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Abstract
Usually, up-conversion (UC) green emission is easily observed by using rare-earth doped fluoride nanocrystals. However, preferential red emission is desired for some actual applications especially in biological field. Here, we demonstrated that the dominant UC red emission can be realized by preparing TiO2:Yb,Er nanocrystals under 980 nm exciation. By controlling the crystal symmetry and size via the annealing temperature and Yb3+ ions concentration, the enhanced UC red emission is achieved. The multi-photon relaxation and cross-relaxation mechanisms may be responsible for the energy transform process and in turn the UC emission.
© 2017 Optical Society of America
1. Introduction
Essentially, up-conversion (UC) is an anti-Stokes process in excited condensed matters, where the sequential absorption of two or more photons leads to the radiation of light at shorter wavelength than the excitation wavelength [1–3]. Start from the late 1960s, the field of photon UC has undergone significant expansion and gained remarkable achievements [4,5]. More recently, novel photon UC materials are increasingly emerging and related potential applications extend rapidly to several developing areas, such as biomedical applications, solar cells, photocatalysis and sensors [6–10]. Moreover, UC materials have been proposed as a promising new class of biological luminescent labels and as alternatives to conventional labels, due to their strong penetration abilities under near-infrared (NIR) radiation, low background light, high detection sensitivity and low toxicity [11,12]. Generally, the red region (600-700 nm) and the NIR spectral range (700-1100 nm) are known as the “optical window” of biological tissues due to the minimum absorption in tissues and the subsequent maximum penetration depth [13,14]. Therefore, tuning both the excitation and emission peaks into the “optical window” is essential for the deep tissue imaging of fluorescent labels. Up to now, the β-NaYF4 nanocrystals doped with the rare-earth (RE) ions of Yb3+ and Er3+ are still one of the most efficient UC materials [15,16]. However, these nanocrystals usually give a bright green emission (around 550 nm) along with a weak dark red emission (around 660 nm), which greatly limits their tissue imaging applications because of the shallow penetration depth of green light and the low intensity of the red emission whose signal is too weak to be detected [17,18]. It is reported that the green emission could not escape from the deep tissue and may also cause many undesired effects which will reduce the imaging sensitivity [13]. Consequently, it is important to obtain the strong and single-band red emission from the Yb- Er couple under the NIR irradiation.
Until now, many efforts have been made to achieve the dominant red emission. For instance, it is reported that the red to green emission intensity ratio of NaYF4:Yb,Er nanoparticles gradually increased from 0.83 to 163.78 with increasing the dopant content of Mn2+ ions due to the non-radiative energy transfer from the 2H9/2 and 4S3/2 levels of Er3+ ions to the 4T1 level of Mn2+ ions, followed by back-energy transfer to the 4F9/2 level of Er3+ ions [12]. Moreover, dominant red emission was achieved by doping Ce3+ ions in the NaYF4 system due to the cross-relaxation of energy between Ce3+ and Ho3+ ions [19]. Subsequently, Yi et al. reported that the red-to-green emission ratio was enhanced more than 10 times by doping 30 mol% Ce3+ ions in BaLnF5:Yb3+/Ho3+ (Ln3+ = Gd3+, Y3+, Yb3+) systems [20]. Additionally, the single-band red emission can be obtained in many other host materials such as GdF3, Gd2O3, KMnF3, ZrO2, TiO2, via changing the concentration of doping ions, morphology and size [21–25]. Among these host materials, TiO2 is one of the ideal hosts for UC emission due to low toxicity, high chemical stability, good optical property and low phonon energy (639 cm−1). In our previous work, the energy transfer processes were deeply investigated in the RE-doped silica films with oxide nanocrystals, such as SnO2, InO2, ZnO [26–30]. Here, we fabricate the TiO2 nanocrystals doped with and Yb3+ and Er3+ ions by traditional sol-gel method and spin-coating technique to obtain efficient red emission with the excitation wavelength at 980 nm. The effects of annealing temperature and doping concentration on the UC characteristics under NIR excitation are investigated. The Yb3+ and Er3+ co-doped TiO2 nanocrystals show dominant red emission under 980 nm excitation. The crystalline phase, size and doping concentration have great effects on the red to green emission intensity ratio. By controlling the annealing temperature and Yb3+ ions doping concentration, the enhanced UC red emission is achieved.
