Freestanding Metallic and Polymeric Nanostructures: Directed Self-Assembly

Suspended, high aspect ratio structures can be extremely flexible, to the point where they are overly delicate, difficult, and expensive to fabricate on wafers by standard undercut and release. However, capillary forces that normally destroy such structures can be redirected in ways to directly self-assemble nanoscale structures on top of fabricated wafers. One class of methods reviewed uses capillary forces to transform droplets of liquid polymers into the suspended fibers and membranes. Another set of methods spontaneously grows freestanding intermetallic nanowires at selected locations on a device or wafer. Both approaches are one-step procedures that can be used to directly add three-dimensional (3-D) nanomechanical functionality to waferscale fabricated devices. These methods have the additional desirable property that they can be performed at room temperature in ambient air. The nanoscale structures provide high-resolution features that can be used in place of top-down nanolithography to build up complex 3-D nanoelectromechanical systems (NEMSs) devices and microsystems through templated growth. These new structures and geometries can provide potentially useful functions in novel forms. This point about novel forms and functions is illustrated through the presentation and consideration of possible applications of a few novel microsystems that could be built up from the basic nanostructures. INTRODUCTION: DEFINING SELF-ASSEMBLY AND DIRECTED SELF-ASSEMBLY Self-assembly refers to reactions or transformations that take place spontaneously. This is to say that excess energy in a system drives a transformation of the system from its current state into a lower energy state. Consider the simple case of pushing a ball up a hill which adds potential energy and is a non-spontaneous transformation. Releasing the ball at the top of the hill can produce the spontaneous reaction of falling as the ball seeks the minimum potential energy and can be viewed as self-assembly. In reality, the ball might not reach the minimum for a number of reasons, such as the rolling resistance is too high for the ball to start rolling, or the ball could be trapped in a local depression. A reaction that reaches an intermediate metastable state could also be viewed as a type of self-assembly. A metastable state can have a short or long lifetime depending on the height of the surrounding energy barrier and the magnitude of disturbances (e.g., in the case of a ball, wind, or ground vibrations and in the case of material systems, thermal fluctuations, and the related kinetics). Based on this broad definition of self-assembly, one can classify all systems as either being in equilibrium, being pushed, or held away from equilibrium by applied forces, or in the process of spontaneously returning to equilibrium. Therefore, one might conclude that self-assembly is no more than a thermodynamic or kinetic concept. While this is true physically, there is a philosophical difference. Our engineering view is that self-assembly uses spontaneous transformations in nature to automatically make desired structures. That is, self-assembly is an alternative to the precisely specified (often called, “top-down”) manufacturing of a desired structure [We do not use the term “bottomup” manufacturing to refer to self-assembly, since some self-assemblies are top-down (larger structures evolving into smaller, more precise structures, e.g., spinodal decomposition or capillary thinning of liquid threads) and others are bottom-up (smaller structures evolving into larger structures, e.g., nucleated growth of crystals and raindrops).].We believe that self-assembly methods have the potential to replace top-down nanomanufacturing processes with simpler, faster, and lower-cost processes of comparable uniformity and precision. Of particular interest is the potential to use self-assembly processes that tend to converge toward a single common result. For instance, in polishing or in multilayer deposition, depending on the materials used and the processing conditions, a small defect can either grow or decrease in size with each processing step. Processes such as the latter, where uniformity and precision improves with processing time, are especially appealing. As an example illustrating this definition of selfassembly, growth of silicon boules from a melt, while a tightly controlled growth process, can be considered as a Enironm etal– Fretanding Dekker Encyclopedia of Nanoscience and Nanotechnology, Third Edition DOI: 10.1081/E-ENN3-120050118 1450 Copyright © 2014 by Taylor & Francis. All rights reserved. D ow nl oa de d by [ U ni ve rs ity o f L ou is vi lle ], [ R ob er t C oh n] a t 0 7: 57 3 0 O ct ob er 2 01 4 type of self-assembly that produces a crystalline material with atomic-level perfection. Another example is the formation of micelles in a solution that