Surface Science 318 (1994) L1181-L1185 Surface Science Letters STh4 investigations of C,Br, on HOPG and MoS, R. Strohmaier *, C Ludwig, J. Petersen, B. Gompf, W. Eisenmenger 1. Phpikalkhes Institut, Universitiit Stuttgart, O-70550 Stuttgart, Germany Received 6 May 1994; accepted for publication 26 July 1994 Abstract Monolayers of the organic molecule hexabromo~~ene on HOPG and MoS, have been imaged with scanning tunneling microscopy. The flat lying molecules form an ordered dose-packed array with unit cell parameters corresponding to their van der Waals radii. In both cases the overlayers are incommensurable to the substrate. In the images with submolecular resolution, the molecules appear different on the two substrates. On. HOPG the observed STM image contrast shows a detailed submolecular pattern depending on the tunneling voltage, whereas on MoS, the molecules appear with a less pronounced inner structure. Neither on HOPG nor on MoS, the observed submolecular contrast depends on the adsorption site. 1. Introduction Since the invention of scanning tunneling mi- croscopy in 1982, it is increasingly used for the characterisation f organic and bioorganic materials [l], but the STM image contrast of organic molecules is still under discussion [2-41. One reason for this is the small number of systematic investigations of the submolecul~ contrast of aromatic molecules. Benzene coadsorbed with CO on Rh( 111) forms an ordered commensurable overlayer of chemisorbed molecules, which were imaged by Othani et al. IS]. At low temperatures Weiss and Eigler [6] showed the influence of the adsorption site on the image contrast of statistically distributed benzene molecules on Pt(lll). Hallmark et al. [7] have imaged naphtha- lene, azulene and a series of related molecules on * Conesponding author. Fax: +49 711 685 4886; E-mail: stroh@pil.physik.uni-stuttgarrde. Pt(ll1) in an irregular arrangement. They compared their images with simple extended Hiickel calcula- tions and found good agreement with the experimen- tal data. For chemisorbed benzene on Rh(ll1) the STM image contrast was calculated by Sautet and Joachim [8], who used an electron scattering quan- tum chemistry treatment. Recently Fisher and Bliichl [9] have published the first calculations for benzene physisor~d on the. most commonly used subtrates for STM, graphite and MoS,. They found that the image contrast on graphite should depend on the adso~tion site and the ~~el~g voltage. In this work we report about the first high resolu- tion images of a physisorbed benzene derivative. On weak ~teracting substrates at room temperature, the immobilisation of molecules necessary for high reso- lution STM images can only be achieved by an ordered two-dimensional arrangement of the molecules. Because of its high vapor pressure ben- zene does not form ordered arrays on graphite at room temperature [lo]. To overcome this difficulty 0039~6028/94/$07.00 Q 1994 Blsevier Science B.V. All rights reserved SsflI 0039-6028(94)00478-l we studied hexabromobenzene (C,Br,) on graphite and MoS,. Since the melting point of C,Br, lies above 3OO”C, the preparation of well ordered mono- layers by organic molecular beam epitaxy becomes possible even at room temperature. Thin films of C,Br6 on MoS, were also characterized by transmis- sion electron microscopy from Kobayashi [ 1 l]. 2. Experimental methods STM investigations were carried out with a mi- croscope specially developed for the characterisation of physisorbed molecules. For fast sample inspection and in order to increase the signal to noise ratio, it operates with a scan rate up to 20 frames/s. In connection with an on-line averager this leads to high resolution images even at noisy conditions. All measurements were made with a mechanically cut Pt/Ir-tip in a nitrogen atmosphere at room tempera- ture. For an improvement of the image quality we substracted a constant background in the Fourier- transformed images. However no digital or analog low or high pass filtering was used. The voltage sign in the figure captions refers to the sample. The molecular layers were prepared by evapora- tion from a graphite effusion cell at a pressure less than low7 mbar. Before evaporation the freshly cleaved substrates were heated to 250°C in order to desorb contaminations and then cooled to 10°C for preparation. Evaporation rates were typically 0.05 monolayers/s which corresponds to a temperature of the effusion cell of about 150°C. The positions of the monolayer molecules relative to the substrate were determined by the “scrape off” technique, i.e. imaging of the underlying substrate after monolayer emoval by slowly approaching the tip to the substrate, as described previously [12]. 3. Results and discussion Fig. 1 shows a 200 A X 200 A image of C,Br, on HOPG recorded in the constant height mode with a frame rate of 4 frames/s and averaged over 4 frames. The hexagonal arrangement of the molecules is clearly visible. Due to different lattice constants and different orientations of the lattice vectors of adsor- Fig. 1. 