Freestanding Luminescent Films of Nitrogen-Rich Carbon Nanodots

Article pubs.acs.org/cm

Freestanding Luminescent Films of Nitrogen-Rich Carbon Nanodots toward Large-Scale Phosphor-Based White-Light-Emitting Devices Woosung Kwon,† Sungan Do,† Jinuk Lee,† Sunyong Hwang,‡ Jong Kyu Kim,‡ and Shi-Woo Rhee*,† †

System on a Chip Chemical Process Research Center, Department of Chemical Engineering and ‡Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea S Supporting Information *

ABSTRACT: In this work, nitrogen-rich carbon nanodots (CNDs) are prepared by the emulsion-templated carbonization of polyacrylamide. The formation mechanism and chemical structure are investigated by infrared, nuclear magnetic resonance, and X-ray photoelectron spectroscopies. Transmission electron microscopy also reveals that the obtained CNDs have welldeveloped graphitic structure and narrow size distribution without any size selection procedure. We vary the molecular weight of the polymer to control the size of the CNDs and finally obtain the CNDs rendering bright visible light under UV illumination with a high quantum yield of 40%. Given that the CNDs are worth utilizing in phosphor applications, we fabricate large-scale (20 × 20 cm) freestanding luminescent films of the CNDs based on a poly(methyl methacrylate) matrix. The polymer matrix can not only provide mechanical support but also disperse the CNDs to prevent solid-state quenching. For practical application, we demonstrate white LEDs consisting of the films as color-converting phosphors and InGaN blue LEDs as illuminators. Such white LEDs exhibit no temporal degradation in the emission spectrum under practical operation conditions. This study would suggest a promising way to exploit the luminescence from solid-state CNDs and offer strong potential for future CND-based solid-state lighting systems. KEYWORDS: carbon nanodots, polyacrylamide, phosphor, freestanding film, white-light-emitting devices

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INTRODUCTION Carbon nanodots (CNDs), luminescent graphitic nanomaterials, are emerging as potential alternatives for traditional semiconductor nanocrystals (CdSe, PbS, etc.) because of their distinct benefits such as chemical inertness, high thermal stability, low photobleaching, no optical blinking, and excellent biocompatibility.1 Thus far, various methods to synthesize CNDs, which are categorized into “top-down”2−11 and “bottom-up”,12−24 have been developed. Although top-down and bottom-up differ from each other in the direction of fabrication, both of them offer quasi-spherical graphitic nanoparticles (mean diameter < 10 nm) with strong deep-UV absorption and excitation-wavelength-dependent emission. However, these methods generally require unpractical sizeselection procedures that would result in low product yield. Recently, several research groups have attempted to use CNDs as an active material in optoelectronic devices such as light-emitting devices (LEDs).25 The Liu and Ma groups demonstrated electroluminescence of CNDs prepared by the noncoordinating solvent method for LEDs, but very high operation voltage (ca. 9 V) was required to obtain reasonable brightness.25a This was attributed to not only the insulating capping layer as per the authors’ indication but also the inherent poor conductivity according to our recent publication.26a Chen and co-workers also reported phosphor-based LEDs with CNDs derived from photonic crystals.25b They successfully render three different colors (bright blue, orange, and © 2013 American Chemical Society

warm white), but the obtained CNDs showed limited absorption in the deep-blue (long-UV, 360−400 nm) region. Thus, InGaN blue LEDs, that are the most used base LEDs, cannot be very effective for this system. Furthermore, aggregation of solid-state CNDs typically leads to serious luminescence quenching, which would significantly limit the performance of CND-based LEDs.26b Considering that there are increasing demands for white LEDs as a potential replacement for traditional light sources, CNDs could offer prospects for phosphor technology due to their strong and broad visible light emission, nontoxicity, and high thermal stability. In this work, we derive nitrogen-rich CNDs from polyacrylamide (PAA) via emulsion-templated carbonization. By varying the molecular weight (MW) of polyacrylamide, we control the size of the CNDs and study the effect of the size on their optical property. The obtained CNDs produce bright visible light under UV illumination, which is very suitable for phosphor applications. To realize their potential, we demonstrate the fabrication of large-area freestanding films of the CNDs based on a poly(methyl methacrylate) (PMMA) matrix. This luminescent film, as a novel type of phosphor, can intrinsically prevent luminescence quenching to gain strong solid-state luminescence as well as Received: February 13, 2013 Revised: April 11, 2013 Published: April 11, 2013 1893

dx.doi.org/10.1021/cm400517g | Chem. Mater. 2013, 25, 1893−1899

Chemistry of Materials

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have several physical benefits such as flexibility, nontoxicity, and mechanical robustness.

