J. Phys. Chem. C 2010, 114, 14489–14495
14489
Effect of Surface Potential Relief on Forming Molecular Arrays: Tryptanthrin Adsorbed on Various Si(111) Reconstructions Dimitry V. Gruznev,*,† Dmitry N. Chubenko,† Andrey V. Zotov,†,§ and Alexander A. Saranin†,‡ Institute of Automation and Control Processes, 5 Radio Street, 690041 VladiVostok, Russia Faculty of Physics and Engineering, Far Eastern National UniVersity, 690000 VladiVostok, Russia, and Department of Electronics, VladiVostok State UniVersity of Economics and SerVice, 690600 VladiVostok, Russia ReceiVed: April 23, 2010; ReVised Manuscript ReceiVed: July 12, 2010
To explore the effect of the substrate surface potential relief on the self-assembly of the adsorbed organic molecules, adsorption of tryptanthrin molecules on the various reconstructed Si(111) surfaces has been studied using scanning tunneling microscopy. The set of the Si(111) surface reconstructions under consideration includes atomically clean adsorbate-free Si(111)7 × 7 and metal-induced Si(111)4 × 1-In, Si(111)’5.5 × 5.5’-Cu, and Si(111)3 × 3-Ag surfaces, each having specific surface potential relief. It has been found that on the Si(111)7 × 7 surface, which is characterized by a high density of dangling bonds, the tryptanthrin molecules are randomly trapped by dangling-bond Si adatoms without forming any ordered array. On the quasi-onedimensional Si(111)4 × 1-In surface, the tryptanthrin molecules are self-assembled into the meandering molecular chains aligned along the In-atom rows of the 4 × 1-In reconstruction. On the Si(111)’5.5 × 5.5’Cu surface, having a honeycomb-like potential relief associated with its discommensurate structure, the tryptanthrin molecules form an array of the supramolecular complexes displaying a ring-shaped STM appearance and containing presumably two molecules each. On the atomically smooth and inert Si(111)3 × 3-Ag surface with very shallow potential relief, the adsorbed tryptanthrin forms at room temperature a two-dimensional gas of highly mobile molecules, which condensates into random disordered molecular islands upon cooling sample to 110 K. Introduction Adsorption of organic molecules on inorganic surfaces is of interest for their potential applications for molecular electronics but also interesting from a fundamental point of view, in terms of self-assembly of molecules in the potential relief of a substrate. Knowledge of such interfacial chemistry is very important due to their impact on the forming nanostructures, which directly affects the properties of molecular layer. As for organic molecules on Si(111) surfaces, the overwhelming majority of the recent studies (e.g., refs 1-6) have employed pristine Si(111)7 × 7 surface as a substrate. The 7 × 7-reconstructed Si(111) surface shows up as a rich array of multireactive sites exhibiting different electronic and geometric structures. Due to a high reactivity of the Si(111)7 × 7 surface, the molecule-substrate interaction there typically prevails over molecule-molecule interaction. Among variety of metalinduced Si(111) reconstructions,7 the Si(111)3 × 3-Ag surface has appeared to be the most popular template for growing molecular layers.8-17 This surface is atomically smooth and chemically inert, hence there adsorbing molecules are free to move and self-assembly into the ordered two-dimensional nanostructures. Other metal-induced reconstructions (e.g., Si(111)3 × 3-Au,18 Si(111)5 × 2-Au,19,20 Si(111)3 × 2-Sm,21 and Si(111)4 × 1-In22) have been used for this purpose much seldom. * To whom correspondence should be addressed. E-mail: gruznev@ iacp.dvo.ru. † Institute of Automation and Control Processes. ‡ Far Eastern National University. § Vladivostok State University of Economics and Service.
Figure 1. Ball model of indolo[2,1-b]quinazolin-6,12-dione (tryptanthrin) molecule.
In the present investigation, taking indolo[2,1-b]quinazolin6,12-dione(tryptanthrin) (see Figure 1) as a prototype organic material, we have systematically studied behavior of the tryptanthrin molecules on a set of various Si(111) reconstructions to elucidate the effect of surface potential relief on the selfassembly of the adsorbed molecules. The chosen reconstructions, Si(111)7 × 7, Si(111)4 × 1-In, Si(111)’5.5 × 5.5’-Cu, and Si(111)3 × 3-Ag, fit two criteria. Atomic structure of each reconstruction is well-established and each reconstruction possesses a specific potential relief, differing from that of the other surfaces. The Si(111)7 × 7 presents a highly reactive surface, the Si(111)4 × 1 presents a surface with a quasi-onedimensional relief, the Si(111)’5.5 × 5.5’-Cu presents a surface with a honeycomb-like relief, and the Si(111)3 × 3-Ag presents a smooth surface with a very shallow potential relief. Additional motivation for choosing tryptanthrin as a sample molecule resides in the fact that it demonstrates potent activity against a wide variety of pathogenic organisms that cause variety of diseases.23 Hence, regularities of tryptanthrin self-assembly
10.1021/jp103645c 2010 American Chemical Society Published on Web 08/05/2010
14490
J. Phys. Chem. C, Vol. 114, No. 34, 2010
Gruznev et al.
