Article pubs.acs.org/JPCC
Substrate Reconstruction Mediated Unidirectionally Aligned Molecular Dipole Dot Arrays Tianchao Niu,† Chenggang Zhou,§ Jialin Zhang,‡ Shu Zhong,† Hansong Cheng,*,† 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 § Sustainable Energy Laboratory, China University of Geosciences (Wuhan), Wuhan, 430074, China ‡
S Supporting Information *
ABSTRACT: We report the fabrication of unidirectionally aligned molecular dipole dot arrays of vanadyl phthalocyanine (VOPc) on Au(111). Low-temperature scanning tunneling microscopy (LT-STM) studies reveal that isolated VOPc molecules adsorb predominantly at the elbow sites of the 22 × √3 herringbone reconstructed Au(111) surface, and adopt an O-down configuration with the phthalocyanine plane parallel to the substrate. It is proposed that the low-coordinated Au atom at the elbow sites on the reconstructed Au(111) can interact strongly with the O-down oriented VOPc molecule, and hence leads to the formation of the unidirectionally aligned VOPc molecular dipole dot arrays.
with the substrate. The former is of paramount importance for characterizing single molecular properties;6 with the latter is viewed as an essential ingredient for the reliability and reproducibility of single-molecule devices.3 In this article, we use a dipole molecule of vanadyl phthalocyanine (VOPc, molecular structure shown in Figure 1A) as a model system to demonstrate the fabrication of unidirectionally aligned molecular dipole dot arrays on the 22 × √3 herringbone reconstructed Au(111) surface and to investigate the moleculemetal binding geometry at the atomic scale. VOPc, representing a class of versatile π-conjugated macrocyclic organic molecules, has drawn considerable attention in the applications of molecular and organic electronics.34−36 The central oxygen atom in VOPc protrudes outside the molecular plane by 1.617 Å, resulting in a permanent molecular dipole of 2.79 D perpendicular to the molecular plane.37 Moreover, this protruded oxygen atom can also serve as an atomic-precise binding site of the molecule to form well-defined geometry for the construction of functional molecular arrays.38,39 The adsorption of single VOPc molecules on the 22 × √3 reconstructed Au(111) surface was characterized using in situ low-temperature scanning tunneling microscopy (LT-STM). Figure 1B shows the formation of unidirectionally aligned molecular dipole dot arrays at 0.01 monolayer (ML, referring to a closely packed layer of flat lying VOPc molecules fully covered the Au(111) surface with their molecular π-plane parallel to the substrate) coverage. Isolated VOPc molecules
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tilizing single molecules as basic functional components in molecular nanodevices is one of the ultimate objectives in microelectronics minaturization.1−4 To realize singlemolecule devices,5 a number of issues should be addressed, such as spatially accurate atomic contact geometry,6 macroscopically ordered molecular arrangement,7,8 good reproducibility and single-molecule addressability,9,10 non cross-talked single-molecule electronic properties,11,12 configurational uniformity, and structural robustness.13 It is also known that functions of a single molecule, such as conductance or rectification,14−16 conformational or electrical switchability,17−19 magnetic properties20,21 and so on, greatly rely on the surrounding medium of the functional units on solid substrates.22 Molecular self-assembly on surfaces offers great promise as an efficient and flexible way for creating long-range ordered two-dimensional (2D) supramolecular nanostructure arrays, steered and sustained via various intermolecular interactions at different length and strength scales, such as hydrogen bonding, metal−ligand coordination, dipole−dipole interactions, van der Waals forces and even covalent bonds.23−29 Static surface nanotemplates with long-range ordered preferential adsorption sites, such as various reconstructed metal30 or semiconductor surfaces,31 boron nitride nanomesh,32 carbon nanomesh,33 and so on, can selectively accommodate individual functioning molecules and hence the formation of molecular nanostructure arrays over the macroscopic area. Typically these approaches are confronted with two issues, including how to ensure the functional elements free from the perturbation via the interactions with surrounding molecules or environment and how to form welldefined binding geometry for individual functioning molecule © 2012 American Chemical Society
Received: February 15, 2012 Revised: May 3, 2012 Published: May 18, 2012 11565
dx.doi.org/10.1021/jp301510a | J. Phys. Chem. C 2012, 116, 11565−11569
The Journal of Physical Chemistry C
Article
To explore the distinct characteristics of unidirectional alignment of this dipole phthalocyanine at the elbow sites, the adsorption features of VOPc at the unfaulted face-center cubic (FCC) region were also probed. Figure 2 displays bias-
Figure 1. (A) Top view and side view of VOPc molecular structure. The O atom of the vanadyl group protruding outside the molecular plane exhibits a permanent dipole moment. (B) STM image of large scale (200 nm × 200 nm) unidirectionally aligned molecular dipole dot arrays of VOPc on the reconstructed Au(111) surface. (C) Magnified STM image (45 nm × 45 nm) shows isolated VOPc molecules predominantly adsorbed at the elbow sites; the red dotted lines act as a guide to the eyes for the herringbone reconstruction. The distances between the nearest adjacent VOPc molecules are highlighted. Two types of elbow sites are labeled as pinched and bulged elbow sites. (D) Molecularly resolved STM image shows the O-down oriented single VOPc molecules at bulged and pinched elbow sites. The red circle with an arrow inside highlights the in-plane molecular orientation of the molecule at bulged elbow site. Scanning parameters: (B) and (C) Vtip = 1.5 V and Iset = 100 pA; (D) 15 nm × 15 nm, Vtip = 2.2 V, Iset = 100 pA.
