1+9.a Comparative Study of the Polyaniline Thin Films Produced by Cluster Beam Deposition and Laser...

8/7/2019 1+9.a Comparative Study of the Polyaniline Thin Films Produced by Cluster Beam Deposition and Laser Abation Me

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A comparative study of the polyaniline thin films producedby the cluster beam deposition and laser ablation methods

Hyuna Lim and Jong-Ho Choia

Department of Chemistry and Center for Electro- and Photo-Responsive Molecules, Korea University,Anam-Dong, Seoul 136-701, Korea

Received 8 September 2005; accepted 2 November 2005; published online 5 January 2006

Polyaniline PANI thin films have been prepared by applying the novel neutral and ionized clusterbeam deposition NCBD and ICBD methods and the pulsed laser deposition PLD technique to thePANI samples of half-oxidized emeraldine base EB-PANI and protoemeraldine base forms in ahigh-vacuum condition. Characterization of the oxidation states and structural changes of pristineand doped thin films has been performed by Fourier transform infrared spectroscopy,ultraviolet-visible spectroscopy, and x-ray photoelectron spectroscopy. Spectroscopic measurementsdemonstrate that the dominant structure of NCBD and ICBD thin films corresponds to the reducedleucoemeraldine base state, whereas the chemical composition of PLD thin films depends criticallyon the laser fluence and the molecular weight of PANI target. The congruent deposition is onlyobtained for the PLD films deposited by the laser-induced decomposition of thelow-molecular-weight targets in the low to intermediate fluence regime below 100 mJ/ cm2 witha pulse duration of 7 ns. The surface morphology examined by atomic force microscopy

measurements shows that the cluster and laser beams are effective in producing smooth, uniformpolymeric thin films. After I2 and HCl doping, the electrical conductivities of the NCBD, ICBD, andparticularly PLD thin films are increased significantly. The higher conductivity of PLD filmsis ascribed to higher amounts of quinoid di-imine doping sites in the EB-PANI state, and the overallstructure-conductivity characteristics are consistent with the spectroscopic observations. 2006 American Institute of Physics. DOI: 10.1063/1.2141508

I. INTRODUCTION

Conducting conjugated polymers have attracted much at-tention in future commercial optoelectronic devices as flex-ible and economical alternatives to traditional silicon-basedinorganic devices. In all of the conducting polymers beinginvestigated, the environmentally stable polyaniline PANIfamily has been recognized as one of the promising organicpolymers due to the excellent electrical, optical, and mag-netic characteristics in the potential applications to displays,microelectronic devices, secondary batteries, and molecularsensors.14 PANI has five distinct base forms represented bythe following molecular formula:5

C6H4 NH C6H4 NH y

C6H4 Nv C6H4vN 1yn,

where y =0 corresponds to the fully oxidized pernigraniline

base, y =0.25 to the nigraniline base, y =0.5 to the half-oxidized emeraldine base EB, y =0.75 to the protoemeral-dine base PEB, and y =1 to the fully reduced leucoemeral-dine base LEB. In particular, the EB form has been knownto show the highest electric conductivity after the dopingprocesses with a protonic acid such as HCl and H 2SO4.

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In the development of polymeric thin-film devices, thefilms are, in general, prepared by spin casting, chemical syn-

thesis, or electrochemical methods.1017 However, in employ-ing such wet procedures most conventional methods undergounavoidable incompatibility with the existing integrated-circuit fabrication processing. The lack of suitability is as-cribed mainly to poor solubility in water and most organic

solvents, unwanted incorporation and/or contamination ofsolvent molecules, the difficulties in controlling the growthparameters for producing high-quality thin films, etc. Manyof such obstacles have been overcome by adopting thesimple physical vapor deposition PVD method, in whichthe sample molecules evaporated through resistive heatingare condensed onto the substrate in a high-vacuumcondition.1823 Even though the PVD method is effective inproducing thin films of low-molecular-weight materials,however, only the limited number of adjustable parameterssuch as the evaporation and substrate temperatures remainsas drawbacks.

