Luminescent Properties of Eu-Doped Lanthanum Oxyfluoride Sol

photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and ... Lidia Armelao , Daniela Belli Dell'Amico , Luca Bellucci , Grego...
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J. Phys. Chem. C 2009, 113, 14429–14434

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Luminescent Properties of Eu-Doped Lanthanum Oxyfluoride Sol-Gel Thin Films Lidia Armelao,*,†,‡,§ Gregorio Bottaro,*,‡,| Laura Bovo,‡,§ Chiara Maccato,‡,§ Michele Pascolini,†,‡,§ Cinzia Sada,⊥ Evelyn Soini,‡,§ and Eugenio Tondello‡,§ ISTM-CNR, Via Marzolo, 1 35131 PadoVa, Italy, INSTM UdR PadoVa, Via Marzolo, 1 35131 PadoVa, Italy, Department of Chemistry, PadoVa UniVersity, Via Marzolo, 1 35131 PadoVa, Italy, IMIP-CNR, Via Orabona, 4, 70126 Bari, Italy, and Department of Physics, PadoVa UniVersity, Via Marzolo, 8 35131 PadoVa, Italy ReceiVed: April 28, 2009; ReVised Manuscript ReceiVed: June 5, 2009

Highly luminescent Eu3+-doped LaOF thin films have been prepared by a sol-gel procedure using La(hfa)3 diglyme (Hhfa ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; diglyme ) bis(2-metoxyethyl)ether) as a singlesource precursor for lanthanum and fluorine. Europium doping (Eu/La ) 3 and 10 atom %) has been achieved by adding the proper amount of europium acetate hydrate (Eu(CH3CO2)3 · xH2O, x e 4) to the precursor solution. After single-layer deposition, the obtained coatings were heat treated in air up to 700 °C. All the samples, under UV irradiation, presented a bright red luminescence clearly visible to the naked eye notwithstanding the low europium content and the limited film thickness (e50 nm). The microstructure, composition, and morphology of the samples and their interplay with the synthesis conditions were investigated by glancing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and field emission scanning electron microscopy (FE-SEM). The luminescence properties of Eu3+:LaOF in the energy and time domains are reported and discussed as a function of europium content and synthesis conditions. The observed variations in the light-emitting behavior of Eu3+ ions have been correlated to the structural and compositional characteristics of the host oxyfluoride matrices. Introduction The peculiar luminescence properties of rare-earth (RE)-doped nanomaterials, originated from the transitions within the 4f shell, can be effectively employed in a large variety of modern technological applications ranging from optical and photonic materials for the development of displays, lasers, and lighting devices, to diagnostic tools in nanobiomedicine.1-5 As far as solid-state light sources for illumination and display purposes are concerned, oxide-based thin film phosphors have received considerable attention for use in flat panel displays because of their good luminescent characteristics, stability in high vacuum, and absence of corrosive gas emission under electron bombardment when compared to currently used sulfide-based phosphors.6,7 Moreover, compared with fine grain luminescent powders used in conventional displays, phosphor films exhibit superior thermal conductivity, higher degree of uniformity, and better adhesion factor.8-10 In phosphor films, the uniform thickness combined with smoother surface morphology and smaller grain size makes it possible to define smaller pixel spot size to achieve a higher resolution.11,12 As a general rule, the controlled dispersion of the optically active RE species (guest) in suitable host matrices plays a key role in the preparation of such systems, since the local environment around the luminescent centers strongly affects the light-emission performances of the materials.13,14 The radiative decay of lanthanide ions may suffer from quenching effects in the case of RE clustering and/or vibronic coupling within the * Corresponding author. E-mail: [email protected] (L.A.); [email protected] (G.B.). † ISTM-CNR. ‡ INSTM. § Department of Chemistry, Padova University. | IMIP-CNR. ⊥ Department of Physics, Padova University.

