Eur. Phys. J. Appl. Phys. 43, 149–153 (2008) DOI: 10.1051/epjap:2008101
THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS
Yb3+ -Er3+-Tm3+ co-doped nano-glass-ceramics tuneable up-conversion phosphor J. M´endez-Ramos1, V.D. Rodriguez1,a , V.K. Tikhomirov2 , J. del-Castillo3 , and A.C. Yanes3 1 2 3
Dpto. F´ısica Fundamental y Experimental, Electr´ onica y Sistemas, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain Chemistry Department, Katholieke Universiteit Leuven, Leuven, 3001 Belgium Dpto. F´ısica B´ asica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain Received: 21 December 2007 / Received in final form: 29 April 2008 / Accepted: 13 May 2008 c EDP Sciences Published online: 19 July 2008 – Abstract. Transparent Yb3+ -Er3+ -Tm3+ co-doped nano-glass-ceramics have been prepared, 32(SiO2 ) 9(AlO1.5 ) 31.5(CdF2 ) 18.5(PbF2 ) 5.5(ZnF2 ): 3.5(Yb-Er-TmF3 ) mol%, where the co-dopants partition mostly to the fluoride PbF2 -based nano-crystals. A comparative study of the up-conversion luminescence in nano-glass-ceramics and its precursor glass indicates that these materials can be used as blue/green/red tuneable up-conversion phosphor, in particular for white light generation. A ratio between blue, green and red emission bands of the Tm3+ and Er3+ can be widely varied with nano-ceramming of the precursor glass and with changing a pump power of luminescence. The change in the ratio between the blue, green and red emission bands is explained to be due to substantial lowering phonon energy and shortening of inter-dopant distances with nano-ceramming of the precursor glass and due to change in the ratio of 2and 3-photon up-conversion processes with pump power. PACS. 81.05.Pj Glass-based composites, vitroceramics – 61.46.-w Structure of nanoscale materials
1 Introduction Nano-glass-ceramics comprising of RE doped nanoparticles embedded in glassy matrixes have proved to be useful for applications in optical devices due to their high transparency, compositional variety and easy mass production [1–10]. The transparent nano-glass-ceramics (GC) can be prepared by either heat treatment of the precursor oxyfluoride glass or by direct laser writing within the bulk of the precursor glass [1–3], when nano-crystalline rare-earth doped fluoride PbF2 -based phase nucleates and growths within the glass matrix [4–10]. Specifically, in this work we use for the precursor oxyfluoride glass the composition 32(SiO2 ) 9(AlO1.5 ) 31.5(CdF2 ) 18.5(PbF2 ) 5.5(ZnF2 ): (2.5YbF3 -0.5ErF3-0.5TmF3 ) mol% [11] which was used earlier for single doping by 3.5ErF3 [4] and 3.5TmF3 [12] mol%, respectively. Fluoride crystalline environment is commonly known to enhance up-conversion luminescence intensity and we use such property in this work. In spite of the high volume fraction of the nanocrystalline phase, high optical transparency from about 350 nm down to 5 µm is a property of this GC due to very small, at about 8 nm, diameter of rare-earth a
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doped PbF2 -based nano-crystals [4], and good matching of refractive index of these nano-crystals with hosting glassnetwork at about n = 1.75, resulting in negligible Rayleigh scattering. This GC emerge then as an excellent medium for rare-earth dopant ions, combining a nano-scaled low energy phonon fluoride crystalline host and consequently high quantum efficiencies with the benefit and easy elaboration and manipulation of a robust oxide glass matrix, such as transparency, chemical and mechanical durability [1–11]. In addition, Tm3+ and Er3+ dopant ions are known of emitting blue (Tm3+ ), green and red (Er3+ ), luminescence bands. Therefore, combining these emission bands in a specific proportion will generate total emission of any desirable colour. Since the phonon energy in the PbF2 nanocrystals hosting the majority of rare-earth dopants is very low (at about 250 cm−1 ), the lifetimes of the dopants are long and up-conversion luminescence efficiency of rare-earth dopants is high [1–11], thus motivating the choice of glass-ceramics host for rare-earth dopants in this work. Tuneability of the infrared-to-visible up-conversion phosphor with changing the pump power/duration and/or by laser-induced ceramming of the precursor material has been attracted recently substantial interest due to possible applications, e.g., in three-dimensional colour
Article published by EDP Sciences
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2 Experimental
3
I15/2→ F9/2
3+
Tm 3 H6→ H4
3
GC G
600
700 Wavelength (nm)
800
(b)
2
2
F7/2 → F5/2 Yb3+
Absorbance (arb. units)
The samples used in this study have been prepared by melting the batch consisting of 32(SiO2 ) 9(AlO1.5 ) 31.5(CdF2 ) 18.5(PbF2 ) 5.5(ZnF2 ) 2.5(YbF3 ) 0.5 (ErF3 ) 0.5 (TmF3 ), mol%. The procedures used for preparation of the parent precursor glass and the corresponding GC have been described elsewhere [4]. In particular, this transparent GC has been obtained by heat treatment of the precursor glass at 440 ◦ C for 30 min. The X-ray diffraction pattern of the precursor glass shows the broad halos characteristic of the amorphous state. Contrary, X-ray diffraction pattern of the corresponding GC shows narrow peaks typical of cubic lead fluoride crystalline phase [4]. The diameter of nano-crystals has been calculated using the Scherrer formula and was found to be about 8 nm. Procedures for measurements of the emission and absorption spectra have been described elsewhere [9,13]. Upconversion emission spectra have been excited by the laser diode operating at 980 nm wavelength and up to 200 mW power. The laser beam was focused within the bulk of the sample with a microscope objective providing the power density in the focal point up to 20 W/cm2 . CCD digital camera was used to take color pictures of up-conversion luminescence emitted by the samples.
