Refractive-Index Tuning of Highly Fluorescent Carbon Dots - ACS

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Refractive-Index Tuning of Highly Fluorescent Carbon Dots Vijay Bhooshan Kumar,*,†,∥ Amit Kumar Sahu,‡,∥ Abu S. M. Mohsin,‡ Xiangping Li,‡,§ and Aharon Gedanken*,† †

Department of Chemistry, Bar-Ilan Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 5290002, Israel ‡ Centre for Micro-Photonics, Swinburne University of Technology, John Street, P.O. Box 218, Hawthorn, Victoria 3122, Australia § Institute of Photonics Technology, Jinan University, Guangzhou 510632, China S Supporting Information *

ABSTRACT: In this manuscript, we report the refractive-index (RI) modulation of various concentrations of nitrogen-doped carbon dots (N@C-dots) embedded in poly(vinyl alcohol) (PVA) polymer. The dispersion and size distribution of N@C-dots embedded within PVA have been investigated using electron microscopy. The RI of PVA-N@Cdots can be enhanced by increasing the doping concentration of highly fluorescent C-dots (quantum yield 44%). This is demonstrated using ultraviolet−visible (UV−visible), photoluminscence, Raman, and Fourier transform infrared (FTIR) spectroscopy measurements. The Mie scattering of light on N@C-dots was applied for developing the relationship between RI tuning and absorption cross section of N@Cdots. The extinction cross section of N@C-dot thin films can be rapidly enhanced by either tuning the RI or increasing the concentration of N@ C-dots. The developed method can be used as effective RI contrast for various applications such as holography creation and bioimaging. KEYWORDS: C-dots, N@C-dots, PVA, refractive index, Mie scattering photovoltaic cells,21 and organic light emitting diodes (OLEDs).22 Therefore, hybrid materials with a high RI are required for photonic devices.23,24 The efficiency or image quality of a device can be enhanced by effectively guiding more light transiting between the high and low RI value. The strategy for increasing the polymer RI affects the fabrication of these composites with different kinds of additives, such as organic compounds, metallic salts and acids, cations of heavy metals, and nanoparticles.25−28 There is plenty of research on RI modulation of polymer metals29 and metal oxide nanocomposites,30 but in most of the cases the light illumination was not uniform. C-dots can be used as additives for improving the optical activity of the host hybrid material.31,32 This is due to their capability of showing unique quantum electrooptical effects.33,34 It is suggested that the choice of doped C-dots is one of the most important steps for tuning the RI of polymers, by exploring the optical activity of hybrid materials. PVA is an optically active transparent material consisting of several positive and negative groups that can help to align the three different configurations of PVA with [email protected] This is motiviated by the capability of PVA to permit the

1. INTRODUCTION Carbon is a well-known material. When nanosized carbon structures are prepared, their properties become obviously different from the classical bulk carbon. Carbon nanomaterials such as fullerenes,1 carbon nanotubes,2 and graphene3 have attracted significant attention due to their unique chemical and physical properties. Among the carbon-based materials, carbon dots (C-dots) are one of the best types of fluorescent nanomaterials, owing to their superior solubility in water,4 excellent biocompatibility5 and optical property,6 chemical inertness,7 neglectable cytotoxicity,8 and capacity for functionalization with different chemicals. Therefore, highly fluorescent C-dots have acquired increasing research attention in the past few years.9−11 C-dots are quasi-zero-dimensional nanomaterials, which have less than 10 nm size in all three dimensions. Polymer and carbon nanomaterials have a significant role in optics-based applications due to several benefits, such as being cheaper, easy handling, and easy modifications into nanocomposites. The polymers have lots of applications and can be used for making polymer C-dot nanocomposites for photonic devices.12,13 There are several methods for the development of two-dimensional and three-dimensional photonic crystals in PVA, such as the two-photon polymerization method12,14−18 and the laser-induced microexplosion method.16−18 However, organic polymers exhibit a smaller RI13 for applications in optical devices, as compared to optical data storage,19,20 © 2017 American Chemical Society

