Modification in surface properties of poly-allyl-diglycol-carbonate (CR-39) implanted by Au⁺ ions at different fluences

Transparent, flat sheet of poly-allyl-diglycol-carbonate, CR-39, with thickness of 2.75 mm was cut into rectangular pieces with dimension 2.3 cm × 1 cm. These samples were ultrasonically cleaned with deionized water. The ion implantation of polymer samples was performed under vacuum ~1.33 × 10⁻⁶ Pa at room temperature by using 400 keV Au⁺ ions with ion fluences 5 × 10¹³ ions/cm², 1 × 10¹⁴ ions/cm², 5 × 10¹⁴ ions/cm², 1 × 10¹⁵ ions/cm², and 5 × 10¹⁵ ions/cm² using 400 kV NEC ion implanter. The total beam current on the target was set at 0.85 μΑ so that the desired ion fluence could be achieved and thermal degradation of the samples could be avoided.

The pristine and ion implanted samples were characterized by Raman, Fourier transform infrared (FT-IR) spectroscopy, atomic force microscopy, UV-Visible (UV-Vis) spectroscopy and four-point probe technique in order to explore the modifications in structural and chemical properties, surface morphology, optical and electrical properties after ion implantation. Raman spectra were recorded in the range of 400 cm⁻¹ to 3300 cm⁻¹ by Ramboss Raman spectrometer with argon laser operating at 514 nm. The FT-IR studies were carried out in transmission mode in the range of 500 cm⁻¹ to 4000 cm⁻¹ using ALPHA FT-IR spectrometer with an attenuated total reflectance (ATR) assembly. Surface morphology of CR-39 after implantation was analyzed by JSPM-5200, JEOL atomic force microscope in tapping mode. The optical properties of Au⁺ ion implanted CR-39 were investigated in the range of 300 nm to 900 nm using Hitachi UV-Vis U-2800 spectrometer. The DC conductivity of implanted samples was measured using four-point probe apparatus with Keithley 6220 current-source and Keithley 2182 nanovoltmeter.

In order to proceed systematically and to avoid material wastage, it is preferable that before ion implantation, parameters of the material and the process are simulated by using the codes like SRIM/TRIM codes.

The value of electronic energy losses (S_e), nuclear energy losses (Sn) and projected range of 400 keV Au⁺ ions in CR-39 samples were calculated using SRIM 2008 software [20] and are tabulated in Table 1.

When 400 keV Au⁺ ions are incident on CR-39, they transfer their energy to the polymer. As CR-39 with chemical formula (C₁₂H₁₈O₇)_n, has carbon, hydrogen and oxygen atoms in its structure, the energy of incident ions is absorbed by these atoms. As a result, they are knocked out from their lattice sites, leaving vacancies there. When the displaced atoms collide with other target atoms along their trajectories, they cause collision cascade in the material which dominates the damage process [21]. TRIM simulation has been performed to analyze the damages produced in CR-39 after Au⁺ ion implantation [20]. Fig. 1a presents the estimated ion trajectories for 1000 Au⁺ ions. It is to be mentioned here that different colors have been used to show the tracks of the moving C, H and O atoms in the material, as labeled in Fig. 1a. The vacancies-depth distribution for 400 keV Au⁺ ion implanted CR-39, estimated using TRIM simulation, is presented in Fig. 1b. The incident ions have a mean projected range of 220 nm, as given in Table 1. This is the most probable projected range of 400 keV Au⁺ ions in CR-39. According to the TRIM simulations, the maximum of hydrogen vacancy distribution is at a depth of 167 nm. Much smaller number of vacancies per incident Au⁺ ion is created by displacements of heavier atoms: carbon and oxygen, as shown in Fig. 1b. Due to the damage process that occurred in Au⁺ ion implanted CR-39, the chemical structure of the polymer has changed.

Fig. 1

The visual examination of implanted samples revealed that the color of the samples was changed after ion implantation. It was observed that the pristine CR-39 sample was transparent and colorless. The sample implanted at fluence 5 × 10¹³ ions/cm² appeared to have creamy color. For the sample implanted at ion fluence 1 × 10¹⁴ ions/cm², the color became pale yellow. Then the color transformed to yellowish brown, light brown and dark brown for samples implanted with ion fluences 5 × 10¹⁴ ions/cm², 1 × 10¹⁵ ions/cm², 5 × 10¹⁵ ions/cm², respectively. This color change is related to the structural modifications produced in the ion implanted polymer [22].

In order to analyze the chemical modifications produced in CR-39 after Au⁺ ion implantation, the Raman spectroscopy has been performed.

