Band-Gap Modulation in Single Bi3+-Doped YttriumâScandium

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Band-Gap Modulation in Single Bi3+-Doped Yttrium−Scandium− Niobium Vanadates for Color Tuning over the Whole Visible Spectrum Fengwen Kang,†,‡ Haishan Zhang,† Lothar Wondraczek,†,§ Xiaobao Yang,† Yi Zhang,† Dang Yuan Lei,‡ and Mingying Peng*,† †

China-German Research Center for Photonic Materials and Devices, the State Key Laboratory of Luminescent Materials and Devices, School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China ‡ Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China § Otto Schott Institute of Materials Research, University of Jena, 07743 Jena, Germany ABSTRACT: The use of rare-earth (RE) (e.g., Eu2+/Ce3+) ions as single luminescent centers in phosphors with tailorable emission properties has been extensively studied for their potential use in white LEDs. However, significant limitations remain, in particular, for red-emitting phosphors due to the inherently broad excitation bands which result from the underlying d−f transitions and span large parts of the visible spectral region. Guided by density functional theory calculations on the ligand structure of the non-RE Bi3+ ion, we report here on an alternative class of phosphors, [(Y,Sc)(Nb,V)O4:Bi3+], which exhibit homogeneous Bi3+ luminescence. In these materials, adjustment of the cation fractions enables dedicated tailoring of the excitation scheme within the spectral range of ∼340−420 nm and, in the meanwhile, allows for tunable emission spanning from about 450 nm (blue) to 647 nm (orange-red). The practical absence of any overlap between the emission and excitation spectra addresses the issues of emission color purity and visible reabsorption. Tailoring through band-gap modulation is achieved by single or parallel substitution of Nb by V and Y by Sc. Such topochemical design of the ligand configuration enables modulation of the electronic band gap and thus provides a new path toward tunable phosphors, exemplarily based on Bi3+ single doping.

1. INTRODUCTION Spectrally tunable inorganic phosphors are a material class in which emission color, excitation schemes, and/or other spectral characteristics of photoluminescence are adjustable through specific routes of material design or processing. Such spectral tuning can be achieved in a variety of ways, e.g., through adjusting the excitation wavelength,1 tailoring energy transfer processes between different active species,1−4 adjusting the ligand configuration and crystal field splitting,5−7 adjusting the doping content,8−10 controlling the type of neighboring cations11,12 or host composition in general,13,14 controlling the distribution of defects and electronic traps,15,16 activator aggregation and segregation,17 core−shell−shell management in nanocomposite particulates18,19 or, in a simplistic way, controlling structural morphology.4,20 Besides, a variety of physical approaches such as the use of ionizing radiation,21 mechanical stress,22,23 magnetic24,25 and electric fields,26,27 or plasmonic resonances28,29 became available over recent decades for generating specific photoemission behavior from solid materials. In consequence, a large number of tunable materials with photoluminescence (PL) ranging from the visible (vis) to the near-infrared (near-IR) spectral region are now available © 2016 American Chemical Society

and are increasingly suggested for use in, e.g., display devices,3,5,6 optical sensing,22−27,30 and biomedicine and biomedical imaging.18,19,31 However, a remaining problem of nearly all Eu2+-doped phosphors, for instance nitrides or oxyonitrides,32,33 is the overlap of their excitation bands with parts of the visible spectral region. This gives rise to distortion or imbalance of the emission color, for example, by luminescence reabsorption, once these phosphors are applied in white light LEDs. In this context, an important task is the consideration of materials in which not RE but other optically active species act as the luminescent center. Among these non-RE PL activator species, bismuth stands out primarily due to the multiplicity of redox states which can be stabilized in inorganic host materials.1,5,34 Each of these redox states, by itself, provides a complex electronic structure which is strongly interacting with the respective ligand environment, thus leading to broadly tailorable PL emission. Received: January 21, 2016 Revised: March 28, 2016 Published: March 29, 2016 2692

DOI: 10.1021/acs.chemmater.6b00277 Chem. Mater. 2016, 28, 2692−2703

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Chemistry of Materials

Figure 1. Density of states in bulk (Y1−y,Scy)(Nb1−x,Vx)O4 (a and c) and in Bi-doped compounds (b, d), derived from DFT calculations. The insets of panels a and b represent structural information on YVO4, YVO4, and ScVO4 samples without and with Bi doping, respectively. (e and f) Absorption spectra of bulk (Y1−y,Scy)(Nb1−x,Vx)O4 and of the corresponding Bi-doped compounds. Typical body colors of Bi-doped samples are shown in the insets of panel d. 2693


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Chemistry of Materials Table 1. Nominal Chemical Compositions of Experimentally Studied (Y1−y,Scy)(Nb1−x,Vx)O4:0.01Bi3+ Compounds no. composition no. composition no. composition no. composition no. composition no. composition

1−1 YVO4 2−1 YNb0.2V0.8O4 3−1 YNb0.4V0.6O4 4−1 YNb0.6V0.4O4 5−1 YNb0.8V0.2O4 6−1 YNbO4

1−2 Y0.8Sc0.2VO4 2−2 Y0.8Sc0.2Nb0.2V0.8O4 3−2 Y0.8Sc0.2Nb0.4V0.6O4 4−2 Y0.8Sc0.2Nb0.6V0.4O4 5−2 Y0.8Sc0.2Nb0.8V0.2O4 6−2 Y0.8Sc0.2NbO4




