Self-Assembled Two-Dimensional Heteromolecular Nanoporous

Dec 24, 2013 - The resulting adlayer is in registry with the underlying graphene substrate and possesses a characteristic domain size of 40–50 nm. T...
1 downloads 8 Views 887KB Size
Download PDF
Letter pubs.acs.org/JPCL

Self-Assembled Two-Dimensional Heteromolecular Nanoporous Molecular Arrays on Epitaxial Graphene Hunter J. Karmel,† TeYu Chien,†,§ Vincent Demers-Carpentier,† John J. Garramone,† and Mark C. Hersam*,†,‡ †

Department of Materials Science & Engineering and ‡Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3108, United States S Supporting Information *

ABSTRACT: The development of graphene functionalization strategies that simultaneously achieve two-dimensional (2D) spatial periodicity and substrate registry is of critical importance for graphene-based nanoelectronics and related technologies. Here, we demonstrate the generation of a hydrogen-bonded molecularly thin organic heteromolecular nanoporous network on epitaxial graphene on SiC(0001) using room-temperature ultrahigh vacuum scanning tunneling microscopy. In particular, perylenetetracarboxylic diimide (PTCDI) and melamine are intermixed to form a spatially periodic 2D nanoporous network architecture with hexagonal symmetry and a lattice parameter of 3.45 ± 0.10 nm. The resulting adlayer is in registry with the underlying graphene substrate and possesses a characteristic domain size of 40−50 nm. This molecularly defined nanoporous network holds promise as a template for 2D ordered chemical modification of graphene at lengths scales relevant for graphene band structure engineering. SECTION: Physical Processes in Nanomaterials and Nanostructures

G

The generation of precisely ordered structures with accuracy approaching the molecular limit is beyond the scope of current top-down fabrication methods. In contrast, bottom-up chemical assembly is attractive at this length scale due to the dimensions of the constituent molecular components. For example, noncovalent chemical interactions available between many organic molecules promote the formation of self-assembled adlayers with highly ordered 2D structures.10−13 The resulting structure of the organic adlayers depends on three primary factors, (1) the size parameters and symmetry that characterize the molecules and the substrate; (2) the degree of mismatch between the lattice constants that define the organic film and the surface reconstruction; and (3) the relative strengths of the adsorbate−adsorbate and adsorbate−substrate interactions. Under favorable conditions, it is possible to rationally select organic molecules with the appropriate intermolecular and molecule−substrate interactions to form highly ordered 2D crystals on graphene.14 While extensive experimental effort has been devoted to the exploration of 2D supramolecular phases on HOPG,15−17 metal,18−20 and semiconductor surfaces,15,21−24 only a few examples have been reported on graphene.25−31 Nanoporous supramolecular architectures are especially attractive in that they can serve as a molecularly thin template for subsequent periodic functionalization of the substrate. These 2D nanoporous networks can be subdivided into both homomolecular

raphene is widely considered to be a promising candidate for next-generation nanoelectronic devices due to its exceptional charge carrier mobility and two-dimensional (2D) nature.1 However, the intrinsic electronic structure of pristine graphene lacks a band gap, which has inhibited efforts to successfully integrate this material into digital electronic devices. Theoretically, it has been predicted that an energy gap of practical utility can be achieved by constraining the lateral dimensions of the 2D graphene sheet into onedimensional (1D) graphene nanoribbons at the sub-5 nm length scale.2 Other strategies for modifying the electronic properties of graphene include covalent functionalization methods that disrupt the sp2-hybridized structure of the graphene hexagonal carbon lattice.3−7 In both of these cases, however, issues related to structural control and/or the stochastic nature of the chemical processes involved have limited their success by introducing significant trade-offs. For instance, sufficiently precise control of the atomic configuration of the edges of graphene nanoribbons is beyond the capability of current lithographic techniques, while covalent modification of the basal plane of the graphene sheet results in a disordered arrangement of chemisorbed species.3−6 This disorder has a detrimental impact on the carrier mobility of modified graphene due to defect scattering off of the randomly distributed edge states and covalent adsorbates, respectively.8,9 Chemical functionalization schemes that achieve spatially periodic structures at the sub-5 nm length scale in registry with the underlying graphene thus remain of high interest for their potential to yield band structure modification with minimal degradation in charge carrier mobility. © 2013 American Chemical Society

