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Yellow-Emitting Carbon Nanodots and Their Flexible and Transparent Films for White LEDs Tak H. Kim,†,‡ Alan R. White,§ Joseph P. Sirdaarta,§ Wenyu Ji,⊥ Ian E. Cock,§ James St. John,∥ Sue E. Boyd,§,∥ Christopher L. Brown,†,§ and Qin Li*,†,‡ †

Queensland Micro- and Nano-Technology Centre, 170 Kessels Road, Nathan, Queensland 4111, Australia School of Engineering, §School of Natural Sciences, and ∥The Eskitis Institute for Drug Discovery, Griffith University, Nathan, Queensland 4111, Australia ⊥ College of Physics, Jilin University, Changchun 130023, China ‡

S Supporting Information *

ABSTRACT: We report carbon nanodots that can be utilized as effective color converting phosphors for the production of white light-emitting diodes (LEDs). Blue-excitable and yellowemitting carbon nanodots, functionalized with 3-(imidazolidin2-on-1-yl)propylmethyldimethoxysilane (IPMDS)-derived moieties (IS-CDs), are synthesized by a novel one-pot reaction in which the products from the initial reaction occurring between urea and 3-(2-aminoethylamino)propylmethyl-dimethoxysilane (AEPMDS) are further treated with citric acid. Distinctive from the majority of carbon nanodots reported previously, IS-CDs emit at 560 nm, under 460 nm excitation, with a quantum yield of 44%. Preliminary toxicity studies, assessed by the Artemia franciscana nauplii (brine shrimp larvae) bioassay, indicate that IS-CDs are largely nontoxic. Furthermore, the IS-CDs form flexible and transparent films without the need of encapsulating agents, and the solid films retain the optical properties of solvated IS-CDs. These features indicate an immense potential for the IS-CDs as an environmental-friendly, blue-excitable carbon nanodot-based phosphor in solid-state lighting devices. KEYWORDS: carbon nanodots, surface functionalization, surface characterization, solid-state nanomaterials, photoluminescence, organosilane

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of white light.4,5 Recently, quantum dots (QDs) encapsulated with silica have demonstrated effective color conversion for phosphor-LEDs.6,7 Commercial products, including display and lighting devices, are expected to appear on the market soon. However, due to their intrinsic toxicity and hazardous environmental impact, the suitability of QDs for inclusion in widespread commercial products remains questionable.8 Moreover, in order to apply the phosphor coating, both YAG and QDs need to be powdered and embedded in polymer or silicone matrices. Thus, the inclusion of an extra processing step is required. Consequently, there is a strong demand for developing new types of phosphors that (i) are especially designed for inclusion in LEDs, (ii) are efficient color converters, (iii) are cost-effective, and (iv) are environmentally responsible/sustainable technologies. Owing to their bright luminescence, water solubility, biocompatibility, and low cost, photoluminescent carbon nanodots (CDs) have drawn significant attention as a new member of the nanocarbon family.9,10 They offer a potential

INTRODUCTION The significance of light-emitting diodes (LEDs) as solid-state lighting devices, affording global energy savings and CO2 emission reduction,1 has been duly acknowledged by the Nobel Prize of 2014. However, GaN based LEDs, without modification, only emit blue light. In order to emulate natural light, white light needs to be artificially produced either by combining blue LEDs with red and green LEDs (RGB-LEDs) or by employing color converting phosphors. Phosphorconverted LEDs, consisting of a monochromatic emitter with a color converting phosphor, are generally preferred over RGBLED systems, due to their simpler processing requirements and production cost advantages.1−3 Phosphor-LED fabrication approaches require UV or blue light emitting semiconductor LED chips, coated with polymer- or silica-based encapsulants in which the color-converting phosphors are embedded. Thus, far, embedded phosphors have generally been transition metals or rare-earth metal-ion-doped metal-oxide powders. For example, cerium doped yttrium aluminum garnet (YAG, Y3Al5O12:Ce3+) was originally developed as the phosphor for cathode ray tubes.4 Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue (from the LED chip) and yellow light (from the phosphor mixture) gives the appearance © 2016 American Chemical Society

Received: September 23, 2016 Accepted: November 15, 2016 Published: November 15, 2016 33102

DOI: 10.1021/acsami.6b12113 ACS Appl. Mater. Interfaces 2016, 8, 33102−33111

Research Article

ACS Applied Materials & Interfaces

initially remove excess AEPMDS and then to fractionate organosilanefunctionalized CDs (OS-CDs). The OS-CDs could be isolated from the combined hexane fractions by evaporation at reduced pressure. The hexane-insoluble residue remaining after the hexane washes contained the IS-CDs. For characterization, the IS-CDs were prepared by dissolution of the IS-CDs in a polar solvent such as MeOH or DMSO. In order to enable Raman spectroscopy and understand the role of the surface functional groups in the PL emission, surface functional group stripped IS-CDs were prepared using iron(III) chloride (FeCl3) catalysis to remove silyl groups from the surface of the IS-CDs.33 The IPMDS-derived intermediates were prepared for chemical and optical analyses. AEPMDS was treated with urea under conditions identical to those used in the first step of the preparation of the IS-CDs. Accordingly, urea and AEPMDS were refluxed under a dry nitrogen atmosphere (0.5 h). The reaction mixture was then cooled to room temperature and washed copiously with hexane (7 × 50 mL) affording the IPMDS-derived intermediates.

