pH controlled Ho3+–Yb3+ codoped Y2O3 nanowires for display devices

pH controlled Ho3+–Yb3+ codoped Y2O3 nanowires for display devices

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 213–217 Contents lists available at ScienceDirect Spectrochimica Acta...

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 213–217

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

pH controlled Ho3+–Yb3+ codoped Y2O3 nanowires for display devices Riya Dey, Vineet Kumar Rai ⇑ Laser and Spectroscopy Laboratory, Department of Applied Physics, Indian School of Mines, Dhanbad 826004, Jharkhand, India

3+

g r a p h i c a l a b s t r a c t

 Preparation of Ho –Yb

3+

codoped Y2O3 phosphor at different pH.  Change in morphology due to pH variation.  Correlate the morphology and UC luminescence behaviour.

Effect of pH variation on the UC emission intensities. Upconversion intensity (arbitrary unit)

h i g h l i g h t s

500000 5

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F4, S2→ I8

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9 pH 10 pH 11 pH

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100000 5

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F4 , S2→ I7

F5→ I8

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Wavelength (nm)

a r t i c l e

i n f o

Article history: Received 17 November 2014 Accepted 16 June 2015 Available online 20 June 2015 Keywords: Optical materials Electron microscopy X-ray diffraction Luminescence

a b s t r a c t The wire like structure with particle size in the nanometer range of Ho3+–Yb3+ codoped Y2O3 phosphors have been synthesized through hydrothermal synthesis route by controlling the pH value. The Field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) analysis of the synthesized phosphor powders have been studied which confirms the formation of nanowires in the prepared materials and the formation of proper crystalline structure respectively. The frequency upconversion emission spectra under 980 nm excitation have been recorded and an efficient green emission has been observed. The excitation energy corresponding to the NIR photon seems to be fully utilized for the emission lying in the green region. The Fourier transform infrared (FTIR) spectroscopic analysis for existence of the impurities in the developed material has also been performed. The experimental observation proves the utility of the prepared material in the display devices and diagnosis purposes. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction In recent time the luminescent nanomaterials having size 6100 nm are of significant interest to the researcher community as they endow excellent optical, magnetic, electrical behaviour as a consequence of quantum confinement effect of nanostructured materials. The high surface to volume ratio of the nanomaterials viz. nanowires, nanoflowers, nanorods, nanoprisms encourages ⇑ Corresponding author. E-mail (V.K. Rai).

addresses:

[email protected],

http://dx.doi.org/10.1016/j.saa.2015.06.047 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

[email protected]

the researcher to switch over from microparticles to nanoparticles synthesis [1–7]. Among the different class of nanomaterials including polymer based dyes, inorganic fluorescent emitters, semiconductors nanocrystals known as ‘quantum dots’, the inorganic luminescent nanomaterials are beneficial because of several shortcomings associated with the others. Generally, the fluorescence arising from the dye molecules in the solution phase show poor intensity due to the interaction between the solvent molecules as well as with the oxygen. In spite of a number of advantageous features associated with the semiconductor quantum dots the toxicity corresponding to them restricts their use [8]. Therefore, the inorganic luminescent nanomaterials are of momentous interest.

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These materials owe sharp emission with better luminescence efficiency, less photobleaching, low toxicity. The inorganic nanomaterials are applicable in a variety of class viz. in solar cells, optoelectronic devices, medical and biological fields, low power laser fabrications, security purposes, display devices, etc. [9–14]. In most of the luminescent materials the lanthanide elements are generally used as a source of the emitting centers which may be described by their 4fn transitions. The transitions within the 4f electron state manifold results sharp emission peaks due to presence of the undisturbed energy states caused by the shielding from the local environment by the outer 5s and 5p orbitals. The dimension within which the energy levels are distributed is very small compared to the size of the nanoparticles [8]. Therefore, in the lanthanides doped nanostructured phosphors the energy level positions of the rare earth ions are not affected by the size of the material. As the low phonon frequency of the Y2O3 host opens up the possibility for getting improved luminescence. Therefore, researchers are motivated for synthesizing rare earth doped Y2O3 luminescent material with better luminescence efficiency in recent time. In order to get the better shape and size of the luminescent phosphor control over the synthesis process is essential. In the present article we report the variation in the structural and luminescent behaviour by changing the pH of the Ho3+–Yb3+ codoped Y2O3 nanophosphor synthesized by hydrothermal process. 2. Experimental 2.1. Synthesis process The Ho3+–Yb3+ codoped Y2O3 phosphors having nanowire like shapes have been synthesized by the hydrothermal synthesis. The Ho2O3, Yb2O3 and Y2O3 powders were used as the starting materials. The host and the dopant elements were mixed according to the following equation

