Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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671

Nanoscale Charge Dynamics

and

Nanostructure

Formation

in Polymers

Akinori Saeki

Osaka University

Osaka,

Japan

Shu Seki

Osaka University

Osaka,

Japan

Kazuo Kobayashi

Osaka University

Osaka,

Japan

Seiichi Tagawa

Osaka University

Osaka,

Japan

Contents

25.1 Introduction ..........................................................................................................................672

25.2 Nanoscale

Dynamics of Charged Species Studied by Pulse Radiolysis .............................. 672

25.2.1

Pulse

Radiolysis Studies on Charged Species of Polymers...................................... 672

25.2.1.1

Relaxation

Processes of Polymer Radical Ions.......................................... 673

25.2.1.2

Highly Sensitive Decomposition of Polymers through

CombinationofDissociative Electron Attachment with Geminate

Ion Recombination................................................................................. 673

25.2.1.3 Highly

Sensitive Electron Beam, X-Ray, and EUV Resists through

theCombination

of Dissociative Electron Attachment with Geminate

Ion

Recombination..................................................................................... 674

25.2.2

Pulse Radiolysis Studies on Dynamics of Charged Species

inσ-ConjugatedSystems..........................................................................................675

25.2.3 Nanometer-Scale

Dynamics of the Hole and the Electron in DNA .........................684

25.2.3.1

Radical

Cation............................................................................................684

25.2.3.2

Radical

Anion............................................................................................687

25.3

Nanoscale Dynamics of Charge Carriers by Time-Resolved Microwave Conductivity......689

25.4 Nanostructure

Formation......................................................................................................694

References...................................................................................................................................... 702

672 Charged Particle and Photon Interactions with Matter

25.1 introduCtion

The rst systematic study of irradiation of polymers was carried out in order to determine the

radiation damage to polymers for use in radiation elds of the Manhattan project during World

War II. The Manhattan project had a very great inuence on all elds of nuclear research, includ-

ing radiation chemistry. The peaceful uses of the nuclear reactor in the eld of radiation effects on

polymers started before the “Atoms for Peace” address delivered by President Eisenhower in 1953.

The rst systematic study of radiation effects on polyethylene thin lms by Dole and Rose, just after

WorldWar II, was carried out using nuclear reactors. Their research was published as a master’s

thesis in 1948 and, later, as a published paper (Dole et al., 1954). However, non-soluble and non-

melting cross-linked polyethylene irradiated by a nuclear reactor was demonstrated by Charlesby in

1952, and it had a huge impact on industry (Charlesby, 1952). Today, the economic scale of utiliza-

tion of radiation in industry is very large and comparable to the economic scale of utilization of

nuclear energy in Japan and the United States (Tagawa et al., 2002; Yanagisawa et al., 2002). The

economic scale of radiation use in polymer elds, such as radiation cross-linking for the production

of improved cables, heat-shrinkable packaging lms and tubes, cross-linked automobile tire com-

ponents, resist materials for semiconductor device fabrication, polymers for biomedical applica-

tions, radiation sterilization of medical plastic supplies, radiation curing etc., is now well established

(Tagawa

et al., 2002; Yanagisawa et al., 2002).

This

chapter is presented in four sections. Section 25.2 reviews the nanoscale dynamics of

charged species studied by pulse radiolysis. Pulse radiolysis studies on relaxation processes of poly-

mer radical ions, highly sensitive nanospace reactions through the combination of dissociative elec-

tron attachment with geminate recombination, recent progress in σ-conjugated polymer research

based on the direct observation of radical anions and cations of σ-conjugated polymers, and the

rst detection and dynamics of radical cations and anions of DNA in an aqueous solution at room

temperature in connection with the initial step of the radiation damages to DNA are reviewed.

