Physics of Nanocrystalline Luminescent Ceramics

Transcription

1 Solid State Phenomena Vol. 94 (2003) pp (2003) Trans Tech Publications, Switzerland doi: / Physics of Nanocrystalline Luminescent Ceramics Ulrich Herr 1, Harald Kaps 1 and Armin Konrad 2 1 Materials Division, Faculty of Engineering, Ulm University Albert-Einstein-Allee 47, Ulm, Germany 2 OSRAM GmbH, Berliner Allee 65, Augsburg, Germany Keywords: Nanocrystalline Materials, Luminescent Materials, Chemical Vapour Reaction Abstract. In this paper we present an overview of recent work on size-dependent optical properties of nanocrystalline ceramics. Nanocrystalline Y 2 O 3 :Eu particles prepared by a clean chemical vapour reaction (CVR) route are used as a model system. Coarse-grained Y 2 O 3 :Eu is the very efficient red phosphor material in conventional fluorescent lamps. New applications for this wellestablished material may open up if the optical properties can be suitably modified in nanoparticles. The experiments show that the emission characteristics, which are dominated by the inner shell transitions of the Eu 3+ ions, remain unchanged with nanoparticles. However, absorption properties and quantum efficiency are size dependent. No significant modification of the emission lifetime is found. The properties of the Y 2 O 3 host lattice are described by a configuration coordinate model which takes into account the localized character of the excitation. Quantum size effects play only a minor role in these materials. The nanocrystalline luminescent ceramics are different from semiconductors with significantly larger exciton radius which show pronounced quantum size effects at comparable crystal size. Introduction Nanocrystalline materials are characterized by having crystal sizes below 100 nm. In this regime, the physical properties may be strongly influenced by the presence of a large number of interfaces in compact materials,or the large surface to volume ratio in nanostructured particles of thin films. One may therefore expect not only gradual changes of the properties, such as a hardness increase in nanocrystalline metallic materials due to the Hall-Petch effect, but also a change in the mechanism of e.g. mechanical deformation. A similar behaviour is found in the nanocrystalline soft magnetic materials, where the usual trend of magnetic hardening with decreasing grain size in polycrystalline materials is completely reversed. As soon as the crystal size falls below the characteristic magnetic length scale, a dramatic softening is observed as explained by the random anisotropy model. Semiconductor nanocrystals [1,2] with exciton radius in the range of several nanometers show pronounced changes in the optical spectra in comparison with their single crystal or polycrystal counterparts. These materials do not follow the text book solid state physics: they do not have a dense spectrum of allowed electron wave vectors. The observations have been termed quantum confinement effects and offer new opportunities to tailor physical properties such as the colour of the materials. More efficient optoelectronic devices on the basis of nanostructured indirect band gap materials appeared within reach after the discovery of luminescence in porous silicon. Ceramic luminescent materials or phosphors represent a class of functional materials with various applications; these range from fluorescent lamps over cathode ray tubes (CRT) for television to X- ray scintillators or solid state laser materials [3]. In all of these materials, energy is absorbed and converted to visible light. The emission is generally bound to the presence of specific sites, which may be crystal defects or impurity atoms. The emission process is more localized than in the undoped semiconductor nanocrystals; thus quantum size effects will not occur in the same size range but rather at the atomic or molecular level. Many of these materials, such as the red lamp All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, United States of America-03/06/14,22:45:23)

