Article pubs.acs.org/JPCC
Controlling Nonradiative Transition Centers in Eu3+ Activated CaSnO3 Nanophosphors through Na+ Co-Doping: Realization of Ultrabright Red Emission along with Higher Thermal Stability Subhajit Saha,† Swati Das,‡ Uttam Kumar Ghorai,† Nilesh Mazumder,‡ Debabrata Ganguly,§ and Kalyan Kumar Chattopadhyay*,†,‡ †
School of Materials Science and Nanotechnology and ‡Department of Physics, Jadavpur University, Kolkata-700032, India Department of Industrial Chemistry and Applied Chemistry, Swami Vivekananda Research Center, Ramakrishna Mission Vidyamandira, Belurmath, Howrah-711202, India
§
S Supporting Information *
ABSTRACT: Solid-state lighting (SSL) and field emission based display (FED) devices collectively encompass a major fraction of contemporary research efforts and thus development of a newer generation of highly luminescent nanophosphors presently defines a critical juncture for further development of this still somewhat-nascent field. However, the low efficiency of red phosphors constitutes a principal bottleneck for commercialization of such devices. Herein, we present a red light emitting highly luminescent Na+ codoped CaSnO3:Eu3+ nanophosphor with an average particle size of 32 nm that has been synthesized by a modified sol−gel technique. The resultant nanophosphor exhibits bright red emission under both UV and low voltage electron beam excitations. Furthermore, quantitative assessments of photoluminescence (PL) signatures for Eu3+ doped and Na+ codoped CaSnO3:Eu3+ nanophosphors conclusively demonstrate that Na+ codoping facilitates an almost 4-fold increase in the luminescence intensity coupled with significant improvement in thermal stability. In addition, charge compensation by incorporation of Na+ leads to an increased order of radiative transition and thereby increases the color purity and lifetime of radiative transition. Obtained results firmly and unambiguously establish the bright and revolutionary prospects of this new type of nanophosphor in the rapidly emerging field of solid state lighting and FED devices.
1. INTRODUCTION The escalating demand of energetically efficient high resolution displays and lighting devices in recent times has resulted in a significant thrust in the development of high quality phosphors with enhanced brightness and good color purity. Versatile physical properties of the phosphors facilitate their usage in varied applications such as field emission displays (FEDs), light emitting diodes (LEDs), and plasma display panels (PDPs).1−3 Among these, phosphor converted white LED (PC-WLED) is a supreme pinnacle for solid state lighting technology. In PCWLEDs, the ultraviolet (UV) light generated from a InGaN based LED chip is utilized to excite a phosphor or collection of phosphors to harvest white light. Mixing of tricolor phosphors (i.e., red, green and blue) in a proper ratio is the most popular method to realize white light. However, currently red phosphors perform rather poorly in comparison to their blue and green counterparts, and consequently WLEDs thus fabricated exhibit high color temperature and low color rendering index that is harsh to human eyes. In order to solve this obnoxious problem, researchers have lately started paying keen attention to the development of highly luminescent red phosphors with phenomenal spectral qualities which can be activated in the near-UV regime. © XXXX American Chemical Society
Incorporating rare-earth (RE) elements in an appropriate host has always remained as the most prevalent protocol to realize brilliant luminescence properties.4−6 Also, efficient transfer of energy from the host crystal to the RE activators can be exploited to realize exotic optical phenomena. As the partly occupied 4f orbitals of RE ions are largely screened from the contiguous chemical surroundings by completely occupied 5s and 5p orbitals, the luminescence originating from 4f →4f transition usually consists of sharp lines with comparatively high lifetimes.7,8 Consequently, the characteristic 5D0 → 7F2 transition of trivalent europium (Eu3+) remains one of the most utilized for numerous practical applications in display and solid state lighting regimes. Moreover, the nature of this transition line strongly depends on the local environment and can be exploited to probe the site symmetry. Already it is widely accepted that size reduction of a crystal can alter optical properties radically due to the formation of enhanced surface states. Again, from an application perspective, nanophosphors can be coated more times than bulk phosphors, Received: April 11, 2015 Revised: June 30, 2015
A
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The Journal of Physical Chemistry C which enable fine-grain control over thickness of the phosphor film, and consequently this helps to enrich the homogeneity as well as uniformity of the resultant film. On the other hand, even higher resolution displays can be realized by means of nanophosphors. These striking advantages motivated various research groups to extensively investigate different nanophosphor systems from both experimental and theoretical perspectives. Additionally, development of this esoteric branch of study has been further fueled by subsequent realization of RE doped nanophosphors at relatively lower temperature in comparison with their bulk counterparts. This report primarily aims to design a highly luminescent nanophosphor having enhanced brightness and greater stability. It is well-known that the brightness of a nanophosphor is greatly modulated by migration of energy from the host to the RE activator. A host system with large band gap and lower vibrational energy can facilitate more efficient transfer of energy from host to activator.9,10 Hence, in addition to the choice of activator, the choice of the host also plays a significant role in extracting rich luminescent behavior from a nanophosphor. Among a plethora of inorganic hosts, alkaline earth stannates have garnered the attention of the researchers owing to their numerous advantages such as lower chemical toxicity, lack of radioactive elements, and greater thermal and chemical stability.11 In particular, CaSnO3 is of great interest due to its phenomenal optical properties, excellent resistance to chemical vulnerabilities, and