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Molecular Beam Deposition of Perylene on Copper: Formation of Ordered Phases S. So¨hnchen, K. Ha¨nel, A. Birkner, G. Witte,* and C. Wo¨ll Physikalische Chemie I, Ruhr-UniVersita¨t Bochum, 44780 Bochum, Germany ReceiVed June 2, 2005. ReVised Manuscript ReceiVed August 10, 2005
The growth of ultrathin perylene films on Cu(110) and Cu(100) surfaces has been studied by means of He atom scattering, low-energy electron diffraction, scanning tunneling microscopy, thermal desorption spectroscopy, and X-ray photoelectron spectroscopy with a special emphasis on the interface structure. In addition to several ordered submonolayer phases, two distinctly different monolayer structures were found at substrate temperatures below 380 K and at about 450 K on Cu(110). The rather open saturation structure formed at elevated temperatures reveals an enhanced density of substrate steps, which indicates an adsorption-induced modification of the Cu(110) surface. In contrast to that, on Cu(100), only a closepacked c(8 × 4) monolayer structure is formed without any less-dense-packed structures.
I. Introduction Organic semiconductors are presently receiving an increasing amount of attention because of their promising application potential for optoelectronic devices1 or organic field effect transistors.2 A particularly interesting class of organic materials for the latter applications are polycyclic aromatic hydrocarbons (PAHs) because they can be grown as single crystals with partly remarkable high charge carrier mobilities. For example, in the case of rubrene (C42H28), room-temperature mobilities as high as µ ) 15 cm2/Vs have recently been obtained.3 Compared to inorganic semiconductors, the electronic transport properties of crystalline organic semiconductors reveal a pronounced anisotropy4 and depend strongly on the molecular orientation and packing.5 For this reason, there is a large interest in a precise control of the molecular orientation and crystalline microstructure occurring in thin organic films used for organic thin film transistor devices. While a rich variety of structures has been observed for organic semiconductor films upon growth on inorganic substrates,6 a detailed understanding of the principles governing these film structures has not yet been achieved. Of particular importance in this respect is, however, the molecular interaction with the substrate, which in the case of a metallic electrode also decisively determines the charge carrier injection in a device. In the case of perylene (C20H12, see Figure 1a), a planar PAH molecule with high charge carrier mobility in the single* Author to whom correspondence should be addressed. E-mail: witte@ pc.ruhr-uni-bochum.de.
(1) Hung, L. S.; Chen, C. H. Mater. Sci. Eng. Rep. 2002, 39, 143. (2) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (3) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 1644. (4) Karl, N. in Organic Electronic Materials; Farchioni, R., Grosso, G., Eds.; Springer-Verlag: Berlin, 2001. (5) Cornil, J.; Calbert, H. P.; Bredas, J. L. J. Am. Chem. Soc. 2001, 123, 1250. (6) Witte, G.; Wo¨ll, Ch. J. Mater. Res. 2004, 19, 1889.
Figure 1. (a) Structure of perylene (C20H12) and (b) scattering geometry of the HAS apparatus.
crystal phase,7 Chen et al. reported a new crystalline structure for films grown by organic molecular beam deposition (OMBD) on a Cu(110) surface.8,9 On the basis of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and electron energy loss spectroscopy (EELS) data, they proposed a new orthorhombic structure with a planar stacking of all perylene molecules parallel to the surface. In contrast to that, we have demonstrated by using near-edge X-ray absorption spectroscopy (NEXAFS) and tapping mode atomic force microscopy (AFM) that, after completion of the first monolayer consisting of molecules with a planar adsorption geometry, subsequent growth proceeds with an upright orientation of the molecular planes. With increasing film thickness, islands are formed where perylene molecules grow in a bulklike structure with their long axes oriented upright to the Cu(110) surface10 and, thus, resemble a growth mode observed previously for pentacene on the same surface.11,12 The apparent differences found for such perylene films thus give rise to the question about the precise structure of the metal-organic interface and thin film formation. Here, we present the results of a comprehensive study on ultrathin films of perylene grown by OMBD on Cu(110). (7) Karl, N. in Landolt-Bo¨rnstein, New Series; Madelung, O., Schulz, M., Weiss, H., Eds.; Springer-Verlag: Heidelberg, Germany, 1985; Vol 17i. (8) Chen, Q.; Rada, T.; McDowall, A.; Richardson, N. V. Chem. Mater. 2002, 14, 743. (9) Chen, Q.; McDowall, A.; Richardson, N. V. Chem. Mater. 2003, 15, 4113. (10) Ha¨nel, K.; So¨hnchen, S.; Lukas, S.; Beernink, G.; Birkner, A.; Strunskus, T.; Witte, G.; Wo¨ll, C. J. Mater. Res. 2004, 19, 2049. (11) Lukas, S.; So¨hnchen, S.; Witte, G.; Wo¨ll, C. ChemPhysChem 2004, 5, 266. (12) So¨hnchen, S.; Lukas, S.; Witte, G. J. Chem. Phys. 2004, 121, 525.
