Molecular Ordering and Dipole Alignment of Vanadyl Phthalocyanine

Article pubs.acs.org/JPCC

Molecular Ordering and Dipole Alignment of Vanadyl Phthalocyanine Monolayer on Metals: The Effects of Interfacial Interactions Tianchao Niu,† Jialin Zhang,§ and Wei Chen*,†,§,‡ †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore ‡ National University of Singapore (Suzhou) Research Institute, Suzhou, 215123, China §

S Supporting Information *

ABSTRACT: We present an in situ low-temperature scanning tunneling microscopy (LTSTM) study to elucidate the effects of interfacial interactions on the molecular ordering and dipole alignment of dipolar vanadyl phthalocyanine (VOPc) monolayer on metal surfaces, including Cu(111), Ag(111), Au(111), and graphite. The adsorption of VOPc on the relatively inert graphite surface leads to the formation of well-ordered molecular dipole monolayer with unidirectionally aligned O-up configuration. In contrast, VOPc on Cu(111), Ag(111), and Au(111) adopts both O-up and O-down configurations. The VOPc strongly chemisorbs on Cu(111), leading to the formation of one-dimensional molecular chains, and two-dimensional molecular islands comprising pure O-down adsorbed VOPc molecules at low and high coverage, respectively. In contrast, VOPc physisorbs on Au(111) and results in an orientation transition from flat-lying to inclined molecular islands. Regarding the interfacial interaction strength, the Ag(111) represents an intermediate case (weak chemisorption), which enables the formation of disordered phase and ordered islands, as well as the orientation transition within the disordered phase.

■

INTRODUCTION Comprehensive understanding of the interfacial properties between organic molecules and the supporting substrates is important for improving the performance of organic electronics,1−5 including the understanding of the energy level alignment,6 the coupling strength between the molecule and substrate,7 charge transport,8 and interfacial electronic charge transfer.9 Such factors influence device performance in organic field effect transistors (OFETs) and light emitting diodes (OLEDs).10,11 Extensive studies of the interfaces between functional π-conjugated molecules and substrates have been performed so as to optimize the preparation of wellordered organic structures, including the metal-phthalocyanine,12 porphyrins, and many other π-conjugated molecules with different functional groups.13−16 Among these, the dipole phthalocyanines,17 such as vanadyl phthalocyanine (VOPc),18 titanyl phthalocyanine (TiOPc),19 chloroaluminum phthalocyanine (ClAlPc),20,21 chlorogallium phthalocyanine (ClGaPc),22 chlorosub phthalocyanine (SubPc),23 and tin phthalocyanine (SnPc),24 in which the central group protrudes outside the molecular plane and possesses an intrinsic dipole moment, are promising functional materials for practical applications as well as for fundamental studies. In particular, their dipole-up and dipole-down adsorption configurations allow the tuning of energy level alignments25 and also enable them to serve as basic elements in molecular switches.26,27 VOPc has long been regarded as a promising candidate for OFET due to its © 2014 American Chemical Society

favorable bulk triclinic crystal structure, in which the significant π−π orbital overlap facilitates efficient charge transport along the π-stacking direction.28,29 However, for such nonplanar dipole molecules, both the molecular ordering and dipole alignment can significantly influence interface electronic properties of molecule/inorganic metal interfaces30 and are essential for the optimization of new hybrid organic−organic heterostructures.31 Long-range ordered two-dimensional unidirectionally aligned molecular dipole dot arrays can be formed by annealing the as-grown ClAlPc molecules on highly oriented pyrolytic graphite (HOPG) and Au(111).27,32 The donation and back-donation of electrons between Sn-down adsorbed SnPc and Ag(111) result in the temperature and coverage dependent order/disorder phase transition.33 The nonplanar threefold symmetric SubPc molecules adopt predominant Cldown configuration on the Au(111) surfaces, possessing coverage dependent packing structures from honeycomb to diamond-like layer.34,35 In view of the effects of interfacial interactions on the molecular ordering, the nonplanar dipole phthalocyanines, which can adopt either up or down configurations, would exhibit dipole alignment dependent packing on different substrates. The nonplanar VOPc molecules on face-centered-cubic (FCC) (111) noble metal surfaces Received: October 14, 2013 Revised: January 29, 2014 Published: February 11, 2014 4151

dx.doi.org/10.1021/jp4101653 | J. Phys. Chem. C 2014, 118, 4151−4159

The Journal of Physical Chemistry C

Article

Figure 1. (a) Molecular structure of dipole VOPc molecule. (b) Triclinic crystal structure of VOPc, phase II. (c) Well-ordered monolayer structure of VOPc on HOPG surface taken at positive tip bias. (d) STM image under negative tip bias showing the unidirectionally aligned dipole arrays with O-up configuration. Scanning parameters: (c) 25 × 25 nm2, Vtip = 2.36 V, I = 65 pA; (d) 18 × 18 nm2, Vtip = −2.6 V, I = 76 pA.

