Preparation and upconversion luminescence of YAG(Y3Al5O12): Yb3+, Ho3+ nanocrystals

Preparation and upconversion luminescence of YAG(Y3Al5O12): Yb3+, Ho3+ nanocrystals

JOURNAL OF RARE EARTHS, Vol. 27, No. 1, Feb. 2009, p. 66 Preparation and upconversion luminescence of YAG(Y3Al5O12): Yb3+, Ho3+ nanocrystals LIU Min ...

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JOURNAL OF RARE EARTHS, Vol. 27, No. 1, Feb. 2009, p. 66

Preparation and upconversion luminescence of YAG(Y3Al5O12): Yb3+, Ho3+ nanocrystals LIU Min ( )1,2,3,4, WANG Shiwei ()1, TANG Dingyuan( )2, CHEN Lidong ( ) 3, MA Jian ( )4 (1. Structural Ceramics Engineering Research Center, Energy Materials Research Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China; 2. School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798; 3. Energy Materials Research Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China; 4. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798) Received 6 March 2008; revised 19 November 2008

Abstract: Cubic YAG: Yb3+, Ho3+ pure phase nanocrystals were synthesized by using coprecipition nitrate and ammonium hydrogen carbonate as raw materials. After calcining the precipitates at 800 °C, the resultant YAG: Yb3+, Ho3+ nanocrystals were nearly spheric and the particle size was about 40 nm. Intense upconversion spectra were observed on the powder compact pumped by a 980 nm continuous wave diode laser, and green emission centered at 549 nm, red emission centered at 667 nm, and NIR centered at 760 nm were all due to two photons process, which originated from 5S2 (5F4)→5I8, 5F5→5I8, and 5S2 (5F4)→5I7 transitions, respectively. Keywords: chemical synthesis; nanoparticle; upconversion luminescence; rare earths

Increasing attention has been given recently to upconversion luminescence materials[1,2] due to their fascinating potential as biological fluorescence labels, infrared sensitive phosphors, or as upconversion laser. Upconversion is the generation of visible or UV light from lower energy radiation, usually NIR or IR, through the use of lanthanide and transition metal ions, such as Er3+, Tm3+, Ho3+, Nd3+, and Cr3+, doped into a solid state host (e.g., single crystals and glasses). However, single crystals are expensive and it is difficult to culture large single crystals, and glasses have poor chemical stability; therefore, it is important to search for a new solid state host. Recently, oxide nanocrystals as upconversion hosts have been intensively studied due to higher melting point and chemical stability than glasses and lower synthetical temperature and lower cost than single crystals, such as Y2O3 and Lu2O3[3–7]. Crystalline yttrium aluminum garnet YAG (Y3Al5O12) is not only a high-temperature structure material but also a very important laser material and a fluorescence material. Furthermore, YAG can be sintered into transparent ceramics, which makes it additionally attractive for practical applications[8–10]. At present, the low-temperature synthesis methods of nanocrystals YAG include coprecipitation[11–13], hydrothermal[14,15], sol-gel[16–18], and combustion reaction[19]. All these chemical processes achieve intimate mixing of re-

actant cations on the atomic level, leading to an increase in reaction rate and lowering synthesis temperature. But concerning the practical application of YAG powder, a large scale and inexpensive preparation process is required. Coprecipitation is one of the most promising techniques because air-stable and moisture-stable precursor powders can be prepared on a large scale in water rather than in organic chemicals without using special apparatus. The upconversion has been examined in YAG: Yb3+, Ho3+ single crystal pumping with a 978 nm diode laser but as a source of loss for the 2 μm Ho3+ emission[20]. In this article, YAG: Yb3+, Ho3+ nanocrystals were synthesized by a coprecipition method, and upconversion luminescence of the nanocrystals was investigated, which was similar to that in single crystals.

1 Experimental 1.1 Chemical preparation The Y3(0.95–x)Yb0.15Ho3xAl5O12 (x=0.005, 0.01, 0.02, 0.03, 0.05) nanocrystals were prepared by using a coprecipitation method. To obtain homogenous precipitate, reverse-strike (adding salt solutions to the precipitant solution) technique was adopted.

