Surface layer morphology of the high fluence Fe implanted polyethylene - Correlation with the magnetic and optical behavior

Journal Pre-proof Surface Layer Morphology of the High Fluence Fe Implanted Polyethylene Correlation with the Magnetic and Optical Behavior Danilo D. Kisić, Miloš T. Nenadović, Jelena M. Potočnik, Mirjana Novaković, Pavol Noga, Dušan Vaňa, Anna Závacká, Zlatko Lj. Rakočević PII:

S0042-207X(19)32270-5

DOI:

https://doi.org/10.1016/j.vacuum.2019.109016

Reference:

VAC 109016

To appear in:

Vacuum

Received Date: 8 July 2019 Revised Date:

10 October 2019

Accepted Date: 14 October 2019

Please cite this article as: Kisić DD, Nenadović MT, Potočnik JM, Novaković M, Noga P, Vaňa D, Závacká A, Rakočević ZL, Surface Layer Morphology of the High Fluence Fe Implanted Polyethylene - Correlation with the Magnetic and Optical Behavior, Vacuum, https://doi.org/10.1016/ j.vacuum.2019.109016. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Table 1. The values of implantation fluences (F.) measured by current integrator (C. I.), RBS, surface Fe concentrations, and maximum Fe concentrations of the Fe concentration profiles. F. C. I. [1017 cm-2]

F. RBS [1017 cm-2]

S. Fe Concentration [at.%]

Max. Fe Concentration [at.%]

0.5

0.53

0.16

5.5

1

0.97

0.30

9.9

2

1.84

0.78

20.7

5

3.14

19.87

46.1

Ion beam implantation of Polyethylene by 56Fe+ ions, using different fluences Fluence dependent changes in morphology of the Fe/Polyethylene surface layer Implantation induced superparamagnetic like behavior and ferromagnetic behavior Implantation induced absorption peak in the UV region of the UV-VIS spectra

Declaration of interests ☒The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Surface Layer Morphology of the High Fluence Fe Implanted Polyethylene - Correlation with the Magnetic and Optical Behavior

Danilo D. Kisića, Miloš T. Nenadovića, Jelena M. Potočnika, Mirjana Novakovića, Pavol Nogab, DušanVaňab, Anna Závackáb and Zlatko Lj. Rakočevića a) University of Belgrade, Vinča Institute of Nuclear Sciences, Laboratory of Atomic Physics, Mike Petrovića Alasa 12-14, 11351, Belgrade, Serbia b) Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology in Trnava, Advanced Technologies Research Institute, Jána Bottu 25, 91724 Trnava, Slovakia Corresponding Author: Miloš T. Nenadović, e-mail: [email protected], Mike Petrovića Alasa 12-14, 11351, Belgrade, Serbia Abstract Fe/Polyethylene nanocomposite was synthesized by ion beam implantation of 56Fe+ into bulk high-density polyethylene. Nanoscale surface morphology along with magnetic and optical behavior was investigated. The aim of the research was to investigate changes of polyethylene's surface layer morphology with the change of Fe implantation fluence in the high fluence range and to find correlations with the magnetic and optical behavior. Four implantation fluences were applied: 5×1016, 1×1017, 2×1017 and 5×1017 cm-2, while the

implantation energy was 95 keV. Concentration profiles of implanted Fe were analyzed by Rutherford backscattering spectrometry, showing Fe concentration profile maxima closer to the surface with increasing implantation fluence. Cross-sectional transmission electron microscopy showed the formation of metallic nanoparticles with sizes in a range from below 1 nm up to few tens of nanometers, depending on the fluence, and for the highest implantation fluence, a continuous layer was formed. Magneto-optic Kerr effect magnetometry demonstrates weak ferromagnetic behavior for the 2 higher fluences, and superparamagnetic for the 2 lower fluences. The UV-VIS remission function spectra show the peak in the UV region, which we attribute to iron nanoparticles.

Keywords: Ion implantation; metal-polymer nanocomposite; Cross-sectional morphology; magnetization curve; UV-VIS spectra.

