STM Image Formation of Organic Thin Films: The Role of Water Shell

Analysis of the water role in STM image formation processes for organic thin films was performed in this work. The contribution of the water shell cov...
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Langmuir 2000, 16, 6577-6582

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STM Image Formation of Organic Thin Films: The Role of Water Shell Sandro Carrara,*,† Victor Erokhin,‡ and Claudio Nicolini† Di.S.T.Bi.M.O., University of Genova, Corso Europa 30, 16152 Genoa, Italy, and Foundazione E.L.BA., Corso Europa 30, 16152 Genoa, Italy Received July 22, 1999. In Final Form: May 31, 2000 Analysis of the water role in STM image formation processes for organic thin films was performed in this work. The contribution of the water shell covering the film was considered under the assumption that no water can be inside the film molecules. Conducting thin films were chosen as objects and they were measured under different humidity conditions. The images were processed for calculating their average corrugation. The relationship of the corrugation with the relative humidity was outlined and compared with a theoretical model of water adsorption. Finally, nanogravimetric and contact angle measurements were performed as independent proofs of the water adsorption model.

Introduction Since the invention of the scanning tunneling microscope (STM),1,2 the mechanisms responsible for the STM image formation are still not completely clear. Some years ago, a theory for the image formation of gold surfaces was proposed by Thersoff and Hamann3,4 using first-order perturbation theory to calculate the tunneling current. The theory of Thersoff and Hamann was extended to include the STM imaging of molecules adsorbed on graphite, taking into account possible current pathways inside the molecules mediated by the overlapping of atomic orbitals.5 This extension explains STM measurements of small molecules, such as butane or benzene5 or thin alkane layers,6 but it has an internal limit due to the fact that large molecules do not have such overlapping orbitals. Conversely, the possibility of imaging large insulating objects, such as large areas (hundred nanometers) of uncoated layered alkane (paraffin), was demonstrated in the literature.7 Moreover, STM microscopy was applied for biological studies, where the main interest is to measure large molecules.8 Successful imaging of uncoated DNA9,10 and proteins11-13 was reported. Therefore, mechanisms other than the Thersoff-Hamman theory must be taken * Corresponding author: Telephone: +39/010/3537429. Fax: +39/010/3538346. E-mail: [email protected]. University of Genova Internet web page: http://www.ibf.unige.it/ † University of Genova. ‡ Foundazione E.L.BA. (1) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Appl. Phys. Lett. 1982, 40, 178. (2) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett. 1982, 49, 57. (3) Thersoff, J.; Hamann, D. R. Phys. Rev. Lett. 1983, 50, 1998. (4) Thersoff, J.; Hamann, D. R. Phys. Rev. B. 1985, 31, 805. (5) Kurnicov, I. V.; Sivozhelezov, V. S.; Redchenko, V. V.; Gritcenko, O. V. Mol. Eng. 1992, 1. (6) McGonigal, G. C.; Bernhardt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990, 57, 28. (7) Michel, B.; Travaglini, G.; Rohrer, H.; Joachim, C.; Amrein, M. Z. Phys. B 1989, 76, 99. (8) Cricenti, A.; Generosi, R.; Selci, S. Rev. Sci. Intrum. 1994, 65, 80. (9) Keller, D.; Bustamante, C.; Keller, R. W. Proc. Natl. Acad. Sci. 1989, 86, 5356. (10) Keller, R. W.; Dunlap, D. D.; Bustamante, C.; Keller, D. J.; Garcia, R. G.; Gray, C.; Maestre, M. F. J. Vac. Sci. Technol. A 1990, 8, 706. (11) Arakawa, H.; Umemura, K.; Ikai, A. Nature 1992, 358, 171. (12) Facci, P.; Erokhin, V.; Nicolini, C. Thin Solid Films 1994, 243, 403. (13) Alekperov, S. D.; Vasil’ev, S. I.; Kononenko, A. A.; Lukashov, E. P.; Panov, V. I.; Semenov, A. E Ä . Sov. Phys. Dokl. 1988, 33, 828.

