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Applied Surface Science 212–213 (2003) 501–507 Orientation of perylene derivatives on semiconductor surfaces T.U. Kampen*, G. Salvan, A. Paraian, C. Himcinschi, A.Yu. Kobitski, M. Friedrich, D.R.T. Zahn Institut für Physik, Technische Universität Chemnitz, D-09107 Chemnitz, Germany Abstract The orientation of the perylene derivatives 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,N0 -dimethyl3,4,9,10-perylenetetracarboxylic diimide (DiMe-PTCDI) on sulphur passivated GaAs(1 0 0) surfaces and its impact on the optical properties were studied by means of near-edge X-ray absorption fine structure spectroscopy (NEXAFS), Raman spectroscopy, and variable angle spectroscopic ellipsometry (VASE). NEXAFS shows that PTCDA molecules lie flat on the substrate with their molecular plane parallel to the substrate surface. DiMe-PTCDI molecules grown on the same type of substrates are tilted with respect to the substrate surface and are predominantly oriented with their long axis parallel to the [1 1 0] direction. The optical properties of these films investigated by VASE show that the DiMe-PTCDI films exhibit a much stronger optical anisotropy than the PTCDA films. # 2003 Elsevier Science B.V. All rights reserved. PACS: 78.66.Qn; 78.30.Jw; 68.55.-a Keywords: Organic films; PTCDA; Structural properties; Optical anisotropy 1. Introduction The two perylene derivatives 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,N0 -dimethyl-3,4,9,10-perylenetetracarboxylic diimide (DiMePTCDI) form monoclinic crystalline structures with a C2h symmetry. The unit cell contains 2 molecules almost perpendicular and parallel to each other in PTCDA and DiMe-PTCDI crystals, respectively. Due to their symmetry crystals show anisotropies in their physical properties. On substrates like Ag(1 1 1) [1], Au(1 1 1) [2], and HOPG [3] PTCDA is adsorbed in a ‘‘herring bone’’ structure with two molecules almost * Corresponding author. Tel.: þ49-371-531-3079; fax: þ49-371-531-3060. E-mail address: [email protected] (T.U. Kampen). perpendicular to each other and parallel to the substrate surface in each unit cell. This molecular arrangement of the molecules is similar to the one in the (1 0 2) plane of a PTCDA crystal. On Ag(1 1 0) a phase transition is observed from a single domain oriented homogenous monolayer with a ‘‘brick wall’’ structure into a more condensed ‘‘herring bone’’ structure [4]. On InAs(0 0 1) substrates the monolayer PTCDA interacts relatively strongly with the substrate and two-dimensional overlayers are formed [5]. For larger coverages a phase transition occurs where bulk-like three-dimensional PTCDA clusters begin to form. The strong interaction between PTCDA and semiconductor surfaces is considerably reduced by a passivation of the surface prior to the growth of the organic film [6,7]. Changing the structure of the molecule, e.g. by an imide group with different side groups instead of the 0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00390-8 502 T.U. Kampen et al. / Applied Surface Science 212–213 (2003) 501–507 anhydride group, the growth properties are changed drastically. Depending on the preparation procedure di(2,6-isopropylphenyl)-3,4,9,10-perylenetetracarboxylic diimide molecules lie parallel or tilted with their perylene core with respect to the Ag(1 1 1) substrate [8]. In epitaxial films grown on cleaved KCl(0 0 1) surfaces the molecular planes of DiMePTCDI molecules are cofacially stacked parallel to the KCl substrate, whereas dibutyl-3,4,9,10-perylenetetracarboxylic diimide molecules are oriented standing upright on the surface [9]. In this work we determined the orientation of PTCDA and DiMe-PTCDI molecules grown on sulfur passivated GaAS(1 0 0) surfaces by NEXAFS and Raman spectroscopy. The impact of molecular orientation on the optical properties is investigated by VASE. 2. Experimental All samples were prepared under ultra-high vacuum (UHV) conditions on S-passivated Te-doped n-type GaAs(1 0 0) (Freiberger Compound GmbH, n ¼ 2 1017 cm3). Prior to the sulfur passivation, the substrates were degreased by ultrasonic baths in acetone, ethanol, and de-ionized water. S-passivation was performed by etching in a diluted solution of S2Cl2 and CCl4 (1:3), followed by rinsing in CCl4, acetone, ethanol, and de-ionized water. Immediately after the chemical treatment the samples were transferred into the UHV chamber and annealed at 430 8C. This results in the formation of Ga2S3 like layer terminated by single S atoms [10]. Onto these surfaces PTCDA and DiMe-PTCDI films were grown by thermally evaporation of pre-purified material. NEXAFS spectroscopy was performed at the PM1 beamline of the synchrotron light source BESSY II. The data