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2009, 113, 5895–5898 Published on Web 03/24/2009
Engineering Dislocation Networks for the Directed Assembly of Two-Dimensional Rotor Arrays Darin O. Bellisario, Ashleigh E. Baber, Heather L. Tierney, and E. Charles H. Sykes* Department of Chemistry, Tufts UniVersity, Medford, Massachusetts 02155-5813 ReceiVed: February 12, 2009; ReVised Manuscript ReceiVed: March 11, 2009
Two-dimensional arrays of molecular rotors may provide entirely new approaches to signal processing, sensing, the development of dielectrics, and energy modulation, all based on rotational motions in the molecular adlayers. To this end, we have engineered a bimetallic surface system with a regular array of dislocations and studied the adsorption of a molecular rotor, dibutyl sulfide. Because of a size difference between the atoms, a single layer of Ag deposited onto Cu{111} reconstructs the Cu surface into a regular hexagonal array of hexagonally close-packed domains with an average spacing of 2.6 ( 0.1 nm surrounded by face-centered cubic closepacked areas. Our data demonstrates that the affinity of adsorbates for these different domains can be used to spatially control single molecule adsorption; individual dibutyl sulfide rotors have been arranged in a hexagonal pattern via their binding preference for hcp sites in a manner analogous to placing cogs on a pegboard. Introduction The design and construction of ordered arrays of interacting molecular rotors will permit novel methods for signal processing, sensing, and energy modulation at molecular length scales.1-7 Rotor systems designed for these applications would ideally employ rotors with tunable dipole moments positioned in a regular arrangement, with each rotor situated in an identical environment. Work is underway constructing three-dimensional (3D) crystals in which custom synthesis allows solids to be built with control over rotor spacing, dipole moment and rotational barrier.3-10 However, regularly ordered 2D arrays of interacting molecular rotors have yet to be reported. Modification of the substrate provides one method for patterning, but the difficulty of forming arrays without distorting the motion of the rotors themselves has so far hindered its realization.3-5,11,12 Herein we report thermodynamically driven 2D ordering of single molecule thioether rotors.13 To accomplish this a substrate that contains a regular array of stacking faults, one monolayer (ML) of Ag on Cu{111}, was employed as a template on which the rotors could be hexagonally arrayed onto identical sites with a rotorrotor spacing of 2.6 nm. Achieving 2D ordering via subtle substrate packing differences allows rotors to be assembled and studied in identical environments. Substrates with heterogeneous surface structure, such as alternating face-centered cubic (fcc) and hexagonally closepacked (hcp) domains, present adsorbates sensitive to these regional differences with a network of preferred adsorption sites. Numerous examples of such substrates exist14 including those involving stacking faults such as Au{111},15,16 and stacking fault overlayers such as Au on Ni{111},17 Cu on Ru{0001},18 Ag on Ru{0001},19 Cu on Pt{111},20 and Ag on Pt{111}.21 Deposition and equilibration of submonolayer coverages of site* To whom correspondence should be addressed. E-mail: Charles.Sykes@ tufts.edu.
