Promising Er3+/Yb3+-Codoped GdBiW2O9 Phosphor for Temperature Sensing by Upconversion Luminescence.

Yb3+/Er3+-codoped GdBiW2O9 phosphors are prepared via the solid-state route for application in upconversion temperature sensors. The structural analyses indicate that all phosphors possess a single-phased orthorhombic structure. Upon the excitation of a laser wavelength of 980 nm, Yb3+/Er3+-codoped GdBiW2O9 phosphors emanate green emission peaks, endorsed to the emission to the 4I15/2 state from the 4S3/2 and 2H11/2 states, respectively, and the weak emission (red) from the 4F9/2 state to the 4I15/2 state of Er3+ ion. The upconversion mechanism has been elucidated via the scheme of energy levels conferred from the pump power-induced upconversion characteristics. The temperature-dependent upconversion of GdBiW2O9 phosphors was investigated in detail along with the estimation of the stability and repeatability of the measurement. The obtained sensitivity data for the present materials with the corresponding sensing parameters show their probable outlook in temperature sensing applications.

Introduction

With the advancement of technology from macrodimension to micro- and nanoelectronics, the temperature in reduced dimensionalities because of the Joule heating effect becomes non-negligible and severe to degrade the device performance.[1−5] Therefore, the assessment of temperature in micro/nano aspects with an elevated precision is an immense confrontation to avert these thermal spikes.[6−8] As the enormity and spot of these temperature spikes mainly depend on the device design and the accuracy of the integrated circuits, it is difficult to get the idea about the location of these hotspots. Moreover, different physical properties including refractive index and optical gain, which control the performance of the devices, are strongly temperature-dependent and hence can influence their utility, even leading to irretrievable smash up.[9,10] This requires thermometry at micro-/nanoscale for the electronic devices during real operating conditions. However, the conventional thermometers, based on the expansion of metal or liquid with temperature, are unfeasible to be used in the micro-/nanoelectronic devices.[4,11] Therefore, the recent development of noninvasive thermometers including nanoscale thermocouples, IR thermometers, and thermometers based on Raman spectroscopy, which can be incorporated into different micro-/nanoelectronic devices to provide the obligatory thermal information, has quickly become a reality.[12−17] In recent years, thermometers based on luminescence are extensively used as noncontact thermometers, owing to their own advantages.

To date, several research works have been conceded on the temperature sensors based on luminescence thermometry, that is, the association between temperature and luminescence characteristics to accomplish thermal sensing.[18−22] However, there exist two different approaches to quantify the temperature sensing performance of luminescent materials,[23,24] including the measurement of fluorescent intensity ratio (FIR) and the decay time. On the basis of these approaches, various quantum dot materials, organic materials, pigments and dyes, and rare-earth ions were used to develop luminescent thermometers. Recently, FIR-based temperature sensors of the Er3+/Yb3+-doped luminescent materials, which exhibit efficient upconversion (UC) green emissions to the 4I15/2 state from the 4S3/2 and 2H11/2 states, draw a lot of research attention.[25−27] Various research works are available with Er3+/Yb3+-codoped materials for the thermal sensing properties with different hosts such as glass materials and phosphor materials. Yi et al.[25] reported the influence of Er3+/Yb3+ codoping of the NaYF4 host matrix for temperature sensing applications. Dong et al.[28] reported the efficient temperature sensitivity of the CaF2 host. All these researches obtained efficient temperature sensing with relative thermal sensitivity in the range of 1–2.1% K–1. However, the temperature range for this temperature sensing was as low as 293–318 K. Liao et al.[29] developed Gd2TiO5 phosphors for thermometry applications and obtained maximum sensitivities as 40.76 × 10–4 K–1 at 565 K. Y2WO6 crystals were synthesized by the hydrothermal method for thermometry applications by Chen et al.[30] Other researchers reported the relative temperature sensitivity of the phosphor materials doped with rare-earth ions as 2–3% K–1 (333–375 K), and to date, the utmost relative sensitivity was obtained as 5.53% K–1.[28,31,32] Some recent researches by the author’s group on different glass materials and oxide-based phosphors for temperature sensing applications were reported, and the optimum relative sensitivity was observed as 2.22% K–1 in the temperature range 300–550 K.[33] Though the temperature range of operation is elevated, still the relative sensitivity requires a large modification. Therefore, the present research work aims to develop new Er3+/Yb3+-based luminescent materials for temperature sensor applications.

