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Near-ultraviolet excitation-based bluish-green emitting K2ZnSiO4: Eu2+ nanophosphors for white light-emitting applications
Accepted Manuscript Near-ultraviolet excitation-based bluish-green emitting K2ZnSiO4: Eu nanophosphors for white light-emitting applications
2+
L. Krishna Bharat, Sk Khaja Hussain, Jae Su Yu PII:
S0143-7208(17)30797-0
DOI:
10.1016/j.dyepig.2017.05.044
Reference:
DYPI 6006
To appear in:
Dyes and Pigments
Received Date: 11 April 2017 Revised Date:
23 May 2017
Accepted Date: 23 May 2017
Please cite this article as: Krishna Bharat L, Hussain SK, Yu JS, Near-ultraviolet excitation-based bluish2+ green emitting K2ZnSiO4: Eu nanophosphors for white light-emitting applications, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.05.044. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Near-ultraviolet excitation-based bluish-green emitting K2ZnSiO4: Eu2+ nanophosphors for white light-emitting applications *
L. Krishna Bharat, Sk. Khaja Hussain, and Jae Su Yu
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Department of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, Yongin-si, Gyeonggi-do 17104, Republic of Korea
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Abstract
The K2ZnSiO4: Eu2+ nanophosphor materials were synthesized via a sol-gel method. The
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morphology of the synthesized materials was examined by field-emission scanning electron microscope images. The structural properties were analyzed from X-ray diffraction (XRD) pattern and Fourier transform infrared spectrum. The K2ZnSiO4 host lattice crystallizes in a cubic phase, and the XRD pattern matches well with the standard JCPDS card values and does not any
impurity
peaks.
The
luminescence
properties
were
studied
using
the
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photoluminescence (PL) excitation and PL emission spectra. The broad band excitation in the near-ultraviolet (NUV) region and the broad band emission in the visible region make this
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phosphor material suitable for the application of NUV-based white light-emitting diodes. The thermal stability of the material was tested by calculating the activation energy value which was
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found to be 0.296 eV. The synthesized K2ZnSiO4: Eu2+ nanophosphor was then coated onto the NUV chip along with a red-emitting phosphor to get visible white-light emission with a color rendering index value of 81 and a correlated color temperature value of 5730 K.
Keywords: NUV excitation, Silicates, Bluish-green emission, White light-emitting diodes * Corresponding author: [email protected] (J. S. Yu)
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1. Introduction Currently, development of energy-efficient devices is spreading in each and every field including
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the solid-state lighting industry. Making of energy-efficient light-emitting diodes (LEDs) which can replace traditional incandescent lamps is believed as a major step for energy consumption worldwide.[1, 2] LED can only emit a single color,[3-5] on the other hand, white light covering the whole visible region (400-700 nm) is required for general illumination and lighting
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applications. Thus, to obtain white light emission, phosphor-converted LED (pc-LED) is a
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typical approach. The commercially available white LED (WLED) is an integration of blue chip and yellow light-emitting garnet material (Y3Al5O12: Ce3+; YAG: Ce).[6-8] However, these WLEDs give poor color rendering index (CRI) and correlated color temperature (CCT) values, which are not suitable for general illumination. To obtain better CRI and CCT values, the nearultraviolet LED (NUV-LED) chip with different color-emitting phosphors is a promising idea.
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The NUV-LED chips (350-420 nm) offer a better excitation efficiency which is nearly equal to that of the fluorescent lamps.[9, 10] In this context, present-day phosphor research mainly
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focuses on the synthesis of NUV excitable phosphor materials for the fabrication of WLEDs. Generally, different kinds of materials like oxides, nitrides, etc. have been chosen as host
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materials.[11, 12] Among all these materials, oxides are one kind of materials which are easy to be prepared and obtained at lower temperatures.[13, 14] Recently, silicate materials have been extensively investigated due to their thermal and chemical stabilities, air insensitivity, and water resistance.[15-17] So, we chose K2ZnSiO4 as the host material. Similarly, the choice of dopant material is also a prerequisite to get the desirable emission color. Europium is one of the widely used dopant materials which played a key role in the display and lighting fields due to its existence in distinct 2+ and 3+ oxidation states. Europium (Eu) in 3+ state gives strong and sharp
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peaks in the red and/or orange spectral regions,[18-20] and in 2+ state, it provides versatile broadband emissions in different crystal fields.[21-24] Up to now, there were only few reports found on the synthesis and the study of luminescent properties of rare-earths doped
doped K2ZnSiO4 and its application for WLEDs.
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K2ZnSiO4.[25, 26] To the best of our knowledge, there was no report on the study of Eu2+ ions
In this work, the sol-gel synthesis of K2ZnSiO4: Eu2+ nanophosphors and the study of
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structural and optical properties were reported. The K2ZnSiO4: Eu2+ nanophosphors provide a
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bluish-green emission under NUV excitation. The structure and morphology of the synthesized material were studied using x-ray diffraction (XRD) pattern and field-emission scanning electron microscope (FE-SEM) images. The optical properties were investigated in detail by the photoluminescence (PL) excitation (PLE) and PL emission spectra. The thermal stability of the materials was characterized using the temperature-dependent PL emission spectra. The
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K2ZnSiO4: Eu2+ nanophosphors were finally used for the preparation of NUV-based WLEDs.
