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Scanning tunneling microscopy investigation of tricycloquinazoline liquid crystals on gold
Thin Solid Films 358 (2000) 241±249 www.elsevier.com/locate/tsf
Scanning tunneling microscopy investigation of tricycloquinazoline liquid crystals on gold Renate Hiesgen a,*, Holger SchoÈnherr b,1, Sandeep Kumar b,2, Helmut Ringsdorf b, Dieter Meissner c a
b
Technische UniversitaÈt MuÈnchen, James-Franck-Strasse 1, D-85748 Garching, Germany Institut fuÈr Organische Chemie, J.-Gutenberg-UniversitaÈt Mainz, D-55099 Mainz, Germany c AQR, Wendelinusstrasse 85, D-52428 JuÈlich, Germany
Received 19 March 1999; received in revised form 14 July 1999; accepted 25 August 1999
Abstract Self-assembled monolayers (SAMs) of hexaalkylthioether derivatives of tricycloquinazoline (TCQ) on Au(111) and tungsten diselenide (WSe2) were investigated by scanning tunneling microscopy (STM). The Au(111) surfaces were found to be etched by the thioether containing solutions. Corroded surfaces which are similar to gold surfaces that were coated with SAMs of thiols or disul®des were revealed by STM. Atomic adsorption spectroscopy proved that an amount of gold that corresponds to ca. 30% of a monolayer was dissolved in the assembly solutions. On gold, the aromatic cores of the molecules were found to be in face-on orientation. The alkyl substituents were in most cases folded upwards and shielding the aromatic cores. Only after long immersion times were crystalline areas observed locally, in which the alkyl chains were lying ¯at on the gold surface. On WSe2 mono- and multilayers of TCQ molecules in face-on orientation could be imaged by STM. The observed columnar structures displayed long-range order. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Monolayers; Self assembled monolayers; Scanning tunnelling microscopy; Liquid crystals
1. Introduction Organic materials with interesting electronic properties are becoming more and more commercially important. They are e.g. widely used as photoconducting materials in xerographic applications. Organic solar cells based on semiconducting organic ®lms have been reported [1]. Liquid crystal displays are another example of excellent performance of organic materials in electronic devices [31]. Some discotic liquid crystals possess extremely high hole mobilities which are comparable to inorganic semiconductors [2]. Therefore also these discotic liquid crystals are promising materials for electronic devices. A novel class of discotic liquid crystals with a tricycloquinazoline (TCQ) core has been synthesized by one of us and his co-workers [3]. These hexaalkylthioethers of TCQ possess interesting electronic properties. TCQs are consid* Corresponding author. Tel.: 149-49-089-28912540; fax: 149-089289-12536. E-mail address: [email protected] (R. Hiesgen) 1 Current address: Faculty of Chemical Technology, MTP, P.O. Box 217, 7500 AE Enschede, The Netherlands. 2 Current address: Centre for Liquid Crystal Research, P.O. Box 1329, Jalahalli, Bangalore-560 013, India.
ered to be electron de®cient molecules, while most of the known discotic liquid crystals possess an electron-rich core [3,4]. The structure of the hexahexadecylthioether derivative of TCQ is shown in Fig. 1. The electronic conductivity of thick ®lms of these molecules, which were oriented in a magnetic ®eld, can be enhanced by the addition of potassium or aluminum chloride [4]. The resistivity of these ®lms is anisotropic resulting in a higher conductivity parallel to the columns. In thin ®lms of TCQ on graphite the molecules were shown by scanning tunneling microscopy (STM) [5±7] to be arranged in a highly ordered layer in which the molecules are oriented face-on [8]. Self-assembled monolayers (SAMs) of organosulfur compounds, namely thiols and disul®des, have attracted much interest in recent years [9± 11]. While usually thiols and disul®des are used to modify gold surfaces due to the rather strong interaction of the sulfur atoms with the gold (chemisorption), thioethers have been considered less favorable to form stable, highly oriented monolayers owing to the rather weak interaction with the substrate (physisorption) [12]. By using multiple anchor groups per molecule however, this drawback has been overcome by Reinhoudt et al. [13±15]. SAMs of tetrathioether derivatives of calixarenes or resorcinarenes were shown by a variety of techniques including atomic force
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Fig. 1. Chem Of®ce (CambridgeSoft, Cambridge MA, USA) images of a TCQ-16 molecule with spread out alkyl chains.
microscopy to be indeed highly ordered and stable [16]. A similar strategy of multiple attachment points was followed in an earlier study on the formation and the structure of SAMs of several discotic and discoid triphenylene hexaalkylthioethers as well as TCQ hexaalkylthioethers on gold [17]. Different analytical and spectroscopic techniques such as FTIR and contact angle measurements were used to obtain indirect and integral information on the structure of these layers [17]. In particular hexadodecylthiotricycloquinazoline (TCQ-12) and hexahexadecylthiotricycloquinazoline (TCQ-16) were shown to be assembled face-on onto gold surfaces with long hydrocarbon substituents covering the aromatic core. In this current study, mono- and multilayers of TCQ-12 and TCQ-16 on Au (111) on WSe2 were investigated by STM in order to obtain structural information in real space, including orientation, order and interaction of the molecules with the substrate.
