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Characterization of nanopores in PFA thin films
Surface & Coatings Technology 203 (2009) 2493–2496
Contents lists available at ScienceDirect
Surface & Coatings Technology 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 / s u r f c o a t
Characterization of nanopores in PFA thin films Renato Amaral Minamisawa 1, Robert Lee Zimmerman, Daryush Ila ⁎ Center for Irradiation of Materials, Alabama A&M University, P.O. 1447, AL, USA
a r t i c l e
i n f o
Available online 3 March 2009 Keywords: Nanopores Porous Membranes PFA Ion Beam Nanofabrication AFM
a b s t r a c t Porous membranes using synthetic polymers have been applied in several research methods and devices such as hydrophobic filters for removal of microorganisms and particles from air and other gases, and chromatography. We report in this paper a recent study of nanopore characterization fabricated in PFA thin film membranes using a homemade feedback controlled ion beam system adapted to a MeV Pelletron accelerator. Fluorpolymer membranes were bombarded with gold ions producing nanopores in a few minutes, with a diameter distribution controlled by the feedback system. The fabrication process is fast when compared with chemical etching techniques. Micro-Raman and AFM were used to characterize the pore properties. © 2009 Published by Elsevier B.V.
1. Introduction Porous membranes include thin sheets and hollow fibers generally formed of a continuous matrix structure containing a range of open pores or conduits of small size. Porous membranes having open micro and nanosized pores, thereby improving permeability, are applied in selective filtration, chemical detectors and molecular electronics, while with closed pores are impermeable and useful for thermal insulation. Poly (tetrafluoroethylene-co-perfluoro (alkyl vinyl ether)) (PFA) polymer is a desirable filtration membrane material because of its excellent chemical and thermal stabilities [1]. However, their inherent inert nature also enables them to be cast into membranes by conventional solution immersion casting processes. Several chemical and physical methods, including stretching [2] and internal melting mold processing [3] of other fluorpolymers, have proven impossible to fabricate well shaped microporous membranes. Moreover, these polymers have inferior stability when compared to PFA. A few authors obtained by chemical methods the fabrication of microporous PFA membranes via thermally induced phase separation [4,5]. However, the described processes are time consuming and difficult to control the pores to the nanosize scale. Successful methods using ion track/etching techniques and focused ion beam drilling followed by closure of the pores using broad plasma beam irradiation can be found in the literature [6–13], but to the best of our knowledge there is no report about the fabrication of pores in PFA thin films using ion bombardment techniques. Despite much progress,
⁎ Corresponding author. Center for Irradiation of Material, Alabama A&M University, 4900 Meridian Street, Normal, Alabama 35762, USA. Tel.: +1 256 372 5877; fax: +1 256 372 8708. E-mail address: [email protected] (D. Ila). 1 U.S. patent pending. 0257-8972/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2009.02.045
there is still a lack of studies in the creation of pores in different materials and over a wide range of diameters. The potential for such studies includes application in biological sensors for objects with different dimensions and functionalities. The ion bombarded induced damage in PFA and other fluorpolymers has been extensively reported in the literature [14–17]. Recently, N. Patterson et al. demonstrated the use of low energy ion irradiation for the direct tailoring of nanopores in silicon nitride membranes, avoiding radiation induced hole closure methods [18]. At the same time, we reported the fabrication of nanopores in PFA membranes by direct high energy ion beam drilling, using a homemade gas based feedback control system [19]. In this investigation we characterized the nanoporous membranes in PFA thin films fabricated using an innovative ion beam controlled feedback system. 2. Materials and methods PFA polymer (DuPont) has demonstrated a high level of drilling yield when bombarded with MeV Au ions due to thermal sublimation in equilibrium and non-equilibrium processes, and to molecular sputtering [14–17,19]. Mass loss of ~8 × 1014 CF2 molecules per incident ion were observed in previous experiments for 300 nA current at a fluence of 1 × 1013 ions/cm2 [20]. Samples were bombarded in a Pelletron accelerator. Films with 2 × 2 cm2 dimensions and 12.5 mm thickness were mounted as a covering window for a He gas chamber in a homemade ion beam controlled feedback system. Scheme of the system is shown in Fig. 1. The ion beam was collimated by a 2000 squares/in. Cu mask in physical contact with the polymer surface, which defines the pore distribution and avoids induced cracks during scanned ion bombardment. While bombarded with 5 MeV Au3+ ions, the He gas diffuses through the film and is detected by the residual gas analyzer (RGA) in the leakage mode. The gas flow signal increases during pore
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Fig. 3. AFM image of fabricated micropores with ~ 2 μm diameter uniformly distributed in space on the side opposite to the side bombarded with gold ions. Fig. 1. Schematic of the feedback-controlled ion beam system. The RGA detection of the He gas that leaks through the bombarded film membrane provides the signal to control the ion beam blocker (IBB) and is used as the feedback for the pore size control. When calibrated with the RGA signal, the charge signal from the beam current integrator counter can be also used to control the IBB.