In the Raman spectrum of pristine sample (Fig. 2), several peaks occur at 828 cm⁻¹, 894 cm⁻¹, 960 cm⁻¹, 1026 cm⁻¹, 1129 cm⁻¹, 1292 cm⁻¹, 1453 cm⁻¹, 1646 cm⁻¹, 1742 cm⁻¹, 2908 cm⁻¹, 2952 cm⁻¹. The bands at 828 cm⁻¹, 894 cm⁻¹, 960 cm⁻¹ are originated from =C–H bending mode. The bands at 1026 cm⁻¹, 1129 cm⁻¹, 1292 cm⁻¹ are due to C–O–C stretching vibrations. At 1453 cm⁻¹, the medium intensity band is correlated to –C–H– bending mode. The bands at 1646 cm⁻¹, 1742 cm⁻¹ indicate the presence of C=C and C=O bonds, respectively. The highest intensity bands at 2908 cm⁻¹, 2952 cm⁻¹ are attributed to symmetric and asymmetric CH2 stretching, respectively. From the presence of these bands, the monomer structure of CR-39 is confirmed [23].

Fig. 2

As a result of Au⁺ ion implantation, all the bands are eliminated (Fig. 3) which points towards the polymer chain scissoring. The disappearance of bands at 2908 cm⁻¹, 2952 cm⁻¹ after implantation indicates the reduction of hydrogen content in the surface of implanted samples [24]. Such dehydrogenation may result in the formation of unsaturated bonds and sp² carbon clusterization in implanted CR-39.

Fig. 3

As a result of implantation (Fig. 3, curve a), the appearance of a broad band at 1577 cm⁻¹, with a big shoulder at 1391 cm⁻¹, is noticed. These two bands are named as the well-known graphite-like G and disorder D bands of hydrogenated amorphous carbon structure. The G band is correlated to the bond stretching of all pairs of sp²-atoms in both rings and chains, whereas the D band refers to the breathing mode of sp²-atoms in rings only [1]. The value of intensity ratio I_D/I_G for the ion fluence of 5 × 10¹⁴ ions/cm² was found to be 0.52. This indicates that the carbonaceous structure with small amount of sp² C–C bonding is formed in ion implanted CR-39 [1].

With further increase in ion fluence to 5 × 10¹⁵ ions/cm² (Fig. 3, curve b), the peak positions of G and D bands shift to 1582 cm⁻¹ and 1402 cm⁻¹, respectively. The ratio I_D/I_G is found to decrease to the value of 0.46. This behavior suggests that sp³ bonds are formed at high implantation fluence. This results in the ordered structure of implanted layer of CR-39 at high fluences [25].

In order to study the changes produced in the functional groups of implanted polymer samples, FT-IR spectroscopy analysis has been carried out, as shown in Fig. 4.

Fig. 4

In the spectrum of pristine CR-39, three strong intensity bands appear at 788 cm⁻¹, 1234 cm⁻¹ and 1745 cm⁻¹ corresponding to =C–H bending modes [26], C–O–C stretching [27] and C=O stretching, respectively [26, 27]. Other weak intensity bands at 1394 cm⁻¹, 1458 cm⁻¹ relate to C–H bending modes, whereas the bands in the region of 1000 cm⁻¹ to 1200 cm⁻¹ are attributed to C–O stretching mode [26]. The band at 2935 cm⁻¹ is due to C–H stretching [28]. The appearance of these bands at their particular wave number confirms the monomer structure of CR-39.

In the spectra of CR-39 implanted by Au⁺ ions with different ion fluences, the intensity of all bands of implanted samples is noticed to decrease at all ion fluences, suggesting degradation of polymer samples [28]. However, no change in the position of the bands is observed. Moreover, no prominent changes in terms of new peaks emergence are observed in the FT-IR spectra which may be due to lower sensitivity of FT-IR measurements for very thin implanted subsurface layer as only a subsurface layer with the thickness of 220 nm (SRIM estimation) is modified during implantation in comparison with the sample thickness (2.75 mm).

In addition to the overall decrease in intensity, a new broad peak appears at 3450 cm⁻¹ at all ion fluences. This band corresponds to the formation of alcohol or phenol group. This suggests that free radicals are formed after chain scission at carbonate site as a result of ion implantation. These radicals react with oxygen present in air, producing OH groups. Even though the implantation was carried out in vacuum, oxidized compounds are supposed to be formed when the samples are put in air after implantation [29]. Simultaneously, a change in percentage transmittance is observed in the region of 1550 cm⁻¹ to 1650 cm⁻¹ which is caused by the formation of carbon-rich structures [30]. This is in agreement with the results of Raman analysis.

Thus, Raman and FT-IR spectra reveal that Au⁺ ion implantation of CR-39 results in bond breaking, chain scissoring, formation of conjugated bonds and carbonaceous clusters due to which the structure of Au⁺ ion implanted CR-39 is altered.

