- Email: [email protected]
or Yb3+
ARTICLE IN PRESS
Journal of Luminescence 117 (2006) 1–12 www.elsevier.com/locate/jlumin
Upconversion spectroscopy and properties of NaYF4 doped with Er3þ, Tm3þ and/or Yb3þ J.F. Suyver, J. Grimm, M.K. van Veen, D. Biner, K.W. Kra¨mer, H.U. Gu¨del Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3000 Bern 9, Switzerland Received 8 February 2005 Available online 23 May 2005
Abstract A spectroscopic investigation of NaYF4 powders doped with several different concentrations of Er3þ , Tm3þ and/or Yb3þ is described. Rare earth-doped NaYF4 is known to be a very efficient near-infrared to visible upconverter. The overview emission spectra for all samples are presented and from these the upconversion efficiency is calculated. Raman spectroscopy of undoped NaYF4 is presented here for the first time, demonstrating that the dominant phonon modes in NaYF4 lie between 300 and 400 cm1 . The fact that these phonon modes are also the optically active ones is further demonstrated by temperature-dependent excitation spectroscopy. These surprisingly low-energy phonon modes explain the extraordinarily high upconversion efficiency of the rare earth-doped NaYF4 samples. Excitation spectroscopy up to 70000 cm1 in an NaErF4 sample reveals a multitude of Er3þ 4f excitations, including the illustrious 2F(2)5/2 one that has not been observed in excitation spectroscopy before. From the low-temperature power-dependence of the emission intensities for an Er3þ , Yb3þ codoped NaYF4 sample, it is concluded that the dominant upconversion mechanism at low temperature is a different one than at room temperature. From direct excitation, the lifetimes of the Yb3þ 2 F5=2 ! 2 F7=2 , Er3þ 4 F9=2 ! 4 I15=2 and Er3þ 4 S3=2 ! 4 I15=2 emissions are determined as a function of temperature for all samples. At elevated temperatures, a significant decrease in the green lifetime is observed, which is correlated to a simultaneous quenching in the luminescence intensity. This quenching is ascribed to cross-relaxation between two nearby Er3þ ions. r 2005 Elsevier B.V. All rights reserved. PACS: 78.20.e; 42.65.Ky; 42.70.Nq; 78.55.m; 78.55.Fv Keywords: Near-infrared to visible upconversion; Rare earth luminescence; NaYF4; Raman spectroscopy
1. Introduction Corresponding author. Present address: Philips Research Laboratories, Eindhoven, The Netherlands. Tel.: +41 31 631 4254; fax: +41 31 631 4399. E-mail address: [email protected] (J.F. Suyver).
There are several processes by which nearinfrared (NIR) radiation can be converted into the visible spectral range [1]. The most famous
0022-2313/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2005.03.011
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J.F. Suyver et al. / Journal of Luminescence 117 (2006) 1–12
one, frequency conversion in nonlinear materials [2], requires high-intensity coherent radiation. As such, this process is not very well suited for possible applications that utilize low-intensity incoherent NIR excitation radiation, such as conversion of the NIR ‘‘waste-light’’ generated by an incandescent lamp to the visible [3]. Another interesting suggestion, application of upconverting phosphors to enhance a silicon solar cell photoresponse in the NIR spectral range [4], will also be difficult to achieve using frequency conversion in nonlinear materials. Fortunately, a completely different nonlinear process exists that is more suitable for applications not requiring high-intensity coherent radiation: photon upconversion (UC) [5,6]. Due to the prerequisite of more than one metastable excited state to be present for UC to occur [7], typically rare earth ions show this effect. The many interesting applications for (rare earth) upconversion, such as lasers [8,9], infrared quantum counters [10,11] and lighting or displays [3,12], clearly indicate that this expanding field warrants significant attention. There exist a large number of materials in which upconversion has been observed, with very large differences in the actual upconversion efficiency. Most of these materials are low phonon-energy halide host-lattices [1,13–15] or glassy hosts [16–19], both with trivalent rare earth ions present as dopants. Already for quite some time, the NaYF4 host lattice has been widely recognized as one of the most efficient upconversion lattices [20,21]. Even though efficient UC in this lattice has been known since the 1970s (specifically when it is doped with Er3þ , Yb3þ or Tm3þ , Yb3þ ) only a few fundamental research papers have appeared regarding this particular material [22–24]. This is probably related to difficulties in the synthesis of phase-pure NaYF4, usually resulting in the presence of both the (efficiently upconverting) hexagonal b-phase and the (comparably poorer upconverting) cubic a-phase. Recently, our group investigated and optimized the reproducible synthesis of high-quality micrometer-sized NaYF4 powders in the hexagonal bphase doped with trivalent lanthanides (substituting for Y3þ ) [25]. It was found that with this new synthesis route, highly stable materials could be
grown. Besides the phase purity, the efficiency also depends strongly on the dopant concentration, the ratio of Na to Y in the starting materials, and the preparation temperature. In the pursuit to find the most efficiently upconverting sample, the ideal concentrations for NIR to green upconversion were found to be 2% Er3þ and 18% Yb3þ . These values were obtained through systematic variation of the Er3þ and Yb3þ concentrations present in the NaYF4 host-lattice. As a result of this investigation, several less-efficient Er3þ -doped NaYF4 samples, with Er3þ concentrations ranging between 2% and 20%, were also obtained. An extensive investigation and comparison of these samples and the efficient (Yb3þ codoped) one will be discussed here. Furthermore, NaYF4: 0.3% Tm3þ , 25% Yb3þ (one of the most efficient NIR to blue/violet upconverters known) will also be included in this discussion. All of these samples were grown under identical synthesis conditions and only the dopant ions and concentrations were varied.
2. Experimental The powders were synthesized using the synthesis route described in Ref. [25]. The powders were excited with a multimode, standing wave Ti:sapphire laser (Spectra Physics 3900S), pumped by the second harmonic of a Nd:YVO4 laser (Spectra Physics Millennia CS-FRU). The wavelength control of the Ti:sapphire laser was achieved by an inchworm driven (Burleigh PZ-501) birefringent filter and a wavemeter (Burleigh WA2100). Sample cooling was achieved using a quartz helium gas flow tube. The sample luminescence was dispersed by a 0.85 m double monochromator (Spex 1402) with gratings blazed at 500 nm (1200 grooves/mm). The signal was detected with an cooled photomultiplier tube (Hamamatsu R3310) and a photon counting system (Stanford Research SR400). The luminescence spectra were corrected for the instrument response, the refractive index of air and are subsequently displayed as a photon flux per constant energy interval [26]. To measure the power dependence, the beam was attenuated with a series of neutral density filters (Balzers). The
ARTICLE IN PRESS J.F. Suyver et al. / Journal of Luminescence 117 (2006) 1–12
3. Results and discussion
1 4
25
20 Energy (103 cm-1)
temporal evolution of the luminescence signal was obtained using as a excitation source the Ramanshifted (Quanta Ray RS-1 equipped with a 340 psi H2 gas cell) output of a dye laser (Lambda Physik FL 3002) pumped with the second harmonic of a Nd:YAG laser (Quanta Ray DCR 3). The luminescence was dispersed through a 0.75 m single monochromator (Spex 1702) with a grating blazed at 750 nm (600 grooves/mm) and detected with a photomultiplier tube (Hamamatsu R3310) coupled to a multichannel scaler (Stanford Research SR 430). Raman measurements were performed on a Bomem DA8 Fourier transform infrared spectrometer equipped with a Raman attachment and a liquid nitrogen-cooled InGaAs detector. Excitation of the samples was done using the 9394:5 cm1 radiation from a Nd3þ :YAG laser (Coherent Compass 1064-2500MN). The laser line was removed using two holographic supernotchplus filters (Kaiser Optical Systems). A resolution of 4 cm1 was used. Duran glass capillaries served as sample holders.
3
15
3/2 4 5/2 F 7/2 2
1
4
3
F 9/2
3
I9/2
4
2
I11/2
3
I13/2
3
5
4
2
I15/2 Er
3+
3
F 7/2
3+
Yb
F 2,3 H4
F 5/2
4
0
G4
H 11/2 4 S 3/2
4
10
D2
G 11/ 2 2 H 9/2
Tm
H5 F4
H6
3+
Fig. 1. Schematic energy level diagrams, upconversion excitation and visible emission schemes for the Er3þ , Tm3þ and Yb3þ ions. Full, dotted, dashed and curly arrows indicate radiative, nonradiative energy transfer, cross-relaxation and multiphonon relaxation processes, respectively. Only those emissions that lie in the visible spectral region are shown here. Further emissions exist, as can be seen in Fig. 2.
3.1. Upconversion emission spectra Fig. 1 shows a schematic diagram of the positions of the energy levels of the three trivalent rare earth ions that are relevant for this investigation. The diagram also indicates several of the most important energy transfer upconversion, cross-relaxation and nonradiative multiphonon mechanisms that occur in Er3þ , Tm3þ and Yb3þ when they are incorporated into NaYF4. Note that, besides the visible emission transitions indicated in the figure, strong infrared emissions also occur. The relative importance of all these transitions will be discussed below. Fig. 2 shows the room temperature emission spectra of five representative samples under NIR excitation. No significant emission was observed outside the region shown in the figure. The energy transfer mechanism involved in excitation of the Er3þ , Yb3þ codoped sample, was described in the literature recently [27]. From Fig. 2 it is clear that all samples show strong emissions in the visible
spectral range upon NIR excitation. Furthermore, of all samples shown, the NaYF4:Er3þ , Yb3þ sample shown in Fig. 2(a) is by far the most efficient upconverter. Upon addition of more Er3þ to the NaYF4 lattice there is a strong increase in the red to green emission ratio, as can be seen by comparison of Figs. 2(b)–(d). This effect is related to the increased probability of the two-ion Er3þ cross-relaxation process j4 I15=2 ; 4 S3=2 i ! 4 4 j I13=2 ; I9=2 i that occurs at higher Er3þ concentrations. This process depopulates the green-emitting 4 S3/2 state, thereby increasing the red to green ratio. Furthermore, the cross-relaxation also results in an increasing population of the 6500 cm1 emitting state (4I13/2), as can be seen from the data presented in the figure, as well as from the values shown in Table 1. For the discussion of the upconversion efficiency of these samples, Fi will denote the number of photons emitted in band i (i.e.: the integral of
ARTICLE IN PRESS J.F. Suyver et al. / Journal of Luminescence 117 (2006) 1–12
4
the emission band as shown in Fig. 2) and pi is the number of NIR excitation photons required to excite this emission (values shown in Tables 1 and 2). With these definitions
(a)
, pi F i
X
! pj F j
(1)
8j
(b)
x100
x50
Photon flux (a.u.)
