Spectral analysis of synthesized nanocrystalline aggregates of Er 3+ :Y 2 O 3

Transcription

1 JOURNAL OF APPLID PHYSICS 101, Spectral analysis of synthesized nanocrystalline aggregates of r 3+ :Y 2 O 3 John B. Gruber, Dhiraj K. Sardar, a Kelly L. Nash, and Raylon M. Yow Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas Waldemar Gorski and Maogen Zhang Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas Received 17 November 2006; accepted 11 April 2007; published online 15 June 2007 The absorption and fluorescence spectra of nanocrystalline aggregates of Y 2 O 3 doped with r 3+ are reported between 8 K and room temperature. The nanocrystalline particles were synthesized from a homogenous solution of the metal ions and urea at elevated temperatures in order to control the precipitation of the mixed hydroxides by a slow uniform reaction throughout the solution. The morphology of the calcinated materials revealed uniformly spherical aggregates 200 nm or less depending on the ratio of the metal ions in the initial solution. Spectra obtained from these particles were analyzed in detail for the crystal-field splitting of the 2S+1 L J multiplet manifolds of r 3+ 4f 11 including the ground-state manifold 4 I 15/2, and excited manifolds 4 I 9/2, 4 F 9/2, 4 S 3/2, 2 H 11/2, 4 F 7/2, 4 F 5/2, and 4 F 3/2. Fluorescence lifetimes and results from an analysis of the intensities of manifold-to-manifold transitions are also reported. The sharp-line absorption and emission spectra are comparable to spectra reported earlier for r 3+ :Y 2 O 3 grown as large single crystals by a flame fusion method. The results described in the present study suggest that the simple, inexpensive method of preparation that is reported will lead to further investigation of these nanocrystals for their optical properties, especially in the near infrared region of the spectrum American Institute of Physics. DOI: / I. INTRODUCTION Synthesis and optical characterization of luminescing materials in their nanocrystalline form have led to their growing use in numerous military and medical applications, such as imaging, range finding, flash lidar, and remote sensing. 1 3 The fluorescing properties of some of these nanoparticles provide labels that allow for time resolved fluorescence imaging for quantitative detection for antigens, as well as tissue-specific transcripts for noninvasive, real time, in vivo diagnosis of various diseases. 4 In addition to the widespread use of luminescent organic dyes, semiconductor quantum dots have also attracted attention as fluorescent tags, but the toxicity of the host limits its usefulness in biological applications. 5,6 An alternative to the use of fluorescing organic dyes and quantum dots that have the prerequisite properties as optical probes, yet lack the toxic nature of the organic or semiconducting host, is the nanoparticles and their aggregates of rare earth R ion-doped oxides that are capable of attaching to a variety of surfaces, physically and chemically. 3,7 Among the R ions, some have strong sharp-line emission in the near infrared produced by excitation of energy levels in the visible and ultraviolet. The large energy gap between absorption and emission allows for high sensitivity detection at wavelengths transparent in tissue. The high quantum yields, relatively long fluorescence lifetimes, and photostability of R ion-doped oxide nanoparticles have been demonstrated recently using u 3+ as the taggant. 8 Different R ions in a Author to whom correspondence should be addressed; electronic mail: dsardar@utsa.edu Y 2 O 3 and Gd 2 O 3 nanoparticle aggregates excited by a common excitation source demonstrate a useful approach to multiwavelength immunoassay analyses. 9,10 In the present study we report a simple inexpensive method to produce aggregates of nanoparticles of r 2 O 3 and r 3+ doped into Y 2 O 3 that are spherical in shape with dimensions as large as 200 nm. Aggregate sizes can be reduced depending on the relative concentration of r 3+ and Y 3+ in the starting solutions. Synthesis of the nanocrystalline aggregates begins with the precipitation of the metal hydroxides/ oxides from a homogenous solution containing the metal ions and urea that is stirred and heated to 95 C for 40 min. The elevated temperature provides reaction control to the precipitation process so that a slow uniform reaction is achieved throughout the solution. The precipitates are then washed, dried, and calcinated to convert the material to the oxide