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Oct 14, 2010 - Editor: Charles P. Norris, pp. 147-193 ...... Co+ ion implantation with fluence of 1.25×1017 cm-2 at j = 8 and 12 µA⋅cm-2 leads to a significant change in .... [28] A. Meldrum, L.A. Boatner, and C.W. White, Nucl. Instrum. Meth B ...

In: Surface Science Research, Editor: Charles P. Norris, pp. 147-193

ISBN 1-59454-159-0 © 2005 Nova Science Publishers, Inc.

Chapter 7

COMPOSITIONAL AND STRUCTURAL ALTERATIONS OF POLYMERS UNDER LOW-TOMEDIUM-ENERGY ION IMPLANTATION Vladimir N. Popok Department of Physics, Gothenburg University, 41296 Gothenburg, Sweden

ABSTRACT Numerous studies of polymers modified by low-to-medium-energy ion beams are generalised. The accent is put on ion stopping in a polymer matrix, latent track formation and on complex process of polymer degradation. Main trends in the changes of the structure and composition of the polymers as a function of implantation conditions are discussed. In particular, the effects of radiothermolysis, degassing and carbonisation are under examination. Recent results on the high fluence and high ion current density implantation are reviewed. Post-implantation oxidation phenomenon of the radiationdamaged polymers is described in connection to the structural alteration. Depth distribution of various implanted species in polymers is one of the significant items under consideration. Main aspects of nucleation and growth of the metal nanoparticles (NPs) under the implantation conditions leading to the exceeding the metal solubility limit in the dielectric matrix are described as a special case of ion-beam treatment. The implantation-induced polymer modification is found to be originating a spectrum of new properties, i.e. significant changes in conductance, optical and magnetic characteristics, surface hardness, adhesion and other parameters that is reviewed with relation to practical applications.

INTRODUCTUON The discovery of conducting polymers in the 70’s and the development of the synthesis of polymers with n- and p-type conductance [1] stimulated the fabrication of organic-based functional electronic devices. In recent years, conducting polymers have been widely used for many research and application purposes. Ion implantation has become one of the effective technological methods to turn dielectric polymers into semiconductors [2-4]. This approach is

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based on a radiation-induced formation of conducting carbon-rich structures due to the disruption of chemical bonds, their subsequent cross-linking and conjugation followed by degassing of volatile components [5-8]. With fluence increase, the conductance rises up to 18 orders of magnitude depending on the polymer type and the implantation regime [2,4,6,9]. To reach a high-conduction value one should use a high-fluence implantation. In this case the implantation process becomes very time consuming. A possible solution for decreasing the time is the use of a high ion current density. It is also found that implantation at higher ion current density leads to higher conductance at the same fluence [10,11]. The use of implanted polymers is not restricted only to electronics applications. Investigations of optical properties of implanted polymer materials to produce passive devices, in particular, light filters, waveguides and electro-optical modulators are promising [12-16]. It is found that the electrical parameters of ion-modified polymers correlate well with the optical ones. Moreover, there is a possibility to estimate some basic parameters, e.g. the band gap width, and predict the conductivity of the implanted polymer using optical methods [17]. The radiation-induced alteration of a polymer changes also surface tribological and mechanical properties, for instance, smoothness, hardness, adhesion, wear and chemical resistance etc [18-21]. Ion bombardment can become one of the possibilities to control biocompability of polymers by the formation of surface pores and creation of the centres of cell adhesion [10,22-25]. The above-mentioned changes of polymer properties are mainly originated by the structural and compositional alterations of the implanted material. These effects are slightly dependent on the type of implanted impurity. Situation is significantly different for a metal ion implantation when the metals imbedded into the dielectric matrices can cause unique magnetic and optical properties [26,27]. Metal NPs can be synthesised in polymers under high fluence bombardment forming the nanostructured composite materials for various applications [28]. These materials can be distinguished as a separated area of implanted polymers because of their specific characteristics and application purposes. In particular, optical plasmon resonance and high values of third-order optical susceptibility of the dielectrics with gold, silver and copper NPs attract a lot of research attention [29-31]. These effects are considered to be promising for nanoscale plasmonics [32] and fabrication of nonlinear optical devices [30]. Ion synthesis of transition metal NPs in various dielectric substrates has been studied extensively last two decades [28,33,34] because of interest in ferromagnetic properties and effect of giant magnetoresistance that can be utilised for developing of magnetic date storage media, magneto-sensors and magneto-optical electronics. Recent data on magnetic and structural properties of the metal/polymer composites formed by the implantation can be found, for example in [35-37]. However, the ion synthesis of magnetic nanocomposites on polymer base is still poor studied since first publication on the subject [26] that is caused by complexity of the radiation-induced transformation of organic materials and multiple related phenomena. It is obvious from the above-presented extraction on the progress in implantation of polymers that the applications of ion-modified polymers require a detailed study of structural and compositional changes depending on implantation regimes. In the present paper, the effects of ion stopping and related to them polymer modification are critically analysed for the case of low-to-medium-energy implantation which is considered to be in the range from tens to a few hundreds keV/ion. Peculiarities of the depth ion distribution as well as the metal NP formation in polymer materials are discussed. The recent data on the high fluence and

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high ion current density implantation regimes are under particular consideration. Variety of aspects on the radiation-induced modification of polymer properties is under reviewing as well.

POLYMER DEGRADATION UNDER NUCLEAR AND ELECTRONIC STOPPING OF IONS Effect of ion implantation on the structure and composition of polymers is a sophisticated complex of physical and chemical processes and phenomena originated by the interaction of the impacting ions with polyatomic target. The processes depend on the energy transferred to the polymer at the ion bombardment, on the composition and structure of virgin polymer and its interaction with the environment after the implantation. The density of energy can be as high as tens or hundreds of eV per 1 nm of ion track length, depending on an ion mass, even for the above-mentioned low-to-medium-energy regime. Taking into account that the bond dissociation energy in polymers does not exceed 10 eV [9], the energy deposited by the projectile leads to multiple breakage of the chemical bonds within and around the latent ion track and formation of the core of the damaged region. The presence of two principally different mechanisms of the energy transfer from ions to polymer, which proceeds by nuclear collisions and electron excitation, influences the course of the radiation-induced processes taking place in the polymer material. Both mechanisms act simultaneously during a very short time period (10-16-10-15 s) of the surface impact and further (10-14-10-12 s) during the ion movement in the target [3]. Their contributions to the total energy loss of the incoming ion are characterised by stopping power Sn for the nuclear mechanism and Se for the electronic one. The values of Sn and Se depend on the ion energy and mass as well as on the type of polymer. The nuclear stopping dominates for heavy ions while the ionisation process – for light ions. By the term light ion one should assume the ion with mass typically below 20 a.m.u. Ratio of the stopping mechanisms in the total energy loss changes as the ion slows down. The electronic stopping dominates at the beginning of the projectile pass while the nuclear collisions prevail nearly the depth where the ions stop. Typical depth distributions of the stopping powers calculated using SRIM-2003 code [38] are presented in Fig. 1. The process of energy transfer to the target is concentrated within the ion track volume originating the direct bonds breakage mainly due to the binary nuclear collisions. Since the ion energy is much higher compared to the binding energy of the target atoms, the ion imparts enough energy to the primary replaced atoms (recoils) for the following replacements thus producing non-linear collision cascades [8]. Energy deposition by means of the electronic stopping results in excitation of the polymer units. Taking into account the lifetime of the “electronic events” up to 10-12 s [3], one assumes that the excitation can migrate to a relatively long distance from the core of the track (up to 100 polymer units [6]) forming electron excitation cascades, so-called penambra region [39]. Relaxation of the excited states causes the selective scission of the weakest bonds. Hence, in contrast to inorganic materials, breakage of the chemical bonds in polymers occurs by means of both the nuclear and electronic stopping.

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Fig. 1. Depth profiles of electronic (Se) and nuclear (Sn) stopping powers for 100 keV B+ (a) and As+ (b) ions in polyethylene calculated using SRIM code [38].

The bond rupture by the electron excitation is especially significant in the case of polymers with heteroatom-containing functional groups [9,11]. For instance, the implantation of polyimide with 150 keV Ne+ or 90 keV N+ ions (when Se/Sn > 6.5) leads first to degradation of the ether linkages [40] and then to gradual converting of the imide groups into amide ones [41] with CO as a major released gaseous product [42]. Similarly, the irradiation of poly(ether sulfone) under the conditions when the electronic stopping prevails causes selective reduction of sulfone groups to sulfoxide ones and then, under high fluences, to sulfide groups [3,43]. Example on the effect of electron excitation for the case of poly(ethylene terephthalate) can be find in [44]. In general, the energy transferred to the polymer host during the implantation as a result of the electronic stopping is mainly released in the reactions of dehydrogenation and weak bond breakage. These processes cause the formation of low-mass fragments and their yield has been found to be an increasing function of the electronic stopping power [45]. Heterocyclic groups are found to be more resistant to the electron excitation. However, because of the asymmetric system of delocolised πelectrons, they can also be transformed. For example, the electron beam irradiation of poly(2vinylpyridine) results in destruction of pyridine rings and formation of amino groups [43] while the aromatic rings in polyimide do not degrade under the electronic stopping [3,43]. The last fact is important in term of further formation of polyaromatic structures with πelectrons responsible for the increase in conductance. Increase of nuclear part in the Se/Sn ratio fundamentally influences the character of the processes occurring in the polymer target. Massive and random rupture of the chemical bonds carries out. For instance, under the implantation of poly(ether sulfone) with 50 keV As+ ions (Se/Sn < 0.2) not only the sulfone group is broken but acetylene splits out and gives rise to the formation of 1.4-substituted butadiene [46]. The ion implantation of polyimide at low Se/Sn ratio results in both the disruption of phenyl rings and degradation of imide groups yielding a number of products: iminic and pyridinic-like groups as well as tertiary amines [3,47]. The

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scheme of the polyimide transformation under high-fluence 40 keV Ar+ ion implantation, proposed in [11], yielding in the formation of extended polycondensed structures is presented in Fig. 2.

Fig. 2. Chemical formula of polyimide elementary unit (a) and polymer structure transformation upon implantation (b).

