INTRODUCTION
Ultrathin films of organosilane have been identified as promising nanocoating for micro- and nanoscale technologies such as electronic devices and micromachines.[1-3] As the silanol groups of organosilane monolayer prepared from organotrichlorosilane or organotrialkoxysilane strongly interact with the substrate surface, the monolayer is thermally and chemically robust compared with conventional amphiphilic monolayers. Because the chain length of organosilane is approximately 1-3 nm, the or-ganosilane forms a uniform ultrathin film on the substrate surface.
Two methods have been proposed for the preparation of organosilane monolayers. One is chemisorption from organosilane solution,[4-7] and the other is the Langmuir-Blodgett (LB) method.[8-22] Fig. 1 shows the film-formation mechanisms of the organosilane by the LB method (a) and the chemisorption method (b). In the case of the LB method Fig. 1a, the toluene solution of organo-trichlorosilane was spread on the water surface (pH = 5.8) at a controlled subphase temperature. To attain the quasi-equilibrium state of the monolayer, the monolayer was kept on the water subphase under a given constant surface pressure for 15 min. The monolayer was transferred and immobilized onto the Si-wafer substrate surface by the LB method. In the case of chemisorption Fig. 1b, organosi-lane molecules were deposited either from the solution or from the vapor phase. It has been clarified that the aggregation state of the organosilane monolayers prepared by the LB method shows a higher packing density than the chemisorbed monolayers.[15,22]
Organosilane monolayers, which have surfaces terminated by various functional groups, are useful for manipulation of the physicochemical properties of solid surfaces such as wettability, nanotribology, and protein adsorption behavior. A key to fabricating functional orga-nosilane monolayers is controlling the distribution of surface functional groups. Fabricating micro- and nanode-vices using a bottom-up approach requires building blocks with a precisely controlled and tunable chemical composition, morphology, and size that can be fabricated virtually at will. The organosilane monolayer is a candidate for such a building block because of its stability and ease of fabrication. Patterned microfeatures of organosilane monolayers can be fabricated on the substrate, allowing surface physicochemical properties to be area-selectively controlled. Two methods will be discussed in this article. One of them utilizes crystallization of organosilane of the binary component monolayer at the air/water interface.1-8-11-1 Because the diffusion of organosilane molecules at the air/water interface is slow, macroscopic phase separation is inhibited, even with the alkylsilane and fluoroalkylsilane mixed monolayers. The phase-separated monolayer is transferred to the Si-wafer substrate by the LB method. Another method utilizes photolithography by a vacuum ultraviolet (VUV) ray source.[23,24] In the case of a VUV source with l =172 nm, photodecomposition of the organic moiety occurs because of the higher photon energy of the VUV ray compared with the bond energy of a typical C-C linkage. Using photolithography, one can prepare a micropatterned surface with various organosilane monolayers by repeating the photodecomposition and chemisorption processes. By changing the shape and area ratio of the patterns of the photomask, this technique enables one to control the area ratio and the wettability gap of different organosi-lane monolayers.
PATTERNING OF ORGANOSILANE MONOLAYERS VIA CRYSTALLIZATION AT THE AIR/WATER INTERFACE
Formation of Organosilane Monolayers at the Air/Water Interface
Fig. 2 shows the chemical structure of organosilanes used for monolayer preparation. Organotrichlorosilane is primarily used for monolayer preparation at the air/water interface. Lignoseric acid [LA, CH3(CH2)22COOH] is also used to prepare the mixed monolayers. LA is not polymerized and is also nonreactive against the silicon substrate. The chlorine groups of organosilane on the water surface were found to be substituted by hydroxyl groups. At a surface pressure of 10-30 mN m-1, the hydroxyl groups in organosilane molecules reacted with those in adjacent molecules in the case of the highly condensed monolayer, resulting in formation of a polymerized monolayer. The polymerized monolayer was easily transferred onto a Si wafer by the LB method, and the residual hydroxyl groups could be covalently bonded with silanol groups on the Si wafer surface.
Fig. 1 Film formation mechanism of organosilane by Langmuir-Blodgett (LB) method (a) and chemisorption method (b).
Fig. 2 Chemical structure of organosilanes.
Fig. 3 shows the surface pressure-area (p-A) isotherms for the n-octadecyltrichlorosilane (OTS), n-dodecyltri-chlorosilane (DDTS), [1H,1H,2H,2H-perfluorododecyl-oxy]propyltriethoxysilane (FDOPTES), and [2-perfluoro-octyl]ethyltrichlorosilane (FOETS) monolayers on the water surface at a subphase temperature of 293 K, as well as electron diffraction (ED) patterns of the monolayers transferred onto the hydrophilic SiO substrate on the EM grid at a surface pressure of approximately 20 mN m-1. The p-A isotherms of the OTS and FDOPTES mono-layers showed a steep increase in surface pressure with decreases in the surface area. The molecular occupied areas were determined to be 0.24 and 0.29 nm2 molecule-1 for the OTS and FDOPTES monolayers, respectively. Electron diffraction patterns of both the OTS and the FDOPTES monolayers showed hexagonal crystalline arcs at 293 K. The (10) spacings of the OTS and the FDOPTES monolayers were calculated to be ca. 0.42 and 0.50 nm based on ED patterns, respectively.1-18-1 These ED results make it clear that the hydrophobic alkyl and fluoroalkyl chains in the crystalline OTS and FDOPTES monolayers were closely packed in the hexagonal crystal lattice at 293 K.
