Understanding of nitrogen-doped carbon nanoparticles based solid phosphors for white light emitting diodes

Abstract

The luminescence of carbon nanoparticles (CNPs) in solid form is believed to be quenched due to aggregation. Herein, we have reported the luminescence of N-doped CNPs (N-CNPs) in solution as well as in solid form. N-CNPs in solution exhibit excitation-dependent emission in solution and the luminescence intensity increases with nitrogen doping as well as with dilution. In the solid form, the luminescence is shown to be dependent on the power and follow a power dependency. The carrier recombination process are investigated in solid N-CNPs using a power dependent luminescence study. Finally, N-CNPs as a solid phosphor have been exploited for white light emission with CIE coordinates of (0.34, 0.34).

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1 Introduction

Phosphors that exhibit both downconversion and upconversion with high efficiency have drawn considerable attention in a variety of applications such as phosphors in white-light-emitting diodes (WLEDs), light harvesters in solar cells, tracking materials in bioimaging, gas sensing, etc.^1–10 One of the principles of WLEDs relies on wavelength conversion and a yellow phosphor is coated over a blue LED to generate white light. Ultraviolet (UV) light based WLEDs can also be achieved and semiconductor quantum dot (QD) based phosphors are being exploited. Unfortunately, QDs consist of toxic substances like lead or cadmium,¹¹ and it is required to develop non toxic new phosphor materials for efficient WLEDs.

Carbon nanoparticles (CNPs) are a new type of nanomaterials¹² that emit visible light and the quantum yield (QY) is reported to be as high as 95% for N-doped CNPs (N-CNPs),¹³ which is comparable with best inorganic semiconductors.¹⁴ However, luminescence of N-CNPs is well-known to be quenched in the solid form which is believed to be due to aggregation,^15,16 though report exists on the luminescence of CNPs in solid form also. Usually, the CNPs are embedded in a polymer matrix for the white light applications.

As luminescence of CNPs and N-CNPs in solid form is very much essential for light-emitting phosphors,^17,18 it is, therefore, prime importance to investigate the luminescence in solid form and understand the possible origin of recombination process. In this letter, we report the luminescence of N-CNPs in solution as well as in solid form. Effect of N at% on luminescence QY has also been reported. The N-CNPs in solution exhibit intense bluish-white emission when exposed to UV light and the luminescence intensity increases with dilution. The QY increases with the N at% and is found to be comparable with broad-luminescent semiconductor nanostructures. One of the interesting observations is that N-CNPs also exhibit luminescence in the solid form that depends on the power of the excitation and sheds light on the recombination process. Finally, we have demonstrated the white light emission from the UV LED coated with N-CNPs embedded in poly vinyl pyrrolidine (PVP) polymer matrix.

2 Experimental section

2.1 Materials

Coffee powder was purchased from Dharshan Pvt and Millipore water was in all experiments.

2.2 Synthesis

Herein, we adopted a hydrothermal method for the preparation of N-CNPs with different N at% (Fig. S1†) from the filter coffee powder by varying reaction time.¹⁹ Briefly, 0.8 g of filter coffee powder was mixed in 40 mL de-ionized water. The resultant mixture is then transferred into 50 mL Teflon-lined stainless steel autoclave and heated at 200 °C for different durations in a muffle furnace. After naturally cooling to room temperature, by using centrifugation, the settled black solid at the bottom of the Teflon flask was separated from the yellow solution of N-CNPs. The prepared N-CNPs at 3, 6, 9 and 12 h are referred as D3, D6, D9 and D12, respectively with different N at% (Table 1).

Table 1 Samples prepared from coffee powder and the variation of C, O, N at% and the QY

Sample	Time (h)	Diameter (nm)	C at%	O at%	N at%	N/C at% ratio	Quantum yield, ΦN-CNPs (%)
D3	3	2.8	75.5	20.5	4.0	4.9	4.6
D6	6	3.2	73.5	21.7	4.8	5.4	4.9
D9	9	3.9	72.8	22.2	5.0	6.0	6.6
D12	12	4.2	68.7	24.5	6.8	8.0	13.7

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For N-CNPs/PVP film preparation, 100 mg of PVP in 10 mL of N-CNPs solution is mixed followed by stirring for 30 min. Then the solution is dropcasted on UV LED and dried under incandescent lamp that leads to the formation of N-CNPs/PVP film.

2.3 Characterization

N-CNPs are characterized by Hitachi U-2900 UV-visible spectrophotometer. Horiba Jobin Yvon fluoromax-4 spectrofluorometer with Xe lamp is used for downconversion as well as upconversion PL measurements. For upconversion PL measurements; excitation wavelength range 950–1100 nm and emission wavelength range 450–700 nm. Optical characteristics such as color rendering index (CRI), Commission Internationale de l’Eclairage (CIE) chromaticity coordinates, correlated color temperature (CCT), and luminous efficacy were calculated using an integrating sphere (Avantes Avaspec-2048 spectrometer) under a forward bias of 30 mA (applied to UV LED). WITec system with 355 nm excitation laser is employed for the photoluminescence (PL) studies.

