Reduced bleaching in organic nanofibers by bilayer polymer/oxide coating

Reduced bleaching in organic nanofibers by bilayer polymer/oxide coatingL. Tavares, J. Kjelstrup-Hansen, H.-G. Rubahn, and H. Sturm Citation: J. Appl. Phys. 107, 103521 (2010); doi: 10.1063/1.3427561 View online: http://dx.doi.org/10.1063/1.3427561 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v107/i10 Published by the American Institute of Physics. Related ArticlesPatterned optical anisotropy in woven conjugated polymer systems Appl. Phys. Lett. 101, 171907 (2012) Interference rings formation inside cellulose from a back-reflected femtosecond laser pulse J. Appl. Phys. 112, 066101 (2012) Anisotropic optical absorption of organic rubrene single nanoplates and thin films studied by μ-mappingabsorption spectroscopy Appl. Phys. Lett. 101, 113103 (2012) Raman and low temperature photoluminescence spectroscopy of polymer disorder in bulk heterojunction solarcell films APL: Org. Electron. Photonics 5, 185 (2012) Raman and low temperature photoluminescence spectroscopy of polymer disorder in bulk heterojunction solarcell films Appl. Phys. Lett. 101, 083302 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

Downloaded 13 Nov 2012 to 130.225.29.254. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

Reduced bleaching in organic nanofibers by bilayer polymer/oxide coatingL. Tavares,1,a� J. Kjelstrup-Hansen,1 H.-G. Rubahn,1 and H. Sturm2

1NanoSyd, Mads Clausen Institute, University of Southern Denmark, Alsion 2, DK-6400 Sønderborg,Denmark2Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, D-12205 Berlin,Germany

�Received 16 March 2010; accepted 13 April 2010; published online 21 May 2010�

Para-hexaphenylene �p-6P� molecules exhibit a characteristic photoinduced reaction �bleaching�resulting in a decrease in luminescence intensity upon UV light exposure, which could render thetechnological use of the nanofibers problematic. In order to investigate the photoinduced reaction innanofibers, optical bleaching experiments have been performed by irradiating both pristine andcoated nanofibers with UV light. Oxide coating materials �SiOx and Al2O3� were applied onto p-6Pnanofibers. These treatments caused a reduction in the bleaching reaction but in addition,the nanofiber luminescence spectrum was significantly altered. It was observed that somepolymer coatings �a statistical copolymer of tetrafluoroethylene and 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxole, P�TFE-PDD�, and poly�methyl methacrylate�, PMMA� do not interferewith the luminescence spectrum from the p-6P but are not effective in stopping the bleaching.Bilayer coatings with first a polymer material, which should work as a protection layer to avoidmodifications of the p-6P luminescence spectrum, and second an oxide layer used as oxygen blockerwere tested and it was found that a particular bilayer polymer/oxide combination results in asignificant reduction in bleaching without affecting significantly the emission spectrum from thenanofibers. © 2010 American Institute of Physics.�doi:10.1063/1.3427561�

I. INTRODUCTION

Organic semiconductors based on small molecules arereceiving increased attention due in part to their applicationpotential within various optoelectronic devices such astransistors,1 light-emitting diodes,2,3 and solar cells,4 but alsodue to their relative ease of processing, low price, and tun-ability through synthetic chemistry.5,6 Phenylene-based mol-ecules are of particular interest due to their ability to self-assemble into elongated, nanoscale, crystalline aggregates or“nanofibers.”7,8 For example, para-hexaphenylene �p-6P�molecules deposited on a mica substrate under well-controlled conditions assemble into nanofiber structures withtypical dimensions of macroscopic length �up to millime-ters�, nanoscopic width �hundred to several hundred of na-nometers� and height of several tens of nanometers.9 Suchp-6P nanofibers emit polarized, blue light upon UV excita-tion with peak wavelengths of the emitted light of �401,�422, �448, and �473 nm due to the radiative decay fromthe vibrational ground state of the first excited electronicstate to various vibrational levels of the electronic groundstate10 ��0→0�, �0→1�, �0→2�, and �0→3�, respectively�.Due to the nanofiber geometry, the emitted light has a spa-tially anisotropic distribution,11 and the nanofibers have beenshown to act as waveguides12 and random lasers.13,14 Takentogether with the ability of both p-6P thin films and nanofi-bers to emit light through electrical stimulation,15 these fea-tures could enable future optoelectronic applications. How-ever, the nanofibers exhibit a characteristic photoinduced

