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Rubidium Doped Metal-Free Phthalocyanine Monolayer Structures on Au(111) Katharina Nilson,† John Åhlund,† Masumeh-Nina Shariati,† Emmanuelle Go¨thelid,†,‡ Pål Palmgren,†,§ Joachim Schiessling,| Simon Berner,†,‡ Nils Mårtensson,†,| and Carla Puglia*,† Department of Physics and Astronomy, Uppsala UniVersity, Box 516, SE-751 20 Uppsala, Sweden, Department of Biology and Chemical Engineering, Ma¨lardalens UniVersity, Box 325, SE- 631 05 Eskilstuna, Sweden, Materials Physics, Royal Institute of Technology, Electrum 229, SE-164 40 Kista, Sweden, and MAX-lab, UniVersity of Lund, Box 118, SE- 221 00 Lund, Sweden ReceiVed: October 13, 2009; ReVised Manuscript ReceiVed: June 9, 2010
Scanning tunneling microscopy (STM) studies of monolayer of metal-free phthalocyanine (H2Pc) adsorbed on Au(111) have shown ordered arrangement of the molecules on the surface. Evaporation of H2Pc onto the Au(111) surface and post annealing of the sample to 670 K results in a densely packed structure of the molecules. The monolayer is characterized by molecules adsorbed with the molecular plane parallel to the substrate surface in a square adsorption unit cell. Furthermore, the high resolution images revealed the orientation of individual molecules. The H2Pc/Au(111) system has also been doped by rubidum and compared to the undoped layers. The Rb affects the molecular adsorption geometry, and a hexagonal unit cell is found for the coadsorption of H2Pc and Rb. Upon doping, highly ordered Rb-induced protrusions are observed at the benzene site of adsorbed molecules. I. Introduction Phthalocyanine molecules have been the object of a number of investigations due to their many possible applications in optical and electronic devices.1,2 The chemical structure of this group of molecules (see Figure 1) is similar to the active sites of chlorophyll, hemoglobin, and different enzymes, making them interesting in biomimetic applications such as in oxidation reactions.3,4 Their chemical and thermal stability makes them suitable for ultra high vacuum (UHV) studies resulting in many spectroscopic investigations.5-15 Several studies, using scanning tunneling microscopy (STM)12,16-24 and other experimental techniques,7,25 have shown that different Pc molecules adsorb on a variety of substrates with their molecular planes lying parallel to the substrate. However, ZnPc has been found to adsorb with the molecular plane normal to the HOPG surface.26 Different Pc’s adsorbed on Au surfaces have been characterized. STM studies of FePc,20,27,28 CoPc,29-32 NiPc,20 CuPc,24,31,32 SnPc,18 and VOPc30,31 on Au(111) have been reported in the literature. In addition systems of Pc’s adsorbed on Au(100), Au(110), Au(111), and polycrystalline Au surfaces have been studied with a number of other techniques, as X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), low energy electron diffraction (LEED), and electron energy loss spectroscopy (EELS).9,25,33-38 The electronic structure of phthalocyanine molecular films can be modified by doping, an interesting effect since this may facilitate tuning of specific properties. Substantial differences of the electronic characteristics of Pc films induced by alkali doping have been reported by, e.g., Craciun et al.39,40 In one of these studies40 the authors observed, by scanning tunneling spectroscopy (STS), that a CuPc film, originally insulating, * To whom correspondence should be addressed. † Uppsala University. ‡ Ma¨lardalens University. § Royal Institute of Technology. | University of Lund.
became metallic for certain K doses. This result was predicted in a theoretical study by Tosatti et al. for several metal-Pc’s (MePc’s).41 After these results alkali doping of CuPc as well as other MePc molecular films have been investigated in several studies, but no metallic phase has been observed in any of these.42-47 In the present paper, we examine the adsorption of metalfree phthalocyanine (H2Pc) onto the Au(111)-(3 × 22) in situ by STM, before and after doping with rubidium (Rb). In the H2Pc monolayer (ML), the molecules adsorb with their molecular planes parallel to the surface in a square unit cell. The orientation of the molecules compared to the substrate and their adsorption direction depend highly on the acquisition temperature. Moreover, upon Rb doping, the molecular adsorption is significantly altered and an adsorption unit cell of hexagonal symmetry is found for the coadsorbed system.