Other less popular but promising methods in depositingpolymeric thin films are applying neutral and ionized clusterbeam deposition NCBD and ICBD and pulsed laser depo-sition PLD methods.2431 In the case of the cluster beamdeposition, the neutral cluster beam produced by the adia-batic expansion of sample vapor molecules into a highvacuum is utilized. Whereas in NCBD the neutral clusterbeam is directly deposited onto the target substrate, in ICBDpartial ionization is made by electron impact and then theresulting ionized cluster beam is accelerated and deposited.The cluster beam consisting of weakly bound molecules

aAuthor to whom correspondence should be addressed. FAX: 82-2-3290-3121. Electronic mail: [email protected]

THE JOURNAL OF CHEMICAL PHYSICS 124, 014710 2006

0021-9606/2006/1241 /014710/10/$23.00 2006 American Institute of Physics124, 014710-1

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shows the unique advantages such as the high directionalityand high translational kinetic energy. Therefore, after the col-lisions with the target substrate, the clusters undergo facilefragmentation into individual molecules followed by activesurface migration resulting in well-defined smooth and uni-form thin films. Especially, in ICBD the electric charge ofthe ionized cluster provides more adjustable deposition pa-rameters such as ionization and acceleration voltage, ion cur-

rent density, etc. In the case of the PLD method, the prepa-ration of thin films of hardly processable materials usinglaser ablation is definitely a promising new scheme and avariety of polymer materials have been studied.3137 It hasbeen known in polymer ablation that whereas the chemicalcomposition and velocity distribution in the ejected plumeare quite complicated and depend upon the absorption crosssection of target polymer and irradiation conditions, the ad-vantages of PLD include inherent simplicity, reactive depo-sition, congruent evaporation, and stoichiometric transfer.30

In consideration of the possible control of deposition param-eters through the selection of the wavelength and power ofthe laser beams used for ablation, the PLD method might be

applied to the formation of PANI thin films.In this paper, we present the characterization studies of

the structures, surface morphology, and electrical propertiesof the novel PANI thin films prepared by the NCBD, ICBD,and PLD methods. The possibility of utilizing those methodsis attractive from both fundamental and practical interests.From a fundamental viewpoint, it remains unclear whetherhigh-molecular-weight PANI can be evaporated or ablatedwithout significant structural degradation. From a practicalpoint of view, deposited films maintaining the desirablephysical and chemical properties of original PANI samplesmight be available, which cannot be easily processed throughconventional solution or thermal techniques. We have also

investigated the structural changes and enhancement of theelectrical conductivity after doping the as-deposited noncon-ducting PANI thin films with iodine I2 and a protonic acidsuch as HCl.

II. EXPERIMENT

The starting PANI powder Aldrich Co, PEB form ofmolecular weight Mw =5000 and EB form of Mw =65 000was deposited in a high-vacuum condition by the homemadeCBD and PLD systems. The schematic diagrams of the twosystems employed in this work are shown in Fig. 1.

The CBD system was described in detail elsewhere, andhere only a brief relative account is presented.25 The CBDapparatus consists of the evaporation crucible cell, the ion-ization and extraction electrodes, the drift region, and thesubstrate. The CBD chamber was pumped by a 10-in.3000 l/s baffled diffusion pump, and the average basepressure was kept below 1106 Torr. The starting materialPANI powder was placed inside the enclosed cylindricalgraphite crucible with a 1.0 mm diameter and 1.0-mm-longnozzle and was resistively heated up to 350 C. The evapo-rated PANI sample underwent an adiabatic supersonic expan-sion through the nozzle. The conversion of random, thermalenergies to highly directional translational motion and the

subsequent condensation resulted in the formation of weaklybound neutral clusters at the working pressure of ca.105 Torr. In the NCBD scheme, the neutral PANI clusters

