Photoluminescence characterization of porous YAG: Yb3+–Er3+ nanoparticles

Journal of Luminescence 153 (2014) 21–28

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Photoluminescence characterization of porous YAG: Yb3 þ –Er3 þ nanoparticles H. Desirena a,n, L.A. Diaz-Torres a, R.A. Rodríguez b, O. Meza c, P. Salas d, C. Angeles-Chávez e, E.H. Tobar b, J. Castañeda-Contreras b, E. De la Rosa a,n a

Centro de Investigaciones en Óptica, A. P. 1-948, León 37150, Guanajuato, Mexico Centro Universitario de Los Lagos, Universidad de Guadalajara, Lagos de Moreno, Jalisco, Mexico c Instituto de Física, Benemérita Universidad Autónoma de Puebla, Apartado Postal J-48, Centro Historico 72570, Puebla, Mexico d Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Apartado Postal 1-1010, Querétaro 76000, Querétaro México e Instituto Mexicano del Petróleo, Ciudad México, D.F. 07730, México b

art ic l e i nf o

a b s t r a c t

Article history: Received 20 November 2013 Received in revised form 20 February 2014 Accepted 4 March 2014 Available online 12 March 2014

Yb3 þ /Er3 þ codoped yttrium aluminium garnet (YAG) porous nanocrystals were prepared by glycolate method assisted with poly-vinyl alcohol (PVA) and urea. The typical cubic structure for YAG was confirmed from XRD with crystallite average size of  40 nm, calculated from Scherrer formula and corroborated by TEM. Strong green and red upconversion emissions are observed readily with the naked eyes, and the color coordinates were obtained from emission spectra. A theoretical model to calculate CIE coordinate as a function of donor (Yb3 þ ) and acceptors (Er3 þ ) concentration is proposed. The eyesafe near infrared emitted signal and fluorescence lifetime were also measured and results show lifetime as large as 8.5 ms. The maximum energy transfer efficiency from Yb3 þ to Er3 þ was 72% for 20 mol% of Yb2O3. The proposed mechanisms for signal emitted are explained in terms of direct and energy back transfer processes, and cross relaxation. & 2014 Elsevier B.V. All rights reserved.

Keywords: Upconversion Energy transfer Yb3 þ /Er3 þ YAG nanocrystal Decay time Color coordinate

1. Introduction Rare earth doped materials with low phonon energy have been reported to be highly efficient to produce visible emission by the upconversion (UC) process [1–4]. It is of great interest due to the potential application such as visible emission lasers, medical diagnostic, light emitting devices, color-displays, among many others [5–11]. UC process is a mechanism by which at least two low-energy excitation photons, typically in the near infrared (NIR), are converted into one visible emission photon of higher energy. Such process commonly is enhanced with the introduction of Yb3 þ ions as sensitizer, which exhibit a much larger absorption cross section and broad absorption band between 850 and 1080 nm compared to that presented for other ions such as Pr3 þ , Ho3 þ , Tm3 þ and Er3 þ . In fact, its presence make possible to obtain visible emission from lanthanides such as Ho3 þ and Tm3 þ whereas in its absence that is impossible [12]. On the other hand, significant changes in the optical properties have been observed with decreasing particle size. This new class of materials exhibits notable particle-size-dependence affecting the

n

Corresponding authors. Tel.: þ 52 477 441 4243. E-mail addresses: [email protected] (H. Desirena), [email protected] (E. De la Rosa).

http://dx.doi.org/10.1016/j.jlumin.2014.03.012 0022-2313/& 2014 Elsevier B.V. All rights reserved.

emission lifetime, quantum efficiency and concentration quenching of rare earth dopant. Electron confinement effect is not expected for lanthanides due to the localization of electrons in atomic orbital of active ions. However, the excitation dynamics is influenced by the nanoscopic interaction that can change the signal emitted due to modifications on the surface-related defects. Therefore, size and shape play an important role in the spectroscopic properties of such nanophosphor and deserve a careful analysis [13–15]. Based in these characteristics, nanocrystals offers significantly advantages over single crystals. One of them is the higher dopant concentration that in turn allow the fabrication of devices with a relatively small volume. In addition their easy fabrication of large amount of material and no special equipment with high cost is required for the fabrication. YAG (Y3Al5O12) is a well-known host widely used for laser applications and its low phonon energy (550–600 cm  1) makes this nanophosphor very interesting for photonic applications, in particular for the UC processes [16]. High phonon energy of the host that induces fast relaxation of excited states avoiding promotion to an upper level. Thus, low phonon energy glasses are necessary to get an efficient UC process. It is well know that there are many other hosts with lower phonon energy, which increase the probability for UC. However, YAG nanopowder and YAG ceramic, continues to be one of the most favorable laser host