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RESULTS AND DISCUSSION Synthesis and Chemical Analysis. The formation mechanism of the CNDs is schematically described in Scheme 1. The emulsion is initially opaque or “milky”, which indicates Scheme 1. Schematic Representation of Formation of Oleylamine-Capped Carbon Nanodots (CNDs) from Carbonization of Polyacrylamide Emulsions in Oleylamine

Figure 1. IR spectra of (a) PAA, (b) an intermediate (water soluble) extracted during the reaction, and (c) the oleylamine-capped CNDs (water insoluble).

the COOH frequencies (3020 and 960 cm−1), as indicated in previous studies on carbonaceous materials.13−22 Evolution of the bonded (secondary) NH bands (3300 and 1100 cm−1) is attributed to the fact that oleylamine is chemically “anchored” with the surface carbonyl groups. This capping layer greatly influences the surface energy; hence, the CNDs are completely water insoluble (hydrophobic), whereas the intermediates are water soluble (hydrophilic). The broad amide band (1630− 1690 cm−1 for the CO stretch, 1500−1600 cm−1 for the NH bending, and 1100−1360 cm−1 for the C−N stretch) is shared by all spectra. NMR spectra shown in Figure 2 reveal the surface chemistry of the final product.12,13 Formation of the water-soluble intermediates indicates that the reaction (or growth) would be terminated when capping molecules (oleylamine) are grafted onto the surface of the CNDs. From the 1H NMR spectrum (Figure 2a), we find a sharp peak at about 7.9 ppm corresponding to the amide hydrogen (−CONH−) that results from surface capping. The 8.4 ppm peak is assigned to the hydrogens ortho to the surface carbonyl groups. A sharp peak near 7.3 ppm assigned to the amide group of PAA indicates that unreacted polymer chains may be dangled on the surface of the CNDs. Figure 2b shows the 13C NMR spectrum with a sharp peak at about 130 ppm and a collection of peaks between 165 and 180 ppm. The 130 ppm peak is assigned to the alkene carbons of oleylamine. The later peaks arise from the amide carbon with various bonding states, as depicted in the inset. The chemical structure of the CNDs is further explored by a series of XPS analyses (Figure 3). Figure 3a (C1s spectrum) shows that the CNDs are composed of mainly graphitic carbon (CC) and partly heterogeneous atoms (nitrogen and oxygen) that may be bonded to the surface. As noted in connection with Figures 1 and 2, the presence of C−N and CO groups stemming from the surface carbonyls such as amide, carboxylic acid, etc., is confirmed. N1s and O1s spectra shown in Figure 3b and 3c likewise provide clues to identification of these surface functional groups. The N−H peak indicates that the C−N groups arise from chemical bonding of primary amine capping molecules (oleylamine) on the surface of the CNDs.

that the size of the water droplets may fall into the micrometer range.27 To supply heat, the water is evaporated and the emulsion becomes transparent because the size of the water droplets is decreased to the nanometer range.27 Accompanying water evaporation, the emulsion shortly reaches a critical supersaturation condition and polyacrylamide chains are then carbonized. Since our experiment using poly(acrylic acid) as a precursor results in no carbonaceous product even at higher temperature (>300 °C), it is thought that the amide functional group of polyacrylamide may play the key role in the reaction. Oleylamine can hereby serve as both a solvent (oil phase) and a capping agent because this molecule exists in a liquid form at room temperature and has a sufficiently high boiling point as well as a highly reactive amine functional group. To further understand the chemistry, we performed infrared (IR) spectroscopy, nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS) measurements of the asobtained CNDs. Figure 1 shows the IR spectra of PAA and the carbogenic product formed via thermal carbonization. The vinyl (R CHCH2) bending frequency (980−1000 cm−1) is assigned to vinyl termination groups on PAA. This band is strongly diminished as the reaction proceeds and completely absent from the CNDs. The amide NH2 stretching band (3400−3500 cm−1) is also diminished and then vanished from the final product. These results indicate that PAA is partially decomposed with the aid of the acid prior to formation of the CNDs. The role of the acid is further examined by varying the concentration of the acid, and it is found that the acid could expedite carbonization of PAA (Figure S1, Supporting Information). Nonetheless, the reaction path still remains unclear due to the presence of diverse undefined intermediates such as ions, radicals, and other complex organic compounds. However, we may deduce a meaningful result from comparison of the final product with the intermediate. Formation of carbonyl groups on the surface of the CNDs is confirmed by 1894



