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Direct observation of long chain alkane bilayer films on graphite by scanning tunneling microscopy
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Surface Science Letters 281 (1993) L297-L302 North-Holland
surface s c i e n c e letters
Surface Science Letters
Direct observation of long chain alkane bilayer films on graphite by scanning tunneling microscopy G. Watel, F. T h i b a u d a u a n d J. C o u s t y DSM / DRECAM / SRSIM, Centre d'dtudes de Saclay, F 91191 Gif-sur-Yvette Cedex, France
Received 15 July 1992; accepted for publication 5 November 1992
The interface between a solution of hexatriacontane (n-C36H74) in decane and the basal plane of graphite has been studied by scanning tunneling microscopy at room temperature. For the first time, we show that a second layer which is rotated by 60° with respect to the first layer may grow at the interface. We demonstrate that the carbon skeleton of the molecule is parallel to the graphite surface. We propose a model for the arrangement of the molecular layers and an explanation for the origin of the second layer rotation.
Normal alkane molecules consist of linear chains of saturated hydrocarbons. The adsorption of these simple chains on the basal plane of graphite provides a model for molecular adsorption. The structure of these molecular layers is of considerable interest. For short alkane molecules (number of carbon atoms less than ten), the molecular arrangement at the v a c u u m / g r a p h i t e interface has been studied by various techniques [1]. For long chain molecules, measurements of adsorption isotherms have shown that molecules adsorb from non-polar solvent as a densely packed layer [2,3]. Recently, scanning tunneling microscopy (STM) has been used to obtain with varying degrees of resolution, images of long chain alkanes and related molecules adsorbed at the l i q u i d / g r a p h i t e interface. Up to now, only observations of the monolayer have been reported [4-6]. In this Letter, we report a STM study of the growth of n-hexatriacontane layers from a solution of n-C36H74 in decane onto graphite. We show that the arrangement of the first layers differs from the bulk structure of this alkane. A saturated solution in decane was prepared at room t e m p e r a t u r e from high purity solid n-C36HT4 and pure decane. We used diluted so-
lutions which are characterized by their concentration (c) which we define as the ratio of the saturated solution volume to the pure decane volume. A droplet of a diluted solution was applied onto the freshly cleaved surface of graphite (HOPG). The tip of our commercial microscope scans the l i q u i d / g r a p h i t e interface through the solution. Because of the evaporation of decane, most of the observations have been made within 30 min after deposition in order to minimize changes in the solution concentration. Within similar tunneling conditions, STM images of the interface layer depend on the initial n-f36HT4 concentration. From hundreds of images obtained at room temperature, two kinds of images can be distinguished. For low concentrations (1% < c < 3%), the images show a wellordered molecular layer with a single orientation of strips for scan areas over 500 x 500 nm 2. The overlayer is formed by parallel strips separated by troughs. The strip width (4.7 nm) corresponds well to the molecule length. In each strip, the long axis of the molecule is oriented parallel to the surface graphite, parallel to each other and perpendicular to the troughs. The measurement of the intermolecular distance gives 0.44 + 0.02 nm. For higher concentrations (5% < c < 10%),
0039-6028/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
L298
G. Watel et al. / STM study of hmg chain alkane bilayer films on graphite
Fig. 1. E x a m p l e of S T M i m a g e s of an interface b e t w e e n a 5 % solution of h e x a t r i a c o n t a n e in d e c a n e and the g r a p h i t e surface at r o o m t e m p e r a t u r e . (200 × 200 n m 2, Z scale = 0.9 nm, V = 692 mV, l = 540 pA).
Fig. 2. T h e s e STM i m a g e s show the two kinds of boundaries. E a c h small stick b e t w e e n t r o u g h s c o r r e s p o n d s to a single molecule. (47 × 47 n m 2, Z scale = 0.9 nm, V = 859 mV, 1 = 280 p A for (a); 20 × 20 nm 2, Z scale = 0.5 nm, V = 316 mV, 1 = 450 p A for (b).
