LT-STM study of self-organization of b-carotene molecular layers on Cu (1 1 1)

Transcription

1 Chemical Physics Letters 369 (2003) LT-STM study of self-organization of b-carotene molecular layers on Cu (1 1 1) A.M. Baro *, Saw-Wai Hla 1, K.H. Rieder Institut f ur Experimentalphysik, Freie Universit at Berlin, Arnimallee 14, D Berlin, Germany Received 26 September 2002; in final form 21 November 2002 Abstract All-trans b-carotene molecules have been deposited at room temperature on Cu (1 1 1) and subsequently studied by STM at liquid nitrogen. They form a self-organized multilayer structure. The STM images allowto unambiguously identify single molecules and to resolve important aspects of their internal structure like its curved backbone polyene chain, its attached methyl groups and the b-ionone rings. We suggest that the molecules lose their center of symmetry, the molecule being flexible enough to allowthe formation of large three-dimensional periodic extensions. Ó 2003 Elsevier Science B.V. All rights reserved. * Corresponding author. Permanent address: Departamento de Fisica de la Materia Condensada, C-III, Universidad Autonoma de Madrid, E Madrid, Spain. Fax: address: arturo.baro@uam.es (A.M. Baro). 1 Permanent address: Department of Physics and Astronomy, Ohio University, Clippinger 252 C, Athens, OH 45701, USA. The potential of STM for molecular imaging has been demonstrated in recent years [1,2]. In this direction, studies on objects of larger sizes in the 1 10 nm range are currently performed. By this way, the capability of Scanning Probe techniques to deal with more complex systems has been demonstrated. Moreover both imaging and manipulation by the STM tip became possible [3,4]. A good reviewon the basic and applied interest of this research can be found in [5]. The present work is the first LT-STM study of b-carotene, performed under well-controlled surface science conditions. At the same time the system is a molecular multilayer structure, where we expect to retain some of the properties which are necessary to understand the important biological activity of this and other related molecules like vitamin A [6]. From chemical evidence, C 40 H 56 (alltrans b-carotene), is a planar, all-trans polyene chain characterized by a set of conjugated double bonds with attached methyl groups, terminated with two b-ionone carbon rings also with attached methyl groups and symmetric with respect to its center [7]. Taylor first studied single crystals of b-carotene [8] and the crystal structure was solved by Sterling [9]. The crystals of b-carotene are monoclinic, spatial group P2 1 =c with the unit-cell dimensions determined. A newx-ray determination reported in [10] and confirmed in [11] is at variance with the precedent one. In particular the b angle changes from 105 to 94. b-carotene belongs to a larger family, the carotenoids, one of the most known pigments, naturally present in the /03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi: /s (02)

2 A.M. Baro et al. / Chemical Physics Letters 369 (2003) vegetal and animal world. The more important property of b-carotene is its chain of conjugated double bonds. This supposedly makes the molecule stable and rigid, though the chain axis is curved [9]. Moreover, the p-electrons in the chain are delocalized, loosely held and easily excited by low energy visible light. This study shows that for a multilayer structure, stable STM working conditions can be found, being the b-carotene molecule identified with high resolution. The images show slight differences between individual molecules in the ordered layer, being the variations important to understand the process of self-organization. We believe that the present work demonstrates the importance of studying the multilayer regime, as intermediate between the monolayer and the crystal, but endowed with specific properties relevant for surface and interface multidisciplinary studies in the fields of physics, organic chemistry and biomedicine. We have used a lowtemperature STM system in UHV as already reported elsewhere [12]. Electrochemically etched W wires were used as tips. Cu (1 1 1) single crystal surface was prepared by repeated cycles of Ne ion sputtering and annealing to 800 K. Bare Cu (1 1 1) surface condition was checked by STM imaging. We have verified that the surface structure shows big terraces, separated in some parts by bunches of steps. About 10 Langmuir exposure of all-trans b-carotene molecules were then deposited on Cu (1 1 1) sample using vapor deposition technique at room temperature. The temperature of the oven was controlled to prevent denaturalization of the molecule. The sample was then transferred to the STM cryostat filled with liquid nitrogen. Already at this temperature, the STM images showthe formation of a perfect ordered layer, which extends all over the whole surface. STM images of this surface showdifferent kinds of features. We have been able to find large terraces containing well-ordered structures, step edges showing single molecules and point defects in the structure like vacancies. We start by showing an image corresponding to a well-ordered structure (Fig. 1a). The first task is to locate the molecule. In this particular case, this can be done because of the presence of an elongated hole going Fig. 1. (a) STM image corresponding to an ordered layer of b-carotene molecules. (Image size: 15:6 15:6 nm 2, z amplitude: 1.23 nm). Notice the elongated hole approximately situated in the center of the image, corresponding to a molecular vacancy. Two different unit cells aband a 0 b 0 have been superimposed in the image. The two molecules marked with an arroware adjacent to the molecular vacancy, showing a distinct change in their internal structure. (b) Once the molecule has been recognized, its internal structure can be divided in four parts designed as A, A 0 for head and tail, and B, B 0 for the conjugated chain. (c) Line profile taken along the molecular axis, containing three aligned molecules.

