Bulk and surface modifications of insulating poly (paraphenylene sulphide) films by ion bombardment

Surface and Coatings Technology, 70 (1995) 197-202

197

Bulk and surface modifications of insulating poly(paraphenylene sulphide) films by ion bombardment M . R. R i z z a t t P ,

M . A. d e A r a f i j o b a n d R. P . L i v i a*

aInstituto de Fisica, Universidade Federal do Rio Grande do Su191501-970 Porto Alegre RS (Brazil) bInstituto de Qulmica, Universidade Federal do Rio Grande do Sul 91501-970 Porto Alegre RS (Brazil)

Abstract Thin poly(paraphenylene sulphide) (PPS) films (2 grn thick), bombarded with He + (380 keV), B ÷ (350 keV) and Ar2+ (700 keV) at fluences ranging from 1012 to 2 x 1016 ions cm -z, were analysed by X-ray diffractometry, UV-visible absorption spectroscopy, electrical resistance measurements and solubility tests. The polymer gradually underwent an amorphization process indicated by the decrease in the main X-ray diffraction peak area and also lost its solubility with increasing ion fluence. This behaviour revealed the formation of cross-linking and amorphous structures. A red shift of the optical absorption threshold with increasing fluence was also observed by UV-visible spectroscopy. This trend is usually attributed to the conjugation of unsaturated carbon bonds which gives rise to non-localized n electrons. From these spectral data the gap between valence band and conduction band (optical gap Eg) using a model for amorphous semiconductors can be obtained. The conjugation process increased with fluence resulting in a decrease in the optical gap. For Ar2÷-bombarded samples the optical gap saturated at 0.9 eV for fluences around 1015cm- 2. The decrease in Eg with increasing fluence is a direct indication that a new group of conducting structures is being formed in the polymeric material. Electrical measurements made immediately after 2 × 1016Arz+ cm-2 bombardment revealed that the electrical resistivity decreased by 18 orders of magnitude in relation to the original PPS. When the bombarded samples were exposed to air the electrical conductivity decreased. This electrical instability was assigned to the free radicals present in the polymer chain after ion bombardment. This work shows that different hydrogenated amorphous carbon films can be obtained from one common polymeric matrix by judicious choice of the projectile ion stopping mechanism.

1. Introduction

2. Experimental details

W h e n ionizing energy is applied to a polymeric material, it is expected that b o t h physical and mechanical structures will respond to such excitation in several ways [1,2]. C o m m o n l y , the generation of free radicals by a b o n d - b r e a k i n g process is observed, and further structural reorganization is then expected. These reactive post-radiation structures exhibit, in several cases, extensive conjugation, carbonization and disorder at b o t h micro and m a c r o scales. The present work is a part of a wider investigation of the modification of optical and electrical properties as well as aspects related to the mechanism of degradation of p o l y ( p a r a phenylene sulphide) (PPS). Specifically, we developed a systematic study of the effects of ion energy in the physical structure of P P S as a function of the fluence and nature of the incident ions, using as m o n i t o r i n g techniques UV-visible spectroscopy, X-ray diffractometry (XRD), electrical resistance measurements and solubility tests.

Commercial grade 2 gm, 6 g m and 125 g m thick P P S films p r o d u c e d by T o r a y C o m p a n y , Japan, were m o u n t e d in annular aluminium suports. The degree of crystallinity of the pristine film is of the order of 40% and its glass transition temperature Tg is a r o u n d 92 °C. The samples were b o m b a r d e d by 1H+ (300 keV), 4He ÷ (380 keV), l°B+ (350 keV) and 4°Ar2+ (700 keV) ions using the H V E E 400 kV ion implanter at the I F U F R G S , P o r t o Alegre, in a v a c u u m better than 10 -6 Torr. The ion beam current density was maintained constant and equal to 20 nA cm -2, for fluences less than or equal to 1015 ions cm -z, to avoid sample heating and excessive outgassing. For 2 g m films, the 1H+ and 4He+ ions ran t h r o u g h the entire sample thickness and the X°B+ and 4°Ar2 + b o m b a r d m e n t s were carried out on b o t h sides of the samples in order to modify them t h r o u g h o u t their thickness. The fluences ranged from 1012 to 3 × 1016 c m - 2 (Table 1). X R D was performed with a Siemens D-500 diffractometer, using Cu K s radiation and a graphite m o n o c h r o m a t o r in the step scanning m o d e (10 s at each 0.1 ° step) t h r o u g h an angular range from 10 ° to 40 °. The chemical degradation process was monitored by

*Author to whom correspondence should be addressed.

