Design of a Novel Near-Infrared Phosphor by Controlling the

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Design of A Novel Near-Infrared Phosphor by Controlling Cationic Coordination Environment Xiangyu Meng, Zhijun Wang, Keliang Qiu, Yuebin Li, Jinjin Liu, Zhipeng Wang, Simin Liu, Xue Li, Zhiping Yang, and Panlai Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00672 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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Crystal Growth & Design

Design of A Novel Near-Infrared Phosphor by Controlling Cationic Coordination Environment Xiangyu Meng, Zhijun Wang*, Keliang Qiu, Yuebin Li, Jinjin Liu, Zhipeng Wang, Simin Liu, Xue Li, Zhiping Yang, Panlai Li* College of Physics Science & Technology, Hebei Key Lab of Optic-Electronic Information and Materials, Hebei University, Baoding 071002, China Abstract: Phosphors with the emission spectra located at the biological window I (650nm~950nm) are significant for biological imaging. In this work, a series of deep red and near infrared phosphors In1-xMgGaO4: xCr3+ and In0.9-yMgGaO4: 0.1Cr3+, yAl3+ are designed and successfully synthesized by a high temperature solid state method. InMgGaO4 is selected as the host considering its special crystal structure that one of the Mg/Ga-O bond is impressible to the surrounding environment. Therefore, when Cr3+ substituting into the lattice, the longer Mg/Ga-O bonds are easy to be broken, which provide a tunable crystal field. The emission spectra of InMgGaO4: xCr3+ cover from 650nm to 1200nm including one sharp line emission peak (peak1) and two broad emission bands (peak2 and peak3). The Racha parameters Dq/B and the decay curves are analyzed to distinguish the origins these three peaks. Meanwhile, these three emission peaks show different degrees of red shift, which is related to the covalency, crystal field splitting (Dq), bond breaking of Mg/Ga-O and the decrease of band gap. However, comparing with the luminescent property of Cr3+ single doped samples, In0.9-yMgGaO4: 0.1Cr3+, yAl3+ show a contrasting luminescence property and the reason is analyzed. In summary, the emission spectra of these samples can be tuned between narrow peak and broad band continuously by controlling the concentration of Cr3+ ions or Al3+ ions, which show a potential application in biological imaging. 1. Introduction Since the proposal about using red to near-infrared (NIR) persistent particles as an optical label is put forward by Chermont et al in 2007, phosphors with the emission band ranging in the NIR I (650-950 nm) and NIR II (1000-1450 nm) have received considerable attentions[1]. Biological tissue exhibits low absorption and high scattering properties in the near infrared region, so near-infrared (NIR) can penetrate biological tissue in a depth of several centimeters

[2-4]

. In biological tissues,

there are many chromophores, such as oxyhemoglobin (HbO2) and de-oxyhemoglobin (HbR), which are closely related to oxygen metabolism and show different absorption in the near-infrared 1

ACS Paragon Plus Environment

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spectrum. So researching for the NIR phosphors with tunable wavelength is necessary. At present, the most widely studied near-infrared fluorescent materials are mainly concentrated in inorganic quantum dots and organic fluorescent dyes

[5-6]

. Inorganic quantum dots usually release harmful

heavy metal ions in an oxidizing environment, which limits their practical application [7]. In addition, organic fluorescent dyes usually have a small stokes shift, which leads to the overlap with background light easily and not conducive to application in the biological imaging

[8]

. Therefore, it

remains a challenge that searching for the near-infrared fluorescent probes with high brightness, non-toxic and good optical stability. Trivalent chromium ion (Cr3+) is a common doping ion for red and NIR luminescence due to its wide-range emission from 650 to 1400 nm, including the spin-forbidden emission around 700 nm from the spin-forbidden transition 2E(2G)→4A2(4F) transition and the spin-allowed transition of 4

T2(4F)→4A2(4F) relying on the host lattices crystal field strength [9]. The luminescence properties of

trivalent Cr3+ ions are strongly influenced by the crystal field around it due to the 3d3 electron configuration of Cr3+ ions. Generally, Cr3+ ion show sharp line emission or broadband emission in strong or weak crystal field strength, and these two kinds of emission exist simultaneously in the intermediate crystal field strength

[10-13]

