Near infrared to visible upconversion studies on Er3+/Yb3+/Tm3+ doped PbZrTiO3 multifunctional ceramics

Prasenjit Prasad Sukul; Amitabh; Kaushal Kumar

doi:10.1364/OSAC.1.001087

1. Introduction

Lead zirconate titanate is considered as one of the primitive piezoelectric material, which is perovskite in nature. After the discovery of Pb(Zr_1-xTi_x)O₃ or PZT ceramics in the starting of 1950’s, it has been extensively investigated because of their ultra-high piezoelectric properties. At room temperature lead zirconate titanate is a combinational solid solution of PbTiO₃ (PT) and PbZrO₃ (PZ) which is the most commonly used piezoelectric ceramic material as transducer, sensors and actuator in many electromechanical systems, medical & aerospace technological aspects [1–6]. The important properties like phase transition (tetragonal-rhombohedral), exceptional response in di-electricity & piezo-electricity and other semiconductor properties observed in lead zirconate titanate, makes it one of the most actively investigated perovskite in early 60’s & 70’s. Hence PZT is widely explored from many years for their interesting electrical properties, but surprisingly their luminescence property has not been explored much. There are very few reports reporting luminescence of PZT ceramic. One of the rare reports in our knowledge is made by liu et al. [7]. PbZrTO₃ films have been synthesized using Rf-sputtering technique by liu et al. which has been further taken to heat-treatment at 750°C in Ar/H₂ environment to create oxygen vacancies in the sample & observe luminescence.

Rare earth doped BaTiO₃ (which is considered to be first piezoelectric ceramic) and other perovskite structured oxides were actively reported by researchers during 60’s periods, subsequently explored for their visible light emission properties [8]. The lead zirconate titanate which is piezoelectric in nature can be turned into a multifunctional material via doping of lanthanide ions. Goel et al. [9] tried a series of rare earth (Eu³⁺, Gd³⁺, Sm³⁺, Yb³⁺, Nb³⁺) modified PZT and reported the change in piezoelectric coefficients, pyroelectric coefficients, dielectric constant and loss with or without RE dopant. Neodymium doped Pb_1.05(Zr_0.53Ti_0.47)O₃ or (PZT) ceramic thin films prepared via sol-gel method are studied by Majumdar et al. for their phase formation behavior & electrical characterization [10]. The incorporation of Nd³⁺ into lead zirconate (PZT) thin films creates disorder in the B-site of the ABO₃ lattice, as a result the sample shows dielectric relaxation, a reduced transition temperature, improved piezoelectricity. Fu et al. [11] reported huge increase in upconversion emission intensity with change in crystal size & subsequent increase in crystallinity. They reported the shift of Er³⁺ position from B-site to A-site in PZT ceramics. They further observed the emission colour shifts from green to orange for different Er³⁺ concentration. Recently, Yao et al. [12] shown an efficient phase probe method using photoluminescence of Er³⁺ doped in Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ where, the emission intensity(green to red ratio) of erbium changes with crystal phase transformation. These literature reports reveal the suitability of PZT as a multifunctional host.

Erbium has significant role as dopant in perovskite oxides because of energy level structure with two favorable absorption transitions (⁴I_15/2→⁴I_11/2 & ⁴I_15/2→⁴I_9/2) that can be efficiently pumped with high power semiconductor lasers. Yb³⁺ ions are efficient for their large absorption for near infrared light and used as sensitizers whereas due to weak ground state absorption of Er³⁺ ions, they have low absorption to near infrared. However, this Yb³⁺-Er³⁺ duo participating in energy transfer from Yb³⁺ to Er³⁺ serves very well as sensitizer-activator pair in upconversion process. The role of acceptors can also be played by Ho³⁺, Tm³⁺ along with Er³⁺ ions. Thus the Er³⁺/Yb³⁺ or Er³⁺/Yb³⁺/Tm³⁺ as dopant will be the suitable choice for upconversion emission in lead zirconate titanate. There are several reports reporting the structural & electrical characteristics of lead zirconate titanate with and without rare earth dopant. Even pyroelectric & piezoelectric studies in rare earth doped lead zirconate titanate have been done in early years. But photoluminescence of Er³⁺/Yb³⁺ or Er³⁺/Yb³⁺/Tm³⁺ doped PZT systems are rarely reported. Somehow the optical properties of this combination have been neglected during its electrical study, whereas structural studies show the combination as a promising candidate of luminescent material.