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M. R. Rizzatti et al. / Ion bombardment of poly(paraphenylene sulphide)

198

TABLE 1. Ion b o m b a r d m e n t parameters Ion

Energy (keV)

H+ 300 KeY

Ion fluence (ions cm -2)

Current density (nA cm -2 )

1012-8 × 1014 1012-10 is 10 ] z - 4 x 1014 1012-1015 1015- 3 x 10 TM

20 20 20 20 250

He+380KeY 2.5

2,5

/ 1 0 Iz

H+ He r B+ Ar 2+

300 380 350 a 700 a

2.0

2.0

4xlO 14

/ BxlO 14

/

1.5

~//~/4xl0B

1.5

1015

aThe B ÷ and Ar 2÷ were implanted on both sides of the 2 ~m thick polymeric film.

1013

1.0~'~

UV-visible spectroscopy, performed in Intralab DMS 80 equipment. Electrical resistance measurements were made using the four-point method for the less resistive samples. For low ion fluence effects a two-point probe was required. Solubility tests were performed around 180°C, using 1,2,4-trichlorobenzene (TCB) as solvent; these tests allowed a qualitative first evaluation of the process of structural reorganization, by which cross-linked chains (gel-like material) are progressively formed.

3. Results and discussion

The PPS optical properties are easily altered by the ion bombardment (Fig. 1). Even at the lowest fluence the material already presents an increase in and a red shift of the optical absorption threshold with increasing fiuence. This phenomenon is usually attributed to the conjugation of unsaturated carbon bonds, which increases the non-localized n electron population. It seems plausible that the conjugation process not only is a function of the total deposited energy but also depends on the overall stopping power. This additional dependence is expected since both nuclear stopping power S, and electronic stopping power Se, which combination constitutes the overall stopping power, are directly proportional to the ion mass I-3]. However, this is not to say that the value of Sn is a measure of the ion's effectiveness in degrading the polymeric material, because bombarding ions such as 1H+ and 4He ÷ dissipated their energy basically by electronic excitation and ionization processes (Table 2). Inelastic events favoured bond breaking and, consequently, led to the formation of free radicals. These highly reactive species undergo reconbination generating conjugated segments along the polymeric chain, therefore changing irreversibly the overall structural organization (both amorphous and crystalline domains). Finally, it can then be concluded that inelastic events are not necessarily involved in structural damage. It can also be noticed that the polymer gradually undergoes an amorphization process

1.0

4xlO14

//~/,Sx|O 13 0.5 ~ 1 0 1 3

350 400

A

0.5

~0

6OO

I

700

350 400

X(nm)

I

I

X(nm)

2.5

2.0

KeY

Z.5

2.0

1

IIII II }.,o,4 1

1.5

1.5 I.O 1.0

0.5 0.5

350 400

500

600

700

350 400

X(nm)

500

600

700

X(nm)

Fig. 1. UV-visible spectroscopy curves for samples bombarded using different fluences of 1H+, 4He +, ]°B+ and 4°Ar2+.

TABLE 2. The proportion of energy deposited by nuclear and electronic processes (simulated by PRAL program) Ion

Energy (keV)

S t°t~l (%)

SteOral (%)

H+ He + B÷ Ar 2 +

300 380 350 700

0.1 8 26 38

99.9 92 74 62

M. R. Rizzatti et al. / Ion bombardment of poly(paraphenylene sulphide) 300

with increasing fluence as indicated by decrease in the main X-ray diffraction peak area in Fig. 2. From the UV-visible absorption spectra, the optical gap Eg can be evaluated using the Tauc formula [4]:

(c~E)1/2 G ( E - Eg)

( 1)

