Langmuir 1993,9, 341-346
341
A Comparative Atomic Force Microscopic Study of Liquid Crystal Films: Transferred Freely-Suspended vs Langmuir-Blodgett. Morphology, Lattice, and Manipulation R. M. Overney, E. Meyer, J. Frommer; and H.-J. Giintherodt The Physics Institute of the University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
G. Decher, J. Reibel, and U. Sohling The Physical Chemistry Institute of the Johannes Gutenberg University at Mainz, Welder Weg 11,D-65500Mainz, Germany Received September 28,1992 The atomicforcemicroscope (AF'M) provides directstructuralinformationon transferred freely-suspended
filmeof liquid crystals, rmging from morphology on the "large, scaleof micrometersdown to intermolecular spacing on the angstrom scale. Multilayer film thickness is measured, 88 well ae monolayer step heighta and unit cellparameters. We show here that these valuesfor film thicheas, stepheights,and intermolecular spacingare often in good agreementwith electrondiffractionand X-rayreflectivitymeaeurementa. However, in some ~ 8 8 8 8the AFM data reveal previously undetected lattices and defecta. AFM meaeurementa are also performed on LaugmukBlodgett (LB) films of the same class of molecules, allowing comparison of the different film-formingtechniques. The AFM has also been used as a tool to intentionallydeform the films, creating features of tailored dimensions. The forces required to deform the f h are d i e d .
Introduction Severalstudieshavenow demonstratedthat atomicforce microscopy (AFM) can provide an important new view of ultrathin, well-ordered organic multilayer films.lJ With very smallloadw, on the order of 10-9-10-8 N (1-10 nN), it has been shown to be poseible to image LangmuircBlodgett films.39' Film thickness, mono- and multilayer steps, and the arrangement of molecules in the surface of the film have been rec0rded.s-8 AFM is a tool that collects data down to the nanometer scale and, therefore, can resolve localized defects on the submiemeter scale.' It produces images of topography of materials by scanning a sharp 4 p mounted on a very soft spring over a sample surface. The spring deflects in responae to ita interaction with surface features. Typical
scan ranges and corresponding resolution are on the micrometer, nanometer, and angstrom scales.1 A novel method for the preparation of ultrathin multilayer films of smectic liquid crystalline compounds has recently been developed to produce transferred freely suspended films.9 T h e films are formed by drawing a thin film of two-dimensional fluid acroas an aperture to create a freely suspended film. These freely suspended films of thermotropicliquid crystalsare then transferred onto solid substrates. The method takes advantageof the high degree of ordering obtainable in the freely-suspended films since they are in a state of equilibrated two-dimensional fluids at the elevated preparation temperatures. In the same highly mobile phase they can be annealed, yielding a fairly homogeneous