formation, which is used as the feedback signal. From the difference of the slopes of the gas partial pressure versus time at initial and final conditions, and atomic force microscopy (AFM) images of the fabricated pores, one can calibrate the system. The ion beam is blocked when the desired diameter of the nanopores is obtained. Vibrational molecular spectroscopy of the bombarded sample was made with a Micro-Raman scattering analysis, performed in LabRam equipped with a He–Ne laser (λ = 632 nm) and focused inside the micro pores. 3. Results and discussion We require the fabrication of porous PFA polymer membranes with control of the spatial distribution and pore diameters. It is also desirable that the polymer membrane keeps its properties after the ion sculpturing modification, specially remaining chemically inert. Fig. 2a shows the drilled hole in the surface of the PFA bombarded film at 1 × 1013 ions/cm2. The bombardment using the Cu mask defines the square shape at the entrance of the hole, which becomes circular as the depth increases because the mask acts as a heat sink and the heat concentrates in the center of the irradiated area. This mechanism is
Fig. 4. AFM surface scan of the back surface of the PFA membrane, showing a ~ 100 nm diameter ion drilled nanopore. The image is a single pore of more than a thousand regularly spaced identical holes produced simultaneously after a few minutes ion bombardment of one PFA sample.
discussed elsewhere [20]. In order to evaluate the chemical changes in the pores induced by the ion bombardment, Raman scattering spectra were measured inside the cavity described above, as show in
Fig. 2. a) AFM image of the bombarded (top) surface of the PFA film and b) Raman spectra measured on the surface of a non bombarded sample and measured inside the micropore shown in a).
R.A. Minamisawa et al. / Surface & Coatings Technology 203 (2009) 2493–2496
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Fig. 5. AFM images of nanopores with different diameters scanned in the back surface of different PFA thin film membranes.
Fig. 2b. One can notice the non destruction of the C–F and C–O bonds of the bombarded area when compared with the non bombarded sample. Few or no indications of damage were observed for 1 × 1014 ions/cm2 accumulated fluence. At bombardment fluences higher than 5 × 1014 ions/cm2 the polymer shows indication of a carbonization process because of the mass loss by carbon–fluorine radicals emission, mainly CF3 as indicated by residual gas measurements [21]. At 1 × 1014 ions/cm2, circular micropores with ~2 μm diameter and uniform distribution in space were fabricated in the non bombarded (back) surface of the PFA thin film membrane as shown in the AFM images on Fig. 3. Distances between adjacent pores have an average of approximately 10 μm, which matches with the center of the collimation squares in the Cu masks. Nanopores with ~ 100 nm diameter are fabricated in the back surface of the bombarded films when decreasing the accumulated ion fluence to 1 × 1013 ions/cm2, as shown in the AFM image on Fig. 4. Distances between the nanopores are not shown in the figure because the scan scale necessary to obtain a suitable magnification to observe a single pore is lower than the distances observed on Fig. 3. Fig. 5 shows the topographic images and profiles of nanopores with 50, 100, 300 and 500 nm diameters fabricated with different accumulated fluence of MeV gold bombardment. Although have produced smaller pores, the observation of nanopores with diameters less than ~ 50 nm is limited by our AFM tip resolution. Valleys observed in the nanopore edges are due to AFM artifacts and were change with scanning direction. A detailed statistics of the pore diameter dependence on ion bombardment fluence will be presented elsewhere [22]. As the nanopores are an extension of the micropores drilled in the bombarded surface of the thin films, low aspect ratios are obtained that are desirable for some detection applications. The technique is able to more rapidly fabricate porous membranes when compared with focused ion beam techniques and ion track etching methods. 4. Conclusion Control of the size of pores in PFA synthetic thin film is important for each specific membrane application and may open new fields for nanotechnology studies. The homemade feedback controlled ion
