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Langmuir 1993,9, 2778-2781
Scanning Tunneling Microscopy of Alkane Adsorbates at the Liquid/Graphite Interface A. Wawkuschewski, H.-J. Cantow, and S. N. Magonov; Materials Research Center (F-MaF),Albert-Ludwigs University, Stefan-Meier-Strasse 31A, Freiburg 0-79104, Germany Received May 3,1993. In Final Form: August 3,1993" Scanning tunneling microscopy (STM) revealed a complex multilayered structure of the adsorbate, formed at the liquidfgraphite interface from a saturated dodecane solution of the cyclic alkane, (CH2)48. Surface etching of the adsorbate was observed when the scanning area was diminished from large areas of several micrometers to submicrometer range. T w o layers of adsorbate, each of ca. 0.6 nm in thickness, were removed by the scanningtip before well-ordered lamellar sheets were reached. STM images of such lamellar layers exhibit features related to the molecular structure of individual alkane molecules. After addition of (CH2)48to C36H74 solution,a stable ordered bilayer of cyclic and normal alkanes was observed. The observation of a sheet of cyclic alkanes lying beneath a layer of normal alkanes was explained by stronger adhesion of (CH2)48molecules to the graphite surface.
Introduction Adsorption of normal alkanes a t the liquid/graphite interface has been intensively examined by scanning tunneling microscopy (STM).l A two-dimensional lamellar structure of the adsorbed layer is observed in the molecular scale images. Sequences of STM patterns, showing extended chain molecules, are aligned perpendicular to the lamellar boundaries, and the width of the lamellaeis determined by the length of the alkanemolecule. These observations supported an earlier assumption that normal alkanes within an adsorbed layer are oriented parallel to the basal plane of graphite.2 So far STM studies of alkanes a t the liquid/graphite interface were mainly conducted on a small scale, where molecular resolution can be achieved. Only few observations in the micrometer range were r e p ~ r t e d . I~t becomes evident,however, that measurementson different scales are important for STM analysis. In the large scale images one can document surface morphology and surface etching. Surface etching, which is induced by tip-sample interactions, was observed with many compounds. A removal of topmost layers was found in STM of layered transition metal chalcogenides and organic conductor^.^ Though this phenomenon is not well understood, the experimental results indicate that such etching depends on the nature of the surface, and its rate increases with the time spent by the tip in a particular area. New STM observations of alkane adsorbates at the liquid/graphite are presented in this Letter. They reveal the complex structure of adsorbates, which has been assumed earlier on the basis of calorimetric ~ t u d i e s . ~ Adsorbed layers of normal and cyclic alkanes were examined on different scales. Besides normal alkanes, e Abstractpublishedin Advance ACSAbstracts, October 15,1993. (1) (a) McGonigal,G. C.;Bernhardt, R. H.; Thomson,D. J. Appl.Phys.
Lett. 1990,57,28. (b) Rabe, J. P.; Buchholz, S. Science 1991,253,424. (c) Rabe, J. P.; Buchholz, S.Phys. Reu. Lett. 1991,66,2096. (d) Watel, G.; Thibaudau, F.; Cousty, J. Surf. Sci. 1993,281,L297. (2)(a) Everett, D. H.; Findenegg, G. N. Nature 1969,223, 52. (b) Groczek, A. J. Proc. R.Soc. London, A 1970,314,473. (c) Findenegg, G. N.; Lippard, M. Carbon 1987,25,119. (3)Eng, L. M.; Fuchs, H.; Buchholz, S.; Rabe, J. P. Ultramicroscopy 1992,42-44, 1059. (4)(a) Parkinson, B. J.Am. Chem. SOC. 1990,112,7498.(b) Harmer, M.A.; Fincher, C.R.; Parkinson, B.A. J. Appl. Phys. 1991,70,2760.(c) Magonov, S.;Bar, G.; Gorenberg, A.;Yagubskii, E. B.; Cantow,H.-J. Adv. Mater. 1993,5,453. (5)(a) Kern, H. E.; Findenegg, G. H. J. Colloid Interface Sci. 1980, 75,346. (b) Kern, H. E.; von Rybinski, W.; Findenegg, G. H. J. Colloid Interface Sci. 1977,59, 301.
