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Electrical Field-Induced Alignment of Nonpolar Hexabenzocoronene Molecules into Columnar Structures on Highly Oriented Pyrolitic Graphite Investigated by STM and SFM Anna Cristadoro,† Min Ai,‡ Hans Joachim Ra1 der,† Ju1 rgen P. Rabe,*,‡ and Klaus Mu1 llen*,† Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany and Department of Physics, Humboldt UniVersity Berlin, Newtonstrasse 15, D-12489 Berlin, Germany ReceiVed: December 13, 2007; In Final Form: January 23, 2008
Scanning tunneling microscopy (STM) and scanning force microscopy (SFM) have been used to study ultrathin films of a kind of nanographene, hexa(p-n-dodecylphenyl)hexabenzocoronene (HBC-PhC12), on a highly oriented pyrolitic graphite (HOPG) surface. An electrical field, applied parallel to the substrate surface during adsorption from solution and subsequent drying, has been found to align columns of HBC-PhC12 molecules with their long axes perpendicular to the field direction, independently from the HOPG lattice orientation underneath. The molecules, under field influence, adopt a tilted edge-on arrangement on the HOPG surface. On the contrary, the corresponding drop-cast films, dried in the absence of an electrical field, shows a faceon arrangement of the extended aromatic π-system of the HBC-PhC12 discs, and no columnar structures ordered parallel to the graphite surface are recognized. Formation of unidirectionally aligned columns of HBC-PhC12 molecules is, therefore, related to the influence of the electrical field during self-assembly of HBC-PhC12 molecules on top of the conducting substrate. The electrical field competes favorably with the strong intermolecular interactions between HOPG and HBC, avoiding the epitaxial growth of thin films with the molecules in face-to-face arrangements parallel to the substrate. Our results constitute an important step toward control of the order and arrangement of functional conjugated molecules in ultrathin layers using electrical fields.
Introduction During recent years organic-based electronic devices, such as photodiodes, organic field effect transistors, and organic light emitting transistors, have been the subject of numerous investigations.1-4 Solution processing and deposition techniques have been developed to improve the molecular order in organic thin films5-7 since it has been clearly demonstrated that electronic performance is morphology dependent.8 Molecules, which form discotic mesophases with significant long-range order and anisotropic electrical and optical properties, continue to be sought for electronic applications. Side-chain-modified hexabenzocoronenes (HBC) are an example of soluble graphenetype molecules that form columnar assemblies exhibiting high charge mobility along the columns,9 which therefore can be considered as self-assembled nanowires with strong π-π interactions. In particular, the good solubility and high aggregation tendency of hexa(p-n-dodecylphenyl)hexabenzocoronene (HBC-PhC12) molecules have made them suitable candidates for novel electronic devices.1 Recently, highly ordered HBCPhC12 films have been obtained by means of electrical fields.10 The field was used to induce a dipole moment along the planar core of the HBC-PhC12 molecules. As a consequence, the HBCPhC12 molecules oriented their aromatic discs along the direction of the field lines and, due to the strong aggregation tendency, created columns perpendicularly aligned to the external electrical * To whom correspondence should be addressed. K.M.: fax, (+49)6131-379350; e-mail,
[email protected]. J.P.R.: fax, (+49)30-20937632; e-mail,
[email protected]. † Max Planck Institute for Polymer Research. ‡ Humboldt University Berlin.
field. The HBC-PhC12 cores, assembling in columnar aggregates, created an angle with the surface of 90°, defined as edge-on arrangement, which seems to be, independently from the presence of an external electrical field, the most stable HBCPhC12 packing on a glass surface.10 Here we present a combined STM and SFM study of HBCPhC12 molecules (Chart 1) deposited on a conducting surface (HOPG) and dried in the presence of an electrical field. At solid-liquid interfaces, STM studies have shown symmetrically substituted and parent HBC molecules with a face-on arrangement on HOPG,11-13 defined as the arrangement of the molecules with their aromatic disc lying flat on the graphitic surface. So far, only nonsubstituted HBC molecules deposited from the gas phase on HOPG via soft-landing mass spectrometry showed a different behavior.14-15 In this case, the molecules adopt an edge-on arrangement, where the aromatic discs are nearly oriented perpendicularly to the surface plane. During a soft-landing deposition, HBC ions are formed by a matrixassisted laser desorption ionization (MALDI) source, accelerated, separated, and then decelerated and focused on a conducting surface by means of electrical fields. Most likely dipole moments are induced in the molecules, which orient their planar cores along the electrical field lines. Hypothesizing that this mechanism allows the molecules to adopt an edge-on arrangement on the HOPG surface, electrical fields could be used to increase the order and control the arrangement of the molecules on the surface. Thus, the field influence on the assembly of HBC-PhC12 molecules is investigated. Additionally, a comparative STM study on HBC-PhC12 films drop cast in the absence of an electrical field at the solid-liquid interface and solid-air interface is conducted.
