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Enhanced up-conversion luminescence intensity in single-crystal SrTiO3: Er3+ nanocubes by codoping with Yb3+ ions
Journal of Alloys and Compounds 724 (2017) 139e145
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Enhanced up-conversion luminescence intensity in single-crystal SrTiO3: Er3þ nanocubes by codoping with Yb3þ ions Zhen Xiao a, Jiawei Zhang a, Lei Jin a, Yang Xia b, *, Lei Lei a, Huanping Wang a, Junjie Zhang a, Shiqing Xu a, ** a b
College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2017 Received in revised form 28 June 2017 Accepted 3 July 2017 Available online 4 July 2017
A series of Er3þ doped and Er3þ/Yb3þ codoped SrTiO3 nanocubes with varying dopant concentrations have been successfully synthesized by a facile hydrothermal method. The doping effects on phase purity, morphology and microstructure of samples were identified by power X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results showed that SrTiO3 nanocubes have a typical perovskite structure with the edge length ranging 20e50 nm. The Er3þ and Yb3þ ions were successfully doped into the crystal lattice of SrTiO3, but did not alter crystal structure of SrTiO3. The photon up-conversion (UC) photoluminescence (PL) measurements indicated that the intense green light emission of Er3þ ions around 525 nm, 550 nm and red light emission centered at 670 nm in UC processes populated by 980 nm laser excitation have been achieved in SrTiO3 nanocubes. Moreover, the PL intensity of Er3þ-doped SrTiO3 nanocubes was greatly enhanced while introducing the Yb3þ to the crystal structure, especially the red emission. The mechanisms for these emissions probably due to the excited state absorption (ESA) and energy transfer (ET) process. Er3þ/Yb3þ-codoped single-crystal perovskite SrTiO3 nanocubes could be interesting objects for applications in mechanicalelectrical luminescence such as electric field modulation luminescence and mechanoluminescence. © 2017 Elsevier B.V. All rights reserved.
Keywords: SrTiO3 Up-conversion Excited state absorption Energy transfer
1. Introduction In recent years, photon up-conversion (UC) from the nearinfrared (NIR) to visible light in rare-earth (RE) ions doped materials has attracted intense interests because of their outstanding properties and practical applications, such as photovoltaics, solar cells, color displays, and biological imaging [1e5]. The UC performance of a material could be significantly enhanced by the suitable selection of host matrix [6]. Moreover, tremendous researches on the energy transition determined by doping ions were emerged, aimed at optimizing the properties and extending the applications of UC materials. ABO3 perovskite oxides (SrTiO3, BaTiO3, PbTiO3, etc.), exhibit many intriguing physical properties, such as piezoelectricity and ferroelectricity, affording wide applications from electromechanical systems to data storage devices [7,8]. Owing to large-
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Xia), [email protected] (S. Xu). http://dx.doi.org/10.1016/j.jallcom.2017.07.024 0925-8388/© 2017 Elsevier B.V. All rights reserved.
range toleration with chemical doping, trivalent lanthanide ions (Ln3þ) were introduced into the perovskite-related oxides to adjust their luminescence properties. Nowadays, various PL and electroluminescence (EL) have been achieved in perovskite-type oxides, from crystalline powders, thin films to amorphous particles, offering the opportunity to lighting and flat-plane displaying. Generally, the trivalent rare earth ions, including Er3þ, Ho3þ, Tm3þ and Yb3þ, have been introduced into host matrix as absorption or emission centers to design the UC PL properties [9e13]. Among them, Er3þ ions are of particular attraction as a luminescent activator because of its long lifetime, sharp emission bandwidth and tunable emission [14]. For example, Gong et al. synthesized Er3þ doped preperovskite and perovskite PbTiO3 nanofibers, respectively. It has been found that the UC PL properties were significantly different in preperovskite and perovskite PbTiO3 since the substitution sites and chemical environments of Er3þ ions were different in their structures [15]. However, in singly doped nanocrystals, two major parameters that effect UC processes are the distance between two neighboring activator ions and the absorption cross-section of the ions. Therefore, the concentration of activator ions should be
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kept low and precisely adjusted to avoid the quenching effect. Otherwise, the overall UC efficiency of singly doped nanocrystals would be relatively low [14,16]. To enhance UC luminescence efficiency, a sensitizer with a sufficient absorption cross-section in the NIR region is usually co-doped along with the activator to take advantage of the efficient energy transfer upconversion (ETU) process. For example, Sun et al. reported the infrared-to-visible UC PL properties of BaTiO3 nanocrystals with dopant of Er3þ ions, and their intensities have enhanced while Liþ and Er3þ were codoped [17]. Furthermore, the sensitization of rare earth Yb3þ ion, is a wellknown one for increasing the optical pump efficiency, as well as its application in optical devices [18,19]. SrTiO3, a typical perovskite oxide, has many fascinating properties, such as high dielectric constant, high charge storage capacity, excellent chemical and physical stability, and good transparency in the visible range [20e23]. In particular, the vibrational frequency of SrTiO3 is quite low. So, it is suitable to be host matrix for UC excitation phosphors. It has been observed that nanoparticles have different properties compared with bulk compounds, such as light emissions, which increases in particles on the nanoscale. For example, the intensity of the emitted radiation in Er3þ/Yb3þ: SrO$TiO2 glass ceramic is much higher than in the glass. Because the glass ceramic has Er3þ/Yb3þ-doped nanocrystalline phase in it, which is benefited to the UP PL emissions [24]. In addition, Souza et al. reported that the photoluminescence of SrTiO3, which is excited by 350 nm visible-UV, could be significantly affected by the particle size and morphology [25]. It has been demonstrated that the particles with cubic morphology have more intense photoluminescent emissions because of its close to single-crystal feature and small dimension. However, the UC PL of Er3þ/Yb3þ doped SrTiO3 with single-crystalline and uniform dimensions is rarely reported. Herein, in this work, Er3þ doped and Er3þ/Yb3þ codoped SrTiO3 nanocubes with various dopant concentrations were synthesized by a facile hydrothermal method. The intense green light emission of Er3þ ions around 525 and 550 nm and red light emission centered at 660 nm in UC processes populated by 980 nm laser excitation have been achieved in SrTiO3 nanocubes. Interestingly, UC emission intensity of Er3þ/Yb3þ codoped SrTiO3 nanocubes was greatly enhanced compared to that of Er3þ doped SrTiO3. 