high mechanical strength.12,13 Apart from this, CaSnO3 also possess some special inherent properties that readily substantiates its suitability as a host matrix for designing a good phosphor material. First, its large band gap (∼4.4 eV) warrants excellent transparency in the total visible spectrum, which strongly disregards any reabsorption possibility of the emitted light and thereby reduces the chance of any kind of emission loss. Moreover, its low phonon energy minimizes the possibility of multiphonon relaxation, which is a critical parameter for realizing high emission efficiency. Another benefit of the CaSnO3 host is the uniqueness of its crystal structure. In perovskite structured CaSnO3, Sn ions form SnO6 octahedra which are linked to each other by corner sharing oxygen. In CaSnO3, the bond angle of Sn−O−Sn deviates from ideal 180°, and due to this octahedral tilting, the overall symmetry of the structure is reduced.14 Due to this lower symmetry feature, the Ca2+ sites are very favorable for 4f → 4f transition of the rare earth activators, especially for Eu3+.13,15 Being motivated by these highly advantageous traits, numerous research groups have devoted substantial effort to designing RE doped CaSnO3 phosphors using various synthesis techniques.16−19 However, previous reports have failed to improve the PL brightness of Eu3+ doped CaSnO3 nanophosphors over a discernible limit that can be detected by the naked eye. Improvement of PL brightness has always remained one of the most important challenges in the case of RE doped nanophosphors. It is found that the nanophosphors’ brightness is directly proportional to the concentration of the rare earth ions up to a certain limit, and after that, critical concentration PL intensity decreases.20,21 Increase of RE ion over the optimum concentration can also generate some impure phases as well. Hence, it is a challenging task to design highly luminescent nanophosphors by keeping the concentration of the RE ion within the quenching limit. Lately, codoping of alkali metal ions (Li+, K+, Na+) has gained traction as a proficient approach to increase the PL brightness and efficiency of the nanophosphors within the
window of critical activator concentration.22−24 Advantages of using alkali metal ions in such a manner are multifold. Primarily, they increase the luminescence properties of trivalent RE ions by solving the charge imbalance problem. In addition, they also play a vital role in improving the morphology and crystallinity of the nanophosphors.25,26 However, until now, to the best of the authors’ knowledge, no prior report exists in the literature regarding luminescence properties of CaSnO3:Eu3+ nanophosphors with Na+ codoping. To address the aforementioned void, in the current work, the photoluminescence and cathodoluminescence (CL) characteristics of CaSnO3:Eu3+, Na+ nanophosphor developed by the sol−gel type citrate− nitrate technique are investigated. Modulation of PL properties with Na+ codoping has been extensively investigated for a wide range of Na+ concentration. Moreover, addition of Na+ coactivator in the CaSnO3:Eu3+ nanophosphor not only increases the PL intensity almost four times, but higher thermal stability was also achieved. The observed phenomenal PL and CL characteristics imply this nanophosphor to be highly suitable for cutting-edge solid state lighting and field emission display devices.
2. EXPERIMENTAL SECTION 2.1. Synthesis of the CaSnO3:Eu3+ and Na+ Codoped CaSnO3:Eu3+ Nanophosphors. CaSnO3:Eu3+ and CaSnO3:Eu3+, Na+ nanophosphors were developed by a modified Pechini-type sol gel technique. The actual synthesis technique is analogous to the one described in our previous report.24 In the typical experimental procedure, the concentration of Eu3+ were varied from 0.5% to 2.5% (x = 0.005, 0.01, 0.015, 0.02, 0.025) for Eux:Ca(1‑x)SnO3 samples, and for Ca(1‑x‑y)SnO3:Eux, Nay samples, the concentration of Na+ were varied from 1% to 5% keeping Eu3+ concentration at a constant 2% (x = 0.02, y = 0.01, 0.015, 0.02, 0.025, 0.05). Analytical reagent grade Ca(NO3)2·6H2O (Merck, 99.9%), SnCl4·9H2O (Merck,99.9%), Eu(NO3)3 (prepared by dissolving Eu2O3, Sigma-Aldrich 99.99% in HNO 3 solution), NaNO 3 (Merck,99.9%), citric acid (Merck, 99.9%), and ethylene diamine (Merck, 99.9%) precursors were used as raw materials in the current experiments. The synthesis procedure is detailed in the following section. At first, stoichiometric amounts of calcium nitrate, tin chloride, europium nitrate, and sodium nitrate were dissolved in 30 mL deionized water (DI), and the solution was vigorously stirred with a magnetic stirrer for 30 min. Then, aqueous citric acid solution was introduced in the resultant metal nitrate solution in Cit3−/ (Ca2+ + Sn4+) = 2 molar ratio and the solution was agitated using a magnetic stirrer for a further 30 min, which resulted in a homogeneous transparent solution. In the next step, ethylenediamine was added in this solution to raise the pH of the solution to 12. The alkaline solution was then slowly evaporated in a water bath and a highly viscous colloidal gel was obtained. The subsequent steps consisted of drying this gel in a vacuum oven at 120 °C for 12 h and calcining the dried gel at 350 °C for 2 h to remove the organics. The porous, solid material obtained in the last step was then finely ground in a mortar and heated at 700 °C for 5 h in a muffle furnace to finally obtain ultrafine Eu3+, Na+ doped calcium stannate powders. A similar approach was adopted to synthesize Eu3+ doped CaSnO3 nanophosphors also; however, in that case NaNO3 was not incorporated. 2.2. Characterization. To analyze structural parameters and phase, the as-synthesized nanophosphors were examined B
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Figure 1. (a) XRD pattern of CaSnO3:Eu3+ nanophosphors. (b) Representation of higher angle shift of (121) XRD peak with increasing concentration of Eu3+. (c) Evolution of XRD pattern with increasing concentration of Na+ in CaSnO3:2%Eu3+ nanophosphors. (d) Demonstration of octahedral tilting in the unit cell structure of CaSnO3 host. (e,f) HRTEM images depicting the distribution of particles and interplanar spacing of (121) lattice plane in CaSnO3: 2% Eu3+, 2.5% Na+ nanophosphors, respectively.
