**3.1. SRIM simulation data**

**Figure 1** shows the typical SRIM simulation results for B+ -implantation into PMMA at energy of 40 keV [36, 45]. Numerical values of SRIM simulation for 40 keV He+ , B+ , O+ , P+ , Cl+ , Cu+ , Ag+ , Xe+ , and Au+ ions into PMMA are gathered in **Table 1** [48].

**Figure 1.** Depth profiles of implanted ions (top, left), introduced vacancies (top, right), 3D image of ion distribution (bottom, left) and depth vs. Y-axis (bottom, right) for B:PMMA. Adapted from [36, 45].


**Table 1.** SRIM simulation results for 40 keV ion implantation into PMMA (*<sup>a</sup>* data from the work [36]; *<sup>b</sup>* data from www.webelements.com; *<sup>c</sup>* data are estimated with error ± 5 nm) [48].

The SRIM data, presented in **Table 1**, were used for estimation a predicted thickness of implanted layer [48]. Let us to explain it on the example of the modified PMMA by 40 keV accelerated B+ ions. A mean penetration range (*R*<sup>p</sup> B) is about 253 nm with a longitudinal straggling (Δ*R*<sup>p</sup> B) of 58 nm in the Gaussian depth distribution. The assumed predicted thickness of the modified PMMA surface layer (*R*<sup>p</sup> <sup>B</sup> + 2Δ*R*<sup>p</sup> B) is about 369 nm, and the maximum penetration depth (*R*maxB) is about 400 nm. At the same time, the vacancy distribution gives a maximum damage around *R*<sup>p</sup> V,B = 220 nm and maximum depth up to around *R*maxV,B = 380 nm. Thus, the estimations performed for various ions, listed in **Table 1**, indicate the possible modification of PMMA surface upon low-energy ion implantation in dependence on the ion mass. These values could also be useful in practice for evaluation of geometrical parameters of ion-implanted layers in PMMA matrix.

#### **3.2. Positron annihilation spectroscopy data**

The 40 keV B:PMMA polymers with different ion doses have been studied for the first time using PAS techniques such as SPBS [36] and temperature-dependent PALS [37].

SPBS, often called variable-energy PAS, is a powerful experimental tool widely used for evaluation of defects in solids as a function of depth (defect depth profiling) by varying the positron energy in the range of a few eV to tens keV (for review, see Ref. [49]). Thus, SBPS could be a very effective for the detection of defects induced by ion implantation, which are localized near the surface of material. At first, the un-implanted PMMA and implanted B:PMMA were characterized by Doppler broadening of annihilation gamma rays (or DBAL) as a function of incident positron energy in the range of 0–30 keV. And then, using Doppler broadening results, the PAL spectra at incident positron energy of 2.15 keV were measured and analyzed for the investigated samples. **Figure 2** shows the typical variable-energy DBAL and PAL results obtained, the explanation of which has been done in [36].

**Ion** *<sup>b</sup>*

292 Radiation Effects in Materials

*a*

*a*

*a*

www.webelements.com; *<sup>c</sup>*

accelerated B+

straggling (Δ*R*<sup>p</sup>

**Atomic weight**

*c R***maxIon, nm**

**Table 1.** SRIM simulation results for 40 keV ion implantation into PMMA (*<sup>a</sup>*

of the modified PMMA surface layer (*R*<sup>p</sup>

of ion-implanted layers in PMMA matrix.

**3.2. Positron annihilation spectroscopy data**

maximum damage around *R*<sup>p</sup>

*R***p Ion, nm**

He+ 4.0026 880 593 102 820 560

B+ 10.81 400 253 58 380 220

O+ 15.999 300 169 45 280 150 P+ 30.974 200 99 29 190 70 Cl+ 35.45 160 85 23 170 60 Cu+ 63.546 130 69 18 140 40 Ag+ 107.87 100 56 11 95 35 Xe+ 131.29 90 56 10 90 30

Au+ 196.97 75 54 7 60 25

The SRIM data, presented in **Table 1**, were used for estimation a predicted thickness of implanted layer [48]. Let us to explain it on the example of the modified PMMA by 40 keV

<sup>B</sup> + 2Δ*R*<sup>p</sup>

penetration depth (*R*maxB) is about 400 nm. At the same time, the vacancy distribution gives a

