Peng Lia,
Linna Guo*a,
Chenxi Lianga,
Tiesheng Li*a,
Penglei Chenb,
Minghua Liu*b and
Yangjie Wu*a
aCollege of Chemistry and Molecular Engineering, Zhengzhou University, The Key Lab of Chemical Biology and Organic Chemistry of Henan Province, The Key Lab of Nano-information Materials of Zhengzhou, Zhengzhou, 450001, P. R. China. E-mail: guolinna@zzu.edu.cn; lts34@zzu.edu.cn
bBeijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China
First published on 3rd November 2017
In this paper, ScVO4:10%Yb3+/2%Er3+ nano/micro-particles doped with optical-inert metal ions including the alkali metal ions (Li+/Na+/K+), alkaline-earth metal ions (Mg2+/Ca2+/Sr2+/Ba2+) and lanthanide ions (Y3+/Gd3+/Lu3+) were synthesized by a conventional solid-state method. X-ray diffraction studies show that the prepared ScVO4:10%Yb3+/2%Er3+ whether single-doping, codoping or tridoping optical-inert metal ions are highly crystalline in nature with tetragonal phase structure when the doping concentration ≤ 10%. Under a 980 nm laser diode excitation, the upconversion luminescence was enhanced significantly by single doping of Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Y3+, Gd3+ or Lu3+, showing the strongest green emission with 5 mol% Li+ dopant. For codoping optical-inert metal ions system, it is found that Li+/Gd3+ couple is the most effective codopant, leading to an drastic increase of the UC luminescence centered at 554 nm by a factor of 15.3 compared to optical-inert metal ions free sample, while the factor of Li+/Ca2+/Gd3+ tridoping system is only 4.6. This work aims to investigate the origin of UC luminescence enhancement for ScVO4:10%Yb3+/2%Er3+ after codoping optical-inert ions based on the systematical analyses of the structures, morphologies, chemical states of elements, oxygen defects, optical absorption properties, etc. Furthermore, temperature-sensing performance was also investigated using the fluorescence intensity ratio technique. This opens a new window for studying the cooperation of the optical-inert ions doping effect on improving UC luminescence and temperature sensitivity properties.
For example, Chen et al.23 discovered a 10 times increase in blue emissions (1D2 → 3F4, 1G4 → 3H6) in Gd2O3:Yb3+,Tm3+ nanoparticles by doping with 6% Li+. Song's group24 demonstrated 10 and 4 times increases in red (5F4, 5S2 → 5I8) and green (5F5 → 5I8) emissions in Y2O3:3%Li+,4%Yb3+,1%Ho3+ nanoparticles. Hom et al.25 reported that the NIR to NIR UC emission intensity of 10 mol% K+ substituted ZnMoO4:Tm3+,Yb3+ nanocrystals increased by 21-fold compared with K+ free sample. Our group26 found that white UC emission was achieved in the Lu6O5F8:6%Yb3+,0.3%Er3+,0.4%Tm3+, 5%Li+ compared to Li+ free sample with the same activator concentration, besides, the integrated UC emission intensity of Lu6O5F8:20%Yb3+,1%Er3+, 3%Li+ is 5.5 times as strong as that of commercial UC phosphor (NaYF4:20%Yb3+,2%Er3+). Aside from alkali metal ions, alkaline-earth metal ions can also be used to tailor the local crystal field. Chen and Wang27 explored that UC luminescence intensities of NaGdF4:Yb3+/Er3+ were enhanced by about 200 times upon introducing Ca2+ dopants into the phosphors, probably due to a modification of the crystal structure of NaGdF4 and an improvement in the crystallinity. Haase and co-workers28 prepared CaAl12O19:Mg2+,Yb3+,Er3+ UCPs, in which the green and red emissions of the Er3+ ions was improved 4 and 1.5 times, respectively, compared with the counterparts without Mg2+ ions. On the other hand, not only the local site symmetry, but also phase transition may be induced with a higher concentration of host-ion substitution by non-active Ln3+. Liu and co-workers29 investigated a NaYF4:Er3+,Yb3+ system in which host ion substitution influences the nanocrystal growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. It was demonstrated that NaYF4 nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and UC emission color (green to blue) as well as intensity by replacing Y3+ with Gd3+. Zhang et al.30 discovered that the UC enhancement with size decrease has been realized in β-NaLuF4:Yb3+/Er3+ nanocrystals (NCs) through doping with Y3+ ions. Compared with β-NaLu0.78Yb0.2Er0.02F4 and β-NaY0.78Yb0.2Er0.02F4 prepared under