonds. The peak at 1450 cm−1 corresponds to C N stretching (amide III) and that at 1560 cm−1 is characteristic of amide II. Several absorption bands between 900 and 1300 cm−1 are due to C O vibrations. The absorption peak at 1050 cm−1 and adjacent band at 982 cm−1 represent the asymmetric stretching and bending modes of the epoxy group. The broad band at 600 cm−1 typically occurs in many carbonaceous solids and it is ascribed to graphitic domains [23]. Last but not least, the peak at 2072 cm−1 is characteristic of (C ≡ C) [45,46]. The emergence of C ≡ C indicates the presence of non-hexagonal carbon rings. When the QCQDs are in a solid film, this peak becomes weak and the vibrations related to C O (in the blue box of Fig. 3b) become more abundant and stronger, indicating that the oxygen related surface states increase and topological defects decrease. Hence, the PL peak at 650 nm of the QCQD film originates from oxygen-related
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Fig. 2. PL emission spectra: (a and b) QCQDs aqueous solution, (c and d) QCQD film on Si, and (e and f) PEG60 /QCQD composite film on Si excited by different wavelengths. (g) Photos taken from the PEG60 /QCQDs composite film excited by different wavelengths (from left to right): 360, 420, 460, 500, 520, and 570 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
states but the topological defect state emission does not disappear. With the exception that vibrations of the CH2 and CH3 groups of the betaine ligand become more obvious, the vibrations due to amide only change slightly, indicating that the amide linkage still exists but it cannot passivate the CQDs effectively. Considering the hydroxyl groups which can react with epoxy and carbonyl under acidic condition to form different surface structures from amino passivation, PEG OH is used to further passivate the QCQDs. The PL and PLE spectra from the PEG60 /QCQD composite film are shown in Fig. 2e and f. The solid film shows strong
tunable PL in the blue–red region. The PL from the PEG60 /QCQD composite solid film is so intense that the emission spots with different wavelengths (colors) are visible to the naked eye even when excited by a Xe lamp. The fluorescent quantum yield of the solid film reaches 12.6% under 420 nm excitation and it is comparable to those of many liquid CQDs [8,34,35]. Fig. 2 g depicts six representative photos taken at excitation wavelengths of 360, 420, 460, 500, 520, 570 nm on a Nikon digital camera. Here, different filters are used to avoid scattering of the excitation light background.
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(a)
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(b) CH2 stretching
Transmittance (%)
PL Imax position (nm)
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QCQDs aquaous solution QCQDs film PEG60-QCQDs film
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Fig. 3. (a) PL intensity maximum positions at different excitation wavelengths and (b) FT-IR spectra of the as-prepared QCQDs (black line) and QCQD film on Si (red line) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
PL Intensity (a.u.)
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analysis, the fixed blue emission originates from the topological defects in the sp2 cluster and the long wavelength emission shifting with excitation wavelength arises from quantum confinement due to the size of the QCQDs. That is, after further passivation with PEG OH, most of the non-radiative recombination defects are removed and consequently, the intrinsic state emission (band to
PL Intensity (a.u.)
Similar to Fig. 2a, as the excitation wavelength is increased, the blue peaks are almost fixed but the long wavelength emissions red shift substantially and uniformly, as shown in Fig. 3a. The largest difference is observed from the two sets of PL spectra with one maximum PL peak at 450 nm excited by 360 nm (Fig. 2a), and the other at 540 nm excited by 420 nm (Fig. 2e). According to the previous
0
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Mass of PEG-OH (mg)
Fig. 4. PL spectra: (a) PEG20 /QCQDs, (b) PEG40 /QCQDs, and (c) PEG60 /QCQD aqueous solution. (d) Optimal emission and excitation wavelengths of QCQDs obtained at different masses of PEG OH.
Please cite this article in press as: Y. Hao, et al., Poly(ethylene glycol)/carbon quantum dot composite solid films exhibiting intense and tunable blue–red emission, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.095
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PL Intensity (a.u.)
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Fig. 5. (a and b) PL emission spectra and (c and d) PLE spectra of PEG20 /QCQD and PEG40 /QCQD composite films.