Received: June 22, 2017 Accepted: August 10, 2017 Published: August 10, 2017 28930

DOI: 10.1021/acsami.7b08985 ACS Appl. Mater. Interfaces 2017, 9, 28930−28938

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Representation of the Fabrication of PVA@N@C-dot Nanocomposites and the Corresponding Hydrogen Bonding Interaction between PVA and N@C-dots Leading to Cross-Linking of PVA Chains To Form the Three-Dimensional PVA@N@ C-dot Film

stirring for 2 h to obtain homogeneous mixing. In each case, a transparent light-yellow slightly viscous solution was obtained. The corresponding hybrid materials were termed PVA-N@C-dots and PVA-C-dots. The pH levels of the PVA solution and the aqueous solution of N@C-dot nanoparticles were kept at similar levels and were maintained between 6 and 8 to obtain more stable nanocomposites. The prepared composite solutions were used for making a thin film of PVA-N@C-dot nasnocomposites by the drop-cast method, and the solvent was evaporated at ambient temperature. The thickness of the prepared films on the glass slide was controlled to be ∼30 μm. The content (by weight) of the embedded C-dot nanoparticles in the nanocomposite films was determined through UV−vis absorption spectroscopy (AAS). A Bruker diamond FTIR spectrometer was used to record the IR spectra of PVA and its nanocomposite films. 2.4. RI Measurements. In order to determine the RI n(k) values after different concentrations of dots were embedded into the polymer, the thin films with and without embedding of C-dots on the roughly 30 μm thick film were deposited on the glass slide, and the roughness of the film was found to be around 30 nm. A spectroscopic ellipsometer was used to record the data for different concentrations of N@C-dots embedded into PVA polymer, and the optical properties were fitted by modeling with a Cauchy function. 2.5. Characterization Techniques. UV−vis absoption spectra of PVA-N@C-dots were measured using a Cary 100 spectrophotometer (Varian Cary Eclipse, Lab-sphere softwere). The photoluminescence of PVA-N@C-dots was analyzed by a fluorometer (Varian Cary Eclipse). The chemical nature of PVA-N@C-dots was recorded using the FTIR technique (Bruker TENSOR 27, platinum ATR F tip) between 4000 and 400 cm−1. The morphology of PVA-N@C-dots was analyzed by high resolution transmission electron microscopy (HRTEM, JEOL 2100 microscope) that was operated at 200 kV (accelerating voltage). The TEM samples of PVA-N@C-dots were prepared by dispersion of PVA-N@C-dots (200 μL) in isopropanol solution (800 μL) using the bath sonication, and a few drops of the resultant mixture were poured on a TEM Cu grid and then dried in vacuum for 12 h. The Raman spectroscopy measurements of the PVAN@C-dots were performed with a Jobin-Yvon Labram spectrometer. Ellipsometry (nanofilm Ep4) was used to determine the thickness of the films and to measure the RI using polarized light, which is shined on the surface and reflected and received by the detector.