L299
G. Watel et al. / STM study of long chain alkane bilayer films on graphite
there are several domains in the overlayer. Each domain exhibits the same lamellar structure formed by parallel strips separated by troughs as shown in fig. 1. The angle between the troughs in neighbouring domains is equal to 60°. Such an image raises the question: what is the nature of the boundary between these domains? Is it a wall between domains within the monolayer? In order to identify their nature, several boundaries have been imaged. Two examples of these boundaries are shown in fig. 2. Although the two domains shown in figs. 2a and 2b present the same relative orientation, the morphologies of the boundaries are very different. In fig. 2a, the boundary (A) is straight and merged with a trough of the domain for which the molecules are fully visible. In fig. 2b, the boundary (B) looks like a staircase with parts parallel to the main axis of the fully imaged molecules. Moreover, the behaviour of boundaries as a function of time depends on their morphologies. The boundary A remains stable over several images. In contrast, the boundary B may exibit large fluctuations of position which means that tens of molecules are moving. As the domains separated by boundaries A and B are rotated by the same angle, we expect both similar morphologies and dynamic behaviours if these boundaries are walls between domains in the same layer. It is clearly not the case. A carefull examination shows that some molecules near the boundary exhibit shorter lengths on the same side of the boundary. For instance in fig. 2a, all the molecules in the left domain are fully imaged while some molecules in the vicinity of the boundary appear shorter in the right domain. Such an apparent shorter length can be only explained by a partial screening of the first layer by an island of the second layer. These images provide strong evidence that these boundaries correspond to the edge of second layer islands. From the arrangement of the molecules in the vicinity of the boundary we can identify the second layer: in fig. 2a, the second layer is in the upper left corner while in fig. 2b it is in the lower right one. The growth of a second layer is consistent with the influence of the solution concentration on the interface as shown before.
However, no relief is detected in the STM images at the boundary of second layer. Usually, one considers that the relief in STM images is related to the surface topography of a conducting material. For thin films of insulating materials deposited on conducting substrates, the correspondence between STM images and the topography of this film is less obvious and depends on the imaging mechanism. Thus, the absence of relief related to a step in the STM images we present does not exclude the existence of such a step. In the next paper [7], we propose an imaging mechanism which explains the absence of relief at the step edge. When decane is evaporating from the solution, the images show some small bumps on the alkane layer. After complete evaporation large crystalshaped protusions are observed. These protusions are interpreted as small 3D crystals of n-Ca6H74 as previously observed [8]. The growth mode of alkane crystals on graphite from a decane solution can therefore be interpreted as StranskiKrastanov. The growth of 3D crystals of long chain alkane from a solution has been studied for a long time [9]. For n-C36H74 molecules, monoclinic and orthorhombic phases have been identified from Xray diffraction at room temperature. It is therefore clear that the double layer we observe cannot be a seed for any of these structures since the axis of the molecules in the second layer are rotated by 60 ° with respect to the first layer. The relative arrangement of two layers of linear saturated hydrocarbon chains has been reviewed by Segerman [10]. Neglecting the influence of the molecular ends, he found 8 kinds of packing arrangements: for 6 arrangements the chain axes are parallel while in the others the chain axes are rotated. Futhermore, he showed that if a rotated structure exists, both the intermolecular distance (d) and the rotation angle (w) verify the relations: d2 -- m ( n - m ) s 2,
w = 180 ° - 2 t a n - l [ ( m / n
-- m)l/2],
where s is the alternate C - C distance along the carbon chain, m is integral if the skeleton plane
L300
G. Watel et al. / STM study of long chain alkane bilayer films on graphite
of the carbon chain is parallel to the substrate plane or half-integral if the skeleton is perpendicular and n is an integer. As s is known (0.251 nm [5]) and suffers negligible variation with the molecule length, we obtain from both the measured intermolecular distance (0.44 nm) and angle (60°): m = 3.04 and n = 4.05. As these values are very close to integers, the above relations are fully verified. Taking m as an integer, we conclude that the n-C36H74 molecules in the monolayer lie on the graphite with their carbon skeleton plane parallel to the substrate. This is in agreement with previous models deduced from macroscopic studies [2,3], but in contrast with molecular dynamic calculations [11]. There are several possible explanations to this discrepancy. The first one could be that molecular dynamic calculations simulate alkane molecules on graphite without solvent. Up to now the influence of the solvent on the layer structure was neglected. The other point is that these simu-
lations depend on the accuracy of the interaction potentials. Fig. 3 shows an example of a high resolution image of the molecular monolayer. The troughs between molecules strips are not so clearly detected as in larger scaled images. However, the ends of each molecule can be identified since the molecules in neighbouring strips are interdigitated as previously reported [5,6]. By counting the number of white dots in between neighbouring troughs in fig. 3, we obtain 38 dots. As we found that the skeleton plane of C36H74 molecules lies flat on graphite, it is tempting to consider Groseck's model [2]. Each CH 2 group in the molecule is attached to one graphite hexagon. As the ends of the molecule (CH3) are bigger, extra hexagons are required in order to keep the molecule flat on the graphite. This means that one molecule occupies 38 graphite cells. This agrees well with the 38 dots we have counted instead of the 36 carbon atoms of the hexatria-
Fig. 3. High resolution STM image showing a detailed view of the molecular layer. A double periodicity is clearlyvisible along the direction parallel to the troughs (10 × I0 nm2, Z scale = 0.4 nm, V= 254 mV, I = 400 pA).
