Role of Eu2 + on the blue‐green photoluminescence of In2O3:Eu2 + nanocrystals

    Role of Eu2 + on the blue–green photoluminescence of In 2 O3 :Eu2 + nanocrystals Konsam Reenabati Devi, Sanoujam Dhiren Meete, Shougaijam Dorendrajit Singh PII: DOI: Reference:

S1044-5803(16)30016-X doi: 10.1016/j.matchar.2016.01.025 MTL 8172

To appear in:

Materials Characterization

Received date: Revised date: Accepted date:

23 July 2015 12 January 2016 17 January 2016

Please cite this article as: Devi Konsam Reenabati, Meete Sanoujam Dhiren, Singh Shougaijam Dorendrajit, Role of Eu2 + on the blue–green photolumidoi: nescence of In2 O3 :Eu2 + nanocrystals, Materials Characterization (2016), 10.1016/j.matchar.2016.01.025

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Role of Eu2+ on the blue-green photoluminescence of In2O3:Eu2+ nanocrystals

Department of Physics, Manipur University, Imphal-795003, Manipur, India

Department of Physics, North Eastern Regional Institute of Science & Technology, Nirjuli,

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Konsam Reenabati Devi1,*, Sanoujam Dhiren Meete1,2, Shougaijam Dorendrajit Singh1

Itanagar-791109, Arunachal Pradesh, India

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*Corresponding Author

Electronic mail: [email protected] (KRD); [email protected] (SDM) &

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[email protected] (SDS)

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ABSTRACT:

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Blue-green light emitting undoped and europium doped indium oxide nanocrystal were synthesized by simple precipitation method. X-ray diffraction (XRD) pattern confirmed

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the cubic phase of undoped and europium doped samples. Further, transmission electron microscopy (TEM), scanning electron microscopy (SEM) , energy dispersive analysis of xrays (EDAX), Fourier transform infra-red (FT-IR), photoluminescence (PL), electron paramagnetic resonance (EPR) studies were performed to characterise the samples. PL

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analysis of the samples is the core of the present research. It includes excitation, emission and CIE (Commission Internationale de l’e´ clairage) studies of the samples. On doping europium to In2O3 lattice, ln3+ site is substituted by Eu2+ thereby increasing the concentration of singly ionized oxygen vacancy and hence blue-green emission from the host is found to increase. Further, this increase in blue-green emission after doping may also be attributed to 4f→5d transitions of Eu2+. However, the blue-green PL emission is found to decrease after an optimum dopant concentration (Eu2+ = 4%) due to luminescence and size quenching. CIE coordinates of the samples are calculated to know color of light emitted from the samples. It suggests that this blue-green light emitting In2O3: Eu2+ nanocrystals may find application in lighting such as in generation of white light. KEYWORDS: Indium oxide; Nanocrystal; Photoluminescence; Oxygen vacancy; Blue-green

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1. INTRODUCTION:

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Metal oxide nanocrystals represent a field of research which attracts considerable interest due to its potential technological applications. In recent years, the implications of these materials on fields such as medicine, information technology, catalysis, energy storage

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and sensing has driven much research in developing synthetic pathways to such

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nanostructures. Among the metal oxide, semiconducting nanocrystals are most promising. Because they have many advantages such as high specific area, more grain boundaries, low

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power consumption, high compatibility with microelectronic processing, etc. [1] Further, it is

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reported that, oxygen vacancies in the metal oxide semiconductors contribute to improvement

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in gas response of the material [2].

Among the semiconducting metal oxides, indium oxide (In2O3), an n-type semiconductor is reported to be very attractive as it has a wide band gap (direct gap of 3.55–

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3.75 eV) [3-7]. It is highly transparent in the visible range and possesses high free carrier mobility. It finds application in diverse fields such as UV photodetection, thin film transistors, optoelectronic devices, tunnel barriers in spintronics devices, solar cells, gas sensor, etc. [8–10] And hence synthesis and characterization of In2O3 is reported recurrently. Few methods that have been reported for synthesis of indium oxide are co-precipitation [5, 11], laser ablation physical evaporation [12], surfactant free room temperature soft chemical, modified hydrothermal route [13]. Wu et al. [14] have synthesized indium oxide nanowires by carbothermal reduction reaction and studied their photoluminescence (PL) property. They reported that the prepared

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nanowires emitted blue light at 416 nm and 435 nm due to the presence of vacancies in the material. Seetha et al. [3] also reported blue emission for their indium oxide samples.

