Appl Phys B (2009) 97: 805–809 DOI 10.1007/s00340-009-3606-9
Visible upconversion and infrared luminescence investigations of Al2 O3 powders doped with Er3+ , Yb3+ and Zn2+ ions V. Singh · V.K. Rai · I. Ledoux-Rak · L. Badie · H.-Y. Kwak
Received: 19 March 2009 / Revised version: 25 May 2009 / Published online: 18 June 2009 © Springer-Verlag 2009
Abstract Alumina (Al2 O3 ) powders doped with Er3+ , Yb3+ and Zn2+ ions have been prepared by a low-temperature combustion synthesis technique. The phase purity and crystalline structure of the combustion products are confirmed by powder X-ray diffraction. An efficient frequency upconversion in the visible region and the emission in the infrared (IR) region respectively corresponding to the 2 H11/2 , 4 S3/2 → 4 I15/2 , 4 F9/2 → 4 I15/2 and 4I 4 13/2 → I15/2 transitions upon direct excitation with a CW laser lasing at ∼980 nm are discussed. The enhancement observed in the intensity of the upconversion emission bands in the visible region and the emission band in the IR region due to the presence of Yb3+ and Zn2+ in Er3+ :Al2 O3 powders is reported and explained in detail. PACS 32.50.+d · 81.20.Ka · 78.55.-m · 42.62.-b
V. Singh () · I. Ledoux-Rak · L. Badie Laboratoire de Photonique Quantique et Moleculaire, UMR CNRS 8537, Institut d’Alembert, Ecole Normale Superieure de Cachan, 61 av. du President-Wilson, 94235 Cachan, France e-mail:
[email protected] V.K. Rai Department of Applied Physics, Indian School of Mines University, Dhanbad 826004, India V. Singh · H.-Y. Kwak () Mechanical Engineering Department, Chung-Ang University, Seoul 156-756, Korea e-mail:
[email protected]
1 Introduction Investigations on triply ionized rare-earth doped materials are very important as they emit intense radiation in the visible, near infrared (NIR) and infrared (IR) regions under suitable excitation conditions. The luminescent properties of rare-earth ions depend on the host matrix in which these ions are incorporated. Recently, attention has been devoted to the upconversion of NIR into visible light because of its different applications in color displays, biomedical diagnostics, upconvertors, optical amplifiers, sensors, telecommunications, etc. [1–5]. Studies on Er3+ doped in different host matrices have made known its efficient upconversion emissions in the green and red spectral ranges after pumping with low-cost diode lasers working in the NIR regions [6–10]. It is well known that the codoping of the host with suitable ions enhances the pump efficiency and excitation absorption to achieve efficient laser gain. Many researchers have worked on pairs of rareearth ions, e.g. Er3+ /Yb3+ , Sm3+ /Eu3+ , Tm3+ /Yb3+ , Ho3+ /Yb3+ , Er3+ /Ho3+ , Tb3+ /Eu3+ , Nd3+ /Ho3+ and Pr3+ /Yb3+ doped in different host matrices [11–16]. They discussed the phenomena of radiative and nonradiative transitions in the rare-earth ions and succeeded in recognizing energy transfer in codoped systems, on the basis of improvement observed in the fluorescence intensity or variation in the lifetime of the emitting levels. In recent times, the search for new host materials suitable for frequency upconversion and emission in the IR region has attracted researchers very much. The NIR luminescence and laser emission of Er3+ doped inorganic materials at 1.5 µm is of great interest due to its role in fibre optical communication and many other applications [17–20]. Studies on luminescence signals at 1.5 µm, due to the 4 I13/2 → 4 I15/2 transition of Er3+ ions, have been reported by different researchers [17–23].
