Toward Bi3+ Red Luminescence with No Visible Reabsorption

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Toward Bi3+ Red Luminescence with No Visible Reabsorption through Manageable Energy Interaction and Crystal Defect Modulation in Single Bi3+-Doped ZnWO4 Crystal Jin Han,†,# Lejing Li,†,# Mingying Peng,*,† Bolong Huang,*,‡ Fengjuan Pan,§ Fengwen Kang,‡ Liyi Li,† Jing Wang,∥ and Bingfu Lei⊥ †

The State Key Laboratory of Luminescent Materials and Devices, and Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China ‡ Department of Applied Physics, and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong 999077, China § College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ∥ School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China ⊥ The College of Materials and Energy, South China Agricultural University, Guangzhou 510640, China ABSTRACT: The last decades have witnessed the discovery of tens of thousands of rare earth (RE) (e.g., Eu2+) and non-RE (e.g., Mn2+) doped photonic materials for nearultraviolet (NUV) and blue converted white light-emitting diodes (wLEDs), but the future development of wLEDs technology is limited greatly by the intrinsic problems of these traditional dopants, such as the insurmountable visible light reabsorption, the weak absorption strength in NUV or blue region, and so on. Here we report a feasible strategy guided by density functional theory (DFT) calculation to discover novel Bi3+ red luminescent materials, which can solve the above problems eventually. Once the untraditional ion of bismuth is doped into ZnWO4 crystal, multiple defects can be possibly created in different charge states such as BiZn, BiW, interstitial Bi, and even defect complexes of 2 BiZnVW among others, and they, as DFT calculated results illustrate, have the potential to produce emission spanning from visible to near-infrared. As confirmed by experiment, tunable emission can be led to cover from 400 to 800 nm after controls over temperatures, defect site-selective excitation schemes, and the energy transfer between these defects and host. A novel red luminescence was observed peaking at ∼665 nm with a broad excitation in the range of 380−420 nm and no visible absorption, which is evidenced by the temperature-dependent excitation spectra and the diffuse reflection spectra. DFT calculation on defect formation energy shows that BiZn3+, the valence state of which is identified by X-ray photoelectron spectroscopy, is the most preferentially formed and stable defect inside a single Bi-doped ZnWO4 crystal, and it produces the anomalous red luminescence as confirmed by the single-particle level calculations. Calculation based on dielectric chemical bond theory reveals that the high covalency of the lattice site which Bi3+ prefers to occupy in ZnWO4 is the reason why the emission appears at longer wavelength than the previously reported compounds. On the basis of this work, we believe that future combination of DFT calculation and dielectric chemical bond theory calculation can guide us to efficiently find new phosphors where Bi3+ can survive and emit red light upon NUV excitation. In addition, the DFT calculation on Bi defects in different charge states will help better understand the longstanding as yet unsolved problem on the mechanism of NIR luminescence in bismuth-doped laser materials.

1. INTRODUCTION Ever since the first success in phosphor-converted white lightemitting diodes (pc-wLEDs) through depositing a layer of yellow-emitting YAG:Ce3+ phosphor on an InGaN blue-emitting LED chip in 1997,1 pc-wLEDs technology has been rapidly recognized as one of the best strategies for the warm white lighting. As compared to conventional lighting technologies, for instance, fluorescent and incandescent lamps, pc-wLEDs technology is featured in smaller and more compact size, faster switching, higher output luminous efficiency, less energy consumption, lower working temperature, and so on.2−4 As a result, it has been found in a wide range of applications such as © 2017 American Chemical Society

general indoor and outdoor lighting, screen backlight source, traffic signal, architectural decoration, agriculture, automobiles, among others.5−9 Obviously, this technology is changing and will continue to change our daily life. As the design concept shows, the most popular pc-wLEDs cannot work well without phosphors. Because of this, extensive efforts have been devoted over the past decades to new phosphors that can match well with commercial NUV or blue LED chips. Tens of thousands of Received: July 16, 2017 Revised: August 31, 2017 Published: September 6, 2017 8412

DOI: 10.1021/acs.chemmater.7b02979 Chem. Mater. 2017, 29, 8412−8424

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Chemistry of Materials

Density functional theory (DFT) is a powerful tool to quantitatively evaluate which types of defects can be produced once a dopant is built into a crystal host, and it can determine which defect can be stabilized and at the same time predict the electronic properties of the doped crystal. The guidance of DFT calculation can therefore avoid the blind attempts. It can improve the possibility to find new optical material by synthesis of only limited but most promising doped compounds. For the dopant, we select bismuth in this study since the transitions of bismuth involve 6s and 6p electrons which are naked directly to ambient, and they are susceptive to the changes of local crystal field.30−32 Because of such unique behavior, Bi3+ can emit various colors spanning from UV to blue, green, yellow, or even red.30 Intentional modulation of the local crystal field around Bi3+ can induce the tunable emission in the whole visible range.31,32 For all single Bi3+-doped phosphors reported so far, they show unique excitation feature in UV but not in visible region, implying that they can possibly solve the problems traditional phosphors suffer from.33−38 Challenges for Bi3+ phosphors are that few of them exhibit strong absorption in NUV region and emission in red spectral range. According to previous works on Bi3+-doped crystals, red luminescence perhaps tends occur readily as Bi3+ is located on a site which average chemical bond covalency is higher than 0.15. To narrow down the candidate compounds, we, therefore, have calculated the covalency of chemical bonds which are connected to a potential lattice site Bi may substitute for in a series of compounds with the dielectric chemical bond theory.39−41 After attempts on different crystals, we noticed wolframite-type zinc tungstate ZnWO4 can be one of possible crystals that can allow Bi3+ emitting red luminescence. In this work, we chose ZnWO4 as the crystal host for Bi3+ doping. We first calculated the average chemical bond covalency with the dielectric chemical bond theory, the projected partial density of states (PDOSs), formation energy, and electronic configurations on all possible types of defects due to Bi substitutions. Overall analyses reveal the potential to produce the emission of visible to near-infrared if all types of Bi related defects can survive, which have been demonstrated by our consequent experiments. Tunable emission has been observed from 400 to 800 nm as ambient temperatures, defect site selective excitation plans, and energy transfer processes have been managed. A novel red luminescence at ∼665 nm was observed to be excitable by light in the range of 380−420 nm and confirmed due to the defect of BiZn3+, whose formation energy, as DFT results show, is the lowest in all involved defects. Singleparticle level calculation illustrate the levels of such defect lies approximately within the range of 600−700 nm, and it is consistent with our observation. Guidance under DFT and the dielectric chemical bond theory helps to find novel photonic materials more efficiently.

