J. Phys. Chem. C 2009, 113, 14407–14410
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A Low-Temperature Scanning Tunneling Microscope Investigation of a Nonplanar Dysprosium-Phthalocyanine Adsorption on Au(111) Yan-Feng Zhang,† Hironari Isshiki,†,‡ Keiichi Katoh,‡ Yusuke Yoshida,‡ Masahiro Yamashita,‡ Hitoshi Miyasaka,‡ Brian K. Breedlove,‡ Takashi Kajiwara,‡ Shinya Takaishi,‡ and Tadahiro Komeda*,†,§ Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, 2-1-1, Katahira, Aoba-Ku, Sendai, 980-0877, Japan, Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aramaki-Aza-Aoba, Aoba-Ku, Sendai 980-8578, Japan, and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ReceiVed: May 17, 2009; ReVised Manuscript ReceiVed: June 24, 2009
Self-assemblies of a nonplanar dysprosium-phthalocyanine (DyPc) molecule on the reconstructed Au(111) substrate have been examined with a low-temperature scanning tunneling microscope (STM). A four-lobed structure with a dark center hole is imaged as an isolated DyPc molecule, where the Dy atom is expected to be positioned below the Pc plane and bound to the Au substrate. Careful measurements reveal that the axes of isolated DyPc molecules align well with the high symmetry directions of Au. This fact illustrates a strong molecule-substrate interaction. In a monolayer film, a square molecule lattice is observed, where the geometries of the molecules can be determined by our submolecularly resolved STM images. The deduced lattice vectors and the azimuthal angles of the molecules account for a dominant molecule-molecule interaction. In a bilayer growth regime, the bonding configurations of the molecules in the second layer coincide with that of the first layer. A similar azimuthal angle appearing in the two layers may indicate a columnar packing geometry of DyPc molecules. Introduction Due to their novel properties, Phthalocyanine (Pc) and its derivatives have been widely used in the area of gas sensing devices, photovoltaic materials, light-emitting diodes, solar and fuel cells, and so on.1-3 As we know, the properties of molecule devices are dramatically influenced by the quality of thin films, thus precise control of the surface assembly is much needed. In recent years, a scanning tunneling microscope (STM) has been successfully utilized to explore the intriguing properties of metal Pcs with an atomic level precision. The first STM observation that revealed the inner structure of an isolated molecule was reported in CuPc/Cu(100), followed by an attempt to distinguish CuPc and CoPc molecules by dark or bright contrasts in the molecule center.4 The central bright contrast was explained to be due to the d-orbital of the Co atom, which enhanced the STM tunneling.5,6 Similar investigations about the surface assembly or the physical properties were pursued on some planar metal-Pcs, such as CuPc, CoPc, and FePc adsorption on various substrates.7-15 Nonplanar metal-substituted molecules like PbPc and SnNc (Sn-naphthalocyanine) constructed other ideal candidates, where the central metal atom can be replaced or an additional benzene group can be added in each benzopyrrole group of Pc.16-18 For PbPc/MoS2, two adsorption geometries were observed with Pb atoms lying above or below the Pc plane, and corresponding STM images reported a bright or a dark contrast in the molecule * To whom correspondence should be addressed. E-mail: komeda@ tagen.tohoku.ac.jp. † Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. ‡ Department of Chemistry, Graduate School of Science, Tohoku University. § CREST, Japan Science and Technology Agency (JST).
