Copyright 2009 American Chemical Society
JANUARY 20, 2009 VOLUME 25, NUMBER 2
Letters Experimental Three-Dimensional Description of the Liquid Hexadecane/Graphite Interface L. Pham Van, V. Kyrylyuk,† J. Polesel-Maris, F. Thoyer, C. Lubin, and J. Cousty* CEA-Saclay, baˆt. 462, F-91191 Gif sur YVette Cedex, France ReceiVed NoVember 4, 2008. ReVised Manuscript ReceiVed December 3, 2008 By using an atomic force microscope based on a quartz tuning fork sensor, a 3-dimensional description of the interface between liquid hexadecane and a highly oriented pyrolytic graphite surface can be achieved at room temperature. The C16H34 monolayer in contact with the substrate surface exhibits a lamellar structure whereas no observation at the liquid/graphite interface by scanning tunnelling microscopy was reported for this alkane. The second layer shows very weak corrugations corresponding to lamella boundaries. Force/distance curves show at least four oscillations separated by 0.4 nm except for the first period with a 0.38 nm distance that corresponds to the layer closer the substrate. Such a description agrees well with molecular dynamics results obtained on alkane/solid interfaces.
Introduction Bulk and surface properties of n-alkanes (CH3(CH2)n - 2CH3, hereafter denoted Cn) have attracted intense research effort for nearly a century because of their relevance to lipids, surfactants, liquid crystals, lubricants, and fuels.1 The organization of alkanes in thin layers or in monolayers on several substrates has also been investigated in detail. From an analysis of all of the results obtained at room temperature, the molecule packing appears to be determined by several factors such as the growth mode of the overlayer, the alkane length, and the nature of the substrate. Let us first consider the structure of free surfaces. When alkanes are deposited onto a graphite surface by the method of using a very dilute mixture in a volatile solvent, the remaining molecules after solvent evaporation form a monolayer. This monolayer consists of an arrangement of parallel lamellae composed of the close packing of side by side parallel molecules lying flat on the substrate. Such a molecular close-packing arrangement was * Corresponding author. E-mail:
[email protected]. † Present address: Department of Radiophysics, National Taras Shevchenko University of Kyiv, Volodymyrska 64, Kyiv 33, 252033 Ukraine. (1) Small D. M. In The Physical Chemistry of Lipids; Handbook of Lipid Research; Hanahan, D. J., Ed.; Plenum Press: New York, 1986; Vol. 4.
recently observed by scanning tunnelling microscopy (STM) for several alkanes on graphite including “short” alkanes such as C13 at room temperature.2 To study the free surface of thicker alkane layers, STM can no longer be employed because the multilayer packing blocks the tunnelling current. Therefore, the surface structure of such thick layers was investigated by using atomic force microscopy (AFM).3-8 For overlayers obtained by the condensation of an alkane vapor, molecules appear to form lamellae sitting roughly parallel to the surface. In some cases, domains are recognized at the layer surface.5 Molecular resolution was achieved on C36 at room temperature.3 In that case, the surface structure shows lamellae with a similar packing to that observed for a monolayer. (2) Chen, S. Q.; Yan, H.-J.; Yan, C.-J.; Pan, G.-B.; Wan, L.-J.; Wen, G.-Y.; Zhang, D.-Q. Surf. Sci. 2008, 602, 1256. (3) Valdre, G.; Allessandrini, A.; Muscatello, U.; Valdre, U. Philos. Mag. Lett. 1998, 78, 255. (4) Martin, D. S.; Weightman, P.; Gauntlett, J. T. Surf. Sci. 1998, 398, 308. (5) Magonov, S. N.; Yerina, N. A. Langmuir 2003, 19, 500. (6) Tracz, A.; Ungar, G. Macromolecules 2005, 38, 4962. (7) Trogisch, S.; Simpson, M. J.; Taub, H.; Volkmann, U. G.; Pino, M.; Hansen, F. Y. J. Chem. Phys. 2005, 123, 154703. (8) Bai, M.; Trogisch, S.; Magonov, S.; Taub, H. Ultramicroscopy 2008, 108, 946.