aligned molecular dipole dot arrays with O-up configuration (Figure 1c and d) dominated by interfacial π−π interaction, i.e., electrostatic and dispersion forces.37 The four-lobe featured with a central bright dot represents an individual VOPc adopting the O-up configuration. The central O-atom protruding outside the molecular plane appears much brighter at negative tip bias, further confirming the O-up configuration of monolayer VOPc on HOPG (Figure 1d). The adsorption of VOPc on the reactive Cu(111) shows distinct features. Figure 2a shows an overview of the adsorption behavior of the VOPc on Cu(111) at ∼0.2 ML. It is clear that VOPc molecules can adopt both O-up and O-down configurations with a relative ratio of 1:1. The four-lobe featured with a central bright dot can be ascribed to O-up adsorbed VOPc molecule with the O-atom pointing toward vacuum, while the molecules with brighter four-lobe and without central bright dot can be ascribed to the O-down configuration. It is worth mentioning that the O-up adsorbed VOPc molecules are quite stable without being disturbed under different scanning conditions, while the isolated O-down adsorbed VOPc molecules appear blurry and diffuse during the sequential scans as highlighted by the yellow ellipsoids in Figure 2b (Supporting Information Figure S1). Moreover, the O-down adsorbed VOPc molecules tend to aggregate into linear chains and be stabilized. Figure 2b shows two linear chains comprising five and three O-down adsorbed VOPc

represent prototypical models to study the substrate effects on the molecular adsorption, packing geometry, and dipole alignment. However, detailed study on these systems is still limited.22 We carried out in situ low-temperature scanning tunneling microscopy (LT-STM) studies of VOPc monolayer structure on Cu(111), Ag(111), Au(111), and HOPG, to reveal the effects of interfacial interactions on the molecular packing of the VOPc monolayer. The dipole alignment (O-up or Odown), molecular ordering (well-ordered and densely packed, disordered phase), and molecular orientation (flat-lying domains, referred to as molecular π orbital parallel to the substrate; inclined island, referred to as molecule tilts with respect to the surface) vary from each other depending on the interfacial interaction strength.

■

RESULTS AND DISCUSSION The molecular structure and triclinic crystal structure36 of VOPc are shown in Figure 1a and b, respectively. The molecule exhibits a dipole moment of 2.27 D perpendicular to the molecular π-plane.17 The crystal structure comprises convex and concave pairs, and hence leads to effective π−π coupling along the stacking direction as well as intermolecular dipole− dipole attraction stemming from the opposite dipole configurations.36 VOPc on HOPG forms unidirectionally 4152



Article

Figure 2. VOPc on Cu(111) at ∼0.2 ML. (a) STM image showing the random distribution of the molecules, with several chains comprising O-down adsorbed VOPc; as highlighted in (b) one chain with an intermolecular distance of ∼1.75 nm, within which the O-down adsorbed VOPc molecules adopt the identical orientation. (c) and (d) Enlarged STM images showing the topography of the O-up and O-down adsorbed VOPc respectively. Scanning parameters: (a) 50 × 50 nm2, Vtip = 1 V, I = 85 pA; (b) 10 × 10 nm2, Vtip = −0.1 V, I = 80 pA; (c) and (d) Vtip = 0.1 V, I = 80 pA.

HOPG. Moreover, the reserved fourfold symmetry of O-up adsorbed VOPc suggests the pseudo-identical atomic environment underneath, which is similar to the case of Cl-up adsorbed ClAlPc on Cu(111).40 Densely packed, short-range ordered islands comprising pure O-down adsorbed VOPc appear after increasing the coverage to ∼0.8 ML as indicated by the yellow arrows in Figure 3a. The Oup adsorbed VOPc molecules still randomly disperse over the other areas as seen from Figure 3b. Figure 3c shows a boundary between an ordered island and the disordered O-up adsorbed VOPc molecules. The molecular chains comprising symmetryreduced O-down adsorbed VOPc molecules alternately pack into a well-ordered pattern, possessing a rectangle unit cell with a = 1.77 nm ± 0.02 nm, b = 2.88 nm ± 0.02 nm as highlighted by a red rectangle. The intermolecular distance within the same chain is 1.77 nm (along the direction as indicated by the red arrow), while the intermolecular distance between the molecules in the neighboring chains decreases to 1.55 nm. Submolecularly resolved STM image of Figure 3d shows the asymmetric charge distribution of the five-member C4N rings in the B-lobes and NB-lobes of these O-down adsorbed VOPc molecules due to the lifted degeneracy. Further increasing the coverage can generate more ordered molecular patterns occupying nearly ∼70% of the surface, as shown in Figure 4a. Besides these ordered islands comprising completely O-down adsorbed VOPc molecules, another ordered pattern comprising mixed O-up and O-down adsorbed molecules is formed at the coverage of ∼1 ML (indicated by the red arrows). The yellow and red dashed arrows highlight these two different types of ordered molecular patterns, which can be clearly observed from the enlarged STM images in