10.1021/jp711707w CCC: $40.75 © 2008 American Chemical Society Published on Web 03/19/2008
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Figure 1. Setup of apparatus for electrical field alignment of HBCPhC12 molecules.
CHART 1. Chemical Structure of HBC-PhC12
Figure 2. STM current images of HBC-PhC12 at the HOPG-solution interface. Sample bias Us ) 1 V; average tunneling current It ) 0.1 nA. (a) Large-scale images with three patterns marked by A, B, and C. (b) B and C patterns. (c) Oblique structure, ascribed to pattern A. (d) Dimer structure B. (e) Trimer structure C.
Experimental Details The synthesis of hexa(p-n-dodecylphenyl)hexabenzocoronene (HBC-PhC12) has already been reported elsewhere.16 The molecular structure of the HBC-PhC12 molecules is shown in Chart 1. 1,2,4-Trichlorobenzene was purchased from Sigma Aldrich (99% Munich, Germany) and used without further purification. The experiments to increase the degree of supramolecular order in films of HBC-PhC12 molecules were carried out in a home-built apparatus. As reported in Figure 1, two stainless steel electrodes (area ) 3 × 3 cm2, thickness ) 0.5 mm) were fixed parallel to each other on a Teflon support. This geometry was selected to create a uniform electrical field (0-24 kV/cm) by charging one electrode and grounding the other one with a high-voltage power supply (Fug Electronic HCN GmbH 0-24 kV). The sample plate was placed between the electrodes on the Teflon support. Dry films were prepared by drop casting a solution of HBC-PhC12 molecules in 1,2,4trichlorobenzene (15 mg/mL) at room temperature onto freshly cleaved HOPG, while an electrical field in the range of 4-5 × 105 V/m was applied parallel to the surface.10 The experiments were performed in a small glass box to control the evaporation rate of the solvent. Crystallization in an atmosphere saturated with the vapor of the organic solvent was employed to improve the alignment of HBC-PhC12 molecules under electrical field influence. STM and SFM analyses were performed subsequently on the dry films at the solid-air interface. For comparison, films were also prepared without the presence of an electrical field and
then characterized by STM. Moreover, HBC-PhC12 molecules in 1,2,4-trichlorobenzene (15 mg/mL) were drop cast on freshly cleaved HOPG without the presence of an external electrical field. STM was then performed at the solid-liquid interface. STM measurements were performed using a home-built beetle-type STM interfaced to a commercial control unit (Omicron Nanotechnology GmbH). STM tips were prepared by mechanically cutting a 0.25 mm thick Pt/Ir (80%/20%) wire. SFM experiments were carried out with a Nanoscope III (Digital Instruments, Inc., Santa Barbara, CA) in the tapping mode with an E-type scanner (12 × 12 µm2) in air at room temperature. Commercial silicon tips (Digital instruments) on cantilevers with spring constants between 17 and 64 N m-1 were used at typical resonance frequencies in the range between 280 and 320 kHz. Results and Discussion STM Imaging of HBC-PhC12 Molecules at the SolidLiquid Interface. The assembly behavior of HBC-PhC12 molecules at the interface of a solution in 1,2,4-trichlorobenzene with HOPG was investigated by in-situ STM. HBC-PhC12 molecules were physisorbed at the interface between the HOPG and a solution in 1,2,4-trichlorobenzene. In the STM current images (Figure 2), the bright features (corresponding to high tunneling probability) can be attributed to π-conjugated aromatic rings because the energy difference between their frontier orbitals and the Fermi level of HOPG is rather small.17 The aliphatic side chains, which are attributed to the dark parts of the images, could not be resolved, probably due to their high conformational mobility on a time scale faster than the STM imaging. Three different types of the 2D unit cells are presented in Figure 2, marked with A, B, and C. The two-dimensional lattice A (Figure 2c) is an oblique structure described by a unit cell with parameters a ) (2.01 ( 0.11) nm, b ) (3.30 ( 0.17) nm, and R ) (76 ( 4)°. In case B (Figure 2d), the 2D unit cell is a dimeric structure (a ) (3.41 ( 0.05) nm, b ) (5.54 ( 0.19) nm, and R ) (74 ( 1)°). The lattice constant of the trimer C, containing three molecules in a unit cell, is a ) (3.48 ( 0.04) nm, b ) (7.50 ( 0.34) nm, and R ) (75 ( 2)°. Most abundant is the oblique pattern, while the trimer structure is rarest.