. And the position of 4T2(4F) level shifts depending on the

crystal field around it. In the strong crystal field, the 4T2(4F) level is beyond the 2E(2G) level and the sharp line emission peak from the spin-forbidden transition is dominant. In the weak crystal field, the 4T2(4F) level is below the 2E(2G) level and the broad emission band from the spin-allowed transition is dominant. And in the intermediate crystal field, the 4T2(4F) level and 2Eg level are overlapped, which leads to the coincidence of these two kinds transition. In addition, the crystal field stabilization energy of Cr3+ ions in tetrahedral sites is almost three times weaker than in octahedral sites[14], therefore, the Cr3+ occupy preferentially into the octahedral sites and produce red or near-infrared emission. However, Cr3+ usually show a narrow band emission in many gallate which does not match well with the biological window I (650nm~950nm) [15-17]. In order to provide a tune crystal field for Cr3+, the substrate with tunable structure is required. Compounds with the same molecular formula but different structures and properties are called isomers. In general, compounds with the structural formula of AB2O4 (where A is a divalent cation and B is a trivalent cation) can be classified into four structures: K2NiF4, CaFe2O4, YbFe2O4 and spinel structure, which are determined by the coordination of cations. In these four structures, the 2


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divalent cation and trivalent cation are located in nine/ten and six folds, six and eight folds, five and six folds, four and six folds, respectively. All this structures belong to isomers, and in this work, the luminescence characteristic of Cr3+ is controlled by forming isomers. In the ideal crystalline structure of InMgGaO4, there are two kinds of cationic sites with five-fold and six-fold coordinate at 3a and 6h sites, so it belongs to the YbFe2O4 structure. However, one of the Mg/Ga-O bonds is impressible to the environment, which is easy to be broken to form the spinel structure

[18]

. So the

isomers of YbFe2O4 structure and spinel structure are concomitant in InMgGaO4 structure, which is uniform and can be controlled by the external environment. With doping Cr3+ into the lattice, it is easy to break the impressible Mg/Ga-O bonds, which has a great influence on the crystal field strength. Therefore, InMgGaO4 is selected as the host to provide a tunable crystal field environment for Cr3+. 2. Experimental 2.1. Materials and synthesis In1-xMgGaO4: xCr3+ (0≤x≤0.1) and In0.9-yMgGaO4: 0.1Cr3+, yAl3+ (0≤y≤0.9) are synthesized by a high temperature solid state method in the air. Raw materials In2O3 (99.99%), MgO (99.99%), Ga2O3 (99.99%), Cr2O3 (99.99%) and Al2O3 (99.99%) are mixed and ground for more than 30 min in an agate mortar to ensure a completely uniform distribution. Then, the mixtures are transferred into alumina crucible and heated at 1500°C for 6h (in the air). Finally, the samples are cooled to room temperature and ground into a powder for measurements. 2.2. Materials characterization Phase formation of samples are measured by X-Ray Powder Diffraction (XRD) (a Bruker D8 X-ray diffractometer with Ni-filtered Cu Kα radiation (λ=0.15405 nm), operating at 40 mA, 40 kV), and the step length and diffraction range are 0.05° and 20°-80°, respectively. Software of Find it, general structure analysis system (GSAS) program and Crystalmaker are used to ensure phase information. Crystal structures are examined by high resolution transmission electron microscopy (HRTEM) which was operated at an acceleration voltage of 200 kV (JEOL- 2010UHR). Spectral property and the luminescence decay curves of samples are determined using a FLS920 (Edinburgh Instruments) fluorescence spectrometer. Absorption and reflectance spectra are measured by a Hitachi U4100 UV-VIS-NIR Spectroscopy with the reference of BaSO4 ranging from 200 nm to 800 nm. 3. Result and discussion 3