2. Experimental section 2.1. Synthesis of Er3þ doped and Er3þ/Yb3þ codoped SrTiO3 nanocubes Strontium nitrate (Sr(NO3)2), erbium nitrate (Er(NO3)3$5H2O) and P25 were chemistry grade and used as received without further purification. Potassium hydroxide (KOH) were used as mineralizer. The Er-doped SrTiO3 samples were synthesized via a hydrothermal method. Er3þ doping concentration was set as atomic ratio of 0 mol %, 1 mol%, 3 mol%, 5 mol%, respectively. In a typical procedure, 6.25 mmol Sr(NO3)2 and stoichiometric Er(NO3)3$5H2O with different concentrations were dissolved in 10 mL deionized water with stirring until it transformed to a transparent solution. Meanwhile, 13.466 g KOH was dissolved in 20 mL deionized water under stirring at room temperature. Then, 0.3994 g P25 was slowly added to KOH solution. Subsequently, the above two precursor solutions were mixed together with violently stirring for 1 h. The resulting precursor suspension was transferred into 50 mL stainless-steel Teflon-lined autoclave for the hydrothermal treatment. The autoclave was sealed and held at 180 C for 12 h. After cooling down to room temperature naturally, the resultant products were filtered and washed with deionized water and ethanol for several times, and subsequently air-dried at 80 C for the characterization. For the Er3þ/Yb3þ codoped SrTiO3 nanocubes, the synthetic procedure is
similar to that of Er3þ doped SrTiO3 sample. However, the concentration of Er3þ ion was fixed at 3 mol%. Meanwhile, Yb3þ ion concentration in the co-doped SrTiO3 were 0, 1 mol%, 2 mol%, 3 mol %, 5 mol%, respectively. The samples were named as SrTiO3: Er 3%, SrTiO3: Er 3%, Yb 1%, SrTiO3: Er 3%, Yb 2%, SrTiO3: Er 3%, Yb 3% and SrTiO3: Er 3%, Yb 5%. 2.2. Characterizations X-ray diffraction (XRD) patterns were collected at room temperature on a Bruker D8 Advance power diffractometer with BraggBrentano geometry by Cu Ka radiation (l ¼ 1.54056 Å). Scanning electron microscopy (SEM) tests, used to observe the morphology of samples, were conducted with a Hitachi SU8010. Transmission electron microscopy (TEM) images, high resolution TEM (HRTEM) images, scanning transmission electron microscopy (STEM) images and energy-dispersive spectroscopy (EDS) results were taken on FEI F20 using an accelerating voltage of 200 kV. The UC spectra were obtained at room temperature by using a PL3-211-P spectrometer (HORIBA JOBIN YVON, America). The emitted UC spectra was collected by a lens-coupled monochromator of a 1 nm spectral resolution. For UC steady-state spectra investigation, a 980 nm continuous diode laser (maximum output power: 2 W) was used to pump the samples. 3. Results and discussions The room temperature UC emission spectra of SrTiO3 nanocubes doped with different Er3þ concentrations are shown in Fig. 1. Excited by the 980 nm diode laser with a power of 500 mW, the strong green emissions peaking at the center of 525 nm and 550 nm and the faint red emission around 670 nm were observed, which come from the 4H11/2 / 4I15/2, 4S3/2 / 4I15/2 and the 4F9/2 / 4I15/2 transitions of Er3þ ions, respectively. From the emission spectra, it is clear that the relative intensities of the green and red emission band change with the Er3þ ions concentration. The intensity of these up-converted emissions are increased as the dopant Er3þ ions concentration increases to 3 mol%. Especially, the luminescence intensity of SrTiO3: Er3þ 3% is several times higher than that of SrTiO3: Er3þ 1%. In Er3þ -doped SrTiO3 system, a resonant crossrelaxation mechanism of the type 4F7/2 / 4F9/2 and 2I11/2 / 4F9/2 type could directly populate the red emitting 4F9/2 state when exciting the 2I11/2 state with 980 nm. This cross-relaxation mechanism is depending on the concentration, so the increasing content of Er3þ ions would increase the efficiency of the mechanism. However, when the doping concentration reaches to 5 mol%, the up-converted emissions reduce sharply. This may be due to the interionic interactions as the substitution concentration of Er3þ ions increasing. Therefore, an optimum concentration of Er3þ ions to realize the intense up-converted luminescence is approximately 3 mol% in SrTiO3 matrix. In order to understand the upconversion mechanism of the observed luminescence bands, the power dependent UC intensity in SrTiO3: Er3þ 3% has been provided in Fig. 1b. The results are excited by the 980 nm diode laser with the power from 500 mW to 900 mW. And the interval of power is 50 mW. In up-conversion process, I is proportional to the nth power of P, IfP n . Where n is the number of pump photons absorbed per up-conversion photon emitted. A plot of log I versus log P yields a straight line with slope n. The slopes n are 1.88, 1.69 and 1.52 for 525, 550 and 670 nm emissions, respectively. These results vividly show that the green and red emission of Er3þ-doped SrTiO3 are two photon process. Fig. 2a presents XRD patterns of Er-doped SrTiO3 samples. As shown in Fig. 1a, when Er dopant concentration is ranging from 1 mol% to 5 mol%, all the diffraction peaks can be indexed as the
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Fig. 1. (a) UC emission spectra of Er3þ (0, 1, 3 and 5 mol%) doped SrTiO3 upon 980 nm excitation with the power of 500 mW. (b) The power dependent green and red emission intensity changes in SrTiO3: Er 3% sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) XRD patterns of Er-doped SrTiO3 nanocubes. (b) A shift of (110) diffraction peak.
cubic phase of SrTiO3 (PDF 35-0734). No impurity phase can be observed, indicating Er dopants do not cause the secondary phase or alter phase structure of SrTiO3. Moreover, a careful comparison on the location of (110) diffraction peak in the range of 30e35 (Fig. 2b) reveals that the (110) peak obviously shifts toward a higher 2q value when Er dopant concentration increases. It is well known that the different substitution sites not only influence the chemical environment around Er3þ, but also determined the UC photoluminescence processes [26,27]. According to Shannon effective ionic radii [28], the radii of Er3þ, Ti4þ and Sr2þ are 0.89, 0.745 and 1.18 Å, respectively. Obviously, the radius of Er3þ ions is larger than Ti4þ ions, but smaller than Sr2þ ions. Therefore, if the substation of Sr2þ by Er3þ happens at the A site of SrTiO3, the crystal lattices will shrink. And the diffraction peaks should shift a higher angle. Whereas the substation of Ti4þ by Er3þ occurs at the B site of SrTiO3, it will cause the expansion of crystal lattices, and the peak will shift to a lower angle. In this work, XRD results vividly illustrate that (110) peaks obviously shifts towards a higher angle along with increasing Er3þ concentration. Consequently, it can be easily inferred that Er3þ ions mainly occupy the A site in SrTiO3, which also is matching well with previous literatures [12,15,29]. Table 1 shows the nominal and measured concentration of Er3þ ions in SrTiO3 nanocubes. Obviously, experimental Er
