Copyright © 2002 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 81 (2002) and may be found at http://link.aip.org/link/?APPLAB/81/1955/1We present the optical properties of a directly patterned light-emitting polymer. The patterned poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-paraphenylenevinylene film is fabricated using hot embossing lithography. The effect of the embossed microstructure on the light emitted from the polymer is examined by measuring the angle-dependent photoluminescence and its photonic band structure. The imposed grating modifies the emitted light by Bragg scattering into free space light that would otherwise be trapped as waveguide modes. This simple patterning technique may find application in improving the performance of light-emitting polymer devices

G. A. Turnbull

I. D. W. Samuel

J. R. Lawrence

M. Buck

P. Andrew

Turnbull G. A.

W. L. Barnes

Applied Physics Letters

Crossref

Optical properties of a light-emitting polymer directly patterned by soft lithography

We present the optical properties of a directly patterned light-emitting polymer. The patterned poly(2-methoxy-5-(3('),7(')-dimethyloctyloxy)-paraphenylenevinylene film is fabricated using hot embossing lithography. The effect of the embossed microstructure on the light emitted from the polymer is examined by measuring the angle-dependent photoluminescence and its photonic band structure. The imposed grating modifies the emitted light by Bragg scattering into free space light that would otherwise be trapped as waveguide modes. This simple patterning technique may find application in improving the performance of light-emitting polymer devices. (C) 2002 American Institute of Physics.</p

Lawrence, JR

Andrew, P

Barnes, WL

Buck, Manfred

Turnbull, Graham Alexander

Samuel, Ifor David William

University of St. Andrews - Pure

Lawrence, J. R.

Andrew, P.

Barnes, William L.

Buck, M.

Turnbull, G. A.

Samuel, I. D. W.