Recently, tungstate materials have fascinated huge curiosity for luminescence applications because of the strong and broad absorption band of the W–O bond and its chemical and thermal stability.[34−36] Owing to this, the tungstate materials including CdWO4, CaWO4, and KGd(WO4)2 can be used for various applications in the fields of scintillators, phosphors, and laser materials.[37−39] However, there is a lack of proper research work reported on the luminescent thermometric characteristics of tungstate-based phosphors. Therefore, in this paper, a new Er3+/Yb3+-codoped phosphor based on the GdBiW2O9 host was successfully developed via a solid-state approach, and the structure and UC characteristics were investigated. The influence of temperature on the UC characteristics was used to estimate the sensing parameters, including sensitivity and resolution, for the first time for this phosphor.

Experimental Section

Material Preparation

The synthesis of the novel GdBiW2O9:Er3+,Yb3+ phosphors was carried out by the well-known high-temperature solid-state method using Er2O3, Yb2O3, Gd2O3, Bi2O3, and WO3 as the starting ingredients. The key ingredients were grinded homogeneously using a mortar–pestle, and their stoichiometric ratios were taken. The raw precursors were then transferred to alumina crucibles and were heated in a furnace at 600 °C for 10 h. The samples were cooled to room temperature, grinded to fine powder, and again sintered for 8 h at 900 °C.

Material Characterization

The X-ray diffraction (XRD) patterns of the as-prepared phosphors were recorded by using an X-ray diffractometer (Ultima IV, Rigaku) with Cu Kα radiation. The particle size distributions along with the morphologies were explored by using a Hitachi S-800 scanning electron microscope. A continuous-wave 980 nm diode laser was used to excite the phosphors, and the UC spectra were evidenced with a PMT detector directly attached to a monochromator. A heating attachment was linked to this instrument with a temperature controller to measure the temperature-induced UC.

Results and Discussion

Phase Identification, Structural Analysis, and Morphology

A series of UC phosphors Gd1–ErBiW2O9 (x = 0.005, 0.01, 0.015, 0.02, 0.025) were synthesized, and to identify the phases of the prepared UC materials, the XRD patterns of the as-prepared Gd1–ErBiW2O9 were measured, which is shown in Figure . All the materials were obtained as single-phased compounds that matched well with the isostructurally similar orthorhombic EuBiW2O9,[40] having the ICSD database no. 183444.

Figure 1

(a) XRD patterns of Gd1–ErBiW2O9 phosphors for (a) x = 0.005, (b) x = 0.01, (c) x = 0.015, (d) x = 0.02, and (e) x = 0.025.

For a better analysis of the crystal structure of the synthesized material, the Rietveld refinement was accomplished for Gd0.99Er0.01BiW2O9 by utilizing the crystallographic data of the isotopic single-crystal structure of EuBiW2O9 (ICSD no. 183444) as an initial model with the help of FullProf Suit software[41] and is shown in Figure . The reliability factors of refinement were converged as Rp = 7.38% and Rwp = 9.58%, and it indicated the quality of the refinement. From the XRD refinement, it was clear that the crystal possessed an orthorhombic phase with the space group Pnma and lattice parameters: a = 31.802 Å, b = 5.594 Å, and c = 3.952 Å. The values of the results were smaller than those reported for the polymorphic EuBiW2O9 of GdBiW2O9. It could be explained on the basis of the difference in the ionic radius of Eu3+ (1.066 Å) and Gd3+ (1.053 Å). When Gd3+ ions are replaced by Eu3+, the volume of the unit cell becomes smaller and hence the lattice parameters.[42]

Figure 2

Rietveld refinement analysis pattern of Gd0.99Er0.01BiW2O9 phosphor.