the same condition, the green UC emission is enhanced by a factor of 1.8 and 16, respectively, for β-NaLu0.48Y0.3Yb0.2Er0.02F4. UC luminescence of NaY0.95−xYb0.03Er0.002ScxF4 was enhanced obviously by tridoping Sc3+ ions, contrasted to the untridoped one, especially for higher energy emission.21 Comprehensively, it shows a rising trend, indicating that optical-inert ions are becoming a more and more important and hot topic in the field of UCL enhancement. It can be concluded from above reports, most researchers have studied the effect of co-doping with alkali metal ions, alkaline-earth metal ions and inactive Ln3+ ions individually on the structural and UCL properties of phosphors.31–33 Comparatively, there is rare research about the effect of combination these optical inert metal ions doping and lack of knowledge on the intercommunication between codopants on the UC luminescence in controlling the properties of UCPs to the best of our knowledge.
In addition, reasons for the UC emission intensity enhancement through optical-inert co-dopants is yet to be determined with complete certainty, and this investigation is still an important and challenging research field. Most reports described the experiment results to the following fact. Some optical inert ions such as Li+ may directly act as a flux or sensitizer, and the others could enter into the host lattice, creating oxygen vacancies or altering the crystal field surrounding the activator, then affecting the luminescence performances of the phosphors.34–39 It is very important to inquire into the characteristics of codopants so as to further understand the mechanism of enhanced luminescence, and also help us to look for some more effective codoping ions. Such studies would be further helpful to understand the mechanisms involved in enhanced luminescence of optical-inert ions and RE co-doped materials.
In our previous work,40 it is demonstrated that ScVO4:Yb3+/Er3+ is a green-emitting phosphor with good monochromaticity, while its emission efficiency still needs for the further improvement. So in this work, we focused on the UCL enhancement of ScVO4:Yb3+/Er3+ through different kinds of optical-inert ions (Li+/Na+/K+, Mg2+/Ca2+/Sr2+/Ba2+ and Y3+/Gd3+/Lu3+) single doping as well as combination doping. In order for horizontal comparison, the concentrations of Li+/Na+/K+, Mg2+/Ca2+/Sr2+/Ba2+ and Y3+/Gd3+/Lu3+ are chosen to be with the same value, respectively. Meanwhile, combination these optical inert metal ions is a wishful and challengeable task since their radius and valance are different. Overall, the present work provides a comparative study including some powerful evidences and a deep understanding on the origin of UCL enhancement for the ScVO4:Yb3+/Er3+ phosphors after optical-inert ions doping in either way.
On the other hand, during the operation of electronic and photonic devices, temperature is needed to be monitored for the best performance. Therefore, accurate sensing and mapping of temperature in a non-invasive way is a challenging field of research.41–44 Hence, this research-need motivated us to tailor the structural and UCL properties of ScVO4:Yb3+/Er3+ nano/microcrystals with optical-inert ions incorporation and to study the temperature sensing performance. Based on the above points, in this work, we employed a modified molten salt method to prepare Yb3+/Er3+-codoped ScVO4 UCPs, adjusting UC luminescence and temperature-sensing performance by codoping optical-inert ions (Li+/Na+/K+, Mg2+/Ca2+/Sr2+/Ba2+ and Y3+/Gd3+/Lu3+), to achieve the purpose of kill two birds with one stone.
Comprehensively, it shows a rising trend, indicating that optical-inert ions are becoming a more and more important and hot topic in the field of UCL enhancement. It can be concluded from above reports, most researchers have studied the effect of co-doping with alkali metal ions, alkaline-earth metal ions and inactive Ln3+ ions individually on the structural and UCL properties of phosphors. Comparatively, there is rare research about the effect of combination these optical inert metal ions doping and lack of knowledge on the intercommunication between codopants on the UC luminescence in controlling the properties of UCPs to the best of our knowledge.