Figure 1. Metal-free phthalocyanine molecule, H2Pc, as calculated in a single molecule computation.6
10.1021/jp910180y 2010 American Chemical Society Published on Web 06/28/2010
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II. Experimental Section The STM measurements have been performed with an Omicron variable temperature, UHV based instrument in Uppsala, Sweden. This instrument consists of a STM chamber and a preparation chamber. The latter is used for sputtering and annealing the samples. The molecular depositions on the substrates have been performed in a separate evaporation chamber connected to the preparation chamber. During most measurements reported here, the samples have been cooled with liquid helium to a temperature of 70 K. All STM pictures have been imaged with negative bias (probing the occupied states) in constant current mode, using a tungsten tip. The instrument has been calibrated in the scanning directions using a HOPG (highly oriented pyrolytic graphite) substrate with a lattice constant of 0.246 nm ((0.02 nm) as a standard. All uncertainty calculations have been performed using the standard deviation with a 95% confidence interval. The Au(111) substrates have been prepared by thermal evaporation of Au onto freshly cleaved mica, held at a temperature of 460 K. The obtained Au(111) substrates have been cleaned by cycles of sputtering and annealing, resulting in large, flat, clean areas exhibiting the characteristic herringbone reconstruction,48,49 as viewed by STM. H2Pc has been purchased from Aldrich (98% dye content). The molecular deposition has been performed in the evaporation chamber under UHV conditions, using a home-built evaporator positioned a few centimeters from the substrate surface and carefully outgassed prior to the experiments. The evaporator consists of a quartz glass tube (with a hole of 3.5 mm diameter) with a tungsten wire winded around it for heating. The evaporation rate has been controlled by careful adjustment of the current used to heat the glass tube. The sample has been kept at room temperature during deposition of H2Pc and has thereafter been annealed to 670 K. The Rb doping has been done in the preparation chamber from a SEAS getter source. Rb has been deposited onto the adsorbed molecular monolayer, on samples kept at room temperature. III. Results and Discussion A. H2Pc Adsorbed on Herringbone Reconstructed Au(111). As mentioned previously, the sample preparations have been done by evaporation of molecules onto a substrate kept at room temperature. After molecular deposition the samples have been annealed to about 670 K resulting in the formation of densely packed two-dimensional (2D) ordered molecular structures on the gold surface. The sample was subsequently cooled to 70 K inside the microscope. In the ML the H2Pc ’s form a densely packed structure, as shown in Figure 2. The molecules are found to arrange in an ordered fashion on the substrate, with the molecular planes parallel to the surface, in agreement with other studies on phthalocyanine monolayers12,18-24,27,29,32,50 and substituted phthalocyanines.25 The resolution of our pictures allows the identification of the adsorption unit cell and the intermolecular orientation. In Figure 2 it is seen that the molecules arrange in an (almost) square unit cell, with average unit vectors b1 ) 1.43 ( 0.04 nm and b2 ) 1.38 ( 0.07 nm, defined as the distance between the center of two neighboring molecules. The angle β formed by the unit vectors is almost 90° (87.7° ( 1.0°). In addition the azimuth angle, δ, between the molecular axis (x or y direction in Figure 1) and the unit cell vector b1 is determined to be about 30° (27.4° ( 1.9°). Finally, the angle θ formed between the
Figure 2. H2Pc adsorbed on herringbone reconstructed Au(111) at 70 K. Molecules on the soliton walls, along the 〈112j〉 direction of the substrate, appears brighter due to the corrugation of the surface. The molecular adsorption unit cell is indicated in the figure, the distances between the neighboring molecules are labeled by b1 and b2. Also the angles β, θ, and δ, defined in the text, are marked. (-1.45 V, 0.17 nA, and 10.4 nm ×10.4 nm.)
