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High-resolution luminescence of epitaxial organic films: quaterthiophene on Ag(111)
S U lfllTflHl Trlll NfTfNtLN ELSEVIER
Synthetic Metals 83 (1996) 227-230
High-resolution luminescence of epitaxial organic films: quaterthiophene on Ag ( 111 ) W. Gebauer *, M. BS.131er,A. Soukopp, C. V~iterlein, R. Fink, M. Sokolowski, E. Umbach Experimentelle Physik Il, Universit~'tWfirzburg, Am HubIand, D-97074 Wu'rzburg, Germany
Abstract
Thin films (less than 40 ,~.) of quaterthiophene (4T) were vapour deposited onto a Ag(111) surface under ultrahigh vacuum (UHV) and were investigated concerning their geometrical structure and photoluminescence (PL) properties by low-energy electron diffraction (LEED) and optical spectroscopy, respectively. Two different structural phases (c~ and [3) were observed, which exhibit high structural order, with crystalline domains that are oriented with respect to the Ag ( 111 ) surface. Phase c~is formed if 4T is deposited onto a 200 K cold Ag surface; phase 13forms after an additional annealing step at 300 K. Due to the high order, the PL spectra of both phases show very narrow lines (phase [3: FWHMo_o < 40 c m - 1) and highly resolved fine structures, which are discussed in detail. In comparison with 4T films on glass, PL efficiency of phase [3 is larger by a factor of at least five. Keywords: Luminescence; Films; Quaterthiophene
1. Introduction
Thin films of conjugated semiconducting polymers and oligomers are subject to numerous investigations with respect to a basic understanding of their electrical, optical and structural properties [ 1-3 ], as well as for possible technological applications, for example, organic light-emitting devices [4 ]. Among the oligomers, oligothiophenes (OTs) are of particular interest, since these molecules can be chemically 'tailored' for specific applications. In addition, OTs can be thermally evaporated, like most other oligomers; therefore vapour deposition under ultrahigh vacuum ( U H V ) conditions can be utilized as an ideal technique for preparing ultraclean films with varying thickness from a (sub) monolayer to thick films [5]. For thin OT films it is well established that the optical properties, especially the photoluminescence (PL), strongly depend on the detailed preparation conditions [ 1,6-8], since these may lead to different crystalline structures and to specific structural defects (e.g. grain boundaries) which may significantly influence the PL spectra. For instance, as a consequence of defects, polycrystalline OT films, obtained, e.g., by vapour deposition on glass, usually show only broad * Corresponding author. Tel.: +49 931 888 5742; fax: +49 931 888 5158; e-mail: [email protected]. 0379-6779/96/$15.00 @ 1996 Elsevier Science S.A. All rights reserved PILS0379-6779(96)04478-5
vibronic bands (FWt-IM--500 cm-1) and low PL quantum yields [1,8]. This has prohibited detailed spectroscopic investigations so far. In addition, further complications arise from the fact that most OT films exhibit PL spectra with more than one PL component [ 8 ]. Thus, a correlation of the optical properties and structural/morphological properties is difficult. In order to obtain films with low defect concentrations, we deposited the OT films on clean single-crystal surfaces under UHV. In this work we report on the geometrical structures and corresponding PL spectra of thin quaterthiophene (4T) films deposited on a clean Ag( 111 ) surface. On the one hand, 4T is a model oligomer which is well suited for preparing stable films by vapour deposition; and on the other hand, 4T :is small enough to be dissolved in standard solvents, and thus high-resolution spectra of matrix-isolated 4T exist for comparison [9]. The A g ( l l l ) surface was used as a substrate, because we have already observed that 4T forms a highly ordered commensurate monolayer of 4T molecules where the molecular planes are oriented parallel to the Ag(111) ('flat lying') [ 10]. As we shall demonstrate, we were also able to obtain highly ordered 4T multilayers, i.e. thin films. Because of the achieved structural order the PL spectra exhibit highly resolved vibrational fine structures, which, to our knowledge, have not been observed for thin OT films and which can be analysed in detail.