2 86 Interfacial Effects and Novel Properties of Nanomaterials phosphor Y 2 O 3 :Eu, are very efficient for energy conversion as expressed by a quantum efficiency close to 100 %, which is defined as the ratio of emitted and absorbed photons. Nevertheless, there has been considerable effort in the last years to investigate nanocrystalline phosphors. This has been driven by the challenge of new applications such as fast and fine-structured displays or mercury free discharge lamps which require new properties such as fast emission or larger excitation in the deep UV. After an introduction to the prospects of nanocrystalline luminescent materials, the results of a detailed study of the model phosphor Y 2 O 3 :Eu are presented. Clean nanocrystalline materials have been obtained via a chemical vapour reaction route. By characterization of both the host lattice alone and the doped material, a more complete picture of the processes in the nanocrystalline material is obtained. Nanocrystalline luminescent materials Considerable interest in the properties of nanocrystalline phosphors has been raised by the work of Bhargava et al. [4,5]. In a study of ZnS doped with Mn, an increase in luminescent efficiency was reported with decreasing crystal size. This seems astonishing at first in view of the role of surface defects for recombination. If the surface is a source of non-radiative electron-hole recombination, then these processes should be more dominant in nanocrystalline material than in coarse grained powders. An apparent explanation seemed to be the dramatic shortening of the luminescence lifetime: the reduction of the lifetime by 5 orders of magnitude would make the slower surface recombination effects less efficient and thereby lead to an overall enhancement of the luminescence efficiency. New applications for fast display applications became conceivable. The origin of the lifetime shortening was attributed to localization effects in the confined structures. In principle, lifetime changes may occur via relaxation of selection rules for transitions by modification of the local environment around the luminescent ions. In a more recent study, however, Bol and Meijerink [6] found that the observed fast decay was related to defects in the ZnS host and not to a faster decay of the Mn 2+ emission. In fact, the Mn 2+ emission showed no significant lifetime change in their measurements. The authors of [6] do therefore question the usefulness of these materials for the projected new applications. The nanocrystalline Y 2 O 3 :Eu has been selected in this study because of its potential for mercury free discharge lamps. The conventional, coarse grained (several micron crystal size) material is well established as the red phosphor ( 5 D 0-7 F 2 Eu line at λ = 610 nm) of the trichromatic fluorescent lamp. The phosphor layer inside the lamp converts the invisible, primary emission of the discharge into visible light; the phosphor layer of the trichromatic lamp consists of three individual components generating red, green and blue light, respectively. Y 2 O 3 :Eu has a high efficiency for the mercury emission line at λ= 254 nm so that there is no need to improve the material for present applications. However, future mercury-free discharge lamps might operate at even shorter wavelength far in the UV region. For these applications it would be very useful to adapt the absorption properties of the phosphor to the new primary emission. Therefore a considerable interest exists for the investigation of the optical properties of nanocrystalline Y 2 O 3 :Eu materials. Experimental Nanocrystalline Y 2 O 3 :Eu particles were synthesized in a tubular flow reactor by chemical vapour reaction as described in detail in [7]. We used organo-metallic Y(C 11 H 19 O 2 ) 3 and Eu(C 11 H 19 O 2 ) 3 precursors which were kept at constant temperature under flowing argon carrier gas. The vaporized precursors were then introduced into a reaction chamber with a flow rate of 200 ml/min together with 800 ml/min of pure oxygen. The reaction chamber consisted of an alumina tube with 90 mm diameter and 900 mm length. At a temperature of 1273 K and a pressure of 50 mbar, the precursors