3 242 A.M. Baro et al. / Chemical Physics Letters 369 (2003) from left to right, approximately located in the center of the image. This hole corresponds to the lack of a single molecule or in other words to a single vacancy defect of the structure. Moreover this hole helps to identify unambiguously the molecular units in the ordered structure. Similar to the hole, the molecules are elongated units oriented approximately along the fast STM scanning direction. Each molecular unit shows a characteristic structure (Fig. 1b): going from left to right, we find a head of almost spherical shape (labeled A), an elongated chain with two lobes (labeled B and B 0 ) and a tail (labeled A 0 ). We attribute the head and the tail to the two b-ionone rings, whereas the rest of the structure is due to the conjugated chain of alternate single-double bonds. In addition, lobes B and B 0 extend laterally as we will analyze later. It is also interesting to tell about the unit cell of the structure. The surface lattice is oblique as can be easily seen by eye inspection. As primitive unit cell we take one having a molecule at each vertex. This unit cell is close to a rhombus, though the sides are slightly different. More precisely, the fundamental vectors of the structure are a, b with lengths of 2.7 and 3.1 nm, respectively, forming an angle c ¼ 31. The slight difference in length between a, b is the same that distinguishes the actual oblique lattice from being rectangular centered. This is better seen by taking a second choice of unit cell, i.e., a 0, b 0 which has an angle of 94 between the axes. Both primitive cells have been superimposed in the STM image. The angle of 94 coincides within our precision with the angle b of the monoclinic unit cell determined by X-ray analysis [10]. The length of the molecules is measured by taking a line profile along their axis (Fig. 1c). The total length is 3.8 nm and the height is 0.5 nm. The diameter of the lobes corresponding to the two b-ionone rings is of 0.7 nm each; this gives a chain length of 2.4 nm. The chain length is in good coincidence with that obtained from molecular geometry. The total length is however larger which can be attributed to the influence of the tip shape being convoluted with that of the molecule. The distance between two adjacent molecules is 5.5 nm, leaving in between an unfilled space. This has, however, a tiny maximum, which we tentatively attribute to the imaging of a subsurface layer. We like to present nowdata of single molecules adsorbed at step edges of the ordered layer. This is illustrated in the STM image of Fig. 2a, consisting on a top view representation of a region with two steps running approximately parallel to a diagonal of the image. Some instability is probably due to the tip dragging a molecule along the scanning direction. In spite of that, it is possible to see various reproducible features, which can be attributed to molecules bonded to the step edge. They have an elongated shape with four lobes of higher corrugation placed along the molecular axis, similar with what is observed when the molecules are placed in an ordered layer. The geometry of the step is quite irregular, not directly connected with the step geometry expected for the Cu (1 1 1) substrate surface. In fact the position of the step follows the end of the adsorbate molecules, suggesting that the steps correspond to the adsorbate layer and not to the Cu (1 1 1) substrate. This is confirmed by measuring the step height giving a value of 1.1 nm much larger than the value corresponding to a Cu (1 1 1) step (0.21 nm). This step height value has been verified on other images with different stepped structures. We conclude from this analysis that we are dealing with a b-carotene multilayer structure. The layers are perfectly flat with no indication of clustering or contamination in extensions as large as 0.2 lm. They end by bunches of steps, being the total number of layers variable 6 10 in a 0:25 0:25 lm maximum image dimension. The height corrugation of the molecules bonded to the step edge is substantially larger than that observed in the terraces. This indicates that the binding to the step is very specific, being associated to the different atomic structure around the step and/or to the different electronic structure. This kind of observation has also been reported for other molecules like C 60 adsorbed on Au (1 1 1) [13]. In our case, however, the step does not correspond to the substrate, but to the multilayer. In contrast to the ordered structure, the molecules at step edges are placed parallel to each other, with their heads A (tails A 0 ) aligned. Their