10.1021/cm051183x CCC: $30.25 © 2005 American Chemical Society Published on Web 09/28/2005
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While multilayer films of perylene reveal a pronounced dewetting and island formation, which is described in detail elsewhere,13 the present study focuses on the structure of the interface, which was characterized by combining highresolution He-atom scattering (HAS) with low-current LEED and STM. Moreover, thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS) were used to investigate the thermal stability and coverage of the layers. In addition to various ordered submonolayer phases, two distinct monolayer structures were observed depending on the actual substrate temperature. A comparison with the corresponding packing density and thermal stability obtained for a saturated perylene monolayer on Cu(100) indicates a particularly strong molecule-substrate interaction on Cu(110), leading further to an enhanced substrate step density. II. Experimental Section The present experiments were carried out on three different ultra high vacuum (UHV) instruments. LEED patterns were recorded by using a microchannel plate LEED system (OCI), which allows operation at incident beam currents as low as 0.05-0.1 nA/mm2 to minimize electron beam damage.14 A quadrupole mass spectrometer (Balzers QMS200, mass range 0-300 amu) with a Feulner cup was used to record TDS spectra by employing a computer controlled linear heating ramp.15 The apparatus is further equipped with an X-ray photoelectron spectrometer. HAS was employed to analyze the lateral structure of the films. The scattering apparatus used for these measurements has been described in detail elsewhere.16 Basically, the apparatus consists of a nearly monoenergetic beam (∆E/E ≈ 2%) of thermal energy (E ) 20-80 meV) He atoms, which is directed at the surface, and the scattered atoms are detected at a fixed total scattering angle θtot ) 90° with respect to the incident beam, as shown schematically in Figure 1b. Rotating the sample about the axis perpendicular to the scattering plane for different azimuth directions (φ) thus allows the recording of He-atom diffraction scans (angular distributions) with a wide range of accessible parallel momentum transfers ∆K ) ki[sin(θtot - θi) sin(θi)], where θi denotes the angle of incidence with respect to the surface normal and ki ) x2mHeE/p is the incident wave vector. The STM measurements were performed at room temperature using an Omicron-Micro-STM instrument. On all three instruments, Cu(110) samples were used which had been polished to within (0.2° of the desired orientation. Before each film preparation, the substrates were cleaned in situ by Ar+ sputtering (800 eV) and subsequent annealing (up to 1000 K). These cleaning cycles were repeated until a sharp (1 × 1) LEED pattern with a low diffuse background signal was observed and the corresponding XPS, STM, or HAS measurements revealed no traces of contamination. Perylene (Fluka, purity g99%) was deposited from resistively heated and differentially pumped Knudsen cells. Some of the evaporators were further surrounded by a cooling shield with a small aperture to provide perylene deposition only at the sample surface without contamination of the chamber. In that way, precise studies of submonolayer phases without additional adsorption from the (13) Witte, G.; Ha¨nel, K.; So¨hnchen, S.; Wo¨ll, C.; Witte, G. Appl. Phys. A in print. (14) Loepp, G.; Vollmer, S.; Witte, G.; Wo¨ll, C. Langmuir 1999, 15, 3767. (15) Lukas, S.; Vollmer, S.; Witte, G.; Wo¨ll, C. J. Chem. Phys. 2001, 114, 10123. (16) Fouquet, P.; Witte, G. Surf. Sci. 1998, 400, 140.
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Figure 2. Thermal desorption spectra for multilayer films of perylene on Cu(110) recorded with (a) a QMS for different coverages and a heating rate of β ) 3 K/s and (b) by measuring the He-atom reflectivity during a linear heating rate for a multilayer (black curve) and a monolayer film (grey curve).
residual perylene pressure in the chamber after nominal deposition were made possible. To maintain a constant flux upon deposition, which was monitored by a quartz microbalance (Leybold Inficon XTC2), the temperature of the perylene containing aluminum crucible was stabilized within (1 K at typical operation temperatures of 380-440 K. Before deposition, all Knudsen cells were carefully outgassed at temperatures slightly above the deposition temperature.