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2. Experimental Procedure
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Materials: Potassium nitrate (KNO3), zinc nitrate hexahydrate (Zn(NO3)2 ∙6H2O), tetra ethyl orthosilicate (TEOS), europium nitrate pentahydrate (Eu(NO3)2 ∙ 5H2O) and citric acid (HCO(COOH)(CH2COOH)2). All the chemicals were purchased from Sigma Aldrich Co., South Korea and were used as received without further purification. The de-ionized (DI) water was obtained from a Milli-Q synthesis system (resistivity ~18.2 MΩ-cm).
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Synthesis: The K2ZnSiO4 nanophosphors were prepared by sol-gel method using citric acid as a complexing agent. Firstly, stoichiometric amount of TEOS was added to 5 ml of ethanol and stirred continuously for 15 min. Secondly, the metal nitrates were added to 200 ml of DI water
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and stirred continuously. Later, the TEOS solution was added to the metal nitrates solution, and after 15 min citric acid was added (1:2 metal to citric acid ratio). The final solution was capped and stirred for 1 h without heating. The solution was then heated to a temperature of 80 °C with
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the help of hotplate. After 1 h of continuous heating and stirring, the beaker was uncapped and the solution was made to evaporate slowly. The gel formed after complete evaporation of
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solution was collected and dried in the oven at 120 °C for few hours. The dried gel was further calcined at 900 °C for 5 h. For the conversion of europium ions from its 3+ state to 2+ state, the sample was further heated in thermal CO reducing atmosphere at 800 °C for 2 h. Characterizations: The prepared phosphor powders were characterized by using a FE-SEM (FE-
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SEM: LEO SUPRA 55, Carl Zeiss) attached with an energy dispersive X-ray spectrometer (EDX), XRD (M18XHF-SRA, Mac Science), Fourier transform infrared spectrometer (FTIR: Spectrum 100, PerkinElmer), and fluorescence spectrometer (FluroMate FS-2, Scinco, South
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Korea) attached with a temperature controlled heating holder (25-250 °C).
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Phosphor Converted-LEDs Fabrication: For making the pc-LEDs, a UV LED chip with an excitation wavelength of 376 nm was utilized. The UV LED chip was attached on an LED frame using glue and slightly pressed to avoid the formation of gaps/air bubbles (formed after dispensing the phosphor mixed silicone encapsulant) between the chip and LED frame, which may cause a damage to the LED when a forward bias current is applied. The LED frame is then kept on a hot surface for few seconds for the hardening of glue and good attachment of LED. Afterwards, the silicone epoxy mixture (1:2 ratio) and small amount of phosphors were added
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and mixed well with the help of a mixer. Later, the phosphor epoxy was kept in a vacuum desiccator to remove the air bubbles and then taken into a dispenser tube. Finally, the phosphor epoxy was encapsulated onto the LED frame using a dispenser and allowed to harden by heating
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(60 °C) for 120 min.
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3. Results and Discussion
Figure 1 shows the morphological properties and elemental analysis of the synthesized K2ZnSiO4
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nanophosphors. Figure 1(a) shows the low-magnification FE-SEM image of the prepared K2ZnSiO4 nanophosphor and the inset shows the high-magnification image. The FE-SEM image revealed that the formed material is in the nanometer size and almost spherical in shape which is useful for its application in making WLEDs due to high packing density and good slurry
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property. The elemental analysis of the K2ZnSiO4 material is shown in Fig. 1(b) and the inset show the atomic and weight percentage distribution of elements. The EDX spectrum exhibited the existence of elements, i.e., K, Zn, Si and O of the host lattice. The presence of carbon (C) and
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platinum (Pt) is due to the use of carbon tape and Pt coating for measuring the sample. The elements of the host lattice occupied the K shell at 3.30 eV (K), 8.63 eV (Zn), 1.74 eV (Si) and
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0.52 eV (O). Small quantity of Zn occupied the L shell at 1.01 eV as shown in Fig. 1(b). The elemental mapping images of elements are shown from Fig. 1(c) -1(f) and the two-dimensional (2D) mapping images revealed the uniform distribution of elements in the sample. The area scanned for elemental mapping overlapped with the mapping images of all the elements is presented in the inset of Fig. 1(b).
