Directed Self-Assembly: Expectations and Achievements
© The Author(s) 2010
Received: 7 May 2010
Accepted: 1 July 2010
Published: 21 July 2010
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© The Author(s) 2010
Received: 7 May 2010
Accepted: 1 July 2010
Published: 21 July 2010
Nanotechnology has been a revolutionary thrust in recent years of development of science and technology for its broad appeal for employing a novel idea for relevant technological applications in particular and for mass-scale production and marketing as common man commodity in general. An interesting aspect of this emergent technology is that it involves scientific research community and relevant industries alike. Top–down and bottom–up approaches are two broad division of production of nanoscale materials in general. However, both the approaches have their own limits as far as large-scale production and cost involved are concerned. Therefore, novel new techniques are desired to be developed to optimize production and cost. Directed self-assembly seems to be a promising technique in this regard; which can work as a bridge between the top–down and bottom–up approaches. This article reviews how directed self-assembly as a technique has grown up and outlines its future prospects.
Nanotechnology promises to revolutionize the way we think about, but more importantly create new materials. The key to making this promise a reality is a commitment to fundamental research in critical areas including synthesis, fabrication, and characterization of nanoscale components. Nanoparticles have attracted wide attention as such components due to their unique size-dependent properties including, superparamagnetism, chemilumiescence, and catalysis. To fully harness the potential capabilities of nanoparticles, we need to develop new methods to assemble them into useful patterns or structures. These self-assembled structures promise new opportunities for developing miniaturized optical, electronic, optoelectronic, and magnetic devices.
As the size of device features becomes increasingly smaller, conventional lithographic processes are limited. Alternative routes need to be developed to circumvent this difficulty. As conventional fabrication technologies, such as optical lithography, develop, they begin to run up against fundamental limits. New measurement methods are needed to understand and help mitigate the effects of those limits. In addition, novel fabrication techniques are required to help extend both the lifetime and range of application of existing techniques. Directed self-assembly is an emergent technology of current interest [1–10]. Directed self-assembly approach is still going through developmental phases, and leverages existing patterning methods by combining them with self-organizing systems, to create manufacturing techniques that can be readily integrated into existing processes. Directed self-assembly technique can be appropriately employed to yield functional nanostructures e.g. nanowires and organized array of nanodots.
As the size scale of device features becomes increasingly smaller, conventional lithographic processes are limited. Alternative routes need to be developed to circumvent this hard stop. As conventional fabrication technologies, such as optical lithography, develop, they begin to run up against fundamental limits. New measurement methods are needed to understand and help mitigate the effects of those limits. In addition, novel fabrication techniques are required to help extend both the lifetime and range of application of existing techniques. Directed self-assembly is one of the emergent technologies which find interest to the researchers currently [1–10]. Directed self-assembly approach is still going through developmental phases and leverages existing patterning methods by combining those with self-organizing systems to create manufacturing techniques that can be readily integrated into existing processes. Directed self-assembly technique can be appropriately employed to yield functional nanostructures e.g. nanowires and arrays of organized nanodots.
Spontaneous self-assembly is introduced as an evaporation-induced phenomenon that yields random patterns. Among guided self-assembly approaches (employing some guiding agent to nanoparticles or vapour of atoms), template-guided and field-guided assemblies are two approaches. For template-guided assembly, existing surface atomic pattern or nano/micro features as templates are made use of. Among field guided assembly, use of pressure gradient, magnetic field, electric field, electron beam, light and laser, etc. are few to count with. The present article reviews the progress so far in the direction of establishment of directed self-assembly as a reproducible and robust technique and its future prospects for its usage at industrial scale.
As shown in Fig. 1a, there is no apparent visible spatial orderliness of nanoparticles in the nickel thin film of thickness 50 nm coated onto borosilicate glass substrate in the process of resistive thermal evaporation. The metal particle size and size distribution depends on the deposition conditions, e.g., electrical power used for thermal evaporation (which determines remaining energy of the adatoms when it lands onto the substrate), wettability offered by the substrate to the thin film material (substrate and thin film material), diffusivity of thin film material atoms on the substrate (substrate temperature) etc.
After transport the substrates and conditions on the substrate surface play an important role in determining the microstructural evolution of the films. It is pertinent at this point to recall the process of condensation of vapor into thin films on substrates . Initially small nuclei, depending on the effective surface energy available, form on the substrate. These satisfy the condition of nucleation (supersaturation ratio > 1), which in turn is dependent on the substrate material itself. Once a few nuclei form, they work as nucleation centers. Coalescence between nuclei occurs, and this finally gives rise to the growth of continuous layers. Nanoparticulate formation in particular can be attributed to the metal–substrate interactions. Energetics decides the contact angle of the condensate onto the substrate, residual strain, and size and shape of the nanoparticles deposited. The capillary model predicts that free energy of formation of condensed aggregate goes through a maximum . With heating of the substrate, densification occurs, and the grain wall boundary width is thinned. At RT deposition conditions, because sufficient energy is not available for mobility of adatoms on the substrate surface, the size is not enhanced much due to coalescence.
Occasionally, some short distance orderlinesses (as shown in Fig. 1b for nickel thin film growth on  silicon substrate by resistive thermal evaporation at room temperature) have been observed in thin films achieved by thermal evaporation; whose origin can be traced in the atomic scale linear edges formed while cutting the substrate in particular plane which virtually works as template for few layers of thin film growth.