were recorded in total yield mode and the light incidence angle was varied between normal and near-grazing incidence (08 and 708). For the normalization spectra of a Ag film of 100 nm thickness were taken and used as reference spectra. Raman measurements were performed with a Dilor XY 800 triple monochromator spectrometer equipped with a CCD detector. As excitation energy the 2.54 eV emission line of an Arþ laser was chosen which is resonant with the first maximum of the absorption spectra of both molecules. The films were measured in a backscattering geometry with a spectral resolution of 2.5 cm1. The incident beam having a power of 30 mW was focused onto a spot of 300 mm in diameter. For comparison Raman measurements were taken from a crystal obtained by sublimation using an Olympus microscope with 100 magnification objective in backscattering geometry with a spectral resolution of 1.2 cm1. The spot size in this case was 1 mm in diameter and the power measured in the focus was 0.08 mW. Two polarization configurations were used for each sample position. In the Porto notation zðxxÞz and zðxyÞz denote the cases where the electric field vector of scattered light is parallel/perpendicular to that of the incident light (parallel/crossed polarization configuration). The position of the polarization analyser was maintained fixed in all experiments, in order to eliminate the systematic error which might be induced by the different response of the spectrometer to differently polarized light. For the measurements performed on the molecular films the laboratory axes coincide with the substrate axis (x ¼ xs ¼ ½1 1 0 , y ¼ ys ¼ ½1 1 0 , z ¼ zs ¼ ½100 ) when the angle of rotation around the surface normal is g ¼ 0 . The orientation of molecules is determined from the depolarization ratio of the breathing mode at 220 cm1 obtained upon rotation of the sample around the surface normal. For a simulation of the depolarisation ratio the Raman tensor specific for the Ag modes is determined from density functional calculations using Gaussian98 package at the B3LYP level of theory with a standard 6-31G(d) basis set [11]. The symmetry of the highest occupied molecular orbital (HOMO) of DiMe-PTCDI is Au, and that of the lowest unoccupied molecular orbital (LUMO) is Bg. Therefore, a resonant Raman effect that involves a HOMO-LUMO transition couples vibrational transitions with an electronic transition dipole of which is orientated along the xm axis (where xm denotes the long axis within the molecular plane while ym and zm denote short axis and the direction perpendicular to the molecular plane, respectively). Under resonant excitation the off-diagonal components disappear and for the lowest frequency internal mode (220 cm1) the Raman tensor reads 0 1 1 0 0 @ 0 0:04 0 A (1) Am g ¼ 0 0 0 T.U. Kampen et al. / Applied Surface Science 212–213 (2003) 501–507 The spectroscopic ellipsometry measurements were performed ex situ using a variable angle spectroscopic ellipsometer (WVASE from J.A. Woollam Co., Inc.) equipped with vertical sample mounting stage, continuously rotating analyser and auto-retarder and a Xelamp source. The measured ellipsometric data are expressed in terms of the effective dielectric function hei ¼ sin2 F þ sin2 Ftan2 F½ð1 rÞ=ð1 þ rÞ 2 where r ¼ rp =rs tanðCÞeiD is the complex reflectance ratio [12], F is the angle of incidence of light, C and D are so-called ellipsometric angles, and rp and rs are the Fresnel coefficients for light polarized parallel and perpendicular to the plane of incidence, respectively. 503 3. Results and discussions In NEXAFS absorption of the incident synchrotron light takes place due to the excitation of an electron from a core shell into the lowest unoccupied states. In perylene derivatives the lowest unoccupied molecular orbitals are derived from the p-electron system with its orbitals oriented perpendicular to the molecular plane. Due to selection rules excitation between the core shell and the p-orbitals takes place if the electric field vector of the incident light has a component parallel to the p-orbitals. Fig. 1a presents selected C-K shell spectra of PTCDA for different angles of incidence. Fig. 1. NEXAFS spectra of the C-K edge of PTCDA (a) and the O-K edge of DiMe-PTCDI (b) as a function of the angle of incidence of synchrotron light. Also the intensity ratio max1/max2 is plotted in b) as a function of angle of incidence. 