Figure 2. (a) 90 × 80 Å2 empty-state (+1.6 V, 1.0 nA) STM image of the Si(111)7 × 7 surface and (b) dimer-adatom-stacking fault (DAS) structural model of this reconstruction. The 7 × 7 unit cell is outlined. The unfaulted and faulted 7 × 7 half unit cells are indicated as UH and FH, respectively. The corner and edge adatoms are shown by pink and blue circles, respectively. For comparison, tryptanthrin molecule is shown in the same scale as that of the Si(111)7 × 7 reconstruction.
on various surfaces might be useful not only for prospective molecular electronics but also for molecular biomedicine. Experimental Section Experiments were performed in the UHV chamber equipped with the Omicron STM-VT25 operated in vacuum better than ∼2.0 × 10-10 Torr. Atomically clean Si(111)7 × 7 surfaces were prepared in situ by flashing to 1280 °C after the samples were first outgassed at 600 °C for several hours. High purity synthesized tryptanthrin was sublimated from a home-built Mo crucible resistively heated to about 70 °C, while the sample was kept at room temperature. The deposition rate was estimated to be about 0.02 ML/min. [1 ML (monolayer) ) 7.8 × 1014 cm-2, the top Si atom density of the unreconstructed Si(111)1 × 1 surface.] The flux of molecules was calibrated by counting the density of the molecule-induced features developing at Si(111)7 × 7 surface upon adsorption or counting the density of molecules on Si(111)4 × 1-In surface, on which individual molecules can be resolved in STM. Prior to tryptanthrin adsorption, one of the four surface reconstructions were formed on the Si(111) surface, including atomically clean Si(111)7 × 7 and three metal-induced reconstructions, Si(111)4 × 1-In, Si(111)’5.5 × 5.5’-Cu, and Si(111)3 × 3-Ag. For preparation of the metalinduced reconstructions, a necessary amount of metal adsorbate was deposited onto the Si(111)7 × 7 surface at appropriate temperature. Indium was deposited from a Ta crucible, copper and silver from metal-covered tungsten filaments. For STM observations, electrochemically etched tungsten tips cleaned by in situ heating were employed. All STM images were acquired in a constant-current mode at both room temperature and 110 K. Results and Discussion Reconstructed Si(111) Template Surfaces. To set the stage, let us consider atomic arrangement of the reconstructed Si(111) surfaces used in the present study as templates for self-assembly of the adsorbed tryptanthrin molecules. Si(111)7 × 7. Clean adsorbate-free Si(111)7 × 7 surface is known to have a structure described by the DAS (dimeradatom-stacking fault) model24 (see Figure 2). The 7 × 7 unit cell consists of two triangular half unit cells (7 × 7 HUCs), one being in the faulted orientation (F-HUC) and the other in the unfaulted orientation (U-HUC) with respect to the underlying Si(111) substrate. Each 7 × 7 HUC contains six Si adatoms,
Figure 3. (a) 170 × 250 Å2 filled-state (-2.14 V, 0.9 nA) STM image of the Si(111)4 × 1-In surface. (b) Enlarged STM image of the area outlined by a frame in (a). (c) Structural model illustrating an area of the Si(111)4 × 1-In reconstruction shown in (b). Indium atoms are shown by red circles, topmost Si atoms in the π-bonded chains by gray circles, substrate Si atoms by white circles. The 4 × 1 unit cell is outlined.
which are seen as bright round protrusions in the STM images. The Si adatoms are subdivided into the corner and edge adatoms, as illustrated in the model in Figure 2b by the pink and blue circles, respectively. The Si(111)7 × 7 represents the surface with a relatively high density of dangling bonds. There are 19 dangling bonds per 7 × 7 unit cell, including twelve associated with the adatoms, six with the rest atoms, and one with the corner hole. Si(111)4 × 1-In. Si(111)4 × 1-In reconstructed surface is formed by saturating adsorption of 1 ML of In onto Si(111)7 × 7 surface held at about 400 °C. It shows up as a quasi-onedimensional reconstruction from the viewpoint of both atomic structure and properties.25-32 According to the accepted structural model,25 the 4 × 1-In reconstruction is built of In rows with quasihexagonal packing of In atoms located in the gaps between zigzag chains of Si atoms as shown in Figure 3. In the filledstate STM images the In rows are seen as bright stripes, while dark furrows correspond to π-bonded Si chains. Due its structural anisotropy, the Si(111)4 × 1-In reconstruction has
Tryptanthrin Adsorbed Si(111) Reconstructions
Figure 4. (a) 180 × 290 Å2 filled-state (-2.6 V, 0.5 nA) STM image of the Si(111)’5.5 × 5.5’-Cu surface. (b) Enlarged STM image of the area outlined by a frame in (a). The quasi-periodic honeycomb-like ’5.5 × 5.5’ structure (outlined by dashed line) is associated with the domain-boundary network which develops to relieve the stress due to the lattice mismatch between the silicide and silicon. (c) Structural model of the Cu2Si/Si(111) monolayer constituting the Si(111)’5.5 × 5.5’-Cu surface. Copper atoms are shown by red circles and Si atoms by white circles. The 1 × 1 unit cell is outlined by dotted line.