Fig. 1 (a) XRD patterns, (b) shifting of main diffraction peaks, (c) changes in main diffraction peaks position of Li+, Ca2+, Gd3+ doped ScVO4:10%Yb3+/2%Er3+ (“No” represents ScVO4:10%Yb3+/2%Er3+). |
Fig. 2 SEM of (a) optically-inert ions free, (b) Gd3+, (c) Li+, (d) Ca2+, (e) Li+/Gd3+, (f) Ca2+/Gd3+, (g) Li+/Ca2+, (h) Li+/Ca2+/Gd3+ co-doped ScVO4:10%Yb3+/2%Er3+. |
Optically-inert ions | Morphology | Size (nm) |
---|---|---|
No | Chips-like | Length: 750–1770; diameter: 140–440 |
Li+ | Smooth stone | 971–1208 |
Ca2+ | Cobblestone | 176–317 |
Gd3+ | Chips-like | Length: 1134–1879; diameter: 344–707 |
Li+/Ca2+ | Smooth stone | 400–800 |
Li+/Gd3+ | Stone fragments | 400–1400 |
Ca2+/Gd3+ | Smooth stone | 200–600 |
Li+/Ca2+/Gd3+ | Rough stone | 200–500 |
Fig. 3 XRD patterns (a), UC emission spectra (b) of Li+ (1–10 mol%) doped ScVO4:10%Yb3+/2%Er3+ samples. |
As well as Li+, other alkali metal ions like Na+ or K+ might possibly influence the final crystal structure and UC luminescence, so Li+, Na+ or K+ with the same doping concentration 5 mol% were studied and compared in a similar way. The XRD patterns of different alkali metal ions doped ScVO4:10%Yb3+/2%Er3+ are presented in supporting Fig. S1a.† All of the XRD patterns could clearly be indexed to the pure tetragonal phase of ScVO4 (JCPDS No. 06-0260), and no trace of other phases or impurities were observed, indicating all the optical-inert ions and Yb3+/Er3+ ions are incorporated into the ScVO4 host matrix and formed a solid solution structure. The UC luminescent performance of alkali metal ions doping ScVO4:10%Yb3+/2%Er3+ was studied, as exhibited in Fig. S1b.† Obviously, addition of alkali metal ions significantly intensified the UC emission intensities of all the two emission bands and the trend of increment is the same for the two emission bands as: Li+ > Na+ > K+ > No alkali metal ions. It is also observed that all the UC bands are splitted into several Stark components. The UC luminescence splitting from 4S3/2 → 4I15/2 and 2H11/2 → 4I15/2 transitions of Er3+ was observed results from the coordination field effect of host matrices. Similar phenomenon were also observed in previous reports.45
Fig. 4 XRD patterns (a), UC emission spectra (b) of Ca2+ (1–10 mol%) doped ScVO4:10%Yb3+/2%Er3+ samples. |
Similarly, other alkaline earth metal ions (Mg2+, Sr2+ or Ba2+) with doping concentration 1 mol% were also studied to investigate their influence on the final crystal structure and UCL properties. Fig. S2a† depicts XRD patterns of the ScVO4:10%Yb3+/2%Er3+ doped with alkaline earth metal ions (Mg2+, Sr2+ or Ba2+), all the diffraction peaks of samples still correspond to the tetragonal structure (JCPDS No. 06-0260) and no other impurity phase was detected. The normalized UC emission spectra of calcined ScVO4:10%Yb3+/2%Er3+ doped with alkaline earth metal ions (Mg2+, Ca2+, Sr2+ or Ba2+) are shown in Fig. S2b.† Obviously, addition of alkaline earth metal ions enhanced the UC emission intensities and the trend of increment is as: Sr2+ > Ca2+ > Mg2+ > Ba2+ > No alkaline earth metal ions. Doping with alkaline earth metal ions (Mg2+, Ca2+, Sr2+ or Ba2+) intensified the UC emission by almost 2.5, 3.7, 4.7 and 2.3 fold to that of optically inert ions-absent sample, respectively.
