Ordered arrangement of 9-aminoanthracene on Au(1 1 1) surfaces: A scanning tunneling microscopy study

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Surface Science 601 (2007) 5533–5539 www.elsevier.com/locate/susc

Ordered arrangement of 9-aminoanthracene on Au(1 1 1) surfaces: A scanning tunneling microscopy study Peter Lauffer a, Ralf Graupner

a,*

, Adrian Jung b, Andreas Hirsch b, Lothar Ley

a

a

b

Technische Physik, Universita¨t Erlangen, Erwin-Rommelstrasse 1, 91058 Erlangen, Germany Institut fu¨r Organische Chemie, Universita¨t Erlangen, Henkestrasse 42, 91054 Erlangen, Germany Received 20 April 2007; accepted for publication 17 September 2007 Available online 21 September 2007

Abstract We present a scanning tunneling microscopy (STM) investigation of 9-aminoanthracene (AA) on the reconstructed Au(1 1 1) surface. The bare Au(1 1 1) surface shows the herringbone reconstruction which is conserved upon deposition of the organic molecules. Most of the AA molecules are found to decorate the regions of fcc-stacking of the gold surface where a periodic linear arrangement is observed. The orientation of the long molecule axis of individual molecules is along the h0 1  1i-directions of the Au substrate. In addition, for individual domains of the surface reconstruction, one of the three possible orientations is preferred. On substrate areas which exhibit a high step density, the steps are completely decorated by AA molecules. A detailed analysis of the STM images reveals that the molecules are located on top terrace levels. The fine structure of individual molecules on the terrace shows a clear dependence on the tunneling voltage and resembles the molecular orbitals of the free AA molecule.  2007 Elsevier B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Adsorption; Au(1 1 1); Organic molecules; Self assembly

1. Introduction One of the ‘‘bottom-up’’ approaches to control matter on the nanoscale relies on the self-assembly of molecules on surfaces [1]. By exploiting non-covalent interactions between individual molecules, ordered patterns of organic molecules can be obtained [2–8]. Although it has been pointed out that insulating substrates would be desirable if not mandatory for molecular electronics [9], in most cases the underlying substrate surfaces were metallic. Recently it has been shown that the electronic states of a mono layer of aromatic molecules may form a band with a free-electron like dispersion and small effective mass (meff = 0.47me), leading to a bandwidth which is more than 10 times that of the corresponding molecular solid [8]. The formation of the free-electron like band was traced back to *

Corresponding author. Tel.: +49 (0)9131 8528335; fax: +49 (0)9131 8527889. E-mail address: [email protected] (R. Graupner). 0039-6028/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.09.023

the interaction mediated by the metallic substrate. Another favorably aspect is the large variety in reconstruction patterns of metallic surfaces which allow a patterning of molecules along the underlying substrate structure [10]. In this work, we study the decoration of the Au(1 1 1) surface by 9-aminoanthracene (AA) molecules (Fig. 1). The AA molecule was chosen because due to its polycyclic benzene rings a flat adsorption geometry can be expected. It has furthermore a pronounced T-like structure as indicated in Fig. 1. The long axis helps to identify its orientation in STMmicrographs and the side group breaks the mirror symmetry along this axis. The Au(1 1 1) substrate shows one of the most complex reconstruction patterns, the chevron phase or herringbone reconstruction [11–14,10]. In our scanning tunneling microscopy (STM) study, a regular arrangement of AA molecules on the Au surface is observed. The molecules decorate the surface in the fcc regions and the hcp-elbows of the herringbone reconstruction. The axes of the molecules are oriented according to the nearest neighbour directions of the Au(1 1 1) surface. These orientations are

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P. Lauffer et al. / Surface Science 601 (2007) 5533–5539 NH2

0.74 nm Fig. 1. (a) Structure of 9-aminoanthracene (AA); (b) schematic used to designate the orientation of AA molecules on the gold surface.