Received: November 24, 2013 Accepted: December 24, 2013 Published: December 24, 2013 270

dx.doi.org/10.1021/jz4025518 | J. Phys. Chem. Lett. 2014, 5, 270−274

The Journal of Physical Chemistry Letters

Letter

and heteromolecular categories.13,32−35 Although the heteromolecular nanoporous networks are more challenging to realize experimentally, they provide additional chemical means by which the molecular interactions governing the assembly can be controlled, thereby enabling the networks to be tuned to meet specific design objectives. While heteromolecular surface assemblies have been investigated on a wide variety of substrates,36−40 nanoporous networks comprised of multiple molecular species remain essentially unexplored on graphene. A notable exception is the recent work by Dichtel and co-workers, which demonstrated the solvothermal condensation of a twocomponent covalent organic framework,41 although the several nanometer film thickness in this case is well beyond the monolayer level that is most feasible to act as a chemical template. Herein, we report the successful intermixing of perylenetetracarboxylic diimide (PTCDI) and melamine molecules on epitaxial graphene on SiC(0001) to form a noncovalent 2D nanoporous network in registry with the underlying substrate lattice. The PTCDI−melamine system was selected because the self-assembly characteristics of PTCDI, melamine, and PTCDI−melamine networks have each been independently investigated on a diverse array of surfaces. In particular, PTCDI, as shown schematically in Figure 1a, is known to form brickwall structures on HOPG,42 MoS2,42 and hydrogen-passivated Si(111).43 On the other hand, melamine, as shown schematically in Figure 1b, forms structures with three-fold symmetry on both Au(111)44 and Ag(111).45 As illustrated in Figure 1c, the well-matched hydrogen bonding arrangements between PTCDI and melamine promote the formation of structures with directional noncovalent bonding. Taking into account the two-fold symmetry of the PTCDI molecule and the three-fold symmetry of melamine, the expected nanoporous architecture for the intermixed assembly is hexagonal, as depicted in Figure 1d. While the hexagonal nanoporous network resulting from the coadsorption of these two molecules has been studied previously on Au(111)46,47 and Ag/Si(111),48,49 it has not been demonstrated on HOPG or graphene. The assembly of PTCDI and melamine on epitaxial graphene was characterized at the molecular scale with a home-built ultrahigh vacuum (UHV) scanning tunneling microscope (STM) with a base pressure of ∼6 × 10−11 Torr.50 Topographic STM images were gathered in constant-current mode with the bias applied to the sample while electrochemically etched tungsten probes were grounded through a current preamplifier. STM images were rendered using Gwyddion SPM analysis software. As described elsewhere,28 epitaxial graphene surfaces, consisting of a mixture of single-layer and bilayer graphene regions, were prepared in UHV via the thermal decomposition of Cree 6H-SiC(0001) n-type single-crystal wafers. The quality of the resulting surface is shown in Figure 1e, where atomically flat terraces are readily observed. PTCDI (98% purity) and melamine (99% purity) were purchased from Alfa Aesar and used without further purification. The molecules were individually loaded into distinct alumina-coated tungsten boats, brought into the UHV preparation chamber, and degassed overnight below the sublimation point of each molecule. To generate the heteromolecular nanoporous network on graphene, a procedure similar to the one reported by Beton and co-workers was used.48 Initially, self-assembled PTCDI islands (Figure 1f) were grown by raising the temperature of the tungsten boat to ∼360 °C and exposing the epitaxial graphene surface to the resulting

Figure 1. Molecular structure of (a) PTCDI and (b) melamine. (c) Model of the intermolecular bonding between melamine and PTCDI. (d) Model of the PTCDI−melamine network unit cell, with lattice vectors indicated by the blue arrows. (e) STM image of a clean epitaxial graphene surface (sample bias (Vs) = −2.0 V, tunneling current (It) = 0.08 nA). The inset shows atomic resolution of the graphene lattice (Vs = 0.50 V, It = 0.10 nA). (f) STM image acquired following the deposition of a submonolayer coverage of PTCDI on epitaxial graphene (Vs = −1.0 V, It = 0.04 nA).