Thus, the estimations performed for various ions, listed in **Table 1**, indicate the possible modification of PMMA surface upon low-energy ion implantation in dependence on the ion mass. These values could also be useful in practice for evaluation of geometrical parameters

The 40 keV B:PMMA polymers with different ion doses have been studied for the first time

SPBS, often called variable-energy PAS, is a powerful experimental tool widely used for evaluation of defects in solids as a function of depth (defect depth profiling) by varying the positron energy in the range of a few eV to tens keV (for review, see Ref. [49]). Thus, SBPS could be a very effective for the detection of defects induced by ion implantation, which are localized near the surface of material. At first, the un-implanted PMMA and implanted B:PMMA were characterized by Doppler broadening of annihilation gamma rays (or DBAL) as a function of incident positron energy in the range of 0–30 keV. And then, using Doppler broadening results, the PAL spectra at incident positron energy of 2.15 keV were measured and analyzed for the investigated samples. **Figure 2** shows the typical variable-energy DBAL

using PAS techniques such as SPBS [36] and temperature-dependent PALS [37].

and PAL results obtained, the explanation of which has been done in [36].

B) of 58 nm in the Gaussian depth distribution. The assumed predicted thickness

V,B = 220 nm and maximum depth up to around *R*maxV,B = 380 nm.

data are estimated with error ± 5 nm) [48].

ions. A mean penetration range (*R*<sup>p</sup>

**Δ***R***<sup>p</sup> Ion, nm**

*c R***maxV, nm**

data from the work [36]; *<sup>b</sup>*

B) is about 253 nm with a longitudinal

B) is about 369 nm, and the maximum

*c R***p V, nm**

data from

**Figure 2.** Dose dependence of Doppler *S* and *W* parameters as a function of incident positron energy in the range of 0– 30 keV (top) and *S* parameter and *o*-Ps intensity at incident positron energy of 2.15 keV (bottom) for B:PMMA. The error bars are within the size of the symbol. Adapted from [36].

It should only be emphasized here that there are two different processes seen from the *S*-*E* and *W*-*E* curves in dependence on the ion implantation dose [36]. In particular, (i) *S*(*E*) increases and *W*(*E*) decreases at lower fluences (6.25 × 1014–3.13 × 1015 ions/cm2 ), while (ii) *S*(*E*) decreases and *W*(*E*) increases at higher fluences (2.5 × 1016–5.0 × 1016 ions/cm2 ) in the range up to 400 nm in a good agreement with the maximum penetration depth *R*maxB = 400 nm after SRIM simu‐ lation; positron energy of 2–3 keV, showing the extreme values of *S*(*E*) and *W*(*E*), corresponds to a mean depth of 100–200 nm in consistent with the maximum damage *R*<sup>p</sup> V,B = 220 nm after SRIM simulation (**Figure 2**, top). A mean penetration depth of positrons (*z*m) was estimated as *z*m = (40/*ρ*)*En*, where *z*m is presented in nm, *E* the positron energy in keV, *ρ* is the density (1.18 g/cm3 for PMMA), and *n* = 1.6 [50]. The results of variable-energy DBAL and PAL measure‐ ments at incident positron energy of 2.15 keV were found to be in consistence (**Figure 2**, bottom) [36]. That is, the decreasing suddenly the *S*(*E*) values and an absence of any observable *o*-Ps yield (intensity *I*3 ~ 0) for the implanted samples at higher ion doses were detected to be explained due to carbonization effect, taking into account that no *o*-Ps yield has been observed in carbon-based materials such as, for instance, fullerene C60 cage [51] and carbon molecular sieve membranes [52].

Thus, the expected two processes of polymer structure modification upon low-energy ion implantation—formation of free radicals at lower fluences (< 1016 ions/cm2 ) and carbonization at higher fluences (>1016 ions/cm2 )—are plausibly confirmed [36].

The results of temperature dependent PALS measurements of the investigated pristine PMMA and implanted B:PMMA samples at lower ion dose (3.13 × 1015 ions/cm2 ) and at higher ion dose (3.75 × 1016 ions/cm2 ) are demonstrated in **Figure 3**. The detail description and possible explanation of these data have been presented in [37]. Two structural transitions in the vicinity of ~150 and ~250 K, ascribed to *γ* and *β* transitions, respectively, should be noted here, which are observed in the both PMMA and B:PMMA. These structural transitions are found to be in consistent with reference data for PMMA [53, 54].