In order to reveal the concentration-dependent UCL properties and obtain the optimum concentration of Gd3+ ions doping in host lattice, the Gd3+ concentration dependent UCL spectra of the ScVO4:10%Yb3+/2%Er3+ samples are shown in Fig. 5b. As can be evidently seen that the doping of Gd3+ ion (even Gd3+ ion concentration reached 100 mol%) cannot change the position of typical emission of Er3+. The UC emission intensity enhanced with rising Gd3+ doping content from 0 to 30 mol%, and then started to weaken when the Gd3+ concentration exceeds 30 mol%. The UC emission intensities at 554 nm in ScVO4:10%Yb3+/2%Er3+ nanocrystals doped with 30 mol% Gd3+ are about 3.2 times than that of Gd3+-absent sample. The red emission intensity has a little change after Gd3+ introducing. In stark contrast, the UC luminescence intensity of GdVO4:10%Yb3+/2%Er3+ (Gd3+ content reaches up to 100 mol%) is 1.2 times lower than that of ScVO4:10%Yb3+/2%Er3+ (Gd3+ concentration 0 mol%), indicating that ScVO4 is more suitable as UCL host than GdVO4.
From the analysis above, when the Gd3+ content is less than or equal to 10 mol%, all the X-ray diffraction peaks of the sample can be well indexed as pure ScVO4 phase. Therefore, the 10 mol% doping content of Y3+, Lu3+ was chosen for the following investigations. Firstly, XRD patterns of these samples are shown in Fig. S3.† All the diffraction peaks can be indexed to those of the tetragonal phase ScVO4 (JCPDS card No. 06-0260). Influences of non-luminescent Y3+, Gd3+ or Lu3+ dopant on the UCL properties of ScVO4:10%Yb3+/2%Er3+ phosphors were compared and studied, as shown in Fig. S3b.† Obviously, addition of Y3+/Gd3+/Lu3+ significantly intensified the UC emission at the green as well as red region and the order of increment was Lu3+ > Gd3+ > Y3+ > inactive ions free. It is worthwhile pointing out that substitution of 10 mol% Lu3+ intensified the UC emission by almost 3.4 fold than that of non-active RE ions free-sample.
Fig. 6 summaries the UCL intensity of ScVO4:10%Yb3+/2%Er3+ with various optical-inert ions single substitution. It can be seen clearly that optical-inert ions doping enhanced the UCL intensity in different degree. Among them, 5 mol% Li+ doping shows the biggest enhancement, by almost 6.1-fold compared to that of ScVO4:10%Yb3+/2%Er3+
No matter in which kind of doping way, the UC emission peak positions remain unaltered, while the UCL intensity varied differently. Among these samples, the Li+/Gd3+ codoped ScVO4:10%Yb3+/2%Er3+ phosphor has the highest emission intensity, and the intensity of UC luminescence enhanced by a factor of 15.3 compared to the optical-inert ions free sample, and the codoping of Ca2+/Gd3+ ions followed. That is to say the combination of Li+/Gd3+ or Ca2+/Gd3+ present more excellent UCL intensity than that of corresponding optical-inert ions single doped or free samples. While, the combination of Li+/Ca2+/Gd3+ or Li+/Ca2+ weakened the UCL intensity compared to that of the Li+ or Ca2+ single doped samples, respectively.
Fig. 8 shows the emission colors of optical-inert ions doped ScVO4:10%Yb3+/2%Er3+ samples under excitation at 980 nm. The apparent color difference in the digital images (collected using a Canon Power Shot G7) also show that optical-inert ions doped ScVO4:10%Yb3+/2%Er3+ samples has bright green emission under NIR laser excitation. Furthermore, the green light spot is biggest in the Li+/Gd3+ codoped system, and smallest in ScVO4:10%Yb3+/2%Er3+ samples, which is agree well with the measured spectra results.