host matrix. In general, the total RE amount has to be maximized while maintaining a high guest dispersion to prevent aggregation phenomena that can be detrimental for the functional properties of the materials. As for the host, the luminescence properties of RE ions can be further improved by tailoring the matrix characteristics in order to minimize the quenching of the lanthanide excited states. In comparison with conventional oxide-based luminescent systems, RE fluorides and oxyfluorides are attractive host matrices for fluorescent materials thanks to both the high optical transparency from UV through IR, and the low phonon energy that enables the radiative transitions of lanthanide ions to occur with high efficiency.1,3,15-18 Recently, we have developed a simple sol-gel route for the synthesis of lanthanum oxyfluoride thin films19 using La(hfa)3 · diglyme (Hhfa ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; diglyme ) bis(2-metoxyethyl)ether) as a single-source precursor for both lanthanum and fluorine. The present work is devoted to the study of the photophysical properties of Eu3+-doped LaOF thin films and their interrelation with composition and microstructural features. The choice of europium as the luminescent doping species is undoubtedly related to the high efficiency of Eu3+ ions as red-light emitters. Moreover, due to the relatively simple energy level structure and to the presence of nondegenerate ground (7F0) and emitting (5D0) states, the emission and excitation transitions of Eu3+ ions can be suitably exploited to monitor their location within the host lattice.3,20-23 In the following, the sol-gel preparation of Eu-doped lanthanum oxyfluoride (Eu/La ) 3 and 10 atom %) thin films and the evolution of their composition, microstructure, and morphology as a function of annealing temperature, up to 700 °C, are presented. Thermal treatments at higher temperatures were not performed since they result in the formation of silicate compounds originated from chemical reactions between the spincoated layers and the silica glassy substrate.19

10.1021/jp903902h CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

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The concomitant variation in the luminescence properties of the samples both in the energy and time domains are discussed and correlated to the synthesis conditions. All the samples were characterized by a bright red luminescence clearly visible at the naked eye, notwithstanding the low europium content and the limited film thickness (e50 nm). The results reported here point out the effectiveness of LaOF as a transparent matrix for the realization of RE-based host-guest luminescent layers. Experimental Section Lanthanum oxyfluoride thin films were prepared by sol-gel spin coating as reported in ref 19. Chemicals. La(hfa)3 · diglyme was synthesized according to the literature24-26 and employed as a single source precursor for lanthanum and fluorine. Commercial (Sigma-Aldrich, 99.9%) europium acetate hydrate (Eu(CH3CO2)3 · xH2O, x e 4) was used as the europium source. Synthesis. La-based thin films were prepared starting from ethanolic solutions of La(hfa)3 · diglyme. The sol-gel reactions occurred under acidic conditions by adding water and trifluoroacetic acid to the precursor solutions in the following molar ratio: 1La/87C2H5OH/3H2O/0.8CF3COOH. Eu3+ doping has been achieved by adding the proper amount of (Eu(CH3CO2)3 · xH2O, x e 4) to the precursor solution (3 and 10 atom %). The solutions were stirred at 60 °C for 6 h and successively employed for film deposition by spin-coating (SCS P6700 SpinCoater) on Herasil silica slides (Heraeus, Quarzschmelze, Hanau, Germany). The films were subsequently annealed from 300 up to 700 °C in air for 1 h. Structural Characterization. Glancing incidence X-ray diffraction (GIXRD) measurements were carried out by means of a Bruker D8 Advance diffractometer equipped with a Go¨bel mirror and a Cu KR source (40 kV, 40 mA), in the 20-55° 2θ range at a fixed incidence angle of 0.5°. The average crystallite dimensions were estimated from line broadening by means of the Scherrer equation. Scanning Electron Microscopy (SEM) Measurements. The morphological features of the samples were investigated by field emission SEM (FE-SEM) analysis. SEM measurements were performed by a Zeiss SUPRA 40 VP instrument operated at an acceleration voltage of 5 kV, equipped with an Oxford INCA x-sight X-ray detector. Chemical Composition. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Perkin-Elmer Φ 5600ci spectrometer using a nonmonochromatized Al KR radiation (1486.6 eV), at a working pressure lower than 10-9 mbar. The specimens, mounted on steel sample holders, were introduced directly into the XPS analytical chamber by a fast entry lock system. Survey scans were run in the 0-1350 eV range. Detailed spectra were recorded for the following regions: La3d, F1s, FKLL, O1s, C1s, Si2s, and Eu3d. The reported binding energies (BEs, standard deviation ) (0.2 eV) were corrected for charging effects, assigning to the adventitious C1s line a BE of 284.8 eV.27 The analysis involved Shirley-type background subtraction, and, whenever necessary, spectral deconvolution, which was carried out by nonlinear least-squares curve fitting, adopting a Gaussian-Lorentzian sum function. The atomic composition of the samples was calculated by peak integration, using sensitivity factors provided by the spectrometer manufacturer (Φ V5.4A software) and taking into account the geometric configuration of the apparatus. Secondary Ion Mass Spectrometry (SIMS) depth profiles were obtained by an IMS 4f mass spectrometer using a 14.5 keV Cs+ primary beam (10 nA) and negative secondary ion detection. Beam blanking