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among all the oxide ceramics. Several works have reported the red and green upconverted signals in YAG:Yb3 þ ,Er3 þ nanocrystals. Emission dependence on dopant ion concentration, the presence of direct energy transfer (ET) and energy back transfer (EBT) between donor (Yb3 þ ) and acceptor (Er3 þ ) ion in nanoparticles has already been reported [17,18]. Here in this work, it is reported the synthesis of porous YAG: Yb3 þ ,Er3 þ nanoparticles by using the precipitation method with PVA and urea as additive, and a detailed analysis of its photoluminescent properties is presented. We focus this study to analyze the visible emission and the physical mechanisms that lead to the observable photoluminescence be explained in terms of direct and energy back transfer and cross-relaxation processes based on a steady-state rate equation analysis. Dependence of energy transfer with Yb3 þ concentration and how such concentration changes the population dynamics is also presented. Color emission was adjusted from green to red light by changing the red to green emission bands ratio via the Yb3 þ concentration. A set of equations to predict color coordinate in terms of ion concentration is proposed. In addition to this, the spectroscopic properties in near infrared are studied in order to evaluate their properties as a laser material.

970 nm, a 2300i spectrograph from Acton Research, a R955 Hamamatsu photomultiplier tube and an InGaAs detector (Thorlabs DET10C). The decay profile (lifetime) corresponding to 554, 655, 1030 and 1534 nm emission was measured by using 970 nm LD at low power and recorded using an SR540 chopper (Stanford Research System) and a Tektronix TDS 3025B oscilloscope. In the experiment, special care was taken to produce a resolution of 20 ms by the right combination of the beam waist of the laser, special modification in the slots of the chopper wheel, the angular velocity of the chopper wheel and coupling impedance of oscilloscope. Samples were supported in 1 mm capillary tubes in order to guarantee the same excitation volume. Special care was taken to maintain the alignment of the set-up in order to compare the intensity of the upconverted signal between different characterized samples. In order to have a better overview of the emission intensity and fluorescence lifetime, we calculated the spectrally average of such parameters and error bar were added to the Figs. 3,4,9 and 10 and represent to standard deviation obtained from quadruplicate experiment. All optical measurements were performed at room temperature.

3. Results and discussion 2. Experimental

3.1. Structural and morphological characterization

2.1. Sample preparation

The diffraction peak at 33.331 dominates the XRD patterns for different Yb3 þ concentrations in Fig. 1 and corresponds to the (4 2 0) crystalline plane of the YAG crystalline structure. All observed peaks are in correspondence with the standard JCPDS #33-0040 of the cubic YAG crystalline phase with space group Ia-3d. The peaks located at 16.811 and 44.681 correspond to (0 0 2) and (1 0 4) crystalline planes of YAlO3 hexagonal phase [19], in correspondence with the standard JCPDS# 74-1334 with space group P63/mmc. The segregated phase represents less than 5 wt%

Porous nanocrystalline YAG:Yb3 þ ,Er3 þ codoped samples were prepared by a simple precipitation method assisted with PVA and urea as additive. All chemicals were reactive grade from SigmaAldrich. In a typical synthesis for the preparation of 1 g of nanophosphor, 3.96 g of poly-vinyl alcohol (PVA) and 2.25 g of urea were dissolved in 100 ml of CO2-free distilled water at 60 1C; then yttrium nitrate (Y(NO3)3–5H2O) and acetic acid (CH3CO2H) were added after 30 min of stirring. In a second beaker, aluminum nitrate (Al(NO3)3–5H2O), Erbium nitrate (Er(NO3)3–5H2O) and ytterbium chloride (YbCl3–6H2O) were dissolved in 50 ml of CO2-free distilled water. Both solutions were mixed and 2 ml of 1,2-ethanediol (ethylene glycol) was added and stirred for 30 min. The resulting solution was dried at 70 1C and then grinded in an agate mortar to obtain a fine powder. This powder was annealed at 300 1C for 2 h, then temperature was raised to 500 1C for 2 h and then to 1000 1C for 4 h, in all cases temperature was increased at a rate of 5 1C/min. The dopant concentration of Er2O3 was 1 mol% and different concentrations of Yb2O3, 1, 2, 5, 10, 20 and 30 mol%, were used. 2.2. XRD, TEM and photoluminescence measurements The crystalline structure of the samples was characterized by using the X-ray diffraction (XRD) of Siemens D-5005 equipment with Cu Kα radiation at 1.5626 ˚. The recorded XRD spectra were obtained from 101 to 701 2θ range with increments of 0.021 and a swept time of 8 s. For transmission electron microscopy (TEM), a JEM-2200FS microscope with accelerating voltage of 200 kV was used. The microscope is equipped with a Schottky-type field emission gun. Local chemical analysis from single nanoparticles was performed with a Noran energy dispersive X-ray (EDX) spectrometer attached to the microscope. Selected samples were suspended in isopropanol at room temperature and dispersed with ultrasonic agitation. Aliquots of the solution were dropped on 3 mm diameter lacey carbon copper grids and left to dry at room temperature. The photoluminescence characterization was performed using a CW semiconductor laser diode (LD) with 350 mW centered at

Fig. 1. XRD spectra for nanocrystalline YAG:Yb3 þ :Er3 þ samples where the dopant concentrations are 1:1, 1:10 and 1:20 mol%.