504 T.U. Kampen et al. / Applied Surface Science 212–213 (2003) 501–507 The incident synchrotron light is linear polarized with the electric field vector lying in the plane of incidence. For an angle of incidence of 08 only broad features above 280 eV photon energy are observed. When changing the angle of incidence to 38 and 708, however, sharp structures below 280 eV photon energy appear. The features above and below 280 eV are attributed to unoccupied p- and s-states, respectively. The intensity increase of the p-resonances with increasing angle of incidence is clear evidence for the parallel orientation of the molecular plane with respect to the substrate surface. For DiMe-PTCDI the p-resonances depend quite differently on the angle of incidence. Fig. 1b shows the O-K shell spectra as a function of the angle of incidence and the intensity of the p-resonances normalized to the intensity of the s-resonances. Here, a maximum in intensity of the p-resonances is obtained for an angle of incidence of about 408. From this result the angle between the DiMe-PTCDI molecular plane and the surface is determined to be about 508. While PTCDA films grown on S-passivated GaAs show the same molecular orientation as for the growth on other substrates, e.g. the (1 0 2) plane of the crystal is parallel to the substrate surface, the orientation of molecules in DiMe-PTCDI films is more complicated and will now be further evaluated by Raman spectroscopy. In Fig. 2 Raman spectra of a DiMe-PTCDI single crystal and a DiMe-PTCDI thin film are shown for the two different polarization geometries. As a consequence of crystal formation with two molecules per unit cell external vibrational modes can be observed in the Raman spectra. The vibrational representation for the external modes is: G ¼ 3Ag þ 3Bg þ 2Au þ Bu . The six modes having even symmetry will show Raman activity at frequencies below 125 cm1. As can be seen in Fig. 2 phonons appear in the Raman spectra of single phase crystals as well as in those of films exhibiting similar polarization response. The phonon intensity relative to that of the internal modes is larger in the crystal compared to film spectra. The difference can be explained by the larger size of the crystals compared to that of the grains in the films. Moreover, the spot size in the case of films allows to probe more than 105 grains with various sizes. The strong polarization response for both external and internal modes is remarkable and indicates a preferred orientation of the grains with respect to the substrate axes. Fig. 2. Comparison between the Raman spectra of DiMe-PTCDI crystal and film obtained in zðxxÞz (upper curves) and zðxyÞz (lower curves) polarization configurations. The packing of the molecules into a crystalline environment is expected to affect the internal modes due to in-phase and out-of phase coupling. This effect called Davydov splitting depends on the number of molecules in the unit cell and their dipole and quadrupole interaction. The former determines the multiplicity and the latter the amount of splitting. For DiMe-PTCDI with two molecules in the unit cell there should be a two-fold splitting. However, due to the fact that the angle between the molecules in the unit cell is 368 the dipole interaction which is proportional to the cosine of that angle is so small that the amount of splitting lies at the resolution limit of the Raman experiment. Therefore, the use of a single molecule Raman tensor is likely to be justified in this particular case. For a quantitative analysis of the polarization response one defines the depolarisation ratio as the intensity ratio between the Raman signal obtained in crossed polarization configuration to that obtained in parallel configuration. The experimental depolarisation T.U. Kampen et al. / Applied Surface Science 212–213 (2003) 501–507 Fig. 3. Experimental (symbols) and simulated (lines) depolarization ratios of the breathing mode at 220 cm1 obtained upon rotation around the sample normal with angle g for a DiMe-PTCDI film. ratio of the 220 cm1 mode for a film is depicted as a function of rotation angle g in Fig. 3. In order to extract the geometrical arrangement of the molecules from the depolarisation ratios there are three coordinate systems to take into account: molecular (xm, ym, zm), substrate (xs, ys, zs) and laboratory (x, y, z). Two consecutive transformations are required to transform the molecular Raman tensor to the Raman tensor Ag,l in the laboratory reference frame. The first orthogonal transformation can be applied using the Eulerian angles (j,y,c) which were previously applied by Aroca et al. to Raman study of orientations in a highly symmetric molecular system [13]. The second transformation is from the substrate to the laboratory coordinate system and implies a clock-wise rotation around the substrate normal (zs) with the angle g. The Raman intensity is then calculated as: I ¼ ðes Ag;l ei Þ2 , where ei and es are the electric field vectors for the incident and the scattered light. A least square fit of the experimental ratio was performed