been used as a template surface for self-assembled growth of the quasi-one-dimensional nanostructures built of metal adsorbates (e.g., Ag,33-38 Pb,39,40 Co,41 and In42) or organic molecules (e.g., pentacene22). Si(111)’5.5 × 5.5’-Cu. The Si(111)’5.5 × 5.5’-Cu reconstructed surface could be prepared by depositing 2-3 ML of Cu onto the Si(111)7 × 7 surface held at room temperature (RT) followed by heating the sample to ∼550 °C. Figure 4, panels a and b, shows filled-state STM images from the Si(111)’5.5 × 5.5’-Cu surface illustrating its large-scale and high-resolution appearance. At a large scale, the surface shows up as a hexagonal-like array formed by the regular domainboundary network (Figure 4a), while at the atomic scale its characteristic features are the 1 × 1 -like structure in the interior of the domains and the presence of the vacancy-like crater defects (Figure 4b). Domains have a shape of nonregular hexagons with crater defects in the corners, the hexagon sizes are close (but not identical), being ∼5.5a0 in average. The latter coins the ’5.5 × 5.5’ notation of the reconstruction. [a0 ) 3.84 Å, the lattice constant of the nonreconstructed Si(111)1 × 1 surface.] For the local atomic arrangement of the 1 × 1 structure at the ’5.5 × 5.5’-Cu/Si(111) interface, the Cu2Si model43 is recently accepted by most of the researchers. According to the model (Figure 4c), the Cu2Si-layer structure is formed via Cu adsorbing in the H3 sites and substituting for Si in the upper half of the Si(111) double layer. Si atoms remaining in the lower half of the (111) double layer tie the Cu2Si monolayer to the Si(111) substrate via Si-Si bonds. Hence, the Si(111)’5.5 × 5.5’Cu reconstruction is essentially a Cu2Si-silicide monolayer in which a quasi-periodic incommensurate domain-boundary network develops to relieve the stress due to the lattice mismatch between the silicide and silicon. Similarly to the other incommensurate surfaces (e.g., 2 ML-Ag/Pt(111)44,45), the Si(111)’5.5 × 5.5’-Cu reconstruction exhibits a honeycomb-like potential relief in which adsorbate atoms or molecules are repelled by the domain boundaries and are accumulated in the domain interior, thus forming regular cluster arrays.46
J. Phys. Chem. C, Vol. 114, No. 34, 2010 14491
Figure 5. (a) 120 × 180 Å2 filled-state (-1.6 V, 0.7 nA) STM image of the Si(111) 3 × 3-Ag surface. (b) Enlarged STM image of the area outlined by a frame in (a). (c) Honeycomb-chained trimer structural model of the Si(111)3 × 3-Ag reconstruction Silver atoms are shown by red circles, topmost Si atoms forming trimers by gray circles, Si(111) substrate atoms by white circles. The 3 × 3 unit cell is outlined.
Si(111)3 × 3-Ag. The Si(111)3 × 3-Ag surface is prepared by saturating adsorption of 1 ML of Ag onto the Si(111)7 × 7 surface held at about 500 °C. In the STM images, the Si(111)3 × 3-Ag reconstruction displays a honeycomblike appearance (Figure 5, panels a and b). Its basic structure is described by the honeycomb-chained-trimer (HCT) model,47,48 shown in Figure 5c. According to the model, the topmost layer of the surface is built of Ag atoms with a HCT arrangement and at 0.75 Å below the Ag layer there exists a layer of Si trimers. The 3 × 3-Ag surface is chemically inert for many adsorbed molecules, hence on this surface the intermolecular interaction typically prevails over molecular-substrate interaction. As a result, the Si(111)3 × 3-Ag template is widely used for observing self-assembly of the various organic molecules8-17 and fullerenes49-54 into the ordered arrays. Tryptanthrin Adsorption on Reconstructed Si(111) Template Surfaces. Tryptanthrin on Si(111)7 × 7. Figure 6 shows empty-state STM images of the Si(111)7 × 7 surface after RT adsorption of tryptanthrin onto it. Like other similar organic molecules,1-4 tryptanthrin molecule adsorbed on the Si(111)7 × 7 surface manifests itself in the empty-state STM image by disappearance of the corresponding Si adatom by which dangling bond the molecule was trapped. Tryptanthrin demonstrates a preference for adsorbing on the edge adatoms rather than on the corner adatoms (see Figure 2c), the occupation probability ratio being ∼1.5 (Figure 6c). Meanwhile, there is a negligible difference for adsorption probability in the faulted and unfaulted 7 × 7 HUCs. With increasing coverage, the tryptanthrin plausibly starts to disrupt the original Si(111)7 × 7 surface reconstruction leading to formation of the disordered regions and appearance of the random shapeless clusters (Figure 6b). It could be concluded that self-assembly of the tryptanthrin molecules on the Si(111)7 × 7 is hampered due to a pronounced chemical activity of this surface with a high density of dangling bonds. The molecules are trapped by these dangling bonds and become immobile already at RT. Note that such a type of behavior is not a specific feature of only the tryptanthrin but it is typical for many other organic molecules adsorbed on Si(111)7 × 7.
