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Substrate-Mediated Electronic Structure and Properties of

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                 HE 8811 infrared light emitting diode (emitting at 810 nm) as a light source,       promising blue light-emitting diode,[2] and it is also considered
                 and a fiber-optic Hewlett-Packard HP 8153 A power meter.
                                                                                                     as a promising material for solar cell applications. 6P is well
                                                                         Received: May 23, 2003      suited to controlled experimental studies, as clean and well
                                                                      Final version: July 24, 2003
                                                                                                     defined coverages can be obtained relatively easily by physi-
                 ±                                                                                   cal vapor deposition in ultra-high vacuum (UHV). Conse-
                  [1] C. R. Mayer, V. Cabuil, T. Lalot, R. Thouvenot, Adv. Mater. 2000, 12, 417.     quently its electronic structure both in the pristine[3±5] and
                  [2] M. Breulmann, H. Cölfen, H. P. Hentze, M. Antonietti, D. Walsh,
                      S. Mann, Adv. Mater. 1998, 10, 237.                                            doped states[5,6] has been extensively studied, as has its film
                  [3] C. ChanØac, E. Tronc, J. P. Jolivet, J. Mater. Chem. 1996, 6, 1905.            growth, crystal structure, and crystallite orientation on a vari-
                  [4] F. del Monte, M. P. Morales, D. Levy, A. Fernµndez, M. Ocaæa, A. Roig,
                                                                                                     ety of substrates.[7±10] When deposited at room temperature
                      E. Molins, K. O'Grady, C. J. Serna, Langmuir 1997, 13, 3627.
                  [5] C. Cannas, D. Gatteschi, A. Musino, G. Piccaluga, C. Sangregorio, J. Phys.     the molecules have been seen to be oriented near parallel to
                      Chem. B 1998, 102, 7721.                                                       the substrate, while growth at elevated temperatures leads to
                  [6] S. Solinas, G. Piccaluga, M. P. Morales, C. J. Serna, Acta Mater. 2001, 49,
                      2805.
                                                                                                     molecules with a perpendicular orientation. It has recently
                  [7] A. Bourlinos, A. Simopoulos, D. Petridis, H. Okumura, G. Hadjipanayis,         been shown that with parallel oriented molecules, increased
                      Adv. Mater. 2001, 13, 289.                                                     electroluminescence with a narrower spectrum at remarkably
                  [8] P. Tartaj, T. Gonzµlez-Carreæo, C. J. Serna. Adv. Mater. 2001, 13, 1620.
                  [9] R. F. Ziolo, E. P. Giannelis, B. A. Weinstein, M. P. O'Horo, B. N.             low driving voltages is achieved relative to devices where the
                      Ganguly, V. Mehrotra, M. W. Russel, D. R. Huffman, Science 1992, 257,          molecules are perpendicular to the substrate. This was consid-
                      219.
                                                                                                     ered to be due to the anisotropic distribution of polarized
                 [10] M. Hayashi, M. Susa, K. Nagata, J. Appl. Phys. 1999, 85, 2257.
                 [11] J. Qiu, K. Hirao, Jpn. J. Appl. Phys. 1996, 35, L1677.                         light emission as well as the efficiency of carrier transport in
                 [12] The Faraday effect is a magneto-optical phenomenon, in which the polar-        ordered molecular arrays with different orientation.[8] Here,
                      ization plane of an electromagnetic wave is rotated under the influence of
                      a magnetic field. This rotation of the polarization azimuth, hF, occurs
                                                                                                     we show that growth at different substrate temperatures not
                      when an optical beam propagates through a Faraday rotator under the            only leads to different molecular orientation but also to differ-
                      influence of a magnetic field H(x) and is given by the following equation:     ent electronic structure of the molecules in the 6P films, as
                      hF = V òL H(x)dx, where V is the material-dependent Verdet constant and
                              0
                      L is the thickness of the material. A high optical transmission, together      evidenced directly in the valence band electronic structure
                      with good magnetic properties of the composite, is essential for the perfor-   obtained by angle-resolved ultraviolet photoemission spec-
                      mance of Faraday effect based devices.