Armelao et al. mode was used to improve the depth resolution. The charge compensation was achieved by means of an electron gun. For each sample, the erosion speed was evaluated at various depths measuring the corresponding crater by means of a Tencor Alpha Step profiler. The dependence of the sputtering rate on the material composition was therefore taken into account in the film thickness determination. Optical Measurements. An ultrafast laser (Clark CPA2110) was used to generate 150 fs pulses with an energy of 1 mJ at a repetition rate of 1kHz. Pulses, centered at 770 nm, were introduced into an optical amplifier (Light Conversion TOPAS C) by which laser pulses at wavelengths ranging from 220 up to 1200 nm were produced. Pulses were focused by means of a lens inside the samples, and the luminescence was recorded with a Horiba Jobin Yvon T64000 triple monochromator coupled with an intensified charge-coupled device (CCD; Andor iStar). The spectrometer was used to both record the luminescence spectra and perform time-resolved experiments. For the latter analysis, the repetition rate of the laser was decreased to 250 Hz, and the timing unit was used to trigger the acquisition of the iCCD, whose delay was adjusted in order to collect spectra integrated over multiple shots for each delay unit. Filters were used to cut the undesired spectral regions. The effective emission decay times, τeff, have been calculated using the equation13

τeff )

∫ tI(t)dt ∫ I(t)dt

where I(t) represents the luminescence intensity at time t corrected for the background, and the integrals are evaluated in a 0 < τ < τmax range where τmax . τeff. All luminescence measurements were performed in air at room temperature. Results and Discussion The variation in the microstructure of europium-doped lanthanum oxyfluoride thin films closely resembles that of the corresponding undoped layers, irrespective of the europium content.19 The GIXRD spectra for the doped samples annealed in air at different temperatures are displayed in Figure 1. The formation of crystalline phases is evidenced at temperature as low as 300 °C. In fact, in the corresponding X-ray pattern, two weak and broad peaks centered at 2θ ) 24.3° and 44.6° are present, which can be indexed as the [(002), (110)] and [(300), (113)] reflexes of the hexagonal structure of LaF3.28 The observed unusual high intensity of the (002) peak with respect to the 100% intensity expected for the (111) reflection could be addressed to some degree of preferential orientation already evidenced in this kind of systems.25,29 Moreover, the presence of nanocrystalline structures and/or structural disorder is likely at the origin of the large full width at half-maximum (fwhm) of the diffraction peaks, thus preventing their straightforward attribution. Annealing at 500 °C promoted system crystallization and the conversion of the lanthanum fluoride phase (LaF3) into oxyfluoride (LaOF). As a matter of fact, the diffraction patterns of the samples after heating (Figure 1) are dominated by the presence of an intense sharp peak centered at 2θ ) 26.7°, indexed as the (110) reflex of the rhombohedral LaOF crystal phase.30 Moreover, the (211), [(332), (10-1)], and [(433), (11-1), (321), (200)] reflexes, located at 2θ ) 31.0, 44.6, and 52.8° respectively, are associated to the oxyfluoride crystal structure as well. As evaluated from the Scherrer equation, the

Luminescent Properties of Eu-Doped LaOF Thin Films

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Figure 1. GIXRD patterns for Eu-doped (10 atom %) lanthanumbased thin films annealed in air for 1 h at different temperatures: (a) 300 °C, (b) 500 °C, and (c) 700 °C. The subscripts F and OF in the (hkl) indexing refer to LaF3 and LaOF, respectively.

average diameter of the LaOF crystallites is ca. 10 nm. The lanthanum oxyfluoride phase remains stable up to 700 °C, and the segregation of Eu-containing crystalline phases (such as EuF3, EuOF, or Eu2O3) has never been detected. However, because of peak broadening and the low amount of europium in the specimens, the presence of the above-mentioned compounds can not be unambiguously ruled out. Moreover, the evolution of the system microstructure as a function of the treatment temperature is not apparently influenced by the europium presence. The development of well-defined crystalline phases upon annealing at T > 300 °C is accompanied by a progressive variation of surface morphology as displayed in Figure 2. In fact, samples annealed at low temperature (300 °C) are characterized by the absence of peculiar morphological features, thus resulting in smooth and flat deposits, typical for nearly amorphous materials; on the other hand, films treated at increasing temperatures present globular-shaped aggregates distributed on the surface. In the case of the 700 °C heated sample, the observed aggregates (ca. 100 nm) are characterized by an homogeneous porosity with regular 30-40 nm wide pores. Such a morphology could be responsible for the presence of a higher amount of surface sites that, in turn, could affect the emission properties of europium ions. Information on sample chemical composition was obtained by XPS. In agreement with GIXRD results, pointing out the formation of oxygen-rich crystalline phases upon increasing the annealing temperature, XPS analysis evidenced a progressive increase of the oxygen content in the films, at the expense of fluorine. This trend is evident also looking at the band shape of La3d photoelectron peak. Figure 3 reports the La3d region for the sample annealed at 700 °C; the observed double-peak structure of each spin-orbit split component (j ) 5/2 and 3/2), where peak and satellite show a comparable intensity, is typical for oxygen-rich La(III) species. It is worth highlighting that, for highly fluorinated lanthanum compounds, the intensity of the satellite peaks (mainly attributed to final-state effects and/ or to charge-transfer (CT) coexcitations)31-34 are sensibly lower due to a less efficient F2p f RE4f CT process with respect to the O2p f RE4f one. In addition, for samples annealed between 500 and 700 °C, the La3d peak-shape and position (La3d5/2 )