(a)
3+
Tm3 H6→ F3,2
3+ 4
Er
4
Absorbance (arb. units)
optical recording/displays, white light generation for ambient lighting and biological labels [13–16]. Here we study up-conversion luminescence in Yb3+ Er3+ -Tm3+ co-doped GC and its precursor glass. These GC and glass emit red, green and blue (RGB) upconversion emissions bands. Moreover, the ratio between intensity of these bands varies with nano-ceramming of the precursor glasses and also with change of pump power of luminescence.
4
4
3+
I15/2 → I11/2 Er
GC 3+
3+
Tm 3 H6 → H5
Er 4 I15/2 → I13/2
4
3
3+
Tm 3 H6 → F4
3
G 1000
1200 1500 1600 Wavelength (nm)
1700
Fig. 1. Absorption spectra of Yb3+ -Er3+ -Tm3+ co-doped precursor glass (G) and nano-glass-ceramics (GC) in the visible (a) and near infrared (b).
3 Results
4
F5/2, 3/2
1
G4
20
F7/2
2
H
4 11/2 4
3
F3,2
H4
4
H5
3
F4
0
2
F5/2
2
3 3+
Tm
H6
3+
Yb
F7/2
F9/2
I9/2 I11/2
4
660 nm
3
5
S3/2
3
550 nm
700 nm
475 nm
10
650 nm
3
15
Pump 980 nm
-1
Energy (x 10 cm )
Figure 1 shows absorption spectra of the precursor glass (G) and GC in the visible (a) and near infrared (b). The absorption bands corresponding to transitions from the ground to excited states for Er3+ , Tm3+ and Yb3+ ions, respectively, are indicated in the Figure 1, and the involved states of the ions can be recognised in respective energy level diagrams in Figure 2. It is seen that after nano-ceramming of the precursor glass the absorption bands show higher Stark-split structure, indicating that the dopant ions are incorporated into a crystalline environment in the GC, as it was argued in [4–10]. The crystalline structure is of type of solid solution of approximate chemical composition Re10 Pb25 F65 , as found by means of transmission electron microscope with energy dispersion analysis (TEM EDS), as described in [4,7]. Therefore the spectra in GC are somewhat broader when in corresponding monocrystals. Figure 3 shows the visible up-conversion luminescence spectra in Yb3+ -Er3+ -Tm3+ co-doped precursor glass and
4
4
I13/2
4
3+
Er
I15/2
Fig. 2. (Color online) Energy level diagram of the Er3+ , Yb3+ and Tm3+ co-dopants. Solid arrows indicate pump and upconversion emissions transitions. Dash lines indicate 2- and 3-photon up-conversion processes. Dot lines indicate crossrelaxation process.
J. M´endez-Ramos et al.: Yb3+ -Er3+ -Tm3+ co-doped nano-glass-ceramics tuneable up-conversion phosphor
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2
4
3+
S3/2( H11/2)→ I15/2
y
4
F9/2→ I15/2 : Er 3 3+ 1 G4→ F4 :Tm
3
4
Er
F3,2→ H6 : Tm
4 3+
3+
Tm 1 3 G4→ H6
GC
0.4
G
0.2 0.0 0.0
G
400
500
600
700
Wavelength (nm)
Fig. 3. Room temperature up-conversion luminescence spectra excited at 980 nm and pump power density of 20 mW/cm2 in the Yb3+ -Er3+ -Tm3+ co-doped precursor glass (G) and Yb3+ Er3+ -Tm3+ co-doped nano-glass-ceramics (GC). Spectra are normalized to the maximum of the green emission band at 550 nm.