center.16 However, much effort is still necessary to explore the internal mechanisms of nonplanar metal-Pcs growth on solid substrates, since many factors, such as the chemical reactivity of central metal atoms, the structure of ligand Pc, and the property of the substrate, may influence the surface adsorption behavior. In this work, we have examined the growth of a nonplanar DyPc molecule on the reconstructed Au(111) substrate and as a function of coverage. Due to the application of a ∼5 K STM, individual molecules can be immobilized and imaged at a very low concentration. The adsorption orientations of individual molecules and thus the molecule-substrate interactions can be illustrated. As two-dimensional molecule films can be achieved at a higher coverage, important parameters inside a square lattice, including the lattice vector, the azimuthal angle, and the domain orientation, were determined experimentally. The research on the second layer growth of DyPc on Au was also accomplished, so as to achieve a detailed understanding about the current system. Experimental Section All the experiments were carried out with a low-temperature STM, which is composed of a sample preparation chamber and an STM chamber. The sample can be transferred between them in the UHV conditions with pressure better than 4 × 10-10 mbar. A low-temperature STM head (Unisoku, Japan) is placed in a tube-like stainless chamber that can be inserted into a Dewar. Thus, the sample can be cooled to ∼5 K by filling the Dewar with liquid He and ∼77 K by filling the Dewar with liquid N2. The Au(111) substrate was processed by a standard method of Ar+ sputtering and then annealing at ∼900 K. The reconstructed surface of Au(111) 22 × 3 was confirmed by
10.1021/jp9045935 CCC: $40.75 2009 American Chemical Society Published on Web 07/15/2009
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Figure 2. (a) Close-view image (8.8 × 8.8 nm2; -0.95 V, 0.4 nA) of DyPc molecules adsorption on Au(111) with a 5 K STM. Two sets of adsorption directions are obtained and marked with white and blue arrows, respectively. (b) Corresponding orientations ([1j10] and [2j11]) deduced by an atomic resolution image of Au(111).
Figure 1. (a) Top and side view structures of a nonplanar DyPc molecule. (b) STM image (15 × 15 nm 2; -0.95 V, 0.4 nA) about the initial molecule adsorption. Individual molecules are imaged as fourleaved structures. (c) Line profile of a single molecule captured along the diagonal direction. (d) Preferential accumulation of DyPc molecules (17.3 × 17.3 nm 2; 0.5 V, 0.4 nA) in the fcc region of Au(111).
subsequent STM observations. The DyPc2 molecule was synthesized following a reported procedure but with a slight modification.19 The degassing of the molecules was performed carefully by heating a Ta container for several hours, and then DyPc was left behind due to the thermal decomposition. The target molecule was evaporated under a flux rate of ∼0.1 monolayer (ML) per minute, as calibrated by the reading of an in situ thickness monitor. In addition, the Au sample was kept at ∼300 K during the molecule deposition. Results and Discussion We have previously examined the surface assembly of some double decker metal-Pc molecules including TbPc2 and DyPc2 which might not be so stable through the thermal evaporation process.20,21 The DyPc molecule shown in this paper should correspond to the one after the removal of a Pc group from a double decker DyPc2. The molecule structure is displayed in Figure 1a with top and side views. Here, we only display one of the possibilities with the Dy atom lying below the Pc ring; the other possibility with the Dy atom residing above the Pc plane is not shown. First, we examined the initial adsorption of DyPc molecules on the reconstructed Au(111) surface by using a ∼5 K STM. Figure 1b demonstrates the surface morphology after a ∼0.15 ML molecule deposition. The reason why we select such a lowtemperature STM to check the surface adsorption is that the individual molecules are mobile on the surface at an even higher sample temperature. In Figure 1b, we can notice some randomly dispersed bright contrasts with a four-lobed shape. These protrusions or crossing-like shapes exhibit an average height of ∼0.16 nm and a diagonal length of ∼1.5 nm, as measured by the STM section view (Figure 1c). These typical values for DyPc are comparable with that of single decker metal-Pcs of a planar geometry. In this case, we can infer that these four-lobed STM contrasts should be the individual DyPc molecules.