                                                                                                     troscopy (ARUPS). For growth on atomically clean Al(111)
                 [13] H. Guerrero, G. Rosa, M. P. Morales, F. del Monte, E. M. Moreno,
                      D. Levy, R. PØrez del Real, T. Belenguer, C. J. Serna, Appl. Phys. Lett.       at elevated temperatures, the molecules are oriented near
                      1997, 71, 2698.                                                                perpendicular to the substrate and have a considerably larger
                                                        Â
                 [14] F. Bentivegna, J. FerrØ, M. Nyvlt, J. P. Jamet, D. Imhoff, M. Canva,
                                                         Â,
                      A. Brun, P. Veillet, S. Visnovsky F. Chaput, J. P. Boilot, J. Appl. Phys.
                                                                                                     p-band width and 0.6 eV lower ionization potential than those
                      1998, 83, 7776.                                                                grown at room temperature. The former can be associated
                                           Â                                              Â,
                 [15] F. Bentivegna, M. Nyvlt, J. FerrØ, J. P. Jamet, A. Brun, S. Visnovsky R. Ur-   with 6P in the commonly accepted planar conformation, while
                      ban, J. Appl. Phys. 1999, 85, 2270.
                 [16] M. P. Morales, S. Veintemillas-Verdaguer, M. I. Montero, C. J. Serna,          the latter is associated with a hitherto unexpected 6P film,
                      A. Roig, Ll. Casas, B. Martínez, F. Sandiumenge, Chem. Mater. 1999, 11,        where the molecules have a twisted conformation and con-
                      3058.
                                                                                                     comitant lower conjugation similar to that in the gas phase. If
                 [17] X. Batlle, A. Labarta, J. Phys. D 2002, 35, R15.
                 [18] L. D'Amico, F. D'Orazio, J. L. Dorman, D. Fiorani, F. Lucari, E. Tronc.        oxygen is present at the aluminum surface, films with planar
                      Mater. Sci. Forum 1995, 195 173.                                               6P form even at room temperature (RT). Finally it is shown
                 [19] http://www.corning.com/lightingmaterials/images/Vycor_7930.pdf
                                                                                                     that the band alignment of 6P films on Al can be tuned over a
                                                                                                     2 eV range depending on the concentration of oxygen on the
                                                                                                     Al surface and the conformation of the molecules in the 6P
                 Substrate-Mediated Electronic Structure
                                                                                                     films.
                 and Properties of Sexiphenyl Films**                                                   Figure 1 summarizes the valence band spectra for the
                                                                                                     growth of 6P on atomically clean Al(111) surfaces at RT and
                 By Jan Ivanco, Barbara Winter, Falko P. Netzer
                                                                                                     high temperature of 430 K (HT). Included in the inset is the
                 and Michael G. Ramsey*
                                                                                                     workfunction as a function of increasing nominal thickness.
                                                                                                     The workfunction behavior at both RT and elevated tempera-
                   Oligomers of conjugated organic materials such as sexiphe-
                                                                                                     ture for increasing coverage are identical, displaying a rapid
                 nyl, sexithiophene, and pentacene are attracting considerable
                                                                                                     decrease up to a nominal thickness of 5±6 Š; beyond this no
                 interest on the one hand as models for their associated poly-
                                                                                                     significant workfunction changes were observed up to the
                 mers, and on the other hand, as active elements for organic
                                                                                                     maximum nominal film thickness of this study (350 Š). For
                 devices in their own right.[1] para-Sexiphenyl (6P) sandwiched
                                                                                                     molecular adsorption, the workfunction changes arise from
                 between indium tin oxide (ITO) and aluminum contacts is a
                                                                                                     the interface dipole induced by the interaction of molecules in
                 ±                                                                                   the first monolayer,[11] and thus a nominal 6P film thickness
                  [*] Prof. M. G. Ramsey, Dr. J. Ivanco, B. Winter, Prof. F. P. Netzer               5±6 Š is considered to correspond to the completion of the
                      Institut für Experimentalphysik, Karl-Franzens-Universität Graz                first molecular layer. For surface coverages up to a mono-
                      A-8010 Graz (Austria)
                                                                                                     layer, the ARUPS signatures for growth at both substrate
                      E-mail: michael.ramsey@uni-graz.at
                 [**] Supported by the Austrian Science Foundation through the SFB: Electro-
                                                                                                     temperatures are identical. In this coverage regime strong
                      active Materials.                                                              angular effects are observed in the molecular emission bands.


                 1812             Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim                   DOI: 10.1002/adma.200304993   Adv. Mater. 2003, 15, No. 21, November 4
                                                                                                                                                      COMMUNICATIONS
                                                                                  mono- and multi-layers are distinctly different, the emission
                                                                                  from the upper p-band between 2 to 5 eV below EF has a sig-
                                                                                  nificantly altered appearance (see Fig. 1, 30 Š HT spectrum).
