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Low-Temperature Scanning Tunneling Microscopy Investigation of Bis(phthalocyaninato)yttrium Growth on Au(111): From Individual Molecules to Two-Dimensional Domains 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: March 15, 2009; ReVised Manuscript ReceiVed: April 17, 2009
We show a 4.8 K STM observation of a double-decker bis(phthalocyaninato)yttrium (YPc2; Pc ) phthalocyanine) molecule adsorption on Au(111) substrate. An eight-lobed structure was imaged as the submolecule STM contrast of a single molecule both in an isolated state and in a molecule film. This feature arises from the top Pc group, where both sides of the four phenyl rings are highlighted. As an isolated molecule, the adsorption orientation is determined by the lower Pc, the diagonal axis of which aligns parallel to the close-packed direction of Au(111). In a 2D film, a near-square molecule lattice appears with a unit of ∼1.47 × 1.38 nm2, and one of the lattice vectors is rotated by ∼15° from the close-packed direction. A tentative model is provided to illustrate the molecule array where neighboring molecules are rotated by ∼30° from each other. In this way, the lower Pcs should align along the [101j] and [2j11] directions (or their equivalent directions) alternately. All these facts illustrate the molecule-substrate and the molecule-molecule interactions in the initial adsorption and in the film accumulation. Introduction Phthalocyanine (Pc) and its derivatives, including doubledecker and triple-decker Pcs with metal atoms as linkers, have attracted much interest in recent decades, because of their wide applications in the area of gas-sensing devices, photovoltaic materials, light-emitting diodes, solar and fuel cells, and so on.1-3 For most of the applications, the properties of the molecule devices are dramatically influenced by the quality of thin films. The introduction of scanning tunneling microscopy/ spectroscopy (STM/STS) enables the characterization of the surface assembly and the electronic structure of such molecules with an atomic-scale precision. A report on CuPc/Cu(100) can be found in the early stage of STM development.4 The work was followed by research to distinguish the coadsorbed CuPc and CoPc on Au(111). It was realized by observing the differing STM contrast in the center of the molecule, where the bright spot was attributed to the d orbital of Co.5,6 Similar investigations were performed on the surface assembly of MPc (M ) Cu, Co, Fe, Pb, Pd, and Mn) on different substrates, and novel physical properties such as Kondo effects in magnetic ion Pc molecules were explored.7-16 Large portions of these results were obtained by using the thermal evaporation method in the ultrahigh vacuum (UHV) condition. For investigations under solution and ambient condi* Corresponding author. E-mail:
[email protected]. † Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. ‡ Department of Chemistry, Graduate School of Science, Tohoku University. § JST.
tions, alkane lamellae play an important role in immobilizing some Pc molecules.17-19 The double-decker or triple-decker Pcs, which sandwich metal atoms in between, are very important components in realizing single molecule magnets (SMMs).20-22 Since there are some concerns about the decomposition of such molecules in the process of thermal evaporation, most of the STM observations about double-decker or triple-decker molecules were executed by using molecule solutions, or a dry imprint method in TbPc2/Cu(111).23-28 However, the use of the thermal evaporation method for thin film preparation has various advantages such as less contamination on the surface. In this work, we have demonstrated that the YPc2 doubledecker molecule can be deposited on Au(111) by using a thermal evaporation method under the UHV condition. Submolecular resolution images of a single molecule were captured with a novel eight-bright-spot contrast with the STM working at ∼4.8 K. The orientation of the isolated molecule is deduced by an atomic resolution image of Au(111), which gives the substrate crystallographic directions. In the molecule film, several parameters for the two-dimensional (2D) assembly are determined, including the lattice vector, the azimuthal angle, and the adsorption orientation with respect to Au. Based on these results, we show a tentative model of the surface assembly of YPc2/ Au(111). The driving forces for the surface adsorption such as the molecule-substrate and the molecule-molecule interactions are illustrated. Experimental Setup All the experiments were carried out with a Unisoku lowtemperature STM which was composed by an STM chamber
10.1021/jp902410q CCC: $40.75 2009 American Chemical Society Published on Web 05/11/2009
STM of YPc2 Growth on Au(111)
Figure 1. (a) Structure of a double-decker bis(phthalocyaninato)yttrium (YPc2) molecule. (b-d) STM images of YPc2 molecules adsorbed on Au(111) obtained at a sample bias of -0.80 V and tunneling current of 0.4 nA: (b) 20 × 20, (c) 16.8 × 16.8, and (d) 32 × 32 nm2. The adsorbed molecules elongate along the Au step edges at a small coverage or accumulate into molecule islands at a higher coverage.
