White upconversion emission in Y2O3:Er3+–Tm3+–Yb3+phosphor

White upconversion emission in Y2O3:Er3+–Tm3+–Yb3+phosphor

Materials Research Bulletin 48 (2013) 2232–2236 Contents lists available at SciVerse ScienceDirect Materials Research Bulletin journal homepage: www...

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Materials Research Bulletin 48 (2013) 2232–2236

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

White upconversion emission in Y2O3:Er3+–Tm3+–Yb3+phosphor Vineet Kumar Rai *, Riya Dey, Kaushal Kumar Laser and Spectroscopy Laboratory, Department of Applied Physics, Indian School of Mines, Dhanbad 826004, Jharkhand, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 26 July 2012 Received in revised form 4 December 2012 Accepted 18 February 2013 Available online 5 March 2013

Er3+–Tm3+–Yb3+ codoped Y2O3 phosphor has been synthesized by optimized combustion synthesis process and its white light upconversion emission property is investigated using cheap 980 nm diode laser excitation. Efficient red, green and blue light emission bands, necessary for attaining white light emission, are observed in the codoped sample. The concentration of each rare earth ion is adjusted to get the required emission. In this phosphor, interestingly, emission colour coordinates are found to almost independent on the excitation power density. The temperature sensing behaviour of the prepared samples has also been studied using fluorescence intensity ratio (FIR) technique. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: C. X-ray diffraction D. Luminescence

1. Introduction Triply ionized rare earth doped oxide materials are of particular interest due to their splendid applications in lasers, colour displays, biomedical diagnostics, upconverters, optical amplifiers, sensors, telecommunication, etc. [1–7]. White light emission from rare earth ions can also be obtained and is an interesting area of research [6,8]. An incandescent bulb can also generate broad white light spectrum but due to good colour contrast, high efficiency and wide colour tunability RGB based white light emitting devices are found more advantageous over the incandescent lamps. These light sources are enviable because of their low power consumption [8,9]. In recent years, light emitted from the rare earth doped phosphors have used extensively in medical science and intensive research is moving ahead in this direction [9]. Moreover, rare earth ions are desirable candidates for upconversion luminescence process as they possess large number of energy levels, many of them have metastable nature [10] and they can be easily populated by infrared excitation sources. Solid materials having lower phonon energies are suitable candidates for good upconversion emission [11–16]. The choice of the host material is an important factor for achieving high emission intensity and colour purity. Y2O3 is considered as good host for the triply ionized lanthanides over other oxide hosts because of several reasons, viz. low cut off phonon energy (380 cm1) [17,18], high refractive index (1.8), higher melting point (2400 8C), wide transparency range, high chemical durability and thermal conductivity [18].

* Corresponding author. Tel.: +91 326 223 5404/5282. E-mail addresses: [email protected], [email protected] (V.K. Rai). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.02.064

Now-a-days intensive studies on white light generation in rare earth doped materials through upconversion process are going on [19–21]. Depending upon the dopants (i.e., rare earth ions) it is possible to get different upconversion emissions upon NIR excitation. To obtain good white light it is necessary to have efficient absorption and controlled emission of three primary colours (i.e., blue, red and green). For obtaining optimized energy upconversion some parameters are responsible such as molar concentration of the dopants, crystallite size, surface area, and excitation density of source [22–24]. Dependence of colour purity of white light emission on excitation power is one of the constraints behind the practical applicability. So a phosphor is required which is free from this limitation. In the present work, we have synthesized Er3+–Tm3+–Yb3+ codoped Y2O3 phosphor using low temperature solution combustion process, since solution combustion route is considered as efficient one due to the homogeneous mixing and uniform particle size of the prepared sample. With 980 nm laser diode excitation three upconversion emission bands in blue, green and red region have been investigated in order to achieve white light emission. Using the FIR technique temperature sensing behaviour of the synthesized sample has also been explored.