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Emission of light in the visible region due to its defects is one of an interesting factor in

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studying the material. In general, vacancy in the material may be either In or O vacancy and

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defects in the material such as anti-site O, In or interstitial oxygen can also induce emission in the visible region [13-16].

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Generally, rare earth ions are incorporated in semiconductors to improve luminescence efficiency [17-19]. However, there are very few reports on synthesis and

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optical properties of Eu2+ doped In2O3 nanocrystal. Keeping this in mind, undoped (In2O3) and europium doped indium oxide (In2O3:Eu2+) were synthesized by simple precipitation

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method. The nanocrystals have been synthesized, characterized and studied exhaustively.

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Role of Eu2+ on the blue-green PL of In2O3:Eu2+ nanocrystals are reported herein.

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2. EXPERIMENTAL:

Indium nitrate, In(NO3)3 (99.999%, Aldrich) and Europium nitrate, Eu(NO3)3

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(99.99%, Aldrich) were used as starting materials without further purification. The stoichiometric quantity of reagents were made to dissolve in distilled water and stirred for 15 min. Then

0.4 M NaOH pellet was added to the solution and stirred for another 30 min

and the precipitate so formed was collected by continuous washing with distilled water and acetone. The precipitate so obtained were dried, ground and annealed at 900 °C for 3 hours. The possible reaction mechanism which leads to In2O3 formation is given below. In(NO3)3 + 3NaOH → In(OH)3 + 3Na(NO3) In(OH)3 → In2O3

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Similar procedures were performed on synthesising samples containing different percentages of europium.XRD data of the samples are recorded in X’Pert PANalytical

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diffractometer at 40 kV and 30 mA with 30 min counting time. Wavelength of the x-ray used

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is 1.54 Å (Cu-Kα). TEM images and SAED pattern were recorded using a JEM-2100

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microscope (JEOL) at 200 kV. FTIR spectra are recorded in MB102 spectrometer (BOMEN). Percentages of elements present in the sample are quantified by EDAX (AMETEK) attached to scanning electron microscope, SEM (Quanta250) at 20 kV. JEOL, JES-FA200 ESR

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spectrometer is used to identify the nature of oxygen vacancy present in the samples.

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Photoluminescence (PL) emissions are recorded on LS55 Fluorescence Spectrometer (PerkinElmer).

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3. RESULTS AND DISCUSSION:

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Fig.1 shows the XRD patterns of In2O3 and In2O3:Eu2+ (1 at.%, 2 at.%, 4 at.%, 5 at.%,

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6 at.%) samples. X’pert HighScore’s search match analysis shows all undoped and doped samples as cubic phase (Ref. Code: 00-006-0416). The average crystallite sizes were calculated from XRD data by using Scherrer formula: t = (0.9λ)/(βcos θ) where λ is the

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wavelength of the X-ray, 0.9 is shape factor, β is full width at half maximum (FWHM) in radian and θ is the Bragg angle. The average crystallite size was found to be 36.5 nm for undoped sample. However, the average crystallite sizes are found to decrease with increasing doping concentrations, as it is shown in Fig. 2. Moreover, with the increase of doping concentration, the shifts in diffraction peaks position toward lower Bragg angle are observed, Fig.1. These are attributed to the substitution of smaller In3+ (ionic radius = 94 pm) by larger Eu2+ (ionic radius = 94.7 pm). Similar, reports are also found in the references [20, 21]. Unit cell parameters of the samples were calculated from the XRD data by using the relation, dhkl = a/(h2+k2+l2)1/2; where dhkl is the inter planner spacing corresponding to Miller

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indices (hkl) and a, lattice constant. Table 1 represents the unit cell parameters of the prepared samples along with the parameters of the reference. It is found that unit cell volume

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of the samples increase as the concentration of Eu2+ increases, as it is shown in Fig. 3. This is

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attributed to the substitution of smaller In3+ by larger Eu2+.