Alignment of Nonpolar Hexabenzocoronene Molecules
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Figure 4. (a and b) Tapping-mode SFM phase image of HBC-PhC12 films on HOPG, prepared within electric fields. (c) Schematic of single HBC-PhC12 columns. (d) Schematic of the stripes formed by two HBCPhC12 columns.
Figure 3. (a and b) STM current images of the surface of a dry electric field-oriented HBC-PhC12 film in air. Sample bias Us ) 3 V; average tunneling current It ) 0.1 nA; unit cell parameters a ) (1.03 ( 0.11) nm, b ) (3.60 ( 0.16) nm, and R ) (79 ( 4)°. (c) Schematic representation of the HBC disc arrangement with respect to the surface. (d) STM current image of a dry film adsorbed without electric field; sample bias Us ) 1 V; average tunneling current It ) 0.1 nA; unit cell parameters a ) (2.01 ( 0.11) nm, b ) (3.30 ( 0.17) nm, and R ) (76 ( 4)°.
Interestingly, a random distribution of two dissimilar contrasts of the aromatic core in all of the three domains at the HOPGliquid interface is recorded.18 It could possibly be attributed to a conformational change of the HBC core with the lateral phenyls from planar to nonplanar, which causes a change in the electronic properties of the HBC cores, resulting in different contrasts in the STM images. Another interpretation would be different positions of the HBC cores with respect to the STM tip and the substrate. In fact, the asymmetric position of the molecules between the tip and substrate influences the contrast in STM current images, in accord with tunneling spectroscopy. STM Characterization of Field-Oriented HBC-PhC12 Films. In order to study the electrical field influence on the arrangement of the HBC-PhC12 molecules on HOPG, dry HBCPhC12 films were prepared. Electric fields in the range of 4-5 × 105 V/m were applied during adsorption of the HBC-PhC12 molecules from a solution in 1,2,4-trichlorobenzene and maintained until solvent evaporation. Then, the films were characterized by STM at the solid-air interface. Figure 3a displays an STM image exhibiting characteristic stripes of ∼3.6 nm width, oriented predominantly perpendicularly to the direction of the previously applied electric field. The stripes can be attributed to HBC-PhC12 columns within ordered domains as large as 300 nm across. No epitaxial growth was recognized,19 indicating that the structure of the deposited film was not dominated by the lattice structure of the HOPG substrate. On the contrary, the film displayed unidirectionally aligned stripes oriented largely perpendicularly to the direction of the previously applied electrical field. From the highly resolved STM image (Figure 3b) the spacings within the stripes and perpendicular to them are (1.03 ( 0.10) nm (s) and (3.6 ( 0.16) nm, respectively. They are attributed to π-π-stacked HBC-PhC12 cores, assembled in a tilted edge-on arrangement
on the graphitic surface (Figure 3c) with an angle γ, which can be estimated from d/s ) sin γ ) 0.34 to be ∼22°.20 The HBCPhC12 molecules could be visualized at an average tunneling impedance of 30 GΩ. Upon reducing the tip-sample separation (average impedance of 10 GΩ), no layer of face-on HBC-PhC12 molecule was found. At even smaller impedance of ∼0.1 GΩ, the graphitic substrate itself was visualized.21 However, even though an additional layer of face-on molecules was not recordable, its presence cannot be completely excluded. The organization of the HBC-PhC12 molecules, unidirectionally aligned by means of an electrical field, differed substantially from the arrangement, which the molecules adopt in dry films prepared in the absence of an electrical field. Figure 3d displays an image of HBC-PhC12 discs face on, arranged in a unit cell with parameters a ) (2.11 ( 0.10) nm, b ) (3.12 ( 0.15) nm, and R ) (70 ( 2)° in an oblique structure, which is indistinguishable from the oblique structure in the molecular monolayer obtained from the solid-liquid STM measurement (Figure 2c). The face-on HBC-PhC12 molecules could be