228
W. Gebauer et aI. / Synthetic Metals 83 (1996) 227-230
2. Experimental Cleaning of the Ag(111) substrate, 4T film preparation, and all measurements were carried out in a UHV chamber ( p = 10 - l ° mbar). The Ag(111) substrate was cleaned by Ar + sputtering and subsequent annealing. Low-energy electron diffraction (LEED) was used for the control of the structural quality of the clean Ag (111) surface and for structural investigations of the 4T films. The 4T films were prepared by vapour deposition at deposition rates below 1 A/ min. Film thicknesses were controlled by a quartz microbalance, which was calibrated by thermal desorption measurements and by a Dektak surface profilometer. During the deposition the substrate was always kept at 200 K. The PL spectra were recorded at 25 K (liquid He cooling) [6] and are not corrected for the instrumental response function. Additional X-ray absorption (NEXAFS) measurements were performed at the BESSY Synchrotron [ 10].
3. Results and discussion In the following we shall first report on the geometric structures of the 4T films. Thereafter, a detailed description of the PL spectra in correlation with the structural properties will be given. 3.1. Geometric structures
Fig. 1 (a) shows a LEED pattern taken at 20 eV electron energy and recorded directly after the deposition of a 20 .~. thick 4T film onto the 200 K cold A g ( l l l ) substrate. The obtained LEED pattern with sharp spots is clear evidence that a long range ordered, crystalline structure of 4T has been formed, even at these low temperatures. From the size of the sharp LEED spots we can deduce that the ordered 4T domains are at least 100 A wide, and that the orientation of the domains is determined by the single-crystalline Ag surface since, otherwise, rings rather than discrete LEED spots would have been observed. The low electron energy (long wavelength), of course, corresponds to the large size of the 4T unit cell. Due to the symmetry of the Ag( 111 ) surface the 4T domains have nucleated with six symmetry equivalent azimuthal orientations with respect to the Ag surface, which is also reflected by the sixfold symmetry of the LEED pattern of Fig. 1 (a). NEXAFS measurements reveal that the 4T molecules are on average inclined with respect to the Ag(111) surface. This phase will be termed phase cx in the following. The full structural analysis will be reported elsewhere [ 11 ]. Fig. 1 (b) shows a LEED pattern of the same 4T film which was additionally annealed at a temperature of about 320 K. At about 300 K the pattern of phase cx changes irreversibly to this completely different LEED pattern, which can be described by the super-structure matrix (6, 0; - 1, 2.5). The parameters of the corresponding real space unit cell are: a = ( 1 7 . 3 _ 1 . 5 ) A, b = ( 6 . 3 + 0 . 7 ) A and 7 = ( 8 3 . 5 + 4 ) ° .
Fig. I. LEED patterns of a 20 ~ thick 4T layer deposited on Ag( I 11 ): (a) as deposited at 200 K (phase cQ; (b) after annealing at 300 K (phase [3). The reciprocal lattice vectors a*. b* are indicated. The electron energy was 20 eV in both cases.
This phase will be termed phase [3 in the following. Contrary to the findings for phase c~, NEXAFS measurements of this phase indicate that the molecules are preferentially oriented with their molecular planes parallel to the substrate surface. Since the LEED pattern of phase [3 did not depend on thickness, the film must consist of parallel planes of 4T molecules arranged within the (2D) unit cell described above. From the area of the unit cell ( 108 .~a) we further deduce that there is only one 4T molecule in this (2D) unit cell, since the area required for a flat-lying 4T molecule can be estimated to be about 120 ,~?. This is illustrated by the model of Fig. 2. The orientation of the 4T molecule within the unit cell, which
W. Gebauer et aI. / Synthetic Metals 83 (1996) 227-230
229
Wavelength (nm) 700
600
550
500
I
I
I
I
4T PL
~
Ag(111):
~
T=25K
,,< ,.:'
."
