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Study of ion tracks by micro-probe ion energy loss spectroscopy
Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Study of ion tracks by micro-probe ion energy loss spectroscopy J. Vacik a,⇑, V. Havranek a, V. Hnatowicz a, P. Horak a, D. Fink a, P. Apel b a b
Nuclear Physics Institute ASCR, v.v.i., 25068 Rez, Czech Republic Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Russia
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Ion energy loss spectrometry Single ion track Microprobe Tomography
a b s t r a c t Structural changes of ion-irradiated polymers can be visualized by wet chemical etching. In this process, the latent tracks can be developed into pores of various shapes and sizes. To nondestructively determine the 3D structure (spatial density distribution) of an individual pore, a new approach, utilizing energy loss spectroscopy of ions transmitted through a pore and its vicinity has been tested. To evaluate the density alterations in the pore, a new Monte Carlo simulation code was developed. The code calculates residual energy of ions passing through the pore of a selected shape and compares the data with the measured energy spectra. The best fit determines the appropriate geometrical parameters of the pore. In the paper, details of a tomographic examination of a single pore in a thin polymer foil are presented and discussed for the first time. Ó 2014 Published by Elsevier B.V.
1. Introduction Energetic heavy ions (with high LET values of about 10 keV/nm) passing through thin films of polymers trigger formation of latent tracks consisting of an amorphized core (of several nanometers in diameter) and a less damaged ‘penumbra’ (100 nm). It is supposed that the latent tracks have a tube-like form and exhibit irreversible alterations in their molecular structures and changes in their macroscopic (e.g., chemical, electrical and optical) properties [1,2]. The swift heavy ions cause molecular bond cleavage producing free radical species, which are responsible for the main chemical transformations in ion irradiated polymers, e.g., chain scission, cross-linking, double and triple bond formation, and also emission of volatile molecules and molecular fragments [3,4]. The latent tracks can be visualized by selective (wet) chemical etching that transforms the latent tracks into hollows that can acquire a wide variety of (axially symmetric) shapes, e.g., long cylinders, short cones, hemispheres, etc. In order to prepare nuclear tracks (i.e., pores) with a certain shape, the process of chemical etching must satisfy certain conditions [5]: long UV exposition (e.g., 24 h) before etching, appropriate etching medium (e.g., 3– 5 M NaOH, KOH, LiOH, etc.), appropriate etching temperature (typically 40–80 °C) and time (e.g., 10 min to 10 h), etc. The etching process is performed either on one or both sides of the polymers, continuously or steppedly, with or without surfactants, etc. In ⇑ Corresponding author. Address: Nuclear Physics Institute, Academy of Sciences of the Czech Republic, 250 68 Husinec – Rez, Czech Republic. Tel.: +420 266173129. E-mail address: [email protected] (J. Vacik).
the etching process, the track to bulk etch rate ratio VT/VB is the determining factor for the pore shape (VB depends on material properties and etching conditions; VT on bombarding particles, material properties, etching conditions, etc.), [5]. Generally, the nuclear pores can be utilized in different applications, e.g., in filter membranes, catalysis, optoelectronics, microelectronics, biotechnology, etc. [6,7]. Recently, also systems with single pores have been applied, e.g., in the study of nonlinear electrochemical processes [8], single molecule or nanoparticle transportation [9–10], single-pore ion detectors [11], voltage sensors [12], biosensors [13–15], etc. For all applications, information of the pores’ basic parameters (i.e., their size and area density) is important; for certain purposes (e.g., for single-pore biosensors) information on the pore’s 3D geometry is significant. Though the density and pore size distribution can be analyzed by various microscopic (SEM, HRTEM, AFM), scattering (SANS, SAXS) or other (e.g., conductometric) nondestructive techniques, the 3D shape of the pores is difficult to obtain (several techniques, such as FIB-SEM [16] or electrochemical replication [17], enable reconstruction of the 3D geometry of the individual pores; they are however fully destructive and for the repeated analysis inapplicable). Over the last decade, an additional approach for a direct and nondestructive analysis of the pore shape, i.e., ion energy loss spectroscopy, has been introduced [18–21]. This technique is based on analysis of the residual energy of monoenergetic ions passing through the thin foils with nuclear tracks. As was shown in the first report [18], the ion transmission energy spectrum usually consists of 2 dominant lines: FEP – full energy peak (corresponding to the
http://dx.doi.org/10.1016/j.nimb.2014.02.084 0168-583X/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: J. Vacik et al., Study of ion tracks by micro-probe ion energy loss spectroscopy, Nucl. Instr. Meth. B (2014), http:// dx.doi.org/10.1016/j.nimb.2014.02.084
2
J. Vacik et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
ions that pass through the pores without energy loss) and REP – reduced energy peak (corresponding to the ions transmitted through the pristine part of the foil). Important, however, is the part of the spectra between the FEP and REP peaks that is determined by the geometry of the pores (and ion beam divergence). To reconstruct the 3D geometry of the pores from the experimental data, the transmitted energy spectra have to be fitted with the MC simulation patterns based on an appropriate pore shape. The ion energy loss spectroscopy can utilize any ion source producing low-intensity (<103 s 1) (quasi) monoenergetic ion beams. In papers [18–20] a thin 241Am radioactive source was utilized to demonstrate the feasibility of the novel method. In [21] the authors tested milli-probes to analyze the process of gradual etching, and well-etched or disrupted pores. As a milli-probe, alpha particles backscattered from 20 nm thin Au foil, were utilized. In the paper, a first attempt to study individual pores using a micro-probe facility (with a spatial resolution 1 lm) was also tested. In this work, the ion energy loss spectroscopy is applied for a study of individual, micrometer-sized pores. This study was carried out for the first time. For this sake, a low intensity micro-probe with a specially developed detector system was prepared. In the paper, details on ion energy loss spectroscopy of porous foil and analysis of an individual pore with a newly-developed simulation code are given.