and defect-free film. After transfer onto a substrate, they are cooled to ambient temperatures and imaged with the AFM. The properties of the films both (1) For recant review of AFM,we: (a) Rugar, D.;H"a, P. Phys. before and after transfer, such asfilm thickness,structure, Today 1990, October. (b) Frommer, J.; Meybr, E.J. Phys.: Condene. and stability as well as phase assignmenta and transition Matter 1991, 3, S1. (c) Ultramicroscopy 1992, 42-44 (entire volumes devoted to STM and AFM). (d) Heckl, W. Thin Solid F i h 1992,210temperatures, have been determined using small angle 211,840. (e) Frommer, J. Angm. Chem., Znt. Ed. Engl. 1992,31,1298. X-ray scattering (SAXS)and optical micro"py.10J1 (2) For recant revimof thin,organic f h ,888: (a) Ulman, A. In An In this study we present AFM resulta on the traderred Zntrodrretion to Ultrathin Organic Film. FromLangmuir-Blodgett to Sa@lmmbly; Academic PteM. San biego, CA, 1991. (b) Swalen, J. films of ethyl 4'-(n-odyloxy)biphenyl-4-carbo.ylate (1) A m . Rev. Mater. Sci. 1991,21,373. and on LB f h of methyl 4'-(n-heptyloxy)biphenyl-4(3) Meyer, E; Howald, L.; Owrwy,R.; Winzelmann,H.; Frommer, carboxylate (2) and compare them. A comparison is also J.; Gllnthemit, H.; Wagner,T.;Bchier, H.; h t h , S. Nature 1991,349, 398. made between AFM data and electron diffraction and (4) Ovemey, R,Meyer, E.; Frommey,J.; Brodbeck, D.;LMhi, R.; small angle X-ray reflectivity data from the same films, Howald,L.;Gllntherodt,H.;Fujihira,M.;TaLhlro,H,Gotoh,Y.Natrcre,focusing on the topics of morphology and molecular 1992,369,133. Meyer,E, Ovemey,R.; La&, R; Brodbeck, D.;Howald, structure. Finally, the AFM is used as an invasive probe L.; Fmmmer, J.; Gllnthemdt, H.;Wdtar, 0.;Fujihira, M.;Takano, H.; Gotoh, Y. Thin Solid F i l m 1992,220. to deform the film.From the forces required to dmturb (6)(a) Bourdieu,L.;Silbe~,P.;Chatenay,D.Phy8.Rev.Lett. 1991, the films and from the nature of the modifications, 67,2029. (b) Alm, C.; Smith, E.; Porter, M. J. Am. Chem. SOC.1992, conclusions are drawn about the cooperative motion of 114,1222. (c) Schwartz, D.;Garnaes, J.; Viswamthan, J.; Zaeadzineki, J. Science 1992,287, 508. assembled molecules in these films. ~~
~
(6) Fucb, H.; Chi,L.; Eng, L.;Craf, K.Thin Solid Film 1992,210211,656. (7) Meyer, E.; Howald, L.; Ovemey, R.; Hehlmann, H.; Frommer, J.;GOnthemdt, H. Ultramicroscopy 1992,42-44,274. Meyer,E.;Overney, R.;Howald,L.;Brodbeck,D.;L[Lthi,R;Gllntherodt,H. InFundamentals of Friction; Singer, I., Pollock, H., Ede.; Kluwer Academic Publishers: DoIdrecht, 1992. (8)Meyer, E.; Ovemey, R.; Brodbeck, D.; Howald, L.; Lllthi, R.; Frommer, J.; GOnhrodt, H. Phy8. Rev. Lett. 1992.69, 1777.
(9) Maclennan, J.; Decher, G.; Sohling, U.Appl. Phy8. Lett. 1991,69, 917. (10) Decher, C.; Maclennan, J.; Reibel, J.; Sohling, U . Adv. Mater. 1991, 3, 617. Decher, G.; Maclennan, J.; Reibel, J. Ber. hn8en-Ge8. Phy8. Chem. 1991,%, 1620. (11) Decher,G.;Maclennan,J.; Sohling,U.;Reibel,J. ThinSolidfilm l992,210/211,M)4.