homogeneous distribution of N@C-dots, serving as an excellent stable system for embedded N@C-dots. Moreover, since the N@C-dot-PVA composites reveal a higher quantum yield (QY) as compared to C-dots, as well as a high optical transparency and a good crystallinity, it was chosen as the material for our studies. The goal of our investigation was to develop new active optical materials for RI modulation, namely, PVA modified with N@C-dots, and to analyze their optical behavior. Motivated by the unique physical and chemical properties of C-dots/N@Cdots, we believed that the presence of N@C-dots in PVA matrices could improve their RI properties. To the best of our knowledge, no investigation has been conducted on the impact of N@C-dots as RI modulators. Mie scattering was performed on N@C-dots in order to determine a relationship between RI tuning and absorption by N@C-dots, and our calculations yielded values close to the experimental data.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Polyethylene glycol (PEG-400), bovine serum albumin (BSA, purity 99.9%), and poly(vinyl alcohol) (PVA) were obtained from Sigma-Aldrich Ltd., Israel. These chemicals were used without further purification. Deionized water was used in all the experiments. 2.2. Formation of C-dots and N@C-dots. C-dots were produced by sonication, as described in our previous reports.36,37 The N@Cdots were prepared from an aqueous solution of BSA according to a previous report.38 The resultant yellow transparent C-dots/N@C-dots suspension was analyzed by various physiochemical techniques and subsequently exploited for RI modulation. 2.3. Preparation of PVA-N@C-dot and PVA-C-dot Nanocomposites. The nanocomposites of PVA-N@C-dots and PVA-Cdots were prepared through vigorous stirring of the transparent aqueous suspension of N@C-dots at varying concentrations in PVA solution. Briefly, 0.16−0.8 g of PVA was added to 5 mL of water in a small beaker. The PVA−water mixture was kept at continuous stirring at 60 °C for 2 h to obtain a transparent aqueous solution. This transparent PVA solution was cooled to 30 °C, and an amount of 5 mL of an N@C-dot aqueous solution was added under continuous 28931


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3. RESULTS AND DISCUSSION 3.1. Fabrication of Functionalized PVA-N@C-dot Hybrid Nanocomposites. Three different hybrids of PVAC-dots/N@C-dots were fabricated. The surfaces of C-dots/ N@C-dots have different functional groups (−COOH, − COOR, −OH, CO, −NHR) offering several chemical modifications of the surface reactions. Scheme 1 shows a possible protocol for the synthesis of functionalized PVA-Cdots/N@C-dot hybrid nanocomposites. Furthermore, it also shows the chemical interactions via hydrogen bonding between the PVA long chain and different surface functional groups of C-dots/N@C-dots in the three-dimensional structures. 3.2. Physical Charecterizations. Fluorescence, XRD, and TEM studies of the functionalized C-dot/N@C-dot−PVA hybrid nanocomposites were carried out to confirm the C-dots and N@C-dots embedding in PVA. HRTEM results (see electronic Supporting Information Figure S1a) demonstrate that the size of the C-dots is around 4−8 nm, whereas the N@ C-dots are around 3−7 nm (Figure S1b). The photoluminescence of C-dots/N@C-dots was recorded at several excitation wavelengths from 330 to 490 nm, and the emission was detected from 425 to 590 nm. The fluorescence spectra of C-dots and N@C-dots are shown in Figures S1c and S1d, respectively. Detailed fluorescence and HRTEM analyses of N@C-dots were reported in our previous paper.38 HRTEM micrographs reveal that both C-dots and N@C-dots are highly crystalline with d-spacing around 0.210 and 0.215 nm, respectively. To analyze the optical design of N@C-dots, we added the PVA to stabilized films of N@C-dots and cerated the patterning. Furthermore, the values of d-spacing estimated from both the HRTEM and the selected area electron differaction (SAED) pattern were 0.215 nm. After embedding these nanoparticles with varying concentrations into PVA, the recorded TEM images (Figure S2) depict their almost uniform distribution in PVA. Furthermore, no change in the size of the N@C-dots was observed after dispersing them into PVA, confirming their stability after embedding inside the host matrix. A pale yellow color was observed for the synthesized C-dots, N@C-dots, PVA-C-dots, and PVA-N@C-dots under daylight, while under UV light (365 nm) it apears whitish blue (Figure 1). Figure 1e and Figure 1f show the fluorescence of films with 30 μm thickness of N@C-dot-PVA nanocomposites in daylight and UV light, respectively. It is observed that there is no change in the fluorescence of the films and the solutions. This proves that the N@C-dots are stable in the PVA-N@C-dot nanocomposites. The C-dots and N@C-dots gave similar results, but drying of PEG-400 from C-dots was difficult. Therefore, the patterning and RI measurements were continued with N@C-dots. The samples were also analyzed by fluorescence spectroscpy. Figure 2a presents the fluorescence spectra of N@C-dots at various excitation wavelengths, including 350, 370, 390, 410, 430, 450, and 470 nm. The excitation-dependent emission was observed between 460 and 490 nm. When the N@C-dots were mixed with the PVA, a similar fluorescence pattern was observed (Figure 2b). The aqueous solution of PVA alone shows no fluorescence (Figure 2c). The UV−visble spectra (Figure 2d) also show the same absorbance peak for N@C-dot and PVAN@C-dot nanocomposites. Both spectroscopies reveal that there were no changes in the chemical structure of the N@Cdots. The UV−vis absorption spectra of the N@C-dots are

