Epitaxial growth of copper phthalocyanine monolayers on Ag(111)

surface science ELSEVIER

Surface Science 366 (1996) 403-414

Epitaxial growth of copper phthalocyanine monolayers on Ag(lll) J.-Y. Grand, T. Kunstmann, D. Hoffmann, A. Haas, M. Dietsche, J. Seifritz, R. M611er * 4. Physikalisches lnstitut, Pfaffenwaldring 55, D-70550 Stuttgart, Germany Received 16 January 1996; accepted for publication 30 May 1996

Abstract Monolayer growth of copper phthalocyanine on A g ( l l l ) has been studied by STM. Three different superlattices with oblique or rectangular unit cells have been identified. The four-fold symmetry of the molecules is maintained for two of the superstructures. For the third, the observed topography indicates a deformation of the molecules, which may be attributed to the interaction with the substrate. Grain boundaries between domains of the same type of superstructure, as well as between domains of different superstructures, have been resolved. The monolayer ordering persists across monoatomic steps of the substrate surface. In some cases these are modified by the interaction with the molecules.

Keywords: C~phthalocyanine; Epitaxy; Monolayer growth; Scanning tunneling microscopy

1. Introduction Ordered layers of organic molecules on metallic surfaces were investigated by STM for the first time by Ohtani et al. [ 1] for the system of benzene coadsorbed with CO on R h ( l l l ) . The adsorption of, for example, perylen tetracarboxylic acid dianhydride (PTCDA) and different thiophene molecules on metallic surfaces has been reviewed by Umbach et al. [2]. The (111) surfaces of cubic crystals with their hexagonal symmetry are generally not regarded as a suitable substrate for the epitaxial growth of molecular layers if the molecule has a four-fold symmetry. Nevertheless several examples were reported. Epitaxial layers of CuPc on C u ( l l l ) * Corresponding author. Fax: +49 711 6855097; e-mail: [email protected]

were studied by Buchholz and Somorjai [3] by LEED. Using STM, Gompf et al. [4] investigated monolayers of CuPc on graphite and MoSz. On the latter substrate they found two different phases, depending on coverage. First studies of the CuPc molecule by STM were performed on polycrystalline silver by Gimzewski et al. [5]. Further investigations of CuPc by STM have been performed by Lippel et al. [6] on Cu(100) who imaged individual molecules as well as ordered layers. Recently influences of CuPc molecules on the formation and displacements of steps on Ag(110) were reported by B6hringer et al. [7]. In the present paper we report on ordered monolayers of CuPc on Ag(111). The interaction with the substrate leads to well-defined superlattices which seem to be commensurate to the substrate lattice. Two rectangular unit cells have been observed which align parallel to a principal

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direction of the substrate, i.e. steps on the substrate, with either the short or the long axis. An oblique unit cell provides the closest packing of the molecules. It strongly resembles a unit mesh found for CuPc multilayers [8]. In principle, six equivalent domain orientations can be obtained by rotation or reflection of one unit cell, due to substrate symmetry. On the investigated area, some of these domains have been observed. At grain boundaries small areas are found which are not perfectly ordered, but leave a few molecules in a mobile position. 0

2. Experimental The experiments have been performed by a selfbuilt STM operated in ultrahigh vacuum at a base pressure of 5 x 10 -11 mbar. The coarse approach of the STM tip is performed by spring reduction, and the fine motion is performed by a piezo tube; vibrational damping is provided by a stack of metal plates separated by viton spacers. Tip and sample can be changed in situ. The tips were prepared by electrochemical etching of a polycrystalline tungsten wire of 0.5 mm in diameter. They were cleaned in situ by head-on sputtering with Ar ÷ ions of 2 keV at a current density of 150 #A cm -z for several hours. Shortly before the measurement an additional preparational step was performed by field desorption. The linearity of the current-to-voltage characteristic was verified on clean metal surfaces. Furthermore, it was checked that dl/dz corresponds to the appropriate work function of several eV. The Ag( 11 l ) substrate was prepared by epitaxial growth of Ag on mica. The latter was heated to 550 K 12 h before and during the evaporation. The silver was evaporated at a rate of 10A s -1 as determined by a quartz microbalance. A total thickness of 2000 .A was chosen, providing a film which exhibits large flat terraces with ( 111 ) orientation. These parameters for the preparation agree with those reported by other groups [9]. An STM image of a small area of the Ag( 111 ) surface been obtained by this preparation is shown in Fig. 1. Initially only a small interaction of the CuPc molecule with Ag(ll 1) was expected: the results