molecules, respectively. All the molecules within the chains adopt identical in-plane orientation with an intermolecular distance of ∼1.75 nm, which is approximately 4√3 times the Cu(111) substrate lattice (4√3 × 0.257 nm = 1.78 nm). The enlarged STM images of isolated single O-up (Figure 2c) and O-down (Figure 2d) adsorbed VOPc molecules show that the lengths of the lobes in both cases are equal, while one pair of the lobes of the O-down adsorbed is brighter and wider than the perpendicular counterpart under the scanning tip voltage ranging from −2.0 to 2.0 V (seen from Figure 2b). The observed symmetry reduction of the lobes on the O-down adsorbed VOPc molecules can be ascribed to the lifted degeneracy of Pc lowest unoccupied molecular orbital (LUMO, π*). It is known that the electronic structure of VOPc is characterized by a highest occupied molecular orbital (HOMO, π) and a doubly degenerate LUMO (π*) which are highly delocalized on the organic ligand.38 These are the main characters of the phthalocyanine conjugated macrocycle, accompanied with one unoccupied molecular orbital composed of a V 3d atomic orbital and an O 2p atomic orbital (Supporting Information Figure S2). After the adsorption of VOPc on the Cu(111) substrate with the O-down configuration, the degeneracy of the Pc LUMO is lifted,40 and hence exhibits the symmetry reduction in STM topography. In the case of O-up adsorbed VOPc molecule, the interaction between the molecule and the Cu substrate would be mainly through the inner aza-N-atoms and the outermost benzene rings.39 The STM topography of these O-up adsorbed VOPc molecules on Cu(111) surface is dramatically changed due to the strong chemisorption on the substrate as compared with that on 4153



Article

Figure 3. VOPc on Cu(111) at ∼0.8 ML. (a) Large STM image showing the coexistence of ordered islands (highlighted by the red arrows), and the randomly distributed molecules. (b) Enlarged STM image showing that the randomly distributed VOPc molecules are predominant O-up adsorbed VOPc molecules. (c) Boundary between the ordered island and the randomly distributed molecules. (d) High-resolution STM image showing the symmetry-reduced topography of these O-down adsorbed VOPc molecules within which the ordered island, the unit cell, and the orientation of the molecules are indicated. Scanning parameters: (a) 80 × 80 nm2, Vtip = 2 V, I = 80 pA; (b) 40 × 40 nm2, Vtip = 2 V, I = 80 pA; (c) 12 × 12 nm2, Vtip = 1.1 V, I = 80 pA; (d) 8 × 8 nm2, Vtip = −0.2 V, I = 80 pA.

configurations can be observed for VOPc on the Au(111) surface at ∼1 ML as shown in Figure 5a. The underlying Au(111) herringbone reconstruction is still visible on monolayer VOPc (Figure 5a). The domain boundaries highlighted by the orange dotted lines are along the elbow sites. Within the single reconstruction domain, VOPc molecules pack along the [1−10] and [11−2] into a highly ordered pattern. The molecules within different domains exhibit mirror symmetry of 25° with respect to the [1−10] direction of the Au(111) substrate (along the line of the elbows, Figure 5a). Further deposition of VOPc under the same experimental conditions leads to the orientation transition from flat-lying to inclined molecular islands, which covers nearly 90% of the whole surface as shown in Figure 5b. A line profile crossing the inclined islands and the first layer of flat-lying molecules shows a height of ∼0.38 nm for these inclined islands with respect to the monolayer flat-lying VOPc (Figure 5c). Figure 5d shows the detailed packing structure of the brighter islands, displaying different topographic features compared with the flat-lying first

Figure 4c and d, respectively. It is noteworthy to mention that mixed ordered patterns comprising both the O-up and O-down VOPc molecules would occupy nearly 80% of all the ordered islands. The intermolecular distance decreases to ∼1.5 nm (∼6 × Cu−Cu(111) lattice = 1.542 nm) in such well-ordered islands as shown in Figure 4d. The strong interfacial interaction with the Cu(111) surface leads to the symmetry reduction of O-down adsorbed molecules, and the randomly dispersed O-up adsorbed molecules. VOPc undergoes physisorption on Au(111), but the existence of the herringbone reconstruction and the elbow sites can significantly affect the molecular adsorption behaviors. Site-specific adsorption of VOPc on the herringbone reconstructed Au(111) surface at 0.05 ML reveals that the VOPc molecules predominantly reside at the elbow sites with O-down configuration, exhibiting unidirectionally aligned molecular dipole arrays.18 Further deposition of VOPc can occupy the face-centered-cubic (FCC) region with dominant O-up configuration.18 However, both the O-up and O-down 4154



Article

Figure 4. (a) STM image of ∼1 ML VOPc molecules on Cu(111), more ordered islands appear (pointed by the yellow and red arrows). (b) Domain boundary between two different types of ordered molecular pattern. (c) One ordered pattern comprising entirely O-down adsorbed VOPc molecules. Yellow rectangle shows the unit cell, the yellow arrows show the molecular orientation, while the green ones indicate the intermolecular distances. (d) Ordered islands comprising O-up and O-down adsorbed VOPc molecules with intermolecular distances (a and b) of ∼1.5 nm. Scanning parameters: (a) 60 × 60 nm2, Vtip = 0.5 V, I = 75 pA; (b) 40 × 40 nm2, Vtip = −1.5 V, I = 75 pA; (c) 20 × 20 nm2, Vtip = −1.0 V, I = 75 pA; (d) 10 × 10 nm2, Vtip = −0.5 V, I = 70 pA.

