Article pubs.acs.org/cm
Efficient Room-Temperature Phosphorescence from Nitrogen-Doped Carbon Dots in Composite Matrices Qijun Li,† Ming Zhou,*,†,‡ Qingfeng Yang,† Qian Wu,† Jing Shi,† Aihua Gong,§ and Mingyang Yang† †
State Key Laboratory of Tribology, School of Mechanical Engineering, Tsinghua University, Beijing 100084, China Department of Industrial Engineering, Purdue University, 225 South University Street, West Lafayette, Indiana 47907, United States § School of Medicine, Jiangsu University, Zhenjiang, Jiangsu 212013, China ‡
S Supporting Information *
ABSTRACT: Carbon dots (CDs) have various attractive properties and potential applications, but there is much less attention paid to their phosphorescent phenomenon and mechanism. Herein, we prepared a kind of highly efficient CD-based phosphorescent material by a subtle design method that incorporated N-doped CDs (NCDs) into composite matrices (the melting recrystallization urea and biuret from the heating urea) by a one-pot heating treatment for the mixture of urea and NCDs. Through systematic investigation, CN bonds on the surface of the NCDs can create new energy level structures, and for the first time, evidence that shows they are the origin of phosphorescence is presented. Composite matrices play a dual role to suppress the vibrational dissipation of long-lived triplets by combining the rigidity of the melting recrystallization urea and hydrogen bonding between biuret and NCDs, which have obvious advantages over a single-component matrix. The results show the obtained materials have an ultralong phosphorescent lifetime of 1.06 s under 280 nm excitation and a high phosphorescent quantum yield of 7% under 360 nm excitation in air, which are the highest values recorded for the CDbased materials. These CD-based room-temperature phosphorescent materials have also shown potential in white light-emitting diodes and data security.
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materials by embedding CDs into a poly(vinyl alcohol) (PVA) matrix, and its phosphorescent lifetime was up to the subsecond (380 ms).21 Subsequently, some visible room-temperature phosphorescence was also observed from CDs dispersed into KAl(SO4)2·xH2O or polyurethane (PU).22,23 However, these CD-based materials have very low emission efficiencies, and persistent emission with a long lifetime (>1 s) has never been achieved, which impede the practical applications. More notably, the phosphorescence mechanism of CDs has not been fully exploited, significantly hindering the development of phosphorescence. Among all the CD-based RTP materials,21−24,36 the phosphorescence is attributed to only carbonyl functional groups of CDs. According to the phosphorescence mechanism, there are two critical preconditions for phosphorescent centers. (1) They must have energy level structures for electron transitions, and (2) they must possess a small energy gap between singlet and triplet states so that electrons can produce effective intersystem crossing. Like aromatic carbonyl groups, nitrogen heterocycles not only have a degree of spin− orbit coupling30,37 but also can create a kind of surface state on CDs.38−40 Therefore, we deduce that nitrogen heterocycles on CDs may be responsible for phosphorescent emission.
arbon dots (CDs), as an important class of photoluminescent (PL) nanomaterials, have received much attention in bioimaging,1−6 light-emitting diodes (LEDs),7−11 photocatalysis,12−15 and sensors16−19 because of their low toxicity, eco-friendly preparation, high chemical stability, and low level of photobleaching. However, nearly all research focuses on the development of CDs based on fluorescence, and much less attention is paid to the phosphorescent phenomenon and their related applications.20−24 Purely organic roomtemperature phosphorescent (RTP) materials have broader opportunities (for example, long lifetime imaging25,26 and electroluminescent devices27) compared to those of their fluorescent counterparts due to the involvement of triplet excited states and comparatively slower decay rates.28 However, they are usually difficult to obtain because of the bonded nature of electrons in organic RTP materials, leaving them with little freedom and less impetus to emit from triplet states.29 In recent years, some molecules have been successfully designed to achieve effective room-temperature phosphorescence by introducing particular moieties (for example, aromatic carbonyls and nitrogen heterocycles)30,31 or using some concepts,29,32−35 such as heavy atom effects, hydrogen bonding, surface plasmon-enhanced absorption, coupling of H-aggregation, engineered crystals, and matrix-assisted isolation. The phosphorescence of CDs was also discovered recently. In 2013, Zhao et al. reported the first example of CD-based RTP © 2016 American Chemical Society
Received: July 25, 2016 Revised: October 10, 2016 Published: November 4, 2016 8221
DOI: 10.1021/acs.chemmater.6b03049 Chem. Mater. 2016, 28, 8221−8227
