Arrangement of Nanosized Ceramic Particles on ...

Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 4596–4600 Part 1, No. 7B, July 2000 c °2000 The Japan Society of Applied Physics

Arrangement of Nanosized Ceramic Particles on Self-Assembled Monolayers Yoshitake M ASUDA, Won Seon S EO and Kunihito KOUMOTO ∗ Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan (Received November 1, 1999; accepted for publication April 11, 2000)

The fabrication of novel micro/nano-sized devices by assembling inorganic particles is anticipated for future microelectronics which will make use of their attractive functions. The surface modification of self-assembled monolayers (SAMs) was studied to prepare templates for sphere assembly. Phenyl groups of SAM were modified into silanol groups by UV irradiation through a photomask, by applying an electric current using an atomic force microscope (AFM) probe, or by contact pressure with a diamond tip. They were used as templates to arrange fine inorganic particles. In addition to the formation of ester bonds, siloxane bond formation between spheres and SAMs was also found to be effective for sphere arrangement. Low-dimensional close-packing of SiO2 spheres was achieved through the formation of siloxane bonds. The two-dimensional arrangement of functional particles on SAMs in a controlled manner through the formation of strong chemical bonds, such as ester bonds or siloxane bonds, can be applied to the microfabrication of ceramic devices. KEYWORDS: self-assembled monolayer, silica, sphere, microdevice, patterning, arrangement

1. Introduction Novel processes for the fabrication of advanced devices and functional inorganic materials are urgently required.1, 2) Various methods of micro/nanometer-scale assembly of metals, inorganic crystals, polymer thin films,3) and functional particles have been investigated.2, 4) A number of organic systems have been demonstrated to be effective as templates for arranging materials two-dimensionally.5, 6) In particular, selfassembled monolayers (SAMs) have been used as templates for a variety of advanced applications, including the fabrication of nonlinear optics multilayer assemblies,7–10) the alignment of liquid crystals,11–13) for conducting polymers,14, 15) and metals such as Cu16) and Ni,17) as well as the growth of oriented mineral structures.18–22) Crystal nucleation of CaCO3 was controlled using patterned SAMs, and ordered crystallization of CaCO3 in the polar regions was achieved.1) Micropatterned TiO2 films on SAMs of phenyltrichlorosilane (PTCS) were successfully fabricated using a liquid-phase deposition process involving the direct precipitation of TiO2 from aqueous solutions con22) A multilayered ferroelectric capacitor taining TiF2− 6 ions. consisting of a Pt/Pb(Zr, Ti)O3 /Pt thin-film structure was fabricated using SAMs by means of metalorganic chemical vapor deposition. Thin films of Cu, Pd, LiNbO3 , (Pb, La)TiO3 and Ta2 O5 have also been deposited successfully by this method.23) Ultrafine particles can be applied to numerous future micro/nano-sized devices such as single-electron transistors,24–26) quantum-effect devices,27) optical devices,28–30) and nanosized magnetic recording media.31, 32) The fabrication of novel micro/nano-sized devices by assembling inorganic particles is thus anticipated because of their multiple functions. Recently, various techniques of arranging micro/nano-sized spheres have been studied by a variety of researchers. Gold particles (50–100 µmφ) were manipulated using a tungsten microprobe with a tip 2 µm in radius. Low voltage was applied between the probe and a gold substrate, and the gold particles were adhered by electric fusion.33, 34) This technique can be applied to nanosized patterning by using an atomic force microscope (AFM) tip instead of a tungsten tip. An ∗ E-mail