All the refinement data are used to draw the crystal structure of the GdBiW2O9 phase using VESTA software,[43] as shown in Figure . Figure a displays the crystal structure of Gd0.99Er0.01BiW2O9 based on the three-dimensional network of alternating (BiO2)− and (Gd2W2O12)6– layers. These two layers are connected by the chains of the WO6 octahedron, which are linked in the corner.[42] The Gd atoms are coordinated by seven oxygen atoms, as shown in Figure b, and formed the GdO7 polyhedra. These polyhedrons are attached to a chain by edge-sharing O atoms. The W atoms are coordinated to five and six oxygen atoms to form a trigonal bipyramidal WO5 and an octahedrally coordinated WO6 polygon,respectively, as shown in Figure b. Figure b also presents the coordination of Bi atoms with six oxygen atoms to form a distorted triangular prism as BiO6, and multiple BiO6 polygons are interconnected into a (BiO2)− layer.[42]Figure c displays the unit cell of the GdBiW2O9 phase.

Figure 3

(a) Crystal structure of Gd0.99Er0.01BiW2O9; (b) coordination geometry of various elements Gd, Bi, and W; and (c) unit cell structure of the Gd0.99Er0.01BiW2O9 phosphor.

Figure presents the XRD data of the Yb-codoped Gd0.99Er0.01BiW2O9, with the Yb concentration varying as 5, 10, 15, 20, and 25 mol %. The XRD patterns were similar to the ISCD pattern no. 183444 and represent a single phase, devoid of any unidentified diffraction peaks from impurity, implying the easy incorporation of Yb3+ in the framework of the Gd0.99Er0.01BiW2O9 phosphor. It was owing to the fact of the similarities of the ionic radii of the host cation to that of the doping cation.

Figure 4

XRD patterns of Gd0.99–Er0.01YbBiW2O9 phosphors for (a) y = 0.05, (b) y = 0.1, (c) y = 0.15, (d) y = 0.2, and (e) y = 0.25.

Figure a,b shows the scanning electron microscopy (SEM) images of the Er3+-ion-doped and Er3+-/Yb3+-ion-codoped GdBiW2O9 phosphors synthesized via the solid-state reaction route. The distribution of particle size for both phosphors indicated the presence of particles less than 1 μm with agglomerated morphology. Figure c displays the transmission electron microscopy (TEM) image of the Er3+-/Yb3+-ion-codoped GdBiW2O9 phosphor, and the inset image indicates the corresponding diffraction pattern of the visible crystal.

Figure 5

SEM images of (a) Gd0.99–Er0.01BiW2O9 and (b) Gd0.84Er0.01Yb0.15BiW2O9 phosphors. (c) TEM image of the Gd0.84Er0.01Yb0.15BiW2O9 phosphor; inset: corresponding TEM diffraction pattern. (d) HRTEM image of the Gd0.84Er0.01Yb0.15BiW2O9 phosphor.

Figure d displays the high-resolution TEM (HRTEM) image of the corresponding crystal. In the HRTEM and diffraction patterns, the well-crystallized phosphors are clearly visible, and the calculated interplanar spacing was obtained as 0.32 nm. It corresponds to the lattice constants of the customary GdBiW2O9 materials in the (111) plane.

Figure a shows the thermogravimetric differential scanning calorimetry (TG-DSC) curves of the Er3+-/Yb3+-ion-codoped GdBiW2O9 phosphors heat-treated in N2 atmosphere up to 1000 °C with a heating rate of 10 °C/min. One band at ∼80 °C in the DSC curve during the heating process was clearly observed (Figure b), which can be attributed to the evaporation of water. For the TG curve, there also existed one stage of weight loss of nearly 0.963% from 70 to 400 °C, owing to the evaporation of water. Above 400 °C, the weight remained about the same. It indicated the thermal stability of the Er3+-/Yb3+-ion-codoped GdBiW2O9 phosphors.

Figure 6

TG-DSC thermal analysis curves of (a) Gd0.84Er0.01Yb0.15BiW2O9 phosphors and (b) close view of the same curve in the temperature range from 40 to 175 °C.