10.1021/jp901317v CCC: $40.75
sensitive species on these surfaces can lead to the templated growth of adsorbates. This approach has been employed to direct metal epitaxy20,22-24 and self-assembled monolayer formation.25-27 In this work, we adopted this strategy to direct the growth of 2D ordered arrays of single molecule thioether rotors using a Ag on Cu{111} system.28-30 Experimental Section The Cu{111} surface substrate (MaTeck) was cleaned prior to Ag deposition by cycles of Ar+ sputtering (1.0 keV/18 µA) for 30 min followed by 2 min anneals to 820 K. After cooling to 300 K, the Ag layer was deposited via the resistive heating of a Ag-wrapped W wire at a rate of 0.1 ML/min. The substrate was then transferred in less than 5 min in vacuum to a scanning tunneling microscope (STM) stage precooled to 78 K. All images were recorded using an Omicron NanoTechnology lowtemperature ultra-high vacuum STM with etched W tips. Dibutyl sulfide (99.9% purity) was obtained from Sigma Aldrich and further purified by cycles of freeze/pump/thaw prior to introduction to the STM chamber via a leak valve. The molecules were deposited on the sample by a collimated molecular doser. Results and Discussion Physical vapor deposition of 1 ML of Ag onto Cu{111} at room temperature reconstructs the Cu substrate surface. The Ag adatoms form a continuous, hexagonally packed monolayer on top of the reconstructed Cu surface. The uppermost Cu layer, which interfaces with the Ag adlayer, reconstructs to form a regular array of triangular regions wherein the three topmost Cu layers adopt an hcp configuration surrounded by fcc packed areas.28-30 A schematic of the atomic layout of the Ag on Cu reconstruction is shown in Figure 1 along with STM images of the system acquired at 78 K. Ag domains nucleate at step edges and grow in a regular close-packed arrangement with a Ag-Ag 2009 American Chemical Society
5896 J. Phys. Chem. C, Vol. 113, No. 15, 2009
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Figure 1. (A) Schematic of the Ag monolayer on Cu{111} system. Ag adatoms (open black circles) adopt a close-packed arrangement over the whole surface while the Cu underlayer reconstructs into fcc (brown) and hcp (purple) domains. (B) Atomically resolved STM image of an hcp domain of the Ag/Cu{111} substrate. While a continuous hexagonally packed Ag layer is present over the whole surface, the apparent distortion of Ag adatoms at the center of the hcp domain results from differences in electronic structure of the hcp and fcc Cu domains underneath (see also Supporting Information S2). (C) STM image of a typical terrace on the Ag/Cu{111} surface with a Fourier transform inset showing the regular periodicity of the hcp sites which appear as depressions in the large scale image. The 1 ML of Ag on Cu{111} surface reconstruction contains two types of hcp area, namely 6 and 10 Cu atom domains. In this image the larger 10 Cu atom domains image as triangles, while the smaller 6 Cu atom domains appear more circular in shape. Image Conditions: (B), Vtip ) 0.8 V, I ) 0.15 nA, T ) 78 K; (C), I ) 0.7 nA, Vtip ) 0.1 V, T ) 78 K.
atom spacing of 0.280 nm.28,31 The mismatch between the Cu{111} surface and the overlayer of larger Ag atoms (bulk dCu ) 0.256 nm, bulk dAg ) 0.289 nm) strains the surface.14 This instability makes it energetically favorable for the Cu surface layer to expel atoms and reorganize into regularly spaced triangular hcp packed domains surrounded by unreconstructed fcc domains.28,29 The hexagonally packed Ag adlayer remains geometrically unperturbed by the formation of this dislocation array in the Cu layer, despite variations in substrate electronic structure which give rise to the image contrast between hcp and fcc regions seen in all the STM images (see also Supporting Information S2). The hcp equilateral triangles are not completely uniform in size; 3, 4, or 5 Cu atoms can be ejected to form triangles of 3, 6, or 10 Cu atoms, respectively, with corresponding differences in the superstructure unit cell.28,29,32 These differences in hcp domain size can be resolved through the Ag adlayer as seen in Figure 1C (see also Supporting Information S3). While this variation in the triangle-triangle spacing introduces an irregularity in the array, we have found this effect to be minimal with the standard deviation of an average unit cell spacing of 2.6 nm being only 0.1 nm, or 4%. This regularity derives from the rarity of the 3 Cu atom hcp domains, resulting in the array being essentially a random mixture of 6 or 10 Cu atom triangles. Our STM results and the data from previous investigations28,33 further suggest that the incidence of these two structures is almost equal. A key point, as seen in Figure 1C, is that the array of triangular hcp regions extends over very wide areas; typical terraces contain ∼10 000 nm2 areas of the ordered structure, making the surface an ideal template for molecular adsorption. This type of self-assembled template with a unit-cell spacing (2.6 nm) an order of magnitude larger than an atom, provides an ideal geometry for the formation of rotor arrays. Our previous work has shown that surface-supported thioethers constitute a simple, robust system with which to study many aspects of molecular rotation (Supporting Information S4).13 Among the many thioethers investigated thus far, the rotational energetics of dibutyl sulfide are by far the most quantified. The onset of rotation of dibutyl sulfide molecules adsorbed on Au{111} was found to occur at a temperature of 15 ( 2 K and at 78 K the molecule rotates at ∼1 × 107 Hz and appears in the STM image as a hexagonal protrusion.13 Figure 2 shows an STM image of individual dibutyl sulfide molecules adsorbed on Au{111} at 78 K. Even though the Au{111} 22 × 3 surface contains a naturally occurring network of fcc and hcp packing structures,
Figure 2. Dibutyl sulfide molecular rotors adsorbed and imaged on the Au{111} surface at 78 K. Molecules appear randomly distributed on all areas of the Au{111} reconstruction. Imaging Conditions: Vtip ) 0.2 V, I ) 0.2 nA, T ) 78 K.
