Low temperature thermoluminescence behaviour of Y2O3 nanoparticles

Accepted Manuscript Low temperature thermoluminescence behaviour of Y2O3 nanoparticles S. Delice, M. Isik, N.M. Gasanly PII:

S1002-0721(18)30099-1

DOI:

10.1016/j.jre.2018.06.005

Reference:

JRE 261

To appear in:

Journal of Rare Earths

Received Date: 6 February 2018 Revised Date:

23 June 2018

Accepted Date: 26 June 2018

Please cite this article as: Delice S, Isik M, Gasanly NM, Low temperature thermoluminescence behaviour of Y2O3 nanoparticles, Journal of Rare Earths (2018), doi: https://doi.org/10.1016/ j.jre.2018.06.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Submitted to: Journal of Rare Earths

Low temperature thermoluminescence behaviour of Y2O3 nanoparticles

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S. Delicea,*, M. Isikb, N.M. Gasanlyc,d a

Department of Physics, Hitit University, 19040 Çorum, Turkey Department of Electrical and Electronics Engineering, Atilim University, 06836 Ankara, Turkey c Department of Physics, Middle East Technical University, 06800 Ankara, Turkey d Virtual International Scientific Research Centre, Baku State University, 1148 Baku, Azerbaijan

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Abstract

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Y2O3 nanoparticles were investigated using low temperature thermoluminescence (TL) experiments. TL glow curve recorded at constant heating rate of 0.4 K/s exhibits seven peaks at around 19, 62, 91, 115, 162, 196 and 215 K. Activation energies and characteristics of traps responsible for observed curves are revealed under the light of results of initial rise analyses and Tmax–Tstop experimental methods. Analyses of TL curves obtained at different stopping temperatures result in presence of one quasi-continuously distributed trap with

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activation energies increasing from 18 to 24 meV and six single trapping centers at 49, 117, 315, 409, 651 and 740 meV. Activation energies of all revealed centers were reported in the present paper. Structural characterization of Y2O3 nanoparticles is accomplished using X-ray

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diffraction and scanning electron microscopy measurements.

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Keywords: Y2O3; luminescence; thermoluminescence; defects; Tmax−Tstop.

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Corresponding author. Tel: +90 364 2271694; fax: +90 364 2277005. E-mail address: [email protected]

1.

Introduction

ACCEPTED MANUSCRIPT Luminescence properties of rare earth oxide materials have forced the researchers to deeply investigate the applications of these materials in a whole range of technology. The physical and optical characteristics of these materials provides them usability in X-ray imaging, bioimaging, lasers, cathode-ray tubes, display technology, lighting and sensing

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applications [1-5]. Among the rare earths, yttrium oxide (Y2O3) has been one of the most studied materials since it presents excellent chemical and physical stability, wide transparency range (0.2−8.0 µm), broad band gap (~5.6 eV), high refractive index (>1.9), good thermal

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conductivity, low phonon energy and high melting point [6-17]. As an electronic application, Y2O3 is employed in MOS transistor and takes role in the production of light emitting devices.

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Luminescence efficiency makes it significant material as a host doped with various rare earth elements. Eu3+ doped Y2O3 is a perfect red-emission phosphor and utilized in lamp and display applications [1,2,18]. Photoluminescence (PL) and thermoluminescence (TL) studies of Y2O3:Eu3+ nanophosphor were reported in Refs. [19,20]. The PL emission spectra showed

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several emissions in the range of 540 and 630 nm. Very low green emission and strong red emission were observed in the PL spectra at 540 and 612 nm, respectively [19]. TL measurements were carried out as a function of wavelength and related temperature. TL

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emission also had strong red emission (611 nm) at 417 K. TL curve exhibited six peaks at 393, 418.5, 445, 498, 561 and 606 K. The activation energies were found between 1.05 and

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1.40 eV [20]. PL studies on Er3+ doped Y2O3 was previously carried out at room temperature [21]. The spectra revealed sharp and strong emission at 1535 nm. Y2O3:Gd3+ phosphor was investigated by means of PL and TL measurements [20]. PL emission spectra showed peaks in the visible range of 400–650 nm. TL curve exhibited one peak centered at 95 °C. Activation energy of trapping state was found to be increasing from 0.67 to 0.73 eV as the UV excitation time was increased. Although there are studies on Y2O3 doped with different rare earth elements in literature,

ACCEPTED MANUSCRIPT the studies investigating the physical and chemical properties of pure Y2O3 nanoparticles are very rare. Determination of existence of defects in such materials is crucial to understand the luminescence mechanism of possible transition centers. For this purpose, in the present study low temperature (10-300 K) TL properties of Y2O3 nanoparticles have been explored for the

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first time. Shallow trapping levels existing in the forbidden band gap of Y2O3 were determined by Tmax−Tstop method suggested by McKeever [23]. The activation energies of

Experimental

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2.