14492
J. Phys. Chem. C, Vol. 114, No. 34, 2010
Figure 6. 250 × 250 Å2 empty-state STM images of the Si(111)7 × 7 surface after adsorbing of (a) 0.02 and (b) 0.06 ML of tryptanthrin at RT. Tunneling parameters: (a) +1.12 V, 0.7 nA and (b) +1.14 V, 1.2 nA. (c) Schematic histogram showing probability for tryptanthrin molecule occupying various types of Si adatoms, i.e., corner and edge adatoms in unfaulted and faulted 7 × 7 HUCs (see Figure 2b).
Figure 7. (a) 500 × 500 Å2 filled-state (-2.18 V, 3.1 nA) STM image of the Si(111)4 × 1-In surface after adsorbing of 0.03 ML of tryptanthrin at RT. The STM image was acquired after cooling the sample to 200 K. (b) illustrates of a plausible correspondence of the ball model of the tryptanthrin molecule to its STM image. (c) 135 × 200 Å2 filled-state (-2.16 V, 0.7 nA) STM image of a dense tryptanthrin molecular array on the Si(111)4 × 1-In surface.
Tryptanthrin on Si(111)4 × 1-In. Compared to Si(111)7 × 7 surface, the Si(111)4 × 1-In reconstruction is more inert, hence interaction of the tryptanthrin molecules with this surface is more weak. As the result, the Si(111)4 × 1-In surface remains intact upon tryptanthrin adsorption and tryptanthrin molecules preserve their entity. Figure 7a shows filled-state STM images of the Si(111)4 × 1-In surface after adsorption of 0.03 ML of tryptanthrin. Note that tryptanthrin molecules were adsorbed at RT, while the STM image was acquired at 200 K to suppress the molecule thermal motion. One can see that each of the molecule is clearly resolved. It shows up as a pair of two bright protrusions about 8 Å apart and a third less bright protrusions in between producing a bean-shaped appearance of the whole molecule. To illustrate the plausible correspondence of the observed STM features to the molecule, Figure 7b shows the schematic diagram of the tryptanthrin molecule superposed on the STM image of the same scale.
Gruznev et al.
Figure 8. (a) 1000 × 500 Å2 filled-state (-1.97 V, 4.6 nA) STM image illustrating the STM-tip-induced motion of tryptanthrin molecules along the rows of the Si(111)4 × 1-In recosntruction. Moving molecule manifests itself by producing a blurred trace in the STM image. The molecules indicated by circles in (b) after moving along the rows toward the top of the image are absent in (c). All STM images were acquired at 200 K.
What is clearly seen is that tryptanthrin molecules are adsorbed in between bright rows of the 4 × 1-In reconstruction, which are known to correspond to the rows composed of In atoms. In other words, the tryptanthrin molecules reside exclusively above the π-bonded Si chains of the reconstruction. There is plausibly an attractive interaction between the molecules within rows, as occurrence of isolated molecules is seldom and most of the molecules are arranged into the chains separated by the regions of the bare Si(111)4 × 1-In surface. One can see that the tryptanthrin molecules, being inclined by about 30° to the row direction, can occur in four possible orientations. They often form meandering wavy chains due to the ordered sequence of molecules in the appropriate orientations. At higher coverages, the tryptanthrin molecules become arranged into the dense arrays with some molecules being adsorbed also in between the rows and atop the molecular layer (Figure 7c). Though at 200 K thermal migration of tryptanthrin molecules on Si(111)4 × 1-In is suppressed, their motion can be induced by the electrical field of the STM tip. The effect is especially apparent at relatively low tryptanthrin coverage, when there are many isolated molecules and a plenty of a free area is left on the surface. The tip-induced motion of the tryptanthrin molecules is illustrated in Figure 8. As one can see, a moving molecule leaves a narrow blurred trace along the furrow between the In rows. Origin of the trace could be understood if one anticipates that a tryptanthrin molecule experiences an attraction to the STM tip and its motion is confined within a single furrow. The STM tip is scanning from left to right and scans are repeated being shifted from bottom toward the top of the image. As a result, each time when tip approaches a given molecule the tip moves it one step along the furrow toward the top of the image. The molecule motion proceeds until it reaches another molecule, step edge or boundary of the scanning area. In the sequential
Tryptanthrin Adsorbed Si(111) Reconstructions
Figure 9. (a) 500 × 500 Å2 filled-state (-0.97 V, 1.4 nA) STM image of Si(111)4 × 1-In after adsorbing 0.03 ML of tryptanthrin at RT. STM image was acquired also at RT. (b) Enlarged STM image of the surface area outlined in (a). One can see that though all tryptanthrin molecules are in fast continuous thermal motion, but their motion is still confined by the area in between In rows.