beam system developed at the Center for Irradiation of Materials at Alabama A&M University is promising for nanopore ion beam fabrication and detection. The system consists in the detection of pore formation using the He gas leakage from a reservoir through a bombarded PFA film window. Bombardment using Au ions at 5 MeV energy demonstrates little damage in the chemical structure of the thin film polymer and a high sputtering yield for the effective fluence necessary to fabricate the nanopores. Diameters of the pores are easily controlled and range from micrometers to few nanometers. The technique has the advantage to produce rapidly porous membranes in relatively thick polymer films using scanned ion beam and is an alternative to focused ion beam methods. Acknowledgements This research was sponsored by the Center for Irradiation of Materials, Alabama A&M University and by the AAMURI Center for Advanced Propulsion Materials under the contract number NNM06AA12A from NASA, and by National Science Foundation under Grant No. EPS-0447675. References [1] DuPont PFA — Properties Bulletin: http://www2.dupont.com/Teflon_Industrial/ en_US/assets/downloads/h04321.pdf. [2] U.S. Pat. Nos. 3,953,566; 3,962,153; 4,096,227; 4,110,392 and 4,187,390. [3] U.S. Pat. Nos. 4,623,670 and 4,702,836. [4] U.S. Pat. No. 4,906,377. [5] M.R. Caplan, C.Y. Chiang, D.R. Lloyd, L.Y. Yen, J. Membr. Sci. 130 (1997) 219. [6] J. Li, D. Stein, C. McMullan, D. Branton, M.J. Aziz, J.A. Golovchenko, Nature 412 (2001) 166. [7] J. Li, M. Gershow, D. Stein, E. Brandin, J.A. Golovchenko, Nature Mater 2 (2003) 611. [8] D. Fologea, M. Gershow, B. Ledden, D.S. McNabb, J.A. Golovchenko, J. Li, Nanoletters 5 (2005) 1905. [9] R.R. Henriquez, T. Ito, L. Sun, R.M. Crooks, Analyst 129 (2004) 478. [10] T. Schenkel, V. Radimilovic, E.A. Stach, S.J. Park, A. Persaud, J. Vac. Tech. B 21 (2003) 2720. [11] J.A. Veerman, A.M. Otter, L. Kuipers, N.F. Van Hulst, Appl. Phys. Lett. 44 (1984) 502. [12] Z. Siwy, A. Fuliski, Phys. Rev. Lett. 89 (2002) 198103. [13] M.E. Mochel, J.A. Eades, M. Metzger, J.I. Meyer, J.M. Mochel, Appl. Phys. Lett. 44 (1984) 502. [14] M.A. Parada, R.A. Minamisawa, M.V. Moreira, A. de Almeida, I. Muntele, D. Ila, Surf. Coat. Technol. 201 (2007) 8246. [15] R.A. Minamisawa, A. de Almeida, S. Budak, V. Abidzina, D. Ila, Nucl. Instrum. Methods Phys. Res. B 261 (2007) 1159.
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R.A. Minamisawa et al. / Surface & Coatings Technology 203 (2009) 2493–2496
[16] R.A. Minamisawa, A. de Almeida, V. Abidzina, M.A. Parada, I. Muntele, D. Ila, Nucl. Instrum. Methods Phys. Res. B 257 (2007) 568. [17] M.A. Parada, R.A. Minamisawa, A. de Almeida, C. Muntele, R.L. Zimmerman, I. Muntele, D. Ila, Braz. J. Phys. 34 (2004) 1. [18] N. Patterson, V.C. Hodges, M.J. Vasile, D.P. Adams, Z. Chen, C.J. Brinker, Mat. Res. Soc. Symp. Proc. (2007) 983. [19] R.A. Minamisawa, R.L. Zimmerman, C. Muntele, D. Ila, Mat. Res. Soc. Symp. Proc. (2007) 983.
[20] R.A. Minamisawa, R.L. Zimmerman, D. Ila, Nucl. Instrum. Methods Phys. Res. Sect. B 266 (8) (2008) 1273. [21] R.A. Minamisawa, R.L. Zimmerman, S. Budak, D. Ila, Nucl. Instrum. Methods B 266 (2008) 1269. [22] Minamisawa R. A., Zimmerman R. L. and Ila D., (to be published).