for surface characterization and a separate preparation chamber for substrate cleaning and molecule deposition. The vacuums in both chambers were better than 4 × 10-10 mbar. The Au(111) substrate was processed by a standard way of Ar+ sputtering and then annealing. The reconstructed surface of Au(111) 22 × 3 was then confirmed by STM observations. The YPc2 molecule was synthesized by some special method, and the details can be seen in ref 29. The degassing of the molecules was performed carefully by heating a container of Ta boat for several hours prior to evaporation. The actual molecule deposition was completed under a flux rate of ∼0.1 monolayer (ML) per minute with the Au substrate kept at room temperature. The growth rate was acquired by the reading of an in situ thickness monitor. After molecule evaporation in the preparation chamber, the sample was instantly transferred to the STM chamber for further sample cooling and surface characterization. The sample holder was cooled with liquid helium, and the calibrated temperature for STM observations was measured to be ∼4.8 K. Results and Discussion A schematic model of the structure of YPc2 is displayed in Figure 1a, which is composed by a Y atom sandwiched by two Pc ligands rotated by ∼45° with each other; thus the Y ion has an 8-fold coordination with the N atom. In the following, we show the morphologies of YPc2 molecules adsorbed on Au(111), with STM working at a temperature of ∼4.8 K. The introduction of the low-temperature measurement is due to the mobile nature of YPc2 molecules, and no clear image can be obtained at the very initial adsorption even at ∼77 K examination. This is also true for a medium coverage region, where the possibility of capturing a molecularly resolved image is not very high. The STM images in Figure 1b, 1c, and 1d correspond to the surface morphologies with coverages of 0.05, 0.1, and 0.2 ML,
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Figure 2. (a) (11 × 11 nm2) brighter and darker contrast showing an eight-lobed shape and a four-lobed shape. They correspond to YPc2 and the Pc molecule, respectively. An atomic resolution image of Au(111) is inserted in (a) with the three white arrows indicating the close-packed directions and the blue arrow delegating the [1j21j] direction. (b) Top-view model fitted in a single YPc2 molecule. The upper Pc ligand (molecule axis L2) aligns with its normal benzene groups along the four pairs of bright spots, and the axis of the lower Pc (L1) is rotated by ∼45° with L2. (c) (9.7 × 9.7 nm2) STM image and (d) section view of (c) along the arrow direction.
respectively. As isolated molecules, two types of STM contrast were obtained; one is a four-lobed structure and the other is an eight-lobed structure. The four-lobed molecule was found both on the Au terrace and at the step edge, while the eight-lobed molecule prefers to absorb along the Au step edge as shown in Figure 1b,c. Molecule islands with a quasi-square shape appear at an even higher coverage, whose typical image is shown in Figure 1d. These small islands usually dominate around the elbow sites of Au, and these nucleation sites have been observed easily to bind some single-decker Pcs or other impurities. In larger islands, the herringbone reconstructions of Au will appear on the film surfaces. The magnified image in Figure 2a reveals the coexistence of the four-lobed and eight-lobed structures. The height of the fourlobed molecule was measured as ∼0.16 nm, while that of the eight-lobed molecule is ∼0.46 nm. The former value has been proven to be the typical height of some single-decker Pc molecules including both metal-Pcs and metal-free Pcs. The latter height of ∼0.46 nm should be more than 2 times higher than a single Pc; thus it is reasonably regarded as a doubledecker YPc2. For the YPc2 molecule, we expect a weak interaction between the upper Pc and the lower Au substrate, due to the presence of the Y atom and the lower Pc which binds to the Au surface directly. The eight-lobed shape of YPc2 in the STM contrast is showing the molecule orbital of the Pc ligand. This assumption can be confirmed by measuring the dI/dV mapping at the sample bias whose energy coincides with the energy level of the molecule orbital of Pc. A previous photoemission study for a variety of metal-Pc molecules shows that energy positions of molecule orbitals are less sensitive to the center metal atoms.28 We have performed dI/dV mapping