Table 1. Variation in fwhm of (121) Peak, Crystallite Size, Lattice Parameter, and Unit Cell Volume with Different Doping Concentration of Activator and Coactivator lattice parameter (Å) sample name CaSnO3: CaSnO3: CaSnO3: CaSnO3: CaSnO3: CaSnO3: CaSnO3: CaSnO3: CaSnO3: CaSnO3:
3+
0.5% Eu 1% Eu3 1.5% Eu3+ 2% Eu3+ 2.5% Eu3+ 2% Eu3+,1% Na+ 2% Eu3+,1.5% Na+ 2% Eu3+,2% Na+ 2% Eu3+,2.5% Na+ 2% Eu3+,5% Na+
fwhm of (121) peak (degree)
crystallite size from Scherrer formula (nm)
a
b
c
unit cell volume (Å3)
0.283 0.295 0.306 0.314 0.329 0.302 0.275 0.269 0.245 0.252
29.19 28.01 26.81 26.34 25.11 27.35 30.04 30.72 33.70 32.63
5.661 5.656 5.642 5.635 5.617 5.641 5.653 5.658 5.675 5.667
7.887 7.864 7.845 7.832 7.813 7.852 7.867 7.875 7.903 7.895
5.521 5.513 5.502 5.495 5.476 5.509 5.521 5.530 5.544 5.538
246.50 245.21 243.53 242.51 240.32 244.01 245.53 246.39 248.64 247.76
PL spectra were taken on Shimadzu RF 5301 spectrofluorometer, whereas for the time-resolved PL spectra, an Edinburgh FLSP-980 luminescence spectrometer was used in which excitation was provided by a microsecond xenon flash lamp. A Gatan Mono CL3 device attached to the FESEM was used to record the room temperature CL spectra using a beam accelerating voltage of 5 kV.
by the means of X-ray diffraction (XRD) patterns, using a Rigaku-Ultima-III X-ray powder diffractometer with monochromatic Cu Kα radiation (λ = 1.5404 Å). FTIR analysis were performed in mid-IR region (i.e., 450−4000 cm−1) using a Shimadzu IRPrestige-21 spectrometer. Chemical composition and chemical state of each of the constituent elements were analyzed by X-ray photoelectron spectroscopy (SPECS, Germany) using monochromatic Al Kα (hν = 1486.6 eV) Xray source and a hemispherical analyzer (HSA 3500). The C 1s line of the adventitious carbon at 284.6 eV was used as a reference for charge correction of the as-recorded spectra. A field emission scanning electron microscope (FESEM, Hitachi S-4800) was used to investigate the surface morphologies of the nanophosphors. A high resolution transmission electron microscope (JEOL JEM 2100) was employed to probe the structural details of the crystalline nanophosphors. Steady state
3. RESULTS AND DISCUSSION 3.1. Structural, Compositional, and Microstructural Analysis. The X-ray diffraction patterns of CaSnO3:Eu3+ and Na+ codoped CaSnO3:Eu3+ nanophosphors are shown in the Figure 1a,c, respectively. All the nanophosphors exhibit a similar kind of diffraction pattern indexed as (002), (121), (202), (141), (042), (123), and (004), which are in good agreement with the ICDD PDF card no. 31−0312. The C
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which is further demonstrated by their respective PL spectra. Detailed analysis of the microstructure is represented by the TEM and HRTEM image of Na+ codoped CaSnO3:Eu3+ nanophosphor as shown in Figure 1e,f, respectively. Though the TEM image displays the particles with slightly agglomerated condition, from the figure it is evident that the particles are nearly spherical for which the size lies within 20 and 30 nm. This is also well corroborated by the crystallite sizes calculated by the Debye−Scherrer equation in the current investigations. The typical HRTEM image clearly demonstrates well-resolved lattice fringes having estimated interplanar spacing of 0.281 nm that is consistent with the (121) plane of Na+ codoped CaSnO3:Eu3+ nanophosphors with orthorhombic perovskite structure. X-ray photoelectron spectroscopy (XPS) is a popular technique for investigating the surface chemical composition of the synthesized nanophosphors. Figure 2 represents the XPS
presence of no impurity peaks confirms the formation of single phase perovskite structure having space group Pnma. The easiest way to visualize this type of distorted perovskite structure is the three-dimensional framework of corner sharing Sn−O6 octahedra shown in Figure 1d. The Ca2+ ions are surrounded by eight SnO6 octahedra, giving a 12-fold oxygen coordination. A closer observation to the (121) XRD peak (Figure 1b) reveals that due to an increase in Eu 3+ concentration, the diffraction peaks shift to the higher angle side. Usually, Eu3+ occupies the position of Ca2+ in the CaSnO3 lattice. Since the ionic radius of Ca2+ and Eu3+ is 0.99 and 0.95 Å, respectively, the shrinkage of the unit cell caused by this substitution is responsible for the observed higher angle peak shifting.27 Replacement of Ca2+ with Eu3+ is also associated with a charge imbalance problem in which Ca2+ vacancies are generated in the crystal. The observed higher angle peak shifting is reflected again from the calculated values of lattice parameters (a,b,c) and unit cell volume (V) presented in Table 1. The table also clarifies that the crystallite size of the asprepared phosphors are restricted between 25 to 33 nm which ensures the formation of CaSnO3:Eu3+ crystals within the nano regime even up to an addition of 5% Na+. From Table 1 it can be readily noted that crystallite size and lattice parameter monotonically increase with increasing Na+ doping concentration, whereas the fwhm of the (121) peak gradually decreases with increasing Na+ doping concentration up to 2.5% and then increases. Since the ionic radii of Na+ and Ca2+ are almost comparable (1.02 and 0.99 Å, respectively),28 Na+ easily substitutes for Ca2+ in the CaSnO3 lattice. This substitution efficiently removes the Ca2+ vacancies formed inside the crystal because of charge imbalance created by doping of Eu3+ in Ca2+ sites. The reduction of fwhm in tandem with an increase in the (121) peak’s sharpness with increasing Na+ concentration indicates that Na+ acts like a self-promoting media for the enriched crystallization of Eu3+:CaSnO3 lattice. However, enhancement of the crystallization is sustained up to a certain concentration of Na+ (2.5% in the present work), beyond which it is difficult to replace the Ca2+ sites by Na+. Hence again, charge imbalance is created in the system and the excess Na+ ions introduce defects which finally decrease the crystallinity of the phosphors. Also, the formation of huge oxygen vacancies beyond 2.5% of Na+ doping strongly contributes in the reduction of the crystallinity of the phosphor. Moreover, since the ionic radius of Na+ is slightly greater than Ca2+, substitution of Ca2+ by Na+ results in an expansion of unit cell which in turn increases the crystallite size of the nanophosphors. Signature of this unit cell expansion is also prominent from a slight lower angle shift of diffraction peaks with the increase in Na+ incorporation (clearly notable in Figure S1 of SI). This phenomenon is also supported by earlier reports with different hosts, which indicates that the degradation of crystallinity may occur due to the substitution of alkaline metal ions having a high degree of concentration.24,29 Realization of phase pure CaSnO3:Eu3+ and Na+ codoped CaSnO3:Eu3+ nanophosphor has also been verified by FTIR analysis (shown in SI Figure S2). The presence of a sharp peak at 645 cm−1 corresponding to the symmetric and antisymmetric vibrations of the Sn−O−Sn inorganic network ensured the formation of the CaSnO3 perovskite structure.30 The microstructural characteristics of all the synthesized nanophosphors play a crucial part in PL features, because enhanced crystallinity of the host−activator system results in more efficient transfer of energy from the host to activator,
Figure 2. (a) XPS survey scan of the CaSnO3:Eu3+ and CaSnO3:Eu3+, Na+ nanophosphors. (b,c) High resolution spectra of Eu 3d and Na 1s core level in CaSnO3:2%Eu3+, 2.5%Na+ nanophosphors, respectively.