were directly deposited onto the glass, quartz, or KBr pelletsubstrates, where the distance between the crucible cell andthe substrate was 190 mm. On the other hand, in the ICBDscheme, the partially ionized clusters were generated by theelectron impact source composed of a cylinder-shaped gridanode surrounded by a filament cathode. The extent of ion-ization was determined by the cathode emission current. Theresulting ionized cluster beam was extracted by the extrac-tion electrode, where the ionized beam was accelerated totravel the drift region and then deposited onto the substrate.The acceleration voltage Va determined by the potentialdifference between the grid anode and the extraction elec-trode was adjusted between 0 and 1 kV. The ion current

FIG. 1. Schematic diagrams of a cluster beam deposition CBD and bpulsed laser deposition PLD apparatus.

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measured by a picoammeter Keithley Co. was typically ca.30 nA. In both NCBD and ICBD schemes, the substrate waskept at room temperature throughout the deposition pro-cesses and normally seven substrates were deposited simul-taneously. The growth rate of the PANI film was generallydominated by the temperature of the crucible cell and wasmonitored by a thickness monitor Maxtek Inc.. The typicalrate was measured to be ca. 0.7 /s.

The PLD system consists of the pulsed Nd:YAG yttriumaluminum garnet laser and the deposition vacuum chambercontaining the rotating target holder and the substrate. ThePLD chamber was pumped by a 6-in. 1400 l/s baffled dif-fusion pump, and the average base pressure was maintainedbelow 1106 Torr. The third harmonic output 355 nm ofa Q-switched Nd:YAG laser beam Continuum Surelite II-10, 7 ns pulse duration, 10 Hz repetition rate was looselyfocused to an approximately 2.5 mm diameter spot with a600 mm focal length fused silica lens at the surface of thePANI pellet target, 10 mm in diameter and 1 mm thick. Theangle of incidence was maintained at 45 and the laser flu-ence ranging from 40 to 200 mJ/cm2 was used for 30 min.For good shot-to-shot ablation stability, the target pellet wasattached to the holder coupled to the motor MDC BRM-

133-01 in a constant rotary motion and was simultaneouslyraster scanned by the laser beam through the focusing lensmounted in a precision translator. The PANI thin films weredeposited on the substrate placed at a distance of 4 cm awayfrom the center of the target at the working pressure of ca.105 Torr. The substrate was kept at 40 C and the thicknesswas measured by an alpha step surface profile monitor Ten-cor Corp..

The conventional doping methods with iodine I2 andhydrochloric acid HCl were utilized to improve the electri-cal conductivity of as-deposited nonconducting NCBD,ICBD, and PLD thin films. The I2- and HCl-doped PANIfilms were prepared by exposing the films in a glass vial

filled with I2 for 48 h and with HCl for 2 min, respectively.The HCl vapor was generated from the reaction of ammo-nium chloride NH4Cl with sulfuric acid H2SO4. The ex-posure to air and water vapor was minimized during the dop-ing processes.

The Fourier transform infrared FTIR spectroscopyBomen MB-104, ultraviolet-visible UV-VIS HewlettPackard 8452A spectroscopy, and x-ray photoelectron spec-troscopy XPS Physical Electronics PHI 5700 were ap-plied to examine the as-deposited and doped PANI thin films.Especially, for the XPS analysis, the monochromatic x-raygenerated from an Al anode at the K line 1486.6 eV at12 kV and 350 W was employed in an ultrahigh-vacuumcondition at 21010 Torr. The surface morphology of thethin films was examined by atomic force microscopy AFMPSIA XE-100 and surface profiler Veeco DEKTAK3. Theelectrical conductivity for the undoped and doped PANI thin-

FIG. 2. a FTIR spectra of PANIMw =5000 pellet and NCBD andICBD thin films. Laser fluence depen-dence of FTIR spectra for PLD thinfilms ofb PANI Mw =5000 and cPANI Mw =65 000.