between the overlapped benzene ring is ∼0.33 nm. Therefore, an inclining angle of ∼25° can be derived. A rectangle-like unit cell with the size of a = 1.51 nm, b = 1.83 nm is indicated, comprising two tilted VOPc molecules. The well-ordered and densely packed monolayer of VOPc on Au(111), and in particular the orientation transition from flatlying to inclined after increasing the coverage clearly, indicates the relatively weak interactions between the flat-lying VOPc molecules and the Au (111) surface. In comparison with the monolayer VOPc on HOPG surface (physisorption), the molecule−substrate interaction would be weaker than that on Au(111). However, there is no orientation transition in the VOPc/HOPG system under the similar growth conditions. The main difference would be the existence of defects near elbow sites and/or grain boundaries in the monolayer VOPc on the Au(111) surface. We propose that these defects around the elbow sites act as the nucleation sites for these tilted molecules, and further induce the orientation transition. Moreover, these inclined islands are mainly stabilized by the intermolecular dipole−dipole attraction, and efficient π−π stacking (inter-

layer and bilayer VOPc on HOPG and Au(111) (Supporting Information Figure S3). These bright dots regularly arrange into a hexagonal network. Similar STM topography of these inclined molecules has also been observed in the multilayer FePc on Au(111)41 and SnPc on NaCl film on Au(111),42 both of which are ascribed to the inclined phthalocyanine with two lobes pointing toward the vacuum. The two outside tilted isoindole groups of individual molecules are visible as bright protrusions.42 Figure 5f schematically depicts the packing structure of these inclined molecules based on the high-resolution STM image (Figure 5e). The distances between the bright dots are in the range of ∼0.68 to ∼0.83 nm. The neighboring two spots with the distance of ∼0.68 nm can be ascribed to one tilted VOPc (yellow ellipsoid), which is identical with the size of inclined single SnPc on NaCl/Au(111).42 Efficient π−π stacking can be found between the up-lobes and the bottom-lobes of the neighboring inclined VOPc molecules. The distance between the neighboring spots measured from the STM topography is ∼0.83 nm (highlighted in Figure 5f), and the nearest distance 4155



Article

Figure 5. Monolayer structure and orientation transition of VOPc on Au(111) surface. (a) ∼0.95 ML/Au(111), ordered flat-lying molecular in different domains of the reconstructed Au(111) surface. (b) Further deposition of ∼0.3 ML results in the orientation transition from flat-lying to inclined molecular islands. (c) Line-profile taken along the line in b showing the height of the inclined islands. (d) Enlarged STM image shows the structure of inclined islands. (e) High-resolution STM image of the inclined VOPc island on Au(111). (f) Schematic diagram of the packing structure of VOPc molecules in the inclined islands. The cofacial overlap of the lobes is highlighted by the gray color. Bottom: the side view showing the π−π stacking with a distance of ∼0.33 nm. Scanning parameters: (a) 40 × 40 nm2, Vtip = 2.2 V, I = 100 pA; (b) 80 × 80 nm2, Vtip = 2.7 V, I = 90 pA; (d) 15 × 15 nm2, Vtip = 2.7 V, I = 90 pA; (e) 7.5 × 7.5 nm2, Vtip = 2.7 V, I = 90 pA.

Figure 6. (a) ∼0.9 ML VOPc on Ag(111), ordered and disordered flat-lying island. (b) Enlarged STM image showing the square unit cell and the orientation of VOPc molecules within the ordered island, two stripes of molecules adopt different configurations with a misorientation angle of 15° are indicated by α and β. (c) High resolution STM image showing the reserved fourfold symmetry of both the O-up and O-down adsorbed VOPc molecules. (d) Further deposition of ∼0.3 ML VOPc molecules on Ag(111) results in orientation transition from flat-lying to tilting, and the flatlying second layer, the line profile (e) crossing the two islands shows the different height difference. (f) High-resolution STM image showing the domain boundary of the tilting and flat-lying second layer, the topology of the second layer was almost identical to that on HOPG surface. Scanning parameters: (a) 32 × 32 nm2, Vtip = 2.2 V, I = 100 pA; (b) 10 × 10 nm2, Vtip = 1.2 V, I = 75pA; (c) 10 × 10 nm2, Vtip = −0.2 V, I = 80 pA; (d) 80 × 80 nm2, Vtip = 2.0 V, I = 90 pA; (f) 15 × 15 nm2, Vtip = 2.7 V, I = 90 pA. 4156