III. Results A. Thermal Stability. In view of the rather high vapor pressure of perylene, which increases from 10-5 Torr at 348 K to about 10-2 Torr at 410 K,17 first, the thermal stability of perylene films deposited under UHV conditions was characterized by employing thermal desorption spectroscopy. Figure 2a shows a series of typical TDS spectra, which were recorded for different film thicknesses at the mass of the molecule ion (m/z ) 252 amu). A common signature in all spectra is a pronounced desorption peak starting at about 350 K, with a maximum around 380 K. Because the peak area increases linearly with the perylene film thickness, it had been assigned to multilayer desorption.10 Moreover, it was shown that the desorption is well-described by a kinetics of order zero, which yields an intensity of the ascending flank of the desorption peak according to ln I ∝ -∆Edes/RT (see inset in Figure 2a). From the slope of the linear curve, an activation energy for desorption of ∆Edes ) 1.26 eV was derived, which is in close agreement with the standard sublimation enthalpy of perylene Hsub ) 1.37 eV.18 At higher temperatures, no further desorption peaks were observed which can be related to the monolayer. Instead, further information on the thermal stability of the perylene layer was derived by monitoring the He-atom reflectivity of the surface during a linear increase of the sample temperature. As shown in Figure 2b, the HAS reflectivity recorded for a multilayer film reveals a sudden increase at about 350 K, (17) Inokuchi, H.; Shiba, S.; Handa, T.; Akamatu, H. Bull. Chem. Soc. Jpn. 1952, 25, 299. (18) Oja, V.; Suuberg, E. M. J. Chem. Eng. Data 1998, 43, 486.
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Figure 3. Cu2p to C1s intensity ratio obtained after deposition of about 5 nm perylene on Cu(110) at 150 K and subsequent annealing.
which concurs with the onset of the desorption peak obtained by the mass spectrometer and can be related to the different Debye-Waller factors of the chemisorbed monolayer and the rather soft van der Waals bond multilayers. This is evident when comparing the He reflectivity of mono- and multilayers at surface temperatures below 380 K (gray and black curves in Figure 2b, respectively). At temperatures above about 600 K, the He reflectivity again increases rapidly, which is attributed to the monolayer desorption. This assignment is corroborated by XPS measurements, which reveal a pronounced decrease of the C 1s signal when heating above 600 K. The distinctly different thermal stabilities of mono- and multilayer films can further be utilized to prepare welldefined saturated monolayers. This is demonstrated by temperature-dependent XPS measurements. For this purpose, thin perylene films of about 5 nm were deposited at low temperatures (150 K) on Cu(110) and the Cu2p to C1s photoelectron intensity ratio was measured as a function of the annealing temperature. As shown in Figure 3, the substrate attenuation becomes rather weak upon annealing above 350 K and yields a values of IC/ICu ) (128 ( 15) × 10-4. The same ratio was also obtained when saturating the copper substrate with perylene at that temperature, where multilayer formation is thermally excluded, while at temperatures above 430 K, a further decrease in the coverage takes place. B. Diffraction Measurements. A closer inspection of the He reflectivity curve shown in Figure 2b also reveals a small kink at about 450 K, thus, indicating a structural change of the monolayer film, which is also evidenced by the XPS data. To characterize the corresponding structures, thin films were prepared by saturating the Cu(110) surface with perylene at 380 and 450 K. The corresponding LEED patterns, which are displayed in Figure 4, clearly indicate some differences in the film structure for these preparation temperatures. Films prepared at elevated temperatures reveal a number of diffraction spots with a rectangular arrangement and a 5-fold superstructure along the [11h0] direction, whereas only a few diffraction spots and some hexagonal or ringlike structures were visible in the LEED pattern for the 380 K preparation (Figure 4a). The apparent image distortion is due to the planar channel plates of the MCP-LEED system (the dashed lines indicate the unit cell of the bare copper surface measured at identical beam energies). The rather small number of diffraction spots, which is attributed to out-of-phase conditions and the weak cross section of the carbon atoms, does not render a clear structural analysis.
Figure 4. LEED patterns of saturated monolayers on Cu(110) prepared (a) at 380 K (217 eV) and (b) at 450 K (198 eV) together with the corresponding schematic diffraction pattern (right side). The LEED patterns were recorded at 120 K to reduce inelastic scattering and to enhance the contrast.