ð100  x  yÞY2 O3 þ x Ho2 O3 þ y Yb2 O3 where x and y were fixed to 0.2 mol% and 3.0 mol% respectively. The concentration of the dopants have been fixed to their optimized values as reported in our previous work [15]. For the preparation of the phosphor powder the starting materials Y2O3 (99.999% pure, form Otto), Ho2O3 (99.99% pure, purchased from Otto) and Yb2O3 (99.9% pure brought from CDH) were taken. All the reagent chemicals were converted into nitrates by the reaction with nitric acid. Then the nitrate mixtures were blended together. In the nitrate mixture NH4OH was added dropwise and the pH value of the solution was controlled to the prerequisite value by checking it with digital pH meter. Because of the addition of alkaline solution to the nitrate mixture solution the precipitate starts appearing. The solution with the precipitate was transferred into a stainless steel autoclave with a 100 ml Teflon liner and placed inside a heating oven at 180 °C temperature for 24 h. After cooling down to the room temperature the precipitate was taken outside and filtered with the help of filter paper. Afterwards the residue was washed with distilled water and ethanol in order to remove the unwanted chemical contents as much as possible. The product was dried out at 70 °C for several hours. Then the as synthesized samples were annealed at 900 °C temperature for 4 h. All the structural and optical studies were performed with these annealed samples at room temperature (27 °C). 2.2. Characterization The X-ray diffraction (XRD) analysis of Ho3+–Yb3+ codoped Y2O3 phosphor has been done by using the X-ray diffractometer with Cu–Ka (1.5405 Å) radiation within 10–80 degree range of 2h. For

the morphological study of the prepared materials the FESEM analyses have been performed. The presence of excess impurity contents has been confirmed with the help of FTIR spectroscopy. The UC luminescence measurements have been performed by a spectrometer attached to a PMT with a 980 nm CW diode laser excitation. All the measurements have been made at 27 °C temperature. 3. Results and discussion 3.1. XRD study For the study of the crystalline nature and the determination of the crystallite size of the prepared material the XRD analysis of the prepared material has been performed and the XRD pattern of the prepared material is shown in the Fig. 1. The XRD spectra matches well with the standard cubic phase Y2O3 (JCPDS file No.-25-1200). The standard pattern is also provide in Fig. 1. We have calculated the crystallite size of the prepared phosphors using the Debye–Scherrer’s formula for the XRD peaks at 20.49°, 29.14°, 33.76°, 48.49° and 57.61°. The average crystallite size was found to be 24 nm. 3.2. FESEM analysis Fig. 2 represent the FESEM micrograph of the Ho3+–Yb3+ codoped Y2O3 phosphor powders prepared at different pH values. From the figure it is clearly visualized that the all the three samples exhibit wire like morphology having diameter in the nanometer regime with different aggregation state. At pH  9, the nanowires are not formed properly; some nanoflakes are coexisting and are compactly aggregated together. As the pH value is increased the improvement in the morphology of the prepared phosphor is observed. The size of the nanowire is reduced when the pH value is varied from 9 to 11. Very fine naowires are visible when the pH value is changed to 11. The aggregations are loosely bound when the alkaline nature is more prominent in the sample (i.e., for pH 10 and 11). Therefore, The FESEM images signify that the phosphor nanowires are successfully prepared. 3.3. FTIR analysis During the preparation of the phosphors some impurities are retained with the phosphor which may act as quencher for optical

Ho3+-Yb3+:Y2O3 Intensity (arbitrary unit)

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JCPDS-25-1200

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2θ (degree) Fig. 1. The X-ray diffraction spectra of Ho3+–Yb3+ codoped Y2O3 phosphor annealed at 900 °C temperature.

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intensity of the different vibrational bands are different for different samples. In case of the phosphor powder prepared at 11 pH shows most intense peaks. This indicates that the more amount of impurities are present in this sample while the FTIR spectra of the phosphor prepared at 9 pH signifies the presence of less impurity contents. 3.4. UC emission study The UC emission spectra of three different samples at 980 nm excitation are shown in Fig. 4. From the Fig. 4 it is observed that the emission intensity is gradually decreased as the pH value is increased. Due to increase in the pH value the impurity content mainly the –OH group peaking around 3144 cm1 appears to increase (Fig. 3). It is well known that the impurity contents like –OH group is sufficient to quench the intensities of the UC emission bands [17]. Also due to increase in the pH value, the surface defect would be expected to increase. Some of these defect centres may play the role of the nonradiative recombination centers and therefore, may reduce the emission intensity [19]. Under the NIR excitation three UC emission bands at 551 nm, 668 nm and 758 nm are observed which are assigned as the 5F4, 5S2 ? 5I8,

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9 pH 10 pH 11 pH

Intensity (arbitrary unit)

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Fig. 2. FESEM image of Ho3+–Yb3+ codoped Y2O3 phosphors prepared at (a) pH  9, (b) pH  10 and (c) pH  11.

properties of the prepared phosphors. As the quantity of NH4OH was increased in order to get improved morphology the impurity contents may also increase. Fig. 3 shows the FTIR spectra of the Ho3+–Yb3+ codoped Y2O3 phosphors prepared at different pH values. Intense bands around 3144 cm1, 1398 cm1 have been observed which are assigned as the stretching vibrational frequency of –OH and the characteristic vibration of –NO3 [16]. The peak around 2379 cm1 is because of the —CO2 stretching 3 vibration [17]. Another peak 1058 cm1 may be due to the rocking vibration of –CH3 [18]. Less intense peak 574 cm1 is arising may be due to the Y–O vibration [16]. It is further observed that the

Upconversion intensity (arbitrary unit)

Fig. 3. FTIR spectra of Ho3+–Yb3+ codoped Y2O3 phosphors prepared at different pH values.