spectra of the synthesized materials. Charge correction of all the spectra was carried out using the C 1s line at 284.6 eV which appeared due to the presence of adventitious carbon on the surface of the samples during atmospheric exposure. The survey scan of CaSnO3:Eu3+ and Na+ codoped CaSnO3:Eu3+ nanophosphors is shown in Figure 2a, which clearly depicts the presence of Ca, Sn, O, and Eu in all samples. In addition, Na was observed in the survey spectra of codoped samples as expected. Additionally, a high resolution scan of each element was carried out for the confirmation of exact valence state and electronic structure of the constituent elements present in the sample. The high resolution (HR) spectra of Eu 3d core level for the Eu3+, Na+ codoped CaSnO3 nanophosphor is shown in Figure 2b. The presence of four core-level peaks in the binding energy (BE) range 1110−1170 eV can be clearly noted in the D
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between 350 and 480 nm that arises due to intraconfigurational 4f−4f transitions of dopant Eu3+ present in the host system. These sharp peaks, arising as a consequence of Eu3+ ground state being directly excited to the higher energy levels in the 4f6 configuration, can be indexed as7F0 → 5D4 (362 nm), 7F0 → 5 L7 (382 nm), 7F0 → 5L6 (395 nm), 7F0 →5D3 (418 nm), and 7 F0 → 5D2 (466 nm) transitions, respectively. Among these, the 395 nm peak displays the highest intensity. The intense excitation lines at 395 and 466 nm facilitate this nanophosphor to be potentially exploited in designing PC-WLEDs that can be excited by both commercial near-UV and blue-LED chips. The excitation spectra also reveal that Na+ codoping triggers a domino effect which increases the intensity of excitation spectra, which in turn further boosts the PL emission intensity significantly. The excitation spectra also reveals that, in Na+ codoped samples, CTB slightly red shifts due to inflation of particle size triggered by incorporation of Na+ in CaSnO3 matrix.36 This phenomenon was also previously evidenced by XRD (Table 1) in the present investigations. Room temperature photoluminescence emission spectra of CaSnO3:Eu3+ and Na+ codoped CaSnO3:Eu3+ nanophosphors, excited at 395 nm wavelength, are demonstrated in Figure 5a,b, respectively. Due to comparable ionic radii, it is generally observed that, in CaSnO3, Eu3+ replaces Ca2+. Hence, Eu3+ belongs to a 12-fold oxygen coordination in the CaSnO3 lattice. In this configuration, the energy levels of Eu3+ get split as a consequence of the crystal field exerted by 12 O2−. As a result, the PL spectra of both Eu3+ and Eu3+, Na+ codoped samples essentially contain sharp and intense lines ranging from 580 to 710 nm. The five principal peaks observed at 580, 593, 615, 653, and 702 nm are attributed to the 5D0 → 7Fj (0−4) transitions of Eu3+ ions, respectively. It is well-known that 5D0 → 7F1 lines originate from magnetic dipole transition, while the 5D0 → 7F2 lines arise from electric dipole transition. Generally, the magnetic dipole transition does not depend on the symmetry or the site preference of Eu3+ ions in the host lattice, whereas the electric dipole transition is hypersensitive to those conditions. However, it is observed that if Eu3+ occupies a noninversion symmetric site, then the electric dipole transition dominates. In CaSnO3, large degree of octahedral tilting along with the reduced Sn−O−Sn bond angle leads to high distortion in the structural symmetry.37 In both Eu3+ and Eu3+, Na+ codoped systems Eu3+ prefers to occupy a noninversion symmetric site, and consequently, electric dipole transition dominates and the 615 nm peak becomes most prominent in all the samples. The PL mechanism is schematically illustrated in Figure 4 which represents the possible luminescence pathways in the background of the energy level diagram of Eu3+ ions. In CaSnO3, the 2p orbitals of O2− form the valence band and the conduction band is composed of Sn 5s and Sn 5p orbitals. In such a system, the excitation of electrons can occur in two alternate methods: by jumping from valence band to the CTB (275 nm peak), or by transferring to even higher levels of Eu3+ (395 nm peak). After excitation, an excited electron at the CTB jumps down to the lowest excited level of Eu3+, i.e., 5D0 by a nonradiative transition. Then, it can undergo one of the five transitions to the lower levels. These transitions give rise to five lines in the emission spectrum at 580, 593, 615, 653, and 702 nm, respectively. From Figure 5a it is also evident that the intensity of the 615 nm transition increases steadily with the increase in Eu3+ concentration up to 2% and then it decreases again for 2.5%. This kind of concentration quenching phenomenon may be
HR scan; the two strong peaks at 1164.5 and 1134.6 eV are assigned to the core-level spin−orbit splitting components of Eu 3d3/2 and Eu 3d5/2, respectively. The difference of BE between these two peaks is in agreement with prior reports regarding oxygen-surrounded Eu ions,31 from which the presence of the 3+ valence state of Eu ions in the synthesized nanophosphors32 can be confirmed. X-ray photoelectron spectra of the RE compounds usually demonstrate a characteristic satellite structure, which arises primarily because of final state effects and/or charge transfer coexcitations from O 2p to Eu 4f owing to the presence of a partially occupied 4f subshell. Consequently, two additional weak peaks are also observed at the lower binding energy sides of each spin−orbit split component. The BE separation of each satellite peak from their parent counterpart is 9.8 eV, which corroborates the previous reports of other Eu3+ doped oxide materials.33 The existence of monovalent Na is also confirmed from the HR scan of 1s spectra observed at 1075.1 eV (shown in Figure 2c). Apart from activator and coactivator, a detailed HR scan analysis of other elements like Ca, Sn, and O is also shown in Figure S3 of the SI. The appearance of a doublet structure is prominent in the core-level spectra of Ca 2p and Sn 3d along with the separation of 3.4 and 8.5 eV in the spin−orbit split components, respectively. The oxygen 1s core-level spectra show the characteristic peak of lattice oxygen at 530.2 eV. These spectra clearly demonstrate +2 and +4 oxidation state of Ca and Sn in CaSnO3, respectively.34 3.2. Spectroscopic Analysis. The PL characteristics of asprepared CaSnO3:Eu3+ and Na+ codoped CaSnO3:Eu3+ nanophosphors were investigated by analyzing PL excitation and emission spectra in which identical amounts of samples were used for each measurement. Figure 3 presents the PL excitation
Figure 3. Excitation spectra of CaSnO3:2%Eu3+ and CaSnO3:2% Eu3+,2.5%Na+ nanophosphors monitored at 615 nm emission.