We emphasize that the low contrast of the present LEED pattern is not related to electron beam damage since partly very sharp spots are observed of which width does not change even after 1 h of LEED measurements. On the other hand, additional measurements which were carried out with a standard LEED system operated at beam currents of 100500 nA/cm2 revealed a rapid decay of the diffraction pattern within a few seconds. This demonstrates certain difficulties and restrictions in the application of LEED measurements to determine the structure of such fragile molecular films. More detailed information on the evolution and structure of ordered perylene layers has been derived from HAS. Figure 5 displays a series of typical HAS angular distributions which were recorded along high-symmetry directions of the substrate for increasing perylene dosage. To exclude the formation of multilayers or islands, all perylene layers were deposited at 380 K while the actual angular distributions were taken at low temperatures of about 110 K to reduce the Debye-Waller factor and inelastic scattering of these soft adlayers. Initial exposure leads to a significant broadening of the substrate diffraction peaks and a strong reduction of the HAS reflectivity. In addition, some weak intensity oscillations were observed in the angular distributions along the less-corrugated [11h0] direction of the substrate (indicated by the dashed line in Figure 5a). Since no evidence for any ordered structure at this coverage was found in the angular distributions for other azimuth directions, these features are attributed to Fraunhofer oscillations arising from interferences between scattering from protruding isolated molecules and reflection from the bare metal surface.19 With further exposure, a firstordered phase appears which was identified as a c(8 × 5) structure. The corresponding angular distributions, which are displayed in Figure 5c and d, show further that the firstorder substrate diffraction peaks along the [001] direction and the Fraunhofer oscillations for the [11h0] azimuth are still visible. This fact clearly indicates that the c(8 × 5) phase appears in islands which coexist with the initial dilute phase. Upon further exposure, the 4-fold superstructure along the [11h0] direction did not change, while a 2-fold periodicity is formed along the [001] azimuth (see Figure 5f). On the basis (19) Lahee, A. M.; Manson, J. R.; Toennies, J. P.; Wo¨ll, C. Phys. ReV. Lett. 1986, 57, 471.
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Figure 7. HAS angular distributions measured along different azimuth directions for a saturated perylene monolayer on Cu(110) prepared at 450 K (a-d), together with a schematic representation of the reciprocal lattice of the (5 × 5) phase (e). For scattering conditions, see Figure 5. Figure 5. Summary of HAS angular distributions of ordered submonolayer phases of perylene on Cu(110): (a, b) isolated molecules, (c, d) c(8 × 5) structure, and (e, f) c(8 × 4) structure, together with the corresponding reciprocal lattices of the superstructures. All angular distributions were recorded at a sample temperature of about 110 K with an incident wave vector of ki ) 6.6 Å-1 and are displayed in terms of the reciprocal lattice vectors g[11h0] ) 2.46 Å-1, g[001] ) 1.74 Å-1, and g[11h2] ) 4.26 Å-1. The gray squares denote the reciprocal lattice of the clean Cu(110) substrate.
Figure 6. HAS angular distributions for the saturated perylene monolayer on Cu(110) (a, b) prepared at 380 K, together with a schematic representation of the reciprocal lattices of (c) the (4.25 × 5) and (d) the (8.5 × 5) phase. For scattering conditions, see Figure 5.
of additional angular distributions, which reveal an 8-fold periodicity for the [11h2] direction, this phase was unambiguously identified as a c(8 × 4) structure. Finally, when saturating the Cu(110) surface with perylene at 380 K, another structure was observed. As shown in Figure 6, the saturated monolayer is characterized by regular and sharp diffraction peaks, revealing a 5-fold and 4.25-fold periodicity relative to the substrate surface along the [001] and [11h0] directions, respectively. On the basis of the precise position of the measured diffraction peaks and the absence of additional diffraction along other high-symmetry azimuth
directions, we conclude a (4.25 × 5) saturation structure. A closer inspection of the angular distribution recorded along [11h0] reveals an additional set of weaker diffraction peaks with a doubled periodicity (marked by arrows in Figure 6b), which indicates a coexisting (8.5 × 5) structure. The LEED and XPS data discussed before indicated a change of the monolayer structure at elevated temperatures. In fact, a different structure was observed by HAS when annealing the saturated monolayer at 450 K for several minutes or when saturating the clean Cu(110) surface with perylene at a sample temperature of 450 K. Figure 7 displays the corresponding angular distributions which were recorded along various high-symmetry directions. The diffraction peaks reveal a 5-fold periodicity relative to the clean surface for all four measured azimuth directions and, thus, yield a (5 × 5) monolayer structure at elevated temperatures. Note that this diffraction data excludes the presence of a c(10 × 10) structure, which also would show a 5-fold periodicity along the [001] and [11h0] directions, whereas a 10-fold superstructure is expected along the [11h1] azimuth. Compared to all other perylene layers, this structure appears to be particularly well-ordered. From the full width at half maximum of the diffraction peaks, a coherence length of more than about 230 Å was determined for this phase. Note that, in contrast to X-ray diffraction, a precise analysis of the diffraction intensities of He atoms scattered from these films is hampered by the lack of the atomic form factors and the precise knowledge of a reliable interaction potential. Moreover, such an analysis would require a more elaborate scattering theory, for example, close coupling calculations. Additional information about the vertical structure of the perylene layer (i.e., effective layer thickness and step height) could be obtained by measuring the He-atom reflectivity as a function of the corresponding scattering vector ∆k⊥ ) 2ki cos(θi) during a controlled variation of the incident beam
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Figure 9. (a) STM image (50 × 50 nm, V ) 2.0 V, I ) 2 nA) of a perylene monolayer on Cu(110) prepared by saturating the surface at 420 K, together with two magnified drift-corrected regions (7.5 × 7.5 nm) showing the local structures of (b) phase I and (c) phase II. Figure 8. Variation of the specular He-atom intensity as a function of the scattering vector ∆k⊥ recorded during a controlled variation of the incident beam energy for (a) the clean Cu(110) surface, (b) the c(8 × 5) phase, and (c) the (5 × 5) monolayer of perylene. The appearance of maxima (marked by the dashed arrows) is due to the constructive interference of He atoms reflected from upper and lower terraces (see inset a). To reduce the DebyeWaller factor, the HAS drift measurements were recorded at 110 K.