Foundation item: Project supported by the National Natural Science Foundation of China (50372075), Shanghai Light-Tech Project (036105021), and Singapore A*Star SERC (052 101 0039) Corresponding author: TANG Dingyuan (E-mail: [email protected]; Tel.: +65-67904337) DOI: 10.1016/S1002-0721(08)60193-3

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LIU M et al., Preparation and upconversion luminescence of YAG(Y3Al5O12): Yb3+, Ho3+ nanocrystals

According to the above, the aqueous solutions of aluminum, yttrium, ytterbium, and holmium nitrate were mixed together. This mixed aqueous solution was added dropwise into an aqueous solution of ammonium hydrogen carbonate under stirring. Then, filtration and washing with water and ethanol were repeated for several times. After drying at 70°C for 36 h, the precipitates were calcined at 800, 1000 1100, and 1200 °C in air for 1 h, respectively. The corresponding nanocrystals were obtained. 1.2 Characterization of physical properties Phase compositions of the nanocrystals were checked with powder X-ray diffraction (XRD, D/Max-2550V, Rigaku, Japan) using Cu Kα radiation (0.15406 nm) and nickel as the filter. The crystalline sizes of the nanocrystals were calculated following the Scherrer’s relation: D=kλ /(βcosθ) (1) where k=0.89, D represents crystalline size; λ is the wavelength of Cu Kα radiation; β is the corrected half width of the diffraction peak. The morphologies of nanocrystal powders were observed with field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan). The as-prepared nanocrystal powders were pressed into smooth flat pellet, the thickness of which was about 1.5 mm. The photoluminescence (PL) spectrum of the powders was measured at room temperature by spectrofluorometer (Fluorolog-3, Jobin Yvon, France). A 980 nm continuous wave diode laser was used as the the pump source. The induced light deviated a little from the focus onto the pellet, and the diameter of the exciting beam on the pellet was about 2 mm in order to avoid the influence of temperature caused by the exciting beam on upconversion luminescence.

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listed in Table 1, the average crystalline size of the powders calcined at 800, 1000, 1100, and 1200 °C is 40, 58, 71, and 90 nm, respectively. Fig.2 shows FESEM images of the nanocrystals calcined at 800, 1000, 1100, 1200 °C, respectively. Nanocrystals are spherical, and the particle size is uniform and increases from about 40 nm to 90 nm with the increase of temperature from 800 to 1200 °C. This result is in accordant with that of XRD calculated by Scherrer’s relation. Soft agglomeration can be observed in the samples heat-treated below 1100 °C. Nanocrystals are sintered and necks appear at 1200 °C, which decreases sinterability of powders, so it is concluded that 1100 °C is the maximal calcination temperature. 2.2 Upconversion spectrum Upconversion emission spectrum was measured on the YAG: Yb3+, Ho3+ powder calcined at 1100 °C under 980 nm excitation at room temperature. The excitation power is tuned to 306 mW. It can be seen in Fig.3 that YAG: Yb3+, Ho3+ emits green, red, and NIR light. The green emission centered at 549 nm corresponded to the 5S2 (5F4)→5I8 transitions of Ho3+ ions, and red emission centered at 667 nm and NIR emission centered at 760 nm originated from 5F5→5I8 and 5S2 (5F4)→5I7 transitions, respectively. The intensity of red and NIR emission is 5.9, 3.3 times smaller than that of green emission, respectively. Furthermore, the green luminescence can be observed clearly by naked eye even when the excitation power was as low as 25 mW. Fig.4 shows the influence of Yb3+ concentration on the intensity of upconversion luminescence in YAG: 5at.%Yb3+,

2 Results and discussion 2.1 Structural investigation X-ray diffraction patterns of the powders calcined at various temperatures are shown in Fig.1. The characteristic peaks of YAG appeared for the sample calcined at 800 °C, which was the lowest temperature reported for the synthesis of pure YAG phase. The peaks grew stronger, and the full width at half maximum of the peaks became narrower with the increase of temperature, reflecting the growth of YAG crystalline particle. Besides, XRD patterns showed that YAG was the only phase detected after heat treatment, and no YAP (YAlO3) and YAM (Y4Al2O9) phases were present, the formation of which was easy during other synthesizing method of YAG[21]. The crystalline sizes of the nanocrystals calcined at various temperatures are calculated by Scherrer's relation. As

Fig.1 XRD patterns of YAG: Yb3+, Ho3+ nanopowders calcined at various temperatures Table 1 Crystalline size calculated from Scherrer’s relation Peak

(4 2 0)

(2 1 1)

(6 3 1)

Avg.