band transition) plays an important role. Compared to the PLE spectra obtained from the QCQD solution, there is no new peak in the PLE spectra from the PEG60 /QCQD composite solid film (Fig. 2f) implying that passivation of PEG OH does not introduce new excitation processes. However, the variable bands (400–550 nm in Fig. 2f) which correspond to band to band excitation process are more intense than the fixed band stemming from topological defects. This also demonstrates that further passivation with PEG OH can improve the degree of excitation due to size and enhance the sizedependent emission. The CQDs generally have a normal size distribution and the strongest emission should originate from the CQDs with the most probable size. When the excitation wavelength is varied from 360 to 420 nm, the QCQDs with the probable size can always be excited. Consequently, the PL intensity in this range increases with the excitation wavelength. When the excitation wavelength exceeds 420 nm, the number of QCQDs that can be excited diminishes thereby leading to a continuous decrease in the PL intensity. According to the most probable CQDs diameter, the optimal emission wavelength in the PL spectrum can be estimated by the following relationship [47,48]: E ∗ = Eg +
h2 1.8e2 − 4ε0 εr 8r 2
in which Eg = 0.04 eV is the band gap of bulk graphite [28,29], = me mh /(me + mh ) is the reduced mass of the exciton (electron and hole). According to Ref. [49], the effective mass of electron
me = 0.42 m0 and that of hole mh = 0.069 m0 , where m0 is the freeelectron mass. Hence, = 0.05926 m0 . ε is the high-frequency dielectric constant of graphite (12–15, selected to be 15 here) and r is the radius of the particle. E* is estimated to be ∼2.27 eV (∼546 nm) which is in good agreement with the observed value (540 nm). The result is also consistent with the band gap derived from the absorption spectrum. The above analysis suggests that the CQD size has an important influence on the PL energy and intensity. In order to confirm the effects of the PEG OH, the degree of passivation of CQDs is changed by varying the amount of the PEG OH. The luminescent properties of the PEG20 /QCQD, PEG40 /QCQD, and PEG60 /QCQD solutions are displayed in Fig. 4a–c. As the amount of PEG OH increases, the optimal emission and excitation peak wavelengths (i.e. the maximum intensity emission peak and corresponding excitation wavelength) of the QCQDs red shift showing that the size of the QCQDs with quantum confinement recombination becomes bigger [50]. When the amount of PEG OH exceeds 60 mg, the PL spectral distribution remains unchanged, as shown in Fig. 4d. The collective band gap of the CQDs corresponds to the most probable radius and therefore, the most probable radius of the PEG-QCQDs increases with the amount of PEG. Since passivation at 110 ◦ C does not change the size of the as-prepared QCQDs, it is believed that the most probable radius is not related to the unpassivated CQDs. It is easy to understand because when the amount of PEG OH is small, the smallest QCQDs are passivated completely but larger QCQDs are passivated only partially. However, when the amount of PEG OH is sufficient, the largest QCQDs are also
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(e)
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(a) 1650
982 1057 1091
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Wavenumbers (cm ) Fig. 6. FT-IR spectra: (a) As-prepared QCQDs, (b) PEG20 /QCQDs, (c) PEG40 /QCQDs, (d) PEG80 /QCQDs, and (e) Pure PEG OH.
passivated completely and the completely passivated QCQDs account for the most probable radius. The less the amount of PEG OH, the smaller is the most probable radius. The amount of PEG OH that can completely passivate the QCQDs with the most probable diameter is estimated to be 66 mg. When the amount of PEG OH is more than 66 mg, the most probable diameter is not changed and so the optimal emission wavelength hardly changes. The luminescent properties of the PEG20 /QCQD and PEG40 /QCQD composite solid films are shown in Fig. 5. Comparing the PL and PLE spectra from the PEG20 /QCQD, PEG40 /QCQD, and PEG60 /QCQD composite solid films, it can be observed that as the amount of PEG OH increases, the size-dependent excitation and emission become stronger and the topological defects related excitation and emission weaken. The more effective the passivation, the more intense is the green–red emission and hence, complete passivation of the QCQDs with PEG OH is necessary in order to achieve strong and tunable blue–red emission. In order to investigate the changes when the amount of PEG OH is increased gradually, FT-IR spectra are acquired from the asprepared QCQDs, PEG20 /QCQDs, PEG40 /QCQDs, and PEG80 /QCQDs and indeed, noticeable changes are observed as the amount of PEG OH is increased. As shown in Fig. 6, an unambiguous drop in the intensity of the epoxide peak at 1050 cm−1 is observed as the amount of PEG OH is increased and the drop is almost eliminated when the amount of PEG OH reaches 40 mg. The bending mode of the epoxy group at 982 cm−1 and C O band at 1720 cm−1 vanish completely when the amount of PEG OH is 20 mg. At the same time, an obvious band at 1091 cm−1 characteristic of alkoxy vibration emerges. Compared to the 1450 cm−1 (amide III) band, the intensity of the 1650 cm−1 band (amide I and C C stretching) shows progressive and continuous decrease but does not disappear completely from the PEG80 /QCQD sample. The C C stretching mode comes from the core of the CQDs. It is not affected by the PEG OH and the decrease in the intensity at 1650 cm−1 indicates that C O stretching of amide is affected by PEG OH. The important information conveyed by FT-IR is that the epoxide groups, carbonyl bonds, and amide linkages transform to alkoxy linkages. As aforementioned, passivation of PEG OH does not introduce new excitation and only greatly enhances the excitation efficiency of the band to band transition. The amide linkage functionalized CQDs have been reported and the CQDs solid films do not exhibit tunable emission [10,11]. Therefore, the tunable blue–red emission observed here results from the mutual effects of size, alkoxy linkages, as well as PEG. The theory postulated for glycerol bonded SiC [51] provides insights into the tunable blue–red