into consideration to explain the results of such experiments on imaging of large insulating molecules.7,9,10,12 For example, in the case of large protein molecules, the role of water contents is not negligible for many aspects of its functioning14 and, in particular, the role of water in their electrical properties was reported more than thirty years ago.15 This fact could affect the image formation process in each kind of microscopy related to molecular conductivity and, in particular, is important for scanning tunneling microscopy in air.16 As it was well established, STM imaging of large molecules, such as globular catalase, is difficult to do below 30% value of air relative humidity, while image contrast increases with the increase of the humidity.17Generally speaking, it seems that the water layer adsorbed from the air exists in thin films18 and that this layer could enable us to perform STM imaging of hydrophilic insulators in humid air.19Therefore, two effects must be taken into consideration when working with large biological molecules: the water contained in the molecules and the water layer covering the sample. The aim of this work is to study the second effect of these phenomena and to characterize the role of the water shell adsorbed from humid air to the surface of a layer of rather small molecules, supposing that they have no internal water. The image contrast variation upon changing the relative air humidity was considered both from theoretical and experimental points of view in order to understand the process of the STM image formation with respect to the amount of adsorbed water. Mathematical Formulation For understanding the role of water in the image formation process during STM experiments, it is necessary to determine some image property as a physical parameter (14) Di Primo, C.; Sligar, S. G.; Hui Bon Hoa, G.; Douzou, P. FEBS 1992, 312, 252. (15) Rosemberg, B. Nature 1962, 27, 364. (16) Guckenberger, R.; Hartmann, T.; Wiegra¨be, W.; Baumeister, W. In Scanning Tunneling Microscopy; Gu¨ntherodt, H. J., Wiesendanger, R., Eds.; Springer-Verlag 1992, vol II, pp 85-98. (17) Leggett, G. J.; Davies, M. C.; Jackson, D. E.; Roberts, C. J.; Tendel, S. J. B.; Williams, P. M. J. Phys. Chem. 1993, 97, 8852. (18) Herdt, G. C.; Czanderna, A. W.; King, D. E. Appl. Sci. 1996, 355, L371. (19) Guckenberger, R.; Heim, M.; Cevc, G.; Knapp, H. F.; Wiegra¨be, W.; Hillebrand, A. Science 1994, 266, 1538.

10.1021/la9909852 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/14/2000

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This is the average corrugation inside a segment of the length p along the line l. To obtain the corrugation averaged on the whole image, it is enough to mediate this quantity for all p-segments of the entire line l. Therefore, we can define the average corrugation of an STM image of a periodical structure as the following integral

C h )

Figure 1. Steric model referring to the water shell inside and over an organic film. (A) represents a situation when the water molecules do not cover totally the organic ones; in this case, the measured corrugation of the sample is decreased by the presence of water molecules placed between organic film. (B) represents a situation when water molecules cover the whole organic film; in this case, the measured corrugation of the sample is mainly due to the water molecules covering the film.

referring to the phenomenon. This parameter, according to other authors, is the corrugation of the image, defined as20,21

C ) hmax - hmin

(1)

where hmax is the maximum height measured on the image and hmin is the minimum height. The absolute value of corrugation on STM images taken in a small arbitrary area is meaningless because this value could be related to some impurities presented in the sample or to some noise occurring during the measurements. Therefore, it is better to consider the average of this parameter calculated over the areas of the images with rather ordered structure and without evident nose regions. If the image is modeled by a scalar field h(x b) where b x(t,l) is the Cartesian form with parameter t defined as movements along the line l, then the periodicity is achieved by the projection of the primitive vector b a of the Bravais lattice,22 as

p)b a‚

x (t2,l) b x (t1,l) - b |x b(t1,l) - b x (t2,l)|

(2)