molecular flux until 30−50% of the surface was covered by PTCDI molecular domains. Following STM validation of the PTCDI surface coverage, melamine was deposited via a similar procedure with the exception that the temperature of its tungsten boat was increased to ∼100 °C. While the surface coverage of PTCDI must be carefully controlled in order to establish conditions favorable for network formation, precision control of the melamine dose is less important because excess melamine is desorbed from the surface during the annealing process that ultimately drives network formation. In order to promote assembly of the PTCDI−melamine honeycomb architecture, two separate methods of heat treatment were explored. In the first of these procedures, melamine was sublimated onto a room-temperature epitaxial graphene substrate, which had been prepared in advance with a submonolayer coverage of PTCDI. Alternatively, a substrate with a similar PTCDI coverage was first annealed at ∼170 °C and then exposed to the melamine dose while at elevated 271

dx.doi.org/10.1021/jz4025518 | J. Phys. Chem. Lett. 2014, 5, 270−274

The Journal of Physical Chemistry Letters

Letter

shown in Figure 2b, the PTCDI−melamine nanoporous network extends in a conformal manner along the length of the topmost step while extending no further than ∼20 nm from that edge. Additionally, Figure 2c illustrates how the network can become enclosed within the boundaries of a region of the surface constrained on all sides by step edges. In such circumstances, the growth of the network appears to become sterically frustrated due to the limited space available, leading to an increase in the presence of pentagonal defects. Similarly, Figure 2d demonstrates how homomolecular PTCDI islands can also act as constraints for the PTCDI−melamine nanoporous network. While the PTCDI−melamine network is occasionally observed to seamlessly cross step edges on the epitaxial graphene surface (Figure 3a), this case is relatively rare and thus is not considered to be a characteristic feature of this system, in contrast to other adsorbates such as 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA).25,28 Moreover, when the nanoporous network encounters one of the sporadically dispersed 1D defects on the epitaxial graphene substrate, it

temperature. In both cases, the sample holder was then sandwiched between a pair of copper blocks that could be heated via two independently controlled tungsten filaments.50 It was observed that the PTCDI−melamine network formation on graphene exhibited a high degree of sensitivity to the annealing temperature (150−165 °C) but was less susceptible to variations in the annealing duration (2−8 h). As shown in Figure 2, PTCDI and melamine form a highly periodic hexagonal nanoporous network with a lattice constant

Figure 2. (a) STM image of a PTCDI−melamine honeycomb domain (Vs = 1.0 V, It = 0.04 nA). The blue arrows are the unit cell lattice vectors. (b) PTCDI−melamine network closely tracking an adjacent step edge (Vs = −1.0 V, It = 0.05 nA). (c) PTCDI−melamine network domain enclosed within a topographic depression defined by the step edges of the underlying substrate (Vs = −1.0 V, It = 0.05 nA). (d) PTCDI−melamine network confined within a region bound by both step edges and PTCDI islands (Vs = −1.0 V, It = 0.05 nA).

of 3.45 ± 0.10 nm and a typical maximum domain size of 40− 50 nm. When the surface is inspected on a larger length scale, three distinct regions can be readily identified, (1) PTCDI− melamine heteromolecular nanoporous networks; (2) persistent PTCDI homomolecular domains identical to those observed prior to melamine deposition; and (3) flat open areas of graphene that appear streaky and indistinct. The streaky character of the exposed graphene regions is attributed to unbound melamine and/or PTCDI−melamine pairs that freely diffuse across the surface under the influence of the STM probe at room temperature. These streaky features are absent prior to melamine deposition and also disappear when following postannealing at temperatures greater than 170 °C, which corresponds to the condition where the melamine has been desorbed and PTCDI−melamine networks are no longer observed on the substrate. The crisp nature of both the PTCDI−melamine heteromolecular networks and the PTCDI homomolecular domains, as shown in Figure 2b−d, suggests that the rapidly diffusing molecules are primarily confined to the open graphene regions. The PTCDI−melamine networks are often found in close proximity to step edges and local topographic depressions. As

Figure 3. (a) A PTCDI−melamine network traverses a substrate step edge. (b) A PTCDI−melamine network circumvents a 1D surface defect. (c) Two distinct PTCDI−melamine domains that share a common orientation with respect to each other and the underlying substrate. (d) Model of the alignment of the lattice vector of the PTCDI−melamine network with respect to the underling graphene lattice. (e) STM image of a PTCDI−melamine network containing a dislocation defect (Vs = 1.0 V, It = 0.04 nA for all STM images). 272