**Figure 3.** *o*-Ps lifetime and intensity as a function of temperature in the range of 50–298 K for (top) PMMA, (middle) B:PMMA (3.13 × 1015 ions/cm2 ), and (bottom) B:PMMA (3.75 × 1016 ions/cm2 ). The dashed lines are drawn as a guide for the eye. Adapted from [37].

The threshold temperatures nearby ~150 and ~250 K for PMMA are also detected on Doppler parameters *S*/*S*0 vs. *W*/*W*0 at different temperatures, normalized to the *S*0 and *W*<sup>0</sup> values at room temperature, using low-temperature DBAL measurements [39] as shown in **Figure 4**. The observed correlation in temperature dependences of *S* and *W* parameters and *o*-Ps data (see **Figure 3**) is found to be in a good agreement with literature PALS and DBAL data for PMMA as well [33].

Thus, the expected two processes of polymer structure modification upon low-energy ion

The results of temperature dependent PALS measurements of the investigated pristine PMMA

explanation of these data have been presented in [37]. Two structural transitions in the vicinity of ~150 and ~250 K, ascribed to *γ* and *β* transitions, respectively, should be noted here, which are observed in the both PMMA and B:PMMA. These structural transitions are found to be in

**Figure 3.** *o*-Ps lifetime and intensity as a function of temperature in the range of 50–298 K for (top) PMMA, (middle)

), and (bottom) B:PMMA (3.75 × 1016 ions/cm2

)—are plausibly confirmed [36].

) are demonstrated in **Figure 3**. The detail description and possible

) and carbonization

) and at higher ion dose

). The dashed lines are drawn as a guide for

implantation—formation of free radicals at lower fluences (< 1016 ions/cm2

and implanted B:PMMA samples at lower ion dose (3.13 × 1015 ions/cm2

at higher fluences (>1016 ions/cm2

consistent with reference data for PMMA [53, 54].

(3.75 × 1016 ions/cm2

294 Radiation Effects in Materials

B:PMMA (3.13 × 1015 ions/cm2

the eye. Adapted from [37].

**Figure 4.** The normalized parameters *S*/*S*0 vs. *W*/*W*0 at different temperatures in the cycles of heating and cooling for PMMA. The solid lines are drawn as a guide for the eye. Adapted from [39].

New results are also obtained from the PALS study of B:PMMA on the free-volume voids distribution at room temperature [37] as shown in **Figure 5**. Namely, estimating the distribu‐ tion of *o*-Ps lifetime (or the distribution of free volume detected by *o*-Ps) by MELT program similarly as it has been done in work [55], it is found that B+ -ion implantation leads to the shortening the lifetime distribution and decreasing molecular weight in PMMA at lower ion dose (3.13 × 1015 ions/cm2 ), while at higher ion dose (3.75 × 1016 ions/cm2 ) the broadening the lifetime distribution is probably caused by local destruction of PMMA matrix and generation of additional free volumes.

**Figure 5.** *o*-Ps lifetime distributions in B:PMMA at different ion doses at room temperature. Adapted from [37].

#### **3.3. Optical spectroscopy data**

Application of UV-visible optical absorption spectroscopy for investigation of ion-implanted polymeric materials has already been reported for the Ag+ -implanted PMMA, ORMOCER, and Epoxy resin [31, 34, 56–59] as well as for the C+ , N+ , and Ar+ -implanted PMMA [29, 30]. It has been suggested by the authors that ion irradiation creates compact carbonaceous clusters in polymers, which may also be responsible for a narrowing of optical band gap, enhanced electrical conductivity, and increasing optical absorbance (for example, see [31, 34, 56]). In the case of the investigated B:PMMA, it is found the gradual increase of absorbance at lower fluences (<1016 ions/cm2 ) and saturation of absorbance at higher fluences (>1016 ions/cm2 ) as shown in **Figure 6** [36].

**Figure 6.** The UV-visible optical absorption spectra of B:PMMA. Adapted from [36].

The observed increasing absorbance for the B:PMMA samples in the course of the ion implan‐ tation should be also interpreted as the signature on the formation of carbonaceous clusters which are plausibly confirmed by slow positrons [36].

