Coverage-dependent superstructures in chemisorbed NTCDA monolayers: a combined LEED and STM study

Surface Science 414 (1998) 423–434

Coverage-dependent superstructures in chemisorbed NTCDA monolayers: a combined LEED and STM study U. Stahl, D. Gador, A. Soukopp, R. Fink *, E. Umbach Experimentelle Physik II, Universita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany Received 6 April 1998; accepted for publication 18 June 1998

Abstract The long-range order and local geometric arrangement of 1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) monolayers on Ag(111) have been studied with low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM ). Two distinct superstructures are detected which appear separately or may even coexist, depending on coverage. Moreover, the unit cells of these two phases are correlated to each other, leading to an arrangement of parallel stripes of alternating superstructures. The structures can reversibly be transferred into one another by additional dosing or by thermal desorption. For certain tunneling conditions it is possible to image the substrate through the monolayer without disturbing it. Thus, for one structure, the exact adsorption site could be determined by the simultaneous observation of contrast from substrate and adsorbate. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Low-energy electron diffraction (LEED); Scanning tunneling microscopy (STM ); Thermal desorption spectroscopy; Adsorption kinetics; Chemisorption; Epitaxy; Aromatics

1. Introduction Thin films of large, organic, p-conjugated molecules condensed on metal substrates have been subject of many experimental studies in recent years [1–5]. This is due in part to the potential applications of organic materials in, for example, optoelectronic devices or sensors in the field of molecular electronics [6 ]. The advantage of such materials is the tailoring of electronic and optical properties by changing size or shape, or by substituting functional groups in the organic molecule. Present activities in fundamental research focus on the electronic and optical properties. In many cases * Corresponding author. Tel: +49-931-888-5163; Fax: +49-931-888-5158; e-mail: [email protected]

organic thin films are prepared on rather inert substrates, such as highly oriented pyrolytic graphite (HOPG) or dichalcogenides [3,7]. The structural properties are then dominated by the intermolecular interactions, which in many cases lead to the formation of incommensurate monolayers and/or polycrystalline films. However, since it has been found that the physical properties depend largely on the structural properties of the films (see, e.g., [8]), more effort has been spent on the preparation of structurally well-defined, in some cases even epitaxial films. With respect to the preparation of such films, the formation of a well-defined, long-range ordered monolayer at the organic/substrate interface is of fundamental interest. The formation of highly oriented thin films is

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generally favored for planar, highly symmetric molecules. We have therefore chosen 1,4,5,8naphthalene-tetracarboxylic-dianhydride (NTCDA), which is an organic semiconductor similar to, but in some interesting details different from, 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and 1,8-naphthalene-dicarboxylicanhydride (NDCA). The latter have been studied extensively as adsorbates and thin films by our group and others [4,5,7,9,10]. Moreover, NTCDA – in combination with the similar molecule PTCDA – has been used to prepare multiple quantum-well structures [10]. Furthermore, the Ag/NTCDA interface might even become technologically important, as it can be used in highly sensitive photosensors [11]. In a recent study of the molecule–substrate interaction for NTCDA on Ag(111) utilizing X-ray photoelectron ( XPS) and near-edge X-ray absorption fine structure (NEXAFS ) spectroscopy, it was concluded that NTCDA is covalently bound to the substrate [12]. The relatively strong adsorbate–substrate interaction results in a flat molecular orientation parallel to the substrate in the monolayer and an average tilt angle of 5° in the bilayer. However, the molecular orientation in multilayers (>10 monolayers) could be manipulated by variation of the preparation parameters (in this case different substrate temperatures during film preparation), which was interpreted in terms of different adsorption kinetics [12]. The formation of highly oriented multilayer films necessitates a well-defined organic/substrate interface, as was concluded from the NEXAFS data. To get a deeper insight into the lateral geometric structure, low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM ) have been employed to study NTCDA monolayer films. The combination of both techniques allows the detailed determination of most geometric parameters and hence represents the best choice of techniques for structural characterization. Previous STM studies of NTCDA monolayers have been carried out solely on inert substrates (HOPG, MoS ) by Strohmaier et al. [13]. Because 2 the molecules are not strongly bound to such substrates, a different behavior – i.e., a different