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distributed close to the mean without any size-selection procedure; in particular, the size of CND-1500 lies within a nearly monodisperse range, whereas CND-10000 shows a broad bell-shaped size distribution (Figure 4c). This can be attributed to the fact that a high-MW polymer may have high polydispersity. High-resolution TEM images (Figure 4d−g) indicate that the CNDs, regardless of the MW, have highcrystalline structures showing typical lattice spacings of graphite, e.g., 0.25 nm for (100) and 0.34 nm for (002). Optical Properties. Figure 5 shows the UV−vis absorption and photoluminescence (PL) spectra of CND-1500 and CND10000. Here, CND-10000 shows broader absorption and PL peaks than CND-1500, which can be attributed to its broad and bell-shaped size distribution. We find that the PL spectra of CND-10000 are red shifted by maximum 20 nm relative to those of CND-1500. In connection with this red shift, the PL intensity of CND-10000 is superior to that of CND-1500 under the same measurement conditions (temperature, concentration, etc.). Figure 5c indicates that CND-10000 could produce almost five times more photons than CND-1500 regardless of the excitation wavelength. In Figure 5d, we compare the two CNDs by dividing their PL spectra taken at 360 nm excitation into six colors (violet, blue, green, yellow, orange, and red). As the size of the CNDs increases, the intensity of individual color is increased by 3−6 times over the entire wavelength range. The increase in intensity is even more significant in the blue to red range, and the ratio of the blue to red intensity to the total intensity is increased by 11%. This feature would make CND10000 suitable for phosphor applications. One plausible explanation for the size-related changes in the PL spectra is the size dependence of the electronic structure of the CNDs.11,25a It has been demonstrated that the PL of luminescent carbon nanoparticles arises from radiative recombination of photoexcited electrons at the surface trap states and as-generated holes at the HOMO level.28 We may assume that the energy levels of the surface trap states, which are closely related to surface chemistry, are independent of the size because the IR and NMR analyses confirm the chemical similarity of CND-1500 and CND-10000. Thus, it is concluded that the HOMO level should vary with size. In the quantum mechanical regime, a larger particle size leads to a higher density of states to raise the HOMO level (Figure 5e).11 As a result, CND-10000 could render more intensive emission in the long-wavelength range than CND-1500. In this regard, the quantum yield of CND-10000 was recorded as high as 38% (360 nm excitation), superior to that of CND-1500 (19%) (Figure S5, Supporting Information).

Figure 2. (a) 1H and (b) 13C NMR spectra of the purified CNDs in toluene-d8.

Structure Characterization. We performed controlled experiments varying the polymer MW and coded each sample as CND-(polymer MW). In Figure 4a and 4b, transmission electron microscopy (TEM) images of CND-1500 and CND10000 indicate that the size of the CNDs is a function of the MW of PAA (see Figure S3 for TEM images of other samples). The mean diameter of CND-10000 (2.4 nm) is ca. twice larger than that of CND-1500 (1.2 nm). It has been reported that formation of carbon nanoparticles would not conform the Ostwald ripening mechanism,21,22 and also, our experimental study reveals that the concentration of PAA only influences the product weight (Figure S4, Supporting Information). Thus, we believe that the MW or the “chain length” plays the key role in tuning the size of the CNDs.24 This can be attributed to the fact that the size of a polymer emulsion is proportional to the change length of the polymer. Since PAA used in this study has a fairly narrow MW distribution, the size of the CNDs is

Figure 3. (a) C1s, (b) N1s, and (c) O1s XPS data of the purified CNDs spin cast on silicon substrates. 1895



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Figure 4. TEM images of (a) CND-1500 and (b) CND-10000 (scale bars = 10 nm). (c) Size histogram of the CNDs measured from lowmagnification TEM images (Figure S2, Supporting Information) for over 200 particles. High-resolution TEM images of (d, e) CND-1500 and (f, g) CND-10000 (scale bars = 2 nm). Lines and arrows indicate the lattice plane distance.

Figure 5. UV−vis absorption (dotted) PL spectra (solid lines) of (a) CND-1500 and (b) CND-10000. Color coding represents the excitation wavelength and is the same for both graphs. All measurements were conducted under controlled temperature and concentration (0.05 mg mL−1 in octane). (c) Integrated intensities (the mathematical area under each PL spectrum) as a function of the excitation wavelength and (d) the spectral composition (360 nm excitation). (e) Schematic representation of the electronic structure of the CNDs.