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cyclic alkanes, (CHd,, are attractive for adsorption studies. Crystallographic analysis of several homologues with n > 2lSshowsa similarity in their molecular geometry. A cyclic alkane consists of two all-trans stems connected by molecular folds, 1. The zigzag planes of linear all-trans segments are oriented perpendicular to the molecular plane, which contains two stems and two folds. It can be expected that cyclicalkanesexhibit an adsorptionbehavior differing from that of normal alkanes due to a stiffer molecular construction and the absence of CH3end groups.
1 Experimental Section STM measurements were carried out with a Nanoscope II at ambient conditions. Cyclicalkane(CH2)~8,'lnolPLLI1alkane C&I.N (EGA Chemie), and dodecane (Aldrich) were used in our experiments. A drop of saturated alkane solution in dodecane was positioned on a freshly cleaved graphite surface, and the tip-a mechanically sharpenedPtIIr wire-was immersed in the drop during scanning. We used the following parameters: tunneling current It, = 0.3 nA,bias voltage ubb = -1.8 V for observations of adsorbate layers,and It, = 1 nA,ubk = 0.14.03 V for imaging of underlying graphite. Scanning line frequency of 2 Hz was applied in the large scale measurementa (6pm X 6 pm) and was increased gradually at smaller scanning areas up to 19Hz. Despite the increase in scanning rate, the particular area during the small scale scan was exposed longer to tipsample interactions than during the large scale scan. Thus, the probability of destructive tipsample interactions is higher in small scale scans. All imageswere recorded in the height imagingmode and are presented without any off-line filtering. Results and Discussion Large scale STM images of (CH2)48alkane on graphite, Figure 1,were recorded on stepwise increased areas using (6)(a)Shearer, H. M. M.; Vand, V. Acta Crystallogr. 1956,9,379. (b) Kay, H.F.; Newman, B. A. Acta Crystallogr. 1968,EM,615. (c) Groth, P.Acta Chem. Scand. 1979,A33,199. (7)MBller, M.;Cantow, H.-J.; Drotloff, H.;Emeis, D.; Lee, K.-5.; Wegner, G. Makromol. Chem. 1987,187,1237.
0 1993 American Chemical Society
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Langmuir, Vol. 9,No. 11,1993 2779 ~~
. Figure 1. Successive STM height images (A-C) of (CH2)48 adsorbate on graphite in different scales obtained with the zoomed-out procedure. The vertical gray-scale bar indicates the height variations in the images in nanometers. Tunneling conditions: Itm= 0.3 nA, v b i m = -1.8 V. T h e insert in (B) shows the cross-section profile along the line A-A.
Figure 2. Successive STM height images of (CH2)a on graphite in different scales (A, B, C) obtained with a zoom-out procedure. The vertical gray-scale bar indicates the height variations in the images in nanometers. Tunneling conditions: It,,,,= 0.30 nA, Vbm = -1.8 V. T h e insert in (B) shows the cross-section profile along the line A-A.
Figure 3. Molecular-scale height image of (CHz).,* layer. Tunneling conditions: It,, = 0.27 nA, vbim = -1.3 V. T h e vertical gray-scale bar indicates the height variations in the images in nanometers.