2. Experiment
TiO2:Yb,Er nanocrystals were prepared by traditional sol-gel method and spin-coating technique. Firstly, 36.8 ml ethyl alcohol, 22.6 ml tetrabutyl-titanate and 8 ml acetylacetone were poured into a quartz conical flask under slightly stirring to form primrose solution A. Then, 18.4 ml alcohol, 5 ml de-ionized water and a few drops of Hydrochloric acid were mixed to form clarify solution B. Tan precursor could be obtained when solution B were slowly poured into solution A. Before the next operation, Yb3+ and Er3+ ions were introduced into the precursor by adding some Yb(NO3)3·5H2O and Er(NO3)3·7H2O under rigorous stirring according to the calculated amount. Subsequently, the mixture was placed into a digital heating circulating oil bath, and then stirred at 60 °C for 4 h for complete hydrolysis, aged at room temperature for 24 h to form wet uniform gel. Using the obtained precursors, we fabricated titanium dioxide thin film on single polished n-type silicon (100) substrate by spin coating technique. The manipulation was performed in a rotate speed of 7000 r/min for 40 s by using a KW-4A spin coater. After spin coating, the prepared films were annealed at different temperatures under nitrogen ambient. During the process, the amount of Er3+ ions was fixed at 2 mol% relative to Ti4+ ions, whereas the amount of Yb3+ ions was changed from 0 to 20 mol%, the annealing temperature covered from 700 °C to 1000 °C.
The crystalline structures of samples were analyzed by X-ray diffractomer (XRD, Rigaku Ultima III) with Cu-Kα radiation (λ = 1.5406 Å). The microstructures of TiO2:Yb,Er nanocrystals were investigated by transmission electron microscopy (TEM, Tecnai G2 F20) operating at 200 kV. The steady state UC spectra of the TiO2:Yb,Er nanocrystals were measured by use of a fluorescence spectrophotometer (Edinburgh Photonics, FLS980) equipped with a 980 nm CW laser with tunable power of 0~1 W. All the measurements were carried out at room temperature.
3. Results and discussions
The XRD patterns of TiO2:Yb,Er nanocrystals annealed at different temperatures under nitrogen ambient are shown in Fig. 1. For sample annealed at 700 °C, the XRD patterns show an amorphous structure with the weak peak of A (101) illustrates the emergence of TiO2 nanocrystalline in anatase phase (JCPDS, No.21-1272). This indicates the crystallization performance is poor with a small amount of anatase phase at 700 °C. With further increase of the annealing temperature from 800 to 1000 °C, the anatase peak gradually weakened, while the diffraction peak related to rutile phase (JCPDS, No. 21-1276) greatly enhanced, which demonstrates the phase transform process from anatase to rutile at high temperature. It is worth noting that, the pyrochlore phase of ErxYb2-xTi2O7 (Er2Ti2O7, Yb2Ti2O7) appeared in the XRD spectra with the increase of annealing temperature. The peaks of ErxYb2-xTi2O7 are in well agreement with the JCPDS cards of Er2Ti2O7 (No.54-0181) and Yb2Ti2O7 (No.17- 0454) due to the similar ionic radius of Yb3+ and Er3+ ions, therefore they have the same structure and similar lattice constant [31]. The appearance of pyrochlore phase indicates that Yb3+ and Er3+ ions are fully integrated into the TiO2 host. On the basis of the XRD spectra, the high temperature is beneficial to improve phase crystallinity of both rutile and pyrochlore phase.
Fig. 1 XRD patterns of TiO2:Yb,Er (Yb:10 mol%, Er:2 mol%) nanocrystals annealed at different temperatures.