3.1. Phase formation The crystal structure of InMgGaO4 is rhombohedral, space group R-3m (No.166) with lattice parameters of a=3.304 Å, b=3.304 Å, c=25.81 Å, and V=244.51 Å3, Mg2+ and Ga3+ located at the 3a site with five coordination and In3+ located at the 6h site with six coordination[18]. Fig.1 shows the crystal structure diagram of InMgGaO4 that one of the Mg/Ga-O bonds is much longer than others, which is impressible and unstable. The X-ray diffraction patterns of In1-xMgGaO4: xCr3+ (0≤x≤0.1) are shown in Fig.1, and the diffraction peaks of InMgGaO4 can be indexed to the standard card pattern of PDF#38-1106. However, as Cr3+ doping into the In3+ site, there will be a shrink of the octahedron due to the smaller radius of Cr3+ than In3+ and the hexahedron of [Mg/GaO5] will be pulled towards to the octahedron, which leads to the breakage of the longer Mg/Ga-O bonds to form the spinel structure. As a result, the X-ray diffraction peaks belonging to the spinel structure present at 35 degrees and 30 degrees, which can be indexed to the standard card patterns of PDF#16-0215. The changes of the XRD patterns are in our expectation, which is caused by the bond breaking of Mg/Ga-O and kept in single phase all the way. In addition, with increasing Cr3+ concentration, the amounts of the broken Mg-O bonds increases and the intensity of the impure diffraction peak increases, which indicate that the spinel structure is more and more predominant. When the Cr3+ concentration up to 0.1, all the impressible Mg-O bonds are broken and the spinel structure consisted by tetrahedrons and octahedrons are formed. Fig.2 shows the results of Rietveld refinement of InMgGaO4: xCr3+ (x=0, 0.005, 0.03 and 0.07), which are conducted by the general structure analysis system (GSAS) program [19]. The red lines present calculated intensities; the black crosses stand for the observed intensities; the blue solid lines below the profiles are the difference between the observed and calculated intensities; the short gray vertical lines show the position of Bragg reflections of the calculated pattern. The processes were stable and ended with low R-factors and the refined crystallographic data of these samples are listed in Table.1. It can be seen that the average bond length of the octahedron (R) is shortened due to the smaller ionic radii of Cr3+ (0.61 Å, N=6) than that of In3+ (0.8 Å, N=6). At the same time, the longer Mg/Ga-O bond length of the hexahedron increase along with the shrinkage of the octahedron. When the Cr3+ ion concentration is 0.03, the Mg-O bonds are broken firstly and still maintains a part of [GaO5] structure, which attributes to the stronger electronegativity of Ga3+ (1.81) than Mg2+ (1.31). The more electronegative of the element is, the greater ability to attract electrons, therefore, the Ga-O bond is more stable than the Mg-O 4


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bond. When Cr3+ concentration increase to 0.07, most the [Mg/GaO5] hexahedrons turn to [Mg/GaO4] tetrahedrons.

Fig.1. Crystal structure of InMgGaO4 and X-ray diffraction patterns of InMgGaO4: xCr3+.

Table.1 Rietveld refinement parameters of InMgGaO4: xCr3+ Cr3+

χ2

WRP

RP

R(octahedron)

concentration

Coordination

Coordination

Number of Mg

Number of Ga

0

3.585

0.0965

0.0702

2.205

5

5

0.005

2.581

0.0869

0.0653

2.197

5

5

0.03

3.808

0.1154

0.0788

2.195

4

5

0.07

6.605

0.1524

0.0982

2.189

4

4

Fig.2. Rietveld refinement of InMgGaO4: xCr3+ (x=0, 0.005, 0.03 and 0.07). 5



Fig.3. X-ray diffraction patterns of In0.1MgGaO4: 0.1Cr3+, yAl3+.

Fig.3 shows the X-ray diffraction patterns of In0.9-yMgGaO4: 0.1Cr3+, yAl3+, the diffraction peaks of these samples tend to large degree with increasing the Al3+ concentration from 0 to 0.9 and finally indexed to the standard card patterns of MgAlGaO4 (PDF#54-0986). MgAlGaO4 belongs to the spinel structure with a space group of Fd-3m (No.227), which shows a spinel structure. According to the Bragg's law [20]: 2dsinθ= nλ

(1)

where d is the distance between parallel lattice planes, θ is the diffraction angle (Bragg angle), n is the order of reflection (integer), and λ is the wavelength of X-rays. The main peaks shift to the higher degree with Al3+ substituting for In3+ sites due to the smaller radius of Al3+ (0.53 Å, N=6) than In3+ (0.8 Å, N=6). Finally, the diffraction peaks can be indexed to the standard pattern of MgAlGaO4. The fine structures of these samples are further examined by high resolution transmission electron microscopy (HRTEM). The TEM, HRTEM and the fast Fourier transform (FFT) images of these samples are shown in Fig.4. As we can see from the selected area electron diffraction (SAED) patterns, the regularly arranged diffraction dots suggest that these samples have a single-crystalline structure. As shown in Fig.4 (a) and (b), the continuous lattice fringes measurements with d spacing for InMgGaO4: 0.01Cr3+ and InMgGaO4: 0.07Cr3+ are calculated to be 0.96Åand 4.29 Å which agree with the theoretical value of 0.99 Å for (2 1 10) and 4.30 Å for (0 0 6) of InMgGaO4. In Fig.4(c) and 6


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(d), the d spacing of SAED for In0.7MgGaO4: 0.07Cr3+, 0.2Al3+ and In0.1MgGaO4: 0.07Cr3+, 0.8Al3+ are calculated to be 4.75 Å and 4.81 Å which are consistent with the theoretical value of 4.73 Å for (1 1 1) of MgAlGaO4. That is to say, the compositions InMgGaO4: xCr3+ and In0.9-yMgGaO4: 0.1Cr3+, yAl3+ are in single phase.

