17110
J. Phys. Chem. C 2007, 111, 17110-17117
Enhancing the Up-Conversion Emission of ZrO2:Er3+ Nanocrystals Prepared by a Micelle Process T. Lo´ pez-Luke,†,‡ E. De la Rosa,*,† P. Salas,§ C. Angeles-Chavez,§ L. A. Dı´az-Torres,† and S. Bribiesca‡ Centro de InVestigaciones en Optica, A. P. 1-948, Leo´ n Gto., 37160, Me´ xico, IIM, UMSNH, C. U., Morelia, Michoacan 58060, Me´ xico, and Instituto Mexicano del Petro´ leo, Ciudad Me´ xico D. F. 07730, Me´ xico ReceiVed: April 16, 2007; In Final Form: August 10, 2007
Erbium doped ZrO2 nanocrystals ranging from 30 to 80 nm in size were prepared by a sol-gel process with the presence of cationic (CTAB) and nonionic (PLURONIC P-127) surfactants, and the up-conversion emission was characterized. A strong green emission was produced by the transition 2H11/2 + 4S3/2 f 4I15/2 and enhanced 1 order of magnitude with the presence of PLURONIC P-127 at a molar ratio Mrp ) 0.0082 annealed 5 h at 1000 °C. A molar ratio of Mrc ) 0.2 CTAB annealed 0 min at 1000 °C induced an increment of 300% of the signal emitted. In both cases the enhancement was partly attributed to the high content of monoclinic phase and crystallite size. However, the dominant parameter for the enhancement of the luminescence was the elimination of residual contaminants and defects produced for surface recombination.
1. Introduction Nanostructured materials exhibit physical properties different from bulk counterparts that are very attractive for photonics applications.1,2 Nanomaterials doped with rare earth ions presenting up-conversion processes (UPCs) provide great challenges for fundamental research and applications. The excitation dynamic of active ions is influenced by the nanoscopic interaction that can induce an enhancement of the fluorescence emission. Up-converted nanophosphors produce visible emission in a two or more photon process and create opportunities for new technology development such as optoelectronics that include lighting, displays, lasers, and security.3-5 Special opportunities are found in biotechnology.6-8 In this case, the higher penetration through the tissue of a near-infrared pumping source is an advantage compared with the UV source used for a down-conversion process, in particular with quantum dots.9,10 Therefore, special attention has been paid to the study of rare earth doped oxide nanocrystals producing visible emission by UPCs. The Er3+ and codoped Yb3+/Er3+ system is one of the most studied in different matrixes.11-15 The UPC process has received special attention in rare earth doped ZrO2 nanocrystals. The low phonon energy (470 cm-1) of this host increases the probability of radiative transitions of rare earth ions enhancing the cooperative processes. Strong visible emission on Er3+ and Yb3+/Er3+ doped nanocrystals has been demonstrated.16-20 More recently, strong blue and green luminescence in the Yb3+/Tm3+ and Yb3+/Ho3+ systems and tunability between red and green in the Yb3+/Er3+ system have been reported.20-22 Oxide nanocrystals have been prepared for different processes, namely, sol-gel process,18 hydrothermal process,23 flame spray pyrolysis,24 combustion,25 and microemulsions.26 Most of those cases have focused on optimizing the phase composition, crystallite size, and morphology in order to enhance lumines* Corresponding author. E-mail:
[email protected]. † Centro de Investigaciones en Optica. ‡ UMSNH. § Instituto Mexicano del Petro ´ leo.
cence. Perhaps the most controversial is the crystallite size effect on the luminescence. It has been reported that the luminescence is increased by reducing the crystallite size.27-29 Such behavior has been attributed to the absence of contaminant impurities, mainly OH and atmospheric CO2. However, if impurities are present the opposite occurs; i.e., luminescence increases as crystallite size increases.30,31 The former process was explained considering that for the same concentration of activator most of the ions are close to or at the surface of the nanocrystal; then the interaction of luminescent centers with the surface will be stronger and the presence of traps due to contaminants will quench luminescence. The latter process was explained in terms of the elimination of contaminants that usually happens increasing the annealing temperature that in turn increases the crystallite size. The increment of signal emitted by reducing the crystallite size was also explained by arguing a more efficient excitation of activators close to the surface, in opposition to a large nanocrystal where activators can be located deep inside the nanoparticle.29 This explanation is acceptable for electron excitation where penetration depends on its energy. The large surface area for small nanocrystals induces a large concentration of traps or defects associated with the presence of impurities and surface recombination defects; those defects are reduced by increasing the crystallite size, which means a higher signal emitted as was described above. More recently, it has been reported that both crystallite size and contaminants did not enhance the luminescence but instead the removal of strain did.25 Thus, at this stage it is not completely understood how the luminescence is increased on reducing the crystallite size. In the meantime, different surfactants have been used to obtain monodispersed nanoparticles controlling the shape and size and acting against aggregation, looking for the optimization of the signal emitted.32-35 However, to our knowledge, no systematic characterization has been provided for rare earth doped oxide nanocrystals. In a recent work we reported preliminary results showing that the uses of cetyltrimethylammonium bromide (CTAB) during the synthesis process enhance the up-converted green emission of ZrO2:Er3+ nanocrystals.36 In the present work,
