European Polymer Journal 44 (2008) 2506–2515

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Contents lists available at ScienceDirect

European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Macromolecular Nanotechnology

Layered self-organized structures on poly(3-octylthiophene) thin filmsstudied by scanning probe microscopy

Jose Abad a,*, Beatriz Pérez-García a, Antonio Urbina b, Jaime Colchero a, Elisa Palacios-Lidón a

a Departmento de Física, Facultad de Química (Campus Espinardo), Universidad de Murcia, E-30100 Murcia, Spainb Departmento de Electrónica, Tec. de Computadoras y Proyectos, Universidad Politécnica de Cartagena, E-30202 Cartagena, Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 November 2007Received in revised form 22 April 2008Accepted 22 May 2008Available online 2 June 2008

Keywords:P3OT (poly-(3-octylthiophene))SFM (scanning force microscopy)Layered structuresSelf-organisation

0014-3057/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.eurpolymj.2008.05.019

* Corresponding author. Tel.: +34 968 39 8551; faE-mail address: jabad@um.es (J. Abad).

The morphology and mechanical properties of poly-(3-octylthiophene) P3OT thin films hasbeen studied by scanning force microscopy techniques. On these films we find self-orga-nized layered structures that appear regardless of the preparation conditions, that is,spin-coating or drop-casting, of the solvent concentration or of the type of substrate. Usingthe drop-casting method for sample preparation these layered structures are hardly visibledue to the high surface roughness, while using spin-coating these structures are the maintopographic feature on the surface. These structures have typically one or two layers, eventhough occasionally up to four layers have been observed. Each layer has a height of 4–5 nm, which is associated to crystalline P3OT domains and lay on the polymer film. The sizeof these structures increases with increasing concentration of the P3OT in the solvent. Wefind well differentiated morphological, electrostatic as well as mechanical properties forthe self-assembled structures as compared to the rest of the polymer film. Finally, thegrowth rate of these structures has been studied.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, the physical and chemical properties ofp-conjugated polymers have been the focus of many inves-tigations due to their potential applications in (opto-) elec-tronic devices such as plastic solar cells [1], thin filmtransistors [2], lasers [3] and light-emitting diodes [4]. Inparticular, poly(3-alkylthiophenes) (PAT) have stimulatedmuch interest among researchers [5–9], because of theirgood environmental stability, easy processability, and easymodification of optical and electronic properties. Many ofthese devices – most importantly the case of organic solarcells- are based on the concept of the bulk heterojuction,where blends of different polymers [10–12], derivates offullerenes [13,14], carbon nanotubes [15,16], or graphiticnanoparticles [17] are used. The bulk heterojunction can

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x: +34 968 36 4148.

be considered a dispersed p–n junction, where donor andacceptor materials are blended on a nanometer scale andeach material forms a percolation network. The efficiencyand performance of a device based on the heterojunctionconcept is determined mainly by the electronic propertiesof the individual materials, as well as the morphology ofthe heterojunction [18]. Understanding organic optoelec-tronic devices from ‘‘first principles” is a huge effort and,at the moment, not possible. Therefore, the problem is gen-erally separated into the investigation of the percolationnetwork on the one hand and the properties of the individ-ual materials on the other. Due to relevant length scales -the exciton diffusion length is of the order of 10 nm, there-fore techniques which obtain nanometer resolution arerequired. Scanning force microscopy (SFM) has proven tobe a very powerful instrument to study the nanoscaleproperties of polymers [19]; accordingly several studieshave used this technique to investigate blends or compos-ites of P3OT with others materials [5,17,20,21]. However,

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there are a few articles that discuss the morphology of sim-ple P3OT thin films by SFM, taking into account differentpreparation methods and different substrates with distinctwetting properties [22].

Polyalkythiophenes have been studied using a variety oftechniques, including X-ray diffraction [23], transmissionelectron microscopy [24], differential scanning calorimetry[25] and optical methods [26]. In addition to the detailedstructure of the polyalkythiophenes, important issues thathave been investigated are the degree of crystallinity[27,28], the electrical and optical properties and theiranisotropy [29,30], as well as how these properties varywith the degree of crystallinity, the molecular weight[31] or the regularity of the alkythiophene monomersalong the polymer (regiorandom/disordered versus regio-regular/ordered). While the details of macromoleculararrangement are still investigated, it is generally acceptedthat polyalkythiophenes are semi-crystalline with crystal-line domains surrounded by amorphous material. Thecrystalline domains have a orthorhombic unit cell (seeFig. 1) with lattice parameters in case of P3OT moleculesa = 4.13 nm, b = 0.76 nm and c = 0.77 nm [23]; where – fol-lowing the usual convention – the lattice parameter a isalong the direction of the alkyl ‘‘arms” of the molecule,the parameter b along the stacking direction of the mole-cules (perpendicular to the projection shown in Fig. 1)and the direction c is in the direction of the backbone ofthe molecule. The degree of crystallinisation of the samplesdepends on the regularity of the monomers along thepolymer – regioregular leads to higher crystallinisation[27] – and on the chain length [28]. For thin films(6100 nm) confinement of the PAT molecules is important[32], leading to additional structuring of the molecules andhighly anisotropic samples.