energy (so-called drift spectrum, see, e.g., ref 20). In the presence of steps, this curve reveals characteristic interference oscillations resulting from He atoms reflected from the upper and lower terrace of a step (see inset, Figure 8). From the positions of the interference maxima given by ∆k⊥ ) n2π/ h, the effective height h of steps between adjacent layers can be determined. Figure 8 displays some typical drift spectra which have been recorded for the clean Cu(110) surface, the submonolayer c(8 × 5) phase, and the (5 × 5) monolayer of perylene. In the case of the bare copper surface, only a small variation of the He reflectivity with two weak intensity maxima was observed, which yields a step height of 1.3 ( 0.2 Å. This value agrees favorably with the (110)plane separation of d(110) ) 1.27 Å and, thus, indicates only a small density of monatomic steps on the Cu(110) surface. In contrast to that for the c(8 × 5) phase, which forms islands (see above), pronounced oscillations in the reflectivity were observed. From the positions and separation of the intensity maxima, an effective layer thickness of 2.1 ( 0.2 Å was derived. This value is in close agreement with the monolayer thickness of 1.9 ( 0.2 Å which has been obtained for pentacene chemisorbed on the same surface.12 Note that this height of the chemisorbed molecular monolayer is distinctly smaller than the typical layer spacing of about 3.6-3.8 Å (20) Witte, G.; Braun, J.; Nowack, D.; Bartels, L.; Neu, B.; Meyer, G. Phys. ReV. B: Condens. Matter Mater. Phys. 1998, 58, 13224.
appearing in van der Waals bond PAH crystals where the molecules frequently adopt a herringbone-type face-to-edge alignment6,7 and the separation of graphene layers (3.34 Å). Moreover, additional drift measurements were carried out for the (5 × 5) monolayer phase of perylene. Although the corresponding angular distributions indicate an excellent long-range ordering for this phase (see Figure 7), the He reflectivity reveals a pronounced variation with increasing scattering vector ∆k⊥ and the appearance of two distinct maxima. The analysis of their position again yields a step height of 1.3 ( 0.2 Å. Because the presence of multilayer islands is safely excluded by the elevated substrate temperature upon deposition, such oscillations can unambiguously be attributed to steps at the Cu(110) surface. Considering further that the amplitude of these reflectivity oscillations is related to the step density, a comparison with the corresponding curve for the clean surface indicates an enhanced substrate step density for the (5 × 5) structure. C. STM Measurements. To derive additional information on the local structure of the organic films and to provide a comparison with the data reported by Chen et al., the perylene layers were also investigated by STM. Attempts to image the submonolayer phases at room temperature failed because no stable tunneling contact could be achieved, which is attributed to the rather high mobility of isolated perylene molecules on the surface. For the saturated monolayer, the situation improved significantly. Figure 9 shows a STM image which was recorded at room temperature after saturating the Cu(110) surface with perylene at about 420 K. Three different structures (labeled as I, II, and III) were observed. Phase I reveals a stripelike structure oriented along the [11h0] direction with an apparent periodicity of 13.5 ( 1
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Figure 11. (a) LEED pattern (E ) 183 eV) of the saturated perylene monolayer on Cu(100) together with (b) a schematic diffraction pattern of a c(8 × 4) structure with two domains. Panel c displays a hard sphere structure model of the c(8 × 4) with two possible molecular orientations.
