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Atomic Force Microscopy of Ordered Monolayer Films from Discotic Liquid Crystals V. V. Tsukruk**fand D. H. Reneker Institute of Polymer Science, The University of Akron, Akron, Ohio 44325
H. Bengs and H. Ringsdorf * Institute of Organic Chemistry, Mainz University, J.J.-Becher- Weg, Mainz 6500,Germany Received February 1,1993. In Final Form: May 28,1993 Ordered monolayer films from discotic liquid crystalline donor-acceptor twin compounds formed from dilute solution by physical adsorption at the surface of silicon waferswere studied by atomicforcemicroscopy. The roughness of flat areas of the films is in the range of 0.4-0.9nm and thickness is 1.8 nm. It was shown that the monomolecular films are formed by columns lying parallel to the solid support and the molecules are arranged in edge-on position. Parameters of columnar ordering in monolayers correspond to the structure observed in the bulk liquid crystalline phase by X-ray diffraction.
Introduction The formation of ordered anisotropic supramolecular structures on a macroscopic scale with controllable spatial distribution of the components is one of the attractive properties of mesomorphic macromolecular and low molecular mass organic compounds.’ The LangmuirBlodgett (LB) technique is used to transfer monolayers onto a solid support to fabricate ultrathin films and to investigate them for optical applications. A variety of organic molecules has been explored, including polyglutamates, azobenzenes, ladder polyimides, and diacetylenes.w Recently the formation of LB films from densely packed molecular columns was demonstrated for discotic liquid crystals (LQ6 and for mixtures of donor and acceptor discotic molecules with charge-transfer interactions.6 In such systems, the columns with alternating donor and acceptor molecules lie parallel to a solid support. Photogenerationof charge carriers and formation of photoinduced two-dimensional charge-density distributions were observed for LB films consisting of columns formed by charge-transfer c~mplexes.~Novel discotic molecules with chemicallylinked donor and acceptor parts were explored as an advanced alternative to physical mixtures.s It was demonstrated that such “twin”molecules form columnar phases in the bulk state with various kinds of intra- and intercolumnar ordering.
* To whom correspondence should be sent. + Permanent address: Institute of Bioorganic Chemistry, Academy of Science, Kiev 253094, Ukraine. Present address: College of Engineering and Applied Science, Western Michigen University, Kalamazoo, MI 49008. (1) Williams, D. J. Angew. Chem., Znt. Ed. Engl. 1964,23,690. (2) Mathy, A.;Mathauer, K.; Wegner, G.;Bubeck, C. Thin Solidh’lma 1992,215, 98. (3) Sawodny, M.; Schmidt, A.; S t a ” , M.; Knoll, W.; Urban, C.; Ringsdorf, H. Thin Solid F i l m 1992,210/211,500. (4) Conjugated Polymeric Materia&: Opportunities in Electronics,
Optoelectronics and Molecular Electronics;Bredas, J. L., Chance, R. R., W.; Kluwer Academic Press: London, 1990. (5) Karthaus, 0.; Ringedorf, H.; Tsukruk, V. V.; Wendorff, J. H. Langmuir 1992,8,2279. Albouy,P. A.;Vandevyver,M.;Perez, X.;Ekoffet, C.; Markovitsi,D.; Veber, M.; Jallabert, C.; Stnelecka, H. Langmuir 1992,
8, 2262.
(6)Tsukruk, V. V.; Wendorff, J. H.; Karthaus, 0.;Ringsdorf, H. Langmuir 1993,9,614. (7) Karthaus, 0. Thesis, Mainz University, 1992. Catry, C.; Van der Auweraer, M.; de Shyrer, F. C.; Hiuseling, L.; Karthaus, 0.; Ringsdorf, H. Submitted for publication in Makromol. Chem. (8) Mtiller, M.; Tsukruk, V. V.; Wendorff, J. H.; Bengs, H.; Ringsdorf, H. Liq. Cryst. 1992,12,17.