200 w X 200 A image of C,Br6 on HOPG recorded with a scanning speed of 4 frames/s and averaged over 4 frames. The dark spots are a moir6 pattern due to the incommensurability of the overlayer (U = 1.2 V, f = 1.9 IA). bate and substrate the overlayer shows a moire pat- tern. The dark spots of the moire pattern form a hexagon pith a distance between adjacent spots of about 80 A. By comparison of this image with the image of the underlying substrate, which becomes visible after scraping off the molecules by the tip, we obtained the unit cell parameters of the C,Br, layer with respect o the substrate. The molecules form a closed-packed hexagonal structure with a distance corresponding to their van der Waals radii, which is incommensurable to the substrate. To determine the crystallographic structure with high accuracy we start modelling the adsorption geometry with the unit cell parameters obtained by the scrape off technique and then correct these pa- rameters until the observed moire pattern is repro- duced by the calculation in a self-consistent way. Fig. 2 shows a detailed model of the ordering of C,Br, on HOPG obtained in this way. To indicate the position of each individual molecule the underly- ing substrate is drawn in lines with the center of each molecule transparent, so that the substrate under- neath becomes visible. The crossing points of the hexagonal graphite lattice represent maxima in the STM image of graphite with a distance of 2.46 A. The circles in Fig. 2 mark regions with equivalent adsorption sites. We obtained for the lattice constant R. Strohmaier et al./Surjace Science 318 (1994) LI181-L1185 Fig. 2. Structural model of the ordering of CsBr, on HOPG. The center of each molecule is transparent toemphasize the different adsorption sites. 9.14 f 0.04 A and for the angle between the sub- strate and the overlayer lattice vector @= 14.5 + 0.1”. The nearest commensurable structures with similar lattice constants are the p($?? X v%> and a p(4 X 4) superstructure with a lattice constant of 8.87 and 9.84 A, respectively. This would require a lattice mismatch of the close-packed structure of more than 3%. These values are far away from our margins of error, so that we are sure that the C,Br, overlayer is incommensurable to the graphite lattice. The crystal- lographic results are summarized in Table 1. Fisher and BlGchl[9] calculated the binding ener- gies and the vertical separation of benzene from graphite and MoS,. They found that the separation between molecul$ and substrate xhibits a corruga- tion of 0.2-0.3 A, with the smallest value for ben- zene centered on a site with threefold symmetry Table 1 Unit cell parameters of C,Brs on HOPG and MoS, l&l (3 lal (A) @ = &(a, g) (de& C,Br, on HOPG 2.46 9.14*0.4 14.5*0.1 C,Br6 on MoS, 3.16 9.16f0.4 19.7*0.1 above a substrate atom. In the framework of this calculation the reason for the dark moir6 areas of Fig. 1 could be a vertical distance modulation. But in the proposed superstructure of Fig. 2 this would be valid only for the substrate C-atoms visible in the STM. The theoretical model used for the calculations however did not regard the fact that only half of the carbon sites at the graphite surface is imaged. Never- theless we expect hat the origin of the dark spots is topographic, but electronic perturbations cannot be excluded. Fig. 3a shows a constant height image of C,Br, on HOPG with submolecular resolution. The individ- ual molecules appear in this image with threefold symmetry and a pronounced minimum in the center of each molecule. The image contrast of individual molecules ioes not vary significantly over the scan area of 50 A, although the overlayer is ~~mmensu- rable to the substrate. This implies that the specific adsorption site does not influence the submolecular contrast, which is in contradiction to the calculations of Fisher and Bliichlf9]. In their model the benzene molecules appear in the low voltage range used here with threefold symmetry when centered at a carbon atom and with twofold symmetry when the benzene ring is located between two carbon sites. Not in all cases it was possible to reproduce the image contrast of Fig, 3a. Sometimes we observed under the same tunneling conditions a tunneling current distribution with fourfold symmetry as can be seen in Fig. 3b. However, in all cases the sub- molecular image contrast did n?t vary o!er the whole scan area extending over 100 A X 100 A, so that we assume that the difference between Figs. 3a and 3b is possibly caused by a tip influence and not by the influence of the specific adso~tion site. At higher voltages we obtained an image contrast with the full sixfold symmetry of the molecule, which is shown in Fig. 4. Here the six outer maxima are located at the positions of the bromines. The lobes are oriented along the three s~et~ equiva- lent directions of the graphite lattice. For this voltage range Fisher and Bliichl 191 expected a tunneling current distribution showing the highest molecular orbitals with near sixfold symmetry, but with no pronoun~d submolecular details. For C,Br, on MoS, we also observe a moire pattern, but in this