local molecular arrangement – is expected to occur in this case. Two configurations were reported. In the low-coverage regime, the molecules form superstructures that are nearly commensurate with the respective substrate with two molecules per unit cell. The molecules are oriented parallel to the substrate. For higher coverages, a herringbone arrangement is formed with upright standing molecules, which resembles the geometry in NTCDA crystals [14]. Such a molecular arrangement allows a denser packing in the monolayer and, in addition, it seems to represent an energetically favorable configuration if the intermolecular interaction is dominating over the substrate–adsorbate bonding. It is thus interesting to study how the stronger interaction with a metallic substrate influences the local geometric structure. We have therefore employed an Ag(111) single-crystal substrate, which has proved to be advantageous for the formation of long-range ordered domains when using other organic adsorbates (e.g., oligothiophenes [4,5,15]) or other anhydrides (such as PTCDA [16 ]). The covalent bonding to this metal substrate [17] may even enable the formation of metastable structural phases in organic thin films for thicknesses above one monolayer, as long as the interlayer interaction is strong enough to keep the metastable structure [12]. Although the NTCDA molecules are covalently bound to the silver substrate, a coverage-dependent arrangement of the molecules is observed in both LEED and STM investigations. In contrast to what was found for NTCDA monolayers on inert substrates [13], the molecular plane is always oriented parallel to the silver substrate, which is due to the interaction of the molecular p-system with the substrate electrons. Besides this, the observation is interesting that the two superstructures observed can be transferred reversibly into one another by thermal desorption or additional dosing. Even more astonishing is the fact that the two phases coexist in an intermediate coverage range, in which the domains with different molecular arrangements form parallel stripes. From the knowledge of the local geometry, an explanation of this structural correlation is almost straightforward. Furthermore, substrate atoms and adsorbate

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molecules have been observed simultaneously, and thus the actual adsorption site could be deduced.

2. Experimental The experiments, i.e., sample preparation as well as analysis, were performed in ultrahigh vacuum ( UHV ) at a base pressure of about 2×10−10 mbar. The Ag(111) single-crystal substrate was prepared by repeated cycles of sputter-cleaning with 500 eV Ar+ ions, followed by annealing at temperatures up to 1080 K, until the surface showed high structural quality as controlled by LEED and STM. NTCDA was evaporated from a well-outgased Knudsen cell, while the purity of the evaporant and the deposition rate were monitored by a quadrupole mass spectrometer (QMS ). During deposition the pressure increased slightly to about 6×10−10 mbar for the deposition rates used, which ranged from 0.05 to 1 monolayer (ML) per min. The substrate temperature was held at room temperature during sample preparation. The STM images were taken with a DME Rasterscope 4000 UHV-STM working at room temperature. For this study mechanically ground Pt–Ir tips supplied by the manufacturer were used. The STM data were taken in the constant-current mode. Thermal desorption spectroscopy ( TDS ) was used for thickness calibration as well as for the study of the adsorption behavior and the molecule–substrate interaction.

(a)

3. Results 3.1. Low-energy electron diffraction In the low-coverage regime (H=0.1 to about 2 ML; H=1 ML corresponds to a densely packed NTCDA monolayer with the molecules oriented parallel to the substrate), two different superstructures are observed by LEED. Fig. 1 shows the two diffraction patterns, which were obtained for H= 0.7 ML (Fig. 1(a), denoted as pattern A) and H= 1.0 ML (Fig. 1(b), denoted as pattern B). The diffraction patterns represent an incoherent superposition of six symmetry-equivalent domains

(b) Fig. 1. LEED diffraction patterns for NTCDA on Ag(111) for two different coverages. For each diffraction pattern, a unit cell of one of the six symmetry-equivalent domains is indicated. (a) Structure A, H=0.7 ML, E =12 eV; (b) structure B, H= kin 1.0 ML, E =12 eV. kin

(three rotational times two reflectional domains) according to the symmetry of the Ag(111) surface. For clarity the reciprocal unit cells of one domain have been marked in both patterns. The pattern of structure A can be observed for H ranging from

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0.1 to 0.9 ML, whereas pattern B is detected in the range between H=0.8 ML and H=2 ML. There is thus an intermediate coverage range in which both structures exist simultaneously. From a detailed analysis of the diffraction patterns, the respective superstructures can be derived. By using the matrix description for the real-space superstructures, we find

A B

4 0 M = A 3 6 and 6 −1 M = B 1 7

A

B

for phase A and B, respectively. Because of the integer numbers of the matrices, both structures represent commensurate superstructures; i.e., the periodicity in the adsorbate layer coincides with that of the substrate. From the size of the respective unit cells, two and four molecules per unit cell have been derived for phase A and B, respectively, as will be verified by the STM data. (Note that the molecules in the monolayer are oriented parallel to the substrate, as stated above and proved in [12].) Although one can obtain a real-space model for the molecular arrangement in these superstructures on the basis of a minimization of molecular overlap, we shall discuss the real-space structures in connection with the presentation of the STM data (see below). Note that both superstructures proved to be largely insensitive to electron-beam irradiation in the energy range below 150 eV. After 1 h of continuous LEED analysis, structure B revealed only a slight fading while structure A did not change at all. This observation is consistent with previous LEED investigations on covalently bound monolayers for other organic substances and may be explained by a rapid relaxation of any excitation into the substrate, which is highly effective for chemisorbed molecules [3–5,15].

