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Langmuir 1999, 15, 6490-6494
Influence of Preparation Conditions on the Monolayer Structure: Bi- and Terthiophenes in Solution and UHV Ralf Stecher, Frank Drewnick, and Bruno Gompf* 1. Physikalisches Institut, Universita¨ t Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany Received February 23, 1999. In Final Form: May 17, 1999 In this work we report on STM and LEED investigations of donor-acceptor-substituted bi- and terthiophene monolayers on weakly interacting substrates prepared under two different conditions: by evaporation in UHV and by adsorption at the liquid/solid interface. A detailed comparison of the results shows that the ordering is mainly given by the intermolecular interactions, but the observed molecular structures can also depend on the specific preparation conditions. In all cases the orientation with respect to the substrate is strongly influenced by the environment. At the liquid/solid interface the moleculesubstrate interaction seems to be reduced by the solvent, leading to close-packed structures with no phase relation with respect to the substrate, whereas for UHV-prepared monolayers well-defined azimuthal orientations were found.
Introduction The formation of highly ordered organic thin films has attracted considerable attention in the past few years, mainly due to their potential application in electronic and optical devices. One widely used preparation technique for the formation of well-defined organic thin films is sublimation under UHV conditions.1-3 For this method the molecules have to be thermally stable up to their sublimation temperature, a condition which could be a problem, especially for very large organic molecules. The advantage of this method is that due to the vacuum deposition the films are very clean, they exhibit a high degree of order, and there exist no restrictions for the substrates. A not so widely used technique is the formation of laterally ordered monolayers at the liquid/solid interface by adsorption from a solvent.4 This technique is only possible on weakly interacting substrates, but there seems to be no specific restrictions for the molecular systems which can be used. For vacuum sublimation the molecular order on weakly interacting substrates is given by a complex balance between the intermolecular interactions and the moleculesubstrate interaction,5 whereas for monolayers at the liquid/solid interface additionally the influence of the solvent has to be considered. Until now there exists no direct comparison between the molecular order obtained by these two different preparation techniques for the same molecule/substrate systems. The growing interest in oligothiophenes is motivated by their possible use as charge-transfer systems in molecular electronic devices. Furthermore, oligothiophenes are considered to be model systems for molecular wires of polythiophenes for which electric conductivity has been observed.6 Nevertheless, in order (1) Umbach, E.; Seidel, C.; Taborski, J.; Li, R.; Soukopp, A. Phys. Status Solidi B 1995, 192, 389. (2) Fenter, P.; Eisenberger, P.; Burrows, P.; Forrest, S. R.; Liang, K. S. Physica B 1996, 221, 145. (3) England, C. D.; Collins, G. E.; Schuerlein, T. J.; Armstrong, N. R. Langmuir 1994, 10, 2748. (4) Mu¨ller, H.; Petersen, J.; Strohmaier, R.; Gompf, B.; Eisenmenger, W.; Vollmer, M. S.; Effenberger, F. Adv. Mater. 1996, 8, 733. (5) Hillier, A. C.; Ward, M. D. Phys. Rev. B 1996, 54, 14037.
to generate materials with the desired properties, they should be incorporated in supramolecular structures, such as, for example, ordered monolayers. Scanning tunneling microscopy (STM) in solution revealed that the crystallographic order of nonpolar alkylthiophene derivatives is determined by the length of their alkyl chains.7-9 Even polythiophenes modified by attachment of alkyl chains built up ordered structures at the liquid/solid interface.10 In previous papers we have shown that monolayer structures can also directly be influenced by the attachment of polar formyl substituents.4,11 STM measurements of thiophenes under ultrahigh vacuum (UHV) conditions were also performed on nonpolar thiophene derivatives on Ag.12-14 In the present work we report on the monolayer structure of donor-acceptor substituted bi-11 and terthiophenes (Figure 1) prepared by vacuum sublimation and the structure of the same systems prepared by adsorption at the liquid/solid interface. The resulting monolayers were characterized mainly by STM and, for the vacuum-deposited films on MoS2, additionally by lowenergy electron diffraction (LEED). Experimental Section For all molecules shown in Figure 1 it was possible to prepare ordered monolayers at the liquid/solid interface from solution as well as by sublimation under UHV conditions. Monolayers in solution were obtained by adsorption of the molecules on freshly cleaved highly oriented pyrolytic graphite (HOPG) from saturated solutions using phenyloctane (Merck) and dodecane (Aldrich) as solvent. Evaporation of the molecules from a Knudsen cell onto (6) Ba¨uerle, P. Adv. Mater. 1993, 5, 879. (7) Stabel, A.; Rabe, J. P. Synth. Met. 1994, 67, 47. (8) Ba¨uerle, P.; Fischer, T.; Bidlingmaier, B.; Stabel, A.; Rabe, J. P. Angew. Chem. 1995, 107, 335; Angew. Chem., Int. Ed. Engl. 1995, 34, 303. (9) Fukunaga, T.; Harada, K.; Takashima, W.; Kaneto, K. Jpn. J. Appl. Phys. 1997, 36, 4466. (10) Bonfiglio, A.; Paradiso, R.; Di Zitti, E.; Ricci, D.; Bolognesi, A.; Porzio, W. Adv. Mater. Opt. Electron. 1993, 2, 295. (11) Vollmer, M. S.; Effenberger, F.; Stecher, R.; Gompf, B.; Eisenmenger, W. Chem. Eur. J. 1999, 5, 96. (12) Frank, E. R.; Chen, X.; Hamers, R. J. Surf. Sci. 1995, 334, L709. (13) Chen, X.; Frank, E. R.; Hamers, R. J. J. Vac. Sci. Technol. B 1996, 14, 1136. (14) Soukopp, A.; Glo¨ckler, K.; Ba¨uerle, P.; Sokolowski, M.; Umbach, E. Adv. Mater. 1997, 11, 902.