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Wavelength (nm) Fig. 4. UC emission spectra of Ho3+–Yb3+ codoped Y2O3 phosphors with excitation at 980 nm.

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F5 ? 5I8 and 5F4, 5S2 ? 5I7 transitions respectively. The power law relation between the UC emission intensity (IUC) and the excitation power (P) as predicted theoretically can be given as

20 green red NIR

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5 F 5 5 I 4 5 I 5 5 I 6 668 nm

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551 nm

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Yb

Fig. 6. Energy level scheme for understanding the upconversion mechanism in Ho3+–Yb3+ codoped system.

ð1Þ

where, ‘n’ gives the number of photons involved in the UC process. The ln–ln plot of excitation power versus UC emission intensity is shown in Fig. 5. The exponents are determined as 2.1, 1.5 and 1.6 for the green, red and NIR band respectively which allow us to believe that the green, red and NIR emission bands are observed due to two photon absorption processes. The plot of logarithmic of UC emission intensity as a function of logarithmic of pump excitation power does not represent any sigmoidal type nature therefore, the probability of occurrence of photon avalanche process is excluded from the picture. Hence, the UC emissions are generating because of the sequential energy absorption by the Ho3+ ion and/or sequential energy transfer from Yb3+ ion to Ho3+ ion. The detail of the process involved is reported elsewhere [15]. In light of the observed UC emissions, the possible mechanisms and the responsible channels could be discussed with the help of energy level diagram (Fig. 6). In the first step, the Ho3+ ions in the ground state absorb the 980 nm photons and through ground state absorption (GSA) process lifted up to the 5I6 level and the excess energy of excitation is adjusted by transferring it to the host lattice in the form of phonon energy. For the re-excitation of the Ho3+ ions from the 5I6 level to the 5F4, 5S2 level, the second photon of same energy (10,200 cm1) is absorbed by the active ions lying in the metastable level (5I6). The other way by which some of the excited Ho3+ ions in the 5I6 state may be utilized is the non radiative relaxation to the 5I7 level by loosing 9 phonons [20]. The process of decaying the Ho3+ ions from the 5I6 level to the 5I7 is a slow process. Therefore, most of the population of 5I6 state are employed to increase the population of the 5F4, 5S2 state by absorbing the 980 nm photon. Also the role of Yb3+ ion in the UC process is to sensitize the luminescence emission by transferring its excitation energy to the activator ions. The energy difference between the 2F7/2 and 2 F5/2 is matching well with the energy of the 980 nm photon. So, the probability of absorbing 980 nm photon by the Yb3+ ion is very high. As more number of photons are absorbed by the Yb3+ ion so, the chance of sensitizing the UC emissions corresponding to the Ho3+ ion by efficient energy transfer becomes more probable. Hence, due to the energy transfer from the Yb3+ ions population of all the emitting levels (5I6, 5F5 and 5F4, 5S2) of Ho3+ ion is enhanced significantly.

Fig. 7. Colour coordinates of Ho3+–Yb3+ codoped Y2O3 phosphor (a) at different excitation power and (b) prepared at different pH value. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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To verify the colour purity of the prepared materials and to propose their applicability in different purposes the colours emitted by the materials have been tested out with the help of CIE chromaticity diagram. The colour co-ordinates of the optimized samples at different pump excitation powers have been calculated and shown in Fig. 7(a). The colour emitted by the sample is lying in the green region irrespective of the excitation power. Not only the pump power dependence also, the pH dependent UC luminescent behaviour has been studied (Fig. 7b). From Fig. 7(a) and (b) it is clearly observed that the CIE coordinates are of almost constant which signifies the appropriateness of the developed materials to be used for making display devices. 4. Conclusion The effect of pH variation in the Ho3+–Yb3+ codoped Y2O3 phosphor powder on the structural as well as the luminescent behaviour have been successfully verified. The effects of the structural disorder as well as the existence of impurities on the UC luminescence behaviour have been explored and discussed in detail. Based on the observed results it may be proposed that the developed phosphor finds its application in making display devices, green upconverter and in cancer localization. Acknowledgements Authors are grateful to Indian School of Mines, Dhanbad for the financial support. Authors acknowledge the financial assistance from Department of Science and Technology (DST), New Delhi, India. References [1] A. Ghezelbash, M.B. Sigman Jr., B.A. Korgel, Solventless synthesis of nickel sulfide nanorods and triangular nanoprisms, Nano Lett. 4 (2004) 537–542. [2] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Room-temperature ultraviolet nanowire nanolasers, Science 292 (2001) 1897–1899.

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