Thus, ion implantation of polymers results in degradation of the organic host due to significant modification of chemical bonds. There are two possible competing processes: (i) scission of molecular polymer chains resulting in fractionating and (ii) free radicals formation (branching) leading to cross-linking and bonds conjugation [5]. Efficiency of the scission or branching is closely connected to the type of polymer. For example, the chain fraction formation is the most typical for polyisobutylene, whereas polyethylene and polystyrene are mainly characterised by the cross-linking [48,49]. When the number of cross-links attains a certain critical value, the gel fractions with a three-dimensional network of bonds between macromolecules may form [49]. The behaviour of these two groups of polymers is recognised in lithography. Polymers of the main-chain-scission type are designated as “positive” resists because the irradiated zones dissolve more easily than the unirradiated domains, whereas polymers of the cross-linking type are called “negative” resists because the irradiated zones become less soluble [50].

LATENT TRACKS AND RADIOTHERMOLYSIS Polymer degradation occurs along the ion path where the disordered area with high density of radiation defects is formed. This area is called an ion latent track. It is experimentally shown that the ion tracks are stable in most cases and they are not recovered even for long period after implantation. This fact is used for formations of filters with tiny pores (submicron diameter) by high-energy through polymer implantation with subsequent etching of the radiation-damaged track volumes (positive resists), see for example [51]. After a number of experimental and theoretical investigations on the track formation, the following picture became the most common accepted. Radial structure of the latent track represents a core with surrounding shell or “hallo”. The cores are often experimentally identified with the

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cavities or craters formed on the surface at the spots where ions penetrate inside the polymer [52]. The core radius ranges ca. from 1 to 10 nm depending on implantation regime, ion specie and type of polymer [52-56]. The core is characterised by lower material density compared to the pristine polymer because of the intensive bond breakage and formation of the low-mass fragments some of them could be volatile and escape out from the polymer by diffusion. The phenomenon of degassing will be discussed later. The surrounding “halo” presents less damaged, usually cross-linked material. With increasing distance from the track axis the concentration of the cross-links declines slowly and the composition tends to that characteristic for the pristine polymer. Density of the radiation defects also changes along the track, i.e. in longitudinal direction, depending on the implantation energy and ion mass. Since the implantation-induced polymer transformation turns out to be confined by a small track volume for every ion, a number of ions bombarding the surface or, in other words, ion fluence is crucial parameter for material modification. Taking into account such simple geometric factor as the track radius it is easy to see that there is a threshold fluence, at which the sample surface appears to be completely filled with ion tracks. Separated damaged volumes start overlapping and further implantation thus carries out into already modified material. Therefore, one can distinguish two implantation regimes: (i) a single-track regime when the tracks are isolated from each other and (ii) a track overlapping regime. According to the above-mentioned data on the ion track parameters and results presented in [3,56,57], the transition from the single track regime to the overlapping one occurs for fluence range of 5×1012-5×1013 cm-2 in the case of light ions and for lower fluences in the case of heavy ions. The density of energy released in the track core is rather high that leads not only to pure radiation defect formation but also to heating process. Part of the ion stopping power causes the vibration excitation of the polymer atoms on the time scale of 10-14-10-12 s after the ion impact [3,8]. Then the excitation energy converts into thermalisation of the ion track volume resulting in abrupt local temperature increase. Dependence of the local temperature T(r,t) in the cylindrical track region on the radial distance r from its axis and on the time elapsed since the ion passage t can be described in the framework of the “thermal spike” model developed by Seitz and Koehler [58] with further refinements introduced by Sigmund [59], Kelly [60] and Bitensky with co-workers [61]. According to these approaches:

T (r ,t ) =

 (r / r0 ) 2  T0 exp − 2  1 + 4tδ r02  1 + 4tδ r0 

(1)

where r0 is the track core radius, δ is the thermal diffusivity of the medium and T0 is the initial temperature given by

T0 =

γ S πρC V r02

(2)

where ρ is the polymer density, CV is the heat capacity, S is the total stopping power and γ is the part of the deposited energy converted to heat in the collision spike. The dependence of normalised temperature on the distance from the track core and its time evolution evaluated using eq. (1) are presented in Fig. 3. This is qualitative approach. One can see widening of the

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heated zone with the time. The T0 values corresponding to the implantation of ions with energies of a few hundreds keV are found to be ranging in between 103-104 K, while quenching of the track down to the typical temperature of a polymer target under implantation (350-400 K) occurs for 10-10-10-9 s. Molecular dynamic simulations performed for 240 keV C+ ions implanted into Makrofol E (C16H14O3) show temperature of 1300 K in the track core [55]. The track area gradually cools down in radial direction and the temperature decreases to 370 K at 7-8 nm distance from the track axis. Thus, the thermal spike phenomenon causes the rapid local heating to temperatures much higher than the glass transition point that leads to the polymer degradation additional to that originated by the pure radiation effects or so-called radiolysis. Further approaches of the thermal spike model to polymers show its applicability and allow estimation of the tracks radii where good agreement between the theory and experiment is achieved. However, most of simulations are carried out for high-energy irradiation (MeV and GeV ranges) when the electronic stopping prevails [62,63]. There are only few attempts to apply the modified thermal spike model to low-energy implantation, in particular, for track radius evaluation [64,65]. Further development of the model criteria is necessary to establish the spatial energy and damage distribution for the low-energy projectiles. It is also necessary to come to the agreement in definition of the latent track because it has certain structure. Unfortunately, different models use the term track radius for either the core only or the core including the shell that leads to the discrepancy in experimental and theoretical data.

Fig. 3. Normalised temperature versus normalised radius of ion track. τ = 4tδ/r2 is specific time.

Thermal and radiative components of the implantation process interplay in a complex manner and represent a unified process of radiothermolysis. Products of the radiation damage are involved in the following thermalisation, which is very similar to conventional pyrolisis (high-temperature treatment in vacuum or inert atmosphere) of polymers. Thus, one can

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expect the similarity in the structural changes. In general, the process of pyrolitical decomposition can be divided into few stages. Temperature intervals for them are different depending on specific chemical reactions. For instance, the pyrolisis of cellulose leads to: (i) dehydration and formation of fragments with carbonyl groups (430-520 K); (ii) breaking of C-C and C-O bonds followed by degassing of CO, CO2, H2 and nucleation of four-atomic carbon groups (520-680 K); (iii) aromatisation and formation of polycondensed hexagonal network (680-1000 K) [66]. Similar scheme, with variations taking into account the nature of particular organic material, can be applied to any polymer. Thus, the pyrolisis is finalised by production of a pyrocarbon phase with conjugated bonds. For majority of polymers this process is completed at temperatures of about 1100 K. Strong analogy of the polymer degradation to the pyrolysis is shown for various implanted polymers, for instance, by studying the volatile products using infrared (IR) and mass spectroscopy [67]. In the case of implanted polymers, the high temperature in the track favour the cyclisation of the radiationinduced unsaturated chain fragments by the intramolecular Diels-Alder mechanism according to which aromatic hydrocarbons are more stable compared to linear ones at temperatures above 1100 K [9]. As the track cooled down, the aromatic fragments are tended to be linked together and stabilised due to extension of the conjugated system [6]. Orientation of the polymer chains is tended to be parallel to the track axis [68].

EFFECTS OF DEGASSING AND CARBONISATION Since the radiothermolysis leads to massive rupture of the chemical bonds, this process is accompanied by the emission of large amount of gaseous compounds from the damaged layer. The residual gas analysis during ion implantation reveals the yield of H2, CH4, C2H2, C3H5 etc. from the polyethylene and polystyrene bombarded by 100 keV He+ and 200 keV Ar+ ions [2]. Large amount of saturated hydrocarbons (methane, ethane) is produced by the ion irradiation of polypropylene and polybutylene [69]. Typical molecules and fragments emitted by implanted Tefzel (copolymer of tetrafluoroethylene and ethylene) and polyimide are H2, HF, C2H2, CF, CF2, CF3 and H2, C2H2, CO, CO2, respectively [18,42]. Therewith, the escape of H2 results in dehydrogenation, CO – in amidisation etc. depending on the polymer host. In particular, during the implantation of polyvinyl chloride an effective dehydrochlorination occurs leading to polyenes with following development into cyclic aromatic structures [2,70,71]. Since the thermolysis of polyvinyl chloride starts at relatively low temperatures, about 470 K, [72] the disordered carbonaceous phase with a high number of the cross-links can be expected under the implantation. Despite the high exhaust rate of the gases evolved during ion implantation only part amount of them escapes from the polymer. Some fraction of gases has no time to leave the thermal spike zone and react with the radiation-induced products. However, the contribution of this effect to the final polymer alteration is still to be clarified. The degassing process is not linear because of the dependence on the ion fluence, ion projected range and ratio of the electronic to nuclear stopping. Comparison of the boron and antimony implantation into polyimide and polyamide-6 [73] shows that in the case of light ions, when the energy transferred mainly by the electronic stopping, the significant surface depletion of oxygen and nitrogen together with rupture of imide groups occur. A strong surface carbonisation, i.e. enrichment with carbon, is the result of the degradation and

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degassing processes. Similar decrease of the surface concentration of oxygen and nitrogen followed by the considerable carbonisation is also observed under very low-energy (0.5-3.0 keV) implantation of N+, O+ and Ar+ ions into polyimide [74]. Conversely, in the case of heavy Sb+ ion implantation, when the energy is deposited mainly by the nuclear collisions at certain depth, the depletion of gaseous elements in the very surface layer is less pronounced and the surface is carbonised slightly [73]. The emission of the volatile compounds from the deeper polymer layers can occur towards the surface by the latent tracks. Mechanisms of formation of the volatile compounds can provide important insight into the primary mechanisms of the polymer modification on molecular scale. Several models are developed, for instance, for the molecular hydrogen formation and release [75]. The carbonisation of polymers under high-fluence implantation is of considerable interest because of drastic influence on all their properties. The enrichment of carbon can be observed directly by a number of experimental methods. For instance, in Rutherford back-scattering (RBS) spectra of the implanted polymers there is a bump on the background of the signal corresponding to the carbon content characterising the virgin sample. The depth profile of an “excess” in carbon concentration in the polymer can be reconstructed from such spectra. The carbon “excess” is especially well pronounced in the case of implantation of heavy ions and located at a certain depth under the surface depending on the implantation energy and type of polymer [76-78]. The example of the carbon “excess” profile for the case of 100 keV Sb+ ion implantation into polyethylene is shown in Fig. 4, curve 3.