L T Cp(x b(t,l))dtdl ∫l)0 ∫t)0

1 LT

(4)

where L and T define the dimensions of the image. Using eq 4 it is possible to obtain two integrals over the total image in order to calculate the maximum and the minimum in eq 3. On the other hand, it is possible to consider an ideal case in order to outline the possible formulation of the theoretical corrugation. In fact, it is possible to consider the steric model presented in Figure 1 considering the hypothesis that a monolayer could be represented by a set of single molecules placed over the solid substrate and that an ideal STM tip could be considered as a very sharp one able to penetrate inside the film until tunneling contact with the substrate. Obviously, the steric model of Figure 1 cannot pretend to be absolutely correct as any other model. Nevertheless, it could be very useful for understanding the imaging of thin films in real cases. Considering the suggested model and supposing high vacuum in the measuring chamber, it is possible to state that the highest point in the STM image corresponds to the top of the organic molecule in the layer, while the lowest is related to the substrate position, e.g.,

C h ) hmol - hgraph ) ∆h0

(5)

However, when we are working in humid air we must take into account the role of the water shell in the measured corrugation. Referring to the situation of an organic hydrophilic film exposed to the humid air, we can identify two limit situations which can account for the image formation in scanning tunneling microscopy. Figure 1 shows the model situations representing the molecules as solid objects without internal structure. When the humidity is such that, only few water molecules can be adsorbed and placed between the organic ones (Figure 1A), the measured corrugation will be decreased because of the conductivity of water. This fact is reflected in the corrugation as

C h ) ∆h0 - hW

(6)

On the other hand, when the water molecules cover the whole film rather than only the space between organic molecules (Figure 1B), the measured corrugation is also related to the conductivity of water molecules present on the top of organic molecules. Therefore, the corrugations must contain another term, namely,

This periodicity inside the single line can be used to calculate the average corrugation over the period p

C h ) ∆h0 - hW(H) + hWM(H)

b(t,l)),t0 e t < t0 + p} Cp(t0,l) ) max{h(x min{h(x b(t,l)), t0 e t < t0 + p} (3)

The two terms h(H) are connected to the humidity of the air, and their conducting contributions follow the rule predicting an increase of current with humidity.19 The two ideal situations presented above represent two different situations for placing water in the organic film, and they can be handled by a function ϑ(H - H0) such that

(20) Facci, P.; Nicolini, C. In From Neural Networks and Biomolecular Engineering to Bioelectronics; Nicolini, C., Ed.; EL.B.A. Forum Series, vol. 1, Plenum Press: New York 1995, p 167. (21) Labani, B.; Girard, C.; Courjon, D.; Van Labeke, D. J. Opt. Soc. Am. 1990, B7, 936. (22) Ashcroft, N. W.; Mermin, N D. Solid State Physics; W. B. Saunders Company: Orlando, 1976; p 64.

limϑ(H - H0) ) ϑ(H - H0) f∞

(7)

(8)

STM Image Formation of Organic Thin Films

Langmuir, Vol. 16, No. 16, 2000 6579 Table 1: List of Salts and Relative Humidity Values Which They Maintain in the Chamber

Figure 2. Chemical structure of the C16H33-BEDT-TTF molecules.

where the Heaviside function ϑ(H) is dominant when the second mechanism takes place. The following function was chosen as it satisfies eq 8:

ϑ(H) )

(H-H0)

e

e(H-H0) + e-(H-H0)

(9)

Parameter  in this formula determines the width of the humidity range, intermediate between two different situations of the water distribution over the film, and H0 is the humidity value in the middle of this range. In conclusion, considering eqs 7 and 6, it is possible to write an expression for the STM average corrugation describing the effects due to the water shell adsorbed from the humid air, namely,

C h ) ∆h0 - hW(H) + ϑ(H - H0)hWM(H)