2. Experimental details TCQ-12 and TCQ-16 were synthesized and highly puri®ed following a route developed by Kumar and coworkers [3]. The composition of the substances was controlled by 1H NMR, 13C NMR, mass spectrometry, and elemental analysis and the phase transitions have been determined by differential scanning calorimetry. The results indicated a high purity of the samples [17]. A contamination of the substance with thiols has been ruled out based on gas chromatography results. Freshly vacuum evaporated gold ®lms of 200 nm thickness, deposited on heated glass, have been used as substrates for one part of the STM experiments. The resulting gold samples exhibit gold crystals with about 100±500 nm wide terraces in (111) orientation. For another part of the measurements a gold crystal was prepared from a gold wire of 1 mm diameter. The gold wire was heated in the
R. Hiesgen et al. / Thin Solid Films 358 (2000) 241±249
reducing part of a gas ¯ame following the method developed by Clavilier [18]. After melting the tip the resulting gold ball was quenched in ultra pure water. An STM image of this ¯ame-annealed gold crystal before adsorption of an organic layer is shown in Fig. 2. In addition, a freshly cleaved tungsten diselenide crystal has been used as substrate for some of the measurements. After cleaving with adhesive tape the surface of the WSe2 sample is extremely clean due to its van der Waals character. It is atomically ¯at over distances up to micrometers. Monolayers of the TCQ molecules were prepared from 1:0±5:0 £ 1024 M solutions in CH2Cl2. Films were deposited by two different techniques. One set of samples was coated by immersing gold substrates into the solution of TCQ for 12 to 24 h (self-assembled ®lms). This procedure has previously been shown to yield self-assembled monolayers due to the strong physisorption of the six sulfur atoms of the thioether molecules [17]. The samples were rinsed with pure solvent and dried in a high purity nitrogen stream. A second set of samples was prepared by placing a drop of TCQ solution onto the gold or WSe2 substrate. After the evaporation of the solvent the samples were carefully rinsed with pure CH2Cl2 and dried (solution-cast ®lms). The STM measurements were performed with a Nanoscope III scanning tunneling microscope in ambient air and in dry nitrogen environment. As tunneling tips mechanically cut PtIr (80:20) tips were used. Small tunneling setpoint currents of typically 100 to 200 pA were chosen in order to keep a large distance between the substrate and the tunneling tip and thereby avoiding mechanical damage of the organic layer with the tip [19]. The bias voltages were
Fig. 2. STM image of the surface of a ¯ame-annealed gold sample before adsorption of the tricyclochinazoline molecules. The imaged area is 1 mm 2.
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controlled not to exceed 1 V to prevent damage of the organic coating by high electric ®elds between the tunneling tip and the gold surface. The polarity of the bias voltage was found to have no detectable in¯uence on the images. The AAS (atomic absorption spectrometry) measurements were made with a Perkin Elmer 3030 spectrometer in a graphite oven after dissolution in HCl.
3. Results and discussion 3.1. Interface reactions After the deposition of thin ®lms of TCQ all the gold substrates clearly showed corrosion of the initial smooth surface of the Au(111) terraces (Figs. 2 and 3). The corrosion of the gold terraces was observed by STM for samples prepared by both methods, self-assembly from dilute solution (Fig. 3a,c,d) and solvent casting with thorough rinse, respectively (Fig. 3b). The observed surface structure is reminiscent of gold surfaces modi®ed with self-assembled monolayers of alkanethiols, see [19±26] and references therein, disul®des [25], and hydrogen sul®de [27] and thiophene [28] on Au(111). For thiols, the corrosion of the gold is observed for self-assembly from solution as well as the gas phase. In addition, Larsen et al. reported the same depressions for SAMs prepared by microcontact printing with thiol-inked PDMS stamps [22]. The initial interpretation of the observed pits was an etching process by the thiol solution as gold could be detected spectroscopically in the assembly solution [19±21]. The depressions were shown to be depressions in the underlying Au(111) [19± 21]. Recent STM studies on self-assembly of thiols from the gas phase present evidence that a surface relaxation phenomenon is very probably the origin of the depressions for assembly from the gas phase [23,24]. Strikingly, etching by the thiol in solution can be ruled out a priori for gas phase assembly as well as for microcontact printed SAMs [22]. Although the process in solution is not completely understood, a complexation of the gold atoms by the sulfur would be a possible explanation for both phenomena. It would raise the mobility of the surface gold atoms leading to surface relaxation in the gas phase and to etching of the surface due to the dissolution of gold in liquid environment. For the SAMs of TCQ-12 and TCQ-16 investigated here, the corrosion of the gold surface after immersion of the electrode into the dye solution for 12 h results in the dissolution of ca. 1/3 of the top gold layer as detected by atomic absorption spectroscopy of the assembly solutions. In pure methylene chloride the concentration of gold was determined to be 2 mg/l which is the detection limit of the method. STM revealed no depressions in Au samples that were in contact with dichloromethane only. In solutions used for self assembly of TCQ on gold the concentration was found to be 20 mg/l. This result proves that the TCQ
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Fig. 3. STM images of different gold surfaces after contact with a TCQ-16 solution in dichloromethane: (a) evaporated gold (111) surface after a 12 h immersion and subsequent rinsing with pure CH2Cl2; (b) ¯ame annealed gold (111) crystal after letting a drop of the TCQ-16/methylene chloride solution evaporate on the surface. The arrows show the starting of hole formation on the terraces; (c) the ¯ame annealed gold (111) surface shown in Fig. 2 after 12 h of adsorption in TCQ-16 solution; (d) enlarged part of (c).
containing solution effectively etches away the top layers of the gold. The process seems to be rather fast as the depressions could be detected by STM after less than 30 min of contact between the thioether solution and the gold. At this point we can only speculate that the process described here is similar to the thiol solution deposition onto Au(111) and may be caused by a complexation of gold atoms by the sulfur. To our knowledge this is the ®rst time that etching of the gold is directly observed for self-assembly of thioethers on gold.