alternative to traditional semiconductor QDs such as CdSe, CdTe, and PbS. Progress has been made in synthetic approaches to CDs,11,12 including improvements in the CDs’ quantum yields (QY),13 their spectral tunability,14,15 and their solid-state optical performance.16,17 Promising applications have been demonstrated in the field of biological imaging,18−20 light harvesting,21 sensing,22−24 and solid-state lighting.25−30 The potential of CDs as the color-converting phosphors for producing white-light LEDs has been demonstrated previously.25−27 However, those systems are not ideal, due to nanoparticle aggregation and suboptimal spectral matching between the emission of the LED chip and the absorption of the CDs. Issues such as significant spectral red-shifts and broadening have been observed. Moreover, in all of these systems, the CDs needed to be dispersed in a solid-state matrix, typically acrylic glass or epoxy.25−27 Clearly, new types of CDs that are less prone to aggregate in the solid state, and can therefore retain the inherent properties of the nanoparticles, are highly desirable for LED phosphor development.31 Recently, it has been demonstrated that organosilane-functionalized CDs retain the optical properties of the CDs in the solid state.16 However, the organosilane-functionalized CDs described thus far have shown photoluminescence (PL) emissions limited to the blue spectral range (by UV excitation) and are therefore not suitable for the production of white-light LEDs. Herein, we report a new reaction that yields CDs functionalized with tethered imidazolidinones (IS-CDs). These new CDs emit in the yellow spectral range under blue excitation. The IS-CDs form films that are flexible in shape and easy to process. Importantly, the solid films retain their optical properties in the solid state owing to the tethered imidazolidinones. The IS-CDs were synthesized in a one-pot procedure, by adding citric acid to the reaction mixture obtained after heating urea with 3-(2-aminoethylamino)propylmethyldimethoxysilane (AEPMDS, 1). The resultant nanoparticles are comprised of nanometer sized particles, with tethered 3-(imidazolidin-2-on-1-yl)propylmethyldimethoxy-silane (IPMDS, 2) derived units on the surface. The IS-CDs show broad, yellowish PL emission over a range of 480−700 nm, with the highest PL emission occurring at blue (460 nm) excitation. Upon solidification, the IS-CDs form a flexible and transparent freestanding film that retains the optical properties of the solvated IS-CDs. No additional encapsulants or additives are required for the film formation. Moreover, a preliminary toxicity test shows IS-CDs are nontoxic (or have very low toxicity). We further demonstrate that the IS-CDs can be used directly as the white-emitting color converting agents for blue GaN LEDs, without any additional encapsulating agents. This represents a new strategy for manufacturing CD-based LEDs.

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PREPARATION OF SOLID CDS AND LEDS Freestanding solid IS-CDs were prepared by drop-casting ISCDs onto glass slides (25 mm × 55 mm). Freshly synthesized IS-CDs were washed copiously with hexane and then dried (evaporation at reduced pressure; 40 °C). Next, 400 μL aliquots of neat, gelatinous, IS-CDs were then dropped onto the slides. The slides were then oven-dried (70 °C, 0.5 h) and then left to age in a dust-free environment (7 days). The solid films were extracted from the glass substrates by scrapping with a utility knife blade. Solid IS-CDs, for optical characterizations, were prepared by drop-casting the IS-CDs (40 μL) onto glass slides (10 mm × 12 mm). The coated glass slides were then oven-dried (70 °C, 0.5 h). LEDs were prepared by dropping 1 to 15 μL of IS-CDs on a Surface-Mount LED (SMD5050, 470 nm, purchased from a local electronic supplier), which was then oven-dried (70 °C 0.5 h). The solid films (25 mm × 55 mm) could be dissolved in MeOH (25 mL) with a constant stirring over several hours. Artemia f ranciscana Nauplii Toxicity Screening. The Artemia f ranciscana nauplii (brine shrimp larvae) lethality bioassay was employed to examine the toxicity of all materials. A. f ranciscana cysts were obtained from North American Brine Shrimp, LLC, USA (harvested from the Great Salt Lake, Utah). The cysts were hatched in synthetic seawater, and the hatched nauplii were separated from the shells and used for the assay. In a typical experiment, 400 μL aliquots, containing approximately 100 A. f ranciscana nauplii, were transferred into wells of a 48well plate and used for bioassays. A volume of 400 μL of homogeneous solutions of each CD sample, in Milli-Q water (2 mg sample per 1 mL of Mili-Q water), were added to the wells; 400 μL of seawater and potassium dichromate solution (1 mg· mL−1, Chem-Supply, Australia) were used as negative and positive controls, respectively. The treated A. f ranciscana nauplii were incubated at 25 °C under artificial light (1000 Lux). All treatments were performed in at least triplicate. The wells were checked at regular intervals, and the number of dead counted. The nauplii were considered dead if no movement of the appendages was observed within 10 s. After 48 h, all nauplii were sacrificed to facilitate counting, by adding 100 μL of acetic acid (concentrated, Chem-Supply, Australia) to the wells. The total number of nauplii were counted to determine the total percent mortality. Characterization. Atomic force microscopy (AFM) images were captured on a NT-MDT NTEGRA Spectra AFM in semicontact mode. Hydrodynamic particle size was measured

EXPERIMENTAL SECTION

Synthesis of IS-CDs. IS-CDs were synthesized by reacting 3-(2aminoethyl-amino)propyldimethoxymethylsilane (AEPMDS, Beijing Shenda Fine Chemical, China)32 with urea (Chem-Supply, Australia) and the nanoparticle scaffolding synthon, citric acid (CA), in a molar ratio of AEPMDS/urea/CA = 19:5:1. Typically, urea (1.5 g) was added to AEPMDS (20 mL in a 50 mL three-necked flask) and the mixture heated to reflux (220 °C), under N2. After 30 min, anhydrous CA (1 g) was introduced carefully, via a solid addition device. Caution! The reaction between CA and the reaction mixture is highly exothermic. Caution must be taken during the addition of CA. The reaction temperature (220 °C) was maintained for a further 30 min. The cooled reaction mixture, containing the synthesized crude CDs was washed with hexane (7 × 50 mL, RCI Labscan, Australia) in order to 33103

DOI: 10.1021/acsami.6b12113 ACS Appl. Mater. Interfaces 2016, 8, 33102−33111

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Figure 1. (a) AFM image, (b) histogram of height distribution observed in the AFM, (c) hydrodynamic sizes obtained for the IS-CDs (the raw correlation data acquired from DLS is inset), and (d) HRTEM image of the IS-CDs.