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Figure 2(a) shows the XRD pattern of the K2ZnSiO4 sample in the 2θ range of 10-72° along with the standard values. The diffraction peaks of the sample matched well with the JCPDS card#39-0268 values. All the peaks were assigned to the respective planes indexed with the
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corresponding (hkl) values. No other impurity peaks were observed, implying the formation of pure phase K2ZnSiO4. Figure 2(b) shows the FTIR spectrum of the prepared K2ZnSiO4 sample. The FTIR spectrum was examined in the wavenumber range of 4000-425 cm-1. The spectrum
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showed a broadband in the range of 3500 to 2700 cm-1 due to the O-H vibrations.[27] The IR absorption bands due to the SiO4 units can be observed at lower wavenumber. The bands are
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usually divided into four groups of symmetric and antisymmetric stretching and bending. The strong and intense bands with the band maxima at the wavenumbers of 893 and 989 cm-1 belong to the symmetric (ν1) and antisymmetric (ν3) stretchings, respectively. The antisymmetric bending (ν4) was observed at the wavenumber of 490 cm-1 and the symmetric bending is usually
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observed near to 400 cm-1 which is absent due to the instrument limit.[28, 29] The absorption band at the wavenumber of 534 cm-1 is due to the symmetric ZnO stretching in ZnO4 units.[30] Figure 3 shows the PLE and PL emission spectra of the 1 mol% Eu2+ ions doped K2ZnSiO4
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(here after referred as K2ZnSiO4: Eu2+) nanophosphor observed with 512 and 375 nm emission and excitation wavelengths, respectively. The PLE spectrum (Fig. 3(a)) was taken in the
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wavelength range of 200-450 nm and noticed a broadband. The broad absorption band range from 200-400 nm and centered at 375 nm which is due to the 4f-5d transitions of Eu2+ ions. This wavelength matches well with the emission light of a NUV chip which is essential for fabricating NUV-based WLEDs. Figure 3b shows the broad PL emission spectrum in the wavelength range of 400-700 nm at a NUV excitation of 375 nm. The emission of Eu2+ ions varies for each host material and gives broad emission bands in red, green, and blue regions, as mentioned in the
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introduction section. In the present work, we observed a bluish-green emission when excited with a NUV chip. The broadband emission corresponds to the 4f65d1 4f7 transition of Eu2+ ions. The inset of Fig. 3(b) shows the emission intensity variation with respect to the
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concentration of Eu2+ ions. As the concentration of Eu2+ ions increased the emission intensity increased and reach maximum at 1 mol% and then decreased further with the increase of Eu2+ ion concentration. As the concentration of Eu2+ ions increased, the energy transfer between the
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Eu2+ ions became extensive, causing the quench in emission intensity. The PL emission spectrum in Fig. 3(b) did not show any peaks corresponding to the Eu3+ emission, indicating the complete
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reduction of europium ions into its divalent state under reduced atmosphere. The inset of Fig. 3(b) shows the emission picture of the phosphor showing the bluish-green emission color. The Commission International De I'Eclairage (CIE) values calculated from the emission data are presented in Table 1.
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Figure 4 shows the 3D surface and counter line plots for the K2ZnSiO4: Eu2+ sample in the excitation wavelength range of 270-400 nm and emission wavelength range of 400-700 nm. The 3D surface plot (Fig. 4(a)) was measured for every 1 nm in the selected region. The counter line
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plot (Fig. 4(b)) exhibited the emission due to the 4f65d1 4f7 transition of Eu2+ ions. From both the 3D surface and counter line plots, the strong emission intensity was observed only due to the
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Eu2+ ions indicating the complete reduction of europium ions form its trivalent state to divalent state. The counter line plot suggests that the K2ZnSiO4: Eu2+ nanophosphor is a promising candidate in the NUV excitation range where it provides the strong bluish-green emission. The temperature-dependent PL emission properties of the prepared material were further characterized to study the thermal stability of the material. The thermal stability is one of the important parameters to be considered for developing high-power WLEDs for general
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illumination which can affect the durability and light output. Hence, the temperature-dependent PL emission spectra were measured for the material in the temperature range of 30-210 °C (interval of 20 °C). Figure 5(a) shows the temperature-dependent PL emission spectra of the
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K2ZnSiO4: Eu2+ sample at 375 nm of excitation wavelength. The PL emission intensity decreased as the temperature increased due to the thermal quenching. The thermal quenching is due to the relaxation of phonons by a non-radiative process from higher energy levels. The
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temperature at which the PL emission intensity drops to 50% of its initial value is called as the thermal quenching temperatures, which was more or less equal to 85 °C. For better
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understanding of the thermal quenching phenomena, the thermal activation energy (Eg) is calculated for the K2ZnSiO4: Eu2+ nanophosphor using well-known Arrhenius equation:[31] ܫ
−ܧ ൚ 1 + ܿ exp ቌ ൘ ܶ ܭቍ
,
(1)
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≈ܫ
where I and Io are the emission intensities at temperature t and room temperature, c is the constant, KB is the Boltzmann’s constant in eV and Eg is the thermal activation energy. A plot is
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drawn between 1/KBT and ln ((Io/I)-1) and a straight line is obtained, and the Eg value is
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calculated from the slope of the linear plot. From Fig. 5(b) and Arrhenius equation, the Eg value calculated was found to be 0.296 eV and this value is much higher than some of the reported silicates.[32, 33] The decay time which is the rate of relaxation of electrons from the excited state to the ground state as a function of time for the K2ZnSiO4: Eu2+ sample is shown in Fig. 5(c). The decay curve was obtained using 375 nm and 512 nm excitation and emission wavelengths. The decay curve was fitted using a single exponential function given as below:[34] ܫ = ܫ + ݁ܣ
ି௧ൗ ఛ
,
(2)
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where Io and I are the emission intensities at time 0 and t, τ is the decay time and A is the constant. The decay time value was obtained to be 8.54 µs. This short decay time avoids
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saturation at high excitation flux, which makes the material suitable for fabricating WLEDs. Two LEDs were fabricated to test the practical applicability of the prepared phosphor material. Here, we took 376 nm NUV chip for fabricating the LED. LED1 was fabricated with the K2ZnSiO4: Eu2+ nanophosphor and LED2 was fabricated using the K2ZnSiO4: Eu2+ along with a
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red-emitting phosphor (Y2Mo4O15: Eu3+). Here, we chose the reported Y2Mo4O15: Eu3+ phosphor
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as the red-emitting source because the LED wavelength matches well with the excitation wavelength of the phosphor.[35] The Y2Mo4O15: Eu3+ phosphor was prepared using a conventional solid-state reaction method as reported and further used in the LED. The PL emission and PLE spectra corresponding to the synthesized Y2Mo4O15: Eu3+ phosphor is as shown in Fig. 6(a). The fabrication process of LED was mentioned in detail in the experimental
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section; in summary desired amount of phosphor powders was mixed in the silicone encapsulant using a mixer and then the mixture was degassed and dispensed on the LED frame. The ratio of K2ZnSiO4: Eu2+ and Y2Mo4O15: Eu3+ phosphor materials taken was 4:1. Later, they were kept in
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an oven for drying and cooled to room temperature and the electroluminescence (EL) spectra
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were further obtained. The EL spectra obtained from both the LED1 and LED2 are shown in Fig. 6(b). The NUV LED chip attached to the frame, the LED frame with the silicone encapsulant are shown in Fig. 6(c) and 6(d), respectively. The LEDs under forward bias current (50 mA) exhibited respective emission color; the LED1 emitted the bluish-green emission (inset of Fig. 6(e)) and the LED2 gave dazzling white light emission (Fig. 6(e)) due to the presence of the redemitting phosphor. The CIE, color rendering index (CRI) and correlated color temperature
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(CCT) values for the fabricated LEDs with the K2ZnSiO4: Eu2+ nanophosphor were calculated
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and the data are shown in Table 1.