PL spectra of CND-1500 and CND-10000 also share several features consistent with previous studies on luminescent carbon nanomaterials.5−24 These common features comprise the strong deep-UV absorption (near 250 nm) and excitationwavelength-dependent emission. On the other hand, our CNDs have one distinctive feature, that is the broad absorption peak in the deep-blue (long-UV) region (350−400 nm). This peak, not reported in the past to the best of our knowledge, may arise from highly inhomogeneous surface chemistry. It has been demonstrated that such surface inhomogeneity could be induced by surface functional groups, capping molecules,

surface defects, etc., and play the key role in trapping photoexcited electrons.28 Our CNDs have tethered polymer chains and nitrogen-rich environment that could offer surface trap states with various trap energies responsible for the prominent deep-blue absorption. Film Morphologies. Given that CND-10000 is worth utilizing because of its strong long-UV absorption, bright visible-light emission, and high quantum yield, we fabricate freestanding luminescent films of CND-10000 using a PMMA matrix. Figure 6 shows photographs of a large-scale (20 × 20 cm) CND-10000 film with uniform thickness of ca. 50 μm. 1896



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Figure 6. Photographs of a large-scale (20 × 20 cm) CND-10000 film under room light (left) and UV (365 nm) light (middle). Cross-sectional SEM image of a CND-10000 film (right).

Short-wavelength light emitted from one point of the film may be absorbed by surrounding CNDs, from which red-shifted long-wavelength light is re-emitted. This process is repeated until the emitted light completely passes through the film and gives rise to the “cut-off” of short-wavelength emission followed by enhancement of long-wavelength emission.30 As a result, the PL intensity near 420 nm gradually diminishes and the peak near 550 nm simultaneously evolves with the increase of the concentration of the in-film CNDs. The decrease in the UV PL intensity is more prominent than the increase in the visible PL intensity because the CNDs show higher absorbance in the UV region and more efficiently cutoff the UV light. The films show the same trend under illumination of varied wavelengths (360, 380, and 420 nm), as described in the Supporting Information (Figure S6). LED Demonstration. For practical application, the most concentrated film (1.65 mg cm−2) was chosen to maximize the light-conversion efficacy. By combining the CND-10000 film and an InGaN blue (400 nm) LED, we present a white LED rendering the emission spectrum shown in Figure 8. Applying a forward-bias current of 50 mA (∼3 V, optimized), we obtain cool white light with the CIE (Commission Internationale d’Éclairage) coordinates of (0.3664, 0.4484) and the correlated color temperature (CCT) of 5080.4 K. This light is almost identical to that emitted from conventional tubular fluorescent lamps. The luminous efficacy of radiation is 108.19 lm Wopt−1, comparable to nanocrystal or compound phosphors.31 Thermal stability of phosphors is also one of the key issues in phosphorbased lighting systems. We thereby examined the stability of the white LED during operation over 12 h and found no change in the emission spectrum. Thus, it is concluded that the freestanding luminescent films are thermally stable under operation conditions and promising for practical phosphor applications.

Since the CNDs are “dispersed” in the polymer matrix, we find no significant reduction in the luminescence due to any type of quenching. The as-obtained films also have physical advantages such as full flexibility, air stability, mechanical strength, and harmlessness. Film Luminescence. Figure 7 shows the PL spectra of CND-10000 films as a function of the concentration of the in-

Figure 7. PL spectra of CND-10000 films as a function of the concentration of the in-film CNDs. Concentration of the in-film CNDs is assumed to be the weight of the CNDs per unit area of the film. Most concentrated film (1.65 mg cm−2) is close to saturation.

film CNDs under deep-blue (400 nm) illumination. The films show a broad emission peak (near 560 nm) in the visible-light regime, which is not perceived in the solution (diluted) phase. This observation suggests that a certain type of energy transfer occurs between the in-film CNDs. 25,30 One plausible explanation is the well-known inner-filter effect that arises from reabsorption of the emitted light in a condensed matter.

Figure 8. Emission spectrum of a white LED using a CND-10000 film as a color-converting phosphor and an InGaN blue (400 nm) LED as an illuminant (left). CIE chromaticity coordinates (x, y) calculated from the emission spectrum (middle). Photograph of an exemplary device under operation (right). 1897



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image Cs-corrector at an accelerating voltage of 200 kV. Scanning electron microscopy was performed using a Jeol JSM-7401F. Emission spectra of white LEDs were recorded using a Stellarnet Black C-50 spectrometer with a Keithley 4200-SCS sourcemeter.