In the present work, the Er³⁺/Yb³⁺/Tm³⁺ doped PbZrTO₃ ceramic has been prepared through solid state reaction method and the effect of various Tm³⁺ co-doping on the emission properties has been studied. Four different samples with Tm³⁺ concentration of 0.05, 0.10, 0.15 and 0.20 mol% were prepared. The structural and morphological properties of the synthesized sample have been studied using x-ray diffraction and FE-SEM techniques. The upconversion emission measurements of Er³⁺/Yb³⁺/Tm³⁺ doped PbZrTO₃ ceramic at room temperature has been done and samples were optimized to give maximum luminescence intensity. The lifetime measurements also support the enhancement in the emission intensity in the presence of Tm³⁺ ions. Thus, the Er³⁺/Yb³⁺/Tm³⁺ doped PbZrTO₃ ceramic phosphor may be a potential candidate for display and other optical devices.

2. Experimental

2.1 Materials and method

Er³⁺/Yb³⁺/Tm³⁺ doped Pb(Zr_0.53Ti_0.47)O₃ powders are synthesized through solid-state reaction route. All the chemicals PbCO₃, ZrO₂, TiO₂, Er₂O₃, Yb₂O₃ and Tm₂O₃ are purchased from Sigma Aldrich, Germany of 99.0% purity. At first, all the raw materials are mixed in a agate mortar via isopropanol media and grinded together according to the stoichiometric ratio. After 6 h of grinding, the homogeneously mixed materials are calcined at 1000°C and 1200 °C for 6 h and 5 h, with excess PbZrO₃ (PZ) packing powder. After calcination the process follows sintering at 1250 °C for 1 h to get the final sample. Trace of excess Zr was absent in the synthesized sample and confirmed by X-ray diffraction and FESEM techniques. Except pure PZT phases, no secondary pyrochlore phases were not found in the sample, though stoichiometric Zr/Ti ratio is well maintained in the sample.

2.2 Characterizations

For studying the structural properties, the x-ray diffraction pattern of the sample was recorded on a Bruker D8 advanced x-ray diffractometer over the angular range 10° ≤ 2θ ≤ 90° using Cu-k_α (1.5405Å) radiation with a scan rate of 2 degree per min. UC luminescence spectra of the ceramic phosphors were recorded by a compact spectrophotometer with dual mode F980, Edinburgh Instruments, UK at ambient room temperature. The behavior of UC emission intensity of the sample under varying pump excitation powers from very low to high have been monitored. Dielectric measurement of the sintered sample in the form of a pellet has been measured using a Gain phase analyzer (HP4294A, Agilent, USA). Evaluation of the temperature dependent dielectric measurement was carried out in a temperature regulated chamber which was connected to a precision LCR meter (TH2827, Tonghui, Changzhou, china).

3. Results and discussion

3.1 Structural measurements

3.1.1 X-Ray diffraction measurement

X-ray diffraction pattern measured for the Er³⁺/Yb³⁺/Tm³⁺ doped Pb(Zr_0.53Ti_0.47)O₃ ceramic of the optimized sample is shown in the Fig. 1

Fig. 1 X-Ray diffraction pattern of Er³⁺/Yb³⁺/Tm³⁺: PZT ceramic sintered at 1250°C.

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. In observation, pure perovskite phase has been found within detection range of diffractometer. The samples sintered at 1250°C shows tetragonal phase (P4mm) confirmed by JCPDF No. 33-0784.

