Capillary Deposition and AFM Oxidation

Nov 6, 2008 - Lithography for Close-Packed Directed Assembly of Colloids ... close-packed assembly of colloids on SiOx patterns fabricated on silicon ...
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Langmuir 2008, 24, 13254-13257

Combining Convective/Capillary Deposition and AFM Oxidation Lithography for Close-Packed Directed Assembly of Colloids L. Ressier,*,† B. Viallet,† A. Beduer,† D. Fabre,† L. Fabie,†,‡ E. Palleau,† and E. Dague‡ UniVersite´ de Toulouse, INSA-CNRS-UPS, LPCNO, 135 aVenue de Rangueil, 31077 Toulouse Cedex 4, France, and LAAS-CNRS, UniVersite´ de Toulouse, 7 aVenue du Colonel Roche, 31077 Toulouse Cedex 4, France ReceiVed September 2, 2008. ReVised Manuscript ReceiVed September 25, 2008 We combine convective/capillary deposition and oxidation lithography by atomic force microscopy to direct the close-packed assembly of colloids on SiOx patterns fabricated on silicon substrates previously functionalized with a hydrophobic monolayer of octadecyltrimethoxysilane. The efficiency of this original generic method, which is well adapted to integrate colloids into silicon devices, is demonstrated for 100 nm colloidal latex nanoparticles and Escherichia coli bacteria in aqueous suspensions. A three-step mechanism involving convective flow and capillary forces appears to be responsible for these close-packed assemblies of colloids onto SiOx patterns.

Studying the specific properties of colloidal micro- or nanoobjects and exploiting them into functional devices require their organized assembly onto specific areas of substrates. Convective/ capillary deposition appears to be an efficient way to form compact arrays of nanoparticles from a drop of colloidal suspension onto large surfaces.1 This technique can be associated with topographical or chemical patterning of substrates to direct the nanoparticle assembly onto specific areas. Isolated particles or the association of a few particles with controlled geometry can be assembled on topographical patterns.2-4 Chemical patterns are usually made either by combining photolithography5 or nanoimprint lithography6 with self-assembled monolayer (SAM) deposition or by microcontact printing.7 Nanoparticles are assembled by convective/capillary deposition onto these patterns, taking advantage of a wettability contrast. A specific interaction between the nanoparticle ligands and the chemical functions of the pattern is also usually involved.6,7 Besides, in the past decade, nanolithography by atomic force microscopy (AFM) has been shown to be a very flexible tool for material structuring and patterning with nanometer precision.8 We report here on an original generic approach for colloid directed assembly combining convective/capillary deposition and AFM oxidation lithography. This method, which is well adapted to the integration of colloids into silicon devices, is demonstrated for 100 nm colloidal latex nanoparticles and Escherichia coli bacteria in aqueous suspensions. Figure 1 gives a schematic description of this method. The first step of the process consists of functionalizing a silicon substrate with a hydrophobic SAM of octadecyltrimethoxysilane * Corresponding author. E-mail: [email protected]. † Universite´ de Toulouse, INSA-CNRS-UPS, LPCNO. ‡ LAAS-CNRS, Universite´ de Toulouse.

(1) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303–1311. (2) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. J. Am. Chem. Soc. 2001, 123, 8718– 8729. (3) Kraus, T.; Malaquin, L.; Schmid, H.; Riess, W.; Spencer, N. D.; Wolf, H. Nat. Nanotechnol. 2007, 2, 570–576. (4) Gordon, M. J.; Peyrade, D. Appl. Phys. Lett. 2006, 89, 053112–3. (5) Fustin, C. A.; Glasser, G.; Spiess, H. W.; Jonas, U. AdV. Mater. 2003, 15, 1025–1028. (6) Maury, P.; Escalante, M.; Reinhoudt, D. N.; Huskens, J. AdV. Mater. 2005, 17, 2718–2723. (7) Fan, F.; Stebe, K. J. Langmuir 2004, 20, 3062–3067. (8) Xie, X. N.; Chung, H. J.; Sow, C. H.; Wee, A. T. S. Mater. Sci. Eng. 2006, R54, 1–48.