"
I"
i;~
l~
,I ^
Fig. 2. Model of the unit cell of the 4T planes which constitute phase 13.The underlying Ag( I 11) is drawn to illustrate the orientation relative to the substrate. For further details see text. cannot be determined from the LEED data, was chosen to exclude a mutual overlap of the molecules. From these findings we conclude the 4T structure of phase ~3must be different from that which was found for polycrystalline 4T films on glass by Porzio etal. [2], because this structure exhibits a herringbone arrangement with four 4T molecules per unit cell. It is remarkable that the long axis of the unit cell (a) is oriented parallel to the close-packed A g ( l l 1) rows. This clearly indicates the influence of the corrugation of the surface potential of the A g ( 1 1 1 ) on the azimuthal ordering of the 4T. The observed parallel orientation of the molecular planes with respect to the substrate surface arises from the covalent bonding of the first 4T layer (monolayer) to the Ag(111) surface via the v-electrons of the flat-lying molecules [ 10]. If the 4T molecules on top of this first layer have sufficient mobility, e.g. during the annealing process, the first strongly bound 4T layer constitutes an efficient nucleation plane for the 4T layers, leading to a parallel arrangement of all molecules in the film. By this mechanism the geometrical structure of phase B is thus determined by the Ag(111) surface, at least for film thicknesses up to 40 * (about 12 layers) as here investigated. Therefore, we describe these films as 'epitaxial'.
3.2. PL spectra and efficiencies For a 20 ,~ thick 4T film ordered in phase c~ we observed a PL spectrum with a highly resolved fine structure (see Fig. 3 ( a ) ) . The 0 - 0 transition is clearly resolved at 20 985 cm - 1 with a half-width of the high-energy side of about 30 c m - 1. For phase ~3we found a slightly blue-shifted PL spectrum with even narrower lines, e.g. with a FWHMo_o < 40 c m - i at 25 K. These narrow PL lines are attributed to the high structural order which is achieved in both phases. The first peak at 21 025 cm - 1 (see Fig. 3 ( b ) ) is identified as the 0 - 0 transition for phase 9, since the lowest energy absorption line is observed at 21 035 cm -1 [11]. The Stokes shift is thus negligibly small and no marked polaronic or self-trapping effects due to lattice interactions are observed. The coincidence of the onset of the PL and absorption indicates that
b) -phase
p
I
i
I
i
I
r
14
16 18 20 22 Wavenumbers ( 1000 cm "1) Fig. 3. PL spectra of 4T films (Tm. . . . . =25 K): (a) 20 ~. thick 4T film
deposited at 200 K on Ag( 111 ) before the annealing step (phase cO; (b) after the annealing step (phase 13); (c) 250 .~ thick 4T film deposited on glass. the pure electronic transition is dipole allowed for both phases o f 4 T (c~ and f3). For comparison a typical spectrum of a 250 A thick 4T film deposited on glass is also shown in Fig. 3(c). In this spectrum all lines are broad (wider than 1000 c m - 1), no fine structure is resolved, and the intensity of the 0 - 0 band is very Small, if discernible at all. From our above results we conclude that inhomogenous broadening of the lines due to structural disorder and many defects are present in the 4T films deposited on glass. The absence of any further fine structure has been also found for several other OT films [ 1,3,8]. We now turn to the details of the PL spectra. The PL spectra of 4T and most OTs are mainly dominated by one vibronic progression, which is attributed to a ring-breathing mode with an energy of about 1470 cm - 1 [ 6,8,9 ]. This progression can be found in all three spectra of Fig. 3, e.g. as an intense peak around 19 500 cm - 1. Remarkably, the fine structure observed for phase c~ is rather different from that observed for matrixisolated 4T [9], especially in the region of the low-energy deformation modes (e.g. at 20 400-20 950 c m - 1), probably because of specific new vibrational modes in this phase 13. From infrared spectroscopy (FTIR) we have evidence that the 4T molecules in this phase are not in a full all-trans conformation, since several infrared transitions which are forbidden for the all-trans conformation are observed for this phase [ 12]. We thus suggest, in agreement with Ref. [ 12], that the changes in the fine structure of the PL spectrum of phase c~ arise from twisted or bent 4T molecules. Moreover, we note that there is a second broad and intense PL component visible in the spectrum of phase c~at about 18 000 cm - 1 This second PL component strongly increases with decreasing temperature and/or with decreasing film thickness; its origin is not fully understood yet.