using a Levenberg-Marquard algorithm. The angles are determined with an of 68. The best match between the calculated depolarisation ratios using a one-molecule approximation is represented in Fig. 3 by the dashed line. Here, the molecular plane is tilted with respect to that of the substrate by 538, and the angle between the projection of xm onto substrate plane and [1 1 0] substrate axis is 78. However, even though the main maxima of the experimental data are reproduced well, the steep minima and the same height of all maxima indicate that a more complex 505 model is required. For the molecular planes of the two molecules tilted by þ5 and 58 with respect to the (xuyu) plane, and the long axis of the two molecules are rotated by þ18 and 188 with respect to the xu axis, several solutions provide a reasonable fit to the experimental data. The one which approaches mostly the relative orientation of the two molecules in the crystal unit cell is presented in Fig. 3 (full line) and provides the following Euler angles: j ¼ 116 ; y ¼ 58 , c ¼ 25 . This means, that the crystal plane (1 0 2) forms an angle of 588 with the substrate plane, and the projections of the long molecular axis are deviating from the [1 1 0] direction of the substrate by 9 and 488 respectively. The impact of the different orientation of molecules in PTCDA and DiMe-PTCDI films on the optical properties is demonstrated in Fig. 4. Here the imaginary part of the effective dielectric function (Imhei) for two azimuthal orientations is presented. The PTCDA has already been discussed in detail and is now compared with data obtained from DiMe-PTCDI films [14]. For the PTCDA film the ellipsometry spectra show characteristic absorption features of PTCDA at 2.21 eV and also between 2.47–2.57 eV in agreement with the results of absorption spectroscopy [15,16]. The spectra measured in both azimuthal directions differ slightly as can be seen in the spectral region around 3.75 eV. At this spectral position a simple shift is observed for the [1 1 0] direction while the shift is accompanied by a change in magnitude of Imhei in the case of [110]. The DiMe-PTCDI films, on the other hand, show a very strong angular dependence on the angle of incidence. This is accompanied by large differences in Imhei between different azimuthal orientations of the sample. As interference induced feature below 2 eV can be seen for the [110] direction, but not for the [1 1 0] direction. On the other hand, the feature around 3.2 eV attributed to the E1 gap in GaAs is enhanced due to interference in the [1 1 0] direction. The presence of negative values Imhei is also induced by interference. With the help of absorption spectroscopy one can also identify electronic transitions characteristic for DiMe-PTCDI in the spectra of Imhei. The feature around 2.14 eV corresponds to the first absorption maximum in the optical absorption spectrum. Its Imhei values are larger for the 506 T.U. Kampen et al. / Applied Surface Science 212–213 (2003) 501–507 Fig. 4. Imginary part of the effective dielectric constant for PTCDA (150 nm) and DiMe-PTCDI (120 nm) films grown on S-passivated GaAs(1 0 0) surfaces. Spectra are taken for two different azimuths and different angles of icidence. [110] azimuth. The higher transitions in the DiMePTCDI film observed in the absorption spectrum are not very well resolved in the ellipsometry spectra for this film also due to the interference effects which may cover some of the transitions of the film itself. These results clearly indicate that the optical anisotropy is larger in the case of DiMe-PTCDI films. be modelled considering the Raman tensors of two molecules arranged in orientation similar to the single crystal. The angle between the (1 0 2) plane of crystalline domains are tilted with respect to the substrate plane by 588 and the long molecular axis are deviating from the [1 1 0] direction of the substrate by 9 and 488 respectively. The macroscopically extremely high order of molecules in DiMe-PTCDI films leads to a larger optical anisotropy compared to PTCDA films. 4. Summary The orientation of molecules in PTCDA and DiMePTCDI films is found to be parallel and tilted with respect to the sample substrate surface, respectively. Raman-active internal modes of DiMe-PTCDI exhibit a strong polarization dependence and their depolarisation ratio varies with a period of 1808 as the sample is rotated around substrate normal. This behaviour can Acknowledgements The authors gratefully acknowledge the financial support provided by the Graduiertenkolleg ‘‘Dünne Schichten und nichtkristalline Materialien’’ at Technical University Chemnitz and the EU DIODE network (HPRN-CT-1999-00164). T.U. 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