G. Watel et aL / STM study of long chain alkane bilayer filrns on graphite
contane molecule. Fig. 4 shows a model for the arrangement of molecules in the first layer. We point out that our model differs slightly from the model proposed by McGonigal et al. [5] (38 graphite cells per molecule instead of 39, respectively). We note that the hydrogen atoms on top of the monolayer adsorbed on graphite form the same lattice as that formed by the adsorbtion sites of graphite (centers of the hexagon). The trigonal site on this lattice is probably the adsorption site for a hydrogen atom of alkane. As for the graphite surface, the lattice of these trigonal sites matches remarkably the distance between hydrogen atoms attached to the carbon chains of alkane. As a result, the same molecular structure as that observed for the first layer can grow on top of the monolayer. Two kinds of orientation for the second layers may be considered: one with molecules
L301
parallel to those in the first layer, the other with molecules rotated by 60 °. The main difference between these second layers is the presence o f some molecules above troughs for the rotated layer. We can roughly compare the adsorption energy of these second layers by counting the number of H - H interactions between H on top of the first layer and the nearest H in the second layer. For the rotated layer, one part of the molecules does not cross first layer troughs and they therefore have the maximum of H - H interactions (nn_ H = 108). The other part of the molecules crosses the troughs and they have fewer H - H interactions. For the rotated second layer, the average number of H - H interactions per molecule is 102.5. For the parallel overlayer, all molecules are equivalent (nil_ H = 106). We cannot be sure that the H - H interaction energy is negative since the substrate/second layer inter-
Fig. 4. Model of both the first layer of C36H74 on a graphite lattice (Groszek's model) and the second C36H74 layer. The honeycomb array is for graphite, white circles are for hydrogens in the first layer and black circles for hydrogens of the second layer. The sticks schematize the carbon skeletons of molecules. The hydrogen atoms within the skeletal plane of the molecules (third atom of CH 3) are localized in the trough well above the substrate. They are not represented.
L302
G. Watel et aL / STM study of long chain alkane bilayer films on graphite
action is not negligible (Stranski-Krastanov growth mode). However, there are some indications of its sign from the particular geometry of stable boundaries. As shown in fig. 1, the step edges of stable staircase-shaped boundaries are always pinned by the troughs in the first layer. This geometry indicates that the less stable arrangement for molecules in the second layer corresponds to crossing the troughs of the first layer. As these molecules have fewer H - H interactions, we deduce that this interaction energy is negative. Hence, the second parallel layer which has more H - H interactions has a lower energy than the rotated layer. But we never observe the parallel second layer. We explain this contradiction by the nucleation process. As noted above, a molecule within the rotated layer which does not cross a trough has a lower energy than a molecule in a parallel overlayer (108 H - H interactions per molecule instead of 106, respectively). As a consequence, an incoming molecule in the first stage of nucleation of the second layer will prefer to adsorb at 60 ° which prevents the nucleation of the parallel second layer. In conclusion, this STM study has shown that: - Two layers may grow at the interface between graphite and a solution of C 3 6 H 7 4 in decane at room temperature. The second layer is rotated by 60 ° with respect of the first layer. - The carbon skeleton of the molecules is parallel to the graphite surface. -The growth of the interface crystal corresponds to a Stranski-Krastanov mode. All these results are consistent with a strong m o l e c u l e / g r a p h i t e interaction. As a consequence
of this interaction, the intermolecular distance (0.426 nm) in the first layer is compressed when compared to the bulk (0.47 nm) at room temperature. Because of this compression, the relative arrangement of the two first layers is different from the 3D crystal. Furthermore, the arrangement with the lowest energy does not grow because of an unfavorable nucleation energy.