1−6 ScVO4 2−6 ScNb0.2V0.8O4 3−6 ScNb0.4V0.6O4 4−6 ScNb0.6V0.4O4 5−6 ScNb0.8V0.2O4 6−6 ScNbO4

Table 2. Bond Lengths (Å) of Bulk LnMO4 (Ln = Y, Sc; M = Nb, V) Samplesa compound polyhedra coordn no. bond type av bond length

YNbO4 [YO8] 8 Y−O(4) 2.405

YVO4 [NbO6] 6 Nb−O(3) 2.09

[YO8] 8 Y−O(2) 2.41

ScVO4 [VO4] 4 V−O(1) 1.73

[ScO8] 8 Y−O(2) 2.26

[VO4] 4 V−O(1) 1.72

a

Numbers in parentheses denote the bond length of the polyhedral coordination polyhedra, e.g., the number of 4 in Y−O(4) denotes that there are four different Y−O bonds in [YO8]).

In the present study, we focus on trivalent bismuth, Bi3+. While the spectral properties of Bi3+ are generally known, in practical terms, the identification and characterization of suitable crystalline host materials has remained rather limited. Similarly, knowledge on the dedicated tailoring of PL from Bi3+ and its use as a versatile activator species is scarce. As a consequence, these challenge researchers for how to precisely realize the spectral tuning of Bi3+ and then to acquire the spectral wavelength we expected as well as to control the excitation tail to avoid the visible light reabsorption issue. Here, we start from first-principles calculations of the electronic properties of Bi3+ in different ligand situations to identify host species of potential interest. After consulting a variety of candidate species, we select YNbO4, YVO4, and ScVO4 hosts for Bi3+ doping, since they exhibit an electronic band gap of 3.76, 2.96, and 2.71 eV, respectively (Figure 1a). Through modulating the V/Nb and Sc/Y ratios, the band gap can be tuned continuously between 3.63 and 2.58 eV (Figure 1c). YNbO 4 , YVO 4 , and ScVO 4 precipitate in monoclinic fergusonite (YNbO4)35 and in the zircon-type heterostructure,36 respectively. In these, the dopant pairs of Nb−V and Y− Sc possess similar valence electrons. Thus, the influence factor of electron gain or loss is minimized in the process of substitution. The results have been evidenced by the bulk mixed oxides (Figure 1a,c). We experimentally verify this reasoning by demonstrating the full-color tunability of Bi3+ photoemission in (Y,Sc)(Nb,V)O4:Bi3+ compounds, at the same time addressing the issue of luminescence reabsorption. We show that this design strategy provides an efficient and generalist tool to generate tunable phosphors, using a broad variety of non-RE activator species.

replacement by Bi on the band-gap energies and absorption spectra in order to reveal the physical mechanism of the spectral tuning. All calculations were performed using the DFT engine implemented in the Vienna ab initio simulation package (VASP). Projector-augmented wave (PAW) potentials with generalized gradient approximation (GGA) were used in the Perdew−Burke−Ernzerhof (PBE) format for exchange correlation potentials.37−40 A 2 × 2 × 2 supercell with 192 atoms was initially adopted to construct the model of the three YNbO4, YVO4, and ScVO4 bulk samples, in which one Bi atom was assumed to substitute one Y or Sc atom. A 1 × 1 × 1 k-point grid was used to screen an optimal structure based on conjugate gradient minimization, with the related force criterion of 0.05 eV/Å. After full relaxation, we increased the k-point grid to 2 × 2 × 2 so as to obtain a more accurate energy and the density of states (DOS) with fixed atoms. In order to minimize the computational cost, the Bi fraction in the modeling situation was kept comparable to the typical maximum Bi3+-doping concentration of about 3.0 at. % (in ScVO4),41 though nominal Bi3+-doping content in this study was kept to 1.0%. With these essential items, a range of cases were simulated where the degree of substitution was 0, 25%, 75%, and 100% for V in Y(Nb1−x,Vx)O4:Bi and Sc in (Y1−x,Scy)VO4:Bi. To more accurately describe the 3d electrons of Sc, V, and Nb, the GGA+U extension approach was employed,42,43 where the values of U and J were defined as 3.0 and 0.8 eV, respectively. The energy cutoff has been repeatedly tested to ensure the respective convergence formation energy located within an acceptable range. A plane-wave energy cutoff of 520 eV for all samples was eventually employed in this work for modeling convenience.

