www.elsevier.com/locate/optmat

Optical Materials 27 (2005) 1235–1239

Thermoluminescence properties of new ZnO nanophosphorsexposed to beta irradiation

C. Cruz-Vazquez a,*, R. Bernal b, S.E. Burruel-Ibarra a,H. Grijalva-Monteverde a, M. Barboza-Flores b

a Departamento de Investigacion en Polımeros y Materiales de la Universidad de Sonora, Apartado Postal 130, Hermosillo, Sonora 83000, Mexicob Centro de Investigacion en Fısica de la Universidad de Sonora, Apartado Postal 5-088, Hermosillo, Sonora 83190, Mexico

Received 18 October 2004; accepted 10 November 2004

Abstract

Novel ZnO nanophosphors were synthesized by thermal annealing of ZnS powders obtained by precipitation in a chemical bath

deposition reaction. Pellet-shape samples were exposed to beta radiation in order to investigate their dosimetric capabilities under

ionizing radiation. The dependence of thermoluminescence response in the 0.15–10.5 kGy dose range increased as the radiation dose

increased. The composition and structure of the ZnO samples are dependent on the annealing time and temperature. Energy-dis-

persive X-ray spectrometry analyses and X-ray diffraction patterns, confirmed the change from amorphous ZnS to nanocrystalline

ZnO (zincite) structure. The samples were beta irradiated and their thermoluminescence response as a function of dose exhibited

good linear ranges, which make them very promising detectors and dosimeters suitable for ionizing radiation.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Dosimetry; Thermoluminescence; Radiation detectors; Zinc oxide; Nanophosphors

1. Introduction

Zinc oxide (ZnO) is a direct band gap semiconductor

with many attractive features. Its 3.4 eV band gap en-ergy at room temperature may be modified introducing

impurities; for instance, diminishes by Cd doping and

increases when doped with Mg. The most common crys-

talline structure in ZnO is hexagonal (zincite) [1].

Among other defects existing in ZnO are the interstitial

Zn ions, oxygen vacancies and hydrogen [1].

ZnO exhibits striking features useful for the develop-

ment of components for optoelectronic applications.Thus, the physical properties of ZnO have been inten-

sively investigated focused mainly in the characteriza-

tion of its optical and electrical properties, for which it

0925-3467/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2004.11.016

* Corresponding author.

E-mail address: cathy@correom.uson.mx (C. Cruz-Vazquez).

has rapidly emerged as a promising optoelectronic mate-

rial suitable to be used in numerous potential technolog-

ical applications. Examples of applications include thin

film gas sensors, varistors, ultraviolet and visible lasersand solar cells components [1–4].

The thermally stimulated luminescence technique,

commonly termed as thermoluminescence (TL), is

widely accepted as a useful and reliable technique to

study defects in insulators and semiconductors materi-

als, but the more widely spread and successful applica-

tion of the TL is in the field of radiation dosimetry [5].

Many phosphor materials, synthetic as well as natural,have been characterized to evaluate its feasibility as

thermoluminescence dosimeters (TLD), applicable in

several low dose dosimetry areas, such as environmental

dosimetry, clinical dosimetry, among others. High doses

applications, involving doses greater than 102 Gy, may

be found inside nuclear reactors or during food steriliza-

tion and materials testing [5,6].

1236 C. Cruz-Vazquez et al. / Optical Materials 27 (2005) 1235–1239

A particular material may or not be useful for radia-

tion dosimetry depending on the kind of radiation to be

used and on the dose range values to be measured. If the

goal is to measure very low doses, a high sensitivity dosi-

metric material is required. In high dose dosimetry, it is

an important feature that the TL response as a functionof dose does not exhibit a superlinear or sublinear

behavior, because it is possible to have an underestima-

tion or overestimation of the actual absorbed dose.

Many materials, in particular the conventional thermo-

luminescence dosimeters, suffer from severe superlinear-

ity at high dose levels, and therefore the number of

materials available for these applications is limited [5,6].

ZnO exhibits TL under irradiation with differ-ent sources and striking radiation hardness [7–10].

Moreover, ZnO is inert to environmental conditions,

non-toxic and insoluble in water. In spite of these char-

acteristics, there is not much information related to the

potential applicability of ZnO in TL dosimetry. The lack

of interest to use ZnO as dosimetric material is due

perhaps to the great amount of other important applica-

tions, for instance in optoelectronics, and also to the lowefficiency of the TL emission reported for the samples

previously studied [7–9].

The physical properties of a material depend upon

the procedure followed to make it. To growth ZnO thin

films on distinct substrates is a major goal in materials

chemistry. Among the methods to synthesize ZnO and

other compounds thin films, the chemical bath deposi-

tion (CBD) method is well recognized because it is ver-satile and non-expensive. During the deposit of thin

films through the CBD technique, a quantity of material

precipitates in the bottom of the solution and can be col-

lected as powder.