230
W. Gebauer et al. / Synthetic Metals 83 (1996) 227-230
The vibrational structure of phase [3 is even better resolved than that of phase (x. The observed fine structure is very similar to that observed earlier for 4T films deposited on graphite (HOPG) [ 6] and for matrix-isolated 4T molecules [9]. This finding leads to the conclusion that the 4T molecules must have the same conformation (all-trans) in these three cases. This PL spectrum can be fully explained by five vibrational modes as published earlier [6]. Thus, we suppose that the molecular distortions, observed for phase (x disappear with the structural phase transition to phase [3, probably because of relaxation of stress. Furthermore, no defect or trap luminescence is found, which also points to a (nearly) perfect geometrical structure with a very low defect concentration. As discussed above, the packing of the molecules in the films on Ag differs from that on glass. However, the comparison of the PL spectra recorded for the two 4T films on Ag and the polycrystalline film on glass reveals that the energy positions of the dominant PL bands are very similar for all three films (see Fig. 3). This is remarkable, because it leads to the conclusion that the intermolecular interactions which generally influence the PL spectra are either very similar for these three films, or are only very small, causing line shifts which are at most of the order of 100 c m - 1. Finally, we briefly report on the PL efficiencies (integrated PL intensities) which are rather high here, since the quenching of the PL by the Ag surface is not as efficient as commonly expected [ 13]. During the phase transition from phase c~ to phase [3 a strong increase of the efficiency by a factor of about two to five is observed. In comparison with 4T films deposited on glass, the PL efficiency of phase [3 is at least a factor of five larger. In an additional experiment using HOPG as a substrate, we observed an even higher increase (factor of about 20) of the PL efficiency by a similar annealing process [ 11 ]. In both cases we explain this finding by a significant decrease of the concentration of structural defects which act as non-radiative recombination centres. In addition, the PL efficiency of phase [3 is found to be nearly independent of temperature, which also points to a low concentration of nonradiative defects. This is likely due to the high structural order. Although the low defect concentration alone may explain the high PL efficiencies, a second effect may also contribute. Fig. 3 shows that for the 4T films on Ag(111) the 0-0 transition is much stronger than that of the 4T film on glass, which exhibits an unusual vibronic progression with a very weak 0-0 band. This could indicate that the electronic transition for the 4T films on glass is only very weak due to the specific molecular packing. For the 4T films on Ag the transition gains oscillator strength because of a modified packing and improved structural order of the molecules, thus leading to higher PL efficiencies.
4. Conclusions We have demonstrated that 4T films with high structural order can be 'epitaxially' grown on a Ag( 111 ) surface up to a thickness of at least 40 ,~,. Two structurally different phases were found: one after adsorption at 200 K and the other after annealing at 300 K. In both of them the packing of the molecules is different to that found for polycrystalline 4T films. Due to the high structural order, the PL lines are much narrower than observed for OT films before, which enabled us to analyse the PL spectra in detail and to correlate them with the structural properties.
Acknowledgements We are grateful to Professor D. Oelkrug and Dr H.-J. Egelhaaf for helpful discussions and to Dr Naarmann, BASF AG, for providing the 4T. The project has been financially supported by the Deutsche Forschungsgemeinschaft (Um 6/41). E.U. thanks the Fond der Chemischen Industrie for financial support.