visualized using an average impedance of 10 GΩ. By increasing the impedance up to 30 GΩ, no edge-on HBC-PhC12 molecules were observed. At even smaller impedance of ∼0.1 GΩ, the graphitic substrate itself was visualized.22 This leads to the conclusion that no upper layers made of edge-on HBC-PhC12 molecules are present. In fact, the HBC-PhC12 molecules, deposited on a graphitic surface in the absence of any external force, can overlap their aromatic orbitals with the ones of the HOPG. Thus, the face-on arrangement results in the energetically most stable molecular assembly. The observed tilted edge-on packing of the HBC-PhC12 molecules on the HOPG surface can be explained considering that, in the presence of an electrical field, the molecules are subjected not only to the strong π-π interactions between nanographene and graphene but also to an external electrical force. The electrical field influence on the HBC-PhC12 molecules strongly competes with the graphene-nanographene affinity, forces the molecules to adopt an edge-on arrangement, and orients them in its direction avoiding epitaxial growth phenomena. SFM. To investigate the degree of order of the field-oriented HBC-PhC12 films on HOPG surface on a larger scale, SFMtapping-mode analyses were performed (Figure 4a and b). Two kinds of domains can be recognized in the film, indicated by A and B. In the zoomed image (Figure 4b) domain A exhibits
5566 J. Phys. Chem. C, Vol. 112, No. 14, 2008 narrow parallel stripes with a distance of (3.43 ( 0.12) nm. In domain B, wider stripes (6.42 ( 0.09) nm are visible. Both stripe patterns are on average oriented perpendicularly with respect to the direction of the formerly applied electric field (marked by the white arrow and E h ). The width of the narrow stripes in domain A is in agreement with the size of a HBCPhC12 molecule with totally stretched alkyl chains (3.46 nm). In domain B, the width of the wide stripes is about twice that in domain A. As for field-oriented HBC-PhC12 molecules on a glass surface,10 the stripes of 3.4 nm width can be attributed to HBC-PhC12 molecules, oriented in the field direction and assembled in unidirectionally aligned columns (Figure 4c). The 6.4 nm bright stripes may be formed by ordered dimers of HBCPhC12, which are shown in Figure 4d. Formation of the wide stripes can be attributed to the high tendency of the HBC-PhC12 molecules to assemble into dimer structures in 1,2,4-trichlorobenzene solvent, as the STM images of the molecules at the solid-liquid interface already proved (Figure 2). However, dimers of HBC-PhC12 molecules assembled into a columnar structure were not visualized in the STM images of the field-oriented films (Figure 3). This could be explained by a lower formation probability of the dimeric pattern. Conclusions This work demonstrates the ability of electrical fields to influence the order and arrangement in thin films of graphene molecules on a HOPG surface. The presence of tilted edge-on molecules creating unidirectionally aligned domains demonstrates that electrical fields can be used to influence the strong interactions between HOPG and HBC-PhC12, avoiding the epitaxial growth of thin films of these molecules in face-toface arrangements parallel to the substrate. As morphology, crystal structure, and molecular ordering of the first organic monolayers are essential determinants of carrier transport phenomena,23 the results of this investigation could be helpful for the basic understanding of novel electronic devices such as field effect transistors. Furthermore, we expect that this study could be used for designing novel supermolecular structures with well-defined geometry even in acceptor-donor monolayers,24 which nowadays represent a great challenge. Acknowledgment. The authors thank Mr. Dirk Richter and the members of the Max-Planck-Institute electronic laboratory for their assistance. Additionally, we thank Ali Rouhanipour and Stephan Tu¨rk for technical help and scientific discussion.