H. Desirena et al. / Journal of Luminescence 153 (2014) 21–28

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Fig. 2. TEM images of nanodendrites for scale of, (a)100 nm, (b) High resolution image showing the atomic resolution lattice of the dendritesc) low magnification showing the nanodentrites network and the points where was realized the local chemical analysis, Marked by numbers and (d) high magnification showing a single nanodentrite. Notice that the nanodendrites are formed by de coalescence of nanoparticles due to de high temperature during the reaction.

of the overall phase composition and is probably produced by the presence of PVA and the low annealing temperature used at the final synthesis stage [20]. No segregation of dopant oxides was observed even for the larger concentration of Yb3 þ . As has been reported, Yb3 þ (ionic radius of 86 pm) ions substitute at Y3 þ positions (ionic radius of 106 pm) [21], such difference in radius leads to smaller lattice parameter, that is corroborated by the shift to larger angles of diffraction peaks as is observed in Fig. 1. The average crystallite size of nanocrystals calculated from the XRD pattern with the Scherrer formula is  40 nm, and confirmed from TEM micrograph displayed in Fig. 2a and b. Interesting, such nanoparticles coalesce forming highly porous secondary particles with an intricate network, as is displayed in Fig. 2c and d. The atomic resolution lattice (Fig. 2b) confirms the well-crystallized structure. Chemical composition of the sample was analyzed by EDX analysis of several single particles indicated in Fig. 2c. Characteristics peaks of O, Al, Y, Er and Yb were observed in the EDX spectra. The elemental concentration of each component was calculated using a Gaussian fit and correction method Cliff– Lorimer without absorbance. The obtained average chemical composition was 33.15 wt% Y, 55.83 wt% Al, 0.48 wt% Er and 10.53 wt% Yb. It is important to mention that the O was excluded from the concentration calculus because it is a light element that cannot be detected efficiently when is analyzed with a microscope of 200 kV. Then, the nominal composition was adjusted to these elements and the values obtained were 31.4 wt% Y, 55.2 wt% Al,

1.16 wt% Er and 12.1 wt% Yb, very close to the nominal composition of the sample. The particular porous network formation can be explained because the acetic acid and ethylene glycol used during the samples preparation form a resin through condensation reactions. The acetic acid acts as a chelating agent, which chemically binds the cations (produced by nitrate ions) dissolved in the solutions. PVA and urea begin a polymerization process as polymeric network forming the resin and due to the chelating action of acetic acid the cations have a low mobility that alters the precipitation process. In combination with ethylene glycol, PVA and urea operate an entrapment mechanism in the organic–inorganic solution that depends on how large the chain molecule is being formed [22]. The properties of such polymerized networks depend of the degree of polymerization, degree of hydrolysis, and distribution of the hydrolyzed groups. Now, at the drying and calcinations stages, PVA acts as a fuel for nanocrystal formation and as dispersing medium forming chains that control the agglomeration of particles, while urea acts as a fuel agent that increases locally the temperature. Furthermore, the combined fuel system controls the adiabatic flame temperature where the crosslinking of the polymeric network plays a very important role. The linear chains of PVA can be cross-linked for the presence of urea [23]. The PVA chains in this situation may provide small cages that during the reaction can form very small nanoparticles. This means, the cages formed may offer resistance to the agglomeration of the

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H. Desirena et al. / Journal of Luminescence 153 (2014) 21–28

Fig. 3. Emission spectra of 4I13/2-4I15/2 transition of Er3 þ for 1, 10, 20 and 30 mol% of Yb3 þ . The inset shows the integrated intensity as a function of Yb3 þ concentration. Error bar are less than 2% and represent to standard deviation obtained from quadruplicate experiment.