                                                                                  In particular this band displays more structure and the
                                                                                  HOMO now appears at a 0.6 eV lower binding energy. This is
                                                                                  in contrast to the higher binding energy emission bands
                                                                                  (> 6 eV), dominated by intra-ring orbitals, whose energies and
                                                                                  appearance are not significantly different. This new electronic
                                                                                  structure is not a result of the temperature of growth per se.
                                                                                  The top spectrum of Figure 1 is for a film grown at room tem-
                                                                                  perature on the Al(111) surface, which had been exposed to
                                                                                  oxygen (2500 s of oxygen at 10±6 torr). The spectrum is similar
                                                                                  to the HT film grown on clean Al(111), albeit rigidly shifted
                                                                                  by 1 eV with respect to (w.r.t.) the Fermi level due to the
                                                                                  workfunction difference. It should be noted that for all 6P
                                                                                  films studied, angle-resolved spectra were taken over a large
                                                                                  range of experimental geometries in a search for band struc-
                                                                                  ture effects. No significant energy shifts that could be asso-
                                                                                  ciated with bulk band dispersion were detectable and the
                                                                                  observed binding energy difference for the HOMO are not
                                                                                  due to bulk band structure effects.
                                                                                     The results presented in Figure 1 clearly demonstrate that
                                                                                  two different electronic structure types are possible in 6P
                                                                                  films. In the seminal work by Seki et al. on the intramolecular
             Binding Energy w.r.t. EF (eV)                                        band structure of solid 6P[3] the electronic structure studied
                                                                                  was equivalent to those obtained here for growth at high sub-
Fig. 1. Comparison of monolayer and multilayer ARUPS spectra for sexiphenyl
films grown at RT and HT (430 K) on atomically clean Al(111). The spectra
                                                                                  strate temperatures or at RT on Al, if it was modified with
with open circles are taken in normal emission, while those with points are for   oxygen. Consequently we will call this type I sexiphenyl. A
spectra taken for electrons at 50 emission angles. The upper spectrum is of a    solid 6P with a different electronic structure to this has not
multilayer film grown at RT on Al(111) that had been exposed to 2500 Lang-
muir (1 Langmuir = 1”10±6 torr s) of oxygen. The insert shows the plot of the     been suggested until now in the literature; films with p-band
workfunction (u) as a function of 6P coverage on clean Al.                        structures such as those for RT grown films on atomically
                                                                                  clean Al will therefore be named type II sexiphenyl. In Fig-
These are particularly pronounced in the upper p-band, where                      ure 2, the p-bands of typical type I and II sexiphenyl films are
the highest occupied molecular orbital (HOMO) and                                 displayed in comparison to the UPS spectra of gas phase 6P
HOMO-1, at 2.5 and 3.0 eV, respectively, have zero intensity                      from Seki et al.[3]
in normal emission but become prominent at higher emission                           In photoemission spectroscopy of solids the Fermi level (EF)
angles. Such ARUPS behavior is indicative of an highly ori-                       is the experimental reference level; however, for comparison
ented molecular species.[12] Indeed, low energy electron dif-                     purposes the appropriate reference level is the vacuum level
fraction (LEED) of these surfaces confirm that the mono-                          (Evac). This not only enables the comparison of solid state
layers are ordered. The large surface unit cell indicated by                      spectra with gas phase spectra, which is necessarily referenced
LEED, and the low nominal film thickness required for com-                        to Evac, but also removes the rigid energy shifts arising from
pletion of the first monolayers suggest that the molecules                        differences in interface dipole for films grown on different sub-
have their molecular axes parallel to the Al(111) surface.[13]                    strates,[11,14] i.e., the differences in band alignment. The Evac
   Although the monolayers are identical irrespective of sub-                     reference is made by shifting the spectrum of a film by the
strate temperature during growth, for thickness beyond this                       workfunction measured for that particular film. The gas-phase
the molecular films that develop are significantly different                      spectrum in Figure 2 has been shifted by 1.2 eV to lower bind-
from each other. For RT growth the orbital emission intensi-                      ing energy to account for extra-molecular screening of the
ties increase and become slightly less well resolved. The angu-                   photohole, which occurs in the solid state. The type II spectrum
lar behavior, although somewhat less pronounced, persists up                      is very similar to that of gas phase 6P with a similar p-band
to the highest nominal thickness studied (350 Š), suggesting                      spread and ionization potential. The type I on the other hand
that an ordered film with a similar molecular orientation to                      has a distinctly different spectral fingerprint, a greater p-band
the monolayer grows at RT. This is confirmed by LEED,                             energy spread, and a 0.6 eV lower binding energy of the
which has shown the same diffraction pattern as the mono-                         HOMO (i.e., a significantly lower ionization potential).