Fig. 4. Depth distribution of oxygen in polyethylene implanted by 100 keV Sb+ ions with fluences of 1×1014 (1) and 5×1015 cm-2 (2). Depth profile of “carbon excess” for the highest fluence is presented as well (3).

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Ion bombardment does not result in the complete carbonisation of the polymer surface. Under the implantation of polyethylene with 150 keV F+ ions, carbon concentration in the implanted layer comes to saturation at level of 40 at. % [41] compared to 33 at. % in the initial polymer. The extent of polymer carbonisation caused by the implantation increases as the nuclear stopping becomes more important in the total energy transfer from the penetrating ions to the polymer. In other words, massive rupture of chemical bonds upon nuclear collisions is more efficient to provide deep dehydrogenation and carbonisation compared to the electron excitation causing only selective breakage. For example, the carbon content reaches 65-85 at. % (depending on ion fluence) for the polyethylene implanted with 150 keV As+ ions and 75 at. % in the case of the iodine implantation into polypropylene [76,77,79]. When the initial content of carbon is higher, as in the case of polyimide (78 at. %) and polyamide-6 (77.5 at. %), the carbon concentration in a few-nm-thick surface layer is found to be saturated at values of about 87-89 at. % under high-fluence (5×1016-1×1017 cm-2) 100 keV B+ ion implantation [73]. Increase in the carbon content from 52 to 83 at. % under the implantation of As+ ions with the fluence of 1×1016 cm-2 in polyamidimide is also found [80]. The process of the ion-induced carbonisation of polymers under implantation of heavy ions is practically accomplished at the fluence level of (1-5)×1015 cm-2. In the case of light ions, this occurs at higher fluences, about (1-5)×1016 cm-2 [81-83]. The composition of the implanted polymer change insignificantly under further fluence increase [82] excepting the sputtering phenomenon which becomes important at fluences higher than 1×1016 cm-2. Hence, implanted polymer layer represents a carbon-enriched material. At low implantation fluences, single track regime, the carbon-enriched zones are formed in the ion track areas. Because of similarity of the radiothermolysis to pyrolysis this zones are called pyrocarbon “drops” or clusters [81,83]. This structural rearrangement occurs through the condensation of the aromatic and unsaturated fragments and results in sp2 bonded carbon atoms. Nucleation of the carbon clusters is confirmed by a number of experiments. According to the electron microscopy and neutron scattering measurements performed on the polymers implanted by high-fluence boron, nitrogen and carbon ions, the size of the inclusions varies from a few to a few tens of nm [84,85]. The optical spectroscopy study and calculations made on its basis show that the nucleated carbon clusters can be about 2 nm in size for the polymers implanted by boron and nitrogen ions [17,83]. With fluence increase, the latent tracks overlap and the π-bonded carbon clusters grow and aggregate forming the network of conjugated C=C bonds. The incorporation of heteroatoms in the carbon network is possible for polymers containing heterocycles or heteroatom-containing functional groups [3]. Delocalisation of electrons in the π-systems of the carbonised phase of polymer leads to appearance of the characteristic paramagnetic signal and concentration of the paramagnetic centres (PCs) correlates with the fluence increase [86-89]. Stronger exchange interaction of the unpaired electrons found after the higher-fluence implantation confirms the carbon networking [83,90]. Under the high-fluence implantation conditions, it is found to be able forming a quasicontinuos carbonaceous layer buried under the polymer surface [91]. Despite the abovementioned saturation of carbon concentration and stabilisation of polymer composition at certain ion fluences, the significant change in properties is observed under further fluence increase (above 1×1016 cm-2). The decrease of resistance, concentration of the PCs and width of optical gap is registered [11,92-94]. This can be explained by the structural alteration, namely, by the transition of the carbonised phase, which is characterised by the agglomerates

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of carbon clusters and conjugated aromatic network in the polymer-like structure, to the phase mostly consisting of amorphous carbon or graphite-like material with individual polymerchain fragments. Thus, the following stages of carbonaceous phase formation with increase of ion fluence can occur in polymers under implantation: (i) degassing, transformation of functional groups and cross-linking within the latent track areas of the polymer resulting in formation of “precarbon” structures; (ii) nucleation and growth of the carbon-enriched clusters with possibility of incorporation of heteroatoms, the cluster size is determined by the ion energy; (iii) aggregation of the carbon clusters up to formation of the quasi-continuos carbonaceous buried layer characterised by the network of conjugated bonds; (iv) transition of the carbonised phase to amorphous carbon or graphite-like material. The corresponding diagram is presented in Fig. 5. It should be noted that the shown fluence ranges for different stages of the carbonisation are approximate depending on a number of implantation parameters. With all this going on, the energy loss of ions (especially at high fluences) effects the composition and structure of the formed carbonaceous phase to a greater extent than the nature or type of the initial polymer.

Fig. 5. Diagram of polymer carbonisation versus implantation fluence. Details are in the text.

When discussing the polymer alteration under ion implantation, one should notice that one more important parameter along with the ion energy and ion fluence, determining the power transferred to the organic matrix during the ion impact and stopping, is an ion current density. It is found that increase of the ion current density at the same fluence leads to change in polymer properties, for example, to the rise of conductance [10, 11]. Hence, the use of high current density in the ion beam is a possible solution for decreasing the implantation time to reach the high fluence and to modify the polymer in a desirable way. However, very high ion

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current density increases drastically the energy flow transmitted to the polymer, which may be destroyed due to its relatively low radiation and thermal resistance. It is shown that for one of the highest thermally resistant polymers – polyimide (glass-transition temperature is 570770 K) – the highest possible current density in the continuous beam of 40 keV Ar+ ions lays in between 16 and 18 µA/cm2 [11]. While, the ion current density can be of the order of mA/cm2 for the pulsed ion beams as shown for the polyethylene implanted by 20 keV N+ ions [95]. As the ion current density increases, the composition and the structure of the nearsurface layer are significantly affected by the local heating of the polymer around the ion tracks due to the intensive energy transfer. The thermolysis process becomes more efficient compared to the case of low and moderate current densities. The formation of more regular structures of the carbonised polymer is assumed that provides more favourable conditions for the electron transport [11]. The effect of ion current density on the electro-physical properties of polymers will be discussed in more details below in this chapter.

DECORATION OF RADIATION DAMAGE It was already mentioned in this chapter that a salient feature of the implanted polymer layer is the latent tracks. The tracks are especially good pronounced in the case of highenergy implantation [96]. The core of the track represents low-density straight and almost cylindrical in shape area that favourites the nanopore appearance [97,98]. This phenomenon is used for the production of polymer membranes by following etching [51,52]. In the case of low-to-medium energy implantation the changes in the polymer density along the latent tracks are sometimes not sufficient to ensure the pore formation but the inlets (craters) can be found on the polymer surface [11]. Because of the tracks, the implanted layer is highly permeable to diffusion of various components from a gas phase or liquid solution, which is in contact with the polymer surface. For polymer matrices not containing oxygen, the oxidation of the radiation-damaged layer is observed because of the oxygen diffusion either during the implantation (residual O2 in vacuum chamber) or after that when the sample is exposed to air [5,99]. The content of oxygen and its depth profile depend on the implantation fluence and energy as well as on the implanted ion species and polymer. For example, in the case of implanted polypropylene the oxidation propagates with a velocity not exceeding 4×10-5 nm/s and it takes a several weeks to complete the oxidation [77,100,101]. While for the polyethylene implanted under the same conditions, oxygen fills the radiation-damaged layer immediately [76-78]. RBS spectroscopy shows that the concentration of the incorporated oxygen gradually increases with the implantation fluence until the later reaches some threshold (1-5)×1014 cm-2 for heavy As+, Sb+ and I+ ions [76-78,101] and (1-5) ×1015 cm-2 for light B+, N+and F+ ions [77,81,83,100]. The total amount of oxygen trapped in the implanted layer can be rather large. It is found for the polyethylene (which does not contain oxygen at all) implanted by 150 keV F+ ions that for the fluience of 5×1015 cm-2 the oxygen/carbon ratio is 1 to 4 in the radiationmodified layer. The oxygen depth distribution is not homogeneous within the layer. For implantation fluences ≤ 1×1013 cm-2, the depth profile follows quite well the profile of ion energy loss on the electronic stopping. With further fluence increase for the heavy ions, the oxygen depth distribution exhibits maximum while the profile shape and its location

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correspond to that typical for the nuclear stopping (Fig. 4, curve 1). For light ions, shape of the oxygen depth profile remains unchanged, i.e. follows the profile of electronic stopping, up to the above-mentioned threshold fluence level. Since oxygen is trapped on the broken bonds and radicals, its profile decorates the depth distribution of the radiation defects as well as shows a contribution of electronic and nuclear mechanisms in the damage formation with the fluence increase. The oxygen penetrating into the ion-damaged layer creates relatively stable products and compounds. According to the IR data, major part of the incorporated oxygen forms carbonyl [89,102] and hydroxyl [103] groups. For the fluences higher than the above-mentioned threshold values, the oxygen depletion in the implanted layer is observed. In the case of heavy ions typical profile is presented in Fig. 4, curve 2. The minimum concentration of the trapped oxygen corresponds to the maximum enrichment of carbon, in other words to the above-called “carbon excess” (curve 3). At high implantation fluences, the carbonaceous phase is represented by the overlapped clusters, i.e. by more regular structures with conjugated bonds and minority of radicals. This reduces the concentration of oxygen scavengers and decreases efficiency of the carbonyl group formation. Presence of the small oxygen surface peak in the concentration profile let us to conclude that the carbonaceous layer is buried under the surface, i.e. it is separated by a thin layer of the low-damaged polymer. This effect can be rationalised by high energy released at the ends of ion tracks, i.e. at certain depths where the thermoradiolysis effect is most prominent. The surface oxygen maximum disappears under high-fluence implantation with lower energies, for example, in the case of 40 keV Fe+ or Co+ ions imbedded into polyimide [104]. It occurs because of rather short ion projected ranges when the energy is transferred in a thin surface layer causing its significant carbonisation from the very top. Fig. 6a shows similar drastic decrease in the oxygen concentration in the surface layer of polyimide implanted by Ar+ ions with high fluences. The other panel of this figure represents dehydrogenation of the polymer.