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emission reported here. With regard to the CQDs without effective passivation, the band to band recombination efficiency is quite low and the quantized levels of the conduction band are separated due to the quantum size effect [51]. Electron relaxation from high to low quantized levels and radiative recombination with holes in the valence band provides the PL emission with a fixed energy position. Thus, it is difficult to tune the wavelength of the emitted photons by changing the frequency of the excitation light from a solid film. After effective passivation by PEG OH via alkoxy linkages, the intrinsic state emission (band to band transition) plays the leading role and by gradually increasing the number of alkoxy linkages as well as PEG, the quantized levels in the conduction band gradually transform to a continuum. With increasing excitation energy, electrons can be pumped to higher levels in the upper quasi-continuous band and then relax to a higher level in the lower quasi-continuous band. Consequently, effective passivation of the CQDs by PEG OH provides the possibility of tuning the wavelengths of the emitted photons by changing the excitation frequencies. 4. Conclusion PEG/CQD composite solid films exhibiting strong and tunable emission in blue–red region are prepared. The hydroxyl groups at the end of the PEG can passivate the QCQDs further by removing the nonradiative defects/surface states effectively. In theory, bonding of PEG via alkoxy linkage causes strong surface modification giving rise to a quasi-continuous band in the conduction band which replaces isolated levels in the CQDs that are not passivated effectively to provide continuously tunable emission. The origins of the PL are identified. The fixed blue emission from the QCQDs originates from the topological defects in the carbon network and the long wavelength emission that shifts with excitation wavelength can be attributed to quantum confinement imposed on the QCQDs. The PL peak of the QCQD film is pinned at 650 nm and may originate from oxygen-related states. The PL mechanism provides insights into the fluorescence mechanism of CQDs. Acknowledgments This work was supported by National Basic Research Programs of China under Grants Nos. 2011CB922102 and 2013CB932901 and National Natural Science Foundation of China (Nos. 11374141 and 21203098). Partial support was also from Science and Technology Foundation of Guizhou Province, China (No. J[2013]2277), Hong Kong Research Grants Council (RGC) General Research Fund (GRF) No. CityU 112212. References [1] H.T. Li, Z.H. Kang, Y. Liu, S.T. Lee, Carbon nanodots: synthesis, properties and applications, J. Mater. Chem. 22 (2012) 24230–24253. [2] S.T. Yang, L. Cao, P.G. Luo, F.S. Lu, X. Wang, H.F. Wang, Carbon dots for optical imaging in vivo, J. Am. Chem. Soc. 131 (2009) 11308–11309. [3] S.K. Bhunia, A. Saha, A.R. Maity, S.C. Ray, N.R. Jana, Carbon nanoparticle-based fluorescent bioimaging probes, Sci. Rep. 3 (2013) 1473–1480. [4] X. Wang, L. Cao, S.T. Yang, F. Lu, M.J. Meziani, L. Tian, Bandgap-like strong fluorescence in functionalized carbon nanoparticles, Angew. Chem. Int. Ed. 49 (2010) 5310–5314. [5] M. Shen, L.P. Zhang, M.L. Chen, X.W. Chen, J.H. Wang, The production of pH-sensitive photoluminescent carbon nanoparticles by the carbonization of polyethylenimine and their use for bioimaging, Carbon 55 (2013) 343–349. [6] L. Cao, S. Sahu, P. Anilkumar, C.E. Bunker, J. Xu, K.A.S. Fernando, Carbon nanoparticles as visible-light photocatalysts for efficient CO2 conversion and beyond, J. Am. Chem. Soc. 133 (2011) 4754–4757. [7] H.T. Li, X.D. He, Z.H. Kang, H. Huang, Y. Liu, J.L. Liu, Water-soluble fluorescent carbon quantum dots and photocatalyst design, Angew. Chem. Int. Ed. 49 (2010) 4430–4434. [8] Y. Liu, C.Y. Liu, Z.Y. Zhang, Synthesis of highly luminescent graphitized carbon dots and the application in the Hg2+ detection, Appl. Surf. Sci. 263 (2012) 481–485.
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Poly(ethylene glycol)/carbon quantum dot composite solid films exhibiting intense and tunable blue–red emission Yanling Hao a,b , Zhixing Gan a , Jiaqing Xu a , Xinglong Wu a,∗ , Paul K. Chu c a b c
Key Laboratory of Modern Acoustics, MOE, Institute of Acoustics, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China Department of Physics, XingYi Normal University for Nationalities, Xingyi 562400, Guizhou, People’s Republic of China Department of Physics and Materials Science City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 23 January 2014 Received in revised form 13 May 2014 Accepted 14 May 2014 Available online xxx Keywords: Carbon quantum dots Solid films Tunable emission
a b s t r a c t Although carbon quantum dots (CQDs) possess excellent luminescence properties, it is a challenge to apply water-soluble CQDs to tunable luminescent devices. Herein, quaternary CQDs are incorporated into poly(ethylene glycol) to produce poly(ethylene glycol)/CQD composite solid films which exhibit strong and tunable blue–red emission. The fluorescent quantum yield reaches 12.6% which is comparable to that of many liquid CQDs and the photoluminescence characteristics are determined to elucidate the fluorescence mechanism. The CQD solid films with tunable optical properties bode well for photoelectric devices especially displays. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Carbon quantum dots (CQDs), a new class of photoluminescence (PL) nanoparticles [1–20], possess unique advantages including low toxicity, simple preparation, chemical inertness, biocompatibility and easy functionalization and have the potential of replacing traditional semiconductor quantum dots comprising toxic and/or expensive heavy metals [1]. These superior properties thus spur new applications such as bio-imaging devices [2–6], photocatalysts [6,7], fluorescence sensors [8], light-emitting diodes [9], and temperature probes [10]. However, since fluorescence from solid state CQDs is quenched significantly, the performance of luminescent devices based on CQD films is not satisfactory and only a few examples of strong fluorescence have been reported [11–14]. Pan and coworkers [11] fabricated carbon thin films with high fluorescence efficiency but the emission was not tunable. Wang et al. [12] synthesized organosilane-functionalized CQDs which could be easily fabricated into CQDs fluorescent solid-state materials but they emit blue light primarily. Qu et al. [13] reported that lens paper printed with CQDs ink showed green to red emission, but the green–red emission was suspected to originate from the lens paper. Wang et al. [14] showed blue to orange emission from a mixture of CQDs and fluorescent dyes printed on paper but it
∗ Corresponding author. Tel.: +86 025 83686303; fax: +86 025 83595535. E-mail address: [email protected] (X. Wu).
