Luminescence properties of sodalite-type Zn4B6O13:Mn2+

Journal of Luminescence 199 (2018) 154–159

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Luminescence properties of sodalite-type Zn4B6O13:Mn2+ a

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Cuili Chen , Peiqing Cai , Lin Qin , Jing Wang , Shala Bi , Yanlin Huang , Hyo Jin Seo a b

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Department of Physics and Interdisciplinary Program of Biomedical, Mechanical and Electric Engineering, Pukyong National University, Busan 608737, Republic of Korea College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Green phosphor Zero phonon line Vibrational sidebands

Luminescence properties of sodalite Zn4B6O13:Mn2+ are investigated by optical and laser excitation spectroscopy in the temperature range 10 – 300 K. The samples of Zn4B6O13:Mn2+ are prepared in the carbon reducing atmosphere. The green emission at 540 nm are observed in Zn4B6O13:Mn2+ for various Mn2+ concentration. Optimum Mn2+ concentration and critical distances between Mn2+ ions in Zn4B6O13 are obtained by the luminescence intensity as functions of Mn2+ concentration. The decays of Mn2+ emission depend strongly on Mn2+ concentration due to the energy transfer and energy diffusion in Zn4B6O13:Mn2+. The fine structures consisting of the intense zero phonon line (ZPL) and weak vibrational sidebands are observed at low temperature. The relevant mechanism of excitation bands, ZPL, vibrational sidebands and the bandwidth as functions of temperature are interpreted in detail by energy level diagram of the 3d5 transition metal ions and configuration coordinated diagram. The high thermal stability of the Zn4B6O13:Mn2+ phosphor are observed in the temperature range 10–300 K.

1. Introduction The luminescence properties of transition metal ions are characterized by the incompletely filled d electrons shell with non-negligible electron-phonon interaction. Luminescence centers of the transition metal ions are usually influenced by the crystal field strength depending almost on the structure of host lattices. Mn2+ as one of the transition metal ions has the electronic configuration of 3d5 and the luminescence from Mn2+ ions could be adjusted by the different coordination environments [1]. The Mn2+ ions substitute for the sites with different coordination numbers including 4, 6, 7, 8, 9, 10, for examples, Li2ZnGeO4 (4) [2], Ca2P2O7 (6) [3], Ca5(PO4)3F (7, 9) [4], NaScSi2O6 (8) [5], and NaCaPO4 (8, 10) [6]. The red light could be obtained from the Mn2+ ions occupied in the octahedral coordination sites due to the relatively strong crystal field strength, whereas, the green light from the Mn2+ ions occupied in the tetrahedral sites, for examples, Li2ZnGeO4:Mn2+ [2] [ii] as green- and CaSr2Al2O6:Mn2+ [7] as red-emitting materials. The Mn2+-doped materials are considered as research topics for decades. ZnSiO4:Mn2+ is known as famous non-rare earth green phosphor for commercial application. The energy transfer between Mn2+ and Eu2+, Ce3+, Tb3+ is attracted researcher's attention because of its color tunable process such as in Ca9Mg(PO4)6F2:Eu2+, Mn2+ [8] and Mg2Y8(SiO4)6O2:Ce3+/Mn2+/Tb3+ [9]. Materials doped with Mn2+ ions are also used as persistent luminescence phosphors such as

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Corresponding author. E-mail address: [email protected] (H.J. Seo).

https://doi.org/10.1016/j.jlumin.2018.03.022 Received 26 January 2018; Received in revised form 2 March 2018; Accepted 11 March 2018 Available online 14 March 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.