Room-Temperature UC and Energy-Level Diagram

The UC spectra of the GdBiW2O9:Er3+ phosphors excited via a 980 nm laser display several emission peaks around 525, 545, and 655 nm, as presented in Figure a. The emission band in the green region peaking at 525 nm (G1 band) and 545 nm (G2 band) is owing to the transition to the 4I15/2 level from the 2H11/2 and 4S3/2 states of Er3+ ions, respectively, and the band in the red region peaking at 655 nm (R band) is due to the 4F9/2 → 4I15/2 transition.[44] With an increasing Er3+ concentration, the UC intensity increased upto 1 mol % and then decreased, owing to the self-concentration quenching.[45]Figure b shows the corresponding CIE coordinate, and it reveals the invariance of the color coordinate with the Er3+ concentration. When the Yb3+ ions are codoped in the optimized GdBiW2O9:Er3+ phosphors, the UC intensity increased manifold, owing to the Yb3+ ion to Er3+ ion energy transfer. However, the G1 band enhanced about 70 times, G2 band about 1.3 times, and R band enhanced about 60 times. The UC intensity of the Er3+-/Yb3+-coactivated phosphor was seen to increase with the Yb3+ concentration up to the Er3+/Yb3+ ratio of 1/15 and then decreased, as shown in Figure c. The increase in UC intensity pointed to the presence of more population in the excited levels from the ground-state level. The population might be increased because of the energy transfer from the Yb3+ to Er3+ ions. The decrease in UC intensity with the increase in the Er3+/Yb3+ ratio over 1/15 could be explained with the concentration quenching phenomenon. The corresponding CIE coordinates (Figure d) indicated that the color coordinate position remained about the same with the variation of the Yb3+ concentration. However, the reason behind the dissimilar variation of different emission bands can be discussed on the basis of the energy-level scheme.

Figure 7

(a) UC spectra of Gd1–ErBiW2O9 phosphors with varying concentrations of Er3+ ions (x), (b) corresponding CIE coordinate diagram of Gd1–ErBiW2O9 phosphors, (c) UC spectra of Gd0.99–Er0.01YbBiW2O9 phosphors vs the concentration (y) of Yb3+ ion, and (d) corresponding CIE coordinate diagram of Gd0.99–Er0.01YbBiW2O9 phosphors with varying y.

To explicate the UC phenomena and the energy plot, the UC emission of the optimized GdBiW2O9:Er3+,Yb3+ phosphor was evidenced with the variation of pump power (P), as viewed in Figure a. The corresponding CIE coordinate is shown in Figure b.

Figure 8

(a) UC spectra of Gd0.84Er0.01Yb0.15BiW2O9 phosphors, varying the pump power of the laser diode from 400 to 1330 mW, (b) corresponding CIE coordinate diagram of Gd0.84Er0.01Yb0.15BiW2O9 phosphors, (c) logarithmic plot of pump power with UC intensity of Gd0.84Er0.01Yb0.15BiW2O9 phosphors, and (d) energy-level diagram of Er3+ and Yb3+ in the GdBiW2O9 host matrix.

It indicated that the pump power had a negligible effect on the CIE color coordinates. This was owing to the consistent doping and successful energy transfer from Yb3+ to Er3+ in the host. It demonstrated the stability of UC emission from the present host materials. The integral intensity (I) follows the relation I ∝ P, where n is the number of incident photons involved in the present UC.[46] From the logarithmic plot of integral UC intensity with pump power, the n value can be calculated. Figure c displays the linear variation for three UC emission bands, including G1, G2, and R. The n value for the G1 and G2 transitions were calculated as 1.97 and 2.27, and for the R emission band, it was estimated as 2.04. These results exhibited the contribution of two-photon excitation in the UC mechanism.[47]

Figure d displays the schematic diagram of the energy-level scheme for the GdBiW2O9:Er3+/Yb3+ phosphor based on the two-photon mechanism.[47] Initially, the electrons from the ground-state 4I15/2 in the Er3+ ions were stimulated to the higher excited state 4I11/2, with the help of ground-state absorption. It is accomplished via the Yb3+ to Er3+ ion energy transfer. Then, the electrons were moved to the 4F7/2 level from the 4I11/2 state via excited state absorption as well as via the energy transfer route. Then, nonradiative relaxation to the lower levels of 4S3/2 and 2H11/2 occurred, which thereby influenced the 2H11/2 → 4I15/2 (G1) and 4S3/2 → 4I15/2 (G2) transitions. The probability of nonradiative relaxation to the 4S3/2 and 2H11/2 levels secures the emission intensity of the G1 and G2 bands. However, the population of the 2H11/2 level increased more than that of the 4S3/2 level after the codoping of the Yb3+ ions, which results in the higher variation of G1 band compared to the G2 band. Therefore, a dissimilar variation of different emission bands was observed. The electron which relaxed to the 4I13/2 level and then stimulated to the 4F9/2 level can help in the 4F9/2 → 4I15/2 transition (R).