In this paper, we use SFM to investigate the morphologyof spin-coated and drop-cast P3OT thin films. We generallyfind that on their surface layered structures are formedwith sizes of several hundred nanometer up to a fewmicrometers, which we relate to the formation of crystal-line P3OT domains. The properties of these structures areinvestigated as the concentration of the polymer in the sol-vent and the type of substrate is varied. These structureshave different morphology, as well as different electro-

Fig. 1. Schematic representation of the molecular arrangement of regio-regular head to tail P3OT. The parameters are taken from reference [23].

static and morphological properties as compared to therest of the P3OT film.

2. Experimental

Regioregular P3OT was purchased from Sigma–Aldrich,with 98.5% head to tail couplings, molecular weightMn = 54,000 and polydispersity index D = 2.6. Differentsubstrates have been used in this study: gold thin films(20–40 nm of thickness) evaporated onto a glass cover slip,highly ordered pyrolytic graphite (HOPG) and glass coverslips. The gold thin film and the glass cover substrates werecleaned by rinsing with acetone in an ultrasonic bath for5 min and subsequent rinsing again with ethanol in anultrasonic bath for another 5 min. The HOPG was cleavedby standard procedure with a scotch tape. These substratesare, on the one hand, typical in SFM experiments due totheir (relative) flatness, and on the other hand they havedifferent wetting properties for the P3OT precursor solu-tion. P3OT samples were prepared by two methods, eithersimple drop-casting or by spin-coating. For drop-casting asolution of 6 mg/ml P3OT in toluene was prepared. Then a10 ll drop of this solution was deposited on the substrateand left until the drop evaporated. To prepare the samplesby spin-coating, a similar droplet (10 ll) of P3OT/toluenewas deposited on the substrate and then it was acceleratedto about 3000 rpm. During this process, the drop is homo-geneously spread out and a thin film P3OT sample isformed on the substrate. For spin-coating, P3OT solutionsin toluene were prepared with concentrations of 9, 18and 39 mg/ml. SFM pictures of scratched polymer filmson to glass cover samples were taken to determine the filmthickness. We found that the concentration of P3OT in thesolvent scales quite well with the film thickness: 40–45 nm, 75–90 nm and 180–205 nm, respectively. The poly-mer concentrations for spin-coating were chosen higher ascompared to the case of drop-casting since spin-coatinggives thinner films.

The morphology and mechanical properties of the thinfilms were studied at room temperature and ambient con-ditions using SFM. A Nanotec Electronica SFM system witha phase locked loop (PLL)/dynamic measurement board[33] was used with Olympus OMCL-AC-type cantilevers(nominal force constant: 2 N/m; resonance frequency:70 kHz). Unless specified otherwise, imaging was per-formed in non-contact dynamic SFM (NC-DSFM) usingthe oscillation amplitude as feedback parameter. Typicalfree oscillation amplitudes were 10 nm (peak to peak),and a 5–10% reduction of the free oscillation amplitudewas chosen as feedback parameter in order to maintainthe tip–sample system in the ‘‘attractive” part of the inter-action and avoid tip–sample contact. Unlike most otherSFM experiments performed in air, a PLL system is usedto keep the cantilever always at resonance and track theresonance frequency when it changes due to tip–sampleinteraction. The variation of resonance frequency is re-corded as a local measure of tip–sample interaction. Forsmall tip–sample interaction the frequency shift signal isproportional to the usually acquired ‘‘phase signal”. Animportant advantage of measuring the interaction as

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‘‘frequency shift” is that it is physically well calibrated andcan be related directly to tip–sample interaction [34].

3. Results and discussion

In the next subsections, we will discuss separately thefollowing topics: First, how the surface roughness varieswith the preparation method used: drop-casting versusspin-coating. Second, for the case of spin-coating, we willstudy how the general properties of the P3OT samples varywith the nature of the substrate and with the concentra-tion of the precursor solution. Third, we will focus on thediscussion of the most relevant features found on our sam-ples: self-organized layered structures. Finally, the timeevolution of these structures will be determined.

3.1. Preparation methods: Drop-casting versus spin-coating

Fig. 2a–c shows topography and frequency shift imagesof a P3OT thin film prepared by drop-casting a 6 mg/mlP3OT/toluene solution on a cover glass slip. The topogra-phy images show a very irregular morphology with a highroot mean square (RMS) roughness of about 120 nm for animage area of 25 lm2. This large roughness is dominatedby large scale features, that is, by features with a lateralsize of 0.5–2 lm. Compared to the surface roughness, thefrequency shift image acquired simultaneously has muchless ‘‘roughness”. As discussed previously differences intip sample interaction (at constant tip–sample distance)are usually interpreted as due to differences in chemicalcomposition. Therefore, since the thin films prepared arecomposed of the same material, the frequency shift imageis expected to be flat. Nevertheless, we observe some dis-

Fig. 2. A series of SFM images showing (a–c) the morphology of a 6 mg/ml P3OT9 mg/ml P3OT thin film spin-coated on to a glass cover slip. (a) Topographic imagethe Dz is changed to Dz = 50 nm for distinguish better the layered structures. (cimage Dz = 80 nm. (e) Topographic image Dz = 80 nm and (f) frequency shift im

tinct contrast in these images. Interestingly, when compar-ing the thinner structures in the frequency shift image weconclude that they are related with structures in the topo-graphic images: they define the borders of island-like lay-ered structures which are almost ‘‘lost” in the largecorrugation of the topographic image. Moreover, eventhough the irregular morphology renders the measure-ment of height differences extremely difficult, we find thatthe height of these layered structures is approximately 4–5 nm. This corresponds to one of the lattice parameters of acrystalline P3OT structure.