a zoom-out procedure. Terraces of graphite, decorated with the adsorbate, are distinguished in the upper right corner of these images. Bright contour lines are the main features seen in Figure la. They are probably related to domain boundaries within the observed layer. Dark rectangular spots, which are observed in the centers of
parts B and C of Figure 1,indicate that a part of adsorbate was removed by surface etching in precedingscans. Thus, it can be concluded that the bright STM patterns outside the central parts belong to a nondestroyed topmost layer of adsorbate. According to the nonhomogeneous contrast in these areas, the topmost layer consists of numerous islands, being less compact than the underlying layer observed in Figure 1A. The cross-section profile in the Figure 1B shows that thickness of the topmost layer is ca. 0.5-0.6 nm. Surface etching of the adsorbate was also found in the submicrometer scale. After a repeated scanning of the area, 250 mm X 250 nm, an image with periodic bright strips was recorded, Figure 2a. By analogy to the images of normal alkanes' these strips can be assigned to an ordered lamellar structure. Large scale images obtained with a zoom-out procedure,Figure 2B-C, revealed that a second adsorbate layer was removed prior to the appearance of the ordered sheet. A step height between the terraces of the second and the third layer (Figure 2B) roughly equals to the thickness of the topmost layer (Figure 1B). It is worth noting that the borders of the etched region in the second layer are less rectangular than those mostly observed on the surfacesof inorganicand organic crystals.4 This finding might be related to a partial readsorption of alkanes from the solution. Thus, the observations on the different scales revealed a layer-by-layer etchingof cyclic alkane adsorbate in STM. In this case surface etching brings important information about the adsorbate structure. As a result a multilayered
2780 Langmuir, Vol. 9, No. 11, 1993
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Figure 4. (A) STM height image of CMH,, layer on graphite, and in the insert the STM height image of graphite. (B)STM height image of an ordered bilayer, consisting of (CH2)a sheet (darker regions) and CsH71 layer (brighter regions). (C) The zoomed part of the image (B).The vertical gray-scale bar indicates the height variations in the images in nanometers. Tunneling conditions: 1= 0.3 nA, Vum = -1.8 V in (A) and ( C ) and It, = 1.3 nA, Vbm = -0.03 V in (B).
structure of the alkane adsorbate at the liquid/graphite interface can be proposed, 2. Three different layers were
2 found in the images. Two upper layers are less stable with respect to tip-sample interactions. Probably, the molecular organization within such layers could be studied by applying tunneling currents in the picoampere range. In such a case one can expect a decrease of the destructive interaction due to a larger tip-sample separation. The third and most stable layer is exposed after the largest part of adsorbate is removed by etching. The molecularscale STM studies on alkanes and other adsorbed organic molecules are carried out on such layers, which are stabilized by intermolecular interactions of alkane molecules within the layer and with a substrate.8 A stable layer might have a complex structure being composed of several monomolecular sheets due to a possible homoepitaxy. Two examples of STM images obtained on such ordered layers are presented below. In the first example we will consider the molecularscale STM image of (CH2)48,Figure 3, which was recorded at higher magnification than that in Figure 2A. The proposal that the bright strips in Figure 2a correspond to the lamellar boundaries is supported by the arrangement of STM patterns in the molecular scale image. The distance between the bright strips equals to the length of the (CH2)48molecule, 3.1 nm. The STM patterns precisely reproduce the molecular features of cyclic alkanes, which are lying flat with respect to the graphite plane being (8) (a) Chiang, S. In Scanning tunneling microscopy l;Wieaendangler,
R,Giintherodt, H.-J., Eds.; Springer Verlag: Heidelberg 1992; p 181. (b) Frommer, J. Angew. Chem., Int. Ed. En& 1992, 104, 1298. Sano, M.; Sasaki, D. Y.; Kunitake, T. Science 1992,258,41.