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Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) observations of the samples were carried out. TEM and HRTEM

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images (inset SAED pattern) of In2O3 are shown in Fig.4 (a, b, c). The average crystallite size

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of In2O3 determined from TEM micrographs are in the range 30-50 nm. It is in close

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agreement with that obtained from XRD line broadening analysis i.e. 36.5 nm. The clear fringes observed in HRTEM image Fig. 4(c) demonstrate that the synthesized In2O3 sample is single crystalline. Further, the separation between the lattice fringes is determined and it is

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found to be 2.96 Å (Fig. 4c). It is in good agreement with the lattice spacing d222 (2.92 Å) of body centred cubic indium oxide (Ref. Code: 00-006-0416). High crystallinity of the sample is also confirmed from the selected area electron diffraction (SAED) pattern (inset fig.4c). Diffraction spots corresponding to the (200), (222) and (400) lattice planes are identified. Similarly, Fig. 5 (a, b) and Fig 5(c) shows the TEM images and HRTEM image (inset SAED pattern) of the In2O3:Eu2+ (Eu2+ = 2 at.%) respectively. The TEM micrographs show average crystallite size ranges from 20-40 nm which is in close agreement with that obtained from XRD line broadening analysis i.e. 25.2 nm. Moreover, the separation of lattice fringes 2.85 Å (Fig. 5c) is in good agreement with the spacing d222 of the reference (Ref. Code: 00-

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006-0416). Diffraction spots corresponding to the (200), (211) and (222) lattice planes are identified here, as shown in the SAED pattern (inset fig.5c).

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To validate the crystalline phase observed from XRD data, FT-IR spectra of the

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samples were recorded. Fig. 6 shows the FTIR spectra of the undoped sample. The observed

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vibrational peaks for undoped sample are at 3457, 1630, 1357, 601, 565and 547 cm-1. The band around 3457 cm-1 corresponds to O-H stretching and the O-H-O bending vibrational

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peak at 1630 cm-1 indicates the presence of water. The absorption band around 1357 cm-1 is assigned to nitrate group and the appearance of peaks at 601, 565 and 557 cm-1 are attributed

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to the presence of In-O phonon vibration [5]. Similar observations are also found in doped

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samples.

Elemental compositions of the samples are recorded by energy-dispersive analysis of

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x-ray (EDAX) attached on SEM. Fig. 7(a) represents the EDAX spectrum obtained from the In2O3 sample. The spectrum shows the presence of carbon (C), oxygen (O), indium (In) in the

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sample. Similarly, Fig. 7(b) represents the EDAX spectrum obtained from the In2O3:Eu2+ (Eu2+ = 2 at. %) sample. The spectrum shows the presence of carbon (C), oxygen (O), indium (In) and europium (Eu) in the sample. The percentage of In and Eu present in the sample, calculated from EDAX, ZAF Quantification, are observed to be 97.6% and 2.4% respectively. This clearly indicates the concentration of Eu2+ introduced in the host In2O3 is in agreement with the data from EDAX observation. Similar observations are found in other samples also. It is to be noted that presence of C in the spectra, is due to carbon-coated tape used in recording the spectra. Similar report is also found in the reference [20]. Figure 8(a) shows the excitation spectra of undoped sample on monitering emission wavelength at 487 nm. Broad hump around 234 nm, 254 nm, 268 nm and 358 nm are

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observed, which are attributed to the charge transfer processes. Similar excitation spectra are also observed for doped samples, as it is shown in Fig. 8(b).While exciting the undoped

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sample at 234 nm, a broad emission at 440 nm (blue), 487nm (blue), and a weak emission at

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540 nm (green) were observed, Fig. 9. Similar emissions of indium oxide nanoparticles have

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been observed by Zhou el al. [16], Wu et al. [14] and Zhang et al. [7]. Generally, the emission spectra can be divided into two broad categories: the near-band-edge

(NBE)

emissions and deep- level (DL) emissions. The NBE emissions can be favored by the high

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crystal quality and quantum confinement effect, and the DL emissions can be enhanced by

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impurities or defects of the crystal [22-24]. It can be noted that PL emssions by nanocrystals in the visible range was mainly attributed to the presence of oxygen vacancies [25]. Hence