2. Experimental For synthesis of the triply ionized rare earth doped phosphor powder we have used solution combustion technique where nitrates of the precursors were used as reagents and urea as organic fuel. Purchased chemicals were in oxide form, so first they were converted into nitrate form. The concentration of Tm3+ and Yb3+ was kept fixed at 0.1 mol% and 2 mol% respectively while that

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of Er3+ was varied from 0.1 mol% to 0.9 mol%. The composition used for the synthesis of the phosphor was the following: ð100xyzÞY2 O3 þ xEr2 O3 þ yTm2 O3 þ zYb2 O3 where x = 0.1, 0.3, 0.6, 0.9 mol%, y = 0.1 mol% and z = 2.0 mol%. According to the designed composition the oxide chemicals were taken and dissolved in nitric acid to form the nitrates then they were finally mixed in urea solution. Then the entire solution was stirred vigorously for 2 h for getting homogeneous mixture with no residue. The stirring was carried out at 60 8C. By this process the whole mixture was transformed into transparent gel after the evaporation of excess water content. The gel was then transferred to alumina crucible and kept inside a closed furnace preheated at 500 8C. At this high temperature combustion took place spontaneously with the evolution of gases. The entire reaction was completed within 2 min resulting fluffy mass like product. The product was then brought outside the furnace and crushed to get fine powder. As-synthesized product was then heat treated at 750 8C for 2 h to get more crystallization. The heat treated samples were then used for further characterization. The X-ray diffraction pattern of the heat treated sample was recorded to know about the crystallite size and the phase. Room temperature upconversion luminescence spectra of the phosphors were recorded by a monochromator connected with a PMT. For excitation process power tunable 980 nm laser diode was used. The sample was optimized to get the maximum upconversion intensity at green region. 3. Results and discussion

Fig. 2. Upconversion luminescence spectra for fixed 0.1 mol% Tm3+, 2.0 mol% Yb3+ and varying Er3+ concentration: (a) 0.1 mol% Er3+, (b) 0.3 mol% Er3+, (c) 0.6 mol% Er3+ and (d) 0.9 mol% Er3+ codoped Y2O3 phosphor powders heat treated at 750 8C.

of dopants were recorded at room temperature within the 200– 750 nm wavelength region upon 980 nm continuous wave laser diode excitation (Fig. 2). Three primary colour bands at blue (440 nm–508 nm), green (514–578 nm) and red (623–710 nm) regions are visible along with weak bands in UV (351–377 nm) region. These bands are assigned as follows:

3.1. X-ray diffraction studies

1

Blueband 3+

G4 ! 3 H6 ðTm3þ Þ

3+

X-ray diffraction pattern of the heat treated Y2O3:Er –Tm – Yb3+ phosphor is represented in Fig. 1. It is observed that the peaks are matching well with the JCPDS data file (no. 25-1200) and the sample coincides with the cubic phase of Y2O3 having lattice parameter a = b = c = 10.60 A˚ and a = b = g = 908. From the XRD data we have calculated the average crystallite size of the prepared phosphor sample using the standard Debye–Scherrer equation and it comes out to be 19 nm.

2

Greenband Redband

4

H11=2 ;4 S3=2 ! 4 I15=2 ðEr3þ Þ

F9=2 ! 4 I15=2 ðEr3þ Þ;1 G4 ! 3 F4 ðTm3þ Þand3 F2;3 ! 3 H6 ðTm3þ Þ

3.2. Frequency upconversion measurements

UVband

The frequency upconversion luminescence spectra of the Y2O3:Er3+–Tm3+–Yb3+ phosphor having different concentrations

The upconversion emission bands in Fig. 2 are showing large stark splitting due to the higher crystal field of Y2O3 host.

Fig. 1. The X-ray diffraction pattern of 0.3 mol% Er3+ + 0.1 mol% Tm3+ + 2 mol% Yb3+codoped Y2O3 phosphor heat treated at 750 8C.

Fig. 3. Intensity variation of various upconversion emission bands with increasing laser power.