(c)
x25
(d)
x100
(e)
5
10 15 20 25 Emission energy (103 cm-1)
Fig. 2. Emission spectra of the samples: (a) NaYF4: 2% Er3þ , 18% Yb3þ , (b) NaYF4: 2% Er3þ , (c) NaYF4: 10% Er3þ , (d) NaYF4: 20% Er3þ , (e) NaYF4: 0.3% Tm3þ , 25% Yb3þ . The excitation energy (indicated by the arrow on the top) was at 10238 cm1 for the spectra shown in (a), (e) and at 10311 cm1 for the other spectra. The measurements were recorded at room temperature and in the high-power limit (80 W=cm2 , unfocussed). The assignments of these transitions can be found in Tables 1 and 2.
represents the fraction of all NIR photons absorbed by the material that contribute to emission band i when one assumes that there is no nonradiative multiphonon-relaxation from the lowest excited 4f state to the ground state [27]. This assumption is reasonable for the samples that are discussed here as the luminescence lifetime of the first excited state has been measured to be in the millisecond range under direct excitation, as will be shown in Section 3.5. Table 1 shows for each of the significant Er3þ related emission bands shown in Fig. 2 the fraction of all NIR photons absorbed that contribute to emission in this band, calculated using Eq. (1). The data is shown for the high-power limit, where all energy transfer steps are saturated [28]. From the data it is clear that the NaYF4: 2% Er3þ , 18% Yb3þ sample is indeed an exceptionally efficient upconverter as 50% of all NIR excitation photons contribute to the upconversion emission bands, mainly in the two bands in the visible. The UC efficiency of the samples only doped with Er3þ are considerably lower than that of the codoped sample, which is related to the much weaker absorption strength of the Er3þ 4 I15=2 ! 4 I11=2 transition compared to the Yb3þ 2 F7=2 ! 2 F5=2 one, combined with the much higher Yb3þ concentration relative to Er3þ . Nevertheless, also the Er3þ only doped NaYF4 materials can be considered to be efficient upconverters, especially when compared to other upconverting phosphors [14] or glasses [18]. Also the NaYF4: 0.3% Tm3þ , 25% Yb3þ sample is evidently a very good upconverter, as Table 2 shows that 40% of all NIR excitation photons are upconverted in this sample at high excitation powers. However, since the energy level structure of the Tm3þ ion is such that it requires at least three NIR excitations on Yb3þ in order to obtain
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Table 1 Fraction of excitation photons absorbed that are subsequently used to excite each of the emission bands in the high power limit, as calculated using Eq. (1), for four differently doped NaYF4:Er3þ samples. The doping concentrations are indicated in the table. Also shown are the assignments of the emission bands and the (minimum) number of excitation photons needed to excite these emission bands (denoted by pi ) i
Energy (cm1 )
Electronic transition
pi
2% Er3þ , 18% Yb3þ
2% Er3þ
10% Er3þ
20% Er3þ
1
6500
1
13%
15%
34%
56%
2
10000
Er3þ : 4 I13=2 ! 4 I15=2 8 < Yb3þ : 2 F5=2 ! 2 F7=2
1
36%
83%
63%
37%
3þ
4
4
: Er : I11=2 ! I15=2 3
11800
Er3þ : 4 S3=2 ! 4 I13=2
2
3.1%
0.5%
0.5%
1.9%
4
15000 18200
Er3þ : 4 F9=2 ! 4 I15=2
2
24%
0.2%
1.4%
4.6%
Er3þ : 4 S3=2 ! 4 I15=2
2
23%
0.7%
0.5%
0.3%
3
1.1%
7 105 %
4 104 %
2 102 %
18800 6
3þ
2
4
Er : H11=2 ! I15=2 Er3þ : 2 H9=2 ! 4 I15=2
24000
Table 2 Fraction of excitation photons absorbed that are subsequently used to excite each of the emission bands in the high power limit, as calculated using Eq. (1), for a NaYF4: 0.3% Tm3þ , 25% Yb3þ sample. Also shown are the assignments of the emission bands and the (minimum) number of excitation photons needed to excite these emission bands (denoted by pi ) i
Energy (cm1 )
1 2 3
5500 6800 9900
4 5 6 7 8
12450 15400 21050 22200 27500
Electronic transition
pi
Tm3þ : 3 F4 ! 3 H6 Tm3þ : 3 H4 ! 3 F4 Yb3þ : 2 F5=2 ! 2 F7=2
1 2 1
13% 4% 46%
Tm3þ : Tm3þ : Tm3þ : Tm3þ : Tm3þ :
2 3 3 4 4
35% 0.2% 1.1% 0.4% 0.04%
3
H4 ! 3 H6 F2 ; 3 F3 ! 3 H6 1 G4 ! 3 H6 1 D2 ! 3 F4 1 D2 ! 3 H6 3
Intensity (a.u.)
5
0
200
400
600
800
1000
Raman shift (cm-1) Fig. 3. Raman spectrum of an undoped NaYF4 powder. The data was measured at room temperature with a Fourier Transform Infrared Spectrometer using 9394:5 cm1 excitation radiation. The three dominant phonon modes are found to be 298, 370 and 418 cm1 , respectively. The measurement was recorded at room temperature.
3.2. Raman and excitation spectroscopy one visible Tm3þ emission, the NIR to visible UC efficiency of the sample is only 2% (mainly into the two blue/violet emission bands). Most of the UC emission (35% of all absorbed NIR photons) is in the NIR, into the 3H4 band centered at 12450 cm1 (880 nm). This extremely high UC efficiency suggests that even lasing might be achievable on this transition when one is able to grow a macroscopic single crystal of NaYF4:Tm3þ , Yb3þ that is of optical quality.
Fig. 3 shows the Raman spectrum of a NaYF4 sample. The sample did not contain rare earth dopants in order to prevent their photoluminescence obscuring the much weaker Raman signal. The intensity found in the tail of the spectrum (above 550 cm1 ) is due to an instrumental feature. Three strong Raman peaks are observed, along with a few weaker ones that are not fully resolved due to the instrumental resolution. The
ARTICLE IN PRESS
F5/2|0〉
2
Photon flux (a.u.)
10220
10230
10240
10250
2
Excitation energy (cm-1)
10200
10300
10400
10500
Excitation energy (cm-1)
Excitation energy maximum (cm-1)
Fig. 4. Excitation spectrum of the 18200 cm1 emission for the NaYF4:Er3þ , Yb3þ sample, recorded at 5 K. The inset shows a zoom-in of the 2 F7=2 j0i ! 2 F5=2 j1i transition in Yb3þ , recorded at 75 K, as well as a Lorenzian fit to the data. The spectrum was linearly corrected for the power-dependence of the laser emission wavelength. The relatively poor signal-to-noise ratio is related to the strong quenching of the upconversion luminescence on reducing the temperature in this sample.
10250
10246
10242
10238 0
50
(a)
100
150
200
150
200
Temperature (K)
60 -1
strongest three phonon modes in NaYF4, which are also the only fully-resolved ones, are found to be 298, 370 and 418 cm1 , respectively. The weighted average position of the overlapping band system is 360 cm1 . This is considerably lower than that of comparable fluoride host lattices, such as LiYF4 (570 cm1 ) [29]. Probably the dominant electron-phonon coupling to this surprisingly lowenergy phonon mode is for a large part responsible for the fact that NaYF4 is such a suitable host lattice for efficient upconversion with different rare earth ions (most notably Er3þ and Tm3þ ). This low value for the phonon energy may be attributed to the higher coordination number for Y in NaYF4 with respect to LiYF4 (9 rather than 8). Furthermore, the shorter Y–F distances in LiYF4 as compared to NaYF4 can also play a role. An effect of the larger mass of Na as compared to Li, as well as a possible weaker bond-strength in NaYF4 relative to LiYF4 cannot be excluded either. Further research will be required to elucidate this phenomenon in detail. In order to verify the influence of the low energy phonon modes in NaYF4 on its spectroscopic characteristics, the 5 K excitation spectrum of the 4 S3/2 emission in the NaYF4:Er3þ , Yb3þ sample was measured. The data is shown in Fig. 4. The two lowest-energy excitation lines are due to transitions into two crystal field components of the 2F5/2 excited state of Yb3þ , as indicated in the figure [27]. The other Er3þ and Tm3þ related emission bands shown in Fig. 2(a), (e) all have comparable excitation spectra. The inset in Fig. 4 shows a zoom-in of the 2 F7=2 j0i ! 2 F5=2 j1i excitation line of Yb3þ , recorded at 75 K, as well as a Lorenzian fit to the data. Clearly, the fit accurately describes the data and the peak position and full-width at half-maximum (FWHM) can be determined accurately. Fig. 5 shows the temperature dependence of the peak energy (a) and FWHM (b) determined from Lorenzian fits to the temperature-dependent excitation spectra. At 5 K, the linewidth is inhomogeneous with a value of 2 cm1 . At temperatures above 50 K the linewidth becomes essentially homogeneous. This effect is demonstrated by the good Lorenzian fit to the data at elevated temperatures (such as in the inset in Fig. 4).
F5/2|1〉
J.F. Suyver et al. / Journal of Luminescence 117 (2006) 1–12
Excitation linewidth (cm )
6
50 40 30 20 10 0 0
(b)
50
100
Temperature (K)
Fig. 5. Temperature dependence of the (a) energy EðTÞ and (b) width GðTÞ of the Yb3þ 2 F7=2 j0i ! 2 F5=2 j1i excitation in a NaYF4:Er3þ , Yb3þ sample. The emission observed was at 9980 cm1 . The lines through the data are fits using Eqs. (2) and (3) with an effective phonon energy of _o ¼ 350 cm1 .