CONCLUSIONS We synthesized nitrogen-rich CNDs via emulsion-templated carbonization of polyacrylamide. The obtained CNDs produce broad and bright visible light under UV illumination that would be worth utilizing in phosphor applications. To realize their potential, we fabricate large-scale (20 × 20 cm) freestanding luminescent films of the CNDs based on a poly(methyl methacrylate) matrix. The polymer matrix can not only provide mechanical support but also disperse the CNDs to prevent solid-state quenching. The obtained films are low cost, fully flexible, easily scalable, thermally stable, eco-friendly, and mechanically robust so that they can play an important role in utilizing large-scale flexible solid-state lighting systems. We finally demonstrate white LEDs by combining the films and an InGaN blue (400 nm) LED. The correlated color temperature is 5080.4 K, which is almost identical to light emitted from conventional tubular fluorescent lamps. Furthermore, the films are thermally stable, so that the white LEDs can show no temporal degradation in their emission spectra under practical operation conditions. Thus, this study would suggest a promising way to exploit the luminescence from solid-state CNDs and offer strong potential for future CND-based solidstate lighting systems.

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ASSOCIATED CONTENT

* Supporting Information S

Acid effects, additional TEM images, additional PL spectra of the CND films, and details in quantum yield measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-54-279-2265. Fax: 82-54-279-8619. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Research Laboratory Project of the Korea Research Foundation (KRF). W.K. is grateful to Hyun-Jin Park (NCNT) for assistance with TEM and Hyemin Kim (POSTECH) for access to a UV illuminator.

METHODS

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Synthesis of Nitrogen-Rich CNDs. All chemicals were purchased from Sigma-Aldrich and used as received unless otherwise specified. To 10 mL of oleylamine was added 1 mL of the polyacrylamide (PAA) solution (50 wt % in water) and 1 mL of 0.5 M nitric acid. Controlled experiments were performed by varying the MW of the polymer (1500, 4000, 10 000, 40 000, and 100 000) and polymer concentration (10, 20, 30, and 40 wt %). The mixture was transferred into a reaction flask and vigorously stirred at 50 °C under argon atmosphere for 30 min, thereby forming a milky emulsion. The emulsion was heated to 250 °C under argon atmosphere for 2 h and then cooled to room temperature. The resulting solution was precipitated with methanol and centrifuged at 3000 rpm for 10 min. Precipitant was dispersed in hexane, precipitated with methanol, and isolated by centrifugation. This process was repeated three times to remove residuals. Final product was dispersed in octane or toluene for further use. Fabrication of Freestanding Luminescent Films. Freestanding films were fabricated according to a modified version of a previously described method.31 A 5 mL amount of the purified CNDs in toluene (5, 10, 25, and 50 mg mL−1) was mixed with 5 mL of PMMA A15 (MW = 950 000, Microchem) and 5 mL of anisole. The solution was vortexed for 10 min and then centrifuged to remove air bubbles. We drop cast an aliquot of the solution on a cleaned glass substrate (1 mL per 10 cm2). The coated glass substrate was placed on a flat table and left overnight under ambient condition for drying the solvent. The resulting 50 μm thick freestanding film was then peeled off from the glass substrate and stored in a desiccator for further use. Characterization. Infrared spectroscopy was performed using a Nicolet 6700 FT-IR spectrometer. A demountable cell (Part No. 1623600) with KBr windows (Pike Technologies) was used for sampling. 1 H and 13C nuclear magnetic resonance spectra were obtained by a Bruker DRX500. Samples were dissolved in toluene-d8 (Cambridge Isotope Laboratories). X-ray photoelectron spectroscopy was performed using an Escalab 250 spectrometer with an Al X-ray source (1486.6 eV). From the above analyses, we found no change in the chemical structure of the CNDs as a function of polymer MW and, hence, provided the CND-10000 data only. UV−vis absorption spectra were recorded on a Mecasys Optizen POP spectrophotometer. Photoluminescence (PL) spectra were recorded on a Jasco FP-8500 fluorometer. Quantum yield was measured using quinine sulfate in 0.1 M H2SO4 as a standard (Supporting Information). Transmission electron microscopy was performed using a Jeol JEM-2200FS with the

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Freestanding Luminescent Films of Nitrogen-Rich Carbon Nanodots

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