FIG. 3. FTIR spectra of a iodine- and b HCl-doped PANI Mw=65 000 thin films produced by NCBD, ICBD, and PLD methods.

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film samples was measured at room temperature through ap-plying the four-point probe technique called the van derPauw method.

III. RESULTS AND DISCUSSION

A. Fourier transform infrared spectroscopy

FTIR spectroscopy is an extremely useful tool to exam-ine the oxidation states and structural changes in the as-deposited thin films produced through several depositionmethods in this study. Typical FTIR spectra of several un-doped PEB-PANI Mw =5000 samples are displayed in Fig.

2. The initial sample pellet shows five strong absorptionbands at 832, 1168, 1304, 1506, and 1597 cm1. Those peaksare assigned to the out-of-plane bending vibration of phenylCH, characteristic vibration of quinoid di-imine, andstretching vibrational modes of CN, benzenoid diamine,and quinoid di-imine, respectively.12,13,15,22,38,39 Two weakbands near 740 and 690 cm1 are attributed to the out-of-plane wagging modes of five adjacent CH on the end phe-nyls and appear clearly resolved in the PANI samples withmolecular weight below 50 000.5 The vibrational spectra ofthe EB-PANI samples with Mw =65 000 not shown havebeen found to be very similar except the broadened out-of-plane wagging bands over the 650800 cm1 range.

In the cases of the slightly violet PANI films producedusing the NCBD and ICBD at acceleration voltage Va of250 V methods shown in Fig. 2a, there are almost nochanges in the absorption bands due to the benzenoid di-amine units compared to those observed in the initial powderpellets Mw =5000 and 65 000. However, the relative inten-sities of characteristic quinoid di-imine bands at 1168 and1597 cm1 decrease significantly in the NCBD and particu-larly ICBD films, indicating that the dominant structure ofthe films corresponds to the reduced LEB-PANI form. Theobserved chemical reduction might be ascribed to the higherreactivity of the quinoid di-imine moieties of the thermallyevaporated chain segments formed during the heating pro-

cess, which results in the formation of the thermodynami-cally more stable LEB-PANI state. In addition, the persistentobservation of two out-of-plane wagging bands over the650800 cm1 range for all NCBD and ICBD films revealsthat the degree of polymerization occurring on the substrateis not believed to be extensive during the evaporation anddeposition processes, leading to the shortening of the averageconjugation length in the LEB-PANI films.

On the other hand, the FTIR spectra of the PLD filmsproduced by ablation at 355 nm show strong dependence onboth the molecular weight of the initial PANI sample and thelaser fluence. For the PEB-PANI target with Mw =5000, thestructural change can be obviously discerned in Fig. 2b. Inthe low to intermediate fluence regime 40, 60, and100 mJ/cm2, little fluence dependence can be observed inthe IR spectra and the bands are in almost the same positionas those in the initial target sample. In the high fluence re-

FIG. 4. Geometric structures of LEB-PANI and EB-PANI before and after doping.

FIG. 5. a UV-VIS spectra of PANI Mw = 65 000 samples dissolved inN-methyl pyrrolidinone NMP solution and NCBD and ICBD thin films.b Laser fluence dependence of UV-VIS spectra of PLD thin films of PANIMw =5000.




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gime 200 mJ/cm2, however, the band intensity due to thecharacteristic vibration of quinoid di-imine at 1597 cm1 in-creases with a little shoulder at 1713 cm1 which is assignedto CvO stretching. Also a new band attributed to CwNstretching appears at 2218 cm1. Carbonyl groups must beformed through the reaction of radical fragments with oxy-gen when the PLD chamber is exposed to air. For the EB-PANI target with Mw =65 000 in Fig. 2c, the deposition ofthe thin film begins to be observed at 60 mJ/cm 2 due to thehigher molecular weight. As the laser fluence increases, theintensities of the 748, 1597, and 2218 cm1 bands are gradu-ally increased and the band broadening occurs over the1000 1500 cm1 range.