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Fig. 1. Topography of A g ( l l l ) obtained by STM. The experimental parameters are Vt = - 1 mV for the tunneling bias at the tip and I t = 250 pA for the tunneling current. The orientation of the substrate lattice is given by the vector al. The vertical corrugation is about 0.03 nm.

showed later that this is not necessarily true. Experiments where approximately one monolayer was evaporated on a clean Ag(111) showed large areas where the molecules are too mobile to allow imaging by STM at room temperature. Only small areas revealed a stable molecular ordering. Therefore nucleation sites were added in the form of small iron clusters. From previous experiments it is known that a small amount of iron, the equivalent to 0.5 ,~, evaporated onto the silver substrate at 540 K leads to a decoration of step edges with iron clusters. By investigation of the thermovoltage across the tunneling barrier, it could be verified that the flat areas were not covered by a continuous monolayer of iron [ 10]. The analysis of the mass spectra of the molecules leaving the Knudsen cell used to evaporate the CuPc revealed that the substance had to be purified. At the beginning, several smaller molecules, probably subunits of CuPc, were detected in great abundance. However, the quality could be greatly improved by heating the cell with the CuPc to a temperature slightly below the sublimation temperature of about 500 K for several days. For the preparation of the monolayer a procedure was applied which had been suggested by results of

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X-ray photoelectron spectroscopy and by thermal desorption spectroscopy [ 11 ]. Instead of evaporating the necessary amount of CuPc for monolayer coverage, several layers were evaporated onto the silver surface. The additional layers were subsequently desorbed at 540 K for 10 min until only a monolayer was left. Measurements by a quadrupole mass spectrometer revealed that the desorbing molecules were not fragmented. This method allows us to obtain an exact monolayer with more reliability than controlled monolayer deposition [12].

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3. Results

The data displayed in the following figures represent only a small representative fraction of the images which have been taken at various places on the sample. It should be noted that no image processing has been applied to the data, except that a planar background has been subtracted to correct for a tilt of the sample. For Fig. 2a, a partially shaded representation has been chosen to visualize the molecules on that large-area scan. All other images are in standard gray-scale representation. The large-area scan of Fig. 2a shows the topography of a CuPc monolayer film on A g ( l l l ) . The applied bias voltage is - 0 . 7 V at the tip, the tunneling current is 5 pA. The large protrusions are iron clusters with a typical diameter of 20 ,~ and a height of 10 A. Several lines of monoatomic steps of the silver substrate surface are visible. Most of them are oriented relative to each other with angles which are multiples of 60 ° as can be seen, for example, at the lower left corner of Fig. 2a. This yields the orientation of the hexagonal lattice of Ag( 111 ), as indicated by the lattice vector a~ in Fig. 2b. One has to be careful, however, since the orientation of some steps are induced by the molecular lattice, as will be discussed later. On the fiat areas ordered arrays of molecules are visible. An analysis of all the collected data leads to three different superstructures, which will be labeled I, II and III in the following. All the observed domains derive from these superlattices by rotation or reflection according to the symmetry of the

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Fig. 2. (a) Large-area scan of a monolayer of CuPc on Ag( 111). The protrusions are iron clusters deposited before the preparation of the CuPc film. Severalmonoatomic steps of the substrate are visible. The experimental parameters are Vt =-0.7 V for the tunneling bias at the tip and I t = 5 pA for the tunneling current. (b) Identificationof different domains labeled according to the superstructures defined in Fig. 7. The orientation of the substrate lattice is given by the primitivevector a~. The notation (M} stands for reflection at al (R=240 °) for a ccw rotation of 240°. The corresponding unit cells are linearly magnified by a factor of ten and indicated for the different superstructures.