form. Absorption and fluorescence spectra of r 3+ :Y 2 O 3 were obtained from the synthesized aggregates of nanoparticles at wavelengths between 400 and 820 nm between 8 K and room temperature. The detailed structure in the observed bands was identified as transitions between individual energy Stark levels associated with the 2S+1 L J manifolds of the electronic energy configuration of r 3+ 4f 11. The intensities of both the emission and absorption spectra were investigated along with the crystal-field splitting of individual manifolds. The measured room temperature fluorescence lifetime at 1.5 m 4 I 13/2 4 I 15/2 is reported and compared with a calculated value. The lifetime of the manifold transition between 4 S 3/2 and 4 I 15/2 is calculated and compared with the value found in the literature. Other spectroscopic properties obtained in this study of r 3+ :Y 2 O 3 are also compared to /2007/ /113116/6/$ , American Institute of Physics

2 Gruber et al. J. Appl. Phys. 101, TABL I. Absorption spectrum of r 3+ in nanocrystalline r 3+ :Y 2 O 3. Spectrum obtained at approximately 25 K; 2 H 11/2 manifold obtained at room temperature to show hot-band transitions from 4 I 15/2. 2S+1 L J a nm b O.D. c Trans. d cm 1 e cm 1 f cm 1 expt. g cm 1 calc h N n i 4 I 9/ Z 4 W Z 3 W Z 2 W Z 1 W W Z 4 W Z 3 W Z 2 W Z 4 W Z 1 W W Z 2 W Z 4 W Z 3 W sh 0.12 Z 1 W W Z 4 W Z 3 W Z 2 W Z 2 W Z 1 W W Z 1 W W 5 4 F 9/ Z 3 V Z 2 V Z 4 V Z 1 V V Z 2 V Z 1 V V Z 2 V Z 3 V Z 1 V V Z 2 V Z 1 V V Z 2 V Z 1 V V 5 4 S 3/ Z 4 U Z 3 U Z 2 U Z 1 U U Z 3 U Z 2 U Z 1 U U 2 2 H 11/ Z 7,8 T 1, Z 7,8 T Z 7,8 T Z 7,8 T Z 6 T 1, Z 6 T Z 6 T Z 5 T Z 5 T Z 5 T Z 4 T sh 0.40 Z 3 T Z 3 T Z 4 T Z 2 T sh 0.72 Z 2 T Z 1 T T 1

3 Gruber et al. J. Appl. Phys. 101, TABL I. Continued. 2S+1 L J a nm b O.D. c Trans. d cm 1 e cm 1 f cm 1 expt. g cm 1 calc h N n i 525 sh 0.56 Z 1 T T Z 1 T T sh 0.68 Z 4 T Z 3 T Z 2 T sh 0.78 Z 2 T Z 1 T T sh 0.43 Z 2 T Z 1 T T Z 1 T T 6 4 F 7/ Z 4 S Z 3 S Z 2 S Z 1 S S Z 3 S Z 2 S Z 1 S S Z 2 S Z 1 S S Z 2 S Z 1 S S 4 4 F 5/ Z 4 R Z 3 R Z 2 R Z 1 R R Z 1 R R Z 1 R R 3 4 F 3/ Z 1 Q Q Z 1 Q Q 2 a Multiplet manifolds of r 3+ 4f 11. b Wavelength in nanometers. c Optical density. d Transitions from Stark levels in the ground-state manifold, 4 I 15/2, to excited Stark levels. e nergy of transitions in vacuum wave numbers. f nergy difference representing the energy of the ground-state 4 I 15/2, splitting. g xperimental energy level. h Calculated energy level. i Label of terminal Stark level. values reported for the same material grown in bulk amounts by a flame fusion method. 11 The similarity between the two sets of r 3+ energy levels, intensities, and lifetimes indicates that synthesized nanocrystalline aggregates of 200 nm or less have the same spectroscopic properties as crystals grown in bulk. The photostability of r 3+ in Y 2 O 3 is an important property in its application in nanocrystalline form. II. SAMPL PRPARATION, MORPHOLOGY, AND OPTICAL CHARACTRIZATION Nanoparticles of r 2 O 3, Y 2 O 3, and r 3+ -doped Y 2 O 3 were prepared by a slow precipitation method from a homogenous solution of dissolved rcl 3 and YCl 3 Aldrich, 99.99% and urea. The generation of OH ions comes from the decomposition of urea, CO NH H 2 O CO 2 +2NH OH, as the solution is heated and stirred at 95 C for 40 min. The slow generation of hydroxide ions in a homogenous solution results in a precipitation of aggregates of mixed metal hydroxide/oxide nanoparticles. The precipitates were filtered from the solution, washed with distilled water and ethanol, dried at 80 C, and calcinated at 900 C for over an hour to convert all solid material to the oxide form. The method so described is easy and inexpensive relative to other methods reported The presence of erbium and yttrium and their relative amounts in the calcinated material were determined using energy dispersive x-ray microanalysis. Separate chemical and optical methods confirmed these results and indicated that the r/y ratios were within 10% relative to the proportions of the ions in the initial solutions. The detailed spectral analysis reported in the present study confirms the crystal 1