References [1] D. Oelkrug, H.-J. Egelhaaf, D.R. Won:all and F. Wilkinson, J. Fluorese., 5 (1995) 165. [2] W. Porzio, S. Destri, M. Mascherpa and S. Brtickner, Acta Polym., 44 (1993) 266. [3] A. Yassar, G. Horowitz, P. Valat, V. Wintgens, M. Hmyene, F. Deloffre, P. Srivastava, P. Lang and F. Gamier, J. Phys. Chem., 99 (1995) 9155. [4] C. Adachi, S. Tokito, T. Tsutsui and S. Salto, Jpn. J. Appl. Phys., 27 (1988) L629. [5] E. Umbach, C. Seidel, J. Taborski, R. Li and A. Soukopp, Phys. Status Solidi (b), 192 (1995) 389. [6] W. Gebauer, C. V~iterlein,A. Soukopp, M. Sokolowski andE. Umbach, Thin Solid Films, 284-285 (1996) 576. [7] F. Deloffre, F. Gamier, P. Srivastava, A. Yassar and J.-L. Fave, Synth. Met., 67 (1994) 223. [8] W. Gebauer, C. V~iterlein, A. Soukopp, M. Sokolowski, H. Port, P. B~.uerle and E. Umbach, Synth. Met., in press. [9] D. Bimbaum, D. Fichou and B.E. Kohler, J. Chem. Phys., 96 (1992) 165. [10] A. Soukopp, C. Seidel, R. Li, M. Biissler, M. Sokolowski and E. Umbach, Thin Solid Films, 284-285 (1996) 343. [II] W. Gebauer, M. B~il3Ier,C. V/iterlein, A. Soukopp, M. Sokolowski and E. Umbach, J. Chem. Phys., to be submitted. [12] R. Li, P. B~iuerleand E. Umbach, Surf Sci., 331-333 (1995) 100. [13] R.R. Chance, A. Prock and R. Silbey, in S.A. Rice and I. Prigogine (eds.), Advances in Chemical Physics, Vol. 37, Wiley-Interscience, New York, I971.
Synthetic Metals 83 (1996) 227-230
High-resolution luminescence of epitaxial organic films: quaterthiophene on Ag ( 111 ) W. Gebauer *, M. BS.131er,A. Soukopp, C. V~iterlein, R. Fink, M. Sokolowski, E. Umbach Experimentelle Physik Il, Universit~'tWfirzburg, Am HubIand, D-97074 Wu'rzburg, Germany
Abstract
Thin films (less than 40 ,~.) of quaterthiophene (4T) were vapour deposited onto a Ag(111) surface under ultrahigh vacuum (UHV) and were investigated concerning their geometrical structure and photoluminescence (PL) properties by low-energy electron diffraction (LEED) and optical spectroscopy, respectively. Two different structural phases (c~ and [3) were observed, which exhibit high structural order, with crystalline domains that are oriented with respect to the Ag ( 111 ) surface. Phase c~is formed if 4T is deposited onto a 200 K cold Ag surface; phase 13forms after an additional annealing step at 300 K. Due to the high order, the PL spectra of both phases show very narrow lines (phase [3: FWHMo_o < 40 c m - 1) and highly resolved fine structures, which are discussed in detail. In comparison with 4T films on glass, PL efficiency of phase [3 is larger by a factor of at least five. Keywords: Luminescence; Films; Quaterthiophene
1. Introduction
Thin films of conjugated semiconducting polymers and oligomers are subject to numerous investigations with respect to a basic understanding of their electrical, optical and structural properties [ 1-3 ], as well as for possible technological applications, for example, organic light-emitting devices [4 ]. Among the oligomers, oligothiophenes (OTs) are of particular interest, since these molecules can be chemically 'tailored' for specific applications. In addition, OTs can be thermally evaporated, like most other oligomers; therefore vapour deposition under ultrahigh vacuum ( U H V ) conditions can be utilized as an ideal technique for preparing ultraclean films with varying thickness from a (sub) monolayer to thick films [5]. For thin OT films it is well established that the optical properties, especially the photoluminescence (PL), strongly depend on the detailed preparation conditions [ 1,6-8], since these may lead to different crystalline structures and to specific structural defects (e.g. grain boundaries) which may significantly influence the PL spectra. For instance, as a consequence of defects, polycrystalline OT films, obtained, e.g., by vapour deposition on glass, usually show only broad * Corresponding author. Tel.: +49 931 888 5742; fax: +49 931 888 5158; e-mail: [email protected]. 0379-6779/96/$15.00 @ 1996 Elsevier Science S.A. All rights reserved PILS0379-6779(96)04478-5
vibronic bands (FWt-IM--500 cm-1) and low PL quantum yields [1,8]. This has prohibited detailed spectroscopic investigations so far. In addition, further complications arise from the fact that most OT films exhibit PL spectra with more than one PL component [ 8 ]. Thus, a correlation of the optical properties and structural/morphological properties is difficult. In order to obtain films with low defect concentrations, we deposited the OT films on clean single-crystal surfaces under UHV. In this work we report on the geometrical structures and corresponding PL spectra of thin quaterthiophene (4T) films deposited on a clean Ag( 111 ) surface. On the one hand, 4T is a model oligomer which is well suited for preparing stable films by vapour deposition; and on the other hand, 4T :is small enough to be dissolved in standard solvents, and thus high-resolution spectra of matrix-isolated 4T exist for comparison [9]. The A g ( l l l ) surface was used as a substrate, because we have already observed that 4T forms a highly ordered commensurate monolayer of 4T molecules where the molecular planes are oriented parallel to the Ag(111) ('flat lying') [ 10]. As we shall demonstrate, we were also able to obtain highly ordered 4T multilayers, i.e. thin films. Because of the achieved structural order the PL spectra exhibit highly resolved vibrational fine structures, which, to our knowledge, have not been observed for thin OT films and which can be analysed in detail.