Fig.4 TEM images and the corresponding HRTEM images of (a), (b) for InMgGaO4: xCr3+(x=0.01, 0.07) and (c), (d) for In0.9MgGaO4: 0.1Cr3+, yAl3+(y=0.2, 0.8). And the corresponding enlarged lattice fringes and their FFT patterns of HRTEM images are given in the respective inset.

3.2 Crystal field analysis The luminescence properties of trivalent Cr3+ ions are strongly influenced by the crystal field around it due to the 3d3 electron configurations of Cr3+ ions. According to the crystal field theory, the ground term 4F of Cr3+ will be split into three levels, where 4A2(4F) is the ground state, 4T1(4F) and 4

T2(4F) for the excited states. Therefore, Cr3+ has two absorption bands of 4A2(4F)→4T1(4F) and

4

A2(4F)→4T2(4F) in octahedron. According to the hamiltonian matrix of d3 electron configuration,

taking the energy of the 4A2(4F) state equal to zero, the energy for the 4T1(4F), 4T2(4F), 2E(2G) levels can be calculated to be: [21]

E ( 4 T2 ) = 10Dq E ( 4 T1 ) = 15 Dq + 7 .5 B − 0 .5 (10 Dq ) 2 − 18 Dq * B + 225 B 2

E (2 E ) = 9 B + 3C −

72B 2 18B 2 − 10Dq + 14B + 3C 10D q +5C

(2)

7



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where Dq is the crystal field strength, B and C are Racha parameters, which can be calculated by the relative energy (∆E) between 4T1(4F) and 4T2(4F) levels: [22]

∆E 2 ∆E ) − 10( ) Dq Dq B= Dq ∆E 15( − 8) Dq (

C=

E ( 2 E ) − 7.9 B + 1.8B 2 / Dq

(3)

3.05

According to equation (2), it is obvious that the energy of 4T1(4F) and 4T2(4F) levels closely depended on the crystal field value of Dq, and the energy of 2E(2G) level is slightly influenced by the value of Dq, which can be reflected from the Tanabe-Sugano diagram of Cr3+ in Fig.5 (b). TanabeSugano diagram shows that the energy level of 4T1(4F) and 4T2(4F) risen with the increasing of Dq/B [23]

. There is a crossover point (M) of 4T2(4F) and 2E(2G) levels, which is the standard to distinguish

the crystal field around Cr3+. When the value of Dq/B is in the right or left of M, the Cr3+ is located in strong or weak crystal field, respectively. When the value of Dq/B is close to M, the Cr3+ should be located in intermediate crystal field. The excitation spectra monitored at 729 nm of InMgGaO4: xCr3+ are displayed in Fig.5 (a), and it consists of three broad excitation bands peaking around 352 nm, 416nm and 581 nm, which belong to the d-d transitions of 4A2(4F)→4T1(4P), 4A2(4F)→4T1(4F) and 4A2(4F)→4T2(4F), respectively. Fig.5 (a) depicts that the 416 nm excitation peak has no shift with increasing Cr3+ concentration. However, the excitation peak around 581 nm show a red shift from 581nm to 597nm, which attributes to the great influence of the crystal field on the 4T2(4F) level. According to equations (2) and (3), the values of Dq, B and Dq/B of InMgGaO4: xCr3+ are calculated and shown in Table.2. As we can see, the values of Dq/B decrease with the increasing of Cr3+ concentration, which indicate that the crystal field around Cr3+ is weakened. As shown in Fig.5 (b), when Cr3+ concentration is lower than 0.01, the value of Dq/B locates at the right of M point, when Cr3+ concentration is greater than 0.01, the value of Dq/B locates at the left of M point, which mean that the crystal field around Cr3+ decreases from the strong crystal field to the weak crystal field.

8


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Fig.5. (a): Excitation spectra of InMgGaO4:xCr3+ (λem=729nm); (b): Tanabe-Sugano diagram of Cr3+.

Cr3+

0.001

Table.2 Changes of Dq/B values. 0.005 0.01 0.03

0.07

0.1

concentration λ1 (nm)

416

416

416

416

416

416

λ2 (nm)

581

586

588

589

590

597

Dq

1721.17

1706.48

1700.68

1697.79

1694.91

1675.04

B

680.77

701.50

711.32

715.77

720.24

752.16

Dq/B

2.55

2.42

2.39

2.37

2.35

2.22

3.3 Luminescence property of InMgGaO4:xCr3+ Under 581 nm excitation, the emission spectra of InMgGaO4: xCr3+ at room temperature is shown in Fig.6 (a). The emission spectra of Cr3+ cover from 650 nm to 1200 nm including one sharp emission peak and two broad emission bands. Generally, the luminescence properties of trivalent Cr3+ ions are strongly influenced by the crystal field, because the 3d3 electron configuration of Cr3+ ions is impressible to the crystal field around it. There are three luminescence conditions for Cr3+: (1) the spin-forbidden transition 2E(2G)→4A2(4F) in strong crystal field; (2) the spin-allowed transition 4