(10)

in which H0 accounts for the value of the humidity when the water shell begins to cover totally the film. Now, it is possible to check the validity of such considerations about the STM image formation mechanism comparing the measured average corrugation from eq 4 with the relationship modeled by eq 10. Materials and Methods The structure of the C16H33-BEDT-TTF molecule is shown in Figure 2. LB films were prepared with an LB trough (MDT, Russia) using water purified with a Milli-Q system (18.2 MΩ cm). The LB films of C16H33-BEDTTTF molecules were prepared on the subphase, containing iron(III) chloride, according to a well-known procedure24 and transferred onto a solid substrate of highly oriented pyrolitic graphite (HOPG). STM images were acquired in air using a scanning tunneling microscope (MM-MDT Co., Russia). The measurements were performed in a constant current mode, the tip-sample voltage was equal to 0.1 V and the allocated tunneling current was close to 0.1 nA. Of course, variation in the humidity will result in differences of the tip-sample distance at fixed voltage and current values. However, it must not be critical for the purposes of this study, as the considered parameter is not a distance, but an average corrugation. The microscope tungsten tips were prepared by chemical etching (Park Scientific Instruments). Atomic resolution on HOPG was obtained before imaging of LB films to check the tip quality and for recovering eventual decalibrations due to scan conditions.25 The humidity of the chamber air was controlled using saturated solutions of some salts reported in Table 1 with the correlated relative humidity. Equilibrium was reached by leaving the film in the chamber for 1 h before the acquisition of the images. Measurements were performed in both increasing and decreasing humidity conditions. The small volume of the working chamber did not allow (23) Mule`, M.; Stussi, E.; de Rossi, D.; Berzina, T. S.; Troitsky, V. I. Thin Solid Films 1994, 237, 225. (24) Carrara, S.; Gussoni, A.; Erokhin, V.; Nicolini, C. Journal of Material Science: Materials in Electronics 1995, 6, 79. (25) Carrara, S.; Facci, P.; Nicolini, C. Rev. Sci. Intrum. 1994, 65, 2860.

farms

salt

relative humidity value [%]

Sigma Fluka Aldrich Sigma Fluka Fluka

KC2-H3O2 CaCl2‚6H2O Zn(NO3)‚6H2O NaHSO4‚H2O NaNO2 KHSO4

20 32.3 42 52 66 86

Table 2: Mean Values and Standard Deviations of the Average Corrugations Calculated with Equation 4 from STM Images of LB Films of C16H33-BEDT-TTF Molecules Acquired at Different Relative Humidity of Air relative humidity value [%]

mean average corrugation

standard deviation

20 32.3 42 52 66 86

2.75 1.75 1.56 2.07 1.97 1.96

0.34 0.19 0.19 0.19 0.30 0.21

for measurements of the humidity during the STM image acquisition. Therefore, the humidity was checked in another chamber with the same volume before making the experiments, therefor determining the amount of saturated salt solution and time required for coming to equilibrium humidity. A gravimetric gauge described previously26 was used for measuring the mass of the adsorbed water. The gauge was calibrated by successive deposition of cadmium arachidate bilayers.26 For contact angle measurements, the samples were placed inside a closed chamber for 1 h to reach equilibrium with humid air at fixed humidity. A water drop was put on the film surface and a photographic image was immediately acquired. The photograph was digitized and the contact angle was measured. Fitting. The fitting procedure was performed in order to minimize the root-mean-square error between the data calculated from eq 10 and the experimental data presented in Table 2. The fitting method was performed with optimization code by non linear GRG2 method by c¸ eon Lasoon (Texas University) and by Allan Warren (Cleveland State University). In their work, ∆h0, H0, and  were fitted in order to estimate their values in comparison with the physical interpretation of the parameters. Experimental Results of STM Imaging To compare the theoretical consideration of the previous section with some experimental results, we must consider periodicity in the features hypothesised in the corrugation definition (eq 4). The considered organic molecules are rather small and cannot contain internal water, which allows us to concentrate our attention on the role of the water shell. To obtain reliable values of the average corrugation, the work must be performed on well-organized layers, such as LB films. Therefore, we have used LB films of conductive molecules already resolved with STM microscopy.23 Figure 3 shows an image of an LB film of C16H33BEDT-TTF molecules acquired at 20% relative humidity. Its features are in good agreement with previous STM images reported by other authors,23 and it gives the (26) Facci, P.; Erokhin, V.; Nicolini, C. Thin Solid Films 1993, 230, 86-89.