reaction during illumination with UV light that causes a de-crease in luminescence intensity �bleaching�, which is partlyattributed to a photooxidation reaction.16,17 For poly�1,4-phenylenevinylene�, for example, it has been suggested thatas the luminescence intensity decreases, the concentration ofcarbonyl groups increases, and the concentration of thedouble bonds that link adjacent phenyl groups decreases.18

The bleaching must be avoided since this reaction destroysthe nanofibers and makes the technological use of the nanofi-bers difficult. A promising solution is to apply a special coat-ing onto the organic material to encapsulate and protect itfrom the ambient surroundings. In this paper, we report oninvestigations aimed at finding an appropriate coating forp-6P nanofibers that maintains the spectrum from uncoatedp-6P fibers and eliminates or at least significantly reducesthe bleaching. Various materials as, for example, SiOx,16

Al2O3,19 and the bilayer Al2O3 /SiO2,20 P�TFE-PDD� /SiOx,P�TFE-PDD� /Al2O3, PMMA /SiOx, and PMMA /Al2O3

�Refs. 21–23� have been investigated as a protecting layeragainst oxidation for organic materials. Once an appropriatecoating material was found, surface analyses were made byatomic force microscopy �AFM� to investigate how nanofi-bers and coating were affected by UV exposure and in addi-tion uncoated and coated samples were investigated with Ra-man spectroscopy.

II. EXPERIMENTAL DETAILS

The p-6P nanofibers were grown on cleaved muscovitemica by vapor deposition under high-vacuum conditions.7

The nanofibers are crystalline with the �1-1-1� face parallela�Electronic mail: tavares@mci.sdu.dk.

JOURNAL OF APPLIED PHYSICS 107, 103521 �2010�

0021-8979/2010/107�10�/103521/6/$30.00 © 2010 American Institute of Physics107, 103521-1

Downloaded 13 Nov 2012 to 130.225.29.254. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

to the substrate surface. After nanofiber growth, differentcoatings were applied to study their ability to reduce thenanofiber bleaching. The SiOx and Al2O3 films were appliedby electron beam evaporation using an Edwards Auto500thin film deposition system. A mixture of silicon and silicondioxide �SiO2� was used as the deposition material to createSiOx films16 and the evaporation conditions were a pressureof 1�10−5 mbar and a maximum evaporation rate of about0.3 nm/s. Activated alumina �Al2O3� was used as the depo-sition material to produce the Al2O3 films �pressure of 1�10−5 mbar and maximum rate of about 0.1 nm/s�. Thedeposition thicknesses were monitored by a quartz crystalmicrobalance and the film thicknesses were verified using aprofilometer. The polymer films �P�TFE-PDD� and PMMA�were applied by spin coating. Some drops of PMMA 950solution were rotated at 1000 rpm for 5 s followed by 8000rpm for 45 s. Immediately after spin coating, the substrateswere baked at 95 °C for 4 min creating a 100–150 nm thicklayer. P�TFE-PDD� �DuPont� films were made from perfluo-rocarbon solution �FC-77, 3M, USA� by spin coating anddrying at room temperature resulting in a thickness of 300nm as measured using a profilometer.

For the bleaching experiments, the samples were irradi-ated with an Hg lamp �emission line of 365 nm selected by aband pass filter� using an epifluorescence microscope with aNikon UV-2A filter cube to separate the excitation and lumi-nescence light, and an Ocean Optics Maya2000 spectrometercoupled to the microscope with a collecting glass fiber with100 �m core diameter. In this setup, the UV light irradiatedthe nanofibers under normal incidence and the resulting lu-minescence was observed under normal incidence, too. TheUV light was focused on the sample surface with a 100�objective giving a spot size of around 820 �m and an inten-sity at the sample surface of about 0.2 W /cm2. As theUV-2A filter blocks wavelengths below 420 nm, some addi-tional bleaching experiments were performed using a HeCdlaser with a wavelength of 325 nm and without filters toobserve the development of the full p-6P spectrum, whichextends below 420 nm. In this latter case, the spot size wasaround 1 mm and the intensity at the sample surface wasapproximately 0.1 W /cm2. Raman spectroscopy measure-ments were made for structural investigations of the p-6Pmolecules24,25 before and after coating. This was done usinga laser with a wavelength of 532 nm and 130–200 mWcoupled to a microscope with a 10� objective. Finally,tapping-mode AFM was used to study the morphology of thecoated nanofibers before and after UV illumination.

