Langmuir 1998, 14, 3967-3970
Selective Deposition of Langmuir-Blodgett Films of a Phthalocyanine onto Patterned Substrates Jean-Philippe Bourgoin* and Serge Palacin Service de Chimie Mole´ culaire, CEA Saclay, Gif sur Yvette, France Received January 29, 1998. In Final Form: March 18, 1998
Introduction The deposition of Langmuir-Blodgett (LB) films, either by the classical vertical method or by the Schaeffer method, generally involves substrates with uniform surface energy. Indeed, the deposition of the first layer, either on upstroke or on downstroke, is controlled by the wetting tension at the three-phase line. The actual contact angle at the threephase line depends on the surface energy of the substrate, the characteristics of the monolayer spread at the airwater interface, and the speed of the up- and downstrokes of the substrate through this interface.1-4 The situation is far more complicated if nonuniform substrates are used. Although some general principles of wetting of periodic surfaces are well-known,5,6 the behavior of the three-phase line during the LB deposition onto a substrate patterned at the micron scale has not yet been investigated. In the present note, we study the LB deposition of a fully hydrophobic molecule onto substrates patterned at the micron scale with periodic hydrophobic and hydrophilic areas. Using a molecule which does not deposit on large hydrophilic areas is expected to give rise to a selective deposition on the patterned substrate. It has been shown that nickel tetrakis(hexyloxycarbonyl)phthalocyanine (PcE6) is easily transferred onto purely hydrophobic substrates with a 0.975 transfer ratio and does not deposit on hydrophilic substrates.7 In the LB films, the Pc rings stand on the edge and form columns almost parallel to the transfer axis (dipping direction). Tapping mode AFM revealed that the one-layer-deep defects observed in the topmost layer are also oriented by the transfer.7 The major mechanism of orientation of the columns has been shown to be the so-called flow orientation effect, which occurs during the transfer.8-10 The main purposes of the present study are two-fold: (i) investigate the possibility of selective deposition of an LB film onto a chemically patterned substrate and (ii) see if such a patterned substrate may influence the final inplane orientation of the transferred molecules. (1) Yaminsky, V.; Nylander, T.; Ninham, B. Langmuir 1997, 13, 1746. (2) Petrov, J. G.; Kuhn, H.; Mo¨bius, D. J. Colloid. Interface Sci. 1980, 73, 66. (3) Egusa, S.; Gemma, N.; Azuma, M. J. Phys. Chem. 1990, 94, 2512. (4) De Gennes, P. G. Colloid Polym. Sci. 1986, 264, 463. (5) De Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (6) Drelich, J.; Wilbur, J. L.; Miller, J. D., Whitesides, G. M. Langmuir 1996, 12, 1913. (7) Bourgoin, J. P.; Doublet, F.; Palacin, S.; Vandevyver, M. Langmuir 1996, 12, 6473. (8) (a) Ogawa, K.; Kinoshita, S.; Yonehara, H.; Nakahara, H.; Fukuda, K. J. Chem. Soc. Chem. Commun. 1989, 477-479. (b) Ogawa, K.; Yonehara, H.; Maekawa, E. Thin Solid Films 1992, 210/211, 535-537. (9) Minari, N.; Ikegami, K.; Kuroda, S.; Saito, K.; Saito, M.; Sugi, M. J. Phys. Soc. Jpn. 1989, 58, 222. (10) Schwiegk, S.; Vahlenkamp, T.; Xu, Y.; Wegner, G. Macromolecules 1992, 25, 2513.
