SCIENCE CHINA Physics, Mechanics & Astronomy

© Science China Press and Springer-Verlag Berlin Heidelberg 2012 phys.scichina.com www.springerlink.com

*Corresponding author (email: congyan@dlnu.edu.cn)

• Article • August 2012 Vol.55 No.8: 1417–1421

doi: 10.1007/s11433-012-4813-7

Effects of Mo on phase structure and up-conversion emissions of Er:Al2O3 nanocrystals

LIU Dan & CONG Yan*

School of Physics and Materials Engineering, Dalian Nationalities University, Dalian 116600, China

Received April 9, 2012; accepted May 25, 2012; published online June 21, 2012

A simple and efficient approach was presented to enhance up-conversion emissions significantly for the Er:Al2O3 nanocrystals by Mo support (Er-Mo:Al2O3) with a 976 nm laser diode excitation. Mo support had evident effects on the phase structure and up-conversion emissions for the Er:Al2O3 nanocrystals, which promoted the -(Al,Er)2O3 transformed to-(Al,Er,Mo)2O3 phase. Compared with the Er:Al2O3, the maximal green and red up-conversion emissions intensities increased about 3×103 and 1.4×102 times for the Er-Mo:Al2O3 nanocrystals, respectively. It suggests that the enhancement of up-conversion emissions is caused by the high excited state energy transfer process from |4I15/2,

3T2> state of the Er3+-MoO42 dimer to the 4F7/2 level of

Er3+.

Er doped Al2O3, up-conversion emissions, Mo support, sol-gel method

PACS number(s): 81.20.Fw, 81.20.Ev, 81.20.Ka

Citation: Liu D, Cong Y. Effects of Mo on phase structure and up-conversion emissions of Er:Al2O3 nanocrystals. Sci China-Phys Mech Astron, 2012, 55: 14171421, doi: 10.1007/s11433-012-4813-7

1 Introduction

Up-conversion emissions of Er doped nanocrystals attracted much focus because of their potential applications, such as color display, optoelectronics, sensor technology, laser cooling and bioimaging [1–11]. In the 1960s, Auzel proved that optical pump efficiency of Er ions can be significantly enhanced by Yb ions sensitization [12]. With more than four-decades of investigation, the high efficient green up-conversion emissions have been realized successfully in the Er-Yb codoped fluorides due to the relatively low lattice phonon energy [13–15]. More recently, much research has been focused on the size/shape/phase-controlled synthesis, and the application potential in bio-imaging of the Er-Yb:NaYF4 nanocrystals [14,15].

Compared with fluorides, oxides have high chemical du-

rability, thermal stability and mechanical strength [16–19]. However, the up-conversion emissions efficiency of Er doped oxide materials is still relatively low because their corresponding high phonon energies failed to suppress the non-radiative loss, thus resulting in weak up-conversion emissions which has greatly hampered their applicability [20]. Thus, developing an alternative strategy to overcome the phonon quenching effect in Er doped oxide based up- conversion luminescent materials is of paramount im-portance for practical applications. In order to increase up-conversion emissions of Er ions in oxide matrix, many important investigations have been done, such as finding a new route to enhance the green UC emission by tailoring the Er ions local environment with Li ions [21], and in-creasing the 520 nm UC emissions through the excitation of surface plasmons supported by Ag islands [22].

We suggest a simple and efficient approach to enhance up-conversion emissions for Er doped Al2O3 nanocrystals by molybdenum support. The aim of this paper is not only

1418 Liu D, et al. Sci China-Phys Mech Astron August (2012) Vol. 55 No. 8

to develop a material with high up-conversion emissions efficiency and high stability, but also to explore a new up-conversion sensitizing approach for high-efficient up- conversion emissions of rare-earth ions doped oxides.

