PHILOSOPHICAL MAGAZINE B 2002 VOL 82 NO 4 485plusmn495

Growth by supersonic molecular-beam epitaxy ofoligothiophene reglms with controlled properties

T Toccoli A Boschetti S IannottayCentro di Fisica degli Stati Aggregati Consiglio Nazionale delle RicercheplusmnIstituto Trentino di Culturia Via Sommarive 18 38050 Povo Trento Italy

P Scardi

Dipartimento di Ingegneria dei Materiali UniversitaAacute di Trento 38050 MesianoTrento Italy

G Barbarella and G Sotgiu

Consiglio Nazionale delle Ricerche Istituto Composti Carbonio ContenentiEteroatomi e Applicazioni Via Gobetti 101 I-40129 Bologna Italy

AbstractSupersonic molecular beam epitaxy can be fruitfully used to grow reglms of

organic materials controlling their degree of ordering and their morphologiesThis is due to the regne control on the initial state of the compounds in the beamVery sharp and `tuneablersquo energy distributions in the beam can be achieved sothat the growth is not governed by processes at thermodynamic equilibrium butby the kinetics This allows us in some cases to `forcersquo the growth in the desireddirections instead of leaving this only to the interplay between the weakinteraction forces and to the equilibrium thermodynamics to drive the growthComparing the optical response morphology and structure of reglms grown underdiŒerent degrees of supersonicity of the beam we show that one can control thegrowth in details The grain size and density of defects can be drastically changedby appropriately tuning the beamrsquos parameters X-ray diŒraction and pole reggureanalysis of the reglms are reported and discussed

1 IntroductionThe growing interest in p-conjugated organic molecular materials and polymers

driven by the wide potential technological impact in electronics and photonics(Farges 1994) still faces severe limitations Applications would be much morefavoured by improving the control of the morphology and structure in the solidstate (Bredas and Silbey 1991 Taliani and Gebauer 1999) Standard growth methodsare based on the self-assembly of the molecules resulting more or less from theinteraction with the substrate Very often the resulting reglms show inadequate order-ing These problems become severe as the thickness increases to more than afew monolayers as is necessary for some major applications Vacuum depositionmethods have improved control over the ordering in the reglms eliminating in several

Philosophical Magazine B ISSN 1364plusmn2812 printISSN 1463plusmn6417 online 2002 Taylor amp Francis Ltd

httpwwwtandfcoukjournals

DOI 10108013642810110085442

y Author for correspondence Email iannottacefsaitcit

cases the presence of impurities A way to improve control of the reglm structurefurther is to take advantage of the moleculeplusmnsubstrate interactions which could betuned to the required strength by selecting the appropriate molecule and surfacepartners (ie silver substrates (Gebauer et al 1998)) When the inmacruence of thesubstrates vanishes (a few monolayers) the growth becomes more and more disor-dered This is related to the random orientation with which the molecules arrive atthe surface in standard vacuum deposition so that if there is no an eŒective forcethat orients the molecules in the `correctrsquo positions there could be simultaneousformation of diŒerent polymorphs and the presence of defects in the growth

We have considered the major role played by the initial state of the molecules atearly stages of growth and we approached the problem by combining a supersonicfree jet which permits us to control the kinetic energy momentum and macrux with anultrahigh-vacuum (UHV) deposition apparatus (Milani and Iannotta 1999) Weperformed the deposition by simultaneously controlling and characterizing the initialstate of the seeded organic molecules by varying the parameters of the supersonicexpansion (the dilution temperature of the source form and diameter of nozzle etc)We prepared a series of reglms of a-quaterthiophene (a-4T) which are then character-ized by optical measurement tapping mode (TM) atomic force microscopy (AFM)and X-ray diŒraction (XRD) We report overall strong evidence of unprecedentedcontrol of the morphology structure and optical response that correlate well to thebeamrsquos parameters Photoluminescence (PL) spectra at low temperatures show thevibronic molecular structure to be very well resolved and present features stronglydepending on the initial state of the oligomer in the beam which could be related tothe ordering in the reglms Films several hundreds of nanometres thick show a highdegree of ordering with surface morphologies characterized by layered structures ofmolecular height XRD conregrms the high degree of ordering induced by the growthfrom highly supersonic beams Correlation between morphology degree of orderingand the optical response of these reglms is discussed

2 ExperimentThe experimental set-up at the Centro di Fisica degli Stati Aggregati consists

essentially of three diŒerentially pumped chambers separated by two skimmers Thesystem is designed with adequate pumping speeds to overcome the macrux of the gasused in the supersonic expansion and to work in the last chamber in UHV conditionsduring the deposition In the regrst chamber the hyperthermal supersonic beam sourceis housed It has been previously described (Toccoli et al 1999) In a supersonic free-jet experiment the regnal state of the molecules seeded in the beam is determined by thesupersonic expansion (Scoles 1988) which is controlled by several parameters suchas geometric factors (ie the shape and diameter of the nozzle and of the skimmerthe type and pressure of the carrier gas the temperature of the source etc) All theseparameters can be controlled and the source and the skimmer have been carefullydesigned in order to optimize the performance of the source and to avoid the pro-blems of clogging the nozzle and of skimmer interferences In the second vacuumchamber a sample manipulator and a chopper are housed The regrst is used to test theinitial parameters for the deposition The second is very useful to characterize thebeam properties (the kinetic energy and velocity spread in particular) in combinationwith the time-of-macright (TOF) mass spectrometer present in the last chamber In thethird chamber is housed the heart of the system besides the TOF mass spectrometernamely a microbalance and the sample manipulator on the beam path The

486 T Toccoli et al

manipulator supports up to eight samples all of these can be heated independentlyand their temperature controlled (from room temperature to 700K) The substratesused in the present experiments are CaF2 or silica glass optical grade bought fromLOT Oriel After insertion in the UHV chamber they are outgassed at temperatureshigher than 600 K for more than 2 h The ionization of the compounds in the beamnecessary for the TOF spectroscopy is made by the fourth harmonic of a neody-mium-doped yttrium aluminium garnet laser or using a tuneable titanium sapphirelaser The polarization of the laser light is usually oriented horizontally and parallelto the beam direction changing the orientation of the polarization of the laser lightby means of a Pockel cell it is possible to study the degree of alignment of theinertial axis of molecules with respect to the beam direction The TOF mass spectro-meter is mounted properly aligned to the beam path to characterize the kineticenergy and the velocity spread of the molecules in the supersonic beam (Iannottaet al 2000)

As in previous experiments (Toccoli et al 1999) the oligothiophene used is the a-4T derived by the impurity present in a-terthiophene powder bought from TokyoKasey Company we also used material synthesized and purireged by two of thepresent authors (G Barbarella and G Sotgiu) obtaining very similar results eventhough the source operating conditions were obviously quite diŒerent The techniqueused to characterize the reglms grown by supersonic molecular-beam epitaxy(SuMBE) are ultravioletplusmnvisible absorption spectra PL at room temperature andat low temperatures (excitation with the 4579 nm line of the ArDagger laser) TM AFMand XRD

XRD patterns were collected in the BraggplusmnBrentano geometry using a RigakuPMG-VH powder diŒractometer equipped with a copper tube and a graphitecurved-crystal analyser in the secondary beam Owing to problems of illuminationof the sample surface at a small angle the peak intensity was not reliable below2sup3 ˆ 10deg

3 Results and discussionAs mentioned one of the major advantages of supersonic beams is that they

allow one to control and to characterize the energy state of the molecules in thebeam To this end as a regrst step in our experiments we carried out an analysis of theproperties of oligothiophene in a free-jet expansion in order to study the correlationbetween the molecular state and the type of expansion Once the beam parameterswere deregned we deposited several reglms of a-4T to determine the best depositionconditions and to correlate the optical and electronic properties of reglms to the stateof the molecules in the beam Changing the source conditions (the type and length ofnozzle) and the parameters of expansion (the type of gas dilution and source tem-perature) we observed that the kinetic energy of the molecules and their velocityspread in the beam are the most important parameters inmacruencing the growth pro-cess As previously reported (PodestaAacute et al 2000 Iannotta et al 2001) and discussedin more detail in the following the kinetic energy plays an important role giving theopportunity to overcome some of the limits inherent in growth governed by equili-brium thermodynamics so that more ordered and speciregc structures can be preparedEven the velocity spread of the molecules in the supersonic beam which aŒects thealignment of their backbone along the beam axis gives rise to reglms with diŒerentdefect densities

Supersonic molecular-beam epitaxy of oligothiophene reglms 487

Supersonic expansion can be modelled in a regrst approximation as an isoenthalpicprocess where the internal degrees of freedom of the molecules are subject to a veryfast cooling process and because of conservation of energy their kinetic energyincreases For compounds that are solid at room temperatures the vapour pressureis so low that it is almost impossible to produce a highly supersonic expansion thatrequires mean free paths in the source much smaller than the nozzle diameter (Scoles1988 Milani and Iannotta 1999) The problem is overcome by using a lighter carriergas in which the oligomer is seeded This solution gives the advantages of producinga good supersonic expansion with the important possibility of tuning the kineticenergy of the molecules by exploiting the so-called aerodynamic accelerations dueto the collision with the faster-macrowing lighter particles of the carrier gas By changingthe relative concentrations of the two species the heavier molecules if su cientlydiluted could be accelerated to the velocity of the lighter carrier gas We previouslyexploited such a process to accelerate C60 to about 60 eV (Biasioli et al 1997) Therange of energies achievable is of great interest for the physicalplusmnchemical processesinvolved in the growth of organic materials

We characterized our seeded supersonic expansion and reggure 1 shows the kineticenergy and the velocity spread for the a-4T using two diŒerent types of carrier gas

488 T Toccoli et al

Figure 1 Kinetic energy of a-4T molecules seeded in helium and H2 as a function of thepressure of the carrier gas With the same abscissa is reported the full width at half-maximum (FWHM) of peaks of a-4T in the TOF mass spectra This value gives anidea of the rotovibrational cooling process due to the supersonic expansion

(H2 and helium) at the same source temperature and as a function of dilution Thevelocity of the seeded oligomer can be easily varied from a few hundred millielec-tronvolts to about 20 eV while the cooling of the molecules increases as remacrected bythe narrowing of the full width at half-maximum of the TOF mass spectrometerpeak

On the basis of this kind of characterization we grew several reglms of a-4T and a-quinquethiophene (a-5T) using several diŒerent conditions of the supersonic expan-sion We then studied their optical response morphology and structure correlatingthem with the properties of the molecules in the beam Figure 2 reports the ultra-violetplusmnvisible spectra of a-4T and a-5T grown by SuMBE changing the degree ofseeding of the compounds in the carrier gas The reglms grown at the same depositionrate but in diŒerent source regimes (highly supersonic and quasi-eŒusive) showinteresting diŒerences in the form of the absorption band located at about 37 eVfor a-4T and at 355 eV for a-5T In particular reglms grown by SuMBE show amuch sharper band typical of the formation of H aggregates where the exciton is

Supersonic molecular-beam epitaxy of oligothiophene reglms 489

Figure 2 Ultravioletplusmnvisible spectrum of an a-4T reglm grown by SuMBE in the high super-sonic regime for which the thickness is estimated to be about 250 nm (ETHETH) togetherwith the ultravioletplusmnvisible spectrum of an a-4T reglm grown in an eŒusive regime with asimilar thickness (ETHETH) and the ultravioletplusmnvisible spectrum of an a-5T reglm grown bySuMBE in the high supersonic regime for which the thickness is estimated to be about300 nm (- - - -) (au arbitrary units)

delocalized on more than one molecule This is an indication that the structurespresent in the reglms grown with this technique are better ordered