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The aluminate (Al2 O3 ) is of particular interest in the present study. In general, this aluminate has been prepared traditionally by solid state reactions, which demand, usually, high annealing temperatures and long times of reaction [24, 25]. Thus, there is a great demand for economical viable synthesis methods, instead of the direct solid state reaction. In the present work, we have succeeded in preparing Al2 O3 at furnace temperatures as low as 500°C in a few minutes using a novel combustion process. The main objective of the present work, for the first time to the best of our knowledge, is to analyze and characterize the effect of the codoping with Yb3+ and Zn2+ in Al2 O3 :Er3+ phosphor on the frequency upconversion in the visible region and the emission in the IR region upon excitation with a CW laser lasing at ∼980 nm. 2 Experimental section 2.1 Preparation of composites Powders of Al1.97 Er0.03 O3 , Al1.92 Er0.03 Yb0.05 O3 and Al1.90 Er0.03 Zn0.02 Yb0.05 O3 were prepared using combustion synthesis based on the reaction of Al (NO3 )3 ·9H2 O with urea (NH2 CONH2 ). The nitrates of erbium, ytterbium and zinc were used as a dopant. The oxidizer-to-fuel ratio was calculated based on oxidizing (O) and fuel (F) valences of the reactants, keeping O/F = 1, as reported earlier [26]. The process involves the exothermic reaction of an oxidizer (metal nitrate) and an organic fuel (urea (NH2 CONH2 )). The energy released due to the exothermic reaction between the metal nitrates and fuel can rapidly heat the system to high temperatures without an external heat source. The combustion method, in fact, makes possible the rapid synthesis of several inorganic materials, without the prolonged hightemperature treatment of sintering. The precursor chemicals were mixed in a Pyrex dish with a minimum quantity of de-ionized water to form a solution. Afterwards, the solution was introduced into a pre-heated muffle furnace maintained at 500 ± 10°C. Combustion took place with the introduction of the solution along with the evolution of gases. The reaction is self-propagating and is able to sustain a high temperature, from 1 to 5 s typically, to form the desired product. The entire combustion process was over in about 5 minutes. Then the dish was taken out from the furnace and the solid mass was crushed into a fine powder. The as-prepared product was then studied without further treatment. 2.2 X-ray diffraction technique A powder X-ray diffraction study was performed on a Phillips X-ray diffractometer with graphite monochromatized CuKα radiation (λ = 0.15418 nm) in the 2θ range of 20° to 80°.
Fig. 1 Powder XRD pattern for Al1.97 Er0.03 O3
2.3 Optical measurement The upconversion emission spectrum was studied using an Acton Research Corporation (ARC) Spectra Pro 275 (0.275 m focal length) monochromator with a 600 grooves/ mm grating coupled to a Hamamatsu Photonics R712 photomultiplier tube connected to a PC, while for the IR photoluminescence emission spectrum a Jobin-Yvon TRIAX 180 monochromator with a 150 grooves/mm grating coupled to a multi-channel InGaAs CCD detector was utilized. Samples were illuminated by a diode (CW) laser operating at ∼980 nm. The luminescence observed was collected in a direction perpendicular to the direction of the incident beam.
3 Results and discussion 3.1 X-ray diffraction (XRD) studies The representative powder XRD pattern for the Al1.97 Er0.03 O3 is shown in Fig. 1. The obtained XRD pattern clearly indicates that the pure hexagonal phase diffraction peaks of parent Al2 O3 are matched well with the JCPDS data file (no. 78-2427). No other phase or unreacted starting material was observed. This confirms that by using combustion synthesis Al2 O3 was formed even if the furnace temperature was only 500°C. The calculated lattice parameters for the hexagonal crystal system were a = 4.74 Å and c = 12.97 Å. 3.2 Upconversion luminescence The frequency upconversion spectra of Er3+ :Al2 O3 powders doped/codoped with Yb3+ , Zn2+ and Yb3+ :Zn2+ ions
Visible upconversion and infrared luminescence investigations of Al2 O3 powders doped with Er3+ , Yb3+