228
W. Gebauer et aI. / Synthetic Metals 83 (1996) 227-230
2. Experimental Cleaning of the Ag(111) substrate, 4T film preparation, and all measurements were carried out in a UHV chamber ( p = 10 - l ° mbar). The Ag(111) substrate was cleaned by Ar + sputtering and subsequent annealing. Low-energy electron diffraction (LEED) was used for the control of the structural quality of the clean Ag (111) surface and for structural investigations of the 4T films. The 4T films were prepared by vapour deposition at deposition rates below 1 A/ min. Film thicknesses were controlled by a quartz microbalance, which was calibrated by thermal desorption measurements and by a Dektak surface profilometer. During the deposition the substrate was always kept at 200 K. The PL spectra were recorded at 25 K (liquid He cooling) [6] and are not corrected for the instrumental response function. Additional X-ray absorption (NEXAFS) measurements were performed at the BESSY Synchrotron [ 10].
3. Results and discussion In the following we shall first report on the geometric structures of the 4T films. Thereafter, a detailed description of the PL spectra in correlation with the structural properties will be given. 3.1. Geometric structures
Fig. 1 (a) shows a LEED pattern taken at 20 eV electron energy and recorded directly after the deposition of a 20 .~. thick 4T film onto the 200 K cold A g ( l l l ) substrate. The obtained LEED pattern with sharp spots is clear evidence that a long range ordered, crystalline structure of 4T has been formed, even at these low temperatures. From the size of the sharp LEED spots we can deduce that the ordered 4T domains are at least 100 A wide, and that the orientation of the domains is determined by the single-crystalline Ag surface since, otherwise, rings rather than discrete LEED spots would have been observed. The low electron energy (long wavelength), of course, corresponds to the large size of the 4T unit cell. Due to the symmetry of the Ag( 111 ) surface the 4T domains have nucleated with six symmetry equivalent azimuthal orientations with respect to the Ag surface, which is also reflected by the sixfold symmetry of the LEED pattern of Fig. 1 (a). NEXAFS measurements reveal that the 4T molecules are on average inclined with respect to the Ag(111) surface. This phase will be termed phase cx in the following. The full structural analysis will be reported elsewhere [ 11 ]. Fig. 1 (b) shows a LEED pattern of the same 4T film which was additionally annealed at a temperature of about 320 K. At about 300 K the pattern of phase cx changes irreversibly to this completely different LEED pattern, which can be described by the super-structure matrix (6, 0; - 1, 2.5). The parameters of the corresponding real space unit cell are: a = ( 1 7 . 3 _ 1 . 5 ) A, b = ( 6 . 3 + 0 . 7 ) A and 7 = ( 8 3 . 5 + 4 ) ° .
Fig. I. LEED patterns of a 20 ~ thick 4T layer deposited on Ag( I 11 ): (a) as deposited at 200 K (phase cQ; (b) after annealing at 300 K (phase [3). The reciprocal lattice vectors a*. b* are indicated. The electron energy was 20 eV in both cases.