In this work, we report the synthesis and thermolumi-

nescence properties of new pellet- shape ZnO nanophos-

phors, obtained by thermal annealing of ZnS powder

precipitated during de deposition of ZnS thin films,employing a CBD method recently reported [11]. The

employed method is easy and non-expensive. The sam-

ples were exposed to beta radiation to study their TL

and dosimetric characterization and the results obtained

show that these phosphors are very suitable as detectors

and TL dosimeters.

2. Experimental

A controlled chemical bath deposition reaction was

carried out to synthesize ZnS powder as follow: 80 ml

of a 0.1 M CS(NH2)2 (thiourea) solution was added to

250 ml of an 8 mM Zn(en)3SO4 solution with stirring.

Then, 50 ml of a 4 M NaOH solution was added. The

reaction was allowed to proceed at 60 �C by stirring for10 h. White-grayish ZnS powder was collected by filtra-

tion. It was washed with deionized water and dried in

vacuum [11]. Then, 0.06 g of the synthesized ZnS powder

were weighted to make each pellet-shaped sample, and

next placed into a 7 mm diameter cylindrical mold. Fi-

nally, the sample was comprised at 0.5 ton during

3 min using an hydraulic press. With this procedure,

0.8 mm thickness pellets were obtained. Afterwards, thepellets just obtained were subjected to a sintering process

at temperatures of 500 or 700 �C during 10 or 24 h underair atmosphere using a Thermolyne 4000 furnace.

The zinc (II) complex used, Zn(en)3SO4, was synthe-

sized by adding an aqueous ethylenediamine solution to

an aqueous ZnSO4 Æ7H2O solution in a mole ratio of 3:1.The colorless zinc (II) complex obtained was re-crystal-

lized from water.ZnS JMC Puratronic, 99.99%, was sintered and its

TL measured to be used for comparison with the synthe-

sized ZnS samples.

A Risø TL/OSL model TL/OSL-DA-15 unit

equipped with a 90Sr beta source was used to perform

beta irradiations and the TL measurements. All irradia-

tions were accomplished using a 5 Gy/min dose rate at

room temperature (�22 �C). The TL measurementswere carried out under N2 atmosphere using a heating

rate of 5 �C/s. The X-ray diffraction (XRD) patternswere collected with a Rigaku Geigerflex diffractometer

equipped with a graphite monochromator by using

CuKa radiation (k = 1.542 A). Scanning electron micro-scope (SEM) images and the samples composition were

obtained using a JEOL JSM-5410LV scanning electron

microscope equipped with an Oxford EDS analyzeroperating at 15 keV.

3. Results and discussion

Energy-dispersive X-ray spectrometry (EDS) analy-

ses carried out on the non-sintered pellets reveled a pro-

portion of 69:31 weight percent for Zn:S, too close tothat of the ZnS (67.1:32.9). Non-sintered ZnS pellets

were beta irradiated up to 100 Gy dose with no detec-

tion of TL emission. When ZnS pellets were sintered

for 10 h at 500 �C, clearly defined TL glow curves wereobtained after exposure to similar irradiation doses. The

TL efficiency did not significantly increased when the

24 h thermal treatments were performed at the same

temperature. The shape of TL curves revealed that it iscomposed of several overlapped TL peaks.

There is a noticeable enhancement of the TL effi-

ciency when the annealing temperature is increased from

500 �C up to 700 �C. Together with the enhancement ofthe TL efficiency, a change in the relative intensities of

the peaks appears. Samples sintered at 700 �C during

10 and 24 h exhibit no meaningful differences in the

TL intensity glow peaks. Thus, the TL efficiency is moredependent on the annealing temperature rather than on

the sintering time.

Fig. 2. Scanning electron microscopy (SEM) image of a pellet before

being sintered (a), and after sintered at 700 �C during 24 h (b).

C. Cruz-Vazquez et al. / Optical Materials 27 (2005) 1235–1239 1237

Fig. 1 shows the X-ray diffraction (XRD) pattern of a

pellet sintered at 700 �C during 24 h together with thereference lines of zincite (JCPDS #36-1451). The pellet

pattern coincides with that of the mentioned reference,

except for the weaker peaks at angles lower than 30�,which can be associated with impurities in the material.These small signals could be associated to the thermal

decomposition of a small part of the material when

the samples are annealed, induced by the evaporation

of thiourea and the Zn(en)3SO4 complex, which precip-

itate together with the ZnS powder during the reaction

[12]. ZnO occurs naturally as the mineral zincite [1].

The XRD pattern of a sample annealed 24 h at 500 �Calso coincides with zincite. Fig. 1 exhibits no peaks asso-ciated to the presence of crystalline ZnS, neither are ob-

served in XRD patterns from samples annealed at

500 �C. This in spite that EDS analyses of samples re-vealed mixtures of a molar ratio ZnS:ZnO of

0.46:0.54, after annealing at 500 �C for 24 h, and

ZnS:ZnO of 0.1:0.9 if annealed at 700 �C during 24 h.There is no crystallization of ZnS with the performed

thermal annealing and, thus it is not observed in theXRD patterns. By increasing the amount of crystalline

ZnO an improvement of the TL efficiency was observed.

