Excellent color rendering index single system white light emitting carbon dots for next generation lighting devices

Recently, quantum dots (QDs) are finding enormous application in white light emitting diodes (WLEDs) and WLEDs with high color rendition are in high demand. QD-WLEDs use different color (Red, Blue, Green) emitting QDs to obtain white light. Use of different color emitting QDs affect purity of white light due to self-absorption losses and QD degradation, in the long run affecting color rendering index (CRI) of WLEDs. Herein, we report low cost, environment friendly, open air atmosphere synthesis of single system white light emitting carbon dots (CDs) with broad emission bandwidth ranging 116 –143 nm and quantum yields (QY) ~ 5 – 13 % in colloidal state by modifying CD surface. Furthermore, carbon dot polymer phosphor (CD-PDMS phosphor) is fabricated which emits white light under UV illumination with a record emission bandwidth of ~ 154 nm and QY ~ 16 % in solid state. Moreover, CD-PDMS phosphor exhibit excellent color rendering index (CRI) ~ 96, the highest reported so far with CIE co-ordinates (0.31, 0.33) that are quite akin to pure white light. Such high performances are achieved due to high quality of CDs and CD-PDMS polymer phosphors by precise control in passivation/functionalization of nanoparticle surface. This work will set platform for the application of CD-phosphor based WLEDs in lighting systems.

www.nature.com/scientificreports/ Considering these shortcomings, there arises a high demand for the fabrication of broad emitting, highly stable, non-toxic, low cost QD phosphor using single white light emitting system, yielding high CRIs and device efficiencies with negligible re-absorption losses. Carbon dots (CDs), on the other hand are one of the most significant class of fluorescent carbon nano materials with least toxicity, stable high luminous intensities and broad emission spectra with outstanding resistance to photo bleaching and photo blinking [20][21][22] , making them excellent phosphor materials for WLEDs. Recently several reports have been published on the fabrication of carbon dot based WLEDs involving tri color WLEDs with CRI 92.9 and 81.1 with CIE color co-ordinate (0.3305, 0.3320) and (0.4046, 0.4028) respectively 23 , tri-color CD-WLED with CRI 92 and CIE (0. 33, 0.34) 24 , orange emissive CD-WLED with CIE (0.41, 0.45) 25 , Orange emissive carbon dot phosphors for WLEDs with CRI 91 and CIE (0.41, 0.39) 26 , tri-CD films integrated with UV-LED chip based WLED with CRI of 96.5 and CIE (0.362, 0.370). These tri-color CD based WLEDs suffer from self-absorption losses and degradation of CDs with time, thus affecting CRI and quality of white light generated from these WLEDs.
In this paper we report in detail, one step fabrication of single system white light emitting CDs in open air atmosphere with quantum yields (QY) of 5 -13 % in solution phase. Further, solid state white light emitting CD phosphor has been successfully fabricated and a QY of 16% is obtained. Furthermore, the fabricated CDs and CD polymer phosphor show broad emission bandwidth of ~ 116-154 nm, one of the highest reported so far for single white light emitting system 27,28 . This broad emission band leads to enhanced CRI of the phosphor 29 . The fabricated CD phosphor exhibited high quality white light with exceptional CRI ~ 96, the highest reported so far for single system white light emitting CD phosphor based WLED and CIE co-ordinates of (0.31, 0.33) that are akin to day light with CIE (0.32, 0.32). This work will set a platform for the fabrication of high CRI single system white light emitting carbon dot based WLEDs for artificial lighting systems.

