Journal ArticleWe have used a variety of optical probes to investigate the changes occurring upon low-temperature thermal treatment to the emissive properties of dialkyl  substituted polyfluorene thin films. We found that the low-temperature-driven morphological changes involving aggregates formation, which are observed in the absorption and photoluminescence spectra at low excitation intensities, give rise at high excitation intensities to laser action in the blue spectral range. In some cases, switching We have used a variety of optical probes to investigate the changes occurring upon low-temperature thermal treatment to the emissive properties of dialkyl substituted polyfluorene thin films. We found that the low-temperature-driven morphological changes involving aggregates formation, which are observed in the absorption and photoluminescence spectra at low excitation intensities, give rise at high excitation intensities to laser action in the blue spectral range. In some cases, switching between two stimulated emission bands was also observed. between two stimulated emission bands was also observed

Shkunov, M. N.

Vardeny, Zeev Valentine

English

The University of Utah: J. Willard Marriott Digital Library

Laser action in polydialkylfluorene films: Influence of low-temperaturethermal treatmentM. N. ShkunovDepartment of Physics, University of Utah, Salt Lake City, Utah 84112-0830R. O¨ sterbackaa)Department of Physics, University of Utah, Salt Lake City, Utah 84112-0830and Department of Physics, A˚ bo Akademi University, Porhansgatan 3, Fin-20500 Turku, FinlandA. Fujii and K. YoshinoDepartment of Electronic Engineering, Osaka University, Suita, Osaka 565-0871, JapanZ. V. VardenyDepartment of Physics, University of Utah, Salt Lake City, Utah 84112-0830~Received 16 December 1998; accepted for publication 25 January 1999!We have used a variety of optical probes to investigate the changes occurring upon low-temperaturethermal treatment to the emissive properties of dialkyl substituted polyfluorene thin films. We foundthat the low-temperature-driven morphological changes involving aggregates formation, which areobserved in the absorption and photoluminescence spectra at low excitation intensities, give rise athigh excitation intensities to laser action in the blue spectral range. In some cases, switchingbetween two stimulated emission bands was also observed. © 1999 American Institute of Physics.@S0003-6951~99!02412-2#The recent demonstration of emission spectral narrowing~SN! in thin solid films of several p-conjugated polymers1–3has generated wide-spread interest in the phenomenon of la-ser action in these materials. It is now generally believed thatSN in neat polymer films is caused by amplified spontaneousemission ~ASE! enhanced by optical waveguiding in thepolymer films.4 SN occurs at the moderately high excitondensity of 1017–1019 cm23, at wavelengths where the stimu-lated emission ~SE! overcomes self-absorption and is accom-panied by substantial lifetime shortening.3 Extensive studiesof various lasing cavities using these polymers as active ma-terials have been directed towards the realization of electri-cally driven organic solid-state lasers.1,5–8 Yet, the SNmechanism itself is not very well understood. For example,the reason why some conjugated polymers which have a highphotoluminescence quantum yield nevertheless lack SN isnot exactly known. Also, the influence of morphologicalchanges involving aggregates formation in the polymer filmon its laser action at high excitation intensities has not, asyet, been thoroughly studied.In this work, we investigate the optical emission proper-ties of dialkyl-substituted polyfluorene ~PDAF! thin films,which undergo morphological changes upon low-temperature thermal treatment,9 using both low and high ex-citation intensities. In particular, we studied films of di-octylsubstitution, PDAF8, the backbone of which is shown in Fig.1~a!, inset. We note that the side groups responsible for thepolymer solubility are well separated from the side groups onthe adjacent fluorene units and this dramatically reduces thesteric interaction that twists the conjugated repeat units fromthe planar configuration. In addition, PDAF films can be con-trollably modified without changes in the polymer chemicalstructure. For example, PDAF melts into a liquid-crystallinephase at '170 °C and can be quenched as a glass or crystal-lites by either fast or slow cooling from the melt.10 More-over, this polymer is a blue emitter, which has recently at-tracted much interest for use in electroluminescencea!Also at the Graduate School of Materials Research, Turku, Finland.FIG. 1. Changes in the absorption ~a! and photoluminescence ~b! spectra ofPDAF8 films upon low-temperature thermal treatment, as measured at roomtemperature. The ‘‘10 K’’ and ‘‘80 K’’ films were cooled to 10 and 80 K,respectively, whereas the ‘‘300 K’’ film was never cooled below room tem-perature. The inset in ~a! shows the polymer repeat unit, whereas the inset in~b! gives the respective films PL quantum efficiency h.APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 12 22 MARCH 199916480003-6951/99/74(12)/1648/3/$15.00 © 1999 American Institute of PhysicsDownloaded 24 Nov 2009 to 155.97.11.183. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspdevices11 and light-emitting electrochemical cells.12 Prelimi-nary studies have shown that dramatic changes occur in theoptical spectra of PDAF films upon low-temperature thermaltreatment.9 In addition, using PDAF as the active medium,lasing in solutions, and ASE in thin films were alsoobserved.13,14 However, the influence of the changes inducedby the low-temperature thermal treatment on the laser-actionproperties, has not been investigated to the best of ourknowledge.We have measured the changes induced upon low-temperature thermal treatment in the absorption ~a! and pho-toluminescence ~PL! spectra at low excitation intensities, andSN at high excitation intensities. The a~v! spectrum wasmeasured using a Perkin–Elmer Lambda 9 UV–Vis spec-trometer. The PL was measured using a 1/4 met Jarell–Ashspectrometer, where the excitation beam was from an Ar1laser operating at 363 nm. All measurements were done atroom temperature. We measured the PL quantum yield, hPLper absorbed photon by an integrating sphere with 363 nmexcitation.For the laser-action measurements at high excitation in-tensities, we used a laser beam with 100 ps pulses at 100 Hzrepetition rate with the energy per pulse ranging from 0.1 to50 mJ, provided by a Nd:YAG regenerative laser amplifier,which was frequency tripled at 355 nm. The pump beam wasfocused onto a circular illuminated area of about 1 mm2 onthe PDAF8 film. The emission was collected by a lens andanalyzed using a 0.6 met spectrometer and a charge-coupleddevice camera with ;3 Å spectral resolution.The PDAF8 was synthesized by a method published inRef. 15 using a Suzuki coupling reaction.16 Thin films for thespectroscopic studies were prepared by spin coating fromsolution of THF, which is known to be a good solvent forPDAF polymers.9 Three initially identical films were pre-pared, where two of them were subject to low-temperaturethermal treatment. The three films are dubbed here as fol-lows: the ‘‘300 K’’ sample was kept at room temperature;the ‘‘80 K’’ sample was cooled down to 80 K in a cold fingercryostat followed by a slow warm-up to room temperature;and the ‘‘10 K’’ sample was heat treated similar to the ‘‘80K’’ films, but at helium atmosphere at a temperature of 10 K.The optical changes in the films reported here are all irre-versible on a time scale of several months, but can be elimi-nated by heating the samples up to the glass transition tem-perature at ;170 °C.In Fig. 1~a! we show the room-temperature absorptionspectra of the ‘‘300 K’’ and ‘‘10 K’’ films; similar resultswere obtained with the ‘‘80 K’’ film. It is seen that the low-temperature thermal treatment enhances an absorption bandat energies below the initial PDAF8 optical gap.9 This bandis better seen in the absorption difference spectrum, Da5a(‘‘10 K’’)2a(‘‘300K’’); the new band peaks at ;3eV with an onset at ;2.88 eV. From the Da spectrum weconclude that a fraction of the material, estimated at ;8%,has been transformed by the thermal treatment into a newmorphology; for the ‘‘80 K’’ sample this fraction is less than1%. The nature of the new molecular structure is not exactlyknown, but from the redshifted absorption spectrum wespeculate that it corresponds to a well-defined state of supe-rior order with enhanced conjugation. Such an order statemay be formed, for example, by chain aggregation where thepolymer adopts a more planar conformation. We note similaraggregation discovered in other polymer films made from thedialkoxy derivatives of poly ~p-phenylene vinylene! andpolythiophene.17,18The corresponding changes occurring in the PL spectraupon the thermal treatment are much clearer, as shown inFig. 1~b!. All three PL spectra are structured at room tem-perature with up to five phonon replica. However, the PLspectra of the ‘‘80 K’’ and ‘‘10 K’’ films show much morepronounced phonon replica. In addition, the PL spectra ofthese two films also show a redshift of order 30 meV, inagreement with the redshift seen in the Da spectrum @Fig.1~a!#. We note that the 0–0 PL band is weaker in the ‘‘10K’’ film compared to the ‘‘80 K’’ and ‘‘300 K’’ films, andthis is caused by the enhanced self-absorption due to theredshifted absorption in this film @Fig. 1~a!#. The PL quan-tum yields also follow the spectroscopic changes in PL ob-served upon thermal treatment, with hPL(‘‘10 K’’).hPL(‘‘80 K’’).hPL(‘‘300K’’). We note that hPLreaches 21% for the ‘‘10 K’’ sample in agreement with thesuperior order of this film.The laser-action properties of the three PDAF8 films athigh excitation intensities I are shown in Fig. 2. The ‘‘300K’’ film @Fig. 2~a!