3. EXPERIMENTAL DETAILS Phosphors with the nominal compositions of (Y1−y,Scy) (Nb1−x,Vx)O4:0.01 Bi3+ were synthesized with x and y changing across the sequence of 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0. For this, a sintering procedure at 1100 °C for 3 h was carried out on raw materials of Y2O3 (99.99%), Sc2O3 (99.99), Nb2O5 (99.99%), NH4VO3 (99.95%), and Bi2O3 (99.999%). Intermediate grinding during the synthesis process was performed to improve the sample homogeneity. All materials were used as purchased without further purification. Details on the chemical compositions and denomination of all samples are provided in Table 1. A Rigaku D/max-IIIA diffractometer (operating at 40 kV, 40 mA, and 1.2°·min−1) with Cu Kα radiation (1.5418 Å) was used for phase analyses by X-ray diffraction (XRD). Photoluminescence excitation and emission spectra as well as the lifetimes at ambient temperature

2. COMPUTATIONAL DETAILS Ab initio computational simulations were performed for a variety of host materials in order to identify a champion material for the experimental realization of the Bi3+-doped phosphor with broad photoemission tunability and negligible absorption in the visible spectral range. We calculated the electronic band-gap energies of YNbO4, YVO4, and ScVO4 and studied the influence of the Y and Sc 2694


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Chemistry of Materials were acquired on a high-resolution spectrometer (FLS 920 spectrofluorometer, Edinburgh Instruments) which was equipped with a red-sensitive photomultiplier (Hamamatsu R928 P) in Peltier air-cooled house in the single photon counting mode. Internal quantum efficiencies (QE) were approximated with an integrating sphere attached to the spectrometer FLS 920. All excitation and emission spectra were corrected over the lamp intensity with a silicon photodiode, and further normalized by the PMT spectral response. Specimen morphologies and chemical compositions of the samples were analyzed with a Hitachi S-3700N scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer. Similar to previous studies of Xie et al.,44−47 we recorded the particle luminescence maps under 254 nm UV-C irradiation, by a 0.300 m imaging triple grating monochromator spectrograph which was connected to an imaging spectrograph (Acton SP 2300, Princeton Instruments).

4. RESULTS AND DISCUSSION 4.1. Band-Gap Analysis. As shown in the inset of Figure 1a, the three optimized configurations from the DFT results consist of a Y or Sc atom surrounded by eight O atoms, forming eight-coordinated [YO8] and [ScO8] polyhedral structures in bulk YNbO4, YVO4, and ScVO4 samples. The band-gap (Eg) energy, defined as the difference between the valence band maximum (VBM) and conduction band minimum (CBM), steadily decreases from 3.76 eV (for YNbO4) to 2.96 eV (for YVO4) and then to 2.71 eV (for ScVO4). This behavior suggests that through controlling the cationic ratios, the band-gap energy can be flexibly adjusted. Detailed inspection on the structural configurations reveals that YNbO4 contains four different Y−O bonds with an average length of 2.405 Å, while both YVO4 and ScVO4 show only two Y−O and Sc−O bonds with average lengths of 2.41 and 2.26 Å, respectively. Comparison of these values indicates that the average Y−O length of YNbO4 is very close to that found in bulk YVO4, while a significant difference between the average lengths of Y−O and Sc−O in YVO4 and bulk ScVO4 is observed. The other two cations connecting the oxygen anions form the 6-fold-coordinated [NbO6] and 4-fold-coordinated [VO4] polyhedral structures, exhibiting a somewhat opposite situation as compared to the Y−O and Sc−O linkages. Here, there are three different Nb−O bonds with an average length of 2.09 Å, but there is only one V−O in YVO4 as well as in ScVO4, with lengths of 1.73 and 1.72 Å, respectively. This underlines that the band-gap difference between YNbO4 and YVO4 is associated with the average bond length of Nb−O, Y−O, and Sc−O. On the other hand, the difference between the Y−O and Sc−O bonds is responsible for the band-gap difference in bulk YVO4 and ScVO4 (Figure 1a and Table 2). One may further note that the VBM of YNbO4, YVO4 and ScVO4 (with almost constant location in the three species) is all dominated by the oxygen p nonbonding. However, the CBM for the three samples which is primarily determined by the integral electrons of Y−Nb, Y−V, and Sc−V varies dramatically as shown in Figure 1c. These results, in combination with the above analysis, point out the important role of the V/Nb and Sc/Y atomic ratios in narrowing the band-gap energy. For this reason, a variety of median values with a regular decrease of Eg value are anticipated in bulk Y(Nb,V)O4 and (Y,Sc)VO4 hosts (Figure 1a,c). Once the Bi ions are incorporated into YNbO4, YVO4, and ScVO4, a further slight decrease of the relevant Eg values occurs in comparison to the pristine samples, i.e., 3.63 eV for YNbO4:Bi, 2.98 eV for YVO4:Bi, and 2.58 eV for ScVO4:Bi

Figure 2. (a) Double lattice cell of YNbO4 (i) and YMO4 (M = Nb, V) (ii), coordination environments of Nb, V and Y, Sc sites (iii), and the related bond lengths (iv), based on data from the Inorganic Crystal Structure Database (ICSD), cards YNbO4 (ICSD No. 20335), YVO4 (ICSD No. 78074), and YNbO4 (ICSD No. 78073). (b) XRD patterns of Y(Nb1−x,Vx)O4:Bi (i) and (Y1−y,Scy)VO4:Bi (ii) compounds. The phase variation trend between YNbO4 and YVO4is illustrated through the symbols ♀ and ♂, respectively. (c) Refined XRD result () of a typical YNbO4:Bi sample, Rietveld refining results (×), Bragg reflections (|), and profile difference between experimental and calculated data (−). 2695


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Figure 3. (a, b) Excitation spectra of Y(Nb1−x,Vx)O4:Bi and (Y1−y,Scy)VO4:Bi compounds. (c, d) Dependence of the excitation tail position on the V/Nb and Sc/Y ratios. Lines in panels c and d represent the result of a linear fit of the data.

