DBS films

Journal of Non-Crystalline Solids 356 (2010) 2429–2432

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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l

Gamma radiation effects on absorbance and emission properties of layer-by-layer PPV/DBS films Marcia Dutra Ramos Silva ⁎, Antônio Ariza Gançalves Jr., Raigna A. Silva, Alexandre Marletta Physics Institute, Universidade Federal de Uberlândia, CP 593, 38400-902, Uberlândia — MG, Brazil

a r t i c l e

i n f o

Article history: Received 15 October 2009 Received in revised form 14 May 2010 Available online 18 September 2010 Keywords: Organic semiconductors Gamma irradiation Dosimeter UV–vis spectroscopy

a b s t r a c t This work addresses the effects of Gamma radiation on the absorbance and emission properties in layer-bylayer (LbL) poly(p-phenylene vinylene) (PPV)/dodecylbenzenesulfonate (DBS) thin films. The LbL PPV/DBS films were irradiated with 2 Gy and 100 Gy doses, using a 60Co source of Gamma radiation. The effects of irradiation on absorbance and emission of PPV were discussed. The new methodology applied to analyze the effects of radiation on luminescent conjugated polymers provides the possibility to use this material in largearea, low-cost dosimeter sensors. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The interaction of ionizing radiation with matter can produce chemical, mechanical and physical modifications, since the resulting energy is higher than the atomic bonds [1]. The changes occurring in the irradiated material can be correlated with the radiation dose, these changes characterize a dosimeter. Nowadays, the most widely used personal dosimeters are still relatively expensive, mainly because the available dose measurement needs specific thermoluminescence(TL) experiment laboratories. In addition, the TL spectral range depends on the material, for instance, the emission of heat from Thermo-luminescent Detectors (TLDs) is in the ultraviolet region, adding to the experimental difficulty [2–4]. Therefore, the development of new measurement methods and materials for dosimeter application represents an important and broad research area. Conjugated polymers with non-localized π-electrons have been under attention due to its electronic structure, which is similar to inorganic semiconductors. After Heeger, Macdiarmid and Shirakawa discovered the conducting polymers in 1970 [5], there was a great advance in the organic semiconductors, chemistry and physics, mainly in their technological application in optical-electronic devices: polymeric light emission diodes, displays, solar cells, field-effect transistors, etc. [6] In addition, conjugated polymers have been used as an active layer of sensors: radiation [7,8], microfluids [9], metals [10], nitric oxide [11], among others. In a specific case, PPV (poly p-phenylene vinylene) and its derivatives and, recently, polyfluorenes have become attractive

⁎ Corresponding author. Tel.: + 55 34 32394190; fax: + 55 34 32394106. E-mail address: [email protected] (M.D.R. Silva). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.099

materials for electroluminescence applications [12]. On the other hand, the change in absorbance band of PPV derivatives has been employed as an indirect parameter to detect the primary effects of, for example, the UV–vis radiation dose in neonatal phototherapy [7] or in ionizing radiation (b1 kGy) for radiotherapy [8]. The PPV synthesis, in particular, consists of heat-induced elimination of tetrahydrothiophenium from the polyelectrolyte poly(xylylidene tetrahydrothiophenium chloride) (PTHT). In this process, the polyelectrolyte film must be kept at approximately 250 °C for a long time (~ 6 h) in vacuum conditions in order to eliminate the tetrahydrothiophene groups [13]. However, higher temperatures, above 150 °C, induce structural defects such as oxidative process. The main consequence is the decrease in effective polymeric conjugation length, which is associated to the luminescence quenching [14]. However, when the reaction is carried out at lower temperatures, the polyelectrolyte may not be efficiently converted to the conjugated polymer. Despite the difficulty, PPV presents better thermal properties, chemical synthesis facility and film manufacturing by diverse techniques when compared to its derivatives, for instance. Alternatively, due to PTHT polyelectrolyte properties, layer-by-layer technique can be used to produce homogeneous and ultra-thin PPV films. Thus, it is possible to grow PTHT and dodecylbenzenesulfonate (DBS) bi-layers in a non-limited method, with a layer thickness of ~3 nm [15]. An immediate result of the alternative route can be observed in the luminescence efficiency enhancement due to the rapid PPV thermal conversion (~ 30 min) at lower temperatures (110 °C) and in atmosphere environment (normal conditions of pressure and temperature). It decreases the structural defects along the main chain, principally, the thermo oxidative process. Fig. 1 shows the conventional chemical route for PPV (Fig. 1(i)) and the alternative route using DBS (Fig. 1(iii)).

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M.D.R. Silva et al. / Journal of Non-Crystalline Solids 356 (2010) 2429–2432 Table 1 Wavelength of spectral mass center (λSMC) and spectral area for LbL1 and LbL2 films for absorbance spectra in Fig. 2.