10.1021/la803665k CCC: $40.75 2009 American Chemical Society Published on Web 12/15/2008
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When the layer results from the solvent evaporation of a solution containing alkanes, some 3D islands may grow up on top of a thin layer of alkanes lying parallel to the substrate.3,7,8 AFM observations in contact mode or in amplitude-modulated mode (tapping) provide information on the molecular organization of thick islands. Considering the island height, the alkanes are deduced to be roughly perpendicular to the substrate surface. X-ray diffraction measurements on the same sample give evidence that these islands are 3D crystals coexisting with a thin layer of molecules covering the substrate.3,7-9 Numerous experimental and theoretical studies were also reported on the alkane organization at the liquid/solid interface. Different techniques such as X-ray diffraction, neutron diffraction, and STM have been used to explore the molecular packing.10-20 From mixtures in various solvents, alkanes are seen to form self-assembled monolayers with molecules parallel to the graphite surface.10-12 STM investigations at room temperature12-20 reveal that the monolayer consists of parallel lamellae, the width of which is related to the alkane length. For n-alkanes with fewer than 23 to 24 carbon atoms, no experimental evidence of molecular self-organization was found by STM.17 However, X-ray diffraction has recently provided evidence of the existence of a C16 monolayer adsorbed on graphite at room temperature.12 This monolayer is shown to melt at 327.5 K. STM studies of the lamella structure built from a liquid mixture of C38 and C17 reveal intense exchanges of molecules between the monolayer and the liquid at room temperature.19 Because thick alkane layers become too insulating, the molecular arrangement for the layers above the first adsorbed monolayer is difficult to study by STM. Nonetheless, alkane 3D arrangements at the liquid/solid interface were investigated by molecular dynamics simulations.21-28 A layered packing of molecules at the interface between different liquid alkanes and the solid surface is deduced from all of these studies for temperatures near the bulk melting temperature. When the distance to the solid surface increases, the density of the molecular mass center exhibits periodic oscillations.21 Typically, three to six oscillations were reported depending on the alkane length and the temperature. The distance between the oscillation maxima ranges from 0.24 22 to 0.45 nm27,28 in relation to the model used to describe the molecules. (9) Bai, M.; Knorr, K.; Simpson, M. J.; Trogisch, S.; Taub, H.; Ehrlich, S. N.; Mo, H.; Volkmann, U. G.; Hansen, F. Y. Eur. Phys. Lett. 2007, 79, 26003. (10) Arnold, T.; Thomas, R. K.; Castro, M. A.; Clarke, S. M.; Messe, L.; Inaba, A. Phys. Chem. Chem. Phys. 2002, 4, 345. (11) Hansen, F. Y.; Criswell, L.; Fuhrmann, D.; Herwig, K. W.; Diama, A.; Dimeo, R. M.; Neumann, D. A.; Volkmann, U. G.; Taub, H. Phys. ReV. Lett. 2004, 92, 046103. (12) Espeau, Ph.; White, J. W.; Papoular, R. J. Appl. Surf. Sci. 2005, 252, 1350. (13) McGonigal, G. C.; Bernhardt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990, 57, 28. (14) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (15) Watel, G.; Thibaudau, F.; Cousty, J. Surf. Sci. Lett. 1993, 281, 297. (16) Wawkuschewski, A.; Cantow, H.-J.; Magonov, S. N. Langmuir 1993, 9, 2778. (17) Couto, M. S.; Liu, X.-Y.; Meekes, H.; Bennema, P. J. Appl. Phys. 1994, 75, 627. (18) Bucher, J. P.; Roeder, H.; Kern, K. Surf. Sci. 1993, 289, 370. (19) Cousty, J.; Pham Van, L. Phys. Chem. Chem. Phys. 2003, 5, 599. (20) Gosvami, N. N.; Sinha, S. K.; O’Shea, S. J. Phys. ReV. Lett. 2008, 100, 076101. (21) Xia, T. K.; Ouyang, J.; Ribarsky, M. W.; Landman, U. Phys. ReV. Lett. 1992, 69, 1967. (22) Balasubramanian, S.; Klein, M. L.; Siepmann, J. I. J. Chem. Phys. 1995, 103, 3184. (23) Tsuchiya, Y.; Hasegawa, H.; Iwatsubo, T. J. Chem. Phys. 2000, 114, 2484. (24) Ungerer, P.; Beauvais, C.; Delhommelle, J.; Boutin, A.; Rousseau, B.; Fuchs, A. H. J. Chem. Phys. 2000, 112, 5499. (25) Yamamoto, T.; Nozaki, K.; Yamaguchi, A.; Urakami, N. J. Chem. Phys. 2007, 127, 154704. (26) Pint, C. L. Surf. Sci. 2006, 600, 921. (27) Pint C. L. arXiv:cond-mat/0602478v1.