phosphors, according to the ISI web of science, thus, have been developed. These phosphors are activated either by RE (e.g., Eu2+, Ce3+, Dy3+, Tb3+, Eu3+, Sm3+, Pr3+, etc.) or non-RE (e.g., Mn2+, Mn4+, Cr3+, etc.). Diverse emission colors have been reported such as (i) green, NaCaPO4:Tb3+ (∼547 nm),10 Ca3Sc2Si3O12:Ce3+ (∼505 nm),11 Ba2LiSi7AlN12:Eu2+ (∼515 nm);12 (ii) blue, Ba2Y5B5O17:Ce3+ (∼443 nm);13 (iii) red, Y2O2S:Eu3+ (∼616 nm),14 Sr[LiAl3N4]:Eu2+ (650 nm),15 NaGdTiO4 :Pr 3+ (∼624 nm),16 Sr2 CaMoO6 :Sm 3+ (∼650 nm), 17 ZnGa 2 O 4 :Cr 3+ (695 nm); 18 CaMg 2 Al 16 O 27 :Mn 4+ (∼655 nm),19 Ca2.5Sr0.5Al2O6:Mn2+ (∼610 nm);20 and (iv) yellow, Ca1.5Ba0.5Si5O3N6:Eu2+ (∼590 nm),21 SrAlSi4N7:Ce3+ (∼555 nm),22 Sr3In(PO4)3:Mn2+ (∼573 nm).23 Colors can be tuned over the visible spectral region as multiple activators are introduced into a single system simultaneously, for instance, (Gd−Y−Bi−Eu)VO4 (green → red),24 BaMg2Al6Si9O30:Eu2+, Tb3+, Mn2+ (full visible color),25 NaGd(WO4)2:Tm3+, Dy3+, Eu3+ (purple → blue → white → red),26 KNaCa2(PO4)2:A (A = Ce3+, Eu2+, Tb3+, Mn2+, Sm3+) (blue → white → reddish/orange),27 Ba1.55Ca0.45SiO4:Eu2+, Mn2+ (blue → yellow).28 Energy transfer processes have to be carefully managed between these dopants and the host to generate these tunable colors. However, these phosphors doped with the traditional activators have some intrinsic problems due to their luminescence nature. For trivalent RE (i.e., Tb3+, Eu3+ and Pr3+) phosphors, the forbidden transitions between well-shielded 4f electrons allow the absorptions in NUV and blue regions but with very weak oscillator strength. This will not fully utilize the chip emission and lower the overall device efficiency. The unabsorbed emission from the chips will be leaked out of the lamp, and the leakage especially in NUV will be detrimental to human health in the long run. For Eu2+ longer-wavelength phosphors, Eu2+ usually is located in a higher covalent crystallographic site, for instance, nitrides or oxynitrides, and the strong crystal field around it will split the 5d levels further and lower the centroid of these levels and eventually leads to the red emission. The stretch of 5d levels enables the extension of the excitation into red spectral range. The longer wavelength excitation unavoidably leads to the reabsorption of the justproduced emission from the device, and it, therefore, decreases the device luminous efficiency. These problems are derived from the nature of the luminescence transition of these phosphors, and they cannot be solved in frame of traditional dopant-based phosphors. This limits the future development of pc-wLEDs technology. One solution to this is obvious to find new phosphor candidates, which is never an easy task. In the past, search on them was performed mainly by trials and errors. It was not only time-consuming but also costly. Xie et al. proposed a singleparticle diagnosis technique, and now it has developed into an approach to discover novel luminescent materials from samples where multiple crystal phases coexist.29 Nevertheless, this still relies on luck and the instrumentations which can in situ detect the luminescence and phase structure of micrometer-size particles. In this work, we are going to propose an alternative approach to efficiently find the red luminescent material. We focus on a red phosphor, particularly for the NUV chip, because so far there are very few reports on it, which can fully meet the requirements for the application. This is also because the phosphor is the key component for high-quality lighting and ultrahigh definition display, such as with high color rendering, wide color gamut.

2. DFT COMPUTATIONAL DETAILS In our work, the geometry relaxation of the lattice was performed at PBE + U in DFT through CASTEP codes,42 which has been proved that it is reliable on many d/f-orbital based oxides.43 To improve the accuracy of the results of electronic properties, we introduced the recently developed method on ab initio determination of Hubbard U parameters to avoid extra spurious error from orbital self-interactions.44−47 We chose different orbitals projectors for different atoms to represent their valence states. For instance, Zn with (3d, 4s, 4p), W with (5s, 5p, 5d, 6s) states, and O with (2s, 2p) states were chosen for 8413


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morphologies were measured with scanning electron microscopy (SEM, FEI Nova NanoSEM 430) equipped with an energy dispersive X-ray spectroscopy (EDS). The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded by JEOL JEM-2100F. X-ray photoelectron spectroscopy (XPS) was measured with Kratos Axis Ultra DLD spectrometer with a focused monochromatic Al Kα X-ray beam (1486.6 eV, 5 mA × 10 kV, ∼5 × 10−9 Torr). The binding energy was calibrated with the reference to the C 1s peak at 284.8 eV. Static and dynamic excitation and emission spectra as well as emission decay curves were measured in the temperature range of 10−300 K with a high-resolution FLS 920 spectrofluorometer, which is equipped with a thermoelectric cooled redsensitive photomultiplier tube (Hamamatsu R928 P) in the singlephoton counting mode, a 450 W xenon lamp and a 60 W μF900 flash lamp. All excitation spectra were corrected over the lamp intensity with a silicon photodiode, and the emission spectra were calibrated by the spectral response of photomultiplier.

pseudopotential generation, respectively. The DFT + U method is developed by Anisimov-type rotational invariant scheme.48 For the electronic minimization process, the ensemble DFT (EDFT) method reported by Marzari et al. was used for solving Kohn− Sham equation,49 in order to prevent the charge-spin out-sync sloshing effect and guarantee the electronic minimization and convergence. With a combined convergence test, we selected the Baldereschi special k-point (1/4, 1/4, 1/4) in the cubic 2 × 2 × 2 supercell for fast convergence in energy, to converge the total energy below the tolerance of 5.0 × 10−7 eV per atom.50 The on-site Coulomb potential energy of Zn-3d10 orbital is usually a challenge in ordinary DFT calculation. The t2g level of Zn-3d10 orbital stays usually too high overlapping with O-2p orbital estimated by the routine local density formalism or gradient corrected approximation. Meanwhile, W-site that has semicore localized 5d-electrons is hardly distinguished by the density variation based the functional. This results in an evident underestimation in the optical band gap and the electronic orbital levels induced by the impurity dopants or intrinsic defects. Describing the electron−electron interaction is the key to solving the electronic structures and understanding related properties of strongly correlated solids. The DFT + U method presents a simple way to describe electrons in semicore d or f orbitals in the transition-metal or rare-earth compounds. In this method, the Hubbard parameter U represents the on-site Coulomb repulsive potentials of localized d/f orbitals. The estimation of U by the conventional linear response (CLR) method under the density functional theory (DFT) framework has been widely used in solids with partially occupied orbitals (open shell). However, the CLR method is problematic for fully occupied orbitals (closed shell) and does not give a U in the physical range within 1 Ryd. This discrepancy arises from the charge response (q) of perturbation being extremely underestimated by the Lagrange multiplier potential (α) in CLR calculations. As a result, the inverse of the response function, U = (−∂α/∂q), between localized electrons in closed shells, is overestimated (i.e., U → ∞ with the response Δq → 0). These issues have been observed in many DFT + U calculations. The U parameters were determined in a transferability test using both PBE and hybrid density functional methods, and the results showed that this method is independent of the function. Moreover, this model can not only quickly give electronic structures of the eigen-bulk properties but also provide satisfactory native defect levels of bulk or low dimensional structures.