In addition, the different annealing temperatures affect not only the crystal phase of TiO2:Yb,Er nanocrystals, but also the size of crystal grain. As shown in Fig. 1, the full width at half maximum (FWHM) decreases with increasing annealing temperature. Based on the XRD patterns, the average particle diameter can be roughly estimated according to the Scherrer Formula [32], as shown in Eq. (1):
where D is the average diameter, λ = 1.5406 Å is the wavelength of incident X-ray, k = 0.89 is a constant, β and θ represent FWHM and angle of the strongest diffraction peaks. The estimated diameter is 14.5 nm, 17.6 nm and 41.2 nm for sample annealed at 800 °C, 900 °C and 1000 °C, respectively. It indicates that the size of the crystal is gradually increased with the annealing temperature. We have also estimated the size distribution based on AFM and SEM images of Yb-Er co-doped TiO2 nanocrystals on the Si substrates annealed at 800 °C and 900 °C, respectively. The average size is 16 nm for sample annealed at 800 °C and the deviation is about 4.2 nm, while the average size of TiO2 nanocrystals annealed at 900 °C is 24 nm with the size deviation of 9.1nm. It seems that the size distribution becomes broaden with the annealing temperature since the average dot size is enlarged.In order to further understand the structure characteristics of the prepared TiO2:Yb,Er nanocrystals, TEM images of the samples annealed at different temperatures are illustrated in Fig. 2. To identify the exact crystalline phase, details of the selected area with enlarged scale and its spatial Fast Fourier Transform(FFT) image were analyzed. As shown in inset Fig. 2(a), the inter-planar distances are calculated as 0.35 nm and 0.23 nm, which is corresponding to the lattice planes of (101) and (112) of anatase TiO2 nanocrystals, respectively. The above illustrates that crystalline phase of TiO2:Yb,Er nanocrystals formed at 700 °C is anatase, which coincided well with previous XRD results. In contrast, the interplanar distance of TiO2:Yb,Er nanocrystals formed at 800 °C is calculated as 0.32 nm which is corresponding to the lattice planes of (110) of rutile TiO2 crystals. That indicates the phase of TiO2:Yb,Er nanocrystals is changed from anatase to rutile with the increase of annealing temperature. Moreover, with further increase the temperature, samples annealed at 900 °C and 1000 °C, the crystal plane spacing can be calculated to be 0.25 nm, corresponding to the lattice planes of (101) of rutile crystal as shown in inset in Fig. 2(c) and (d). The match of the lattice indicated that Yb3+ and Er3+ ions might be homogeneously accommodated within TiO2 lattice. And the structure of TiO2 host may not be damaged by the doping ions.
Fig. 2 TEM images of TiO2:Yb,Er (Yb:10 mol%, Er:2 mol%) nanocrystals annealed at different temperatures: (a) 700 °C, (b) 800 °C, (c) 900 °C and (d) 1000 °C. The insets of (a), (b), (c) and (d) are high resolution images and Fast Fourier Transform images of the selected area in (a), (b), (c) and (d), respectively.
Figure 3(a) shows the UC emission spectra of the TiO2:Yb,Er nanocrystals after annealing at various temperatures, the content of doped Er3+ and Yb3+ ions are kept at 2 mol% and 10 mol%, respectively. The excitation source is a 980 nm near-infrared CW laser with power of 656 mW. Obviously, UC emissions are observed which include three major emission bands at 529 nm (green light), 550 nm (green light), and 661 nm (red light), both of which are derived from the 4fn energy level of the Er3+ ions active centers. As shown in Fig. 3(a), the UC emission is first enhanced and then decreased as the annealing temperature increased from 700 to 1000 °C. We have not measured the exact quantum efficiency (QE) of red light emission in our samples. However, it is reported that the QE of green light emission from NaYF4 is about 3-4% [33] and the integrated luminescence intensity of red light emission is about two orders of magnitude lower compared with it in the present stage. In our present work, we found that the red light emission can be enhanced by 50 folds via controlling the annealing temperature though the QE is still lower than that of green light emission from rare-earth doped NaYF4 film.
Fig. 3 (a) UC emission spectra of TiO2:Yb,Er (Yb:10 mol%, Er:2 mol%) nanocrystals with different annealing temperatures. (b) Intensity ratio of IR/IG (where IR and IG represent the intensities of red and green UC emission, respectively) with different annealing temperatures. The dotted curves of (b) are the PL intensities of red and green UC emission respectively.
It looks like that both the crystal structures and dot sizes can affect the UC properties of TiO2 nanocrystals. Many groups reported that lower symmetric crystal symmetry is generally favorable for higher UC efficiency, since intermixing of the lanthanide ion’s f states with higher electronic configurations can be more pronounced [34–36]. The UC property of anatase phase TiO2 may be better than that of rutile phase due to the lower crystal symmetry of anatase phase. However, the sample annealed at 700 °C exhibits the lowest UC emission intensity as shown in Fig. 3(a) and (b) though it has an anatase phase. The UC emission intensity is incaresed obviously for sample annealed at 800 °C and 900°C. The possible explanation can be described below. For 700 °C annealed sample, the dot size is quite small and the film quality is poor as we discussed in Fig. 1, so that both the luminescence intensity as well as the IR/IG is low. With increasing the annealing temperature to 900 °C, the dot size is increased and the crystal quality is improved as revealed by XRD spectra. The larger-sized nanocrystals with good crystal quality can enhance both the UC green and red light emission due to smaller surface-to-volume ratio and less defects states [16].