4 A.M. Baro et al. / Chemical Physics Letters 369 (2003) Fig. 2. (a) STM image (31:3 31:3 nm 2 ) with two steps decorated with single molecules. (b) Zoom in the previous image showing a set of three individual molecules. The internal structure of the molecule appears by taking line profiles (a) and (b). Labeling of the features A, B, B 0,A 0 is marked. Notice the maximum in B 1 and the minimum in B 0 r indicative of an asymmetry in the electronic structure of the molecule. orientation is also parallel to that of the molecules in the terrace. The distance between nearestneighbor molecules is 1.2 nm, as we can measure directly in a group of three situated in the right lower part of the image. A zoom in on this area, presented as Fig. 2b, helps to better obtain the internal molecular structure of the molecule. More precisely, what we have labeled as lobe B in the ordered molecule becomes nowresolved into two protrusions, called B 1 and B r.b 1 is situated to the left if we look at the molecule along its axis from its head, whereas B r is situated to the right. Lobe B 0 is also divided into two features, though this is not easy to see in the image because B 0 r is nowa minimum. We have verified this point by tracing profiles along the right and left-hand sides of the molecule (Fig. 2b). This strong change in corrugation accompanied by the observation of a minimum is certainly due to an electronic effect. The asymmetry in the electronic density of states between the two parts of the molecule is in contrast with the symmetric nature of the free molecule. Therefore, we suggest that this effect is induced by the formation of the self-assembled layer. An interesting point is about the type of electronic levels responsible of this behavior. Since the data were obtained at lowvoltage bias (+155 mv) and the structure is located in lobes B 1 and B 0 r, we suggest that they correspond to the loosely held p-electrons of the conjugated chain. Another

5 244 A.M. Baro et al. / Chemical Physics Letters 369 (2003) consequence of this analysis is to attribute lobes B r and B 0 1 to the CH 3 groups attached to the conjugated chain. The next evident step is trying to pursue the precedent analysis on the molecules in the ordered layer. Since we are dealing with an electronic effect it is very convenient to consider images taken at several bias voltages. The presentation of the ordered structure of b-carotene at V ¼ 0:8 V bias voltage was done on purpose since that image was very helpful to identify the molecule. STM imaging is also stable at both lower and higher positive voltages. We went up to 2.2 V and down to 0.15 V and still recognize the main features of the molecule as shown at 0.8 V. For lower positive voltages, the imaging was stable, the same unit cell can be observed but the molecule cannot be recognized. At negative voltages the imaging is generally more difficult. In fact, we could only solve the molecule at )1.4 and )1.5 V. At lower values of the negative voltage, the layer becomes distorted in similar way as explained for lowpositive voltages. At this point it is important to notice that the tunneling process depends on three factors: (i) tip status including shape and electronic structure; (ii) topography and electronic structure of the sample including the transport of the tunneling electrons and (iii) interaction force between tip and sample [14]. All these three factors are mixed in the image and difficult to distinguish. We believe that the cases when stable imaging is possible but the molecule cannot be recognized are due to the tip perturbing the molecular layer by the high interaction force. It is interesting that by going from 0.3 to 0.05 V and back to 0.5 V, the structure can be recovered indicating a reversible process, or in other words that the molecular layer is flexible. Irrespective of its true origin, we do observe changes in the images taken at different bias voltages. We will concentrate our attention to those taken at +0.3 and +1.5 V (Fig. 3). In comparison with the image taken at 0.8 V (Fig. 1a), the molecules appear with higher resolution. We would like first to emphasize the observation of a curved line in the molecular image at 1.5 V, situated in what we have labeled as position B 0 r. We suggest that this line corresponds to the right-hand side of the polyene chain. The same line extends to the left-hand side of the chain, in lobe B 1, but there, it is added to the higher corrugation of the structure. This observation can be correlated with the X-ray determination of the molecular structure in a single crystal of b-carotene, showing that the chain is slightly curved. This curved line in position B 0 is observed also at 0.3 V and other bias voltages (not shown), so that we are confident that it is a reproducible feature of the molecule internal structure. From this analysis, we also deduce that the protrusions at B r and B 0 1 are due to the methyl groups attached to the conjugated chain. Additional evidence can be obtained by measuring the distance of the onset of B r and B 0 1 to the center of the molecule, obtaining 0.8 nm in coincidence with the distance expected from the molecular geometry. The question now is whether we can find a reasonable coincidence with the asymmetric behavior of the corrugation along the polyene chain clearly resolved in the molecule at the step. Even if the images are certainly different, we find a similarity with the observation that lobe B has higher corrugation than lobe B 0 and therefore our answer is positive. This analysis of the electronic structure shows that there is a maximum of the unoccupied density of states at position B and a minimum at B 0. We do not see changes in this behavior in the range from 0.2 to 1.5 V. Unfortunately, our data at negative voltages are not sufficient to drawany conclusion about the occupied density of states. From the precedent analysis, we like to present an elementary schematic drawing of the molecule based on four experimental observations (Fig. 4): (i) the resolution of the backbone polyene chain curved according to the STM image shown in Fig. 3 (better resolved in Fig. 3b at V ¼ 1:5 V), (ii) the attribution of lobes B r and B 0 1 to the CH 3 groups attached to the polyene chain, (iii) the attribution of lobes A and A 0 to the b ionone rings; they are placed centered with respect to the polyene chain since the STM images do not resolve their internal structure, and (iv) the asymmetry of the unoccupied density of p-electron states, leading to a positive corrugation in B and a negative corrugation in B 0.(B 1 and B 0 r by analogy with the data of the molecule at the step). We discuss in the following the different points of this model. In general it is worth to stress that