For laser ablation of polymers, photochemical and pho-tothermal mechanisms are generally proposed depending onthe absorption cross section of the target material at the ab-lation wavelength. In the photochemical model, the strongabsorption of laser photons induces the direct photodecom-position of target molecules, and the ejected fragments leav-ing behind the cleanly etched surface result in the smoothdeposition of congruent films and the decrease in the mo-lecular weight. In the case of the photothermal ablation, thelaser energy is converted into heat of evaporation via therovibrational motions of the target molecules, leading to themelting of the charred target surface. Since the PANI targetin this study absorbs strongly the 355 nm photons through

the -*

transition to be discussed in the next section, theablation process is mainly governed by the photochemicalmechanism. For the low-Mw PANI target, the laser-induceddecomposition of principal chains in the low to intermediatefluence regime is believed to generate the plume of fragmentradicals, which undergoes repolymerization on the substrateto form the PEB-PANI thin films with the same chemicalcomposition with the target. As the laser fluence increases,however, additional portions of the chains are broken in thequinoid di-imine and particularly benzenoid diamine unitsdue to the weaker chemical binding, and also the surpluslaser energy induces some chemical changes such as the for-mation of nitrile groups. Therefore, it is presumed that the

structure for the resulting PANI film is highly likely to be thePEB-PANI state with CvO, CwN, and the larger amountof quinoid di-imine units as observed in Fig. 2b. For thehigh-Mw PANI target, the ablation process is more difficult toproceed due to the higher mass of the sample target. Byincreasing laser fluence, the portion of quinoid di-imine unitsis similarly enhanced. However, the observation of both theband broadening and the increase of the 2218 cm1 CwNstretching band suggest clearly that both significant structuralchange and the complicated incongruent deposition are in-duced. Such non-PANI-like PLD films formed from the ab-

FIG. 6. UV-VIS spectra of a iodine- and b HCl-doped PANI Mw =65 000 thin films produced byNCBD, ICBD, and PLD methods.

FIG. 7. N1s core-level XPS spectra ofa an initial EB-PANI sample and bNCBD, c ICBD, and d PLD thin films.




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lation of high-Mw PANI target are not within the scope ofthis study and are ruled out in the following sections. Theabove photochemical model is also consistent with the UV-VIS and XPS results below. A similar broadening in the highfluence region has also been reported in the ablation process

of several high-Mw polymers, where the extensive develop-ment of various functional groups and aromatic ring conden-sation might take place.31

Figure 3 demonstrates the FTIR spectra of three differentNCBD, ICBD Mw =65 000, and PLD obtained from thelow-Mw PANI ablation at 40 mJ/cm2 thin films after dopingthe as-deposited LEB- and EB-PANI films with iodine andHCl. The geometric structures of various doped PANI statescontaining the semiquinone cationic radicals are demon-strated as the polaron lattice in Fig. 4.12,13 The structures aredescribed in more detail in the following section. Comparedwith the spectra of the as-deposited pristine films, most ofthe major bands in the doped films are redshifted with more

uniform intensities. In particular, the shift of the bands as-signed to the quinoid di-imine units appears to be particu-larly considerable, indicating that the doping reactionsmostly occur at the quinoid di-imine units in the PANI mo-lecular chains. Such a spectral shift is in good agreement

with the previous work conducted by Zeng and Ko.12,13

B. UV-VIS spectroscopy

The optical UV-VIS spectrum of commercial PANIMw =65 000 sample dissolved in N-methyl pyrrolidinoneNMP solution is displayed in Fig. 5. The strong absorptionband observed at 329 nm which is common to all forms ofPANI is associated with the -* transition giving rise to theband gap. The characteristic 638 nm absorption band isassigned to a molecular exciton band due to a charge-transfer excitonlike electronic transition from the highest oc-cupied molecular orbitals HOMOs centered on the ben-

FIG. 8. N1s core-level XPS spectra ofac iodine- and df HCl-doped NCBD, ICBD, and PLD thinfilms.