Moreover, they are adsorbed with their Pc planes parallel to the Au surface. If the herringbone structure and the molecules can be imaged simultaneously, we can judge the preferential adsorptions of DyPc molecules on the Au(111) substrate. In Figure 1d, the wider and the narrower regions separated by bright stripes correspond to the fcc or the hcp parts of Au. It is clear to see that the fcc regions of Au present more molecule adsorptions than that of the hcp ones. To understand the STM contrast of the DyPc molecule, we should recall some important results reported in metal-Pc adsorption on solid substrate. In a planar CoPc growth on Cu(100), the d-orbital of Co was explained to contribute to the STM tunneling and thus the bright center of the four-lobed shape.5 For a nonplanar PbPc on MoS2, two adsorption geometries with the Pb atom above or below the Pc plane were assumed, and they were characterized with a bright or a dark center by the STM imaging.16 As for our DyPc/Au(111), we suggest relatively strong interactions between Dy and Au, and the f-orbital of Dy should imply almost no effect on the STM tunneling. Only in this case, the weakened center of the molecules by STM observations can be understandable. It is worth saying that further scanning tunneling spectroscopy measurement is very necessary to confirm the electronic state of the Dy atom. Second, we discuss the bonding configurations of isolated DyPc molecules on Au(111). A magnified image is captured in Figure 2a, where the marked white and blue arrows delegate the diagonal directions (or adsorption orientations) of individual molecules. Further careful measurements confirm that there are only two types of directions. For each type, their orientations can be expressed by a multiple rotation of ∼60° with each other, while the two typical orientations (white and blue) in Figure 2a manifest a ∼30° rotation. A tentative model illustrating the bonding configurations of the adsorbed molecules is plotted in Figure 2b, where schematic structures are superimposed on an atomically resolved Au(111) surface. The center of the molecule is tentatively proposed to dominate the on-top site of the substrate. The white and blue arrows are directing in the same direction as those in Figure 2a, or [011j] and [112j] directions. In general, the molecules usually align along these two typical directions (or their equivalent directions) within an error of (2°. This result manifests a good correspondence with a previous one of CoPc/ Au(111), where CoPc molecules prefer to point to the high symmetry directions of Au. A strong molecule-substrate interaction is explained to be the reason for the preferential
Dysprosium-Phthalocyanine Adsorption on Au(111)
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Figure 4. (a) High-resolution STM image of DyPc molecules growth on Au(111) (4.35 × 4.35 nm2; 0.35 V, 0.5 nA). (b) Molecule model of the DyPc lattice. The unit cells are marked with vectors b A and b B oriented along [101j ] and [1j 21j ], respectively. The azimuthal angle (R) is defined as the angle between the molecule axis (c b) and one b). of the lattice vectors (B Figure 3. Constant current STM morphologies of DyPc films: (a) 17.34 × 17.34 nm2; 0.55 V, 0.5 nA; (b) 7 × 7 nm2; 0.32 V, 0.5 nA; and (c) 11.3 × 11.3 nm2; 0.71 V, 0.5 nA. For panels a and c, the phase orientations are [1j21j] and [1j10], respectively. (d) Nearly defect free film with a submolecule resolution (6 × 6 nm2; 0.32 V, 0.5 nA).
orientations of CoPc/Au(111), and this deduction fits well with our DyPc/Au(111) system.12 In the following, DyPc films evolve after increasing the molecule concentration. STM morphologies are obtained in Figure 3a-d with a sample temperature of ∼77 K. From these images, we can acquire a near square molecule lattice. The scattered molecules on top of the films should originate from the second layer adsorption. And they have a little bit different STM contrast with the individually adsorbed molecule with a four-lobed shape (seen in Figure 2a). That is, the second layer molecules seem to be eight-lobed (Figure 3b,c). We show a molecularly resolved STM image of a DyPc film in Figure 3d, which shows a much more complicated structure as compared with the normal crossing-like shape. Each molecule is characterized with eight bright spots in the inner circle and eight symmetrical spots in the outer part. Considering of the molecule structure, we can infer that the inner eight bright spots should correspond to the conjugated porphyrin ring and the surrounding lobe-like regions should be in line with the phenyl groups. It is interesting to see that the domains shown in Figure 3a,c are oriented in some high symmetry directions of [1j21j] and [1j10]. Further experiments certify that their other ten equivalent directions can also be observed to be the domain orientations. Therefore, a mixing effect from the 3-fold symmetry of the substrate and the 2-fold symmetry of the square lattice will result in a 6-fold symmetry in DyPc domains prepared on Au(111). A magnified image of 4.35 × 4.35 nm2 was captured to illustrate the molecule-molecule interactions inside the overlayer lattice (Figure 4a). As a guide, the crystallographic directions of Au are indicated in the lower left corner. We superimpose the unit vectors in Figure 4b by b A and b B, and they coincide with the [101j] and [1j21j] directions, respectively. The dimension of the unit cell can be measured by the STM section view, which is ∼1.40 × 1.40 nm2. This scale seems to be comparable with the van der Waals dimension of a Pc ligand (∼1.50 × 1.50 nm2). As seen in Figure 4b, the reduction of the unit cell requires an azimuthal rotation of the molecules inside a lattice. We then suppose the molecule model by making the phenyl groups of each Pc directing to the N atoms of the neighboring molecules. To make a qualitative illustration of these relative rotations of neighboring molecules, an azimuthal angle (R) is defined as