2

G9=2 ! 4 I15=2 ðEr3þ Þ

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Fig. 4. Schematic energy level diagram for Er3+–Yb3+–Tm3+ energy transfer system.

To identify the number of photons involved for emission process the input pump power dependence of upconversion emission intensity of various bands is studied. The upconversion emission intensity is proportional to the nth power of the pump power as Iup a Pn, where n is the number of NIR photons involved for a particular upconversion emission band. So, log(Iup) versus log(P) plot gives the straight line with slope n. The values of n for UV (365 nm), blue, green and red emission bands come out to be nearly 2.5, 1.98, 1.86 and 1.58, respectively as from Fig. 3. These values are indicating that UV emission occurs due to three photon absorption process while the blue, green and red emissions occur due to two photon absorption process. The upconversion mechanisms can be explained with the help of energy level diagram as shown in Fig. 4. In Tm3+ singly doped phosphor sample, the upconversion emission is not observed at low excitation power but at very high pump power excitation density (64.94  104 W/m2) very weak blue upconversion emission is observed. This excitation process is due to the absorption of three NIR photons only at high pump power of the incident radiation [16]. But in case of Yb3+–Tm3+ codoped Y2O3 phosphor even at very low pump power density (1.29  104 W/m2) the UC emission in the blue region is observed. This is possible due to the two photon absorption process. In this case, the ground state Yb3+ ion after absorbing a NIR photon transits upward to the 2F5/2 level. The excited Yb3+ ions in 2F5/2 state transfer their energy cooperatively and go to a virtual level V and then transfer their excitation energy efficiently to the ground state Tm3+ ions and pump them to the 1G4 level. A radiative transition from the 1G4 level to the 3H6 level emits a photon in the blue region. In this triple doped ion system Yb3+ ions energize both the Er3+ and Tm3+ ions. With 980 nm excitation, two Yb3+ ions in the ground state make transition to excited level and a part of excited Yb3+ ions cooperatively transfer their energy to the Tm3+ ions in 1G4 level [6]. The 1G4 level depopulates through radiative transition to the ground 3H6 level and through nonradiative transitions to lower excited levels. The transition from 1G4 level to 3H6 level gives rise the blue emission band. Red emission may generate due to following transitions between the Tm3+ levels.

(i) 1G4 ! 3F4 (ii) 3F2,3 ! 3H6 Remaining part of excited Yb3+ ions transfer their energy to the Er ions in 4F7/2 and 4I11/2 levels. Two green bands are observed in the upconversion spectra which are due to the 2H11/2 ! 4I15/2 and 4 S3/2 ! 4I15/2 transitions of Er3+ ions. Er3+ ions itself absorb incident photons, in the first step, with the absorption of one NIR photon (980 nm) the 4I11/2 level is populated. After that the excited ions in the 4I11/2 level further get excited to the 4F7/2 level via excited state absorption process. The ions in the 4F7/2 level relax nonradiatively to the 2H11/2, 4S3/2 levels. The upconversion emission corresponding to the 4F9/2 ! 4I15/2 transition within Er3+ ions is also responsible for the generation of red photon. In case of Yb3+ and 3+

Fig. 5. Colour coordinates for fixed 0.1 mol% Tm3+, 2 mol% Yb3+and (a) 0.1 mol% Er3+, (b) 0.3 mol% Er3+, (c) 0.6 mol% Er3+ and (d) 0.9 mol% Er3+ codoped Y2O3 phosphor powders at 34.87  104 W/m2 input excitation power density.