10.1021/jp072957v CCC: $37.00 © 2007 American Chemical Society Published on Web 10/19/2007
Up-Conversion Emission of ZrO2:Er3+ Nanocrystals
J. Phys. Chem. C, Vol. 111, No. 45, 2007 17111
we report a systematic study and demonstrate that surfactant (CTAB and PLURONIC P-127) helps to reduce residual contaminants, avoid aggregation, control crystallite size, and in the proper concentration enhance 1 order of magnitude the up-converted signal emitted. 2. Experimental Section 2.1. Sample preparation. Samples were prepared by using the sol-gel method. All chemicals used were of reactant grade and were supplied by Aldrich, Inc. Nanoparticles of ZrO2:Er were obtained by mixing zirconium n-propoxide and erbium nitrate pentahydrate with a molar composition of 0.2 mol % Er2O3 with respect to ZrO2. Care was taken in the addition of Er to guarantee the same concentration in all prepared samples. In a typical preparation, 0.043 g of erbium nitrate was dissolved in 57 mL of ethanol and 10.9 mL of zirconium n-propoxide. After complete dissolution, 1.5 mL of nitric acid, 0.6 mL of hydrochloric acid, and 1.7 mL of distilled water were added. The surfactant, cetyltrimethylammonium bromide (CTAB) or PLURONIC P-127, was added 10 min later under strong stirring conditions. The former is a well-known cationic surfactant with one positive charge termination, and PLURONIC is a nonionic surfactant with two hydrophilic terminations. A detailed explanation of characteristics is provided later. Surfactant was added at different molar ratios (Mrc ) CTAB/ZrO2 ) 0, 0.2, 0.4, 0.6, 0.8, and 1.0; Mrp ) PLURONIC/ZrO2 ) 0, 0.0008, 0.0017, 0.0041, 0.0082, and 0.01). The mixed solution was stirred for 1 h, and the resulting suspension was transferred into sealed autoclaves. Hydrothermal treatment was carried at 80 °C for 24 h. After that, the autoclave was allowed to cool naturally and the gel was washed twice with absolute ethanol. All samples were annealed at 1000 °C and removed from the furnace as soon as a temperature was reached; we will denote this annealing time 0 min. A couple of samples were annealed at the same temperature for 5 h. A sample with Mrc ) 0.2 was also annealed at 800 and 940 °C for 0 min. In all cases, the heating rate was 5 °C/min and stayed at 300 and 500 °C for 2 h. The temperature was raised again to reach the final temperature as explained above. 2.2. Structural and Morphology Characterization. The X-ray diffraction (XRD) patterns were obtained using SIEMENS D-5005 equipment provided with a Cu tube with KR radiation at 1.5405 Å, scanning in the 15°-100° 2θ range with increments of 0.02° and a sweep time of 8 s. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed in a Tecnai G2 F30 S-Twin transmission electron microscope operating at 300 kV. The microscope is equipped with a Schottky-type field emission gun and an S-Twin objective lens (Cs ) 1.2 mm; Cc ) 1.4 mm; point-to-point resolution, 0.20 nm). Chemical analysis was performed by X-ray energy dispersive spectroscopy (EDAX). Chemical mapping was obtained combining scanning transmission electron microscopy and X-ray energy dispersive spectroscopy techniques. Mapping was performed with a spectral resolution of 1.486 keV, spatial resolution of 3 nm, beam diameter of 1 nm, and detection limit of 1 wt %. Samples were suspended in isopropyl alcohol at room temperature and dispersed with ultrasonic stirring, and then aliquots of the solution were dropped on a 3 mm diameter lacey carbon copper grids. The Fourier transform infrared (FTIR) spectra were obtained using a Spectrophotometer Spectrum BX, FTIR system from Perkin-Elmer with a DTGS detector at 4 cm-1 spectral resolution and Beer-Norton anodization. Measurements were performed in the attenuated total reflectance (ATR) mode using 100 mg of ZrO2:Er powder covering the
Figure 1. X-ray diffraction patterns. (a) Mr ) 0, 0 min at 1000 °C; (b) Mrc ) 0.2, 0 min at 800 °C; (c) Mrc ) 0.2, 0 min at 1000 °C; (d) Mrp ) 0.0082, 5 h at 1000 °C.
whole active area of the ATR device. The spectrum was obtained in the medium-infrared region from 1000 to 4000 cm-1 with 20 scans per spectra. Before measurements, the equipment was calibrated by verifying that the power energy was ∼80% and the spectral response was calibrated with a polystyrene film as a reference. 2.3. Photoluminescence Characterization. The photoluminescence (PL) characterization was performed using a CW semiconductor laser diode with a 350 mW pumping source centered at 968 nm. The fluorescence emission was analyzed with an Acton Pro 500i monochromator and a R955 Hamamatsu photomultiplier tube connected to a mode-locking amplifier SR360 (Stanford). All measurements were done at room temperature. Nanopowder was supported in a capillary tube with a diameter of 1 mm in order to guarantee the same quantity of excited material. Special care was taken to maintain the alignment of the setup in order to compare the intensities of the up-converted signal between different characterized samples. The reproducibility was proven, and based on this an uncertainty of (7% was estimated in the measurement of the signal emitted. 3. Results and Discussion 3.1. Structural Characterization. The representative XRD patterns for different samples are shown in Figure 1. According to these patterns and for samples annealed at 1000 °C for 0 min, the crystalline phase for samples prepared without surfactant is dominated by the monoclinic (∼90 wt %) with a small amount (
