Band gap and trapping parameters of color tunable Yb3+/Er3+

codoped Y2O3 upconversion phosphor synthesized by combustionroute

S. Som • M. Chowdhury • S. K. Sharma

Received: 25 April 2013 / Accepted: 23 September 2013 / Published online: 10 October 2013

� Springer Science+Business Media New York 2013

Abstract This paper reports the structural, optical and

luminescence properties of Yb3?/Er3? codoped Y2O3

phosphor synthesized by combustion method. The prepared

phosphor was characterized by X-ray diffraction (XRD).

XRD studies confirm the body-centered cubic structure of

the phosphor. The optical properties such as diffuse

reflectance (DR), photoluminescence and thermolumines-

cence were studied. DR spectra were used to determine the

bandgap of the phosphor. Mechanism of upconversion by

two-photon and energy transfer processes are interpreted

and explained. The color coordinates were measured and

the color tunability was analyzed as a function of the

980 nm excitation source power. Different trapping

parameters associated with the glow peak were calculated

by various glow curve methods.

Introduction

Rare earth doped upconversion materials have been attrac-

ted intensive research attention from the past decade for

their excellent performance in the field of colorful display,

lighting source, optical communications, fluorescent labels,

biomedical imaging and upconversion lasers [1]. Recently,

a lot of research work is going on to achieve new upcon-

version materials with trivalent erbium ion (Er3?) due to

their potential applications in upconversion and near infra-

red lasers. In particular, color tunable lasers [2] have played

an important role in advancement of fundamental physics

and science. Among basic fields that employ tunable lasers

are astronomy, atomic physics, Bose–Einstein condensa-

tion, laser cooling and atomic/molecular spectroscopy. With

the advancements in nanomaterials synthesis, several com-

plementary methods have been developed to fine-tune the

upconversion emission in a broad range of color output

under single wavelength excitation (980 nm) [3]. These

methods mainly include three categories (i) lanthanide

dopant/codopant combination and their doping level

(ii) morphology and size control and (iii) input pump power.

The main interests of Er3? ion lay in its electronic energy

level scheme and long excited state lifetimes which allow for

many radiative transitions to occur and increased population

of electrons in meta-stable states by the absorption of pho-

tons in the near-infrared (NIR) region. It makes Er3? as a

suitable candidate to convert infrared light to visible effi-

ciently, which is called upconversion. Due to these unique

properties, Er3? doped materials are widely used in eye-safe

lasers, laser medical treatments and full color solid-state

displays [4, 5]. Since the absorption cross section of Er3? is

small [6] and concentration quenching happens to be at high

doping, Yb3? ion has been used as a very efficient sensitizer

[7] due to its large absorption cross section at 980 nm exci-

tation resulting an efficient energy transfer to Er3? ion. As a

result, the upconversion emission of Yb3?/Er3? codoped in

different host lattices (especially in yttrium oxide) has been

investigated extensively [8–10]. For efficient NIR to visible

frequency upconversion the host plays an important role.

Host matrices with less phonon energies are preferable for

this purpose as the energy loss due to non-radiative transi-

tions is reduced. Over the past years, yttrium oxide (Y2O3)

has been one of the best host materials owing to its good

chemical durability, thermal stability and broad optical

spectra. Its relatively low phonon cut-off energy reduces the

multiphonon relaxation process and leads to higher emission

efficiency [1]. It is one of the best host materials for Er3?

S. Som � M. Chowdhury � S. K. Sharma (&)

Department of Applied Physics, Indian School of Mines,

Dhanbad 826004, India

e-mail: sksharma.ism@gmail.com

123

J Mater Sci (2014) 49:858–867

DOI 10.1007/s10853-013-7769-8

because Er2O3 and Y2O3 have the same crystal structures

with very similar lattice constants, and Y3? and Er3? have

nearly the same ionic radii. Combustion synthesis is an

effective, low-cost method for production of Yb3?/Er3?

codoped Y2O3 upconversion phosphor. The extensive

research carried out in last 5 years emphasize the advantage

of materials development by combustion synthesis such as

energy saving and environmental protection [11]. Though,

combustion synthesis is easy method to prepare nanomate-

rials, a lot of attention is required especially in terms of

repeatability and the variation related to position of atoms/

ions in a sample.