Figure 1. (a) C-dots and PVA C-dots in daylight and (b) UV light. (c) N@C-dots and PVA-N@C-dots in daylight and (d) UV light. (e) Film of N@C-dot-PVA nanocomposites after drying in daylight and (f) in UV light.

similar to the PVA nanocomposites and show that the N@Cdot excitonic absorption strongly depends on their mixing ratios with PVA (Figure 2e, Figure 2f). Our results demonstrate that the N@C-dot-to-PVA ratio changes the absorbance intensity without shifting the peak wavelength. When the mass ratio of the PVA and N@C-dots is around 1:2, the N@Cdots possess the maximum absorption. FTIR spectroscopy analysis of well-dispersed PVA and PVAN@C-dot nanocomposites was conducted to reveal the chemical and molecular interactions of PVA and N@C-dots. The surface functional groups of the N@C-dots (−OH, −COOH, −NH2) at the circumference can create hydrogen bonding with the PVA matrix. The FTIR measurements for pure PVA and PVA-N@C-dot nanocomposites are presented in Figure 3. Absorption peaks of PVA (Figure 3a) were observed at about 3247.5 cm−1 (−OH stretching) and at about 1082 and 1429 cm−1 for the −CO group. The PVA film was transparent and showed a similar FTIR spectrum after drying (Figure 3b). The PVA spectrum shows a broad absorption band at 3080−3630 cm−1, which is assigned to the −OH symmetrical stretching vibration. Moreover, the characteristic absorption peaks of N@C-dots at 3285 cm−1 are due to the surface −OH functional group, and the peak at 2916 cm−1 is attributed to the CH stretching vibration (Figure 3c). The broad band at 1668 cm−1 is assigned to deformation vibration of the water molecules adsorbed on the N@C-dot particles. The characteristic absorption bands at 1668 and 1082 cm−1 can be attributed to the stretching vibration of amino functional groups present in N@C-dots. These IR peaks suggest that the as-synthesized N@C-dots are rich in −OH, −CO, −NH2, and −COOH groups on the surface, which make them soluble in aqueous medium or other polar solvents. Figure 3d−f present the spectra of the PVA-N@C-dot nanocomposite film. The −OH stretching band is very sensitive to hydrogen bonding. Therefore, the −OH stretching band of PVA-N@Cdots shifts to a higher wavenumber as compared with pristine PVA. The CO stretching band at 1672 cm−1 is more intense in the PVA-N@C-dots than in the N@C-dots, indicating the 28932


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Figure 2. (a) Fluorescence spectra of N@C-dots. (b) Fluorescence spectra of PVA-N@C-dots. (c) Fluorecence spectra of PVA. (d) Absorbance spectra of water, PVA, N@C-dots, PVA-N@C-dots. (e) Absorption spectra of PVA-N@C-dots with different ratio of wt %. (f) Magnified portion of part e from 300 to 400 nm.

changes in PVA-N@C-dots clearly suggest the possibility of N@C-dots attached to OH groups in the side chain of PVA, leading to an increase in conjugation between PVA chains. This conjugation may cause the increased density of PVA and thus induces the increase in RI with increasing concentrations of embedded N@C-dots. Raman analysis was also used to find out the chemical interaction between the PVA and surface functional groups of N@C-dots. Figure 4 illustrates the Raman spectroscopy of the

formation of H-bonding between the PVA and N@C-dots. The H-bonding interaction explains the uniform distribution of N@ C-dots in the PVA matrix. However, the band intensity at 3285 cm−1 decreases with an increase in concentration of the N@Cdots, which can explain the scarcity in the planar configuration of PVA due to formation of H-bonds between PVA and N@Cdots. This phenomenon may lead to an abundance of delocalized electrons for enhancing the physical properties such as thermal, optical, and electrical properties.31,39 The slight 28933