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measurements of single YPc2 molecules, where the highest occupied molecule orbital (HOMO) or the lowest unoccupied molecule orbital (LUMO) presents a feature similar to the eightlobed STM topography. The details about the spectroscopy measurements will be reported elsewhere.30 It is worth noting that this eight-lobed structure has been obtained in TbPc2/Cu(111).28 Similar eight-lobed STM contrasts were observed in single-decker metal-porphine or metal-Pc adsorption on substrates decorated with thin insulating films. The appearance of the eight-lobed feature in the STM images has been attributed to a weak molecule-substrate interaction, where the intrinsic electronic structure of molecules can be emphasized.31-33 At this stage, the origin of the four-lobed molecule is not clear. The possible sources are (1) metal-free Pcs formed in the synthesis process and (2) daughter molecules formed by a decomposition of YPc2 molecules. In case 2, both YPc and Pc are probable. This impurity (or so-called four-lobed molecule) is seldom found and usually has a ratio of ∼2% in the entire experiment. We judge from these facts that the thermal evaporation method should work well, and most of the YPc2 molecules are kept intact throughout the evaporation process and then deposited on the Au substrate. The understanding of the orientation and the geometry of the adsorbed molecule are crucial to knowing the driving forces in the molecule assembly such as the molecule-substrate or the molecule-molecule interactions. We obtained close-up STM images of YPc2 and Pc molecules in a single panel. In addition, an atomic resolution image of the Au substrate was captured in the same area to calibrate the substrate orientation. Then, we could analyze the adsorption configuration of the YPc2 molecule with respect to the substrate crystallographic orientation. We show the atomic resolution image of Au and its high symmetry directions of [1j10] (white arrow) and [1j21j] (blue arrow) in the inset of Figure 2a. One of the diagonal directions of the cross-like Pc can be deduced to align with the [1j10]direction (or with the other two equivalent crystallographic directions). In a separate experiment, we observed metal-free Pc (H2Pc) molecule growth on Au(111) under a similar condition. In that experiment, the four-lobed shape is characteristic for an isolated H2Pc molecule which is oriented along the close-packed direction of Au. For the eight-lobed structure of YPc2, the [1j10] direction coincides with the node positions of the eight bright spots (dark area between the bright spots). We consider the following tentative model to explain the observed configuration. The lower Pc ligand should have an adsorption orientation similar to that of the four-lobed molecule. This is schematically illustrated with a superimposed Pc owning an axis of L1 in Figure 2b. Consequently, the upper Pc is rotated by ∼45° from the lower one and has an orientation of L2 in Figure 2b. We can see that the eight-lobed feature should locate on both sides of the four phenyl rings of the upper Pc. We have confirmed that there are molecules with the lower Pcs aligned with [01j1] and [1j01], which are crystallographically equivalent to the [1j10] direction. The experimental facts described above support that isolated YPc2 molecules prefer to align with their lower Pc axis along the close-packed directions of Au. These preferential orientations of YPc2 molecules correspond well with the 3-fold symmetry of the substrate, which may imply strong molecule-substrate interactions. In Figure 2c, a molecule island was captured with a tunneling current of 0.4 nA and a bias voltage of -0.80 V, the section view of which (shown in Figure 2d) reveals an average molecule
Zhang et al.
Figure 3. (a) STM image of YPc2 film (V ) -0.8 V, 6.4 × 6.4 nm2). (b) Schematic model showing an array of upper Pcs with ∼30° rotation in neighboring molecules. In (b), the dots around molecules 1 and 2 indicate the positions of the highlighted spots by the STM image, which are well reproduced by the fitted model (both sides of the phenyl rings are highlighted). (c) STM morphology taken at V ) 0.8 V. The arrows marked with A and B are the film lattice vectors, which are rotated by ∼15° with the [101j] and [1j21j] directions of Au, respectively. c1 and c2 stand for the axes of the upper Pcs in different orientations. (d) Test model with all molecules pointing to the same direction. The marked points correspond to different parts of the upper Pcs in molecules 1 and 2.