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4. Conclusion
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In summary, we successfully synthesized the NUV excitable K2ZnSiO4: Eu2+ bluish-green emitting nanophosphors by the sol-gel method. The morphology of the material showed that these particles are in the nanometer scale and are almost spherical in shape. The materials crystallize in a cubic phase and the XRD pattern did not show any impurity peaks. The synthesized K2ZnSiO4: Eu2+ materials revealed blueish-green broadband emission under 375 nm
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excitation. The excitation band of the material matched well with the emission of the NUV chip which makes these materials more preferable for fabricating the NUV-based WLEDs. The temperature-dependent PL emission spectra showed that the material possesses good thermal
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stability with an activation energy of 0.296 eV. The LED made with a combination of red-
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emitting phosphor exhibited dazzling white-light emission with good CRI and CCT values. Therefore, the K2ZnSiO4: Eu2+ nanophosphors are expected to be a promising candidate for the development of NUV-based WLEDs for solid-state lighting applications.
Acknowledgments
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This work was supported by the National Research Foundation of Korea (NRF) Grant funded by
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the Korea government (MSIP) (No. 2015R1A5A1037656).
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References:
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[35] Janulevicius M, Marmokas P, Misevicius M, Grigorjevaite J, Mikoliunaite L, Sakirzanovas S, et al. Luminescence and luminescence quenching of highly efficient Y2Mo4O15:Eu3+ phosphors and ceramics. Sci Rep. 2016;6:26098.
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Table 1. CIE, CRI and CCT values of the K2ZnSiO4: Eu2+ nanophosphors and the fabricated LEDs with K2ZnSiO4: Eu2+ nanophosphor. Name
K2ZnSiO4: Eu2+
CRI --70.5 81
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LED1 LED2
CIE (x, y) (0.248, 0.427) (0.296, 0.378) (0.326, 0.383)
CCT (K) --6899 5730
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Figure 1. (a) FE-SEM (inset shows the high-magnification image), (b) EDX spectrum (inset shows the elemental distribution table), and (c)-(f) elemental mapping images of the K2ZnSiO4 nanophosphor.
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(220)
(044)
(224)
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(133)
(222)
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(004)
K2ZnSiO4
(111)
Intensity (a. u.)
(a)
JCPDS # 39-0268
30
40
50
2θ (degree)
60
A Transmittance C C (%) EP TE D
20
70
(b)
ν4
OH
Zn-O
ν3
K2ZnSiO4
3750
3000
2250
ν1
1500
-1
Wavenumber (cm )
750
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Figure 2. (a) XRD pattern and (b) FTIR spectrum of the K2ZnSiO4 nanophosphor.
λEmiss= 512nm
200
250
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2+
K2ZnSiO4: Eu
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Intensity (a. u.)
(a)
300
350
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Wavelength (nm)
400
450
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Figure 3. (a) PLE and (b) PL emission spectra of the K2ZnSiO4: Eu2+ nanophosphor. Inset of (b)
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shows the intensity versus Eu2+ ion concentration.
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Figure 4. (a) 3D surface plot and (b) counter line plot for the K2ZnSiO4: Eu2+nanophosphor.
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Intensity (a. u.)
(a) K2ZnSiO4: Eu2+
210 °C
400
500
600
700
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Wavelength (nm) (b)
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2
ln((I0/I)-1)
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30 °C
Slope = -0.296
1 0
Experimental Fitted Line
-1
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24 26 28 30 32 34 36
1/KBT (e/V)
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Intensity (a. u.)
(c)
K2ZnSiO4: Eu2+ λExci = 375 nm λEmiss = 512 nm τ = 8.54 µs
0 50 100150200250300 Decay Time (µ µS)
Figure 5. (a) Temperature-dependent PL emission spectra, (b) ln((Io/I)-1) versus 1/KBT plot and (c) Luminescence decay time profile for the K2ZnSiO4: Eu2+ nanophosphor.
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Figure 6. (a) PL emission and PLE spectra of Y2Mo4O15: Eu3+ phosphor (inset shows the emission picture), (b) EL spectra of LED1 and LED2, (c) LED frame with NUV LED, (d) LED
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frame filled with silicone encapsulant, and (e) LED2 under a forward bias current of 50 mA
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(inset shows the LED1 under forward bias current).
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Highlights:
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The K2ZnSiO4: Eu2+ nanophosphors were prepared using sol-gel method. The NUV excitation (375 nm) gives broadband bluish-green emission The samples have good thermal stability with an activation energy value of 0.296 eV. The LED made with K2ZnSiO4: Eu2+ and red-phosphor give cool white light emission.