particles and the particle growth. Here, the adiabatic flame temperature (inherent to the reaction) promote the coalescence of the nanoparticles and introduce in the case of PVA a small YAlO3 phase [24]. 3.2. Luminescence properties of Yb3 þ /Er3 þ codoped YAG nanocrystals 3.2.1. Near infrared and visible emission Yb3 þ ions are used commonly as sensitizers to enhance the pumping efficiency of 970 nm LD emission. Such ions exhibits a large absorption cross section and a broad absorption band between 850 and 1080 nm compared with weak absorption of Er3 þ ions [4]. The overall result is the increment of population in the level 4I13/2 of Er3 þ . The eye-safe near infrared (NIR) emission associated to 4I13/2-4I15/2 transition in the YAG:Yb3 þ /Er3 þ nanocrystals was centered at 1.53 μm as is displayed in Fig. 3. It is well-known that the ion concentration modifies the intensity of the signal emitted, in our samples the signal increase monotonically with the increment of Yb3 þ , showing a maximum at 10 mol% and then decrease for higher concentration, as is indicated in the inset of Fig. 3. Such concentration dependence remark the importance to optimize the concentration of both Er3 þ and Yb3 þ ions to improve the signal emitted. Concentration should be appropriate to minimize the up-conversion emission and non-radiative processes due to cluster formation of ions. Both phenomena reduce the fluorescence lifetime of 4I13/2 level. Although larger concentrations of sensitizer ion help to disperse Er3 þ ion and enhance its excitation, it also enhance the up-conversion emission. Furthermore, larger concentration of sensitizer promotes energy migration Yb3 þ -Yb3 þ quenching the overall signal emitted. The brilliant visible emission from Er3 þ obtained after 970 nm excitation is shown in Fig. 4. The emission bands centred at 524, 554 and 655–670 nm are assigned to 2H11/2, 4S3/2, and 4F9/2-4I15/2 transitions, respectively. These visible emissions bands are the result of the well-known upconversion process and depend on the concentration of Er3 þ and Yb3 þ ions. In our case, we keep constant Er3 þ and change the Yb3 þ concentration. The overall intensity of the upconverted signal increases with Yb3 þ concentration decreasing after 20 mol% as is observed in the inset of Fig. 4. However, the maximum for green and red band was obtained at 10 and 20 mol%, respectively. Before the concentration for maximum signal emitted, both visible bands increase with Yb3 þ concentration but the red band increase faster as displayed in Fig. 5a. This behavior is in agreement with other reports where

Fig. 4. Upconversion emission for 1, 10, 20 and 30 mol% of Yb3 þ . The inset shows the total intensity as a function of Yb3 þ concentration. Error bar are less than 2% and represent to standard deviation obtained from quadruplicate experiment.

the red band increases faster with the increase of Yb3 þ concentration [25]. Thus, a change in the sensitizer concentration induces a change of the color of upconverted signal emitted as is observed in Fig. 5b where a photograph of signal emitted at different concentration of Yb3 þ is presented. Such visible emission was easily observed at the naked-eyes. A green emission was observed for sample doped with 1 mol% of Er3 þ and 1 mol% of Yb3 þ that becomes stronger when Yb3 þ concentration increase from 2 to 10 mol%, changes to orange for 20 mol% and finally turn to red with 30 mol% of Yb3 þ . The increment of upconverted signal is explained in terms of the energy transfer efficiency due to the increment of donor (Yb3 þ ) while the decrement is presumable due to energy migration among Yb3 þ -Yb3 þ ions and energy back transfer from Er3 þ to Yb3 þ . Dependence of the integrated upconverted signal (Iupc) as a function of the pumping power (Ipp) was calculated according to the expression Iupc ¼ kInpp , and the number of photons involved in the process was n 2 for green emission at different concentration of Yb3 þ confirming the expected two-photon process. The same mechanism is expected for the red band, however such value deviate to n  1.6 and for larger concentration of sensitizer n diminishes to 1. Such changes suggest the presence of additional processes strongly related with the presence of Yb3 þ . It can be attributed to the linearization of UC process because of the saturation of 4I13/2 energy level due to the combined effect of cross relaxation (CR) and energy back transfer (EBT) process, as will be discussed below. According to this, the proposed mechanism for both upconverted and NIR signal emitted is displayed in the energy diagram of Fig. 6. The pumping photons (970 nm) populate the 4I11/2 intermediate excited state of Er3 þ ion (acceptor) via energy transfer (ET1) from Yb3 þ (donor) according to the equation 2F5/2 (Yb) þ 4I15/2 (Er)-2F7/2 (Yb) þ 4I11/2 (Er), excited directly (2F7/2 þ hv-2F5/2) by the pumping source. Direct excitation from ground state of Er3 þ is also possible. But, energy transfer is most probably due to the larger absorption cross section of Yb3 þ (10  20 cm2 for Yb3 þ ) [26]. Cross section no calculated here, is strongly affected by the ion environment provided by the host matrix. Such parameter quantify the ability of an ion to absorb or emit light, large emission cross section means high gain coefficient and low threshold energy of laser pump. Thus, in order to obtain the best performance of laser and optical amplifiers, it is necessary to obtain the stimulated emission cross section as high as possible. Several works have calculated this parameter and the obtained results show identical values with small variations either, ceramic or single crystal [26–28]. Since the host matrix used in this work