III. RESULTS AND DISCUSSION

First, SiOx, Al2O3, PMMA, and P�TFE-PDD� monolayerand multilayer coatings were deposited onto a sample ofpristine mica for recording the background. Transparent andcolorless films without measurable autofluorescence wereobserved.

Figure 1 shows the normalized, initial spectra �at thebeginning of UV illumination� for uncoated nanofibers andcoated nanofibers with 200 nm SiOx, 10 nm Al2O3, 300 nmP�TFE-PDD�, and 100–150 nm PMMA, respectively. As

seen from Fig. 1, the two polymer coatings �P�TFE-PDD�and PMMA� do not change the p-6P emission spectrum. Thespectrum of the SiOx coating, however, shows a broad emis-sion peak in the green region around 500 nm. High SiOx

evaporation rates ��1 nm /s� generate an even broaderemission from 425 to 625 nm and the luminescence of thenanofibers cannot be observed anymore. However, even us-ing a fairly low SiOx deposition rate, p-6P nanofibers coatedwith 300 nm SiOx show meanderlike breaks16 in addition tothe peak in the green. It is thus not suitable to use more than200 nm SiOx directly on p-6P nanofibers and it must beconcluded that the evaporation rate for SiOx films must bevery low �rate �0.3 nm /s�.

In previous studies, Al2O3 has been applied as a coatingmaterial for organic solar cells as it is not permeable tooxygen.19,20 Figure 1 shows that the Al2O3 coating also in-duces a broad peak around 500 nm and the characteristicpeaks from the nanofibers are no longer distinguishable. Thereason for the parasitic emission is not clear yet. However, itexcludes both inorganic coatings in direct contact with thep-6P nanofibers so far as for a usage as oxygen barrier, al-though an atomic layer deposition process at temperature aslow as 33 °C has the potential to coat thermally fragile sub-strate such as organic materials.26

Figure 2 shows bleaching decay curves for the spectradisplayed in Fig. 1. For the 200 nm SiOx coating, the lumi-nescence intensity decays by about 40% after 7 min of UVillumination. For the 10 nm Al2O3 coating, the luminescenceintensity does almost not change even after 13 min UV illu-mination. The spectra after UV illumination �not shown� re-veal that the broad peak around 500 nm is significantly re-duced for SiOx coatings and the p-6P spectrum is at leastpartially recovered16 but for Al2O3 coatings, the p-6P spec-trum is not recovered. The bleaching of p-6P nanofiberscoated with P�TFE-PDD� and PMMA is similar to thebleaching for uncoated p-6P nanofibers. This clearly indi-cates that the polymers alone cannot be used as coating ma-terials since they are ineffective in reducing the bleaching.

The bilayer SiOx /Al2O3 was tested to investigate its use

FIG. 1. �Color online� Normalized luminescence spectra of uncoated nanofi-bers �a�, black line �–�, and coated p-6P nanofibers on mica. The coatingsare 200 nm SiOx �b�, red circle ���, 10 nm Al2O3 �c�, green triangle ���,300 nm P�TFE-PDD� �d�, blue cross ���, and 100–150 nm PMMA �e�,purple star ���, respectively. Excitation at �ex=365 nm.