4 Gruber et al. J. Appl. Phys. 101, structure as cubic by its similarity to the spectra of r 3+ :Y 2 O 3 in bulk crystals, whose structure was investigated in detail earlier by Kisliuk et al. 11 The cubic structure of Y 2 O 3 also called the bixbyite structure has two different metal cation sites: 1 the C 2 site, which has noninversion symmetry that allows for forced electric-dipole transition within the 4f 11 configuration, and 2 the C 3i site which has inversion symmetry, so that only magnetic electronic transitions are allowed, which are further reduced by selection rules. In Y 2 O 3 there are three times as many C 2 sites 24 as C 3i sites 8 in the unit cell. Chang et al. 15 and Gruber et al. 16 reported observing transitions in both sites. In r 3+ magnetic transitions were observed only between 5 I 15/2 and 4 I 13/2. 16 Chang et al. 15 analyzed the r 3+ spectra in the C 2 sites and postulated that the r 3+ ions are statistically distributed between the two sites based on their study of intensities and lifetimes. The agreement between our spectroscopic data and theirs suggests that the r 3+ ion is also found randomly in the two sites in the nanocrystalline aggregates. The morphology of the particles was determined by methods of scanning electron microscopy SM, which showed the particles to be uniform in size and spherical in shape, suggesting that polycrystalline aggregates of nanocrystals are formed. Particles having a diameter of about 200 nm could be made smaller depending on the relative proportion of the rcl 3 and YCl 3 salts in the solution prior to the onset of precipitation as well as the rate of formation of the OH ions. The size of individual nanocrystals and the interpretation of both the SM and the transmission electron microscope TM results that pertain to the morphology of the structures investigated in the present study are reported in a separate publication. 14 The aggregates were prepared for optical studies by first mixing the aggregate powder with epoxy in a 1 to 2 ratio by mass. The mixture was then spread on a 1.0 mm glass microscope slide, cured, and polished for optical measurements. Absorption spectra were obtained with an upgraded Cary Model 14R controlled by a desktop computer. The spectral bandwidth was set to 0.5 nm and the internal calibration of the instrument was 0.5 nm. Data were obtained between 400 and 820 nm. Difficulties with our detector precluded reporting the wavelength measurements further into the infrared to observe the 4 I 11/2 manifold 975 nm and the 4 I 13/2 manifold 1550 nm. However, the similarity between the crystal-field splitting reported in this study and the splitting of energy levels of r 3+ in large single crystals of Y 2 O 3 Ref. 15 suggests that the manifold splitting of 4 I 11/2 and 4 I 13/2 in this study is similar to the splitting for each of these manifolds in large single crystals. 11 The fluorescence spectra reported in this study also confirm this conclusion. Fluorescence was obtained using a SPX Model 1250M monochromator that had a resolution of 0.01 nm. For lowtemperature studies, the samples were mounted on a cold finger of a CTI Model 22 closed-cycle helium cryogenic refrigerator that operated between 8 and 300 K. The sample temperature was monitored by a Lake Shore control system. Fluorescence lifetimes were measured by chopping the 488 nm emission line from an argon-ion laser. The signal FIG. 1. The absorption spectrum of the 4 I 9/2 manifold obtained at approximately 25 K. Transitions are observed from Stark levels Z 1, Z 2, Z 3,andZ 4 to Stark levels in excited manifolds of r 3+. was detected by a photomultiplier tube attached to the exit slit of the monochromator and sent to a 150 MHz Tektronix oscilloscope Model 2445A having a resolution of 10 ns. III. ANALYSIS OF TH SPCTRA AND SUMMARY The absorption spectrum of nanocrystalline r 3+ :Y 2 O 3 is reported between 400 and 820 nm in Table I at a sample temperature of approximately 25 K. The spectrum represents transitions from individual energy Stark levels of the ground-state manifold, 4 I 15/2 to excited manifolds 4 I 9/2, 4 F 9/2, 4 S 3/2, 2 H 11/2, 4 F 7/2, 4 F 5/2, and 4 F 3/2. In addition to transitions from the ground-state Stark level Z 1, transitions are also observed from excited Stark levels Z 2 =39, Z 3 =77, and Z 4 =89, in units of cm 1. Transitions from excited Stark levels are described as temperature-dependent or hot band spectra and are identified by changes in intensity of the absorption bands due to population changes in the excited Stark levels as spectra are recorded at higher temperatures 80, 120, 200, and 300 K. The energy difference given in col- FIG. 2. The absorption spectrum of the 4 F 9/2 manifold obtained at approximately 25 K. Transitions are observed from Stark levels Z 1, Z 2, Z 3,andZ 4 to Stark levels in excited manifolds of r 3+.