was suspected that the green, yellow–green, and orange emission stemmed from calcein, CdTe QDs, and RhB, respectively, in lieu of the CQDs. Although CQDs in an aqueous solution exhibit excellent color-tunable PL properties, solid films composed of CQDs with tunable emission have not been reported, but in order to realize their full potential, it is crucial to obtain CQDs films with tunable emission characteristics. In addition, owing to the lack of experimental evidence, the PL mechanism of CQDs is not well understood. Based on the PL characteristics of CQDs in solutions, several PL mechanisms have been proposed [4,6,15,16]. However, since the solution environment and the structure of the CQDs are quite complex, it is difficult to determine the origins of PL from CQDs. In this case, an investigation of the PL properties of solid CQD films can provide insights to the mechanism. Diamine-terminated poly(ethylene glycol) (PEG NH2 ) has been used to passivate CQDs by amidation of the carboxyl groups in order to attain large quantum yield. The electrons and holes are generated via photo-induced charge separation in the carbon nanoparticles and surface passivation is believed to stabilize the surface sites facilitating radiative recombination due to the smaller number of nonradiative centers [15,17]. However, the passivation mechanism is still controversial. First of all, in semiconductor quantum dots, passivation can remove the surface/defect/nonradiative recombination states and so quantum confined PL can be achieved. Second, transforming some surface-bound carboxyl groups into amide groups may not restore the sp2 network to repair the defects. Third, surface passivation does not necessarily lead to a higher
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quantum yield in many cases [7,18–20]. Therefore, it is possible that poly(ethylene glycol) (PEG) diamine (or amine linkages) does not passivate the CQDs effectively. PEG, a simple linear polymer chain made of (CH2 CH2 O)n , has the hydroxyl group on one end (referred to as PEG OH here) which can be grafted [21,22] and may produce passivation effects different from PEG NH2 . In this work, quaternized CQDs (QCQDs) are produced and passivated with PEG to produce QCQD and PEG/QCQD composite solid films. The PEG/QCQD composite solid films show intense tunable blue–red emission and the mechanism is discussed. 2. Experimental 2.1. Materials Betaine hydrochloride (99%), tris(hydroxymethyl) aminomethane (99.9%), and PEG with a molecular weight of 1500 Da was purchased from Aladdin (Shanghai, China). Other chemicals were of analytical grade and all the chemicals were used as received without purification. 2.2. Preparation of QCQD and QCQD films The QCQDs were synthesized by a bottom-up approach proposed by Giannelis et al. [23] with some modifications. The obtained syrup after extraction by isopropanol was dried in an oven at 70 ◦ C for 24 h. The white powder was calcined at 250 ◦ C in air for 2 h at a heating rate of 10 ◦ C/min in a quartz tube. After cooling to room temperature naturally, a brown solid was obtained. It was ground in an agate mortar for 20 min and the powder was dispersed in 50 ml of deionized water, ultrasonically processed for 20 min, and centrifuged at 9000 rpm/min for 15 min. The suspension was collected and further dialyzed in a dialysis bag with a molecular weight cut-off of 500 Da for 3 days to remove the residual tris and betaine hydrochloride. Finally, the suspension was dried in a vacuum oven to obtain the pure luminescent QCQD powder. The QCQD film was fabricated on a silicon wafer by conventional drop casting. After the concentrated QCQDs solution was dropped on the safer, it was dried carefully in a vacuum oven at 70 ◦ C. 2.3. Preparation of PEG/QCQD composite films One milligram of the QCQDs were dispersed in 20 ml of distilled water, with the pH adjusted to 4–5 with diluted HCl. Various amounts (0, 20, 40, 60, and 80 mg) of PEG OH were added to the solution prior to sonification for 10 min to form a homogenous suspension. The suspension was transferred to a three-necked flask, heated to 110 ◦ C, and vigorously stirred under N2 for 24 h. Afterwards, the liquid was cooled to room temperature naturally. According to the different weights of the PEG OH, the samples were referred to as PEG0 /QCQD, PEG20 /QCQD, PEG40 /QCQD, PEG60 /QCQD, and PEG80 /QCQD. In order to prepare the PEG/QCQD composite solid films, these samples were dried for one day at 70 ◦ C in a vacuum oven to obtain a sticky colloid. The conventional drop casting was employed to prepare the PEG20 /QCQD, PEG40 /QCQD and PEG60 /QCQD composite solid films. 2.4. Passivation of QCQDs with PEG OH The extent of passivation of the QCQDs can be varied by the amount of PEG OH. The amount of PEG OH that can completely passivate the QCQDs is estimated according to the atomic percent of C and O. It should be noted that the maximum emission efficiency requires an optimized density of QCQDs, otherwise fluorescence quenching results in if the concentration of QCQDs is too large, and the optimal concentration of QCQDs is about 0.05 mg/ml. According