concentrations in SrTiO3 are increasing as the nominal values increased. And the measured rare earth ion contents are close to the nominal ones. These results clearly demonstrate that Er3þ ions are successfully doped into the crystal lattice of SrTiO3. Besides, the unaltered phase structure and increasing concentration also imply that the UC luminescence intensity change mainly originates from Er dopants that will be discussed later, instead of the impurity. Although the doping of Er3þ ions in SrTiO3 nanocubes can enhance its green and red emissions, the corresponding transitions of Er3þ ions have weak ground state absorption [30]. Hence, Yb3þ ions as the sensitizer are introduced to increase the luminescence efficiency in SrTiO3: Er3þ system. In this work, Er3þ concentration is
Table 1 The nominal and measured concentration of Er3þ ions in Er-doped SrTiO3 nanocubes. Sample
Nominal concentration (Er3þ)
Measured concentration (Er3þ)
SrTiO3 SrTiO3: Er 1% SrTiO3: Er 3% SrTiO3: Er 5%
0 1 mol% 3 mol% 5 mol%
0 0.89 mol% 2.77 mol% 4.96 mol%
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fixed at 3 mol%, and a series of Er3þ/Yb3þ codoped SrTiO3 samples with Yb3þ concentration from 0 to 5 mol% are further synthesized. Fig. 3 presents XRD patterns of Er3þ/Yb3þ codoped SrTiO3 samples. When the concentration of Yb3þ dopant is low (<3 mol%), all the diffraction peaks can be indexed as the cubic SrTiO3 (PDF 35-0734) (Fig. 3a). However, the doping concentration increases to 5 mol%, the impurity phase marked by small circles appears. This result indicates that there is a narrow solid solubility for Yb3þ ions that is about 3 mol% in SrTiO3: Er3þ 3% system. Furthermore, as shown in Fig. 3b, the (110) diffraction peaks of Er3þ/Yb3þ codoped SrTiO3 gradually shift to lower angle, implying that doping introducedlattice expansion occurs in Er3þ/Yb3þ codoped SrTiO3 since Yb3þ ions (0.868 Å) possess an ionic radius larger than that of Ti4þ (0.745 Å). Fig. 4 displays the morphology of Er3þ/Yb3þ codoped SrTiO3 samples. Interestingly, Er3þ/Yb3þ codoped SrTiO3 samples are monodispersed nanocubes with the edge length ranging from 20 to 50 nm as well. Moreover, the particle size of Er3þ/Yb3þ codoped SrTiO3 samples are reduced along with increasing Yb3þ concentration. Owing to the un-equivalent substitution of Ti4þ by Yb3þ, the charge balance in SrTiO3 might be disturbed. In order to establish the charge compensation, vacancies will be formed, which may hinder the diffusion of crystal nucleus and retard the growth of SrTiO3 [31]. As a result, the particle size of Er3þ/Yb3þ codoped SrTiO3 samples are reduced along with increasing Yb3þ concentration. TEM and EDS tests are further performed to investigate the microstructure and element distribution of SrTiO3: Er 3%, Yb 3% nanocubes. As depicted in Fig. 5a, SrTiO3: Er 3%, Yb 3% have a uniform cube-like morphology, which is matching well with SEM results. The clear interval of lattice fringes measured to be 0.194 nm in HRTEM image (Fig. 5b) can be indexed to the (200) plane of cubic SrTiO3. In addition, these lattice fringes are continuous and clear, suggesting that the nanocubes are single-crystalline characteristic in nature. To verify the composition of SrTiO3: Er 3%, Yb 3%, highangle annular dark-field (HAADF-STEM) and area-scan elemental mapping images are supplied. Fig. 5c exhibits the element distribution in SrTiO3: Er 3%, Yb 3% nanocubes. It can be found that Sr, Ti, O, Yb and Er are homogeneously distributed in the randomly selected area, demonstrating Yb and Er are successfully doped into SrTiO3. Moreover, as shown in Table 2, the measured concentration of Er ion in all the samples are close to the nominal values. And the measured Yb concentrations are increasing when the nominal concentration in SrTiO3: Er 3% nanocubes increased from 0 to 3%, and the values are matched well with the nominal ones. These
Fig. 4. SEM images of SrTiO3: Er 3% nanocubes with different Yb doping concentrations. (a) 0 mol%, (b) 1 mol%, (c) 2 mol%, (d) 3 mol%.
results further confirm that Yb and Er ions are uniformly incorporated into SrTiO3 as expected. Fig. 6 shows the UC emission of Er3þ/Yb3þ codoped SrTiO3 samples during 980 nm laser excitation at room temperature. The green and red UC bands centered at 525, 550 and 670 nm can also be observed from the Er3þ/Yb3þ codoped SrTiO3 samples as illustrated in Fig. 6a. The characteristic green and red emissions can be ascribed to the 4H11/2 / 4I15/2, 4S3/2 / 4I15/2 and 4F9/2 / 4I15/2 transitions of Er3þ ions, respectively. Moreover, both green and red up-conversion emission spectra of Er3þ ions in all Er3þ/Yb3þ codoped SrTiO3 samples present significant differences in emission intensity. The red emission intensity at 670 nm is stronger than that of the green emissions. The intensity of green and red emissions both increase as Yb3þ concentration increases from 0 to 3 mol%, and the red one increase dramatically. Quantitative comparisons of green and red emission intensities versus the concentration of Yb3þ, are shown in Fig. 6b. A significant increase of the red emission
Fig. 3. (a) The XRD patterns of SrTiO3: Er 3% nanocubes with different Yb doping concentrations. (b) The enlarged (110) diffraction peak for the pattern.
Z. Xiao et al. / Journal of Alloys and Compounds 724 (2017) 139e145
Fig. 5. (a) TEM, (b) HRTEM and (c) HAADF-STEM images and the corresponding areascan elemental mappings of SrTiO3: Er 3%, Yb 3%.
compared to the green one causes a decrease of the green/red intensity ratio. In this work, the average distance between neighboring Er ion and Yb ion is about 3.382 Å, which is smaller than the average distance between Er ion and Er ion (3.905 Å). It is reported that introduction of an elevated amount of Yb3þ dopant in the host lattice would decrease Yb-Er inter-atomic distance and thus facilitate back-energy-transfer from Er3þ to Yb3þ [32]. When the Yb3þ ions doped into SrTiO3: Er3þ host lattice, it would induce enhanced back-energy-transfer from Er3þ to Yb3þ. Then, the energy transfer should subsequently suppress the population in the excited levels of 2H11/2 and 4S3/2, resulting in the decrease of green light emission at 525 nm and 550 nm. In contrast, the decreased Yb-Er interatomic distance and enhanced back-energy-transfer from Er3þ to Yb3þ would promote the population in the 2H9/2 level, thereby leading to a relative increase in the intensity of red emission of Er3þ at 670 nm. In an attempt to analyze the observed luminescence bands, the power dependent UC intensity in SrTiO3: Er3þ 3%, Yb3þ 3% was carried out (Fig. 6c). The plot of the 4H11/2 / 4I15/2, 4S3/2 / 4I15/2 and 4F9/2 / 4I15/2 upconverted transitions in the graph of logI vs logP exhibit gradients of 1.77, 1.67 and 1.97, respectively. The slopes are approximately equal to 2, indicating the upconversion occurs via a two-photon process. This behavior is the same as observed in Er/Yb codoped SrTiO3 nanocrystals, but different from the three