10.1021/la990196k CCC: $18.00 © 1999 American Chemical Society Published on Web 08/12/1999
Bi- and Terthiophenes
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Figure 1. Investigated bi- and terthiophenes (1a, 5-(dimethylamino)-5′-nitro-2,2′-bithiophene; 1b, 5-(dimethylamino)5′-dicyanovinyl-2,2′-bithiophene; 2a, 5-(dimethylamino)-5′′nitro-2,2′:5′,2′′-terthiophene; 2b, 5-(dimethylamino)-5′′-dicyanovinyl-2,2′:5′,2′′-terthiophene). freshly cleaved HOPG and MoS2 leads to monolayers under UHV conditions (base pressure ) 10-8 mbar). The evaporation temperatures were 117 °C for 1a, 178 °C for 1b, 192 °C for 2a, and 211 °C for 2b, and the evaporation rates were about 0.1-0.3 monolayer/s. The layer thickness during evaporation was controlled with a quartz microbalance. The STM investigations were carried out in two nearly identical home-built video-scanning tunneling microscopes: one operating in air (for the solution experiments) and the other integrated in a UHV chamber. All images shown are recorded in the constantheight mode with a scanning frequency of 1 kHz, corresponding to 4 frames/s, with mechanically cut Pt/Ir tips. For noise reduction, 4-8 frames were averaged on-line. All STM measurements were recorded at room temperature in situ directly after preparation. For the LEED measurements we used a four-grid combined LEED/Auger system (WA Technologies). The evaluation of the LEED images has been done by calculating the expected diffraction pattern from the unit cell parameters obtained by STM. A geometric approximation was used for the calculation including all symmetry equivalent domains and taking into account glide symmetry elements.
Results Due to the large number of investigated systems in this systematic study, only one STM image is shown for each molecule. Only in the cases where different molecular
Figure 2. High-resolution STM image of a monolayer of 1a on MoS2 in UHV (U ) 1.42 V, I ) 0.21 nA, 5 nm × 5 nm).