well-known persistent material of MgGeO3:Mn2+ [10]. In this paper, we focus the study on the narrow emission band and fine structure of Mn2+ in Zn4B6O13:Mn2+. Zn4B6O13 is isostructural with sodalite Na8(Al6Si6O24)Cl2 in which the Na atoms are replaced by Zn atoms, the Si and Al atoms by B atoms and the Cl atoms by oxygen. All the B atoms are coordinated by 4 oxygen atoms and linked each other to form the framework [11]. Zn4B6O13 has relatively simple crystal structure with BO4 tetrahedrons linked to structural framework with a large cavity in which the ZnO4 tetrahedrons occupy. This is beneficial to study the fine structure of the Mn2+-doped Zn4B6O13 phosphor. We investigate luminescence properties of Mn2+-doped Zn4B6O13 with sodalite structure as an efficient green phosphor. The luminescence performance of Zn4B6O13:Mn2+ with various temperature and the related mechanism are described in detail. Specially, the fine structure consisting of the distinct zero phonon line and vibrational sidebands are observed at low temperature. 2. Experiment The phosphors of Zn4B6O13:Mn2+ were synthesized via simple solid-state reaction using starting materials of ZnO (99.99%), H3BO3 (99.9%) and MnCO3 (99.99%). All the starting reagents were mixed in stoichiometric ratios and ground in an agate mortar for half an hour to obtain the homogeneous mixture. Then, the mixture was calcined in the

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muffle furnace. The heating procedure is as follows: firstly, calcined at 500 °C for 5 h in air atmosphere; then calcined at 700 and 900 °C for 7 h in reduced carbon atmosphere. Thus the Zn4B6O13:Mn2+ crystals were successfully obtained as cooling down to room temperature. The sample characterization of the powders was carried out using the X-ray diffraction (XRD) with Philips XPert/MPD operating with CuKα radiation as an incident beam. The excitation and emission spectra were recorded with a Photon Technology International (PTI) spectrofluorimeter. The temperature dependent emission spectra are obtained by the excitation of a continuous wave 458 nm argon laser (The Coherent Innova 60/70 Series Ion Laser). The decay curves were recorded by the excitation of a 266 nm-pulsed laser with a pulse width of 5 ns (Spectron Laser Sys. SL802G). The sample temperature was controlled by a closed-cycle helium cryostat in the temperature region 10–300 K.

mentioned above, all the Zn2+ ions are coordinated by four O2- ions and form the tetrahedral sites. The result of the green emission at 540 nm is in accordance with the theoretical prediction. The excitation spectrum exhibits the strongest band at 240 nm and several weak bands at 442, 422, 376 and 340 nm attributed to the excitation from the ground 6A1 state to the excited 4T2(G), 4A1(G), 4T2(D) and 4E(D) states, respectively. The strongest excitation band at 240 nm cannot be observed in most of the Mn2+ doped materials such as NaZnPO4:Mn2+ [14] and NaCaPO4:Mn2+ phosphor [6]. The 240 nm band could be ascribed to the ionization of Mn2+ to Mn3+ or the transition from d5 to d4s [15]. Similarly, strong excitation band shorter than 280 nm is found in Zn2SiO4:Mn2+ phosphor [16]. The excitation at 250 nm is in favor of the Zn4B6O13:Mn2+ used as an efficient lamp phosphor since the mercury lamp as the excitation source emits the ultraviolet line at 253.7 nm. In addition, the excitation band at 423 nm attributed to the 6A1(S)→4A1(G) transition is relatively narrow which is in accordance with the prediction of the Tanabe-Sugano diagrams. Since the 4A1(G) level is almost parallel to the abscissa axis, which means 4A1(G) level is insensitive to the influence of the surroundings [17]. Fig. 3(a) shows emission spectra of Zn4B6O13:Mn2+ as functions of the Mn2+ concentration. The emission intensity increases quickly up to 2 mol% and then decreases gradually. It was confirmed that the intensity quenches totally for the Mn2+ concentration of 70 mol%. The critical distance for luminescence quenching could be evaluated by the equation [18].