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photon process at 525 nm UC emission of Er/Yb codoped SrTiO3 glass ceramics [24]. This may attribute to the difference of crystal field between nanocrystals and glass ceramics, which would result in the different influence on the energy levels of Er3þ ions. The excited states for UC can be populated by several wellknown mechanisms: (i) exited state absorption (ESA), (ii) energy transfer (ET) and (iii) photon avalanche [33,34]. Photon avalanche is ruled out, because no inflection point is observed in the power study. Fig. 6d presents the energy level diagram of Er3þ and Yb3þ ions as well as the proposed UC mechanisms for the green and red emissions. For Er3þ-doped SrTiO3 sample, Er3þ ions are excited initially from the ground state 4I15/2 to 4I11/2 energy level by the ground state absorption (GSA) under the excitation by 980 nm laser. Subsequently, some Er3þ ions in the 4I11/2 level absorb another 980 nm photon and raise to 4I7/2 level by the ESA process. There is a possible ET route occurring during and after the UC excitation process which can also populate to the 4I7/2 level. An excited ion relaxes from 4I11/2 level to 4I15/2 level nonradiatively and transfer the excitation energy to a neighboring ion in the same level, promoting the latter to 4I7/2 level: 4I11/2þ 4I11/2 / 4I15/2þ 4I7/2. After that, ions at the 4I7/2 level decay nonradiatively to 4I11/2, 4S3/2 or 4I9/2 levels by multi-phonon relaxation. And then, the ions at 4I11/2, 4S3/2 and 4I9/2 return back to the ground state and produce the green and red lights. For the Er3þ/Yb3þ codoped SrTiO3 system, Yb3þ ions are excited to the 4I5/2 state via 980 nm photons and transfer the energy to Er3þ ions in the ground state. Thereby the Er3þ ions can be excited to the 4 I11/2 intermediate excited state by GSA and ET process: 4I15/2 (Er3þ) þ 4I5/2 (Yb3þ) /4I11/2 (Er3þ) þ 4I7/2 (Yb3þ). Subsequently, energy transfer from another Yb3þ ion also in the excited state results in population of the 4I7/2 state of Er3þ ion. Based on the ESA and the following ET process: 4I11/2 (Er3þ) þ 4I5/2 (Yb3þ) / 4I7/2 (Er3þ) þ 4I7/2 (Yb3þ). This process is particularly dominant in the Er3þ/Yb3þ codoped samples, due to the larger absorption cross section of the 4I5/2 excited state of Yb3þ compared to that of the 4I11/ 3þ 3þ ions in the 4I7/2 level relaxes 2 excited state of Er . Then, Er rapidly and nonradiatively to the next lower levels 4I11/2 and 4S3/2 green emitting levels. And the 4I9/2 level can also be populated by nonradiative relaxation from the 4S3/2 level as the Er3þ-doped SrTiO3 sample. Additionally, it can be seen that the intensity of red emission increases fast than that of green emissions with increasing Yb3þ concentration (Fig. 6b). The most likely cause of this phenomenon is due to the ET process that only populated to the 4I9/2 level: 4I13/2 (Er3þ) þ 4I5/2 (Yb3þ) / 4I9/2 (Er3þ) þ 4I7/2 (Er3þ). Additionally, the populated 4I13/2 level might be excited to the 4I9/2 red-emitting level in Er3þ ions by cross-relaxation process: 4I11/2 (Er3þ) þ 4I13/2 (Er3þ) / 4I9/2 (Er3þ) þ 4I7/2 (Er3þ). 4. Conclusions Single-crystalline Er3þ-doped and Er3þ/Yb3þ codoped SrTiO3 nanocubes are successfully synthesized via a facile hydrothermal method. Er3þ and Yb3þ ions are substituted to Sr2þ in A site and Ti4þ in B site of SrTiO3, respectively. The UC emissions of Er3þ-doped and Er3þ/Yb3þ codoped SrTiO3 nanocubes both reveal that green and
Table 2 The nominal and measured concentration of Er3þ and Yb3þ ions in Er3þ/Yb3þ-codoped SrTiO3 nanocubes. Sample SrTiO3: SrTiO3: SrTiO3: SrTiO3:
Er Er Er Er
3%, 3%, 3%, 3%,
Yb Yb Yb Yb
0 1% 2% 3%
Nominal concentration (Er3þ)
Measured concentration (Er3þ)
Nominal concentration (Yb3þ)
Measured concentration (Yb3þ)
3 3 3 3
2.92 2.89 2.94 2.93
0 1 mol% 2 mol% 3 mol%
0 0.78 mol% 1.89 mol% 2.95 mol%
mol% mol% mol% mol%
mol% mol% mol% mol%
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Fig. 6. (a) UC emission spectra of Yb3þ (0, 1, 2 and 3%), Er3þ (3%) codoped SrTiO3 nanocubes upon 980 nm excitation with the power of 500 mW. (b) Plots of the green and red emission intensities and the green/red intensity ratio versus the concentration of Yb3þ ions. (c) The power dependent green and red emission intensity changes in SrTiO3: Er 3%, Yb 3%. (d) An energy level diagram showing the 980 nm excited UC mechanism of Yb3þ and Er3þ in SrTiO3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
red emissions centered at 525, 550 and 670 nm are observed under 980 nm excited, which can be ascribed to the 4H11/2 / 4I15/2, 4S3/ 4 4 4 3þ 3þ 2 / I15/2 and F9/2 / I15/2 transitions, respectively. For Er /Yb codoped SrTiO3 system, the intensity of both green and red emissions significantly increased as Yb3þ concentration increases, but the red emission increases faster than that of green ones. The decreased Yb-Er inter-atomic distance and enhanced back-energytransfer from Er3þ to Yb3þ could greatly promote the population in the 2H9/2 level, thereby leading to a relative increase in the intensity of red emission of Er3þ at 670 nm. As a result, the introduced Yb3þ ions can effectively enhance the emission intensity of Er3þ-doped SrTiO3 for the energy transfer process. Acknowledgement This work was supported by Zhejiang Provincial Natural Science Foundation of China (LQ14E020005, LY17E020007 and LY17E020010), National Nature Science Foundation of China (21403196) and Science and Technology Department of Zhejiang Province (2016C31012). References [1] J.C. Goldschmidt, S. Fischer, Upconversion for photovoltaics e a review of materials, devices and concepts for performance enhancement, Adv. Opt. Mater. 3 (2015) 510e535. [2] G.E. Arnaoutakis, J. Marques-Hueso, A. Ivaturi, S. Fischer, J.C. Goldschmidt, €mer, B.S. Richards, Enhanced energy conversion of up-conversion K.W. Kra solar cells by the integration of compound parabolic concentrating optics, Sol. Energy Mater. Sol. C 140 (2015) 217e223. [3] J. Zhou, J. Deng, H. Zhu, X. Chen, Y. Teng, H. Jia, S. Xu, J. Qiu, Up-conversion luminescence in LaF3:Ho3þvia two-wavelength excitation for use in solar cells,