2 Experimental

The 0.1 mol% Er doped Al2O3 powders were prepared by a sol-gel method. The sol-gel method was performed by hy-drolyzing the aluminium isopropoxide [Al(OC3H7)3], which was given by chemical reaction between pure aluminium (99.99%) and isopropanol [(CH3)2CHOH] at 90°C under reflux and strong agitation for 2 h with a molar ratio of

110:1 of distilled water and Al(OC3H7)3. The reaction con-tainer was open in order to remove the (CH3)2CHOH pro-duced by hydrolysis reaction under strong agitation at 90°C. Nitric acid (HNO3) was added as catalyst to a molar ratio of 0.15:1 for H:Al ions. After the reaction further continued under reflux and strong agitation at 90°C for 16 h, a clear boehmite (-AlOOH) sol was obtained. Finally, Er ions were introduced by addition of Er(NO3)3·5H2O with the

molar ratio of 0.01:1 for Er:Al ions, and the Er doped -AlOOH sols were produced. The Er doped -AlOOH sols were dried at 100°C for 8 h to remove the solvent from the sols. The xerogels obtained were sintered at the tempera-tures of 1000°C with a heating rate of 5°C/min. The sinter-ing temperature was maintained for 1 h, and cooled down to room temperature in a furnace. The Er doped Al2O3 sintered were then milled into nanocrystals. The 0.1 mol% Er doped Al2O3 nanocrystals with 0–1 mol% Mo support were pre-pared as follows: Er:Al2O3 nanocrystals impregnated with aqueous solution of (NH4)6Mo7O24·4H2O (0.001 g/mL), and the wet samples were dried at 100°C, then heated to 1000°C, maintained for 1 h, and cooled down to room tem-perature in a furnace.

The phase structure of the samples was analyzed by a SHIMADZU XRD-6000 X-ray diffractometer (XRD) with Cu K radiation. The up-conversion emissions spectra in the wavelength range of 500–750 nm were obtained from the samples by a 976 nm semiconductor LD excitation. The up-conversion emissions from the samples were focused onto a single-monochromator, detected with a CR131 pho-tomultiplier tube associated with a lock-in amplifier. The spectral resolution of the experimental set-up is 0.1 nm. All spectroscopic measurements were carried out at room tem-perature.

3 Results and discussion

Figure 1 shows the XRD patterns of the 0.1 mol% Er:Al2O3

nanocrystals without and with Mo support. For the 0.1 mol% Er:Al2O3 nanocrystals, only the -(Al,Er)2O3 phase

(JCPDS No. 11-0517) was detected. After Mo support, the diffraction peaks of -(Al, Er, Mo)2O3 phase (JCPDS No. 43-1484) appeared to coexist with the -(Al, Er, Mo)2O3 phase. With the increase of Mo concentration above 0.5 mol%, the intensities of diffraction peaks of -(Al, Er, Mo)2O3 phase increased compared with that of -(Al, Er, Mo)2O3 phase, indicating that Mo support promoted the phase transformed to phase. It can be seen that Er and Mo existed in the - and -Al2O3 phases as the solution ele-ments. No precipitation of the other compounds composed of Er, Mo, Al and O was detected. The decrease of full width of half maximum (FWHM) of and phases indi-cated good crystallization with Mo support.

Figure 2 shows the up-conversion emissions spectra in the wavelength range of 500–750 nm for the Er:Al2O3 nanocrystals without and with Mo support. For the 0.1 mol% Er:Al2O3 nanocrystals, the green and red up-conversion emissions originated from the transitions of 2H11/2,

4S3/2→ 4I15/2 and 4F9/2→4I15/2 of Er ions, respectively.

After Mo support, the green and red up-conversion emis-sions band has no evident change, but stark split appeared in the characteristics of green and red up-conversion emissions

Figure 1 XRD patterns of the Er:Al2O3 nanocrystals with Mo support.

Figure 2 Up-conversion emissions spectra for the Er:Al2O3 nanocrystals with Mo support.