Generally, the factors affecting luminescence intensity are various and complex, including its crystal structure, shape, size, phonon modes, etc.1,14 In our work, it can be seen above that phase of all prepared samples is the same with the standard pattern of ScVO4 (JCPDS Card, File No. 06-0260), indicating that those optical-inert ions get incorporated into the ScVO4 matrix without phase separation, so the benefit of phase effect on the luminescence augmentation can be excluded. However, doping those optical-inert ions leads to the change of lattice cell. From the enlarged spectra which are dominated by the shifting of main diffraction peaks (220) of tetragonal phase ScVO4 as shown in Fig. 1b and c, we can see that the peak shifts changed as the different optical-inert ions doping. To see how crystal structure quantitatively evolves along with the addition of optical-inert ions (Li+, Ca2+ or Gd3+), these raw data of XRD were analyzed by Rietveld refinement method.46–49 Fig. 9 shows the Rietveld analysis pattern of ScVO4:Yb3+/Er3+ sample and Li+, Ca2+ or Gd3+ single doped ScVO4:Yb3+/Er3+ samples, in which the black crosses, red solid lines, green solid lines, blue lines and magenta bars characterized experimental patterns, calculated patterns, background patterns, differences and Bragg position, respectively. The deviation between the calculated and experimental results are expressed by blue lines, which are shown between the background line and the Bragg reflection line. The refinement results and the main refinement parameters are shown in Table 2 and Table 3. Profile factor Rp lies between 8.24% to 9.91%, weighted profile factor Rwp is 6.79–8.03%.
Fig. 9 The Rietveld analysis pattern of (a) ScVO4:10%Yb3+/2%Er3+, (b) Li+, (c) Ca2+, (d) Gd3+ doped ScVO4:10%Yb3+/2%Er3+. |
Bond lengths and angles | No | Li+ | Ca2+ | Gd3+ |
Crystal system | Tetragonal | Tetragonal | Tetragonal | Tetragonal |
Space group | I41/amd | I41/amd | I41/amd | I41/amd |
Z | 4 | 4 | 4 | 4 |
Cell parameters/Å | a = b = 6.8289, c = 6.1709 | a = b = 6.8375, c = 6.1705 | a = b = 6.8454, c = 6.1728 | a = b = 6.8148, c = 6.1597 |
Cell volume/Å3 | 287.77 | 288.48 | 289.25 | 286.07 |
Profile factor Rp | 9.89% | 8.24% | 9.91% | 9.77% |
Weighted profile factor Rwp | 7.97% | 6.79% | 7.77% | 8.03% |
Bond lengths and angles | No | Li+ | Ca2+ | Gd3+ |
Sc1–O1 | 2.377(8) Å | 2.408(4) Å | 2.376(16) Å | 2.365(5) Å |
Sc1–O2 | 2.129(11) Å | 2.116(6) Å | 2.134(21) Å | 2.139(7) Å |
Sc1–O–Sc1′ | 112.4(5)° | 111.83(29)° | 112.2(7)° | 112.16(33)° |
These results indicate that the crystal structure data of these samples by Rietveld structural refinement are well matched with experimental data. Due to similarity between refined patterns, we list the refined profile of the sample of ScVO4:Yb3+/Er3+ as an example. For ScVO4:Yb3+/Er3+, Rp = 9.89%, Rwp = 7.97%. The refinement results indicate that ScVO4:Yb3+/Er3+ and Li+, Ca2+ or Gd3+ single doped ScVO4:Yb3+/Er3+ phosphors belong to tetragonal phase, and its space group is I41/amd with Z = 4. The refined unit cell parameters are a = b = 6.8289 Å, c = 6.1709 Å and cell volume V = 287.77 Å3 for ScVO4:Yb3+/Er3+ sample. For Li+, Ca2+ or Gd3+ single doped ScVO4:Yb3+/Er3+ samples, the unit cell parameters are a = b = 6.8375 Å, c = 6.1705 Å, cell volume V = 288.48 Å3; a = b = 6.8454 Å, c = 6.1728 Å, cell volume V = 289.25 Å3 and a = b = 6.8148 Å, c = 6.1597 Å, cell volume V = 286.07 Å3, respectively. Obviously, the trend of increment for the cell volume V as: Ca2+ > Li+ > No optical-inert ions > Gd3+. The emergence of the result may be caused by that these optical-inert ions can be doped into the