Although Yb3?/Er3? doped Y2O3 is one of the well-

known luminescent materials, there is a comparative lack

of information about the structural and defect properties in

these materials. The luminescence properties of the mate-

rials are markedly influenced by the structure and the

presence of intrinsic and extrinsic defects/traps [12]. To

know the information about the luminescence mechanism

in this well-known upconversion material, the knowledge

of defect/trap centers, their distribution in the band struc-

ture and different trapping parameters associated with the

defects/traps are very important. Diffuse reflectance (DR)

and thermoluminescence (TL) techniques play a vital role

in getting the information about the structure, bandgap,

distribution of defects/traps within the forbidden gap

present in a material and the associated trapping parame-

ters. Keeping this in view, DR and the TL characterization

of Yb3?/Er3? codoped Y2O3 phosphor synthesized by

combustion route along with the color tunable upconver-

sion studies were carried out first time. An attempt was

made to calculate the structural parameters, bandgap and

different trapping parameters associated with the defects/

traps.

Experimental

Yb/Er codoped Y2O3 phosphor was prepared by the well-

known combustion synthesis method using erbium oxide

(Er2O3), ytterbium oxide (Yb2O3), Y2O3, nitric acid

(HNO3) and urea (CO(NH2)2) as the starting raw mate-

rials. The stock solutions of Y(NO3)3, Er(NO3)3 and

Yb(NO3)3 were prepared by dissolving Y2O3, Er2O3 and

Yb2O3 in HNO3 and diluting with deionized water. The

stock solutions of Y(NO3)3, Er(NO3)3 and Yb(NO3)3 were

mixed according to the formula (Y0.98Er0.01Yb0.01)2O3. A

suitable amount of urea was added to the mixture of

corresponding nitrate solution keeping urea to metal

nitrate molar ratio as 2.5 [13]. The corresponding mixture

was then dissolved properly to achieve a uniform solution

and dried by heating at 80 �C using magnetic stirrer.

Finally, the solid residue was transferred to silica crucible

and heated at 600 �C in a furnace for an hour. The syn-

thesis reaction was [13]:

2� 2x� 2yð ÞY NO3ð Þ3þ2xEr NO3ð Þ3þ2yYb NO3ð Þ3þ 5 NH2ð Þ2CO ! Y1�x�yErxYby

� �2O3 þ 5CO2

þ 8N2 þ 10H2O x ¼ y ¼ 0:01ð Þ:

X-ray diffractogram of the prepared phosphor was

recorded in a wide range of Bragg angle 2h using a Bruker

D8 advanced X-ray diffraction (XRD) measuring

instrument with Cu target radiation (k = 0.154056 nm).

The DR spectra were recorded using Perkin-Elmer

Lambda 35, UV–Vis spectrophotometer in the

wavelength range 200–1100 nm. The photoluminescence

upconversion spectra of the phosphor were recorded by

exciting the phosphor samples with CW 980 nm diode

laser through a monochromator attached with a photo

multiplier tube. TL studies were carried out using Harshaw

TL analyzer at Inter University Accelerator Center

(IUAC), New Delhi.

Results and discussion

Structural analysis

XRD

XRD patterns of the Yb/Er codoped Y2O3 phosphor were

recorded to investigate the structure of the prepared com-

pound and compared with the JCPDS file No. 83-0927, as

shown in Fig. 1a. The sharp and single diffraction peaks of

the XRD pattern confirm the formation of single phase

compound, which is attributed to the high in situ temper-

ature generated during the combustion reaction. The XRD

peaks were identified and indexed according to the JCPDS

database [14]. The (hkl) values are labeled on the peaks.

This implies that the codoped phosphor is of body-centered

cubic structure with space group Ia-3 and point-group

symmetry m-3. The peak at 29.5� was observed as the

strongest peak corresponding to the plane (2 2 2) and no

peaks were observed due to dopant.