a)
0.6
3+
GC
3
Intensity (arb. units)
0.8
b)
c)
Fig. 4. Colour tuneability of infrared up-conversion phosphor consisting of Yb3+ -Er3+ -Tm3+ co-doped precursor glass and GC: (a) white (slightly blue) emission from Yb3+ -Er3+ -Tm3+ co-doped precursor glass excited at 980 nm and 20 W/cm2 ; (b) green emission from Yb3+ -Er3+ -Tm3+ co-doped GC excited at 980 nm and 20 mW/cm2 ; (c) red emission from Yb3+ -Er3+ Tm3+ co-doped GC excited at 980 nm and 0.5 mW/cm2 .
GC, when excited at 980 nm, i.e. in the absorption bands of the Yb3+ and Er3+ , as seen from the absorption spectrum in Figure 1, and indicated by up-headed arrows in Figure 2. The emission spectra are dominated by red, green and blue emission bands which are assigned to transitions of Er3+ and Tm3+ ions; these emission transitions are shown in Figure 2 by the correspondingly coloured down-headed solid arrows. In particular, in the precursor glass the emission spectrum shows blue, green and red emission bands around 475, 550 and 660 nm, respectively. Contrary, the up-conversion luminescence spectrum of GC consists of only green and red emission bands centred around 550, 660 and 700 nm, respectively. Most striking difference in the spectra of glass and GC is that GC loses a blue emission band of the Tm3+ at 480 nm and gets a new red emission band of the Tm3+ at 700 nm. In additions, up-conversion emission bands in GC show higher Starksplit structure pointing out, as absorption spectra in Fig-
0.2
0.4 x
0.6
0.8
Fig. 5. Colour coordinates in the CIE standard chromaticity diagram corresponding to total visible up-conversion emission of the Yb3+ -Er3+ -Tm3+ co-doped precursor glass (G) and nano-glass-ceramics (GC). Excitation power density was at 20 W/cm2 .
ure 1, that the dopants partition into structurally ordered crystalline phase. When excited at 20 W/cm2 , the colour of the total emitted luminescence changes from mostly white (slightly blue) in the precursor glass to mostly green in the GC as pictured in Figures 4a and 4b, respectively. Quantitatively, the colour of the total visible upconversion emission in GC can be represented by the respective point in the CIE (Commission Internationale ´ d’Eclairage) standard chromaticity diagram [17], as in Figure 5. CIE diagram represents a colour of visible luminescence emitted by the sample (a convolution of blue, green and red emission bands) seen by the human eye when corrected to sensitivity of blue, green and red receptors of the eye. The x and y-axis of CIE diagram are the respective projective coordinates of the total visible luminescence [17]. In particular, the edges and the centre of the CIE diagram correspond to monochromatic light and white light, respectively. Intensity dependence of the up-conversion luminescence spectrum is seen in Figure 6, where excitation power density varies from 0.5 to 20 W/cm2 . In particular, the emission spectrum of GC changes substantially with pump power from mostly green at highest pump to mostly red at the lowest pump. The pictures in Figures 4b and 4c illustrate this change of the total visible luminescence colour in GC. CIE diagram coordinates in Figure 7 indicate quantitatively how the colour gradually changes with pump power from 0.5 to 20 W/cm2 in the GC and precursor glass (G), showing up the tuneability of emission colour from green to red in the GC and from white to greenorange in the precursor glass.
4 Discussion 4.1 Change of up-conversion luminescence mechanism with nano-ceramming of precursor glass The energy level diagram in Figure 2 illustrates how the up-conversion luminescence routes are changed with nanoceramming of the precursor glass and with increase of
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Intensity (arb. units)
(G)
3+
3+
3+
Er S3/2( H11/2)
Tm 3 G4 H6
4
1
2
4
I15/2
Er 4 F9/2 I15/2
4
3+
Tm 3 G4 F4
1
2
20 W/cm
2
15 W/cm
2
10 W/cm 2
5 W/cm
2
1 W/cm
2
0.5 W/cm
400
450
500 550 600 Wavelength (nm)
700
3+
(GC) Intensity (arb. units)
650
Er 4 F9/2→ I15/2
4 3+
Er 2 4 S3/2( H11/2) → I15/2
4
2
20 W/cm
3+
Tm 3 F3,2→ H6
3
3+
Tm 3 G4→ F4
1
2
15 W/cm
2
10 W/cm 2
5 W/cm
2
1 W/cm
2
0.5 W/cm
400
450
500 550 600 Wavelength (nm)
650
700
Fig. 6. Up-conversion emission spectra excited at 980 nm in Yb3+ -Er3+ -Tm3+ co-doped precursor glass (G) and (GC) for different excitation power density. The emission bands are assigned according to diagrams of Figure 2. Spectra are normalized to the maximum of the red emission band at 660 nm.