J. Mater. Chem. C 1 (2013) 8023e8027. [4] L.M. Jin, X. Chen, C.K. Siu, F. Wang, S.F. Yu, Enhancing multiphoton upconversion from NaYF4:Yb/Tm@NaYF4 coreeshell nanoparticles via the use of laser cavity, ACS Nano 11 (2017) 843e849. [5] F. Wang, X. Liu, Multicolor tuning of lanthanide-doped nanoparticles by single wavelength excitation, Acc. Chem. Res. 47 (2014) 1378e1385. [6] F. Wang, X.G. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38 (2009) 976e989. [7] M.E. Lines, A.M. Glass, Principles and Applications of Ferroelectrics and Related Materials, Oxford university press, 1977. [8] J. Scott, Applications of modern ferroelectrics, Science 315 (2007) 954e959. ^ men, R. Sakamoto, N. Sakamoto, S. Kunugi, M. Itoh, Photoluminescence [9] T. Kyo properties of Pr-doped (Ca, Sr, Ba) TiO3, Chem. Mater. 17 (2005) 3200e3204. [10] T.C. Ozawa, K. Fukuda, K. Akatsuka, Y. Ebina, T. Sasaki, Preparation and characterization of the Eu3þ doped perovskite nanosheet phosphor: La0. 90Eu0. 05Nb2O7, Chem. Mater. 19 (2007) 6575e6580. [11] S. Ida, C. Ogata, M. Eguchi, W.J. Youngblood, T.E. Mallouk, Y. Matsumoto, Photoluminescence of perovskite nanosheets prepared by exfoliation of layered oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: lanthanide ion), J. Am. Chem. Soc. 130 (2008) 7052e7059. [12] S.Y. Gong, M. Li, Z.H. Ren, X. Yang, X. Li, G. Shen, G.R. Han, Polarizationmodified upconversion luminescence in Er-Doped single-crystal perovskite PbTiO3 nanofibers, J. Phys. Chem. C 119 (2015) 17326e17333. [13] H. Suo, C. Guo, T. Li, Broad-scope thermometry based on dual-color modulation up-conversion phosphor Ba5Gd8Zn4O21: Er3þ/Yb3þ, J. Phys. Chem. C 120 (2016) 2914e2924. [14] F. Wang, X. Liu, Upconversion multicolor fine-tuning: visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles, J. Am. Chem. Soc. 130 (2008) 5642e5643. [15] S.Y. Gong, Z.H. Ren, S. Jiang, M. Li, X. Li, X. Wei, G. Xu, G. Shen, G.R. Han, Phasemodified up-conversion luminescence in Er-Doped single-crystal PbTiO3 nanofibers, J. Phys. Chem. C 118 (2014) 5486e5493. [16] F. Wang, X. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38 (2009) 976e989. [17] Q. Sun, X.Q. Chen, Z.K. Liu, F.P. Wang, Z.H. Jiang, C. Wang, Enhancement of the upconversion luminescence intensity in Er3þ doped BaTiO3 nanocrystals by codoping with Liþ ions, J. Alloy. Compd. 509 (2011) 5336e5340. [18] Z.G. Zang, Y.J. Zhang, Low-switching power (<45 mW) optical bistability based
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced up-conversion luminescence intensity in single-crystal SrTiO3: Er3þ nanocubes by codoping with Yb3þ ions Zhen Xiao a, Jiawei Zhang a, Lei Jin a, Yang Xia b, *, Lei Lei a, Huanping Wang a, Junjie Zhang a, Shiqing Xu a, ** a b
College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2017 Received in revised form 28 June 2017 Accepted 3 July 2017 Available online 4 July 2017
A series of Er3þ doped and Er3þ/Yb3þ codoped SrTiO3 nanocubes with varying dopant concentrations have been successfully synthesized by a facile hydrothermal method. The doping effects on phase purity, morphology and microstructure of samples were identified by power X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results showed that SrTiO3 nanocubes have a typical perovskite structure with the edge length ranging 20e50 nm. The Er3þ and Yb3þ ions were successfully doped into the crystal lattice of SrTiO3, but did not alter crystal structure of SrTiO3. The photon up-conversion (UC) photoluminescence (PL) measurements indicated that the intense green light emission of Er3þ ions around 525 nm, 550 nm and red light emission centered at 670 nm in UC processes populated by 980 nm laser excitation have been achieved in SrTiO3 nanocubes. Moreover, the PL intensity of Er3þ-doped SrTiO3 nanocubes was greatly enhanced while introducing the Yb3þ to the crystal structure, especially the red emission. The mechanisms for these emissions probably due to the excited state absorption (ESA) and energy transfer (ET) process. Er3þ/Yb3þ-codoped single-crystal perovskite SrTiO3 nanocubes could be interesting objects for applications in mechanicalelectrical luminescence such as electric field modulation luminescence and mechanoluminescence. © 2017 Elsevier B.V. All rights reserved.
Keywords: SrTiO3 Up-conversion Excited state absorption Energy transfer
1. Introduction In recent years, photon up-conversion (UC) from the nearinfrared (NIR) to visible light in rare-earth (RE) ions doped materials has attracted intense interests because of their outstanding properties and practical applications, such as photovoltaics, solar cells, color displays, and biological imaging [1e5]. The UC performance of a material could be significantly enhanced by the suitable selection of host matrix [6]. Moreover, tremendous researches on the energy transition determined by doping ions were emerged, aimed at optimizing the properties and extending the applications of UC materials. ABO3 perovskite oxides (SrTiO3, BaTiO3, PbTiO3, etc.), exhibit many intriguing physical properties, such as piezoelectricity and ferroelectricity, affording wide applications from electromechanical systems to data storage devices [7,8]. Owing to large-
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Xia), [email protected] (S. Xu). http://dx.doi.org/10.1016/j.jallcom.2017.07.024 0925-8388/© 2017 Elsevier B.V. All rights reserved.
range toleration with chemical doping, trivalent lanthanide ions (Ln3þ) were introduced into the perovskite-related oxides to adjust their luminescence properties. Nowadays, various PL and electroluminescence (EL) have been achieved in perovskite-type oxides, from crystalline powders, thin films to amorphous particles, offering the opportunity to lighting and flat-plane displaying. Generally, the trivalent rare earth ions, including Er3þ, Ho3þ, Tm3þ and Yb3þ, have been introduced into host matrix as absorption or emission centers to design the UC PL properties [9e13]. Among them, Er3þ ions are of particular attraction as a luminescent activator because of its long lifetime, sharp emission bandwidth and tunable emission [14]. For example, Gong et al. synthesized Er3þ doped preperovskite and perovskite PbTiO3 nanofibers, respectively. It has been found that the UC PL properties were significantly different in preperovskite and perovskite PbTiO3 since the substitution sites and chemical environments of Er3þ ions were different in their structures [15]. However, in singly doped nanocrystals, two major parameters that effect UC processes are the distance between two neighboring activator ions and the absorption cross-section of the ions. Therefore, the concentration of activator ions should be
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kept low and precisely adjusted to avoid the quenching effect. Otherwise, the overall UC efficiency of singly doped nanocrystals would be relatively low [14,16]. To enhance UC luminescence efficiency, a sensitizer with a sufficient absorption cross-section in the NIR region is usually co-doped along with the activator to take advantage of the efficient energy transfer upconversion (ETU) process. For example, Sun et al. reported the infrared-to-visible UC PL properties of BaTiO3 nanocrystals with dopant of Er3þ ions, and their intensities have enhanced while Liþ and Er3þ were codoped [17]. Furthermore, the sensitization of rare earth Yb3þ ion, is a wellknown one for increasing the optical pump efficiency, as well as its application in optical devices [18,19]. SrTiO3, a typical perovskite oxide, has many fascinating properties, such as high dielectric constant, high charge storage capacity, excellent chemical and physical stability, and good transparency in the visible range [20e23]. In