dx.doi.org/10.1021/jz4025518 | J. Phys. Chem. Lett. 2014, 5, 270−274

The Journal of Physical Chemistry Letters

■

preferentially circumvents such topographic obstructions (Figure 3b). Finally, it should be noted that, unlike PTCDI− melamine nanoporous assemblies on Ag/Si(111) 48 or Au(111),44 network formations near step edges on epitaxial graphene do not attach to the step. This observation suggests that while the nucleation of the nanoporous PTCDI−melamine hexagonal phase on graphene is favored in confined regions of the surface, it does not directly involve the step edges themselves. Figure 3c displays two spatially isolated PTCDI−melamine assemblies, which share an identical orientation with respect to each other and thus the underlying substrate. Indeed, this orientation is shared by every PTCDI−melamine network domain observed in this study, suggesting that the strength of the π−π stacking interactions between the molecules and the basal plane of the graphene sheet are sufficient to promote substrate registry. Determining the specific orientation of the hexagonal molecular array with respect to the graphene lattice is complicated by the fact that the open graphene regions are obfuscated by the presence of quickly diffusing physisorbed species. However, it has been demonstrated that the highly faceted step edges of epitaxial graphene on SiC(0001) are closely aligned with the graphene ⟨1 −1⟩ direction.51 Consequently, the orientation of the step edges shown in Figure 3c serves as a reference point that enables determination of the orientation of the graphene lattice. Analysis of the molecular domains displayed in Figure 3c indicates that the associated lattice vectors of the PTCDI−melamine network are rotationally offset from the step edges by ∼30°, which implies that the nanoporous array is itself aligned with the graphene ⟨1 0⟩ direction, as summarized in Figure 3d. Additional evidence for this conclusion can be found in Figure 1e (inset), where high-resolution STM imaging of the epitaxial graphene lattice taken after the substrate has been annealed above 170 °C corroborates the assigned network orientation. Lattice mismatch between crystalline substrates and adsorbed molecular thin films results in the accumulation of strain during domain growth until a crystallographic defect is formed, such as the dislocation shown in Figure 3e. The lattice mismatch of the PTCDI−melamine network on epitaxial graphene thus limits the observed domain size to 40−50 nm, in contrast to domain sizes exceeding 100 nm on Ag/Si(111).48 In summary, a hydrogen-bonded 2D periodic nanoporous network has been realized on epitaxial graphene through the successful intermixing of PTCDI and melamine. Roomtemperature STM imaging confirms that this organic heteromolecular assembly possesses registry with the underlying graphene lattice. Specifically, the 3.45 ± 0.10 nm periodicity of the network permits the formation of continuous, dislocation-free, domains at the 40−50 nm length scale. Furthermore, the PTCDI−melamine nanoporous domains are frequently localized within surface topographic depressions, suggesting that lithographic prepatterning of the substrate can enable assembly into predetermined patterns and geometries. Overall, this work establishes a method to functionalize graphene in a 2D spatially periodic manner, thus presenting opportunities for further chemical templating at the sub-5 nm length scales that are relevant for graphene band structure engineering.

Letter

ASSOCIATED CONTENT

S Supporting Information *

Additional STM images of epitaxial graphene, PTCDI domains, and network defects. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

T.C.: Department of Physics and Astronomy, University of Wyoming, Laramie, Wyoming 82071, U.S.A. Author Contributions

All authors have contributed and given approval to this manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Department of Energy (Award Number DE-FG02-09ER16109), the Office of Naval Research (Award Number N00014-11-1-0463), and a W. M. Keck Foundation Science and Engineering Grant. V.D.-C. acknowledges a NSERC Postdoctoral Fellowship. The authors thank J. W. Lyding for the use of his STM control software.