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the emission peaks of the present work can be referred to the deep level (DL) or trap state

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emissions due to oxygen vacancies [4,26,27]. That is, oxygen vacancies induce the formation of new energy levels in the band gap and as a result the emissions involving these trap states

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attribute to the radiative recombination of a photo-excited hole with an electron thereby emitting light at room temperature. In the present case, oxygen vacancies are expected to be

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formed while calcinating the sample in static atmosphere i.e. box type muffle furnace. During calcination the hydroxyl group in the indium hydroxide lattice must have been removed in the form of water. Further, some of the lattice oxygen may also released in the form of O2 gas, thus leaving oxygen vacancies [11]. Furthermore EPR spectra ( Fig.10) reveals that the nature of oxygen vacancy is singly ionized oxygen vacancies with g = 1.9985 because other oxygen vacancies are not paramagnetic [4,11,27,28].Thus singly ionized oxygen vacancy is responsible for PL emission of undoped indium oxide nanocrytal. To investigate further, PL emission spectra of Eu doped samples excited at 234 nm were measured at room temperature. The emission peak intensity of the host is found to increase after doping

europium but there is no distinct peak around 610 nm, i.e., red

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emission, which corresponds to well known emission of Eu3+ (Fig.9). This suggests that the europium in the host is Eu2+ and not Eu3+. Moreover,Fig. 11 shows that intensity of EPR

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signal (g = 2.0448) is found to increase for doped sample (Eu2+ = 4 at.%). This may be due to

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increase of oxygen vacancies in the doped sample resulting to the enhancement of host

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emission intensity. The enhancement of emission around 450-560 nm and almost negligible emission around 600-650 nm thus reaffirms that the doped europium ion is in +2 oxidation state, i.e., Eu2+. It was also suggested that Eu2+ replaces the In3+ in In2O3:Eu2+ lattice thereby

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creating singly ionized oxygen vacancies [21]. And this additional oxygen vacancies helps in

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enhancing the emission around 450-560 nm. Further this increase in blue-green PL emission intensity after doping may also be due to the 4f-5d transitions of Eu2+ [29,30].

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From the Fig. 9 it is clearly observed that the peak intensity for the doped samples

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increases up to Eu2+ = 4 at.% ;beyond that,it decreases as the concentration of doping increases. This can be attributed to luminescence quenching. Hence, the optimum dopant

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concentration is found to be 4 at.% in this study. At low concentration of Eu2+ ions within the indium oxide host, the ions are distributed randomly and the distances between Eu

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is

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larged. Therefore, the probability of energy transfer between Eu2+ ions is low. However, as the concentration increases, the distance between the Eu2+ ions are shortened and hence the probability of energy transfer between the Eu2+ ions increase. Consequently, concentration quenching of luminescence occurs i.e., some amount of excitation energy is dissipated nonradiatively which result in decrease of luminescence intensity [20,29]. Besides the concentration quenching discussed above, the decrease in emission intensity can also be explained due to decrease in crystallite size or size quenching. It is clearly observed from Table 1. that the crystallte sizes decrease on increasing the doping concentration. As a result, the surface areas of the nanocrystals increase on decreasing the crystallite size. And therefore, increase in surface area of the nanocrystals can also lead to significant increase in non-

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radiative decay and hence decrease in the emission intensity. Similar reports are also found in the references [20].

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Emission intensity (at 487 nm) as a function of Eu2+ concentration for the synthesized

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In2O3:Eu2+ nanocrystals is shown in the Fig.12. The figure also shows that the maximum

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emission intensity at 487 nm first increases and then found to decrease after 4 at.% of Eu2+ concentration.

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The Commission Internationale de l’e´ clairage (CIE, 1931) co-ordinate is calculated in order to evaluate the phosphors’ performance of the synthesized nanocrystals, as shown in

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Table 2. All the co-ordinates (x, y) in the Table 2 fall on blue-green region which are marked

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on CIE colour co-ordinate diagram Fig. 13.