Figure 2. SEM micrographs for Eu-doped (10 atom %) lanthanumbased thin films annealed in air for 1 h at 300 °C, 500 °C, and 700 °C.

834.9 and 838.3 eV, La3d3/2 ) 851.0 and 855.4 eV) are in agreement with previously reported data regarding the synthesis of LaOF thin films by sol-gel and chemical vapor deposition (CVD).19,24 Unfortunately, because of the low intensity of europium-related signals and the severe spectral overlap between La3p3/2 and Eu3d5/2 regions (both located at BE ≈ 1130 eV),13,35 europium quantification within lanthanum oxyfluoride films was prevented. Information about europium presence throughout the layers was obtained by SIMS depth profiling. The in-depth distributions of La, O, F, Si, and Eu for the samples annealed

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Figure 3. La3d XPS surface peak for the Eu-doped (10 atom %) LaOF thin film annealed at 700 °C for 1 h. Spin-orbit components and satellite peaks are highlighted in the figure. Figure 5. Schematic representation of configurational diagram illustrating nonradiative transitions feeding the 5D levels from CT state. R is the configurational coordinate. Only a few parabola of the 4f6 configuration have been drawn.

Figure 4. SIMS depth profiles for Eu-doped (3 atom %) lanthanum oxyfluoride thin films annealed in air for 1 h at (a) 300 °C, (b) 500 °C, and (c) 700 °C.

at different temperatures are shown in Figure 4. As a general trend, despite the limited film thicknesses (that is close to 25 nm in the sample annealed at 300 °C), homogeneous profiles for all elements are observed. By increasing the annealing temperature, a clear interdiffusion of the species occurs, thus smoothing the film-to-substrate interface. Moreover, the remarkable similarity in the distribution shape of La, F, and Eu suggests

a common chemical origin of these species, pointing out the high and effective dispersion of Eu3+ ions in the host matrix. It is worth noting that the integrated intensity of the fluorine-related SIMS signals progressively decreases upon raising the annealing temperature, indicating the formation of oxygen-rich compounds, in agreement with XPS and GIXRD results. These findings are consistent with the results reported in a previous study concerning the thermal stability of LaOF in air. It has been evidenced that oxyfluoride species undergo hydrolysis by exposure to moisture at high temperatures, leading to fluorine release and to the formation of pure oxide compounds.36,37 To excite the strongly absorbing CT bands of Eu3+ ions, UV irradiation at λ ) 260 nm was employed. In this way, the luminescent 5D0 state of the lanthanide centers can be effectively populated by exploiting nonradiative decay processes from the upper lying excited states of Eu3+ ions.38,39 After excitation, emission can be observed in the red region of the spectrum mainly resulting from the 5D0 f 7F0, 7F1, 7F2 transitions. The sequence of excitation, relaxation, and emission properties can be explained taking into account the configurational coordinate model (Figure 5).40-42 The excitation of Eu3+ takes place from the bottom of the 7F0 curve, rising along the straight vertical line, until it crosses the CT state. Relaxation occurs along the CT curves. Near the bottom of the CT curve, the excitation is transferred to 5Dj states. Relaxation to the bottom of the 5Dj states is followed by light emission downward to the 7Fj states.40-42 The room-temperature photoluminescence (PL) spectra from samples characterized by different europium content (3 and 10 atom %) and thermally treated under different conditions are reported in Figure 6. The emission, dominated by the 5D0f7F2 transition (λmax ≈ 612 nm), undergoes severe modifications upon increasing the treatment temperature, pointing out the occurrence of structural rearrangements around the Eu3+ centers.13,21 While, for samples treated at temperatures as low as 300 °C, the 5 D0f7F2 emission appears broad and not well resolved, upon increasing the annealing temperature up to 700 °C the presence of sharper peaks and better resolved bands is clearly evidenced in accordance with the development of a more ordered structure. In the luminescence spectra of samples annealed between 500 and 700 °C (Figure 6), the emission band associated with the

Luminescent Properties of Eu-Doped LaOF Thin Films

Figure 6. Emission spectra (λexc ) 260 nm) in the 570-660 nm range for Eu-doped (3 and 10 atom %) LaOF films treated in air for 1 h at different temperatures. The peaks marked by a star in the figure are relative to emissions from higher excited states.