A morphological characterization of the same types of reglm shows features thatcorrelate nicely to this picture Figure 3 shows the AFM picture of the same reglmsThe reglms grown using beams in a high supersonic regime typically show lamellae

490 T Toccoli et al

Figure 3 (a) AFM picture of an a-4T reglm 250 nm thick grown by SuMBE in the highlysupersonic regime (b) Picture of an a-4T reglm grown in the quasi-eŒusive regime

structures characterized by tiles parallel to the substrate When these lamellae areanalysed in detail a terraced structure where the steps correspond to the height ofthe a-4T molecules becomes clearly discernible The picture emerging from the reglmsgrown from beams in an eŒusive regime is very diŒerent Here the morphologyshows the typical feature of dendritic growth which when analysed by high-resolu-tion TM AFM shows only lobular shapes (PodestaAacute et al 2000)

A very sensitive way to assess the degree of ordering of the oligothiophene reglmsis to study the PL spectra at low temperatures Since the homogeneous contributionto the bandwith at low temperatures can be neglected these spectra give directinformation about the presence of defects and impurities in the reglms Our reglmsgrown by SuMBE show a well resolved vibronic structure where the FWHM ofthe 0plusmn0 band in the best reglms is about 40 cm 1 at 10 K We obtained particularlyinteresting results by comparing the PL from reglms grown by changing the beamparameters slightly As shown in reggure 4 the reglms grown with pressures of thecarrier gas (H2) of 200 and 700 mbar in the source give rise to quite similar spectra

Supersonic molecular-beam epitaxy of oligothiophene reglms 491

Figure 4 PL spectra at about 10 K of several a-4T reglms about 300 nm thick grown indiŒerent regimes of supersonic expansion Note the increase in the presence of C-type defects with decreasing dilution of the oligothiophene molecules in the beam

both corresponding to a very good degree of crystallinity A very signiregcant diŒer-ence however is the growth of the bands at about 20 600 19 200 and 17 800 cm 1These bands previously observed by the Umbach group (Hopmaier et al 1999) canbe assigned to the presence of defects corresponding to diŒerent orientation of themolecules close to the grains The spectra in reggure 4 show that these type of defectdecrease with increasing supersonicity of the beam and with decreasing velocityspread of the oligothiophene molecules in the beam A possible microscopic inter-pretation is that the highly supersonic beam could produce longer crystallites

We have also characterized the structure of some of the reglms Figure 5 shows theXRD pattern collected in BraggplusmnBrentano geometry The adopted powder geometryimplies that only crystalline planes parallel to the sample surface contribute to thecoherently diŒracted signal The observed remacrections are compatible (for positionsand relative intensities) with the (00l ) lines of the low-temperature a-T4 structurereported by Siegrist et al (1998) Since no other diŒracted peaks or signiregcant back-ground were collected we can suppose that the main part of the a-T4 thin reglm grewin the low-temperature crystalline form with the growth axis along [00l ] that is withthe c axis perpendicular to the sample surface The intensity of the line and therelatively low FWHM of these lines indicate that the reglms present a high orderthat is deregnitely much better than for reglms grown at similar thicknesses (greaterthan 300 nm) with the standard vacuum techniques

From this analysis however nothing deregnitive can be said about the growthmechanism and this is especially true for the in-plane growth For a more completepicture it is necessary to study pole reggures collected in four-circle geometry Figure 6shows the pole reggure for the remacrection at 233deg ((008) low temperature structure) Itis evident that the diŒracted signal is concentrated in the Aacute-tilting region about thenormal (Aacute ˆ 0deg) and no other measurable signals are visible up to Aacute ˆ 85deg Fromthe detail in reggure 7 (b) we can see that the (008) pole is centred about the surfacenormal but it is not sharp Figure 7 (d) shows an average Aacute-tilting plot of the

492 T Toccoli et al

Figure 5 XRD of an a-4T reglm 400 nm thick grown by SuMBE

diŒracted intensity obtained by averaging all the constant-iquest (variable-Aacute) patternsthat compose the pole reggure in reggure 7 (b) Here we can clearly see that growthdirections are spread over a rather uniform angular region about the surface normalwe can quantify this mosaicity by means of the half-width at half-maximum(HWHM) of the trend in reggure 7 (d) which is of about 3deg

To study the in-plane growth mechanism we need information along some crys-tallographic direction diŒerent from the c axis of the low-temperature a-T4 struc-ture The powder pattern simulated on the basis of the structural data of Siegrist etal 1998) indicates that the low-temperature a-T4 structure gives an intense (11113)plusmn(1113) remacrection (at 2sup3 ˆ 2021deg) which according to the c-axis growth mechanismshould be present at an angle of 6583deg with respect to the surface normal The (11113)plusmn(1113) pole reggure is shown in reggure 7 (b) whereas the corresponding average Aacute-tiltingplot is in reggure 7 (c) The pole reggure was collected in the range 55deg lt Aacute lt 75deg with amuch longer counting time than that used for the (008) pole reggure In spite of this thediŒracted signal is very weak but visible The ring-shaped maximum is for theexpected angle of 658deg and it is distributed over a relatively large Aacute-tilting rangeas observed for the (008) pole as can be seen in reggure 7 (c) the HWHM is about 3degas in the (008) pole reggure The symmetry around the iquest-rotation axis clearly indicatesthe presence of a regbre texture that is a preferred growth orientation along the c axisbut lack of any order in the growth plane This a very frequently observed feature innon-epitaxial thin reglms even if in this case observation was made quite di cult bythe considerable mosaicity and low scattering power of the material (at least withrespect to typical inorganic thin reglms) This is to our knowledge the regrst pole reggure

Supersonic molecular-beam epitaxy of oligothiophene reglms 493

Figure 6 (008) pole reggure

study of organic reglms of oligothiophenes and conregrms the high level of crystallinitythat could be achieved by SuMBE

4 ConclusionWe have shown that SuMBE is a very eŒective and promising technique to grow

thin reglms of oligothiophene with a high degree of ordering over an unprecedentedrange of thicknesses Optical structural and morphological characterizations give apicture coherent with the control that SuMBE promises for the growth process Weenvisage the possibility of controlling with this method the polymorphism byforcing kinetically preferential growth in reglms This opens up new opportunitiesfor studying the correlation between structure morphology and optical properties

494 T Toccoli et al

Figure 7 (a) (11113)plusmn(1113) pole reggure and (b) detail of the (008) pole reggure in reggure 2 (c) (d)average Aacute-tilting plot for (c) the (11113)plusmn(1113) and (d) the (008) pole reggures (auarbitrary units)

of the reglms as well as new possible developments in the organic based electronics andoptoelectronics

ACKNOWLEDGEMENTS

The valuable technical support of C Corradi and M Mazzola is acknowledgedThanks are due to A PodestaAacute for the atomic force micrograph We are grateful to LAversa M Ferrari P Milani S Ronchin and R Verucchi for stimulating discus-sions This work is supported by the `Atomic Detectors Development and optimiza-tion via synthesis of reglms by supersonic beams of clusters and organic materialsrsquoproject regnanced by Provincia Autonoma di Trento

ReferencesBiasioli F Boschetti A Barborini E Piseri P Milani P and Iannotta S 1997

Chem Phys Lett 270 115Bredas J L and Silbey R 1991 Conjugated Polymers The Novel Science and Technology

of Highly Conducting and Nonlinear Optically Active Materials (Dordrecht Kluwer)Farges J P 1994 Organic Conductors Fundamentals Applications (New York Marcel

Dekker)Gebauer W Sokolowski M and Umbach E 1998 Chem Phys 227 33Hopmeier M Gebauer W Oestreich M Sokolowski M Umbach E and Mahrt

R F 1999 Chem Phys Lett 314 9Iannotta S Toccoli T Boschetti A and Scardi P 2001 Synth Metals 122 221Iannotta S Toccoli T Ferrari M Ronchin S PodestaAacute A Milani P and

Scardi P 2000 Organic Photonic Materials and Devices II Proceedings of theSPIE Vol 3939 (Bellingham Washington SPIE) p 135

Milani P and Iannotta S 1999 Synthesis of Nanophase Materials by Cluster BeamDeposition (Berlin Springer)

PodestaAacute A Toccoli T Milani P Boschetti A and Iannotta S 2000 Surf SciLett 464 L673

Scoles G 1988 Atomic and Molecular Beam Methods (Oxford University Press)Siegrist T Kloc C Laudise R A Katz H E and Haddon R C 1998 Adv

Mater 379 10Taliani C and Gebauer W 1999 Handbook of Oligo and Polythiophenes (Weinheim

WileyplusmnVCN)Toccoli T van Opbergen M Boschetti A Ciullo G Ronchin S and Iannotta

S 1999 Phil Mag B 79 2157

Supersonic molecular-beam epitaxy of oligothiophene reglms 495

manipulator supports up to eight samples all of these can be heated independentlyand their temperature controlled (from room temperature to 700K) The substratesused in the present experiments are CaF2 or silica glass optical grade bought fromLOT Oriel After insertion in the UHV chamber they are outgassed at temperatureshigher than 600 K for more than 2 h The ionization of the compounds in the beamnecessary for the TOF spectroscopy is made by the fourth harmonic of a neody-mium-doped yttrium aluminium garnet laser or using a tuneable titanium sapphirelaser The polarization of the laser light is usually oriented horizontally and parallelto the beam direction changing the orientation of the polarization of the laser lightby means of a Pockel cell it is possible to study the degree of alignment of theinertial axis of molecules with respect to the beam direction The TOF mass spectro-meter is mounted properly aligned to the beam path to characterize the kineticenergy and the velocity spread of the molecules in the supersonic beam (Iannottaet al 2000)

As in previous experiments (Toccoli et al 1999) the oligothiophene used is the a-4T derived by the impurity present in a-terthiophene powder bought from TokyoKasey Company we also used material synthesized and purireged by two of thepresent authors (G Barbarella and G Sotgiu) obtaining very similar results eventhough the source operating conditions were obviously quite diŒerent The techniqueused to characterize the reglms grown by supersonic molecular-beam epitaxy(SuMBE) are ultravioletplusmnvisible absorption spectra PL at room temperature andat low temperatures (excitation with the 4579 nm line of the ArDagger laser) TM AFMand XRD

XRD patterns were collected in the BraggplusmnBrentano geometry using a RigakuPMG-VH powder diŒractometer equipped with a copper tube and a graphitecurved-crystal analyser in the secondary beam Owing to problems of illuminationof the sample surface at a small angle the peak intensity was not reliable below2sup3 ˆ 10deg

3 Results and discussionAs mentioned one of the major advantages of supersonic beams is that they

allow one to control and to characterize the energy state of the molecules in thebeam To this end as a regrst step in our experiments we carried out an analysis of theproperties of oligothiophene in a free-jet expansion in order to study the correlationbetween the molecular state and the type of expansion Once the beam parameterswere deregned we deposited several reglms of a-4T to determine the best depositionconditions and to correlate the optical and electronic properties of reglms to the stateof the molecules in the beam Changing the source conditions (the type and length ofnozzle) and the parameters of expansion (the type of gas dilution and source tem-perature) we observed that the kinetic energy of the molecules and their velocityspread in the beam are the most important parameters inmacruencing the growth pro-cess As previously reported (PodestaAacute et al 2000 Iannotta et al 2001) and discussedin more detail in the following the kinetic energy plays an important role giving theopportunity to overcome some of the limits inherent in growth governed by equili-brium thermodynamics so that more ordered and speciregc structures can be preparedEven the velocity spread of the molecules in the supersonic beam which aŒects thealignment of their backbone along the beam axis gives rise to reglms with diŒerentdefect densities