0743-7463/93/2409-0341$04.00/0(9 1993 American Chemical Society
342 Langmuir, Vol. 9, No. 1, 1993
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Experimental Section Films of 1are prepared according to the literature procedure, J ~ liquid crystal is drawn into briefly described as f o l l o ~ s . ~The a freely-suspended film and then transferred to a hydrophobized quartz substrate in its fluid, smectic A phase at 100 "C. Before the AFM measurementsare performed,the transferred films are cooled to their crystalline phase at room temperature. The preparation of the LB films is described elsewhere.12 The fused quartz and silicon substrates are cleaned using step SC-1of the RCA procedure13 and then treated with a mixture (v:v) of 1 part concentrated hydrochloric acid (32% (w/w)) and 7 parts Milli-Qwater for 20 min. The substrates are then rendered hydrophobic by treatment with octadecyltrichlorosilane (OTS) (3h in a solution of 1vol % OTS in a mixture of 70 mL of decalin, 20 mL of CCL, and 10 mL of CHCl3) as adapted from the 1iterat~re.l~ Alternatively, surfaces are rendered hydrophobic by treatment with hexamethyldisilazane (HMDS) for 24 h. Small angle X-ray scattering (SAXS) is performed with a Siemens D500 powder diffractometer using copper K a radiation with a wavelength of 1.542 A. Scattering data are acquired using a DACO-MP interface connected to a personal computer. The same samples prepared for SAXS can also be used in the AFM measurements. Transmissionelectron diffraction is performed with a Philips EM 300, using 80 kV. Samples are prepared for these measurements by transfer of the freely suspended films onto amorphous carbon- or polymer-coated electron microscope grids, prepared according to literature procedures.15 The uncertainty in the electron diffraction measurements is less than 1% for the determination of lengths and less than lo for the determination of angles. A commerciallyavailableAFM outfitted with 100-pmand 1-pm scanners is used. The lever deflection is measured by laser beam deflection. Cantilever-like springs are V-shaped, 100 and 200 pm long, with integrated tips of Si3N4and spring constants of 0.3 and 0.12 N/m.16 All measurements are carried out at room temperature in ambient atmosphere. Under normal scanning conditions (10-9-10-8 N, 1 3 pm/s) images of unperturbed planes of molecularly-resolved film surfaces are obtained. Intentional "scribing" into the film surface is performed by controlled variation of the parameters of applied load from the AFM tip, scan velocity, and feedback loop frequency4.5c+8and is described in detail below. The images presented here are unfiltered, raw data with a lateral uncertainty of 5 % and vertical uncertainty of 10%. The instrument is calibrated on the micrometer scale with silicon grids having micrometer-scale features, and on the nanometer scale with graphite and mica standards.
Results and Discussion Morphology and Structure of Transferred Freely Suspended Films. The AFM images of the transferred f i i of 1 reveallarge,flat planes,micrometers in dimension (Figure 1). These planes are irregularly shaped and (12) Decher, G.; Sohling, U. Ber. Bunsen-Ges. Phys. Chem. 1991,95, 1538. (13)Kern, W. Semicond. Int. 1984,94. (14) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100,465. (15) Reimer, L. Elektronenmikroskopische Untersuchungs- und Pruepurutionsmethoden, 2nd ed.; Springer-Verlag: Berlin, 1967; p 322. (16)The cantilever force constant is chosen to be "soft" enough to detectably respond to interactions with the scanned surface, yet "stiff" enough to reversiblymaintain ita integrity,i.e., to be able to return to an equilibriumpositionbetweeninteractions. TypicalAFM cantilevershave spring constants of 0.1-100 Nom-'. By modifying the cantilever specifications and/or the scanning conditions, the AFM can be transformed from a relatively passive imaging tool into an active modifying tool.
Figure 1. 5 X 5 pm2AFM images taken at various points on the surface of an -&layer film of 1: (top, middle) 2D displaysof the AFM data; (bottom) 3D display of the AFM data. Large, flat, overlaid planes, micrometersin lateral dimension and 18-22 nm in total height, are recorded. The step size of 24 A measured as the difference in height between two vertically adjacent planes, corresponds to a monomolecular layer (see text).
overlaid on each other, probably due to crystallizationof the film during cooling. The surfaces of these films are pore-free, unlike the surfaces of thin organic films in previous AFM studiesG8and unlike the LB films of a
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Langmuir, Vol. 9, No. 1, 1993 343
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Figure 2. 13 X 13 pm2 AFM image of the edge of a TFS-film of 1, 20 nm high, on a quartz substrate dotted with small islands (see text). 1os
0
.
0
3
0
7
1
,Inm,
0.025
0.020 10'
0.015
0.010 1.0
2.0
3.0
4.0
Scattering Angle 2 0 [des.]