Structural parameters

The structural parameters such as Miller indices, lattice

constants, crystallite size, density, dislocation density, mi-

crostrain and inter-planar spacing were determined from

XRD data. The Miller indices (hkl) were calculated by an

analytical method described elsewhere [15]. The calculated

(hkl) values were matched with the JCPDS database. The

J Mater Sci (2014) 49:858–867 859

123

lattice constants were calculated using the following for-

mula [15]:

sin2 h ¼ k2

4a2ðh2 þ k2 þ l2Þ; ð1Þ

where the terms have their usual meanings.

For estimating the crystallite size (D), Scherrer formula

[15] was used:

D ¼ 0:9kb cos h

; ð2Þ

where k is the wavelength of the X-ray radiation, b is the

full-width at half-maximum (FWHM) and h is the angle of

diffraction. The estimated crystallite size of Yb3?/Er3?

codoped Y2O3 was found to be in the range of 10–20 nm.

In the present case, broad XRD peaks were obtained.

Broadening of XRD lines was associated with small

crystallite size of coherently diffracting crystallites or

strains present within the sample or both. Small crystallites

had relatively few lattice planes that contribute to the

broadening of the diffraction lines. Broadening of the peak

might also occur due to microstraining of the crystal

structure arising from defects like dislocation and twinning

etc. These defects were considered to be associated with

the chemically synthesized materials because they grew

spontaneously during chemical reaction. As a result, the

chemical ligands got almost negligible time to be

associated with an energetically favorable site. In some

cases this might be arising due to insufficient energy that

was needed for an atom to move a proper site in forming

Fig. 1 a XRD. b Hall–Williamson plot. c TEM image for the Yb3?/Er3? codoped phosphor (Color figure online)

860 J Mater Sci (2014) 49:858–867

123

the crystallite. The strain and the crystallite size were also

calculated from the FWHMs of the diffraction peaks. The

FWHMs (b) can be expressed as a linear combination of

the contributions from the strain (e) and crystallite size (D)

through the Hall and Williamson relation [15]

b cos hk¼ 1

Dþ e sin h

k: ð3Þ

Figure 1b shows the plot of bcosh/k versus sinh/k for the

codoped Y2O3, which is almost a straight line. The

reciprocal of intercept on the bcosh/k axis gave the

average crystallite size as 18 nm. It is in good agreement

with that calculated by Scherrer formula. The slope of this

equation e implies the microstrain (e) present in the samples.

The density (Dx), dislocation density (d) and microstrain (e)were calculated using the following relations [14, 15]:

Dx ¼16M

Na3; ð4Þ

d ¼ 1

D2; ð5Þ

e ¼ b cos h4

; ð6Þ

where M is the molecular mass and N is Avogadro’s

number. The inter-planar spacing was calculated using the

following formula [15]:

d ¼ affiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffih2 þ k2 þ l2p : ð7Þ

All these calculated structural parameters are summarized

in Table 1.

TEM studies

Figure 1c depicts the TEM micrograph for the Yb/Er

codoped Y2O3 phosphor. The corresponding selected area

electron diffraction (SAED) pattern and frequency plot of

the size distribution are shown as the inset of Fig. 1c. The

phosphor shows agglomerated particles with particle size

*17 nm. The SAED pattern of the phosphor exhibits the

diffraction spots indicating a high degree of crystallinity.

The frequency plot of the size distribution was obtained by

measuring the size of a large number of particles in the

phosphor. Size distribution of the particles was found to

obey a log normal nature [16]

PðdÞ ¼ 1

drffiffiffiffiffiffi2pp exp

� ln2ðd=dÞ2r2

� �ð8Þ

Here d and r are related to the average size and the size

distribution of the particles. By fitting the frequency plot

using the above equation (solid blue line in Fig. 1c), the

average particle size of the codoped phosphor was esti-

mated 17 nm with a wide size distribution of 0.20.