6 A.M. Baro et al. / Chemical Physics Letters 369 (2003) Fig. 3. (a, b) STM images taken at 0.3 V (size 15:6 15:6 nm 2, z amplitude 1.08 nm) and 1.5 V (size 15:2 14:6 nm 2, z amplitude 0.88 nm). STM is a microscopic technique, giving information about the ordered layer, but also about every single molecule in the structure, and by extension to local or extended defects of the structure. This is important in connection with X-ray diffraction techniques where a molecular single crystal is measured and the data statistically analyzed, giving a model of the molecule obtained as an average over many molecules supposed to be perfectly ordered in the crystal. We already anticipate that a careful look to the molecules in the ordered layer shows that almost every molecule is distinct. This is clearly seen concerning point (i), i.e., the curvature of the conjugated chain (Fig. 3b). The

7 246 A.M. Baro et al. / Chemical Physics Letters 369 (2003) Fig. 4. Schematic model of the b-carotene molecule showing the curved backbone of the polyene chain, the methyl groups attached to it, the asymmetric corrugation attributed to p-electrons and the b-ionone rings. curvature is gradually changing up to the point that some molecules showit very close to a straight line, i.e., with almost zero curvature. We have even seen a molecule (image taken at 1.2 V not shown), where the convexity of the curve is reversed. Concerning the corrugation of the methyl groups (ii), if we consider the high corrugation of lobe B r, we infer that the methyl group points in the direction out of the surface, while in the B 0 1 lobe the direction of the methyl group points towards the surface (Fig. 1a). This strongly suggests that the plane of the molecule does not coincide with the plane of the surface. For a single monolayer, we would expect that the molecule lies planar above the surface, with the p-electrons overlapping the Cu (1 1 1) surface orbitals. For a multilayer, however, the rotation of the plane of the molecule is not surprising, since it provides with additional free space for other molecules to form a more compact structure. Notice that the molecule to molecule mean distance in the ordered layer is 0.8 nm in contrast with 1.2 nm at step edges. This issue is closely related with the analysis of lobes A and A 0, attributed to the b-ionone rings. It is unfortunate that their rich structure, i.e., the cyclohexene ring and the attached methyl groups is not resolved by STM. Nevertheless, we can measure their corrugation and their diameter. About the diameter, even if it changes within molecules, it is always smaller than the radius of the methyl groups attached to the conjugated chain. This leads to the conclusion that the b-ionone rings are oriented normal to the surface being the p-electrons situated parallel to the surface. Another consequence of this orientation of the molecule with respect to the surface is that lobes A and A 0 should have a different corrugation due to the s-cis conformation at the connection to the conjugated chain [9]. That is what we observe since lobes A and A 0 appear generally with different corrugation. This is of course qualitative and we cannot give any number for this rotation. Summarizing briefly, the b-ionone rings are oriented normally to the surface, while the methyl groups attached to the conjugated chain extend laterally in opposite directions. The last point is about the asymmetry of the unoccupied density of states. This can of course be attributed to an asymmetry of the molecule, being the density of states dominated by the spatial dependence of the surface electronic structure. There would actually be a more symmetric way to place the molecules in a surface lattice, which is quite close to the actual structure. As we already mentioned, the unit cell a, b is almost, but it is not a rhombus. The reason is the following: if we take two next nearest neighbor (nnn) molecules, the closest parts of them, are lobe B of one molecule to lobe A of the other situated below(see Figs. 1a and 3a). This could be the origin of the asymmetry of the molecule. It is however difficult to know whether this happens by the existence of an attractive interaction and/or a steric interference. We are aware that the asymmetric structure of the molecule does not agree with the symmetric nature attributed to the molecule in a crystal. We take this contradiction as an indication that a multilayer is substantially different from a crystal. A consequence is also the asymmetry in the distribution of the electric charge in the molecule, i.e., the development of a permanent dipole moment. This is important concerning the optical activity of the carotene layer, being an efficient absorbent of visible light (notice that the levels