2.4 Quantum yield (QY) calculation

For the estimation of QY, quinine sulphate (QS) is dissolved 0.05 M H₂SO₄ solution and as-prepared N-CNPs are diluted with water for optimum luminescence intensity. An UV-visible Hitachi 2900 absorption spectrometer is used to record absorption values of QS and N-CNPs, and Horiba Jobin Yvon fluoromax-4 spectrofluorometer is used to record PL at 365 nm excitation at room temperature. The QY of N-CNPs is calculated by using below equation

Φ_N-CNPs = Φ_QS(θ_N-CNPs/θ_QS)(η_QS/η_N-CNPs)(α_QS/α_N-CNPs),

where Φ_N-CNPs and Φ_QS are quantum yield of N-CNPs and QS and θ, η, α are area under PL curve, refractive index of solvent, absorption value, respectively.

3 Results and discussion

To explore the optical properties, we performed the UV-visible spectroscopy studies on D3–D12 and we observed absorption bands between 200 and 350 nm (Fig. S2†), similar to GQDs.²⁰ The peaks between 200 and 280 nm are due to π–π* transition of C=C while the peaks between 280 and 350 nm correspond to n–π* transition of C=O functional groups present at the edges. Due to incorporation of N atom in the carbon conjugated system with the increase of reaction time, peak at 280 nm (π–π* transition of C=C) is gradually decreasing. A detailed PL study of N-CNPs in solution has been carried out by using different excitation wavelengths at room temperature and the PL spectra of D12 are shown in Fig. 1a. It is evident that D12 (Fig. 1a) and D3–D9 (Fig. S3†) exhibit excitation-dependent luminescence which is due to different energy levels associated with various surface states^18,20,21 as is the case with GQDs and N-GQDs.²⁰ An increase in PL intensity (∼six times) is observed as the colloidal solution is diluted with water (Fig. 1b), which is due to minimization of the self-quenching of emitted light among N-CNPs.²² With further increase in dilution, the concentration of luminescence centers (N-CNPs) decrease, thereby the intensity decreases²³ as shown in Fig. 1c. At a dilution of 30 mL, D12 exhibited bright cool white luminescence under 365 nm illumination as shown in the insets of Fig. 1c. Unlike similar size of 2–5 nm green luminescent GQDs²⁴ and blue luminescent N-GQDs,²⁵ our D12 emits intense bluish white luminescence. It may be noted here that blue luminescence is due to the intrinsic states, while the green luminescence is due to oxygeneous group²⁶ and bluish white luminescence is obtained when exposed to UV light. Bright cool white light is also obtained when a UV light emitting diode (LED) is dipped in as-prepared as well as in diluted D12 as shown in Fig. 1d and e.

Fig. 1 (a) Excitation-dependent PL spectra of as-prepared D12 and (b) PL of diluted D12 at an excitation wavelength of 365 nm. (c) PL intensity as a function of dilution. Insets show the photographs of D12 with UV (365 nm) illumination. (d and e) As-prepared and diluted D12 exposed to a UV LED. (f and g) PL spectra of D3–D12 at 365 nm excitation with optimum dilution and normalized PL of (f).

For comparison of luminescence intensity for D3–D12, the solutions are diluted to obtain optimum intensity. As the particle size increases from D3 to D12, a red shift was expected. Interestingly, it is found that the PL intensity increases (Fig. 1f and g) and an emission peak blue-shifts from D3 to D12. Therefore, the enhancement in the intensity and the blue-shift are attributed to the increase in the N-doping concentration.²⁷ We evaluate the QY by taking quinine sulfate as a reference. The QY is found to be 4.6, 4.9, 6.6, and 13.7% for D3–D12, respectively, which are higher than that reported for carbon quantum dots,^28–30 but lower than N-GQDs prepared from citric acid and EDA²⁸ and N-CNPs from glycine.³⁰ Overall, the QY increases with N at% (inset of Fig. 1g) due to formation of new surface states labelled as N-states which trap the electrons thereby facilitating the high yield of radiative recombination,¹³ and is comparable to that for semiconductor nanostructures.^31–38

Now, we examine the upconversion PL as it is an important feature and it was observed frequently in CNPs and GQDs when excited by Xe lamp^1,39 or lasers^40–45 as excitation sources. In addition to normal PL, N-CNPs (D3–D12) exhibit excellent excitation-dependent upconversion, which is similar to downconversion behavior (Fig. S4a–d†). As shown in Fig. S4,† the upconverted emission peak of D3–D12 is red-shifted as the excitation wavelength changes from 950 to 1100 nm and is similar to CNPs.^42,43 Interestingly, all the N-CNPs exhibit a linear relation between the energy of upconverted emission (E_m) and excitation (E_x) as shown in Fig. 2. The fitting of the experimental data yields E_m = 1.0E_x + δE with δE = ∼1.0 eV (Table 2). This clearly indicates that the energy δE difference between π and σ orbitals is ∼1.0 eV that is in excellent agreement with earlier reports.^40,46,47

Fig. 2 Excitation energy versus emission energy of upconversion.