ARTICLE IN PRESS J.F. Suyver et al. / Journal of Luminescence 117 (2006) 1–12
EðTÞ ¼ E 0 þ C T 4
Z
_oD kB T
dx
0
ex
x3 , 1
(2)
where E 0 is the energy of the transition at 0 K, oD is the Debye cut-off frequency (i.e.: the energy of an effective phonon mode that couples to the optical transition) and C is a (positive) constant. Similarly, the temperature dependence of the homogeneous linewidth of the excitation band has been determined in the weak-coupling limit [30] 0
GðTÞ ¼ G0 þ C T
7
Z 0
_oD kB T
dx
x6 ex , 1Þ2
ðex
(3)
where G0 is the experimental linewidth at 0 K, oD is the Debye cut-off frequency and C0 is a (positive) constant. G0 contains both a homogeneous and an inhomogeneous (2 cm1 in this case) contribution. Eqs. (2) and (3) were used to simultaneously fit the temperature dependence of the spectral position EðTÞ and the linewidth GðTÞ of the excitation of the Yb3þ 2 F7=2 j0i ! 2 F5=2 j1i transition. This particular transition was chosen as it has the largest oscillator strength. The fits are shown together with the data in Fig. 5. The fits provide an effective phonon energy of 350 cm1 , in very good agreement with the Raman data presented earlier in this section. The complimentary results of the two very different sets of data shown in this section convincingly demonstrate that the energies of the phonon modes that dominate the electron-phonon coupling in Er3þ , Tm3þ and Yb3þ -doped NaYF4 are indeed extraordinarily low. It is postulated that this is one of the determining factors for the highly
efficient upconversion characteristics in these materials. 3.3. High-energy excitation spectrum Fig. 6 shows the 10 K excitation spectrum of the 18200 cm1 emission related to the 4 S3=2 ! 4 I15=2 transition in the high-energy regime for a NaErF4 sample. The spectrum was measured at the Double Ring Store III beam line of the Synchrotronstrahlungslabor HASYLAB at the Deutsche Elektronen-Synchrotron DESY in Hamburg (Germany). In the excitation spectrum the ultimate spectral resolution was better than 0.3 nm. The spectrum was corrected for the spectral intensity distribution of the synchrotron radiation using a spectrum provided by DESY. The excitation spectrum in Fig. 6, in the range of 30000–67500 cm1 , shows a multitude of sharpline excitations that can all be assigned to transitions to Er3þ related 4f11 multiplets, see Table 3. In the same table the theoretically predicted values for Er3þ -doped LiYF4 are presented [31,32]. The theoretical values were obtained from the literature based on energy level calculations using D2d symmetry, and the theoretical values include d-function correlation crystal field
40
Photon flux (a.u.)
Theoretical work on the shift of zero-phonon lines has been done using approximations that are valid in the weak-coupling limit [30]. The very small Huang-Rhys parameter associated with the excited states of trivalent rare earth ions implies that the weak-coupling limit can be used in this case [37]. Under the assumption of a Debye distribution for the phonon density of states, the temperature dependence of the energy of the zerophonon line will be given by [30]
7
30 62500
20
63500
64500
10
0 30000
40000
50000
60000
Excitation energy (cm -1 )
Fig. 6. High-energy excitation spectrum of the 4 S3=2 ! 4 I15=2 emission in a NaErF4 sample. The data was recorded at 10 K using a synchrotron radiation excitation source and the assignment of all transitions is given in Table 3. The inset shows a high-resolution zoom-in of the 2F(2)5/2 excitation line in the range of 62500–64500 cm1 .
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Table 3 Experimentally observed high-energy levels of Er3þ in NaYF4:Er3þ as well as the theoretically predicted levels for LiYF4:Er3þ . The experimental uncertainty in the data increases from 15 to 50 cm1 with increasing excitation energy Experiment (cm1 )
Theory (cm1 )
Assignment
31630 33044 33176 33442 34125 34945 36509 38668 39353 41147 41610 41763 42350 42946 43564 47208 47886 48181 49009 51191 54727 55278 63345
31656a 33076a 33176a 33458a 34131a 34933a 36509a 38694a 39343a 41145a 41596a 41768a 42368a 42953a 43507a 47235b 47911b 48218b 49082b 51240b 54713b 55336b 63369b
2
P3/2 K13/2 2 P1/2 4 G5/2 4 G7/2 2 D5/2 2 H(2)9/2 4 D5/2 4 D7/2 2 I11/2 2 L17/2 2 L17/2 4 D3/2 2 P3/2 2 I13/2 4 D1/2 2 L15/2 2 H(1)9/2 2 D(2)5/2 2 H(1)11/2 2 F(2)7/2 2 D(2)3/2 2 F(2)5/2 2
a
Data taken from Ref. [31]. b Data taken from Ref. [32].
For the energy range shown in Fig. 6 all expected Er3þ -related excitation transitions have been observed. Most interestingly, also direct excitation into the 2F(2)5/2 multiplet is observed (see inset of Fig. 6). This particular excitation has never been presented in the literature before. This is related to the fact that in previous experiments, performed on LiYF4:Er3þ , the broad 4f 10 5d excitations are at too low energy to allow the 2 F(2)5/2 multiplet to be resolved [32]. The observation of all important Er3þ excitation bands up to 67500 cm1 proves that the (micrometer sized) NaErF4 crystals are of good optical quality. Furthermore, the crystals do not contain significant defects or impurities that would generate optically active states within the bandgap of the material. Finally, this result suggests that, under suitable pumping conditions, upconversion to the vacuum ultraviolet spectral range may be achievable in NaYF4 as well, similar to what was observed in the case of LiYF4 crystals under XeF laser excitation [35]. 3.4. Power dependence of the low temperature emission intensity In the low-power limit, the upconversion intensity for any emission band I depends nonlinearly on the laser power P. It is typically written as [36] IðPÞ / Pn ,
contributions [31–33]. There is good agreement between the calculated and experimental values. This is also expected since the energies of the rare earth multiplets in NaYF4 are close to those in LiYF4. It should be noted that the 2Sþ1 LJ assignments are not always unambiguous, but can depend on the energy level calculations that are used in their determination [34]. Here the assignments most accepted in the literature are used without further investigation [31,32]. At energies above 65000 cm1 a broad excitation band is observed, which stretches until at least 90; 000 cm1 . This broad band is most likely attributed to 4f 11 ! 4f 10 5d excitations in Er3þ . However, F to Er3þ charge-transfer transitions could also contribute.
(4)
where n denotes the number of NIR photons that must be absorbed to excite one upconversion photon. In the specific case of Er3þ or Tm3þ , these values of n are identical to the pi shown in Tables 1 and 2. Fig. 7 shows the measured power dependence of the three major Er3þ upconversion emissions in NaYF4:Er3þ , Yb3þ at 5 K and room temperature. The data are recorded in the low-power limit, as at higher powers a decrease of the slopes is observed that is related to a change in the main depopulation mechanism of the excited states [28]. It is important to note that the upconversion emission intensity at 5 K is many orders of magnitude weaker than that at room temperature, which is related to the absence of direct Yb3þ 2 F5=2 ! Er3þ
ARTICLE IN PRESS
Slope = 2
10 10
S3/2 Emission intensity (a.u.)
11
10
10
9
8
10
Slope = 3
7
2
3
4
5 6 7 8 9
2
3
1
4
10 14 Slope = 2
10 13 10 12 10 11 10 10 10 9
5 6 7 8 9
Slope = 2 6
10
2
Excitation density (W/cm )
10 9
2
8
0.1
(b)
3
4
6
8
2
1
3
Excitation density (W/cm2)
4
6
8
10
Slope = 3
10 8
Slope = 4
10 7
2
H9/2 Emission intensity (a.u.)
(a)
9
4
10
4
F9/2 Emission intensity (a.u.)
J.F. Suyver et al. / Journal of Luminescence 117 (2006) 1–12
5
6
7 8 9
2
1
(c)
3
4
5
2
6
7 8 9
10
Excitation density (W/cm )
Fig. 7. Power dependence of the (a) 4 F9=2 ! 4 I15=2 , (b) 4 S3=2 ! 4 I15=2 and (c) 2 H9=2 ! 4 I15=2 transitions of the NaYF4:Er3þ , Yb3þ sample in the low power regime. Both the data recorded at room temperature (&) as well as that recorded at 5 K () is shown. Excitation was at 10238 cm1 and the vertical scales cannot be compared. Note the logarithmic axes. The slopes indicated are fits using Eq. (4).
4
I11/2 energy transfer at low temperature [27]. Clearly, Fig. 7 shows that the green 4S3/2 UC remains a two-photon process regardless of the temperature while the red 4F9/2 and violet 2H9/2 emissions show a different power-dependence at room temperature and at 5 K: at room temperature they are normal two- and three-photon processes, respectively. But at 5 K they are threeand four-photon processes, respectively. The highly puzzling observation that at 5 K the 4 F9=2 emission is a three-photon process while the 4 S3/2 emission remains a two-photon process must be explained. For this, it must be noted that at low temperature there is roughly an order of magnitude more green upconversion emission as compared to the red emission. This excludes any energy transfer upconversion process bypassing the Er3þ 4I11/2 state in the first Yb3þ ! Er3þ step,
as such a bypass would imply a significantly reduced green upconversion emission intensity. Therefore, it seems reasonable to ascribe the threephoton process populating the 4F9/2 through involvement of the 4S3/2 for the first and second excitation steps. The final mechanism is then explained as the following: after the normal twophoton process into the 4S3/2, a cross relaxation occurs j4 S3=2 ; 4 I15=2 i ! j4 I9=2 ; 4 I13=2 i. This is followed by an energy transfer upconversion step from Yb3þ 2F5/2, bringing Er3þ from the 4I13/2 into its 4F9/2 state. Interestingly, this implies that the main population process of the Er3þ 4I13/2 state is a two-photon process at low temperatures. This seems reasonable, and in-line with our earlier observation that the main population process of the Er3þ 4I13/2 state is also a two-photon process at room temperatures in the high-power limit [27].