4. RESULTS AND DISCUSSION 4.1. Chemical Bond Covalency. As we look up the Inorganic Crystal Structure Database (ICSD) which was produced cooperatively by the Fachinformationszentrum Karlsruhe and the National Institute of Standards and Technology, ICSD has collected more than 120 000 compounds or mineral samples. It is practically impossible to calculate all the compounds. Nevertheless, our previous works can give us the clue.51 As we noticed, higher covalent lattice site bismuth resides usually leads to longer wavelength emission of Bi. For instance, in solid solution compounds of (Y,Lu,Sc)VO4:Bi, as the mean chemical bond covalency fc of Ln−O increases from 0.1551 to 0.1588, the emission λem of Bi shifts from 566 to 635 nm. A linear equation (fc = 0.1259 + 5.176 × 10−5 λem) was found between fc and λem.31 This implies the sensitivity of Bi to the surrounding environment. Inspired by this, we believe it is meaningful to calculate the average chemical bond covalency fc of the bonds which are connected to the potential lattice sites. In the first view on size and charge matches, we think Bi will prefer to replace Zn rather than W. Therefore, we calculated the chemical bond covalency fc of each Zn−O bond, which was listed as Table 1. The Table 1. Bond Length (d) and Calculated Chemical Bond Covalency ( fc) for the Zn Site in ZnWO4 and the Sc Site in ScVO4 compounds

3. EXPERIMENTAL DETAILS

ZnWO4 (ICSD no. 1520641)

3.1. Sample Preparation. Conventional solid state reaction at high temperature was employed to synthesize all the targeted samples of ZnWO4:Bi3+. The chemical reagents involved in this work were ZnO (A. R., “A. R.” denotes “analytical reagent”), WO3 (A. R.), and Bi2O3 (99.999%), and they were used as purchased without further purification. Since we could not confirm which sites in ZnWO4 crystal could be substituted readily by Bi3+ ions, we designed the targeted samples with the nominal chemical compositions of ZnWO4:xBi3+ (x = 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%). We weighed chemical reagents first according to the nominal compositions, mixed them evenly in an agate mortar, and then moved them to high-purity alumina crucibles for consequent sintering. The samples were sintered in a tube furnace at 1000 °C for 4 h in air. After they were cooled naturally to room temperature, samples were white and they were taken out of the furnace and crushed and ground once again for consequent measurements. 3.2. Characterization Details. Phase purity of all samples ZnWO4:xBi3+ was examined by Bruker D8 ADVANCE powder diffractometer (operating at 40 kV, 40 mA, and 1.2°·min−1) with Cu Kα radiation (λ = 1.54059 Å). Element mappings and sample

ScVO4 (ICSD no. 78073)

bond

bond length d (Å)

fc

Zn−O(1) × 2 Zn−O(2) × 2 Zn−O(2) × 2 average Sc−O(1) × 4 Sc−O(2) × 4 average

1.9801 2.1072 2.2227 2.1033 2.1289 2.3672 2.2481

0.3711 0.3601 0.3516 0.3609 0.1634 0.1549 0.1591

fc of Sc−O bonds of only red phosphor ScVO4:Bi were compiled in the same table as reference.52 The fc of Zn−O in ZnWO4 lies between 0.3516 and 0.3711, and the average value is 0.3609, which is even higher than 0.1591 of Sc−O in ScVO4:Bi (see Table 1). This implies that once Bi is doped successfully into ZnWO4, it is highly possible to produce red luminescence at even longer wavelength than ScVO4:Bi. 4.2. DFT Calculations on Possible Types of Defects. Encouraged by the above analysis, we started to employ DFT to calculate all possible types of defects inside ZnWO4, which are produced along with Bi substitution, and to exclude the 8414


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Figure 1. (a) Projected electronic density of states (PDOSs) of blank ZnWO4 crystal; (b) The electronic band structure of ZnWO4 along the path of high symmetrical points in the reciprocal Brillouin zone; (c) The PDOSs properties of the different types of BiW defects; (d) The PDOSs properties of the different types of BiZn defects. The 0 eV denotes the highest occupied electronic level, while other denotations are labeled beside each curve; (e) The single-particle levels of Bi substitution doping (BiW and BiZn), intrinsic defect VZnO, and defect complex models (2 BiZnVW and 2 BiW3 VZn) in ZnWO4 with different charge states; The red lines denote the localized hole levels, while the black lines represent the electronic levels, i.e., within the optical fundamental band gap area, empty states = red lines, filled states = black lines.

impossible substitutions and study how the electronic properties of ZnWO4 change after Bi doping. 4.2.1. Blank Sample ZnWO4. Bandgap represents the barrier for electrons to jump from top of the valence band to bottom of the conduction band. It is the energy required to promote a valence electron bound to an atom to become a conductive electron, which is free to move within the crystal lattice and serve

as a charge carrier to conduct electric current. In this way, bandgap is the dominant reason that determines the electrical conductivity for materials. The larger the bandgap, the larger the energy barrier for electrons to move in the conduction band, meaning that the material is more possible to be insulated. Figure 1a,b illustrate the projected density of states (PDOSs) and the reciprocal electronic band structure of blank ZnWO 4 , 8415