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Ag(111) surface. Fig. 2b indicates the different domains which can be distinguished in Fig. 2a. A higher magnified image of superstructure I is given in Fig. 3. The lattice is rectangular, the long axis is parallel to a primitive lattice vector of the substrate which is 60 ° clockwise from a r The size of the unit cell has been deduced from large-area scans which were carefully checked for the absence of thermal drift, etc. Within the experimental uncertainties [13] superstructure I can described by the following matrix

The area of the monomolecular unit cell is A-det M . A s = 2 5 A s = 181 ~2, where A s is the area of the unit cell of the substrate. The structure is coincident, and the unit cell of the coincidence lattices comprises two unit cells of the superstructure. As can clearly be seen, the molecules are not symmetrically arranged within this unit cell. A reasonable agreement with the measurement is found for an angle of about 30 ° between one 10

molecular axis, as defined by c 2 in Fig. 7, and the longer side of the unit cell, denoted b2. However, a simple structural model for the arrangement of the molecules in the unit cell suggests a slightly different angle of 27 °. For this model, the size of the molecule has been taken from Ref. [3] and the hydrogen atoms were drawn with a van der Waals radius of 1.1 ,~. Assuming that the free molecules are of quadratic symmetry and not distorted, for example by the interaction with the substrate, an angle of 27 ° leads to the smallest overlap of the electronic wave functions. The molecular axis cl would then be almost aligned with the lattice vector al of the substrate. The proposed position and orientation of the molecules of one unit cell have been overlaid onto the STM image in Fig. 2. It should be noted that the position of the silver atoms is arbitrary with respect to translations; only the orientation of the substrate relative to the superstructure can be given since it was not possible to obtain images of the substrate and the molecules on the same scan frame. If the tunneling current is increased to more than 30 pA in order to visualize the substrate, the tip usually picks up molecules and the "clean" vacuum barrier between tip and sample is lost. The second rectangular superstructure (II) is displayed in detail in Figs. 4 and 5. For this structure the short axis is parallel to the substrate vector al. The size of the unit cell has been determined on the basis of many independent scans. Within the experimental uncertainties a primitive superstructure is obtained that can be represented by the following matrix

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Fig. 3. Small area with a molecularordering accordingto superstructure I. The proposed structure accordingto Fig. 7 has been overlaid. The experimentalparameters are Vt= -0.7 V for the tunneling bias at the tip and It = 5 pA for the tunneling current.

Assuming that this assignment is correct, the length of the short axis is only 11.6 A, and the long axis is 15.0 A. Again the molecules are not oriented in a symmetric fashion, Figs. 4 and 5 show examples for the two possible orientations resulting from the reflection at the a 1 direction. As can be seen clearly in Fig. 5, the four-fold symmetry of the molecule is broken in the topography observed by the STM. The angle [ 14] between the molecular axis cl and c2 is about 98 ° instead of 90 °, and the

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Fig. 4. Small area with a molecular ordering according to superstructure II (reflected at al). The proposed structure according to Fig. 7 has been overlaid. The experimental parameters are Vt= -0.7 V for the tunneling bias at the tip and It=5 pA for the tunneling current.

molecular shape can no longer be circumscribed by a square as for structure I, but by a rectangle. The different apparent forms of the molecules will be referred to as "quadratic" and "rectangular" shape in the following. As will be discussed later, this is no artifact of the S T M due to thermal drift, etc. since quadratic and rectangular shaped molecules are found within the same scan frame at d o m a i n boundaries (see Fig. 8). Since the S T M maps the density of electronic states contributing to the electronic tunneling current the observation of non quadratic molecules not necessarily implies that the molecule itself, i.e. the position of the nuclei is distorted. A distortion of the molecule is suggested, however, by the length of the short axis of the unit cell. Two fiat-lying phthalocyanine molecules c a n n o t a p p r o a c h each other so close in one plane without assuming either an unreasonable overlap of the molecular wave functions or a bending of the molecule. O n e might also consider that on the surface the molecules are inclined. This question c a n n o t yet be answered unambiguously