Supersonic molecular-beam epitaxy of oligothiophene reglms 487

Supersonic expansion can be modelled in a regrst approximation as an isoenthalpicprocess where the internal degrees of freedom of the molecules are subject to a veryfast cooling process and because of conservation of energy their kinetic energyincreases For compounds that are solid at room temperatures the vapour pressureis so low that it is almost impossible to produce a highly supersonic expansion thatrequires mean free paths in the source much smaller than the nozzle diameter (Scoles1988 Milani and Iannotta 1999) The problem is overcome by using a lighter carriergas in which the oligomer is seeded This solution gives the advantages of producinga good supersonic expansion with the important possibility of tuning the kineticenergy of the molecules by exploiting the so-called aerodynamic accelerations dueto the collision with the faster-macrowing lighter particles of the carrier gas By changingthe relative concentrations of the two species the heavier molecules if su cientlydiluted could be accelerated to the velocity of the lighter carrier gas We previouslyexploited such a process to accelerate C60 to about 60 eV (Biasioli et al 1997) Therange of energies achievable is of great interest for the physicalplusmnchemical processesinvolved in the growth of organic materials

We characterized our seeded supersonic expansion and reggure 1 shows the kineticenergy and the velocity spread for the a-4T using two diŒerent types of carrier gas

488 T Toccoli et al

Figure 1 Kinetic energy of a-4T molecules seeded in helium and H2 as a function of thepressure of the carrier gas With the same abscissa is reported the full width at half-maximum (FWHM) of peaks of a-4T in the TOF mass spectra This value gives anidea of the rotovibrational cooling process due to the supersonic expansion

(H2 and helium) at the same source temperature and as a function of dilution Thevelocity of the seeded oligomer can be easily varied from a few hundred millielec-tronvolts to about 20 eV while the cooling of the molecules increases as remacrected bythe narrowing of the full width at half-maximum of the TOF mass spectrometerpeak

On the basis of this kind of characterization we grew several reglms of a-4T and a-quinquethiophene (a-5T) using several diŒerent conditions of the supersonic expan-sion We then studied their optical response morphology and structure correlatingthem with the properties of the molecules in the beam Figure 2 reports the ultra-violetplusmnvisible spectra of a-4T and a-5T grown by SuMBE changing the degree ofseeding of the compounds in the carrier gas The reglms grown at the same depositionrate but in diŒerent source regimes (highly supersonic and quasi-eŒusive) showinteresting diŒerences in the form of the absorption band located at about 37 eVfor a-4T and at 355 eV for a-5T In particular reglms grown by SuMBE show amuch sharper band typical of the formation of H aggregates where the exciton is

Supersonic molecular-beam epitaxy of oligothiophene reglms 489

Figure 2 Ultravioletplusmnvisible spectrum of an a-4T reglm grown by SuMBE in the high super-sonic regime for which the thickness is estimated to be about 250 nm (ETHETH) togetherwith the ultravioletplusmnvisible spectrum of an a-4T reglm grown in an eŒusive regime with asimilar thickness (ETHETH) and the ultravioletplusmnvisible spectrum of an a-5T reglm grown bySuMBE in the high supersonic regime for which the thickness is estimated to be about300 nm (- - - -) (au arbitrary units)

delocalized on more than one molecule This is an indication that the structurespresent in the reglms grown with this technique are better ordered

A morphological characterization of the same types of reglm shows features thatcorrelate nicely to this picture Figure 3 shows the AFM picture of the same reglmsThe reglms grown using beams in a high supersonic regime typically show lamellae

490 T Toccoli et al

Figure 3 (a) AFM picture of an a-4T reglm 250 nm thick grown by SuMBE in the highlysupersonic regime (b) Picture of an a-4T reglm grown in the quasi-eŒusive regime

structures characterized by tiles parallel to the substrate When these lamellae areanalysed in detail a terraced structure where the steps correspond to the height ofthe a-4T molecules becomes clearly discernible The picture emerging from the reglmsgrown from beams in an eŒusive regime is very diŒerent Here the morphologyshows the typical feature of dendritic growth which when analysed by high-resolu-tion TM AFM shows only lobular shapes (PodestaAacute et al 2000)

A very sensitive way to assess the degree of ordering of the oligothiophene reglmsis to study the PL spectra at low temperatures Since the homogeneous contributionto the bandwith at low temperatures can be neglected these spectra give directinformation about the presence of defects and impurities in the reglms Our reglmsgrown by SuMBE show a well resolved vibronic structure where the FWHM ofthe 0plusmn0 band in the best reglms is about 40 cm 1 at 10 K We obtained particularlyinteresting results by comparing the PL from reglms grown by changing the beamparameters slightly As shown in reggure 4 the reglms grown with pressures of thecarrier gas (H2) of 200 and 700 mbar in the source give rise to quite similar spectra

Supersonic molecular-beam epitaxy of oligothiophene reglms 491

Figure 4 PL spectra at about 10 K of several a-4T reglms about 300 nm thick grown indiŒerent regimes of supersonic expansion Note the increase in the presence of C-type defects with decreasing dilution of the oligothiophene molecules in the beam

both corresponding to a very good degree of crystallinity A very signiregcant diŒer-ence however is the growth of the bands at about 20 600 19 200 and 17 800 cm 1These bands previously observed by the Umbach group (Hopmaier et al 1999) canbe assigned to the presence of defects corresponding to diŒerent orientation of themolecules close to the grains The spectra in reggure 4 show that these type of defectdecrease with increasing supersonicity of the beam and with decreasing velocityspread of the oligothiophene molecules in the beam A possible microscopic inter-pretation is that the highly supersonic beam could produce longer crystallites

We have also characterized the structure of some of the reglms Figure 5 shows theXRD pattern collected in BraggplusmnBrentano geometry The adopted powder geometryimplies that only crystalline planes parallel to the sample surface contribute to thecoherently diŒracted signal The observed remacrections are compatible (for positionsand relative intensities) with the (00l ) lines of the low-temperature a-T4 structurereported by Siegrist et al (1998) Since no other diŒracted peaks or signiregcant back-ground were collected we can suppose that the main part of the a-T4 thin reglm grewin the low-temperature crystalline form with the growth axis along [00l ] that is withthe c axis perpendicular to the sample surface The intensity of the line and therelatively low FWHM of these lines indicate that the reglms present a high orderthat is deregnitely much better than for reglms grown at similar thicknesses (greaterthan 300 nm) with the standard vacuum techniques

From this analysis however nothing deregnitive can be said about the growthmechanism and this is especially true for the in-plane growth For a more completepicture it is necessary to study pole reggures collected in four-circle geometry Figure 6shows the pole reggure for the remacrection at 233deg ((008) low temperature structure) Itis evident that the diŒracted signal is concentrated in the Aacute-tilting region about thenormal (Aacute ˆ 0deg) and no other measurable signals are visible up to Aacute ˆ 85deg Fromthe detail in reggure 7 (b) we can see that the (008) pole is centred about the surfacenormal but it is not sharp Figure 7 (d) shows an average Aacute-tilting plot of the

492 T Toccoli et al

Figure 5 XRD of an a-4T reglm 400 nm thick grown by SuMBE

diŒracted intensity obtained by averaging all the constant-iquest (variable-Aacute) patternsthat compose the pole reggure in reggure 7 (b) Here we can clearly see that growthdirections are spread over a rather uniform angular region about the surface normalwe can quantify this mosaicity by means of the half-width at half-maximum(HWHM) of the trend in reggure 7 (d) which is of about 3deg

To study the in-plane growth mechanism we need information along some crys-tallographic direction diŒerent from the c axis of the low-temperature a-T4 struc-ture The powder pattern simulated on the basis of the structural data of Siegrist etal 1998) indicates that the low-temperature a-T4 structure gives an intense (11113)plusmn(1113) remacrection (at 2sup3 ˆ 2021deg) which according to the c-axis growth mechanismshould be present at an angle of 6583deg with respect to the surface normal The (11113)plusmn(1113) pole reggure is shown in reggure 7 (b) whereas the corresponding average Aacute-tiltingplot is in reggure 7 (c) The pole reggure was collected in the range 55deg lt Aacute lt 75deg with amuch longer counting time than that used for the (008) pole reggure In spite of this thediŒracted signal is very weak but visible The ring-shaped maximum is for theexpected angle of 658deg and it is distributed over a relatively large Aacute-tilting rangeas observed for the (008) pole as can be seen in reggure 7 (c) the HWHM is about 3degas in the (008) pole reggure The symmetry around the iquest-rotation axis clearly indicatesthe presence of a regbre texture that is a preferred growth orientation along the c axisbut lack of any order in the growth plane This a very frequently observed feature innon-epitaxial thin reglms even if in this case observation was made quite di cult bythe considerable mosaicity and low scattering power of the material (at least withrespect to typical inorganic thin reglms) This is to our knowledge the regrst pole reggure

Supersonic molecular-beam epitaxy of oligothiophene reglms 493

Figure 6 (008) pole reggure

study of organic reglms of oligothiophenes and conregrms the high level of crystallinitythat could be achieved by SuMBE

4 ConclusionWe have shown that SuMBE is a very eŒective and promising technique to grow

thin reglms of oligothiophene with a high degree of ordering over an unprecedentedrange of thicknesses Optical structural and morphological characterizations give apicture coherent with the control that SuMBE promises for the growth process Weenvisage the possibility of controlling with this method the polymorphism byforcing kinetically preferential growth in reglms This opens up new opportunitiesfor studying the correlation between structure morphology and optical properties

494 T Toccoli et al

Figure 7 (a) (11113)plusmn(1113) pole reggure and (b) detail of the (008) pole reggure in reggure 2 (c) (d)average Aacute-tilting plot for (c) the (11113)plusmn(1113) and (d) the (008) pole reggures (auarbitrary units)

of the reglms as well as new possible developments in the organic based electronics andoptoelectronics

ACKNOWLEDGEMENTS

The valuable technical support of C Corradi and M Mazzola is acknowledgedThanks are due to A PodestaAacute for the atomic force micrograph We are grateful to LAversa M Ferrari P Milani S Ronchin and R Verucchi for stimulating discus-sions This work is supported by the `Atomic Detectors Development and optimiza-tion via synthesis of reglms by supersonic beams of clusters and organic materialsrsquoproject regnanced by Provincia Autonoma di Trento

ReferencesBiasioli F Boschetti A Barborini E Piseri P Milani P and Iannotta S 1997

Chem Phys Lett 270 115Bredas J L and Silbey R 1991 Conjugated Polymers The Novel Science and Technology

of Highly Conducting and Nonlinear Optically Active Materials (Dordrecht Kluwer)Farges J P 1994 Organic Conductors Fundamentals Applications (New York Marcel

Dekker)Gebauer W Sokolowski M and Umbach E 1998 Chem Phys 227 33Hopmeier M Gebauer W Oestreich M Sokolowski M Umbach E and Mahrt

R F 1999 Chem Phys Lett 314 9Iannotta S Toccoli T Boschetti A and Scardi P 2001 Synth Metals 122 221Iannotta S Toccoli T Ferrari M Ronchin S PodestaAacute A Milani P and

Scardi P 2000 Organic Photonic Materials and Devices II Proceedings of theSPIE Vol 3939 (Bellingham Washington SPIE) p 135