1. Introduction Ion beam implantation is one of the widely used methods for the synthesis of nanocomposites and modification of surface properties [1]. This technique can provide the formation of metallic nanoparticles in different kinds of media. Implantation of metals in polymers has been established as an effectual method for synthesis of metallic nanoparticles in dielectric media [2-4]. Finite-size particles incorporated in dielectric media, significantly influence macroscopic properties of composites [5-7]. Magnetic properties of such systems are of particular interest, both fundamentally and practically,

considering possible applications in the development of new-generation information storage devices and magnetic sensors [8,9]. On the other hand, organic-based media are seen as attractive for the use in spintronics and as magnetic media [10,11]. Formation of nanoparticles is mostly influenced by local concentration and diffusion of an implanted element [12]. In order to achieve metal nanoparticles formation by use of ion beam implantation, it is important to reach concentrations above the solubility limit in the given material. For that reason, high implantation fluences are necessary, and the threshold fluence for the formation of nanoparticles in most polymers is found to be 1×1016 cm-2 [12]. Polymers are quite sensitive to irradiation, so the high fluence implantations lead to significant damage to the material, due to ion stopping [13-15]. The substrate material is important because of its interaction with the implanted element. This interaction determines a concentration profile of an implanted element, and it is highly important regarding nanoparticles formation [11,15-17]. Highly important factors that determine nanoparticles formation in polymers are specific density and viscosity of a substrate [11,16]. Thus, nanoparticles of the same metal are smaller if the substrate's specific density is higher [11]. Furthermore, nanoparticles of the same metal are larger if the polymer is in the viscous state [16]. In their pristine state, polymers are typically diamagnetic. Implantation of ferromagnetic transition metals (Fe, Ni, Co) can lead to the emergence of ferromagnetic behavior. Magnetic properties of such nanocomposite systems are very dependent on a particle size distribution. Depending on the particle size and morphology, these systems can vary from paramagnetic and superparamagnetic, to ferromagnetic [18-21]. Ion beam irradiation of polymers leads to changes in optical

properties, as well. Generally, an increase of absorption in the UV-VIS spectra of polymers is expected as a consequence of ion implantation [22-27]. On the other side, an absorption peak in the UV-VIS spectra of metallic nanoparticles or other nanostructures, can appear as a consequence of the localized surface plasmon resonance (LSPR) effect [28,29]. Thus, implantation of metals can lead to an appearance of an absorption peak in the UV-VIS spectra, as a consequence of the LSPR effect, only if the implanted atoms are aggregated into nanoparticles or other nanostructures [30-32]. Nevertheless, ion implantation in polymers can lead to a shift of an existing absorption band from UV towards higher wavelengths in the visible range [22], or an appearance of new bands in the visible range [33], as a consequence of the radiation damage. These bands could be related to the formation of new bonds (double bonds, triple bonds, conjugated domains, etc.), as the consequence of the implantation induced chain scissions and cross-linking [34-37]. So far, there have not been many studies of the Fe/polyethylene nanocomposite layers, synthesized by such high implantation fluences. Most ambiguities of these nanocomposites are related to surface and nanoparticles’ morphology, and their correlation to magnetic and particularly optical properties. Therefore, this study aimed to comprehensively investigate changes of surface layer morphology in dependence of the implantation fluence and to find the correlation with the changes of the magnetic and optical properties, in a broad fluence range. Since there is a broad field of applications for these kinds of systems, finding connections between these properties would be very beneficial in the production of different types of magnetic recording media, electrical components, or sensors [38-40].

2. Materials and Methods 2.1. Synthesis Commercial 2 mm thick sheet of high-density polyethylene (HDPE) (0.945 g/cm3, BASF) was cut into (3 cm × 3 cm) square pieces, mechanically polished with diamond suspension, and then cleaned by the ultra-sonic bath with 18.2 MΩ deionized water. The prepared HDPE square pieces were implanted by iron (56Fe+) with an energy of 95 keV, at 0o beam incidence with respect to the sample surface normal, and average flux of 0.79 µA cm-2, using 500 kV ion beam implantation system (High Voltage Engineering Europa, Amersfoort) at the ATRI MTF STU ion beam laboratory [41]. Four different implantation fluences were applied: 5×1016 (S1), 1×1017 (S2), 2×1017 (S3) and 5×1017 cm-2 (S4). The pristine HDPE sample will be designated as S0 in the further text. The samples were implanted at room temperature (298 K), while the sample holder was water-cooled to prevent polymer overheating and thermal degradation. Vacuum level during implantation was 10-6 mbar (10-4 Pa). The pumping system consists of turbomolecular pumps and oil-free forepumps (scroll type). Hydrocarbon contamination is commonly neglected in literature for such pumping systems. As the ion flux was kept at 0.79µA.cm-2, at 95 keV, the average power delivered to the sample was 0.0745 W cm-2. The solution of the 1D [42] steady state equation provided the temperature of the HDPE surface at steady-state to be 35.38 oC (interface resistance value is estimated to be 100.00 W m-2 K-1, heat conductivity is 0.45 W m-1 K-1, and holder temperature is 24.54 oC.