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This effect was explained through the same crossrelaxation step mentioned previously. After the three-photon excitation-path into the red-emitting Er3þ 4F9/2 state, only the four-photon excitation of the violet-emitting 2H9/2 state at 5 K remains to be explained. From energetic considerations it is seen that an energy transfer upconversion pathway from 4F9/2 into 2H9/2 must involve the Yb3þ 2F5/2 state directly. No Yb3þ 2 F5=2 ! Er3þ 4 I13=2 energy transfer can take place for this case, because the remaining energy would be insufficient to excite Er3þ into the 2H9/2. Therefore, an energy transfer upconversion step is proposed from the Yb3þ 2F5/2 to the Er3þ 4F9/2 bringing the Er3þ into 4G9/2, followed by nonradiative multiphonon relaxation to 2H9/2. This explains the four-photon power dependence of the 24000 cm1 emission. At room temperature, the very efficient direct Yb3þ 2 F5=2 ! Er3þ 4 F11=2 energy transfer becomes much more important [27] and this completely dominates the inefficient three-photon upconversion mechanism into the 4F9/2 state. As a result, the room-temperature mechanism that populates the red emitting state will be a twophoton process, as is also shown by the data in Fig. 7 and conforms to the schematic representation shown in Fig. 1.
3.5. Lifetime and intensity quenching Direct excitation into the infrared (Yb3þ 2F5/2 or Er3þ 4I11/2 at 10200 cm1 ), red (Er3þ 4F9/2 at 15100 cm1 ) and green (Er3þ 4S3/2 at 18300 cm1 ) emitting states of Yb3þ and Er3þ was performed by exciting with 10 ns laser pulses at roughly 400 cm1 higher energy than the respective emissions. Table 4 shows the lifetimes obtained from single-exponential fits to the decay data for the transitions and samples indicated. The set of data includes all four Er3þ doped samples and the lifetimes are shown for temperatures ranging from 5 K to room temperature. For all samples, the data could be fitted well using a single-exponential decay for at least the first 1–2 orders of magnitude of the signal intensity. The data shown in Table 4 reveal that for increased temperatures, there is virtually no change in the lifetime of the Yb3þ 2F5/2 and Er3þ 4 I11/2 emissions. Similarly, only a small change in the lifetime of the Er3þ 4F9/2 emission at around 15100 cm1 is observed. In contrast, a drastic decrease in the lifetime of the Er3þ 4S3/2 emission at 18300 cm1 is observed, especially in the samples with a higher Er3þ concentration. Due to the strong dependence on Er3þ concentration, this reduction of the lifetime is attributed to the
Table 4 Lifetimes of the significant emission bands in NaYF4:Er3þ , Yb3þ under direct excitation (400 cm1 higher energy than the observed emission) as a function of sample temperature for the Er3þ and Yb3þ concentrations shown in the table. The lifetimes, indicated in milliseconds, were obtained from single-exponential fits to the data over at least two orders of magnitude of signal intensity Electronic transition
4
I11=2 ! 4 I15=2
2
F5=2 ! 2 F7=2
4
F9=2 ! 4 I15=2
4
S3=2 ! 4 I15=2
½Er3þ
½Yb3þ
Sample temperature (K) 5
25
50
100
150
200
250
300
2% 10% 20% 2%
0% 0% 0% 18%
7.8 7.9 4.2 2.1
7.8 7.8 4.1 2.1
7.9 7.8 2.9 2.1
7.8 7.7 2.1 2.0
7.9 7.8 2.0 2.0
7.9 7.8 2.0 2.0
7.9 7.7 2.0 1.9
7.9 7.3 2.0 1.9
2% 10% 20% 2% 2% 10% 20% 2%
0% 0% 0% 18% 0% 0% 0% 18%
0.60 0.60 0.69 0.55 0.71 0.75 0.63 0.46
0.61 0.70 0.73 0.59 0.72 0.74 0.64 0.43
0.67 0.80 0.73 0.71 0.71 0.75 0.64 0.42
0.70 0.86 0.75 0.77 0.71 0.74 0.27 0.43
0.68 0.84 0.73 0.75 0.70 0.29 0.09 0.39
0.62 0.75 0.66 0.69 0.66 0.064 0.025 0.32
0.53 0.63 0.54 0.57 0.50 0.022 0.0088 0.23
0.43 0.48 0.39 0.47 0.36 0.010 0.0026 0.14
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stronger contribution of cross-relaxation processes. From temperature-dependent emission measurements, a simultaneous quenching of the 4S3/2 upconversion intensity is found that follows the reduction of the lifetime for each of the samples. The fact that both data sets are strongly correlated indicates a temperature and concentration dependent depopulation process of the Er3þ 4S3/2 state. It is reasonable to ascribe this to the two-ion Er3þ cross-relaxation processes j4 I15=2 ; 4 S3=2 i ! j4 I13=2 ; 4 I9=2 i and j4 I15=2 ; 4 S3=2 i ! j4 I9=2 ; 4 I13=2 i. Note that these processes also increasingly populate the 6500 cm1 emitting state (4I13/2), which will therefore show an increased luminescence intensity with increasing temperature. As this quenching is present in all Er3þ only doped samples, it is logical to extrapolate these results to the NaYF4:Er3þ , Yb3þ sample. Therefore, it is concluded that the strong increase in the green upconversion emission intensity that was observed [27] between 5 K and room temperature for NaYF4: 2% Er3þ ,18% Yb3þ actually underestimates the increase in the efficiency of the underlying upconversion process by roughly a factor of 2. The fact that the effect of cross-relaxation is not so pronounced in the codoped sample, is related to the strong competition between cross-relaxation on Er3þ and energy transfer processes involving Yb3þ .
4. Conclusions The material NaYF4 has been known for many years to be one of the most efficient host-lattices for NIR to visible photon upconversion when doped with Er3þ or Tm3þ . Despite this fact, only few papers have been published that explore the spectroscopic properties of doped NaYF4 in detail. In the present paper, an extensive set of data regarding rare-earth doped NaYF4 is presented. The focus here is mainly on understanding the luminescent characteristics of Er3þ or Tm3þ doped NaYF4 in terms of the properties of the NaYF4 host-lattice itself. In order to achieve this goal, material-specific data has been collected, such as a Raman spectrum of undoped NaYF4, as well as
11
spectroscopic data that is specific for the luminescent ions present in the material. On comparison with LiYF4 it is found that the available phonon modes are at significantly lower energy in NaYF4. This is an important reason why NaYF4 is such an exceptionally suitable hostlattice for lanthanide upconversion. This high efficiency is exemplified by the series of measurements that show that up to 50% of all NIR excitations can be upconverted in lanthanidedoped NaYF4 when suitable dopant concentrations are chosen. Measurements on the excitationpower dependence of the upconversion emissions show a distinctly different (albeit inefficient) upconversion mechanism active at low temperature than the well-known (efficient) one that is dominant at room temperature: the excitation of the red emission becomes a three-photon process, even though the green emission is excited by two photons. The data presented in this paper constitutes an interesting overview of some of the fascinating properties of the NaYF4 upconversion host-lattice.
Acknowledgements Pieter Dorenbos is acknowledged for his expertise and assistance in obtaining and interpreting the highenergy excitation spectrum. This work was financially supported by the Swiss National Science Foundation. MKvV acknowledges financial support by the Swiss National Science Foundation through Project NFP47 (4047-057481).
References [1] F. Auzel, Chem. Rev. 104 (2004) 139 and references therein. [2] L.R. Dalton, A.W. Harper, R. Ghosn, W.H. Steier, M. Ziari, H. Fetterman, Y. Shi, R.V. Mustacich, A.K.Y. Jen, K.J. Shea, Chem. Mater. 7 (1995) 1060. [3] J.F. Suyver, A. Aebischer, D. Biner, P. Gerner, J. Grimm, S. Heer, K. Kra¨mer, C. Reinhard, H.U. Gu¨del, Opt. Mater. 27 (2005) 1111. [4] A. Shalav, B.S. Richards, T. Trupke, K.W. Kra¨mer, H.U. Gu¨del, Appl. Phys. Lett. 86 (2005) 013505. [5] F. Auzel, F.C.R. Acad. Sci. (Paris) 262 (1966) 1016.
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[6] V. Ovsyankin, P.P. Feofilov, JEPT Lett.-USSR 3 (1966) 322. [7] D.R. Gamelin, H.U. Gu¨del, Top. Curr. Chem. 214 (2001) 1 and references therein. [8] R. Scheps, Prog. Quant. Electron. 20 (1996) 271. [9] M.F. Joubert, Opt. Mater. 11 (1999) 181. [10] N. Bloembergen, Phys. Rev. Lett. 2 (1959) 84. [11] J.S. Chivian, W.E. Case, D.D. Eden, Appl. Phys. Lett. 35 (1979) 124. [12] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Science 273 (1996) 1185. [13] R.A. Hewes, J.F. Sarver, Phys. Rev. 182 (1969) 427. [14] F. Auzel, D. Pecile, J. Lumin. 8 (1972) 32. [15] N.J. Cockroft, G.D. Jones, D.C. Nguyen, Phys. Rev. B 45 (1992) 5188. [16] D.C. Yeh, W.A. Sibley, M. Suscavage, M.G. Drexhage, J. Appl. Phys. 62 (1987) 266. [17] T. Catunda, L.A.O. Nunes, A. Florez, Y. Messaddeq, M.A. Aegerter, Phys. Rev. B 53 (1996) 6065. [18] M.P. Hehlen, N.J. Cockroft, T.R. Gosnell, A.J. Bruce, Phys. Rev. B 56 (1997) 9302. [19] V.K. Bogdanov, D.J. Booth, W.E.K. Gibbs, J.S. Javorniczky, P.J. Newman, D.R. MacFarlane, Phys. Rev. B 63 (2001) 205107. [20] N. Menyuk, K. Dwight, J.W. Pierce, Appl. Phys. Lett. 21 (1972) 159. [21] T. Kano, H. Yamamoto, Y. Otomo, J. Electrochem. Soc. 119 (1972) 1561. [22] J.L. Sommerdijk, J. Lumin. 6 (1973) 61. [23] A. Bril, J.L. Sommerdijk, A.W. de Jager, J. Electrochem. Soc. 122 (1975) 660.