References [1] H. Taub, in: The Time Domain in Surface and Structural Dynamics, Eds. G.J. Long and F. Grandjean, Vol. 288 (Kluwer, Dordrecht, 1988) 467. [2] A.J. Groszek, Proc. Roy. Soc. London A 314 (1970) 473. [3] G.H. Findenegg and M. Lipard, Carbon 25 (1987) 119. [4] D.P.E. Smith, J.K.H. H6rber, G. Binnig and H. Nejoh. Nature 344 (1990) 641. [5] G.C. McGonigal, R.H. Bernhardt and D.J. Thomson, Appl. Phys. Lett. 57 (1990) 28, G.C. McGonigal, R.H. Bernhardt, Y.H. Yeo and D.J. Thomson, J. Vac. Sci. Technol. B 9 (1991) 1107. [6] J.P. Rabe and S. Buckholz, Phys. Rev. Lett. 66 (1991) 20%; Science 253 (1991) 424; Makromol. Chem. Macromol. Symp. 50 (1991) 261; S. Buckholz and J.P. Rabe, Angew. Chem. Int. Ed. Engl. 31 (1992) 189. [7] F. Thibaudau, G. Watel and J. Cousty, Surf. Sci. Lett. 281 (1992) L303. [8] B. Michel, G. Travaglini, H. Rohrer, C. Joachim and M. Amrein, Z. Phys. B: Condensed Matter 76 (1989) 99. [9] V. Vand, Acta Cryst. 6 (1953) 797; H.M.M. Shearer and M. Vand, Acta Cryst. 9 (1956) 379: R. Boistelle, B. Simon and G. P~pe, Acta Cryst. B 32 (1976) 1240. [10] E. Segerman, Acta Cryst. 19 (1965) 789. [11] R. Hentschke, B.L. Schiirmann and J.P. Rabe, J. Chem. Phys. 96 (1992) 6213.
Surface Science Letters 281 (1993) L297-L302 North-Holland
surface s c i e n c e letters
Surface Science Letters
Direct observation of long chain alkane bilayer films on graphite by scanning tunneling microscopy G. Watel, F. T h i b a u d a u a n d J. C o u s t y DSM / DRECAM / SRSIM, Centre d'dtudes de Saclay, F 91191 Gif-sur-Yvette Cedex, France
Received 15 July 1992; accepted for publication 5 November 1992
The interface between a solution of hexatriacontane (n-C36H74) in decane and the basal plane of graphite has been studied by scanning tunneling microscopy at room temperature. For the first time, we show that a second layer which is rotated by 60° with respect to the first layer may grow at the interface. We demonstrate that the carbon skeleton of the molecule is parallel to the graphite surface. We propose a model for the arrangement of the molecular layers and an explanation for the origin of the second layer rotation.