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obtained after lyophilization. Figure S3a shows the daylight images of a solution of N@C-dots, and Figure S3b shows the same powder under UV light. This phenomenon reveals that the N@C-dots have high stability, which is sustained even after lypholyzation. The same powder material was analyzed by the XRD technique and is depicted in Figure S4. In this figure, the XRD pattern demonstrates a peak at 2θ = 22°, which is due to the graphitic nature of carbon.40 3.3. RI Modulation. The RI values of PVA-N@C-dot nanocomposite films at different filler concentrations in pure PVA were calculated as a function of wavelength, and the results are presented in Figure 5. The RI (n) values of N@Cdots are shown in Figure 5a with different concentrations of PVA polymer matrix, based on the UV−visible spectroscopy analysis, whereas Figure 5b shows similar measurements performed using an ellipsometer at wavelengths of 400 and 700 nm. It is clear from Figure 5b that the RI decreases initially with an increase in wavelength and after 500 nm roughly saturates for pure N@C-dots and PVA films, which is explaned by the normal dispersion behavior above 500 nm. The synthesized PVA-N@C-dots were fabricated on the glass slide as a thin film with thickness of 30 μm. Figure 6a shows a schematic representation of femtosecond laser patterning in N@C-dot dispersed PVA thin films, whereas Figure 6b shows large-area femtosecond laser patterning of PVA-N@C-dot thin films by spin coating, and Figure 6c shows the grating formation in PVA-N@C-dot thin films. A femtosecond pulsed laser (a pulse duration of 100 fs and repetition rate of 80 MHz) of 800 nm was focused on the Cdot-embedded thin films by an objective with NA of 0.7. The fluorescence of PVA-N@C-dots decreases with the laser power, which provides a mechanism for potential photonic applications in optical storage, patterning and holograms. By laterally translating the sample across the focus point, a grating of reduced PVA-N@C-dot nanocomposites with a period of 5 μm and a duty cycle of 1/2 was patterned at a power of 0.5 mW. These materials will be further used to measure the RI modulation by C-dot- and N@C-dot-based PVA film (thickness of 30 μm). The normal dispersion charecteristics obtained for the RI in this region can be elucidated on the basis of Sellmeier’s dispersion theory. 3.4. Theoretical Calculation. Calculation of the optical absorption and scattering cross section of C-dots embedded in PVA polymers was performed by employing Mie scattering. In

Figure 3. FTIR spectra for (a) pure PVA, (b) PVA film, (c) N@Cdots, and nanocomposite films of (d) PVA-N@C-dots (1:2 wt %), (e) PVA-N@C-dots (1:1 wt %), and (f) PVA-N@C-dots (2:1 wt %).

Figure 4. Raman spectra of (a) PVA film, (b) N@C-dots, and nanocomposite films of (c) PVA-N@C-dots (1:2 wt %), (d) PVA-N@ C-dots (1:1 wt %), and (e) PVA-N@C-dots (2:1 wt %).

as-synthesized dried N@C-dot and PVA-N@C-dot nanocomposites with different weight ratios. There are two major Raman peaks: the D band at around 1341 cm−1 is assigned to the sp3 carbon, and the band at around 1582 cm−1 is related to the carbon G band and stretching vibration of CC bonds (sp2). The intensity of the D and G peaks decreases with increasing PVA concentrations, which is expected as the relative amount of C-dots is reduced. A solid powder of N@C-dots was

Figure 5. (a) RI (n) values of N@C-dots at different concentrations of embedding into the PVA polymers were measured using an ellipsometer at wavelengths of 400 and 700 nm. (b) The relation of RI (n) to the wavelength for PVA, N@C-dots, and nanocomposite films of N@C-dots at different concentrations of embedding into the PVA polymers (1:2, 1:1, and 2:1 wt %). 28934