3.2. Structure and orientation in SAMs of TCQ on Au(111) 3.2.1. Solution cast ®lms Solution cast ®lms of TCQ-16 were imaged with STM before and after rinsing with methylene chloride. Prior to rinsing the surface is completely covered with liquid crystal molecules (Fig. 4a). The islands seen in this ®gure can be attributed to a ®rst and a second overlayer of TCQ molecules in face-on orientation. The step height of these elevated features was measured to be 0.5 nm for the ®rst
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Fig. 4. (a) STM image of a TCQ-16 covered ¯ame annealed gold (111) crystal after evaporation of a drop of the TCQ-16/methylene chloride solution surface. The sample has not been rinsed afterwards. The imaged area is l00 nm 2; (b) stepped area on the same sample as in (a), an area of the left bottom part is enlarged and displayed below; (c) higher resolution image from another part of the sample; (d) side view of a TCQ-16 molecule with upwards folded alkyl chains; (e) top view of a TCQ-16 molecule.
layer, whereas the diameter was found to be between 2.5 nm and 6.3 nm. The steps in the Au(111) lattice were estimated to be 0.24 nm which is in agreement with the literature value of 0.236 nm [22]. Therefore it can be concluded that the observed islands are not gold islands but a ®rst or sometimes two overlayers of the TCQ-16 molecules. The second overlayer can be removed by carefully rinsing the samples as con®rmed by STM (Fig. 5). These observations are in full agreement with earlier QCM and FTIR results which showed that the initial adsorption of molecules leads to loosely bound multiple layers which can be removed by carefully rinsing with the solvent [17].
3.2.2. TCQ monolayer on gold In Fig. 4b an image with higher resolution is shown. The gold terraces are covered with liquid crystals. Here ring-like structures can be observed. No long-range order is visible in the ®lm. The diameter of the rings were found to be between 1.4 nm and 2.5 nm. In Fig. 4c a higher resolved image from another part of the sample is shown where the arrows mark three ring-like structures with a dark spot in the middle which are attributed to aromatic ring systems of TCQ molecules with a diameter of 1.4 nm. These values are consistent with a TCQ-16 layer composed of face-on oriented molecules with upfolded alkane chains as is schematically drawn
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Fig. 5. (a) STM image of TCQ-16 molecules on ¯ame-annealed gold (111) after subsequent rinsing with the pure solvent. The imaged area is 80 nm 2. (b) Model of a TCQ-16 molecule with parallel spread out alkyl chains. An appropriately sized image as in (b) is positioned beside the enlarged region. (c) STM image of a TCQ-16 molecular layer on ¯ame-annealed gold (111) after 12 h of adsorption and subsequent rinsing with the pure solvent. The imaged area is 10 nm 2.
as a side view in Fig. 4d and as a top view in Fig. 4e. The alkyl chains are of course ¯exible, so that the diameters of the rings formed by the ends of the alkyl chains can vary within certain limits. If one assumes this diameter to be a little larger than the width of the aromatic ring, which is 1.3 nm and to be smaller than the width of the ¯at molecule given in Fig. 1 (4.4 nm), the measured values are in good agreement with these numbers. A top view of molecules arranged in this way would display a ring-like surface struc-
ture. The FTIR and contact angle measurements made by SchoÈnherr et al. are in good agreement with these results [17]. The observations indicate that the alkyl chains are folded upwards over the aromatic cores of the molecules at least to some extent. In Fig. 5a, a STM image of a stepped area of the TCQ-16 covered ¯ame-annealed gold crystal is displayed. On most parts of the surface no obvious ordering of the molecules is visible. Only on the upper left side of the image there are
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Fig. 6. STM images of TCQ-16 molecules on WSe2 (a) after evaporation of a drop of TCQ-16 containing methylene chloride solution. The imaged area is 150 nm 2, in the inset the pro®le along the drawn line is presented. (b) Height pro®le along the line, (c) after evaporation of a drop of TCQ-16 containing methylene chloride solution; the imaged area is 200 nm 2.
regions with parallel structures, which can be assigned to small ordered regions formed by the TCQ-16 molecules. For a better view a magni®ed image of this region is displayed on the left side of this ®gure. The stripe-like features have a length between 1.5 nm and 3.5 nm and are assumed to be formed by the alkyl chains of the TCQ-16 molecules. Here the alkyl chains seem to be arranged parallel to the surface as can be found in thin ®lms of TCQ on graphite surfaces, where the alkyl chains crystallize forming large ordered domain structures [8]. A model of the proposed arrangement of the side chains of the molecules is presented in Fig. 5b. An appropriately sized model of a molecule has been positioned beside the STM images
for a comparison between the dimensions of the molecule chains and the structures in the image. For a two-dimensional structure model the resolution is not high enough to really distinguish between individual molecules. Similar structures have been found with monolayers formed by hexakisalkoxy-triphenylene molecules on graphite, which show a comparable structure to that of the TCQ molecules used here [29]. On gold correspondingly crystallized regions in TCQ-16 layers found with the STM are only small. After 12 h of immersion and subsequent rinsing of the sample some higher degree of order of the surface structure can be found here, too. In Fig. 5c a high resolution STM
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image is displayed, which shows an area of 10 nm 2 on the annealed gold crystal. Here, two parallel oriented bundles are visible, which have dimensions comparable to those of the TCQ-molecule drawn for comparison on the image. These parallel structures consist of a repeating unit with a distance of 0.25 nm, which is consistent with the repeating distance of the methylene groups along the alkyl chains [30]. These structures may therefore be attributed to alkyl chains of the TCQ-16 molecules oriented parallel to the surface. We can conclude that STM images revealed that the TCQ molecules are arranged with some short-range order in both solution-cast and self-assembled ®lms on Au(111). In these areas the alkyl chains of the face-on adsorbed discotic molecules were found to be lying in parallel bundles ¯at on the gold surface. 3.3. TCQ on WSe2 TCQ molecules have also been adsorbed on tungsten diselenide crystals. On most parts of this surface not even traces of the liquid crystal molecules have been found which may be caused by sweeping away the loosely bound ®lms with the STM tip. Since the tungsten diselenide surface is a non-reactive van der Waals surface, the adhesion of the TCQ-16 molecules to the surface is weak. However, on more stepped parts of the surface which provide anchor points for the molecules well de®ned structures formed by the TCQ molecules were observed as shown in Fig. 6. Fig. 6a shows a STM image of the surface of a tungsten diselenide crystal, which had been immersed in a TCQ-16 containing solution of methylene chloride for 12 h. In Fig. 6a a completely covered area is exhibited. Here a drop of TCQ-16 containing solution has been evaporated on the surface without rinsing the sample. The molecules are ordered in parallel rows with a periodicity of 15 nm and a height of 2 to 3 nm with a minimum width of the rows of 5 nm. The height pro®le measured along the line is shown in Fig. 6b. These dimensions indicate that this TCQ-16 coverage is a multilayer ®lm. The appearance of the rows resembles the surface structure on gold as described above. The image is not clear enough to distinguish an internal ordering of the molecules, but at some regions the surface of the rows exhibits a spotted structure, where round features with the dimensions of molecules are arranged. Therefore, it is more likely that the TCQ-16 molecules form stacks parallel to the surface than perpendicular. This would be in accordance with the observation reported by Boden et al. [4] for a 200 mm thick ®lm of hexakis(hexylthio) TCQ molecules. From the geometrical dimensions of the rows it can be assumed that 1 to 3 molecules are arranged side by side. There are also some defects visible, where parts of the ®lm are missing, presumably due to scratching by the STM tip. Another area on the WSe2 crystal, where TCQ-16 molecules have been imaged, is displayed in Fig. 6c. Here the molecules also form rows which are parallel to each other.