Normal alkane molecules consist of linear chains of saturated hydrocarbons. The adsorption of these simple chains on the basal plane of graphite provides a model for molecular adsorption. The structure of these molecular layers is of considerable interest. For short alkane molecules (number of carbon atoms less than ten), the molecular arrangement at the v a c u u m / g r a p h i t e interface has been studied by various techniques [1]. For long chain molecules, measurements of adsorption isotherms have shown that molecules adsorb from non-polar solvent as a densely packed layer [2,3]. Recently, scanning tunneling microscopy (STM) has been used to obtain with varying degrees of resolution, images of long chain alkanes and related molecules adsorbed at the l i q u i d / g r a p h i t e interface. Up to now, only observations of the monolayer have been reported [4-6]. In this Letter, we report a STM study of the growth of n-hexatriacontane layers from a solution of n-C36H74 in decane onto graphite. We show that the arrangement of the first layers differs from the bulk structure of this alkane. A saturated solution in decane was prepared at room t e m p e r a t u r e from high purity solid n-C36HT4 and pure decane. We used diluted so-
lutions which are characterized by their concentration (c) which we define as the ratio of the saturated solution volume to the pure decane volume. A droplet of a diluted solution was applied onto the freshly cleaved surface of graphite (HOPG). The tip of our commercial microscope scans the l i q u i d / g r a p h i t e interface through the solution. Because of the evaporation of decane, most of the observations have been made within 30 min after deposition in order to minimize changes in the solution concentration. Within similar tunneling conditions, STM images of the interface layer depend on the initial n-f36HT4 concentration. From hundreds of images obtained at room temperature, two kinds of images can be distinguished. For low concentrations (1% < c < 3%), the images show a wellordered molecular layer with a single orientation of strips for scan areas over 500 x 500 nm 2. The overlayer is formed by parallel strips separated by troughs. The strip width (4.7 nm) corresponds well to the molecule length. In each strip, the long axis of the molecule is oriented parallel to the surface graphite, parallel to each other and perpendicular to the troughs. The measurement of the intermolecular distance gives 0.44 + 0.02 nm. For higher concentrations (5% < c < 10%),
0039-6028/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
L298
G. Watel et al. / STM study of hmg chain alkane bilayer films on graphite
Fig. 1. E x a m p l e of S T M i m a g e s of an interface b e t w e e n a 5 % solution of h e x a t r i a c o n t a n e in d e c a n e and the g r a p h i t e surface at r o o m t e m p e r a t u r e . (200 × 200 n m 2, Z scale = 0.9 nm, V = 692 mV, l = 540 pA).
Fig. 2. T h e s e STM i m a g e s show the two kinds of boundaries. E a c h small stick b e t w e e n t r o u g h s c o r r e s p o n d s to a single molecule. (47 × 47 n m 2, Z scale = 0.9 nm, V = 859 mV, 1 = 280 p A for (a); 20 × 20 nm 2, Z scale = 0.5 nm, V = 316 mV, 1 = 450 p A for (b).
L299
G. Watel et al. / STM study of long chain alkane bilayer films on graphite
there are several domains in the overlayer. Each domain exhibits the same lamellar structure formed by parallel strips separated by troughs as shown in fig. 1. The angle between the troughs in neighbouring domains is equal to 60°. Such an image raises the question: what is the nature of the boundary between these domains? Is it a wall between domains within the monolayer? In order to identify their nature, several boundaries have been imaged. Two examples of these boundaries are shown in fig. 2. Although the two domains shown in figs. 2a and 2b present the same relative orientation, the morphologies of the boundaries are very different. In fig. 2a, the boundary (A) is straight and merged with a trough of the domain for which the molecules are fully visible. In fig. 2b, the boundary (B) looks like a staircase with parts parallel to the main axis of the fully imaged molecules. Moreover, the behaviour of boundaries as a function of time depends on their morphologies. The boundary A remains stable over several images. In contrast, the boundary B may exibit large fluctuations of position which means that tens of molecules are moving. As the domains separated by boundaries A and B are rotated by the same angle, we expect both similar morphologies and dynamic behaviours if these boundaries are walls between domains in the same layer. It is clearly not the case. A carefull examination shows that some molecules near the boundary exhibit shorter lengths on the same side of the boundary. For instance in fig. 2a, all the molecules in the left domain are fully imaged while some molecules in the vicinity of the boundary appear shorter in the right domain. Such an apparent shorter length can be only explained by a partial screening of the first layer by an island of the second layer. These images provide strong evidence that these boundaries correspond to the edge of second layer islands. From the arrangement of the molecules in the vicinity of the boundary we can identify the second layer: in fig. 2a, the second layer is in the upper left corner while in fig. 2b it is in the lower right one. The growth of a second layer is consistent with the influence of the solution concentration on the interface as shown before.