structures for the two preparation conditions were found is a direct comparison shown. A comprehensive overview of all results is given in Table 1. Figure 2 shows an STM image of a monolayer of 1a prepared under UHV conditions on MoS2. A structural proposal for the ordering is given by two molecules drawn with their van der Waals radii. The arrows mark the dimension and orientation of the unit cell. Obviously, the molecules lie parallel to each other and form lamellae. The polar end groups cannot be identified in the STM images, but due to the dipole moment of the molecules an alternating arrangement within the lamellae is assumed, leading to two molecules per unit cell. Additionally, in Figure 3 a LEED image of 1a on MoS2 together with a simulated LEED pattern is shown. The positions of the LEED reflexes are calculated on the basis of unit cell parameters obtained from the STM investigations; the intensities are adapted to the measured LEED pattern. This comparison shows that both methods yield the same molecular arrangement. As can be seen from Table 1, for all investigated systems the LEED and STM results agree well within the accuracy of the measurements ((5% for the unit cell vectors A B and B B , and (5° for angles). This
Table 1. Comparison of the Unit Cell Parameters (Length of the Unit Cell Vectors A B and B B , Angle between Them, Angle between A B and a Substrate Lattice Vector a b, Area per Molecule, and Number of Molecules per Unit Cell) of the Investigated Thiophenes Obtained with STM and LEED for Certain Combinations of Underlying Substrate and Surrounding Medium 1a
1b
2a
2b
method
medium
substrate
STM STM STM LEED STM STM STM LEED STM STM STM LEED STM STM STM LEED
solution UHV UHV UHV solution UHV UHV UHV solution UHV UHV UHV solution UHV UHV UHV
HOPG HOPG MoS2 MoS2 HOPG HOPG MoS2 MoS2 HOPG HOPG MoS2 MoS2 HOPG HOPG MoS2 MoS2
A (nm)
B (nm)
∠(A,B)
∠(A,a)
area (nm2)
molec
1.36 1.34 1.36 1.31 1.22 1.24 1.23 1.33 1.23 1.35 1.30 1.42 1.43 1.37 1.45 1.52
1.37 1.37 1.37 1.34 1.65 1.62 1.59 1.73 1.94 3.31 3.26 3.52 1.82 3.66 3.56 3.80
115° 116° 116° 117° 95° 95° 95° 101° 98° 92° 92° 91° 95° 91° 93° 94°
arbitrary 10° 26° 24° arbitrary 0° 27° 30° several 28° 3° 1° several 29° 14° 17°
0.84 0.83 0.84 0.78 1.01 1.00 0.97 1.13 1.19 1.12 1.06 1.25 1.30 1.25 1.29 1.42
2 2 2 2 2 2 2 2 2 4 4 4 2 4 4 4
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Figure 4. Highly resolved STM image of a monolayer of 1b on MoS2 in UHV (U ) 1.42 V, I ) 0.38 nA, 8 nm × 8 nm).
Figure 3. Comparison of measured LEED image (above) and calculated LEED pattern (below) of 1a on MoS2 (E ) 22 eV).
indicates that for evaporated monolayers the ordering on a nanometer scale found with STM is also present over macroscopic dimensions (the diameter of the electron beam of the LEED system is in the millimeter range) and not influenced by the STM tip. On HOPG, monolayers of 1a prepared by evaporation show the same molecular arrangement as that on MoS2 with the same unit cell parameters. As can be seen from Table 1, for all molecular systems prepared under UHV conditions we found the same unit cell parameters on HOPG and MoS2. The ordering of molecular monolayers seems not to be affected by weakly interacting substrates, especially when electrostatic interactions between the molecules cannot be neglected.16,17 STM investigations of 1a prepared at the liquid/solid interface on HOPG in phenyloctane also show the same molecular ordering as that for those under UHV conditions, if again an alternating arrangement of the molecules is assumed. In Table 1 all values for 1a agree well within the accuracy of the measurements. In this case the
Figure 5. Molecular resolution STM image of 2a on MoS2 in UHV (U ) 1.10 V, I ) 0.18 nA, 10 nm × 10 nm).
interaction with the solvent molecules seems to have no significant influence on the monolayer ordering. The ordering of 1b is shown in Figure 4. The image shows a monolayer on MoS2 prepared in UHV. The results for HOPG as substrate as well as for monolayers prepared at the liquid/solid interface show the same overall structure. Again, the molecules arrange in lamellae, but now the angle between A B and B B is reduced and, due to the larger polar end group, the area per molecule is larger. To study the influence of the thiophene core on the molecular arrangement, terthiophene derivatives with the same donor and acceptor end groups have been investigated. For example in Figure 5 a STM image of 2a prepared by evaporation on MoS2 is shown. The molecules again form lamellae, but now their orientation in adjacent lamellae is no longer parallel to each other. Always two
Bi- and Terthiophenes
Figure 6. High-resolution STM image of 2a on HOPG in phenyloctane (U ) 0.68 V, I ) 4.55 nA, 9 nm × 9 nm).
