Bioinspired Ceramic Thin Film Processing: Present Status and

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CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 5 1983-2017

Review Bioinspired Ceramic Thin Film Processing: Present Status and Future Perspectives Yanfeng Gao† and Kunihito Koumoto* Nagoya University, Graduate School of Engineering, Nagoya 464-8603, Japan Received November 3, 2004

ABSTRACT: Nature has ingeniously succeeded in producing an impressive variety of inorganic functional structures with a designed shape and size on specific sites through a biologically controlled biomineralization process, usually at near room temperature and in aqueous solutions. The most important principle understood from biomineralization processes is that nucleation and growth of the biomineral phase are almost always carefully and exquisitely controlled by complex organic matrix biopolymersspreorganized supramolecular templates, which are associated to regulate a single, precise step in either the nucleation or the growth portion of the production of the mineral phase. The interaction at a molecular-level solid-liquid interface between a specific surface chemistry and a solution supersaturated with respect to the inorganic material is one key feature of natural biomineralization. The study of biomineralization offers valuable insights into the scope and nature of materials chemistry at the inorganic-organic interface, which represents an inspiration toward future innovations in seeking highly efficient and/or unique materials synthesis strategies. So-called bioinspired ceramics processing has been developed to produce ceramic thin films, to create specific microstructures, or to control crystallization. In the present review, attention is drawn toward the recent increase in research activities involving the preparation of functional ceramic thin films induced by a specific chemical surface modification. This review provides a brief description of the bioinspired process for in situ patterning of ceramic oxides using a template derived from a self-assembled monolayer (SAM) for site-selective nucleation and growth from solutions under normal conditions in terms of pressure and temperature, emphasizing the fundamental knowledge of the chemistry of solutions and interfaces. Our discussion is limited to methods that grow films in a liquid phase by control of the supersaturation of the solution. Sol-gel and methods equipped with additional energy sources, such as hydrothermal synthesis and electrochemical deposition, are excluded. The following issues are addressed: preparation, photocleavage (for structural/lateral modification) and characterization of SAMs, surface-interface chemistry and solution chemistry for the deposition of ceramic oxides from solutions, and patterning of ceramic oxides on the template of SAMs from aqueous solution under mild conditions. We start with a brief overview of the present status of the fundamental methodology for the synthesis of ceramic thin films from solutions and patterning techniques. Then we discuss the biomineralization process and its inspiration to create novel approaches for the production of engineering materials, focusing on ceramic films. Then we discuss chemical aspects of the deposition of films from solutions, including solution chemistry, modification of surfaces, and the physics and chemistry of interfaces. Next, several experimental examples are given to explain how these aspects influence the formation of films and their properties. Finally, we summarize and discuss future techniques for this field. 1. Introduction Ceramics materials, along with metals, polymers, and composites, are the most important basic building blocks for the technology world. Ceramic technology has been developed for thousands of years, resulting in numerous strategies for producing ceramics in the form of bulk * To whom correspondence should be addressed. E-mail: koumoto@ apchem.nagoya-u.ac.jp. † Present address: Advanced Research Laboratories, Musashi Institute of Technology, Todoroki 8-15-1, Setagaya, Tokyo 158-0082 Japan. E-mail: [email protected].

materials, films, fibers, powders, and so on. With the increasing development in nanoscience and nanotechnology, ceramic nanotechnology has been one of the most active areas of research in recent years. Besides a general trend toward miniaturization, nanosized ceramic oxide materials, such as semiconductor nanoparticles,1-8 nanowires,9-11 nanotubes,12 and various types of artificial supramolecular entities13 along with integration materials including inorganic/organic hybrid materials14,15 and thin films,16 are interesting because of the size-/thickness-dependent properties of materials

10.1021/cg049624x CCC: $30.25 © 2005 American Chemical Society Published on Web 07/26/2005

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and because of their potential for a broad range of applications. As an important aspect, functional ceramic thin films exhibit unique properties compared to bulk materials, are fundamental for electronic, coatings, displays, sensors, optical equipment, and numerous other technologies, and suit the need for scaling devices to small sizes, and are therefore attractive both in theory and practice. Various growth techniques have been applied to synthesize ceramic thin films. These techniques fall into at least two categories: vacuum-based methods such as chemical vapor deposition (CVD),17 sputtering,18 metal organic chemical deposition (MOCVD),19 atomic layer deposition (ALD),20 and chemical solution methods including sol-gel, electrochemical deposition, and hydrothermal synthesis.21,22 Vacuum-based techniques are an expensive investment, are limited to line-of-sight production, and usually require high-vapor-pressure chemicals or high-purity targets as starting materials. For conventional CVD methods, precursors exhibiting volatility are needed, which requires that the precursor are liquid or solid with a low melting point. For wet MOCVD, precursors are usually transferred to reactors by the liquid delivery technique, which makes it possible to use relatively nonvolatile metal-organic precursors but leads to difficulties in controlling the stoichiometric composition of the films. Wet chemical methods mentioned above can overcome some of the defects of vacuum-based methods by use of a homogeneous solution, but there are still processing issues, such as the control of the film thickness at the nanometer scale of length, professional equipment, and complex processing. 1.1. Solution Deposition Techniques. Even when well-established methods exist for the production of high-quality ceramic films, there is still considerable interest in alternative approaches that may be simple, cheap, and environmental friendly or that are capable of producing films with novel or improved properties. Evidently, a simple, cheap process emphasizes not only the simplicity of the production of a specific thin film but also emphasizes that the process can be applied to the production/processing of many different types of products with only small variations in conditions and equipment.23 When considering the preparation of thin films through a chemical route, one should realize that modern chemistry as a major branch of science and industry should be developed to emphasize low consumption of raw materials and energy, low generation of waste, and producer/user friendliness.24 These processes are the basis of a series of concepts,25 so-called “green chemistry” (formal nomenclature in USA, U.K., Italy; common name in Japan), “green and sustainable chemistry” (GSC; formal nomenclature in Japan), and “sustainable chemistry” (in Germany), and their derivatives such as “environmentally benign chemistry”, “environmentally friendly chemistry”, or “clean chemistry”. Techniques for the synthesis of ceramic thin films from aqueous solutions at low temperatures meet with some of the requirements mentioned above and are emerging as possible alternatives to vapor-phase and chemical-precursor techniques. The main techniques, including chemical bath deposition (CBD), successive ion layer adsorption and reaction (SILAR), liquid phase deposition (LPD) methods, and electroless deposition