to the shell approximation [24,25], the number of C atoms in one nanocrystal (NC) is given by NC = 4d3 /3a3 , where d and a are the NC diameter and lattice constant (0.246 nm for hexagonal graphite), respectively. For QCQDs with a diameter of 3.3 nm, there are about 10,070 C atoms. After integration and correcting for the carbon and oxygen contents in a reference sample (a clean Si wafer), the C/N/O atomic ratio is 1:0.04:0.1 and hence, there are 1,007 O atoms surrounding the QCQD. It is assumed that all the oxygen atoms on the QCQD react with PEG OH and the diameter of the QCQDs is 3.3 nm. Thus, the amount of PEG OH (M) which can completely passivate the QCQDs with the most probable diameter can be calculated according to the following relation: M = Mr × 1007 × m/NA × , where Mr is the average molecular weight of PEG OH (1500), m is the mass of the QCQD in 20 ml of aqueous solution (1 mg), NA is Avogadro’s constant, is the density of graphite (2 g/cm3 ), and is the QCQD volume with a diameter of 3.3 nm. Accordingly, M is calculated to be 66 mg. 2.5. Characterization Transmission electronic microscopy (TEM) was conducted on a JOEL TEM-2100F at 200 kV and PL and PL excitation (PLE) were performed on an Edinburgh FLS920 fluorescence spectrometer with the photos taken by a Nikon digital camera. The PL quantum yields were determined on a Hamamatsu absolute PL quantum yield spectrometer. The UV–vis spectra were recorded on a Shimadza UV–vis–NIR spectrophotometer and the Fourier transform infrared spectroscopy (FTIR) spectra were acquired from a Nicolet Nexus 870 spectrometer. X-ray photoelectron spectroscopy (XPS) was performed on the K-Alpha spectroscopy (Thermo Fisher Scientific Corporation, USA) with an Al K˛ X-ray source. 3. Results and discussion The transmission electron microscopy (TEM) image (Fig. 1a) discloses that the QCQDs have a nearly spherical shape and a mean diameter of 3.3 nm ± 0.5 nm (one standard deviation) as shown in Fig. 1b. The high-resolution TEM (HR-TEM) images in Fig. 1c and d reveal the high crystallinity of QCQDs. The lattice spacings of 0.34 and 0.21 nm correspond to the (0 0 2) and (1 0 0) faces of hexagonal graphite, respectively. The data suggest that the CQDs are graphite quantum dots. Fig. 1e displays the UV–vis absorption spectrum of the QCQDs revealing the absence of distinctive peaks in the UV–vis region except the one at about 270 nm which has been observed from other UV–vis absorption results of CQDs with a graphitic structure [26,27]. Experimental and theoretical studies suggest that the ideal graphite is a semiconductor with a narrow band gap of about 40 meV [28,29] and the electronic states of nanocrystalline carbon particles are size dependent. Meanwhile, passivation (e.g. hydrogenation) of the dangling bonds is expected to influence the properties of the carbon particles [30]. Similar to that of an amorphous semiconductor, the energy gap can be calculated from the UV–vis absorption spectra according to the following relationship known as Tauc’s expression [31,32]:
˛hv = A hv − Eg
2
where ˛ is the optical absorbance coefficient, hv is the photon energy, A is a constant, and Eg is the optical √ band gap. As demonstrated in previous studies [30,33], plotting ˛hv versus the photon energy (hv) and extrapolating the linear region of the curve to the x-axis yield the optical band gap of the QCQDs to be 2.46 eV (Fig. 1e). The PL and PLE spectra of the QCQDs in an aqueous solution are displayed in Fig. 2a and b. As previously reported, the PL spectra exhibit excitation wavelength-dependent emissions but as the excitation wavelength increases, the emission intensity drops
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Fig. 1. (a) TEM and (c and d) HR-TEM images of the QCQDs with the size distribution histogram of the QCQDs √ in (b). (e) UV–vis absorption spectrum of the QCQDs and the inset showing the ˛h versus photon energy curve.
rapidly. It should be mentioned that when excited by 300–340 nm, the position of the PL bands are almost fixed in the blue region and have an asymmetric shape. However, when the excitation wavelength is changed from 360 to 500 nm, the long wavelength emission bands shift evenly. The PL intensity maxima for different excitation wavelengths are shown in Fig. 3a. This phenomenon has been observed from other experiments in spite of the different sample preparation methods [3,5,34,35]. Based on this PL feature, it can be concluded that the origins of fixed blue and shifted long wavelength emissions from the QCQDs aqueous liquid are different. In the PLE spectra of the QCQDs aqueous liquid, except for the fixed band at about 285 nm which may be attributed to the −* transition of aromatic C C bonds, there is another fixed band at about 370 nm. A gradually broadened and variable band (400–500 nm) is observed when the monitoring emission wavelength is varied. In general, the redshift excitation process (400–500 nm) corresponds to long wavelength emissions which also redshift with increasing excitation wavelengths. The results are consistent with the quantum confinement effect [36,37] and hence, the shift in the long wavelength emission with increasing excitation wavelength observed from the QCQDs aqueous liquid can be attributed to quantum confinement on the QCQDs [3,6,38]. The energy gaps of the QCQDs are determined by the size of the QCQDs. The smaller the size, the wider is the energy gap. The distribution of the energy gaps
3
based on the size distribution results in emission wavelengths in the range of 450–560 nm. Dual fluorescence bands (blue and green) have recently been observed from CQDs. The blue emission is assigned to amide-containing fluorophores [39]. According to the formation mechanism, if the CQDs are prepared by a bottom-up approach, organic molecules undergo polymerization first to form intermediate polymers prior to carbonization. The organic fluorophores may account for the blue emission. However, it should be mentioned that blue emission can always be observed regardless of sample preparation [6,9,10,16,34,35]. Therefore, the blue emission band ascribed to the organic fluorophores is incorrect. It is easy to understand when the bottom-up method is adopted. The irregular carbon rings or topological defects (non-hexagonal rings) in the carbon network must be present. Although the top-down approaches do not have a polymerization step, non-hexagonal rings cannot be avoided due to the severe synthetic conditions for example, laser ablation, reflux in concentrated acid, electrochemical etching. That is, both the top-down and bottom-up methods produce topological defects within the carbon network inevitably. It has been reported that the strong blue emission (about 440 nm) from reduced graphene oxide (rGO) stems from carbon defect states [40] and so we