The assembly of nanoparticles into ordered architectures is a potential route to achieve further construction and miniaturization of electronic and optical devices. Among guided self-assemblies, (a) template-guided self-assemblies and (b) field-guided self-assemblies are two broad divisions.
Among template-guided self-assemblies, use of physical templates, chemical templates and biological templates are three ways to achieve orientation in the growth features. Physical template has to do with physical existence of ridge, depth, patterning on the substrate surface. Physical templates can be an atomic pattern or ridges, nano/micro-scale pre-existing pattern on the substrate in the form of pores or linear features or two-dimensional architectures. Unsatisfied bonds can in principle work as chemical templates. Use of DNA as a biological template for guided self-assembly has been attracting attention to biochemists and biophysicists.
Advantages with physical template-assisted fabrication of nanowires lie in the fact that they combine fabrication with organization and solve integration issues (eliminating the need to manipulate individual nanowires). Issues related to contacts for electrical and magnetotransport are also solved. Moreover, physical vapour deposition techniques such as evaporation, sputtering and Pulsed Laser Deposition (PLD) are well-known industrially applicable techniques, and hence fabrication of nanowires using these approaches is also expected to be very useful.
Brown et al.  have achieved nanowires inside the lithographically fabricated trenches using nanocluster source. The nanoclustered nanowires usually grow at the apex of the trench (as shown in Fig. 5b).
Gold-tipped CdSe rods (nanodumbbells) were solubilized in an aqueous phase and self-assembled in a head-to-tail manner using biotin disulphide and avidin . The disulphide end of the biotin molecule attaches to the gold tip of the nanodumbbell, and the biotin end of the molecule is able to conjugate to an avidin protein. The avidin can strongly conjugate up to four biotin molecules. Changing the ratios of biotin to nanodumbbells leads to the formation of dimers, trimers, and flowerlike structures. To further improve the distribution of chain lengths, a separation method based upon weight was applied using a concentration gradient. The gold tips provide effective anchor points for constructing complex nanorod structures by self-assembly. Metal-directed self-assembly of two- and three-dimensional synthetic receptors has been reviewed recently .
Among field-guided self-assemblies, use of pressure gradient, electric field, magnetic field, light, laser, etc. are some to count with.
Ordering induced by shear flow can be used  to direct the assembly of particles in suspensions. Flow-induced ordering is determined by the balance between a range of forces, such as direct interparticle, Brownian, and hydrodynamic forces. The latter are modified when dealing with viscoelastic rather than Newtonian matrices. In particular, 1D stringlike structures of spherical particles have been observed to form along the flow direction in shear thinning viscoelastic fluids, a phenomenon not observed in Newtonian fluids at similar particle volume fractions. Here, we report on the formation of freestanding crystalline patches in planes parallel to the shearing surfaces.
Directed self-assembly has been proven to be quite handy for chemists, physicists and biologists alike and more importantly to materials scientists. Various kinds of materials starting from elemental materials to oxides, nitrides, superconductors, magnetic materials, dielectrics have been self-assembled using some template or field as a guide. Cracks, nanopores, V-grooves and various other surface patterns have already been used as physical templates. One such example is the use of carbon nanotube as template for further vapour growth . It has to be kept in mind that lithography itself can conveniently be used to direct the growth. Unsatisfied bonds have been conveniently used as chemical templates. Similarly, biologists too have employed bio-alignments (DNA as template is one example). Electric field, excimer laser, light, magnetic field, pressure gradient, shear gradient and various fields have been employed to achieve functional nanostructures by field-directed self-assembly. Here in this review, use of electric field and laser has been described in detail. However, magnetic field [89, 90] and focused ion beam  are the other two fields as competitive for the purpose of directed self-assembly of materials. Directed self-assembly of nanomaterials as a discipline is quite versatile in nature. Guise et al.  has achieved patterning of sub-10-nm Ge islands on Si(100) by directed self-assembly. Greve et al.  have dealt with the directed self-assembly of amphiphilic regioregular polythiophenes on the nanometer scale.
Xu et al.  have demonstrated directed self-assembly of block copolymers on two-dimensional chemical patterns fabricated by electro-oxidation nanolithography. Gupta et al.  have described the entropy-driven segregation of nanoparticles to cracks in multilayered composite polymer structures. Adam et al.  have used NH···O hydrogen bonding for directed self-assembly and achieved a trilayered supramolecular array formed between 1,2-diaminoethane and benzoic acid. Lee et al.  have achieved self-assembly of 2,6-dimethylpyridine on Cu(1 1 0) directed by weak hydrogen bonding. Kinge et al.  have reviewed self-assembling of nanoparticles at surfaces and interfaces. Sitti  has demonstrated high aspect ratio polymer micro/nanostructure manufacturing using directed self-assembly.
Even though science and technology of directed self-assembly has advanced manifold, this part of nanotechnology as a pursuit of research and development is still young. To industrialize this technique, control of material growth has to be understood at atomic scale. Accurate prediction and tailoring of physical properties at nanoscale are still a challenge.
Author would like to acknowledge the moral support and encouragements from Prof. P. Sen at School of Physical Sciences, Jawaharlal Nehru University, New Delhi, Dr. M. G. Krishna from School of Physics, University of Hyderabad and Prof. C. N. R. Rao at Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bangalore. Facilities at University of Hyderabad were instrumental in carrying out various aspects of research. Funding from University grants Commision (UGC) and Department of Science and Technology (DST), Govt. of India are also acknowledged.
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