Table 2 The parameters obtained by fitting the experimental data represented in Fig. 2

Sample	Slope	Intercept (δE)	R^2
D3	1.01	0.98	0.9960
D6	0.99	1.05	0.9985
D9	0.98	1.07	0.9972
D12	1.03	1.04	0.9980

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It is well known that the luminescence of N-CNPs in solid form is quenched due to aggregation. Here, we have shown that luminescence depends on the laser power that sheds light on the carrier recombination mechanism. The PL spectrum of D12 (coated on a glass slide) with different laser power is shown in the Fig. 3a. As shown in the Fig. 3b, the variation of integrated PL intensity (I) with different laser power (P) follow a power law,^48–50

I ∼ P^α

where α is a power factor, value of α lies between 1 and 2 i.e. 1 < α < 2 for bound exciton emission, and 0 < α < 1 for free to bound radiative recombination.⁴⁹ It can be noted from the Fig. 3b that the α = 0.14 for P < 0.22 mW which indicates free hole and donor recombination (h, D). However, α = 01.43 for P > 0.22 mW indicating the dominance of donor bound exciton emissions due to the formation of more number of excitons. As the laser power is increased from low to high, sudden increase in the number of radioactive recombinations and the minimization of absorption process results in the exponential increase in the intensity.

Fig. 3 (a) PL spectra of N-CNPs with different laser power, (b) log–log plot of integrated PL intensity as function of laser power. The excitation of the laser is 355 nm.

For practical applications, it is essential to evaluate the luminescence in solid state. In order to do so, N-CNPs drop-casted onto a glass slide and dried. The PL spectrum of the drop-casted film is shown in Fig. 4a and the optical photograph is shown in the inset. The optical photograph appears to be bright which clearly suggests that N-CNPs can be used as white light phosphors. Though the PL studies of the drop-casted film are carried out with 355 nm excitation, we choose UV LED that emits around 385 nm because of its availability. The emission spectra of UV LED and N-CNPs coated UV LED are shown in Fig. 4b. Similarly, the emission spectra of UV LED without and with N-CNPs and PVP film are shown in Fig. 4c. These results confirm that the solid film can convert UV light into visible light effectively and can be made suitable for white LED applications. The 1931 Commission Internationale l’Eclairage (CIE) diagram with coordinates for solid N-CNPs film (inset of Fig. 4a), UV LED and thin coating of N-CNPs on UV LED (inset of Fig. 4b) are (0.24, 0.29), (0.23, 0.20) and (0.41, 0.46), respectively. Interestingly, it was found that CIE coordinates for thin and thick coating of solid film of N-CNPs in PVP polymer on UV LED (Fig. 4d) are (0.34, 0.34) and (0.34, 0.35) which are close to that of pure white light (0.33, 0.33) and the corresponding cool white color is desirable for fluorescent lamps.⁵¹ Correlated color temperature (CCT), color rendering index (CRI) and luminous efficacy (LE) are found to be 5175 K, 84.7, and 39.5 lm W⁻¹, respectively, for WLED (fabricated by coating N-CNPs/PVP on UV LED) with CIE color coordinate of (0.34, 0.34). Compared to the UV LED coated with mixture of N-CNPs and PVP, UV LED coated only with N-CNPs is displaying emission in the yellow region (inset of Fig. 4b) which is believed to be due to aggregation and strong reabsorption.

Fig. 4 (a) PL spectrum of N-CNPs film. Insets show the optical photograph and CIE diagram. (b) The luminescent spectra of UV LED and N-CNPs coated UV LED. The emission wavelength of UV LED is 385 nm at 3.5 V. Insets show the CIE diagram of UV LED and N-CNPs coated UV LED. (c) The luminescent spectra of UV LED without and with N-CNPs and PVP film. Insets show the optical photographs of UV LED coated with N-CNPs and PVP film (d) CIE diagram of UV LED and UV LED coated with N-CNPs and PVP films.

4 Conclusions

In summary, we have reported the luminescence of N-CNPs in solution as well as in solid form. The aqueous solution of N-CNPs exhibit excitation-dependent luminescence which is due to different energy levels associated with various surface states. Interestingly, bluish-white emission is observed when N-CNPs solution exposed to UV light and the intensity is enhanced with nitrogen concentration and dilution. The QY increases with the N at% and is found to be comparable to that for broad-luminescent semiconductor nanostructures reported in the literature. We show that the luminescence is due to the hole and donor recombination (h, D) at low power of excitation and the dominance of donor bound exciton emissions at high power. Finally, we demonstrate white light emission with CIE coordinates (0.34, 0.34) when an UV LED is coated with N-CNPs and PVP film.

Acknowledgements

The authors acknowledge Department of Science and Technology (DST) and Defence Research and Development Organization (DRDO) for the financial support. The authors also acknowledge I.I.Sc for the financial assistance and Satish Laxman Shinde for his help in the PL studies of solid N-CNPs.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: More structural characterization, XRD, XPS, upconversion PL information. See DOI: 10.1039/c6ra11208k

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