Figure 10. Large-scale STM image (85 × 85 nm, V ) -2.0 V, I ) 0.5 nA) of a perylene monolayer prepared by saturating the Cu(110) surface and subsequent annealing for 60 min at 420 K together with a line scan along A-B.
Å and a lateral stripe separation of 19.5 ( 1 Å. These dimensions are in close agreement with those of a (5 × 5) unit cell. Moreover, the appearance of this structure is corroborated by the corresponding LEED pattern. A closer inspection of the STM data indicated that a detailed analysis of the various structures is largely limited by the image distortions due to (thermal) drift. This is particularly evident when looking at the lateral registration of the stripes of phase I. On the other hand, this phase could be accurately identified as a (5 × 5) structure on the basis of the present highresolution HAS data and, thus, allowed a compensation of this drift distortion. After distortion correction, phase II can clearly be recognized as a c(8 × 4) structure (see Figure 9c). The third phase appears to be more closely packed, but a detailed analysis of the microscopic structure was hampered by the rather poor ordering. In contrast, a more uniform ordering was observed after annealing the saturated monolayer at 420 K for about 60 min. As displayed in Figure 10, subsequently recorded STM micrographs reveal only the (5 × 5) phase together with a large density of steps. A quantitative analysis (see line scan in Figure 10) yielded a step height of h ) 1.25 ( 0.2 Å, which is in close agreement with the value derived independently from the HAS drift measurements. Moreover, this step height is in very close agreement with the separation of (110) planes of the copper substrate [d(110) ) 1.27 Å] and, thus, indicates a high density of substrate steps underneath the organic layer. D. Perylene Films on Cu(100). For comparison, the adsorption of perylene on Cu(100) was also studied by MCPLEED and XPS. Figure 11a displays a typical LEED pattern recorded after saturating the surface with perylene at elevated
temperatures. The same diffraction pattern was observed upon saturating the Cu(100) surface at temperatures between 350 and 450 K and clearly can be identified as a c(8 × 4) structure appearing in two rotational domains according to the substrate symmetry (see Figure 11b). After heating above about 500 K, the LEED pattern gradually disappeared without the formation of any further structures. A comparison of the real space c(8 × 4) unit cell with the van der Waals dimensions of planar adsorbed perylene molecules (11.3 × 8.7 Å) demonstrates that, despite various possible in-plane orientations, the structure can only be primitive because no second molecule can fit into the unit cell (see Figure 11c). Corresponding XPS measurements for the saturated perylene monolayer on Cu(100) yielded a ratio of the C1s to Cu2p peak intensities of IC/ICu ) (133 ( 12) × 10-4. Because the saturated monolayer adopts a primitive structure, the corresponding coverage is well-defined. Therefore, the measured intensity ratio can be correlated to an absolute coverage and, hence, allows a comparison with the corresponding perylene layer on the Cu(110) surface (see below). IV. Discussion On the basis of the results of the present TDS and XPS measurements, which revealed an onset of multilayer desorption already at sample temperatures of 350 K, welldefined monolayer films have been prepared by OMBD of perylene on Cu(110) at slightly higher temperatures of 380 K. Under these conditions, multilayer growth is thermally impossible and, thus, enables a detailed characterization of the metal-organic interface, that is, the perylene monolayer. Depending on coverage and substrate temperature, various ordered perylene phases were identified, the structure of which has been determined precisely by means of highresolution HAS and STM. With increasing exposure, the successive appearance of a c(8 × 5) and a c(8 × 4) phase and, finally, a (4.25 × 5) saturation structure was observed upon deposition at 380 K. We note that both intermediate phases could only be stabilized and subsequently studied in detail by HAS when rather small growth rates of about 1 ML/h were used, which effectively exclude additional adsorption from the residual perylene pressure in the chamber after nominal deposition.
Molecular Beam Deposition of Perylene on Copper
Figure 12. Structure models of successive perylene superstructures appearing upon saturation of a Cu(110) surface at 380 K derived from the present data (a). After annealing at 450 K, a (5 × 5) monolayer structure is formed (b).