ascribe the fixed blue emission from the QCQDs to the optical transition in the topological defects in the carbon network and the fixed PLE band in Fig. 2b (at 370 nm) to defect state excitation. When the QCQDs are in the form of a thin film on a Si wafer, the PL peak is pinned at about 650 nm and there is another weak fixed peak at 460 nm (Fig. 2c in the red circle). This indicates that the effect of the quantum confinement disappears, but the defect states still exist because the topological defects are stable and common in CQDs. To identify the origin of the 650 nm PL emission band, the PLE spectra of the QCQD film (Fig. 2d) are examined in details. All the PLE spectra are pinned at 365 nm showing no dependence on the monitoring emission wavelength. The intensity of the fixed PLE band is much smaller than that of the QCQD aqueous liquid, indicating that the fixed PLE band at 365 nm in Fig. 2d is different from the 370 nm band in Fig. 2b. The fixed PLE feature has also been observed from porous Si [41] indicating that the photo-excited carriers for the 650 nm radiation are not from the carbon nanocrystal core [42] but rather from some possible oxygen-related states [43]. Hence, it is supposed that the red emission from the QCQD film is associated with oxygen-related structures, similar to the red emission from graphene oxide (GO) assigned to oxygen-related structures [44]. The FT-IR spectra impart more information about the changes in the structure and surface functionalities between the as-prepared QCQD and QCQD film on the Si wafer (Fig. 3b). The as-prepared QCQDs show a prominent absorption band between 3100 and 3500 cm−1 for the hydroxyl ( OH) groups due to the presence of moisture. The intense and broad peak at 1650 cm−1 is attributed to amide I and C C stretching mode, whereas the shoulder at 1720 cm−1 is assigned to carbonyl (C O) bonds. The peak at 1450 cm−1 corresponds to C N stretching (amide III) and that at 1560 cm−1 is characteristic of amide II. Several absorption bands between 900 and 1300 cm−1 are due to C O vibrations. The absorption peak at 1050 cm−1 and adjacent band at 982 cm−1 represent the asymmetric stretching and bending modes of the epoxy group. The broad band at 600 cm−1 typically occurs in many carbonaceous solids and it is ascribed to graphitic domains [23]. Last but not least, the peak at 2072 cm−1 is characteristic of (C ≡ C) [45,46]. The emergence of C ≡ C indicates the presence of non-hexagonal carbon rings. When the QCQDs are in a solid film, this peak becomes weak and the vibrations related to C O (in the blue box of Fig. 3b) become more abundant and stronger, indicating that the oxygen related surface states increase and topological defects decrease. Hence, the PL peak at 650 nm of the QCQD film originates from oxygen-related
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Fig. 2. PL emission spectra: (a and b) QCQDs aqueous solution, (c and d) QCQD film on Si, and (e and f) PEG60 /QCQD composite film on Si excited by different wavelengths. (g) Photos taken from the PEG60 /QCQDs composite film excited by different wavelengths (from left to right): 360, 420, 460, 500, 520, and 570 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
states but the topological defect state emission does not disappear. With the exception that vibrations of the CH2 and CH3 groups of the betaine ligand become more obvious, the vibrations due to amide only change slightly, indicating that the amide linkage still exists but it cannot passivate the CQDs effectively. Considering the hydroxyl groups which can react with epoxy and carbonyl under acidic condition to form different surface structures from amino passivation, PEG OH is used to further passivate the QCQDs. The PL and PLE spectra from the PEG60 /QCQD composite film are shown in Fig. 2e and f. The solid film shows strong
tunable PL in the blue–red region. The PL from the PEG60 /QCQD composite solid film is so intense that the emission spots with different wavelengths (colors) are visible to the naked eye even when excited by a Xe lamp. The fluorescent quantum yield of the solid film reaches 12.6% under 420 nm excitation and it is comparable to those of many liquid CQDs [8,34,35]. Fig. 2 g depicts six representative photos taken at excitation wavelengths of 360, 420, 460, 500, 520, 570 nm on a Nikon digital camera. Here, different filters are used to avoid scattering of the excitation light background.
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650
(a)
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(b) CH2 stretching
Transmittance (%)
PL Imax position (nm)
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450
QCQDs aquaous solution QCQDs film PEG60-QCQDs film
ν( C C)
epoxy ν( C=O)
O-H or N-H
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Fig. 3. (a) PL intensity maximum positions at different excitation wavelengths and (b) FT-IR spectra of the as-prepared QCQDs (black line) and QCQD film on Si (red line) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
PL Intensity (a.u.)
320nm 340nm 360nm 380nm 400nm 420nm 440nm 460nm 480nm 500nm 520nm 540nm
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(b)
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PL Intensity (a.u.)
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(d)
optimal emission wavelength optimal excitation wavelength
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2.6
460 2.8
440 420
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Wavelength (nm)
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Phonon energy (eV)
(a)
analysis, the fixed blue emission originates from the topological defects in the sp2 cluster and the long wavelength emission shifting with excitation wavelength arises from quantum confinement due to the size of the QCQDs. That is, after further passivation with PEG OH, most of the non-radiative recombination defects are removed and consequently, the intrinsic state emission (band to
PL Intensity (a.u.)