Fig. 1. Chemical structures and thermal conversion of precursor polymer PTHT for PPV synthesis. (i) Conventional route, (ii) exchange of counter-ion Cl by DBS, and (iii) alternative conversion route [15].

This paper addresses the study of low dose Gamma radiation (b100 Gy) on the absorbance and emission of LbL PPV/DBS films. Absorbance and emission measurements were performed before and after the gamma irradiation process. Finally, a new methodology to evaluate the irradiation dose for luminescent conjugated polymers thin films is proposed. 2. Materials and methods Fluoride Tin Oxide (FTO) substrates were washed using a solution containing H2O(Milli-Q):H2O2:NH4OH in the ratio of 5:1:1 (v/v) at 80 °C for 1 h. The substrates were rinsed with ultrapure water (MilliQ) to remove chemical residues. The PTHT precursor polymer was purchased from ALDRICH®. The LbL PTHT/DBS films were processed by substituting chloride anions of poly(xylylidene tetrahydrothiophenium chloride) (PTHT) for a long chain sodium dodecylbenzenesulfonate (DBS) anion and deposited on hydrophilic substrates with 20 bi-layers following the procedures described in reference [13]. The LbL PPV/DBS films were thermally converted (Tconv) at 110 °C for 30 min under atmosphere environment; the sample was labeled as LbL1. For comparison, the conventional thermal procedure was performed at 200 °C during 120 min in vacuum environment (10 mbar); this sample was labeled as LbL2.

Dose (Gy)

bλSMCN (nm)

Area (a.u.)

LbL1 LbL1-LD LbL1-HD LbL2 LbL2-LD LbL2-HD

–

431.7 431.5 431.8 429.9 428.8 422.0

152.3 143.2 118.9 142.8 135.7 91.3

3. Results and discussions Fig. 2 shows UV–vis absorbance spectra for LbL1 (Fig. 2(a)(i)) and LbL2 (Fig. 2 (b)) films non-irradiated and irradiated at 2 and 100 Gy Gamma radiation doses. For both films and low dose (LD), the absorbance line shape and intensity did not change substantially. On the other hand, the intensity of the absorbance spectrum decreases and the blue spectrum shifts 14 nm for LbL2-HD. In order to quantify the effects of radiation on absorbance, Table 1 displays the area and the spectral mass center (λSMC). The λSMC was calculated by using Eq. (1). For LbL1 film, the λSMC does not change regardless of the radiation dose, but for LbL2 this parameter decreases from ~430 nm to ~422 nm following the maximum rate of absorbance. The mean difference between films is the thermal conversion temperature. For LbL2, the increase in thermal-oxidative process is expected due to the formation mainly of carbonyl groups [16]. As a result, the effect observed in this work is similar for MEH–PPV in solution reported by Silva et al. in reference [7], where the presence of oxygen in solvent increases polymeric degradation, i.e., a reduction in effective

(b) Tc=200oC

0.40

0.40

0.35

0.35

0.30

0.30

0.25

0.25

0.20 0.15

0.00 350

0.20 0.15 0.10

0.10 0.05

2 100 – 2 100

The samples (LbL PPV/DBS films) were irradiated at 2 Gy (LD) and 100 Gy (HD) Gamma radiations by using a Theratron 780C Phoenix 60 Co-g-ray source at room temperature. The ultraviolet–visible (UV– vis) absorption measurements were performed using an Ocean OpticsUSB4000 spectrometer and a DT-Mini light source. Photoluminescence (PL) spectra were obtained by exciting the films at 458 nm from Ar ion laser, model Stabilite 2017 — Spectra Physics Inc. The samples were closed in a cryostat in vacuum environment (10− 2 mbar) and PL signal was carried out by Ocean Optics-USB2000 spectrometer.

Absorbance

Absorbance

(a) Tc=110oC

Sample

LbL1 LbL1-LD 2Gy LbL1-HD 100Gy

400

450

500

Wavelength (nm)

0.05

550

600

0.00 350

LbL2 LbL2-LD 2 Gy LbL2-HD 100 Gy

400

450

500

550

600

Wavelength (nm)

Fig. 2. UV–visible spectra for LbL1 (a) and LbL2 (b) PPV/DBS films before (—) and after Gamma radiation at 2 Gy (-o-) and 100 Gy (-•-).