However, no relief is detected in the STM images at the boundary of second layer. Usually, one considers that the relief in STM images is related to the surface topography of a conducting material. For thin films of insulating materials deposited on conducting substrates, the correspondence between STM images and the topography of this film is less obvious and depends on the imaging mechanism. Thus, the absence of relief related to a step in the STM images we present does not exclude the existence of such a step. In the next paper [7], we propose an imaging mechanism which explains the absence of relief at the step edge. When decane is evaporating from the solution, the images show some small bumps on the alkane layer. After complete evaporation large crystalshaped protusions are observed. These protusions are interpreted as small 3D crystals of n-Ca6H74 as previously observed [8]. The growth mode of alkane crystals on graphite from a decane solution can therefore be interpreted as StranskiKrastanov. The growth of 3D crystals of long chain alkane from a solution has been studied for a long time [9]. For n-C36H74 molecules, monoclinic and orthorhombic phases have been identified from Xray diffraction at room temperature. It is therefore clear that the double layer we observe cannot be a seed for any of these structures since the axis of the molecules in the second layer are rotated by 60 ° with respect to the first layer. The relative arrangement of two layers of linear saturated hydrocarbon chains has been reviewed by Segerman [10]. Neglecting the influence of the molecular ends, he found 8 kinds of packing arrangements: for 6 arrangements the chain axes are parallel while in the others the chain axes are rotated. Futhermore, he showed that if a rotated structure exists, both the intermolecular distance (d) and the rotation angle (w) verify the relations: d2 -- m ( n - m ) s 2,
w = 180 ° - 2 t a n - l [ ( m / n
-- m)l/2],
where s is the alternate C - C distance along the carbon chain, m is integral if the skeleton plane
L300
G. Watel et al. / STM study of long chain alkane bilayer films on graphite
of the carbon chain is parallel to the substrate plane or half-integral if the skeleton is perpendicular and n is an integer. As s is known (0.251 nm [5]) and suffers negligible variation with the molecule length, we obtain from both the measured intermolecular distance (0.44 nm) and angle (60°): m = 3.04 and n = 4.05. As these values are very close to integers, the above relations are fully verified. Taking m as an integer, we conclude that the n-C36H74 molecules in the monolayer lie on the graphite with their carbon skeleton plane parallel to the substrate. This is in agreement with previous models deduced from macroscopic studies [2,3], but in contrast with molecular dynamic calculations [11]. There are several possible explanations to this discrepancy. The first one could be that molecular dynamic calculations simulate alkane molecules on graphite without solvent. Up to now the influence of the solvent on the layer structure was neglected. The other point is that these simu-
lations depend on the accuracy of the interaction potentials. Fig. 3 shows an example of a high resolution image of the molecular monolayer. The troughs between molecules strips are not so clearly detected as in larger scaled images. However, the ends of each molecule can be identified since the molecules in neighbouring strips are interdigitated as previously reported [5,6]. By counting the number of white dots in between neighbouring troughs in fig. 3, we obtain 38 dots. As we found that the skeleton plane of C36H74 molecules lies flat on graphite, it is tempting to consider Groseck's model [2]. Each CH 2 group in the molecule is attached to one graphite hexagon. As the ends of the molecule (CH3) are bigger, extra hexagons are required in order to keep the molecule flat on the graphite. This means that one molecule occupies 38 graphite cells. This agrees well with the 38 dots we have counted instead of the 36 carbon atoms of the hexatria-
Fig. 3. High resolution STM image showing a detailed view of the molecular layer. A double periodicity is clearlyvisible along the direction parallel to the troughs (10 × I0 nm2, Z scale = 0.4 nm, V= 254 mV, I = 400 pA).
G. Watel et aL / STM study of long chain alkane bilayer filrns on graphite
contane molecule. Fig. 4 shows a model for the arrangement of molecules in the first layer. We point out that our model differs slightly from the model proposed by McGonigal et al. [5] (38 graphite cells per molecule instead of 39, respectively). We note that the hydrogen atoms on top of the monolayer adsorbed on graphite form the same lattice as that formed by the adsorbtion sites of graphite (centers of the hexagon). The trigonal site on this lattice is probably the adsorption site for a hydrogen atom of alkane. As for the graphite surface, the lattice of these trigonal sites matches remarkably the distance between hydrogen atoms attached to the carbon chains of alkane. As a result, the same molecular structure as that observed for the first layer can grow on top of the monolayer. Two kinds of orientation for the second layers may be considered: one with molecules
L301
parallel to those in the first layer, the other with molecules rotated by 60 °. The main difference between these second layers is the presence o f some molecules above troughs for the rotated layer. We can roughly compare the adsorption energy of these second layers by counting the number of H - H interactions between H on top of the first layer and the nearest H in the second layer. For the rotated layer, one part of the molecules does not cross first layer troughs and they therefore have the maximum of H - H interactions (nn_ H = 108). The other part of the molecules crosses the troughs and they have fewer H - H interactions. For the rotated second layer, the average number of H - H interactions per molecule is 102.5. For the parallel overlayer, all molecules are equivalent (nil_ H = 106). We cannot be sure that the H - H interaction energy is negative since the substrate/second layer inter-
Fig. 4. Model of both the first layer of C36H74 on a graphite lattice (Groszek's model) and the second C36H74 layer. The honeycomb array is for graphite, white circles are for hydrogens in the first layer and black circles for hydrogens of the second layer. The sticks schematize the carbon skeletons of molecules. The hydrogen atoms within the skeletal plane of the molecules (third atom of CH 3) are localized in the trough well above the substrate. They are not represented.