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Fig. 5. Small area with a molecular ordering according to superstructure II. The proposed structure according to Fig. 7 has been overlaid. The dashed lines indicate the position of the height profiles shown in Fig. 11. The experimental parameters are Vt = - 1.0 V for the tunneling bias at the tip and I t = 8 pA for the tunneling current.

since some measurements indicate a tilt (eventually due to an asymmetric tip); however, there are other results where different superstructures are imaged simultaneously which do not show the slightest indication of a tilt. F u r t h e r m o r e the observed height of the molecule equals the one of structure I (or 1II). To achieve a reasonable agreement between the observation and a model for the structure, we assume a distortion of the molecule which can be described by a compression of 8% along an axis at 45 ° between ct and c2, and an extension of 8% along an axis perpendicular to the previous one. The new molecular axes are then given by

c'z/

\ 0.08

0.994 / \c'2/

The determinant of this matrix is unity, the "area" of the molecule is maintained. As can be seen from Fig. 7 the proposed form of the molecule leads to a very small overlap of the van der Waals radii of

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two neighbouring molecules which is comparable to the situation found for structure I and for structure III, as will be seen later. The angle between e~ and c~ is 98 °, which is in agreement with the topography observed by STM. The direction of c~ is very close to a primitive lattice vector of the substrate (al + 60 ° clockwise rotation). The proposed structure has been overlaid in Fig. 4 (reflected at at) and Fig. 5 to allow a comparison. The area of the unit cell is A = d e t M'A~=24As=174,~ 2, and hence the packing is slightly denser than that of structure I. Fig. 6 shows a detailed scan of superstructure III. Again the structure is coincident with the substrate if one takes the range of experimental errors into account. It can be described by the following matrix

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93) _

.

This unit cell is oblique with the two lattice vectors not differing very much in length bl = 12.6 A and b2 = 13.8 A. The corresponding coincidence lattice contains two unit cells of the superstructure. The area of the unit cell is A = d e t M'As= 23.5As = 170 ~2, and hence it represents the densest structure observed in our experiments [ 15]. As for the two other structures, one axis (e2) of the molecules almost aligns with a lattice vector of the substrate (a2). A model of the structure, which is displayed in Fig. 7, reveals that this configuration permits a very efficient packing of the molecules without significant overlap of the molecular wave functions or a distortion of the molecule. The angle ~, for the orientation of the molecule given is deduced from the model rather than from the observed topography. It minimizes the interaction of the molecules; even small changes which are still in good agreement with the experimental uncertainties [13] would lead to a substantial increase of the overlap. This structure is also observed in an orientation which is obtained by reflection at ax, for example in the lower right part of Fig. 2. Besides the well-ordered area of molecules, Fig. 2 shows several effects resulting from the interaction

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Fig. 6. Small area with a molecular ordering according to superstructure III. The proposed structure according to Fig. 7 has been overlaid. The experimental parameters are Vt = - 1.0 V for the tunneling bias at the tip and I t = 5 pA for the tunneling current.

of the molecular superlattices with monoatomic steps of the substrate. The intrinsic step directions for the bare substrate are in [ 1i-0] or symmetrically equivalent directions. 1-16]. As observed in previous experiments [10], the iron clusters may pin the steps but they do not alter the direction. One lattice vector of structures I and II may align with one of structure III. However, this is not possible for the second lattice vector of the superstructure. In this particular situation the superstructure may induce a step in [112] or an equivalent direction, i.e. at a right angle to the first step line. An example is found in Fig. 2a in the region of structure I on the left side. It has been marked in Fig. 2b. Similar effects are observed for structure III, which cannot align with any of the natural step directions of the substrate. In the upper right corner of Fig. 2a a step parallel to bl of the superlattice is found at an angle of 37 ° relative to a~. A more complicated step formation arises if the b2 direction interferes with a step on the substrate which is misaligned by only 5 °. As can