The different rows are shifted against each other and there are defects visible in the layer. The width of these rows is also about 15 nm, but their height is only 0.9 nm, which is just the length of the respective alkyl chain. If it is assumed that the chains are folded up, this TCQ-16 layer is a monolayer of the liquid crystal molecules. In this case the molecules are also oriented with their aromatic core ¯at on the surface. From the STM image no additional information can be obtained on the ®ne structure of these rows. 4. Conclusions Hexaalkylthioether derivatives of TCQ were shown to form self-assembled monolayers on Au(111) by both selfassembly from dilute solution and solution casting with a subsequent thorough rinse. Atomic adsorption spectroscopy proved that the Au(111) surfaces are etched by the TCQ containing solutions. STM revealed a corroded surface which is similar to gold surfaces that were coated with self-assembled monolayers of thiols or disul®des. During the assembly multilayers of the organic molecules are formed on the surface. These multiple layers can be removed by thoroughly rinsing the surface with pure solvent. The remaining monolayer is ®rmly attached to the surface due to the multiple attachment points per molecule. The aromatic cores of the molecules were found to be oriented in face-on orientation. The alkyl substituents were in most cases folded upwards and thus shielding the aromatic cores. After long immersion times crystalline areas were revealed by STM. In these areas the alkyl chains were lying ¯at on the gold surface. This order was found only locally. On WSe2 mono- and multilayers of TCQ molecules in face-on orientation were imaged by STM as well as columnar structures which displayed long range order. Acknowledgements We gratefully acknowledge ®nancial support from the VW foundation under contracts No. I/69 880 and I/72 365, the Commission of the European Community under contract No. JOR3-CT 96-0106, the Bundesminister fur Bildung und Forschung (BMBF) under contract No. 03 M 4084 AO, and the Deutsche Forschungsgemeinschaft (DFG) under contracts No. ME 855/3-1 and STI 74/9-2. References [1] D. WoÈhrle, D. Meissner, Adv. Mater. 3 (1991) 129±138. [2] D. Adam, P. Schuhmacher, J. Simmerer, L. HaÈussling, K. Siemensmeyer, K. Etzbach, H. Ringsdorf, D. Haarer, Nature 65 (1994) 456± 459. [3] E. Keinan, S. Kumar, S.P. Singh, R. Ghirlandos, E. Wachtel, J. Liq. Cryst. 1 (1992) 157. [4] N. Boden, R.C. Borner, R.J. Bushby, J. Clements, J. Am. Chem. Soc. 116 (1994) 10807±10808. [5] J.S. Foster, J.W. Frommer, Nature 333 (1988) 542.
R. Hiesgen et al. / Thin Solid Films 358 (2000) 241±249 [6] W. Mizutani, M. Shigeno, M. Ohne, M. Suginoya, K. Kajimura, M. Ono, J. Vac. Sci. Technol. B9 (1991) 1102. [7] D.P.E. Smith, H. HoÈrber, V. Gerber, G. Binnig, Science (1989) 24543. [8] J. Rego, private communication. [9] A. Ulman, Introduction to Ultrathin Films, From Langmuir±Blodgett Films to Self-Assembly, Academic Press, Boston, MA, 1991. [10] L.H. Dubois, R.G. Nuzzo, Annu. Rev. Phys. Chem. 43 (1992) 437± 463. [11] E. Delamarche, B. Michel, H.A. Biebuyck, Ch. Gerber, Adv. Mater. 8 (1996) 719. [12] D.L. Mlara, Biosensors Bioelectron. 10 (1995) 771. [13] E.U. Thoden van Velzen, J.F.J. Engbersen, D.N. Reinhoudt, J. Am. Chem. Soc. 116 (1994) 3597±3598. [14] B.H. Huisman, D.M. Rudkevich, F.C.J.M. van Veggel, D.N. Reinhoudt, J. Am. Chem. Soc. 118 (1996) 3523±3524. [15] M.W.J. Beulen, B.-H. Huisman, P.A. van der Heijden, et al. Langmuir 12 (1996) 6170. [16] H. SchoÈnherr, G.J. Vancso, B.-H. Huisman, F.C.J.M. van Veggel, D.N. Reinhoudt, Langmuir 13 (1997) 1567±1570. [17] H. SchoÈnherr, F.J.B. Kremer, S. Kumar, et al. J. Am. Chem. Soc. 118 (1996) 13051. [18] J. Clavilier, J. Electroanal. Chem. 107 (1980) 211.