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Experimental Section Materials. Gold (99.99%) was obtained from Engelhard-Clal (Paris, France). Chromium (99.996%) was obtained from Johnson-Matthey (Paris, France). Silicon wafers were purchased from ACM (Villiers, France). Microcontact printing stamps were made of PDMS (Sylgard 184, Dow-Corning) according to published procedures.11 Chloroform (puriss, Fluka) and 2-mercaptoethanesulfonic acid (MES, Aldrich) were used without further purification. Hexadecanethiol (HT, Aldrich) was distilled under reduced pressure before use. The synthesis and properties of nickel tetrakis(hexyloxycarbonyl)phthalocyanine (PcE6) were described elsewhere.7 Substrates. Gold films (10 nm to 1 µm) were thermally evaporated on 500-µm-thick silicon wafers that had been primed with chromium (2-4 nm) to promote gold adhesion. We used microcontact printing11 (µCP) immediately after the evaporation to pattern the substrates with hydrophobic and hydrophilic areas. Parallel lines alternately covered by monolayers of HT (4.1-µm width) and MES (1.4-µm width) were created by (i) stamping the substrate with HT and (ii) dipping it into a MES solution in ethanol/water for 15 s. The rinsed samples were immediately used as substrates for the LB deposition. LB Deposition. The LB deposition was performed at room temperature in a class 100 clean room on a LB 105 trough from Atemeta (France). The PcE6 monolayer was built from a 1.3 × 10-4 M solution of PcE6 in pure chloroform. A pressure-area isotherm was recorded before each deposition. The transfer pressure was 14 mN‚m-1. The transfer speed v was variable (see text). The angle i, between the dipping direction and the patterned lines, was adjusted manually. The classical dippingwithdrawing method of deposition was used. The substrate plane was kept vertical and parallel to the moving barrier throughout the whole deposition process. Characterization. AFM (tapping mode, 125-µm cantilever) was performed in air at room temperature with a Nanoscope IIIa (Digital Instruments). SEM images were collected on a JEOL JSM-840A scanning electron microscope operating at 5 kV, with a typical beam current of 100 pA. Linear dichroism in the infrared range was performed on a Nicolet Magna 860 FTIR spectrometer.
Results and Discussion Selectivity of the Deposition. Figure 1 shows a typical tapping mode AFM image recorded on a sample obtained by deposition of ten monolayers of PcE6 on a pattern of lines oriented at an angle i ) 45°, at a transfer speed below 0.5 cm‚min-1. It is clear that the transfer ratio is very different between the two different chemical surfaces and that transfer occurs preferentially onto the hydrophobic areas. The cross-section shown in Figure 1b indicates that the step height between covered and uncovered areas matches the number of deposited monolayers (with some uncertainty coming from the original height difference between HT areas and MES areas, which is less than 2 nm). Hence we conclude that the transfer occurs selectively on the hydrophobic lines and is not significantly affected by the hydrophilic ones in between. We believe that only the hydrophilic/hydrophobic character of the substrate is significant here, not the exact chemical nature of the hydrophilic area, because (i) it is consistent with what is observed on uniform substrates, (ii) the molecule PcE6 has no specific reactivity with MES, and (iii) attempts with other hydrophilic groups (carboxylic acid, ammonium salt, alcohol) showed equivalent results. In addition to the striped pattern reported here, other patterns were tested: we found that even very narrow (11) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498.
S0743-7463(98)00112-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/03/1998
3968 Langmuir, Vol. 14, No. 14, 1998
Notes
Figure 1. (a) Tapping mode AFM on an 10-layer PcE6 LB film built up with i ) 45° and v ) 0.4 cm‚min-1. (b) Vertical profile of the same sample; the vertical period of the PcE6 LB film is 2.2 nm.6
MES lines, down to at least 0.5-µm wide, were able to prevent the LB deposition. On the contrary, square islands of MES of micrometer size were generally covered by the multilayers. A tentative explanation is given in the next paragraph. Three different parameters were screened during the present study: the transfer speed v, the angle i between the transfer axis and the parallel lines of the pattern, and the total number of layers n deposited on the substrates. We did not study the effect of temperature on the deposition. The transfer pressure of 14 mN‚m-1 was selected because it was shown to give the best deposition on uniformly hydrophobic substrates. Effect of the Angle i. For a small number of layers (e12) and a low deposition speed (e0.5 cm‚mn-1), we found no influence of the angle i on the selectivity of the deposition; that is, no significant bridging of the hydrophilic areas by the LB multilayers was observed. However, as shown in Figure 2 some bridging was observed for i ) 90°, but this phenomenon was quite rare throughout the whole sample. This difference between the cases i ) 0°
and i ) 90° may originate from a difference in the contortion of the three-phase line, as shown by Drelich et al., on the wetting of patterned substrates by a liquid droplet.6 Our result indicates that the creep process of the three-phase line is probably dominated by the very fast dewetting of the hydrophilic areas and the nucleation of jogs at defects along the border between hydrophilic and hydrophobic areas, as described by De Gennes for the wetting of parallel grooves.5 It should be noted that the bridging lines, when present, are very narrow (∼80-nm to 300-nm wide and ∼20-nm high), highlighting that each jog is limited in width by the fast dewetting occurring in its immediate vicinity. The behavior of square islands of MES of micrometer size probably originates from similar phenomena, but the whole area of the square can be covered because no fast dewetting process is possible in those very small hydrophilic areas. Effect of the Transfer Speed v. As observed originally by Langmuir,12 there is a critical transfer speed (12) Langmuir, I. Science 1938, 87, 493.