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2+
L. Krishna Bharat, Sk Khaja Hussain, Jae Su Yu PII:
S0143-7208(17)30797-0
DOI:
10.1016/j.dyepig.2017.05.044
Reference:
DYPI 6006
To appear in:
Dyes and Pigments
Received Date: 11 April 2017 Revised Date:
23 May 2017
Accepted Date: 23 May 2017
Please cite this article as: Krishna Bharat L, Hussain SK, Yu JS, Near-ultraviolet excitation-based bluish2+ green emitting K2ZnSiO4: Eu nanophosphors for white light-emitting applications, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.05.044. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Near-ultraviolet excitation-based bluish-green emitting K2ZnSiO4: Eu2+ nanophosphors for white light-emitting applications *
L. Krishna Bharat, Sk. Khaja Hussain, and Jae Su Yu
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Department of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, Yongin-si, Gyeonggi-do 17104, Republic of Korea
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Abstract
The K2ZnSiO4: Eu2+ nanophosphor materials were synthesized via a sol-gel method. The
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morphology of the synthesized materials was examined by field-emission scanning electron microscope images. The structural properties were analyzed from X-ray diffraction (XRD) pattern and Fourier transform infrared spectrum. The K2ZnSiO4 host lattice crystallizes in a cubic phase, and the XRD pattern matches well with the standard JCPDS card values and does not any
impurity
peaks.
The
luminescence
properties
were
studied
using
the
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show
photoluminescence (PL) excitation and PL emission spectra. The broad band excitation in the near-ultraviolet (NUV) region and the broad band emission in the visible region make this
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phosphor material suitable for the application of NUV-based white light-emitting diodes. The thermal stability of the material was tested by calculating the activation energy value which was
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found to be 0.296 eV. The synthesized K2ZnSiO4: Eu2+ nanophosphor was then coated onto the NUV chip along with a red-emitting phosphor to get visible white-light emission with a color rendering index value of 81 and a correlated color temperature value of 5730 K.
Keywords: NUV excitation, Silicates, Bluish-green emission, White light-emitting diodes * Corresponding author: [email protected] (J. S. Yu)
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1. Introduction Currently, development of energy-efficient devices is spreading in each and every field including
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the solid-state lighting industry. Making of energy-efficient light-emitting diodes (LEDs) which can replace traditional incandescent lamps is believed as a major step for energy consumption worldwide.[1, 2] LED can only emit a single color,[3-5] on the other hand, white light covering the whole visible region (400-700 nm) is required for general illumination and lighting
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applications. Thus, to obtain white light emission, phosphor-converted LED (pc-LED) is a
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typical approach. The commercially available white LED (WLED) is an integration of blue chip and yellow light-emitting garnet material (Y3Al5O12: Ce3+; YAG: Ce).[6-8] However, these WLEDs give poor color rendering index (CRI) and correlated color temperature (CCT) values, which are not suitable for general illumination. To obtain better CRI and CCT values, the nearultraviolet LED (NUV-LED) chip with different color-emitting phosphors is a promising idea.
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The NUV-LED chips (350-420 nm) offer a better excitation efficiency which is nearly equal to that of the fluorescent lamps.[9, 10] In this context, present-day phosphor research mainly
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focuses on the synthesis of NUV excitable phosphor materials for the fabrication of WLEDs. Generally, different kinds of materials like oxides, nitrides, etc. have been chosen as host
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materials.[11, 12] Among all these materials, oxides are one kind of materials which are easy to be prepared and obtained at lower temperatures.[13, 14] Recently, silicate materials have been extensively investigated due to their thermal and chemical stabilities, air insensitivity, and water resistance.[15-17] So, we chose K2ZnSiO4 as the host material. Similarly, the choice of dopant material is also a prerequisite to get the desirable emission color. Europium is one of the widely used dopant materials which played a key role in the display and lighting fields due to its existence in distinct 2+ and 3+ oxidation states. Europium (Eu) in 3+ state gives strong and sharp
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peaks in the red and/or orange spectral regions,[18-20] and in 2+ state, it provides versatile broadband emissions in different crystal fields.[21-24] Up to now, there were only few reports found on the synthesis and the study of luminescent properties of rare-earths doped
doped K2ZnSiO4 and its application for WLEDs.
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K2ZnSiO4.[25, 26] To the best of our knowledge, there was no report on the study of Eu2+ ions
In this work, the sol-gel synthesis of K2ZnSiO4: Eu2+ nanophosphors and the study of
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structural and optical properties were reported. The K2ZnSiO4: Eu2+ nanophosphors provide a
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bluish-green emission under NUV excitation. The structure and morphology of the synthesized material were studied using x-ray diffraction (XRD) pattern and field-emission scanning electron microscope (FE-SEM) images. The optical properties were investigated in detail by the photoluminescence (PL) excitation (PLE) and PL emission spectra. The thermal stability of the materials was characterized using the temperature-dependent PL emission spectra. The
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K2ZnSiO4: Eu2+ nanophosphors were finally used for the preparation of NUV-based WLEDs.
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2. Experimental Procedure
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Materials: Potassium nitrate (KNO3), zinc nitrate hexahydrate (Zn(NO3)2 ∙6H2O), tetra ethyl orthosilicate (TEOS), europium nitrate pentahydrate (Eu(NO3)2 ∙ 5H2O) and citric acid (HCO(COOH)(CH2COOH)2). All the chemicals were purchased from Sigma Aldrich Co., South Korea and were used as received without further purification. The de-ionized (DI) water was obtained from a Milli-Q synthesis system (resistivity ~18.2 MΩ-cm).