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mesh cells) to inject the fields because the fields are not physically meaningful within this region. Hence, the monitor should not be placed in this restricted region. The rate at which energy is removed from the incident plane wave, hence the net power flow into the particle, is considered as absorption cross section, which can be calculated by the analysis group located inside the TFSF source using an optical theorem. On the contrary, the net power scattered from the particle, hence the scattering cross section, can be calculated by another analysis group located outside the TFSF source. This group measures the net power scattered from the particle. Afterward, the absorption or scattering cross-section can be calculated considering the geometrical area πr2 and the size r parameter 2π λ . For higher accuracy, simulation mesh refinement was set to “conformal variant 1” to achieve subcell resolution followed by convergence testing. Mesh override mesh size was set to 0.5 nm. Simulation span was set to 2 μm to avoid the interaction of evanescent tails of the resonant surface plasmon modes with perfectly matched layer (PML boundary) conditions. To reduce the light reflections by PML layers, more PML layers were considered. Setting the X min boundary condition to symmetric and Z min boundary condition to antisymmetric, the simulation memory and time were improved by a factor of 4. To calculate the absorption cross section of the N@C-dot sheet, we used the Mie theory for an arbitrary shaped spherical particle that can solve Maxwell’s equation with the correct boundary conditions. The cross section depends on the radius of the nanosphere R, the vacuum number of the incident light k, and the dielectric function of the nanosphere, where εp and its surrounding εm are expressed as

Figure 6. (a) Schematic diagram of femtosecond laser patterning in N@C-dot dispersed PVA thin films. (b) Large area femtosecond laser patterning of carbon dot thin films. (c) Grating formation in PVA-N@ C-dot thin films.

our study, we employed the commercial finite difference time domain (FDTD) simulation software 8.15 to simulate the N@ C-dot sphere structure. In the layout editor we simulated objects, total field, scattered field, and polarization direction. The carbon dot sphere was simulated in the middle, a position that can be moved easily with the mouse. Surrounding this sphere, the yellow rectangular box is the total field, followed by total field scattered field source (white rectangular box) and scattered field (outer yellow rectangular box). The pink arrow shows the direction of propagation, the k vector. The blue dot represents the direction of the electric field (Figure 7).

2π k2

σscat = σext =

∞

∑ (2n + 1)(|an|2 + |bn|2 ) n=1

2π Re(an + bn) k2

(1)

(2)

The coefficients an and bn can be calculated from the following equation an =

mΨn(mx) Ψn(x) − Ψn(mx) Ψ′n (x) mΨ′n (mx) ξn(x) − Ψn(mx) ξ′n (x)

(3)

bn =

Ψ′n (mx) Ψn(x) − mΨn(mx) Ψ′n (x) Ψ′n (mx) ξn(x) − mΨn(mx) ξ′n (x)

(4)

Here, x =

Figure 7. Image of simulated carbon dot thin films.

εm kR is a size parameter where R is the radius of

the sphere and m =

To simulate the N@C-dot sphere, a total field scattered field (TFSF) source was utilized which surrounds the C-dot sphere. Two analysis groups, one in the total field region and the other in the scattered field region, were used. This analysis group provides absorption and scattering cross sections and angular distribution of scattered radiation. To calculate the electric field, an enhancement frequency profile monitor can be included. We used RI values of 1.55, 1.65, 1.7, and 1.8. The mesh override region was intentionally kept large enough to resolve the locations of N@C-dot interfaces accurately, especially for a curved surface and for TFSF sources (wavelength ranging from 400 to 1000 nm), which works best in uniform meshed regions. In addition, sources require a certain amount of space (∼2