M.D.R. Silva et al. / Journal of Non-Crystalline Solids 356 (2010) 2429–2432

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Fig. 3. Photoluminescence spectra for LbL1 (a) and LbL2 (b) films before being irradiated (—) and after irradiation at 2 Gy (-o-) and 100 Gy (-•-).

conjugation degree and blue shift in the absorbance spectra could be observed.

hλSMC i =

∫AðλÞλdλ ∫AðλÞdλ

ð1Þ

Selectively, the emission occurs in long PPV segments and lower gap energy after energy transfer and/or charge diffusion from excited chronophers (PPV segments with lower conjugation degree and higher gap energy). The photoluminescence (PL) of PPV presents, commonly, three bands: i) pure electronic transitions (or zerophonon transition) and ii) two bands at lower energy of electron– phonon transition. In the latter case, the electron–phonon coupling can be characterized by Huang–Rhys—S. The S factor is correlated to the disorder and/or the presence of defects along the main polymeric chain, being more pronounced in more localized electronic states (smaller size conjugated segments) [7,16]. The parameter S is approximately given by the following: S=

I1 I0

ð2Þ

where I0 is the intensity of the zero-phonon peak (~510 nm) and I1 is the intensity of the first electron–phonon transition peak (~550 nm). Fig. 3 presents the PL spectra for LbL1 (Fig. 3a) and LbL2 (Fig. 3b) before and after the Gamma irradiation (2 and 100 Gy). For nonirradiated film, the spectra for both samples are quite different due to the increase in structural defects for PPV films which are thermally converted at higher temperature (N150 °C); which has been correlated to thermal-oxidative reaction with carbonyl formation [14,15]. The former effect is evidenced for LbL2 film (Fig. 3b) where the peak intensity for first-phonon replica is greater than the peak intensity for zero-phonon peak. For lower dose, the PL spectra do not substantially change in comparison to the non-irradiate ones in both films; this is in agreement with the absorbance experiment, see Fig. 2. When the dose rises, the PL intensity substantially decreases; but parameter S does not change and the spectra do not shift. Table 2 shows S values obtained by using Eq. (2) for all PL spectra in Fig. 3. It is important to observe the decrease in PL intensity, ~35% for LbL1 and ~ 70% for LbL2

film after irradiation at 100 Gy. The resulting behavior shows that HD dose is not enough to extensively degrade the PPV films; PL peak positions and parameter S do not change significantly in comparison to PL spectra before irradiation. Finally, our results indicate the importance of oxygen groups for Gamma radiation detection using conjugation polymer and the possibility of using PL intensity as a parameter to correlate with the radiation dose. 4. Conclusions This work addressed the study of Gamma radiation effects on absorbance and emission properties in LbL PPV/DBS thin films. Considering the physical model for PPV polymer, where there is a distribution of conjugated PPV segments along the main polymer chain and the energy relaxation process, it was possible to qualitatively correlate the irradiation dose with polymeric electronic structure. The analysis of absorbance spectra shows that spectral mass center is not a good parameter to correlate to the radiation dose. This observation is equivalent to the Huang–Rhys parameter and spectral position for PL experiment (Table 2). Therefore, spectral intensity can be considered the best parameter, since the considerable variation in PL can be correlated with the emission quantum efficiency. As a result, the radiation increases the non-radioactive process due to the increase of structural defects along the polymer chain caused by oxidative process, carbonyl groups, in PPV thermal conversion procedures at higher temperature (N150 °C). Therefore, absorbance is not able to detect this effect. Additionally, selectivity, PL is able to

Table 2 Huang–Rhys parameter (S — Eq. (2)) and PL intensity (IPL) for LbL1 and LbL2 films for spectra emission (Fig. 3) to non-irradiated (—) and irradiated gamma doses at 2 Gy and 100 Gy. Sample

Dose (Gy)

S ± 0.01

IPL (u.a.)

LbL1 LbL1-LD LbL1-HD LbL2 LbL2-LD LbL2-HD

– 2 100 – 2 100

0.87 0.86 0.81 1.12 1.13 1.16

100.0 94.9 64.5 100.0 92.5 29.3

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indirectly infer the increase of non-radioactive process channel. As a result, it is possible to use thin films as Gamma ionizing radiation detection when considering the PL properties of these films. Finally, the new methodology developed in this work opens the opportunity to produce large-area low-cost dosimeter sensors. Acknowledgments The authors are very grateful to the Brazilian Agencies (CAPES, CNPq and FAPEMIG), and also to MCT/INEO for financial support. References [1] N. Souza, A.N. Farag, Dosimetric studies based on radiation induced of sudan blue dyes in organic solutions, Int. J. Appl. Radiat. Isot. 41 (8) (1990) 739. [2] M.S. Akselrod, V.S. Kortov, D.J. Kravetsky, V.I. Gottlib, Highly sensitive TL aniondefect _-Al2O3: C single crystal detectors, Radiat. Prot. Dosim. 33 (1990) 119–122. [3] M.S. Akselrod, V.S. Kortov, D.J. Kravetsky, V.I. Gottlib, Radiat. Prot. Dosim. 33 (1990) 119.

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