■

REFERENCES

(1) Avouris, Ph. Graphene: Electronic and Photonic Properties and Devices. Nano Lett. 2010, 10, 4285−4294. (2) Son, Y.-W.; Cohen, M. L.; Louie, S. G. Energy Gaps in Graphene Nanoribbons. Phys. Rev. Lett. 2006, 97, 206803. (3) Hossain, Md. Z.; Walsh, M. A.; Hersam, M. C. Scanning Tunneling Microscopy, Spectroscopy, and Nanolithography of Epitaxial Graphene Chemically Modified with Aryl Moieties. J. Am. Chem. Soc. 2010, 132, 15399−15403. (4) Hossain, Md. Z.; Johns, J. E.; Bevan, K. H.; Karmel, H. J.; Liang, Y. T.; Yoshimoto, S.; Mukai, K.; Koitaya, T.; Yoshinobu, J.; Kawai, M.; et al. Chemically Homogeneous and Thermally Reversible Oxidation of Epitaxial Graphene. Nat. Chem. 2012, 4, 305−309. (5) Mandeltort, L.; Choudhury, P.; Johnson, J. K.; Yates, J. T., Jr. Reaction of the Basal Plane of Graphite with the Methyl Radical. J. Phys. Chem. Lett. 2012, 3, 1680−1683. (6) Johns, J. E.; Hersam, M. C. Atomic Covalent Functionalization of Graphene. Acc. Chem. Res. 2013, 46, 77−86. (7) Gogotsi, Y. Controlling Graphene Properties Through Chemistry. J. Phys. Chem. Lett. 2011, 2, 2509−2510. (8) Nicholas, R. J.; Mainwood, A.; Eaves, L. Carbon-Based Electronics: Fundamentals and Device Applications. Philos. Trans. R. Soc. London, Ser. A 2008, 366, 189−193. (9) Mao, H. Y.; Lu, Y. H; Lin, J. D.; Zhong, S.; Wee, A. T. S.; Chen, W. Manipulating the Electronic and Chemical Properties of Graphene via Molecular Functionalization. Prog. Surf. Sci. 2013, 88, 132−159. (10) Barth, J. V.; Costantini, G.; Kern, K. Engineering Atomic and Molecular Nanostructures at Surfaces. Nature 2005, 437, 671−679. (11) Cicoira, F.; Santato, C.; Rosei, F. Two-Dimensional Nanotemplates as Surface Cues for the Controlled Assembly of Organic Molecules. Top. Curr. Chem. 2008, 285, 203−267. (12) Miyashita, N.; Kurth, D. G. Directing Supramolecular Assemblies on Surfaces. J. Mater. Chem. 2008, 18, 2636−2649. (13) Elemans, J. A. A. W.; Lei, S.; De Feyter, S. Molecular and Supramolecular Networks on Surfaces: From Two-Dimensional Crystal Engineering to Reactivity. Angew. Chem., Int. Ed. 2009, 48, 7298−7332. (14) Wee, A. T. S.; Chen, W. Molecular Interactions on Epitaxial Graphene. Phys. Scr. 2012, T146, 014007.