Milani P and Iannotta S 1999 Synthesis of Nanophase Materials by Cluster BeamDeposition (Berlin Springer)

PodestaAacute A Toccoli T Milani P Boschetti A and Iannotta S 2000 Surf SciLett 464 L673

Scoles G 1988 Atomic and Molecular Beam Methods (Oxford University Press)Siegrist T Kloc C Laudise R A Katz H E and Haddon R C 1998 Adv

Mater 379 10Taliani C and Gebauer W 1999 Handbook of Oligo and Polythiophenes (Weinheim

WileyplusmnVCN)Toccoli T van Opbergen M Boschetti A Ciullo G Ronchin S and Iannotta

S 1999 Phil Mag B 79 2157

Supersonic molecular-beam epitaxy of oligothiophene reglms 495

Supersonic expansion can be modelled in a regrst approximation as an isoenthalpicprocess where the internal degrees of freedom of the molecules are subject to a veryfast cooling process and because of conservation of energy their kinetic energyincreases For compounds that are solid at room temperatures the vapour pressureis so low that it is almost impossible to produce a highly supersonic expansion thatrequires mean free paths in the source much smaller than the nozzle diameter (Scoles1988 Milani and Iannotta 1999) The problem is overcome by using a lighter carriergas in which the oligomer is seeded This solution gives the advantages of producinga good supersonic expansion with the important possibility of tuning the kineticenergy of the molecules by exploiting the so-called aerodynamic accelerations dueto the collision with the faster-macrowing lighter particles of the carrier gas By changingthe relative concentrations of the two species the heavier molecules if su cientlydiluted could be accelerated to the velocity of the lighter carrier gas We previouslyexploited such a process to accelerate C60 to about 60 eV (Biasioli et al 1997) Therange of energies achievable is of great interest for the physicalplusmnchemical processesinvolved in the growth of organic materials

We characterized our seeded supersonic expansion and reggure 1 shows the kineticenergy and the velocity spread for the a-4T using two diŒerent types of carrier gas

488 T Toccoli et al

Figure 1 Kinetic energy of a-4T molecules seeded in helium and H2 as a function of thepressure of the carrier gas With the same abscissa is reported the full width at half-maximum (FWHM) of peaks of a-4T in the TOF mass spectra This value gives anidea of the rotovibrational cooling process due to the supersonic expansion

(H2 and helium) at the same source temperature and as a function of dilution Thevelocity of the seeded oligomer can be easily varied from a few hundred millielec-tronvolts to about 20 eV while the cooling of the molecules increases as remacrected bythe narrowing of the full width at half-maximum of the TOF mass spectrometerpeak

On the basis of this kind of characterization we grew several reglms of a-4T and a-quinquethiophene (a-5T) using several diŒerent conditions of the supersonic expan-sion We then studied their optical response morphology and structure correlatingthem with the properties of the molecules in the beam Figure 2 reports the ultra-violetplusmnvisible spectra of a-4T and a-5T grown by SuMBE changing the degree ofseeding of the compounds in the carrier gas The reglms grown at the same depositionrate but in diŒerent source regimes (highly supersonic and quasi-eŒusive) showinteresting diŒerences in the form of the absorption band located at about 37 eVfor a-4T and at 355 eV for a-5T In particular reglms grown by SuMBE show amuch sharper band typical of the formation of H aggregates where the exciton is

Supersonic molecular-beam epitaxy of oligothiophene reglms 489

Figure 2 Ultravioletplusmnvisible spectrum of an a-4T reglm grown by SuMBE in the high super-sonic regime for which the thickness is estimated to be about 250 nm (ETHETH) togetherwith the ultravioletplusmnvisible spectrum of an a-4T reglm grown in an eŒusive regime with asimilar thickness (ETHETH) and the ultravioletplusmnvisible spectrum of an a-5T reglm grown bySuMBE in the high supersonic regime for which the thickness is estimated to be about300 nm (- - - -) (au arbitrary units)

delocalized on more than one molecule This is an indication that the structurespresent in the reglms grown with this technique are better ordered

A morphological characterization of the same types of reglm shows features thatcorrelate nicely to this picture Figure 3 shows the AFM picture of the same reglmsThe reglms grown using beams in a high supersonic regime typically show lamellae

490 T Toccoli et al

Figure 3 (a) AFM picture of an a-4T reglm 250 nm thick grown by SuMBE in the highlysupersonic regime (b) Picture of an a-4T reglm grown in the quasi-eŒusive regime

structures characterized by tiles parallel to the substrate When these lamellae areanalysed in detail a terraced structure where the steps correspond to the height ofthe a-4T molecules becomes clearly discernible The picture emerging from the reglmsgrown from beams in an eŒusive regime is very diŒerent Here the morphologyshows the typical feature of dendritic growth which when analysed by high-resolu-tion TM AFM shows only lobular shapes (PodestaAacute et al 2000)

A very sensitive way to assess the degree of ordering of the oligothiophene reglmsis to study the PL spectra at low temperatures Since the homogeneous contributionto the bandwith at low temperatures can be neglected these spectra give directinformation about the presence of defects and impurities in the reglms Our reglmsgrown by SuMBE show a well resolved vibronic structure where the FWHM ofthe 0plusmn0 band in the best reglms is about 40 cm 1 at 10 K We obtained particularlyinteresting results by comparing the PL from reglms grown by changing the beamparameters slightly As shown in reggure 4 the reglms grown with pressures of thecarrier gas (H2) of 200 and 700 mbar in the source give rise to quite similar spectra

Supersonic molecular-beam epitaxy of oligothiophene reglms 491

Figure 4 PL spectra at about 10 K of several a-4T reglms about 300 nm thick grown indiŒerent regimes of supersonic expansion Note the increase in the presence of C-type defects with decreasing dilution of the oligothiophene molecules in the beam

both corresponding to a very good degree of crystallinity A very signiregcant diŒer-ence however is the growth of the bands at about 20 600 19 200 and 17 800 cm 1These bands previously observed by the Umbach group (Hopmaier et al 1999) canbe assigned to the presence of defects corresponding to diŒerent orientation of themolecules close to the grains The spectra in reggure 4 show that these type of defectdecrease with increasing supersonicity of the beam and with decreasing velocityspread of the oligothiophene molecules in the beam A possible microscopic inter-pretation is that the highly supersonic beam could produce longer crystallites

We have also characterized the structure of some of the reglms Figure 5 shows theXRD pattern collected in BraggplusmnBrentano geometry The adopted powder geometryimplies that only crystalline planes parallel to the sample surface contribute to thecoherently diŒracted signal The observed remacrections are compatible (for positionsand relative intensities) with the (00l ) lines of the low-temperature a-T4 structurereported by Siegrist et al (1998) Since no other diŒracted peaks or signiregcant back-ground were collected we can suppose that the main part of the a-T4 thin reglm grewin the low-temperature crystalline form with the growth axis along [00l ] that is withthe c axis perpendicular to the sample surface The intensity of the line and therelatively low FWHM of these lines indicate that the reglms present a high orderthat is deregnitely much better than for reglms grown at similar thicknesses (greaterthan 300 nm) with the standard vacuum techniques

From this analysis however nothing deregnitive can be said about the growthmechanism and this is especially true for the in-plane growth For a more completepicture it is necessary to study pole reggures collected in four-circle geometry Figure 6shows the pole reggure for the remacrection at 233deg ((008) low temperature structure) Itis evident that the diŒracted signal is concentrated in the Aacute-tilting region about thenormal (Aacute ˆ 0deg) and no other measurable signals are visible up to Aacute ˆ 85deg Fromthe detail in reggure 7 (b) we can see that the (008) pole is centred about the surfacenormal but it is not sharp Figure 7 (d) shows an average Aacute-tilting plot of the

492 T Toccoli et al

Figure 5 XRD of an a-4T reglm 400 nm thick grown by SuMBE

diŒracted intensity obtained by averaging all the constant-iquest (variable-Aacute) patternsthat compose the pole reggure in reggure 7 (b) Here we can clearly see that growthdirections are spread over a rather uniform angular region about the surface normalwe can quantify this mosaicity by means of the half-width at half-maximum(HWHM) of the trend in reggure 7 (d) which is of about 3deg

To study the in-plane growth mechanism we need information along some crys-tallographic direction diŒerent from the c axis of the low-temperature a-T4 struc-ture The powder pattern simulated on the basis of the structural data of Siegrist etal 1998) indicates that the low-temperature a-T4 structure gives an intense (11113)plusmn(1113) remacrection (at 2sup3 ˆ 2021deg) which according to the c-axis growth mechanismshould be present at an angle of 6583deg with respect to the surface normal The (11113)plusmn(1113) pole reggure is shown in reggure 7 (b) whereas the corresponding average Aacute-tiltingplot is in reggure 7 (c) The pole reggure was collected in the range 55deg lt Aacute lt 75deg with amuch longer counting time than that used for the (008) pole reggure In spite of this thediŒracted signal is very weak but visible The ring-shaped maximum is for theexpected angle of 658deg and it is distributed over a relatively large Aacute-tilting rangeas observed for the (008) pole as can be seen in reggure 7 (c) the HWHM is about 3degas in the (008) pole reggure The symmetry around the iquest-rotation axis clearly indicatesthe presence of a regbre texture that is a preferred growth orientation along the c axisbut lack of any order in the growth plane This a very frequently observed feature innon-epitaxial thin reglms even if in this case observation was made quite di cult bythe considerable mosaicity and low scattering power of the material (at least withrespect to typical inorganic thin reglms) This is to our knowledge the regrst pole reggure

Supersonic molecular-beam epitaxy of oligothiophene reglms 493

Figure 6 (008) pole reggure

study of organic reglms of oligothiophenes and conregrms the high level of crystallinitythat could be achieved by SuMBE

4 ConclusionWe have shown that SuMBE is a very eŒective and promising technique to grow

thin reglms of oligothiophene with a high degree of ordering over an unprecedentedrange of thicknesses Optical structural and morphological characterizations give apicture coherent with the control that SuMBE promises for the growth process Weenvisage the possibility of controlling with this method the polymorphism byforcing kinetically preferential growth in reglms This opens up new opportunitiesfor studying the correlation between structure morphology and optical properties

494 T Toccoli et al

Figure 7 (a) (11113)plusmn(1113) pole reggure and (b) detail of the (008) pole reggure in reggure 2 (c) (d)average Aacute-tilting plot for (c) the (11113)plusmn(1113) and (d) the (008) pole reggures (auarbitrary units)

of the reglms as well as new possible developments in the organic based electronics andoptoelectronics

ACKNOWLEDGEMENTS

The valuable technical support of C Corradi and M Mazzola is acknowledgedThanks are due to A PodestaAacute for the atomic force micrograph We are grateful to LAversa M Ferrari P Milani S Ronchin and R Verucchi for stimulating discus-sions This work is supported by the `Atomic Detectors Development and optimiza-tion via synthesis of reglms by supersonic beams of clusters and organic materialsrsquoproject regnanced by Provincia Autonoma di Trento

ReferencesBiasioli F Boschetti A Barborini E Piseri P Milani P and Iannotta S 1997

Chem Phys Lett 270 115Bredas J L and Silbey R 1991 Conjugated Polymers The Novel Science and Technology

of Highly Conducting and Nonlinear Optically Active Materials (Dordrecht Kluwer)Farges J P 1994 Organic Conductors Fundamentals Applications (New York Marcel

Dekker)Gebauer W Sokolowski M and Umbach E 1998 Chem Phys 227 33Hopmeier M Gebauer W Oestreich M Sokolowski M Umbach E and Mahrt