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electron beam and an ion beam were used to form a pattern on an insulating substrate (CaTiO3 )35, 36) to which SiO2 spheres (5 µm in diameter) were adhered. However, no strong chemical bonds formed between particles and the substrate only gave rise to a weak adhesion force. A two-dimensional, anisotropically conductive matrix was fabricated by transferring an array of Ni-coated Al2 O3 particles (47 µm in diameter) from a template to an adhesive tape using magnetic attractive force.37) The monolayer deposition of nanoscale Au colloidal particles (20 nmφ and 2 nmφ) was demonstrated by employing aminofunctional silane as a coupling agent.38) Surface modification techniques for SAMs and micro/nanosized particles are expected to be useful for fabricating a twoor three-dimensional arrangement of particles. The ability to alter the chemical reactivity of the surface of SAMs through exposure to UV light,39–45) electron46) or ion47–49) beams, or plasma or low-energy electrons generated by employing proximal probes such as STM probes50) and AFM probes51, 52) is an additional advantage in that it provides a means to create spatially well-defined reactive templates. Muller et al.’s pioneering work demonstrated that scanning with a platinumcoated AFM tip over a SAM surface containing terminal azide groups in the presence of H2 led to the reduction of azide groups to primary amino groups.52) Derivatization of the resulting amine surface with aldehyde-modified latex beads (29 nm in diameter) resulted in specific labeling of the reduced areas. We have proposed a concept for two-dimensionally arranging fine particles using chemical reactions, and reported the initial research results.53) PTCS-SAM was modified to have a phenyl/silanol-pattern by UV irradiation using a photomask. Parallel to this reaction, the surfaces of silica spheres were modified to have carboxyl groups with an organosilane reagent through chemical reactions. Ester bonds were formed from carboxyl groups of silica spheres and silanol groups of SAM by the use of a condensing agent, and consequently, silica spheres were selectively adhered to silanol surfaces of SAM. Even though the selectivity of the silica sphere arrangement was fairly good, the number density of silica spheres on the silanol surface was rather low compared to our initial expectation for a two-dimensional close-packed structure. Moreover, a small number of silica spheres were also observed on hydrophobic phenyl surfaces.

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Accordingly, more effective chemical reactions between spheres and SAMs must be developed in order to arrange spheres in a controlled manner for device fabrication. Although UV irradiation was used to modify the surface of SAMs in our previous study,53) it is necessary to develop new techniques for the surface modification of SAMs in order to realize an ideal arrangement of nanosized spheres. Here we propose a new chemical reaction for arranging silica spheres on SAMs to achieve close packing of spheres: the formation of siloxane bonds between spheres and SAMs in the presence of an acid or base. Furthermore, we have attempted to modify the surfaces of SAMs by means of the AFM lithography technique or a mechanical modification technique with a diamond tip to prepare templates for nanosize patterning. We report the results of our initial attempts in this paper. 2. Experimental 2.1 SAM preparation P-type Si (100) wafers were employed for the substrates. They were cleaned ultrasonically in deionized water (>17.6 M·cm), immersed in a 1:1 (vol.) HCl:CH3 OH solution for 30 min, and again cleaned in deionized water. They were further immersed in conc. H2 SO4 for 30 min and then in boiling water for 5 min, and were subsequently cleaned with acetone. PTCS-SAM was prepared by immersing the substrate in an anhydrous toluene (99.8%, Aldrich) solution containing 1 vol% PTCS (Aldrich) for 5 min under a N2 atmosphere. The substrate with a PTCS-SAM was then baked at 120◦ C for 5 min to remove residual solvent and to promote chemisorption of the SAM.

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easily through dehydration in the presence of an acid or base.54) We attempted to form siloxane bonds between silanol groups of silica spheres and silanol groups of SAMs using (a) hydrochloric acid, (b) sulfuric acid, and (c) sodium hydroxide. Silica spheres (500 nmφ, powder, Admatechs Co., Ltd., SO-E2) were sonicated in a hydrochloric acid solution, a sulfuric solution, or a sodium hydroxide solution for 10 min for dispersion. Then, patterned SAMs were immersed in the solution for 5 min. After having been rinsed in acetone, the patterned-SAM substrates were observed with a scanning electron microscope (S-3000N, Hitachi Ltd.). Silica spheres were observed predominantly in the silanol regions of the SAM in all cases (Fig. 2), which indicates that silanol groups on silica sphere surfaces were adhered to silanol groups of the SAM selectively. The arrangement with hydrochloric acid exhibits the finest patterning of all our experiments, it was much denser than the previously reported one,53) and spheres with different sizes appear to be closely packed twodimensionally. This close-packed structure is very interesting and must be associated with the formation of siloxane bonds among silica spheres themselves as well as between spheres and a SAM. In contrast, many by-products were observed se-