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(Bii) is very unstable and costs more energy to form within the host lattice, which is 7.83 eV under the Bi-rich chemical potential limit. We may further deduce that there will be a spontaneous process for Bii transformation into BiZn or BiW via local migration to the nearest neighboring VZn or VW site. 4.2.3. Charge-Compensated Defect Complex. If the BiZn and BiW substitutions were involved simultaneously, the probability of the Bi substitution for the same cationic site may increase with the increasing of Bi doping content, which indicates us to focus on the effect of the local clustering on native point defects and dopants. Under DFT consideration, we initially suppose that the local short-range disorder in the lattice will occur spontaneously because the local charge distribution needs to be balanced. When the BiZn type is formed, the most stable valence charge state is BiZn3+, which is also the case for BiW3+. To compensate the +3 charge, a (1/2)VW6− (where V is denoted as vacancy) with opposite charge should be formed in order to keep the local neutrality. Since this tends to be more energetically favorable, the Bi dopants have closer crossover distances in ZnWO4 crystal lattice. In consequence, two charge-compensated complexes, 2 BiZnVW and 2 BiW3 VZn, which derive from the BiZn and BiW doping types, respectively, are possibly formed. Our results show that the formation energy is 0.95 eV for 2 BiZnVW and 9.95 eV for 2 BiW3 VZn, and they are sharply different from each other. Based on these values, the latter is more difficult to form than the former. More remarkably, comparing the PDOSs properties of Figure 1c (iv) and Figure 1d (iv) with respect to that of Figure 1c (i−iii) and Figure 1d (i−iii) reveals the orbital levels that dominantly contribute to VB and CB do not change. In the figures, the energetic interval between the lowest hole level and highest electronic occupied level (∼1.93 eV) for 2 BiW3 VZn (Figure 1c (iv)) coincides basically with that (∼2.20 eV) without considering the crystal defect (Figure 1c (i-ii)), but the corresponding energetic interval (1.00 eV) for 2 BiZnVW (Figure 1d (iv)) is 1.98, 2.69, and 0.69 times less than that of ∼1.98 eV for BiZn3+ (Figure 1d (iii)), ∼2.69 eV for BiZn+ (Figure 1d (ii)), and ∼0.69 eV for BiZn0 (Figure 1d (i)). Due to the large difference of ions radii and valence between W and Bi, the PDOSs properties for 2 BiW3 VZn should change as compared with that of blank ZnWO4, but DFT results show the situation is not like this. Doping Bi into Zn sites always allows the PDOSs properties changing more significant than W sites. This illustrates from the other side that doping W site with Bi ion seems impossible in Bidoped ZnWO4 crystal. 4.2.4. VZnO in ZnWO4 Crystal. As ZnWO4 phase appears, it may experience a series of reversible reaction-decomposition reactions such as ZnO + WO3 ⇌ ZnWO4 ⇌ ZnO + WO3. This process will become more prominent as temperature rises up. Typically as the synthesis temperature is higher than 1100 K, ZnO trends to decompose into the O2 and Zn vapor gas easily. We calculated the formation enthalpies for ZnO, WO3, and ZnWO4, which are −3.67, −10.80, and −15.47 eV, respectively. Obviously, these values are in a large contrast, typically between the ZnO and ZnWO4. According to similar theoretical works reported by Huang et al. in CaZnOS crystal,53 the vacancy formation of the ZnO local motif (VZnO) costs rather low energy. In view of this, formation of ZnO vacancy (VZnO) within the ZnWO4 host lattice seems possible. We calculated the formation energy for VZnO in charge states −2, −1, 0, +1, +2 as 7.75, 4.88, 2.47, 2.14, 1.65 eV, respectively. The energy decreases monotonically as the charge state increases. Based on formation energy within charge states of −2 to +2, VZnO is a negative-Ueff effect in the defect reaction of 2 VZnO+ → VZnO0 + VZnO2+, and the

respectively. The optical band gap (Eg), which defines as the difference between the valence band maximum (VBM) and the conduction band minimum (CBM), is ∼4.019 eV (see Figure 1a,b). This agrees with 4.203 eV which we can get from the diffuse reflectance spectrum. The O-2p and the W-5d orbital levels have dominantly contributed to VB and CB, and their energy band widths are about 6.5 and 2.0 eV, respectively (see Figure 1a). The position of Zn-3d orbital levels, which starts from about 7.5 eV below the top of VB, is deeply below VB, and its energy band width is about 1.0 eV. From the width of these orbital levels, Zn-3d is smaller than O-2p and W-5d, and the position of the orbital levels between O-2p, W-5d, and Zn-3d is different from each other. 4.2.2. All Types of Bi-Related Defects in ZnWO4:Bi. The Bi doping in ZnWO4 has been considered in different cases, including Zn-substituted (BiZn), W-substituted (BiW), interstitial (Bii), and charge-compensated complex (2 BiZnVW and 2 BiW3 VZn), among others. We discuss and screen them in terms of formation energy by our calculations. Since the samples were prepared in air, an oxidizing atmosphere, we performed the calculation under O-rich chemical potential limit. At ground state (0 K), the formation energy was calculated as −0.38, −0.52, −0.64, and −1.92 eV for BiW in charge states 0, +1, +2, +3, respectively. Similarly, for BiZn0, BiZn+, BiZn2+, BiZn3+, the formation energy is −1.55, −5.73, −7.25, and −9.06 eV, respectively. For both cases of substitutions, the formation energy decreases as the entity charge state increases. The negative values mean that both BiZn and BiW formation are more energetically favorable rather than the other native point defects formed in the host lattice. Clearly, the formation energy of BiZn is lower than that of BiW, indicating that the BiZn has more stable energetic configuration with local lattice relaxation. We further calculated the Ueff of the defect reactions of 2 BiZn2+ → BiZn+ + BiZn3+ and 2 BiW2+ → BiW+ + BiW3+ as −0.30 eV and −1.16 eV, respectively. The negative Ueff means the BiZn2+ and BiW2+ are all unstable charge states, and the lifetime in lattice will be rather short; meanwhile, the charge state spontaneously undergoes the transition and redistributes the charges into the single and triple positively charged doping state. In view of this, only 0, +1, and +3 charge states are stable valence for Bi substitution in ZnWO4 host lattice. The lowest value was found for BiZn3+, and it should be the most stable defect which can come into being after Bi substitution. Therefore, for BiZn and BiW doping types, we screen the six possibilities through the DFT calculations, i.e., the W-substituted Bi0 (i.e., BiW0), W-substituted Bi+ (i.e., BiW+), Wsubstituted Bi3+ (i.e., BiW3+), Zn-substituted Bi0 (i.e., BiZn0), Znsubstituted Bi+ (i.e., BiZn+), and Zn-substituted Bi3+ (i.e., BiZn3+). The PDOSs properties for BiZn and BiW doping types are depicted in Figure 1c,d (i−iii). The PDOSs properties are aimed to project possible individual and synergetic contributions on the electronic properties within the band gap, and the computational data are fetched from the various atomic sites near the defect region. In the figures, replacing the W site with different Bi valence do not change the energetic interval between the lowest hole level and highest electronic occupied level significantly (i.e., the value keeps at ∼2.60 eV), and the O-2p and W-5d levels also contribute dominantly to VB and CB. For Zn-substituted Bi type (Figure 1d (i−iii)), however, the size of energetic interval as the Bi decreases from high valence (+3) to low valences (+1 and 0) exhibits a sharp difference, i.e., ∼ 1.98 eV for BiZn3+, ∼ 2.99 eV for BiZn+, and ∼0.69 eV for BiZn0. PDOS results indicate that the influence of Bi valence on BiZn doping type is stronger than BiW doping type. Meanwhile, we found that the Bi interstitial doping 8416


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Figure 2. (a) Lattice cell of ZnWO4 crystal redrawn according to ICSD No. 156483; (b) XRD patterns of the samples ZnWO4: xBi3+ (x = 0, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%); (c) Transmission electron microscopy (TEM) image for blank sample ZnWO4; (d) TEM image for sample ZnWO4:1.0% Bi3+; (e) TEM image for sample ZnWO4:2.0%Bi3+; (f) TEM image for sample ZnWO4:3.0%Bi3+; (g,h) High-resolution TEM (HRTEM) images for sample ZnWO4:1.0%Bi3+; inset shows the selected area electron diffraction patterns.