Notes
Langmuir, Vol. 14, No. 14, 1998 3969
Figure 2. SEM image of a 10-layer sample built up with v ) 0.4 cm‚min-1 and i ) 90°. A detail is given in the inset. The direction of the transfer is shown by the arrow.
Figure 4. SEM images of 16-layer samples built up with v ) 0.4 cm‚min-1 and i ) 45° (top) and i ) 90° (bottom). The direction of the transfer is shown by the arrow.
Figure 3. SEM images of 8-layer samples built up with i ) 45° and v ) 1 cm‚min-1 (top) and v ) 2 cm‚min-1 (bottom). The direction of the transfer is shown by the arrow.
above which the entrainment of water between the monolayer and the substrate begins. This critical transfer speed can be theoretically calculated4 or measured for any given couple of monolayer and substrate.2 The transfer speed is also a key parameter in the occurrence of in-plane anisotropy governed by the so-called flow effect.9,10 For this series of experiments, the angle i was 45° and the number of deposited layers was 8. Figure 3 shows that v dramatically affects the quality of the deposition: the number of defects and bridging areas obviously increased with the transfer speed. When a different pattern exhibiting very thin hydrophilic areas (0.5-µm wide) was used, all the thin MES areas were bridged by the PcE6 multilayers when the transfer speed reached 1 cm‚min-1 (not shown). Thus, the critical transfer speed for our system lies between 0.5 and 1 cm‚min-1. This figure is far lower than the values measured for “classical” systems such as arachidic acid, cadmium arachidate, or octadecylamine deposited on glass slides under various water bath conditions,2 but the present case does not involve high-surface-energy substrates or highly polar molecules. Effect of the Number of Layers n. Figure 4 shows a comparison between two samples prepared at i ) 45° and i ) 90°, with a high number of layers. The selectivity of the deposition of PcE6 multilayers decreased with the number of deposited layers. Unselective deposition on the hydrophilic areas increased with the angle i (especially i ) 90°) when the number of layers was higher than 12. No significant unselective deposition was observed on samples up to 16 layers when i ) 0° (not shown). We have no definite explanation for the degradation of the selective
3970 Langmuir, Vol. 14, No. 14, 1998
deposition. It may originate (i) from pinning on very small hydrophobic defects (too small to be clearly detected by SEM, see Figure 2) left within the hydrophilic areas by the former monolayer deposition or (ii) from an accumulation of matter at the border between hydrophilic and hydrophobic areas (this accumulation of matter at the borders was actually detected by AFM and might become progressively unstable as the transfer goes on) or (iii) from a perturbation of the Langmuir film in the immediate vicinity of the substrate. Effect of the Patterning on the In-Plane Orientation. Finally, we checked the effect of the patterning on the orientation of the molecules within the LB films. IR linear dichroism performed on the patterned areas did not reveal any specific effect of the patterning on the inplane orientation of the Pc rings. Tapping mode AFM shows that the orientation of the defects in the topmost layer of the LB films was influenced by the hydrophilichydrophobic interface rather than by the axis of the patterned lines.13 The same conclusion about the crucial role of the interface between the hydrophilic and hydro(13) It cannot be excluded that more suitable geometries could induce some in-plane orientation. New patterns are presently being considered to this end.
Notes
phobic lines is reached from the inspection of Figures 2-4, when one considers the orientation of the fibrils-like bridging defects: when they completely bridge the hydrophilic gaps, they are mostly perpendicular to the interface; when they are linked to only one hydrophobic line, they are also mostly perpendicular to it, while when they are not linked to any hydrophobic line, they are oriented mostly along with the dipping direction. Conclusion We have investigated the transfer of LB films of PcE6 onto substrates patterned with alternating hydrophilic and hydrophobic lines. Provided the transfer speed is kept at a value below 1 cm‚mn-1 and the number of layers is kept below 12, the deposition occurs selectively on the hydrophobic lines. The selectivity of the deposition appears to degrade with increasing speed, number of layers, and angle between the transfer axis and the patterned lines. No macroscopic influence of the pattern on the orientation of the molecules was found. The interface between the hydrophobic and hydrophilic areas might however induce a local order. LA980112S