Article

molecular distance of ∼0.33 nm) to form the preferred crystal structure. On Cu(111) (strong chemisorption), the flat-lying O-down adsorbed VOPc molecules show an attractive intermolecular interaction, whereas the randomly dispersed O-up adsorbed VOPc molecules imply an intermolecular repulsion. With respect to strong interfacial interaction induced intermolecular attraction between O-down adsorbed VOPc on Cu(111), and the high coverage of the inclined islands stemming from the orientation transition of VOPc physisorbed on Au(111), the growth of VOPc on Ag(111) may represent an intermediate case. As demonstrated in Figure 6a, one striking feature of VOPc on Ag(111) compared with that on Au(111) is the disordered arrangement at the coverage of ∼0.9 ML. It is clear that the Odown adsorbed VOPc molecules dominate the disordered phase. Zoomed-in scan of the ordered pattern reveals a square unit cell of a2 = b2 = 2.12 nm, comprising two molecules with a different rotation angle of 15° (Figure 6b). Although there is no dipole ordering observed within these ordered islands, alternatively packed molecular stripes with different in-plane orientation can be found, indicated by α and β, respectively. Furthermore, high-resolution STM image shows that both the O-up and O-down adsorbed VOPc molecules reserve the fourfold symmetry (Figure 6c). Further deposition of VOPc onto Ag(111) leads to the formation of both the inclined molecular islands and the flat-lying second layer VOPc molecules, as shown in Figure 6d and the line profile in Figure 6e. The flat-lying second layer molecules adopt the same configuration as that on the HOPG surface with the O atom pointing toward the substrate (Figure 6f, left side), while the inclined molecular islands have an identical packing structure with that on the Au(111) surface (right side in Figure 6f). Statistical analysis of the surface structure after increasing coverage reveals that the disordered first layer disappears after the formation of these tilted islands, whereas the flat-lying second layer VOPc molecules predominantly decorate the top of the ordered first layer islands. The orientation transition from flat-lying to inclined VOPc molecules on Ag surface mainly occurs at the disordered phase. The formation of disordered phase comprising predominantly O-down adsorbed VOPc molecules, followed by the orientation transition after increasing the coverage, may indicate an intermolecular repulsion stemming from the interfacial charge transfer.33 However, ordered islands comprising mixed O-up and O-down adsorbed VOPc molecules in a short-range ordered pattern can compensate the repulsion and be stabilized on the surface without the orientation transition. Repulsive intermolecular interaction has been reported in a few cases. The illustrative examples are the nonplanar SnPc and planar CuPc on Ag(111). In contrast to the electrostatic dipole−dipole interaction as the origin of repulsion for dipole molecules (tris[2-phenylpyridinato-C2,N]iridium(III), Ir(ppy)3 on Cu(111))43 or molecules with induced dipole after attached to the surface (tetrathiafulvalene, TTF on Au(111)),44 the origin of repulsion between neighboring SnPc-down on Ag(111) is the interfacial areas with overlapped Sn and Ag orbitals originating from the donation and backdonation effect. This effect is also the origin of repulsion between CuPc molecules on Ag(111) as well. However, CuPc on the Cu(111) can form linear chain at low coverage and ordered islands at high coverage.45 These phenomena have been ascribed to the intermolecular attraction between the neighboring symmetryreduced CuPc with an induced in-plane quadrupole moment.

The observed differences of molecular ordering (flat-lying, inclined), dipole alignment (O-up, O-down) of VOPc on different substrates can be correlated to the different molecule/ surface interaction strength. It is clear that the trend of interaction strength is observed to increase from Au−Ag−Cu, while on the relatively inert graphite surface, the π−π interaction (physisorption), i.e., dispersion and electrostatic forces, drives the formation of flat-lying monolayer with all the molecules having the same O-up configuration. On the noble metal surfaces, the degree of interfacial charge transfer between the substrate and the molecule can strongly affect the orbital overlap with the surface state, and consequently the lateral intermolecular interaction.46 The interaction of phthalocyanine on Cu(111) is the strongest among the FCC(111) noble metals due to the enhanced charge transfer.44 The strong chemisorption between O-down adsorbed VOPc and Cu(111) induces the symmetry reduction of these O-down adsorbed VOPc molecules. For the physisorbed VOPc on Au(111), the intermolecular interaction may exceed the interfacial molecule−substrate interaction, and hence orientation transition from flat-lying to inclining occurs after increasing the coverage. The concave and convex pair can maximize the dipole−dipole attraction and the efficient π−π stacking between the molecular lobes. However, in the case of weakly chemisorbed VOPc on Ag(111), the disordered phase comprising O-down adsorbed VOPc molecules may imply intermolecular repulsion, while the coexistence of O-up and Odown compensates the repulsion and stabilizes the ordered pattern.33

■

CONCLUSION

In conclusion, the monolayer structures of VOPc on Au(111), Ag(111), and Cu(111) were systematically investigated using submolecularly resolved LT-STM. The chemically inert HOPG surface was chosen as a comparison to show the formation of unidirectionally aligned 2D molecular dipole arrays. Both O-up and O-down configurations can be found on the three noble metal surfaces. The interfacial interaction between VOPc and Cu(111) is strong enough to confine the molecules with favorable configuration to form commensurate patterns, such as one-dimensional molecular chains, and two-dimensional molecular islands comprising pure O-down adsorbed VOPc molecules at low and high coverage, respectively. In contrast, the weak physisorption of VOPc on the Au(111) results in an orientation transition from flat-lying to inclined molecular islands which decorate ∼90% of the whole surface. Regarding the interfacial interaction strength, the Ag(111) represents an intermediate case which enables the formation of both disordered phase and molecularly ordered islands, as well as the orientation transition within the disordered phase after increasing the coverage. This comprehensive study on these prototypical model systems enables us to optimize the molecular ordering and dipole alignment to achieve better efficiency in functional organic and molecular electronic devices.