2.2. Characterization Methods

Verification of the implanted fluence, as well as concentration profiles, was done by Rutherford backscattering spectrometry (RBS) using the experimental setup at the 6 MV Tandetron accelerator in the ATRI MTF STU ion beam laboratory [41]. The geometry of the RBS experiment setup was the same for all samples: incidence angle 0°, exit angle 10°, scattering angle 170° in ion beam mixing (IBM) geometry (the detector is mounted in the plane which contains the beam line and is perpendicular to the tilt axis). The primary beam was He+ with 2.0 MeV energy and H+ with 650 keV, acquired charge in both cases 10 µC. The iron concentration profiles were evaluated using the SIMNRA software [43]. Cross-sectional morphology of the implanted layers was analyzed by transmission electron microscopy (TEM) of the focused ion beam (FIB) prepared lamellas. The acceleration voltage was set to 200 kV. The images were recorded on a Talos F200X electron microscope from FEI Company, equipped with a CCD camera with a resolution of 4096×4096 pixels using the User Interface software package. An energy-dispersive X-ray spectroscopy (EDX) system attached to the TEM operating in the scanning transmission (STEM) mode was used for elemental profiling and element color mapping. High-angle annular dark-field (HAADF) images were captured in nanoprobe-TEM mode with probes of below 1 nm size and a camera length of ~200 mm. The samples were examined in crosssection, and TEM lamella preparations were done by an FEI Scios2 Dual Beam System

The magnetization measurements were performed by Magneto-optical Kerr effect (MOKE) magnetometry (evico magnetics Kerr-Microscope & Magnetometer) in longitudinal mode (the applied field is parallel with the sample's surface plane). The applied field was in the DC regime. From the magnetization curve measured by the MOKE magnetometer, the coercive field was estimated. The applied field was in the range (-500 - 500) Oe. There was no dependence of the magnetization curves on the in-plane rotation of the samples. UV–VIS diffuse reflectance spectra of all samples were recorded using a UV–VIS spectrophotometer Shimadzu UV-3600 in the wavelength range from 180 nm to 600 nm. 3. Results and Discussion 3.1. Rutherford Backscattering Spectrometry Fe atoms were detected, and their corresponding depth profiles analyzed by RBS. The concentration depth profiles of the Fe/HDPE samples are given in Fig. 1. It is observable that the position of the concentration peak varies with the implantation fluence. For S1, the peak position is at 88.3 nm below the surface, while for S2 and S3, it is positioned at 67.8 nm and 63.7 nm, respectively. The concentration peak of S4 is positioned at 61.6 nm. The concentration profile peak is closer to the surface with the increase of the implantation fluence which is in a great part the consequence of sputtering. The peak of the projected range calculated by SRIM 2013 code [44] is at 140 nm. This difference with the RBS results is also the consequence of sputtering and change of the substrate material during implantation, which is not regarded by SRIM code. It is also worth noting that

implanted fluences measured by RBS do not differ much from those measured by a current integrator, except for the highest implantation fluence (S4), where it reaches only 3.3×1017 cm-2. The explanation for this outcome could be found in the surface Fe concentrations. For S4, the surface Fe concentration is 19.9 %, which shows that the initial part of the depth profile, with very low concentrations, is missing due to high sputtering. The values of fluences measured by current integrator and RBS, as well as the surface and maximum Fe concentrations of the Fe concentration depth profiles, are given in Table 1. Table 1. The values of implantation fluences (F.) measured by current integrator (C. I.), RBS, surface Fe concentrations, and maximum Fe concentrations of the Fe concentration profiles. F. C. I. [1017 cm-2]

F. RBS [1017 cm-2]

S. Fe Concentration [at.%]

Max. Fe Concentration [at.%]

0.5

0.53

0.16

5.5

1

0.97

0.30

9.9

2

1.84

0.78

20.7

5

3.14

19.87

46.1

Some studies have shown that the shape of an implanted element's concentration profile is dependent on the implantation fluence and that for very high fluences (∼ 1×1016 cm-2) the distribution of elements is dominantly influenced by diffusion [45,46]. As a consequence, the depth profile could exhibit inward tails with local maxima. However, these inward tails could be connected to local heating and consequential degassing of the substrate material [12]. Iron concentration profiles of the Fe/HDPE samples are slightly

asymmetrical, but without multiple maxima, which is an indication that the distribution of elements was not dominantly influenced by diffusion, but nuclear collisions. Similar shapes of depth profiles to that in our study were obtained in the study of Fe implanted polyether ether ketone (PEEK) and polyethylene terephthalate (PET), but for much lower fluences [47]. However, besides different substrates, ion current density was much higher (4 µA cm2

) than in our study, so that it can be another reason for such concentration profiles at low

implantation fluences.