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Chemistry of Materials In addition, the transition from the triplet to the ground state is easily quenched by the nonradiative processes. Previous studies have shown that matrices can effectively protect their triplet states from being quenched by restricting intramolecular motions and isolating atmospheric triplet O 2 molecules.21,34,41−43 However, matrices of CD-based materials are usually single-component materials such as PVA, KAl(SO4)2· xH2O and PU, which hardly offer strong rigidity, dense hydrogen bonding sites, and good oxygen barrier performance simultaneously to reduce the nonradiative transition [such as the inevitable vibration/diffusion motion of PVA,32 insufficient hydrogen bonding sites of KAl(SO4)2·xH2O,22 and oxygen permeability of PU23]. Two-component matrices offer advantages over single-component matrices by combining their individual ingredient role. Niday et al. demonstrated that the addition of selected sugars and salts dramatically enhanced the lifetime of the room-temperature phosphorescence of 2-naphthalenesulfonate on cellulose paper.44 They attributed this phenomenon to an increase in the rigidity of matrices or an improvement in oxygen barrier performance by the added substances. Richmond et al. investigated cyclodextrin/salt mixtures as solid matrices for obtaining roomtemperature phosphorescence from adsorbed compounds.45 However, to date, composite matrices for CD-based materials have never been reported. In addition, simply physical blends of two kinds of materials hardly exert their respective role for suppression of the nonradiative processes. Herein, we prepared a kind of highly efficient CD-based phosphorescent material by a subtle design method that incorporates N-doped CDs (NCDs) into composite matrices (the melting recrystallization urea and biuret from the heating urea) by a one-pot heating treatment of the mixture of urea and NCDs. The melting urea can firmly adhere and capture the NCDs during the evaporation process, so that the NCDs can be homogeneously dispersed into composite matrices without apparent aggregation and delamination, which does not need to be further postprocessed; this is different from previously published methods.46,47 We studied the effects of different surface N contents on the phosphorescence and for the first time proved that CN bonds of CDs are responsible for the phosphorescence in view of both creative energy level structures and the small energy gap between singlet and triplet states. Furthermore, we first introduced composite matrices to suppress the nonradiative quenching processes of CDs. Composite matrices play a dual role to suppress the vibrational dissipation of long-lived triplets by combining the rigidity of the melting recrystallization urea and hydrogen bonding between biuret and NCDs, which have obvious advantage over a singlecomponent matrix (Figure 1). The results show the obtained NCD-based materials have an ultralong phosphorescence lifetime of 1.06 s under 280 nm excitation, and a high phosphorescence quantum yield (PQY) of 7% under 360 nm excitation in air, which are the highest values recorded for CDbased materials. Such excellent properties make it possible for them have great potential in white LEDs (WLEDs) and data security.
Figure 1. (a) NCDs are embedded into melting recrystallization urea and biuret matrices. (b) Schematic illustration of possible energy structures of CN bonds and phosphorescent emission processes.
exhibit good dispersion with average sizes of 3.41, 3.46, and 4.21 nm for the HN-CDs, MN-CDs, and LN-CDs, respectively (Figure S1). The high-resolution TEM images provided in the insets reveal that all the NCDs exhibit an identical lattice fringe with a spacing of 0.20 nm, in agreement with the basal spacing of graphite.48 Atomic force microscopy (AFM) images further demonstrate the topographic morphology of the NCDs (Figure S2). All of the three NCDs exhibit similar topographic heights of approximately 2−6 nm. The results of X-ray photoelectron spectroscopy (XPS) and elemental analysis (Figure 2a and Table S1) indicate that all the NCDs mainly consist of carbon, nitrogen, and oxygen. The high-resolution spectra of C 1s (Figure S3a−c) show three peaks at 284.6, 285.6, and 288.1 eV for C−C, C−N, and CN/CO, respectively.38 N 1s bands (Figure 3a) reveal the presence of N atoms mainly in the form of pyridine N (398.8 eV), pyrrolic N (399.6 eV), and graphite N (400.8 eV).12 The contents of total pyridine N and pyrrolic N for HN-CDs are up to 90% (Table S1), implying a large number of the CN bonds on the surface of HN-CDs. However, with greater degrees of reaction, the N atom content gradually decreases from HN-CDs to LN-CDs; meanwhile, more and more pyrrolic N and pyridine N translate into graphite N. Such a trend means a corresponding decrease in the number of CN bonds in the LN-CDs. The Fourier transform infrared (FTIR) spectra (Figure 2b) further indicate the decrease in the number of CN bonds supported by the reduction of the stretching vibration peak at 1580 cm−1 from HL-CDs. Moreover, the CO stretching vibration band at 1680 cm−1 in HN-CDs and MN-CDs shifts to 1710 cm−1 in LN-CDs, and an enhancement of the stretching vibration of C−O bonds at 1410 cm−1 is observed from LN-CDs, indicating an increase in the degree of oxidation. It should be noted that CO groups are almost unchanged, although the surface atomic concentration of oxygen gradually increases (Figure S3a−f and Table S1). The UV−visible absorption spectra of HN-CDs, MN-CDs, and LN-CDs are compared in Figure 3b. All samples exhibit similar absorption peaks at 228, 280, and 330 nm. Differently, in MN-CDs, the peak at 280 nm is weaker than that of HNCDs and nearly disappears in LN-CDs. The different types of electron transitions correspond to the absorption peaks. Similar to literature reports,49 the characteristic absorption at 228 nm is attributed to the π−π* transition of aromatic sp2 domains.