maintained even at the step edges, where a complete decoration of the steps on the upper terrace levels is observed. STM images of individual AA molecules resemble the molecular orbitals of the free AA molecule. 2. Experimental As substrates, we used commercially available 200 nm thick gold films on a thin chromium layer evaporated on glass. To prepare atomically flat Au(1 1 1) surfaces, the substrates were first annealed with a conventional propane gas flame under ambient conditions. The substrate is thereby heated to 850 C (measured by an optical pyrometer) on a quartz plate and this temperature is held for 20 min. After inserting these substrates into ultrahigh vacuum, the surfaces were sputtered using Ar ions accelerated by 500 V and subsequently annealed at 430 C. This process was repeated several times. The detailed preparation of the aminoanthracene (AA) is given in the appendix. For the STM sample AA was evaporated from a small crucible heated to 65 C. This resulted in a partial pressure of 1 · 106 mbar and the Au surface was exposed to the AA vapor for about 1 s. The desired coverage for the STM measurements was then adjusted by keeping the sample at 200 C in ultrahigh vacuum for various periods of time. All microscopy experiments were carried out in an Omicron LT-STM at a temperature of 4.7 K and a base pressure of less than 1 · 1010 mbar. STM tips are made by electrochemically etching of a Pt–Ir wire [15]. Typical tunneling parameters are U = 1 V, I = 3 pA. This corresponds to the smallest tunneling currents which provided stable STM current images. 3. Results and discussion 3.1. Clean Au(1 1 1) surface Fig. 2 shows the topographical image of the Au(1 1 1) surface prior to the evaporation of AA. The image displays two atomically flat regions, separated by a step of monoatomic height. To enhance the details of the Au(1 1 1) surface reconstruction, the corrugation due to the different terrace levels has been subtracted in Fig. 2. The surface exhibits the typical herringbone or chevron phase reconstruction [11–14,10] where regions of fcc and hcp stacking are separated by the discommensuration lines (DLs). The areas of fcc and hcp stacking are not of equal size

Fig. 2. STM topographic image (U = 1.4 V, I = 0.5 nA) of the sputterannealed Au(1 1 1) surface prior to the evaporation of AA. The corrugation due to the mono-atomic step which is running from the lower left to the middle right has been subtracted in order to enhance the visibility of the reconstruction pattern.

[11,12], the larger areas with fcc and the smaller areas of hcp stacking can be identified. Most of the DLs are found to cross the mono-atomic step in Fig. 2 and continue on the lower terrace. In previous STM studies of the Au(1 1 1) surface it was realized that the crystallographic orientation of the steps determines whether the DLs continue over the step edge or not [16–18]. In the ½2 1 1-direction, the DLs are found to be continuous over a descending step, while they do not cross an ascending step in this direction. On this basis we have identified the crystallographic directions in Fig. 2. 3.2. AA on terrace areas After evaporation of AA and subsequent annealing, the AA molecules decorate the Au(1 1 1) surface while leaving the herringbone reconstruction intact (Fig. 3, top). All AA molecules visible in Fig. 3 appear as pairs with the connection line between the molecules pointing in the same direction. This direction is off from the symmetry axis of the surface (the ½1 2 1-direction). A closer look reveals that among all pairs both molecules are oriented in the same way (Fig. 3, bottom). An obvious explanation for the appearance of the molecule as pairs in Fig. 3 is the presence of an STM double tip whose apexes are a distance of 1.2 nm apart. Nevertheless, even if this effect of a double tip it is taken into account, it is evident that the overwhelming majority of the AA molecules decorate the fcc regions of the reconstructed surface and not the hcp regions which was also observed for other organic molecules [2,19,20]. In addition, the elbows of the reconstruction, i.e. the point dislocations in ½1 2 1-direction are decorated as well [5,21–23]. From the close up, shown in Fig. 3, bottom, in conjunction with high resolution images to be discussed

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Fig. 4. Position and orientation of the AA molecules from Fig. 3. The position of the amino group is marked by the upstrokes of the Ts. The artifact due to a double tip has been removed here.