lower valence states such as 0, +1 or even cluster ions. This is strong evidence that lower valence state bismuth ions are the origin of the NIR luminescence. These near-infrared emission are due to the transitions between the localized electronic and hole levels within the optical fundamental band gap area (Figure 1e). For the defect complex model of 2 BiZnVW, it exhibits a wide range of emission wavelengths from 500 to 827 nm. For BiW defects, the emission lies between 477 and 810 nm, while for VZnO emission, they appear at 484−816 nm. The most stable defect among VZnO species is VZnO2+, which can show emission at 484, 512, and 785 nm. The transition corresponding to the emission at 512 nm shows the strongest oscillator strength. As Figure 1e illustrates, emission from ZnWO4:Bi crystal are very complicated, and they are broadly distributed from visible to near-infrared. It shows the possibility to tune the emission in a wide spectral space if energy transfer processes could be well tailored, or specific defects can be stabilized and excited properly. 4.4. Experimental Confirmation. 4.4.1. Sample Crystallographic Structure and Morphology. Figure 2a illustrated the crystal structure of ZnWO4, which was draw according to Inorganic Crystal Structure Database (ICSD) file No. 156483. We can clearly see from this figure that both Zn site (2f) and the W site (2e) are coordinated by six oxygen atoms and ZnO6 and WO6 layers are in parallel to plane ac and they are packed along axis b, and these octahedra are linked to each other by sharing the common oxygen. Within the crystal of ZnWO4, there is only one type of Zn2+, W6+, and O2−. To study the influence of Bi doping on the phase purity and microstructural morphology, we measured XRD patterns and SEM images of samples with and without Bi doping, as shown in Figure 2b and Figure 2c−f, respectively. From Figure 2b, we can find that all the diffractions

remaining charge state transition shows the positive-Ueff effects, indicating that the most stable charge state for VZnO is +2. 4.3. Single-Particle Levels of Bi doping. DFT analysis shows that if only formation energy is considered, BiZn3+ should be most stable species after Bi is doped into ZnWO4 since its energy is the lowest among all possible types of defects. Nevertheless, the rest of defects such as VZnO, 2 BiZnVW, BiW may also become stable as temperature or the dopant concentration increases. Figure 1e summarizes single-particle levels of Bi substitution doping (BiW and BiZn), intrinsic defect VZnO, and defect complex models (2 BiZnVW and 2 BiW3 VZn) in ZnWO4 with different charge states. The different energy transition levels, which represent the wavelength of the luminescence, are classified on the basis of the oscillator strength of the electronic transition process. Generally, these defects have the potential to emit visible to near-infrared light if they could be stabilized in the compound. From Figure 1e, we can see that the electronic transitions between BiZn3+ doping levels can lead to photoluminescence with both 611 and 629 nm wavelengths at the same single BiZn3+ doping state with two steps of photon emission. The oscillator strengths of these transitions are more than two times stronger than BiZn3+. This is a very good sign that doping Bi can be possible to generate red luminescence. We also noticed that lower valence state of BiZn can give out near-infrared emission. For instance, BiZn0 can emit at 1634 or 1107 nm, BiZn+ can be at 1634, 1228, or 785 nm, and BiZn2+ can emit at 1016 or 785 nm. This is an important calculated result for NIR luminescence bismuth-doped laser materials. For these materials, there is a longstanding open problem on which valent bismuth ions are responsible for the NIR luminescence. Some proposed higher valence state such as Bi5+, and others suggested 8417


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Figure 3. (a) Energy-dispersive X-ray spectroscopy (EDS) spectrum for sample ZnWO4:1.0%Bi3+; (b) Diffused reflectance spectra of blank sample ZnWO4 (black line) and ZnWO4:1.0%Bi3+ (red line); (c) Elemental maps of two representative particles of sample ZnWO4:1.0%Bi3+.

4.4.2. Sample Homogeneity. Phosphors, which are prepared by the high temperature solid state method, always exhibit irregularly shaped particles, as vividly illustrated in Figures 2c−f. To check whether the Bi dopants would induce the inhomogeneity such as element aggregation in Bi-doped ZnWO 4 samples, we have selected the particles of ZnWO4:1.0%Bi sample as the representative and measured the energy-dispersive X-ray spectroscopy (EDS) and elemental mapping images, respectively. Two examples are shown in Figure 3a,b, and both of them comprise the elements Zn, W, O, and Bi (Figure 3a). All the elements Zn, W, O, and Bi are distributed homogeneously throughout the particles (Figure 3b), without any traceable element aggregation and phase separation in ZnWO4:1.0%Bi sample. The random distribution of these elements suggests that the hypothesis we made during DFT is reasonable to randomly substitute Bi for host elements. 4.4.3. Static Excitation and Emission at Room Temperature. When the Bi-doped samples were illuminated by 365 nm lamp, red luminescence can be easily observed. This proves our calculations of covalency and DFT. In the following, we will try to understand the dynamic and nature of the red luminescence. First of all, we achieved the emission spectrum of the red luminescence upon the excitation at 365 nm (see Figure 4a). It covers a very broad spectral range from 450 to 850 nm, and it is peaked around 665 nm, which is basically consistent with the single-particle level of BiZn3+ if calculation uncertainty is considered (see Figure 1e). To confirm this, we checked over each sample of ZnWO4:Bi with XPS, and these spectra appear very similar. We, therefore, listed the spectrum of ZnWO4:1.0% Bi as one of the representatives and α-Bi2O3 as a reference for trivalent bismuth of Bi3+ (see Figure 5a). Comparison of our sample to α-Bi2O3 shows that trivalent bismuth dominates in the sample, as reflected by characteristics peaks of Bi3+ at ∼159.3 eV

consist well with ZnWO4 standard pattern (ICSD no. 156483). This implies that on the one hand incorporating Bi into ZnWO4 has not induced detectable impure phase, and on the another hand, all samples crystallize in the monoclinic wolframite-type structure with the space group P12/c1 (no. 13). However, inspection on the XRD results reveals that the diffraction intensity distribution for Bi-doped samples is quite different from that of the standard pattern of ZnWO4 and blank sample of ZnWO4. For example, the planes (−111), (111), and (020) correspond to the diffraction peaks at 30.47°, 30.72°, and 31.26°, respectively. For blank sample ZnWO4, the relative intensities of (−111) and (111) are stronger than (020). As soon as bismuth was doped into the compound, the intensity of the (020) peak becomes more than 4 times stronger than (−111) and (111). Such change can be also found between other diffraction peaks such as (011) and (110), (022) and (200). To understand why these changes occur, we measured SEM, TEM, and HRTEM images for these samples. The blank sample appears as irregular sphere-like particles in several micrometers while the particles in the doped samples are much bigger than the blank sample. They are usually larger than 10 μm (see Figure 2c−f). As Bi content increases, the sample morphologies do not change obviously (see Figures 2b−f). High-resolution TEM (HRTEM) technique with the Fast Fourier Transform (FFT) pattern was employed to measure the thinnest fringe of the selected ZnWO4:1.0%Bi particle. As shown in Figure 2g,h, two different interplanar distances can be found as 0.487 and 0.285 nm, which correspond to (020) and (100), respectively. The sharp diffraction spots can be found in the selected area electron diffraction patterns (see inset of Figure 2h). Doping Bi into the sample can partially act as flux to promote crystallization and help the particles growing into big ones with the preferred orientations along the planes such as (0k0), (h00), and so on. 8418


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Figure 4. (a) Emission spectra of the samples ZnWO4: xBi3+ (x = 0.5%, 1.0%, 1.5%, 2.0%, 2.5% and 3.0%), λex = 365 nm; (b) Excitation spectra of the samples ZnWO4: xBi3+ (x = 0.5%, 1.0%, 1.5%, 2.0%, 2.5% and 3.0%), λem = 665 nm; (c) Emission spectra of the samples ZnWO4: xBi3+ (x = 0, 0.5%, 1.0%, 1.5%, 2.0%, 2.5% and 3.0%), λex = 305 nm; (d) Excitation spectra of the samples ZnWO4: xBi3+ (x = 0, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%), λem = 485 nm; (e) Emission spectra for sample ZnWO4:1.0%Bi3+ upon different excitation wavelength from 250 to 400 nm; (f) The dependence of emission peak wavelength on excitation wavelength for the samples ZnWO4: xBi3+ (x = 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%).