Here, the cationic radii decrease continuously,5,35,36 i.e., from Nb5+ (CN = 6, 0.780 Å) to V5+ (CN = 4, 0.495 Å) and from Y3+ (CN = 8, 1.019 Å) to Sc3+ (CN = 8, 0.870 Å), respectively. This effect is then used for precisely modulating the relevant cell volume, coordination number ratio, and bond length by adjusting the ratios of Nb/V and Sc/Y. It is noteworthy that the electronic tunability is also directly reflected in the absorption spectra of both pristine and Bi-doped samples (Figure 1e,f), where the absorption tails are shifted to lower energy with increasing V/Nb and Sc/Y ratios. In consequence, this tunable band-gap property guides us to design and achieve the following spectral tuning. 4.2. Structural Analysis. Regarding the size and charge of the cationic species in all samples, the larger Bi3+ dopant ion assumedly prefers to occupy the 8-fold-coordinated Y or Sc site rather than the 4-fold-coordinated V or the 6-fold-coordinated Nb site (Figure 2a). In addition to the above-mentioned DFT calculations (Figure 1), XRD analyses and Rietveld refinement (Figure 2b,c) were employed to confirm this expectation. We find that edge-sharing or oxygen-sharing in [YO8], [ScO8], [NbO6], and [VO4] units leads to chains of Y···O···Nb, Y···O··· V, and Sc···O···V, in which the Y and Sc sites are partially substituted by Bi3+, transforming the Y···O···Nb or Y···O···V chain to Bi···O···Nb or Bi···O···V entities. As we do not expect very high solubility between the YNbO4 and YVO4 endmembers, a continuous increase of the V/Nb ratio controls the

(Figure 1b). This is because the transformation of Y−O and Sc−O bonds to the Bi−O bond increases the average length (Figure 1b (inset)). The average length of the Bi−O bond is found to be 2.453 Å for YNbO4:Bi, 2.445 Å for YVO4:Bi, and 2.375 Å for ScVO4:Bi. Similar to the observations in the undoped counterparts, the difference in the Bi−O bond length between YVO4:Bi and ScVO4:Bi is very obvious, whereas the bond lengths of YNbO4:Bi and YVO4:Bi are very close. Again, the Nb−O and V−O bond lengths follow an opposite trend in YNbO4:Bi, YVO4:Bi, and ScVO4:Bi samples. It is clear that adjusting the V/Nb and Sc/Y ratios allows for modulation of the band-gap energies in the Bi-doped samples (Figure 1d), showing adjustment behaviors similar to those in Figure 1c. As a result, it is concluded that the VBM in the doped samples is mainly determined by the Bi atoms, which clearly differs from the pristine samples where the oxygen atoms play the key role. Additionally, the CBM location remains almost unaffected, still being dominated by the contribution from the electronic configuration of the Y−Nb, Y−V, and Sc−V pairs. The above results demonstrate that the Bi doping directly contributes to band-gap narrowing and can, hence, be used for band-gap tuning. At the same time, the Nb/V and Sc/Y ratios can also affect the width of the electronic band gap. For both contributions, it is obvious that the fundamental design parameter is the ratio between the cationic radii of the participating ions in their respective coordination environment. 2696


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Table 3. Emission Positions, FWHM, QE, Lifetimes, and CIE Chromaticity Coordinates (x, y) of Selected (Y1−y,Scy)(Nb1−x,Vx)O4:0.01 Bi3+ Compoundsa no.

compounds

Em/nm

fwhm/nm

QE/%

CIE(x,y) value

6−1 5−1 4−1 3−1 2−1 1−1 1−2 1−3 1−4 1−5 1−6

YNbO4:Bi YNb0.8V0.2O4:Bi YNb0.6V0.4O4:Bi YNb0.4V0.6O4:Bi YNb0.2V0.8O4:Bi YVO4:Bi Y0.8Sc0.2VO4:Bi Y0.6Sc0.4VO4:Bi Y0.4Sc0.6VO4:Bi Y0.2Sc0.8VO4:Bi ScVO4:Bi

456 514 548 560 564 566 579 588 604 621 647

120.0 214.7 174.4 171.8 169.5 164.1 180.9 189.2 205.8 225.9 230.2

45.6 47.1 47.5 52.4 64.4 74.8 71.2 57.2 53.8 49.4 37.8

(0.191,0.226) (0.312,0.365) (0.375,0.457) (0.407,0.479) (0.409,0.480) (0.413,0.481) (0.439,0.480) (0.454,0.477) (0.484,0.468) (0.510,0.455) (0.521,0.443)

a

Sample denomination has been listed in Table 1 and the excitation wavelengths for QE measurement correspond to the maximum excitations of Figure 3a,b.