In general, samples prepared by drop-casting on the dif-ferent substrates studied here (glass cover, gold thin filmand HOPG) have a very rough surface morphology, of theorder of 100 nm for an image area of 25 lm2 (RMS rough-ness of 120, 50 and 30 nm, for glass, gold and HOPG,respectively). In addition, the P3OT thin film samples pre-pared on gold and HOPG present holes, in particular forHOPG. These holes, and the difference of the measuredroughness, are assumed to be related with the wettingproperties of the P3OT precursor solution on the differentsubstrates: the P3OT solution wets graphite more easily(lower contact angle) than gold and glass cover.

As expected, samples prepared by spin-coating result inmuch smoother surface morphology than those preparedby drop-casting. Fig. 2d–f shows typical images of a sampleprepared by spin-coating a 9 mg/ml P3OT/toluene solutionon a glass substrate; substrate and solution are thus com-parable to the preparation just discussed for drop-casting.The morphology of this sample is much flatter, with a sur-face RMS roughness of about 2 nm for an image area of25 lm2, almost two orders of magnitude lower than forsamples prepared by drop-casting. Moreover, in Fig. 2d–f

thin film drop-cast on to a glass cover slip, and (d–f) the morphology of aDz = 1000 nm. (b) Topographic image Dz = 200 nm, in the region outlined

) Frequency shift image of the (b) image Dfreq = 300 Hz. (d) Topographicage of the (e) image Dfreq = 70 Hz.

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two distinct regions are clearly distinguished: a lower re-gion which usually covers most of the sample and a secondhigher region with a characteristic layered structure,which will be discussed in detail below. The measured sur-face roughness is determined by the height differences be-tween these two regions. The structures corresponding tothe higher regions have one or two layers, even thoughoccasionally up to four layers have been observed. Eachlayer has a height of 4–5 nm, and thus essentially the sameas the layered structures observed in the samples preparedby drop-casting. We therefore conclude that while the lay-ered structures are observed in samples prepared by bothmethods, drop-casting and spin-coating, only for the lattercase the layered structures are clearly distinguished, whilefor samples prepared by drop-casting these structures arehidden in the rough morphology of the surface. Since themain focus of this work will be the structures correspond-ing to the higher regions, we will limit our further discus-sion to samples prepared by spin-coating, where thesestructures are more easily recognized.

3.2. Morphology of the self-organized layered structures

Fig. 3 shows a comparison of spin-coated samples(18 mg/ml P3OT/toluene) prepared on different substrates:glass, gold and HOPG. The main features are essentially thesame: islands on top of a homogeneous polymer back-ground. Due to their characteristic appearance, we havenamed these islands ‘‘jellyfish” structures, since they pres-ent rounded bodies and many elongated ‘‘legs”. Thesestructures have been found on all substrates used and alsofor all concentrations prepared. We therefore concludethat they are characteristic of the P3OT thin film, and not

Fig. 3. Large scale images of P3OT thin films prepared on three different substra18 mg/ml P3OT in toluene. Images on top (a, b and c) correspond to the topogrfrequency shift (interaction) of the resonance frequency, Dfreq = 100 Hz.

induced by the substrate or by the concentration of theprecursor solution. On a large scale, the variation betweenP3OT films prepared on the different substrates is mainlyrelated to their thickness. As is observed in Fig. 3c steps,which are typically found on graphite, can be clearly dis-tinguished through the polymer film. This indicates thatthe interaction between the polymer–graphite interfaceand the polymer–air interface (disjoining pressure) is lar-ger than the molecular interactions which induce flatten-ing of the film (surface energy). This can only be the caseif the film thickness is of the order of a few nanometers,since the disjoining pressure falls of as 1/t3, where t isthe film thickness [35]. For films prepared on other sub-strates the characteristic features of the substrate – grainsfor the case of gold, and holes for the case of glass – are notobserved through the polymer film, which is in agreementwith the larger thickness measured on the glass substrateas discussed above. We attribute the different thicknessof the samples to the different wetting behavior of the pre-cursor P3OT/toluene solution. To study possible variationsof the jellyfish structures due to the substrate, their aver-age size was calculated for samples prepared from thesame precursor solution (see Table 1). The correspondingstatistical analysis leads to a similar mean size of the jelly-fish structures on graphite and gold substrates (4 ± 1 lm2

for gold, 5 ± 1 lm2 for graphite) and a mean size which isabout twice as large for structures on a glass substrate(11 ± 1 lm2).