(a)

(b) Fig. 2. STM images of NTCDA adsorbed on Ag(111) (image size: 11 nm×11 nm). (a) Molecular arrangement of structure A in the low-coverage regime, H≤0.8 ML (U =−0.5 V, sample I=−0.3 nA); (b) structure B for H=1 ML (U =1.0 V, sample I=0.5 nA).

3.2. Scanning tunneling microscopy Large domains of both superstructures A and B were observed by scanning tunneling microscopy with (sub)molecular resolution. Fig. 2 displays

STM images of both structures (image size: 11 nm×11 nm). The unit cell of structure A, identically and independently derived from LEED and STM, is rectangular with the base vectors having

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 ˚ and  ˚ . From a length of |a |=11.6 A |a |=15.0 A 1 2 the analysis of STM images with submolecular resolution (see Fig. 3(a)) and making use of the D symmetry of the molecule, the orientation and 2h relative positions of the molecules were derived. The real-space model thus derived for structure A is presented in Fig. 3(b). The molecules are arranged in a (slightly twisted) brick-wall arrangement with two molecules per unit cell. The molecular shapes shown in Fig. 3(b) were obtained in the following way. The intramolecular binding distances were calculated by a force-field molecular modelling program [18] and compare well with X-ray data [14]. The contour lines of the molecules indicate the van der Waals’ radii of the outermost atoms (r =120 pm, r =140 pm [19]). For the H O ˚ along the long axis overall size one obtains 9.1 A ˚ along the anhyof the naphthalene core and 9.5 A dride–anhydride axis. For certain tunneling conditions, every second  row along the a direction appears brighter (see, 1 e.g., Fig. 2(a)). While for NTCDA on HOPG and MoS [13] a similar phenomenon was attributed 2 to the different orientation of the molecules within the unit cell, this cannot be the reason here. There is no doubt that all NTCDA molecules are oriented parallel to each other within one domain. From simultaneous imaging of the adsorbate and the substrate atoms (see below) we find that the molecules we chose as corner molecules in the unit cell (Fig. 2(a)) are located such that their center is in a ‘‘top’’ site, while the center molecules of the unit cell are found on ‘‘bridge’’ sites. This difference in adsorption sites leads to the tunneling contrast between the two inequivalent molecules in the unit cell, thus yielding the bright and dark rows in the STM images. From the STM images with submolecular resolution (Fig. 3), the molecular orientation cannot be derived as directly as, for example, for the experiments on HOPG [13] or for PTCDA on Ag(111) [4,16 ], for which the shapes of the tunneling maxima are very similar to those of the lowestunoccupied molecular orbital. This may be due to the fact that the (sub)molecular contrast depends largely on the tunneling parameters and the bonding to the substrate [4,16 ]. Thus the difference of the present data from those for NTCDA on graph-

(a)

(b) Fig. 3. (a) STM image of NTCDA on Ag(111), representing structure A in the low-coverage regime (U =0.2 V, I= sample 2.0 nA, image size: 2.8 nm×2.8 nm). The unit cell is indicated by the base vectors  a and  a . (b) Model for the real-space 1 2 arrangement of the molecules (the ‘‘envelope’’ is given by the van der Waals’ radii). Owing to the relatively low density, this structure is denoted the ‘‘relaxed monolayer’’. The short lines in the upper left corners represent one of the symmetry planes of the molecule, which was used for evaluation of the molecular orientation.