Optical properties

DR

The DR spectra of the codoped Y2O3 powder phosphor were

measured against a white reference standard Spectralon. In

DR spectra, a sharp band at 209 nm was observed for the

prepared sample, as shown in Fig. 2. This corresponds to the

fact that light having this particular wavelength was absor-

bed. The band at 209 nm was due to the bandgap of the

codoped Y2O3 phosphor. Other weak bands beyond 300 nm

were also observed in the prepared sample. These bands were

due to meta-stable energy states formed between the valence

band and the conduction band by the Er3? ions.

Fig. 2 DR spectra of Yb3?/Er3? codoped phosphor (inset Bandgap

calculation)

Table 1 Structural parameters of Yb3?/Er3? codoped phosphor

Lattice constants a (A) Crystallite size D (nm) Density (Dx)

(gm/cm-3)

Dislocation density (d)

(91015 m-2)

Microstrain e (910-3)

JCPDS Calculated Debye–Scherrer Hall–Williamson’s Calculated Hall–Williamson’s

10.60 10.61 10–20 18 5.02177 3.09 2.22 5.13

J Mater Sci (2014) 49:858–867 861

123

Bandgap

The K–M theory [17] was used to calculate the bandgap of

the codoped Y2O3 phosphor using a DR spectrum. In a DR

spectrum, the ratio of the light scattered from a thick layer

of sample and an ideal non-absorbing reference sample is

measured as a function of the wavelength k, R? = Rsample/

Rreference [17]. The relation between the DR of the sample

(R?), absorption coefficient (K) and scattering coefficient

(S) is given by the K–M function

F R1ð Þ ¼ 1� R1ð Þ2

2R1¼ K

S: ð9Þ

The bandgap Eg and the linear absorption coefficient a of a

material are related through the well-known Tauc relation:

ahm ¼ C1 hm� Eg

� �n=2; ð10Þ

where m is the photon energy and C1 is a proportionality

constant. When the material scatters in perfectly diffuse

manner (or when it is illuminated at 60� incidence), the

K–M absorption coefficient K becomes equal to 2a(K = 2a). Considering the scattering coefficient S as

constant with respect to wavelength, and using Eqs. (8)

and (9), the following expression can be written as:

F R1ð Þhm ¼ C2ðhm� EgÞn: ð11Þ

The value of n is 1 for direct allowed transitions, 2 for

non-metallic materials, 3 for direct forbidden transitions,

4 for indirect allowed transition and 6 for indirect for-

bidden transition. Now, among the plot of [F(R?)hm]2,

[F(R?)hm], [F(R?)hm]2/3, [F(R?)hm]1/2, [F(R?)hm]1/3 as a

function of photon energy hm, the best fitting was

obtained with n = 1 in Eq. (10), i.e., [F(R?)hm]2 as a

function of hm indicating that the band transitions

occurred are direct in nature, as suggested by Tauc et al.

[18].

From the plot of [F(R?)hm]2 versus hm, the value of Eg is

obtained by extrapolating the linear fitted regions to

[F(R?)hm]2 = 0. The inset curve of Fig. 2 exhibits non-

linear and linear portions. The nonlinear portion corre-

sponds to a residual absorption involving impurity states

and the linear portion characterizes the fundamental

absorption. The bandgap calculated from the DR spectra

using the K–M function F(R?) was found to be 5.8 eV for

the prepared sample.

Upconversion spectra

Figure 3a shows upconversion emission spectra of Y2O3:

Yb3?, Er3? phosphor excited by a laser diode CW,

980 nm. It can be seen that there are three typical emission

bands around green and red region [19]. Each band is

composed of numerous narrow emission peaks. The sharp

peaks in the green region of 520–541 and 545–565 nm are

assigned to the transitions 2H11/2 ? 4I15/2 and 4S3/2 ? 4I15/2

of Er3? ions, respectively, while the peaks in the red

region of 650–685 nm correspond to the 4F9/2 ? 4I15/2

transition.