Liu D, et al. Sci China-Phys Mech Astron August (2012) Vol. 55 No. 8 1419

spectra. Because the -Al2O3 phaseis a defect spinel struc-ture and the -Al2O3 phase as a distortional structure of-Al2O3, the Er ions stochastically located in the octahe-dral interstitials of the O2 sublattice by replacement of Al ions and/or occupancy of vacant sites in the phase. The Er doping in the phase has multiplicity of the sites and envi-ronments because of the disordered nature of Er ions, lead-ing to the inhomogeneous broadening of the up-conversion emissions peaks. Thus, no appearance of Stark splitting in the emission spectra was observed. Moreover, compared with -Al2O3 phase, theparticles of-Al2O3 phase are larg-er and have a lower site symmetry [23], which may increase the transition probabilities of Er ions [24], thus leading to the enhancement of the up-conversion emissions intensities. The green and red up-conversion emissions intensity for the Er:Al2O3 nanocrystals with Mo support is shown in Figure 3. The emission intensities increased with Mo support. For the 0.1 mol% Er:Al2O3 nanocrystals with 0.5 mol% Mo support concentration, the green and red up-conversion emissions intensity enhanced to about 3×103 and 1.4×102 times higher than that of the Er:Al2O3 nanocrystals, respectively. It should be noted that the intense green up-conversion emis-sions were obtained, which can be observed by the unaided eye even at a laser pump power of only about 2 mW. By increasing the Mo support concentration up to 10 mol%, the pairing or aggregation of Mo ions may turn some of them into quenchers and thus induce a decrease of energy transfer (ET) efficiency and an increase of back ET.

The up-conversion mechanism for Er doped materials is well known. We propose a new up-conversion mechanism for the Er-Mo:Al2O3 in Figure 4. In the case of Er-Mo sys-tem, the sensitization through the Er3+-MoO4

2 dimer com-plex entails both ground state absorption (GSA) and excited state absorption (ESA). The Er3+-MoO4

2 dimer ground state

is represented by |4I15/2, 1A1>, the intermediate excited state

in the NIR by |4I11/2, 1A1>, and the relevant higher excited

states by |4I15/2, 3T1>, |4I15/2,

3T2>, |4I15/2, 1T1>, and |4I15/2,

1T2>,

respectively. Here the GSA (|4I15/2, 1A1>→ |4I11/2,

1A1>) is

followed by the ESA (|4I11/2, 1A1>→ |4I15/2,

1T1→). This combination of GSA+ESA is consistent with the recent finding in the Rb2MnCl4:Yb system [25,26]. However, un-like the Rb2MnCl4:Yb system where luminescence transi-tion occurs within the Yb-Mn dimer, here the two-photon absorption is followed by a high excited state energy trans-fer (HESET) to the Er ions. In the case of Er ions, the HESET from |4I15/2,

3T2> state of the Er3+-MoO42 dimer to

the 4F7/2 level of Er3+ occurs, resulting in intense green up-

conversion emissions. It suggests the HESET process has advantage in the phonon quenching avoidance, as well as the exchange enhancement of two-photon absorptions in the sensitization complex [27], which contributes to the ex-traordinary enhancement of green up-conversion emissions.

Figure 5 shows the intensity ratio of the green to red up-conversion emission (Igreen/Ired) for the Er:Al2O3 nano-crystals with Mo support. For Er:Al2O3, Igreen/Ired is about 0.7. After Mo support, the Ired/Igreen had an evident

Figure 3 Green and red up-conversion emissions intensity for the Er:Al2O3 nanocrystals with Mo support.

Figure 4 Up-conversion mechanism for the Er-Mo system.

Figure 5 Intensity ratio of the green to red up-conversion emission (Igreen/Ired) for the Er:Al2O3 nanocrystals with Mo support.

1420 Liu D, et al. Sci China-Phys Mech Astron August (2012) Vol. 55 No. 8

Figure 6 Green up-conversion emissions intensity of Er-Mo:Al2O3 and Er-Yb: Al2O3 relative to the pump power.

increase. Igreen/Ired is approximately 59 for Er:Al2O3 nano-crystals with 10 mL Mo support, which was almost entirely green up-conversion emission. The effect of Mo support on the up-conversion emission characteristic of Er ions is in-consistent of Yb on that of Er ions, which was reported in the Er-Yb codoped Y2O3 and TiO2 [28,29]. Herein, the evi-dent enhancement in the green up-conversion emission was caused by the high excited state energy transfer process from |4I15/2,

3T2> state of the Er3+-MoO42 dimer to the 4F7/2

level of Er3+. Figure 6 shows the green up-conversion emissions inten-

sity of Er-Mo:Al2O3 and Er-Yb: Al2O3 relative to the pump power. With the increasing pump power from 1100 mW to 1500 mW, the green up-conversion emissions intensity in-creased only about 3 times for the Er-Yb: Al2O3. However, when the pump current increased from 52 mW to 60 mW, the green up-conversion emissions intensity increased about 90 times to that of Er-Mo:Al2O3, which indicates that the Mo is an efficient sensitizer.