In a previous study, we employed NEXAFS to determine the orientation of perylene molecules upon growth on Cu(110). It was shown that, in the monolayer regime, the molecules are orientated with their molecular plane parallel to the copper surface.10 Such an adsorption geometry is commonly found for the chemisorption of PAHs on metal surfaces6 and is attributed to the interaction of the molecular π system with the metal d band. A comparison of the unit cells of the two intermediate phases with the van der Waals dimensions of perylene (11.3 × 8.7 Å) shows that no additional molecules can fit in the unit cell with a planar adsorption geometry (see Figure 12). Comparing further the unit cell size of the intermediate c(8 × 4) phase with that of the (4.25 × 5) saturation structure appearing at further exposure indicates the presence of a second molecule in the unit cell of the saturated monolayer. Such a molecular packing is further corroborated by the relative coverage derived from our XPS data. Generally, a precise determination of adsorbate coverage solely based on photoelectron intensities of the adsorbate and the substrate is frequently hampered by uncertainties in the knowledge of the transfer function or the mean free path through monolayer films. In the present case, however, the relative adsorbate coverage could be derived from a comparison of the corresponding XPS measurements for the saturated monolayer of perylene on Cu(100). On that surface, perylene forms a well-ordered c(8 × 4) monolayer structure (see Figure 11b) with a molecular area of 104 Å2. A comparison of the relative C1s to Cu2p peak intensities measured for the saturated monolayer on both copper surfaces, thus, yields a molecular area of about 100 Å2 for perylene on Cu(110). This value is in very close agreement with a molecular area of 98 Å2 expected for a (4.25 × 5) unit cell containing two molecules. Figure 12a summarizes the structure models for the various perylene phases which have been identified upon deposition at 380 K. While the precise molecular in-plane orientation could not be derived from the present data for the intermedi-
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ate phases, we assume an orientation of the molecular axis along the [11h0] direction because of a close match between the molecular width and the substrate row separation. A similar molecular alignment has been observed before for pentacene and anthracene layers on Cu(110).21 In the case of the saturated (4.25 × 5) monolayer, such an alignment is, however, no longer possible, and instead, the molecules must adopt an alternating (herringbone-type) arrangement, which is presumably stabilized by the quadrupole moments of the molecules. The resulting packing density is similar to those obtained for saturated perylene monolayers on other metal surfaces [Ag(110): 103 Å2 and Au(111): 119 Å2]22 and, thus, indicates a close packing according to the van der Waals dimensions of perylene. A different monolayer structure was observed after annealing the saturated monolayer at 450 K or, alternatively, by saturating a clean Cu(110) surface with perylene at that temperature, which results in a well-ordered (5 × 5) phase. This phase is stable up to temperatures of about 550 K, as inferred from the HAS reflectivity and XPS measurements. Accompanied STM data (see Figure 10) reveal a distinct molecular contrast and a rectangular arrangement according to the size of the (5 × 5) unit cell. All STM images which were recorded at various tunneling conditions showed no evidence of further molecules within this unit cell and, hence, indicate a primitive structure, as depicted schematically in Figure 12b. A closer inspection of the STM data recorded for the (5 × 5) phase revealed, further, a significantly enhanced step density. A systematic analysis of many STM images yielded a step height of 1.25 ( 0.1 Åsa value which was also obtained independently from HAS drift measurements for the (5 × 5) phase (see Figure 8). This step height is significantly smaller than the effective height of 2.1 Å derived for chemisorbed perylene molecules but is in close agreement with the (110)-plane separation in copper [d(110) ) 1.27 Å] and, thus, reflects the presence of steps at the substrate surface. Again, molecular steps can safely be excluded since multilayer films are thermally not stable at the elevated temperatures used for the preparation of this phase. Compared to the saturated monolayers of perylene on Cu(110) and Cu(100), the (5 × 5) phase on Cu(110) reveals a distinctly higher thermal stability. The thermally induced transition from the saturated (4.25 × 5) monolayer toward the (5 × 5) phase by heating at 450 K is reversible since the (4.25 × 5) structure was again observedsalthough with a reduced long range orderingsby subsequent perylene exposure onto the (5 × 5) structure at 380 K. A possible explanation for this high stability is a mutual repulsion in the close-packed film, which can either be caused by a direct, for example, multipolar, interaction or by a displacement of the molecules from the preferential adsorption site upon close packing. This is evident when realizing that, in the saturated monolayer, the molecules can no longer be aligned along a high azimuth direction of the substrate, which is only possible (21) Lukas, S.; Witte, G.; Wo¨ll, C. Phys. ReV. Lett. 2002, 88, 028301. (22) Seidel, C.; Ellerbrake, R.; Gross, L.; Fuchs, H. Phys. ReV. B: Condens. Matter Mater. Phys. 2001, 64, 195418.