=

where c~ is the linear absorption coefficient, E is the photon energy and G is a constant. These results can be plotted as function of the fluence ~b (ions cm -2) or as function of the energy fluence ~b*. The energy fluence is a measure of the mean deposited energy per repeating unit at the implanted area. It does not take in account how this energy is deposited (for example in electronic or in nuclear processes), and it is given by the following equation as proposed by R6ssler [5]: ~b*-

e~

200

100

(2)

n

where E (eV) is the total energy deposited by the incident ion, ~b (ions cm -2) is the ion fluence in and n is the number of repeating units per square centimetre at the implanted area. In Fig. 3 are plotted (ctE) 1/2 vs. E curves for bombarded PPS samples (Ar 2 +, 700 keV), where the full lines are the fits of eqn. (1). From Fig. 3, the fluences that are best suited for the determination of Eg a r e those near or higher than the amorphization fluence, as evaluated from several techniques (XRD, differential scanning calorimetry, solubility test) [6]. The optical gap g g decreases from the value of the pristine sample (above 3 eV) to the saturation value ft. These results are better understood when the experimental points for Eg obtained from Fig. 3 are fitted by an exponential curve of the form Eg=Aexp(-ac~)+fl, which gives the conjugation cross-section ac (Fig. 4 and

0

0.5

1.0

1.5

2.0 2.5 Energy (eV)

3.0

3.5

Fig. 3. (c~E)1/2 vs. E curves for bombarded PPS samples (Ar 2+, 700 keV):---, fitting of eqn. (1).

4.0 I

a He[ <> B + Ar

3.0 z~

2000

j~ . . . . .

pristine ~

1500 -.

. . . .

[ \~-~- - - - -

03 z

D

199

1000 -

4xlO ~

2.0

o

8x101~

1014

o (J

1.0

500 -

0.0

i

0 15

17

19

21

23

25

2e

Fig. 2. Main X-ray diffraction peak corresponding to several different He + bombardment fluences.

20

i

i

i

40 60 80 Ion fluence (xl 013cm 2)

i

100

120

Fig. 4. Optical absorption threshold as function of the ion fluence for the PPS of 2 ~tm thickness when bombarded by He + (380 keV), B + (350 keV) and Arz+ (700 keV).

200

M. R. Rizzatti et al. / Ion bombardment of poly(paraphenylene sulphide)

Fig. 5). Table 3 shows the cross-section a~ for the conjugation process and the saturation value fl for different ions. Experimental values of fl, from 0.9 to 1.6 eV, are between those verified for amorphous carbon (a-C) (0.4-0.7eV) and hydrogenated amorphous carbon (a-C: H) (1.6-2.7 eV), where hydrogenation introduces several sp 3 sites, thus leading to higher optical gaps [7]. It is well established that carbon allows three different hybridizations: sp, sp 2, sp 3. The structures of a-C and a - C : H were investigated by Robertson and O'Reilly [7]; their basic assumption was that these structures were formed by a number of model sites with sp 2 and sp 3 configurations, which in turn consist of a series of separate and disordered clusters. The electronic behaviour of these clusters is analogous to n states in organic molecules. These researchers reported that the n electron energetic state favoured the organization of the sp 2 sites and that the most stable arrangement is in the form of clusters of isolated or fused benzene-type rings. Among 4.0

I ,0,, BHe I + Ar 3.0 ,x /X

Q..

2.0 o

o

1.0

L

0.0 0

i

i

i

i

1 O0

200

300

400

500

these the most probable structures are compact clusters made up of fused rings, where the largest will tend to assemble in graphite-like layers. Such structural organization will minimize the value of E v this quantity being inversally proportional to the size of the graphite-like clusters. If M is the number of rings in a cluster, we can write

Eg ~ 6/M 1/2

(3)