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[19] C. SchoÈnenberger, J.A.M. Sondag-Huethorst, J. Jorritsma, L.G.J. Fokkink, Langmuir 10 (1994) 611±614. [20] K. Edinger, A. GoÈlzhaÈuser, K. Demota, C. WoÈll, M. Grunze, Langmuir 9 (1993) 4±8. [21] J.P. Bucher, L. Santesson, K. Kern, Langmuir 10 (1994) 980±983. [22] N.B. Larsen, H. Biebuyck, E. Delamarche, B. Michel, J. Am. Chem. Soc. 119 (1997) 3017. [23] G.E. Poirier, Langmuir 13 (1999) 2019. [24] M.H. Dishner, J.C. Hemminger, F.J. Feher, Langmuir 13 (1997) 2318. [25] M. Jaschke, H. SchoÈnherr, H. Wolf, H. Ringsdorf, M.K. Besocke, E. Bamberg, J.-J.J. Butt, Phys. Chem. 100 (1996) 2290±2301. [26] R.L. McCarley, D.J. Dunaway, R.J. Willicut, Langmuir 9 (1993) 2775±2777. [27] I. Touzov, C.B. Gorman, Langmuir 13 (1997) 4850±4854. [28] M.H. Dishner, J.C. Hemminger, F.J. Feher, Langmuir 12 (1996) 6176±6178. [29] L. Askadskaya, C. Boeffel, J.P. Rabe, Ber. Bunsenges. Phys. Chem. 97 (1993) 517±521. [30] R. Kassing (Ed.), Scanning Microscopy Springer, Berlin, 1992, pp. 125. [31] C. Weder, C. Sarwa, A. Montali, C. Bastiaansen, P. Smith, Science 279 (1998) 835.
Scanning tunneling microscopy investigation of tricycloquinazoline liquid crystals on gold Renate Hiesgen a,*, Holger SchoÈnherr b,1, Sandeep Kumar b,2, Helmut Ringsdorf b, Dieter Meissner c a
b
Technische UniversitaÈt MuÈnchen, James-Franck-Strasse 1, D-85748 Garching, Germany Institut fuÈr Organische Chemie, J.-Gutenberg-UniversitaÈt Mainz, D-55099 Mainz, Germany c AQR, Wendelinusstrasse 85, D-52428 JuÈlich, Germany
Received 19 March 1999; received in revised form 14 July 1999; accepted 25 August 1999
Abstract Self-assembled monolayers (SAMs) of hexaalkylthioether derivatives of tricycloquinazoline (TCQ) on Au(111) and tungsten diselenide (WSe2) were investigated by scanning tunneling microscopy (STM). The Au(111) surfaces were found to be etched by the thioether containing solutions. Corroded surfaces which are similar to gold surfaces that were coated with SAMs of thiols or disul®des were revealed by STM. Atomic adsorption spectroscopy proved that an amount of gold that corresponds to ca. 30% of a monolayer was dissolved in the assembly solutions. On gold, the aromatic cores of the molecules were found to be in face-on orientation. The alkyl substituents were in most cases folded upwards and shielding the aromatic cores. Only after long immersion times were crystalline areas observed locally, in which the alkyl chains were lying ¯at on the gold surface. On WSe2 mono- and multilayers of TCQ molecules in face-on orientation could be imaged by STM. The observed columnar structures displayed long-range order. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Monolayers; Self assembled monolayers; Scanning tunnelling microscopy; Liquid crystals
1. Introduction Organic materials with interesting electronic properties are becoming more and more commercially important. They are e.g. widely used as photoconducting materials in xerographic applications. Organic solar cells based on semiconducting organic ®lms have been reported [1]. Liquid crystal displays are another example of excellent performance of organic materials in electronic devices [31]. Some discotic liquid crystals possess extremely high hole mobilities which are comparable to inorganic semiconductors [2]. Therefore also these discotic liquid crystals are promising materials for electronic devices. A novel class of discotic liquid crystals with a tricycloquinazoline (TCQ) core has been synthesized by one of us and his co-workers [3]. These hexaalkylthioethers of TCQ possess interesting electronic properties. TCQs are consid* Corresponding author. Tel.: 149-49-089-28912540; fax: 149-089289-12536. E-mail address: [email protected] (R. Hiesgen) 1 Current address: Faculty of Chemical Technology, MTP, P.O. Box 217, 7500 AE Enschede, The Netherlands. 2 Current address: Centre for Liquid Crystal Research, P.O. Box 1329, Jalahalli, Bangalore-560 013, India.
ered to be electron de®cient molecules, while most of the known discotic liquid crystals possess an electron-rich core [3,4]. The structure of the hexahexadecylthioether derivative of TCQ is shown in Fig. 1. The electronic conductivity of thick ®lms of these molecules, which were oriented in a magnetic ®eld, can be enhanced by the addition of potassium or aluminum chloride [4]. The resistivity of these ®lms is anisotropic resulting in a higher conductivity parallel to the columns. In thin ®lms of TCQ on graphite the molecules were shown by scanning tunneling microscopy (STM) [5±7] to be arranged in a highly ordered layer in which the molecules are oriented face-on [8]. Self-assembled monolayers (SAMs) of organosulfur compounds, namely thiols and disul®des, have attracted much interest in recent years [9± 11]. While usually thiols and disul®des are used to modify gold surfaces due to the rather strong interaction of the sulfur atoms with the gold (chemisorption), thioethers have been considered less favorable to form stable, highly oriented monolayers owing to the rather weak interaction with the substrate (physisorption) [12]. By using multiple anchor groups per molecule however, this drawback has been overcome by Reinhoudt et al. [13±15]. SAMs of tetrathioether derivatives of calixarenes or resorcinarenes were shown by a variety of techniques including atomic force
0040-6090/99/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00685-9
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Fig. 1. Chem Of®ce (CambridgeSoft, Cambridge MA, USA) images of a TCQ-16 molecule with spread out alkyl chains.
