Recife, PE, Brazil Recife, PE, Brazil Recife, PE, Brazil Rio de Janeiro, RJ, Brazil

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1 Radiation Protection Dosimetry (2006), Vol. 119, No. 1 4, pp doi: /rpd/nci684 Advance Access published on April 27, 2006 HIGHLY SENSITIVE THERMOLUMINESCENT CARBON DOPED NANOPOROUS ALUMINIUM OXIDE DETECTORS W. M. de Azevedo 1,, G. B. de Oliveira 1, E. F. da Silva Jr 2, H. J. Khoury 3 and E. F. Oliveira de Jesus 4 1 Departamento de Química Fundamental, Universidade Federal de Pernambuco Cidade Universitária, Recife, PE, Brazil 2 Departamento de Física, Universidade Federal de Pernambuco Cidade Universitária, Recife, PE, Brazil 3 Departamento de Energia Nuclear, Universidade Federal de Pernambuco Cidade Universitária, Recife, PE, Brazil 4 Laboratório de Instrumentação Nuclear, Universidade Federal do Rio de Janeiro Caixa Postal 68509, Rio de Janeiro, RJ, Brazil In this work we present the synthesis, characterisation and the thermoluminescence (TL) response of nanoporous carbon doped aluminium oxide Al 2 O 3 :C produced by anodic oxidation of aluminium in organic and inorganic solvents. The X-ray and scanning electron microscopy (SEM) measurements reveal that the synthesised samples are amorphous and present highly ordered structures with uniform pore distribution with diameter of the order 50 nm. The photoluminescence and spectroscopic analysis in the visible and infrared regions show that the luminescence properties presented by the samples prepared in organic acid are due to carboxylate species, incorporated in anodic alumina films during the synthesis process. After an annealing treatment, part of the incorporated species decomposes and is incorporated into the structure of the aluminium oxide yielding a highly thermoluminescent detector (TL). The results for X-ray irradiation in the range from 21 to 80 kev indicate a linear TL response with the dose in the range from 5 mgy to 1 Gy, suggesting that nanoporous aluminium oxide produced in the present route of synthesis is a suitable detector for radiation measurements. INTRODUCTION Nanotechnology research, to a great extent is based on fabricating functional nanoscale structures and devices in a well-controlled way. This represents one of the most difficult challenges that researchers and engineers have to face nowadays. The way to organise nanoelements into device structures, in order to realise their desired functionalities, using inexpensive fabrication techniques, is essential from the technological point of view. Owing to the small dimensions of these nanoelements, a bottom-up selfassembly process often provides a viable approach to overcome such technological challenges (1 3). Anodic porous alumina (4), a typical self-ordered nanochannel material formed by anodisation of Al in an appropriate acid solution, has recently attracted increasing interest as a key material for the fabrication of nanometre-scale structures (5 7). The structure of anodic porous alumina can be described as a close-packed array of columnar cells, each containing a central pore of which the size and interval can be controlled by changing the forming conditions (8). The nanochannel-array materials have been used as a host or template structure for nanometre devices, such as magnetic, electronic and optoelectronic devices (9,10). Corresponding author: wma@ufpe.br On the other hand, another interesting problem to be solved is regarding the manufacture of reliable dosemeters to be used in the ionising radiation field. The increased use of ionising radiation for the diagnosis and therapy of some diseases have made necessary the use of highly reliable dosemeters which would be able to measure lower and lower doses of ionising radiation. One of the former materials studied for possible use as a dosimeter is aluminium oxide (Al 2 O 3 ) (11) However, the study of this material was forgotten for a long time, because of its low sensibility as compared with that of thermoluminescent detector (TLD)-100. Recently the interest on this material has increased because of the development of anion defective Al 2 O 3 :C single crystals developed by the group at the Urals Polytechnical Institute, Russia (12,13). This material opened the possibility of several promising application for ultra high sensitivity dosimetry, particularly for short-term exposure in personal, environmental and extremity dosimetry. The indication for this material is that it possesses a TL sensitivity some times greater than that of LiF TLD- 100 (12), making it a strong candidate for low dose application. However, the well-established crystal growth technique requires a sophisticated laboratory infrastructure and high temperature processing, using a highly reducing atmosphere, some times not easily available in common laboratory structure. In this paper, we present the development of a straightforward route to prepare a highly sensitive Ó The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oxfordjournals.org