In this work, novel charge-transfer compounds possessing columnar LC phases in the bulk state are used for fabrication of ordered films by a self-organizationprocess, at silicon surfaces, that occurs in a drop of dilute solution during evaporation of the solvent. Self-assembled monomolecular films, obtained through chemisorption and physical adsorption of alkyl compounds with various functional end groups, are well-kn0~11.QJ0 Scanning tunneling microscopy and atomic force microscopy (AFM) were used for investigation of the surface morphology and molecular ordering in ordered molecular films.”& Until now, there is no information about the structural ordering in molecular films from discotic compounds which have mesomorphic order in the bulk state.’6b Application of AFM techniques to ordered mono- and multilayers of organic compounds on solid supports provides structural information about the surface morphology on a micrometer scale, average roughness of the films and local roughness of separate layers and support, friction properties, arrangement of the molecular structures at solid surfaces, conformation and shape of the molecules, symmetry, and parameters of positional ordering in two-dimensional lattices of m o l e ~ u l e s . l ~Domain -~~ morphology and local defects such as holes, edges, and d i ~ l o c a t i o n s ~are 0~~~~l~~~ (9) Abbott, N. L.; Folkere, J. P.; Whitesides, G. M. Science 1992,257 (September 4) 1380. (10) Rabe,J. P.; Buchholz, S. Phys. Rev. Lett. 1991,66,2096. Rabe, J. P.; Buchholz, S. Science 1991, 253 (July 26), 424. Rabe, J. P.; Askadskaya, L. Phys. Reu. Lett. 1992,69,1395. (11)Frommer, J. Angew. Chem., Znt. Ed. Engl. 1992,31, 1298. Chi, L. F.; Anders, M.; Fuchs, H.;Johnston, R. R.; R i o r f , H. Science 1995, 259,213. Chi, L. F.; Eng,L. M.; Graf, K.; Fuchs, H. Langmuir 1992,8, 2255. Peltonen, J. P. K.; He, P.; Rosenholm, J. B. J. Am. Chem. SOC. 1992, 114, 7637. Sano, M.; S a d , D. Y.;Isayama, M.;Kunitake, T. Langmuir 1992,8, 1893. (12) Kim, Y.-T.;Bard, A. J. Langmuir 1992,8, 1096. (13) Sarid, D. Scanning Force Microscopy; Oxford University Press: New York, 1991. (14) Schwarz, D. K.; Garnaes, J.; Viswanathan, R.; Zaeadziiki, J. A. Science 1992,257 (July 24), 608. (15) Viswanathan, R.; Schwarz, D. K.; Garnaes, J.; Zaaadzinski, J. A. Langmuir 1992,8, 1603. (16) (a) Hamma, H. G.; Could, S. A.; Hamma, P. K.; Gaub, H. E.; Longo, M. L.; Zaaadziiski, J. A. Lrrngmuir 1991, 7, 1051. (b) First observation of columnar order in LB films by AFM from diecotia, wae reported recently: h f o w i c z , J. Y.; et al. Science 1993,260,323. (17) Binniig G.; Quate C. F.; Gerber, Ch. Phys. Rev. Lett. 1986,12, 980.
(18) See The Proceedings of Six International Conference in STM. Ultramicroscopy 1 9 9 2 , 4 2 4 (JuIY). (19) Drake,B.; Prater, C. B.; Weiacmhom, A. L.; Gould, S. A. C.; Albrecht, T. R.; et al. Science 1989,24;3 (March) 1586.
0743-7463/93/2409-2141$04.00/00 1993 American Chemical Society
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Figure 1. AFM mesoscale images of the ordered monomolecular films from 1: (a) the surfaces of the films from 1 near the central part of the film, 7 pm X 7 pm; (b) 1pm X 1 pm scale image with a square hole (300 nm X 300 nm across and 1.8 nm deep) near the center made by selected scanning with higher forces.
also observed. The present paper describes the morphology and molecular ordering in the ordered monomolecular
films from discotic LC compounds investigated by AFM.