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Figure 3. STM image of a C16H33-BEDT-TTF film acquired at 20% of relative humidity with a tip-sample voltage equal to 0.1 V, a tunneling current of 0.1 nA, and a scanning frequency of 500 Å/s. Image size is 25 Å per 24 Å.

Figure 4. STM images of a C16H33-BEDT-TTF film acquired at the same conditions of Figure 3 and shown in 3-D view with comparable scale: (A) acquired at 20% of the air relative humidity with the box height equal to 3.3 Å; (B) acquired at a 42% of relative humidity with the box height equal to 2.2 Å.

following unit cell parameter values, after calibration corrections:25 r1 ) 3.23 Å, r2 ) 2.46 Å, and γ ) 97.1°.23 Figure 4 shows the images of two acquisitions in different conditions of humidity. The role of environmental water is easily evident in the observed corrugation; it is much higher at 20% (Figure 4A) than at 42% (Figure 4B) relative humidity. The average corrugation was estimated according to the definition (eq 4) by carrying out the measurements at different relative humidities, as specified in Table 1. In the case of real images, eq 4 also ensures a better noise rejection as it is averaged over the whole image. Furthermore, values of the corrugations from different images are included in a statistical analysis and the results of the mean values and the standard deviations of these average corrugations are summarized in Table 2. Figure 5 represents these results. It is clearly a decrease in corrugation following the increasing air humidity. This fact is in contrast to other conclusions proposed by authors rep-

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Figure 5. Average corrugations estimated from a statistics on different images acquired at different humidity conditions (black box) in comparison with model of image formation mechanisms form eq 9 (continuous line). The images are acquired for relative humidity value equal to: 20, 32.3, 42, 52, 66, 86. An increase of the measured corrugation takes place between 42% and 52% of the relative humidity; after that, the curve continues to decrease further.

resenting STM experiments on biological samples.17 The reason for this fact is connected to the choice of the sample under investigation. For example, in the case of covalently immobilized globular catalase, the contrast decreases when the humidity conditions are changed from 33% to 10%, and the corrugation was reversed when the humidity became less than 5%.17 This fact was explained referring to the higher conductivity of the hydrated proteins as compared to the dehydrated ones. In our case, the used molecules do not have the possibility to increase their internal conductivity with the increase of the humidity of the chamber, because the hydration of the LB films increases the water contents between organic molecules or covers the whole film, as suggested by the model presented in Figure 1. In other words, when the humidity is low, the space between film molecules is practically empty and the STM tip can come closer to the conductive substrate, while filling of this space by water molecules at higher humidity decreases this penetration, resulting in lower corrugation (Figure 1B). As it is clear from the Table 2 and Figure 5, the average corrugation in the STM images decreased to 43% of its highest value in the range of relative humidity from 20% up to 42%. Conversely, the average corrugation was slightly increased at 52% relative humidity and then goes to a further decrease. These observed data are over the standard deviations of the statistics, therefore it is possible to consider that these measurements are a good estimation of the phenomenon, even if this kind of microscopy is very sensitive to the noise during acquisitions. On the other hand, similar experiments performed over a clean graphite sample resulted in a variation of the mean corrugations, comparable to the standard deviations. The observed decrease of the film corrugation, which is not simply monotonic, seems to be related to the two different water distributions over the film, discussed in the previous section. In fact, Figure 5 presents two decreasing trends, but the second one is acting after a slight reincrease of the average corrugation near 50% relative humidity. Furthermore, fitting of the eq 10 with the experimental data of Table 2 (Figure 5) confirms the validity of the hypothesis and, moreover, provides an independent way for checking the possibility of two situations of water adsorption as it will be discussed in the next section.

STM Image Formation of Organic Thin Films

Figure 6. Number of water molecules per each molecule of a C16H33-BEDT-TTF adsorbed from the environment by the film conditioned at different relative humidity. The presence of two plateaux confirms two distributions of adsorbed water over the film (Figure 1).