3 Solid State Phenomena Vol were reacted with the pure oxygen gas to form the desired oxides. The resulting powders were collected at a water cooled trap. Optical spectroscopy was performed using a SPEX Fluoromax II spectrometer. Nanocrystalline Y 2 O 3 host lattice properties In the first part of the investigations the properties of the pure Y 2 O 3 host material have been measured [7]. The samples have been prepared via the CVR route described above. X-ray diffraction showed only Bragg peaks of the cubic Y 2 O 3 phase. The lattice parameter of the nanocrystals is identical with that of the coarse grained standard within the accuracy of the measurement. From the width of the maxima, a crystal size of 10 nm is calculated. This is in good agreement with transmission electron microscopy observations. The powders are only weakly agglomerated as can be seen in the scanning electron microscopy pictures (see Fig. 1). Larger crystal sizes were obtained after annealing the powders at elevated temperatures (1173 K and above). Fig. 1 High resolution scanning electron microscope (SEM) images of the as-prepared nanocrystalline powders. The image to the left shows an agglomerate of individual particles attached to the SEM sample holder. The image to the right was taken after dispersing the powders in an ultrasonic bath under acetone and spreading of the suspension on a silicon wafer. The samples were pressed to pellets with a thickness of 1 mm and diffuse reflection spectra were recorded. The recorded intensity was normalized using an alumina standard as a reference. The resulting curves are shown in Fig. 2. The main characteristic of the data is the reduction of reflectivity in the region of exciton absorption in the region below 220 nm. The onset of the exciton absorption is shifted to shorter wavelengths (blue shift) in the nanocrystalline samples. A systematic increase of the shift with decreasing crystal size is observed. The maximum shift obtained from a fit of the absorption edge using a gaussian is 0.2 ev at room temperature. Low temperature measurements at a temperature of 80 K give a smaller shift of the absorption edge of only 0.07 ev. This already indicates the influence of thermal vibrations on the observed absorption edge shift. Even without Eu doping the Y 2 O 3 host lattice shows a weak emission mainly in the near UV region with a maximum around 340 nm. Valuable information about the host lattice properties in both coarse grained and nanocrystalline material can be obtained from this emission. The emission curves of powders with different crystal size have been recorded at room temperature and 80 K, respectively. As an example, the emission at room temperature of a sample with 50 nm grain size is shown in comparison with that of the coarse grained reference material in Fig. 3. It is found that

4 88 Interfacial Effects and Novel Properties of Nanomaterials the nanocrystalline powders show reduced intensity of this emission, making it impossible to record high quality spectra at room temperature for the 10 nm samples. At 80 K, a weak emission is found even for the 10 nm sample. Fig. 2 Diffuse reflection spectra of nanocrystalline Y 2 O 3 and coarse grained reference [7] Fig. 3 Emission spectrum of nanocrystalline (50 nm grain size) pure Y 2 O 3 and the coarse grained (10 µm) reference at room temperature [8]. These measurements show that the host lattice properties are modified significantly by the nanocrystalline state of the material. A configurational coordinate model was developed to account for the optical properties of the host material. This seems appropriate in view of the large Stokes shift between the absorption and the emission of the pure Y 2 O 3. The model, which is given in detail in [8], describes a self-trapped exciton inside the Y 2 O 3 lattice. The configurational coordinate describes the interaction of electronic excitation and local distortion of the lattice. The fast electronic excitation is followed by a slower relaxation of the lattice resulting in a decrease of energy. The emission occurs from the distorted, lower energy state into a ground state with the

