Tb3+-codoped monodisperse NaYF4 nanocrystals

Optics Communications 282 (2009) 3028–3031

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Upconversion luminescence in Yb3+/Tb3+-codoped monodisperse NaYF4 nanocrystals Huijuan Liang, Guanying Chen, Long Li, Yuan Liu, Feng Qin, Zhiguo Zhang * Department of Physics, Harbin Institute of Technology, 150001 Harbin, PR China

a r t i c l e

i n f o

Article history: Received 8 December 2008 Received in revised form 14 March 2009 Accepted 6 April 2009

a b s t r a c t Upconversion (UC) luminescence in monodisperse NaYF4:Yb3+/Tb3+ nanocrystals was observed under diode laser excitation of 970 nm, which were synthesized by a hydrothermal method. UC emissions at 380, 413, 436 nm and at 488, 542, 584, 620 nm arise from transitions 5D3(5G6) ? 7FJ(J = 6, 5, 4) and 5 D4 ? 7FJ(J = 6, 5, 4, 3) of Tb3+ ions, respectively. UC mechanisms are proposed based on spectral, kinetic, decay time measurements, and pump power dependence analyses. Blue, green and red emissions originate from the same long-lived (milliseconds) upper 5D4 state, which promises the potential applications of these monodisperse Yb3+/Tb3+-codoped NaYF4 nanocrystals in the field of photonics, lasers and biomedicine. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Rare-earth-ion-doped upconversion (UC) nanocrystals are of particular interest to be used in photonics [1], lasers, [2,3] and biomedical systems [4,5] due to their unique spectral property to transform the infrared (IR) light into visible and stable fluorescence as well as due to the facile availability of cost-effective and high power IR diode laser that is transparent for biomolecules. Compared with well-characterized rare earth ions, e.g. Er3+, Ho3+ and Tm3+ [6,7], the Tb3+ ion deserves special attention due to its distinctive energy level structure, which can deploy a number of promising applications. In photonics system, the Tb3+ ion has already been utilized in various kinds of traditional phosphors for the generation of green light [8,9]. In the field of lasers, the long lifetime of the excited state 5D4 make it very attractive, since the long lifetime can allow substantial and efficient energy storage, which is convenient for population inversion and realize highpower UC lasers. Particularly, all the blue, green and red UC emissions are generated from the same long-lived upper 5D4, and simultaneous blue, green and red lasers pumped by diode lasers of a single IR wavelength could be envisaged. The Tb3+doped UC nanocrystals can have substantial applications in biomedicine due to the following reasons: firstly, the IR laser can not induce autofluorescence, which thereby is not harmful to tissues and can allow a high penetration depth in tissues. Secondly, nanocrystals can enter into the living tissue without harm. Thirdly, Tb3+-doped UC nanocrystals can offer several simultaneous channels as compared to conventional bio-labels due to its unique spectral properties. It is known that spectral parameters * Corresponding author. Tel.: +86 0451 86402639. E-mail address: [email protected] (Z. Zhang). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.04.006

of tissues, e.g., scattering and absorption coefficients are important or required for scientists to understand, classify, and differentiate complex human diseases. Unfortunately, until now, there is no any method that can acquire these tissues spectral parameters in vivo. Tb3+-doped UC nanocrystals might can offer such an opportunity, since the proportion of the blue, green and red UC emissions that radiated from the same 5D4 state is unchanged for all pump powers and its variety induced by tissues can be used to probe these parameters. Therefore, investigating UC emissions of Tb3+doped nanocrystals is expected for biomedical fields. Unfortunately, UC emissions of Tb3+-doped nanocrystals were scarcely reported in literature, since there are no intermediate levels of Tb3+ ions that can absorb IR excitation. To circumvent this problem, femtosecond IR laser was previously used to excite the Tb3+ ions in single LiNbO3 crystal, which, however, is expensive and nonconvenient for users [10]. Additionally, to widen application areas of Tb3+ ions, the Yb3+/Tb3+-codoped bulk materials such as, silica glasses, fluoroindate glass, and glass ceramics [11–14], have also been investigated, since the Tb3+ ions can be excited by cooperative processes from Yb3+ ions that have large ground-state absorption in the spectroscopic range of 900–1000 nm [15,16]. However, UC emissions of Yb3+/Tb3+-codoped nanocrystals has never been reported previously, which may arise from the fact that nanocrystals usually have less UC efficiency than the bulk material due to surface contaminations, the defects in the host lattice, etc. Compared to oxides, fluorides are more suitable to be luminescence materials because of their lower phonon energies, relatively high optical damage threshold, and low non-linear refractive indices. For example LiYF4, NaYF4, LiScF4 have been investigated widely [17,18]. Among them, NaYF4 has been recognized as the most efficient host lattice for UC systems due to its low phonon energy of about 370 cm1 [19] and its ordering of the cations and their