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zenoid rings to the lowest unoccupied molecular orbitalsLUMOs centered on the quinoid rings. As the molecularweight increases, the two absorption bands are redshifted tolonger wavelengths due to the extension of the -con-

jugation system.15

The absorption spectra of the as-deposited films shownin Fig. 5 further support the chemical reduction observed inthe infrared spectra, as pointed out in the previous section.The very weak molecular exciton band in the as-depositedNCBD and ICBD Mw =65 000 films can be definitely un-derstood by the chemical reduction of the quinoid di-imineunits to the stable benzenoid diamine units, resulting in the

stable leucoemeraldine base form. Such a change in the oxi-dation state is consistent with the significant decrease in theintensity of characteristic quinoid di-imine bands at 1168 and1597 cm1 displayed in the FTIR spectra. In addition, theblueshift of the absorption bands due to the -* band-gaptransition suggests the shortening of the average conjugationlength in the NCBD and ICBD films, which is also in goodagreement with the persistent observation of two out-of-plane wagging bands over the 650800 cm1 range. On theother hand, in the case of the PLD films Mw =5000 theclear observation of the molecular exciton band in the lowfluence regime implies the polymer deposition with the samechemical composition with the pristine PEB-PANI materials.

The congruent deposition is also consistent with the infraredspectra, where the bands due to both the quinoid di-imineand benzenoid diamine moieties clearly appear. By increas-ing the laser fluence, the gradual decrease in the exciton bandas well as the blueshift in the -* absorption band demon-strate the significant structural change such as formation ofCwN and the incongruent deposition due to the large sur-plus laser energy, as suggested in the infrared spectra.

Figure 6 shows the optical UV-VIS spectra of I2-andHCl-doped PANI thin films. For all doped thin films, thestrong -* absorption bands at 310 nm appear as before,but the molecular exciton band at 630 nm associated with

the quinoid di-imine units is eliminated. Instead, for I2-dopedPANI thin films a strong absorption band at 380 nm and abroad band at 850 nm and below are observed, whereas forHCl-doped PANI thin films a band at 415 nm and a strong,broad band at 1050 nm and below are observed. New ab-sorption bands can be understood in terms of the electronicstructure associated with the polaron lattice of the oxidizedPANI salts formed after doping shown in Fig. 4 and areinterpreted as optical excitations to the only half-occupiedpolaron band located within the band gap. Besides, the broadfeature continuing to the far-infrared region is, in fact, attrib-uted to another low-frequency intraband through the opticalabsorption excited within the finite width of the polaron

FIG. 9. ac I3d 5/2 and df Cl2p 3/2 core-levelXPS spectra of the doped NCBD, ICBD, and PLD thinfilms.




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band. The overall band characteristics in the observed UV-VIS spectra are also consistent with the combined theoreticaland optical investigations performed by Stafstrom et al.40

C. X-ray photoelectron spectroscopy

In combination with the FTIR and UV-VIS spec-troscopies, XPS is another powerful method to probe the

structures and properties of the various PANI samples withdifferent oxidation levels. Figure 7 shows the N1s core-levelXPS spectra of the initial EB-PANI sample and NCBD,ICBD, and PLD thin films. Each spectrum can be best de-convoluted into four peaks, 398.5, 399.4, 400.4, and402.2 eV, corresponding to the four nonequivalent nitrogensites of quinoid di-imine, benzenoid diamine, oxidizedamine, and protonated imine, respectively.4144 As clearly