2.2 Surface modification of SAM by UV irradiation PTCS-SAM was exposed for 2 h to UV light (184.9 nm) from a Hg lamp through a photomask. The UV-irradiated regions became hydrophilic due to Si–OH group formation, while the nonirradiated part remained unchanged, i.e., it was composed of hydrophobic phenyl groups, which gave rise to a patterned PTCS-SAM.6–9) In order to check for successful film formation and functional group change, the water drop contact angle was measured for both irradiated and nonirradiated surfaces. Initially deposited PTCS-SAM showed a water contact angle of 74◦ , but the UV-irradiated surface was wetted completely (contact angle < 5◦ ). These SAMs were used for the sphere arrangement in the presence of hydrochloric acid.

OH

2.3 Micropatterning of SiO2 spheres on SAM using acid or base Silanol groups were reported to form siloxane (Si–O–Si)

HO HO

OH

HO

OH SiO2

OH OH

+ UV irradiation photomask = phenyl group

= OH group

Fig. 1. Conceptual process of arranging silica spheres on a SAM.

Fig. 2. SEM photographs of silica spheres arranged on patterned SAMs using (a) hydrochloric acid, (b) sulfuric acid and (c) sodium hydroxide.

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lectively on silanol surfaces for the case of sodium hydroxide; energy-dispersive X-ray analysis (EDAX Falcon, EDAX Co. Ltd.) indicated that sodium hydrogencarbonate (NaHCO3 ) was possibly precipitated on silanol surfaces. The substrates were further sonicated in acetone for 10 min, but the removal of silica spheres was not observed. This observation strongly suggests that the bonds between silica spheres and silanol groups of SAM, possibly siloxane bonds, are strong enough to keep them adhered to each other during sonication. Ester bonds between silica spheres and SAM could not maintain their bonds during sonication.53) 2.4 Surface modification of SiO2 spheres Silica spheres (500 nmφ) were immersed in a dicyclohexyl solution and sonicated for 10 min under a N2 atmosphere for good dispersion. One volume percent trichlorocyanoethylsilane (TCES) was added to the dicyclohexyl solution under a N2 atmosphere, and the solution was stirred gently for 10 min in order to chemisorb TCES to SiO2 sphere surfaces. SiO2 spheres with TCES were centrifuged several times to remove unreacted TCES with dicyclohexyl. SiO2 spheres with TECS were then baked at 120◦ C for 5 min to remove residual solvent and to promote chemisorption of the TCES. SiO2 spheres with TCES were further dispersed in a tetrahydrofuran solution containing potassium tert-butoxide (t-BuOK) and 18-crown 6-ether for 24 h under an ambient atmosphere in order to oxidize CN-groups to carboxyl groups.55) This reaction was initiated by deprotonation at a C–H adjacent to the nitrile, and an oxidative cleavage transformed the carbon α to the nitrile into the carboxyl group. The solution was centrifuged to remove tetrahydrofuran, and brown precipitate was obtained. It was further centrifuged several times using distilled water to remove t-BuOK, 18-crown 6-ether, and tetrahydrofuran; carboxylgroup-terminated SiO2 spheres were finally obtained after centrifugation. These spheres were arranged on patterned SAMs using AFM lithography and a diamond tip. 2.5 Surface modification of SAMs with AFM probe and arrangement of SiO2 spheres A source measure unit (SMU Model 236, Keithley) was installed in the AFM (Nanoscope E, Digital Instruments) in order to control the current flowing through the probe and the SAM. PTCS-SAMs were biased positively, and scanned with the AFM probe in constant current mode (50 nA), and the scanned area was used as a template for sphere arrangement (Fig. 3). Phenyl groups were modified to silanol groups by an electric current.51) In order to verify the modification of the SAM surface, cyano-group-terminated SAM was prepared using TCES, and scanned with the AFM probe in constant current mode. SAMs were evaluated by X-ray photoelectron spectroscopy (XPS) (ESCALAB 210, VG Scientific Ltd.). The X-ray source (MgKα, 1253.6 eV) was operated at 15 kV and 18 mA, and the analysis chamber pressure was 1–3 × 10−7 Pa. The spectrum peak corresponding to the N 1s binding energy centered at 399.7 eV was observed for the surface of the TCES-treated Si substrate on which TCES-SAM was formed (Fig. 4). The intensity of this peak decreased greatly after the substrate was scanned by the AFM probe. This showed that ethylene chains and/or cyano groups were decomposed by AFM lithography.