Fig. 3. Top: STM topography image of AA molecules on a flat Au(1 1 1) terrace area taken at U = 0.4 V, I = 4.5 pA. The bottom image shows a close up of the STM topography image. The fine structure allows the determination of the orientation of the AA molecules shown schematically in the inset (schematic drawing not to scale).

later, the orientation of the individual molecules and the position of the amino-group can be inferred as shown by the ball-and-stick model in the inset. It is found that all molecules are oriented with their long molecule axis in the h0 1  1i directions (Fig. 4), i.e. along the nearest neighbour directions on the Au(1 1 1) surface. Moreover, a periodic arrangement of the molecules with a mean distance of d ¼ 2:1 nm is observed on the fcc areas of the Au(1 1 1) reconstruction along the ½ 1 1 2- and the ½2  1 1-directions in the respective domains. This regular arrangement can be induced by an indirect interaction mediated by the surface state band of the Au(1 1 1) surface. The scattering of the surface state electrons by adsorbed molecules induces a circular standing wave pattern around the adsorbate. Adjacent molecules tend to adsorb in the minima of the oscillating adsorbate–adsorbate interaction energy caused by this standing wave pattern [24,25,27,26,28]. In the approach of Hyldgaard and Persson [27], the interaction energy for two adsorbates at a dis-

 2 sinð2qF dþ2dF Þ FÞ tance d is given by DEint ðdÞ  F 2 sinðd , p ðqF dÞ2 where F is the Fermi energy of the surface state band (measured from the bottom of the band), qF is the Fermi wavevector and dF is the Fermi level phase shift. The scattering phase shift dF depends on the adsorbate and can be determined independently from qF if, starting from one adsorbate, the site occupation probabilities as a function of distance is analyzed [26,28]. In our case the adsorption sites appear to be equidistant, which implies that only the first minimum of the mutual adsorbate–adsorbate interaction potential energy is occupied. If one determines dF by taking the Au(1 1 1) surface state band structure from high resolution photoemission data [29] (F = 487 meV, ˚ 1) values of dF  0 are obtained. This is in qF = 0.18 A contrast to the evaluation of the adsorbate pattern of S and Cu atoms on Cu(1 1 1) surfaces, where values of dF = 0.3p–0.5p [26–28] were obtained. The small scattering phase shift in our case would imply that the disturbance of the surface state electrons by the AA molecules is small. One possible explanation is that the adsorption pattern of the AA molecules observed in Fig. 3 is formed by a standing wave pattern of the Au(1 1 1) surface state electrons upon scattering at the domain walls of the herringbone reconstruction. In addition, direct interactions may also contribute to the observed adsorption patterns, as was recently shown for iridium complexes adsorbed on Cu(1 1 1) surfaces [30] where the adsorbate separations were found to be governed by indirect, substrate mediated interactions as well as by dipole–dipole interactions. In our case, an inspection of the orientations of the individual molecules reveals that within a single Au(1 1 1) domain most of the AA molecules are oriented in the same way (Fig. 4). Here, the long axes of the molecules are arranged predominantly perpendicular to the DLs while the amino-groups tend to face the aromatic backbone of the adjacent molecules. These preferential orientations show

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that direct molecule–molecule interactions play a role in the observed adsorption patterns as well. 3.3. AA on regions of high step density Fig. 5 shows the topographical (top) and current image (bottom) of AA molecules in a region of high step density. The corrugation due to the terrace levels has been subtracted in the topographical image to reveal the underlying Au(1 1 1) reconstruction. Steps of single atomic height with a general direction along ½0 1  1 cross the image and the terraces are descending from the lower right to the upper left. Again, the DLs of the Au(1 1 1) surface cross the steps. On the terraces, individual AA molecules are identified which are again located exclusively on the wider regions of fcc stacking. In contrast, the step edges are completely decorated by AA molecules irrespective of the stacking of the Au atoms. A careful investigation of the AA molecules at