Lopez-Mata et al. [22] report that an increase of theP3OT concentration induces changes in the morphologyof the films. To check this issue, a statistical analysis simi-lar to the one discussed before was done. In this case, theaverage size of the jellyfish structures was calculated for

tes, glass (a, d), gold (b, e) and graphite (c, f) from a precursor solution ofaphy of the samples, Dz = 70 nm; and images on the bottom (d–f) to the

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Table 1Average size of the jellyfish structures for different spin-coated samples

Variation: substrate Variation: concentration

Substrate Concentration(mg/ml)

Size(lm2)

Substrate Concentration(mg/ml)

Size(lm2)

Gold film 18 4 ± 1 Glasscover

9 4 ± 1Graphite 5 ± 1 18 11 ± 1Glass

cover11 ± 1 39 18 ± 3

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samples prepared on the same substrate from differentprecursor solutions. For the analysis shown P3OT/toluenesolutions of 9, 18, and 39 mg/ml were spin-coated on toglass. As summarized in Table 1, the mean size of the jelly-fish structures increases with increasing concentration.

In some images we have found a preferential orienta-tion of the jellyfish structures analyzing the 2D FFT of thecorresponding images. These orientations have been foundfor samples prepared from 9 and 18 mg/ml concentrationson glass and gold substrates. However, for the graphitesubstrate no preferential orientation has been found. Thiscould be due to the very thin film on graphite, whichmay hinder the formation of ordered domains inside ofthe polymer film. The precise physical reason for this ori-entation is, however, not yet known.

3.3. Non-contact dynamic scanning force microscopy of theself-organized structures: molecular arrangement andintermolecular interaction

As discussed previously, in our experiments the jellyfishstructures appear for all the preparation methods, all sub-

Fig. 4. Images showing the morphology of 18 mg/ml P3OT thin films spin sampshown corresponds to regions with jellyfish structures at medium and high rescorresponds to 30 nm (a and c), while the high resolution images corresponding toall frequency images is 100 Hz (b, d, f and h).

strates and all concentrations used. In general they haveone or two layers, however, also structures with up to fourlayers have been found. Within the experimental error ofSFM, the height of the different layers (1st, 2nd...) is thesame and does not depend on the substrate used for thinfilm deposition. From the different images acquired withno-contact dynamic scanning force microscopy we esti-mate a height of 4.5 ± 0.5 nm for each of these layers.

Fig. 4 presents medium and high resolution images offilms prepared on gold and graphite. In these images, a sin-gle jellyfish structure as well as an enlarged area corre-sponding to the border (step) between the jellyfishstructure and the lower region is shown. As observed inFig. 4, at high resolution samples prepared on different sub-strates are indistinguishable. This is also the case for sam-ples prepared on glass (image not shown). For allsamples, the jellyfish structures present a much flattertopography (Fig. 4a, c, e and g) as compared to the lower re-gions and the frequency shift image shows essentially nosubstructure. In the topography, the lower regions of thesamples show some granular disordered structure with atypical lateral dimension of 20–50 nm and a typical heightvariation of 1–2 nm. Interestingly on these regions, also thefrequency images (Fig. 4b, d, f and h) shows some disor-dered structures. The variation of frequency shift is about20 Hz, which corresponds to a force gradient difference ofabout 1 mN/m and is induced by differences of tip–sampleinteraction. We note that this difference in tip–sampleinteraction is not induced by errors of the feedback loop,since the oscillation amplitude images are flat (imagesnot shown), indicating that the feedback loop is fast enoughto compensate for topographic height differences. In allcases, the border region of the jellyfish structures appears

les prepared on gold (a, b, e and f)) and graphite (c, d, g and h). The areaolution. For medium resolution topographic images, the total grey scalethe areas outlined has a total z-scale of 15 nm (e and g). The grey scale for

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brighter in the frequency shift images (width 20–30 nm,variation 20–40 Hz). Usually a similar but thinner rim canalso be observed in the topographic images. We remarkthat with the exception of these border regions and someareas at their border the frequency shift is very homoge-neous on the jellyfish structures. These areas with differentfrequency shift (see lower right in Fig. 5b here the contrastis�20 Hz as compared to about�40 Hz for rest of the moreextended jellyfish regions) can be identified in the topogra-phy images as lower areas (about 1.5 nm) as compared tothe rest of the jellyfish structure. These lower areas couldbe related to a single plane of P3OT molecules correspond-ing to half a lattice parameters (half a conventional layersee Fig. 1).

As discussed in the introduction, one of the latticeparameters of crystalline P3OT samples is a = 4.13 nm[23], corresponding to the distance between the mainpolymer chains along the alkyl ‘‘arms” of the molecule(see Fig. 1). We conclude that the jellyfish structures areordered crystalline layers where two molecule backbonesare stacked on top of each other forming parallel planesseparated by the octyl-side chains; therefore, there aretwo backbones per layer. These structures seem to lay ona polymer film with very different structure. In a XRDstudy of regioregular poly-(3-hexylthiophene) thin filmsspin-coated onto glass substrates Aasmundtveit et al.[29] find that the polymer close to the solution–air inter-face is oriented with the side chains normal to the sub-strate. Those results are in agreement with the presence

Fig. 5. SPM images showing the morphology of a 39 mg/ml P3OT thinfilm spin-coated on to a gold thin film. (a) Topographic image Dz = 15 nm.(b) Frequency shift image of the (a) image Dfreq = 100 Hz. (c) and (d)response of the lock-in amplifier to the modulation of the tip–sample biasvoltage. This bias voltage of frequency mel induces small variations of thetip–sample resonance frequency at the frequencies mel and 2mel. Theresponse of the lock-in to the frequency component at mel (c) isproportional to the surface potential between tip and sample, while thatat 2mel (d) is proportional to the tip–sample capacitance.