5.0
3
4
5
6
7
8
Order of Minimum n
Figure 3. (a, left) X-ray diffractogram of the same TFS film of 1 imaged by AFM (Figures 1,2,4, and 5). The marked change of baseline a t 2.2' is caused by the footprint of the X-ray beam suddenly becoming smaller than the quartz substrate. The peak a t 3.71' corresponds to a layer spacing of 2.38 nm. It is strongly broadened because the lattice of the film extends over only -8 layers. (b, right) Assignment of the fringesto their corresponding order. From the slope of the linear fit, a film thickness of 19.8 i 0.4 nm is calculated, corresponding to -8 layers of 1 on the underlying layer of OTS.
homologous alkoxybiphenyl ester liquid crystal, 2, discussed below. A fairly uniform film thickness of 18-22 nm is measured with the AFM, the range being due to inhomogeneity in the film structure, clearly seen in Figure 1. Steps with a height of 2.4 nm, corresponding to the thickness of a monomolecular layer, are observed. The film thickness is measured at the edge of the film as the difference in height between the top surface of the film and the quartz substrate, Figure 2. A measured, local film thickness of 20 nm implies that the film is comprised of -8 molecular layers. Small angle X-ray measurements have been performed with the same sample as in the AFM measurements. Figure 3a is a diffractogram exhibiting a broad peak at 3.71°, equivalent to a layer spacing of 2.38 nm. This value is in remarkably good agreement with the step height of 2.4 nm obtained by AFM for a monomolecular step. The region below 3.5O shows fringes that arise from the interference of X-ray beams at the film/air and film/ substrate interfaces. Figure 3b shows the assignment of the minima of these fringes to their corresponding order. From the slope of the linear fit, a film thickness of 19.8 f 0.4 nm is calculated, corresponding to 8 layers of 1 on an underlying layer of OTS. Since the AFM measurement is confined to a more localized area (5 X 5 pm2) and the
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Figure 4. Surface lattice data of a transferred, freely suspended film of 1 (same film of 1 as in previous Figures 1,2, and 3): (a) 5 X 5 nm2AFM image with molecular resolution on the surface of the film; (b) two-dimensional Fourier spectrum of (a), corresponding to a unit cell of 7.3 A X 5.8 A, y = 71'.
X-ray measurement represents a larger sampling area of the whole film, a small difference in the overall film thickness is not unexpected, due to the inhomogeneity evident in Figure 1. Beyond the edges of the organic film, the quartz substrate is recorded by the AFM as a fairly flat surface, dotted with small islands, Figure 2. These islands are also observed on glass and quartz substrates, alone. Their origin and effect on the overlying film are not known. On scanning over a smaller area on the film surface, 5 X 5 nm2, individual molecules that comprise the film are resolved in a close, regular packing, Figure 4a. As seen in the Fourier spectrum (Figure 4b), higher spectral orders are also imaged, further evidence of the order in the film. (The central spot in the spectrum is an artifact due to frame effects and low frequency noise.) The 2-D Fourier spectrum provides unit cell dimensions of 0.73 nm X 0.58
344 Langmuir, Vol. 9, No. 1, 1993 Table I. method AFM
ED
SAXS
ED ED
SAXS
AFM ED
SAXS SAXS
ED
SAXS
AFM
and Electron Diffraction (ED) and
compound
1 1 1 3 3 3 2
2 2 1 3 3
Ouerney et al.