3. Results and discussion 3.1. The crystal structure of sodalite Zn4B6O13 The crystal phases of Zn4B6O13:Mn2+ for various Mn2+ concentrations were confirmed by the XRD patterns as displayed in Fig. 1(a). All the diffraction peaks match well with the cubic phase of the Zn4B6O13 structure (JCPDs PDF#36–1451). There exists no obvious change in XRD patterns after doping of Mn2+ ions. Lattice constants of samples are calculated according to the XRD data in order to investigate the change of unit cell parameters with increasing Mn2+ concentrations. The lattice parameters of cubic Zn4B6O13:Mn2+ are 7.4266, 7.4277 and 7.4634 Å when Mn2+ concentrations are 0.1, 1.0 and 20 mol%, respectively. The lattice constants slightly increase with increasing Mn2+ concentration due to the ionic radius of Mn2+ is larger than that of Zn2+ in the same coordinate number. Fig. 1(b) displays the details of the crystal framework for the sodalite Zn4B6O13. Generally, the stability of metaborate demands threefold coordination of the boron atoms at atmospheric pressure, which reported by Murray et al. [12]. But for the cubic zinc metaborate Zn4B6O13, all the boron atoms are coordinated by four oxygen ions. The Zn4B6O13 crystal has a sodalite-like structure of Na8(Al6Si6O24)Cl2 in which the Zn atoms coordinated by a center O atom and three other O atoms shared by B atoms. The center O atom is replaced by the position of Cl atom in Na8(Al6Si6O24)Cl2 structure. As seen in Fig. 1(b), a large cavity is formed by B-O tetrahedrons in which boron atoms coordinated by four O2- ions. The cavities are occupied by the Zn2+ sites. The Zn2+ ions are also coordinated by four O2- ions and every four Zn-O tetrahedrons aggregate as Zn clusters by point-shared. There are 8 Zn2+ sites in every unit cell and all the Zn2+ sites are identical in the cubic Zn4B6O13 structure.

1/3

3V ⎤ Rc ≈ 2 ⎡ ⎢ 4πXc N ⎦ ⎥ ⎣

(1)

Where, V represents the unit cell volume, N is the number of doping site in the unit cell, Xc is the critical concentration of the dopant ions. For Zn4B6O13:Mn2+ crystals, V is 418.17 Å3, the N is 8 and the Xc is 2% for Mn2+. Thus the critical distance (Rc) of the Zn4B6O13:Mn2+ is evaluated to be 17.09 Å. The decays of Zn4B6O13:Mn2+ for various Mn2+ concentrations are shown in Fig. 3(b). The decay curves consist of two decay components: fast and slow. The decays are fast in the early times and then become slow with single exponential as depicted by the solid lines in Fig. 3(b). The fast components become stronger with increasing Mn2+ concentration and dominant over the Mn2+ concentration of 10 mol%. The slow components for all the Mn2+ concentrations have the same single exponential with decay time of 12.0 ms. The parity and spin forbidden transition of Mn2+ in Zn4B6O13 is responsible for the relatively long decay time of milliseconds scale. As discussed above about the crystal structure in Fig. 1(b), only one type of the optical active site exists in Zn4B6O13:Mn2+. Thus the slow components correspond to the electronic transition of 4T1(G) → 6A1(S) of Mn2+ in the single Zn2+ site. The fast components are attributed to the fast energy transfer from the Mn2+ ions to defect centers (quenching centers) or other Mn2+ ions. For the high Mn2+ concentrations of 10 and 20 mol%, the energy diffusion among the Mn2+ ions dominates resulting in fast decays. The average decay time is estimated to be 1.70 ms at the concentration of 20 mol% Mn2+.

3.2. The Mn2+ luminescence and concentration dependence of Zn4B6O13:Mn2+ Fig. 2(a) shows excitation and emission spectra of the Zn4B6O13:Mn2+ phosphor. The emission spectrum was obtained under excitation at 250 nm and the excitation spectrum was obtained by monitoring the 540 nm emission at room temperature. The excitation spectrum consists of several absorption bands corresponding to the transitions from the 6A1(S) state to the 4T2(G), 4A1(G), 4T2(D) and 4E(D) states. The single emission band at 540 nm corresponds to the transition from the 4T1(G) state to the 6A1(S) state. The luminescence mechanism of the transition metal Mn2+ ions can be illustrated in detail through the Tanabe-Sugano energy level diagram for the d5 configuration as shown in Fig. 2(b) [13]. The vertical coordinates on the left is the energy levels of free Mn2+ ion and the energy levels split by the crystal field acting on the Mn2+ ion. The substitution of Mn2+ ions for the tetrahedral sites with relatively weak crystal field strength leads to the green light, while it will emit orange to red light as the Mn2+ ions doped into the octahedral sites. As