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ite could be due to the stronger coupling to the silver substrate. A similar effect has been observed for PTCDA on Ni(111) as compared with Ag(111), in that no submolecular contrast could be seen in STM images in the stronger bonding PTCDA/Ni case while a good submolecular contrast was found for PTCDA/Ag [20,21]. Moreover, investigations of PTCDA on Ag(111) have shown that, for tunneling voltages below 0.3 V, the molecules look more and more distorted with decreasing tunneling voltage, which is most probably due to a tip–adsorbate interaction [21]. Such a behavior might also explain the asymmetric molecular shape in Fig. 3(a). The unit cell of structure B is hexagonal with   base vectors b and b , each with a length of 2 1 ˚ , and contains four 19.0 A molecules ( Fig. 4). Note that the molecular arrangement along base vector  b corresponds to that of the diagonal of structure 1   A; i.e., to the vector sum a +a . Structure B also 2 1 exhibits a different tunneling contrast for every  second row parallel to the b direction ( Fig. 4). In 1 this case the molecules with different contrast differ in both orientation and adsorption site. Superstructure B is again consistent with the data obtained from the LEED analysis. However, from the STM images alone (i.e., by ignoring the substrate), one might be tempted to choose a rectangular ‘‘unit cell’’ marked in Fig. 4(b) by dashed lines. Although the corner molecules of this (incorrect) unit cell seem to be nearly equivalent in most STM measurements, they have slightly different local surroundings, if the substrate geometry is also taken into consideration. This can also be seen by (additional ) characteristic spots in the Fourier transform of STM images taken from structure B (not shown). Therefore the hexagonal unit cell is the correct description for this substrate–adsorbate complex. The interpretation of images with submolecular resolution (Fig. 4(a)) is less obvious compared with the case of structure A and hence does not allow us to extract the exact molecular orientation  directly with respect to the base vector b . The 1 model presented for structure B ( Fig. 4(b)), however, makes use of the already noted relationship    +a and thus between the base vectors: b =a 1 2 1  assumes that the molecules along b are arranged 1

(a)

(b) Fig. 4. (a) STM image of NTCDA on Ag(111), representing structure B in the saturation-coverage regime (U =−0.3 V, I=−0.3 nA, image size: 5.5 nm×5.5 nm). sample (b) Model for the real-space arrangement of the molecules. This structure, which is more densely packed than structure A, is denoted the ‘‘compressed monolayer’’. The ‘‘incorrect’’ unit cell indicated by dashed lines is discussed in the text. The short lines in the left part of the images represent one of the molecular symmetry planes.

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Table 1 Geometric parameters for the superstructures A and B for NTCDA adsorbed on Ag(111) obtained by LEED and STM measurements, compared with to literature STM data for NTCDA on HOPG [13] and MoS [13]. The parameters of structure B are closest to 2  those of the inert substrates when using the ‘‘incorrect’’ representation.  g is a unit-cell vector of the respective substrate lattice, b 1 1      and b are the unit-cell vectors of the adsorbate lattices (a and a in the case of structure A). C is the angle between b and b , while 2 1 2 1 2 W describes the rotation of the adsorbate unit cell with respect to the substrate lattice NTCDA on various substrates

 Base vector |b | 1 Base vector |b | 2 Molecules per cell  C=%(b ,b ) 1 2 W=%(b ,g ) 1 1 Area per molecule Matrix notation

 |=2.89 A ˚) Ag(111) (|g 1 Structure A

B correct

B ‘‘incorrect’’

 |=2.46 A ˚) (|g 1

MoS [13] 2  |=3.16 A ˚) (|g 1

˚ 11.6 A ˚ 15.0 A 2 90° 0° ˚2 86.8 A

˚ 19.0 A ˚ 19.0 A 4 120° 7.6° ˚2 77.8 A

˚ 9.5 A ˚ 16.4 A 2 90° 7.6° ˚2 77.8 A

˚ 9.8 A ˚ 16.9 A 2 90° 19.1° ˚2 82.8 A

˚ 9.6 A ˚ 16.7 A 2 90° 25.3° ˚2 80.2 A

6 −1 1 7

7 1 2 2 5 13 2 2

3 −3 2 6 15 2

7 3 2 2 1 11 2 2

A B 4 0 3 6

A B

 . This assumption in the same way as along  a +a 1 2 is further justified by the coexistence of structures A and B in parallel stripes (see below). The geometric data for both structures obtained from LEED and STM measurements are summarized in Table 1. STM data for NTCDA physisorbed on HOPG and MoS [13] are listed for 2 direct comparison. The density of structure A, i.e., the number of molecules per unit area, is 89.6% of the density of structure B. Therefore, we describe structure B as a compressed monolayer and A as a relaxed monolayer. The domain size of the long-range ordered regions is mainly limited by the terrace width of the silver substrate, therefore extending up to more ˚ in each direction. Boundaries between than 1000 A rotational domains, however, have only been observed in phase B. Moreover, as stated above, the coexistence of both phases was observed for coverages below saturation (‘‘compressed’’ monolayer). This is consistent with the LEED results, which showed the superposition of both diffraction patterns in the coverage range between 0.8 ML and 0.9 ML. In the STM investigations, however, small domains of structure B are detected even at lower coverage (H<0.8 ML). More astonishing is the fact that the different domains are correlated to each other, as shown in Fig. 5. Both structures coexist in a ‘‘stripy’’ arrangement as clearly seen in Fig. 5(a) and (b).