Luminescence Thermometry and Temperature Sensing

To check the dependence of the temperature on UC, a series of UC spectra were recorded with the optimized GdBiW2O9:Er/Yb materials (Gd0.84Er0.01Yb0.15BiW2O9) as a temperature function and displayed in Figure a. It is observed that with an increase in temperature from 303 to 498 K (30–225 °C), the intensity of the G1 band and the G2 band changed in opposite ways, whereas the peak positions remained unchanged. At room temperature (303 K), the intensity of the G1 band corresponding to the 2H11/2 → 4I15/2 transition was comparable to the G2 band intensity corresponding to that of the 4S3/2 → 4I15/2 transition. As the temperature increases, the luminescent intensity of the G1 band at about 525 nm shows a gradual increase, whereas the corresponding intensity of the G2 band first increases and then slightly decreases. It is owing to the low transition rate of 2H11/2 → 4I15/2 compared to 4S3/2 → 4I15/2 at high temperatures. The integral intensity of the phosphor decreased because of the increased lattice vibration, and, therefore, the upconverted green emission shows a small variation with a change in temperature. The variation in color emission with temperature was clearly observed in the CIE color coordinate diagram, as shown in Figure b. The integral intensity ratio between the 525 and 545 nm transitions [(2H11/2 → 4I15/2)/(4S3/2 → 4I15/2)], generally known as fluorescence intensity ratio (FIR), was also measured with different temperatures and shown in Figure c. It is well-known that the 2H11/2 and 4S3/2 energy states are very close and separated by a very low energy gap (699 cm–1), thus being thermally coupled.

Figure 9

(a) UC spectra of Gd0.84Er0.01Yb0.15BiW2O9 phosphors, varying the temperature from 303 to 498 K; (b) corresponding CIE coordinate diagrams of Gd0.84Er0.01Yb0.15BiW2O9 phosphors; (c) variation of the FIR value with temperature. The blue line indicates the Boltzmann fitting, and (d) logarithmic plot of the FIR value with the inverse of temperature for Gd0.84Er0.01Yb0.15BiW2O9 phosphors.

Therefore, the electron population on these levels can be governed by Boltzmann’s distribution. However, with the rise in temperature, the population of the 2H11/2 and 4S3/2 levels changes in a different manner because of which the intensity ratio of FIR transitions might vary. The variation of this ratio is generally described as per the Boltzmann formula. It is in complete agreement with the present results. The variation of FIR shows a gradual increase with temperature, following the Boltzmann behavior according to the equation[48]where B is a constant factor (pre-exponential), ΔE is the energy gap involving the two levels, T is the temperature (K), and k is the Boltzmann constant.

It can be understood from this equation that FIR is only temperature-dependent, therefore making the phosphor a suitable material for temperature sensing applications. The plot of FIR with the inverse of temperature (Figure d) gives a near-linear curve, thus verifying the phosphor’s suitability in the temperature sensing field.

The slope of the linear plot was obtained as −1091.71 K and further used for the calculation of the ΔE value. It was obtained as 752 cm–1, which was close to the energy gap sandwiched between the 2H11/2 and 4S3/2 levels, as observed from the emission energy-level diagram in the present case.

To know more about the sensing ability of the GdBiW2O9:Er,Yb phosphor, the temperature-induced variation of the rate of FIR (R)[3,4]

In the present case, it was estimated to be 0.0107 K–1. This value was seen to increase to 0.0174 K–1, with an increase in temperature up to 450 K, and then decreased.