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RESULTS AND DISCUSSION The three different N contents of NCDs were prepared by hydrothermal treatment of folic acid at 260 °C for 2 h (labeled as HN-CDs), 6 h (labeled as MN-CDs), and 6 h by adding HCL to the precursor (labeled as LN-CDs). Typical transmission electron microscopy (TEM) images of the NCDs 8222
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Figure 2. (a) XPS survey spectra. (b) FTIR spectra.
Then, three NCD powders (labeled as HN powders, MN powders, and LN powders) were fabricated by heating a mixed solution of the urea and HN-CDs, MN-CDs, and LN-CDs, respectively, via the same method. During the heating process, one part of urea transformed into the biuret, and the others undergo melting recrystallization. The melting urea can firmly adhere and capture NCDs during the evaporation process, so that the NCDs can be homogeneously dispersed into composite matrices without apparent aggregation and delamination. Under UV (365 nm) light illumination, the HN powders emit bright cool white light. Interestingly, upon removal of the ultraviolet source, phosphorescence that lasts several seconds could be observed at room temperature (Movies S1 and S2). The steady-state PL and phosphorescence spectra of HN powders indicate that there is a large Stokes shift (75 nm) between fluorescence and phosphorescence (Figure S4b). The phosphorescence decay spectrum of HN powders is shown in Figure 4a. It can be fitted into a triexponential function, and the average lifetime is 0.93 s (detailed results of fitting are listed in Table S3). The multiple lifetimes of the phosphorescence may be due to a wide range of chemical environments on the surface of CDs for CN and CO bonds. Furthermore, its PQY is up to 5% using BaSO4 as the reflectance standard under 280 nm excitation. In contrast, the phosphorescence lifetime of LN powders is only 0.62 s, and its absolute PQY is only 0.4% under the same condition. Combined with the results presented above, surface CN bonds on HN-CDs could create new energy levels for electron transitions. It is simultaneously considered to have a small energy gap between singlet and triplet states. Thus, we conclude that surface CN bonds may be an origin of phosphorescence to promote the production of the triplet excitons. To confirm this possible explanation, the phosphorescence excitation (PLE) spectra of the three powders were detected under 490 nm emission (Figure 3b). In PLE spectra of HN powders, two main excitation bands at 280 and 330 nm nearly overlap with their absorption peaks, suggesting the phosphorescence comes from the CN and CO bonds of HN-CDs. While the content of CN bonds decreases, the PLE peak at 280 nm becomes weaker and nearly disappears in LN powders, which is consistent with their UV/vis absorption spectra. In view the almost unchanged number of CO groups during the
Figure 3. (a) Deconvolution of high-resolution N 1s XPS spectra of NCDs. (b) Phosphorescence excitation spectra (blue lines) of NCD powders with emission at 490 nm and absorption spectra of NCDs dispersed in water (black lines).