particular, the vibrational frequency of SrTiO3 is quite low. So, it is suitable to be host matrix for UC excitation phosphors. It has been observed that nanoparticles have different properties compared with bulk compounds, such as light emissions, which increases in particles on the nanoscale. For example, the intensity of the emitted radiation in Er3þ/Yb3þ: SrO$TiO2 glass ceramic is much higher than in the glass. Because the glass ceramic has Er3þ/Yb3þ-doped nanocrystalline phase in it, which is benefited to the UP PL emissions [24]. In addition, Souza et al. reported that the photoluminescence of SrTiO3, which is excited by 350 nm visible-UV, could be significantly affected by the particle size and morphology [25]. It has been demonstrated that the particles with cubic morphology have more intense photoluminescent emissions because of its close to single-crystal feature and small dimension. However, the UC PL of Er3þ/Yb3þ doped SrTiO3 with single-crystalline and uniform dimensions is rarely reported. Herein, in this work, Er3þ doped and Er3þ/Yb3þ codoped SrTiO3 nanocubes with various dopant concentrations were synthesized by a facile hydrothermal method. The intense green light emission of Er3þ ions around 525 and 550 nm and red light emission centered at 660 nm in UC processes populated by 980 nm laser excitation have been achieved in SrTiO3 nanocubes. Interestingly, UC emission intensity of Er3þ/Yb3þ codoped SrTiO3 nanocubes was greatly enhanced compared to that of Er3þ doped SrTiO3. 2. Experimental section 2.1. Synthesis of Er3þ doped and Er3þ/Yb3þ codoped SrTiO3 nanocubes Strontium nitrate (Sr(NO3)2), erbium nitrate (Er(NO3)3$5H2O) and P25 were chemistry grade and used as received without further purification. Potassium hydroxide (KOH) were used as mineralizer. The Er-doped SrTiO3 samples were synthesized via a hydrothermal method. Er3þ doping concentration was set as atomic ratio of 0 mol %, 1 mol%, 3 mol%, 5 mol%, respectively. In a typical procedure, 6.25 mmol Sr(NO3)2 and stoichiometric Er(NO3)3$5H2O with different concentrations were dissolved in 10 mL deionized water with stirring until it transformed to a transparent solution. Meanwhile, 13.466 g KOH was dissolved in 20 mL deionized water under stirring at room temperature. Then, 0.3994 g P25 was slowly added to KOH solution. Subsequently, the above two precursor solutions were mixed together with violently stirring for 1 h. The resulting precursor suspension was transferred into 50 mL stainless-steel Teflon-lined autoclave for the hydrothermal treatment. The autoclave was sealed and held at 180 C for 12 h. After cooling down to room temperature naturally, the resultant products were filtered and washed with deionized water and ethanol for several times, and subsequently air-dried at 80 C for the characterization. For the Er3þ/Yb3þ codoped SrTiO3 nanocubes, the synthetic procedure is
similar to that of Er3þ doped SrTiO3 sample. However, the concentration of Er3þ ion was fixed at 3 mol%. Meanwhile, Yb3þ ion concentration in the co-doped SrTiO3 were 0, 1 mol%, 2 mol%, 3 mol %, 5 mol%, respectively. The samples were named as SrTiO3: Er 3%, SrTiO3: Er 3%, Yb 1%, SrTiO3: Er 3%, Yb 2%, SrTiO3: Er 3%, Yb 3% and SrTiO3: Er 3%, Yb 5%. 2.2. Characterizations X-ray diffraction (XRD) patterns were collected at room temperature on a Bruker D8 Advance power diffractometer with BraggBrentano geometry by Cu Ka radiation (l ¼ 1.54056 Å). Scanning electron microscopy (SEM) tests, used to observe the morphology of samples, were conducted with a Hitachi SU8010. Transmission electron microscopy (TEM) images, high resolution TEM (HRTEM) images, scanning transmission electron microscopy (STEM) images and energy-dispersive spectroscopy (EDS) results were taken on FEI F20 using an accelerating voltage of 200 kV. The UC spectra were obtained at room temperature by using a PL3-211-P spectrometer (HORIBA JOBIN YVON, America). The emitted UC spectra was collected by a lens-coupled monochromator of a 1 nm spectral resolution. For UC steady-state spectra investigation, a 980 nm continuous diode laser (maximum output power: 2 W) was used to pump the samples. 3. Results and discussions The room temperature UC emission spectra of SrTiO3 nanocubes doped with different Er3þ concentrations are shown in Fig. 1. Excited by the 980 nm diode laser with a power of 500 mW, the strong green emissions peaking at the center of 525 nm and 550 nm and the faint red emission around 670 nm were observed, which come from the 4H11/2 / 4I15/2, 4S3/2 / 4I15/2 and the 4F9/2 / 4I15/2 transitions of Er3þ ions, respectively. From the emission spectra, it is clear that the relative intensities of the green and red emission band change with the Er3þ ions concentration. The intensity of these up-converted emissions are increased as the dopant Er3þ ions concentration increases to 3 mol%. Especially, the luminescence intensity of SrTiO3: Er3þ 3% is several times higher than that of SrTiO3: Er3þ 1%. In Er3þ -doped SrTiO3 system, a resonant crossrelaxation mechanism of the type 4F7/2 / 4F9/2 and 2I11/2 / 4F9/2 type could directly populate the red emitting 4F9/2 state when exciting the 2I11/2 state with 980 nm. This cross-relaxation mechanism is depending on the concentration, so the increasing content of Er3þ ions would increase the efficiency of the mechanism. However, when the doping concentration reaches to 5 mol%, the up-converted emissions reduce sharply. This may be due to the interionic interactions as the substitution concentration of Er3þ ions increasing. Therefore, an optimum concentration of Er3þ ions to realize the intense up-converted luminescence is approximately 3 mol% in SrTiO3 matrix. In order to understand the upconversion mechanism of the observed luminescence bands, the power dependent UC intensity in SrTiO3: Er3þ 3% has been provided in Fig. 1b. The results are excited by the 980 nm diode laser with the power from 500 mW to 900 mW. And the interval of power is 50 mW. In up-conversion process, I is proportional to the nth power of P, IfP n . Where n is the number of pump photons absorbed per up-conversion photon emitted. A plot of log I versus log P yields a straight line with slope n. The slopes n are 1.88, 1.69 and 1.52 for 525, 550 and 670 nm emissions, respectively. These results vividly show that the green and red emission of Er3þ-doped SrTiO3 are two photon process. Fig. 2a presents XRD patterns of Er-doped SrTiO3 samples. As shown in Fig. 1a, when Er dopant concentration is ranging from 1 mol% to 5 mol%, all the diffraction peaks can be indexed as the
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Fig. 1. (a) UC emission spectra of Er3þ (0, 1, 3 and 5 mol%) doped SrTiO3 upon 980 nm excitation with the power of 500 mW. (b) The power dependent green and red emission intensity changes in SrTiO3: Er 3% sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) XRD patterns of Er-doped SrTiO3 nanocubes. (b) A shift of (110) diffraction peak.