T2(4F)→4A2(4F) in weak crystal field; (3) the spin-forbidden transition 2E(2G)→4A2(4F) and

spin-allowed transition 4T2(4F)→4A2(4F) occur simultaneously in the intermediate crystal field. According to the calculation results of Dq/B, Cr3+ is located in the intermediate crystal field, so the two kinds of transition will occur simultaneously. In addition, the 2E(2G) level is slightly influenced by the surrounding crystal filed, so the spin-forbidden transition2E(2G)→4A2(4F) is a sharp line emission peak. However, the 4T2(4F) level is greatly influenced by the surrounding crystal field , so the spin-allowed transition 4T2(4F)→4A2(4F) is a broad emission band. For the emission spectra of 9



Page 10 of 21

InMgGaO4: xCr3+, it is obvious that peak1 is much narrower than that of peak2 and peak3, which indicates that peak1 may come from the spin-forbidden transition 2E(2G)→4A2(4F), peak2 and peak3 may come from spin-allowed transition 4T2(4F)→4A2(4F). In order to further confirm the origin of these three peaks, the fluorescence decay curves monitored at 712 nm, 736 nm and 915 nm with the excitation of 581 nm are measured and shown in Fig.6 (d), (e) and (f), respectively. It can be found that the decay curves conform to a second-order exponential decay, which can be fitted by the equation [24-25] I(t)=I0+A1e-t/τ1+A2e-t/τ2

(4)

where I0 and I(t) are the luminous intensity at moments of 0 and t, A1 and A2 are constants. The average lifetime τ can be calculated by the following equation [26-27] τ=(A1×τ12+A2×τ22)/( A1×τ1+A2×τ2)

(5)

According to equations (4) and (5), the lifetimes of these three peaks are 340 us, 280us and 49.5us. There is an obvious distinction between the decay times of the sharp line emission peak and the broad emission band, which is often used as the important evidence to distinguish between the two transitions of 2E(2G)→4A2(4F) and 4T2(4F)→4A2(4F). The spin allowed transition 4T2(4F)→4A2(4F), whose energy corresponds to the crystal field (CF), is strongly coupled to the lattice providing short-lived broadband PL. However, the spin forbidden transition 2E(2G)→4A2(4F) does not involve any change in electronic configuration and is weakly coupled to the lattice, giving rise to long-lived narrow-line emission. Obviously, there is a prodigious difference between the lifetimes of peak1, peak2 and peak3 that the lifetime of peak1 is much longer than that of peak2 and peak3, which further illustrates that peak1 comes from the spin forbidden transition 2E(2G)→4A2(4F), peak2 and peak3 originate from the spin allowed transition 4T2(4F)→4A2(4F). In addition, there is an obvious difference between the lifetimes of peak2 and peak3, which indicates that there are two kinds of luminescence centers. Only in the octahedral environment, Cr3+ exhibits red or near-infrared emission. In InMgGaO4 crystal structure, there is only one kind of [InO6] octahedron, however, there are three emission band in the emission spectra of InMgGaO4: xCr3+, which mean that a part of Ga3+ and Mg2+ may take the place of In3+ in the octahedral coordination with the decreasing of crystal field, as shown in Fig.7. That is to say, there are two kinds of octahedrons: [InO6] regular octahedron and [Mg/GaO6] inversed octahedron. So, it is necessary to analyze the occupation of Cr3+. The crystal field strength is inversed to the torsion resistance, so the crystal field around the inversed 10


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[Mg/GaO6] octahedron is weaker than that of the regular [InO6] octahedron. From the Tanabe-Sugano diagram of Cr3+, the energy position of 4T2(4F) level decline with the weakening of crystal field. Thus, the long wavelength emission band (peak3) comes from the Cr3+ occupied in the inversed [Mg/GaO6] octahedron, and the short wavelength emission band (peak1 and peak2) corresponds to the Cr3+ occupied in the regular [InO6] octahedron.

Fig.6. (a) Emission spectra of InMgGaO4:xCr3+ (λex=581 nm). (b) Decay curves of InMgGaO4:0.01Cr3+ (λem=712 nm, 736 nm and 915 nm).

Fig.7. Diagram of the inversion between cation sites and the co-existence of two transitions.