height of ∼0.46 nm. This value is similar to the height of an individual YPc2 molecule. The detailed structure of the YPc2 film is displayed in Figure 3a. It is clear to see that the shape of each molecule is similar to the one observed in the isolated case. In Figure 3a, we can notice a periodic alternation of the STM contrast within the film, which is most obvious at the intersection points of the lattice. A pair of elongated bright spots from the molecule edge changes its directions in an alternative way. In this case, we anticipate that the adsorbed molecule may not be arranged in a simple parallel geometry. In Figure 3b, a tentative model of the upper Pc ordering is superimposed in the same image of Figure 3a. For the molecules denoted “1” and “2”, we made dots at the bright areas in the rim of the molecules. The ordering of the bright spots can be best described if we assume an alternative variation of the axial orientation of the neighboring molecules. That is, the neighboring molecules are rotated by ∼30° with each other. The highlighted positions of both cases are placed on both sides of the four phenyl rings with this model, which is also consistent with the model discussed for an isolated molecule. We have taken a series of STM images with Vsample ) -0.8, -0.4, 0.4, and 0.8 V, and the surface contrasts show a similar feature. Here, we only show one of the images taken at Vsample ) 0.8 V in Figure 3c. The crystallographic directions of Au are depicted in Figure 3c with black arrows. The unit cell of the molecule lattice can be specified by the vectors A and B (marked with white arrows), the absolute values of which are carefully measured to be ∼1.47 and ∼1.38 nm, respectively. If we suppose the Au lattice constant (a0) to be ∼0.286 nm, the
STM of YPc2 Growth on Au(111) lattice vectors |A| ≈ 5.15a0 and |B| ≈ 4.85a0. The angle between the two vectors is 90 ( 2°. Thus, the overlayer film should own a nearly square lattice. Meanwhile, vector A is rotated by ∼15° from the [101j] direction. That is, the molecule unit cell is rotated with respect to the substrate lattice, and a noncommensurate lattice evolves. The axial directions of the two types of upper Pcs are rotated by ∼45° for c1 and ∼75° for c2 from the [101j] direction. As we know, the actual binding of the adsorbed molecule with the substrate is made by the lower Pc which is rotated by ∼45° from the upper Pc. For the molecule with c1, the lower Pc arranges its diagonal axis almost parallel to the [101j] direction, which is similar to the case of the isolated YPc2 molecule. The other type of molecule with c2 has its lower Pc axis parallel to the[1j21j] direction. An ordering in which all molecules are pointing to the same direction is usually considered to be a more straightforward model for the assembly of single-decker metal-Pcs. However, as shown in Figure 3d, the correspondence between the superimposed model and the actual STM image seems not very perfect with this model. An example can be seen in the molecule indicated “2” on top. Four black dots are marked on the bright areas of the molecule, which are right on the fitted phenyl rings. This case differs from that of molecule 1, where the bright spots reside on both sides of the phenyl rings. Based on this schematic illustration, we can infer that the model with all molecules pointing in one direction does not correspond well with the surface assembly of YPc2/Au(111). As described above, the average size of the unit cell is measured to be ∼1.47 nm × 1.38 nm, while the diagonal length of the ligand Pc is ∼1.50 nm. In case the diagonal axis of each molecule is aligned with the lattice vector, there should be a steric repulsion. This is expected to be one of the driving forces for the azimuthal rotation of the molecule in reference to the lattice direction. In general, the typical interactions that make surface assemblies should be derived from some noncovalent interactions such as the van der Waals, electrostatic, and hydrogen bonding interactions. In the report of SnNc and PdPc molecules grown on a graphite surface, an azimuthal angle (R) was used as a scale to estimate the strength of the molecule-molecule interaction.34,35 The parameter is defined as the smallest angle between the molecule axis and one of the lattice vectors. The packing density approaches a low value if R ∼ 0 and reaches a maximum when R ∼ 45° for a single-decker Pc molecule. In reality, there should be a competition effect between the van der Waals attractive interaction through the π-electrons and the repulsive steric interaction. The reported value of R is 22° for CuPc/Ag(111), 36° for PdPc/HOPG, and 30° for CoPc/Au(111).34,35 For the double-decker Pc, the molecule-molecule interaction should work in both the lower and upper Pc ordering. An example in which all molecules are aligned with R ) 30° is proposed in Figure 4a and 4b, which correspond to the upper and the lower Pc ordering, respectively. This angle of 30° is actually deduced from the STM morphology. The lattice vectors (A, B), azimuthal angles (R), axes of upper Pcs (c1, c2), and substrate crystal directions of [101j] and [1j21j] are indicated on top of the Pc ordering. In this model, different phenyl-phenyl distances can be seen in both layers. On the other hand, for the other ordering we assigned for the YPc2 film, the global arrangements of the upper Pc (Figure 4c) and the lower Pc (Figure 4d) are similar. In each Pc plane, neighboring molecules are rotated by ∼30° with each other. The azimuthal angle should be same for neighboring molecules, that is, R ) 30°. We speculate that the suggested configuration will result in an energy
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Figure 4. (a and b) Test model of upper and lower Pc arrays for YPc2/ Au(111). The Pcs are arranged in parallel in (a) and (b) which correspond to different phenyl-phenyl distances. (c and d) A similar model suggesting an azimuthal rotation of neighboring Pcs by ∼30° with each other. The distance of phenyl-phenyl is similar in an average in (c) and (d). The lattice vectors, molecule axes, azimuthal angles, and substrate directions are indicated on the Pc array.