HOPG [13]

A B

A B

A B

A model of the boundary between the two structures, which is seen in detail in Fig. 5(b), is given in Fig. 5(c). No direct observation of a rearrangement of molecules during STM imaging was made. However, the average width of stripes of structure B increased at the expense of those of structure A with increasing coverage as expected. By additional deposition of molecules, structure A could always be changed to structure B, while thermal desorption of a small amount of molecules led back to structure A. This cycle was completely reproducible, indicating a reversible phase transition. Furthermore, no difference could be detected, neither in STM nor in LEED, between layers prepared directly by adsorption and those prepared by thermal desorption, as long as the same coverages were compared. We further note that the formation of stripes of different superstructures is facilitated by the fact that the diagonal of the unit cell of structure A has exactly the same length and orientation as the  base vector b of the unit cell of structure B, if an 1 appropriate domain is chosen. This direction hence represents the direction of the phase boundaries between the two superstructures (see Fig. 5(b) and (c)) and may thus explain why the stripy arrangement within one orientational domain is formed.  One row of molecules parallel to b is shared by 1 the two phases, leading to a commensurate phase boundary. Note, however, that since the exact

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(a)

(b)

(c) Fig. 5. (a) STM image of NTCDA on Ag(111) for H=0.9 ML. Structures A and B coexist in parallel stripes (image size: 44 nm×44 nm, U =1.0 V, I=0.4 nA). (b) Phase boundary between structure A (darker area) and B (11 nm×11 nm, sample U =−1.0 V, I=−1.0 nA). The corresponding unit cells are indicated. Note the point defect (vacancy) in the lower right corner, sample which obviously does not affect the local molecular arrangement. (c) Real-space model of a phase boundary (as in Fig. 5(b)) containing both unit cells and the corresponding base vectors. An ‘‘incorrect’’ unit cell for structure B (dashed lines) is also given (see text).

adsorption sites of the molecules of structure B are unknown, the model is not yet completely proven. Finally, it is very noticeable that for structure A it was possible to determine the adsorption site

by just using different voltages, because a change of the bias voltage from above 0.2 V to values between 0.15 V and 0.2 V (tunneling current #1.8 nA) led to a change in contrast. At the higher voltages the molecules were imaged, show-

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ing the structure described above. For the lower voltages atomic resolution of the substrate was obtained. By changing the bias back to higher values, the undestroyed monolayer structure could be recovered. For certain tunneling conditions, it was even possible to achieve contrast of the adsorbate and the substrate simultaneously, leading to a superposition of NTCDA molecules and silver atoms. This is shown in Fig. 6, from which the adsorption sites used in our model of structure A could thus be derived. We can unambiguously exclude multiple-tip artefacts, because the contrast change proved to be reproducible on different spots of the sample and at different times. Moreover, the adsorbate covered the substrate completely, eliminating the possibility that a microtip imaged an uncovered part of the substrate.

(a)

3.3. Thermal desorption spectroscopy As mentioned before, both structures can be transferred into one another either by additional dosing or by desorption. The latter should be observable by thermal desorption spectroscopy, which can also be employed to study the bonding of the NTCDA monolayer. Since only the desorption of the monolayer is interesting in the present context, only the corresponding TDS peaks are depicted in Fig. 7. (Multilayer desorption will be discussed elsewhere [22].) Fig. 7 shows TDS curves for varying coverages of NTCDA for which the QMS signal of NTCDA (molecular mass=268 u) was recorded while the sample temperature was increased linearly from 300 to 500 K at a rate of 1.0 K/s. NTCDA multilayers show zero-order desorption kinetics with a desorption maximum at about 370 K which, of course, depends on the initial coverage (not contained in Fig. 7). The TDS signal corresponding to desorption of the monolayer shows a broad main structure (denoted by (a)) and an additional peak with much smaller intensity ( b), which appears around H#0.8 ML and becomes most intense when the monolayer is saturated (1 ML). These two distinct desorption structures have to be interpreted in terms of differently bound molecules. Of course, it is straightforward to attribute peak (a) to desorption from phase A, while peak

(b) Fig. 6. (a) STM image showing a superposition of NTCDA molecules in structure A and atomic resolution of the Ag(111) substrate. The image was Fourier-filtered by suppression of Fourier components with an amplitude of less than 10% of the maximum (image size: 11 nm×11 nm, U =0.14 V, I= sample 1.8 nA). (b) Only the basic Fourier components of substrate and molecular superstructure were used to synthesize this image. The adsorption sites ‘‘top’’ and ‘‘bridge’’ can be seen clearly.