It is interesting to find that, much stronger red light emission relative to green light is observed in our case, which is different from that of fluoride nanocrystals, which usually show the strong green light emission. As shown in Fig. 3 (b), the IR/IG ratio (where IR and IG represent the intensities of UC red and green light emission, respectively) increases with the annealing temperatures from 700 °C to 800 °C and 900 °C, then decreases fast at 1000 °C. It is noted that the crystalline structure is changed from anatase phase to rutile phase as demonstrated by the XRD and TEM observations. The low crystal symmetry of anatase phase favors the UC process to emit the green light but the high crystal symmetry of rutile phase may promote the cross-relaxation process such as the relax process from 4F7/2 to 4F9/2 and 4I13/2 levels, which can cause the enhancement of the red light emission as observed in our case. It is noted that the IR/IG ration is almost the same for 800 °C and 900 °C annealed sample though the size is obviously enlarged in 900 °C annealed one, it suggests that the crystal symmetry plays a dominant role in controlling the red-to-green intensity ratio. As for sample annealed at 1000 °C, the lowest IR/IG ratio can be attributed to the formation of ErxYb2-xTi2O7, which decreases the activator of Er3+ ions.
According to our previous work [16,37], the intrinsic mechanism of the dominant UC red emission in TiO2:Yb,Er nanocrystals can be discussed in detail, as shown in Fig. 4. The electron of Yb3+ ion is first excited from 2F7/2 to 2F5/2 level which can promote Er3+ ion from 4I15/2 level to the 4I11/2 level due to the energy transfer process. Then, a second 980 nm photon transferred by the adjacent Yb3+ ions can excite Er3+ ions from 4F11/2 to 4F7/2 level by the two-photon process. Finally, the Er3+ ion can relax non-radiatively to the 2H11/2, 4S3/2 and 4F9/2 levels. Then they return to the ground state 4I15/2 level, emitting 529 nm, 550 nm, and 661 nm photons respectively. This is the general mechanism of green and red UC emissions. Moreover, there are two other mechanisms for promoting the UC red light emission. One is that the excited Er3+ ions at 4I11/2 level relax to 4I13/2 level through a multi-photon relaxation, and then absorb the pump photons and populate from 4I13/2 to 4F9/2 level. Another is that two adjacent excited Er3+ ions at 4F7/2 and 4I11/2 levels populate to 4F9/2 level by a cross-relaxation. After that, radiative transfer from 4F9/2 to 4I15/2 level resulted in red emission (661 nm) [38]. The strong 661nm light emission achieved from TiO2 nanocrystals as in our case can be ascribed to the probability of cross-relaxation and energy back-transfer processes in different host materials. As we know, the phonon energies of NaYF4 (350 cm−1) is much lower than that of TiO2 (690 cm−1). The higher phonon energies may induce more non-radiative recombination such as the multi-photon relaxation and cross-relaxation, which can contribute to the red light emissions. Additionally, NaYF4 and TiO2 have different energy transfer processes due to the different environments of luminous centers which affect the UC properties [39–41].
In order to deeply investigate the dominant red emission, the UC emission spectra with different concentration of Yb3+ ions are studied. Figure 5(a) demonstrates the UC emission spectra of TiO2:Yb,Er nanocrystals with different doping ratios, the annealing temperature is kept at 900 °C. The concentration of Yb3+ ions increases from 0 to 20 mol% while the concentration of Er3+ ions remains 2 mol%. The UC emission intensity is first enhanced and then weakened with the increase of Yb3+ ions. The TiO2:Yb,Er nanocrystals have the strongest UC emission when the concentration of Yb3+ ions is 10 mol%. The IR/IG ratio also first increases and then decreases with the increase of Yb3+ ions, as shown in Fig. 5(b). The red light emission still dominates the UC process. As we know, the Yb3+ ions used as sensitizers to increase the absorption cross-section, absorb the incident light of 980 nm laser and then transfer the energy to the active centers Er3+ ions, which gives rise to the enhancement of the UC emission with higher concentration of Yb3+ ions. And the increase of Yb3+ ions can reduce the distance between Er3+ and Yb3+ ions, which can strongly influence the UC rate. And possibly induce saturation of Er3+ ions in the 4I13/2 level, the Er3+ ions were then jump from 4I13/2 to 4F9/2 level, and finally return to the ground state 4I15/2 to emit 661 nm red light. As a consequence, a high IR/IG ratio can be achieved. However, the Yb3+ ions may act as trapping centers and dissipate energy non-radiatively in high Yb3+ concentration. Moreover, the energy transform from Er3+ back to Yb3+ ions will hinder the increase of UC emission efficiency with excess Yb3+ ions doping.