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of layered structures with a stacking distance of 4.13 nmon the polymer surface.

Fig. 5 shows a pair of high resolution topography andfrequency images where the frequency shift image has par-ticularly high signal to noise ratio, possibly due to a verygood tip. In the frequency image, the contrast betweenthe darker jellyfish structure (which is brighter/higher inthe topography) and the lower region is about �40 Hzand thus about twice as high as in other images. Interest-ingly, in the frequency images the lower region does notappear completely disordered as in the correspondingtopographic image. Instead, some stripes with quite welldefined width (20–30 nm) appear, a length of up to250 nm and a difference of interaction of about 20 Hz ascompared to the somewhat darker material around it.We propose that these stripes are due to an ordering ofthe P3OT molecules in such a way that the molecules formlamella by folding over themselves after this distance. Inthis kind of lamellas the c-axis of the molecule would bealong the width of the stripes, with the a-axis either paral-lel to it and the b axis normal to the surface, or, the b-axisparallel to the width of the stripes and the a-axis normal tothe surface. These result are in agreement with the foldedchain model [36], which describes the polymer morphol-ogy as consisting of a continuous amorphous region (dar-ker region between stripes) with disperse crystallites(stripes). This kind of structure has been discussed in Ref.[31,37] and the authors report that for low molecularweight of the molecules the thickness of these lamellasare defined by the length of the molecules while for largemolecules this width saturates at a value of about 30 nmbecause the molecules fold. For single molecules this hasin fact been observed by scanning tunneling microscopy[38].

In order to study the electrostatic response we haveperformed electrostatic force microscopy on the P3OT thinfilms. A detailed description on how we implement thistechnique is presented elsewhere [39,40]; essentially analternating bias voltage is applied (frequency mel ffi m0/10)between tip and sample in order to measure the frequencyshift induced by the alternating electric field between tipand sample. This frequency shift is measured using lock-in techniques, which allows to determine the response atthe electrical driving frequency mel as well as at twice thatfrequency. As shown in detail elsewhere [40], the firstharmonic is proportional to the contact potential, whilethe second harmonic is proportional to the capacitance ofthe tip–sample system. Fig. 5c shows the response of thelock-in amplifier at the first harmonic (local surface poten-tial map) while Fig. 5d shows the response at the secondharmonic (local capacitance map). In order to reduce thenoise in these images, some Fourier filtering has been ap-plied to the raw data corresponding to these images. Thejellyfish structures have a different capacitance as com-pared to the lower amorphous region. Moreover, on eachof the phases, the capacity signal is – within the noise levelof our detection scheme – quite homogeneous. For the con-tact potential we find a completely different behavior:quite large contrast variations are observed on the jellyfishstructures. In fact, the variations of surface potential aremuch larger on different areas of the jellyfish structures

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than the variation between the bright regions of the jelly-fish and the lower region of the polymer film. We note thatthis large variation of surface potential on the jellyfishstructures has not been observed only once, it is observedconsistently on a large portion of these structures. Whilethe precise physical origin for this variation is not yetunderstood, we believe that it may be related to local di-poles. In summary, also for the case of electrostatic proper-ties we clearly find a different behavior of the jellyfishstructures as compared to the lower amorphous regions.

Finally, we found that these layered structures growwith the time at ambient and room temperature condi-tions. Fig. 6 shows the first (a) and the last frame (c) of amovie taken during 17.5 h, for a 18 mg/ml sample spin-coated onto a glass cover slip. The difference image be-tween the first and the last frames is shown in Fig. 6b.We estimate a growth rate of about 2 lm2/day. As is ob-served in the difference image the growth occurs mainlyat the dendritic sides of the jellyfish structures, an observa-tion that is general for all movies analyzed so far. We alsoobserve the growth of the second layer in the jellyfishstructures (not shown). In general we analyzed severalmovies and the growth rate varies between 1 and 5 lm2/day. We thus conclude that surface disordered chains areable to move relative to one another to rearrange in the jel-

Fig. 6. (a) first frame and (c) last frame of a topographic movie of a 18 mg/ml spin-coated P3OT thin film on to a glass cover slip, (b) difference imagebetween the first and the last frame of the movie to show the growth ofthe jellyfish structures. The movie was taken during 17.5 h. Dz = 40 nm.

lyfish structure at room temperature, because the surfaceenhances the mobility of the chains [41].