R F 1999 Chem Phys Lett 314 9Iannotta S Toccoli T Boschetti A and Scardi P 2001 Synth Metals 122 221Iannotta S Toccoli T Ferrari M Ronchin S PodestaAacute A Milani P and

Scardi P 2000 Organic Photonic Materials and Devices II Proceedings of theSPIE Vol 3939 (Bellingham Washington SPIE) p 135

Milani P and Iannotta S 1999 Synthesis of Nanophase Materials by Cluster BeamDeposition (Berlin Springer)

PodestaAacute A Toccoli T Milani P Boschetti A and Iannotta S 2000 Surf SciLett 464 L673

Scoles G 1988 Atomic and Molecular Beam Methods (Oxford University Press)Siegrist T Kloc C Laudise R A Katz H E and Haddon R C 1998 Adv

Mater 379 10Taliani C and Gebauer W 1999 Handbook of Oligo and Polythiophenes (Weinheim

WileyplusmnVCN)Toccoli T van Opbergen M Boschetti A Ciullo G Ronchin S and Iannotta

S 1999 Phil Mag B 79 2157

Supersonic molecular-beam epitaxy of oligothiophene reglms 495

(H2 and helium) at the same source temperature and as a function of dilution Thevelocity of the seeded oligomer can be easily varied from a few hundred millielec-tronvolts to about 20 eV while the cooling of the molecules increases as remacrected bythe narrowing of the full width at half-maximum of the TOF mass spectrometerpeak

On the basis of this kind of characterization we grew several reglms of a-4T and a-quinquethiophene (a-5T) using several diŒerent conditions of the supersonic expan-sion We then studied their optical response morphology and structure correlatingthem with the properties of the molecules in the beam Figure 2 reports the ultra-violetplusmnvisible spectra of a-4T and a-5T grown by SuMBE changing the degree ofseeding of the compounds in the carrier gas The reglms grown at the same depositionrate but in diŒerent source regimes (highly supersonic and quasi-eŒusive) showinteresting diŒerences in the form of the absorption band located at about 37 eVfor a-4T and at 355 eV for a-5T In particular reglms grown by SuMBE show amuch sharper band typical of the formation of H aggregates where the exciton is

Supersonic molecular-beam epitaxy of oligothiophene reglms 489

Figure 2 Ultravioletplusmnvisible spectrum of an a-4T reglm grown by SuMBE in the high super-sonic regime for which the thickness is estimated to be about 250 nm (ETHETH) togetherwith the ultravioletplusmnvisible spectrum of an a-4T reglm grown in an eŒusive regime with asimilar thickness (ETHETH) and the ultravioletplusmnvisible spectrum of an a-5T reglm grown bySuMBE in the high supersonic regime for which the thickness is estimated to be about300 nm (- - - -) (au arbitrary units)

delocalized on more than one molecule This is an indication that the structurespresent in the reglms grown with this technique are better ordered

A morphological characterization of the same types of reglm shows features thatcorrelate nicely to this picture Figure 3 shows the AFM picture of the same reglmsThe reglms grown using beams in a high supersonic regime typically show lamellae

490 T Toccoli et al

Figure 3 (a) AFM picture of an a-4T reglm 250 nm thick grown by SuMBE in the highlysupersonic regime (b) Picture of an a-4T reglm grown in the quasi-eŒusive regime

structures characterized by tiles parallel to the substrate When these lamellae areanalysed in detail a terraced structure where the steps correspond to the height ofthe a-4T molecules becomes clearly discernible The picture emerging from the reglmsgrown from beams in an eŒusive regime is very diŒerent Here the morphologyshows the typical feature of dendritic growth which when analysed by high-resolu-tion TM AFM shows only lobular shapes (PodestaAacute et al 2000)

A very sensitive way to assess the degree of ordering of the oligothiophene reglmsis to study the PL spectra at low temperatures Since the homogeneous contributionto the bandwith at low temperatures can be neglected these spectra give directinformation about the presence of defects and impurities in the reglms Our reglmsgrown by SuMBE show a well resolved vibronic structure where the FWHM ofthe 0plusmn0 band in the best reglms is about 40 cm 1 at 10 K We obtained particularlyinteresting results by comparing the PL from reglms grown by changing the beamparameters slightly As shown in reggure 4 the reglms grown with pressures of thecarrier gas (H2) of 200 and 700 mbar in the source give rise to quite similar spectra

Supersonic molecular-beam epitaxy of oligothiophene reglms 491

Figure 4 PL spectra at about 10 K of several a-4T reglms about 300 nm thick grown indiŒerent regimes of supersonic expansion Note the increase in the presence of C-type defects with decreasing dilution of the oligothiophene molecules in the beam

both corresponding to a very good degree of crystallinity A very signiregcant diŒer-ence however is the growth of the bands at about 20 600 19 200 and 17 800 cm 1These bands previously observed by the Umbach group (Hopmaier et al 1999) canbe assigned to the presence of defects corresponding to diŒerent orientation of themolecules close to the grains The spectra in reggure 4 show that these type of defectdecrease with increasing supersonicity of the beam and with decreasing velocityspread of the oligothiophene molecules in the beam A possible microscopic inter-pretation is that the highly supersonic beam could produce longer crystallites

We have also characterized the structure of some of the reglms Figure 5 shows theXRD pattern collected in BraggplusmnBrentano geometry The adopted powder geometryimplies that only crystalline planes parallel to the sample surface contribute to thecoherently diŒracted signal The observed remacrections are compatible (for positionsand relative intensities) with the (00l ) lines of the low-temperature a-T4 structurereported by Siegrist et al (1998) Since no other diŒracted peaks or signiregcant back-ground were collected we can suppose that the main part of the a-T4 thin reglm grewin the low-temperature crystalline form with the growth axis along [00l ] that is withthe c axis perpendicular to the sample surface The intensity of the line and therelatively low FWHM of these lines indicate that the reglms present a high orderthat is deregnitely much better than for reglms grown at similar thicknesses (greaterthan 300 nm) with the standard vacuum techniques

From this analysis however nothing deregnitive can be said about the growthmechanism and this is especially true for the in-plane growth For a more completepicture it is necessary to study pole reggures collected in four-circle geometry Figure 6shows the pole reggure for the remacrection at 233deg ((008) low temperature structure) Itis evident that the diŒracted signal is concentrated in the Aacute-tilting region about thenormal (Aacute ˆ 0deg) and no other measurable signals are visible up to Aacute ˆ 85deg Fromthe detail in reggure 7 (b) we can see that the (008) pole is centred about the surfacenormal but it is not sharp Figure 7 (d) shows an average Aacute-tilting plot of the

492 T Toccoli et al

Figure 5 XRD of an a-4T reglm 400 nm thick grown by SuMBE

diŒracted intensity obtained by averaging all the constant-iquest (variable-Aacute) patternsthat compose the pole reggure in reggure 7 (b) Here we can clearly see that growthdirections are spread over a rather uniform angular region about the surface normalwe can quantify this mosaicity by means of the half-width at half-maximum(HWHM) of the trend in reggure 7 (d) which is of about 3deg

To study the in-plane growth mechanism we need information along some crys-tallographic direction diŒerent from the c axis of the low-temperature a-T4 struc-ture The powder pattern simulated on the basis of the structural data of Siegrist etal 1998) indicates that the low-temperature a-T4 structure gives an intense (11113)plusmn(1113) remacrection (at 2sup3 ˆ 2021deg) which according to the c-axis growth mechanismshould be present at an angle of 6583deg with respect to the surface normal The (11113)plusmn(1113) pole reggure is shown in reggure 7 (b) whereas the corresponding average Aacute-tiltingplot is in reggure 7 (c) The pole reggure was collected in the range 55deg lt Aacute lt 75deg with amuch longer counting time than that used for the (008) pole reggure In spite of this thediŒracted signal is very weak but visible The ring-shaped maximum is for theexpected angle of 658deg and it is distributed over a relatively large Aacute-tilting rangeas observed for the (008) pole as can be seen in reggure 7 (c) the HWHM is about 3degas in the (008) pole reggure The symmetry around the iquest-rotation axis clearly indicatesthe presence of a regbre texture that is a preferred growth orientation along the c axisbut lack of any order in the growth plane This a very frequently observed feature innon-epitaxial thin reglms even if in this case observation was made quite di cult bythe considerable mosaicity and low scattering power of the material (at least withrespect to typical inorganic thin reglms) This is to our knowledge the regrst pole reggure

Supersonic molecular-beam epitaxy of oligothiophene reglms 493

Figure 6 (008) pole reggure

study of organic reglms of oligothiophenes and conregrms the high level of crystallinitythat could be achieved by SuMBE

4 ConclusionWe have shown that SuMBE is a very eŒective and promising technique to grow

thin reglms of oligothiophene with a high degree of ordering over an unprecedentedrange of thicknesses Optical structural and morphological characterizations give apicture coherent with the control that SuMBE promises for the growth process Weenvisage the possibility of controlling with this method the polymorphism byforcing kinetically preferential growth in reglms This opens up new opportunitiesfor studying the correlation between structure morphology and optical properties

494 T Toccoli et al

Figure 7 (a) (11113)plusmn(1113) pole reggure and (b) detail of the (008) pole reggure in reggure 2 (c) (d)average Aacute-tilting plot for (c) the (11113)plusmn(1113) and (d) the (008) pole reggures (auarbitrary units)

of the reglms as well as new possible developments in the organic based electronics andoptoelectronics

ACKNOWLEDGEMENTS

The valuable technical support of C Corradi and M Mazzola is acknowledgedThanks are due to A PodestaAacute for the atomic force micrograph We are grateful to LAversa M Ferrari P Milani S Ronchin and R Verucchi for stimulating discus-sions This work is supported by the `Atomic Detectors Development and optimiza-tion via synthesis of reglms by supersonic beams of clusters and organic materialsrsquoproject regnanced by Provincia Autonoma di Trento

ReferencesBiasioli F Boschetti A Barborini E Piseri P Milani P and Iannotta S 1997

Chem Phys Lett 270 115Bredas J L and Silbey R 1991 Conjugated Polymers The Novel Science and Technology

of Highly Conducting and Nonlinear Optically Active Materials (Dordrecht Kluwer)Farges J P 1994 Organic Conductors Fundamentals Applications (New York Marcel

Dekker)Gebauer W Sokolowski M and Umbach E 1998 Chem Phys 227 33Hopmeier M Gebauer W Oestreich M Sokolowski M Umbach E and Mahrt

R F 1999 Chem Phys Lett 314 9Iannotta S Toccoli T Boschetti A and Scardi P 2001 Synth Metals 122 221Iannotta S Toccoli T Ferrari M Ronchin S PodestaAacute A Milani P and

Scardi P 2000 Organic Photonic Materials and Devices II Proceedings of theSPIE Vol 3939 (Bellingham Washington SPIE) p 135

Milani P and Iannotta S 1999 Synthesis of Nanophase Materials by Cluster BeamDeposition (Berlin Springer)

PodestaAacute A Toccoli T Milani P Boschetti A and Iannotta S 2000 Surf SciLett 464 L673

Scoles G 1988 Atomic and Molecular Beam Methods (Oxford University Press)Siegrist T Kloc C Laudise R A Katz H E and Haddon R C 1998 Adv

Mater 379 10Taliani C and Gebauer W 1999 Handbook of Oligo and Polythiophenes (Weinheim

WileyplusmnVCN)Toccoli T van Opbergen M Boschetti A Ciullo G Ronchin S and Iannotta

