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Synthetic Metals 161 (2012) 2516– 2520

Contents lists available at SciVerse ScienceDirect

Synthetic Metals

j o ur nal homep ag e: www.elsev ier .com/ locate /synmet

maging nanostructures in organic semiconductor films with scanningransmission X-ray spectro-microscopy

enjamin Wattsa,∗, Christopher R. McNeill b, Jörg Raabea

Swiss Light Source, Paul Scherrer Institut, 5232 Villigen, SwitzerlandDepartment of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia

r t i c l e i n f o

rticle history:eceived 1 June 2011eceived in revised form 22 August 2011ccepted 2 September 2011vailable online 20 October 2011

eywords:-rayicroscopyEXAFSTXM

a b s t r a c t

Thin films of organic semiconducting materials are of increasing technological importance in optoelec-tronic devices such as light emitting diodes (LEDs), lasers, field effect transistors (FETs) and solar cells.However, the morphology of such films is complex, often displaying three dimensional compositionstructure or molecular alignment effects. The structure of the polymer film incorporated into a devicecan strongly affect its performance characteristics, e.g. via the connectedness of polymer domains and tothe device electrodes, or due to anisotropic material properties due to molecular alignment.

Scanning transmission X-ray spectro-microscopy (STXM) has been demonstrated to be an excellenttool for the study of organic materials due to its high spatial resolution (down to about 20 nm) andstrong contrast based on a variety of spectroscopic mechanisms. In particular, tuning the probing X-ray beam to resonances in the near edge X-ray absorption fine structure (NEXAFS) spectra provides a

olymerolecular orientation

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mechanism for molecular-structure based contrast which is a very powerful tool for studying blends oforganic components. A further advantage of STXM is that strategic use of spectroscopic information allowsquantitative compositional analysis of imaged areas. Recent work at the PolLux STXM has demonstratedtwo new developments in the imaging of thin organic films: simultaneous surface and bulk imagingvia an additional channeltron electron detector, and molecular orientation mapping via anisotropic nearedge resonances.

. Introduction

Thin films of organic semiconducting materials are of increas-ng technological importance in optoelectronic devices such asight emitting diodes (LEDs) [1,2], lasers [3], field effect transis-ors (FETs) [4–6] and solar cells [7,8]. However, the morphologyf such films is complex, often displaying three dimensional com-osition structure or molecular alignment effects. The structure ofhe polymer film incorporated into a device can strongly affect itserformance characteristics, e.g. via the connectedness of polymeromains [9] and to the device electrodes, or due to anisotropicaterial properties due to molecular alignment [10,11]. For such

easons, knowledge of the structures within organic semiconduc-or thin films provides a clear path towards further advancementf organic optoelectronic device technologies. However, imaging

he structure of thin organic films is challenging due to a combina-ion of small length scales and the limited variation in elementalomposition and density amongst organic materials. For example,

∗ Corresponding author. Tel.: +41 56 3105516; fax: +41 56 3103151.E-mail addresses: benjamin.watts@psi.ch, benjamin.watts@gmail.com

B. Watts).

379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2011.09.016

© 2011 Elsevier B.V. All rights reserved.

ultra-violet, visible and infra-red microscopy techniques are lim-ited in resolution by the wavelength of the probe beam, whileelectron microscopy and scanning probe microscopy struggle toachieve significant (and unambiguous) chemical contrast. Scanningtransmission X-ray spectro-microscopy (STXM) provides an excel-lent opportunity for imaging nanostructures in organic thin filmsdue to its combination of high resolution (down to about 20 nm[12,13]) and strong, natural contrast via X-ray spectroscopy [13].In particular, near-edge X-ray absorption fine structure (NEXAFS)spectroscopy can be utilised to achieve strong contrast betweenorganic materials based on differences in the types of chemicalbonding present in the molecules [14].