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Science 366 (1996) 403-414

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Phase I

Ii,/= 12.6A lgzl= 13.8A P=L(&,&)=102” y=L(?,&)=400 6=L(Sz,C?S)=550 A=l&, x& 1=23.5.4,=170 A2 Ag(lll) I;, ) = [Li’, / = 2.89 B, a=L(L?,,ii,)=120”

1; ON

ecu

A,I.= / ii, x si, I= 7.23 A* Fig. 7. Proposed superstructures. The substrate lattice may differ from the indicated positions by an unknown translational vector.

be seen in the lower right corner of Fig. 2a, a ragged form of the step is formed, locally it aligns with the molecules for about 15 unit cells. The overall direction, however, remains along the original step direction in the absence of the molecule. The kinks correspond to a displacement of one row of molecules.

Due to the large number of defects on the surface of the substrate, several areas are found where the molecules fill the available area in a local arrangement which cannot be described by the superstructures discussed above. This is observed particularly at domain boundaries between different superstructures. An example is displayed in Fig. 8. The

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Fig. 8. Domain boundary between superstructures III (upper part) and II (lower part). The experimental parameters are V t = - - 1 . 0 V for the tunnelingbias at the tip and I t = 5 pA for the tunnelingcurrent. upper part of Fig. 8 shows superlattice III, the lower part superlattice II. According to the proposed structure, the corresponding unit cells have been overlaid. The appearance of the molecules clearly differs between the two structures. While within the experimental uncertainties the molecules in structure III show the four-fold symmetry of the free molecule, those of structure II appear deformed under the same experimental conditions, i.e. thermal drift, piezo creep, etc. Since the following image repeats this difference between the two structures, a change of the outermost part of the tunneling tip, which might cause a different appearance, can be excluded. Molecules at the domain boundary have more space available than those within the ordered domains. The perturbations and fuzzy parts in the topography are due to the lateral motion of those molecules. To analyze the influence of the tunneling parameters on the observed topography the tunneling voltage and the tunneling current were varied. Fig. 9 shows four images of structure III measured at different bias voltages. However, the topography of the molecule is almost unaffected. Even changing

the polarity does not lead to important changes. Not only the bias voltage but also the tunneling current has been varied. However, it has been found that stable imaging requires a tunneling current below 30 pA. At higher settings, current "jumps" occur which are associated with changes of the resolution, the apparent length, and the spectroscopic properties of the tip, etc. For comparison with the data of other groups the STM has been operated in the "constant height mode" by choosing a very long time constant for the feedback loop which maintains a constant tunneling current. Fig. 10 shows a small-area image of structure III. The displayed gray scale corresponds to the tunneling current, which varies between 0.1 and 15 pA. The topography looks very similar to the one obtained by the "constant current mode". However, the width of the four lobes of the molecule seems to be smaller. All the data have in common that the central part of the molecule appears as high as the lobes of the molecule. However, in Fig. 4 one molecule was found where the central part was as low as the minima between the molecules. Many repeated scans show that the topography of this particular molecule remains unchanged even if the bias voltage is varied, including a change of polarity. During the whole period of experiments, images of several thousand molecules have been acquired, but only this single molecule showed this type of topography. The abundance of this particularity can be estimated to less than 100 ppm. Fig. 11 shows two cross-sections taken from Fig. 5 along the el-axis of the molecules; the upper trace in Fig. 11 displays the normal appearance of a molecule, the lower trace is for the particular molecule. Since it is not clear how closely the tunneling tip reaches towards the substrate between the molecules, the apparent height of about 2 A is only a lower limit to the value which would be observed for an isolated molecule. To learn more about the electronic states contributing to the tunneling current, tunneling spectroscopy was performed. Fig. 12 shows the tunneling current versus bias voltage as observed for the investigated CuPc layer. For a clean metallic sample the same tip leads to a linear I - V curve. Hence the deviation from the linearity may be