#### **3.4. Raman spectroscopy data**

**Figure 7** shows the Raman spectra of the investigated samples excited by 514 nm and 785 nm diode laser lines [43]. Identification of the observed Raman bands has been reported in [43]. The ion-irradiation-induced structural changes as revealed from Raman study can be sum‐ marized as follows. New C=C and C−C bands in the vicinity of ~1590 and 1322 cm−1, respec‐ tively, are formed for the as-implanted samples at higher fluences (>1016 ions/cm2 ). At the same time, the decreasing intensity of CH2 band at ~815 cm−1, C=O band at ~1730 cm-1, O−CH3 band at ~991 and 2845 cm-1, and C−H band at ~1455, 2955, and 3001 cm−1 is detected.

Ion-Irradiation-Induced Carbon Nanostructures in Optoelectronic Polymer Materials http://dx.doi.org/10.5772/62669 297

**3.3. Optical spectroscopy data**

296 Radiation Effects in Materials

fluences (<1016 ions/cm2

shown in **Figure 6** [36].

polymeric materials has already been reported for the Ag+

**Figure 6.** The UV-visible optical absorption spectra of B:PMMA. Adapted from [36].

which are plausibly confirmed by slow positrons [36].

**3.4. Raman spectroscopy data**

The observed increasing absorbance for the B:PMMA samples in the course of the ion implan‐ tation should be also interpreted as the signature on the formation of carbonaceous clusters

**Figure 7** shows the Raman spectra of the investigated samples excited by 514 nm and 785 nm diode laser lines [43]. Identification of the observed Raman bands has been reported in [43]. The ion-irradiation-induced structural changes as revealed from Raman study can be sum‐ marized as follows. New C=C and C−C bands in the vicinity of ~1590 and 1322 cm−1, respec‐

time, the decreasing intensity of CH2 band at ~815 cm−1, C=O band at ~1730 cm-1, O−CH3 band

tively, are formed for the as-implanted samples at higher fluences (>1016 ions/cm2

at ~991 and 2845 cm-1, and C−H band at ~1455, 2955, and 3001 cm−1 is detected.

and Epoxy resin [31, 34, 56–59] as well as for the C+

Application of UV-visible optical absorption spectroscopy for investigation of ion-implanted

has been suggested by the authors that ion irradiation creates compact carbonaceous clusters in polymers, which may also be responsible for a narrowing of optical band gap, enhanced electrical conductivity, and increasing optical absorbance (for example, see [31, 34, 56]). In the case of the investigated B:PMMA, it is found the gradual increase of absorbance at lower

, N+

, and Ar+

) and saturation of absorbance at higher fluences (>1016 ions/cm2



) as

). At the same

**Figure 7.** Raman spectra of PMMA and B:PMMA for ion doses from 6.25 × 1014 to 5.0 × 1016 ions/cm2 excited by (top) 514 nm and (bottom) 785 nm laser lines. Adapted from [43].

In particular, Raman spectroscopy data for excitation by the laser wavelength of 785 nm seem to be additional confirmation for the carbonization processes in the B:PMMA. Indeed, new Raman bands at ~1325 and 1590 cm-1, attributed to C−C and C=C vibrations, respectively, are detected only for the ion-irradiated samples with higher fluences (>1016 ions/cm2 ). These two new Raman bands may also be attributed to the so-called D and G peaks in the region of 1300-1600 cm−1 which are the main peaks characteristic for graphite and graphene structure [60–66]. The intensity ratio of the D and G peaks *I*D/*I*<sup>G</sup> is a measure of the size of the sp2 phase organized in rings [67]. The sp2 phase is mainly organized in chains, when *I*D/*I*G is negligible

[63]. The relation between *I*D/*I*G and the size of the sp2 phase *La* is given by the equation of Tuinstra and Koenig [60]: *I*D/*I*G = *C*(*λ*)/*La*. Here, *C*(*λ*) is a wavelength dependent factor [29, 68]: *C*(*λ*) = −126 + 0.033*λ*, where *λ* is the excitation wavelength in (Å) at which the Raman spectra were recorded.