5 Gruber et al. J. Appl. Phys. 101, FIG. 3. The absorption spectrum of the 4 F 7/2 manifold obtained at approximately 25 K. Transitions are observed from Stark levels Z 1, Z 2, Z 3, and Z 4 to Stark levels in excited manifolds of r +3. umn 6 of Table I represents the energy of the individual Stark levels in the 4 I 15/2 manifold relative to the ground-state Stark level Z 1. xamples of transitions from the excited Stark levels of the 4 I 15/2 manifold are shown in the spectra of 4 I 9/2 Fig. 1, 4 F 9/2 Fig. 2, and 4 F 7/2 Fig. 3 for samples at 5 K. That the absorption spectra remain sharp and well defined at higher temperatures can be seen in Fig. 4, the spectrum of the 2 H 11/2 manifold recorded at room temperature. In this figure one observes transitions from all the Stark levels of the ground-state manifold to the six Stark levels of the 2 H 11/2. The assignments made to the splitting of the 4 I 15/2 manifold in Fig. 4 are confirmed by an analysis of the fluorescence spectrum of the sample from 4 S 3/2 to 4 I 15/2 obtained at approximately 9 K. A 280 mw argon-ion laser emitting at FIG. 4. The absorption spectrum of the 2 H 11/2 manifold obtained at room temperature. Transitions are observed from all Stark levels of 4 I 15/2 to the six Stark levels of the 2 H 11/2 manifold. 488 nm was used to excite the 4 F 7/2 manifold shown in Table I between 491 and 486 nm. Rapid relaxation of the energy in the nanocrystals to the 4 S 3/2 manifold results in emission from the two Stark levels U 1 and U 2 to the Stark levels of the 4 I 15/2 manifold. Table II lists the spectra, energy of the transitions, and the assignments given to the fluorescence shown in Fig. 5. The analyses of the spectra presented in Tables I and II give a manifold Stark splitting in agreement with the splitting reported for the same manifolds of r 3+ in large single crystals of r 3+ :Y 2 O 3 grown by a flame fusion technique reported by Kisluik et al. 11 We conclude that the optically active local symmetry environment of r 3+ is the same in the nanocrystals as it is in the bulk phase. In Table I, col. 8 we list the calculated crystal-field split- TABL II. Fluorescence spectrum from 4 S 3/2 to 4 I 15/2. Sample temperature 9 K; excitation at 488 nm; 260 mm Ar ion laser. 2S+1 L J a nm b I c Trans. d cm 1 e cm 1 f cm 1 expt. g cm 1 calc h 4 I 15/ U 2 Z U 2 Z U 2 Z U 2 Z U 1 Z U 2 Z U 2 Z sh 4.28 U 1 Z U 1 Z U 1 Z U 2 Z U 1 Z U 2 Z 7, , U 1 Z 7, , 507 a Terminal manifold. b Wavelength in nanometers. c Relative intensity. d nergy of transitions in vacuum wave numbers. e nergy difference between initial and terminal levels. f Transition between energy Stark levels. g xperimental Stark energy level. h Calculated Stark energy level.