( b) is caused by desorption of the excess molecules during the phase transition from B to A. Note that only those molecules contribute to peak ( b) which

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4. Discussion and conclusions

Fig. 7. TDS spectra in the coverage range from 0.6 to 1 ML. Besides the broad peak (a) indicating desorption from domains of structure A, a relatively sharp desorption peak, denoted ( b), at 425 K reflects the desorption of excess molecules upon reorientation from phase B to A.

desorb from structure B until it is completely transformed into structure A (about 10% of 1 ML). The ratio of the intensities of (a) and ( b) fits well that value, which is derived from the geometrical data, e.g., from the comparison of the areas per molecule for structures A and B (see Table 1). However, the desorption maxima are not clearly separated in the spectrum and therefore no precise quantitive evaluation of desorption energies is possible ( leading-edge analysis for H<0.6 ML yielded a desorption energy of 112±10 kJ/mol ). Finally, we emphasize that the comparison of the deposited coverage (calculated as the time integral of the partial pressure of NTCDA during deposition) with the integrated desorption spectra yields excellent proportionality, thereby confirming the complete desorption of intact molecules. This observation is corroborated by in situ XPS and NEXAFS measurements which also show a complete desorption of NTCDA from Ag(111) [12]. This is an important result since, for many similar systems (e.g., PTCDA/Ag, Ni, Si; NDCA/Ni; oligothiophenes/Ag), (partial ) molecular cracking is observed for the monolayer upon heating due to the strong covalent interaction of the molecules with the substrate [4,5,9,15,16,20].

From LEED, STM and TDS experiments, it is obvious that two different commensurate superstructures exist for NTCDA on Ag(111) in the (sub)monolayer regime. In a narrow coverage regime (with LEED, H#0.8–0.9 ML; with STM in an even wider range) the coexistence of both superstructures is observed. Even more important, the coexisting phases are structurally correlated to each other. As derived from photoemission and X-ray absorption experiments, NTCDA molecules are covalently bound to the Ag(111) substrate [12]. New hybrid orbitals are formed at the interface from the mixing of metal states with molecular p-type orbitals. The bonding occurs mainly via the molecular p-system of the naphthalene core and thus forces the NTCDA molecules into a flat geometry; i.e., with their molecular planes parallel to the substrate surface [12]. This observation is in accordance with the present STM results, which are only compatible with flat-lying molecules. The coupling via the extended naphthalene p-orbitals may also explain the high degree of rotational freedom which is, in addition to translational movement, necessary to transform structure A into structure B, and vice versa. The observation of LEED patterns (which ˚ diameter) and requires islands of at least 100 A large ordered domains in the STM investigations even for very low NTCDA coverages clearly indicates a very high mobility of the adsorbed molecules, although the bonding to the silver substrate is much stronger than in the case of inert substrates. Moreover, island formation also requires an attractive intermolecular interaction which in the present case is provided by van der Waals’ as well as electrostatic forces (NTCDA has a quadrupole moment) and probably also by indirect interaction via the substrate. The island formation observed here is consistent with the results of various other investigations where long-range ordered domains have been detected even for low substrate temperatures [4,16,22]. The small corrugation of the surface potential of the Ag(111) substrate and the ‘‘averaging’’ of the adsorbate bond over several substrate atoms owing to the relatively large size of the molecule are the most likely reasons for the high mobility of the molecules.