Review

(ED) with a catalyst, along with their variations, were summarized in a recently published review article.26 CBD27,28 can produce a solid film in a single immersion through control of the formation kinetics of the solid, typically without changing the metals’ oxidation states. This technique is used mainly to deposit sulfide and selenide thin films, while some relatively new reports involve preparation of oxides.29 Unlike the CBD method, SILAR can deposit a solid by alternating the adsorption of metal ions and cationic ions from the corresponding solutions.30 The LPD method refers to the formation of an oxide thin film by the hydrolysis of a metal-fluoro complex in an aqueous solution. Boric acid or aluminum metal can react with F- to generate stable species and is added to accelerate the hydrolysis rate. The LPD method was first developed for depositing SiO2 thin films and was later used to prepare other oxides, such as TiO2, V2O5, VO2, FeOOH (Fe2O3), and multicomponent metal oxide films.31 Variations of the main techniques, such as photochemical deposition,32 ferrite plating,33 and deposition using a functional surface,34-44 are being investigated by various groups. The approaches using a functional surface to induce the film formation is inspired from the biomineralization process. It represents a typical low-temperature approach for ceramic films and has attracted much attention during the past decade. 1.2. Patterning Techniques. On the other hand, microfabrication, the generation of small parts, is essential to much of modern science and technology; it supports information technology and permeates society by the integration of microelectronic and optoelectronic functionalities within a very small area, where “smaller” has meant bettersless expensive, more components per chip, fast operation, high performance, and lower consumption of energy and materials.45 Further, integration of a range of these small parts has resulted in portability; reduction in time, cost, reagents, sample size, and power consumption; and improvements in detection limits and types of functions.45a The dominant contributor to the cost of manufacturing chips is patterning technology, often known as photolithography, which is the most successful technology for high-volume manufacturing in microfabrication.45,46 Technique is based on a projection-printing system (usually called a stepper) in which the image of a reticle pattern is focused and projected onto a thin film of a photoresist or light-sensitive polymer that is spin-coated on a wafer through a high numerical aperture lens system.46 Selectively dissolving away either the exposed or the unexposed regions of the resist leaves a relief image on the silicon, which can then be transferred, by etching, to the underlying material to make the components that comprise the chip. A sequence of between 4 and 30 overlaid photolithographic levels is needed to make a complete chip.46a The resolution of this traditional topdown technique is approximately the wavelength of the light used (according to the Rayleigh equation), that is, 110 nm for ultraviolet light with a wavelength of about 193 nm.45a Hence, illuminating sources with shorter wavelengths are progressively introduced.45,46 However, in the foreseeable future, this method will soon become increasingly difficult and expensive for industrial application. As alternatives, e-beam lithography and soft

Review

lithography, such as conventional, mechanical printing techniques, have been developed to create patterns with submicrometer to nanometer features.45,47,48 Among these techniques, the self-assembly of molecular and colloidal building blocks to create larger, functional devices has attracted much effort and interest. These materials are formed from interatomic and intermolecular interactions other than the traditional covalent, ionic, and metallic bonding forces. In contrast to the technological world, organisms develop abundant micropatterned inorganic materials by a biomineralization process, which are usually under the assistance of a macromolecule template.48c,d This macromolecule controls the nucleation, structure, morphology, crystal orientation, and spatial confinement of the inorganic phase. The understanding of biomineralization will catalyze new enthusiasm for bioinspired materials processing, which is important for the design, control, and optimization of materials synthesis at both the molecular and the macroscopic levels. For nanoscience and technology, it is necessary to develop new bottom-up approaches to self-assemble the chemically synthesized nanoblockssnanoparticles, nanowires, nanodots, nanotubessinto large, functional ensembles. 2. Biomineralization and Bioinspired Ceramic Processing 2.1. Biomineralization. Much effort has gone into the development of new green solution processes for the deposition of ceramic oxides. With evolution, however, nature has ingeniously succeeded in producing an impressive variety of inorganic functional structures with designed shapes and sizes on specific sites through a biologically controlled biomineralization process, usually at near room temperature and in aqueous solutions.49 These structures are formed through templateassisted self-assembly, in which a self-assembled organic material serves as the structure scaffolding for the deposition of inorganic material. Most importantly, this mineralization process is usually accomplished under mild conditions in terms of low temperature (usually χA or χ/M < χB,z, condensation in the solution cannot occur and the element will exist as a monomeric cation or anion. χB,z < χ/M < χOl,z limits a domain where cations may condense through olation only and form stable hydroxides (e.g., Al, Zn). If χOl,z < χ/M < χPA,z, condensation may occur through both olation and oxolation and lead to oxyhydroxides [Fe(III)] or to oxides [Ti(IV)] depending upon the relative rate of olation and oxolation. Elements for which χPA,z < χ/M < χA,z will condense through oxolation, leading to polyacid formation. According to this model, the calculated partial charge for the water molecules (δ(H2O)) is closely related to the tendency of the metal ions to form either an oxide precipitate or a mixed hydroxide/oxyhydroxide precipitate.58a Take the Zr(IV) as an example. For the aqua ion of zirconium, δ(H2O) is positive, such that the water