Fig.8 (a) displays the normalized emission spectra of InMgGaO4: xCr3+, and the changes in emission intensity and position of the three peaks are shown in Fig.8 (b), (c) and (d), respectively, the intensities of the three peaks rise first and then decrease with increasing the Cr3+ concentration. As shown in Fig.8 (b), when Cr3+ concentration is greater than 0.01, the sharp emission peak (peak1) around 710 nm is despaired due to that the crystal field have been weakened in this concentration, which is consistent with the calculation values of Racha parameter Dq/B. In Fig.8 (c) and (d), the 11



broad emission bands (peak2 and peak3) increase first and then decrease with increasing the Cr3+ concentration. The inversion between In3+ and Mg2+/Ga3+ ions increases with increasing Cr3+ concentration, so the broad emission band (peak3) from Cr3+ in inversed [Mg/GaO6] octahedron increases and then decreases due to the concentration quenching effect. In addition, these three emission peaks show various degrees of red shift, in which the shift of peak1 (6 nm) is great shorter than that of peak2 (57 nm) and peak3 (21 nm) due to the minor crystal field effect on the spin-forbidden transition of 2E(2G)→4A2 (4F) and the great effect on the spin allowed transition 4

T2(4F)→4A2(4F). The reason for the red shift can be explained by position shift of the excited and

ground energy levels. However, the 2E(2G) level is slightly affected by the crystal field around, and the 4T2(4F) level is strongly influenced by the crystal field. Therefore, the reason for the red shift of the sharp line emission peak and the broad emission band is different.

Fig.8: (a) Normalized emission spectra of In1-xMgGaO4:xCr3+ (λex=581nm). (b) (c) and (d) are the changes in emission intensity and position of these three peaks.

The red shift of the sharp line emission peak is mainly affected by the nephelauxetic effect, which can be expressed by the following equation: 1-β=hk

(6)

where β is the nephelauxetic ratio; h and k are the parameters of anion ligands and metal, respectively. The decrease of interelectron repulsion and the increase of h parameter are attributed to covalency between the activator and surrounding ligands. The position of the excited states decreases with the increase of covalency, and the difference between the excited states and the ground state decrease, which lead to a red shift. The covalency is related to the electronegativity of 12


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the center ion and the ligands. The greater covalency is, the more obvious the nephelauxetic effect will be. In this work, the nephelauxetic of Cr3+ (1.66) is smaller than In3+ (1.78), which lead to the increase of covalency. So the excited levels will decrease with increasing the Cr3+ concentration. And the sharp line emission from the spin-forbidden transition of 2E(2G)→4A2 (4F) shift to the red slightly.

Fig.9. Reasons for the red shift of emission spectra.

In addition, the red shift of the broad emission band is mainly affected by the crystal field around it, which is mainly affected by two aspects: the changes of octahedral bonds length and the breaking of Mg/Ga-O bonds. On the one hand, the crystal field splitting is related to the average bond length between the activator and the ligands, and the crystal field splitting (Dq) can be determined by the following equation [28-29]:

Dq =

ze2 r 4 6R 5

(7)

where Dq is measurement of the crystal field strength, z is the charge of the anion, R is the distance between the central ion and its ligands, r is the radius of the activators and e is the charge of an electron. According to the Rietveld refinement, the average bond length of the octahedron decreases with increasing the Cr3+ concentration due to the smaller radius Cr3+ (0.615Å, N=6) than In (0.8 Å, N=6), thus the crystal field splitting around Cr3+ becomes stronger. On the other hand, the breaking of the Mg/Ga-O bonds has a great effect on the crystal field, which will lead to the decrease of the crystal field. Comparing with these two aspects, ion-bond breaking has a greater impact on the 13



crystal field, so the crystal field around Cr3+ is decreased. From the Tanabe-Sugano diagram, the excited state position will decrease with the decreasing of the crystal field, and the distance between excited state and ground state will be decreased, which leads to the red shift of the emission spectrum. The diagram of the reasons for the red shift of the emission spectra is shown in Fig.9.

Fig.10. Diffuse reflection spectra of InMgGaO4:xCr3+ (x=0, 0.001, 0.005, 0.01, and 0.07), and the band gap (Eg).

