Conventional Assay

(e.g., bulk thermal conductivity)

e.g. an entire ternary composition diagram (Fig. 34.2). A primary screen specifically developed to evaluate the very large number of compositions within that library identifies a particular composition or range of compositions that is of further interest. Primary screens are typically designed to eliminate a large fraction of the compositions studied in the first library, while secondary and follow-up libraries examine a more narrow range of compositions as well as additional chemical substitutions and processing conditions. Optical and electronic properties such as capacitance or luminescence are examples of physical properties that may be efficiently examined in high-throughput primary screens.

At the follow-up level, more detailed information can be obtained from high-throughput secondary screens because the number of compounds that must be screened has been greatly reduced. This process of synthesis, screening, and optimization continues until a manageable number of compositions has been reached.

These selected compositions, all of which have passed previous screens, can now be studied using conventional methods to obtain the more precise chemical and physical data necessary to characterize a material completely. Combinatorial methods can thus act as an efficient filter for conventional methods by selecting only the best candidates for further, more detailed study.

Materials Library Synthesis

Several synthesis techniques have been developed for combinatorial materials library formation. Some materials can be made using solution deposition methods, while others are more suited for thin-film deposition [33-38]. By modifying technologies similar to those used to make integrated circuit (IC) chips, materials libraries or integrated materials (IM) chips were first developed and utilized by Schultz and coworkers [39]. The choice of synthesis technique is based on both the material being prepared as well as the primary screen employed after synthesis. Unlike the case of drug discovery, however, the synthesis of solid-state materials often relies on processing temperatures in excess of 400 °C. High-temperature reaction conditions have been addressed through the creation of two-dimensional, spatially addressable arrays of samples deposited on thermally stable substrates.

Vapor Deposition Techniques

Vapor deposition is commonly used in the semiconductor industry to deposit thin films of material onto a substrate. Vapor deposition techniques that have been utilized in combinatorial library synthesis include sputtering [21, 22, 40, 41] and thermal evaporation [23, 42-46], electron-beam evaporation [29], pulsed laser ablation [21, 47, 48], ion-beam implantation [49-52], molecular-beam epitaxy [24, 53-56], and chemical vapor deposition [57].

One of the most straightforward thin-film approaches is the continuous composition spread (CCS) technique, which utilizes two or three off-axis sources to co-deposit material on a substrate [41]. This technique relies on the nonuniform deposition of materials formed by the geometric arrangement between the sources and the substrate. The relative concentration of each component at a specific location on the substrate decreases with the distance from the source. As materials spread from the sources, they mix in the vapor and are deposited on the substrate creating atomic-level mixing that reduces or eliminates the need for high-temperature postprocessing of the library. This technique is also amenable to the isolation of meta-stable or low-temperature phases that are crystalline on deposition. The lack of precise stoichiometric control and limited compositional range have relegated this technique primarily to optimization and exploration of systems with only two independent variables.

A typical vacuum deposition system for combinatorial materials science has several source materials and is used in conjunction with masking techniques (physical or shadow masks, movable shutters, or photolithography) to deposit different

Fig. 34.3. Binary and quaternary masking strategies for combinatorial material library synthesis. See text for details.

materials, sequentially or simultaneously, in particular areas of the substrate. The design of the masks and the sequence in which they are employed determine which materials are deposited at any given location on the substrate. By altering the sequence, time, and rate of deposition, it is possible to control the exact chemical composition of each element in the library.

The efficiency with which a particular compositional landscape may be examined is dictated by the masking strategy employed [58]. Simple binary and gradient masks are useful for optimizing the composition of a known material. In the binary masking strategy, half of the substrate is exposed to different masking patterns (1, 2, 4,... strips oriented in two different directions) in each step (Fig. 34.3). After N steps, the number of different compositions is 2 N, including all possible combinations of N elements. Gradient or x/ y shutter masking utilizes movable masks that either expose or block certain areas of the substrate, allowing for controlled concentration and/or thickness variations in the deposited films.