3.3. The low temperature luminescence properties of the Zn4B6O13:Mn2+ Fig. 4(a) shows the emission spectrum of Zn3.92B6O13:0.08Mn2+ (2 mol%) under excitation at 458 nm at 10 K. The spectral feature at 10 K is totally different from that at room temperature. The intense sharp line at 534 nm is assigned to the zero-phonon line (ZPL) of the 4 T1→6A1 transition. The ZPL accompanies vibrational sidebands at 538, 548 and 560 nm. The emission intensities of the vibrational sidebands are regarded as the intensities borrowing from the ZPL [19]. Similarly, the ZPLs of Mn2+ are also observed in other materials at low temperature like the well-known silicate phosphor Zn2SiO4:Mn2+ [20]. The 155

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Relative Intensity(a.u.)

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PDF: 36-1451 10

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2 theta

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Fig. 1. (a) XRD Patterns of the Zn4B6O13 and Zn4B6O13:Mn2+ crystals; (b) the graphic representation of the Zn4B6O13 crystal structure: the cavity framework formed by the boron atoms, the B-O tetrahedrons, a full unit cell of Zn4B6O13 and the Zn-O tetrahedrons clusters.

10 − 300 K. The red solid line in Fig. 4(c) is the best fit result according to Eq. (2). The phonon energy ћω and Huang-Rhys parameter S are calculated to be 502 cm−1 and 0.652, respectively. For comparison, the phonon energies of other materials are listed in Table 1. The phonon energy of Zn4B6O13:Mn2+ is larger compared to other oxide materials like Mg4Ta2O9:Mn2+ but similar to the fluoride KZnF3:Mn2+. This is because the energy of a symmetric breathing vibration (often as the dominant mode) depends only on the mass of the ligands and the force constant of bond [22]. The host material Zn4B6O13 is composed of light boron element, thus the phonon energy is relatively high. In addition, the difference in electron-phonon coupling in different materials can be characterized by the Huang-Rhys parameter S. All the intensity belongs to the ZPL for S = 0. The intensity of the ZPL

emission spectra of Zn4B6O13:Mn2+ as functions of temperature are shown in Fig. 4(b). The emission intensity of the ZPL decreases, while the intensities of the vibrational sidebands increase with increasing temperature. The band shape changes gradually and becomes broad and asymmetric up to room temperature. The intensity of the ZPL as a function of temperature can be written as follows [21]:

I = I0exp (− Scoth

ћω ) 2kT

(2)

Where, the S is Huang-Rhys parameter, ћω is phonon energy, the k is Boltzmann constant and the T is temperature. Eq. (2) describes that the intensity of the ZPL decreases and those of the vibrational sidebands increase with increasing temperature. Fig. 4(c) shows the ZPL intensity of the Zn4B6O13:Mn2+ as a function of temperature in the range of 156

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Fig. 2. (a) Excitation and emission spectra of the Zn3.92B6O13:0.08Mn2+ crystals under excitation at 250 nm and by monitoring at 540 nm, respectively; (b) The energy level diagram of the transition metal ions with d5 configuration.