Figure 1. Principle of the colloid-directed assembly by combining convective/capillary deposition and AFM oxidation lithography: (a) grafting of a hydrophobic OTMS monolayer by CVD, (b) fabrication of hydrophilic SiOx patterns by AFM oxidation lithography, and (c) colloid-directed assembly by convective/capillary deposition.

(OTMS) by chemical vapor deposition (CVD) (Figure 1a). The experimental conditions used for this silanization were optimized in previous work.9 They lead, without any need for post-CVD treatment, to a 2.3-nm-thick monolayer, presenting a water contact angle of about 100°. Hydrophilic SiOx patterns are then fabricated by AFM oxidation lithography in contact mode under ambient conditions (20 °C, 60% RH) using a negative dc bias (Figure 1b). These patterns are topographically and chemically characterized by AFM in contact mode and lateral force microscopy (LFM), respectively. The last step in the process involves the convective/capillary deposition of the desired colloids onto this chemically patterned substrate (Figure 1c). This deposition, performed with a homebuilt setup, consists of dragging a drop of the colloidal suspension onto the substrate at a fixed speed using a motorized linear stage. The temperature is kept constant at 20 °C thanks to a thermoregulated substrate holder. AFM observations are finally carried out under ambient conditions to characterize the colloid-directed assemblies. To demonstrate the versatility of this technique, two kinds of colloids in aqueous solution were used: commercial 100 nm latex nanoparticles in water (Fluka, ref 90517) with a concentra(9) Ressier, L.; Martin, C.; Viallet, B.; Grisolia, J.; Peyrade, J. P. J. Vac. Sci. Technol., B 2007, 25, 17–20.

10.1021/la8028726 CCC: $40.75  2008 American Chemical Society Published on Web 11/06/2008

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Figure 2. (a) Dependence of the protruding height and the trace friction contrast of oxidized squares of 11 × 11 µm2 pattern size with the AFM tip bias (both parameters are measured in comparison with the surrounding OTMS layer). (b) AFM topographic and friction (trace and retrace) images of a SiOx square of 11 × 11 µm2 pattern size fabricated by AFM oxidation lithography with a tip bias of -10 V.

tion of 2% by volume and E. coli bacteria. The E. coli DH5 alpha cells (exponential phase) were grown from a single colony in Luria-Bertani broth in a rotary shaker incubator at 37 °C overnight. They were then centrifuged at 5000g for 2 min. The supernatant was removed, and the bacteria were finally resuspended in water. The resulting bacteria concentration was about 7 × 108 bacteria/mL. The elaboration of hydrophilic SiOx squares of 11 × 11 µm2 pattern size by AFM oxidation lithography was optimized from AFM and LFM observations. Figure 2a presents the protruding height and the trace friction contrast for a set of oxidized square patterns formed at various tip biases. These two parameters are measured in comparison with the surrounding OTMS monolayer. In agreement with previous work,10,11 this graph clearly shows three regimes: in the first regime (tip biases above -6 V), no change in topography and no friction contrast can be noticed. In this case, no modification of the OTMS monolayer is induced by the AFM tip bias. For a tip bias of about -6 V, a slight negative relief associated with a positive trace friction contrast is observed. This regime corresponds to the conversion of the top hydrophobic methyl groups of the OTMS molecules in the monolayer into hydrophilic carboxylic acid end groups. In the third regime (tip biases below -6 V), two effects are observed simultaneously: the local degradation of the OTMS monolayer and the formation of silicon oxide in the silicon substrate. The presence of a stable water meniscus in the tip-sample gap is crucial for the oxidation of the silicon substrate. Indeed, water (10) Sugimara, H.; Hanji, T.; Hayashi, K.; Takai, O. Ultramicroscopy 2002, 91, 221–226. (11) Wouters, D.; Willems, R.; Hoeppener, S.; Flipse, C. F. J.; Schubert, U. S. AdV. Funct. Mater. 2005, 15, 938–944.