4. CONCLUSION:

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In summary, the outcomes of the present implies that cubic undoped and europium doped indium oxide nanocrystals were synthesized by cost effective and easy production simple precipitation method. EPR analysis reveals that the doped europium is in +2 oxidation state. This causes the increase of singly ionized oxygen vacancy as the percentage of doping increases and hence PL enhancement of host blue-green emission. Increase in blue-green emission intensity after doping may also be attributed to the 4f-5d transitions of Eu2+. However, this emission intensity decreases after 4 at. % of Eu2+ which may be due to concentration quenching and size quenching. The blue-green photoluminescence emitted from these nanocrystals may find application in lighting such as in generation of white light.

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ACKNOWLEDGEMENTS:

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The authors thank and acknowledged Department of Chemistry, Manipur University,

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India for providing FT-IR and EPR (ESR) facility and SAIF, NEHU, Shillong, Meghalaya,

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India for providing TEM facility.

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Ree FIGURE CAPTIONS:

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Fig. 1: XRD patterns of the In2O3 and In2O3:Eu2+ (Eu2+ = 1, 2, 4, 5, 6 at.%) samples.

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Fig. 2: Average crystallite size of the In2O3 and In2O3:Eu2+ samples as a function of Eu2+ concentration.

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Fig. 3: Unit cell volumes of the In2O3 and In2O3:Eu2+ samples as a function of Eu2+ concentration.

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Fig. 4: (a) and (b) TEM images and (c) HRTEM image of the In2O3 showing

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interplanner spacing d222 (inset show its SAED pattern).

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Fig. 5: (a) and (b) TEM images and (c) HRTEM image of the In2O3:Eu2+ (Eu2+ = 2

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at.%) showing interplanner spacing d222( inset shows its SAED pattern). Fig. 6: FT-IR spectra of the In2O3 sample.

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Fig. 7: EDAX spectra of the (a) In2O3 and (b) In2O3:Eu2+ (Eu2+ = 2 at.%) samples. Fig. 8: PL excitation spectra of the (a) In2O3 and (b) In2O3:Eu2+ (Eu2+ = 2 at.%) monitored at emission wavelength 487 nm. Fig. 9: PL emission spectra of the In2O3 and In2O3:Eu2+ on monitoring excitation wavelength at 234 nm. Fig. 10: EPR spectra of the In2O3 sample. Fig. 11: EPR spectra of the In2O3:Eu2+ (Eu2+ = 4 at.%) sample. Fig. 12: PL emission intensity (at 487 nm) of the samples as a function of Eu2+ concentration.

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Fig. 13: CIE chromaticity diagram showing the region of colour co-ordinates of the

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samples.

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Intensity (counts)

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840 In2O3:Eu2+(6 at.%) 560 280 0 960 In2O3:Eu2+(5 at.%) 640 320 0 1170 In2O3:Eu2+(4 at.%) 780 390 0 1170 In2O3:Eu2+(2 at.%) 780 390 0 1500 In2O3:Eu2+(1 at.%) 1000 500 0 1440 In2O3 960 480 0 990 Ref.code(00-006-0416) (222) 660 (622) (440) (400) 330 (211) (431) (611) (444) 0

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Eu concentration (at.%) Fig.2

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c

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CE P

TE

D

MA

5 nm

Fig.4(a, b, c)

Page 20 of 34

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AC

20nm

CE P

TE

D

MA

b

NU

100nm

SC R

IP

T

a

Page 21 of 34

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SC R

IP

T

c

NU

0.285nm

AC

CE P

TE

D

MA

2nm

Fig.5(a, b, c)

Page 22 of 34

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T IP SC R

80 1630 cm -1 1357 cm

0

-1

1000

1500

2000

2500 -1

D

500

-1

CE P

TE

Wavenumber(cm )

AC

3457 cm

-1

NU

601 cm -1 565 cm

MA

20

-1

60

557 cm

Transmittance (counts)

100

40

Page 23 of 34

Fig.6

3000

3500

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Page 24 of 34

3000

IP

T

(a) X-ray counts

In

In

1000

In

NU

O CN

100

200

300

MA

In In

0 0

SC R

2000

400

500

600

700

800

TE

D

Energy(eV)

CE P

1000

(b)

In

In

AC

X-ray counts

800

In

600 400 200

O In

N C

In Eu

Eu Eu

0 0

100

200

300

400

500

Energy (eV) Fig. 7(a, b)