Contents lists available at ScienceDirect
Surface & Coatings Technology 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 / s u r f c o a t
Characterization of nanopores in PFA thin films Renato Amaral Minamisawa 1, Robert Lee Zimmerman, Daryush Ila ⁎ Center for Irradiation of Materials, Alabama A&M University, P.O. 1447, AL, USA
a r t i c l e
i n f o
Available online 3 March 2009 Keywords: Nanopores Porous Membranes PFA Ion Beam Nanofabrication AFM
a b s t r a c t Porous membranes using synthetic polymers have been applied in several research methods and devices such as hydrophobic filters for removal of microorganisms and particles from air and other gases, and chromatography. We report in this paper a recent study of nanopore characterization fabricated in PFA thin film membranes using a homemade feedback controlled ion beam system adapted to a MeV Pelletron accelerator. Fluorpolymer membranes were bombarded with gold ions producing nanopores in a few minutes, with a diameter distribution controlled by the feedback system. The fabrication process is fast when compared with chemical etching techniques. Micro-Raman and AFM were used to characterize the pore properties. © 2009 Published by Elsevier B.V.
1. Introduction Porous membranes include thin sheets and hollow fibers generally formed of a continuous matrix structure containing a range of open pores or conduits of small size. Porous membranes having open micro and nanosized pores, thereby improving permeability, are applied in selective filtration, chemical detectors and molecular electronics, while with closed pores are impermeable and useful for thermal insulation. Poly (tetrafluoroethylene-co-perfluoro (alkyl vinyl ether)) (PFA) polymer is a desirable filtration membrane material because of its excellent chemical and thermal stabilities [1]. However, their inherent inert nature also enables them to be cast into membranes by conventional solution immersion casting processes. Several chemical and physical methods, including stretching [2] and internal melting mold processing [3] of other fluorpolymers, have proven impossible to fabricate well shaped microporous membranes. Moreover, these polymers have inferior stability when compared to PFA. A few authors obtained by chemical methods the fabrication of microporous PFA membranes via thermally induced phase separation [4,5]. However, the described processes are time consuming and difficult to control the pores to the nanosize scale. Successful methods using ion track/etching techniques and focused ion beam drilling followed by closure of the pores using broad plasma beam irradiation can be found in the literature [6–13], but to the best of our knowledge there is no report about the fabrication of pores in PFA thin films using ion bombardment techniques. Despite much progress,
⁎ Corresponding author. Center for Irradiation of Material, Alabama A&M University, 4900 Meridian Street, Normal, Alabama 35762, USA. Tel.: +1 256 372 5877; fax: +1 256 372 8708. E-mail address: [email protected] (D. Ila). 1 U.S. patent pending. 0257-8972/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2009.02.045
there is still a lack of studies in the creation of pores in different materials and over a wide range of diameters. The potential for such studies includes application in biological sensors for objects with different dimensions and functionalities. The ion bombarded induced damage in PFA and other fluorpolymers has been extensively reported in the literature [14–17]. Recently, N. Patterson et al. demonstrated the use of low energy ion irradiation for the direct tailoring of nanopores in silicon nitride membranes, avoiding radiation induced hole closure methods [18]. At the same time, we reported the fabrication of nanopores in PFA membranes by direct high energy ion beam drilling, using a homemade gas based feedback control system [19]. In this investigation we characterized the nanoporous membranes in PFA thin films fabricated using an innovative ion beam controlled feedback system. 2. Materials and methods PFA polymer (DuPont) has demonstrated a high level of drilling yield when bombarded with MeV Au ions due to thermal sublimation in equilibrium and non-equilibrium processes, and to molecular sputtering [14–17,19]. Mass loss of ~8 × 1014 CF2 molecules per incident ion were observed in previous experiments for 300 nA current at a fluence of 1 × 1013 ions/cm2 [20]. Samples were bombarded in a Pelletron accelerator. Films with 2 × 2 cm2 dimensions and 12.5 mm thickness were mounted as a covering window for a He gas chamber in a homemade ion beam controlled feedback system. Scheme of the system is shown in Fig. 1. The ion beam was collimated by a 2000 squares/in. Cu mask in physical contact with the polymer surface, which defines the pore distribution and avoids induced cracks during scanned ion bombardment. While bombarded with 5 MeV Au3+ ions, the He gas diffuses through the film and is detected by the residual gas analyzer (RGA) in the leakage mode. The gas flow signal increases during pore
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Fig. 3. AFM image of fabricated micropores with ~ 2 μm diameter uniformly distributed in space on the side opposite to the side bombarded with gold ions. Fig. 1. Schematic of the feedback-controlled ion beam system. The RGA detection of the He gas that leaks through the bombarded film membrane provides the signal to control the ion beam blocker (IBB) and is used as the feedback for the pore size control. When calibrated with the RGA signal, the charge signal from the beam current integrator counter can be also used to control the IBB.