manifested, the N1s peaks of the initial EB-PANI Mw=65 000 sample consist of mainly quinoid di-imine andbenzenoid diamine with the ratio of 1:1. In the cases of theNCBD and especially ICBD films, the relative intensities ofthe quinoid di-imine peaks decrease, whereas the intensitiesof the benzenoid diamine peaks increase significantly. Suchdrastic changes are attributed to the chemical reduction ofthe initial EB-PANI form to the LEB-PANI state, and thehigher reactivity of ionized species participating in the ICBDscheme appears to enhance the reduction to a greater extent.On the other hand, such an obvious chemical reduction is notdemonstrated in the spectra of the PLD films producedthrough the ablation of the initial PEB-PANI Mw =5000

pellet at 40 mJ/cm2. The peak intensities for the PEB-PANIsample which correspond to the quinoid di-imine and ben-zenoid diamine units with the ratio of 2:1 are still main-tained, suggesting that the congruent stoichiometric transferis apparently involved in the formation of the PLD films. Theoverall N1s-XPS results for the structural changes of the as-deposited NCBD, ICBD, and PLD films above are in verygood agreement with the results examined through the IRand UV-VIS spectroscopies described in the previous sec-tions.

Figure 8 shows that after doping the as-deposited thinfilms with iodine and HCl, the relative intensities of the

quinoid di-imine peaks decrease drastically, whereas the in-tensities of the benzenoid diamine peaks remain almost un-changed and the intensities of the oxidized amine peaks in-crease considerably. The N1s core-level XPS results revealthat the quinoid di-imine nitrogens of the PANI molecularchain are preferentially doped by iodine and HCl, as in thescheme demonstrated in Fig. 4. As a result, a large number ofoxidized amines of semiquinone cationic radicals areformed, confirming the optical transitions in the UV-VISspectroscopic results due to the formation of the polaron lat-tice in the previous section.

Figure 9 shows the I3d 5/2 and Cl2p 3/2 XPS spectra of thedoped thin films. The I3d 5/2 spectra can be resolved into two

peaks at about 618.7 and 620.9 eV, corresponding to the I 3

and I5 anions, respectively. In comparison to the work per-

formed by Zeng and Ko on the iodine-doped PANI filmsprepared in the solution phase, the smaller contents of I5

anions relative to that of I3 anions indicate that the doping

level for the three films is not a significant proportion due tothe insufficient exposure doping to the iodine vapor presentin gas phase.12,13 In the Cl2p 3/2 spectra, two major compo-nents lying at about 197.6 and 198.9 eV are attributed to thetwo different types of chloride anion Cl species and theminor peak at about 200.0 eV is related to the covalentlybonded chlorine Cl species. It should be noted that thehigher binding-energy peak at about 198.9 eV signifies the

FIG. 10. 2D AFM images of a NCBD thickness=817 , Rrms=56.5 ,b ICBD thickness=637 , Rrms=88.5 , and c PLD thin filmsthickness=227 , Rrms=11.8 .




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presence of the Cl species in a more positive local environ-

ment associated with the polaron lattice in Fig. 4.

D. Atomic force microscopy

The characterization of surface morphology for the as-deposited NCBD, ICBD, and PLD thin films on the quartzsubstrates has been performed by recording AFM images.Figure 10 shows typical two-dimensional micrographs forthe three thin films taken over 33 m2. Both NCBD andICBD films consist of densely packed grains. The quantita-tive values of the root-mean-square roughness Rrms have

been obtained through conducting section analysis using thebuilt-in software of the AFM apparatus. Typical roughnessestimates for the NCBD and ICBD films are 55 and 90 ,respectively. The observed large lateral dimensions of thegrains suggest that they are not individual polymer chains oroligomers. Since the thermally evaporated chain segmentsproduced during the heating process of the initial EB-PANIsamples Mw =65 000 would be less than two or three re-peating units, the small energetic segments undergo thecross-linking reactions and the reduction of molecular weightupon contact with the substrate, as manifested in the afore-mentioned spectroscopic investigations. Furthermore, theweakly bound cluster beam is effective in transforming the

translational kinetic energy into the surface migration energy,which induces the formation of smooth thin films composedof LEB-PANI dominant grains.