spectra of 2% Eu3+ doped CaSnO3 and 2% Eu3+, 2.5% Na+ doped CaSnO3 nanophosphors, which were measured by observing the emission of Eu3+ at 615 nm. The excitation spectra typically consist of two parts: a broad peak and a few narrow sharp peaks. The broad peak, observed at 275 nm, can be attributed to the charge transfer band (CTB) which appears due to transfer of electron from the O2− (2p) orbital to the empty 4f states of Eu3+ (Eu3+−O2− transition).35 This type of ligand to metal charge transfer transition is strongly related to the covalency between O2− and Eu3+ and the coordination environment around Eu3+. The other part of the excitation spectra consists of several narrow and sharp lines in the higher wavelength domain E
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Na+ concentration, the emission intensity was observed to decrease. The 2.5% Na+ codoped nanophosphor shows an almost 4-fold increase in the emission intensity than the highest intense Eu3+ doped sample. As a more straightforward demonstration of the enhancement of PL brightness, digital images of the samples kept under the UV lamp are shown in the bottom left inset images of Figure 5a,b. The Na+ codoped sample shows a strong emission which is uniform over a large area. It is to be noted that enhanced surface states of the nanocrystals often quench PL brightness in solid condition due to partial aggregation and energy transfer mediated nonradiative recombination.41,42 Hence, it is very crucial for a good nanophosphor to achieve high brightness in the solid condition. From the digital images, it is evident that incorporation of Na+ increases the PL brightness considerably by overcoming the enhanced surface state related problems. Enhancement of the PL characteristics with the addition of Na+ can be attributed to several different factors. Charge compensation is one of the most prominent among these. In CaSnO3:Eu3+ nanophosphor, Eu3+ occupies the Ca2+ site. But due to dissimilar valency of Eu3+ and Ca2+, the effect of this kind of substitution usually manifests as a gain of net positive charge in the system. Hence, if the concentration of Eu3+ increases further in the CaSnO3 lattice, the repulsive force among these net positive charges also increases, and further substitution of Eu3+ in the lattice gets strongly hindered. As a result, some of the Eu3+ ions tend to aggregate instead of entering in the lattice sites properly. The localized aggregation centers of these Eu3+ ions act as quenching sites and decrease the PL intensity by multipolar interaction mediated energy migration among them. Hence, to realize an electrically neutral state, two Eu3+ ions would have to be substituted for three Ca2+ ions, and consequently, a Ca2+ vacancy (VCa ″ ) would also be created according to the possible process 3CaCa → 2EuCa + V″Ca. However, these Ca2+ vacancies behave as defect centers and reduce the net PL intensity owing to efficient transfer of energy from Eu3+ to the Ca2+ vacancies.43 Therefore, to avoid such a charge imbalance and vacancy formation, some alkali metal ions are usually introduced into the system along with the rare earth activators. Due to the introduction of Na+ ions in the CaSnO3:Eu3+ lattice, charge balance is realized via the following equation: 2V″Ca + Eu3+ + Na+ → EuCa + Na′Ca. Thus, the incorporation of Na+ reduces the probability of nonradiative transition and significantly enhances the efficiency of radiative transition as shown in Figure 5b. However, as a consequence of different ionic radii of Ca2+ and Na+ ions, in spite of the effective mitigation of charge imbalance problem, alkali metal ions cannot completely eradicate the volume imbalance problem. However, volume imbalance can effectively be ignored up to a certain degree of alkali metal ion concentration, which is found to be ∼2.5% in the current case. Hence, incorporation of Na+ beyond this limit distorts the crystal lattice severely, which in turn diminishes the crystallinity of the phosphor and consequently quenches the PL intensity. It is also important to mention that replacement of Ca2+ by Na+ always leads to the formation of an oxygen vacancy in the lattice which indeed affects the PL brightness of Na+ codoped CaSnO3:Eu3+ nanophosphors. Generally, the formation of these oxygen vacancies takes place on the nanophosphor surface and they act as sensitizers. These sensitizers efficiently promote the transfer of energy from the host to Eu3+ ions which causes intense overlap between the charge transfer states. Due to this strong mixing, oscillator strength of the radiative transition
Figure 4. Schematics of energy levels demonstrating the possible transition pathways in Na+ coactivated CaSnO3:Eu3+ nanophosphors.