temperature (CCT) were measured by an USB4000 Ocean Optics spectrometer with an integrating sphere, and the current (voltage) across the devices are recorded by a Keithley Model 2400 SourceMeter power supply. Fluorescence microscopy images of the A. franciscana nauplii were captured on an Olympus IX70 epifluorescence microscope at the excitation of 405, 488, and 594 nm; exposure times for each excitation were 450, 1500, and 1000 ms, respectively.

on a Malvern Zetasizer Nano ZS, using the dynamic light scattering (DLS) technique. High-resolution transmission electron microscopy (HRTEM) images were captured on a Philips Tecnai F20 (200 kV), and TEM images were captured on a JEOL JEM-1010 (80 kV). Raman spectra were measured on a Renishaw inVia Raman microscope equipped with 514 nm laser. Nuclear magnetic resonance (NMR) spectra were acquired on a Varian 400 MHz Unity INOVA spectrometer in d6-DMSO, CDCl3, and CD3OD solvent mixtures, at the temperatures quoted. 1H and 13C{1H} NMR chemical shifts are referenced to solvent residuals, taken as 2.49 and 39.5 ppm (d6DMSO), 3.30 and 49.0 (CD3OD), or 7.23 and 77.0 ppm (CDCl3), respectively. 29Si NMR chemical shifts are referenced to external tetramethylsilane (TMS), taken as 0 ppm (CDCl3/ d6-DMSO solution 2:2:1 v/v). All 29Si NMR spectra were locked on deuterium signal of d6-DMSO.34 2D gradient filtered COSY, heteronuclear single quantum correlation (gHSQC), heteronuclear multiple bond correlation (gHMBC), and nuclear Overhauser effect spectroscopy (NOESY) were acquired using the standard sequences implicit in the VNMRJ 2.1b software package. All NMR spectra were processed using the MestReNova software package (v8.1.4). Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer Spectrum Two IR spectrometer equipped with a diamond attenuated total reflectance (ATR) attachment. X-ray photoelectron spectroscopy (XPS) spectra were acquired using a Kratos Axis ULTRA X-ray photoelectron spectrometer incorporating a 165 mm hemispherical electron energy analyzer. Calculations of atomic concentrations and peak fitting of the high-resolution data were carried out using the CasaXPS software package (v2.2.73). UV visible (UV−vis) absorption spectra for dispersed samples were measured on an Agilent 8453 UV−vis spectrometer. UV−vis absorption spectra of solid samples were acquired from a Jasco V-650 UV−vis spectrometer equipped with a 60 mm integrating sphere. Photoluminescence (PL) spectra were measured by a Thermo Lumina fluorescence spectrometer. Photoluminescence lifetimes were recorded by using 374 nm/443 nm picosecond pulse laser excitation sources in an Edinburgh Photonics FLS920 photoluminescence spectrometer. Quantum yields were measured by comparing quantum yield standards at 360 nm excitation (quinine sulfate, 54%)35 and 470 nm excitation (rhodamine 6G, 94%).36 All samples for quantum yield were prepared by diluting the original samples until their absorbance was below 0.05 for each excitation wavelength. Spectra and CIE 1931 color space coordinates of the LEDs were recorded by an Ocean Optics USB4000 with a fiber optic probe, in a closed light chamber. Solid-state quantum yield (QY), color rendering index (CRI), luminous efficacy (LE), and correlated color

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RESULTS AND DISCUSSION Synthesis. IS-CDs are synthesized in a one-pot procedure in two steps (Scheme S1). Initially, urea reacts with the ethylenediamine moiety on AEPMDS affording the corresponding imidazolidinone, (IPMDS, 2),37,38 with ammonia being released during the reaction.38,39 In the second step, citric acid (CA) is added to the hot reaction mixture. A vigorous reaction, accompanied by intense gas evolution, is observed and the color of the reaction mixture turns dark brown. On cooling, the reaction mixture is washed copiously with hexane, providing two fractions of CDs. Evaporation of the solvent from the hexane soluble fractions afforded, predominantly, CDs functionalized with AEPMDS-derived moieties (OS-CDs). This fraction is attributed to the statistically favorable direct reaction of CA with unreacted AEPMDS. The second, hexaneinsoluble fraction afforded carbon nanoparticles functionalized with a high proportion of IPMDS-derived moieties (IS-CDs). Since we have found the properties of the OS-CDs obtained here to be consistent with those previously reported for silane functionalized CDs,16,40 we will focus our discussions here on the novel IS-CDs isolated from the hexane insoluble fraction. However, comparative data from the OS-CDs is provided in the Supporting Information. AFM, DLS, TEM, and Raman Analyses. IS-CDs were assessed by AFM, on a mica substrate using semi contact mode, DLS, and TEM (Figure 1). AFM characterization indicated that the majority of the particles were between 1.8 and 6 nm, while DLS measurement indicated that the hydrodynamic particles size of dispersed IS-CD particles was between 2 and 10 nm. DLS also showed a small amount of agglomerates present in the solution, with a size of ∼250 nm (which also observed in AFM). The slight relative reduction in particle size determined in the AFM measurements is likely attributable to the removal of hydration layer from the peripheral functionality of the particles on the mica substrate. TEM characterization further shows that the particles are spherical and size distribution are between 2 and 5 nm, consistent with the AFM characterization. Furthermore, the HRTEM in Figure 1d suggests that the carbon structure is somewhat crystalline. Successive planes of carbon lattices can be observed, and the planes were separated 33104

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Figure 2. Expansions of the gHMBC spectrum (400 MHz, 298 K, d6-DMSO) of the IPMDS-derived intermediates isolated from reaction of 1 and urea ((i) 493 K, 0.5 h (ii) hexane wash). 1H and 13C NMR assignments are as illustrated. The general numbering schemes used in the assignments of the 1H and 13C resonances of the two silane fragment types, 1 (red) and 2 (green), are adapted from the corresponding dimethoxysilane parent species (1 and 2) and are illustrated (inset). 1D 1H (400 MHz, 298 K, d6-DMSO) and 13C{1H} (100 MHz, 298 K, d6-DMSO) are provided as axes references. Key 2JHC and 3JHC correlations, supporting the formation of an imidazolidinone ring, are highlighted (inset).