microscopy to be indeed highly ordered and stable [16]. A similar strategy of multiple attachment points was followed in an earlier study on the formation and the structure of SAMs of several discotic and discoid triphenylene hexaalkylthioethers as well as TCQ hexaalkylthioethers on gold [17]. Different analytical and spectroscopic techniques such as FTIR and contact angle measurements were used to obtain indirect and integral information on the structure of these layers [17]. In particular hexadodecylthiotricycloquinazoline (TCQ-12) and hexahexadecylthiotricycloquinazoline (TCQ-16) were shown to be assembled face-on onto gold surfaces with long hydrocarbon substituents covering the aromatic core. In this current study, mono- and multilayers of TCQ-12 and TCQ-16 on Au (111) on WSe2 were investigated by STM in order to obtain structural information in real space, including orientation, order and interaction of the molecules with the substrate.
2. Experimental details TCQ-12 and TCQ-16 were synthesized and highly puri®ed following a route developed by Kumar and coworkers [3]. The composition of the substances was controlled by 1H NMR, 13C NMR, mass spectrometry, and elemental analysis and the phase transitions have been determined by differential scanning calorimetry. The results indicated a high purity of the samples [17]. A contamination of the substance with thiols has been ruled out based on gas chromatography results. Freshly vacuum evaporated gold ®lms of 200 nm thickness, deposited on heated glass, have been used as substrates for one part of the STM experiments. The resulting gold samples exhibit gold crystals with about 100±500 nm wide terraces in (111) orientation. For another part of the measurements a gold crystal was prepared from a gold wire of 1 mm diameter. The gold wire was heated in the
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reducing part of a gas ¯ame following the method developed by Clavilier [18]. After melting the tip the resulting gold ball was quenched in ultra pure water. An STM image of this ¯ame-annealed gold crystal before adsorption of an organic layer is shown in Fig. 2. In addition, a freshly cleaved tungsten diselenide crystal has been used as substrate for some of the measurements. After cleaving with adhesive tape the surface of the WSe2 sample is extremely clean due to its van der Waals character. It is atomically ¯at over distances up to micrometers. Monolayers of the TCQ molecules were prepared from 1:0±5:0 £ 1024 M solutions in CH2Cl2. Films were deposited by two different techniques. One set of samples was coated by immersing gold substrates into the solution of TCQ for 12 to 24 h (self-assembled ®lms). This procedure has previously been shown to yield self-assembled monolayers due to the strong physisorption of the six sulfur atoms of the thioether molecules [17]. The samples were rinsed with pure solvent and dried in a high purity nitrogen stream. A second set of samples was prepared by placing a drop of TCQ solution onto the gold or WSe2 substrate. After the evaporation of the solvent the samples were carefully rinsed with pure CH2Cl2 and dried (solution-cast ®lms). The STM measurements were performed with a Nanoscope III scanning tunneling microscope in ambient air and in dry nitrogen environment. As tunneling tips mechanically cut PtIr (80:20) tips were used. Small tunneling setpoint currents of typically 100 to 200 pA were chosen in order to keep a large distance between the substrate and the tunneling tip and thereby avoiding mechanical damage of the organic layer with the tip [19]. The bias voltages were
Fig. 2. STM image of the surface of a ¯ame-annealed gold sample before adsorption of the tricyclochinazoline molecules. The imaged area is 1 mm 2.
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controlled not to exceed 1 V to prevent damage of the organic coating by high electric ®elds between the tunneling tip and the gold surface. The polarity of the bias voltage was found to have no detectable in¯uence on the images. The AAS (atomic absorption spectrometry) measurements were made with a Perkin Elmer 3030 spectrometer in a graphite oven after dissolution in HCl.
3. Results and discussion 3.1. Interface reactions After the deposition of thin ®lms of TCQ all the gold substrates clearly showed corrosion of the initial smooth surface of the Au(111) terraces (Figs. 2 and 3). The corrosion of the gold terraces was observed by STM for samples prepared by both methods, self-assembly from dilute solution (Fig. 3a,c,d) and solvent casting with thorough rinse, respectively (Fig. 3b). The observed surface structure is reminiscent of gold surfaces modi®ed with self-assembled monolayers of alkanethiols, see [19±26] and references therein, disul®des [25], and hydrogen sul®de [27] and thiophene [28] on Au(111). For thiols, the corrosion of the gold is observed for self-assembly from solution as well as the gas phase. In addition, Larsen et al. reported the same depressions for SAMs prepared by microcontact printing with thiol-inked PDMS stamps [22]. The initial interpretation of the observed pits was an etching process by the thiol solution as gold could be detected spectroscopically in the assembly solution [19±21]. The depressions were shown to be depressions in the underlying Au(111) [19± 21]. Recent STM studies on self-assembly of thiols from the gas phase present evidence that a surface relaxation phenomenon is very probably the origin of the depressions for assembly from the gas phase [23,24]. Strikingly, etching by the thiol in solution can be ruled out a priori for gas phase assembly as well as for microcontact printed SAMs [22]. Although the process in solution is not completely understood, a complexation of the gold atoms by the sulfur would be a possible explanation for both phenomena. It would raise the mobility of the surface gold atoms leading to surface relaxation in the gas phase and to etching of the surface due to the dissolution of gold in liquid environment. For the SAMs of TCQ-12 and TCQ-16 investigated here, the corrosion of the gold surface after immersion of the electrode into the dye solution for 12 h results in the dissolution of ca. 1/3 of the top gold layer as detected by atomic absorption spectroscopy of the assembly solutions. In pure methylene chloride the concentration of gold was determined to be 2 mg/l which is the detection limit of the method. STM revealed no depressions in Au samples that were in contact with dichloromethane only. In solutions used for self assembly of TCQ on gold the concentration was found to be 20 mg/l. This result proves that the TCQ
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Fig. 3. STM images of different gold surfaces after contact with a TCQ-16 solution in dichloromethane: (a) evaporated gold (111) surface after a 12 h immersion and subsequent rinsing with pure CH2Cl2; (b) ¯ame annealed gold (111) crystal after letting a drop of the TCQ-16/methylene chloride solution evaporate on the surface. The arrows show the starting of hole formation on the terraces; (c) the ¯ame annealed gold (111) surface shown in Fig. 2 after 12 h of adsorption in TCQ-16 solution; (d) enlarged part of (c).