The crystallite size (D) of the prepared samples has been calculated using Deby-Scherrer relation [13]:

D = K λ / β * \cos θ

where β is the full width at half maximum (FWHM), θ is the diffraction angle for h k l plane, D is the crystallite size in nm, λ is the wavelength of the x-ray (0.15406 nm), K is shape factor whose value is 0.89. The crystallite size (D) of Er³⁺/Yb³⁺/Tm³⁺:PZT ceramic for 0.05, 0.10, 0.15 and 0.20 mol% Tm³⁺ co-doping is found to be 80, 105, 120 and 148 nm, respectively. Thus, the crystallite size increases as the concentration of Tm³⁺ ions increases. It means the crystallinity increases in the presence of Tm³⁺ ions.

3.1.2 Microstructure analysis

The field emission scanning electron micrographs (FESEM) of four samples at different Tm³⁺ concentrations are shown in Fig. 2(a)–2(d)

Fig. 2 FESEM images of the sample at 40.00kx magnification for different thulium concentration.

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. The observation shows the shape of the particles is not much affected by the increment of sintering temperature of the ceramic but size of the particle definitely increases with increasing Tm³⁺ concentration. It is clearly seen that the particles are in an agglomerated form and have the sizes in a range of 70-200 nm. The homogeneity of particles may be seen in the FESEM image.

3.2 Optical characterizations

3.2.1 Upconversion emission spectra

The upconversion emission spectra of the samples were measured in the 350–900 nm range upon 980 nm laser excitation is shown in Fig. 3

Fig. 3 Upconversion emission spectra of the sample sintered at 1250°C for different thulium concentration.

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. These UC emission bands are the results of 4f-4f electronic transitions of the Ln³⁺ ions. The upconversion emission spectrum under 980nm excitation consists of emission wavelength peaking at 525(²H_11/2→⁴I_15/2) & 545(⁴S_3/2→⁴I_15/2) in the green region and 655(⁴F_9/2→⁴I_15/2) in the red region, which are the results of specified Er³⁺ transitions. The emission bands at 479 nm, 680 nm and 800 nm are attributed to the activator Tm³⁺ transitions from ¹G₄ →³H₆, ¹G₄ →³F₄ and ³H₄ →³H₆ respectively. The presence of Er³⁺ and Tm³⁺ emissions in photoluminescence spectra proves the existence of the energy transfer from Yb³⁺ to Er³⁺ and Tm³⁺ after excitation at 980 nm, as has been reported elsewhere [14,15].

The concentration of sensitizer to activator ratio plays an important role in the upconversion emission intensity. Here the Er³⁺ & Yb³⁺ concentrations is fixed at 0.5 mol% & 2.0 mol% respectively, while variation in Tm³⁺ concentrations has been observed for the samples. Most of the reports comprising Yb³⁺ as sensitizer, the optimal concentration was reported to be 2.0 mol% [16]. It was commonly reported by researchers that higher concentration of Yb³⁺ ions shows quenching in UC emission intensity. Yb³⁺ ions forms clusters to dissipate the excess energy when back energy transfer (BET) occurs from Ln³⁺ to Yb³⁺ ions [17]. The Tm³⁺ added as another activator ion serves the property of efficiently absorbing the energy transferred from Yb³⁺. For the present sample optimum emission is measured for 0.15 mol% of Tm³⁺ which is shown in the Fig. 3. The luminescent properties also strongly depend on particle size and morphology, larger particle size has greater surface defects and deteriorating exciting cross-section.

The upconversion emission intensity at different pump power gives better understanding of UC mechanism. The excitation power density & the emission intensity have the following relation in the low power input approx.

I_{U C} \propto P^{n}

where n is the number of photons involving in the upconversion process that must be absorbed for the emission of one UC photon. Hence, the logarithmic intensity & logarithmic power gives a straight line whose linear slope gives the number of photons (n) absorbed in the upconversion process [18].

