ARTICLE pubs.acs.org/JPCC
Optical and Structural Characterization of Terbium-Doped Y2SiO5 Phosphor Particles P. C. Ricci,*,† C. M. Carbonaro,† R. Corpino,† C. Cannas,‡ and M. Salis† † ‡
Dipartimento di Fisica, Universita di Cagliari, s.p. n 8 Km 0.700, 09042 Monserrato (Cagliari), Italy Dipartimento di Scienze Chimiche, Universita di Cagliari, Cittadella Universitaria, 09042 Monserrato (Cagliari), Italy
bS Supporting Information ABSTRACT: The structural and the optical properties of terbium-doped yttrium oxyorthosilicate phosphor particles were analyzed. The samples were prepared through an aqueous solgel route and investigated in a large doping concentration range (ranging from 0.001% to 10%). The structural properties were studied by joint approach of X-ray measurements and Raman analysis, revealing the same principal structural phase (C2/c space group) independently of the Tb concentration. Radioluminescence measurements, excitation of photoluminescence, and time-resolved luminescence indicated a very efficient charge transfer mechanism from the excited level 5D3 to 5D4 level of Tb3+. A simple three-level kinetics model was applied to explain the experimental data and to give an estimation of the efficiency of the charge transfer process as a function of terbium doping. Finally, we give an estimation of the high absolute fluorescence quantum yield (0.95) for the 5D4 recombinations, almost independent of the Tb concentrations, showing that, from this level, there are no efficient nonradiative channels. The results disclose the heavy Tb-doped Y2SiO5 matrix as good candidate for the development of efficient UV light-emitting-diode-excited green phosphor in mercury-free fluorescent lamps and for a new generation of scintillator panels for X-ray radiography.
’ INTRODUCTION In recent years, a considerable effort was dedicated to the research of luminescent nanoparticles for the development of new types of displays, such as plasma panels, field-emission displays, electroluminescent panels, and scintillator panels for X-ray radiography. In order to achieve devices with high resolution, high brightness, and long operating time, researchers have extensively investigated various inorganic luminescent materials and, in this framework, rare earth (Re)-doped nanoscaled samples have attracted most of the studies.14 Different crystal hosts were examined with excellent results, such as alkali earth orthophosphates,5 aluminum garnet (such as Y3Al5O12)6 and Redoped oxyorthosilicates (Re2SiO5), mainly doped with Eu3+, Ce3+, Sm3+, and/or Tb3+.1,7,8 Among others, Tb-doped yttrium oxyorthosilicates show interesting and promising properties related to the combined chemical-physical characteristics of the matrix and the emission properties of the doping element. In particular, Y2SiO5 (YSO) crystals present a high chemical and thermal stability and elevated mass absorption coefficient with high stopping power, and it is already known as an excellent host material for cathodoluminescent phosphors.4 In addition, terbium ions display efficient radiative recombination channels, mainly observed in the green region of the visible spectral range. Since the maximum of the human eye sensibility falls in this region, it is of fundamental interest for the development of red green blue (RGB) panels. However, when the Tb ions are excited by high-energy radiation, such as soft X-ray, they provide an additional emission in the blue r 2011 American Chemical Society
region that can reduce the efficiency of the green emission and, as a consequence, the possible application of Tb:YSO as good phosphor for the development of new emitting devices. Actually, Tb3+ ions present two main radiative recombination channels in the UVvisible spectral range: 5D3f7Fj and 5D4f7FJ (J = 1 to 6); the former appears in the UV and blue regions, and the latter is responsible for the above-discussed green emission. Moreover, it was recently shown that an interesting cross relaxation process between the two excited levels occurs as a function of the host matrix crystal structure and Tb concentration.9 The cross relaxation mechanism can affect the emission efficiency and the relative photo- and cathode luminescence intensity of the two different radiative channels.10,11 In this paper, the structural and the optical properties of terbium-doped yttrium oxyorthosilicates were investigated with the aim to assess the effects of doping concentration by analyzing them over a large doping concentration range (ranging from 0.001 atm % to 10 atm %). The novelty of the presented results is the capability to achieve inorganic phosphor of different colors in the bluegreen range by changing the Tb ions concentration but leaving the crystal structure unaltered. Beside the interest on the fundamental parameters of the high-energy states of Re ions, the proposed research is technologically driven, since Received: April 15, 2011 Revised: July 18, 2011 Published: July 18, 2011 16630
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new efficient UV-excited luminescent materials could be used in mercury-free fluorescent lamps and in scintillator panels for X-ray radiography.