In NEXAFS spectroscopy, X-ray absorption resonance peaks areobserved at photon energies close to the binding energy of a coreelectron shell. These resonances correspond to an electronic tran-sition from the core shell to an unoccupied (i.e. anti-bonding) statewhose energy follows from the associated bonding orbital and thusthe molecular structure of the sample material (as illustrated inFig. 1) [14,15]. For organic materials, the rich structure of the C K-

edge NEXAFS spectrum provides a good fingerprint of the materialand plentiful opportunities for strong contrast (by tuning the pho-ton energy of the STXM probe beam) between organic materialswithout the need for labeling [13,16].

B. Watts et al. / Synthetic Metal

Fig. 1. Schematic of the electronic transition associated to a NEXAFS resonance. InNEXAFS, photoabsorption creates excited states that can be described by electronicconfigurations in which a core electron is promoted to an energy level (orbital orband) that is empty in the ground state. The excited atom then relaxes via an electronf(

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alling into the core hole with the excess energy producing either an Auger electronpreferred in light elements) or a fluorescent X-ray.

The utility of STXM for imaging organic nanostructures is furtherxtended via the ability to exploit linear X-ray dichroism in order torovide sensitivity to molecular order and orientation [14,17,18].ince anti-bonding orbitals have a limited spatial extent, electronicransitions to such states also have a preferred direction and thushe intensity of the associated NEXAFS resonances depend on thelignment between the electric field vector of the X-ray beam andhe transition dipole moment. Thus, by performing STXM measure-

ents with a linearly polarized X-ray beam with a photon energyuned to that of a dichroic NEXAFS resonance, contrast is observed

etween chemically identical molecules due to differing orienta-ions with respect to the polarisation axis [18].

In this article, we present the advantages of STXM inmaging nanostructures in organic semiconductor films using

ig. 2. Example C K-edge NEXAFS spectra and molecular structures of PFB and F8BT, wndicate the photon energies used for images in Fig. 4.

s 161 (2012) 2516– 2520 2517

the example polymers poly(9,9′-dioctylfluorene-co-bis(N,N′-(4,butylphenyl))bis(N,N′-phenyl-1,4-phenylene)diamine) (PFB)and poly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT). Themolecular structures and NEXAFS spectra of PFB and F8BT arepresented in Fig. 2. Further details of the STXM studies showcasedhere can be found in the respective previous publications [19–23].

2. Experimental

As the name suggests, a scanning transmission X-ray spectro-microscope (STXM) operates by raster-scanning a sample acrossa focused X-ray beam while the number of X-rays transmittedthrough the sample is measured at each scan position. Thus, eachpixel in a STXM image represents the X-ray transparency of thesample one scan position of the sample. The X-ray beam is focusedby a Fresnel zone plate (a diffractive optic) and passes through an“order sorting” aperture (OSA) that is positioned so that it allowsthe first-order focused X-rays to pass, while blocking the other zoneplate diffraction orders. After being transmitted by the sample, theX-rays are converted to visible photons by a phosphor, which arein turn detected by a photo-multiplier tube (PMT). Extra detectors,such as a channeltron [25] or fluorescent X-ray detector for exam-ple, can be mounted beside the standard PMT detector in order tomeasure complementary data about the electrons (for surface sen-sitivity) and fluorescent X-rays (for sensitivity to trace elements)that are ejected from the sample. An interferometer provides pre-cise positioning of the sample relative to the zone-plate over a widearea, allowing distortion free imaging over millimeter, micron andnanometer length scales.

The PolLux STXM at the Paul Scherrer Institute, Switzerland is

situated on a bend-magnet type synchrotron beamline that pro-vides a linearly polarized X-ray beam with a photon energy rangebetween approximately 250 eV and 1600 eV [26,27]. For measure-ments at the C K-edge (∼300 eV), a higher order suppression mirror

ith inset showing details of the �* resonances near 285 eV [19,24]. Vertical lines

2 Metals 161 (2012) 2516– 2520

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Fig. 3. Schematic of the experimental setup for STXM, showing both the standard,phosphor coated photo-multiplier tube detector (measuring transmitted X-rays)and an auxiliary channeltron detector (measuring emitted electrons).

Reproduced from [19].