H. Desirena et al. / Journal of Luminescence 153 (2014) 21–28

Fig. 5. (a) Green, red and integrated signal. (b) Luminescence photograph of YAG nanocrystals as a function of Yb3 þ concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Energy diagram of the Yb3 þ /Er3 þ system and the mechanism proposed to explain the visible and IR emission. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

is YAG, it is expect to obtain similar values for cross section. The probability for ET increase much more due to the resonance between 2F5/2-2F7/2 (10400) and 4I15/2-4I11/2 (10,100 cm  1) transitions of Yb3 þ and Er3 þ , respectively, as is shown in the energy diagram. Part of the population on 4I11/2 excited states relaxes non-radiatively (phonon coupling) to 4I13/2 level and from here relaxes to ground state producing the 1.532 μm emission band. And part was promoted to 4F7/2 level by absorbing a second pumping photon (ESA) or by ET2 from a neighbor ion (either Er3 þ or Yb3 þ ). The population on 4F7/2 level is phonon coupled ( 3 phonons) to the mixed level 2H11/2 þ 4S3/2 that are thermally coupled (  800 cm  1). From here, part of the population relaxes to ground state producing the green emissions centered at 524 and 554 nm, and part decay non-radiatively to 4F9/2 that in turn decay to ground state producing the red band as shown in Fig. 6. However, such relaxation apparently is not dominant because require five phonons to couple the energy gap (  3200 cm  1) between the 4 S3/2 and 4F9/2 energy levels. Another probable mechanism is the

25

phonon coupling by OH  (3450 cm  1) considering the matching with the energy gap, but was not possible to correlate changes on the intensity of the red band with the OH  content observed in the infrared spectra (not showed here). Therefore, the strong enhancement of the red band with the increment of Yb3 þ concentration suggest the dominance of a mechanism associated to the presence of such sensitizer. The proposed mechanism to explain such dependence is based on the CR process associated to the high concentration of acceptors, the ET from donor to acceptor and the EBT from Er3 þ -Yb3 þ that in addition increase the population on 4I13/2 also increase the population of Yb3 þ available to transfer the energy to some neighbor acceptor. The CR process is described by the equation 4S3/2 þ 4I15/2-4I9/2 þ 4I13/2 as is shown Fig. 6, induces a decrement of population on 4S3/2 and an increment on 4I13/2 level. This is a well-known phenomenon strongly dependent of concentration. It means, for larger concentration of Er3 þ the green band is quenched while the red one is enhanced because the population on 4I13/2 is promoted to 4F9/2 by ET3 [2]. Furthermore, the decrement of population on 4S3/2 level (green band) as a function of Yb3 þ concentration suggest the presence of energy back transfer (EBT) strongly promoted by the high concentration of Yb3 þ (acceptor in this case) expressed by the equation 4S3/2 (Er)þ 2F7/2 (Yb)-4I13/2 (Er)þ 2F5/2 (Yb) as shown in Fig. 6 [29]. This process increase the population on 4I13/2 and more excited Yb3 þ available to promote such population to 4F9/2 via ET3 resulting on the quenching of green band and the enhancement of red band as is confirmed from Fig. 5. EBT limits seriously the efficiency of green emission because most of the energy delivered to 4S3/2 level is reversely transferred to Yb3 þ ions being much more pronounced with the increment of concentration [30]. It is unidirectional due to the difference in the energy gap between 4S3/2 and 4I13/2 (11,850 cm  1) levels of Er3 þ slightly larger than that between 2F5/2 and 2F7/2 of Yb3 þ (10,400 cm  1) [31,32]. Therefore, the dominance of 4F9/2 level is mainly due to ET from donor (Yb3 þ ) to acceptors (Er3 þ ) because of the high concentration of the donor but the reduction of red emission, and the overall signal emitted, for larger concentration of donor is due to the energy migration among Yb3 þ -Yb3 þ ions, this phenomenon reduces the direct energy transfer to 4F9/2 of Er3 þ . The contribution of CR and EBT process on the green and red emission was determined by analyzing the behavior of the red/ green ratio that can also be expressed by N4/N5 associated to the population on transitions producing the red and green emission. It was modeled according to transitions described in the energy diagram of Fig. 6 and by using the steady-state rate equation dN i =dt ¼ 0. This set of equation was resolved in detail and reported previously for the same ions in ZrO2 and Y2O3 host [29]. Considering low pump power, assuming N0 ENEr and Na ENYb are the nominal ions densities and the absence of non-radiative relaxation between 4S3/2 and 4F9/2, the ratio N4/N5 was obtained and expressed by N4 C 5b N Yb C 51 NEr  þ ; N5 W 43 þ W 4 W 43 þ W 4