2 thermoluminescence (TL) dosemeter based on carbon doped nanoporous aluminium oxide, at low temperature, using the anodisation process in different solvents. The photoluminescent results as a function of the synthesis medium and the TL response with the radiation dose are also discussed. W. M. DE AZEVEDO ET AL. MATERIALS AND METHODS The anodising aluminium oxide (AAO) template was generated using high purity aluminium foil (99.99%) with typical dimensions of 20 mm 5mm1mmin acid solution using a two step process (4). The process was carried out either at a constant voltage of 40 V in 0.3 mol organic acid solution at 10 Corat18Vin 0.5 mol of inorganic acid. After the first anodisation, the oxide film was removed and the newly patterned aluminium substrate was anodised again. The anodising time was determined by the required thickness of the AAO film, finally the AAO was annealed in argon or open air. The photoluminescence and infrared characterisation were performed using a Jobin Yvon Ramanor model U-1000 spectrometer with either a 150 W Xe Lamp or an argon laser as excitation source and a FTIR Bruker IF 66 spectrophotometer respectively. A Siemens D-5000 X ray diffractometer with a Cu target was used to obtain the X-ray diffraction patterns of the samples, and the surface microstructure of anodic aluminium oxide films were analysed using a scanning electron microscope (SEM) JEOL model JSM 5600 LV. Previously to dosimetric measurements, the samples was annealed at 400 C for 60 min in order to erase any remaining information. All samples were irradiated at the same time with a TLD-100, with X-ray beams from 21 to 80 kev, in a range from 5 mgy to 1 Gy. The TL response was measured in a Victoreen TLD reader, Model 2800M using the heating rate of 8 Cs 1 in a nitrogen atmosphere and the heating temperature was up to 593 K (320 C). RESULTS AND DISCUSSION The structural characteristics of nanoporous aluminium oxide films, synthesised by the anodisation process in oxalic acid are illustrated in Figure 1. The patterns show several peaks associated with Al and Al 2 O 3 crystalline phases on top of a broad peak centred at 2y 25. The broad peak is an indication of a highly disordered and/or amorphous aluminium oxide compound Figure 2 shows the SEM surface micrographs of the alumina film prepared in oxalic acid as described in the experimental setup. The figure shows a typical view of the alumina surface, where the dark areas are the pores with a quite regular hexagonal distribution pattern, with average diameter of 50 nm, and Figure 1. X-ray diffractogram of anodized aluminum oxide synthesized in oxalic acid 0.3 M. Figure 2. Scanning electron micrographs of fragments of the surface side of the alumina film prepared in oxalic acid. the surrounding lighter areas is alumina These results are in agreement with literature reports (14). The infrared spectra of aluminium oxide film prepared in oxalic acid (solid line) and sulphuric acid (dotted line) are compared in Figure 3. Both spectra have absorption bands at about 3400 cm 1 and in the region cm 1. The former is because of O H stretching vibration of the bound water within the film. The latter owes to the intrinsic vibrations of the alumina constituting the bulk of the film. The band with a peak at 1246 cm 1 observed with the sulphuric acid film is assigned to the vibration arising from sulphate species incorporated into the film during anodisation, and the band observed at 1107 and 1032 cm 1 in oxalic acid can be assigned to the 202