In Section 25.3, nanoscale dynamics of charge carriers by time-resolved microwave conductivity

(TRMC) is discussed. TRMC is an electrodeless method that detects the intrinsic nature of charge

carrier transport in the nanospace of organic semiconductor materials, comparable with eld-effect

transistor (FET) and time-of-ight (TOF) methods, which are dependent on grain boundary, impu-

rities, and structural defects. Section 25.4 deals with the “unique” ion beam (IB)-induced nano-

structure formation in polymers, that is, nanostructure formation by high-energy IBs. A limited

spatial distribution of the transient species induced by IBs gives “unique” nanomaterials as the nal

products via nonhomogeneous chemical processes, which could not be realized by other chemical

reactions.

We can produce nanostructures based on any cross-linkable polymer materials.

25.2 nanosCale dynamiCs oF Charged speCies

studied

by p

ulse

adiolysis

25.2.1 pulSe radiolySiS StudieS on charged SpecieS of polyMerS

After the start of systematic studies on industrial applications of radiation of polymers (Charlesby,

1952; Dole et al., 1954), the mechanisms of radiation-induced polymer reactions have been extensively

studied. A direct detection of reactive intermediates is essential to elucidate the detailed mechanisms

of radiation-induced polymer reactions. Pulse radiolysis is the most powerful method for detecting

radiation-induced reactive intermediates of polymers directly and for investigating the dynamics of

reactive charged species (Tagawa, 1986, 1991a,b, 1993). A large number of unsuccessful pioneering

researches had been carried out to detect the transient absorption of both polymer and monomer

radical ions as initiators of both polymer and polymerization reactions by pulse radiolysis techniques

after the introduction of pulse radiolysis in radiation chemistry in the late 1950s. The rst detection

of the radical ions of polymers by pulse radiolysis was carried out for polyvinylcarbazole in 1974

Nanoscale Charge Dynamics and Nanostructure Formation in Polymers 673

(Tagawa et al., 1974). Also, the rst detection of the monomer radical cation as the initiator of polym-

erization was carried out in 1972 (Tagawa et al., 1972). After the rst detection of polymer radical ions

(Tagawa

et al., 1974), many kinds of reactive radical ions of polymers (Tagawa, 1986, 1991a,b, 1993),

as initiators of radiation-induced polymer reactions, have been observed by pulse radiolysis at room

temperature, including IB pulse radiolysis of polymers (Tagawa, 1993) as well as electron-beam (EB)

pulse radiolysis of polymers (Tagawa, 1991a,b). The spectroscopic studies on radical ions of typical

polymers, such as polystyrene- and polyethylene-related polymers, polysilanes, and polymethylmeth-

acrylate (PMMA), have been reviewed (Tagawa et al., 2004). Here, we review pulse radiolysis studies

on nanospace dynamics of charged species in polymers: at rst, nanospace relaxation processes of

polymer ion radicals, and then, highly sensitive decomposition of polymers and highly sensitive reac-

tions of EB, x-ray, and extreme ultraviolet (EUV) resists through nanospace reactions. Polystyrene-

related polymers have been extensively studied in connection with industrial applications such as

highly sensitive radiation-induced cross-linking and degradation initially, and highly sensitive resist

materials later. Pulse radiolysis and laser ash photolysis have been extensively employed to clarify the

detailed mechanisms of highly sensitive radiation-induced cross-linking and degradation and highly

sensitive resist processes. In all cases, the combination of highly effective dissociative electron attach-

ment with geminate ion recombination plays an important role in highly sensitive nanospace reactions.

This combination is only one large-scale industrial application of radiation-induced nanoscale charge

dynamics, as far as we know.

25.2.1.1 relaxation

rocesses

of p

olymer

adical

ons

After the rst detection of polymer radical cation (Tagawa et al., 1974), the detailed dynamics of

polyvinylcarbazole radical cation was studied by nanosecond pulse radiolysis, and the difference

between the reactivity of ethylcarbazole (the monomer unit of polymer) and polyvinylcarbazole

radical cations was clearly shown (Washio et al., 1981). The stabilization of the polymer radical

cation decreases with its reactivity (Washio et al., 1981). Relaxation kinetics of the polymer excited

states of polyvinylcarbazole studied by picosecond pulse radiolysis (Tagawa et al., 1979) was stud-

ied in tandem with the intramolecular excimer formation of polyvinylcarbazole in the late 1970s.