0.8 20 W/cm
0.6 y
10 W/cm
0.4
20 W/cm
2
10 W/cm
2
2
5 W/cm
2
2
0.5 W/cm
2
0.5 W/cm
2
4.2 Change of up-conversion luminescence mechanism with pump power of luminescence
0.2 0.00.0
As seen in Figure 6a, the blue emission at 475 nm originating from the 1 G4 level of the Tm3+ substantially increases with pump power in precursor glass, while it is absent in GC at any pump power (Fig. 6b). This points out that the 1 G4 level of the Tm3+ is completely depopulated in GC, i.e. with nano-ceramming of the precursor glass. This depopulation can be due to substantial shortening of inter-dopant distances in GC compared to the precursor glass, because most of the dopants (up to 90%) [10] are densely embedded in tiny nano-crystals in GC while they are homogeneously distributed in the precursor glass. This shortening results in efficient cross-relaxation process of the Tm3+ ions, shown by dot lines in Figure 2, which depopulates the 1 G4 level of the Tm3+ (down headed dot line) and additionally populates 3 F3,2 level of the Tm3+ adding to the experimentally observed increase of intensity of the red emission of the Tm3+ at 700 nm. The 3 F3,2 level of the Tm3+ is quenched nonradiatively by emission of phonons in precursor glass and therefore the 700 nm emission band of the Tm3+ is absent in the emission spectrum of glass, Figure 3. While the phonon energy in precursor glass is relatively high at about 900 cm−1 , it drastically decreases in PbF2 -based nano-crystals to about 250 cm−1 [9,13,21,22] and references therein. This precludes a non-radiative decay of the 3 F3,2 level of the Tm3+ to the first lower lying 3 H4 level of the Tm3+ thereby also adding to the intensity of 700 nm emission band of the Tm3+ , in agreement with data of Figure 3. The quantum yield of up-conversion luminescence in GC and its precursor glass is out of the scope of this article and it will be addressed in our future work. The Raman spectra of the GC and precursor glass have been investigated in [21,22] and references therein, and they prove that phonon spectrum of nanoparticles in GC differs substantially from the spectrum of the embedding glass network. Additionally, luminescence measurements of phonon side bands carried out in these nano-GCs in previous works [9] clearly reveals the reduction of phonon energy in the GC compared to precursor glass, resulting in higher upconversion efficiencies.
0.2
0.4 x
0.6
0.8
Fig. 7. Colour coordinates in the CIE standard chromaticity diagram corresponding to total visible up-conversion emission of the Yb3+ -Er3+ -Tm3+ co-doped precursor glass (◦) and GC (•). Excitation power density is postsigned.
pump power of luminescence. The 980 nm pump (shown by up-headed arrow in Fig. 2) excites Yb3+ ions, which transfer energy to Tm3+ and Er3+ ions by identified routes (shown by up-headed dash lines) [14,18–20]. The Er3+ ions are also excited directly by the 980 nm pump as shown by dash up-headed arrow in Figure 2.
The intensity of up-conversion luminescence IUP is proportional to a power n of the excitation intensity, IIR , IUP ≈ (IIR )n , where n is the number of infrared photons absorbed per visible photon emitted. Therefore, 3-photon up-conversion process shows stronger intensity dependence than 2-photon process. The 2- and 3-photon absorption processes are indicated by up-headed dash lines in Figure 2; these processes have been discussed elsewhere [18–20], and references therein. It is seen that the 1 G4 level of the Tm3+ can be populated only by 3-photon process, and therefore the blue emission from the 1 G4 level at 475 nm occurs only at high pump power of luminescence accounting for experimental data of Figure 6a, where the blue band is completely absent at the lowest pump power.
J. M´endez-Ramos et al.: Yb3+ -Er3+ -Tm3+ co-doped nano-glass-ceramics tuneable up-conversion phosphor
The increase in ratio of 2- to 3-photon processes with decrease of pump power also accounts for gradual red shift of total luminescence colour observed both in GC (Figs. 4b, 4c, 6b, 7) and precursor glass (Figs. 4a, 4b, 6a, 7) with decreasing the pump power. In the investigated range of intensities, we have not noted the heating of sample by laser radiation.
5 Conclusion In summary, the glass and nano-glass-ceramics presented in this work have potential use as infrared tuneable phosphor. Observed changes in the spectra of the up-conversion luminescence provide a tool for tuning the colour of the up-conversion luminescence by nanoceramming of the precursor glass and changing incident excitation power for both nano-glass-ceramics and precursor glass.
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