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present in the meniscus is dissociated by the negative tip bias, and the O- and OH- oxidative ions diffuse through the OTMS layer and react with the silicon substrate to create localized oxide patterns.12,13 This oxidation leads to raised nanopatterns because the molar volume of the silicon oxide is about twice as large as that of the silicon substrate. The increasing height of the silicon oxide patterns with decreasing tip bias, as shown in Figure 2a, is consistent with previous work.11,14 As expected, this graph also reveals that the created silicon oxide patterns present a frictional coefficient larger than that of the surrounding intact OTMS SAM. Indeed, the capillary force effect on such a hydrophilic oxide surface is larger than that on the hydrophobic organic monolayer surface. In this third regime, the friction contrast remains constant with the tip bias, which means that the hydrophilic/hydrophobic contrast is independent of the protruding height of the silicon oxide patterns. In the present study, all the SiOx patterns used for colloid directed assembly were elaborated with a tip bias of -10 V. Figure 2b presents typical AFM topography and friction images of such a pattern. This silicon oxide pattern protrudes from the surrounding intact OTMS monolayer of about 2 nm. It presents a mean roughness of about 0.1 nm, which is very similar to that of the OTMS SAM. The opposite LFM contrast on trace and retrace ensures that this friction contrast is a chemical contrast and not a topographical one. Latex nanoparticles (100 nm each) were assembled by convective/capillary deposition onto these chemically patterned substrates. Figure 3 presents typical AFM images in tapping mode of the resulting directed assembly and shows that the SiOxpatterns are covered by a close-packed assembly of latex nanoparticles. These assemblies have a pyramidal shape presenting a bi- or trilayer of nanoparticles at the center of the patterns and a monolayer at their edges. The number of deposited nanoparticle layers increases with decreasing meniscus dragging speed (two layers for a dragging speed of 10 µm · s-1 (Figure 3a) and three layers for 5 µm · s-1 (Figure 3b)). The immobilized particles are strongly attached to the substrate. Indeed, we observed that rinsing in water after deposition does not remove the grafted particles. The selectivity of these directed assemblies is excellent because no colloid is deposited outside the patterns. Real-time observations of convective/capillary deposition by optical microscopy allowed us to evidence a three-step mechanism to explain these directed colloid assemblies on SiOx patterns (Figure 4). The first two steps are very similar to those previously reported in the literature in the case of colloid deposition onto silicon substrates, which were chemically patterned by photolithography and silanization.15,16 These two steps are dominated by convective flow phenomena. In the first step, the colloidal solution drop is dragged onto the unmodified OTMS monolayer (Figure 4a). In this case, no deposition of nanoparticles occurs because the hydrophobic behavior of this SAM leads to a high dynamic contact angle of the dragging colloid drop on the surface. The concentration of colloids in the dragging drop is increased near the meniscus, but the colloids stay in the suspension drop and are embedded with the convective flow. (12) Marchi, F.; Bouchiat, V.; Dallaporta, H.; Safarov, V.; Tonneau, D.; Doppelt, P. J. Vac. Sci. Technol., B 1998, 16, 2952–2956. (13) Xie, X. N.; Chung, H. J.; Sow, C. H.; Wee, A. T. S. Mater. Sci. Eng. 2006, R54, 1–48. (14) Park, J.; Lee, H. Mater. Sci. Eng. 2004, C24, 311–314. (15) Jonas, U.; del Campo, A.; Kru¨ger, C.; Glasser, G.; Boos, D. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5034–5039. (16) Fustin, C. A.; Glasser, G.; Spiess, H. W.; Jonas, U. Langmuir 2004, 20, 9114–9123.

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Figure 3. AFM images in tapping mode of the directed assembly of 100 nm latex nanoparticles onto SiOx patterns: meniscus dragging speeds of (a) 10 µm · s-1 and (b) 5 µm · s-1.

Figure 5. AFM images in contact mode of the directed assembly of E. coli bacteria onto SiOx patterns: meniscus dragging speeds of (a) 1 µm · s-1 and (b) 0.5 µm · s-1

Figure 4. Optical top views of the convective/capillary deposition of 100 nm latex nanoparticles taken at different times with schematic diagrams of the associated mechanism.