attributed to the nonradiative energy transfer between the same rare earth ions. It is to be noted that nonradiative energy transfer may take place either by exchange interaction or by multipole−multipole interaction.38 In case of exchange interaction, if the concentration of Eu3+ is such that the distance between two nearest Eu3+ ions is sufficiently low, then the interaction between the two identical Eu3+ ions increases the nonradioactive relaxation, and consequently the PL intensity decreases. The nature of the interaction mechanism can be recognized from the critical distance (Rc) between the activator ions which can be obtained directly from the Blasse equation39 expressed as ⎡ 3V ⎤1/3 R c = 2⎢ ⎣ 4πcN ⎦⎥
where V is the volume of the unit cell, c is the optimum concentration of the activator ion, and N is the number of ions in the unit cell. In the present work, for CaSnO3 host, by taking N = 4, c = 0.02, and V = 246.19 Å3, the value of Rc was obtained as 18.05 Å. Since the value of Rc is greater than 5 Å, it can be conclusively stated that exchange interaction between the Eu3+ ions is not the cause of concentration quenching, but rather multipolar interaction is involved in it. The type of multipole− multipole interaction can be identified straight through Dexter’s theory of energy transfer. Generally, in rare-earth doped perovskite luminophores dipole−dipole interaction plays a major role in concentration quenching.40 Effect of Na+ Doping. Figure 5b shows the modulation of the emission spectra as a consequence of Na+ codoping. The concentration of Eu3+, in each sample, was kept fixed at 2%. It is evident from the above figure that when the concentration of Na+ was changed from 1% to 5%, no major changes in the position of the emission peaks occurred; however, it can also be clearly noted that, as compared to Eu3+ doped CaSnO3 nanophosphors, the intensity of all peaks increased remarkably. In addition, Figure 5b clearly demonstrates that as the Na+ concentration is gradually increased from 1% to 2.5%, the emission intensity also increased sequentially; but post-2.5% F
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Figure 5. (a) PL emission spectra of CaSnO3:xEu3+ nanophosphors recoded at 395 nm excitation. Lower left and right insets show the digital image of feeble glow and corresponding color coordinate of the CaSnO3:2%Eu3+ nanophosphor irradiated under a UV lamp, respectively. (b) PL emission spectra of CaSnO3:Eu3+,yNa+ nanophosphors recoded at 395 nm excitation for different concentration of Na+ keeping the Eu3+ concentration fixed at 2%. Lower left and right insets show the digital image of bright glow and the corresponding color coordinate of the CaSnO3:2%Eu3+,2.5%Na+ nanophosphor irradiated under a UV lamp, respectively. (c) Schematic representation of the alkaline metal ion induced charge compensation mechanism in CaSnO3:Eu3+,Na+ nanophosphors.
intensifies and consequently the PL intensity also increases.44,45 The enhancement of the radiative transition probability was quantitatively estimated from the values of intensity parameters and quantum efficiencies which were calculated using JuddOfelt formalism. The entire calculation method is elaborately documented in the SI. The Judd-Ofelt intensity parameters (Ω2, Ω4) and quantum efficiency (η) of 2% Eu3+ and 2% Eu3+, 2.5% Na+ codoped CaSnO3 phosphors are provided in Table 2. The table clearly shows that due to the advent of charge
compensation, incorporation of Na+ promotes quantum efficiency by enhancing the probability of radiative transition. But, if the Na+ concentration is increased beyond a certain degree (2.5%), an excessive amount of oxygen vacancy is formed in the host lattice. The net result of this manifests as the collapse of the crystal lattice and subsequent severe degradation of the luminescence intensity. It is well-known that the electric dipole transition dominates the emission spectra when Eu3+ is situated at a low symmetry site, which we have already shown in Figure 5a. Na+ codoped CaSnO3:Eu3+ nanophosphors are not an exception to this trend. In all the Na+ codoped samples, it was observed that the intensity of 615 nm emission was greater than that of 593 nm emission. Therefore, the intensity ratio of the 5D0 → 7F2 to 5D0 → 7F1 transition represents the degree of distortion from the inversion symmetry of the local environment around Eu3+ in the host matrix and is known as the asymmetric ratio.24 The asymmetric ratio of Na+ codoped CaSnO3:Eu3+ nanophosphors for different concentrations of Na+ doping was computed by estimating the area under the peak of 615 and 593 nm PL bands, respectively. Variation of the asymmetric ratio for various concentration of Na+ contents are shown in Figure S4
Table 2. Decay rates of Radiative (Arad), Nonradiative (Anrad) Processes of 5D0 → 7FJ Transitions, Luminescence Lifetimes (τ), Intensity Parameters (Ω2, Ω4), and Quantum Efficiency (η) Determined from PL Spectra Recorded for CaSnO3:Eu3+ and CaSnO3:Eu3+, Na+ Nanophosphors
sample CaSnO3: 2% Eu3+ CaSnO3: 2% Eu3+,2.5% Na+
Arad [s−1]
Anrad [s−1]
τ [ms]
Ω2 [10−19 cm2]
Ω4 [10−19 cm2]
η [%]
0.156
1.23
0.83
1.12
0.75
52.67
0.378
0.458
1.46
1.63
1.26
87.34
G
DOI: 10.1021/acs.jpcc.5b03500 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C ⎛ t⎞ ⎛ t ⎞ I(t ) = A1exp⎜ − ⎟ + A 2 exp⎜ − ⎟ ⎝ τ1 ⎠ ⎝ τ2 ⎠