the step edges reveals that they are all oriented parallel to the ½0 1 1-direction. This holds even if the edge direction deviates from the ½0 1 1-direction, as marked by the arrows in Fig. 5, bottom. Fig. 6 shows a close-up of the region near a step. The step is completely decorated by AA while some of the AA molecules are located on the flat terrace area. Whereas the AA molecules located at the step edges are fixed in their position, some of the molecules on the terraces are moving during the scanning process as evidenced by their blurred appearance. A tip induced movement of anthracene molecules on Ag(1 1 0) surfaces was also observed and studied in detail by Bo¨hringer et al. [31]. In our case this is most evident for the molecule in the upper right part as well as for those in the center of the STM image. On the lower right part of the STM image a cluster of AA molecules is observed, where due to the mutual AA–AA interaction stable STM imaging conditions are obtained. The difference in stability upon tip-induced movements for molecules on the step edge and on the terraces points – not surprisingly – towards a higher substrate–molecule interaction at the step edges compared to that on the terraces. A closer look

fcc [011] hcp fcc

B hcp fcc

A 5nm

0.1

B

0.0

0.9 Å

Height (nm)

-0.1 -0.2 0.4 0.3

1.8 Å

0.2

A

0.1 0.0 Fig. 5. STM image (U = 1.3 V, I = 4.3 pA) of gold step edges after sublimation of AA. The top image is the topographical image, where the corrugation due to the terrace levels has been subtracted. The terraces are descending from the lower right to the upper left. The bottom image displays the current signal.

0

1

2 Position [nm]

3

Fig. 6. Top: STM topography image of AA molecules at a step edge (U = 1.2 V, I = 3.5 pA). Bottom: Line profiles taken at the indicated positions in the top STM image.

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DL

DL

Height [nm]

at the appearance of the molecules in the STM images (compare the AA molecule marked by the circle in the lower right of the image to a molecule at the edge) also reveals a different fine structure for AA on the terraces compared to those on the step edges. Whereas the fine structure of the terrace molecule resembles an occupied molecular orbital of AA, as will be discussed below, the molecules at the edges appear quite different in the STM image resembling two parallel rows of three beads each. A difference in the AA–substrate interaction for the two sites is not only reflected in the fine structure of the AA molecules, but also in their measured height. In Fig. 6, bottom, two line profiles are compared, one from the AA molecule located on the terrace (A) and one on the step (B). The measured ˚, height of the molecule on the terrace amounts to 1.8 A which is close to the measured heights of nitro naphthalene ˚ [19]) and somewhat higher than the on Au(1 1 1) (1.5–1.6 A ˚ [31]). In conheight of anthracene on Ag(1 1 0) (1.2–1.3 A ˚ high, trast, the molecules at the steps appear only 0.9 A measured from the upper terrace level. It is important to note that these heights are obtained from the same STM image, ruling out influences of different tunneling potentials or tip configurations. Although the height of the AA at the edges of the steps is lower compared to those on the terraces, we nevertheless suggest that the AA molecules are actually located on the upper terrace close to the edge. This is for several reasons. A planar adsorption geometry on the lower terrace seems unlikely as the height of the ˚, molecule in the STM image would then amount to 3.0 A which would be almost twice the height measured for the molecules on the terrace. Another possibility would be that the AA molecules are standing upright on the lower terrace, leaning against the step edge and thus appear lower in height with respect to the upper terrace level. In this case, however, it is hard to understand why the AA molecules orient according to the substrate lattice in ½0 1 1direction, even if the edge does not follow this direction. For nitro naphthalene on Au(1 1 1), Bo¨hringer et al. also concluded a preferential adsorption geometry on top of the terraces [19]. Here, a significantly lower height for the ˚ ) compared to those on molecules at the step edges (1.1 A ˚ ) was reported as well. In our case the terraces (1.5–1.6 A the steps are covered completely by AA molecules in contrast to the case of nitro naphthalene on Au(1 1 1), therefore we cannot infer the true position of the step edge from our STM images. In the study mentioned above [19] it was suggested that the molecules may be tilted over the edge. This effect may also be present in our case, which accounts for the fact that the AA molecules do not show a broken C2-symmetry in the STM images, in contrast to the molecules on the terraces (see below). A further support of a planar adsorption geometry on the top terrace level can be deduced from a closer inspection of the STM images of the molecules close to the step edges (Fig. 7). Little dips are observed in the immediate vicinity of the AA molecules adsorbed at the step edges in the fcc and hcp regions, which are missing or less pro-