The second step begins when the meniscus arrives on the hydrophilic silicon oxide patterns (Figure 4b). At this point, the contact angle of the dragging drop decreases because of the wettability contrast between the unmodified OTMS SAM and the silicon oxide patterns. The meniscus is thus elastically deformed onto the SiOx patterns. The contact angle on the SiOx patterns is low enough (the thickness of the colloidal film at the edge of the drop is comparable to the colloid size) to enable the colloid assembly in front of the meniscus by convective flux and

capillary forces (Figure 4c). In the present study, the topography and the roughness of the SiOx patterns do not significantly contribute to the decrease in the contact angle of the colloid dragging drop because they are negligible compared to the colloid size. The third step is dominated by capillary phenomena. The drop of colloidal solution is elastically deformed by the silicon oxide patterns up to a threshold above which the meniscus line breaks. The drop then jumps ahead of the patterns, leaving a small volume of suspension that completely wets the already deposited colloids (Figure 4d). The remaining solvent finally evaporates, and the colloids are rearranged by capillary forces with respect to the shape of the remaining drop to obtain a pyramidal compact arrangement. It is interesting to notice that the entire convective/capillary deposition lasts only a few seconds.

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Very similar results are obtained with aqueous solutions of E. coli bacteria. Figure 5 shows typical AFM images in contact mode under ambient conditions of directed assemblies of bacteria onto SiOx patterns using the same procedure. It reveals a close-packed monolayer of bacteria (for a meniscus dragging speed of 1 µm · s-1 (Figure 5a) and a monolayer/bilayer pyramidal structure of bacteria (for a meniscus dragging speed of 0.5 µm · s-1 (Figure 5b) covering the silicon oxide patterns with excellent selectivity. Contrary to the methods usually used,17 this bacterial cell immobilization method induces very weak physical tension and no chemical contamination on the bacterial cell wall. It can be applied to all kinds of bacteria (round or cylindrical). Moreover, the deposited bacteria are sufficiently stable to be imaged by AFM in contact mode. They can be reimmersed in solution immediately after convective/capillary deposition (before the bacterial drop left on the silicon oxide patterns completely evaporates) without being removed from the SiOx patterns. Any damaging air exposure of bacteria can thus be avoided. This original bacterial cell immobilization should therefore become a powerful tool in the nanomicrobiology toolbox18 because it (17) Vadillo-Rodriguez, V.; Busscher, H. J.; Norde, W.; De Vries, J.; Dijkstra, R. J. B.; Stokroos, I.; Van der Mei, H. C. Appl. EnViron. Microbiol. 2004, 70, 5441–5446.

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makes possible nanomechanical,19 single-molecule recognition,20 or protein unfolding21 experiments on a single bacteria. In conclusion, we showed that combining convective/ capillary deposition and AFM oxidation lithography is an efficient generic method for directing the assembly of any kind of colloid (independently of their nature, size, charge, or chemical surface function) onto specific areas of silicon substrates. The versatility of this process was demonstrated for 100 nm latex nanoparticles and E. coli bacteria. We evidenced that a three-step mechanism involving both convective flow and capillary forces that is responsible for these directed colloid assemblies on SiOx patterns. Acknowledgment. We thank F. Durand from the Biotechnology & Bioprocedures Laboratory (LBB Toulouse) for his help in providing E. coli bacteria. LA8028726 (18) Dufreˆne, Y. F.; Mu¨ller, D. J. Nat. Nanotechnol. 2008, 3, 261–269. (19) Francius, G.; Tesson, B.; Dague, E.; Martin-Je´ze´quel, V.; Dufreˆne, Y. F. EnViron. Microbiol. 2008, 10, 1344–1356. (20) Hinterdorfer, P.; Dufreˆne, Y. F. Nat. Methods 2006, 3, 347–355. (21) Oesterhelt, F.; Oesterhelt, D.; Pfeiffer, M.; Engel, A.; Gaub, H. E.; Muller, D. J. Science 2000, 288, 143–146.