Combining the structural characteristic and spectral changes of three kinds of NCDs, we can ascribe the absorption peak at 280 nm to the n−π* electron transition for CN, while the peak at 330 nm is due to n−π* transition CO bonds. Under UV excitation, the dilute aqueous solution of HN-CDs exhibits excitation-dependent PL behavior (Figure S3g). Such a phenomenon usually is attributed to quantum effects or surface states. However, the absorption at 228 nm of the HN-CDs produces nearly no observed PL signal, excluding the quantum effects (Figure S4a).39 Interestingly, excitation-dependent FL behaviors of HN-CDs turn into excitation-independent behaviors of LN-CDs as the duration of oxidation increased (Figure S3i). Meanwhile, lifetime components change from the three of HN-CDs to the two of LN-CDs (Table S2). These results further verify surface CN bonds on HN-CDs can efficiently introduce new energy levels for electron transitions. 8223
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Figure 4. (a) Lifetime decay profiles of the emission band at 490 nm excited at 280 nm. (b) Flow diagram of verification experiments. (c) Lifetime decay profiles of the emission band at 490 nm of sample 3, sample 4, and HN powders excited at 280 nm. (d) Phosphorescence spectra of sample 3, sample 4, and HN powders excited at 280 nm. (e) Lifetime decay profiles of the emission band at 490 nm of HN powders at different reaction times excited at 280 nm.
Figure 5. (a) Average lifetimes and PQYs of HN powders at different HN-CDs contents. The lifetimes were monitored by the emission wavelength at 490 nm. (b) Lifetime decay profiles of HN powders in nitrogen and air excited at 280 nm. (c) Repeated excitation illumination at 280 nm for 40 min under the continuous switch fluorescence and phosphorescence mode (voltages of 400 and 700 V for fluorescence and phosphorescence modes, respectively). (d) Lifetime decay profiles of the emission of HN powders at 490 nm at temperatures from 78 to 301 K. (e) Emission color coordinates of assembled LEDs. (f) Security protection applications.
process of reaction, such a characteristic further proves that CN bonds are responsible for the phosphorescence. Although surface CN and CO bonds are responsible for phosphorescence, similar room-temperature phosphorescence cannot be observed from the HN-CDs dispersed in water, PEG-4000, and PVP, implying the important role of the matrix. Our composite matrices are mainly composed of the unreacted melting recrystallization urea and the biuret from heating urea. The biuret comes from urea during the heating process, as confirmed by the FTIR spectrum (Figure S4c). There is a large
number of amino groups on biuret molecules, so we suggest that these groups could effectively form hydrogen bonds to rigidify CN/CO bonds on NCDs, leading to effective phosphorescence emission. To understand the role that hydrogen bonding plays in determining the optical properties of HN-powders, sample 3 (redissolving and drying of the HN powders) and sample 4 (the physical blends of the HN-CD solution, urea, and biuret in the same proportion as in sample 3) were prepared (Figure 4b and details in the Supporting Information). Although the HN-CDs may undergo a chemical 8224
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HN powders have a high total QY (fluorescence and phosphorescence) of 36% and a PQY of 7% with an average lifetime of 0.83 s under 360 nm excitation. The HN powders with a dual-emissive property could be used to fabricate WLEDs. Three types of LEDs were fabricated, and their CIE coordinates are (0.213, 0.204), (0.338, 0.363), and (0.485, 0.483) for types 1−3, respectively (Figure 5e). In addition, the HN powders with distinguishable lifetime codes could be developed for promising applications in data encryption33 (Figure 5f). The patterned security feature of “2468” was mixed ultralong phosphorescent HN powders and fluorescent HN starch, while “1357” was only filled with fluorescent HN starch. The “2468” was encrypted in “12345678” under 365 nm UV light excitation because of the fluorescence background of the HN starch. However, when the UV light is turned off, the encryption of “2468” could be readily visualized.