■

METHODS Freshly cleaved HOPG substrate was thoroughly degassed in ultra-high-vacuum (UHV) at around 800 K for more than 4 h to get a clean surface before deposition.19 The Au(111), Ag(111), and Cu(111) single crystal substrates were cleaned in situ by several cycles of Ar ion sputtering (700 eV energy, 10 4157



Article

μA/cm2 ion current density) and thermal annealing (40 min at 800 K).16 The surface cleanliness was checked by STM before deposition. The VOPc molecules were purified twice by gradient vacuum sublimation prior to the deposition on the substrate. They were thermally evaporated from low-temperature Knudsen cells (MBE-Komponenten, Germany) onto the surface held at room temperature (298 K) in a separated growth chamber after degassing the pure material to 520 K for 24 h. The deposition rate was about 0.2 ML/min as approximated from STM images (1 ML refers to one full monolayer of closely packed VOPc with their conjugated πplane oriented parallel to surface). In situ LT-STM experiments were carried out in a custom-built multichamber UHV system having a base pressure better than 1.0 × 10−10 mbar and housing an Omicron LT-STM.47 The STM imaging was performed in constant-current mode under 77 K using a chemically etched tungsten (W) tip.

■

(8) Li, L.; Hu, W.; Fuchs, H.; Chi, L. Controlling Molecular Packing for Charge Transport in Organic Thin Films. Adv. Energy Mater. 2011, 1, 188−193. (9) Tseng, T.; Urban, C.; Wang, Y.; Otero, R.; Tait, S. L.; Alcamí, M.; Écija, D.; Trelka, M.; Gallego, J. M.; Lin, N.; et al. Charge-TransferInduced Structural Rearrangements at Both Sides of Organic/Metal Interfaces. Nat. Chem. 2010, 2, 374−379. (10) Pegg, L. J.; Hatton, R. A. Nanoscale Geometric Electric Field Enhancement in Organic Photovoltaics. ACS Nano 2012, 6, 4722− 4730. (11) Chen, W.; Qi, D. C.; Huang, H.; Gao, X. Y.; Wee, A. T. S. Organic-Organic Heterojunction Interfaces: Effect of Molecular Orientation. Adv. Funct. Mater. 2011, 21, 410−424. (12) Zhao, A. D.; Li, Q. X.; Chen, L.; Xiang, H. J.; Wang, W. H.; Pan, S.; Wang, B.; Xiao, X. D.; Yang, J. L.; Hou, J. G.; et al. Controlling the Kondo Effect of an Adsorbed Magnetic Ion Through its Chemical Bonding. Science 2005, 309, 1542−1544. (13) Zhang, J. L.; Niu, T. C.; Wee, A. T. S.; Chen, W. Self-Assembly of Binary Molecular Nanostructure Arrays on Graphite. Phys. Chem. Chem. Phys. 2013, 15, 12414−12427. (14) Jiang, Y. H.; Xiao, W. D.; Liu, L. W.; Zhang, L. Z.; Lian, J. C.; Yang, K.; Du, S. X.; Gao, H.-J. Self-Assembly of Metal Phthalocyanines on Pb(111) and Au(111) Surfaces at Submonolayer Coverage. J. Phys. Chem. C 2011, 115, 21750−21754. (15) Huang, H.; Wong, S. L.; Chen, W.; Wee, A. T. S. LT-STM Studies on Substrate-Dependent Self-Assembly of Small Organic Molecules. J. Phys. D: Appl. Phys. 2011, 44, 464005. (16) Li, Z. Y.; Li, B.; Yang, J. L.; Hou, J. G. Single-Molecule Chemistry of Metal Phthalocyanine on Noble Metal Surfaces. Acc. Chem. Res. 2010, 43, 954−962. (17) Fukagawa, H.; Hosoumi, S.; Yamane, H.; Kera, S.; Ueno, N. Dielectric Properties of Polar-Phthalocyanine Monolayer Systems with Repulsive Dipole Interaction. Phys. Rev. B 2011, 83, 085304−1−8. (18) Niu, T. C.; Zhou, C. G.; Zhang, J. L.; Zhong, S.; Cheng, H. S.; Chen, W. Substrate Reconstruction Mediated Unidirectionally Aligned Molecular Dipole Dot Arrays. J. Phys. Chem. C 2012, 116, 11565− 11569. (19) Burson, K. M.; Wei, Y.; Cullen, W. G.; Fuhrer, M. S.; ReuttRobey, J. E. Potential Steps at C60-TiOPc-Ag(111) Interfaces: Ultrahigh-Vacuum-Noncontact Scanning Probe Metrology. Nano Lett. 2012, 12, 2859−2864. (20) Huang, Y. L.; Wang, R.; Niu, T. C.; Kera, S.; Ueno, N.; Pflaum, J.; Wee, A. T. S.; Chen, W. One Dimensional Molecular Dipole Chain Arrays on Graphite via Nanoscale Phase Separation. Chem. Commun. 2010, 46, 9040−9042. (21) Niu, T. C.; Huang, Y. L.; Sun, J. T.; Kera, S.; Ueno, N.; Wee, A. T. S.; Chen, W. Tunable Two-Dimensional Molecular Dipole Dot Arrays on Graphite. Appl. Phys. Lett. 2011, 99, 143114. (22) Gerlach, A.; Hosokai, T.; Duhm, S.; Kera, S.; Hofmann, O. T.; Zojer, E.; Zegenhagen, J.; Schreiber, F. Orientational Ordering of Nonplanar Phthalocyanine on Cu(111): Strength and Orientation of the Electric Dipole Moment. Phys. Rev. Lett. 2011, 106, 156102. (23) Beaumont, N.; Cho, S. W.; Sullivan, P.; Newby, D.; Smith, K. E.; Jones, T. S. Boron Subphthalocyanine Chloride as an Electron Acceptor for High-Voltage Fullerene-Free Organic Photovoltaics. Adv. Funct. Mater. 2012, 22, 561−566. (24) Wang, Y. F.; Kröger, J.; Berndt, R.; Hofer, W. Structural and Electronic Properties of Ultrathin Tin-Phthalocyanine Films on Ag(111) at the Single-Molecule Level. Angew. Chem., Int. Ed. 2009, 48, 1261−1265. (25) Toader, M.; Hietschold, M. Tuning the Energy Level Alignment at the SnPc/Ag(111) Interface Using an STM Tip. J. Phys. Chem. C 2011, 115, 3099−3105. (26) Wang, Y. F.; Kröger, J.; Berndt, R.; Hofer, W. A. Pushing and Pulling a Sn Ion through an Adsorbed Phthalocyanine Molecule. J. Am. Chem. Soc. 2009, 131, 3639−3643. (27) Huang, Y. L.; Lu, Y.; Niu, T. C.; Huang, H.; Kera, S.; Ueno, N.; Wee, A. T. S.; Chen, W. Reversible Single-Molecule Switching in an