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crystalline surrounding [17]. NaYF4 thereby is selected for host material in this work. In this letter, we report on the UC emissions in monodisperse Yb3+/Tb3+-codoped NaYF4 nanocrystals under diode laser excitation of 970 nm.

3. Results and discussion XRD patterns of these nanocrystals are shown in Fig. 1. As can be seen, these nanocrystals mainly have a cubic structure, which

35 30

111

220

20

422

400

222

5

331 420

10

311

15

200

Intensity (a.u.)

25

0 0

20

40

60

80

100

2θ (degree) Fig. 1. X-ray diffraction patterns of Yb3+/Tb3+-codoped NaYF4 nanocrystals. The inset is a recording of TEM image of these nanocrystals.

20

5 7 D3, G → F4 6

λP=970 nm

5 7 D4→ F6 5 7 5 7 D4→ F3 D4→ F4

5

10

NaYF4:Yb20Tb1

5

30

5 7 D3, G → F5 6

5 7 D3, G → F6 6

40

5

NaYF4 nanocrystals codoped with 1 mol% Tb3+ and 20 mol% Yb3+ ions were synthesized according to a hydrothermal process described briefly as follows. Firstly, 6.4 mmol sodium fluoride and 1.6 mmol EDTA were completely dissolved in 10 ml deionized water. Secondly, solutions contain Tb(NO3)3, Yb(NO3)3, Y(NO3)3 with proper molar ratio were dropped into the previous EDTA solution according to the (Tb3+ + Yb3+ + Y3+)/NaF molar ratio of 1:4. Subsequently, the two solutions were mixed together under strong stirring. The mixture was then transferred into a teflon vessel (50 ml) which were then added up to the 2/3 of the total volume by use of deionized water. Lastly, the pH value of the solution was adjusted to 3 by use of dilute HNO3 and NH3OH solutions. The vessel was tightly sealed and treated at 200 °C for 20 h and then naturally cooled down to the room temperature. The products were achieved by filtrated the solutions using membrane. The morphology of the powders was characterized by a HITACHI H-8100 transmission electron microscope (TEM) operating at 200 kV. Powder X-ray diffraction (XRD) analysis was carried out using a Rigaku D/Max-2550/pc diffractometer (Cu Ka radiation) in the 2h range of 10°–90°. The dried powders were pressed into a smooth and flat disk, where were then irradiated by a focused 970 nm diode laser with a maximum power of about 300 mW. The room temperature UC fluorescence was collected by a lens-coupled monochromator (Zolix Instruments Co. Ltd., Beijing) of 3-nm spectral resolution with an attached photomultiplier tube (Hamamatsu CR131). Decay profiles of visible (VIS) and ultraviolet (UV) radiation were measured by square-wave-modulation of the electric current input to the 970 nm diode laser, and by recording the signals via a Tektronix TDS 5052 digital oscilloscope. All spectral measurements were performed at room temperature.

50

Intensity (a.u.)

2. Experimental

7 5 D4→ F5

60

0 350

400

450

500

550

600

650

700

Wavelength (nm) Fig. 2. Upconversion spectrum for the 1 mol% Tb3+ and 20 mol% Yb3+ codoped NaYF4 under 970 nm diode laser excitation.