Fig. 6. Depletion of oxygen (a) and hydrogen (b) in implanted layer of polyimide implanted by 40 keV Ar+ ions with various fluences at ion current density of 8 µA/cm2. According to [11].

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Diffusion of both organic and inorganic substances into implanted polymers was carried out on purpose. The observed concentration depth profiles of the dopants in radiationdamaged polymers differ significantly from the conventional Fickian law being rather close to the profiles of the ion electronic and nuclear stopping and depending on the ion species and fluence as well as on the type of doping agent. For example, it is found that the diffusion of complex organic compound (metallocarborane) from water solution into the fluorineimplanted polyethylene occurs by the decomposed anions and cations [105]. Their resulting profiles follow the profile of electronic stopping for fluences below 1×1015 cm-2 that is in agreement with the threshold fluence mentioned for the oxidation process. For higher fluences the depletion in the dopant concentration is found. Similar effects are shown in the case of Pb diffusion from 0.5M aqueous solution of lead acetate into the polyethylene implanted by 150 keV F+ and As+ ions [79]. An increased diffusion rate is observed for the samples implanted to lower fluences and treated at boiling temperature of the solution. The resulting Pb depth profiles follow closely the theoretical profile of electronic energy losses. For fluences of (1-5)×1014 cm-2, concentration of the diffused Pb drops significantly because of the decreased permeability as a result of the carbonisation. Effective incorporations of molecular iodine in the implanted polypropilene [106] and polyethylene [107,108] are good examples of decoration of the radiation damage. The absorption capacity of the implanted layer is determined by the density of the stopping power along the tracks. Fig. 7a shows how the iodine depth distribution profile is diverted in shape from that typical for the electronic stopping to that characteristic to the nuclear one with the change of As+ ion fluence from 1×1012 to 1×1013 cm-2 [107]. Next panel (b) of the figure represents the iodine depletion in the layer enriched by carbon due to the high-fluence implantation. Formation of quite similar anomalous depth profiles is also found for the Li diffusion from water solution of LiCl into the poly(ethylene terephtalate) implanted by 150 keV Ar+ ions [109]. Ion track doping by Li is also used as an efficient technique for the damage profile decoration in the case of high-energy implantation with following use of a neutron depth profiling (NDP) method [110,111]. The nature of adsorption centres in the implanted polymers remains unclear. However, it is apparent that different doping agents can be trapped by different centres or defects. This assumption finds the confirmation by the results on diffusion of Fe3+ and Cl- ions from FeCl3 aqueous solution into the nitrogenimplanted polyethylene [112]. By contrast to the previous cases, the dopants depth distributions do not follow the ion stopping profiles. No depletion is observed for high fluences. Experimentally estimated Fe/Cl ratio in the implanted layer is from 2 to 3 which is significantly differ from the initial atomic densities. Hence, pronounced adsorption ability is inherent in polymers implanted with low fluences corresponding to the stages of the “pre-carbon” structures and nucleation of the carbon-enriched clusters. The observed depletion in the dopant depth profiles is the evidence of formation of the regular carbonaceous phase or buried layer. Therefore, the diffusive decoration can be use as efficient means for the fluence optimisation to obtain the required carbonisation level for practical applications of the implanted polymers.

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Fig. 7. Depth profiles of iodine diffused into polyethylene implanted by 150 keV As+ ions. The profiles are fitted by normalised curves corresponding to electronic (Se) and nuclear (Sn) stopping powers (a). Depth depletion of iodine in polyethylene implanted with fluence of 1×1015 cm-2 corresponds to “carbon excess” (b). According to [107].

DEPTH DISTRIBUTIONS OF IMPLANTED IMPURITIES Projected Ranges and Diffusion In most cases the experimentally obtained depth distributions of the implanted species in polymers differ significantly from those simulated, for example, by TRIM or its later version SRIM [38]. For the implantation of medium and heavy ions (conditionally, mass > 20), shape of the experimental profiles is close to that theoretically predicted, i.e. regular (see Fig. 8a). However, projected ranges Rp are found to be 10-30 % shorter and range stragglins are up to 120 % higher in the experiments compared to the calculations [113-115]. It has been proved that in the case of implantation of heavy ions into “light matrix”, which is polymer in our case, the correction ∆Se should be introduced into the Se/Sn ratio respecting to the known ZBL (Ziegler, Biersack, Littmark) potential used in the TRIM code [116]. This correction allows improving the correlation of the theory and experiment. Nevertheless, the origin shortening the projected ranges and widening the stragglings is most probably related to the gradual carbonisation of polymer that changes density of the implanted layer. Hence, the final depth profile is a sum of particular depth profiles accumulated during various phases of the implantation. For high implantation fluences (>1×1015 cm-2), effects of ion mixing and surface sputtering are noticeable and should be included into the simulations. Codes taking into account change of the near-surface layer composition due to cascade atom mixing as well as sputtering of the surface layer lead to more precise predictions of the depth distribution profiles of the implanted species. One of the examples is DYNA code [117].

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Fig. 8. Experimental depth distributions of Sb implanted with energy of 100 keV and fluence of 5×1016 cm-2 into polyethylene (a) and Xe implanted with energy of 80 keV and fluence of 1×1015 cm-2 into photoresist AZ1350 (b) (calculated profiles are shown as histograms). According to [83,113].

Difference from the above-described peculiarities in atom depth distributions is found for the polymers bombarded with inert gases (Ar, Xe and Kr) at room temperature, when the experimental Rp values are more than 2 times higher compared to the calculated ones and shapes of the profiles are anomalous (Fig. 8b) [113, 118]. The experiments at low temperature (90 K), when the measured depth profile of Xe is found to be corresponding to the simulated one with an accuracy of 10 %, give the answer [119]. Room temperature is a parameter leading to diffusion of the impurity, which is slow in the radiation-damaged layer and enhanced in the intact region. This diffusion is responsible for the inward “tail” in the depth distribution. As found, the experimentally obtained values of the diffusion coefficient follow an Arrhenius type of behaviour. In the case of very high-fluence (> 1×1017 cm-2) and low-energy (50 keV) Xe+ ion implantation, when the projected range in the polymer is rather short, the layer concentration of the implanted gas is found to be much lover (5.5×1015 cm-2) than the fluence value [80]. This phenomenon can not be explained by only sputtering of the surface layer. It is suggested that some part of the implanted gas atoms undergoes diffusion towards the surface and escapes from the polymer. This mechanism is especially probable taking into account the low penetration depth of the Xe+ ions and the high level of disorder and radiation damage of the near-surface layer. The effect of diffusion towards surface is also found for the polyimide implanted by 80 keV Ar+ ions to very high fluences [11]. With fluence increase over 2.5×1016 cm-2, the drastic drop of Ar concentration in the implanted layer is observed (Fig. 9) that is explained by the polyimide restructuring causing the Ar to escape. Furthermore, the Ar atoms implanted with 40 keV (Rp ≈ 56 nm according to SRIM-2000) are not found in the polymer at all using RBS even for the samples bombarded with the highest fluences [11]. Atomic force microscopy (AFM) images show craters formation at the sports of ion penetration, confirming

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the possibility of effective argon volatilisation via the tracks at low fluences (Fig. 10a). At high fluences the surface disorder with a vertical roughness of about 30 nm (Fig. 10b), which is comparable with the Rp, explains the Ar disappearance.

Fig. 9. Argon depth profiles for polyimide implanted by 80 keV Ar2+ ions with different fluences at ion current density of 4 µA/cm2. The profile calculated by SRIM-2000 code is presented as well (in rel. un.). According to [11].

Fig. 10. AFM images of polyimide surfaces implanted by 40 keV Ar+ ions with fluences of 1.0×1015 (a) and 7.5×1016 cm-2 (b) at ion current density of 8 µA/cm2. Circles in panel a show craters of ion inlets. According to [11].

Another case of anomalous depth distribution is found for the polymers implanted with extremely high fluences (1016-1017 cm-2) of metal ions (cobalt and iron) at high ion current densities (4-12 µA/cm2) [104,120]. Penetration of the Fe and Co atoms beyond the ionprojected range is observed (Fig. 11). The inward tail exhibits two other peaks with maximum concentrations by about two orders of magnitude lower compared to the surface one. The inward tail and the presence of the side maxima could be related to both an appearance of

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strain waves and a heating of polymer surface under the high fluence and high ion current density stimulating rapid metal diffusion into the polymer bulk. The strains could appear as a result of “free volumes” forming due to the degassing of the volatile compounds on the one hand and compacting of the surface layer due to the carbonisation on the other hand. That, in turn, can cause a formation of cracks to the depth exceeding the implanted region. Similar surface damages were observed, for example, on polyetherimide and poly(tetrafluoroethylene) implanted to high fluences [121, 122]. The inward diffusion of Fe and Co could also be facilitated by radiation-induced formation of metal-carbonyl compounds [104] possessing relatively high mobility. Nevertheless, other studies on high-fluence implantation of various metals (Ag, Cu, Pd and W) do not report such anomalous depth profiles with the side maxima [123]. Hence, the mechanism of the radiation-stimulated inward metal diffusion depends in complex manner on the implantation parameters and the details are not clear yet. Nucleation of the metal NPs should be also taken into account while discussing the metal diffusion under the implantation with fluences higher that 1×1016 cm-2 (will be discussed in next section of this chapter).

Fig. 11. Depth profiles of Co atoms in polyimide implanted with energy of 40 keV and different fluences at the ion current density of 4 µA/cm2. The profile calculated by SRIM-2000 code is presented as well (in rel. un.). According to [120].

Diffusion has an especially significant effect on the spatial distribution of light species implanted into polymers. For instance, early experiments on the 6Li+ and 10B+ ion implantation showed that a fraction of the implant diffuses towards the surface and the resulting depth profiles are close in shape to those predicted for the electronic stopping of the ions [124]. The impurities diffuse towards the surface and they are captured by the radiation defects produced due to the ionisation effects dominating in the total stopping energy loss. It is shown that the transition from the regular depth distribution to the “ionised” one takes place when the contribution of the electronic stopping is significantly high compared to the

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nuclear stopping (Se ≈ (2-5) Sn) [125]. Later models show that the implanted atoms are redistributed immediately after their ballistic slowing-down. The mobility of the implant is enhanced in the radiation-damaged layer and the local diffusion enhancement as well as trapping being controlled by the electronic stopping [126]. It is also found that the shape of the depth profiles changes with increase of both the fluence and implantation energy [126, 127]. By the example of boron implantation into various polymers, it is observed that the profiles become bimodal at high fluences [83, 128]. The boron depth profile consists of a bulk maximum and a surface peak (Fig. 12). The bulk part of the profile resembles more or less the distribution of collisional energy transfer (due to the nuclear stopping). The surface peak appears as a result of the diffusion presumably via the latent track. The formation of volatile alkylboron compounds is suggested: they are oxidised in the very surface layer under the interaction with the environmental air [73].