Figure 4

Fig. 4 Upconversion emission intensity as a function of pump power dependence for optimized sample at room temperature

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shows the pump power dependence of emission bands at 525,550, 656, 690 and 800 nm, when the material was excited upon 980 nm diode lasers with tunable power. Figure 5

Fig. 5 Logarithmic plot of pump power versus integrated UC intensity of the Er³⁺/Yb³⁺/Tm³⁺: PZT ceramic

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shows the double logarithmic plot of upconversion emission intensity versus excitation power density. For investigating upconversion mechanism, it is necessary to take into account the upconversion emission intensities with different power, as it gives the information regarding the photons involved in the upconversion process. The slope (n) obtained from the relation between the pump intensity (I_P) and the integrated upconversion emission intensity (I_UC), in a double logarithmic scale, specify the number of photons are involved in populating the excited levels. The linear fitted slopes (n) from the log-log plot are n₅₂₅ = 1.80, n₅₄₅ = 1.34, n₆₅₅ = 2.15, n₆₈₀ = 2.04 and n₈₀₀ = 1.27 respectively. These values confirm the two photon absorption process for green emission. On the contrary, the value of n₆₅₅ & n₆₈₀ suggests, the involvement of three photons in the UC process, resulting the red emissions. The details of the origin of these emission bands are discussed below. These results has consistency with the other interpretations for similar Er³⁺/Yb³⁺/Tm³⁺ doped YP₅O₁₄ [19], GdPO₄ [20] and Y₂BaZnO₅ [21] systems.

3.2.2 Upconversion mechanism

Figure 6

Fig. 6 Schematic energy level diagram with proposed pathways.

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shows a schematic energy level diagram of Er³⁺, Yb³⁺ and Tm³⁺ and the proposed upconversion mechanisms accounting possible energy match for the observed UC emission bands. When excited at 980 nm, the Yb³⁺ ions are excited from the ²F_7/2 ground state to the ²F_5/2 level. Since the Yb³⁺ concentration is higher than the Er³⁺ and Tm³⁺ ions, the probability of energy transfer (ET) mechanism from Yb³⁺ ions to the Er³⁺ and Tm³⁺ goes higher [22,23]. Here we can neglect the Cooperative upconversion processes as their efficiency is much lower for Yb³⁺ ions. Yb³⁺ ion absorbs single photon and transfers its whole energy to the activator (Er³⁺ & Tm³⁺) ions. At first, the Er³⁺ ion is excited from the ⁴I_15/2 ground state to the ⁴I_11/2 level and subsequently to the ⁴F_7/2 level. The population of ²H_11/2 level from ⁴F_7/2 level is governed by a non-radiative process, which is thermalized with respect to the ⁴S_3/2 one, and ⁴F_9/2 emission to the low lying excited states can occur shown in the Fig. 6. For Tm³⁺ ions, a three-step energy transfer (ET) process is required to populate the ¹G₄ exited level, the excitation of Tm³⁺ in the ³H₅ is realized by means of the energy transfer (ET) mechanism of excited Yb³⁺ to Tm³⁺ and then it relaxes non-radiatively to the ³F₄ level. Another photon from Yb³⁺→Tm³⁺ populates ³F₂ level of the Tm³⁺ ion via ET process which further decays to ³H₄ through non-radiative processes. The radiative transfer to the ³H₆ ground state from the ³H₄ level emits in the NIR region (800 nm). The third photon efficiently pumped to ¹G₄ from ³H₄ and then relaxes to the ¹H₅ and to the ³H₆ ground state giving visible emissions (650 nm & 479 nm).

4. Dielectric property studies

The dielectric response measurements of the sample have been discussed in terms of complex permittivity i.e. real permittivity and imaginary permittivity. Figure 7

Fig. 7 Temperature dependent dielectric constant as a function of frequency (log scale) for the optimized sample.