This equation is only valid for a-C, assuming that the clusters are interconnected by sp 3 sites. Taking eqn. (3) and optimizing Eg one can calculate approximately 21, 30 and 44 fused rings per cluster, for samples bombarded with He +, B ÷ and Ar 2÷ respectively. On the contrary, for a-C : H, where hydrogen is preferentially linked to sp 3 sites, it is possible that sp 2 sites are arranged in unsaturated structures in such way that 30% of these sites are forming benzene-type clusters. From the equation Eg~2.8/M t/2 one obtains approximately 5,6 and 10 fused tings per cluster, for samples bombarded with He +, B + and Ar 2+ respectively. Therefore, the change in Eg for bombarded PPS samples shows that a wide variety of a - C : H films can be obtained as a result of the different mechanisms of energy loss in the polymeric material. The decrease in the energy difference between the valence and conduction bands with increasing ionic fluence leads to a higher probability to find charge carriers in the conduction band, which favours a decrease in electrical resistivity, thus generating a new class of conducting material. Although exhibiting a relatively low Eg at the optical gap saturation fluence, the material still has insulating characteristics (p > 102~) cm) (Table 4). Only for fluences from 2 x 1015 Ar 2÷ cm -2, at which the carbon clusters should be sufficiently close or interconnected, were semiconductor-like values found (Table 5). This result is in accordance with the well-established model of electrical conduction in high fluence irradiated polymers [8]. In Fig. 6 can be seen the logarithm of the ratio P j P m i n plotted against Ar 2 ÷ ion fluence, where po is the electrical resistivity of a sample bombarded with a certain fluence and Pmin is the minimum sample resistivity (6.5 ~)cm)

Energy fluence (eV/mer)

Fig. 5. Optical absorption threshold as function of the energy fluence (per repeating unit) for the PPS of 2 lam thickness when bombarded by He + (380 keV), B ÷ (350 k e V ) a n d Ar 2+ (700 keV). TABLE 3. Cross-sections for the optical gap change process in poly(paraphenylene sulphide) and E s saturation values fl Thickness (gm)

Ion

a¢ (x 10 -15 cm 2)

fl (eV)

2 2 2

He B Ar

1.0 4.16 10.7

1.6 1.3 0.9

TABLE 4. Electrical resistivity change as function of Ar z+ ion fluence (electrical measurements were made seven days after bombardment) Ion fluence (Ar e+ cm 2)

/9 (f~ cm)

PPS 1015 2 x 1015 4 × 1015 6 x 10 is 8 x 10 is 1016 2 x 1016

1017 2.6 x 104 65 16 10.3 7 7.5 6.5

M. R. Rizzatti et al. / Ion bombardment of poly(paraphenylene sulphide) TABLE 5. Resistivities Material

p (f~ cm) ( T = 3 0 0 K)

Highly oriented pyrolytic graphite Graphite Carbon Semiconductors, doping PPS-1016 Ar z+

1.5 x 10 .4 3.3 x 10 .3 1.7 x 10 2 102 10 4 6.9 x 10 1

17.0

14.0

11.0 E

8.0

5.0

2.0 ×

4

-1.00. 0

0.4

X

X

--

l

r

I

+

- -

0.8

1.2

1.6

2.0

Ion fluence (xlO TM cma) Fig. 6. Logarithm of the ratio PcffPminas a function of the bombarding ion fluence.

corresponding to the 2× 1016Ar 2+ cm -2 fluence bombardment. By fitting the equation p4ffPmin = A exp(-ascq~)+ B, one can obtain the region where the stronger conjugation or carbonization process ocurred. It is important to emphasize that the electrical resistivity of the bombarded PPS samples depends of the time of exposure to the atmosphere. For example the resistivities of PPS bombarded by 1016Ar 2+ cm -2 (700keV) are as follows: after 3 min, 0.69 f~ cm; after 6 days, 13 f~ cm; after 6 months, 22 f~ cm.