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Synthesis: The K2ZnSiO4 nanophosphors were prepared by sol-gel method using citric acid as a complexing agent. Firstly, stoichiometric amount of TEOS was added to 5 ml of ethanol and stirred continuously for 15 min. Secondly, the metal nitrates were added to 200 ml of DI water
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and stirred continuously. Later, the TEOS solution was added to the metal nitrates solution, and after 15 min citric acid was added (1:2 metal to citric acid ratio). The final solution was capped and stirred for 1 h without heating. The solution was then heated to a temperature of 80 °C with
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the help of hotplate. After 1 h of continuous heating and stirring, the beaker was uncapped and the solution was made to evaporate slowly. The gel formed after complete evaporation of
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solution was collected and dried in the oven at 120 °C for few hours. The dried gel was further calcined at 900 °C for 5 h. For the conversion of europium ions from its 3+ state to 2+ state, the sample was further heated in thermal CO reducing atmosphere at 800 °C for 2 h. Characterizations: The prepared phosphor powders were characterized by using a FE-SEM (FE-
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SEM: LEO SUPRA 55, Carl Zeiss) attached with an energy dispersive X-ray spectrometer (EDX), XRD (M18XHF-SRA, Mac Science), Fourier transform infrared spectrometer (FTIR: Spectrum 100, PerkinElmer), and fluorescence spectrometer (FluroMate FS-2, Scinco, South
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Korea) attached with a temperature controlled heating holder (25-250 °C).
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Phosphor Converted-LEDs Fabrication: For making the pc-LEDs, a UV LED chip with an excitation wavelength of 376 nm was utilized. The UV LED chip was attached on an LED frame using glue and slightly pressed to avoid the formation of gaps/air bubbles (formed after dispensing the phosphor mixed silicone encapsulant) between the chip and LED frame, which may cause a damage to the LED when a forward bias current is applied. The LED frame is then kept on a hot surface for few seconds for the hardening of glue and good attachment of LED. Afterwards, the silicone epoxy mixture (1:2 ratio) and small amount of phosphors were added
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and mixed well with the help of a mixer. Later, the phosphor epoxy was kept in a vacuum desiccator to remove the air bubbles and then taken into a dispenser tube. Finally, the phosphor epoxy was encapsulated onto the LED frame using a dispenser and allowed to harden by heating
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(60 °C) for 120 min.
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3. Results and Discussion
Figure 1 shows the morphological properties and elemental analysis of the synthesized K2ZnSiO4
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nanophosphors. Figure 1(a) shows the low-magnification FE-SEM image of the prepared K2ZnSiO4 nanophosphor and the inset shows the high-magnification image. The FE-SEM image revealed that the formed material is in the nanometer size and almost spherical in shape which is useful for its application in making WLEDs due to high packing density and good slurry
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property. The elemental analysis of the K2ZnSiO4 material is shown in Fig. 1(b) and the inset show the atomic and weight percentage distribution of elements. The EDX spectrum exhibited the existence of elements, i.e., K, Zn, Si and O of the host lattice. The presence of carbon (C) and
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platinum (Pt) is due to the use of carbon tape and Pt coating for measuring the sample. The elements of the host lattice occupied the K shell at 3.30 eV (K), 8.63 eV (Zn), 1.74 eV (Si) and
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0.52 eV (O). Small quantity of Zn occupied the L shell at 1.01 eV as shown in Fig. 1(b). The elemental mapping images of elements are shown from Fig. 1(c) -1(f) and the two-dimensional (2D) mapping images revealed the uniform distribution of elements in the sample. The area scanned for elemental mapping overlapped with the mapping images of all the elements is presented in the inset of Fig. 1(b).
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Figure 2(a) shows the XRD pattern of the K2ZnSiO4 sample in the 2θ range of 10-72° along with the standard values. The diffraction peaks of the sample matched well with the JCPDS card#39-0268 values. All the peaks were assigned to the respective planes indexed with the
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corresponding (hkl) values. No other impurity peaks were observed, implying the formation of pure phase K2ZnSiO4. Figure 2(b) shows the FTIR spectrum of the prepared K2ZnSiO4 sample. The FTIR spectrum was examined in the wavenumber range of 4000-425 cm-1. The spectrum
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showed a broadband in the range of 3500 to 2700 cm-1 due to the O-H vibrations.[27] The IR absorption bands due to the SiO4 units can be observed at lower wavenumber. The bands are
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usually divided into four groups of symmetric and antisymmetric stretching and bending. The strong and intense bands with the band maxima at the wavenumbers of 893 and 989 cm-1 belong to the symmetric (ν1) and antisymmetric (ν3) stretchings, respectively. The antisymmetric bending (ν4) was observed at the wavenumber of 490 cm-1 and the symmetric bending is usually
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observed near to 400 cm-1 which is absent due to the instrument limit.[28, 29] The absorption band at the wavenumber of 534 cm-1 is due to the symmetric ZnO stretching in ZnO4 units.[30] Figure 3 shows the PLE and PL emission spectra of the 1 mol% Eu2+ ions doped K2ZnSiO4
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(here after referred as K2ZnSiO4: Eu2+) nanophosphor observed with 512 and 375 nm emission and excitation wavelengths, respectively. The PLE spectrum (Fig. 3(a)) was taken in the
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wavelength range of 200-450 nm and noticed a broadband. The broad absorption band range from 200-400 nm and centered at 375 nm which is due to the 4f-5d transitions of Eu2+ ions. This wavelength matches well with the emission light of a NUV chip which is essential for fabricating NUV-based WLEDs. Figure 3b shows the broad PL emission spectrum in the wavelength range of 400-700 nm at a NUV excitation of 375 nm. The emission of Eu2+ ions varies for each host material and gives broad emission bands in red, green, and blue regions, as mentioned in the
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introduction section. In the present work, we observed a bluish-green emission when excited with a NUV chip. The broadband emission corresponds to the 4f65d1 4f7 transition of Eu2+ ions. The inset of Fig. 3(b) shows the emission intensity variation with respect to the
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concentration of Eu2+ ions. As the concentration of Eu2+ ions increased the emission intensity increased and reach maximum at 1 mol% and then decreased further with the increase of Eu2+ ion concentration. As the concentration of Eu2+ ions increased, the energy transfer between the
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Eu2+ ions became extensive, causing the quench in emission intensity. The PL emission spectrum in Fig. 3(b) did not show any peaks corresponding to the Eu3+ emission, indicating the complete
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reduction of europium ions into its divalent state under reduced atmosphere. The inset of Fig. 3(b) shows the emission picture of the phosphor showing the bluish-green emission color. The Commission International De I'Eclairage (CIE) values calculated from the emission data are presented in Table 1.