isomorphous tetragonal zircon structure in which the diffraction peaks shift to higher angles (i.e., larger structural features) as y increases. This reflects the continuous replacement of the bigger Y3+ ion by the smaller Sc3+ (Figure 2b). The associated lattice contraction can then be used to again control the ligand field strength in the environment of the Bi3+ dopant. 4.3. Spectral Tuning of Bi3+ Photoemission. Upon exposure to a UV-C lamp, all blank YNbO4, YVO4, and ScVO4 samples exhibit a characteristic blue photoemission. When excited with monochromatic light at 265 nm, the origin of this visual appearance can be traced to a single broad emission band at 412, 435, and 465 nm, respectively, corresponding to the electronic transitions of 1T2 → 1A1 in NbO43− and 1B(1T2) → 1 A1 in VO43−. In these observations, the excitation energy corresponds well to the electronic band gap of the bulk compounds as calculated through DFT (Figure 1a). Also the red shift of the emission positions in the sequence of decreasing ionic radii, i.e., Nb5+ → V5+ and Y3+ → Sc3+, is clearly observed. Noteworthy, the small underestimation between the energy values derived by DFT versus the experimental data, which is also frequently observed in previous works,41,48 is considered as not significant here. When doping YNbO4 with Bi3+, the excitation spectrum contains two overlapping bands with maximum intensities at ∼263 and ∼298 nm, corresponding to the 1A1 → 1T2 and 1A1 → 3T1 transitions in NbO43− groups. Similarly, for the YVO4:Bi3+ and ScVO4:Bi3+ samples, excitation bands at ∼263 and ∼328 nm and at ∼263 and ∼330 nm, respectively, reflect the 1A1 → 1E(1T1) and 1A1 → 1E(1T2) transitions of VO43−. The spectral shifts which are seen in these excitation (and emission) bands between the different compounds correspond to the previously discussed DFT and XRD data. The compounds of Y(Nb1−x,Vx)O4:Bi3+ and (Y1−y,Scy)VO4:Bi3+ were studied in the following. The respective excitation spectra were obtained by monitoring the maximum emission intensity, as shown in Figure 3a,b. Here, regular replacement of Nb5+ in YNbO4:Bi3+ by V5+ leads to a proportional shift of the excitation tail from its original position at ∼356 nm (for YNbO4:Bi3+) to significantly larger wavelength up until ∼370 nm in YVO4:Bi3+ (Figure 3a,c). Starting from YVO4:Bi3+ and gradually replacing Sc3+ for Y3+ further extends this shift of the excitation tail to ∼420 nm in ScVO4:Bi3+ (Figure 3b,d). Hence, the full transition enables a tuning range

Figure 4. (a(i)) Normalized emission spectra of (Y1−y,Scy) (Nb1−x,Vx)O4:Bi compounds. The Bi-doping content is 0.01 mol %, and the sequence for x and y values is 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0. (a(ii)) Selected CIE chromaticity coordinates as calculated from the emission spectra of panel a(i) and the corresponding digital photographs taken during exposure to a UV-C lamp. (a(iii)) Dependence of the emission band position on the V/Nb and Sc/Y ratios. The fitted curves (red) are based on the equation m = a + A exp(−n/k) for Y(Nb1−x,Vx)O4:Bi compounds (curve 1) and m = a + kn for (Y1−y,Scy)VO4:Bi compounds (curve 2), respectively. (iv) Dependence of the Bi3+ emission band position on the average Y−O and Sc−O covalency in (Y1−y,Scy)VO4:Bi compounds, where the fitting curve (red) is based on y = a + kx. (b) Measured and fitted (bright green) decay curves of Y(Nb1−x,Vx)O4:Bi and (Y1−y,Scy) VO4:Bi samples, where x values are 0.0, 0.2, 0.4, 0.6, and 1.0 and y values are 0.0, 0.2, 0.4, 0.6, and 0.8. The excitation and monitored wavelengths are shown in the panel.

ratio between 4-fold-coordinated [VO4] and 6-fold-coordinated [NbO6] polyhedral structures. Then, the dominating phase shifts from the monoclinic fergusonite (YNbO4) to the zircontype monoclinic-tetragonal heterostructure (YVO4) with increasing x in Y(Nb1−x,Vx)O4:Bi3+ (Figure 2a). This structural transition can subsequently be employed to tailor the ligand environment of the Bi3+ species, whereby at least two different potential Bi3+ sites are available. In the mixed (Y1−y,Scy)VO4:Bi3+, however, the situation is somewhat different. Here, the diffraction patterns, for all y > 0, can be assigned to the 2697


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Chemistry of Materials Table 4. Lifetimes of Selected (Y1−y,Scy)(Nb1−x,Vx)O4:0.01Bi3+ Compoundsa

a

No.

6−1

5−1

4−1

3−1

2−1

1−1

1−2

1−3

1−4

1−5

1−6

γ2 τ1 /μs τ2 /μs

0.997 0.581 4.462

0.995 0.628 6.828

0.996 0.607 7.348

0.997 0.646 7.261

0.997 0.595 7.735

0.998 0.583 8.239

0.997 0.582 8.561

0.998 0.663 7.947

0.998 0.643 9.135

0.997 0.561 7.816

0.997 0.617 7.972

The fitted τ1 and τ2 Values Is Based on the Double-Exponential Equation of I(t) = A1 exp(−t/τ1) + A2 exp(−t/τ2).