layer.[13] Clearly the electronic structure of the monolayers                        The p-band of 6P can be considered to develop from combi-
and the RT multilayer films are essentially identical. In con-                    nations of the two degenerate e1g orbitals of benzene and thus
trast, for growth at elevated temperatures the spectra for                        consists of twelve distinct orbitals.[3±5] These are indicated


Adv. Mater. 2003, 15, No. 21, November 4         http://www.advmat.de              Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim          1813
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                                                                                                   phase can however also be grown by condensing 6P onto sub-
                                                                                                   strates at 90 K. If the substrate is oxygen modified the amor-
                                                                                                   phous type II converts to type I on warming to RT, while if
                                                                                                   the substrate was atomically clean its p-band fingerprint
                                                                                                   remains type II.
                                                                                                      To prove that the conformational differences and related
                                                                                                   electronic structure differences between RT (twisted) and HT
                                                                                                   grown (planar) 6P films are not merely differences at the sur-
                                                                                                   face, nominally 35 nm thick films of both were grown in situ,
                                                                                                   and ex-situ photoluminescence (PL) was performed on them.
                                                                                                   Apart from the intensity differences expected due to the ori-
                                                                                                   entational differences, the high temperature grown films PL
                                                                                                   spectra showed a shift to ~ 0.35 eV lower emission energy.
                                                                                                   Such a shift can be seen to be in accord with a difference
                                                                                                   between planar and twisted molecules. Indeed, a similar shift
                                                                                                   in PL features has been reported for 6P under high pressure
                                                                                                   and was attributed to a planarization of the molecules.[20]
                                                                                                      That till now only a single type of solid state sexiphenyl has
                                                                                                   been reported in the literature is due to a number of probable
                                                                                                   reasons apart from the general expectation that conjugated
                                                                                                   oligomers are believed to be ªrigid and rod likeº in the solid
                                                                                                   state. It is likely that type I is more common. On the one
                                                                                                   hand, high substrate temperatures are often used to try and
                                                                                                   promote crystallinity, which will lead to type I. On the other
                                                                                                   hand, substrate contamination can also lead to type I even for
                                                                                                   RT growth, as demonstrated here by the Al-oxide substrate
                 Fig. 2. The upper p-band of a 6P film grown at elevated substrate temperature     result. This was probably the case in Seki et al.'s original study
                 (type I) and RT (type II) in comparison to the gas-phase valence band spectrum    of 6P on an evaporated Cu substrate.[3] Despite this, type II 6P
                 of 6P [3]. The energetic positions of the twelve orbitals that make up the band
                 are indicated. The spectra have been referenced to the vacuum level.
                                                                                                   can be found on close inspection of some published valence
                                                                                                   band spectra. For instance the 6P films of Schroeder et al. on
                                                                                                   HOPG graphite,[21] and Koch et al. on Sm and SiO2[22] clearly
                 on the gas phase spectrum; p1 to p3 are the inter-ring anti-                      have type II spectral fingerprints and ionization potentials
                 bonding, p10 to p12 are the bonding, and p4 to p9 are the six                     (6.4 eV w.r.t. Evac in all three cases). Anomalous behavior of
                 near-degenerate non-bonding orbitals. The relationship of the                     6P observed in the structural study of Baker et al.[19] led them
                 p-band spread and the torsional angle between phenyl rings is                     to suggest that 6P might have other polymorphs and changing
                 well understood in phenyl oligomers[3,5,15±18]Ða lower twist                      conditions of crystallization could influence the crystal struc-
                 angle results in a greater inter-ring orbital overlap and a great-                ture. Moreover, it was suggested that the valence band spectra
                 er energy spread of p orbitals, resulting in a reduction of the                   of Seki et al.[3] might therefore not be uniqueÐa prediction
                 bandgap and ionization potential. The band structure calcula-                     that is borne out in the results here.