Fig. 5 (a) UC emission spectra of TiO2:Yb,Er nanocrystals annealed at 900 °C with different concentrations of Yb3+ ions under 980 nm laser diode excitation; (b) Intensity ratio of IR/IG with different concentration of Yb3+ ions. The dotted curves of (b) are the PL intensities of red and green UC emission respectively.
In order to further elucidate the dominated red emission and photon UC process of TiO2:Yb,Er nanocrystals, we have measured the log-log plot of UC red emission intensity as a function of pump power under 980 nm excitation, as shown in Fig. 6. It is well known in the UC process that the UC emission intensity depending on the excitation power can be described by the following Eq. (2):
Im is the emission intensity, Ppump is the power of pump laser, n is the number of the excitation photons required to produce the UC emission. The value n can be deduced by fitting log-log plot for the UC red emission at 661 nm (4F9/2 to 4I15/2). As shown in Fig. 6, the deduced n value (1.1) is much lower than 2, which suggests that there may probably be a saturation effect of Er3+ ions in 4I13/2 level resulted in one-photon process for producing the UC red light emission [24]. And the above-mentioned cross-relaxation process of the Er3+ ions (2H11/2 + 4I15/2 → 4I9/2 + 4I13/2) may contribute to the saturation effect of Er3+ ions in 4I13/2 level. It is worth noting that the minimum required energy (upon CW excitation) for our samples to get red light emissions is about 127 mW as shown in the inset of Fig. 6.Fig. 6 Log-log plot of the UC red emission(661 nm) intensity versus pump power for the TiO2:Yb,Er (Yb:10 mol%, Er:2 mol%, annealed at 900 °C) nanocrystals under 980 nm laser diode excitation. Inset is the UC emission intensity of TiO2:Yb,Er nanocrystals under 980 nm excitation with the power of 127 mW.
4. Conclusion
In summary, the dominant UC red light emission instead of the usually green light emission in fluoride nanocrystals has been achieved in TiO2:Yb,Er nanocrystals prepared by traditional sol-gel method under 980 nm excitation, which can be mainly attributed to multi-photon relaxation and cross-relaxation. It is found that both the annealing temperature and Yb3+ ions doping concetrations can strongly affect the UC emission intensities. On one hand, with increasing the annealing temperature from 700 °C to 900 °C, the size of nanocrystal is enlarged which enhance both the red and green light emission due to the improved crystal quality while the crystalline phase is changed from anatase to rutile which results in the enhanced red-to-green emission intensity ratio. The further increasing the annealing temperature to 1000 °C, the formation of pyrochlore phase (ErxYb2-xTi2O7) degrades the UC emission intensity. On the other hand, by adding the Yb3+ ions into the films as sensitizers, the more incident photons can be absorbed and then transfer the energy to the active centers Er3+ ions to enhance UC emission. However, the exssive introduction of Yb3+ ions results in serious energy back transfer process to suppress the UC emission. In our case, the red light emission can be enhanced by 50 folds via controlling the annealing temperature and additional Yb3+ ions concentrations compared with the reference sample. Our present study provides a interesting way to control the UC emission benhaviors in oxide semiconductor nanocrystals and there are still many interesting issues such as the dynamic luminescence process, the effect of ambient conditions as well as the actual applications in bio-field, need further studies.
Funding
National Natural Science Foundation of China (NSFC) (61735008, 11774155); Program 973 (2013CB632101); “333 project” of Jiangsu Province (BRA2015284); Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
Acknowledgments
The authors would like to thank Zewen Lin and Dongke Li from Nanjing University for their technical support about PL.
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