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2nm

0.2 0.1 0.0 0

1

2 Position [nm]

3

Fig. 7. Close-up of AA molecules close to the step edge. Close to the molecules depressions in the STM images are observed (arrows). The bottom image displays the line profile along in the dashed line. The bottom ˚ than the surrounding terrace of these depressions (arrow) is lower by 0.1 A area.

nounced for molecules located at the DLs of the Au(1 1 1) reconstruction. A line profile across one of these depressions (shown in Fig. 7, bottom) reveals that the bottom ˚ than the surroundof these depressions is lower by 0.1 A ing gold terrace. A lower height in the STM topography image can be caused by a perturbation of the electronic structure of the surrounding gold substrate. Comparable effects were observed in STM images of TCNQ molecules adsorbed on Cu(1 1 1) surfaces, where this effect was attributed to a charge transfer from the Cu surface to the TCNQ molecules in a flat adsorption geometry on the substrate [32,33]. Here, the molecules are adsorbed on the bottom terrace close to the step edge which also resulted in the formation of depressions in the STM images of the surrounding substrate surface on the bottom terrace. In our case, however, the depressions are located on the top terrace which in turn backs our interpretation that the AA molecules are located on the upper terrace level. 3.4. Individual AA molecules In Fig. 8 two close-ups of individual AA molecules are shown, taken at different tunneling voltages. At positive sample voltages (Fig. 8, top), where tunneling occurs from occupied electronic states of the tip into unoccupied molecular states, the observed fine structure in the topographical

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fact reflects unoccupied tip states as the barrier transmission is lower for the tip states than for the occupied sample states. Nevertheless it can be excluded that the tip states dominate the STM image entirely as the corresponding structures, rotated by 180, are observed in the same image. Furthermore it has to be kept in mind that the interaction with the Au(1 1 1) surface has to be taken into account which results in the fact the fine structure observed in the STM images is in general not comparable to the molecular orbitals of a free molecule [34]. 4. Conclusion

Fig. 8. STM topography images of individual AA molecules. Top: taken at U = 1.05 V, I = 5 pA, the inset represents the calculated LUMO of the free AA-molecule. Bottom: taken at U = 1.1 V, I = 5 pA. The inset shows the HOMO-16 orbital of the free AA-molecule. Both molecular orbitals are drawn to scale.

image resembles the calculated lowest unoccupied molecular orbital of a free AA molecule, shown in the inset of Fig. 8, top.1 Reversing the tunneling voltage results in a quite different image of the AA molecule (Fig. 8, bottom). The most striking detail is the fact that in the STM images of the AA molecules at negative tunneling voltages a node in the orbital structure along the symmetry axis is observed, indicated by the dashed line in Fig. 8, bottom. The HOMO orbital of the free AA molecule, however, belongs to a symmetric representation. Instead, the topographical image is closer to the calculated HOMO-16 orbital (inset of Fig. 8, bottom) which, however, should be too low in energy to take part in the tunneling process. One possible explanation might be that the STM image in

1

The molecular orbitals are the results of a semi-empirical AM1 calculation using HyperChem7.5. The presented surfaces are the wave˚ 3. functions squared at an iso-value of 5 · 105 A