Figure 10. (a) 500 × 500 Å2 filled-state (-1.93 V, 1.8 nA) STM image showing Si(111)’5.5 × 5.5’-Cu surface at initial stage of tryptanthrin adsorption at RT. Insets illustrate location of a single tryptanthrin molecule and tryptanthrin ring-shaped supramolecular complexes with respect to the domain-boundary network (outlined by solid lines). (b) 250 × 250 Å2 empty-state STM image showing Si(111)’5.5 × 5.5’-Cu surface with an array of tryptanthrin ring-shaped supramolecular complexes.
STM images taken from the same area, a given molecule with its trace is absent, as shown in Figure 8b and c. At room temperature, all the adsorbed tryptanthrin molecules are in continuous motion and their speed exceeds that of the STM tip scanning. As a result, the STM image displays a fuzzy picture like shown in Figure 9 where tryptanthrin molecules manifests themselves by short bright segments. This type of appearance is a sequence of the fact that a molecule leaves until being recorded completely in STM image (which requires several scans over the molecule). Though it was impossible to take the images of the molecules, the location of their trace segments clearly demonstrates that at RT tryptanthrin molecules still prefer to occupy the furrows between the In rows. Tryptanthrin on Si(111)’5.5 × 5.5’-Cu. Similar to the Si(111)4 × 1-In surface, the Si(111)’5.5 × 5.5’-Cu reconstruction is also inert and tryptanthrin molecules have a possibility for self-assembly in the potential relief of this surface. One should be reminded that potential relief here resembles the surface domain structure with potential basins of attraction inside the hexagonal domains and potential barriers along the domain boundaries.46 Figure 10a shows the early stages of the tryptanthrin adsorption onto Si(111)’5.5 × 5.5’-Cu surface at RT. Note that in this experiment the STM images were taken also at RT but they were similar to those taken at 110 K. One can see that following the general tendency the molecules are trapped inside the hexagonal domains as illustrated in the insets in Figure 10a. One can see that there are two types of features associated with the tryptanthrin. The first one is believed to correspond to the single tryptanthrin molecules, since they have STM appearance
J. Phys. Chem. C, Vol. 114, No. 34, 2010 14493
Figure 11. Tip-induced migration of tryptanthrin on Si(111)’5.5 × 5.5’-Cu surface during STM image acquisition. (a) 280 × 400 Å2 filledstate (-1.12 V, 1.0 nA) STM image of Si(111)’5.5 × 5.5’-Cu surface with tryptanthrin ring-shaped supramolecular complexes. The uncomplete nanorings (cut off from their bottom side) are those which appear within given hexagonal domains during their scanning by STM tip. (b) 700 × 950 Å2 filled-state (-1.67 V, 0.9 nA) STM image of Si(111)’5.5 × 5.5’-Cu surface with tryptanthrin ring-shaped supramolecular complexes acquired after scanning first the surface area outlined by dotted frame (at +1.67 V, 0.9 nA). Within the prescanned region, the surface density of nanorings are much greater than on the surrounding surface.
identical to that of tryptanthrin molecules on Si(111)4 × 1-In. The second one shows up as a ring having a diameter of ∼20 Å and apparent height of ∼2 Å. At higher tryptanthrin coverages, these nanorings prevail (Figure 10b). The nanoring is plausibly a supramolecular complex consisting of a few tryptanthrin molecules. By analogy with the Si(111)4 × 1-In, where a quasi-one-dimensional surface relief forces the tryptanthrin to self-assembly into molecular chains, a few tryptanthrin molecules being confined within a basin of hexagonal shape might self-assembly into the ring-like complex. To estimate the number of molecules constituting the ring structure, we prepared an uncompleted Si(111)’5.5 × 5.5’-Cu phase, i.e., that containing patches of the Si(111)‘5.5 × 5.5′-Cu phase surrounded by original Si(111)7 × 7 reconstruction and adsorbed tryptanthrin molecules onto it. We counted the density of molecule-induced features on the 7 × 7 area (which yields essentially a coverage of the deposited molecules) and the density of rings on ’5.5 × 5.5’-Cu patches. The density of the features appears to be about twice greater than the density of the rings. Hence, each ring contains presumably two molecules. It should be noted that individual molecules within a ring are not resolved in the STM images. It is possible that molecules trapped in a well might be also involved in cyclic motion similar to that recently reported for single-molecule rotor.55 However, our STM observations at 110 K did not reveal any difference from the STM appearance of the nanorings at RT, but this temperature might be not low enough. For conclusive elucidation of the nanoring origin, STM image simulation for the various structural models together with the STM observations at lower temperatures are desirable. While the interdomain thermal migration of the tryptanthrin molecules on Si(111)’5.5 × 5.5’ at RT is suppressed, the STMtip-induced motion is significant. The effect is especially pronounced with a negative potential on the tip, i.e. at recording the empty-state STM images. In this case, the tryptanthrin molecules are attracted to the surface region under the tip, as illustrated in Figure 11. One can see in Figure 11a that there are many nanorings which are cut off from their bottom side. Taking into account that STM-tip slow scanning direction is
14494
J. Phys. Chem. C, Vol. 114, No. 34, 2010
Gruznev et al.