Fig. 12. Depth distributions of boron implanted with energy of 100 keV and fluence of 5×1016 cm-2 into polyamide-6 (PA), cellulose (CE) and polyethylene (PE). The profile calculated by TRIM-95 code is presented as well. According to [83].

Following model of the diffusive redistribution of the implanted light species is suggested. Stopping ions break the chemical bonds and displace the host atoms forming socalled “free volumes” or nanocavities in polymer. Increase of ion fluence rises the “free volume” that stimulates the enhanced impurity diffusion. Free radicals and other radiation defects serve as the trapping centres for the diffusing atoms, i.e. a sort of neutralisation of the radiation defects occurs. Next portion of incoming ions causes more displacements and defects. Thus, the dynamic process of the radiation-stimulated diffusion based on the trapping-detrapping mechanism can take place. To model the diffusion, the following equations are used:

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∂N f ∂2 Nf =D − αN f ρ + fd a ( x) ∂t ∂x 2 ∂ρ = βfd t ( x) − αN f ρ ∂t

(3)

∂N c = αN f ρ ∂t where Nf and Nc are the atomic densities of free and trapped implanted atoms, D is the diffusion coefficient for the dopant, f is the fluence, da(x) and dt(x) are the functions of depth distributions of the implanted atoms and created trapping centres, respectively, ρ is the immediate density of unoccupated centres, α and β are constants for the trapping efficiency (in dimension of cm3/s) and the mean number of the trapping centres created by single ion, respectively. The system of eqs (3) is solved numerically by standard finite difference method using successive approximations of α, β and D. This approach gives good agreement of the simulated profiles with the experimental ones for the boron implantation with exception of the near-surface peak [129,130]. Increase of fluence leads to drop of diffusion because of the carbonisation and formation of more regular conjugated systems decreasing the number of the trapping centres. Depletion in the boron profiles in Fig. 12 is caused by this effect. Subsequent annealing of the boron-implanted polymer samples up to the glass transition temperature causes the rapid inward migration of a significant fraction of the boron [129,130]. Hence, under implantation into polymers the depth profiles of dopants are significantly different in most cases from the regular ones predicted theoretically which is caused by low resistance of organic matrices against the radiation phenomena leading to the drastic change of their composition and structure. Diffusion to be found playing important role in the redistribution of the implanted light and gaseous species. Effects implying final distribution of the implanted atoms are of great importance for the desirable modification of polymer properties.

Metal Nanoparticle Formation Special case of high-fluence implantation of metal ions into dielectrics is NPs synthesis. Ion implantation enables a high metal filling factor to be reached in a solid matrix beyond the equilibrium limit of solubility. The system relaxes by precipitation of metal as the NPs. Advantage of the method is a possibility to form any composite of metal/dielectric. The unique mechanical, optical, electronic and magnetic properties of these nanocomposites have stimulated considerable research interest in study of the metal ion implantation into various crystalline and amorphous materials including polymers. Since the first publications on the formation of small ferromagnetic clusters in polymers under the high-fluence iron implantation [26,131], number of studies has been gradually increased forming the separated area of material science. Therefore, in this chapter only main aspects on the metal NP

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formation in polymers are discussed. More data on the specific properties of the nanostructured metal/polymer composites will be presented in next section of this chapter. Threshold fluences when the particles start nucleating are found to be about 1×1016 cm-2 for majority of polymers and the NPs can be directly observed using transmission electron microscopy (TEM) [121,123,132]. Metal clustering in polymers results from the high metal cohesive energy compared to much lower cohesive binding energy between the metal atoms and polymer components. Formation of the NPs in the implanted layer includes a few stages: metal accumulation up to supersaturation, formation of few-atoms nuclei and their growth [31]. Assuming that the NP growth occurs by successive joining of the single atoms one can conclude that the process is governed by both the local concentration of metal and diffusion coefficient. The particles nucleated at fluences just slightly above the threshold one are usually spherical in shape. Typical picture is presented in Fig. 13a. Mean size (diameter) depends on type of both the metal and polymer. For instance, for Ag and Cu the size is found to be about few nm in the epoxy resin [132] and poly(methyl methacrylate) (PMMA) [133] while for Fe – tens of nm in the PMMA and polyimide [35,133,134]. It is also experimentally found that mean size of the NPs of the same metal has tendency to decrease with increase of the specific density of virgin polymer substrate [135]. Disadvantage of ion implantation is statistically non-uniform distribution of the metal atoms over the depth. This leads to a wide size distribution of the NPs. Larger in size particles are formed at the depth corresponding to highest concentration, i.e. to the mean projected range of the metal ions. Increase of ion fluence leads to growth of the NPs in size followed by widening of the size distribution. The growth of the metal NPs is effected by many factors: not only by the metal concentration and atoms mobility but also by the parameters of the polymer media such as composition and structure, which undergo drastic alteration under high-fluence implantation. The carbonisation and radiation-induced disordering of the polymer effect the metal diffusion. The presence of radiation defects can be the origin of twomaximum function in size distribution of the NPs, which is, for example, observed for the cases of Fe+ and Co+ ion implantation into polyimide and epoxy resin [135]. Polymer viscosity is found to be playing important role for the NP nucleation and growth. Implantation into viscous polymers is more favourable for the coagulation of metal atoms and coalescence of the NPs [136]. Ostwald ripening, when smallest particles dissociate and released metal atoms enlarge other NPs, is one of the mechanisms playing important role in the growth process, especially at high ion current density implantation regimes. At fluences higher than 1×1017 cm-2 either a buried quasi-continuos granular metal layer can be synthesised or an agglomeration process can result in formation of the needle- or worm-like structures (Fig. 13b). Depth of occurrence of the structures or layer and width of the layer depend on the implantation energy. However, not all implanted atoms contribute into formation of the metal NPs. For instance, it is measured that in the case of Fe and Co the NPs contain only up to 65 % of atoms implanted into the polyimide with fluence of 1.25×1017 cm-2 [134,135]. Phase analysis of the metal/polymer composites synthesised by the implantation shows that: iron preferably forms NPs of α-Fe with some contribution of Fe3O4 phase in various polymers [133,135]; cobalt NPs are pure metallic with small fraction bounded to carbonyl group in polyimide [104] (this fraction is most probably formed by atomic Co, not by Co of the NPs); silver NPs has fcc structure, no chemical compounds with silver atoms are found in PMMA [31]; copper NPs are formed from both the pure metallic phase and Cu2O in PMMA [133].

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Fig. 13. TEM in-plane images of polymers implanted by 40 keV Fe+ ions at ion current density of 4 µA/cm2: polyimide, fluence 7.5×1016 cm-2 (a) and poly(ethylene terephtalate), fluence 1.5×1017 cm-2 (b).

PROPERTIES OF ION-MODIFIED LAYERS Trybological Properties and Biocompability It has been shown that the ion beam modification has great potential in improving of mechanical properties and trybological behaviour of polymers. First results on these parameters were published in the 80’s when a wide spectrum of changes of polymer properties under implantation was reported in a number of publications. In the beginning of the 90’s special study of the mechanical properties was carried out by a few groups. Rao, Lee and co-workers published a lot of results showing the significant change of hardness, elastic modulus, friction coefficient and wear resistance for a number of polymers implanted by different ion species under various conditions, for example [18,84,121,137,138]. In the case of certain implantation conditions, the hardness increases for tens times reaching the values typical for stainless steel (3-12 GPa) [18,138]. The case of polysterene implanted by Ar+ ions is presented as example in Fig. 14. It is found that surface hardness is increasing function of the ion fluence and beam energy [137,139]. Moreover, the hardening effect is also dependent on the ion species and type of polymer. It is observed that in the case of metal implantation the nucleated metal NPs improve surface mechanical parameters [121,140]. Structure and composition of virgin polymer effects the processes of its radiation change as described in the above sections of this chapter. The improvement of hardness and elastic modulus are originated from the formation of three-dimensional conjugated networks through the carbonisation and cross-linking of polymer chains in the implanted layer. This mechanism is found to be realising very well under the dual ion implantation when the mechanical parameters reach twice- or thrice-higher values compared to the conventional single regime [141]. The increase of polymer hardness and stiffness results in improving of such trybological characteristic as abrasive wear resistance [20]. Higher adhesive wear resistance is also conferred because of the limitation of plastic deformation and the promotion of elastic deformation as well as due to the augment of wettability improving lubrication [122]. However, the carbonisation may also lead to rise of brittleness that decreases the abrasive

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wear resistance. It is also found that there is no specific correlation between friction values and improvements in wear behaviour for the implanted polymers: the boron implantation into polyethylene exhibits reduce of the friction coefficient while the oxygen implantation of polycarbonate leads to its rise [20]. That is why, the optimisation of implantation regimes with its relation to required compositional and structural changes determining the surface polymer properties is still under study [21,142].

Fig. 14. Dependence of polystyrene microhardness on implantation energy and fluence of Ar+ ions. According to [138].

Significant modification of surface under implantation: change of topology and polar component of the surface energy, formation of pores, stable radiation defects (free radicals), new chemical states and adsorption centres leads to the increase of chemical reactivity and adhesion of surface [143]. Formation of specific centres can be used for the cell adhesion and synthesis of biocompatible polymers. First experiments proved the change of adsorption of plasma proteins and the improvement of antithrombogenicity of the silicon rubber implanted by H2+, N2+, Na+ and Ne+ ions [144]. Later studies show a possibility of the efficient endothelial cell adhesion, protein coating of detachable coils for endovascular treatment [10,25] and colonisation with vascular smooth muscle cell [145,146] using the ion implantation of various polymers. These results play important role in production of the functional tissue substitutes by combining biologically active cells with suitable polymer materials. It is suggested for implanted polyethylene that increase in the surface polarity (wettability) facilitates cell adhesion [147]. However, in general, physics and chemistry of the firm cell adhesion on surface of implanted polymers is poor studied so far.