host lattice through the substitution or occupation of the interstitial. As shown in Fig. 10, for tetragonal phase ScVO4, each Sc3+ ion is eight-coordinated by O atoms. In 10%Yb3+/2%Er3+-doped ScVO4 sample, the two kinds of Sc–O bond lengths are 2.377(8) and 2.129(11) Å, respectively. The angles of Sc–O–Sc is 112.4(5)°. The corresponding bond lengths and bond angles of Li+, Ca2+ or Gd3+ single doped ScVO4:Yb3+/Er3+ samples are deposited in Table 2. Compared with the ScVO4:Yb3+/Er3+ sample, the decreased Sc–O average bond length will change the surrounding environment of Yb3+ and Er3+ and break the local crystal field symmetry around the Er3+ ions, leading to a low symmetric site of the Er3+ ions, which can make an enhancement in UC efficiency. It is worthwhile pointing out that Sc–O average bond length of Li+ doping sample changed the most, in accordance with the UCL intensity of this sample is the highest.
Besides, it is considered that the larger size and more regular morphology has higher UCL intensity, while it is noticed from the Table 1 that the size are not consistant with the UC luminescence intensity perfectly, thus, the benefit of the crystalline size effect on the luminescence enhancement is not an important factor. Thus, the enhancement mechanisms of optical-inert ions should be searched from other directions.
On one hand, to investigate chemical composition of the material surface, a well-known, extensively used X-ray photon spectroscopy (XPS) technique was used.50–52 Fig. 11a and b shows the XPS spectra of Sc(2p), V 2p3/2 and V 2p1/2 regions (between 455–467 eV) for ScVO4:Yb3+/Er3+ sample and Li+, Ca2+, Gd3+ single doped ScVO4:Yb3+/Er3+ samples. These results confirm the +3 oxidation state of Sc and V in its +5 oxidation state.51,53 Moreover, the XPS spectra of O 1s are used as a probe for investigating the presence of oxygen ion vacancies on the surface of sample. The peaks were de-convoluted using Lorentzian function. In the case of ScVO4:Yb3+/Er3+ sample, the two peaks well fitted to BE ∼529.57 (P1) and 531.97 eV (P2) with FWHM ∼1.08 and 1.32 eV, respectively. Upon optical-inert ions doping, all the peaks showed asymmetric behaviour towards higher BE. Moreover, the decrease extent of the P2/P1 ratio of optical-inert ions doped samples is as follows: 1%Ca2+ < 5%Li+ < 10%Gd3+, as shown in Fig. 11c. This is probably because of the creation of oxygen ion vacancies and/or surface defects through the sample surface with introduction of optical-inert ions into the host matrix. It can be concluded from the analysis of XPS spectra that the creation of appropriate amounts of oxygen ion vacancies and/or surface defects through the sample surface with the introduction of optical-inert ions into the host matrix, which are beneficial to the stronger UCL. This is because of a lower or optimal proportion of optical-inert ions incorporation into the host lattice, thus inducing a fast ET from the host to the Er3+ ion. This may create the vacancies that act as the sensitizer, mixing the charge-transfer states. Optical-inert ions addition increased the UCL intensity by increasing the radiative transition probability. However, an increase in the optical-inert ions concentration or type over a certain limit (such as 10%Gd3+ in this work) generates a significant amount of oxygen ion vacancies in the lattice. Consequently, the crystal lattice collapses, and the luminescence intensity decreases. Therefore, the brightness increases with oxygen ion vacancies concentration to certain extent, if above this point, the luminescence begins to decrease, then quenching behavior appears as a result.