[24] R.H. Page, K.I. Schaffers, P.A. Waide, J.B. Tassano, S.A. Payne, W.F. Krupke, W.K. Bischel, J. Opt. Soc. Am. B 15 (1998) 996. [25] K.W. Kra¨mer, D. Biner, G. Frei, H.U. Gu¨del, M.P. Hehlen, S.R. Lu¨thi, Chem. Mater. 16 (2004) 1244. [26] E. Edjer, J. Opt. Soc. Am. 59 (1969) 223. [27] J.F. Suyver, J. Grimm, K. Kra¨mer, H.U. Gu¨del, J. Lumin., 2005, in press. [28] J.F. Suyver, A. Aebischer, S. Garcı´ a-Revilla, P. Gerner, H.U. Gu¨del, Phys. Rev. B 71 (2005) 125123. [29] S.A. Miller, H.E. Rast, H.H. Caspers, J. Chem. Phys. 52 (1970) 4172. [30] D.E. McCumber, M.D. Sturge, J. Appl. Phys. 34 (1963) 1682. [31] M.A. Couto dos Santos, E. Antic-Fidancev, J.Y. Gesland, J.C. Krupa, M. Lemaıˆ tre-Blaise, P. Porcher, J. Alloys and Compounds 275–277 (1998) 435. [32] R.T. Wegh, E.V.D. van Loef, G.W. Burdick, A. Meijerink, Molecular Physics 101 (2003) 1047. [33] G. Burdick, F.S. Richardson, Chem. Phys. 228 (1998) 81. [34] W.T. Carnall, G.L. Goodman, K. Rajnak, R.S. Rana, A Systematic Analysis of the Spectra of the Lanthanides Doped into Single Crystal LaF3, Argonne National Laboratory, Argonne Illinois, 1988. [35] D. Lo, V.N. Makhov, N.M. Khaidukov, J.C. Krupa, J.Y. Gesland, J. Lumin. 106 (2004) 15. [36] M. Pollnau, D.R. Gamelin, S.R. Lu¨thi, M.P. Hehlen, H.U. Gu¨del, Phys. Rev. B 61 (2000) 3337. [37] B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Solids, Clarendon Press, Oxford, 1989.
Journal of Luminescence 117 (2006) 1–12 www.elsevier.com/locate/jlumin
Upconversion spectroscopy and properties of NaYF4 doped with Er3þ, Tm3þ and/or Yb3þ J.F. Suyver, J. Grimm, M.K. van Veen, D. Biner, K.W. Kra¨mer, H.U. Gu¨del Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3000 Bern 9, Switzerland Received 8 February 2005 Available online 23 May 2005
Abstract A spectroscopic investigation of NaYF4 powders doped with several different concentrations of Er3þ , Tm3þ and/or Yb3þ is described. Rare earth-doped NaYF4 is known to be a very efficient near-infrared to visible upconverter. The overview emission spectra for all samples are presented and from these the upconversion efficiency is calculated. Raman spectroscopy of undoped NaYF4 is presented here for the first time, demonstrating that the dominant phonon modes in NaYF4 lie between 300 and 400 cm1 . The fact that these phonon modes are also the optically active ones is further demonstrated by temperature-dependent excitation spectroscopy. These surprisingly low-energy phonon modes explain the extraordinarily high upconversion efficiency of the rare earth-doped NaYF4 samples. Excitation spectroscopy up to 70000 cm1 in an NaErF4 sample reveals a multitude of Er3þ 4f excitations, including the illustrious 2F(2)5/2 one that has not been observed in excitation spectroscopy before. From the low-temperature power-dependence of the emission intensities for an Er3þ , Yb3þ codoped NaYF4 sample, it is concluded that the dominant upconversion mechanism at low temperature is a different one than at room temperature. From direct excitation, the lifetimes of the Yb3þ 2 F5=2 ! 2 F7=2 , Er3þ 4 F9=2 ! 4 I15=2 and Er3þ 4 S3=2 ! 4 I15=2 emissions are determined as a function of temperature for all samples. At elevated temperatures, a significant decrease in the green lifetime is observed, which is correlated to a simultaneous quenching in the luminescence intensity. This quenching is ascribed to cross-relaxation between two nearby Er3þ ions. r 2005 Elsevier B.V. All rights reserved. PACS: 78.20.e; 42.65.Ky; 42.70.Nq; 78.55.m; 78.55.Fv Keywords: Near-infrared to visible upconversion; Rare earth luminescence; NaYF4; Raman spectroscopy
1. Introduction Corresponding author. Present address: Philips Research Laboratories, Eindhoven, The Netherlands. Tel.: +41 31 631 4254; fax: +41 31 631 4399. E-mail address: [email protected] (J.F. Suyver).
There are several processes by which nearinfrared (NIR) radiation can be converted into the visible spectral range [1]. The most famous
0022-2313/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2005.03.011
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one, frequency conversion in nonlinear materials [2], requires high-intensity coherent radiation. As such, this process is not very well suited for possible applications that utilize low-intensity incoherent NIR excitation radiation, such as conversion of the NIR ‘‘waste-light’’ generated by an incandescent lamp to the visible [3]. Another interesting suggestion, application of upconverting phosphors to enhance a silicon solar cell photoresponse in the NIR spectral range [4], will also be difficult to achieve using frequency conversion in nonlinear materials. Fortunately, a completely different nonlinear process exists that is more suitable for applications not requiring high-intensity coherent radiation: photon upconversion (UC) [5,6]. Due to the prerequisite of more than one metastable excited state to be present for UC to occur [7], typically rare earth ions show this effect. The many interesting applications for (rare earth) upconversion, such as lasers [8,9], infrared quantum counters [10,11] and lighting or displays [3,12], clearly indicate that this expanding field warrants significant attention. There exist a large number of materials in which upconversion has been observed, with very large differences in the actual upconversion efficiency. Most of these materials are low phonon-energy halide host-lattices [1,13–15] or glassy hosts [16–19], both with trivalent rare earth ions present as dopants. Already for quite some time, the NaYF4 host lattice has been widely recognized as one of the most efficient upconversion lattices [20,21]. Even though efficient UC in this lattice has been known since the 1970s (specifically when it is doped with Er3þ , Yb3þ or Tm3þ , Yb3þ ) only a few fundamental research papers have appeared regarding this particular material [22–24]. This is probably related to difficulties in the synthesis of phase-pure NaYF4, usually resulting in the presence of both the (efficiently upconverting) hexagonal b-phase and the (comparably poorer upconverting) cubic a-phase. Recently, our group investigated and optimized the reproducible synthesis of high-quality micrometer-sized NaYF4 powders in the hexagonal bphase doped with trivalent lanthanides (substituting for Y3þ ) [25]. It was found that with this new synthesis route, highly stable materials could be
grown. Besides the phase purity, the efficiency also depends strongly on the dopant concentration, the ratio of Na to Y in the starting materials, and the preparation temperature. In the pursuit to find the most efficiently upconverting sample, the ideal concentrations for NIR to green upconversion were found to be 2% Er3þ and 18% Yb3þ . These values were obtained through systematic variation of the Er3þ and Yb3þ concentrations present in the NaYF4 host-lattice. As a result of this investigation, several less-efficient Er3þ -doped NaYF4 samples, with Er3þ concentrations ranging between 2% and 20%, were also obtained. An extensive investigation and comparison of these samples and the efficient (Yb3þ codoped) one will be discussed here. Furthermore, NaYF4: 0.3% Tm3þ , 25% Yb3þ (one of the most efficient NIR to blue/violet upconverters known) will also be included in this discussion. All of these samples were grown under identical synthesis conditions and only the dopant ions and concentrations were varied.
2. Experimental The powders were synthesized using the synthesis route described in Ref. [25]. The powders were excited with a multimode, standing wave Ti:sapphire laser (Spectra Physics 3900S), pumped by the second harmonic of a Nd:YVO4 laser (Spectra Physics Millennia CS-FRU). The wavelength control of the Ti:sapphire laser was achieved by an inchworm driven (Burleigh PZ-501) birefringent filter and a wavemeter (Burleigh WA2100). Sample cooling was achieved using a quartz helium gas flow tube. The sample luminescence was dispersed by a 0.85 m double monochromator (Spex 1402) with gratings blazed at 500 nm (1200 grooves/mm). The signal was detected with an cooled photomultiplier tube (Hamamatsu R3310) and a photon counting system (Stanford Research SR400). The luminescence spectra were corrected for the instrument response, the refractive index of air and are subsequently displayed as a photon flux per constant energy interval [26]. To measure the power dependence, the beam was attenuated with a series of neutral density filters (Balzers). The
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3. Results and discussion
1 4
25
20 Energy (103 cm-1)
temporal evolution of the luminescence signal was obtained using as a excitation source the Ramanshifted (Quanta Ray RS-1 equipped with a 340 psi H2 gas cell) output of a dye laser (Lambda Physik FL 3002) pumped with the second harmonic of a Nd:YAG laser (Quanta Ray DCR 3). The luminescence was dispersed through a 0.75 m single monochromator (Spex 1702) with a grating blazed at 750 nm (600 grooves/mm) and detected with a photomultiplier tube (Hamamatsu R3310) coupled to a multichannel scaler (Stanford Research SR 430). Raman measurements were performed on a Bomem DA8 Fourier transform infrared spectrometer equipped with a Raman attachment and a liquid nitrogen-cooled InGaAs detector. Excitation of the samples was done using the 9394:5 cm1 radiation from a Nd3þ :YAG laser (Coherent Compass 1064-2500MN). The laser line was removed using two holographic supernotchplus filters (Kaiser Optical Systems). A resolution of 4 cm1 was used. Duran glass capillaries served as sample holders.