TABLE 1: Values for the Parameters of H2Pc Adsorbed on Au(111) at Low and Room Temperature, As Seen in Figures 2 and 4 unit cell
H2Pc LT
H2Pc RT
b1 b2 β θ δ
1.43 ( 0.04nm 1.38 ( 0.07nm 87.7° ( 1.0° 29.7° ( 1.5° 27.4° ( 1.9°
1.51 ( 0.08 nm 1.44 ( 0.03 nm ∼90° (60° varying
molecular adsorption directions, defined by the b1 vector, and the soliton walls of the surface reconstruction along the 〈112j〉, is found to be about 30° (29.7° ( 1.5°). The values of the parameters defined in Figure 2 are listed in Table 1. With the angle values mentioned above, the adsorption direction given by b1 is aligned with one of the close-packed 〈11j0〉 like directions of the surface. Thus, the observed length of the b1 vector is close to five time the nearest neighbor distance of Au atoms in this direction (0.288 nm). This confirms the commensurism of the layer in these directions, in agreement with what has been observed for FePc on the same surface.28 The vector b2 is in turn parallel to 〈112j〉 directions. Its length is at best equal to 2.75 time the gold nearest neighbor distance in this direction 3 × 0.288 nm, indicating that every fourth molecule in the 〈112j〉 direction will occupy a gold lattice site. Then these discrepancies observed for b1 and b2 might either be caused by a tip-scanning effect, or result from delicate balance between molecule-substrate and molecule-molecule interactions. Moreover, it can also result from a slight tilting of the molecules, due to the corrugation of the substrate. It can be pointed out here that, at this temperature, the b1 and b2 unit vectors are always defined along the same direction compared to the soliton walls of the surface reconstruction. It can be further observed in Figure 2 that the molecules are oriented with the benzene rings of one molecule close to the aza-bridging nitrogen atom (N3 in Figure 1) of the neighboring molecules. This orientation is similar to what was earlier found for H2Pc adsorbed on graphite,23 where the H2Pc intermolecular interaction was found to be weak, of van der Waals type. Moreover, a recent study about the adorption of FePc on graphite, where the molecule show a similar adsorption geometry, has indicated that the molecular layer is stabilized by weak hydrogen bonding between the bridging nitrogen atom on one molecule and the benzene hydrogen on the neighbor molecule.51 In Figures 2 and 3 the herringbone reconstruction of the underlying substrate is clearly visible as rows of brighter
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Figure 3. Adsorption of H2Pc on herringbone reconstructed Au(111) at 70 K. The underlying surface reconstruction is clearly visible as brighter rows (the so-called soliton walls). It can also be seen that the adsorption direction of the molecules is dependent on the substrate structure, since it changes at the elbow sites of the substrate. However, the symmetry axes of the H2Pc molecules remain parallel to each other from one domain to the other, as indicated by two white crosses. (-1.78 V, 0.03 nA, and 54.8 nm ×54.8 nm.)