Experimental details
Bare CDs are synthesized in open air atmosphere at temperatures as high as 250 °C using citric acid (1 g) as carbon precursor and octadecene (30 ml) as a non-coordinating solvent. Formation of bare CDs involve breakdown of citric acid molecules at higher temperatures accompanied by carbonization of carbon core (Fig. 1a). The fabricated CDs are purified using ethanol as a purifying agent and dispersed in chloroform for further analysis. Bare CDs have dangling bonds which leads to non-emissive trap states on the surface that decrease the luminescence intensity and QY of CDs. Henceforth, in order to enhance the emission characteristics and QY of CDs, CD surface is passivated using hexadecyl amine (HDA) as a surface functionalizing agent that provides −NH 2 groups to CD surface (Fig. 1b). In order to check the affect of surface functionalization, SFCD samples with varied degree of surface functionalization are synthesized in open air atmosphere at 250 °C by maintaining constant amount of citric acid (1 g), octadecene (30 ml) and varying HDA concentration from 1.5 to 4 g in the reaction mixture. SFCD samples with HDA/ Citric acid molar concentration ratio ranging from 1.2 (1 g citric acid + 1.5 g HDA), to 3.5 (1 g citric acid + 4 g HDA) are synthesized. HDA/Citric acid molar concentration 1.2, 1.8 and 3.5 are labelled as SFCD 1.2 , SFCD 1.8 and SFCD 3.5 respectively throughout this report. Formation of SFCDs involve breakdown of citric acid molecules accompanied by carbonization of carbon core followed by surface functionalization of CD surface.
In order to enhance the stability of colloidal CDs and to ease their use in device applications, they are sealed in transparent polymer to protect them from environmental moisture 30 . Proper choice of polymer for packing CDs is made, such that it enhances the emission intensity, QY 31 and quality of white light generated [32][33][34][35][36][37][38] . In this work CDs have been embedded in Polydimethylsiloxane (PDMS) polymer, considering its flexibility, high transparency, good dispersity [39][40][41][42] and compatibility with CDs. For the fabrication of CD-PDMS polymer phosphor, PDMS and SYLGARD 184 curing agent are mixed in 10:1 ratio and stirred continuously for about ~ 5 min in a vial until the constituents get thoroughly mixed. The mixture is then degassed in a vacuum desiccator for about 15 min until a clear transparent mixture is observed. To this mixture 2 ml of fabricated CDs dispersed in chloroform are added and the entire solution is heated at 90 °C until chloroform evaporates from the mixture completely. The mixture is then transferred to a prefabricated plastic mold and allowed to rest at ambient conditions for about 48 h. After 48 h, the plastic mold is removed, and the fabricated CD polymer phosphor is ready for further characterization.

Results and discussion
To explore the nanostructures, High Resolution Transmission Electron microscopy (HRTEM) and Selective Area Electron Diffraction (SAED) images are taken. HRTEM images confirm the formation of spherical CD nanoparticles ( Figure S5) with prominent lattice fringes of 0.32 nm observed both in bare CDs (Fig. 2a) and SFCDs (Fig. 2b) that correspond to (002) lattice planes of graphitic crystal structure. SAED images of CDs and SFCDs (inset Fig. 2a,b) exhibit spot diffraction pattern supporting highly crystalline nature of fabricated CDs. High magnification image of CDs showing hexagonal crystal structure in the fabricated CDs (Fig. 2c). Additional HRTEM images of bare CDs (Fig. 2d) and SFCDs (Fig. 2e) showing planar lattice fringes with d spacing of 0.17 nm, 0.21 nm, 0.29 nm and 0.32 nm that correspond to (004), (100), (101) and (002) planes of graphitic crystal structure. Furthermore, the quality of graphitic structure in bare CDs is investigated by estimating the ordered and disordered phases using Raman spectra (Fig. 2f) 43,44 . Peaks at 1302 cm −1 and 1441 cm −1 correspond to the D and G bands of graphitic structure. D band represent defects (disordered sp 3 hybridized carbon) and G band represents vibration of sp 2 hybridized carbon atoms (ordered sp 2 hybridized carbon) in graphitic structure. Intensity ratio i.e., I D /I G represents graphitization in fabricated CDs. It is observed that I D /I G ratio in fabricated CDs is ~ 0.86, which indicates that fabricated CDs have ordered graphitic structure with lesser number of defects. Obtaining Raman spectra for SFCDs was unsuccessful due to overlapping of strong fluorescence peaks with Raman signals.  