A correlation between slow positron beam and Raman spectroscopy results for B:PMMA is mentioned in our recent works [69]. **Table 2** gives the SPBS data, exemplified by *o*-Ps lifetimes and intensities at incident positron energy of 2.15 keV and Raman data, exemplified by *I*D/*I*<sup>G</sup> and *L*a, for the same B:PMMA samples. A good correlation between these data is clearly observed. Thus, the expected carbonization processes or formation of carbon nanostructures at higher fluences (>1016 ions/cm2 ) can independently be identified using SPBS and Raman spectroscopy, providing their combination as a powerful experimental tool in the investigation of ion-implanted polymers.


**Table 2.** *o*-Ps lifetimes and intensities at incident positron energy of 2.15 keV and the intensity ratio of the D and G peaks, and calculated sizes of sp2 phase *La* [69].

#### **3.5. Electrical measurements data**

The un-implanted PMMA and as-implanted B:PMMA samples were measured at 300 K and 360 K with DC method [43]. It was supposed that the conductive layer in ion-implanted polymer is dependent on the temperature and at a higher temperature the effect of increasing conductivity with temperature will be more pronounced. In order to avoid a possible structural change in polymer matrix, the maximum temperature at 360 K was selected in the electrical measurement experiment, not exceeding the glass transition temperature *T*g of PMMA ranging from 358 to 438 K; the range is so wide because of the vast number of commercial compositions which are copolymers with co-monomers other than methyl methacrylate [70]. Similar value *T*g ≅ 355 ± 18 K of PMMA was obtained using temperature dependent positron annihilation lifetime measurements [53].

**Figure 8** shows the obtained *I*-*V* dependences, fitted by the linear regression line [43]. As an example, the *I*-*V* characteristic of as-implanted B:PMMA (5.0 × 1016 ions/cm2 ) sample is presented with noise (as-implanted + noise (II)) and linear regression line (linear fit of asimplanted (II)). While for other samples only, the linear regression lines of the DC measure‐ ments are demonstrated. More details of the electrical measurement experiment have been reported in [43].

[63]. The relation between *I*D/*I*G and the size of the sp2

were recorded.

298 Radiation Effects in Materials

**Dose [B+ /cm2 ]**

at higher fluences (>1016 ions/cm2

of ion-implanted polymers.

peaks, and calculated sizes of sp2

lifetime measurements [53].

reported in [43].

**3.5. Electrical measurements data**

Tuinstra and Koenig [60]: *I*D/*I*G = *C*(*λ*)/*La*. Here, *C*(*λ*) is a wavelength dependent factor [29, 68]: *C*(*λ*) = −126 + 0.033*λ*, where *λ* is the excitation wavelength in (Å) at which the Raman spectra

A correlation between slow positron beam and Raman spectroscopy results for B:PMMA is mentioned in our recent works [69]. **Table 2** gives the SPBS data, exemplified by *o*-Ps lifetimes and intensities at incident positron energy of 2.15 keV and Raman data, exemplified by *I*D/*I*<sup>G</sup> and *L*a, for the same B:PMMA samples. A good correlation between these data is clearly observed. Thus, the expected carbonization processes or formation of carbon nanostructures

spectroscopy, providing their combination as a powerful experimental tool in the investigation

0 1.765 35.0 No peaks 6.25 × 1014 1.752 34.6 No peaks 3.13 × 1015 1.752 34.8 No peaks

phase *La* [69].

2.5 × 1016 No *o*-Ps or ~0 1.42 9.4 5.0 × 1016 No *o*-Ps or ~0 1.40 9.5

**Table 2.** *o*-Ps lifetimes and intensities at incident positron energy of 2.15 keV and the intensity ratio of the D and G

The un-implanted PMMA and as-implanted B:PMMA samples were measured at 300 K and 360 K with DC method [43]. It was supposed that the conductive layer in ion-implanted polymer is dependent on the temperature and at a higher temperature the effect of increasing conductivity with temperature will be more pronounced. In order to avoid a possible structural change in polymer matrix, the maximum temperature at 360 K was selected in the electrical measurement experiment, not exceeding the glass transition temperature *T*g of PMMA ranging from 358 to 438 K; the range is so wide because of the vast number of commercial compositions which are copolymers with co-monomers other than methyl methacrylate [70]. Similar value *T*g ≅ 355 ± 18 K of PMMA was obtained using temperature dependent positron annihilation