TABLE 2: Values of the Unit Cells Parameters of CoPc29 and FePc28 Adsorbed on Au(111) unit cell
CoPc
FePc
b1 b2 β θ
1.47 nm 1.47 nm ∼90° ∼(60°
1.43 nm 1.49 nm ∼90° ∼(60°
molecules. As in studies of other Pc’s (such as CoPc,29 FePc,27,28 VOPc,30,31 or CuPc24,52) adsorbed on Au(111), the reconstruction is not lifted by the H2Pc ML. Note that in some other cases, such as NiPc/Au(111),20 the images presented in the articles are not conclusive. The molecular overlayer is evidently influenced by the substrate reconstruction. Indeed, as seen in Figure 3, the molecules on the surface build ordered domains, with welldefined boundaries that coincide with the elbow sites of the reconstructed surface. The orientation of the ML compared to the Au(111)-(3 × 22) surface defined by the angle θ (see Figure 2) defined earlier and found to be about 30° is constant from one H2Pc domain to another. This effect seems to be specific for H2Pc at this measuring temperature, as it was observed neither for CuPc,52 CoPc,29 nor for FePc27 adsorbed on the reconstructed Au(111) surface (all measured at low temperature). In these cases, the molecular adsorption directions defined by b1 and b2 seemed not to be affected by the elbow sites of the surface reconstruction, even if these systems are characterized by rather similar unit cells as for H2Pc (see Table 2). The angles formed by the adsorption direction and the substrate crystallographic directions differ significantly, as θ goes from 30° in the case of H2Pc to (60° for the MePc’s (Me ) Co or Fe). Moreover, it is seen Figure 3 that the symmetry axes of the H2Pc molecules, x and y, remain parallel to each other from one domain to the other; that is, the absolute orientation of the molecules is unchanged through the domain edges, as shown by white crosses added in Figure 3. For the MePc’s (Fe, Co) the symmetry axes x and y are always aligned with the substrate crystallographic ridge directions 〈112j〉 and 〈11j0〉. In two studies of MePc’s (FePc28 and CuPc24) line defects within the ML have been observed at the position of the substrate elbow sites, while the molecular adsorption direction remaines unchanged.
Nilson et al. Apparently, for the H2Pc and MePc’s, there are differences on the intermolecular and adsorbate-substrate interactions resulting in different molecular adsorption arrangement at low temperature. In this study, we cannot attribute this phenomenon exclusively to the influence of the metal or metal-free molecular center, as the preparation methods of the Pc monolayers are different. H2Pc has been deposited at RT, the substrates have thereafter been annealed to about 670 K, whereas CoPc and FePc have been deposited at substrates with a temperature of 350 and 390 K, respectively. Either the annealing of the H2Pc/ Au(111) affects the adsorption geometry, and/or the interaction between the Pc metallic center (CoPc and FePc) and the gold substrate plays a significant role for the molecular adsorption. Several previous investigations claimed a rather weak interaction between Pc’s and Au substrates.9,18 However, since the CuPc/ Au(111) ML has been prepared with a method similar to ours, the metal center is likely to be largely influencing the ML formation. Takada et al.29 also reported in their study a strong interaction between the CoPc ML and the Au substrate, as also found for a ML of CuPc on Au(111).33 For the FePc/Au(111) system,27 it was found that for subML coverage the molecules adsorb in two distinct configurations, with respect to the Au(111) surface structure. A strong interaction between the central iron atom and the gold atom, placed directly under the molecular metal center, was claimed for both configurations. For ML coverage, instead only one molecular orientation, compared to the substrate, was observed. According to the authors this would indicate the importance of the intermolecular interaction, for the formation of the ordered ML, whereas the molecule-surface interactions are less important. In the investigation of H2Pc/Au(111) presented here, we propose as in the case of FePc/Au(111)27 and FePc/HOPG22 a dominant intermolecular interaction, which results in the formation of the H2Pc/Au(111) ML. This is supported by the observed well ordered densely packed structure where the molecules do not change their orientation with respect with each other even through the elbow sites. However, also the molecule-substrate interaction is significant. In fact the adsorption of the molecules is found to be stable under measurement conditions, indicating quite strong bonding to the surface. Moreover, the molecules reveal less internal structures than the ones observed in our previous STM study of H2Pc adsorbed on graphite.23 High resolution images of the submolecular structure as well as different molecular orbitals as a function of tunneling bias could be obtained in the graphite study but not in the case of H2Pc/ Au(111). This is interpreted as induced by a significant interaction between the molecules and the Au substrate resulting in a more significant modification of the molecular electronic structure. A similar observation was made by Takada et al. for CoPc.29 In order to clarify the influence of the measuring temperature, the H2Pc monolayer was also imaged at room temperature (RT). The results obtained, displayed in Figure 4, show a different organization of the H2Pc layer onto the Au surface. The adsorption parameters, as defined for the LT phase, are summarized in Table 1. Not only are the molecules themselves oriented differently compared to the substrate but very few domain boundaries are now observed at the elbow sites of the surface reconstruction. The molecular layer is now similar to the results reported with the metal phthalocyanines.24,28 At RT the adsorption orientation b1 and the substrate 〈112j〉 direction form an angle θ of about (60°. The molecular symmetry axes (x and y) are now aligned along the substrate direction 〈112j〉
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Figure 5. H2Pc covered Au(111) surface, doped with Rb, observed at LT. Well ordered bright protrusions, induced by the Rb doping, are observed on top of the molecular layer. The bright protrusions are arranged in pairs; however, occasionally a single bright unit is observed, as here marked by a white ellipse (-2.61 V, 0.14 nA, and 37.8 nm × 37.8 nm).