                 tions for a single polyparaphenylene chain indicate a 0.5 eV                         Figure 3 displays typical p-band spectra for 6P multilayer
                 lowering of the ionization potential on reduction of the tor-                     films grown on Al under various substrate and temperature
                 sional angle from 50 to 0.[15] Sexiphenyl is known to be                        conditions. All spectra are referenced to Evac, while the Fermi
                 twisted in the gas phase with a torsional angle of 40±45. In                     level and HOMO energy positions are indicated. The two
                 the solid state it is generally accepted that the molecule is pla-                types of 6P electronic structures are clearly recognized by
                 nar, and it has been shown to be on average planar in a single                    their p-band spectral fingerprints. The most objective distin-
                 crystal X-ray diffraction study of Baker et al.[19] The coplanar-                 guishing criterion is the ionization potential and, over many
                 ity of type I 6P is certainly indicated by its large p-band                       different film preparations on Al, the range of ionization
                 spread.[3] The type II 6P on the other hand, with its p-band                      potentials for type I was 5.7 to 6.0 eV, while type II lay in the
                 spread and ionization potential similar to that of the gas                        range 6.3 to 6.6 eV.
                 phase, is due to a solid 6P with molecules with a high torsional                     The band alignment, HOMO w.r.t. EF, is a critical determi-
                 angle near that of the gas phase. This is not due to a lack of                    nant for charge injection in an organic device. As can be seen
                 crystallinity. The type II obtained at RT on clean Al is crystal-                 in Figure 3, it can have a range a values of up to 2 eV depend-
                 line as indicated by its ARUPS behavior, LEED,[13] X-ray dif-                     ing on the Al substrate conditions. This can be partitioned
                 fraction, and the fact that it is stable and does not make a                      into the effects of molecular conformation and the effects of
                 phase change to type I even after a many hours annealing                          the interface dipole. Due to the conformational difference up
                 above 400 K and a month in UHV. Type II in amorphous                              to 0.9 eV difference in the ionization potential between type I


                 1814            Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim                   http://www.advmat.de         Adv. Mater. 2003, 15, No. 21, November 4
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                                                                                 Experimental
                                                                                    The experiments were performed in situ in an UHV VG-ADES 400 angle
                                                                                 resolving electron spectrometer, with a base pressure better than 10±10 mbar,
                                                                                 with facilities for ARUPS using an unpolarized He discharge lamp, work func-
                                                                                 tion measurements via the secondary electron cut-off, LEED, and Auger spec-
                                                                                 troscopy. The substrate was an Al(111) crystal, mounted such that it could be
                                                                                 heated resistively or cooled to 90 K, which was cleaned by Argon ion bombard-
                                                                                 ment and annealing cycles. Sexiphenyl (Tokyo Chemical Industry Co., Ltd.) was
                                                                                 deposited in situ from a thoroughly degassed evaporator such that the pressure
                                                                                 in the system remained in the 10±10 mbar range during film growth. A nominal
                                                                                 growth rate of 2 Š min±1, as monitored by a water cooled quartz microbalance
                                                                                 assuming a density 1 g cm±3, was used.

                                                                                                                                    Received: February 14, 2003
                                                                                                                                      Final version: July 2, 2003



                                                                                 ±
                                                                                  [1] J. Fraxedas, Adv. Mater. 2002, 22, 1603.
                                                                                  [2] S. Tasch, C. Brandstätter, F. Meghdadi, G. Leising, G. Froyer, L. Athouel,
                                                                                      Adv. Mater. 1997, 9, 33.
                                                                                  [3] K. Seki, U. O. Karlsson, R. Engelhardt, E.-E. Koch, W. Schmidt, Chem.
                                                                                      Phys. 1984, 91, 459.
                                                                                  [4] S. Narioka, H. Ishii, K. Edamatsu, K. Kamiya, S. Hasegawa, T. Ohta,
                                                                                      N. Ueno, K. Seki, Phys. Rev. B 1995, 52, 2362.
                                                                                  [5] M. G. Ramsey, D. Steinmüller, M. Schatzmayr, M. Kiskinova, F. P. Netzer,
                                                                                      Chem. Phys. 1993, 177, 349.
                                                                                  [6] M. G. Ramsey, M. Schatzmayr, S. Stafström, F. P. Netzer, Europhys. Lett.
                                                                                      1994, 28, 85.
Fig. 3. The p-band spectra of 6P films grown on atomically clean and oxygen       [7] R. Resel, N. Koch, F. Meghdadi, G. Leising, W. Unzog, K. Reichmann,
modified Al(111). The temperature of growth is indicated. The positions of the        Thin Solid Films 1997, 305, 232.