agrees well with the standard pattern JCPDS 77-2042 with a = 5.470 A. The inset of Fig. 1 displays the TEM image of NaYF4: 20%Yb3+, 1%Tb3+ nanocrystals, which illustrate that the synthesized nanocrystals are monodisperse with a mean size of about 70 nm. Considering their applications in biomedicine a core-shell structure is needed, which provide a core NaYF4 environment for rare earth ions to emit efficient UC luminescence and a shell for surface modifications that can make them water soluble and biocompatible [20]. Such morphology constitute a suitable basis for bio-labels and for the investigation of tissue optics properties. The UC emission spectrum of 1 mol% Tb3+ and 20 mol% Yb3+ doped NaYF4 is shown in Fig. 2 in the spectroscopic range 350– 700 nm under 970 nm laser irradiation of 300 mW. As shown in the figure, the UC emission bands centered at 380, 413, 436, 488, 542, 584 and 620 nm can be observed. In order to learn the origin of these radiations, the decay profiles of these emissions have been measured, shown in Fig. 3 and the inset. These decay profiles were fitted with exponential, the decay time of these radiations centered at 380, 413, 436, 488, 542, 584 and 620 nm are 0.24(2), 0.24(2), 0.25(2), 5.4(5), 5.6(5), 5.5(5), 5.7(5) ms, respectively. So the emission bands centered at 380, 413, 436 and 488, 542, 584, 620 nm are assigned to the transitions 5D3(5G6) ? 7FJ(J = 6, 5, 4) and 5 D4 ? 7FJ(J = 6, 5, 4, 3), respectively. Such assignments arise from the following reasons: firstly, all the emissions coming from the state 5D3 have the same decay time about 0.24 ms and all the emissions radiated from the state 5D4 have the same decay time about 5.5 ms. Secondly, the above conclusion coincides with the energy level structure. It should be noted that the long decay time of the 5 D4 state is very useful for energy storage, which can find applications in UC lasers. Particularly, all the blue, green, and red were radiated from the same long-lived 5D4 state, which thereby can possibly realize multicolor laser outputs under one IR pump source. More importantly, the long-lived 5D4 state can also be used as an intermediate state for pumping, which might be of significance for developing UC lasers e.g. the lasers of 380, 410, 436 nm UC UV lasers. Additionally, the UC spectra have been measured under the laser power ranging from 30 to 300 mW. It is found that the intensity ratio of 488, 542, 584, 620 nm emission remain unchanged, which arise from the fact that they are radiated from the same excited state. If these nanocrystals are injected into the tissue and excited by a 970 nm diode laser from outside, the escaped UC emissions from the tissue can then be detected. By comparing such recorded UC spectra with the UC spectra in pure nanocrystals, the optical parameters of the tissue, e.g. scattering and the absorption coefficient might can be given.

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0.4 0.2

436 nm 410 nm 380 nm

0.6 0.4 0.2 0.0 0

20

40

Time (ms)

30

N3

5 D 5 15 D3 G 6

20

N2

5

2

F5/2

10

7 7 F0-2 F 7 3

970 nm 2

F7/2

0

0.0

N1

D4

486 nm 542 nm 584 nm 620 nm

0.6

620 nm 584 nm 542 nm 488 nm

380 nm 413 nm 436 nm

Intensity (a.u.)

Intensity (a.u.)

0.8

0.8

3 -1 Energy (10 cm )

1.0

1.0

3+

3+

Yb -0.2 2

Fig. 3. Decay profiles of the radiations centered at 436 nm, 410 nm and 380 nm in 1 mol% Tb3+ and 20 mol% Yb3+ codoped NaYF4 nanocrystals. The inset shows the decay profiles of the radiation centered at 620 nm, 584 nm, 542 nm and 488 nm in these nanocrystals.

In order to know the UC dynamics of Yb3+/Tb3+-codoped NaYF4 nanocrystals, the power dependence of the UC luminescence have also been measured and shown in Fig. 4. The photon processes involved can be achieved, according to the relation, Iupc / Pn, where Iupc is the measured UC intensity, P is the incident pump intensity, and n is the number of pump photons that required populating the upper emitting state [21]. As shown in Fig. 4, the quadratic dependences illustrate that blue, green and red emissions all arise from two-photon UC processes. Similarly, three-photon processes are involved to generate the UV UC emission centered at 380 nm. The photon processes of the 413 and 436 nm UC emissions were not given, since the signal to noise ratio of them is bad at low laser pumps. However, three-photon processes are still suggested for them, since the 380, 413 and 436 nm arise from the same 5D3(5G6)(Tb3+) state and three-photon process is demonstrated for the UV radiation. Based on the analysis above, UC mechanisms are proposed and shown in Fig. 5. The incident pump photons at 970 nm can only be absorbed by Yb3+ ions at the ground state 2F7/2 and promoted to the excited 2F5/2 state, since Tb3+ ions have no matching energy levels.

Intensity (a.u.)

100

542nm 486nm 584nm 620nm 380nm

2. n=

n= 10

6 1 .8

6 1 .8

(

n

9 = 1.

dN1 N1 ¼ dt s1 dN2 N2 ¼ dt s2 dN3 1 ¼ W 2 N2 N1  dt s3

" N3 ðtÞ ¼ N3 ð0Þ 

( 3)

3 (5

After excitation of two Yb3+ ions, they can simultaneously transfer their energy to a Tb3+ ion at the ground state 7F6, which then goes up to an excited 5D4 level. From the 5D4 levels, the Tb3+ ions radiatively relax to the 7FJ(J = 6, 5, 4, 3) lower levels generating the observed 488, 542, 584, and 620 nm UC emissions (see Fig. 2). As for the 380, 413 and 436 nm emission, they are generated by radiative decays from the 5D3(5G6) state to the lower 7FJ(J = 6, 5, 4) levels, which is populated by nonradiative relaxations from the 5D1 state. There exist three possible routes to populate the 5D1 levels: the multiphoton absorption, the excited state-absorption (ESA) from state 5D4, and energy-transfer (ETU) from one Yb3+ ions to one Tb3+ in the state 5D4. Among these possible routes, simultaneous multiphoton absorption is most unlikely here, since the laser pump power density used here is very low and on the order of 100 W/ cm2. In order to learn either the ESA or the ETU process plays a major role in exciting a Tb3+ ion from the 5D4 to the 5D1 level, the rate equations will be used. At first, the ETU process from one Yb3+ ions to one Tb3+ in the state 5D4 has been supposed, the rate equations at the cease of laser pumping is given as follows:

5 1 .9 n=

ð1Þ ð2Þ ð3Þ

where Ni(si) is the energy level populations (decay time) denoted in Fig. 5, W2 is the energy transfer rate from the excited Yb3+ ion to the Tb3+ ion in the state 5D4. According to Eqs. (1)–(3), we can easily get,

5)

)

100

Pump power (mW) Fig. 4. Log–log plot of the UC emission as a function of the excitation laser power at 970 nm.

1 1

s3

( 3)

(3)

n= 1

68

F4

F 7 5 F6

Fig. 5. Energy level diagram of Yb3+ and Tb3+ ions as well as the proposed mechanisms.

4

Time (ms)

1000

Tb

7

þ

 s11  s12

1 1

s3

 s11  s12

# W 1 N2 ð0ÞN1 ð0Þ et=s3

W 1 N2 ð0ÞN1 ð0Þe



t s1 þs1 1 2

 ð4Þ

From Eq. (4), if the ETU process occurs, the decay profile of the state 5 D3(5G6) will be double-exponential, and the decay profile will be consisting of two parts: one is its own decay profile with the decay time s3 and the other one is the profile with the decay time ss11þss22 . If these two exponentials have the same decay time, they can not be distinguished. So the decay time of the exponentials were estimated firstly. The decay time of state 2F5/2 of Yb3+ ion is about 2 ms[19], and s2 is about 5 ms, so ss11þss22 will be 1.4 ms. s3 is the real lifetime of state 5D3(5G6), which will be distinguished if it is not millisecond. The decay time of state 5D3(5G6) have been measured to make sure whether they are single-exponential or double-exponential. As

H. Liang et al. / Optics Communications 282 (2009) 3028–3031

shown in Fig. 3, the decay time of the state 5D3(5G6) is an exponential rather than double-exponential, and the decay time is 0.24 ms, which is the real lifetime of state 5D3(5G6), shorter than the order of millisecond. So the ETU process is impossibly to occur to populate the 5D1 state. The ESA process from state 5D4 can be considered to play a major role in populating the state 5D1, which is in agreement with that reported in the literature [12]. 4. Conclusions In conclusion, UV and VIS UC emissions of Yb3+/Tb3+-codoped monodisperse NaYF4 nanocrystals have been observed under a 970 nm diode laser excitation. Two-photon UC emissions at 488, 542, 584, 620 nm and three-photon UC emissions at 380, 413, 436 nm have been observed. The spectral, kinetic, decay time measurement, and pump power dependence analyses show that two excited Yb3+ ions can simultaneously transfer their energy to a Tb3+ ion at the ground state 7F6, which then goes up to an excited long-lived (milliseconds) upper 5D4 level, from which the two-photon emissions radiated. The rate equations at the cease of laser pumping proved that the ESA process from state 5D4 to state 5D1 plays significant role in populating the 5D3(5G6) level. Acknowledgements The work is supported by the SIDA Asian-Swedish Research Partnership Programme and the 863 Hi-Tech Research and Development Programs of People’s Republic of China.

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