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Figure 4 shows the 3D surface and counter line plots for the K2ZnSiO4: Eu2+ sample in the excitation wavelength range of 270-400 nm and emission wavelength range of 400-700 nm. The 3D surface plot (Fig. 4(a)) was measured for every 1 nm in the selected region. The counter line
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plot (Fig. 4(b)) exhibited the emission due to the 4f65d1 4f7 transition of Eu2+ ions. From both the 3D surface and counter line plots, the strong emission intensity was observed only due to the
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Eu2+ ions indicating the complete reduction of europium ions form its trivalent state to divalent state. The counter line plot suggests that the K2ZnSiO4: Eu2+ nanophosphor is a promising candidate in the NUV excitation range where it provides the strong bluish-green emission. The temperature-dependent PL emission properties of the prepared material were further characterized to study the thermal stability of the material. The thermal stability is one of the important parameters to be considered for developing high-power WLEDs for general
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illumination which can affect the durability and light output. Hence, the temperature-dependent PL emission spectra were measured for the material in the temperature range of 30-210 °C (interval of 20 °C). Figure 5(a) shows the temperature-dependent PL emission spectra of the
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K2ZnSiO4: Eu2+ sample at 375 nm of excitation wavelength. The PL emission intensity decreased as the temperature increased due to the thermal quenching. The thermal quenching is due to the relaxation of phonons by a non-radiative process from higher energy levels. The
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temperature at which the PL emission intensity drops to 50% of its initial value is called as the thermal quenching temperatures, which was more or less equal to 85 °C. For better
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understanding of the thermal quenching phenomena, the thermal activation energy (Eg) is calculated for the K2ZnSiO4: Eu2+ nanophosphor using well-known Arrhenius equation:[31] ܫ
−ܧ ൚ 1 + ܿ exp ቌ ൘ ܶ ܭቍ
,
(1)
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≈ܫ
where I and Io are the emission intensities at temperature t and room temperature, c is the constant, KB is the Boltzmann’s constant in eV and Eg is the thermal activation energy. A plot is
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drawn between 1/KBT and ln ((Io/I)-1) and a straight line is obtained, and the Eg value is
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calculated from the slope of the linear plot. From Fig. 5(b) and Arrhenius equation, the Eg value calculated was found to be 0.296 eV and this value is much higher than some of the reported silicates.[32, 33] The decay time which is the rate of relaxation of electrons from the excited state to the ground state as a function of time for the K2ZnSiO4: Eu2+ sample is shown in Fig. 5(c). The decay curve was obtained using 375 nm and 512 nm excitation and emission wavelengths. The decay curve was fitted using a single exponential function given as below:[34] ܫ = ܫ + ݁ܣ
ି௧ൗ ఛ
,
(2)
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where Io and I are the emission intensities at time 0 and t, τ is the decay time and A is the constant. The decay time value was obtained to be 8.54 µs. This short decay time avoids
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saturation at high excitation flux, which makes the material suitable for fabricating WLEDs. Two LEDs were fabricated to test the practical applicability of the prepared phosphor material. Here, we took 376 nm NUV chip for fabricating the LED. LED1 was fabricated with the K2ZnSiO4: Eu2+ nanophosphor and LED2 was fabricated using the K2ZnSiO4: Eu2+ along with a
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red-emitting phosphor (Y2Mo4O15: Eu3+). Here, we chose the reported Y2Mo4O15: Eu3+ phosphor
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as the red-emitting source because the LED wavelength matches well with the excitation wavelength of the phosphor.[35] The Y2Mo4O15: Eu3+ phosphor was prepared using a conventional solid-state reaction method as reported and further used in the LED. The PL emission and PLE spectra corresponding to the synthesized Y2Mo4O15: Eu3+ phosphor is as shown in Fig. 6(a). The fabrication process of LED was mentioned in detail in the experimental
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section; in summary desired amount of phosphor powders was mixed in the silicone encapsulant using a mixer and then the mixture was degassed and dispensed on the LED frame. The ratio of K2ZnSiO4: Eu2+ and Y2Mo4O15: Eu3+ phosphor materials taken was 4:1. Later, they were kept in
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an oven for drying and cooled to room temperature and the electroluminescence (EL) spectra
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were further obtained. The EL spectra obtained from both the LED1 and LED2 are shown in Fig. 6(b). The NUV LED chip attached to the frame, the LED frame with the silicone encapsulant are shown in Fig. 6(c) and 6(d), respectively. The LEDs under forward bias current (50 mA) exhibited respective emission color; the LED1 emitted the bluish-green emission (inset of Fig. 6(e)) and the LED2 gave dazzling white light emission (Fig. 6(e)) due to the presence of the redemitting phosphor. The CIE, color rendering index (CRI) and correlated color temperature
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(CCT) values for the fabricated LEDs with the K2ZnSiO4: Eu2+ nanophosphor were calculated
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and the data are shown in Table 1.