containing solution effectively etches away the top layers of the gold. The process seems to be rather fast as the depressions could be detected by STM after less than 30 min of contact between the thioether solution and the gold. At this point we can only speculate that the process described here is similar to the thiol solution deposition onto Au(111) and may be caused by a complexation of gold atoms by the sulfur. To our knowledge this is the ®rst time that etching of the gold is directly observed for self-assembly of thioethers on gold.
3.2. Structure and orientation in SAMs of TCQ on Au(111) 3.2.1. Solution cast ®lms Solution cast ®lms of TCQ-16 were imaged with STM before and after rinsing with methylene chloride. Prior to rinsing the surface is completely covered with liquid crystal molecules (Fig. 4a). The islands seen in this ®gure can be attributed to a ®rst and a second overlayer of TCQ molecules in face-on orientation. The step height of these elevated features was measured to be 0.5 nm for the ®rst
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Fig. 4. (a) STM image of a TCQ-16 covered ¯ame annealed gold (111) crystal after evaporation of a drop of the TCQ-16/methylene chloride solution surface. The sample has not been rinsed afterwards. The imaged area is l00 nm 2; (b) stepped area on the same sample as in (a), an area of the left bottom part is enlarged and displayed below; (c) higher resolution image from another part of the sample; (d) side view of a TCQ-16 molecule with upwards folded alkyl chains; (e) top view of a TCQ-16 molecule.
layer, whereas the diameter was found to be between 2.5 nm and 6.3 nm. The steps in the Au(111) lattice were estimated to be 0.24 nm which is in agreement with the literature value of 0.236 nm [22]. Therefore it can be concluded that the observed islands are not gold islands but a ®rst or sometimes two overlayers of the TCQ-16 molecules. The second overlayer can be removed by carefully rinsing the samples as con®rmed by STM (Fig. 5). These observations are in full agreement with earlier QCM and FTIR results which showed that the initial adsorption of molecules leads to loosely bound multiple layers which can be removed by carefully rinsing with the solvent [17].
3.2.2. TCQ monolayer on gold In Fig. 4b an image with higher resolution is shown. The gold terraces are covered with liquid crystals. Here ring-like structures can be observed. No long-range order is visible in the ®lm. The diameter of the rings were found to be between 1.4 nm and 2.5 nm. In Fig. 4c a higher resolved image from another part of the sample is shown where the arrows mark three ring-like structures with a dark spot in the middle which are attributed to aromatic ring systems of TCQ molecules with a diameter of 1.4 nm. These values are consistent with a TCQ-16 layer composed of face-on oriented molecules with upfolded alkane chains as is schematically drawn
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Fig. 5. (a) STM image of TCQ-16 molecules on ¯ame-annealed gold (111) after subsequent rinsing with the pure solvent. The imaged area is 80 nm 2. (b) Model of a TCQ-16 molecule with parallel spread out alkyl chains. An appropriately sized image as in (b) is positioned beside the enlarged region. (c) STM image of a TCQ-16 molecular layer on ¯ame-annealed gold (111) after 12 h of adsorption and subsequent rinsing with the pure solvent. The imaged area is 10 nm 2.
as a side view in Fig. 4d and as a top view in Fig. 4e. The alkyl chains are of course ¯exible, so that the diameters of the rings formed by the ends of the alkyl chains can vary within certain limits. If one assumes this diameter to be a little larger than the width of the aromatic ring, which is 1.3 nm and to be smaller than the width of the ¯at molecule given in Fig. 1 (4.4 nm), the measured values are in good agreement with these numbers. A top view of molecules arranged in this way would display a ring-like surface struc-
ture. The FTIR and contact angle measurements made by SchoÈnherr et al. are in good agreement with these results [17]. The observations indicate that the alkyl chains are folded upwards over the aromatic cores of the molecules at least to some extent. In Fig. 5a, a STM image of a stepped area of the TCQ-16 covered ¯ame-annealed gold crystal is displayed. On most parts of the surface no obvious ordering of the molecules is visible. Only on the upper left side of the image there are
R. Hiesgen et al. / Thin Solid Films 358 (2000) 241±249
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Fig. 6. STM images of TCQ-16 molecules on WSe2 (a) after evaporation of a drop of TCQ-16 containing methylene chloride solution. The imaged area is 150 nm 2, in the inset the pro®le along the drawn line is presented. (b) Height pro®le along the line, (c) after evaporation of a drop of TCQ-16 containing methylene chloride solution; the imaged area is 200 nm 2.