Our STM results show that AA molecules which decorate the Au(1 1 1) surface leave the herringbone reconstruction intact. The individual molecules are located in the fcc regions of the reconstructed surface as well as on the hcp elbows of the reconstruction and are oriented along the h0 1 1i directions of the Au(1 1 1) surface. In the fcc regions AA forms a periodic linear arrangement with the individual molecules an average distance of 2.1 nm apart. Although this distance is typical for an indirect, substrate mediated interaction, additional interactions seem to exist which lead to the preferred orientation among the individual molecules. At present the physical origin of this preferred orientation is unclear, as direct molecule–molecule interactions are usually of a shorter range [35]. On regions of high step densities the AA molecules are found to decorate the step edges completely. Here, the AA molecules are located on the upper terrace level. Individual AA molecules exhibit a fine structure which resembles the molecular orbitals. Although some of the calculated orbitals resemble the appearance of the molecules STM images, a more detailed calculation would have to take the influence of the substrate into account. Appendix For the preparation of aminoanthracene, 2.0 g (11.22 mmol) of finely powdered anthracene was suspended in 150 mL glacial acetic acid in a 250 mL round-bottomed flask. The flask was immersed in an ice-water bath and the mixture was intensively stirred using a magnetic motordriven stirrer. 0.9 mL (12.96 mmol) of 65% nitric acid was added drop wise during the next 30 min. The mixture was then allowed to reach room temperature and the solvent was removed using a rotary evaporator. The resulting orange–yellow solid was dissolved in dichloromethane and washed with saturated NaHCO3 and deionized water. Subsequently, the mixture was dried over MgSO4 and the solvent was removed under reduced pressure. The crude product was recrystallized from boiling ethanol to yield bright yellow 9-nitroanthracene in 89% isolated yield (2.23 g, 9.99 mmol). 0.5 g (2.24 mmol) of the finely powdered 9-nitroanthracene was suspended in 100 mL of ethanol under N2-atmosphere in a 250 mL round-bottomed flask. After addition of 200 mg Pd/C, 1.0 mL (20.5 mmol)

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N2H4 Æ H2O was added drop-wise and the mixture was heated under reflux for 6 h. The solvent was removed under reduced pressure and the resulting solid was taken up in 100 mL of dichloromethane and washed with 10 mL deionized water. The organic phase was subsequently dried over MgSO4 and the solvent was removed under reduced pressure. The crude product was purified by column chromatography (SiO2, CH2Cl2, RF = 0.5) to yield 9aminoanthracene in 80% isolated yield (0.35 g, 1.79 mmol). References [1] J.V. Barth, G. Costantini, K. Kern, Nature 437 (2005) 671. [2] M. Bo¨hringer, K. Morgenstern, W.-D. Schneider, R. Berndt, F. Mauri, A. De Vita, R. Car, Phys. Rev. Lett. 83 (1999) 324. [3] J. Weckesser, A. De Vita, J.V. Barth, C. Cai, K. Kern, Phys. Rev. Lett. 87 (2001) 096101. [4] R. Otero, Y. Naitoh, F. Rosei, P. Jiang, P. Thostrup, A. Gourdon, E. Lægsgaard, I. Stensgaard, C. Joachim, F. Besenbacher, Angew. Chem. Int. Ed. 43 (2004) 2092. [5] T. Yokoyama, S. Yokoyama, T. Kamikado, Y. Okuno, S. Mashiko, Nature 413 (2001) 619. [6] J.V. Barth, J. Weckesser, G. Trimarchi, M. Vladimirova, A.D. Vita, C. Cai, H. Brune, P. Gu¨nter, K. Kern, J. Am. Chem. Soc. 124 (2002) 7991. [7] A. Kraft, R. Temirov, S.K.M. Henze, S. Soubatch, M. Rohlfing, F.S. Tautz, Phys. Rev. B 74 (2006) 041402. [8] R. Temirov, S. Soubatch, A. Luican, F.S. Tautz, Nature 444 (2006) 350. [9] J. Repp, G. Meyer, S.M. Stojkovic, A. Gourdon, C. Joachim, Phys. Rev. Lett. 94 (2005) 026803. [10] A.L. Va´zquez De Parga, R. Miranda, Basic properties of metal surfaces, in: P. Gru¨tter, W. Hofer, F. Rosei (Eds.), Properties of Single Organic Molecules on Crystal Surfaces, Imperial College Press, London, 2006. [11] C. Wo¨ll, S. Chiang, R.J. Wilson, P.H. Lippel, Phys. Rev. B 39 (1989) 7988. [12] J.V. Barth, H. Brune, G. Ertl, R.J. Behm, Phys. Rev. B 42 (1990) 9307.

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