ð1Þ

where C5b and C51 stand for the coefficient of EBT and CR, respectively. W4 is the radiative relaxation rates of the N4 level to the ground state and W43 is the non-radiative relaxation of 4 F9/2-4I9/2 transition of Er3 þ than can be neglected considering the large energy coupling (2450 cm  1) compared with the phonon energy of the host. Eq. (1) is dominated by the first term, N4/ N5 EC5bNYb/W4, corresponding to the EBT process for a population of Yb3 þ much larger than Er3 þ (NYb c NEr), but in the opposite case (NEr c NYb) is dominated by the CR process, N4/N5 EC51NEr/W4. The experimental data of red to green ratio (N4/N5) were fitted with the proposed model from Eq. (1) with a precision of 97% considering C5bNYb /(W43 þW4) 0.1889 and C51NEr/(W43 þW4)0.1904. This in

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H. Desirena et al. / Journal of Luminescence 153 (2014) 21–28

and red emitted band, IðλÞ ¼ N5 I5 ðλÞð2 H11=2 þ 4 S3=2 Þ þ N4 I4 ðλÞ 4

ð4Þ

ð F9=2 Þ

where I4 and I5 are the normalized spectra in the corresponding red and green band range, N4 and N5 stand for the population on 4 F9/2 and 2H11/2 þ 4S3/2 energy levels, respectively. Replacing Eq. (4) in Eq. (2) and substituting in Eq. (3), a general expression for the color coordinate (x,y) was obtained as a function of the green and red upconverted signal x¼

ax N4 N 5 þ bx

  1 bx

1 4 ∑ bi  N N 5 þ b ∑ ai ; x

i

i

R1

Fig. 7. Performance concentration ratio.

of

CR

and

EBT

term

as

function

of

Er3 þ /Yb3 þ

turn confirms that the increment of Yb concentration leads to an increase in the red and a decrement of green emission. The CR and EBT contribution term are complements, the enhancement of one means the decrement of the other one, such as is displayed in Fig. 7 where both terms of Eq. (1) were plotted calculated from the experimental data. Equal contribution of both coefficients was obtained for a concentration ratio NYb/NEr ¼ 1.208 and describes the limit approximation of N4/N5 as was described above. Once each term was calculated for each composition, the CR and EBT coefficient were calculated for different donor concentration. For the same concentration of donor and acceptor the calculated coefficient were C51 4.02  10  18 and C5b  4.06  10  18 but C5b changes to 4.01, 4.44, 4.96, 7.40 and 16.11  10  18 for 2, 5, 10, 20 and 30 mol% of Yb3 þ . Such values are smaller than that reported for values in LiNbO3, 1.8  10  16 for EBT and 4.8  10  16 for CR [33]. 3.2.2. Color tunability as a function of Yb3 þ concentration The color emission of codoped samples as a function of sensitizer concentration can be represented using the CIE tristimulus values X, Y and Z. For a color with a visible (VIS) emission, such values are expressed in terms of the standard observer by using the expression [34], Z X¼ IðλÞxðλÞdλ Z

vis

Z

vis

Y¼ Z¼

vis

IðλÞyðλÞdλ

IðλÞzðλÞdλ

ð2Þ

where xðλÞ, yðλÞ, zðλÞ are the color-matching functions defined by CIE 1931 and I(λ) is the emission spectrum as a function of wavelength. In this system, Y measures the brightness or luminance of a color. Then, the chromaticity of a color can be specified by two normalized parameters x and y derived from the tristimulus values X, Y, and Z, x¼

X ; X þY þZ

y¼

Y ; X þY þZ

ð3Þ

known as the CIE xyY color space and represent the total visible emission (ratio between blue, red and green emission). The center of CIE xyY diagram correspond to white light and are expressed by the coordinates, x ¼0.33 and y ¼0.33. In order to calculate (x,y) color coordinate in terms of acceptor and sensitizer concentration, the experimental spectrum can be rewritten as a sum of the green

1 b y

R1

N4 N5

ð5Þ

a

þ by y

N

∑ bi N 4 þ b1 ∑ ai ; i

5

y i

where bi ¼ 0 I4  i  dλ; ai ¼ 0 I5  idλ; with i ¼ x; y; z. Integrating the corresponding signal from the emission spectra, the CIE xyY color space for YAG:Er3 þ /Yb3 þ nanocrystals is expressed by x¼