A large number of picosecond and femtosecond laser ash photolysis studies on the dynamics of

polymer excited states have been carried out in 1980s, 1990s, and 2000s. A comprehensive survey

on this subject is beyond the scope of this chapter. Recently, systematic picosecond pulse radiolysis

studies on the relaxation of cation radicals of polystyrene and related polymers have been performed

(Okamoto et al., 2001, 2006). Deprotonation reactions have also been studied with respect to the

development of EUV resists (Okamoto et al., 2008, 2009). Picosecond pulse radiolysis studies on

two “unique” relaxation processes for both polymer excited states and radical ions have been carried

out (Matsui et al., 2002b; Ohnishi et al., 2009). The intermolecular energy migration between conju-

gated segments in a polymer chain of σ-conjugated polymer excited states (Matsui et al., 2002b) and

the conformational relaxation of σ-conjugated polymer radical anions, which was observed for the

rst time by nanosecond pulse radiolysis in 1987 (Ban et al., 1987), have also been clearly observed

picosecond pulse radiolysis in 2009 (Ohnishi et al., 2009).

25.2.1.2

highly

ensitive

ecomposition

of p

olymers

through Combination

of

issociative

electron a

ttachment

with g

eminate

ecombination

Polystyrene-related polymers in a solid state or in a solution of dioxane and cyclohexane are generally

not sensitive to radiation. But these become very sensitive to radiation and are easily decomposed

by irradiation in some chlorinated solvents. The reason why radiation-resistant polystyrene-related

polymers change to radiation-sensitive polymers in some chlorinated solvents was made clear by

the researches in laser ash photolysis (Tagawa and Schnabel, 1980a,b, 1983; Tagawa et al., 1984;

Tagawa, 1986) and pulse radiolysis (Tagawa et al., 1981; Itagaki et al., 1983, 1987, 1992; Washio

etal., 1983; Tagawa, 1986; Zezin et al., 1995; Zhang and Thomas, 1996; Okamotoet al., 2001, 2006).

674 Charged Particle and Photon Interactions with Matter

In the case of photolysis of polystyrene and polystyrene-related polymers in cyclohexane, the pho-

todecomposition of polymers does not occur effectively, and singlet monomer and excimer states

(Tagawa and Schnabel, 1980a) and also triplet states (Tagawa et al., 1984) of polystyrene are observed

by laser ash photolysis. But, in the case of photolysis of polystyrene and poly-α-methylstyrene

in chlorinated solvents, the photodecomposition of polymers occurs very effectively and polymer

excited states are converted to charge-transfer (CT) complexes. The CT complex was observed by

laser ash photolysis (Tagawa and Schnabel, 1980b, 1983). In the case of radiolysis of polystyrene-

related polymers in cyclohexane, the photodecomposition of polymers does not occur effectively,

and singlet monomer and excimer states of polystyrene and poly-α-methylstyrene are observed by

picosecond pulse radiolysis (Tagawa et al., 1981; Itagaki et al., 1983, 1987, 1992). The CT complex

was also observed in the case of pulse radiolysis of polystyrene and poly-α-methylstyrene in chlo-

rinated solvents (Tagawa et al., 1981; Washio et al., 1983). Based on these studies, the combination

of very effective dissociative electron attachment with geminate ion recombination produces a CT

complex and then polymer radicals, and plays an important role in the initial reactions of highly

sensitive decomposition of polystyrene in chlorinated solvents with oxygen.

25.2.1.3 highly

ensitive

electron b

eam,

-ray,

and euv r

esists

through

the Combination of d

issociative

electron

ttachment

with g

eminate

ecombination

Photo and radiation induced decomposition of polymers occurs very effectively in polystyrene

related polymer solutions in chlorinated solvents, but photo and radiation induced polymer cross-

linking occurs very effectively in chlorinated polystyrene-related polymers in the solid state.

Chlorinated polystyrene-related polymer lms were used for EB and excimer negative resists.