arranged in ordered lamella. Sequences of patterns oriented perpendicular to lamellar boundaries present the electroniccontributionsof the most protruded hydrogens, which belong to the CH2 groups of linear stems. Due to the geometry of the cyclic alkane, hydrogens of molecular folds are more elevated than those of linear stems, and they appear as the brightest spots in the STM image. This rational interpretation of the molecular-scaleimage of cyclic alkane was obtained in the interplay between theory and experiment in the analysis of STM images of normal and cyclic alkanes adsorbed on g r a ~ h i t e .The ~ surface partial electron density plots of the propane/graphite bilayer were calculated in the frame of the extended Htickel tight binding electron band structure method. This approach shows that the STM images of normal and cyclic alkane monolayers as well as those of multilayers are dominated by the contributions of topmost hydrogen atoms.a The second example is related to the observation of ordered layers adsorbed on graphite from a mixed solution of normal and cyclic alkanes. First, a drop of normal alkane (CsH74) solution was positioned on the substrate and an ordered layer of normal alkane was observed (a). Then a drop of (CH2)48solution was added, and measurements were continued in the mixed solution (b). (a)The lamellar arrangement found in the C36H7.4sheet, Figure 4A, is typical for the adsorbed normal alkane l a ~ e r s . ~ Perfectly ordered extended features related to lamellar boundaries and two homoepitaxial adsorbates (bright patterns) are seen in this image. The periodicity in the perpendicular direction (ca. 4.9 nm) corresponds to the lamellae width, and it coincides with the length of the CsH74 chain. Thin bright strips within the lamellae are related to individual chain molecules, which are oriented perpendicular to the boundaries. Alkane molecules are aligned along one of the principal directionsof the graphite image (insert in Figure 4A). The image of graphite was registered in the same area a t a lower tunneling gap resistance, when the tip comes closer to the substrate. Such observations of substrate and adsorbate at different tunneling conditions are reversible, and this might be considered as a proof that we are imaging a first alkane layer(s) lying on graphite. The coincidence of the main directions of alkane chain and graphite supports the Groszek model,2bwhere each CH2 group of an extended (9) (a) Liang,W.; Whangbo, M.-H.; Wawkuscheweki, A.; Cantow, H.J.; Magonov, S. N. Adu. Mater. 1993,5, 817. (b) Wawkuechewski, A;
Cantow,H.-J.; Magonov, S. N.; Mcller, M.; Liang, W.; Whangbo, M.-H. Ado. Mater. 1993,5,821.
Letters
alkane occupying one hexagon of an underlying graphite plane. Consequently, the intermolecular interaction between alkanes and graphite is one of the driving forces leading to formation of ordered adsorbed layers. (b)After addition of (CH& solution a stable imaging was possible only 30 min later. In the images recorded on different places, two different linear structures were observed, as shown in Figure 4B. Strips with a shorter period, ca.3.1 nm, were found in the darker regions. They can be assigned to the (CH2)a layer. Strips with a larger separation of ca. 4.9 nm are related with lamellar order in the layer of normal alkane. They exhibit a brighter contrast than those assigned to cyclic alkanes, Figure 4C. Such contrast difference can be assigned to formation of a bilayer, consisting of an elevated C36H74 layer and an underlying (CH2)a sheet. This conclusion is consistent with recent observations of homoepitaxial layers of cyclic alkanes and the aforementioned theoretical cons i d e r a t i o n ~ .Thus, ~ ~ the STM image in Figure 4C can be explained as the observation of an ordered bilayer, consisting of an elevated C3sH74 layer and an underlying (CH2)a sheet. The height difference (ca. 0.3nm) between the normal and cyclic alkane layers in Figure 4C correlates to the expected thickness of a normal alkane sheet, when the zigzag molecular plane is parallel to the substrate. As in the case of the normal alkane the all-trans stems of cyclic alkane are lying parallel to one of the principal graphite directions. This makes evident that the inter-
Langmuir, Vol. 9, No. 11,1993 2781 action sites of the CH2 groups with graphite are similar for both alkanes. The chains of normal alkanes in the top layer (Figure 4C) are also parallel to these stems, Figure 4C. Small deviations of the lamellar orientation in these layers are seen in the lower left corner of this image. Various structural defects within adsorbed cyclic alkane layers and their homoepitaxy are described e l s e ~ h e r e . ~ ~ Thus, we found that cyclic alkanes replace linear moleculea in the first monolayer on graphite. This finding might be explained by a stronger adsorption of (CH& cyclic alkane on graphite in comparison with the C36H74 linear alkane. The absence of CHJ groups and a rigid molecular construction of cyclic alkanes are, probably, the favorable factors for their adsorption. The additional indirect confiiation of the strong adhesion of cyclic alkanes to graphite arises from the experimental fact that the repeatability of STM images of (CH2)a adsorbates on graphite is higher than that of linear alkanes. In conclusion the presented results demonstrate a complex multilayered structure of alkane adsorbates at the liquid/graphite interface and a stronger adsorption of the (CH& cyclic alkane on graphite in comparison with the normal alkane, C36H74.
Acknowledgment. We acknowledge Professor M. MBller ( U h University) for the presented crystals of (CHzh.