In order to further investigate the CDs and SFCDs, their emission and absorption characteristics are studied. Absorption spectra of the CDs were recorded from 269 to 700 nm range. Bare CDs exhibit a broad absorption window ranging from 270 to 400 nm as observed in the absorption spectra (Fig. 5a). This broad absorption spectrum is a cumulative effect of electronic transitions between intrinsic carbon domains (π-π* electronic transitions) and between oxygenated surface groups on CD surface with intrinsic carbon domains (n-π* electronic transitions). Open air atmosphere synthesis of CDs favour oxygenation of CD surface that involve oxygen containing groups getting attached to CD surface supported by XPS and FTIR analysis. These oxygen containing groups (such as C=O, C−O) act as emissive trap sites enhancing the luminescent properties of these dots. However, SFCDs absorb from 270 to 500 nm, exhibiting an absorption peak at 363 nm which intensifies and becomes prominent with increase in degree of surface functionalization from SFCD 1.2 to SFCD 3.5 . This peak corresponds to electronic transitions (n-π* electronic transitions) between surface groups (−NH 2 created by HDA) and intrinsic carbon domain. In order to further investigate the nanoparticles, emission characteristics of bare CDs and SFCDs are studied. As fabricated nanoparticles emit white light when exposed to UV lamp (peak wavelength of 350 nm) (Fig. 1c,d). Considering this, bare CDs are excited at 350 nm excitation wavelength and emission spectra recorded. CDs exhibit broad emission spectrum (Fig. 5b) ranging from 360 to 700 nm with a bandwidth of 133 nm which is expected for white light emitting sources. SFCDs emit from 360 to 700 nm on excitation with 350 nm source. Emission bandwidth changes from 116 to 143 nm with increase in degree of surface functionalization from SFCD 1.2 to SFCD 3.5 . Both bare CDs and SFCDs show broad emission spectra due to the presence of different surface groups (C=O, C−O, N−H) which excite at different energies and emit correspondingly that favours bright white light emission under UV illumination (Fig. 1c,d). Emission spectra of SFCD 3.5 is red shifted by 10 nm when compared to bare CDs (Fig. 5b). The red shift is accounted to surface states (surface oxygenation, surface functionalization) and size (carbonization) effects. Increase in degree of surface functionalization of CD surface leads to an increase in electron cloud around CD which increases the electronic conjugation of lone pair of electrons from the surface groups with π orbitals of carbon core. Increased electronic conjugation of lone pair of electrons from the surface groups with π orbitals of carbon core decreases the energy gap between n (state due to surface groups) and π * (carbon core anti bonding orbital) electronic states. Additionally, in SFCDs, increased degree of carbonization of CD leads to increase in electron cloud in CD core that increases the electronic conjugation of π-π * orbitals of carbon core. Increase in electronic conjugation of   www.nature.com/scientificreports/ π-π * orbitals of carbon core decrease the energy gap between π-π * electronic transitions. Thus, the red shift is attributed to a cumulative effect of functionalization and carbonization in SFCDs.
In order to have more insight into the functional groups attached to CD surface and the effect of degree of surface functionalization on carbonization of carbon core and surface oxygenation of CD surface, Fourier Transform Infrared Spectroscopy of bare CDs and SFCDs (Fig. 6a) is recorded. FTIR spectra of bare CDs reveal characteristic peaks of C=O stretching vibrations of conjugated acid at 1779 cm −1 , 1768 cm −1 and 1708 cm −1 , C=C stretching vibration at 1638 cm −1 and a broad band of C−O stretching vibrations at 1218 cm −1 which support that bare CD surface is embellished with C=O and C−O surface groups as indicated by XPS analyses. SFCDs show signature peak of N−H stretching vibrations at ~ 3436 to 3218 cm −1 , C=O stretching vibration of primary amide at 1698 cm −1 and a band of CONH stretching vibrations at 1233 cm −1 . With increase in surface functionalization (SFCD 1.2 to SFCD 3.5 ) an increase in the peak width and intensity of N−H stretching vibrations (Fig. 6b) and an increase in the peak intensity of C=O stretching vibrations (Fig. 6c) is observed. This increase in peak intensities upon surface functionalization support increase in number of emissive sites as observed in absorption spectra of SFCDs (also observed in XPS analyses). Peaks observed in bare CDs and SFCDs at 2918 cm −1 , 2847 cm −1 correspond to C−H stretching vibrations and peaks at 1468 cm −1 , 908 cm −1 correspond to C−H bending and C=C bending modes respectively. However, in SFCDs an increase in the intensity of C=C stretching vibrations is observed compared to bare CDs which support an increase in degree of carbonization of carbon core upon surface functionalization (Fig. 6d). Henceforth, functionalization of CD surface increases the number of surface emissive states on CD surface and intrinsic carbon emissive states in CD core which further enhances the quality of CD emitting white light.