Quaternary masking strategies have been developed to enable efficient generation of diverse libraries containing materials with very different compositions [30]. In the quaternary masking scheme, deposition is carried out using a series of N different masks that successively subdivide the substrate into a series of nested quadrant patterns (Fig. 34.3 and Fig. 34.4). Each mask is used for up to four depositions, but after each deposition the mask is rotated by 90°. With N different masks, this process will generate up to 4N different compositions in just 4N deposition steps. The rth (1 < r < N) mask contains 4r—1 windows, where each window exposes one-quarter of the area deposited using the preceding (r — 1)th mask. Within each window is an array of 4N—r openings, which can be provided by means of an underlying contact mask or created directly on the substrate by photolithographic techniques [33, 34, 59-61]. Each section of the substrate is thus exposed to a different combination of precursors by depositing each layer through a different mask.

Fig. 34.4. The quaternary masking system uses six fractal masks, each of which can be rotated 90° to allow four choices of materials at each level.

1024 total possibilities

Fig. 34.4. The quaternary masking system uses six fractal masks, each of which can be rotated 90° to allow four choices of materials at each level.

Thin-film deposition methods are synthetically quite versatile; they have progressed to enable atomic- and molecular-layer epitaxy and offer the ability to construct artificial lattices, epitaxial overlayers, and patterned films of a variety of materials. Dopants are usually sandwiched between layers of the host material to avoid evaporation and to assure proper interdiffusion. Subsequent thermal processing results in a library of materials or devices the physical properties of which can be screened with either contact or noncontact screening probes. The number of compounds that can be simultaneously synthesized by this technique is limited by the spatial resolution of the masks and by the degree to which synthesis can be carried out on a microscale.

One can use either physical shadow masks [39] or photolithographic lift-offs [61] to carry out masking. Photolithography, a standard process in the semiconductor industry, has a high level of spatial resolution and accuracy and can generate chips with a high density of diverse compositions (up to 106 per square inch). The multilayer thin-film deposition may result in the synthesis of multiple binary phases rather than the desired multielement single-phase material, a result that may be avoided through the use of an effective two-step annealing process [33]. Early nu-cleation and thus binary-phase formation is avoided by deposition at relatively low temperatures. If the thickness of the deposited layer is less than a critical value (material dependent) of typically 1-10 nm, diffusion is dominant over nucleation for sequential precursor layers. Johnson and coworkers have advanced this elegant technique based on the sequential controlled deposition of thin films [62]. Using Johnson's method, an extended period oflow-temperature (100-400 °C) annealing is performed for proper interdiffusion of thin-film precursors. Subsequently, crystallization of the intermediate, amorphous material is induced by a high-temperature annealing process allowing growth of entire integrated materials chips [63]. Growing entire integrated materials chips in this fashion is crucial for many materials, where the material properties are closely tied to the crystalline quality of the films.

Reflection high-energy electron diffraction (RHEED) has been proposed for in situ monitoring of molecular beam epitaxial (MBE) deposition [35].

Pessaud and coworkers utilized pulsed laser ablation of two compound targets made of superconducting YBa2Cu3O7 (YBCO) and the double-chain insulator MCuO2 (M = Ca, Sr) in an attempt to create new metastable superconducting compounds [47]. The goal of this exploration was to deposit m consecutive layers of YBCO with n layers of MCuO2 in an attempt to use the two-dimensional structure of YBCO as a structural template so that MCuO2 would form additional conducting CuO2 planes upon deposition. The technique has led to the identification of a number of new phases with enhanced critical temperatures and extends the multilayer deposition concept from elemental targets to compound targets.

Combinatorial thin-film methods are also being used to optimize the performance of multiwavelength emitting chips that show potential applications in wavelength-divisible multiplexing (WDM). Multiwavelength emission has been achieved using selective area epitaxy and postgrowth selective region intermixing. Layer intermixing techniques have achieved wavelength shifting through implantation-induced intermixing, which is difficult to control.