Upconversion mechanism

To understand the upconversion mechanism, the emission

intensity (I) of the band was recorded as a function of laser

pump power (P). Intensity of the upconversion bands fol-

lows the relation Iemission / Pn where n denotes the number

of incident photons involved in the emission of an up-

converted photon [19]. Inset of Fig. 3a shows the power

dependence for different bands. Slope of the straight line of

the log–log plot of I versus P gives the value of n. The

slope (n) was found as 1.94 for the red transition and

1.95 for the green transition. This clearly indicates the

involvement of two photons in upconversion process.

The excited states for upconversion can be populated by

several well-known mechanisms: (1) excited-state absorp-

tion (ESA), (2) energy transfer (ET) and (3) photon ava-

lanche (PA). PA was ruled out as a possible mechanism for

upconversion because no inflection point was observed in the

power dependence study [19]. Since Yb3? ions have much

larger absorption cross section compared to Er3? ions at

980 nm, the ET process dominates. Figure 3b shows the

energy level diagram of Er3? and Yb3? ions as well as the

probable upconversion mechanisms accounting for the green

and red emissions under 980 nm excitation. A 980 nm

photon excites an Yb3? ion from the ground state 2F7/2 to the

excited state 2F5/2, which may transfer the energy to a nearby

Er3? ion, promoting the Er3? ion from 4I15/2 to the 4I11/2 state,

and if the latter is already populated, the Er3? ion may transit

from the 4I11/2 to the 4F7/2 states. Subsequent nonradiative

relaxations could populate the 2H11/2 and 4S3/2 states that are

the emitting levels for the green luminescence.

4I15/2 ? 4I11/24I11/2 ? 4F7/24F7/2 ? 2H11/2/4S3/22H11/2/4S3/2 ? 4I15/2

The 4I11/2 states of Er3? ions could be depopulated by an

alternative relaxation route to 4I13/2 states, which may be

further excited to the red emitting levels 4F9/2, and the

corresponding transition to the ground state 4I15/2 produces

red emission.

4I15/2 ? 4I11/24I11/2 ? 4I13/24I13/2 ? 4F9/24F9/2 ? 4I15/2

862 J Mater Sci (2014) 49:858–867

123

Another way is also possible for red emission. The 4I11/2

states of Er3?ions could be depopulated 4F7/2 states, which

may be further excited to levels 4F7/2. Subsequent nonra-

diative relaxations could populate the red emitting levels4F9/2 which further emit red light.

4I15/2 ? 4I11/24I11/2 ? 4F7/24F7/2 ? 2H11/2/4S3/22H11/2/4S3/2 ? 4F9/24F9/2 ? 4I15/2

Photometric characterization

To measure the color of the visible emission, color coor-

dinates (x, y) were calculated by the spectrophotometric

method using the spectral energy distribution of the chro-

maticity diagram [20]. In addition, using the color coor-

dinates calculated above, the correlated color temperatures

(CCTs) of the mixed upconversion fluorescence corre-

sponding to different excitation powers were given by the

McCamy empirical formula [21]

CCT ¼ �473n3 þ 3601n2 � 6861nþ 5514:31; ð12Þ

where n = (x - xe)/(y - ye) is the inverse slop line, and the

CCT epicenter is xe = 0.3320 and ye = 0.1858 [17]. The

CIE (Commission Internationale de l’Eclairage) parameters

such as color coordinate, color correlated temperature

(CCT) and luminescence efficacy of radiation (LER) were

calculated to know the photometric characteristics of the

prepared phosphor. The CIE parameters under different

excitation powers are given in Table 2 and the color

Fig. 3 Upconversion spectra of Yb3?/Er3? codoped phosphor

J Mater Sci (2014) 49:858–867 863

123

coordinates are marked in Fig. 3c. A wide range of color

from red to yellow was achieved on varying the laser power.

The power dependence of both x and y coordinates is

plotted in Fig. 3d. The x and y CIE coordinates can be

fitted with a cubic function of the form x, f(x) = A ?

Bx ? Cx2 ? Dx3. The power dependence shows that the

x-coordinate decreases and the y-coordinate increases with

power [17]. The color tuning range of the phosphor is

shown in Fig. 3d where it can be seen that the emission

color can simply be tuned from 575 to 594 nm by

changing the excitation power from 537 to 1410 mW.