Open Research Exeter

APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 11 9 SEPTEMBER 2002Optical properties of a light-emitting polymer directly patternedby soft lithographyJ. R. LawrenceUltrafast Photonics Collaboration & Organic Semiconductor Centre, School of Physics & Astronomy,University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, United KingdomP. Andrew and W. L. BarnesThin Film Photonics Group, School of Physics, University of Exeter, Stocker Road, Exeter, Devon,EX4 4QL, United KingdomM. BuckOrganic Semiconductor Centre & School of Chemistry, University of St. Andrews, North Haugh,St. Andrews, Fife KY16 9ST, United KingdomG. A. Turnbull and I. D. W. Samuela)Ultrafast Photonics Collaboration & Organic Semiconductor Centre, School of Physics & Astronomy,University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, United Kingdom~Received 19 February 2002; accepted for publication 29 June 2002!We present the optical properties of a directly patterned light-emitting polymer. The patternedpoly~2-methoxy-5-~38,78-dimethyloctyloxy!-paraphenylenevinylene film is fabricated using hotembossing lithography. The effect of the embossed microstructure on the light emitted from thepolymer is examined by measuring the angle-dependent photoluminescence and its photonic bandstructure. The imposed grating modifies the emitted light by Bragg scattering into free space lightthat would otherwise be trapped as waveguide modes. This simple patterning technique may findapplication in improving the performance of light-emitting polymer devices. © 2002 AmericanInstitute of Physics. @DOI: 10.1063/1.1502187#Semiconducting conjugated polymers are a very promis-ing class of materials for use in optoelectronics. They areeasily-processible and are suitable for a range of applicationsdue to their high photoluminescence ~PL! quantum yield,large gain cross section, broad emission spectrum and theirability to be electrically pumped.1–3 The performance of la-sers, light emitting diodes, and displays made from thesematerials can be further improved by incorporating wave-length scale microstructures.4 At present such microstruc-tured devices are made using one or a combination of com-plex techniques such as photolithography, electron beamlithography, and chemical etching. New fabrication methodsthat retain the ease of processing that is a key advantage ofpolymeric materials would therefore be very attractive.In this letter we use a simple technique known as hotembossing lithography5,6 to fabricate sub-wavelength scalemicrostructures in a light-emitting polymer film. We measureangle-dependent PL to show how the microstructure affectsthe light emission from the film and explain the results bymeasuring the photonic band structure and by theoreticalmodeling.The conjugated polymer used in this study is poly@~2-methoxy-5-~38,78-dimethyloctyloxy!-1,4 paraphenylenevi-nylene# (OC1C10-PPV) and its structure is shown in Fig. 1.Thin films of OC1C10-PPV were formed by spin coatingfrom chlorobenzene onto flexible plastic substrates ~over-head transparencies!. This results in films of about 100 nm inthickness. These films are then patterned using a soft litho-a!Author to whom correspondence should be addressed; electronic mail:idws@st-andrews.ac.uk1950003-6951/2002/81(11)/1955/3/$19.00Downloaded 18 Mar 2005 to 144.173.6.75. Redistribution subject tographic technique known as hot embossing similar to themethod used to microstructure a photodiode.6 The polymerwas heated to 20 °C above its glass transition temperature~Tg;200 °C7!, that is, the temperature at which it begins tosoften and flow. A master copy of the desired structure waspressed into the polymer with a pressure of 48 N/cm2. Thepressure needed for embossing was relatively low comparedto other embossing methods. In order to prevent theOC1C10-PPV from sticking, the silica master was first coatedwith a monolayer of octadecyltrichlorosilane. Under pressurefrom the master, the polymer indents and flows around themaster conforming to its shape. After 20 min the polymerwas allowed to cool with the master still in place. This cool-ing allows the polymer to drop below its glass transitiontemperature effectively ‘‘freezing in’’ the imprinted structure.The master was then removed from the polymer leaving thereplicated structure embossed into the polymer.In the present work the master structures were crossedgratings formed in photoresist by multiple exposure to inter-fering laser beams and etched into silica as shown in Fig.2~a!. The angle f between the two gratings was 90°, andeach grating had a period of 400 nm and a peak-to-troughdepth of 30 nm. The entire procedure was carried out in air.In order to characterize the embossed structure, the patternedpolymer films were examined using an atomic force micro-scope. An image of the embossed OC1C10-PPV film obtainedusing atomic force microscopy is shown in Fig. 1. The im-printed crossed grating structure can be clearly seen in thefilm. The embossed structure has a period of 400 nm and adepth of 20 nm. The period matches exactly that of theetched silica master but the embossed grating is shallower.5 © 2002 American Institute of Physics AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp1956 Appl. Phys. Lett., Vol. 81, No. 11, 9 September 2002 Lawrence et al.In order to understand how the microstructure affects thelight emission, the photonic mode structure of the film wasmeasured by angle-dependent transmission as shown in Fig.2~b!.8,9 The impact of the embossed microstructure on thephotonic band structure is shown in Fig. 3. Figure 3~a! showsthe modes observed when the incident beam is s-polarizedand Fig. 3~b! shows p-polarization. When the incident lightis s-polarized, two dark bands are observed, both starting atv51.63 mm21 and ending at 1.55 and 1.7 mm21, respec-tively. For p-polarized incident light there is a single hori-zontal dark band showing little dispersion and centered atv51.63 mm21. These dark bands correspond to a drop intransmission, which indicates that the incident light has beencoupled into a waveguide mode. The incident light is dif-fracted by the periodic microstructure at an angle greaterFIG. 