# does not show SN at all I. The two otherfilms, however, show SN at 442 and 463 nm, respectively,where the broad PL band of 120 nm collapses at I.I0 into anarrow band of 4 nm spectral width. It is notable that thepristine PDAF8 film does not show SN, but aggregates for-mation in the heat-treated film results in SN. We speculatethat this is due to the superior order in the film that is gen-erated upon thermal treatment. The excitation intensitythreshold I0 for SN is 7.4 mJ for the ‘‘80 K’’ film and 1.5 mJfor the ‘‘10 K’’ film. That I0 is lower for the ‘‘10 K’’ film isin agreement with its higher PL quantum yield @Fig. 1~b!#.The most interesting behavior at high I, however, is theSN switching in the ‘‘80 K’’ film from a SE band centered at463 nm to another SE band at 442 nm, which occurs at I1511mJ.I0 ; the ‘‘10 K’’ film, on the other hand, showsonly a single SE band at 463 nm. The SN switching betweenthe two wavelengths in the ‘‘80 K’’ film may be explainedby an interplay between optical gain, coupled with an opticalabsorption saturation at I1 as follows. At moderately high I,the effective optical gain geff in the polymer film follows therelation:19geff~v !5Ns~v !2a~v !, ~1!where N(;I) is the exciton density, s~v! is their opticalgain cross-section spectrum that is mainly proportional to thePL spectrum, and a~v! is the absorption loss in the tail. SEoccurs at I0 and v0 such that geff(I0 ,v0).0. Since Ns(v) inEq. ~1! increases linearly with I, then SE switching from onev0 to the other must be driven by nonlinear changes ingeff(v), and thus, can only occur if some changes in a~v!happen at high I. This may occur, for example, if a~v! in thetail is saturated or photobleached at high I due to excitonenergy migration into lower-energy states and, in particular,into the aggregate states.20 We note that the aggregate ab-sorption band in the ‘‘80 K’’ film is quite small ~smaller than1% of the total a!, and therefore, this film is more suscep-1649Appl. Phys. Lett., Vol. 74, No. 12, 22 March 1999 Shkunov et al.Downloaded 24 Nov 2009 to 155.97.11.183. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsptible to optical saturation at high I, compared to that of the‘‘10 K’’ film where the aggregate absorption band is of theorder of 8% @Fig. 1~a!#.From the PL spectrum at low I @Fig. 1~b!#, it is seen thatin the ‘‘80 K’’ film the 0–0 emission band ~;442 nm! isstronger than that of 0–1 band at ;463 nm; this is carriedover to s~v! of this film. Nevertheless, geff.0 first occurs atthe 0–1 emission band rather than at the 0–0 band becauseof a~v! @Eq. ~1!#, which is larger at 442 nm @Fig. 1~a!#.However, at sufficiently high I, a~442 nm! is partially pho-tobleached, resulting in geff(442 nm).geff(463 nm) and thiscauses SE switching between these two wavelengths. Wealso note from Fig. 1~b! that the 0–0 emission band in the‘‘10 K’’ film is much weaker than the 0–1 emission band,and this is caused by the higher a in the tail for this sample@Fig. 1~a!#. Under these conditions geff.0 occurs first at the0–1 not at the 0–0 band, and since photobleaching of a inthe tail does not occur in this film because a is too high, thenno SE switching is observed here.The SE switching phenomenon from one band to theother at a certain threshold excitation intensity is a very in-teresting effect with potential applications ranging from op-tical switches, to saturable and inverse saturable absorbers.We note that the SE switching effect is unique forp-conjugated polymers, and stems from the pronounced pho-non replica in the PL spectra of these materials.This work was partially financed by the NSF, DMR 97-32820. One of the authors ~R.O¨ .! acknowledges fundingfrom the Swedish Academy of Engineering Sciences in Fin-land and the Neste, Ltd. Foundation.1N. Tessler, G. 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Fu, T. M. Swager, and A. G.MacDiarmid, Phys. Rev. B 54, R3683 ~1996!.19S. V. Frolov, Z. V. Vardeny, and K. Yoshino, Phys. Rev. B 57, 9141~1998!.20R. Kersting, U. Lemmer, R. F. Mahrt, K. Leo, H. Kurz, H. Ba¨ssler, and E.O. Go¨bel, Phys. Rev. Lett. 70, 3820 ~1993!.FIG. 2. The uncorrected emission spectra of ~a! ‘‘300 K,’’ ~b! ‘‘80 K,’’ and~c! ‘‘10 K’’ PDAF8 films at high laser excitation intensities of 100 ps at 355nm. The excitation intensity is given in energy per pulse, where the illumi-nated area on the films was 1 mm2. Narrow SE bands are formed in ~b! and~c! at excitation intensity thresholds of 7.4 and 1.5 mJ, respectively.1650 Appl. Phys. Lett., Vol. 74, No. 12, 22 March 1999 Shkunov et al.Downloaded 24 Nov 2009 to 155.97.11.183. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Laser action in polydialkylfluorene films: influence of low-temperature thermal treatment

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Laser action in polydialkylfluorene films: influence of low-temperature thermal treatment

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