Langmuir 1993,9,649-651
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Structural Phase Transition of a 1-Dodecanol Monolayer Physisorbed at the Liquid/Graphite Interface by Scanning Tunneling Microscopy Y. H.Yeo, G. C. McGonigal, and D.J. Thomson* Department of Electrical and Computer Engineering, Univeristy of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 Received September 30,1992. In Final Form: January 25,1993 The adsorption of a l-dodecanolmonolayer at the liquid/graphiteinterface was investigated using the scanning tunneling microscope. Two distinct adsorbed structures were observed between the bulk l-dodecanol melting temperature (297 K) and the adsorbed layer melting temperature (333 K). The molecular packing arrangement shows a herringbone structure at 303 and 308 K, while at 313 K the monolayer exhibits a structure similar to that of the bulk crystal of l-dodecanol. This monolayer phase transition may proceed via a rotator phase similar to those observed in solid l-alkanolsat higher temperature. The formation of a physisorbed organic monolayer on solid surfaces has been a subject of great interest because of its importancein relation to processessuch as adhesion, lubrication,corrosion, and adsorption. Usingthe scanning tunnelingmicroscope(STM),we have previouslyobserved the formation of a highly ordered n-alkane monolayer physisorbed at the liquid/graphiteinterface.lq2 To explore further the interesting phenomena of molecular ordering at the liquid/solid interface, we have examined the adsorption of l-dodecanol [CH&H2)&H2OH J on a graphitesurface. Previoussurfacemass studiesof l-dodecanol at the liquid/graphite interface as a function of temperature have suggested the formation of an ordered monolayer in the range of 303-333 K, 6-36 K above the bulk melting point at 297 Here we present the STM images of the l-dodecanolmonolayer obtained at 303,308, and 313 K. Surprisingly,two distinct structural orderings of the adsorbed monolayer are observed. The sample was prepared by directly applying l-dodecanol crystals onto a freshly cleaved highly oriented pyrolytic graphite surface at room temperature (294 K). The surface temperature was controlled to within k0.5O by means of a Pelltier device situated directly underneath the graphite substrate. In each experiment the sample starts at room temperature and is heated and imaged at progressively higher temperatures. Investigation of this system was performed with an electrochemically etched W tip immersed in the liquid and the STM operating in the constant-current mode. It was observed that a threshold of 0.3 V must be exceededto image the molecules; below this threshold the graphite lattice was imaged. Images of the graphite substrate were taken at each temperature. This indicates that a voltage-dependent imaging mechanismis involved in imagingthe l-dodecanol molecules. Fourier transforms of the graphite lattice at 303,308, and 313 K show that the substrate orientation and the properties of the piezoelectric scanner remain constant throughout an experiment. Figure l a shows an 80 A X 80 A STM image of l-dodecanolphysisorbed onto the graphite at 303 K. The K . 3 9 4
(1)McGonigal, G.C.;Bemhardt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990,57,28-30, (2) McGonigal, G. C.; Bemhardt, R. H.; Yeo, Y. H.; Thomson, D. J. J. Vac. Sci. Technol. 1991,B9,1107-1110. (3) Findenegg, G. H. J. Chem. SOC.,Faraday Trans. I 1972,68,17991806. (4) Bein-Vogelsang,U.;Findenegg, G. H. Colloids Surf. 1986,21,469-
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Figure 1. (a, top) STM image of l-dodecanolphysisorbed at the liquid/graphite interface at a substrate temperature of 303 K. The field of view is 80 A X 80 A. (b, bottom) Long-range ordering of the adsorbed monolayer at 303 K, revealing the moire pattern. The field of view is 160 A X 160 A.
bias voltage is 0.44 V sample positive, and the tunneling current is 0.5 nA. A two-dimensional array of molecules is observed. Note the high degree of molecular ordering with each molecule clearly resolved. The length of each molecule is measured to be approximately 17 from the STM image. This implies that the l-dodecanol molecules are lying parallel to the graphite substrate, as the chain length of l-dodecanol is approximately 17A. The parallel orientation of the molecule is consistent with previous studies of this and other molecules adsorbed on the
0 1993 American Chemical Society
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650 Langmuir, Vol. 9, No. 3, 1993
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(a)T= 303 K, 308 K
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(b) T= 313 K
Figure 3. Schematic representation of the structures of the l-dodecanolmonolayer physisorbed (a) at 303 and 308 K and (b) at 313 K. Open circles are the OH head groups.