ħω 2kT

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Fig. 3. (a) Emission spectra of the Zn4B6O13:Mn2+ crystals as functions of Mn2+ concentration. The inset is the relative integrated emission intensity of the Zn4B6O13:Mn2+ crystals for different Mn2+ concentration (b) Decay Curves of Zn4B6O13:Mn2+ for various Mn2+ concentration obtained by monitoring the 540 nm emission excited with a pulsed laser at 266 nm.

decreases as the S value increases and the vibrational sidebands appears for the intensity compensation. The ZPL will disappear as the S value increases to a sufficiently large value. Therefore, the Huang-Rhys parameter S for the electron-phonon coupling strength is a crucial parameter for luminescence materials to determine the band shape of luminescence spectra. The fit result (S=0.652) indicates that the S value is less than 1, which means the relatively weak electron-phonon coupling in the material of Zn4B6O13:Mn2+. Therefore, in this work the ZPL intensity is strong and the intensity of vibrational sideband is relatively weak as shown in Fig. 4a and b. Besides, it is worth to note that the emission band of Zn3.92B6O13:0.08Mn2+ expands to the short wavelength side of the ZPL with increasing temperature as shown in Fig. 4(b). The bandwidth as a function of temperature is given by the equation [19]:

Г(T ) ≅ Г(0) coth

0.02

Decay Time ( s )

respectively. As shown in Fig. 5(a), the maximum peak intensity occurs at the location of ZPL and the vibrational sidebands only occur at the longer wavelength side of the ZPL at 10 K, which is in accordance with the case of the materials with weak electron-phonon coupling (S < 1). However, the emission intensities of vibrational sidebands from the short wavelength side increase and they are unseparated at high temperature (Fig. 5(b)), which causes the broadening of emission bands with increasing temperature. The shapes of emission spectra can be explained by the coupling of electronic transitions to vibrational levels as shown in Fig. 5(a) and (b). When the luminescent ions with surrounding ligands are located at the equilibrium position, the system is at the lowest electronic energy. The equilibrium position could displace as the transition occurs from the ground state to the excited state. The displacement ΔR depends on the strength of the electron-phonon coupling, which is characterized by Huang-Rhys parameter S. The quantum mechanical description of the energy potential results in the occurrence of the discrete vibrational state. The energy difference between the adjacent vibrational levels is so-called phonon energy ћω. For the case of the Zn3.92B6O13:0.08Mn2+, it shows small displacement ΔR as the Huang-Rhy parameter S = 0.652 which is less than 1 as shown in Fig. 5(a) and (b). When the transition occurs from the ground state to higher vibrational levels of the excited state, it decays quickly to the lowest vibrational level of the excited state by multiphonon relaxation. Subsequently, it returns to the vibrational level of the ground state by radiative decay. The radiative decay from v′ = 0 to v = 0 is the zero-phonon transition. The radiative decay from v′ = 0 to

(3)

Eq. (3) represents the emission bandwidth which increases with increasing temperature. The bandwidth as a function of temperature is displayed in Fig. 4(d) and it is well fitted for temperature to the Eq. (3). The fitting parameter of the phonon energy ћω is calculated to be 500 cm−1, which is in accordance with the above discussed result (502 cm−1) of the ZPL intensity as a function of temperature. The vibrational sidebands at shorter and longer wavelength sides of the ZPL are responsible for the increase in bandwidth of Zn3.92B6O13:0.08Mn2+ with increasing temperature as shown in Fig. 4(b). These vibrational sidebands at the short wavelength side are called anti-Stokes sidebands. Fig. 5(a) and (b) display the configuration coordinate diagram to illustrate the band shape and the transition mechanism of the Zn3.92B6O13:0.08Mn2+ at low temperature and room temperature, 157

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Fig. 4. (a) Emission spectrum of Zn3.92B6O13:0.08Mn2+ excited with an Ar-ion laser at 458 nm at 10 K; (b) Emission spectra of the Zn3.92B6O13:0.08Mn2+ as functions of temperature from 10 to 300 K; (c) Temperature dependent emission intensity of the zero phonon line; (d) Temperature dependent bandwidth of the emission spectrum.

of Zn3.92B6O13:0.08Mn2+ are not significantly influenced by the temperatures. The Mn2+-doped materials, for instance, NaCaPO4:Mn2+ phosphor shows also non-significant changes in decay curves at different temperatures [6]. Besides, Mn2+-doped quantum dots (ZnS:Mn2+) also exhibits excellent thermal stability at temperature range from 100 to 500 K [25]. Generally, decay times usually decrease with increasing temperature due to thermal quenching effects, such as multiphonon relaxation, thermally activated nonradiative transition or thermally activated photoionization from the excited state to the host conduction band. However, the Zn4B6O13:Mn2+ displays good thermal stability with low thermal quenching effects. The unusual performance in thermal stability indicates the peculiar character in Mn2+ doped materials.