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for the (5 × 5) phase (see Figure 12). The increased adsorbate-substrate interaction in such a particular adsorption geometry may cause an increasing surface stress, which can partly be released by the creation of defect steps at the substrate surface. In fact, adsorbate-induced changes or even reconstructions of the Cu(110) surface have been observed upon adsorption of different organic molecules,23 for example, hexa-tert-butyl-decacyclene (C60H66),24 Lander molecules (C90H98),25 or aminobenzoic acid (C7H7NO2).26 In previous work, Chen et al. also studied the growth and structure of thin films of perylene on Cu(110) by using STM, LEED, and EELS.8,9 From the intensity of vibrational modes measured by EELS, they concluded that perylene multilayer films remain thermally stable up to 465 K, while the monolayer desorbs between 540 and 600 K. For monolayer coverage, they reported a c(8×4) structure which transforms into a (- 32 13) phase upon annealing at 450 K.9 After additional deposition, they observed an ordered stripelike structure with characteristic molecular rows along the [11h0] azimuth direction.8 While some of these structures appear to be rather similar to those observed in the present study, our interpretation is somewhat different. When comparing these results with the present study, we emphasize that our XPS and TDS data revealed a significantly lower thermal stability, with an onset of multilayer desorption occurring already at 350 K. We note further that the precise structural analysis presented here is mainly based on high-resolution He-atom diffraction data. Compared to that, our LEED and STM measurements demonstrated certain limitations in a precise structural analysis because of a rather poor contrast in the LEED pattern (see Figure 4) and image distortions caused by thermal drifts in the piezo (see Figure 9). For the stripelike structure, Chen et al. observed STM data8 which are very similar to those recorded for the present (5 × 5) phase and also revealed a substantial density of steps. According to their analysis, a step height of 1.7 Å was derived, and these steps were attributed to molecular terraces in multilayer films, while their STM data clearly showed a step height of 1.2 Å (see Figure 9 in ref 8), which indicates some problems with the calibration of the piezo scanner. V. Conclusions The present results demonstrate a rather complex growth scenario for the formation of thin perylene films on Cu(110). Because of the rather different thermal stability of chemi(23) Chen, Q.; Richardson, N. V. Prog. Surf. Sci. 2003, 73, 59. (24) Schunack, M.; Petersen, L.; Ku¨hnle, A.; Legsgaard, E.; Stensgaard, I.; Johannsen, I.; Besenbacher, F. Phys. ReV. Lett. 2001, 86, 456. (25) Rosei, F.; Schunack, M.; Jiang, P.; Gourdon, A.; Laegsgaard, E.; Stensgard, I.; Joachim, C.; Besenbacher, F. Science 2002, 296, 328. (26) Chen, Q.; Frankel, D. J.; Richardson, N. V. Langmuir 2001, 17, 8276.
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sorbed monolayer films and perylene multilayers, the evolution and structure of the metal-organic interface can be studied in detail by using growth conditions which thermally exclude the growth of multilayers. With increasing coverage, the successive appearance of various intermediate phases and, finally, the formation of a close-packed (4.25 × 5) saturated monolayer structure is found. This structure is characterized by a planar adsorption geometry and a close packing according to the van der Waals dimensions of perylene. Upon heating at 450 K, a partial desorption of perylene and the formation of a highly ordered (5 × 5) structure takes place. The appearance of this phase is accompanied by a significantly enhanced density of substrate steps, which is attributed to a thermally activated and adsorbate-mediated relaxation of the Cu(110) surface. The presently reported structures were observed independently for three different crystals so that artifacts such as misaligned samples can be excluded. Further growth of perylene on that surface proceeds in an upright molecular orientation and is accompanied by pronounced island formations.10,13 A similar situation was reported previously for the growth of pentacene on Au(111), where, on the basis of STM data, a cofacial π stacking of pentacene molecules was suggested.27 Using XPS, NEXAFS, and AFM, we have shown recently that, again, a molecular reorientation from a substrate-mediated monolayer phase toward a bulklike film structure with an upright orientation without any π stacking in multilayer films but a pronounced island formation takes place.28 The present study emphasizes the importance of a precise knowledge of the thermal stability and film morphology when characterizing the microstructures of such organic thin films by STM. Moreover, it is also demonstrated that the structure of metallic surfaces can be modified upon the growth of molecular soft matter. Generally, a large diversity of organic thin film structures have been reported6, resulting essentially from the balance between molecule-molecule and molecule-substrate interactions. While a number of different adsorbate phases have been identified upon completion of the saturated monolayer of perylene on Cu(110), these structures are not transferred to multilayer films.13 This is presumably related to the rather weak mutual interaction of the small molecule perylene. Acknowledgment. This work has been funded by the Deutsche Forschungsgemeinschaft (DFG, focus program OFET and Grant Wi 1361/3-3). CM051183X (27) Kang, J. H.; Zhu, X. Y. Appl. Phys. Lett. 2003, 82, 3248. (28) Beernink, G.; Strunskus, T.; Witte, G.; Wo¨ll, C. Appl. Phys. Lett. 2004, 85, 398.