103521-2 Tavares et al. J. Appl. Phys. 107, 103521 �2010�

Downloaded 13 Nov 2012 to 130.225.29.254. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

as an appropriate coating material for reducing the bleachingeffects since a 200 nm SiOx coating does not drown thecharacteristic fluorescence peaks from nanofibers completely�Fig. 1� and since a 10 nm Al2O3 coating seems to stopbleaching �Fig. 2�. However, the bilayer 100 nmSiOx /40 nm Al2O3 shows a huge peak at about 500 nm andthe peaks of the p-6P nanofibers are significantly reduced asseen in Fig. 3. The fact that polymer coatings do not interferewith the luminescence spectrum from the p-6P �Fig. 1� andoxide coatings appear to reduce bleaching reactions �Fig. 2�indicates that bilayer coatings, with an initial polymer mate-rial as a protection layer to avoid the green fluorescence andan oxide layer as oxygen blocker would be favorable. Dif-ferent polymer/oxide bilayer combinations were tested withthe resulting spectra shown in Fig. 3.

The P�TFE-PDD�/40 nm Al2O3 and P�TFE-PDD�/200nm SiOx spectra are not so distorted if compared to the p-6P

spectrum �Fig. 3� but the coatings show a fast bleachingreaction �see Fig. 4�. Another disadvantage of using P�TFE-PDD� is that the surface of the P�TFE-PDD�/200 nm SiOx

coated sample shows the appearance of meanderlike breakson the surface in addition to the region exposed to UV lightappearing partly burned as can be observed in Fig. 5.

In terms of reducing the bleaching, the combinations of100 nm SiOx /40 nm Al2O3, PMMA/200 nm SiOx, andPMMA/40 nm Al2O3 are most promising, as the bleaching issignificantly suppressed as seen in Fig. 4. From Fig. 3, how-ever, it is clear that the bilayer PMMA/200 nm SiOx, al-though inducing a broad peak around 500 nm, has the leastdegrading effect on the spectrum. In addition, it is interestingto investigate the spectra after bleaching. Figure 6 shows thatafter extended UV exposure, the peak around 500 nm is nowless pronounced and the “pure” p-6P spectrum is partly re-covered for the PMMA /SiOx coating, while the other twobilayer coating samples still exhibit a substantial green emis-sion peak.

Of the tested coating candidates, the combination ofPMMA /SiOx appears to be an appropriate coating for p-6P

FIG. 2. �Color online� Normalized luminescence intensity for emitted lightat �em=422 nm as a function of time from uncoated nanofibers �a�, blackline �–�, and coated p-6P nanofibers on mica. The coatings are 200 nm SiOx

�b�, red circle ���, 10 nm Al2O3 �c�, green triangle ���, 300 nm P�TFE-PDD� �d�, blue cross ���, and 100–150 nm PMMA �e�, purple star ���,respectively. Excitation at �ex=365 nm.

FIG. 3. �Color online� Normalized luminescence spectra of uncoated nanofi-bers �a�, black line �–�, and coated p-6P nanofibers on mica. The coatingsare 100 nm SiOx /40 nm Al2O3 �b�, red circle ���, P�TFE-PDD�/200 nmSiOx �c�, green triangle ���, P�TFE-PDD�/40 nm Al2O3 �d�, blue cross ���,PMMA/200 nm SiOx �e�, purple star ���, and PMMA/40 nm Al2O3 �f� pinkx ���, respectively. Excitation at �ex=365 nm.

FIG. 4. �Color online� Normalized luminescence intensity for emitted lightat �em=422 nm from uncoated nanofibers �a�, black line �–�, and coatedp-6P nanofibers on mica. The coatings are 100 nm SiOx /40 nm Al2O3 �b�,red circle ���, P�TFE-PDD�/200 nm SiOx �c�, green triangle ���, P�TFE-PDD�/40 nm Al2O3 �d�, blue cross ���, PMMA/200 nm SiOx �e�, purple star���, and PMMA/40 nm Al2O3 �f� pink x ���, respectively. Excitation at�ex=365 nm.

FIG. 5. �Color online� Fluorescence microscope image of the sample coatedwith P�TFE-PDD�/200 nm SiOx. The nanofiber film exhibits meanderlikebreaks and after 7 min the spot exposed to UV light appears burned.

103521-3 Tavares et al. J. Appl. Phys. 107, 103521 �2010�

Downloaded 13 Nov 2012 to 130.225.29.254. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

nanofibers, so additional samples were prepared to search forthe best SiOx thickness and to verify the effectiveness of thiscoating �see Fig. 7�.