8 A.M. Baro et al. / Chemical Physics Letters 369 (2003) participating in the asymmetry should have a very lowenergy, so that we expect transitions in the visible range). It is also worth to notice that tunneling currents close to 1 na can flowthrough several layers of b-carotene giving stable tunneling conditions. We believe that this is due to the electrons flowing easily from molecule to molecule in the same layer and the layer below, thanks to the attractive interaction that connects the p-electron orbitals from adjacent molecules. Therefore this work sustains the idea that b-carotene is a good molecule for electron transfer, when it is in the multilayer regime. Carotene has been proposed as molecular wire by attaching a thiol group that adsorbs on gold [15]. In this geometry the carotenethiol molecule behaves like a 4.2 GX resistor much larger than some of our tunneling resistances. This discrepancy can be due to the different geometry of our layer and/or to the influence of the attached thiol. In summary, we have presented high-resolution STM images of b-carotene molecules forming a self-organized multilayer structure. Analysis of the data allows identifying the curved backbone polyene chain and its attached methyl groups. As a consequence of self-organization, the molecules lose their center of symmetry. In general we find a flexible adaptation of single molecule structure leading to the completion of a large periodic multilayer. We gratefully acknowledge the Alexander von Humboldt Stiftung for financing the research stay in the FU Berlin, through a Humboldt Award (AMB), SFB 290/TP A5 and EFRE (SWH, KHR) and US-DOE BES nanoscience grant (SWH). References [1] J.K. Gimzewski, E. Stoll, R.R. Schlittler, Surface Sci. 181 (1987) 267. [2] P.H. Lippel, R.J. Wilson, M.D. Miller, Ch. W oll, S. Chiang, Phys. Rev. Lett. 62 (1989) 171. [3] S.-W. Hla, L. Bartels, G. Meyer, K.-H. Rieder, Phys. Rev. Lett. 85 (2000) [4] F. Moresco, G. Meyer, K.-H. Rieder, H. Tang, A. Gourdon, C. Joaquim, Phys. Rev. Lett. 86 (2001) 672. [5] J.K. Gimzewski, C. Joachim, Science 283 (1999) [6] J.T. Dingle, J.A. Lucy, Biol. Rev. 40 (1965) 422. [7] H. Beyer, W. Walter, Lehrbuch der organischen Chemie, 21.Aufl., S. Hirzel Verlag, Stuttgart, [8] W.H. Taylor, Z. Kristallogr. 96 (1937) 150. [9] C. Sterling, Acta Cryst. 17 (1964) [10] M.O. Senge, H. Hope, K.M. Smith, Z. Naturforsch. C 47 (1992) 474. [11] H. Hashimoto, Y. Sawahara, Y. Okada, K. Hattori, T. Inoue, R. Matsushima, Jpn. J. Appl. Phys. 37 (1998) [12] G. Meyer, Rev. Sci. Instrum. 67 (1996) [13] C. Rogero, J.I. Pascual, J. Gomez-Herrero, A.M. Baro, J. Chem. Phys. 116 (2002) 832. [14] R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cambridge University Press, Cambridge, [15] G. Leatherman, E.N. Durantini, D. Gust, T.A. Moore, A.L. Moore, S. Stone, Z. Zhou, P. Rez, Y.Z. Liu, S.M. Lindsay, J. Phys. Chem. B 103 (1999) 4006.