3
15
3/2 4 5/2 F 7/2 2
1
4
3
F 9/2
3
I9/2
4
2
I11/2
3
I13/2
3
5
4
2
I15/2 Er
3+
3
F 7/2
3+
Yb
F 2,3 H4
F 5/2
4
0
G4
H 11/2 4 S 3/2
4
10
D2
G 11/ 2 2 H 9/2
Tm
H5 F4
H6
3+
Fig. 1. Schematic energy level diagrams, upconversion excitation and visible emission schemes for the Er3þ , Tm3þ and Yb3þ ions. Full, dotted, dashed and curly arrows indicate radiative, nonradiative energy transfer, cross-relaxation and multiphonon relaxation processes, respectively. Only those emissions that lie in the visible spectral region are shown here. Further emissions exist, as can be seen in Fig. 2.
3.1. Upconversion emission spectra Fig. 1 shows a schematic diagram of the positions of the energy levels of the three trivalent rare earth ions that are relevant for this investigation. The diagram also indicates several of the most important energy transfer upconversion, cross-relaxation and nonradiative multiphonon mechanisms that occur in Er3þ , Tm3þ and Yb3þ when they are incorporated into NaYF4. Note that, besides the visible emission transitions indicated in the figure, strong infrared emissions also occur. The relative importance of all these transitions will be discussed below. Fig. 2 shows the room temperature emission spectra of five representative samples under NIR excitation. No significant emission was observed outside the region shown in the figure. The energy transfer mechanism involved in excitation of the Er3þ , Yb3þ codoped sample, was described in the literature recently [27]. From Fig. 2 it is clear that all samples show strong emissions in the visible
spectral range upon NIR excitation. Furthermore, of all samples shown, the NaYF4:Er3þ , Yb3þ sample shown in Fig. 2(a) is by far the most efficient upconverter. Upon addition of more Er3þ to the NaYF4 lattice there is a strong increase in the red to green emission ratio, as can be seen by comparison of Figs. 2(b)–(d). This effect is related to the increased probability of the two-ion Er3þ cross-relaxation process j4 I15=2 ; 4 S3=2 i ! 4 4 j I13=2 ; I9=2 i that occurs at higher Er3þ concentrations. This process depopulates the green-emitting 4 S3/2 state, thereby increasing the red to green ratio. Furthermore, the cross-relaxation also results in an increasing population of the 6500 cm1 emitting state (4I13/2), as can be seen from the data presented in the figure, as well as from the values shown in Table 1. For the discussion of the upconversion efficiency of these samples, Fi will denote the number of photons emitted in band i (i.e.: the integral of
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4
the emission band as shown in Fig. 2) and pi is the number of NIR excitation photons required to excite this emission (values shown in Tables 1 and 2). With these definitions
(a)
, pi F i
X
! pj F j
(1)
8j
(b)
x100
x50
Photon flux (a.u.)
(c)
x25
(d)
x100
(e)
5
10 15 20 25 Emission energy (103 cm-1)
Fig. 2. Emission spectra of the samples: (a) NaYF4: 2% Er3þ , 18% Yb3þ , (b) NaYF4: 2% Er3þ , (c) NaYF4: 10% Er3þ , (d) NaYF4: 20% Er3þ , (e) NaYF4: 0.3% Tm3þ , 25% Yb3þ . The excitation energy (indicated by the arrow on the top) was at 10238 cm1 for the spectra shown in (a), (e) and at 10311 cm1 for the other spectra. The measurements were recorded at room temperature and in the high-power limit (80 W=cm2 , unfocussed). The assignments of these transitions can be found in Tables 1 and 2.
represents the fraction of all NIR photons absorbed by the material that contribute to emission band i when one assumes that there is no nonradiative multiphonon-relaxation from the lowest excited 4f state to the ground state [27]. This assumption is reasonable for the samples that are discussed here as the luminescence lifetime of the first excited state has been measured to be in the millisecond range under direct excitation, as will be shown in Section 3.5. Table 1 shows for each of the significant Er3þ related emission bands shown in Fig. 2 the fraction of all NIR photons absorbed that contribute to emission in this band, calculated using Eq. (1). The data is shown for the high-power limit, where all energy transfer steps are saturated [28]. From the data it is clear that the NaYF4: 2% Er3þ , 18% Yb3þ sample is indeed an exceptionally efficient upconverter as 50% of all NIR excitation photons contribute to the upconversion emission bands, mainly in the two bands in the visible. The UC efficiency of the samples only doped with Er3þ are considerably lower than that of the codoped sample, which is related to the much weaker absorption strength of the Er3þ 4 I15=2 ! 4 I11=2 transition compared to the Yb3þ 2 F7=2 ! 2 F5=2 one, combined with the much higher Yb3þ concentration relative to Er3þ . Nevertheless, also the Er3þ only doped NaYF4 materials can be considered to be efficient upconverters, especially when compared to other upconverting phosphors [14] or glasses [18]. Also the NaYF4: 0.3% Tm3þ , 25% Yb3þ sample is evidently a very good upconverter, as Table 2 shows that 40% of all NIR excitation photons are upconverted in this sample at high excitation powers. However, since the energy level structure of the Tm3þ ion is such that it requires at least three NIR excitations on Yb3þ in order to obtain
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Table 1 Fraction of excitation photons absorbed that are subsequently used to excite each of the emission bands in the high power limit, as calculated using Eq. (1), for four differently doped NaYF4:Er3þ samples. The doping concentrations are indicated in the table. Also shown are the assignments of the emission bands and the (minimum) number of excitation photons needed to excite these emission bands (denoted by pi ) i
Energy (cm1 )
Electronic transition
pi
2% Er3þ , 18% Yb3þ
2% Er3þ
10% Er3þ
20% Er3þ
1
6500
1
13%
15%
34%
56%
2
10000
Er3þ : 4 I13=2 ! 4 I15=2 8 < Yb3þ : 2 F5=2 ! 2 F7=2
1
36%
83%
63%
37%
3þ
4
4
: Er : I11=2 ! I15=2 3
11800
Er3þ : 4 S3=2 ! 4 I13=2
2
3.1%
0.5%
0.5%
1.9%
4
15000 18200
Er3þ : 4 F9=2 ! 4 I15=2
2
24%
0.2%
1.4%
4.6%
Er3þ : 4 S3=2 ! 4 I15=2
2
23%
0.7%
0.5%
0.3%
3
1.1%
7 105 %
4 104 %
2 102 %
18800 6
3þ
2
4
Er : H11=2 ! I15=2 Er3þ : 2 H9=2 ! 4 I15=2
24000
Table 2 Fraction of excitation photons absorbed that are subsequently used to excite each of the emission bands in the high power limit, as calculated using Eq. (1), for a NaYF4: 0.3% Tm3þ , 25% Yb3þ sample. Also shown are the assignments of the emission bands and the (minimum) number of excitation photons needed to excite these emission bands (denoted by pi ) i
Energy (cm1 )
1 2 3
5500 6800 9900
4 5 6 7 8
12450 15400 21050 22200 27500
Electronic transition
pi
Tm3þ : 3 F4 ! 3 H6 Tm3þ : 3 H4 ! 3 F4 Yb3þ : 2 F5=2 ! 2 F7=2
1 2 1
13% 4% 46%
Tm3þ : Tm3þ : Tm3þ : Tm3þ : Tm3þ :
2 3 3 4 4
35% 0.2% 1.1% 0.4% 0.04%
3
H4 ! 3 H6 F2 ; 3 F3 ! 3 H6 1 G4 ! 3 H6 1 D2 ! 3 F4 1 D2 ! 3 H6 3
Intensity (a.u.)
5
0
200
400
600
800
1000
Raman shift (cm-1) Fig. 3. Raman spectrum of an undoped NaYF4 powder. The data was measured at room temperature with a Fourier Transform Infrared Spectrometer using 9394:5 cm1 excitation radiation. The three dominant phonon modes are found to be 298, 370 and 418 cm1 , respectively. The measurement was recorded at room temperature.
3.2. Raman and excitation spectroscopy one visible Tm3þ emission, the NIR to visible UC efficiency of the sample is only 2% (mainly into the two blue/violet emission bands). Most of the UC emission (35% of all absorbed NIR photons) is in the NIR, into the 3H4 band centered at 12450 cm1 (880 nm). This extremely high UC efficiency suggests that even lasing might be achievable on this transition when one is able to grow a macroscopic single crystal of NaYF4:Tm3þ , Yb3þ that is of optical quality.
Fig. 3 shows the Raman spectrum of a NaYF4 sample. The sample did not contain rare earth dopants in order to prevent their photoluminescence obscuring the much weaker Raman signal. The intensity found in the tail of the spectrum (above 550 cm1 ) is due to an instrumental feature. Three strong Raman peaks are observed, along with a few weaker ones that are not fully resolved due to the instrumental resolution. The
ARTICLE IN PRESS
F5/2|0〉
2
Photon flux (a.u.)
10220
10230
10240
10250
2
Excitation energy (cm-1)
10200
10300
10400
10500
Excitation energy (cm-1)
Excitation energy maximum (cm-1)
Fig. 4. Excitation spectrum of the 18200 cm1 emission for the NaYF4:Er3þ , Yb3þ sample, recorded at 5 K. The inset shows a zoom-in of the 2 F7=2 j0i ! 2 F5=2 j1i transition in Yb3þ , recorded at 75 K, as well as a Lorenzian fit to the data. The spectrum was linearly corrected for the power-dependence of the laser emission wavelength. The relatively poor signal-to-noise ratio is related to the strong quenching of the upconversion luminescence on reducing the temperature in this sample.