3 2.0 NANOPOROUS ALUMINUM OXIDE DETECTORS Reflection (a.u.) Wavenumber cm 1 Figure 3. Comparison between the IR reflection spectra of the oxalic acid (solid line) and sulphuric acid films (dotted line). coupling of the C C stretching vibration and the O C¼O bending vibration. The double absorption band with peaks at 1634 and 1485 cm 1, observed with the oxalic acid film (and absent for films formed in sulphuric acid electrolytes) can be reasonably assigned to the anti-symmetric O C O stretching vibration; and to the coupling band arising from the symmetric O C O stretching vibration, and the C C stretching vibration respectively. Accordingly, it seems quite reasonable to assume that the double absorption band is inherent to the films formed in aliphatic carboxylic acid solutions and is because of the carboxylate species incorporated into the films during anodisation. The photoluminescence emission spectra for the nanoporous aluminium oxide prepared in sulphuric acid and oxalic acid are shown in Figure 4. One can see that, for sulphuric acid samples in Figure 4a, no fluorescence at all is presented when excited with an ultraviolet lamp or at 488 nm from argon laser. On the other hand, the sample synthesised in oxalic acid presents two strong, broad fluorescent peaks, one at 431 nm and the other at 491 nm when excited at 320 nm (Figure 4b). Theses peaks increase their intensity when the sample is annealed for 4 h in argon atmosphere (Figure 4c). Also, we observed that for the sample annealed in open air for the same period (Figure 4d), the first peak increases in intensity and is shifted to 420 nm, whereas the second peak shifts to 510 nm, with a third peak appearing at 550 nm. The origin of these fluorescent and phosphorescent peaks seems to be related to the presence of F-centres and/or free radicals of carbon related centres, as it is shown in the infrared spectra, or because of interstitial aluminium Al i þ ions (15,16). After the annealing treatment probably the organic radicals decomposes and the two valent ions of carbon impurity are supposed to replace three valent cations of Al in the oxide lattice, which leads to the formation of Figure 4. Room temperature photoluminescence emission from nanoporous aluminum oxide synthesized in (a) sulfuric acid, and (b) oxalic acid, (c) same as (b) but annealed in argon atmosphere and (d) same as (b) but annealed in air. Figure 5. TL glow curves for carbon doped nanoporous Al 2 O 3 :C samples annealed for 24 h and irradiated with X-ray 20 kev, at room temperature. hole trapping centres. Comparing our results to the a-al 2 O 3 :C data, we can see that the nanoporous aluminium oxide photoluminescence process is not only because of F-centres (12). Theses samples present more emission lines due to the radical incorporation during the syntheses process that can be converted by the annealing treatment, as shown in Figure

4 Figure 6. Dose response for different thickness of carbon doped nanoporous Al 2 O 3 :C annealed at 873 K for 24 h and irradiated with X-ray 20 kev. (a) Not annealed sample thickness 48 mm, (b) 16 mm, (c) 32 mm and (D) 96 mm. Figure 7. Dose response of carbon doped nanoporous Al 2 O 3 :C irradiated with X-ray beam. For the annealing treatment in open air we observed that the emission of the F-centre at 420 nm increases in intensity (Figure 4d), these fluorescent enhancements are responsible for the increasing sensibility on the TL results shown in Figure 5. The glow curves for carbon doped nanoporous Al 2 O 3 :C samples irradiated with X ray is shown in Figure 5. The TL glow curve presents two peaks, one low temperature peak at 50 C sometimes observed just upon irradiation which decays at room temperature and it does not introduce errors into the measurements and the other intense which presents no W. M. DE AZEVEDO ET AL. noticeable fading, at 190 C, both peak positions coincide with the one reported for carbon doped single crystal Al 2 O 3 :C in the literature (12). Another characteristic present by these samples can be seen in Figure 6. For samples irradiated in air with X ray of 76 kev, the sensibility of the sample increases with the sample thickness. For a sample that is 48 mm thick (without annealing treatment), the sensibility is too low, probably because of the low doping carbon content, however for the annealed sample as the thickness increases we observe an increase in the TL response. The reason for the response enhancement TL in the annealed samples may be related to free radicals or carbon related centres adsorbed on the nanoporous Al 2 O 3 surface. For the sample prepared in inorganic acid the TL glow is too low as compared with the doped sample. The dosimetric response for carbon doped nanoporous aluminium oxide Al 2 O 3 :C exposed to different X ray air kerma values are shown in Figure 7. The curve was obtained by plotting the integrated TL intensity as a function of dose for irradiations in the dose range from 5 to 500 mgy for 20, 40, 61 and 76 kev X-ray beam. The main feature is its linearity as the X-ray dose increases, presenting a correlation coefficient (r 2 ) of for the curve of 76 kev excitation energy. CONCLUSION Carbon doped nanoporous aluminium oxide has been synthesised by anodic oxidation of aluminium foil in organic and inorganic acids with subsequent thermal treatment. This straightforward route of synthesis proved that a highly sensitive Al 2 O 3 :C dosemeter can be developed, at quite low temperatures and with any laboratory facility. The nanoporous aluminium oxide shows excellent sensibility to X rays when compared with conventional dosemeters taking in account their thickness, and this sensibility depends upon their thickness The TL results for X rays of different energies, in the dose interval from 5 up to 500 mgy present linear response with the dose, which strongly suggest that this material presents a high potential to be used for X-rays dosimetry. ACKNOWLEDGEMENTS The authors would like to thank Geysa Keylla dos Santos for dosimetric readings. We acknowledge the financial support received during the development of this work from CNPq under contract Nano- SemiMat/CNPq # /01-9 and CNPq #47314/

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