film type
TFS TFS TFS TFS TFS TFS LB
lattice conetanta
unit cell area (nm2)
subetrat@
A E
0.73 0.78
0.58
71 90
0.40 0.44
D E A
0.78
90
0.806
0.55 0.55
71
0.41 0.42
C
0.5 0.75
0.5 0.61
80 90
0.25 0.46
0.56
layer SDaChlZ (nm)
2.4
A
D
LB LB LB
SAXS Data on Transferred, Freely-&upended and LB Filmr
2.38 2.27 4.0
BandC
B D B
LB LB
4.04 2.41'
0.78
0.57
90
0.45 2.39
*
0 Subatratee: A = quarte/oT5;B = silicon/OTS; C = silicon/HMDS;D = Formvar 5/95E E = amorphous carbon. Standard deviation here a0.04 nm. Value for the etable phase at ambient temperature.12
nm, with an angle y of 71' between the two measured lattice vectors (Table I). Electron diffraction data on a TFS film of the same molecules are in reasonable agreement with the AFM values for the a- and b-lattice vectors, 0.78 nm X 0.56 nm; however, the angle y is 90°, 19' larger than that recorded with the AFM. This discrepancy could be due to the AFM measurement's confinement to a very localized nonconforming domain in the film, such as the uppermost surface layer or a single crystalline domain in a multicrystallinefilm. We favor the former explanation, a surface "reconstruction", since AFM images have been recorded over many sites on the surface of the sample and consistently give an angle of 71' (*3O). The AFM-recorded angle of 71' has been observed before in the transferred freely suspended films of a homologue of 1: the heptyloxy derivative, 3. Electron diffraction analysisof 3 produces lattice parameters (0.80 nm X 0.55 nm,y = 71') somewhat similar to the AFMdetermined parameters of 1,0.73 nm X 0.68 nm, y = 71' (Table I).l2 In comparing electron diffraction and AFM lattice data, it is important to keep in mind that the two methods require different substrata properties; therefore, differences in lattice parameters could arise from the different underlying substrate as well.18 Heating the AFM sample of 1 gently to temperatures of -50 "C during soanning results in a change in tacticity of the surface. At this temperature, an unassignedphase transition ie recorded on bulk samples by DSC." Further investigation of these more fluid films was impeded by the stickinem of the heatad film. Additional development of scanning conditions is required before continuing work on more fluid phases. Comparisonof LB and TFS Films. AFM imageshave been recorded on LB films of this class of molecules to compare the producta from the two film preparation methods. These Langmuir-Blodgett filmsare comprised of a homologous molecule, 2, to that of the TFS f h , 1. This analogue, methyl 4'-(n-heptyloxy)biphenyl-4-carboxylate, 2, is deposited as an LB film of 12 bilayers on hydrophobized silicon using standard LB techniques.12 This film is deemed to be of good quality as judged by the conventionaltechniques for film characterization such as transfer ratio, optical transparency,homogeneityof optical interference color, optical microscopy, and X-ray diffraction.12 One of the most striking differences between the AFM images of the TFS and LB f h is the nature of the defects. ~
~~~
(.l7) Decher, G.; Honig, M.; Reibel, J.; Voight-Martin,I. University of M w , manwript in preparation. (18) Engel, M.; Merle, H.; Peterson,1.; Riegler, H.;Stsitz, R. BunsenCea. Phys. Chem. 1991,96, 1614.
The AFM images of the LB film of 2 show a continuous, flat film occasionally interrupted by pores of a bilayer (-40 A) depth, Figure Sa. These pores are characteristic of other LB filmsstudied with the AFM.leIm The standard analytical tool of optical microscopy fails to resolve this pore-defect in thin crystalline LB films of alkoxybiphenyl esters.12 The difference between the LB and TFS f i i defects could stem from their different thermal histories. LB films are transferred onto substrates at room temperature in a highly ordered state. They are expected to be highly strained, since it is not feasible to transfer the monolayer onto the substrate under ideal thermodynamic equilibrium conditions. The pores could be one mechanism for release of this strain. In contrast, TFS films are created in a highly annealed state and gain in-plane order on cooling. Defects in TFS films would likely arise from the strain caused by the different thermal expansion coefficientsof film and substrate. Interestingly,TFS and LB films of 1 exhibit similar layer structures as can be seen by SAX5 measurements, but the TFS f i i has a superior thermal stability.ll Another noteworthy departure of the AFM data from conventional analysis is again found in the lattice parameters. As determined by AFM, an LB film of 2 displays molecular spacing of a = 0.5 nm, b = 0.5 nm, and y = No, Figure Sb. This is in contrast to the electron diffraction data of a = 0.75 nm, b = 0.61 nm, and y = 90' (see Table I) from the same material. The source of this discrepancy could be a surface reconstruction, a "deviant" crystalline subdomain, or different substrates, as invoked earlier for the TFS films of 1. The AFM has already been shown to be capable of documenting surface reconstructions that have gone undetected by conventional structural analysis, e.g., on free-standing organic crystals such as pyrene.10 Since the electron diffraction sample population is made up primarily of molecules beneath the surface layer, it is not unreasonable to propose that the AFM measurements are from a different (superficial) population with a different intermolecular arrangement. Manipulation of TFS Films. Scanning conditione can be modified to intentionally manipulate the film and create localized structures.4b~5+8*20 As mentioned earlier, the AFM image is affected by the experimental param(19) Ovemey, R.; Howald, L.; Frommer, J.; Meyer, E.; Brodbeck, D.; Giintherodt, H. Ultramicroscopy 1992,42-44,983. (20) Examples of using the AFM to manipulats organic clurfaw, in additionto refs 4,6c, and 6-8 (a) J u g ,T.;M-r, A.; Hug,H.; Brodbeck, D.;Hofer,R.;Hidber,H.;Schwarz,U. Ultramicroscopy 1992,1244,1446; (b) Hamada, E.; Kaneko, R. J. Phys. D, Appl. Phys. 1992,26,AM.