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associated trap levels were reported in the present study.

Y2O3 nanoparticles used for TL investigations were supplied from Alfa Aesar. The structural characterization of samples were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. XRD experiments were carried out in the 15o ≤ 2θ ≤ 80o region using a Rigaku Miniflex diffractometer with Cu Kα radiation (λ =

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0.154049 nm) working at a scanning speed of 0.02 (º)/s. Surface morphology of Y2O3 nanoparticles was obtained using a JSM-6400 scanning electron microscope.

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In the TL experiments, Y2O3 was fixed to sample holder in the closed cycle helium gas cryostat (Advanced Research Systems, Model CSW 202) which is enable to change

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temperature between 10 and 300 K. Then, temperature of the environment was lowered to 10 K and the UV source (~215 nm) attached to the optical access port of the cryostat with quartz window was activated for 15 min to fill the trap levels. An expectation time (3 min) was waited for equilibrium after excitation. In order to observe TL spectrum, the temperature of the sample was increased to room temperature with a constant heating rate of 0.4 K/s by employing a temperature controller (Lakeshore Model 331). In order to separate the TL peaks exhibited in TL spectrum, the same process was repeated for different stopping temperature (Tstop) ranging from 10 to 170 K. Here, the sample was illuminated with UV source at the

ACCEPTED MANUSCRIPT stopping temperature and heating process was achieved between 10 and 300 K. During heating process emitted luminescence was collected by a lens, which focuses the light to the photomultiplier (PM) tube (Hamamatsu R928, spectral response: 185-900 nm). PM tube pulses due to emitted luminescence were converted into TTL pulses (0-5 V) by a fast

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amplifier/discriminator (Hamamatsu Photon Counting Unit C3866). Counter of a data acquisition module (National Instruments, NI 6211) counted the TTL pulses. The whole experimental setup were controlled by a software improved with LabView (National

Results and Discussion

3.1. Structural properties

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Instruments).

Fig. 1 shows the XRD pattern of Y2O3 nanoparticles in the 2θ range of 15º–80º. Existence of sharp diffraction peaks in the pattern is an indication of good crystallinity of the

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nanoparticles. The lattice parameters of the crystal structure were determined using software “DICVOL 04”. The analyses revealed the crystal structure of the Y2O3 nanoparticles as cubic structure with lattice parameter of a = 1.061 nm. Moreover, Miller indices of the peaks were

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also obtained from the analyses and are presented on the diffraction peaks. The obtained lattice parameter and Miller indices are well correlated with those given in JCPDS 86-1326

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(cubic structure with a = 1.059 nm). The crystallite size (D) of the nanoparticles was also estimated using XRD diffraction pattern under the light of well-known Debye-Scherrer’s Equation [24] D=

0.9λ β cos θ

(1)

where β is full-width-half maximum, λ = 0.154049 nm is wavelength of X-rays and θ is diffraction angle. The size of the nanoparticles was calculated between 25–35 nm using Eq. (1) for observed diffraction peaks.

ACCEPTED MANUSCRIPT Scanning electron microscopy (SEM) technique was used to obtain information about the surface morphology of Y2O3 nanoparticles. Figure 2 shows the SEM images of Y2O3 nanoparticles used as powder forms in measurements for two different resolutions. As can be seen from the figure, used samples are in the form of nanoparticles having particle size

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generally in the range of 50–80 nm. The statistic distribution of particle size plotted according to SEM image is given in the inset of Fig. 2(b). According to statistical distribution, most of the nanoparticles (~76%) have particle size between 40–160 nm.