Fig. 3. Schematic illustration of AFM lithography and sphere arrangement on patterned SAM.

Fig. 4. XPS spectrum of N 1s for the surface of a TCES-treated Si substrate (a) before and (b) after AFM lithography.

Carboxyl-group-terminated silica spheres were sonicated for 10 min in dichloromethane, and this solution was refrigerated to −20◦ C for 1 h. N,N0 -dicyclohexylcarbodiimide (DCC, Kishida Chemical Co., Ltd.) was added to this solution as a condensing agent,56, 57) and the temperature of the solution was increased slowly to 20◦ C. A phenyl/silanol-patterned SAM substrate was then immersed in this solution for 2 h. The solution temperature was further raised and held at 25◦ C for 2 h. After rinsing in acetone, the patterned SAM substrates were observed with a scanning electron microscope. Silica spheres were observed predominantly in the silanol regions (Fig. 5), which indicates that carboxyl groups on the silica sphere surfaces were adhered selectively to the silanol groups of the SAM. The AFM lithography technique was shown to be useful for sphere arrangement in this experiment. 2.6 Surface modification of SAMs with diamond tip and arrangement of SiO2 spheres The diamond tip was contacted to a SAM surface lightly


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nique and mechanical modification with a diamond tip were also successfully applied in preparing templates for the selective arrangement of silica spheres on silanol surfaces through ester bond formation.

Fig. 5. SEM photograph of silica spheres arranged on patterned SAMs by AFM lithography.

Fig. 6. SEM photograph of silica spheres arranged on patterned SAMs by a diamond tip.

and traced with low contact pressure in order to modify the SAM surface. Phenyl groups were broken mechanically by contact pressure with the diamond tip, and they possibly changed into silanol groups. The diamond tip was used to avoid contamination from a metal tip and the influence of a chemical reaction between the tip and the SAM. After modification using the diamond tip, the patterned SAM was immersed into a dichloromethane solution containing carboxylgroup-terminated silica spheres. DCC was then added in the same manner as that for the patterning using AFM lithography. After rinsing in acetone, the patterned SAM substrates were observed with a scanning electron microscope. Silica spheres were observed predominantly on silanol surfaces (Fig. 6), which indicates that carboxyl groups on silica sphere surfaces were strongly adhered selectively to silanol groups of the SAM. This result suggests the possibility of mechanical surface modification using a diamond tip, and this technique can be applied to modification of the SAM for sphere arrangement. 3. Conclusions Silica spheres were arranged on silanol surfaces of SAMs selectively in the presence of an acid or base. A two-dimensional close-packed arrangement of spheres was achieved by using hydrochloric acid to form siloxane bonds among silica spheres and SAMs. The AFM lithography tech-

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