Similar to Fig. 2a, as the excitation wavelength is increased, the blue peaks are almost fixed but the long wavelength emissions red shift substantially and uniformly, as shown in Fig. 3a. The largest difference is observed from the two sets of PL spectra with one maximum PL peak at 450 nm excited by 360 nm (Fig. 2a), and the other at 540 nm excited by 420 nm (Fig. 2e). According to the previous
0
20
40
60
80
Mass of PEG-OH (mg)
Fig. 4. PL spectra: (a) PEG20 /QCQDs, (b) PEG40 /QCQDs, and (c) PEG60 /QCQD aqueous solution. (d) Optimal emission and excitation wavelengths of QCQDs obtained at different masses of PEG OH.
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PL Intensity (a.u.)
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250
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Fig. 5. (a and b) PL emission spectra and (c and d) PLE spectra of PEG20 /QCQD and PEG40 /QCQD composite films.
band transition) plays an important role. Compared to the PLE spectra obtained from the QCQD solution, there is no new peak in the PLE spectra from the PEG60 /QCQD composite solid film (Fig. 2f) implying that passivation of PEG OH does not introduce new excitation processes. However, the variable bands (400–550 nm in Fig. 2f) which correspond to band to band excitation process are more intense than the fixed band stemming from topological defects. This also demonstrates that further passivation with PEG OH can improve the degree of excitation due to size and enhance the sizedependent emission. The CQDs generally have a normal size distribution and the strongest emission should originate from the CQDs with the most probable size. When the excitation wavelength is varied from 360 to 420 nm, the QCQDs with the probable size can always be excited. Consequently, the PL intensity in this range increases with the excitation wavelength. When the excitation wavelength exceeds 420 nm, the number of QCQDs that can be excited diminishes thereby leading to a continuous decrease in the PL intensity. According to the most probable CQDs diameter, the optimal emission wavelength in the PL spectrum can be estimated by the following relationship [47,48]: E ∗ = Eg +
h2 1.8e2 − 4ε0 εr 8r 2
in which Eg = 0.04 eV is the band gap of bulk graphite [28,29], = me mh /(me + mh ) is the reduced mass of the exciton (electron and hole). According to Ref. [49], the effective mass of electron
me = 0.42 m0 and that of hole mh = 0.069 m0 , where m0 is the freeelectron mass. Hence, = 0.05926 m0 . ε is the high-frequency dielectric constant of graphite (12–15, selected to be 15 here) and r is the radius of the particle. E* is estimated to be ∼2.27 eV (∼546 nm) which is in good agreement with the observed value (540 nm). The result is also consistent with the band gap derived from the absorption spectrum. The above analysis suggests that the CQD size has an important influence on the PL energy and intensity. In order to confirm the effects of the PEG OH, the degree of passivation of CQDs is changed by varying the amount of the PEG OH. The luminescent properties of the PEG20 /QCQD, PEG40 /QCQD, and PEG60 /QCQD solutions are displayed in Fig. 4a–c. As the amount of PEG OH increases, the optimal emission and excitation peak wavelengths (i.e. the maximum intensity emission peak and corresponding excitation wavelength) of the QCQDs red shift showing that the size of the QCQDs with quantum confinement recombination becomes bigger [50]. When the amount of PEG OH exceeds 60 mg, the PL spectral distribution remains unchanged, as shown in Fig. 4d. The collective band gap of the CQDs corresponds to the most probable radius and therefore, the most probable radius of the PEG-QCQDs increases with the amount of PEG. Since passivation at 110 ◦ C does not change the size of the as-prepared QCQDs, it is believed that the most probable radius is not related to the unpassivated CQDs. It is easy to understand because when the amount of PEG OH is small, the smallest QCQDs are passivated completely but larger QCQDs are passivated only partially. However, when the amount of PEG OH is sufficient, the largest QCQDs are also
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(e)
Transmittance (%)
(d) (c) (b)
(a) 1650
982 1057 1091
1711
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-1
Wavenumbers (cm ) Fig. 6. FT-IR spectra: (a) As-prepared QCQDs, (b) PEG20 /QCQDs, (c) PEG40 /QCQDs, (d) PEG80 /QCQDs, and (e) Pure PEG OH.