From Fig. 7, it can be observed that the thicker SiOx

coating in the PMMA /SiOx bilayer has the more intensepeak at around 500 nm, while it was observed that they giverise to almost the same slow bleaching decay �in Fig. 4, thebleaching decay for PMMA/200 nm SiOx is shown� evenafter 30 min of UV illumination �the luminescence intensitydecays by about 25% for 50, 100, and 300 nm SiOx and onlyby 14% for 200 nm SiOx for the samples shown in Fig. 7�.After irradiation the spectra in Fig. 7 are similar to the spec-trum for uncoated nanofibers as observed in Fig. 6. There-fore, to better identify the behavior of the PMMA/200 nmSiOx, the full spectrum was recorded using a 325 nm wave-length laser system to excite the photoluminescence to studythe time evolution of the full spectrum �Fig. 8�.

The similar appearance of the spectra excited by 325 nmand 365 nm wavelengths, respectively, indicates that the ad-ditional photon energy in the 325 nm photons is not causingany significant difference. From Fig. 8, it can be observedthat the peak at around 500 nm decreases while the peaks at�401 and �422 nm do not suffer from significant variationsin wavelength and intensity. The changes of the peak at�448 nm are due to the changes on the large peak around500 nm, which has a broad tail into the 448 nm peak. Thepeak around 500 nm appears to be the only peak that de-creases significantly after UV illumination for thePMMA /SiOx coating. After 40 min of UV illumination, thefull spectrum of the p-6P nanofibers coated with PMMA/200nm SiOx looks similar to the full spectrum for uncoated p-6Pnanofibers �Fig. 8�.

In Fig. 9�a�, the residual difference between the spectrafrom coated p-6P nanofibers �PMMA/200 nm SiOx� and thespectrum from uncoated p-6P is presented and in Fig. 9�b�the evolution of the prominent peak at 500 nm derived fromthe residual difference in Fig. 9�a�. It appears that the spec-trum from the coated samples tends to stabilize with time.

D. Vollath et al.21,22 have shown a large emission peakfor alumina nanoparticles coated with PMMA around 450nm and the authors have suggested that the combination ofnonfluorescent oxide material with nonfluorescent polymercoating may lead to a nanocomposite with strong fluores-cence. In order to investigate if the additional peaks observedin our measurements were due to a similar effect, a puremica substrate was coated with the PMMA/200 nm SiOx

bilayer, however no fluorescence could be observed.Kadashchuk et al.27 have observed a similar peak in

p-6P spectra and found that it was caused by structural de-fects within the p-6P material. We propose the followingexplanation to account for our observations: upon depositionof either SiOx or Al2O3 �which are both deposited by elec-tron beam evaporation�, structural defects are generated inthe nanofibers either by the impinging metal atoms/clustersor by the thermal stress, thereby causing the significant de-fect peak around 500 nm. A similar spectral change is not

FIG. 6. �Color online� Normalized luminescence spectra of uncoated nanofi-bers �a�, black line �–�, after 410 s of UV illumination and coated p-6Pnanofibers on mica. The coatings are 100 nm SiOx /40 nm Al2O3 �b�, redcircle ���, after 1865 s of UV illumination, PMMA/200 nm SiOx �e�, purplestar ���, after 880 s of UV illumination and PMMA/40 nm Al2O3 �f� pink x���, after 1995 s of UV illumination, respectively. Excitation at �ex

=365 nm.

FIG. 7. �Color online� Normalized luminescence spectra of uncoated nanofi-bers �a�, black line �–�, and coated p-6P nanofibers on mica. The coatingsare PMMA/50 nm SiOx �b�, red circle ���, PMMA/100 nm SiOx �c�, greentriangle ���, PMMA/200 nm SiOx �d�, purple star ���, PMMA/300 nmSiOx �e�, blue cross ���, respectively. Excitation at �ex=365 nm.

FIG. 8. �Color online� Normalized luminescence spectra of uncoated nanofi-bers, black line �–�, and p-6P nanofibers on mica coated with PMMA/200nm SiOx at the beginning of illumination, purple circle ���, and after 40 minof UV illumination, blue star ���. Excitation at �ex=325 nm, spectra arenormalized using the emission wavelength �em=422 nm.