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The comparison of our results with STM data for NTCDA on the basal planes of MoS and 2 HOPG [13] yields additional information on the adsorbate–adsorbate and adsorbate–substrate interaction. On each of these two latter substrates, only one superstructure with the molecular plane parallel to the respective substrate is found in addition to a bulk-like phase. Because the interaction with the substrate is very weak on the inert substrates, the adsorbate–adsorbate interaction is the dominant one, leading to very similar structural parameters (see Table 1). The structures A and B deviate from these structures as a consequence of the stronger adsorbate–substrate coupling, leading to a significantly lower or higher packing density for A and B, respectively, as can be derived from the comparison of the areas per molecule ( Table 1). If the lengths of the base vectors are compared (for structure B the ‘‘incorrect’’ representation has to be taken) clearly structure B comes closer to the structures found on the inert substrates than structure A. Furthermore, structure B shows a herringbone arrangement of the molecules (as found on the inert substrates), while structure A reveals a brick-wall arrangement. Herringbone structures were also observed for the larger PTCDA molecule adsorbed on inert surfaces (HOPG, MoS ) and Ag(111) [16 ]. This 2 arrangement is energetically favorable because of the electric quadrupole moment of the PTCDA molecule induced by the polar anhydride groups. For PTCDA monolayers the angle between the long molecular axes of nearest-neighbor molecules is about 90°, thus bringing the parts of the molecule with opposite polarity as close together as possible. For NTCDA the angles are different in the herringbone structure B (Fig. 4(b)), which seems reasonable as the quadrupole moment is probably smaller in the shorter molecule. The molecular density of structure A is significantly smaller than that of structure B. This is, however, true for only one direction (Fig. 5(c)).  With respect to the b direction, the molecular 1 density is exactly the same for both structures. Also the relative angle between neighboring molecules within the densely packed rows in structure A agrees with that of every second row in the compressed monolayer B. The phase transition from A to B upon addi-

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tional dosing may thus be considered as occurring in the following way. In structure A the rows of  densely packed molecules parallel to b move  1 slightly along the b direction (see Fig. 5(c)), and 1 the molecules of every second row are rotated by about 60° in addition to the translation. Because of this rotation the space between the ‘‘A-like’’ rows is used more effectively such that the ‘‘A-like’’ rows can move closer to each other, thus increasing the density. For the reverse transition from B to A upon desorption, one may think of the reverse process taking place. Note that a direct observation of the reorientation process, e.g., by STM, was not possible. A structural reorientation upon adsorption or thermal desorption of excess molecules in monolayers of large organic molecules has previously been observed for elongated oligothiophenes ( EC3T, EC4T ) on Ag(111) substrates [23–25]. Oligothiophenes are also covalently bound to the silver substrate. For EC3T, translational and rotational movements are necessary to explain the transfer from an incommensurate compressed superstructure to a commensurate relaxed superstructure [25]. In contrast, for end-capped quaterthiophene ( EC4T ), mainly translations along the molecular axis are sufficient for the respective phase transition [24]. In both examples the amount of desorbed molecules is of the same order (about 10% of 1 ML) as in the present study. For HOPG and MoS the transition to the bulk2 like structure occurs upon additional dosing; i.e., when the total coverage exceeds the coverage of a monolayer of parallel-oriented molecules [13]. The resulting geometric structure agrees well with the bulk structure of NTCDA crystals along the crystal cleavage plane. In contrast, no bulk-like phase with the molecular plane about perpendicular to the substrate, was observed in the present investigation because of the covalent bonding of the NTCDA monolayer. This again reflects the very weak substrate coupling for the inert substrates, which allows more orientational freedom, at least in the monolayer regime. Thus, for inert substrates, the intermolecular interaction may become dominant and may force the molecules into a bulk-like configuration. Finally, we note that an imaging of the substrate through the monolayer without destroying its

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order – as we found for structure A – has previously been reported in the literature. This ‘‘transparency effect’’ was also observed on HOPG, using the liquid crystals 7CB and 8CB [26,27] and behenic acid [28] as adsorbates. For Cu-phthalocyanine on Si(111)-7×7 transparency was also reported on semiconductors [29]. As theoretical models to explain the transparency, a simple resonant tunneling model [26 ] and a more elaborate theory based on the transfer matrix formalism [30] were proposed. In summary, we have shown that NTCDA forms two distinct superstructures when adsorbed in the submonolayer range on a Ag(111) substrate. NTCDA monolayers are covalently bound to the silver substrate, but thermal desorption of intact molecules occurs around 440 K. The superstructures of the ‘‘relaxed’’ and ‘‘compressed’’ monolayer depend on the coverage and coexist within a small coverage range. Domains of different superstructures show a long-range correlation leading to a stripy arrangement of the domains. The domain walls are formed along rows of densely packed molecules, which are present in both superstructures. The transparency of the NTCDA overlayer was demonstrated for specific tunneling conditions, which allowed us to determine the positions of the molecules relative to the substrate; i.e., the exact adsorption sites for structure A.

Acknowledgements This work was funded by the Bundesminister fu¨r Bildung und Forschung (project 05 625 WWA 9). We are indebted to Professor Dr N. Karl ( Universita¨t Stuttgart) for the ultrapure organic substance. One of us ( E.U.) is grateful to the Fond der Chemischen Industrie for support.