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ystem improves the spectral purity of the beam by preferentiallybsorbing 2nd order (∼600 eV) and 3rd order (∼900 eV) photonslso passed by the monochromator [28]. A 35 nm outer zone widthresnel zone width provides a focus spot size of about 25 nm (higheresolution has been demonstrated with different zone plates [12]).

STXM samples need to be thin enough to transmit X-rays. Theequired thickness varies strongly with X-ray energy and the ele-ental composition of the sample, but for polymer materials,hich are chiefly carbon, investigated at the C K-edge, a sample

hickness between 50 nm and 200 nm is usually optimal [13]. Sam-les can be supported in a number of ways, with TEM grids and-ray transparent silicon nitride membranes (which are also com-ercially available) being the most common.While radiation damage is always an issue for techniques util-

sing high energy probes, X-rays are generally less damaging thanlectron beams [29] and STXM is designed to control and minimizehe radiation dose received by the sample. The scanning naturef STXM lends itself well to controlling the radiation dose a sam-le receives simply by varying the data acquisition time for each

mage pixel. One can check for radiation damage through repeatedcans of the same area, set a limit according to the accumulatedose at which damage is observed (via imaging or NEXAFS spec-roscopy) and then avoid damage by limiting the measurementwell time below the set limit. However, organic semiconductorsave been observed to be quite resilient against radiation dam-ge, possibly due to their delocalized electronic structure. Notehat while the conductivity of organic semiconductors has beenbserved to be extremely susceptible to photo-oxidation [24] (viaeduced conjugation length [30]), STXM measurements do not relyn the sample conductivity and so are only affected by photo-xidation when a significant proportion of the sample material haseen chemically altered. Reasonable sample handling and transportrocedures (minimizing exposure to oxygen and light) are easilynough to avoid photo-oxidation concerns.

.1. Composition mapping

Fig. 4 shows raw STXM data of the same area on the PFB:F8BT:1 blend film imaged (10 �m × 10 �m, with 150 nm steps) at X-raynergies of 280 eV, 284.5 eV, 285 eV and 320 eV [19]. As can be seenrom Fig. 2, 280 eV and 320 eV correspond to non-resonant sectionsf the component spectra and so show little difference between

he two polymers. Hence, the images at 280 eV and 320 eV showontrast based only on variations in the film thickness. An F8BTesonance occurs at 284.5 eV, causing an increase in absorption of-rays at this energy by F8BT, meanwhile PFB sees no significant

ig. 4. (Right) 10 �m × 10 �m (150 nm pixels) quantitative composition and thickness malend film with strategically chosen X-ray energies. The X-ray energies at which the imag

eproduced from [19].

increase in X-ray absorption relative to 280 eV. At 285 eV, however,there exists a PFB resonance that is much stronger than the F8BTresonance at this energy. The resulting strong absorption by PFBreverses the contrast of the finer features seen in the 285 eV imageof Fig. 4, revealing areas of high PFB concentration.

Utilising mathematical techniques such as singular valuedecomposition, the information contained within the set of images,in addition to the spectra of the component materials, can be re-expressed as maps quantifying the composition and thickness ofthe sample in each pixel (Fig. 4) [31]. Spectra of the componentmaterials are easily measured in the STXM from separate pure sam-ples. Also, a number of databases of NEXAFS spectra of insulatingpolymers, [32,33] conjugated polymers, [24] and small molecules[34,35] are available. The composition and thickness maps shownto the right of Fig. 4 reveal a micron-scale structure of thinner PFB-rich domains (∼90 nm thick and ∼77% pure) enclosed by a thickerF8BT-rich matrix (∼175 nm thick and ∼72% pure), with furthernano-scale subdomains residing within the micron-scale domains.More detailed discussion of the segregated film structures can befound in previous publications [19–22].

In principle, any number of components can be separatelymapped providing that enough information is recorded by measur-ing more STXM images at X-ray energies at which the componentsdisplay some difference between their spectra. In other words, N + 1STXM images are required to calculate quantitative compositionmaps for N components. On the other hand, if only one desires just

location information for a specific component, the required numberof images can be relaxed to just 2 STXM images.

ps calculated from (left) multiple images of the same sample area of a 1:1 PFB:F8BTes were measured are indicated against the component spectra in Fig. 2.