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Langmuir, Vol. 9, No. 1, 1993 346
Figure 6. 7.5 X 7.5 pm2 AFM image of a two-stage process to deform a TFS-film of 1. The stages are performed successively
H
H Figure 5. AFM images of a 12-bilayer LB film of 2: (a) 3 X 3 pm2image of the surface of the film shows a continuous,flat film, interrupted occasionally by holes of a bilayer (40 A) depth; (b) 4 X 4 nm2 image with molecular resolution, showing a unit cell of a = 0.5 nm, b = 0.5 nm, and y = 80”.
etersof applied force, scan speed,feedbackloop frequency, and relative tip and sample geometries. In this study, a stepwise process of scribing and imaging the TFS film of 1 is repeatedly performed and the threshold conditions necessary to deform the sample are determined. With an increase of the scan speed or appliedload above the usual “passive”imaging settings, the film undergoes plastic deformation. Rectangular holes are intentionally created under these conditions. This is confirmed by returning the instrument to its scan settings where noninvasive imaging occurs and scanning the area where the hole has been scribed. This process is illustrated in
over scan ranges of 2.5 and 5 pm, at scan speeds of 10 and 4 pm/s, respectively, creating 14 and 5 nm deep holes, respectively. A final, slower scan (3 pm/s) over 7.5 pm does not further alter the sample and provides an overview of the destructive effect of the two previous scans (see text).
the image of Figure 6 taken after a series of 1,4, and 25 pm2 scans, described as follows. The first scan, over an area of 2.5 X 2.5 pm2,is performed at a fast scan speed of 10 pm/s (normal scan speeds are 53 pm/s). It results in the removal of a layer of the film, 14 nm thick (-6 layers) over the whole scan area. The second scan is performed over an area of 5 X 5 pm2 surroundingthe first scan area, at a scan speed of 4 pm/s. At this scan speed only a 5-nm-deep sheet (-2 layers) is removed. A further decrease in scan speed does not result in film removal. Scanning at a speed of 3 pm/s over an area 7.5 X 7.5 pm2 that includes the previous two scans does not further deform the surface and provides an overview of the individual steps in the film deformation process. The displaced material can be seen in Figure 6, displaced to the sides of the hole. The uncovered surfaces at the bottom of the square “holes” are remarkably flat, indicating that the molecules are removed in the conformation of sheets. Similar cooperative behavior has been observed in our labs, in which moleculesmove in “packets” and are transferred collectively.8 Similar film modifications can be effected by changing the applied force and maintaining the scan speed constant. We believe that the mechanics of this AFM-mediated deformation involve a relatively slow response of the feedbackloop. The modificationof the film surfaceoccurs when the response of the feedback loop is too slow to respond to higher tip slidingvelocities. Thus, the tip does not necessarily follow the surface contour, rather it plows into surface features and deforms them.21 The quantification of forces applied in the AFM experiment, performed cautiously and conservatively,l* produces values that are consistentwith the observed film deformation results, as follows. The force exerted by the measuring AFM probe is in the range of 10-9 to N. In the AFM experiment,applied pressuresare estimatedfrom an approximated contact area between probe and sample: 10-9-10-8 N applied over 10-100 nm2,giving pressures
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(21) To a firstapproximation,the temperatures generatedunder these scanning conditionsare not sufficient to cause local melting of the film. The increase in temperature due to the sliding of the AFM tip on the surface is estimated to be less than 1 0C.22 (22) Johnson, K. Contact Mechanics; Cambridge University Press: Cambridge, 1985; p 380.