2. Experimental The experiments were performed at the CANAM infrastructure [22] in NPI ASCR Rez, i.e., on the Tandetron accelerator (HV Engineering, 4130 MC) utilizing the 4 MeV alpha particles in the micro-probe facility (Oxford Instruments) with an intensity <103 particles s 1, scanning area from 5 5 lm2 to 500 500 lm2, and spatial resolution of 0.6 lm. The ion beam divergence was <1°, the energy resolution of the system 18 keV. The transmitted ions were detected with the (windowless) Hamamatsu PIN diode (with the restricted sensitive area of 8 8 mm2), set in 10 cm behind the foil (the solid angle of the foil-detector system was <10 3 sr) in a shiftable frame allowing for change of the detector’s position and thus avoiding its radiation damage. The energy spectra were measured by the STIM mode (Scanning Transmission Ion Microscopy, Oxford Microbeams Ltd., OMDAQ 2007) that provides information on the density variation within the foil (but does not provide information on the 3D pore shape). For the transmission tests, polyethylene terephthalate [PETP – (C10H8O4)n] foils, with a thickness of 11 lm, have been utilized. The foils were irradiated with the 11.4 MeV/u Xe+ ions up to the fluence 5 105 cm 2 in JINR Dubna [23], and etched in standard NaOH solution (6 mol/l) to form large pore with the lateral size of several micrometers. For reconstruction of the pore geometry, a Monte Carlo (MC) tomographic code was newly developed. As proposed, the code calculates the energy loss spectra of charged particles, transmitted through the foil, by numerical integration. The code sets conditions for several following parameters: (quasi) monoenergetic ion beam with assigned energy E in the MeV range, definable beam divergence d (an additional small divergence surplus of the beam, caused by the multiple scattering of ions passing through the examined foil, was not taken into account), known composition of foils, thickness d in the 100–102 lm range, known area density of the pores n (cm 2), and pore chaotic dispersion and orientation (parallel or divergent). It is supposed that ions can randomly penetrate the foil either through the pores, or virgin parts or their combinations in directions given by the ion beam divergence (see
ION SOURCE OPTION
Y X FRONT SIDE
r0
PORE
FOIL
d r1
REAR SIDE
RD
PARTICLE DETECTOR Fig. 1. Principle conditions of the MC code. Ion source: monoenergetic ions, energy E in the MeV range, definable beam divergence d. Polymer foil: composition in atomic concentration, thickness d in the 100–102 lm range, area density of the pores n (cm 2), pore chaotic dispersion and orientation. Numerical integration: ions can penetrate the foil through the pores, virgin parts or their combinations in directions given by the ion beam divergence; the partial energy spectra depend on the pore shape, and the relative position of the ion source, pores and the detector; geometry of pores is selected for simulation; final spectrum is a sum of all partial spectra, partial stopping powers are calculated using the SRIM code data.
Fig. 1). The partial energy spectra depend on the foil thickness d, pore shape (e.g., on the entrance and exit radii, r0 and r1, in the case of a simple conical form), and the relative position of the ion source, the individual pore and the detector (with the radius R). The resulting spectrum is obtained as a sum of all particular spectra, taking into account the density of the pores n as an input parameter. The composition of the foil is given in atomic concentrations and the particle stopping powers are calculated using the SRIM code [24]. The MC simulation is a procedure with several steps: in the first step, certain (assumed) geometry of the pore is selected and the MC computation (with other E, d, d, R assigned and unvarying parameters) is carried out. The first evaluation is usually not satisfying, so other steps with better specified pore parameters (e.g., entrance, inner and exit radii) have to be performed till a most appropriate fit is achieved. Generally, the code enables simulation of the energy spectra for ions passing through thin films with pores of different shapes (e.g., conical, cylindrical, parabolic, etc.), partly etched, empty or also filled with material of different densities. Comparing the simulated data with the experimental spectra, the pore 3D geometry can be obtained. The code can be used for analysis of both (i) single pores, using a micro-probe set-up with a small scanning area (to find geometrical parameters of the individual pores), or (ii) a multiple pore system, using a milli-probe beam arrangement (in this case an average pore parameters can be determined). Here, ion energy loss spectroscopy was carried out for inspection of a single pore only. 3. Results and discussion Fig. 2 shows the energy spectrum of the 4 MeV alpha particles transmitted through a small scanning area (10 10 lm2) of a thin
Please cite this article in press as: J. Vacik et al., Study of ion tracks by micro-probe ion energy loss spectroscopy, Nucl. Instr. Meth. B (2014), http:// dx.doi.org/10.1016/j.nimb.2014.02.084
J. Vacik et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
105
4 MeV α
Energy slices
01 02 03 04 05 06 07 08 09 10 11 12
REP
FEP
COUNTS
104 103 102 101
experimental data MC simulation
100 2100 2400 2700 3000 3300 3600 3900 4200
ENERGY [ keV ] Fig. 2. Energy spectrum of the 4 MeV alpha particles transmitted through an 11 lm thin PETF foil with a single large pore. The spectrum consists of the FEP and REP peaks, corresponding to the ion transmission through the pore or virgin surroundings, respectively. To reconstruct the pore shape, the spectrum was analyzed by the MC code. The appropriate fit (see the full line) determines the parameters of the pore (Fig. 4).
(11 lm) PETP foil with a large-size single pore. The spectrum consists of 2 peaks (FEP and REP) with energies of 4 MeV (FEP) and 2.6 MeV (REP) that correspond to the transmission of the 4 MeV alphas either through the empty etched pore (FEP), or through its virgin surroundings (REP). As can be seen, the FEP peak is less pronounced than REP – this is because the pore occupies only 15% of the scanning area, though the virgin part occupies the rest. The widths of the peaks are also different: for the FEP the FWHM (20 keV) is close to the energy resolution of the spectrometer (18 keV), much higher FWHM (66 keV) for the REP is due to the energy straggling of the transmitted ions. The energy part between the FEP and REP peaks, spanning the diapason of 1.4 MeV, corresponds to the transmission of the alpha particles through and around the wall of the pore. This part of the spectrum is relatively flat with low-level values (300), compared to the REP (15 000) or FEP (8000) peaks, which indicates that the wall of the pore is relatively smooth and narrow. This outcome is confirmed by
3
the STIM analysis (Fig. 3) that shows the area distribution of the transmitted ions with a certain portion of the energy loss. In the series of the 12 maps (that correspond to the equiparted energyloss slices, depicted in the energy spectrum, see Fig. 2), the area of the wall is seen as narrow rings (maps #05–#11). The rings (because of a slight, erroneous inclination of the foil against the ion beam, they look like ovals) are changing size – the largest is in the map #05, the smallest in #11 (the difference is about 20%). It is assumed that the reason should be a gradual change of the inner pore diameter – the largest is, obviously, at the entrance port, the smallest at the exit. Interesting are certain inhomogeneities, seen in distribution of the wall rings – one can see several ‘hot spots’ changing position around. The existence of the ‘hot spots’ indicates that the wall is semiporous with ‘hot spot’ zones of varying density. Reconstruction of the pore shape was made with the help of the MC code, developed for simulation of the ion energy loss spectra (see above). In Fig. 2 is seen the most appropriate fit (black full line) of the spectrum that was achieved after several steps of the fitting procedure. The fit copies the spectrum in all parts with a high precision (a maximal difference of 5% is estimated). It was computed with an assumption that the pore has a simple cylindrical-conical form with a smooth wall and plain front and rear surfaces without humps that might appear after etching of the ion irradiated foils. The result of the MC simulation is shown in the Fig. 4, where is seen the dependence of the local pore radius r on the depth of the pore. It is evident that the examined pore has a slightly conical spatial form with the anterior port radius r0 = 2.1 lm, and the rear port radius r1 = 1.65 lm. The inner part of the pore is formed with a straight wall spanning the front and rear ports with no sharp alteration. In the ion energy loss tests, also other pores, etched into different geometries (e.g., cylindrical, double-conical, etc.), or filled with the Al and Cu metals (by mechanical or electrochemical methods), have been examined. It was confirmed that the energy spectra are very dependent (i) on the shape of the pores, and (ii) on presence of different materials in the pores. The transmission spectrum pattern depends on the quality of the pore hollow, especially on smoothness of the wall, the presence of remnants (from etching), or any collapse of the hollow. Imperfection may result in a dramatic
01
02
03
04
05
06
07
08
09
10
11
12
Fig. 3. A series of the 12 STIM (Scanning Transmission Ion Microscopy) maps corresponding to the equiparted energy-loss slices of the energy spectrum.
Please cite this article in press as: J. Vacik et al., Study of ion tracks by micro-probe ion energy loss spectroscopy, Nucl. Instr. Meth. B (2014), http:// dx.doi.org/10.1016/j.nimb.2014.02.084
3.5 3.0
REAR
4.0
FRONT
J. Vacik et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
PORE DEPTH PROFILE
2.5 2.0
1.0 0.5 0.0
r1= 1.65μm
1.5
randomly-shaped material inhomogeneities, however, the method has to carry out a set of energy loss tomographic scans from different inclining angles. This is technically more complicated but also a challenging goal. Acknowledgements
r0= 2.10μm
LOCAL PORE RADIUS r[ μm ]
4
d = 10.75 μm
0
1
2
3
4
5
6
7
The authors acknowledge financial support from the Grant Agency of the Czech Republic, Grant Project No. P108/12/G108. The experiments were carried out at the CANAM infrastructure of the NPI ASCR Rez. Reference
8
9
10 11 12
DEPTH [ μm ] Fig. 4. Dependence of the local pore radius on the depth of the pore, obtained by the MC simulation of the energy spectrum (full line in Fig. 2).
alteration of the FEP peak and especially the FEP–REP part. It was found that the simple MC code is applicable only for high-quality pores with no (or only small) structural deficiency. To analyze individual pores with defects, the MC code should accept a tomographic approach with multiple energy loss scans from different inclining angles. 4. Conclusions In the paper, a nondestructive approach (i.e., ion energy loss spectroscopy) for the study of individual (or also multiple) nuclear tracks (pores) in polymers was discussed. The method enables determination of the 3D spatial form of the pore, and also study of dynamic processes, such as gradual pore evolution in the etching procedure, or filling of the pore hollow with materials of different densities. The technique is based on the MC simulation of energy loss spectra of ions passing through the pore system of examined foils. The pore geometry is given by the best fit of the transmitted energy spectra that is achieved by gradual variation of the pore shape parameters. In conclusion, the ion energy loss spectroscopy with a microprobe beam is an effective (but still novel) method that enables to obtain information about individual (or multiple) nuclear tracks in polymers (e.g., 3D geometry) that cannot be achieved nondestructively by other common techniques. The study of the nuclear tracks is simplified due to an axial symmetry of the tracks. For
[1] D. Fink (Ed.), Fundamentals of Ion-Irradiated Polymers, Springer Series in Material Science, Springer-Verlag, Berlin, 2004. [2] R. Spohr, Ion Tracks and Microtechnology, Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig, 1990. [3] J. Davenas, G. Boiteux, X.L. Xu, E. Adem, Nucl. Instr. Meth. B 32 (1988) 136. [4] S. Bouffard, B. Gervais, C. Leroy, Nucl. Instr. Meth. B 105 (1995) 1. [5] D. Fink (Ed.), Transport Processes in Ion-Irradiated Polymers, Springer Series in Material Science, Springer-Verlag, Berlin, 2004. [6] P. Buford, Price, Radiat. Meas. 40 (2003) 146. [7] A. Weidinger, Europhys. News 35 (5) (2004) 152. [8] M.R. Powell, M. Sullivan, I. Vlassiouk, D. Constantin, O. Sudre, C.C. Martens, R.S. Eisenberg, Z.S. Siwy, Nat. Nanotechnol. 3 (2008) 51. [9] A. Mara, Z. Siwy, C. Trautmann, J. Wan, F. Kamme, Nano Lett. 4 (3) (2004) 497. [10] M. Davenport, K. Healy, M. Pevarnik, N. Teslich, S. Cabrini, A.P. Morrison, Z.S. Siwy, S.E. Letant, ACS Nano 6 (9) (2012) 8366. [11] E.B. Kalman, O. Sudre, I. Vlassiouk, Z.S. Siwy, Anal. Bioanal. Chem. 394 (2) (2009) 413. [12] Z. Siwy, Y. Gu, H.A. Spohr, D. Baur, A. Wolf-Reber, R. Spohr, P. Apel, Y.E. Korchev, Europhys. Lett. 60 (2002) 349. [13] A. Oukhaled, L. Bacri, M. Pastoriza-Gallego, J.M. Betton, J. Pelta, ACS Chem. Biol. 7 (12) (2012) 1935. [14] D. Fink, H. Gerardo Muñoz, L. Alfonta, Y. Mandabi, J.F. Dias, C.T. de Souza, L.E. Bacakova, J. Vacik, V. Hnatowicz, A.E. Kiv, D. Fuks, R.M. Papalo, NATO Sci. Peace Secur. Ser. B: Phys. Biophys. 2 (2012) 269. [15] G.L. Hou, Z.J. Peng, Y. Tian, H.C. Zhang, L. Juany, Chin. Sci. Bull. 58 (13) (2013) 1473. [16] W. Xu, M. Ferry, N. Mateescu, J.M. Cairney, F.J. Humphreys, Mater. Charact. 58 (10) (2007) 961. [17] D. Dobrev, J. Vetter, R. Neumann, N. Angert, J. Vac. Sci. Technol. B: Microelectron. Nanometer Struct. 19 (4) (2001) 1385. [18] J. Vacik, J. Cervena, V. Hnatowicz, D. Fink, P.Yu. Apel, P. Strauss, Nucl. Instr. Meth. B 146 (1998) 475. [19] D. Fink, M. Muller, J. Vacik, J. Cervena, V. Hnatowicz, Appl. Phys. A 68 (1999) 87. [20] J. Vacik, J. Cervena, V. Hnatowicz, S. Posta, D. Fink, R. Klett, P. Strauss, Surf. Coat. Technol. 123 (2000) 97. [21] J. Vacik, V. Havranek, V. Hnatowicz, V. Lavrentiev, P. Horak, AIP Conf. Proc. 1525 (2013) 663. [22]
Please cite this article in press as: J. Vacik et al., Study of ion tracks by micro-probe ion energy loss spectroscopy, Nucl. Instr. Meth. B (2014), http:// dx.doi.org/10.1016/j.nimb.2014.02.084
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Study of ion tracks by micro-probe ion energy loss spectroscopy J. Vacik a,⇑, V. Havranek a, V. Hnatowicz a, P. Horak a, D. Fink a, P. Apel b a b
Nuclear Physics Institute ASCR, v.v.i., 25068 Rez, Czech Republic Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Russia
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Ion energy loss spectrometry Single ion track Microprobe Tomography
a b s t r a c t Structural changes of ion-irradiated polymers can be visualized by wet chemical etching. In this process, the latent tracks can be developed into pores of various shapes and sizes. To nondestructively determine the 3D structure (spatial density distribution) of an individual pore, a new approach, utilizing energy loss spectroscopy of ions transmitted through a pore and its vicinity has been tested. To evaluate the density alterations in the pore, a new Monte Carlo simulation code was developed. The code calculates residual energy of ions passing through the pore of a selected shape and compares the data with the measured energy spectra. The best fit determines the appropriate geometrical parameters of the pore. In the paper, details of a tomographic examination of a single pore in a thin polymer foil are presented and discussed for the first time. Ó 2014 Published by Elsevier B.V.
1. Introduction Energetic heavy ions (with high LET values of about 10 keV/nm) passing through thin films of polymers trigger formation of latent tracks consisting of an amorphized core (of several nanometers in diameter) and a less damaged ‘penumbra’ (100 nm). It is supposed that the latent tracks have a tube-like form and exhibit irreversible alterations in their molecular structures and changes in their macroscopic (e.g., chemical, electrical and optical) properties [1,2]. The swift heavy ions cause molecular bond cleavage producing free radical species, which are responsible for the main chemical transformations in ion irradiated polymers, e.g., chain scission, cross-linking, double and triple bond formation, and also emission of volatile molecules and molecular fragments [3,4]. The latent tracks can be visualized by selective (wet) chemical etching that transforms the latent tracks into hollows that can acquire a wide variety of (axially symmetric) shapes, e.g., long cylinders, short cones, hemispheres, etc. In order to prepare nuclear tracks (i.e., pores) with a certain shape, the process of chemical etching must satisfy certain conditions [5]: long UV exposition (e.g., 24 h) before etching, appropriate etching medium (e.g., 3– 5 M NaOH, KOH, LiOH, etc.), appropriate etching temperature (typically 40–80 °C) and time (e.g., 10 min to 10 h), etc. The etching process is performed either on one or both sides of the polymers, continuously or steppedly, with or without surfactants, etc. In ⇑ Corresponding author. Address: Nuclear Physics Institute, Academy of Sciences of the Czech Republic, 250 68 Husinec – Rez, Czech Republic. Tel.: +420 266173129. E-mail address: [email protected] (J. Vacik).
the etching process, the track to bulk etch rate ratio VT/VB is the determining factor for the pore shape (VB depends on material properties and etching conditions; VT on bombarding particles, material properties, etching conditions, etc.), [5]. Generally, the nuclear pores can be utilized in different applications, e.g., in filter membranes, catalysis, optoelectronics, microelectronics, biotechnology, etc. [6,7]. Recently, also systems with single pores have been applied, e.g., in the study of nonlinear electrochemical processes [8], single molecule or nanoparticle transportation [9–10], single-pore ion detectors [11], voltage sensors [12], biosensors [13–15], etc. For all applications, information of the pores’ basic parameters (i.e., their size and area density) is important; for certain purposes (e.g., for single-pore biosensors) information on the pore’s 3D geometry is significant. Though the density and pore size distribution can be analyzed by various microscopic (SEM, HRTEM, AFM), scattering (SANS, SAXS) or other (e.g., conductometric) nondestructive techniques, the 3D shape of the pores is difficult to obtain (several techniques, such as FIB-SEM [16] or electrochemical replication [17], enable reconstruction of the 3D geometry of the individual pores; they are however fully destructive and for the repeated analysis inapplicable). Over the last decade, an additional approach for a direct and nondestructive analysis of the pore shape, i.e., ion energy loss spectroscopy, has been introduced [18–21]. This technique is based on analysis of the residual energy of monoenergetic ions passing through the thin foils with nuclear tracks. As was shown in the first report [18], the ion transmission energy spectrum usually consists of 2 dominant lines: FEP – full energy peak (corresponding to the
http://dx.doi.org/10.1016/j.nimb.2014.02.084 0168-583X/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: J. Vacik et al., Study of ion tracks by micro-probe ion energy loss spectroscopy, Nucl. Instr. Meth. B (2014), http:// dx.doi.org/10.1016/j.nimb.2014.02.084
2
J. Vacik et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
ions that pass through the pores without energy loss) and REP – reduced energy peak (corresponding to the ions transmitted through the pristine part of the foil). Important, however, is the part of the spectra between the FEP and REP peaks that is determined by the geometry of the pores (and ion beam divergence). To reconstruct the 3D geometry of the pores from the experimental data, the transmitted energy spectra have to be fitted with the MC simulation patterns based on an appropriate pore shape. The ion energy loss spectroscopy can utilize any ion source producing low-intensity (<103 s 1) (quasi) monoenergetic ion beams. In papers [18–20] a thin 241Am radioactive source was utilized to demonstrate the feasibility of the novel method. In [21] the authors tested milli-probes to analyze the process of gradual etching, and well-etched or disrupted pores. As a milli-probe, alpha particles backscattered from 20 nm thin Au foil, were utilized. In the paper, a first attempt to study individual pores using a micro-probe facility (with a spatial resolution 1 lm) was also tested. In this work, the ion energy loss spectroscopy is applied for a study of individual, micrometer-sized pores. This study was carried out for the first time. For this sake, a low intensity micro-probe with a specially developed detector system was prepared. In the paper, details on ion energy loss spectroscopy of porous foil and analysis of an individual pore with a newly-developed simulation code are given.
2. Experimental The experiments were performed at the CANAM infrastructure [22] in NPI ASCR Rez, i.e., on the Tandetron accelerator (HV Engineering, 4130 MC) utilizing the 4 MeV alpha particles in the micro-probe facility (Oxford Instruments) with an intensity <103 particles s 1, scanning area from 5 5 lm2 to 500 500 lm2, and spatial resolution of 0.6 lm. The ion beam divergence was <1°, the energy resolution of the system 18 keV. The transmitted ions were detected with the (windowless) Hamamatsu PIN diode (with the restricted sensitive area of 8 8 mm2), set in 10 cm behind the foil (the solid angle of the foil-detector system was <10 3 sr) in a shiftable frame allowing for change of the detector’s position and thus avoiding its radiation damage. The energy spectra were measured by the STIM mode (Scanning Transmission Ion Microscopy, Oxford Microbeams Ltd., OMDAQ 2007) that provides information on the density variation within the foil (but does not provide information on the 3D pore shape). For the transmission tests, polyethylene terephthalate [PETP – (C10H8O4)n] foils, with a thickness of 11 lm, have been utilized. The foils were irradiated with the 11.4 MeV/u Xe+ ions up to the fluence 5 105 cm 2 in JINR Dubna [23], and etched in standard NaOH solution (6 mol/l) to form large pore with the lateral size of several micrometers. For reconstruction of the pore geometry, a Monte Carlo (MC) tomographic code was newly developed. As proposed, the code calculates the energy loss spectra of charged particles, transmitted through the foil, by numerical integration. The code sets conditions for several following parameters: (quasi) monoenergetic ion beam with assigned energy E in the MeV range, definable beam divergence d (an additional small divergence surplus of the beam, caused by the multiple scattering of ions passing through the examined foil, was not taken into account), known composition of foils, thickness d in the 100–102 lm range, known area density of the pores n (cm 2), and pore chaotic dispersion and orientation (parallel or divergent). It is supposed that ions can randomly penetrate the foil either through the pores, or virgin parts or their combinations in directions given by the ion beam divergence (see
ION SOURCE OPTION
Y X FRONT SIDE
r0
PORE
FOIL
d r1
REAR SIDE
RD
PARTICLE DETECTOR Fig. 1. Principle conditions of the MC code. Ion source: monoenergetic ions, energy E in the MeV range, definable beam divergence d. Polymer foil: composition in atomic concentration, thickness d in the 100–102 lm range, area density of the pores n (cm 2), pore chaotic dispersion and orientation. Numerical integration: ions can penetrate the foil through the pores, virgin parts or their combinations in directions given by the ion beam divergence; the partial energy spectra depend on the pore shape, and the relative position of the ion source, pores and the detector; geometry of pores is selected for simulation; final spectrum is a sum of all partial spectra, partial stopping powers are calculated using the SRIM code data.
Fig. 1). The partial energy spectra depend on the foil thickness d, pore shape (e.g., on the entrance and exit radii, r0 and r1, in the case of a simple conical form), and the relative position of the ion source, the individual pore and the detector (with the radius R). The resulting spectrum is obtained as a sum of all particular spectra, taking into account the density of the pores n as an input parameter. The composition of the foil is given in atomic concentrations and the particle stopping powers are calculated using the SRIM code [24]. The MC simulation is a procedure with several steps: in the first step, certain (assumed) geometry of the pore is selected and the MC computation (with other E, d, d, R assigned and unvarying parameters) is carried out. The first evaluation is usually not satisfying, so other steps with better specified pore parameters (e.g., entrance, inner and exit radii) have to be performed till a most appropriate fit is achieved. Generally, the code enables simulation of the energy spectra for ions passing through thin films with pores of different shapes (e.g., conical, cylindrical, parabolic, etc.), partly etched, empty or also filled with material of different densities. Comparing the simulated data with the experimental spectra, the pore 3D geometry can be obtained. The code can be used for analysis of both (i) single pores, using a micro-probe set-up with a small scanning area (to find geometrical parameters of the individual pores), or (ii) a multiple pore system, using a milli-probe beam arrangement (in this case an average pore parameters can be determined). Here, ion energy loss spectroscopy was carried out for inspection of a single pore only. 3. Results and discussion Fig. 2 shows the energy spectrum of the 4 MeV alpha particles transmitted through a small scanning area (10 10 lm2) of a thin
Please cite this article in press as: J. Vacik et al., Study of ion tracks by micro-probe ion energy loss spectroscopy, Nucl. Instr. Meth. B (2014), http:// dx.doi.org/10.1016/j.nimb.2014.02.084
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ENERGY [ keV ] Fig. 2. Energy spectrum of the 4 MeV alpha particles transmitted through an 11 lm thin PETF foil with a single large pore. The spectrum consists of the FEP and REP peaks, corresponding to the ion transmission through the pore or virgin surroundings, respectively. To reconstruct the pore shape, the spectrum was analyzed by the MC code. The appropriate fit (see the full line) determines the parameters of the pore (Fig. 4).