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molecule is repelled by the metal center and allows the oxo bridge formation. Hence, zirconium will hydrolyze and condense to generate zirconium oxide or oxyhydroxide depending on the solution conditions. The electronegativity-pH diagram58a for the aqua ion of [Zr4(OH)8(OH2)16]8+ suggests that Cl- is retained in the solid of zironia when it is precipitated at higher pH. The sulfate ion (SO42-) is easier to complex with zirconium than is Cl- and can form various complexes of zirconium over a wide range of pH values. Significant amounts of impurities such as Cl- or SO42- should be easily embedded in the ZrO2 when prepared under these conditions, which has been proven experimentally.147,186 At higher pH, the sulfate group, weakly coordinated to the metal ion, is easily replaced by other anions.147 In an aqueous solution containing zirconium sulfate and urea, a basic zirconium carbonate (Zr2(OH)6CO3‚2H2O) is possible to precipitate.81 F- also has a strong ability to form complexes in solution with many metal ions such as Si(IV), Ti(IV), and Zr(IV); therefore, precipitation of F into the corresponding oxides via the LPD method has been well-known.82 Conversely, the nitrate ion does not form complexes with zirconium except in concentrated nitric acid, which has been confirmed experimentally in our research. We further explain the effects of solution conditions on the deposition of TiO2 thin films in the following section. 2.4.5. Chemical Reactions for the Solution Deposition of Ceramic Oxides: An Example of TiO2. Different reactions have been employed in direct deposition of thin films in solution. Among them, synthesis of thin films by the controlled hydrolysis-condensation reaction is often involved. Take direct deposition of TiO2 thin films as an example. TiO2 thin films have been prepared by controlled hydrolysis of titanium species using an aqueous solution34 or in an organic solvent.35 Amorphous TiO2 thin films were successfully synthesized on the silanol regions of UV-irradiated octadecyltrichlorosilane (OTS) SAMs under nitrogen atmosphere by using a series of organic solutions of titanium compounds.35 The hydrolysis of TiCl4,83 TiF4,84 or (NH4)2TiF634,85 can yield either amorphous or crystallized TiO2 thin films depending on the synthetic conditions employed. For a moisture-sensitive precursor such as TiCl4, the hydrolysis rate was suppressed by lowering the pH. For a relatively low-reactivity precursor such as (NH4)2TiF6, boric acid or aluminum was added to accelerate the hydrolysis reaction to effectively synthesize highquality thin films. Other solution systems were also selected to control the hydrolysis rate. A complex peroxo precursor of titanium was employed for the deposition of TiO2 thin films by the electrochemical method,86 SAM-induced film formation,87 and the sol-gel-based method.88 Tada et al.88b fabricated a TiO2 thin film from a chlorine-free aqueous solution by a dip-coating method using a watersoluble peroxotitanate precursor, ammonium citratoperoxotitanate(IV), (NH4)8[Ti4(C6H4O7)4(O2)4]‚8H2O, which was prepared by dissolving citric acid into a transparent titanium peroxo aqueous solution. In this case, addition of H2O2 and H2O2 with citric acid makes it possible to prepare a transparent aqueous solution in air and postpone the occurrence of precipitation, which was further employed to deposit TiO2 thin films.

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We also researched the preparation of TiO2 thin films on Si, glass, or SAMs through an H2O2-added solution route.87a,89 We initiated the fabrication of a transparent Ti-containing solution by dissolving titanic acid (H2TiO3) into a solvent mixture composed of an appropriate amount of H2O2 and ammonia. The product of dissolution is a peroxotitanate solution, which is stable under high pH and low temperatures. Lowering the pH makes peroxotitanate unstable by a series of deprotoning processes. Deposition of a TiO2-based thin film was therefore achieved at room temperature. By use of a nonaqueous solvent, the hydrolysis rate of titanium organics was controlled and appropriate supersaturation was maintained, permitting deposition of thin films. The reactions in this case are closely dependent on the precursor chemistry of titanium(IV). Ti(IV)-organic precursors have been well studied because of the development of sol-gel, MOCVD, and other chemical routes to materials synthesis. The most probable precursors for the preparation of TiO2 are molecules having Ti-O bonds, namely, titanium alkoxides Ti(OR)n or oxoalkoxides TiO(OR)n (R ) saturated or unsaturated organic group, alkyl or aryl), b-diketonates Ti(b-dik)n (b-dik ) RCOCHCOR′), and titanium carboxylates M(O2CR)n. Among them, titanium alkoxides are commonly used due to their commercial availability and to the high lability of the Ti-OR bond allowing low-temperature hydrolysis and facile tailoring of reactivity by means of in situ modification with organic solvents. Titanium alkoxides Ti(OR)n react easily with the protons of a large variety of molecules. This allows easy chemical modification and thus tuning of properties by organic hydroxyl compounds such as alcohols, silanols R3SiOH, glycols OH(CH2)nOH, carboxylic and hydroxycarboxylic acids, hydroxyl surfactants, etc.90,91 The “modifying” ligand should have a pka lower than that of the alcohol eliminated in the process (corresponding to the alkoxide ligand). Complexation of Ti(IV) alkoxides by neutral ligands (L) is limited due to the poor stability of Ti(OR)nLx adducts. Lewis bases with hard O or N donor sites are required for coordination. One of the best ligands is the parent alcohol giving Ti(OR)n(ROH)x solvates. Such solvates are those of the isopropoxides of tetravalent metals (Zr, Hf, Sn, Ce), and their stability is assisted by hydrogen bonding.90,91 Ti(IV) alkoxides show good properties in terms of solubility in organic solvents, reactivity with organic ligands, and high hydrolysis ability, compared to other Ti(IV)organic precursors. The hydrolysis proceeds first by formation of unstable hydroxyalkoxides [Ti(OH)(OR)n-1] and then by polycondensation reactions via olation (preferential elimination of water) or oxolation (preferential elimination of alcohol), which have been discussed earlier. Ti(IV) along with most metal alkoxides is prone to hydrolyze, leading to uncontrolled precipitation. Their handling therefore requires specific care, usually under inert conditions (atmosphere and anhydrous solvents), or by modification to lower the hydrolysis rate. The hydrolysis rate also can be slowed by changing the nature of the organic group R: alkoxides with primary organic groups such as n-butoxides are less sensitive to hydrolysis than secondary ones such as isopropoxides; increasing the metal coordination number thus hindering attack of water and formation of the metal hydroxyl