3þ

y¼ 

N 4 =N 5 þ 2:67 ; 1:37N4 =N 5 þ 8:64

y¼

N4 =N 5 þ 15:9365 : 3:67N 4 =N5 þ 23:06

ð6Þ

Considering the fitting of experimental data with Eq. (1), the red to green ratio is expressed by N 4 =N 5 ¼ 0:1889N Er ðmol%Þ þ0:1904NYb ðmol%Þ . Then, by substituting the expression for N4/N5 in Eq. (6) the color coordinates space become, þ 0:1904NEr þ 0:1889N Yb x ¼ 2:67 8:64 þ 0:2617NEr þ 0:2596N Yb

y¼

15:93 þ 0:1904N Er þ 0:1889N Yb 23:06 þ 0:6991N Er þ 0:6936N Yb

ð7Þ

From this equation, it is possible to predict the color emission by changing the concentration of donor Yb3 þ and acceptor Er3 þ ions. The color coordinate calculated from the experimental results with Eq. (3) moves from green to greenish when Yb3 þ increase from 1 to 10 mol% and move from orange to red when increase from 20 to 30 mol%, as displayed in Fig. 8. There, it is also shown the calculated values from the proposed theoretical model expressed in Eq. (7). Both, experimental and theoretical, values are listed in Table 1. The excellent matching between the experimental data and the calculated theoretical values of color coordinates x and y for the lowest and highest Yb3 þ concentration confirm the validity of the proposed model as a tool to adjust the color emission as a function of dopant concentration. 3.2.3. Luminescence lifetime The luminescence lifetime depend strongly on the ion concentration. For laser applications, it is necessary to increase lifetime as high as possible, the longer the lifetime the lower the threshold of laser oscillation. The lifetime of all samples under study, for green (554 nm), red (655 nm) and near infrared (1.03 and 1.532 mm) emissions were measured and listed in Table 1. The fluorescence intensity I0 versus the time τ was approximated numerically by the function I ¼ I0 expð  t=TÞ. In our samples the lifetime of 4 I13/2-4I15/2 transition increase from 7.89 to 8.50 ms when Yb2O3 increase from 1 to 20 mol% and then decrease to 8.31 ms for 30 mol%, see Fig. 9. The obtained values are in agreement with other results reported recently [35,39]. Such increment is the result of the higher concentration of sensitizer (Yb3 þ ) and the better excitation process of Er3 þ via the energy transfer from Yb3 þ , as has been reported previously [36]. Furthermore, because the ionic radius of Yb3 þ is similar to that of Er3 þ ion, then sensitizer disperse acceptors enhancing the ET and avoiding cluster formation that in turn avoids luminescence quenching. Moreover, the increase in lifetime is also possible due to the radiation trapping effect. In this phenomenon, photons spontaneously relaxed from 4I13/2

H. Desirena et al. / Journal of Luminescence 153 (2014) 21–28

Fig. 8. Experimental and theoretical color coordinates as a function of Yb3 þ content. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

27

lifetime. This effect increase with the sample size, refractive index and spectral overlap between fluorescence and absorption as has been reported by Sumida et al. [37]. For a YAG single crystal the refractive index is 1.82, consequently all spontaneous emission photons traveling at an angle larger than 33.31 with respect to the normal will experience total internal reflection at the interface between the crystals and air, then most of the spontaneous emission will be trapped into the crystal [38]. Decay time of 4I13/2 transition decrement for concentration larger than 30 mol% of Yb3 þ is explained as a result of energy migration among sensitizer ions and the energy back transfer from Er3 þ to Yb3 þ . These contribution could larger than the radiation trapping effect and could be one of the reason for the decrement of decay time. The behavior is not clear at this stage and more experiments are in progress to elucidate this point. Energy migration, in combination with direct ET, also explain the continuous decrease lifetime of 2 F5/2 energy level from Yb3 þ from 960 to 130 ms, as is observed in the plot of Fig. 9 and listed in Table 1. Furthermore, the calculated lifetime of 4S3/2 transition of Er3 þ was 341 ms for 1 and 2 mol% of Yb3 þ and decrease monotonically to 86 ms when Yb2O3 increase from 2 to 30 mol% while 4F9/2 decrease from 718 to 149 ms, see Fig. 10. Such decrement suggest an strong depopulation of 4 S3/2 and 4F9/2 level probably due to the energy back transfer from 4 S3/2-4I13/2(Er3 þ ) to 2F7/2-2F5/2(Yb3 þ ) transition and energy migration between Yb3 þ ions. 3.3. The energy transfer efficiency

Table 1 Fluorescence lifetime of 4S3/2(Er3 þ ), 4F9/2(Er3 þ ), 4I13/2(Er3 þ ) and 4F5/2(Yb3 þ ) levels and experimental and predicted coordinates chromaticity values. Yb content

Fluorescence lifetime (ms)

x, y Values

(mol%)