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Fig. 9. Variation of the topography of the superstructure III for different bias voltages. The proposed structure according to Fig. 7 has been overlaid. (a) V,= 1.7 V and It=5 pA. (b) V,= 1.0 V and I,=5 pA. (c) V~=0.7 V and It=5 pA. (d) Vt= -1.4 V and It=8 pA,

a t t r i b u t e d to the molecules. It should be noted, however, t h a t this is only o b s e r v e d with a w e l l - p r e p a r e d tip. S o m e t i m e s it h a p p e n e d that

molecules were p i c k e d up by the tip, leading to a c o m p l e t e l y " s e m i c o n d u c t i n g " I - V curve where no tunneling c u r r e n t is f o u n d within a g a p of 0.7 eV.

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0,15 0,1 0,05

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Fig. 10. "Constant height" image of superstructure III. The proposed structure according to Fig. 7 has been overlaid. The experimental parameters are lit = - 1.0 V for the tunneling bias at the tip and I t = 5 pA for the average tunneling current.

4. Discussion

The topographical images displayed in Figs. 1-7 do not directly allow the conclusion that the observed layers are monolayers of CuPc on Ag(lll). However, there are several arguments that the investigated films really represent only one layer. First of all, a complementary study by thermal desorption and X-ray photoelectron spectroscopy revealed that heating an A g ( l l l ) substrate previously covered with several layers of CuPc to 540 K leaves a residual layer which is stable up to much higher temperatures [ 11 ]. This is further supported by the spectroscopic data of Fig. 12, which reveal a strong contribution of the metallic states of the substrate to the electronic tunneling process, as can be seen from the linear part of the I - V characteristic between approximately -0.3 and +0.3 V. This interval may correspond to energy levels of the molecules. The electronic states which have to be attributed to the molecule are superimposed on the linear background. In contrast, the contribution of the sub-

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11133 Fig. 11. Line scans of the topography of CuPc molecules of structure II. (a) "Normal" molecule. The position and direction of the cross section is indicated in Fig. 5 by the left dashed line. (b) Molecule with a dip in the center. The position and direction of the cross-section is indicated in Fig. 5 by the right dashed line. 15 10 5

-I - o , 5 . , , . . - - ' ' -

,,/f f

o,5

I

, -10 -15 tip voltage [V]

Fig. 12. l-Vcharacteristic measured for the monolayer of CuPc on A g ( t l l ) .

strate completely vanishes for multilayers of CuPc investigated on Ag(111) [8]. This is also substantiated by the different structure of the superlattices

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which is observed for the multilayer. Finally, an investigation of the thermovoltage across the vacuum barrier between the tunneling tip and the sample surface may be used to distinguish electronic states which are specific for different materials, molecules, etc. [17] Detailed results of these investigation will be presented elsewhere [ 18]. They reveal that in the trenches between the molecule a thermopower of about - 50/~V K - 1 is observed, which is characteristic for the bare silver substrate. If the tip is above a molecule, the thermopower is significantly different and amounts to about - 3 0 /~V K-1. Since the influence of one single molecule on the thermovoltage is so strong, it is not conceivable that between the molecules the value of the thermopower of the bare substrate should be observed if there were an additional layer of molecules below the observed structures. Hence we conclude that the presented data refer to a monolayer of CuPc on A g ( l l l ) . The fact that the CuPc monolayer forms superlattices on A g ( l l l ) in well-defined orientations relative to the substrate only reveals that the interaction of the molecules with the substrate plays an important role. In contrast to the results which were reported for C u ( l l l ) , the molecules arrange in superstructures with asymmetric unit cells, i.e. the lengths of the two lattice vectors bl and b2 are different for all three superstructures. Assuming that the molecules are symmetric, this asymmetry must be induced by the interaction with the substrate. For the primitive superstructure (II) the interaction is so strong that the centers of two neighbouring molecules approach to a little less than 12 A. Assuming that these molecules lie in the same plane, this is only conceivable with a deformation of the molecules in order to avoid a strong overlap of the molecular electronic wavefunctions. The experimentally observed topography suggests a distortion of the right angle between two lobes of the molecule to an angle of 98 ° (82°). Since this deformation is independent of the direction of the scan and it is found in the same scan together with undistorted molecules of another superstructure, experimental artifacts are very unlikely. Such a distortion has not so far been reported for comparable systems like CuPc on graphite or MoS2 [4] or CuPc on Cu(001) [6].