reaction with biuret or urea in sample 3 (Figure S4c), the lifetime decay curves of samples 3 and 4 are triexponential with similar average lifetimes (Figure 4c and Table S3), indicating a similar luminescence process. Moreover, their steady-state PL and phosphorescence spectra (Figure 4d and Figure S4d) are also almost identical. The result reveals that the hydrogen bonding between HN-CDs and biuret plays a major role in emissive triplet relaxation. It should be noted that the urea has no effect on phosphorescence because no phosphorescence was observed from the physical blends of the HN-CDs and urea. Moreover, by contrast, via analysis of sample 3 and HN powders, there are no evident differences except urea from the melting recrystallization state of HN powders to the nature of the crystallization state of sample 3, but their optical results are markedly different (Figure 4c,d and Figure S4d). The average lifetime is lowered from 0.93 s of HN powders to 0.68 s of sample 3 (detailed results of fitting are listed in Table S3). Moreover, steady-state PL and phosphorescence intensities of sample 3 exhibit remarkable reduction compared to that of HN powders. These results clearly demonstrate that melting recrystallization urea, as a rigid matrix, exhibits strong suppression in the nonradiative deactivation pathways, which is similar to the matrix packing mechanism.44 According to the analyses described above, we conclude that our composite matrices play a dual role in protecting the triplet state from a long lifetime quenched by suppressing the nonradiative processes. To further prove the dual role of the composite matrices, we prepared HN powders at 155 °C at different reaction times from 1 to 8 h. Optimal phosphorescence is found to be excited for 6 h at 280 nm (Figure 4e, Figure S5a,b, and Table S4). This phenomenon is ascribed to a synergy effect of the melting recrystallization urea and biuret. At short reaction times, only a small portion of urea is converted into biuret, which leads to low emission due to insufficient hydrogen bonds between biuret and HN-CDs. At long reaction times, almost all urea is depleted so that they cannot offer strong rigidity to restrict the vibration and/or diffusional motion of the matrices and HNCDs, resulting in relatively poor phosphorescence. The optical spectra of HN powders at various HN-CD contents were recorded (Figure 5a, Figure S5c,d, and Table S5). At low concentrations, the phosphorescence lifetime and PQY are almost unchanged, but they gradually decrease with an increase in HN-CD content, which is due to an increase in the level of self-absorption of HN-CDs. As is well-known, oxygen is a good quencher for the triplet states. We investigated the effects of different media (nitrogen and air) on the optical properties of HN powders (Figure 5b and Figure S5e,f). Almost identical optical curves were obtained in air and nitrogen, revealing a good oxygen barrier performance of the composite matrices. A photostability experiment with the HN powders revealed no obvious photobleaching as expected by repeated excitation illumination at 280 nm for 40 min (Figure 5c). In a further set of experiments, we investigated the effects of temperature on the photoluminescence of HN powders in the range of 78−301 K (Figure 5d, Figure S6a, and Table S6). There is an initial rapid decay of phosphorescence followed by a much slower decay of the lifetime, and the lifetime decay is almost identical in the range of 78−301 K, which indicates the very effective nature of the hydrogen bonds in suppressing vibrational dissipation at all temperatures.50 For practical purposes, we studied the optical properties of the HN powders under 360 nm excitation (Figure S6b,c). The
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CONCLUSIONS
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ASSOCIATED CONTENT
In summary, we have prepared a kind of highly efficient CDbased phosphorescent material by a subtle design method that incorporated NCDs into composite matrices (the melting recrystallization urea and biuret from the heating urea) by a one-pot heating treatment of a mixture of urea and NCDs. The NCDs can be uniformly dispersed into composite matrices without apparent aggregation or delamination attributed to a firm adhesion force of melting urea. Through systematic investigation, CN bonds on the surface of NCDs can promote the production of the triplet excitons through the n−π* transition, which is considered an origin of phosphorescence. In addition, our composite matrices greatly suppressed the vibrational dissipation of long-lived triplets by the considerable rigidity of the melting recrystallization urea combined with hydrogen bonding between biuret and NCDs. The results show that HN powders have an ultralong phosphorescence lifetime of 1.06 s under 280 nm excitation. Moreover, they have a high PQY of 7% and a TQY of 36% under 360 nm excitation. HN powders show great potential in phosphor-based WLEDs and data security. Our results are greatly important for the rational design of CD-based materials to realize efficient phosphorescence emission.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03049. Experimental section, TEM images and size distribution of the NCDs, deconvolution of high-resolution and fluorescence spectra of NCDs, element composition percentage of the NCDs by element analysis and XPS, fit parameters of the fluorescence decay curves of the NCDs, fluorescence, phosphorescence, and lifetime decay profiles of HN powders, and the emission spectrum of assembled LEDs (PDF) Movie showing the ultralong phosphorescence excited by an UV lamp (in a dish) (MP4) Movie showing the ultralong phosphorescence excited by an UV lamp (as a powder) (MP4) 8225
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Telephone: +86 18801105908. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This research was supported by the National Basic Research Program of China (973 Program, Grant 2011CB013004) of the Ministry of Science and Technology of China and Major Project of Chinese State Key Laboratory of Tribology (Grant SKLT2014A01).
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DOI: 10.1021/acs.chemmater.6b03049 Chem. Mater. 2016, 28, 8221−8227