With the increase in pump power, the intensity of red and

green colors is greatly enhanced (Fig. 3a). However, the

integrated intensity ratio of red to green color decreases

significantly from 28.3 to 13.5 (Fig. 4). Figure 4 indi-

cates the increment of green color intensity at the cost

of decrement of red color intensity and so the tunability

of color is achieved from red to green with pump

power [2].

TL

To characterize the trap levels, the codoped Y2O3 phosphor

was irradiated with different c-doses and the TL glow

curves of the irradiated phosphor were recorded at different

heating rates. Thus, the effects of c-dose and heating rate

on the TL glow curves along with the determination of

trapping parameters were studied in detail.

Effect of c-dose TL glow curve of Y2O3:Yb3?, Er3?

phosphor was recorded after the c-ray irradiation with

different c-doses ranging from 1 to 5 kGy to check the

dose dependence effect on the peak position. The TL glow

curve of different c-doses for the codoped phosphor at a

constant heating rate of 5 �C s-1 is shown in Fig. 5. The

glow curve of the prepared phosphor exhibits single broad

glow peak around 320 �C for different c-doses. The shape

of the glow peak did not alter significantly with an increase

in c-ray dose. The only effect observed is the shifting of

glow peak towards lower temperature region along with an

increase in intensity of glow peak with dose.

The variation in the intensity of glow peak with irradiation

dose is shown as an inset in Fig. 5. The peak height increases

linearly with the increase of irradiation dose. The intensity of

the glow peak also increases with increasing c-doses sug-

gesting that an increasing number of traps responsible for

these glow peak were getting filled with an increase of irra-

diation dose and these traps release the charge carriers on

thermal stimulation to finally recombine with their coun-

terparts, thus giving rise to different glow peaks [22].

Effect of heating rate Figure 6 shows the TL glow curves

of the prepared Y2O3 phosphor recorded at different heat-

ing rates (3, 5 and 7 �C s-1) for the c-dose of 5 kGy. The

shift in peak temperature of the glow peak was observed

around 20 �C with heating rates and the peak shifts towards

higher temperature.

Table 2 CIE parameters of Yb3?/Er3? codoped phosphor

Power (mW) Emission color (nm) x y CCT LER

537 594 0.5917 0.4001 3199 75

618 592 0.5777 0.4145 2629 81

694 589 0.5697 0.4218 2099 84

860 585 0.5429 0.4482 1810 95

1030 580 0.5066 0.4815 1758 113

1410 575 0.4716 0.5113 1710 133

Fig. 4 Plot of pump power versus red to green ratio Fig. 5 TL glow curves of Yb3?/Er3? codoped phosphor

864 J Mater Sci (2014) 49:858–867

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Trapping parameters

The trap levels formed after c-irradiation were character-

ized by trapping parameters such as order of kinetics, trap

depth and frequency factor [22]. TL glow curves were used

to calculate these trapping parameters. The mechanism of

recombination of detrapped charge carriers with their

counterparts is known as the order of kinetics. The loss of

dosimetry information stored in the materials after irradi-

ation is strongly dependent on the position of trapping

levels within the band gap which is known as trap depth or

activation energy (E). The frequency factor (s) represents

the product of the number of times an electron hits the wall

and the wall reflection coefficient, treating the trap as a

potential well [22]. A reliable dosimetry study of an up-

conversion phosphor is based on its trapping parameters. In

the present paper, the trapping parameters associated with

the trap levels of upconversion phosphor were calculated

by different methods.