103521-4 Tavares et al. J. Appl. Phys. 107, 103521 �2010�

Downloaded 13 Nov 2012 to 130.225.29.254. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

observed with the single-layer PMMA and P�TFE-PDD�coatings, as the spin-coating process is gentler. ThePMMA /SiOx bilayer coating appears to be the best compro-mise of minimum bleaching and the least pronounced defectpeak, which is even reduced with UV illumination. The rea-son for the reduction is at present not clear, but we speculatethat the UV illumination could somehow “anneal” some ofthe defects.

Raman spectroscopy measurements show the character-istic peaks from p-6P nanofibers24,25 even after coating withPMMA/200 nm SiOx which suggests that at least the localatomic arrangement of the nanofibers has not been affectedby this bilayer coating.

Figures 10�a� and 10�b� show AFM images of aPMMA /SiOx coated sample before and after 1 h of UV il-lumination, respectively, and Fig. 10�c� shows the linescansfor the lines indicated in Figs. 10�a� and 10�b�. The surfaceroughness average for the coated but unbleached region onthe PMMA /SiOx sample �Fig. 10�a�� is 113 nm. For thecoated and bleached �during 1 h� region �Fig. 10�b��, thesurface roughness average is 110 nm. Obviously, the bleach-

ing has very little effect on the surface morphology of thecoated samples, since the surface roughness is essentially thesame. Hence, the coating is not affected by the UV illumina-tion and the nanofibers are preserved and no material is re-moved from the nanofibers.

IV. CONCLUSIONS

SiOx coatings on p-6P nanofibers result in a huge lumi-nescence peak at 500 nm but after about 7 min of UV irra-diation the spectra exhibit a reduction in this peak and thespectral features look more similar to the uncoated p-6Pnanofibers. However, the luminescence intensity still decaysat a relatively high rate. Al2O3 coatings also generate a hugepeak around 500 nm and even after long UV illumination,the spectra do not change and it has not been possible toidentify the characteristic peaks from p-6P nanofibers.

Bilayer polymer/oxide coatings show different behav-iors. PMMA/200 nm SiOx appears to be the best coating.Only weak bleaching has been observed even after about 30min of UV illumination, the uncoated p-6P spectrum hasbeen to a large degree recovered after UV exposure, andRaman measurements suggest that at least no abrupt changesin p-6P nanofibers occurs. AFM images show that the sur-face morphology of this coating is almost not altered afterlong time of UV illumination. In addition, the appearance ofthe coated sample with PMMA/200 nm SiOx is not alteredmuch as seen for example in Fig. 5, even after 30 min of UVillumination. As conclusion we suggest PMMA/200 nm SiOx

against oxygen and UV light-induced photoreactions for pro-tecting p-6P nanofibers.

FIG. 9. �Color online� �a� Residual difference between spectra of coatedp-6P nanofibers with PMMA/200 nm SiOx and uncoated p-6P nanofibersrecorded at 5 min intervals and �b� evolution of the prominent peak at 500nm observed from the residual difference between spectra of coated p-6Pnanofibers with PMMA/200 nm SiOx and uncoated p-6P nanofibers duringUV illumination using a 325 nm laser.

FIG. 10. �Color online� �a� AFM image from mica / p-6P /PMMA /200 nmSiOx sample and �b� AFM image from mica / p-6P /PMMA /200 nm SiOx

sample after 1 h UV illumination. �c� Linescans for the images in Figs. 10�a�and 10�b�.

103521-5 Tavares et al. J. Appl. Phys. 107, 103521 �2010�

Downloaded 13 Nov 2012 to 130.225.29.254. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

ACKNOWLEDGMENTS

The authors thank Martin Aage Barsøe Hedegaard andKit Drescher Jernshøj �Institute of Sensors, Signals and Elec-trotechnics, University of Southern, Odense, Denmark� thatprovided support with Raman experiments.

1M. Mas-Torrent and C. Rovira, Chem. Soc. Rev. 37, 827 �2008�.2C.-C. Chang, M.-T. Hsieh, J.-F. Chen, S.-W. Hwang, and C. H. Chen,Appl. Phys. Lett. 89, 253504 �2006�.

3Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, and S. R.Forrest, Nature 440, 908 �2006�.

4B. P. Rand, J. Genoe, P. Heremans, and J. Poortmans, Prog. Photovolt:Res. Appl. 15, 659 �2007�.