The variation of sensitivity with temperature is shown in Figure a. Various researches were reported on luminescence-based thermometry, and Table summarizes the results with the present research. These comparisons indicate the suitability of the present phosphor for temperature sensing applications. The sensitivity value is used further to estimate the relative sensor sensitivity via the normalization of the temperature-dependent sensitivity as follows[3]

Figure 10

(a) Variation of the sensitivity of the Gd0.84Er0.01Yb0.15BiW2O9 phosphors with respect to the temperature change from 303 to 498 K and (b) corresponding temperature-induced variation of the relative sensor sensitivity of Gd0.84Er0.01Yb0.15BiW2O9 phosphors.

Table 1

List of Materials with Maximum Sensitivity and Temperature Range

phosphor	sensitivity (K–1)	temperature range (K)	refs
Gd2O3:Er3+/Yb3+	0.0039	300–900	(50)
NaYF4:Er3+/Yb3+/Gd3+/Nd3+	0.0026	288–328	(51)
NaYF4:Er3+/Yb3+	0.0029	93–673	(52)
TeO2–WO3:Er3+/Yb3+	0.0028	300–690	(33)
NaGdF4:Er3+/Yb3+	0.0037	303–563	(53)
CaMoO4:Ho3+/Yb3+/Mg3+	0.0066	303–543	(54)
YVO4:Er3+/Yb3+	0.0117	302–582	(55)
CaWO4:Ho3+/Yb3+	0.005	303–923	(56)
La2CaZnO5:Er3+/Ho3+/Yb3+	0.00625	303–573	(47)
GdBiW2O9	0.0174	303–498	this work

The variation of relative sensitivity with temperature is shown in Figure b.

The reproducibility and stability of the sensors are the indispensable parts of sensitivity estimation.[49] The variations of the FIR value with the increase in temperature from 300 to 500 K and the corresponding decrease in temperature from 550 to 300 K for the consecutive two cycles are shown in Figure a.

Figure 11

(a) Variation of FIR with temperature and reproducibility measurement for two cycles and (b) temperature modification on the FIR value at three different temperatures, 498, 398, and 303 K, for the 120 min cycle test with 20 min intervals to estimate the stability of the present estimation.

An insignificant hysteresis behavior was found in the present case. It indicated an exceptional reversible character of the present phosphor. The FIR values were also recorded at 303, 398, and 498 K incessantly for the 120 min cycle test with 20 min intervals. During this time, the sample position was kept undisturbed.

The FIR value remained about the same throughout the 2 h experiment time and is shown in Figure b. These two characteristics also vouch for the use of the present phosphor as a suitable material for practical appliances.

Conclusions

In summary, GdBiW2O9 phosphors doped with Er3+ ions and Er3+/Yb3+ ions were synthesized by the solid-state method. The structural refinement analysis indicated that the phosphors crystallized in an orthorhombic crystal system and the doping/codoping of RE ions keep the crystal structure unchanged. Using the laser excitation of 980 nm, the Er3+ phosphor exhibited strong green UC emissions from the 2H11/2, 4S3/2 → 4I15/2 transitions of Er3+ ions, respectively, which were enhanced manifold after the codoping of Yb3+, and the explanation was given on the basis of the energy exchange mechanism between the Er3+ and Yb3+ ions. The contribution of two-photon absorption was verified from the power dependence studies. The near-linear curve obtained from the logarithmic plot of the FIR as a function of temperature inverse establishes a possibility of the usability of the phosphors for temperature sensing applications. The phosphor sensitivity was estimated to be 0.0107 K–1, which increased with increasing temperature. An insignificant hysteresis behavior was found for the variations in FIR value with the increase in temperature from 300 to 500 K and the corresponding decrease in temperature from 550 to 300 K for the consecutive two cycles. It indicated an exceptional reversible character of the present phosphor. The recorded FIR values at 303, 398, and 498 K incessantly for the 120 min cycle test with 20 min intervals remained about the same throughout the 2 h experiment time, indicating a good stability of the present materials. The high sensor response over a broad range of temperature with great temperature resolution, good reproducibility, and stability of the phosphor sensors indicate the suitability of the GdBiW2O9:Er3+,Yb3+ phosphors for temperature sensing applications.