Liu and coworkers used arsenic and proton implantation to create a multi-wavelength-emitting library of doses and ''species-dependent'' intermixing, which they analyzed by microphotoluminescence at room temperature [50]. Al0 35Ga0 65As/GaAs single quantum wells were grown on GaAs (001) substrates by MBE. Ion implantation was accomplished using four different pattern masks on an 8 x 8 hollow circle-patterned blank mask in sequence. Four rows of elements implanted with different As doses at an ion energy of 90 keV were obtained. The four masks were then rotated 90° for proton implantation at an ion energy of 40 keV. The fifth column and the fifth row were implanted with protons only and As only. The implanted chip was annealed in a rapid thermal annealer (RTA) at 950 °C, and the microphotoluminescence measured at room temperature. Using this technique Liu and coworkers were able to generate more than 20 different wavelengths from a single chip.

Amorphous materials have been investigated in a combinatorial optimization of hydrogenated amorphous silicon (a-Si:H)-based thin-film transistor (TFT) devices by Koinuma and coworkers using plasma-enhanced chemical vapor deposition [57b]. A contact mask placed over the indium-tin oxide substrate and sequential depositions were carried out through a moving slit mask. A 90° substrate rotation followed by deposition of hydrogenated amorphous silicon nitride (a-SiN:H) allows for systematic investigation of the effects of thickness and compositional variation across the library. Source and drain contacts were deposited on the top of the bi-

layers by aluminum evaporation to allow the current-voltage characteristics of the devices to be characterized.

Alternative Library Synthesis Techniques

Synthesis for many materials, e.g. zeolites, polymers, polycrystalline phosphors, and many other oxides and sulfides is best accomplished using solution-phase methods. When applicable, solution-phase methods provide a large number of libraries at a significantly lower capital investment relative to automated vapor deposition equipment. Solution-phase methods may result in improved compositional control. One significant complication associated with materials discovery in thin-film format is the occasional lack of correlation with bulk properties owing to differences caused by film microstructure, strain, and so on. Physical and chemical properties of compounds prepared using solution techniques generally show excellent correlation with bulk properties regardless of the quantity of material prepared. Thus, library-based data may be easily confirmed in bulk.

Solution techniques allow mixing at the molecular level, reducing the need for high-temperature interdiffusion and also facilitating the isolation of metastable phases. Xiang, Schultz, and coworkers have demonstrated that a scanning multi-head inkjet delivery system can be used to perform automated microsynthesis of solid-state libraries, enabling rapid delivery and accurate control of nanoliter deposition volumes [34, 64]. Droplets are delivered sequentially to single reaction wells; the droplet size is on the order of 500 pL with reproducibility better than 1% and a maximum delivery speed of 2000 droplets s_1. The system has been successfully used to generate libraries of 100 members per inch2 (100 members per 6.49 cm2), and a system to generate 1000 members per inch2 (1000 members per 6.49 cm2) is under development.

Sol-gel processing of glasses and ceramics provides an effective method for fabricating inorganic nanostructured materials that is amenable to combinatorial methods. Molecular precursors, such as metal alkoxides, are typically used as starting materials [65, 66]. The process begins with the formation of arrays of homogeneous sols with desired compositions through the use of automated liquid-dispensing robots. The sol is then converted to a gel, while maintaining its homogeneity and purity. A solid network is obtained through hydrolysis and condensation reactions. The technique has been applied in the investigation of phosphors [65] and oxide semiconductors [66], and in the synthesis of polymer/ vanadium oxide nanocomposites [67].

Multicomponent zeolite synthesis is another area of advanced solid-state materials research in which combinatorial discovery and optimization can be successfully applied. A multiautoclave system capable of performing at least 100 crystallizations under hydrothermal conditions has been reported by Akporiaye and colleagues [68]. In its simplest form, the reactor consists of a Teflon block with cylindrical holes, designed to accommodate Teflon-coated septa, which is sandwiched between two steel plates. The capability to stack identical synthesis blocks allows for the parallel synthesis of in the order of 1000 combinations in one experiment. The system was tested by reproducing the phase diagram of the intensively studied ternary phase system Na2O/Al2O3/SiO2/H2O in a single experiment and under identical reaction conditions. With a total volume of each sample gel of not more than 0.5 mL, crystalline phases of zeolite A, faujasite, and gmelinite were obtained, with the region of zeolite A coinciding almost perfectly with earlier results for bulk materials. Good agreement was also obtained in the case of faujasite. The formation of sodalite was only observed in a more recent investigation. This partial agreement was attributed to better control of the water content in the parallel autoclave.