273

dx.doi.org/10.1021/jz4025518 | J. Phys. Chem. Lett. 2014, 5, 270−274

The Journal of Physical Chemistry Letters

Letter

(15) Walzer, K.; Sternberg, M.; Hietschold, M. Formation and Characterization of Coronene Monolayers on HOPG(0001) and MoS2(0001): A Combined STM/STS and Tight-Binding Study. Surf. Sci. 1998, 415, 376−384. (16) Piot, L.; Marchenko, A.; Wu, J.; Mullen, K.; Fichou, D. Structural Evolution of Hexa-peri-hexabenzocoronene Adlayers in Heteroepitaxy on n-Pentacontane Template Monolayers. J. Am. Chem. Soc. 2005, 127, 16245−16250. (17) Xu, L.-P.; Gong, J.-R.; Wan, L.-J.; Jiu, T.-G.; Li, Y.-L.; Zhu, D.B.; Deng, K. Molecular Architecture of Oligothiophene on a Highly Oriented Pyrolytic Graphite Surface by Employing Hydrogen Bondings. J. Phys. Chem. B 2006, 110, 17043−17049. (18) Weckesser, J.; De Vita, A.; Barth, J. V.; Cai, C.; Kern, K. Mesoscopic Correlation of Supramolecular Chirality in One-Dimensional Hydrogen-Bonded Assemblies. Phys. Rev. Lett. 2001, 87, 096101. (19) Lackinger, M.; Griessl, S.; Heckl, W. M.; Hietschold, M. Coronene on Ag(111) Investigated by LEED and STM in UHV. J. Phys. Chem. B 2002, 106, 4482−4485. (20) Humblot, V.; Lorenzo, M. O.; Baddeley, C. J.; Haq, S.; Raval, R. Local and Global Chirality at Surfaces: Succinic Acid versus Tartaric Acid on Cu(110). J. Am. Chem. Soc. 2004, 126, 6460−6469. (21) Upward, M. D.; Moriarty, P.; Beton, P. H. Double Domain Ordering and Selective Removal of C 60 on Ag/Si(111)(√3×√3)R30°. Phys. Rev. B 1997, 56, R1704−R1707. (22) Upward, M. D.; Beton, P. H.; Moriarty, P. Adsorption of Cobalt Phthalocyanine on Ag Terminated Si(111). Surf. Sci. 1999, 441, 21− 25. (23) Nakayama, T.; Onoe, J.; Takeuchi, K.; Aono, M. Weakly Bound and Strained C60 Monolayer on the Si(111)√3×√3R30°-Ag Substrate Surface. Phys. Rev. B 1999, 59, 12627−12631. (24) Keeling, D. L.; Oxtoby, N. S.; Wilson, C.; Humphry, M. J.; Champness, N. R.; Beton, P. H. Assembly and Processing of Hydrogen Bond Induced Supramolecular Nanostructures. Nano Lett. 2003, 3, 9− 12. (25) Wang, Q. H.; Hersam, M. C. Room-Temperature MolecularResolution Characterization of Self-Assembled Organic Monolayers on Epitaxial Graphene. Nat. Chem. 2009, 1, 206−211. (26) Pollard, A. J.; Perkins, E. W.; Smith, N. A.; Saywell, A.; Goretzki, G.; Phillips, A. G.; Argent, S. P.; Sachdev, H.; Muller, F.; Hufner, S.; et al. Supramolecular Assemblies Formed on an Epitaxial Graphene Superstructure. Angew. Chem., Int. Ed. 2010, 49, 1794−1799. (27) Roos, M.; Uhl, B.; Kunzel, D.; Hoster, H. E.; Groβ, A.; Behm, R. J. Intermolecular vs Molecule-Substrate Interactions: A Combined STM and Theoretical Study of Supramolecular Phases on Graphene/ Ru(0001). Beilstein J. Nanotechnol. 2011, 2, 365−373. (28) Johns, J. E.; Karmel, H. J.; Alaboson, J. M. P.; Hersam, M. C. Probing the Structure and Chemistry of Perylenetetracarboxylic Dianhydride on Graphene before and after Atomic Layer Deposition of Alumina. J. Phys. Chem. Lett. 2012, 3, 1974−1979. (29) Yang, H.; Mayne, A. J.; Comtet, G.; Dujardin, G.; Kuk, Y.; Sonnet, Ph.; Stauffer, L.; Nagarajan, S.; Gourdon, A. STM Imaging, Spectroscopy and Manipulation of a Self-Assembled PTCDI Monolayer on Epitaxial Graphene. Phys. Chem. Chem. Phys. 2013, 15, 4939−4946. (30) Mann, J. A.; Dichtel, W. R. Noncovalent Functionalization of Graphene by Molecular and Polymeric Adsorbates. J. Phys. Chem. Lett. 2013, 4, 2649−2657. (31) Li, B.; Tahara, K.; Adisoejoso, J.; Vanderlinden, W.; Mali, K. S.; De Gendt, S.; Tobe, Y.; De Feyter, S. Self-Assembled Air-Stable Supramolecular Porous Networks on Graphene. ACS Nano 2013, 7, 10764−10772. (32) Yang, Y.-L.; Wang, C. Hierarchical Construction of SelfAssembled Low-Dimensional Molecular Architectures Observed by Using Scanning Tunneling Microscopy. Chem. Soc. Rev. 2009, 38, 2576−2589. (33) Bonifazi, D.; Mohnani, S.; Llanes-Pallas, A. Supramolecular Chemistry at Interfaces: Molecular Recognition on Nanopatterned Porous Surfaces. Chem.Eur. J. 2009, 15, 7004−7025.