of the SI. The figure shows that Na+ codoping in the CaSnO3:Eu3+ nanophosphor system enhances the asymmetric ratio to an extensive amount. The enrichment of this 5D0 → 7F2 transition is also associated with the enhancement of color coordinates, which can be distinctly identified from the CIE chromaticity diagrams which are demonstrated in the bottom right inset of Figure 5a,b. An increase in the x coordinate accompanied by a decrease in the y coordinate in the chromaticity diagram indicates superior color purity of the Na+ codoped nanophosphors. To confirm this phenomenon, we have calculated the color purity of the synthesized nanophosphors by using the formula46 Color purity =
(xs − xi)2 + (ys − yi )2 (xd − xi)2 + (yd − yi )2
(1)
where τ1 and τ2 are the decay lifetimes of the luminescence, and A1 and A2 are the weighting parameters. The biexponential decay indicates that more than one decay channel is involved in the total decay process. Generally, at higher activator concentration, energy migration to the Ca2+ vacancies along with multipolar interaction is a major source of nonradiative transitions. These nonradiative decay centers strongly affect the decay process and no longer leave a decay curve of monoexponential nature. The average lifetime for biexponential decay can be written as47
× 100%
τav =
where (xs,ys) are the coordinates of a sample point, (xd,yd) are the coordinates of the dominant wavelength, and (xi,yi) are the coordinates of the illuminant point. In the current work, taking (xd,yd) = (0.67, 0.32) for the dominant wavelength at 615 nm and (xi,yi) = (0.3101, 0.3162) for the C illuminant point, the color purity of the 2% Eu3+ and 2% Eu3+, 2.5% Na+ codoped CaSnO3 nanophosphors was found to be 87.2% and 94.5%, respectively. Moreover, after Na+ codoping, the color coordinate reaches very close to the ideal red chromaticity for the National Television Standard Committee (NTSC) system. The observed high PL brightness along with excellent color purity indicates Na+ codoped nanophosphors to be highly suitable for lighting and display applications. Reduction of nonradiative transition processes with the addition of Na+ coactivator has been further ensured by monitoring the luminescence decay profile of both 2% Eu3+ and 2% Eu3+, 2.5% Na+ codoped CaSnO3 nanophosphors as shown in Figure 6. The room temperature decay characteristics of the
A1τ12 + A 2 τ2 2 A1τ1 + A 2 τ2
(2)
The measured average lifetime of 2% Eu3+ doped CaSnO3 nanophosphor is found to be 0.83 ms. But with the addition of Na+ coactivator, remarkably the decay profile changes and can be well fitted with a monoexponential decay function expressed as ⎛ t⎞ I = I0 exp⎜ − ⎟ ⎝ τ⎠
(3)
This decay profile strongly confirms that only one kind of luminescent center governs the PL emission, and the coordination environment of Eu3+ is homogeneous48 within the Na+ codoped CaSnO3:Eu3+ lattice. In this case, the lifetime τ of the 615 nm emission is found to be 1.46 ms. An enhancement of the radiative lifetime as a direct consequence of Na+ doping also implies that codoping of Na+ decreases the surface defects as well as nonradiative recombination centers in the system.24 As the efficiency of the radiative transition is always found to be directly proportional to the decay time of a specific transition, longer lifetimes are always favored for reallife applications. Lifetimes of the order of milliseconds for all the nanophosphors reported in the current work indicate that they are highly suitable for using in various types of display and lighting applications. In order to verify the capacity of the current nanophosphors for field emission display applications, CL properties of the assynthesized samples have also been explored. The CL spectra of 2% Eu3+ and 2% Eu3+, 2.5% Na+ doped nanophosphors are shown in Figure S5 of the SI. The CL spectra clearly corroborate the spectral features acquired from PL measurement in the current investigations. Moreover, the consequences of Na+ incorporation in the CaSnO3:Eu3+ crystal is also evident from greater CL intensity demonstrated by those samples. Consequently, the observed strong red light emission and enhanced CL properties of the present nanophosphor conclusively affirms that it could be an ultimate choice for designing field emission display devices and solid state lighting systems as well. Thermal Stability and Electron Phonon Coupling. Thermal stability of the nanophosphors is an important criterion for real-life LED applications because the elevated temperature at the phosphor-UV LED chip interface affects the light output as well as the performance of the LED device.49 Hence, in order to better understand the high temperature emission behavior of the Eu3+ luminescence center in CaSnO3 host, temperature dependent emission spectra of the nanophosphors were also investigated. The temperature dependent
Figure 6. Photoluminescence decay profiles of 5D0 → 7F2 transition for CaSnO3:Eu3+ and CaSnO3:Eu3+,Na+ nanophosphors excited under 395 nm transition.
nanophosphors was measured for 615 nm emission of Eu3+ excited at 395 nm UV excitation. The figure shows that both samples exhibit completely different trends of PL decay profiles. The decay curves for the 5D0 → 7F2 transition of 2% Eu3+ activated CaSnO3 nanophosphor were well fitted into a biexponential function which can be expressed as follows: H
DOI: 10.1021/acs.jpcc.5b03500 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 7. (a) Temperature dependent PL emission spectra of CaSnO3:2%Eu3+ and 2.5%Na+ coactivated CaSnO3:2%Eu3+ nanophosphors within the temperature range from 27 to 227 °C. (b) Configuration coordinate diagram illustrating the thermal quenching behavior of the nanophosphors. (c) Activation energy plot of the corresponding nanophosphors.