Fig. 3 p-A isotherms for the OTS, DDTS, FDOPTES, and FOETS monolayers on the water surface at a subphase temperature of 293 K, as well as the AFM images and ED patterns of the monolayers transferred onto the substrate at the surface pressure of around 20 mN m-1.
High-resolution atomic force microscopy (AFM) was applied to observe the molecular arrangement of both the crystalline OTS-C18 and FDOPTES monolayers. Fig. 3 also displays AFM images on a molecular scale for the crystalline OTS-C18 and FDOPTES monolayers transferred onto a Si wafer at a surface pressure of 15 mN m-1 at 293 K.[18] The (10) spacing was evaluated to be ca. 0.42 and 0.50 nm by two-dimensional fast Fourier transform (2-D-FFT). These values are in good agreement with the (10) spacings determined from the ED patterns for the crystalline OTS and FDOPTES mono-layers. Thus, it is conceivable that the higher portions (the brighter dots) in the AFM images of Fig. 3 correspond to the individual methyl group of the OTS molecule and the fluoromethyl group of the FDOPTES molecule in the monolayers, respectively. In contrast, the p-A isotherms for the DDTS monolayer and the FOETS monolayer with a shorter alkyl chain showed a gradual increase in surface pressure with decreasing surface area. In general, this can be interpreted as an indication that the monolayer is in a liquid-condensed or liquid-expanded state. In addition, the ED patterns of the DDTS and the FOETS monolayers showed an amorphous halo at 293 K. Hence, it can be envisaged that the chain lengths of hydrophobic groups of the DDTS and the FOETS molecules were not long enough to crystallize on the water subphase at 293 K.
Phase Separation of Mixed Monolayers at the Air/Water Interface
Phase separation is expected to occur in the binary component monolayer of crystalline OTS and amorphous FOETS because of the incompatibility of OTS and FOETS. Fig. 4 shows the p-A isotherm and the AFM image of the scanned area 10 x 10 mm2 for the OTS/ FOETS (50/50 mol/mol) mixed monolayer, which was transferred onto the Si wafer substrate by the LB method at a surface pressure of 25 mN m-1 The height profile along the line shown in the AFM image revealed that the brighter and darker portions in the AFM image correspond to the higher and the lower regions of the monolayer surface, respectively. The molecular occupied area (the limiting area) of 0.28 nm2 molecule- 1 for the OTS/FOETS (50/50) mixed monolayer appears to be almost equal to the average of the molecular occupied area for the OTS (0.24 nm2 molecule-monolayer and the FOETS (0.31 nm2 molecule- 1) monolayer based on the molar fraction of OTS and FOETS. The OTS/FOETS mixed monolayer can be transferred onto the Si wafer substrate over a wide range of surface pressure. The transfer ratio of the OTS/ FOETS mixed monolayer was ca. 1.0 at a surface pressure of 25 mN m- 1, indicating that the substrate surface is almost completely covered with the immobilized mixed monolayer. Also, the transfer of the OTS/FOETS mixed monolayer on the silicon substrate was confirmed by using attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Because the area occupied by the circular flat-topped domains increases with increases in the OTS content, it is expected that the circular domains correspond to the OTS domain. The AFM line profile revealed that the circular domains were 1.1-1.3 nm higher than the surrounding area. Because the difference in molecular lengths between OTS and FOETS was ca. 1.3 nm, it can be concluded that the higher circular domains and the surrounding flat matrix regions were composed of OTS and FOETS molecules, respectively. OTS molecules formed circular domains even if the molar percent of OTS molecules was 75%. It is apparent from the ED pattern of the OTS/FOETS (75/25) mixed monolayer that the OTS domain is in a crystalline state, as the ED pattern showed a Debye ring and the magnitude of spacing corresponds to the (10) spacing of the OTS monolayer. In the case of the mixture of fluoroalkane and alkane, macroscopic phase separation was observed. However, in the case of organosilanes at the air/water interface, macroscopic phase separation such as that occurring with coalescence of the crystalline domain is inhibited because of the limited diffusion at the air/water interface.
Fig. 4 p-A isotherm and AFM image of the scanned area 10 x 10 mm2 for the OTS/FOETS (50/50) mixed monolayer, which was transferred onto the Si wafer substrate by the LB method at the surface pressure of 25 mN m-1.