Letters
Besides these investigations, several studies using a surface force apparatus (SFA) have experimentally shown the layered structure of the liquid/solid interface.29,30 For C16 molecules, the typical distance between each neighboring layer near the interface is 0.5 nm,29 in good agreement with bulk data and simulations. However, the principle of SFA prevents any observation of the molecular arrangement within a layer, including the monolayer physisorbed onto the solid surface. Finally, similar results have been obtained near the liquid/solid interface by AFM but without any imaging of the molecular arrangement in a layer.31,32 In this letter, we report the first direct observation of both the monolayer and the second layer of C16 adsorbed on graphite by using an AFM equipped with a quartz tuning fork. Periodic features in the phase signal/distance curve obtained on the same sample will provide additional insight into the layered structure of the C16 liquid/solid interface at room temperature.
Experimental Conditions AFM observations were made using a quartz tuning fork sensorbased microscope working in air and developed in the laboratory. Using this probe instead of a conventional cantilever enables us to keep the sensor away from the liquid because only the tip is immersed. To improve the sensitivity, the tuning fork was run in the so-called zero-phase mode described elsewhere.33 This technique allows us to image the surface in frequency-modulation mode with the enhanced stability and speed of phase-mode AFM.34 The tip is prepared by the electrochemical etching of a tungsten wire (∼70 µm diameter) in a KOH (3 M) solution. The sharpened wire, with a typical length of 1.5 mm, is then glued to one prong of the tuning fork. The other prong receives some extra glue to diminish the loss of resonance quality factor Q. Hence, using commercial watch quartz crystals resonators (32 768 Hz, k ) 15 kN/m), it is possible to reduce the oscillation amplitude of the tip to few tenths of a nanometer and maintain Qbetter than 3000 in liquid. Our samples were made of freshly cleaved HOPG (0001) surfaces rapidly covered with a droplet of hexadecane (as received from Sigma-Aldrich). After C16 is spread on the surface, a liquid film exhibiting a thickness of few micrometers is formed. The topography of the samples was acquired in frequencymodulated AFM mode (FM AFM) at different frequency detunings. On several points of these images, phase and excitation voltage spectroscopy curves have been acquired in phase-mode AFM (PM AFM) while approaching the tip. Both experiments were performed on the same sample with the same tip, keeping the same oscillation amplitude (typically 0.7-1 nm). Note that the microscope features an automatic gain control that regulates the oscillation amplitude. It was shown that it prevents instabilities occurring in the approach phase curves.35
Results and Discussion The topography images were recorded for different detuning values in the repulsive regime (∆f > 0). Figure 1a-c shows a series of topographic downward images recorded at different values of detuning. At ∆f ) +0.23 Hz (Figure 1a), the contrast is weak at the beginning of the scanning (upper part). Some instability starts to develop in the lower half of the image, and (28) Gao, J.; Luedtke, W. D.; Landman, U. J. Chem. Phys. 1997, 106, 4309. (29) Christenson, H. K.; Gruen, D. W. R.; Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1987, 87, 1834. (30) Georges, J. M.; Millet, S.; Loubet, J. L.; Tonck, A. J. Chem. Phys. 1993, 98, 7345. (31) Klein, D. L.; McEuen, P. L. Appl. Phys. Lett. 1995, 66, 2478. (32) Gosvami, N. N.; Sinh, S. K.; Hofbauer, W.; O’Shea, S. J. J. Chem. Phys. 2007, 126, 14708. (33) Pham Van, L.; Kyrylyuk, V.; Thoyer, F.; Cousty, J. J. Appl. Phys. 2008, 104, 074303. (34) Albrecht, T. R.; Gru¨tter, P.; Horne, D.; Rugar, D. J. Appl. Phys. 1991, 69, 668.
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Figure 2. Averaged approach phase curves (four approach curves used) exhibiting oscillations induced by the layered structure of the solid/ liquid interface. The phase signal corresponds to the difference between the excitation and the measured signal at constant frequency excitation. The Z origin was arbitrarily chosen. At least four oscillations can be observed (arrows). Their separation distance is nearly constant (∼0.4 nm) and demonstrates the stacking of four molecular layers.