4 Conclusions

A simple and efficient approach was presented to greatly enhance up-conversion emissions for the Er:Al2O3 nano-crystals by Mo support with a 976 nm laser diode excitation. Mo support has evident effects on the phase structure and up-conversion emissions, which may promote the -Al2O3 transformation to-Al2O3 phase, and thus enhance the up-conversion emissions. Compared with Er:Al2O3,

the maximal green and red up-conversion emissions intensity increased about 3×103 and 1.4×102 times for the Er- Mo:Al2O3 nanocrystals, respectively. It suggests the great enhancement of up-conversion emissions may be caused by the highly excited state energy transfer process from |4I15/2, 3T2> state of the Er3+-MoO4

2 dimer to that of the 4F7/2 level of Er3+.

This work was supported by the National Natural Science Foundation of China (Grant No. 11004021) and the Fundamental Research Funds for the Central Universities (Grant Nos. DC12010117 and DC120101174).

1 Boyer J C, Cuccia L A, Capobianco J A. Synthesis of colloidal up-converting NaYF4: Er3+/Yb3+ and Tm3+/Yb3+ monodisperse nano-crystals. Nano Lett, 2007, 7: 847–852

2 Sivakumar S, van Veggel F C J M, May P S. Near-infrared (NIR) to red and green up-conversion emission from silica sol-gel thin films made with La0.45Yb0.50Er0.05F3 nanoparticles, hetero-looping-enhanced energy-transfer (Hetero-LEET): A new up-conversion process. J Am Chem Soc, 2007, 129: 620–625

3 Wang X, Liu X G. Upconversion multicolor fine-tuning: Visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles. J Am Chem Soc, 2008, 130: 5642–5643

4 Dong B, Cao B S, He Y Y, et al. Temperature sensing and in-vivo-imaging by molybdenum sensitised visible upconversion lu-minescence of rare-earth oxides. Adv Mater, 2012, 24(15): 1987– 1993

5 Wageh S, Zhao S L, Xu Z. An optical and structural investigation in-to CdTe nanocrystals embedded into the tellurium lithium borophos-phate glass matrix. Sci China-Phys Mech Astron, 2010, 53 (5): 818–822

6 Ding B F. Investigation of structural and magnetic properties of Ni implanted rutile. Sci China Phys Mech Astron, 2012, 55 (2): 247– 251

7 Wang M, Mi C C, Wang W X, et al. Immunolabeling and NIR-excited fluorescent imaging of HeLa Cells by using NaYF4:Yb, Er upconversion nanoparticles. ACS Nano, 2009, 3: 1580–1586

8 Wang X J, Yuan G, Isshiki H, et al. Photoluminescence enhancement and high gain amplification of ErxY2xSiO5 waveguide. J Appl Phys, 2010, 108: 013506

9 Qi H Z, Yan B, Li C K, et al. Synthesis and characterization of wa-ter-soluble magnetite nanocrystals via one-step sol-gel pathway. Sci China-Phys Mech Astron, 2011, 54(7): 1239–1243

10 Feng Z Q, Bai L, Cao B S, et al. Er3+-Yb3+ codoped borosilicate glass for optical thermometry. Sci China-Phys Mech Astron, 2010, 53: 848–851

11 Liu C B, Wei K F, Yao C F, et al. Investigation of microstructure modification of C-doped a-SiO2/Si after Pb-ion irradiation. Sci Chi-na-Phys Mech Astron, 2012, 55(2): 242–246