formation, which is used as the feedback signal. From the difference of the slopes of the gas partial pressure versus time at initial and final conditions, and atomic force microscopy (AFM) images of the fabricated pores, one can calibrate the system. The ion beam is blocked when the desired diameter of the nanopores is obtained. Vibrational molecular spectroscopy of the bombarded sample was made with a Micro-Raman scattering analysis, performed in LabRam equipped with a He–Ne laser (λ = 632 nm) and focused inside the micro pores. 3. Results and discussion We require the fabrication of porous PFA polymer membranes with control of the spatial distribution and pore diameters. It is also desirable that the polymer membrane keeps its properties after the ion sculpturing modification, specially remaining chemically inert. Fig. 2a shows the drilled hole in the surface of the PFA bombarded film at 1 × 1013 ions/cm2. The bombardment using the Cu mask defines the square shape at the entrance of the hole, which becomes circular as the depth increases because the mask acts as a heat sink and the heat concentrates in the center of the irradiated area. This mechanism is
Fig. 4. AFM surface scan of the back surface of the PFA membrane, showing a ~ 100 nm diameter ion drilled nanopore. The image is a single pore of more than a thousand regularly spaced identical holes produced simultaneously after a few minutes ion bombardment of one PFA sample.
discussed elsewhere [20]. In order to evaluate the chemical changes in the pores induced by the ion bombardment, Raman scattering spectra were measured inside the cavity described above, as show in
Fig. 2. a) AFM image of the bombarded (top) surface of the PFA film and b) Raman spectra measured on the surface of a non bombarded sample and measured inside the micropore shown in a).
R.A. Minamisawa et al. / Surface & Coatings Technology 203 (2009) 2493–2496
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Fig. 5. AFM images of nanopores with different diameters scanned in the back surface of different PFA thin film membranes.
Fig. 2b. One can notice the non destruction of the C–F and C–O bonds of the bombarded area when compared with the non bombarded sample. Few or no indications of damage were observed for 1 × 1014 ions/cm2 accumulated fluence. At bombardment fluences higher than 5 × 1014 ions/cm2 the polymer shows indication of a carbonization process because of the mass loss by carbon–fluorine radicals emission, mainly CF3 as indicated by residual gas measurements [21]. At 1 × 1014 ions/cm2, circular micropores with ~2 μm diameter and uniform distribution in space were fabricated in the non bombarded (back) surface of the PFA thin film membrane as shown in the AFM images on Fig. 3. Distances between adjacent pores have an average of approximately 10 μm, which matches with the center of the collimation squares in the Cu masks. Nanopores with ~ 100 nm diameter are fabricated in the back surface of the bombarded films when decreasing the accumulated ion fluence to 1 × 1013 ions/cm2, as shown in the AFM image on Fig. 4. Distances between the nanopores are not shown in the figure because the scan scale necessary to obtain a suitable magnification to observe a single pore is lower than the distances observed on Fig. 3. Fig. 5 shows the topographic images and profiles of nanopores with 50, 100, 300 and 500 nm diameters fabricated with different accumulated fluence of MeV gold bombardment. Although have produced smaller pores, the observation of nanopores with diameters less than ~ 50 nm is limited by our AFM tip resolution. Valleys observed in the nanopore edges are due to AFM artifacts and were change with scanning direction. A detailed statistics of the pore diameter dependence on ion bombardment fluence will be presented elsewhere [22]. As the nanopores are an extension of the micropores drilled in the bombarded surface of the thin films, low aspect ratios are obtained that are desirable for some detection applications. The technique is able to more rapidly fabricate porous membranes when compared with focused ion beam techniques and ion track etching methods. 