Review

bond, M-OH necessary for the development of the network; decreasing the functionality of the precursor by partial substitution of the OR ligands with anionic ligands such as carboxylates or b-diketonates leading to M-Z bonds less susceptible to hydrolysis (and to M(OR)n-xZx. species); or condensation through a nonhydrolytic method.91 2.4.6. Characterization of Nucleation and Growth Process. To date, particle growth (mostly with respect to crystallization) has been a well-established research area, especially in situ investigation with AFM, dynamic microscopy, or dynamic light scattering (DLS), which can support space- and/or time-resolved information of the crystallization process. Dynamic processes at the solid-liquid interface are crucial in many areas of science and technology, but they are difficult to study. Electrochemically deposited copper films, for example, are used as interconnects in integrated circuits, but as miniaturization continues, a detailed understanding of nucleation, growth, and coalescence that control the growth of these thin films is essential in optimizing the final microstructure. However, the difficulty in studying solid-liquid interfaces leaves understanding of these processes less complete. Measurement of current transients provides an indirect means of monitoring the nucleation and growth of Cu film. Conventional scanning probe microscopy enables direct in situ observation, but it can only support a resolution of about 30 s92-95 and misses the crucial, subsecond processes at the beginning of nucleation and growth. X-ray diffraction96 or Rutherford backscattering,97 with limited lateral resolution, are confined to layer-by-layer growth on homogeneous substrates. Rapid imaging technique can support several frames of pictures per second, whereas the method is limited to specific single-crystal substrates and cannot take threedimensional images.98,99 Recently, a dynamic observationsrecorded in situ using a novel TEM techniquesof the nucleation and growth of nanoscale copper clusters during electrodeposition was reported.100 This method enables the researcher to follow in real time the evolution of individual clusters and to compare their development with simulations incorporating the basic physics of electrodeposition during the early stages of growth. In a newly developed technique, the researchers used a specially designed liquid cell to study the growth of Cu clusters in real time.100 Controlled deposition took place in the transmission electron microscope with images captured at a rate of 30 frames per second and a spatial resolution of about 5 nm. On polycrystalline Au, the researchers observed nucleation at random locations, indicating that many equivalent sites are available. After nucleation, cluster growth depended on its share of material flux, which is in turn, determined by the surface area. The initial growth period lasted about 2 s and then slowed because of the depletion of ions in solution. Conventional scanning electronic microscopy (SEM) techniques have been developed toward high magnification, high-resolution imaging, whereas the resolution and operating conditions are still beyond the requirement for detecting nucleation and early stages of growth. Conventional SEM usually requires a vacuum to minimize the interface with the electron beam, and

Review

careful preparation of samples is essential. This treatment involves thorough dehydration of any sample and increasing the conductivity of specimen, especially for those insulators. More modern environmental SEM (ESEM) allows samples to be examined in a lowpressure gaseous (usually water) environment (10 Torr in the specimen area), which means that hydrated samples (including those of biological origin) can be imaged in their native state without significant specimen preparation.101 It is also possible to observe insulators without coating with a conductive layer before imaging in ESEM because of its specific design. Therefore, in theory the ESEM can be employed for the observation of growth in some conditions. However, it is still difficult to measure fully hydrated samples under natural conditions, such as in situ observation of crystal growth. Recent developments in SEM focus on finding a barrier material that is strong enough to withstand the vacuum yet transparent enough to not impede the passage of electrons. Moses et al. reported the success of this aspect.102 A membrane that is transparent to electrons protects the fully hydrated sample from the vacuum. The result is a hybrid technique combining ease of use and ability to see into cells using optical microscopy with the higher resolution of electron microscopy. The resolution of low-contrast materials is 100 nm, whereas the resolution for high-contrast materials can reach 10 nm.102 Using such a wet-SEM, one can observe in situ crystal growth in solutions, which may be attractive when the crystal grows beyond a size limit, where using high-speed scanning AFM is difficult to produce high-resolution images due to the large noise. For film deposited on SAMs, Niesen et al. has observed using AFM that the film is prepared by deposition of crystallites with dimensions in the range of a few nanometers.103 However, it is still unclear whether crystals form by the classical theory or by the deposition of preformed precursor crystallites; compared to that introduced above, this method cannot record the nucleation process in the beginning stage, which must be monitored at a subsecond scale. High scan speed AFM has been developed, which can be operated in a solution at a scanning speed of 0.6 s for 1 frame.104 Although the speed is about 200 times higher than that of the conventional types, it is still not high enough for the observation of nucleation in solutions. For a solution deposition process, deposition is usually dominated by both the surface characteristics and the solution chemistry. Therefore, characterization of nucleation and growth in solutions is important to understand how solution conditions affect the film properties, such as morphology, growth mechanism, surface roughness, thickness, density, and so on. Dynamic light scattering (DLS), analytical ultracentrifugation (AUC), and quartz crystal microbalance (QCM) are effective techniques to follow in situ the particle formation/ growth process. DLS can be used to investigate the growth of particles with a hydrodynamic radius larger than 5 nm. The present technique using DLS enables the precise measurement of particle sizes ranging from several nanometers to a few micrometers. Some research105 has demonstrated that DLS really gives information on nucleation and growth occurring in the solution. AUC is an excellent tool to investigate particles