5 Solid State Phenomena Vol same lattice distortion and, therefore, an elevated energy. The lattice relaxation can therefore explain the large Stokes shift often observed in luminescent materials [3]. The model of the configurational coordinate can explain well the observed absorption and emission properties of the bulk or nanocrystalline material. The main result is that the modified properties of the nanocrystalline material originate from an increased lattice relaxation contribution of the excited state. This may be attributed to an increase of the exciton-phonon coupling, which has already been proposed in the case of CdSSe nanocrystals before [9]. A similar temperature behavior, i.e. a weaker temperature dependence of the excitation energy in the nanoparticles, has also been observed in nanocrystalline CdSSe. This modified exciton-phonon coupling may be due to the influence of possible extrinsic defects in nanocrystals [9]. A simple estimate of possible quantum confinement effects in the Y 2 O 3 host lattice, which would lead to an increase of the band gap and the exciton energy, shows that the expected effects are too small to account for the observed shifts in the nanocrystalline Y 2 O 3 particles [8]. This can be made using the particle-in-a-box model by [10,11]: 2 2 h π 1 E = 2µ R 2 where R is the particle size and µ is the reduced effective mass of the exciton. With reasonable values for the effective masses of electrons and holes in Y 2 O 3, this leads to an energy shift of only 0.02 nm for the 10 nm Y 2 O 3 particles which is much smaller than the observed effects. However, quantum confinement has recently been claimed to be responsible for band gap energy effects observed in nanocrystalline ZrO 2 :Y thin films [12] which are of similar magnitude as the effects observed in this study. Luminescence of nanocrystalline Y 2 O 3 :Eu The CVR process is especially suited for the introduction of dopands since the mixing of the constituents occurs already in the vapor phase in a very controlled manner. Nanocrystalline Y 2 O 3 :Eu powders with different Eu concentrations and grain sizes of about 10 nm have been prepared. The samples presented here contained 7 wt% Eu 2 O 3. For comparison, coarse grained (5 µm) Y 2 O 3 :Eu powders with 2 or 4 wt % have also been measured [13]. In addition, samples with grain sizes of 21 and 63 nm have been prepared by annealing of the powders at 1173 and 1373 K, respectively [14]. The nanocrystalline samples have the cubic equilibrium structure as shown by X-ray diffraction. This is also reflected in the emission spectra which show all characteristic lines of Eu 3+ in a cubic lattice. Only for smaller particles below 5 nm, traces of the characteristic emission of monoclinic Y 2 O 3 :Eu [15] have been observed. The non-equilibrium monoclinic structure has been obtained by others using the gas-phase condensation method [15,16]. The samples have been excited with a wavelength of 254 nm corresponding to the mercury emission. In the range of nm, the excitation of Eu via charge transfer between oxygen and Eu is dominant. The emission spectrum of the nanocrystalline material is very similar to the emission of the Y 2 O 3 :Eu coarse grained standard material; in particular, no additional lines are found on the region 550 to 700 nm, and no shifts of the line positions are observed. However, the intensity of the emission from the nanocrystalline material is lower than that of the standard. This reduction is most obvious for the 10 nm crystal size (see Fig. 4). The emission is found to increase again in the annealed powders with larger grain size.

6 90 Interfacial Effects and Novel Properties of Nanomaterials In order to obtain more information about the excitation properties, excitation spectra have been measured using primary wavelengths between 190 and 320 nm [13]. The emission of the 5 D 0-7 F 2 Eu line at λ = 610 nm was used to monitor the efficiency. The results are shown in Fig. 5. The excitation spectrum is dominated by the host lattice absorption in the range nm and the charge transfer region from nm. In the nanocrystalline samples, the ratio of host lattice to charge transfer excitation is larger than in the standard. Similar observations have been made by Schmechel et al. [17] on nanocrystalline Y 2 O 3 :Eu of comparable crystal size. These authors propose that the difference may be attributed to the different absorption coefficients for host lattice or charge transfer absorption which leads to quenching of the host lattice absorption process in the coarse grained standard. It is also observed that the excitation of the 10 nm and 21 nm samples is much lower than for the coarse grained reference material. Schmechel et al. have also determined the quantum efficiency for different nanocrystalline samples in various environments. They do also find a reduction of quantum efficiency with decreasing grain size. Fig. 4 Emission spectra of the Y 2 O 3 :Eu samples with different grain sizes after excitation at 254 nm [14]. Fig. 5 Excitation spectra for the 5 D 0-7 F 2 emission at 610 nm for different Y 2 O 3 :Eu samples [13].