Table 1 The phonon energy of different materials. Materials

ћω (cm−1)

Reference

Cs2NaYBr6:Cr3+ Cs2NaInCl6:Cr3+ Al2O3:Cr3+ Mg4Ta2O9:Mn2+ KZnF3:Mn2+ Zn4B6O13:Mn2+

183 298 250 121 540 502

[23] [23] [19] [20] [24] This work

other higher vibrational levels results in the vibrational sidebands as shown in Fig. 5(a). The intensity of the vibrational sidebands are relatively weak compared to the ZPL due to the low phonon energy at low temperature. As temperature increases, the increase in phonon density of state induces the increase in vibrational sidebands and simultaneously the decrease in the ZPL intensity. In addition, radiative decays from higher vibrational levels of the excited state occur, which lead to the vibrational sidebands at shorter wavelength side of the ZPL as shown in Fig. 5(b). Fig. 6 shows the decay curves of Zn3.92B6O13:0.08Mn2+obtained by monitoring 540 nm emission under excitation at 266 nm in the temperature range 10–300 K. No significant change in decay curves with temperature is observed in the temperature range 10–300 K. As discussed in Fig. 3(b), the decays exhibit nonexponential with fast decays in the early times and then single exponential in the late times. The calculated average decay times including fast and slow components are shown in the inset of Fig. 6. The result indicates that the decay times are insensitive to the temperatures in the range of 10–300 K. It remains several milliseconds: the longest 7.89 ms and the shortest 6.16 ms. This means that the electronic transition rates in the luminescence material

4. Conclusions The sodalite-type Zn4B6O13:Mn2+ is successfully synthesized by the solid state method at a relatively low temperature in the carbon reducing atmosphere. The non-rare earth Zn4B6O13:Mn2+ phosphor emits green light due to relatively weak crystal field strength of the tetrahedral site. The concentration dependence of emission spectra shows that the optimized concentration is 2 mol% with the critical quenching distance of 17.09 Å. The decay curves with various Mn2+ concentrations consist of slow and fast components. The fast component is attributed to fast energy transfer to defect centers or other Mn2+ ions. The emission spectra of the Zn3.92B6O13:0.08Mn2+ phosphor displays different luminescence performance at various temperatures in the range of 10 – 300 K. The fine structures with the sharp and intense ZPL at 534 nm and several vibrational sidebands are observed at 10 K. The distinct shapes of the emission band at low and room temperatures are interpreted in detail by configuration coordinate diagram. The 158

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e Luminescence Intensity (arb.units)

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relaxation 3' 2' 1' v'=0'

vibrational sidebands Ex

g

ZPL

0.005

0.020

0.025

0.030

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Fig. 6. Temperature dependent decay curves of the Zn3.92B6O13:0.08Mn2+ under excitation at 266 nm. The inset shows temperature dependent average decay times of the Zn3.92B6O13:0.08Mn2+.

Wavelength (nm)

2 1 v=0

0.015

Decay Time ( s )

emission

3

0.010

through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03029432).

R

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emission 3' 2'

1'

0'

vibrational sidebands

vibrational sidebands Ex

g

Wavelength (nm)

relaxation

ZPL

3 2 1

0

R Fig. 5. Configuration coordinate diagrams with emission spectra at 10 (a) and 300 K (b).

insensitive decays of Zn3.92B6O13:0.08Mn2+ to the temperature indicates high thermal stability of Mn2+-doped Zn4B6O13. Acknowledgements This research was supported by Basic Science Research Program

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