(11 lm) PETP foil with a large-size single pore. The spectrum consists of 2 peaks (FEP and REP) with energies of 4 MeV (FEP) and 2.6 MeV (REP) that correspond to the transmission of the 4 MeV alphas either through the empty etched pore (FEP), or through its virgin surroundings (REP). As can be seen, the FEP peak is less pronounced than REP – this is because the pore occupies only 15% of the scanning area, though the virgin part occupies the rest. The widths of the peaks are also different: for the FEP the FWHM (20 keV) is close to the energy resolution of the spectrometer (18 keV), much higher FWHM (66 keV) for the REP is due to the energy straggling of the transmitted ions. The energy part between the FEP and REP peaks, spanning the diapason of 1.4 MeV, corresponds to the transmission of the alpha particles through and around the wall of the pore. This part of the spectrum is relatively flat with low-level values (300), compared to the REP (15 000) or FEP (8000) peaks, which indicates that the wall of the pore is relatively smooth and narrow. This outcome is confirmed by
3
the STIM analysis (Fig. 3) that shows the area distribution of the transmitted ions with a certain portion of the energy loss. In the series of the 12 maps (that correspond to the equiparted energyloss slices, depicted in the energy spectrum, see Fig. 2), the area of the wall is seen as narrow rings (maps #05–#11). The rings (because of a slight, erroneous inclination of the foil against the ion beam, they look like ovals) are changing size – the largest is in the map #05, the smallest in #11 (the difference is about 20%). It is assumed that the reason should be a gradual change of the inner pore diameter – the largest is, obviously, at the entrance port, the smallest at the exit. Interesting are certain inhomogeneities, seen in distribution of the wall rings – one can see several ‘hot spots’ changing position around. The existence of the ‘hot spots’ indicates that the wall is semiporous with ‘hot spot’ zones of varying density. Reconstruction of the pore shape was made with the help of the MC code, developed for simulation of the ion energy loss spectra (see above). In Fig. 2 is seen the most appropriate fit (black full line) of the spectrum that was achieved after several steps of the fitting procedure. The fit copies the spectrum in all parts with a high precision (a maximal difference of 5% is estimated). It was computed with an assumption that the pore has a simple cylindrical-conical form with a smooth wall and plain front and rear surfaces without humps that might appear after etching of the ion irradiated foils. The result of the MC simulation is shown in the Fig. 4, where is seen the dependence of the local pore radius r on the depth of the pore. It is evident that the examined pore has a slightly conical spatial form with the anterior port radius r0 = 2.1 lm, and the rear port radius r1 = 1.65 lm. The inner part of the pore is formed with a straight wall spanning the front and rear ports with no sharp alteration. In the ion energy loss tests, also other pores, etched into different geometries (e.g., cylindrical, double-conical, etc.), or filled with the Al and Cu metals (by mechanical or electrochemical methods), have been examined. It was confirmed that the energy spectra are very dependent (i) on the shape of the pores, and (ii) on presence of different materials in the pores. The transmission spectrum pattern depends on the quality of the pore hollow, especially on smoothness of the wall, the presence of remnants (from etching), or any collapse of the hollow. Imperfection may result in a dramatic
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Fig. 3. A series of the 12 STIM (Scanning Transmission Ion Microscopy) maps corresponding to the equiparted energy-loss slices of the energy spectrum.
Please cite this article in press as: J. Vacik et al., Study of ion tracks by micro-probe ion energy loss spectroscopy, Nucl. Instr. Meth. B (2014), http:// dx.doi.org/10.1016/j.nimb.2014.02.084
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randomly-shaped material inhomogeneities, however, the method has to carry out a set of energy loss tomographic scans from different inclining angles. This is technically more complicated but also a challenging goal. Acknowledgements
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The authors acknowledge financial support from the Grant Agency of the Czech Republic, Grant Project No. P108/12/G108. The experiments were carried out at the CANAM infrastructure of the NPI ASCR Rez. Reference
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DEPTH [ μm ] Fig. 4. Dependence of the local pore radius on the depth of the pore, obtained by the MC simulation of the energy spectrum (full line in Fig. 2).
alteration of the FEP peak and especially the FEP–REP part. It was found that the simple MC code is applicable only for high-quality pores with no (or only small) structural deficiency. To analyze individual pores with defects, the MC code should accept a tomographic approach with multiple energy loss scans from different inclining angles. 4. Conclusions In the paper, a nondestructive approach (i.e., ion energy loss spectroscopy) for the study of individual (or also multiple) nuclear tracks (pores) in polymers was discussed. The method enables determination of the 3D spatial form of the pore, and also study of dynamic processes, such as gradual pore evolution in the etching procedure, or filling of the pore hollow with materials of different densities. The technique is based on the MC simulation of energy loss spectra of ions passing through the pore system of examined foils. The pore geometry is given by the best fit of the transmitted energy spectra that is achieved by gradual variation of the pore shape parameters. In conclusion, the ion energy loss spectroscopy with a microprobe beam is an effective (but still novel) method that enables to obtain information about individual (or multiple) nuclear tracks in polymers (e.g., 3D geometry) that cannot be achieved nondestructively by other common techniques. The study of the nuclear tracks is simplified due to an axial symmetry of the tracks. For
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Please cite this article in press as: J. Vacik et al., Study of ion tracks by micro-probe ion energy loss spectroscopy, Nucl. Instr. Meth. B (2014), http:// dx.doi.org/10.1016/j.nimb.2014.02.084