Similarly, the lowest fwhm and the lowest variance of Bi3+ emission energy are found in the proximity of the end-member compounds. The bright color of photoemission then evolves from a convolution of all spectral contributions. On the other hand, in the mixed samples of (Y1−y,Scy)VO4:Bi3+, replacement Y3+ by Sc3+ does not only increase the fwhm (Table 3) but also produces a proportional red shift of the position of the emission maximum (Figure 4a(iii), curve 2). In this case, reconsidering the structural features (Figure 2a), it is obvious that the Y···O and Sc−O bond lengths adjust during the transformation of YVO4:Bi3+ to ScVO4:Bi3+. This is accompanied by a regular transition in the field strength which surrounds the Bi3+ dopant species in each composition in the join. For better evaluation, the average covalency of Y−O and Sc−O bonds based on the dielectric chemical bond theory of complex crystals was performed along the series of (Y1−y,Scy)VO4:Bi3+.52,53 As expected, a linear increasing of the Y−O and Sc−O covalence is found (Figure 4a(iv)). This is taken as the direct reason for the red-shifting emission band (noteworthy at this point, in ScVO4:Bi3+, the formation of oxygen vacancies has been identified as an additional factor for generating unusually deep red luminescence41). Finally, one may expect energy transfer from NbO43− and VO43− to Bi3+ at room temperature, noting that monochromatic excitation at, e.g., 300 nm triggers only Bi3+-related emission bands but does not stimulate emissions from NbO43− and VO43−. Considering the excitation spectra of all these entities (Figure 3a,b), as the possible mechanism for such energy transfer, we propose a resonant nonradiative reaction, even though there is some overlap between the expected NbO43−/VO43− emissions and the absorption regime of Bi3+ (Figure 3a,b and Figure 4a(i)). Considering the practical application of these phosphors, we recorded the internal quantum efficiencies (QE) upon maximum excitations (see Figure 3a,b). QE values of Y(Nb1−x,Vx)O4:Bi3+ and (Y1−y,Scy)VO4:Bi3+ compounds are listed in Table 3. Sequential substitution of V by Nb and Y by Sc gives rise to increasing and then decreasing the QE values. Maximum QE of ∼74.8% is found for the YVO4:Bi3+ endmember compound. The initial increase of the QE value is ascribed to the structural transition from YNbO4 to YVO4, where the luminescence intensity of the Bi3+-doped sample is much stronger in YVO4 than in YNbO4. As for the (Y1−y,Scy)VO4:Bi3+ compounds, due to the increased defect probability, the decrease of QE occurs accordingly. As compared to commercial phosphors such as Ba3MgSi2O8:Eu (λem = ∼440 nm, blue, 65.3%),54 (Ba,Sr)MgAl10O17:Eu2+ (λem = ∼454 nm, blue, 83%),55 YAG:Ce (under blue LED excitation, yellow, 85%),50,56 ZnS:Cu (λem = ∼506 nm, green, 48%),57 SrS:Eu (λem = ∼616 nm, red, 60.8%),55 S2Si5N8:Eu2+ (λem = ∼636 nm, red, 70%),32 and (Ba,Ca,Sr)MgSi2O8:Eu,Mn (λem = ∼657 nm, red, 52%),55 Some of the observed QEs are smaller and some are comparable (see Table 3). Considering previous studies which pointed to a significant dependence of emission properties on processing conditions, we may expect strong