This phase will be termed phase [3 in the following. Contrary to the findings for phase c~, NEXAFS measurements of this phase indicate that the molecules are preferentially oriented with their molecular planes parallel to the substrate surface. Since the LEED pattern of phase [3 did not depend on thickness, the film must consist of parallel planes of 4T molecules arranged within the (2D) unit cell described above. From the area of the unit cell ( 108 .~a) we further deduce that there is only one 4T molecule in this (2D) unit cell, since the area required for a flat-lying 4T molecule can be estimated to be about 120 ,~?. This is illustrated by the model of Fig. 2. The orientation of the 4T molecule within the unit cell, which
W. Gebauer et aI. / Synthetic Metals 83 (1996) 227-230
229
Wavelength (nm) 700
600
550
500
I
I
I
I
4T PL
~
Ag(111):
~
T=25K
,,< ,.:'
."
"
I"
i;~
l~
,I ^
Fig. 2. Model of the unit cell of the 4T planes which constitute phase 13.The underlying Ag( I 11) is drawn to illustrate the orientation relative to the substrate. For further details see text. cannot be determined from the LEED data, was chosen to exclude a mutual overlap of the molecules. From these findings we conclude the 4T structure of phase ~3must be different from that which was found for polycrystalline 4T films on glass by Porzio etal. [2], because this structure exhibits a herringbone arrangement with four 4T molecules per unit cell. It is remarkable that the long axis of the unit cell (a) is oriented parallel to the close-packed A g ( l l 1) rows. This clearly indicates the influence of the corrugation of the surface potential of the A g ( 1 1 1 ) on the azimuthal ordering of the 4T. The observed parallel orientation of the molecular planes with respect to the substrate surface arises from the covalent bonding of the first 4T layer (monolayer) to the Ag(111) surface via the v-electrons of the flat-lying molecules [ 10]. If the 4T molecules on top of this first layer have sufficient mobility, e.g. during the annealing process, the first strongly bound 4T layer constitutes an efficient nucleation plane for the 4T layers, leading to a parallel arrangement of all molecules in the film. By this mechanism the geometrical structure of phase B is thus determined by the Ag(111) surface, at least for film thicknesses up to 40 * (about 12 layers) as here investigated. Therefore, we describe these films as 'epitaxial'.
3.2. PL spectra and efficiencies For a 20 ,~ thick 4T film ordered in phase c~ we observed a PL spectrum with a highly resolved fine structure (see Fig. 3 ( a ) ) . The 0 - 0 transition is clearly resolved at 20 985 cm - 1 with a half-width of the high-energy side of about 30 c m - 1. For phase ~3we found a slightly blue-shifted PL spectrum with even narrower lines, e.g. with a FWHMo_o < 40 c m - i at 25 K. These narrow PL lines are attributed to the high structural order which is achieved in both phases. The first peak at 21 025 cm - 1 (see Fig. 3 ( b ) ) is identified as the 0 - 0 transition for phase 9, since the lowest energy absorption line is observed at 21 035 cm -1 [11]. The Stokes shift is thus negligibly small and no marked polaronic or self-trapping effects due to lattice interactions are observed. The coincidence of the onset of the PL and absorption indicates that
b) -phase
p
I
i
I
i
I
r
14
16 18 20 22 Wavenumbers ( 1000 cm "1) Fig. 3. PL spectra of 4T films (Tm. . . . . =25 K): (a) 20 ~. thick 4T film
deposited at 200 K on Ag( 111 ) before the annealing step (phase cO; (b) after the annealing step (phase 13); (c) 250 .~ thick 4T film deposited on glass. the pure electronic transition is dipole allowed for both phases o f 4 T (c~ and f3). For comparison a typical spectrum of a 250 A thick 4T film deposited on glass is also shown in Fig. 3(c). In this spectrum all lines are broad (wider than 1000 c m - 1), no fine structure is resolved, and the intensity of the 0 - 0 band is very Small, if discernible at all. From our above results we conclude that inhomogenous broadening of the lines due to structural disorder and many defects are present in the 4T films deposited on glass. The absence of any further fine structure has been also found for several other OT films [ 1,3,8]. We now turn to the details of the PL spectra. The PL spectra of 4T and most OTs are mainly dominated by one vibronic progression, which is attributed to a ring-breathing mode with an energy of about 1470 cm - 1 [ 6,8,9 ]. This progression can be found in all three spectra of Fig. 3, e.g. as an intense peak around 19 500 cm - 1. Remarkably, the fine structure observed for phase c~ is rather different from that observed for matrixisolated 4T [9], especially in the region of the low-energy deformation modes (e.g. at 20 400-20 950 c m - 1), probably because of specific new vibrational modes in this phase 13. From infrared spectroscopy (FTIR) we have evidence that the 4T molecules in this phase are not in a full all-trans conformation, since several infrared transitions which are forbidden for the all-trans conformation are observed for this phase [ 12]. We thus suggest, in agreement with Ref. [ 12], that the changes in the fine structure of the PL spectrum of phase c~ arise from twisted or bent 4T molecules. Moreover, we note that there is a second broad and intense PL component visible in the spectrum of phase c~at about 18 000 cm - 1 This second PL component strongly increases with decreasing temperature and/or with decreasing film thickness; its origin is not fully understood yet.