passivated completely and the completely passivated QCQDs account for the most probable radius. The less the amount of PEG OH, the smaller is the most probable radius. The amount of PEG OH that can completely passivate the QCQDs with the most probable diameter is estimated to be 66 mg. When the amount of PEG OH is more than 66 mg, the most probable diameter is not changed and so the optimal emission wavelength hardly changes. The luminescent properties of the PEG20 /QCQD and PEG40 /QCQD composite solid films are shown in Fig. 5. Comparing the PL and PLE spectra from the PEG20 /QCQD, PEG40 /QCQD, and PEG60 /QCQD composite solid films, it can be observed that as the amount of PEG OH increases, the size-dependent excitation and emission become stronger and the topological defects related excitation and emission weaken. The more effective the passivation, the more intense is the green–red emission and hence, complete passivation of the QCQDs with PEG OH is necessary in order to achieve strong and tunable blue–red emission. In order to investigate the changes when the amount of PEG OH is increased gradually, FT-IR spectra are acquired from the asprepared QCQDs, PEG20 /QCQDs, PEG40 /QCQDs, and PEG80 /QCQDs and indeed, noticeable changes are observed as the amount of PEG OH is increased. As shown in Fig. 6, an unambiguous drop in the intensity of the epoxide peak at 1050 cm−1 is observed as the amount of PEG OH is increased and the drop is almost eliminated when the amount of PEG OH reaches 40 mg. The bending mode of the epoxy group at 982 cm−1 and C O band at 1720 cm−1 vanish completely when the amount of PEG OH is 20 mg. At the same time, an obvious band at 1091 cm−1 characteristic of alkoxy vibration emerges. Compared to the 1450 cm−1 (amide III) band, the intensity of the 1650 cm−1 band (amide I and C C stretching) shows progressive and continuous decrease but does not disappear completely from the PEG80 /QCQD sample. The C C stretching mode comes from the core of the CQDs. It is not affected by the PEG OH and the decrease in the intensity at 1650 cm−1 indicates that C O stretching of amide is affected by PEG OH. The important information conveyed by FT-IR is that the epoxide groups, carbonyl bonds, and amide linkages transform to alkoxy linkages. As aforementioned, passivation of PEG OH does not introduce new excitation and only greatly enhances the excitation efficiency of the band to band transition. The amide linkage functionalized CQDs have been reported and the CQDs solid films do not exhibit tunable emission [10,11]. Therefore, the tunable blue–red emission observed here results from the mutual effects of size, alkoxy linkages, as well as PEG. The theory postulated for glycerol bonded SiC [51] provides insights into the tunable blue–red
7
emission reported here. With regard to the CQDs without effective passivation, the band to band recombination efficiency is quite low and the quantized levels of the conduction band are separated due to the quantum size effect [51]. Electron relaxation from high to low quantized levels and radiative recombination with holes in the valence band provides the PL emission with a fixed energy position. Thus, it is difficult to tune the wavelength of the emitted photons by changing the frequency of the excitation light from a solid film. After effective passivation by PEG OH via alkoxy linkages, the intrinsic state emission (band to band transition) plays the leading role and by gradually increasing the number of alkoxy linkages as well as PEG, the quantized levels in the conduction band gradually transform to a continuum. With increasing excitation energy, electrons can be pumped to higher levels in the upper quasi-continuous band and then relax to a higher level in the lower quasi-continuous band. Consequently, effective passivation of the CQDs by PEG OH provides the possibility of tuning the wavelengths of the emitted photons by changing the excitation frequencies. 4. Conclusion PEG/CQD composite solid films exhibiting strong and tunable emission in blue–red region are prepared. The hydroxyl groups at the end of the PEG can passivate the QCQDs further by removing the nonradiative defects/surface states effectively. In theory, bonding of PEG via alkoxy linkage causes strong surface modification giving rise to a quasi-continuous band in the conduction band which replaces isolated levels in the CQDs that are not passivated effectively to provide continuously tunable emission. The origins of the PL are identified. The fixed blue emission from the QCQDs originates from the topological defects in the carbon network and the long wavelength emission that shifts with excitation wavelength can be attributed to quantum confinement imposed on the QCQDs. The PL peak of the QCQD film is pinned at 650 nm and may originate from oxygen-related states. The PL mechanism provides insights into the fluorescence mechanism of CQDs. Acknowledgments This work was supported by National Basic Research Programs of China under Grants Nos. 2011CB922102 and 2013CB932901 and National Natural Science Foundation of China (Nos. 11374141 and 21203098). Partial support was also from Science and Technology Foundation of Guizhou Province, China (No. J[2013]2277), Hong Kong Research Grants Council (RGC) General Research Fund (GRF) No. CityU 112212. References [1] H.T. Li, Z.H. Kang, Y. Liu, S.T. Lee, Carbon nanodots: synthesis, properties and applications, J. Mater. Chem. 22 (2012) 24230–24253. [2] S.T. Yang, L. Cao, P.G. Luo, F.S. Lu, X. Wang, H.F. Wang, Carbon dots for optical imaging in vivo, J. Am. Chem. Soc. 131 (2009) 11308–11309. [3] S.K. Bhunia, A. Saha, A.R. Maity, S.C. Ray, N.R. Jana, Carbon nanoparticle-based fluorescent bioimaging probes, Sci. Rep. 3 (2013) 1473–1480. [4] X. Wang, L. Cao, S.T. Yang, F. Lu, M.J. Meziani, L. Tian, Bandgap-like strong fluorescence in functionalized carbon nanoparticles, Angew. Chem. Int. Ed. 49 (2010) 5310–5314. [5] M. Shen, L.P. Zhang, M.L. Chen, X.W. Chen, J.H. Wang, The production of pH-sensitive photoluminescent carbon nanoparticles by the carbonization of polyethylenimine and their use for bioimaging, Carbon 55 (2013) 343–349. [6] L. Cao, S. Sahu, P. Anilkumar, C.E. Bunker, J. Xu, K.A.S. Fernando, Carbon nanoparticles as visible-light photocatalysts for efficient CO2 conversion and beyond, J. Am. Chem. Soc. 133 (2011) 4754–4757. [7] H.T. Li, X.D. He, Z.H. Kang, H. Huang, Y. Liu, J.L. Liu, Water-soluble fluorescent carbon quantum dots and photocatalyst design, Angew. Chem. Int. Ed. 49 (2010) 4430–4434. [8] Y. Liu, C.Y. Liu, Z.Y. Zhang, Synthesis of highly luminescent graphitized carbon dots and the application in the Hg2+ detection, Appl. Surf. Sci. 263 (2012) 481–485.
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Please cite this article in press as: Y. Hao, et al., Poly(ethylene glycol)/carbon quantum dot composite solid films exhibiting intense and tunable blue–red emission, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.095