S 1999 Phil Mag B 79 2157

Supersonic molecular-beam epitaxy of oligothiophene reglms 495

delocalized on more than one molecule This is an indication that the structurespresent in the reglms grown with this technique are better ordered

A morphological characterization of the same types of reglm shows features thatcorrelate nicely to this picture Figure 3 shows the AFM picture of the same reglmsThe reglms grown using beams in a high supersonic regime typically show lamellae

490 T Toccoli et al

Figure 3 (a) AFM picture of an a-4T reglm 250 nm thick grown by SuMBE in the highlysupersonic regime (b) Picture of an a-4T reglm grown in the quasi-eŒusive regime

structures characterized by tiles parallel to the substrate When these lamellae areanalysed in detail a terraced structure where the steps correspond to the height ofthe a-4T molecules becomes clearly discernible The picture emerging from the reglmsgrown from beams in an eŒusive regime is very diŒerent Here the morphologyshows the typical feature of dendritic growth which when analysed by high-resolu-tion TM AFM shows only lobular shapes (PodestaAacute et al 2000)

A very sensitive way to assess the degree of ordering of the oligothiophene reglmsis to study the PL spectra at low temperatures Since the homogeneous contributionto the bandwith at low temperatures can be neglected these spectra give directinformation about the presence of defects and impurities in the reglms Our reglmsgrown by SuMBE show a well resolved vibronic structure where the FWHM ofthe 0plusmn0 band in the best reglms is about 40 cm 1 at 10 K We obtained particularlyinteresting results by comparing the PL from reglms grown by changing the beamparameters slightly As shown in reggure 4 the reglms grown with pressures of thecarrier gas (H2) of 200 and 700 mbar in the source give rise to quite similar spectra

Supersonic molecular-beam epitaxy of oligothiophene reglms 491

Figure 4 PL spectra at about 10 K of several a-4T reglms about 300 nm thick grown indiŒerent regimes of supersonic expansion Note the increase in the presence of C-type defects with decreasing dilution of the oligothiophene molecules in the beam

both corresponding to a very good degree of crystallinity A very signiregcant diŒer-ence however is the growth of the bands at about 20 600 19 200 and 17 800 cm 1These bands previously observed by the Umbach group (Hopmaier et al 1999) canbe assigned to the presence of defects corresponding to diŒerent orientation of themolecules close to the grains The spectra in reggure 4 show that these type of defectdecrease with increasing supersonicity of the beam and with decreasing velocityspread of the oligothiophene molecules in the beam A possible microscopic inter-pretation is that the highly supersonic beam could produce longer crystallites

We have also characterized the structure of some of the reglms Figure 5 shows theXRD pattern collected in BraggplusmnBrentano geometry The adopted powder geometryimplies that only crystalline planes parallel to the sample surface contribute to thecoherently diŒracted signal The observed remacrections are compatible (for positionsand relative intensities) with the (00l ) lines of the low-temperature a-T4 structurereported by Siegrist et al (1998) Since no other diŒracted peaks or signiregcant back-ground were collected we can suppose that the main part of the a-T4 thin reglm grewin the low-temperature crystalline form with the growth axis along [00l ] that is withthe c axis perpendicular to the sample surface The intensity of the line and therelatively low FWHM of these lines indicate that the reglms present a high orderthat is deregnitely much better than for reglms grown at similar thicknesses (greaterthan 300 nm) with the standard vacuum techniques

From this analysis however nothing deregnitive can be said about the growthmechanism and this is especially true for the in-plane growth For a more completepicture it is necessary to study pole reggures collected in four-circle geometry Figure 6shows the pole reggure for the remacrection at 233deg ((008) low temperature structure) Itis evident that the diŒracted signal is concentrated in the Aacute-tilting region about thenormal (Aacute ˆ 0deg) and no other measurable signals are visible up to Aacute ˆ 85deg Fromthe detail in reggure 7 (b) we can see that the (008) pole is centred about the surfacenormal but it is not sharp Figure 7 (d) shows an average Aacute-tilting plot of the

492 T Toccoli et al

Figure 5 XRD of an a-4T reglm 400 nm thick grown by SuMBE

diŒracted intensity obtained by averaging all the constant-iquest (variable-Aacute) patternsthat compose the pole reggure in reggure 7 (b) Here we can clearly see that growthdirections are spread over a rather uniform angular region about the surface normalwe can quantify this mosaicity by means of the half-width at half-maximum(HWHM) of the trend in reggure 7 (d) which is of about 3deg

To study the in-plane growth mechanism we need information along some crys-tallographic direction diŒerent from the c axis of the low-temperature a-T4 struc-ture The powder pattern simulated on the basis of the structural data of Siegrist etal 1998) indicates that the low-temperature a-T4 structure gives an intense (11113)plusmn(1113) remacrection (at 2sup3 ˆ 2021deg) which according to the c-axis growth mechanismshould be present at an angle of 6583deg with respect to the surface normal The (11113)plusmn(1113) pole reggure is shown in reggure 7 (b) whereas the corresponding average Aacute-tiltingplot is in reggure 7 (c) The pole reggure was collected in the range 55deg lt Aacute lt 75deg with amuch longer counting time than that used for the (008) pole reggure In spite of this thediŒracted signal is very weak but visible The ring-shaped maximum is for theexpected angle of 658deg and it is distributed over a relatively large Aacute-tilting rangeas observed for the (008) pole as can be seen in reggure 7 (c) the HWHM is about 3degas in the (008) pole reggure The symmetry around the iquest-rotation axis clearly indicatesthe presence of a regbre texture that is a preferred growth orientation along the c axisbut lack of any order in the growth plane This a very frequently observed feature innon-epitaxial thin reglms even if in this case observation was made quite di cult bythe considerable mosaicity and low scattering power of the material (at least withrespect to typical inorganic thin reglms) This is to our knowledge the regrst pole reggure

Supersonic molecular-beam epitaxy of oligothiophene reglms 493

Figure 6 (008) pole reggure

study of organic reglms of oligothiophenes and conregrms the high level of crystallinitythat could be achieved by SuMBE

4 ConclusionWe have shown that SuMBE is a very eŒective and promising technique to grow

thin reglms of oligothiophene with a high degree of ordering over an unprecedentedrange of thicknesses Optical structural and morphological characterizations give apicture coherent with the control that SuMBE promises for the growth process Weenvisage the possibility of controlling with this method the polymorphism byforcing kinetically preferential growth in reglms This opens up new opportunitiesfor studying the correlation between structure morphology and optical properties

494 T Toccoli et al

Figure 7 (a) (11113)plusmn(1113) pole reggure and (b) detail of the (008) pole reggure in reggure 2 (c) (d)average Aacute-tilting plot for (c) the (11113)plusmn(1113) and (d) the (008) pole reggures (auarbitrary units)

of the reglms as well as new possible developments in the organic based electronics andoptoelectronics

ACKNOWLEDGEMENTS

The valuable technical support of C Corradi and M Mazzola is acknowledgedThanks are due to A PodestaAacute for the atomic force micrograph We are grateful to LAversa M Ferrari P Milani S Ronchin and R Verucchi for stimulating discus-sions This work is supported by the `Atomic Detectors Development and optimiza-tion via synthesis of reglms by supersonic beams of clusters and organic materialsrsquoproject regnanced by Provincia Autonoma di Trento

ReferencesBiasioli F Boschetti A Barborini E Piseri P Milani P and Iannotta S 1997

Chem Phys Lett 270 115Bredas J L and Silbey R 1991 Conjugated Polymers The Novel Science and Technology

of Highly Conducting and Nonlinear Optically Active Materials (Dordrecht Kluwer)Farges J P 1994 Organic Conductors Fundamentals Applications (New York Marcel

Dekker)Gebauer W Sokolowski M and Umbach E 1998 Chem Phys 227 33Hopmeier M Gebauer W Oestreich M Sokolowski M Umbach E and Mahrt

R F 1999 Chem Phys Lett 314 9Iannotta S Toccoli T Boschetti A and Scardi P 2001 Synth Metals 122 221Iannotta S Toccoli T Ferrari M Ronchin S PodestaAacute A Milani P and

Scardi P 2000 Organic Photonic Materials and Devices II Proceedings of theSPIE Vol 3939 (Bellingham Washington SPIE) p 135

Milani P and Iannotta S 1999 Synthesis of Nanophase Materials by Cluster BeamDeposition (Berlin Springer)

PodestaAacute A Toccoli T Milani P Boschetti A and Iannotta S 2000 Surf SciLett 464 L673

Scoles G 1988 Atomic and Molecular Beam Methods (Oxford University Press)Siegrist T Kloc C Laudise R A Katz H E and Haddon R C 1998 Adv

Mater 379 10Taliani C and Gebauer W 1999 Handbook of Oligo and Polythiophenes (Weinheim

WileyplusmnVCN)Toccoli T van Opbergen M Boschetti A Ciullo G Ronchin S and Iannotta

S 1999 Phil Mag B 79 2157

Supersonic molecular-beam epitaxy of oligothiophene reglms 495

structures characterized by tiles parallel to the substrate When these lamellae areanalysed in detail a terraced structure where the steps correspond to the height ofthe a-4T molecules becomes clearly discernible The picture emerging from the reglmsgrown from beams in an eŒusive regime is very diŒerent Here the morphologyshows the typical feature of dendritic growth which when analysed by high-resolu-tion TM AFM shows only lobular shapes (PodestaAacute et al 2000)

A very sensitive way to assess the degree of ordering of the oligothiophene reglmsis to study the PL spectra at low temperatures Since the homogeneous contributionto the bandwith at low temperatures can be neglected these spectra give directinformation about the presence of defects and impurities in the reglms Our reglmsgrown by SuMBE show a well resolved vibronic structure where the FWHM ofthe 0plusmn0 band in the best reglms is about 40 cm 1 at 10 K We obtained particularlyinteresting results by comparing the PL from reglms grown by changing the beamparameters slightly As shown in reggure 4 the reglms grown with pressures of thecarrier gas (H2) of 200 and 700 mbar in the source give rise to quite similar spectra

Supersonic molecular-beam epitaxy of oligothiophene reglms 491

Figure 4 PL spectra at about 10 K of several a-4T reglms about 300 nm thick grown indiŒerent regimes of supersonic expansion Note the increase in the presence of C-type defects with decreasing dilution of the oligothiophene molecules in the beam

both corresponding to a very good degree of crystallinity A very signiregcant diŒer-ence however is the growth of the bands at about 20 600 19 200 and 17 800 cm 1These bands previously observed by the Umbach group (Hopmaier et al 1999) canbe assigned to the presence of defects corresponding to diŒerent orientation of themolecules close to the grains The spectra in reggure 4 show that these type of defectdecrease with increasing supersonicity of the beam and with decreasing velocityspread of the oligothiophene molecules in the beam A possible microscopic inter-pretation is that the highly supersonic beam could produce longer crystallites

We have also characterized the structure of some of the reglms Figure 5 shows theXRD pattern collected in BraggplusmnBrentano geometry The adopted powder geometryimplies that only crystalline planes parallel to the sample surface contribute to thecoherently diŒracted signal The observed remacrections are compatible (for positionsand relative intensities) with the (00l ) lines of the low-temperature a-T4 structurereported by Siegrist et al (1998) Since no other diŒracted peaks or signiregcant back-ground were collected we can suppose that the main part of the a-T4 thin reglm grewin the low-temperature crystalline form with the growth axis along [00l ] that is withthe c axis perpendicular to the sample surface The intensity of the line and therelatively low FWHM of these lines indicate that the reglms present a high orderthat is deregnitely much better than for reglms grown at similar thicknesses (greaterthan 300 nm) with the standard vacuum techniques