Figure 1. FM AFM topographic images (23 nm × 23 nm) acquired in PM AFM for three different detunings: (a) +0.23, (b) +19.83, and (c) +36.53 Hz. Some tip instabilities indicated by dotted lines mainly affect image a.The three images exhibit a contrast forming an arrangement of stripes. On image c, the measured width between darker stripes corresponds to the hexadecane length. The corrugation due to boundaries between lamella is approximately 0.1 nm.
it can be attributed to tip changes induced by the probable presence of graphite flakes. As the tip recovers, some stripes can clearly be resolved (lower part of the image). We have checked that both the upper and lower parts of the image exhibit the same height. By progressively increasing the detuning, the stripe contrast vanishes. It reappears by ∆f ) +19.8 Hz (Figure 1b) and improves while increasing ∆f. Figure 1c shows the stripe pattern at ∆f ) +36.5 Hz. In that image, the distance between the stripes is approximately 2.4 nm, and the corrugation settles to 0.1 nm. In other experiments, the width of the lamellae formed from C14 deposited on graphite reduces to ∼2.1 nm, as expected from the molecular length (AFM image not shown). In a second set of experiments on the C16 solid interface, we have recorded the variations of the phase difference between the
measured signal and the excitation as the mean distance between the tip and the surface is decreased. During the approach, the excitation frequency is kept constant. Figure 2 represents a phase approach curve obtained by averaging four approach curves. We clearly observe oscillations in the phase shift as the tip approaches the surface. These oscillations, which correspond to changes in the tip/surface interaction, signal the presence of a layered structure at the solid/liquid interface. The distance separating the extremes, the periodicity, is on the order of 0.4 nm. Its value, however, decreases slightly for the layer closest to the graphite substrate. Remarkably, the measured 0.4 nm value agrees well with the 0.45 nm distance separating the mass centers of the molecules in the neighboring layers, recently calculated by molecular dynamics for C24 molecules on graphite27 and with the interlayer distance of confined C16 films.28 The distance between neighboring alkane layers perpendicular to the main molecular axis is relatively constant (∼0.45 nm) in alkane crystals independent of the molecular length.1 To scale the approach distance of the topographical images taken at a constant frequency detuning and identify which layer is actually seen, we utilize the excitation voltage (damping) and phase-shift curves simultaneously recorded for a single approach (Figure 3). Our aim is to compare the dissipation voltage measured during FM AFM imaging to the damping approach curve obtained in PM AFM. From a nonlinear control theory treatment,36,37 the phase φ and the excitation signal w0 can be expressed as
[
φ ) -arctan
ω Qω0
(
)]
Ξ ω2 1- 2 +Ψ ω0
(1)
and
w0 ) s1k
ω Ξ Qω0 sin φ
(2)
where Ξ and Ψ are two dimensionless quantities associated with the dissipative and the conservative contributions of the tip/ sample interaction averaged over one oscillation period, respec(35) Sugawara, Y.; Kobayashi, N.; Kawakami, M.; Li, Y. J.; Naitoh, Y.; Kageshima, M. Appl. Phys. Lett. 2007, 90, 194104. (36) Polesel-Maris J. Modeling and Experiments in Dynamic Force Microscopy in Ultra High Vacuum. Ph.D. Thesis, Toulouse, 2005;. http://tel.archivesouvertes.fr/tel-00009666/en/. (37) Polesel-Maris J.; Venegas de la Cerda M. A.; Martrou D.; Gauthier S. submitted to Phys. ReV. B.