the angle between the diagonal direction of the molecule and one of the lattice vectors. In a sense, this parameter can be introduced to evaluate the packing density of a metal-Pc film.16,17 If R has a large value, a smaller steric repulsion from the lobes of neighboring molecules will happen. Reversely, if R is small, the molecule lobes will be arranged close to each other and a stronger steric-repulsion interaction will dominate. In the accumulation of thin films, there should be a competition effect between the attractive van der Waals interaction and the repulsive steric interaction. Briefly, a certain R will correspond to some kind of surface assembly. For DyPc/Au(111), we determine R to be 30((2)°, which falls in the medium range of previous systems such as SnPc (R ) 14((2)°) and PdPc (R ) 36((2)°) layers prepared on graphites.16,17,22 That is to say, relatively strong moleculemolecule and weak molecule-substrate interactions contribute to the fabrication of the close-packed DyPc superstructures. Finally, the surface morphologies for the early stage of the second layer growth were also researched by a 77K STM (Figure 5). As discussed above, the second layer adsorbed molecules possess a higher resolution than that of the first layer. In Figure 5b, the eight-lobed molecules belonging to the second layer and the ground layer can be obtained at the same image. In this case, the azimuthal angle (R) of a second layer molecule can be deduced to be 30((2)°. A further growth of the film is exhibited in Figure 5c,d, at coverages of ∼1.8 and ∼1.4 ML, respectively. Here, the second layer lattice evolves after the completion of the first layer (or at least large domains of several tens of nanometers take shape). Similarly, we can deduce the lattice vectors of the quasisquare lattice of the second layer film. They are indicated in B2, which point to [01j1] and [2j11] Figure 5d with b A2 and b directions, respectively. This case is identical with the orientations of the ground layer lattice. In addition, we can notice that the molecules of the second layer should be positioned at the on-top sites of the ground layer and oriented with a similar azimuthal angle. Namely, the central Dy atom of the second layer molecule should be trapped by the cavity of the ground layer Pc. The similar azimuthal angles in both layers may be realized so as to gain the π-π interactions, by orienting their molecule planes parallel to each other. Our result presents a significant difference with a recent report of planar FePc growth on Au(111), where the molecules of the second layer are imaged with three-lobed shapes owing to protrusions in the centers. That was explained to be due to the molecules being tilted by ∼40° from the flat-lying configuration, and one of the lobes pointing to the substrate and being trapped
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Zhang et al. Acknowledgment. We acknowledge financial support from the following: Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research on Priority Areas, 448, 2005 (T.K.); International Collaborative Research Grant by the National Institute of Information and Communication Technology of Japan (T.K.); and Japan Society for the Promotion of Science (Y.F.Z.). References and Notes
Figure 5. STM morphologies showing the initial adsorption of the second layer: (a) 10.4 × 10.4 nm2; 0.35 V, 0.5 nA and (b) 8.7 × 8.7 nm2; 0.35 V, 0.5 nA. The azimuthal angle of a second layer molecule is identified with R. (c and d) Bilayer growth of DyPc on Au(111) (11.3 × 11.3 nm2; -0.86 V, 0.4 nA). The lattice vectors of the second layer are indicated with b A2 and b B2, respectively.
in the hole of the first layer unit cell.12 On the contrary, Tada et al. reported an on-top adsorption of the second layer, which appeared to have no lateral displacement with that of the first layer film.23 This case is very similar to our data. Summary A low-temperature STM (5 and 77 K) was utilized to examine a nonplanar DyPc molecule growth on a reconstructed Au(111) substrate. The DyPc molecule was imaged as a four-leaved structure in an individual case by 5 K STM. More details showing the inner porphyrin ring and the outer benzene groups were obtained in an even higher resolution image of a 2D film. A flat lying geometry of the DyPc molecule was justified, with Dy residing below the Pc plane and binding directly to the Au substrate. This adsorption configuration agrees well with our STM observation, which shows a dark hole in the molecule center. At a low concentration, strong molecule-substrate interactions are expected to induce preferential orientations of isolated DyPc molecules. Molecule-molecule interactions should dominate some important parameters in the accumulation of the 2D film, such as the lattice vector and the azimuthal angle (or the packing density) and so on.
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