References [1] B.G. Frederick, Q. Chen, S.M. Barlow, N.G. Condon, F.M. Leibsle, N.V. Richardson, Surf. Sci. 352 (1996) 238. [2] T.J. Schuerlein, N.R. Armstrong, J. Vac. Sci. Technol. A 12 (1994) 1992.

[3] M. Mo¨bus, N. Karl, T. Kobayashi, J. Cryst. Growth 116 (1992) 65. [4] E. Umbach, M. Sokolowski, R. Fink, Appl. Phys. A 63 (1996) 565. [5] E. Umbach, C. Seidel, J. Taborski, R. Li, A. Soukopp, Phys. Stat. Sol. B 192 (1995) 389. [6 ] R.F. Service, Science 278 (1997) 383. [7] C. Ludwig, B. Gompf, J. Petersen, R. Strohmaier, W. Eisenmenger, Z. Phys. B 93 (1994) 365. [8] W. Gebauer, M. Ba¨ßler, R. Fink, M. Sokolowski, E. Umbach, Chem. Phys. Lett. 266 (1997) 177; E. Umbach, W. Gebauer, M. Sokolowski, J. Luminesc. in press. [9] J. Taborski, P. Va¨terlein, H. Dietz, U. Zimmermann, E. Umbach, J. Electr. Spectrosc. 75 (1995) 129. [10] F.F. So, S.R. Forrest, Y.Q. Shi, H.W. Steier, Appl. Phys. Lett. 56 (1994) 674. [11] T. Katsume, M. Hiramoto, M. Yokoyama, Appl. Phys. Lett. 69 (1996) 3722. [12] D. Gador, C. Buchberger, R. Fink, E. Umbach, Europhys. Lett. 41 (1998) 231. [13] R. Strohmaier, C. Ludwig, J. Petersen, B. Gompf, W. Eisenmenger, Surf. Sci. 351 (1996) 292. [14] L. Born, G. Heywang, Z. Kristall. 190 (1990) 147. [15] A. Soukopp, K. Glo¨ckler, P. Ba¨uerle, M. Sokolowski, E. Umbach, Adv. Mater. 8 (1996) 902. [16 ] K. Glo¨ckler, C. Seidel, A. Soukopp, M. Sokolowski, E. Umbach, M. Bo¨hringer, R. Berndt and W.-D. Schneider, Surf. Sci. in press. [17] W. Gebauer, M. Ba¨ßler, A. Soukopp, C. Va¨terlein, R. Fink, M. Sokolowski, E. Umbach, Synth. Met. 83 (1996) 227. [18] PCMODEL (version 3.0), Serena Software, Bloomington, IN 47402-3076, USA. [19] L. Pauling, The Nature of the Chemical Bond and The Structure of Molecules and Crystals, 2nd edn, Cornell University Press, Ithaca, NY, 1948. [20] E. Umbach, M. Sokolowski, K. Glo¨ckler, Surf. Sci. in press. [21] K. Glo¨ckler, Doctoral thesis, Universita¨t Wu¨rzburg, 1997. [22] D. Gador, U. Stahl, C. Buchberger, R. Fink, E. Umbach, to be published. [23] A. Soukopp, Doctoral thesis, Universita¨t Wu¨rzburg, 1997. [24] C. Seidel, A. Soukopp, R. Li, P. Ba¨uerle, E. Umbach, Surf. Sci. 374 (1997) 17. [25] A. Soukopp, K. Glo¨ckler, P. Kraft, S. Schmitt, M. Sokolowski, E. Umbach, E. Mena-Osteritz, P. Ba¨uerle, E. Ha¨dicke, Phys. Rev. B in press. [26 ] W. Mizutani, M. Shigeno, M. Ono, K. Kajimura, Appl. Phys. Lett. 56 (1990) 1974. [27] J.-C. Poulin, H.B. Kagan, C.R. Acad. Sci. Paris, Ser. II 313 (1991) 1533. [28] E. Kishi, H. Matsuda, R. Kuroda, K. Takimoto, A. Yamano, K. Eguchi, K. Hatanaka, T. Nakagiri, Ultramicroscopy 42–44 (1992) 1067. [29] M. Kanai, T. Kawai, K. Motai, X.D. Wang, T. Hashizume, T. Sakura, Surf. Sci. 329 (1995) L619. [30] M. Tsukada, K. Kobayashi, N. Isshiki, Appl. Surf. Sci. 67 (1993) 235.