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A further manifestation of the adsorbate-substrate interaction is the orientation of the molecules within the unit cells. It seems that the molecules tend to align with one of their molecular axis along a primitive lattice vector of the substrate. The most striking example of the interaction is the formation of monoatomic steps on the Ag substrate which do not coincide with a natural direction of steps on the hexagonal substrate, but which are parallel to one axis of the molecular superlattice. A close inspection of the experimental data suggests that several steps on the substrate have moved or have been created during the formation of the monolayer. Since for the particular substrate the steps have been decorated with clusters of iron which are fairly immobile, one may assume that the arrangement of these clusters indicates where the steps on the substrate have been located before the formation of the CuPc film. Those steps which are not decorated (see Fig. 2) are probably newly formed or modified under the CuPc coverage. Similar effects have been reported by B6hringer et al. for CuPc on A g ( l l 0 ) [7]. The appearance of the CuPc molecules resembles that observed for the Cu(100) substrate [6], but differs from the results which have been reported for other substrates in so far as the center of the molecule shows the same height as the surrounding parts of the molecule. On Ag, G a A s ( l l 0 ) [19], graphite, etc. STM images show a minimum in the middle of the molecule. Since the topography observed by STM reflects the contribution of the different electronic states to the tunneling current, one might expect that this observation should depend on the tunneling parameters. However, a variation of the bias voltage does not yield a significant difference. Unfortunately the tunneling current could only be varied in a very limited range since currents of more than 30-100 pA lead to unstable imaging conditions. If the STM is operated in the so-called "constant height mode" the lobes of the molecule appear a little bit narrower but the central part is not changed. On the other hand, experimental details which might prevent us finding a minimum in the central part of the molecule, i.e. a blunt tip, can be excluded, since for one single molecule a hole in the middle was

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indeed found. To explain this particularity, which is very rare - one out of several thousands - one may consider that this is due to another phthalocyanine, for example H2-Pc. This is a known impurity of CuPc which sublimates at an only slightly lower temperature, simultaneously with the CuPc. It might also be possible that a CuPc molecule has been modified in the process of the formation of the ordered structure, for example by transferring the copper atom into the silver surface.

5. Conclusions In summary, the investigation of monolayers of CuPc on A g ( l l l ) reveals significant interactions between the adsorbate and the substrate. The molecules arrange in a few well-defined superstructures which are either coincident or commensurate to the substrate. The unit cells contain one molecule which lies flat on the surface, and which tends to align with one molecular axis to a primitive lattice vector of the substrate. The interaction with the substrate leads to unequal lengths of the lattice vectors of the unit cells. For one superstructure the topography observed by STM shows a distortion of the molecule. This may be understood considering the center-to-center distance between neighbouring molecules, which is so small that a deformation or a tilt is needed to accommodate the molecules. The molecular superlattice of CuPc may induce monoatomic steps on the A g ( l l l ) surface running parallel to a lattice vector of the superstructure. This direction is not found on the bare substrate. In contrast to observations on other substrates, the CuPc molecules show no depression at center where the Cu 2 ÷ ion is located.

Acknowledgements The authors would like to thank Professor N. Karl for many stimulating discussions. This work has been supported by the Deutsche Forschungsgemeinschaft within the SFB 329.

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