0

50

100

150

200

250

100

150

200

250

6 4

a)

Fe concentration [at. %]

2 0 10

b)

5 0 20

c)

10 0 40

d)

Fe (Ions)

20 0 1500 1000

e)

500 0 0

50

Depth [nm]

Fig. 1. Concentration depth profiles of iron in Fe/HDPE, measured by RBS, for: a) S1; b) S2; c) S3; d) S4. e) SRIM calculated projected range.

3.2. Transmission Electron Microscopy (TEM)

Cross-sectional morphology of the Fe/HDPE samples was analyzed by TEM. In Fig. 2, bright field TEM micrographs of the samples S1, S2, S3, and S4 are presented.

Fig. 2. Surface cross sections of: a) S1, b) S2, c) S3 and d) S4. The brightness of the micrographs is approximately reversely proportional to the atomic number Z so that areas filled with heavier elements are darker. The dark regions at the right side of each image present Pt protective layers deposited prior to FIB preparation of lamellas.

In the case of the lowest applied implantation fluence (S1), very small nanoparticles of iron are formed ranging in size from 0.8 nm to 2.7 nm. As the fluence increases, the size of the iron nanoparticles increases, as well. For S2, nanoparticles are still very small, with diameters ranging from 2 nm to 5 nm. Further increase of implantation fluence (S3), leads to the formation of significantly larger nanoparticles, with diameters ranging from 5 nm to 37 nm. Eventually, the highest implantation fluence (S4) leads to the formation of a continuous layer, about 50 nm thick. However, this layer is not homogeneous, and it is comprised of two characteristic parts. The first part (going from the surface) is about 15 nm thick, and it is somewhat brighter in comparison to the other part of the continuous Fe layer. From the RBS measurements in Section 3.1, it can be concluded that it is probably the initial part of the Fe concentration profile, in which the atomic concentration of Fe is in the range from 20 % to 25%. The second part of the Fe continuous layer is about 35 nm thick. Below this layer, nanoparticles with a broad distribution of sizes and diameters from few nanometers up to 20 nm are observable. Nevertheless, the depth distribution of Fe nanoparticles in each sample is inhomogeneous. There are 3 characteristic zones for samples S1, S2, and S3: near-surface zone (zone 1), middle zone (zone 2), and the posterior zone (zone 3). In zone 1, for S1, a small number of Fe nanoparticles with diameters below 1 nm are observable, so that most of Fe atoms in this zone are probably isolated. The increase of implantation fluence (S2 and S3) leads to an increase in the number of nanoparticles, with diameters up to a few nm. This zone extends down to about 45 nm from the surface for S1, about 40 nm for S2, and about 30 nm for S3. In zone 2, the largest nanoparticles are formed. This zone extends from about 45 nm to 125 nm in respect to the surface for S1, from about 40 nm to

120 nm for S2, while for S3 it extends from 30 nm to 110 nm. Zone 3 is very similar to zone 1 for S1 and S2, while for S3, nanoparticles from zone 3 are somewhat greater in comparison to nanoparticles from zone 1, and with a broader size distribution. In Fig. 3, the average nanoparticles' diameters in dependence of implantation fluence are shown. Only the nanoparticles from zone 2 were taken into account. S2 nanoparticles have approximately 2 times larger diameters in comparison to S1 nanoparticles, which is in agreement with the increase of fluence. On the other hand, nanoparticles of S3 have diameters about 4 times larger than S2 nanoparticles. This sudden increase shows that there is a critical fluence between 1×1017 and 2×1017 cm-2, for which the formation of Fe nanoparticles changes its proportion to implantation fluence so that much larger nanoparticles form. If we compare TEM results with RBS depth profiles in Section 3.1, we can see that for atomic concentrations below 10 %, an increase of the nanoparticles’ size is directly proportional to an increase of Fe concentration. In the regions with atomic concentrations above 10 % (Fig. 1c and Fig. 2 c), the proportion of the nanoparticles' size to the atomic concentration becomes higher, so that much larger nanoparticles are formed. Eventually, for the highest fluence, the atomic concentration of Fe in the subsurface region varies from 20 % to 45%, so that a continuous layer is formed. The reason for the relatively broad distribution of sizes could be found in the mechanism of metal clustering. It is known that the process of metal nanoparticles nucleation consists of 3 phases: metal accumulation up to supersaturation, the formation of the nuclei, and their growth [48]. The process of nanoparticles growth could be directed by both, the local metal concentration, and diffusion. In the next stage, when the local atomic concentrations