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The vibrational structure of phase [3 is even better resolved than that of phase (x. The observed fine structure is very similar to that observed earlier for 4T films deposited on graphite (HOPG) [ 6] and for matrix-isolated 4T molecules [9]. This finding leads to the conclusion that the 4T molecules must have the same conformation (all-trans) in these three cases. This PL spectrum can be fully explained by five vibrational modes as published earlier [6]. Thus, we suppose that the molecular distortions, observed for phase (x disappear with the structural phase transition to phase [3, probably because of relaxation of stress. Furthermore, no defect or trap luminescence is found, which also points to a (nearly) perfect geometrical structure with a very low defect concentration. As discussed above, the packing of the molecules in the films on Ag differs from that on glass. However, the comparison of the PL spectra recorded for the two 4T films on Ag and the polycrystalline film on glass reveals that the energy positions of the dominant PL bands are very similar for all three films (see Fig. 3). This is remarkable, because it leads to the conclusion that the intermolecular interactions which generally influence the PL spectra are either very similar for these three films, or are only very small, causing line shifts which are at most of the order of 100 c m - 1. Finally, we briefly report on the PL efficiencies (integrated PL intensities) which are rather high here, since the quenching of the PL by the Ag surface is not as efficient as commonly expected [ 13]. During the phase transition from phase c~ to phase [3 a strong increase of the efficiency by a factor of about two to five is observed. In comparison with 4T films deposited on glass, the PL efficiency of phase [3 is at least a factor of five larger. In an additional experiment using HOPG as a substrate, we observed an even higher increase (factor of about 20) of the PL efficiency by a similar annealing process [ 11 ]. In both cases we explain this finding by a significant decrease of the concentration of structural defects which act as non-radiative recombination centres. In addition, the PL efficiency of phase [3 is found to be nearly independent of temperature, which also points to a low concentration of nonradiative defects. This is likely due to the high structural order. Although the low defect concentration alone may explain the high PL efficiencies, a second effect may also contribute. Fig. 3 shows that for the 4T films on Ag(111) the 0-0 transition is much stronger than that of the 4T film on glass, which exhibits an unusual vibronic progression with a very weak 0-0 band. This could indicate that the electronic transition for the 4T films on glass is only very weak due to the specific molecular packing. For the 4T films on Ag the transition gains oscillator strength because of a modified packing and improved structural order of the molecules, thus leading to higher PL efficiencies.
4. Conclusions We have demonstrated that 4T films with high structural order can be 'epitaxially' grown on a Ag( 111 ) surface up to a thickness of at least 40 ,~,. Two structurally different phases were found: one after adsorption at 200 K and the other after annealing at 300 K. In both of them the packing of the molecules is different to that found for polycrystalline 4T films. Due to the high structural order, the PL lines are much narrower than observed for OT films before, which enabled us to analyse the PL spectra in detail and to correlate them with the structural properties.
Acknowledgements We are grateful to Professor D. Oelkrug and Dr H.-J. Egelhaaf for helpful discussions and to Dr Naarmann, BASF AG, for providing the 4T. The project has been financially supported by the Deutsche Forschungsgemeinschaft (Um 6/41). E.U. thanks the Fond der Chemischen Industrie for financial support.
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