of about 72 nm. This far exceeds our previously reported range of ∼55 nm which was observed in (Y,Lu,Sc)VO4:Bi.5 On closer inspection, substituting Nb5+ with V5+ and then Y3+ with Sc3+ enables dedicated excitation tailoring in two different regimes of wavelengths, i.e., starting from ∼298 nm for YNbO4:Bi3+ and from ∼328 nm for YVO4:Bi3+ (here considering the positions of the peaks of the secondary excitation band). This is a direct result of the absorption properties of the Bi3+ species which are affected by its incorporation into the NbO43−/VO43− environment. At the same time, the excitation peak at shorter wavelength (which is apparently not affected by the presence of trivalent Bi) remains at the fixed value of ∼263 nm. Moreover, red-shifting of the excitation tails for Bi3+-doped samples (Figure 3a,b), which is consistent with the absorption tail shifting (Figure 1f), could also be reflected by the body color variation (Figure 1f (inset)). A point worth emphasizing here is that current UV-converting phosphors as they are considered for application in WLEDs present either relatively low utilization of UV light (e.g., Eu3+-/Pr3+-doped phosphors3,49,50) or strong and sometimes broad absorbance in the visible spectral region (e.g., Eu2+-/Ce3+-doped phosphors33,34,51). The former is of disadvantage in terms of conversion efficiency but also in terms of eventually harmful UV straylight, while the latter causes color distortion due to reabsorption. As for the present case, all excitation spectra which were recorded for Y(Nb1−x,Vx)O4:Bi3+ and (Y1−y,Scy)VO4:Bi3+ exhibit, if any, only very weak activity in the visible region (Figure 3a,b), what is taken as a clear advantage. Similar to the above excitation tuning, increasing the V/Nb and Sc/Y ratios also enables the tuning of photoemission behavior (Figure 4a(i)), though varying of the excitation wavelength does not change the emission band positions. That is, tunability of the emission band maximum between 456 nm (YNbO4:Bi3+), 566 nm (YVO4:Bi3+), and 647 nm (ScVO4:Bi3+) has been achieved here, i.e., over a tuning range of ∼191 nm (as compared to the previously reported value of ∼69 nm in (Y,Lu,Sc)VO 4:Bi5). CIE chromaticity coordinates of all phosphors basing on the emission spectra of Figure 4a(i) are given in Table 3. Some representative CIE data are plotted in Figure 4a(ii) in order to illustrate the tunability of photoemission in terms of generated color. Color tuning is achieved from blue (0.191, 0.226) to yellow (0.413, 0.481), and further to deep orange-red (0.521, 0.443). Further inspecting the spectral details of the emission data, all spectra cover almost the visible region. While the full width at half-maximum (fwhm) of the emission band gradually decreases, increasing of the x value from 0.2 to 1.0 in Y(Nb1−x,Vx)O4:Bi3+ leads to a red shift of photoemission by only ∼52 nm (Figure 4a(i) and Table 3). This stands in apparent contrast to the red shift between YNbO4:Bi3+ and YNb0.8V0.2O4:Bi3+ which is ∼58 nm. However, in the endmember compounds of YNbO4:Bi3+ and YVO4:Bi3+ only one single Bi3+ emission center is presented (Figure 2b(i)), whereas in the mixed species, at least two centers are simultaneously responsible for the observed photoemission (Figure 4a(i)). 2698


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Figure 5. continued

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Figure 5. (a) Typical SEM images of as-prepared compounds (i−v) and size distribution of the Y0.6Sc0.4Nb0.4V0.6O4:Bi sample (vi); (b(i−iv)) Elemental maps of four typical particles selected from panel a. (c and d) Microscopic luminescence images of phosphors without and with 100 °C heat treatment, respectively.

potential for further optimization of QE toward application in WLED devices. To further understand Bi3+ luminescence in the present solid-state compounds, the decay curves of Y(Nb1−x,Vx)O4:Bi3+ and (Y 1−y ,Sc y )VO 4 :Bi 3+ samples were considered. The excitation and monitored wavelengths correspond to the maximum excitations seen from Figure 3a,b and from the corresponding maximum emission intensity shown in Figure 4a(i) and Table 3, respectively. Due to curve overlap, Figure 4b provides only eight typical decay curves. Best fits of these curves are obtained with a double-exponential decay function,6,41

I(t ) = A1 exp( −t /τ1) + A 2 exp(−t /τ2)

(1)

where τ1 and τ2 correspond to short- and long-decay components, respectively; parameters A1 and A2 are fitting constants. Based on the fitted results, it is found that changing the V/Nb and Sc/Y ratios can strongly influence the τ2 value, but not the τ1 value (which stays at around 600 ns; Table 4). 4.4. Sample Morphology and Homogeneity. Hightemperature solid-state reactions are a popular route for the production of inorganic phosphors. Also this study involves such a reaction which, in the present case leads to irregularly shaped particulates. In Figure 5a, a series of exemplarily SEM micrographs is given for YNbO 4 :Bi, YNb 0.6 V 0.4 O 4 :Bi, 2700