Scheme 1. Potential Architectures for the Tethered IS-CDsa

a

Surface functionalities on (a) 1mer, (b) 2mer, (c) 3mer, and (d) 4mer. N-Termini can be imidazolidinone as illustrated or unreacted diamines.

at a distance of ∼0.36 nm. Raman spectrum (Figure S3) obtained from the stripped IS-CDs demonstrates that the ISCDs have carbon cores that consist of both graphitic and amorphous carbons.41 NMR Spectroscopy. The isolated IPMDS-derived intermediates and derived IS-CDs were examined using NMR spectroscopy. The 1H and 13C{1H} NMR spectra acquired for the samples obtained from the reaction of AEPMDS, 1, and urea, with and without subsequent addition of CA, are remarkably similar (Figure S4). Mixtures of closely related materials are obtained. These materials are derived, presumably, from either elaboration of the products formed from the reaction of the ethylenediamine moiety of 1, directly with urea (i.e., IPMDS, 2), and/or from a subsequent reaction of diamine moieties of already elaborated silanes with urea.42,43

The conversion of the diamine fragment to an imidazolidinone is evidenced in the 1H and 13C{1H} NMR44 and the gHMBC spectrum of the IPMDS-derived intermediates (Figure 2). A new carbonyl resonance is seen in the 13C{1H} NMR at δ 162.3 ppm (C2′), and new methylene resonances are also seen in the 13C{1H} NMR and the gHSQC spectrum at δ 44.4 (C5′) and 37.5 ppm (C4′). Furthermore, the carbonyl resonance correlates with 1H resonances at δ 6.21 (NH), 3.30−3.16 (H5′/H4′) and 2.97 ppm (H3′) in the gHMBC spectrum, confirming cyclization at the N terminus. The clustering of resonances observed for the carbons of the propyl chains and methyl groups and the broadening of the corresponding resonances of the 1H NMR spectrum suggest the formation of a number of closely related species. It is notable that resonances attributable to the dimethoxy substituents initially attached to the Si (and evidenced in the 33105

DOI: 10.1021/acsami.6b12113 ACS Appl. Mater. Interfaces 2016, 8, 33102−33111

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ACS Applied Materials & Interfaces spectra of 1) are significantly reduced in intensity in the spectra of the IPMDS-derived intermediates and IS-CDs (1H, 13C{1H}, gHSQC, and gHMBC spectra of the intermediates and IS-CDs; Figures 2 and S4). This suggests IS-CD formation is likely to involve initial hydrolysis of the methoxysilane substituents of 1 and/or 2, affording SiOH groups. These groups can then participate in subsequent condensations, resulting in dot tethering. Alternatively, siloxane forming reactions could occur between tethered and/or untethered silanes, resulting in the formation of families of surface groups such as those illustrated in Scheme 1. These tethered or clustered chains may be terminated in a mix of uncapped ethylenediamine and/or imidazolidinone functionality, with the latter representing the majority of N termini in the IS-CDs.44 1 H/29Si gHMBC experiments were conducted to probe the chemical environments of Si nuclei in the AEPMDS synthon, the IPMDS-derived intermediates, and the IS-CDs (Figure 3).

Figure 4. Expansions of the NOESY matrices (400 MHz, 298 K, τmix = 600 ms, CD3OD/d6-DMSO (4:1 v/v) obtained for (a) the IPMDSderived intermediates and (b) the IS-CDs. Expansions of the corresponding regions of the 1H NMR spectra of the same samples are provided as axes references. The relative phases of the cross-peaks linking protons of the silyl substituents and the propyl chains in the two species are highlighted. *ωo is the Larmor frequency, 400 MHz; τc is the rotational correlation time.

these regions, post dot tethering (yellow highlights). In contrast, however, NOESY cross-peaks, correlating the protons on the substituents directly attached to the Si with adjacent protons, are all negative.50,51 This indicates there is lower mobility near the silicon termini in the IS-CDs (long τc; blue highlights). Therefore, these regions of the chains are more macromolecular in character; thus, dot tethering of the IPMDSand AEPMDS-derived chains occurs at the silyl positions in the IS-CDs. IR Spectroscopy. The infrared spectra (IR) obtained for the AEPMDS, the IPMDS-derived intermediates, and the ISCDs are shown in Figure 5. All materials exhibit N−H stretches

Figure 3. Expansions of the 1H/29Si gHMBC experiments (400 MHz, 296 K, d6-DMSO) conducted on (a) AEPMDS, (b) IPMDS-derived intermediates, and (c) IS-CDs. 29Si NMR assignments are as illustrated and referenced to external TMS (0 ppm). The corresponding 1D 1H spectra (400 MHz, 296 K, d6-DMSO) are provided as axes references.

Clustered 29Si resonances were observed between δSi −18 and −23 ppm in both the intermediates and IS-CDs. Resonances occurring in this region of the 29Si spectrum are consistent with those previously observed for siloxane bridged species, similar to those illustrated in Scheme 1.45−48 Additionally, cross-peaks were also observed in the spectra of the intermediates and the IS-CDs in the region δSi −13 and −16 ppm. These resonances are assigned to Si−OH terminated 2-mers, 3-mers and so on, of the general formulation, R′Si(R)(CH3)−O-Si(R)(CH3)− OH.46,47 The starting synthon showed the expected 29Si resonance at δSi −0.9 ppm. This shift is directly comparable to shifts previously reported for (CH3O)2(CH3)2Si, δSi (1.25 to ca. −2.5 ppm).48,49 The orientation of the IPMDS and AEPMDS derived chains, with respect to the surface of the nanoparticles, is in evidence in the NOESY spectra of the isolated IPMDS-derived intermediates and IS-CDs (Figure 4). The phase of the cross-peaks observed in the NOESY matrix of the intermediates are all positive (Figure 4a). Positive NOE is expected for mobile small molecules with short molecular rotational correlation times (τc). Similarly, the cross-peaks linking protons on the propyl chains and the N-terminal groups of the IS-CDs are also positive (Figure 4b). This indicates comparable high mobility in