case the distance between the dark R. Strohmaier et al./Surfack Science 318 (1994) Ll181-L1185 spots is only 39 A due to the Iarger lattice constant of MO&. Here we obtained in the way mentioned above for the lattice constant 9.16 + 0.04 A and for the angle @ 19.7 + 0.1”. As on HOPG the overlayer is incommensurable to the MoS, lattice. Fig. 5 shows a high resolution image recorded with a frame rate of 14 frames/s and averaged over 8 frames. Due to the Fig. 4. High resolution images of C,Br, recorded at higher tunneling voltages showing the full sixfold symmetry of the molecule. (a) On HOPG the molecules appear with a more detailed inner structure (50 AX 50 A, U = - 1.8 V, I = 1.8 r~4, 14 frames/s, 8 frames averaged). (b) On MoS, the submolecular image contrast is less pronounced (50 I(X 50 A, U = 1.4 V, I = 0.27 nA, 14 frames/s, 8 frames averaged). Fig. 3. High resolution images of C,Br, on HOPG. The image contrast of individual molecules i tip dependent. (a) The molecules appear with threefold symmetry with a pronuonced minimum in the center. (b) The molecules appear with a more quadratic tunnel current distribution. Over the entire scan area there is no signifi- cant influence of the specific adsorption sites on the image contrast visible in both images. (a) U = - 390 mV, I = 1 nA, (b) U= -250 mV, I=&6 nA. small scan area only a part of the hexagonal moire pattern is visible. The molecules show a sixfold symmetry with the lobes oriented parallel to the symmetry directions of the substrate, but with less submolecular details. No dependence of the image contrast of individual molecules on the specific ad- sorption site is observed. However, as the calcula- tions from Fisher and B&h1 [9] show, on MoS, the R. Strohmaier et al. /S&ace Science 318 (19941 LIIH-LlI85 interaction with the substrate broadens the tunneling current distribution. On HOPG we obtained molecular resolved im- ages for both polarities, but the images with the highest resolution were always recorded with a nega- tive sample bias. Because of its rectifying behavior, on MoS, only images with positive polarity could be recorded. With respect to the comparison of our results with the calculations of Fisher and Blijchl[9], the main difference between C,Br, and C,H, is the much larger van der Waals radius of the bromines. This may lead to a different epitaxial growth [lo]. However, the symmetry of the molecular orbitals remains unchanged. Only the energy levels are slightly shifted, which should not change the sub- molecular contrast significantly. The larger van der Waals radius of the bromines can lead to a larger molecule-substrate distance and thereby to a smaller interaction of the rr-electrons of the aromatic ring with the substrate. Our results agree qualitatively with the calculations regarding the image contrast on MoS, as well as for the vertical variation of the molecule-substrate distance caused by different ad- sorption sites. A detailed comparison of the experimental data with theoretical predictions requires a quantitative analysis of the image contrast. 4. Conclusion In summary, we have shown that it is possible to prepare well ordered monolayers of C,Br, on HOPG and MoS, by organic molecular beam epitaxy. The molecules form a close-packed hexagonal overlayer, which is in both cases incommensurable the sub- strates. In the high resolution images the submolecu- lar contrast is different for the two substrates. On HOPG the individual molecules appear with three- or fourfold symmetry depending on tip conditions in the low voltage range and with the expected sixfold symmetry with a detailed inner structure at voltages above - 1.5 V. The accurate knowledge of the orien- tation of the monolayers relative to the substrate allows us to exclude an influence of the specific adsorption site on the observed submolecular image contrast. This is in contradiction to calculations pub- lished previously [9]. On MoS, the molecules how in the high resolution images also the expected six- fold symmetry but with less submolecular details. The authors would like to thank the Deutsche Forschungsgemein~h~ for financial support under SFB 329. References [l] J. Frommer, Angew. Cbem. 104 (1992) 1325. [2] W. Mizutani, M. Shigeno, K. Kajimura and M. Ono, Ultra- microscopy 42-44 (1992) 236. [3] P. Sautet and C. Joachim, Ul~~icro~py 42-44 (1992) 115. [4] C. Ludwig, B. Gompf, J. Petersen, R. Strobmaier and W. Eisemnenger, Z. Phys. B 93 (1994) 365. [5] H. Othani, R.J. Wilson, S. Cbiang and C.M. Mate, Pbys. Rev. Lett. 60 (1988) 2398. [6] P.S. Weiss and D.W. Eigler, Phys. Rev. Lett. 71 (1993) 3139. [7] V.M. Hallmark, S. Cbiang, K.-P. Meinbardt and K. Hafner, Pbys. Rev. Lett. 70 (1993) 3740. [8] P. Sautet and C. Joachim, Chem. Pbys. Lett. 185 (1991) 23. [9] A.J. Fisher and P.E. BlochI, Pbys. Rev. Lett. 70 (1993) 3263. [lo] I. Gameson and T. Rayment, Chem. Phys. Lett. 123 (1986) 150. [ill T. Kobayasbi, in: Crystals, Vol. 13 (Springer, Berlin, 1991). [12] C. Ludwig, B. Gompf, W. Glatz, J. Petersen, W. Eisen- menger, M. M6bus, U. 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