The diffuse reflectance spectra of InMgGaO4: xCr3+ (x=0, 0.001, 0.005, 0.01, and 0.07) are shown in Fig.10, which are measured at room temperature ranging from 300 nm to 800 nm. These absorption bands root in the absorption transitions from the ground state 4A2(4F) to the excited 4T1(4F), 4T2(4F) states of Cr3+ ion. And the band gap (Eg) of InMgGaO4: xCr3+ can be determined by Kubelka-Munk function as follow [30-33]: F(R∞) = S×(1－R)2/(2×R)

(8)

where R is the diffuse reflectance of spectrum, R∞ is the reflectance of the infinitely thick sample. S represents the scattering coefficient, which is irrelevant to wavelength. Therefore, the (F(R∞)hυ)2-hυ plot can be made by[34-36]: (F(R∞)hυ)2 = A×(hυ－Eg)

(9)

A is a constant, and the values of the band gap energy Eg are listed in the inset of Fig.10. We can see that band gap energy Eg decreases gradually with the increasing of the Cr3+ concentration, which suggests that the incorporations of Cr3+ may lead to decline of the band gap. Therefore, the energy

14


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difference between the excited state and ground state decreases, which contribute to the red shift. Comprehensively, all the nephelauxetic, crystal field splitting, breaking of Mg/Ga-O bonds, and decreasing of the band gap contribute to the red shift of the emission spectra. 3.3 Influence of doping Al3+ into In0.9MgGaO4: 0.1Cr3+ Considering the ionic radius and valence, Al3+ is selected to introduce into the In3+ sites to tune the luminescence properties of Cr3+. Fig.11 (a) shows the emission spectra of In0.9-yMgGaO4: 0.1Cr3+, yAl3+ (0≤y≤0.9), and the change of the emission intensity are shown in Fig.11 (b) and (c). With increasing the Al3+ concentration from 0 to 0.9, there is a continuous enhancement on the intensity of short wavelength emission peak, and the intensity of long wavelength emission band decreases gradually.

Fig.11. Emission spectra of In0.9-yMgGaO4:0.1Cr3+, yAl3+ (0≤y≤0.9) (λex=581 nm).

As the smaller Al3+ (0.533Å, N=6) introducing into the In3+ (0.8 Å, N=6) sites, according to the crystal field theory equation (7), the crystal field around Cr3+ is enhanced. So the phenomenon of inversion between In3+ and Mg2+/Ga3+sites will be weakened and the crystal structure becomes more symmetrical. So the spin-allowed transition of 4T2(4F)→4A2(4F) of Cr3+ in inversed [Mg/GaO6] octahedron is weakened, and the spin-forbidden transitions of 2E(2G)→4A2(4F) of Cr3+ in a regular octahedral environment will be enhanced with increase the Al3+ concentration. As a result, the emission intensity around longer wavelength disappears gradually, but the emission intensity around

15



shorter wavelength increases obviously. In addition, the 4T2(4F) level move up and eventually higher than 2E(2G) level, which leads to the dominant of spin-forbidden transition 2E(2G)→4A2 (4F), as shown in Fig.12. On the other hand, the energy difference between 4T2(4F) and 2E(2G) levels decreases with the increase of Dq/B in the weak crystal field. When the value of Dq/B up to the crossover point, there is an overlap between the two energy levels, and the electrons in 4T2(4F) level will transfer to the 2E(2G) level, which also contributes to the enhancement of emission intensity of 2

E(2G)→4A2(4F).

Fig.12. Reason for the enhancement of the sharp emission peak.

4. Conclusions In summary, the luminescence properties of Cr3+ can be tuned between deep red and near infrared depending on the especial crystal structure of InMgGaO4, whose Mg/Ga-O bond is impressible to the surrounding environment, and Al3+ is introduced into In3+ sites to tune the spectra further. These samples have been proved to be single phase by the HRTEM and fast Fourier transform (FFT) images. The Racha parameters Dq/B of Cr3+ are calculated to be decreased with increasing the Cr3+ concentration, which indicates that the crystal field around Cr3+ is weakened. As a result, a part of impressible Mg/Ga-O bonds are broken to form the spinel structure and the inversion between Mg/Ga sites and In sites are increased. So there are two kinds of octahedral: regular [InO6] octahedron and inversed [Mg/GaO6] octahedron. The emission spectra of InMgGaO4: xCr3+ includes one sharp line emission peak (peak1) and two broad emission bands (peak2 and peak3). According to the decay curves of the three peaks, peak1 and peak2 come from the spin-forbidden transition 2

E(2G)→4A2(4F) and spin-allowed transition 4T2(4F)→4A2(4F) of Cr3+ occupied in the regular [InO6]

octahedron, and peak3 comes from the spin-allowed transition 4T2(4F)→4A2(4F) of Cr3+ occupied in 16