In order to examine the change in surface morphology and to measure roughness values of CR-39 implanted by 400 keV Au⁺ ions of different fluences, atomic force microscopy study was performed. Fig. 5 shows topographic scans of pristine and Au⁺ ion implanted samples with fluences ranging from 5 × 10¹³ ions/cm² to 5 × 10¹⁵ ions/cm².

Fig. 5

In Fig. 5a, a relatively rough surface of pristine CR-39 is observed. Upon implantation by Au⁺ ions at 5 × 10¹³ ions/cm², a significant change in the surface topography is obtained. Compared to the virgin sample, the one implanted by Au at 5 × 10¹³ ions/cm² shows the appearance of newly formed grain-like features and consequently increase in surface roughness. The surface swelling of the implanted sample is also noticed, as is presented in Fig. 5b. During implantation, Au⁺ ions are being neutralized along their way in the sample. As a result, Au atoms are produced which occupy a near surface region up to the target depth [31]. Since the kinetic energy of implanted Au⁺ ions is rather high along the longitudinal direction of implanted sample, it contributes to local temperature increase in addition to the diffusion of Au atoms. It causes the surface swelling of the implanted sample [32]. Increasing the implantation fluence to 5 × 10¹⁴ ions/cm², spiky cone phases with average diameter of 100 nm to 200 nm and height of 5 nm to 12 nm, become visible on the implanted sample as shown in Fig. 5c. According to thermal spike model, when the temperature of cylindrical volume in the ion track reaches the melt phase, the material pushes out from the surface and is quenched by thermal conduction [33]. For the highest implantation fluence, the spikes are slightly revealed but their surface density is increased as observed in Fig. 5d.

The root mean square roughness (RMS) of pristine and Au⁺ ion implanted CR-39 at different fluences is given in Table 2.

It is clear from the Table 2 that RMS roughness of 8.95 nm is achieved already for the lowest implantation fluence. At ion fluence of 5 × 10¹⁴ ions/cm², the roughness value decreased to 1.02 nm and finally at the ion fluence of 5 × 10¹⁵ ions/cm², the root mean square roughness was reduced to 0.66 nm. These observations are in good agreement with the Raman analysis described earlier, where at highest implantation fluence, the implanted layer of CR-39 is more diamond-like carbon than that implanted at 1 × 10¹⁵ ions/cm². Upon interaction of Au ions with the polymer at high fluences, the large chains in the structure are broken and side chains are formed which easily pack and recombine. As a result, the implanted surface becomes smoother, leading to the decrease in surface roughness [32]. Such a two-stage behavior was also observed in case of high density polyethylene implanted by Ag+ ions at different fluences [31].

Due to the changes observed in the structure and surface morphology of CR-39 after ion implantation, its optical and electrical properties in the implanted layer are modified.

In order to investigate the modification produced in the electrical properties of metal ion implanted CR-39, the DC electrical conductivity was measured for pristine and Au⁺ ion implanted CR-39. The obtained results are, thus, presented in Table 4.

It is observed that the electrical conductivity of implanted samples is enhanced significantly in comparison with that of pristine sample. With the increase in ion fluence, the electrical conductivity of implanted samples goes on increasing and approaches a value of 6.52 × 10⁻⁰⁶ (Ω･cm)⁻¹ at maximum ion fluence of 5 × 10¹⁵ ions/cm². Since the electrical conductivity of semi-insulating materials lies in the range of 10⁻⁶ (Ω･cm)⁻¹ to 10⁻⁹ (Ω· cm)⁻¹, it can be inferred that Au⁺ ion implanted CR-39 can be utilized in different semi-insulating devices.

The reason for the observed increase in electrical conductivity of implanted samples with respect to that of pristine sample is the formation of extended system of conjugated bonds in the implanted samples [1], as already has been confirmed from Raman studies. The delocalized π-electrons are formed in the carbonaceous clusters as a result of sp³-sp² rehybridization [41]. Thus, carbon clusters act as charge carriers/conductive paths in the implanted samples. With the increase in ion fluence, the growth of carbon clusters is increased. Thus, charge carrier density is increased. In these conductive islands, the delocalized π-electrons move through hopping between the islands. This hopping current enhances the electrical conductivity of implanted samples [1].

A correlation has been established between the optical and electrical properties of implanted samples with the increase in ion fluence. The carbonenriched domains are formed due to disordering produced in the implanted CR-39 as a result of ion implantation. The optical band gap of implanted CR-39 is decreased, while electrical conductivity of the polymer is increased after ion implantation. On the basis of these results, it can be inferred that ion implanted CR-39 can be utilized in various opto-electronic devices.