4

Experimental

Model

1 2 5 10 20 30

341 345 312 279 187 86

0.349, 0.650 0.358, 0.641 0.363,0. 636 0.396, 0.603 0.462, 0.537 0.516, 0.480

0.333, 0.666 0.344, 0.655 0.373, 0.626 0.413, 0.586 0.470, 0.529 0.510, 0.489

S3/2

4

F9/2

718 681 545 507 311 149

4

I13/2

7890 8060 8420 8480 8520 8310

4

F5/2(Yb)

960 810 520 430 280 130

The energy transfer (ET) efficiency from Yb3 þ to Er3 þ , Yb3 þ (2F5/2) þEr3 þ (4I15/2)-Yb3 þ (2F7/2) þEr3 þ (4I11/2), depends strongly on the donor concentration and can be evaluated by using the expression [39]. n ¼ 1

τYb  Er ; τYb

ð8Þ

where τYb  Er and τYb is the lifetime of 2F5/2 level of Yb3 þ for Yb3 þ doped and codoped Yb3 þ –Er3 þ nanoparticles. The calculated ET was 37% for a concentration of donor (1 mol% Yb) lower than acceptors (2 mol% Er), increase up to 72% for 20 mol% of donors and decrease to 67% when concentration increases to 30 mol%, see Fig. 10. The decrement of ET is probably due to energy back transfer from Er3 þ to Yb3 þ and the energy migration between Yb– Yb ions induced by the agglomeration of such ions. The distance between donors (Yb3 þ ) and acceptors (Er3 þ ) decrease and the

Fig. 9. Decay time of 4S3/2(Er3 þ ), 4F9/2(Er3 þ ), 4I13/2(Er3 þ ) and 4F5/2(Yb3 þ ) levels. Error bar represent to standard deviation obtained from quadruplicate experiment.

level are re-absorbed by the neighboring ions in the ground state (4I15/2 level). This process of re-absorption and re-emission is repeated several times and the overall result is an increase in the

Fig. 10. Compositional dependence of energy transfer efficiency on Yb3 þ concentration in YAG nanocrystals. Error bar represent to standard deviation obtained from quadruplicate experiment.

28

H. Desirena et al. / Journal of Luminescence 153 (2014) 21–28

probability for energy back transfer increase with the increment of Yb3 þ content. This process has been reported previously in the literature in Er3 þ /Yb3 þ codoped tellurite glasses [25]. The obtained ET in the nanocrystal is in agreement with other reports published recently [40], however is lower compared to other host, i.e. silicate and phosphate glasses where efficiencies larger than 96% has been reported [36]. Gapontsev et al. have reported that the ET efficiency from Yb3 þ to Er3 þ is mainly dependent on the ratio, W BT =W MR , energy back transfer efficiency (WBT) from Er3 þ (4I11/2-4I15/2) to Yb3 þ (2F5/2-4F7/2) and multi-phonon relaxation (WMR) of the 4I11/2-4I13/2 transition of Er3 þ [41]. The highest phonon energy of YAG crystals is around 550–600 cm  1 [16], while that of silicate and phosphate glasses is around 1000 and 1100 cm  1, respectively [42]. This fact makes that the multiphonon relaxation rate in YAG nanocrystals decrease in comparison to silicate and phosphate glasses. The overall result reduces the ratio W BT =W MR and this is the reason why the ET efficiency is lower in YAG nanocrystals. 4. Conclusions YAG:Er3 þ /Yb3 þ nanocrystals with average size of 40 nm were prepared and the spectroscopic properties as a function of Yb3 þ concentration were analysed. The use of urea acts as a fuel during the annealing process promoting the coalescence of nanoparticles forming intricate dendrimers-like particles. Based on the experimental results it is concluded that intensity ratios between emissions bands can be adjusted either by choosing properly the Yb3 þ concentration. Below 10 mol% of Yb3 þ strong green emission was observed while above 20 mol% of Yb3 þ color turn to orange– red. Color coordinate can be controlled by adjusting properly the red/green ratio. Such ratio is dominated by CR and EBT process and the contribution depend on Er3 þ and Yb3 þ concentration, respectively. The calculated values for CR and EBT were C51  4.02  10  18 and C5b 4.06  10  18, respectively. The EBT become dominant for a concentration ratio Yb3 þ /Er3 þ Z 1.208. Fluorescence lifetime of 4 I13/2 level increases with an increment of Yb3 þ concentration, partly because help to reduce quenching of Er3 þ and partly because energy transfer was enhanced. The large fluorescence lifetime of this level suggest that this nanocrystals present strong possibilities to be used in lasers and amplifiers design at the eye safe emission. The maximum energy transfer efficiency was 72% for 20 mol% of Yb3 þ , however from 5 to 20 mol% of Yb3 þ content the ET just increase 4%, this suggest the energy back transfer take place strongly. Acknowledgements This work was partly supported by CONACyT, México through grant 134111 and and from the European Community Seven Framework Programme (FP7-NMP-2010-EU-MEXICO).

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