regions with parallel structures, which can be assigned to small ordered regions formed by the TCQ-16 molecules. For a better view a magni®ed image of this region is displayed on the left side of this ®gure. The stripe-like features have a length between 1.5 nm and 3.5 nm and are assumed to be formed by the alkyl chains of the TCQ-16 molecules. Here the alkyl chains seem to be arranged parallel to the surface as can be found in thin ®lms of TCQ on graphite surfaces, where the alkyl chains crystallize forming large ordered domain structures [8]. A model of the proposed arrangement of the side chains of the molecules is presented in Fig. 5b. An appropriately sized model of a molecule has been positioned beside the STM images
for a comparison between the dimensions of the molecule chains and the structures in the image. For a two-dimensional structure model the resolution is not high enough to really distinguish between individual molecules. Similar structures have been found with monolayers formed by hexakisalkoxy-triphenylene molecules on graphite, which show a comparable structure to that of the TCQ molecules used here [29]. On gold correspondingly crystallized regions in TCQ-16 layers found with the STM are only small. After 12 h of immersion and subsequent rinsing of the sample some higher degree of order of the surface structure can be found here, too. In Fig. 5c a high resolution STM
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image is displayed, which shows an area of 10 nm 2 on the annealed gold crystal. Here, two parallel oriented bundles are visible, which have dimensions comparable to those of the TCQ-molecule drawn for comparison on the image. These parallel structures consist of a repeating unit with a distance of 0.25 nm, which is consistent with the repeating distance of the methylene groups along the alkyl chains [30]. These structures may therefore be attributed to alkyl chains of the TCQ-16 molecules oriented parallel to the surface. We can conclude that STM images revealed that the TCQ molecules are arranged with some short-range order in both solution-cast and self-assembled ®lms on Au(111). In these areas the alkyl chains of the face-on adsorbed discotic molecules were found to be lying in parallel bundles ¯at on the gold surface. 3.3. TCQ on WSe2 TCQ molecules have also been adsorbed on tungsten diselenide crystals. On most parts of this surface not even traces of the liquid crystal molecules have been found which may be caused by sweeping away the loosely bound ®lms with the STM tip. Since the tungsten diselenide surface is a non-reactive van der Waals surface, the adhesion of the TCQ-16 molecules to the surface is weak. However, on more stepped parts of the surface which provide anchor points for the molecules well de®ned structures formed by the TCQ molecules were observed as shown in Fig. 6. Fig. 6a shows a STM image of the surface of a tungsten diselenide crystal, which had been immersed in a TCQ-16 containing solution of methylene chloride for 12 h. In Fig. 6a a completely covered area is exhibited. Here a drop of TCQ-16 containing solution has been evaporated on the surface without rinsing the sample. The molecules are ordered in parallel rows with a periodicity of 15 nm and a height of 2 to 3 nm with a minimum width of the rows of 5 nm. The height pro®le measured along the line is shown in Fig. 6b. These dimensions indicate that this TCQ-16 coverage is a multilayer ®lm. The appearance of the rows resembles the surface structure on gold as described above. The image is not clear enough to distinguish an internal ordering of the molecules, but at some regions the surface of the rows exhibits a spotted structure, where round features with the dimensions of molecules are arranged. Therefore, it is more likely that the TCQ-16 molecules form stacks parallel to the surface than perpendicular. This would be in accordance with the observation reported by Boden et al. [4] for a 200 mm thick ®lm of hexakis(hexylthio) TCQ molecules. From the geometrical dimensions of the rows it can be assumed that 1 to 3 molecules are arranged side by side. There are also some defects visible, where parts of the ®lm are missing, presumably due to scratching by the STM tip. Another area on the WSe2 crystal, where TCQ-16 molecules have been imaged, is displayed in Fig. 6c. Here the molecules also form rows which are parallel to each other.
The different rows are shifted against each other and there are defects visible in the layer. The width of these rows is also about 15 nm, but their height is only 0.9 nm, which is just the length of the respective alkyl chain. If it is assumed that the chains are folded up, this TCQ-16 layer is a monolayer of the liquid crystal molecules. In this case the molecules are also oriented with their aromatic core ¯at on the surface. From the STM image no additional information can be obtained on the ®ne structure of these rows. 4. Conclusions Hexaalkylthioether derivatives of TCQ were shown to form self-assembled monolayers on Au(111) by both selfassembly from dilute solution and solution casting with a subsequent thorough rinse. Atomic adsorption spectroscopy proved that the Au(111) surfaces are etched by the TCQ containing solutions. STM revealed a corroded surface which is similar to gold surfaces that were coated with self-assembled monolayers of thiols or disul®des. During the assembly multilayers of the organic molecules are formed on the surface. These multiple layers can be removed by thoroughly rinsing the surface with pure solvent. The remaining monolayer is ®rmly attached to the surface due to the multiple attachment points per molecule. The aromatic cores of the molecules were found to be oriented in face-on orientation. The alkyl substituents were in most cases folded upwards and thus shielding the aromatic cores. After long immersion times crystalline areas were revealed by STM. In these areas the alkyl chains were lying ¯at on the gold surface. This order was found only locally. On WSe2 mono- and multilayers of TCQ molecules in face-on orientation were imaged by STM as well as columnar structures which displayed long range order. Acknowledgements We gratefully acknowledge ®nancial support from the VW foundation under contracts No. I/69 880 and I/72 365, the Commission of the European Community under contract No. JOR3-CT 96-0106, the Bundesminister fur Bildung und Forschung (BMBF) under contract No. 03 M 4084 AO, and the Deutsche Forschungsgemeinschaft (DFG) under contracts No. ME 855/3-1 and STI 74/9-2. References [1] D. WoÈhrle, D. Meissner, Adv. Mater. 3 (1991) 129±138. [2] D. Adam, P. Schuhmacher, J. Simmerer, L. HaÈussling, K. Siemensmeyer, K. Etzbach, H. Ringsdorf, D. Haarer, Nature 65 (1994) 456± 459. [3] E. Keinan, S. Kumar, S.P. Singh, R. Ghirlandos, E. Wachtel, J. Liq. Cryst. 1 (1992) 157. [4] N. Boden, R.C. Borner, R.J. Bushby, J. Clements, J. Am. Chem. Soc. 116 (1994) 10807±10808. [5] J.S. Foster, J.W. Frommer, Nature 333 (1988) 542.
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