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tive compounds such as YNbO 4 :Bi, YNb 0.6 V 0.4 O 4 :Bi, Y0.8Sc0.2NbO4:Bi, and Y0.4Sc0.6NbO4:Bi to observe the homogeneous Bi3+ luminescence on the microlevel under the microscope with 10 times upon UV illumination. The design method is similar to that previously reported by Xie et al.44−47 Upon a UV-C irradiation, the homogeneous Bi3+ luminescence reflected by digital photographs is observable (Figure 5c), exhibiting the same color as that of a powder mixture (Figure 6b). In addition, when treating the four samples at 100 °C for 3 h followed by naturally cooling to room temperature, we still observe the bright Bi3+ luminescence analogous to the powder mixtures and the unheated scenarios (Figure 5d). In combination with our previous results concerning the excellent thermal quenching of Bi3+ luminescence,15,41,59 the above analysis has unraveled the homogeneous distribution of the elements (i.e., Y, Sc, Bi, V, Nb, and O) in (Y,Sc)(Nb,V)O4:Bi3+ compounds. This is consistent with our idea on DFT modeling, where Bi dopants randomly replace the Y and Sc sites in the compounds. 4.5. Design Concept for Excitation and Emission Tuning. Based on the discussion of band-gap tunability in section 4.1 (Figure 1) and the double-exponential decay of Bi3+ in section 4.3 (Figure 4b), we propose a schematic mechanism which enables the chemical design of photoexcitation and emission from the exemplary Bi3+-doped compound of Y−Sc− Nb vanadates (Figure 6a). The ground state of 1S0 and the lowest 3P0 position of Bi3+ are determined on the ion’s relative location in Figure 1b. As can be seen, exciting at the host absorption, i.e., 3.76 eV for YNbO4, 2.96 eV for YVO4, and 2.71 eV for ScVO4, causes blue photoemission in all samples, with increasing emission peak position in the sequence of 412, 435, and 465 nm. Upon Bi3+doping, photoemission occurs via the newly introduced localized Bi3+ centers, due to an energy transfer process from the host to Bi3+. This is accompanied by a change in the emission color (413 nm for YNbO4:Bi, blue; 566 nm for YVO4:Bi, yellow; 647 nm for ScVO4:Bi, orange/red). An increase of the V/Nb ratio and further increase of the Sc/Y ratio lead to a continuous decrease of the electronic band gap of the host, i.e., from 3.57 to 2.89 eV, and then to 2.62 eV (Figure 1b), with various emission features emerging accordingly (Figure 4a(i)). Spectral tuning in (Y,Sc)(Nb,V)O4:Bi3+ is hence very closely linked to band-gap modulation. Singly or in parallel varying the cationic ratios of Y, Sc, Nb, and V enables a broad variety of emission colors when exciting with UV light (Figure 6b). This second step illustrates the importance of Bi3+ ligand tuning, which goes hand in hand with the aforementioned band-gap modulation. As a consequence, we achieve a singly non-RE-doped phosphor with full visible color tunability. Furthermore, we also give the configurational coordinate model of Bi3+ for interpreting the possibly intrinsic mechanism of the double-exponential decay resulting from the two transitions of 3 P1 → 1S0 and 3P0 → 1S0 that correspond to the short- and long-decay components, respectively. This is similar to the explanation we previously reported.16,41,59

Figure 6. (a) Design concept for spectral tuning and mechanism for the double-decay behavior of Bi3+ photoemission in (Y,Sc)(Nb,V)O4:Bi3+. The energy unit on the profile is based on using energy and wavelength data from Figures 1, 3, and 4. (b) Exemplary photographs of Bi3+ luminescence in some (Y,Sc)(Nb,V)O4:Bi3+ compounds during exposure to a UV-C lamp.

Y0.4Sc0.6VO4:Bi, YNb0.8V0.2O4:Bi, and Y0.6Sc0.4Nb0.4V0.6O4:Bi. The sample surfaces appear generally smooth, and the particulate size remains in the scale of one or a few micrometers, regardless of the substitution of Nb, V, Y, or Sc. For example, for sample Y0.6Sc0.4Nb0.4V0.6O4:Bi3+, the average particle size is 1.5 ± 1.0 μm (see Figure 5a(iv)). Compared to liquid-phase synthesis (for instance Pechini sol−gel or hydrothermal methods),57,58 solid-state reactions more easily lead to the inhomogeneity in sample composition and dopant distribution. However, we actually did not observe the noticeable phase or element segregation with the help of the techniques of energy dispersive X-ray spectroscopy (EDS), element maps, and fluorescence scattering images and spectroscopy. Four typical particles, which correspond to the samples of YNbO4:Bi, YNb0.6V0.4O4:Bi, Y0.4Sc0.6VO4:Bi, and YNb0.8V0.2O4:Bi and are marked with red circles in Figure 5a, are selected to perform the elemental mapping measurement (see Figure 5b(i−iv)). This vividly tells us that all aimed elements have been detected and there is no phase or dopant elements segregation in designed compositions, showing the homogeneous distributions. We further select the representa-

5. CONCLUSIONS AND OUTLOOK In summary, we reported on an alternative class of phosphors of the type [(Y,Sc)(Nb,V)O4:Bi3+]. Guided by density functional theory calculations on the band-gap energy and the ligand structure of Bi3+, we showed that, in this exemplary material, adjustment of the cation fractions enables dedicated tailoring of the excitation scheme within the spectral range of ∼340−420 nm. At the same time, an emission range of about 2701


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450 nm (blue) to 647 nm (orange-red) is achieved. The practical absence of any overlap between emission and excitation spectra addresses the issues of emission color purity and visible reabsorption. This combination of properties is achieved by the dedicated modulation of the electronic band gap in the host material, and simultaneously of the ligand arrangement around the Bi3+ dopant. The approach based on the DFT guidance may act as a model for designing a variety of new, broadly tunable phosphor materials toward application in solid-state lighting. Furthermore, we showed the homogeneous luminescence of Bi3+ in our solid-state phosphors, which, together with the obtained QE results, indicated the possible applicability in white LEDs.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 20 87114204. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 51322208 and 11474100), Guangdong Natural Science Foundation for Distinguished Young Scholars (Grant No. S20120011380), the Department of Education of Guangdong Province (Grant No. 2013gjhz0001), Fundamental Research Funds for the Central Universities, Key Program of Guangzhou Scientific Research Special Project, Hundred, Thousand and Ten Thousand Leading Talent Project in Guangdong Program for Special Support of Eminent Professionals, and the Hong Kong Research Grants Council (GRF Grant No. 153014).

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on April 7, 2016, with panels c, d, e, and f missing from Figure 1. The corrected version was reposted on April 8, 2016.

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