Figure 7. Emission spectra (5D0 f7F0) transition for Eu-doped LaOF (10 atom %) treated in air at different temperatures.

F2 transitions (612 - 630 nm range) results from the overlap of the crystal field (CF) components of the 7F2 multiplets43,44 with the emission from higher excited states (5D1 f 7F4).43 In these samples, depending on the europium content, emissions from higher excited states (5D1 f 7F3) can be also observed. Such emissions, usually quenched by multiphonon relaxations, are often present in oxyfluoride matrices because of their low phonon energy (e550 cm-1).15,39,45 In fact, in this case, multiphonon relaxations should involve the occurrence of a highly improbable 4-phonon process in order to fill the ∼2000 cm-1 energy gap between 5D1 and 5D0 states.15,45 Further information about the local surrounding of Eu3+ ions can be gained from the analysis of the 5D0 f 7F0 transition (Figure 7) since, being nondegenerate (J ) 0 for both levels), any broadening can be attributed to site variations in the local field acting on the ions.46 In particular, it is likely that the observed band broadening could be attributed to the presence of some Eu3+ ions located in the peripheral region of the LaOF grains, i.e. on the crystallite surface. In this way, the different coordination exhibited by europium centers originates from different local fields, which are responsible for the observed band-shape. In addition, the contribution to peak broadening due to Eu ions on non stoichiometric lanthanum oxyfluoride can not be completely ruled out.45 As far as peaks position is concerned, a small progressive red-shift is evidenced upon increasing the annealing temperature up to 700 °C, in agreement with the formation of oxygen-rich phases at the expense of fluorine content. Consequently, the variation in the ligand field 7

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Figure 8. Luminescence decay curves for Eu-doped (10 atom %) LaOF specimens obtained by monitoring the Eu3+ emission at a wavelength of 612 nm (λexc) 260 nm). The annealing temperatures and the effective decay times are reported.

can be considered at the origin of the observed peak shift.46 A similar variation in the distribution of the luminophores in the host matrix is also in agreement with the trend observed in the 5 D0 excited-state lifetime. As an example, the room-temperature emission decay curves for the Eu3+-doped LaOF (10 atom %) thin films obtained monitoring the Eu3+ emission at a wavelength of 612 nm and the value of the effective decay times13 are reported in Figure 8. All the curves are non-exponential and present at least two time constants. The τeff decreases from 715 to 420 µs upon raising the annealing temperature from 300 up to 700 °C, in agreement with the strong reduction of fluorine content.19 Moreover, the shortening of the 5D0 excited-state lifetime may also be related to the contribution of the surfacelocated Eu3+ sites in the nanocrystalline layers and to vibronic quenching from -OH moieties in the oxygen-rich matrix formed at the higher temperatures. Conclusions Homogeneous and highly luminescent Eu3+-doped lanthanum (Eu/La ) 3 and 10 atom %) oxyfluoride thin films have been prepared through a simple sol-gel procedure using La(hfa)3 · diglyme as single-source precursor for lanthanum and fluorine. Pure single-phase LaOF films are obtained by annealing at temperatures ranging from 500 to 700 °C. The layers are nanocrystalline and present a similar surface morphology characterized by nanoporous globular aggregates. A highly homogeneous dispersion of the Eu3+ ions is accomplished within the lanthanum oxyfluoride layers that produce a bright red luminescence clearly visible at the naked eye notwithstanding the low europium content and the limited film thickness (e50 nm). These results point out the effectiveness of LaOF films as matrices for the realization of host-guest lanthanide-based luminescent nanosystems. Acknowledgment. This work was financially supported by research projects CNR-INSTM PROMO, FIRB-MIUR RBNE033KMA “Molecular compounds and hybrid nanostructured materials with resonant and non resonant optical properties for photonic devices”, and CARIPARO 2006 “Multi-layer optical devices based on inorganic and hybrid materials by innovative synthetic strategies”.

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