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Figure 3. Simultaneous damping signal (dotted gray line) and calculated corrected damping (solid gray line) are plotted along with phase signal (black) during a single approach (see text). The Z-scale origin was arbitrarily adjusted to the maximum of the corrected damping voltage. The corrected damping signal allows us to compare data obtained in FM AFM and PM AFM. In particular, the respective dissipation figures corresponding to the images in Figure 1 were accordingly positioned on the corrected damping graph (arrows) and enable us to discriminate which layer is observed on the topographic images.
tively. The quantity Ψ was originally proposed by Giessibl,38 and a similar expression for Ξ was also derived by Sader et al.39 s1 is the oscillation amplitude, k is the spring constant, ω is the angular frequency (ω ) ω0 at the free resonance), and Q is the quality factor. We consider ΞFM and ΞPM, the dissipative terms associated with frequency-modulation mode and phase mode, respectively, with (w0)FM and (w0)PM being the corresponding excitations. Here, because the oscillation amplitude is constant and the relative frequency change is negligible, we can assume ΞFM and ΞPM to be quasi-equal for the same distance to the substrate. Thus, from eq 2 we infer the simple equivalence
(w0)FM )
ω (w0)PM (w0)PM ≈ ω0 sin φ sin φ
In Figure 3, we have plotted the corrected damping, which merely corresponds to the excitation voltage curve (black dotted) normalized by sin φ (solid gray curve). The average excitation values of 0.12, 0.22, and 0.30 mV corresponding to the images presented in Figure 2a-c, respectively, are highlighted by the arrows. Thus, a straightforward identification of the layer can be made. Figure 1a corresponds to the second layer (II) whereas parts a and b of Figure 1 are related to the first layer (I). As mentioned in the Introduction, STM observations of alkane organization in the monolayer at the liquid/graphite interface were reported only for molecules longer than C22-C24 at room temperature. In contrast, the results presented above demonstrate that C16 (and C14) molecules do self-organize on graphite at (38) Giessibl, F. J. Phys. ReV. B 1997, 56, 16010. (39) Sader, J. E.; Uchihashi, T.; Higgins, M. J.; Farrell, A.; Nakayama, Y.; Jarvis, S. P. Nanotechnology 2005, 16, S94.
Letters
room temperature. Furthermore, the oscillations detected by using AFM provide evidence of a layered interface between C16 molecules and the graphite surface. A possible origin of the discrepancy between STM and AFM observations is the lateral force induced by the STM tip when it probes the self-assembled molecules. As shown two decades ago, the STM tip distorts the graphite surface when an insulating layer lies between the tip and the surface.40 Because of this deformation, the scanning tip applies a shear force on the monolayer, squeezing out molecules off the surface. This hypothesis could explain the absence of any self-assembled alkanes shorter than C22 on graphite in STM images because the alkane/graphite interaction increases with the molecular length.41 STM images of self-assembled monolayers of C12, C14, and C16 at the liquid/Au (111) interface42 support this model. For this system, the STM tip does not alter the metal surface. This explanation is also consistent with the presence of a C16 monolayer on graphite as detected by X-ray diffraction.12 The existence of the layered structure of C16 near a solid surface evidenced by the AFM measurements agrees well with results of molecular dynamics simulation. We believe that two nonexclusive reasons could explain the absence of molecular resolution in AFM images. The first one concerns the diameter of the AFM tip. Scanning electron microscopy observations of the tip apex show a diameter of less than 50 nm. We do not know the exact value of the apex diameter, but we think that above 10 nm it is difficult to detect the molecules in lamellae because the intermolecule distance in a lamella is 0.5 nm. The other explanation could be some thermal mobility of molecules in the monolayer preventing good contrast in the AFM images.
Conclusions A 3D description of the interface between liquid hexadecane and a graphite surface can be achieved at room temperature by using an AFM based on a quartz tuning fork sensor. The lamellar structure of the C16 monolayer in contact with the substrate is clearly imaged whereas no observation by STM for this alkane on graphite was reported in the literature. The second layer exhibits very weak corrugation corresponding to lamellae. Phase/distance curves show at least four oscillations separated by 0.4 nm, except for the first period (0.38 nm) that corresponds to the layer closer to the substrate. Such a description agrees well with molecular dynamics calculations performed on alkane/solid interfaces. Finally, this study demonstrates that an AFM based on a quartz tuning fork sensor can display surprisingly good sensitivity suitable for exploring the liquid/solid interface. That opens avenues for studying self-assembled molecules and also biological objects in physiological solutions. LA803665K (40) Soler, J. M.; Baro, A. M.; Garcia, N.; Rohrer, H. Phys. ReV. Lett. 1986, 57, 444. (41) Paserba, K. R.; Gellman, A. J. J. Chem. Phys. 2001, 115, 6737. (42) Marchenko, O.; Cousty, J. Phys. ReV. Lett. 2000, 84, 5363.