(34) Kudernac, T.; Lei, S.; Elemans, J. A. A. W.; De Feyter, S. TwoDimensional Supramolecular Self-Assembly: Nanoporous Networks on Surfaces. Chem. Soc. Rev. 2009, 38, 402−421. (35) Gunasinghe, R. N.; Reuven, D. G.; Suggs, K.; Wang, X.-Q. Filled and Empty Orbital Interactions in a Planar Covalent Organic Framework on Graphene. J. Phys. Chem. Lett. 2012, 3, 3048−3052. (36) de Wild, M.; Berner, S.; Suzuki, H.; Yanagi, H.; Schlettwein, D.; Ivan, S.; Baratoff, A.; Guentherodt, H.-J.; Jung, T. A. A Novel Route To Molecular Self-Assembly: Self-Intermixed Monolayer Phases. ChemPhysChem 2002, 3, 881−885. (37) Spillmann, H.; Dmitriev, A.; Lin, N.; Messina, P.; Barth, J. V.; Kern, K. Hierarchical Assembly of Two-Dimensional Homochiral Nanocavity Arrays. J. Am. Chem. Soc. 2003, 125, 10725−10728. (38) Xu, B.; Tao, C.; Cullen, W. G.; Reutt-Robey, J. E.; Williams, E. D. Chiral Symmetry Breaking in Two-Dimensional C60−ACA Intermixed Systems. Nano Lett. 2005, 5, 2207−2211. (39) Gonzalez-Lakunza, N.; Canas-Ventura, M. E.; Ruffieux, P.; Rieger, R.; Mullen, K.; Fasel, R.; Arnau, A. Hydrogen-Bonding Fingerprints in Electronic States of Two-Dimensional Supramolecular Assemblies. ChemPhysChem 2009, 10, 2943−2946. (40) Huang, Y. L.; Chen, W.; Li, H.; Ma, J.; Pflaum, J.; Wee, A. T. S. Tunable Two-Dimensional Binary Molecular Networks. Small 2010, 6, 70−75. (41) Colson, J. W.; Woll, A. R.; Mukherjee, A.; Levendorf, M. P.; Spitler, E. L.; Shields, V. B.; Spencer, M. G.; Park, J.; Dichtel, W. R. Oriented 2D Covalent Organic Framework Thin Films on SingleLayer Graphene. Science 2011, 332, 228−231. (42) Ludwig, C.; Gompf, B.; Petersen, J.; Strohmaier, R.; Eisenmenger, W. STM Investigations of PTCDA and PTCDI on Graphite and MoS2: A Systematic Study of Epitaxy and STM Image Contrast. Z. Phys. B 1994, 93, 365−373. (43) Uder, B.; Ludwig, C.; Petersen, J.; Gompf, B.; Eisenmenger, W. STM Characterization of Organic Molecules on H-Terminated Si(111). Z. Phys. B 1995, 97, 389−390. (44) Perdigao, L. M. A.; Perkins, E. W.; Ma, J.; Staniec, P. A.; Rogers, B. L.; Champness, N. R.; Beton, P. H. Bimolecular Networks and Supramolecular Traps on Au(111). J. Phys. Chem. B 2006, 110, 12539−12542. (45) Schmitz, C. H.; Ikonomov, J.; Sokolowski, M. Two Commensurate Hydrogen-Bonded Monolayer Structures of Melamine on Ag(111). Surf. Sci. 2011, 605, 1−6. (46) Silly, F.; Shaw, A. Q.; Porfyrakis, K.; Briggs, G. A. D.; Castell, M. R. Pairs and Heptamers of C70 Molecules Ordered via PTCDI− Melamine Supramolecular Networks. Appl. Phys. Lett. 2007, 91, 253109. (47) Madueno, R.; Räisänen, M. T.; Silien, C.; Buck, M. Functionalizing Hydrogen-Bonded Surface Networks with SelfAssembled Monolayers. Nature 2008, 454, 618−621. (48) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Controlling Molecular Deposition and Layer Structure with Supramolecular Surface Assemblies. Nature 2003, 424, 1029− 1031. (49) Theobald, J. A.; Oxtoby, N. S.; Champness, N. R.; Beton, P. H.; Dennis, T. J. S. Growth Induced Reordering of Fullerene Clusters Trapped in a Two-Dimensional Supramolecular Network. Langmuir 2005, 21, 2038−2041. (50) Foley, E. T.; Yoder, N. L.; Guisinger, N. P.; Hersam, M. C. Cryogenic Variable Temperature Ultrahigh Vacuum Scanning Tunneling Microscope for Single Molecule Studies on Silicon Surfaces. Rev. Sci. Instrum. 2004, 75, 5280−5287. (51) Alaboson, J. M. P.; Sham, C.-H.; Kewalramani, S.; Emery, J. D.; Johns, J. E.; Deshpande, A.; Chien, T.; Bedzyk, M. J.; Elam, J. W.; Pellin, M. J.; et al. Templating Sub-10nm Atomic Layer Deposited Oxide Nanostructures on Graphene via One-Dimensional Organic Self-Assembled Monolayers. Nano Lett. 2013, 13, 5763−5770.

274

dx.doi.org/10.1021/jz4025518 | J. Phys. Chem. Lett. 2014, 5, 270−274