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Optical Characteristics The ion-irradiation-induced alteration of polymer changes its optical properties. For optically transparent polymers the implantation causes induced coloration at fluences as low as about 1013 cm-2 [12]. The colour changes from pale yellow to deep brown or grey with the fluence; metallic lustre appears at high fluences (about 1×1015 cm-2) [148]. As seen from Fig. 15 the change is colour correlates with the shift of the absorption edge. The absorption in UVvisible range of the spectrum is determined by the formation of carbonaceous phase and extension of the conjugated system. Additional information on the radiation-induced transformation of implanted polymers is obtained from IR spectroscopy data. In the case of polyethylene implanted with boron, a band at about 1640 cm-1 is observed in the absorption spectrum for low fluence [17]. The band is attributable to stretching vibrations of unsaturated groups, in particular dienes. This band is gradually transformed into broader one at 15801600 cm-1 with the increase of fluence that probably corresponds to the absorption on polyenes (Fig. 16). The other band at 1705 cm-1 confirms oxidation of the uncompensated carbon bonds and formation of the carbonyl groups. Infrared spectra of the PMMA and polystyrene irradiated by 500 keV He+ ions also show that the implanted layers have lost most of memory of the polymer structure and exhibit two absorption bands at 1600 and 1670 cm-1 attributed to the conjugated –C=C– bonds and to the oxidised carbonaceous material [149]. These results are in fair agreement with the data on formation of unsaturated conjugated bonds under low-fluence implantation of polyolefins [19,150].

Fig. 15. Optical transmission spectra of polyamide-6 implanted by 100 keV B+ ions with different fluences. According to [17].

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Fig. 16. IR optical transmission spectra of virgin polyethylene and one implanted by 100 keV B+ ions with fluence of 5×1016 cm-2. According to [17].

The presented in Fig. 15 red shift originated from the nanodimensional carbon-enriched clusters can be considered as a nanosize-related effect. This approach allows extracting the cluster size from the optical data [17,83]. Assuming that thr finite carbon clusters are composed of some number of fused benzene rings the following relation derived for amorphous carbon [151] can be used:

E g = 2 β N −1 / 2

(4)

where Eg is the optical gap, which can be evaluated using the Tauc plot [152] for the optical spectrum, β is the resonance integral (Hückel theory gives β = 2.9) and N is the number of benzene rings forming the cluster. By known value of the optical gap one can extract the number of rings, i.e. approximate cluster size, from eq. (4). For the implanted polypropylene [153], polystyrene [154], polyethylene [17,95] and polyamide-6 [17] the optical gap decreases with fluence and saturates at value of about 0.6 eV for the fluences (1-2)×1016 cm-2 (Fig. 17). For the polycarbonate implanted by 50 keV Ar+ ions with fluence of 1.2×1016 cm-2 the optical gap value is found to be 0.4 eV [155]. According to eq. (4) value of 0.6 eV corresponds to a carbon cluster comprising about 100 benzene rings, i.e. of ~2.0-2.5 nm in size [17,83]. This estimate is very rough because the equation is valid only for compact clusters. Moreover, the Hückel theory can overestimate the energies of the optical transitions in the π-systems. Nevertheless, optical spectroscopy enables to trace the major stages of the carbonaceous phase formation and shows the difference in the carbonisation for various polymers related to their structure. For example, the formation of smaller clusters (wider optical gap) in polyamide-6 compared to other polymers (see Fig. 17) can be a result of the incorporation of

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heteroatoms (nitrogen) into the cluster reducing the size of extended region of the π-electron conjugated system.

Fig. 17. Dependence of optical gap width on ion fluence for various polymers implanted by different species and energies. According to [17,95,153,154].

Change in the optical gap for the implanted polymers is in good correlation with change in the electrical conductance and paramagnetic properties, which is strongly connected to the chemical and structural modifications of polymers. This correlation is found for variety of polymers and good illustrated, for instance, for the polyimide implanted by Ar with highfluence and high ion current density [11]. Under fluence of 1×1017 cm-2 the optical gap is as small as 0.25 eV that is much closer to the values typical for amorphous carbon than to those for polymers confirming the significant polymer carbonisation. From the correlation of the optical, paramagnetic and electrical parameters it is suggested that the optical gap can approximately correspond to the band gap of the semiconducting material formed due to the polymer alteration under implantation. Optical parameters such as refractive index and extinction coefficient can be calculated from the absorption and reflection spectra of the implanted polymers [156]. They change significantly under irradiation: refractive index is increasing function of fluence [16,157]. It should be noted that rather small fluences (1013-1014 cm-2) are used for efficient control of the optical parameters. The control of the refractive index by implantation allows using this method for formation of the planar waveguides in the polymer films. By using masking technology or photolithography it is possible to make either the surface or buried waveguides depending on the implantation energy [15,16]. The specimens of Y-branches and interferometers are already produced [15]. Technology for formations of Mach-Zehnder modulator using the reactive ion beam etching of polyimide is suggested [14]. The nonlinear properties such as an electronic nonlinear refractive index and high values of a third order

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susceptibility are expected for the implanted polymers due to the conjugation systems sharing π-electrons that could be highly polarizable and could lead to Kerr effect in an intense electric field [13]. Despite the intensive study of optical properties of non-organic dielectrics with the synthesised by ion implantation metal NPs, optical properties of the polymers implanted by metal ions are only beginning to be investigated. Interest to noble metals and copper is caused by the effect of plasmon resonance for the nanosize particles embedded into dielectric media. This effect is related to strong interaction of the NP with visible light at its dipole surface plasmon frequency due to the excitation of a collective electron motion inside the particle. It is shown that electromagnetic energy can be guided in a coherent fashion via the arrays of closely spaced metal NPs due to near-field coupling, which creates basis for prospective nanophotonics and active plasmonics [158]. One of the first publications on study of optical properties of polymers with the metal particles synthesised by ion implantation appeared in the middle of the 90’s and it was related to the formation of Ag NPs in epoxy resin [132]. Later on, a few articles discussing the synthesis of Ag particles in PMMA and viscous silicon polymer [31,159,160] and Cu ones in polyethylene, polystyrene and polycarbonate [161] as well as optical properties of these composites formed by the ion implantation has been published. The polymers implanted by silver show absorption band in the optical spectra for fluences > 2×1016 cm-2. For the Cuimplanted polymers the absorption band characteristic for the plasmon resonance appears at higher fluences (ca. 1×1017 cm-2). Position of maximum and width of the band on wavelength scale are found to be dependent on the implantation fluence, type of implanted metal and polymer. One can see in Fig. 18 that the absorption maximum shifts to longer wavelengths and it becomes wider and less pronounced with the fluence increase. It is suggested that the absorption band is a superposition of the absorption caused by the NPs (plasmon reconance) and carbonaceous phase of the radiation-modified polymer. This can be seen by comparison of the Xe- and Ag-implanted PMMA (panels a and b in Fig. 18). Theoretical modelling of extinction spectra of the Ag NPs covered by carbon shells or embedded into amorphous carbon shows the similar “red shift” and broadening of the absorption band confirming the affecting role of carbonisation on the optical parameters of the metal/polymer composites [160]. Position and shape of the band are also found to be different for epoxy resin, PMMA and viscous silicon polymer implanted by Ag+ ions [132,159,160]. This reflects the influence of the polymer matrix on the growth of the NPs and on the final optical properties of the composites. However, the research area of the NPs formation by ion implantation in polymers is still under development. There are more question marks than understood effects and phenomena concerning the control of optical properties of the synthesised nanostructured metal/polymer composites and production of the materials with required parameters.

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Fig. 18. Optical absorption spectra of PMMA implanted by 30 keV Xe+ (a) and Ag+ (b) ions with fluences of 3×1015 (1), 6×1015 (2), 2.5×1016 (3), 5×1016 (4) and 7.5×1016 cm-2 (5). Spectra of virgin PMMA and silica glass with Ag NPs formed by implantation are presented for comparison. According to [160].

Paramagnetic and Ferromagnetic Phenomena Majority of unmodified polymers are diamagnetic, they do nor exhibit paramagnetic behaviour. Only in some cases, for example, for the polyimide-6 and poly(ether sulfone) a weak signal of electron paramagnetic resonance (EPR) with g = 2.0025 is observed due to a nonhomogeneous electron interaction caused by the heteroatoms in the polymer chain [46,162]. The radiothermolysis processes accompanying the implantation of polymers result in the massive rupture of chemical bonds, formation of free radicals and conjugated systems associated with the carbonaceous phase. Therefore, EPR measurements can provide valuable information on the structural alteration and electron states of the polymers undergoing the implantation. The EPR spectra of the polymers ion-implanted with fluences of about 1014 cm-2 and higher show an isotropic singlet with g-value of 2.0025 which is close to that of free electron (2.0023) [2,86-90]. This value also coincides (within the error) with the values of 2.0026 and 2.0027 that are characteristic of the conducting and pyrolised polymers [163-165]. This fact indicates the identity of the PCs in various carbon-based materials and for different methods of treatment.

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Intensity and width of the EPR line depend on implantation parameters (ion species, fluence, density of ion current) and type of polymer. For fluences of 1014-1015 cm-2 the linewidth ∆H varies in between 5-10 G and the intensity corresponds to concentration of the PC NPC about 1016-1017 spin/g [89]. With increase of ion fluence the NPC rises reaching saturation (Fig. 19) for fluences of 5×1015-5×1016 cm-2 depending on the ion species that is in good agreement with the threshold fluence for the polymer restructuring (see section “Effects of Degassing and Carbonisation”). The NPC value is found to be as high as (1-3)×1020 spin/g for the boron-implantated polyethylene and polyamide-6 and its slightly lower for the case of implantation of heavier ion species [73,90]. The highest NPC corresponds to one unpaired electron per ∼100 carbon atoms that is in agreement with typical NPC for the pyrolised polymers [163]. While the NPC value increases and saturates the ∆H narrows and also saturates with fluence. The narrowing of the line can be interpreted in terms of extension of the regions of electron delocalisation and strengthening of the π-electron exchange interactions [17,83]. For very high implantation fluences the intensity of the EPR signal tends to diminish after the saturation (like it is shown in Fig. 19) [11,88,92,166]. It is also found that the NPC is decreasing function of ion current density for the fixed high ion fluence [11].