Figure 5. FTIR spectra of the samples prepared and isolated throughout the IS-CDs syntheses.

between 3400 and 3200 cm−1; however, additional bands are observed in the spectra of the intermediates and the IS-CDs (CO stretch at 1685 cm−1, C−N bend at 1490 cm−1), supporting the presence of urea like functionality in these materials.40 In addition, bands are observed at 1010 and 900 cm−1 in the spectra of the intermediates and IS-CDs; these 33106

DOI: 10.1021/acsami.6b12113 ACS Appl. Mater. Interfaces 2016, 8, 33102−33111

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bright yellow fluorescence under 460 and 480 nm excitation. In comparison, the OS-CDs reported previously (and verified in this study; see Supporting Information) all exhibit strong blue fluorescence centered around 460 nm (λEx = 360 nm). The significant red-shift of the emission peak of IS-CDs is presumably attributable to the incorporation of imidazolidinone functionality into the nanoparticles. These pendant groups will have introduced different surface states compared to OS-CDs. IS-CDs quantum yields were quantified as 28% at 360 nm and 44% at 470 nm excitation in methanol. A slight PL emission blueshift (∼5 nm) and increase in quantum yields were observed when the IS-CDs were dispersed in DMSO (29% at λEx = 360 nm, 52% at λEx = 470 nm, Figure S9). Solid-state QY of IS-CDs was measured as 30% at 460 nm excitation. IS-CDs in methanol also displayed distinctive absorption properties (Figure 7a, * peaks); two noticeable peaks are observed at 302 and 318 nm, a characteristic band at 360 nm, as well as a noticeable absorption shoulder, starting at ∼500 nm. This prominent, strong, UV−vis absorption in the blue spectral range is consistent with the yellow daylight color of the sample. In an effort to gain a better understanding of the role of the imidazolidinones in the PL of IS-CDs, we examined the UV− vis and PL of the IPMDS-derived intermediates and the surface functionality stripped IS-CDs (Figure S10). A comparison between the UV−vis absorbance of the intermediates, stripped IS-CDs and IS-CDs reveals that the strong absorption in the range of 300−500 nm is unique to the IS-CDs (the intermediates show no absorption in this range while the stripped IS-CDs shows no shoulder ∼450 nm). This indicates that the absorption in this range is related to the nanoparticles and/or the surface attachment and silane elaboration in the ISCDs. A comparison between the PL emission spectra of the intermediates and IS-CDs shows a significant bathochromic shift in IS-CDs (550 nm IS-CDs; cf. 440 nm intermediates). Another comparison between the PL emission spectra of the IS-CDs and the stripped IS-CDs also shows a notable PL emission property changes (550 nm IS-CDs at λEx = 460 nm; cf. 470 nm intermediates at λEx = 380 nm). The three sets of emission peaks are, in general, different in their spectral shape and trend (Figure S10). The quantum yields of the intermediates were 7% at 360 nm and less than 1% at 470 nm excitation, and the stripped IS-CDs were 22 and 10% at λEx = 360 and 470 nm, respectively, significantly lower than those of IS-CDs. The absence of yellow emissions in samples of the isolated IPMDS-derived intermediates and the significantly lower yellow emission from the functional group stripped ISCDs strongly suggest that the 550 nm emission observed in the untreated IS-CDs are attributable to additional surface states made available when the IPMDS-derived functionality is tethered to the carbon core (Figure S10).41 The photoluminescent excitation (PLE) spectra of the samples, at their maximum emission intensity (550 nm), further confirms that IS-CDs can be effectively excited in the blue spectral range (i.e., 420 to 470 nm). As presented in Figures 7e (and S11), Gaussian sub-bands were extracted from the emission and excitation spectra of IS-CDs by deconvolution. The emission spectrum of IS-CDs, resulting from λEx = 460 nm, shows a broad, asymmetric band (λEm = 480−700 nm). This spectrum can be further deconvoluted to 528, 557, and 602 nm (Figure S11). Best-fit splitting of the excitation spectrum (λEx = 320−500 nm), resulting from λEm = 550 nm, of the IS-CDs, shows eight components at 301, 320, 364, 391, 419, 445, 469, and 488 nm. The deconvoluted components in

bands are not observed in the AEPMDS spectrum. These are assignable to Si−O−Si stretches and Si−O(H) bends, respectively. The emergence of these bands confirms that the methoxysilane functionality, present in the AEPMDS, is at least partially converted into siloxane linkages in the intermediates and IS-CD products.52 Elaboration of the silane moieties is further supported by changes in the CH3 (between 1275 and 1150 cm−1). The spectra of the intermediates and IS-CDs show substantial changes in these regions: The band at 2833 cm−1 is absent, and there is increased intensity at 1255 cm−1 and decreased intensity at 1190 cm−1. This agrees with our observations made in the NMR spectroscopy studies; functional group conversion of the dimethoxysilane units has occurred, resulting in the formation of siloxane linkages. XPS Spectroscopy. XPS survey spectra (Figure 6) indicate that the majority of the samples consist of carbon (69%, Table

Figure 6. (a) XPS survey spectrum. High-resolution spectrum of (b) C 1s, (c) N 1s, and (d) Si 2p of the IS-CDs.