Figure 2. (a, top) Image of the l-dodecanol monolayer physisorbed at 313 K. The field of view is 80 A X 80 A. Open circles are the OH head groups. (b, bottom) 160 A X 160 A view of the long-range ordering of the adsorbed monolayer at 313 K.
graphite surface. The molecular packing arrangement is such that molecules are linked together in a pair by the hydrogen bonding of the OH head groups in adjacent rows; see Figure 3a.3 Recent neutron and X-ray diffraction studiesof ethanol and methanol adsorbed on graphite show a similar hydrogen-bonded ~ t r u c t u r e . The ~ * ~most striking feature observedin this experiment is that the l-dodecanol moleculesare packed in a herringbonestructure. A similar structure has also been observed in the STM of longer chain l-alkanols adsorbed out of solution onto graphite surface^.^ This herringbone structure is completely different from that of the l-dodecanol bulk c r y ~ t a l .A~view of the long-range ordering of the molecules is shown in Figure lb. We observe a pattern of alternating areas of high and low contrast repeated on a length scale much larger than the molecular size. This moir6 pattern results from neighboringmoleculesadsorbed at slightly differing substrate sites, thus yielding higher contrast at different locationson the substrate. This suggeststhat the adsorbed monolayer may be incommensurate with the substrate lattice. A similar herringbonestructure and moire pattern are also observed at a 308 K substrate temperature. When the substrate temperature is raised to 313 K, a dramatic change in the structure of the adsorbed mono(5) Herwig, K. W.; Trouw, F. R. Phys. Reu. Lett. 1992,69, 84-92. (6)Morishige, K.; Kawamura, K.; Kose, A. J . Chem. Phys. 1990,93, 5267-5270. (7) Rabe, J. P. Buchholz, S. Science 1991,253, 424-427. (8) Tanaka,K.; Seto,T.; Watanabe,A.; Hayashida,T. Bull. Znst.Chem. Res., Kyoto Uniu. 1959,37,281-293.
layer takes place. Figure 2a is taken at a bias voltage of 1.7 V, and the tunneling current is 0.4 nA. Images of the monolayer are difficult to obtain below 1.7 V. While retaining the high degree of two-dimensional ordering, the herringbone structure has now transformed into a structure similar to the bulk crystal of l-dodecanol. Each bright band in the STM image corresponds to molecules linked together by the hydrogen bonding of the OH head groups and oriented approximately 60' with respect to the lamella; see Figure 3b. At 313 K, the long-range ordering of this monolayer is also different from that at lower temperatures, as shown in Figure 2b. Clearly no moire pattern is observed at 313 K, thus, it is possible this is a commensurate adsorbed structure. It is possible the structural phase transition is due to the presence of rotator phases at higher temperature. Figure 3 showsschematic representations of the two types of structures observed. Intermolecular spacingsmeasured along the a axis at 303 and 313 K are approximately 4.8 and 6.2 A, respectively. The increase in the intermolecular spacing is consistent with the existence of a rotational mode at the higher temperature. The rotator phase has previously been observed in the X-ray studies of a Langmuir monolayer and solid l-alkanol~.~J~ At 313 K, the molecules could rotate about their C-O axes due to the increase in thermal energy, and the effect of this rotational mode transforms the l-dodecanol monolayer structure from herringbone to a structure similar to that of the bulk crystal. It is important to note that the herringbone structure reappears when the substrate temperature is lowered from 313 to 303 K. We do not observe the adsorbed monolayer above 318 K. This could be due to the instability of the monolayer at higher temperature. It has been suggested that in the ethanol/ graphite system, the melting of the ethanol monolayer takes place via a two-step mechanism. First is the melting of the alkyl tails, while the hydrogen bondings are maintained to ensure the stability of the monolayer. At sufficiently high temperature, the hydrogen bonds break (9) Yamamoto, T.; Nozaki, K.; Hara, T. J. Chem. Phys. 1990,92,631641. (10) Barton, S. W.; Thomas, B. N.; Flom, E. B.; Rice, S. A.; Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P. J . Chem. Phys. 1988,89,22572270.
Letters and an order-disorder transition 0ccur8.6 Our results also indicate such a stepwisenature of increased disorder,with alkyl tail transition occurring at lower temperature. In conclusion, we have observed the molecular packing arrangement of 1-dodecanol physisorbed at the liquid/ graphite interface. The results show that, over a range of temperatures, the adsorbed monolayer can exhibit two distinct highly ordered structures. This provides further
Langmuir, Vol. 9, No. 3, 1993 661 insight into the behavior of two-dimensional system processes such as the monolayer order-order transition and “two-dimensionalmelting”.
Acknowledgment. We gratefullythank J. E. Frommer at the University of Basel for helpful conversations. This work is supported by the Natural Scienceand Engineering Research Council of Canada.