become sufficiently high, any further implantation contributes to the growth of larger nanoparticles, which is known as Ostwald ripening. Therefore, with the increase of the implantation fluence, significantly larger nanoparticles were formed, and finally, after the implantation with a very high fluence of 5×1017 cm-2, a coalescence of nanoparticles takes place, so that the continuous layer was formed. Fe concentration in the continuous layer varies from 20 % to 45 %. It is the indication that the threshold atomic concentration for the formation of a continuous layer of iron in HDPE is about 20 %. However, the effect of sputtering and change of the substrate composition during the implantation should be considered as the relevant factor for the growth of metallic nanoparticles. From many previous studies, it can be concluded that the type of substrate is also an important factor for the growth of metallic nanoparticles. Thus, in the study of 40 keV Fe implanted Poly(methyl methacrylate) (PMMA) with the fluence of 4×1017 cm-2 (ion current density 4 µA cm-2) [49], it was shown that 2 groups of metallic nanoparticles form - one with diameters in the range from 60 nm to 70 nm, and the other with diameters below 10 nm. In the other study [50], for the same system and with the same parameters, implantation with the fluence of 2.5 ×1017 cm-2 brought formation of nanoparticles with a broad range of sizes, up to 100 nm, but the most numerous were those with diameters up to 20 nm, and from 20 nm to 40 nm.

In the series of studies of Fe/silicone polymer composites

synthesized by ion beam implantation with the same parameters as in the previously mentioned studies [16, 51-53], it was found that drop like Fe nanoparticles with diameters up to 30 nm form, even for fluences lower than 5×1016 ions/cm2. Also, with increasing of the fluence up to 1.8×1017 cm-2, the number of nanoparticles increased, causing the formation of elongated agglomerates of nanoparticles up to 100 nm in size. On the other

hand, the implantation with the same parameters in polyimide (PI) provided formation of a continuous layer similar to that in our study, for the fluence of 1.25 ×1017 cm-2 [46]. In another study [20], implantation of Fe6+ ions in low-density polyethylene (LDPE), with the energy of 90 keV up to the fluence of 1×1017 cm-2 (ion current density of 3.2 µA cm-2), provided similar morphology to that in our study, for the same fluence.

Particle Diameter [nm]

25

20

15

10

5

0

0.5

1.0

1.5

2.0 17

-2

Implantation Fluence [x10 cm ]

Fig. 3. The average diameter of the Fe nanoparticles in dependence of implantation fluence. To determine the distribution of elements around nanoparticles with high precision, HAADF micrograph, together with EDS elemental mapping, the high-resolution HAADF micrograph and S3 single particle EDX line scan, are presented in Fig. 4. Contrast variations in the HAADF images indicate differences in the atomic number (the brightness is approximately proportional to the square of the atomic number, Z2). Thus, the thin bright layer in the top left part of Fig. 4. a), represents the protective platinum layer, and bright spherical spots some 50 nm below the surface, represent iron nanoparticles. The red, blue

and green color images in Fig. 4 b) represent the spatial mapping of C, Fe, and O, using the K-line spectra of the elements. In Fig. 4 c), a high-resolution HAADF micrograph of the selected area from 4 a), is given, with a framed straight line designating the scanning path of an electron beam, for the EDX elemental analysis.

Fig. 4. Detailed analysis of the S3 sample nanoparticles: a) HAADF micrograph; b) high resolution HAADF micrograph of the selected area with elemental mapping of C, Fe, and O; c) high resolution HAADF micrograph with the green line indicating the scanning path of an electron beam; d) EDX elemental analysis of the line profile, taken along the nanoparticle, marked with a framed straight line in (c). According to the elemental mapping in Fig. 4. (b), carbon concentration is lower in the areas where the iron nanoparticles are formed. On the other hand, it is not that obvious for

oxygen, because of its generally low concentration, so that oxygen appears to be distributed evenly across the implantation zone. However, from the EDX elemental profiles (Fig. 4. d), it is observable that oxygen concentration is also lower in the area occupied by the iron nanoparticle. On the other hand, iron is almost completely localized inside nanoparticles, which suggests that nanoparticles are dominantly comprised of pure iron Fe (0). Based on some previous studies, it can be expected that part of the implanted Fe is outside of nanoparticles, in the form of isolated atoms [11,19]. However, it is expected that nanoparticles are preferably comprised of α-Fe, with some contribution of Fe3O4 [11, 54]. Considering that oxygen concentration does not drop at the edges of the nanoparticles' EDX line profile, it can be expected that Fe3O4 or other iron-oxides' phase is formed at surface regions of nanoparticles, while inner regions of nanoparticles are comprised of pure iron Fe (0).

3.3. MOKE Magnetometry Magnetic behavior of the Fe/HDPE samples was analyzed by MOKE magnetometry. In Fig. 5, magnetic hysteresis loops of the samples S2, S3 and S4 are shown.

a) 1

Hc = 0 Oe

0

Mn [Dimensionless - normalized]

-500

0

500

0

500

0

500

-1

b) Hc=(20.2 ± 0.5) Oe

1

0 -500 -1

c) Hc=(57 ± 1) Oe

1

0 -500 -1

Field [Oe]

Fig. 5. Magnetization curves at room temperature for a) S2; b) S3; c) S4.