from the bottom to the top, it means that a given hexagonal domain, which was initially free of molecules, accumulates them during recording STM image. Figure 11b demonstrates the result of the experiment in which the empty-state STM image was acquired first in the limited surface region and then the scanning area was increased. One can clearly see that density of nanorings in the prescanned area considerably exceeds that on the surrounding surface. Interestingly that in the most cases the accumulated tryptanthrin appears already in the form of the nanoring complexes but not individual molecules. The tip-induced displacement of the adsorbed molecules is generally described by the interaction of the inhomogeneous electrostatic field b E with the dipole moment of the molecule56 given to first order in E as
b + ... b p )b p 0 + RE
(1)
b is an induced dipole where b p0 is a static dipole moment and RE moment with R being the polarizability of the adsorbate. The spatially dependent energy of the adsorbate molecule is then given by
1 U(b) r ) -b p0 · b E(b) r - RE2(b) r + ... 2
(2)
As the field is spatially nonuniform (with maximal strength underneath the tip apex), the adsorbate would experience a potential gradient, i.e., a force. When the second (polarizability) term dominates, the adsorbate will always be attracted toward the region underneath the tip apex irrespective of the polarity of the applied bias voltage. When the first (static-dipole) term dominates, the orientation of the dipole remains unchanged, hence the direction of adsorbate motion changes with the change of the bias polarity. Tryptanthrin molecules on Si(111)’5.5 × 5.5’-Cu are attracted underneath the tip at both polarities, that means that induced dipole moment always dominates. A more pronounced effect for the empty-state STM observations as compared to the filled-state observations indicates that in the former case the static and induced dipole moments are oriented in the same direction, while in the latter case they are in the opposite directions. Tryptanthrin on Si(111)3 × 3-Ag. Compared to the reconstructions considered before, the Si(111)3 × 3-Ag has the most shallow potential relief. Hence, it is expected to have a negligible effect on the self-assembly of the tryptanthrin molecules. Figure 12 shows filled-state STM images of Si(111)3 × 3-Ag surface after adsorption of 0.03 ML of tryptanthrin acquired at RT and 110 K. The STM image taken at RT (Figure 12a) shows a surface covered by a blurred almost featureless layer, which corresponds to the two-dimensional gas of the highly mobile molecules. Upon cooling to 110 K, the tryptanthrin molecules condensate into the islands baring intact Si(111)3 × 3-Ag surface (Figure 12b). The molecular islands demonstrate a preference for decorating the step edges, but in all cases, they have a random shape and size and do not display noticeable ordering of molecules within the islands. Conclusion To explore the template effect of the substrate potential relief on the self-assembly of organic molecules, we have studied adsorption of the tryptanthrin molecules on the various welldefined Si(111) reconstructions, including Si(111)7 × 7, Si(111)4 × 1-In, Si(111)’5.5 × 5.5’-Cu, and Si(111) 3 × 3-
Figure 12. 500 × 500 Å2 filled-state STM images of Si(111)3 × 3-Ag surface after adsorption of 0.03 ML of tryptanthrin at RT. STM images were acquired at (a) RT and (b) 110 K. Tunneling parameters: (a) -1.45 V, 3.9 nA and (b) -2.7 V, 2.1 nA. The images display two terraces with Si(111)3 × 3-Ag reconstruction separated by a single atomic step. At RT tryptanthrin represents a two-dimensional gas of highly mobile molecules which condensate into random islands upon cooling to 110 K.
Ag. The Si(111)7 × 7 and Si(111) 3 × 3-Ag reconstructions represent two limiting cases, the one with extremely strong molecular-substrate interaction and the other where the molecules are almost free, respectively. On the Si(111)7 × 7 surface, the tryptanthrin molecules are randomly trapped by the danglingbond Si adatoms. On the Si(111)3 × 3-Ag surface, the tryptanthrin molecules form at RT a two-dimensional gas of highly mobile molecules, which are condensed into the random molecular islands. Thus, no self-ordering of the molecules takes place in the both cases, though due to different reasons. In contrast, the Si(111)4 × 1-In and Si(111)’5.5 × 5.5’-Cu reconstructions represent an an intermediate case when both intermolecular interaction and surface potential relief simultaneously affect self-assembly of adsorbed molecules. On the quasi-one-dimensional Si(111)4 × 1-In surface, the tryptanthrin molecules reside exclusively above π-bonded Si chain in between In atomic rows of the 4 × 1-In reconstruction and at 200 K they self-assemble themselves into the meandering molecular chains. On the Si(111)’5.5 × 5.5’-Cu surface having a honeycomb-like potential relief, the tryptanthrin molecules are trapped within hexagonal wells of the reconstruction forming there identical supramolecular nanorings. Additional possibility to control the resultant molecular arrays arises from using an STM-tip effect on the adsorbed molecules, namely, the tip was found to attract the tryptanthrin molecules into the surface area under the tip. Acknowledgment. Part of this work was supported by Russian Foundation for Basic Research (Grant No. 09-02-00094) and Russian Federal Agency for Science and Innovations (Grant Nos. 02.740.11.0111 and 4634.2010.2). The authors are grateful to V. Ph. Anufriev and V. A. Stonik for synthesizing high-purity tryptanthrin and for stimulating discussions. References and Notes (1) Yong, K. S.; Zhang, Y. P.; Yang, S. W.; Xu, G. Q. Surf. Sci. 2008, 602, 1921–1927. (2) Horn, S. A.; Patitsas, S. N. Surf. Sci. 2008, 602, 630–637. (3) Andersen, T. H.; Svenum, I. H.; Gabrielsen, A.; Borg, A. Surf. Sci. 2009, 603, 84–90. (4) Sakulsermsuk, S.; Sloan, P. A.; Theis, W.; Palmer, R. E. J. Phys.: Condens. Matter 2010, 22, 084002-5. (5) Martı´nez-Blanco, J.; Klingsporn, M.; Horn, K. Surf. Sci. 2010, 604, 523–528. (6) Guan, D.; Mao, H.; Chen, M.; Dou, W.; Song, F.; Zhang, H.; Li, H.; He, P.; Chen, H.; Bao, S. J. Chem. Phys. 2009, 130, 174712–6.