12 Auzel F. Compteur quantique par transfert denergie entre deux ions de terres rares dans un tungstate mixte ET dans un verre. C R Acad Sci (Paris), 1966, 262B: 1016–1019

13 Krämer K W, Biner D, Frei G, et al. Hexagonal sodium yttrium fluo-ride based green and blue emitting upconversion phosphors. Chem Mater, 2004, 16: 1244–1251

14 Wang G F, Peng Q, Li Y D. Upconversion luminescence of mono-disperse CaF2:Yb3+/Er3+ nanocrystals. J Am Chem Soc, 2009, 131: 14200–12401

15 Heer S, Kompe K, Güdel H U, et al. Highly efficient multicolour up-conversion emission in transparent colloids of lanthanide-doped NaYF4 nanocrystals. Adv Mater, 2004, 16: 23–24

16 Dong B, Feng Z Q, Zu J F, et al. Strong visible up-conversion emis-sions and thermometric applications of Er3+-Yb3+ codoped Al2O3 prepared by the sol-gel method. J Sol Gel Sci Tech, 2008, 48: 303–307

17 Dong B, Cao B S, Feng Z Q, et al. Optical temperature sensing through extraordinary enhancement of green up-conversion emissions for Er-Yb-Mo:Al2O3. Sens Actuators B, 2012, 165: 34–37

18 Li Z P, Dong B, He Y Y, et al. Selective enhancement of green up-conversion emissions of Er3+:Yb3Al5O12 nanocrystals by high excited state energy transfer with Yb3+-Mn2+ dimer sensitizing. J Lumin, 2012, 132: 1646–1648

19 Dong B, Li C R, Wang X J. Two color up-conversion emissions of Er3+-doped Al2O3 nanopowders prepared by non-aqueous sol-gel method. J Sol Gel Sci Technol, 2007, 44: 161–166

20 Dong B, Liu D P, Wang X J, et al. Optical thermometry through in-

Liu D, et al. Sci China-Phys Mech Astron August (2012) Vol. 55 No. 8 1421

frared excited green upconversion emissions in Er3+-Yb3+ codoped Al2O3. Appl Phys Lett, 2007, 90: 181117

21 Chen G Y, Liu H C, Somesfalean G, et al. Enhancement of the up-conversion radiation in Y2O3:Er3+ nanocrystals by codoping with Li+ ions. Appl Phys Lett, 2008, 92: 113114

22 Aisaka T, Fujii M, Hayashi S. Enhancement of upconversion lumi-nescence of Er doped Al2O3 films by Ag island films. Appl Phys Lett, 2008, 92: 132105

23 Ayyub P, Palkar V R, Chattopadhyay S, et al. Effect of crystal size reduction on lattice symmetry and cooperative properties. Phys Rev B, 1995, 51: 6135–6138

24 Patra A, Friend C S, Kapoor R, et al. Effect of crystal nature on up-conversion luminescence in Er3+:ZrO2 nanocrystals. Appl Phys Lett, 2003, 83: 284–286

25 Reinhard C, Valiente R, Güdel H U. Exchange-induced upconversion in Rb2MnCl4: Yb3+. J Phys Chem B, 2002, 106: 10051–10057

26 Gerner P, Reinhard C, Güdel H U. Cooperative near-IR to visible photon upconversion in Yb3+-doped MnCl2 and MnBr2: Comparison with a series of Yb3+-doped Mn2+ halides. Chem Eur J, 2004, 10: 4735–4741

27 Tanabe Y, Moriya T, Sugano S. Magnon-induced electric dipole transition moment. Phys Rev Lett, 1965, 15: 1023–1025

28 Vetrone F, Boyer J C, Capobianco J A, et al. Significance of Yb3+ concentration on the upconversion mechanisms in codoped Y2O3:Er3+, Yb3+ Nanocrystals. J Appl Phys, 2004, 96: 661–667

29 Chen S Y, Ting C C, Hsieh W F. Comparison of visible fluorescence properties between sol-gel derived Er3+-Yb3+ and Er3+-Y3+ co-doped TiO2 films. Thin Solid Films, 2003, 434: 171–177