3.4. Jumping mode scanning force microscopy: Adhesion andmechanical properties

In order to further characterize the differences betweenthe lower regions and the jellyfish structures we havestudied their mechanical properties. Different modes canbe used to study these properties, including scanning fric-tion force microscopy [42,43], as well as the local variationof force vs. distance curves [44]. In the present work weuse jumping mode SFM (JM-SFM) and locally resolvedforce versus distance curves. In JM-SFM [45], which is sim-ilar to ‘‘pulsed force microscopy” [46], essentially a forcevs. distance curve is acquired and analyzed in real timeat every position of the image. From the analysis of thesecurves, at each image point the height of the sample(topography), the minimum force (adhesion force) andthe slope of each force vs. distance curve (related to localstiffness) can be determined. Fig. 7 shows such a set oftopography, adhesion and stiffness images. The jellyfishstructures show a different adhesion, as well as a differentstiffness, which clearly indicates that the physico-chemicalproperties of the two regions are well differentiated. Thetopography images are very similar to those taken withNC-DSFM. The height of the jellyfish structure measuredby JM-SFM is about 4–5 nm, and thus similar to the heightobserved in NC-DSFM. As for the case of the topographicimages acquired in NC-DSFM the jellyfish structures showless roughness than the lower regions.

The adhesion images show not only a (small) differenceof adhesion force between the jellyfish structure and thelower polymer (see below), but also a different texture ofthe images. On the one hand, some roughness is observed– as in the topographic images – on the lower regions ofthe sample while the adhesion images are much flatteron the jellyfish structures. On the other hand the jellyfishstructures look ‘‘streaky” (dark horizontal lines in Fig. 7e)which we relate to sudden changes in tip–sample interac-tion due to a variation of the structure of the tip. This var-iation is induced either by pick-up of material from thesample or by loss of tip-material. In any case, the jellyfishstructures present a less stable tip–sample interaction ascompared to the lower material. Another interesting issueis related to the correlation of the roughness in the topo-graphic and adhesion images. No significant correlation isfound for the data acquired on the jellyfish structureswhile a significant correlation is found on the lower region.As can be seen in Fig. 7i, high topographic regions in thetopographic image have lower adhesion. This effect isattributed to the local curvature of the sample. The adhe-sion is, in the simplest model, described by the relation[47]

Fad ¼ 4pcReff cos h ð1Þ

where c is the surface energy of water, h the (mean) contactangle of water on tip and sample and 1/Reff = 1/Rtip + 1/Rsam-

ple is the effective curvature of the system, determined by thecurvature Rtip of the tip and the local curvature Rsample ofthe sample. We note that for a flat surface (Rsample =1)

Fig. 7. Images showing JM-SPM data of a P3OT thin film prepared from a solution of 39 mg/ml on gold. Top: topography (a), adhesion (b) and stiffnessimages (c). Images (d)–(g): enlarges images of the regions marked in (a) and (b) showing the topography of the samples on the jellyfish structures (d) and onthe lower region (f) as well as the adhesion on the on the jellyfish structures (e) and on the lower region (g). Graph (h): Histograms of the adhesion forcemeasured on the jellyfish structure (black histogram) and the lower region (dashed histogram). Finally, the group of for images (i) shows a further enlargedview of the jellyfishes (top 2 images of the group, left: topography, right: adhesion) and the lower region (bottom 2 images of the group, left: topography,right: adhesion) in a similar arrangement as the images (d)–(g). The lines in the lower two images (lower region) connect features that are connected in thetopography and adhesion images. Gray scale of the images: total topography in (a) is 50 nm, in (d) and (f) 15 nm and in (i), images of adhesion correspond toabout 5 nN. For the discussion regarding the calibration of normal forces see main text.

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Y

the effective tip–sample curvature is that of the tip, fordepressions (concave surface, Rsample < 0) it is larger andfor summits (convex surface, Rsample > 0) it is smaller thanthe tip. Local differences in adhesion are therefore inducedeither by local variations of the physicochemistry of thesample – this is why a different mean adhesion is measuredon the layered structures as compared to the lower regions– or to local variation of curvature [48], which is why theadhesion varies locally on the lower region, being loweron high points and higher on low points.

In order to calibrate the force vs. distance curves and tofurther study the differences in mechanical properties ofthe jellyfish structures and the lower regions of the poly-mer films force vs. distance curves have been acquired onthese areas, see Fig. 8. From the calibration of the normalforce signal obtained from these curves and from the histo-grams shown in Fig. 7h we find an adhesion force of 11 nNon the lower region, and about 5% more on the jellyfishareas, which is a rather small but nevertheless statisticallysignificant difference. In addition, we find that the slope of

Fig. 8. Force vs. distance curves on the polymer lower region and on thejellyfish structure (first and second layers). Data was acquired using acantilever with a force constant of 3 N/m.

Table 2Adhesion force, energy loss, stiffness and hysteresis obtained from the dataof Fig. 8 for the polymer lower region and the jellyfish structure (first andsecond layers)

Adhesionforce (nN)

Stiffness(N/m)

Hysteresis(nm)

Energyloss (aJ)

Polymer 11.0 2.2 4.0 461st Layer 11.5 2.4 1.1 222nd Layer 11.6 3 1.0 12

2514 J. Abad et al. / European Polymer Journal 44 (2008) 2506–2515

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

the force vs. distance curve is different on these two areas:the slope is higher on the jellyfish areas, indicating a higherstiffness of the tip–sample contact, and thus a larger Elasticmodulus of the sample material. This agrees with the differ-ent stiffness as measured by JM-SFM (Fig. 7c). Finally weobserve that the visco-elastic behavior of the two regionsis different: on the lower region, the force vs. distance curvedoes not coincide during the forward and backward cycle inthe contact region, instead, the force vs distance curve‘‘opens” inducing hysteresis and energy loss. This effectcan be characterized either by the horizontal distance ofthe two cycles at zero force, or by the area enclosed bythe forward and backward cycle for positive forces (inden-tation of the sample) [49]. As seen in Table 2, both the hor-izontal distance at zero force, as well as the energydissipated during the indentation process is largest on thelower material, and lowest on the second layer of the jelly-fish structures.