εp εm

, where εp and εm are the dielectric

functions of the sphere and medium, respectively. Also, ψ and ξ are the Riccati−Bessel cylindrical functions of order n, and ψ and ξ prime indicate differentiation with respect to argument x. The summations of absorbed and scattered energy graphs are shown in Figure 8, known as the extinction energy. This phenomenon can be expressed as σabs = σext − σscat (5) whereas Figure 5 shows the experimental verification of PVAN@C-dot nanocomposites. Mie theory was employed to calculate the absorption cross section of the N@C-dots embedded in PVA polymers. Mie theory can give the exact analytical solution of arbitrary shaped particles, where particles 28935


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Figure 8. Calculated absorption (a), extinction (b), and scattering (c) cross sections of C-dots with a diameter of 20 nm.

the construction of channel waveguides, optical fibers, etc. This work provides a new technique for the measurement of the RI of N@C-dots and carbon-based materials. The demonstrated PVA-N@C-dot nanocomposites open up potential application in multimode optical recording, optical security, and ultrasecure encryptions.

are much smaller than the wavelength of incident light. In our study, we employed the commercial finite difference time domain (FDTD) simulation software 8.15 to simulate the N@ C-dot sphere. We observed the absorption of the N@C-dot sphere which increased from 0.1 to 0.9 times for the change of refractive index n = 1.55 to 1.88. We also find the absorption peak at a wavelength of 340 nm (Figure 8a), which was experimentally found at 350 nm in the UV−visible absorption spectrum (Figure 2f). The relative scattering contribution to the extinction can be increased by increasing the RI. This study points out that the RI of PVA can be regulated by embedding N@C-dots in the polymers, enabling their use in the construction of channel waveguides, optical fibers, etc. The unique character of PVA-N@C-dot nanocomposites at the subwavelength scale for the RI modulation, together with the photoluminescence property of N@C-dots, suggests a method for multimode optical recording and optical security applications.

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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.7b08985.

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TEM, HRTEM, fluoresce spectra, XRD of N@C-dots and PVA-N@C-dots (PDF)

AUTHOR INFORMATION

Corresponding Authors

*V.B.K.: e-mail, [email protected]. *A.G.: e-mail, [email protected]; phone, 972-3-5318315; fax, 972-3-7384053.

4. CONCLUSIONS In summary, a simple synthesis of PVA-N@C-dot nanocomposites using an ex situ technique was carried out. The RI behavior of these films was measured from UV−visible spectra by surface reflectance and transmittance. The obtained values of RI of N@C-dot-PVA nanocomposites increase in conjunction with increasing concentrations of N@C-dots embedded in PVA matrix. The measured values of RI of such nanocomposites have been found to be in close agreement with the values predicted from Lorentz−Lorenz effective medium theory. FTIR, UV, and Raman spectroscopies were used to analyze the structures of PVA and N@C-dots before and after embedding the dots in the polymer. The results suggest a possible reason for the changes in the RI in the nanocomposite films. This study points out that the RI of PVA can be regulated by embedding N@C-dots in the polymers, enabling their use in

ORCID

Vijay Bhooshan Kumar: 0000-0001-7899-1463 Author Contributions ∥

V.B.K. and A.K.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Ortal Lidor-Shalev for helping with the TEM measurements, Daniel Raichman for Raman measurements, and Merav Muallem for the HRSEM measurements, conducted at BINA at the Department of Chemistry in Bar-Ilan University, Ramat-Gan, Israel. 28936


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ABBREVIATIONS C-dots, carbon dots N@C-dots, nitrogen-doped carbon dots PVA, poly(vinyl alcohol) fwhm, full-width at half-maximum HRTEM, high-resolution transmission-electron microscopy FDTD, finite difference time domain TFSF, total field scattered field RI, refractive index

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ACS Applied Materials & Interfaces (40) Jiang, C.; Wu, H.; Song, X.; Ma, X.; Wang, J.; Tan, M. Presence of Photoluminescent Carbon Dots in Nescafe® Original Instant Coffee: Applications to Bioimaging. Talanta 2014, 127, 68−74.

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Refractive-Index Tuning of Highly Fluorescent Carbon Dots - ACS

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