Cristadoro et al. Funding for this research was provided by the Marie Curie Marie Curie Host Fellowship for Early Stage Research Training, by the DFG “Optoelektronik”, and by the Sfb 658 “Elementary processes in molecular switches at surfaces”. References and Notes (1) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science, 2001, 293, 1119-1122. (2) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.; Woo, E. P.; Grell, M.; Bradley, D. D. C. Appl. Phys. Lett. 2000, 77, 406-408. (3) Liu, C.-y.; Bard, A. J. Nature, 2002, 418, 162-164. (4) Hassheider, T.; Benning, S. A.; Lauhof, M. W.; Oesterhaus, R.; Alibert-Fouet, S.; Bock, H.; Goodby, J. W.; Watson, M. D.; Mu¨llen, K.; Kitzerow, H.-S. Proc. SPIE-Int. Soc. Opt. Eng. 2003, 5003 (Liquid Crystal Materials, Devices, and Applications IX), 167-174. (5) Briseno, A. L.; Aizenberg, J.; Han, Y.-J.; Penkala, R. A.; Moon, H.; Lovinger, A. J.; Kloc, C.; Bao, Z. J. Am. Chem. Soc. 2005, 127, 1216412165. (6) Chen, J.; Leblanc, V.; Kang, S. H.; Benning, P. J.; Schut, D.; Baldo, M. A.; Schmidt, M. A.; Bulovic, V. AdV. Funct. Mater. 2007, 17, 27222727. (7) Tracz, A.; Jeszka, J. K.; Watson, M. D.; Pisula, W.; Mu¨llen, K.; Pakula, T. J. Amer. Chem. Soc. 2003, 125, 1682-1683. (8) Paulsson, M.; Stafstro¨m, S. J. Phys. Condens. Matter 2000, 12, 9433-9440. (9) Van de Craats, A. M.; Warman, J. M.; Fechenko¨tter, A.; Brand, J. D.; Harbison, M. A.; Mu¨llen, K. AdV. Mater. 1999, 11, 1469-1472. (10) Cristadoro, A.; Lieser, G.; Ra¨der, H. J.; Mu¨llen, K. ChemPhysChem 2007, 8, 585-591. (11) Stabel, A.; Herwig, P.; Mu¨llen, K.; Rabe, J. P. Angew. Chem. 1995, 34, 1609-1611. (12) Ito, S.; Wehemeier, M.; Brand, D. J.; Ku¨bel, C.; Epsch, R.; Rabe, J. P.; Mu¨llen, K. Chem. Eur. J. 2000, 6, 4327-4342. (13) Ja¨ckel, F.; Watson, M. D.; Mu¨llen, K.; Rabe, J. P. Phys. ReV. B 2006, 73, 045423/1-045423/6. (14) Ra¨der, H. J.; Mu¨llen, K. Nachr. Chem. 2006, 54, 746-750. (15) Ra¨der, H. J.; Rouhanipour, A.; Talarico, A. M.; Palermo, V.; Samori, P.; Mu¨llen, K. Nat. Mater. 2006, 5, 276-280. (16) Fechtenko¨tter, A.; Saalwa¨chter, K.; Harbison, M. A.; Mu¨llen, K.; Spiess, H. W. Angew. Chem., Int. Ed. 1999, 38, 3039-3042. (17) Lazzaroni, R.; Calderone, A.; Bredas, J. L.; Rabe, J. P. J. Chem. Phys. 1997, 107, 99-105. (18) Samorı´, P.; Fechtenko¨tter, A.; Ja¨ckel, F.; Bo¨hme, T.; Mu¨llen, K.; Rabe, J. P. J. Am. Chem. Soc. 2001, 123, 11462-11467. (19) Cheng, Z. H.; Gao, L.; Deng, Z. T.; Liu, Q.; Jiang, N.; Lin, X.; He, X. B.; Du, S. X.; Gao, H.-J. J. Phys. Chem. C 2007, 111, 2656-2660. (20) Samori, P.; Engelkamp, H.; de Witte, P.; Rowan, A. E.; Nolte, Roeland J. M.; Rabe, J. P. Angew. Chem., Int. Ed. 2001, 40, 2348-2350. (21) Piot, M. L. Doctor Thesis, Paris, 2006; p 51. (22) Ja¨ckel, F.; Ai, M.; Wu, J.; Mu¨llen, K.; Rabe, J. P. J. Am. Chem. Soc. 2005, 127, 14580-14581. (23) Sheraw, C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. AdV. Mater. 2003, 15, 2009-2011. (24) Samori, P.; Severin, N.; Simpson, C. D.; Mu¨llen, K.; Rabe, J. P. J. Am. Chem. Soc. 2002, 124, 9454-9457.