Figure 4. H2Pc adsorbed on herringbone reconstructed Au(111) observed at RT. The molecules show a different orientation compared to the substrate with respect to the LT results. (a) 21 nm ×18.6 nm, the geometric parameters mentioned in Figure 2 are repeated for comparison; (b) 35.6 nm ×33.6 nm (-1.74 V, 1.01 nA for both images).
even in different domains as shown by the white crosses in Figure 4b, giving a varying value for δ. This comparison suggests that the substrate-molecule interaction is, at RT, more significant than in the low T case when instead the moleculemolecule interaction plays a major role. B. Coadsorption of H2Pc and Rb on Herringbone Reconstructed Au(111). After Rb doping of the H2Pc/Au(111) system, substantial differences are observed. A picture of the coadsorbed system is displayed in Figure 5. Rb-induced bright protrusions can be observed on top of the molecular layer, resulting in a well ordered system with couples of bright protrusions evenly distributed over the surface. However, single bright units are also observed in a few cases, as indicated in Figure 5. By probing the protrusion positions with respect to the H2Pc layer on different region of the sample, we are able to conclude that the Rb-induced enhancements of the density of the states (DOS) are always about 1 Å above the molecular plane and are located on top of one of the Pc benzene rings in two adjacent molecules. The intensity enhancement clearly indicates that the Rb is not interacting with the H2Pc molecular center differently from the metal atom in a metal Pc (MePc). Our STM images do not show any metallization of the central ring of the macrocycle as it is always imaged as a dip, independently by the used bias and differently from other study dedicated to the metallization of H2Pc by Fe, which instead fits in the molecular center.53 The different behaviors observed for Fe and Rb can be most likely attributed to the different electronic structure of the two metals, as well as by simple geometrical considerations reminding that the radius of the alkaline ion would probably not fit the molecular center. Important also to
take in mind that the metallic center in a MePc is usually a Me+2 ion while it is known that alkali are usually forming ionic state +1. Moreover the preferential electrostatic cation-π interaction between alkali and aromatic systems54 seems to support the different location of the alkali on the phenyl rings with respect to the metal in a MePc center. The important result that we want to enlighten with this work is the rearrangement of the molecular overlayer after alkali doping. In Figure 6, a comparison between the undoped and the Rb doped H2Pc/Au(111) system is shown. The H2Pc/ Au(111) system is characterized by a square unit cell (marked by white lines) as described in section III.A. For the Rb/H2Pc/ Au(111) system, the molecular adsorption geometry has changed. The unit cell of the coadsorbed system (marked by white lines to the right in the figure) is characterized by a hexagonal symmetry. For the H2Pc/Au(111) system there is one molecule in the unit cell, with a size of 1.43 nm × 1.38 nm, resulting of in an area of 1.97 nm2 /molecule. For the Rb/H2Pc/Au(111) system the unit vectors are determined to be c1 ) 2.92 ( 0.06 nm and c2 ) 2.82 ( 0.04 nm. Taking into account that there are four molecules in each unit cell, the area for one molecule is 1.77 nm2. Consequently, the H2Pc molecules are slightly more densely packed after Rb doping. The geometrical arrangement of the H2Pc molecules, with respect to the nearest neighbors, is highly affected by the Rb doping. The former arrangement with the benzene group of one molecule close to an aza-bridging nitrogen atom in the neighboring molecule is no longer observed. In the hexagonal molecular adsorption unit cell, after Rb doping, the molecules are instead arranged with the benzene groups of neighboring molecules close together, i.e., interacting via the benzene groups. For the Rb doped