7 Solid State Phenomena Vol The lifetime of the 5 D 0-7 F 2 emission of the Y 2 O 3 :Eu has also been determined after excitation with an ArF excimer laser. For the samples with 10 nm crystal size, a value of 1.3 ms is found which does not deviate significantly from the value of 1.4 ms of the co measured with the same experimental setup (see Fig. 6). Schmechel et al. [17] find slightly longer lifetimes in their Y 2 O 3 :Eu nanocrystals ranging from 2 to 3.7 ms. In any case, no dramatic shortening of lifetime is observed. The origin of the reduced quantum efficiency of the nanocrystals could be a reduced absorption of the material, or additional non-radiative relaxation processes in the nanocrystals reducing the energy transfer to the luminescent center. It is interesting in that respect to study the reflectivity of the Y 2 O 3 :Eu nanopowders as has been done in [14]. The result is shown in Fig. 7. It is found that much more intensity is reflected in the charge transfer range ( nm) from the nanocrystalline material than from the coarse grained standard. One has to consider here possible changes due to modified light scattering resulting from the different grain sizes. It is clear, however, that only a lower fraction of the incident energy can be converted into visible light in case of the nanocrystals. However, this effect is not sufficient to account for the reduction of efficiency by more than one order of magnitude found by Schmechel et al.. Fig. 6 Decay of the 5 D 0-7 F 2 emission at 610 nm for 10 nm Y 2 O 3 :Eu samples. Apart from a smaller absorption due to modified scattering, the efficiency of the nanopowders may also suffer from a reduced transfer of the absorbed energy to the luminescent center, represented by the Eu ions here. This charge transfer process competes with other energy transfer processes which do not lead to the characteristic 5 D 0-7 F 2 Eu emission. If these non-radiative transitions are bound to the presence of surface-related defects, they will be more frequent in nanocrystals due to the large surface-to-volume ratio. First results indicate that grain growth induced by annealing of the Y 2 O 3 :Eu powders increases the quantum efficiency. Whether this increase is associated with the reduction of the surface-to-volume ratio or the annealing of defects has to be clarified in further investigations. Schmechel et al. [17] conclude that the reduced quantum efficiency of the nanoparticles is due to a reduced radiative decay rate and an increased rate of non-radiative transitions. Recently, Song et al. [18] have demonstrated that irradiation with UV light modifies the charge transfer band in 5 nm Y 2 O 3 :Eu nanoparticles but not in the coarse grained standard of the same composition. The efficiency of emission from the nanopowders decreased with increasing irradiation time by up to 15 %. The effect has been attributed to optical activated rearrangements of

8 92 Interfacial Effects and Novel Properties of Nanomaterials local environment of Eu 3+ ions in the surface region of the nanoparticles which may lead to nonradiative transitions again. Fig. 7 Diffuse reflection spectra of nanocrystalline and coarse grained Y 2 O 3 :Eu [14]. Non-radiative transitions may also be treated within the configurational coordinate model of an isolated luminescent center. Within this model, these processes occur as transitions from vibrational levels of the excited and the ground state. As a result, the energy of the excitation is dissipated within the lattice as heat [3]. This model leads to a strongly temperature dependent quantum efficiency with a quenching of the luminescence at high temperature. Since modifications of the configurational coordinate diagram have been found already in the case of the pure Y 2 O 3, it is reasonable to assume modified parameters in the case of the nanocrystalline Y 2 O 3 :Eu, too. Further investigations are necessary to clarify the influence of modified coupling parameters on the quantum efficiency of Y 2 O 3 :Eu. Conclusions We presented recent results concerning the optical properties of nanocrystalline luminescent ceramics. The red phosphor Y 2 O 3 :Eu was chosen as a model system. Careful characterization of the pure Y 2 O 3 host revealed that size effects are present already in the undoped material. These effects, namely the blue shift of the absorption edge, have been interpreted in the framework of a configurational coordinate model which seems more appropriate than the assumption of quantum confinement effects. The luminescence properties of nanocrystalline Y 2 O 3 :Eu have been studied in detail. Whereas emission line positions and lifetimes are not significantly modified in the nanocrystalline materials, the quantum efficiency is lower than in coarse grained materials of the same composition. Different possible contributions to this decrease of the efficiency have been discussed. Future studies will have to concentrate on methods to increase the efficiency of these new materials in order to make them more useful for applications. Passivating surface coatings may provide a way to reduce surface related non-radiative processes and lead to increased efficiency.

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