6 Gruber et al. J. Appl. Phys. 101, FIG. 5. The fluorescence spectrum representing transitions from the Stark levels, U 1 and U 2 of 4 S 3/2 to the Stark levels of the 4 I 15/2 manifold. Sample temperature was approximately 9 K. ting obtained for large crystals of r 3+ :Y 2 O 3 reported by Chang et al., 15 who listed the details of the crystal-field splitting calculation. They also listed the set of phenomenological crystal-field parameters obtained by assuming that the r 3+ ion replaces the Y 3+ ion in the C 2 metal site in Y 2 O 3. The second metal site having C 3i inversion symmetry is presumed to be statistically occupied as well. However, the inversion symmetry of this site precludes the observation of forced electric-dipole transitions between the Stark levels of the manifolds reported in this study. The intensities of the room temperature absorption spectrum of the r 3+ 4f 11 manifolds listed in Table I were used in a Judd-Ofelt analysis to determine the radiative lifetimes of the states whose fluorescence is of interest for potential applications. 17,18 The authors have reported the methodology for such an analysis earlier. 19,20 Since the spectroscopic properties of the nanocrystals and the large single crystals of r 3+ :Y 2 O 3 are similar, we used the index of refraction reported in the literature 15 and the concentration of r 3+ as described earlier in this study. The phenomenological intensity parameters were determined as 2 = cm 2, 4 = cm 2, and 6 = cm 2 for the r 3+ :Y 2 O 3 nanoparticles. These values can be compared to those reported by Krupke 21 for large crystal samples grown by a flame fusion technique as: 2 = 4.59± cm 2, 4 = 1.21± cm 2, and 6 = 0.48± cm 2. The reasonable agreement between the two sets of parameters within experimental error also supports the validity of our method to establish the sample density and the number of r 3+ ions/cm 3 in the nanocrystalline aggregates. Also, following the Judd-Ofelt analysis, 17,18 we have calculated the radiative lifetimes for the manifold transition 4 S 3/2 to the ground-state manifold 4 I 15/2 and the manifold transition 4 I 13/2 to the ground-state manifold 4 I 15/2, and found them to be approximately 4.6 and 76 ms, respectively. If instead, we use Krupke s parameters 21 for the 4 I 13/2 4 I 15/2 manifold-to-manifold transition, we obtain a somewhat longer lifetime of about 96 ms. These calculated values can be compared with experimental values. The measured lifetime for the 4 S 3/2 4 I 15/2 transition reported by Jia et al.. 22 is approximately 25 s, which suggests that nonradiative mechanisms are involved. On the other hand, for the 4 I 13/2 4 I 15/2 manifold-to-manifold transition, we have measured a room temperature lifetime of 45 ms. For this transition, it appears that the nonradiative contribution to the lifetime is relatively small, and that a quantum efficiency of about 60%, using the calculated value based on our parameters, is quite acceptable for numerous applications. Comparing the measured fluorescence lifetime for the 4 I 13/2 transition to 4 I 15/2 with r 3+ in other oxide hosts, such as Y 2 Al 5 O ms, suggests the potential for the ion not only as an activator ion in the near infrared, but also its potential as a phosphor. In summary, we report an easy and inexpensive method to prepare nanocrystalline aggregates of Y 2 O 3 doped with r 3+ ions. A detailed spectroscopic analysis of the aggregates indicates that the material has optical properties similar to those reported for single crystals grown by a flame fusion method. 11,15 Photostability is maintained in the nanocrystalline aggregates. The ability of these aggregates to attach to different surfaces by either chemical or physical means renders them useful in numerous technologies. 3 ACKNOWLDGMNT This research was supported by the National Science Foundation Grant No. DMR A. A. Kaminskii, Crystalline Lasers: Physical Processes and Operating Schemes CRC, New York, R. C. Powell, Physics of Solid-State Laser Materials AIP, New York/ Springer, New York, J. B. Gruber, DARPA Review AM03, Arlington, VA, D. Dosi, M. Lui, B. Guo, G. Y. Lui, B. D. Hammock, and I. M. Kennedy, J. Biomed. Opt. 10, A. M. Derfus, W. C. W. Chan, and S. N. Bahatia, Nano Lett. 4, F. Meiser, C. Cottey, and F. Caruso, Angew. Chem., Int. d. 43, J. K. Jaiswol, and S. M. Simon, Trends Cell Biol. 14, J. Feng, G. 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