Experimental Section The chemical structure of the twin molecules (1 and 2) consisting of donor (triphenylene) and acceptor (trinitrofluorenone (TNF)) chemically connected via flexible spacers of various lengths is presented in Chart I. Synthesis,phase behavior, oriented structures, and the models of molecular packing in the bulk LC state were described in detail.* Samples for investigations were obtained by putting of a drop of dilute solution of compounds in chloroform (0.005 w t %) on the surface of a silicon wafer at ambient conditions. Substrates were cleaned by ultrasonication, first in hot chloroform for 15 min and then in a hot 1:1:5 solution of NHIOH/H202/H20 for 30 min. After rinsing in Milliporewater the surfaces were rendered hydrophobic by etching in an Ar/O2 plasma and etching further with an N H P solution. Images of the surfaces in ambient conditions were obtained with an atomic force microscope, the Nanoscope I1 (Digital Instruments, Inc.), using a pyramidal Si& tip accordingto wellknown procedwes.lSm Heads D and A were used for scanning on scales from 10 p m to 4 nm with applied forces in the range of 5-40 nN. AFM data on the micrometer scale (head D) are presented after removingsample tilt. Molecular scale AFM data were processed only by high-frequencies filtration. All images presented were obtained repeatedly and were stable under experimental conditions.
Results and Discussion The silicon support has a flat surface with a few randomly distributed bumps of 1-4 nm height and 50-100 nm width in a 2 pm X 2 pm area. The microroughness for a square of 500 nm x 500 nm for various flat parts of sample is in the range of 0.14.3 nm, which correspondsto molecularly (20) Reneker, D. H.; Patil, R.; Kim, S. J.; Tsukruk, V. V. In Polymer
~ b ~ ~ ~ t D&ere, ~ n ' M., ~ ~ t Kluwer i ~ ~ Academic ; press:
1993.
London,
flat surfaces with random deviations of the heights on an atomic scale and is consistent with the literature data.1333 Optical microscopy of the films far from the center of the evaporated drop showssurfacesconsistingof concentric ridges separated about some tens of millimeters by mirrorflat surfaces. Near the center of these rings the reflection is distorted by the presence of a large amount of the material. In Figure l a typical AFM image of films studied in the vicinity of the centeris is presented. The concentration of ridges is high and their thicknesses are in the range of 50-500 nm. The ridges are formed by the dynamics of surface tension during solvent evaporation. For the inner rings not so far from the center, we observed a random distribution of materials with the flat areas between neighboring ridges up to 3pm. Between the outer rings a very flat surface with occasional bumps was observed with flat areas extending over 10 pm. Their roughness is in the range of 0.4-0.9 nm, which is only slightly rougher than the value determined for the silicon surface. The outer ridges have a height of 100-200 nm and width of 0.5-2 pm. They are aggregatesof precipitated material. The ridges are weakly attached to the substrate and can be pushed away by applying higher forces during scanning. On the images with higher magnification (300 nm X 300 nm) for the flat surfaces of films of both compounds random "clusterlike" deviations of the heights of 0.1-0.5 nm on areas with lateral sizes of 30-100 nm are visible. Microroughness of the film surfaces on this scale is only 0.2-0.3 nm, which shows that these films have molecularly flat surfaces with random, atomic scale variations of thickness. In order to determine the thickness of the films formed in the flat areas, we followed known proced~re.~~J~,20 Selected regions of film from 1 were scanned with high forces, and the organic material coveringthe solid support was scraped away. As a result of this process, rectangular holes with depths corresponding to the thickness of the films are formed. These holes can be observed by scanning over the same areas a t lower magnification and with lower forces. The result of this test of the films of 1 is presented in Figure lb. The depth of the 300 nm X 300 nm hole is 1.8 f 0.5 nm. Small ridges on the edges of the hole were formed by the scraped material. The depth corresponds quite well to the diameter of one discotic molecule standing on edge and to the thickness of a single column, known from X-ray data (from 1.6 to 2.5 nm). Thus, this is the first evidence that the film Of the compound studied is a monomolecular film formed by a single layer of columns
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ATF of F i l m from Discotic Liquid Crystah
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