Figure 2. (A−D) Bias-dependent 15 nm × 15 nm STM images of various structures of VOPc molecules on the Au(111) surface. The VOPc molecules adopt O-down configuration at the elbow sites (i), whereas it oriented with O-up configuration at the FCC region (ii). (E and F): Schematic illustrations of experimentally observed configuration for (i) at elbow site and configuration (ii) at FCC region. Scanning parameters: (A) Vtip = 3.0 V, Iset = 90 pA; (B) Vtip = 2.2 V, Iset = 90 pA; (C) Vtip = 1.5 V, Iset = 90 pA; and (D) Vtip = 1.0 V, Iset = 90 pA.
predominantly adopt an O-down configuration with the phthalocyanine plane parallel to the substrate at the elbow sites, as indicated in the enlarged STM image in Figure 1C. The molecular orientation and the adsorption site can be identified in the atomically resolved STM image as shown in Figure 1D. The four-lobe feature with a central dark hole represents the Odown oriented single VOPc molecule. Two types of elbow sites exist on the surface and are classified as pinched and bulged elbows.40,41 Intensive theoretical and experimental results have revealed that an additional atomic row terminates with a lowcoordinated gold atom at both types of elbow sites (Figure S1).42−44 Hence, the Au(111) surface with these dislocation sites can provide a platform for the fabrication of well-aligned molecular arrays with atomic-scale accuracy.45−47 The unidirectional alignment of the molecular dipoles can be facilitated through the site- and configuration-specific adsorption of individual VOPc molecules on these elbow sites. The separation between the nearest adjacent VOPc molecules at the elbow sites in these unidirectionally aligned molecular dipole dot arrays is approximately 81.6, 147, and 172 Å along [11−2], [−110], and [1−10] direction respectively, indicating that there is no direct or indirect (substrate-mediated) intermolecular interactions occurred (Figure 1C).48−50
dependent molecularly resolved STM images showing the VOPc molecules adsorbing on the elbow sites and the unfaulted FCC region. The four-leaf pattern with a central bright protrusion represents an O-up oriented single VOPc molecule with the Pc plane parallel to the substrate at the FCC region, labeled as (i); while the molecules at the pinched elbow sites with a central dark hole possess the O-down configuration, labeled as (ii). The observed molecular contrast for O-up and O-down oriented single VOPc molecules at different adsorption sites can be preserved under various tip bias. This means that the features of unidirectionally aligned VOPc molecules at the elbow sites are due to the distinct molecular configuration (i.e., O-down configuration), rather than the effect of the electric filed51,52 or the different conductance channels of the adsorbed molecules.53 The observed configurations at different sites are schematically shown in Figure 2, panels E and F, based on the atomic-resolved herringbone reconstructed background. This clearly demonstrates that the isolated VOPc preferred O-down configuration at the elbow 11566
dx.doi.org/10.1021/jp301510a | J. Phys. Chem. C 2012, 116, 11565−11569
The Journal of Physical Chemistry C
Article
VOPc molecule among the dipole dot arrays can be ascribed to the distinct atomic packing geometry of the elbow sites.40,42 The dislocation at the elbow gives rise to a much reactive site which can accommodate adsorbate (Figure S1)58 and hence interact strongly with the O-down oriented VOPc molecules. Furthermore, the dislocation sites of the reconstructed surface can transfer the periodicity through the site specific adsorption to form the unidirectionally aligned molecular dipole dot arrays.59
sites. The VOPc molecules at unfaulted FCC region with the O pointing toward the vacuum are mainly steered by the interaction between the molecular π-orbitals and d-electrons of Au.54 To further verify the structural stability of these O-down oriented VOPc molecules at the elbow sites, we increase the VOPc coverage to ∼0.9 ML. As can be seen in Figure 3A, the
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CONCLUSION We demonstrate the fabrication of unidirectionally aligned molecular dipole dot arrays upon the site-specific adsorption of VOPc molecules on the elbow sites of the 22 × √3 herringbone reconstructed Au (111) surface. The unidirectional alignment of the isolated VOPc molecules as the O-down configuration was revealed by molecularly resolved and biasdependent STM images. It is proposed that the lowcoordination elbow sites on the reconstructed Au(111) can strongly interact with the O-down oriented VOPc molecule, facilitating the formation of unidirectionally aligned VOPc molecular dipole dot arrays. Our study can be extended to the precise control of the spatial position of functional elements with conformational uniformity and structural robustness for this class of π-conjugated organic molecule.
Figure 3. (A) STM image of 0.9 ML VOPc molecules on the Au(111) surface. The lattice vector of the substrate is given for comparing the relative orientation of molecules at different domains. The red circles indicate that the molecular configuration of the O-down oriented VOPc molecules at the bulged elbow sites was unchanged after further increasing the coverage. (B) Enlarged STM image of the closely packed VOPc islands in domain A. Rectangular unit cell size: a1 = 15.2 ± 0.2 Å and b1 = 17.3 ± 0.2 Å. Scanning parameters: (A) 20 nm × 20 nm, (B) 10 nm × 10 nm, Vtip = 2.2 V, Iset = 100 pA for both images.
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METHODS Au(111) substrate was cleaned by several cycles of Ar+ ion bombardment (700 eV energy, 10 μA/cm2 ion current density) and thermal annealing (40 min at 800 K). The surface was checked by STM before deposition and displayed a typical 22 × √3 reconstruction. 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 Au(111) surface held at room temperature (298 K) in a separated growth chamber (base pressure