B. Watts et al. / Synthetic Metals 161 (2012) 2516– 2520 2519

Fig. 5. 10 �m × 10 �m STXM images of a 1:1 PFB:F8BT blend film simultaneously measured at an X-ray energy of 285 eV (strong absorption by PFB) via (A) transmittedX-rays (bulk information) and (B) emitted electrons (surface sensitive; normalized to incident X-ray flux). (C) An isometric rendering of the blend film (PFB in purple; F8BTi ted byt e.)

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n yellow), showing the calculated compositions and thickness variations (exaggerao colour in this figure legend, the reader is referred to the web version of the articl

eproduced from [19].

.2. Surface/bulk imaging

Fig. 5 demonstrates simultaneous imaging (at 285 eV; strongbsorption by PFB) of the surface and bulk of a 1:1 PFB:F8BT blendlm through the use of a channeltron detector mounted beside theMT detector (see Fig. 3) [19]. Note that the images in Fig. 5 are ofhe same sample area as in Fig. 4, but measured with higher resolu-ion (scanned with 50 nm steps instead of 150 nm steps) in order toetter show fine structures (at the cost of a longer scan time). In theulk sensitive image of Fig. 5(A), we see more clearly the nano-scaleubdomains discussed above, while the surface sensitive image inig. 5(B) shows a very different nano-scale structure. The contrastf Fig. 5(B) is provided by the emission of electrons from the first2.5 nm [36] of material on the rear, detector-facing surface, nor-alized by the X-ray flux reaching this surface (handily provided

y the transmission signal in Fig. 5(A)). With the X-ray energyuned to the strong PFB C 1s → �* resonance at 285 eV, PFB rep-esents both a strong X-ray absorber (dark) in Fig. 5(A) and a stronglectron emitter (bright) in Fig. 5(B). Therefore the partial cappingayer we see on top of the F8BT-rich matrix is composed chiefly ofFB. We can further see that the partial capping layer is of similarhickness to the depth sensitivity of ∼2.5 nm and is pinned to the

nderlying PFB-rich subdomains. While other techniques such asFM have observed this PFB-rich partial capping layer previously,

he unambiguous chemical sensitivity and simultaneous imagingf the surface and bulk provided by STXM allow for much deeper

ig. 6. (A) A series of STXM images of an annealed F8BT film measured at 285 eV displayinf the X-ray beam and the C 1s → �* transition moment of the F8BT molecules. (B) Plot of

nferred orientation (and estimated fully extended length) of the F8BT backbone (black linnformation from the out-of-plane direction. (For interpretation of the references to colou

eproduced from [23].

a factor of 200) of the surface and bulk layers. (For interpretation of the references

insight into the connectedness of the polymer domains. Anotherinteresting feature of Fig. 5 is the F8BT-rich subdomains enclosedwithin the large PFB-rich domains, which we can clearly see bythe few electrons emitted are connected to the upper surface ofthe film. Further measurements on an underside of the same film(not shown) clearly demonstrated a PFB-rich wetting layer overthe entire film/substrate interface except for the F8BT-rich subdo-mains, which penetrated through. Fig. 5(C) illustrates relationshipbetween compositions of the surface and bulk as well as variationsin the film thickness in an isometric rendering.

2.3. Molecular conformation

The molecular orientation and degree of order and withinpolycrystalline domains are known to affect the charge trans-port and optical properties of an organic semiconductor material[23]. The interest in such molecular conformation properties isdemonstrated by the range of experimental techniques aimed atmeasuring them that have emerged in the past 5 years [37]. STXMpresents strong advantages with its combination of high resolution,chemical specificity and high sensitivity to molecular conformationvia linear dichroism.

Fig. 6(A) displays a series of STXM images of an annealedF8BT film measured at the X-ray energy (285 eV) corresponding tothe dichroic C 1s → �* transition moment and at sample rotationangles of 0◦ (red), 45◦ (green) and 90◦ (blue) with respect to the

g linear dichroic contrast due to the relative angle between the electric field vectorthe in-plane orientation of the F8BT C 1s → �* resonance at 285 eV (colour) and thees). (C) Plot of the calculated percentage of local molecular order, including inferredr in this figure legend, the reader is referred to the web version of the article.)