’ EXPERIMENTAL SECTION Four set of samples containing different amount of terbium were prepared trough an aqueous solgel route. Yttrium nitrate (Y(NO3)3 *6H2O, Aldrich, 99.9%), terbium nitrate (Tb(NO3)3 *5H2O, Aldrich, 99.9%), tetraethoxysilane (TEOS, Aldrich, 98%), and absolute ethanol (Carlo Erba 99.8%) were used as reactants for the preparation (Supporting Information, Figure S1). To this end, aqueous solutions containing appropriate concentrations of yttrium and terbium nitrates were mixed with an ethanolic solution of TEOS and acidified with nitric acid (pH = 1). The resulting transparent sols were stirred for 180 min at room temperature and then allowed to gel at 50 °C for 3 days. The X-ray powder diffraction (XRD) pattern recorded (here not reported for the sake of brevity) indicated the amorphous structure of the prepared gels, which were successively powdered in an agate mortar and gradually thermally treated (4 °C/min) up to 1200 °C, leaving the samples at this final temperature for 4 h. Steady time radioluminescence (RL) spectra were carried out by exciting the samples with a standard X-ray source operating at 25 kV and 20 mA (Cu anode). The RL signal was collected by means of a photonic multichannel spectrum analyzer (Hamamatsu PMA-11) with a spectral bandwidth of 1 nm. Time-resolved photoluminescence (TR-PL) measurements were performed using an optical parametric oscillator with a frequency doubler device, excited by the third harmonic (355 nm) of a pulsed Nd:YAG laser (Quanta-Ray Pro 730). The excitation pulse width at half-maximum was 8 ns with 10 Hz of repetition rate, with a power density of 25 mW/cm2 on the samples and spectral bandwidth less than 0.3 cm1. The PL signal was dispersed by a spectrograph (ARC-SpectraPro 300i) with a spectral bandpass 0.99). The large decrease of the decay time as the Tb concentration increases indicates the presence of concentration-dependent nonradiative de-excitation 16633
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Figure 6. Scheme of energy levels and illustrations showing the energy transfer process between Tb3+ ions in heavily doped YSO crystals. The excited state is shown in purple, the red arrows and the letter A indicate the nonradiative energy transfer process and the probability factor of the cross relaxation between 5D3f5D4, and the emission from the 5D4 level is shown in green.
pathways, such as the cross relaxation mechanism between blue and green bands previously reported.9,23,10 To better characterize the charge transfer mechanism, the PL time profiles from the 5D4 level, with selective excitation on the 5D3 level, was studied as a function of Tb concentrations (Figure 5). A definite PL rise time appears for all the samples; however, it presents a strong dependence with Tb concentrations: the luminescence intensity (5D4 f 7F5 transition) increases for about 1 ms in the 0.001 atm % case, while the rise time is only of about 3 μs in the sample with Tb at 10 atm %. On the contrary, the decays do not present the same dependence on Tb content, and for the samples with 0.2 atm %, 1 atm %, and 10 atm %, a time constant of about 1.7 ms can be calculated by a single exponential fitting procedure. However, the low signal-to-noise ratio and, mainly, the very slow rise time do not permit an accurate analysis on time decay of the sample with the lowest concentration (Tb at 0.001 atm %). Nevertheless, the concentration-dependent rise time of the 5D4 PL intensity in all the examined samples is a further indication of the charge transfer mechanism indicated by the PLE spectra and a simple model, which consider that the coupling between Tb3+ ions can be applied to explain the experimental data (Figure 6):9,23,10 the absorption process brings up the system to the higher excited level (5D3 or 5D4 depending on the excitation energy) from which a radiative transition to the ground state is allowed with a probability γ1 or γ2, respectively. In addition, since a coupling mechanism among nearest neighbor Tb3+ions could populate the 5D4 level from the 5D3, a suitable probability factor A for the charge transfer processes needs to be considered. It is worth nothing, that since the decay time in samples with different Tb3+ concentrations was shown to be constant irrespective of the Tb content, the reverse nonradiative process from the 5D4 level to the 5D3 level can be reasonably omitted. The solution of the kinetics equations can be obtained in a simplified form:23 dn1 ¼ ðγ1 þ AÞn1 dt
ð1Þ
dn2 ¼ γ2 n2 þ An1 dt
ð2Þ
where n1 and n2 stand for the populations of the higher and lower excited levels. In the final form we can obtain9 I1 ðtÞ ¼ expð λ1 tÞ I0
ð3Þ
I2 ðtÞ γ A ¼ ½expð λ2 tÞ expð λ1 tÞ 2 I0 γ 1 λ1 λ2
ð4Þ