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with a radius < 1 nm.The quartz crystal microbalance (QCM) is a simple, cost-effective, high-resolution mass sensing technique, based upon the piezoelectric effect.106 As a methodology, QCM evolved for solution measurements largely in analytical chemistry and electrochemistry applications due to its sensitive surface-interface measurement capability. QCM with a measurement range of 8-10 MHz is extremely sensitive to mass change loaded onto a crystal surface and possesses a wide detection range, typically from 0.1 to 2 mg for a ∼9 MHz crystal.106b At the low mass end, it can detect monolayer surface coverage by small molecules or polymer films; therefore, it can be used in various fields for mass and/or thickness control, which includes CVD, sputtering, molecular adsorption, layer-by-layer deposition, especially for those cases in which careful control of the film growth process is needed. At the upper end, it is able to detect much larger masses bound to the surface.106b In the measurement of solution-based deposition, QCM can give information on the particle formation/adhesion onto a specific surface, which in turn can be attributed to specific surface-interface interactions.106a Compared to DLS, it can directly monitor the nucleation and growth on the substrate rather than in the solution. Therefore, it is a tool to investigate various deposition behaviors with respect to different surfaces, and it is convenient to be able to understand the effect of surface functional groups on the deposition process. 3. Experimental Procedures and Examples 3.1. Experimental. 3.1.1. SAM Preparation. In general, SAMs can be fabricated through chemisorption of organic molecules to the solid surfaces through either the liquid phase or the vapor phase.107-109 For example, organothiols can bond to metals such as Au and Ag, while organosilanes can bond to -OH terminated surfaces such as silica, glass, ceramic oxides, or silicon with a native oxide or a thermally grown oxide layer. SAMs can usually be prepared by the solutionphase107,108a-c,109 or the vapor-phase deposition method.108d-n,110 The hydrolyzed organosilane bonds to the native silica layer of p-Si substrate by the formation of a covalent siloxane (Si-O-Si) bond, and they condense with each other, creating a SAM with a functional terminal group.107,109a The cross linking and van der Waals interactions between chains stabilize the highly ordered, two-dimensional arrangement of SAMs.107,109a In a silanization reaction, the hydrolysis of silanes determines the overall reaction speed and depends on the reactive groups bonding to Si atoms in silanes. It decreases in the order Cl > OCH3 > OCH2CH3.108e In the vapor-phase deposition process, SAM molecules are vapored, hydrolyzed at 110 to 150 °C and transported to the reaction chamber usually under inert atmospheric conditions, where precleaned substrates are placed. The vapor phase deposition can be simply carried out using a drybox at elevated temperatures.108d-n,110 The deposition may continue over 1-3 h depending on the precursors and process conditions.108d Sugimura et al. used an ellisometer to measure the thickness of films after deposition for different times and observed that the thickness increased with deposition time until it reached a maximum, which is smaller compared to the corresponding molecular length. Thus,

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Figure 3. Illustration of SAM formation and subsequent film formation. (a) Octadecyltrichlorosilane hydrolyzed by the liberation of HCl. (b) SAM fabrication: the hydrolyzed organosilane bonded to the native silica layer of the p-Si substrate by the formation of covalent siloxane (Si-O-Si) bonds, creating a SAM with a terminal CH3 group. (c) Selective modification of SAMs by exposing them to UV light through a photomask; photocleavage at the Si-C bond formed volatile (or removable) products, and an extremely reactive Si radical left on the surface immediately reacted with trace water to form polar functional silanol (Si-OH) groups. This functionalized surface should be reactive to a variety of chemical coupling reactions, such as chemical adsorption of ceramic precursors from a solution. (d) The difference in physical and chemical properties of a patterned SAM shows different behavior when it was immersed in a deposition solution. (e) Selective nucleation and growth in specific functional groups. (f) A dense film was selectively formed in the silanol regions.

the formed film was considered to be a monomolecular layer.108d The thickness change with time may also suggest that the deposited molecules at the interface in the beginning stage declined more than those after a monolayer formed, implying a self-assembled process based on the interaction of neighboring molecules. In the liquid-phase deposition process, substrates are soaked in a solution containing organosilane or organothiol for a period of time, typically from several minutes to tens of hours.107,108a-c,109 The solvents used are usually water, ethanol, toluene, methanol, and so on. The temperature ranges from room temperature to near 100 °C. The liquid-phase deposition method is usually carried out at relatively low temperature, and the soaking time is generally short, whereas the aggregates of organosilane molecules may be deposited in some cases, which affects the surface roughness. The vaporphase deposition is comparably conducted at high temperatures, and the deposition time is relatively long, but deposition of aggregates of organosilane molecules may be avoided. Moreover, the method is limited to materials that can be vaporized and is therefore not widely used for SAM preparation compared to liquid-phase deposition. A schematic description of the SAM formation process by a liquid-phase deposition method, modification, and fabrication of ceramic patterns is shown in Figure 3. Typically, SAM surfaces used for the preparation of SrTiO3 were prepared as follows. P-type Si (100) wafers (Shin-Etsu; resistivity: 5-10 Ω cm) employed for the substrates were cleaned ultrasonically in acetone, ethanol, and deionized water (>17.6 M Ω cm), followed by immersion in boiling water for 5 min. After being dried at 50 °C in air, the substrate was exposed to UV light for 2 h. SAM materials were organosilanes such as heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane

(HFDTS, Aldrich Chem. Co., USA), octadecyl-trichlorosilane (OTS, Acros, New Jersey, USA), and phenyltrichlorosilane (PTCS, Aldrich). SAMs were prepared by immersing the cleaned and dried substrates into an anhydrous toluene (99.8%, water < 0.002%, Aldrich) solution containing 1% vol SAM materials for 5 min under an N2 atmosphere. The hydrolyzed organosilane bonded to the native silica layer of p-Si substrate by the formation of a covalent siloxane (Si-O-Si) bond, creating a SAM with a terminal CH3 group.109 After being dried under N2 atmosphere, the substrates with SAMs were then baked at 120 °C for 5 min to remove residual solvent and to promote chemisorption of the SAMs. A detailed description of the SAM fabrication process appears in previous reports.34,35,37,39 Selective modification of SAMs was conducted by exposing the sample to UV light through a photomask; photocleavage at the Si-C bond formed volatile (or removable) byproducts, and an extremely reactive Si radical left on the surface immediately reacted with trace water to form polar functional silanol (Si-OH) groups.109 The difference in physical and chemical properties of a patterned SAM suggests different nucleation/growth behaviors when the substrate was immersed in a deposition solution. 3.1.2. SAM Characterization. SAMs are extremely thin layers with a typical thickness of several nanometers depending on the molecular structure. Characterization of the SAM surface usually involves a series of surface analysis techniques, structural observations, and chemical/physical property measurements. Scanning electron microscopy (SEM) can be used for surface observations; patterned SAMs with different functional regions show image contrasts due to the difference in the lowest unoccupied molecular orbital (LUMO).110 However, because of resolution limits, SEM can usually

Review

be used at low magnification. For further detailed structure determination, AFM or scanning tunnel microscopy (STM) is needed.107 TEM can also be used to confirm the success in preparation of SAMs and determination of the SAM thickness.111 These observations, however, require special pretreatment of the specimen because SAM is weak with respect to thermal and electron beam irradiation; the thermal stability of various SAMs in air has been reported to be 160-330 °C, which should increase a little in a vacuum or inert atmosphere.112 An ellipsometer, however, can operate at ambient atmosphere for measuring the thickness along with the refractive index of SAMs.110 Powerful synchrotron radiation techniques such as near edge X-ray adsorption fine structure (NEXAFS), X-ray adsorption near edge structure (XANES), and extended X-ray adsorption fine structure (EXAFS) also may be used for determining surface structures. NEXAFS and XANES are effective for investigating the energy regions between the absorption edges and the regions where EXAFS oscillations begin. NEXAFS has particular applications for chemisorbed molecules on surfaces. Information concerning the orientation of the molecule can be inferred from the polarization dependence. NEXAFS is sensitive to bond angles, whereas EXAFS is sensitive to interatomic distances. NEXAFS spectra are frequently dominated by intramolecular resonance or sigma symmetry. The energy, intensity, and polarization dependence of these resonances can be used to determine the orientation and intramolecular bond lengths of the molecule on the surface. XANES can provide information about vacant orbital, electronic configuration, and site symmetry of the absorbing atom. Some common equipment in a chemical laboratory can be employed for the understanding of surface physics and chemistry of a SAM, including attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT-IR) and grazing-angle spectroscopy,113 secondary ion mass spectroscopy (SIMS),114 ultraviolet (UV) spectroscopy,115 X-ray photoelectron/Auger electron spectroscopy (XPS/AES),116 fluorescence probes,117 and scanning probe microscopy such as lateral force microscopy (LFM)118 and Kelvin force microscopy (KFM).119 Among these techniques, FTIR and XPS are commonly used for the analysis of surface composition and structures with the lowest limit of analysis not below 0.01 monolayer (ML, the maximum surface concentration of packed alkyl chains, 4.2 × 1014 cm-2), where concentrations are on the order of 1014 cm-2. The fluorescence probe technique enables the detection of surface chemical groups in the range of 1011 to 1013 molecules/cm2 (10-410-2 ML) by specific covalent attachment of fluorescent chromophores to surface functionalities, and it does not require an ultra-high-vacuum environment commonly needed for XPS or SIMS. LFM and KFM provide surface friction and surface potential distribution in micro/ nanoregions, respectively. These analyses are much more useful to investigate the chemical change of the surface after heating, UV irradiation, and plasma processing, all of which are usually related to the surface modification. Additionally, a high-resolution mass sensing technique with nanogram sensitivity, quartz crystal microbalance (QCM),120 is also involved in the characterization of the kinetics and dynamics of the formation

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of SAMs. In many of these applications, QCM is not a mass sensor but provides valuable information about reactions and conditions at the liquid-solid or gas-solid interfaces. Measurements of zeta potential121 and (water) contact angles122 are simple and widely used methods for the confirmation of the SAM formation and characterization of their surface properties. Other analysis techniques for characterization of SAMs are discussed in ref 107b. 3.1.3. Patterning of SAMs. There are several methods to pattern a SAM surface in the laboratory; they can be divided into two categories: site-selective modification34,35,37,39,54,109,110 and direct patterning.45,123 In site-selective modification processes, a SAM layer is prepared on the whole surface of a substrate first by chemisorption of molecules in either the gaseous phase or liquid solutions. The patterning of the SAM layer can be accomplished in situ without disrupting the monolayer by a number of techniques using additional energy sources. These techniques almost share the same mechanism; that is, organic molecules of SAM on the substrate surface undergo changes (energy conversion and/ or chemical reactions, often irreversible) in a siteconfined manner. In practice, these changes can be induced by mechanical, physical, optical, electrical, chemical, or magnetic means or a combination of them. Deep UV irradiation employs different lamps with wavelengths ranging from 354 to 157 nm. Mercury (Hg), excimer, krypton fluoride, argon fluoride, and F2 lamps irradiate UV light of 354, 170, 193, 248, and 157 nm in center light wavelength, respectively. Selective photocleavage and/or photoinduced ozone can decompose the exposed areas, creating new functional terminals different from the original ones.34,35,37,39,54,109,110 Recently, a variation, photocatalyst-assisted photolithography, was also reported to be effective for the photopatterning of SAMs using a TiO2 layer to improve the photocleavage efficiency.54g Another variation of a site-selective photooxidation process based on near-field scanning optical microscopy (NSOM) uses a transparent, aluminum-coated probe tip at the end of an optical fiber that functions as a waveguide to direct light from a laser source to the SAM layer on a substrate surface.109b The resolution is improved because the tip is sufficiently close to the substrate, lowering the diffraction effects in the far-field. A similar modification can be also achieved using X-ray,51 electron beam,51a,52 ion bombardment53 as an additional energy source or by mechanical displacement of unneeded areas using a probe tip equipped to either an STM or an atomic force microscope (AFM).109c-h In a mechanical writing technique, the pen of the probe tips moves in a spatially controlled way across the substrate surface and engraves a pattern. Mechanically removing the SAM materials from the writing areas generates the pattern. No chemical reaction occurs. Focused beams of energetic particles comprised of electrons, ions, and other highenergy particles can induce physical or chemical changes in the irradiated areas of the SAM surface, specifically altering the properties of the SAM.109i-l With the application of an additional electromagnetic field in scanning tips, physicochemical transformation will be induced in the scanned areas of SAM.109m