Fig. 11 XPS and their peak fitting curves of (a) Sc 2p, (b) V 2p, (c) O 1s, (d) Li 1s, (e) Ca 2p, and (f) Gd 4d of Gd3+, Li+, Ca2+ doped ScVO4:10%Yb3+/2%Er3+, respectively. |
Based on the experiment results described above, in ScVO4 phosphor, it can be concluded that optical-inert ions doping induced change of local symmetry and oxygen vacancy generated should be the main reason that is responsible for UC emission enhancement. In addition, morphology and size of the obtained samples also contributes to the enhanced UC emission. In order to interpret the effects of these optical-inert ions doping on the UCL process, the dependence of emission intensity on the pump power for the red, green emission was measured, in order to better verify the role of the optical-inert ions played in the energy transfer of UC emissions. As a commonly used method, the relationship between integrated emission intensity I and excitation power P, Iem = Pn is often used to provide the information of n photons involved in the UC process.14 For comparison, the power dependence of S0–S4 samples are shown in Fig. 12. Considering the energy transfer UC process, the population of emissive levels will be greatly influenced by the energy transfer process showing a changeable n value. It is demonstrated whether in ScVO4:10%Yb3+/2%Er3+ (S0) or optical-inert ions doped ScVO4:10%Yb3+/2%Er3+ (S1–S4), the slop value n are all approximate to 2 both for the green and red. As the slope denotes the number of NIR photons absorbed to generate one frequency upconverted photon under unsaturated conditions, the green and red emissions are two-photon processes both in the UCPs doping optical-inert ions or not. The UC mechanism of this system is similar to our previous work.40
Fig. 12 Pump power dependence of the fluorescent bands centered at 523 nm, 554 nm and 659 nm in (a) Gd3+/Ca2+, (b) Li+, (c) Gd3+, (d) Ca2+, (e) without co-doped ScVO4:10%Yb3+/2%Er3+. |
(1) |
(2) |
The UC emission spectra at various temperatures and curves of the emission intensity ratio (R) versus temperature (T/K) for ScVO4:10%Yb3+/2%Er3+ and ScVO4:10%Yb3+/2%Er3+/5%Li+/10%Gd3+ are presented in Fig. 13a and b, respectively. The intensity of the UC emission bands around 522 nm and 554 nm seemed to drastically vary with increasing temperature of the sample. The plots of Ln(R) versus 1/T are exhibited in Fig. 13c and d, the slope (−ΔE/k) is a very important parameter to judge the optical temperature sensing ability of Er3+ doped materials, and the linear fitting of the experimental data gave slope and intercept equal to −655.15 ± 16.22 and −511.12 ± 38.57 for both of these two samples, respectively. Besides, the sensor sensitivity is another important coefficient of a sensing material. The sensor sensitivity (S) can be defined as
(3) |
Fig. 13 Green UC emission spectra of ScVO4:10%Yb3+/2%Er3+ (a) and ScVO4:10%Yb3+/2%Er3+/5%Li+/10%Gd3+ (b) phosphors on increasing temperatures and the variation of emission intensity ratio (R) as function of absolute temperature (inset). (c, d) Monolog plots of R as a function of inverse absolute temperature for these two samples, fitted by eqn (2). |
Actually, with increasing temperature, the absolute sensitivity for both two samples first increases, then reach a certain temperature it starts decreasing (Fig. 14). It is noteworthy that the sensitivity increases dramatically in the ScVO4:10%Yb3+/2%Er3+/5%Li+/10%Gd3+ sample compared to the Li+/Gd3+ free sample. At the temperature of 260 K, the sensitivity of ScVO4:10%Yb3+/2%Er3+/5%Li+/10%Gd3+ reached its maximum value of about 0.0092 K−1, while the maximum sensitivity of ScVO4:10%Yb3+/2%Er3+ only 0.0073 K−1 at 330 K. The results indicated that the multifunctional optical-inert ions Li+/Gd3+ could be used not only to enhance the temperature sensor sensitivity but also the UCL intensity.
Fig. 14 Relative sensitivity of the as-prepared ScVO4:10%Yb3+/2%Er3+ and 5%Li+/10%Gd3+ codoped ScVO4:10%Yb3+/2%Er3+ samples for optical thermometry as a function of temperature, fitted by eqn (3). |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra10035c |
This journal is © The Royal Society of Chemistry 2017 |