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3.2. Thermoluminescence properties

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Emitted luminescence from the Y2O3 nanoparticles during thermally stimulating process was recorded in the below room temperature region (10−300 K) using constant heating rate of 0.4 K/s and observed TL spectrum of Y2O3 nanoparticles is shown in Fig. 3. As seen from the figure, there exist overlapping TL peaks in a wide range of temperature

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between 10 and 240 K. Although the TL process was achieved up to room temperature, there was no peak beyond 240 K. Therefore, the figure was presented in the temperature range exhibiting TL peaks. TL intensities of the peaks arising in the 10–150 K range were relatively

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low as compared with the other peaks. In order to better view, these peaks are presented in the inset of Fig. 3. The TL peaks were observed at temperatures (Tmax) around 19, 62, 91, 115 and

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196 K. Moreover, there are shoulders at both sides of the most intensive TL peak with Tmax values around 162 and 215 K. TL spectrum presented in Fig. 3 is composed of at least seven individual peaks which

corresponds to the number of local peak maximum temperatures. In the literature, there are some analyses methods to determine the activation energies of trapping centers. Analyses of many overlapped TL peaks situated in such a wide range of temperature cannot give reliable results. Among the many analyses techniques, initial rise method based on the analyses of the initial portion of TL spectrum is one of the powerful techniques for the purpose of

ACCEPTED MANUSCRIPT determination of activation energies. Since this method is applied on the initial portion of the glow curve, obtained activation energy by this way is related to firstly appearing peak. McKeever reported a well-known experimental method called Tmax−Tstop to determine the activation energies and characteristics of trapping centers [23]. Tmax−Tstop method is also very

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effective in understanding whether the sequentially overlapping TL peaks are individual peaks corresponding to a single energy levels or quasi-continuously distributed. In this method, the shallower trapping levels are cleaned and charge carriers are not allowed to

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occupy these levels, by performing TL experiments for higher stopping temperatures (Tstop) instead of initial temperature (T0 = 10 K). That is to say, environment of the sample is brought

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to Tstop and is kept at this temperature until equilibrium. Then, stimulation of charge carriers from valence band to conduction band with a light/radiation source is achieved. Following the decrease of temperature to initial value in dark, the sample is heated up to room temperature and TL emission from the occupied deeper levels is recorded simultaneously. McKeever

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investigated the behavior of Tmax−Tstop dependency of the TL peaks and suggested two different cases for overlapped peaks in consequence of this method. If the plot exhibits a “continuous line” with a slope of nearly 1.0, this points out the existence of a quasi-

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continuous distribution of TL peaks and if the plot has a “staircase” structure, this means that overlapped TL peaks are each responsible for well separated peaks which are associated with

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discrete trap levels [23]. In our study, we applied this experimental process to obtain the successive TL curves for different Tstop values ranging between 10 and 170 K. Fig. 4(a) illustrates the TL peaks observed for Tstop values ranging from 10 to 18 K. As seen from the figure, the TL intensity of the peak (labelled as peak A) decreases and Tmax shifts towards higher temperatures with increasing Tstop. The peak A disappears substantially at Tstop = 18 K. Increase of Tmax with Tstop is an indication that peak A cannot be denoted for a discrete, single energy level. Instead, it is quasi-continuously distributed in the forbidden band gap. Fig. 4(b)

ACCEPTED MANUSCRIPT shows the experimental TL curve (labelled as peaks B, C and D) detected for increasing Tstop values between 20 and 100 K. TL curve obtained for 10 K is represented in the figure as a reference curve. Since the employed Tstop except 90 and 100 K did not affect the TL peaks (labelled as peaks E, F and G) arising at higher temperature region remarkably, remaining

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parts of the curves are given in next figure. As can be seen from the figure, there is a shoulder at the initial tail of the peak B. This arises because of the existence of energy levels within the quasi-continuously distributed traps associated with peak A. Peak B is available for Tstop

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values of 20, 30 and 40 K and vanishes as the Tstop = 50 K is used. Peak C survives at Tstop values between 20 and 60 K and completely depletes at 70 K. Peak D appears at Tstop values

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between 20 and 90 K and it disappears at 100 K. Fig. 4(c) indicates the Tstop behaviors of remaining TL peaks (peaks E, F and G) of the initially observed TL spectrum with T0 = 10 K. The TL intensity of peak E gradually decreases with increasing Tstop. This peak exists at Tstop values ranging from 90 to 120 K and it dies as Tstop = 130 K is experienced. Peak F having the

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highest luminescence intensity arises for Tstop values increasing from 90 to 150 K and depletes at 160 K. After this temperature the TL peak G related to the deepest trapping level is separated entirely and it almost runs out at Tstop = 170 K.