Tryptanthrin Adsorbed Si(111) Reconstructions (7) Lifshits, V. G.; Saranin, A. A.; Zotov, A. V. Surface Phases on Silicon; Wiley: Chichester, U.K., 1994; p 450. (8) Hughes, G.; Carty, D.; McDonald, O.; Cafolla, A. A. Surf. Sci. 2005, 580, 167–172. (9) Guaino, P.; Cafolla, A. A.; Carty, D.; Sheerin, G.; Hughes, G. Surf. Sci. 2003, 540, 107–116. (10) Tsai, C. S.; Wang, J. K.; Skodje, R. T.; Lin, J. C. J. Am. Chem. Soc. 2005, 127, 10788–10789. (11) Swarbick, J. C.; Ma, J.; Theobald, J. A.; Oxtoby, N. S.; O’Shea, J. N.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2005, 109, 12167– 12174. (12) Sheerin, G.; Cafolla, A. A. Surf. Sci. 2005, 577, 211–219. (13) Perdiga˜o, L. M. A.; Fontes, G. N.; Rogers, B. L.; Oxtoby, N. S.; Goretzki, G.; Champness, N. R.; Beton, P. H. Phys. ReV. B 2007, 76, 245402–6. (14) Li, Q.; Yamazaki, S.; Eguchi, T.; Hasegawa, Y.; Kim, H.; Kahng, S. J.; Jia, J. F.; Xue, Q. K. Nanotechnology 2008, 19, 465707–5. (15) Krasnikov, S. A.; Hanson, C. J.; Brougham, D. F.; Cafolla, A. A. J. Phys.: Condens. Matter 2007, 19, 446005–10. (16) Teng, J.; Wu, K.; Guo, J.; Wang, E. Surf. Sci. 2008, 602, 3510– 3514. (17) Zhang, H. M.; Gustafsson, J. B.; Johansson, L. S. O. Chem. Phys. Lett. 2010, 485, 69–76. (18) Honda, M.; Matsui, F.; Daimon, H. Surf. ReV. Lett. 2006, 13, 197– 200. (19) Thayer, G. E.; Sadowski, J. T.; zu Heringdorf, F.; Meyer; Sakurai, T.; Tromp, R. M. Phys. ReV. Lett. 2005, 95, 256106–4. (20) Zheng, F.; Barke, I.; Liu, X.; Himpsel, F. J. Nanotechnology 2008, 19, 445303–7. (21) Palmino, F.; Duverger, E.; Labrune, J. C.; Mossoyan, M.; Themlin, J. M. Surf. Sci. 2007, 601, 2588–2591. (22) Zhang, H.; Li, Y.; Li, B.; Fang, X.; Ji, H.; Wang, B.; Zeng, C.; Hou, J. G. Surf. Sci. 2009, 603, L70–L73. (23) Novak, M. J.; Baum, J. C.; Buhrow, J. W.; Olson, J. A. Surf. Sci. 2006, 600, L269–L273. (24) Takayanagi, K.; Tanishiro, Y.; Takahashi, S.; Takahashi, M. Surf. Sci. 1985, 164, 367–392. (25) Bunk, O.; Falkenberg, G.; Zeysing, J. H.; Lottermoser, L.; Johnson, R. L.; Nielsen, M.; Berg-Rasmussen, F.; Baker, J.; Feidenhans’l, R. Phys. ReV. B 1999, 59, 12228–12231. (26) Yeom, H. W.; Takeda, S.; Rotenberg, E.; Matsuda, I.; Horikoshi, K.; Schaefer, J.; Lee, C. M.; Kevan, S. D.; Ohta, T.; Nagao, T.; Hasegawa, S. Phys. ReV. Lett. 1999, 82, 4898–4901. (27) Nakamura, J.; Watanabe, S.; Aono, M. Phys. ReV. B 2001, 63, 193307–4. (28) Wang, S.; Lu, W.; Schmidt, W. G.; Bernholc, J. Phys. ReV. B 2003, 68, 035329-5. (29) Kanagawa, T.; Hobara, R.; Matsuda, I.; Tanikawa, T.; Natori, A.; Hasegawa, S. Phys. ReV. Lett. 2003, 91, 036805-4. (30) Fleischer, K.; Chandola, S.; Esser, N.; Richter, W.; McGilp, J. F. Phys. ReV. B 2007, 76, 205406–7. (31) Chandola, S.; Hinrichs, K.; Gensch, M.; Esser, N.; Wippermann, S.; Schmidt, W. G.; Bechstedt, F.; Fleischer, K.; McGilp, J. F. Phys. ReV. Lett. 2009, 102, 226805–4.