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4. Conclusion
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In summary, we successfully synthesized the NUV excitable K2ZnSiO4: Eu2+ bluish-green emitting nanophosphors by the sol-gel method. The morphology of the material showed that these particles are in the nanometer scale and are almost spherical in shape. The materials crystallize in a cubic phase and the XRD pattern did not show any impurity peaks. The synthesized K2ZnSiO4: Eu2+ materials revealed blueish-green broadband emission under 375 nm
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excitation. The excitation band of the material matched well with the emission of the NUV chip which makes these materials more preferable for fabricating the NUV-based WLEDs. The temperature-dependent PL emission spectra showed that the material possesses good thermal
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stability with an activation energy of 0.296 eV. The LED made with a combination of red-
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emitting phosphor exhibited dazzling white-light emission with good CRI and CCT values. Therefore, the K2ZnSiO4: Eu2+ nanophosphors are expected to be a promising candidate for the development of NUV-based WLEDs for solid-state lighting applications.
Acknowledgments
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This work was supported by the National Research Foundation of Korea (NRF) Grant funded by
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the Korea government (MSIP) (No. 2015R1A5A1037656).
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References:
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[35] Janulevicius M, Marmokas P, Misevicius M, Grigorjevaite J, Mikoliunaite L, Sakirzanovas S, et al. Luminescence and luminescence quenching of highly efficient Y2Mo4O15:Eu3+ phosphors and ceramics. Sci Rep. 2016;6:26098.
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Table 1. CIE, CRI and CCT values of the K2ZnSiO4: Eu2+ nanophosphors and the fabricated LEDs with K2ZnSiO4: Eu2+ nanophosphor. Name
K2ZnSiO4: Eu2+
CRI --70.5 81
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LED1 LED2
CIE (x, y) (0.248, 0.427) (0.296, 0.378) (0.326, 0.383)
CCT (K) --6899 5730
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Figure 1. (a) FE-SEM (inset shows the high-magnification image), (b) EDX spectrum (inset shows the elemental distribution table), and (c)-(f) elemental mapping images of the K2ZnSiO4 nanophosphor.
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(220)
(044)
(224)
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(133)
(222)
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K2ZnSiO4
(111)
Intensity (a. u.)
(a)
JCPDS # 39-0268
30
40
50
2θ (degree)
60
A Transmittance C C (%) EP TE D
20
70
(b)
ν4
OH
Zn-O
ν3
K2ZnSiO4
3750
3000
2250
ν1
1500
-1
Wavenumber (cm )
750
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Figure 2. (a) XRD pattern and (b) FTIR spectrum of the K2ZnSiO4 nanophosphor.
λEmiss= 512nm
200
250
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2+
K2ZnSiO4: Eu
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Intensity (a. u.)
(a)
300
350
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Wavelength (nm)
400
450
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Figure 3. (a) PLE and (b) PL emission spectra of the K2ZnSiO4: Eu2+ nanophosphor. Inset of (b)
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shows the intensity versus Eu2+ ion concentration.
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Figure 4. (a) 3D surface plot and (b) counter line plot for the K2ZnSiO4: Eu2+nanophosphor.
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Intensity (a. u.)
(a) K2ZnSiO4: Eu2+
210 °C
400
500
600
700
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Wavelength (nm) (b)
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2
ln((I0/I)-1)
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30 °C
Slope = -0.296
1 0
Experimental Fitted Line
-1
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24 26 28 30 32 34 36
1/KBT (e/V)
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Intensity (a. u.)
(c)
K2ZnSiO4: Eu2+ λExci = 375 nm λEmiss = 512 nm τ = 8.54 µs
0 50 100150200250300 Decay Time (µ µS)
Figure 5. (a) Temperature-dependent PL emission spectra, (b) ln((Io/I)-1) versus 1/KBT plot and (c) Luminescence decay time profile for the K2ZnSiO4: Eu2+ nanophosphor.
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Figure 6. (a) PL emission and PLE spectra of Y2Mo4O15: Eu3+ phosphor (inset shows the emission picture), (b) EL spectra of LED1 and LED2, (c) LED frame with NUV LED, (d) LED
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frame filled with silicone encapsulant, and (e) LED2 under a forward bias current of 50 mA
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(inset shows the LED1 under forward bias current).
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Highlights:
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The K2ZnSiO4: Eu2+ nanophosphors were prepared using sol-gel method. The NUV excitation (375 nm) gives broadband bluish-green emission The samples have good thermal stability with an activation energy value of 0.296 eV. The LED made with K2ZnSiO4: Eu2+ and red-phosphor give cool white light emission.
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