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Unlike the modification processes mentioned above, in which no ink is involved, microcontact printing (µCP), nanotransfer printering (nTP), and dip pen nanolithography (DPN) represent methods that combine highresolution “stamps” or “pens” with SAM “inks” for patterning through direct writing of SAMs in the requisite areas.45,123,124 In a µCP process, an elastomeric, patterned stamp is first inked by immersing the patterned face in a ink solution, and then it is dried and makes contact with the surface of a substrate. Ink, which may be SAMs, catalysis, colloid particles, proteins, and other chemical species, attaches to specific sites of a substrate, really replicating the pattern of the stamp. The µCP process was first introduced to patterning of alkanethiols on Au. Alkanethiol reacts with Au via a S-Au chemical bond and further self-assembles into a dense, highly ordered monolayer by strong van der Waals’ interactions between the long alkyl chains. It can be simply extended to the selfassembly of alkanethiols on other metals along with alkylsiloxanes on hydrolxyl-terminated substrates or alkylphosphonic acids on aluminum. The technique of nTP can print metals such as Au, Ag, Ti, which are evaporated to a stamp as inks, on a patterned alkylthiolSAM via the similar chemical interaction.123 Compared to µCP, nTP is not limited by surface diffusion or edge disorder in patterned SAM “inks”, nor does it require post-printing etching or deposition steps to produce structures of functional materials. The use of SAMs as covalent “glues” and “release” layers allows transfer of material from relief features on a stamp to a substrate. It is a purely additive technique that can generate complex patterns of single or multiple layers of functional materials with nanometer resolution over large areas in a single step. Although the nTP is primarily aimed at patterning a metal film, the areas not covered by metal are left to be able to prepare a pattern of SAMs from the liquid phase. DPN can be used to print liquid-based inks on a defined site of a surface.124 The pen employed is an AFM tip, which scans the surface along a designed route and downloads ink of SAM compound onto the surface through a water meniscus formed between the tip and the substrate. The capillary force drives the transportation of materials onto confined areas of a target surface. These methods enable the patterning of a SAM surface with sizes ranging from micrometer to deep nanometer scale. Among the introduced methods for patterning of SAMs, almost all modification-related techniques and DPN methods show difficulties in patterning on unplanar surfaces. This problem has been partially resolved by using µCP or nTP. In addition, after SAM patterning the surface functionality of SAMs can be in situ transformed to other groups with various physical and/or chemical properties,60a,121c,125 which may enable optimization of film deposition processes by the design of surface reactivity and allow an understanding of the interrelationships between film properties and surface functionality as well as structure configuration. In a recent paper,60a Shyue et al. reported a wide variety of surfaces could be obtained by starting with a single surfactant, which was used to prepare alkyl bromide SAMs on Si. Various surfaces were created with terminal groups, including

Review

thioacetate, sulfonate, thiol, nitrile, dodecanoic acid, amines, which show different acid-base properties and zeta potentials of the surface. Recently Li et al. reviewed methods for the transformation of functional groups of SAMs on flat gold surfaces.121c SAMs with various types of functional groups (headgroups) were bond to a Au surface and transformed to requisite groups by in situ chemical reactions. Transformation was started with functional groups such as amino, carboxylic acid, anhydride, and hydroxyl. Some SAMs that were electroactive or photochemically active were easily modified through corresponding electrochemical or photochemical reactions. 3.2. Examples of the Preparation of Single Ceramic Oxides on SAMs. 3.2.1. General Introduction. Various kinds of thin films of ceramic oxides have been deposited on SAMs with different functional groups by the bioinspired process at temperatures lower than 100 °C. The representatives are TiO2,34-36,38,41a,42a,44,89,126-140 ZrO2,37,141-147 SnO2,112,148-153,185 Y2O3,154 ZnO,155-163 La2O3,164 Ta2O3,165 iron oxides166-171 including FeOOH,56,48f,168-170 Fe2O3,171 and Fe3O4,166,167 along with complex oxides such as SrTiO3,39 CaCO3,57,172,189 apatite,40,106a,106c,173a-c and others.173d-g In general, synthesis of monometallic oxides is usually a result of inorganic polycondensation involving the hydrolysis of metal ions in solution and condensation of hydroxylated complexes. The reaction behavior for different elements is significantly different because of the specificity in element chemistry. Preparation of polymetallic oxides, however, needs control of the formation of the polymetallic complex, which is crucial for the control of the stoichiometry. The growth of these films was mostly induced using a sulfonate-SAM with a terminal SO3H group, an amino- or phenyl-group-functioned surface, thiol-anchored SAMs on Au, or others, especially silanol groups derived from UV-modification of SAMs. Rieke et al. prepared goethite films from iron(III) nitrate solutions and akaganeite films from iron(III) chloride solutions onto sulfonate (-SO3H)-terminated SAMs and sulfonated polystyrene, reporting a polymorph on counteranion.56,48f,181 Nagtegaal et al. synthesized iron oxyhydroxide films on thiol-anchored Au and on -SO3H-SAMs using similar solution systems but under conditions (deposition temperature, pH, concentration) different from those Rieke et al. employed.168,169 Growth was found to be improved on sulfonated-SAMs, on thiolcovered gold, but not on surfaces functionalized by alcohol, methyl, carboxylate, phosphonate, and amine hydrochloride.168,169 DeGuire and Niesen et al. fabricated a series of ceramic oxides on -SO3H-terminated SAMs from aqueous solutions with or without thermal treatment. Typically, the film is in an amorphous or crystalline state with thickness < 100 nm. The film is usually deposited by hydrolysis of metal salts in acidic conditions. In their processes, ZrO2 film was formed on -SO3H-terminated SAMs by enhanced hydrolysis of zirconium sulfate Zr(SO4)2‚4H2O solutions in the presence of HCl at 70 °C.141-147 The film was a mixture of two phases: nanocrystalline tetragonal and amorphous basic zirconium sulfate measured by TEM.147 By controlling the reaction conditions, a random-oriented crystal ZrO2 film with no