is saturated with a surfactant, which can produce essentially monodisperse spheres. A liquid jet at a specific diameter and velocity can break up into a continuous stream of monodisperse droplets (via Plateau–Rayleigh instability). Thin films of liquids can dewet from solid surfaces to form periodic arrays of droplets (via Rayleigh–Taylor instability). The dewetting instability proceeds at a preferred spatial frequency (in mathematically analogous form to spinodal decomposition). A few other examples are wellstructured solidification growth fronts due to the constitutional supercooling of metal alloys (e.g., hexagonal cellular growth of Sn–Pb eutectic) and the various (lamellar, spherical, cylindrical, gyroidal) phases of block copolymers. An appealing capability would be the ability to include self-assembly processes in the fabrication of devices. One or more of the fabrication steps might include self-assembly processes that occur at selected locations on the device. We refer to these types of processes as “directed self-assembly.” Especially appealing are those directed self-assembly processes that with approximate or simple directions cause macrostructured materials to evolve into more precise and uniform nanostructures. While there are numerous methods to grow nanostructures, there are far fewer methods to selectively grow or (after growth) place these same nanostructures at user-designated locations on a device. This entry reviews the recent developments in these types of directed self-assemblies. One major class of methods uses viscoelastic extensional flow and capillary thinning of polymeric liquids to form “air-bridges,” which are nanofibers or membranes that are suspended over free-space or air-gaps. A second class of methods stimulates at selected locations the nucleated growth of freestanding metal alloy nanowires. This entry first reviews these methods. Then, we consider the broader use of the structures in the three-dimensional (3-D) fabrication of microand nanoelectromechanical systems (MEMSs and NEMSs). Then, applications of the structures that are enabled by the resulting high aspect ratio structures are considered. SELF-ASSEMBLY STRATEGY I: CAPILLARY FORCE-DRIVEN SELF-ASSEMBLY OF POLYMERIC LIQUIDS Capillary forces, which are well known for causing the undesirable collapse of overly flexible microcantilevers and beams during wet-etching, can be redirected to directly self-assemble the suspended fiber air-bridges from liquid polymers. The basic “brush-on” method, as shown in Fig. 1, can produce well-ordered arrays of nanometer diameter fibers by simply hand-brushing a solution of dissolved polymer over an array of microscale pillars. Due to its large surface area, the initially drawn liquid film is unstable. It breaks up into threads that span the pillars in the direction of brushing. Then, these liquid bridges, which are extended beyond their limit of stability, continue to thin due to capillary forces (specifically, the Laplace pressure, γ/r, where γ is the surface tension of the liquid and r is the radius of the fiber). Rather than breaking, evaporation of the solvent causes the fibers to reach a stable diameter as the polymer solidifies. Very high aspect ratio fibers (sometimes as high as 20,000:1) have been produced this way (Fig. 2A), including a hand-brushed array of fibers of 37 ± 6 nm diameter. The fibers, as shown in Fig. 2A, can often form on the sidewalls of the pillar array. Other interesting nanostructures that resemble trampolines (Fig. 2B) and tennis nets (Fig. 2C) have been formed (by a modified process in which monomers polymerize as the liquid is being brushed over the substrate). The potential of the brush-on approach to be developed into a manufacturable batch fabrication process is illustrated by the last array where over 1000 contiguous tennis-net-shaped membranes formed without an intervening defect. The fabrication process is quite robust, enabling defect-free arrays of identical structures to be produced, even with crude hand applications. The level of uniformity and repeatability demonstrated for the fabrication of nanofiber arrays currently recommends them for prototyping of various suspended MEMS and NEMS devices, especially those that are very flexible. Nanofiber air-bridges have been produced in seconds with numerous types of organic polymers and block copolymers, nanomaterial–polymer composites, and biopolymers—including actin, fibrin, and DNA. En vi ro nm en ta l– Fr ee st an di ng Fig. 1 Self-assembly of nanofiber air-bridges by brushing polymeric liquids over micropillar arrays. The process can yield uniform arrays of structures even when the liquids are brushed over the array by hand. Source: From Rathfon, Yan, et al. © 2011 IEEE. Freestanding Metallic and Polymeric Nanostructures: Directed Self-Assembly 1451 D ow nl oa de d by [ U ni ve rs ity o f L ou is vi lle ], [ R ob er t C oh n] a t 0 7: 57 3 0 O ct ob er 2 01 4