**Figure 8** shows the obtained *I*-*V* dependences, fitted by the linear regression line [43]. As an

presented with noise (as-implanted + noise (II)) and linear regression line (linear fit of asimplanted (II)). While for other samples only, the linear regression lines of the DC measure‐ ments are demonstrated. More details of the electrical measurement experiment have been

example, the *I*-*V* characteristic of as-implanted B:PMMA (5.0 × 1016 ions/cm2

*o***-Ps lifetime,** *τ***3 [ns]** *o***-Ps intensity,** *I***3 [%]** *I***D/***I***<sup>G</sup>** *La* **[nm]**

phase *La* is given by the equation of

) sample is

) can independently be identified using SPBS and Raman

**Figure 8.** *I*-*V* dependences for PMMA and B:PMMA (3.75 × 1016 and 5.0 × 1016 ions/cm2 ) at temperatures (top) 300 and (bottom) 360 K. Adapted from [43].

The numerical values of the *I*-*V* characteristics are presented in **Table 3** [43]. It is evidently proven that the pristine PMMA does not exhibit any conductivity neither at higher tempera‐ ture applied. At room temperature, the slope of linear regression line of *I*-*V* dependence remains unchanged and has even negative value. In contrary to unimplanted sample, the results of the *I*-*V* measurements for the B:PMMA (3.75 × 1016 and 5.0 × 1016 ions/cm2 ) samples revealed the change of slope of the linear regression line of *I*-*V* dependence at 360 K. It means that the as-implanted samples have created a very thin conductive layer or conductive joints due to carbonization processes or formation of carbon nanostructures in consistent with the results of slow positron beam and Raman spectroscopy measurements [36, 43].


**Table 3.** The evaluated values of *I*-*V* measurements of PMMA and B:PMMA with higher fluences (3.75 × 1016 and 5.0 × 1016 ions/cm2 ) at temperatures 300 and 360 K: Slope is value of slope of line of linear regression, Error is the error of linear regression, CC is correlation coefficient, and SD is standard deviation of the linear regression [43].

#### **3.6. Nanoindentation test data**

A first time the results of investigation of the influence of low dose (6.25 × 1014 ions/cm2 ) B+ ion-irradiation on the mechanical properties (hardness and elastic modulus) of PMMA probed by nanoindentation with UNHT in the range of 300–1100 nm indentation depth have been reported in [45]. It has been established that the hardness and elastic modulus versus maximum indentation depth illustrate the main difference between the un-implanted (pristine) and ionimplanted samples in the range up to about 400 nm. The same value of the maximum pene‐ tration depth of B+ -ions into PMMA has been found using SPBS and SRIM simulation [36]. As a continuation of the nanoindentation test of the B:PMMA, the averaged values of indentation hardness versus maximum indentation depth for the un-implanted and as-implanted samples with ion doses of 6.25 × 1014, 1.25 × 1016, 2.5 × 1016, and 3.75 × 1016 ions/cm2 are plotted in **Figure 9** [46].

**Figure 9.** Indentation hardness versus maximum indentation depth for PMMA and B:PMMA (6.25 × 1014, 1.25 × 1016, 2.5 × 1016, and 3.75 × 1016 ions/cm2 ). Adapted from [46].

One may see that the hardness dependence on the maximum indentation depth demonstrates the difference between the PMMA and B:PMMA samples in the entire range studied up to 1100 nm with the largest changes in the vicinity of 300-400 nm in consistence with the maxi‐ mum penetration depth of B+ -ions into PMMA as revealed from SPBS measurements and SRIM simulation [36]. The observed improving of surface-sensitive mechanical properties of B:PMMA by ion beam processing is obviously detected to be more significant as ion dose increases, that may be suitable for hard-materials applications. According to Lee et al. [4] these properties are apparently related to the effectiveness of cross-linking. Besides, the abovemen‐ tioned formation of carbon nanostructures upon high-dose ion implantation (>1016 ions/cm2 ) seems to be also important. But further increase of hardness with deeper penetration of indenter up to 1100 nm for higher fluences (3.75 × 1016 B+ /cm2 ) was found to be very interesting and not fully understood yet (see **Figure 9**). Actually, similar results that the hardness of B+ implanted polycarbonate increased with increasing ion dose has also been observed in Ref. [2] but only for penetration of indenter up to 400 nm, that is not so far from the implanted layer as in our case. A deeper understanding the low-energy ion-irradiation-induced processes in polymeric materials exemplified by PMMA irradiated by accelerated light, middle, and heavy ions is still required.