800 °C

40

40

41

40

1000 °C

58

58

59

58

1100 °C

70

71

72

71

1200 °C

90

91

91

91

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JOURNAL OF RARE EARTHS, Vol. 27, No. 1, Feb. 2009

Fig.2 FESEM micrography of YAG: Yb3+, Ho3+ nanocrystals calcined at (a) 800 °C, (b) 1000 °C, (c) 1100 °C and (d) 1200 °C, respectively

Ho3+ nanocrystals. The intensity of upconversion luminescence was the strongest when 1at.% Ho3+ ions were codoped, and then the intensity decreased above this concentration. The possible reason was that the interaction between Ho3+ ions would transfer the energy to lattice defects, such as OH– and CO32–, and lattice defects may trap energy. To understand deeper the upconversion of YAG:Yb3+, Ho3+ nanocrystals, the luminescence intensity versus pump power for YAG:Yb3+, Ho3+ nanocrystals was investigated and is shown in Fig.5. The intensity of upconversion luminescence became stronger with the increase of pump power.

Fig.3 Upconversion emission spectra of YAG: Yb3+, Ho3+ nanocrystals calcined at 1100 °C under the excitation of 980 nm continuous wave diode laser

According to the following equation[22]: lnInlnP (2) where I represents the intensity of the visible upconverted luminescence; P, the near-infrared excitation power; index n, the number of photons required to populate the visible emitting states. lnI versus lnP for the Yb3+ and Ho3+ doped nanocrystals was plotted in Fig.5. The slopes are approximately equal to 2 for green emission centered at 549 nm, red emission centered at 667 nm, and infrared emission centered at 760 nm, respectively. It indicates that the upconversion luminescence involves a two-photon process. The mechanism of upconversion luminescence in Yb3+,

Fig.4 Upconversion luminescence of YAG: Yb3+, Ho3+ nanocrystals with different concentration of Ho3+ excited at 980 nm

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LIU M et al., Preparation and upconversion luminescence of YAG(Y3Al5O12): Yb3+, Ho3+ nanocrystals

Ho3+ doped materials pumped by a 980 nm continuous wave diode laser has been extensively studied[23,24], which is sketched in Fig.6. The wavelength of the diode laser (980 nm) matches the absorption transition between the ground state 2F7/2 and the excited state 2F5/2 of Yb3+ ions. Therefore, the Yb3+ ion absorbs one 980 nm photon and is excited to the 2F5/2 state, then transfers its energy to a Ho3+ ion in the ground state, thereby excites Ho3+ ion to 5I6 intermediate excited state, and the excess energy (about 1600 cm–1) is released to the matrix. Then, a second energy transfer from Yb3+ ion excites the same Ho3+ ion from the 5I6 level to the 5 S2 (5F4) emitting level. Consequently, bright green (centered at 549 nm) and NIR (centered at 760 nm) emissions are observed because of the radiative transitions of 5S2 (5F4)→5I8 and 5S2 (5F4)→5I7. The band around 667 nm was rather weak, which was assigned to 5F5→5I8 transition of Ho3+ ion. There are two possible ways for the population of 5F5 level. One possibility is the Ho3+ ion relaxed to the 5F5 state from 5S2 (5F4) state by a nonradiative process. The other is that the Ho3+ ion in the 5I6 state relaxed to the 5I7 state, then energy transfer from Yb3+ ion populates the 5F5 state. Finally, it emits red light centered at 667 nm by 5F5→5I8 transition.

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In the present study, red and NIR light were largely suppressed and green emission appeared bright to be seen by the naked eyes. As we know, materials doped with Yb3+ and Ho3+ have fascinating potential application in green upconversion laser, whereas the concentration of Yb3+ ions relative to Ho3+ ions imposes important effect on the properties of upconversion luminescence. Therefore, it is possible to obtain completely green emission by changing the concentration of Yb3+ ions relative to Ho3+ ions. Studies of this subject are currently underway.

3 Conclusion Pure YAG: Yb3+, Ho3+ nanocrystals were prepared by a coprecipition method at 800 °C, which was the lowest temperture reported. The crystal was spherical and the crystalline size was 40 nm. Under the excitation of 980 nm laser, YAG: Yb3+, Ho3+ nanocrystals emitted green, red, and NIR light. The green emission centered at 549 nm was the strongest, corresponding to the transition of 5S2 (5F4)→5I8 of Ho3+ ion. The intensity of red emission centered at 667 nm and NIR emission centered at 760 nm was 5.9 and 3.3 times smaller than that of green emission, respectively, which originated from the transitions of 5F5→5I8 and 5S2 (5F4)→5I7 of Ho3+ ion, respectively.

References:

Fig.5 Graph of lgI vs lgP for the Yb3+, Ho3+ codoped nanocrystals

Fig.6 Energy level of YAG: Yb3+, Ho3+ nanocrystals

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