Computerized glow curve deconvolution (CGCD)

The recorded glow curves of the prepared phosphor

exposed to 5 kGy c-irradiation dose at different heating

rates 3, 5 and 7 �C s-1 seem to be composite in nature as

they exhibit a broad peak. So, CGCD method was used to

deconvolute and calculate the trapping parameters. Here in

the present case, the glow curves of maximum c-dose

irradiated phosphors were deconvoluted by CGCD method

using TLanal computer program [23]. This computer pro-

gram is based on Halperin and Braner equations which

describe the flow of the charges between the various energy

levels during a trap emptying by thermal heating. Chung

et al. [23] derived the equation given below for TL inten-

sity by applying some physical assumptions to Halperin

and Braner equations. Assuming the free carrier concen-

tration in the conduction band (nc) to be very small com-

pared to trapped carrier concentration (n) and also the rate

of change of nc to be much smaller than the rate of change

of n, the TL intensity can be expressed as:

IðtÞ ¼ �adn

dt¼ a

n2p

nþ RðN � nÞ ; ð13Þ

where R is the ratio of the retrap and recombination coef-

ficient, N is the concentration of the available electron traps

of depth E below the conduction band and a is a scaling

factor of the detecting system. Here, p = s exp (-E/kT) is

defined as the probability per unit time of the release of an

electron from the trap and s is the frequency factor for the

escape from the traps to the conduction band. In the case of

the linear temperature rise, the above equation is called the

general one trap (GOT) equation. Although this method is

based on the GOT model, there is no restriction in the

temperature profile; this program can be used in the case

where the temperature changes linearly as well as arbi-

trarily. The software works for the deconvolution of the TL

glow curves having mixed order, first-order, second-order

and general order kinetics. It was shown that this method/

software is efficient and fast in generating TL glow peaks

and can be adopted into a numerical curve fitting [23].

The trapping parameters of trap levels were then

determined for each deconvoluted peak by this program.

The theoretically generated glow curves were fitted with

the experimental glow curves and the quality of fitting was

checked by calculating the figure of merit (FOM) for each

fitting defined by

FOM ¼P

TLExp � TLThePTLThe

ð14Þ

Fig. 6 TL glow curves at different heating rates

Fig. 7 Deconvolution of TL glow curves

J Mater Sci (2014) 49:858–867 865

123

Here TLExp and TLThe represent the TL intensity of

experimental and theoretical glow curves, respectively.

The summation extends over all the available experimental

data points. Quality of fitting and choice of appropriate

number of peaks were refined by repeating the process of

fitting to get the minimum FOM with minimum number of

possible peaks. The fits were considered adequate when the

FOM values were observed below 5 %, with most actually

being below 2 %. In the present studies, FOM was found

around 1 % which confirms a very good agreement

between theoretically generated and experimentally recor-

ded glow curves [23]. The fitted TL glow curves for ion

irradiated phosphors are shown in Fig. 7.

The order of kinetics for each deconvoluted peaks was

found as second order (b = 2). Other trapping parameters

namely trap depth (E) and frequency factor (s) for c-irra-

diated phosphor are summarized in Table 3.

Glow curve shape methods

The various methods based on the shape of the glow curve

were used to verify the above calculated trapping param-

eters of the deconvoluted peaks.

Order of kinetics (b)

According to Chen method, the following shape parameters

were calculated to get the order of kinetics [24]:

Total half intensity width (x = T2 - T1)

High temperature half width (d = T2 - Tm) and

Low temperature half width (s = Tm - T1).

Here Tm is the peak temperature, T1 and T2 are the tem-

peratures on either side of Tm corresponding to half peak

intensity. Chen introduced a symmetry factor (lg) to differ-

entiate between first- and second-order TL glow peaks:

lg = d/x = (T2 - Tm)/(T2 - T1)

lg = 0.42 for first-order kinetics

lg = 0.52 for second-order kinetics

In the present case, the value of lg for the deconvoluted

glow peaks for c-irradiated Y2O3: Yb3?/Er3? phosphors

was found to be around 0.52 indicates the existence of

second-order kinetics. Trapping parameters calculation was

done for both the deconvoluted peaks only.

Trap depth (E)

General formulae [24] for calculating trap depth (E) by

various glow curve shape methods are given by:

Ec ¼ ccðkT2

m

cÞ � bcð2kTmÞ Chenð Þ ð15Þ

Ec ¼ cckTmT1

cGrossweinerð Þ ð16Þ

Ec ¼ cckT2

m

cLuschikð Þ; ð17Þ

where c is s, d or x. Thus, trap depth was calculated by

averaging the Es, Ed and Ex values. The constants cc and bc for

the three methods for second-order kinetics are given in [24].