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the inversed [Mg/GaO6] octahedron. When Cr3+ concentration is more than 0.01, the value of Dq/B is located in the left of the crossover point M, which indicates that Cr3+ is located in weak crystal field, meanwhile, the sharp line emission peak (peak1) disappears and the broad emission band enhances gradually. In addition, the three peaks show different degrees of red shift, which have been explained by the nephelauxetic, crystal field splitting (Dq), breaking of Mg/Ga-O bonds, and decreasing of the band gap. On the contrary, as Al3+ entering to the In3+ sites, the crystal structure becomes more stable, which lead to the weakening of the inversion between In3+ ions and Mg2+/Ga3+ ions sites. So the emission intensity of broad band decreases and the emission intensity of sharp line peak increases gradually. Moreover, the luminescence intensity of 2E(2G)→4A2(4F) is significantly improved with Al3+ substituting for In3+ sites because the electrons in 4T2(4F) level transfers to the 2

E(2G) level. Finally, the emission spectra of these samples can be tuned between deep red and near

infrared continuously which can match well with the NIR window I. Corresponding Author *[email protected] (Panlai Li). *[email protected] (Zhijun Wang). Notes The authors declare no competing financial interest. Acknowledgements The work is supported by the National Natural Science Foundation of China (No.51672066), the Funds for Distinguished Young Scientists of Hebei Province, China (No.A2018201101), and the personnel training project of Hebei Province, China (No.A2016002013). References [1] Chermont Q L M D.; Chanéac, C.; Seguin, J.; Pellé, F.; Maîtrejean, S.; Jolivet, J. P.; Gourier, D.; Bessodes, M.; Scherman, D. Nanoprobes with Near-Infrared Persistent Luminescence for in vivo Imaging. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 9266-9271. [2] Welsher, K.; Sherlock, S. P.; Dai, H. Deep-tissue anatomical imaging of mice using carbon nanotube fluorophores in the second near-infrared window. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 8943-8948. [3] Smith, B. R.; Gambhir, S. S. Nanomaterials for In Vivo Imaging. Chem. Rev. 2017, 117, 901-986. 17



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[29] Huang, C. H.; Wu, P. J.; Lee, J. F.; Chen, T. M. (Ca, Mg, Sr)9Y(PO4)7:Eu2+, Mn2+: Phosphors for white-light near-UV LEDs through crystal field tuning and energy transfer. J. Mater. Chem. 2011, 21, 10489-10495. [30] Som, S.; Mitra, P.; Kumar, V.; Terblans, J. J.; Swart, H. C.; Sharma, S.K. The energy transfer phenomena and colour tunability in Y2O2S: Eu(3+)/Dy(3+) micro-fibers for white emission in solid state lighting applications. Dalton Trans. 2014, 43, 9860-9871. [31] Kumar, A.; Dhoble, S. J.; Peshwe, D. R.; Bhatt, J.; Terblans, J. J.; Swart, H. C. Crystal structure, energy transfer mechanism and tunable luminescence in Ce3+/Dy3+, co-activated Ca20 Mg3Al26Si3O68, nanophosphors. Ceram. Int. 2016, 42, 10854-10865. [32] Morales, A. E.; Mora, E. S.; Pal, U. Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures. Rev. Mex. Fis. 2007, 53, 18-22. [33] Peter, A. J.; Banu, I. B. S. Synthesis and luminescent properties of Tb3+, activated AWO4, based (A=Ca and Sr) efficient green emitting phosphors. J. Mater. Sci.: Mater. Electron. 2014, 25, 2771-2779. [34] Bedyal, A. K.; Kumar, V.; Prakash, R. A near-UV-converted LiMgBO3:Dy3+, nanophosphor: Surface and spectral investigations. Appl. Surf. Sci. 2015, 329, 40-46. [35] Clark, C. J.; McGlone, V. A.; Jordan, R. B. Detection of Brownheart in ‘Braeburn’apple by transmission NIR spectroscopy. Postharvest Biol. Technol. 2003, 28, 87-96. [36] Wang, T.; Xiang, Q. C.; Xia, Z. G.; Chen, J.; Liu, Q. L. Evolution of Structure and Photoluminescence by Cation Cosubstitution in Eu2+-Doped (Ca1-xLix)(Al1-xSi1+ x)N3 Solid Solutions. Inorg. Chem. 2016, 55, 2929-2933.

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For Table of Contents Use Only

Design of A Novel Near-Infrared Phosphor by Controlling Cationic Coordination Environment Xiangyu Meng, Zhijun Wang*, Keliang Qiu, Yuebin Li, Jinjin Liu, Zhipeng Wang, Simin Liu, Xue Li, Zhiping Yang, Panlai Li*

TOC graphic

Emission spectra of Cr3+ can be tuned between narrow peak and broad band continuously relying on the covalency, crystal field splitting (Dq), and bonds breaking of Mg/Ga-O.

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Design of a Novel Near-Infrared Phosphor by Controlling the

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