Figure 5. (a and b) Large-scale STM images (24 × 24 nm2, 48 × 48 nm2) demonstrating different molecule domains with their orientations marked by dashed arrows. The close-packed direction of [1j10] is indicated with a solid arrow.
minimization in the building up of the molecule film through the molecule-molecule interaction. Further theoretical calculations are very necessary for understanding the exact orientations of the YPc2 molecules inside a lattice. With the increase of coverage, compact domains with sharp boundaries appear. A typical STM image is shown in Figure 5a; here a 2D molecule domain coexisting with some randomly dispersed molecules is captured. A large-scale image including several islands with different orientations is also recorded in Figure 5b. As a guide, the solid arrow indicates the [1j10] direction of Au. Careful measurements reveal that the domain orientations are rotated by a multiple of 30° with each other. Again, they have an offset of ∼15° from the substrate closepacked directions. As the Au substrate has a 3-fold symmetry, it is not hard to understand that the YPc2 film with a square lattice will result in a 6-fold symmetry in the molecule islands. Summary We have shown a 4.8 K STM observation of a double-decker YPc2 molecule adsorption on Au(111), with the surface
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morphology varying from individual molecules to 2D thin films. An eight-bright-spot structure is observed to be characteristic for a single YPc2 molecule. The overlayer lattice in a square geometry is rotated by ∼15° with the substrate lattice; thus a noncommensurate ordering can be deduced. A molecule model is proposed to illustrate the relative orientations of the molecules inside a lattice. Large-scale STM images are obtained to capture the orientations of molecule islands, which reflect a strong influence from the substrate symmetry. As a summary, a strong molecule-substrate interaction determines the preferential orientations of YPc2 molecules in the initial adsorption, and the molecule-molecule interaction implies a strong effect on the 2D assembly. The double-decker YPc2 molecule has been proven to be suitable for sample preparation by the thermal evaporation method. This is a very important premise for further research on the electronic structure or the physical properties. Acknowledgment. This work was financially supported in part by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research on Priority Areas, 448, 2005. Y.F.Z. acknowledges the financial support of JSPS (Japan Society for the Promotion of Science). This work was also supported by an International Collaborative Research Grant from the National Institute of Information and Communications Technology of Japan. References and Notes (1) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature (London) 2000, 408, 541–548. (2) Cracium, M. F.; Rogge, S.; Morpurgo, A. F. J. Am. Chem. Soc. 2005, 127, 12210–12211. (3) Papageorgiou, N.; Salomon, E.; Angot, T.; Layet, J. M.; Giovanelli, L.; Lay, G. L. Prog. Surf. Sci. 2004, 77, 139–258. (4) Lippel, P. H.; Wilson, R. J.; Miller, M. D.; Woll, Ch.; Chiang, S. Phys. ReV. Lett. 1989, 62, 171–174. (5) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207–11210. (6) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197–7202. (7) Chizhov, I.; Scoles, G.; Kahn, A. Langmuir 2000, 16, 4358–4361. (8) Takada, M.; Tada, H. Jpn. J. Appl. Phys. 2005, 44, 5332–5335. (9) Chen, L.; Hu, Z. P.; Zhao, A. D.; Wang, B.; Luo, Y.; Yang, J. L.; Hou, J. G. Phys. ReV. Lett. 2007, 99, 146803(1-4) . (10) Zhao, A. D.; Li, Q. X.; Chen, L.; Xiang, H. J.; Wang, W. H.; Pan, S. A.; Wang, B.; Xiao, X. D.; Yang, J. L.; Hou, J. G.; Zhu, Q. S. Science 2005, 309, 1542–1544.
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JP902410Q