10250
10246
10242
10238 0
50
(a)
100
150
200
150
200
Temperature (K)
60 -1
strongest three phonon modes in NaYF4, which are also the only fully-resolved ones, are found to be 298, 370 and 418 cm1 , respectively. The weighted average position of the overlapping band system is 360 cm1 . This is considerably lower than that of comparable fluoride host lattices, such as LiYF4 (570 cm1 ) [29]. Probably the dominant electron-phonon coupling to this surprisingly lowenergy phonon mode is for a large part responsible for the fact that NaYF4 is such a suitable host lattice for efficient upconversion with different rare earth ions (most notably Er3þ and Tm3þ ). This low value for the phonon energy may be attributed to the higher coordination number for Y in NaYF4 with respect to LiYF4 (9 rather than 8). Furthermore, the shorter Y–F distances in LiYF4 as compared to NaYF4 can also play a role. An effect of the larger mass of Na as compared to Li, as well as a possible weaker bond-strength in NaYF4 relative to LiYF4 cannot be excluded either. Further research will be required to elucidate this phenomenon in detail. In order to verify the influence of the low energy phonon modes in NaYF4 on its spectroscopic characteristics, the 5 K excitation spectrum of the 4 S3/2 emission in the NaYF4:Er3þ , Yb3þ sample was measured. The data is shown in Fig. 4. The two lowest-energy excitation lines are due to transitions into two crystal field components of the 2F5/2 excited state of Yb3þ , as indicated in the figure [27]. The other Er3þ and Tm3þ related emission bands shown in Fig. 2(a), (e) all have comparable excitation spectra. The inset in Fig. 4 shows a zoom-in of the 2 F7=2 j0i ! 2 F5=2 j1i excitation line of Yb3þ , recorded at 75 K, as well as a Lorenzian fit to the data. Clearly, the fit accurately describes the data and the peak position and full-width at half-maximum (FWHM) can be determined accurately. Fig. 5 shows the temperature dependence of the peak energy (a) and FWHM (b) determined from Lorenzian fits to the temperature-dependent excitation spectra. At 5 K, the linewidth is inhomogeneous with a value of 2 cm1 . At temperatures above 50 K the linewidth becomes essentially homogeneous. This effect is demonstrated by the good Lorenzian fit to the data at elevated temperatures (such as in the inset in Fig. 4).
F5/2|1〉
J.F. Suyver et al. / Journal of Luminescence 117 (2006) 1–12
Excitation linewidth (cm )
6
50 40 30 20 10 0 0
(b)
50
100
Temperature (K)
Fig. 5. Temperature dependence of the (a) energy EðTÞ and (b) width GðTÞ of the Yb3þ 2 F7=2 j0i ! 2 F5=2 j1i excitation in a NaYF4:Er3þ , Yb3þ sample. The emission observed was at 9980 cm1 . The lines through the data are fits using Eqs. (2) and (3) with an effective phonon energy of _o ¼ 350 cm1 .
ARTICLE IN PRESS J.F. Suyver et al. / Journal of Luminescence 117 (2006) 1–12
EðTÞ ¼ E 0 þ C T 4
Z
_oD kB T
dx
0
ex
x3 , 1
(2)
where E 0 is the energy of the transition at 0 K, oD is the Debye cut-off frequency (i.e.: the energy of an effective phonon mode that couples to the optical transition) and C is a (positive) constant. Similarly, the temperature dependence of the homogeneous linewidth of the excitation band has been determined in the weak-coupling limit [30] 0
GðTÞ ¼ G0 þ C T
7
Z 0
_oD kB T
dx
x6 ex , 1Þ2
ðex
(3)
where G0 is the experimental linewidth at 0 K, oD is the Debye cut-off frequency and C0 is a (positive) constant. G0 contains both a homogeneous and an inhomogeneous (2 cm1 in this case) contribution. Eqs. (2) and (3) were used to simultaneously fit the temperature dependence of the spectral position EðTÞ and the linewidth GðTÞ of the excitation of the Yb3þ 2 F7=2 j0i ! 2 F5=2 j1i transition. This particular transition was chosen as it has the largest oscillator strength. The fits are shown together with the data in Fig. 5. The fits provide an effective phonon energy of 350 cm1 , in very good agreement with the Raman data presented earlier in this section. The complimentary results of the two very different sets of data shown in this section convincingly demonstrate that the energies of the phonon modes that dominate the electron-phonon coupling in Er3þ , Tm3þ and Yb3þ -doped NaYF4 are indeed extraordinarily low. It is postulated that this is one of the determining factors for the highly
efficient upconversion characteristics in these materials. 3.3. High-energy excitation spectrum Fig. 6 shows the 10 K excitation spectrum of the 18200 cm1 emission related to the 4 S3=2 ! 4 I15=2 transition in the high-energy regime for a NaErF4 sample. The spectrum was measured at the Double Ring Store III beam line of the Synchrotronstrahlungslabor HASYLAB at the Deutsche Elektronen-Synchrotron DESY in Hamburg (Germany). In the excitation spectrum the ultimate spectral resolution was better than 0.3 nm. The spectrum was corrected for the spectral intensity distribution of the synchrotron radiation using a spectrum provided by DESY. The excitation spectrum in Fig. 6, in the range of 30000–67500 cm1 , shows a multitude of sharpline excitations that can all be assigned to transitions to Er3þ related 4f11 multiplets, see Table 3. In the same table the theoretically predicted values for Er3þ -doped LiYF4 are presented [31,32]. The theoretical values were obtained from the literature based on energy level calculations using D2d symmetry, and the theoretical values include d-function correlation crystal field
40
Photon flux (a.u.)
Theoretical work on the shift of zero-phonon lines has been done using approximations that are valid in the weak-coupling limit [30]. The very small Huang-Rhys parameter associated with the excited states of trivalent rare earth ions implies that the weak-coupling limit can be used in this case [37]. Under the assumption of a Debye distribution for the phonon density of states, the temperature dependence of the energy of the zerophonon line will be given by [30]
7
30 62500
20
63500
64500
10
0 30000
40000
50000
60000
Excitation energy (cm -1 )
Fig. 6. High-energy excitation spectrum of the 4 S3=2 ! 4 I15=2 emission in a NaErF4 sample. The data was recorded at 10 K using a synchrotron radiation excitation source and the assignment of all transitions is given in Table 3. The inset shows a high-resolution zoom-in of the 2F(2)5/2 excitation line in the range of 62500–64500 cm1 .
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Table 3 Experimentally observed high-energy levels of Er3þ in NaYF4:Er3þ as well as the theoretically predicted levels for LiYF4:Er3þ . The experimental uncertainty in the data increases from 15 to 50 cm1 with increasing excitation energy Experiment (cm1 )
Theory (cm1 )
Assignment
31630 33044 33176 33442 34125 34945 36509 38668 39353 41147 41610 41763 42350 42946 43564 47208 47886 48181 49009 51191 54727 55278 63345
31656a 33076a 33176a 33458a 34131a 34933a 36509a 38694a 39343a 41145a 41596a 41768a 42368a 42953a 43507a 47235b 47911b 48218b 49082b 51240b 54713b 55336b 63369b
2
P3/2 K13/2 2 P1/2 4 G5/2 4 G7/2 2 D5/2 2 H(2)9/2 4 D5/2 4 D7/2 2 I11/2 2 L17/2 2 L17/2 4 D3/2 2 P3/2 2 I13/2 4 D1/2 2 L15/2 2 H(1)9/2 2 D(2)5/2 2 H(1)11/2 2 F(2)7/2 2 D(2)3/2 2 F(2)5/2 2
a
Data taken from Ref. [31]. b Data taken from Ref. [32].
For the energy range shown in Fig. 6 all expected Er3þ -related excitation transitions have been observed. Most interestingly, also direct excitation into the 2F(2)5/2 multiplet is observed (see inset of Fig. 6). This particular excitation has never been presented in the literature before. This is related to the fact that in previous experiments, performed on LiYF4:Er3þ , the broad 4f 10 5d excitations are at too low energy to allow the 2 F(2)5/2 multiplet to be resolved [32]. The observation of all important Er3þ excitation bands up to 67500 cm1 proves that the (micrometer sized) NaErF4 crystals are of good optical quality. Furthermore, the crystals do not contain significant defects or impurities that would generate optically active states within the bandgap of the material. Finally, this result suggests that, under suitable pumping conditions, upconversion to the vacuum ultraviolet spectral range may be achievable in NaYF4 as well, similar to what was observed in the case of LiYF4 crystals under XeF laser excitation [35]. 3.4. Power dependence of the low temperature emission intensity In the low-power limit, the upconversion intensity for any emission band I depends nonlinearly on the laser power P. It is typically written as [36] IðPÞ / Pn ,
contributions [31–33]. There is good agreement between the calculated and experimental values. This is also expected since the energies of the rare earth multiplets in NaYF4 are close to those in LiYF4. It should be noted that the 2Sþ1 LJ assignments are not always unambiguous, but can depend on the energy level calculations that are used in their determination [34]. Here the assignments most accepted in the literature are used without further investigation [31,32]. At energies above 65000 cm1 a broad excitation band is observed, which stretches until at least 90; 000 cm1 . This broad band is most likely attributed to 4f 11 ! 4f 10 5d excitations in Er3þ . However, F to Er3þ charge-transfer transitions could also contribute.
(4)
where n denotes the number of NIR photons that must be absorbed to excite one upconversion photon. In the specific case of Er3þ or Tm3þ , these values of n are identical to the pi shown in Tables 1 and 2. Fig. 7 shows the measured power dependence of the three major Er3þ upconversion emissions in NaYF4:Er3þ , Yb3þ at 5 K and room temperature. The data are recorded in the low-power limit, as at higher powers a decrease of the slopes is observed that is related to a change in the main depopulation mechanism of the excited states [28]. It is important to note that the upconversion emission intensity at 5 K is many orders of magnitude weaker than that at room temperature, which is related to the absence of direct Yb3þ 2 F5=2 ! Er3þ
ARTICLE IN PRESS
Slope = 2
10 10
S3/2 Emission intensity (a.u.)
11
10
10
9
8
10
Slope = 3
7
2
3
4
5 6 7 8 9
2
3
1
4
10 14 Slope = 2
10 13 10 12 10 11 10 10 10 9
5 6 7 8 9
Slope = 2 6
10
2
Excitation density (W/cm )
10 9
2
8
0.1
(b)
3
4
6
8
2
1
3
Excitation density (W/cm2)
4
6
8
10
Slope = 3
10 8
Slope = 4
10 7
2
H9/2 Emission intensity (a.u.)