From this analysis however nothing deregnitive can be said about the growthmechanism and this is especially true for the in-plane growth For a more completepicture it is necessary to study pole reggures collected in four-circle geometry Figure 6shows the pole reggure for the remacrection at 233deg ((008) low temperature structure) Itis evident that the diŒracted signal is concentrated in the Aacute-tilting region about thenormal (Aacute ˆ 0deg) and no other measurable signals are visible up to Aacute ˆ 85deg Fromthe detail in reggure 7 (b) we can see that the (008) pole is centred about the surfacenormal but it is not sharp Figure 7 (d) shows an average Aacute-tilting plot of the

492 T Toccoli et al

Figure 5 XRD of an a-4T reglm 400 nm thick grown by SuMBE

diŒracted intensity obtained by averaging all the constant-iquest (variable-Aacute) patternsthat compose the pole reggure in reggure 7 (b) Here we can clearly see that growthdirections are spread over a rather uniform angular region about the surface normalwe can quantify this mosaicity by means of the half-width at half-maximum(HWHM) of the trend in reggure 7 (d) which is of about 3deg

To study the in-plane growth mechanism we need information along some crys-tallographic direction diŒerent from the c axis of the low-temperature a-T4 struc-ture The powder pattern simulated on the basis of the structural data of Siegrist etal 1998) indicates that the low-temperature a-T4 structure gives an intense (11113)plusmn(1113) remacrection (at 2sup3 ˆ 2021deg) which according to the c-axis growth mechanismshould be present at an angle of 6583deg with respect to the surface normal The (11113)plusmn(1113) pole reggure is shown in reggure 7 (b) whereas the corresponding average Aacute-tiltingplot is in reggure 7 (c) The pole reggure was collected in the range 55deg lt Aacute lt 75deg with amuch longer counting time than that used for the (008) pole reggure In spite of this thediŒracted signal is very weak but visible The ring-shaped maximum is for theexpected angle of 658deg and it is distributed over a relatively large Aacute-tilting rangeas observed for the (008) pole as can be seen in reggure 7 (c) the HWHM is about 3degas in the (008) pole reggure The symmetry around the iquest-rotation axis clearly indicatesthe presence of a regbre texture that is a preferred growth orientation along the c axisbut lack of any order in the growth plane This a very frequently observed feature innon-epitaxial thin reglms even if in this case observation was made quite di cult bythe considerable mosaicity and low scattering power of the material (at least withrespect to typical inorganic thin reglms) This is to our knowledge the regrst pole reggure

Supersonic molecular-beam epitaxy of oligothiophene reglms 493

Figure 6 (008) pole reggure

study of organic reglms of oligothiophenes and conregrms the high level of crystallinitythat could be achieved by SuMBE

4 ConclusionWe have shown that SuMBE is a very eŒective and promising technique to grow

thin reglms of oligothiophene with a high degree of ordering over an unprecedentedrange of thicknesses Optical structural and morphological characterizations give apicture coherent with the control that SuMBE promises for the growth process Weenvisage the possibility of controlling with this method the polymorphism byforcing kinetically preferential growth in reglms This opens up new opportunitiesfor studying the correlation between structure morphology and optical properties

494 T Toccoli et al

Figure 7 (a) (11113)plusmn(1113) pole reggure and (b) detail of the (008) pole reggure in reggure 2 (c) (d)average Aacute-tilting plot for (c) the (11113)plusmn(1113) and (d) the (008) pole reggures (auarbitrary units)

of the reglms as well as new possible developments in the organic based electronics andoptoelectronics

ACKNOWLEDGEMENTS

The valuable technical support of C Corradi and M Mazzola is acknowledgedThanks are due to A PodestaAacute for the atomic force micrograph We are grateful to LAversa M Ferrari P Milani S Ronchin and R Verucchi for stimulating discus-sions This work is supported by the `Atomic Detectors Development and optimiza-tion via synthesis of reglms by supersonic beams of clusters and organic materialsrsquoproject regnanced by Provincia Autonoma di Trento

ReferencesBiasioli F Boschetti A Barborini E Piseri P Milani P and Iannotta S 1997

Chem Phys Lett 270 115Bredas J L and Silbey R 1991 Conjugated Polymers The Novel Science and Technology

of Highly Conducting and Nonlinear Optically Active Materials (Dordrecht Kluwer)Farges J P 1994 Organic Conductors Fundamentals Applications (New York Marcel

Dekker)Gebauer W Sokolowski M and Umbach E 1998 Chem Phys 227 33Hopmeier M Gebauer W Oestreich M Sokolowski M Umbach E and Mahrt

R F 1999 Chem Phys Lett 314 9Iannotta S Toccoli T Boschetti A and Scardi P 2001 Synth Metals 122 221Iannotta S Toccoli T Ferrari M Ronchin S PodestaAacute A Milani P and

Scardi P 2000 Organic Photonic Materials and Devices II Proceedings of theSPIE Vol 3939 (Bellingham Washington SPIE) p 135

Milani P and Iannotta S 1999 Synthesis of Nanophase Materials by Cluster BeamDeposition (Berlin Springer)

PodestaAacute A Toccoli T Milani P Boschetti A and Iannotta S 2000 Surf SciLett 464 L673

Scoles G 1988 Atomic and Molecular Beam Methods (Oxford University Press)Siegrist T Kloc C Laudise R A Katz H E and Haddon R C 1998 Adv

Mater 379 10Taliani C and Gebauer W 1999 Handbook of Oligo and Polythiophenes (Weinheim

WileyplusmnVCN)Toccoli T van Opbergen M Boschetti A Ciullo G Ronchin S and Iannotta

S 1999 Phil Mag B 79 2157

Supersonic molecular-beam epitaxy of oligothiophene reglms 495

both corresponding to a very good degree of crystallinity A very signiregcant diŒer-ence however is the growth of the bands at about 20 600 19 200 and 17 800 cm 1These bands previously observed by the Umbach group (Hopmaier et al 1999) canbe assigned to the presence of defects corresponding to diŒerent orientation of themolecules close to the grains The spectra in reggure 4 show that these type of defectdecrease with increasing supersonicity of the beam and with decreasing velocityspread of the oligothiophene molecules in the beam A possible microscopic inter-pretation is that the highly supersonic beam could produce longer crystallites

We have also characterized the structure of some of the reglms Figure 5 shows theXRD pattern collected in BraggplusmnBrentano geometry The adopted powder geometryimplies that only crystalline planes parallel to the sample surface contribute to thecoherently diŒracted signal The observed remacrections are compatible (for positionsand relative intensities) with the (00l ) lines of the low-temperature a-T4 structurereported by Siegrist et al (1998) Since no other diŒracted peaks or signiregcant back-ground were collected we can suppose that the main part of the a-T4 thin reglm grewin the low-temperature crystalline form with the growth axis along [00l ] that is withthe c axis perpendicular to the sample surface The intensity of the line and therelatively low FWHM of these lines indicate that the reglms present a high orderthat is deregnitely much better than for reglms grown at similar thicknesses (greaterthan 300 nm) with the standard vacuum techniques

From this analysis however nothing deregnitive can be said about the growthmechanism and this is especially true for the in-plane growth For a more completepicture it is necessary to study pole reggures collected in four-circle geometry Figure 6shows the pole reggure for the remacrection at 233deg ((008) low temperature structure) Itis evident that the diŒracted signal is concentrated in the Aacute-tilting region about thenormal (Aacute ˆ 0deg) and no other measurable signals are visible up to Aacute ˆ 85deg Fromthe detail in reggure 7 (b) we can see that the (008) pole is centred about the surfacenormal but it is not sharp Figure 7 (d) shows an average Aacute-tilting plot of the

492 T Toccoli et al

Figure 5 XRD of an a-4T reglm 400 nm thick grown by SuMBE

diŒracted intensity obtained by averaging all the constant-iquest (variable-Aacute) patternsthat compose the pole reggure in reggure 7 (b) Here we can clearly see that growthdirections are spread over a rather uniform angular region about the surface normalwe can quantify this mosaicity by means of the half-width at half-maximum(HWHM) of the trend in reggure 7 (d) which is of about 3deg

To study the in-plane growth mechanism we need information along some crys-tallographic direction diŒerent from the c axis of the low-temperature a-T4 struc-ture The powder pattern simulated on the basis of the structural data of Siegrist etal 1998) indicates that the low-temperature a-T4 structure gives an intense (11113)plusmn(1113) remacrection (at 2sup3 ˆ 2021deg) which according to the c-axis growth mechanismshould be present at an angle of 6583deg with respect to the surface normal The (11113)plusmn(1113) pole reggure is shown in reggure 7 (b) whereas the corresponding average Aacute-tiltingplot is in reggure 7 (c) The pole reggure was collected in the range 55deg lt Aacute lt 75deg with amuch longer counting time than that used for the (008) pole reggure In spite of this thediŒracted signal is very weak but visible The ring-shaped maximum is for theexpected angle of 658deg and it is distributed over a relatively large Aacute-tilting rangeas observed for the (008) pole as can be seen in reggure 7 (c) the HWHM is about 3degas in the (008) pole reggure The symmetry around the iquest-rotation axis clearly indicatesthe presence of a regbre texture that is a preferred growth orientation along the c axisbut lack of any order in the growth plane This a very frequently observed feature innon-epitaxial thin reglms even if in this case observation was made quite di cult bythe considerable mosaicity and low scattering power of the material (at least withrespect to typical inorganic thin reglms) This is to our knowledge the regrst pole reggure

Supersonic molecular-beam epitaxy of oligothiophene reglms 493

Figure 6 (008) pole reggure

study of organic reglms of oligothiophenes and conregrms the high level of crystallinitythat could be achieved by SuMBE

4 ConclusionWe have shown that SuMBE is a very eŒective and promising technique to grow

thin reglms of oligothiophene with a high degree of ordering over an unprecedentedrange of thicknesses Optical structural and morphological characterizations give apicture coherent with the control that SuMBE promises for the growth process Weenvisage the possibility of controlling with this method the polymorphism byforcing kinetically preferential growth in reglms This opens up new opportunitiesfor studying the correlation between structure morphology and optical properties

494 T Toccoli et al

Figure 7 (a) (11113)plusmn(1113) pole reggure and (b) detail of the (008) pole reggure in reggure 2 (c) (d)average Aacute-tilting plot for (c) the (11113)plusmn(1113) and (d) the (008) pole reggures (auarbitrary units)

of the reglms as well as new possible developments in the organic based electronics andoptoelectronics

ACKNOWLEDGEMENTS

The valuable technical support of C Corradi and M Mazzola is acknowledgedThanks are due to A PodestaAacute for the atomic force micrograph We are grateful to LAversa M Ferrari P Milani S Ronchin and R Verucchi for stimulating discus-sions This work is supported by the `Atomic Detectors Development and optimiza-tion via synthesis of reglms by supersonic beams of clusters and organic materialsrsquoproject regnanced by Provincia Autonoma di Trento

ReferencesBiasioli F Boschetti A Barborini E Piseri P Milani P and Iannotta S 1997

Chem Phys Lett 270 115Bredas J L and Silbey R 1991 Conjugated Polymers The Novel Science and Technology

of Highly Conducting and Nonlinear Optically Active Materials (Dordrecht Kluwer)Farges J P 1994 Organic Conductors Fundamentals Applications (New York Marcel

Dekker)Gebauer W Sokolowski M and Umbach E 1998 Chem Phys 227 33Hopmeier M Gebauer W Oestreich M Sokolowski M Umbach E and Mahrt