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orizontal electric field vector of the linearly polarized X-ray beam.he images show varying levels of X-ray absorption in the sameample areas that derive from the set of molecular orientations con-ained in the ensemble of F8BT part-molecules (the estimated fullyxtended length of the polymer molecules is ∼400 nm, comparedo the 50 nm pixel size) observed in each pixel.

Fitting the observed angular dependence of the X-ray absorptiono the known cosine squared relationship [14,18] produces the mapf the in-plane resonance orientation shown in Fig. 6(B). Since the Cs → �* transition moment measured is known to be oriented per-endicular to the plane of the aromatic ring structures (see Fig. 2)nd the film is significantly thinner (∼100 nm thick) than the esti-ated fully extended length of the polymer molecules (∼400 nm),e can infer that the F8BT backbones are oriented both perpendic-lar to the observed resonance direction and within the plane ofhe film (indicated in Fig. 6(B) by black lines). Fig. 6(C) shows a plotf the percent degree of local molecular order within the F8BT film,ncluding information involving the out-of-plane direction that wasot directly probed by the STXM measurements, but deduced by

everaging knowledge of the material and the nature of NEXAFSpectroscopy. A very high degree of order can be seen in manyomains, even in regions where the molecules are observed to flowmoothly between domains, while other domain boundaries areharacterized by ∼200 nm wide highly amorphous regions. Inter-stingly, the pattern of the highly amorphous domain boundariess very similar to the electroluminescence map of a polycrystallineight-emitting field-effect transistor measured by Zaumseil et al.6], indicating that the recombination of charges is facilitated byharge trapping in these disordered regions.

. Summary and outlook

Thin PFB:F8BT spin-cast films contain a number of differentypes of structures that have been imaged and quantified by STXM.ateral structures include PFB-rich domains enclosed in an F8BT-ich matrix, which have different thickness and purity levels thatepend on parameters such as molecular weight, annealing andolvent drying time [20–22]. Within these domains reside smallerub-domains that have been observed to evolve significantly witholvent drying time [20]. STXM observations have also unambigu-usly demonstrated vertical structures with a PFB-rich wettingayer covering the entire film/substrate interface except for the PFB-ich subdomains and a partial PFB-rich capping layer on top of the8BT-rich matrix [19].

The combination of high resolution imaging and NEXAFS spec-roscopy provided by STXM offers many advantages to the study ofano-structured organic materials. The strong contrast achievedy STXM, based on the molecular structure and molecular con-ormation of the sample materials, is especially well suited to theolymer samples described above where the lack of differentiationy density or elemental composition hinder competing techniques.owever, the details discussed here of the ability of STXM to

mage nanostructures applies equally to all organic materials. STXMan quantitatively characterize organic nanostructures in terms ofomposition, local order and molecular orientation both in the bulks well as surface layers. These are all important aspects in the oper-tion and performance of organic optoelectronic devices and it isikely that STXM investigations of organic semiconductor films willontinue to play a leading role in understanding the nanostructuresf these materials. An interesting recent use of the quantitativeomposition capabilities of STXM has been to determine the misci-ility and diffusion of PCBM molecules in P3HT:PCBM blend films

38,39].

While STXM instruments are only available at synchrotron facil-ties, the PolLux STXM and many of its sister instruments are openlyccessible by outside researchers, with beamtime awarded on the

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s 161 (2012) 2516– 2520

basis of proposals which are reviewed by an international com-mittee [40]. PolLux receives user groups from a diverse range ofscientific disciplines and welcomes new users proposing novelexperiments. PolLux users can expect enthusiastic support in plan-ning and setting up experiments as well as learning to operatethe STXM instrument. The PolLux staff look forward to workingwith you to characterize the nanostructures of the organic semi-conductor films that will power the next generation of organicoptoelectronic devices.

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