J. Phys. Chem. C, Vol. 114, No. 34, 2010 14495 (32) Snijders, P. C.; Weitering, H. H. ReV. Mod. Phys. 2010, 82, 307. (33) Ryjkov, S. V.; Lifshits, V. G.; Hasegawa, S. e-J. Surf. Sci. Nanotechnol. 2003, 1, 72–79. (34) Nagamura, N.; Matsuda, I.; Miyata, N.; Hirahara, T.; Hasegawa, S.; Uchihashi, T. Phys. ReV. Lett. 2006, 96, 256801–4. (35) Uchihashi, T.; Ohbuchi, C.; Tsukamoto, S.; Nakayama, T. Phys. ReV. Lett. 2006, 96, 136104–4. (36) Uchihashi, T.; Nakayama, T.; Aono, M. Jpn. J. Appl. Phys. 2007, 46, 5975–5980. (37) Okuda, T.; Takeichi, Y.; He, K.; Harasawa, A.; Kakizaki, A.; Matsuda, I. Phys. ReV. B 2009, 80, 113409–4. (38) Li, Y.; Liu, M.; Ma, D.; Yu, D.; Chen, X.; Ma, X. C.; Xue, Q. K.; Xu, K.; Jia, J. F.; Liu, F. Phys. ReV. Lett. 2009, 103, 076102-4. (39) Hupalo, M.; Tringides, M. C. Phys. ReV. B 2006, 73, 041405-4. (40) Yakes, M.; Chen, J.; Hupalo, M.; Tringides, M. C. Appl. Phys. Lett. 2007, 90, 163117–3. (41) Liu, C.; Uchihashi, T.; Nakayama, T. Phys. ReV. Lett. 2008, 101, 146104–4. (42) Xu, M.; Okada, A.; Yoshida, S.; Shigekawa, H. Appl. Phys. Lett. 2009, 94, 073109-3. (43) Zegenhagen, J.; Fontes, E.; Grey, F.; Patel, J. R. Phys. ReV. B 1992, 46, 1860–1863. (44) Brune, H.; Giovannini, M.; Bromann, K.; Kern, K. Nature 1998, 394, 451–453. (45) Aı¨t-Mansour, K.; Ruffieux, P.; Gro¨ning, P.; Fasel, R.; Gro¨ning, O. J. Phys. Chem. C 2009, 113, 5292–5299. (46) Gruznev, D. V.; Olyanich, D. A.; Chubenko, D. N.; Gvozd, I. V.; Chukurov, E. N.; Luniakov, Y. V.; Kuyanov, I. A.; Zotov, A. V.; Saranin, A. A. Surf. Sci. 2010, 604, 666–673. (47) Katayama, M.; Williams, R. S.; Kato, M.; Nomura, E.; Aono, M. Phys. ReV. Lett. 1991, 66, 2762–2765. (48) Takahashi, T.; Nakatani, S.; Okamoto, N.; Ishikawa, T. Surf. Sci. 1991, 242, 54–58. (49) Upward, M. D.; Moriarty, P.; Beton, P. H. Phys. ReV. B 1997, 56, R1704–R1707. (50) Nakayama, T.; Onoe, J.; Takeuchi, K.; Aono, M. Phys. ReV. B 1999, 59, 12627–12631. (51) Iizumi, K.; Ueno, K.; Saiki, K.; Koma, A. Appl. Surf. Sci. 2001, 169/170, 142–146. (52) Tsuchie, K.; Nagao, T.; Hasegawa, S. Phys. ReV. B 1999, 60, 11131–11136. (53) Butcher, M. J.; Nolan, J. W.; Hunt, M. R. C.; Beton, P. H.; Dunsch, L.; Kuran, P.; Georgi, P.; Dennis, T. J. S. Phys. ReV. B 2001, 64, 195401– 6. (54) Wang, L.; Schulte, K.; Woolley, R. A. J.; Kanai, M.; Dennis, T. J. S.; Purton, J.; Patel, S.; Gorovikov, S.; Dhanak, V. R.; Smith, E. F.; Cowie, B. C. C.; Moriarty, P. Surf. Sci. 2004, 564, 156–164. (55) Gao, L.; Liu, Q.; Zhang, Y. Y.; Jiang, N.; Zhang, H. G.; Cheng, Z. H.; Qiu, W. F.; Du, S. X.; Liu, Y. Q.; Hofer, W. A.; Gao, H. J. Phys. ReV. Lett. 2008, 101, 197209–4. (56) Oura, K.; Lifshits, V. G.; Saranin, A. A.; Zotov, A. V.; Katayama, M. Surface Science. An Introduction; Springer-Verlag: Berlin, 2003; p 398.
JP103645C