the rotor molecules show no preference for adsorption on either area when deposited at 78 K. Figure 2 illustrates that the rotor molecules are essentially randomly distributed across the surface. Attempts to direct the assembly of dibutyl sulfide rotors on specific areas of the Au{111} 22 × 3 reconstruction by annealing the surface to 120 K resulted in coalescence of the molecules into large islands. As a direct comparison to Au{111}, dibutyl sulfide rotors were deposited on the Ag/Cu{111} substrate at 78 K. Figure 3 shows that dibutyl sulfide rotors adsorb on both the fcc (molecule a) and hcp (molecule b) Cu domains. A series of 30 images containing over 1000 dibutyl sulfide molecules were analyzed and by counting the population of molecules in each of these sites at a range of coverages, a nearly 7-fold (6.92 ( 0.03) preference for adsorption in the hcp domains was found (Supporting Information S5). This corresponds to a Boltzmann distribution energy difference of 1.25 ( 0.01 kJ/mol between these adsorption sites. These observations demonstrate that adsorption of dibutyl sulfide rotors at the hcp sites, depicted in Figure 3, is thermodynamically favorable. The differences in electronic structure that occur in different areas of a surface reconstruction may be responsible for
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J. Phys. Chem. C, Vol. 113, No. 15, 2009 5897 adsorb at the center of the hcp region, most likely at 3-fold adsorption sites. In contrast, rotors adsorbed at all other positions on the surface (other than the center of hcp domains) never appeared as the hexagonal shape characteristic of thermally driven rotation.13 Instead these molecules appeared streaky, as shown in the lower left corner of Figure 3, implying that the dibutyl sulfide rotors adsorbed in fcc regions of the reconstruction (molecule a) are more weakly bound and are subject to frustrated translational motion as opposed to thermally driven rotation. Conclusions
Figure 3. Dibutyl sulfide molecules adsorbed on fcc (a) and hcp (b) sites of the Ag/Cu{111} surface. Image Conditions: Vtip ) 0.1 V, I ) 1 nA, T ) 78 K.
Figure 4. (A) Atomically resolved STM image of the Ag/Cu{111} substrate. In this image, all the atoms of the continuous, hexagonally packed Ag monolayer that lies above the reconstructed Cu substrate can be clearly seen. The darker appearance of the hcp domains arises from differences in electronic structure of the hcp and fcc regions. (B) Identical area of the surface with dibutyl sulfide molecules preferentially adsorbed in the center of the hcp domains that again appear depressed as compared with the fcc domains. White parallelogram depicts the Cu reconstruction unit cell in both images. Image Conditions: (A), Vtip ) 0.4 V, I ) 0.7 nA; (B), Vtip ) 0.1 V, I ) 1 nA; T ) 78 K.