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shows the dielectric dispersion in lower frequency region and frequency independent behavior in higher frequency region for three different sets of temperatures (390K, 450K and 510 k). The upconversion luminescence is closely related to the interaction between the RE ions and crystal field environment. The ionic occupation and distribution of RE ions in the hosts have major influence on the local lattice distortion (LLD) which distorts the local symmetry [24–26]. This observation can be explained introducing the fact that the tendency of dipoles to orient along the direction of applied electric field. Later when frequency increases, it was difficult to orient themselves in the field direction and dielectric values attain the minimum [27,28]. Majority of researcher’s reports the dielectric constant is strongly correlated with the crystal structure (lattice constant and unit cell volume) as Ɛ’ vastly depends on the ionic and atomic polarization [29]. These studies also revealed that the dielectric constant increases with increase in temperature. Here, the study shows increase in dielectric constant value with the increase in temperature for the Er³⁺/Yb³⁺/Tm³⁺:PZT(Tm _{x = 0.15mol%}) ceramic. The authors explained this behavior introducing the fact, Yb ions substituting the Pb ions, has a tendency to contract the unit cell volume resulting reduced value in ionic polarization. Such a rise in dielectric constant was due to the increase in polarization as an effect of distorting crystal symmetry that allowed dipole orientation along the field direction, for a strong dielectric nature. This observation leads to the fact, not only optical property is changing also the electrical property is modifying with the Tm³⁺ concentration variation.

5. Conclusion

PbZrTiO₃ ceramic doped with Er³⁺, Yb³⁺ and Tm³⁺ for different thulium molar concentration have been successfully prepared using solid state reaction technique. The synthesized samples were characterized by X-ray diffraction, Field emission scanning electron microscopy (FESEM) and upconversion luminescence measurements. The x-ray diffraction pattern confirmed the samples to be in pure perovskite phase [tetragonal symmetry (P4mm)] & no other secondary phases are found. FESEM images show the increment of crystal size and grain boundaries in the sample with change in thulium concentration. Under 980 nm laser excitation, red (Tm³⁺: (³F₃, ³F₂→³H₆) Er³⁺: ⁴F_9/2 →⁴I_15/2), green (Er³⁺: ²H_11/2, ⁴S_3/2→⁴I_15/2), and blue (Tm³⁺: ¹G₄→³H₆) upconversion emission bands are observed. UC intensities as a function of Pump power dependence exhibit a well-fashioned pattern. On the basis of possible energy match of Erbium, ytterbium and thulium, UC mechanism is discussed. The high dielectric response of the sample makes it a promising candidate for operating in the microwave region. In summary, PZT ceramic has rich electric properties but current study shows great optical emissive properties with rare earth combination. Hence Er³⁺/Yb³⁺/Tm³⁺ doped PbZrTiO₃ ceramic can be a potential host for various multifunctional applications.

6. Supplementary data

See Fig. 8

Fig. 8 Power dependent upconversion spectra for wide range of power.

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for additional data.

Funding

DST-SERB, New Delhi (EMR/2017/000228/18); Council for Scientific and Industrial Research (CSIR), New Delhi under project [No. CSIR 03(1303)/13/EMR-II/2013].

Acknowledgment

Prasenjit Prasad Sukul is thankful to the Indian Institute of Technology (ISM), Dhanbad, India, for providing a research fellowship. The authors would like to acknowledge the financial support from DST-SERB, New Delhi (EMR/2017/000228/18). The authors also acknowledge the financial support from the Council for Scientific and Industrial Research (CSIR), New Delhi under project [No. CSIR 03(1303)/13/EMR-II/2013].

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Near infrared to visible upconversion studies on Er³⁺/Yb³⁺/Tm³⁺ doped PbZrTiO₃ multifunctional ceramics

Abstract

1. Introduction

2. Experimental

2.1 Materials and method

2.2 Characterizations

3. Results and discussion

3.1 Structural measurements

3.1.1 X-Ray diffraction measurement

3.1.2 Microstructure analysis

3.2 Optical characterizations

3.2.1 Upconversion emission spectra

3.2.2 Upconversion mechanism

4. Dielectric property studies

5. Conclusion

6. Supplementary data

Funding

Acknowledgment

References

Cited By

Figures (8)

Equations (2)

OSA Continuum