where I represents the PL intensity, λ1 = γ1 + A and λ2 = γ2. Details on the kinetics equations are reported in the Supporting Information file. By assuming data reported in ref 23 and 10 for comparable systems where the probability factor of the radiative transition γ1 was calculated (γ1 = 767 s1), it is possible to directly calculate the probability factor (A) of the cross relaxation between 5 D3f5D4 levels from the time-resolved luminescence spectra. Indeed the decay profiles of the recombinations from the 5D3 level (main peaks at 380, 416, and 438 nm) were successfully fitted with a single exponential law, and from the inverse of the obtained decay constants the parameter λ1 can be retrieved (Figure 4). It is worth nothing that there is an abrupt increase of the probability of charge transfer to the 5D4 levels (see Table 1) in the samples with 10% of Tb3+ ions: indeed in these samples the excitation spectrum of the 5D4 level fully overlaps the PLE spectrum of the 5D3 level, increasing the effective excitation channels of the green luminescence. Indeed, the direct excitation spectrum of the 5D4 levels is limited in the spectral range 360520 nm (Figure 3); by increasing the Tb concentration, the probability of the charge-transfer process increases (“A” factor in table 1); and the PLE range of 5D4 recombinations can be extended to the excitation channel of the 5D3 level. 16634
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Table 1. Time Decay Constants (τ) Obtained from the Fitting Procedure of PL Experimental Data (R2 > 0.98), and Probability Factor (A) of Nonradiative Cross Relaxation between 5D3f5D4 Tb3+ Levels Calculated Considering 1/τ = λ1 = γ1 + A, where γ1 = 767 ( 1 s1 from References 23 and 10 τ (ms)
λ1 (s1)
A (s1)
YSO:Tb 0.001% 1.17 ( 0.04 855 ( 29 88 ( 30 YSO:Tb 0.2% 0.99 ( 0.02 1010 ( 20 243 ( 21 YSO:Tb 1% 0.72 ( 0.04 1388 ( 77 621 ( 78 YSO:Tb 10% (1.57 ( 0. 09) 103 636942 ( 36512 636175 ( 36513
The quenching of the blue emission and the presence of new and efficient excitation channels are fundamental for the development of monochromatic green phosphors for X-ray scintillator or in light-emitting diode (LED)-based devices especially shaped as luminescent thin films or panels. These possible applications could be further boosted by the high efficiency of the emission processes in Tb:YSO calculated by measuring the time decays as a function of the sample temperature (the TR-PL measurements are reported in the Supporting Information, Figure S4 and Table S1). Indeed, we have calculated, for the 5D4 levels (after direct excitation at 490 nm), a time constant at room temperature τRT = 1.59 ms and at 10 K τLT = 1.67 ms, both in the samples with Tb at 0.2 atm % and 10 atm %. As a consequence, the absolute fluorescence quantum yield η, defined as28 Γ τRT η¼ = Γ þ knr τLT
ð5Þ
where Γ = 1/τLT is the radiative rate constant and knr is the non radiative rate constant (which vanishes at low temperature), was estimated to be 0.95 and resulted almost independent of the Tb concentrations. This result discloses that there are no efficient nonradiative channels for the 5D4 levels. Moreover, the time decays from the 5D3 level in Tb-doped YSO samples (excited at 260 nm) does not sensibly depend on the temperature, being 1.1 ms and 1.8 μs at 10 K for the samples with Tb at 0.2 atm % and 10 atm %, respectively, and were estimated to be 0.99 ms and 1.57 μs at room temperature (see Table 1). As a consequence the quantum yield is η = 0.89 and 0.87, confirming that the 5D3f5D4 energy transfer mechanism is independent of the temperature, at least in the investigated temperature range. These findings strongly support the Tb:YSO crystals as efficient phosphors in the visible range, in particular for the green region where an intense and very efficient emission is obtained by increasing the Tb concentration up to 10 atm %.
’ CONCLUSIONS Terbium-doped yttrium oxyorthosilicate phosphor particles were prepared through the solgel route, and their optical and structural properties were analyzed as a function of doping concentration. The samples present the same principal structural phase (C2/c space group) and a low secondary cation-deficient phase identified as apatite type (Y4.67(SiO4)3O,) independently from the Tb concentration. However, the increase in the oxyapatite phase in the sample with the largest Terbium concentration (10%) suggests that higher doping concentration can affect the stability of yttrium silicate crystalline phase. The optical properties were studied through RL spectroscopy, PLE in the visible/UV range, and time-resolved luminescence. The analysis evidences a very efficient charge transfer mechanism
from the 5D3 excited level to the 5D4 level of Tb3+ ions that generates almost total quenching of the blue emissions (5D3f7FJ) and a large increase of the green PL (5D4f7FJ) in the RL spectrum of the sample with 10% Tb3+ concentrations. A simple three-level kinetics model was applied to explain the experimental data as a function of terbium doping, allowing us to give an estimation of the efficiency of the charge transfer process. Besides the interest in the fundamental parameters in the highenergy states of Re ions, the proposed research indicates the heavy Tb-doped YSO matrix as a good candidate for the development of efficient UV LED-excited green phosphors in mercury-free fluorescent lamps and for a new generation of scintillator panels for X-ray radiography.
’ ASSOCIATED CONTENT
bS
Supporting Information. Optical image of the samples without the excitation, TEM images of the sample Tb:YSO (Tb content at 10 atm %), PLE and PL spectra of the YSO sample with the lowest concentration (0.001 atm. %), time-resolved luminescence at 10 and 290 K, and the calculated values of the quantum yield of the 5D3 and 5D4 levels in the samples with Tb contents at 0.2 atm % and 10 atm %. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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