R F 1999 Chem Phys Lett 314 9Iannotta S Toccoli T Boschetti A and Scardi P 2001 Synth Metals 122 221Iannotta S Toccoli T Ferrari M Ronchin S PodestaAacute A Milani P and

Scardi P 2000 Organic Photonic Materials and Devices II Proceedings of theSPIE Vol 3939 (Bellingham Washington SPIE) p 135

Milani P and Iannotta S 1999 Synthesis of Nanophase Materials by Cluster BeamDeposition (Berlin Springer)

PodestaAacute A Toccoli T Milani P Boschetti A and Iannotta S 2000 Surf SciLett 464 L673

Scoles G 1988 Atomic and Molecular Beam Methods (Oxford University Press)Siegrist T Kloc C Laudise R A Katz H E and Haddon R C 1998 Adv

Mater 379 10Taliani C and Gebauer W 1999 Handbook of Oligo and Polythiophenes (Weinheim

WileyplusmnVCN)Toccoli T van Opbergen M Boschetti A Ciullo G Ronchin S and Iannotta

S 1999 Phil Mag B 79 2157

Supersonic molecular-beam epitaxy of oligothiophene reglms 495

diŒracted intensity obtained by averaging all the constant-iquest (variable-Aacute) patternsthat compose the pole reggure in reggure 7 (b) Here we can clearly see that growthdirections are spread over a rather uniform angular region about the surface normalwe can quantify this mosaicity by means of the half-width at half-maximum(HWHM) of the trend in reggure 7 (d) which is of about 3deg

To study the in-plane growth mechanism we need information along some crys-tallographic direction diŒerent from the c axis of the low-temperature a-T4 struc-ture The powder pattern simulated on the basis of the structural data of Siegrist etal 1998) indicates that the low-temperature a-T4 structure gives an intense (11113)plusmn(1113) remacrection (at 2sup3 ˆ 2021deg) which according to the c-axis growth mechanismshould be present at an angle of 6583deg with respect to the surface normal The (11113)plusmn(1113) pole reggure is shown in reggure 7 (b) whereas the corresponding average Aacute-tiltingplot is in reggure 7 (c) The pole reggure was collected in the range 55deg lt Aacute lt 75deg with amuch longer counting time than that used for the (008) pole reggure In spite of this thediŒracted signal is very weak but visible The ring-shaped maximum is for theexpected angle of 658deg and it is distributed over a relatively large Aacute-tilting rangeas observed for the (008) pole as can be seen in reggure 7 (c) the HWHM is about 3degas in the (008) pole reggure The symmetry around the iquest-rotation axis clearly indicatesthe presence of a regbre texture that is a preferred growth orientation along the c axisbut lack of any order in the growth plane This a very frequently observed feature innon-epitaxial thin reglms even if in this case observation was made quite di cult bythe considerable mosaicity and low scattering power of the material (at least withrespect to typical inorganic thin reglms) This is to our knowledge the regrst pole reggure

Supersonic molecular-beam epitaxy of oligothiophene reglms 493

Figure 6 (008) pole reggure

study of organic reglms of oligothiophenes and conregrms the high level of crystallinitythat could be achieved by SuMBE

4 ConclusionWe have shown that SuMBE is a very eŒective and promising technique to grow

thin reglms of oligothiophene with a high degree of ordering over an unprecedentedrange of thicknesses Optical structural and morphological characterizations give apicture coherent with the control that SuMBE promises for the growth process Weenvisage the possibility of controlling with this method the polymorphism byforcing kinetically preferential growth in reglms This opens up new opportunitiesfor studying the correlation between structure morphology and optical properties

494 T Toccoli et al

Figure 7 (a) (11113)plusmn(1113) pole reggure and (b) detail of the (008) pole reggure in reggure 2 (c) (d)average Aacute-tilting plot for (c) the (11113)plusmn(1113) and (d) the (008) pole reggures (auarbitrary units)

of the reglms as well as new possible developments in the organic based electronics andoptoelectronics

ACKNOWLEDGEMENTS

The valuable technical support of C Corradi and M Mazzola is acknowledgedThanks are due to A PodestaAacute for the atomic force micrograph We are grateful to LAversa M Ferrari P Milani S Ronchin and R Verucchi for stimulating discus-sions This work is supported by the `Atomic Detectors Development and optimiza-tion via synthesis of reglms by supersonic beams of clusters and organic materialsrsquoproject regnanced by Provincia Autonoma di Trento

ReferencesBiasioli F Boschetti A Barborini E Piseri P Milani P and Iannotta S 1997

Chem Phys Lett 270 115Bredas J L and Silbey R 1991 Conjugated Polymers The Novel Science and Technology

of Highly Conducting and Nonlinear Optically Active Materials (Dordrecht Kluwer)Farges J P 1994 Organic Conductors Fundamentals Applications (New York Marcel

Dekker)Gebauer W Sokolowski M and Umbach E 1998 Chem Phys 227 33Hopmeier M Gebauer W Oestreich M Sokolowski M Umbach E and Mahrt

R F 1999 Chem Phys Lett 314 9Iannotta S Toccoli T Boschetti A and Scardi P 2001 Synth Metals 122 221Iannotta S Toccoli T Ferrari M Ronchin S PodestaAacute A Milani P and

Scardi P 2000 Organic Photonic Materials and Devices II Proceedings of theSPIE Vol 3939 (Bellingham Washington SPIE) p 135

Milani P and Iannotta S 1999 Synthesis of Nanophase Materials by Cluster BeamDeposition (Berlin Springer)

PodestaAacute A Toccoli T Milani P Boschetti A and Iannotta S 2000 Surf SciLett 464 L673

Scoles G 1988 Atomic and Molecular Beam Methods (Oxford University Press)Siegrist T Kloc C Laudise R A Katz H E and Haddon R C 1998 Adv

Mater 379 10Taliani C and Gebauer W 1999 Handbook of Oligo and Polythiophenes (Weinheim

WileyplusmnVCN)Toccoli T van Opbergen M Boschetti A Ciullo G Ronchin S and Iannotta

S 1999 Phil Mag B 79 2157

Supersonic molecular-beam epitaxy of oligothiophene reglms 495

study of organic reglms of oligothiophenes and conregrms the high level of crystallinitythat could be achieved by SuMBE

4 ConclusionWe have shown that SuMBE is a very eŒective and promising technique to grow

thin reglms of oligothiophene with a high degree of ordering over an unprecedentedrange of thicknesses Optical structural and morphological characterizations give apicture coherent with the control that SuMBE promises for the growth process Weenvisage the possibility of controlling with this method the polymorphism byforcing kinetically preferential growth in reglms This opens up new opportunitiesfor studying the correlation between structure morphology and optical properties

494 T Toccoli et al

Figure 7 (a) (11113)plusmn(1113) pole reggure and (b) detail of the (008) pole reggure in reggure 2 (c) (d)average Aacute-tilting plot for (c) the (11113)plusmn(1113) and (d) the (008) pole reggures (auarbitrary units)

of the reglms as well as new possible developments in the organic based electronics andoptoelectronics

ACKNOWLEDGEMENTS

The valuable technical support of C Corradi and M Mazzola is acknowledgedThanks are due to A PodestaAacute for the atomic force micrograph We are grateful to LAversa M Ferrari P Milani S Ronchin and R Verucchi for stimulating discus-sions This work is supported by the `Atomic Detectors Development and optimiza-tion via synthesis of reglms by supersonic beams of clusters and organic materialsrsquoproject regnanced by Provincia Autonoma di Trento

ReferencesBiasioli F Boschetti A Barborini E Piseri P Milani P and Iannotta S 1997

Chem Phys Lett 270 115Bredas J L and Silbey R 1991 Conjugated Polymers The Novel Science and Technology

of Highly Conducting and Nonlinear Optically Active Materials (Dordrecht Kluwer)Farges J P 1994 Organic Conductors Fundamentals Applications (New York Marcel

Dekker)Gebauer W Sokolowski M and Umbach E 1998 Chem Phys 227 33Hopmeier M Gebauer W Oestreich M Sokolowski M Umbach E and Mahrt

R F 1999 Chem Phys Lett 314 9Iannotta S Toccoli T Boschetti A and Scardi P 2001 Synth Metals 122 221Iannotta S Toccoli T Ferrari M Ronchin S PodestaAacute A Milani P and

Scardi P 2000 Organic Photonic Materials and Devices II Proceedings of theSPIE Vol 3939 (Bellingham Washington SPIE) p 135

Milani P and Iannotta S 1999 Synthesis of Nanophase Materials by Cluster BeamDeposition (Berlin Springer)

PodestaAacute A Toccoli T Milani P Boschetti A and Iannotta S 2000 Surf SciLett 464 L673

Scoles G 1988 Atomic and Molecular Beam Methods (Oxford University Press)Siegrist T Kloc C Laudise R A Katz H E and Haddon R C 1998 Adv

Mater 379 10Taliani C and Gebauer W 1999 Handbook of Oligo and Polythiophenes (Weinheim

WileyplusmnVCN)Toccoli T van Opbergen M Boschetti A Ciullo G Ronchin S and Iannotta

S 1999 Phil Mag B 79 2157

Supersonic molecular-beam epitaxy of oligothiophene reglms 495

of the reglms as well as new possible developments in the organic based electronics andoptoelectronics

ACKNOWLEDGEMENTS

The valuable technical support of C Corradi and M Mazzola is acknowledgedThanks are due to A PodestaAacute for the atomic force micrograph We are grateful to LAversa M Ferrari P Milani S Ronchin and R Verucchi for stimulating discus-sions This work is supported by the `Atomic Detectors Development and optimiza-tion via synthesis of reglms by supersonic beams of clusters and organic materialsrsquoproject regnanced by Provincia Autonoma di Trento

ReferencesBiasioli F Boschetti A Barborini E Piseri P Milani P and Iannotta S 1997

Chem Phys Lett 270 115Bredas J L and Silbey R 1991 Conjugated Polymers The Novel Science and Technology

of Highly Conducting and Nonlinear Optically Active Materials (Dordrecht Kluwer)Farges J P 1994 Organic Conductors Fundamentals Applications (New York Marcel

Dekker)Gebauer W Sokolowski M and Umbach E 1998 Chem Phys 227 33Hopmeier M Gebauer W Oestreich M Sokolowski M Umbach E and Mahrt

R F 1999 Chem Phys Lett 314 9Iannotta S Toccoli T Boschetti A and Scardi P 2001 Synth Metals 122 221Iannotta S Toccoli T Ferrari M Ronchin S PodestaAacute A Milani P and

Scardi P 2000 Organic Photonic Materials and Devices II Proceedings of theSPIE Vol 3939 (Bellingham Washington SPIE) p 135

Milani P and Iannotta S 1999 Synthesis of Nanophase Materials by Cluster BeamDeposition (Berlin Springer)

PodestaAacute A Toccoli T Milani P Boschetti A and Iannotta S 2000 Surf SciLett 464 L673

Scoles G 1988 Atomic and Molecular Beam Methods (Oxford University Press)Siegrist T Kloc C Laudise R A Katz H E and Haddon R C 1998 Adv

Mater 379 10Taliani C and Gebauer W 1999 Handbook of Oligo and Polythiophenes (Weinheim

WileyplusmnVCN)Toccoli T van Opbergen M Boschetti A Ciullo G Ronchin S and Iannotta

S 1999 Phil Mag B 79 2157

Supersonic molecular-beam epitaxy of oligothiophene reglms 495