Fig. 19. Concentration of paramagnetic centres versus ion fluence for 80 keV argon-implanted polyimide. According to [11].

Assuming the above-presented results on the paramagnetic properties, the following model in relation to the carbonisation process is developed. The process of accumulation of the PC strongly correlates with the growth of the carbonaceous phase under implantation: electrons are delocolised in the π-system of the carbon-enriched clusters; electron exchange interaction becomes stronger with overlapping of the clusters and expanding of the delocalisation region. The saturation in the PC concentration and line-width corresponds to the fluences causing completion of the cluster percolation: the unsaturated carbon bonds tend to recombination and formation of the more regular structures, however, there is no significant qualitative change in the electronic system yet. Implantation with highest fluences

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originates further structural alteration resulting in the percolation transition to the phase mostly consisting of amorphous carbon or graphite-like material that causes the observed depletion of the PC followed by the EPR line broadening due to weakening of the electronexchange interaction in the newly-formed material. Study of temperature dependence of the EPR parameters shows that for fluences above the percolation threshold the signal intensity drops (Fig. 20a) while there is no change in the line-width with temperature: ∆H ≈ 1.1-1.2 G (Fig. 20b), i.e. constant for temperature range of 320-460 K [17]. The case of weak influence of temperature on the width of the EPR line corresponds to electron states described within the framework of two-dimensional systems [167]. Thus, formation of a quasi-two-dimensional electron gas in the buried carbonised layer is possible. This possibility is confirmed when the EPR spectra are measured under stagnation pressure of oxygen [83]. The oxygen penetrating into the implanted layer recombines with the free radicals and destroys the electron system that is resulted in the broadening of the line with g = 2.0025 and appearance of additional spectral component with lower g-value and anisotropy behaviour. This effect is found to be reversible when the samples are pumped out of oxygen.

Fig. 20. Temperature dependences of EPR signal intensity (a) and line-width (b) for polyethylene (PE) and polyamide-6 (PA) implanted by 100 keV N+ ions with fluence of 5×1016 cm-2. According to [90].

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Similar to the study of optical properties of the metal/polymer composites synthesised by ion implantation, the investigation of magnetic properties of such systems is still under development. After first publications on the ferromagnetic properties of the metal-implanted polymers in the middle of the 80’s [26,168] next generation of results on this subject showed up only in the middle of the 90’s [35,169]. Since that time ferromagnetic properties of the transition metal NPs synthesised in various polymers by implantation has been under intensive study.

Fig. 21. FMR spectra of polyimide implanted by 40 keV Fe+ ions with different fluences. The left panel corresponds to parallel orientation and the right one – to perpendicular orientation of applied magnetic filed with respect to sample plane.

As mentioned in section “Metal Nanoparticle Formation” of this chapter, metal ion implantation with fluences higher than 1×1016 cm-2 results in formation of the NPs in polymers. When polymers are implanted by transition metals, like iron or cobalt, a ferromagnetic resonance (FMR) signal can be detected [35,36,136,170]. However, the signal appears only at some threshold fluences and depends on type of polymer as well as on the ion species. For the Fe-implanted silicone polymer and polyimide, the FMR signal is detectable for the fluence as low as 2.5×1016 cm-2 that correlates with the TEM data on the NPs nucleation. Intensity of the signal increases with fluence while the spectra gains strong anisotropy (Fig. 21) [134]. The phenomenon qualitatively resembles the anisotropy behaviour of the FMR signal of the continuous thin magnetic films, where the value of resonance field depends on the film orientation in the magnetic field [171]. Measurements of angular dependence of the effective anisotropy allow the conclusion that the iron-implanted polymers exhibit uniaxial out-of-plane type of anisotropy; magnetisation of the composite layer is

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directed in plane with the surface [135]. As mentioned before, the polymer viscosity affects nucleation and growth of the metal NPs that also results in change of the ferromagnetic properties. The resonance line shifts to low- or high-field range depending on the sample orientation in the magnetic field (similar to that shown in Fig. 21) and amplitude of the signal changes non-monotonically. It is also found that the out-of-plane magnetic anisotropy increases with viscosity [135]. From the observed hysteresis behaviour of the FMR spectrum for the polymers implanted by Fe+ ions with fluence of 1×1017 cm-2 [134,135] such property as remanent magnetisation with value of a coercive field about 270 G is established. Magnetic response of the Co NPs synthesised by the implantation of various polymers is much weaker compared to the Fe ones. The cobalt-epoxy nanocomposites show the FMR signal for implantation fluences as high as 1.8×1017 cm-2 at ion current density of 4 µA/cm2 [170,172]. The Co-implanted polyimides represent the ferromagnetic properties only after subsequent thermal annealing or for the case of implantation at high current densities in the ion beam (8 and 12 µA/cm2) [37] that is a sort of equivalence to annealing. Following explanation of these results is suggested. The granular metal layer in the as-prepared sample consists of small cobalt NPs in a superparamagnetic state at room temperature. Orientation of the magnetic moments of the particles is effected by thermal fluctuations. Hence, the signal of magnetic resonance can be observed only when frequency of the fluctuations decreases below the magnetic resonance frequency, i.e. at low temperatures. In fact, the FMR signal appears with cooling down to 100 K in the as-prepared samples implanted with fluences of 1.25×1017 and 1.50×1017 cm-2 at ion current density of 4 µA/cm2. The presents of the FMR signal for the annealed samples is explained by the coagulation and coalescence of the cobalt granules. The magnetic moments of these agglomerated particles are strongly magnetically coupled to each other either by exchange forces (if there is a direct contact between them) or by dipolar forces and Ruderman-Kittel-Kasuya-Yosida type of interactions via conduction electrons [173] (if the particles are distant). Thus, the “effective magnetic” size of the agglomerates exceeds a critical size beyond which orientations of the magnetic moments are nearly static compared to the magnetoresonance measurement time. In general, the ensemble of metal NPs formed in the implanted layer of polymer may behave as a thin layer of ferromagnetic continuum due to strong magnetic dipolar coupling between the particles. The magnetic percolation transition in this film may be observed by FMR measurements. The transition occurs when concentration of the magnetic NPs is high enough and strength of the interparticle coupling is comparable with Zeeman energy of the NPs in the external magnetic field [135]:

mi mi +1 ≈ mi H mean ri,3i +1

(5)

where mi is the magnetic moment of individual NP and ri is the average distance between the NPs, Hmean ~ 3300 G is the mean resonance field of the individual magnetic NP. In the ferromagnetic continuum approximation the resonance field for two limiting orientations of the magnetic field with respect to the sample plane may be determined by Kittel set of equations [174]:

Compositional and Structural Alterations of Polymers…

hν = gβ ⋅ ( H r − 4πM )

θH = 0o

hν = gβ ⋅ H r ⋅ ( H r + 4πM )

179 (6)

θH = 90o

where h is Plank constant, ν is the resonance frequency, Hr is the resonance magnetic field, β is Bohr magneton and M is the magnetisation for two orientations of plane of the implanted layer, parallel (θH = 0o) and perpendicular (θH = 90o), in respect to the magnetic field. These equations give a possibility to extract both the g-value and magnetisation. For the ironimplanted polymers the effective g-value is calculated to be 2.1±0.1 which is close to typical g-value of bulk iron film [135]. The calculated values of the magnetisation for various polymers implanted by Fe+ ions are presented in Fig. 22. For the polymers with the Fe NPs the magnetic percolation transition can be estimated from the dependence shown in the figure which is found to be corresponding to fluence of 6×1016 cm-2. The same result is found for the cobalt-implanted polyimide [37]. For the Co-containing polymers the decrease in the magnetisation is observed for the high-fluence implantation, which is considered to be due to the formation of non-magnetic cobalt phase, in particular carbonyls or carbides [37].

Fig. 22. Fluence dependence of magnetisation for various metal/polymer composites formed by implantation of 40 keV Fe+ ions.

Electrical Conductance Since the first publications on the ion-beam modification of polymers most of them have focused on the changes in conductance. It is of great importance that the conductance of ionirradiated polymers is originated by the formation of carbon-containing structures rather than

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by doping effects with exception of the metal implantation. Therefore, the conductance only slightly depends on the ion species being mainly determined by the energy transfer to polymer matrix during the stopping of ions, see for example [2,87,166]. Depending on type of polymer and implantation parameters (fluence, energy, density of ion current and temperature) it is possible to vary the resistivity within up to 18 orders of magnitude starting from pure dielectrics (ca. 1015-1018 Ω⋅cm) and ending in the range of poor conductors (10-110-3 Ω⋅cm). An example, how the resistance decreases with the increase of ion fluence and ion current density, is shown in Fig. 23 for the case of Ar-implanted polyimide. The abovementioned good correlation of conductance and optical band gap is seen as well. However, for the same implantation conditions the conductance can differ for a few orders of magnitude for diverse polymers, which is originated by the difference in the structural and compositional alterations under the ion-beam treatment.

Fig. 23. Resistivity and optical gap-width versus ion fluence for 40 keV argon-implanted polyimide. According to [11].