the adsorption effects reported here. The surface potential of the Au{111} 22 × 3 reconstruction has been shown to vary among the hcp, fcc, and soliton regions,34 and it has been revealed experimentally that some adsorbates are sensitive to this variation.25-27 While Figure 2 demonstrates that the modulation of the surface potential of Au{111} is insufficient to direct the adsorption of individual dibutyl sulfide rotors, we postulate that a similar adsorbate response to variations in electronic structure of the Ag/Cu surface drives the observed rotor segregation. It may be the case that the potential corrugation is actually enhanced in the Ag/Cu bimetallic surface as compared to Au{111}. The greater stability of rotors in hcp sites is also apparent from high-resolution STM images. As depicted in Figures 3 and 4, the six characteristic lobes of rotating thioethers13 are clearly resolved for dibutyl sulfide molecules adsorbed in the hcp regions, implying that the symmetry of the rotational motion is unbiased by the boundaries of the hcp domain. Figure 4A shows an atomically resolved image of the bare Ag/Cu{111} reconstruction at the same scale as Figure 4B, in which dibutyl sulfide rotors are seen adsorbed at hcp sites. The white parallelograms depict the unit cells of both surfaces and such cross comparisons reveal unambiguously that the rotor molecules
This work demonstrates that substrate dislocation networks can be used to direct two-dimensional rotor assembly, a crucial step toward rotor-based nanotechnology.6,7 We have shown that thioether molecular rotors can be ordered on the Ag/Cu{111} system in a hexagonal pattern with a spacing of 2.6 ( 0.1 nm. The wide range of available substrates with similar dislocation networks14 makes this approach promising for the design and construction of a variety of ordered rotor networks. Such surface-directed segregation also provides opportunities in other areas of nanoscience and in single-molecule spectroscopy. In the field of nanoscience, directed self-assembly is crucial for enabling the design of complex systems. In terms of singlemolecule spectroscopy, this approach will allow molecules to be placed in identical environments, permitting single-molecule measurements unperturbed by inhomogeneities such as different surface binding sites and aggregation. Our future work is aimed at using this system to explore the ground-state configurations of dipolar rotor arrays, the propagation of rotational excitation between adjacent molecules, and expanding this approach to other substrates with different symmetries and spacing. Acknowledgment. The authors thank Research Corporation, NSF (Grant 0844343) and the Beckman Foundation for support of this research. H.L.T. thanks the DOEd for a GAANN fellowship. E.C.H.S. thanks the Usen family for support. Supporting Information Available: Analysis of the Ag/ Cu{111} system, counting statistics, and information on thioether molecular rotors including detailed STM images. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) deJonge, J. J.; Ratner, M. A.; deLeeuw, S. W.; Simonis, R. O. J. Phys. Chem. B 2004, 108, 2666. (2) Rozenbaum, V. M. Phys. ReV. B 1996, 53, 6240. (3) Clarke, L. I.; Horinek, D.; Kottas, G. S.; Varaksa, N.; Magnera, T. F.; Hinderer, T. P.; Horansky, R. D.; Michl, J.; Price, J. C. Nanotechnology 2002, 13, 533. (4) Horansky, R. D.; Clarke, L. I.; Price, J. C.; Khuong, T. A. V.; Jarowski, P. D.; Garcia-Garibay, M. A. Phys. ReV. B 2005, 72, 0143021. (5) Horansky, R. D.; Clarke, L. I.; Winston, E. B.; Price, J. C. Phys. ReV. B 2006, 74, 05430601. (6) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. ReV. 2005, 105, 1281. (7) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007, 46, 72. (8) Dominguez, Z.; Khuong, T. A. V.; Dang, H.; Sanrame, C. N.; Nunez, J. E.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2003, 125, 8827. (9) Khuong, T. A. V.; Nunez, J. E.; Godinez, C. E.; Garcia-Garibay, M. A. Acc. Chem. Res. 2006, 39, 413. (10) Winston, E. B.; Lowell, P. J.; Vacek, J.; Chocholousova, J.; Michl, J.; Price, J. C. Phys. Chem. Chem. Phys. 2008, 10, 5188. (11) Wintjes, N.; Bonifazi, D.; Cheng, F.; Kiebele, A.; Stohr, M.; Jung, T. A.; Spillmann, H.; Deiderich, F. Angew. Chem., Int. Ed. 2007, 46, 4089. (12) Wahl, M.; Stoher, M.; Spillmann, H.; Jung, T. A.; Gade, L. H. Chem. Commun. 2007, 1349.
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