Frequency factor (s)

Frequency factor (s) [24] was calculated by Chen and

Winer equation after getting the trap depth (E) and order of

kinetics (b):

Table 3 Trapping parameters by CGCD method

b Peak 1 Peak 2 FOM

(%)E (eV) s (s-1) E (eV) s (s-1)

3 1.05 6.08 9 108 1.18 1.19 9 109 1.08

5 1.07 8.16 9 108 1.21 1.85 9 109 1.05

7 1.09 8.92 9 108 1.31 9.06 9 109 1.02

Table 4 Trapping parameters

by glow curve shape methodsMethods b Peak 1 Peak 2

E (eV) s (s-1) E (eV) s (s-1)

Chen 3 1.02 2.97 9 108 1.14 5.30 9 108

Grossweiner 1.03 3.71 9 108 1.13 4.31 9 108

Luschik 1.03 3.71 9 108 1.15 6.51 9 108

Chen 5 1.02 2.84 9 108 1.17 9.18 9 108

Grossweiner 1.02 2.84 9 108 1.15 6.14 9 108

Luschik 1.04 4.40 9 108 1.18 1.12 9 109

Chen 7 1.05 4.09 9 108 1.28 5.19 9 109

Grossweiner 1.05 4.09 9 108 1.26 3.52 9 109

Luschik 1.07 6.27 9 108 1.28 5.19 9 109

866 J Mater Sci (2014) 49:858–867

123

bE

kT2m

¼ s 1þ ðb� 1Þ 2KTm

E

� �exp

�E

kTm

� �: ð18Þ

The values of trap depth and frequency factor are sum-

marized in Table 4.

The composite TL glow curve of Yb/Er codoped Y2O3

phosphor in the c-dose range 1–5 kGy can be best described

as a superposition of two glow peaks which indicates two

different sets of traps were being activated having their own

trapping parameters. All the deconvoluted peaks were found

to obey second-order kinetics indicating the occurrence of

retrapping phenomena. The calculation of trapping param-

eters by various glow curve methods shows a close agree-

ment. TL glow peak shift towards higher temperature sides

with the increase in heating rate which is in good agreement

of Garlick–Gibson equation for second-order kinetics [24].

TL glow curves are always related to the defects present in

the phosphors. The two e- traps existing at 1.05 and 1.2 eV

are shown in Fig. 8 as confirmed from TL studies. The

occurrence of detrapping/retrapping phenomena during the

release of the electron is also shown in this figure. The released

electrons recombine with holes at the luminescence center

giving rise to TL. Though in some papers it is reported that the

increase in defect level concentration causes an increase in the

upconversion intensity [2], but in the present work, the up-

conversion intensity is completely related on the position of

energy levels associated with Er3? and Yb3? ions which exist

far below compared to the defect levels created by c-irradia-

tion. And hence the defect levels created by c-irradiation do

not affect the upconversion properties of this phosphor.

Conclusion

• XRD/TEM studies and quantitative calculation of

structural parameters confirm the structure of Yb/Er

codoped Y2O3 phosphor as cubic with crystallite size

10–20 nm.

• The wide bandgap (5.8 eV) of the phosphor calculated

from DR studies indicates that this phosphor can be

used as the promising candidate for applications in

different optoelectronic devices.

• The prepared phosphor offers power dependent color

tuning properties indicating its applications in color

tunable laser.

• Simple glow peak, good linearity in TL dose–response,

excellent TL sensitivity under studied c-dose range

make this upconversion phosphor suitable for radiation

dosimetry.

Acknowledgements The authors are thankful to University Grants

Commission, New Delhi, Government of India for funding this work

under Special Assistance Program No. F.530/1/DRS/2009 (SAP-I).

The authors are also thankful to Dr. S. P. Lochab, IUAC, New Delhi

for TL measurements.

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Fig. 8 Representation of defect levels in energy diagram

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