The increase in conductance is associated with the formation of the π-bonded clusters and net of conjugations. As result of the dehydrogenation and carbonisation the carbon atoms have tendency to clusterisation with sp2 hybridisation. This type of chemical bonding possesses unpaired π-electrons, which become charge carriers within the clusters and their agglomerates [175]. The most probable mechanisms providing the charge carriers transport through potential barriers of the dielectric media (polymer) between the clusters is hopping or tunnelling [80]. Since the conducting phase in the implanted layer is formed of the discrete clusters the conductance has a threshold character upon the fluence showing the percolation transition for the fluence range corresponding to the track overlapping. This percolation behaviour is confirmed by number of publications [2,166,176] and discussed in details, for

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instance, in [6,9]. A stick-slip nature in the conductivity dependence with subsequent saturation (plateau effect), which can be seen in Fig. 23 and found for various implanted polymers implanted to very high fluences [11,70,88,93,177], is in good agreement with the structural alteration of the polymer into the material containing amorphous carbon or graphite-like structures which is described in one of the previous sections of this chapter. Measurements of temperature T dependence of conductance or resistance give more detailed information on the mechanisms of charge transport. In general, the temperature dependence of conductivity σ can be described in terms of following equation

σ (T ) = σ 0 exp(− (T0 / T ) m )

(7)

where σ0 is the conductivity at T → ∞, T0 is the characteristic temperature. The power m is crucial for determining the conduction mechanism. For band conduction in extended states, m = 1 and kT0 is the activation energy, where k is the Boltzmann constant. If states are not extended but Andersen localisation throughout the whole band so that any mobility edge is in a higher energy band, a nearest-neighbour hopping occurs which can also lead to a temperature dependence with m = 1 [80]. For a truly disordered material, Mott has predicted a three-dimensional (3D) variable range hopping effect between localised states [179]. The m value in this case is 1/4. 2D and 1D models represent eq. (7) with the power equal to 1/3 and 1/2, correspondingly. Majority of polymers implanted by various ions with different energies but with low or medium fluences (1014-1015 cm-2) exhibit temperature dependence of conductance well described by eq. (7) with m = 1/2 [46,70,88,166]. It was suggested [88] that 1D hopping mechanism dominates in these cases. However, it is hard to believe that the disordered by implantation polymer can form the structures providing pure 1D conductance. Another possibility, suggested by Wang and co-authors [180], assumes that conduction along the ion tracks would be 1D in nature while conduction in the highly disordered region, toward the mean ion range, may be 3D. This model of the composite conduction gives reasonable good agreement with the experimental results and allows to calculate average characteristic temperature and activation energy, which are found to be decreasing functions of ion fluence. With further fluence increase, the value of power m is observed to be decreasing to 1/3 or 1/4, for example, for the polyimide implanted by N+ and Ar+ ions [41,93] as well as for the polyethylene and polyamide-6 bombarded by B+ and Sb+ ions [91,181] that is evidence of dominant contribution of the 3D hopping. There is also probability for the 2D variable range hopping to contribute into the complex mechanism of carriers transport especially in the case of the buried quasi-continuos carbonised layer for which the possibility of quasi-2D electron gas formation was suggested [83]. Several groups reported a conduction behaviour with m = 1 for the cases of either high-energy (MeV) [178] or high-fluence (1016 cm-2) [92] implantation of polymers. It is also found that m value can be close to 1 (0.7-0.8) but does not reach it even for the high-fluence (1017 cm-2) boron implantation into polyethylene [91]. These results are kind of extreme case when the heavily carbonised layer (possibly graphite-like) represents either the mechanism of conductance with constant activation energy or the “nearestneighbour hopping”. Metal-implanted polymers represent a special case regarding the conductance. Typically, resistivity of the layer implanted by metal ions is lower compared to the implantation of non-

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metal species with the same fluence. The highest values are reported for the Cu- and Agimplanted poly(ethylene terephthalate) reaching 1.5×10-4 Ω⋅cm for the fluence of 2×1017 cm-2 [182]. This allows assuming that in the samples implanted with high-fluence metal ions the electron transport is caused by both the radiation-induced changes of the material and the metal NPs formation. For the Cs- and Ga-implanted polymers, conductivity versus temperature is found to be following the dependence σ(T) ∼ T -1 that corresponds to the activation mechanism [183,184]. However, the variable range hopping is found to be dominating mechanism for the polyimide implanted with high fluences (2.5×1016-1.25×1017 cm-2) of Co+ ions at a relatively low ion current density of 4 µA⋅cm-2 [185]. As can be seen in Fig. 24 the curves corresponding to the lowest fluences follow a linear function in co-ordinates R-(1/T)1/4 but only in the high-temperature interval of the measurements. With fluence increase, the linear function with m = 1/4 extrapolates the experimental dependences down to T ≈ 40 K. Below this temperature, R ∼ (1/T)1/3 (Fig. 24, insertion). The change of m from 1/4 to 1/3 corresponds to the transition from 3D to 2D variable range hopping.

Fig. 24. Temperature dependence of normalized resistance for polyimide implanted by 40 keV Co+ ions with various fluences at ion current density of 4 µA⋅cm-2. The low-temperature interval for fluence 1.25×1017 cm-2 is in insertion. According to [185].

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Co+ ion implantation with fluence of 1.25×1017 cm-2 at j = 8 and 12 µA⋅cm-2 leads to a significant change in the temperature dependence of the resistance (Fig. 25). The dependence with a minimum is typical for disordered (granular) metal films. Calculation of a local activation energy using the method proposed in [186] allows to suggest semimetallic or metallic type of the electron transport in these samples or, in other words, Andersen insulatorto-metal transition due to the agglomeration of the Co NPs forming a percolation way for the charge carriers. It is shown that both quantum effects of weak localisation and electronelectron interaction give a significant contribution to the transport mechanism and conductivity can be described by the equation

σ (T ) = σ 0 + AT 1 / 2 + B ln T

(8)

where А and В are fitting parameters. The transition to metallic type of conduction is also observed for the very high-fluence (up to 3×1017 cm-2) and high-temperature (up to 620 K) implantation of non-metallic Ar+ and N+ ions into polyimide [92] which is probably related to complete restructuring of the implanted layer and ordering of the carbonaceous phase under the intensive radiation and thermal treatment.

Fig. 25. Temperature dependence of normalized resistance for polyimide implanted by 40 keV Co+ ions with fluence of 1.25×1017 cm-2 at various ion current densities. According to [185].

Current-voltage (I-V) dependences of the implanted polymers are found to be linear [91,153,187]. However, a hysteresis-like behaviour for the I-V plots is observed for the polyethylene implanted with medium fluences (Fig. 26) [91]. This effect can be attributed to the aligning of the electric dipoles in the implanted layer by the applied electric field: the orientation of the dipoles being retained due to a relatively high resistivity of the layer. The

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occurrence of the dipole moment is related to the individual carbon clusters enriched by πelectrons and separated from each other by insulating barriers. With fluence increase over 1×1016 cm-2 the effect vanishes that is explained by the extended overlapping of the clusters.

Fig. 26. Current-voltage dependence for polyethylene implanted by 100 keV B+ ions with fluence of 5×1015 cm-2. According to [91].

CONCLUSIONS The present chapter discusses the implantation-induced modification of polymers by keV energy ions. Thermal effects caused by the ion stopping together with the radiation-induced phenomena originate complex radiothermolysis process. The structural changes of polymers are resulted in the scission and cross-linking of polymer chains, formation of volatile lowmolecular fragments followed by their degassing and carbonisation of the implanted layer. The carbonisation process occurs by few stages with increase of implantation fluence or/and ion current density: (i) formation of “pre-carbon” structures; (ii) nucleation and growth of the carbon-enriched clusters; (iii) aggregation of the clusters resulting in the formation of network of conjugated bonds; (iv) transition to amorphous carbon or graphite-like material. The carbonisation process depends mainly on the ion fluence and energy and to a smaller extent on ion species (only in sense of ion mass). The carbonaceous phase formed under the highfluence implantation is slightly dependent on the initial polymer nature. It is emphasised in the chapter that information about depth distribution of the radiationinduced damages can be obtained by means of decoration, i.e. by diffusion and trapping of the organic and non-organic agents on the radiation defects in the implanted polymer layer. This method allows collecting data on peculiarities of the radiation defects formation under various implantation regimes. The phenomenon of carbonisation can be studied and the depth distribution of the “carbon excess” can be evaluated.

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Reviewing the studies on depth distribution of the implanted species show significant difference of the experimental profiles from those theoretically calculated. In general, because of relatively low radiation stability of majority of polymers, entire implantation process carries out by few stages: each of them is characterised by the gradual change of the polymer structure and composition. All cases of implantation are splitted into few specific groups, namely: implantation of light ions and implantation of heavy ions; implantation of gaseous species (mainly inert gases) and implantation of metals. For all cases, the carbonaceous phase formation is found to be significant phenomenon effecting the ion projected ranges and straggling. Thus, the final depth profile of the ion species is a superposition of particular depth profiles for various stages of the implantation or, in other words, for various phases of the modified polymer. Diffusion also influences the final depth distribution of the implanted impurities especially in the cases of inert gases and light ions: model of diffusive redistribution of the implanted species is discussed in the paper. Metal nanoparticle nucleation is a special case of the high-fluence implantation of metal ions. The nanoparticle formation is governed by the local metal concentration and metal diffusion coefficient as well as by parameters of the polymer material such as density (carbonisation effects it during the implantation), composition and viscosity. Structural and composition alterations of the implanted polymer layers result in drastic change of chemical and physical properties. New mechanical and trybological properties of the radiation-modified surfaces show the possibility to use the polymers as protective coatings Sensitivity of the implanted polymer surfaces to moisture opens their application as humidity sensors [188]. Ion-induced modification of some specific polymers like, for example, metallophthalocyanines widens possibility to control the surface absorption which is responsible for the sensitivity to small concentrations of environmental gases (including toxic ones) [189]. Dependence of the conductance on the gas absorption opens a way for improvement of the phthalocyanine gas sensors. Formation of the specific centres for absorption of bioplasts on the implanted polymer surface allows introducing the polymer materials into medicine as implants. Control of polymer conductance by ion implantation is of great importance. Polymer materials can be used as active elements of electronic devices. Disadvantage of the radiationmodified polymers is low mobility of charge carriers. However, low prices and specific properties as plasticity and suppleness as well as stable dependence of the conductance on temperature give them an advantage to be employed for fabrication of resistors, varistors and temperature sensors [4]. Moreover, formation of the buried carbonaceous conductive layer in the polymer matrix by means of implantation makes it possible to fabricate transistor-like electronic switches operating in the AC mode [91,190]. By utilising the piesoresistive properties of the implanted polymers the polymer strain gauges were produced [191]. Possibility of control over optical properties of polymers by implantation gives an impulse for fabrication of passive optical devises as filters, waveguides, coatings for lenses etc. The synthesis of metal nanoparticles in polymer media by ion implantation founds new area of material science with perspectives for nanophotonics, plasmonics and non-linear optics as well as for magnetosensoring and magnetoelectronics. Thus, ion implantation is a powerful and versatile tool for modification of polymers. Control of the implantation energy, fluence and ion species is key point for obtaining of the materials with required parameters. However, capability of this experimental technique as well as details of physical and chemical processes accompanying ion implantation into

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polymers require further study to develop polymer-based devices and application-oriented technologies.

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