Figure 4d, and it peaks at 295 nm, and it is very similar the excitation of blank sample, which is mainly due to the transition of 3T1u → 1A1g of WO66− group (see curve x = 0 in Figure 4d). As soon as Bi is doped into the compound, the host emission is quickly quenched by 50%, and the quenching continues as the content of Bi keeps increasing (see Figure 4c). The quenching is a sign that energy transfer happens from host to Bi3+, though the process is not so efficient. Another evidence for this is the excitation spectra of Bi3+ emission at 665 nm (see Figure 4b). As Bi content increases from 0.5% to 3.0%, the red emission is slightly enhanced first at x = 1.0% and weakened afterward (see Figure 4a). The quenching mechanism by Bi content was analyzed with Ozawa and Jaffe equation, and θ turns out to be

(4f7/2) and 164.6 eV (4f5/2), respectively (see Figure 5a). The peak locations are the same as ref 31. As we monitored the emission at 665 nm, we measured the excitation spectra of ZnWO4:xBi, as illustrated in Figure 4b. The spectra comprise two overlapped broad bands at 295 and 365 nm. The former is more than 10 times stronger than the latter. This, however, does not consist with the diffuse reflectance spectrum, which shows a broad strong absorption at ∼350 nm. This absorption differs from the host absorption at ∼330 nm (see Figure 3c). The shift toward longer wavelength is due to the Bi substitution. The absorption enhancement at 350 nm originates from the Bi3+ transitions of 3P0 to 1S0. As it is excited into the host absorption, we observed a broad emission at ∼485 nm (see Figure 4c). The excitation spectra of host emission are shown as 8419


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Figure 5. (a) XPS spectra for sample ZnWO4:1.0%Bi3+ (blue curve) and α-Bi2O3 (black curve); (b) Emission spectra of sample ZnWO4:1.0%Bi3+ upon excitation in the wavelength range of 250−380 nm at 10 K; (c) Excitation spectra of sample ZnWO4:1.0%Bi3+ upon different emission wavelength from 450 to 700 nm at 10 K; (d) Emission spectra of sample ZnWO4:1.0%Bi3+ at different temperatures of 10−300 K, λex = 270 nm; (e) Emission spectra of sample ZnWO4:1.0%Bi3+ at different temperatures of 10−300 K, λex = 360 nm; (f) Emission spectra of sample ZnWO4:1.0%Bi3+ at different temperatures of 10−300 K, λex = 320 nm; The labels are the same for (d−f), and a different color curve corresponds to different temperatures as the inset of (f) shows.

NUV LED chips. As the excitation wavelength becomes shorter, the red emission soon immerges into the emission at 485 nm. As the excitation wavelength is 250 nm, only the emission can be found at 485 nm (see Figure 4e). The dominant emission wavelength depends tightly on the excitation schemes rather than the content of Bi (see Figure 4f). At the excitation between 320 and 330 nm, two types of emission coexist because both the host and Bi3+ show the absorption in the range. Careful control over site selective excitation can tune the emission in a wide spectral region from 485 to 665 nm. 4.4.4. Static and Transient Photoluminescence at 10−300 K. As the temperature is lowered to 10 K, we found that the emission redshifts gradually from 498 to 672 nm as the excitation wavelength increases from 250 to 400 nm (see Figure 5b). The

3.54. This indicates exchange coupling interaction dominating the quenching process. One may notice that the sample ZnWO4:Bi shows no considerable absorption in the visible spectral region. Since popular NUV LED chip emission usually lies in the range of 380−420 nm, the excitation and diffuse reflectance spectra jointly imply that Bi-doped ZnWO4 samples can address the reabsorption problem in visible range that is encountered by traditional activators doped photonic materials. As Figure 3c shows, the absorption of ZnWO4:Bi matches the NUV chip very well. We have tried to use NUV light to excite the sample ZnWO4:1.0%Bi, and found the emission at ∼665 nm can always be led by excitation within 340−400 nm (see Figure 4e). This means the phosphor can be applicable to pc-WLED based on 8420


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Figure 6. (a) PL decay curve (λex = 305 nm, λem = 485 nm) of blank sample ZnWO4 at different temperatures of 10−300 K; (b) PL decay curve (λex = 305 nm, λem = 485 nm) of sample ZnWO4:1.0%Bi3+ at different temperatures of 10−300 K; (c) Lifetime of blank sample ZnWO4 and Mean lifetime of ZnWO4:1.0%Bi3+ at different temperatures of 10−300 K.

Table 2. Decay Lifetime of ZnWO4:xBi3+ (x = 0, 1.0%) Phosphors at 10−300 K (λex = 305 nm, λem = 485 nm) x=0

temperature

x = 1.0%

T /K

A

τ /μs

A1

τ1 /μs

A2

τ2 /μs

τave /μs

10 50 100 150 200 250 300

0.8216 0.8293 0.8372 0.8454 0.8468 0.8803 0.9523

46.99 45.86 44.13 44.64 44.29 41.38 34.62

0.3915 0.6227 0.4611 0.5004 0.5001 0.5001 0.5004

2.63 1.48 35.54 38.85 40.11 39.48 26.12

1.967 0.5952 0.4611 0.5004 0.5001 0.5001 0.5004

37.01 36.06 35.54 38.85 40.11 39.48 26.12

30.87 34.63 35.54 38.85 40.11 39.48 26.12

longer wavelength emission, and only the peak at ∼485 nm remains at 300 K (see Figure 5f). To illustrate luminescence dynamic, we measured the decay curves of blank ZnWO4 and ZnWO4:1.0%Bi phosphors within the temperature range of 10−300 K, as shown in Figure 6a,b. The excitation wavelength and the monitored emission wavelength are 305 and 485 nm, corresponding to the absorption region and the maximum emission intensity of blank sample ZnWO4, respectively. For ZnWO4, the decay dynamic behavior does not change greatly with the increases in temperature, and it keeps following the single exponential decay equation. Fitting the decay curves with the single-exponential equation of I = Aexp(−t/τ),31 where parameters A, I, and t are the constant, the luminescence intensity and the decay time, respectively, produces host lifetimes. As depicted by Table 2 and Figure 6c (black curve), the lifetimes of blank sample ZnWO4 as the temperature increases reduce gradually from 46.99 to 34.62 μs. However, the sample ZnWO4:1.0%Bi follows a double-exponential decay equation when temperature is below 100 K. For these curves, they can be