In the 1980s, the increasing density of VLSI required the development of submicron lithogra-

phy, such as excimer, x-ray, EB, and IB lithography. At that time, much attention was devoted to

negative EB resists (Imamura, 1979; Imamura et al., 1982; Kamoshida et al., 1983) containing

chlorinated or chloromethylated phenyl side chains. This type of enhancement is observed in

other resists containing chlorinated or chloromethylated silicon-containing resists (Morita et al.,

1983, 1984). EB (Tabata et al., 1984, 1985; Tagawa, 1986, 1987) and IB (Tagawa, 1993) pulse

radiolysis and excimer laser ash photolysis (Tagawa, 1986, 1987) studies on resist materials

were carried out in order to elucidate the detailed cross-linking reaction of chlorinated or chlo-

romethylated excimer, x-ray, EB, and IB resists. The same CT complex is conrmed in both

photo- and radiation-induced chlorinated or chloromethylated resist reactions. Similar reactions

of iodination (Shiraishi et al., 1980) of phenyl rings are used for EB resists. In both photolysis

and radiolysis, the generation of two polymer radicals and acid through dissociative electron

attachment and decomposition of the CT complex is observed. But the initial steps of EB and

x-ray resist reactions are mainly induced by ionization and are different from the initial steps of

photoresists induced by excited states of molecules. The combination of geminate ion recombina-

tion with dissociative electron attachment induces pair production of polymer radicals, which in

turn induces highly effective cross-linking even for chloromethylpoly-α-methylstyrene, although

poly-α-methylstyrene is well known to be decomposed on irradiation. The fundamental radiation

chemistry of nanospace reactions due to the geminate recombination in organic materials, such

as liquid alkanes (Thomas et al., 1968; Sauer and Jonah, 1980; van den Ende et al., 1980; Tagawa et al.,

1983, 1989; Yoshida et al., 1987; Le Motais and Jonah, 1989; Saeki et al., 2002, 2004, 2005a)

and liquid aromatics (Okamoto et al., 2003, 2007), and also the formation processes of the CT

complex of polystyrene-related polymers through dissociative electron attachment and geminate

recombination (Okamoto et al., 2001, 2006) have been studied extensively by nanosecond,

picosecond, and subpicosecond pulse radiolysis.

Recently, a single-component chemically amplied resist based on dehalogenation of polymers

has been reported (Yamamoto et al., 2007a). The reaction mechanism of this resist is acid formation

Nanoscale Charge Dynamics and Nanostructure Formation in Polymers 675

through dissociative electron attachment and geminate recombination (Yamamoto et al., 2005a,b),

similar to the acid formation of halogenated polystyrene-related resists (HPRR) (Tabata et al., 1984,

1985; Tagawa, 1986,1987). Both acid formation processes produce two polymer radicals. Chemically

amplied resists use acid for acid-catalyzed reactions, and HPRRs use a polymer radical for cross-

linking. The difference between the acid formation processes for HPRR and brominated single-

component chemically amplied resist (BSCAR) is explained as follows. In both cases, the initial

steps are ionization, emission of secondary electrons, and ionization and excitation of molecules, and

then thermalization of electrons. The processes of polymer radical cation formation by ionization

and halogen anion formation by dissociative electron attachment are the same. The stability of poly-

mer radical cations is different. Polymer radical cations are stable in HPRR but unstable in BSCAR,

where the protons and polymer radicals are generated through the deprotonation of polymer radical

cations. In both processes, geminate recombination occurs. In BSCAR, geminate pairs are proton

and halogen anion and geminate recombination produces acid. In HPRR, geminate pairs are poly-

mer radical cation and halogen anion and geminate recombination produces a CT complex, which

in turn produces acid and a polymer radical. Thus, the nal products are the same, but the forma-

tion processes and distances between two polymer radicals are different. Distance of two polymer

radicals is very short in HPRR, but is fairly long in BSCAR. The dissociative electron attachment

is competitive between halogenated resist polymers and the acid generators in BSCAR (Yamamoto

et al., 2009).

25.2.2 pulSe radiolySiS StudieS on dynaMicSofchargedSpecieS