600

700

800

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50

(a)

45

T

40 em= 487nm

IP

35 25

234 nm

20

254 nm

15

268 nm

10

SC R

30

358 nm

NU

Intensity (counts)

Page 25 of 34

5

MA

0 220 240 260 280 300 320 340 360 380 400

300 250 200

TE

350

(b)

em nm

234 nm

AC

Intensity (counts)

400

CE P

450

D

Wavelength (nm)

150

254 nm 268 nm

358 nm

100 50 220 240 260 280 300 320 340 360 380 400

Wavelength (nm)

Fig. 8(a, b)

ACCEPTED MANUSCRIPT

45

487 nm

IP SC R

30 25

540 nm

20 15

NU

Relative Intensity

440 nm

10

MA

5 0 400

TE

D

500

CE P AC

(0 at.%) (1 at.%) (2 at.%) (4 at.%) (5 at.%) (6 at.%)

T

ex= 234 nm

40 35

Page 26 of 34

600

700

Wavelength (nm) Fig. 9

800

Page 27 of 34

SC R

Intensity (counts)

IP

T

ACCEPTED MANUSCRIPT

D

MA

NU

g = 1.9985

330

AC

CE P

TE

325

335

Magnetic field (mT) Fig.10

340

IP

Intensity (counts)

MA

NU

SC R

g = 2.0145

200

300

400

CE P

TE

D

Magnetic Field (mT)

AC

Page 28 of 34

T

ACCEPTED MANUSCRIPT

Fig.11

500

Page 29 of 34

ex= 234 nm

T

45

SC R

IP

40 35

NU

30 25 0

MA

Emission Intensity at 487nm

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1

2

3

2+

4

5

AC

CE P

TE

D

Eu concentration (at.%)

Fig.12

6

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Fig. 13

Page 30 of 34

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Page 31 of 34

TABLES CAPTIONS:

IP

T

Table 1. Average crystallite sizes and unit cell parameters of the In 2O3 and In2O3:Eu2+

SC R

along with that of the reference.

AC

CE P

TE

D

MA

NU

Table 2. Calculated CIE chromaticity co-ordinates of the In2O3 and In2O3:Eu2+ samples.

ACCEPTED MANUSCRIPT

Page 32 of 34

Sl.

Sample

Average Crystallite Size (nm)

SC R

No.

IP

T

Table 1

(Å)

6 7

NU

10

1028.42

10

1029.43

10

1029.67

10

1030.29

10

1033.34

10

1034.51

.094

26.0

MA

5

.118

36.5

D

4

In2O3:Eu2+ (1 at.%) In2O3:Eu2+ (2 at.%) In2O3:Eu2+ (4 at.%) In2O3:Eu2+ (5 at.%) In2O3:Eu2+ (6 at.%)

_

TE

3

CE P

2

Ref. Code: (00-0060416) In2O3

AC

1

Unit Cell Parameters Cell volume 3 (Å) 10 1035.82 a

.097

25.2 .098 21.3 .100 18.9 .120 19.0 .114

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Page 33 of 34

Table 2 Sl.

Sample

CIE Chromaticity

x 0.21

In2O3 (Eu2+ = 1

0.21

NU

2

In2O3

at.%) In2O3 (Eu2+ = 2 at.%) In2O3 (Eu2+ = 4

In2O3 (Eu2+ = 5

CE P

at.%)

In2O3 (Eu2+ = 6 at.%)

AC

6

TE

at.%) 5

0.22

0.27

Bluegreen

0.28

Bluegreen

0.28

Bluegreen

0.27

D

4

0.23

MA

3

y

SC R

1

Color

T

Coordinate

IP

No.

CIE

Bluegreen

0.20

0.27

Bluegreen

0.21

0.26

Bluegreen

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Page 34 of 34

Highlight:

T

IP

SC R

NU MA D TE



CE P

 

XRD and TEM study confirms the synthesis of cubic doped and europium doped nanocrystals. EPR study reveals the doped europium is in +2 oxidation state. Enhance PL emission intensity of host material due to increase in singly ionized oxygen vacancy and 4f-5d transitions of Eu2+. CIE co-ordinates suggests the blue-green colour of the samples.

AC