Experimental Results from Other Techniques To provide an additional experimental proof of the two different situations of the water absorption from the humid air, it is possible to estimate the water contents adsorbed from the environmental humidity directly by gravimetric measurements. A gravimetric gauge previously described26 was used for measuring the adsorbed water. Figure 6 reports the experimental results. In the figure, it is clearly noticeable that two different plateaux appeared: one near the 40% relative humidity, the other after 70%. Experimental error in these measurements is determined by the instability of used quartz resonators, which is about 1 Hz. This results in a relative error of 5% when only one water molecule is adsorbed to each film molecule. When the number of adsorbed water molecules is increased, this relative error decreases proportionally. The appearance of two plateaux in Figure 6 can be explained considering two situations of water distribution in the film. Therefore, the resultant curve can be considered as a superposition of two sigmoidal dependencies on the humidity, but with different parameters (namely, their inputs are different in different humidity ranges). It is worthwhile to note that the appearance of the second sigmoidal shape occurs in Figure 6 near the same humidity value at which the slight increase of the average corrugation in STM imaging was observed. Another experimental proof for the hypothesis of two adsorption situations was performed by measuring the contact angle of a water drop on the surface of a film conditioned at different humidity values. If the first step in water absorption is related to its penetration into the internal part of the film (Figure 1A), we can expect that for lower relative humidity values the contact angle will remain constant as the surface of the film will remain the same. On the other hand, when the water begins to cover the film completely (Figure 1B), we can expect an increase of the hydrophilicity of the film surface. The results of the contact angle measurements are shown in Figure 7 and reveal that for relative humidity values under the limit of 52% the contact angle remains quasi the same with small variations within 5% of the average contact angle. Conversely, when the humidity changes from 52% up to 86%, the variation in contact angle is more than 30% toward the increased hydrophilicity of the film surface. These data confirm that two different water absorption situations take place in the film and that only the second one varies the surface hydrophilic properties. Contact angle measurements revealed 50% in relative humidity, as a critical point in

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Figure 7. Contact angle of a water drop on a C16H33-BEDTTTF monolayer surface at different humidity. There are only small variations of the angle when the humidity is lower 52%. Instead, when the humidity overcomes this value, the angle begins to decrease, confirming the formation of the water shell over the film (Figure 1). Table 3: Values of the Parameters in Equation 9 Obtained from Experimental Results Reported on Figure 5 model parameters

value

∆h0 H0 

4.947 51.387 1.113

agreement with nanogravimetric and STM corrugation measurements. Discussion Three different techniques have shown evidence of two different situations in the water adsorption from humid air by a thin organic film. Consideration of steric conditions can also be useful for understanding the STM corrugation data. In fact, the cross sections of the hydrophobic chains are about 21 Å2, and those of water molecules are about 10 Å2. On the other hand, a close molecular packing as that previously seen for this organic film23 is such that the empty space between the organic molecules is about one-third of the area occupied by the film molecules. Therefore, considering the graph in Figure 6, it is not strange to think that two water molecules are packed in the free space between organic molecules for a relative humidity less than 50% (compare with Figure 1). For higher humidity the film becomes covered by water molecules, providing complete coverage up to 90%. Calculation of the total amount of water molecules in the case of a water shell completely covering the film gives 5 water molecules per each film molecule. This is very similar to the situation shown in Figure 6: seven water molecules per each film molecule. The similarity of results obtained from two different approaches (a rough steric estimate and experimental results from mass measurements) seems to confirm once again the two water adsorption situations. Parameters from the fitting of eq 10 with the experimental average corrugations reported in Table 3 were compared with other available information on the film. In particular, the re-increase of the average corrugation at about 50% of relative humidity and further decrease after this value seems to be connected to two previously presented situations of water adsorption. In the framework of the STM image formation model the action of the two situations is described by the function ϑ(H - H0), which drives the corrugation increase when the relative humidity H0 is equal to 51%. This value corresponds to the situation when the water shell begins