Fig. 2 The frequency upconversion spectra of Er3+ :Al2 O3 powders with a CW diode laser excitation at ∼980 nm at room temperature. (A) Er3+ :Al2 O3 , (B) Yb3+ + Er3+ :Al2 O3 , (C) Yb3+ + Zn2+ + Er3+ :Al2 O3 , and the inset shows the green upconversion emission spectra in the wavelength range of 510–570 nm for (C) Yb3+ + Zn2+ + Er3+ :Al2 O3 powder with different pump powers
with a CW diode laser excitation at ∼980 nm at room temperature are shown in Fig. 2. Figure 2 shows three upconversion emission bands corresponding to the 2 H11/2 , 4S 4 4 4 3/2 → I15/2 and F9/2 → I15/2 transitions lying in the green and red regions respectively. The intensity of the upconversion emission peaks lying in the green region appears to be smaller than the intensity of the upconversion emission peak in the red region. The Fig. 2 inset represents the green upconversion emission spectra in the 510–570 nm wavelength range for Er3+ :Al2 O3 powder codoped with the Zn2+ and Yb3+ at different pump powers. It is interesting to note that the change in the intensity at ∼527 nm appears to be larger than that at ∼548 nm with increasing pump power and it ultimately approaches to be of nearly the same intensity without any evidence of PL saturation in the investigated range. Almost the same behaviour was observed for Yb3+ codoped Er3+ :Al2 O3 powder. The upconversion intensity as a function of the laser intensity follows a quadratic behaviour for all the transitions (Fig. 3), which shows that the two NIR laser photons are participating in producing the upconversion emission in the visible region. The excitation mechanisms of such upconversion emissions may be easily understood by considering the energy level diagram of Er3+ as depicted in Fig. 4. In Er3+ doped Al2 O3 powders, the NIR laser photons at ∼980 nm are responsible for the population of the 4 I11/2 level through the ground state absorption (GSA), which can also populate the lower lying 4 I13/2 level by multiphonon decay. The population stored in the metastable 4 I11/2 and 4I 4 4 13/2 levels is promoted respectively to the F7/2 and F9/2 levels through the excited state absorptions (ESAs) of the NIR laser photons. Consequently, due to the smaller energy gap between the 4 F7/2 and (2 H11/2 , 4 S3/2 ) levels, the ions
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Fig. 3 Variation of the frequency upconversion intensity corresponding to the 4 F9/2 → 4 I15/2 transition versus NIR laser intensity
Fig. 4 Energy level diagram of Er3+ : Yb3+
in the 4 F7/2 level relax rapidly via the emission of phonons, leading to the population in the lower-lying (2 H11/2 , 4 S3/2 ) levels, the radiative relaxation from the latter levels to the ground 4 I15/2 level generates the photons in the green region. The nonradiative decay from the 4 S3/2 level can also populate the 4 F9/2 level, supporting the red emission corresponding to the 4 F9/2 → 4 I15/2 transition. The intensity of the upconversion emission band in the red region increases faster than the intensity of the upconversion emission band in the green region with increasing the pump power. This indicates that one should not exclude the nonradiative relaxation from the 2 H11/2 and 4 S3/2 levels to the 4 F9/2 level, the other mode of excitation for the frequency upconversion in the red region. Further, a part of the population stored in the 4 I13/2 level relaxes radiatively to the 4 I15/2 level and produces a pho-
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Fig. 5 IR emission spectra of (A) Er3+ :Al2 O3 , (B) Yb3+ + Er3+ :Al2 O3 , (C) Yb3+ + Zn2+ + Er3+ :Al2 O3 under CW excitation at λexi ∼ 980 nm
ton in the IR region peaking at 1.5 µm corresponding to the 4 I13/2 → 4 I15/2 transition (Fig. 5). The intensity of the emission band peaking at 1.5 µm, with its full width at halfmaximum (FWHM) ∼68 nm, exhibits a linear behaviour with the NIR laser intensity, thereby indicating that only one NIR photon is responsible for this IR emission, which is applicable for an eye-safe telecommunication window to exist. 