Fermi levels and highest occupied molecular orbitals (HOMO) w.r.t. Evac are       [8] H. Yanagi, S. Okamoto, Appl. Phys. Lett. 1997, 71, 2563.
marked to express the great range of band alignment possible on Al.               [9] R. Resel, K. Erlacher, B. Müller, A. Thierry, B. Lotz, T. Kuhlmann,
                                                                                      K. Lischka, G. Leising, Surf. Interface Anal. 2000, 30, 518.
and type II sexiphenyl is observed. For a particular conforma-                   [10] R. Resel, N. Koch, F. Meghdadi, G. Leising, L. Athouel, G. Froyer, F. Ho-
                                                                                      fer, Cryst. Res. Technol. 2001, 36, 47.
tion the amount of oxygen on the aluminum surface can shift                      [11] G. Koller, R. I. R. Blyth, A. Sardar, F. P. Netzer, M. G. Ramsey, Appl.
the Fermi level position by up to 1 eV. With increasing expo-                         Phys. Lett. 2000, 76, 927.
                                                                                 [12] F. P. Netzer, M. G. Ramsey, CRC Crit. Rev. Solid State Mater. Sci. 1992,
sure of the Al(111) surface to oxygen the surface goes from a
                                                                                      17, 397.
predominantly chemisorbed oxygen phase (AlOx), through a                         [13] B. Winter, J. Ivanco, F. P. Netzer, M. G. Ramsey, Thin Solid Films 2003,
mixed phase with both chemisorbed oxygen and oxide                                    433, 269.
                                                                                 [14] R. I. R. Blyth, S. A. Sardar, F. P. Netzer, M. G. Ramsey, Appl. Phys. Lett.
patches, till finally a thin disordered oxide layer forms                             2000, 77, 1212.
(Al2O3). These changes correlate to a steady decrease in the                     [15] M. S. Miao, P. E. Van Camp, V. E. Van Doren, J. J. Ladik, J. W. Mintmire,
workfunction. Studies of benzene films condensed on various                           J. Chem. Phys. 1998, 109, 9623.
                                                                                 [16] J. P. Maier, D. W. Turner, Faraday Discuss. Chem. Soc. 1972, 54, 149.
oxygen exposed Al(111) surfaces showed a HOMO offset                             [17] W. K. Ford, C. B. Duke, A. Paton, J. Chem. Phys. 1983, 78, 4734.
directly related to the workfunction changes induced by the                      [18] M. G. Ramsey, D. Steinmüller, F. P. Netzer, J. Chem. Phys. 1992, 92, 6210.
                                                                                 [19] K. N. Baker, A. V. Fratini, T. Resch, H. C. Knachel, W. W. Adams, E. P.
adsorbed oxygen.[15] An identical effect is observed here for
                                                                                      Socci, B. L. Farmer, Polymer 1993, 34, 1571.
sexiphenyl. This implies that oxygen at the Al±6P interface                      [20] S. Guha, W. Graupner, R. Resel, M. Chanrasekar, H. R. Chandrasekhar,
will improve the electron injection ability at an Al contact, as                      R. Glaser, G. Leising, Phys. Rev. Lett. 1999, 82, 3625.
                                                                                 [21] P. G. Schroeder, C. B. France, B. A. Parkinson, R. Schlaf, J. Appl. Phys.
has been postulated.[15,23]                                                           2002, 91, 9095.
   In conclusion, it has been shown that the substrate condi-                    [22] N. Koch, E. Zojer, A. Rajagopal, J. Ghjisen, R. L. Johnson, G. Leising,
tions of temperature and/or oxygen content can lead to films                          J.-J. Pireaux, Adv. Funct. Mater. 2001, 11, 51.
                                                                                 [23] N. Koch, A. Pogantsch, E. J. W. List, R. I. R. Blyth, M. G. Ramsey, F. P.
of pristine sexiphenyl with significantly different p-band elec-                      Netzer, G. Leising, Appl. Phys. Lett. 1999, 74, 2909.
tronic structures and ionization potentials. This is argued to
be due to different conformations of the molecules in the
films and, as well as the expected planar form, films of 6P can
be grown consisting of molecules with large torsional angles.
This result is particularly significant in the field of organic                                ______________________
devices, as it suggests that growth conditions not only effect
morphology and molecular orientation but can also lead to
different electronic structures of the films. This is illustrated
with the band alignment of 6P films on Al, which is shown to
be variable over almost 2 eV; 0.9 eV arising from conforma-
tional differences and up to 1 eV depending on the oxygen
content at the Al interface.


Adv. Mater. 2003, 15, No. 21, November 4        http://www.advmat.de              Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim                        1815

				
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