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Er3+ ion is responsible for the generation of green and red emission, so with increase in the Er3+ concentration the colour of the upconverted emission is tuned. When the concentration of Er3+ is low the blue colour which is due to Tm3+ ion is the dominant one. The white colour is the mixture of three primary colours. For 0.3 mol% Er3+ + 0.1 mol% Tm3+ + 2 mol% Yb3+ codoped Y2O3 phosphor at 34.87  104 W/m2 excitation power density the CIE coordinates are X = 0.32, Y = 0.34 as shown in Fig. 5. The CIE coordinates for different samples at 34.87  104 W/m2 pump power density are shown in Fig. 5. The CIE colour coordinates of 0.3 mol% Er3+ + 0.1 mol% Tm3+ + 2 mol% Yb3+ doped Y2O3 phosphor at three different pump power densities, viz. 7.08  104 W/m2, 34.87  104 W/m2 and 61.3  104 W/m2 are (0.32, 0.35), (0.32, 0.34) and (0.32, 0.34) respectively, as shown in Fig. 6. The figure suggests that the upconversion emission color from 0.3 mol% Er3+ + 0.1 mol% Tm3+ + 2 mol% Yb3+ codoped Y2O3 phosphor is almost independent on excitation power density. 3.4. Temperature sensing behaviour

Fig. 6. Colour coordinates of 0.3 mol% Er3+, 0.1 mol% Tm3+, 2 mol% Yb3+and codoped Y2O3 phosphor powders at (i) 7.08  104 W/m2, (ii) 34.87  104 W/m2, and (iii) 61.3  104 W/m2 input excitation power density.

Er3+ codoped system, excited Yb3+ ions prefer to transfer their energy via non-cooperative way because the 2F5/2 level of Yb3+ matches well with 4I11/2 level of Er3+. So, cooperative energy transfer from Yb3+ to Er3+ is least probable. 3.3. Colour tunability By changing the concentration of the Er3+ ions with fixed Tm3+ and Yb3+ concentrations the colour of the emitted light can be tuned. With the increase of Er3+ concentration the upconversion emission changes from bluish to yellowish colour. For 0.1 mol% Er3+ + 0.1 mol% Tm3+ + 2 mol% Yb3+ codoped Y2O3 phosphor the upconversion emission appears blue in colour, whereas 0.3 mol% Er3+ + 0.1 mol% Tm3+ + 2 mol% Yb3+ codoped Y2O3 phosphor gives nearly white emission. Green emission is generated by the 0.6 mol% Er3+ + 0.1 mol% Tm3+ + 2 mol% Yb3+ codoped Y2O3 phosphor while in case of 0.9 mol% Er3+ + 0.1 mol% Tm3+ + 2 mol% Yb3+ codoped Y2O3 phosphor yellowish colour emission is observed. The camera images of these colours are shown in inset of Fig. 2. As the

There are several temperature sensing techniques for rare earth doped phosphors. Among them FIR (fluorescence intensity ratio) technique is the most popular technique in which the intensity ratio of the two thermally coupled emission peaks is measured with temperature [4,25]. In Er3+ ions the 2H11/2 and 4 S3/2 levels are found thermally coupled at room temperature and act as temperature sensor. The spacing between these levels is 800 cm1. Here we have checked the FIR of these two peaks with increasing excitation power. The intensity ratio is plotted against input power in Fig. 7. The graph shows a linear behaviour. The change in FIR with increasing excitation power is due to the increase in sample temperature. Higher excitation power produces higher temperature through nonradiative transitions. Measurement suggests that studied phosphor could act as temperature sensor for temperature above the room temperature. 4. Conclusions The Er3+–Tm3+–Yb3+ codoped Y2O3 phosphor with cubic crystal structure has been prepared successfully through cost-effective, low temperature combustion route. The upconversion emission spectrum of the synthesized phosphors has shown multicolour emissions including white light emission. By using the FIR technique the temperature sensing behaviour of the synthesized phosphor has also been investigated. Acknowledgements Authors are grateful to the University Grants Commission (UGC), New Delhi, India for providing the financial assistance. Also, one of the author Riya Dey is very much thankful to Indian School of Mines, Dhanbad for providing the financial assistance. References

Fig. 7. Fluorescence intensity ratio of the emission bands centred at 521 nm and 553 nm as a function of input laser power.

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