cubic phase of SrTiO3 (PDF 35-0734). No impurity phase can be observed, indicating Er dopants do not cause the secondary phase or alter phase structure of SrTiO3. Moreover, a careful comparison on the location of (110) diffraction peak in the range of 30e35 (Fig. 2b) reveals that the (110) peak obviously shifts toward a higher 2q value when Er dopant concentration increases. It is well known that the different substitution sites not only influence the chemical environment around Er3þ, but also determined the UC photoluminescence processes [26,27]. According to Shannon effective ionic radii [28], the radii of Er3þ, Ti4þ and Sr2þ are 0.89, 0.745 and 1.18 Å, respectively. Obviously, the radius of Er3þ ions is larger than Ti4þ ions, but smaller than Sr2þ ions. Therefore, if the substation of Sr2þ by Er3þ happens at the A site of SrTiO3, the crystal lattices will shrink. And the diffraction peaks should shift a higher angle. Whereas the substation of Ti4þ by Er3þ occurs at the B site of SrTiO3, it will cause the expansion of crystal lattices, and the peak will shift to a lower angle. In this work, XRD results vividly illustrate that (110) peaks obviously shifts towards a higher angle along with increasing Er3þ concentration. Consequently, it can be easily inferred that Er3þ ions mainly occupy the A site in SrTiO3, which also is matching well with previous literatures [12,15,29]. Table 1 shows the nominal and measured concentration of Er3þ ions in SrTiO3 nanocubes. Obviously, experimental Er
concentrations in SrTiO3 are increasing as the nominal values increased. And the measured rare earth ion contents are close to the nominal ones. These results clearly demonstrate that Er3þ ions are successfully doped into the crystal lattice of SrTiO3. Besides, the unaltered phase structure and increasing concentration also imply that the UC luminescence intensity change mainly originates from Er dopants that will be discussed later, instead of the impurity. Although the doping of Er3þ ions in SrTiO3 nanocubes can enhance its green and red emissions, the corresponding transitions of Er3þ ions have weak ground state absorption [30]. Hence, Yb3þ ions as the sensitizer are introduced to increase the luminescence efficiency in SrTiO3: Er3þ system. In this work, Er3þ concentration is
Table 1 The nominal and measured concentration of Er3þ ions in Er-doped SrTiO3 nanocubes. Sample
Nominal concentration (Er3þ)
Measured concentration (Er3þ)
SrTiO3 SrTiO3: Er 1% SrTiO3: Er 3% SrTiO3: Er 5%
0 1 mol% 3 mol% 5 mol%
0 0.89 mol% 2.77 mol% 4.96 mol%
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fixed at 3 mol%, and a series of Er3þ/Yb3þ codoped SrTiO3 samples with Yb3þ concentration from 0 to 5 mol% are further synthesized. Fig. 3 presents XRD patterns of Er3þ/Yb3þ codoped SrTiO3 samples. When the concentration of Yb3þ dopant is low (<3 mol%), all the diffraction peaks can be indexed as the cubic SrTiO3 (PDF 35-0734) (Fig. 3a). However, the doping concentration increases to 5 mol%, the impurity phase marked by small circles appears. This result indicates that there is a narrow solid solubility for Yb3þ ions that is about 3 mol% in SrTiO3: Er3þ 3% system. Furthermore, as shown in Fig. 3b, the (110) diffraction peaks of Er3þ/Yb3þ codoped SrTiO3 gradually shift to lower angle, implying that doping introducedlattice expansion occurs in Er3þ/Yb3þ codoped SrTiO3 since Yb3þ ions (0.868 Å) possess an ionic radius larger than that of Ti4þ (0.745 Å). Fig. 4 displays the morphology of Er3þ/Yb3þ codoped SrTiO3 samples. Interestingly, Er3þ/Yb3þ codoped SrTiO3 samples are monodispersed nanocubes with the edge length ranging from 20 to 50 nm as well. Moreover, the particle size of Er3þ/Yb3þ codoped SrTiO3 samples are reduced along with increasing Yb3þ concentration. Owing to the un-equivalent substitution of Ti4þ by Yb3þ, the charge balance in SrTiO3 might be disturbed. In order to establish the charge compensation, vacancies will be formed, which may hinder the diffusion of crystal nucleus and retard the growth of SrTiO3 [31]. As a result, the particle size of Er3þ/Yb3þ codoped SrTiO3 samples are reduced along with increasing Yb3þ concentration. TEM and EDS tests are further performed to investigate the microstructure and element distribution of SrTiO3: Er 3%, Yb 3% nanocubes. As depicted in Fig. 5a, SrTiO3: Er 3%, Yb 3% have a uniform cube-like morphology, which is matching well with SEM results. The clear interval of lattice fringes measured to be 0.194 nm in HRTEM image (Fig. 5b) can be indexed to the (200) plane of cubic SrTiO3. In addition, these lattice fringes are continuous and clear, suggesting that the nanocubes are single-crystalline characteristic in nature. To verify the composition of SrTiO3: Er 3%, Yb 3%, highangle annular dark-field (HAADF-STEM) and area-scan elemental mapping images are supplied. Fig. 5c exhibits the element distribution in SrTiO3: Er 3%, Yb 3% nanocubes. It can be found that Sr, Ti, O, Yb and Er are homogeneously distributed in the randomly selected area, demonstrating Yb and Er are successfully doped into SrTiO3. Moreover, as shown in Table 2, the measured concentration of Er ion in all the samples are close to the nominal values. And the measured Yb concentrations are increasing when the nominal concentration in SrTiO3: Er 3% nanocubes increased from 0 to 3%, and the values are matched well with the nominal ones. These
Fig. 4. SEM images of SrTiO3: Er 3% nanocubes with different Yb doping concentrations. (a) 0 mol%, (b) 1 mol%, (c) 2 mol%, (d) 3 mol%.
results further confirm that Yb and Er ions are uniformly incorporated into SrTiO3 as expected. Fig. 6 shows the UC emission of Er3þ/Yb3þ codoped SrTiO3 samples during 980 nm laser excitation at room temperature. The green and red UC bands centered at 525, 550 and 670 nm can also be observed from the Er3þ/Yb3þ codoped SrTiO3 samples as illustrated in Fig. 6a. The characteristic green and red emissions can be ascribed to the 4H11/2 / 4I15/2, 4S3/2 / 4I15/2 and 4F9/2 / 4I15/2 transitions of Er3þ ions, respectively. Moreover, both green and red up-conversion emission spectra of Er3þ ions in all Er3þ/Yb3þ codoped SrTiO3 samples present significant differences in emission intensity. The red emission intensity at 670 nm is stronger than that of the green emissions. The intensity of green and red emissions both increase as Yb3þ concentration increases from 0 to 3 mol%, and the red one increase dramatically. Quantitative comparisons of green and red emission intensities versus the concentration of Yb3þ, are shown in Fig. 6b. A significant increase of the red emission
Fig. 3. (a) The XRD patterns of SrTiO3: Er 3% nanocubes with different Yb doping concentrations. (b) The enlarged (110) diffraction peak for the pattern.
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Fig. 5. (a) TEM, (b) HRTEM and (c) HAADF-STEM images and the corresponding areascan elemental mappings of SrTiO3: Er 3%, Yb 3%.
