The ultimate challenge of nanotechnology is to control the structure of matter with atomic precision. The better we are at shaping and structuring material on a small scale, the more powerful technology we can dream of. Unfortunately, the atomic scale is entirely out of range for conventional patterning. Researchers now report that they have achieved nanoscale self-assembly within a two-dimensional layer. Dosing of ethylene and borazine near a hot iridium surface, leads for self-organising of a two-dimensional superlattice of graphene dots.
Researchers have now shown that, by varying the shape of magnetite nanoparticles, they can control the nature of the self-assembled structures as the nanoparticles assemble. This new work provides guidelines for the design of new self-assembled materials. Self-assembly of nanoparticles driven by competing forces can result in truly unique structures, the diversity and complexity of which could be particularly striking if the building blocks were simultaneously coupled by short- and long-range forces of different symmetries.
Industrial production processes, by and large, rely on robotic assembly lines that place, package, and connect a variety of disparate components. While the manufacturing world is dominated by robots, there are applications where the established processes of serial 'pick and place' and manipulation of single objects reach scaling limits in terms of throughput, alignment precision, and the minimal component dimension they can handle effectively. By contrast, the emerging methods of engineered self-assembly are massively parallel and have the potential to overcome these scaling limitations.
The long-term vision of revolutionary bottom-up nanotechnology is based on two different concepts of molecular assembly technologies. One follows Nature's blueprint, which uses molecular recognition for self-assembly of nanoscale materials and structures; the other is man-made and uses instruments to assemble nanoscale building blocs into larger structures and devices. In contrast, the most common nanoscale fabrication techniques used today, for instance in the sub 100-nanometer semiconductor industry, are top-down approaches where fabrication technologies such as lithography or stamping are used. Here, you create ever smaller structures by starting with a block of material and remove the bits and pieces you don't want until you get the shape and size you do want. While top-down techniques can be highly parallel (semiconductor industry) it is not feasible to control single molecules with them. Using a hybrid approach that combines the precision of an atomic force microscope with the selectivity of DNA interactions, researchers in Germany have successfully demonstrated a technique that fills the gap between top-down and bottom-up since it allows for the control of single molecules with the precision of atomic force microscopy and combines it with the selectivity of self-assembly.
The key to using self-assembly as a controlled and directed nanofabrication process lies in designing the components that are required to self-assemble into desired patterns and functions. Self-assembly reflects information coded in individual components - characteristics such as shape, surface properties, charge, polarizability, magnetic dipole, mass, etc. These characteristics determine the interactions among the components and the whole essence of self-assembly arises from these dynamic properties. In this respect, many self-assembled nanostructures show to be responsive to external stimuli such as temperature, pH, or solvent polarity. An exciting field for nanotechnology researchers is the challenge of extending the scope of nanostructures with stimulus-responsive properties towards the fabrication of 'smart' nanoscale materials. New work by Korean scientists demonstrates that simple addition of small guest molecules triggers reversible structural transformation. The novelty of this research is that, so far, switching of material properties triggered by external stimuli via nanoscale objects had not been realized yet.
The use of spontaneous self-assembly as a lithography- and external field-free means to construct well-ordered, often intriguing structures has received much attention for its ease of organizing materials on the nanoscale into ordered structures and producing complex, large-scale structures with small feature sizes. These self-organized structures promise new opportunities for developing miniaturized optical, electronic, optoelectronic, and magnetic devices. An extremely simple route to intriguing structures is the evaporation-induced self-assembly of polymers and nanoparticles from a droplet on a solid substrate. However, flow instabilities within the evaporating droplet often result in non-equilibrium and irregular dissipative structures, e.g., randomly organized convection patterns, stochastically distributed multi-rings, etc. Therefore, fully utilizing evaporation as a simple tool for creating well-ordered structures with numerous technological applications requires precise control over several factors, including evaporative flux, solution concentration, and the interfacial interaction between solute and substrate.
Nanotechnology's much-touted notion of bottom-up fabrication - the key concept behind every idea and model of advanced nanotechnologies - so far doesn't have much to do with assembly-line style of manufacturing; rather, it relies on natural self-assembly processes. The stability of covalent bonds enables the chemical synthesis of almost arbitrary configurations of up to 1000 atoms. Larger molecules, molecular aggregates, and forms of organized matter more extensive than molecules cannot be synthesized bond-by-bond. Self-assembly is one strategy for organizing matter on these larger scales. Nevertheless, as a wholly novel way to manufacture and create new materials, self-assembly is of fundamental importance for the future of a myriad of technologies. While there is no doubt self-assembly works, as evidenced by the world around us, scientists have just begun to understand and devise working examples of self-assembly. Much of this work has been at the millimeter and micron scales were it is relatively easy to fabricate components for assembly. A recent paper details a general method, using microcontact printing, for modifying cubic building blocks with nanoscale dimensions. The controlled assembly of metallic nanoparticles remains a challenge and this work provides a novel functional example to study and build upon.
Self-assembly and self-organization are terms used to describe processes in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. Self-organizing processes are common throughout nature and involve components from the molecular (e.g. protein folding) to the planetary scale (e.g. weather systems) and even beyond (e.g. galaxies). Self-assembly has become an especially important concept in nanotechnology. As miniaturization reaches the nanoscale, conventional manufacturing technologies fail because it has not been possible (yet) to build machinery that assembles nanoscale components into functional devices. Until robotic assemblers capable of nanofabrication can be built, self-assembly - together with chemical synthesis - will be the necessary technology to develop for bottom-up fabrication. The key to using self-assembly as a controlled and directed fabrication process lies in designing the components that are required to self-assemble into desired patterns and functions. Self-assembly reflects information coded - as shape, surface properties, charge, polarizability, magnetic dipole, mass, etc. - in individual components; these characteristics determine the interactions among them.