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Fig. 5 shows the Tmax−Tstop graph plotted with different Tstop increasing from 10 to 170 K. As can be seen from the figure, Tmax values of the observed TL peaks in the spectra does

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not change distinctively and the plot creates a staircase structure with increasing Tstop except Tstop values between 10 and 18 K. This indicates that the overlapped TL peaks B, C, D, E, F and G are well separated peaks corresponding to single different energy levels. The Tmax values of peak A as a function of Tstop between 10 and 16 K are represented in a large scale in Fig. 6. This plot gives a straight line yielding a slope of nearly 1.0. This strong evidence indicates the presence of quasi-continuous distribution of trapping levels related to peak A.

ACCEPTED MANUSCRIPT Activation energies (Et) of revealed trap levels in Y2O3 crystals were determined using the initial rise method for TL curves obtained with different Tstop values. Since this method is independent from order of kinetics of trap levels and only based on the analysis of initial tail of the TL peak, it is very applicable to TL curves composed of overlapping TL

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peaks [25]. Under the light of theoretical background of initial rise method, the initial tail of the TL peak arises as proportional to exp(−Et/kT). Therefore, one can plot a logarithmic TL intensity as a function of 1/T. The slope of this plot yields −Et / k. Activation energy found by

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initial rise method is responsible for the shallowest trap level correlated to TL peak appearing at the lowest temperature region in each TL spectrum recorded for gradually increased Tstop.

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In the present study, we applied this successful method to all TL curves detected with various Tstop values ranging from 10 to 170 K. Fig. 7 presents the related plots for initial rise method application for TL peaks A, B, C, D, E, F and G separated completely at Tstop = 10, 30, 50, 70, 100, 130 and 160 K, respectively. Et−Tstop dependencies are plotted as seen in the inset of Fig.

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6 and in Fig. 8. Activation energy values found for TL peak A are shown as a function of increasing Tstop in the inset of Fig. 6. As seen from the inset, activation energy increases from 18 to 24 meV as the Tstop is increased from 10 to 16 K. This result displays that the trap level

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related to peak A exhibits the properties of a quasi-continuous distribution [26,27]. However, Et vs Tstop graph achieved for activation energy values corresponding to Tstop between 30 and

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170 K exhibits a staircase structure (see Fig. 8). This supported the fact that the overlapped TL peaks B, C, D, E, F and G are all individual TL peaks. Each flat region in the figure corresponds to single trap levels related to these peaks. Activation energies of revealed trap levels were found to be 49, 117, 315, 409, 651 and 740 meV. 4.

Conclusions Shallow

trapping

centers

in

Y2O3

nanoparticles

were

investigated

using

thermoluminescence experiments in the below room temperature region. TL spectrum

ACCEPTED MANUSCRIPT obtained at constant heating rate of 0.4 K/s presents overlapped seven peaks around 19, 62, 91, 115, 162, 196 and 215 K. Tmax−Tstop experimental method was used to determine the number of peaks consisting in whole TL curve and characteristics of trapping centers associated with these peaks. Initial rise method analyses and experimental data point out that

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one of the seven trapping center is quasi-continuously distributed within the forbidden gap whereas other six peaks are related to six discrete single trapping centers. Activation energies of distributed trap centers were found as increasing from 18 to 24 meV while single trapping

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centers were located at 49, 117, 315, 409, 651 and 740 meV.

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Figure Captions

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Fig. 1. XRD diffraction pattern of Y2O3 nanoparticles.

Fig. 2. SEM images of Y2O3 nanoparticles. Inset of (b) indicates the statistical distribution of nanoparticle size.

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Fig. 3. Thermoluminescence glow curve of Y2O3 nanoparticles.

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Fig. 4. Experimental TL curves observed for different Tstop temperatures for (a) peak A, (b) peaks B, C and D, (c) peaks E, F and G.

Fig. 5. Tmax−Tstop dependency for Y2O3 nanoparticles.

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Fig. 6. Tmax−Tstop plot for peak A and its linear fit (solid line). Inset shows the Et−Tstop plot for peak A.

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Fig. 7. Initial rise method plots for TL curves associated with revealed trapping centers.

Fig. 8. Et−Tstop plot for peaks B, C, D, E, F and G. Solid line was drawn to show staircase behavior.

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Experimental TL curves observed for different Tstop temperatures for peaks E, F and G

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Graphical Abstract