Review

amorphous phase could be obtained.147 AFM results suggest that the film was formed by aggregation of nanocrystals, typically a few nanometers in diameter.103 However, pinholes of about tens of nanometers to 100 nm in diameter could still be detected, which may be attributed to the imperfections in SAMs or contaminants adsorbed before deposition weakened the function of SAMs during deposition from solutions.103 Similarly, nanocrystal TiO2 film was also deposited on the -SO3H-SAMs from a 0.5 M acidic TiCl4 aqueous solution at 80 °C.126-128 The process can be used to prepare a polycrystalline film consisting of nanocrystalline anatase with a size in the range of 2-4 nm. These crystals were embedded into an amorphous matrix detected by TEM.126 The surface exhibited nanoscale characteristics confirmed by AFM, but no high-magnification images were available.26 These films may be formed by aggregation of preexisted nanoparticles formed in solutions onto the SAM-modified surfaces.126-128 DLVO calculation supported this proposal that the film formation was driven by the interaction between the SAM and the particles in the solutions as well as that between the growing film and particles.42a Deposition of thin films from solutions requires control of the reactive process so that samples can nucleate and grow on surfaces rather than in solutions. For a hydrolysis reaction, low solution temperatures or low concentration of metal ions can decrease the degree of supersaturation, suppressing the nucleation in solutions. In addition, lowering the pH is effective in decreasing the reaction rate of a proton-generated hydrolysis process, as discussed above. The chemistry of precursors also influences the hydrolysis process. Take Ti species as an example. Soluble Ti(IV) inorganic salts, such as TiCl4, TiOSO4, TiF4, M2TiF6 (M ) K, Na, Li, and NH4, etc.), etc. are usually involved in the preparation of TiO2 thin film.31,34,126-128 The reactivity of TiCl4 in water is much higher than that of TiF4 or TiF62-; gases are generated when it is in contact with the wet atmosphere. Rapid and uncontrollable reactions occur when it is directly dissolved in aqueous solution, resulting in the formation of precipitate. For tuning the reactivity and improving the film properties, high acidity,126-128 complexation of Ti(IV) with other ligands,31,34 or formation of peroxotitanium complex86-89,130 are usually feasible means. TiF4 and TiF62- are solid and more stable to moisture than is TiCl4. Their hydrolysis reactions proceed very slowly. To facilitate the reaction, Al metal or boric acid is usually added. Both of them can consume free fluorine ions by the formation of more stable species.31,34 The pH of the solution can be in a wide range up to about 4.34 Koumoto et al. combined the liquid-phase deposition process with a SAM template technique and successfully fabricated a micropattern of anatase TiO2.34 Anatase TiO2 was nucleated and grown on an entire surface of UV-patterned silanol/phenyl template first. Then a micropattern was obtained after sonication cleaning of the substrate in deionized water.34a In this process, deposits on the phenyl and silanol surfaces show different adhesion to the substrate, which may be originated from different interface interactions operating during deposition. Complexation of Ti(IV) to form a peroxotitanate complex in aqueous solution is another

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way to slow the hydrolysis rate.87 Niesen et al. deposited a TiO2 thin film using the TiCl4-H2O2 system; amorphous TiO2 had formed on sulfonated surfaces, while no continuous film was formed on amino- or OHterminated surfaces.87b Note the pH of the solution is low after the complexation reaction because of the generation of HCl as a byproduct. We employed an alkali solution (pH > 9) containing H2O2 to form a peroxotitanate complex, which allows the use of various titanium compounds such as soluble titanium salts, either inorganic or organic, and insoluble Ti(IV) species including titanic acid (H2TiO3 or Ti(OH)4), amorphous TiO2, and TiO2-based sol or gel.87a Some of them are in the solid state and easy to handle and store. The asprepared solution is stable at low temperatures and high pH. Increasing the degree of supersaturation of the solution can be achieved by lowering the pH or by elevating the temperature. Films of amorphous TiO2 can be obtained under either basic or acid conditions at low temperatures.87 Baskaran et al. reported growing TiO2 on -SO3H-functionalized SAMs using a commercially available titanium lactate, (NH4)2(OH)2Ti(C3H4O3)2, in which lactic acid is a chelating agent used to stabilize the titanium ion in solution for controlling the degree of supersaturation.127b A crystalline film composed of anatase as a major phase was obtained at 70 °C. For a glass substrate that is 90 nm thick, the UV transmittance below ∼300 nm in wavelength decreased