system, the distances between neighboring molecules are significantly different along different directions. A Rb-induced enhancement of the DOS is observed at the position of one of the benzene groups, for every second molecule. In Figure 6 (right-hand side), two dashed lines (marked 1 and 2) are drawn to indicate that the molecules are arranged so that they are closer toward one of the pair of bright protrusions. This results in a regular arrangement of “holes” in an otherwise close packed molecular layer. At this stage we cannot address the origin of the bright protrusions induced by the Rb. Since an absolute designation
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Figure 6. To the left: H2Pc/Au(111) (-1.45 V, 0.17 nA, and 10.4 nm ×10.4 nm). To the right: Rb doped H2Pc on Au(111) (-2.62 V, 0.14 nA, and 10.4 nm ×10.4 nm). In both pictures, the molecular adsorption unit cells, quadratic for H2Pc/Au(111) and hexagonal for Rb/H2Pc/Au(111), are indicated by white lines and schematic models marks the positions of single molecules. The geometrical arrangement of the H2Pc molecules is changed after Rb doping and an enhancement of the occupied density of states is observed at the position of one of the benzene groups, for half of the molecules. For the doped system, there are four H2Pc molecules in the unit cell and the distances between neighboring molecules are significantly different along different directions. Two dashed lines (marked 1 and 2) show the length difference between the center of a bright protrusion and the center of a neighboring molecule (line 1 and 2). The distance 2 is considerably smaller than the distance 1, and it seems like the white protrusion attracts the neighboring molecule, resulting in units of four molecules (gray lines) gathered around the Rb-induced protrusions. Both images observed at 70 K.
of the observed structures cannot be obtained from the presented data, a stoichiometric analysis of the coadsorbed system is planned. However, the STM data stimulate the discussion of different possible explanations. Assuming that the bright units are due to single Rb atoms, the appearance of the observed features can be related to several reasons. A single bright protrusion, as observed in Figure 6, is slightly elongated and measures about 0.5 nm in one direction and about 0.7 nm in the other. The size of the outermost orbital of a single Rb has been determined to be 0.2287 nm,55 considerably smaller than the observed protrusions. However, the observed size can either be tip induced (such as tip size effect or a tip-induced mobility of the adsorbed Rb) and/or to a joint effect of the adsorbed Rb together with an enhancement of the occupied DOS at the benzene site of the molecule (see Figure 6). In fact, even if a charge transfer from the Rb atom to the molecular layer would decrease the observed size of the atom considerably, the electronic structure of the molecules would be affected and a superimposed imaging of a modified molecular state and an adsorbed Rb atom would be expected. Another possible explanation could arise from Rb dimerization or cluster formation. The bond length of a Rb dimer in the gas phase is 0.421 nm,56 closer to the size of a single protrusion (as compared to the size of a single Rb atom). Although a dimerization of Rb atoms can occur in the gas phase in a low percentage,56,57 dimer formation of Rb atoms could more likely occur at the surface. If formed in the gas phase, the Rb2 may not dissociate upon adsorption since its relatively strong58 bonding. In Figure 7, the system is shown for a lower doping level. Two noticeable effects are visible in the figure. First, even at a lower doping level, the Rb-induced bright protrusions appear
Figure 7. Low coverage of Rb on H2Pc-covered Au(111) surface imaged at 70 K. Bright protrusions are distributed over the surface, following the soliton walls (along the 〈112j〉 direction) of the underlying surface reconstruction (-1.71 V, 0.14 nA, and 94.5 nm × 94.5 nm).