In the case of the PLD method, the films in Fig. 9cconsist of a relatively flat surface with some localized globu-larlike conglomerates. The typical Rrms value is 12 ,which is considerably lower than those reported in otherPLD works.3337 Such a lower roughness for the PLD filmsimplies that after irradiating the PEB-PANI target Mw=5000 at a fluence of 40 mJ/cm2, the hyperthermal plasmaplume ejected might be efficient in migrating to the favorablepinning sites, where the congruent repolymerization reactionto form the islands of PEB-PANI polymer takes place. Espe-

cially after a long irradiation about 30 min, the gradual

growth of the islands leads to the extremely smooth and flatthin PLD films observed in the micrograph.

E. Electrical conductivity

The four-point probe technique called the van der Pauwmethod has been employed to measure the resistivity forundoped and doped PANI thin films.45 Figure 11 shows theelectrical probe arrangement consisting of a current sourceand a voltmeter, in which two resistances RAB,CD = VDC/IABand RBC,DA = VAD/IBC can be measured separately. For thethin-film samples with a uniform thickness d, the resistivity as the reciprocal of the conductivity is given by the follow-

ing equation:

=d

ln 2

RAB,CD + RBC,DA2

fRAB,CDRBC,DA

,where f is defined as a solution of

RAB,CD RBC,DARAB,CD + RBC,DA

=f

ln 2arccosh expln2/f

2 .

Figure 11 displays the current-voltage characteristics mea-sured at room temperature for the I2- and HCl-doped NCBD,ICBD, and PLD thin films. Before doping, the as-depositedpristine films are found to be insulators with the electrical

conductivities below 1010

S/m. However, all doped filmsdemonstrate that although the absolute values are rather low,the electrical conductivities are increased significantly. Inparticular, the doped PLD films show the highest conductivi-ties, which is ascribed to the EB-PANI states with higheramounts of quinoid di-imine doping sites compared to theNCBD and ICBD films with the LEB-PANI structures. Suchstructure-conductivity characteristics are in very good agree-ment with the spectroscopic results discussed in the previoussections. The doping process of the thin films in gas phaserequires some improvement to enhance the electrical conduc-tivity and more extensive investigations are currently underinvestigation.

FIG. 11. I-V characteristics for iodine- and HCl-dopedPANI thin films produced by NCBD, ICBD, and PLDmethods.




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IV. CONCLUSIONS

The structural changes and oxidation states of pristineand doped PANI thin films prepared by NCBD, ICBD, andPLD methods in a high-vacuum condition have been charac-terized by applying spectroscopic methods such as FTIR,UV-VIS, and XPS. While the dominant structure of NCBDand ICBD thin films corresponds to the reduced LEB-PANIstate, the chemical composition of PLD thin films depends

critically on the laser fluence and the molecular weight ofPANI target. The original chemical composition is only re-tained for the PLD thin films deposited by the laser ablationof the low-Mw PANI targets in the low to intermediate flu-ence regime. Atomic force microscopy measurements showthat three methods are effective in producing smooth, uni-form PANI thin films. The electrical conductivity demon-strates that the electrical conductivities for all doped filmsare increased significantly and the overall structure-conductivity characteristics are very consistent with the spec-troscopic observations. Throughout our experiments, wehope to gain some insights into the preparation of polymericthin films which cannot be easily processed through conven-tional solution or thermal techniques. In addition, severalpolymeric thin-film devices using PANI composites blendedwith nanocrystal such as carbon nanotubes and GaN nano-wires are currently under investigation, which have beenlittle explored so far.

ACKNOWLEDGMENTS

One of the authors H.L. gratefully acknowledges thesupport of BK21 fellowships. This work was financially sup-ported by NRL-KOSEF, MOST.

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