Samples S1 and S2 have zero values of a coercive field, which is characteristic for superparamagnetic (SPM) behavior. For S3, a coercive field has a measurable value of 20 Oe, and almost 3 times higher for S4, which is in good agreement with the raise of implantation fluence. This means that ferromagnetic phases are formed in S3 and S4. The

critical diameter of a spherical metallic iron nanoparticle to show ferromagnetic behavior is near 14 nm, and below this value, metallic iron nanoparticles show superparamagnetic (SPM) behavior [55,56]. Hence, this behavior was expected, as from the TEM micrographs (Fig. 2), it can be seen that for the 2 lower fluences, nanoparticles do not reach diameter sizes over 6 nm. However, the magnetic behavior of S3 is also influenced by SPM particles, although there are a significant number of nanoparticles with diameter size over the critical. Therefore, the value of the coercive field is low, and the shape of the magnetization curve is similar to the shape of the S2 magnetization curve. As the concentration of the implanted iron increases, the hysteresis loop shape becomes more like the shape for pure ferromagnetic materials (Fig. 5. c), and it is known that hysteresis shapes are influenced by the domain nature of samples. It is known from the literature that depending on phase, structure, and shape, iron or iron oxide nanoparticles could behave as single domain (SD) ferromagnetic particles in a certain size range, from the lower size limit for ferromagnetic behavior (~10nm), up to 100 nm or more [57-60]. For that reason, based on the TEM micrographs in Fig. 2, it is expected that in the case of S3, SD particles dominate ferromagnetic phase behavior, while in the case of S4, multidomain (MD) ferromagnetic phase is formed, considering the typical hysteresis shape for a mixture of SD and MD phases [56]. An absence of saturation hints towards a presence of strong interactions among nanoparticles or inside of nanoparticles. In the previous section, the EDX line scan of the single nanoparticle (Fig. 4d) showed that the composition of the nanoparticles' surface may be different from the composition of the inside. This inhomogeneity could be the source of an inner particle interaction which prevents orientation of magnetic moments parallel to the

external magnetic field and causes unsaturated hysteresis loops. It is also possible that the Faraday Effect can be the reason for the absence of saturation in the MOKE measurements, but in this case, the Faraday Effect correction gave very noisy and irregular shapes of the hysteresis loops and therefore was not taken into account. In one of the first studies of Fe/polyethylene composite synthesized by ion implantation, ferromagnetic behavior was reached for the fluence of 3×1016 cm-2 [61]. However, magnetization curves were recorded in a low-temperature regime (2K-150 K), and the implantation energy was only 25 keV. For such low implantation energy, iron concentration is expected to be much higher in the implanted layer, than in our study. In another study, the ferromagnetic behavior of Fe/PET composite was reached for the fluence of 7.5×1016 cm-2 [62]. It is to be noted that besides different substrate and characterization method, implantation parameters such as current density and energy were also different from those in our study. In the other study [20], where implantation parameters and substrate were similar to ours, ferromagnetic behavior at room temperature was not achieved for the fluence of 1×1017 cm-2. It is known that different implantation parameters lead to different morphologies of implanted layers. Furthermore, the magnetic behavior is in great part influenced by the morphology of an implanted layer, and as we can see from the previous studies, differences in the morphology are followed by differences in the magnetic behavior [20,62,63].

3.4 UV-VIS Spectroscopy

The optical behavior of the samples was analyzed by UV-VIS spectroscopy. In Fig. 6, UV-VIS spectra of the samples are shown, as Kubelka-Munk (KM) remission function (f(R)) in dependence of wavelength. The KM theory is the most widely used for the description of the diffuse reflectance of weakly absorbing powdered samples. KubelkaMunk remission function [64] is defined via reflectance as: f(R) = (1-R)2/2R

(1)

whereas R represents the diffuse reflectance. In the KM theory, a powdered material is considered as a continuous medium which absorbs and scatters radiation. For the description of monochromatic illumination of a powdered sample with the radiation of the intensity I0, the following 2 differential equations are used: dI = -(k'+s)Idx + sJdx

(2)

dJ = (k'+s)Jdx - sIdx

(3)

whereas I represents radiation flux in the downward direction (from the sample's surface towards the sample's bottom) at the depth x, J is radiation flux in the upward direction at the depth x, k' is the constant associated with the radiation absorption, and s is the constant associated with the radiation scattering. The approximation that reflectance is constant throughout the sample, which is valid for samples of infinite thickness, simplifies the solution of the equations (2) and (3), so that by further equating and rearranging of them, the following expression for f(R) is obtained [65]: f(R)=k'/s

(4)

As can be seen from the equation (4), f(R) is proportional to radiation absorption (k'), and inverse to radiation scattering (s).

f(R) [A.U.]

200 6

300

400

300

400

a)

4 2

f(R) [A.U.]

0 6

b) 4 2 8 0

f(R) [A.U.]

6

f(R) [A.U.]

f(R) [A.U.]

c) 6 4 2

d)

4 2 1.5

e) 1.0 0.5 0.0 200

Wavelength [nm]

Fig. 6. UV-VIS spectra of Kubelka Munk remission function for: a) S0, b) S1, c) S2, d) S3 and e) S4. In Fig. 6, the peak in the UV part of the spectra for each Fe/HDPE sample is observable. However, there is also a rise of the remission function for the pristine HDPE, but much narrower, and less steep. The steep decrease/increase of the remission function originates partly from the change of the excitation lamp, which is why it is observable for the pristine HDPE, as well. In order to show the absorption which originates only from implanted iron

or the radiation damage in the implantation zone, the spectrum of pristine HDPE was subtracted from the spectra of the other samples, that is shown in Fig. 7.

f(R)-f(R)HDPE

4.0

S2

3.0

S3

f(R)-f(R)HDPE(normalized)

S1

0.0

2.0 1.0

1.0 0.8 0.6 0.4 0.2 0.0

S4

200

300

400

λ [nm]

Fig. 7. UV-VIS spectra of KM remission function of Fe implanted samples, without the contribution of pristine HDPE. The values of f(R) for S4, are normalized because they were negative due to a very low signal of its spectrum in Fig. 6. It can be seen that there are absorption peaks at 234 nm (for S1, S2, and S4) or 236 nm (for S3). However, each peak contains a shoulder, positioned at around 216 nm. Similar shapes and positions of the peaks are found for electrodynamic calculation of spherical iron clusters' extinction spectra, based on the Mie theory [66,67]. According to this theory, positions of the LSPR peaks are very dependent on the dielectric constant of the surrounding medium. Thus, for εm = 1, there is a sharp rise of extinction, positioned at around 168 nm (7.4 eV), while for εm = 3, a clearly observable peak is positioned at 264 nm (4.7 eV), with a shoulder at around 194 nm (6.4 eV). Further increase of the dielectric

constant leads to the redshift of the peak (310 nm for εm=5). Considering that for polyethylene, εm = 2.3[68], the LSPR peak of our samples would be expected in the range from 220 nm to 250 nm, which is in good agreement with our results. Nevertheless, it should be noted that substrate material is expected to be significantly changed by ion irradiation, in the implantation zone. Therefore, the value of the dielectric constant of the surrounding medium in our samples could be expected to be higher than in pristine HDPE. LSPR of iron nanoparticles was also confirmed by several experimental studies [69-74]. Thus, a peak at 357 nm with the following shoulder peak at 325 nm was recorded in the UV-VIS spectra of very small iron nanoparticles (up to 3 nm in diameter) in xylene solution (dielectric constant εr in the range from 2.27 to 2.55 [75]) [69]. A similar peak positioned at ca.255 nm with the following shoulder peak at around 219 nm, was detected for iron nanoparticles (~5nm) embedded in the mesoporous silicate MCM-41 (εr~9 [76]) [70]. In another study, a peak at ca. 285 nm was observed for Fe nanoparticles (with diameters from 2nm up to 9 nm) in silica gel (εr≈2.2 [77]), but without a shoulder peak at a lower wavelength [71]. However, it should be noted that the samples were subjected to thermal and oxidation treatments. In the study of iron nanoparticles suspended in 2propanol (εr=19.4 [78]), an absorption peak was observed at 238 nm [72]. The shape of the peak is asymmetrical so that it is wider in the lower wavelengths half, although shoulder peak is not clearly pronounced. A peak at 235 nm was also observed in the study of Fe nanoparticles embedded in carboxymethyl cellulose (CMC) (εr~2 [79]) [73]. Although a shoulder peak at a lower wavelength was not observed, it appears that the spectrum would be very similar to that in our study if the spectrum of the surrounding medium (CMC) would be subtracted. In the study of iron nanoparticles embedded in carbon films (a-C:H)

(εr≈5.5 [80]), a peak at 235 nm was observed for Fe nanoparticles with diameters in the range from 20 nm to 35 nm [74]. A shoulder peak at a lower wavelength was not observed. Although Mie's theory predicts a strong correlation of the LSPR peaks' positions on the dielectric constant of the surrounding medium, this correlation is not confirmed from the previously mentioned studies. The shift of peaks' positions does not follow any particular trend with the change of the dielectric constant of a medium. On the other hand, the formation of iron-oxide shell around iron nanoparticles' core could be another important factor which determines the optical behavior of iron nanoparticles [71]. In the part of the spectra from 270 nm to 600 nm, a rise of signal with the implantation fluence is also observable, and it probably originates from the radiation damage in the implantation zone. Similar changes in this part of the UV-VIS spectra were detected for polycarbonate (PC) irradiated with noble gases or non-metals [33]. Increasing of the ion irradiation fluence, generally increases absorption in the UV-VIS spectra of polymers, which was confirmed by several studies [22, 23,25,26,33]. However, the increase of fluence does not always lead to an increase of absorption in the whole spectral range. The peak of S2 (at 236 nm) is higher than that of S3, although the signal of S3 is generally higher in the rest of the spectra. A higher absorption could be expected in the whole spectral range, since the iron quantity and a level of irradiation damage increase with increasing of the fluence. Therefore, this peculiar behavior could be the consequence of the Rayleigh scattering, which takes place when particles' diameters d, are much lower than the irradiation wavelength λ (λ/d ≥ 10). It is proportional to the size of a particle and inversely proportional to the irradiation wavelength [81,82]:

Is =I0 (

1+cos2 θ 2

2R

)(

2π 4 n2 -1 d ) ( 2 )( )6 λ n +2 2

(5)

whereas Is represents the intensity of the scattered light, θ is the scattering angle from the particle surface, R is the distance between the particle and the observer, n is the refraction index, λ is the wavelength, and d is the particle diameter. Thus, the signal is lower because of higher scattering from larger nanoparticles, and scattering is even higher for shorter wavelengths. It is also observable that for S4, the values of remission function are very low in comparison to the other samples, which is probably the consequence of the very high reflection and scattering. However, a very weak peak at 234 nm with its shoulder at around 208 nm is still observable. It is also important to note that this sample has a continuous layer structure in the near-surface region, besides nanoparticles which lie beneath this layer, which can be another reason for a weakly pronounced LSPR peak. In the study of gold thin films of different thicknesses, it was demonstrated that nanoparticles' aggregation leads to suppression of LSPR, due to the delocalization of free electrons and consequential increase of the free electron absorption [83].

4. Conclusions

High fluence iron implantation proved to be an effective method for synthesis of Fe/HDPE nanocomposite films with a diverse magnetic and optical behavior. It was shown that a nanocomposite layer with a broad range of nanoparticles sizes, or a continuous layer can be synthesized, depending on the fluence. For the fluences of 5×1016 and 1×1017 cm-2 small nanoparticles of iron up to few nanometers in diameter were formed, while for the

fluence of 2×1017 cm-2, nanoparticles with diameters of up to few tens of nanometers were formed. For the highest implantation fluence, a continuous layer about 50 nm thick was formed in the subsurface region, and nanoparticles with a broad distribution of sizes, beneath it. Ferromagnetic behavior was first recorded for the fluence of 2×1017 cm-2. For the samples synthesized with the fluences of 2×1017 and 5×1017 cm-2, the coercive field was about 20 Oe and 57 Oe, while for the lower fluences, the coercive field was equal to zero. As the morphology of the implanted layer changes with increasing implantation fluence, magnetic behavior changes as well. Implantation of iron caused significant changes in optical behavior, also. A peak in the UV-VIS remission function spectra appeared as a consequence of the iron implantation. This peak probably originates from LSPR of iron nanoparticles. However, there was no substantial change of the peak's shape with implantation fluence, but only of its intensity. The intensity of the peak increased up to the fluence of 1×1017 cm-2, and then it was lower for the 2 higher fluences. This is probably the consequence of increased scattering and reflection of these samples. Iron-polyethylene nanocomposites are very attractive in the field of magnetic recording media, optical sensors or electronic devices. Ion implantation is an effective technique for the production of that kind of materials because it is possible to produce a nanocomposite layer with desirable magnetic or optical properties by choosing appropriate implantation parameters. Correlating magnetic and optical behavior with the morphology of the iron-polyethylene composite is of the high importance for understanding the nature of this system, and the possibility to create materials with desirable properties, by choosing adequate procedures.

Acknowledgement This work was supported by the Ministry of Education and Science of the Republic of Serbia (projects No. III 45005 and 171029), the Serbian-Slovakian bilateral project SKSRB 2016-0002, the European Regional Development Fund under contract No. ITMS: 26220220179, the Slovak Grant Agency VEGA under contract 1/0330/18, and the Slovak Research and Development Agency under contract APVV-15-0049. The authors are grateful to Dr. Violeta Nikolić for useful discussions regarding the magnetic behavior, and MSc. Filip Marinković for his contribution in UV-VIS measurements.

Declarations of Interest: None

References

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