in pairs, with very few exceptions. The other striking effect, observed in Figure 7, is that the ordering of the coadsorbed system appears initially to be influenced by the underlying Au surface. For low Rb coverage, the bright protrusions are clearly aligned with the direction of the soliton walls of the herringbone reconstructed surface, indicating that the ordered arrangement is induced by the surface reconstruction. However, the exact site of the protrusion pairs compared to the substrate structure is still unclear. The surface plane of the herringbone reconstructed surface consists of alternating hcp and fcc regions,48 separated by the soliton walls which are clearly observed as bright areas in the pictures of the H2Pc/Au(111) system (see for example Figure 3). As observed in Figure 7, there are alternating larger and smaller distances between the soliton walls of the substrate (due to the larger area of the fcc regions as compared to the hcp regions). For the higher Rb doping level
H2Pc Monolayer Structures (see Figure 5), the Rb-induced protrusions are uniformly distributed, which would not be the case if the Rb-induced protrusions were exclusively arranged directly above the soliton walls. This implies that, even if the ordering of the coadsorbed system is originally steered by the reconstruction of the surface, there is a rearrangement of the adsorbate around certain adsorption sites resulting in an uniform ordered overlayer. We could also consider the possibility that the reconstruction of the Au surface is lifted after higher Rb doses. However, also after higher Rb doping, we still observe adsorption domains (in analogy with the domains seen in Figure 3) which probably are induced by the original surface reconstruction (image not shown here). IV. Conclusions The metal-free phthalocyanine (H2Pc) adsorption on herringbone reconstructed Au(111) surface has been characterized by STM at 70 K and RT. For the ML of H2Pc, the molecules are densely packed with a molecular adsorption unit cell of square symmetry and the H2Pc oriented with the molecular plane parallel to the surface. The measurements done at 70 K show that the adsorption geometry is affected by the underlying surface structure, and the molecular adsorption direction is found to change at the elbow sites of the herringbone reconstruction. At RT instead, the molecules keep a constant adsorption direction through the elbow sites and reorient themselves to follow the substrate 〈112j〉 direction. This would indicate a stronger molecule-substrate interaction in comparison with the molecule-molecule interaction which dominates at LT. For the Rb/H2Pc/Au(111) system, the molecular adsorption geometry is strongly altered, and the unit cell of the coadsorbed system is characterized by a hexagonal symmetry. By the doping, the interactions present in the system are significantly changed. The balance between adsorbate-adsorbate and adsorbate-substrate interactions in the doped system results in a modified molecular adsorption structure as compared to the undoped system. The previous molecular arrangement with the benzene group of one molecule close to the aza-nitrogen of the neighboring molecule is altered by the alkali, resulting in an ordered molecular adsorption where the benzene groups of neighboring molecules are closely interacting. The Rb doping induces a modification in the occupied density of states, seen by STM as bright protrusions, forming highly ordered structures on the surface. For low Rb doses, the observed bright protrusions are found to be aligned with the underlying substrate reconstructed rows, suggesting that the coupling to the underlying substrate is still significant. Conclusive assignation of such protrusions will be presented in a future publication after the already planned spectroscopic measurements which will also permit a stoichiometric evaluation of the Rb doping of the coadsorbed system. Acknowledgment. We acknowledge the Knut and Alice Wallenberg Foundation for financing our STM equipment. This work has been supported by the Swedish Foundation for Strategic Research (SSF), the Swedish Research Council (VR), the Go¨ran Gustafsson Foundation, and the Uppsala University Unimolecular Centre (U3MEC). Profs. Leif Karlsson and Maria Novella Piancastelli are acknowledged for fruitful discussions. References and Notes (1) Gu, G.; Parthasarathy, G.; Forrest, S. R. Appl. Phys. Lett. 1999, 74, 305. (2) Riad, S. Thin Solid Films 2000, 370, 253. (3) Grennberg, H.; Ba¨ckwall, J.-E. Acta Chem. Scand. 1993, 47, 506.
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