4. Conclusion Control of the size of pores in PFA synthetic thin film is important for each specific membrane application and may open new fields for nanotechnology studies. The homemade feedback controlled ion
beam system developed at the Center for Irradiation of Materials at Alabama A&M University is promising for nanopore ion beam fabrication and detection. The system consists in the detection of pore formation using the He gas leakage from a reservoir through a bombarded PFA film window. Bombardment using Au ions at 5 MeV energy demonstrates little damage in the chemical structure of the thin film polymer and a high sputtering yield for the effective fluence necessary to fabricate the nanopores. Diameters of the pores are easily controlled and range from micrometers to few nanometers. The technique has the advantage to produce rapidly porous membranes in relatively thick polymer films using scanned ion beam and is an alternative to focused ion beam methods. Acknowledgements This research was sponsored by the Center for Irradiation of Materials, Alabama A&M University and by the AAMURI Center for Advanced Propulsion Materials under the contract number NNM06AA12A from NASA, and by National Science Foundation under Grant No. EPS-0447675. References [1] DuPont PFA — Properties Bulletin: http://www2.dupont.com/Teflon_Industrial/ en_US/assets/downloads/h04321.pdf. [2] U.S. Pat. Nos. 3,953,566; 3,962,153; 4,096,227; 4,110,392 and 4,187,390. [3] U.S. Pat. Nos. 4,623,670 and 4,702,836. [4] U.S. Pat. No. 4,906,377. [5] M.R. Caplan, C.Y. Chiang, D.R. Lloyd, L.Y. Yen, J. Membr. Sci. 130 (1997) 219. [6] J. Li, D. Stein, C. McMullan, D. Branton, M.J. Aziz, J.A. Golovchenko, Nature 412 (2001) 166. [7] J. Li, M. Gershow, D. Stein, E. Brandin, J.A. Golovchenko, Nature Mater 2 (2003) 611. [8] D. Fologea, M. Gershow, B. Ledden, D.S. McNabb, J.A. Golovchenko, J. Li, Nanoletters 5 (2005) 1905. [9] R.R. Henriquez, T. Ito, L. Sun, R.M. Crooks, Analyst 129 (2004) 478. [10] T. Schenkel, V. Radimilovic, E.A. Stach, S.J. Park, A. Persaud, J. Vac. Tech. B 21 (2003) 2720. [11] J.A. Veerman, A.M. Otter, L. Kuipers, N.F. Van Hulst, Appl. Phys. Lett. 44 (1984) 502. [12] Z. Siwy, A. Fuliski, Phys. Rev. Lett. 89 (2002) 198103. [13] M.E. Mochel, J.A. Eades, M. Metzger, J.I. Meyer, J.M. Mochel, Appl. Phys. Lett. 44 (1984) 502. [14] M.A. Parada, R.A. Minamisawa, M.V. Moreira, A. de Almeida, I. Muntele, D. Ila, Surf. Coat. Technol. 201 (2007) 8246. [15] R.A. Minamisawa, A. de Almeida, S. Budak, V. Abidzina, D. Ila, Nucl. Instrum. Methods Phys. Res. B 261 (2007) 1159.
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[16] R.A. Minamisawa, A. de Almeida, V. Abidzina, M.A. Parada, I. Muntele, D. Ila, Nucl. Instrum. Methods Phys. Res. B 257 (2007) 568. [17] M.A. Parada, R.A. Minamisawa, A. de Almeida, C. Muntele, R.L. Zimmerman, I. Muntele, D. Ila, Braz. J. Phys. 34 (2004) 1. [18] N. Patterson, V.C. Hodges, M.J. Vasile, D.P. Adams, Z. Chen, C.J. Brinker, Mat. Res. Soc. Symp. Proc. (2007) 983. [19] R.A. Minamisawa, R.L. Zimmerman, C. Muntele, D. Ila, Mat. Res. Soc. Symp. Proc. (2007) 983.
[20] R.A. Minamisawa, R.L. Zimmerman, D. Ila, Nucl. Instrum. Methods Phys. Res. Sect. B 266 (8) (2008) 1273. [21] R.A. Minamisawa, R.L. Zimmerman, S. Budak, D. Ila, Nucl. Instrum. Methods B 266 (2008) 1269. [22] Minamisawa R. A., Zimmerman R. L. and Ila D., (to be published).