S1). The high-resolution C 1s spectrum of IS-CDs could be deconvoluted into six peaks, which corresponds to C−Si at 283.3 eV, sp2 CC at 284.0 eV, sp3 C−C at 284.6 eV, C−N at 285.3 eV, C−O at 286.1 eV, and CO at 287.8 eV, respectively. The high-resolution N 1s spectrum of IS-CDs could be deconvoluted into two peaks, which corresponds to − NH at 398.8 eV and N−C 399.5 eV. The distinct N−C is likely to have originated from imidazolidinone. The high-resolution Si 2p spectrum can be deconvoluted into two peaks, assignable to Si−O−Si (100.9 eV) and Si−C (101.6 eV).53,54 The deconvoluted Si−O−Si peak at 100.9 eV and the absence of dimethoxysilane (∼104 eV) further confirm the changes in the dimethoxy groups and the formation of the siloxane. This observation is consistent with NMR and FTIR results. Optical Properties. As shown in Figure 7a, PL emission of IS-CDs is excitation dependent. Interestingly, however, it is very different from previous reports on organosilane-functionalized CDs.16,40 When excited in the range of 340−420 nm, the emission is relatively weak and broad, ranging from 425−650 nm, with the emission maxima centered at ∼510 nm. When excited under 460 and 480 nm, however, the emission intensity is maximized, peaking at 550 nm. If the excitation wavelength is further red-shifted, then the emission intensity is reduced; however, the emission maxima remain centered at 550 nm. As shown in Figure 7c, the IS-CDs dissolved in methanol exhibit 33107

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Figure 7. UV−vis and PL emission spectra of (a) the IS-CDs in MeOH, (b) in solid state, (c) the integrated PL emission intensities of the IS-CDs in MeOH and solid, (d) the PL emission peak positions of the IS-CDs as a function of excitation wavelength, (e) PL excitation spectrum (λem = 550 nm) with Gaussian sub-bands, (f) schematics of PL mechanisms of the IS-CDs, and photographic images of the IS-CDs under a series of excitation wavelengths (g) in MeOH and (h) in solid states. (Excitation spectrum of solid IS-CDs is presented in Figure S10.)

the UV region are in good agreement with the peaks observed in the UV−vis spectra. It is clear that the most efficient excitation sub-bands of the IS-CDs are 420, 447, 469, and 488 nm. These coincide with the absorption shoulder ranging from 410 to 500 nm (as shown in the UV−vis absorbance). Figure 7f illustrates the PL mechanisms, which is an integration of the π*−π transition of sp2 domain, surface functional groups induced molecular type of emission, and trapping states.15,55−57 The noticeable absorption shoulder in the UV−vis spectrum of the IS-CDs (λAbs = 420−500 nm) and the pronounced PLE peak (at λEx = 469 nm) indicate that the distinctive yellow emission is associated with the surface states15,56,57 (i.e., the surface state transitions are dominant in IS-CDs). This is likely due to the presence of the imidazolidinone functionalities creating new energy levels on the surface, leading to a number of radiative recombinations.15,55−57 Upon drying, the IS-CDs form a yellow-colored, transparent polymer-like film. Notably, the solid-state IS-CD films retain the PL properties of solvated IS-CDs; exhibiting the highest emission at 550 nm under 460 nm excitation, as shown in Figure 7b. The absence of red-shift suggests that the organic− inorganic IS-CD particles are well-separated from one another by the tethered groups. The absorption spectrum of solid-state IS-CDs bears significant resemblance to solvated IS-CDs, with an absorption edge around 250 nm, a peak at 360 nm, and distinctive shoulders around 410 and 500 nm. However, the peaks at 302 and 318 nm were no longer observable in the spectrum of the solid IS-CDs. It is also noticeable that the emission peaks of the solid sample under UV excitations (360 and 380 nm) were blue-shifted compared to those of the solvated sample (Figure 7d). These changes in PL emission peaks of the solid IS-CDs are likely due to the changes in nanoparticle morphology induced by packing in the solid state. PL intensities, however, show no noticeable differences between the solvated and the solid SI-CDs over the range of excitation wavelengths tested (Figure 7c). Furthermore, the PLE spectra of the solid-state IS-CDs (provided in Figure S11)

match well with those of solvated IS-CDs. This indicates that there is no noticeable quenching or reabsorption caused by aggregation of the nanoparticles. PL decay of both solvated and solid samples was measured at excitations of 377 and 443 nm using the time correlated single photon counting (TCSPC) technique, in order to probe the optical property differences between solvated and solid IS-CDs (Figure S12). PL decay of solvated and solid samples, at the same excitation, are identical and the exponential fitting of the decay (Table S2) further confirms that solidification of the ISCDs results in insignificant changes in the optical properties of the nanoparticles. Toxicity of the Samples. Toxicity was tested using a modified A. f ranciscana nauplii lethality assay.58,59 As shown in Figure 8, a crude mixture of the two types of CDs demonstrate a substantial toxic effect after 48 h. Data from the fractionated samples indicate that the mortality on the A. f ranciscana nauplii was attributed to OS-CD exposure, as the IS-CDs showed no significant effect in isolation. Isolated OS-CDs, however, resulted in 20% mortality within 24 h and 100% mortality after 48 h. Fluorescence microscopy images of A. f ranciscana nauplii were captured in order to validate uptake of the IS-CDs by the A. f ranciscana nauplii. The images from the control group (seawater) showed no obvious fluorescence. In contrast, the images from the A. f ranciscana nauplius exposed to IS-CDs displayed clear distributions of the nanoparticles within the A. f ranciscana nauplius, confirming uptake of the samples by the organism. Solid Sample Fabrication and LED Demonstration. Solidified IS-CD samples can be prepared on glass slides. They readily can be peeled off from the slide after aging for a week at room temperature. This provides the solid IS-CDs as freestanding films that are both flexible and transparent (Figure 9a). Unlike other CD films, IS-CDs do not require the addition of encapsulating agents;26 flexible and transparent films are obtained from IS-CDs alone. The films properties are reasonably attributed to interparticle H-bonding occurring 33108

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applied.60 As shown in Figure 9d,e, the color of the IS-CDLEDs varied from blue to yellow with increasing IS-CD coating volumes. The resulting mix of yellow light from the IS-CD coating and blue light from the LED chip provides the appearance of white light (since yellow light stimulates the green and red receptors of human eye). The optimal volume of IS-CDs for producing white light was 5 μL; the CIE 1931 color space coordinates were close to those of daylight in this case (Figure 9d, Table S3, x: 0.305, y: 3.05). Due to the wide spectral distribution from IS-CDs, the CD-LEDs exhibit high CRI value of over 84 (5 and 10 μL CD-LEDs, Table S4). The corresponding luminous efficacies are higher than 20 lm/W. Correlated color temperature (CCT) of the CD-LEDs further confirmed the color tunability of IS-CD-LEDs shifting their values from cool white to warm white region (Figure S13 and Table S4). Furthermore, IS-CD-LEDs showed minimal color changes, over approximately 200 h of operation. Minor changes were observed after 48 h (Figure S14a, CIE 1931 coordinates; x: 0.317, y: 0.324 to x: 0.307, y: 0.311); however, no further changes occurred during the remainder of the testing period. Thermal stability of the IS-CDs was also assessed at 80 °C for 200 h. After 24 h of the heat exposure, 5% intensity changes from the IS-CDs were observed (Figure S14b). However, no significant changes were noted from the IS-CDs after 48 h indicating their good thermal stability.

Figure 8. (a) Lethality of the positive control (potassium dichromate, 1 mg·mL−1), crude CDs, OS-CDs, and IS-CDs (2 mg·mL−1). Fluorescence microscopy images of an A. f ranciscana nauplius in (b) negative control (seawater) and (c) IS-CDs, using excitations at 405, 488, and 594 nm (left to right).

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CONCLUSIONS We have synthesized CDs functionalized with IPMDS derived moieties, by utilizing the reaction between the diamine segment of AEPMDS and urea. Addition of CA to the reaction mixture affords nanoparticles with imidazolidinone tethered on surface via the silyl end (IS-CDs). The distinctive yellow emission of IS-CDs, upon blue excitation, is from the surface states’ absorption and emission. A. f ranciscana nauplii toxicity tests show that IS-CDs are nontoxic. The IS-CDs showed consistent optical performance in the solid state, forming flexible and transparent films, without the need for encapsulating agents. Their high quantum yield and maximum emission in the yellow spectral range, along with the simplicity of solid sample fabrication, enabled utilization of the IS-CDs as phosphors for blue GaN LEDs; affording white light producing IS-CD-LEDs. The IS-CD-LEDs’ color could be controlled by varying the amount of IS-CDs applied. No significant reabsorption was noticed over the amount of coatings tested. This indicates no significant inefficiency occurring in the solid sample. The 5 μLIS-CD-LED presented CIE 1931 color space coordinates at x:

between individual nanoparticles during solidification (Figure 9a). Irreversible aggregation has not occurred, as the solid ISCDs redissolve when stirred in methanol (over several hours). The formation of interparticle H-bonding networks results in a film which inhibits the PL emission red-shifts, in contrast to the systems requiring additives.25−27 The films exhibit bright visible white light upon 1 mW 405 nm laser excitation. The film also illuminates white light under 460 nm blue LED light, operating at 10 mA (Figure 9b). The strong emission and higher quantum yield demonstrated by IS-CD films under blue spectral excitation were utilized to produce white LEDs, by drop-casting IS-CDs onto the surface of commercially available GaN blue LEDs (SMD5050, Figure 9e). IS-CD coated LEDs were prepared, each using different amounts of freshly synthesized, gelatinous, IS-CDs; volumes of 1, 2.5, 5, 10, and 15 μL were used for coating of the LEDs. The resultant IS-CD coated LEDs (IS-CD-LEDs) exhibited color tunability. Figure 9c shows emission spectra obtained for the prepared IS-CD-LEDs, as a function of the amount of IS-CDs

Figure 9. (a) IS-CD solid sample (30 × 10 mm, loop) and schematic illustration of one potential inter particle H-bonding motif. (b) IS-CD solid sample under (1) room light, (2) 405 nm laser, and (3) 460 nm blue light. (c) Emission spectra of the LEDs prepared from IS-CDs. (d) CIE 1931 color space diagram of the LEDs (the coordinates are available in Table S3). (e) LEDs under room light (i.e., off, left) and operating at 10 mA (right), (1) bare blue LED, (2) 1 μL, (3) 2.5 μL, (4) 5 μL, (5) 10 μL and (6) 15 μL CD-LED). 33109

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(5) Smet, P. F.; Parmentier, A. B.; Poelman, D. Selecting Conversion Phosphors for White Light-Emitting Diodes. J. Electrochem. Soc. 2011, 158 (6), R37−R54. (6) Jang, E.; Jun, S.; Jang, H.; Lim, J.; Kim, B.; Kim, Y. White-LightEmitting Diodes with Quantum Dot Color Converters for Display Backlights. Adv. Mater. 2010, 22 (28), 3076−3080. (7) Demir, H. V.; Nizamoglu, S.; Erdem, T.; Mutlugun, E.; Gaponik, N.; Eychmuller, A. Quantum Dot Integrated LEDs Using Photonic and Excitonic Color Conversion. Nano Today 2011, 6 (6), 632−647. (8) Hardman, R. A Toxicologic Review of Quantum Dots: Toxicity Depends on Physicochemical And Environmental Factors. Environ. Health Perspect. 2006, 114 (2), 165−172. (9) Baker, S. N.; Baker, G. A. Luminescent Carbon Nanodots: Emergent Nanolights. Angew. Chem., Int. Ed. 2010, 49 (38), 6726− 6744. (10) Cao, L.; Meziani, M. J.; Sahu, S.; Sun, Y.-P. Photoluminescence Properties of Graphene versus Other Carbon Nanomaterials. Acc. Chem. Res. 2013, 46 (1), 171−180. (11) Zhu, H.; Wang, X.; Li, Y.; Wang, Z.; Yang, F.; Yang, X. Microwave Synthesis of Fluorescent Carbon Nanoparticles with Electrochemiluminescence Properties. Chem. Commun. 2009, 4 (34), 5118−5120. (12) Wang, F.; Pang, S. P.; Wang, L.; Li, Q.; Kreiter, M.; Liu, C. Y. One-Step Synthesis of Highly Luminescent Carbon Dots in Noncoordinating Solvents. Chem. Mater. 2010, 22 (16), 4528−4530. (13) Dong, Y.; Pang, H.; Yang, H. B.; Guo, C.; Shao, J.; Chi, Y.; Li, C. M.; Yu, T. Carbon-Based Dots Co-Doped with Nitrogen and Sulfur for High Quantum Yield and Excitation-Independent Emission. Angew. Chem., Int. Ed. 2013, 52 (30), 7800−7804. (14) Yang, S. T.; Wang, X.; Wang, H.; Lu, F.; Luo, P. G.; Cao, L.; Meziani, M. J.; Liu, J. H.; Liu, Y.; Chen, M.; Huang, Y.; Sun, Y. P. Carbon Dots as Nontoxic and High-Performance Fluorescence Imaging Agents. J. Phys. Chem. C 2009, 113 (42), 18110−18114. (15) Wang, Y.; Kalytchuk, S.; Zhang, Y.; Shi, H. C.; Kershaw, S. V.; Rogach, A. L. Thickness-Dependent Full-Color Emission Tunability in a Flexible Carbon Dot Ionogel. J. Phys. Chem. Lett. 2014, 5 (8), 1412− 1420. (16) Xie, Z.; Wang, F.; Liu, C. Y. Organic-Inorganic Hybrid Functional Carbon Dot Gel Glasses. Adv. Mater. 2012, 24 (13), 1716− 1721. (17) Wang, F.; Xie, Z.; Zhang, B.; Liu, Y.; Yang, W.; Liu, C.-y. Downand Up-Conversion Luminescent Carbon Dot Fluid: Inkjet Printing and Gel Glass Fabrication. Nanoscale 2014, 6 (7), 3818−3823. (18) Liu, R.; Wu, D.; Liu, S.; Koynov, K.; Knoll, W.; Li, Q. An Aqueous Route to Multicolor Photoluminescent Carbon Dots Using Silica Spheres as Carriers. Angew. Chem., Int. Ed. 2009, 48 (25), 4598− 4601. (19) Zhu, A.; Qu, Q.; Shao, X.; Kong, B.; Tian, Y. Carbon-Dot-Based Dual-Emission Nanohybrid Produces a Ratiometric Fluorescent Sensor for In Vivo Imaging of Cellular Copper Ions. Angew. Chem., Int. Ed. 2012, 51 (29), 7185−7189. (20) Ray, S. C.; Saha, A.; Jana, N. R.; Sarkar, R. Fluorescent Carbon Nanoparticles: Synthesis, Characterization, and Bioimaging Application. J. Phys. Chem. C 2009, 113 (43), 18546−18551. (21) Wang, C. F.; Wu, X.; Li, X. P.; Wang, W. T.; Wang, L. Z.; Gu, M.; Li, Q. Upconversion Fluorescent Carbon Nanodots Enriched with Nitrogen for Light Harvesting. J. Mater. Chem. 2012, 22 (31), 15522− 15525. (22) Zhu, S. J.; Zhang, J. H.; Tang, S. J.; Qiao, C. Y.; Wang, L.; Wang, H. Y.; Liu, X.; Li, B.; Li, Y. F.; Yu, W. L.; Wang, X. F.; Sun, H. C.; Yang, B. Surface Chemistry Routes to Modulate the Photoluminescence of Graphene Quantum Dots: From Fluorescence Mechanism to Up-Conversion Bioimaging Applications. Adv. Funct. Mater. 2012, 22 (22), 4732−4740. (23) Dong, Y.; Li, G.; Zhou, N.; Wang, R.; Chi, Y.; Chen, G. Graphene Quantum Dot as a Green and Facile Sensor for Free Chlorine in Drinking Water. Anal. Chem. 2012, 84 (19), 8378−8382. (24) Kim, T. H.; Ho, H. W.; Brown, C. L.; Cresswell, S. L.; Li, Q. Amine-Rich Carbon Nanodots as a Fluorescence Probe for

3.05, y: 3.05 which are close to those of natural white light. Since the interparticle H-bonding on the imidazolidinone moieties may inhibit the particle aggregation in solid state, further increase of the IPMDS functionalities on the CDs’ surface may improve solid-state QY of the IS-CDs as well as performance of the CD-LEDs. In summary, the promising results of these new IS-CDs present new possibilities for the construction of environmentally responsible solid-state lighting technologies, due to their nontoxicity, the ease with which derived devices could be fabricated, and their relative cost effectiveness.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12113. AFM, DLS, FTIR, XPS, optical data of OS-CDs, lowmagnification TEM and Raman spectrum of IS-CDs, expansions of the 13C{1H}, 1H (400 MHz, 298 K, d6DMSO) NMR spectra, gHMBC of IS-CDs, PL spectra in DMSO, UV−vis/PL spectra of IPMDS-derived intermediates and imidazolidinone functional group stripped IS-CDs, excitation/emission spectra of solid IS-CDs and their Gaussian fittings, PL decay of IS-CDs, Emission spectra of IS-CD-LEDs measured in an integrating sphere, CIE 1931 diagram and thermal stability of 5 μL-IS-CD-LEDs. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: qin.li@griffith.edu.au. ORCID

Tak H. Kim: 0000-0002-4495-176X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.K. acknowledges the support of Australian Postgraduate Award and Queensland Smart Futures PhD Scholarship. Q.L. acknowledges Griffith University Research Infrastructure Funding and Griffith School of Engineering Research Seed Funding. We acknowledge Mr. Ehsan Eftekhari for his assistance in TEM, Mr. Christopher R. Merritt for his assistance in Raman characterization, Dr. Barry Wood at the University of Queensland for his assistance in XPS analyses, and Prof. Lanzhou Wang at the University of Queensland for the access to the instrument for solid-state UV−vis and PL decay measurement.

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REFERENCES

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