Further Quantum yield measurements of the fabricated CDs and SFCDs are taken to check the quality of fabricated CDs. It is observed that bare CDs exhibit a QY of ~ 5% and SFCD 1.2 exhibit a QY of ~ 8%, SFCD 1.8 exhibit a QY of ~ 12% and SFCD 3.5 exhibit a QY of ~ 13%. Surface functionalization of CD surfaces passivates the non-emissive surface states and increases the number of emissive groups (such as −NH 2 , C=O) on CD surface, also an increase in number of C=C intrinsic carbon domains (supported by FTIR) which further enhances the QY of CDs.
In order to further enhance the application of these white light emitting colloidal nanomaterials, it is vital to embed them in a solid material. PDMS is the choice of solid material because of its good flexibility, good dispersity, compatibility with CDs and transparency to visible light. CDs embedded in PDMS have been fabricated and analysed for their optical properties (Fig. 7a). To investigate the luminescent behavior of CD polymer www.nature.com/scientificreports/ phosphor, absorption and emission spectra is recorded as shown in (Fig. 7e,f). The CD polymer phosphor shows a broad absorption band ranging from ~ 350 to 700 nm. Under 350 nm excitation, CD polymer phosphor produce very broad white emission band ranging from 360 to 700 nm with a bandwidth of ~ 154 nm (Fig. 7f). This CD polymer phosphor emits bright white light under UV illumination (Fig. 7c) with a quantum yield of ~ 16%. This enhancement in QY in CD-PDMS polymer compared to colloidal CDs (QY of 5%) is due to effective passivation of dangling bonds and non-emissive trap sites on CD surface by PDMS polymer matrix (for FTIR of CD-PDMS polymer phosphor please refer supplementary data ( Figure S4). Further, the quality of white light generated, and the efficiency parameters of the fabricated CD phosphor are examined as shown in Fig. 7b. It is observed that white light emitted from CD polymer phosphor show Commission Internationale de l'Eclairage (CIE) chromatic co-ordinates of (0.31, 0.33) which are very close to day light from sun with CIE chromatic co-ordinates of (0.32, 0.32), indicating that CD phosphor emit high quality pure white light. The emitted broad band white light emission of ~ 154 nm bandwidth exhibit a color rendering index (CRI) of ~ 96 the highest reported so far for single system white light emitting CD phosphor based WLED, making it an excellent phosphor for lighting systems that require very accurate color discrimination (An ideal artificial lighting device is supposed to have CRI > 80). Application of CD polymer phosphor has been tested and demonstrated by coating it on a UV LED (Fig. 7d). It is observed that CD polymer coated LEDs emit excellent bright white light. The demonstration successfully indicates that this CD polymer probably can be used in WLEDs. A detailed study on the important optical parameters (such as CRI, CIE, efficiency) of CD polymer based WLEDs will be a subject of further research.

Conclusions
In summary we report open air atmosphere synthesis of highly luminescent, single system white light emitting carbon dots and carbon dot phosphor for next generation white light emitting devices. Fabricated CDs emit white light with a Quantum yield of 5-13% in colloidal state by modifying the surface of CD fabricated. Further material and optical characterizations of the fabricated CDs reveal the effect of carbonization, oxygenation and surface modifying on the quality of white light emission from these dots. Furthermore, fabricated CD-PDMS polymer phosphor at 350 nm excitation show highly intense bright white light emission with a QY of ~ 16. White light emitted from CD polymer phosphor show chromatic color co-ordinates of (0.31, 0.33) with a color rendering index of ~ 96, the highest reported so far for single system white light emitting CD phosphor based WLED supporting its highest color purity, and quality of white light generated. Henceforth, the fabricated CD polymer phosphor finds its highest potential in fabricating eco-friendly high quality white light emitting devices.