dependence of the emission peak wavelength on the excitation wavelength is the same to that at room temperature. As we monitored the emission wavelength from 450 to 700 nm at 10 K, the shape of the excitation spectrum looks similar. However, the ratio of 295 to 365 nm changes significantly. The 365 nm peak of excitation grows faster at longer emission wavelengths and it extends to 425 nm (see Figure 5c). This confirms again that the red luminescence is excitable by NUV light. When temperature is lowered to 10 K from 300 K, we noticed that the host emission redshifts from 485 to 500 nm upon excitation into host absorption, while the red emission blueshifts from 665 to 625 nm upon 360 nm (see Figure 5d,e). As excited into the joint absorption of host and BiZn3+ for instance at 320 nm, we found the profile of the emission spectrum changes largely as the temperature changes. At 10 K, the emission peaks are around 600 nm, and it is enhanced at 50 K. Further increase to 90 K redshifts the emission peak to ∼660 nm and at the same time another peak shows up as a shoulder at ∼500 nm. At 130 K, the two peaks are almost at the same intensity. However, the temperature rising up to 150 K → 200 K → 250 K → 300 K gradually quenches the 8421


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Chemistry of Materials fitted by the second-order exponential equation I = A1exp(−t/τ1) + A2exp(−t/τ2),52,55,56 where I is the luminescence intensity; A1 and A2 are constants; t is the time; and τ1 and τ2 are rapid and slow lifetimes for exponential components. Further calculation into the two series of values with τave = (A1τ12 + A2τ22)/(A1τ1 + A2τ2)55,56 enables us to obtain the average emission lifetimes (Table 1 and Figure 6c (red curve)). As temperature exceeds 100 K, the decay for ZnWO4:1.0%Bi phosphor follows single exponential equation, and it does not change with further increasing temperature. Clearly, the sample ZnWO4:1.0%Bi differs from blank sample ZnWO4. 4.5. Mechanism on Tunable Emission and Excitation. Tunable emission was observed in a broad spectral range from 400 to 800 nm as the excitation schemes, temperature, and the energy-transfer processes could be controlled (see Figures 4−6). To our surprise, if the blank sample ZnWO4 is not contaminated, it also shows a tunable emission in the spectral range, which, however, is different from the doped sample (see Figures 6f and 7a). The blank sample exhibits a strong red emission at 10 K with a peak at ∼684 nm. At the same temperature, the emission at ∼500 nm becomes a shoulder of the emission at ∼684 nm. When temperature increases, the red emission starts quenched monotonically. At 300 K, only the ∼500 nm emission can be observed (see Figure 7a). The differences in red emission wavelength and the thermal quenching processes imply the different origin of the red emission in the blank sample. DFT calculation on single-particle level shows BiZn3+ can emit at either 611 or 629 nm at 0 K. This is very comparable to the ∼625 nm emission of ZnWO4: Bi (see Figure 5e) at 10 K, and it means that the calculation is reliable within random errors. Comparison between Figure 1e and Figure 7a suggests the ∼684 nm emission in the blank sample is possible from VZnO0, especially as we consider the formation energy and the electronic structure of VZnO in different charges. Since VZnO2+ has the lowest formation energy among all types of VZnO, we cannot fully exclude the contribution of VZnO2+ to the emission at ∼500 nm in both blank and doped samples. Similarly, the contribution of 2 BiZn3 VW to the emission at 500 and 660 nm in ZnWO4:Bi cannot be neglected. The different sources of contribution to the emission at ∼500 nm and ∼665 nm might be one of the reasons why the emission changes so complicatedly along with temperature. The energy transfer between host and the defects can be traced in different aspects. The first evidence is that for samples, regardless of the dopant content, the excitation spectra of the red emission are always made up of the host excitation at ∼295 nm and the excitation due to bismuth at different temperatures (see Figures 4 and 5). The average lifetime of the blank sample is always slightly longer than ZnWO4:1.0%Bi at different temperatures of 10−300 K (see Figure 6c). The reduction in lifetime in the doped sample somehow means the existence of the energy transfer. For ZnWO4:1.0%Bi, the average lifetime is elongated initially as the temperature goes from 10 to 250 K, and it is reduced quickly afterward (see Figure 6c). This indicates the energy transfer depends on temperature. The initial elongation of lifetime perhaps means that partially energy may be transferred from Bi to host. The rapid decrease of lifetime implies the backward energy transfer from host to Bi dominates as the temperature is higher than 250 K. However, in either way, the energy transfer efficiency is not high. Therefore, we can observe the emission from host and/or defects at different temperatures. The dominance of one over the others leads to the tunable

Figure 7. (a) Emission spectra of blank sample ZnWO4 at different temperatures of 10−300 K, λex = 325 nm; (b) Schematic diagram for tunable luminescence in ZnWO4:Bi3+. The tunable emission images are demonstrated below; (c) Transition between different types of defects such as BiZn0, BiZn+, BiZn3+, and charge-compensated complex 2 BiZnVW. The color difference in the orbital contour plot denotes the defect levels within the optical band gap are spin-polarized. The blue color means the spin-up electronic configuration, while the green represents the spindown configuration.

emission as shown in Figure 7b as proper excitation plan is adopted for each type of defects. Previous works on Bi3+ phosphors show that there are two types of Bi3+ transitions 3P1 → 1S0 and 3P0 → 1S0.52,54 The former is Laporte allowed, while the latter transition is spin forbidden. The latter usually takes dominance at lower temperature. Since 3 P0 lies higher than 3P1 in the band gap, the dominate transition of 3P0 → 1S0 will lead to the blueshift of the emission of bismuth at low temperature. We think the population redistribution between 3P0 and 3P1 levels should also be one of the reasons why the emission blueshifts as the temperature is lowered to 10 K (Figure 5e). 8422


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Ten Thousand Leading Talent Project in Guangdong Program for Special Support of Eminent Professionals.

As we examine Figure 1e once again, different charge state defects of BiZn such as BiZn0, BiZn+, BiZn2+, BiZn3+, or 2 BiZnVw can give out the emission in 500−1634 nm, which spans visible and near-infrared. Because ZnWO4:Bi was prepared in an oxidizing condition, it is not favorable to promote the formation of the lower charge state defects. However, if we could stabilize them inside the crystal by utilizing special synthesis techniques such as in situ reduction (see Figure 7c), we could even find Bi-doped new laser materials, which can luminesce in near-infrared. In this way, tunable emission can extend beyond visible to near-infrared by intentional creation of different types of Bi defects (see Figure 7c).