1. An atomic force microscope image and chemical structure of theOC1C10-PPV film patterned by hot embossing.FIG. 2. ~a! Polymer film with imprinted crossed grating. The plane of inci-dence was aligned parallel to one of the sets of gratings. ~b! Experimentalsetup for measurement of photonic mode structure. Bragg scattering whenthe magnitude of the incident wave vector k is ~c! greater than and ~d! lessthan that of the grating vector kg .Downloaded 18 Mar 2005 to 144.173.6.75. Redistribution subject tothan the critical angle, thereby causing the light to wave-guide inside the film. In Fig. 3~a! it can be seen that for eachincident polar angle there are two wavelengths that arestrongly scattered by the microstructure. This behavior is ex-plained by Fig. 2. The scattering process of the grating canbe thought of as adding a wave vector (kg562p/L) to theoptical modes of the waveguide (k52pneff /l), where L isthe period of the corrugation, neff is the effective refractiveindex of the waveguide, and l is the wavelength of the inci-dent light. When a light beam is directed onto the guide fromfree space at a polar angle of u to the substrate normal, thefield has an in-plane wave vector component of (2p sin u)/l.If this component matches the difference uk2kgu, light willbe strongly coupled into the waveguide mode. For eachangle u there are two wavelengths for which this condition issatisfied: uku.ukgu and uku,ukgu, as illustrated in Figs. 2~c!and 2~d!.Having measured the photonic band structure of the film,we now examine the effects of the microstructure on the lightemission. To investigate this we made measurements of theangle-dependent PL. The patterned polymer film was housedin a circular vacuum chamber and excited using the secondharmonic output of a diode pumped continuous waveNd:YAG laser ~532 nm!. Light emitted from the film wascollected by an optical fiber and detected using a CCD spec-trometer. The end of the optical fiber was attached to a free-rotating moveable arm mounted on the vacuum chamber toenable light emitted over a wide range of angles to be de-tected.Figure 4~a! shows the PL spectrum of the directly pat-terned OC1C10-PPV film measured at various angles. As theangle of observation ~u in Fig. 2~a!! changes, the central peaksplits into two peaks which move to longer and shorterwavelengths due to Bragg scattering.10,11 This can be seenmore clearly in Fig. 4~b! where each PL peak is plotted as afunction of its wavelength and angle of observation. All scat-tered features shown in Fig. 4~b! originate from emissioninto the TE0 mode of the waveguide ~shown in Fig. 3!, whichis then scattered by one of the crossed gratings. The featureindicated by the squares is due to light coupling to the wave-FIG. 3. Measured photonic mode structure of directly patternedOC1C10-PPV film when probed with ~a! s and ~b! p-polarized light. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp1957Appl. Phys. Lett., Vol. 81, No. 11, 9 September 2002 Lawrence et al.FIG. 4. ~a! Angle dependent PL spec-trum of a directly patternedOC1C10-PPV film and ~b! PL peak po-sitions as a function of in-planewavevector and frequency ~points! andtheoretical fit ~lines!.guide mode via Bragg scattering arising from the grating thathas grooves running perpendicular to the plane of observa-tion, that is, Bragg vector in plane of incidence. The featureindicated by the triangles is due to light coupling to thewaveguide mode via Bragg scattering arising from the grat-ing that has grooves parallel to the observation plane. Thefrequencies and in-plane wavevectors for optimum couplingin these two cases can be given by Eqs. ~1! and ~2!, respec-tivelyv56k6mkg562pneffl6m2pL, ~1!v56Ak22~mkg!256AS 2pneffl D22S m 2pL D2, ~2!where L is the fundamental grating period and m is an inte-ger. These equations are used to fit the experimental data.The fit is shown as a solid line in Fig. 4~b!. The refractiveindex of OC1C10-PPV as a function of wavelength was mea-sured by spectroscopic ellipsometery. Using this information,neff the effective index of the waveguide could becalculated.12Despite the relatively weak corrugation of the embossedstructures when compared to that used when polymers aredeposited onto microstructured substrates,4,13 significantmodification of the PL spectra is still seen. This arises be-cause the periodic modulation occurs at the polymer air in-terface, which has stronger index contrast than the polymerglass interface. We note a small discrepancy of 0.06 mm21 inthe frequency of the modes observed in PL ~1.57 mm21! andthe photonic mode structure ~1.63mm21! in Fig. 3. It is likelythat this arises from slightly different regions of the samplebeing measured, with the regions being of different thick-ness. A difference in thickness of less than 10% between theareas probed for each experiment would account for this dis-crepancy.In summary, we have shown that directly patterning aconjugated polymer film provides a simple way of control-Downloaded 18 Mar 2005 to 144.173.6.75. Redistribution subject toling the emission of light from within it. The patterned filmwas examined by atomic force microscopy and shown toclosely follow the shape of the master structure. The effect ofthe embossed microstructure on the emitted light was exam-ined by comparing angle dependent PL with the photonicmode structure. It was found that the imposed grating en-ables light trapped in waveguide modes to be Bragg scatteredout into free space. The observations also agree with wave-guide mode structure deduced from measured refractive in-dex data. Hot embossing processes such as this should leadto simpler device fabrication and provide flexibility in newdevice design.We are grateful to EPSRC and SHEFC for financial sup-port, and Covion for the supply of OC1C10-PPV. One of theauthors ~I. D. W. S.! is a Royal Society University ResearchFellow.1 M. D. McGehee and A. J. Heeger, Adv. Mater. 12, 1655 ~2000!.2 R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks,C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund,and W. R. Salaneck, Nature ~London! 397, 121 ~1999!.3 U. Scherf, S. Riechel, U. Lemmer, and R. F. Mahrt, Curr. Opin. Solid StateMater. Sci. 5, 143 ~2001!.4 J. M. Lupton, B. J. Matterson, M. J. Jory, W. L. Barnes, and I. D. W.Samuel, Appl. Phys. Lett. 77, 1 ~2000!.5 S. Y. Chou, P. R. Krauss, and P. J. Renstrom, Appl. Phys. Lett. 67, 3114~1995!.6 L. S. Roman, O. Inganas, T. Granlund, T. Nyberg, M. Svensson, M. R.Andersson, and J. C. Hummelen, Adv. Mater. 12, 189 ~2000!.7 T. Q. Nguyen, B. J. Scwartz, R. D. Schaller, J. C. Johnson, L. F. Lee, L. H.Haber, and R. J. Saykally, J. Phys. Chem. B 105, 5153 ~2001!.8 M. G. Salt and W. L. Barnes, Opt. Commun. 166, 151 ~1999!.9 M. G. Salt, W. L. Barnes, and I. D. W. Samuel, J. Mod. Opt. 48, 1085~2001!.10 B. J. Matterson, M. G. Salt, W. L. Barnes, and I. D. W. Samuel, Synth.Met. 101, 250 ~1999!.11 A. N. Safonov, M. Jory, J. M. Lupton, M. G. Salt, J. A. E. Wasey, W. L.Barnes, and I. D. W. Samuel, Synth. Met. 116, 145 ~2001!.12 A. Yariv, Quantum Electronics ~John Wiley, New York, 1989!.13 G. A. Turnbull, P. Andrew, M. J. Jory, W. L. Barnes, and I. D. W. Samuel,Phys. Rev. B 6412, 5122 ~2001!. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Optical properties of a light-emitting polymer directly patterned by soft lithography

Abstract

Similar works

Full text

Available Versions

Crossref

University of St. Andrews - Pure

Open Research Exeter

University of St. Andrews - Pure