3.3 Effect of the addition of Yb3+ , Zn2+ in Er3+ :Al2 O3 powder: improvement of the upconversion efficiency The visible frequency upconversion in Er3+ :Al2 O3 powder under NIR excitation with a CW laser has been studied. The addition of the Yb3+ in Er3+ :Al2 O3 powder enhances the upconversion emission intensities in the green and red regions, respectively, by ∼4.4 times and ∼21 times compared to the upconversion emission intensity observed for Er3+ :Al2 O3 powder only (Fig. 2). This enhancement is due to the presence of the sensitization effect caused by the Yb3+ as a sensitizer. The Yb3+ ions exhibit a strong absorption cross section compared to that of Er3+ ions when pumped with an NIR (∼980 nm) laser and hence Yb3+ is a suitable candidate for use as a sensitizer for the emission of Er3+ ions [26]. The effect of this sensitization is based on the quasi resonance in energy of the 2 F5/2 level of Yb3+ with the 4 I11/2 level of erbium ions. The oscillator strength of Yb3+ corresponding to the 2 F7/2 → 2 F5/2 transition is larger than the one for Er3+ corresponding to the 4I 4 15/2 → I11/2 transition. We attribute the excitation of the 2H 4 4 11/2 , S3/2 and F9/2 levels of erbium ions due to the energy transfer from Yb3+ ions. Therefore, the pump energy efficiently absorbed by the Yb3+ ions is transferred to Er3+ ions that are promoted from the ground (4 I15/2 ) state to the
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excited (4 I11/2 ) state. The excited Er3+ ions in the 4 I11/2 level through the energy transfer from Yb3+ (see Fig. 4) transit upward to the 4 F7/2 level from where they relax nonradiatively to the 2 H11/2 and 4 S3/2 levels. Afterwards, a radiative relaxation from these levels to the ground (4 I15/2 ) level originates the photons in the green region corresponding to the 2 H11/2 , 4 S3/2 → 4 I15/2 transitions. Before the feeding of the 4 F9/2 level via the nonradiative relaxation from the 4 S3/2 level, the energy transfer process shown by ET3 in Fig. 4 can occur following the nonradiative 4I 4 11/2 → I13/2 transition. The intensity of the red emission corresponding to the 4 F9/2 → 4 I15/2 transition is increased very much compared to the green one due to the energy transfer from Yb3+ . This is due to the smaller lifetime of the 4 I11/2 level compared to the lifetime of the 4 I13/2 level, which provides a dominant path for the channel ET3 over channels ET1 and ET2 . Moreover, the addition of the Zn2+ ions in the Yb3+ − Er3+ :Al2 O3 powder enhances the upconversion emission intensity by ∼7 times and ∼34 times in the green and red regions, respectively, compared to the upconversion emission intensity observed for Er3+ :Al2 O3 powder only (Fig. 2). This is the maximum enhancement observed up to now [27, 28]. This significant enhancement is due to the presence of the sensitization effect caused by the Zn2+ as a sensitizer or due to the presence of the local field around the erbium ions caused by the Zn2+ , which is beyond the scope of the present study. This process of excitation through Yb3+ seems to be much more efficient than the direct pumping of Er3+ ions to the 4 I11/2 level due to the relatively small absorption cross section of Er3+ ions at ∼980 nm. Thus, from Fig. 2, it is clear that the maximum enhancement observed for the upconversion emission in the visible region is due to the presence of both Yb3+ and Zn2+ ions. Moreover, the codoping of Yb3+ and Zn2+ in the 3+ Er :Al2 O3 powder enhances the intensity of the IR emission band by ∼2.5 times compared to the intensity of the IR emission band observed for the Er3+ :Al2 O3 powder only (Fig. 5), which is very much smaller than the enhancement observed for the intensity of the frequency upconversion emission bands observed in the visible region.
4 Conclusions We have synthesized the Al2 O3 :Er3+ phosphor codoped with Yb3+ and Zn2+ with hexagonal structure through a low-temperature, cost-effective, solution combustion route. The analysis of the luminescence spectra results shows a larger sensitivity for the frequency upconversion in the visible region, especially for the red emission in the Er3+ :Al2 O3 powder containing both Yb3+ and Zn2+ . This provides a way for the powder (Er3+ :Al2 O3 ) containing Yb3+ and Zn2+ to be used as a good upconvertor.
Visible upconversion and infrared luminescence investigations of Al2 O3 powders doped with Er3+ , Yb3+ Acknowledgements Vijay Singh gratefully acknowledges the LPQM, ENS de Cachan, (France) for the one-year postdoctoral fellowship. Vijay Singh also gratefully acknowledges the Research Assistant Professorship at Chung-Ang University, Seoul (South Korea) from the BK21 program.
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