the temperature dependent PL intensity was fitted into the above equation, which is shown in the Figure 7c. The plot of ln [(I0/IT) − 1] vs 1/kT yields a straight line whose slope determines the activation energy Ea. The activation energy was obtained to be 0.13 and 0.21 eV for Eu3+ doped and Eu3+,Na+ codoped nanophosphors, respectively. Moreover, the thermally activated nonradiative transition rate is related to the activation energy as52
PL spectra of 2% Eu3+:CaSnO3 and 2.5% Na+ codoped CaSnO3:2%Eu3+ nanophosphors are depicted in Figure 7a. Both nanophosphors show a temperature quenching phenomenon, i.e., the PL intensity of the nanophosphors decreases with the increase in temperature from room temperature to 227 °C. Careful observation also reveals that the PL intensity decays to 38% and 77% of the initial intensity at 227 °C for Eu3+ doped and Eu3+,Na+ codoped nanophosphors, respectively. Since the phosphor converted LEDs are generally operated at ∼150 °C, it is extremely important to check the performance of the phosphor at this temperature. At 150 °C, the PL intensity drops to 57% and 81% of the initial intensity for CaSnO3:Eu3+ and CaSnO3:Eu3+,Na+ nanophosphors, respectively. Therefore, from the temperature quenching behavior it is evident that addition of Na+ also increases the thermal stability of the CaSnO3:Eu3+ nanophosphor. The thermal degradation behavior of both nanophosphors occurs due to the nonradiative transition from the excited state to the ground state. It is well-known that at higher temperature, the Eu3+ ion can reach to the ground state via 5D0-CTB crossover quenching processes.50 This mechanism is explained schematically by typical configuration coordinate model shown in Figure 7b. At low temperatures, the radiative transition from the bottom of the 5D excited state to the 7F ground state is independent of temperature. But when temperature increases, the excited electrons gain sufficient thermal energy to reach the 5 D0-CTB crossover point and then relax to another 7F0-CTB crossover point from which they comes to the ground state of Eu3+ via nonradiative transition. With increasing temperature, these nonradiative processes become more dominant and consequently the luminescence intensity drops down at high temperature. In this thermally activated process, the potential barrier that has to be overcome by the excited electrons is called activation energy. This activation energy in the crossover quenching process can be obtained directly from the modified Arrhenius equation51 IT =
⎛ E ⎞ k nr = s exp⎜ − a ⎟ ⎝ kT ⎠
where s is the frequency factor (s−1). From the equation it is clear that higher activation energy leads to lower probability of nonradiative transition and thereby high thermal stability. Also, with the addition of Na+ coactivator, the nonradiative recombination centers (Ca2+ vacancy) are greatly passivated and thermal stability increases. The results demonstrate that 2.5% Na+ codoped CaSnO3:2%Eu3+ nanophosphor could be a promising candidate for WLED application. Phonon sideband spectroscopy is one of the useful techniques to determine different phonon modes coupled to the Eu3+ions and electron−phonon coupling strength in the host matrix.53 These parameters strongly affect different multiphonon relaxation processes and nonradiative decay of Eu3+. Among all transitions of Eu3+, the 7F0 → 5D2 transition is a purely electric dipole transition and the phonon sideband (PSB) spectrum can be obtained by taking the barycenter wavenumber of this transition as a reference for the zerophonon line. The typical PSB spectrum for the synthesized nanophosphors is shown in Figure 8. It is to be noted that the energy difference between 5D2 and 5D3 of Eu3+ is larger than most vibrational energies of chemical bonds in inorganic compounds. Since any radiative transition between electronic levels in a crystal exhibits emission or absorption sidebands due to the perturbation, the weak bands found between 7F0 → 5D2 and 7F0 → 5D3 transitions represent phonon assisted transition (PAT). The phonon assisted transition for both nanophosphors was found to be located at 1885 cm−1. The extent to which such a particular vibrational phonon mode is coupled to an electronic transition is generally described by the electron−phonon coupling strength which can be calculated from the Huang−Rhys factor (S). According to the theory of
I0 1 + C e−Ea / kT
where I0 stands for the initial PL intensity, IT denotes the luminescent intensity at a given temperature T, C is a constant, k is Boltzman’s constant, and Ea is the activation energy. Hence, for better understanding of thermal quenching phenomenon, I
DOI: 10.1021/acs.jpcc.5b03500 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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efficiency of the nanophosphors. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03500.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors wish to acknowledge the financial help from the Council of Scientific and Industrial Research (CSIR), the Government of India, for awarding a Senior Research Fellowship during the execution of the work. They also wish to thank the Department of Science & Technology (DST), the Government of India and the University Grants Commission, for the University with Potential for Excellence (UPE-II) scheme.
Figure 8. Phonon sideband spectra of CaSnO3:2%Eu3+ (blue line) and CaSnO3:2%Eu3+,2.5%Na+ (red line) nanophosphors.
nonradiative transition, the Huang−Rhys factor can be obtained from the following equation54 I S = PSB IZP
■
where IPSB and IZP stand for the integrated intensities of the PSB and zero-phonon line, respectively. The calculated S values were found to be 0.045 and 0.014 for 2% Eu3+ doped and 2% Eu3+, 2.5% Na+ codoped CaSnO3 nanophosphors, respectively. These parameters again reflect that incorporation of Na+ results in a more homogeneous surrounding environment of Eu3+ and reduced electron phonon coupling, which in turn increases the PL intensity.
■
CONCLUSION We have successfully demonstrated a simple and effective citrate nitrate route to synthesize Eu3+ doped and Na+ codoped CaSnO3:Eu3+ nanophosphors with a wide range of Eu3+ and Na+ concentration. Upon 395 nm excitation, the synthesized nanophosphors exhibit bright red luminescence with an emission maxima at 615 nm. Also under low voltage electron beam excitation (5 kV), all the samples show characteristic 5D0 → 7Fj (j = 0−4) transitions of Eu3+. Furthermore, comparative PL and CL study reveals that incorporation of Na+ coactivator increases the brightness as well as color purity of the nanophosphor significantly. It is also established that Na+ codoping improves the crystallinity of the nanophosphors due to the advent of charge compensation. Addition of coactivator not only enhances the thermal stability of the synthesized nanophosphor with higher activation energy, but also increases the lifetime of the radiative transition remarkably. As a whole, the strategy undertaken successfully stands out to minimize the nonradiative transition centers in CaSnO3:Eu3+ nanophosphors. Therefore, the obtained results suggest that CaSnO3:Eu3+, Na+ nanophosphors might well be an ultimate candidate for solid state lighting and field emission display applications.
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ASSOCIATED CONTENT
S Supporting Information *
Variation of XRD peak position with increasing concentration of Na+, FTIR spectra of 2.5%Na+ coactivated CaSnO3:2%Eu3+ nanophosphors, high resolution core-level spectra of Ca 2p, Sn 3d and O 1s, variation of asymmetric ratio for different Na+ concentration, CL spectra of the nanophosphors, calculation scheme of Judd-Ofelt intensity parameter and quantum J
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DOI: 10.1021/acs.jpcc.5b03500 J. Phys. Chem. C XXXX, XXX, XXX−XXX