ASSOCIATED CONTENT

S Supporting Information *

STM images show the diffusion of VOPc-up molecules on Cu(111) (Figure S1A and B). The DFT calculated HOMO and LUMO of isolated VOPc molecule (Figure S2). STM image of bilayer VOPc on HOPG and Au(111) surface, and the line profile of the height difference between the flat-lying molecules and the inclined molecular islands (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 65-6516 2921; Fax: 65-6777 6126. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We greatly thank Dr. Giuseppe Mattioli for providing us the calculated HOMO and LUMO of VOPc molecule. The authors acknowledge the support from the Singapore ARF grant R-143000-505-112, R143-000-530-112, and R143-000-559-112.

■

REFERENCES

(1) Heimel, G.; Rissner, F.; Zojer, E. Modeling the Electronic Properties of π-Conjugated Self-Assembled Monolayers. Adv. Mater. 2010, 22, 2494−2513. (2) Koch, N. Organic Electronic Devices and Their Functional Interfaces. ChemPhysChem 2007, 8, 1438−1455. (3) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Semiconducting πConjugated Systems in Field-Effect Transistors: a Material Odyssey of Organic Electronics. Chem. Rev. 2012, 112, 2208−2267. (4) Zhong, S.; Zhong, J. Q.; Mao, H. Y.; Zhang, J. L.; Lin, J. D.; Chen, W. The Role of Gap States in the Energy Level Alignment at the Organic−Organic Heterojunction Interfaces. Phys. Chem. Chem. Phys. 2012, 14, 14127−41. (5) Hwang, J.; Wan, A.; Kahn, A. Energetics of Metal−Organic Interfaces: New Experiments and Assessment of The Field. Mater. Sci. Eng., R 2009, 64, 1−31. (6) Hosokai, T.; Machida, H.; Gerlach, A.; Kera, S.; Schreiber, F.; Ueno, N. Impact of Structural Imperfection on the Energy Level Alignment in Organic Films. Phys. Rev. B 2011, 83, 195310−1−7. (7) Tautz, F. S. Structure and Bonding of Large Aromatic Molecules on Noble Metal Surfaces: The Example of PTCDA. Prog. Surf. Sci. 2007, 82, 479−520. 4158