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(1) Nakamura, S. Blue-Green Light-Emitting Diodes and Violet Laser Diodes. MRS Bull. 1997, 22, 29−35. (2) Schubert, E. F.; Kim, J. K. Solid-State Light Sources Getting Smart. Science 2005, 308, 1274−1278. (3) Pimputkar, S.; Speck, J. S.; DenBaars, S. P.; Nakamura, S. Prospects for LED Lighting. Nat. Photonics 2009, 3, 180−182. (4) Pust, P.; Schmidt, P. J.; Schnick, W. A. Revolution in Lighting. Nat. Mater. 2015, 14, 454−458. (5) Shang, M. M.; Li, C. X.; Lin, J. How to Produce White Light in a Single-Phase Host? Chem. Soc. Rev. 2014, 43, 1372−1386. (6) Lin, C. C.; Liu, R. S. Advances in Phosphors for Light-Emitting Diodes. J. Phys. Chem. Lett. 2011, 2, 1268−1277. (7) Hirosaki, N.; Takeda, T.; Funahashi, S.; Xie, R. J. Discovery of New Nitridosilicate Phosphors for Solid State Lighting by the Single-ParticleDiagnosis Approach. Chem. Mater. 2014, 26, 4280−4288. (8) Xia, Z. G.; Ma, C. G.; Molokeev, M. S.; Liu, Q. L.; Rickert, K.; Poeppelmeier, K. R. Chemical Unit Cosubstitution and Tuning of Photoluminescence in the Ca2(Al1‑xMgx)(Al1‑xSi1+x)O7:Eu2+ Phosphor. J. Am. Chem. Soc. 2015, 137, 12494−12497. (9) Wang, B.; Lin, H.; Huang, F.; Xu, J.; Chen, H.; Lin, Z. B.; Wang, Y. S. Non-Rare-Earth BaMgAl10−2xO17:xMn4+,xMg2+: A Narrow-Band Red Phosphor for Use as a High-Power Warm W-LED. Chem. Mater. 2016, 28, 3515−3524. (10) Ratnam, B. V.; Jayasimhadri, M.; Bhaskar Kumar, G.; Jang, K.; Kim, S. S.; Lee, Y. I.; Lim, J. M.; Shin, D. S.; Song, T. K. Synthesis and Luminescent Features of NaCaPO4:Tb3+ Green Phosphor for Near UVBased LEDs. J. Alloys Compd. 2013, 564, 100−104. (11) Liu, Y. F.; Zhang, X.; Hao, Z. D.; Wang, X. J.; Zhang, J. H. Tunable Full-Color-Emitting Ca3Sc2Si3O12:Ce3+, Mn2+ Phosphor via Charge Compensation and Energy Transfer. Chem. Commun. 2011, 47, 10677− 10679. (12) Takeda, T.; Hirosaki, N.; Funahshi, S.; Xie, R. J. Narrow-Band Green-Emitting Phosphor Ba2LiSi7AlN12:Eu2+ with High Thermal Stability Discovered by a Single Particle Diagnosis Approach. Chem. Mater. 2015, 27, 5892−5898. (13) Hermus, M.; Phan, P.-C.; Brgoch, J. Ab Initio Structure Determination and Photoluminescent Properties of an Efficient, Thermally Stable Blue Phosphor, Ba2Y5B5O17:Ce3+. Chem. Mater. 2016, 28, 1121−1127. (14) Guo, C. F.; Luan, L.; Chen, C. H.; Huang, D. X.; Su, Q. Preparation of Y2O2S:Eu3+ Phosphors by a Novel Decomposition Method. Mater. Lett. 2008, 62, 600−602. (15) Pust, P.; Weiler, V.; Hecht, C.; Tücks, A.; Wochnik, A. S.; Henß, A.-K.; Wiechert, D.; Scheu, C.; Schmidt, P. J.; Schnick, W. Narrow-Band Red-Emitting Sr[LiAl3N4]:Eu2+ as a Next-Generation LED-Phosphor Material. Nat. Mater. 2014, 13, 891−896. (16) Zhang, S.; Liang, H. B.; Liu, C. M. Increased 1D2 Red Emission of Pr3+ in NaGdTiO4:Pr3+ Due to Temperature-Assisted Host Sensitization and Its Color Variation. J. Phys. Chem. C 2013, 117, 2216−2221. (17) Wang, L. L.; Noh, H. M.; Moon, B. K.; Park, S. H.; Kim, K. H.; Shi, J. S.; Jeong, J. H. Dual-Mode Luminescence with Broad Near UV and Blue Excitation Band from Sr2CaMoO6:Sm3+ Phosphor for White LEDs. J. Phys. Chem. C 2015, 119, 15517−15525. (18) Maldiney, T.; Bessière, A.; Seguin, J.; Teston, E.; Sharma, S. K.; Viana, B.; Bos, A. J. J.; Dorenbos, P.; Bessodes, M.; Gourier, D.; Scherman, D.; Richard, C. The in Vivo Activation of Persistent Nanophosphors for Optical Imaging of Vascularization, Tumours and Grafted Cells. Nat. Mater. 2014, 13, 418−426. (19) Wang, B.; Lin, H.; Xu, J.; Chen, H.; Wang, Y. S. CaMg2Al16O27:Mn4+-based Red Phosphor: A Potential Color Converter for High-Powered Warm W-LED. ACS Appl. Mater. Interfaces 2014, 6, 22905−22913. (20) Li, M. Q.; Zhang, J. L.; Han, J.; Qiu, Z. X.; Zhou, W. L.; Yu, L. P.; Li, Z. Q.; Lian, S. X. Changing Ce3+ Content and Codoping Mn2+

5. CONCLUSIONS AND OUTLOOK In this work, we prescreen high covalency candidate compound ZnWO4 by the dielectric chemical bond theory calculation, and consequently, we screen all possible types of defects due to Bi substitutions inside ZnWO4 in aspects of PDOSs, formation energy, and electronic configurations. Calculations show Bidoped ZnWO4 can promisingly produce broad-range tunable emission from visible to near-infrared. Consequent experiments confirm this, and tunable emission was observed between 400 to 800 nm as we modulated the environment temperatures, excitation schemes, and energy transfer processes for these defects. The red luminescence from BiZn3+ was observed to be excitable by light in the range of 380−420 nm. Overall, under the guidance of DFT and the dielectric chemical bond theory calculations, we demonstrate a methodology to find efficiently a novel Bi3+ red phosphor ZnWO4:Bi3+, which particularly shows no absorption in the visible spectral range, and, therefore, addresses the intrinsic problems traditional RE phosphors cannot solve. We think the methodology can extend to find bismuth-doped new laser materials which can exhibit luminescence in near-infrared spectral range. This will help to develop laser sources in new spectral range.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mingying Peng: 0000-0002-0053-2774 Bolong Huang: 0000-0002-2526-2002 Fengwen Kang: 0000-0002-6022-655X Jing Wang: 0000-0002-1246-991X Bingfu Lei: 0000-0002-6634-0388 Author Contributions #

J.H. and L.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

We acknowledge financial support from Program for Innovative Research Team in University of Ministry of Education of China (Grant No. IRT_17R38), the Key Program of Guangzhou Scientific Research Special Project (Grant No. 201607020009), the National Natural Science Foundation of China (Grant No. 51672085), National Key Research and Development Plan (Grant No. 2017YFF0104504), Fundamental Research Funds for the Central Universities, and the Hundred, Thousand, and 8423


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