compared to the green one causes a decrease of the green/red intensity ratio. In this work, the average distance between neighboring Er ion and Yb ion is about 3.382 Å, which is smaller than the average distance between Er ion and Er ion (3.905 Å). It is reported that introduction of an elevated amount of Yb3þ dopant in the host lattice would decrease Yb-Er inter-atomic distance and thus facilitate back-energy-transfer from Er3þ to Yb3þ [32]. When the Yb3þ ions doped into SrTiO3: Er3þ host lattice, it would induce enhanced back-energy-transfer from Er3þ to Yb3þ. Then, the energy transfer should subsequently suppress the population in the excited levels of 2H11/2 and 4S3/2, resulting in the decrease of green light emission at 525 nm and 550 nm. In contrast, the decreased Yb-Er interatomic distance and enhanced back-energy-transfer from Er3þ to Yb3þ would promote the population in the 2H9/2 level, thereby leading to a relative increase in the intensity of red emission of Er3þ at 670 nm. In an attempt to analyze the observed luminescence bands, the power dependent UC intensity in SrTiO3: Er3þ 3%, Yb3þ 3% was carried out (Fig. 6c). The plot of the 4H11/2 / 4I15/2, 4S3/2 / 4I15/2 and 4F9/2 / 4I15/2 upconverted transitions in the graph of logI vs logP exhibit gradients of 1.77, 1.67 and 1.97, respectively. The slopes are approximately equal to 2, indicating the upconversion occurs via a two-photon process. This behavior is the same as observed in Er/Yb codoped SrTiO3 nanocrystals, but different from the three
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photon process at 525 nm UC emission of Er/Yb codoped SrTiO3 glass ceramics [24]. This may attribute to the difference of crystal field between nanocrystals and glass ceramics, which would result in the different influence on the energy levels of Er3þ ions. The excited states for UC can be populated by several wellknown mechanisms: (i) exited state absorption (ESA), (ii) energy transfer (ET) and (iii) photon avalanche [33,34]. Photon avalanche is ruled out, because no inflection point is observed in the power study. Fig. 6d presents the energy level diagram of Er3þ and Yb3þ ions as well as the proposed UC mechanisms for the green and red emissions. For Er3þ-doped SrTiO3 sample, Er3þ ions are excited initially from the ground state 4I15/2 to 4I11/2 energy level by the ground state absorption (GSA) under the excitation by 980 nm laser. Subsequently, some Er3þ ions in the 4I11/2 level absorb another 980 nm photon and raise to 4I7/2 level by the ESA process. There is a possible ET route occurring during and after the UC excitation process which can also populate to the 4I7/2 level. An excited ion relaxes from 4I11/2 level to 4I15/2 level nonradiatively and transfer the excitation energy to a neighboring ion in the same level, promoting the latter to 4I7/2 level: 4I11/2þ 4I11/2 / 4I15/2þ 4I7/2. After that, ions at the 4I7/2 level decay nonradiatively to 4I11/2, 4S3/2 or 4I9/2 levels by multi-phonon relaxation. And then, the ions at 4I11/2, 4S3/2 and 4I9/2 return back to the ground state and produce the green and red lights. For the Er3þ/Yb3þ codoped SrTiO3 system, Yb3þ ions are excited to the 4I5/2 state via 980 nm photons and transfer the energy to Er3þ ions in the ground state. Thereby the Er3þ ions can be excited to the 4 I11/2 intermediate excited state by GSA and ET process: 4I15/2 (Er3þ) þ 4I5/2 (Yb3þ) /4I11/2 (Er3þ) þ 4I7/2 (Yb3þ). Subsequently, energy transfer from another Yb3þ ion also in the excited state results in population of the 4I7/2 state of Er3þ ion. Based on the ESA and the following ET process: 4I11/2 (Er3þ) þ 4I5/2 (Yb3þ) / 4I7/2 (Er3þ) þ 4I7/2 (Yb3þ). This process is particularly dominant in the Er3þ/Yb3þ codoped samples, due to the larger absorption cross section of the 4I5/2 excited state of Yb3þ compared to that of the 4I11/ 3þ 3þ ions in the 4I7/2 level relaxes 2 excited state of Er . Then, Er rapidly and nonradiatively to the next lower levels 4I11/2 and 4S3/2 green emitting levels. And the 4I9/2 level can also be populated by nonradiative relaxation from the 4S3/2 level as the Er3þ-doped SrTiO3 sample. Additionally, it can be seen that the intensity of red emission increases fast than that of green emissions with increasing Yb3þ concentration (Fig. 6b). The most likely cause of this phenomenon is due to the ET process that only populated to the 4I9/2 level: 4I13/2 (Er3þ) þ 4I5/2 (Yb3þ) / 4I9/2 (Er3þ) þ 4I7/2 (Er3þ). Additionally, the populated 4I13/2 level might be excited to the 4I9/2 red-emitting level in Er3þ ions by cross-relaxation process: 4I11/2 (Er3þ) þ 4I13/2 (Er3þ) / 4I9/2 (Er3þ) þ 4I7/2 (Er3þ). 4. Conclusions Single-crystalline Er3þ-doped and Er3þ/Yb3þ codoped SrTiO3 nanocubes are successfully synthesized via a facile hydrothermal method. Er3þ and Yb3þ ions are substituted to Sr2þ in A site and Ti4þ in B site of SrTiO3, respectively. The UC emissions of Er3þ-doped and Er3þ/Yb3þ codoped SrTiO3 nanocubes both reveal that green and
Table 2 The nominal and measured concentration of Er3þ and Yb3þ ions in Er3þ/Yb3þ-codoped SrTiO3 nanocubes. Sample SrTiO3: SrTiO3: SrTiO3: SrTiO3:
Er Er Er Er
3%, 3%, 3%, 3%,
Yb Yb Yb Yb
0 1% 2% 3%
Nominal concentration (Er3þ)
Measured concentration (Er3þ)
Nominal concentration (Yb3þ)
Measured concentration (Yb3þ)
3 3 3 3
2.92 2.89 2.94 2.93
0 1 mol% 2 mol% 3 mol%
0 0.78 mol% 1.89 mol% 2.95 mol%
mol% mol% mol% mol%
mol% mol% mol% mol%
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Fig. 6. (a) UC emission spectra of Yb3þ (0, 1, 2 and 3%), Er3þ (3%) codoped SrTiO3 nanocubes upon 980 nm excitation with the power of 500 mW. (b) Plots of the green and red emission intensities and the green/red intensity ratio versus the concentration of Yb3þ ions. (c) The power dependent green and red emission intensity changes in SrTiO3: Er 3%, Yb 3%. (d) An energy level diagram showing the 980 nm excited UC mechanism of Yb3þ and Er3þ in SrTiO3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
red emissions centered at 525, 550 and 670 nm are observed under 980 nm excited, which can be ascribed to the 4H11/2 / 4I15/2, 4S3/ 4 4 4 3þ 3þ 2 / I15/2 and F9/2 / I15/2 transitions, respectively. For Er /Yb codoped SrTiO3 system, the intensity of both green and red emissions significantly increased as Yb3þ concentration increases, but the red emission increases faster than that of green ones. The decreased Yb-Er inter-atomic distance and enhanced back-energytransfer from Er3þ to Yb3þ could greatly promote the population in the 2H9/2 level, thereby leading to a relative increase in the intensity of red emission of Er3þ at 670 nm. As a result, the introduced Yb3þ ions can effectively enhance the emission intensity of Er3þ-doped SrTiO3 for the energy transfer process. Acknowledgement This work was supported by Zhejiang Provincial Natural Science Foundation of China (LQ14E020005, LY17E020007 and LY17E020010), National Nature Science Foundation of China (21403196) and Science and Technology Department of Zhejiang Province (2016C31012). References [1] J.C. Goldschmidt, S. Fischer, Upconversion for photovoltaics e a review of materials, devices and concepts for performance enhancement, Adv. Opt. Mater. 3 (2015) 510e535. [2] G.E. Arnaoutakis, J. Marques-Hueso, A. Ivaturi, S. Fischer, J.C. Goldschmidt, €mer, B.S. Richards, Enhanced energy conversion of up-conversion K.W. Kra solar cells by the integration of compound parabolic concentrating optics, Sol. Energy Mater. Sol. C 140 (2015) 217e223. [3] J. Zhou, J. Deng, H. Zhu, X. Chen, Y. Teng, H. Jia, S. Xu, J. Qiu, Up-conversion luminescence in LaF3:Ho3þvia two-wavelength excitation for use in solar cells,
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