4. Conclusions

Both physical and chemical changes induced by ionic bombardment are responsible for surface and bulk effects in a polymeric material. These effects are directly related to the amount of reactive and/or non-reactive defects generated by unit volume of the material. It is under-

201

stood that non-reactive defects are perturbations of a physical nature specifically generated by disruption of secondary bonds. On the contrary, reactive defects are those rising from primary bond breaking processes; the resulting reactive species are known as free radicals. In turn, these chemical species can either react with external molecules, for example oxygen and nitrogen from the atmosphere, or undergo recombination leading to crosslinked structures. In principle, the cross-linking process can ultimately cause overall structural disorder of the polymeric material. The reactive defects are initially formed along the ion track. As the fluence increases so does the amount of these defects and the probabilities of recombination between free radicals within a given ion track and those belonging to vicinal tracks become larger, accelerating the structural disorder process. XRD revealed a decrease in the main diffraction peak area with increasing fluence, confirming a gradual reduction in crystallinity. Apparently, the crystallites present in semicrystalline PPS films slowly disappear with increasing fluence and after a certain critical fluence the resulting material is completely amorphous. Solubility tests with TCB made on bombarded PPS films were performed in order to verify the formation of cross-linked structures. It was observed that, for example at 4 x 1013 He + cm -2, PPS films become partially insoluble, but largely swollen, in TCB. For higher fluences, near or higher than 4 x 10 x4 He + cm 2, these films are completely insoluble. These results confirm the new structure of cross-linked polymeric chains attained at a given fluence. Bombarded PPS films exhibited an increase in optical absorption in the UV-visible region followed by a red shift of the optical absorption threshold. These effects are associated with a decrease in the optical gap and can be related to a structural rearrangement of aromatic rings, possibly generating a continuous network of conjugated cyclic structures. Conjugation facilitates the intramolecular mobility of charge carriers. As conjugation becomes more extended, the energy difference Eg between valence and conduction bands becomes smaller and electron transport is improved. Eg is related to the number n of unsaturated rings which form the largest clusters. It is noteworthy that such a model is valid only for a-C and a-C:H, and it does not consider the formation of other cyclic structures, generated by free radical recombination, because they are energetically less favourable. After reaching saturation values for the optical gap, the electrical resistivity continues to decrease, probably because the conducting islands grow continually until the percolation limit is attained. For samples bombarded with 4 ° A r 2 + , resistivity saturation occurs for fluences one order of magnitude higher than those corresponding to the optical gap limiting value.

202

M. R. Rizzatti et al. / Ion bombardment of poly(paraphenylene sulphide)

From UV-visible spectroscopy and electrical conductivity data, it is possible to estimate, in terms of crosssections go and asc, the area or the effective radius of the ion central track and the ion efficiency for the induction of conjugation and carbonization processes. The ion-induced optical gap change process (conjugation) occurs in a region around the ion trajectory defined by a characteristic mean radius Re. For the different bombarding ions those radii are 1.8 ~, for 4He +, 3.6 ~, for I°B÷ and 5.8~, for 4°Ar2÷. From the electrical resistivity change data it is possible to define a region of stronger conjugation or carbonization within a radius Rsc = 2.2 A around the 4°Ar2+ trajectory. The presence of free radicals contributes to the electrical instability of the b o m b a r d e d PPS films when they are exposed to air, atoms such as oxygen and nitrogen being incorporated in these samples. The consumption of these reactive defects, either by recombination or by reaction with external molecules, is detected by an

increase in electrical resistivity by more than one order of magnitude in a period of a few weeks.

References 1 J. S. Williams, Rep. Prog. Phys., 49 (1986) 491. 2 R. M. Papal6o, M. A. de Arat~jo and R. P. Livi, Nucl. Instrum. Methods B, 65 (1992) 442. 3 J. P. Biersack, Ion Beam Modification of Materials, Vol. 2, Elsevier, Amsterdam, 1987, Chap. 1. 4 F. W. Smith, J. Appl. Phys., 55 (1984) 764. 5 K. R6ssler, Radiat. Eft., 99 (1986) 505. 6 M. R. Rizzatti, R. M. Papal6o, M. A. de Arafijo and R. P. Livi, in Congresso Brasileiro de Polimeros 1, S~to Paulo, November 5-7, 1991

Vol. 1, Associa~go Brasileira de Polimeros, S~o Paulo, 1991, pp. 286-290. 7 J. Robertson and E. P. O'Reilly, Phys. Rev. B, 35 (1987) 2946. 8 T. Venkatesan, L. Calcagno, B. S. Elman and F. Foti, in P. Mazzoldi and G. Arnold (eds.), Ion Beam Modification of Insulators, Elsevier, Amsterdam, 1986, pp. 301-375.