(a)
9
4
10
4
F9/2 Emission intensity (a.u.)
J.F. Suyver et al. / Journal of Luminescence 117 (2006) 1–12
5
6
7 8 9
2
1
(c)
3
4
5
2
6
7 8 9
10
Excitation density (W/cm )
Fig. 7. Power dependence of the (a) 4 F9=2 ! 4 I15=2 , (b) 4 S3=2 ! 4 I15=2 and (c) 2 H9=2 ! 4 I15=2 transitions of the NaYF4:Er3þ , Yb3þ sample in the low power regime. Both the data recorded at room temperature (&) as well as that recorded at 5 K () is shown. Excitation was at 10238 cm1 and the vertical scales cannot be compared. Note the logarithmic axes. The slopes indicated are fits using Eq. (4).
4
I11/2 energy transfer at low temperature [27]. Clearly, Fig. 7 shows that the green 4S3/2 UC remains a two-photon process regardless of the temperature while the red 4F9/2 and violet 2H9/2 emissions show a different power-dependence at room temperature and at 5 K: at room temperature they are normal two- and three-photon processes, respectively. But at 5 K they are threeand four-photon processes, respectively. The highly puzzling observation that at 5 K the 4 F9=2 emission is a three-photon process while the 4 S3/2 emission remains a two-photon process must be explained. For this, it must be noted that at low temperature there is roughly an order of magnitude more green upconversion emission as compared to the red emission. This excludes any energy transfer upconversion process bypassing the Er3þ 4I11/2 state in the first Yb3þ ! Er3þ step,
as such a bypass would imply a significantly reduced green upconversion emission intensity. Therefore, it seems reasonable to ascribe the threephoton process populating the 4F9/2 through involvement of the 4S3/2 for the first and second excitation steps. The final mechanism is then explained as the following: after the normal twophoton process into the 4S3/2, a cross relaxation occurs j4 S3=2 ; 4 I15=2 i ! j4 I9=2 ; 4 I13=2 i. This is followed by an energy transfer upconversion step from Yb3þ 2F5/2, bringing Er3þ from the 4I13/2 into its 4F9/2 state. Interestingly, this implies that the main population process of the Er3þ 4I13/2 state is a two-photon process at low temperatures. This seems reasonable, and in-line with our earlier observation that the main population process of the Er3þ 4I13/2 state is also a two-photon process at room temperatures in the high-power limit [27].
ARTICLE IN PRESS J.F. Suyver et al. / Journal of Luminescence 117 (2006) 1–12
10
This effect was explained through the same crossrelaxation step mentioned previously. After the three-photon excitation-path into the red-emitting Er3þ 4F9/2 state, only the four-photon excitation of the violet-emitting 2H9/2 state at 5 K remains to be explained. From energetic considerations it is seen that an energy transfer upconversion pathway from 4F9/2 into 2H9/2 must involve the Yb3þ 2F5/2 state directly. No Yb3þ 2 F5=2 ! Er3þ 4 I13=2 energy transfer can take place for this case, because the remaining energy would be insufficient to excite Er3þ into the 2H9/2. Therefore, an energy transfer upconversion step is proposed from the Yb3þ 2F5/2 to the Er3þ 4F9/2 bringing the Er3þ into 4G9/2, followed by nonradiative multiphonon relaxation to 2H9/2. This explains the four-photon power dependence of the 24000 cm1 emission. At room temperature, the very efficient direct Yb3þ 2 F5=2 ! Er3þ 4 F11=2 energy transfer becomes much more important [27] and this completely dominates the inefficient three-photon upconversion mechanism into the 4F9/2 state. As a result, the room-temperature mechanism that populates the red emitting state will be a twophoton process, as is also shown by the data in Fig. 7 and conforms to the schematic representation shown in Fig. 1.
3.5. Lifetime and intensity quenching Direct excitation into the infrared (Yb3þ 2F5/2 or Er3þ 4I11/2 at 10200 cm1 ), red (Er3þ 4F9/2 at 15100 cm1 ) and green (Er3þ 4S3/2 at 18300 cm1 ) emitting states of Yb3þ and Er3þ was performed by exciting with 10 ns laser pulses at roughly 400 cm1 higher energy than the respective emissions. Table 4 shows the lifetimes obtained from single-exponential fits to the decay data for the transitions and samples indicated. The set of data includes all four Er3þ doped samples and the lifetimes are shown for temperatures ranging from 5 K to room temperature. For all samples, the data could be fitted well using a single-exponential decay for at least the first 1–2 orders of magnitude of the signal intensity. The data shown in Table 4 reveal that for increased temperatures, there is virtually no change in the lifetime of the Yb3þ 2F5/2 and Er3þ 4 I11/2 emissions. Similarly, only a small change in the lifetime of the Er3þ 4F9/2 emission at around 15100 cm1 is observed. In contrast, a drastic decrease in the lifetime of the Er3þ 4S3/2 emission at 18300 cm1 is observed, especially in the samples with a higher Er3þ concentration. Due to the strong dependence on Er3þ concentration, this reduction of the lifetime is attributed to the
Table 4 Lifetimes of the significant emission bands in NaYF4:Er3þ , Yb3þ under direct excitation (400 cm1 higher energy than the observed emission) as a function of sample temperature for the Er3þ and Yb3þ concentrations shown in the table. The lifetimes, indicated in milliseconds, were obtained from single-exponential fits to the data over at least two orders of magnitude of signal intensity Electronic transition
4
I11=2 ! 4 I15=2
2
F5=2 ! 2 F7=2
4
F9=2 ! 4 I15=2
4
S3=2 ! 4 I15=2
½Er3þ
½Yb3þ
Sample temperature (K) 5
25
50
100
150
200
250
300
2% 10% 20% 2%
0% 0% 0% 18%
7.8 7.9 4.2 2.1
7.8 7.8 4.1 2.1
7.9 7.8 2.9 2.1
7.8 7.7 2.1 2.0
7.9 7.8 2.0 2.0
7.9 7.8 2.0 2.0
7.9 7.7 2.0 1.9
7.9 7.3 2.0 1.9
2% 10% 20% 2% 2% 10% 20% 2%
0% 0% 0% 18% 0% 0% 0% 18%
0.60 0.60 0.69 0.55 0.71 0.75 0.63 0.46
0.61 0.70 0.73 0.59 0.72 0.74 0.64 0.43
0.67 0.80 0.73 0.71 0.71 0.75 0.64 0.42
0.70 0.86 0.75 0.77 0.71 0.74 0.27 0.43
0.68 0.84 0.73 0.75 0.70 0.29 0.09 0.39
0.62 0.75 0.66 0.69 0.66 0.064 0.025 0.32
0.53 0.63 0.54 0.57 0.50 0.022 0.0088 0.23
0.43 0.48 0.39 0.47 0.36 0.010 0.0026 0.14
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stronger contribution of cross-relaxation processes. From temperature-dependent emission measurements, a simultaneous quenching of the 4S3/2 upconversion intensity is found that follows the reduction of the lifetime for each of the samples. The fact that both data sets are strongly correlated indicates a temperature and concentration dependent depopulation process of the Er3þ 4S3/2 state. It is reasonable to ascribe this to the two-ion Er3þ cross-relaxation processes j4 I15=2 ; 4 S3=2 i ! j4 I13=2 ; 4 I9=2 i and j4 I15=2 ; 4 S3=2 i ! j4 I9=2 ; 4 I13=2 i. Note that these processes also increasingly populate the 6500 cm1 emitting state (4I13/2), which will therefore show an increased luminescence intensity with increasing temperature. As this quenching is present in all Er3þ only doped samples, it is logical to extrapolate these results to the NaYF4:Er3þ , Yb3þ sample. Therefore, it is concluded that the strong increase in the green upconversion emission intensity that was observed [27] between 5 K and room temperature for NaYF4: 2% Er3þ ,18% Yb3þ actually underestimates the increase in the efficiency of the underlying upconversion process by roughly a factor of 2. The fact that the effect of cross-relaxation is not so pronounced in the codoped sample, is related to the strong competition between cross-relaxation on Er3þ and energy transfer processes involving Yb3þ .
4. Conclusions The material NaYF4 has been known for many years to be one of the most efficient host-lattices for NIR to visible photon upconversion when doped with Er3þ or Tm3þ . Despite this fact, only few papers have been published that explore the spectroscopic properties of doped NaYF4 in detail. In the present paper, an extensive set of data regarding rare-earth doped NaYF4 is presented. The focus here is mainly on understanding the luminescent characteristics of Er3þ or Tm3þ doped NaYF4 in terms of the properties of the NaYF4 host-lattice itself. In order to achieve this goal, material-specific data has been collected, such as a Raman spectrum of undoped NaYF4, as well as
11
spectroscopic data that is specific for the luminescent ions present in the material. On comparison with LiYF4 it is found that the available phonon modes are at significantly lower energy in NaYF4. This is an important reason why NaYF4 is such an exceptionally suitable hostlattice for lanthanide upconversion. This high efficiency is exemplified by the series of measurements that show that up to 50% of all NIR excitations can be upconverted in lanthanidedoped NaYF4 when suitable dopant concentrations are chosen. Measurements on the excitationpower dependence of the upconversion emissions show a distinctly different (albeit inefficient) upconversion mechanism active at low temperature than the well-known (efficient) one that is dominant at room temperature: the excitation of the red emission becomes a three-photon process, even though the green emission is excited by two photons. The data presented in this paper constitutes an interesting overview of some of the fascinating properties of the NaYF4 upconversion host-lattice.
Acknowledgements Pieter Dorenbos is acknowledged for his expertise and assistance in obtaining and interpreting the highenergy excitation spectrum. This work was financially supported by the Swiss National Science Foundation. MKvV acknowledges financial support by the Swiss National Science Foundation through Project NFP47 (4047-057481).
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