L302
G. Watel et aL / STM study of long chain alkane bilayer films on graphite
action is not negligible (Stranski-Krastanov growth mode). However, there are some indications of its sign from the particular geometry of stable boundaries. As shown in fig. 1, the step edges of stable staircase-shaped boundaries are always pinned by the troughs in the first layer. This geometry indicates that the less stable arrangement for molecules in the second layer corresponds to crossing the troughs of the first layer. As these molecules have fewer H - H interactions, we deduce that this interaction energy is negative. Hence, the second parallel layer which has more H - H interactions has a lower energy than the rotated layer. But we never observe the parallel second layer. We explain this contradiction by the nucleation process. As noted above, a molecule within the rotated layer which does not cross a trough has a lower energy than a molecule in a parallel overlayer (108 H - H interactions per molecule instead of 106, respectively). As a consequence, an incoming molecule in the first stage of nucleation of the second layer will prefer to adsorb at 60 ° which prevents the nucleation of the parallel second layer. In conclusion, this STM study has shown that: - Two layers may grow at the interface between graphite and a solution of C 3 6 H 7 4 in decane at room temperature. The second layer is rotated by 60 ° with respect of the first layer. - The carbon skeleton of the molecules is parallel to the graphite surface. -The growth of the interface crystal corresponds to a Stranski-Krastanov mode. All these results are consistent with a strong m o l e c u l e / g r a p h i t e interaction. As a consequence
of this interaction, the intermolecular distance (0.426 nm) in the first layer is compressed when compared to the bulk (0.47 nm) at room temperature. Because of this compression, the relative arrangement of the two first layers is different from the 3D crystal. Furthermore, the arrangement with the lowest energy does not grow because of an unfavorable nucleation energy.
References [1] H. Taub, in: The Time Domain in Surface and Structural Dynamics, Eds. G.J. Long and F. Grandjean, Vol. 288 (Kluwer, Dordrecht, 1988) 467. [2] A.J. Groszek, Proc. Roy. Soc. London A 314 (1970) 473. [3] G.H. Findenegg and M. Lipard, Carbon 25 (1987) 119. [4] D.P.E. Smith, J.K.H. H6rber, G. Binnig and H. Nejoh. Nature 344 (1990) 641. [5] G.C. McGonigal, R.H. Bernhardt and D.J. Thomson, Appl. Phys. Lett. 57 (1990) 28, G.C. McGonigal, R.H. Bernhardt, Y.H. Yeo and D.J. Thomson, J. Vac. Sci. Technol. B 9 (1991) 1107. [6] J.P. Rabe and S. Buckholz, Phys. Rev. Lett. 66 (1991) 20%; Science 253 (1991) 424; Makromol. Chem. Macromol. Symp. 50 (1991) 261; S. Buckholz and J.P. Rabe, Angew. Chem. Int. Ed. Engl. 31 (1992) 189. [7] F. Thibaudau, G. Watel and J. Cousty, Surf. Sci. Lett. 281 (1992) L303. [8] B. Michel, G. Travaglini, H. Rohrer, C. Joachim and M. Amrein, Z. Phys. B: Condensed Matter 76 (1989) 99. [9] V. Vand, Acta Cryst. 6 (1953) 797; H.M.M. Shearer and M. Vand, Acta Cryst. 9 (1956) 379: R. Boistelle, B. Simon and G. P~pe, Acta Cryst. B 32 (1976) 1240. [10] E. Segerman, Acta Cryst. 19 (1965) 789. [11] R. Hentschke, B.L. Schiirmann and J.P. Rabe, J. Chem. Phys. 96 (1992) 6213.