molecules in a lamella form a kind of dimer. If again an alternating arrangement of the dipole moments along the lamellae is assumed, this leads to a herringbone structure with four molecules per unit cell. Opposite to the case of the bithiophenes, the molecular ordering of 2a prepared at the liquid/solid interface (Figure 6) is different from that found under UHV conditions. Here the molecules in adjacent lamellae lie parallel to each other with their molecular axes perpendicular to the lamellae direction. This leads to unit cells composed of only two molecules compared to four under UHV conditions. So in this case the interaction of the ordered molecules either with the solvent molecules or with the solved thiophenes above the monolayer seems to induce an arrangement different from that found for UHV preparation. As illustrated by Figure 7, the changing of the polar end group (2b) did not change the ordering of terthiophenes significantly for monolayers prepared under UHV conditions. The molecular arrangement with 4 molecules per unit cell is similar to the structure of 2a, but the area per molecule is much larger. In Figure 8 the result obtained for 2b at the liquid/solid interface is shown. Here again, always two molecules form a kind of dimer, but the molecules in adjacent dimers are parallel to each other, resulting in a structure with only two molecules per unit cell. Remarkable are the frequent defects where always one molecule is missing (shown by the small arrows in Figure 8). Besides the molecular ordering itself, the orientation of the overlayer with respect to the substrate is important. For monolayers prepared under UHV conditions, we found for all molecules well-defined azimuthal orientations with respect to the substrate. The corresponding angles between the unit cell vector A B and one substrate lattice vector B B are also given in Table 1. Whether these overlayers are commensurate or coincident in any sense cannot be decided from our experimental results. At the liquid/solid interface the situation is more complex. Here we found arbitrary azimuthal orientations for the bithiophenes 1a and 1b. However, monolayers of the terthiophene derivatives 2a and 2b prepared at the liquid/solid interface are between these two possibilities.
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Figure 7. Highly resolved STM image of 2b on MoS2 in UHV (U ) 1.26 V, I ) 0.19 nA, 10 nm × 10 nm).
Figure 8. Molecular resolution STM image of 2b on HOPG in phenyloctane (U ) 1.26 V, I ) 0.09 nA, 9 nm × 9 nm). The small arrows mark missing molecules.
They exhibit not one but a small number of well-defined azimuthal orientations with respect to the substrate. In a next step we investigated the influence of different solvents on the observed order. For comparison we used as solvent dodecane, which itself (as phenyloctane) does not build up monolayers at room temperature. One result of this variation is shown in Figure 9. A solution of 2a in dodecane leads to a monolayer structure which obviously cannot be explained by the solved molecule but agrees well with the known STM images of pure aklyl chains.18 (15) Effenberger, F.; Wu¨rthner, F.; Steybe, F. J. Org. Chem. 1995, 60, 2082. (16) Petersen, J.; Strohmaier, R.; Gompf, B.; Eisenmenger, W. Surf. Sci. 1997, 389, 329. (17) Ludwig, C.; Gompf, B.; Petersen, J.; Strohmaier, R.; Eisenmenger, W. Z. Phys. B 1994, 93, 365.
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Figure 9. STM image of 2b on HOPG in dodecane. Obviously not the solved molecules but the solvent dodecane itself builds up a monolayer (U ) 0.88 V, I ) 0.23 nA, 9 nm × 9 nm).
In this case the formation of stable solvent monolayers induced by the solute seems to be the only possible explanation for the observed structure. This phenomenon is reproducible: not caused by an accidental pollution of the solution or the STM tip and not limited to this special case. We observe this solute-induced formation of solvent monolayers also for 2b and a variety of other molecules, such as, for example, diphenylpolyenes. Discussion For well-ordered monolayers the molecules should have a symmetry and shape suitable for close packing in periodic (18) Gilbert, E. P.; White, J. W.; Senden, T. J. Chem. Phys. Lett. 1994, 227, 443.
Stecher et al.
structures.19 For epitaxially ordered monolayers, additionally a complex balance between the intermolecular and molecule-substrate interactions is necessary. On weakly interacting substrates the molecule-to-substrate interaction has no significant influence on the observed unit cell parameters, but it is important for the orientation with respect to the substrate.16,17 In solution three additional interactions have to be considered: the molecule-solvent, the solvent-solvent, and the solventsubstrate interactions. For the solved bithiophenes, all three contributions seem to be too small to have a significant influence on the ordering, but the moleculesolvent interaction leads to a loss of the phase relation between monolayer and substrate. In the case of the terthiophenes, this interaction also leads to a rearrangement of the molecular order. Using dodecane as solvent shows that all three interactions can become important, leading even to the solute-induced formation of solvent monolayers. In conclusion, our investigations have shown that adsorption from solution is an alternative method to the evaporation technique for preparing well-ordered films. This allows the formation of monolayers for a wide variety of molecules even if they are not thermally stable. In some cases the influence of the solvent on the ordering is small, but in general the additional interactions cannot be neglected. However, this opens the possibility to tailor specific molecular arrangements not only by varying the intermolecular interactions but also by varying the surrounding environment. Acknowledgment. We would like to thank Dr. F. Steybe and Prof. F. Effenberger for the synthesis of the investigated molecules and Prof. W. Eisenmenger for helpful discussions. For financial support we thank the Deutsche Forschungsgemeinschaft (SFB 329). LA990196K (19) Karl, N.; Gu¨nther, C. Cryst. Res. Technol. 1999, 34, 243.