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to cover the whole sample. The estimation of H0 is also confirmed by the appearance of the second plateau in the adsorbed water mass (Figure 6) and the significant decrease of the contact angle after 50% (Figure 7). The ratio of the adsorbed water molecules to a film molecule was changed and a drastic change in the hydrophilic properties of the film surface was observed near the same value of the 50% of relative humidity. It is interesting to note, that the critical point of the humidity found in this study (about 50%) is very close to that, described in the literature (55%), when it was observed the formation of the water bridge between the tip and the substrate surface, indicating complete coverage of the sample surface.27 Conversely, the estimation of the value of the theoretical corrugation for the dried films ∆h0 is equal to 4.947 Å and is much less than the length of the molecules used for building-up the LB films. The organic molecules inside our films have 16 carbon atoms in the chain. The polar height range from 3.2 Å to more than 7 Å depending on the orientation.23 Therefore, the total length of a C16H33BEDT-TTF molecule is from 23.2 Å to 27 Å, if the tail is normal to the graphite plane. The height of the dry films must be comparable to the length of the molecules because no water participates the imaging process. Conversely, our estimation of this quantity is much less. This discrepancy cannot be related to the fact that the organic molecules are not in a vertical position with respect to the graphite plane. In fact, even if such tilted models of packing in LB films were proposed,23 the estimated film high must be about 18 Å, and it is not enough to explain why ∆h0 is so small. This fact can be related to tip, which is not ideal. Curvature of the tip will prevent its penetration in the space between molecules, due to the lateral current. This fact can limit the apparent molecule heigh to only 5 Å. Another hypothesis explaining the small value of the parameter ∆h0 is connected with the possibility that the tip might penetrate the film during scanning. It is wellknown that hydrocarbon chains of fatty acids in LB

monolayers deposited onto any substrates are oriented to the air.28 Even if there is some evidence about the possibility of having a current along carbon chains,29,30 it seems to be more likely that the tip penetrates the film in order to provide enough tunneling current. In such a case, the interpretation of the parameter ∆h0 must be quite different. It cannot be the total height of the organic molecules, but it is only comparable with the headgroups of the molecules. If this is the case, our estimated value is correct and comparable with typical dimensions of our molecule headgroup. Finally, the small value of the parameter  informs us that the transition from the first situation of water adsorption (Figure 1A) to the second one (Figure 1B) takes place in rather wide humidity range.

(27) Heim, M.; Eschrich, R.; Hillebrand, A.; Knapp, H. F.; Guckenberger, R.; Cevc, G. J. Vac. Sci. Technol. B 1996, 14, 1498-1502. (28) Kato, T. Jpn. J. Appl. Phys. 1988, 27, 1358. (29) Evenson, J. W.; Karplus, M. Science 1993, 262, 1247. (30) Haran, A.; Waldeck, D. H.; Naaman, R.; Moons, E.; Cahen, D. Science 1994, 263, 948.

Acknowledgment. This work has been supported by EL.B.A. Foundation. The authors are very grateful to Cristina Rando for her technical support.

Conclusion This work is dedicated to elucidating the possible shape of an adsorbed water shell during the scanning tunneling microscopy investigations. The experimental results had shown that the relative humidity of the air plays a quite different role with respect to films of large protein molecules. The experimental results are compared with a model considering that a water shell can cover the whole film only after a particular value of the relative humidity. When the humidity is less than this value, water molecules are placed between film molecules and do not cover them completely. The study revealed 51% to be this critical value of the relative humidity. The estimated value of the monolayer height is less than that expected from chemical consideration of the film molecules. This fact can be due to the real tip shape and, considering the fact that only a very small current can flow along aliphatic chains, we can suppose that the tip penetrates the film in order to be closer to the polar headgroups. The STM image formation model based on two water adsorption situations seems to be confirmed by contact angle measurements as well as by nanogravimetric assay, and it is in agreement with steric considerations related to the area covered by the water molecules.

LA9909852