A similar phase-separated structure was also expected for the mixed monolayer of the crystalline NTS and amorphous FOETS. Crystallization of the n-nonadecenyl-trichlorosilane (NTS) phase was confirmed by ED.[17] Fig. 5 shows the AFM images of the mixed NTS/FOETS and carboxylated NTS (NTSCOOH)/FOETS monolayers. It was clarified that the NTS/FOETS mixed monolayers were in a phase-separated state, and that circular flat-topped domains ca. 1-2 mm in diameter were surrounded by a sealike flat region. The phase separation in the mixed NTS/FOETS can also arise from crystallization of the NTS component. The NTSCOOH/FOETS mixed monolay-er was prepared through oxidation of the vinyl group of the NTS phase in the NTS/FOETS mixed monolayer.[17] The NTSCOOH monolayer showed high surface free energy, with the magnitude being comparable to that of water. The surface morphology of the NTS/FOETS mixed monolayer was not changed even after oxidation because of the presence of a strong interaction between silanol groups of NTS and the Si wafer. The height difference between the NTSCOOH domain and the FOETS matrix phase in the NTSCOOH/FOETS mixed monolayer was almost the same as that for the NTS/FOETS mixed monolayer. In addition, an XPS measurement was performed for the NTS/FOETS and the NTSCOOH/FOETS mixed monolayers to confirm oxidation of the NTS phase. The larger ratio of oxygen/carbon atoms for the (NTSCOOH/FOETS) mixed monolayer than that for the NTS/FOETS monolayer suggests that the vinyl end groups of the NTS molecules were oxidized to carboxyl groups. The magnitude of the lateral force of the NTSCOOH phase was higher than that of the FOETS phase in the case of the NTSCOOH/FOETS mixed monolayer in contrast to the case of the NTS/FOETS mixed monolayer. As the NTSCOOH phase had hydrophilic carboxyl end groups at the surface, these end groups can presumably form intermolecular hydrogen bonds with neighboring NTSCOOH molecules. Therefore, the surface of the outermost NTSCOOH phase is expected to show a higher shear strength than that of the NTS phase because of a difficulty in surface deformation. Also, there is a significant contribution of adhesion force between the sample surface and the tip with regard to lateral force. Because the surface free energy of the NTSCOOH phase is comparable to that of water, the water capillary force interacting between the NTSCOOH monolayer surface and the hydrophilic Si3N4 tip could strongly contribute to the adhesion force of the NTSCOOH phase. It is therefore conceivable that the NTSCOOH phase exhibited higher lateral force than the FOETS phase because of the formation of intermolecular hydrogen bonding and a thicker absorbed water layer as discussed above. Lateral force microscopic (LFM) and XPS measurements revealed that the phase-separated monolayer with a large surface energy gap was successfully prepared.
Fig. 5 AFM images of the mixed NTS/FOETS and carbox-ylated NTS (NTSCOOH)/FOETS monolayers.
To investigate the various types of mixed monolayers, a novel mixed-monolayer system was designed using reactive organosilanes and a nonreactive fatty acid. A phase-separated monolayer can also be prepared from both FOETS and nonpolymerizable and crystallizable amphiphiles such as stearic acid (SA) and lignoceric acid (LA). Fig. 6 shows the AFM images of the LA/FOETS (50/50 mol/mol) mixed monolayer Fig. 6a and after extraction of LA with hexane Fig. 6b. The LA/FOETS mixed monolayer was in a phase-separated state similar to the OTS/FOETS mixed monolayer as shown in Fig. 4. It is reasonable to conclude from Fig. 6a and b that the circular domains are composed of LA molecules, as the circular flat-topped domains were preferentially extracted with hexane. In addition, the FOETS matrix was not extracted with hexane because FOETS molecules were immobilized on the Si wafer surface by the Si-O-Si covalent bond and multiple hydrogen bonding. The electron diffraction of the LA/FOETS mixed monolayer showed a crystalline diffraction from LA domains. In addition, the circular domain of LA is higher than that of FOETS by 2 nm. Because the bare Si surface with Si-OH groups was exposed to the surface, the Si phase can easily be backfilled by another organosilane through chemisorption from its solution. Thus, various types of surface modification are possible by chemisorption of various organo-silanes to the FOETS monolayer with holes shown in Fig. 6b. Fig. 6c shows the AFM topographic image of the OTS/FOETS mixed monolayer prepared by the chemisorption of OTS onto the Si part of the FOETS monolayer. The surface structure of Fig. 6c was very similar to the phase-separated structure observed for the OTS/FOETS mixed monolayer directly prepared by the LB method. The OTS phase is mechanically and chemically very stable because of the polymerization and anchoring to the Si wafer. However, the domain height of the OTS phase in Fig. 6c was less than that observed for the OTS/FOETS mixed monolayer prepared by the LB method in Fig. 6c. In addition, the surface roughness of the OTS domains prepared by chemisorp-tion was more distinct than that of the OTS domains prepared by the LB method. These results indicate that the OTS monolayer prepared by chemisorption is less ordered than that prepared by the LB method. This was also confirmed by grazing incidence X-ray diffraction (GIXD).[15,22] However, the abovementioned procedure to backfill the Si portion of the FOETS monolayer by the chemisorption of organosilane could be applicable to the preparation of two-phase monolayers in which the constituents have different surface free energies or surface chemistries.
Fig. 6 AFM images of the LA/FOETS (50/50 mol/mol) mixed monolayer (a), ghost monolayer after extraction of LA with hexane (b), and after backfill of ghost monolayer with OTS (c).