5K. Müllen and U. Scherf, Organic Light Emitting Devices �Wiley-VCH,Weinheim, 2005�.

6H. Hoppe and N. S. Sariciftci, J. Mater. Res. 19, 1924 �2004�.7F. Balzer and H.-G. Rubahn, Appl. Phys. Lett. 79, 3860 �2001�.8M. Schiek, F. Balzer, K. Al-Shamery, J. R. Brewer, A. Lützen, and H.-G.Rubahn, Small 4, 176 �2008�.

9F. Balzer and H.-G. Rubahn, Adv. Funct. Mater. 15, 17 �2005�.10F. Balzer, A. Pogantsch, and H.-G. Rubahn, J. Lumin. 129, 784 �2009�.11J. Brewer, C. Maibohm, L. Jozefowski, L. Bagatolli, and H.-G. Rubahn,

Nanotechnology 16, 2396 �2005�.12F. Balzer, V. G. Bordo, A. C. Simonsen, and H.-G. Rubahn, Phys. Rev. B

67, 115408 �2003�.13F. Quochi, F. Cordella, A. Mura, G. Bongiovanni, F. Balzer, and H.-G.

Rubahn, J. Phys. Chem. B 109, 21690 �2005�.14F. Quochi, F. Cordella, A. Mura, G. Bongiovanni, F. Balzer, and H.-G.

Rubahn, Appl. Phys. Lett. 88, 041106 �2006�.15J. Kjelstrup-Hansen, X. Liu, H. H. Henrichsen, K. Thilsing-Hansen, and

H.-G. Rubahn, “Conduction and Electroluminescence From Organic Con-tinuous and Nanofiber Thin Films,” Phys. Status Solidi C �in press�.

16C. Maibohm, J. R. Brewer, H. Sturm, F. Balzer, and H.-G. Rubahn, J.Appl. Phys. 100, 054304 �2006�.

17M. S. Weaver, L. A. Michalski, K. Rajan, M. A. Rothman, J. A. Silvernail,J. J. Brown, P. E. Burrows, G. L. Graff, M. E. Gross, P. M. Martin, M.Hall, E. Mast, C. Bonham, W. Bennett, and M. Zumhoff, Appl. Phys. Lett.81, 2929 �2002�.

18R. D. Scurlock, B. Wang, P. R. Ogilby, J. R. Sheats, and R. L. Clough, J.Am. Chem. Soc. 117�41�, 10194 �1995�.

19P. Madakasira, K. Inoue, R. Ulbricht, S. B. Lee, M. Zhou, J. P. Ferraris,and A. A. Zakhidov, Synth. Met. 155, 332 �2005�.

20F. Zhang, H. Zhu, W. Yang, Z. Wu, H. Qi, H. He, Z. Fan, and J. Shao,Appl. Surf. Sci. 254, 3045 �2008�.

21D. Vollath, I. Lamparth, and D. V. Szabó, Mater. Res. Soc. Symp. Proc.703, 303 �2002�.

22D. Vollath, D. V. Szabó, and S. Schlabach, J. Nanopart. Res. 6, 181 �2004�.23Z. G. Wang, X. T. Zu, S. Zhu, X. Xiang, L. M. Fang, and L. M. Wang,

Phys. Lett. A 350, 252 �2006�.24G. Heimel, D. Somitsch, P. Knoll, and E. Zojer, Synth. Met. 139, 823

�2003�.25G. Heimel, D. Somitsch, P. Knoll, J.-L. Brédas, and E. Zojer, J. Chem.

Phys. 122, 114511 �2005�.26M. D. Groner, F. H. Fabreguette, J. W. Elam, and S. M. George, Chem.

Mater. 16, 639 �2004�.27A. Kadashchuk, S. Schols, P. Heremans, Yu. Skryshevski, Yu. Piryatinski,

I. Beinik, C. Teichert, G. Hernandez-Sosa, H. Sitter, A. Andreev, P. Frank,and A. Winkler, J. Chem. Phys. 130, 084901 �2009�.

103521-6 Tavares et al. J. Appl. Phys. 107, 103521 �2010�

Downloaded 13 Nov 2012 to 130.225.29.254. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions