New strategy of designing a novel yellow-emitting phosphor Na4Hf2Si3O12:Eu2+ for multifunctional applications

Journal Pre-proof 2+ New strategy of designing a novel yellow-emitting phosphor Na4Hf2Si3O12:Eu for multifunctional applications Qi Wei, Jianyan Ding, Xiaopeng Zhou, Xicheng Wang, Yuhua Wang PII:

S0925-8388(19)34008-3

DOI:

https://doi.org/10.1016/j.jallcom.2019.152762

Reference:

JALCOM 152762

To appear in:

Journal of Alloys and Compounds

Received Date: 24 July 2019 Revised Date:

20 October 2019

Accepted Date: 21 October 2019

Please cite this article as: Q. Wei, J. Ding, X. Zhou, X. Wang, Y. Wang, New strategy of designing a 2+ novel yellow-emitting phosphor Na4Hf2Si3O12:Eu for multifunctional applications, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152762. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

New Strategy of designing a novel yellow-emitting phosphor Na4Hf2Si3O12:Eu2+ for multifunctional applications Qi Weiab, Jianyan Dingc , Xiaopeng Zhouab, Xicheng Wangab* and Yuhua Wangab* a

Key Laboratory for Special Function Materials and Structural Design of the Ministry of the Education, Lanzhou University. b

National and Local Joint Engineering Laboratory for Optical Conversion Materials and Technology, Lanzhou University, Lanzhou, China

c

College of Chemistry & Materials Science, Longyan University, Longyan, Fujian, 364000, China Corresponding author’ email: [email protected].

Highlights 1.A novel yellow-emitting phosphor Na4Hf2Si3O12:Eu2+ was synthesized for the fist time. 2.A WLED lamp was fabricated with higher Ra(87.5) and lower CCT(3953K) compared with ommercial YAG:Ce3+ combined with blue LED chip. 3.The phosphor shows characteristic thermal stability, which improves new ideas for the development of temperature sensors. 4.The phosphor also exhibits high current saturation and perfect stability under low voltage electron bombardment. Abstract A novel yellow emitting phosphor Na4Hf2Si3O12:Eu2+ (NHSO:Eu2+) has been successfully synthesized through solid-state reaction for the first time. The crystalline structure, morphology, and band structure were researched at length. The average diameter of NHSO is 3.06 µm. Under 365 nm excitation, the NHSO:Eu2+ phosphors exhibit a broad yellow emission peaking at 545 nm with FWHM sbout 126 nm. NHSO:Eu2+ phosphors exhibit an asymmetric emission peak owing to Eu2+ occupying two types of Na+(Na1 and Na2) with different environment.The energy transfer between Eu2+ belongs to dipole-dipole interaction. A WLED lamp with high Ra(87.5) and low CCT(3953K) was obtained by combing NHSO:Eu2+ with BAM, CaAlSiN3:Eu2+ and n-UV chip(λmax=380 nm). Under low voltage and current condition, the phosphors also exhibit perfect stability with high current saturation. All these results suggest that NHSO:Eu2+ phosphor would be a potential candidate for n-UV LED and FED applications. Keywords: phosphor, Na4Hf2Si3O12:Eu2+, photoluminescence, cathodoluminescence, temperature 1

sensor.

1.Introduction On account of the especial electronic structure of rare-earth elements, rare-earth ions doped luminescent materials show abundant emission colours, intrinsic temperature-dependent relationship and so on, which have an extensive application in many fields. It not only has application in white light-emitting diodes(WLEDs) and field emission displays(FEDs), but also has a tremendous prospect in temperature sensor. At present, phosphor-converted white light-emitting diodes (pc-WLEDs) have obtained widespread attention owing to their merits of high efficiency, long operation life as well as energy-saving.[1-5] Nowadays, the universal method for obtaining white light with high-efficiency is to combine YAG:Ce3+ with InGaN-based blue chips. Whereas, this method leads to nethermore colour rendering index (Ra < 80) and upper correlated colour temperature (CCT) due to the shortage of green and red colours.[6-8] In order to avoid these problems, another effective way is to combine near ultraviolet (n-UV) chip with multi-colored phosphors. Therefore, it is urgent to develop novel phosphors with excellent performance.[9-11] Meanwhile, yellow phosphors play a significant part in field emission displays (FEDs), such as warning panels on highways and emergency display, FEDs have been deemed to the most greatest potential technology for next-generation flat panel displays.[12] FEDs possesses numerous superiorities including thin panels, broad viewing angle, fast response time, overlong lifetime, low energy consumption and excellent stability.[13, 14] Under the condition of low excitation voltages(≤10 kV) and high current densities (≥µA cm-2), FEDs phosphors must be highly efficient, wonderful stability and flawless degradation properties.[15] So far, there are a few effective sulfide-based phosphors have been commercialized for FEDs, for instance, [Zn(Cd)S:Cu,Al], [Gd2O2S:Tb3+] and [ZnS:Ag,Cl].[16-18] However, the sulfide-based powders would decompose under the high-energy electron bombardment owing to their instability.[19] In addition, they are environment-friendly because of the existence of S element. The problems limit their development and application in FEDs. Hence, it’s very necessary to find novel efficient phosphors with perfect stability for FEDs. 2

Silicate-based phosphors have a striking potential for luminescent materials owing to their outstanding chemical and physical stability, efficient luminescence performance and so on. Silicates are inexpensive and readily available, besides basic research, they are also used in LED/FED applications. In this work, Eu2+ doped tetrasodium hafnium tri-silicate Na4Hf2Si3O12 (NHSO) was first synthesized via solid-state reaction. The crystalline structure, morphology, photoluminescence and cathodoluminescence performances of Na4Hf2Si3O12:Eu2+ were studied in detail. A WLED lamp was obtained by combing Na4Hf2Si3O12:Eu2+, CaAlSiN3:Eu2+, BAM and a n-UV (λmax=380 nm) chip. These capabilities indicate the phosphor has enormous potential for application in WLEDs and FEDs.

2. Experimental section 2.1Preparation A series of Na4Hf2Si3O12:Eu2+ phosphors have been synthesized by a traditional solid-state reaction method. The raw materials Na2CO3(A.R), SiO2(A.R.), HfO2(A.R.) and Eu2O3 (>99.99%) were weighed in stoichiometric ratio. Then all the raw materials were put in an agate mortar for full grinding and the mixture was placed in alumina crucibles. Afterwards, the powder mixtures were sintered at 1150 oC under a reducing atmosphere (95%N2/5%H2) for 4 h. Finally, the resulting samples cooling to room temperature naturally and were regrounded into powder for further characterization analysis.

2.2Measurements and characterization The phase purity of Na4Hf2Si3O12:Eu2+ were analyzed by Bruker D2 PHASER X-ray diffractometer (XRD) using Cu Kα radiation. In addition, the XRD data were measured in the range of 10–80° with the step size of 0.03 and count time of 0.1 s per step. The Rietveld refinement of NHSO was obtained using the software General Structure Analysis System (GSAS).[20] The electronstructure of NHSO was calculated with density functional theory (DFT) and was performed with CASTEP code.[21] The diffuse reflectance spectra (DRS) were measured on UV-vis spectrophotometer using the BaSiO4 white powder as the reference. The photoluminescence (PL), photoluminescence excitation (PLE) spectra were recorded using Fluorolog-3 spectrofluorometer equipped with a 450W xenon light source. Thermal stability of 3

NHSO:Eu2+ was tested using a heating apparatus (TAP-02) in combination with PL equipment. The PL decay curves were measured by FLS920T fluorescence spectrophotometer with an F900 nanosecond flash hydrogen lamp as the light source. The CL properties of NHSO:Eu2+ phosphors were obtained using a modified Mp-Micro-S instrument.

3. Results and discussion 3.1Phase formation and crystal structure

Figure 1 Rietveld refinement XRD patterns of the NHSO host.

For the purpose of gaining exhaustive crystal structure information of Na4Hf2Si3O12. The XRD Rietveld data was obtained by using the crystal data of Na4Zr2Si3O12 as the original pattern. Figure 1 presents the observed (plus sign), calculated (red line) XRD patterns with difference (blue line) profiles and background (green line) for the Rietveld refinement of NHSO host. And the position of Bragg reflection of the calculated model is expressed by the pink plummet lines. The final refinement converges with the residual factors Rwp = 13.27% and Rp = 9.32%, which indicates the refinement results are reliable. The final refined crystallographic data, atomic coordinates and selected bond lengths of NHSO are listed in Table 1, Table 2 and Table 3, separately.

4

Table 1 Crystallographic data for NHSO Formula

Na4Hf2Si3O12

Crystal system

trigonal

Z

4

Space-group

R -3 c (167) 1611.24 Å3

V

4.48401 g/cm3

Calculated density

Table 2 Atomic coordinates for NHSO Atom

Wyckoff

x/a

y/b

z/c

Na1

36f

0

0

0

Na2

36f

-0.3702

0

0.25

Hf

12c

0

0

0.1469

Si

6b

0.2957

0

0.25

O1

18e

0.1839

0.1709

0.0947

O2

18e

0.1952

-0.0280

0.1936

Table 3 Selected bond lengths (Å) Bond

distance

Na1-O1(×6)

2.6625

Na2-O1(×2)

2.4634

Na2-O1(×2)

2.5848

Na2-O1(×2)

2.6403

5

Figure 2 The crystal structure of NHSO and the coordination environment of two Na+ sites with the bond lengths of Na-O.

As displayed in Figure 2, NHSO crystallizes as a trigonal structure. In this structure, [SiO4]4tetrahedrons are connected with [HfO6]8- octahedrons and isolated Na+ by sharing O2-. There are two kinds of Na+ (Na1 and Na2) environment, both are surrounded by six oxygen atoms. The activator Eu2+ is supposed to replace Na+ sites due to the ionic radius of Eu2+ (1.17 Å, coordination number(CN)=6) is lightly larger than Na+ (1.02 Å, 6CN) and far greater than Hf4+ (rHf4+=0.71 Å, 6CN). From Table 3, the average bond lengths of Na1-O and Na2-O are calculated as 2.6625 Å and 2.5628 Å, respectively.

6

Figure 3. XRD patterns of Na4Hf2Si3O12 with different Eu2+ concentration.

As shown in Figure 3, all the diffraction peaks of the Na4Hf2Si3O12:xEu2+ samples match well with the calculated data, illustrating that the good single phase is obtained. The diffraction peaks of the NHSO:Eu2+ slightly shifted towards a lower 2θ degree with the increase of Eu2+ concentration. According to Bragg equation, the left move of the peak positions could be attributed to lattice expansion. As the ionic radius of Eu2+ (1.17 Å, CN=6) is larger than Na+ (1.02 Å, CN= 6), when Na+ were replaced by the larger Eu2+ ions, the lattice was expanded. In order to achieve the charge balance, one Eu2+ ion would occupy one Na+ site and generates a Na vacancy at the same time. The defects reaction equation is shown as follows:

•

7

+

(1)

Figure 4(a) The SEM image of NHSO; (b) EDX spectrum of NHSO:0.03Eu2+; (c) The size distribution of NHSO particles; (d)SEM mapping(Na,Hf,Si, and Eu) of NHSO

The morphology image of NHSO particle has been studied by SEM. Figure. 4(a) displays the morphology of the typical sample, the particle shape is irregular and distribute across the screen. The elemental composition of NHSO:Eu2+ was shown in Figure. 4(b), the result expresses that the phosphor is consist of Na, Hf, Si, O, Eu and no impurities exist. The size of NHSO particles was also calculated. Figure. 4(c) presents that the average diameter of NHSO is 3.06 µm, which well satisfies the requirement of WLED. Figure. 4(d) exhibits that Na, Hf, Si, O and Eu distribute evenly on the surface of the sample. From the above results, a single-phase phosphor with well crystallinity was obtained.

8

Figure 5(a and b) Band structures of NHSO; (c) the DRS spectra of undoped and Eu2+-doped NHSO; (The inset exhibits the relationship of (hν)2 vs. photo energy hv for NHSO); (d) Total and partial density of the states for NHSO;

Based on the refined crystallographic results, the calculated band structure of NHSO was shown in Figure 5(a and b). The maximal value of valence band(VB) and the minimal value of conduction band(CB) are both at G point, which proves that the NHSO compounds possesse a direct bandgap of 4.33 eV. To verify if the computation is credible, the diffuse reflection spectrum(DRS) of the NHSO host was measured and depicted in Figure 5(c). In this case, the bandgap of the NHSO host could be calculated via the following equations:

(2) !

(3)

Where α refers to the absorption coefficient, hν equals to the incident photo energy, A is a constant, and n = 2 for a direct bandgap.[22] As displayed in Figure 5(c) inset, by extrapolating the value of hν to zero linearly, the obtained Eg value was 4.90 eV, which is close to the calculated value. The discordance is chiefly due to error of related function calculation, lightly 9

from the error from the DRS measurement and the estimation with equation(2).[23] Such a large bandgap indicates that the 4f65d ↔ 4f7 energy levels transition of the activator Eu2+ has little influence on the conduction band and valence band of NHSO. The results manifest that the NHSO would become a potential candidate for luminescent materials. The appropriate electronic structure is very important for phosphors to form the luminescence centers of rare earth ions, Figure 5(d) displays the calculated total density of states and partial density of states of NHSO. The image shows that VB is mainly dominated by Si-3s3p and O-2p orbitals with a bandwidth about 7 eV, while CB is chiefly consist of Hf-5d , Si-3p and O-2p orbitals. The maximal strength of the Na-2p orbital lies at -21.6 eV, which is distant with the top of VB, indicating that there are weak interactions between the Na-2p and O-2p orbitals because of the large bond length of Na-O.

3.2Photoluminescence Property

Figure 6 (a) PL and PLE spectra of NHSO:0.03Eu2+ (The inset shows the image of the NHSO:0.03Eu2+ phosphor under 365 nm excitation); (b) Gaussian peaks fitting for PL spectrum of NHSO:0.03Eu2+ monitored at 365 nm; (c) 10

The PL spectra of NHSO:xEu2+ with varying Eu2+ concentration. (Inset: the Eu2+concentration dependence of the emission peak); (d)The emission intensity of NHSO:xEu2+ phosphors with different Eu2+ concentration. (The inset shows the relationship between log(I/x) and log(x).)

Figure 6(a) depicted the PLE and PL spectra of NHSO with 3% Eu2+ dopant concentration, the inset displays the actual photograph of NHSO:0.03Eu2+ under 365 nm n-UV lamp. Monitoring at 545 nm, the wide excitation band can be detected with the scope covering from 260 to 450 nm. Under 365 nm excitation, the NHSO:0.03Eu2+ appears a bright yellow emission with peak value is 545 nm and the full width at half-maximum (FWHM) is 126 nm, which is ascribed to Eu2+ 4f65d→4f7 transitions.[24] As there are two symmetrically inequivalent Na+ (Na1 and Na2) sites in NHSO, the wide emission could be decomposed into two Gaussian curves centering at 534 nm(labelled as Eu1) and 597 nm(labelled as Eu2), as exhibited in Figure. 6(b). Based on the crystal structure, the average bond length of Na1-O is 2.6625 Å, the value is larger than Na2-O(2.5628 Å). Therefore, the substitution of Eu2+ for Na2 leads to a larger crystalline field intensity. On the basis of the theory of crystalline field splitting, the crystalline field strength could be determined by the following equation: [25] '(

)* ! + , -

.

(4)

where Ze is the charge of the ligand ions and r is the radius of the d wave function. According to eqn (4), the larger distance R corresponds to the weaker crystalline field intensity, which results in a longer wavelength of emission position, and vice versa.[26] Hence, it could draw a conclusion that the Eu1 and Eu2 occupy the positions of Na1 and Na2 in Na4Hf2Si3O12, respectively. Figure 6(c) shows the photoluminescence spectra of NHSO:xEu2+ phosphors with various Eu2+ concentrations (1%≤x≤6%) under 365 nm excitation. The intensity of emission peak increases with the increase of Eu2+ concentration until 3%, beyond which the concentration quenching phenomenon was observed. The concentration quenching could be ascribed mostly to the non-radiative energy transfer between Eu2+, its probability would increase along with the increase of Eu2+ concentration.[27] The critical distance (Rc) between activators is an important argument for concentration quenching, when Rc≤5 Å, the exchange interaction is deemed to be the cause of concentration 11

quenching phenomenon, in the other case, it would be attributed to electric multipolar interaction.[28] The value of Rc could be calculated by the below equation:[29] /0

12

34

5670 8

:/3

9

(5)

where xc is the critical concentration, Z is the number of formula units per unit cell, and V is the volume of the unit cell. As for Na4Hf2Si3O12 host, when Z = 4, xc = 0.03, and V = 1611.24 Å3, the calculated Rc is 29.49 Å. The value is much larger than the closest distance between adjacent Na+ (3.4370 Å), indicating that energy is easily transferred from one Eu2+ to another. Therefore, the content quenching phenomenon appears at a nethermore Eu2+ doping ratio is rational. The interaction type between Eu2+ centers could be determined by using the following equation:[30] <

7

= : > 7

? 3

(6)

where k and β are constants for each type of interaction for Na4Hf2Si3O12 and I/x is the emission intensity of each ion. Based on the outcomes published by Van Uitert, θ=6, 8 and10 stand for dipole–dipole, dipole–quadrupole, quadrupole–quadrupole interaction, separately.[31] As exhibited in Figure 6(d) inset, the relationship between log (I/x) and log(x) for NHSO:Eu2+ was fitted into a straight line. The gradient of the line was calculated as −2.10, meaning that θ= 6.30 and the value is closest to 6. From the above calculation results, it could be seen that the energy transfer among Eu2+ belongs to dipole–dipole interactions. As shown in Figure 6(c) inset, the emission peak manifested red shift first and then little blue shift. There are two main influence factors on peak position shift, one is crystal field strength and the other one is the energy transfer from higher energy levels to lower energy levels. When the doping content is very low, the crystal field strength is dominant. When more Na+ were replaced by larger Eu2+, the lattice would expand, which leads to weaker crystal field strength and emission peak shift to a shorter wavelength. Nevertheless, when doping density reached a larger value, the energy transfer of Eu2+ from higher energy levels to lower energy levels become more possible. Thus, the interaction among Eu2+ would play a leading role, which results in the shift to longer wavelength.

12

Figure 7. The decay curves of NHSO:xEu2+(0.01≤x≤0.06) monitored at 545 nm.

The decay curves of NHSO:Eu2+(0.01≤x≤0.06) were measured detailly to further clarify the energy transfer process between Eu2+. (Figure 7). The phosphors were excited at 365 nm and monitored at 546 nm. The attenuation curves could be primely fitted by a second-order index using the following equation:[32] I t

exp D

E

FG

H+

E

exp D

F!

H

(7)

Where I is the luminescence intensity, t is the time, A1 and A2 are constants, τ1 and τ2 are short and long lifetimes, respectively. Moreover, the average lifetime τ could be determined using the equation:[33] t

τ +

τ /

τ +

τ

(8)

According to the above-mentioned formula, the average lifetimes τ of NHSO:Eu2+ excited 365 nm are 487, 480, 478, 472, 445 and 426 ns corresponding to Eu2+ concentration of 1%, 2%, 3%, 4%, 5% and 6%, severally. Obviously, the value of τ decreases continuously with the increase of Eu2+ concentration, indicating that the non-radiative transition increases which ultimately results in Eu2+ concentration quenching.

13

Figure 8. Temperature dependent luminescence spectra of NHSO:0.03Eu2+ phosphor from 25 to 200 oC (the inset exhibits activation energies of the NHSO:0.03Eu2+ phosphor and the temperature dependence of the emission intensity of NHSO:0.03Eu2+)

Temperature characteristic of phosphors is a crucial coefficient for the application of LEDs. The temperature dependent PL spectra of NHSO:0.03Eu2+ excited at 365 nm is shown in Figure 8. With the increase of the temperature from 25 oC to 200 oC, the emission intensities show a strict linear (R2=0.988) decline which reveals that the phosphor could be used for a temperature sensor. Fluorescence-based temperature sensors could accomplish non-contacting temperature detection of fleet moving substances, strong electromagnetic fields and biological fluids. For now, most temperature sensors are designed by dual-emitting phosphors such as [BaMoO4:Er3+/Yb3+], [SrWO4:Er3+,Yb3+], [CaMoO4:Er3+,Yb3+].[34-36] Dual-emitting phosphors are generally doped with two kinds of RE ions, which results in higher costs. In addition, there are few design methods for single-peak temperature sensors. Hence, developing novel phosphors with distinctive physics and chemistry stability is meaningful. The sensitivity is a key parameter to value the possibility of practical applications, the absolute sensitivity SA is defined as: JK

LM

LN

(9)

Here, I is the relative intensity and T is the temperature. The calculated value is 0.0054/K (298K～473K), which is larger than the maximum sensitivity of previous reported dual-emitting phosphors LaOBr:0.01Ce3+,0.09Tb3+ (0.0042/K, 293～443K).[37] To further interpret the thermal stability and calculate the activation energy of NHSO:Eu2+, the 14

Arrhenius equation was used for calculating the activation energy ∆E:[38] OP

QR

STUV

WX YZ

(10)

Where IT is the intensity at different temperatures, I0 is the initial emission intensity of the phosphor at 250C, c is constant, ∆E is the activated energy, and k is the Boltzmann constant. Based on the above-mentioned formula, the obtained ∆E value of Na4Hf2Si3O12 was 0.1625 eV (Figure 8 inset), the value is small which is consistent with poor thermal stability.

Figure 9. The chromaticity coordinates of NHSO:0.03Eu2+

The chromaticity coordinate of NHSO:0.03Eu2+(0.4079,0.5365) has been displayed in Figure 9. In order to verify the practicality of the novel synthesized phosphors for pc-WLEDs, a LED lamp was fabricated by combing NHSO:0.03Eu2+ phosphor with BAM, CaAlSiN3:Eu2+ and near UV chips. The electroluminescence spectrum and actual picture are shown in the top right corner. It can be seen that the WLED lamp emits a warm white light with correlated color temperature (CCT) of 3953K, color rendering index (Ra) of 87.5, which satisfies the requirements of dwelling lighting. Compared with commercial YAG:Ce3+ combined with blue LED chip(Figure bottom right corner), NHSO:0.03Eu2+ has lower CCT and higher Ra, which indicates the yellow-emitting phosphor NHSO:0.03Eu2+ has a tremendous promising prospect in pc-WLEDs.

15

4.Cathodoluminescence

Figure 10. SEM image(a) and CL mapping image(b) of NHSO:0.03Eu2+ monitored at 545nm of the same area.

The cathodoluminescence (CL) performance of NHSO:0.03Eu2+ phosphor was investigated detailly to further explore its potential applications in FEDs. Figure 10 exhibits the SEM image and the homologous CL mapping image of NHSO:0.03Eu2+ at the same area, the depth of the color represents the intensity of the luminescence. The luminescence intensity is uniform on the surface of particles, indicated that Eu2+ are evenly distributed in the sample.

Figure 11. The cathodoluminescence spectra of NHSO:0.03Eu2+ phosphor with different accelerating voltage (2-8 kV).

From Figure 11, while the probe current is fixed to 50 mA, the CL intensity persistently enhances with the accelerating voltage from 2 to 8 kV (Figure 10 inset) and there is no voltage saturation was observed. To further explain the cause that CL intensity gradually increases with the electronic energy, the electronic penetration depth were calculated by using the empirical 16

equation:[39] [ Å

`

b

250 D H D H , n a √d

.

h. i jk d

(11)

where A is relative molecular mass, ρ is the density of phosphor, Z is the number of atoms or electrons per molecule in a compound, and E is the accelerating voltage in the unit of kV. For NHSO:Eu2+ phosphor, A= 725.1884, Z=4, ρ=4.48401 g/cm3, the calculated penetration depths of 2 kV, 4 kV, 6 kV and 8 kV are 3.4 nm, 20.8 nm, 59.7 nm and 464.0 nm, severally. Therefore, the increase of CL intensity with the increasing excitation voltage is attribute to a deeper electron penetration depth, which results in an increase in luminescence centers. In the circumstances, the deeper electron penetration depth, the more plasma engendered, which leads to more Eu2+ ions being stimulated and the increase of CL intensity.

Figure 12. The CL spectra of NHSO:0.03Eu2+ phosphor in 3D color mapping surface (20-90 mA).

In addition, while the excitation voltage is fixed to 5 kV, the CL intensity prominently increases along with the accelerating current increasing from 20 mA to 90 mA (Figure 12). The reason why CL intensity increases with the increase of accelerating current could be ascribe to the deeper electron penetration depth and the higher electron-beam current density.[40]

17

Figure 13.The degradation performance and the colour stability of NHSO:0.03Eu2+ with various bombarding time.

The degradation performance of phosphors is a key parameter in FEDs applications. The CL degradation stability of NHSO:0.03Eu2+ was measured detailly under prolonged low voltage electron beam excitation(5 kV, 60 mA). Figure 13 presents the relationship between phosphor CL intensity and the bombardment time. Apparently, the CL intensity slowly decreases with the successive electron bombardment from 10 to 90 min. After 90 min electron bombardmen, the CL intensity still keeps about 80% of the initial value, which indicates that the CL stability of NHSO:0.03Eu2+ is perfect. In the process of electron bombardment, carbon would accumulate on the surface which results in the decrease of CL intensity.[41] The accumulation of graphite carbon during electron-beam revealing at a high current density is a proverbial effect. The carbon pollution will not only hinders low-energy electrons from arriving phosphor particles, but also aggravates surface charging which leads to the reduction of CL intensity. As shown at the center of Figure 13, the colour stability of NHSO:0.03Eu2+ under a persistent electron beam bombardment with various bombardment time were estimated as well. The CIE values are almost immobile during continuous electron radiation. The experimental result indicates that the CL degradation performance and colour stability are both perfect, which proves NHSO:0.03Eu2+ phosphor has potential applications in FEDs.

18

5. Conclusions A series of Eu2+ doped NHSO phosphors have been successfully designed and synthesized for the first time. The calculated bandgap value based on DFT is 4.33 eV which is consistent with the value obtained from DRS. The phosphor can be effectively excited in the n-UV range and displays a broad yellow emission peaking at 545 nm, which could be attributed to the 4f7→4f65d transitions of the Eu2+. A WLED lamp was fabricated by combing NHSO:0.03Eu2+ phosphor with BAM, CaAlSiN3:Eu2+ and a n-UV chip, which possesses lower CCT(3953K) and higher Ra(87.5) compared with commercial YAG:Ce3+ combined with blue LED chip. Under prolonged low voltage electron-beam excitation, NHSO:0.03Eu2+ phosphors exhibit excellent degradation performance and perfect colour stability. The detailed basic research provides important value for the future development of novel WLEDs and FEDs.

Acknowledgment This work is supported by the National Natural Science Funds of China (Grant No. 51372105). Thanks for the support of Gansu Province Development and Reform Commission.

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Cathodoluminescent Phosphors

at High Current

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Highlights 1. A novel yellow-emitting phosphor Na4Hf2Si3O12:Eu2+ was synthesized for the fist time. 2.A WLED lamp was fabricated with higher Ra(87.5) and lower CCT(3953K) compared with ommercial YAG:Ce3+ combined with blue LED chip. 3. The phosphor shows characteristic thermal stability, which improves new ideas for the development of temperature sensors. 4. The phosphor also exhibits high current saturation and perfect stability under low voltage electron bombardment.

Declarations of interest: none