Thermal annealing induces the ZnS to ZnO transforma-

tion and crystallization afterwards.

Fig. 2 shows the scanning electron microscopy (SEM)

image of a pellet before being sintered (a) and after sin-

tered at 700 �C during 24 h (b). Fig. 2(b) allows us todiscover the presence of nanosized 50–500 nm ZnOgrains. The improved TL efficiency under thermal

annealing is due to crystallization of nanometric ZnO

particles clearly observed in Fig. 2(b) compared to the

ZnS amorphous structure seen in Fig. 2(a).

10 20 30 40 50 60 702 θ (grados)

Fig. 1. X-ray diffraction pattern of pellets subjected to thermal

annealing at 700 �C during 24 h. The vertical lines correspond to

zincite, JCPDS #36-1451.

Fig. 3 shows the TL glow curves of pellets sintered

during 24 h at 700 �C after exposed to beta radiation

in the dose range from 0.15 to 10.5 kGy. A maximum

100 200 300 4000.0

3.0x105

6.0x105

9.0x105

1.2x106

1.5x106

TL In

tens

ity (a

rb. u

.)

Temperature ( C)

Dose (Gy)

150

600

1500

4500

750010500

o

Fig. 3. TL glow curves of pellets sintered during 24 h at 700 �C afterexposure to beta radiation in the dose range 0.15–10.5 kGy.

100 200 300 4000.0

5.0x104

1.0x105

1.5x105

2.0x105

2.5x105

TL In

tens

ity (a

rb. u

.)

Temperature (°C)

(a) (b)

Fig. 5. TL glow curves of one synthesized ZnS pellet sintered 24 h at

700 �C after 600 Gy of beta exposure (curve (a)), and that obtainedfrom commercially available ZnS powder subjected to the same

thermal treatment (curve (b)) and exposed to 800 Gy of the same

radiation.

1238 C. Cruz-Vazquez et al. / Optical Materials 27 (2005) 1235–1239

of the TL glow emission is located at around 220 �C. Upto the maxima dose investigated no saturation was ob-

served in the behavior of the TL curves but their inten-

sities increased with larger doses as displayed in Fig. 3.

The whole thermogram exhibits a complex structure in

the form of a broad curve indicating the overlap of sev-eral TL glow peaks.

Many materials suffer from severe superlinearity at

high dose levels. The ZnO nanophosphors here reported

exhibited a superlinear dependence of the total TL (inte-

grated TL) as a function of irradiation dose. This incon-

venience was fixed by applying a two steps thermal

annealing previous to irradiation: at 300 �C for 30 minfollowed for a second annealing at 200 �C during30 min. It was verified that this procedure guarantee a

good TL reproducibility. The TL glow curves shown

in Fig. 3 were obtained following this pre-irradiation

protocol.

Fig. 4 shows the integrated TL as a function of dose,

up to 10.5 kGy, of a pellet sintered 24 h at 700 �C. Itexhibits a remarkable good dosimetric response in which

there are no indications of response saturation. It shouldbe noticed that the dosimetric behavior is linear for

doses of the order of 102 Gy. The integrated TL fades

down to 55% of its value just after irradiation in a 2 h

period after which tends to remain with a constant

value. For comparison, ZnS powder commercially avail-

able was subjected to similar thermal treatments as the

synthesized one, obtaining extremely low TL response.

It was not possible to fabricate compressed pellets fromthe commercial ZnS under the same conditions used in

the sintering process, obtaining a very poor TL emission

compared with that of our ZnO sintered samples. Fig. 5

shows the TL glow curves of one synthesized ZnS pellet

sintered 24 h at 700 �C after 600 Gy of beta exposure

(curve (a)), and that obtained from commercially avail-

0 4 8 10 12

0.0

5 0x10 .7

1 0x10 . 8

1.5x108

2.0x108

2.5x108

3.0x108

3.5x108

ITL

(arb

. u.)

Dose (kGy)2 6

Fig. 4. Integrated TL as a function of dose, of pellets sintered 24 h at

700 �C, for doses up to 10.5 kGy of beta radiation.

able ZnS powder subjected to the same thermal treat-

ment (curve (b)) and exposed to 800 Gy of the same

radiation.

4. Conclusions

We have presented experimental evidence about anovel ZnO nanophosphor obtained by thermal anneal-

ing of ZnS powder synthesized by a CBD method. It

exhibited good thermoluminescence properties and

may be considered as a promising material to be used

in thermoluminescence dosimetry. The synthesis of

ZnO nanophosphors enhanced the TL efficiency as com-

pared to that of ZnO obtained from commercially avail-

able ZnS.

Acknowledgments

The authors gratefully acknowledge the financial sup-

port for this work from CONACyT (Mexico), under

Grant J35222-E, from SESIC-SEP (Mexico) Grant

PROMEP-UNISON-PTC-01-01, and from Universidadde Sonora.

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