Therefore, as for the JM images, also from the analysis ofthe force vs. distance curves we conclude that the mechan-ical properties of the lower polymer region are differentfrom that of the jellyfish regions. In addition, the higherstiffness and lower visco-elastic losses (lower hysteresis)of the jellyfish regions results are compatible with a more

compact (macro-) molecular structure of these regions.Similar results have been reported for molecularly thinthiol films on gold substrates [50].

4. Conclusions

The nanometer surface properties of P3OT thin filmswas investigated on different substrates (Gold, Glass coverand Graphite), prepared with different methods (spin-coating and drop-casting) and using various polymer con-centrations. We find, as expected, that the spin-coatingyields much more homogeneous samples than drop-cast-ing. With both preparation methods we observe character-istic self-organized structures – jellyfish structures –having up to four layers lying on top of the P3OT thin film.The height of each of these layers is about 4.5 nm beingessentially the same for the different layers. We concludethat the jellyfish structures are superficial, that is, arereally on top of the film. We have observed the growth ofthese structures with the time and estimate a growth rate.At ambient conditions the jellyfish structures are thereforequite dynamic.

The jellyfish structures have different physico-chemicalproperties as compared to the lower regions of the film: alower surface roughness, a lower interaction with the tip,different electrostatic properties, a higher mechanical stiff-ness, a higher adhesion and the energy dissipation due tovisco-elastic effects is lower. We therefore conclude thatit is a different phase of the same material. While thisphase represents only a very small portion of the bulkmaterial it may represent a much more significant amountof the surface, which may be the relevant parameter insome applications, where electrodes are evaporated ontop of the layers of organic material. As a very rough esti-mate, we consider that the jellyfish structures cover about5% of the surface, and represents 0.1% of the bulk volume ofour samples.

Finally, we believe that these findings are very relevantfor a better understanding of the properties of functionaldevices made from these P3OT or similar conducting poly-mers. In fact, in applications where these materials areused as one of the components of a device (organic light-emitting diodes or organic solar cells) our results showthat these materials might not be homogeneous as is usu-ally assumed, but may themselves be composed of differ-ent phases, resulting in a much more complex structureof the overall device. The fact that the jellyfish structuresare on the surface of the P3OT makes this finding evenmore relevant, since it means that these structures – if alsopresent at the interphase of two different materials – will

J. Abad et al. / European Polymer Journal 44 (2008) 2506–2515 2515

ACR

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OLE

CULA

RN

AN

OTE

CHN

OLO

GY

be at the most critical place of the device, namely the het-erojunction in bilayer devices or the active layer electrodejunction in all kind of devices.

Acknowledgements

The authors thank Dr. J. Abellán for technical assis-tance. This work has been supported by the Spanish gov-ernment under Projects NAN2004-09183-C10-3,MAT2006-12970-C02 and Consolider HOPE CSD2007-00007. E.P. and J.A. acknowledge financial support fromthe Ministerio de Educacion y Ciencia through the Juande la Cierva program and the Project MAT2006-12970-C02, respectively.

References

[1] Brabec CJ, Sariciftci NS, Hummelen JC. Adv Funct Mater 2001;11:15.[2] Sirringhaus V, Brown PJ, Friend RH, Nielsen MM, Bech-gaard K,

Langeveldvoss BMW, et al. Nature 1999;401:685.[3] Hide F, Diaz-Garcia MA, Schwartz BJ, Andersson MR, Pei QB, Heeger

AJ. Science 1996;273:1833.[4] Friend RH, Gymer RW, Holmes AB, Burroughes JH, Marks RN, Taliani

C, et al. Nature 1999;397:121.[5] Camaioni N, Catellani M, Luzzati S, Magliori A. Thin Solid Films

2002;403:489.[6] Prosa TJ, Winokur MJ, Moulton J, Smith P, Heeger AJ. Phys Rev B

1995;51:159.[7] Ong BS, Wu Y, Liu P, Gardner S. Adv Mater 2005;17:1141.[8] Kaniowski T, Niziol S, Sanetra J, Trznadel M, Pron A. Synth Met

1998;94:111.[9] Singh R, Kumar J, Singh RK, Kaur A, Sood KN, Rastogi RC. Polymer

2005;46:9126.[10] Ionescu-Zanetti C, Mechler A, Carter SA, Lal R. Adv Mater

2004;16:579.[11] Chiesa M, Burgi L, Kim JS, Shikler R, Friend RH, Sirringhaus H. Nano

Lett 2005;5:559.[12] Palermo V, Ridolfi G, Talarico AM, Favaretto L, Barbarella G, Camaioni