The attractiveness of combinatorial methods for hydrothermal synthesis was also realized by Klein et al., who claim that the advantage of their reactor over the one described above is their much smaller reaction volume, in the order of 2 mL, as well as the direct preparation of a library of materials the components of which can be identified automatically on the library substrate by X-ray microdiffraction (Fig. 34.5) [69]. The central feature of Klein's reactor is a silicon wafer that, after hydrothermal synthesis and calcination, contains the sintered reaction products and represents the library. The identification of the individual products may be directly performed with a GADDS (General Area Detector Diffraction System) micro-diffractometer. Choi and coworkers designed a centrifuge apparatus that allows quantitative product recovery onto filter paper for X-ray microdiffraction without manipulation of the individual samples [70]. Furthermore, Lai and coworkers demonstrated parallel synthesis of zeolite films in a 21-well reactor using a vertical substrate configuration to provide uniform wetting that favored heterogeneous film growth in an organic-free clear solution [71]. The results of the synthesis of

40 mm

Fig. 34.5. Schematic cross-section of the multireactor autoclave.

40 mm

Fig. 34.5. Schematic cross-section of the multireactor autoclave.

template molecules and metal components, followed by a qualitative indication showing that the materials formed are ''amorphous or crystalline,'' confirm that zeolite synthesis on a microgram scale is possible and justified.

A solution-based combinatorial strategy has been described by Baker and co-workers for synthesis of surfaces exhibiting nanometer-scale variation in mixed-metal compositions and architecture [72]. Continuous or stepped gradients in the size and number density of surface features can be generated simultaneously over different regions of a single substrate. Baker et al. and Kneipp et al. prepared libraries of Ag-clad colloidal Au arrays in which the coverage of colloidal Au and the extent of the reductively deposited Ag coating were varied in an attempt to optimize the enhancement factor for surface-enhanced Raman scattering (SERS) [72, 73]. A sulfide-functionalized glass slide was immersed at a constant rate into an aqueous solution of colloidal gold, and the plate was then rotated by 90° and lowered at a fixed rate into a solution of Ag+. As a result, a library was generated containing approximately 108 colloids that differed in their particle size and Ag coverage. Para-nitrosodimethylaniline was adsorbed onto this surface and the surface-enhancement factor of the Raman scattering of this compound was measured. The values spanned approximately three orders of magnitude across the library and allowed for identification of a region of the library that gave the greatest enhancement. Atomic force microscopy was then used to determine the nanometer scale morphologies of these regions of interest. The availability of dispersible metal-containing nanoparticles and the numerous routes to metal deposition from complexed metal cations should allow for extension of this method to a variety of other metals.

A scheme for generating complex, spatially separated patterns of multiple types of semiconducting and/or metallic nanocrystals has been presented by Vossmeyer and colleagues [38]. Nanocrystals may play an important role in future technologies such as photovoltaics, switches, phosphors, light-emitting diodes, electronic data storage systems, and sensors. Most of the photonics and electronics applications will eventually require parallel schemes for the control of spatial positioning of the nanocrystals. Standard patterning techniques such as laser ablation of the material and deposition through a shadow mask do not work well for nanocrystals since most metal and semiconductor nanocrystals have covalently bound organic surfactants that tend to desorb at temperatures above 100 °C. Consequently, any patterning approach must be carried out at low temperatures. The stepwise preparation of multiple particle arrays is based on lithographic patterning of amino-functionalized organic monolayers that contain a photolabile protecting group covalently bound to SiO2 surfaces. The photosensitive substrate is irradiated through a mask in the near UV, removing the protecting group in the areas exposed to the radiation. To prepare binary micropatterns consisting of Pt or Au and highly luminescent CdSe/ CdS core/shell nanocrystals, the substrate is treated with a solution of amine-stabilized metal-containing nanoparticles, which assemble in the area of depro-tected amino groups. Changing the orientation of the mask, the deprotection step is repeated and the amine-stabilized CdSe/CdS nanoparticles are assembled onto areas of freshly deprotected amino groups, yielding a binary nanoparticle array.

region A

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