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346 Longmuir, Val. 9, No.1, 1993
107-10ePau The energy imparted by the AFM probe is estimated to be in the range of 0.1-60 kJ*mol-1, figured from a force of W-lPN applied over a 1-5 A vertical distance into a surface area of 25-100 nm2(which is 100400 molecules, baeed on an end-on orientation with intermolecular spacing of -5 A, as in Figure Sb). For
-
comparison, chemical bond energies roughly order as follom: van der Waals, 2 kJ mol-1; hydrogen bonding, -20 kJ mol-l; covalent carbon-carbon bonds, -350 kJ mol-’.% Evidence that the AFM is indeed operating within the energy range of 0.1-50 kJ*mol-1 is provided in these and other examples of intentional modification of LB films with an AFM where assemblies of molecules held together by van der Waals f o r m are disruptxi and displaced by careful adjustment of applied forces in this energy range.Wb&W20 From the ease of deformation of these particular TFS films,we conclude that interlayer interactions are relatiuely weak. This conclusion is based, in part, on comparison to AFM experiments performed on LB films of fatty acid salta, wherein it was more difficult to create such def0rmations.4~~~8 In the fatty acid cases, the contribution of ionic bonding to intra- and interplane attractive interactions could account for this difference.The intermolecular forces within the TFS film planes of this study appear to be sufficiently robust, however, to result in the collective and cooperative motion of molecules on being manipulated with an AFM probe. Thisobservation is of relevance in the field of tribology, in studies of wear and friction, and particularly at the l i i i t of boundary lubrication.
Conclusions Good agreement b been demonstrated between AFM and X-ray scattering analyses of transferred freely SUBpended liquid crystalfilms on parametera of film thickness and layer height. With the AFM, these determinations are made under ambient conditions, nondestrwtively,and without additional eample preparation. The AFM data sometimes diverge from the electron difftaction data for lattice parameters. The AFM-determined lattice data might indicate a localized departure from the film’s internal crystal structure, e.g., a surface rearrangement. The AFM resulta also present straightforward resolution of localized defecta in the f h . Thia hae been demonstrated particularly well in a comparative study of films prepared by two different methods TFS films show different types of defecta than their LB counterparts, the most notable of which is a lack of pores in the TFS films, a feature which is always observed by AFM in LangmuirBlodgett films. Manipulationsthat remove discretepackets of molecules enable comparisonof interplanar forcesin these filme with other multilayer film assemblies studied by AFM in our laboratory. The stepwise procedure used to deform the filmsyields information about the relative forcesrequired to break-up these molecular assemblies. “Dissembly” occurs, however, in a rather orderly, cooperative process of collective motion of oriented packets of molecules.
Acknowledgment. We wish to thank P.Gdtter, L. Howald, and J. Maclennan. This work was supported by the Swiss National Science Foundation, the Kommission (23) 1 N.m-2 = 1 Pa (pascal) P 10-5 atm 7.5 X 10-3 mmHg. zur Farderung der wissenschaftlichenForachung, and the (24) (a)Atkine, P. General Chemietry;ScientificAmericanBmke: New York, 1989;pp 364 and 850. (b)Atkins, P. Physical Chemistry, 2nd 4.; Deutsches Bundesministerium ftlr Forschung und Technologie. Oxford Univelaity Press: Oxford, 19&4; pp 23-24.
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