Article

Ordered Monolayer Molecular Dipole Array. Small 2012, 8, 1423− 1428. (28) Ohta, H.; Kambayshi, T.; Hirano, M.; Hoshi, H.; Ishikawa, K.; Takezoe, H.; Hosono, H. Application of a Transparent Conductive Substrate with an Atomically Flat and Stepped Surface to Lateral Growth of an Organic Molecule: Vanadyl Phthalocyanine. Adv. Mater. 2003, 15, 1258−1262. (29) Wang, H.; Song, D.; Yang, J.; Yu, B.; Geng, Y.; Yan, D. High Mobility Vanadyl-Phthalocyanine Polycrystalline Films for Organic Field-Effect Transistors. Appl. Phys. Lett. 2007, 90, 253510. (30) Braun, S.; Salaneck, W. R.; Fahlman., M. Energy Level Alignment at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 2009, 21, 1450−1472. (31) Stadtmüller, B.; Sueyoshi, T.; Kichin, G.; Kröger, I.; Soubatch, S.; Temirov, R.; Tautz, F. S.; Kumpf, C. Commensurate Registry and Chemisorption at a Hetero-Organic Interface. Phys. Rev. Lett. 2012, 108, 106103. (32) Huang, Y. L.; Chen, W.; Bussolotti, F.; Niu, T. C.; Wee, A. T. S.; Ueno, N.; Kera, S. Impact of Molecule-Dipole Orientation on Energy Level Alignment at the Submolecular Scale. Phys. Rev. B 2013, 87, 085205. (33) Stadler, C.; Hansen, S.; Kröger, I.; Kumpf, C.; Umbach, E. Tuning Intermolecular Interaction in Long-Range-Ordered Submonolayer Organic Films. Nat. Phys. 2009, 5, 153−158. (34) Jiang, N.; Wang, Y. L.; Liu, Q.; Zhang, Y. Y.; Deng, Z. T.; Ernst, K.-H.; Gao, H.-J. Polymorphism and Chiral Expression in TwoDimensional Subphthalocyanine Crystals on Au(111). Phys. Chem. Chem. Phys. 2010, 12, 1318−1322. (35) Stadtmüller, B.; Kröger, I.; Reinert, F.; Kumpf, C. Submonolayer Growth of CuPc on Noble Metal Surfaces. Phys. Rev. B 2011, 83, 085416. (36) Kolotovska, V.; Friedrich, M.; Zahn, D. R. T.; Salvan, G. Magnetic Field Influence on the Molecular Alignment of Vanadyl Phthalocyanine Thin Films. J. Cryst. Growth 2006, 291, 166−174. (37) Huang, Y. L.; Chen, W.; Wee, A. T. S. Molecular Trapping on Two-Dimensional Binary Supramolecular Networks. J. Am. Chem. Soc. 2011, 133, 820−825. (38) Mattioli, G.; Filippone, F.; Bonapasta, A. A. Controlling the Magnetic Properties of a Single Phthalocyanine Molecule Through its Strong Coupling With the GaAs Surface. J. Phys. Chem. Lett. 2010, 1, 2757−2762. (39) Mugarza, A.; Lorente, N.; Ordejón, P.; Krull, C.; Stepanow, S.; Bocquet, M.-L.; Fraxedas, J.; Ceballos, G.; Gambardella, P. Orbital Specific Chirality and Homochiral Self-Assembly of Achiral Molecules Induced by Charge Transfer and Spontaneous Symmetry Breaking. Phys. Rev. Lett. 2010, 105, 115702. (40) Niu, T. C.; Zhou, M.; Zhang, J. L.; Feng, Y. P.; Chen, W. Dipole Orientation Dependent Symmetry Reduction of Chloroaluminum Phthalocyanine on Cu(111). J. Phys. Chem. C 2013, 117, 1013−1019. (41) Cheng, Z. H.; Gao, L.; Deng, Z. T.; Liu, Q.; Jiang, N.; Lin, X.; He, X. B.; Du, S. X.; Gao, H.-J. Epitaxial Growth of Iron Phthalocyanine at the Initial Stage on Au(111) Surface. J. Phys. Chem. C 2007, 111, 2656−2660. (42) Wang, Y. F.; Kröger, J.; Berndt, R.; Tang, H. Molecular Nanocrystals on Ultrathin NaCl Films on Au(111). J. Am. Chem. Soc. 2010, 132, 12546−12547. (43) Yokoyama, T.; Takahashi, T.; Shinozaki, K. Surface Diffusion of Ir(ppy)3 on Cu(111). Phys. Rev. B 2010, 82, 155414. (44) Fernandez-Torrente, I.; Monturet, S.; Franke, K. J.; Fraxedas, J.; Lorente, N.; Pascual, J. I. Long-Range Repulsive Interaction Between Molecules on a Metal Surface Induced by Charge Transfer. Phys. Rev. Lett. 2007, 99, 176103. (45) Stadtmüller, B.; Kröger, I.; Reinert, F.; Kumpf, C. Submonolayer Growth of CuPc on Noble Metal Surfaces. Phys. Rev. B 2011, 83, 085416. (46) Rojas, G.; Simpson, S.; Chen, X.; Kunkel, D. A.; Nitz, J.; Xiao, J.; Dowben, P. A.; Zurek, E.; Enders, A. Surface State Engineering of Molecule−Molecule Interactions. Phys. Chem. Chem. Phys. 2012, 14, 4971−4976.

(47) Niu, T. C.; Zhou, M.; Zhang, J. L.; Feng, Y. P.; Chen, W. Growth Intermediates for CVD Graphene on Cu(111): Carbon Clusters and Defective Graphene. J. Am. Chem. Soc. 2013, 135, 8409− 8414.

4159


Molecular Ordering and Dipole Alignment of Vanadyl Phthalocyanine

Recommend Documents