N, et al. Adv Func Mater 2007;17:472.[13] Hoppe H, Glatzel T, Niggemann M, Hinsch A, Lux-Steiner MC,

Sariciftci NS. Nano Lett 2005;5:269.[14] McNeill CR, Watts B, Thomsen L, Belcher WJ, Warwick J, Kilcoyne

ALD, et al. Small 2006;2:1432.[15] Kymakis E, Amaratunga GAJ. Rev Adv Mater Sci 2005;10:300.[16] Phang IY, Liu T, Zhang W, Schönherr H, Vancso GJ. Eur Polym J

2007;43:4136.[17] Palacios-Lidon E, Perez-Garcia B, Abellan J, Miguel C, Urbina A,

Colchero J. Adv Func Mat 2006;16:1975.[18] Halls JJM, Arias AC, MacKenzie JD, Wu W, Inbasekaran M, Woo EP,

et al. Adv Mater 2000;12:498.

M

[19] Vancso GJ, Hillborg H, Schönherr H. Adv Polym Sci 2005;182:55.[20] Gebeyehu D, Gebeyehu D, Brabec CJ, Padinger F, Fromherz T,

Spiekermann S, et al. Synth Met 2001;121:1549.[21] Landi BJ, Castro S L, Ruf HJ, Evans CM, Bailey SG, Raffaelle RP. Sol

Energ Mat Sol Cells 2005;87:733.[22] Lopez-Mata C, Nicho ME, Hu H, Cadenas-Pliego G, Garcia-Hernadez

E. Thin Solid Films 2005;490:189.[23] Prosa TJ, Winokur MJ, Moulton J, Smith P, Heeger AJ.

Macromolecules 1992;25:4364.[24] Esselink FJ, Hadziioannou G. Synth Met 1995;75:209.[25] Liu SL, Cheng TS. Polymer 2000;41:2781.[26] Zhokhavets U, Gobsch G, Hoppe H, Sariciftci NS. Synth Met

2004;143:113.[27] Chen TA, Wu X, Rieke RD. J Am Chem Soc 1995;117:233.[28] Zen A, Saphiannikova M, Neher D, Grenzer J, Grigorian S, Pietsch U,

et al. Macromolecules 2006;39:2162.[29] Aasmundtveit KE, Samuelsen EJ, Guldstein M, Steinsland C, Flornes

O, Fagerno C, et al. Macromolecules 2000;33:3120.[30] Bolognesi A, Botta C, Mercogliano C, Porzio W, Jukes PC, Geoghegan

M, et al. Polymer 2004;45:4133.[31] Brinkmann M, Rannou P. Adv Funct Mater 2007;17:101.[32] Qiao C, Zhao J, Jiang S, Ji X, An L, Jiang B. J Polym Sci B 2005;43:1303.[33] Horcas I, Fernandez R, Gomez-Rodriguez JM, Colchero J, Gomez-

Herrero J, Baro AM. Rev Sci Inst 2007;78:013705.[34] Palacios-Lidon E, Colchero J. Nanotechnology 2006;17:5491.[35] DeGennes PG, Brochard-Wyart FG, Quéré D. In: Capillary and

wetting phenomena. New York: Springer Verlag; 2004.[36] Bower DI. In: An introduction to polymer

physics. Cambridge: Cambridge University Press; 2002.[37] Brinkmann M, Wittmann JC. Adv Mater 2006;18:860.[38] Mena-Osteriz E, Meyer A, Langeveld-Voss BMW, Janssen RAJ, Meijer

EW, Bäuerle P. Angew Chem Int Ed 2000;39:2679.[39] Sorokina KL, Tolstikhina AL. Crystallogr Rep 2004;49:476.[40] Palacios Lidón E, Abellán J, Munuera C, Ocal C, Colchero J. Appl Phys

Lett 2005;87:154106.[41] Jones RAL, Richards RW. In: Polymers at surfaces and

interfaces. Cambridge: Cambridge University Press; 1999.[42] Marti O, Colchero J, Mlynek J. Nanotechnology 1990;1:141.[43] Nisman R, Smith P, Vancso GJ. Langmuir 1994;10:1667.[44] Schönherr H, Hruska Z, Vancso GJ. Macromolecules 2000;33:4532.[45] De Pablo PJ, Colchero J, Gomez-Herrero J, Baro AM. Appl Phys Lett

1998;73:3000.[46] Krotil HU, Stifter T, Waschipky H, Weishaupt K, Hild S, Marti O. Surf

Inter Anal 1999;27:336.[47] Israelachvili JN. In: Molecular and intermolecular forces. San

Diego: Academic Press; 1992.[48] De Pablo PJ, Colchero J, Gomez-Herrero J, Baro AM, Schaefer DM,

Howell S, et al. J Adhes 1999;71:339.[49] Note that the energy dissipation calculated here is not the usual

energy dissipation due to tip sample adhesion – which would beproportional to the area enclosed during the forward and backwardcycle for negative forces, but due to the visco- elastic behaviour ofthe system as the tip sample system is deformed.

[50] Barrena E, Palacios-Lidón E, Munuera C, Torrelles X, Ferrer S, Jonas U,et al. J Am Chem Soc 2004;126:385.