Prog. Reactions   
 

Traditional methods for making micro and nanostructuring of materials require either several rounds of photolithography and precise registration or the use of serial techniques, such as localized electrodeposition methods, reactive-ion etching, proton and ion beam machining, laser ablation, powder blasting, or electrochemical micromachining These procedures are usually laborious, often expensive, and are especially difficult when dealing with hard/brittle materials and features of submicrometer dimensions.

Conceptually, all these methods share the common assumption that in order to make smaller and more precise architectures, one must first increase the resolution of the "parent" fabrication tool ??be it a stamp, or a writing beam, or a milling micro-chisel. Although this "what you print/ablate/drill/mill is what you get" philosophy appears very logical and quite intuitive, it is interesting to note that Nature ??the ultimate builder ??uses it only rarely to make surface patterns and structures. Indeed, skin patterns in fish, zebras and tigers, compositional zones in sea-shells and agates, or the fantastic three-dimensional shells of radiolarians are not meticulously "imprinted" by some underlying template or a writing tool, but rather emerge spontaneously from a self-organization process carried out by the organism as a whole. On the most abstract level, such process can be viewed as a chemical "program" comprising the details of the chemical reactions involved, information about the migration/delivery of different substrates, and the initial/boundary conditions of the process. The execution of these "instructions" is synonymous with the task of fabrication.
 In our research, we apply this biologically-inspired idea of "chemical programming" to the fabrication of technologically relevant micro and nanostructures and devices. The "programs" we execute are based on irreversible inorganic reactions and diffusive transport of the participating chemicals through the supporting "soft" medium. Once the reactions are initiated from well-defined spatial locations defined by our Wet Stamping technique, they "execute programs" contained in their chemical kinetics and their transport properties within the supporting medium. The structures that literally build themselves via complex sequences of reaction and diffusion (RD) events represent stable or metastable points in the process where the "programs" achieve global/local equilibrium.

A variety of chemical "programs" our team has "coded" and executed enabled self-organization of microscopic optical (Nature Mater. 2004, Adv. Mater. 2004, Appl. Phys. Lett. 2004, J. Appl. Phys. 2005) and microfluidic (Langmuir 2005) devices; 3D structures in metals, glasses, crystals, and semiconductors (Adv. Mater. 2005a,b, 2006, Chem. Mater. 2006); systems that display molecular-level events in the form of macroscopic patterns (J. Phys. Chem. B 2004, J. Am. Chem. Soc. 2005a, Chaos 2006); supports for cell spreading and motility studies (Nature Methods 2005, J. Am. Chem. Soc. 2005b); and more (Soft Matter, 2006).

On the fundamental level, this approach poses intriguing questions regarding (i) the limits of its spatial resolution (i.e., the smallest discrete features that can result from spatially continuous reaction-diffusion fields) and (ii) the ability to reverse-engineer the final, self-organized state to determine the necessary reactions and initial conditions. To understand these issues, our group has combined experiment with theory (nonlinear kinetics, statistical mechanics) and succeeded in extending the method down to the nanoscale (remarkably, to 20 nm resolution, where the classical RD theories fail. 

 

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